Marie Coutand

Martin Cyr

Pierre Clastres

Eric Deydier

Richard Guilet

Characteristics of industrial and laboratory meat and bone meal ashes and their potential applications

Keywords: Meat and bone meal ash, Bottom ash, Fly ash, Calcium phosphate, Hydroxyapatite, Whitlockite, Chemical properties, Chemical composition, Heavy metals, Leaching

come

Introduction

Meat and bone meal (MBM) is a by-product of the rendering industry. The total production of MBM for the European Union (EU) exceeds 3,500,000 tons/year, including more than 700,000 tons in France [1]. In Europe, since May 1, 2003, MBM has been classified with other animal by-products according to whether they come from sources defined as categories 1, 2 or 3 [START_REF]1774/2002 of the European Parliament and of the Council of 3 October 2002 laying down health rules concerning animal by-products not intended for human consumption[END_REF].

-Category 1 includes by-products of animals suspected of being infected by TSE (transmissible spongiform encephalopathy) and specified risk materials (SRM). -Category 2 includes by-products of animals presenting a risk of infection other than TSE, animals that have died in ways other than being slaughtered and animals killed to eradicate an epizootic disease. -Category 3 includes by-products arising from the production of goods intended for human consumption using slaughtered * animals not affected by any sign of diseases transmissible to humans or other animals.

The progressive restriction of the use of MBM (including category 3) in applications, such as animal feed [START_REF]European Commission Decision 94/381/EC of 27 June 1994 concerning certain protection measures with regard to bovine spongiform encephalopathy and the feeding of mammalian derived protein[END_REF][START_REF]forbidding the use of certain proteins of animal origin in the manufacture of feed intended for animals of the bovine species[END_REF][START_REF]extending the ban on the use of certain proteins of animal origin in the feed and the manufacture of feed intended for ruminants[END_REF][START_REF]European Council Decision 2000/766/EC of 4 December 2000 concerning certain protection measures with regard to transmissible spongiform encephalopathies and the feeding of animal protein[END_REF] (except pet food [START_REF]1774/2002 of the European Parliament and of the Council of 3 October 2002 laying down health rules concerning animal by-products not intended for human consumption[END_REF][START_REF]French decree of 6 August 2005 establishing the health rules applicable to certain animal sub-products not intended for human consumption[END_REF]) has led to a significant increase in the amount of MBM to be eliminated. Since June 28, 1996 in France [START_REF]French decree of 28[END_REF], MBM from categories 1 and 2 are co-incinerated through specific channels, mainly in the cement industry. Only MBM coming from animals having TSE are subjected to special industrial incineration [START_REF] Afssa | Les risques sanitaires liés aux différents usages des farines et graisses d'origine animale et aux conditions de leur traitement et de leur élimination[END_REF]. Cement kilns, where the temperature reaches 1450 • C, provide good conditions for complete combustion in terms of temperature and time spent in the kiln [START_REF] Conesa | Thermal decomposition of meat and bone meal[END_REF] and should lead to the destruction of all organic matter, including proteins, such as prions. In 2003, the cement industry incinerated 400,000 tons of MBM as alternative fuel in more than 20 cement plants in France (205,000 tons in 2000, 260,000 tons in 2001 and 350,000 tons in 2002) [1,[START_REF] Observatoire De L'énergie | Consommation de combustibles de substitution dans l'industrie cimentière en[END_REF].

Considering that MBM has a heating value ranging between 13 and 30 MJ/kg [START_REF] Conesa | Thermal decomposition of meat and bone meal[END_REF][START_REF] García-Peña | MBM (meat and bone meal) co-gasification in IGCC technology[END_REF][START_REF] Beck | Effects of sewage sludge and meat and bone meal cocombustion, SCR catalysts[END_REF][START_REF] Aho | Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass[END_REF][START_REF] Chaala | Recycling of meat and bone meal animal feed by vacuum pyrolysis[END_REF][START_REF] Conesa | Dioxin production during the thermal treatment of meat and bone meal residues[END_REF][START_REF] Ayllon | Kinetic study of meat and bone meal pyrolysis: an evaluation and comparison of different possible kinetic models[END_REF][START_REF] Gulyurtlu | Co-combustion of coal and meat and bone meal[END_REF][START_REF] Enda | Dispersion modelling and measurement of emissions from the co-combustion of meat and bone meal with peat in a fluidised bed[END_REF][START_REF] Fryda | Experimental investigation of fluidised bed co-combustion of meat and bone meal with coals and olive bagasse[END_REF][START_REF] Fryda | Meat and bone meal as secondary fuel in fluidized bed combustion[END_REF][START_REF] Skodras | Pyrolysis and combustion behaviour of coal-MBM blends[END_REF], incineration in a thermal plant could be one of the most appropriate methods to eliminate this residue. Several studies available on the thermal behavior of MBM confirm this interest. These studies include specific incineration [START_REF] Conesa | Thermal decomposition of meat and bone meal[END_REF][START_REF] Chaala | Recycling of meat and bone meal animal feed by vacuum pyrolysis[END_REF][START_REF] Conesa | Dioxin production during the thermal treatment of meat and bone meal residues[END_REF][START_REF] Gulyurtlu | Co-combustion of coal and meat and bone meal[END_REF][START_REF] Skodras | Pyrolysis and combustion behaviour of coal-MBM blends[END_REF][START_REF] Mcdonnell | Behaviour of meat and bonemeal/peat pellets in a bench scale fluidised bed combustor[END_REF], co-combustion [START_REF] García-Peña | MBM (meat and bone meal) co-gasification in IGCC technology[END_REF][START_REF] Beck | Effects of sewage sludge and meat and bone meal cocombustion, SCR catalysts[END_REF][START_REF] Aho | Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass[END_REF][START_REF] Gulyurtlu | Co-combustion of coal and meat and bone meal[END_REF][START_REF] Enda | Dispersion modelling and measurement of emissions from the co-combustion of meat and bone meal with peat in a fluidised bed[END_REF][START_REF] Fryda | Experimental investigation of fluidised bed co-combustion of meat and bone meal with coals and olive bagasse[END_REF][START_REF] Fryda | Meat and bone meal as secondary fuel in fluidized bed combustion[END_REF][START_REF] Skodras | Pyrolysis and combustion behaviour of coal-MBM blends[END_REF][START_REF] Mcdonnell | Behaviour of meat and bonemeal/peat pellets in a bench scale fluidised bed combustor[END_REF][START_REF] Beck | The behaviour of phosphorus in flue gases from coal and secondary fuel co-combustion[END_REF][START_REF] Beck | The behaviour of phosphorus in the flue gas during the combustion of high-phosphate fuels[END_REF] and pyrolysis [START_REF] Conesa | Thermal decomposition of meat and bone meal[END_REF][START_REF] Chaala | Recycling of meat and bone meal animal feed by vacuum pyrolysis[END_REF][START_REF] Ayllon | Kinetic study of meat and bone meal pyrolysis: an evaluation and comparison of different possible kinetic models[END_REF][START_REF] Skodras | Pyrolysis and combustion behaviour of coal-MBM blends[END_REF] of MBM. However, it should be kept in mind that incineration always produces ashes that must then be managed. It has been shown that between 100 and 310 kg of ash is produced for each ton of MBM that is incinerated [START_REF] Conesa | Thermal decomposition of meat and bone meal[END_REF][START_REF] García-Peña | MBM (meat and bone meal) co-gasification in IGCC technology[END_REF][START_REF] Beck | Effects of sewage sludge and meat and bone meal cocombustion, SCR catalysts[END_REF][START_REF] Aho | Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass[END_REF][START_REF] Chaala | Recycling of meat and bone meal animal feed by vacuum pyrolysis[END_REF][START_REF] Conesa | Dioxin production during the thermal treatment of meat and bone meal residues[END_REF][START_REF] Ayllon | Kinetic study of meat and bone meal pyrolysis: an evaluation and comparison of different possible kinetic models[END_REF][START_REF] Gulyurtlu | Co-combustion of coal and meat and bone meal[END_REF][START_REF] Enda | Dispersion modelling and measurement of emissions from the co-combustion of meat and bone meal with peat in a fluidised bed[END_REF][START_REF] Skodras | Pyrolysis and combustion behaviour of coal-MBM blends[END_REF][START_REF] Mcdonnell | Behaviour of meat and bonemeal/peat pellets in a bench scale fluidised bed combustor[END_REF][START_REF] Deydier | Beneficial use of meat and bone meal combustion residue: an efficient low cost material to remove lead from aqueous effluent[END_REF][START_REF] Deydier | Physical and chemical characterisation of crude meat and bone meal combustion residue: "waste or raw material?[END_REF]. Thermal treatment of the entire production of MBM in Europe would then produce a large amount of ash (350,000-1,000,000 tons/year). Therefore, the fate of such a stock of ashes is a major environmental concern.

The aim of this paper is to present and compare the chemical, mineralogical, physical and leaching characteristics of meat and bone meal ashes (MBM category 3) obtained from one laboratory and one industrial process, in order to classify these wastes and evaluate possible ways of valorization. This paper reports one of the first exhaustive characterizations of industrial MBM ashes, since almost all other studies found in literature concerned ashes obtained on a laboratory scale.

Materials and methods

The experimental methods used for the physical and chemical characterization of the ashes are summarized in Table 1. The measurement of the physical properties included the density by hydrostatic weighting, the particle size analysis (sieving for MBM-BA and laser diffraction analyzer for other ashes) and the specific surface area (Blaine apparatus, . Electron scanning microscopy (SEM) observations was performed to study the morphology of ashes with JEOL JSM-6380 LV, coupled with EDX analysis. The chemical composition, carried on lithium tetraborate/lithium metaborate flux obtained at 1100 • C, was realized using atomic adsorption with flame atomization (Fe, Na, K, Ca, Mg, Si) and inductively coupled plasma coupled with mass spectrometry (ICP-MS: P, heavy metal and lanthanide). Loss on ignition was measured by calcinations at 1000 • C. Mineralogical properties were obtained by X-ray powder diffractometry using a Siemens D5000 diffractometer equipped with a monochromator and using a K␣ (λ = 1.789 Å) cobalt anticathode. Measurements were made with a 2θ step interval of 0.02 • (5 • to 70 • ) and an acquisition time of 10 s/step. Water absorption coefficient of MBM-BA was realized according to standard EN 1097-6. The coefficient is defined as the mass of water absorbed in open pores of dry-surface saturated aggregate over the dry mass of the material. The evaluation of environmental impact was performed with the European standard EN 12-457. The liquid:solid ratio was 10:1, the extraction medium was demineralised water and the extraction time was 24 h. Measurements on the leachates collected after filtration (0.45 m) included major (Ca, Si, Fe, Na, K, Mg) and trace (Ti, V, Cr, Ni, Cu, Zn, As, Cd, Sb, Ba, Pb) element contents using atomic adsorption and ICP-MS, respectively.

Three kinds of ashes were studied, two from an industrial combustion process and one incinerated at laboratory scale. The industrial ashes were bottom ash (MBM-BA) and fly ash (MBM-FA) coming from an incineration plant equipped with an industrial rotary furnace (12 m long). This incinerator has a capacity of 2 tons/h and the maximum temperature reached near the flame is 1000 • C. The incinerator was fed with 95% of meat and bone meal from pork production (category 3), the remaining 5% being composed of other waste, such as plastic bags or sewage sludge. The remaining amount of residue ranged between 10 and 15% by mass of MBM. Two batches of ashes were used and mixed together in order to take into account of the variability of industrial production.

Flue gases (45,000 N m 3 /h) were treated with sodium bicarbonate at the incineration plant before their recovery. Stream treatment produced a significant amount of soluble salts, reaching almost 50% by mass of the total MBM fly ash (MBM-FA). So it was decided to study the crude ash and its washed part separately. MBM-FA was washed with deionized water for 20 min in a stirred container, using a water:ash ratio of 20. The filtered residue was then dried at 40 • C. The filtrate was evaporated and analysis of the residual solid showed that it contained essentially Na + , K + , SO 4 2-, Cl -and some heavy metals (total of 430 mg/kg, mostly Zn-350 mg/kg). Salts could be recovered for industrial applications, as is done for wastewater from the washing of municipal solid waste incineration (MSWI) fly ash [START_REF] Mulder | Pre-treatment of MSWI fly ash for useful application[END_REF][START_REF] Mangialardi | Disposal of MSWI fly ash through a combined washing-immobilisation process[END_REF][START_REF] Aubert | Utilization of municipal solid waste incineration (MSWI) fly ash in blended cement. Part 1: processing and characterization of MSWI fly ash[END_REF].

The properties of the industrial ashes were compared to those of a laboratory ash (MBM-LA). MBM-LA was the incineration residue of meat and bone meal from slaughterhouse waste, combusted in a laboratory electric furnace at 850 • C in air atmosphere. The incineration process was operated in two steps: an initial combustion of 3 h at 600 • C to eliminate the organic fraction of the material, and a second combustion from 25 to 850 • C at a rate of 2 • C/min. During combustion, MBM particles melted and stuck together, inducing pyrolysis of organic matter trap inside. The first combustion gave a black residue (carbon-rich). This residue was mixed manually before a second combustion in order to complete decomposition and obtain clear ashes. Amount of residue represent nearly 24% weight of initial MBM.

Results and discussion

Chemical and mineralogical properties

The chemical compositions of MBM-BA, crude and washed MBM-FA and MBM-LA are given in Table 2 (major elements) and Table 3 (trace elements). Due to the high combustion temperature, the MBM ashes were composed of mineral matter.

Major elements

Industrial ashes (MBM-BA and washed MBM-FA) and laboratory ash (MBM-LA) had high calcium and phosphorus contents (Table 2), up to 47% of the total ash quantity. These results were expected, since the major non-organic fraction of MBM is bone, the mineral part of which is composed of calcium phosphates (mainly hydroxyapatite). MBM-LA had the highest Ca and P contents (47%), probably due to the absence of impurities in the specific incineration process. The lower contents for the industrial ashes may be explained by the fact that up to 5% of other wastes were incinerated with MBM in the industrial incineration plant. Other elements (Si, Al, Mg, Fe, Na, K, SO 3 2-and Cl -) made up 8% of MBM-LA, 13% of MBM-BA and 16% of washed MBM-FA.

Only crude MBM-FA had a notably different composition from other ashes, since it contained a significantly high amount of sulfates, chlorides and alkalis (Na and K). These elements are present as soluble salts in water and they represented almost 50% by mass of the total ash.

Fig. 1 gives a ternary representation of the chemical compositions of the four MBM ashes, compared with the compositions of other MBM ashes found in the literature [START_REF] García-Peña | MBM (meat and bone meal) co-gasification in IGCC technology[END_REF][START_REF] Beck | Effects of sewage sludge and meat and bone meal cocombustion, SCR catalysts[END_REF][START_REF] Aho | Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass[END_REF][START_REF] Fryda | Experimental investigation of fluidised bed co-combustion of meat and bone meal with coals and olive bagasse[END_REF][START_REF] Fryda | Meat and bone meal as secondary fuel in fluidized bed combustion[END_REF][START_REF] Deydier | Physical and chemical characterisation of crude meat and bone meal combustion residue: "waste or raw material?[END_REF][START_REF] Cyr | Low risk meat and bone meal (MBM) bottom ash in mortars as sand replacement[END_REF]. Except for the crude MBM-FA, it can be seen that the ashes from this study had similar compositions with other MBM ashes from different origins. It is interesting to note that the calcium and phosphorus contents in MBM ashes (25-29% Ca and 15-19% P) were of the same order of magnitude with the average contents found by other authors [START_REF] Ma | Lead immobilisation from aqueous solutions and contaminated soils using phosphate rocks[END_REF][START_REF] Martin | TTPIXE analysis of phosphate rocks and phosphogypsum[END_REF][START_REF] Ogunleye | Radioactivity and heavy metal composition of Nigerian phosphate rocks: possible environmental implications[END_REF][START_REF] Tinge | Cadmium and chromium present in calcium sulfate dehydrate produced by the nitrophosphate process[END_REF][START_REF] Da Conceicao | Radionuclides, heavy metals and fluorine incidence at Tapira phosphate rocks, Brazil, and their industrial (by) products[END_REF] in natural phosphate rocks (35 ± 2% Ca and 15 ± 1% P).

Trace elements

Table 3 gives the trace element contents of MBM ashes. The total amounts made up less than 0.6% in all cases, since they reached 2200, 2800, 5500 and 960 mg/kg for MBM-BA, crude MBM-FA, washed MBM-FA and MBM-LA, respectively. Zinc was the main trace element of MBM-FA (0.1-0.3%) and MBM-LA (<0.1%), whereas Ti was the main one of MBM-BA (<0.1%). As expected, trace element analysis showed that meat and bone meal incinerated with other wastes (industrial ashes) contained much more hazardous element, mainly heavy metal, than meat and bone meal obtained from specific incineration (laboratory ash). In the case of industrial ashes (MBM-BA and MBM-FA), most trace elements and especially heavy metals, were present in higher concentrations in fly ash. Moreover, washed MBM-FA contained more trace elements than crude MBM-FA, indicating that heavy metals were mainly present as insoluble species.

A comparison of these results with those of literature [START_REF] Cyr | Low risk meat and bone meal (MBM) bottom ash in mortars as sand replacement[END_REF][START_REF] Ma | Lead immobilisation from aqueous solutions and contaminated soils using phosphate rocks[END_REF][START_REF] Martin | TTPIXE analysis of phosphate rocks and phosphogypsum[END_REF][START_REF] Ogunleye | Radioactivity and heavy metal composition of Nigerian phosphate rocks: possible environmental implications[END_REF][START_REF] Tinge | Cadmium and chromium present in calcium sulfate dehydrate produced by the nitrophosphate process[END_REF][START_REF] Da Conceicao | Radionuclides, heavy metals and fluorine incidence at Tapira phosphate rocks, Brazil, and their industrial (by) products[END_REF][START_REF] Aubert | Use of municipal solid waste incineration fly ash in concrete[END_REF][START_REF] Coutand | Use of sewage sludge ash (SSA) as mineral admixture in cement-based materials[END_REF][START_REF] Clark | The composition of the earth's crust[END_REF][START_REF] Rankama | Isotope Geology[END_REF][START_REF] Mason | Principles of Geochemistry[END_REF][START_REF] Turekian | Distribution of the elements in some major units of the earth's crust[END_REF][START_REF] Vinogradov | Average contents of chemical elements in the principal types of igneous rocks of the earth's crust[END_REF][START_REF] Taylor | The abundance of chemical elements in the continental crust-a new table[END_REF][START_REF] Shacklette | Element concentrations in soils and other surficial materials of the conterminous United States[END_REF][START_REF] Quinby-Hunt | Elemental geochemistry of black shales-statistical comparison of low-calcic shales with other shales[END_REF][START_REF] Alloway | Heavy Metals in Soils[END_REF][START_REF] Merian | Metals and Their Compounds in the Environment: Occurrence, Analysis and Biological Relevance[END_REF][START_REF] Baize | Teneurs totales en éléments traces métalliques dans les sols (France), Références et stratégies d'interprétation[END_REF] showed that MBM ashes from this study: Fig. 2. Heavy metal contents of MBM ashes compared to natural phosphate rocks [START_REF] Ma | Lead immobilisation from aqueous solutions and contaminated soils using phosphate rocks[END_REF][START_REF] Martin | TTPIXE analysis of phosphate rocks and phosphogypsum[END_REF][START_REF] Ogunleye | Radioactivity and heavy metal composition of Nigerian phosphate rocks: possible environmental implications[END_REF][START_REF] Tinge | Cadmium and chromium present in calcium sulfate dehydrate produced by the nitrophosphate process[END_REF][START_REF] Da Conceicao | Radionuclides, heavy metals and fluorine incidence at Tapira phosphate rocks, Brazil, and their industrial (by) products[END_REF] and typical soils and rocks [START_REF] Clark | The composition of the earth's crust[END_REF][START_REF] Rankama | Isotope Geology[END_REF][START_REF] Mason | Principles of Geochemistry[END_REF][START_REF] Turekian | Distribution of the elements in some major units of the earth's crust[END_REF][START_REF] Vinogradov | Average contents of chemical elements in the principal types of igneous rocks of the earth's crust[END_REF][START_REF] Taylor | The abundance of chemical elements in the continental crust-a new table[END_REF][START_REF] Shacklette | Element concentrations in soils and other surficial materials of the conterminous United States[END_REF][START_REF] Quinby-Hunt | Elemental geochemistry of black shales-statistical comparison of low-calcic shales with other shales[END_REF][START_REF] Alloway | Heavy Metals in Soils[END_REF][START_REF] Merian | Metals and Their Compounds in the Environment: Occurrence, Analysis and Biological Relevance[END_REF][START_REF] Baize | Teneurs totales en éléments traces métalliques dans les sols (France), Références et stratégies d'interprétation[END_REF].

-contained more trace elements than other MBM ashes found in literature, especially for industrial ashes. For instance, the hazardous element content of MBM bottom ash studied by Cyr and Ludmann [START_REF] Cyr | Low risk meat and bone meal (MBM) bottom ash in mortars as sand replacement[END_REF] was nearly 375 mg/kg. This difference could be related to the amount of other waste incinerated with meat and bone meal. -Had much lower hazardous element contents than other incinerated wastes, such as municipal solid waste incineration fly ash (MSWI-FA) or sewage sludge ash (SSA), which contain more than 46,000 mg/kg [START_REF] Aubert | Use of municipal solid waste incineration fly ash in concrete[END_REF] and 16,000 mg/kg [START_REF] Coutand | Use of sewage sludge ash (SSA) as mineral admixture in cement-based materials[END_REF], respectively. -Had heavy metal concentrations of the same order of magnitude as those found in natural phosphate rocks [START_REF] Ma | Lead immobilisation from aqueous solutions and contaminated soils using phosphate rocks[END_REF][START_REF] Martin | TTPIXE analysis of phosphate rocks and phosphogypsum[END_REF][START_REF] Ogunleye | Radioactivity and heavy metal composition of Nigerian phosphate rocks: possible environmental implications[END_REF][START_REF] Tinge | Cadmium and chromium present in calcium sulfate dehydrate produced by the nitrophosphate process[END_REF][START_REF] Da Conceicao | Radionuclides, heavy metals and fluorine incidence at Tapira phosphate rocks, Brazil, and their industrial (by) products[END_REF] or typical natural soils and rocks [START_REF] Clark | The composition of the earth's crust[END_REF][START_REF] Rankama | Isotope Geology[END_REF][START_REF] Mason | Principles of Geochemistry[END_REF][START_REF] Turekian | Distribution of the elements in some major units of the earth's crust[END_REF][START_REF] Vinogradov | Average contents of chemical elements in the principal types of igneous rocks of the earth's crust[END_REF][START_REF] Taylor | The abundance of chemical elements in the continental crust-a new table[END_REF][START_REF] Shacklette | Element concentrations in soils and other surficial materials of the conterminous United States[END_REF][START_REF] Quinby-Hunt | Elemental geochemistry of black shales-statistical comparison of low-calcic shales with other shales[END_REF][START_REF] Alloway | Heavy Metals in Soils[END_REF][START_REF] Merian | Metals and Their Compounds in the Environment: Occurrence, Analysis and Biological Relevance[END_REF][START_REF] Baize | Teneurs totales en éléments traces métalliques dans les sols (France), Références et stratégies d'interprétation[END_REF] (Fig. 2).

Mineralogical composition

The mineralogical characteristics of the four MBM ashes are given in Fig. 3. As stated earlier, crude MBM-FA has a different composition compared to other ashes, due to the presence of a significant amount of soluble salts. Industrial ashes (MBM-BA and washed MBM-FA) are mainly composed of whitlockite (␤TCP) Ca 3 (PO 4 ) 2 and hydroxyapatite Ca 5 (PO 4 ) 3 (OH), while laboratory ash (MBM-LA) contains only hydroxyapatite. Fig. 3 gives the main different peaks between these two minerals. These results are in accordance with the chemical compositions (Table 2) reported earlier, since calcium phosphates are the main components of these ashes. However, Ca/P molar ratios (1.38, 1.24 and 1.16 for MBM-BA, washed MBM-FA and MBM-LA, respectively) are lower than the values for hydroxyapatite (1.67) and whitlockite (1.5). These lower ratios could be due to either the presence of calcium-deficient apatite, as observed by Deydier et al. [START_REF] Deydier | Physical and chemical characterisation of crude meat and bone meal combustion residue: "waste or raw material?[END_REF] in their IR structural analysis of MBM ashes, and/or the existence of other compounds containing phosphorus.

Moreover, it was observed that MBM-BA was a blend of white, brown and black grains. XRD analysis of these white and black grains showed they contained essentially hydroxyapatite and whitlockite, respectively (Fig. 4). The brown grains were mixtures of these two minerals, with an XRD pattern similar to that of the bottom ash as a whole (Fig. 3). This composition is similar to the one given by Cyr and Ludmann [START_REF] Cyr | Low risk meat and bone meal (MBM) bottom ash in mortars as sand replacement[END_REF], who worked on MBM-BA from the same incinera-tion plant. Only the proportions of minerals were different, thus confirming the variability inherent in any waste material.

The difference of mineralogical composition between these ashes could be explained by the presence of other waste material in the industrial ashes, and/or by the various incineration processes used as well as the various operating temperatures of incineration. Many authors [START_REF] Monma | Properties of hydroxyapatite prepared by the hydrolysis of tricalcium phosphate[END_REF][START_REF] Young | Reversible high temperature exchange of carbonate and hydroxyl ions in tooth enamel and synthetic hydroxyapatite[END_REF][START_REF] Wu | Sintering of hydroxylapatite-zirconia composite materials[END_REF][START_REF] Ishikawa | Determination of the Ca/P ratio in calcium-deficient hydroxyapatite using X-ray diffraction analysis[END_REF][START_REF] Barralet | Thermal decomposition of synthesised carbonate hydroxyapatite[END_REF] have reported the production of whitlockite/apatite mixtures from the heating of non-stoichiometric hydroxyapatite above 800 • C. This was con-firmed on our products with a sample of MBM-LA (which did not contain whitlockite after laboratory incineration at 850 • C) combusted at 1100 • C for 1 h. Fig. 5, which compares the XRD patterns of MBM-LA incinerated at 850 and 1100 • C, shows the appearance of whitlockite peaks for the ash combusted at the higher temperature.

Physical properties

Meat and bone meal bottom ash (MBM-BA) has the same size as sand particles (Fig. 6), ranging from 0.04 to 4 mm, with a mean diameter (D 50 ) of 0.58 mm (Table 4). Its particle size distribution, measured by sieving, is given in Fig. 7. Some rare grains having a large diameter (over 30 mm) were intentionally removed. As seen on the SEM observations (Fig. 8), MBM-BA particles are mostly heterogeneous, porous and have an irregular form. The large open porosity of the ash leads to a water absorption coefficient (EN 1097-6) of 9%, which is high compared to the value for siliceous or calcareous sands (<1%). Crude and washed fly ashes (MBM-FA) are significantly finer than bottom ash, since their particle sizes range between 1 and 200 m (Fig. 7), with mean diameters of 20 and 70 m, respectively (Table 4).

The coarser particles of washed MBM-FA are probably due to the agglomeration of grains during the drying process. This agglomeration may also explain the large decrease in the Blaine specific surface area: 8000 and 1100 cm 2 /g for crude and washed MBM-FA, respectively (Table 4).

Fig. 9 shows the morphology of MBM-FA. As for MBM-BA, washed MBM-FA is composed of heterogeneous grains having an irregular shape. Only a few particles show a spherical form. SEM observations of crude and washed MBM-FA (Fig. 9) brought out a similar morphology, except for the grain surfaces, which had thin deposits mostly, composed of S, Cl, K and Na (EDX analysis on Fig. 9).

Meat and bone meal laboratory ash (MBM-LA) had a broad particle size distribution (Fig. 7) ranging between 1 m and 2 mm. The grains were heterogeneous and angular (Fig. 10), but no spherical particles were found. This could be due to the incineration process, which was very different than for the industrial by-products (industrial rotary furnace).

The average densities of MBM-BA (2940 kg/m 3 ), washed MBM-FA (2880 kg/m 3 ), MBM-LA (3040 kg/m 3 ) and crude MBM-FA (2610 kg/m 3 ) are in accordance with their mineralogical compositions. MBM-BA and washed MBM-FA are a mixture of hydroxyapatite (3200 kg/m 3 ) and whitlockite (2860 kg/m 3 ), while MBM-LA mainly contains hydroxyapatite. The lower density of crude MBM-FA is due to the presence of salts having lower densities (2200 and 1990 kg/m 3 for NaCl and KCl, respectively).

Leaching behavior

Table 5 reports the concentrations of leached elements for the ashes according to European standard EN 12-457. MBM-LA, obtained from specific incineration, released much smaller amounts of heavy metals than other ashes. Moreover, it can be observed that, for MBM-BA, washed MBM-FA and MBM-LA, the leachate quantities were quite low compared to the total amount of elements initially present in the ashes (Table 3). Only crude MBM-FA released a significantly high amount of heavy metals, due to their higher solubility.

As these ashes are usually considered as waste materials, these results can be compared to the threshold values given in European regulations [START_REF]European Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste[END_REF][START_REF]EC of 19 December 2002 establishing criteria and procedures for the acceptance of waste at landfills[END_REF] concerning the classification of waste to be landfilled. Table 5 categorizes the ashes according to the amount of element leached in regard to European waste acceptance criteria for landfills.

The small quantities of elements leached by MBM-LA (laboratory ash) allowed us to classify this ash as inert waste. MBM-BA (industrial bottom ash) can be considered as inert waste too, except as far as chromium (Cr) is concerned. For washed MBM-FA, the leached concentrations of seven elements are sufficiently low to allow them to be included in the inert category. Only zinc (Zn) and antimony (Sb) are in the non-hazardous (nh) and hazardous (h) categories, respectively. Finally, in the Table 5 Leaching behavior (EN 12-457) of MBM-BA, crude and washed MBM-FA and MBM-LA (g/kg), compared to European waste acceptance criteria in landfill [START_REF]European Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste[END_REF][START_REF]EC of 19 December 2002 establishing criteria and procedures for the acceptance of waste at landfills[END_REF] Waste acceptance criteria [START_REF]European Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste[END_REF][START_REF]EC of 19 December 2002 establishing criteria and procedures for the acceptance of waste at landfills[END_REF] MBM ashes case of crude MBM-FA, a lot more elements are released compared to the other ashes. The release of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni) and lead (Pb) leads to a classification of crude MBM-FA as non-hazardous (nh) waste (hazardous for arsenic). Antimony (Sb) and zinc (Zn) contents are higher than the hazardous (h) criteria. In that case, stabilization or a washing treatment is necessary for these two elements. This preliminary environmental study highlights the advantage of specific incineration (MBL-LA) compared to co-incineration. From above, it can be assumed that, except for crude MBM-FA, ways of reusing these ashes other than disposal in landfill could probably be envisaged.

Inert (i) Non-

Preliminary evaluation of reuse of MBM ashes

According to the major characteristics found in this study, a few examples of the possible reuse MBM ashes are given below:

• Use of MBM ashes as raw materials for the phosphate industry Since the composition, and especially the amount of phosphorus, is similar to the one found in natural phosphate rocks, and the heavy metal content (Pb or Cd) is often lower, MBM ashes could probably be used as a phosphorus source for the industry (phosphoric acid production, manufacturing of fertilizer, animal food, washing powder . . .). Production of phosphoric acid from laboratory MBM ashes has been demonstrated at a laboratory scale [START_REF] Deydier | Les cendres issues de l'incinération de farines animales: déchets ou produits valorisables?[END_REF]. Re-utilization of ashes in fertilizer manufacture could also be possible, since nearly 90% of the phosphoric acid production is used for the manufacture of fertilizer. At last, fertilizing action of soil amended with crude ashes has been observed on tobacco elongation [START_REF] Deydier | Evaluation of meat and bone meal combustion residue as lead immobilizing material for in situ remediation of polluted aqueous solutions and soils: "chemical and ecotoxicological studies[END_REF], probably induced by high phosphate content. Additional studies will be carried out to investigate the influence of combustion condition on the formation of phosphate phases during ash incineration, in order to control phosphates dissolution, and thus phosphates biodisponibility in soil.

• Use of MBM ashes to stabilize waste by phosphatation processes Phosphatation processes are used to chemically stabilize heavy metals contained in materials, such as polluted soils and waste [START_REF] Zupancic | Comparison of various phosphate stabilisation agents for the immobilisation of Ni and Zn in sewage sludge[END_REF]. These heavy metals usually precipitate in new mineral forms, which are less leachable.

• Use of MBM ashes to remove and immobilize heavy metals Deydier et al. [START_REF] Deydier | Beneficial use of meat and bone meal combustion residue: an efficient low cost material to remove lead from aqueous effluent[END_REF] studied the potential for immobilizing lead from aqueous effluents by means of MBM ashes. It was found that, due to its apatite content, MBM ash could remove 257 mg lead/(g ash) in less than 3 h, with a remaining lead concentration in solution lower than 0.1 ppb. Mouchet et al. [START_REF] Mouchet | Assessment of lead ecotoxicity in water using the amphibian larvae (Xenopus laevis) and preliminary study of its immobilization in meat and bone meal combustion residues[END_REF] showed, using Xenopus laevis, that MBM ash led to a reduction or an inhibition of the toxic and genotoxic potential of lead in water. This research could be extended to other hazardous metals, such as cadmium or mercury, and to other applications requesting immobilization of metals. The study of other applications will imply the evaluation of the sorp-tion stability of heavy metal under various environmental conditions, such as low or high pH (for instance in concrete).

• Use of MBM ashes in construction materials

Cyr and Ludmann [START_REF] Cyr | Low risk meat and bone meal (MBM) bottom ash in mortars as sand replacement[END_REF] and Collins [START_REF] Collins | Feasibility of producing concrete products from meat and bone meal ash[END_REF] showed the potential of using MBM as a substitute for sand in mortar and concrete. However, considering their low amounts of Si, MBM ashes cannot be considered as pozzolanic material. Technological and environmental assessments are still needed before any industrial application. This will represent a part of our future work. Other uses, such as geotechnical and roadwork applications could also be envisaged.

Conclusion

This paper has presented a characterization of four meat and bone meal (MBM) ashes from different sources. The following results were found:

• Three of the ashes -MBM-LA (laboratory ash), MBM-BA (industrial bottom ash) and washed MBM-FA (industrial fly ash washed with water) -contain up to 47% of calcium and phosphorus, mainly as whitlockite and hydroxyapatite. Trace element contents are below 0.6% and industrial ashes contain much more hazardous element than laboratory ash. Major and trace compositions are near those found for natural phosphate rocks. The amounts of leached elements are low, especially for laboratory ash. • Crude MBM-FA has a quite different composition than other ashes, since it has a significantly high content of sulfates, chlorides and alkalis (Na and K). These elements are present as soluble salts produced by the stream treatment at the plant and they represent almost 50% by mass of the total ash. High amounts of heavy metals are leachable. • The results of the leaching tests highlighted the benefit of specific incineration of meat and bone meal. According to the European classification of waste to be landfilled, the laboratory ash MBM-LA can be classified as inert waste. MBM-BA and washed MBM-FA are mostly inert. Only crude MBM-FA is highly leachable and needs stabilization or washing treatment to enter the category of hazardous waste at least.

According to the characterization results presented in this paper, other possibilities than landfill seem possible for giving value to the ashes, mainly because of the low hazardous element content of three of the four ashes. However, extensive environmental and technical characterizations of these ashes would be required before any application.

Fig. 3 .

 3 Fig. 3. XRD pattern of MBM-BA, crude and washed MBM-FA and MBM-LA.

Fig. 4 .

 4 Fig. 4. XRD patterns of white and black grains from MBM-BA.

Fig. 5 .

 5 Fig. 5. XRD patterns of MBM-LA incinerated at 850 and 1100 • C.

Fig. 6 .

 6 Fig. 6. Industrial meat and bone meal bottom ash (MBM-BA).

Fig. 7 .

 7 Fig. 7. Particle size distribution of industrial ashes (MBM-BA and MBM-FA) and laboratory ash (MBM-LA).

Fig. 8 .

 8 Fig. 8. Scanning electron microscopy of MBM-BA particles.

Fig. 9 .

 9 Fig. 9. Scanning electron microscopy and EDX analysis of crude and washed MBM-FA particles.

Fig. 10 .

 10 Fig. 10. Scanning electron microscopy of MBM-LA particles.

Table 1 Experimental methods for the physical and chemical characterization of MBM ashes

 1

	Property	Test method/standard
	Density	Hydrostatic weighting
	Particle size distribution	Sieving (MBM-BA), laser granulometry in ethanol (MBM-FA and MBM-LA)
	Specific surface area	Blaine (NF EN 196-6)
	Morphology	Secondary electron microscopy (SEM-JEOL JSM-6380 LV), coupled with elemental
		analysis (EDX)
	Water absorption coefficient	Ratio of the mass of water absorbed in open pores of dry-surface saturated aggregate to the
		dry mass of the material (NF EN 1097-6)
	Chemical analysis
	Tetraborate metaborate lithium flux at 1100 • C
	-Fe, Na, K, Ca, Mg, Si	Atomic absorption with flame atomization
	-P, heavy metal and lanthanide	Inductively coupled plasma-mass spectrometry (ICP-MS)
	Other techniques
	-Chlorides	Potentiometric titration with silver nitrate
	-Sulphates	Gravimetric analysis

Mineralogy

X-ray diffraction (XRD), Siemens D5000, Co K␣ radiation (λ = 1.789 Å) 2θ step interval of 0.02 • (5 • to 70 • ) and acquisition time of 10 s Leaching behavior European Standard EN 12457-2, ratio of liquid:solid, 10:1; extraction medium, demineralised water; one extraction time of 24 h

Table 2

 2 Chemical compositions of MBM-BA, crude and washed MBM-FA and MBM-LA (wt.%)

		Ca	P	Si	Al	Mg	Fe	Na	K	SO 4	2-	Cl -	LOI
	MBM-BA	28.8	16.2	3.7	1.1	1.2	1.5	2.7	1.7	0.8	0.3	0.3
	MBM-FA (crude)	12.7	7.8	1.5	tr	0.5	0.5	15.5	8.1	16.6	11.2	8.6
	MBM-FA (washed)	24.6	15.4	3.6	0.9	1.3	1.4	4.4	2.5	1.7	0.7	5.8
	MBM-LA	28.2	18.9	0.7	tr	0.7	0.1	2.4	1.5	0.4	2.0	3.6

LOI: loss on ignition; tr: trace.

Table 3 Trace

 3

		Ba	Bi	Cd	Ce	Co	Cr	Cs	Cu	Dy	Er
	MBM-BA	208.4	0.5	0.3	10.0	9.5	136.2	1.5	189.5	0.7	0.4
	MBM-FA (crude)	158.0	0.5	1.7	4.9	73.5	115.3	1.5	133.2	0.3	0.2
	MBM-FA (washed)	184.6	0.5	0.4	9.5	24.3	155.3	2.0	213.7	0.6	0.4
	MBM-LA	120.7	<4.10 -3	0.3	0.8	3.7	20.8	1.1	46.6	0.1	5.10 -2
		Ga	Gd	Ge	Hf	Ho	La	Lu	Nb	Nd	Ni
	MBM-BA	3.5	0.7	<3.3	0.1	0.2	17.8	0.1	1.7	4.2	93.0
	MBM-FA (crude)	3.7	0.3	<3.3	0.0	0.1	9.5	0.0	2.9	1.7	97.0
	MBM-FA (washed)	6.3	0.6	<3.3	0.1	0.1	16.5	0.1	2.7	4.0	119.9
	MBM-LA	1.9	0.1	0.1	<8 × 10 -3	0.0	25.5	0.0	1.6	0.3	78.4
		Pb	Pr	Rb	Sb	Sc	Sm	Sn	Sr	Ta	Tb
	MBM-BA	<19.3	1.0	13.7	8.7	25.6	0.9	10.1	273.8	<1 × 10 -3 0.1
	MBM-FA (crude)	<19.3	0.4	93.3	12.1	26.9	<4.10 -2 22.8	104.1	<1 × 10 -3 3.10 -2
	MBM-FA (washed)	<19.3	1.0	29.9	36.9	31.5	0.7	48.3	237.0	<1 × 10 -3 0.1
	MBM-LA	<19.3	0.1	13.1	4.6	17.8	1.10 -2	1.5	172.1	1.10 -2	6.10 -3
		Th	Ti	Tl	Tm	U	V	Y	Yb	Zn	Zr
	MBM-BA	1.0	725.4	0.1	4.10 -2	1.2	206.1	4.7	0.4	262.0	2.8
	MBM-FA (crude)	0.7	385.9	5 × 10 -2	<4 × 10 -3	0.4	177.6	1.8	0.2	1349.0	2.5
	MBM-FA (washed)	1.1	842.2	0.1	4.10 -2	1.1	197.3	4.1	0.4	3372.8	2.6
	MBM-LA	0.6	64.3	8 × 10 -3	0.1	0.2	11.8	0.4	0.1	373.1	1.6

elements in MBM-BA, crude and washed MBM-FA and MBM-LA (mg/kg) Fig.

1

. Ternary diagram of MBM ash and phosphate rock compositions

[START_REF] García-Peña | MBM (meat and bone meal) co-gasification in IGCC technology[END_REF][START_REF] Beck | Effects of sewage sludge and meat and bone meal cocombustion, SCR catalysts[END_REF][START_REF] Aho | Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass[END_REF][START_REF] Fryda | Experimental investigation of fluidised bed co-combustion of meat and bone meal with coals and olive bagasse[END_REF][START_REF] Fryda | Meat and bone meal as secondary fuel in fluidized bed combustion[END_REF][START_REF] Deydier | Physical and chemical characterisation of crude meat and bone meal combustion residue: "waste or raw material?[END_REF][START_REF] Cyr | Low risk meat and bone meal (MBM) bottom ash in mortars as sand replacement[END_REF][START_REF] Ma | Lead immobilisation from aqueous solutions and contaminated soils using phosphate rocks[END_REF][START_REF] Martin | TTPIXE analysis of phosphate rocks and phosphogypsum[END_REF][START_REF] Ogunleye | Radioactivity and heavy metal composition of Nigerian phosphate rocks: possible environmental implications[END_REF][START_REF] Tinge | Cadmium and chromium present in calcium sulfate dehydrate produced by the nitrophosphate process[END_REF][START_REF] Da Conceicao | Radionuclides, heavy metals and fluorine incidence at Tapira phosphate rocks, Brazil, and their industrial (by) products[END_REF]

.

Table 4

 4 Physical properties of MBM-BA, crude and washed MBM-FA and MBM-LA

	Ashes	D 50 (m)	Density (kg/m 3 )	Color	Specific surface area blaine (m 2 /kg)
	MBM-BA	580	2940	White, black and brown	-
	Crude MBM-FA	20	2610	White	800
	Washed MBM-FA	70	2880	Light brown	110
	MBM-LA	300	3040	Light brown	290


N Ibaseta

B Biscans

Ultrafine Aerosol Emission from the Free Fall of TiO2 and SiO2 Nanopowders †

Keywords: Nanopowder, Aerosol, Free-Falling powder, ELPI, Dustiness

Due to the increasing production and development of nanoparticles, it has become necessary to control the exposure to ultrafine particles when handling nanopowders. The use of dustiness tests makes it possible to compare the ability of a given powder to re-suspend particles, and to determine the effect of the different external (powder handling method) or internal parameters (powder properties). A dustiness test associated with an electrical low-pressure impactor (ELPI) device is proposed to study the free fall of nanopowders. Titanium dioxide (TiO2) and fumed silica (SiO2) are the studied nanopowders. The free falling of nanopowders in the test chamber generates bimodal aerosols corresponding to the re-suspension of the micrometric agglomerates that constitute the nanopowders and to the breakage and/or erosion of these agglomerates leading to ultrafine aggregates. The presence of ultrafine aggregates was checked by scanning electron microscopy (SEM). When the height of fall and the dropped mass of the powders are increased, the aerosol concentration increases. Aerosols are mostly generated by the impact of nanopowders on the floor of the experimental chamber. Fumed silica is dustier than titanium dioxide, and its agglomerates break more easily.

Introduction

Nanoparticles, also called ultrafine particles, are defined as par ticles smaller than 100 nm. Their high surface-to-volume ratio gives them interesting properties: magnetic, mechanical, electrical, optical, thermodynamical and thermal properties that are different from those of the source materials [START_REF] Brune | Nanotechnology. Assessment and Perspectives[END_REF] . They thus constitute an enhanced development, due to the wide range of applications in electronics, biomedicine, pharmaceutics, cosmetics, catalysts and materials [START_REF] Aitken | Nanoparticles: An occupational hygiene review[END_REF] . This increasing interest in nanomaterials has led to some questions about the possible adverse effects on human health [START_REF] Aitken | Nanoparticles: An occupational hygiene review[END_REF][START_REF] Maynard | Airborne nanostructured particles and occupational health[END_REF][START_REF] Maynard | Nanotechnology: assessing the risks[END_REF] . Previous works have studied the toxicity of ultrafine particles. Their conclusions can be summarized as follows in a non-exhaustive way: ・ Ultrafine particles induce much higher lung inflammation than an equal mass of larger respirable particles of the same material [START_REF] Oberdörster | Role of the alveolar macrophage in lung injury-studies with ultrafine particles[END_REF] . ・ Lung damages due to agglomerates and aggregates of nanoparticles (henceforth called nanostructured par ticles) is more severe than that caused by particles lacking a nanostructure, even when they have the same size [START_REF] Maynard | Nanotechnology: assessing the risks[END_REF] . ・ Ultrafine particles can pass into the bloodstream [START_REF] Oberdörster | Extrapulmonar y translocation of ultrafine carbon par ticles following wholebody inhalation exposure of rats[END_REF] . ・ Pulmonar y inflammation due to low-solubility nanostructured particles is fairly independent of particle compositions and sizes. On the contrary, it correlates closely with surface area concentration [START_REF] Maynard | Airborne nanostructured particles and occupational health[END_REF] . ・ Particle shape should be taken into account when studying the hazards; anisotropic particles such as fibres or nanotubes enhance pulmonary damage [START_REF] Maynard | Nanotechnology: assessing the risks[END_REF] . ・ Surface chemistr y can modify particle toxicity;

for instance, the inflammation response caused by quartz can be strongly reduced by modifying the surface of the particles (e.g. with aluminium lactate) [START_REF] Duffin | The importance of surface area and specific reactivity in the acute pulmonary inflammatory response to particles[END_REF] . All those studies point out that it is necessary to determine the exposure levels during manufacturing, handling and clean-up operations with nanopowders. Not only nanoparticles, but nanostructured particles as well as aerosolised liquid suspensions containing nanoparticles should be taken into account. Several parameters such as particle shape, size and surface area of the nanostructured particles are properties which should be determined. Two different approaches can be followed. The first one concerns measurement of the exposure levels at work sites in order to obtain representative values of real industrial levels. However, it is difficult in that case to identify the parameters inducing ultrafine dust generation, as well as their influence. A good summary of this kind of approach can be found in Aitken et al [START_REF] Aitken | Nanoparticles: An occupational hygiene review[END_REF] . The other approach concerns dustiness tests applied to nanostructured powders. Dustiness is defined as the trend of a given material to generate dust. In such tests, a powder is subjected to different constraints such as free falling, fluidisation or stirring in a rotating drum [START_REF] Hamelmann | Methods for Estimating the Dustiness of Industrial Powders-A review[END_REF] . Although the values obtained are less representative than measurements at work sites, the identification of key parameters affecting dust emission becomes easier. To our knowledge, the only previous studies using dustiness tests for nanomaterials were implemented with single-walled carbon nanotubes (SWCN) [START_REF] Baron | Evaluation of Aerosol Release During the Handling of Unrefined Single Walled Carbon Nanotube Material[END_REF][START_REF] Maynard | Exposure to carbon nanotube material: aerosol release during the handling of unrefined single-walled carbon nanotube material[END_REF] and fumed silica [START_REF] Hamelmann | Methods for Characterizing the Dustiness Estimation of Powders[END_REF] . Tests with SWCN were carried out in a test tube agitated by a vortex shaker. The emitted aerosol size distribution was measured by two scanning mobility particle sizers, SMPS [START_REF] Flagan | Electrical Techniques[END_REF] , and an aerodynamic particle sizer, APS [START_REF] Baron | Direct-Reading Techniques Using Particle Motion and Optical Detection[END_REF] . The aerosol concentration was found to be quite low and ultrafine particles were not significantly detected, except when applying strong agitation. These results were checked against field measurements where the overall airborne concentration of nanotubes was estimated to be quite low (lower than 53μg/m 3 ). Tests with fumed silica were carried out in a rotating drum [START_REF] Hamelmann | Methods for Estimating the Dustiness of Industrial Powders-A review[END_REF] and in a dropping test (Dust View [START_REF] Hamelmann | Methods for Characterizing the Dustiness Estimation of Powders[END_REF] , from Palas society). Size distributions were not measured in that work. For the rotating drum test, a dustiness index was defined as the mass ratio of the emitted ultrafine aerosol (sucked and collected by a filter) to the initial nanostructured powder. For the dropping test, the characteristic values used were the maximum light extinction of a laser beam placed next to the settling dust stream, as well as the light extinc-tion 30 seconds after the fall of the powder. The aim of the present work is to design an experimental set-up which enables the study of ultrafine dust generation by free-falling nanostructured powders. The results of the test provide the size distribution and the mass concentration of the aerosol. This experiment will therefore make it possible to check the presence (or absence) of ultrafine particles suspended in air due to free-falling powders, and to study the effect of parameters such as the drop height or the dropped mass of powder on both size distribution and concentration. Such an experiment can provide a tool to quantify and to compare the ability of several nanostructured powders to generate ultrafine aerosols for a given situation (free falling). It can also help to correlate the properties of the particles to their behaviour.

Experimental Methods

Experimental set-up

The experimental set-up shown in Fig. 1 is inspired by the MRI test [14]. Powder fall was performed in a stainless steel chamber with internal dimensions 180×50×80cm. The front and lateral sides of the chamber are partially of glass to enhance visualization. The powder is stocked in a silo (a) which can be adjusted in height inside the chamber along a supporting stem (b). The silo comprises a storage vessel that tapers into a beaker of 125cm The second set of experiments consisted of measuring the aerosol concentration and size distribution after a powder fall. The aerosol concentration and size distribution were measured by an electrical lowpressure impactor (ELPI). The ELPI device enables the particles contained in the aerosol to be separated according to their size, and to determine the particle concentration corresponding to each class. The principle of the ELPI measurements is explained in the next section. These experiments were started by placing the nanostructured powder in the silo, closing the front door and switching on the air extraction through the HEPA outlet filter. The HEPA filter placed at the air inlet allowed the particle concentration (coming from the atmosphere) in the chamber to be decreased. At the same time, the ELPI was switched on to equilibrate the temperature in the chamber. After a stabilisation time of at least 20-30 minutes, the air extraction was switched off and the clean air evolution was recorded during 15-20 minutes. The beaker was then turned upside down and the powder fell in the chamber.

The dropped mass of powder can be changed by partially filling the beaker below the silo. In that case, the mass of powder was weighed before placing it in the beaker. Another possibility was to totally fill the beaker. In that case, the method of filling was more reproducible, but the dropped mass of powder was not directly measured before each experiment. It was obtained for each powder by carr ying out preliminary experiments, recovering the dropped powder in a plastic bag and weighing the bag. Finally, a third set of experiments were carried out in order to validate the presence of ultrafine aggregates in the emitted aerosol by scanning electron microscopy (SEM) visualisation of the particles collected on the ELPI stages. These SEM visualisations need specific experiments. They were performed for a given height of fall, fixed at 160 cm, and a given height of sampling: 50 cm.

Principle of the ELPI measurements

The ELPI [START_REF] Keskinen | Electrical Low Pressure Impactor[END_REF][START_REF] Marjamäki | Performance evaluation of the Electrical Low-Pressure Impactor (ELPI)[END_REF][START_REF] Moisio | Real Time Size Distribution Measurement of Combustion Aerosols[END_REF][START_REF] Ltd | [END_REF][START_REF]Elpivi 3.1 Software Manual ver 3[END_REF][START_REF] Virtanen | Fine particle losses in electrical low-pressure impactor[END_REF][START_REF] Marjamäki | ELPI Response and Data Reduction I: Response Functions[END_REF][START_REF] Baltensperger | Dynamic Mass and Surface Area Measurements[END_REF] is a device able to measure aerosol concentration as well as size distribution of the aerosol in the range between 30 nm and 10μm at a given location. The principle of the instrument has been well explained elsewhere [START_REF] Keskinen | Electrical Low Pressure Impactor[END_REF][START_REF] Marjamäki | Performance evaluation of the Electrical Low-Pressure Impactor (ELPI)[END_REF][START_REF] Moisio | Real Time Size Distribution Measurement of Combustion Aerosols[END_REF][START_REF] Ltd | [END_REF] , so that the main elements needed for this work will only be touched upon here.

A schematical view of the ELPI device is shown in Fig. 2. The aerosol is sucked by a vacuum pump. It goes first through a corona charger (a), where particles are charged positively depending on their mobility diameter. The mobility diameter (db) is defined as the diameter of the sphere having the same mobility, i.e. the same velocity-to-driving force ratio as the particle. Since, according to the Stokes-Einstein equation, the particle diffusion coefficient is proportional to the particle mobility [START_REF] Hinds | Aerosol Technology. Properties, Behavior, and Measurement of Airborne Particles[END_REF] , the mobility diameter is the most representative diameter when describing Brownian motion.

The cascade impactor (b) is located downstream of the corona charger. It classifies the particles according to their aerodynamic diameter. The aerodynamic diameter (da) is defined as the diameter of the sphere with a unit density of 1000kg/m 3 having the same settling velocity as the particle. This diameter is the key particle property for characterising respiratory deposition [START_REF] Hinds | Aerosol Technology. Properties, Behavior, and Measurement of Airborne Particles[END_REF] .

When the particles are collected on an ELPI stage, they generate an electrical current that is measured by the electrometers. Teflon insulators are placed between the ELPI stages in order to isolate the stages electrically.

As the particle charge depends on the mobility diameter and the particle deposition on the ELPI stages depends on the aerodynamic diameter, the conversion between both diameters is essential. If the particles are spherical, the diameters are related through equation 1, where ρp is the particle density, ρ0 = 1000kg/m 3 and CC is the slip (or Cunningham) correction factor [START_REF] Hinds | Aerosol Technology. Properties, Behavior, and Measurement of Airborne Particles[END_REF] . The aerodynamic diameters and their corresponding mobility diameters were calculated in Table 1 for the two different particle densities used in this work. If the particle density is 1000kg/m 3 , then the aerodynamic diameter and the mobility di-ameter have the same values.

ρ p d 2 b C C (d b ) = ρ 0 d 2 a C C (d a ) (1)
Measurement data can be treated by ELPI software [START_REF] Moisio | Real Time Size Distribution Measurement of Combustion Aerosols[END_REF][START_REF]Elpivi 3.1 Software Manual ver 3[END_REF] , which conver ts raw current data into aerosol concentration and size distribution. The only required parameters are the particle density and the calibration data. Both the charger and the cascade impactor calibrations are performed by the manufacturer [START_REF] Marjamäki | Performance evaluation of the Electrical Low-Pressure Impactor (ELPI)[END_REF] . Cascade impactor calibration provides the cut-off aerodynamic diameters of the ELPI stages.

Charger calibration provides the charger efficiency (Ech) as a function of the mobility diameter. The charger efficiency represents the current carried by charged particles of a given size leaving the charger, divided by the particle number concentration in the aerosol.

For each stage, ELPI software calculates the average aerodynamic diameter given by the geometric mean of the cut-off diameters of a given stage and the previous one. Then the mobility diameter is calculated by Eq.( 1). The charger efficiency is then calculated according to its calibration data. Finally, the particle number concentration is given by the ratio between the current I' and the charger efficiency Ech:

N = I  E ch ( 2
)
I' is not the measured current I, but a corrected current [START_REF] Moisio | Real Time Size Distribution Measurement of Combustion Aerosols[END_REF] that takes into account the fine particle losses in upper ELPI stages [START_REF] Virtanen | Fine particle losses in electrical low-pressure impactor[END_REF][START_REF] Marjamäki | ELPI Response and Data Reduction I: Response Functions[END_REF] . The rebound of the particles impacting against an ELPI stage is minimised by greasing the ELPI substrates with Vaseline.

Tested nanostructured powders

Two types of nanostructured powders were selected for testing the device: titanium dioxide, TiO2 (G5, produced by Millenium Inorganic Chemicals) and fumed silica, SiO2 (Aerosil 200, produced by Degussa). Although both powders are formed by microscopic agglomerates of nanoparticles, their structures are quite different, as can be seen on the SEM photographs (Fig. 3). TiO2 agglomerates (Fig. 3.a) have a compact (raspberry-like) structure; whereas SiO2 agglomerates (Fig. 3.b) resemble a foam with a high air content.

The main physical proper ties of both powders were measured and reported in Table 2. Agglomerate size measurements were performed by laser diffraction (Mastersizer 2000, Malvern Instruments).

The difference between mass and number median diameters can be explained by the fact that for one given large particle, several fine particles are needed to obtain its weight. Primary particle sizes are provid-ed by the manufacturers and are in agreement with our transmission electron microscopy (TEM) visualisations (same range of sizes). Particle densities were measured by helium picnometr y, whereas tapped densities were obtained using a Hosokawa Micron Powder Characteristics Tester. Finally, the bulk density was calculated from the dropped mass of powder measured when the beaker was completely filled (the beaker volume is 125cm 3 ).

Results

Video recording of the powder fall

In order to analyse the two sources of the aerosols: dust generation from the particles falling and dust generation after impaction on the chamber floor, the free-fall process was visualised by video camera recording. The same volume of dropping powder and same height of fall (130cm) were used for both powders TiO2 and SiO2. Several pictures were taken during the fall: they show that for both powders, the column of the falling powder is neither regular in shape nor in particle concentration. Moreover, when the first particles reach the floor of the chamber, others are still leaving the beaker in the upper part of the chamber. The mean time of the fall, considered as the time during which there was some powder falling from the silo, was found to be less than 2 s for TiO2 and around 9 s for SiO2. For TiO2, the particle cluster impacts on the sur-face of the chamber floor and generates smaller agglomerates. Oblique jets are observed when the powder impacts on a layer of particles previously fallen. These jets can lead to particle re-suspension. Fig. 4 is a view of the impact of TiO2 powder, showing the rising jets. For SiO2 powder, the impact shown in Fig. 5 is different: the small agglomerates seem to bounce on the surface and horizontal air/particle clouds are generated parallel to the surface. Finally, in both cases, particles are re-suspended after the impact and rise along the walls of the chamber. Consequently, the measurements performed on the aerosol with the ELPI instrument correspond to the added contributions of the dust generated from the fall and the dust generated from impact.

Time evolution of the aerosol

The experimental conditions of the quantitative tests performed in the dustiness experimental set-up are reported in Table 3.

Although ELPI raw currents are measured in a continuous way, the results are averaged over at least 5 consecutive seconds [START_REF] Ltd | [END_REF] because of the different residence times of the particles in the cascade impactor. This time (5 seconds) compared with the time of fall (2 s for TiO2 and 9 s for SiO2) shows that the aerosol generation during the free fall and the aerosol generation due to impact cannot be distinguished by ELPI measurements. raw currents measured on ELPI stages during the free fall of TiO2 and SiO2 nanopowders, respectively, the first one corresponding to experiment E2.5 and the second one to E1.5. For both powders, when sampling near the impact surface, the aerosol concentration increases rapidly and then decreases faster than an exponential law. This is due to both settling and convection by the air vortex generated by the powder fall [START_REF] Uchiyama | Three-dimensional vortex simulation for particulate jet generated by free falling particles[END_REF] . After some 10-15 minutes, aerosol concentration (which is proportional to the measured currents) decreases following an exponential law. This is typical of stirred settling [START_REF] Hinds | Aerosol Technology. Properties, Behavior, and Measurement of Airborne Particles[END_REF] . For SiO2 (Fig. 6.b), the fluctuations are much more significant, giving way to two independent peaks. When the second peak is reached, the aerosol concentration decreases in an exponential way. This decrease is faster than for TiO2, since SiO2 particles settle faster than the TiO2 ones because of their larger aerodynamic diameter (see next section).

In the following sections, the results (size distribu- tion and particle concentration) were obtained by considering the maximum of the first peak of the current curve. In order to reduce the drift effect, the currents obtained just before the peak were subtracted from the measured currents.

Aerosol concentration at higher sampling positions (100cm and 150cm) increases more slowly, and begins to decrease following an exponential law. This behaviour can be explained by the fact that the main part of the aerosol is generated during the impact against the solid surface, and then diffuses (by Brownian and especially by hydrodynamic diffusion) to higher positions. Once the aerosol concentration is uniform in the chamber, only settling becomes important. This can be verified in Fig. 7, which shows the particle concentration at different sampling positions for different times. At t=0 (which corresponds to the first peak), the aerosol is much more concentrated near the impact surface (since most of the aerosol is generated during the impact). Aerosol concentration near the sur face decreases rapidly, whereas concentration at higher sampling positions increases. At t=10 min, aerosol concentration is uniform in the chamber. At t=15 min, aerosol concentration begins to decrease homogeneously in the entire chamber.

Characterisation of the emitted aerosol 3.3.1 Size distribution

Fig. 8 shows the size distribution of the aerosols obtained in experiments E1 to E8, corresponding to different heights of fall at the same sampling position. Fig. 9 shows the size distribution of the aerosols in experiments E2, E9 and E10, corresponding to different sampling heights for a same height of fall. Error bars represent the confidence inter val for a confidence level of 95%. In all cases, the size distributions show two maxima, one corresponding to ultrafine particles, the other one to micrometric particles. The maximum corresponding to micrometric particles is located on the 12 th ELPI stage for SiO2 and on the 8 th stage for TiO2. They correspond to aerodynamic diameters of 8.05μm and 1.21μm, respectively. If particle sizes are expressed as mobility diameters, they are equal to 4.57μm for SiO2 and 0.660μm for TiO2. They are in good agreement with the number median diameters of the initial powders, measured by light scattering, which are equal to 5.2μm for SiO2 and 0.800μm for TiO2. Hence, these maxima correspond to the re-suspension of the particles just in the same state as they are in the powder. The ultrafine maximum is located in the 1 st or the 2 nd ELPI stages, showing the presence of primar y particles or at least ultrafine aggregates in the aerosol. This fact has been checked by visualisation of the particles collected on the ELPI stages by a scanning electron microscope (SEM), Section 3.3.2. In order to analyse the reproducibility of the measurements, experiments E1 and E2 were repeated 5 times and reproducibility was defined as the average of the ratio between the standard deviation and the averages of the particle number concentration for each stage (Eq. ( 3)).

Reproducibility = (1/12) • 12  i=1 (σ i /x i ) (3)
The calculated reproducibility values are 0.47 for E1 and 0.62 for E2. The maximum corresponding to micrometric particles is reproducible for both powders (σ/x ¯)12=0.16 for E1, (σ/x ¯)8=0.076 for E2. However, the maximum corresponding to ultrafine particles is variable, especially for TiO2: (σ/x ¯)1=1.5, whereas it is equal to 0.63 for SiO2. This could indicate that the ultrafine aggregate generation by breakage and/or erosion of the micrometric agglomerates could be somewhat dependent on the operating conditions such as temperature or humidity which were not controlled during the experiments.

SEM visualisation

The presence of ultrafine aggregates in the air after the powder fall was checked by removing particles down the ELPI stages and analysing the collected particles with a scanning electron microscope (SEM). In all cases, two main remarks can be made. The first one is that ultrafine aggregates are actually present in the aerosol, as obser ved from the ELPI measurements. This result shows that the peak corresponding to ultrafine particles in the size distributions given by the ELPI measurement in the chamber is representative of this population and is not due to measurement fluctuations. The second one is that aggregates and agglomerates are not spherical. This is especially the case with silica, whose aggregates seem to have a fractal structure in several cases as in Fig. 10.a.

Influence of the height of fall

As shown in Fig. 8, neither the shape of the size distribution nor the position of the maxima is modified by the height of fall. However, increasing the height of fall increases the micrometric agglomerates concentration. On the other hand, the effect of the height of fall on the ultrafine aggregates concentration is not clear due to the problems of reproducibility of the experiments. This problem is stronger when the height of fall is small (50cm, experiments E5 and E8). In this case, the peak in the raw current is small and the drift of the instrument becomes too important. This leads to a larger imprecision for experiments E5 and E8 than for the others. Fig. 11.a shows the influence of the falling height on the mass concentration of the TiO2and the SiO2generated aerosols. For the same volume of dropped powder, the dependence of mass concentration upon the falling height seems to be well described by a straight line for SiO2, whereas for TiO2, this dependence correlates with the square of this height. It is generally supposed in the literature [START_REF] Cheng | Formation of Airborne Respirable Dust at Belt Conveyor Transfer Points[END_REF][START_REF] Sutter | Investigation of Accident-Generated Aerosols: Releases from Free Fall Spills[END_REF][START_REF] Plinke | Experimental Examination of Factors That Affect Dust Generation[END_REF][START_REF] Heitbrink | An Investigation of Dust Generation by Free Falling Powders[END_REF][START_REF] Plinke | Dust Generation from Handling Powders in Industry[END_REF] that the resuspension of micrometric particles depends upon a power of the falling height, h α , where α varies from 0.4 29) to 2.05 [START_REF] Sutter | Investigation of Accident-Generated Aerosols: Releases from Free Fall Spills[END_REF] . Moreover, the value ofαcould depend on the type of the powder. Hence, our results for both powders seem to agree with previous works on micrometric powders.

Influence of the dropped mass

One should not be tempted to directly compare the concentration of the SiO2 aerosol in experiments E1, E3 to E5 with that of the TiO2 aerosol in experiments E2, E6 to E8. One could induce that TiO2 is much dustier than SiO2. In fact, the mass of TiO2 that falls in those experiments is 8 times higher than that of When the dropped mass is reduced, the proportion of powder impacting on a layer of previously dropped particles is reduced. Since this is the main mechanism of particle release, the quantity of re-suspended particles is lower, even when normalising the mass by the dropping mass. This observation agrees with Heitbrink et al's work [START_REF] Heitbrink | An Investigation of Dust Generation by Free Falling Powders[END_REF] .

Discussion

The average number of particles falling each time (nfall) can be estimated from the mass of the falling powder (mfall), the particle density (ρp) and the diameter (dp):

n f all = m f all ρ p • (π/6) • d 3 p (4)
By normalising the particle number concentration on the 1 st ELPI stage by nfall, a value of 10 -8 part/cm 3 in the aerosol per dropped particle was obtained for SiO2, whereas it is 10 -11 for TiO2 (experiments E1 and E2, respectively). Although it is a rough estimation, it is a good indication of the fact that SiO2 agglomerates break more easily than the TiO2 ones. The reason seems to be the much more open structure of the SiO2 agglomerates (see Fig. 2). The void fraction,ε, of TiO2 agglomerates can be estimated from the agglomerate densityρagg (measured by helium picnometry, Table 2) and the primary particle density ρpp, calculated from the crystallographic parameters 30) . It is found to be equal to 11a Absolute values of the generated aerosol.

11b Normalised values (aerosol mass concentration to the mass of the falling powder ratio).

Since fumed silica is amorphous, the void fraction must be estimated using another method. It can be calculated from their fractal dimension, Df, Eq.( 7) [START_REF] Wengeler | Turbulent hydrodynamic stress induced dispersion and fragmentation of nanoscale agglomerates[END_REF] . The average fractal dimension for SiO2 agglomerates can be measured by laser scattering [START_REF] Wengeler | Turbulent hydrodynamic stress induced dispersion and fragmentation of nanoscale agglomerates[END_REF] with Mastersizer 2000, and it has been found to be roughly equal to 2.1; this method, however, cannot be performed with TiO2 since TiO2 agglomerates are too small, and very large scattering wave vectors, not available in the Mastersizer 2000, are needed [START_REF] Sorensen | Light Scattering by Fractal Aggregates: A Review[END_REF] .

The solid-fraction-to-void-fraction ratio for SiO2 can be calculated by Eq.( 6). It is found to be 4・10 -3 for SiO2. This ratio is 1 000 times higher for TiO2.

1 -ε ε ∼ = (d agg  d pp ) D f -3 (6)
If the interparticle forces are attributed to van der Waals interactions [START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF] , then they are roughly equal for both SiO2 and TiO2.

F = A • d pp 24 • z 2 (7)
Finally, the primary particle size for TiO2 is smaller than for SiO2 (see Table 2). Rumpf's theory [START_REF] Reynolds | Breakage in granulation: A review[END_REF] establishes that the tensile strength of an agglomerate (σt) depends upon its void fraction (ε), the interparticle force (F) and the primary particle size (dpp):

σ t = 1 -ε ε • F d 2 pp ( 8
)
Applying Rumpf's theor y, we find that the tensile strength for TiO2 agglomerates is at least 1000 times higher than for the SiO2 ones. This could explain why SiO2 agglomerates break more easily than the TiO2 ones. However, attention should be paid to the fact that the hydrodynamic stresses are not necessarily the same.

In the future, the powder fall will be simulated by an Eulerian-Lagrangian approach, in order to study the mechanism of the particle release.

Nomenclature

Fig. 1

 1 Fig. 1 Free-falling nanostructured powders device for aerosol emission measurement: (a) silo; (b) supporting stem; (c) inlet HEPA filter; (d) outlet HEPA filter; (e) sampling point; (f) latex gloves; (g) vacuum cleaner inlet.

Fig. 2

 2 Fig. 2 Schematical representation of the Electric Low-Pressure Impactor (ELPI) used in this study: (a): corona charger; (b): cascade impactor.To the vacuum pump

Fig. 3a

 3a Fig. 3a Titanium dioxide (TiO 2 ). Fig. 3b Fumed silica (SiO 2 ).

Fig. 3

 3 Fig. 3 SEM photographs of the nanostructured powders.

Fig. 4a Fig. 4

 4a4 Fig. 4a Camera visualisation Fig. 4b Schematic representation of the fall Fig. 4 TiO 2 nanopowder fall.

Fig. 5a Fig. 5

 5a5 Fig. 5a Camera visualisation Fig. 5b Schematic representation of the fall Fig. 5 SiO 2 nanopowder fall.

Fig.s 6

 6 .a and 6.b show the time evolution of the

Fig. 6a Fig. 6 Fig. 7

 6a67 Fig. 6a Experiment E2.5. Fig. 6b Experiment E1.5. Fig. 6 Time evolution of ELPI raw currents during the free fall of nanopowders

Fig. 8a

 8a Fig. 8a SiO 2 (experiments E1, E3 to E5).

Fig. 8b

 8b Fig. 8b TiO 2 (experiments E2, E6 to E8).

Fig. 8

 8 Fig. 8 Number size distribution of the generated aerosols at different heights of fall

Fig.s 10 .

 10 a to 10.c show two ultrafine aggregates of SiO2 and one of TiO2 found on the 1 st , the 2 nd and the 3 rd stage of the ELPI device, respectively.

Fig. 9

 9 Fig. 9 Number size distribution of the generated aerosols at different sampling heights, for TiO 2 nanopowder (experiments E2, E9 and E10).

  SiO2. It seems more judicious to compare the aerosol concentrations normalised by the mass of the fall. Results are shown in Fig. 11.b. Although normalised mass concentrations of TiO2 are within the confidence interval of the SiO2 values for a confidence level of 95%, these values are always smaller than those for SiO2. We can therefore deduce that SiO2 is a little dustier than TiO2. Another way to compare the two powders is to use the same mass of falling powder in the beaker of the experiment (and not the same volume). Fig.s 11.a and 11.b represent the mass concentrations for experiments E11, E12 and E13, for which the dropped mass of TiO2 was equal to the dropped mass of SiO2 used in E1, E3 to E5 experiments. We can see that for the same dropped mass, the SiO2 aerosol mass concentrations are about 5 times higher than those of TiO2 aerosols. This consolidates the affirmation that the tested SiO2 nanopowder is dustier than the tested TiO2 one. Fig. 11.b shows that increasing the dropped mass increases the normalised mass concentration.

Fig. 10a Fig. 10

 10a10 Fig. 10a SiO 2 , 1 st stage (×120000)

Fig. 11

 11 Fig. 11 Influence of the height of fall and the dropped mass on the mass concentration of the generated aerosols.

Table 1

 1 Mean aerodynamic diameters of ELPI stages, and their equivalent mobility diameters

	ELPI stage	da (ρ0=1000 kg/m 3 )	db (ρp=2300 kg/m 3 )	db (ρp=3100 kg/m 3 )
	1	38.3 nm	17.6 nm	13.2 nm
	2	70.9 nm	34.1 nm	25.8 nm
	3	119 nm	60.5 nm	46.6 nm
	4	199 nm	108 nm	85.3 nm
	5	312 nm	180 nm	146 nm
	6	477 nm	289 nm	239 nm
	7	753 nm	471 nm	396 nm
	8	1.21μm	777 nm	660 nm
	9	1.93μm	1.25 μm	1.07μm
	10	3.05μm	2.01μm	1.70μm
	11	5.11μm	3.37μm	2.90μm
	12	8.05μm	5.31μm	4.57μm

Table 2

 2 Specific properties of the nanostructured powders used in the free-fall test

	TiO2	SiO2

Table 3

 3 Operating conditions of the experiments performed in the free-falling powder chamber

	Experiment number	Powder	Height of fall	Height of sam-pling	Dropped mass		Temperature	Humidity
	E1	SiO2	160 cm	50 cm	5.5 ± 0.2 g	E1.1	23.6℃	52.2%
						E1.2	22.8℃	60.8%
						E1.3	23.1℃	56.3%
						E1.4	25.1℃	49.1%
						E1.5	24.5℃	48.4%
	E3	SiO2	120 cm	50 cm	5.5 ± 0.2 g	E3.1	26.1℃	56.6%
						E3.2	23.4℃	65.2%
	E4	SiO2	80 cm	50 cm	5.5 ± 0.2 g	E4.1	26.0℃	58.7%
	E5	SiO2	50 cm	50 cm	5.5 ± 0.2 g	E5.1	22.0℃	52.3%
						E5.2	23.7℃	53.4 %
						E5.3	22.8℃	60.0%
	E2	TiO2	160 cm	50 cm	45.9 ± 0.9 g	E2.1	19.3℃	40.3%
						E2.2	22.5℃	39.1%
						E2.3	20.5℃	40.9%
						E2.4	23.1℃	37.9%
						E2.5	20.5℃	48.0%
	E6	TiO2	120 cm	50 cm	45.9 ± 0.9 g	E6.1	23.7℃	55.8%
	E7	TiO2	80 cm	50 cm	45.9 ± 0.9 g	E7.1	24.2℃	47.1%
	E8	TiO2	50 cm	50 cm	45.9 ± 0.9 g	E8.1	22.7℃	47.3%
	E9	TiO2	160 cm	100 cm	45.9 ± 0.9 g	E9.1	24.8℃	58.8%
	E10	TiO2	160 cm	150 cm	45.9 ± 0.9 g	E10.1	20.6℃	57.8%
	E11	TiO2	160 cm	50 cm	5.5 g	E11.1	23.2℃	61.1%
					5.5 g	E11.2	25.0℃	50.7%
	E12	TiO2	120 cm	50 cm	5.5 g	E12.1	23.6℃	57.3%
	E13	TiO2	80 cm	50 cm	5.5 g	E13.1	24.0℃	54.0%

Acknowledgements

The authors acknowledge the CNRS, Centre National de Recherche Scientifique and the INRS, Institut National de Recherche et Securite for their financial support within the frame of the DINANO project. They also want to acknowledge Lucien Datas and Stéphane Le Blond du Plouy from the TEMSCAN Ser vice of the University of Toulouse for the SEM and the TEM visualisations.

Author's short biography

Nelson Ibaseta

Nelson Ibaseta obtained his B.Eng. degree in chemical engineering from the University of Oviedo, Spain, and his M.Sc. degree in chemical and environmental engineering from the National Polytechnic Institute of Toulouse, France, where he is currently working on his Ph.D. thesis. His research interests are in multiphase systems, ranging from physicochemical and transfer systems to simulation and optimisation aspects.

Béatrice Biscans

Dr Béatrice Biscans is a permanent researcher at CNRS (French National Centre for Scientific Research), in the Chemical Engineering Laboratory in Toulouse, in charge of a group working on crystallisation and nanoparticles. Her research interests are directed towards understanding and predicting the properties of solidform chemical products through control of the process parameters. She develops both theoretical and experimental studies associated with both fundamental and strategic research programmes including industrial collaboration.


A Badev
email: alex_badev@yahoo.com

I Iliev

J P Cambronne

T Lebey

A Barnabe C

V Levcheva

Microwave dielectric properties of the spinel composition Mg 1+x Ti x Al 2-2x O 4

Keywords: Substituted aluminates, Microwave properties, Structure

The objects of our research is magnesium aluminates (spinel), in which the electric charge of Al 3+ cation is partially substituted with constitutive trivalent cation of Mg 2+ and Ti 4+ in the system with the following composition: Mg1+xTixAl2-2xO4 with (x) varying from 0,1 to 0,6. X-ray diffraction data shows that within the above values of (x) the composition is a single phase spinel. The microwave basic parameters were measured as εr, tan δ and τf, respectively with the following values: εr = 6-12, tanδ = (1.5-3.5) x 10 -4 at 10 GHz and Qf = 70 000 -90 000, and τf from -50 to -70 ppm/deg.

Introduction

It is known that for microwave dielectric materials with relatively high ε r in the range of 40, experimental difficulties exist at frequencies above 30 GHz, as the size of the resonator disks becomes very small, and therefore it is difficult to press and polish them. The size of the disks become acceptable when materials like spinels with ε r = 9-14 are used. Monospinels of Zn and Mg have ε r = 8.5 -8.7, Q f = 56000 to 69000 at 12 GHz (respectively tanδ = 2×10 -4 -1.7×10 -4 ). Unfortunately they have relatively high temperature coefficient of frequency τ f = -75 to-79 ppm/deg. Compensation of τ f could be achieved upon additives with opposite sign, like TiO 2 . For example, systems like (1-x) MgAl 2 O 4 . x TiO 2 or (1-x) ZnAl 2 O 4 . x TiO 2 for x = 0.12 to 0.5 have ε r = 9 -14 and τ f = -27 to 6.2 ppm/deg [1][START_REF] Sebastian | List of Microwave dielectric resonator materials[END_REF][START_REF] Alford | [END_REF][4][5]. It is necessary to notice that these compositions are not electron equilibrated. Such materials have a sintering temperature of 1400-1450 °C, at which a spinel solid solution is formed and contains a cation (Ti 4+ ) with different valence than M 2+ and Al 3+ that could cause oxygen defects, and it therefore worsens dielectric losses.

The object of the present work is the study of MgAl 2 O 4 , in which Al 3+ cations are partially substituted with constitutive trivalent cation Mg 2+ -Ti 4+ for the composition Mg 1+x Ti x Al 2-2x O 4 to be electron balanced , which represents a scientific and practical interest. We suppose that this substitution will lead to an increase of ε r and a probable decrease of τ f , keeping dielectric losses at10 -4 rate.

Experimental procedure

As starting materials, MgO Fluka, TiO 2 Kronos, Al 2 O 3 Fluka and CaCO 3 Fluka with purity 98-99 % were used.

The synthesis is made by the conventional mixed oxide route. The ball milling and homogenization are made in agate milling pots and balls in deionized water in planetary ball mill Retsch during one hour. The dried mixture is calcined at 1100°C for three hours in corundum pots. After drying, 0.3 % B 2 O 3 is added to ease the sintering process, and thereafter a secondary milling is made. Press-powder is made from the dried mixture, using PVA (10% solution) as a binder agent. For the pressing process, we use a powder fraction of 0.25-0.5 mm grain size at a pressure of 1.5 t/cm 2 . The sintering is proceeded in super kantal furnace Linn, at different temperatures with isothermal maintaining during three hours. X-ray analysis is made from the powder material using CuKα radiation in CIRIMAT-Toulouse.

The measurements of ε r , tanδ, and τ f are made according Hakki and Koleman' s method [6] modified by Courtney [7].

Results and discussions

X-Ray analysis

According to the ternary diagram of the MgO-Al 2 O 3 -TiO 2 system at sintering temperatures between 1400-1450°C, a solid solution which consists of MgAl 2 O 4 and TiMg 2 O 4 phases with a spinel structure is formed.

Fig. 1 presents the dependence between the lattice parameter calculated for different values of (x) by Végard law and the real measured one. Fig. 1 shows, that for all (x) values a spinel phase with lattice parameter varying from 8.1 to 8.25 is formed. Fig. 2 confirms the appearance of the single phase spinel, as only for x=0.1 a very small quantity of MgO did not react.

Microwave properties

On Fig. 3 is shown the relation between ε r and the composition (x) in the formula Mg 1+x Ti x Al 2-2x O 4 . The figure shows that ε r increases with (x) from 6 -12, which is probably due to the increase of Ti 4+ acting as a polarizing agent. Fig. 4 below shows that with increasing (x),dielectric losses move from 1.5×10 -4 to 3.5×10 -4 Likely for all dielectric materials losses increase with ε r which is due to conductivity phenomenon and electron exchange betweenTi 4+ and Ti 3+ . Therefore the quality factor which is related to dielectric losses, decreases with (x) content (from 90 000 to 33-40 000). Fig 5. shows that the temperature coefficient of frequency τ f linearly decreases (from -70 to -50 ppm/deg) with composition (x). This is due to a certain compensation of the temperature coefficient by the presence of Ti 4+ cations. This compensation is lightly shown due to the presence of Mg 2+ whose oxides have a τ f with an opposite sign than that of Ti 4+ (respectively TiO 2 ). The sintering temperature has a low impact on ε r but reflects on the losses and τ f . It is directly related to the microstructure (size of grains and porosity) and therefore affects losses tan δ. The optimal sintering temperature for our compositions appears to be 1430°C where losses and τ f are minimal.

Conclusions

The following system Mg 1+x Ti x Al 2-2x O 4 (MTA) is studied, in which Al 3+ cations are partially replaced with constitutive trivalent cation Mg 2+ -Ti 4+ to preserve the stochiometry and electronic balance between the positive and negative ions.

The synthesized obtained material is a single phase spinel.

The materials from the MTA system, especially those with x=0,1-0,2 have very good microwave properties as ε r = 9-10, tan δ = (1.5-3) x 10 -4 and Q f = 70 000 -90 000 at 10 GHz, but unfortunately relatively high τ f = -50 to -60 ppm/deg within the same range as Al 2 O 3 and MgAl 2 O 4 . Therefore they are not suitable for microwave resonators, but they can be used as thin films for electronic circuits. One of our additional research showed that the MTA composition could be thermally compensated (50-60 ppm/°C) with substitutions of Ca 2+ , which we will report in another paper.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Evolution of unit cell parameter as a function of the composition (x)

Fig. 3 .

 3 Fig. 3. Evolution of ε r with the composition (x).

Fig. 4 .

 4 Fig. 4. Dielectric losses as a function of the composition (x).

Fig. 5 .

 5 Fig. 5. Evolution of τ f with the composition (x)