Gorazd Žibret
email: gorazd.zibret@geo-zs.si

Bruno Lemiere
email: b.lemiere@brgm.fr

Ana-Maria Mendez
email: anamaria.mendez@upm.es

Carlo Cormio
email: c.cormio@serengeo.com

Danielle Sinnett
email: danielle.sinnett@uwe.ac.uk

Peter Cleall
email: cleall@cardiff.ac.uk

Katalin Szabó
email: szabo.katalin@mbfsz.gov.hu

M Teresa Carvalho

National Mineral Waste Databases as an Information Source for Assessing Material Recovery Potential from Mine Waste, Tailings and Metallurgical Waste

Keywords: tailings, mining, resource recovery, construction materials, metals, raw materials

We examined the national mine waste registries from seven European countries, created to fulfil the requirements of the "Mine Waste Directive" (2006/21/EC), for their potential use as an initial source of information for the valorisation of specific mine waste deposits for their resource recovery. A set of parameters for mine waste valorisation was defined and divided into three groups: the "basic", the "metal-centric" and the "material-centric" group. The "basic" group of 19 parameters considers properties of the mine waste deposit, including the location, history, homogeneity and quantity, among others, while the other two groups relate to the two desired material recovery types. The "metal-centric" group of parameters contains the six parameters needed to preliminarily assess the potential to valorise mine waste for metal extraction, while the "material-centric" group contains the nine parameters needed to consider the use of mine waste for the production of different construction materials. National mine waste registries from Slovenia, France, Spain, Italy, UK, Hungary and Portugal were reviewed to determine whether they contain information about each of the parameters. In line with the objectives of the Mine Waste Directive, the national mine waste registries were developed to reduce or prevent environmental damage, and not to enable resource recovery from mine waste. The registries contain most of the information for the parameters in the "basic" group, less information for the parameters in the "metal-centric" group and almost no information to define the parameters in the "material-centric" group. The conclusion is that national mine waste registries could serve only as an initial source of information, and more detailed information must be obtained from other sources. This misses an opportunity to see these sites as a resource, and not only as a potential source of pollution, given the urgent need to find alternative stocks of metals within the EU (European Union).

Introduction

The global mining industry is facing several challenges today. For example, the decreasing average grade of mined ore, and the inaccessibility of the deposits, whether being in areas unfavourable for humans to work (i.e., ultra-deep deposits, under the sea, etc.) or where current land use prevents mining. The latter, especially, limits the development of the European mining industry in many countries. Although the mining industry in Europe has been declining over the last few decades, Europe has a long-lasting mining tradition. The oldest mines in Europe are more than 5000 years old, and a golden age for mining was the period of industrial revolution in 19th century [START_REF] Coulson | The History of Mining: The Events, Technology and People Involved in the Industry that Forged the Modern World[END_REF]. However, due to the economic and environmental pressures, many mines in Europe have been closed down in the 19th and 20th century, and their legacy are, among others, mined waste rock (the material that is extracted but not processed, mine spoils, overburden), low grade stockpiles, tailings (material rejected during mineral processing) and metallurgical waste deposits (slags). In this paper, all these types of waste are generally classified as "mine waste".

Mineral processing and metallurgical technology in the past were not as efficient as they are today. What was regarded as waste in the past can often be regarded as quality ore today. For example, Mudd [START_REF] Mudd | The Sustainability of Mining in Australia: Key Production Trends and Their Environmental Implications for the Future[END_REF] reported that the average grade of mined Cu ore in Australia was 15%-25% from 1842 to 1880, and then gradually decreased to around 4% between 1880 and 1940, dropping to around 2% by 2008, while during this time, the production of Cu ore and waste rock were steadily increasing. Today, the largest Cu open pit mines can economically extract Cu ores below 1%. A similar pattern is also observed for Au. In Australia, ores with 15-30 g/t Au were extracted during 1850 to 1910, dropping to around 15 g/t during 1910 to 1940 and steadily decreasing to 1-2 g/t in 2008 [START_REF] Mudd | The Sustainability of Mining in Australia: Key Production Trends and Their Environmental Implications for the Future[END_REF]. In addition, current technologies require a greater variety of raw materials than were needed in the past. In ancient times, from the Bronze Age to the beginning of Medieval times, only seven metals were required to meet human needs (Fe, Cu, Ag, Sn, Au, Hg and Pb), with an additional five (Zn, As, Sb, Pt and Bi) being required in Medieval times. Today, almost every naturally occurring element in the periodic table is needed in order to produce all types of goods used by society [START_REF] Habashi | History of[END_REF]. This means that many elements needed to produce new technologies, electronic devices, green technologies, computing, etc., were completely disregarded even 50-60 years ago and were deposited as mine wastes (Figure 1). Prime examples are semiconductors (e.g., Ge and Ga), rare earth elements (e.g., Ce, Nd, Eu, Er and Lu) or so-called energy elements, needed for batteries (graphite, Co, Li) [START_REF] Zepf | Materials Critical to the Energy Industry[END_REF]. The American Chemical Society [START_REF]Endangered Elements[END_REF] has also presented a similar list of endangered and critical elements. Elements in this list are grouped into three groups: those which could face limited availability due to future risks to supply (28 elements), those for which supply is at risk due to increased use (7 elements) and finally elements for which future supply is predicted to be at serious risk in the next 100 years (9 elements). The EU (European Union) also published its own list of critical raw materials [START_REF]The 2017 List of Critical Raw Materials for the EU[END_REF], and this list is regularly updated. The 2017 list contains 27 different non-energy raw materials because risks of their supply shortage and their impacts on the EU economy are higher than those of the other raw materials.

Minerals 2020, 10, 446 3 of 20

. The need of minerals through time (adapted from [START_REF] Lemière | Mining the waste: Prospecting valuable residues optimising processes with modern technology, sustainably remediating legacy sites[END_REF]).

Very efficient mineralogical, pyrometallurgical and hydrometallurgical processes have been developed for the recovery of metals from low-grade ores and wastes in the last two decades [START_REF] Binnemans | Recovery of rare earths from industrial waste residues: A concise review[END_REF][START_REF] Falagán | New approaches for extracting and recovering metals from mine tailings[END_REF][START_REF] Matinde | Mining and metallurgical wastes: A review of recycling and re-use practices[END_REF][START_REF] Valderrama | Modified-three-product-column (3PC) flotation of copper-gold particles in a rougher feed and tailings[END_REF], such as in situ leaching, dump and heap leaching, hydrometallurgical processes and agromining, among others. An alternative area of development has been in relation to contaminated land remediation technologies, and Sapsford et al. [START_REF] Sapsford | In situ resource recovery from waste repositories: Exploring the potential for mobilisation and capture of metals from anthropogenic ores[END_REF] offer a review of many of these approaches and assess their limitations and constraints and technology status. One method of particular interest is the application of electrokinetic techniques, which have potential applicability to the fine-grained materials often found in mine waste deposits. Peppicelli et al. [START_REF] Peppicelli | Changes in metal speciation and mobility during electrokinetic treatment of industrial wastes: Implications for remediation and resource recovery[END_REF] have published the results of an Very efficient mineralogical, pyrometallurgical and hydrometallurgical processes have been developed for the recovery of metals from low-grade ores and wastes in the last two decades [START_REF] Binnemans | Recovery of rare earths from industrial waste residues: A concise review[END_REF][START_REF] Falagán | New approaches for extracting and recovering metals from mine tailings[END_REF][START_REF] Matinde | Mining and metallurgical wastes: A review of recycling and re-use practices[END_REF][START_REF] Valderrama | Modified-three-product-column (3PC) flotation of copper-gold particles in a rougher feed and tailings[END_REF], such as in situ leaching, dump and heap leaching, hydrometallurgical processes and agromining, among others. An alternative area of development has been in relation to contaminated land remediation technologies, and Sapsford et al. [START_REF] Sapsford | In situ resource recovery from waste repositories: Exploring the potential for mobilisation and capture of metals from anthropogenic ores[END_REF] offer a review of many of these approaches and assess their limitations and constraints and technology status. One method of particular interest is the application of electrokinetic techniques, which have potential applicability to the fine-grained materials often found in mine waste deposits. Peppicelli et al. [START_REF] Peppicelli | Changes in metal speciation and mobility during electrokinetic treatment of industrial wastes: Implications for remediation and resource recovery[END_REF] have published the results of an experimental study of the changes in metal speciation and mobility during the electrokinetic treatment of industrial wastes. They also consider the implications of this approach in terms of remediation and resource recovery, noting that this type of approach has the potential to convert waste materials into assets by transforming them into viable ore deposits [START_REF] Peppicelli | Changes in metal speciation and mobility during electrokinetic treatment of industrial wastes: Implications for remediation and resource recovery[END_REF].

It is evident that some old abandoned mine waste deposits could be increasingly relevant as sources for raw materials [START_REF] Bellenfant | Re-processing of mining waste: Combining environmental management and metal recovery? In Mine Closure[END_REF], and some of them could, by today's standards, be considered as low-grade ores. Their easy access (located on the surface without significant overburden), already crushed (no need for primary crushing) and potentially contained ore grades, which can today be economically exploited, makes them interesting materials for the possible future valorisation for resource recovery. If located close to consumption centres, such materials can also potentially be used in construction or as a source for the production of construction materials [START_REF] Ndlovu | Waste Production and Utilization in the Metal Extraction Industry[END_REF]. These deposits are usually located next to historic mines, where the natural environment has undergone many changes in the past and can pose a potential source for the future dispersion of pollutants into the environment. Therefore, there may be an advantage in combining resource recovery from mine waste deposits with site rehabilitation processes. This has also been recognised by the EU, funding several projects dealing with material recovery from mine wastes whether, for example, through the Horizon 2020 programme (i.e., Smart Ground or Remediate projects) or the EIT (The European Institute of Innovation & Technology) RawMaterials network (RIS-RECOVER, RIS-CuRE, RIS-ALiCE and many others).

However, little is generally known about the physical and chemical characteristics of mine wastes, particularly in older deposits. The same is true for their composition below the surface, homogeneity and any secondary processes following deposition. The lack of reliable data about these deposits, combined with the ambiguity in many countries regarding which legislation takes precedence for resource recovery from mine waste deposits (i.e., mineral extraction, waste management, environmental protection, planning, etc.) seems to present a barrier to their large-scale reuse. In order to examine the potential for resource recovery from mine waste, it is therefore essential that a detailed understanding of the composition and properties of these wastes are developed first.

One potential source of the composition data and properties of mine wastes is the inventory of mine waste deposits available for each EU member state. They have been produced according to the EU Directive 2006/21/EC [16] (often referred to as "The Mine Waste Directive"). This directive was a response to two large environmental disasters caused by improper tailings management, the Aznalcollar tailing dam collapse (Spain, 1998) [START_REF] Grimalt | The mine tailing accident in Aznalcollar[END_REF][START_REF] Mcdermott | The Aznalcóllar tailings dam accident-A case study[END_REF] and the Baia Mare cyanide spill (Romania, 2000) [START_REF]Report of the International Task Force for Assessing the Baia Mare Accident[END_REF], and one of its principal aims is to prevent similar disasters in the future. According to Article 20 of this directive, all EU member states are obliged to produce an inventory of closed and abandoned (mine) waste facilities which cause or have a potential to cause serious negative environmental impacts or pose a threat to humans. Such inventories are required to be updated regularly and made publicly available from 1 May 2012.

Another EU document published in 2008 details the Raw Materials Initiative (COM(2008)699) [START_REF]The Raw Materials Initiative-Meeting Our Critical Needs for Growth and Jobs in Europe[END_REF]. This document was not directly linked to the previously mentioned 2006/21/EC directive [16], and was adopted as a response to a perceived potential global threat to the uninterrupted supply of mineral resources which are vital for the EU's economic development. This initiative defines three pillars regarding the sustainable supply of raw materials for the EU economy: a fair and sustainable supply from global markets, a sustainable supply from within the EU and a resource efficiency and sustainable supply from secondary raw materials through recycling. This paper addresses the last pillar of the Raw Materials Initiative because the recycling of old and abandoned mine waste deposits, which are abundant within the EU due to a long-lasting mining tradition, could be an interesting source of raw materials for the EU economy. However, before actual mine waste recycling projects can be undertaken, many steps are required, the first of which consists of collecting basic information about individual mine waste deposits.

Therefore, the main objective of this study is to review a sample of national mine waste inventories, and to provide an assessment of their suitability as an initial source of data for potential resource recovery projects, while a secondary objective of this study is to define the set of most important mine waste valorisation parameters, which could be potentially contained within the abovementioned registries. This information will be useful to policy makers who will benefit from an improved understanding of the most critical national mine waste valorisation data gaps and determine steps forward, while the metallurgical and extractive industry will benefit from an initial assessment of the data availability for a set of EU countries to obtain information about the potential for resource recovery from mine wastes.

Materials and Methods

The materials used in the study were gathered by a literature review, information collected in the extractive waste inventories of seven EU member states, listed in Table 1 (France, Hungary, Italy, Portugal, Slovenia, Spain and UK; this study was conducted in the period between 2017-2019, when the UK was still the European Union member state) and enquiries to different experts in public and private institutions. These countries were chosen to offer coverage of a range of varying geological conditions, geographical locations and mining legacies in the EU. The work was carried out in the European Cooperation in Science and Technology (COST) action Mining the European Anthroposphere (MINEA), working group 2.1 (WG2.1)-"Resource potential in residues from extractive industries". The steps conducted in this research are presented schematically in Figure 2 and are described in more detail in Sections 2.1 and 2.2.

publicly available from 1 May 2012.

Another EU document published in 2008 details the Raw Materials Initiative (COM(2008)699) [START_REF]The Raw Materials Initiative-Meeting Our Critical Needs for Growth and Jobs in Europe[END_REF]. This document was not directly linked to the previously mentioned 2006/21/EC directive [16], and was adopted as a response to a perceived potential global threat to the uninterrupted supply of mineral resources which are vital for the EU's economic development. This initiative defines three pillars regarding the sustainable supply of raw materials for the EU economy: a fair and sustainable supply from global markets, a sustainable supply from within the EU and a resource efficiency and sustainable supply from secondary raw materials through recycling. This paper addresses the last pillar of the Raw Materials Initiative because the recycling of old and abandoned mine waste deposits, which are abundant within the EU due to a long-lasting mining tradition, could be an interesting source of raw materials for the EU economy. However, before actual mine waste recycling projects can be undertaken, many steps are required, the first of which consists of collecting basic information about individual mine waste deposits.

Therefore, the main objective of this study is to review a sample of national mine waste inventories, and to provide an assessment of their suitability as an initial source of data for potential resource recovery projects, while a secondary objective of this study is to define the set of most important mine waste valorisation parameters, which could be potentially contained within the abovementioned registries. This information will be useful to policy makers who will benefit from an improved understanding of the most critical national mine waste valorisation data gaps and determine steps forward, while the metallurgical and extractive industry will benefit from an initial assessment of the data availability for a set of EU countries to obtain information about the potential for resource recovery from mine wastes.

Materials and Methods

The materials used in the study were gathered by a literature review, information collected in the extractive waste inventories of seven EU member states, listed in Table 1 (France, Hungary, Italy, Portugal, Slovenia, Spain and United Kingdom) and enquiries to different experts in public and private institutions. These countries were chosen to offer coverage of a range of varying geological conditions, geographical locations and mining legacies in the EU. The work was carried out in the European Cooperation in Science and Technology (COST) action Mining the European Anthroposphere (MINEA), working group 2.1 (WG2.1)-"Resource potential in residues from extractive industries". The steps conducted in this research are presented schematically in Figure 2 and are described in more detail in Sections 2.1 and 2.2.

Defining Key Mine Waste Valorisation Data Parameters

In order to evaluate the use of national mine waste inventories as a source of data, it was necessary to define the parameters of mine wastes that would be needed to understand their potential for resource recovery. This objective was achieved through a literature review, workshops and review of best practices. A final list was then formulated and refined by experts in the COST action MINEA, WG2.1. The literature review included academic, policy and practice literature, and examples from actual material recovery projects, which were used to develop an initial list of the parameters used to valorise the mineral resources. A key study identified was presented by Panagiotopoulou et al. [START_REF] Panagiotopoulou | Valorisation of Extractive Industries' Wastes[END_REF], who describe several cases of material recovery from mine waste within the EU and defined the most crucial steps from the idea towards realisation. Other publications used for the literature review were different key reports from the topic [START_REF] Lemière | Mining the waste: Prospecting valuable residues optimising processes with modern technology, sustainably remediating legacy sites[END_REF][START_REF] Bellenfant | Re-processing of mining waste: Combining environmental management and metal recovery? In Mine Closure[END_REF][START_REF] Garbarino | Best Available Techniques (BAT) Reference Document for the Management of Waste from Extractive Industries in Accordance with Directive 2006/21/EC[END_REF][START_REF]Recovery of Critical and Other Raw Materials from Mining Waste and Landfills: State of Play on Existing Practices; EUR 29744 EN[END_REF][START_REF] Bodenan | Re-Processing of mine tailings: Discussion on case studies[END_REF][START_REF] Dino | Smart Ground Project: A New Approach to Data Accessibility and Collection for Raw Materials and Secondary Raw Materials in Europe[END_REF][START_REF]Management of Mining, Quarrying and Ore Processing Waste in the European Union. Study Made for DG Environment[END_REF], as well as CRIRSCO [START_REF]International Reporting Template for the Public Reporting of Exploration Targets, Exploration Results[END_REF], JORC [START_REF]Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves (The JORC Code 2012 Edition[END_REF], PERC [START_REF]Pan-European Standard for Reporting of Exploration Results, Mineral Resources and Reserves[END_REF] and UNECE [START_REF]United Nations Framework Classification for Fossil Energy and Mineral Reserves and Resources 2009 Incorporating Specifications for Its Application[END_REF] classification codes for reporting the exploration results, mineral resources and reserves and the references contained in the aforementioned documents. The parameters identified included those related to basic site and commodity information, historical framework, data collection methodology, extractability and accessibility of the secondary resources, the policy and legislative environment, as well as the relevant chemical and physical properties. By accessibility we mean if there are any legal, environmental or societal obstacles for a material recovery project, and by extractability we mean whether the material can be extracted and reallocated without posing a serious risk to workers and the environment. The initial list of parameters was further evaluated and refined by between 40 and 50 experts from the fields of mining, geosciences, material processing and others, who participated in a series of workshops, organised or co-organised by the authors of this study and funded by the MINEA network: Ljubljana (23-24 February 2017), Vienna (14-15 December 2017) and Budapest (26-27 November 2018) working group meetings; the workshop/conference "Knowledge base for material resources/reserves for construction and demolition waste, landfills and waste incineration residues recovery" (Prague, 24-25 January 2019); the workshop "Knowledge base for anthropogenic resource and reserve estimates II" (Brussels, 20 March 2019); and the conference "Recovery of secondary raw materials from mining residuals and waste, case studies and best practices" (Berlin, 23 May 2019). During the meetings and workshops, this list was supplemented with data from case studies of resource recovery from mine waste in the EU and globally, with an emphasis on the experiences from different cases from Greece (e.g., Kassandra, Kirki, Lavrion, Tsagli, Domokos, Zidanio, Mantoudi mining areas and others) and from projects identified as relevant by the French Geological Survey BRGM (Pinto Valley, Arizona, USA; Disputada Mine, Chile; Kaltails Project, Australia; Kasese, Uganda, and others). This process resulted in the final list of the key mine waste valorisation parameters presented in this paper.

Review and Assessment of National Mine Waste Inventories

We carefully examined national mine waste inventories (made according to the EU directive 2006/21/EC obligations [16]) to ascertain the availability of the key data parameters for a preliminary assessment of the resource recovery potential of specific mine wastes. Major data gaps in the critical information for deposit ranking were identified and according to the findings' recommendations were made, to assist a collection of initial information needed for mine waste resource recovery projects in the future. Seven member states' national inventories were reviewed (Table 1) and were compared against the list of parameters needed to evaluate the potential for resource recovery, and, where necessary, experts from national authorities responsible for the inventories were consulted to supplement the review (Table 1).

Results and Discussion

A set of key mine waste valorisation parameters for material recovery were divided into three groups using an approach similar to that presented in Panagiotopoulou et al. [START_REF] Panagiotopoulou | Valorisation of Extractive Industries' Wastes[END_REF]. The "basic" group of parameters describes the characteristics related to the mine waste deposit, i.e., the location, type of material, data collection methods, history of mine, etc., as well as the main drivers and barriers for resource recovery, e.g., legislation, land use restrictions, the availability of data, etc. (Table 2). The "metal-centric" group includes crucial information about the properties of mine waste material that should be considered to further extract valuable minerals/metals. The chemical and mineralogical composition of the mine waste defines the potential metallurgical or chemical extraction process, while the physico-chemical properties mainly define the pre-processing activities (drying, grinding, additives, homogenisation, separation, etc.) needed before the commodity extraction. Finally, the "material-centric" group describes the key parameters required to assess the feasibility of using mine waste for products for the construction sector. The parameters describing the composition of the material allow us to define the materials' ability to form clinker minerals and other binding agents during the production process, as well as the needs of specific additives. Parameters related to the physico-chemical properties allow us to estimate the costs and equipment needed to pre-process the source material (grinding, separation, screening, etc.). The determination of the physical and chemical properties of the potential end-products allows the assessment of the economic viability of resource recovery. During the reviewing of the national mine waste registries, we identified two special cases. Firstly, in Italy, the data for the number of sites with a detailed assessment (Table 1) included sites from the whole country which had been subjected to either research projects, or had been reclaimed (completed or ongoing), and all Sardinian sites (data published in 2017 registry update) which had been subjected to an additional assessment. In 2009, Sardinia region developed the so-called A.R.A.G.N.A. method (Relative Risk Analysis of Abandoned mining sites in Sardinia) which was applied to Sardinian mining sites. The method provides detailed instructions concerning the sampling procedures and methods, data processing, characterisation, etc. The method has been developed for risk assessment, thus there is a lack of information about minerals and elements that could be of industrial interest. Detailed information is not available in the report published by ISPRA (Italian Institute for Environmental Protection and Research), who is responsible for Italian mine waste registry. Secondly, four different authorities are managing the mine waste registry in the UK, with each one responsible for a specific geographic region, thus they also used slightly different risk assessment methodologies. The Northern Ireland risk assessment measured downstream water and sediment quality from sites mined for bauxite, copper, iron ore, lead, coal, lignite and baryte, but no detailed assessment of wastes themselves have been made. The risk assessment was based on the use of hazard quotients, and As, Cd, Cr, Cu, Fe, Ni, Pb and Zn have been tested. In Scotland, England and Wales, the risk assessment was based on the elevated metal concentrations in water catchments and known locations of mines, supplemented with local authority information which identified additional sites to be included due to the explosive risk or instability.

The key valorisation parameters' availabilities in the reviewed national mine waste registries are shown in Table 3 ('basic" group), Table 4 ("metal-centric" group) and Table 5 ("material-centric" group). The analysis of the information in Table 3 shows that the main objective of creating national mine waste inventories was to decrease their potential and actual environmental hazard and impact, with the aim of determining the specific tailings in most urgent need of remediation, and only to a lesser extent for their potential for resource recovery in the future (Figure 3a).

Mine waste deposits were generally classified according to their potential for further mobilisation of harmful substances to deposits where there is no or low risk, and deposits of higher risk. Higher risk sites were then assessed in greater detail (Table 1) and, as a result, have more data available. This approach was taken by the majority of countries (Figure 3b). However, the exceptions are Spain, where only larger deposits were assessed, and UK, where detailed information about specific mine waste deposits are not contained in the national mine waste inventory, but instead in other databases and reports collected on an ad-hoc basis. This demonstrated that national mine waste inventories are generally an incomplete and inconsistent data source for the mine waste valorisation for material recovery. The most useful information contained in the inventories are the location of the deposit, the general description of the material and the estimated quantity of such a material, while the detailed assessment of individual deposits focused more on environmental parameters (i.e., concentrations of potentially toxic elements (PTEs) and the results of leaching tests, sediment mobilisation or similar). It is apparent that only Italy and Spain have more comprehensive information regarding all the listed mining sites in the inventory. In France, Hungary and UK, the amount of data for each site is predominantly linked with assessment of the site's environmental risk. In the UK, the environmental risk determination was based by the presence of receptors, and therefore is not necessarily an indication of the concentrations of metals in the wastes; there are thousands of abandoned metal mines in England and Wales, yet only around 150 in the inventory [START_REF]Inventory of Closed Mining Waste Facilities[END_REF]. In Slovenia, the size of sites was the most relevant parameter (others parameters are linked to the specific type of mine waste), determining a further data gathering protocol, and in Portugal, the data availability is limited to the remediated and active sites.

Another important factor is the local policies and regulations pertaining to resource recovery from mine waste, which is also almost completely disregarded in the national inventories. One very important aspect is the ownership of the tailings (i.e., state, county, municipality, private entity, etc.), as well as the indication of key legislations and policies, which regulate mine waste exploitation and processing (i.e., mining legislation, spatial planning at national and local level, environmental protection, waste management, etc.). Currently, it seems that many relevant regulations apply, including those related to mineral extraction, waste management, land use planning and environmental protection, and practices are not harmonised within the study area (Figure 3c). For example, it is not always apparent whether any resource recovery would be considered as mineral extraction, waste management or remediation, all of which have different regulatory regimes in place. Therefore, it is rarely clear which stakeholders would need to be consulted to examine the potential of resource recovery, and many different regulations could apply, making the permitting process less transparent and potentially much more costly and protracted. Data from Table 4 show that the key "metal-centric" and "material-centric" resource recovery valorisation parameters are generally available for larger and high-risk sites, but are mainly focused on the analysis of PTEs. However, the mine waste deposits could be very heterogeneous, depending on the history of ore extraction and processing, as well as the occurrence of secondary processes in the waste column, but the information on PTE levels are generally available only from surface materials, not only in the UK [START_REF] Crane | Physicochemical composition of wastes and co-located landscape designations at legacy mine sites in south west England and Wales: Implications on resource potential[END_REF], but also elsewhere. This also limits the usefulness of mine waste registries as a source of information for metal extraction beyond providing only brief information in order to plan more detailed investigations. Another drawback is also that national mine waste registries generally do not contain information about metals that are not classified as PTEs. Many of such metals are regarded as critical elements today (e.g., Li, Ge or rare earth elements) and mine waste dumps could be an interesting source of these raw materials.

Even less information about mine waste is provided regarding the key "material-centric" valorisation parameters (Table 5). Except for a few cases from the Italian inventory, the information of the content of alkali ions, redox potential, silicon, organic substance, etc., was not assessed at all. The only exception is the levels of PTEs, measured in larger sites or sites with a higher environmental risk. Therefore, we can conclude that national mine waste registries are not a useful initial source of information to valorise mine waste for potential material-centric recovery (i.e., to produce construction materials), but do perhaps provide a basis for a further search for information from other sources (i.e., papers, reports, projects, archiver, etc.).

During the assessing of the specific datasets from the national registries, it also became evident that different countries had different data access policies (Figure 3d). While most countries provide information about the locations of mine waste deposits and basic characteristics, it is still very hard to access detailed reports, which contain the information needed for the potential assessment of resource recovery from mine waste. Even if such reports are publicly available, they cannot be found in one place, but could be scattered across different locations (i.e., web pages, libraries, etc.). Another potential barrier is language, where the information and reports are, with the only exception of Hungary, provided only in the national language. UK, France or Spain are, of course, a special case here because their languages can be regarded as world languages. However, this is not the case for smaller countries such as, for example, Slovenia or Portugal. This barrier can be overcome by hiring translation services, local experts or use electronic translator services, but the initial identification of potential future mine waste recycling projects by international companies can be made much easier if data are available in a commonly spoken language. Therefore, we can conclude that countries could also contribute to the use of mine waste registries outside the originally designated scope of environmental protection by placing searchable data online in one place, along with all non-confidential details, and to also make them available in English. This would be a significant step forward in providing mine waste data in the context of future resource recovery and reuse, and would move towards a more harmonised approach across the EU. However, the French example represents the best practice in the sense of the amount of data contained in the national mine waste inventory. Investigations on closed or abandoned mine sites and mine waste follow a consistent national framework in France. Sites with a responsible owner are usually documented through the mine lease renunciation document, which establishes the respective obligations of the owner and of the administration. For the much larger number of sites without a responsible party, investigations are led by a public agency (GEODERIS) to identify the potential risks at the site (mainly geotechnical and environmental) via a tiered approach. A first level of investigation on all sites leads to a selection of sites based on potential risks (ranking) that require further assessment. Subsequent investigations (second level) are then performed at these sites where an evidence of risk was demonstrated or in cases where actual damage is recognised. The highest level of investigation is where a significant risk of adverse health or environmental impact was identified in the vicinity of the site. It can even be applied to local housing ("maison sur depot") [START_REF] Geoderis | Suites Données à L'inventaire DDIE Actualisation Méthodologique Introduction de la Catégorisation des Secteurs[END_REF] when the property is located on waste. Even if these investigations are led by a specialised public agency, they are carried out on behalf of, and funded by, the administration, which keeps their results out of the public domain until the full cycle of investigations has been completed. This can result in long delays in the disclosure of site-scale primary information such as waste data.

Within the analysed countries in this study, the UK is a special case. The national mine waste inventories contain only basic data, based on the environmental or human health risk. However, much more data can be obtained from the British Geological Survey which holds a national dataset (BRITPITS) that includes information on every mine location (as point data) in the UK [START_REF] Cameron | Use Guide for the BRITPITS GIS Dataset[END_REF]. However, the mine waste inventories have been compiled, often based on this dataset, by the individual regulatory agencies (Environment Agency, The Scottish Government and the Northern Ireland Department of Environment). The methods used in England and Wales, and Scotland are similar, whereas those used in Northern Ireland differ in their approach. In England and Wales, and Scotland, the data from BRITPITs have been used in conjunction with information held by local authorities and to assess the risk from closed and abandoned mines using a source-pathway-receptor approach. These included leaching, erosion, windblown dust and dermal contact/ingestion, providing a pathway for pollution, heap, dam or pond failure, resulting from instability and the smoke, heat, dust and gases caused by flammable materials. The receptors included human health, surface and groundwater, protected ecological systems, property, crops and livestock. This was tested against a series of criteria for serious environmental risk (e.g., based on Environmental Quality Standards for surface water quality, meeting the definition of contaminated land). Potential sites were sought via a proforma that was sent to all local authorities requesting information on the sites in their areas with the potential for inclusion on the inventory. This was used in conjunction with information on the location and volume of mine wastes associated with abandoned mines, as estimated by the British Geological Survey [START_REF] Palumbo-Roe | The Nature of Waste Associated with Closed Mines in England and Wales[END_REF], and information on water quality taken from a range of sources to develop the inventory. The approach in Northern Ireland was similar except that a hazard quotient was used in the risk assessment process when examining the source-pathway-receptor linkage [START_REF] Palumbo-Roe | Inventory of Closed Mine Waste Facilities in Northern Ireland, Phase 2[END_REF].

Based on the findings of this study, a recommendation for policy makers and regulative bodies would be to make all data which are contained within national mine waste registries available online (if not yet done), together with at least the basic data in English. An EU-wide assessment of the resources available in mine wastes is needed to inform decision makers on the management of mineral resources, whether to prioritise new, efficient methods for extracting resources from wastes or changing policy to simplify resource recovery in the case of expressed interest from the private sector. The detailed information being compiled on a case-by-case basis does not enable a national or EU-wide strategic assessment of the potential value in these sites and whether they could alleviate some of the concerns regarding raw material supply security from domestic sources. Currently, case-by-case data that have been collected make a persuasive case for resource recovery, especially where sites are causing adverse environmental impacts, and the potential release of land being used for waste storage and resource recovery can be combined with site remediation. Without this EU-wide assessment, it is very hard to assess the potential for raw material supply from mine waste deposits. A shift of scope from environmental protection towards including the consideration of material recovery from abandoned mine waste sites is needed in any possible future nation-or EU-wide mine waste data collection. In the case of potential new data collection or national mine waste registry updates, it is suggested that the collection of the following information should be prioritised first: a more detailed assessment of the quantities of specific mine wastes deposits and their homogeneity, the levels of commodity elements in mine wastes, the determination of parameters describing site accessibility and other missing parameters from the "basic" group of parameters. It is also recommended that a case-wise assessment should be made to collect the parameters needed for the consideration of metal-centric or/and material-centric mine waste valorisation and materials recovery.

A further recommendation to those exploring the potential for resource recovery from mine wastes would be to use these national mine waste registries to find basic information about potential suitable sites, and then consult other sources of information (geological surveys, regional or local authorities) to obtain more data in order to preliminarily valorise specific mine waste deposit sites.

Conclusions

In this study, we examined national mine waste registries from seven European member states (France, Hungary, Italy, Portugal, Slovenia, Spain and United Kingdom), developed in accordance with the Mine Waste Directive (2006/21/EC), for their potential use as an initial source of information for the valorisation of specific mine waste deposits for material recovery. The amount of data which have been collected and is now available varies from country to country. Due to the original motivation for their collection being the prevention of PTEs' mobilisation into the environment and the long-term stability of mine waste deposits, potential future resource recovery or reuse was not considered as the priority. This is reflected in the data content, which makes the national mine waste inventories only a potential source of basic information, i.e., location of mine waste, main commodity or estimation of tailing deposit volume. Only limited or even no information is provided in regard to the homogeneity, grain size distribution or content of substances which are not regarded as pollutants, but are important for the potential future use of such materials (alkali and alkaline earth ions, moisture, redox state, etc.). In most cases, there is not sufficient enough data that can be used in assessing projects for resource recovery from national mine waste registries of EU countries. Further, the legislative, regulative and policy frameworks for material recovery from mine waste are not homogeneous across the EU. The results of this study can be useful for policy makers that could benefit from it by obtaining the most critical nation-wide mine waste valorisation data gaps and determine the steps forward, while the metallurgical and extractive industry could find the first glimpse of data availability for a set of EU countries to obtain information about mine waste potentials, especially in the light of the latest developments in tailings and materials reprocessing (including hydrometallurgical processes, bioleaching, in-situ leaching and others).

Figure 1 .

 1 Figure1. The need of minerals through time (adapted from[START_REF] Lemière | Mining the waste: Prospecting valuable residues optimising processes with modern technology, sustainably remediating legacy sites[END_REF]).

Figure 2 .

 2 Figure 2. Methodological approach and the different steps conducted in this study.

Figure 2 .

 2 Figure 2. Methodological approach and the different steps conducted in this study.

Figure 3 .

 3 Figure 3. (a) The main country vision for creating national mine waste registries. (b) Availability of detailed mine waste deposit data. (c) Key legislation which regulates resource recovery from mine waste (simplification). (d) Data availability and language used in national mine waste registries.

Table 1 .

 1 Responsible organisation for preparing and keeping the National mine waste registries and basic information about the registries.

	Country	Abbreviations	Name	Reference	Total No. of Sites	No, of Sites with Detailed Assessment	Measured Substances
	France	GEODERIS	GEODERIS	[31-33]	3144	200	Pb, Zn, As, Cd, Cr, Hg, Cu, V, Mo, Co, Ni, Se, Sb, Tl
	Spain	IGME	Instituto Geológico y Minero de España	[34]	370	370	Pb, Zn, As, Cd, Cr, Hg, Cu, V, Mo, Co, Ni, Se
							Asbestos, Ag, As, Cd, Cr, Co, Cu, Hg,
	Italy	ISPRA	Istituto Superiore per la Protezione e la Ricerca Ambientale	[35]	650	220	Mn, Pb, Ni, Tl, Zn, Sb, Sn, Be, V, CN, Fluoride, Aromatic compounds,
							TPH C > 12
	* UK	BGS EA NIDoE	British Geological Survey Environment Agency (England and Wales) Scottish Government Northern Ireland Department of Environment	[36-40]	404	0	Harmful substances are measured in downstream water and not in waste material
	Hungary	MBFSZ	Magyar Bányászati és Földtani Szolgálat	[41]	1046	71	Ag, Au, As, Be, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Sb, Se, Sn, Th, Tl, U, Zn
	Slovenia	ARSO	Agencija Republike Slovenije za okolje	[42-44]	173	78	As, Ba, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Zn
	Portugal	DGEG, EDM	Direcção Geral de Energia e Geologia	No references	199	39	Cd, Cu, Ni, Pb, Zn, Cr total, Cr 6+ , Hg, Co, Mo, As

Table 2 .

 2 Key identified mine waste valorisation parameters.

		Parameter ID	Short Description	Sub Division
			Location, history, mining
		B1	and processing
			technology of the site
			Volume, area and
		B2	structure of the existing
			waste deposits
		B3	Reason for which the mine was abandoned
	Basic Valorisation Parameters	B4	Homogeneity of the tailings
		B5	Methodology of data collection
		B6.1		Actual physical
		B6.2		Potential physical
		B6.3	Environmental impacts	Actual chemical
		B6.4	of mine waste deposit	Potential chemical
		B6.5		Need for remediation
		B6.6		Remediation costs
		B7.1	Site extractability	Possibility for safe extraction and waste relocation
		B7.2		Revegetation status
		B8.1		Ownership
		B8.2 B8.3	Site accessibility	Special permits required Land use restrictions
		B8.4		Other legislative barriers
		B9.1 B9.2	Data availability	Data managing authority Language(s)
		M10.1	Chemical and	Matrix
		M10.2	mineralogical	Commodity elements
	Metal-Centric	M10.3	composition	Trace elements
	Valorisation Parameters	M11.1 M11.2 M11.3	Physico-chemical properties	Grain size distribution Moisture content Redox state at different pH
		C10.1		Type and content of alkali ions
		C10.2	Chemical and	Type and content of alkaline earth ions
		C10.3	mineralogical	Type and content of silicon
	Material-Centric Valorisation Parameters	C10.4 C10.5	composition	Type and content of potentially toxic elements (PTEs) Type and content of organic substance
		C11.1	Physico-chemical	Moisture
		C11.2	properties	Grain size distribution
		C12.1	Potential final products	Mechanical properties
		C12.2	specifications	Thermal properties

Table 3 .

 3 Key "basic" valorisation parameters (Pa.)-data availability in different EU countries (AV-Available, NA-Not Available, NAS-Not Assessed).

	Pa.	France	Spain	Italy	UK	Hungary	Slovenia	Portugal
	B1	AV only for some sites	AV as short description	AV for most of the sites, detailed for some	AV (1)	AV only in mine closure reports	AV as short description of history of mining site	AV as short description of history of mining site
	B2	AV, data quality depends on potential risk	AV	AV if provided by Regional Geological Survey	NA in inventory, but in various publications	Estimation, AV in survey reports technical operation plans or individual	Estimation	AV
	B3	NAS	AV for larger mines	NA	NA	NA	AV for high risk sites	AV in selected sites
	B4	AV for high risk sites with ongoing remediation	NA	NA	NA in inventory, but in various publications	NA	NA	Visual estimation
				AV for deposits
	B5	AV for high-risk sites (2)	AV (3)	investigated with ARAGNA method	AV as part of the methodology report	NA	AV	AV for remediated sites
				(Sardinia)
	B6.1	AV for potential high-risk sites	AV	AV	AV, based on risk assessment (4)	AV	AV for high risk sites	AV for active mines and remediated sites
	B6.2	AV for high risk sites with ongoing remediation	AV	AV	AV, based on risk assessment (4)	AV	AV for high risk sites	AV for active mines and remediated sites
	B6.3	AV for potential high-risk sites	AV	AV	AV, based on risk assessment (4)	AV	AV for high risk sites	AV for active mines and remediated sites
	B6.4	AV for high risk sites with ongoing remediation	AV	AV	AV, based on risk assessment (4)	AV	AV for high risk sites	AV for active mines and remediated sites
	B6.5	AV for potential high-risk sites	AV	AV	AV, based on risk assessment (4)	AV	AV for highly risk sites	AV for risk sites
	B6.6	AV for high risk sites with ongoing remediation	NAS	AV for specific sites within the remediation projects	Based on risk assessment (4)	NAS	NAS	AV, but possibly confidential
		AV for high risk sites
	B7.1	with ongoing	NAS	NAS	NA	AV for selected deposits	NAS	NAS
		remediation
	B7.2	NA	AV	AV	NA

Table 3 .

 3 Cont.

	Pa.	France	Spain	Italy	UK	Hungary	Slovenia	Portugal
	B8.1	AV for sites with no responsible owner	NAS	Data in the environmental protection and land use plans	NA, but held in BRITPITS and/or the Land Registry	AV for operating mines	Information is AV in land register	AV
	B8.2	According to Mining legislation	NAS	Depending on land use restrictions and site hazard characterisation, involving local and national authorities	Mineral extraction is subject to planning -likely to be complicated as may fall under environmental regulation	According to Mining legislation	Depending on land use restrictions and site hazard characterisation, involving local and national authorities	Mine waste are not classified as waste in the Portuguese environmental law
				Depending on land	There may be land use
	B8.3	According to Mining legislation	NAS	use restrictions and site hazard characterisation, involving local and	restrictions which could include cultural or ecological designations associated with past	According to Mining legislation	Determined in national and municipality spatial plans	Determined in national and municipality spatial plans
				national authorities	mining
	B8.4	If reprocessing includes activities outside the scope of mining regulations	NAS	Depending on land use restrictions and site hazard characterisation, involving local and national authorities	Waste management and pollution control regulations as well as those protecting the natural environment and cultural assets	If reprocessing includes activities outside the scope of mining regulations	Depending on land use restrictions and site hazard characterisation, involving local and national authorities	If reprocessing includes activities outside the scope of mining regulations
	B9.1

Table 4 .

 4 Key "metal-centric" valorisation parameters-data availability in different EU countries (AV-Available, NA-Not Available, NAS-Not Assessed).

	Pa.	France	Spain	Italy	UK	Hungary	Slovenia	Portugal
	M10.1	AV for high risk sites with ongoing remediation	AV, not public	AV for investigated sites	NA, but AV for a limited number of mines in other documents	AV	AV	AV for active sites and selected remediated sites
	M10.2	AV	AV, not public	AV for investigated sites	NA, as above	NA, only for specific sites in other reports	AV for high risk sites	AV for active and selected remediated sites
	M10.3	AV for high risk sites with ongoing remediation	AV, not public	AV for investigated sites	NA, as above	AV for red mud tailings and for other sites containing PTEs	AV for high risk sites	AV for active and selected remediated sites
	M11.1	AV for high risk sites with ongoing remediation	AV, not public	AV for investigated sites	NA, as above	AV	Visual estimation for some sites	AV for active and selected remediated sites
	M11.2	AV for high risk sites with ongoing remediation	NAS	AV for investigated sites	NA, as above	NAS	NAS	AV for active sites
	M11.3	AV for high risk sites with ongoing remediation	pH and electrical conductivity only, not public	AV for investigated sites	NA, as above	NAS	NAS	AV for active sites

Table 5 .

 5 Key "material-centric" valorisation parameters-data availability in different EU countries (AV-Available, NA-Not Available, NAS-Not Assessed).

	Pa.	France	Spain	Italy	UK	Hungary	Slovenia	Portugal
	C10.1	NAS	NAS	AV as potential residues	NA, but AV for a limited other documents number of mines in	NAS	NAS	NAS
	C10.2	NAS	NAS	AV as potential residues	NA, as above	NAS	NAS	NAS
	C10.3	NAS	NAS	AV as potential residues	NA, as above	NAS	NAS	NAS
	C10.4	NAS	AV, not public	AV as potential residues	NA, as above	AV for larger waste sites	AV for high risk sites	NAS
	C10.5	NAS	NAS	AV as potential residues	NA, as above	NAS	Visual estimation for some sites	NAS
	C11.1	AV for high risk sites with ongoing remediation	NAS	AV for investigated sites	NA, as above	NAS	NAS	NAS
	C11.2	AV for high risk sites with ongoing remediation	AV, not public	AV for investigated sites	NA, as above	NAS	Visual estimation for some sites	NAS
	C12.1	NAS	NAS	AV for investigated sites	NA, as above	NAS	NAS	NAS
	C12.2	NAS	NAS	AV for investigated sites	NA, as above	NAS	NAS	NAS

Acknowledgments: Authors would like to specially thank Dimitrios Panias from the School of Mining & Metallurgical Engineering, Greece, and Patrick d'Hugues from BRGM, France, for sharing their mine waste valorisation experiences with the group, and Ulrich Kral from TU Wien for coordinating the MINEA cost action and encouraging the publication. Special regard goes to three reviewers for their valuable comments, which improved the manuscript.

2 National National National National National and English National National (1) AV, but with cross-referencing to BRITPITS which details the type of mine (open cast, underground) but not the technology. (2) A guide with a methodology has been published [46].

Funding: This study was conducted as a part of the MINEA Cost action, CA15115, "Mining the European Anthroposphere", as a part of activities within working group no. 2.1, which covers the travel expenses. Working hour expenses are covered by the following organisations: The Slovenian Research Agency (research core funding No. P1-0025 "Mineral resources"), BRGM, France, (post-mining research activities, Ministry in charge of the Environment), CERENA, Portugal, (through the strategic Project UID/ECI/04028/2019) and the Natural Environment Research Council /Economic and Social Research Council-funded 'INSPIRE: IN Situ Processes In Resource Extraction from waste repositories' project (Grant Reference NE/L013916/1).

Data in Table 1

Conflicts of Interest:

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


Amanda Pieyre

Takeshi Yokomori

Determination of wall temperature in a narrow channel by thermographic phosphor thermometry

Keywords: Mesoscale combustion, Laser induced phosphorescence, Temperature measurements, Laminar flame

The stabilization of a premixed methane/air flame in a narrow quartz tube strongly depends on the thermal environment, especially on convective and conductive heat exchanges between walls and gas. First, to determine heat transfer and better understand the stabilization mechanisms of a flame in a narrow channel, but also to obtain boundary conditions for numerical computations, temperature profile measurements are needed. Several methods for temperature measurements are available but since the outer wall, inner wall and gas flow temperature have to be measured, only a few are compatible. The Laser Induced Phosphorescence (LIP) was then selected for its semi-intrusive characteristic and good accuracy. This method is based on the use of thermographic phosphors. Thus the thermographic phosphor YAG:Pr was selected for its large temperature range. It is composed of trivalent praseodymium (Pr3+) ions doped into an yttrium aluminum garnet (YAG) host. In a first step it is bound to the outer wall surface of a 7mm outer diameter cylindrical quartz tube. A premixed methane/air flame is stabilized inside the tube and emission intensity is captured by an ICCD camera. The intensity ratio approach is then used to determine the temperature map of the wall.

Introduction

In mesoscale combustion, the analysis of thermal exchanges between the walls and the flame is a necessary step toward the understanding of a flame's stabilization mechanism in a narrow channel. The goal of this study is then to obtain a map of temperature of the external wall of a 7mm outer diameter and 1mm thick quartz tube using Laser Induced Phosphorescence (LIP) and intensity ratio method. The LIP method has the advantage of being semi-intrusive and giving a good accuracy [1]. The intensity ratio method exploits the fact that the time-integrated luminescence intensity of thermographic phosphors depends on temperature.

Temperature measurements with the LIP method

Figure 1 : Experimental set up scheme showing the light pathway from the laser to the camera. The red graph displayed above the tube is a temperature profile obtained with a thermocouple (Fig. 4).

The thermographic phosphor YAG:Pr is used for its large range of temperature. It also has two temperature dependent peaks in its emission spectra, which increases the accuracy of the intensity method compared to a phosphor with only one temperature dependent spectral band. Indeed, since the two spectral band luminescence intensity are equally influenced by critical factors, like excitation energy or phosphor layer's homogeneity, the normalization of one spectral band to another limits the influence of these factors [1].

To measure the temperature, this phosphor needs to be fixed on the outer surface of the quartz tube. Therefore, a phosphorescent binder is created by mixing phosphor powder and a high temperature inorganic binder (Aremco Ceramabind 644). This paint is then sprayed on a strip of the outer surface of the quartz tube as shown in Fig. 1. The thickness of the paint layer must remain under 100 µm so that the thermal conductivity is not significantly modified, but also to avoid temperature gradient in the paint layer. It is to be noted that the radiative transfers might also be slightly modified by the change of wall transmittance.

A methane/air premix flame, equivalence ratio of 0.8, is stabilized inside the tube, while the phosphor paint is excited by a Nd:YAG Laser (Quanta-Ray GCR-130) quadrupled at 266 nm with a 10 Hz repetition rate. The emitted luminescence response of the phosphor paint is then recorded by a ICCD camera (Hamamatsu C12550 with image intensifier C9016-21). The emission light pathway is split in two. The two different pathways pass through two band pass filters, which wavelength of 487nm and 611nm corresponds to the two temperature dependent peaks of the emission spectra of the chosen phosphor. The two tube images for the two wavelength are recorded on two areas of the same camera chip as shown in Fig. 1 and Fig. 3.

Then the ratio of luminescence intensity is made between the two images for the two wavelength. This intensity ratio is linked to the temperature through a calibration curve, made by painting a 1mm diameter thermocouple with the same phosphorescent paint used on the tube. This thermocouple is placed in a temperature controlled air flow [2] and at the same position as the tube shown in Fig. 1. As for the quartz tube, the painted thermocouple is excited by the laser and the emission intensity is recorded by the camera. Simultaneously, the temperature displayed by the thermocouple is recorded. The range of temperature for this in-situ calibration goes from 300K to 700K. The calibration curve, shown in Fig. 2

Results

The Fig. 3 shows the camera chip image of intensity distribution with and without the flame in the tube. As the temperature increases, the emission intensity at 611 nm increases. On the contrary, the emission intensity at 487 nm decreases with the increase of temperature. This phenomenon is in accordance with the literature [3].

A comparison is made on Fig. 4 between the temperature profile of the outer wall measured by a thermocouple (blue curve) and an extracted temperature profile from the centerline of the temperature map obtained (red curve) (upper graph) and the temperature map obtained with the LIP method (lower graph). The measurement with the thermocouple was made using a 0.075 mm diameter type N thermocouple (Omega OPON-003), forced against the outer wall of the quartz tube and moved along the tube by a micro-displacement device (Thorlabs MTS50/M-Z8E).

In these graphs, the black line shows the flame front position. This comparison shows good results of temperature distribution and for the preheating of the fresh gas upstream from the flame. Nevertheless, it also shows some limits and inaccuracy of the LIP process: highest temperatures are only located at the center line of the map (lower graph) and the maximum measured by LIP is 50K lower then the one measured by thermocouple Fig. 4 (upper graph). Fixing these inaccuracies will be the object of further research.

Conclusion

The LIP and intensity ratio method are used to draw a temperature map of a 7mm diameter quartz tube, with a premix methane/air flame stabilized inside. This method shows great potential for temperature measurements in mesoscale and microscale combustion.

The next step of this research would be to better approximate the temperature distribution, and to get the temperature map from the inner wall of the quartz tube.

Acknowledgements

The stay of A.Pieyre in Keio University has been supported by the Erasmus Mundus EASED program (Grant 2012-5538/004-001) coordinated by CentraleSupélec.

  , corresponds to the literature [3]. The margins of errors are shown on the Fig.2 taking in account numerical errors and the accuracy of the thermocouple.

Figure 4 :

 4 Figure 4 : Temperature map on a 7mm outer diameter quartz tube outlined by the white dotted rectangle, with a stabilized premix methane/air flame inside the tube, and with an equivalence ratio of 0.8. The flame position is indicated by the black line. The black dotted line represents the start of the preheating zone of the fresh gas by the flame. The gas flow goes from the right to the left.

[ 1 ]

 1 Brübach, J., Pflitsch, C., Dreizler, A., & Atakan, B. (n.d.). On surface temperature measurements with thermographic phosphors: A review. Progress in Energy and Combustion Science, 39(1), 37-6, 2012. [2] T.F. Guiberti, D. Durox, P. Scouflaire, T. Schuller, Impact of heat loss and hydrogen enrichment on the shape of confined swirling flames, Proceedings of the Combustion Institute, Volume 35, Issue 2, 2015, Pages 1385-1392, ISSN 1540-7489.[3] J. Jordan. Thermographic phosphor based planar thermometry using the trivalent praseodymium ion (Pr3+) doped into a yttrium aluminum garnet (YAG) crystal. Spring Technical Meeting of the Central States Section of the Combustion Institute(April 22-24, 2012), pages1-11, 2012.

FlowFigure 2 :Figure 3 :

 23 Figure 2 : The calibration curve for the intensity ratio method shows the dependence of the intensity ratio on the temperature. The intensity ratio of the flame is normalized by the room temperature intensity ratio.


A Pieyre

T Yokomori

F Richecoeur

Thermal characterization of a narrow channel wall by thermographic phosphor thermometry

The stabilization of a flame in a narrow quartz tube strongly depends on the thermal environment. To determine heat transfer and better understand the stabilization mechanisms of a flame in a narrow channel, wall temperature profile measurements are needed. The Laser Induced Phosphorescence (LIP) is used with the intensity ratio method to obtain temperature distribution of the outer surface of a 5mm inner diameter quartz tube within which a lean premix methane/air flame is stabilized. This study evaluates the ability of this method to capture a large temperature gradient on a small volume with a high curvature and compares the use of two phosphors.

Introduction

The recent need for micro-technologies has increased the development of small scale portative devices. In this context, the combustion in narrow channel has a great potential for small mechanical and heat power source devices and also electrical power supply. Indeed, the micro and mesoscale combustion takes advantages of the great energy density of hydrocarbon fuels which is sixty times higher than the one for classical lithium-ion batteries and has the advantage of unlimited and immediate recharge. Based on a 10% energy conversion efficiency, the energy density of a micro-combustor is six times higher than the one of a lithium battery [START_REF] Maruta | Micro and mesoscale combustion[END_REF]. Devices using narrow channel combustors could then be used for MEMS in various applications as biomedical, telecommunications or thrusters [START_REF] Maruta | Micro and mesoscale combustion[END_REF][START_REF] Fernandez-Pello | Micropower generation using combustion: issues and approaches[END_REF]. Nonetheless the size of these micro and mesoscale devices sets some limits to overcome for a broad industrial development. In fact, in those devices the large surface to volume ratio causes massive heat loss, which leads to flame oscillations, quenching and sets flammability limits [START_REF] Kim | Effects of thermal and chemical surface-flame interaction on flame quenching[END_REF].

For tube diameters below the quenching diameter, heat losses are so important that it leads to quenching and the only solutions to counter-balance the large heat losses of the flame consist in adding external energy to the system to assist and sustain the flame. The energy addition is usually over-sized and the total budget is negative (i.e. power used to maintain the flame is larger than power produced by the flame). The lack of knowledge of the interactions between the flame and the wall makes impossible a fine tuning of assistance to the flame. To quantify heat exchanges between the flame and the tube, the present study uses a tube diameter slightly above the quenching diameter so that heat transfers have a significant impact on the flame but the latter is selfmaintained [START_REF] Maruta | Micro and mesoscale combustion[END_REF].

First, to determine heat transfer and better understand the stabilization mechanisms of a flame in a narrow channel, but also to obtain boundary conditions for numerical computations, wall temperature profile measurements are needed.

Several methods for temperature measurements are available, but since the outer wall and inner wall temperatures have to be measured, only a few are compatible. The Laser Induced Phosphorescence (LIP) was then selected for its semi-intrusive characteristic and good accuracy [START_REF] Brübach | On surface temperature measurements with thermographic phosphors: A review[END_REF]. This method is based on the use of thermographic phosphors. Two thermographic phosphors were of interest in our case for their large temperature range and presenting no oxygen dependence, and mostly used in combustion cases: the dysprosium and praseodymium doped yttrium aluminium garnet (Y3Al5O12:Dy and Y3Al5O12:Pr) [START_REF] Brübach | On surface temperature measurements with thermographic phosphors: A review[END_REF][START_REF] Jordan | Thermographic phosphor based planar thermometry using the trivalent praseodymium ion (Pr3+) doped into a yttrium aluminum garnet (YAG) crystal[END_REF]. The use of these two phosphors is compared in accuracy, and according to their sensitivity to the temperature, to obtain a temperature map of the outer surface of a 7mm outer diameter quartz tube within which a premix methane/air flame is stabilized. The intensity ratio method is used to link the phosphor emission intensity to the temperature. We evaluate the ability of this method to measure the temperature on a small volume with a high curvature and subject to high temperature gradient.

The specific objectives of the paper are presented in Section 2. The experimental setup, the diagnostic, the phosphors and the validation method are described in Section 3. The temperature maps and centreline profile obtained and the evaluation of the method applied to small volumes is made in Section 4.

Specific Objectives

In this paper the specific goals are: 1-To evaluate the capacity of the Laser Induced Phosphorescence to measure temperature when applied to small and curved volumes subject to high temperature gradients.

2-To measure the temperature of the meso scale quartz tube outer surface with a lean premix flame stabilized inside at different equivalence ratio.

3-To compare the accuracy and sensitivity of two phosphors (YAG:Dy and YAG:Pr) .

Experimental setup and introduction to LIP

The experimental setup shown in Figure 1 is on purpose rather simplistic to better understand the heat exchanges between the wall and the flame and the flame stabilization mechanism. It is composed of a quartz tube of 5mm inner diameter and 1 mm thickness in horizontal position, inside which a lean methane/air premixture is injected and ignited. The speed and equivalence ratio of the flux are controlled by two massflow meters (REF). At functioning points of equivalence ratio and speed a flame can be stabilized inside the tube. The method of Laser Induced Phosphorescence is used. A thin layer of a rare earth or transition metal doped ceramic material is painted on the tube and reaches the same temperature than the tube surface. When excited by a monochromatic UV light source, the ceramic emits a temperature dependent light. The emission spectrum is recorded and, after calibration, is processed to determine the surface temperature. This method has the advantages of being semi invasive, has a high spatial and temporal resolution, it can be used on broad temperature ranges depending on the phosphors used, it is insensitive to surface properties and robust against interferences [START_REF] Brübach | On surface temperature measurements with thermographic phosphors: A review[END_REF]. The thermographic phosphor thermometry requires three steps: the choice of phosphors, the measurement and calibration strategy and the film deposition method.

To measure the temperature distribution, the phosphors are fixed on the outer surface of the quartz tube using a phosphorescent binder. This binder is created by mixing phosphor powder (YAG:Dy Phosphor Technology, QMK66) and a high temperature inorganic binder (ZYP Coatings, HPC Binder). This paint is then sprayed on a strip of the outer surface of the quartz tube using an airbrush. The thickness of the paint layer must remain under 100 µm so that the thermal conductivity is not significantly modified, but also to avoid temperature gradient in the paint layer. The radiative transfers might also be slightly modified by the change of wall transmittance.

A methane/air premix flame is stabilized inside the tube, while the phosphor paint is excited by a Nd:YAG Laser quadrupled at 266 nm or tripled at 355nm depending on the excitation wavelength of the phosphor with a 10 Hz repetition rate (Quanta-Ray GCR-series). The emitted luminescence response of the phosphor paint is then recorded by an ICCD camera (Hamamatsu C12550 with image intensifier C9016-21). The emission light pathway is split in two. The two different pathways pass through two band pass filters, which wavelength corresponds to the two temperature dependent peaks of the emission spectra of the chosen phosphor. The two tube images for the two wavelengths are recorded on two areas of the same camera chip as shown in Figure 2.

The intensity ratio method is then used to get back to the temperature: the ratio of intensities is made between the two tubes images and linked to temperature through a calibration curve. The calibration curve is made by replacing the tube in Figure 2 by a tube furnace within which a sample of the same phosphorescent binder is placed. As for the tube, the ICCD camera records the subsequent emission of the phosphor and simultaneously the temperature is recorded by a thermocouple placed in contact with the sample. The range of temperature for the calibration goes from 300K to 1000K. (The calibration curve obtained corresponds to the literature. ) 3a). It is broadly used in the literature [START_REF] Brübach | On surface temperature measurements with thermographic phosphors: A review[END_REF]. The YAG:Pr (Y 3 Al 5 O 12 :Pr), less common, is excited at 266nm and has also two temperature dependent peaks at 487nm and 611nm (Figure 3b).

To record the emission spectra of YAG:Dy (Figure 3a) and YAG:Pr (Figure 3b), the camera in Fig. 2 configuration : the phosphor sample inside the tube furnace. The furnace is gradually increased in temperature and the emission spectra of the phosphor are recorded. The spectra are represented in Figure 3 normalised by the maximum of intensity. The doted lines represent the temperature dependent peaks of each spectrum.

These emission spectra allow us to make a first analysis on the sensitivity of the phosphors with temperature. As the temperature increases, the intensity of the 487nm temperature dependent peak of YAG:Pr drops to a very low value. At high temperatures the YAG:Pr is then less accurate than at low temperatures.

Results and Discussion

Thermocouple measurements on the outer surface

The LIP technique is subject to optical misalignments and distortions, and a comparison method is necessary to estimate the accuracy of this method applied to the measurement on the outer surface of a narrow channel. This method chosen was made using a 0.075 mm diameter type N thermocouple (Omega OPON-003), forced against the outer wall of tube and moved along the tube by a micro-displacement device (Thorlabs MTS50/M-Z8E). The temperature profiles obtained at equivalence ratio from 0.7 to 0.85 are drawn on Figure 4. The black doted line highlights the flame front position. As expected, the maximum of temperature and the length of the cooling area both increase with the equivalence ratio. This thermocouple method gives a good accuracy since the theoretical errors for the temperature, the thermocouple and for the position the displacement are both given by constructors. Due to the very small diameter of the thermocouple, another imprecision can occur by the fact that the soldered joint is not directly in contact with the quartz tube's outer surface but with the surrounding air. The inaccuracies in temperature are estimated at +/-10K and at 1mm in displacement. This method is cheap, easy to operate and accurate, but has its limits. As a result, it is used as a comparison method for the LIP technique on the outer surface of the quartz tube.

Temperature maps

Once the intensity ratio method applied with the calibration curve, we obtain a temperature map Figure 5 of the observed tube. Several experimental configurations are tested: two phosphors are compared at different equivalence ratio. In all this third section, the reference point is the flame front position, placed at the zero coordinates and the flow direction in all graphs goes from right to left.

The Figure 5 shows a comparison of temperature maps for equivalence ratio 0.7 (top) and 0.8 (bottom) and for both YAG:Dy (left) and YAG:Pr (right). In both cases we can identify three areas on the temperature maps: the preheating zone located before the flame, the flame area where the temperatures are the highest, and the cooling area after the flame. The maximum of temperature reached with both equivalence ratio is higher for YAG:Dy than for YAG:Pr. Notable optical deformations can be observed on YAG:Pr (Figure 5c &d) temperature maps. Indeed, the flame area can only be observed on the upper part of the tube. This is due to laser misalignment with the tube: the tube must be perfectly orthogonal to the laser sheet.

To compare the thermocouple temperature profiles with the LIP method, the centreline profiles are extracted from the temperature maps. Indeed, to get the centreline profile, an average in made between the lines surrounding the maximum of temperature. As a result, the top graph in Figure 6 shows the centreline profile extracted from the bottom temperature map for YAG:Dy, at equivalence ratio 0.8. Only a region of 75mm around the flame is recorded, limit being set by the camera size. The temperature map and the plotted centreline profile show that the LIP technique correctly reproduces the high temperature gradient near the flame on this small volume. The preheating and cooling zones are well represented, and the flame area can be clearly identified. However, optical imprecisions are also put in relief, when the flame area doesn't cover the entire tube's surface. First to test the repeatability of the measurements, three acquisitions are made successively, in the same conditions at equivalence ratio 0.7, for the YAG:Dy phosphor. The Figure 7 shows the three extracted centreline profiles. It gives indications on the order of magnitude of the imprecisions of the method, but also on the averaging in the centreline profile extraction. Nonetheless, the tendency of the temperature profile is similar between the three graphs.

Centreline profiles comparison with the validation method

The centreline profiles are extracted from the temperature maps for each experimental configuration and compared with the validation method of thermocouple.

The first comparison in Figure 8 is made between the centreline profiles of YAG:Pr (blue line), YAG:Dy (red line) and thermocouple profile (green doted line) at equivalence ratio 0.7. This comparison shows that the YAG:Dy predicts with more accuracy the preheating zone before the flame front, but the maximum of temperature and cooling zone is 50K overestimated. The YAG:Pr centreline is less smooth, and the preheating zone and maximum of temperature is not well reproduced. Nonetheless the cooling zone matches with the thermocouple profile. The second comparison between the same graphs but for equivalence ratio 0.8 in Figure 9 shows confirms the tendencies observed in the previous figure except for the YAG:Pr cooling zone. It can be explained by the fact that since the maximum temperature in more important, the cooling zone is also higher in temperature, and as a result not well predicted by YAG:Pr.

Error bars

With the reproductivity test made in Figure 7 for the temperature inaccuracy, but also by taking in account the optical distortions for the spatial errors, qualitative error bars are represented on the Figure 10. This graphs show that the tendencies are well reproduced by the LIP measurements, but this method needs improvement on accuracy.

Conclusion

The present study aims at measuring the wall temperature distribution along a 7mm outer diameter quartz tube with a stabilized flame inside, in order to evaluate the heat flux around the flame. The study shows that the LIP method can be used on small volumes with a great curvature and high temperature gradient. Expected optical deformations were observed on temperature maps, however the temperature gradient was correctly reproduced. Two phosphors were compared and their different sensitivity with temperature impacts the quality of the measurements according to the distance to the flame and the equivalence ratio. For more accuracy and in further work, the co-doping with these two phosphors is considered.

The intensity ratio method was used in a first step to make sure that measurements could be done on a small volume with a high curvature. For further work and to obtain a better accuracy, the LIP lifetime method has been proven to be more sensitive to temperature and more accurate than intensity ratio but harder and more expensive to set up.

Figure 1 :

 1 Figure 1: Experimental set up composed by a quartz tube within which a lean premixture methane/air flow is injected, and at functioning points a flame can be stabilised.

Figure 2 :

 2 Figure 2: Experimental scheme of the Laser intensity ratio method using LIP and calibration method Two thermographic phosphors were of interest in our case for their large temperature range and presenting no oxygen dependence, mostly used in combustion cases: dysprosium and praseodymium doped yttrium aluminium garnet (Y 3 Al 5 O 12 :Dy and Y 3 Al 5 O 12 :Pr). The YAG:Dy (Y 3 Al 5 O 12 :Dy ) is excited at 355nm and has two temperature dependent peaks at 456nm and 497nm (Figure3a). It is broadly used in the literature[START_REF] Brübach | On surface temperature measurements with thermographic phosphors: A review[END_REF]. The YAG:Pr (Y 3 Al 5 O 12 :Pr), less common, is excited at 266nm and has also two temperature dependent peaks at 487nm and 611nm (Figure3b).To record the emission spectra of YAG:Dy (Figure3a) and YAG:Pr (Figure3b), the camera in Fig.2is replaced by a spectrometer in the calibration

  is replaced by a spectrometer in the calibration

Figure 3 :

 3 Figure 3: Normalised emission spectra of YAG:Dy (a) and YAG:Pr (b) showing the two temperature dependent peaks at various temperatures. The spectrum are normalised by the maximum value.

Figure 4 :

 4 Figure 4: Temperature profiles measured by thermocouple forced against the quartz tube and moved horizontally by micro-displacement for several equivalence ratio. The black doted line marks the flame position. The flow direction is from right to left.

Figure 5 :

 5 Figure 5: Temperature maps for YAG:Dy (a & b) and YAG:Pr (c & d) at equivalence ratio 0.7 (top) and 0.8 (bottom). The white doted line highlights the tube. The flame position is at the length reference point 0. The flow direction goes from right to left.

Figure 6 :

 6 Figure 6: Centreline temperature profile (top) of YAG:Dy at equivalence ratio 0.8 extracted from the temperature map (bottom).

Figure 7 :

 7 Figure 7: Evaluation of the reproductivity of the LIP measurements for YAG:Dy at equivalence ratio 0.7.

Figure 8 :

 8 Figure 8: Comparison of centreline temperature profile of YAG:Pr and YAG:Dy to the thermocouple profile for the equivalence ratio 0.7.

Figure 9 :

 9 Figure 9: Comparison of centreline temperature profile of YAG:Pr and YAG:Dy to the thermocouple profile for the equivalence ratio 0.8 .

Figure 10 :

 10 Figure 10: Qualitative errors bars on the Laser Induced Phosphorescence method using intensity ratio.

Acknowledgements

This study was pursued in the frame of the research project ANR-14-CE05-0030-01 coordinated by EM2C laboratory.

The stay of A.Pieyre in Keio University has been supported by the Erasmus Mundus EASED program (Grant 2012-5538/004-001) coordinated by CentraleSupélec.