Anouar Belahcen

Paavo Rasilo

Thu Trang Nguyen

Stéphane Clénet

Uncertainty Propagation of Iron Loss from Characterization Measurements to Computation of Electrical Machines

This paper presents the uncertainty propagation of iron losses from the material characterization measurements to the computed losses in an induction machine. The iron losses are modelled with an analytical loss model applied locally in finite element simulation. The uncertainty is taken into account by the decomposition into Polynomial Chaos Expansion, i.e., the spectral approach. The results of the simulations show that uncertainties in the measured losses propagate in different ways and to different parts of the machine depending on the measurement frequency and the nature of losses, i.e., hysteresis, eddy current or excess losses. The computations also show that the measurement uncertainties at low frequencies have negligible effect on the accuracy of computed losses whereas those at the supply frequency and higher ones affect very much this accuracy. It is also shown that the most important loss components are those related to the classical eddy current and alternating hysteresis losses especially in the tooth tips of the stator and rotor. The results suggest that more attention should be paid to the measurement and characterization of these sensitive components.

Introduction

Electrical machines consume about 40% of the electrical energy produced in the European Union (SEC 2009). The design and simulation of electrical machines and electromechanical energy conversion devices is nowadays exclusively carried out through finite element (FE) based numerical models. These models can be two-or three-dimensional with different approaches in the formulations and the implementations. The accuracy of the numerical simulations of electrical machines has increased in the last decades through better material models and the coupling of the magnetic field and the circuit equations of the windings of the machine. However, this accuracy depends strongly on the parameters of the material models and the manufacturing processes of both the materials and the machine. The computation of the resistive losses is nowadays very accurate and the only problems related to these losses reside in the effect of circulating current in parallel wires of the random windings (Hämäläinen et al. 2013), skin effect in form windings [START_REF] Islam | Effects of pulse-width-modulated supply voltage on eddy currents in the form-wound stator winding of a cage induction motor[END_REF], and the interbar currents in the case of cage induction machine [START_REF] Dorrell | A Multisliced Finite-Element Model for Induction Machines Incorporating Interbar Current[END_REF]). On the other hand, the i r o n l o s s e s , w h i c h a c c o u n t f o r u p t o h a l f o f t h e t o t a l l o s s e s , a r e s t i l l a s u b j e c t o f e x t e n s i v e investigations and the accuracy of the computed iron losses varies heavily from one author to the other.

Conventionally, the process of iron loss characterization and computation has a waterfall structure as shown in Figure 1. The whole process starts with some more or less complex model of losses in the electrical steel. The model is usually an analytical one where the losses are computed based on some model parameters and the waveform of the magnetic flux density in the sample (Belahcen andArkkio 2008, Fratila et al. 2013) but some more advanced models, e.g. magnetodynamic vector hysteresis models [START_REF] Dlala | Improving loss properties of the Mayergoyz vector hysteresis model[END_REF][START_REF] Handgruber | Three-Dimensional Eddy-Current Analysis in Steel Laminations of Electrical Machines as a Contribution for Improved Iron Loss Modeling[END_REF], are also used. In all cases, the model parameters are estimated from measurements made on electrical steel sheet samples. These measurements could be either standard ones like the Epstein frame or non-standard ones like the Rotational Single Sheet Tester (RSST). Once the model parameters are estimated through some fitting strategy, they are used in conjunction with the numerical simulation of the electrical machine, where the flux density distribution is computed, the iron loss model is applied locally and then the losses are integrated over the volume of the machine core. Such a process is completely deterministic and thus it does not account for the variability introduced by the manufacturing process of both the material and the machine. Indeed, the properties of the same material grade vary within a certain range due to the stochastic nature of the manufacturing process. Further, the punching of iron sheets and the shrink fitting of the iron core as well as the welding deteriorate the material properties in a more or less stochastic way [START_REF] Krings | Iron Losses in Electrical Machines-Influence of Material Properties[END_REF].

The above uncertainties of the manufacturing processes make the iron loss parameters exhibit stochastic behaviour. So far, the numerical models used in the optimization of electrical machines have typically not accounted for these uncertainties. Since the computed optimal design can be within a narrow region of the design space, even a small deviation from the model parameters will result in a large amount of machines with very inefficient operation [START_REF] Kim | Robust Optimization Utilizing the Second-Order Design Sensitivity Information[END_REF], as schematically illustrated in Figure 2. A better solution of the problem would take the uncertainties into account and fall in a flat local minimum with very little variability of the machine performances, thus ensuring regular quality of the production line. In this paper we study the uncertainty propagation of iron losses from the characterization measurements of iron sheets up to the loss computations in induction machines. Two different approaches are taken. In the first one the variability is assumed to be in the loss parameters and in the second one the variability is assumed to be in the measured quantities from which the loss parameters are estimated. In both approaches a stochastic global sensitivity analysis is carried out by assuming uniform probability distributions of the quantities. We show that the uncertainties propagate through the model almost linearly but the distribution of losses in different parts of the machine as well as the segregation of losses depend strongly on the nature of uncertainties. Measurement uncertainties in low frequencies do not affect very much the computed losses whereas uncertainties in the measurements at high frequencies or under rotating fields propagate to either the tooth tips or yoke of the machine.

In Section 2 we present the measurement setup used to characterize the material all together with the measured results. In Section 3 we present the 2D finite element model used to simulate the induction machine under investigations along with the iron loss model. Section 4 explains the stochastic approach used in this work, and Section 5 presents the obtained results and discusses them. At the end a conclusion is given, where the consequences of the presented investigations are summarized and suggestions are given on how the research in this field should be advanced.

Method of iron loss characterization

The characterization of electrical steel sheet in this work is carried out on the RSST shown in Figure 3 [START_REF] Belkasim | Identification of loss models from measurements of the magnetic properties of electrical steel sheets[END_REF]. The device consists of two vertical U-shaped magnetizing iron cores and the corresponding windings. The windings are supplied by a computer-controlled two-phase power amplifier to produce the desired magnetic flux density in the measuring area of the sample. The fieldmetric measurement method is used to pick up the magnetic flux density B and the magnetic field strength H vectors. For this purpose two orthogonal B-coils and two calibrated H-coils are utilized as shown in Figure 3. The frequency and the shape of the magnetic flux density are set by the user and controlled by a LabView-based program in the laboratory computer. In our measurements we used frequencies from 10 to 1000 Hz with alternating and rotating flux densities of different amplitudes from 0.2 T up to 1.8 T, in total 250 measurements, each measurement consisting of not less than 3000 samples of the B and H waveforms. The power loss density in W/kg was then computed from the measured B and H as

0 1 T m pd T HB (1)
where is the mass density of the electrical steel sheet (7600 kg/m 3 ) and T is the period of magnetization.

The measurements will be used later in the paper to estimate the loss model parameters and carry out the sensitivity analysis as to determine how the uncertainty in these measurements will propagate to the computed losses in an induction machine.

Iron loss model and FE simulation

The computation of the iron losses in the software is carried out through an analytical model, where the losses are segregated into four components, i.e. P c classical eddy current, P ha alternating, and P hr rotational hysteresis, and P e excess losses according to [START_REF] Belahcen | Comprehensive Dynamic Loss Model of Electrical Steel Applied to FE Simulation of Electrical Machines[END_REF] and reported here as follows:

2 cc P kd t B (2) h aa P kd t B B (3) 2 1 1 ( 1) s h rr s B P kd t b B B B B (4) 3 2 ee P kd t B ( 5
)
where k c , k a , k r ,k e , and b are constant model parameters, B s is the technical saturation flux density, and is the angular direction of the magnetic flux density vector.

Fitting of the loss model parameters is shown in Figure 5. The x-axis in the figure is an arbitrary number associated with each measurement point. It is seen that the measured and computed losses agree with each other at most of the points. However, at some points the measured and computed losses do not agree very well. This is most likely caused by the fact that the measurements were made in both rolling and transverse directions and the loss parameters for the two directions are not the same. Especially, in case of rotational magnetization, the loss parameters are, in a way, an average of those in rolling and transverse directions. Fig. 5. Measured and fitted power loss density for different rotating and alternating magnetization at different amplitudes and frequencies of the magnetic flux density. The alternating losses were measured in both rolling and transverse directions. A noticeable difference between the directions can be seen as well as its effect on the rotational magnetization case. The x-axis represents the experiment number only.

The simulations of the induction machine under investigation are carried out with an in-house, 2D FE software package. The software is based on the A-formulation and it incorporates the coupling between the field and the circuit equations of the windings of the machine. The permeability of the iron core is modelled as a single-valued nonlinear function of the magnetic flux density, and the Newton-Raphson iteration procedure is used to solve the nonlinear problem. The software performs time-harmonic and transient analysis with both current and voltage supply. In the transient case, the Crank-Nicolson time integration scheme is applied. The parameters of the studied machine are given in Table I.

In the FE model, the losses are computed for each element of the mesh from the time-dependent flux density vector and then summed up for each region of the machine separately. The machine is divided into six regions namely the stator yoke, teeth and tooth tips and the rotor yoke, teeth and tooth tips. The flux density distribution in the cross section of the induction machines at the last time step of the simulation is shown in Figure 6, and the computed losses in different parts of the machine and at different loads are given in Table II. The increase of the total iron losses with loading as reported in (Dlala et al. 2012) can be seen as well as the increase of the losses in the stator teeth and tooth tips. Such a result confirms the validity of the deterministic loss model. In the next section the stochastic approach will be explained and its results presented.

Stochastic approach

To investigate the effect of the measurement uncertainties on the computed losses, we followed two approaches. In the first approach, the loss parameters are supposed to be uncertain and are modelled by uniform random variables, whereas in the second approach the uncertainties are supposed to be due to the measurements and uniform random noises are added to the measured values. In both cases, 10% variation of the quantities is taken as uncertainty range. The first approach thus describes the propagation of uncertainties from the parameters of the model to the computed losses through the FE simulation and loss model, whereas the second approach describes this propagation from measurements to computations as described in [START_REF] Ramarotafika | Stochastic Modeling of Soft Magnetic Properties of Electrical Steels: Application to Stators of Electrical Machines[END_REF]. The uncertainty itself is quantified using the Sobol indices [START_REF] Sobol | Sensitivity Estimates for Nonlinear Mathematical Models[END_REF]) and the stochastic sensitivity analysis. For this purpose we write the loss parameters or the measured losses as a vector of probability distributions () q X and compute the loss probability distribution as

M MM ii i ji j M M i ij ji X X X XX Y X Y f ff (6)
where Y stands for the vector of losses in different regions of the machine at different loads, X i are either the loss coefficient in the first approach or the random noise added to the measurements in the second approach and f i the multidimensional polynomial expansions to be calculated in accordance with the spectral expansion. The first order Sobol indices for each case are then computed as

( ( ( ))) ( ( )) ii i V a rfX S V a rY (7)
where Var stands for the variance of the quantity in question.

Results and discussion

As discussed above, the loss model parameters were first assumed to have a uniform distribution around the deterministic values estimated from the measurements made on the RSST. Meanwhile the induction machine has been simulated with the FE software at no load, half load and full load and the normalized losses at different regions of the machine have been calculated and stored in a separate file. The global sensitivity analysis with the spectral approach has been implemented as a post processing routine in Matlab, where the inputs are the normalized losses and the variability of each of the loss parameters. The outputs of the routine are the first order Sobol indices corresponding to each parameter of the loss model. These indices are shown in Figure 7. Here it can be seen that throughout the machine and independently of the load, the most significant parameters are the ones related to the classical eddy current and the alternating hysteresis losses. The parameter related to the rotational losses has almost no significance. The significance of the eddy current loss parameter increases with the loading in the whole machine and especially in the stator tooth tips. In the second approach, we introduced the uncertainty in the measurements before the estimation of the parameters of the loss model. Here, besides the uncertainty on the measured losses (Ptm in Fig. 8) according to (1), we subdivided the measurements into three different bunches consisting of the measurements of the magnetic flux densities at low, medium and high frequencies. We here assume also a uniform distribution of uncertainty in the measured losses as computed from the magnetic field strength and the magnetic flux density as well as on the three other bunches as they are depending on the magnetic flux density only. This approach, thus assumes that the uncertainty is on one hand in the measured field strength H and magnetic flux density B as well as in the relative angle between the measurement sensors, and on the other hand the uncertainty is in the measurement of the flux density B. A similar post processing routine implementing the spectral approach is used in this section too but the inputs of this routine are now the measured losses and the three sets of quantities derived from the measured magnetic flux densities altogether with the normalized losses computed by the FE analysis. The Sobol indices for this computation are given in Figure 8. Here one can see clearly that the magnetic flux densities at low frequencies (10 -30 Hz) have virtually no effect on the computed losses in the induction machine independently of the loading or the region under investigation. This result although it seems to be obvious is not really expected as the loss parameters are not computed based on any harmonic analysis.

The most significant set is the one coming from the measured losses, which are computed based on the magnetic field strengths and the magnetic flux densities. This result was expected as these are the quantities against which the loss model is identified. The importance of the next set related to the medium frequencies (50 -200 Hz) increases when the load of the machine is different but does not increase anymore from half load to full load. Such a result is emphasised in regions 3-5, i.e. the stator and rotor yokes as well as the stator tooth. The significance of the high frequency components (500 -1000 Hz) is seen in regions 6 and 7, i.e. the rotor teeth and tooth tips and as the loading increases. Such a result should be compared with the results from Figure 7, where it was shown that at these regions the eddy current losses are the most significant.

As a summary of this study, it could be said that the global accuracy of the iron loss model depends on the accuracy of measurements made at the supply frequency of the machine and higher ones, and that the most significant accuracy of the loss components is the one related to the eddy current and alternating hysteresis losses. Further, measurements at lower frequency than the supply frequency of the machine do not have any effect on the accuracy of loss computation. . The left histogram is for no-load case the middle one for half-load and the right for the full load case. The colours describe the variability in the measured losses at different frequency ranges. Note that the column corresponding to the low frequency is hardly seen as its effects are not significant as discussed in the text.

Conclusion and future developments

The above conclusion about the significance of loss components and measurement frequencies are based on assumed probability distributions of the related components. For a better correlation with the real life devices, these probability distributions should be acquired through repetitive measurements of the same grade of the material but from different material lots. This is the only method to obtain realistic data about the variability in the material itself. Further, the effect of manufacturing process variability need real process input that could be retrieved from measurements on constructed machines if the manufacturing series are large enough to allow for statistical analysis. Last but not least, the higher order Sobol indices should be computed for the different stochastic variable in order to quantify their combined effects.

Fig. 1 .

 1 Fig. 1. Illustration of the deterministic waterfall process for iron loss characterization and computation. The equations and other similar texts are only illustrative; they are reported in details at the corresponding sections of the paper.

Fig. 2 .

 2 Fig. 2. Illustration of the effect of uncertainties on the result of optimization. A sharp global minimum might turn out to be a worst design than a flat local minimum when the uncertainties in the model parameters are taken into account.

Fig. 3 .

 3 Fig. 3. The Rotational Single Sheet Tester used to characterize the electrical steel sheet. Actual device (left) and illustrated parts and instrumentation (right). The supply and control system are not shown.

Fig. 4 .

 4 Fig. 4. Examples of measured flux density locus in case of rotating magnetization at 10 Hz and different amplitudes (left) and hysteresis loops for alternating magnetization at 1.6 T and different frequencies (right).

Fig. 6 .

 6 Fig. 6. Geometry of the simulated induction machine altogether with the flux density distribution at the last time step.

Fig. 7 .

 7 Fig. 7. Sobol indices for different regions of the machine (1: whole machine, 2: St tooth tips, 3: St teeth, 4: St yoke, 5: Rt yoke, 6: Rt teeth, 7: Rt tooth tips). The left histogram is for no-load case the middle one for half-load and the right for the full load case. The colours describe the variability in the different loss coefficients. Note that the in some regions the columns corresponding to the alternating and rotating hysteresis are hardly seen as their effects are non-significant as discussed in the text.

Fig. 8 .

 8 Fig. 8. Sobol indices for different regions of the machine (1: whole machine, 2: St tooth tips, 3: St teeth, 4: St yoke, 5: Rt yoke, 6: Rt teeth, 7: Rt tooth tips). The left histogram is for no-load case the middle one for half-load and the right for the full load case. The colours describe the variability in the measured losses at different frequency ranges. Note that the column corresponding to the low frequency is hardly seen as its effects are not significant as discussed in the text.

Table I .

 I Parameters of the simulated induction machine.

	Parameter	Value
	Rated voltage	380 V
	Slip	2.0 %
	Rated current	69 A
	Rated power	37 kW
	Number of pole pairs	2
	Outer diameter of the stator core	310 mm
	Inner diameter of the stator core	200 mm

Table II .

 II Computed losses at different regions of the induction machine at different loads.

	Region	No load [W]	Half load [W]	Rated load [W]
	1: Whole machine	594.88	707.05	836.99
	2: Stator tooth tips	6.63	25.05	53.45
	3: Stator teeth	116.32	171.12	235.05
	4: Stator yoke	315.72	318.65	319.06
	5: Rotor yoke	0.19	0.83	1.57
	6: Rotor teeth	69.09	104.35	140.43
	7: Rotor tooth tips	86.93	83.04	87.42


Huynh Pham

Doan Pham
email: doan.phamminh@mines-albi.fr.

Marta Galera Martinez

Ange Nzihou

Patrick Sharrock

Ecole Des Mines D'albi

Xuan Doan

Pham Minh

Marta Galera-Martinez

Valorization of Calcium Carbonate-Based Solid Wastes for the Treatment of Hydrogen Sulfide from the Gas Phase

Keywords: Solvay process®, solid waste, calcium carbonate, H 2 S removal, regeneration

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Sodium bicarbonate (NaHCO 3 ) and sodium carbonate (Na 2 CO 3 ) are widely used in different applications. [START_REF] Breton | Carbonate de sodium -Procédé Solvay à l'ammoniac[END_REF] They can be used as melting agent in glass and metallurgy industries.

They can be used as alkaline reactant in different processes, for the neutralization of acid gas, or for the synthesis of sodium salts such as sodium phosphates, sodium nitrate, sodium chromate, sodium citrate. Other fields of application of sodium carbonate and sodium bicarbonate may be listed as follows: detergence, steel industry, pharmaceutical, paper mill, water softening, food industry, and many others. Discovered and developed by Ernest Solvay during the 1860s, the Solvay process® became the main process for the fabrication of sodium carbonate and sodium bicarbonate from limestone (CaCO 3 ) and salt brine (NaCl) as the starting materials. [START_REF] Ernest | Ammoniacal process for the manufacture of carbonates of soda. CA7019 (A)[END_REF] This process can be summarized by the following steps: [START_REF] Breton | Carbonate de sodium -Procédé Solvay à l'ammoniac[END_REF] • Step 1: Preparation of a saturated aqueous solution of NaCl (brine).

• Step 2: Saturation of this brine with NH 3 to form an ammoniated brine (H 2 O, NaCl and NH 3(aq) ).

• Step 3: Calcination of limestone :

CaCO 3 → CaO + CO 2 (1)
• Step 4: Preparation of lime milk :

3 CaO + H 2 O → Ca(OH) 2 (2)
• Step 5: Carbonation of the ammoniated brine with CO 2 recovered from Step 3.

Ammonium bicarbonate is formed (Eq. 3), followed by the double decomposition of sodium chloride and ammonium bicarbonate to form sodium bicarbonate (Eq. 4)

NH 3(aq) + H 2 O + CO 2 → NH 4 HCO 3 (3) NH 4 HCO 3 + NaCl → NH 4 Cl + NaHCO 3 (4)

• Step 6: Sodium bicarbonate, which is less soluble, is filtered from the solution.

•

The global equation of this process is the following: 2NaCl + CaCO 3 → Na 2 CO 3 + CaCl 2 (7) The key step of this process is the carbonation of the ammoniated brine and the crystallization of NaHCO 3 , where the yield reaches generally about 70-75% for the transformation from NaCl to NaHCO 3 . The liquid phase (filtrate) generated from Step 6 contains large amounts of NH 4 HCO 3 , (NH 4 ) 2 CO 3 , NaHCO 3 , NH 4 Cl and NaCl, which are soluble in water. [START_REF] Trypuc | CaCO 3 production using liquid waste from Solvay method[END_REF] Thus, during the generation of NH 3 using lime milk (Step 8), a precipitate which contains mainly calcium carbonate (called hereafter PCC) is formed. Taking into account the annual production of sodium carbonate and sodium bicarbonate by the Solvay process® of about 55 millions of tons worldwide (in 2011), [4] large quantities of PCC are generated annually. Upto-date, PCC is considered as a by-product and is directly discharged around the production site or to the sea. However, environmental preservation and production appear as one of the most important challenge of the XXI century. As indicated by the waste management hierarchy, [START_REF] Hyman | Guidelines for national waste management strategies: moving from challenges to opportunities[END_REF] the most favored option for waste management is its prevention, followed by its minimization, its reuse, its recycling, its energy recovery and its disposal.

This work aims to investigate the reuse of two PCC generated by two different sites of Solvay process® for the development of new and low cost sorbents for the treatment of hydrogen sulfide in the gas phase. As we can see later, PCC contains mainly calcium carbonate, but also significant contents of metals such as iron, magnesium, sodium etc. Calcium carbonate, with its basicity, and with the presence of various metals may be favorable for the fixation of hydrogen sulfide (H 2 S). [START_REF] Ozdemir | Hydrogen sulfide removal from coal gas by zinc titanate sorbent[END_REF][START_REF] Montes | Reactive H 2 S chemisorption on mesoporous silica molecular sieve-supported CuO or ZnO[END_REF][START_REF] Yamamoto | Gas-phase removal of hydrogen sulfide using iron oxyhydroxide at low temperature: Measurement of breakthrough curve and modeling of sulfidation mechanism[END_REF] H 2 S is formed as gas pollutant in different processes such as coal combustion, petrochemical processing, fermentation of biomass or gasification of coal/biomass for generating syngas etc. [START_REF] Cullis | Atmospheric sulphur: natural and man-made sources[END_REF][10][START_REF] Wang | Simultaneous removal of hydrogen sulfide and mercury from simulated syngas by iron-based sorbents[END_REF] The efficient removal of H 2 S usually appears as the crucial step for a given process. For example, syngas produced from gasification of biomass has to be purified and the total amounts of H 2 S and COS must be less than 20, 20, 1 and 0.01 mg Nm -3 for engine, turbine, methanol synthesis and Fisher-Tropsch synthesis applications, respectively. [START_REF] Arena | Process and technological aspects of municipal solid waste gasification. A review[END_REF][START_REF] Woolcock | A review of cleaning technologies for biomass-derived syngas[END_REF] 2. Materials and methods

Chemical and physico-chemical analyses

Different chemical and physico-chemical techniques were used for analyzing sorbents before test of H 2 S removal. Thermogravimetry (TG) was performed with a SDTQ600 analyzer (TA Instruments) at a heating rate of 5°C/min under air flux (100 mL/min). X-ray diffraction (XRD) data of the solids were collected using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu Kα (1.543 Å) radiation source. Scanning electron microscopy (SEM) was carried out on a Philips XL30 ESEM apparatus (FEI Company) which was coupled with an energy-dispersive X-ray spectroscopy (EDX analysis). Specific surface area was determined by BET method, using a MICROMERITICS Gemini Vacprep 061). X-ray fluorescence (XRF) was carried out with a PANanalytical (Epsilon E3 XL). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a HORIBA Jobin-Yvon Ultima 2. The pH pzc was determined according to Pham Minh et al. [START_REF] Pham Minh | Degradation of olive oil mill effluents by catalytic wet air oxidation 1. Reactivity of p-coumaric acid over Pt and Ru supported catalysts[END_REF] Calcination experiments as pretreatment of sorbents were done in a static muffle-furnace (Nabertherm P320). For each calcination, about 5 g of solid was introduced in a ceramic open dish. This last one was then placed in the furnace. The furnace was heated by means of 20°C min -1 up to the desired temperature (105 to 850°C), and was kept at this final temperature for 2 h. Then, the furnace was freely cooled down to room temperature. The calcined solid was immediately used for all further characterization and tests.

Sorbents

Two PCC sources were used in this study. The first one was sampled from the Solvay factory at Dombasle-sur-Meurthe (France), which is designated thereafter PCC-F. The second one was taken from the Solvay factory at Torrelavega (Spain), which is called thereafter PCC-S.

Both PCC were in fine powder form. They were firstly dried at room temperature (ca. 20°C) up to constant weight. Then, they were sieved to eliminate particles bigger than 315 µm. In fact, the fraction of particles larger than 315 µm represented only less than 1 wt.% of both PCC. The fraction of particles smaller than 315 µm was thermally treated by calcination at 105-850 °C before sorption test.

Pure calcite powder (CaCO 3 , > 98 wt.%) from Fisher Scientific, and commercial activated carbon (L3S from CECA, France, 900 m 2 g -1 ) in powder form were used as the reference sorbents. They are designated thereafter "Calcite" and "AC" for pure calcite and activated carbon, respectively. This pure calcite was also treated by calcination at 105-850°C before 6 sorption test in order to compare with calcined PCC-F and PCC-S. AC was used as received, without further pretreatment.

Sorption test

The tests of H 2 S removal were carried out at room temperature (ca. 20°C) and ambient pressure using a glass fixed-bed reactor. Figure 1 shows the scheme of the reactor used. This reactor (length 60 mm, internal diameter 11 mm) was equipped with a porous glass disc which allowed keeping the sorbent bed, and the passage of gas through the sorbent bed. The volume of sorbent bed varied from about 1.5 to 3.2 cm 3 , corresponding to the apparent density and the amount of sorbent used for each experiment (0.5 to 1.0 g). A synthetic waste gas, based on dried air and containing 50 ppmv of H 2 S passed through the sorbent bed with a flow rate of 50 mL min -1 and was controlled by a flow meter. The output H 2 S concentration was monitored every 5 seconds using a gas analyser from BW Technologies (Gas Alert QUATTRO). Sorption experiments were stopped when the sorbent was saturated or when the rate of H 2 S sorption slowed down. In this paper, the following terms are used in order to compare different sorbents and different experimental conditions:

• H 2 S Input (mg): the total accumulated quantity of H 2 S injected to the reactor at a given reaction time.

• Accumulated Sorbent (mg): the total accumulated quantity of H 2 S fixed on the sorbent at a given reaction time.

• t 100% (min): the reaction time wherein the removal of H 2 S was total. So, theoretically

H 2 S Input is equal to Accumulated Sorbent within t 100% .
The amount of H 2 S Input and Accumulated Sorbent was calculated with the Eqs. ( 8) and ( 9), respectively:

t C RT PQM S H Input int 6 2 10 = (8)       - = ∫ dt C t C RT PQM d Accumulate t out Sorbent 0 int 6 10 (9)
where Q: inlet flow rate (L min -1 ), M: the molecular weight of H 2 S (34.06 g mol -1 ), W: mass of sorbent used (g), C in : inlet concentration of H 2 S (50 ppmv), C out : outlet concentration of H 2 S (ppmv), t: reaction time (min). Table 1 shows the main elements of both initial PCC-F and PCC-S. Calcium carbonate was found as the main compound of both solid wastes. Other elements were also present, including different metals, sulphur and chlorine. The content of organic compounds was negligible. solid preatreated at 850 °C, the decarbonation was complete after 2 h. Previous studies showed that the kinetic of calcite calcination depends on the particle size and the atmosphere of calcination. [START_REF] Escardino | Kinetic study of calcite particle (powder) thermal decomposition: Part I[END_REF] These results were also found for FCC-S and commercial pure calcite, pretreated at different temperatures (results not shown here).

Results and discussion

Analysis and characterisation of the sorbents before test

Because of the thermal decomposition of calcium carbonate, starting above 580 °C, we studied only the sorption activity of the sorbents which were thermally treated at 580 °C and below. All diffraction peaks could be attributed to calcite, which was found as the only main crystalline phase of this solid waste. The solid powder dried at ambient condition was already highly crystalline. Thus, the calcination step up to 580 °C did not significantly modify its crystallinity.

These results confirmed TG analyses above. And the similar results were obtained with FCC-S and commercial pure calcite (results not shown).

All three initial calcite based sorbents (FCC-F, FCC-S, pure calcite) had the specific surface area lower than 3 m 2 g -1 .

Figure 3. XRD patterns of the FCC-F after thermal treatment at different temperatures.

Sorption activity of the sorbents

The reactivity of the different sorbents was investigated at ambient condition. Figures 4-6 For FCC-F based sorbents, the sorption performance increased with the calcination temperature up to 400 °C (Figure 4). The sorbents calcined at 400 and 580 °C had the similar reactivity. Thermal treatment is known as an efficient treatment for desorbing molecules adsorbed on the surface of a given solid, as well as for the combustion of eventual organic molecules present in the initial sorbents. Thus, calcined solid with clean surface is more appropriate for the fixation of H 2 S. In addition, the thermal treatment allows transforming carbonates of metals such as iron, sodium, magnesium etc. which were present in this solid waste, into the corresponding metal oxides. These last ones may have higher affinity for H 2 S fixation.

When the calcination temperature rose to 580 °C, no further physico-chemical transformation took place, compared to 400 °C, as indicated by TG analysis (Figure 2). This may explain the similar reactivity of the two sorbents calcined at 400 and 580 °C. Taking into account the energy consumed for the calcination step, the pre-treatment at 400 °C was the most efficient. Figure 5 shows the results obtained with FCC-S based sorbents. As previously observed with FCC-F based sorbents, the reactivity of FCC-S treated at 105 °C was smaller than that of the solids treated at 400 and 580 °C. Both the solids calcined at 400 and 580 °C had the similar sorption behaviour during the first 6 min of time-on-stream. Then, the sorbent treated at 580 °C was saturated more quickly than the one treated at 400 °C. This behaviour was also observed for the sorbents starting from pure commercial calcite (Figure 6). Standard characterizations such as TG-DSC, SEM, IR, XRD did not reveal the differences between these calcined solids. From the results in Figures 456, the calcination at 400 °C seemed to be the most suitable pretreatment of both the solid wastes. The comparison of the reactivity of three sorbents, calcined at 400 °C, shows that both the solid wastes were more active than the pure calcite (Figure 7). This was probably due to the presence of various impurities in the solid wastes, as reported in Table 1. Metals oxides such as iron, copper, zinc oxides, are well known as active phase for H 2 S fixation. [START_REF] Huang | Effect of moisture on H 2 S adsorption by copper impregnated activated carbon[END_REF][START_REF] Nassar | Ultradispersed particles in heavy oil: Part II, sorption of H 2 S (g)[END_REF][START_REF] Wu | Microwave effects on the structure of CeO 2 -doped zinc oxide sorbents for H 2 S removal[END_REF] Further study is necessary for understanding the transformation of impurities present in the initial FCC during calcination step, and their role for H 2 S removal. Taking into account the apparent volume of the solid bed of 1.5 mL for 0.5 g of FCC-F or FCC-S, the corresponding gas hourly space velocity used (GHSV) was equal to 2000 h -1 . At this GHSV, despite the encouraging results presented above, neither the solid wastes nor pure calcite were good enough for the effective removal of H 2 S. In fact, t 100% never exceeded 3 min for all the sorbents treated at various temperatures. In order to enhance the sorption performance, the quantity of the sorbent FCC-F_400 was doubled while keeping unchanged the gas flow rate. So, the contact time was found to be a crucial parameter for the purification of trace amounts of H 2 S in dry air.

Influence of iron addition

Figure 9. Sorption behavior of iron-doped sorbents. Sorption conditions: 50 ppmv of H 2 S in dried air, 50 mL min -1 , 1.0 g of sorbent, room temperature and pressure.

The improvement of the performance of a sorbent can also be realized by the addition of highly-dispersed metal-based particles, which have high affinity for H 2 S immobilization. [START_REF] Davydov | Metal oxides in hydrogen sulfide oxidation by oxygen and sulfur dioxide I. The comparison study of the catalytic activity. Mechanism of the interactions between H 2 S and SO 2 on some oxides[END_REF][20] In this study, we chosen iron as the metal active because of its low cost, its availability and its high potential reactivity for H 2 S removal. [START_REF] Nguyen-Thanh | Adsorption of hydrogen sulfide on montmorillonites modified with iron[END_REF][START_REF] Hongrapipat | Investigation of simultaneous removal of ammonia and hydrogen sulphide from producer gas in biomass gasification by titanomagnetite[END_REF] FCC-F and pure calcite were used as supports for the dispersion of iron particles. Iron particles deposition was carried out using the standard incipient wetness impregnation method, as described elsewhere. [START_REF] Pham Minh | Degradation of olive oil mill effluents by catalytic wet air oxidation: 2-Oxidation of p-hydroxyphenylacetic and p-hydroxybenzoic acids over Pt and Ru supported catalysts[END_REF][START_REF] Galera Martínez | Synthesis, characterization, and thermo-mechanical properties of copper-loaded apatitic calcium phosphates[END_REF] An aqueous solution of Fe(NO 3 ) 3 was added to FCC-F or pure calcite powder in order to wet just enough the outer and pore surfaces of the support material. The concentration of Fe(NO 3 ) 3 solution was calculed to reach finally 1 wt.% of iron in the final solid sorbent, which was either Fe/FCC-F or Fe/Calcite. The iron content was chosen at 1 wt.% according to the low specific surface area of FCC-F and pure calcite. After the impregnation step, the solid was dried overnight at 105 °C and calcined at 500 °C under air before use in H 2 S sorption experiment.

Figure 9 shows the sorption behavior of iron-doped sorbents. Firstly, the addition of iron particles to FCC-F solid led to the considerable improvement of its sorption performance. Its t 100% reached 150 min, which was much higher than t 100% of the undoped FCC-F (55 min, Figure 8) under the same sorption conditions. Secondly, using the same preparation method for the deposition of iron particles, FCC-F based sorbent (Fe/FCC-F) was more active than pure calcite based sorbent (Fe/Calcite), which had t 100% equal to 150 and 70 min, respectively. SEM analysis allowed observing the formation of iron-based particles on the surface of these two supports (Figure 10). These particles had higher contrast compared to the surface of the supports, as confirmed by EDX analysis (results not shown). We can see that iron-based particles formed on the surface Fe/FCC-F were smaller than those formed on the surface of Fe/Calcite. In addition, the phenomenon of local agglomeration of iron particles was observed for Fe/Calcite, but was not the case for Fe/FCC-F. In fact, the pH pzc was 8.5 and 9.9 for FCC-F dried at room temperature, and for commercial calcite, respectively. Commercial calcite, with higher pH pzc , led to the local and rapid precipitation of iron oxides during the addition of aqueous solution of Fe(NO 3 ) 3 on its surface. On the other hand, FCC-F solid waste, with lower pH pzc , prevented or limited this local precipitation of iron oxides.

Comparison with an activated carbon

Activated carbon is considered as the reference sorbent for gas treatment, in particular in the case of H 2 S treatment. In this study, the reactivity of the undoped FCC-F_400 and the iron-doped sorbent (Fe/FCC-F) was compared with that of an activated carbon purchased from CECA (France, L3S model), which is specially designed for H 2 S treatment. Figure 11 shows the results obtained. FCC-F solid waste doped with 1 wt.% of iron (Fe/FCC-F) was slightly more active than the activated carbon. The value of t 100% reached around 55 and 60 min for the activated carbon and Fe/FCC-F, respectively, when 0.5 g sorbent were used. By doubling the quantity of the undoped sorbent FCC-F_400, the equivalent sorption performance compared to Fe/FCC-F or the activated carbon could be also obtained. This observation appeared as a major result because FCC-F and FCC-S are currently considered as solid wastes and need to be treated inside the industrial sites of the Solvay process®. adsorption by physisorption interaction, or by chemisorption; this last one is generally not reversible. To verify the reutilization possibility of the used sorbent, desorption test by thermal treatment was tested. The used sorbent was heated at 500 °C for 2 h before the first recycling test. The desorption was repeated again for the second recycling. Figure 12 shows the results obtained with undoped FCC-F, and with iron-doped FCC-F sorbents. For the undoped sorbent (Figure 12 A); t 100% decreased from about 60 min to 30 min at the first recycling, and to 12 min at the second recycling. For the iron-doped sorbent (Figure 12 B), its t 100% decreased from about 150 min to 45 min at the first recycling and to 25 min at the second recycling. The partial loss of reactivity during thermal reactivation of used sorbent has commonly been observed for other kinds of sorbents, such as activated carbon. [START_REF] Bagreev | Thermal regeneration of a spent activated carbon previously used as hydrogen sulfide adsorbent[END_REF][START_REF] Cal | High temperature hydrogen sulfide adsorption on activated Carbon II. Effects of gas temperature, gas pressure and sorbent regeneration[END_REF][START_REF] Fang | Selective oxidation of hydrogen sulfide to sulfur over activated carbon-supported metal oxides[END_REF] This reactivity loss might be due irreversible interaction between H 2 S and metal oxides. This loss might be also due to the oxidation of H 2 S to nonvolatile sulfur compounds. [28] This might be also due to the agglomeration of active sites during the regeneration by heating at 500 °C. The agglomeration of active sites is known as irreversible deactivation. Further analysis is needed for determining sulfur compounds formed during H 2 S removal and for understanding the reason of this irreversible deactivation. Table 1. Elemental analyses of both FCC-F and FCC-S dried at room temperature.

Figure 1 .

 1 Figure 1. Scheme of the reactor used in this work.

  analysis of the initial PCC Elemental analysis of the initial PCC was carried out by XRF and ICP-AES techniques.

Figure 2 .

 2 Figure 2. TG curves of the FCC-F after thermal treatment at different temperatures.

  show the results obtained with FCC-F, FCC-S and pure calcite based sorbents, respectively. For each figure, the Input curve represents the accumulated quantity of H 2 S introduced into the curves represent the accumulated quantity of H 2 S, which was fixed by the sorbents.

Figure 4 .

 4 Figure 4. Quantity of the accumulated H 2 S fed into the reactor (Input curve) and remaining after

Figure 5 .

 5 Figure 5. Quantity of the accumulated H 2 S fed into the reactor (Input curve) and remaining after

Figure 6 .

 6 Figure 6. Quantity of the accumulated H 2 S fed into the reactor (Input curve) and remaining after

Figure 7 .Figure 8 .

 78 Figure 7. Comparison of three sorbents treated at 400 °C. Sorption conditions: 50 ppmv of H 2 S in dried air, 50 mL min -1 , 0.5 g of sorbent, room temperature and pressure.

Figure 8

 8 Figure8show the results obtained. The increase of the sorbent quantity improved strongly the performance of H 2 S removal. The value of t 100% reached up to 55 min at the GHSV of 1000 h -1 .

Figure 10 .

 10 Figure 10. SEM analysis of Fe/FCC-F (A, B) and Fe/Calcite (C, D).

Figure 11 .

 11 Figure 11. Comparison of the activity of FCC-F based sorbents and an activated carbon as reference. Sorption conditions: 50 ppmv of H 2 S in dried air, 50 mL min -1 , 0.5 to 1.0 g of sorbent,

Figure 12 .

 12 Figure 12. Regeneration of used sorbent; (A) undoped FCC-F_400; (B) iron-doped sorbent

Figure and Table captions

  Figure and Table captions

Figure 1 .

 1 Figure 1. Scheme of the reactor used in this work.

Figure 2 .

 2 Figure 2. TG curves of the FCC-F after thermal treatment at different temperatures.

Figure 3 .

 3 Figure 3. XRD patterns of the FCC-F after thermal treatment at different temperatures.

Figure 4 .

 4 Figure 4. Quantity of the accumulated H 2 S fed into the reactor (Input curve) and remaining after

Figure 5 .Figure 6 .

 56 Figure 5. Quantity of the accumulated H 2 S fed into the reactor (Input curve) and remaining after

Figure 7 .

 7 Figure 7. Comparison of three sorbents treated at 400 °C. Sorption conditions: 50 ppmv of H 2 S in dried air, 50 mL min -1 , 0.5 g of sorbent, room temperature and pressure.

Figure 8 .

 8 Figure 8. Influence of the contact time on the performance of the process using FCC-F pretreated at 400 °C. Sorption conditions: 50 ppmv of H 2 S in dried air, 50 mL min -1 , 0.5 to 1.0 g of

Figure 9 .

 9 Figure 9. Sorption behavior of iron-doped sorbents. Sorption conditions: 50 ppmv of H 2 S in dried air, 50 mL min -1 , 1.0 g of sorbent, room temperature and pressure.

Figure 10 .

 10 Figure 10. SEM analysis of Fe/FCC-F (A, B) and Fe/Calcite (C, D).

Figure 11 .

 11 Figure 11. Comparison of the activity of FCC-F based sorbents and an activated carbon as reference. Sorption conditions: 50 ppmv of H 2 S in dried air, 50 mL min -1 , 0.5 to 1.0 g of sorbent,

Figure 12 .

 12 Figure 12. Regeneration of used sorbent; (A) undoped FCC-F_400; (B) iron-doped sorbent

  Step 7: Thermal decomposition of sodium bicarbonate to form sodium carbonate at

	about 150-200°C:
	2NaHCO 3 → CO 2 + Na 2 CO 3 + H 2 O	(5)

• Step 8: Regeneration of NH 3 from lime milk (Step 4) and NH 4 Cl (Step 5): 2NH 4 Cl + Ca(OH) 2 → 2 NH 4 OH + CaCl 2

Table 1 .

 1 Elemental analyses of both FCC-F and FCC-S dried at room temperature.

	Compounds	CaCO 3 Mg	Al	Si	Fe	S	Cl	Na	Others*
	FCC-F, wt.%	98	0.6	0.2	0.5	0.5	0.2	0.1	0.4	0.5
	FCC-S, wt.%	97	0.7	0.2	0.6	0.3	0.6	0.5	Trace 0.3

(* P, Cu, Mn, Zn, Sn) b. Thermogravimetric analysis Figure

2

shows TG curves of different FCC-F solids, which were previously calcined at different temperatures. Small weight losses between 350 and 580 °C were observed for the solids pretreated at 20, 105 and 200 °C. The main weight loss starting at 580-780 °C was due to the decarbonation of calcium carbonate. This weight loss represent 42.8-43.7 wt.% for the solids pretreated at 20-580°C, which means that FCC-F contains mainly calcium carbonate (theoretical weight loss of pure calcium carbonate being 44 wt.%). This confirms again the results of elemental analyses. The calcination at 700 °C for 2 h in a static furnace did not allow the complete decarbonation of calcium carbonate because the weight loss at 580-780 °C of this solid reached again 35.3 wt.% on second thermal treatment in TG analyzer. On the other hand, for the

Page 8 of 27 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research

ACS Paragon Plus EnvironmentIndustrial & Engineering Chemistry Research

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research


Fabien Jobard

CESDIP-CNRS

The 2005 French Urban Unrests. Data-Based Interpretations ANR-ESRC 1 st Seminar on French & English Riots, Sheffield, Febr. 2007

The intensity and duration of the urban unrest which occurred in France in the course of the autumn 2005 left their mark throughout the post-industrial world. Some television broadcasts in the USA like Fox News presented headlines on "the civil war in France", and their counterparts in Russia defined the situation as a race war declared by the Muslims against the main French society. Social scientists were certainly less hot-headed, but the spectre of the dispensable sociological interpretation was particularly bright, though empirical work rather rare (for a presentation of the spectre: [START_REF] Mauger | L'émeute de novembre 2005. Une révolte protopolitique[END_REF]. The following contribution intends to answer questions which are central to both political and social scientific worlds, but it will mainly focus on empirical data (quantitative and qualitative) in order to fulfil this aim. We will leave the root causes and contextual aspects by one side (economic deprivation, urban segregation, conflicts with police forces, etc.) and concentrate on immediate data produced in the wake of the riots, in the little research produced afterwards (see [START_REF] Poupeau | French Sociology Under Fire: A Preliminary Diagnosis of the November 2005 "urban riots[END_REF], for a critical assessment of the sociological work about the riots; more generally the different contextual analysis presented online by the SSRC, 2006, and[START_REF] Zauberman | Police, minorities, and the French republican ideal[END_REF] for a larger assessment on routine conflicts between police forces and minorities). In what follows, I will make use of the term "riots", which implies that I have made the assumption that collective violence is of political nature (Tilly, 2003, 18-19). Firstly, I shall briefly review the course of the events.

I. The 2005 French riots: An exceptional series of disorders throughout the country

First of all, in dealing with the unrest of October-November 2005 we absolutely need to keep in mind that these events were far from being the first of this kind in France.

From the so-called "hot summer" of 1981 in the bleak suburbs of Lyons, to the most recent unrest in Lille, 2000 or Nîmes, in the South of France in 2003 (Duprez, BodyGendrot, 2002) , a recollection of all unrests related in the France Press Agency bulletins between 1990 and 2004 reveals around 10 to 15 local "unrests" every year in different places (Lagrange, 2006a, 44-46). But not only did the 2005 unrest expand into more than two-weeks of disorder, they also spread across almost all the national territory, including some small and peaceful cities on the far less urbanised areas.

On 27 October 2005, three young people (two of African descent and one from Turkey) on their way back from a football game, ran away from police officers responding to a call about a break-in (see the story told by both lawyers of the dead boys' families: [START_REF] Mignard | L'affaire Clichy[END_REF]. They tried to hide in an electrical substation behind a cemetery and the two African children were killed through being electrocuted. For three nights, in the town and two of its neighbouring cities, unrest developed, with 30 cars set alight on each night. Head-to-head confrontations with riot police were numerous, but were strictly bounded within the three cities. On the 1 st of November in Clichy, a tear gas grenade was fired by the riot police into a hall used by the local Muslims as a mosque and where, at the time of the shot, dozens of Muslims were gathered for Ramadan prayers.

This was certainly the reason for the first wave of geographical expansion of the troubles beyond the Clichy area to other parts of the 93rd Department. Until the 4 th of November, only the Northern parts of the Paris suburbs were hit by the unrest, consisting in torching cars and head-to-head confrontations with the riot police. On the 4 th of November, the forms of unrest changed slightly (arson attacks on public goods, such as schools or sport halls, or some warehouses) and, parallel to this, the unrest extended to some middle-ranked cities of less then 50,000 inhabitants, sometimes even 20,000 inhabitants. On the 5 th of November incidents occurred in 200 cities, where 1,300 torched cars are counted. On the 6 th , 1,500 cars are destroyed by fire, in 275 cities. On the 7 th , in 300 cities, with 1200 cars burnt out. On the evening of the 7 th , the Prime Minister declared a kind of state of emergency.

However, the weather turned distinctly colder, and the riots irreversibly decreased.

After the night of the 14 th November, one could not register more than 100 cars burnt each night.

After this 20-day period of unrest, 10 000 cars had disappeared in flames, 200 million Euros damage had been registered by insurance companies, 250 public buildings had been damaged. In contrast with the riots that ever took place for instance in the USA, the unrest in France did not give rise to physical fighting: There had been hardly any use of firearms and despite (except for?) a death provoked by a fight in a town in the North of Paris and the severe injury suffered by a disabled woman in a bus on fire, no important casualty was reported.

II. Interpretations

In the following section, I will focus on the sociological analysis of the events, and not so much on broader political interpretations of them. I will then prioritize here the "fact-bound interpretations".

1) Historical analysis: The timing of the growth and decrease of the unrests.

The time-related development of the unrests needs to be put in a constant parallel with the moves on the national political scene, since both the signs sent out from the political leaders had concrete consequences not only on the forms of collective violence in use on the streets, but also on the meaning of the action held by the rioters or the perpetrators.

Since (because) or While? the unrests were circumscribed to Clichy and its immediate area, neither the Prime minister nor the minister of the Interior displayed the smallest compassion for the two dead minors of Clichy. They both were constantly repeating that the young boys were felons fleeing from the police after an attempted break-in. And two days before the electrocution, on the 25 th of October, the then minister of the Interior Nicolas Sarkozy declared on a trip to a rundown north-west suburb of Paris that he would clear all such cities from their scum (racaille). This word caused uproar and fuelled the protest in Clichy. The French media uncritically echoed the the stigmatization by the Interior Minister. The minister's word stood as a collective symbolic degradation of all the young men living in low-rise urban areas. The throw of the tear gas canister into the Clichy mosque by the riot police (which remained under the immediate command of the Minister of the Interior) enlarged the circle of the population hit by this symbolic degradation by Sarkozy: Not only the youth facing the riot police in Clichy, but all youth living in poor suburbs, adding to them their parents and brothers at Ramadan prayers. This tear gas canister acted as a powerful mechanism of boundary activation, organizing a firm us-them boundary within the French society (Tilly, 2003, 21).

Following the same logic of political ignition of collective violence, a parallel could be made with the night of the National Holiday in July 14 th 2005, as cars were set alight in La Courneuve and two near-by cities, St-Denis (administrative/political centre (?) of the 93rd Departement) and Stains. La Courneuve is a low-income estate about which the plain-speaking Sarkozy had said some weeks before in June that it should be cleaned out with Karcher (which is an industrial cleaner used to clean the mud off tractors). Using the traditional opportunity offered by the National Holiday night of July14 th , youths from these three cities torched cars during the night… A second sign of the link between what happens in the national political arena and on the street-level is the spread of the disorders beyond the limits of the 93rd Department: the apparent dislocation of the government at the national level. The Prime Minister and the President seemed to let Minister Sarkozy dealing with the riots on his own, sending in advance a clear sign of non-solidarity between him and the rest of the government in case of accusations of police brutality or the lack of control over the situation. Some government members even expressed disapproval of Sarkozy, while the head of the government did not react.

This had a tactical consequence on the street-level interactions. Fearing the consequences of police abuse cases, the Minister of the Interior gave the priority to highly trained riot police forces during the first week of the unrests, displaying a low level of arrests and high professionalism in the use of force. This use of massive static police forces created an opportunity for small and highly mobile groups to jump from one spot to the other and to prefer hit-and-run guerrilla tactics rather than headto-head confrontation with heavily armed police forces. Tactical opportunism (more than coordinated violence or violent rituals) is certainly or clearly? one of the main features that characterised the rioters (Tilly, 2003, 14-15).

The second consequence of the awareness of governmental duality by rioters in the suburbs was the first weakening of the president-in-waiting Sarkozy since 2002. For the first (and since then only) time, the widely-read newspaper Le Parisien published, on the 8 th November, a larger picture of Sarkozy with the caption: "Dans la nasse" ("fallen into the trap"). Events were then set on a media spiral: Journalists interviewed people in the suburbs about their feelings, being only interested into what they got ( as in understood? Or received? Or how they reacted to..?) from the national political scene. Everybody seemed then to agree on the need for a dismissal of Sarkozy.

The next step gave rise to a complete shift in political alignments on the government's side. On Nov 4 th , the MPs of the governing conservative party at the National Assembly opposed the Prime Minister and acclaimed Sarkozy: A green light was then given to the Minister of the Interior to stand as the one and only man able to deal with the situation, and pressure was put on the Prime Minister to send a heavy handed response to the rioters. Sarkozy then turned to the use of smaller urban police squads aimed to chase after youngsters and to bring them before the prosecutors. On November 7 th , The Prime Minister revived a law authorising curfew measures, which had originally been adopted in 1955 to quell Algerian rebels (and indeed, curfews were almost only implemented in some Southern cities of France, hardly hit by the riots, but with large populations of former "French Algerians").

From then onwards, the unrests decreased in the face of the cold weather, of the level of judicial repression and of the fact that no move would be made on the government side in order to marginalize the hated minister of Interior. Short-term gains were then not to be reached. (There would be nothing more to gain on the short term level?)

2) Geographical analysis: Where did the unrests take place?

As I said, what set the 2005 unrests apart was, above their duration, their very odd geography. Let us have a look into the territorial specificities of these riots, based on the statistical analysis conducted by Hugues Lagrange (Lagrange, 2006b, 106-121).

First indicator: only 15% of the riot-torn places do not belong to a so-called ZUS project (so-called "Urban Sensitive Area projects", which are implemented in 751 places, among the most deprived urban areas in France, including 157 in Paris and its surroundings, and 36 in the 93d Departmentfor a presentation in English, see [START_REF] Salanié | The Riots in France: An Economist"s View[END_REF].

Meanwhile, if one takes a ratio "days of unrests/size of the population" (on a Department level), and the 93rd Department taken apart, Departments where the duration of unrest was the longest per capita were low-urbanized Departments. The simple equation "size of the agglomeration/risk of unrest" is far from offering a satisfying explanation.

In an effort to specify the "ZUS explanation", Hugues Lagrange sorted out that ZUS + proportion of young male residents (<20 yrs old) significantly enhance the risk of unrest. Furthermore (or additionally), a large part of the riots took place in the West part of France where a large population of migrants from sub-Saharan Africa has recently settled. And, indeed, Hugues Lagrange study shows that ZUS with a wide proportion of large families (i.e. more than 6 members) is the best predicator, all other variables considered considered, of the likelihood of unrests. Large families are here to be considered as a proxy variable for "recent African migrants".

The last indicator used by Lagrange: ZUS where the law of 2003 on housing renovation projects was implemented surprisingly appears to be a strong predictor of the likelihood of unrests. Two interpretations of this matter can be here offered:

-First, the sites chosen for being spots of implementation of the law could be considered to be the most depressed ones among the ZUS. -Second, the concrete consequence of the law for the families living there is the possibility of being relocated somewhere else, combined with a high degree of anxiety as soon as administrations come into the daily lives of these poor people.

Other indicators were tested, but were not statistically significant. This means, to conclude with this geographical analysis, that the combined variable of poverty, highly stressful local situations, and the presence of new migrants from sub-Saharan Africa had by far the heaviest impact on the likelihood for a city to be hit by unrests in 2005.

Meanwhile, this combination of factors did not automatically imply a riot: 30% of the cities in which all factors were available had no such event, 20% of the cities in which none of these determinants were to be found were swept by some riots. Local qualitative factors like the local relationships with the police, the public authorities or the associations working in the neighbourhoods (grassroots organizations) certainly played a significant role. Small scale qualitative studies like these are of great help, as one shall see it in the following sections.

3) Sociological analysis: Who are the rioters?

Following his unbeaten strategy of uninterrupted issue-setting, the head of the National Police declared that 80% of the arrested perpetrators were offenders wellknown by the police as such. Based on this assumption, the interpretation of the events was served on a golden platter: The unrests only consisted in a sudden and anomic aggregation of petty criminals, committed by well-known offenders as such and thus had no political aims at all.

From October 29 th to November 18 th , 3,100 people were arrested and brought up to the prosecutors, among them 950 minors. From the 2,600 adults held in police custody, 560 were sent to jail (either sentenced after an immediate trial, or serving awaiting a pre-sentence decision). 320 minors out of the 950 arrested were already known by the justice system, the majority of them not as offenders, but as so-called "endangered children" (Ministry of Justice, 2006).

Let's look at the adults. Considering the fact that the most predictive (if not the only) determinant for receiving a sentence to jail is to have been convicted once (Aubusson de Cavarlay, 2006, and for some critical insights into the arbitrariness of the judicial system in France: [START_REF] Hodgson | Hierarchy, Bureaucracy and Ideology in French Criminal Justice: Some Empirical Observations[END_REF], one can consider that 20 to 25% of all arrested adults only have been sentenced once before their appearance in court in November 2005, which is some distance away from the 80% assumed by N. Sarkozy.

Adding to this, we must take the modus operandi of the police into consideration. On many occasions, the arrests were made the day following the unrest and, in the Court of 93 rd Department the rate of discharge during this period was as high as 30% (for such attempts like violence against police officers, it normally lays on 3-4% - [START_REF] Jobard | La couleur du jugement. Discriminations dans les décisions judiciaires en matière d"infractions à agents de la force publique (1965-2005)[END_REF]. Two considerations follow:

-The high rate of discharge shows a considerable loss of efficiency beyond the prosecution, which can easily be explained by a will to prosecute even beyond a reasonable doubt about offences attributed to the arrestees, -This is largely due to the way the investigators worked, asking the riot police officers to "identify" the arrestees from the night before on the basis of books of identification (i.e. photographs of local offenders) held in the police stations, which of course lead to a substantial increase of arrestees previously known by the police as offenders…

All these considerations about the way arrests were made show that one can not exactly be sure if these data really describe the rioters, or the ones among the rioters who could not run as fast or as long as the others (and there minors are particularly concerned), or even only the usual suspects known by the police.

Anyway, a study of the 208 arrestees judged in immediate hearings in the 93rd Department has recently been published [START_REF] Mazars | Le traitement judiciaire des violences urbaines de l'automne[END_REF]. 40% of the 270 alleged crimes consisted in crimes against a police officer, 30% in the destruction and damage of public or private goods, and only 6% in riotous assembly. Interestingly, is that the average age is largely comparable to the age of offenders against police officers routinely judged in another Court of Paris suburbs in which I gathered such data over the last 15 years: There is no major difference between the youngsters involved in daily conflicts with the police and the ones who took part in the November 2005 disorders.

Michel Mazars' points to another interesting fact: 60% of the arrestees had no criminal justice record, and only a few of the 40% left had more than 2 records (i.e. 13 out of 78 had more than 2 records). For the 93 minors, the proportion of those who had once endured a criminal proceeding does not reach 40%.

But the main information from this report has to do with the personal situation of the arrestees. First of all, the vast majority of them still live with their parents (91%), and in fact only 10% of them could be considered by the author as having a secure professional activity, the other ones being either still in the school system, or being unemployed (there is hardly any difference between the adults and the minors in this matter).

Second, the families of the arrestees are less often called "traditional ones" (both parents living together with their children): half of the families are "traditional ones" (80% nation-wide), one third are single-parent (20% nation-wide), and 3% of them polygamous. But this strongly contradicts some political positions that were taken by some conservative representatives who considered polygamy as being the cause for urban disorders. Still, the number of large and very large families is quite considerable: Average number of brothers and sisters is 4.6 ; and one-fifth of the families encompass 7 or more children at home.

The social background of the parents is not really precisely recorded, since one cannot distinguish the job actually held by the father or the mother from the profession declared to the judge. Unemployed parents do not seem to be overrepresented in the considered population (around 10% of the fathers). But the number of families with at least one dead parent (13%) or with an absence of any kind of contact between the children and at least one parent (also 13%) is strikingly high. It shows how much it is less the social situation or the situation on the labour market which is relevant here than the socio-affective situation of a non-negligible part of the arrested children (minors and adults together).

4) Political analysis: Can the riots be considered as political protests?

This question is certainly one of the most difficult ones, for the rioters were very reluctant to express themselves about their motivations and aspirations (apart from the desire for the dismissal of Sarkozy) and for the fact that they did not have any kind of leader speaking, not even a group of different local leaders. Post-hoc explanations given by participants or supporters (exact boundaries between both kinds of actors being extremely vague) also mix together categorizations in terms of political protest, violent deviance, war games and nihilist desires [START_REF] Galland | Comprendre les émeutes de novembre[END_REF][START_REF] Kokoreff | Comprendre les émeutes de novembre[END_REF], and on over-estimates of the political dimensions testimonies Marliere, 2006). Beyond (Despite? After?) the silence on the spot and the post-event confusion, one can say that the unrests had a significant impact on policies, politics, and political identities. As such, the unrests definitely deserve to be described as political uprisings. I would here identitfy five aspects of this wave of protest.

First, two different local studies conducted in low-income estates of Saint-Denis town and of Aulnay (Clichy's neighbouring town, which was one of the most severely hit by violent episodes) show how strongly the rioters and their targets are bound to local matters [START_REF] Galland | Comprendre les émeutes de novembre[END_REF][START_REF] Kokoreff | Comprendre les émeutes de novembre[END_REF]. Both studies show how frequently young (indeed very young) rioters decided to act violently in order to prove to themselves and to their local community that they refuse both the state of the society and the attitude of the older people of their housing estates (the ones above, say, 25 yrs old), who resigned in silence either by fear of imprisonment, or because of the need to protect their quiet illegal businesses, or even by an acceptance of their status as low-income, fragile, despairing, and hopelessness youth. They also insist on the importance of parochial factors, even if they are quite difficult to analyse clearly. For instance, a large Renault car garage was spectacularly set on fire on the 2 November in the city of Aulnay (the event was then internationally broadcasted). Interviews conducted then by Galland et al. could not determine if the motivations were bound to employment practices by the local boss (he is said to have refused to employ the local youth), or to local car traffic, or simply to a collateral damage due to the fire set in the neighbouring oil station. In St Denis, the one and only school burnt in the town was lead by a principal under suspected of racism and whose car was set on fire some months ago; in Aulnay, an old people's home (retirement home?) which had been the centre of a conflict between youth organizations and the mayor the month before because the youths wanted to use the place for their own purpose was destroyed.

But all this strongly supports the point that the riots were not a kind of a political movement driven by Islamic militancy or whatever could have played the role of a political leader, but an aggregation of protests driven by larger feelings toward the police and different local actors from the school system or the labour market (the absence of any kind of religious commitment is not only underlined in local studies like Galland et al., 2006, 13, but also in a very interesting National Police report published against the will of the Minister of Interior by the newspaper Le Parisien on 7 December 2005 and by a study conducted by Patrick Haenni for the International Crisis Group, 2006both reports, even if produced by very different institutions, show how small Islamist activists' influence was in the events, but also in the role of peace keepers they pretended to play in some localities). To sum up this aspect in Olivier Roy's terms: "There was nothing Islamic or Arab in the riots. Strangely enough, Palestinian or Algerian flags as well as Arafat-style keffyehs (a must in leftist demonstrations in France) have been totally absent. "Allah akbar" was shouted by the would-be mediators, not the rioters. Attacks on churches and synagogues have been almost absent" [START_REF] Roy | The Nature of the French Riots[END_REF].

Secondly, this consideration should not be separated from the fact that, new waves of migration seem to have played a substantial role in the uprisings (Lagrange, 2006a). In a way, considering that the youth of former migrants (the ones from the Arab countries) were the rioters of the 90s, one could say that the riots are simply part of the history of migration in France and constitute the way in which migrants step into politics in France. As such, riots or urban unrests, even if not expressed as such by the actors, are a specific form of political socialization for a specific population at a specific age.

Thirdly, riots seem to have had a substantial impact on the votes in the housing estates. The next opportunity to vote after the riots came during national presidential elections in April and May 2007. In the second ballot, Sarkozy won against his left opponent Ségolène Royal with a large majority of 53% of the votes. Not surprisingly, Ségolène Royal won in almost all very industrialized and urbanized Départements: she scored 57% of the votes in the 93rd Département (where the riots started), not so surprisingly, since it is traditionally a left-wing territory (the Communist Party still leads the local government there). But in Clichy, or in cities where the then Minister of Interior displayed some of his aggressive words towards the local youth ("scum" in Argenteuil on the 25 October 2005, "Kärcher" in La Courneuve by the end of June 2005), Ms Royal received very high levels of votes (62% in Clichy, 57% in Argenteuil, 64% in La Courneuve). More precisely, Sarkozy received dreadfully low levels of votes in polling stations located in housing estates (16% in the "Kärcher" estates of La Courneuve at first ballot, 23% in the all city, etc.).

The most striking element concerning the possible linkage between collective violence (or riots) and conventional forms of political expression (or election) is the rise in poll registrations in the housing estates from the 2005 events onwards (to put it briefly, registration on the electoral rolls is not automatic in France and people above 18 years old have to go to the City Hall to register in order to vote). Recent studies showed that until 2007, there was a large discrepancy in the deprived estates between the size of the population and the number of voters registered in the electoral roll. Céline Braconnier et Jean-Yves Dormagen (2007b), who published a very convincing qualitative and quantitative study on the long-term dynamic of electoral apathy in La Courneuve's estates known as the "Cité des Cosmonautes" (2007a), show how 2005 was marked by a rise in registration in the housing estates, where electoral participation was dramatically low in the 2002's presidential election [START_REF] Fauvelle-Aymar | Les comportements électoraux dans les ZUS aux élections présidentielles[END_REF]. Braconnier and Dormagen also show that out of a sample of 96 cities 8.83% of registered population in 2005 in the sampled ZUS are newly registered (national average: 5,75%, average in the sample's non-ZUS areas: 6.20%). Moreover, the proportion of "first registered" (in comparison to people who registered in their city after having moved from another place) is at the highest in their sample ZUS (close to 40%, vs. 20% of the newly registered in the sample's non-ZUS areas), and was higher in 2005 than in 2004 and 2003 (a bit less than 30%). "First registered ever" are in their vast majority young people who were never registered either because they have only just reached the legal age for being a voter, or because they simply had never showed any interest previously. Incentives to convert violent action into registration, frequently promoted by rap stars or football players (like Liliam Thuram), certainly remind the numerous calls to civic order sent to the rebellious Parisian workers after the insurgency of June 1848 and the introduction of the one-man-one-vote system in 1848 [START_REF] Traugott | Armies of the Poor. Determinants of Working-Class Participation in the Parisian Insurrection of June 1848[END_REF][START_REF] Offerlé | Mobilisation électorale et invention du citoyen français. L"exemple du milieu urbain à la fin du XIXe siècle[END_REF]. Our conclusion will show that this comparison with 1848 riots is far from being the only one that can be drawn with 2005's collective violence. In any case, the result of this massive registration, specifically by young people (among whom high levels of abstention were observed in the 2002 presidential poll, Muxel, 2003, Braconnier andDormagen, 2007a) and by people in the ZUS contributed to massive electoral participation in the cities hit by riots, specifically in comparison with their low level of participation in 2002. Consequently, Sarkozy's defeat in the cities hit by riots must not only be read in percentages, but also in absolute numbers in the deprived urban areas (5800 people voted on the first ballot in Clichy in 2002, 7500 in 2007, 11000 people voted in La Courneuve in 2007, 8800 in 2002, 40000 people voted in Argenteuil in 2007, 32000 in 2002).

Fourthly, in a study I conducted over some months with young men who had taken part in riots in 1997 and protested on a more conventional basis in 2002 against police violence (two of their peers were shot down in their small suburb town in the area of Paris), I noticed how hard it was for them to gather primary political resources (money to survive, a room to share, a place to be) and to construct a positive political identity apart from their status as being known to the police (they all had been previously convicted). As such, confrontations with judges at the local court or even the appeal court in Paris allocate them both a political identity and a political space.

Their status as being known by the police turns out to be their only political identity and their only political message. Court rooms are at the end the only places they can enjoy meeting a public attention [START_REF] Jobard | La couleur du jugement. Discriminations dans les décisions judiciaires en matière d"infractions à agents de la force publique (1965-2005)[END_REF].

This matter of fact is strongly strengthened by the harsh reaction of the courts during the disorders (harsh in comparison to usual sentences given in France; considering sentences given for instance after Bradford's riots in England [START_REF] Bagguley | The BradFord 'Riot' of 2001: a preliminary analysis[END_REF]Carling et. al., 2004), French courts were very soft on the rioters).

Around one fifth of the arrested adults nationwide were imprisoned, either in a pretrial decision, or as a result of a judgement. Michel Mazars' report about the arrestees in the 93 rd Department is more precise on this. Courts dismissed one third of the cases submitted by the prosecution. But out of the two thirds left, judges in the court of the 93rd Department sentenced two thirds of them to jail, which consists in a heavy use of imprisonment. In the study I conducted on the basis of data from a comparable court in the Paris region [START_REF] Jobard | La couleur du jugement. Discriminations dans les décisions judiciaires en matière d"infractions à agents de la force publique (1965-2005)[END_REF], I found out that the proportion of such sentences for crime against police officers in the most recent years does not reach the level of half of all sentences given. Justice reacted during the crisis against people it considered it could condemn very harshly, and in some places (according to personal statements which were made to me) sentencing was made on a collective basis with no consideration for the individual circumstances of the defendant. This strongly encourages those concerned to feel subsumed under a single political identity, the one of being an object of the police and the justice system.

Fifth and last point. In the wake of the riots, Dominique de Villepin, the then Prime Minister, seeking an alternative political response to the law-and-order stance of his Minister of Interior, promised the year 2006 to be "the year of equality of opportunity". For this sake, he tried to pass a law creating a new kind of labour contract for young people, called CPE ("contrat première embauche"), against which students and youths massively demonstrated in the spring of 2006. This wave of protest was marked by a violent confrontation between the youngsters who were at school or at the university and still had some hope of finding their way on the basis of their abilities, and the youth in the suburbs. The protest marches in Paris provided the occasion for scenes of extreme violence: Protesting youngsters from the schools and universities were robbed, brutalized and beaten by those coming from the social class below them.

This strongly reminds us of the historical meaning of the word "scum", used by Sarkozy. This word is used in usual translation of the German notion of "Lumpen" set by Marx and Engels to describe the unorganized and anarchical forces of the Lumpenproletariat, which was paid by the French government and enrolled into the "Civil Guard" to quell the second wave of uprising of the Parisian workers occurring in June 1848 [START_REF] Marx | The Civil War in France[END_REF][START_REF] Traugott | Armies of the Poor. Determinants of Working-Class Participation in the Parisian Insurrection of June 1848[END_REF]. The spectre of apolitical, ignorant, criminal forces haunted the Spring 2006 protests, and brought a new light to the events of November 2005. Together with Olivier Fillieule, I conducted qualitative interviews with almost all the high-ranking police officers of the Paris police in 1995. These interviews and an historical analysis of riot police files and documents show that the police head in Paris ("Préfecture de police") always displayed negotiation and compromise strategies in order to hold the rebellious crowds out of the city centre and to prevent any kind of disorder [START_REF] Fillieule | The Maintenance of Order in France. Towards a Model of Protest Policing[END_REF]; all that was absolutely not implemented during the events of Spring 2006. All seemed to be in place to give the so-called scum the opportunity to engage collective violence against their peers from one social class above them. And as a consequence, the so-called scum behaved as such: the speech act in which politics consists was effectively completed.

III. Conclusion

How far is violence an act of protest? How far can the authors of collective violence acting in the French deprived urban areas in November 2005 be described as "rioters" or "rebels"? Charles Tilly would refuse to see the events as riots: "because it embodies a political judgment rather than an analytical distinction. Authorities and observers label as riots the damage-going gatherings of which they disapprove but they use terms like demonstrations, protest, resistance or retaliation for essentially similar events of which they approve" (2003,18).

Meanwhile, we think we have shown on the basis of the comparison of the course of the events on the streets and on the national government level that both timelines were inseparably bound together: Molotov cocktails and the government's display of (verbal or factual) actions seemed to answer to each other. In Tilly's terms, the "scattered attacks" (one of his analytical categories in order to describe types of collective violence) were the consequence of a political cycle. Closer to the scene of the violence, the strategies employed by the different actors at the government's level (in a context of pre-presidential race exacerbation of personal ambitions) played the role of a tactical opportunity structure [START_REF] Mcadam | Tactical innovation and the pace of insurgency[END_REF], Fillieule, 1997, 54-57), since police actions were a direct consequence of these conflicts at the government level.

The migrant backgrounds of the rioters (in contrast with those of the non-rioters) and generational claims of the rioters, which are brought to light through quantitative and qualitative studies, clearly show how much political motives were of importance during the events. Moreover, the fact that the riots played the role of encouraging people into the sphere of electoral registration and preferences show how much these unconventional actions did lead to the stage of conventional politics. As such, it was imperative for the government (or the then Minister of Interior) to keep a grip on the way the rioters could be discredited. Tactical battles were effectively led on the streets, resulting, in the aftermath of the riots (during the student's and pupils demonstrations in March 2006) in a new cycle of collective violence, where police and rioters were used as pawns in a game whose real players have then been (were in fact?) the French electorate and a president-in-waiting (one year later). Collective violence undoubtedly appeared to be a profitable (if hazardous) political investment.

Short