Reactivity of Low-Grade Chromite Concentrates towards Chlorinating Atmospheres

The most economically important iron-chromium bearing minerals is chromite. In natural deposits, iron(II) is frequently substituted by magnesium(II) while chromium(III) is replaced by aluminum(III) and/or iron(III) forming a complex chromium bearing material. The majority of mined chromite is intended for the production of ferrochrome which requires a chromite concentrate with high chromium-to-iron ratio. Found mostly in the spinel chromite structure, iron cannot be removed by physical mineral processing methods. In this frame, the present work deals with the reaction of chlorine and chlorine+oxygen with selected samples of chromite concentrates for assessing the reactivity of their components towards chlorinating atmosphere, allowing the preferential removal of iron, hence meeting the chromite metallurgical grade requirements. Isothermal thermogravimetric analysis was used as a reliable approach for the kinetic reactivity investigation. Results indicated a wide difference in the thermal behavior of chromite constituents in a chlorinating atmosphere when considering their respective values of apparent activation energy oscillating from about 60 to 300 kJ/mol as a function of the sample reacted fraction. During the chromite treatment by chlorine in presence of oxygen, chromium was recovered as liquid chromyl chloride by condensation of the reaction gas phase.


Introduction
Chromium is part of the extended group of refractory metals offering beneficial properties for various end-uses and manufacturing applications. As displayed in Figure 1, part of these metals (Nb, W, V, Hf, Ta) belongs to the critical materials, according to the European Union criticality assessment [1]. Although chromium is in the cut-off level to be critical, it ranks third (after Mg and W) from the standpoint of economic importance.
Most refractory metals are found and extracted from their oxide bearing materials, likewise, the only economic source of chromium is chromite (FeCr 2 O 4 ) ore. Nevertheless, in natural deposits, Mg(II) may substitute Fe(II), while Al(III) and Fe(III) often substitute Cr(III) resulting to a complex chromium bearing mineral with the fairly general formula (Fe,Mg)(Cr,Al,Fe) 2 O 4 , encompassing the main end-members such as FeO·Fe 2 O 3 (magnetite), FeO·Cr 2 O 3 (iron chromite), MgO·Cr 2 O 3 (magnesiochromite), MgO·Al 2 O 3 (true/regularly spinel). Being multiple and complete solid solution of the spinel group, the composition of chromite is no fixed, but varies largely and depends on the geographic and geochemical features of its deposits.
The major part of mined chromite goes to the ferrochrome (FeCr) manufacture [2][3][4][5] and in turn, the FeCr is intended to stainless steels and chromium bearing alloys production. It seems that chromium has no substitute for these industrial end-uses. According to available data [6], the average annual growth of the world stainless steels production is about 5.84% reaching 50.7 million metric tons in 2018. Selected reports [7][8][9][10][11][12][13] from numerous recent research works reported to the scientific journals Materials and Metals are focused on the chromium bearing steels and alloys showing the importance of these leading areas for chromium utilization driving the needs for the ferrochrome and chromite production. Chemical industry, foundry sands and refractory segments are other end-uses of chromite, but their weight relative to the total chromite demand is minor.
Materials 2020, 13, x FOR PEER REVIEW 2 of 17 The major part of mined chromite goes to the ferrochrome (FeCr) manufacture [2][3][4][5] and in turn, the FeCr is intended to stainless steels and chromium bearing alloys production. It seems that chromium has no substitute for these industrial end-uses. According to available data [6], the average annual growth of the world stainless steels production is about 5.84% reaching 50.7 million metric tons in 2018. Selected reports [7][8][9][10][11][12][13] from numerous recent research works reported to the scientific journals Materials and Metals are focused on the chromium bearing steels and alloys showing the importance of these leading areas for chromium utilization driving the needs for the ferrochrome and chromite production. Chemical industry, foundry sands and refractory segments are other end-uses of chromite, but their weight relative to the total chromite demand is minor. It must be emphasized that the production of ferrochrome not only requires a chromite ore and/or concentrate with a high Cr2O3 content (46)(47)(48), but also with a chromium-iron ratio above 2 (typically around 2.8). The physical processing of the mineral can be successful for the removal of the chromite ore gangue leading to a concentrate with a satisfactory Cr2O3 content meeting the metallurgical requirement. However, as the major part of iron is found in the lattice structure of chromite, only a chemical broken down of the chromite structure seems appropriate for the removal of iron from chromite. According to Nafziger [14], chemical techniques, such as hydrometallurgical methods, chlorination, roasting and leaching, as well as smelting, are required for increasing the chromium-to-iron ratio of lean chromite ores and concentrates. Recently, a carbo-thermic reduction followed by hydrochloric acid leaching was tried as an efficient method [15] for the extraction of iron from a poor chromium concentrate. Specific research works [16][17][18][19][20][21] regarding the use of various chlorination agents for chlorination of chromite ores and concentrate and their constituents were summarized earlier [2,3].
Our various research reports published previously [2,3,[22][23][24] were focused on the carbochlorination, chlorination and oxychlorination of rich chromite concentrates having a chromium-to-iron ratio of around 3.2 suitable for ferrochrome manufacturing. However, no recent studies were disclosed in the literature regarding to the use of Cl2+O2 for processing lean chromite materials, i.e., having typically a chromium to iron ratio lesser than 2.8. In this regard, the present paper essentially describes the behavior of a poor chromite concentrate with a chromium-to-iron ratio near to 1.5, under Cl2 and Cl2+air atmospheres. The reactivity and behavior of chromite constituents were examined using thermogravimetric analysis (TGA) approach under isothermal It must be emphasized that the production of ferrochrome not only requires a chromite ore and/or concentrate with a high Cr 2 O 3 content (46-48% Cr 2 O 3 ), but also with a chromium-iron ratio above 2 (typically around 2.8). The physical processing of the mineral can be successful for the removal of the chromite ore gangue leading to a concentrate with a satisfactory Cr 2 O 3 content meeting the metallurgical requirement. However, as the major part of iron is found in the lattice structure of chromite, only a chemical broken down of the chromite structure seems appropriate for the removal of iron from chromite. According to Nafziger [14], chemical techniques, such as hydrometallurgical methods, chlorination, roasting and leaching, as well as smelting, are required for increasing the chromium-to-iron ratio of lean chromite ores and concentrates. Recently, a carbo-thermic reduction followed by hydrochloric acid leaching was tried as an efficient method [15] for the extraction of iron from a poor chromium concentrate. Specific research works [16][17][18][19][20][21] regarding the use of various chlorination agents for chlorination of chromite ores and concentrate and their constituents were summarized earlier [2,3].
Our various research reports published previously [2,3,[22][23][24] were focused on the carbochlorination, chlorination and oxychlorination of rich chromite concentrates having a chromium-to-iron ratio of around 3.2 suitable for ferrochrome manufacturing. However, no recent studies were disclosed in the literature regarding to the use of Cl 2 +O 2 for processing lean chromite materials, i.e., having typically a chromium to iron ratio lesser than 2.8. In this regard, the present paper essentially describes the behavior of a poor chromite concentrate with a chromium-to-iron ratio near to 1.5, under Cl 2 and Cl 2 +air atmospheres. The reactivity and behavior of chromite constituents were examined using thermogravimetric analysis (TGA) approach under isothermal conditions. For a better understanding of the processes, the experimental results are compared with those obtained for the treatment of a rich chromite concentrate (of metallurgical grade).

Materials and Methods
The first chromite concentrate sample (low grade) used for this investigation was provided by a European Union manufacturer. A second sample (high grade) provided by an Albanian chromite plant (Bulqiza, Albania) was also used, mostly for comparison purpose. The physico-chemical characterization of the chromite samples and of the reaction products was performed by diverse analytical methods such as chemical analysis (by inductively coupled plasma atomic emission spectroscopy "ICP-AES"), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS, HITACHI S-4800, Hitachi Ltd., Tokyo, Japan) and X-ray diffraction (XRD, Bruker D8 Advance device, Bruker, Karlsruhe, Germany). Their description was given in previous research works [2,25,26]; only the results will be reported hereafter.
Experimental tests of the reaction of chromite concentrates with chlorine were carried out in a vertical microbalance (model CAHN 1000, Cahn Co., Cerritos, CA, USA) operating at a sensitivity of 10 µg and designed to work under corrosive atmosphere. The equipment configuration with the accessory parts is shown schematically in Figure 2. Several milligrams (more often 40 to 50 mg) of sample were spread out in a silica crucible and the whole specimen was heated up to the desired temperature under nitrogen atmosphere. Subsequently, the nitrogen was replaced by Cl 2 and/or Cl 2 +air (O 2 ) and the evolution of the mass loss over time was recorded. The accessory parts of the setup illustrated in Figure 2 are the units for measuring and purifying the inlet gases as well as those for neutralizing the outlet gases.
Materials 2020, 13, x FOR PEER REVIEW  3 of 17 conditions. For a better understanding of the processes, the experimental results are compared with those obtained for the treatment of a rich chromite concentrate (of metallurgical grade).

Materials and Methods
The first chromite concentrate sample (low grade) used for this investigation was provided by a European Union manufacturer. A second sample (high grade) provided by an Albanian chromite plant (Bulqiza, Albania) was also used, mostly for comparison purpose. The physico-chemical characterization of the chromite samples and of the reaction products was performed by diverse analytical methods such as chemical analysis (by inductively coupled plasma atomic emission spectroscopy "ICP-AES"), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS, HITACHI S-4800, Hitachi Ltd., Tokyo, Japan) and X-ray diffraction (XRD, Bruker D8 Advance device, Bruker, Karlsruhe, Germany). Their description was given in previous research works [2,25,26]; only the results will be reported hereafter.
Experimental tests of the reaction of chromite concentrates with chlorine were carried out in a vertical microbalance (model CAHN 1000, Cahn Co., Cerritos, CA, USA) operating at a sensitivity of 10 μg and designed to work under corrosive atmosphere. The equipment configuration with the accessory parts is shown schematically in Figure 2. Several milligrams (more often 40 to 50 mg) of sample were spread out in a silica crucible and the whole specimen was heated up to the desired temperature under nitrogen atmosphere. Subsequently, the nitrogen was replaced by Cl2 and/or Cl2+air (O2) and the evolution of the mass loss over time was recorded. The accessory parts of the setup illustrated in Figure 2 are the units for measuring and purifying the inlet gases as well as those for neutralizing the outlet gases.

Physico-Chemical Characterization of Chromite Concentrate Samples
The chemical composition in the five main constituents of the first chromite sample is given in Figure 3. As shown, this concentrate is characterized by a high iron content (26.9% wt expressed as FeO) and its chromium-to-iron ratio of 1.48 makes it unsuitable for the FeCr manufacturing. This sample is denoted as low-grade chromite concentrate (LGChC). The XRD patterns of the sample matched well with the (Fe,Mg)(Cr,Fe,Al)2O4 phase. Note that the simple constituents of chromite (Fe3O4, FeCr2O4, MgCr2O4 and MgAl2O4) are isomorphs having analogous XRD profile making their individual identification difficult. Based on the chemical analysis and supposing a perfect stoichiometric composition without cation/anion deficiency and/or defect, the general formula of the

Physico-Chemical Characterization of Chromite Concentrate Samples
The chemical composition in the five main constituents of the first chromite sample is given in Figure 3. As shown, this concentrate is characterized by a high iron content (26.9% wt expressed as FeO) and its chromium-to-iron ratio of 1.48 makes it unsuitable for the FeCr manufacturing.  Figure 4). The second sample had a chromium-to-iron ratio close to 3.2 and is labelled as high-grade chromite concentrate (HGChC). Although a rich concentrate with 47.7% Cr2O3 (Figure 3), it contained high amount of gangue (close to 7% SiO2 belonging also to olivine and serpentine minerals). The chromite body was separated from the gangue by successive physical separations (using dense liquor) and was defined as (Fe0.30, Mg0.70)(Cr1.56,Al0.37,Fe0.07)O4 with the average composition of end-members 4.4% Fe3O4, 30.9% FeCr2O4, 51.0% MgCr2O4 and 13.7% MgAl2O4 ( Figure 4). The mean particle size of both chromite samples used for this study is less than 100 μm. As the reactions of HGChC with Cl2+CO, Cl2 and Cl2+O2 gaseous mixtures were studied thoroughly in previous research work [2,3,[22][23][24], its behaviour in the chlorinating atmosphere is used as a reference to explain the phenomena observed during the treatment of the low-grade chromite concentrate.

Behavior of Chromite under Chlorine Atmosphere
Envisaged reactions of the complex chromite constituents (Fe3O4, FeCr2O4, MgCr2O4 and MgAl2O4) and those of simple oxides (FeO, Fe2O3, Cr2O3, MgO and Al2O3) with chlorine may be represented by Equation (1) through (9). The value of standard free energy changes (ΔrG°) at 900 °C is computed from HSC thermochemical database [27] and is indicated beside each reaction. According to these values, the reactions of chromite constituents with Cl2(g) (Equations (1)-(4))  The second sample had a chromium-to-iron ratio close to 3.2 and is labelled as high-grade chromite concentrate (HGChC). Although a rich concentrate with 47.7% Cr2O3 (Figure 3), it contained high amount of gangue (close to 7% SiO2 belonging also to olivine and serpentine minerals). The chromite body was separated from the gangue by successive physical separations (using dense liquor) and was defined as (Fe0.30, Mg0.70)(Cr1.56,Al0.37,Fe0.07)O4 with the average composition of end-members 4.4% Fe3O4, 30.9% FeCr2O4, 51.0% MgCr2O4 and 13.7% MgAl2O4 ( Figure 4). The mean particle size of both chromite samples used for this study is less than 100 μm. As the reactions of HGChC with Cl2+CO, Cl2 and Cl2+O2 gaseous mixtures were studied thoroughly in previous research work [2,3,[22][23][24], its behaviour in the chlorinating atmosphere is used as a reference to explain the phenomena observed during the treatment of the low-grade chromite concentrate.

Behavior of Chromite under Chlorine Atmosphere
Envisaged reactions of the complex chromite constituents (Fe3O4, FeCr2O4, MgCr2O4 and MgAl2O4) and those of simple oxides (FeO, Fe2O3, Cr2O3, MgO and Al2O3) with chlorine may be represented by Equation (1) through (9). The value of standard free energy changes (ΔrG°) at 900 °C is computed from HSC thermochemical database [27] and is indicated beside each reaction. According to these values, the reactions of chromite constituents with Cl2(g) (Equations (1)-(4)) proceed with ΔrG° > 0 indicating a nonspontaneous process in the forward direction; these reactions will absorb energy from its surroundings in order to take place. Among the reactions of simple The second sample had a chromium-to-iron ratio close to 3.2 and is labelled as high-grade chromite concentrate (HGChC). Although a rich concentrate with 47.7% Cr 2 O 3 (Figure 3), it contained high amount of gangue (close to 7% SiO 2 belonging also to olivine and serpentine minerals). The chromite body was separated from the gangue by successive physical separations (using dense liquor) and was defined as (Fe 0. 30 (Figure 4). The mean particle size of both chromite samples used for this study is less than 100 µm. As the reactions of HGChC with Cl 2 +CO, Cl 2 and Cl 2 +O 2 gaseous mixtures were studied thoroughly in previous research work [2,3,[22][23][24], its behaviour in the chlorinating atmosphere is used as a reference to explain the phenomena observed during the treatment of the low-grade chromite concentrate.  (9). The value of standard free energy changes (∆ r G • ) at 900 • C is computed from HSC thermochemical database [27] and is indicated beside each reaction. According to these values, the reactions of chromite constituents with Cl 2(g) (Equations (1)-(4)) proceed with ∆ r G • > 0 indicating a nonspontaneous process in the forward direction; these reactions will absorb energy from its surroundings in order to take place. Among the reactions of simple metals oxides of the chromite with chlorine involving their respective chlorides (Equations (5) to (9)), only the reaction of FeO seems to be a spontaneous reaction from a thermodynamic point of view; Cr 2 O 3 and Al 2 O 3 are the most stable oxides in chlorine atmosphere. 11.54 (1)

Behavior of Chromite under Chlorine Atmosphere
An important point of the thermodynamic findings is that the thermodynamic reactivity of the complex constituents of the chromite in chlorine is decreasing according to the following sequence: The evolution of the vapor pressure of chlorides of main chromite elements (Cr, Fe, Mg, Al, Si) is displayed in Figure 5 [28,29]. It indicates that in the case of the chlorination of chromite constituents, a selective separation of the obtained chlorides is feasible thanks to great differences in their vapor pressure in a selected temperature interval. A special case is the high volatility of chromyl chloride (CrO 2 Cl 2 ), which will be discussed in Section 3.3.
Materials 2020, 13, x FOR PEER REVIEW 5 of 17 metals oxides of the chromite with chlorine involving their respective chlorides (Equations (5) to (9)), only the reaction of FeO seems to be a spontaneous reaction from a thermodynamic point of view; Cr2O3 and Al2O3 are the most stable oxides in chlorine atmosphere.
ΔrG° (900 °C), kJ/mol Cl2 2/9 Fe3O4(s) + Cl2(g) → 2/3 FeCl3(g) + 4/9 O2(g) 11.54 (1) 2/9 FeCr2O4(s) + Cl2(g) → 2/9 FeCl3(g) + 4/9 CrCl3(s) + 4/9 O2(g) 54.03 85.93 (9) An important point of the thermodynamic findings is that the thermodynamic reactivity of the complex constituents of the chromite in chlorine is decreasing according to the following sequence: The evolution of the vapor pressure of chlorides of main chromite elements (Cr, Fe, Mg, Al, Si) is displayed in Figure 5 [28,29]. It indicates that in the case of the chlorination of chromite constituents, a selective separation of the obtained chlorides is feasible thanks to great differences in their vapor pressure in a selected temperature interval. A special case is the high volatility of chromyl chloride (CrO2Cl2), which will be discussed in Section 3.3. Based on these thermodynamic predictions and on the work previously performed [2], experimental tests of LGChC were conducted with chlorine alone with a flow rate of 60 L/h. The recorded data are depicted in Figure 6 as percent mass loss (% ML) of the sample over the reaction time. The somewhat atypical curve shape is a first indication of the complexity of the reactions of Based on these thermodynamic predictions and on the work previously performed [2], experimental tests of LGChC were conducted with chlorine alone with a flow rate of 60 L/h. The recorded data are depicted in Figure 6 as percent mass loss (% ML) of the sample over the reaction time. The somewhat atypical curve shape is a first indication of the complexity of the reactions of chromite with chlorine. As shown by Figure 6b, the first 50% of the sample was quickly chlorinated and volatilized, while the remaining sample appeared more refractory to chlorine. As an example, the isothermal data indicates that 32 min were sufficient to achieve 50% conversion at 950 • C while 75% conversion required 180 min at this temperature. To get an insight about this change in the curve shape, it is helpful to compare several isothermal TGA plots for both samples, i.e., LGChC and HGChC, as illustrated in Figure 7. The chlorination of chromite concentrates at 800 °C ( Figure 7a) tends to an asymptote of the %ML beyond 2 h of treatment corresponding to about 50% and 35% ML for LGChC and HGChC, respectively. To get an insight about this change in the curve shape, it is helpful to compare several isothermal TGA plots for both samples, i.e., LGChC and HGChC, as illustrated in Figure 7. The chlorination of chromite concentrates at 800 • C (Figure 7a) tends to an asymptote of the %ML beyond 2 h of treatment corresponding to about 50% and 35% ML for LGChC and HGChC, respectively.
Gathering this data with the mineralogical composition of chromite concentrates ( Figure 4) and the thermodynamic predictions allows us to hypothesize that only magnetite and iron chromite are chlorinated at 800 • C. It was evaluated that these compounds (Fe 3 O 4 +FeCr 2 O 4 ) represent close to 52.7% and 35.3% of the LGChC and HGChC, respectively. With this evidence, one may also conclude that the TG analysis of chromite reactions with chlorine at low temperature can be an effective method for the fair determination of the amount of (Fe 3 O 4 +FeCr 2 O 4 ) contained in the chromite ores and/or concentrates. Although less wide, the difference between the %ML obtained for the LGChC and HGChC is still evident at 950 • C (Figure 7b) and 1040 • C (Figure 7c).
The direct application of well-known kinetics models [30] for describing the reaction progress and rate expression faces certain difficulties related to successive reactions, altering of the physical and chemical reactivity of the remaining sample, inter-reaction between gaseous reaction products and the working sample, etc. Hence, it was suggested that the best way to evaluate the temperature impact on the chromite reactions with Cl 2 was to calculate the reaction rate in increments of 5% mass losses in the interval ranging from 5.0% to 85.0% ML. This is performed for all isothermal data from 950 to 1040 • C. Shown in Figure 8 is the graphical representation of the processed data at 975 • C. Besides data linearization expressing the reaction rate, the correlation coefficient (R 2 ) of data fitting, for each segment of 5% ML, is also shown.
To get an insight about this change in the curve shape, it is helpful to compare several isothermal TGA plots for both samples, i.e., LGChC and HGChC, as illustrated in Figure 7. The chlorination of chromite concentrates at 800 °C (Figure 7a) tends to an asymptote of the %ML beyond 2 h of treatment corresponding to about 50% and 35% ML for LGChC and HGChC, respectively.  It was stated [2] that the reaction of chlorine with chromite concentrates generated metal chlorides having volatilization rate higher than their formation rate, indicating the %ML of the sample expresses directly the fraction of the sample (α) reacted. Therefore, the Arrhenius diagrams displaying the logarithm of the reaction rate plotted versus the inverse of the temperature for each 5% ML segment were drawn and values of the apparent activation energy (E a ) with standard error were computed. Figure 9 gives Arrhenius' plots for the four chosen reacted fractions. Good fitting of the traced data is obtained with the value of E a increasing from 58±5 kJ/mol at (0.10 ≤ α ≤ 0.15) to 285 ± 8 kJ/mol at (0.75 ≤ α ≤ 0.80) with a good confidence level. Furthermore, the reaction rate at 1000 • C for (0.75 ≤ α ≤ 0.80) is decreased by around 27 times with respect to the reaction rate for (0.10 ≤ α ≤ 0.15) at the same temperature, which seems unusual for the gas-solid reactions with gasification of reaction products and without formation of new solid products. and the working sample, etc. Hence, it was suggested that the best way to evaluate the temperature impact on the chromite reactions with Cl2 was to calculate the reaction rate in increments of 5% mass losses in the interval ranging from 5.0% to 85.0% ML. This is performed for all isothermal data from 950 to 1040 °C. Shown in Figure 8 is the graphical representation of the processed data at 975 °C. Besides data linearization expressing the reaction rate, the correlation coefficient (R 2 ) of data fitting, for each segment of 5% ML, is also shown.  It was stated [2] that the reaction of chlorine with chromite concentrates generated metal chlorides having volatilization rate higher than their formation rate, indicating the %ML of the sample expresses directly the fraction of the sample (α) reacted. Therefore, the Arrhenius diagrams displaying the logarithm of the reaction rate plotted versus the inverse of the temperature for each 5% ML segment were drawn and values of the apparent activation energy (Ea) with standard error were computed. Figure 9 gives Arrhenius' plots for the four chosen reacted fractions. Good fitting of the traced data is obtained with the value of Ea increasing from 58±5 kJ/mol at (0.10 ≤ α ≤ 0.15) to 285 ± 8 kJ/mol at (0.75 ≤ α ≤ 0.80) with a good confidence level. Furthermore, the reaction rate at 1000 °C for (0.75 ≤ α ≤ 0.80) is decreased by around 27 times with respect to the reaction rate for (0.10 ≤ α ≤ 0.15) at the same temperature, which seems unusual for the gas-solid reactions with gasification of reaction products and without formation of new solid products. An overall profile of the evolution of Ea as a function of the chromite conversion fraction is displayed in Figure 10. The beginning of the reaction proceeded with an Ea near to 78 kJ/mol and decreased to about 60 kJ/mol at 0.10 ≤ α ≤ 0.20. Such a trend suggests the formation of an intermediate species, unfortunately unknown, decreasing the potential barrier of the reaction. An overall profile of the evolution of E a as a function of the chromite conversion fraction is displayed in Figure 10. The beginning of the reaction proceeded with an E a near to 78 kJ/mol and decreased to about 60 kJ/mol at 0.10 ≤ α ≤ 0.20. Such a trend suggests the formation of an intermediate species, unfortunately unknown, decreasing the potential barrier of the reaction. Based on the chemical and mineralogical composition of the LGChC the fraction converted at the beginning of reaction may be attributed to the reaction of Fe 3 O 4 with Cl 2 involving ferric chloride as final product as it is highly volatile at this temperature range ( Figure 5). Thereafter, an increase of the E a is observed reaching about 175 kJ/mol at (0.45 ≤ α ≤ 0.55). One may attribute globally this value of E a to the reaction of iron chromite (FeCr 2 O 4 ) with chlorine. Next, at α > 0.60, the apparent activation energy increased again reaching values as high as 304 kJ/mol for α ranging from 0.70 to 0.75. This conversion fraction corresponds most probably to the removal of MgCr 2 O 4 from the chromite body. According to obtained apparent activation energy and reaction rate trends, the decreasing reaction rank of chromite constituents with chlorine follows the sequence: In other words, a higher reaction rate was associated with a lower value of Ea and vice versa. It is hence concluded that the reactivity of chromite constituents towards chlorine is in good agreement with the apparent activation energy and the reaction rates are sufficiently different to achieve a selective elimination of one constituent without affecting the other constituents. Another point is also to be mentioned that this reaction sequence (Equation (11)) seems to match well with the sequence based on thermodynamic predictions and shown in (Equation (10)).

Reactions of Chromite with Chlorine in Presence of Oxygen
Having obtained information on the reaction of chromite with chlorine, it was useful to investigate the impact of oxygen on the reaction kinetic and involved products. The chemical reactions of the two main chromite constituents, FeCr2O4 and MgCr2O4, with chlorine +oxygen can be described by Equations (12) and (13), respectively. The values of ΔrG° are still positive, but they are much lower than those obtained for the chlorination with chlorine alone (Equations (2) and (3)  In other words, a higher reaction rate was associated with a lower value of E a and vice versa. It is hence concluded that the reactivity of chromite constituents towards chlorine is in good agreement with the apparent activation energy and the reaction rates are sufficiently different to achieve a selective elimination of one constituent without affecting the other constituents. Another point is also to be mentioned that this reaction sequence (Equation (11)) seems to match well with the sequence based on thermodynamic predictions and shown in (Equation (10)).
This thermodynamic assessment is completed with a kinetic study of the Cr 2 O 3 and Cl 2 +O 2 interaction using TG isothermal tests and varying the chlorine content from 0% to 100% Cl 2 . Three typical TGA curves at 50%, 80% and 100% Cl 2 are plotted in Figure 11a as evolution of %ML versus reaction time. More than 160 min were required to reach 75% of the Cr 2 O 3 sample reacted in Cl 2 alone, while the reaction time is decreased to about 112 min when an equimolar (50% Cl 2 +50% O 2 ) gas mixture was used and decreased again to about 85 min when the chlorine content in the Cl 2 +O 2 was 80%. Data displayed in Figure 11b demonstrates that the initial reaction rate (0.05 ≤ α ≤ 0.40) of the Cr 2 O 3 chlorination with Cl 2 +O 2 had a maximum value at 80% Cl 2 corresponding to Cl 2 to O 2 ratio equal to 4. Such a result matches well with the stoichiometric coefficients of the reaction described by Equation (14) resulting in chromyl chloride as final reaction product. Shown in Figure 12 is the %ML versus time for the isothermal treatment of the LGChC under the above-mentioned atmosphere between 950 and 1040 °C for reaction time up to 200 min. As in the previous cases ( Figures 5 and 6), there is an abrupt change on the curve shape after 50% ML, it is more obvious at low temperature and reflected on the reaction progress. For instance, the first 50% ML was reached with a reaction time of 36 min at 950 °C, while more than 410 min are needed to reach 75% ML of the sample. According to this analysis, the LGChC is treated under a flowing gas (Cl 2 +air) with total flow rate of 61 L/h containing 28 L/h of chlorine and 33 L/h air (i.e., 26 L/h N 2 and 7 L/h O 2 ) corresponding to Cl 2 /O 2 molar ratio equal to 4. Complementary data for choosing this gas mixture composition to enhance the chromium oxide reaction rate and to lower the reaction of iron oxides with the chlorinating gas mixtures are found in previously published research reports [31][32][33][34].
Shown in Figure 12 is the %ML versus time for the isothermal treatment of the LGChC under the above-mentioned atmosphere between 950 and 1040 • C for reaction time up to 200 min. As in the previous cases ( Figures 5 and 6), there is an abrupt change on the curve shape after 50% ML, it is more obvious at low temperature and reflected on the reaction progress. For instance, the first 50% ML was reached with a reaction time of 36 min at 950 • C, while more than 410 min are needed to reach 75% ML of the sample. Figure 12 is the %ML versus time for the isothermal treatment of the LGChC under the above-mentioned atmosphere between 950 and 1040 °C for reaction time up to 200 min. As in the previous cases ( Figures 5 and 6), there is an abrupt change on the curve shape after 50% ML, it is more obvious at low temperature and reflected on the reaction progress. For instance, the first 50% ML was reached with a reaction time of 36 min at 950 °C, while more than 410 min are needed to reach 75% ML of the sample. For comparison, the acquired TG data for the reactions of LGChC and HGChC with chlorine in presence of oxygen are shown in Figure 13. The main observations are the high difference between %ML of HGChC and LGChC and the slope change of the %ML vs. time (ML rate), which is clearly more pronounced at 950 °C for both materials, although this evidence is also visible at 1025 °C. For comparison, the acquired TG data for the reactions of LGChC and HGChC with chlorine in presence of oxygen are shown in Figure 13. The main observations are the high difference between %ML of HGChC and LGChC and the slope change of the %ML vs. time (ML rate), which is clearly more pronounced at 950 • C for both materials, although this evidence is also visible at 1025 • C. Combining the chemical and mineralogical composition of both concentrates with these isoconversion data lead to assign the kinetic changes to the substantial reaction of MgCr 2 O 4 with chlorine for the conversion higher than 35% and 50 % for HGChC and LGChC, respectively. Both thermodynamic and kinetics reactivity may explain this particular behaviour of magnesiochromite in the Cl 2 +air gaseous mixture. Combining the chemical and mineralogical composition of both concentrates with these isoconversion data lead to assign the kinetic changes to the substantial reaction of MgCr2O4 with chlorine for the conversion higher than 35% and 50 % for HGChC and LGChC, respectively. Both thermodynamic and kinetics reactivity may explain this particular behaviour of magnesiochromite in the Cl2+air gaseous mixture. To have an idea about the evolution of the elemental and mineralogical composition of the treatment residue, the HGChC reacted at various α-values was examined by SEM-EDS ( Figure 14). This method of analysis was preferred to XRD due to the presence of chromite crystalline isomorph phases. To have an idea about the evolution of the elemental and mineralogical composition of the treatment residue, the HGChC reacted at various α-values was examined by SEM-EDS ( Figure 14). This method of analysis was preferred to XRD due to the presence of chromite crystalline isomorph phases. To have an idea about the evolution of the elemental and mineralogical composition of the treatment residue, the HGChC reacted at various α-values was examined by SEM-EDS ( Figure 14). This method of analysis was preferred to XRD due to the presence of chromite crystalline isomorph phases.  Distinct characteristic peaks of Cr, Mg, Al, Fe, Si and O are present in the SEM-EDS spectrum of the initial sample, in good agreement with the HGChC elemental composition afore-mentioned in Figure 3. The product corresponding to the fraction reacted α = 0.40 does not contain iron. As iron bearing compounds of the chromite body are FeO·Fe 2 O 3 and FeO·Cr 2 O 3 , the SEM-EDS analysis of the fraction reacted at α-0.40 is an indirect confirmation for the removal of magnetite and iron chromite from the HGChC. Spectra at α = 0.60 and α = 0.80 with their decreasing chromium peak intensity reflect the evolution of the composition, down to a chromium-free residue at α = 0.88. Accordingly, magnesio-chromite (MgCr 2 O 4 ) had reacted at 0.35 < α < 0.88, while the true spinel (α = 0.88) appeared more refractory to chlorine in presence of oxygen. Reasoning by analogy, the reaction pathways of the LGChC with chlorine in presence of oxygen will be similar, with the conversion fraction agreeing with its mineralogical composition ( Figure 4. The reaction rates are higher due to the fast reaction of 52.7% (Fe 3 O 4 +FeCr 2 O 4 ) with chlorine, hence exhibiting more reactive surface for the progress of the reaction.

Shown in
The Arrhenius plot shown in Figure 15 for the reaction of LGChC with Cl 2 +air at various α-values illustrates large changes in the apparent activation energy, starting from about 130 kJ/mol at beginning of the reaction, followed by an average E a of about 80-85 kJ/mol at 0.15 ≤ α ≤ 0.50 and by a final strong increase up to 300 kJ/mol. reaction of 52.7% (Fe3O4+FeCr2O4) with chlorine, hence exhibiting more reactive surface for the progress of the reaction.
The Arrhenius plot shown in Figure 15 for the reaction of LGChC with Cl2+air at various α-values illustrates large changes in the apparent activation energy, starting from about 130 kJ/mol at beginning of the reaction, followed by an average Ea of about 80-85 kJ/mol at 0.15 ≤ α ≤ 0.50 and by a final strong increase up to 300 kJ/mol. To help to interpret these peculiar changes, isothermal treatments under Cl2+O2 (Cl2/O2 = 4) of the main oxides (Fe2O3, Cr2O3 and MgO) of the chromite constituents (Fe3O4, FeCr2O4 and MgCr2O4) were performed up to 1025 °C. Note that ferrous oxide (FeO) is transformed into FeCl3 and Fe2O3 under a chlorinating atmosphere [32]. The experimental data showed that the reactivity of these oxides towards Cl2+O2 is widely different. As an example, 90% of the Fe2O3 sample was reacted for 10 min at 950 °C. This reaction time, for reaching the same reaction extent, was extended to 60 min and 270 min for Cr2O3 and MgO, respectively at 950 °C. As shown in Figure 16a, this trend of the TGA measurements is still evident during the treatment of these oxides at 1000 °C. Based on the isothermal data recorded, the Arrhenius plots for the reactions of the above-mentioned oxides with Cl2+O2 are reported in Figure 16b. The Ea values are about 148, 46 and 214 kJ/mol for the Fe2O3, Cr2O3 and MgO reactions, respectively. In addition, the decreasing reaction rate ranking of simple oxides with Cl2+O2 follows the sequence represented by Equation (15).  [32]. The experimental data showed that the reactivity of these oxides towards Cl 2 +O 2 is widely different. As an example, 90% of the Fe 2 O 3 sample was reacted for 10 min at 950 • C. This reaction time, for reaching the same reaction extent, was extended to 60 min and 270 min for Cr 2 O 3 and MgO, respectively at 950 • C. As shown in Figure 16a, this trend of the TGA measurements is still evident during the treatment of these oxides at 1000 • C. Based on the isothermal data recorded, the Arrhenius plots for the reactions of the above-mentioned oxides with Cl 2 +O 2 are reported in Figure 16b. The E a values are about 148, 46 and 214 kJ/mol for the Fe 2 O 3 , Cr 2 O 3 and MgO reactions, respectively. In addition, the decreasing reaction rate ranking of simple oxides with Cl 2 +O 2 follows the sequence represented by Equation (15).
Materials 2020, 13, x FOR PEER REVIEW 13 of 17 Assuming that the chromite constituents (FeO·Fe2O3, FeO·Cr2O3, MgO·Cr2O3) are chlorinated only when both constituents (FeO and Fe2O3; FeO and Cr2O3; MgO and Cr2O3) are chlorinated and taking into account that the whole reaction rate of each chromite constituent is governed by the slowest reaction rate of its simple constituent, one may deduce that: • the first compounds to be reacted are iron oxides of Fe3O4, the value of Ea obtained for Fe2O3 reaction are close to that obtained for the beginning of the chromite reaction with Cl2+O2, • the reaction of chromite with Cl2+O2, for 0.15 ≤ α ≤ 0.50, is controlled by the reaction rate of Cr2O3 (contained in FeCr2O4) being the slowest step of the overall reaction rate. The lower value of Ea seems another argument for Cr2O3 impact on the overall reaction of chromite, • the rest of the chromite reaction with Cl2+O2 (α > 0.50) is affected by the reaction of MgO (MgCr2O4) which is characterized by a low reaction rate and a high Ea value. However, the energy of the chemical binding of the simple constituents in the chromite structure may affect the values of the inherent activation energy and the multistep reaction rates.
The comparison of the kinetic parameters for the reaction of chromite with chlorine and Cl2+air showed a difference in the apparent activation energy values (Figures 10 and 15) essentially for α  However, the energy of the chemical binding of the simple constituents in the chromite structure may affect the values of the inherent activation energy and the multistep reaction rates.
The comparison of the kinetic parameters for the reaction of chromite with chlorine and Cl 2 +air showed a difference in the apparent activation energy values (Figures 10 and 15) essentially for α between 0.20 and 0.55. Two factors may explain that. First, from a thermodynamic point of view, the reaction of Cr 2 O 3 with Cl 2 (Equation (7)) is less favourable than that with Cl 2 +O 2 (Equation (14)); the E a values accordingly appear higher for the FeCr 2 O 4 chlorination with Cl 2 alone. Second, the increasing and higher apparent E a with Cl 2 alone may reflect a MgCr 2 O 4 reaction, characterized by a high E a , starting before the FeCr 2 O 4 reaction is finished.
In spite of these differences, the present study showed an atypical temperature impact on the chlorination of chromite due to the combination of different thermodynamic and kinetics aspects of chromite component reactions with chlorine in absence and/or presence of the oxygen.
As described in the previous sections, TGA tests were performed with small powder samples (40-50 mg) and high flowrates (e.g., 60 L/h Cl 2 ) of reactive gases to attenuate the reaction starvation impact and to enhance mass and heat transfers. To be closer to the practical chlorination process, tests using tenth grams of chromite concentrates were also performed in a horizontal setup described earlier [2] under Cl 2 +air atmosphere from 700 to 1000 • C. The obtained data are depicted in Figure 17 as evolution of the chromium and iron content and chromium to iron ratio of the residues showing the function of the treatment temperature. These results show the preferential removal of iron from 700 • C; about 77% of iron and 18% of chromium were extracted during the treatment of HGChC at 900 • C for 2 h. The chromium and iron contents of the obtained residue at 900 • C were 35.6 and 3.2 wt%, respectively. As shown in Figure 17, the chromium to iron ratio increased rapidly from 3.9 at 700 • C to reach values as high as 11.1 at 900 • C.
Materials 2020, 13, x FOR PEER REVIEW 14 of 17 900 °C were 35.6 and 3.2 wt%, respectively. As shown in Figure 17, the chromium to iron ratio increased rapidly from 3.9 at 700 °C to reach values as high as 11.1 at 900 °C.
To gain understanding of the physical state of the chromium bearing phase synthetized during reaction of chromite with chlorine in presence of oxygen, a two-step cooling of the outlet gases was performed, first at room temperature and second at much lower temperature (−35° C). The analyses of the solid condensate obtained at room temperature by SEM-EDS technique indicated the absence of chromium in the solid phase. However, a red liquid was isolated and collected ( Figure 18) in a glassware vessel emerged in a refrigerated alcohol bath. This liquid produced reddish brown fumes in air and seems to correspond to chromyl chloride (CrO2Cl2) Figure 17. Evolution of chromium and iron content and chromium to iron ratio versus temperature during treatment of HGChC in a Cl 2 +air gaseous mixture.
To gain understanding of the physical state of the chromium bearing phase synthetized during reaction of chromite with chlorine in presence of oxygen, a two-step cooling of the outlet gases was performed, first at room temperature and second at much lower temperature (−35 • C).
The analyses of the solid condensate obtained at room temperature by SEM-EDS technique indicated the absence of chromium in the solid phase. However, a red liquid was isolated and collected ( Figure 18) in a glassware vessel emerged in a refrigerated alcohol bath. This liquid produced reddish brown fumes in air and seems to correspond to chromyl chloride (CrO 2 Cl 2 ) characteristics containing chromium at hexavalent state and it is characterized by a high vapor pressure at room temperature ( Figure 5). The CrO 2 Cl 2 synthesis was also reported in other research works [35][36][37][38][39][40] during thermal treatment of various chromium bearing materials. The analyses of the solid condensate obtained at room temperature by SEM-EDS technique indicated the absence of chromium in the solid phase. However, a red liquid was isolated and collected ( Figure 18) in a glassware vessel emerged in a refrigerated alcohol bath. This liquid produced reddish brown fumes in air and seems to correspond to chromyl chloride (CrO2Cl2) characteristics containing chromium at hexavalent state and it is characterized by a high vapor pressure at room temperature ( Figure 5). The CrO2Cl2 synthesis was also reported in other research works [35][36][37][38][39][40] during thermal treatment of various chromium bearing materials.
By analogy, the ability of chlorine to oxidize iron (II,III) into iron(VI) in high alkali medium by gas-solid and solid-solid reactions was also demonstrated in recent investigations [41][42][43][44][45][46]. This research work gave several insights for the evolution of the (Fe,Cr,Mg,Al)-O-Cl system, in the case of chromite, at different temperatures. Nonetheless, more in-depth and detailed studies are needed to complete the current knowledge in such a complex system. By analogy, the ability of chlorine to oxidize iron (II,III) into iron(VI) in high alkali medium by gas-solid and solid-solid reactions was also demonstrated in recent investigations [41][42][43][44][45][46].

Conclusions
This research work gave several insights for the evolution of the (Fe,Cr,Mg,Al)-O-Cl system, in the case of chromite, at different temperatures. Nonetheless, more in-depth and detailed studies are needed to complete the current knowledge in such a complex system.

Conclusions
Thermogravimetric analysis technique provides valuable information to fairly evaluate the constituent composition of complex materials such as chromite and to analyze its reactions with Cl 2 and Cl 2 +O 2 gaseous mixtures.
The reactions of (Fe,Mg)(Cr,Al,Fe) 2 O 4 constituents, for both chromite concentrates (low grade chromite concentrate-LGChC and high grade chromite concentrate-HGChC) with chlorine in isothermal conditions proceeded by gradual scheme starting by the reaction of iron oxides (Fe 3 O 4 ) followed by interaction of iron chromite (FeCr 2 O 4 ). Magnesio-chromite (MgCr 2 O 4 ) appeared stable in Cl 2 and Cl 2 +O 2 atmosphere at temperatures equal to or lower than 800 • C.
The overall reaction of LGChC with chlorine is affected differentially by temperature at 950-1040 • C, resulting in an apparent activation energy strongly dependent on the degree of conversion, e.g., increasing sharply from about 60 kJ/mol to 300 kJ/mol for fractions reacted of 0.15 and 0.75, respectively. Having a low reactivity, the MgCr 2 O 4 compound required high temperature for the reaction to occur. Similar trends were observed for the reaction of chromite with chlorine in presence of oxygen although the values of the apparent activation energy are somewhat different.
Thermodynamic analysis of the envisaged reactions of the chromite constituents with Cl 2 and Cl 2 +O 2 gave complementary elements for further clarifying this particular behavior of chromite in the chlorinating atmosphere.
The kinetics results of the simple oxides (Fe 2 O 3 , Cr 2 O 3 , MgO) reactions with Cl 2 +O 2 in the interested temperature range was another insightful building block for better understanding the multistep process of chromite processing under chlorine in presence of oxygen.
Low temperatures and short times for the interaction chromite-chlorine favor the preferential removal of iron from the low-grade concentrate, giving a chromite with a chromium-to-iron ratio satisfactory for the ferrochrome production. The presence of oxygen in the system favors the synthesis of pure chromyl chloride.