Direct Production of Ferrochrome by Segregation Reduction of Chromite in the Presence of Calcium Chloride

metals

Article Direct Production of Ferrochrome by Segregation Reduction of Chromite in the Presence of Calcium Chloride

Dawei Yu * ID and Dogan Paktunc * ID

CanmetMINING, Natural Resources Canada, 555 Booth Street, Ottawa, ON K1A 0G1, Canada * Correspondence: [email protected] (D.Y.); [email protected] (D.P.); Tel.: +1-613-947-8024 (D.Y.); +1-613-947-7061 (D.P.)

Received: 20 December 2017; Accepted: 16 January 2018; Published: 19 January 2018

Abstract: A solid reduction process is described whereby chromite is reduced with the help of calcium chloride to produce ferrochrome alloy powders with high metal recovery. The process involves segregation reduction of chromite using graphite as the reductant and calcium chloride as the segregation catalyst. Experiments were performed in the temperature range of 1200–1400 ◦C to evaluate the inﬂuences of various design parameters using both a thermogravimetric analyzer and an electric tube furnace with continuous off-gas analysis. The reduced products were characterized by scanning electron microscopy, X-ray powder diffraction, synchrotron X-ray absorption spectroscopy, and were subjected to wet chemical analysis. It was concluded that the addition of calcium chloride not only accelerated the carbothermic reduction of chromite but also promoted the formation and growth of individual ferrochrome alloy particles. The alloy formation within chromite particles was minimized, enabling the effective separation of ferrochrome alloy particles from the unwanted gangue without the need for ﬁne grinding. Majority of the calcium chloride remained in a recoverable form, with a small percentage (<10 wt %) consumed by reacting with the siliceous gangue forming wadalite. Pure ferrochrome alloy powders were successfully produced with high metal recovery using elutriating separation.

Keywords: chromite; ferrochrome; carbothermic reduction; segregation; CaCl2; elutriation

1. Introduction Ferrochromium, which is a critical alloy in the production of stainless steel and high-alloying ferritic steel [1], has been produced worldwide by carbothermic smelting reduction of chromite in submerged electric arc furnaces (SAF) for almost a century [2,3]. More than 90% of the high carbon ferrochrome produced, with a typical composition range of 60–70 wt % Cr, 6–8 wt % C and <5 wt % Si [1], is used for stainless steel production. The SAF process for ferrochrome production is extremely electric energy-intensive [3], with the specific energy consumption (SEC) in the range of 2.4–4.8 MWh per ton of ferrochrome produced [4]. Among the various SAF smelting technologies, pre-reduction of chromite before SAF smelting generally results in a lower SEC, because significant amounts of energy that are required for both heating the feed material and for enabling the endothermic reduction reactions are supplied by burning fuels in the pre-reduction stage, which is generally carried out in rotary kilns [5]. The premus process operates at a pre-reduction degree of ~50% Cr metallization, achieving a low SEC of 2.4 MWh/t [6]. As the most significant mineral source of Cr, chromite can be represented by the general 2+ 3+ formula (Mg,Fe )(Cr,Al,Fe )2O4 [7]. Reduction of chromite is relatively difficult due to its refractory nature, reflected partly by the strong oxygen bonding of the constituent cations. 2+ 3+ Unreduced cations (Mg and Al ) can form a refractory product layer in the form of MgAl2O4

Metals 2018, 8, 69; doi:10.3390/met8010069 www.mdpi.com/journal/metals Metals 2018, 8, 69 2 of 17 around unreduced or partially-reduced residual chromite, inhibiting further reduction [8]. In an effort to improve the carbothermic reduction kinetics of chromite, various fluxes were used as additives, such as CaO [9], SiO2 [10–14], CaF2 [15], combinations of NaF–CaF2, fluorspar–feldspar–silica, and fluorspar–granite [16], etc. Many kinetic studies on the carbothermic reduction of chromite with/without the addition of fluxes [9,10,17–19] revealed that the ionic diffusion through the solid product layer could limit the reduction rate, due mainly to the presence of the unreducible oxides. Addition of some of the fluxes can accelerate the reduction rate by dissolving the solid refractory oxide layer forming molten slags. Although significant research efforts have been made on the carbothermic solid reduction of chromite on the aspect of improving reduction kinetics, little attention was paid to alloy growth during solid reduction. The significance of alloy growth lies in the fact that it could potentially enable the direct production of pure ferrochrome by separation without the need of a SAF or a melting furnace for separating the alloy from the unwanted gangue/slag. Elimination of the SAF smelting or a melting operation means a much higher energy efficiency because the process will operate at the solid-state reduction regime of a much lower temperature. It can also remove the heavy reliance of the ferrochrome production on electric energy. Both of the two aspects bear much significance from the economic point of view. From the perspectives of alloy growth during reduction and its subsequent separation, challenges arise due to the complexity of the carbothermic solid reduction mechanism. Metallic product inevitably formed in various regions of the reduction system, both inside or on the surface of the chromite phase [20], evidenced by the formation of significant amounts of fine alloy particles from both within chromite particles and on the surface [8,10,12,13,21]. A complex spatial relationship between the fine alloy particles and the unwanted materials in the reduced product means excessive grinding/milling is essential to physically liberate the alloy particles before separation (e.g., magnetic and gravitational separation). Excessive grinding of the reduced product not only consumes much electricity, but also likely produces powders too fine to be separable by conventional separation techniques. Therefore, the particle size of the ferrochrome alloys formed during reduction should be large enough to allow subsequent easy liberation and separation. For the first time, we utilized the approach of segregation reduction of chromite with the use of CaCl2 to prevent the alloy formation from within chromite particles while producing individual ferrochrome particles that are amenable for subsequent physical separation. Segregation phenomenon was accidentally discovered in the year 1923 when a particular Chilean copper ore was roasted with a solid fuel at 700 ◦C with the exclusion of oxygen [22]. Outward migration of the copper from finely-divided ore particles was observed. Further investigation revealed the key role of sodium chloride that was naturally present in the ore [22]. Subsequent extensive development lead to the invention of the TORCO (Treatment of Refractory Copper Ores) process [23]. This segregation process incorporates four main steps [23]: (1) formation of HCl from the hydrolyzation of the salt by moisture in the presence of Si-bearing minerals; (2) formation of volatile cuprous chloride from the reaction between HCl and copper oxide; (3) generation of H2 from reduction of water vapor by carbon; (4) reduction of cuprous chloride vapor in the vicinity of carbon particles by H2, forming metallic copper and regenerating HCl. Segregation roasting has subsequently been studied to treat lateritic nickel ores [24–26] and iron ores [27,28]. The reasons for the choice of CaCl2 as the segregation catalyst for chromite direct reduction include the following. First, CaCl2 is readily available in large quantities because it is produced as a by-product of industrial processes [29,30]. Second, it is relatively cheap (300–400 USD/ton) [31]. ◦ Third, the boiling point of CaCl2 is 1935 C[32], indicating its potential for a low evaporation rate during chromite reduction at temperatures higher than 1200 ◦C. In comparison, the boiling point of NaCl is 1465 ◦C[32], meaning that significant evaporation would take place when using NaCl as the additive for chromite reduction. Metals 2018, 8, 69 3 of 17 Metals 2018, 8, 69 3 of 17 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials The chromite ore from the Ring of Fire area of Northern Ontario was sieved to produce four size fractionsThe chromiteused for orethe from experiments. the Ring ofThe Fire averag area ofe formula Northern of Ontario chromite was can sieved be torepresented produce four as (Mgsize0.5 fractionsFe0.5)(Cr1.4 usedFe0.1Al for0.5)O the4 [8]. experiments. Table 1 lists The theaverage compositions formula of the of chromite four size canfractions be represented of chromite. as As(Mg shown,0.5Fe0.5 )(Crthe finer1.4Fe 0.1sizeAl 0.5fractions)O4 [8]. contain Table1 listshigher the amounts compositions of the of gangue the four minerals, size fractions evidenced of chromite. by the lowerAs shown, Cr2O3 theand finer higher size SiO fractions2 concentrations. contain higherXRD (Rigaku amounts D/MAX of the 2500 gangue , Rigaku minerals, Ltd., Tokyo, evidenced Japan) by analysisthe lower of Cr the2O 337–75and higherµm size SiO fraction2 concentrations. (Figure 1) XRD indicates (Rigaku that D/MAX the main 2500, gangue Rigaku minerals Ltd., Tokyo, are clinochloreJapan) analysis (Mg,Fe) of the5Al(Si 37–753Al)Oµm10(OH) size fraction8, magnesite (Figure (MgCO1) indicates3), and that phlogopite the main KMg gangue3(Si3 mineralsAl)O10(OH) are2. Chromiteclinochlore size (Mg,Fe) fraction5Al(Si of 180–3003Al)O10 (OH)µm has8, magnesite much less (MgCOgangue,3 ),with and only phlogopite clinochlore KMg identified3(Si3Al)O 10by(OH) XRD2 . (FigureChromite 1). sizeFlake-shaped fraction of graphite 180–300 µpowdersm has much (99.9995% less gangue, metal basis) with onlywere clinochlore used as the identified reductant. by AnhydrousXRD (Figure calcium1). Flake-shaped chloride (CaCl graphite2) was powders ground (99.9995% quickly in metal a mortar basis) and were pestle used to as a the fine reductant. powder formAnhydrous before calciumuse, while chloride minimizing (CaCl2 moisture) was ground absorption. quickly in a mortar and pestle to a fine powder form before use, while minimizing moisture absorption. Table 1. Chemical composition of the chromite of four size fractions. Table 1. Chemical composition of the chromite of four size fractions. Particle Size (µm) Cr2O3 FeO * MgO Al2O3 SiO2 TiO2 V2O5 NiO K2O CaO

Particle Size37–75 (µ m) Cr2O35.323 FeO 15.92 * MgO18.51 Al 2O10.423 SiO9.792 TiO0.212 V0.182O5 0.29NiO 0.44 K2O 0.17 CaO 37–7575–105 35.3244.36 15.9219.71 18.5114.07 10.4210.84 9.794.54 0.210.27 0.180.20 0.17 0.29 0.21 0.44 0.09 0.17 75–105105–180 44.3646.42 19.7120.50 14.0712.85 10.8411.16 4.543.35 0.270.28 0.200.19 0.12 0.17 0.10 0.21 0.06 0.09 105–180180–300 46.4247.36 20.5020.92 12.8512.71 11.1611.61 3.352.95 0.280.28 0.190.22 0.10 0.12 0.06 0.10 0.04 0.06 180–300 47.36 20.92 12.71 11.61 2.95 0.28 0.22 0.10 0.06 0.04 * FeO: recast as total iron including a small fraction of Fe2O3. * FeO: recast as total iron including a small fraction of Fe2O3.

FigureFigure 1. 1. XRDXRD patterns patterns of of the the chromite chromite or oree of four size fractions.

2.2.2.2. Thermogravimetric Thermogravimetric Analysis Analysis NETZSCHNETZSCH STASTA 449C 449C Simultaneous Simultaneous Thermal Thermal Analyzer Analyzer (Netzsch (Netzsch Gerätebau Gerätebau GmbH, Selb,GmbH, Germany) Selb, Germany)coupled with coupled a NETZSCH with a QMS NETZSCH 403 C Aeolos QMS Mass403 C Spectrometer Aeolos Mass was Spectrometer used for the thermogravimetricwas used for the thermogravimetric analysis (TG-DSC-MS). For each test, the ore sample, graphite and CaCl2 with pre- analysis (TG-DSC-MS). For each test, the ore sample, graphite and CaCl2 with pre-determined mass determinedproportions mass were proportions homogenized were before homogenized a pellet measuring before a pellet 3 mm measuring in diameter 3 mm and in approximately diameter and approximately 70 mg in weight was prepared using a 3 mm die set. The pellet was placed in an

Metals 2018, 8, 69 4 of 17

70 mg in weight was prepared using a 3 mm die set. The pellet was placed in an alumina crucible (outer diameter 6.8 mm), and covered with an alumina lid (with one pin hole). The crucible was placed along with a reference crucible on the sample carrier before the measurement. A vacuum was applied to remove any residual air inside the TG-DSC chamber before a continuous ﬂow of 100 mL/min Ar (5 N) was introduced. The sample was heated in the Ar ﬂow at 20 ◦C/min to the target temperature, and dwell for 3 h before cooling down to room temperature.

2.3. Electric Tube Furnace Tests

The ore sample weighing 10 g was mixed with a pre-determined amount of graphite and CaCl2 before pellets were prepared using a 13 mm die set. Each cylindrical pellet was approximately 2.5 g in weight, 13 mm in diameter, and about 15 mm in height. All pellets were placed in an alumina crucible (inner diameter 4 cm) before being placed inside the chamber of the vertical tube furnace. The samples were heated in an Ar ﬂow of 500 mL/min at a rate of 6.25 ◦C/min to the target temperature for reduction. Samples were cooled as quickly as possible to room temperatures after 2 h dwell. The offgas was continuously measured for its CO and CO2 concentrations by a gas analyzer (ABB EL3020, ABB, Cary, NC, USA).

2.4. Separation of Ferrochrome by Elutriating Tube An elutriating tube similar to the one described by Frost [33] was used for the separation of ferrochrome from other unwanted materials in the reduced product, based on the density difference under a counter flow of water. A water pump with precise control of flowrate was used to continuously feed the water from the bottom of the elutriating tube. At a constant water flow after reaching steady state, denser particles were suspended in the lower and narrower section of the tube, while less dense particles were suspended in the upper and wider section. Separation was achieved by gradually lowering the water flowrate, causing only the alloy particles to settle to the bottom of the tube. The alloy particles were collected by discharging from the bottom into a receiver flask. The unwanted materials were also collected by stopping the water flow and discharging from the bottom of the tube.

2.5. Analytical Methods

As water-soluble CaCl2 was present during reduction of chromite, quantification of the water-soluble species present in the reduced product was performed by water leaching of 1 g sample for 1 h, followed by filtration. The leachate was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES, Varian VISTA RL, Varian, Palo Alto, CA, USA). Quantitative chemical analysis of solid samples was performed by completely dissolving the sample into aqueous solution using Na2O2 fusion technique, followed by analyzing the solution with ICP-AES. Polished sections of the reduced samples were prepared and examined by scanning electron microscopy (Hitachi S-3200N, Hitachi Corporation, Tokyo, Japan), which is coupled with an energy dispersive spectrometer (Quantax EDS, Bruker, Karlsruhe, Germany), with an accelerating voltage of 20 kV. Surface morphology of the reduced sample powders was also examined by firstly removing water-soluble chlorides with water leach, followed by filtration, and drying to remove moisture. The dried powders were then spread onto a double-sided carbon tape as a mono-layer, before SEM-EDS analysis. XRD analysis was performed using a Rigaku D/MAX 2500 (Rigaku Ltd., Tokyo, Japan) rotating-anode powder diffractometer with Cu Kα radiation at 40 kV, 200 mA. X-ray absorption spectroscopy experiments were performed at the 9 BM and 20 BM beamlines of the Advanced Photon Source. Measurements were made on finely ground samples, spread onto tapes as monolayers. X-ray absorption spectra were collected by scanning the Si (111) monochromator in the 5780–6850 eV range (Cr K-edge) and 6911–7964 eV energy range (Fe K-edge). Samples were scanned four to five times in transmission and fluorescence modes using beam sizes of 1 mm × 1 mm and 1 mm × 2 mm. Chromium and iron foils were used for monitoring and calibrating energy shifts. Data reduction and analysis were made by ATHENA [34]. Metals 20182018,, 88,, 69 69 5 of 17

3. Results and Discussions 3. Results and Discussions

3.1. Effect of the Amount of CaCl2 3.1. Effect of the Amount of CaCl2 A series of isothermal TG-DSC-MS tests was firstlyfirstly performed at 1300 ◦°CC using a mixture of chromite (75–105 µm), graphite (25–37 µm, 25 wt % of ore), and varying amount of CaCl2 (0–30 wt % chromite (75–105 µm), graphite (25–37 µm, 25 wt % of ore), and varying amount of CaCl2 (0–30 wt % of of ore). For each test, a 3 mm pellet was prepared from the mixture with a hydraulic press by ore). For each test, a 3 mm pellet was prepared from the mixture with a hydraulic press by employing employing a pressure of 200 MPa. The temperature profile, mass change (TG), heat flow (DSC), and a pressure of 200 MPa. The temperature profile, mass change (TG), heat flow (DSC), and gas analysis gas analysis (MS) results are plotted in Figure 2. In order to allow direct comparison among the tests, (MS) results are plotted in Figure2. In order to allow direct comparison among the tests, each mass each mass change curve (mg) was normalized to represent the test in which the starting sample change curve (mg) was normalized to represent the test in which the starting sample contained exactly contained exactly 50 mg chromite, 12.5 mg graphite, and varying amounts of CaCl2. As seen, the first 50 mg chromite, 12.5 mg graphite, and varying amounts of CaCl2. As seen, the first thermal event thermal event took place at approximately◦ 170 °C, represented by the sharp endotherm peak (DSC), took place at approximately 170 C, represented by the sharp endotherm peak (DSC), H2O peak, and H2O peak, and the corresponding mass loss. Mass loss was absent in the test without CaCl2 addition. the corresponding mass loss. Mass loss was absent in the test without CaCl2 addition. This thermal This thermal event represents the evaporation of moisture which was initially absorbed by event represents the evaporation of moisture which was initially absorbed by anhydrous CaCl2 during anhydrous CaCl2 during preparation of the pellet. A second mass loss took place at approximately preparation of the pellet. A second mass loss took place at approximately 570 ◦C, corresponding to the 570 °C, corresponding to the second H2O peak. This was identified as the thermal dehydration of second H2O peak. This was identified as the thermal dehydration of clinochlore. Detailed discussion clinochlore. Detailed discussion on this thermal event can be found in our previous investigation [8]. on this thermal event can be found in our previous investigation [8]. A small endotherm at 761 ◦C A small endotherm at 761 °C resulted from the melting of CaCl2. The significant mass loss when the◦ resulted from the melting of CaCl2. The significant mass loss when the temperature reached 1300 C temperature reached 1300 °C was due to the carbothermic reduction of chromite, corresponding to was due to the carbothermic reduction of chromite, corresponding to an endotherm and the CO peaks. an endotherm and the CO peaks. This can be represented by the much-simplified reaction Equation This can be represented by the much-simplified reaction Equation (1). A steeper mass loss and sharper (1). A steeper mass loss and sharper CO peak can be observed at a higher CaCl2 addition, indicating CO peak can be observed at a higher CaCl2 addition, indicating that the carbothermic reduction of that the carbothermic reduction of chromite can be greatly accelerated in the presence of CaCl2. chromite can be greatly accelerated in the presence of CaCl2. FeCr2O4 + 4C = Cr2Fe(alloy) + 4CO (1) FeCr2O4 + 4C = Cr2Fe(alloy) + 4CO (1)

Figure 2. Isothermal thermogravimetric analysis (TG-DSC-MS) tests at 13001300 ◦°C,C, varying the amounts of CaCl2 addition. of CaCl2 addition.

Figure 33 illustrates the the cross cross sections sections of of the the reduced reduced products. products. XRD XRD patterns patterns are shown are shown in Figure in Figure4 for comparison.4 for comparison. In the absence In the absence of CaCl of2 (Figure CaCl 2 3a),(Figure each3 a),chromite each chromite particle was particle partially was partiallyreduced, reduced,with a Cr-rich with chromite a Cr-rich core chromite (light coregrey) (light and grey)an outermost and an outermostrefractory spinel refractory (MgAl spinel2O4) layer (MgAl (dark2O4) layergrey). (darkThe grey).representative The representative composition composition of the ofCr-rich the Cr-rich chromite chromite core corecan can be be expressed expressed as (Mg0.95FeFe0.050.05)(Cr)(Cr1.51.5AlAl0.5)O0.54)O based4 based on on semi-quantitative semi-quantitative EDS EDS microanalysis. microanalysis. Significant Signiﬁcant amounts amounts of ferrochrome alloy particles were ﬁnelyfinely disseminateddisseminated in each chromite particle, making it impossible to recover by grinding for liberation followed by physical separation. The presence of residual chromite and appreciable amounts of graphite identified by XRD analysis (Figure 4) confirmed the

Metals 2018, 8, 69 6 of 17

Metalsto recover 2018, 8, by69 grinding for liberation followed by physical separation. The presence of residual6 of 17 Metalschromite 2018, 8 and, 69 appreciable amounts of graphite identified by XRD analysis (Figure4) confirmed6 of the 17 incomplete chromitechromite reductionreduction in in the the absence absence of of CaCl CaCl2.2 With. With the the addition addition of 10of wt10 %wt CaCl % CaCl2 (Figure2 (Figure3b), incomplete3b),a much a much higher chromite higher degree degree reduction of reductionof reduction in the has absence has taken taken of place, plCaClace, evidenced2. evidenced With the byaddition by the the absence absence of 10 wt ofof Cr-rich%Cr-rich CaCl 2 chromite (Figure 3b),cores. a much Most higherferrochrome degree alloys of reduction formed has outside taken of pl the ace, original evidenced chromite by the particlesabsence ofdue Cr-rich to the chromite chloride cores.segregation Most ferrochromeeffect. Some alloyssmall particles formed outside of Fe-rich of theferrochrome original chromite alloy can particles still be observeddue to the inside chloride the segregationoriginal chromite chromite effect. particlesSome particles small (e.g., (e.g., particles the the enlarged enlarged of Fe-rich particle particle ferrochrome in in Figure Figure alloy 3b).3b). Incan In the thestill product productbe observed resulted resulted inside from from the 30 original chromite particles (e.g., the enlarged particle in Figure 3b). In the product resulted from 30 wt30 wt% CaCl % CaCl2 addition2 addition (Figure (Figure 3c),3c), the the completely completely reduced reduced chromite chromite particles particles are are mainly composed ofof wt % CaCl2 addition (Figure 3c), the completely reduced chromite particles are mainly composed of MgAl2O4 andand MgO, MgO, and and are are nearly nearly free free of of ferrochrome ferrochrome inclusions. inclusions. Based Based on on EDS EDS analysis, analysis, the the reduced reduced MgAlchromite2O4 andparticles MgO, contain and are onnearly average free of 1.1 ferrochrome wt % Cr and inclusions. 0.1 wt % Based Fe, which on EDS translates analysis, tothe estimated reduced chromitemetallization particles degrees contain of 98.9 on wt average % Cr and1.1 wt 99.8 % wtCr %and Fe.Fe. 0.1 The wt individual % Fe, which ferrochrome translates alloy to estimated particles metallization degrees of 98.9 wt % Cr and 99.8 wt % Fe. The individual ferrochrome alloy particles formed outside outside of of the the reduced reduced chromite chromite are are mainly mainly the the M7 MC37 typeC3 type carbide carbide (M = (M Cr, =Fe), Cr, with Fe), the with Cr:Fe the formed outside of the reduced chromite are mainly the M7C3 type carbide (M = Cr, Fe), with the Cr:Fe ratioCr:Fe determined ratio determined by that by of that the of starting the starting chromi chromitete composition. composition. In addition In addition to carbide, to carbide, MgAl MgAl2O4, 2andO4, ratio determined by that of the starting chromite composition. In addition to carbide, MgAl2O4, and MgO,and MgO, XRD XRD analysis analysis (Figure (Figure 4) 4also) also identified identified wadalite wadalite (Ca66AlAl5Si5Si2O2O16Cl16Cl3) 3that) that resulted resulted from from the MgO, XRD analysis (Figure 4) also identified wadalite (Ca6Al5Si2O16Cl3) that resulted from the reaction of CaCl2 with the silicate gangue (e.g., clinochl clinochlore)ore) present in the ore, with the possible reactioninvolvement of CaCl of chromite.2 with the silicate gangue (e.g., clinochlore) present in the ore, with the possible involvement of chromite.

Figure 3. Backscattered electron micrographs of the reduced products from the tests with (a) no Figure 3. Backscattered electron micrographs of the reduced products from the tests with (a) no additive, Figureadditive, 3. (Backscatteredb) 10 wt %, and electron (c) 30 wt micrographs % CaCl2 addition. of the reduced products from the tests with (a) no (b) 10 wt %, and (c) 30 wt % CaCl2 addition. additive, (b) 10 wt %, and (c) 30 wt % CaCl2 addition.

Figure 4. XRD patterns of the reduced products from the tests with no additive and 30 wt % CaCl2 Figure 4. XRD patterns of the reduced products from the tests with no additive and 30 wt % CaCl2 addition.Figure 4. XRD patterns of the reduced products from the tests with no additive and 30 wt % addition. CaCl2 addition. 3.2. Effect of the Amount of Reductant 3.2. Effect of the Amount of Reductant 3.2. EffectIn theory, of the no Amount less than of Reductant the stoichiometric amount of carbonaceous reductant should be used in In theory, no less than the stoichiometric amount of carbonaceous reductant should be used in orderIn to theory, allow nothe less complete than the the stoichiometric reduction of amountchromite. of For carbonaceous example, the reductant stoichiometric should amount be used of in order to allow the complete the reduction of chromite. For example, the stoichiometric amount of carbonorder to required allow the for complete the complete the reductionreduction of chromite.the 75–105 Forµm example,chromite thefraction, stoichiometric forming gaseous amount CO of carbon required for the complete reduction of the 75–105 µm chromite fraction, forming gaseous CO andcarbon M7C required3-type ferrochrome for the complete carbide reduction is approximately of the 75–105 19 wt %µ ofm ore. chromite In order fraction, to evaluate forming the influence gaseous andof excessive M7C3-type carbon ferrochrome on the reduction carbide is of approximately chromite, elec tric19 wt tube % offurnace ore. In tests order at to 1300 evaluate °C were the performed influence CO and M7C3-type ferrochrome carbide is approximately 19 wt % of ore. In order to evaluate the ofusing excessive 13 mm carbon pellets, on theprepared reduction at a of pressure chromite, of elec 200tric MPa tube from furnace mixtures tests at of 1300 chromite °C were (75–105 performed µm), usinggraphite 13 (25–37mm pellets, µm, 22 prepared or 28 wt at% ofa pressureore), and ofground 200 MPa anhydrous from mixtures CaCl2 (30 of wt chromite % of ore). (75–105 It was µm), later graphitefound by (25–37 examining µm, 22 the or reduced 28 wt % productof ore), andthat ground certain anhydrousamounts of CaCl ferrochrome2 (30 wt % alloys of ore). were It was attached later foundto the reducedby examining chromite the particles,reduced productdue to the that high certain pressure amounts used ofin ferrochromepreparing the alloys pellets. were This attached created to the reduced chromite particles, due to the high pressure used in preparing the pellets. This created

Metals 2018, 8, 69 7 of 17 influence of excessive carbon on the reduction of chromite, electric tube furnace tests at 1300 ◦C were performed using 13 mm pellets, prepared at a pressure of 200 MPa from mixtures of chromite (75–105 µm), graphite (25–37 µm, 22 or 28 wt % of ore), and ground anhydrous CaCl2 (30 wt % of ore). It ‘was later found by examining the reduced product that certain amounts of ferrochrome alloys were attached to the reduced chromite particles, due to the high pressure used in preparing the pellets. This created difficulties during the subsequent physical separation of ferrochrome alloys from the unwantedMetals materials. 2018, 8, 69 A third test was therefore performed using the pellets containing 19 wt7 of % 17 graphite Metals 2018, 8, 69 7 of 17 (i.e., stoichiometric amount), prepared with a much smaller pressure of approximately 0.7 MPa. All the difficulties during the subsequent physical separation of ferrochrome alloys from the unwanted 13 mm pelletsdifficulties used during in the the subsequent subsequent physical tests were sepa preparedration of ferrochrome at a pressure alloys of 0.7 from MPa. the Theunwanted results from materials. A third test was therefore performed using the pellets containing 19 wt % graphite (i.e., materials. A third test was therefore performed using the pellets containing 19 wt % graphite (i.e., off-gas analysisstoichiometric with amount), the temperature prepared with profiles a much are smaller plotted pressure in Figure of approximately5 for comparison. 0.7 MPa. As All seen,the 13 the CO stoichiometric amount), prepared with a much smaller pressure of approximately 0.7 MPa. All the 13 profilesmm for pellets 22 and used 28 wtin the % Csubsequent are nearly tests identical, were prepared indicating at a pressure that excessive of 0.7 MPa. graphite The results has from little off- influence mm pellets used in the subsequent tests were prepared at a pressure of 0.7 MPa. The results from off- on the reductiongas analysis rate. with Comparatively,the temperature profiles the pellets are plo fortted the in Figure test using 5 for 19comp wtarison. % graphite As seen, were the CO relatively gas analysis with the temperature profiles are plotted in Figure 5 for comparison. As seen, the CO profiles for 22 and 28 wt % C are nearly identical, indicating that excessive graphite has little influence looselyprofiles packed, for because 22 and 28 ofwt a% muchC are nearly smaller identical, pressure indicating used that for excessive pellet preparation. graphite has little This influence would mean on the reduction rate. Comparatively, the pellets for the test using 19 wt % graphite were relatively that largeron the gaps reduction existed rate. between Comparatively, the chromite the pellets and for graphite the test using particles. 19 wt % As graphite a result, were the relatively reduction rate loosely packed, because of a much smaller pressure used for pellet preparation. This would mean appearedloosely slightly packed, slower because based of a on much its COsmaller profile. pressure Cross used sections for pellet of preparation. the products This from would reduction mean with that larger gaps existed between the chromite and graphite particles. As a result, the reduction rate that larger gaps existed between the chromite and graphite particles. As a result, the reduction rate 19 and 22appeared wt % graphiteslightly slower are illustrated based on itsin CO Figure profile.6. Cross The residualsections of chromite the products particles from reduction (grey) in with Figure 6a appeared slightly slower based on its CO profile. Cross sections of the products from reduction with contain19 on and average 22 wt %1.4 graphite wt % are Cr, illustrated and 0.1 in wt Figure % Fe 6. basedThe residual on the chromite EDS microanalyses,particles (grey) in translatingFigure to 19 and 22 wt % graphite are illustrated in Figure 6. The residual chromite particles (grey) in Figure 6a contain on average 1.4 wt % Cr, and 0.1 wt % Fe based on the EDS microanalyses, translating to metallization6a contain degrees on average of 98.6 1.4 wt wt % % Cr, Cr and and 0.1 99.8wt % wtFe based % Fe. on The the EDS presence microanalyses, of residual translating graphite to cores metallization degrees of 98.6 wt % Cr and 99.8 wt % Fe. The presence of residual graphite cores (black) (black) inmetallization ferrochrome degrees alloy of particles98.6 wt % Cr (white) and 99.8 can wt be % Fe. seen The in presence Figure6 ofb, residual because graphite of the cores excessive (black) amount in ferrochrome alloy particles (white) can be seen in Figure 6b, because of the excessive amount of of graphitein ferrochrome used. In alloy comparison, particles (white) reduction can be by seen the in stoichiometric Figure 6b, because amount of the excessive of carbon amount resulted of in the graphite used. In comparison, reduction by the stoichiometric amount of carbon resulted in the graphite used. In comparison, reduction by the stoichiometric amount of carbon resulted in the formationformation of clean of clean ferrochrome ferrochrome alloy alloy particles particles that that are free free from from inclusions inclusions of residual of residual carbon, carbon, as seen as seen formation of clean ferrochrome alloy particles that are free from inclusions of residual carbon, as seen in Figurein6 Figurea. 6a. in Figure 6a.

Figure 5. Temperature profiles and off-gas CO concentrations for the tests with the variation of FigureFigure 5. Temperature 5. Temperature proﬁles profiles and and off-gas off-gas COCO concentr concentrationsations for forthe thetests testswith withthe variation the variation of of graphite addition. graphitegraphite addition. addition.

Figure 6. Backscattered electron micrographs of the products from reduction with (a) 19 wt % graphite Figure 6. Backscattered electron micrographs of the products from reduction with (a) 19 wt % graphite Figure 6.and (b) 22 wt % graphite. a andBackscattered (b) 22 wt % graphite. electron micrographs of the products from reduction with ( ) 19 wt % graphite and (b) 22 wt % graphite. 3.3. Effect of Reduction Temperature 3.3. Effect of Reduction Temperature Growth of ferrochrome alloy particles during the CaCl2-assisted segregation reduction can be 3.3. Effect ofGrowth Reduction of ferrochrome Temperature alloy particles during the CaCl2-assisted segregation reduction can be controlled by two main mechanisms. Firstly, the alloy particle size can be controlled by that of the controlled by two main mechanisms. Firstly, the alloy particle size can be controlled by that of the Growthgraphite of powders ferrochrome used alloyfor reduction particles because during metallization the CaCl -assistedtakes place segregation on graphite reduction particles. can be graphite powders used for reduction because metallization2 takes place on graphite particles. Secondly, sintering/coalescence of nearby alloy particles contributes further to the alloy growth, controlledSecondly, by two sintering/coalescence main mechanisms. of nearby Firstly, alloy the particles alloy particle contributes size further can be to controlled the alloy growth, by that of the which can be promoted at a higher temperature. Therefore, a larger graphite particle size range was graphitewhich powders can be used promoted for reduction at a higher because temperature. metallization Therefore, takesa larger place graphite on graphiteparticle size particles. range was Secondly, used in this study for the promotion of alloy growth. used in this study for the promotion of alloy growth.

Metals 2018, 8, 69 8 of 17 sintering/coalescence of nearby alloy particles contributes further to the alloy growth, which can be promoted at a higher temperature. Therefore, a larger graphite particle size range was used in this study for the promotion of alloy growth. Electric tube furnace reduction tests were performed on the 13 mm pellets composed of 10 g Metals 2018, 8, 69 8 of 17 chromite (75–105 µm), graphite (75–105 µm, 22 wt % of ore), and anhydrous CaCl2 powder (30 wt % of ◦ ore) to evaluateElectric the tube influence furnace ofreduction reduction tests temperature were performed in the on rangethe 13 ofmm 1200–1400 pellets composedC (Figure of 107 ).g As the ◦ main gaseouschromiteMetals reduction2018 (75–105, 8, 69 µm), product graphite at (75–105 temperatures µm, 22 wt higher % of ore), than and 1000 anhydrousC, CO CaCl concentration2 powder (308 of in wt 17 the % off-gas state directlyof ore) to relates evaluate to the influence reduction of reduction rate. As temp seen,erature the overall in the range reduction of 1200–1400 was considerably °C (Figure 7). As slower at 1200 ◦C.the The main reductionElectric gaseous tube reduction rate furnace was reductionproduct comparably attests temperatures were fast pe forrformed 1300 higher andon than the 1400 131000 mm◦C, °C, pellets with CO concentrationcomposed near-complete of 10 in g the reduction chromite (75–105 µm), graphite (75–105 µm, 22 wt % of ore), and anhydrous CaCl2 powder (30 wt % that tookoff-gas place state within directly the relates first hourto the forreduction both reductionrate. As seen, temperatures. the overall reduction Figure was8 illustrates considerably the cross slowerof ore) at 1200to evaluate °C. The the reduction influence rateof reduction was comparably temperature fast in for the 1300 range and of 1200–14001400 °C, with °C (Figure near-complete 7). As sections and surface morphologies of the reduced product at 1200 ◦C and 1400 ◦C. It should be noted reductionthe main that gaseous took placereduction within product the first at temperatures hour for both higher reduction than 1000 temperatures. °C, CO concentration Figure 8 illustrates in the that thethe samples off-gascross sections state for directly surface and surface relates morphology morphologiesto the reduction observations of ratethe .reduced As seen, were product the water-leachedoverall at 1200 reduction °C and was to 1400 remove considerably °C. It should chlorides, as slower at 1200 °C. The reduction rate was comparably fast for 1300 and 1400 °C, with near-complete indicatedbe innoted the that Section the samples2.5. As for shown surface in Figuremorphology8a, each observations chromite were particle water-leached consists to of remove an unreacted reduction that took place within the first hour for both reduction temperatures. Figure 8 illustrates chromitechlorides, core (light as indicated grey) surrounded in the Section by 2.5. a partiallyAs shown reducedin Figure Cr-rich8a, each chromite particle rim (dark consists grey), of an based on the cross sections and surface morphologies of the reduced product at 1200 °C and 1400 °C. It should unreacted chromite core (light grey) surrounded by a partially reduced Cr-rich chromite rim (dark EDS microanalysis.be noted that The the presence samples for of surface siliceous morphology gangue observations minerals caused were water-leached partial sintering to remove among some grey), based on EDS microanalysis. The presence of siliceous gangue minerals caused partial chromite particleschlorides, (e.g.,as indicated the enlarged in the Section particles 2.5. As inshown Figure in Figure8a). The8a, each chloride chromite segregation particle consists effect of an resulted in sintering among some chromite particles (e.g., the enlarged particles in Figure 8a). The chloride the formationunreacted of ferrochrome chromite core metallic (light grey) phase surrounded on the by graphite a partially particles, reduced Cr-rich which chromite appear rim as (dark stripe-shaped segregationgrey), based effect on resulted EDS microanalysis. in the formation The ofpresence ferrochrome of siliceous metallic gangue phase minerals on the graphite caused particles,partial white particles with unreacted graphite (black) core (Figure8a), and as flake-shaped white particles whichsintering appear among as stripe-shaped some chromite white particles particles (e.g., with the unreacted enlarged particlesgraphite in (black) Figure core 8a). (Figure The chloride 8a), and (Figure8asb), segregationflake-shaped which resemble effect white resulted particles the in original the (Figure formation shapes 8b), of whichferrochrome of the resemble starting metallic the graphite phaseoriginal on particles.theshapes graphite of the Theparticles, starting ferrochrome alloy particlesgraphitewhich particles.haveappear an as The stripe-shaped average ferrochrome Cr:Fe white alloy weight particles particles ratiowith have unreacted of an about average graphite 1.4:1, Cr:Fe (black) which weight core is ratio lower(Figure of about 8a), than and 1.4:1, that of the startingwhich chromiteas flake-shaped is lower (i.e., than 2.0:1),white that of particles indicatingthe starting (Figure chromite incomplete 8b), which (i.e., 2.0:1), reductionresemble indicating the of original Cr.incomplete In shapes comparison, reduction of the starting of carbothermic Cr. In comparison,graphite particles. carbothermic The ferrochrome reduction alloyat 1400 particles °C wa haves complete an average and Cr:Fethe high weight temperature ratio of about resulted 1.4:1, in reduction at 1400 ◦C was complete and the high temperature resulted in substantial sintering among substantialwhich is sinteringlower than among that of the startingferrochrome chromite alloy (i.e., particles, 2.0:1), indicating contributing incomplete to the alloy reduction growth of Cr. (Figure In the ferrochrome8c,d).comparison, alloy carbothermic particles, contributingreduction at 1400 to °C the wa alloys complete growth and the (Figure high temperature8c,d). resulted in substantial sintering among the ferrochrome alloy particles, contributing to the alloy growth (Figure 8c,d).

Figure 7. Temperature profiles and off-gas CO concentrations for the tests at different temperatures. Figure 7. Temperature proﬁles and off-gas CO concentrations for the tests at different temperatures. Figure 7. Temperature profiles and off-gas CO concentrations for the tests at different temperatures.

FigureFigure 8. Backscattered8. Backscattered electron electron micrographs micrographs ofof thethe products formedformed at at 1200 1200 °C °C (a (,ba,),b ),and and 1400 1400 °C °C ◦ ◦ Figure 8.(c,dBackscattered().c, dPhotomicrographs). Photomicrographs electron ( a( micrographs,ac,)c ) representrepresent ofpolishedpolished the products section formedsurfacessurfaces at whilewhile 1200 (b(,bC(d,)d )ashow ,bshow), andsurface surface 1400 C(c,d). Photomicrographsmorphologiesmorphologies (ofa, cofthe) the represent reduced reduced products. polishedproducts. section surfaces while (b,d) show surface morphologies of the reduced products.

Metals 2018, 8, 69 9 of 17 Metals 2018, 8, 69 9 of 17

X-ray absorption spectra collected at the Fe K-edge indicate a significant significant reduction of Fe species at all the temperature ranges studied. The least-squa least-squaresres fittingfitting of the measured spectra by a series of ◦ model compounds indicates metallization degrees of about 91.6% for the product formed at 1200 °CC ◦ ◦ and 100% for the sample representing 1300 °CC (Table(Table2 2).).The The simulated simulatedspectrum spectrum ofof 1200 1200 °CC sample does not adequately capture the measured oscillationsoscillations at the extended range around 7185–7230 eV (Figure(Figure9 9a),a), suggestingsuggesting thatthat thethe chromitechromite andand ferrochromeferrochrome modelsmodels areare notnot representativerepresentative ofof thethe FeFe compounds present in the sample. It is likely that chromite is not present in the sample and that the degree of metallization is likely to be near 100% because the observed spectrum (especially the edge region) resembles both both ferrochrome ferrochrome models models shown shown (Figure (Figure 9a).9a). The The least least squares squares fitting fitting of ofthe the Cr Cr K- K-edgeedge X-ray X-ray absorption absorption near near edge edge structure structure (XANES (XANES)) indicated indicated that that the the samples samples are are binary mixtures dominated by ferrochrome and lesser amounts of residualresidual chromitechromite (Table(Table2 ).2). TheThe proportionproportion ofof ferrochrome alloy indicates the degree of Cr metallizationmetallization ranging from about 58.6% for the product ◦ ◦ formed at 1200 °CC to 100% for the 1400 °CC sample.

Table 2. Elemental distribution of Fe and Cr in the ferrochrome alloy and the residual chromite phases, Table 2. Elemental distribution of Fe and Cr in the ferrochrome alloy and the residual chromite produced from least-square ﬁtting of the X-ray absorption spectra as shown in Figure9. phases, produced from least-square fitting of the X-ray absorption spectra as shown in Figure 9.

TemperatureTemperature (◦C) (°C) Elements Elements Ferrochrome Ferrochrome (%) (%) Chromite Chromite (%) χ2 χ* 2 * FeFe 91.6 8.4 8.4 0.014 0.014 1200 1200 CrCr 58.6 41.4 41.4 0.045 0.045 FeFe 100.0 0.0 0.0 0.067 0.067 1300 1300 CrCr 94.2 5.8 5.8 0.010 0.010 14001400 CrCr 100.0 0.0 0.0 0.211 0.211 * *Chi Chi square square goodnessgoodness of of ﬁt. fit.

Figure 9. (a) Normalized Fe K-edge spectra of the products formed at 1200 and 1300 °C. Vertical Figure 9. (a) Normalized Fe K-edge spectra of the products formed at 1200 and 1300 ◦C. Vertical dotted dotted lines show the fitting range (i.e., 7092–7142 eV). Reference spectra used in least-squares fitting lines show the fitting range (i.e., 7092–7142 eV). Reference spectra used in least-squares fitting are shown are shown for chromite (ore) and two Ring of Fire ferrochrome samples (FeCr-1 and FeCr-2); (b) for chromite (ore) and two Ring of Fire ferrochrome samples (FeCr-1 and FeCr-2); (b) Normalized Cr Normalized Cr K-edge spectra of the products formed at 1200, 1300 and 1400 °C. The fitting range of K-edge spectra of the products formed at 1200, 1300 and 1400 ◦C. The fitting range of 5970–6020 eV 5970–6020 eV is marked by vertical dotted lines. Also shown are the spectra of the relevant model or is marked by vertical dotted lines. Also shown are the spectra of the relevant model or reference reference compounds (chromite represents the starting ore material; FeCr-3 represents Cr3C2). compounds (chromite represents the starting ore material; FeCr-3 represents Cr3C2). 3.4. Effect of Chromite Particle Size 3.4. Effect of Chromite Particle Size The influence of chromite particle size was evaluated with electric tube furnace reduction tests The influence of chromite particle size was evaluated with electric tube furnace reduction using 13 mm pellets prepared by mixing 10 g ore in the particle size ranges of 37–75 µm, 105–180 µm tests using 13 mm pellets prepared by mixing 10 g ore in the particle size ranges of 37–75 µm, and 180–300 µm) with 2.2 g graphite (105–150 µm), and ground 3 g anhydrous CaCl2. The mixtures were subjected to reduction at 1300 °C for 2 h (Figure 10). Strictly speaking, these reduction tests were

Metals 2018, 8, 69 10 of 17

Metals 2018, 8, 69 10 of 17 105–180 µm and 180–300 µm) with 2.2 g graphite (105–150 µm), and ground 3 g anhydrous CaCl2. ◦ Theinfluenced mixtures by werea combination subjected of to both reduction the chromite at 1300 particleC for 2size h(Figure and the 10 amount). Strictly of gangue speaking, minerals these reductionpresent in teststhe starting were influenced ore because by the a combination ore with a sm ofaller both size the fraction chromite contained particle size a higher and the proportion amount of ganguethe gangue minerals minerals present (as inseen the from starting Table ore 1 becauseand Figure the ore1). To with further a smaller assess size the fraction influence contained of the apresence higher proportionof gangue ofminerals, the gangue the reduced minerals products (as seen fromwere Tableanalyzed1 and for Figure their1 elemental). To further compositions assess the influenceand were ofalso the leached presence with of gangue water to minerals, quantify the the reduced water-soluble products species were analyzed(Figure 11). for The their percentage elemental compositionsof water-soluble and species were alsoM was leached calculated with waterbased on to quantifyEquation the(2). water-solubleAs seen from speciesFigure 10, (Figure a strong 11). The percentage of water-soluble species M was calculated based on Equation (2). As seen from CO2 peak appeared after about 20 min (500 °C) when the ore sample in the 37–75 µm size fraction ◦ Figurewas reduced. 10, a strong This was CO 2causedpeak appearedby the thermal after aboutdecomposition 20 min (500 of magnesiteC) when present the ore in sample the chromite in the 37–75 µm size fraction was reduced. This was caused by the thermal decomposition of magnesite ore, which can be represented by Equation (3). The corresponding CO2 peak was insignificant for presentboth the in other the chromitetwo tests, ore, as the which chromite can be size represented fractions of by 105–180 Equation µm (3). and The 180–300 corresponding µm contained CO2 µ peakmuch was less insignificant magnesite (Figure for both 1). the The other massive two tests, CO aspeaks the chromiteappeared size after fractions 100 min of were 105–180 due mto andthe µ 180–300carbothermicm contained reduction much of chromite. less magnesite The (FigureCO profile1). The for massive the 37–75 CO peaks µm ore appeared started after at 100a lower min µ weretemperature due to theof approximately carbothermic reduction1000 °C, compared of chromite. with The the COother profile two CO for profiles. the 37–75 Thism was ore likely started due at ◦ ato lower the presence temperature of a much of approximately higher amount 1000 of siliceC, comparedous gangue with (e.g., the clinochl other twoore) CO in profiles.the 37–75 This µm was ore likely due to the presence of a much higher amount of siliceous gangue (e.g., clinochlore) in the (Figure 1). Reaction between the molten CaCl2 and the siliceous-gangue took place in the presence of µ 37–75residualm moisture, ore (Figure leading1). Reaction to the betweenformation the of molten wadalite CaCl and2 and gaseous the siliceous-gangue HCl. With the presence took place of ina therelatively presence high of partial residual pressure moisture, of HCl, leading chloride to the segregation formation ofand wadalite its induced and carbothermic gaseous HCl. reduction With the presenceof chromite of abecame relatively more high effective partial pressureat temperatures of HCl, chlorideas low as segregation 1000 °C. Because and its of induced this, near-complete carbothermic ◦ reduction of of 37–75 chromite µm chromite became moretook place effective within at the temperatures first hour at as 1300 low °C as according 1000 C. to Because its CO profile. of this, µ ◦ near-completeAnother important reduction promoting of 37–75 effectm was chromite the fact took that place a finer within chromite the firstparticle hour size at 1300range Cwas according used in tothis its test. CO Generally profile. Another speaking, important the reduction promoting rate is effect less waslikely the limited fact that by athe finer mass chromite transfer particle within size the rangechromite was particles used in when this test. finer Generally chromite speaking, particles are the used, reduction in addition rate is lessto the likely fact limitedthat greater by the specific mass transfersurface areas within are the available chromite for particles reduction. when finer chromite particles are used, in addition to the fact that greater specific surface areas are available for reduction. Mass of M in water leachate % water-soluble of M = × 100% (2) TotalMass mass of of M M in in water reduced leachate product % water-soluble of M = × 100% (2) Total mass of M in reduced product MgCO3 = MgO + CO2 (3) MgCO3 = MgO + CO2 (3)

Figure 10. Temperature profiles, off-gas CO, and CO2 concentrations for the tests with the variation Figure 10. Temperature proﬁles, off-gas CO, and CO2 concentrations for the tests with the variation of chromiteof chromite particle particle size. size.

Metals 2018, 8, 69 11 of 17 Metals 2018, 8, 69 11 of 17

Figure 11.11. ElementalElemental composition composition of of the the pellets pellets reduced reduce atd 1300 at ◦1300C, and °C, the and percentages the percentages of each elementof each thatelement are water-solublethat are water-soluble (calculated (calculated based on based Equation on Equation (2), and plotted(2), and as plotted an inset). as an inset).

In comparison, comparison, no no appreciable appreciable reduction reduction took took place place below below 1200 1200 °C ◦whenC when using using 105–180 105–180 µm andµm and180–300 180–300 µm µsizedm sized ore oresamples, samples, because because of ofmuch much less less siliceous-gangue siliceous-gangue present present in in both chromite fractions. The reduction also took longer timetime (~2(~2 h) toto reachreach near-completenear-complete reduction.reduction. The limitedlimited effect fromfrom thethe changechange of of chromite chromite particle particle size size can can be be observed observed by by comparing comparing the the two two CO CO profiles profiles for thefor the 105–180 105–180µm µm and and 180–300 180–300µm µm size size fractions fractions because because a slightly a slightly higher higher CO CO peak peak can can be seen be seen from from the usethe use of 105–180 of 105–180µm µm size size fractions. fractions. Based Based on on the the analysis analysis of of the the reduced reduced productsproducts usingusing synchrotron X-ray spectroscopy, Cr metallizationmetallization of 100% was achieved for the test using 37–75 µµmm chromite fraction, while it was 98.9% for the 105–180 µµmm chromite fraction, and 75.8% for the 180–300 µµmm chromite fraction. The resultsresults presentedpresented in in Figure Figure 11 11 confirmed confirmed the the presence presence of of higher higher amounts amounts of gangueof gangue during during the reductionthe reduction of the of 37–75the 37–75µm sample,µm sample, as indicated as indicated by the by higher the higher concentrations concentrations of Si andof Si Mg, and and Mg, lower and concentrationslower concentrations of Cr andof Cr Fe and in the Fereduced in the reduced product. product. Approximately Approximately 86.5 wt %86.5 Ca wt in % this Ca reduced in this productreduced wasproduct water-soluble, was water-soluble, presumably presumably in the form in the of form chloride. of chloride. The remainder The remainder (13.5 (13.5 wt %) wt was %) fixatedwas fixated as water-insoluble as water-insoluble components components (primarily (primar asily wadalite) as wadalite) from from the reactions the reactions between between CaCl CaCl2 and2 theand siliceousthe siliceous gangue. gangue. The percentageThe percentage of water-soluble of water-soluble Ca became Ca became higher higher when when the ore the sample ore sample with thewith highest the highest amount amount of chromite of chromite was was used. used. For For example, example, as as much much as as ~98 ~98 wt wt % % Ca Ca inin thethe reduced product was water-soluble when the sample with th thee highest amount of chromite (i.e., 180–300 µm)µm) was reduced. It also should be noted that appreciableappreciable percentages of Si also became water-soluble, ranging fromfrom 8–28 8–28 wt wt %. %. This This is because is because the silicate the silicate gangue gangue was partially was partially chloridized chloridized during reduction during atreduction high temperatures, at high temperatures, leading to theleading formation to the offormation silicon chloride of silicon (SiCl chloride4), which (SiCl existed4), which possibly existed as bothpossibly vapor as andboth in vapor dissolved and in form dissolved in the moltenform in CaClthe molten2. CaCl2. Figure 12 presents the cross sectionssections inin thethe reducedreduced products,products, whichwhich clearlyclearly demonstratesdemonstrates the effective segregationsegregation in in all all three three tests, tests, forming forming discrete discrete ferrochrome ferrochrome alloy particles. alloy particles. Due to theDue presence to the ofpresence higher of amounts higher ofamounts gangue of in gangue the 37–75 in theµm 37–7 size5 fraction, µm size sinteringfraction, sintering among the among chromite the chromite particles tookparticles place, took evidenced place, evidenced by the irregular by the irregular grey particles grey particles present present in Figure in 12Figurea. A small12a. A fractionsmall fraction of the reducedof the reduced chromite chromite particles particles (dark grey)(dark cangrey) be can found be found to enclose to enclose a residual a residual chromite chromite core (light core (light grey) ingrey) Figure in Figure 12c. One 12c. of One the of two the such two particlessuch particles present present in Figure in Figure 12c is 12c shown is shown in Figure in Figure 13, along 13, along with thewith EDS the EDS point point microanalyses microanalyses along along the the white white arrow arrow across across the the particle. particle. A A clear clear FeFe concentrationconcentration gradient can be seen inside the chromite core, indicating the outward diffusion of Fe towards the oxide layer (the rim). Significant fluctuation of the elemental composition across the rim is due to the presence of finely-mixed multi-components (i.e., mono-oxide MgO, spinel, and chloride). It can also

Metals 2018, 8, 69 12 of 17 gradient can be seen inside the chromite core, indicating the outward diffusion of Fe towards the oxideMetals 2018 layer, 8, (the69 rim). Significant fluctuation of the elemental composition across the rim is due12 to of the 17 presenceMetals 2018, of 8, finely-mixed69 multi-components (i.e., mono-oxide MgO, spinel, and chloride). It can12 alsoof 17 be observed that tiny Fe-rich alloy beads (white) were scattered in a confinedconfined zone surrounding the residualbe observed chromitechromite that tiny core.core. Fe-rich alloy beads (white) were scattered in a confined zone surrounding the residual chromite core.

Figure 12. Backscattered electron micrographs of the reduced products resulted from the ore samples Figure 12. Backscattered electron micrographs of the reduced products resulted from the ore samples Figurewith particle 12. Backscattered size ranges electronof (a) 37–75 micrographs µm, (b) 105–180 of the redu µm, cedand products (c) 180–300 resulted µm. from the ore samples with particle size ranges of (a) 37–75 µm, (b) 105–180 µm, and (c) 180–300 µm. with particle size ranges of (a) 37–75 µm, (b) 105–180 µm, and (c) 180–300 µm.

Figure 13. Cross section of a partially reduced chromite particle (180–300 µm) showing the residual Figure 13. Cross section of a partially reduced chromite particle (180–300 µm) showing the residual Figurechromite 13. coreCross (left section); and ofthe a EDS partially elemental reduced scan chromite along the particle white (180–300arrow (rightµm)). showing the residual chromite core (left); and the EDS elemental scan along the white arrow (right). chromite core (left); and the EDS elemental scan along the white arrow (right). 3.5. Influence of Pelletization 3.5. Influence of Pelletization 3.5. InﬂuenceAgglomeration of Pelletization (e.g., pelletization, briquetting) of the powder feed is a common industrial Agglomeration (e.g., pelletization, briquetting) of the powder feed is a common industrial practiceAgglomeration in pyrometallurgical (e.g., pelletization, plants for briquetting) various reasons, of the powderand requires feed is some a common capital industrial and operational practice practice in pyrometallurgical plants for various reasons, and requires some capital and operational incosts. pyrometallurgical It was, therefore, plants reasonable for various to study reasons, the feasibility and requires of directly some capital treating and the operational powder sample costs. costs. It was, therefore, reasonable to study the feasibility of directly treating the powder sample Itwithout was, therefore, agglomeration. reasonable Both to13 studymm pellets the feasibility and powder of directlys prepared treating from mixing the powder chromite sample (37–75 without µm), without agglomeration. Both 13 mm pellets and powders prepared from mixing chromite (37–75 µm), agglomeration.graphite (105–150 Both µm, 13 22 mm wt pellets % of ore), and and powders ground prepared anhydrous from CaCl mixing2 (30 chromite wt % of (37–75ore), wereµm), subjected graphite 2 graphiteto reduction (105–150 at 1300 µm, °C 22 for wt 2 h% (Figure of ore), 14). and As ground seen, theanhydrous CO2 peaks CaCl that (30 appeared wt % of atore), about were 40 subjected min (500 (105–150 µm, 22 wt % of ore), and ground anhydrous CaCl2 (30 wt % of ore), were subjected to to reduction at 1300 °C for 2 h (Figure 14). As seen, the CO2 peaks that appeared at about 40 min (500 reduction°C) were from at 1300 the◦ Cdecomposition for 2 h (Figure of14 magnesite). As seen, theas discussed CO peaks earlier that appeared (Equation at (3)). about A 40comparatively min (500 ◦C) °C) were from the decomposition of magnesite as discussed2 earlier (Equation (3)). A comparatively werelowerfrom and wider the decomposition CO peak for ofthe magnesite reduction asof discussedthe powder earlier sample (Equation revealed (3)). a slightly A comparatively slower yet lower and wider CO peak for the reduction of the powder sample revealed a slightly slower yet lowercomparable and wider reduction CO peak rate. forThis the difference reduction was of the mainly powder due sample to the revealedloosely packed a slightly nature slower of yetthe comparable reduction rate. This difference was mainly due to the loosely packed nature of the comparablepowder sample, reduction meaning rate. comparatively This difference wider was gaps mainly existed due between to the the loosely chromite packed and nature the graphite of the powder sample, meaning comparatively wider gaps existed between the chromite and the graphite powderparticles. sample, Segregation meaning reduction comparatively involves widertransfer gaps of reducible existed between Cr and theFe as chromite ionic species and the in graphitethe form particles. Segregation reduction involves transfer of reducible Cr and Fe as ionic species in the form particles.of short polymeric Segregation species reduction (i.e., involvesmonomers, transfer dimers of reducibleor trimers Cr of and Cr and Fe as Fe ionic coordinated species in to the O formand/or of of short polymeric species (i.e., monomers, dimers or trimers of Cr and Fe coordinated to O and/or shortCl) from polymeric chromite species to the (i.e., surface monomers, of graphite dimers wher ore trimers metallization of Cr and takes Fe coordinatedplace. The slower to O and/or reduction Cl) Cl) from chromite to the surface of graphite where metallization takes place. The slower reduction fromrate when chromite heating to the the surface powder of graphitesample implies where metallizationthat the reduction takes place.rate was The determined slower reduction or at least rate rate when heating the powder sample implies that the reduction rate was determined or at least whenpartially heating limited the by powder the mass sample transfer implies between that the chromi reductionte and rate graphite was determined particles. orIn ataddition least partially to the partially limited by the mass transfer between chromite and graphite particles. In addition to the limitedreduction by theof chromite mass transfer directly between by solid chromite carbon and (Equation graphite particles.(1)), chromite In addition can also to thebe reductionreduced by of reduction of chromite directly by solid carbon (Equation (1)), chromite can also be reduced by chromitegaseous CO directly forming by solid CO carbon2 (Equation (Equation (4)), resulting (1)), chromite in the can evolution also be reduced of minor by gaseousamounts CO of forming CO2 at 2 2 gaseousapproximately CO forming 150 min CO when (Equation pellets were(4)), heatedresulting (Figure in the 14). evolution The CO 2of can minor be further amounts consumed of CO byat CO2 (Equation (4)), resulting in the evolution of minor amounts of CO2 at approximately 150 min 2 theapproximately Boudouard 150 reaction, min when i.e., Equationpellets were (5). heatedThe absence (Figure of 14). this The minor CO COcan2 bepeak further when consumed the powder by when pellets were heated (Figure 14). The CO2 can be further consumed by the Boudouard reaction, the Boudouard reaction, i.e., Equation (5). The absence of this minor CO2 peak when the powder i.e.,sample Equation was used (5). The(Figure absence 14) strongly of this minor suggests CO thpeakat the when Boudouard the powder reaction sample was was greatly used (Figurepromoted. 14) sample was used (Figure 14) strongly suggests2 that the Boudouard reaction was greatly promoted. stronglyNear-complete suggests reduction that the Boudouardwas achieved reaction for both was test greatlys, indicated promoted. by both Near-complete the EDS analysis reduction on wasthe reducedNear-complete products, reduction and the waslow off-gasachieved CO for concentratio both tests,n indicated(<0.5 vol %) by at both the end the of EDS the two-houranalysis on dwell the reducedat 1300 °C. products, Analysis and by thesynchrotron low off-gas X-ray CO spectroscopyconcentration also (<0.5 suggests vol %) at that the 100% end of Cr the metallization two-hour dwell was achievedat 1300 °C. for Analysis both tests. by synchrotron X-ray spectroscopy also suggests that 100% Cr metallization was achieved for both tests. FeCr2O4 + 4CO = Cr2Fe(alloy) + 4CO2 (4) FeCr2O4 + 4CO = Cr2Fe(alloy) + 4CO2 (4)

Metals 2018, 8, 69 13 of 17 achieved for both tests, indicated by both the EDS analysis on the reduced products, and the low off-gas CO concentration (<0.5 vol %) at the end of the two-hour dwell at 1300 ◦C. Analysis by synchrotron X-ray spectroscopy also suggests that 100% Cr metallization was achieved for both tests. Metals 2018, 8, 69 13 of 17 FeCr2O4 + 4CO = Cr2Fe(alloy) + 4CO2 (4) Metals 2018, 8, 69 13 of 17 CO2 + C = 2CO (5) CO2 + C = 2CO (5) CO2 + C = 2CO (5)

Figure 14. Temperature profiles, off-gas CO and CO2 concentrations for the tests using pellet and Figure 14. Temperature profiles, off-gas CO and CO2 concentrations for the tests using pellet and powder samples. powder samples. Figure 14. Temperature profiles, off-gas CO and CO2 concentrations for the tests using pellet and powderChemical samples. analysis of the reduced products enabled mass balance calculations (Figure 15). This Chemical analysis of the reduced products enabled mass balance calculations (Figure 15). figure illustrates the redistribution of the four elements among the three components after reduction: This(a) figure vaporizedChemical illustrates chlorides analysis the ofduring redistribution the reduced reduction; products of (b) the molten/condensed enabled four elements mass balance chlorides among calculations remaining the three (Figure in components the 15).reduced This after reduction:figureproducts, illustrates (a) which vaporized isthe water-soluble; redistribution chlorides (c) during of gangue the four reduction; component, elements (b) among which molten/condensed the is water-insoluble.three components chlorides It isafter obvious reduction: remaining from in the reduced(a)this vaporized figure products, that chlorides the reduction which during is of water-soluble; reduction;powder sample (b) molten/condensed (c) resu ganguelted in component,higher chlorides vapor losses which remaining in is general, water-insoluble. in the except reduced for It is obviousproducts,Al, which from whichremained this figureis water-soluble; completely that the in reduction(c) the gangue gangue. component, of It powderis suggested which sample that is water-insoluble. agglomeration resulted in higher (e.g., It is obviouspelletization) vapor from losses in general,thisis beneficial figure except that forin termsthe Al, reduction which of reducing remained of powder the vapor completely sample loss, resudue in ltedto theits in closely-packed gangue.higher vapor It is losses suggestednature in and general, thatlower agglomerationexcept porosity. for (e.g.,Al,As pelletization) seen,which approximately remained is beneficial completely 9 wt % in ofin terms thethe addedgangue. of reducing CaCl It is2 wassuggested the consumed vapor that loss, agglomerationby reacting due to itswith closely-packed(e.g., siliceous pelletization) gangue nature isforming beneficial water-insoluble in terms of reducing species (e.g.,the vapor wadalite), loss, dueas discussed to its closely-packed earlier. About nature 60 wt and % remainedlower porosity. in the and lower porosity. As seen, approximately 9 wt % of the added CaCl2 was consumed by reacting with Asreduced seen, productsapproximately as chloride 9 wt % (i.e., of theSoluble), added and CaCl 302 wtwas % consumed evaporated by during reacting reduction, with siliceous both of gangue which siliceous gangue forming water-insoluble species (e.g., wadalite), as discussed earlier. About 60 wt % formingcan possibly water-insoluble be recovered species as chloride (e.g., forwadalite), re-use fromas discussed a practical earlier. point About of view. 60 wt % remained in the remainedreduced in theproducts reduced as chloride products (i.e., as Soluble), chloride and (i.e., 30Soluble), wt % evaporated and 30 wtduring % evaporated reduction, both during of which reduction, bothcan of whichpossibly can be possiblyrecovered be as recoveredchloride for as re-use chloride from for a practical re-use frompoint aof practical view. point of view.

Figure 15. Mass distribution of the elements among three forms, i.e., Vapor: vaporized during reduction; Soluble: water-soluble form in the reduced product; Gangue: water-insoluble form in the Figurereduced 15. product. Mass distribution of the elements among three forms, i.e., Vapor: vaporized during Figurereduction; 15. Mass Soluble: distribution water-soluble of the form elements in the reduced among product; three Gangue: forms, i.e.,water-insoluble Vapor: vaporized form in the during 3.6.reduction; Recoveryreduced Soluble: product.of Ferrochrome water-soluble form in the reduced product; Gangue: water-insoluble form in the reduced product. Approximately 170 g spherical pellets in the size range of 6.7–12.7 mm were produced using a 3.6. Recovery of Ferrochrome laboratory disc pelletizer. These pellets were made from a mixture of chromite (75–105 µm), graphite (150–250Approximately µm, 22 wt % 170 of gore), spherical and ground pellets anhydrous in the size CaClrange2 (30of 6.7–12.7 wt % of mmore), were and wereproduced heated using in an a laboratoryelectrical tube disc furnace pelletizer. at 1300 These °C pellets for 3 h. were made from a mixture of chromite (75–105 µm), graphite (150–250 µm, 22 wt % of ore), and ground anhydrous CaCl2 (30 wt % of ore), and were heated in an electrical tube furnace at 1300 °C for 3 h.

Metals 2018, 8, 69 14 of 17

3.6. Recovery of Ferrochrome Approximately 170 g spherical pellets in the size range of 6.7–12.7 mm were produced using a laboratory disc pelletizer. These pellets were made from a mixture of chromite (75–105 µm), graphite (150–250 µm, 22 wt % of ore), and ground anhydrous CaCl2 (30 wt % of ore), and were heated in an electrical tube furnace at 1300 ◦C for 3 h. The reduced pellets were crushed gently and leached with water for the recovery of chloride. Solid CaCl2 was successfully produced by boiling off water from the leachate. After drying in an oven, 10 g of the reduced powders was taken and subjected to sieving to produce five size fractions (Table3). It was found that the size fraction of <105 µm was predominantly unwanted gangue with little-entrainment of alloy particles, and therefore can be rejected (as tailings in Table3) without much metal loss. This was because the final alloy particle size was largely controlled by the starting graphite particle size (150–250 µm) due to the nature/mechanism of segregation reduction. As presented in Table3, the other four size fractions were individually subjected to elutriating separation to produce (a) concentrate of alloy particles, (b) tailings composed largely of gangue particles, and (c) middlings which would require further processing. Figure 16 illustrates the surface morphologies of the three portions produced from elutriating separation of the 250–500 µm size fraction as an example. As seen, the concentrate fraction is composed essentially of ferrochrome alloy particles that are free of entrained gangue, whereas the tailings fraction contains little entrained alloy particles, demonstrating the effectiveness of the elutriating separation. Flake-shaped ferrochrome alloy particles (Figure 16a,b) resembling the original flake-shaped graphite powders used for the study further confirm the dominance of the segregation reduction mechanism in the presence of CaCl2. Therefore, to allow for reduction it is beneficial to use a larger particle size range of the graphite powders so that they are optimized for forming larger ferrochrome alloy particles allowing easier separation.

Table 3. Mass distribution (wt %) of the products in various size fractions following sieving-elutriating separation.

Size Fraction (µm) Concentrate (wt %) Middling (wt %) Tailings (wt %) Total (wt %) >500 8.38 0 1.19 9.57 250–500 29.75 6.75 15.46 51.95 150–250 12.48 0.49 6.95 19.91 105–150 3.36 0 2.63 5.98 <105 0 0 12.58 12.58 Total 53.96 7.24 38.80 100

The elemental distribution of Cr, Fe, Mg, Al, and Si among the concentrate, middling, and tailings fractions is plotted in Figure 17, based on the chemical analysis. It should be noted that this concentrate fraction represents the combined concentrate produced from all size fractions. The same is true for the middling and tailings fractions. The concentrate fraction, representing 53.96 wt % of the total starting mass, contains impurities of 0.25 wt % Mg, 0.14 wt % Al, and 0.96 wt % Si in their oxide forms. This means the concentrate fraction has only 2.85 wt % oxides/silicates as the entrained gangue. As seen from Figure 17, metal recoveries of 83.5 wt % Cr and 90.6 wt % Fe were achieved in the concentrate fraction, while most of Mg, Al and Si reported to the Tailings fraction. The Middling fraction can be further processed to increase the metal recovery. Metals 2018, 8, 69 14 of 17

The reduced pellets were crushed gently and leached with water for the recovery of chloride. Solid CaCl2 was successfully produced by boiling off water from the leachate. After drying in an oven, 10 g of the reduced powders was taken and subjected to sieving to produce five size fractions (Table 3). It was found that the size fraction of <105 µm was predominantly unwanted gangue with little-entrainment of alloy particles, and therefore can be rejected (as tailings in Table 3) without much metal loss. This was because the final alloy particle size was largely controlled by the starting graphite particle size (150–250 µm) due to the nature/mechanism of segregation reduction. As presented in Table 3, the other four size fractions were individually subjected to elutriating separation to produce (a) concentrate of alloy particles, (b) tailings composed largely of gangue particles, and (c) middlings which would require further processing. Figure 16 illustrates the surface morphologies of the three portions produced from elutriating separation of the 250–500 µm size fraction as an example. As seen, the concentrate fraction is composed essentially of ferrochrome alloy particles that are free of entrained gangue, whereas the tailings fraction contains little entrained alloy particles, demonstrating the effectiveness of the elutriating separation. Flake-shaped ferrochrome alloy particles (Figure 16a,b) resembling the original flake-shaped graphite powders used for the study further confirm the dominance of the segregation reduction mechanism in the presence of CaCl2. Therefore, to allow for reduction it is beneficial to use a larger particle size range of the graphite powders so that they are optimized for forming larger ferrochrome alloy particles allowing easier separation.

Table 3. Mass distribution (wt %) of the products in various size fractions following sieving- elutriating separation.

The elemental distribution of Cr, Fe, Mg, Al, and Si among the concentrate, middling, and tailings fractions is plotted in Figure 17, based on the chemical analysis. It should be noted that this concentrate fraction represents the combined concentrate produced from all size fractions. The same is true for the middling and tailings fractions. The concentrate fraction, representing 53.96 wt % of the total starting mass, contains impurities of 0.25 wt % Mg, 0.14 wt % Al, and 0.96 wt % Si in their oxide forms. This means the concentrate fraction has only 2.85 wt % oxides/silicates as the entrained gangue. As seen from Figure 17, metal recoveries of 83.5 wt % Cr and 90.6 wt % Fe were achieved in theMetals concentrate2018, 8, 69 fraction, while most of Mg, Al and Si reported to the Tailings fraction. The Middling15 of 17 fraction can be further processed to increase the metal recovery.

Figure 16. Backscattered electron micrographs of (a) concentrate, (b) middlings, and (c) tailings Figure 16. Backscattered electron micrographs of (a) concentrate, (b) middlings, and (c) tailings fractions produced from elutriating separation of the 250–500 µm size fraction of the reduced product. fractions produced from elutriating separation of the 250–500 µm size fraction of the reduced product. White or light grey particles are alloy whereas dark particles represent gangue. White or light grey particles are alloy whereas dark particles represent gangue. Metals 2018, 8, 69 15 of 17

FigureFigure 17. 17. ElementalElemental distribution distribution (normalized (normalized to to 100 100 wt wt % % for for each each element) element) among among the the concentrate, concentrate, middling,middling, and and tailings. tailings.

4.4. Conclusions Conclusions

Addition of CaCl2 was effective both in accelerating the carbothermic reduction of chromite and Addition of CaCl2 was effective both in accelerating the carbothermic reduction of chromite and in inpromoting promoting the the formation formation of individual of individual ferrochrome ferrochrome alloy particlesalloy particles from outside from outside of the chromite of the chromite particles particles via the chloride segregation mechanism. With the CaCl2 additive, formation of finely via the chloride segregation mechanism. With the CaCl2 additive, formation of finely divided alloy dividedparticles alloy within particles chromite within was chromite minimized, was which minimized, enabled which the subsequentenabled the separation subsequent of separation ferrochrome of ferrochromefrom the unwanted from the gangue unwanted without gangue the need without of physical the need liberation of physical (e.g., liberation milling). (e.g., The particle milling). size The of particlethe alloys, size which of the was alloys, largely which determined was largely by the particledetermined size ofby the the starting particle carbonaceous size of the reductant starting carbonaceous(i.e., graphite), reductant was large enough(i.e., graphite), to allow efficientwas large separation enough usingto allow the elutriatingefficient separation separation using technique. the elutriating separation technique. CaCl2-assisted carbothermic reduction of chromite by graphite was thermally activated and was relativelyCaCl2 fast-assisted at temperatures carbothermic higher reduction than 1200 of chromite◦C. Near-complete by graphite metallizationwas thermally of activated both Cr andand Fewas at relatively1300 ◦C generally fast at temperatures took place between higher than 1 to 1200 2 h. Compared°C. Near-complete to the powder metallization sample, of pelletization both Cr and ofFe the at 1300 °C generally took place between 1 to 2 h. Compared to the powder sample, pelletization of the mixture of chromite, graphite and CaCl2 resulted in a slightly faster reduction. Pelletization minimized mixturethe evaporation of chromite, of chloride graphite during and reductionCaCl2 resulted because in of a its slightly lower porosity.faster reduction. Only a stoichiometric Pelletization minimizedamount of graphitethe evaporation was required of chloride for complete during reduction. reduction Excessive because carbonof its lower addition porosity. resulted Only in the a stoichiometricpresence of residual amount carbon of graphite cores was in the required ferrochrome for complete alloy particles. reduction. After Excessive segregation carbon reduction, addition resultedthe residual in the chromite presence particles of residual were carbon highly cores porous in andthe ferrochrome mainly composed alloy particles. of finely-divided After segregation monoxide reduction, the residual chromite particles were highly porous and mainly composed of finely-divided (MgO) and spinel (MgAl2O4) phases. The presence of siliceous gangue (e.g., clinochlore) caused monoxide (MgO) and spinel (MgAl2O4) phases. The presence of siliceous gangue (e.g., clinochlore) partial sintering among chromite particles during reduction, which also consumed CaCl2 forming caused partial sintering among chromite particles during reduction, which also consumed CaCl2 wadalite (Ca6Al5Si2O16Cl3). The majority (~60%) of the CaCl2 remained in the reduced product in forminga water-soluble wadalite chloride (Ca6Al form,5Si2O16 andCl3). about The 30%majority evaporated (~60%) during of the reduction. CaCl2 remained Both forms in the representing reduced product in a water-soluble chloride form, and about 30% evaporated during reduction. Both forms representing >90% of the original CaCl2 can be recovered for re-use to minimize the material cost from a practical point of view. By taking advantage of the density difference between the ferrochrome and the unwanted gangue, pure ferrochrome alloy (with 2.85 wt % gangue) in its M7C3 powder form was successfully produced using elutriating separation, with total metal recoveries of 83.5% Cr and 90.6% Fe.

Acknowledgments: The following contributions are acknowledged: Derek Smith for XRD analyses, Judith Price for the preparation of polished sections, and KWG Resources Inc. for providing the chromite ore samples. The study was funded by NRCan under the Rare Earth Elements and Chromite R&D Program. X-ray absorption experiments at the Advanced Photon Source were carried out under a General User Proposal to the senior author and a Partner User Proposal supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through a major facilities access grant. Research at the PNC-CAT beamline of APS, Argonne National Laboratory is supported by the U.S. Department of Energy under Contracts W-31-109-Eng-38 (APS) and DE- FG03-97ER45628 (PNC-CAT). We acknowledge the help provided by the beamline scientists Tianpin Wu and George Sterbinsky (9 BM) and Zou Finfrock (20 BM).

Metals 2018, 8, 69 16 of 17

>90% of the original CaCl2 can be recovered for re-use to minimize the material cost from a practical point of view. By taking advantage of the density difference between the ferrochrome and the unwanted gangue, pure ferrochrome alloy (with 2.85 wt % gangue) in its M7C3 powder form was successfully produced using elutriating separation, with total metal recoveries of 83.5% Cr and 90.6% Fe.

Acknowledgments: The following contributions are acknowledged: Derek Smith for XRD analyses, Judith Price for the preparation of polished sections, and KWG Resources Inc. for providing the chromite ore samples. The study was funded by NRCan under the Rare Earth Elements and Chromite R&D Program. X-ray absorption experiments at the Advanced Photon Source were carried out under a General User Proposal to the senior author and a Partner User Proposal supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through a major facilities access grant. Research at the PNC-CAT beamline of APS, Argonne National Laboratory is supported by the U.S. Department of Energy under Contracts W-31-109-Eng-38 (APS) and DE-FG03-97ER45628 (PNC-CAT). We acknowledge the help provided by the beamline scientists Tianpin Wu and George Sterbinsky (9 BM) and Zou Finfrock (20 BM). Author Contributions: Dawei Yu conceived and designed the experiments; Dawei Yu performed the experiments; Dawei Yu and Dogan Paktunc analyzed the data; Dawei Yu and Dogan Paktunc contributed reagents/materials/analysis tools; Dawei Yu and Dogan Paktunc wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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