Cleaner Production of Chromium Oxide from Low Fe(II)-Chromite

minerals

Article Cleaner Production of Chromium Oxide from Low Fe(II)-Chromite

Qing Zhao 1,2,*, Chengjun Liu 1,2, Peiyang Shi 1,2 , Lifeng Sun 1,2, Maofa Jiang 1,2, Henrik Saxen 3 and Ron Zevenhoven 3 1 Key Laboratory for Ecological Metallurgy of Multimetallic Mineral (Ministry of Education), Northeastern University, Shenyang 110819, China; [email protected] (C.L.); [email protected] (P.S.); [email protected] (L.S.); [email protected] (M.J.) 2 School of Metallurgy, Northeastern University, Shenyang 110819, China 3 Process and Systems Engineering Laboratory, Åbo Akademi University, 20500 Åbo/Turku, Finland; hsaxen@abo.ﬁ (H.S.); rzevenho@abo.ﬁ (R.Z.) * Correspondence: [email protected]; Tel.: +86-024-8368-1478

Received: 14 April 2020; Accepted: 15 May 2020; Published: 19 May 2020

Abstract: Sulfuric acid-based leaching is a promising cleaner method to produce chromium salts, but its feasibility for treating low Fe(II)-chromite still remains to be proven. A Box–Behnken design (BBD)-based set of experiments for sulfuric acid leaching of low Fe(II)-chromite was utilized in this work for generating an experimental dataset for revealing the functional relationships between the processing parameters and the extraction yields of Cr and Fe. The dependent variables were found to exhibit strong intercorrelations and the models developed on the basis of statistical criteria showed excellent prediction accuracy. The optimum process conditions of leaching treatment were found to be a temperature of 176 ◦C, a dichromic acid/chromite mass ratio of 0.12, and a sulfuric acid concentration of 81%. Furthermore, the dissolution behavior of chromite in the leaching process and the eﬀect of dichromic acid were experimentally investigated. It was found that the decomposition eﬃciency was highly dependent on the Fe(II) content of chromite, and that the dichromic acid acted both as an oxidant and a catalyst in the leaching process. On the basis of the results of this study, a novel process for treating low-Fe(II) chromite was proposed.

Keywords: mineral process; cleaner production; chromite; chromium oxide; hydrometallurgy

1. Introduction Chromium (Cr) and its oxides have been widely applied in manufacturing tanning agents, pigments, stainless steel, alloys, and refractory materials [1–3]. As the primary natural source of Cr in nature, chromite consists of a series of spinel of Cr(III) oxide with magnesium oxide (MgO), aluminum oxide (Al2O3), and iron oxides (FeO and/or Fe2O3)[4]. More than 90% of the world’s viable chromite reserves are found in South Africa, Kazakhstan, and Zimbabwe [5]. To extract the Cr for chromium salt preparation, researchers have proposed a leaching of the chromite using calcium, sodium, or potassium oxides/hydroxides in an oxidizing solvent solution [6–8]. In these processes, chromite decomposes caused by an oxidation of chromium from Cr(III) into Cr(VI) [9]. Bolaños-Benítez et al. [10] carried out a bio-leaching treatment of chromite tailings using acidithiobacillus thiooxidans (A. thiooxidans) and pseudomonas putida. Similar to the transformation route of alkali roasting, Cr(III) was initially oxidized and extracted in a hexavalent state, which was then reduced to a trivalent state by Fe and A. thiooxidans. A. thiooxidans has also been employed to remove Cr from tannery sludge [11] and soil [12]. The Cr(VI) has been proved to have toxic effects on organisms [13–15] and affect plant growth and development [16]. Therefore, governments are updating Cr regulations, and massive efforts have been

Minerals 2020, 10, 460; doi:10.3390/min10050460 www.mdpi.com/journal/minerals Minerals 2020, 10, 460 2 of 14 made on remediation of Cr-bearing wastes [17–22]. The dichromic acid generated in the remediation treatments is a kind of strong oxidant [23–25], which can be employed in mineral processing to promote ore decomposition. To avoid the transformation from Cr(III) into Cr(VI) in the chromite processing, sulfuric acid leaching has been proposed and extensively studied [26–29]. In this process, Cr(III) is extracted from the chromite in a heated sulfuric acid solution with the help of recovered dichromic acid, with Cr(III) salts as products [30,31]. A lot of effort is presently being made to understand the dissolution behavior of chromite and improve the extraction yield of Cr. Researchers found that the extraction yield of high Fe(II)-chromite was always higher than that of low Fe(II)-chromite, and the reason was ascribed to the oxidation of ferrous into ferric iron in chromite [32–34]. The feasibility of the sulfuric acid leaching process on treating low Fe(II)-chromite has yet to be further evaluated, and the effect of the dichromic acid is still unclear. With the aim of extracting Cr from low Fe(II)-chromite, sulfuric acid leaching treatment with the help of dichromic acid was investigated in the current work following an experimental plan, i.e., the Box–Behnken design (BBD), which is widely employed in different research fields to produce, in an optimal way, experimental data for response surface modeling [35–38]. By these methods, the combined effects of sulfuric acid concentration (C), dichromic acid/chromite ratio (r), and processing temperature (T) on the Cr and Fe extracting behavior were studied. Furthermore, three chromites with various Fe(II) contents were employed as comparison samples to investigate the dissolution behavior of chromite in the sulfuric acid-based leaching process.

2. Materials and Methods

2.1. Materials Three chromites with various Fe(II) contents were studied in this work. Inductively coupled plasma-optical emission spectrometry (ICP-OES) and chemical analysis was performed to determine the chemical composition of the samples. Results are shown in Table1, indicating the three chromites contained similar Cr2O3 content, and that the FeO contents in Zimbabwean chromite, Pakistani chromite, and South African chromite were about 1.5%, 6.7%, and 18.7%, respectively. On the basis of the FeO content, the three ores were referred to as low Fe(II)-chromite (LFC), medium Fe(II)-chromite (MFC), and high Fe(II)-chromite (HFC), respectively, in the current study. Furthermore, it was noted that the molar ratio between total bivalent metallic cations (Mg2+ + Fe2+) and total trivalent metallic cations (Cr3+ + Al3+ + Fe3+) in LFC was much lower than the theoretical ratio of a corresponding spinel (1:2). Molecular dynamics simulation of vacancy diffusion of chromite undertaken by Vaari [39] indicated that a bivalent metallic cation has a larger diffusion coefficient and a smaller activation energy for migration than other ions. Therefore, the low mole ratio between bivalent and trivalent metallics may be a sign of a crystal lattice defect of LFC. The phase composition of samples was determined by X-ray diffraction (XRD), indicating the principal phases of the chromite were spinel and gangue (cf. Figure1). Scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS) analysis using a Shimadzu SSX-550TM (Shimadzu Corp., Kyoto, Japan) showed that the gangue in chromite consists of a series of silicate species. Dichromic acid prepared from the leachate of chromite ore processing residue was employed as an oxidant [31], while other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

Table 1. Chemical composition (dry wt.%) of chromites.

Chromite FeO Fe2O3 Cr2O3 Al2O3 MgO SiO2 Other Zimbabwean chromite (LFC) 1.52 21.56 45.95 12.05 6.40 2.80 9.72 Pakistani chromite (MFC) 6.67 5.34 42.29 13.01 19.75 6.67 6.27 South African chromite (HFC) 18.69 5.22 45.18 13.25 8.87 6.79 2.00 Minerals 2020, 10, x FOR PEER REVIEW 3 of 14 Minerals 2020, 10, 460 3 of 14

Zimbabwean chromite ☆-Spinel △-Silicate

Pakistani chromite

Intensity

South African chromite ☆ ☆

☆ △ △ ☆ ☆ ☆ △

10 20 30 40 50 60 70 80 2-Theta/°

Figure 1. X-ray diffraction (XRD) patterns of chromites used in this study. Figure 1. X-ray diffraction (XRD) patterns of chromites used in this study. 2.2. Methods 2.2. MethodsA certain amount of dichromic acid (dichromic acid/chromite ratio = 0.08, 0.10, and 0.12, respectively)A certain and amount 150 mL of sulfuric dichromic acid (sulfuricacid (dichromic acid concentration acid/chromite= 70%, ratio 80%, = and0.08, 90%, 0.10, respectively) and 0.12, respectively)was poured into and an 150 Erlenmeyer mL sulfuric flask acid on (sulfuric an automatic acid concentration temperature-controller = 70%, 80%, electric and 90%, heater. respectively) Ten grams wasof chromite poured powderinto an Erlenmeyer (<74 µm) was flask added on an to auto thematic vessel temperature-controller when the solution heated electric to theheater. set point Ten grams(160, 180, of chromite and 200 ◦powderC, respectively). (<74 μm) Afterwas added 60 min to mechanical the vessel when agitation, the solution the leachate heated was to removed the set point from (160,the heater 180, and and 200 a filtration °C, respectively). was carried After out. 60 The min concentrations mechanical agitation, of metallic the ions leachate were determinedwas removed by fromchemical the heater analysis and (for a filtration macro-elements) was carried and out. ICP-OES The concentrations analysis (for micro-elements), of metallic ions andwere the determined extraction byyields chemical of Cr and analysis Fe (mass (for ratio macro-elements) between the filtrate and and ICP-OES the chromite) analysis were (for calculated. micro-elements), The Cr introduced and the extractionfrom dichromic yields acid of Cr into and the Fe solution (mass ratio was between deducted the in thefiltrate calculation and the of chromite) the extraction were yield.calculated. After The this, Crthe introduced effects of the from processing dichromic temperature, acid into the dichromic solution acid was/chromite deducted ratio, in the and calculation sulfuric acid of the concentration extraction yield.on the After recovery this, behaviorsthe effects of Crthe andprocessing Fe were temp investigated.erature, dichromic acid/chromite ratio, and sulfuric acid concentration on the recovery behaviors of Cr and Fe were investigated. 2.3. Data Correlation Analysis Methods 2.3. DataThe Correlation BBD was applied Analysis and Methods quadratic response surface models were developed in the form

The BBD was applied and quadratic response surface models were developed2 in2 the form2 RCr = a0 + a1X1 + a2X2 + a3X3 + a12X1X2 + a13X1X3 + a23X2X3 + a11X1 + a22X2 + a33X3 (1) 222 RaaXaXaXaXXaXXaXXaXaXaX=+ + + + + + + + + (1) Cr01122331212131323231112223332 2 2 RFe = b0 + b1X1 + b2X2 + b3X3 + b12X1X2 + b13X1X3 + b23X2X3 + b11X1 + b22X2 + b33X3 (2) =+ + + + + + +222 + + RFe0112233121213132323111222333 bbXbXbXbXXbXXbXXbXbXbX (2) where RCr and RFe are the extraction yields (%) of Cr and Fe, respectively, a0 and b0 are constant a b i i a b whereterms inRCr the and models, RFe are thei and extractioni ( = 1, yields 2, 3) are (%) the of parametersCr and Fe, respectively, of linear effect a0 ofand the b0 areth factor, constantij andtermsij i j i j i j th a b in( , the= models,1, 2, 3; a,i and) are bi ( thei = 1, interaction 2, 3) are the eff ectparameters parameters of linear of the effectth and of theth i factors, factor, while aij andii bijand (i, j =ii 1,is i th th X X X 2,the 3; quadratici ≠ j) are the eff ectinteraction parameter effect of theparametersth factor. of In the Equations i and j (1) factors, and (2), while1, aii 2and, and bii is3 theare quadratic the coded th C effect(dimensionless) parameter factorsof the i of factor. the original In Equations dependent (1) and variables (2), X1, (ofX2, sulfuricand X3 are acid the concentration coded (dimensionless) in water , factorsdichromic of the acid original/chromite dependent ratio r, and variables temperature (of sulfuricT) of the acid model, concentration given by in water C, dichromic acid/chromite ratio r, and temperature T) of the model, given by C C X = − 0 (3) 1 CC∆- C X = 0 (3) 1 ΔC r r0 X2 = − (4) rr∆- r X = 0 (4) 2 TΔrT X = − 0 (5) 3 ∆T

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where C0, r0, and T0 are the mid-values of the region, and ∆C, ∆r, and ∆T are the intervals between the levels of the variables. This normalization yields variables that take on values 1, 0, or +1. On the − basis of the thermodynamic analysis, the mid-values of C (80%, mass fraction), r (0.10), and T (180 ◦C) and the intervals (10%, 0.02, and 20 ◦C, respectively) were determined. In order to test the statistical significance of parameters and evaluate the predictive ability of the models, analysis of variance (ANOVA) and multiple regression analysis were conducted, and the coefficient of determination (R2), adjusted coefficient of determination (Adj. R2) and predicted coefficient of determination (Pred. R2) were calculated in this study. The 3D response surface plots were used to investigate the interactive effect of parameters on the extraction yields of Cr and Fe.

3. Results

3.1. Modeling

Table2 shows the results of the BBD and the experimental extraction yields of Cr and Fe, RCr(exp) and RFe(exp), respectively.

Table 2. Box–Behnken design (BBD) with actual and coded values (in parentheses) for variables and experimental results.

Actual and Coded Level of Variables Standard Order RCr(exp)/% RFe(exp)/% C/% (X1) r/- (X2)T/◦C (X3) 1 90 (+1) 0.10 (0) 160 ( 1) 56.25 58.26 − 2 70 ( 1) 0.08 ( 1) 180 (0) 51.84 67.97 − − 3 90 (+1) 0.10 (0) 200 (+1) 56.63 62.77 4 80 (0) 0.08 ( 1) 200 (+1) 62.37 68.56 − 5 90 (+1) 0.12 (+1) 180 (0) 70.08 82.92 6 80 (0) 0.08 ( 1) 160 ( 1) 61.22 77.69 − − 7 80 (0) 0.10 (0) 180 (0) 79.97 87.40 8 70 ( 1) 0.10 (0) 200 (+1) 47.21 54.70 − 9 80 (0) 0.10 (0) 180 (0) 77.11 87.24 10 90 (+1) 0.08 ( 1) 180 (0) 49.08 58.26 − 11 80 (0) 0.10 (0) 180 (0) 78.91 85.74 12 80 (0) 0.10 (0) 180 (0) 80.77 88.31 13 70 ( 1) 0.10 (0) 160 ( 1) 49.63 78.9 − − 14 70 ( 1) 0.12 (+1) 180 (0) 60.89 86.44 − 15 80 (0) 0.12 (+1) 160 ( 1) 81.91 93.87 − 16 80 (0) 0.10 (0) 180 (0) 80.82 89.66 17 80 (0) 0.12 (+1) 200 (+1) 75.33 86.21 “-” means dimensionless.

Experimental results indicate that the sulfuric acid concentration (70–90%), dichromic acid/chromite ratio (0.08–0.12), and processing temperature (160–200 ◦C) have influence on the decomposition of chromite. It can be seen in Table2 that the extraction yield of Cr falls in the range 50–80%, and it is always lower than that of Fe. Since the separation of Cr and Fe ions in solution is a challenge, the higher content of Fe in the leachate may cause problems in subsequent purification treatments. On the basis of the experimental results, the coefficients of the two polynomials can be defined by multiple regression analysis, and the Equations (1) and (2) can be expressed as

R = 79.52 + 2.81X + 7.96X 0.93X + 2.99X X + 0.70X X 1.93X X 19.66X2 1.88X2 7.43X2 (6) Cr 1 2 − 3 1 2 1 3 − 2 3 − 1 − 2 − 3

R = 87.67 3.22X + 9.62X 4.56X + 1.55X X + 7.18X X + 0.37X X 15.85X2 + 2.08X2 8.16X2 (7) Fe − 1 2 − 3 1 2 1 3 2 3 − 1 2 − 3 Minerals 2020, 10, 460 5 of 14

3.2. Model Validation ANOVA was used to test the statistical significance of the parameters of the Response surface methodology (RSM) quadratic model, with results listed in Table3. For the model of Cr, the resulting F = 83.70 suggests that the model is clearly significant, with only a 0.01% chance that such a value could occur by chance. Values of probability > F lower than 0.05 means that the terms of model are significant, while values exceeding 0.10 indicate the terms are not significant for the model. According to this, the terms C, r, Cr, C2 and T2 are significant, and terms T, CT, rT, and R2 are not significant. For the model of Fe, the resulting F = 100.07 indicates significance, just as for the Cr model. Significant terms in this model are C, r, T, CT, C2, r2, and T2, and the other terms are not.

Table 3. ANOVA for response surface methodology (RSM) quadratic models developed.

Cr Fe Source Sum of Mean F p-Value Sum of Mean F p-Value df df Square Square Value Prob. > F Square Square Value Prob. > F Model 2608.23 9 289.80 83.70 <0.0001 2607.88 9 289.76 100.07 <0.0001 C 63.11 1 63.11 18.23 0.0037 83.21 1 83.21 28.73 0.0011 r 507.21 1 507.21 146.50 <0.0001 740.36 1 740.36 255.68 <0.0001 T 6.98 1 6.98 2.01 0.1988 166.35 1 166.35 57.45 0.0001 Cr 35.70 1 35.70 10.31 0.0148 9.58 1 9.58 3.31 0.1118 CT 1.96 1 1.96 0.57 0.4763 206.07 1 206.07 71.16 <0.0001 rT 14.94 1 14.94 4.31 0.0764 0.54 1 0.54 0.19 0.6788 C2 1627.52 1 1627.52 470.08 <0.0001 1057.61 1 1057.61 365.25 <0.0001 r2 14.93 1 14.93 4.31 0.0765 18.15 1 18.15 6.27 0.0408 T2 232.16 1 232.16 67.06 <0.0001 280.62 1 280.62 96.91 <0.0001

Detailed descriptive statistical performance indices for the proposed models are shown in Table4. R2, Adj. R2, and Pred. R2 were used to evaluate the predictive ability and the goodness of fit of the model, and the results are listed in Table4 as well. The high values of the coe fficient of determination for Cr and Fe imply that only 1% of the total variations cannot be explained by Equations (6) and (7), meaning the dependent variable shows a strong correlation with the independent variables. The Adj. R2 is usually employed to compare the explanatory power of regression models, which shows high values for the Cr and Fe models in this work. Pred. R2 was calculated to test how well the model predicts responses for new observations. For both models, the differences between Pred. R2 and Adj. R2 < 0.1 implying that Pred. R2 coincides well with the Adj. R2 [40].

Table 4. Detailed descriptive statistical regression analysis for the models.

Element Standard Deviation Mean Coeﬃcient of Variance (C.V.%) R2 Adj. R2 Pred. R2 Cr 1.86 65.88 2.82 0.9908 0.9790 0.9055 Fe 1.70 77.35 2.20 0.9923 0.9824 0.9225

The predicted extraction yields calculated by Equations (6) and (7) and the experimental values are shown in Figure2. The data points fall close to the 45 ◦ line, meaning that the proposed models can accurately predict the leaching yields of Cr and Fe in sulfuric acid-based leaching operated under the conditions studied. MineralsMinerals 20202020,, 10, 460x FOR PEER REVIEW 6 of 146 of 14

Figure 2. Relationships between experimental and predicted extraction yields of (a) Cr and (b) Fe. Figure 2. Relationships between experimental and predicted extraction yields of (a) Cr and (b) Fe. 3.3. Optimization 3.3. Optimization Three-dimensional response surface plots are helpful tools to investigate the overall and coupled effectsThree-dimensional of the inputs on theresponse outputs surface for the plots LFC are sample.helpful tools Figure to investigate3 shows the the plots overall of twoand parameterscoupled effects of the inputs on the outputs for the LFC sample. Figure 3 shows the plots of two parameters determining the extraction yields by keeping the other variable at a certain value, and the optimum determining the extraction yields by keeping the other variable at a certain value, and the optimum conditions for the sulfuric acid leaching of LFC can be easily identified. It can be seen in Figure3 conditions for the sulfuric acid leaching of LFC can be easily identified. It can be seen in Figure 3 that that the extraction yields of Cr and Fe exhibit similar trends with respect to the inputs. The leaching the extraction yields of Cr and Fe exhibit similar trends with respect to the inputs. The leaching efficiency significantly increased with the dichromic acid dosage in the current work. The plots of efficiency significantly increased with the dichromic acid dosage in the current work. The plots of extractionextraction yieldyield versusversus solutionsolution acidityacidity andand temperaturetemperature showshow aa parabolicparabolic dependence,dependence, indicatingindicating that reasonablethat reasonable sulfuric sulfuric acidconcentration acid concentration and temperature and temperature would would be around be around 80% and 80% 180 and◦C. Elevation180 °C. of acidityElevation and of temperature acidity and temperature to a higher-level to a higher-level lead to a decrease lead to a indecrease the recovery in the recovery of Cr and of FeCr whileand Fe more solidwhile residues more solid were residues obtained. were obtained. ItIt was noted noted that that the the color color of ofthe the leaching leaching residues residues changed changed from fromlight gray light to gray green to when green the when thetemperature temperature elevated elevated from from160 to 160 200 to °C. 200 Other◦C. Otherresearchers researchers have observed have observed the same the in sulfuric same in acid- sulfuric acid-basedbased processing processing of chromite, of chromite, and the and results the results indicate indicate that thatthe green the green residues residues were were mixed mixed Cr-bearing Cr-bearing sulfatessulfates [[27,33,34].27,33,34]. The The precipitation precipitation of of these these sulfates sulfates is attributed is attributed to the to thelocal local supersaturation supersaturation induced induced byby aa rapidrapid dissolutiondissolution andand highhigh viscosityviscosity ofof thethe solutionsolution ininthe theconcentrated concentrated sulfuricsulfuric acidacid atat suchsuch aahigh temperature.high temperature. The mixedThe mixed sulfates sulfat precipitatedes precipitated and coveredand covered the chromitethe chromite particles, particles, inhibiting inhibiting further decompositionfurther decomposition of the ore.of the Furthermore, ore. Furthermore, the trivalent the trivalent state state of chromium of chromium in sulfatein sulfate could could be be easily oxidizedeasily oxidized to the toxicto the hexavalent toxic hexavalent state, makingstate, making the residue the residue become become a potential a potential source source of chromium of contamination.chromium contamination. Therefore, Therefore, these sulfate these phases sulfate should phases be should prevented be prevented from precipitating from precipitating by controlling by processingcontrolling parametersprocessing parameters or recovered or viarecovered somespecific via some methods. specific methods. To optimize the leaching treatment of chromite with respect to the Cr recovery, Derringer’s To optimize the leaching treatment of chromite with respect to the Cr recovery, Derringer’s desirability function was employed in the studied range. Results exhibited a maximum value of RCr desirability function was employed in the studied range. Results exhibited a maximum value of = 86.3% when the sulfuric acid leaching process is carried out at 176 °C with a sulfuric acid R = 86.3% when the sulfuric acid leaching process is carried out at 176 C with a sulfuric acid concentrationCr of 81% and dichromic acid/chromite ratio of 0.12. In order to confirm◦ this finding, the concentration of 81% and dichromic acid/chromite ratio of 0.12. In order to confirm this finding, the duplicate confirmatory tests were conducted under these conditions and the extraction yield of Cr duplicate confirmatory tests were conducted under these conditions and the extraction yield of Cr was was found to be 84.2%, demonstrating the proposed model has a good predictive ability for treating foundthis chromite. to be 84.2%, Some demonstrating SEM images of the LFC proposed before model and after has leaching a good unde predictiver the optimum ability for conditions treating this chromite.are shown Some in Figure SEM 4. images As seen of in the this LFC figure, before the andparticle after size leaching of the ore under significantly the optimum decreased conditions after are shownthe leaching in Figure treatment.4. As seen Corrosion in this was figure, found the to particle occur on size the ofsurface the ore of the significantly spinel particles, decreased giving after the the leachingparticles treatment.a rough surface. Corrosion Unreacted was chromite found to was occur the ononly the phase surface that ofcan the be spineldetected particles, in the leaching giving the particlesresidue, which a rough can surface. be recycled Unreacted and used chromite in further was leaching the only process. phase that can be detected in the leaching residue, which can be recycled and used in further leaching process.

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FigureFigure 3.3. Three-dimensionalThree-dimensional response response surfaces surfaces of extraction of extraction yield yieldas a function as a function of (a) sulfuric of (a) sulfuricacid acidconcentration concentration and anddichromic dichromic acid/chromit acid/chromitee ratio ratioat a attemperature a temperature of 180 of 180°C; ◦(C;b) (sulfuricb) sulfuric acid acid concentrationconcentration and and temperature temperature for afor dichromic a dichromic acid/chromite acid/chromite ratio of ratio 0.10; (ofc) dichromic0.10; (c) aciddichromic/chromite ratioacid/chromite and temperature ratio and for temperature a sulfuric acid for a concentration sulfuric acid concentration of 80%. of 80%.

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Figure 4. Scanning electron microscopy (SEM) images of low Fe(II)-chromite (LFC) (a) before and (b) after leaching at 176 °C with a sulfuric acid concentration of 81% and dichromic acid/chromite ratio of 0.12. Figure 4. ScanningScanning electron electron micro microscopyscopy (SEM) (SEM) images images of oflow low Fe(II)-chromite Fe(II)-chromite (LFC) (LFC) (a) (beforea) before and and (b) after leaching at 176 °C with a sulfuric acid concentration of 81% and dichromic acid/chromite ratio 3.4. Effect(b) of after Dichromic leaching Acid at 176 ◦C with a sulfuric acid concentration of 81% and dichromic acid/chromite ratio of 0.12. On the basis of the obtained results, the decomposition efficiency of chromite in sulfuric acid- based3.4. Effect Esolutionffect of Dichromic seems closely Acid associated with the oxidation potential of the solution. Inspired by these findings, three chromites with different contents of Fe(II) (LFC, MFC, and HFC) were used to On thethe basisbasis of of the the obtained obtained results, results, the the decomposition decomposition effi ciencyefficiency of chromite of chromite in sulfuric in sulfuric acid-based acid- investigate the dissolution behavior of chromite and the effect of dichromic acid. With the goal to basedsolution solution seems closelyseems closely associated associated with the with oxidation the oxid potentialation potential of the solution. of the solution. Inspired Inspired by these findings,by these avoid the interference effect of mixed Cr-bearing sulfates, a series of exploratory experiments was findings,three chromites three chromites with different with contents different of contents Fe(II) (LFC, of MFC,Fe(II) and(LFC, HFC) MFC, were and used HFC) to investigatewere used theto first conducted for the three ores. The results indicate that little sulfate was detected when the investigatedissolution behaviorthe dissolution of chromite behavior andthe of echromiteffect of dichromic and the acid.effect Withof dichromic the goal to acid. avoid With the interferencethe goal to leaching treatment was performed at 160 °C for 60 min at a sulfuric acid concentration of 80% and avoideffect ofthe mixed interference Cr-bearing effect sulfates, of mixed a series Cr-bearing of exploratory sulfates, experiments a series of was exploratory first conducted experiments for the threewas dichromic acid/chromite ratio of 0.08. Therefore, these conditions were selected for investigating the firstores. conducted The results for indicate the three that little ores. sulfate The results was detected indicate when that the little leaching sulfate treatment was detected was performed when the at chromite decomposition mechanism. A series of dichromic acid-free leaching tests were carried out 160leaching◦C for treatment 60 min at was a sulfuric performed acid at concentration 160 °C for 60 of min 80% at and a sulfuric dichromic acid acid concentration/chromite ratioof 80% of 0.08.and as well. Figure 5 shows the extraction yields of Cr of the different chromites and final Cr(VI) Therefore,dichromic acid/chromite these conditions ratio were of 0.08. selected Therefore, for investigating these conditions the chromite were selected decomposition for investigating mechanism. the concentrations in the leachates. Achromite series of decomposition dichromic acid-free mechanism. leaching A testsseries were of dichromic carried out acid-free as well. leaching Figure5 testsshows were the carried extraction out yieldsas well. of CrFigure of the 5 dishowsfferent the chromites extraction and yields final Cr(VI) of Cr concentrationsof the different in thechromites leachates. and final Cr(VI) concentrations in the leachates. 100 3.0 Dichromic acid free Dichromic acid/chromite ratio = 0.08 Cr(VI) 2.5 concentration/g Cr(VI) 80100 3.0 Dichromic acid2.26g free⋅L-1 Dichromic acid/chromite ratio = 0.08 Cr(VI) 2.0 2.5 concentration/g Cr(VI) 6080 2.26g⋅L-1 1.5 2.0 4060 1.0 ⋅

1.5L Extraction yield of Cr/% -1 20 40 0.5 1.0 ⋅ L Extraction yield of Cr/% -1 020 0.0 LFC MFC HFC 0.5 Chromite/-

Figure 5. Extraction yields0 of Cr of different chromites and final Cr(VI) concentrations0.0 in the leachates. LFC MFC HFC Figure 5. Extraction yields of Cr of different chromitesChromite/- and final Cr(VI) concentrations in the leachates. As seen in Figure5, chromites of di fferent Fe(II) contents give rise to different results under the sameAs leachingseen in Figure condition. 5, chromites In the dichromic of different acid-free Fe(II) tests,contents more give than rise 20% to different of the Cr results was extracted under the from Figure 5. Extraction yields of Cr of different chromites and final Cr(VI) concentrations in the leachates. sameMFC leaching and HFC, condition. while LFC In hardlythe dichromic decomposed acid-free in the tests, 80% more sulfuric than acid 20% solution of the Cr at 160was◦ C,extracted demonstrating from MFC and HFC, while LFC hardly decomposed in the 80% sulfuric acid solution at 160 °C, its higherAs seen stability. in Figure When 5, chromites using an of oxidizing different solution, Fe(II) contents such as give the rise sulfuric to different acid solution results usedunder here, the same leaching condition. In the dichromic acid-free tests, more than 20% of the Cr was extracted from MFC and HFC, while LFC hardly decomposed in the 80% sulfuric acid solution at 160 °C,

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demonstrating its higher stability. When using an oxidizing solution, such as the sulfuric acid Minerals 2020, 10, 460 9 of 14 solution used here, to dissolve the spinel phase, some Fe(II) in spinel transforms into Fe(III) generating a decrease of ionic radius, so the lattice distortion of crystal structure occurs. This topromotes dissolve the the chromite spinel phase, decomposition some Fe(II) inin spinelacid solutions, transforms which into Fe(III) explains generating why it ais decrease much easier of ionic to radius,dissolve so high the latticeFe(II)-chromite distortion than of crystal that with structure low occurs.Fe(II) content. This promotes Moreover, the Vaari chromite [39] decomposition has proposed inthat acid the solutions, diffusion which coefficient explains of Fe why2+ are it is much much larger easier tocompared dissolve with high Fe(II)-chromitethat of other metallic than that ions with in lowspinel. Fe(II) In the content. oxidizing Moreover, atmosphere, Vaari [Fe39]2+ hasions proposed tend to migrate that the from diffusion the core coe offfi cientspinel of crystals Fe2+ are toward much largerthe reaction compared interface with and that are of otheroxidized metallic into Fe ions3+ producing in spinel. tetrahedral In the oxidizing vacancies atmosphere, [41,42]. Hydrogen Fe2+ ions tendions provided to migrate by from the theacid core attack of spinelthe oxygen crystals skelet towardon of the the reaction lattice, destructing interface and the are spinel oxidized structure into Feand3+ releasingproducing metallic tetrahedral ions. vacancies [41,42]. Hydrogen ions provided by the acid attack the oxygen skeletonFigure of the5 also lattice, shows destructing that adding the spineldichromic structure acid significantly and releasing improves metallic ions.the leaching yield of Cr for allFigure ores,5 which also shows is mainly that attributed adding dichromic to the st acidrong significantlyoxidization potential improves of the dichromic leaching acid. yield When of Cr forthe alldichromic ores, which acid/chromite is mainly attributedratio was to0.08, the about strong 60% oxidization of the Cr potential was extracted of dichromic from LFC, acid. and When the therecovery dichromic rate of acid Cr /inchromite MFC and ratio HFC was tests 0.08, were about high 60%er than of the 90%. Cr The was ion extracted concentrations from LFC, of leachate and the recoverywere analyzed rate of by Cr chemical in MFC andanalysis HFC and tests ICP-OES were higher analysis, than 90%.also reported The ion concentrationsin Figure 5. Results of leachate show werethat no analyzed chromium by chemicalion besides analysis the Cr3+ and and ICP-OES Cr6+ was analysis, found in also the reportedfiltrate. In in test Figure with5 .LFC, Results the Cr(VI) show thatconcentration no chromium was ion2.26 besides g·L−1, showing the Cr3+ aand little Cr reduction6+ was found in the in leaching the filtrate. process. In test As with for LFC,tests with the Cr(VI) MFC concentrationand HFC, very was little 2.26 Cr(VI) g L 1was, showing detected a little in the reduction leachates, in theand leaching almost all process. of the Asiron for ions tests occurred with MFC in · − andthe trivalent HFC, very state. little It may Cr(VI) be wasspeculated detected that in the the dichromic leachates, acid and acts almost both all as ofan the oxidant iron ionsand a occurred catalyst in the decomposition trivalent state. of It chromite. may be speculated When the thatdichromi the dichromicc acid solution acid actsis used both for as treating an oxidant low Fe(II)- and a catalystchromite, in the the catalysis decomposition of dichroma of chromite.te ions Whenplays thea critical dichromic part on acid the solution chromite is used decomposition. for treating lowThe Fe(II)-chromite,transformation of the Cr(III) catalysis into of dichromateCr(IV) may ionsfirstly plays occu a criticalr by Cr(VI) part on oxidizin the chromiteg causing decomposition. the lattice Thedistortion transformation of spinel, and of Cr(III) then the into generated Cr(IV) may Cr(IV) firstly transfers occur back by Cr(VI)to trivalent oxidizing and hexavalent causing the form lattice via distortiondisproportionation of spinel, due and thento the the instability generated of Cr(IV) Cr(IV). transfers This could back also to trivalentexplain why and hexavalentdichromic formacid has via disproportionationless effect on the dissolution due to the process instability of Fe(II)- of Cr(IV). and This Cr(III)-free could also spinel, explain such why as MgAl dichromic2O4 and acid MgFe has less2O4 e[43].ffect On on the dissolutionbasis of these process findings, of Fe(II)- the proposed and Cr(III)-free effectsspinel, of dichromic such as acid MgAl have2O4 beenand MgFe illustrated2O4 [43 in]. OnFigure the 6. basis of these findings, the proposed effects of dichromic acid have been illustrated in Figure6. The resultsresults suggest suggest that that dichromic dichromic acid acid is a suitableis a suitable additive additive for sulfuric for sulfuric acid leaching acid ofleaching chromite, of aschromite, it possesses as it apossesses strong oxidizing a strong capacityoxidizing and capaci canty act and as acan catalyst. act as However,a catalyst. sinceHowever, the dichromic since the aciddichromic is consumed acid is onlyconsumed by Fe(II), only the by dosage Fe(II), usedthe dosage for treating used lowfor treating Fe(II)-chromite low Fe(II)-chromite should be carefully should controlled.be carefully Moreover,controlled. a Moreover, reduction treatmenta reduction for treatm reducingent for residual reducing Cr(IV) residual is required Cr(IV) tois preventrequired the to leachingprevent the products leaching from products containing from Cr(IV)-richcontaining phase.Cr(IV)-rich The phase. Cr cycle The during Cr cycle the during leaching the process leaching of chromiteprocess of is chromite depicted is in depicted Figure7. in Figure 7.

Figure 6. IllustrationIllustration of of the the effects eﬀects of dichromicdichromic acid on chromite decomposition.

Minerals 2020, 10, 460 10 of 14 Minerals 2020, 10, x FOR PEER REVIEW 10 of 14

Chromite Fe(III) Cr(VI) Cr(III)

Cr cycle Cr(IV) Fe(II)

Dichromic Transient acid Cr(VI) Disproportionation Cr(IV) state

Cr(III) Cr(IV) FigureFigure 7. 7. CrCr cycle cycle during during the the leaching leaching process process of of chromite. chromite.

3.5.3.5. Product Product Preparation Preparation AA reducing treatment to to transfor transformm residual residual Cr(IV) Cr(IV) in intoto Cr(III) Cr(III) is is a nece necessaryssary process process before subsequentsubsequent impurity impurity separation. separation. Sc Scalesales stripped stripped in in the the rolling rolling process process of of steel steel sheets sheets were were collected andand used as a reductant for the reducing treatmenttreatment of thethe leachate.leachate. In In the current work, solvent extractionextraction by by 2-ethylhexyl 2-ethylhexyl dihydrogen dihydrogen phosphate (P507) was conducted conducted to to accomplish accomplish the the separation separation ofof Fe Fe from from the the multi-element multi-element leachate. leachate. Some Some H2 HO22O was2 was used used and and added added in the in leachate the leachate to oxidize to oxidize Fe2+ 2+ 3+ toFe Feto3+ Febeforebefore P507 P507 treating treating step. step. Afterwards, Afterwards, the the P507 P507 extractant extractant was was diluted diluted with with sulfonated kerosenekerosene inin aa volume volume ratio ratio of of 2:3 2:3 followed followed by aby saponifying a saponifying using using 10 wt.% 10 NaOHwt.% NaOH solution. solution. The solvent The solventextraction extraction for leachate for leachate was conducted was conducted under atmospheric under atmospheric conditions conditions five times five to achievetimes to a achieve sufficient a sufficientextraction extraction of iron. The of resultsiron. The showed results that showed almost that all almost of the ironall of could the iron be recovered, could be recovered, and the Cr and loss thewas Cr less loss than was 7%. less than 7%. 2+ 3+ MetallicMetallic ions of Mg 2+ andand Al Al3+ inin solution solution were were recovered recovered by by subsequent subsequent oxalic oxalic acid acid and and sodium sodium hydroxidehydroxide treatments. treatments. Figure Figure 8 presents a flowflow sheetsheet ofof thethe processprocess forfor treatingtreating low-Fe(II)low-Fe(II) chromitechromite proposed in in this this work. work. A Aproduct product Cr Cr2O32 Owas3 was prepared prepared from from LFCLFC by using by using the proposed the proposed process, process, with awith chemical a chemical composition composition as follows: as follows: Cr2O Cr3%2 O= 399.36%,% = 99.36%, MgO% MgO% = 0.05%,= 0.05%, and Al and2O Al3%2 O= 30.59%.% = 0.59%. The productThe product can canbe used be used as a as raw a raw material material for for produc producinging chrome chrome pigment pigment in in China. China. A A small small amount amount of of Al in the Cr 2OO33 pigmentpigment is is beneficial beneficial for for the the color color performance, performance, so so it it is is not not necessary necessary to to fully fully remove remove Al from the the product. product. All All wastes wastes discharged discharged to to th thee environment environment meet meet the the Chinese Chinese discharge discharge standard. standard.

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Dichromic acid Chromite Sulfuric acid

Leaching

Scale Leachate

H2O2 Fe recovery P507

Aqueous phase

Mg recovery H2C2O4

Filtrate

Al recovery NaOH

Chromium hydroxide

Roasting

Chromium oxide

Figure 8. FlowFlow sheet sheet of of the the proposed proposed process route for treating low-Fe(II) chromite.

4. Conclusions Conclusions To investigate the feasibility of sulfuric acid-basedacid-based leaching of low Fe(II)-chromite, the optimal experimental conditions for leaching of a Zimbabwean chromite with 1.52% of FeO were determined by aa Box–Behnken Box–Behnken design design (BBD). (BBD). Second-order Second-order polynomial polynomial regression regression models capturingmodels capturing the functional the functionalrelationships relationships between the between processing the parameters processing (sulfuricparameters acid (sulfuric concentration acid concentrationC, dichromic acidC, dichromic/chromite acid/chromiteratio r, and processing ratio r, and temperature processingT )temperature and the extraction T) and the yields extraction of Cr and yields Fe wereof Cr proposed and Fe were and 2 2 validated.proposed and The validated. significance The of significance model terms of was model statistically terms was evaluated, statistically showing evaluated, that C, showingr, Cr, C andthat TC, 2 2 2 werer, Cr, significantC2 and T2 were terms significant for the Cr terms model, for and theC Cr, r, model,T, CT, C and, r C, and, r, TT, CTfor, C the2, r2 Fe, and model. T2 for The the dependent Fe model. variablesThe dependent exhibited variables a strong exhibited inter-correlation a strong inter-correlation of variables, andof variables, the prediction and the accuracy prediction of accuracy the two ofmodels the two was models excellent. was Three-dimensional excellent. Three-dimensional response surfaces response were surfaces plotted were to depict plotted the to overall depict andthe overallcoupled and effects coupled of the effects leaching of parametersthe leaching on paramete the extractionrs on the yield. extraction Optimum yield. conditions Optimum of the conditions leaching processof the leaching were found process at awere temperature found at ofa temperatureT = 176 ◦C, a of dichromic T = 176 °C, acid a dichromic/chromite ratioacid/chromite of r = 0.12, ratio and of a rsulfuric = 0.12, acidand a concentration sulfuric acid ofconcentrationC = 81%. of C = 81%. Three chromiteschromites with with various various contents contents of Fe(II) of Fe(II) were were studied studied to investigate to investigate the dissolution the dissolution behavior behaviorof chromite of inchromite sulfuric acid-basedin sulfuric solution acid-based and thesolution effect ofand dichromic the effect acid. of Resultsdichromic indicated acid. thatResults the indicateddecomposition that the effi decompositionciency of chromite efficiency strongly ofdepended chromite strongly on the Fe(II) depended content. on The the dichromicFe(II) content. acid The acts dichromicboth as an acid oxidant acts andboth a as catalyst an oxidant in the and leaching a catalyst process, in the and leaching the catalysis process, plays and athe principal catalysis role plays on athe principal decomposition role on the of lowdecomposition Fe(II)-chromite. of low The Fe(II)-chromite. Cr cycle during The the Cr leachingcycle during process the leaching of chromite process was offurther chromite illustrated. was further illustrated. Based on the findingsfindings of the work, a novel processprocess for treating low-Fe(II) chromite by sulfuric acid-based leaching is proposed, and a chromium oxide isis obtainedobtained asas aa product.product. No Cr-bearing wastes are discharged to the environment from this process, making it an interesting alternative for scale-up tests.

Minerals 2020, 10, 460 12 of 14

Author Contributions: Conceptualization, Q.Z. and P.S.; Funding acquisition, Q.Z. and L.S.; Methodology, C.L.; Investigation, Q.Z.; Writing—Original Draft Preparation, Q.Z.; Project administration, M.J.; Writing—Review & Editing, R.Z. and H.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by The National Key Research and Development Program of China (No. 2017YFB0304603), the National Natural Science Foundation of China (No. 51704068), the Liaoning Provincial Natural Science Foundation of China (No. 2019-MS-127), and the Fundamental Research Funds for the Central Universities (No. N2025035 and N180725008). Conﬂicts of Interest: The authors declare no conﬂict of interest.

Abbreviations List of nomenclature and abbreviations. TC Turkish chromite RSM Response surface methodology BBD Box-Behnken design C Sulfuric acid concentration r Dichromic acid/chromite ratio T Processing temperature ICP-OES Inductively coupled plasma-optical emission spectrometry LFC low Fe(II)-chromite MFC Medium Fe(II)-chromite HFC High Fe(II)-chromite XRD X-ray diffraction CSM Crystallographica Search-Match PDF Powder Diffraction File ICDD International Centre for Diffraction Data SEM-EDS Scanning electron microscopy-energy-dispersive X-ray spectroscopy ANOVA Analysis of variance R2 Coefficient of determination Adj. R2 Adjusted coefficient of determination Pred. R2 Predicted coefficient of determination RCr Extraction yields of Cr RFe Extraction yields of Fe RCr(exp) Experimental extraction yields of Cr RFe(exp) Experimental extraction yields of Fe C.V.% Coefficients of variance

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