High Temp. Mater. Proc. 2015; 34(3): 237–244

S. Aktas*, C. Eyuboglu, M.H. Morcali, S. Özbey and Y. Sucuoglu Production of from Turkish Concentrate Using

Abstract: In this study, the possibility of chromium ex­ 1 Introduction traction from Turkish chromite concentrate and the pro­ duction of chromium oxide were investigated. For the Chromite ore is an abundant world resource; more than 11 ­conversion of chromium(III) into chromium(VI), NaOH billion tons of chromite ore are known and this supply will was employed, as well as air with a rate of 20 L/min. The be adequate to meet world demand for hundreds of years. effects of the base amount, fusing temperature, and fusing Since the year 2000, chromite ore production has time on the chromium conversion percentage were inves­ been steadily on the rise, increasing from 15 million tons tigated in detail. The conversion kinetics of chromium(III) to 23 million tons in 2011 [1]. This impressive increase can to chromium(VI) was also undertaken. Following the be explained by the swift rise of stainless steel demand in steps of dissolving the chromate in water and fil­ the modern world, as well as by the intensified production tering, aluminum hydroxide was precipitated by adjusting of local chromium alloys in recent years in China [1]. the pH level of the solution. The chromium(VI) solution In 2011, 2% of the world’s total chromite reserves were was subsequently converted to Cr(III) by the combination employed for the production of chromium chemicals [1]. of sulfuric acid and ethanol. Interestingly, it was observed Chromite is also used for refractory purposes and foundry that ethanol precipitated chromium as chromium(VI) at sands. Refractory chromite is used in the metallurgy, mildly acidic pH levels, although this effect is more pro­ cement, and glass industries to line kilns, furnaces, and nounced for K2Cr2O7 than Na2Cr2O7. On the other hand, in reactors that require a lining that can withstand high tem­ the strongly acidic regime, ethanol acted as a reducing perature levels [2–3]. Around 95% of the world’s chromite agent role in that chromium(VI) was converted into Cr(III) ores are employed as ferrochrome alloy, which is 50–70% whereas ethanol itself was oxidized to carbon dioxide and chromium and 30–50% iron, in the metallurgical industry water. Subsequently, chromium hydroxide was obtained [2]. by the help of and converted to chro­ Chromium has a wide range of applications such as mium oxide by heating at 800 °C, as indicated in thermo alloying, chemical, and refractory industries. Its high resis­ gravimetric analysis (TGA). tance to corrosion and extreme strength makes it an opti­ mum metal for using in plating and metal finishing [3–5]. Keywords: chromium, chromite ore, ethanol, reduction Turkey has a vast source of chromite and Eti Krom A.Ş. is a leading ferrochrome producer in Turkey and an DOI 10.1515/htmp-2014-0056 esteemed worldwide leader in the chromium industry [1]. Received March 27, 2014; accepted May 6, 2014; This type of chromite is more suitable for ferrochrome published online June 25, 2014 alloy (FeCr) production than manufacturing of chromium chemicals. Thus, some fraction of domestic chromite ore is used for ferrochrome alloys only. The rest is exported abroad (Soda Sanayii A.Ş., Turkey). Kromsan is the only chromium chemical producing company in Turkey, as well as one of the leading chro­ mium compound producing companies in the world. The company employs South African-based chromite concen­ *Corresponding author: S. Aktas: Metallurgical and Materials trates. According to the new process applied at the plant, Division, Engineering Faculty, Marmara University, Goztepe Campus, is no longer used to eliminate silicon Istanbul, Turkey. E-mail: [email protected] because calcium combines with the chromate anion to Metallurgical and Materials C. Eyuboglu, S. Özbey, Y. Sucuoglu: make CaCrO [6–8]. This compound precipitates due to its Division, Engineering Faculty, Marmara University, Goztepe Campus, 4 −4 Istanbul, Turkey low constant (Ksp = 7.1 × 10 ) [9]. However, at M.H. Morcali: Faculty of Chemical and Metallurgical Engineering, the same time, it re-dissolves to a certain extent in water, Istanbul Technical University, Maslak 34469, Istanbul, Turkey which threatens human life and ecology [8, 10]. 238 S. Aktas et al., Production of Chromium Oxide Using Ethanol

The main purpose of the present study is to outline an effective metallurgical process for the production of chromium oxide (Cr2O3) from Turkish chromite concen­ trate and describe the optimal conditions and parameters for this extraction process. Chromite ores are industrially treated with sodium carbonate (above 1100 °C) however, to ensure that the extraction process is carried out at rela­ tively low temperatures (500–700 °C), sodium hydroxide was employed [11–13]. For this purpose, the following pa­ rameters were studied to investigate their effect on chro­ mium extraction (%): base amount, fusing time, and fusing temperature. Furthermore, the activation energy of the process was calculated in a kinetic study. The products obtained in the study were characterized using X-ray dif­ fraction and TGA.

2 Experimental work

In this work, a chromite concentrate was obtained from an ore dressing plant in Adıyaman District, Turkey. The concentrate’s composition and XRD pattern are shown in Table 1 and Fig. 1, respectively.

Table 1: Composition of chromite concentrate used in the study

Compounds Mass%

Cr2O3 48.88 MgO 15.56 FeO 17.60

Al2O3 15.28

SiO2 2.63 Others 0.05 Fig. 2: The proposed process for production of Cr2O3 from chromite concentrate

The chromite concentrate was ground to −100 µm and produced various fractions; in this research work, a frac­ tion of −75+45 µm was used. The proposed process for the

production of Cr2O3 from chromite concentrate is shown in Fig. 2. It is possible to treat this chromite concentrate

using Na2CO3 above 1100 °C. But the purpose here is to de­ crease the operation temperatures below 700 °C to ensure that operational heating costs decrease. All of the chemicals used in this study were of ana­ lytical grade. The obtained powder was subjected to ­homogenization using a three-dimensional shaker fol­ lowing drying in an oven at 105 °C. For the fusing and leaching experiments, this homogeneous chromite con­ centrate was employed. We next attempted to deter­ Fig. 1: XRD pattern of the chromite concentrate used in the study mine the opti­mal conversion parameters by examining S. Aktas et al., Production of Chromium Oxide Using Ethanol 239

Fig. 3: Titration plot of potential versus titrant volume

the following factors: time, temperature, and the amount Conversion percentage (%) = [MCr6+/(W × 0.3345)] × 100 of sodium hydroxide. The conversion experiments were (1) carried out in a muffle furnace where oxidizing atmo­ sphere was provided by air blowing system. For each ex­ where MCr6+ is the quantity of dis­ periment, 1 g of concentrate and various amounts of solving in the solution in grams; W is the sample weight in NaOH (1–8 g) were employed to dissolve the chromium grams, the total chromium content of the ground chromite incorporated in the concentrate. To ensure an oxidative concentrate was found to be 33.45%. Fig. 3 shows a plot medium in the crucible, air with a flow rate of 20 L/min of potential versus titrant consumed during the titration was also employed. To establish the chromium content process, which enabled us to determine the chromium of the concentrate, five samples were taken and sub­ content. jected to conversion in an excessive amount of sodium For the fusion of chromite ore (−75+45 µm), various ­hydroxide (Merck Extra Pure) to ensure that they were amounts of NaOH were employed for a duration of 15–75 all converted and subsequently dissolved in water. The minutes at 500–700 °C. chromium content in the chromite concentrate was de­ To investigate the conversion behavior of chromium termined by potentiometric titration using a platinum from the concentrate, a kinetic study was undertaken for electrode with a 0.1M sodium thiosulfate solution and temperatures between 500–650 °C. a 10% (w/w) KI solution acidified by sulfuric acid. Next, After the fusing and subsequent leaching with water, the chromium content was measured using a SI Ana­ the impure chromate solution was treated with sulfuric lytics Titroline 6000 Chromium Titrator and the average acid to adjust the pH level of the solution. At around was calculated to be 33.45±0.05%. For manual titrations, pH = 7, Al(OH)3 was precipitated. After the filtration, the starch was used as an indicator, which let us note the solution was acidified with sulfuric acid and treated ­endpoint. with ethanol to ensure that all chromium(VI) ions were Solid/liquid separation was performed following converted to chromium(III). Subsequently, chromium hy­ each run. For the Atomic absorption spectrometer AAS droxide precipitation was performed using a NaOH solu­ (Analytik Jena, ContrAA 300, Germany) analyses, the fil­ tion around pH = 7. After the chromium hydroxide was tered solution was introduced into the machine at an ap­ pre­cipitated, it was heated up to 800 °C to obtain chro­ propriate dilution for the determination concentration of mium oxide. TGA was also employed to determine the the other elements. complete conversion temperature of Cr(OH)3 to Cr2O3. The percentage of the conversion was calculated from XRD analysis was also performed on the samples using the amount of chromium (VI) transformed from the chro­ a Philips X’Pert PW3020 (theta/2theta, 2 motors) X-ray mite concentrate using the following equation: ­diffractometer. 240 S. Aktas et al., Production of Chromium Oxide Using Ethanol

3 Results and discussion of NaOH since 600 °C was not adequate for complete ex­ traction of chromium. In the following section, the effect The process outlined in this paper can be used to convert of temperature was studied in the range between 500 °C almost all of the chromium present in the chromite con­ and 700 °C. centrate employed in this study into a water-leachable A similar trend can be observed with extraction of product, i.e. Cr(VI). Aluminum hydroxide (Al(OH)3) was Al. In Fig. 4, we see that the extraction percentage of alu­ removed from the solution by simply adjusting the pH minum increases with an increased amount of sodium level of the solution. ­hydroxide. This effect is not desirable, but it can be elimi­ nated and selectively removed after adjusting the solution pH, which will be discussed in further sections. 3.1 The effect of the amount of NaOH on the conversionof chromium(III) to Cr(VI) 3.1.1 The effect of fusion temperature Chromite concentrate reacts with NaOH above 500 °C under oxidative conditions. In order for chromium to be In this experimental series, the effect of temperature on oxidized in a higher state, air was employed. In this exper­ the conversion percentage of chromium with a changing imental series, the rate of air flow was kept constant at amount of NaOH was investigated. Fig. 5 displays the 20 L/min whereas the sodium hydroxide amount was effect of temperature on the conversion percentage of changed so as to observe its effect on the conversion chromium. The effect of temperature on the oxidation of ­percentage. chromite ore was considered in the range of 500 °C to During the fusion, the following reactions took place: 700 °C. As shown in Fig. 5, the temperature plays a major

7 role on the chromite dissolution process. The chromium 2FeCr2O4 + 8NaOH + O2 = Fe2O3 + 4Na2CrO4 + 4H2O (2) 2 conversion percentage increases with increasing tempera­ MgCr O + 4NaOH + 3 O = MgO + 2Na CrO + 2H O (3) 2 4 2 2 2 4 2 ture, which is in agreement with the study by Arslan and 1 Orhan, 1997. With 2 g of NaOH at 500 °C, the conversion 2FeAl2O4 + 4NaOH + O2 = Fe2O3 + 4NaAlO2 + 2H2O (4) 2 percentage was found to be 48%, whereas at 700 °C a con­ Fig. 4 shows the change in extraction (%) with regard to version percentage of more than 90% was attained at the the NaOH amount. As can be seen from Fig. 4, the conver­ same quantity. Between 650 °C and 700 °C, there is not sion percentage of chromium increased with an increased much difference in the extraction percentages. It is also amount of sodium hydroxide. However, after certain addi­ evident from the figure that it is possible to extract the tion threshold was reached, the conversion percentage chromium incorporated in the chromite concentrate with did not change considerably. Thus, we concluded that a conversion percentage of 97% under the condition of temperature plays a more important role than the amount 1 hour reaction time, in the mixture of 1 g concentrate +

Fig. 4: Change in extraction (%) of Cr and Al with regard to the NaOH Fig. 5: The effect of temperature on the conversion percentage amount (1 g conc. 600 °C, 1 h) of chromium (1 hour, 1 g conc.) S. Aktas et al., Production of Chromium Oxide Using Ethanol 241

6 g NaOH at 650 °C. This result clearly highlights the im­ As shown in Fig. 6, fusing time plays an important portance of temperature in this conversion process [5]. role in the conversion of chromium from the concen­ This is due to the fact that conversion reaction of chromite trate [5–6]. For instance, a conversion percentage of 29% is chemically controlled, which is proved by kinetic study. was observed after 15 minutes of fusing time using 2 g The significant improvement in the chromium extraction of NaOH at 500 °C. However, under the same conditions, percentage results in the reaction time as well as sodium when the fusing time was extended to 1 hour, a conver­ hydroxide consumption. Detailed kinetic study as well sion percentage of 43% was achieved (see Fig. 6a). At as the effect of fusion time are presented in the following 650 °C, a conversion percentage of 78% was observed after sections. 15 minutes of fusing time using 4 g of NaOH. However, under the same conditions, when the fusing time was ­extended to 1 hour, a conversion percentage of 93% was 3.1.2 The effect of fusion time achieved (see Fig. 6b). It can be concluded that both ­temperature and fusing time affect the chromium ex­ In this experimental series, the effect of time on the con­ traction percentage. It is also worth noting that after 60 version percentage of chromium was studied. Fig. 6a and minutes (in some cases, after only 45 minutes) reactions 6b display the effect of time on the conversion percentage cease in that there is not much difference in the ex­ of chromium with 2 g and 4 g of NaOH, respectively. traction percentages. Fig. 7a and 7b display extraction

Fig. 6: The effect of time on the conversion percentage of chromium: Fig. 7: The effect of fusion time on the extraction percentage (a) 2 g of NaOH, (b) 4 g of NaOH (1 g conc.) of aluminum: (a) 2 g of NaOH, (b) 4 g of NaOH (1 g conc.) 242 S. Aktas et al., Production of Chromium Oxide Using Ethanol percentage of aluminum. It is obvious from Fig. 7 that ions do not precipitate as hydroxide at any pH level, which fusing time plays an important role in the extraction of allowed us to eliminate aluminum. aluminum from the concentrate. For instance, a conver­ sion percentage of 21% was observed after 15 minutes of fusing time using 2 g of NaOH at 500 °C. However, under 3.2 Conversion kinetics of chromium from the same conditions, when the fusing time was extended chromite concentrate to 1 hour, an extraction percentage of 43% was achieved (see Fig. 7a). At 650 °C, a conversion percentage of 40% To determine the rate constant (k) of chromium conver­ was observed after 15 minutes of fusing time using 4 g of sion in the temperature range of 500–650 °C, 1 − (1 − X)1/3 NaOH. However, under the same conditions, when the was plotted against time, as displayed in Fig. 9, where fusing time was extended to 1 hour, an extraction per­ X denotes the conversion percentage of chromium. The centage of 80% was achieved (see Fig. 7b). It can be activation energy, Ea, was calculated using the Arrhenius ­concluded that both temperature and fusing time con­ equation: siderably affect aluminum extraction efficiency. It is also worth noting that after 60 minutes (in some cases, after k = Ae−Ea/RT or ln k = ln A − Ea/RT (5) only 45 minutes) reactions cease and there is not much where k is the rate constant, Ea is the activation energy, R difference in the extraction percentages. Normally, alumi­ is the gas constant, and T is the temperature in Kelvin. num extraction is not desirable since it would present an Fig. 10 displays the Arrhenius plot of ln k versus impurity in the respective chromium compound; how­ 1000/T for the chromium conversion. The activation ever, aluminum was subsequently precipitated as alumi­ energy for chromium conversion was calculated to be num hydroxide by decreasing the pH value of the solution 40.15±2.60 kJ/mol, which indicates that this is a down to 7 by the addition of sulfuric acid, resulting in alu­ ­temperature-controlled process [14]. minum recovery above 99% from the sodium chromate solution. It is worth noting that the ­aluminum hydroxide was in jelly form and we had difficulty filtering it. XRD analysis revealed that the obtained powder is Al(OH)3, as shown in Fig. 8. From this by-­product, it is possible to make other valuable products such as alumina, aluminum sulfate, etc. In this way, aluminum accompanying the chromate solution was removed. The remaining chromium solution was first acidified with sulfuric acid, then treated with ethanol to ensure that chromium(VI) could be converted Fig. 9: Plot of 1 − (1 − X)1/3 with respect to time to Cr(III) and precipitated as chromium hydroxide at pH = 7. This methodology is possible since chromium(VI)

Fig. 8: XRD pattern of Al(OH)3 Fig. 10: Arrhenius plot of ln k vs. 1000/T S. Aktas et al., Production of Chromium Oxide Using Ethanol 243

3.3 Chromium oxide production with the help of ethanol

After chromium was extracted into the solution, the pH of the solution was set to around 7 so that aluminum was pre­ cipitated as aluminum hydroxide. Subsequently, ethanol was deliberately added to the dichromate solution to ensure that all chromium(VI) was converted to chromi­ um(III). Interestingly, it was observed that ethanol pre­ cipitated chromium as Cr(VI) at a mildly acidic level, but at the highly acidic regime, ethanol acted as a reducing agent role in that chromium(VI) was converted into Cr(III).

However, this effect is more profound in K2Cr2O7 than

Na2Cr2O7, where almost 90% precipitation was observed when potassium dichromate solution was employed. Fig. Fig. 12: TGA plot of conversion of the Cr(OH)3 into Cr2O3 11 shows precipitation (%) as a function of the ethanol: solution ratio. The following reaction between­ Cr(VI) and Table 2: Chemical analysis of obtained chromium oxide ethanol takes place in the strongly acidic regime:

2− + 3+ Compounds Mass% 2Cr2O7 + C2H5OH + 16H = 4Cr + 11H2O + 2CO2 (6)

Cr2O3 99.85 In the strongly acidic regime, ethanol acts as a reducing MgO N/A* agent role in that chromium(VI) was converted into Cr(III) Fe2O3 N/A* whereas ethanol itself was oxidized to carbon dioxide and Al2O3 0.04 water, as indicated by Eq. (6) [15]. Subsequently, chro­ SiO2 0.02 Na O 0.09 mium hydroxide was successfully precipitated at around 2 V2O5 N/A* pH = 7 with the help of sodium hydroxide. NiO N/A* MnO N/A* 3.4 Characterization of the obtained * Not Available powders

Chromium hydroxide was obtained with the help of an 800 °C environment. Fig. 12 shows the TGA plot of the sodium hydroxide and converted to chromium oxide in conversion of Cr(OH)3 into Cr2O3. It is evident from Fig. 12

that the conversion of Cr(OH)3 into Cr2O3 starts at around 430 °C and finishes around 700 °C. To determine the purity of the powder that was produced in this study, potentio­ metric titration was carried out. The sample was found to contain 99.85% chromium oxide. The full analysis is dis­ played in Table 2. XRD analysis revealed that the powder was chromi­ um(III) oxide, as shown in Fig. 13.

4 Conclusions

In the present work, a chemical processing scheme was developed to produce chromium oxide from Turkish chro­ mite concentrate. We demonstrated that it is possible to extract the chromium incorporated in the chromite con­ Fig. 11: Precipitation (%) of chromium as a function of the ethanol:solution volume ratio. (Room temp, 0.24 and 0.48 M sol., centrate with a conversion percentage of 97% under the 300 rpm, 1 hour) condition of 1 hour reaction time, in the mixture of 1 g 244 S. Aktas et al., Production of Chromium Oxide Using Ethanol

present study, we employed air as an oxidant to put forth an approach to employ Turkish chromite ores. How­ ever, for industrial applications, the employment of pure oxygen is unavoidable for mixing the mixture of concen­ trate and sodium hydroxide and ensuring a proper oxida­ tive medium.

Acknowledgments: Special thanks are due to Kromsan for providing valuable information regarding the process that is being applied.

Funding: The author wishes to thank Marmara Bapko for financial support under project FEN-A-150513-0165.

Fig. 13: X-ray diffraction pattern of chromium oxide References

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