High Temp. Mater. Proc. 2015; 34(3): 237–244 S. Aktas*, C. Eyuboglu, M.H. Morcali, S. Özbey and Y. Sucuoglu Production of Chromium Oxide from Turkish Chromite Concentrate Using Ethanol 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 sodium 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 sodium hydroxide 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, calcium carbonate 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 solubility 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 hexavalent chromium 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.