Chromium Oxide Was Produced From High-carbon Ferrochrome Alloy Powder

Cr₂O₃ is widely used in national defence, electrical alloys, hard alloys, vacuum galvanization, attrited materials, paint, et al. At present, chromium oxide is commonly produced by thermolysis of sodium bichromate, ammonium sulfate and chromic anhydride, and there is serious pollution of Cr⁶⁺ in the production. In this paper, pollution-free technology method of the preparation chromium oxide was studied. Chromium oxide was successfully produced from high-carbon ferrochrome alloy powder. The main results were as follows.The study showed that 100g high-carbon ferrochrome leached by 120ml sulfuric acid which was not enough, The extraction rate of high-carbon ferrochrome reached above 75%, and the final pH value of the leaching was about 1.0. The residue of unreacted high-carbon ferrochrome was reacted with the next high-carbon ferrochrome, which is full utilization of resource and save the quantity of sodium carbonate in the next experiment procedure of pH adjusting. Two removing iron methods was studied in the paper, Iron (III) removed by goethite and iron (II) removed by oxalate. The effects of Cr³⁺ concentration, pH, and temperature on the removal of iron were examined in the goethite process. The results showed that when Cr³⁺ concentration was 7.0g/L, temperature was 94℃and pH=2.5, rate of agitation was 200 r/min, more than 99% of iron is precipitated and 15% of chromium was lost; The optimal experimental condition of iron removed by oxalate was that pH=2.5, oxalate was 150% of theoretical value, reaction temperature was 30℃, more than 99% of iron was precipitated and loss ratio of chromium was little. The experiment studied and compared goethite process to iron removed by oxalate. The cost of goethite process is lower, but 15% chromium entered residue; By contrast, although oxalate method had more cost compared to goethite process has better separation effect. The process not only did not lost chromium but also produced ferrous oxalate which is main material used as the precusor in synthesizing lithium iron phosphate (LiFePO₄). After iron removed by oxalate, other metal ions in the chromium sulfate solution was removed by sodium dithiocarbamate. The results showed that pH=2.5, rate of agitation was 200 r/min, sodium dithiocarbamate was the theoretical value. The results showed that LiFePO₄/C with uniform particle distribution and good charge-discharge curves and displayed 140 mAh·g^-1 at 1C rate. The ferrous oxalate can be used as the precusor of preparing lithium iron phosphate. Cr(OH)₃ is difficult to filter and wash, which is unfavorable for the industrialized production. The effects of Cr³⁺ concentration pH temperature and polyacrylamide(PAM) additives on the filtering property of Cr(OH)₃ and recovery rate of Chromium were studied. The results of experiment showed that under the optimal conditions of Cr³⁺ concentration below 10 g/L, temperature more than 85℃, pH 7.0 and agitation rate 200 r/min, the recovery rate of Chromium reached 99%, and the filtering capability of Chromium hydroxide was also improved, which was beneficial to industrial production.Finally, the decomposed process of Cr(OH)₃ compound was studied by TG/DSC method and Cr₂O₃ was successfully obtained. The results showed Cr(OH)₃ compund decomposed with three periods. Decomposition of water is between 50¹50℃,and decompose of Cr(OH)₃ is between 200～500℃. The formation process of chromium oxide is between 400～500℃in which Cr(OH)₃ lost the ratio of 26.63%. It is almost the same with the theoretical value which lost the ratio of 26.21%. Cr₂O₃ was prepared at holding temperature of 500℃, and the samples was satisfied standards of metallurgical chromium oxide.