Cr (VI) reduction in aqueous solitions by using pyrite

SUMMARY Chromium, which is used widely as its alloys and its compounds, is an important pollution for enviroment. Cr(VI) is more toxic than the other forms. The permited amount in water is 50 ng/L in order to remove Cr(VI) from aqueous medium, first it must be reduced to Cr(III). So, iron(II)sülphate, S02 and alkaline sulphides are used as reducing agents for this purpose. In this study, the effects of parameters such as acid concentration of the medium, amount of pyrite, initial Cr(VI) concentration, contact time, temperature and particle size of pyrite on the reduction of Cr(VT) by discontinuous experiments using raw pyrite and the pyrite samples roasted at high temperatures in an airless medium were examined systematically. In addition, in order to determine reduction capacites of pyrite samples, the samples were treated with succesive reduction process until their reduction effects lost and thus their reduction capacities were determined. Moreover, to utilize the pyrite samples which had lost their reduction abilities copper and cobalt have been recovered from these samples at high yield. The effects of parameters such as roasting time at projected temperature and sulphur concentration of the medium on recovery yields were examined. This study is systematic and compares the reduction abilities of raw pyrite and the pyrite samples roasted at high temperatures in airless medium. As expected, it was found that the reduction ability of latter was much higher. It was determined that the reduction efficiency of Cr(VI) increased with increasing amount pyrite, contact time and temperature of medium for the solution containing certain amount of Cr(VI), while it decreased with increasing acid concantration of the medium, decreasing initial Cr(VT) concantration and enlarging particle size of pyrite. It was found that at the agitation process at 25 °C for 60 minute by adding 2,5 g (100 mesh) pyrite samples into 50 mL Cr(VI) solution, concentration of which was 14700 mg/L and included stochiometric ratio acid, the pyrite sample roasted at 700 °C in anVI airless medium reduced all CrfVT) in the medium, while raw pyrite sample reduced only 75,40 % of CrCVT) in the medium. Also, at the end of agitation process at 25 °C for 60 minute by adding 0,5 g(100 mesh) pyrite samples into 50 mL Cr(VI) solution, concentration of which was 147 mg/L and included stochiometric ratio acid, the pyrite sample roasted at 700 °C in an airless medium reduced all Cr(VI) in the medium, while raw pyrite sample reduced only 81,30 % of Cr(VI) in the medium. It was found that in the successive experiments with the same pyrite sample the reduction capacities of the pyrite sample roasted in an airless medium at 700 °C in concantrated and diluted medium were 37,43 % and 14,55 % respectively, while those of raw pyrite were 12,00 % and 2,78 % respectively. It was also determined that at the end of precipitation process held at pH 7,7 in reduced solutions, reduced chromium and some heavy metal ions passed from pyrite to solution removed by precipitation. In the experiments related with recovery of copper and cobalt from pyrite samples lost their reduction abilities, the pyrite samples firstly treated with roasting at 600 °C for 2,3,4,5 and 6 hours, respectively it was found that the optimum roasting time of recovery yield of copper and cobalt from the pyrite sample roasted at 700 °C in an airless medium was 5 hours, and after roasting for that time 96,56 % copper and 97,00 % cobalt recovery was accomplished. The optimum roasting time of recovery yield of copper and cobalt from raw pyrite sample was found to be 6 hours, and after roasting for that time 83,06 copper and % 68,08 cobalt recovery was accomplished. In the experiments examining the effect of sulphur concentration of the medium on the recovery yield of copper and cobalt, various amount of pyrite sample roasted at 725 °C in an airless medium was added into the pyrite samples lost their reduction abilities and roasted at 600 °C for 5 hours. Thus, it was found that the more pyrite sample roasted at 725 °C in an airless medium added into the medium, the recovery yield of copper and cobalt were decreased