Economic considerations of chromium recovery from tanning wastewater

Abstract

The chrome recovery from tanning wastewater was investigated in field conditions using a real-sized pilot plant. The tested wastewaters were from the tanning process both without and with tanning additives, whereas the precipitating chemicals of interest were magnesium oxide (MgO) and sodium carbonate (Na₂CO₃). The precipitation with MgO followed by 1-hour sedimentation was the best solution. The optimal dosages were 2 and 4 times that of stoichiometric requirements for conditions without and with the additives, respectively. By redissolving the obtained sludge with 1:1 diluted sulfuric acid and maintaining the pH between 2.5 and 2.8, the chromium recovery were 97.6 and 88.6%, respectively. Another alternative of using Na₂CO₃ as the precipitant was not promising because of its high chemical cost and excessive sludge-settling period (15-20 hours). The MgO precipitation scenario proved to be able to pay back economically within 3 years for the “without-additive” waste stream whereas that of “with-additive” wastewater was not worth the investment. This is because the “with-additive” discharge contains low chromium content whereas it requires high chemical doses.