Characterization of extracting Cr, Ni and Fe from the sludge of stainless steel acid washing wastewater with ammonia and sulfuric acid

Extracting Cr, Ni and Fe from the sludge of stainless steel acid washing wastewater with ammonia and sulfuric acid was carried out. The effects of reaction time, liquid-to-solid ratio, temperature, ammonia concentration and the volume of sulfuric acid on the extraction ratio were investigated. In ammonium leaching process, the maximum leaching rate of Ni is 30.8%, and on the same condition the leaching rate of Cr and Fe is 2.21% and 0.016% respectively. In acid leaching process, the maximum leaching rate of Ni, Fe and Cr is 78.1%, 30.24% and 26.54% respectively. The factor that leads to the low extracting ratio maybe the suspended substance in the sludge which exists as particulate matter in original wastewater before. In acid leaching process metals can lowly be leached out because of the surface passivation of chromium in these particulate matter. It can be concluded that regular leaching reaction is not a proper method for extracting Cr, Ni and Fe from the sludge of stainless steel acid wastewater.