Production of iron chloride from stainless steel swarf via hydrometallurgy

Abstract

Stainless steel swarf is a hazardous waste which is largely deposited in landfills. Recycling of valuable metals from this swarf has been difficult due to its high content of hazardous metalworking fluids. This thesis presents a hydrometallurgic approach for the production of iron chlorides from stainless steel swarf. Iron chlorides are a valuable product and can be used as a flocculant in wastewater treatment. A swarf sample from the production of ball bearings containing 69% iron (Fe) was leached with hydrochloric acid (HCl) at static pH levels. Design of experiments and regression modelling was used to determine the effects of temperature, pH and solid to liquid (S:L) ratio on the leachability of Fe, Cr, Mn, Ni and Mo. Temperature and pH greatly influenced the leachability of Ni and leaching rates of Fe and Mn. Extraction of these metals was generally favored at higher temperatures and acid concentrations. Operating at a high pH was crucial for preventing dissolution of both Cr and Mo. Higher temperatures were also found to inhibit leaching of Cr. While the leachability of most metals was unaffected by S:L ratio, Ni was leached to a lesser extent at high S:L ratios. A very high purity of Fe was achieved when leaching at 60oC, pH 4 and S:L=1:20. After leaching for 4 h at these conditions, a final solution containing 21800 mg/l Fe, 150 mg/l Mn, 12 mg/l Ni and no Cr or Mo was obtained. Moreover, only 1% of the cutting fluid was transferred to the aqueous phase. Over 96% of Fe was extracted as FeCl2 and the remaining fraction as FeCl3. These findings show that metal recovery from stainless steel swarf is possible with minimal extraction of oil based cutting fluids.