Waste control by waste: Metals recovery from electroplating sludge via a chlorination roasting followed by silicothermic reduction using solar-grade silicon cutting waste.

Abstract

Electroplating sludge (ES) is a hazardous waste, because it contains heavy metals. It poses severe environmental and health risk if not properly disposed. This study proposed a combined pyro-metallurgical process to separate and recover copper, nickel, chromium and iron from it. A chlorination roasting was firstly used to selectively recover copper and nickel, in which they were chlorinated and volatilized while chromium and iron retained in the residue in the forms of FeCr₂O₄ and Fe₂O₃. A certain FeS₂ promoted the conversion of the chlorinating agent of NaClO to Cl₂ (g), increasing the copper and nickel chlorination. Though Cr₂S₃ could be chlorinated and volatilized, a high O₂ partial pressure oxidized it to Cr₂O₃ and reduced it chlorination. Under the optimal condition, the chlorination of copper and nickel obtained 99.1 % and 92.6 % respectively, while that of chromium was only 5.7 %. In the followed silicothermic reduction, a silicon cutting waste (Si-CW) was employed as reductant to recover chromium and iron from the roasted residue, due to the reduction capacity of Si and SiC phases in it. The chromium and iron oxides were reduced and recycled in an Fe-Cr alloy ingot, and Si and SiC changed to a refractory SiO₂ and entered into the slag. CaO could be slagged with SiO₂ and converted to a slag-liquid phase, which accelerated the separation between alloy and slag. The chromium and iron yields could obtain 97.6 % and 98.9 %, respectively. This study supplied a new method to co-treat two wastes for recovering nickel, iron, copper and chromium.