Quantitative Assessment and Control of the Environmental Risk of Both Ash and Slag in the Carbothermal Reduction of Electroplating Sludge for Eco-friendly Conversion of Toxic Heavy Metals into Valuable Alloy

The production of alloys from electroplating sludges (ES) by using carbothermal reduction is a short-process and high-value route for its resourceful utilization. However, the process regulation is experiential and coarse, and the environmental risk from slag and ash is ignorant. In this work, total metallization rate (TMR) and overall pollution toxicity index (OPTI) were developed, and a complete set of mathematical tools was introduced for achieving precise and quantitative regulation of this process for the first time. The results showed there was a notable difference in the distribution behaviors in alloy-slag-ash between various heavy metals, the formation of alloy followed by Ostwald formula, including reduction nucleation, deep reduction, and coarsening of metal particles. The resulting slag has no environmental risk due to low OPTI (0.7), whereas ash has notable environmental risk owing to high OPTI (25.2). Models were built using response surface methodology (RSM) to describe the quantitative relationships between TMR of alloy or OPTI of ash and key process factors. The adaptive weight particle swarm optimization (AWPSO) was used to solve the models to get the optimized combination parameters, respectively, acquiring 94.1% of predicted TMR for alloy and 31.2 of predicted OPTI for ash. Both the confirmatory test of laboratory and the field verification of a factory demonstrated the reliability of models. A relatively low OPTI of ash and a high TMR of alloy were simultaneously obtained, thus realizing an eco-friendly translation of toxic heavy metals in ES into valuable alloy by carbothermal reduction process. Graphical abstract: [Figure not available: see fulltext.].