Fixation stability of glass matrix co-existent with crystal phases for heavy metals formed by high-temperature vitrification

Mineral transformation and solidification of heavy metals during co-melting of MSWI fly ash with coal fly ash

Melting is an efficient method to turn municipal solid waste incineration (MSWI) fly ash (FA) into non-hazardous material. Coal fly ash (CFA) was selected as the silica-alumina source to carry out co-melting research with MSWI FA in this work. The effects of the temperature and the CFA content on mineral transformation and the migration characteristics of heavy metals were analyzed. The results showed that the mixtures of MSWI FA and CFA reacted at high temperatures to mainly generate Ca2Al2SiO7, Ca2SiO4, and CaAl2Si2O8 primarily and then melted and formed the amorphous-phase vitreous body when the CFA content was more than 40% and the temperature was higher than 1300 °C. During the melting process, Cd and Pb were almost volatilized, while Cr, Mn, and Ni were almost retained. Besides, the volatilization rates of Cu and Zn fluctuated with the temperature and the CFA content. Suitable treatment temperature and CFA content were conducive to the transformation of the heavy metals in the FA into stable forms, and the melting products were no longer hazardous wastes because the vitreous body could effectively encapsulate heavy metals. This study aims to help reuse the FA and CFA collaboratively and be more environmentally friendly.

A novel method for effective solidifying chromium and preparing crude stainless steel from multi-metallic electroplating sludge

Electroplating sludge (ES) is a globally prevalent hazardous waste that primarily contains Cr, Cu, Ni, and Fe. However, the residual Cr phases within the slag potentially poses an environmental risk in current vitrification. A novel method for effective recovering and solidifying Cr in ES is proposed in this work. ES was desulfurized and subsequently co-treated with ferrosilicon (Fe-Si) and spent carbon anode (SCA) for enhancing the recovery of Cr, Cu, Ni, and Fe to prepare crude stainless steel. Under optimal conditions, the recovery ratios of Cr, Cu, Ni, and Fe reached 96.96%, 99.45%, 99.92%, and 99.20%, respectively, signifying improvements of 21.4%, 0.2%, 1.5%, and 2.8%, respectively, compared with existing research. Meanwhile, the fluoride in SCA yielded CaF2, further progressing to the Si-Ca-F-Na-Al-O phase, with a solidification ratio of 97.87%. The Cr leaching content of the residual Cr-Cu-S phase in the slag remained below 5 mg/L across a pH range of 2-4, demonstrating enhanced stability compared to prior alloy, oxide, and chemically dissolved phases. An innovative approach for solidify Cr by forming matte holds implications for the treatment of Cr-containing solid wastes such as chromium slag, tannery sludge and stainless steel slag.

Heavy metal migration regimes in the production of syngas from solid waste by thermal plasma treatment

In recent years, thermal plasma technology has been widely used in the harmless and resource-efficient treatment of solid waste (SW). This study investigates the migration behaviors of heavy metals during the thermal plasma treats SW to obtain the interphase structure change regimes of heavy metals. The transformation of SW under high-temperature environments was analyzed by Fluent simulation, and the composition of the crystalline phases and heavy metal content of the post-treatment slags were studied through a combination of XRD, SEM, and heavy metal leaching experiments. The results show that the thermal plasma provides a melting zone temperature of more than 4000 K, and the treated slag is mostly an amorphous solid composed of glassy Si-O mesh, which effectively encapsulates heavy metals and reduces their leaching rate. Additional analysis of the migration and transformation of heavy metals during thermal plasma treatment revealed that solid-phase heavy metals primarily took the form of sulphides and sulphates, while liquid- and gas-phase heavy metals were mostly oxides and chlorides. Simultaneously, Economic analysis results showed that the thermal plasma treats SW economically with an Energetic efficiency of up to 76.7%. The results of this study providing new insights into thermal plasma treatment SW research.