Purification of waste liquids from alkaline washing pretreatment of municipal solid waste incineration fly ash using Friedel's salt.

Washing is a widely used pretreatment method for the resource utilization of municipal solid waste incineration fly ash (MSWI FA). However, the high concentrations of heavy metals and chlorides in alkaline washing pretreatment waste liquids can adversely affect the environment and human health, necessitating proper disposal. Therefore, in this study, the heavy metal removal and ultrasonic-assisted two-stage dechlorination process from MSWI FA alkaline washing waste liquids using Friedel's salt were developed. When the end-point pH was 9 and Friedel's salt dosage was 4 g/L, the removal rate of key heavy metals from MSWI FA alkaline washing waste liquids exceeded 95 %. A series of characterization showed that Friedel's salt can effectively remove heavy metals from waste liquids through isomorphic substitution, ion exchange, surface adsorption and precipitation formation. Experimental results demonstrated that when the molar ratio of Ca: Al: Cl was adjusted to 2.5:1.2:1 in the first stage and 7:3:1 in the second stage, combined with an ultrasonic power of 600 W, the ultrasound-assisted two-stage dechlorination method achieved a chloride removal efficiency exceeding 93.8 % from the alkaline washing solution of MSWI FA. And, 272.6 g of Friedel's salt was obtained while treating 1 L of waste liquids. This study revealed that ultrasonic-enhanced dechlorination occurs through mechanisms that promote the dissolution and dispersion of CaO while inhibiting the formation of CaAl(OH). In addition, the combination of alkaline washing pretreatment and waste liquids reuse, an integrated idea was provided, which was helpful for the comprehensive utilization of MSWI FA.