The heavy metal leaching property and cementitious material preparation by treating municipal solid waste incineration fly ash through the molten salt process

Abstract

This research investigated the heavy metal leaching property and cementitious material preparation by treating municipal solid waste incineration fly ash through the molten salt process. The results indicated that the heavy metal thermal evaporation of fly ash in the molten salt was related to molten salt composition, heat treatment temperature and atmosphere. After treatment with sodium chloride molten salts (contains 10–50 wt% calcium chloride) from 900°C to 1000°C for 2 h, the leaching concentrations of lead, cadmium, copper, zinc and other heavy metals in fly ash were decreased more than 90% and they could fully meet with the landfill standard. Moreover, after molten salt treatment, the weight fraction of fly ash was reduced by 50 wt% than the original one, and the fly ash has been changed as a kind of cementitious material, which has excellent cementitious property. The X-ray diffraction result indicated that the main crystal mineral composition of cementitious materials obtained was alite, belite, alinite and calcium sulphate.

Synergistic effect of polyvinyl chloride and coal ash on thermal separation of heavy metals from MSWI fly ash through molten salt process

“…The treatment of FA by molten salt method could not only reduce energy consumption but also make heavy metals reach the leaching toxicity standards. And FA slag after treatment had good cementitious property (Zhao et al, 2020). Furthermore, the volatilized heavy metal chlorides could be collected for recovery and utilization, which is needed for further study.…”

“…The previous research showed that the optimal condition of molten salt treatment was the CaCl 2 /sodium chloride (NaCl) weight ratio 1:1, the FA/molten salt weight ratio 1:10, treatment temperature 1000°C for 2 hours in reducing atmosphere. The leaching concentration of heavy metals in FA after treatment was less than the limit value specified in GB16889-2008(Chinese Ministry of Environmental Protection, 2008, and the treated FA had good gelling properties (Zhao et al, 2020). Therefore, this experiment was carried out according to the previous conditions of molten salt treatment.…”

“…Previous study indicated that there were almost no heavy metals in the drained salt, most were volatilized into the flue gas, and only a small amount of heavy metals were still in the slag (Zhao et al, 2020). Therefore, the volatilization performance was evaluated by the content of heavy metals in the sample before and after treatment.…”

“…The reaction between HCl and CaO/CaCO 3 could also expose the heavy metals wrapped by CaO/CaCO 3 particles to molten salt, thus promoting the further reaction of heavy metals. Zhao et al (2020) have studied the influence of oxidation atmosphere and reduction atmosphere on the chlorination and volatilization of heavy metals in FA, and it was found that the reduction atmosphere was more conducive to the volatilization of heavy metals. As can be seen from Figure 4, PVC would decompose to carbon (C) element after 500°C, and C could further promote the volatilization of heavy metals as a reducing agent.…”

A mini-review of heavy metal recycling technologies for municipal solid waste incineration fly ash

“…They concluded that CaCl 2 and MgCl 2 first react with O 2 and/or water to generate Cl 2 and/or HCl (indirect chlorination), which are more efficacious for the majority of HMs compared to NaCl (direct chlorination). Another study also revealed that compared with the addition of NaCl alone at 900°C, adding 10% CaCl 2 to NaCl reduced the leaching concentrations of Zn, Cu, Cr, and Pb in the treated FA from 1056, 129.5, 67.6, and 1072 μg/L, respectively, to 451, 30.7, 43.9, and 177.5 μg/L, respectively, corresponding to an impressive boost in the HM volatilization rate (Zhao et al, 2020b (Yu et al, 2016). Furthermore, the volatilization of HMs in FA of low alkaline earth metals could also be amended by exerting FA of high alkaline earth metals as chlorine donor, but its effectiveness is inferior to the chlorinating agents (Yu et al, 2015a).…”