Promotion of chloride removal from MSWI fly ash by an accelerated wet-carbonation process to enhance ash recycling in cement manufacture

Chlorides, sulfates, and heavy metals are the three main factors preventing the recycling of municipal solid waste incineration (MSWI) fly ash as an alternative material for cement clinker production, of which chloride plays the main role due to its extremelly hign content in fly ash. This study evaluated three wet-based pretreatment methods (water-washing, CO2-aided washing, and flue gas-aided washing) to remove chlorides using MSWI fly ashes from 13 incineration plants in China. Water washing was suitable for removing Cl- from MSWI fly ash containing a limited amount of less-soluble Cl-containing salts (Ca(OH)Cl, Ca2Al(OH)6(H2O)2Cl, and Ca6(CO3)2(OH)7Cl). However, for fly ash with a significant proportion of these less-soluble Cl-containing salts, injection of CO2/flue gas during washing promoted their decomposition and increased the chloride removal rates by 2–12%. Compared with chloride, the removal rate of sulfate was lower under all treatment scenarios and ranged from 7% to 47%. Therefore, caution should be taken when exploring techniques for the deep removal of chloride from MSWI fly ash, as sulfates may be the limiting factor in fly ash samples with low chloride contents when utilizing the fly ash for cement clinker production. Heavy metals in fly ash were not a limiting factor for any ash samples (either raw ash or treated ash) when utilizing the fly ash for clinker production. However, the dissolution of heavy metals in the washing solution deserves more attention, as a large amount of wastewater will be generated during wet-based pretreatment methods. This study confirmed that CO2/flue gas injection during washing promoted the removal of the less-soluble harmful components of fly ash, expanding the applications of ash materials. In 2021, approximately 5.4 million tons of MSWI fly ash were produced in China, most of which was disposed of in sanitary landfills after stabilization/solidification pretreatment. Ash landfilling has certain problems and environmental risks. First, the heavy metals in fly ash may become remobilized due to invalidation of the cleating agent added during ash stabilization. Second, according to disposal law in China, fly ash should be landfilled in separate landfill cells, however, illegal landfilling of the MSWI fly ash with MSW was frequently discovered in some landfills. Third, most landfill sites experience leakage, which aggravates environmental pollution caused by fly ash landfilling. At present, the environmental risks caused by fly ash landfilling have attracted more and more attention and more voice against landfilling has been heard. Recycling of the MSWI fly ash as alternative raw materials for cement production may serve as a potential way to reduce ash landfilling. The MSWI fly ash can be separated into two fractions (salts fraction and residue faction) via wet-based pretreatment. The waste salt fraction can be recycled after further purification, while the residue faction can be utilized for cement clinker production. Cost-effective separation of the two fractions is a prerequisite for ash recycling. This manuscript provides a novel MSWI fly ash pretreatment method that significantly improves the separation efficiency of the salts fraction from the fly ash matrix.