Removal of iron and chromium from acid leaching solution of gasification slag and preparation of polyaluminum chloride
Gasification slag is a solid waste produced by coal gasification process,and its large amount of emission seriously threatens the ecological environment and restricts the development of coal chemical industry. This study focuses on the characteristics of gasification slag with high calcium,high iron,low aluminum,and chromium content in its phase composition. Through a counter-current mixed acid leaching process,aluminum enrichment and calcium ion removal were achieved in the liquid phase while effectively reducing the residual acid content. Subsequently,iron and chromium impurities were removed from the leaching solution to obtain a purified solution suitable for preparing qualified polyaluminum chloride (PAC). Using this aluminum-rich purified solution as raw material,the effects of aluminum chloride and aluminum hydroxide addition on PAC product performance were further investigated. The results show that the optimal conditions for the mixed acid countercurrent leaching stage are hydrochloric acid concentration of 2.125 mol/L,sulfuric acid concentration of 0.69 mol/L,liquid-solid ratio of 4∶1,temperature of 85 ℃,and reaction time of 4 h,under which the aluminum leaching efficiency reaches 85.39％. The optimal conditions for the removal of impurities by the iron vitriol dreg of yellow sodium method are reaction temperature of 97 ℃,reaction time of 1.5 h,and a endpoint pH of 2.0,under which the iron removal rate and chromium removal rate are 69.46％ and 51.36％,respectively. In the PAC preparation stage,using the purified solution as a raw material,aluminum chloride of 150 g/L and aluminum hydroxide of 100 g/L are added,followed by the hydrolysis process at 60 ℃ for 1 h and polymerization at 85 ℃ for 1.5 h. The liquid PAC products are produced,which is compliant with the national standard GB/T 22627—2022,achieving a turbidity removal efficiency of 94.33％.
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