Abstract:

In the evaporation and crystallization process of extracting alumina from fly ash by hydrochloric acid method,the aluminum chloride solution was repeatedly evaporated,which caused the enrichment of impurity ions such as calcium,magne-sium,silicon and phosphorus in the evaporation mother liquor.When the impurity ions in the mother liquor reached a certain degree,part of the aluminum chloride evaporation mother liquor needs to be discharged,which will cause the loss of 15%~20% of the raw aluminum.The technology of HCl gas salting-out crystallization was used to treat the evaporation mother liquor.After two-stage salting-out crystallization and concentrated hydrochloric acid washing,not only the purpose of recovering aluminum chloride in the evaporation mother liquor was realized,but also the purity of aluminum chloride crystal hexahydrate was increased,so that the purity of alumina recovered reached to 99.38%. Meanwhile,the contents of CaO,MgO and P₂O₅ in alumina were reduced to 2.680×10^-5,2.849×10^-4 and 2.051×10^-4 respectively.In addition,the process of recovering aluminum chloride from mother liquor was put forward,which can be completely incorporated into the existing main process of extracting aluminum oxide by hydrochloric acid method from fly ash.

Key words: fly ash, evaporation mother liquor, aluminum chloride, HCl, salting-out crystallization