TREATMENT OF FLY ASH/APC RESIDUES INCLUDING LEAD SALT RECOVERY

摘要

The present invention relates to a process for treating fly ash/APC residues chlorides, sulphates, earth alkali metals selected from the group consisting of calcium, potassium, and sodium, and heavy metals selected from the group consisting of lead, which residues are obtained from air pollution control processes that utilize dry/semi-dry lime injection, such that said residues may be disposed of as non-hazardous materials. The process comprises the sequential steps of: (a) washing and agitating the fly ash/APC residue (20) with water (50) to form a residue slurry containing solubilized lead salts and calcium chloride; (b) filtering (42) the residue slurry to remove a filtrate (46) having a pH of higher than about 11.8 and containing the solubilized lead salts and calcium chloride; and (c) recovering a first calcium enriched filter cake (44). The solubilized lead salts are then removed from the filtrate by the sub-steps of: (i) adding sufficient hydrochloric acid (58) to the filtrate to lower the pH thereof to between about 10 and 11; (ii) agitating the filtrate until a colour change from white to yellow occurs; (iii) adding additional sufficient hydrochloric acid to the filtrate so agitated to lower the pH to between about 7 and 8; (iv) filtering (82) the filtrate to remove a brine (86) containing calcium chloride; and (v) recovering a second yellow coloured, lead enriched filter cake (84).