Metal and phosphoric acid recovery from metal phosphatisation sludge - by alkaline hydrolysis, solid liquid sepn. and recovery of acid and alkali from liquid

摘要

In the treatment of phosphation sludge contg. metal phosphates (I), formed during phosphation of metals, pref. steel, Zn and Al, by stirring with aq. alkali soln. (II) to convert (I) to metal hydroxides (III), the novel features are that (a) the suspension (IV) is sepd. into (III) and alkali phosphate soln. (V); and (b) (V) is sepd. into H3PO4 and (II). The sludge is treated with (II), pref. aq. NaOH or KOH soln., in 1:(1-10), pref. 1:(3-5) ratio at 60-120, pref. 65-85 deg. C. (IV) is sepd. by solid/liquid sepn., then the filter cake is washed and dried. (V) is sepd. by membrane electrolysis, into H3PO4 and (II), which are recycled. (III) is converted to oxide by heating to 1000-1200 deg. C, pref. in a rotary calciner. Also claimed is the appts. consisting of a lixiviation vessel with stirrer, solid/liquid sepn. appts., e.g., decanter, filter press, band filter, etc., and membrane electrolysis cell. USE/ADVANTAGE - The technique eliminates the need to dump phosphation sludge. The metals are recovered and the chemical solns. are recycled, which minimises the addn. of fresh chemicals. The metal oxides obtd. by heating (III) can be used as raw material in Zn prodn., when all other materials are converted to slag, which is also used in Zn prodn.