The sludge composed chiefly of iron phosphate that accumulates as an insoluble residue in aqueous solutions used for phosphatizing automobile body parts to retard rust, is separated from the phosphate solution by filtering and treated with an aqueous solution containing approximately the amount of alkali, such as naoh, used at 10-30 pct concentration, to convert the sludge phosphates to na3po4. This sludge slurry is dried 2-24 h at 150 deg.-250 Deg. C, then sintered approximately 1 h at 800 deg.-1,200 Deg. C in an atmosphere of CO, ch4, or h2, to volatilize any zn and pb present and reduce fe and ni to metal powder. The pulverized sinter can then be leached with 4-15x its weight of h2o to dissolve na3po4, which can be crystallized out for recovery, while the insoluble residue is treated chemically or magnetically to separate fe and ni, and zn and pb are obtained mostly as fume condensed from gas evolved from the reduction roast. A solid reducing agent, preferably powdered coke, can be used instead of h2 in that roast. With h2 reduction a sludge containing fe 20, zn 7.5, Ni 0.6, PB 0.5, and po4 51 pct gave a product fume containing zn 80, fe 4.1, and pb 4 pct in an amount comprising 8.1 pct of the total, residue containing 86.2 pct fe and 3.55 pct ni comprising 11.8 pct, and na3po4 crystals containing 57.3 pct po4 on a dry basis comprising 78.8 pct, with some po4 remaining uncrystallized in the mother liquor. The residues with reduction by coke were very similar.
U.S. Pat. 3,653,875; Apr. 4, 1972; Chem. Abstr. 77:9,440R