Abstract: To remove lead, cadmium and zinc from dusts produced in the electrostatic precipitator in the electrothermal production of yellow phosphorus, the dusts are first digested by phosphoric acid having a content of at least 25% by weight of P.sub.2 O.sub.5 at temperatures above 60.degree. C. in a ventilated reaction zone. The digest solution is then separated in a filter zone into an essentially lead-, cadmium- and zinc-free solid residue and a phosphoric acid containing dissolved lead, cadmium and zinc.

Abstract: To remove lead and cadmium from phosphoric acid, alkali solution and aqueous sulfide solution are simultaneously introduced with thorough mixing into a phosphoric acid having a P.sub.2 O.sub.5 content of up to 32% by weight, an iron content of up to 0.3% by weight and an alkali/P ratio of up to 0.30. The temperature of the phosphoric acid which is thoroughly mixed with alkali solution and sulfide solution is adjusted to temperatures of 58.degree. to 72.degree. C. with escape of the excess hydrogen sulfide. To complete the sulfide precipitation, the temperature is lowered to below 45.degree. C. Finally, the precipitated sulfides of the lead and cadmium are separated off from the phosphoric acid.

Abstract: Metal ions are reduced from a higher to a lower oxidation stage using ferrophosphorus as the reductant.

Abstract: Alkali metal phosphate solutions containing little vanadium are made from phosphoric acid containing vanadium. To this end, a negative redox potential is established in the phosphoric acid by addition of a reductant, the acid is partially neutralized and an alkali metal-phosphorus-molar ratio between 0.85 and 1.0 is established, the partial neutralization being continued over a period of at least 10 minutes. The partially neutralized acid is not filtered intermediarily and the alkali metal/phosphorus-molar ratio is increased to 1.9 to 2.1 in a second neutralization stage, collected precipitate is separated and processed, if desired.