Abstract: A two-stage oxidation/reduction process for preparing commercially valuable, color enhanced and stabilized low viscosity wet process phosphoric acid is provided which yields improved acid that can be concentrated to superphosphoric acid and/or converted to high quality liquid fertilizer by reaction with ammonia, all without reversion to black color or excessive sludge formation in the fertilizer. The preferred process involves initially oxidizing a starting black acid having a 58-72% P.sub.2 O.sub.5 concentration by addition of an oxidizing agent such as NH.sub.4 NO.sub.3 with heating and agitation until the oxidized acid exhibits an EMF of greater than about 700 mv as compared with a 600 mv standard; the oxidized acid is then subjected to a reduction step to lower the valence of metallic impurities, by aging and/or adding reducing agents such as ferrous iron compounds to the acid.

Abstract: A process for treating wet-process weak phosphoric acid solution is disclosed which significantly lowers the magnesium impurities in the acid (as well as certain other impurities) permitting subsequent concentration of the acid to levels for use in preparation of liquid fertilizers which are less prone to settling out of the impurities during storage than would be the case without such treatment. It has unexpectedly been found that by limiting evaporation of water from the initial 26% to 32% P.sub.2 O.sub.5 wet-process acid to a level such that the acid has a P.sub.2 O.sub.5 content of from about 42% to about 45% and thereafter maintaining the semi-concentrated acid at a temperature within the range of from about 145.degree. F. to about 270.degree. F.