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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. The reduction is carried out until the EMF of the product is less than 700 mv relative to the standard and a stabilized green color is produced in the acid. The final acid product may be concentrated if in the form of orthophosphoric acid, or cooled and directly reacted with ammonia if the product is SPA.

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