Method for removing heavy metals in the manufacture of phosphoric acid

A method for manufacturing phosphoric acid from a calcium- and heavy metal-containing phosphate ore, consisting essentially of the following steps:

• (a) digesting the calcium- and heavy metal-containing phosphate ore with sulfuric acid to give an aqueous mixture of calcium sulfate, phosphoric acid, and heavy metal impurities, wherein the content of calcium sulfate in the aqueous mixture is from about 20 to about 40% weight/weight;
• (b) removing the calcium sulfate from the aqueous mixture to give a wet-process phosphoric acid solution containing from about 20 to about 45% weight/weight P2O5 and having a pre-treated heavy metal content more than about 10 mg M/kg P2O5;
• (c) treating the wet-process phosphoric acid solution with from about 100 to about 5000 ppm by weight on a 100% basis of a complex reagent to form a heavy metal complex-contaminated solid adsorbing material, wherein the solid adsorbing material of the heavy metal complex-contaminated solid adsorbing material is calcium sulfate,
• wherein the complex reagent has the following structure (I) or (II):
• wherein in structure (I)
• Q is a bond or O;
• R1 and R2 are independently H, or a linear or branched C1-4alkyl, wherein at least one of R1 and R2 is not H;
• and
• X is H, an alkali metal or ammonium,
• wherein in structure (II)
• Q is a bond or O;
• R1 is a linear or branched C1-8 alkyl;
• and
• X is H, an alkali metal or ammonium,
• wherein the phosphoric acid solution is maintained during treatment at a temperature between 20-60° C., wherein the temperature is sufficient to allow treatment time to be up to about 90 minutes without significant decomposition of the heavy metal complex, wherein the amount of complex reagent added is sufficient to lower the heavy metal content by at least about 10% at the temperature maintained, and wherein the solid absorbing material is present within the phosphoric acid solution from about 0.5 to about 5% weight/weight and not separately added;
• (d) removing the heavy metal complex-contaminated solid adsorbing material from the phosphoric acid solution to yield a treated phosphoric acid solution having a post-treated heavy metal content at least about 10% lower than the pre-treated heavy metal content;
• and
• (e) wherein the treated phosphoric acid solution of step (d) is recirculated in a heat exchange apparatus and used to cool the untreated wet-process phosphoric acid solution in step (c).