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Chemistry of Gypsum Pond Systems


Executive Summary

Previous studies on management of phosphoric acid gypsum ponds included attempts to describe chemical species and processes that influenced distributions of phosphorus, fluorine, and other elements in these systems. A lack of qualitative and quantitative information about the pertinent chemistry frustrated those efforts. The Florida Institute of Phosphate Research (FIPR) and the Tennessee Valley Authority (TVA) sponsored an investigation to gain information that will assist in better understanding the chemistry of gypsum pond systems.

Samples of gypsum pond return, discharge, and slurry waters were collected from operating Florida phosphoric acid producing plants in May 1988 and in December 1989. Densities, pH’s, and concentrations of P2O5, F, SO4-S, Si, Al, Ca, Fe,K, Mg, Na, NH4-N, and Cl were determined on filtered aliquots of the samples. Results showed a wide variance in pond compositions. For example, approximate ranges of measured pH’s were 1.4 to 1.9, %P2O5 ranged 0.5 to 2.7, %F from 0.3 to 1.3, %SO4-S from 0.12 to about 0.3, and %Si from 0.1 to 0.3.

Inspections of sample data indicated that the operating characteristics of individual acid plants strongly influenced pond water compositions. Results from correlation and factor analyses point to feed rock source, free sulfuric acid, and the presence or absence of ancillary ammoniated phosphate production as typical influential characteristics. Sample type (return, discharge, or slurry waters) had negligible influence on variations in mean sample compositions. The only significant seasonal effects detected for samples collected in this study were the mean density and F, Si, and K concentrations were higher for the samples collected in May 1988 than for samples collected in December 1989. No large, universal variations were observed in compositions of pond water samples collected at different seasons.

Solids suspended in collected samples and sediments produced during storage contained gypsum; the alkali fluosilicates, Na2SiF6 and NaKSiF6; chukhrovite, Ca4SO4SiAlF13 · nH2O; and a complex iron phosphate,
Fe3(NH4,K,H)H8(PO4)6 · 6H2O. Data evaluations suggested the latter compound was mostly, but not exclusively,
present in the ammonium form.

Sample aliquots were evaporated to increase concentrations and force precipitation of the principal saturating solid phases. Phases consistently detected in solids collected after the evaporations were gypsum and the alkali fluosilicates. Attempts to induce chukhrovite precipitation by addition of Al before evaporations were not successful. Results from mass balance experiments, conducted to check for inadvertent material losses that could have influenced distribution among the solution and solid phases, showed that the amounts of P, Si, and F in solutions and solids recovered after evaporations at 60°C equaled the amounts contained in the starting solutions within experimental error.

Pond water samples were treated with a variety of Ca and Al compounds to attempt removal of F by induced precipitation of F-containing solids. As expected, common neutralizing agents such as Ca(OH)2 and a clay-sized fraction from a dolomitic phosphate ore were most effective agents for reducing soluble F levels. Reactions of pond water with Al powder and AlPO4 also reduced soluble F concentrations. Treatments with the clay-sized fraction from a nondolomitic ore, an attapulgite clay, and Al(OH)3 did not cause reductions in soluble F concentrations.

Results from computations with the chemical equilibrium modeling program PHREEQE were in good qualitative agreement with other investigations. The P species with highest computed concentrations in pond water samples were H3PO4 and H2PO4-, consistent with the known dissociation reactions of phosphoric acid and expected distribution of species at low pH’s. The dominant F species predicted by the model computations was SiF6-2; this was the only F-containing species detected by NMR spectroscopy. Saturation indices computed by PHREEQE predicted gypsum and sodium fluosilicate to be the important saturating phases, and these were the most prevalent compounds found in sediments collected from pond water samples. There was good agreement between ion activity products of saturated species and solubility product constant values obtained from other sources.

Solubility product constants were estimated for complex compounds about which no information was available in the literature. Log K is estimated to be -78.77 for chukhrovite dissolution by the reaction

Ca4SO4AlSiF13 · 10H2O = 4Ca+2 + Al+3 + 4H+ + H4SiO4 + 6H2O + SO4-2 + 13F-.

Estimated Log Ksp values for the ammonium and potassium forms of Fe3(NH4,K)H8(PO4)6 · 6H2O are -140.9 and -141.6, respectively.