Evaluating and quantifying the liming potential of phosphate rocks

Abstract

The liming potential of phosphate rock was evaluated with theoreticalcalculations and quantified by laboratory titration and soil incubation. Threeanions present in the carbonate apatite structure of phosphate rock that canconsume protons and cause an increase in pH when dissolved from apatite arePO₄ ³⁻, CO₃ ²⁻, andF⁻. The pKa for HF is so low that F⁻ has verylittle effect on increasing pH. The pKa for 2 protons onH₂PO₄ ⁻ and H₂CO₃are sufficiently high enough to cause an increase in pH withPO₄ ³⁻ and CO₃ ²⁻releasedinto solution if the pH range is between 4 and 6. Because of the greater molarquantity of PO₄ ³⁻ compared toCO₃ ²⁻, PO₄ ³⁻ exerts agreater affect on the liming potential of P rock. For a variety of phosphaterocks with a axes ranging from 9.322 to 9.374 Å in the carbonate apatitestructure, the theoretical % calcium carbonate equivalence (CCE) rangesfrom 59.5 to 62%. With the presence of gangue carbonate minerals from2.5to 10% on a weight basis in the phosphate rocks, the theoretical%CCE ranges from 59.5 to 63.1%. Use of AOAC method 955.01 forquantifying the %CCE of North Carolina phosphate rock (NCPR) and Idahophosphate rock (IDPR) resulted in %CCE ranging from 39.9 to 53.7%which were less than the theoretical values. The lower values measured in theAOAC method was presumed to be due to formation of CaHPO₄ orCaHPO₄·2H₂O precipitates which would result inlessthan 2 protons neutralized per mole of PO₄ ³⁻released from carbonate apatite. The highly concentrated solution formed in themethod was considered not indicative of a soil solution and thus determined%CCE values would be suspect. A soil incubation study was conducted todetermine a more appropriate %CCE value in a soil environment usingCopper Basin, Tennessee soil with a soil pH of 4.2. Agricultural limestone,NCPR, IDPR, and a granulated IDPR were added to 100 g of soil atrates of 0.1, 0.3, 1, 3, and 10 g/kg soil, incubated for 105 daysat field moisture capacity, and analyzed for changes in soil pH and P. The%CCE of each phosphate rock addition was determined using limestone as astandard curve. The relationship between %CCE and % dissolved Pfollowed a quadratic model where%CCE = 8.47 + 0.0078(%dissolved P)² (r² = 0.84).At 0% dissolved P, the model predicted 8.47% CCE which wasprobably due to gangue carbonate minerals. The experimental model showedqualitative agreement with theory showing increased liming ability withincreased dissolved P from the P rock. However, the model showed lower%CCE than theoretical calculations when %P dissolved ranged from20 to 60%.