Laboratory evaluations of feed-grade and agricultural-grade phosphates

Nine samples of pure, feed-grade (FP) and agricultural-grade (AP) phosphate s were evaluated at seven laboratories (six in Brazil and one in the U.S.) for physical and chemical characteristics. Phosphates were one "standard" p ure dicalcium phosphate; four FP, two dicalcium phosphates (FP-1 and FP-2) made in Brazil, one di-monocalcium phosphate (FP-3), and one defluorinated phosphate (FP-4) made in the U.S.; and four AP made in Brazil [single super phosphate (AP-1), triple superphosphate (AP-2) and monoammonium (AP-3), and thermo-magnesium (AP-4) phosphates]. Average analytical values for FP and AP, respectively, were 3.3 and 6.3% moisture, 1.0 and 2.5% insoluble residu e, 16.2 and 28.4% loss on ignition, 6.8 and 4.7 (pH), 1,028 and 1,023 g/L a pparent density, 9.6 and 55.0% P solubility in water, 83.6 and 88.4% P solu bility in 2% citric acid, and 85.2 and 97.0%:P solubility in neutral ammoni um citrate. Based on particle size, six products were classified as "fine," and three were classified as "irregular." Atomic absorption and plasma spe ctrometry determinations were performed for 31 essential and potentially ha rmful or radioactive minerals. The Na level was high in FP-4 (6.03%). Miner al concentrations were safe for all FP as compared with NRC standards. Leve ls in AP were toxic, exceeding the tolerance limits for F, Fe, Mg, and Ba, and were particularly high as compared with FP for S, Ti, and radioactive T h. The AP-1 was high in F, Ba, S, and Tn; AP-2 and AP-3 were high in F and S; and AP-4 was high in F, Ba, Fe, Mg, Ti, and Tn. X-ray diffraction assays detected impurities for all commercial samples and identified as major com ponents CaHPO4. 2H(2)O (standard phosphate), CaCO3 and CaHPO4 (FP-1, FP-2, and FP-3), Ca(H2PO4)(2). H2O (FP-3), Na2Ca3Al2(PO4)(2)(SiO4)(2) and Ca-3(PO 4)(2) (FP-4), CaSO4. nH(2)O and (NH4)Fe3P6O20.(PO4)(2) (AP-1), Ca(H2PO4)(2) . H2O and KFe3P6O20. 10H(2)O (AP-2), (NH4)H2PO4 and CaSO4. nH(2)O (AP-3), a nd no definite molecular structure for AP-4, an amorphous product. The biol ogical consequences of feeding animals a mineral source with no definite mo lecular structure, an amorphous product, is not known. A biological evaluat ion of all phosphates included in this article is being published as a sepa rate report (Fernandes et al., 1999).