Removal of fluoride contaminant in phosphate fertilizers through solid State thermal treatment

[Display omitted] •Solid state thermal de-fluoridation of commercial phosphate fertilizers demonstrated.•57–89 % and 53–93 % residual fluoride eliminated from SSP and DAP, respectively.•Accompanying pH changes observed and rationalized.•Fluoride removal from SSP ascribed to HF/ H2SiF6 formation and volatilization.•Fluoride removal from DAP ascribed to (NH4)2SiF6 decomposition and volatilization.•Scheme for compensation of ammonia loss from DAP proposed. It was reported by us in 2018 that phosphate fertilizers marketed in India are contaminated with fluoride (0.85 % w/w on average). A follow-up study conducted in 2019 did not reveal any improvement in the situation. It is reported herein that 57–89 % w/w residual fluoride in as-received single superphosphate (SSP) powder samples could be removed through thermal treatment at 140 ± 5 °C. Fluoride removal was ascribed to formation and escape of gaseous acidic fluoride constituents, with concomitant increase in the fertilizer pH from 4.7-5.8 to 6.0–6.5. The pH increase was likely due to a shift of the H2PO4- ⇌ HPO42- equilibrium to the right. A marginal reduction in the sample mass (4.2–10.2 % w/w) was also observed. Similar thermal treatment given to as-received granular diammonium phosphate (DAP) samples reduced the fluoride content by 57–93 % w/w. The reduction was ascribed to removal of (NH4)2SiF6, which has lower decomposition temperature than DAP. The pH of the DAP samples reduced from 8.2–8.5 to 6.5–7.7, the sample mass reduced by 7.4–16.6 % w/w, and in one sample subjected to N, P analysis, the N/P ratio reduced from 1.16 to 0.71. These observations were consistent with a partial loss of ammonia and a shift of the HPO42- ⇌ H2PO4- equilibrium to the right. Exposure of a heat-treated DAP fertilizer to ammonia vapour raised the pH to 8.1. This suggested that, if fluoride impurities can be separated out from the gaseous emission released on thermal treatment, the lost ammonia can, in principle, be recycled to regenerate the original DAP through gas-solid reaction.