SSP (single superphosphate)

Single superphosphate (SSP) stands as the first industrially manufactured synthetic mineral fertiliser, emerging in the 1840s in the United Kingdom. It is a straight fertiliser containing phosphate, produced by treating phosphate rock with sulphuric acid. Its chemical make-up combines monocalcium phosphate [Ca(H2PO4)2] and hydrated calcium sulphate (gypsum, CaSO4·2H2O). The product contains 16% water-soluble P2O5, 12% S in sulphate form and 21% Ca^[1]. Other sources cite a range of 15 to 22% P2O5^[2]. SSP thus represents a worthwhile source of secondary and tertiary nutrients.

Manufacture

The process involves finely grinding phosphate rock and reacting it with 65–75% sulphuric acid^[3]. The overall reaction is: Ca10F2(PO4)6 + 7 H2SO4 + H2O → 3 Ca(H2PO4)2·H2O + 7 CaSO4 + 2 HF^[3]. This exothermic reaction reaches 90–100 °C and continues for several days during the curing stage^[3]. Unlike the wet-process route for phosphoric acid manufacture, the SSP process retains calcium sulphate in the finished product^[3]. The paste from the reactor is discharged onto a slow-moving conveyor (the "den") or into a holding tank for 10 to 40 minutes. The superphosphate is then ground, after which it is sent either to a granulation line or to storage for a curing period of one to six weeks^[3]. The quality of the phosphate rock affects phosphorus solubility: the presence of aluminium, iron or magnesium reduces it^[3]. The sulphuric acid may be virgin acid (produced from elemental sulphur, pyrites or recovered from non-ferrous metal metallurgy) or recycled spent acid^[3].

Properties and agronomic uses

SSP has a critical relative humidity of 93.7% at 30 °C and an angle of repose of 26°^[1]. Its high water solubility makes it an effective base fertiliser, used on its own or incorporated into blends^[1]. The calcium it contains is of particular interest for certain crops such as groundnut during pod formation^[1]. The sulphate sulphur is directly available for plant uptake. SSP must not be mixed with calcium ammonium nitrate (CAN) or urea unless the blend is applied immediately without storage^[1].

Market and trade

SSP holds a niche position in the global phosphate industry, which is dominated by phosphoric acid-based fertilisers (DAP, MAP, TSP)^[2]. Superphosphates (SSP and TSP) accounted for one quarter of world phosphate fertiliser output according to early 2000s data^[4]. Figures for 2020 indicate SSP production of 19 Mt and exports of 2 Mt^[2]. These volumes fall well short of the main phosphate fertilisers: in 2023, DAP recorded 85 Mt produced and 8 Mt exported, MAP 35 Mt produced and 16 Mt exported, TSP 30 Mt produced and 12 Mt exported^[2]. Phosphate rock production, the essential raw material, totalled 216 Mt in 2023, including 30 Mt exported^[2]. During the 1999/2000 season, world consumption stood at 6.1 Mt of P2O5 for SSP and 2.2 Mt of P2O5 for TSP^[4].