Phosphate rock is the only source of phosphorus chemicals, including fertilizers. The starting point is fertilizer-grade phosphoric acid (13.833 MMT P2O5 content).27 Of 1988 production, exports—mostly ammonium phosphates—accounted for 2.608 MMT, leaving 10.549 MMT for domestic consumption. Most of the latter, 9.329 MMT, or 88.4 percent, was converted into "wet process" phosphoric acid (H3PO4). Elemental phosphorus production in the United States in 1988 was 0.32 MMT (0.73 MMT P2O 5 equivalent), of which 85 percent was reconverted to pure furnace-grade phosphoric acid for chemical manufacturing. Some of this is used to make triple superphosphate fertilizer, but about 40 percent was used to manufacture sodium tripolyphosphate (STPP, Na5P3O10), a detergent builder. Production of this chemical in 1988 was 0.497 MMT, with a P2O5 equivalent of 0.309 MMT. The detergent industry has been shifting to an alternative, tetrasodium pyrophosphate, which contains less phosphorus. Some phosphoric acid is used as a flavoring agent in the food and soft drink industry.
A minor but growing use of phosphorus is in the manufacture of lubricating-oil additives such as zinc dithiophosphate. This use accounted for 0.015 MMT of phosphorus metal in 1974; we estimate 0.02 MMT in 1988, corresponding to about 0.05 MMT P205 equivalent.
The starting point for organic phosphate synthesis is phosphorus trichloride (PCl3). Production figures are not published, but assuming 1 percent of chlorine output goes to PCl3 production (see discussion of chlorine below), we conclude that about 0.03 MMT of phosphorus metal, or 0.07 MMT P2O5, would have been required. The trichloride is later converted to phosphorus oxychloride (POCl3) by direct reaction with chlorine and phosphorus pentoxide P2O5. The oxychloride, in turn, is the basis of organic phosphate esters that now have many uses. The most important of these uses is in the manufacture of the plasticizer tricresyl phosphate. Phosphate esters are also used as flame retardants and fire-resistant hydraulic fluids. The United States used 0.043 MMT (P2O5) of phosphate esters in 1990 (International Trade Commission, 1991). Usage in 1988 was probably similar.
All of the uses mentioned above are inherently dissipative. No more than 0.2 MMT of elemental phosphorus is embodied in chemicals used to manufacture other chemical products, mainly detergents.
Elemental chlorine and sodium hydroxide (NaOH, or caustic soda) are coproduced by electrolysis of sodium chloride (salt), mainly in the form of brine (Figure 7). In 1988, U.S. salt production was 35 MMT, of which 18.1 MMT was consumed by chlor-alkali producers. The United States produced 10.21 MMT of chlorine and 9.55 MMT of sodium hydroxide (Chemical and Engineering News, 1992).28 The wastes from this process, mostly spent brines, amount to about 15 percent of the weight of the products, or 3 MMT. The mass balance is made up from about 5 MMT water on the input side and 0.3 MMT hydrogen gas released from the electrolytic cells. The latter is generally burned on site for energy recovery.