All phosphates are made from the same compound, phosphoric acid. Production of phosphoric acid or any of the various phosphates discussed here are produced by one of only two general methods:
- The wet process
- The furnace process
The wet process takes phosphate rock through a sulfuric acid digestion. Impurities are precipitated out and after filtering, a “green phosphoric acid” is produced. This is usually used in fertilizers. To produce phosphates by the furnace process, high quality phosphate rock is heated in an electric furnace to vaporize elemental phosphorous. The phosphorous is then burned to produce P2O5. This phosphorous pentoxide is collected and dissolved in water or dilute phosphoric acid to produce strong phosphoric acid solutions. This procedure is energy intensive but produces the highest quality phosphoric acid which is the ortho phosphate starting point for all phosphate salts used in water treatment. Under carefully controlled conditions combining sodium or potassium salts, temperature and time, ortho phosphate is converted to condensed phosphates. Condensed phosphates are sometimes called metaphosphates but are generally called polyphosphates. At 320 degrees F, ortho phosphate is condensed to pyrophosphate. At 460 degrees F and the appropriate time, pyrophosphate will condense further to form trimetaphosphate. Increasing the temperature to 1200 degrees F further condenses the trimetaphosphate followed by a rapid quench to create hexameta and higher metaphosphates.
Condensed phosphates are all phosphates comprised of multiple phosphate groups. The simplest is pyrophosphate with two groups. The term “condensed phosphates” originates from the name of the reaction forming them. Several independent phosphate groups condense into a larger group. During this phosphate condensation, water is driven off. Condensed phosphates are therefore sometimes referred to as dehydrated phosphates.
The extent of the condensation determines the physical appearance of the molecule formed. The physical appearance is sometimes used as a descriptive term. The crystalline phosphates comprise the pyro’s, tripoly’s and trimeta- phosphates. The glassy phosphates comprise the hexameta and longer chain phosphates. The commercial hexametaphosphates are mixtures of various phosphate chain lengths. Most retail suppliers of water treatment products will buy the dry powders from manufacturers and then mix various combinations of ortho, pyro, tripoly and hexametaphosphates together to make powder and liquid formulas.