The Furnace Acid Process

furnace acidThe FAP is also an older process. It dates back to the 1890’s and was one of the first commercial techniques for making phosphoric acid but has since been largely displaced by WAP plants. Furnace acid is now only a few percent of all phosphoric acid.

  • Furnace acid is made by using large amounts of electrical energy to separate phosphorus from phosphate rock.
  • The Furnace Acid Process (FAP) uses carbon to reduce phosphorous to elemental phosphorous (P2); the carbon oxidizes to carbon monoxide (CO) and calcium silicate (CaSiO2) is formed.
  • The elemental phosphorus is subsequently oxidized, to phosphorus gas and is then absorbed in water to produce phosphoric acid (H3PO4).
  • FAP is an energy intensive process owing to the energy required for the high temperature (1500° C) carbothermal reduction to produce P2 and the difficulty to integrate this energy intensive step with the energy produced by the oxidation reaction to form P4O10.
  • This route for making purified phosphoric acid is no longer cost effective in many locations except China owing to electricity costs and environmental restrictions.



Electric phosphate smelting furnace used in the making of elemental phosphorus in a TVA chemical plant in  Muscle Shoals, Alabama

  • Almost all furnace acid plants were built in locations where inexpensive power was available, e.g. Tennessee River, Niagara Falls and Idaho.
  • FAP plants successfully completed with WAP plants until the late 1960’s when the demand for fertilizer accelerated.
  • The WAP scale advantages and the lack of low cost electricity made FAP non-competitive with WAP.
  • The production of elemental phosphorous by FAP is used almost exclusively in high margin markets.
  • Only one plant remains in the US in the state of Idaho.
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