Triple superphosphate economics

The simplest method for producing sodium phosphates involves the neutralization of phosphoric acid with either sodium hydroxide or carbonate, but, unless some waste source of alkali is found, it may not be the most economical. For this reason other methods of producing sodium phosphates were investigated. The two most promising methods involve conversion of monocalcium phosphate (or normal or triple superphosphate fertilizer) to sodium phosphate by ion exchange, and neutralization of phosphoric acid with dilute sodium hydroxide produced electrolytically from brine. 

The use of normal or triple superphosphate as a raw material for phosphate should have a distinct economic advantage over phosphoric acid. Because the procedure yields monosodium phosphate, additional savings would be realized from the decreased ammonia consumption. 

Analyses of compound fertilizer rose further as triple superphosphate replaced ordinary superphosphate, and ammonium nitrate or urea replaced ammonium sulfate or sodium nitrate. Later, still higher analyses were attained by the liberal use of phosphoric acid and ammonia in the formulations. Compound fertilizer preparation was no longer a simple mechanical task but a complex chemical engineering operation. As such, its economics depended on large-scale operation. Some small local mixers expanded to large regional manufacturers others became bulk blenders or went out of business. 

A higher grade of fertiliser known as triple superphosphate is made by the action of wet process phosphoric acid on ground phosphate rock. The reaction may be summarised by Equation 12.3. The product contains a higher proportion of available phosphate than ordinary superphosphate, and its manufacture and use has become more economic than the latter, over the past two decades. On the industrial scale, reactions such as (12.1) and (12.3) do not go to completion immediately, and reaction periods up to 30 days may be required under some conditions. 

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