Phosphate rock and common fertiliser salts
Using resources • The Haber process and fertilisers
Flashcards
Test your knowledge with interactive flashcards
Key concepts
What you'll likely be quizzed about
Phosphate rock composition
Phosphate rock consists mainly of calcium phosphates, commonly written as Ca3(PO4)2. The phosphate anion (PO43−) supplies phosphorus, an essential macronutrient for plant growth. Natural rock is largely insoluble, so chemical treatment with acids converts phosphate into water-soluble forms that plants can absorb. Solubility determines usefulness as a fertiliser because only soluble phosphate is available to plants. The physical form of the rock and impurities influence the yield of soluble phosphate during treatment.
Reaction with nitric acid - calcium nitrate
Reaction of phosphate rock with nitric acid produces calcium nitrate, Ca(NO3)2, and phosphoric acid. The process supplies nitrogen in a soluble nitrate form that plants absorb readily. The nitrate salt functions as a nitrogen fertiliser rather than a phosphate-rich product. Cause → effect: Strong nitric acid reacts with calcium phosphate (insoluble) to form soluble calcium nitrate; soluble nitrate increases available nitrogen in soil while leaving phosphoric acid that can be processed further.
Reaction with sulfuric acid - single superphosphate
Reaction of phosphate rock with sulfuric acid produces single superphosphate (SSP). Single superphosphate consists mainly of water-soluble monocalcium phosphate (the useful phosphate) and calcium sulfate (gypsum) as an insoluble by-product. The gypsum acts as a filler and can improve soil structure in small amounts. Cause → effect: Sulfuric acid partially converts insoluble calcium phosphate to soluble monocalcium phosphate, creating a product that supplies phosphate to plants but includes gypsum, which dilutes the phosphate concentration.
Reaction with phosphoric acid - triple superphosphate
Reaction of phosphate rock with concentrated phosphoric acid produces triple superphosphate (TSP). Triple superphosphate contains a higher proportion of soluble phosphate (for example monocalcium phosphate) and therefore provides more phosphorus per mass than single superphosphate. The product is more concentrated because phosphoric acid introduces phosphate without producing large amounts of insoluble by-products. Cause → effect: Phosphoric acid converts the rock’s phosphate into a soluble phosphate salt with minimal formation of gypsum, so the final fertiliser contains a higher percentage of available phosphorus.
Practical and environmental limiting factors
Availability of phosphate rock limits large-scale production because phosphate rock is a finite natural resource. Acid concentration, impurity levels in the rock, and processing conditions influence yield and product purity. Cost and safety of handling strong acids affect industrial choice of route. Environmental effects influence selection of treatments: nitrate production increases risk of nitrate leaching, sulfates produce gypsum waste, and mining of phosphate rock causes habitat disruption. Recycling and efficient use of phosphorus reduce reliance on mined rock.
Key notes
Important points to keep in mind