1. Ground Phosphate Rock
2. Partially Acidulated Phosphate Rock
3. Enriched Superphosphate
4. Potassium Phosphates
5. Meat & Bone Meal (MBM)
6. Dicalcium Phosphate
7. Use of phosphoric acid in various fertiliser processes

Ground Phosphate Rock

Ground phosphate rock is only classified as a fertiliser when the P₂O₅ has a formic acid solubility of >55% (soft rocks). It is regarded as a “slow-release fertiliser” where the rate of release is very much dependent on the soil acid concentration. The principal advantage of ground phosphate rock is its low cost and its ability to neutralise acidic soils.

The main disadvantages are uncertainty as to agronomic value, inconvenience of handling and applying the fine, dusty material, and relatively low P₂O₅ content compared with Triple superphosphate (TSP) or ammonium phosphates.

It is generally agreed that ground phosphate-rock is effective only on acid soils (pH 6 or less). This statement applies to apatite rocks. The rock should be finely ground and well mixed with the soil.

Ground rock can be granulated after grinding with use of a binder in a blunger type of granulator. The granular products is easier to handle and creates less nuisance.

Partially Acidulated Phosphate Rock

In some instances, an indigenous phosphate rock may prove unsuitable for direct application and require the addition of a more soluble form of phosphate. An alternative is partial acidulation to render its P₂O₅ more available. This option is of particular interest since the crop response is often similar to that obtained when using fully acidulated products such as Single superphosphate (SSP) or TSP. In this situation, less acid is used, and production capacity may be increased.

The partially acidulated phosphate rock (PAPR) process depends on treating ground phosphate rock with only a portion of the stoichiometric value of acid, for example 50% PAPR. The amount of water-soluble phosphate in PAPR varies according to the degree of acidulation.

Sulfuric and phosphoric acids are the most common acids used for partial acidulation.

Enriched Superphosphate

“Enriched” superphosphate is essentially a mixture of SSP and TSP, usually made by acidulation of phosphate rock with a mixture of sulfuric and phosphoric acids.

Enriched superphosphate may be a useful product for application in sulphur-deficient areas where SSP would supply more sulphur than necessary. One advantage is that mixed acid of the proper concentration can be obtained by mixing concentrated sulfuric acid (93% or 98% H₂SO₄) with dilute phosphoric acid (30% P₂O₅), thereby avoiding the need for concentrating the latter.

Potassium Phosphates

(Note: this section is referring to pure products such as MKP, not blended mixes of P and K.) Potassium phosphates are excellent fertilisers, but their use is limited to special purposes for which the high cost can be justified. At present, most of the potassium phosphates used in fertilisers are produced from potassium hydroxide or carbonate and phosphoric acid and are used in liquids for foliar application or other specialty uses. Some of the alternative salts of potassium phosphates are given in Table 1.

In a process used in China (and previously in Israel), potassium chloride is treated with concentrated sulfuric acid to produce potassium bisulphate [1, 2]:

KCl + H₂SO₄ à KHSO₄ + HCl

After removal of hydrochloric acid as a dry gas, the potassium bisulphate is reacted with rock phosphate to produce a monopotassium phosphate (MKP). The latter can be ammoniated to obtain a chloride-free NPK fertiliser, such as 8-48-16, or monopotassium phosphate can be marketed separately.

Meat & Bone Meal (MBM)

A relatively small amount of bone meal is used as fertiliser mainly by home gardeners. It is much too expensive for farm use. After Incineration of MBM it turns into MBM ash (MBMA). This can be the route to get rid of contaminated product  MBMA can be used as a relatively high concentrated and pure substitute for rock phosphate.

Dicalcium Phosphate

Dicalcium phosphate is a common constituent of nitro phosphate fertilisers and of compound fertilisers formed by ammoniation of superphosphates. There is a relatively small but substantial production of straight dicalcium phosphate in Europe, which is based on use of by-product hydrochloric acid and lately also from waste sewage sludge ashes. The process consists of dissolving phosphate rock or P-containing ashes in hydrochloric acid and then precipitating dicalcium phosphate by stepwise addition of limestone and slaked lime. The product is recovered by filtration and washing, and the remaining solution of calcium chloride may be used or discarded.

Various other methods for producing dicalcium phosphate are known, but none are known to be used commercially for fertiliser production. Direct neutralisation of pure or defluorinated phosphoric acid with lime or limestone is used to produce feed- or food-grade dicalcium phosphate.

Use of phosphoric acid in various fertiliser processes

Use of phosphoric acid in granulation processes

Phosphoric acid often is used in granulating compound fertilisers to supply part of the P₂O₅ in formulations in which the remainder of the P₂O₅ is supplied by single or triple superphosphate or solid ammonium phosphates. The acid usually is sprayed into the granulator, and ammonia or ammoniating solution is added during granulation to neutralise it. The heat of reaction promotes granulation and moisture evaporation. Both merchant-grade (54% P₂O₅) and super phosphoric acid (69%-72% P₂O₅) have been used in this way. This approach is described in the section on NPK fertilisers.

Miscellaneous fertiliser uses for phosphoric acid

A relatively small portion of phosphoric acid is used in the production of potassium phosphates (MKP), made from KOH and phosphoric acid. Sometimes phosphoric acid is used for direct application to the soil via drip irrigation, especially in alkaline soils.

Slow-release fertilisers, for example MgNH₄PO₄ and MgKPO₄ (or mixtures of the two salts), represent another use. The products, called ammonium struvite and potassium struvite respectively, are on the market as by-products. In water treatment stations sewage water, containing NH₄^– and PO₄, is mixed with a soluble Mg-salt to precipitate the ammonium struvite. A mixture of potassium struvite and ammonium struvite is sometimes recovered from manure. This technology is getting more attention due to the Circular Economy approach in which finite resources like phosphate are being recovered and brought back into the circle. 

Compounds of the general formula (M)NH₄PO₄, where M may be a divalent metal ion, such as Fe, Mn, Cu, or Zn, have been tested as slow-release sources of micronutrients.

Urea phosphate CO(NH₂)₂• H₃PO₄ is an interesting compound that is produced on a small scale in the middle East and used in glasshouse fertigation.

References

1. Fittel, R. S., and L.A. Hollingsworth. 1977. “Manufacture of Ammonium Phosphates Using a Pipe Reactor Process,” Proceedings of the 27th Meeting of the Fertilizer Industry Round Table, pp. 70-80.

2 “The Cros Fertilizer Granulation Process.” 1977. Phosphorus and Potassium, 87:33-36.

Links to related IFS Proceedings

2, (1947), Granulation of Phosphatic Fertiliser – Theory and Practice, Sven Nordengren

13, (1951), Theory and Practice in the Treatment of Phosphate Rock with Nitric Acid, M H R J Plusjé

21, (1953), Manufacture of Triple Superphosphate, J J Porter, J Frisken

23, (1953), Ammoniation of Superphosphate, J Angus

76, (1963), Potassium Metaphosphate: A Novel Method of Manufacture and a Summary of its Behaviour as a Fertiliser, F J Harris

91, (1966), NPK Fertiliser Production Using Superphosphoric Acid, G Bischofberger, R R Heck

92, (1966), Phosphorus and Potassium Fertilisers: Their Forms and Their Places in Agriculture, G W Cooke

112a, (1969), New Phosphoric Acid Processes – Symposium, S M Janikowski, N Robinson

112c, (1969), Kellogg-Lopker Phosphoric Acid Process, W C Weber, E J Roberts, I S Mangat, E Uusitalo

138, (1973), The Shipment of Phosphatic Materials – Some Technical and Economic Aspects, W T Charlton, J D Crerar, R C Akroyd

237, (1985), Production of Chloride-Free NPK Fertiliser and Feedgrade Dicalcium Phosphate, K C Knudsen

269, (1988), Phosphoric Acid – Wet Process: What Process? A Guide to Process Selection for Phosphoric Acid Manufacture by Sulphuric Acid Dissolution, P A Smith

364, (1995), Partially Acidulated Phosphates – Production, Agronomic and Environmental Aspects, Y Pelovski, M K Garrett.

432, (1999), Speciality Mineral and Organo-Mineral Fertilisers – Products and Markets, A Rainbow

763, (2015), Review of Promising Methods for Phosphorus Recovery and Recycling from Wastewater, C Kabbe, C Remy, F Kraus