Phosphate fertilizers, Chemistry tutorial


Phosphorus intake is necessary to the metabolism of both plants. This is an essential plant nutrient that is available in a native material (that is, phosphate rock), although in low quantity. In order to improve availability of phosphorus to plants, phosphate fertilizers are prepared commercially.

Phosphoric acid: Production and its place in the manufacture of phosphate fertilizers

Phosphoric acid plays an important role in the preparation of phosphorus fertilizers as it is the phase in the production of most commercial phosphate fertilizers. This is by either of two processes: a dry or wet method. In the dry method, rock phosphate is treated in an electric furnace. This treatment generates a very pure and more expensive phosphoric acid (it is generally termed as white or furnace acid) employed mainly in the food and chemical industry. Fertilizers that make use of white phosphoric acid as the P source are usually more expensive due to the costly treatment method.

The wet method comprises the treatment of rock phosphate by sulphuric acid. Phosphoric acid via the wet method is generated via the given steps namely: (a) Dissolution of the phosphate rock in sulphuric acid (b) Acidulated slurry is held till calcium sulphate crystals grow to sufficient size (c) Calcium sulphate crystals are separated from the acid via filtration and (d) Concentration of the acid to the preferred level. 

The phosphoric acid generated by either of these methods (wet or dry) is often heated, driving off water and generating a superphosphoric acid.

Classification of Phosphorous Fertilizers:

Different phosphate fertilizers have different solubility in soil solutions and are thus assimilated via plants differently. Accordingly to their solubility, they are categorized as water soluble, available or insoluble. Simple and double (that is, triple) superphosphates belong to the water soluble fertilizers. Precipitate, calcined phosphate fused phosphates and basic slag are the illustrations of available or insoluble fertilizers. Such insoluble fertilizers have non-readily available phosphorus salts that are soluble merely in strong acids.

Manufacture of Phosphate Fertilizers:

As we are familiar that manufacture of most of the commercial fertilizers begins with the production of phosphoric acid. The different steps used in the manufacture of different phosphate fertilizers are illustrated in the generalized diagram shown below: 

614_Manufacture of various phosphate fertilizers.jpg

Fig: Manufacture of various phosphate fertilizers

Let us in brief illustrate the manufacture of several phosphate fertilizers.

Normal Super Phosphate:

Raw materials needed for the manufacture of normal superphosphate are: a fairly high grade of phosphate that comprises 20%, 33.5% or more available phosphorus pent-oxide (P2O5) and strong sulphuric acid. Although what is a superphosphate? Soluble calcium phosphate made up by treating rock phosphate by sulphuric acid is termed as superphosphate.

Ca3(PO4)2 + 2H2SO4 + 2H2O → Ca(H2PO4)2 + 2CaSO4.2H2

Normal superphosphate is prepared by mixing equivalent quantities of powdered phosphate rock and chamber acid to a cast iron mixer provided by a stirring procedure. The mass is stirred for around 5 minutes and then allowed to remain for a day. The reaction is exothermic and as such temperature increases to around 100 to 110oC. A mixture of fumes comprising of HF, SiF and CO2 are evolved. Such gases form the material porous. As the reaction carries on the mixture stiffens and eventually set to a solid mass. Successive charges from the mixer are introduced to the pits till it is full and then allowed to stand for several days. Whenever it has become perfectly dry, it is dug out of the pit and in the meantime the second pit is filled. The gases from the mixture and pits are washed in two successive towers. The resultant hydrofluoric acid solution is then neutralized either via sodium carbonate or sodium fluoride and lastly treated by washed sand to form hydroflousilicic acid. The latter is further neutralized by sodium carbonate to make sodium silicoflouride (Ni2SiF6) or by magnesium to make magnesium silicoflouride (MgSiF6). The product is employed without further treatment other than breaking it up to the desired size. The major reaction is:

Ca10(PO4)6F2 + 7H2SO4 + 3H2O → 3CaH4(PO4)2.3H2O + 7CaSO4 + 2HF

Ammonium Phosphate:

a) Monoammonium phosphate is made up by the action of sulphuric acid on a mixture of calcium phosphate and ammonium sulphate. This is formed all along by slurry of CaSO4. The phosphate is separated from the slurry and crystallized to obtain fine crystals of Monoammonium phosphate containing around 12% nitrogen and 50% P2O5

Ca3(PO4)2 + (NH)4SO4 + 2H2SO4 → 2NH4H2PO4 + 3CaSO4

Ammonium phosphate is readily soluble in water and thus becomes available to crops readily. This is best appropriated for all the crops and soils and is as well slightly acidic in the nature.

b) Diammonium phosphate the other form of ammonium phosphate is made up by a continuous method in which anhydrous ammonia gas and nearly pure phosphoric acid are passed to saturated mother liquor having mono-ammonium phosphate. The temperature is kept at around 60-70oC and pH of around 6.0. The heat of reaction vaporizes water from the liquor and crystals of pure ammonium phosphate are made. Such are centifused, washed and dried.

NH4H2PO4 + NH3 → (NH4)2HPO4

The other phosphates comprise calcium metaphosphate, bonemeal (appropriate for acidic soils and long duration crops such as sugar cane and fruit crop) and thermal phosphates (P2O5 content differs from 19 to 24%).

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