🔺 Single and Triple Superphosphate

Single and triple superphosphates are key phosphatic fertilizers produced by converting insoluble rock phosphate into more plant-available forms. This lesson explains their manufacturing basis, reactions, and handling considerations.

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manufacturing Super phosphate

The term super phosphate is generally applied to phosphates which are present

in a readily available form to plants. After the resolution passed in the meeting of the

first world International congress on chemical fertilizers, 1938, the term super phosphate

is restricted to refer those phosphates having less than 25% of P2O5 and the term

concentrated SSP for all those phosphates containing more than 25% P2O5.

The super phosphates are generally manufactured from phosphate rock, but

bones and phosphatic guanoes are also extensively used for the manufacture. The

super phosphates prepared from organic sources (bones & guano) are said to be having

good physical condition, but with a lower P2O5 content. In recent times only the

phosphate rock is being utilized for the manufacture of various super phosphate.

Phosphate rock containing less than 50% of tricalcium phosphate are not

suitable for manufacture of phosphatic fertilizers and are not used in the industry.

Phosphate rocks containing > 4% total R2O3 are considered unsuitable and the

phosphate rock should not contain > 2% iron oxide.

The following method are commonly followed to convert the insoluble phosphate

found in RP into more soluble forms.

1. Acid treatment method.

2. Phosphorus volatilization method

3. Decomposition with an alkali or silicate, and

4. Method followed in steel industry.

a. Ordinary super phosphate

This is also called as Den super phosphate. Often the term ‘super’ is used to

refer only to this product. This is the most extensively used fertilizers among the

farmers.

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Manufacture

The principle involved in the process is the treatment of RP with acids, mainly

H2SO4 to convert the insoluble and unavailable tricalcium phosphate to available

dicalcium and monocalcium phosphates. Concentrated HNO3, HCl, H3PO4 and H2SO4

can be used, but sulphuric acid is found to be more suitable because of the fact that

when it reacts with the RP, CaSO4 is formed, which is a dehydrating agent and improves

the physical condition of the final product to a better condition.

The method consisted of mixing equal quantities of RP with H2SO4 in open tanks

and then powdering the resultant product after the reactions are over. The calculated

quantity of H2SO4 and ground phosphate rock (material passing through a four-mesh

sieve only has to be used) are introduced and mixed very thoroughly and then dropped

in a pit or den. The process is also called Den method. Sufficient time of 24 to 36 hrs is

allowed for the completion of the reaction. Temperature rises to 100°C and there is a

loss of volume by 10% due to evaporation of water, CO2 and F. The CaSO4 act as a

dehydrating agent and artificial drying is also done wherever necessary. Then the

product is ground to a very fine powder and stored for sufficient time for the curing of the

material.

(mcp)

Precautions

1. An excess quantity of H2SO4 should not be used.

2. The quality of the RP must be pre decided.

Properties

The commercial super phosphate is grey in colour and it is only partly soluble in

water. It has an acid odour and acid reactions due to the presence of some free acids

and the acid salt monocalcium phosphate. It addition to the presence of phosphate, it

contains gypsum, silica and aluminum sulphate, CaF etc. It contains about 20% Ca and

12% Sulphur which are the secondary nutrient elements required for the plants.

b. Concentrated super phosphate

The main principle involved in the preparation is to first manufacture, phosphoric

acid (H3 PO4) by treating the RP with excess quantity of HCl or HNO3 or H2SO4 and

secondly the H3PO4 so obtained is used for the treatment of another quantity of

phosphate rock to get this fertilizer. Hence, there are two acid treatments involved and

so the product in called double super phosphate (double insulation). Following are the

main steps in the manufacturer of concentrated super phosphate.

1. Preparation of H3PO4 using RP and one of the mineral acids.

2. Preparation of the concentrated super phosphate by treating fresh quantity of

RP with the prepared H3PO4.

PR is ground and allowed to pass through a constant weight feeder to which a

measured flow of H3PO4 under pressure enters. All the mixing is done in the funnel like

mixer and the mixture is allowed to stand for sufficient time for the reaction to take place.

The slurry is conveyed to a troughed belt conveyor where the product sets in a few

seconds to plastic mass. To get a dried material, sufficient retention time is allowed and

at the end of the belt- conveyor, a rotary disintegrator reduces the lumps to less than 2”

size. The disintegrated super phosphate a stored for curing at least for a period of 15

weeks.

Properties

This fertilizer resembles in all respects the ordinary super phosphate. It is grey in

color, a major portion being soluble in water. It contains 45 to 48% water soluble P2O5.

It contains lesser quantity of other ingredients.

Thermophosphates

These are manufactured by heating phosphate rock in varying temperatures.

The main drawbacks in thermal phosphates are

They are generally more expensive than acid derived phosphates. It contains no water

soluble P. They do not have any value in the manufacture of NPK fertilizers

Polyphosphates

Differ slightly from the more common orthophosphate fertilizers. Nearly all of the

liquid fertilizers containing P are of the polyphosphate type. Polyphosphates are

composed of a series of orthophosphate molecules connected by the process of

dehydration (removal of water). Commercial ammonium polyphosphates are usually a

mixture of ortho- and polyphosphate. With prolonged storage, polyphosphates will

hydrolyze to orthophosphates. Solutions of ammonium polyphosphate most commonly

made are 10-34-0 and 11-37-0. The most common dry polyphosphate is 13-52-0.

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Summary Cheat Sheet

Topic	Key exam point
Main products	Single superphosphate (SSP) and triple superphosphate (TSP)
SSP origin	Produced by acidulation of rock phosphate with sulphuric acid
TSP origin	Produced by acidulation of rock phosphate with phosphoric acid
Key difference	TSP has higher phosphorus concentration than SSP
SSP bonus nutrient	SSP also supplies sulphur and calcium along with phosphorus
Product class	Both are water-soluble phosphatic fertilizers
Related groups	Thermophosphates and polyphosphates are separate phosphatic fertilizer categories
Agronomic note	Used as basal phosphatic fertilizers for crop establishment
Exam distinction	SSP and TSP differ mainly in acid used for manufacture and final P concentration
Trap	Do not confuse TSP with DAP or complex fertilizers

References

3 sources • [1] [2] [3]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook

Book

[3]

Department of Agriculture & Farmers Welfare

Website