1. Fundamentals of SSP
2. Suitability of Phosphate Rocks
3. Chemistry of SSP
4. Production Methods

Fundamentals of SSP

Single superphosphate (SSP), also called normal or ordinary superphosphate, has been the principal phosphate fertiliser for more than a century and supplied over 60% of the world’s phosphate in the mid 20th century. Since then its relative importance has declined steadily. The decline in actual tonnage has been small, but most of the new facilities have been built to produce other, higher analysis products.

The growing interest of sulphate (SSP contains 12%S) due to the reduction of SO₂ deposition via air, has kept the market interested in SSP as a three-nutrient product (P, S and Ca).

The advantages of SSP are:

• The process is simple, requiring little technical skill and small capital investment.
• The economies of scale are minor; thus, small plants can be economical.
• The fertiliser effectiveness of SSP is unquestioned. In fact, it is a standard of comparison for other phosphate fertilisers
• There is no by-product gypsum that has to be disposed of.

Despite these impressive advantages, the disadvantage of low analysis, 16% – 22% P₂O₅, and consequent high distribution costs have caused declining interest in its production because the delivered cost at the farm level is usually higher per unit of P₂O₅ than that of Triple superphosphate (TSP) or ammonium phosphates.

SSP will still be a logical choice in several situations such as:

• Where both P₂O₅ and sulphur are deficient, SSP may be the most economical product to meet these needs. This is the case in much of Australia and New Zealand, some parts of the United States, and Brazil and nowadays more and more in Europe.
• In small countries or remote regions where demand is insufficient to justify an economical scale of production of concentrated phosphate fertilisers, and where importation is expensive, SSP can be the most economical means of supplying local needs.
• In many cases, SSP can be an attractive way to use by-product sulfuric acid that cannot be used to produce more concentrated products because the quality or quantity of the acid is unsuitable. Likewise, SSP can use deposits of phosphate rock that are too small to justify a more expensive plant or even from phosphorus containing ashes from sewage sludge, meat & bone meal, and wood.

Suitability of Phosphate Rocks

Since the grade of the rock determines the grade of the product SSP, a high-grade rock is desirable. Reactivity is also important; unreactive rocks must be ground more finely. It is extremely difficult to produce SSP from some igneous apatites. Iron and aluminium compounds can be tolerated up to a point, although they decrease the P₂O₅ water solubility. Silica has no adverse effect other than a decrease in grade and more wear in grinding equipment. An increase in the CaO: P₂O₅ ratio raises the sulfuric acid consumption per unit of P₂O₅ and decreases the grade. High-chloride rocks (up to 0.5% Cl and perhaps higher) can be used without serious disadvantage since corrosion is not a serious problem in SSP production.

Chemistry of SSP

The main overall chemical reaction that occurs when finely ground phosphate rock is mixed with sulfuric acid in the manufacture of SSP may be represented by the following equation.

Ca₃(PO₄)₂ + 2H₂SO₄ + H₂O -> Ca(H₂PO₄)₂ • H₂O + 2CaSO₄ + 108.44 kcal

It is generally agreed that the reaction proceeds in two stages: (1) the sulfuric acid reacts with part of the rock, forming phosphoric acid and calcium sulphate and (2) the phosphoric acid formed in the first step reacts with more phosphate rock, forming monocalcium phosphate. The two reactions take place concurrently, but the first stage is completed rapidly while the second stage continues for several days or weeks.

The calcium sulphate is mainly in the anhydrous form. Most of the phosphate in the rock is fluorapatite, and the fluorine reacts with the sulfuric acid to form hydrogen fluoride. The hydrogen fluoride reacts with silica in most rocks, and part of it is volatilised, usually as SiF₄. The remainder may form fluosilicates or other compounds in the SSP. Usually 25% or more of the fluorine is volatilised and must be recovered to prevent atmospheric pollution. In some cases, recovery as saleable fluorine compounds is feasible, but more often the scrubber liquor is recycled back into the granulation and acidulation stages.

Production Methods

The manufacture of superphosphate involves the following three (or four) operations.

1. Finely ground phosphate rock (90% <100-mesh) is mixed with sulfuric acid. With rock of 34% P₂O₅ content, about 0.58 kg of sulfuric acid (100% basis) is required per kilogram of rock. Sulfuric acid is available commercially in concentrations ranging from 77% to 98% H₂SO₄. The acid usually is diluted to 68% – 75% H₂SO₄ before it is mixed with the rock, or in the case of the cone mixer, the water may be added separately to the mixer. When concentrated sulfuric acid is diluted, much heat is generated; many plants cool the acid in heat exchangers to about 70°C before use in order to avoid a lumpy SSP run-of-pile (ROP).
   
2. The fluid material from the mixer goes to a den where it solidifies. Solidification results from continued reaction and crystallisation of monocalcium phosphate. The superphosphate is excavated from the den after 20-40 minutes. At this time, it is still somewhat plastic, and its temperature is about 100°C.
   
3. The product is removed from the den and conveyed to storage piles for final curing, which requires 2 – 6 weeks, depending on the nature and proportions of the raw materials and the conditions of manufacture. During curing, the reaction approaches completion. The free acid, moisture, and unreacted rock contents decrease, and the available and water-soluble P₂O₅ contents increase. The material hardens and cools. After curing, the product from storage is fed to a disintegrator, usually of the hammer mill or cage mill type. The product from the mill is discharged onto an inclined screen of about 6-mesh size. The material that fails to pass the screen is returned to the mill for further grinding.
   
4. If granular superphosphate is desired, the product is granulated either before or after it is cured. Granulation before curing has the advantage that less water or steam is required. After granulation, the product is dried in a direct contact dryer and screened; the fines are returned to the granulation unit.

For many years SSP was produced only by batch mixing methods; however, most modern plants use continuous mixing and denning processes. There is a wide variety of both batch and continuous mixers and dens; no attempt will be made to describe them all. More details may be found in [1].

Batch mixing has practically disappeared in favour of continuous mixing, but there are two cases where it remains very competitive. They are as follows:

1. When the only rock available is igneous rock, the batch mixing and associated denning system are preferred because they allow precise control of the mixing conditions and because the den can be made tight enough to contain the very fluid slurry delivered by the mixer.
2. When the purpose is to develop a small phosphate resource in a remote country, the batch process can be built at low cost and is easy to operate.

Figure 1 shows a flow diagram of a popular type of continuous den: The Broadfield den is a well-known example: The mixer may be a cone mixer, as shown, a paddle mixer (pugmill), or sometimes a cone mixer with a mixing impeller. Retention time in such dens ranges from 20 to 40 minutes and can be varied by changing the speed of the slat conveyor. This type of den is also suitable for making triple or enriched superphosphate.

The production of 1 tonne of SSP of 20% available P₂O₅ content would require 626 kg of ground phosphate rock (34% P₂O₅), 390 kg of sulfuric acid (93% H₂SO₄), and 90 kg of water. The reaction generates considerable heat. Approximately 8%-10% of the weight of the ingredients (water vapour and volatiles) is lost in the manufacturing and curing steps.

The capital cost for SSP production will vary widely depending on the battery limits adopted. The cost is much greater if the battery limits include the investment for sulfuric acid manufacture; in fact, SSP plants often use by-product sulfuric acid derived from a plant that is not a part of the SSP plant. Also, small SSP plants may purchase sulfuric acid from a larger plant that supplies several customers or use by-product sulfuric acid from other industries (e.g., metal industry, or organic industry).

Whether the SSP should be granulated depends on local preference. In some countries nongranular SSP is acceptable. Also, in many cases the SSP will be used as an ingredient for producing granular compound fertiliser.

When granular SSP is desired, the ex-den granulation system described under non-granular MAP is suitable. According to Sinte Maartensdijk, the recycle ratio in this plant was 0.63:1.0 [2].

References

1. Houghtaling,  J.,  and V. J. Margiotti,  Jr. 1994. “‘Advanced  Ammonia  Phosphate   Scrubbing  With Minimum Water  Discharge,”   IFA Technical  Conference Proceedings,   Amman,   Jordan.

2. Sinte Maartensdijk, A. 1976. “Direct Production of Granulated Superphosphates and PK Compounds From Sulphuric Acid, Phosphoric Acid, Rock Phosphate and Potash,” ISMA 1976 Technical Conference Proceedings, The Hague, Netherlands.

Links to related IFS Proceedings

7, (1949), Slurry Dispersion Methods for the Granulation of Superphosphate Fertilisers, J T Procter

42, (1957), Use of Different Types of Phosphate Rock in Single and Triple Superphosphate Production, T P Dee, R J Nunn, K Sharples