1. Introduction
2. Fundamentals of TSP Production
3. Technology of TSP Production Powder or Granular TSP by the Den Process
4. Slurry Granulation

Introduction

Since the end of the last century, Triple superphosphate (TSP) consumption has declined, primarily as a result of a continuous increase in the use of ammonium phosphates, mainly diammonium phosphate (DAP).

One of the advantages of TSP is that it is the most highly concentrated straight phosphate fertiliser available, with 44% to 48% available P₂O₅ and 40% to 45% water-soluble P₂O₅. Another advantage is that part of its P₂O₅ content is derived directly from phosphate rock, a relatively low-cost source. The percentage of P₂O₅ in TSP that is derived directly from rock varies from about 25% to 30%, depending on the CaO:P₂O₅ ratio in the rock, the impurity content of the rock and acid, and other factors. On the other hand, with a given amount of phosphoric acid, a greater amount of fertiliser P₂O₅ can be produced as TSP than as ammonium phosphate.

The manufacture of TSP is quite similar to that of single superphosphate (SSP) and has the same advantages of simplicity, low technical skill requirement, relatively small capital investment and the ability to use alternative raw materials from water cleaning etc (sewage sludge ashes) instead of rock phosphate.

TSP has three main disadvantages:

1. The total nutrient content (N+P₂O₅) is lower than that of ammonium phosphates.
2. It is acidic character may cause deterioration of some types of bags (hemp and paper) and might have a negative influence on high ammonium nitrate components in a blend.
3. It is not well suited for blending with urea because of reactions that cause deterioration of its physical condition.

Fundamentals of TSP Production

The reactivity of phosphate rock is of more importance in TSP production than in phosphoric acid production. Unreactive rocks may require very fine grinding or long reaction times or both. ·Even so, acceptable completion of the reaction may be difficult to achieve with some igneous apatite.

The reaction of rock and phosphoric acid is as follows:

Ca₃(PO₄)₂.CaF  + 4H₃PO₄ + 3H₂Oà 3CaHPO₄.H₂O + 3H₃PO₄ -> 3Ca(H₂PO₄)₂.H₂O+ 172.58 kcal

        Apatite          phosacid                dicalciumphosphate                  monocalciumphosphate

The reaction will not complete to 100% but is an equilibrium which can change over time (curing will increase the content) Fluorine from the apatite will partly be released in a side reaction as a mixture of HF and SiF₄, the rest remaining in the final product as CaF₂ or CaSiF6.

Rocks containing carbonates or carbonate substitutions in apatite release CO₂ during the reaction. The released gases result in a porous structure of the superphosphate made by den processes. This is considered desirable when the superphosphate is to be used in making compound fertilisers. When the rock contains very little carbonate, the superphosphate may be hard and dense, leading to granulation difficulties and poor ammoniation characteristics.

TSP may be prepared in either granular or nongranular form. The nongranular form is preferred for use as an intermediate for the production of compound fertiliser by granulation processes, whereas the granular form is preferred for direct application or for blending.

For simplicity, the phosphate source in the reaction is shown as tricalcium phosphate. If some of the calcium is converted to calcium fluoride or fluosilicate, somewhat less phosphoric acid is required than indicated by the equation. However, various impurities in the acid and rock cause variations in the optimum acid:rock ratio. The product will contain (in addition to monocalcium phosphate) iron, aluminium, and magnesium phosphates; probably a small amount of dicalcium phosphate; unreacted rock; calcium sulphate originating from free sulfuric acid in the phosphoric acid and sometimes from rock impurities; and various other impurity compounds.

The economically optimum acid:rock ratio is best determined by test methods and will depend on whether the commercial value is based on solubility in water, neutral ammonium citrate, or other solvents: when water solubility is the criterion, both the rock and acid should be as low in iron and aluminium as is economically feasible.

The effects of the CaO:P₂O₅ ratio of the rock and grade of rock on the proportions of rock and phosphoric acid and on the grade of the TSP product are given in Table 1. The calculations are intended only for illustration of general trends; accurate values can be obtained only from experimental data derived during tests with the specific phosphate rock and phosphoric acid in question.

Technology of TSP Production Powder or Granular TSP by the Den Process

The manufacture of TSP by this route involves the following operations:

Reaction

Very finely ground phosphate rock (95% to 98% < 100 mesh) is mixed with phosphoric acid. With rock of 34% P₂O₅ content, about 2.6 kg of acid P₂O₅ is required per 1 kg of rock P₂O₅. The phosphoric acid used is merchant-grade acid at 52% P₂O₅ concentration. This is diluted by scrubber liquid to about 48% P₂O₅. Some plants use the cone-mixer process that was originated by the Tennessee Valley Authority (TVA) [1,2]. Figure 1 shows a flow diagram of the process. The cone-mixer has the disadvantage that mixing is far from perfect and a substantial amount of fine rock phosphate is entrained to the scrubber system. A somewhat similar process is known as the Kuhlmann process; in this case, the mixer is a small cylindrical vessel equipped with a high-speed stirrer. Here the mixing is better than in the cone. Another variety is the ICL process where the mixer is a double shaft horizontal mixer with almost perfect mixing and no entrainment.

Denning

The fluid material from the mixer goes to a den where it solidifies. Solidification results from the continued reaction and crystallisation of monocalcium phosphate and crystal water. The process is faster than with SSP, and denning times of 10-30 minutes are suitable for TSP, as compared with 30 minutes to 2 hours for SSP. Special belt conveyors are sometimes used for TSP rather than conventional dens. In any case, the belt or den must be enclosed and connected to a fume exhaust system to direct fluorine-containing gases to a scrubber.

Storage/Curing

The product is removed from the den and conveyed to storage piles for final curing, which requires 1-6 weeks depending on the nature of the raw materials (more or less soft rock, harmful impurities like Fe₂O₃, Al₂O₃, MgO; useful carbonates). During curing, the reaction approaches completion. The free acid, moisture and unreacted rock contents decrease, and the available and water-soluble P₂O₅ contents increase. Small amounts of fluorine compounds continue to be evolved during storage curing, and good ventilation is needed to remove the fluorine from the working area. Scrubbing of the exhaust gas may be necessary to prevent atmospheric pollution. Curing will increase the content of mono calcium phosphate.

After storage curing, the TSP is reclaimed with a shovel and disintegrated in a cage or chain mill to pass a 6-mesh screen (3.3 mm). The disintegrated TSP (sometimes called run-of-pile TSP or ROP-TSP) may be used for making compound fertiliser by agglomeration granulation, or it may be used as is for direct application. In most countries, farmers prefer a granular form because the TSP is dusty if it is too dry, and it cakes if it is too moist.

Granulation

Granulation of powder or ROP-TSP may take place before or after curing. Cured TSP is granulated by the process shown on Figure 2. After milling and screening, the cured powder TSP is conveyed to a rotary drum granulator and mixed with recycle material (typical recycle ratio recycle:product 1:1.5). Water is sprayed onto the bed of material and steam is sparged underneath the bed to provide wet granular material. The wet granules are discharged to a rotary, co-current dryer. The dried granules are screened, and the oversize is milled and returned with the fines to the granulator or transported back to the feed of the screens to be classified again. In the latter option, the final product tends to contain broken coarse material and its appearance is less round. Dust and fumes from the dryer are scrubbed in a water scrubber. Mostly, dust may be removed by a bag filter or cyclone system prior to the wet scrubbing.

In a process variation, phosphate rock and phosphoric acid are fed to the granulator to supply a portion of the phosphates; production cost is lowered, and granulation is improved, but greater investment is required, and the product is more difficult to handle (less matured, so sticky).

The typical consumption figures per tonne of product are summarised in Table 2.

Ex-Den Granulation

In ex-den granulation or direct granulation, the acidulation and denning steps are similar to those described for producing nongranular TSP except that the rock may be somewhat more finely ground, and the den retention time is longer (25-45 minutes vs 10-30 minutes). Also, the product from the den goes directly to a granulator rather than to storage. After granulation, the product is dried, screened, cooled, and conveyed to storage.

Drying is controlled to yield a product of 4% – 6% moisture (measured at 3hrs 105°C). Under these conditions, some further reaction takes place in storage (curing).

A simplified flow diagram of a typical direct ex-den granulation system is shown in Figure 3. Typical data for the production of TSP by ex-den granulation are given in Table 3.

The plasticity and heat content of fresh TSP (or SSP) make it much easier to granulate than cured TSP; less recycle, water, and steam are required. Recycle ratios (recycle:product) are around 1:1.Presumably, less fuel is required for drying. Total electric power consumption is somewhat lower. The product is said to be superior in hardness, shape, uniformity, and smoothness. However, great care must be put in accurate process control and type of rock since all deviations in the acidulation have a direct impact on the granulation.

Slurry Granulation

When granular TSP is the desired product, it is sometimes preferred to produce it directly from a slurry rather than by granulation of powder TSP.

Some advantages of slurry granulation processes are:

1. Variable cost is usually lower due to the use of diluted phosphoric acid from local production,
2. Granules are denser and stronger.
3. Granulation equipment can be used interchangeably for producing TSP and ammonium phosphates.

There are four main disadvantages of slurry granulation:

1. Owing to the limited reaction time, unreactive rocks are poorly suited for use in the direct granulation process.
2. Greater losses of soluble P₂O₅ may occur owing to incomplete reaction, or a higher ratio of phosphoric acid to phosphate rock may be needed to prevent this loss.
3. Higher energy consumption due to higher recycle ratio
4. Lower output from a given plant due to a higher recycle ratio.

The first slurry-type granulation process developed was the Jacobs-Dorrco process, formerly known as the Dorr-Oliver process. A simplified flow diagram is· shown in Figure 4. Ground phosphate rock and phosphoric acid, 38%-40% P₂O₅, are fed to the first of a series of two or three steam-heated, stirred reaction vessels. The overall retention time is about 30 minutes, and the temperature is about 90°C. The thick slurry is fed to a blunqer or rotary drum granulator together with a high proportion of recycle. The moist granules are dried and screened, and the product size material is cooled and sent to storage.

Recycle ratios (recycle:product) are 10-12:1 for blunger granulation and about 8:1 for rotary-drum granulation. The lower ratio for rotary-drum granulation is ascribed to moisture evaporation in the drum granulator enhanced by a counter current sweep of air.

The estimated utility requirements per tonne of product are about 40 kWh for electric power (including phosphate rock grinding), about 125,000 kcal of fuel oil for drying, and about 20 kg of steam, mainly for heating the reactors.

References

1. Bridger, G. L., RA. Wilson, and R. Burt. 1947. “Continuous Mixing Process for the Manufacture of Concentrated Superphosphate,” Industrial Engineering Chemistry, 39:1265-1269.

2. Bridger, G. L. 1949. “Development of Processes for Production of Concentrated Superphosphate,” Chemical Engineering Report 5, lVA, Muscle Shoals, AL, U.S.A.

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

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