TRIPLE SUPER PHOSPHATE FERTILISERS –MANUFACTURE

INTRODUCTION-

Triple superphosphate is a fertilizer produced by the action of concentrated phosphoric acid on ground phosphate rock. The active ingredient of the product, monocalcium phosphate, is identical to that of superphosphate, but without the presence of calcium sulfate that is formed if sulfuric acid is used instead of phosphoric acid. The phosphorus content of triple superphosphate (17 - 23% P; 44 to 52% P2O5) is therefore greater than that of superphosphate (7 - 9.5% P; 16 to 22% P2O5). It is produced in granular and nongranular form and is used both in fertilizer blends (with potassium and nitrogen fertilizers) and by itself.

TSP (Triple Super Phosphate) represented as 0-46-0, is normally applied where plants are grown in soils with low or average levels of phosphorus. Its importance can be measured by the fact that in absence or it, the root development is weak, growth is stunted, productivity drops, the leaves or the edges of the leaves turn purple and in plants like tobacco and cotton, the leaves turn an abnormal color of dark green; potato tubers develop brown spots etc. Because it is a fertilizer with slightly acidic composition, its effect is limited in neutral or alkali soils. Because the phosphorus in its composition dissolved easily in water, it shows its effects rapidly. TSP is used as a base fertilizer. If it’s applied too early, the phosphorus in it combines with the lime and other elements in the soil and loses its effectiveness. If it is applied after the planting or seeding, it remains on the surface and has little effect. For these reasons, it should be applied either during or immediately after planting, seeding for maximum effect.

DAP (Diammonium Phosphate), MAP (Monoammonium Phosphate) and TSP (Triple superphosphate) are the most commonly traded phosphate fertilizers. TSP is made by reacting wet-process phosphoric acid with phosphate rock. Typically it contains 46% P2O5. TSP is produced in granular and non-granular form and is used both in fertilizer blends (with potassium and nitrogen fertilizers) and by itself.

TSP production amounts to around 5m. tonnes a year, of which around 4m. tonnes are traded internationally. It is produced in very few countries. The largest producers and exporters are Chinese, Tunisian, Moroccan, Israeli, Bulgarian and Egyptian companies. The main consuming countries are Brazil, Bangladesh, Iran the US and northwest Europe.

Triple Superphosphate is a high analysis product of the reaction between phosphate rock and phosphoric acid, containing 45 - 48% available phosphate (AP). Triple superphosphate plants are mainly large units operating as a part of chemical complexes, located near the source of phosphate rock.

PROPERTIES-

CHEMICAL:
Chemical formula :Ca(H2PO4)2•H2O

Fertilizer analysis :45% (0-45-0) P205; 15% Ca
Water-solubility :Generally >90%
Solution pH :1 to 3

PHYSICAL:

TSP is a highly concentrated and water soluble. Greyish, granular phosphate fertilizer with a P2O5 availability of more than 46%.

The properties of the triple superphosphate depend upon the specific process used and on the granular or non-granular nature of the product. The chemical and physical characteristics of the material depend on the nature of the phosphate rock, including composition and degree of fineness the concentration of the phosphoric acid and the method of manufacturing.

PRODUCT SPECIFICATIONS-

|Water- and citric acid soluble phosphate|46.0 |
|(P2O5) min. % | |
|Water soluble phosphate (P2O5) min. % |43.0 |
|Free acid (by water) max. % |3.5 |
|Moisture max. % |4.0 |
|Granulation 1-4 mm min. % |90.0 |

RAW MATERIALS-

• Phosphate rock • Phosphoric acid • Aqueous solution of flourapatite with phosphoric acid

CHEMICAL REACTIONS-

The production of triple superphosphate from raw apatite ore involves a complex series of reactions. These involve phosphoric acid, and also use sulfuric acid. Monocalcium phosphate is only one of the products of the process. Triple superphosphate is made by the reaction of fluorapatite with phosphoric acid in aqueous solution (Hoffmeister, 1993):

Ca10(PO4)]6F2 + 14 H3PO4 + 10 H2O 10Ca(H2PO4)2 • H2O + 2 HF

PROCESS FLOW SHEET-

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PROCESS DESCRIPTION-

There are variations of processes used for the production of TSP, the two major routes are the so-called "solid" and "slurry or quick-cure" processes.
Triple superphosphate, also known as double, treble, or concentrated superphosphate, is a fertilizer material with a phosphorus content of over 40 percent, measured as phosphorus pentoxide (P2O5).

Two processes are generally used to produce triple superphosphate: • Run-of-the-pile (ROP-TSP) • Granular (GTSP).

The ROP-TSP material is essentially a pulverized mass of variable particle size produced in amanner similar to normal superphosphate. Wet-process phosphoric acid (50 to 55 percent P2O5) is reacted with ground phosphate rock in a cone mixer. The resultant slurry begins to solidify on a slow moving conveyer en route to the curing area. At the point of discharge from the den, the material passes through a rotary mechanical cutter that breaks up the solid mass. Coarse ROP-TSP product is sent to a storage pile and cured for 3 to 5 weeks. The product is then mined from the storage pile to be crushed, screened, and shipped in bulk.

Phosphate rock and concentrated phosphoric acid (50-54% P2O5) are brought together into a mixer which discharges directly into a continuously moving den. A very dense slurry is produced which solidifies readily into a porous, honeycomb-like mass. It is carried to a rotating cutter where it is chopped into particles and then conveyed to the storage area for pile curing where the reaction continues for several weeks.
After curing, the material is reclaimed with power equipment and shipped as run-of-pile triple superphosphate or granulated by treatment with water and steam in a rotary drum, followed by drying and screening and shipped as granular triple superphosphate.
In the "slurry" granulation process, the phosphate rock is reacted with relatively weak phosphoric acid (32-40% P2O5). The resulting slurry is mixed with a large quantity of dried, recycled fine granules which are coated with the slurry, thus producing the desired granule size. The coated granules are dried and screened and the product size material is withdrawn to storage, the remainder being recycled to the process. The product is ready for shipment without any further curing required.

GTSP yields larger, more uniform particles with improved storage and handling properties.
Granular Triple Superphosphate is the predomintate product being produced today.

Dorr-Oliver slurry granulation process:

This method is illustrated in the figure -

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In this process, ground phosphate rock or limestone is reacted with phosphoric acid in
1 or 2 reactors in series. The phosphoric acid used in this process is appreciably lower in concentration (40 percent P2O5) than that used to manufacture ROP-TSP product. The lower strength acid maintains the slurry in a fluid state during a mixing period of 1 to 2 hours. A small sidestream of slurry is continuously removed and distributed onto dried, recycled fines, where it coats the granule surfaces and builds up its size.
Pugmills and rotating drum granulators have been used in the granulation process. A pugmill is composed of a U-shaped trough carrying twin counter-rotating shafts, upon which are mounted strong blades or paddles. The blades agitate, shear, and knead the liquified mix and transport the material along the trough. The basic rotary drum granulator consists of an open-ended, slightly inclined rotary cylinder, with retaining rings at each end and a scraper or cutter mounted inside the drum shell. A rolling bed of dry material is maintained in the unit while the slurry is introduced through distributor pipes set lengthwise in the drum under the bed. Slurry-wetted granules are then discharged onto a rotary dryer, where excess water is evaporated and the chemical reaction is accelerated to completion by the dryer heat. Dried granules are then sized on vibrating screens. Oversize particles are crushed and recirculated to the screen, and undersize particles are recycled to the granulator. Product-size granules are cooled in a countercurrent rotary drum, then sent to a storage pile for curing. After a curing period of 3 to 5 days, granules are removed from storage, screened, bagged, and shipped.

Taguchi method:

Triple super phosphate is a fertilizer produced by the reaction of concentrated phosphoric acid with phosphate rock containing 45 to 55% P2O5. The Taguchi method with an L9 orthogonal array has been used to determine the optimum conditions for the production of triple super phosphate
TSP production process has two steps. In the first step the phosphate rock is milled and the phosphoric acid is added to the feed mixture and the reactions occur in the mixed slurry media. When the slurry saturates from calcium phosphate, the reaction finishes. The reactions of phosphate with phosphoric acid are as follow:

[pic] (1)
[pic] (2)
[pic] (3)

By saturating the slurry with the products, the second step (which contains the crystallization of the reaction products) starts and the solid and liquid phases of slurry gradually are bounded from the reaction of phosphate granule with the produced calcium phosphate crystals. Crystallization of solid phase in the second step depends on the initial phosphoric acid concentration (and so solid to liquid ratio in the slurry) and temperature.
Finally, TSP is neutralized with calcium carbonate or ammonia or the other basic material in order to decrease the free acid to 1-2 percent.

USE OF TSP WORLDWIDE-

Concentrated or triple superphosphate (TSP) is used exclusively as a fertilizer material. TSP is a solid single-nutrient phosphate fertilizer material produced from phosphate rock and phosphoric acid. The market for TSP exhibited a downward trend for many years, as ammonium phosphate materials accounted for virtually all of the growth in phosphate fertilizer consumption.
World TSP production is concentrated in the developing world. Africa, Socialist Asia, the Middle East, and Central and South America are the primary producers, together accounting for about 81% of world production in 2009. Several other regions are also large producers. Large production decreases have occurred in Western Europe, Southeast Asia and the United States since 1993. World trade in triple superphosphate is substantial, with imports accounting for about 58% of total world production in 2009.
The following pie chart shows world consumption of triple superphosphate:

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MAJOR ENGINEERING PROBLEMS-

TSP had been losing market share to ammonium phosphates, which accounted for essentially all of the growth in world phosphate fertilizer consumption for many years. Although TSP has a similar phosphate analysis to ammonium phosphates, it contains no other nutrients. Ammonium phosphates, on the other hand, also contain a significant amount of the most commonly used nutrient, nitrogen.

The outlook for the fertilizer market through 2014 is for a growing agricultural market. After farmers reduced applications of P and K fertilizers in the last two fertilizer seasons, P fertilizer demand is expected to grow substantially in the next few years. However, the outlook also points to major uncertainties, the most important being the evolution of the financial crisis, policy priorities in China, and subsidy schemes in India. However, TSP will benefit only to a limited extent from the generally positive outlook for growth of almost 4% annually in total P2O5 consumption. In 2009, only about 7% of all phosphate fertilizers was applied in the form of TSP. During the forecast period, the largest part of the additional capacity will be in the form of DAP.

END USES-

AGRICULTURAL USES-
TSP has several agronomic advantages that made it such a popular P source for many years.
It has the highest P content of dry fertilizers that do not contain N. Over 90% of the total P in TSP is water soluble, so it becomes rapidly available for plant uptake. As soil moisture dissolves the granule, the concentrated soil solution becomes acidic. TSP also contains 15% calcium (Ca), providing an additional plant nutrient.

Other uses:
A major use of TSP is in situations where several solid fertilizers are blended together for broadcasting on the soil surface or for application in a concentrated band beneath the surface. It is also desirable for fertilization of leguminous crops, such as alfalfa or beans, where no additional N fertilization is needed to supplement biological N fixation.

It is used in the formulation of high analysis grade fertilizers and also applied directly to the soil as a single nutrient material.

APPLICATIONS-

Until the 1960s, TSP was the most common fertilizer in the US. Since then, Nitrogenphosphate fertilizer (i.e. MAP and DAP) became more and more popular on the American market. TSP is a highly concentrated phosphate fertilizer, which is used either in direct application or as a raw material in the production of complex fertilizers and blends with a high concentration of P2O5. Due to its high water solubility, TSP can be absorbed easily by the plant.

ENVIRONMENTAL IMPLICATIONS-

The most significant potential environmental hazards are dust (from the grinding of phosphate rock) and gaseous hydrofluosilicic acid (from the reaction between hydrofluoric acid and silica or quartz) emissions. These are both carefully monitored, and a dust catcher and gas scrubber are used.