GTSP Production : How to select the best process. Den route (run of pile TSP) or Slurry route ?

#GTSP #TSP #Fertilizers #Acidulation #Granulation #Den Reactor

I am writing this article to share my thoughts about how to select the most appropriate process for GTSP fertilizer production.

There are two principle processes for production of Granular Triple Superphosphate (GTSP), there are normally called the Den Route and Slurry Route. Each process has its particular advantages and disadvantages, and the process finally chosen is determined mostly by local circumstances.

The Den route description

In the Den Process (also named as run of pile TSP route), finely ground phosphate rock is reacted with about 50% P2O5 phosphoric acid, in a special piece of equipment known as Den (A Den is an elaborate enclosed conveyor). The material flowing from the Den is a solid of wide size range in which reaction between acid and rock is about 80% complete. The material is conveyed to a maturation building, where it remains for about 10 days after which reaction is about 95% completed. At this stage, TSP is suitable for feeding to a low recycle granulation plant where it is formed into granules with size range generally between 1.5-4.0 mm. The final chemical quality is good with water and citrate P2O5 contents maximized (within the limits of rock and acid quality) and free acidity minimized.

The main features of the granulation of powder TSP (produced in the Den reactor) is the very low recycle ratio of about 2:1 in the granulation loop, whilst achieving good physical quality and minimizing utility consumptions. The granulation unit can also be designed to produce other granular NPK fertilizers from the same equipment.

Advantages of the Den Route :

• Low capital cost for the granulation section because of the low process recycle ratio.

• Low operating cost because utility consumptions per ton of product are minimized.

• The Den equipment can be used alternatively and independently of the granulation plant to produce other intermediates such as SSP.

• Production flexibility : the powder TSP can be used directly for GTSP production or as a raw material for granular NPK fertilizers.

• Low design recycle ratio for the granulation section resulting in small equipment and easier/low maintenance cost.

• Large capacities in a single line are possible.

• Removal of fluorine gases from process air streams is simple because it is contained in a single air stream.

In general, the Den route is favored where phosphoric acid is imported (normally 54% P2O5) and/or where client requires superphosphates as an intermediate in NPK production in addition to straight GTSP production.

The Slurry route description

In the slurry process, finely ground phosphate rock is reacted with about 42% P2O5 acid in series (normally two) of stirred tank reactors. The reaction system is operated at chemical equilibrium (P2O5-CaO-H2O) with the rock about 70% reacted at temperature of about 100°C and water content about 20%. Reaction is completed in the granulation section. Under these conditions the resulting reaction slurry is pumpable and is fed directly to the granulator. The high water content in the TSP slurry requires that the granulation plant is designed with a recycle ration of about 8:1. The high recycle ratio results in a large plant in terms of equipment sizes for a relatively low production rate. The product chemical quality is comparable with that obtained from the Den route, and is determined by rock and acid quality. Product size is 90-95% in the range of 2.0-4.0 mm.

Advantages of the Slurry Route :

• Fully integrated process in which rock and acid are fed at one end, and final product flow from the other end.

• Phosphoric acid can be used directly from the hemihydrate process (if rock is fed in dry conditions). Therefore neither an evaporation nor steam for evaporation is required.

• Lower phosphoric acid concentration required (42% compared to 50%). Thus even with a dihydrate acid plant, steam savings are possible.

• An intermediate maturation building is not required.

• No intermediate solid handling.

• Plant plot size is small.

• Lower labor cost.

In general, the Slurry process is more appropriate where production takes place in a fully integrated factory with sulphuric and phosphoric acid production at the same site, and client required only GTSP as a final product.

Do we need to test the phosphate rock before selecting the best process ?

Unless the phosphate source has been already used to produce TSP, laboratory tests are beneficial to understand how the phosphate rock will react in Den route or Slurry route.

The tests are used to evaluate the rock phosphate performances in both Den route or Slurry route. The purpose of the tests are :

• Specify raw materials characteristics, phosphate granulometry, phosphoric acid concentration and temperature.

• Predict phosphoric acid and rock phosphate consumptions, to meet the final product quality.

• Predict final product chemical quality (Total P2O5, soluble P2O5, citrate P2O5, free acidity).

• Determine phosphate rock reactivity index.

It is to be noted that some phosphates with low reactivity index may not be suitable for Slurry route.

Low rock quality will tend to consume more acid, to meet the final product compositions requirements, and will also provide a final product with high free acidity.

Acid quality has also an impact on the reaction conversion. Acid with low impurities will have positive effect on the final product chemical quality.

Conclusion

From the above statements, we can conclude that every project can be unique. Final decision has to be taken, based on local circumstances, lab tests results, production flexibility requirements and a CAPEX/OPEX/profitability exercise.

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