Run of Pile TSP Explained: Simple Chemistry, Big Impact
- Mohamed Lembaid
- Oct 1
- 4 min read
Understanding the fertilizer that set the stage for ammonium phosphates
Triple Superphosphate (TSP) marked a major step forward after Single Superphosphate, made possible by the advent of phosphoric acid production. By the mid-20th century, it became the leading high-phosphorus fertilizer before ammonium phosphates took over. This article looks at Run of Pile TSP (ROP TSP)—its process, product characteristics, and role in modern fertilizer production.
Introduction
Run of Pile TSP (ROP TSP) – was the first major advancement that followed the production of Single Superphosphate. Its development became possible once phosphoric acid units started operating.
The production process is relatively straightforward: ground phosphate rock is reacted with phosphoric acid.
By the 1950s and 1960s, TSP had become the dominant high-phosphorus fertilizer, benefiting from the scaling up of phosphoric acid plants. This era marked its golden age, just before the industry shifted toward ammonium phosphates (DAP and MAP), which gradually took over the market.
Two different processes
Within the industry, two distinct processes are used to produce TSP:
Run of Pile TSP (ROP TSP) – the subject of this presentation.
The final product is a powder containing significant levels of free acidity and moisture.
The “curing” process is carried out in this powdered form.
Once cured, the ROP TSP can be fed to a granulation unit, where it can be used as the sole feedstock or incorporated into the production of NP and NPK fertilizers.
Slurry TSP (GTSP) – this will be the focus of another presentation.
In this process, phosphate rock and phosphoric acid are reacted together in a heated tank and sprayed into a granulator as a slurry.
The final curing occurs in the granular form, making this process capable of producing only granular TSP on an economically viable scale.
Product Specifications
In international trade, the accepted product specification for granular TSP is:
46% available P₂O₅, typically defined as P₂O₅ soluble in Neutral Ammonium Citrate (NAC).
Moisture content: around 2%.
Unlike granular TSP, Run of Pile TSP (ROP TSP) has very limited international trade. Most of it is consumed internally within the same industrial complex where it is produced.
The product quality of ROP TSP is highly dependent on:
The raw materials used,
The final destination/application,
And the economic considerations of the plant.
When ROP TSP is intended for ammoniated granular fertilizers (NP/NPK), it is common practice to manufacture it with a higher level of free acidity. This has two main advantages:
Improves conversion efficiency,
Facilitates ammoniation and granulation.
Phosphoric Acid
Phosphoric acid is the key raw material in the manufacture of run-of-pile Triple Superphosphate (TSP). Its composition and level of impurities directly determine both the chemical and physical quality of the final product, as well as the efficiency of P₂O₅ solubilization from the phosphate rock.
The critical parameters influencing phosphoric acid behavior in the TSP process include:
Concentration: defined by water-soluble P₂O₅ and H⁺ content.
Temperature: governs the kinetics of dissolution and reaction.
Viscosity: affected by soluble impurities and suspended solids.
These factors, together with the balance between primary P₂O₅ (from the acid) and secondary P₂O₅ (from the rock), determine the performance of the reaction system.
Phosphate Rock
The type of phosphate rock plays a decisive role in selecting the appropriate process for Triple Superphosphate (TSP) production. Rocks with lower reactivity, such as igneous phosphates, are better suited for the Run-of-Pile (ROP) TSP route, where longer reaction times are tolerated.
The GTSP process works efficiently with high-reactivity sedimentary rocks such as those from Florida and Morocco. However, when the feedstock is switched to igneous phosphate rock, the process efficiency drops.
Reaction between Rock Phosphate & Phosphoric Acid
In Run-of-Pile (ROP) TSP production, the reaction proceeds through three distinct physical phases: fluid, plastic, and solid.
Fluid phase: This is the initial and most critical stage. The reaction mass must be discharged into the den before this phase is complete. Controlling its duration is essential—the fluid state should last as long as possible within the operational limits of the mixer and den. At the end of the den, the product should already have transitioned into a solid phase.
Plastic phase: The material becomes more viscous and begins to solidify as the reaction progresses, with liquid absorption by solids playing a dominant role.
Solid phase: The final state, where the reaction mass consolidates into the desired product structure.
On a microscopic level, as phosphoric acid contacts phosphate rock, it forms a thin liquid film of reaction solution around the rock particles. The acid must then diffuse through this layer to continue reacting with the unconverted rock beneath. This interplay between reaction kinetics and physical absorption is fundamental to controlling the efficiency and quality of ROP TSP production.
An important parameter is the H⁺ concentration of the acid, since it is the hydrogen that play a key role to the attack on phosphate rock. The difference between the total P₂O₅ content of the acid and its effective acidity can be quite significant.
Temperature is another key factor influencing the reaction between phosphate rock and phosphoric acid.
Conclusion
For more detailed insights into Run of Pile TSP—its production, applications, and optimization—feel free to reach out to us. In our upcoming articles, we will explore the next steps in greater depth, covering process parameters, operating conditions, and equipment design to provide a complete technical perspective on TSP production.
