Phosphoric acid when concentrated above 54% P2O5 forms superphosphoric acid (SPA). This name is given to phosphoric acid in which an appreciable proportion, usually 30% or more, of the P2O5 is in the form of condensed or polyphosphoric acids [general formula Hn+2 (PnO3n+1)]. A variety of concentrations of SPA can be manufactured from wet-process phosphoric acid; generally they are in the range of 69%-76% P2O5. Essentially, production of SPA first involves removal of physical water from the weaker acid and then the removal of chemically bound water. The latter step can be represented by the equations:
2H3PO4 + Heat -> H4P2O7 + H2O (g)
H3PO4 + H4P2O7 + Heat -> H5P3O10 + H2O (g)
where H3PO4 is orthophosphoric acid, H4P2O7 is pyrophosphoric acid, and H5P3O10 is tripolyphosphoric acid. At an SPA concentration of about 70% P2O5, the conversion of H3PO4 to other forms is of the order of 25%-45%.
For concentration from 54% to 70% P2O5, about 1 tonne of high-pressure steam (about 2.7 MPa pressure and 230°C temperature) is required per tonne of P2O5. Power requirements are about 24 kWh/tonne of P2O5. When a Dowtherm heat transfer medium is used, the fuel requirement is about 2.7 GJ/tonne of P2O5.
Several types of evaporators may be used for SPA production, but the most popular is a forced-circulation system (Figure 1). In production of superphosphoric acid (69%-72% P2O5) by concentrating wet-process acid, most of the flourine is volatised so that the acid contains only 0.2%-0.3% F. By adding reactive silica during evaporation to enhance flourine volatilisation, the flourine content can be further decreased to about 0.1%. Such acid is suitable for the manufacture of animal-feed supplement products, such as dicalcium phosphate or ammonium phosphate, and is used for that purpose. Other advantages of superphosphoric acid are:
- Savings in freight per unit of P2O5 compared with 54% acid.
- Sludge is eliminated. The polyphosphoric acids sequester most common impurities; however, in some acids, titanium or magnesium pyrophosphates may precipitate.
- Superphosphonc acid is much less corrosive than acid of lower concentrations.
- Superphosphoric acid is suitable for production of clear liquid fertilisers (ammonium polyphosphate solutions) because the polyphosphate sequesters impurities that otherwise would precipitate upon ammoniation.
- Superphosphoric acid is suitable for production of clear liquid fertilisers with micronutrients because of the ability to sequester metal ions.
The main disadvantages of superphosphoric acid are:
- the energy requirement.
- corrosion in some types of evaporators.
- high viscosity. The viscosity depends on temperature, concentration and impurity content; some superphosphoric acids must be heated to 60°C or higher to permit pumping with centrifugal-type pumps.
An example of viscosity variations due to impurities and temperature is shown in Figure 2.
The compositions of superphosphoric acids produced from several types of phosphate rock are shown in Table 1.
Shipment of Phosphoric Acid
Advantages of phosphoric acid as a source of P2O5 are versatility, rapid loading and unloading of ships, and high concentration. Phosphoric acid may be used to produce any desired phosphate or compound fertiliser to meet local needs; triple superphosphate (TSP) or ammonium phosphates are less versatile.
Most of the acid shipped overseas is at least 52-54% P2O5 concentration although some ‘superphosphoric’ acid, 69%-72% P2O5, has also been shipped.
Compared with importing raw materials, 1.0 tonne of P2O5 as phosphoric acid requires shipment of only 1.85 tonnes of 54% P2O5 or 1.43 tonnes of 70% P2O5 acid versus about 4.3 tonnes of raw materials (3.3 tonnes phosphate rock plus about 1 tonne of sulfur).
Rapid loading and unloading decrease costs by saving labour, decreasing port time, and decreasing congestion in ports. Handling of phosphoric acid (or any liquid) is dust. free, thereby avoiding atmospheric contamination by dust and minimising losses.
Some disadvantages are the requirement for specially equipped ships and special terminals with pumps and storage tanks at both shipping and receiving points. The need for further processing by the importer limits the market to countries that have sufficient demand to warrant such facilities.
Phosphoric acid for shipment should be relatively free of sludge-forming solids, preferably less than 1%. To meet this requirement the acid usually must be cooled and clarified. The amount of sludge depends on the composition of the phosphate rock and the phosphoric acid production process and, in some cases, clarification is unnecessary.
Two means of protection against the corrosion of tank walls are currently in common usage:
- the lining of mild steel tanks with sheets of self vulcanising chloroprene – based rubber;
- the lining of mild steel tanks with a plating of molybdenum-alloyed stainless steel (mainly 317L) or constructing the tank itself entirely from this material.
Under the International Maritime Organisation (IMO) bulk chemical code, merchant-grade phosphoric acid requires a moderate degree of containment due to its corrosive nature and its high specific gravity (typically 1. 7). In addition, a vessel must have a double bottom and wing tanks on the side if the tank section is to be used for acid. Because impurities in the acid tend to form a thick, viscous sludge, which is difficult to remove, tanks used for transport (and storage) must also be equipped with agitators to keep the acid in motion.
Superphosphoric acid is usually heated during transit to avoid a long period of reheating prior to discharge; thus, tankers for SPA transportation require heating equipment.
For construction of storage tanks, rubber-lined steel is common although stainless-steel linings may be used. In some cases, ponds or lagoons are used for storage. The ponds are lined with heavy sheets of rubber or plastic underlaid with gravel with drainage to a sump so that any leakage can be detected and returned to another pond. One such installation near Tampa consisted of four ponds with a total capacity of 11,000 tonnes. The ponds had inflatable plastic covers to protect them from rain or other contamination. Most storage tanks have facilities for agitating the acid occasionally to prevent settling
Links to Related IFS Proceedings
7, (1949), Slurry Dispersion Methods for the Granulation of Superphosphate Fertilisers, J T Procter
21, (1953), Manufacture of Triple Superphosphate, J J Porter, J Frisken
23, (1953), Ammoniation of Superphosphate, J Angus
42, (1957), Use of Different Types of Phosphate Rock in Single and Triple Superphosphate Production, T P Dee, R J Nunn, K Sharples
91, (1966), NPK Fertiliser Production Using Superphosphoric Acid, G Bischofberger, R R Heck
Links to external sources
Becker, P. (1989) Phosphates and Phosphoric Acid: Raw Materials: Technology, and Economics of the Wet Process. Marcel Dekker, Inc., New York, NY, U.S.A.
Havelange, S. et al. (2022). Phosphoric Acid and Phosphates in Ullmann’s Encyclopaedia of Industrial Chemistry.
Slack, A.V. (1968). Phosphoric Acid (Part I and II). Marcel Dekker, Inc., New York, NY, U.S.A.
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