1. Hemihydrate Process (HH]
2. Hemidihydrate Processes
3. Hemihydrate-Dihydrate Process with Intermediate Filtration (HDH)
4. Dihydrate-Hemihydrate Process (DHH)
5. Plant Engineering/Construction

As with dihydrate processes, there are a number of hemihydrate and combined systems developed by various companies, and no attempt is made to describe them all in detail. However, for simplicity it may be taken as a rule that the basic equipment for both hemihydrate and combined processes is the same, and differences exist only in technical parameters, additional equipment, general layout, and to some extent in materials of construction used.

Hemihydrate Process (HH]

The hemihydrate reaction proceeds in two distinct zones. It is therefore necessary to have at least two separate vessels or compartments in the reaction section. The preferred volumetric ratio is 2: 1. The first zone is often divided into two identical compartments or vessels, 1A and 1B.

Phosphate rock is fed to reactor 1A; sulfuric acid and dilute phosphoric acid from the filter are fed to reactor 2 (Figure 1). Slurry from reactor 2 is recycled through a flash cooler to reactor 1A, thus exposing the phosphate rock to sulfate ions under controlled chemical conditions. Slurry overflows from compartment 1A to 1B. Heat is removed by vacuum cooling (or by air with omitting flash cooler) to maintain the reaction slurry temperature at 98°-100°C.

The product acid at between 40% and 50% P₂O₅, depending on the quality and composition of the phosphate, downstream requirements and hemihydrate gypsum, is separated by a horizontal vacuum filter with three countercurrent wash stages, The product acid from the filter passes directly to storage. Most of the time, it does not require clarification or solid removal and may be used directly as concentrated acid without evaporation.

However, there are some disadvantages that have tended to restrict the popularity of the hemihydrate route, among them the following:

• P₂O₅ losses and water balance: The water balance is more critical; thus the amount of wash water that can be used is restricted. Amounts of both soluble and cocrystallised P₂O₅ remaining in the filter cake are greater because of the higher P₂O₅ concentration of the slurry being filtered.
• Scaling: Hemihydrate is not a stable form of calcium sulfate, and it tends to revert to gypsum even before the acid has been filtered off. During washing, conditions are even more in favour of re-hydration, which may lead to scaling of piping and equipment, and filter cloth blinding.
• Corrosion: At the higher temperature and acid concentration in a hemihydrate reaction system, there is more rapid wear of equipment, particularly of agitators and slurry pumps.

However, apart from the reduction or elimination of the evaporation heat requirement, the process has certain advantages:

• Capital saving: Less evaporation equipment is needed, if any.
• Purer acid: The acid contains substantially less free sulfate and suspended solids than the evaporated dihydrate-process acid of the same strength and somewhat lower levels of aluminum and fluorine contents.
• Lower rock grinding requirements: Under more severe reaction conditions, phosphate rock feed reacts much more quickly, and a satisfactory rate of reaction can be achieved from much coarser rock.

Average process inputs for hemihydrate processes are shown in Table 1.

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Hemidihydrate Processes

Hemihydrate-dihydrate processes without intermediate filtration, so-called hemihydrate with recrystallisation (HRC), are widely used in Morocco, the countries of East Asia and Oceania. The plant layouts for these processes resemble those of multiple-reactor dihydrate processes. An exception is that the attack and digestion reactors operate under hemihydrate conditions while succeeding reactors operate under conditions favouring the rehydration of hemihydrate to gypsum, which is encouraged by seed dihydrate crystals recycled in slurry from the filter feed. The product acid is not any more concentrated than that obtained from the dihydrate process because of the need to crystallise easily filterable crystals in the presence of the product acid, but the gypsum is much purer and therefore of greater use in, for example, plaster manufacture or as a cement-setting retarder. This advantage is important for countries where natural gypsum is scarce, wholly or partly imported, and expensive.

The process also provides a very high recovery of P₂O₅ from the rock since losses in the gypsum are very low. The value of the 2%-3% increase in P₂O₅ recovery depends on the cost of the rock. The disadvantages are the higher cost and increased complexity of· the process. Also some phosphate rock contains impurities such as lanthanides that stabilise the hemihydrate, thereby preventing recrystallisation to gypsum at an acceptable rate.

Hemihydrate-Dihydrate Process with Intermediate Filtration (HDH)

The first stage of the process is almost identical to the HH process already described. In the transformation stage the hemihydrate cake is discharged from the filter into an agitated vessel (Figure 2). The operating conditions are controlled to ensure complete transformation of hemihydrate calcium sulfate to dihydrate and to allow sufficient time for the dihydrate crystals to grow.

The rate of transformation is increased by the addition of a small feed of sulfuric acid. Nearly all the lattice P₂O₅ co-precipitated with the hemihydrate is released into the liquid phase. The dihydrate gypsum is then filtered off and the cake washed with process or pond water. The filtered and the released P₂O₅ is returned to the hemihydrate reaction stage as the last wash on the hemihydrate filter.

The extra filtration step increases the cost and complexity of the plant, but this disadvantage may be offset by decreasing or eliminating the concentration step because of the high P₂O₅ concentration of about 45% in the phosphoric acid produced. Moreover, the dihydrate gypsum obtained is much purer than that from HH or DH processes and similar to HRC gypsum and may be used in the production of building materials.

Median hemidihydrate process requirements are given in Table 2.

Dihydrate-Hemihydrate Process (DHH)

Although the attack and digestion sections are run under dihydrate conditions, it is not desirable to effect a very high degree of P₂O₅ recovery during the separation of the acid from the dihydrate because the succeeding dehydration stage requires about 20%-40% P₂O₅ and 10%-20% H₂SO₄. Either a filter or a centrifuge can be used to produce a thickened slurry of gypsum in phosphoric acid of the correct concentration, and it is not of critical importance to produce highly filterable crystals in the first stage. Thus, it is possible to make a product of up to about 35% P₂O₅, compared with a maximum of about 30% for the ordinary dihydrate process. The transformation of dihydrate into hemihydrate is not hindered by the impurities in Kola apatite and other igneous rocks for which the process is very suitable.

The dihydrate-hemihydrate process has about the same advantages and disadvantages as hemihydrate-dihydrate except that the hemihydrate byproduct may be more useful than gypsum in some cases. Also, the hemihydrate exhibits self-drying behaviour. The hemihydrate calcium sulphate after filtration takes up moisture from the cake to recrystallise in dihydrate. On the other hand, the product acid concentration is somewhat lower.

A process without intermediate filtration, so-called dihydrate attack – hemihydrate filtration (DA-HF) has recently been developed. The main advantages of this process are the reduced cost of the plant compared to DHH process with two filtration steps, its higher P₂O₅ acid concentration and P₂O₅ recovery in comparison with the classical single DH process. On the other hand, the product acid contains more sulfate and an additional desulfation step may be required depending on the downstream application.

Plant Engineering/Construction

Materials of Construction

Choice of construction materials is very important since it will affect the degree of maintenance and downtime of a phosphoric acid plant.

Corrosion rates in a plant are variable and depend largely on the chloride and free fluoride content of the acid.

Typical materials of construction for various items of equipment are as follows [23].

Reactors: Concrete, with acid-resistant aggregate such as siliceous sand and gravel, rubber lined or RLMS (rubber lined mild steel) protected by carbon bricks.

Vessels: RLMS, propylene, FRP (fiber-reinforced polypropylene).

Pipework: Polypropylene, FRP, Rubber lined carbon steel.

Agitators: Alloy 20, 904 L, HV9, Sanicro 28, Ferralium 255, 317L, Uranus B6 and Uranus 52N+ (Super Duplex).

Pumps: Alloy 20 and 904L, alloy 28, HV9, Sanicro 28, high-density polyethylene, ferrahum 255, 317 L, Hastelloy C and Uranus 52N+ (Super Duplex).

Heat exchangers: Impregnated graphite reinforced with carbon fiber as Diabon (Sigri), Bigilor, Graphilor BS (Le Carbone- Lorraine)

The most vital and vulnerable portions of the phosphoric acid plant are agitators, pumps, and filters. Corrosive conditions are the most severe for agitators; however, agitators can be replaced with relative ease and low cost. Filters, on the other hand, are large and expensive and not so easily replaced or repaired. Corrosive conditions are somewhat less severe for filters than for agitators because the average temperature is lower and erosion is a lesser factor.

For many years, 316L stainless steel was a common construction material for agitators, filters, and the other equipment coming in direct contact with wet-process phosphoric acid or reaction slurry. This has now replaced by 904L for acid and Uranus 52N+ (Super Duplex) for abrasive slurry.

References

23. ‘Phosphoric Acid Equipment – IL’ 1991. Phosphorus and Potassium, 176:26-40.

Links to Related IFS Proceedings

192, (1980), The Oxy Hemihydrate Phosphoric Acid Process, M B Caesar, H C Smith, L E Mercando

209, (1982), Recent Experiences in Phosphoric Acid Production by Hemihydrate Routes, M L Parker, C McDonald

248, (1986), Relative Merits of Different Filters for Hemihydrate Filtration, M L Parker, J A Hallsworth

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.