1. Fundamentals of ammonium phosphates
2. Technologies for the Manufacture of MAP/DAP

Fundamentals of ammonium phosphates

Ammonium phosphates, particularly DAP and MAP, are the most popular phosphate fertilisers worldwide because of their high analysis and good physical properties. The compositions of the pure salts – monoammonium phosphate (MAP) and DAP – are given in Table 1.

The standard grade for DAP is 18-46-0. This is the “commodity” grade, and products analysing lower in N or P₂O₅ may not be sold as DAP. There is no standard grade for MAP; grades range from 11-55-0 to 10-50-0 made from high-sludge acid. A prevalent median grade is 11-52-0. Grades containing both MAP and DAP also are produced; examples are 13-52-0 and 16-48-0.

Relatively small amounts of pure DAP and MAP are made by crystallisation processes using phosphoric acid made by the electric-furnace process or using wet-process acid purified by any of various processes. The pure, fully soluble ammonium phosphates are used mainly for specialty liquid fertilisers. The nutrient content is higher than in commodity grades due to lower impurity content.

Both MAP and DAP usually have very good physical properties when made from wet-process acid. For both, storage properties and ease of granulation depend on the impurity content; a gel-like structure of impurities, mainly aluminium and iron phosphates, promotes granulation and serves as a conditioner to prevent caking, even at a moderately high moisture content (about 3%). On the other hand, pure ammonium phosphates are difficult to granulate and tend to cake badly in storage, even with a very low moisture content, unless coated with a conditioner. It has been demonstrated that the addition of impurities, particularly compounds containing aluminium, can improve granulation and product quality when there are insufficient impurities in the acid for this purpose. To determine the influence of acid impurities on ammonium phosphate products, a study was made of carefully “spiked” samples of base merchant-grade acid [1]. The conclusion was that the effects of aluminium, iron, and magnesium were simply cumulative and not multiplicative Thus, if one impurity is increasing, its effect can be compensated by a decrease of the other impurities. Another finding of this work is that citrate- insoluble P₂O₅ is more influenced by the operating conditions than by iron, aluminium, and magnesium content. Moreover, higher fluorine levels tend to reduce citrate-insoluble P₂O₅ as well as reactor viscosities.

Technologies for the Manufacture of MAP/DAP

Tennessee Valley Authority (TVA) Basic Process

Granular DAP is commonly produced by a slurry process; the process developed by the Tennessee Valley Authority (TVA) and illustrated in Figure 1 is typical. The average concentration of the wet-process acid is about 40% P₂O₅; however, most plants use part of the acid at 54% P₂O₅ and part at 30% P₂O₅. The acid reacts with ammonia in a pre-neutraliser where the mole ratio of NH₃:H₃PO₄ is controlled at about 1.4. This ratio corresponds to a point of high solubility (Figure 2).

The heat of reaction raises the slurry temperature to the boiling point (about 115°C) and evaporates some moisture. The hot slurry containing about 16% – 20% water is pumped to the granulator where in case of DAP production more ammonia is added to increase the mole ratio to approximately 1.8. Additional heat is generated, evaporating more moisture. The decreased solubility in going from· 1.4 to 1.8 mole ratio assists granulation. The moist granules from the granulator are dried and screened, the product size is cooled, and the undersize and crushed oversize are recycled. The usual ratio of recycle to product is about 5: 1. Ammonia escaping from the granulator dryer, and pre-neutraliser is recovered by scrubbing with weak acid (30% P₂O₅), and the scrubber solution is added to the pre-neutraliser.

The same equipment can be used to make MAP by one of two procedures:

1. The pre-neutraliser is operated at an NH₃:H₃PO₄ mole ratio of 0.6 (a point of high solubility) and the balance of ammonia is added in the granulator.
2. The pre-neutraliser is operated at a mole ratio of about 1. 4, and phosphoric acid is added in the granulator to decrease the ratio to 1. 0.

In producing MAP, ammonia recovery by acid scrubbing is not necessary, but all gaseous effluents are scrubbed to recover dust and fumes.

A basic flowsheet for the slurry granulation process is shown in Figure 3.

Pipe-reactor process.

Pipe reactor technology is now leading in the industry because of its favourable use of the chemical heat coming from the reaction of NH₃ with phosphoric acid.

Ammonia and phosphoric acid react in a pipe inside of the granulator. Steam generated by the reaction is released in the granulator and swept out with a current of air. This arrangement is simpler and less expensive than the use of a tank pre-neutraliser. An additional advantage is that it uses more concentrated phosphoric acid; thus, slurry discharged into the granulator contains less water and the recycle ratio is decreased. A plant using the pipe reactor has a recycle ratio of 3: 1 (as compared with a 5: 1 ratio for tank pre-neutralisation) [2].

A pipe reactor is, literally, just that – a length of pipe into which raw materials are introduced to react. The most common embodiment is the T-reactor, which has a T-shaped mixer at one end, Alternatively, the mixer head may be cross-shaped. In a T-shaped reactor (Figure 4), ammonia (gas or liquid, preferably liquid) is introduced to the mixer in the direction of the horizontal axis; the other feed – phosphoric acid and, sometimes, small quantities of sulfuric acid – is added to the mixer head at a right angle to the ammonia.

While in the pipe, ammonia and phosphoric acid react to produce a slurry, which is discharged from the end of the pipe directly into the granulator. Further ammoniation and incorporation of other solid ‘materials, in the case of NPKs particularly, take place in the granulator, which is generally a drum granulator.

The pipe reactor is, in most instances, a replacement for the pre-neutraliser. It is simpler to operate, avoids the need to pump slurry, and is cheaper too. The main advantage is that the pipe reactor can produce a more concentrated slurry than can a pre-neutraliser, and thus the specific process water requirement is much less. Consequently, less water needs to be introduced with the raw materials – 54% P₂O₅ phosphoric acid may be used instead of 40%-42% acid – and the recycle ratio can thus be reduced. This, in turn, reduces the size of the materials handling equipment needed or, in an existing plant, permits capacity to be increased.

A downside of the pipe reactor can be scaling in case of the use of phosphoric acids with high metal content.

Because the reaction is confined in such a small space, the heat balance of the process is generally improved as well. In that much of the water is vapourised in the reactor itself by the heat of reaction, product leaving the granulator is much drier and energy consumption for drying can be reduced. The main advantages of the pipe reactor process, compared with the pre-neutraliser process, can be summarised as follows [3]:

• Lower investment cost:
  • no pre-neutraliser.
  • low recycle ratio.
• Lower operating cost:
  • close to autothermal operation (no heating energy).
  • low electric power consumption.
  • lower citrate-insoluble P₂O_5.
  • high efficiency in ammonia consumption.
• High adaptability concerning feedstocks:
  • suitable for phosphoric acids from various origins; even some sludges may be used.
  • can be fed exclusively with merchant-grade acid at 52%-54% P₂O₅ and so is well adapted for plants that import their phosphoric acid.
• High operating flexibility and stability:
  • well-proven scrubbing system.
  • effective control of granulation conditions
• Wide variety of product formulations:
  • MAP, DAP, or various types of NPK.
• Low environmental impact:
  • low emission values.

Typical utilities consumptions of the pre-neutraliser and pipe-reactor processes are given in Table 2.

Nongranular MAP

Another process applied mainly in the Far East is the production of nongranular MAP for use as an intermediate in producing compound fertilisers. ln most cases, the product is made in large plants located adjacent to phosphoric acid plants. Often the product is shipped to smaller granulation plants for use as raw material to be granulated with others in an agglomeration granulation of compound fertilisers.

Although production of powdered MAP is declining, it is still produced in some plants as a cheap and efficient method to solidify sludge phosphoric acid or raffinate from cleaned phosphoric acids into a fertiliser intermediate or a feed material.

References

1. Handley, M. M. 1984. “Effects of Impurities .on Production of Diammonium Phosphate,” IFA Technical Conference Proceedings, Paris, France.

2. Marze, L. M., and J. L. Lopez-Ninio. 1986. “Powdered or Granular DAP,” In New Diammonium Phosphate Technology Proceedings 245, The Fertiliser Society, London.

3. Improved Techniques for Ammonium Phosphate Production. 1986. Phosphorus and Potassium, 144 (July-August):27-33.

Links to related IFS Proceedings

23, (1953), Ammoniation of Superphosphate, J Angus

162, (1977), Developments in Ammonium Phosphate Technology, I A Brownlie, E Davidson, T R Dick

216, (1983), Granulation of Ammonium Phosphates – Recent Experiences, K J Barnett, D M Ivell, S F Smith

244, (1986), Adapting a Pipe Reactor to a Blunger for NPK Production, R J Milborne, D W Philip

245, (1986), New Diammonium Phosphate Technology – Powdered or Granular DAP, L M Marzo, J L Lopez-Nino

245, (1986), Dual Pipe Reactor Process for DAP, NP and NPK Production, P Chinal, Y Cotonea, C Debateux, J F Priat