1. Ammonium Polyphosphate
2. Ammonium Phosphate-Sulphates
3. Ammonium Phosphate-Chloride
4. Ammonium Phosphate-Nitrate (APN)
5. Urea-Ammonium Phosphates (UAP)
6. Urea Superphosphate (USP)

Ammonium Polyphosphate

The Tennessee Valley Authority (TVA) developed a process for producing granular ammonium polyphosphate in late 1973. The usual grade produced is 11-55-0; higher grades (such as 12-57-0) are possible.

The principal advantage of the process is that products with very low moisture content are made without drying. Elimination of the rotary dryer and its appurtenances substantially decreases the investment cost and the energy requirement (fuel and electrical energy). Another advantage of the process is that the product has exceptionally good storage properties, which are attributed to the low moisture content and the polyphosphate content of the product.

Ammonium Phosphate-Sulphates

A group of fertilisers known as ammonium phosphate-sulphates has been popular for many years and is still popular in many areas. The best-known grade is 16-20-0, which essentially consists of MAP and ammonium sulphate (AS). One reason for its popularity is that it is relatively non-hygroscopic. Hygroscopicity, as measured by the critical relative humidity (CRH) of some phosphate and nitrogen fertilisers and combinations, is discussed under NPK-fertilisers.

It is evident that MAP is one of the least hygroscopic of the phosphate materials and that AS is the least hygroscopic of the straight nitrogen materials. When a fertiliser with a higher N:P₂O₅ ratio than that of DAP (0.4:1.0) is needed, the MAP plus AS combination is the least hygroscopic.

Another advantage of ammonium phosphate-sulphate is the sulphur content, which is agronomically useful for many crops and soils. Ammonium phosphate-sulphates have been produced by the Dorr-Oliver slurry process since 1933 and more recently by other slurry granulation processes like the AZF-Grande Paroisse dual pipe-reactor process. The usual method involves reaction of sulfuric and phosphoric acid with ammonia, although ammonium sulphate from by-product sources (caprolactam production) can be used. In addition to 16-20-0,several NPK grades are produced such as 14-28-14, 13-36-12 and 13-13-13.

In TVA’s pipe-cross reactor system, illustrated in Figure 1, most of the reaction of phosphoric and sulfuric acids with ammonia is carried out in a pipe, which discharges a melt into the drum granulator. Steam generated by the heat of reaction is swept out of the granulator by an airstream. One advantage of the process is that the heat of reaction is utilised to dry the product, and thus less/no dryer is necessary. Another advantage over slurry processes is the comparatively low recycle ratio, in the range of 2-3:1 for 12-48-0 or 1-2:1 for 13-13-13 or 6-24-24.

Ammonium Phosphate-Chloride

Ammonium chloride is used in some Far East countries, both as a straight nitrogen fertiliser and as an ingredient of compound NP, NPK, and PK fertilisers. Ando described the operation briefly [1]. He mentioned that, as compared with ammonium sulphate, ammonium chloride was 10% cheaper per unit of nitrogen in Japan) and more concentrated (25% vs 21% N). Wet-process phosphoric acid is ammoniated to form a MAP slurry, which is fed to a pugmill along with ammonium chloride, recycle, potassium chloride (optional), and other fertiliser materials. Ando mentioned that granulation in the pugmill is not as easy as with ammonium sulphate but noted that the difficulty had been overcome. The recycle ratio ranges from 2-4: 1. The moisture content of the granulator product ranges from 5% to 6% before drying and 0.5% to 0.8% after drying. The main NP grade is 18-22-0; NPK grades include 14-14-14 and 12-18-14 [34].

Ammonium Phosphate-Nitrate (APN)

There are several processes that produce fertilisers containing ammonium phosphate and ammonium nitrate, but most of them are used to produce NPK grades and are described further in the section on NPK Fertilisers.

The AZF – Grande Paroisse dual pipe-reactor process introduces ammonium nitrate as a solution that is· directly sprayed onto the bed of granules in the granulator.

APN solutions are readily produced by neutralising phosphoric acid with ammonia-ammonium nitrate solutions. The solution is used as supplemental feed for compound fertiliser production. Prilling of APN melts has been carried out by several companies, some of them also adding potash as well. APN-melts may contain some polyphosphate, which lowers the melting point.

Urea-Ammonium Phosphates (UAP)

Fertilisers based primarily on urea and ammonium phosphate have been produced in Japan for many years. Ando described a variety of processes that were in use [1]. Ammonium phosphate was supplied either as a slurry produced in the plant by ammoniation of phosphoric acid or in solid form as MAP or DAP. Urea was supplied as a solid (crushed prllls or crystals), as a melt, or as a concentrated solution. Several types of granulators were in use; rotary drums were the most common, but pan granulators, pug mills, and other types were used in some plants.

A later paper gave a detailed description of a granulation plant that produces urea-ammonium phosphate (UAP) grades, such as 28-28-0, 22-22-11, 18-18-18, etc. [2]. The main raw materials are crushed urea prills, spray-dried ammonium phosphate (12-50-0), and potash salts. One unusual feature of the plant is a methylene-urea reactor, which is used for production of some NPK grades. Part of the urea is fed into the steam-heated reactor with formaldehyde solution to produce a methylene-urea slurry containing unreacted urea, monomethylene diurea, dimethylene triurea, and trimethylene tetraurea. The purpose of this step is to facilitate drying, improve the physical properties of the products, and provide some slow-release nitrogen.

TVA pursued the development of UAP processes for several years. The first approach was to incorporate urea into their DAP slurry granulation process. In pilot-plant tests of this method, UAP grades ranging from 38-13-0 to 21-42-0 were produced. Urea was added in the granulator either as concentrated solution or solids (crystals or prills).  Coromandel Fertilizers Limited (CFL) in India implemented the TVA technology using a pre-neutraliser as well as operation with a pipe reactor [3]. The specification for UAP (28-28-0) produced at CFL is given in Table 1.

The basic concept of the process is that slurry obtained in the reactor (pre-neutraliser or pipe reactor), at a N:P ratio of 1.4-1.45 and water content of 16%-18%, is pumped into the granulator where urea, recycled materials, and ammonia are added, Granules are dried, sieved, and bagged. Unreacted ammonia from the reactor and granulator as well as the fine particles from the dryer and cooler cyclones are recovered in a scrubber by washing with circulated 30% phosphoric acid and water. The continuous stream of weak acid is fed back to the reactor. The parameters of the operation are given in Table 2.

In TVA’s production of UAP by a melt granulation process ammonium polyphosphate melt, and molten urea were co-granulated in a pugmill. The principal grade was 28-28-0, produced at a rate of 16 tph; 35-17-0 was also produced. The production rate was limited by the capacity of the urea synthesis unit. TVA also prilled a molten mixture of UAP in a pilot-plant oil-prilling unit. Yara (previously Norsk Hydro) developed a process on a pilot-plant scale for prilling UAP in air with or without potash addition [4]. In the TVA work, ammonium polyphosphate melt was premixed with urea melt immediately before prilling. In one variation of the Norsk Hydro process, preheated solid MAP is premixed with urea melt before prillinq; alternatively, preheated solid urea is premixed with MAP melt. The prills are 1,8 mm average, hard, and dense. A diagram of the Norsk Hydro process is shown in Figure 2.

Urea Superphosphate (USP)

Introduction

The USP process was prseented by AZF GRANDE PAROISSE at the IFA technical meeting in Amman [5] in 1994.

Even though mixing urea and superphosphate is not recommended because the products react to form lumps and a sticky mixture, GRANDE PAROISSE and their partner ARMINES found that as early as 1934 Dalman [6] had reported that urea and sulfuric acid form the following complexes:

CO(NH₂)₂•H₂SO₄ and 2CO(NH₂)₂•H₂SO₄

There are two eutectics E1 and E2 (Figure 3) that correspond respectively to 3.6 moles of urea to 1 mole of H₂SO₄ and 1.8 mole of urea to 1 mole of acid. While the melting point of urea is 132.7°C, both eutectics have a melting point of about 10°C.

The preparation of the mixtures of urea, sulfuric acid, and water at the mole ratios, 3.6: 1 and 1.8: 1, is· exothermic in both cases. Heat release with the first ratio is lower than with the second one and allows the preparation of the mixture under stable and reliable conditions at an equilibrium temperature of 60°- 70°C, which is ideal to acidulate phosphate rock.

In the manufacture of USP, the reaction of acidulation may be written as follows:

Ca₃(PO₄)₂ + 2H₂SO₄ + (8a + 2f) CO(NH₂)₂ + (e + 2bx) H₂O -> 2a[CaSO₄•4CO(NH₂)₂] + 2b(CaSO₄•xH₂O) + eCa(H₂PO₄)₂•H₂O + f[Ca(H₂PO₄)₂•2CO(NH₂)]

with a + b = 1, and e + f = 1.

It will be noticed that urea is associated with calcium sulphate rather than water of hydration [7]. But the sulfuric acid to rock ratio has not changed.

Identification of the Reaction Products

X-ray analysis of the product showed that:

• There is no more free urea.
• There is a substantial amount of tetra urea calcium sulphate.
• P₂O₅ as monocalcium phosphate may be linked to two ureas.

Quality of the Product:

20-10-0 USP simultaneously supplies urea nitrogen, sulphur, calcium, and phosphate; 95% of the citrate soluble phosphate is soluble in water. A typical analysis of the product after granulation is given in Table 3.

Storage properties

USP can be used as produced, i.e., in powdered form, or as a granular material. In the latter case, its physical properties are quite similar to those of urea-based NP and NPK grades. The product stores well. Its critical relative humidity is 65-70% at 20°C; consequently, it is suitable for bulk storage.

Advantages of the Process

• Zero liquid effluents, near zero fluorine emission. Because the fluorine in the phosphate rock is entirely recovered in the USP, a single stage scrubbing unit satisfies the most stringent standards while SSP requires three or four stages. Moreover, the scrubbing liquor is recycled into the preparation of the urea-sulfuric acid mixture.
• The phosphoric acid route is avoided. This is one of the rare processes that enables the production of a urea-based compound fertiliser without using phosphoric acid, thus avoiding its costs and nuisances.
• Granulation plants can be retrofitted to operate the USP process.
• The USP process is cost effective, because it uses the lowest cost raw materials based on concentrated sulfuric acid, it produces a drier product that requires no additional drying when used in powdered form or saves 40% of the drying energy when it is granulated. The production technology is simple and requires limited capital cost.
• It can be produced in SSP or TSP plants after an easy and cheap revamping.

Nevertheless, there are only a few plants of this type in the world, probably because of the not-ideal handling characteristics of the NP and the fact that carrying urea over a superphosphate site will lead to much contamination with detrimental effects (sludge formation).

References

1. Ando, J. 1970. “Development in Granulation of Mixed Fertilizers in Japan,” Fertilizer Industry Round Table Proceedings, Tokyo, Japan, 85-92.

2. Kuwabara, M., S_ Hayamizu, and A. Hatekeyama.1977. ”Trends in Urea-Based Granular Compound

3. Ramadurai, S. 1990. “Operational Experiences With NP /NPK Granulation at Coromandel Fertilisers Limited – lndia,” IN J.J. Schultz and G. Hoffmeister (Eds.), Urea-Based NPK Plant Design and Operating Alternatives, Workshop Proceedings, pp. 21-26, SP-15, IFDC, Muscle Shoals, AL, U.S.A.

4. “Urea Based NP and NPK Fertilizers.” 1975. Phosphorous and Potassium, 76:48-54.

5 Limousin, L., B.Neveu,and J. B. Peudpiece, P.Achard, and Y. Schwob. 1994 “A New Way to

 Produce Urea-Superphosphate Fertilizers: The AZF USP Process,” pp. 172-186, Proceedings of IFA Technical Conference, Amman, Jordan.

6. Dalman, L. H. 1934. “Ternary Systems of Urea and Acids,” JACS, 56:549-553.

7. Frazier, A. W, J. R. Lehr, and J. P, Smith. 1967. “Urea-Monocalcium Phosphate, a Component of Mixed Fertilizers,” J. Agr, Food Chem., 15(2):345-347.

Links to related IFS Proceedings

67, (1961), Developments in Phosphoric Acid Manufacture, W C Weber, Frank W Edwards

76, (1963), Potassium Metaphosphate: A Novel Method of Manufacture and a Summary of its Behaviour as a Fertiliser, F J Harris

102, (1968), Production of High Nitrogen NPK Granular Fertilisers, W C Weber, I S Mangat

123, (1971), Phosphate Fertiliser Sources: Agronomic Effectiveness in Relation to Chemical and Physical Properties, G L Terman

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

237, (1985), Production of Chloride-Free NPK Fertiliser and Feedgrade Dicalcium Phosphate, K C Knudsen

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

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

783, (2016), Granulation of Complex Fertilisers, H Kiiski and A Kells

821, (2018), Approaches to improving the quality of phosphoric acid, T Henry