In the distant past, compound fertilisers were made in batch mixers in non-granular form. They were sometimes termed ‘pulverised’.

Initially, superphosphates were the preferred sources of phosphate. Chemical reactions did occur, however, in the storage piles, and the piles became very severely caked after a short period (‘pile set’). The continuation of the chemical reactions in superphosphates was termed ‘curing’, which was often accelerated by moving the piles several times during the storage period. It was often necessary to blast the caked piles with explosives and to process the lumped material through heavy-duty crushers.

Granulation occurred more or less accidentally when superphosphate was ammoniated in rotary batch mixers. Nitrogen solutions containing free ammonia were used to ‘ammoniate’ the normal single superphosphate or the triple superphosphate to make various ammonium calcium phosphate compounds. Solutions containing free ammonia were used because these were the least expensive sources of nitrogen and because the resulting compounds were fairly stable. The liquid phase supplied with the ammoniating solution wetted the dry materials. In the rotary mixers, the wet particles rolled together forming small agglomerates. The amount of liquid phase increased as various salts dissolved in the water. The chemical reactions caused the temperature to increase and this further promoted additional liquid phase.

It was discovered that the extent of granulation could be controlled by formulating (varying the relative amounts and/or adding additional substances) the materials, both liquid and solid, such as to yield a maximum number of agglomerates of the desired size. Fertilisers in this form still contained smaller particles, which were generally not separated out from the product as the granules were somewhat wet and soft and screening was nearly impossible. These products were termed ‘semi-granular.’ They usually handled better than ‘pulverised’ fertilisers and became the preferred product form for farmers.

However, these semi-granular products still tended to cake in bulk piles and in bags. Segregation of specific nutrients from the mixtures was a common problem, caused by the non-uniform (irregular in size and shape) sizing. Efforts were then directed toward controlling the processes to maximise proper granule formation. Equipment was designed to produce smoother, more spherical granules, and to dry and cool the granules, and to separate and recycle the off size portion of the process stream. This is how modern granulation units came to be developed.

The exact beginning of fertiliser granulation is difficult to trace precisely. In 1926, I.G. Farben in Germany produced a range of fertilisers called Nitrophoska. In 1931 the United States Department of Agriculture stated ‘The Nitrophoskas differ from ordinary salt mixtures in that they have been subjected to a graining treatment in the process of mixing’. The process involved granulating a slurry of diammonium phosphate (DAP) and ammonium nitrate (AN) in pug mills with additional potash salts in order to obtain more homogeneous particles.

The first production of superphosphate in a form that could properly been called ‘granular’ was in 1935, by the Davison Chemical Company. In this continuous process, the superphosphate was disintegrated from lumps, then agglomerated by spraying water on the crumbed mass when passing through a horizontal, cylindrical drum; the resulting soft and damp agglomerates were subsequently dried and hardened in a direct fired, co-currently operating rotary drier.

However, Nordengren in his work on superphosphates (1935-1960) claims that the idea of granulation originated in 1926, in America, and that the first granular fertilisers were Chilean nitrate of soda. Crystalline soda was melted and prilled for granular form.

Whatever the first country to develop granular fertilizers, it is worth realizing that granulation of fertilizers is a technique developed only during the past century.

In 1931 Abatross Superfosfaatfabrieken (The Netherlands) began using an extrusion process to granulate compound fertilisers from superphosphate, ammonium sulphate, ammonium phosphate, and potash salts. Eirich mixers were used to premix materials. With surprise,  it was discovered that an Eirich mixer was capable of granulating the fertiliser mixtures. The Eirich mixer process was adopted by the Fisons Fertilisers company in 1937.

In the 1930s granulation was considered to be an extra expense which farmers would not be able to pay for. After World War 2 there was a need to convert ammonium nitrate plants, which used to supply the munitions industry, to supply agricultural needs. Ammonium nitrate being very prone to caking, the need for improved particle and product qualities and thus for granulation became evident.

For example, in the Proceedings of the Fertiliser Society (No. 2, 1947) Nordengren summarises:

‘The granulator should be put on top of the drier so that the continuous flow of newly formed moist granules can drop right into the drier. The combustion gases used for drying will pass in the same direction as the material so that the pipe leading the material into the drier will be just in the way of those hot gases. This is a weak point, and we have tried different constructions in order to protect the pipe from being destroyed by the heat. We now use double walled, water cooled pipe constructed to avoid all corners where boiler scale might be collected. Another difficulty lies in the disposition of the moist product to stick to the sides of the said pipe.’

The main development in granulation processes has taken place since the 1950s and the sizes (capacities) of granulation units have steadily increased. Daily production throughputs have increased to 2.000 (NPK) – 5.000 (urea) per granulation per day. This has also become possible due to better management of the recycle ratios by reducing the moisture content of the melts and solutions and by increasing granulation temperatures. Much development in the granulation of straight fertilisers has taken place. The production and granulation of complex NPK fertilisers is mainly based on technologies developed between 1950 and 1980. Product quality has also greatly improved, not the least by reducing the moisture in the final product through better control of granulation conditions.

Tennessee Valley Authority

In early 1950s the Tennessee Valley Authority (TVA) developed a continuous ammoniator for more efficient ammoniation of single and triple superphosphates. It was noted that the product was often granular, and that the granulation could be controlled by controlling the amount of steam and water. In 1953, the production of NPK grades 6-12-12 and 10-20-20 was demonstrated in the pilot plant. Twenty three companies had obtained a license to use the process by 1955 and in USA in 1962 already 164 plants were using the process. By the early 1960s nearly 80% of the mixed fertiliser market was accounted for by ammoniation-granulation plants.

The granulation was mainly controlled by means of the recycle ratios. As well, most of the early nitrophosphate plants used a pug mill (or blunger) for granulating the slurry together with recycle material.

In the 1960s the spherodiser process was developed by C&I Girdler. This particulation technique combines the granulation and (partial) drying of slurries. It includes a rotary drum with flights which  lift up the granules and then drops the particles through a stream of hot air while slurry is sprayed onto the granules. Granulation is obtained through accretion with each granule receiving a thin coating of slurry, which is instantly dried, each time the granule moves through the drum. The obtained granules were hard and round, introducing a new standard for the physical properties of fertilisers.

A prilled nitrophosphate (NP) product, 20-20, was developed by DSM in the 1960s. The ammonium nitrate-phosphate solution was evaporated to an almost anhydrous melt and was then prilled by using a rotating, perforated, conical bucket. This process formed droplets (prills) which solidified as they fell through a stream of ascending air. Albright and Wilson adapted the process by mixing fine, preheated potash with the melt before prilling. Norsk Hydro developed a similar prilling process for NP and NPK nitrophosphate fertilisers that is still used in several large-scale units. As well, granulation processes for homogeneous nitrophosphate NP(K) products were developed by BASF.

TVA’s invention of the pipe cross reactor in 1974 was an important advance in production technology for multinutrient homogeneous and granulated  fertilisers, enabling higher production capacities and lower energy requirements. Largely because of the oil embargo of 1974, methods for decreasing the cost of drying fertilisers received a high priority. This also implied using low cost steam to produce a slurry or melt that is relatively dry and thus requires little or no fossil fuel (gas or oil) for drying.

Links to related IFS Proceedings

2, (1947), Granulation of Phosphatic Fertiliser – Theory and Practice,
S Nordengren.

7, (1949), Slurry Dispersion Methods for the Granulation of Superphosphate Fertilisers, J T Procter.

55, (1959), Mechanism of Granule Formation,
D M Newitt, A L Papadopoulos.

59, (1960), Granulation Techniques – New Developments,
P J van den Berg, G Hallie.

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

127, (1972), Low Recycle NPK Granulation – Design and Practical Aspects,
G B Whyte.

Links to external resources

Ammonia and Fertilizers, H. Górecki, Wroclaw, Poland, pages 132 – 164, in Chemical Engineering and Chemical Process Technology, Chemical Engineering Education and Main Products, Volume V, edited by R. Pohorecki, J. Bridgwater, M. Molzahn, R. Gani, and C. Gallegos, 2010.

Heck, R.R. (1968). ‘Modified Diammonium Phosphate Production in a TVA Type Granulation Plant,’ ISMA Technical Conference, Brussels, Belgium, 10-13 September.

Heudier, C. (1953). ‘Granulation of Superphosphate,’ ISMA Technical Conference, Cambridge, United Kingdom, 15-17 September.

Moraillon, P. (1959). ‘Manufacture and Granulation of Enriched Superphosphate,’ ISMA Technical Conference, Stockholm, Sweden, 14-18 September.

Nielsson, F. T. (ed). (1986). Manual of Fertilizer Processing, Fertilizer science and technology series – volume 5.

Thiessenhausen, W. (1955). ‘The Granulation of Superphosphate and Mixed Fertilisers,’ ISMA Technical Conference, Aarhus, Denmark, 19-22 September.