Nitrophosphates

Nitrophosphates form a family of nitrogen and phosphate compound fertilisers, manufactured by digesting phosphate rock with nitric acid, either alone or blended with sulphuric or phosphoric acid, with optional neutralisation by ammonia^[1]. Their nitrogen is divided between ammoniacal and nitric forms, whilst their phosphorus shows partial water solubility and complete solubility in neutral ammonium citrate^[1].

Manufacturing process

The nitrophosphate route, also known as the Odda process, is distinguished by the use of nitric acid to attack phosphate rock, followed by cooling the digestion solution to separate calcium ions as calcium nitrate tetrahydrate crystals^[2]. This method avoids phosphogypsum formation, unlike sulphuric acid-based processes^[2]. The facility is typically integrated with ammonia, nitric acid and calcium ammonium nitrate production units^[2]. In Europe, five sites operate this process: BASF at Antwerp, AMI at Linz, Lovochemie at Lovosice, and two Yara plants at Porsgrunn and Glomfjord^[3]. Plant capacities range from a few hundred to over 3,000 tonnes per day; a standard-sized plant produces roughly 50 tonnes per hour, i.e. 1,200 tonnes per day or 350,000 tonnes per year^[3].

Agronomic characteristics and uses

Nitrophosphates have their nitrogen fraction split equally between ammoniacal and nitric forms^[4]. Water solubility of phosphorus ranges from 30 to 85% depending on grade, with the remainder being citrate soluble^[1][4]. Products with low water-soluble phosphate content prove more effective in acid soils or those with medium phosphate fertility, particularly for long-duration crops, whilst water solubility of at least 60% is generally preferred in neutral to alkaline soils for short growing cycles^[4]. The 23–23–0 grade, frequently cited as an example, illustrates the technical specifications of this fertiliser type: under common international standards, the maximum moisture content is set at 1% by mass, whilst specifications developed from Indian experience raise this threshold to 1.5%^[1]. For this same grade, total nitrogen must reach 23% minimum, with at least 11.5% ammoniacal nitrogen and at most 11.5% nitric nitrogen; P₂O₅ soluble in neutral ammonium citrate must be at least 23%, including 18.5% water-soluble minimum^[1].

Market and trade

The nitrogen fertiliser market, the segment to which nitrophosphates belong, is marked by considerable fragmentation owing to the wide availability of the fundamental raw materials, natural gas and air^[5]. This dispersion contrasts with the greater concentration of phosphate and potash markets, dominated by a handful of countries holding phosphate rock or potash mineral deposits^[5]. In 2023, the three leading potash-producing countries accounted for roughly 70% of the world market, whilst the nitrogen industry remained split amongst numerous players including CF Industries, Yara, Nutrien, Ostchem, OCI, TogliattiAzot, Koch and EuroChem^[5]. Compound fertiliser consumption in Western Europe stood, in the early 2000s, at 2,171 kt of nitrogen units, 1,739 kt of P₂O₅ and 2,253 kt of K₂O for NPKs, to which were added 461 kt of N and 807 kt of P₂O₅ for NPs^[3][6].

Environmental and regulatory aspects

The nitrophosphate route generates specific emissions differing from those of the mixed-acid route. Discharges from phosphate rock digestion and calcium nitrate tetrahydrate conversion amount to 1.2 kg of total nitrogen and 0.4 kg of P₂O₅ per tonne of product^[7]. The neutralisation, granulation, drying and coating stage emits 0.2 kg of total nitrogen and 0.03 kg of fluorides per tonne of NPK, figures identical to the mixed-acid route for these items^[7]. Cadmium content in digestion section discharges reaches 0.0014 kg per tonne, i.e. 1.4 g/t^[7]. The absence of phosphogypsum production constitutes a notable environmental advantage of this route over the sulphuric acid process^[2].