Odda process

The Odda process denotes the nitrophosphate route for producing compound nitrogen-phosphate-potassium (NPK) fertilisers, based on attacking phosphate rock with nitric acid rather than sulphuric or phosphoric acid. This technology, referred to as "nitrophosphate route (ODDA process)" in European industrial nomenclature, is distinguished by the absence of phosphogypsum formation, an unavoidable by-product of conventional sulphuric acid-based processes^[1][2].

Manufacturing principles

Digestion of the phosphate rock is carried out using nitric acid. The resulting solution is then cooled to induce crystallisation and separation of calcium ions as calcium nitrate tetrahydrate (CNTH, Ca(NO₃)₂·4H₂O)^[2]. This crystallisation step forms the heart of the process: it enables removal of the bulk of calcium from the phosphate solution, thereby preventing calcium sulphate precipitation. The filtered nitrophosphoric acid, termed "NP acid", undergoes neutralisation and evaporation before final composition adjustment through incorporation of ammonia, urea, ammonium sulphate, ammonium nitrate solutions, sulphuric or phosphoric acids, as well as potassium salts and other nutrients^[1]. Particle formation and finishing yield the final NPK fertiliser.

The Odda process generally requires integration with production units for ammonia, nitric acid and calcium ammonium nitrate, owing to the raw materials required and the co-products generated^[2]. Liquid ammonia may additionally be used economically for cooling the nitrophosphoric section, notably during CNTH crystallisation^[2].

Calcium co-product valorisation

The calcium nitrate tetrahydrate isolated during crystallisation constitutes a major by-product of the process. Two main valorisation routes exist. The first involves converting CNTH into commercial calcium nitrate fertiliser^[3]. The second, more frequently implemented in integrated complexes, transforms CNTH into ammonium nitrate and calcium carbonate (lime) by reaction with ammonia and carbon dioxide. The carbon dioxide typically comes from the adjacent ammonia unit, where it is produced in excess relative to the stoichiometric requirements of urea synthesis^[3][4]. The reaction takes place in a carbonation column where dissolved ammonia and CO₂ form ammonium carbonate, which reacts with dissolved calcium nitrate to produce ammonium nitrate and precipitated calcium carbonate. Following separation on a belt filter, the ammonium nitrate solution at approximately 65% is concentrated by two-stage evaporation; the calcium carbonate may be reused for manufacturing calcium ammonium nitrate^[3].

Environmental and regulatory aspects

Aqueous discharges from the Odda process exhibit specific characteristics. For a production capacity of 1,200 t/d of NPK fertiliser, effluents after neutralisation show notably a flow rate of 3,450 m³/d, a total nitrogen content of 0.2 kg/t of product, and fluoride concentrations of 0.29 kg/t^[5]. The effluents originate from the nitrophosphoric section and from CNTH conversion. Fluorine, present in the phosphate rock, constitutes a particular monitoring parameter: wastewater from washing nitric acid-digested rock and from sand washing contains phosphorus, fluorine and nitric nitrogen^[5].

At European regulatory level, legislation on fertilising products governs the placing on the market of fertilisers, without specific distinction of manufacturing process; requirements relate to composition, health safety and criteria for valorisation of secondary raw materials.

Industrial position and market

The Odda process occupies a niche position in the global NPK fertiliser industry, dominated by the mixed acid route (sulphuric and phosphoric). Its industrial implementation requires integrated chemical complexes simultaneously possessing ammonia, nitric acid and calcium ammonium nitrate capacities, which limits its geographical spread^[2]. The main competitive advantage lies in the absence of phosphogypsum to store or valorise, a major environmental constraint of sulphuric acid-based processes, and in the flexibility to formulate certain NPK grades rich in nitrate nitrogen. In Europe, a few historical sites still operate this technology, notably in configurations integrated with nitrate and CAN production.