# Heavy inorganics: salt, sulphur, nitrogen and phosphorus: From ammonia and rock to fertiliser: nitric acid, urea, ammonium nitrate and phosphates Source page: https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p4/ [Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 4: Heavy inorganics: salt, sulphur, nitrogen and phosphorus](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/) Part 4 of 5 ## From ammonia and rock to fertiliser: nitric acid, urea, ammonium nitrate and phosphates Little ammonia is applied to fields as it is. It is turned into nitric acid and ammonium nitrate, into urea with the CO2 from its own plant, or into ammonium phosphates with phosphoric acid made from rock and sulphuric acid. Each step is simple chemistry with a recycle or waste that sets its cost. ### 4.1 Nitric acid: ammonia burns on platinum gauze in a millisecond, then absorption takes the time The Ostwald process, discovered in 1900, has three steps (Kent). First, 4NH3 + 5O2 → 4NO + 6H2O on a pad of woven platinum-rhodium gauze. In Shreve's example, air with about 10% ammonia passes through about 30 gauze layers. The reaction is over in under a millisecond, and the gas leaves at about 940 C with 94-95% of the ammonia converted to nitric oxide (NO). Second, NO is oxidised to NO2 as the gas cools, a slow step that speeds up with pressure. Third, NO2 is absorbed in water in a tall column with cooled trays: 3NO2 + H2O → 2HNO3 + NO. The product is acid of 50-70%, below the 68.4% azeotrope (the strength at which boiling no longer concentrates it). The design conflict is that oxidation of ammonia prefers low pressure, and absorption high pressure. Single-pressure plants, at about 10 bar, cost up to 30% less to build and were preferred in the US. Dual-pressure plants burn the ammonia at medium pressure and compress the gas for absorption. They give better ammonia yield and lose less platinum, so they were preferred in Europe, where ammonia was dearer (Kent). Side reactions turn some ammonia into nitrogen and some into nitrous oxide (N2O). Kent gives 300-3,500 ppmv of N2O in absorber tail gas. N2O is a greenhouse gas, although Kent's chapter discusses only NOx limits, such as 1.5 kg of NOx per tonne in the US and 200 ppm in the EU. These are met by extra absorption and catalytic reduction with ammonia. Nitric acid plants export steam. About 65% of the world's nitric acid, 53 Mt in 1999, went into ammonium nitrate fertiliser (Kent). Process flowchart Dual-pressure nitric acid plant, per tonne of 100% HNO3 Inputs **Ammonia**0.279 t (0.285-0.291 t in single-pressure plants) **Air**about 3,000 Nm3 **Platinum, net of recovery**30 mg **Power**about 2 kWh net (compressors driven by steam and tail-gas turbines) **LP steam import**0.06 t **Cooling water**110 m3 Process steps 1 **Ammonia vaporiser and air compressor** air and NH3 mixed to about 10% NH3 2 **Converter** Pt-Rh gauze, about 850-900 C at 4.5 bar; about 96% yield to NO 3 **Waste heat boiler and cooler-condenser** raises steam; weak acid condenses 4 **NOx compressor** from about 4.5 to 12 bar for absorption 5 **Absorption column** cooled sieve trays; water at top; 3NO2 + H2O → 2HNO3 + NO 6 **Bleacher** secondary air strips dissolved NOx from the acid 7 **Tail gas treatment and expander** catalytic NOx reduction to about 200 ppmv; turbine recovers power Outputs **Nitric acid**1 t (as 60-68% acid) **HP steam export**0.54 t at 42 kg/cm2, 400 C **Tail gas**NOx about 200 ppmv; N2O 300-3,500 ppmv before any N2O control Kent's dual-pressure case runs 210 days between gauze changes. Source: Kent (2012), Ch 22 Synthetic Nitrogen Products, Table 22.18; Dryden (1997), Ch IIE Nitrogen Industries, Kent pdf pp. 912-916; Dryden pdf p. 141 Worked example #### How efficient is 0.28 t of ammonia? Each tonne of HNO3 contains nitrogen equal to 17/63 = 0.270 t of ammonia. Kent's plant data show 0.279-0.291 t used, so 93-97% of the ammonia's nitrogen ends up in the acid. The rest is lost as N2 or N2O, or destroys NOx in the tail gas. With ammonia the main cost, each point of yield matters, which is why European plants paid more for dual-pressure designs. - Kent (2012), Ch 22 Synthetic Nitrogen Products · pdf pp. 910-917 - Shreve (1984), Ch 18 Nitrogen Industries · pdf pp. 332-336 - Dryden (1997), Ch IIE Nitrogen Industries · pdf pp. 138-142 ### 4.2 Urea joins ammonia to the CO2 from the same plant, and ammonium nitrate neutralises ammonia with nitric acid Urea forms in two steps (Kent). Ammonia and CO2 combine quickly, giving off heat, to form ammonium carbamate: 2NH3 + CO2 → NH2COONH4. The carbamate then slowly loses water to give urea, NH2COONH4 → CO(NH2)2 + H2O. This second step absorbs heat and stops well short of completion, at 50-80% conversion of the CO2 per pass. Shreve describes a reactor at 14 MPa and 170-190 C. Most of the plant handles the unconverted carbamate, which is very corrosive. Total-recycle plants now dominate. In the Stamicarbon process, the incoming CO2 strips the reactor effluent at full synthesis pressure, so recovered gases return with only a pump, not a gas compressor, and the heat of carbamate formation raises steam (Shreve). Over 99% of the ammonia and CO2 become urea. Dryden's figures show why recycle won. A once-through plant used 1.15 t of ammonia and 210 kWh per tonne; total recycle needs about 0.6 t of ammonia, 0.77 t of CO2 and 145 kWh, close to the theoretical 0.567 t of ammonia and 0.733 t of CO2. Long heating forms biuret, which harms some crops. Urea plants sit beside ammonia plants, needing both their ammonia and their CO2. Kent's rule is that the cash cost of urea is 1.4-1.6 times the price of ammonia. At 46% nitrogen it is the most concentrated solid nitrogen fertiliser. Ammonium nitrate is simpler. Ammonia gas neutralises 50-70% nitric acid, and the heat of reaction boils off much of the water. An evaporator concentrates the melt to 96-99%, and it is prilled in towers or granulated (Kent). Dryden gives 0.22 t of ammonia and 1.38 t of 60% acid per tonne. At 33-35% nitrogen it acts fast, but it absorbs water from air and becomes explosive when mixed with fuel oil. Process flowchart Total-recycle urea plant with CO2 stripping, per tonne of urea Inputs **Liquid ammonia**about 0.6 t (theoretical 0.567 t) **Carbon dioxide**0.77 t (theoretical 0.733 t) **Power**about 145 kWh (Dryden, total recycle) **Steam**about 2.4 t (Dryden, total recycle), for stripping and evaporation Process steps 1 **CO2 compressor** a little air added to protect stainless steel from corrosion 2 **High-pressure carbamate condenser and reactor** 14 MPa, 170-190 C; carbamate forms, then 50-80% goes on to urea 3 **CO2 stripper at synthesis pressure** decomposes unreacted carbamate; gases return without recompression 4 **Low-pressure decomposer** about 400 kPa; last carbamate removed and recycled as solution 5 **Vacuum evaporation** to under 1% water, briefly, to limit biuret 6 **Prilling tower or granulator** melt sprayed and cooled; granulators can exceed 3,000 t/day Outputs **Urea**1 t (46% N; prills 0.8-1.5% biuret) **Process steam**raised from carbamate heat **Process water**about 0.3 t formed (stoichiometric) Kent: single-train designs of 2,300 to over 3,000 t/day. Source: Kent (2012), Ch 22 Synthetic Nitrogen Products; Shreve (1984), Ch 18 Nitrogen Industries; Dryden (1997), Ch IIE Nitrogen Industries, Kent pdf pp. 919-922; Shreve pdf pp. 330-331; Dryden pdf pp. 146-150 Common misreading #### Urea does not lock up CO2 Urea uses CO2 from the ammonia plant's reformer, and Shreve put urea at 40% of US CO2 use. But Kent notes that urea dissolved in water slowly breaks down to carbamate and then to ammonia and CO2. Once urea is spread on a field, the CO2 returns to the air. Using CO2 in urea delays its release rather than storing it. - Kent (2012), Ch 22 Synthetic Nitrogen Products · pdf pp. 885, 917-921 - Shreve (1984), Ch 18 Nitrogen Industries · pdf pp. 328-330 - Dryden (1997), Ch IIE Nitrogen Industries · pdf pp. 144-155 ### 4.3 Wet-process phosphoric acid trades sulphuric acid for gypsum, about five tonnes per tonne of P2O5 Phosphate rock is mostly fluorapatite, Ca10(PO4)6F2. In the wet process the rock reacts with sulphuric acid in a slurry of recycled phosphoric acid and gypsum crystals: Ca3(PO4)2 + 3H2SO4 + 6H2O → 2H3PO4 + 3CaSO4.2H2O (Kent). The skill is to grow gypsum crystals that filter well without coating the rock. In the dihydrate process the slurry is held at 80-85 C with 2-3% free sulphuric acid. About 10-15 volumes of slurry are recycled for every volume sent to the filter, and reactors hold 2-6 hours of slurry. The acid leaves the filter at 26-32% P2O5 and is evaporated to 40%, or to 52-54% merchant-grade acid for trade. Hemihydrate processes run hotter, at 95-100 C, and stronger, giving 38-42% acid directly. This saves 2.3-3.5 GJ of evaporation steam per tonne (Shreve). Yields through the filter are 95-97% for dihydrate, 91-94% for single-stage hemihydrate and 98-99% for two-stage processes (Kent). Dihydrate still leads: it accepts wet rock slurry, runs reliably and is proven at over 2,000 t/day of P2O5. The waste is enormous. Kent gives about 5 t of dry gypsum per tonne of P2O5. It carries P2O5, fluorine, low-level radioactivity and traces of heavy metals. Most is stacked; only in Europe and Japan, where natural gypsum is dear, has much gone into building products. Fluorine, about 3.5% of the rock, is partly recovered as fluosilicic acid. Pure acid once came from the electric furnace, which smelts rock, silica and coke to phosphorus vapour that is burned and hydrated. Shreve's figures are 18,840 MJ of electricity per tonne of 85% acid. By 2001 only one US furnace plant remained. Emission rules, high costs and cheaper purification of wet acid by solvent extraction closed the rest (Kent). Process flowchart Wet-process (dihydrate) phosphoric acid, per tonne of P2O5 Inputs **Phosphate rock (32% P2O5)**about 3.45 t **Sulphuric acid (93-98%)**about 2.8 t **Water**process and recycled pond water **Steam**for vacuum evaporation Process steps 1 **Rock grinding** often wet, as 65-68% solids slurry 2 **Reactor** 80-85 C; 2-3% free H2SO4; 2-6 hours; vacuum or air cooling 3 **Filter** tilting pan or belt; two or three countercurrent washes; 95-97% yield 4 **Vacuum evaporation** from 26-32% to 40% or 52-54% P2O5 5 **Fluorine scrubbing** vapours give 20-25% fluosilicic acid Outputs **Phosphoric acid**1 t P2O5 (as 28-54% acid) **Phosphogypsum**about 5 t dry **Fluosilicic acid**by-product Rock and acid converted from Dryden's figures per tonne of H3PO4 (2.5 t and 2.0 t) using 0.724 t P2O5 per t H3PO4; Dryden's gypsum figure is lower: 2.7 t per t H3PO4, about 3.7 t per t P2O5. Source: Kent (2012), Ch 23 Phosphorus and Phosphates; Dryden (1997), Ch IID Fertilizer Industry and IIF Phosphorus Industries, Kent pdf pp. 953-961; Dryden pdf p. 163 Case #### Paradeep and India's rock imports Kent shows the Oswal plant at Paradeep, Orissa, under construction in 1999 as the world's largest phosphoric acid plant: 2,650 t/day of P2O5, with a 4,350 m3 concrete reactor and five large belt filters. Dryden explains why it sits on the coast: about 75% of Indian deposits are low grade, and in 1990-91 India mined about 0.55 Mt of rock and imported 3.5 Mt. Rock and sulphur both arrive by sea. - Kent (2012), Ch 23 Phosphorus and Phosphates · pdf pp. 945, 951-962 - Shreve (1984), Ch 16 Phosphorus Industries · pdf pp. 294-298 - Dryden (1997), Ch IID Fertilizer Industry and IIF Phosphorus Industries · pdf pp. 156-157, 163-165 ### 4.4 DAP, MAP, superphosphates and NPK: the choice is about nutrient per tonne shipped Fertiliser buyers pay for nutrients, not tonnes, so concentration matters. Single superphosphate (SSP), the oldest product, is made by mixing ground rock with sulphuric acid, about 0.6 kg of acid per kg of rock. It holds only about 20% available P2O5, because the gypsum stays in. Kent notes it is cheaper to ship 32% rock to local SSP works than to ship SSP itself. Triple superphosphate (TSP) attacks the rock with phosphoric acid instead, so no gypsum is added, and reaches 45-46% P2O5. Ammonium phosphates took over. In the TVA process, wet-process acid at about 40% P2O5 is part-neutralised with ammonia in a preneutraliser to a mole ratio of about 1.4 NH3 to H3PO4. At that ratio the salt is most soluble, so the slurry stays fluid with only 16-20% water. It is sprayed onto a rolling bed of recycled granules in a rotary drum, where more ammonia brings the ratio to 1.85-1.94. Solubility drops, the slurry solidifies on the granules, and the product is dried gently (Kent). The product grade is 18-46-0 (per cent N, P2O5 and K2O). MAP, stopped near a ratio of 1.0, carries 52-54% P2O5 and so more phosphate per tonne shipped. The rise was fast: 0.5 Mt of P2O5 as ammonium phosphates in 1960, 16.7 Mt in 1999, and two-thirds of world phosphate fertiliser (Kent). Nitrophosphates attack rock with nitric acid instead of sulphuric, which suits countries short of sulphur; India's Trombay plant used this route (Dryden). Policy shaped India's mix. Dryden records that no new superphosphate plants were licensed after 1962, to push farmers towards higher-analysis mixed fertilisers. Kent shows India using 28.1 Mt of nutrients in 2010/11, second only to China. Process flowchart Granular DAP by the TVA process, per tonne of 18-46-0 Inputs **Ammonia**220 kg **Phosphoric acid**465 kg (fits the grade only if read as P2O5; fed at 40-45% P2O5) **Fuel**525 MJ **Electricity**200 MJ Process steps 1 **Preneutraliser** NH3:H3PO4 mole ratio about 1.4; slurry 16-20% water; reaction heat evaporates water 2 **Ammoniator-granulator drum** more NH3 sparged under the bed; ratio 1.85-1.94; recycle 5-7 kg per kg 3 **Scrubber** unabsorbed NH3 caught in 28-30% phosphoric acid 4 **Dryer** gentle drying to 180-190 F product temperature 5 **Screens, crusher and cooler** oversize crushed; fines recycled Outputs **DAP**1 t (18% N, 46% P2O5; 1.5-2% moisture) **Recycle fines**returned to granulator **Scrubbed off-gas**vented Kent: many plants make about 50 t/hour, some 70-100 t/hour. Source: Shreve (1984), Ch 16 Phosphorus Industries; Kent (2012), Ch 24 Fertilizers and Food Production, Shreve pdf pp. 302-303; Kent pdf pp. 980-981 Worked example #### Reading a DAP recipe Shreve's flowchart gives 220 kg of ammonia and 465 kg of phosphoric acid per tonne of 18-46-0 DAP, plus 525 MJ of fuel and 200 MJ of electricity. Check it: 220 kg of NH3 holds 220 x 14/17 = 181 kg of nitrogen, 18% of a tonne. Read as P2O5, which is the only reading that fits the grade, 465 kg is 46.5%. The recipe is the grade in kilograms. The plant's own energy is small; most embedded energy sits upstream in the ammonia and sulphuric acid. - Kent (2012), Ch 24 Fertilizers and Food Production · pdf pp. 969, 975-981 - Shreve (1984), Ch 16 Phosphorus Industries · pdf pp. 293, 302-303 - Dryden (1997), Ch IID Fertilizer Industry and IIF Phosphorus Industries · pdf pp. 122, 172-176 ### See it in the table [H₂O Water](https://www.materialinsights.org/commodities/water/)[P Phosphate rock](https://www.materialinsights.org/commodities/phosphate/)[Pt Platinum](https://www.materialinsights.org/commodities/platinum/)[Rh Rhodium](https://www.materialinsights.org/commodities/rhodium/)[NaCl Salt](https://www.materialinsights.org/commodities/salt/)[Gyp Gypsum](https://www.materialinsights.org/commodities/gypsum/)[H2SO4 Sulphuric acid](https://www.materialinsights.org/chemicals/sulphuric_acid/)[HNO3 Nitric acid](https://www.materialinsights.org/chemicals/nitric_acid/)[H3PO4 Phosphoric acid](https://www.materialinsights.org/chemicals/phosphoric_acid/)[NH3 Ammonia](https://www.materialinsights.org/chemicals/ammonia/)[Ur Urea](https://www.materialinsights.org/chemicals/urea/)[AN Ammonium nitrate](https://www.materialinsights.org/chemicals/ammonium_nitrate/) ### Check your understanding Why do European nitric acid plants tend to use dual-pressure designs? Ammonia oxidation is more efficient at lower pressure while absorption needs high pressure. Dual-pressure plants cost more but save ammonia and platinum, which pays where ammonia is expensive. In the TVA DAP process, why is the acid first ammoniated only to a mole ratio of about 1.4? At about 1.4 the slurry is pumpable with only 16-20% water. Ammoniating further in the drum lowers solubility, so the slurry solidifies on the granules. Roughly how much phosphogypsum does a wet-process plant make per tonne of P2O5, according to Kent? Kent gives about 5 t of dry gypsum per tonne of P2O5, mostly stacked as waste. This is why gypsum disposal is a major cost and environmental issue. [← Part 3: Ammonia: fixing nitrogen with natural gas](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p3/)[Mark read · Part 5: Industrial gases, and the heavy inorganics side by side →](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p5/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/index.md) - [Sulphur and sulphuric acid: the workhorse acid](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p1/index.md) - [Salt: chlor-alkali, hydrochloric acid and soda ash](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p2/index.md) - [Ammonia: fixing nitrogen with natural gas](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p3/index.md) - [Industrial gases, and the heavy inorganics side by side](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/review/index.md)