Fertilisers and inorganic chemicals

In short

This topic covers the heavy inorganic chemical industry: ammonia and its downstream nitrogen fertilisers (urea, ammonium nitrate, CAN, ammonium sulphate), phosphatic fertilisers (SSP, TSP, DAP, NPK complexes), potash, and the "big four" industrial inorganics — sulphuric acid, nitric acid, caustic soda/chlorine and soda ash. India is the world's second-largest consumer of fertilisers and a very large importer of urea, DAP, rock phosphate, sulphur and potash, so these plants are among the biggest chemical employers in the country. In exams it appears as Chemical Technology / Process Industries (flow sheets, operating conditions, catalysts, material balances) and inside Stoichiometry, Reaction Engineering and Mass Transfer as equilibrium conversion, recycle and absorption problems. Employers who test it: IFFCO, KRIBHCO, RCF, NFL, GSFC, GNFC, FACT, Coromandel International, Chambal (Zuari), Paradeep Phosphates, Deepak Fertilisers, Tata Chemicals, GHCL, Aditya Birla Chemicals, DCM Shriram, plus PSU recruitment through GATE.

Explained

Ammonia: feedstock to synthesis loop

• Overall route (natural-gas based, ~80% of Indian capacity): desulphurisation → primary steam reforming → secondary (air-blown) reforming → high- and low-temperature shift → CO₂ removal → methanation → compression → synthesis loop.
• Desulphurisation: ZnO bed (plus CoMo hydrogenator for organic sulphur) brings S below ~0.1 ppm because nickel and copper catalysts downstream are sulphur-poisoned.
• Primary reformer: CH₄ + H₂O ⇌ CO + 3H₂ (endothermic, ΔH ≈ +206 kJ/mol) over Ni/α-alumina in vertical tubes, 800–900 °C outlet, 30–40 bar, steam-to-carbon ratio 3.0–3.5 to suppress carbon laydown.
• Secondary reformer: process air is added to burn residual methane and simultaneously supply the nitrogen, fixing the H₂:N₂ ratio at 3:1; outlet CH₄ slip 0.3–0.5%.
• Shift conversion: CO + H₂O ⇌ CO₂ + H₂ (exothermic). HTS on Fe₃O₄–Cr₂O₃ at 350–450 °C, LTS on Cu–ZnO–Al₂O₃ at 200–250 °C to take CO down to ~0.2–0.3%.
• CO₂ removal: chemical absorption in hot potassium carbonate (Benfield) or activated MDEA; the stripped CO₂ is the feed for urea — the two plants are always built as a pair.
• Methanation: residual CO and CO₂ are hydrogenated back to CH₄ over Ni because they are oxygen-bearing poisons for the iron synthesis catalyst.
• Synthesis: N₂ + 3H₂ ⇌ 2NH₃, ΔH ≈ −92 kJ/mol, Δn negative. Promoted magnetite (Fe with K₂O, Al₂O₃, CaO) catalyst, 400–500 °C, 150–250 bar; conversion per pass only 15–20%, so ammonia is condensed out and unconverted gas recycled, with a purge stream to control argon and methane build-up.
• Le Chatelier reading (standard exam question): high pressure and low temperature favour ammonia, but low temperature kills the rate — so the plant runs at an economically optimum intermediate temperature with inter-bed cooling or quench.
• Other feedstocks: naphtha (older FACT/Madras units), fuel oil/coke partial oxidation, and coal gasification (Talcher Fertilizers, under the New Investment Policy 2012, along with the revived Ramagundam, Gorakhpur, Sindri and Barauni urea units). Energy consumption: modern gas plants about 7.5–8.0 Gcal per tonne NH₃; coal routes are far higher.

Urea and the other nitrogen fertilisers

• Bosch–Meiser, two steps in one reactor:
• 2NH₃ + CO₂ → NH₂COONH₄ (ammonium carbamate), fast and strongly exothermic;
• NH₂COONH₄ → NH₂CONH₂ + H₂O, slow, endothermic, liquid-phase, equilibrium-limited.
• Operating conditions: 180–210 °C, 140–200 bar, N/C (NH₃:CO₂ mole) ratio 3–4 to push step 2; CO₂ conversion per pass 55–65%.
• Recycle schemes: once-through → partial recycle → total recycle → modern stripping processes: Stamicarbon CO₂-stripping and Saipem (Snamprogetti) NH₃-stripping, where the unconverted carbamate is decomposed in the stripper and returned as condensate, cutting steam use.
• Finishing: evaporation/concentration to ~99.7% melt, then prilling (molten urea sprayed from the top of a tower, cooled by counter-current air) or granulation (fluid-bed/pan — larger, harder granules, better for bulk blending and less dust).
• Biuret (2NH₂CONH₂ → NH₂CONHCONH₂ + NH₃) forms at high temperature and long residence; kept below 1% (below ~0.3% for foliar grades) because it is phytotoxic.
• Materials of construction: ammonium carbamate is viciously corrosive — urea-grade stainless (316L mod. UG), duplex, titanium or zirconium liners, with passivating oxygen injection.
• Nitric acid (Ostwald): 4NH₃ + 5O₂ → 4NO + 6H₂O on Pt–Rh gauze, 850–900 °C, contact time milliseconds, yield 94–96%; then 2NO + O₂ → 2NO₂ and 3NO₂ + H₂O → 2HNO₃ + NO absorbed counter-currently in a sieve-tray tower to give 55–65% acid; concentration above the 68% azeotrope needs extractive distillation with Mg(NO₃)₂ or H₂SO₄.
• Ammonium nitrate: NH₃ + HNO₃, neutraliser then evaporation and prilling; 35% N but an oxidiser — in India its manufacture, storage and movement fall under the Ammonium Nitrate Rules, 2012 (Explosives Act, 1884). CAN (calcium ammonium nitrate, 26% N) dilutes it with limestone/dolomite to make it safe and non-detonable.
• Ammonium sulphate: 21% N, 24% S; made from NH₃ + H₂SO₄ or as a by-product of caprolactam and coke-oven gas.

Phosphates and potash

• Rock phosphate — fluorapatite Ca₅F(PO₄)₃ — is the only primary source; India imports the bulk of it (Jordan, Morocco, Togo), with limited domestic ore at Jhamarkotra (Rajasthan).
• Acidulation routes:
• SSP (single superphosphate, 16% P₂O₅ + 11% S): rock + H₂SO₄ → monocalcium phosphate + gypsum, in a cone mixer and den, then 2–4 weeks curing. Cheap, carries sulphur, but low analysis.
• TSP (46% P₂O₅): rock + phosphoric acid; no gypsum diluent.
• DAP (18-46-0): ammoniation of phosphoric acid in a pre-neutraliser to NH₃:H₃PO₄ ≈ 1.4, then granulation in a rotary drum/ammoniator, drying, screening, cooling, coating.
• Wet-process phosphoric acid: rock + H₂SO₄ → H₃PO₄ + CaSO₄·nH₂O; dihydrate (most common, ~70–75 °C), hemihydrate and hemi-dihydrate routes differ in filtration and P₂O₅ recovery. Filtered on a tilting-pan or table filter, concentrated 28% → 54% P₂O₅ in vacuum evaporators. Produces roughly 4.5–5 tonnes of phosphogypsum per tonne of P₂O₅ and needs fluoride scrubbing.
• Thermal (furnace) phosphoric acid: electric-furnace P₄ burnt and hydrated — food/technical grade, far more power-intensive.
• Potash: India has no commercial potash reserves; MOP (KCl, 60% K₂O) and SOP (K₂SO₄, 50% K₂O, chloride-free for tobacco, fruits) are imported.
• NPK complexes versus blends: complexes (10-26-26, 12-32-16, 20-20-0-13S) are chemically produced granules of uniform composition; bulk blends are physical mixtures of DAP/MOP/urea and can segregate. Each granule of a complex carries all nutrients — the usual interview distinction.

Sulphuric acid, chlor-alkali and soda ash

• Sulphuric acid — contact process: sulphur burning (or pyrites/smelter gas) → gas cleaning and drying → catalytic oxidation 2SO₂ + O₂ ⇌ 2SO₃ over V₂O₅ on silica with K₂SO₄/K₂S₂O₇ promoter, 4 beds, 400–440 °C inlet with inter-pass cooling → absorption in 98–98.5% H₂SO₄ (never in water, which creates an unabsorbable acid mist). DCDA (double contact double absorption) removes SO₃ after bed 3 and returns the gas for a 4th pass, taking overall conversion from ~98% (single absorption) to 99.7%+. Oleum is made by further SO₃ absorption. The process is a net steam/power exporter.
• Caustic soda and chlorine (brine electrolysis): 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
• Mercury cell: 50% caustic of very high purity, but mercury emissions — phased out in India.
• Diaphragm cell: asbestos diaphragm, dilute caustic with salt, needs evaporation.
• Membrane (ion-exchange) cell: perfluorosulphonic/carboxylic membrane passes only Na⁺, gives 32–35% caustic, lowest energy — the Indian standard today. Needs ultra-pure brine (Ca+Mg below ~20 ppb) via primary chemical and secondary ion-exchange treatment.
• Cl₂ : NaOH are produced in a fixed 0.886 : 1 mass ratio — the industry's chronic "chlorine balance" problem, solved by making HCl, PVC/EDC, chlorinated solvents, bleaching powder and water-treatment hypochlorite.
• Soda ash — Solvay (ammonia–soda): NaCl brine saturated with NH₃, carbonated with CO₂ from lime kiln → NaHCO₃ precipitates (lower solubility) → calcined to Na₂CO₃ + CO₂ + H₂O; ammonia recovered by treating filter liquor with lime: 2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + 2H₂O. Only Na and CO₂ efficiency matter commercially (Na utilisation ~70–75%); the CaCl₂ effluent is the main environmental issue. Dual/modified (Asahi) process recovers NH₄Cl as a fertiliser instead. India has no trona, so all soda ash is synthetic (Tata Chemicals Mithapur, GHCL, DCW, Nirma).
• Others commonly asked: HCl (from H₂ + Cl₂ burner, or as chlorination by-product), calcium carbide, titanium dioxide (sulphate and chloride routes), aluminium sulphate/alum, sodium silicate, hydrogen peroxide (anthraquinone auto-oxidation), industrial gases by air separation.

Indian policy, standards and safety

• Fertiliser (Control) Order, 1985, issued under the Essential Commodities Act, 1955, is the governing law: it specifies grades, nutrient tolerances, packing, marking, dealer licensing and sampling. Quality tests follow IS/FCO methods; selling below specification is an offence.
• Urea is under statutory price control (MRP fixed by the Centre, with the producer paid a subsidy under the New Urea Policy/NPS); P&K fertilisers are under the Nutrient Based Subsidy (NBS) scheme, where a per-kg rate is notified for N, P, K and S each season and the MRP is nominally decontrolled. Rates are revised every season — check the current notification rather than quoting a number.
• Neem-coated urea has been mandatory for domestic urea since 2015 (slows release, discourages diversion to industrial use). "One Nation One Fertiliser" (PMBJP) requires the *Bharat* brand on bags since 2022. Nano urea/nano DAP (IFFCO) are notified under the FCO as separate categories. PM-PRANAM incentivises states that cut chemical fertiliser use.
• Environment: CPCB/MoEFCC standards under the Water Act 1974, Air Act 1981 and Environment (Protection) Act 1986 — for sulphuric acid plants, SO₂ of about 4 kg per tonne of acid for single conversion and 2 kg/t for double conversion, with acid-mist limits; urea plants have limits on urea dust and ammonia from prilling towers; fertiliser units need Consent to Operate and are listed for environmental clearance.
• Hazard control: large ammonia and chlorine inventories make these MAH (major accident hazard) units under the Manufacture, Storage and Import of Hazardous Chemical Rules, 1989 and the Chemical Accidents (EPPR) Rules, 1996 — on-site/off-site emergency plans, safety reports, HAZOP. Occupational safety, hours and welfare are now governed by the four Labour Codes in force from 21 November 2025, the OSH & Working Conditions Code having replaced the Factories Act, 1948. Pressure equipment follows the relevant IS/ASME codes and SMPV(U) Rules for ammonia and chlorine vessels; PESO regulates chlorine and ammonium nitrate licensing.
• GST: fertilisers sit in the lowest 5% slab while several inputs (ammonia, sulphuric acid, machinery) are taxed higher, so refund accumulation is a standing industry issue.

Key points

• Haber–Bosch — N₂ + 3H₂ ⇌ 2NH₃, exothermic, volume-reducing; promoted iron catalyst, 400–500 °C, 150–250 bar, 15–20% per-pass conversion with recycle.
• Steam reforming — endothermic CH₄ + H₂O → CO + 3H₂ on Ni at 800–900 °C; steam/carbon 3.0–3.5 prevents coking.
• Shift + methanation — HTS (Fe–Cr) then LTS (Cu–Zn) convert CO to CO₂ and H₂; methanation removes the last oxygenates that poison the iron catalyst.
• Secondary reformer supplies nitrogen from air and sets the 3:1 H₂:N₂ ratio.
• Urea = carbamate formation (fast, exothermic) + dehydration (slow, endothermic); high N/C ratio and 180–210 °C/150–200 bar drive it.
• Biuret — condensation by-product of urea, phytotoxic, kept under 1%.
• Prilling vs granulation — prills are smaller and softer; granules are stronger, larger and better for blending.
• Ostwald process — NH₃ oxidation on Pt–Rh gauze at 850–900 °C, then NO₂ absorption to 55–65% HNO₃; 68% is the azeotrope.
• DCDA — intermediate SO₃ absorption raises sulphuric acid conversion to 99.7%, shifting equilibrium by removing product.
• SO₃ is absorbed in 98.5% acid, not water, to avoid acid mist.
• Membrane chlor-alkali — lowest energy route, needs ultra-pure brine; fixed Cl₂:NaOH:H₂ mass ratio 0.886 : 1 : 0.0252.
• Solvay — NaHCO₃ precipitates because of low solubility; ammonia is recycled using lime; CaCl₂ is the waste.
• Nutrient grades — urea 46% N, AN 35% N, CAN 26% N, ammonium sulphate 21% N, SSP 16% P₂O₅, TSP 46% P₂O₅, DAP 18-46-0, MOP 60% K₂O, SOP 50% K₂O.
• P and K are reported as oxides — multiply %P by 2.29 and %K by 1.20.
• FCO 1985 governs fertiliser grades, labelling and sale; urea is price-controlled, P&K run on NBS.
• Phosphogypsum — 4.5–5 t per tonne of P₂O₅ from wet-process acid; the industry's biggest solid-waste stream.
• Ammonia and chlorine inventories make fertiliser and chlor-alkali sites MAH units with statutory emergency plans.

Formulas and examples

Example 1 — Urea plant stoichiometry (3,500 TPD). 2NH₃ + CO₂ → NH₂CONH₂ + H₂O. M: NH₃ 17.03, CO₂ 44.01, urea 60.06, H₂O 18.02.

• NH₃ per tonne urea = 2 × 17.03/60.06 = 0.567 t → 3,500 × 0.567 = 1,985 t/day NH₃.
• CO₂ per tonne urea = 44.01/60.06 = 0.733 t → 2,565 t/day CO₂ (this is why the ammonia plant's CO₂ removal section is sized to the urea plant).
• Process water formed = 18.02/60.06 = 0.300 t/t → 1,050 t/day.
• Nitrogen delivered = 3,500 × 0.4665 = 1,633 t N/day; at 45 kg of urea per bag the output is 77,778 bags/day.

Example 2 — Hydrogen, nitrogen and gas demand per tonne of ammonia. 1,000 kg NH₃ = 1,000/17.03 = 58.72 kmol.

• H₂ = 1.5 × 58.72 = 88.08 kmol = 88.08 × 2.016 = 177.6 kg = 88.08 × 22.414 = 1,974 Nm³.
• N₂ = 0.5 × 58.72 = 29.36 kmol = 822.4 kg = 658 Nm³ (ratio 3:1 by volume ✓).
• Energy check: at 8.0 Gcal/t = 8.0 × 4.1868 = 33.5 GJ/t, with natural gas LHV 35.5 MJ/Nm³ → 33,500/35.5 = 944 Nm³ of gas per tonne NH₃ (feed + fuel). The thermodynamic minimum is about 21 GJ/t, so a good plant runs near 60% of ideal.

Example 3 — Sulphuric acid plant, 1,000 TPD (100% H₂SO₄ basis), DCDA at 99.7%.

• Sulphur chemically fixed = 1,000 × 32.06/98.08 = 326.9 t/day.
• Sulphur burnt = 326.9/0.997 = 327.9 t/day (≈0.328 t S per tonne of acid).
• Unconverted S = 327.9 − 326.9 = 1.0 t/day → SO₂ emitted = 1.0 × 64.06/32.06 = 2.0 t/day, i.e. 2.0 kg SO₂ per tonne of acid — just at the double-conversion standard, which is why DCDA is effectively compulsory for new plants.
• A single-absorption plant at 98% would emit 0.328 × 0.02 × 2 × 1,000 = 13 kg SO₂/t — more than six times higher.

Example 4 — Making 1 tonne of 20-20-0 by bulk blending DAP (18-46-0) and urea (46-0-0).

• P₂O₅ required = 200 kg → DAP = 200/0.46 = 434.8 kg.
• N from DAP = 434.8 × 0.18 = 78.3 kg; balance N = 200 − 78.3 = 121.7 kg → urea = 121.7/0.46 = 264.6 kg.
• Filler/sand = 1,000 − 434.8 − 264.6 = 300.6 kg.
• Cost of nutrient check: a 50 kg DAP bag at ₹1,350 gives 9 kg N and 23 kg P₂O₅; a 45 kg urea bag at ₹266 gives 20.7 kg N → urea N ≈ ₹12.8 per kg N, which is why urea is over-applied and the N:P:K ratio is skewed. (Bag prices are policy-set and change — use the current notified MRP.)

Example 5 — Membrane chlor-alkali unit, 300 TPD caustic (100% NaOH), 96% current efficiency, 3.1 V per cell.

• Equivalents per tonne NaOH = 1,000,000/40.00 = 25,000 → charge = 25,000 × 26.80 = 670 kA·h/t theoretical; actual = 670/0.96 = 698 kA·h/t.
• DC energy = 698 × 3.1 = 2,164 kWh per tonne NaOH (≈2,440 kWh per tonne Cl₂) → for 300 t/day, 649 MWh/day ≈ 27 MW — power is the dominant cost, typically 55–65% of it.
• Co-products: Cl₂ = 300 × 35.45/40.00 = 265.9 t/day; H₂ = 300 × 1.008/40.00 = 7.56 t/day ≈ 84,000 Nm³/day, usually burnt for steam or used to make HCl.
• Salt (pure NaCl) = 300 × 58.44/40.00 = 438 t/day theoretical.

Where it is used

• Nitrogenous fertiliser complexes: IFFCO (Kalol, Phulpur, Aonla), KRIBHCO Hazira, RCF (Trombay, Thal), NFL, Chambal Gadepan, Matix — roles in ammonia/urea operations, DCS panel, catalyst and energy management.
• Phosphatic and complex fertilisers: Coromandel International (Kakinada, Vizag), Paradeep Phosphates, GSFC, Zuari, Deepak Fertilisers — phosphoric acid plants, granulation, gypsum handling, imported-rock blending.
• Heavy inorganics: Tata Chemicals and GHCL (soda ash), Aditya Birla Chemicals, DCM Shriram, Gujarat Alkalies & Chemicals, Chemplast Sanmar (chlor-alkali), Hindustan Zinc and Sterlite smelters (by-product sulphuric acid).
• Downstream users: HCl and chlorine into PVC and water treatment; sulphuric acid into fertilisers, detergents, metal pickling and batteries; soda ash into glass, detergents and silicates; nitric acid into explosives (nitro-compounds), adipic acid and nylon.
• Job roles: process/production engineer on an ammonia-urea or sulphuric acid train, process design and technical services, catalyst and energy audit engineer, EHS officer handling MAH compliance, technical sales in agri-inputs, and quality control against FCO specifications.
• Transition work: green ammonia from electrolytic hydrogen, coal-gasification urea at Talcher, CO₂ utilisation, and nano-fertiliser formulation.

Interview questions

• Q: Why does ammonia synthesis run at high pressure but only moderate conversion per pass? A: The reaction is exothermic and volume-reducing, so pressure helps and low temperature helps equilibrium, but the rate needs 400–500 °C; the compromise gives only 15–20% per pass, so ammonia is condensed out and the gas recycled.
• Q: Why is a methanator placed after CO₂ removal? A: Residual CO and CO₂ (a few thousand ppm) are oxygen-containing poisons for the iron synthesis catalyst; converting them to inert methane is cheaper than removing them completely.
• Q: What fixes the 3:1 H₂:N₂ ratio in an ammonia plant? A: The amount of process air admitted to the secondary reformer, trimmed by the synthesis-gas analyser.
• Q: Why is a purge stream needed in the synthesis loop? A: Argon from the process air and unconverted methane are inert and would accumulate indefinitely, lowering the partial pressures of the reactants.
• Q: Urea formation has one exothermic and one endothermic step — which controls the reactor design? A: The slow, endothermic dehydration of carbamate; it needs long liquid residence time at 180–210 °C and a high NH₃:CO₂ ratio, while the exothermic carbamate step supplies much of the heat.
• Q: What is biuret and why is it limited? A: A urea condensation product formed at high temperature and long hold-up; it is toxic to plants, especially in foliar sprays, so it is held below about 1%.
• Q: Why is SO₃ absorbed in 98.5% sulphuric acid rather than water? A: Absorption in water or weak acid generates a fine sulphuric acid mist that is extremely hard to capture; strong acid absorbs SO₃ efficiently with no mist.
• Q: How does DCDA raise conversion beyond the equilibrium of a single pass? A: Removing SO₃ between beds shifts the equilibrium of the remaining gas forward, so the fourth bed converts most of the leftover SO₂.
• Q: Why did India move from mercury to membrane cells? A: Mercury emissions and effluent (Minamata concerns) plus higher energy use; membrane cells give 32–35% caustic at the lowest power per tonne, though they need very pure brine.
• Q: What is the chlorine balance problem? A: Electrolysis fixes Cl₂ and NaOH output at a ratio of about 0.886:1, so when caustic demand is strong the chlorine must be absorbed into derivatives, and when it is weak the plant must cut load.
• Q: Why does the Solvay process precipitate bicarbonate rather than carbonate? A: NaHCO₃ is far less soluble in the ammoniated brine, so it crystallises out and is then calcined to Na₂CO₃.
• Q: DAP is sold as 18-46-0 but the pure salt calculates to 21-54-0 — why? A: Commercial DAP is partly ammoniated with mono-ammonium phosphate, impurities from wet-process acid and a moisture/coating allowance, so the guaranteed grade is lower than the stoichiometric value.
• Q: A farmer applies only urea for years. What goes wrong? A: Phosphorus, potassium and sulphur get depleted and soil acidifies, so the N response falls — the reason NBS, soil-health cards and sulphur-bearing grades exist.

Common mistakes

• Reporting phosphorus and potassium as the element instead of P₂O₅ and K₂O (or applying the 2.29 and 1.20 factors the wrong way round).
• Treating the ammonia reactor as a once-through conversion and forgetting the recycle, so the material balance on fresh feed and purge never closes.
• Saying "lower temperature is better" for ammonia or SO₂ oxidation without mentioning that the rate collapses and a catalyst light-off temperature exists.
• Mixing up where the CO₂ for urea comes from — it is recovered from the ammonia plant's shifted gas, not bought in.
• Confusing wet-process phosphoric acid (fertiliser grade, gypsum by-product) with the furnace/thermal route (food grade, power-intensive).
• Quoting single-absorption conversion or emission figures for a modern DCDA plant, or forgetting that gas must be dried before the converter.
• Using 96,485 C/mol with current in amperes and time in hours without converting — use 26.80 A·h per mole of electrons — and ignoring current efficiency.
• Calling a bulk blend a "complex fertiliser"; a complex has every nutrient in each granule, a blend can segregate in the bag.

Related topics

Petrochemicals and polymers · Pharmaceutical and specialty chemicals · Chemical reaction engineering (equilibrium, catalysis and recycle reactors) · Stoichiometry and material/energy balances · Mass transfer and absorption · Process safety, HAZOP and environmental regulation · Hydrogen and the energy transition.