Phosphatic fertilisers and phosphoric acid

Wet-process phosphoric acid from fluorapatite (dihydrate and hemihydrate routes, phosphogypsum, fluorine recovery) and SSP, TSP and DAP fertilisers, with P2O5-basis balances.

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Why it matters

Phosphorus is the second major plant nutrient after nitrogen, and India imports most of its rock phosphate and phosphoric acid, so efficient conversion matters for both cost and foreign exchange. Wet-process phosphoric acid is the intermediate for DAP, MAP, NP/NPK complexes and triple superphosphate, and it is the largest consumer of sulphuric acid.

Key ideas

Rock phosphate. The mineral is mainly fluorapatite, Ca₅(PO₄)₃F (or written 3Ca₃(PO₄)₂·CaF₂). Commercial rock contains roughly 28–38 % P₂O₅ along with CaO, fluorine, silica, iron, aluminium, organics and traces of cadmium and uranium. Phosphate content is always reported as % P₂O₅ (and in fertilisers as available P₂O₅), not as % P. Insoluble tricalcium phosphate is unavailable to plants; all processing converts it to water- or citrate-soluble forms.

Wet process phosphoric acid. Ground rock is digested in sulphuric acid in recycled slurry: Ca₅(PO₄)₃F + 5H₂SO₄ + 10H₂O → 3H₃PO₄ + 5CaSO₄·2H₂O + HF

  • Dihydrate process (most common): 70–80 °C, product filtrate about 26–32 % P₂O₅, gypsum as CaSO₄·2H₂O. Recovery about 94–96 %.
  • Hemihydrate and hemi-dihydrate processes: higher temperature (90–100 °C), stronger acid (40–48 % P₂O₅), less evaporation, but more demanding operation.
  • Sulphate control: a small excess of free sulphate is kept in the slurry. Too little gives poor gypsum crystals and P₂O₅ locked in the lattice; too much coats rock particles with gypsum and blocks attack. Either way recovery falls.
  • Filtration: tilting-pan or belt vacuum filters with counter-current washing separate gypsum. Crystal size and shape decide filtration rate.
  • Concentration: vacuum evaporation to 52–54 % P₂O₅ (merchant-grade acid). Fluorine leaves as HF/SiF₄ and is scrubbed to fluorosilicic acid (H₂SiF₆).
  • By-product: about 4.5–5 t of phosphogypsum per tonne of P₂O₅. It carries residual acid, fluoride and radioactivity, and its stacking or use (cement retarder, plasterboard, soil amendment) is a major environmental issue.

Thermal (furnace) process. Rock, silica and coke are reduced in an electric furnace to elemental phosphorus, which is burnt to P₄O₁₀ and hydrated. Very pure acid for food and technical uses, but energy-intensive.

Phosphatic fertilisers.

  • Single superphosphate (SSP): rock + H₂SO₄ without gypsum separation; about 16 % P₂O₅ (water-soluble) and gypsum stays in the product. Ca₅(PO₄)₃F + H₂SO₄ gives Ca(H₂PO₄)₂ + CaSO₄ in the mix; it is cured in a den and stored for weeks.
  • Triple superphosphate (TSP): rock + phosphoric acid; about 46 % P₂O₅, no gypsum diluent.
  • Diammonium phosphate (DAP, 18-46-0) and monoammonium phosphate (MAP, 11-52-0): phosphoric acid ammoniated in a pre-neutraliser or pipe reactor, then granulated, dried and screened. Pure (NH₄)₂HPO₄ is 21.2 % N and 53.7 % P₂O₅; the commercial grade is lower because of impurities from wet acid.
  • NPK complexes combine these with urea or potash.

Grade notation. N-P-K = % N, % P₂O₅, % K₂O. The P₂O₅ "equivalent" is a reporting convention; there is no P₂O₅ in the bag.

Formulas

Ca₅(PO₄)₃F + 5H₂SO₄ + 10H₂O → 3H₃PO₄ + 5CaSO₄·2H₂O + HF P₂O₅ + 3H₂O → 2H₃PO₄ (conversion of the P₂O₅ basis) H₃PO₄ (100 %) = P₂O₅ × (2 × 97.995 / 141.94) = 1.381 × P₂O₅ H₂SO₄ for digestion ≈ CaO in rock × (98.08 / 56.08) (mol CaO = mol H₂SO₄) Acid mass at strength w = P₂O₅ mass / w

  • w: mass fraction of P₂O₅ in the acid. Water evaporated = acid mass at w₁ − acid mass at w₂.

Molar masses: P₂O₅ 141.94, H₃PO₄ 97.995, H₂SO₄ 98.08, CaO 56.08, CaSO₄·2H₂O 172.17 g/mol.

Worked examples

Example 1 (standard): P₂O₅ basis. How much P₂O₅ is contained in 1000 kg of 100 % H₃PO₄, and how much rock (30 % P₂O₅) is needed at 90 % recovery?

  1. P₂O₅ = 1000 × 141.94 / (2 × 97.995) = 724.2 kg.
  2. Rock P₂O₅ needed = 724.2 / 0.90 = 804.7 kg.
  3. Rock = 804.7 / 0.30 = 2682 kg.

Answer: 724 kg P₂O₅; about 2.68 t of rock.

Example 2 (GATE level): dihydrate plant per tonne of rock. Rock: 32 % P₂O₅, 50 % CaO. P₂O₅ recovery 95 %. All CaO is converted to gypsum. Filter acid is 30 % P₂O₅ and is evaporated to 54 % P₂O₅. Find H₂SO₄ (100 %) required, gypsum formed, and water evaporated.

  1. CaO = 500 kg = 500 / 56.08 = 8.916 kmol, so H₂SO₄ = 8.916 kmol × 98.08 = 874.5 kg.
  2. Gypsum = 8.916 × 172.17 = 1535 kg CaSO₄·2H₂O.
  3. P₂O₅ recovered = 320 × 0.95 = 304 kg.
  4. Filter acid = 304 / 0.30 = 1013.3 kg; product acid = 304 / 0.54 = 563.0 kg.
  5. Water evaporated = 1013.3 − 563.0 = 450.4 kg.

Answer: about 875 kg H₂SO₄, 1.54 t gypsum, 450 kg water evaporated per tonne of rock.

Common mistakes

  • Confusing % P with % P₂O₅ (P₂O₅ is 43.6 % P by mass).
  • Using 1 mol P₂O₅ → 1 mol H₃PO₄; it gives 2.
  • Thinking more H₂SO₄ always means more acid: excess sulphate coats the rock and lowers recovery.
  • Forgetting the water of crystallisation when computing gypsum mass.
  • Assuming SSP removes gypsum; it stays in the product, which is why SSP is only about 16 % P₂O₅.

For GATE CH

Expect questions on the digestion reaction and its by-products (gypsum, HF/H₂SiF₆), dihydrate vs hemihydrate conditions, why SSP is low-grade, the composition of DAP and TSP, and fertiliser grade notation. Numericals involve P₂O₅ ⇄ H₃PO₄ conversion, rock and sulphuric acid consumption, gypsum generation and evaporation duty. Practise doing every balance on a P₂O₅ basis.

Quick check

  1. What is the main mineral in phosphate rock?
  2. What is the by-product of wet-process acid, and roughly how much per tonne P₂O₅?
  3. Why is SSP lower in P₂O₅ than TSP?
  4. What does the grade 18-46-0 mean?

Answers: 1. fluorapatite, Ca₅(PO₄)₃F; 2. phosphogypsum, about 4.5–5 t; 3. SSP keeps the gypsum from the sulphuric acid in the product, while TSP uses phosphoric acid and adds no diluent; 4. 18 % N, 46 % P₂O₅, 0 % K₂O (DAP).

Interview questions

All Chemical Technology interview questions

Try answering each one aloud before you open it.

  1. 1.What are phosphatic fertilizers, and why are they important in agriculture?Concept

    Phosphatic fertilizers are fertilizers that contain phosphorus as a major nutrient. They are important in agriculture because phosphorus is essential for plant growth, playing a key role in energy transfer, photosynthesis, and nutrient movement within the plant. Phosphorus deficiency can lead to stunted growth and poor crop yields.

  2. 2.Explain the process of manufacturing phosphoric acid.Concept

    Phosphoric acid is commonly manufactured using the wet process, which involves reacting phosphate rock with sulfuric acid to produce phosphoric acid and gypsum as a by-product. The reaction is typically carried out in a series of reactors, and the phosphoric acid is then separated from the gypsum by filtration. The acid is concentrated by evaporation to achieve the desired concentration.

  3. 3.What is the chemical formula of phosphoric acid, and what are its main uses?Concept

    The chemical formula of phosphoric acid is H₃PO₄. It is mainly used in the production of phosphatic fertilizers, as a food additive, in the treatment of water and metal surfaces, and in the manufacture of detergents and cleaning agents.

  4. 4.Why is sulfuric acid used in the production of phosphoric acid?Application

    Sulphuric acid is a strong, cheap acid that releases phosphoric acid from fluorapatite and, crucially, binds the calcium as nearly insoluble gypsum (CaSO4.2H2O). Because the calcium leaves as a solid, the phosphoric acid can be separated by simple vacuum filtration. Hydrochloric or nitric acid would leave soluble calcium salts in the acid, which is why they are used only in special processes.

  5. 5.What happens if excess sulfuric acid is used in the phosphoric acid production process?Application

    The amount of gypsum is fixed by the calcium in the rock, not by excess acid. Too high a free-sulphate level coats unreacted rock particles with gypsum, blocking further attack and lowering P2O5 recovery; it also wastes acid and leaves sulphate in the product. Too little sulphate gives poor, hard-to-filter gypsum crystals with P2O5 locked in their lattice, so plants control free sulphate within a narrow band.

  6. 6.Calculate the amount of phosphate rock needed to produce 1000 kg of phosphoric acid (100% H3PO4), assuming the rock contains 30% P2O5 and the process recovery is 90%.Numerical

    P2O5 + 3H2O -> 2H3PO4, so P2O5 in 1000 kg H3PO4 = 1000 x 141.94 / (2 x 97.995) = 724.2 kg. Allowing for 90% recovery, the rock must supply 724.2 / 0.90 = 804.7 kg P2O5. Rock needed = 804.7 / 0.30 = 2682 kg, i.e. about 2.68 t.

  7. 7.What are the environmental concerns associated with the production of phosphoric acid?Application

    The production of phosphoric acid can lead to environmental concerns such as the release of fluorine compounds, heavy metals, and radioactive elements present in phosphate rock. Additionally, the disposal of gypsum by-products can pose challenges, and the use of sulfuric acid can lead to acidification of nearby water bodies if not managed properly.

  8. 8.What safety precautions should be taken when handling phosphoric acid in an industrial setting?Application

    When handling phosphoric acid, it is important to wear appropriate personal protective equipment such as gloves, goggles, and acid-resistant clothing. Adequate ventilation should be ensured to avoid inhalation of fumes. Emergency showers and eyewash stations should be accessible, and proper storage and labeling of the acid should be maintained to prevent accidental exposure.

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