Steel-grade ammonium sulphate is recovered from coke-oven gas. Ammonia liberated during the coking of coal is scrubbed from the raw gas with sulfuric acid in a saturator or an acid-wash tower, and the resulting liquor is crystallised. Because the ammonia is stripped from a gas stream that also carries tar, oils, phenols and hydrogen sulfide, the impurity profile and appearance differ systematically from petrochemical by-product routes. This is the single most important honest distinction in the ammonium sulphate market.
Substance identity
| Chemical name | Ammonium sulfate |
| Synonyms | Ammonium sulphate, coke-oven ammonium sulphate, by-product ammonium sulphate |
| Formula | (NH₄)₂SO₄ |
| CAS No. | 7783-20-2 |
| PubChem CID | 6097028 |
| Molar mass | 132.14 g/mol |
| Appearance | Light grey, beige, tan or brown crystalline solid; colour varies between plants and between campaigns at one plant |
Specification
— typical commercial ranges for this route; not a guaranteed specification
| Parameter | Typical range | Note |
|---|---|---|
| Total nitrogen (N), dry basis | 20.5 – 21.0 % min | stoichiometric 21.2 %; lower guarantees are usual for this route |
| Sulfur (S) | 23 – 24 % | stoichiometric 24.3 % |
| Moisture (H₂O) | ≤ 0.5 – 1.0 % | fine crystal holds more surface water |
| Free acid (as H₂SO₄) | ≤ 0.10 – 0.20 % | higher than caprolactam grade; acid-saturator operation leaves residual acidity |
| Water-insoluble matter | ≤ 0.10 % or contract-specific | tar/oil and coke fines |
| pH, 5 % aqueous solution | 4.5 – 6.0 | lower end reflects free acid |
| Predominant particle size | 0.2 – 1.0 mm | fine to medium crystal; significant fines fraction |
| Colour | light grey to brown | not a nutrient parameter |
Trace species reported for this route, depending on the gas-cleaning train, include iron, thiocyanate, residual phenols and tar/oil residues. Where any of these matters for the end use, it must be named and analysed in the contract rather than inferred.
Properties
The compound behaves as ammonium sulphate: same solubility, same critical relative humidity near 79–81 % at 30 °C, same decomposition behaviour above roughly 235 °C, same acidifying effect on nitrification. Route-specific behaviour is physical and cosmetic: fine crystal dusts more, packs denser, cakes more readily under load and humidity because there is more contact area per tonne, and spreads over a narrower width from a spinning disc. Higher free acid means slightly more aggressive corrosion of unprotected mild steel handling equipment when the material is damp.
Applications
Direct field application where colour is irrelevant — the great majority of agricultural use. Its nitrogen and sulfur are chemically identical to those of any other route, and no agronomic response difference should be expected on the nutrients themselves. It is a common feed for compaction into granules, which is the standard remedy for its dust and spreading limitations (see the compacted granular steel and hydraulic granular steel pages).
It is generally the wrong choice for: clear-solution fertigation and drip systems, where insolubles block emitters and filters; spray-tank water conditioning, where insolubles and colour are unacceptable; retail-bagged consumer product; and any food-adjacent or technical application with a colour or organic-residue specification. For those, specify caprolactam or MMA grade.
Handling and storage
Dry, covered storage; the fines fraction makes caking under stack load a real risk, so limit stack height and store on pallets. Use dust extraction at transfer points and provide respiratory and eye protection. Avoid prolonged contact between damp material and unprotected mild steel. Keep away from strong alkalis, which liberate ammonia. Not classified as dangerous goods for transport.
Notes
Colour varies lot to lot for this route and is not a reliable indicator of assay. Judge each lot on nitrogen, sulfur, free acid and insolubles. If a specific end use is sensitive to organic residues, request the analysis explicitly — a general fertiliser certificate of analysis will not cover it.