Caprolactam-grade ammonium sulphate is recovered from the manufacture of caprolactam, where the Beckmann rearrangement of cyclohexanone oxime in oleum is neutralised with ammonia. The neutralisation liquor is crystallised under controlled supersaturation, which produces comparatively large, well-formed crystals. This route is the reference point for crystal size and colour among by-product ammonium sulphates.
Substance identity
| Chemical name | Ammonium sulfate |
| Synonyms | Ammonium sulphate, diammonium sulfate, sulfuric acid diammonium salt, AMSUL, SA; mineral form: mascagnite |
| Formula | (NH₄)₂SO₄ |
| CAS No. | 7783-20-2 |
| PubChem CID | 6097028 |
| Molar mass | 132.14 g/mol |
| Appearance | White to off-white crystalline solid, odourless or with faint ammoniacal note |
Specification
— typical commercial ranges for this route, not a guaranteed specification; confirm against the certificate of analysis for the delivered lot
| Parameter | Typical range | Note |
|---|---|---|
| Total nitrogen (N), dry basis | 21.0 % min (20.5 % min in some contracts) | stoichiometric value 21.2 % |
| Sulfur (S) | 24 % min | stoichiometric value 24.3 %, present as sulfate |
| Moisture (H₂O) | ≤ 0.1 – 0.5 % | large crystal drains and dries well |
| Free acid (as H₂SO₄) | ≤ 0.03 – 0.05 % | low, because the liquor is neutralised with ammonia |
| Water-insoluble matter | ≤ 0.05 % | |
| pH, 5 % aqueous solution | 5.0 – 6.0 | |
| Iron (Fe) | tens of ppm (commonly quoted ≤ 0.007 %) | low; no ferrous process equipment carry-over of the kind seen in coke-oven material |
| Predominant particle size | 2 – 4 mm | coarse crystal; finer cuts screened out |
| Bulk density | ≈ 0.9 – 1.05 t/m³ |
Properties
Freely soluble in water — roughly 74–76 g per 100 g water at 20–25 °C, rising to about 104 g per 100 g at 100 °C; practically insoluble in ethanol and acetone. Dissolution is mildly endothermic, so a stock tank cools slightly on make-up. Not hygroscopic under normal warehouse conditions: the critical relative humidity is approximately 79–81 % at 30 °C, higher than urea (roughly 72–75 %) and much higher than ammonium nitrate. It has no true melting point — on heating it loses ammonia slowly above about 100 °C and decomposes in the 235–280 °C region toward ammonium bisulfate, then to ammonia, sulfur oxides, nitrogen and water. It is not combustible and not an oxidiser.
In soil the ammonium fraction is nitrified by soil bacteria, releasing protons; ammonium sulphate is therefore a strongly acidifying nitrogen source. Textbook neutralisation requirement is on the order of 5.4 kg CaCO₃ per kg of applied N, i.e. roughly 110 kg CaCO₃ per 100 kg of product — treat this as an order-of-magnitude planning figure, not a soil-specific value.
Applications
The large, clean crystal habit is the practical reason to specify this route. Coarse crystal spreads more predictably than fine crystal, generates less dust in bag filling and field handling, and screens to a narrower size band for blending. Colour close to white matters where the product is bagged for retail sale, dyed for identification, or used in non-fertiliser applications where visible contamination would be rejected.
Agronomically the product delivers 21 % N and 24 % S in one particle, both immediately plant-available — ammonium-N and sulfate-S. Sulfate is the form roots take up; elemental sulfur must first be oxidised by soil microbes and is not equivalent in-season. High-sulfur-demand crops (oilseed rape and other brassicas, alliums, alfalfa, tea, milling wheat) respond to the sulfur component independently of the nitrogen.
Handling and storage
Store under cover in a dry, well-ventilated building, away from direct wetting and off bare concrete floors. Keep away from strong alkalis — lime, caustic, basic slag and calcium cyanamide liberate ammonia gas on contact. Do not co-store or blend with ammonium nitrate-based products where regulations separate them. Not classified as dangerous goods for transport; nevertheless, fire involving the product releases ammonia and sulfur oxides, so use breathing apparatus in an enclosed space. Standard dust control and eye protection apply during bagging and blending.
Notes
Off-white lots from this route carry a trace of organic material from the process liquor. This affects appearance and occasionally a faint odour, not the nitrogen or sulfur assay — see the dedicated off-white page. For granulated versions of this material, see the compacted granular and hydraulic granular pages.