“Crystal ammonium sulphate” describes the physical form as it comes from the crystalliser, before any granulation. Within that form, particle size varies widely — from powdery fines below 0.3 mm to well-formed crystals of 4 mm — and size, more than any other single variable, determines how the product handles. This page covers crystal habit and size grading; for solution behaviour see the crystalline ammonium sulphate page.
Substance identity
Ammonium sulfate · (NH₄)₂SO₄ · CAS 7783-20-2 · PubChem CID 6097028 · 132.14 g/mol · orthorhombic crystals; colour white to brown depending on production route.
Specification
— typical commercial size cuts; nutrient values apply to all cuts
| Size cut | Typical range | Where it comes from and how it behaves |
|---|---|---|
| Fine / powder | < 0.5 mm | Common from coke-oven and some MMA plants. Dusty, dissolves fastest, cakes most, spreads worst. |
| Medium | 0.5 – 2 mm | Common from MMA and screened coke-oven material. General-purpose; segregates from granular blend partners. |
| Coarse | 2 – 4 mm | Characteristic of caprolactam-route crystallisation with long residence time. Low dust, best spreading of the crystal forms. |
| Parameter | Typical range | |
| — | — | |
| Total nitrogen (N) | 20.5 – 21.0 % min | |
| Sulfur (S) | 23 – 24 % | |
| Moisture (H₂O) | ≤ 0.1 – 1.0 %, rising as crystal size falls | |
| Bulk density | ≈ 0.9 – 1.1 t/m³, rising as crystal size falls |
Properties
Crystal size is set in the crystalliser, not afterwards. Long residence time, controlled low supersaturation and gentle agitation grow large crystals; rapid cooling and high supersaturation nucleate many small ones. That is why route and crystal size correlate — different parent processes deliver different liquor conditions and different economics for crystalliser residence time.
Size then drives four practical behaviours. Dust: proportional to the fine fraction. Caking: small crystals have far more contact area per tonne, so bridges form more readily under stack load and humidity cycling; coarse crystal is markedly more free-flowing. Spreading: kinetic energy per particle scales with mass, so fine crystal lands short and drifts; coarse crystal spreads wider and more evenly. Dissolution rate: inversely related to size — fines dissolve fastest.
Note that the moisture figure and the bulk density figure both drift upward as crystal size falls; a low moisture guarantee on a fine product is harder to hold than the same guarantee on coarse crystal.
Applications
Direct field application, where coarse crystal is preferred and fine crystal is workable only with equipment set for it. Dissolving-tank feed, where fine and medium crystal are preferred for speed. Compaction feed, where a narrow, consistent size distribution matters more than absolute size. Blending — only with size-matched partners.
Handling and storage
Match stack height and storage duration to crystal size: fine material should be stored lower and turned over faster. Keep dry and covered; the critical relative humidity of roughly 79–81 % at 30 °C is the threshold above which moisture uptake and subsequent caking accelerate. Dust extraction where fines are present.
Notes
Ask for the actual sieve analysis, not just a nominal size. “Crystal” on a specification sheet spans an order of magnitude in particle diameter, and two lots described identically can behave completely differently in the same spreader.