Buy on Al₂O₃, not on tonnage
Aluminium sulphate (Al₂(SO₄)₃, CAS 10043-01-3 anhydrous) is sold as a hydrated solid or as a solution. The active ingredient is aluminium, conventionally expressed as Al₂O₃ content, and that figure varies widely.
| Grade | Al₂O₃ | Iron | Form | Primary use |
|---|---|---|---|---|
| Standard commercial | 15.6–17.0% | ≤750 ppm | Granule, crystal, lump | Municipal water, wastewater, construction |
| Industrial liquid | 8.0–8.5% | ≤1,000 ppm | Solution | Continuous dosing systems |
| Low iron | 15.6–17.0% | ≤100 ppm | Granule | Textile dyeing of light shades |
| Iron-free | 15.6–17.0% | ≤10 ppm | Granule, powder | Paper sizing, food processing |
| Food (FCC / E520) | 15.0–17.0% | ≤10 ppm | Powder, crystal | Food additive, pickling, starch |
Convert every quotation to cost per kilogram of Al₂O₃ delivered before comparing. A liquid at 8.3% Al₂O₃ needs roughly twice the tonnage of a 17% granule to deliver the same aluminium, and the freight difference is substantial.
The iron limit decides which grade you need
Iron content is what separates the grades commercially, and it is set by the manufacturing route. Alum made from bauxite carries iron; alum made from purified aluminium hydroxide does not.
| Route | Aluminium source | Typical iron | Resulting grade |
|---|---|---|---|
| Bauxite + sulfuric acid | Bauxite ore | High, 750 ppm and above | Standard water treatment and construction |
| Aluminium hydroxide + sulfuric acid | Purified Bayer hydroxide | Very low, ≤10 ppm | Iron-free for food, paper, textile |
- For municipal drinking water and wastewater, standard grade is normally acceptable and is much cheaper
- For paper sizing, iron causes discoloration of the sheet — specify iron-free
- For textile dyeing of light or white goods, iron causes staining that shows on the finished fabric — specify low iron or iron-free
- For food use, iron-free food grade against FCC is the requirement, not a preference
- Do not pay for iron-free where standard will do; the price gap is significant
Key physical properties
| Property | Value |
|---|---|
| Chemical formula | Al₂(SO₄)₃, commercially as the 18-hydrate |
| CAS number | 10043-01-3 anhydrous; 7784-31-8 octadecahydrate |
| Appearance | White to off-white crystals, granules or powder |
| pH (1% solution) | 3.0–4.0 — acidic |
| Density (solid) | 1.69 g/cm³ |
| Density (liquid, ~8.3% Al₂O₃) | ~1.33 g/cm³ |
| Solubility in water | 31.2 g/100 mL at 0 °C; 89 g/100 mL at 100 °C |
Solid or liquid
- Solid carries roughly twice the Al₂O₃ per tonne, so freight per unit of treatment is much lower — the right choice where transport distance is long
- Solid needs a dissolving station, and dissolution is slower in cold water, which matters in winter
- Liquid dilutes and doses directly, removing the dissolving step and the dust
- Liquid can crystallise in cold weather, so it needs insulated or heated storage in northern Pakistan in winter
- Solid alum is hygroscopic and cakes; store sealed, off the floor and under cover
Establishing your dose
There is no universal alum dose. It depends on raw water turbidity, alkalinity, temperature and pH, and the only reliable way to set it is a jar test on your actual water.
- Run jar tests seasonally, not once — monsoon turbidity and winter temperature both change the optimum substantially
- Alum consumes alkalinity and depresses pH, so low-alkalinity water often needs lime or soda ash alongside it
- Coagulation performance falls off below about 5 °C, which is where PAC often outperforms alum
- Overdosing wastes chemical, raises residual aluminium and increases sludge volume
- Record the jar test result against raw water conditions so that dose can be adjusted predictively rather than reactively
What to put on the RFQ
- Al₂O₃ content with a minimum, not just the product name
- Iron limit, expressed in ppm
- Form — granule, lump, powder or solution — and for solid, the particle size if your dissolver depends on it
- Water-insoluble matter limit, which affects dissolver fouling and sludge
- Quantity per delivery and annual volume, expressed in tonnes of Al₂O₃ where possible
- Pack — 25 kg or 50 kg bags, jumbo bags, IBC or tanker
- Delivery city, and drinking-water suitability certification for potable use
What drives the price
- Bauxite and sulfuric acid feedstock costs
- Al₂O₃ content, which is what you are really buying
- Iron specification, with iron-free carrying a substantial premium over standard
- Form, since liquid ships roughly twice the weight for the same treatment capacity
- Freight and delivery city
- Monsoon season demand, when raw water turbidity rises and coagulant consumption follows
Sourcing considerations in Pakistan
- Standard grade is widely available from both domestic production and import; iron-free is more often a specialty import
- It is not classified as a dangerous good for transport, so logistics are straightforward
- For potable water treatment, keep the drinking-water suitability documentation on file for regulatory review
- Plan monsoon stock ahead of the season, since both demand and raw water turbidity peak together