Basicity is the parameter that decides performance
PAC (CAS 1327-41-9) is a family of pre-hydrolysed aluminium polymers rather than a single compound. Basicity — the ratio of hydroxide to aluminium, expressed as a percentage between roughly 40% and 90% — describes how far that pre-hydrolysis has gone, and it changes how the product behaves.
- Low basicity, roughly 40–55% — behaves more like alum, consumes more alkalinity, generally better in high-turbidity water
- Medium basicity, roughly 55–70% — the general-purpose range for municipal drinking water
- High basicity, roughly 70–90% — consumes least alkalinity, works well in low-alkalinity and low-temperature water, produces less sludge
- The right basicity depends on your raw water alkalinity and temperature, not on which number is highest
- Two products at the same Al₂O₃ and different basicity are genuinely different coagulants, and should be jar tested separately
This is the most common specification gap in PAC procurement. Buyers name Al₂O₃, take the cheapest, and then find the dose does not translate. Name both figures.
Key properties and grades
| Property | Value |
|---|---|
| Chemical formula | Alₙ(OH)ₘCl₍₃ₙ₋ₘ₎ — a polymeric mixture |
| CAS number | 1327-41-9 |
| Al₂O₃ (powder) | 28–32% industrial; ≥30% for drinking water grade |
| Al₂O₃ (liquid) | 10–18% |
| Basicity | 40–90% |
| pH (1% solution) | 3.5–5.0 |
| Density (liquid at 10% Al₂O₃) | ~1.19 g/cm³ |
| Solubility | Fully water-soluble |
| Form | Al₂O₃ | Best suited to |
|---|---|---|
| Liquid | 10–18% | Large plants with continuous dosing and short freight |
| Spray-dried powder | 28–32% | Drinking water, long shelf life, long freight |
| Plate and frame powder | 28–30% | Municipal sewage and industrial effluent |
| Granular | 28–31% | Remote sites and manual dosing — low dust, good flow |
Spray-dried powder is the most stable and the cheapest to freight per unit of aluminium, which matters for inland Pakistani plants. Plate and frame material is cheaper but carries more water-insoluble matter, which is acceptable in sewage treatment and less so in drinking water.
Comparing PAC against alum honestly
PAC costs more per kilogram than alum and often less per unit of water treated. Whether that holds for your plant depends on your raw water, so it is worth running the comparison properly rather than accepting either claim.
| Parameter | PAC | Alum |
|---|---|---|
| Effective pH range | 5.0–9.0 | 6.0–8.0 |
| Cold water performance | Holds up well | Falls off below about 5 °C |
| Dose required | Lower | Higher |
| Floc settling | Fast | Slower |
| Sludge volume | Lower | Higher |
| Alkalinity consumed | Less | More |
| Residual aluminium | Lower | Higher |
| Cost per kilogram | Higher | Lower |
- Run jar tests on your own raw water with both, at seasonal extremes, and compare cost per megalitre treated rather than per tonne bought
- Include sludge handling and disposal cost in the comparison — lower sludge volume is a real saving that never appears on the chemical invoice
- Include alkalinity chemicals; if alum forces you to dose lime or soda ash and PAC does not, that belongs in the comparison
- PAC is generally less corrosive to dosing equipment than ferric chloride and comparable to alum
What to put on the RFQ
- Al₂O₃ content with a minimum, and basicity with a range — both, not one
- Form — spray-dried, plate and frame, granular or liquid
- Water-insoluble matter limit, which matters for drinking water and for dosing equipment fouling
- Heavy metals limits where the end use is potable
- Quantity per delivery and annual volume, expressed in tonnes of Al₂O₃ where possible
- Pack — 25 kg bags, jumbo bags, IBC or tanker
- Delivery city, and drinking-water suitability certification for potable use
Storage
- Powder is hygroscopic — store sealed, off the floor, under cover, and reseal opened bags
- Liquid PAC has a finite shelf life and can gel or drop sediment on long storage, particularly if it gets cold; ask for the shelf life and the storage temperature range
- Suitable materials — HDPE, FRP, PVC and rubber-lined steel; avoid carbon steel and aluminium
- Solutions are acidic at pH 3.5–5.0, so treat dosing lines as acid service
- Keep away from alkalis, which precipitate aluminium hydroxide and block dosing lines
What drives the price
- Aluminium hydroxide and hydrochloric acid feedstock costs
- Al₂O₃ content and basicity, since higher basicity requires more processing
- Form, with spray-dried powder costing more to produce than plate and frame but less to freight per unit of aluminium
- Drinking-water grade certification, which carries a premium over industrial
- Freight and delivery city
- Monsoon season demand, when turbidity and coagulant consumption rise together
Sourcing considerations in Pakistan
- PAC is largely imported, so confirm lead time alongside price and plan monsoon stock in advance
- It is not classified as a dangerous good for transport in the usual solid forms
- For potable use, require drinking-water suitability documentation and heavy metals results on the batch certificate
- Where you are switching from alum, budget for a jar testing programme and a transition period rather than a straight substitution