HANDLING & STORAGE
PAC Shelf Life and Storage: What Actually Degrades the Product
PAC does not spoil the way food does, but it does change. Powder takes up moisture and cakes, liquid slowly hydrolyses, and both lose performance well before anyone notices that a bag has gone hard. Most of the avoidable loss happens in the buyer's warehouse rather than at the factory.
Three mechanisms, one result
Everything that goes wrong with stored PAC goes wrong through one of these three routes.
- Moisture. Powder PAC is hygroscopic. Above roughly 60 - 70% relative humidity the surface of the granules absorbs water, particles stick, and the mass sets. Caked product dissolves slowly and never completely, so part of what you paid for stays in the make-up tank
- Heat. Above about 35 °C the distribution of aluminium species shifts toward larger, less reactive aggregates and basicity drifts. A container parked in tropical sun sits well above that for weeks
- Time in solution. A working solution is a partially hydrolysed system that keeps moving. Three to five days is the practical limit before performance falls away, and warm weather shortens it
Shelf life by form
| Form | Typical shelf life | What limits it |
|---|---|---|
| Solid powder, sealed | 12 - 24 months | Moisture uptake once the liner is opened |
| Solid granular | 18 - 24 months | Caking and lump formation |
| Liquid, 10% Al2O3 | 6 - 12 months | Gradual hydrolysis and sedimentation |
| Working solution, 5 - 10% | 3 - 5 days | Hydrolysis and biological growth |
The clock starts at production, not at delivery
Shelf life is counted from the production date on the certificate, not from the day the container reaches your yard. A shipment that spent six weeks on a quay in July has already spent part of its life.
Product past its nominal shelf life is not automatically unusable. It needs a jar test against a retained sample before it goes into a critical treatment stage, and the result of that test is what should decide, not the date stamp.
The warehouse checklist
None of this is expensive, and all of it is cheaper than a rejected batch.
- Keep the store between 5 and 35 °C, with relative humidity below 60 - 70%
- Palletise: 10 - 15 cm off the floor, at least 30 cm from walls, so air can move
- Stack no more than ten bags high - the bottom layer takes the load and splits
- Reseal the inner liner after every part-used bag; an open bag can cake overnight in humid weather
- Rotate stock first in, first out, and label every pallet with receipt date and batch number
- Keep away from caustic soda, hypochlorite, lime and strong oxidants; PAC is acidic and reacts with all four
- No direct sun on bags or totes, and no storage against a steam line or in a boiler room
Liquid PAC: tanks, materials and frost
Liquid PAC is acidic, pH 3.5 - 5.0, so the tank and every wetted part should be HDPE, polypropylene, FRP or rubber-lined steel. Carbon steel and aluminium corrode, and the corrosion products contaminate the product before the tank itself fails.
Bund the tank to 110% of the largest container volume, keep it out of direct sunlight, and protect it from frost. Freezing changes the polymer structure; thawing and mixing may restore the appearance but not always the performance.
When liquid goes wrong
| What you see | What it usually means | What to do |
|---|---|---|
| Crystals or slush after cold weather | Freezing, not contamination | Warm slowly to 15 - 20 °C, mix gently, jar test before full-scale use |
| Sediment or cloudiness in a clear product | Hydrolysis has progressed | Check the 1% solution pH and Al2O3; discard if out of specification |
| Product darkens over months | Normal ageing within limits | Jar test against a fresh sample; dose may need a small increase |
| Line freezes in winter | Outdoor run without protection | Lag or trace-heat the line; the line is often the real limit, not the tank |
Caking: what can be recovered
Light caking - a bag with hard lumps you can break by hand - is recoverable. Dissolve with longer mixing and water at 30 - 40 °C, and expect a higher insoluble residue; strain the solution before it reaches a pump.
A bag that has set into a single hard block is not worth saving. Breaking it up produces coarse, slow-dissolving fragments and a high insoluble load, which is how dosing lines and strainers get blocked. Discard it and fix the storage instead.
An opened bag is the commonest cause of both cases. Ten kilograms taken out and the liner left open is enough to form a hard crust overnight in a humid climate.
What importers should ask about shipping and port storage
Storage problems are often created before the container reaches the plant, and they are easier to prevent than to fix.
- Container interiors in tropical sun reach well above 50 °C. Ask for a stow away from the door end and unload promptly rather than leaving the box on the quay
- Insist on intact inner liners and palletised bags; loose bags in a container abrade and split in transit
- Container rain - condensation on the underside of the roof - is a real risk on long sea routes with temperature swings. Desiccant sachets and a moisture-barrier liner are cheap insurance
- Jumbo bags need a forklift and a dry floor; a yard that cannot provide either will produce caked product regardless of specification
- Check the COA against the batch on arrival, and keep a retained sample from each shipment for later comparison
A ten-minute monthly inspection
A short walk through the store once a month prevents almost every storage failure that reaches the dosing pump.
- Look for split bags, swollen totes, stains and corrosion on shelving
- Pick a bag from the bottom of the stack and feel for hardness
- Test the pH of a 1% solution - above 5 suggests hydrolysis has started
- Jar test the oldest batch in stock against a retained sample
- Check the temperature and humidity log, and the FIFO labels
- Clear any product within three months of its shelf life into current production before it becomes a disposal problem
Frequently asked questions
Does PAC expire?
It has a practical shelf life rather than a hard expiry date: 12 - 24 months for sealed powder, 6 - 12 months for liquid. Product beyond that is not necessarily unusable, but it needs a jar test before it goes into a critical stage.
Can I store PAC and PAM in the same warehouse?
Yes, in separate sealed packaging and away from moisture. Never in the same solution tank or the same make-up line - the two react on contact and the mixture is useless.
My liquid PAC has crystals in it. Is it finished?
Usually that is freezing or long storage in the cold, not contamination. Warm it slowly to 15 - 20 °C, mix gently, and jar test. If sediment remains after warming, treat the batch as degraded and do not dose it.
Is PAC a dangerous good for transport?
At commercial concentrations it is generally not classified as dangerous goods, but it is acidic and irritant, and it corrodes carbon steel and aluminium. Confirm the classification with your freight forwarder for your route, and keep the SDS with the shipping documents.