APPLICATION
PAC in Papermaking: Sizing, Anionic Trash and Retention
Paper mills were using aluminium chemistry long before water treatment borrowed it. In the wet end, PAC works as an anionic-trash scavenger, a sizing promoter and a drainage improver - three roles that need different dose points and different dose sizes. Treating them as one knob is the most common way mills waste chemical.
Three roles, three dose regimes
| Role | Where it is added | Typical scale (set by trial) |
|---|---|---|
| Anionic trash scavenger | Thick stock or machine chest, ahead of cationic additives | a few kg per tonne of fibre |
| Sizing promoter with AKD / rosin | Near the headbox, with the sizing agent | fractions of a percent on dry fibre |
| Retention and drainage aid | Combined with cationic starch or PAM at the fan pump | low kg per tonne, tuned on first-pass retention |
Anionic trash: the quiet budget leak
Mechanical pulps and broke-heavy furnish carry dissolved anionic colloids that bind cationic additives before they reach the fibre. The mill responds by over-adding cationic starch or polymer, which shows up as poor first-pass retention and foamy white water. A modest PAC dose ahead of the cationic additives neutralises that trash, and the cationic programme suddenly needs less to do the same job - the savings usually exceed the PAC cost.
The tell-tale sign that trash scavenging is missing is a retention figure that will not hold despite rising additive dose, plus white-water cationic demand that never settles. A zeta-potential or cationic-demand titration on white water makes the diagnosis objective.
Sizing and runability
Under neutral and alkalimal sizing with AKD, PAC supplies the cationic bridge and the aluminium species that anchor the sizing agent to the fibre, letting mills size at higher pH without losing pick resistance. It also buffers the wet end against pH drift from furnish changes, which stabilises everything downstream from drainage to drying.
As with coagulation, more is not better: an overshot wet end goes cationic, retention aids stop anchoring, and deposits rise. Set the PAC dose by trial on first-pass retention and white-water demand, then hold it - wet ends reward steadiness, not enthusiasm.
Frequently asked questions
Should the paper mill use PAC or alum?
PAC gives the same aluminium chemistry with a higher basicity: less pH sag, less sulphate load in the white water loop, and steadier behaviour across furnish changes. Mills running neutral-alkalimal sizing generally find PAC the easier fit; acid-sizing mills on rosin may keep alum where it already works.
Which PAC grade suits the wet end?
Industrial-grade PAC at 28% Al2O3 (GB/T 22627-2022) is the normal choice for wet-end roles; paper-grade white powder with low insolubles matters more than a content premium. Confirm low insolubles on the COA - wet-end screens punish dirty product.
Can PAC replace my retention polymer?
No - they do different jobs. PAC neutralises and bridges at low dose; polyacrylamide builds the actual floc network for retention. Mills get the most from a small PAC dose fixing the environment and a PAM programme doing the retention, tuned together rather than one against the other.