Inorganic coagulants are used in two critical parts of the paper production process: sizing and retention.

Sizing
Sizing controls the amount of liquid the cellulosic fibres in paper can absorb. Appropriate sizing regulates the amount of ink or water that can penetrate into the paper. Without it, our printed papers would end-up a soggy mass.
A combination of aluminium salts and organic compounds have been used to control the amount of sizing in paper since the 1800s. Aluminium sulfate was initially used, but it was replaced in the 1990s by poly aluminium chloride (PAC).

Retention
Retention is a papermaking term which describes the process of separating fibres from water to form a paper sheet. Good retention is essential for economical and quick paper production.
Retention is one of the most controlled parameters in the papermaking process. Improving retention reduces the amount of papermaking compounds, fibres and fillers in the waste water and improves profitability.
To increase fibre and filler retention, aluminium salts, sodium aluminate (SAL) and poly aluminium chloride (PAC) are used. Aluminium salts enhance the effect of other process chemicals (for example, poly-electrolytes) and remove unwanted anionic trash.