How wood pulp is made

A pulp mill burns the leftover wood goo it cooks out of the logs, and runs on the energy.

stations: 6
things upstream: 11
raw sources: 6
steps from the ground: 3

The short version

Wood pulp is wood with its glue removed. Logs are debarked and chipped, then cooked in a pressure vessel with a hot, harsh alkaline liquid that dissolves the lignin holding the fibers together. The loose fibers are washed, usually bleached, and dried into sheets. The spent cooking liquid is burned to recover the chemicals.

Debark and chip

Logs are stripped of bark, which fuels the mill, and chopped into small uniform chips. Only the heartwood and sapwood carry usable fiber.

Goes in
Timber
Cook the chips

Chips go into a digester, a huge pressure cooker, with a hot liquid of sodium hydroxide and sodium sulfide. The liquid dissolves lignin, the natural glue that binds the cellulose fibers, and leaves the fibers intact. About half the weight of the dry wood survives as pulp.

170 to 176 °C

Goes in
Caustic soda Water
Blow out and wash

The cooked chips are blown into a tank at normal pressure, where they fall apart into brown fibers. A chain of washers then rinses the dark liquor out, moving the pulp one way and the wash water the other.

Goes in
Water
Burn the dark liquor

The used cooking liquor is boiled down to a thick syrup and burned in a recovery boiler. The sodium chemicals melt out of the ash and are mixed with lime to rebuild fresh cooking liquid. The same fire raises steam and electricity for the mill.

Goes in
Quicklime
Bleach to white

For white paper, the brown fibers pass through several bleaching stages. Mills now mostly use chlorine dioxide, oxygen, ozone or hydrogen peroxide instead of chlorine gas. Packaging pulp often skips this step.

Goes in
Oxygen gas Water Hydrogen peroxide
Dry into sheets

The clean pulp is pressed and dried into sheets or rolls, and shipped as bales to paper mills. Air-dry pulp keeps about 10 percent moisture so it is easy to break up in water later.

about 10% moisture

End of the lineWood pulp Next it goes into nail polish, kitchen sponge, sanitary pad and 7 more.

Worth repeating

The kraft process was patented in Danzig, yet its first mill started in Sweden in 1890, a location attributed to the German paper industry not accepting the new process.
It takes about 24 trees to make a ton of printing and writing paper by the kraft process, and about 12 to make a ton of mechanical pulp.
Tree products make up more than 95 percent of the world's pulp.