Pulp and Paper: Freeing Wood Fibres with Chemistry

0:00How do we turn solid wood into soft, white paper?

0:06Wood contains long fibres of cellulose, a natural polymer made from glucose.

0:12Lignin, another natural polymer, glues the fibres together.

0:17One way to make paper is to dissolve the lignin with chemicals and free the fibres.

0:23In the kraft process, wood chips go into a digester, a sealed pressure cooker, and cook at about 170 degrees Celsius.

0:32The cooking liquid, called white liquor, is sodium hydroxide and sodium sulfide dissolved in water.

0:40These chemicals break the lignin into small pieces that dissolve, so the fibres come apart.

0:46The freed fibres are called pulp.

0:49The used liquid, now dark with dissolved lignin, is called black liquor.

0:55Black liquor is burned in a recovery boiler, and the heat makes steam and electricity.

1:01The sodium and sulfur compounds left behind are turned back into white liquor and used again.

1:07The pulp is still brown, because a little lignin is left on the fibres.

1:12Bleaching removes most of this leftover lignin, so the pulp turns white.

1:18The white pulp is then pressed and dried into paper.

1:22The kraft process can release sulfur gases, such as hydrogen sulfide, which smell like rotten eggs.

1:29Mills also use lots of water, so the wastewater is cleaned in a treatment plant before it is released.

1:37So the kraft process dissolves lignin to free the fibres, and recovers its chemicals to use again.