Generally, when raw materials are cooked with sodium hydroxide in alkaline papermaking, dark brown plant pulp will be produced, and black liquor will be produced when washed with water, and the black liquor contains sodium hydroxide. In order to recover sodium hydroxide, black liquor is evaporated and concentrated, and then burned. The ash produced by combustion contains sodium sulfate and sodium hydroxide, which is absorbed by water. After absorption, the liquid is green, which is called green liquid, and then quicklime is added to generate calcium salt precipitation, which is filtered in vacuum to form white slurry, the main components of which are calcium carbonate and calcium sulfate.

After washing and bleaching, pulp will be produced, and the remaining water after bleaching is basically called white liquor.