The sulfate process (also known as “Kraft digestion” because of the strength of the paper made using it) is the most common process for making paper pulp from cellulose. The cell walls are digested by boiling in an alkaline medium containing sodium sulfide and sodium hydroxide (“white liquor”) and the lignin contained in the plant material is separated. The pulp is sorted, washed and bleached. By clever combination of process steps, the chemicals used can be largely recovered, with gases increasingly being used here.