Many methods are used in the modern day pulp and paper industry. From 1973 to 1984 the total pulp capacity went from 68% to 72% sulfite pulp, and the bleached and semi-bleached increased from 31% to 38%. In 1992, the total world wood pulp capacity consisted of 68% chemical (kraft and sulfite), 26% mechanical, and 6% chemimechanical (CTMP) pulping.
Chemical pulping uses chemicals to separate the cellulose from the lignin. This creates pulps with different properties that can be used for higher quality paper. The three types of chemical pulping are Kraft, Sulfite, and semichemical.
The Kraft process, also called the sulfate process, involves an alkaline treatment with solutions of sodium sulfide and sodium hydroxide. In 1987 Kraft pulping was 95% of the total chemical pulping.
Sulfite pulping began with the use of calcium as the sulfite liquor base. Today the base has been changed to magnesium, ammonia, or sodium. This allows for the recovery of spent liquor.
The semichemical pulping process involves cooking wood chips in a neutral solution of sodium sulfite and sodium carbonate or sodium hydroxide. Only some of the lignin is removed, and then the pulp undergoes mechanical disintegration.
The two processes of mechanical pulping are stone groundwood (fiber is torn from the side of short logs with grindstones) and refiner groundwood (wood chips are passed through disc refiners). Mechanical pulping does not separate the cellulose fiber from the lignin in the wood, while chemical pulping does.
In chemimechanical pulping, the wood is softened with chemicals before grinding. There is also a process called thermomechanical pulping, where the wood is softened with heat and then disc-refined under pressure.
| Kraft: chemical | Sulfite: chemical | CTMP: chemimechanical | Groundwood: mechanical | ||
| yield of pulp | 40-42% | same as Kraft | 90-92% | about 80% | |
| strength of fibers | higher than mechanical, lower than CTMP | lower than Kraft | highest | lowest strength | |
| color/ brightness | more effective bleaching methods have been developed | lower than Kraft | high | poor | |
| cost | higher than mechanical or CTMP | lower than Kraft | lower than chemical | low | |
| water use | depends on water reuse, higher than mechanical or CTMP | same as Kraft | lower than chemical | low | |
| uses | fine paper, paperboard, cartons, magazines, etc. | same as Kraft | same as Kraft | low quality paper, newspaper, tissues, towels, etc. | |
| advantages | better than Groundwood and sulfite, better recovery of chemicals | better than Groundwood technique | less processing required, range of wood species used | printing | |
| disadvantages | toxic waste streams (caused by bleaching, odor | same as Kraft | highly concentrated effluents, worse than chemical | low strength, impermanent paper (reactive) |