Generally, the chemical pulp produced in the world today is based on one of two processes: sulphate or sulphite. Most chemical pulp is made by the alkaline kraft or sulphate process which uses caustic soda and sodium sulphate to "cook" the woodchips. In the unbleached stage, a dark brown, but very strong pulp results which can be bleached to a high brightness if required.The acid, or neutral sulphite mills produce easily bleached pulps, generally with hydrogen peroxide. These pulps are fulfilling today’s demand for "chlorine free" products in the disposables sector and also in printing and writing papers.The yield in both chemical processes is much lower than in the manufacture of groundwood, as the lignin is completely dissolved and separated from the fibres. However, the waste lignin from the kraft, and some sulphite processes, can be burnt as a fuel oil substitute. In modern mills, recovery boiler operations and the controlled burning of bark and other residues makes the chemical pulp mill a net energy producer which can often supply power to the grid, or steam to local domestic heating plants.

What is Wood Pulp? Mechanical Pulping Thermo-Mechanical-Pulp and Chemi-Thermo-Mechanical-Pulp Source of Fibre for UK Papermaking

Chemical Pulping The British Wood Pulp Association