Chemical pulping involves treating wood chips with chemicals to remove the lignin and hemicellulose, thus separating and cleaning the fibers. Delignification gives the fibers greater flexibility, resulting in a substantially stronger paper (because of greater contact between the fibers in the finished sheet) than can be manufactured from high-lignin fibers produced by mechanical pulping. Paper strength and durability is gained at the expense of fiber yield. Chemical processes may yield only half the fiber that can be recovered by the use of mechanical pulping techniques.
Most chemical pulp is made by the sulphate process (or Kraft process), in which caustic soda and sodium sulphate ‘cook’ the woodchips. In the unbleached stage, a dark brown but very strong pulp results and this can be bleached to a high brightness if required.
The sulphite pulping process is an alternative method best suited for speciality pulp which can be easily bleached, generally with hydrogen peroxide. These pulps fulfil the demand for ‘chlorine-free’ products in the hygiene paper sector and also in printing and writing papers.
Chemical pulping process consists of three steps:
Step 1 – Pulp Cooking Process
Cooking removes lignin and separates the wood into cellulose fibres. The resulting slurry contains loose but intact fibres which maintain their strength. During the process, approximately half of the wood dissolves into what is called black liquor. The cooked pulp is then washed and screened to achieve a more uniform quality. The black liquor is separated out from the pulp before the bleaching process.
Step 2 – Pulp Washing Process
The pulp is washed with water to wash out the cooking chemicals and lignin from the fiber so that they will not interfere with later process steps.
Used chemical recover is the most important part for Kraft pulping process. The chemicals are separated from fiber as black liquor. Concentration of the black liquor is very significant for recovery; on the other hand pulp should be well washed. In one word, the liquor should be minimum dilution to get better evaporation benefits and better wash pulp.
Step 3 – Pulp Bleaching Process
The purpose of bleaching is to remove remaining dark coloured lignin impurities in the pulp and thus meet certain quality criteria. In other words, bleaching is the treatment of cellulosic fiber with chemicals to increase brightness. Brightness may be achieved by either lignin removal (delignification) or lignin decolonization. Lignin remains a major constituent of pulp even after digestion by chemical pulping. For example, Kraft pulp may contain up to 6 percent lignin based on its dry weight. Unbleached groundwood spruce pulp may contain 27 percent lignin.