At the end of the eighteenth century chlorine and hypochlorite became available and the commercial sun bleaching of textiles was soon replaced by bleaching with hypochlorite. Calcium hypochlorite, called bleaching powder, became the most useful bleaching compound because it could be prepared and transported as a powder. For a long time calcium hypochlorite was the only bleaching agent for rag and other fibers use for papermaking. The next change in bleaching came about 1930 when elementary chlorine was commercially applied as a bleaching agent for pulp. Multistage of sulfite pulp, using chlorination, alkaline extraction, and hypochlorite expanded rapidly. The bleaching of kraft pulp by this method came after it was discovered that hot alkali extraction dissolved the products formed by chlorination and that several bleaching stages would bleach kraft pulp to a high brightness.The effectiveness of chlorine as an agent for lignin-removal in pulp had been known for a long time, but there had been no suitable equipment for carrying out the chlorination process on an industrial scale. The breakthrough of chlorination came with development of stainless steel , which could resist the attack chlorine. The next major advance in pulp bleaching, the introduction of chlorine dioxide, was also closely connected with development of more corrosion-resistant. construction materials. Chlorine dioxide had been known as bleaching agent for pulp from about 1920, but the first commercial application was not until 1946. Besides the problem with equipment materials there also were problems arising from the fact the chlorine dioxide is highly toxic and explosive when the concentration in the gas phase exceeds 15%.
Bisulfite was used in the nineteenth century for the brightening of mechanical pulp. Dithionite (hydrosulfite) was introduced later for the brightening of mechanical pulp and also to give a slight brightening to kraft pulp. Peroxide, as sodium peroxide or hydrogen peroxide, was introduced as a bleaching agent for pulp about 1940.
Oxygen has been will known for many years as a strong oxidizing agent. Separation of oxygen from air is also relatively cheap is large-scale plant. It was logical, therefore, that oxygen should be tried as a pulping and bleaching agent , but the first really promising result with oxygen bleaching were not obtained until about 1955. The first commercial installation for oxygen bleaching was made in 1970, and there are significant number of other plants in operation or under construction.
Bleaching of pulp is in a changing state. The effluents from bleaching plants are among the most troublesome ones in the pulp and paper industry and nobody can at the present time predict what the final solution will be. There is general arrangement that we should see great changes in the next decade. New bleaching agents will probably be introduced. Ozone appears to be on the threshold of industrial application and chlorine monoxide is a potential bleaching agent. New methods for the application of the conventional bleaching agents are being introduce and even newer methods can be expected in the future.
(Source: Casey, J.P., (1980). Pulp and Paper Chemistry and Chemical Technology)
(Source: Casey, J.P., (1980). Pulp and Paper Chemistry and Chemical Technology)
