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Paper bleaching

The pulp and paper industry is a large and growing industry in the US. However this impressive economic impact is diminished by the environmental damage resulting from the pulp bleaching section of the industry.

The primary reason of pulp bleaching is to remove the lignin content to obtain a lighter pulp. Lignin is degraded during the process into chromophoric groups, which account for 90% of the dark colour associated with unbleached pulp.

Vinings Industries Process of Paper and Pulp Bleaching

Conventional pulp bleaching process consists of a sequence of steps using alkaline, acid, hydrogen and sodium peroxide, oxygen, dithionite salts, sodium bisulfite and water wash processes, along with a variety of chlorinating treatments. The chlorinating treatments use chlorine gas, hypochlorite salts and chlorine dioxide (ClO₂).

Here are the two types of pulp and how they appear after bleaching.

Environmental effects of bleaching

       Unfortunately, bleaching causes a grave impact on our earth.

Release of waste bleach waters is the most serious problem of the paper and pulp industry. Examples of the pollutants are dioxin and chlorine compounds.

Dioxin

Dioxin

The green spheres represents chlorine atoms.

The grey spheres represents the arbon atoms

The red spheres represent the oxygen atom.

Dioxin causes great damage to our health by building up in our bodies in large amounts.

Case Study

The Seveso incident focused world attention on the dangers connected with the chemical industry. A chemical plant in Seveso, Italy, employed 200 people in the production of 2,4,5-trichlorophenol for the Swiss Givaudan Corporation, which is a subsidiary of Hoffman-LaRoche. On 10 July 1976, a build-up of pressure in a reaction vessel caused a safety valve to rupture, and a cloud of 2,4,5-trichorolphenol and other chemicals was released into the atmosphere.

One of the chemicals released was 2,3,6,7-tetrachlorobenzo-4-dioxin, also known as dioxin for short. It is a by-product of the manufacture of 2,4,5-trichlorophenol which is an important intermediate in the manufacture of 2,4,5-trichlorophenoxyethanoic acid (2,4,5-T), a herbicide used to control brushwood and hexachlorophene, a bactericide which is used for the treatment of acne, for sterilizing wounds and in skin cleansers.

Dioxin is stable to heat, acids and alkalis, is almost insoluble in water but soluble in some organic solvents. It is 10 000 times more poisonous than the cyanide ion. When a person is exposed to dioxin over a long period of time, dioxin residues accumulate in the liver and fat cells. Symptoms are cirrhosis of the liver, damage to the heart, kidney, spleen, central nervous system, lungs and pancreas, memory and concentration disturbances and depression. The skin disease, chloracne, is caused by the body’s attempt to get rid of the poison through the skin. Dioxin also has a teratogenic effect, an effect of the genes, which result in birth defects.

Among the children in Seveso, 134 were confirmed to have chloracne and 250 out of 730 pregnant women opted for abortion. The whole town was evacuated. Hoffman-LaRoche agreed to pay for all the material damage and set up a fund to pay compensation to individuals affected. Over a period of many years, many layers of topsoil contaminated by dioxin were removed and buried 10m beneath plastic and clement.

Many countries now banned the use of the herbicide 2,4,5-T because it contains dioxin and replaces it with other more costly but less hazardous herbicide. The Seveso accident moved the European Community (EC) to bring out a set of guidelines aimed at preventing similar accidents. This legislation, known as the Seveso Directive, laid down regulations for the control of hazardous industrial activities in member countries of the EC.

1. H₂O₂ is rapidly replacing chlorine as the bleaching agent to increase the brightness of paper pulp. However in the absence of a stabilizer, a significant proportion of the peroxide is wasted as a result of decomposition catalyzed by heavy metals in both wood pulp and the processing equipment. Hence many products are manufactured to remove these transitions metals which act as the catalysts.
2. Another alternative method is the use of ozone O₃, in bleaching by passing ozone through a high voltage electric field. There are no harmful byproducts formed using this process.
3. We, as consumers, can also help to save the environment by using unbleached or totally chlorine free paper.