Hydrogen peroxide bleaching

Hydrogen peroxide is a total chlorine free (TCF) bleaching agent. It is appropriate for chemical, mechanical and recycled pulp to achieve a wide range of brightness. It has both oxidizing and reducing properties which is pH depended. The chemical formula of this bleach is H₂O₂. Pure hydrogen peroxide is very light blue color but colorless in solution. Recently, the use of this bleaching agent increased significantly in the pulp and paper industry due to easy to use, lower production costs, improved paper quality, increased yield percentage and environmental friendly. The brightness of the paper that is made from hydrogen peroxide bleach is more stable. A color reversion of about 1% is obtained over a long period. The results obtained from hydrogen peroxide bleaching depend upon the species of wood, age of wood, lignin content etc.

Hydrogen peroxide bleaching is a single-stage or two-stage process. This bleaching agent can be used in extraction stage of the bleaching sequence. However, it is also useful as a terminal stage. When it is used, it can be saved a great amount of ClO₂ chemical in the subsequent stage. Some bleaching plant design the system as the first tower is at medium consistency whereas the second tower at high consistency. Ours bleaching plant is two stages and medium consistency system. We used it in extraction stage. At both stages we used 3-4 kg/ton whereas the magnesium sulfate is 1-1.5 kg/ton. At first extraction stage we also used oxygen with it. A two-stage process has an advantage in providing a cleaner pulp. The chemistry is very similar to that involved in oxygen delignification.

HOO^– ion is the active agent of hydrogen peroxide bleaching. This ion is originated by the ionization process.

It is a reversible reaction. As the front reaction produce H⁺, so high pH will increase the HOO^– ion. This ion little effect on cellulose, but selectively reacts with the organic coloring matter. It reacts with lignin through the carbonyl groups.

Peroxide is catalytically decomposed in the presence of metal ions such as iron, copper, lead and manganese. In order improvement the efficiency of hydrogen peroxide bleaching sodium silicate and magnesium sulfate are added during preparation of the peroxide bleach. It controls the metal levels.

Peroxide bleaching introduces in the year 1907, but commercial development took place at 1941. After that the development comes in rapidly.

Effect of pH
The pH is an important factor for hydrogen peroxide bleaching. Maximum brightness is obtained at pH 10.5 or higher. The consumption rate of peroxide is also maximum at this pH because of OH_– ion. This ion forwards the reaction rate into right side and helps to produce more perhydroxyl ion (HOO^–).
H₂O₂ + OH^– → HOO^– + H₂O

Usually the pH level is maintained by addition of sodium hydroxide and sodium silicate. Here, the sodium silicate acts as both a stabilizer and a buffering agent. It is usually added with magnesium sulfate. Higher pH has one drawback that it can cause the development of yellow color and reversion of brightness. This drawback can be arise if the pulp left alkaline after the peroxide is all consumed; otherwise not. So, to get better benefit from hydrogen peroxide bleach, it is requires a careful control of alkali application to the pulp. Generally, it is applied so that the pulp will be neutral or slightly acidic at the end point of this bleaching.

Higher concentrations would harmful to human skin and corrode many materials. Over 75 ppm of its vapor is risk to human health. It should be stored in a cool, dry, well-ventilated area and away from any flammable substances. The stored container materials should be non-reactive.

Effect of temperature
Temperature plays an important role on the rate of bleaching action. Lower temperature can increase the reaction time. On the other hand, higher temperature accelerates the reactions rate but can lead to a slight color reversion due to undesirable side reactions. Moreover, in the presence of heavy metal ions the stability of peroxide is badly affected. Generally, the temperature is maintained from 40 to 6o⁰C to obtain maximum brightness. At moderate temperatures hydrogen peroxide bleach improving the brightness of pulps without significant yield loss.

Effect of Consistency
The consistency of the pulp has noticeable effects on the reaction rate during hydrogen peroxide bleaching. High consistency can reduce reaction time and effectiveness of the bleaching. It is also economical. At first, most of the bleaching plant installed lower consistency (3 to 6%), but at present higher consistency is desirable. Now, most of the bleaching plant operates at 10 to 15% consistency whereas some are operate at 40 to 50% consistency. It reduces the consumption of the bleach and steam requirement. Lower consistency is mostly carried out as a batch wise in a beater, pulper or circulating tank. High consistency is great for maximum brightness, retention time and lower chemical dosage, but it can create maintenance problems and the equipment costs are higher. So it is better to keep medium consistency from 10 to 15%. It requires the retention time 45 minutes to 5 hours and 40 to 6o⁰C temperature.