Chlorination bleaching process

Chlorination bleaching is the second stage of the pulp bleaching process. It is the cheapest bleaching agent for eliminating lignin. Elemental chlorine (EC) reacts selectively with the non carbohydrate compounds. It is another significant delignification process. It ruptures the lignin bonds and make important amount of chlorinated organic compounds. All most all the products of chlorination stage are soluble in water or in alkaline solution. The pulp chlorination bleaching process is not environmental friendly; it produces many toxic compounds as like organic halide compounds. Hence many of the industry are not use this elemental chlorine. Moreover, a lot of industries are used modified chlorination for many years. The vital function of the chlorination bleaching is to change the lignin residues remaining in the pulp into an alkaline soluble structure to remove.

Variables:
The important variables of chlorination bleaching process are permanganate number, method of pulping, viscosity, Time of chlorination, pH value, Consistency, temperature, available chlorine, degree of mixing, stock color etc.

Condition for chlorination bleaching process
As like others bleaching stages, chlorination bleaching process is not required medium or high consistency. Although upper consistencies accelerate the reactions but create mechanical difficulties. It is required low consistency and better to keep 2-4%. It helps to mixing the gaseous chlorine with the pulp stock properly. The stock is dilute through the filtrate of same stage or fresh water. It does not also require elevated temperatures and frequently maintained between 20 and 30⁰C, whereas some paper mills operate up to 60 ⁰C. It is better to keep the retention time between 30 and 90 minutes and pH value around 2. Sufficient amount of chlorine ought to be applied into the system to get better results.

Application of chlorine
Chlorine can be applied into the pulp by two ways; directly as chlorine gas or as a chlorine water solution. Since the gaseous chlorine is easy to control and can be managed accurately, therefore it is more popular. Elemental Chlorine is not well soluble into the water at normal temperature. On the other hand, it is rapidly absorbed in the presence of pulp. Dry chlorine is not corrosive to iron but hydrous chlorine is corrosive to iron.

Chlorination bleaching reactions

pH is very significant for chlorination bleaching process. Normally, the pH value of the chlorine-water system is about 2; in presence of pulp this pH becomes lower due to HCl production. Moreover it is further lower for using chlorination effluent. Chlorine and hypochlorous acid are more active bleaching agents than hypochlorite ion. The pH determines that which compound is dominant in the chlorine-water system. The reactions of chlorination bleaching process can be classified into substitution and oxidation reactions.

In acidic medium, Elemental chlorine shows electrophilic substitution reactions and reacts selectively with the non carbohydrate compounds like lignin. Chlorine replaces hydrogen of non carbohydrate compounds and produces organic halide compounds and hydrochloric acid.
RH + Cl₂ → RCl +HCl

This reaction is the desired reaction. About 50% of the applied chlorine reacts by substitution. These substitution reactions are extremely effective for eliminate lignin, although it produces polychlorinated aromatics and dibenzofurans.
The equilibrium condition for acidic condition is:

At very low pH value as low as 1.0, the reactions not selective. In this condition hypochlorous acid (HOCl) can attack the cellulose.

Almost neutral condition or alkaline condition, the oxidation reaction is dominant. These reactions are not selective. While this reaction can help eliminate lignin but it also degrades the polysaccharides, which is not desired. In this case, the elemental oxygen is produced and reacts with the pulp and lignin. This reaction also helps to stay acidic to support substitution.
H₂O + Cl₂ → [O] + 2HCl

The equilibrium condition for base condition is:

Without these two types of reactions, another possible reaction is addition, which is little part. In addition reaction, Chlorine reacts with the double bonds of lignin and generates dichlorinated compounds. These products are stable in successive alkali washing.

Environmental problem
Polychlorinated dioxins and dibenzofurans are found in effluents of chlorination bleaching process. These are highly toxic and carcinogens chemicals. Without dioxans and furans the others problematic chemicals that are produce in this process are organic halides or AOX. Therefore many industries are moving away from using this elemental chorine; they started to use ECF or TCF such as oxygen, peroxide, chlorine dioxide, ozone bleaching process.