CHLORINATION BLEACHING

SUBIMITTED BYKRISHNA PRATIM KONCHPPT- III SemesterROLL NO :- 11

Schematic Diagram of the Chlorination Process

IntroductionChlorination, which comprises the first stage of most bleaching sequences, is intended to Delignify rather than simply to decolorize pulp and therefore constitutes an extension of the pulping process.

Chlorine reacts smoothly and rapidly with the “Non-Carbohydrate” component of chemical pulps to form a highly colored, partially water soluble material usually referred to as “Chloro-Lignin.”

Chloro-Lignin becomes readily removable by Alkali Extraction in subsequent bleaching stages. Availability of liquid Chlorine at Low cost is the main factor contributing to its extensive use in bleaching processes.

About Chlorine

Chlorine is a chemical element with symbol “Cl” and atomic number 17. It has a relative atomic mass of about 35.5. Chlorine is in the halogen group (17) and is the second lightest halogen, following fluorine. The element is a yellow-green gas under standard conditions, where it forms diatomic molecules. Chlorine has the highest electron affinity and the third highest electro negativity of all the reactive elements. For this reason, chlorine is a strong oxidizing agent.

Its Oxidation States are 7, 6, 5, 4, 3, 2, 1, 0, −1 (Strongly Acidic Oxide) .

Fundamental Aspects of ChlorinationChlorine-Water System :- When chlorine gas is dissolved in water, a reversible hydrolysis occurs, according to the following equilibrium ;

Cl2 + H2O HOCl + HCl (H+) + (Cl –).

When a Base (e.g. Sodium Hydroxide) is present, the additional reaction occurs ;

HOCl + NaOH NaOCl + H2O

(Na+) + (OCl - )

Both H+ & Cl- are responsible for bleaching.

The proportion of Chlorine, Hypochlorous Acid, and Hypochlorite Ion in solution depends upon the “pH” of the solution .

Thus pH plays an important role in Chlorine-water systems, since each of these forms of Chlorine attack Cellulose and Lignin in a different manner.

One factor is the oxidation Potential or oxidizing activity of the three forms of Chlorine & the other is the manner in which each form combine with Lignin, Cellulose and other materials.

Forms of chlorine Oxidation Potential, Volts

OCl – + 0.94 Cl2 + 1.35 HOCl + 1.50

Hypochlorous Acid is the most active and Hypochlorite the least Active of the three Forms.

It is determined by KH = (H+) (Cl-) (HOCl) = 4.84 X (10)-4 (Cl2)

Reaction of Chlorine with residual Lignin

1. The Addition involves reaction of Cl2 with Lignin at Unsaturated double and triple bond, thereby form no Byproduct.

ROH CHR’ + Cl2 ROHCl CHClR’

2. The Substitution involves the replacement of Hydrogen atom in the Lignin molecule with Chlorine forming one molecule of HCl acid as Byproduct.

RH + Cl2 RCl + HCl

3. The Oxidation involves the reaction with Nascent Oxygen with Lignin two molecules of HCl acid per molecules of Cl2 consumed .

Reaction of Chlorine with residual Lignin RH + Cl2 + H2 RHO + 2 HCl

Oxidation is the principal reaction competing with Aromatic Substitution in the Chlorination of Lignin. During Technical pulp Chlorination , about 50% to 70% of the applied chlorine is consumed by Oxidation reactions.

Solublization of Lignin occurs due to Substitution and Oxidation Reactions.

Factors affecting removal of Residual Lignin during Chlorination of Pulp

1 . Physical hindrance owing to the morphological structure structure of the fiber.2 . A high proportion of Methoxyl groups that add resistance to the solubilizing action of Chlorine.

Reaction of Chlorine with Carbohydrates in Chlorination Stage

The action of Chlorine solution on cellulose (and other polysaccharides) is not selective, i.e. they oxidize more than one functional group in the structural saccharide building units.

Furthermore, oxidation may be Preceded, accompanied or followed by hydrolysis.

Hence Hydrolytic & Oxidative Cleavage of the Acetal linkages in Cellulose can result in a direct loss of Fiber Strength.

Chlorination reactions in bleaching of Pulp

Variables in Chlorination Process

1 . Chlorine Concentration :- The optimum amount of Chlorine to be applied depends on the type of Pulp and number of stages in bleaching; usually calculated with the help of Permanganate Number (TAPPI Method T 214) or the Kappa Number (TAPPI Method T236) and Roe Chlorine Number (TAPPI Method T 202).

2 . Consistency :- The Chlorination Stage is usually carried out at low pulp consistencies of 3 to 4 % . It is done mainly because of low solubility of Chlorine in water and also because of greater ease of mixing and pumping operation in the mill.

3 . Temperature and Time :- The Chlorination Stage proceeds freely at ordinary temperature (20 to 30 *C), and close control of temperature is therefore not much important. It is governed by the existing temperature of the water supply at the mill. It is an Exothermic Process and evolves heat of about 500 Calories for every 1Kg chlorine Added with a rise of 1 to 3 *C.

Generally it is carried out for 45 to 60 mins.

4 . pH :- The suitable pH range for Chlorination of Pulp is pH less than 2. Lowering the Chlorination pH from 3 to 1, result in consumption of 90 % chlorine during the first 5 to 10 minutes.

5 . Mixing :- Successful Chlorination is the product of adequate mixing, insuring intimate contact of chlorine gas with the fiber in an aqueous medium. Lack of mixing results in high gas concentration in the retention tower, degradation of Pulp due to “chlorine burn” and pockets of unchlorinated Pulp within the tower.

Disadvantages / Demerits in Chlorination Process

At this time, the majority of bleaching processes rely onthe use of molecular chlorine and chlorine dioxide as theoxidizing agent to remove residual lignin from the cellulosefibres. Traditionally, molecular chlorine, with a smallpercentage (10%) of chlorine dioxide was favoured. However,more recently, mills have switched to 100% substitutionof molecular chlorine with chlorine dioxide. Thischange in the industry has been driven by the need toreduce both the amount and the degree of chlorinatedcompounds released from the bleaching process. Thereasons for the shift to chlorine dioxide technology arefound in the chlorination reactions that occur in thebleaching process and the extent to which these contributeto the formation of potentially hazardous by-products, Inducers of Mixed Function Oxidase and Dibenzofurans.

Of the organochlorine compounds produced in the bleaching of pulp, those of greatest concern arepersistent, highly bioaccumulative, and toxic like Dioxins Organochlorides and Chlorinated organic compounds that result in higher risks to the environment. The formation of these compounds is theresult of the nature of the reactions in the bleaching process and use of molecular chlorine

Conclusion

On the basis of the evidence from studies on the use of chlorine and chlorine dioxide in the bleaching ofpulp and paper, it is clear that the use of molecular chlorine as a bleaching agent results in the formationand release of chlorinated organic compounds, sometimes at concentrations that present an unacceptablerisk to the receiving environment and the food chain. Alternatives to the use of molecular chlorine, such aschlorine dioxide, result in reductions in the quantities of organochlorines produced, and the degree ofchlorine substitution in the organochlorines formed. This, in turn, leads to reduced persistence, reducedpotential for bioaccumulation and food chain transfer, reduced toxicity and reductions in adverse ecologicaleffects. And efforts were made to Use TCF & ECF Bleach.

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