Pulping and Bleaching PSE 476/Chem E 471 Lecture #17 Introduction to Bleaching

Agenda Brightness General Bleaching Principles Chemistry Process Chemicals Description Advantages/Disadvantages

Why Bleach? Improve brightness. Improve brightness stability. Clean up pulp (impurities). Wood based (bark, resins, sand, shives). Process based (carbon specs, rust, rubber). External sources based (plastics, grease, ash). Increase capacity of paper to accept printing.

The purpose of bleaching

Bleach plant

Brightness Determination (1) Light shinning on a sheet of paper is either transmitted, adsorbed, or reflected. Light is scattered by fibers at air/fiber interfaces Light is adsorbed by certain chemicals in the fibers (lignin) Light Transmitted Reflected Absorbed

Brightness Determination (2) Brightness is measurement of how much light is reflected from a sheet of paper. Whiteness does not mean brightness. Whiteness is a physical phenomena related to how the eye views the paper. A very white looking piece of paper may not have high brightness. Example: blue dye added to a yellow tinged sheet of paper will give a white sheet of paper with low brightness.

Brightness Determination (3) Brightness determination method: Light reflectance measured and compared to light reflectance from MgO. MgO assumed to reflect 100% light. Brightness is reported as % of MgO reflectance (85 brightness is equivalent to 85% of MgO). Variables: Angle of light: Light is applied to sheet at 45° angle. Wavelength: 457 nm (blue light most sensitive). Pine kraft Unbleached-ISO 23-28% Semi bleached-ISO 60-80% Bleached-ISO 88-91%

General Principles Two types of bleaching: Lignin removing: chemical pulps. Lignin retaining: mechanical pulps. Bleaching is used because at a certain point in the pulping process, carbohydrate degradation becoming greater than lignin removal. Bleaching chemicals are more selective for lignin. Bleaching chemicals much more expensive than pulping chemicals so they are not used in pulping.

General Principles: Chemistry Pulping Pulping typically involves cleavage of ether linkages and some substitution (sulfonation). Bleaching Bleaching involves attacks on aromatic rings, olefinic structures, and carbonyl groups. Substitution reactions play a big role.

Multiple stages of bleaching

General Principles: Process Bleaching uses a combination of chemicals in series. One chemical alone will not remove residual lignin. Each step reacts with material modified in previous step. NaOH NaOH ClO2 ClO2 O2 O2 Unbleached D EO EO D Bleached Pulp Pulp

Washing

General Principles: Chemicals (1)

General Principles: Chemicals (2)

General Principles: Chemicals (3a)

General Principles: Chemicals (3b)

General Principles: Chemicals (3c)

Groups of bleaching chemicals (2) Bleaching chemicals can be divided into three groups according to their function: 1 Group The chlorine (Cl2), ozone (O3) reacts with all aromatic lignin units (phenolic groups and their =bonds) 2 Group The chlorine dioxide(ClO2) and oxygen (O2) reacts in general with lignin structures that have free phenolic hydroxyl groups 3 Group The hypochlorite (H) and hydrogen peroxide (H2O2) reacts only with certain functional groups, for example carbonyl groups

Bleaching reactions Bleaching chemicals are used primarily as oxidants, to break down residual lignin and to increase its solubility. Mode of operation: Electrophiles (oxidative reactions, low pH, involve cations) Nucelophiles (reductive reactions, high pH, anions) Radicals

Bleaching Generalities It is important to note that when bleaching with a specific reagent, it will be converted into a number of different reactive species which will react with lignin and carbohydrates differently. A simple example is when chlorine gas is added to water; both hypochlorous acid and/or hypochorite is formed depending on the pH.