Bleaching Process

The main purpose of bleaching chemical pulp is to make it whiter or brighter. Bleaching is a multistage process with the pulp being washed between stages. The extent of bleaching is determined by the brightness required. This is achieved by using one of a number of bleaching sequences described as CEH, CEDED, CEHDED, etc. Most bleaching technology is multi-sequence and may involve up to six stages in all.

Chlorine

The principal active chemicals in an aqueous chlorine solution are elemental chlorine and hypochlorous acid. In bleaching, the relative amounts of these chemicals are determined mainly by the elemental chlorine and hydrogen ion concentrations.

Cl2 + H2O ↔ HOCl + H+ + Cl-

Molecular chlorine is an ideal chemical for delignification

There is a limit to the degree of lignin removal that further oxidation can achieve and another extraction step is often required to reactivate the pulp for a final oxidation stage.

The first alkaline extraction stage contributes the largest potential pollutant load released from the pulp bleach plant. It may be possible to reduce the pollutant loss from the extraction stage considerably by substituting oxidative extraction, particularly sodium hypochlorite, for the first alkaline extraction stage.

Oxygen

Strictly the process is one of delignification rather than brightening or bleaching. Alkaline oxygen delignification (O) is ideal for kraft operation as it takes oxidized white liquor as an alkali source and it spent liquor is reused in the kraft digester counter wash. Oxygen is poorly soluble in alkali so needs a stable gas/liquid/solid dispersion, all reactions occur at the solid-liquid interface preferably in a high consistency pulp. Oxygen delignification is complex and involves free radical mechanisms. The treatment will remove half the residual lignin without serious cellulose

It is a special form of oxygen association produced by the discharging an electrical current to oxygen gas. While oxygen atoms normally occur in pairs, the electrical discharge makes three atoms associate with one another, thus giving extraordinary oxidative properties to ozone. Its decomposition to oxygen after bleaching produces neither a residue nor undesirable inorganic byproducts. The strongly electrophilic character of ozone promotes its reaction with functional groups in the remained lignin. Ozone likely reacts with the aromatic rings in lignin to form muconic acid derivatives and other compounds containing carboxylic acid groups. Ozone appears to be more efficient than chlorine dioxide, hydrogen peroxide and dimethyldioxirane in inducing the formation of carboxylic acid and eliminating condensed phenolic structures in solvent extracted residual kraft lignin.

The most critical bleaching stage of TCF bleaching process of dissolving pulps is the ozone treatment which is necessary for removal of residual lignin. As ozone delignification is accompanied by serious cellulose damage, the ozone charge in the bleaching stage should be as low as possible. (Kordsachia O., S. Roßkopf, R. Patt. 2004. Production of spruce dissolving pulp with the pre-hydrolysis alkaline sulfite process (PH-ASA). Lenzinger Berichte, 83 (2004)