Method for bleaching wood pulp including dissolving oxygen into the dilution water of an extraction stage
AssigneeThe BOC Group, Inc.
InventorRobert J. Stawicki
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Abstract
Wood pulp is bleached in a process which includes passing pulp slurry from a chlorine tower through a washer to a downflow extraction tower which is open to the ambient atmosphere. Caustic and steam are supplied to the extraction tower as is dilution water which is introduced into the lower portion thereof. Oxygen is dissolved in the dilution water which may be at least partially comprised of recycled washer filtrate. This addition of oxygen to the dilution zone of an extraction stage enables the consumption of chemicals such as hypochlorite and/or chlorine dioxide to be reduced while still meeting required levels of pulp brightness and strength. Oxygen may be added to the extraction or other alkaline stages of a bleaching process.
Claims9
A method for bleaching wood pulp wherein said pulp is passed as a slurry downwardly through an extraction stage in the form of a tower the top of which is exposed to ambient conditions and which has a dilution zone in the lower portion thereof comprising the steps of adding alkali to said stage;
- maintaining the temperature of pulp in said stage between approximately 40.degree.-80.degree. C., introducing dilution water into said dilution zone and introducing oxygen into said dilution water to thereby at least partially dissolve said oxygen in said water with the oxygenated dilution water being introduced into said stage generally cocurrently with the flow of said pulp therethrough, and retaining said pulp slurry in said dilution zone for an average residence time of less than approximately 5.0 minutes prior to removal from said tower, wherein the oxygen is introduced in an amount effective to increase the viscosity of the bleached pulp.
Dependencies are inferred from the claim text.