Sequential bleaching of de-inked paper pulp with sodium bisulfite, dithionite and borohydride
First Claim
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1. A multi stage process for bleaching de-inked paper pulp which comprises:
- treating the pulp sequentially,(1) by adding and mixing sodium bisulfite to said pulp to cause bleaching of the said pulp in a first stage,(2) bleaching said pulp in a second stage with sodium dithionite by adding sodium borohydride which reacts with unreacted residual sodium borohydride from the first stage to form sodium dithionite generated in situ, and(3) bleaching said pulp in a third stage with unreacted sodium borohydride added in said second stage, wherein said borohydride is added in stage (2) after a time interval of at least 2 minutes from the termination of the mixing of the sodium bisulfite addition of stage (1).
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Abstract
Paper pulp, especially recycled paper pulp which has been deinked, is bleached in a synergistic two-stage process first with sodium bisulfite and then with sodium borohydride. The borohydride is preferably added in an aqueous mixture with sodium hydroxide. A weak acid such as acetic acid may be added to the pulp to advance further the brightness gain.
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13 Claims
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1. A multi stage process for bleaching de-inked paper pulp which comprises:
treating the pulp sequentially, (1) by adding and mixing sodium bisulfite to said pulp to cause bleaching of the said pulp in a first stage, (2) bleaching said pulp in a second stage with sodium dithionite by adding sodium borohydride which reacts with unreacted residual sodium borohydride from the first stage to form sodium dithionite generated in situ, and (3) bleaching said pulp in a third stage with unreacted sodium borohydride added in said second stage, wherein said borohydride is added in stage (2) after a time interval of at least 2 minutes from the termination of the mixing of the sodium bisulfite addition of stage (1). - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
Specification