TH10307A - Mechanical bleaching process for paper pulp. - Google Patents

Abstract

It claims a two-step process for mechanical pulp bleaching, with the first step being a reduction process followed by a second, peroxygen-based step.

Claims (9)

1. The process for mechanical bleaching of pulp in aqueous slurry consists of (a) mechanical contact of the aqueous pulp slurry with a selective reduction bleach from sodium hydrosulfite, zinc hydrosulfite, and sodium morohydride in the first step; (b) rinsing the pulp slurry from the first step to remove any remaining reduction bleach; (c) viscosity adjustment of the slurry to a minimum thickness of 10%; and (d) adding sufficient peroxide bleaching solution to the slurry from step (d) to achieve a hydrogen toxide concentration of at least 1%, using the dry weight of the pulp in the slurry as a reference, and maintaining the slurry temperature at a high temperature for a sufficient period to increase the brightness of the pulp in the slurry. 2. The process of claim 1 is characterized by the use of a reducing agent, a 0.5% to 1% aqueous hydrosulfite solution, in the form of sodium hydrosulfite at pH 5 and 6. 3. The process of claim 1 is characterized by the presence of a reducing agent, sodium monohydride, at a pH of 10 to 12. 4. The process of claim 1 is characterized by the presence of a reducing agent, aqueous hydrosulfite solution of 0.5 to 1% zinc hydrosulfite at pH 5 and 6. 5. The process of claim 1 is characterized by the presence of an oxygen-containing diluent solution consisting of an aqueous solution containing at least 1% hydrogen peroxide at a pH of 10 to 11, and 1% to 3% sodium silicate and 0.5% to 0.2% magnesium sulfate. 6. The process of claim 1 is characterized by the presence of an oxygen-containing diluent solution consisting of an aqueous solution containing at least 1% hydrogen peroxide at a pH of 10 to 11, and 1% to 3% sodium silicate and 0.5% to 0.2% magnesium sulfate. 7. The process of claim 3 is characterized by the presence of an oxygen-containing diluent solution consisting of an aqueous solution containing at least 1% hydrogen peroxide at a pH of 10 to 11, and 1% to 3% sodium silicate and magnesium sulfate. 0.5% to 2% 9. The process of claim 4 is characterized by the presence of an oxygen-containing diluent solution consisting of an aqueous solution containing at least 1% hydrogen peroxide at a pH of 10 to 11, and 1% to 3% sodium silicate and 0.5% to 2% magnesium sulfate. (Clause 9, page 2, Figure)