It is known that the brightness gain of a mechanical pulp subjected to hydrogen peroxide bleaching increases with both the amount of peroxide applied to the pulp and the amount of peroxide consumed by the pulp. In an effort to obtain mechanical pulps of high brightness, the utilization of greater concentrations of hydrogen peroxide is being widely investigated. Most commonly, bleaching of mechanical pulps is carried out in a single stage, but as more and more peroxide is applied in a single stage, the amount of unused peroxide that does not contribute to the bleaching increases, and this results in an increase in the wasteful non-bleaching reactions of the peroxide, and/or a high peroxide residual remaining at the end of the bleach.
Two stage bleaching processes were developed in order to more effectively utilize hydrogen peroxide for bleaching to obtain higher brightnesses, i.e. to optimize the consumption of peroxide by the pulp for the purpose of bleaching. Processes for one stage, two stage and three stage bleaching have been compared in an article by C. Gagne, M. C. Barbe and C. Daneault, Tappi, November 1988, p. 89. In the two stage bleaching process with peroxide described therein, and which process is typical of two stage bleaching with peroxide, there are two bleaching towers connected in series. Displacement-washing screw presses are included for thickening the pulp either prior to the inlet of the first bleaching tower or between the first and second towers. The residual peroxide from the second tower is reused in the first tower, and the whole white water system is countercurrent for washing or pretreating the pulp. This process is reported to allow for better use of peroxide and to provide brightness gains at lower cost. In such conventional types of multi-stage peroxide bleaching configurations, there is a sequential treatment of the entire feedstock pulp, and the residual liquor containing peroxide from the final peroxide bleaching tower is separated from the bleached pulp by pressing.
In these processes, the quality of the resultant bleached pulp is governed by the sequential treatment that the entire batch of pulp receives, and the process is thus limited to the production of a single quality of product at a time. The residual liquor containing peroxide is separated by pressing, and there are practical limitations to the volume of liquor that can be reasonably separated from a pulp by pressing means. As the charge of peroxide utilized in the final bleaching stage increases to obtain a higher brightness pulp, which as a consequence thereof the amount of peroxide in the residual liquor increases, then a proportionately greater absolute amount of peroxide will remain on the bleached pulp after pressing, and will be lost. Any attempt at washing the bleached pulp to more completely remove and collect the peroxide would afford a filtrate liquor containing peroxide that is of too low a concentration to be effectively utilized in a primary bleaching stage.
U.S. Pat. No. 4,915,785 in the name of C-I-L Inc., issued Apr. 10, 1990, discloses a rapid single stage process for the bleaching of mechanical pulp to enhanced brightness levels with hydrogen peroxide. In the process disclosed therein, substantially greater charge of hydrogen peroxide and accompanying additives are utilized in a single bleaching stage to provide pulps of enhanced brightness in a short period of time. This process provides a residual liquor that contains a substantial amount of peroxide. In a preferred embodiment of the invention described therein the residual liquor is recycled to a pulping or bleaching process.