In the pulp and paper industry, there are basically two fundamentally different processing methods for converting lignocellulosic material, being wood or nonwood, into pulp used in papermaking. One processing method is chemical pulping, which uses chemicals such sodium hydroxide, sodium sulfide, sodium sulfite or different solvents, to break down bonding between each individual fiber. The other processing method is mechanical pulping, which uses mainly mechanical means such as a pair of rotating discs commonly referred to as a refiner, or a rotating grinding stone, to separate the lignocellulosic fibers from one another. The process of using mainly mechanical means for separating lignocellulosic fibers from one another is commonly called defiberization. In some mechanical pulping processes chemicals are used before, during, and/or after the mechanical defiberization in order to modify the pulp properties and/or reduce energy consumption. Applying chemicals before and/or during refiner mechanical defiberization, is commonly referred to as Chemical Mechanical Pulping (CMP) process.
In CMP, there are three fundamentally different concepts used to produce pulp. The first of these CMP concepts is to treat the material, normally in the form of chips, with chemicals and complete the treatment, or most of it, before the refiner defiberization step. The treatment may be a high temperature cooking for an extended period of time, as in a conventional CMP process, or high temperature for a relatively short period of time, as in Chemi-Thermal Mechanical Pulping (CTMP), or relatively low temperature and long period of time as in Cold Caustic Soda (CCS), Alkali-Peroxide Mechanical Pulping (APMP), Alkali-Peroxide Pulping (APP) processes. Exemplary APMP processes are disclosed in U.S. Patent Application Publication 2004/0200586 and WO 05/042830 (the entire content of each being expressly incorporated hereinto by reference). Some of these CMP processes also use a compression device to squeeze the lignocellulosic material before the chemical application to improve the desired chemical treatment effect.
The second CMP concept is to add chemical to the chips during refiner defiberization, as proposed in U.S. Pat. Nos. 3,023,140; 3,069,309; 4,187,141; 4,311,553; 4,270,976; 5,129,987 (the entire content of each such patent being incorporated expressly hereinto by reference). This type of treatment uses the refiner not only to provide defiberization but also as a mixer for chemical distribution and reaction, although in some cases, a chip pretreatment is mentioned. The pretreatment step is only for stabilizing hydrogen peroxide (H2O2), as described in U.S. Pat. No. 4,311,553,or softening the chips as described in U.S. Pat. No. 3,069,309. In such a pretreatment step the principal chemicals, in most cases peroxide, have an effect on the development of pulp properties, such as brightness, are applied at the refiner.
The third CMP concept is to combine the chemical pretreatment (in this case referred as Preconditioning) before refiner defiberization, and chemical treatment during the refiner defiberization and fibrillization process and before the final refining step (referred to as Refiner Chemical treatment) and is referred to as P-RC, (Preconditioning, followed by Refiner Chemical treatment). In P-RC APMP pulping, two different chemical strategies had been introduced. The first is to add a significant amount of alkali peroxide chemicals immediately prior to the primary refining step, which is most suitable for atmospheric refining (e.g., as described in China Patent Number: CN ZL02814472.4,the entire content of which is incorporated fully hereinto by reference); and the second is to apply the main alkali peroxide chemicals immediately after the primary refiner, which is most suitable for pressurized refining (e.g., as described in United States Patent Application Publication US 2004/0069427 A1,the entire content of which is incorporated fully hereinto by reference).