The invention relates to a method and apparatus for distribution and mixing of high concentration or consistency (5% or higher) cellulose pulp with a treatment fluid, such as chlorine or chlorine dioxide.
The object of the invention is to make such distribution and mixing as effective as possible, so that the treatment fluid(s) is distributed as evenly as possible in the pulp suspension when introduced thereto, so that mixing of the pulp suspension and treatment fluid is effected, so that even a relatively small quantity of a treatment fluid is distributed evenly in and around all particles or fibers of the pulp suspension.
The effectiveness of such distribution and mixing depends on many factors, such as the pulp concentration in relation to the quantity of liquid or gas which is to be added, the solubility of the added liquid or gas in the suspension liquid, and to the reaction speed of the added treatment fluid with the particles of the pulp suspension. Generally, it can be said that the higher the concentration of solids or fibers in the pulp suspension, the more difficult it is to mix in treatment fluids so that they are evenly distributed in the suspension. Generally, it can also be said that the faster the added fluids react with the pulp, the more important it is that the fluids are distributed and mixed in as quickly and as evenly as possible. Since chlorine reacts quickly with pulp, and since it is desirable to treat high solids concentration pulp during bleaching, it is especially important to quickly mix chlorine with pulp. Since chlorine has an especially quick initial reaction with pulp and since it is undesirable to dilute the pulp with additional quantity of liquid, chlorine is most often added as gas dispersed in a relatively small quantity of liquid which, however, in turn means that problems can easily arise in the distribution and mixing of such a relatively small quantity. An object with the invention is therefore to solve this problem and also to solve the problems which arise when the pulp suspension has relatively high consistency of fibers, preferably above 5%, e.g., about 8-20% or about 10%.
In the pulp industry bleaching of pulp with chlorine liquid has hitherto preferably been done at 3-4% concentration mainly due to mechanical difficulties with mixing in and distribution [gas phase chlorination may be done with a pulp concentration in the range of 20-50%]. Since in other treatment stages of industrial bleach plants the pulp concentration normally is kept around 10%, it is desirable also to be able to effect the treatment of pulp with chlorine at this same concentration so that one can use uniform equipment in the bleach plant. This has special importance for the washing apparatus which is used between the treatment stages. Since the treatment with chlorine most often takes place in the beginning of the bleach plant and the pulp therefore must be thickened to about 10% concentration before the pulp goes on to the next treatment state, simplification and bulk reduction of equipment can be obtained if this first chlorine treatment also can take place at about the same high concentration.
The present invention allows chlorine treatment with proper mixing of high concentration pulp, as will become clear from an inspection of the detailed description of the invention and the appended claims.