Method of Oxygen Delignifying Wood Pulp With Between Stage Washing

There is intense interest in the pulp and paper art for the reduction of the amount of chlorine used in bleaching pulp. Chlorine has been shown in many situations to generate poisonous compounds, such as dioxins, in the bleach plant effluent.

A number of steps have been taken to try and minimize the amount of chlorine utilized for bleaching. Some approaches utilize oxygen pre-treatment, and high chlorine dioxide use. Such procedures are not entirely effective, however, because there are limitations in the final brightness that can be achieved, and because chlorine dioxide is a much more expensive bleaching chemical and significantly increases bleaching chemical and significantly increases bleaching costs. To overcome the final brightness limitations, peroxide is sometimes used. However peroxide is also very expensive. A typical old sequence, and a new sequence in which chlorine use is minimized, are C.sub.E EDED, and OC.sub.D E.sub.o DEDP, respectively, wherein O is oxygen treatment, D chlorine dioxide, C chlorine, P peroxide, and E caustic extraction.

Another way in which chlorine usage can be minimized is to use more oxygen--for example a stronger E.sub.O stage. Unfortunately this causes strength (viscosity) losses. However, according to the present invention it has been found that the viscosity loss caused by utilizing more oxygen can be overcome by providing a series of oxygen stages with washing between the stages. It has also been found that while chlorine can be used as the first stage to affect acid removal of metals, pre-treatment of the pulp with a chelating agent, such as DTPA, and/or adding another chelating agent, such as EDTA, and the oxygen reactor, also allows one to achieve better bleaching (a lower Kappa number) without undue loss of viscosity or yield. The lower pH's caused by the oxygen stage combined with the chelating removes metals, which allows operation of the process to lower Kappa numbers.

According to the present invention it is possible to minimize or eliminate chlorine usage in bleaching by utilizing a two (or more) stage oxygen treatment process with washing between the stages and with the first stage operated to control pH between the stages, and with the chelating agent utilized for pre-treatment and/or added to the counter-flow of wash liquid in the wash between the oxygen stages. The gains in brightness, without subsequent viscosity loss, by utilizing the between stage washing are dramatic, being substantially equal to such gains as can be obtained utilizing pre-treatment with DTPA.

According to one aspect of the present invention there is provided a method of bleaching paper pulp comprising the steps of: (a) Effecting oxygen bleaching of the pulp in at least two consecutive stages; and (b) Effecting washing of the pulp between each of said at least two consecutive stages to maximize viscosity for a given degree of bleaching. It is also desirable to practice the step (c), before step (a), of pre-treating the pulp with a chelating agent such as DTPA, and to provide the further step (d), during the practice of step (b), of simultaneously treating the pulp with a chelating agent such as EDTA. For example the EDTA may be added to a countercurrent flow of wash liquid to the pulp in the between stage washing. Exactly two oxygen stages may be utilized, or a number of different oxygen stages. Step (a) is practiced to control the pH between stages so that it is at an acidic level conducive to effective EDTA chelating.

According to another aspect of the present invention a high viscosity bleached paper pulp is produced by practicing the steps of: (a) Effecting oxygen bleaching of the pulp in at least two consecutive stages; and (b) Effecting washing of the pulp between each of said at least two consecutive stages to maximize viscosity for a given degree of bleaching.

The invention also contemplates a method of delignifying a suspension of comminuted cellulosic fibrous material (pulp) at a consistency of about 6-15% comprising the steps of sequentially and continuously: (a) Subjecting the suspensions, while at a consistency of about 6-15%, to a first oxygen delignification treatment. (b) Washing the suspension, while at a consistency of about 6-15%. And, (c) subjecting the suspension, while at a consistency of about 6-15%, to a second oxygen delignification treatment. Step (b) is practiced by a countercurrent flow of wash liquid, and during the practice of step (b) a chelating agent is preferably added to the wash liquid. Prior to step (a) there preferably is also a step of pre-treating the suspension with a chelating agent.

It is the primary object of the present invention to provide a method for bleaching paper pulp or the like in which the amount of chlorine usage is minimized, or chlorine is eliminated entirely, while the viscosity of the paper pulp is maximized and an adequate degree of bleaching (low Kappa number) is achieved.

FIG. 1 is a schematic illustration of the exemplary method steps that may be utilized in the practice of the present invention; and

FIG. 2 is a graphical approximation of results achievable in the practice of the present invention compared to other procedures, showing the viscosity of the pulp at various Kappa numbers.

According to the preferred method of the present invention, comminuted cellulosic fibrous material, e.g. paper pulp, from a source 10 (such as a digester or storage vessel) is subjected to DTPA pre-treatment in a vessel 12. DTPA, or like chelating agent, is added to pulp that has a fairly low pH. For example in the DTPA pre-treatment stage 12 the pulp can be held at about 20.degree.-23.degree. C. at a pH of about 7 for 30 minutes. This results in a considerable removal of metals which allows a greater degree of bleaching without viscosity loss. For example compared to feed stock not treated with the chelating agent, the viscosity is the same at two Kappa points lower.

While the consistency of the pulp may vary widely, it is desirable to perform the steps while the consistency of the pulp is maintained between about 6-15%, i.e. medium consistency. The medium consistency pulp is fed from the pre-treatment stage 12 to a conventional oxygen stage (oxygen reactor) 14 where oxygen bleaching takes place. The temperature and pressure conditions in the oxygen stage 14 are conventional (e.g. about 90.degree. to 100.degree. C. at substantially atmospheric pressure), and caustic, e.g. NaOH, is added to the pulp. Other pressure and temperature conditions can be utilized, but it is desirable to maintain the pressure and temperature conditions as close as feasible to conventional systems.

After bleaching in the first oxygen stage 14, the pulp is passed to a countercurrent wash stage 16 (or the washing can take place at one end of the oxygen reactor 14). While a countercurrent wash flow is preferred, a wide variety of different washes may be utilized in order to effectively remove metals or the like. In FIG. 1 the countercurrent wash liquid is shown introduced at 18 with the spent wash liquor removed at 20. Also for maximum results it is desirable to use another chelating agent, such as EDTA, in the wash liquid, as by adding it to the countercurrent flow 18 as illustrated in FIG. 1.

After the wash stage 16 the pulp, still of medium consistency, is passed to a second oxygen stage 22, substantially identical to the first stage 14. While under many circumstances exactly two bleaching stages 14, 22 will achieve the desired results, any number of bleaching stages, as indicated schematically at 24 in FIG. 1, may be provided, as long as a wash is provided between each. For example a second wash stage 26 with countercurrent wash liquor introduction at 28 and removal at 30, and third oxygen bleaching stage 32, may be utilized. After the last oxygen bleaching stage, the pulp may be washed, passed to storage, or otherwise treated depending upon the desired end use.

A graphical representation of the results that are achievable according to the invention is illustrated in FIG. 2. In FIG. 2 the scan viscosity has been plotted against Kappa number (i.e. strength vs. degree of bleaching). Line 40 is a rough approximation of the results achieved when there is no chelating agent used and no between stage washing during oxygen treatment. Line 42 is a rough approximation of when there is treatment with the chelating agent but no between stage washing. Line 44, which is roughly equivalent to line 42, illustrates the results when there is no chelating agent treatment but between stage washing. Line 46 illustrates the results when there is a pre-treatment with a chelating agent and between stage washing, and line 48 illustrates the results when there is pre-treatment with a chelating agent, treatment in each of the oxygen stages with chelating agent, and between stage washing (the optimum results). As FIG. 2 clearly illustrates, between stage washing very significantly increases pulp viscosity especially at lower Kappa numbers (higher degrees of bleaching).

The following tables illustrates the results achievable by practicing the invention compared to other procedures. Table 1 is an index of the different samples run in the testing set forth in Tables 2 through 8. Note that there are 13 samples.

Tables 2 shows the parameters at the various stages for each of the first 11 samples. Table 3 shows the parameters at each of the stages and the results achieved for sample 13. Table 4 illustrates the parameters between stages and the results achieved for sample 12; sample 12 is a test merely to determine whether or not there is any impact from the cooling between mixing stages of multi-stage trials. This test was run utilizing a conventional mixer, namely one sold under the trademark "MC.RTM." by Kamyr, Inc. of Glens Falls, N.Y. and Kamyr AB of Karlstad, Sweden. In this sample since the mixer was the reactor, oxygen and caustic are added without any cooling resulting. No significant difference in results was obtained in sample 12 compared to others, indicating that cooling between mixing stages does not play any significant role in the results achieved.

Tables 5 through 8 have self-explanatory titles. In Table 5 note that for the last sample the ph was adjusted by adding black liquor. All of the results indicate the improved results achieved according to the invention, utilizing inter-stage washing. The practice of the invention allows one to minimize the amount of chlorine added in a first chlorine stage prior to oxygen bleaching, or to eliminate chlorine bleaching all together. In all the tests, the consistency of the pulp was between about 6-15%, although the invention can be practiced utilizing pulp of other consistencies.

The between stage washing and multiple oxygen stage treatment before any chlorination reduces the pH sufficiently so as to make chelating agents effective.

While the following examples used EDTA in some circumstances and DTPA in others, it should be understood that either--or some other conventional chelating agent--may be used at any particular point in the process, the choice of agent depending upon the pH and/or other conditions at that point.

It will thus be seen that according to the present invention a method for bleaching paper pulp, and a high viscosity bleached paper pulp resulting from the method, are provided which allow minimization or elimination of chlorine during bleaching by using multiple oxygen bleaching stages with washing between stages. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadset interpretation of the appended claims so as to encompass all equivalent methods and products.