FI69656B - FOERFARANDE FOER FOERBAETTRAD TVAETTNING AV UR LIGNOCELLULOSAMATERIAL FRAMSTAELLDA CELLULOSAMASSOR - Google Patents

Abstract

To provide a cleaner pulp and to obtain a higher chemical recovery during washing of chemically cooked and optionally defibrated cellulose pulp, the partially washed pulp is subjected to a mild mechanical treatment e.g. in a device (10) adapted for high-consistency treatment and provided with screwsrotating relative to each other, said treatment taking place between at least one pair of conventional washing stages (4,14).

Description

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This invention relates to a process for washing cellulosic pulps made from lignocellulosic material in a more efficient manner than before for better recovery of chemicals and better recovery of combustible organic material from waste liquors. Cellulose pulps refer primarily to chemical, semi-chemical and chem-10 mimechanical pulps made from both softwood and hardwood chips.

In the lignin removal of a lignocellulosic material by chemicals, a residue is obtained which consists of inorganic chemicals and dissolved organic matter, which later consists mainly of lignin and smaller amounts of cellulose, as well as extractants such as resin. Unwanted residual materials are washed away. The resulting waste liquor and wash liquor are usually evaporated and incinerated so that the chemicals can be recovered. In order to facilitate recovery, the highest possible dry matter content in the washing liquid is always sought, i.e. an attempt is made to use as little water as possible in the washing.

When producing cellulosic pulps with a wood yield of more than about 55%, the wood chips are usually mechanically defibered into single fibers before washing and / or screening the pulp. The screening yields a concentrate of non-fibrous wood residues, the so-called wreck, which is usually defibered in a separate step into individual fibers, which are then recombined with the mass flow.

The washing of the pulp suspension obtained after lignin removal can be carried out in various known ways. The oldest method is washing in so-called diffusers, i.e. batch displacement washing in large tanks. Today, the usual method is washing with a drum filter, usually 35 in several steps. The number of drum filters used may vary, but usually rises to 3-4 2 69656 pieces. Another newer pulp washing method utilizes presses to remove larger amounts of liquid than filters, resulting in more efficient recovery of chemicals and organic substances.

However, the washing technique described above leaves considerable amounts of residual chemicals and undesirable organic matter in the pulp, leading to an unnecessarily high use of bleaching chemicals if the pulp is subsequently bleached, as well as causing undesirably high emissions of pollutants into the environment. From both an economic and an environmental point of view, it is still desirable to increase the recovery rate of chemicals used in cellulose processes.

The present invention solves the above-mentioned problems and makes it possible to increase the degree of chemical recovery.

Accordingly, the invention relates to a process for the improved washing of unbleached cellulose pulp made of a lignocellulosic material, in which the starting material is ligninized by chemical decomposition (cooking) and, after possible mechanical defibering, washed in a known manner in several steps with water. The process is characterized in that the pulp, after being partially washed, is subjected to a mild mechanical treatment between two or more washing steps.

In the application of the method according to the invention, the cleaning effect has proved to be particularly good if, in the partial washing preceding the mild mechanical treatment, at least 10% of the impurities originally present in the pulp suspension are removed. In other words, at least 10% of the dry matter content of the waste liquor must be washed off before the pulp suspension is subjected to a slight mechanical treatment.

It has further proved particularly suitable to increase the pulp concentration 35 to between 10 and 50%, preferably between 14 to 40% and suitably between 11 3 69656 and 20 to 35% before mild mechanical treatment. After mild mechanical treatment, the pulp is diluted to a concentration of between 0.5 and 13% with mixing of the washing liquid in the subsequent washing step. If the washing is carried out with a filter, the most suitable pulp concentration is between 0.5 and 6%, while the washing-dewatering, for example in a press, is suitably carried out with a pulp outlet concentration between 2 and 13 S.

The mild mechanical treatment according to the invention can be carried out at virtually all temperatures.

1C However, a temperature range of 20 to 110 ° C and preferably a range of 35 to 90 ° C is particularly suitable.

By mild mechanical treatment in this case is meant a repeated pressure, whipping and shear treatment of the pulp suspension at the above-mentioned pulp concentrations. Wherein the electrical energy input for carrying out this processing is kept in the range of 7 to 200 kWh per ton of dry weight and preferably in the range of 10 to 100 kWh per ton of dry weight.

It has proved particularly suitable to carry out a mild mechanical treatment according to the invention in a screw fiber dryer of the type comprising two rotating blades with blades arranged parallel to each other in a housing with an inlet and outlet and in adjustable contact with each other in the mild mech described above. machining achievements. The screw blades of the rotating screws also suitably have bends on the outer circumference of at least some of the screw threads to form teeth between said bends. The screw fiberizers described above are marketed by MoDoMekan Ab under the name FROTAPULPEJ ^.

The new method of mechanically treating the pulp slurry mechanically in a two or more stage wash series has quite surprisingly been shown to give a cleaner pulp and lead to a higher recovery rate, lower impurity emissions and a higher dry matter content than in the wash water. when applying conventional washing techniques.

Thus, in the application of the invention, a pulp with better purity can be obtained, which is a great advantage. This 5 is identified e.g. a substantial reduction in the extract content of the pulp and its number of pieces.

Advantages also include the possibility to increase the dry matter content, i.e. the content of organic material as well as the content of inorganic chemicals in the liquid leaving the pulp wash. This increases the calorific value of the washing liquid, which provides valuable energy savings. To the extent that more pollutants go to evaporation and incineration and more chemicals are recovered, the amount of pollutants released into the environment is reduced, which is an essential benefit and worth pursuing from an environmental point of view. The costs of handling and degrading environmentally harmful substances released into the environment can thus be reduced by applying the invention.

A further advantageous and surprising effect achieved by the application of the invention is that the consumption of bleaching chemicals such as chlorine, chlorine dioxide, hypochlorite, hydrogen peroxide and oxygen gas is significantly reduced in the possible bleaching of the pre-washed pulp.

Another surprising effect that has occurred in connection with the introduction of the mild mechanical treatment according to the invention is that the dewatering of the obtained pulp suspension is significantly improved - a fact which provides obvious advantages in the further processing of the pulp.

The advantages of the procedure according to the invention will become more apparent from the following embodiment described below.

The figure schematically shows a suitable embodiment of the process according to the invention used in the washing tests of birch sulphate pulp and chemimechanical spruce pulp in three stages, all of which are equipped with a press. Between the first and second washing stages is in these experiments

II

5 69656 installed screw fiber, October is of the type marketed under the trade name FROTAPULPER ".

Experiments according to a preferred embodiment of the described procedure have been performed and can be found in the 5 implementation examples below together with the results obtained therein.

In the parallel experiments described below, birch sulphate pulp and chemimechanical spruce pulp were washed in three steps partly according to the procedure of the invention using a mild mechanical treatment between the first and second washing steps (Examples 1 and 2), partly according to conventional technique (comparative experiments 1 and 2) without any mechanical treatment of pulp suspension between the different stages of the washing series.

The process diagram shown in the figure was used in all 15 experiments, which is why the numerical markings below refer to the corresponding markings in the figure.

Example 1

The unbleached and unwashed birch sulfate pulp was passed through line 1 to tank 2, where the pulp suspension-20 was diluted with press waste liquor fed through line 22 until a pulp concentration of about 8% was obtained.

From the tank 2, the pulp suspension was passed via line 3 to a first washing step 4 consisting of a press from which the pulp suspension treated to a pulp concentration of 33.3% was passed through line 5 to a screw conveyor which fed the pulp suspension further via line 7 and line 8 to the screw fiber. 10, which was provided with two rotating screws with screw blades with bends in which the pulp suspension was subjected to a slight mechanical treatment, after which it was passed through a line 11 to another screw conveyor 12. The pulp suspension temperature was measured at 68 ° C and 68 ° C. The electrical energy consumption during mild mechanical treatment in the screw fiberizer 35 10 was measured to be 16 kWh per ton of dry mass. In this case, line 9 was closed.

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In the screw conveyor 13, the treated pulp suspension was diluted to 8% pulp concentration with the press waste liquor fed through line 21, which was effectively mixed with the pulp suspension as it passed through the screw conveyor 12.

The diluted pulp suspension was then passed via line 12 to a second washing step 14 consisting of a press similar to that used in the first washing step. In press 14, the pulp suspension was washed and re-thickened to a pulp concentration of 3.3%. The compression waste liquor from the press-10 was passed through line 22 to tank 2, where it was used to dilute the incoming pulp suspension. In the second washing step, the washed and thickened pulp suspension was then passed through line 15, screw conveyor 16 and line 17 to a third washing step 15 equipped with a similar press 18 as in both previous washing steps. The pulp suspension was diluted and mixed in a screw conveyor 16 in the same manner as in the previous screw conveyor 12, but with pure water fed through line 20 to a pulp concentration of 8%. After washing and thickening to a pulp concentration of 33.3% in the press 18, the pre-washed pulp suspension was taken out via line 19.

During the experiment, compression waste liquors were taken from lines 21 and 22. In addition, 25 samples of pre-washed pulp were taken from line 19. The analysis results are summarized in Table 1. Compression waste liquor from the first washing step in press 4 was taken via line 23 and passed to a recovery plant (not shown). . This compression contained waste liquor, because the washing liquid had been passed countercurrently and through the mechanical treatment of the pulp suspension, contained a high content of organic matter (about 13%) and at the same time had a high calorific value.

Comparative Experiment 1 35 In this experiment, a birch sulphate pulp of 7 69656 was taken out at the same point and the same equipment was used; as in Example 1, except that the pulp suspension was not subjected to any mild mechanical treatment between the first and second washing steps. Thus, all the flow of the pulp suspension through the line 8 was cut off, so that the pulp suspension was instead passed directly via the line 9 to the second screw conveyor 12.

After washing and thickening the pulp suspension in the press 14 of the second washing step, a pulp concentration of 26 10 was obtained in this case. After washing and thickening, a pulp concentration of 26% was similarly obtained in the press 18 of the third washing step. It should be noted here that the mass concentration of the pulp suspension as it entered the presses 14 and 18 was also 8% in this experiment and that the set point 11 of the presses was the same as in the experiment of Example 1.

Analytical samples were taken in this experiment from the same sites as in the experiment of Example 1. The analysis results are summarized in Table 1 below.

Table 1 20 Dry matter content of the pulp suspension Comparative Example (mk): _______ experiment 1____1______ after press of 2 washing steps,% 26.0 33.3 3 washing steps. after press,% 26.0 33.3 li Amount of press waste liquor obtained: 3 in 2 washing presses, m / 'ton 8.65 9.50 in 3 washing presses, m-3 / ton 8.65 9.50

Dry matter content of press waste liquor: 30 out of 2 washing presses,% 3.50 3.70 out of 3 washing presses,% 1.62 1.89

Sodium content of press waste liquor: from 2 washing presses, g / l Na 6.55 7.02 35 From 3 washing presses, g / l Na 2.83 3.38 8 69656

Comparative Example ____test 1_1 _

Total washed out of the pulp suspension in 2 and 3 washing steps: dry matter, kg / ton dry 450.0 532.0 5 intended pulp

Na, kg / ton dry mass 80.9 98.5

Properties of the washed mass:

Extract content (SCAN-C7: 62 to mold), DCM% 0.34 0.18 10 Viscosity (according to SCAN-Cl 5:62), crn ^ / g 972 969

Number (according to SCAN-Cl: 59) 17.5 14.7

As can be seen from the tables, in the application of the process according to the invention, quite surprisingly, a significantly better pulp suspension has been obtained than in the washing according to the prior art. Likewise, for example - despite the fact that the mild mechanical treatment has been performed only on the partially washed pulp suspension - the process according to the invention has surprisingly resulted in a pulp with a significantly lower extract content than can be obtained in conventional washing without mechanical treatment between washing steps.

An important energy advantage in the increased purification of organic matter is that the calorific value of the waste liquor increases. Further, a higher recovery rate for cooking chemicals is obtained.

Example 2

The experiment described in Example 1 was repeated with a chemimechanical spruce pulp cooked to a pulp yield of 73% based on absolute dry wood. After washing and thickening the pulp suspension in the press 14 of the second washing stage, a pulp concentration of 34% was obtained in this experiment. After washing and thickening, a pulp concentration of 34% was similarly obtained in the press 18 of the third washing step. In this experiment, 35 had lead 9 closed.

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Comparative Experiment 2 In this experiment, the same type of chemimechanical spruce pulp was washed and the same equipment as in Example 2 was used, except that the pulp suspension 5 was not subjected to any mild mechanical treatment between any of the washing steps. Thus, all flow of the pulp suspension through line 8 was shut off so that the pulp suspension was instead passed through line 9 directly to the second screw conveyor 12. After the press 14 of the second washing step, a pulp concentration of 30% was measured in this experiment. The same pulp concentration was measured after the press 18 of the third washing step.

A comparison between the prior art wash and the present invention wash shows that the process of the invention results in better dewatering and better chemical recovery. This conclusion is based on the fact that the volume of recovered waste liquor in the application of the invention became higher despite the fact that the mass concentrations going to the presses were kept unchanged at 7% in both experiments. According to the measurements, the washing waters in Example 2 still contained slightly higher amounts of dry matter than the washing waters in Comparative Experiment 2. Thus, it can be stated that the calorific value of the Stunning obtained by the process according to the invention was also higher than with corresponding lyes in chemimechanical pulp washing according to conventional technique. .

Claims (5)

10 69656 An improved washing method for bleaching unbleached cellulosic pulp from lignocellulosic material, wherein the starting material is ligninized by chemical cooking and, after possible mechanical defibering, washed in a known manner in several steps with water, characterized in that the pulp, after partial subjected to 10 mild mechanical treatments between two or more washing steps. Process according to Claim 1, characterized in that at least 10% of the dry matter content of the cooking liquor is removed in a partial wash which precedes a mild mechanical treatment. Process according to Claims 1 and 2, characterized in that the mild mechanical treatment is carried out at a mass concentration of between 10 and 50%, preferably between 14 and 40% and suitably between 20 and 35%. Method according to Claims 1 to 3, characterized in that the supply of electrical energy in the mild mechanical treatment is kept between 7 and 200 kWh / ton and preferably between 10 and 100 kWh / ton. Method according to Claims 1 to 4, characterized in that the mild mechanical treatment is carried out in a screw fiberizer of the type comprising two rotating screws with blades arranged parallel to one another in a housing provided with an inlet and outlet and in contact with each other. for slightly shaping the material and having screw holes with bends on the outer circumference of at least some screw threads to form teeth between said bends.