FI86895C - Method for countercurrent washing and bleaching of cellulose boiling pulp - Google Patents

Description

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5,86895

The present invention relates to a process for countercurrent washing and bleaching of pulp from pulp cooking, and in particular to a process in which the pulp in a washing line is washed countercurrently with hot water. is passed to evaporation and chemical recovery and in which the washed pulp is then washed with still warm water before it is fed to a bleaching line, the bleaching line having several successive bleaching steps and in which the bleached pulp is washed after each bleaching step.

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Bleaching of chemical pulp is generally performed using oxidizing and temporal chemicals. The most common chemicals today are chlorine, chlorine dioxide, oxygen and sodium hydroxide. In the bleaching process, the wood 20 is dissolved together with the bleaching chemical in the bleaching process water. These waters are usually discharged into water bodies after wastewater treatment. The process produces harmful compounds under the influence of bleaching chemicals, which are not completely removed in wastewater treatment. The dissolved wood 25 also contributes poorly to degradable in conventional biological purification processes. In nature, waste materials also decompose and leave very slowly.

The organic matter contained in the bleaching waters would be usable as fuel, for example, if it could be enriched and concentrated. The sodium base in wastewater would also be useful. However, the effluents are very dilute and it does not make sense to evaporate them.

35 One useful method is to direct the bleaching water in the process upstream in the pre-bleaching pulp washing process. This known method has been used in some factories and a considerable amount of 2,86895 studies have been done. There are several different disadvantages associated with the method. One of them is the disadvantage of the water balance of the process.

The bleaching of the pulp is carried out in several different steps, between which the pulp is washed either with pure water or with the filtrates of the other bleaching step. So-called washing filters or washing diffusers have been used as the washing device, from which the dry matter content of the incoming pulp is about 10-15%. Similar equipment as well as so-called washing presses have been used in pre-bleaching pulp washing. Washing presses, on the other hand, due to their high cost, have not been used as bleach cleaners, as there has been no need for them.

It is therefore an object of the present invention to provide a method of the type mentioned in the introduction for countercurrent washing and bleaching of pulp from pulp cooking, in which the amounts of waste water and the water balance of the pulp mill are made suitable for bleaching water recovery.

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The main features of the invention appear from the appended claim 1.

The object of the present invention is thus achieved by removing considerably more, about twice as much filtrate from the washed pulp after the first bleaching step and the second bleaching step as before, so that when the pulp has a dry matter content of at least about 30%, the excess filtrate is small and at least partially the filtrate thus obtained is derived into the first wash water of the last stage of the countercurrent wash, which is displaced by hot water before the post-wash.

In a preferred embodiment of the invention, the filtrate from the bleaching line is passed to the oxygen stage displacement wash as the last step 35 of the countercurrent wash as its first wash water. Alternatively, the filtrate from the bleaching line can be passed to the final stage of the countercurrent washing of the brown mass and displaced by the hot water mentioned therein after post-washing.

In a particularly preferred embodiment of the invention, the filtrate of the first 5 and / or second bleaching stages is fed entirely to the countercurrent washing line and in the post-wash so much wastewater is removed from the pulp that the dry matter content of the pulp fed to the bleaching line is at least about 30%.

10 In order to achieve the required high dry matter content in the washing after the first and second bleaching stages and possibly in the washing of the washing line, the pulp washing solution is pressed by pre-bleaching the functionality of.

In the process according to the invention, the first 20 bleaching steps preferably use a so-called chlorine step, in which the bleaching is carried out with chlorine, chlorine dioxide or a mixture thereof, and the second bleaching step uses a so-called alkali step.

The invention will be described in more detail below with reference to the accompanying drawings, in which Figure 1 shows a process diagram of the connection of a modern bleached cellulose mill fiber line and Figures 2 and 3 show two embodiments of the method according to the invention.

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In the accompanying drawings, the soup is generally designated by reference numeral 1 and the subsequent brown pulp wash by reference numeral 2. The sorting and precipitation of the pulp from the brown pulp wash is generally denoted by reference numeral 3 and the subsequent oxygen step by reference numeral 4. The countercurrent washing with hot water after the oxygen step After countercurrent washing 2-5, a further washing 6 is carried out with warm water, after which 4,86895 the washed pulp is fed to a bleaching line 7-11, the first stage, chlorine stage of which is indicated by reference number 7 and its washing stage of reference number 8, second stage, alkali stage of reference number 9 and subsequent washing with reference number 5 10 and final bleaching with reference number 11 in general.

The mass flow is marked with a bold line while the liquid flows are marked with a thinner line. The filtrate from washing brown mass 2 is passed to evaporation and recovery of chemicals. Liquid flows are numerically expressed in cubic meters per tonne of mass. The mass flows are equal in all figures, so that the consistency of the pulp is 10% and the liquid content is 9 m3 per ton of pulp, except after press washes, where the consistency of the pulp is considerably higher and the liquid content considerably lower.

Figure 1 shows, for comparison, the connection of the fiber line of a modern bleached cellulose mill. In this process known per se, the pulp is washed with pure water before bleaching with chlorine chemicals.

From the wash after the oxygen step 4, the liquid stream is countercurrent to the pulp stream and the concentrated broth 25 is finally passed from the brown pulp wash 2 to evaporation and chemical recovery. After this series of washes 2-5, there is still a post-wash 6, which is carried out with warm water, but the filtrate obtained is discharged into waste water. The washed pulp is passed to bleaching 7-11 with at least 3 bleaching steps. The figure shows in more detail only the first two stages, the so-called chlorine step 7 and the so-called. alkaline step 9. These are essential to the invention. The final bleaching 11 is generally shown as a screen which may have many steps. After all steps, there is a pulp scrubber 8, 10, which typically produces 35 pulps at a consistency of about 10%. The excess washing filtrate from the chlorine 7 and alcohols stages 9 is discharged into the wastewater.

The scrubbers of these stages use as filtrate the filtrate from the scrubbers of the different stages of the final bleaching 11.

5,86895

According to the balance sheet, the total amount of wastewater is 34 m3 per ton of mass and in practice 30..40 m3 / ton of mass. Bleaching filtrates are not directed countercurrently in the process to wash the unbleached pulp.

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In the process according to the invention shown in Figure 2, the so-called the waters of the alkaline stage 9 are passed to the recovery of cooking chemicals. The washing press is used as scrubbers 8, 10 for the first two bleaching steps, chlorine 7 and alkali-10 steps 9. Before bleaching, filters or diffusers are used instead as washing devices. The last washing step is the so-called. post-wash 6 from which no filtrates are recovered. In the example case, the pulp is removed from the washing presses at a consistency of 35%, in which case it has a liquid of 1.86 m3 per pulp-15. In order to achieve a washing effect, 8 and 10 approx. 1 m3 of water per ton of pulp are fed to the washing press in excess of the amount in the leaving pulp. Also taking into account the water contained in the bleaching chemical, the amount of filtrate from the alkali stage 9 washing press 10 is about 3 m3 / ton of pulp.

20 This water is passed to the last scrubber 5 for the recovery of cooking chemicals. It uses this water as the first washing water. As the second washing water, pure water is used, the amount of which is larger and approximately equal to the amount of liquid in the pulp leaving this washing device. If the consistency of the pulp is 10%, the amount of liquid is 9 m3 / ton of pulp and the total amount of washing water is 12 m3 / ton of pulp. This provides conditions that are characteristic of good pulp washing.

30 The method in Figure 2 is estimated to utilize 60..70% of the most harmful part of the bleach effluent in the mill's processes, i.e. alkaline phase filtrate. This reduces the amount of substance dissolved in the wastewater accordingly. The total amount of wastewater is about 2/3 of the amount of wastewater in the modern plant-35.

In the embodiment according to Figure 3 and the first so-called the filtrates of the chlorine stage 7 and the second alkali stage 9 are passed to the washing chemicals recovery wash 2-5. In this case, the washing press is also used as the last washing step 6 before the bleaching plant 7-11. Also in this application, this washer 6, which precedes the bleaching plant 7-11, operates openly so that its filtrate is discharged from the process. Due to the higher amount of chemical water in the chlorine step 7, the amount of liquid to be fed from the bleaching plant 7-11 to the cooking chemical recovery wash 2-5 is about 2 m3 / ton of pulp in the case of Figure 2, but in this case also suitable for the process.

10 It is thus possible to transfer most of the waste generated in the first two stages 7, 9 of the bleaching process to the process of recovery of cooking chemicals. At the same time, the amount of wastewater is still reduced to less than half the amount of wastewater from a modern plant, ie only 15 m3 / ton of pulp.

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It will be apparent to one skilled in the art that the present invention may be varied and modified within the scope of the appended claims. Thus, for example, oxygen step 4 is by no means necessary but can be omitted if necessary.

Claims (5)

7 86895 A method for countercurrent washing and bleaching of pulp from cellulose broth (1), wherein the pulp in the washing line (2-5) is washed countercurrently with hot water, the filtrate 5 of which is finally passed to evaporation and chemical recovery and the washed pulp is then washed (6) with warm with water before it is passed to a bleaching line (7-11) having several successive bleaching steps and in which the bleached pulp is washed after each bleaching step, characterized in that so much filtrate is removed from the pulp washed after at least the first two bleaching steps (7, 9) that the pulp has a dry matter content of at least about 30% and at least a part of the filtrate thus obtained is passed to the first washing water of the last step of the countercurrent washing (5), which is displaced by said hot water before the post-washing (6). A method according to claim 1, characterized in that the filtrate from the bleaching line (7-11) is led to the displacement wash (5) of the oxygen step (4) as the last step of the countercurrent wash as its first wash water. Process according to Claim 1, characterized in that the filtrate from the bleaching line (7-11) is passed to the final stage of the countercurrent washing of the brown mass and is displaced ... by the hot water mentioned therein before post-washing. Method according to Claim 1, 2 or 3, characterized in that the filtrate of the first bleaching stage (7) and / or the second bleaching stage (9) is fed in its entirety to the countercurrent washing line (2-5) and so much wastewater is removed from the pulp in the post-wash (6): the pulp fed to the bleaching line (7-11) has a dry matter content of at least about 30%. Method according to one of the preceding claims, characterized in that in the washing (8, 10) after the first (7) and second bleaching steps (9) and optionally in the washing (6) of the washing line, the pulp is pressed to achieve the required high dry matter content. 8 86895