CA2033855A1

CA2033855A1 - Process for countercurrent washing and bleaching of a pulp from chemical pulping

Info

Publication number: CA2033855A1
Application number: CA002033855A
Authority: CA
Inventors: Risto Silander
Original assignee: Individual
Current assignee: Jaakko Poyry Oy
Priority date: 1990-01-11
Filing date: 1991-01-09
Publication date: 1991-07-12
Also published as: FI86895C; JPH04333684A; JP3001990B2; BR9100093A; FI900162A; FI900162A0; FI86895B; SE510558C2; AU6859891A; SE9100024D0; SE9100024L

Abstract

Abstract The invention relates to a process for countercurrent washing and bleaching of a pulp from chemical pulping (1). The pulp is washed on the washing line (2-5) countercurrently with hot water, the filtrate being ultimately directed to evaporation and to chemicals recovery, and the pulp thus washed is further after-washed (6) with warm water before being directed to the bleaching line (7-11), which has successively a chlorine -chlorine dioxide step (7), an alkali step (9) and a final bleaching (11), the bleached pulp being washed after each of the above bleaching steps. According to the invention, after the chlorine - chlorine dioxide step (7) and the alkali step (9) so much filtrate is removed from the washed pulp, for ex-ample, by pressing, that the solids content of the pulp will be at least 30 %, and at least a portion of the filtrate thus obtained is directed to the last step (5) of the countercurrent wash as its first washing water, which is displaced with the said hot water before the after-wash (6).

Description

~ ~ S.~ t~ " 1 5 A process for countercurrent washing and bleaching of a pulp from chemical pulping The present invention relates to a process for countercurrent washing and bleaching of a pulp from chemical pulping, and in particular to a process in which the pulp is washed in the washing line countercurrently with hot water, the filtrate is ultimately directed to evaporation and to chemicals recovery, and the pulp thus washed is further after-washed with warm water before the pulp is directed to a bleaching line which comprises a plurality of successive bleaching steps and in which the bleached pulp is washed after each bleaching step.

The bleaching of chemical pulp is in general carried out using oxidizing and alkaline chemicals. Nowadays the most common chemicals are chlorine, chlorine dioxide, oxygen, and sodium hydroxide. In the bleaching process, some wood becomes dis-solved, together with the bleaching chemical, in the process waters of the bleach. After purification of the waste waters, these waters are in general directed into watercourses. Under the effect of the bleaching chemicals, hazardous compounds are formed in the process, and these compounds are not completely removed in the purification of the effluents. The dissolved wood for its part also decomposes poorly in conventional bio-logical purification processes. In nature the decomposition and elimination of the waste materials is likewise very slow.

The organic matter present in the effluents from bleaching would be usable, for example, as fuel, if it could be enriched and concentrated. The sodium alkali present in the effluents would also be usable. However, the waste waters are very di-lute, and therefore there is no sense in evaporating them.

One applicable method is to direct the bleaching liquors in the process countercurrently in the pulp-washing process preceding 2 9 ~

the bleaching. This known method has been used in certain mills, and it has been subject to a considerable amount of research. The process involves a number of detrimental factors.
One of them is the disadvantageous water balance of the process.

The bleaching of pulp is carried out in a number of steps, be-tween which the pulp is washed either with pure water or with filtrates from another bleaching step. The washers used are so-called washing filters or washing diffusers, the solids content of the pulp coming from them being approx. 10-15 %. Similar devices, and also so-called washing presses, have been used in the washing of pulp which precedes bleaching. On the other hand, owing to their high price, washing presses have not been used as washers in the bleaching plant, because there has deemed to be no need for such use.

The object of the present invention is thus to provide a proc-ess, of the type referred to in the preamble, for countercur-rent washing and bleaching of a pulp from chemical pulping, process in which the effluent amounts and water balance ob-tained for the pulp mill are suitable with respect to the re-covery of the bleaching li~uors.

The principal characteristics of the invention are given in accompanying Claim 1.

The object of the present invention is thus achieved by remov-ing after the first bleaching step and the second bleaching step considerably more, approximately twice more, filtrate from the washed pulp than has been done so far, so that when the solids content of the pulp is at minimum approx. 30 %, the excess of filtrate is low, and at least a portion of the fil-trate thus obtained is directed to the last step of the coun-tercurrent wash as its first washing water, which is displaced with hot water before the after-wash.

2 ~ '? ~ Q s -In a preferred embodiment of the invention the filtrate from the bleaching line is directed as the first washing water to the displacement wash which constitutes the last step of the countercurrent wash. Alternatively, the filtrate from the bleaching line can be directed to the last step of the counter-current washing of brown pulp and be displaced in it by the said hot water before the after-wash.

In an especially preferred embodiment of the invention, the filtrate of the first andtor the second bleaching step is di-rected in its entirety to the countercurrent-washing line, and so much waste water is removed from the pulp in the after-wash that the solids content of the pulp directed to the bleaching line is at minimum approx. 30 %.

In order to achieve the required high solids content in the wash after the first and second bleaching steps, and possibly in the after-wash on the washing line, washing solution is pressed out of the pulp by means of washing presses, known E~
se, used in the pulp wash preceding the bleaching, the use of which presses in this case as washers in the bleaching plant is, however, highly justified, since their higher price will be compensated for many-fold by the improved functioning of the process.

In the process according to the invention, preferably a so-called chlorine step is used as the first bleaching step, in which the bleaching is carried out using chlorine, chlorine dioxide, or a mixture of these, and the second bleaching step is a so-called alkali step.

The invention is described below in greater detail with ref-erence to the accompanying drawings, in which Figure 1 depicts a process flow diagram of the fiber line of a mill which pro-duces bleached chemical pulp, and Figures 2 and 3 depict two embodiments of the process according to the invention.

2 ~ ~J 3 ,~ ~S ~

In the accompanying drawings, the pulping process in general is indicated by reference numeral 1, and the subsequent washing of brown pulp is indicated by reference numeral 2. The grading and precipitation of the pulp from the washing of brown pulp is in-dicated in general by reference numeral 3, and the subsequent oxygen step by 4. The countercurrent wash with hot water, sub-sequent to the oxygen step, is indicated by reference numeral 5. After the countercurrent wash 2-5 there is further carried out an aft~r-wash 6 with warm water, whereafter the washed pulp is directed to the bleaching line 7-11, in which the flrst step, i.e. the chlorine step, is indicated by 7 and its washing step by 8, the second step, i.e. the alkali step, by reference numeral 9 and the subsequent wash by 10, and the final bleach-ing in general by reference numeral 11.

The pulp flow is indicated by a thickened line, whereas the liquid flows are indicated by a thinner line. The filtrate from the washing 2 of brown pulp is directed to evaporation and to chemicals recovery. The liquid flows are indicated numerically in cubic meters per onP metric ton of pulp. The pulp flows are equal in all figures, so that the consistency of the pulp is 10 % and its liquid content 9 m3 per one metric ton of pulp, ex-cept after the press washes, at which time the consistency of the pulp is considerably higher and its liquid content consid-erably lower.

Figure 1 shows, for comparison, the flow diagram of the fiber line of a present-day mill producing bleached chemical pulp. In this process, known per se, the pulp is washed using pure water before a bleach carried out using chlorine chemicals.

From the wash subsequent to the oxygen step 4 onwards, the li~uid flow is countercurrent to the pulp flow, and the con-centrated liquor is directed from the washing 2 of brown pulp ultimately to evaporation and to chemicals recovery. After this 2~ 9 j~
series of washes there is furthermore an after-wash 6, which is carried out with warm water, but ~he filtrate obtained is di-rected to the effluents. The washed pulp is directed to the bleaching 7-11, which has at least 3 bleaching steps. The fig-ure shows in greater detail only the first two steps, i.e. the so-called chlorine step 7 and the so-called alkali step 9.
These are essential in terms of the invention. The final bleach 11 is indicated generally as a block, which may comprise a plu-rality of steps. After all the steps there is the pulp washer 8, 10, which typically produces pulp at a consistency of ap-prox. 10 %. The washing filtrate excess from the chlorine 7 and alkali 9 steps is directed to the effluents. The washers in these steps use as the washing water a filtrate which is de-rived from the washers of the various steps of the final bleach 11. According to the balance, the total amount of waste water is 34 m3 per one metric ton of pulp, and in practice 30...40 m3/one metric ton of pulp. The filtrates from the bleach are not directed countercurrently in the process to the washing of unbleached pulp.

In the process according to the invention, depicted in Figure 2, the liquors from the second step of the bleaching plant, the so-called alkali step 9, are directed to the recovery of cook-ing chemicals. Washing presses are used as washers 8, 10 in both of the first bleaching steps, i.e. the chlorine step 7 and the alkali step 9. On the other hand, the washers used before the bleaching are filters or diffusers. The last washing step is a so-called after-wash 6, from which the filtrates are not recovered. In the example case the pulp leaves the washing presses at a consistency of 35 %, its liquid content being 1.86 m3 per one metric ton of pulp. In order to produce the washing effect, water is introduced to the washing presses 8 and 10 at approx. 1 m3 per one metric ton of pulp in excess of the water present in the pulp leaving the press. Taking also into account the water present in the bleaching chemical, the amount of fil-trate from the washing press 10 of the alkali step 9 is approx.

2 ~

3 m3/one metric ton of pulp. This water is directed to the last washer 5 serving the recovery of cooking chemicals. In it this water is used as the first washing water. The second washing water used is pure water, the amount of which is high r and approx. equal to the amount of liquid present in the pulp leav-ing this washer. If the consistency of the pulp is 10 %, the liquid amount is 9 m3/one metric ton of pulp, and the total amount of washing water is 12 m3/one metric ton of pulp. Thus conditions are produced which are characteristic of good wash-ing of pulp.

It is estimated that the process of Figure 2 exploits in the mill processes 60...70 % of the most hazardous portion of the bleaching plant effluents, i.e. the filtrate of the alkali step. Thus the amount of material dissolved in the waste water is respectively reduced. The total quantity of effluents is approx. 2/3 of the effluents of a present-day mill.

In the embodiment according to Figure 3, the filtrates from both the first, so-called chlorine step 7, and of the second, the alkali step 9, are directed to the wash 2-5 of recovery of the cooking chemicals. In this case, a washing press is used also as the last washing step 6 preceding the bleaching plant 7-11. In this embodiment, also, this washer 6 preceding the bleaching plant 7-11 operates as an open washer, so that its filtrate is directed out of the process. Owing to the higher quantity of water in the chemicals in the chlorine step 7, the amount of liquid to be directed from the bleaching plant 7-11 to the cooking chemicals recovery wash 2-5 is approx. 2 m3/one metric ton of pulp higher than in the case of Figure 2, but even in this case suitable in terms of the process. Thus it is possible to transfer to the cooking chemicals recovery process most of the waste material produced in the first two steps 7, 9 of the bleaching plant. At the same time the effluents amount is further reduced to less than one-half of the effluents of a present-day plant, i.e. to only 15 m3/one metric ton of pulp.

7 2~ ;3.~

It is evident for an expert in the art that the present inven-tion can be varied and modified within the accompanying claims.
Thus, for example, the oxygen step 4 is by no means indispen-sable, but can, when necessary, be omitted.

Claims

1. A process for countercurrent washing and bleaching of a pulp from chemical pulping (1), wherein the pulp is washed in the washing line (2-5) countercurrently with hot water, the filtrate being ultimately directed to evaporation and to chemi-cals recovery, and the pulp thus washed is further after-washed (6) with warm water before being directed to the bleaching line (7-11), which comprises a plurality of successive bleaching steps and in which the bleached pulp is washed after each bleaching step, characterized in that, at least after the first two bleaching steps (7, 9), so much filtrate is removed from the washed pulp that the solids content of the pulp will be at least approx. 30 %, and at least a portion of the filtrate thus obtained is directed to the last step (5) of the countercurrent wash, as its first washing water, which is displaced with the said hot water before the after-wash (6).

2. A process according to Claim 1, characterized in that the filtrate from the bleaching line (7-11) is directed to the displacement wash (5) of the oxygen step (4) which is the last step of the countercurrent wash, as its first washing water.

3. A process according to Claim 1, characterized in that the filtrate from the bleaching line (7-11) is directed to the last step of the countercurrent wash of brown pulp and is dis-placed in it with the said hot water before the after-wash.

4. A process according to Claim 1, 2 or 3, characterized in that the filtrate from the first bleaching step (7) and/or the second bleaching step (9) is in its entirety directed to the countercurrent washing line (2-5), and in the after-wash (6) so much waste water is removed from the pulp that the solids con-tent of the pulp directed to the bleaching line (7-11) is at least 30 %.

5. A process according to any of the above claims, charac-terized in that in the washes (8, 10) subsequent to the first (7) and the second (9) bleaching steps, and possibly in the after-wash (6) of the washing line, washing solution is pressed out of the pulp in order to achieve the required high solids content.