PROCESS FOR BLEACHING PULP IN CONJUNCTION WITH ADSORPTION OF METALS

Abstract: The following invention relates to a process for bleaching pulp, in which, after defibration, a first filtrate (2) containing metals, principally in ionic form, is separated off from pulp, in which the separated metals are supplied to a certain fibre quantity (1), which adsorbs the metals, which fibre quantity, during its further treatment, is at least not significantly negatively affected by the presence of the metals.

Description: Title: Process for bleaching pulp in conjunction with adsorption of metals Technical field The present invention relates to a process for bleaching pulp preferably comprising several bleaching stages, in which at some stage after defibration a first filtrate containing metals, principally in ionic form, is separated off, in which the metals are first separated from a pulp stream and the pulp stream is subsequently bleached, in which, in a preferred case, the metals are reintroduced to the fibre stream from which they had been separated. Using the invention, a problem, which is otherwise difficult to solve, namely the problem of catering for released metals, is solved in a very simple manner. State of the art and problems The environmental authorities are placing ever greater demands on the pulp industry to decrease the use of chlorine gas for bleaching. Permitted discharges of organic chlorine compounds (AOX) with the waste water from bleaching plants have been gradually decreased and are now at such a low level that the pulp factories have in many cases stopped using chlorine gas. Instead, only chlorine dioxide is used as a bleaching agent. Chlorine dioxide forms smaller quantities of AOX than chlorine gas while achieving the same bleaching effect. However, even the use of chlorine dioxide has been questioned. On the one hand, the environmental authorities in certain countries demand that the discharges of organic chlorine compounds be reduced to such a low level that these demands can scarcely be met even if only chlorine dioxide is used for bleaching. On the other hand, customers in many countries have begun to demand paper products which are bleached entirely without using either chlorine gas or chlorine dioxide. The pulp industry is therefore searching for methods which permit bleaching of pulp without using chlorine chemicals. Methods which have been successfully tested involve removing metals in an acid stage (A) , or possibly by addition of chelating agents (Q) , e.g. EDTA, to an oxygen- delignified pulp. The pulp is washed and is further bleached using, for example, hydrogen peroxide (P) and/or ozone (Z) in different sequences. One example is the method which is described in SE-A-8902058 (EKA NOBEL), the so-called Lignox method. Another known bleaching process includes the bleaching sequences AZ (EOP) where A is an acid stage without chelating agent. It is a significant feature of these different methods that certain metal ions have a negative effect on the bleaching process in the form of impaired pulp quality and/or greater consumption of chemicals. In these methods, the decision is taken to wash out the metals by means of an open A/Q stage. A problem which arises in conjunction with these methods is that, as a result, a liquid flow is obtained from the washing stage after release of the metals which contains, on the one hand, a certain amount of released substance and, on the other hand, dissolved metal ions, which situation is difficult to manage from the point of view of waste and recovery. According to conventional technology, this filtrate is treated by means of external purification, which can be extensive and costly, after which it is released into the receiver. Solution and advantages The present invention is characterised in that the released metals are supplied to a certain fibre quantity, which adsorbs the metals, in which connection the further treatment of the fibre quantity is to some considerable extent not negatively affected by the presence of metals. Using the process according to the invention, it has thus been possible to solve the problems associated with the metal-containing liquid flow, which problems, with currently known methods, cause both increased work and increased costs. With the aid of the invention, this somewhat troublesome problem has thus been solved in a very simple manner by supplying the separated metals to a fibre flow whose further treatment is at least not appreciably affected negatively by the presence of the metals. In what probably will become the most usual application of the invention, there is only one fibre line, i.e. only one fibre flow line, and in this case the metals are therefore reintroduced to the same fibre flow line from which they had been separated at an earlier stage. It can, however, be advantageous to have at least two parallel fibre lines whereby the separated metals are preferably supplied to one and the same fibre flow, as a result of which it is possible to obtain as final products one or more lateral flows, which may be bleached or unbleached and which have not been supplied with metals, and at least one final product which then has an enriched quantity of metals. The addition of the metals can take place in a pulp tank or the like or else as washing liquid for the washing apparatus of the last bleaching stage. In order to ensure that the metals are adsorbed, the pH in the pulp mixture is preferably adjusted in a suitable manner. In addition, supplementary chemicals can be used (for example a retention agent) . In a preferred case, the filtrate, after giving up the metals, i.e. adsorption of the metals by the pulp, should be supplied to another part of the process, preferably to an earlier stage. In this way, the degree of closure can be significantly increased, i.e. the effluent quantity, and consequently the need for fresh water, decreased. This can conveniently be done by the .pulp being thickened and washed in a washing apparatus, for example a wash press, for further transport to an additional bleaching (which is not negatively affected by the metals) with, for example, chlorine dioxide, or to a drying machine, storage tower or paper mill. The filtrate from this washing apparatus is then conveyed, in this preferred case, in a counter- current manner back into the process, preferably to the washing apparatus of either the Q stage or the A stage. In addition, the filtrate can be divided up so that one part can be conveyed back to the washing apparatus of one of the bleaching stages. The invention will be clarified here in more detail with the aid of the attached figures in which: Figure 1 shows a block diagram of a fibre line in which the invention is used, Figure 2 shows a block diagram of two parallel fibre lines in which the invention is used Figure 3 shows a preferred bleaching plant in conjunction with the use of the invention. Figure 1 shows a block diagram constructed around a flow line 1 for pulp which has been prepared chemically by cooking. In the preferred case the cooking is continuous. After cooking and washing, the pulp is conveyed through the first part of the flow line 1 to an additional washing and sieving stage, followed by oxygen-delignification and subsequent washing of the pulp. The next stage is a Q stage. In the Q stage, metals are separated from the fibres in the pulp. The filtrate 2, containing the metals, from the Q stage is thus separated from the pulp stream 1, which stream is thereafter conveyed to sequences in a bleaching plant in which processes take place which are negatively influenced by metals in the pulp. Because the metals have been separated in an earlier filtrate 2, these component processes can thus be carried out in an improved and more efficient manner. A bleaching sequence which is negatively affected by metals is, for example, a PZP bleaching plant, comprising an initial and a terminal hydrogen peroxide stage and an intermediate ozone stage. After the pulp has been bleached there is a stage which is designated R in the figure. In this stage, the metals which were separated off in the earlier filtrate stream 2 are returned/reintroduced. Released organic substances, such as lignin residues, are not adsorbed by the fibres and are washed away in the subsequent washing press or the like. The most preferred method for reintroducing the metals is by addition of a suitable medium for adjusting the pH (for example by means of NaOH) , so that the fibres included in the fibre stream can adsorb the reintroduced metals. As is evident from Figure 1, a bleach plant according to the invention is, in the preferred case, one in which the plant can be totally closed, i.e. there is no effluent from the bleach plant. According to Figure 1 this is achieved by conducting the filtrate 4 back from the actual bleaching sequences PZP to the oxygen-delignification and by allowing a first filtrate stream 5 from the R stage to be recirculated to the bleaching sequences and, finally, by allowing a second filtrate stream 6 from the R stage (in principle the filtrate 2 conveyed to the R stage, but without the metals) to be reintroduced into the Q stage. New washing liquid 3 is added to the R stage. The final product obtained is a bleached pulp containing metals. Experiments which have been carried out showed that more than 98% of Mh 2+ ions from an A stage were adsorbed onto a fully-bleached pulp at pH values greater than 7. In squeezed filtrate after the mixing, the concentration of Mn 2+ ions was undetectable, i.e. less than 0.05 mg/1. The concentration in the A filtrate was 3.6 mg/1. Figure 2 shows a block diagram of the outline solution when two parallel flow lines 1A and IB, respectively, are present. The one flow line 1A is for a chemical pulp whose final product should have as low a content of metals as possible while the second flow line IB is for a completely different type of pulp, for example unbleached pulp for which the metal content is of relatively minor importance. As the block diagram shows, the filtrate 2 from the Q stage in the first line 1A is conducted to an R stage in the second line IB. The metals which were separated in the Q stage of the first line 1A are conveyed to the R stage in the second line IB. In order to ensure liquid balance between the two lines, a filtrate 3 is conveyed back from the R stage in the second line IB to a sieving or washing stage in the first line 1A. This latter filtrate is naturally a filtrate which does not contain metals. Figure 3 shows in more detail a preferred bleach plant with only one flow line for pulp being present. Thus, a diffuser bleach plant is shown which is constructed for the sequence AZ(EOP). The beginning of the bleaching plant consists of a chute 2 with an MC-pump to which an acidifier, preferably in the form of sulphuric acid (H2SO4) , can be added via an inlet conduit 3. Next there is a washing apparatus 4 which appropriately comprises a KAMYR ® wash diffuser 4. In this washing apparatus 4, the metals contained in the pulp are separated from the fibres and the filtrate 5, which contains the metals, is taken out via a separate conduit 5. Wash liquid for the washing apparatus 4 is supplied via a conduit 7, which comes from a later stage in the process, and, if necessary, also via a separate conduit 6. After the washing apparatus 4 there follows a chute 35 with an MC-pump, in which chute pH- adjusting substances can be supplied via a conduit 8. Ozone and oxygen are supplied in a subsequent mixer unit 9 and are allowed to react with the pulp in a reactor unit 11. Thereafter the gas and pulp are separated in a separating device 12 from which the gas is conducted away through an upper conduit 13. The pulp is then pumped to a second washing apparatus 14 which is supplied with washing liquid via a conduit 15 from a later bleaching stage. The filtrate which is separated off in the second washing apparatus 14 is conveyed via 16 to a washing stage prior to the bleach plant. After the washing apparatus there follows a further chute with an MC-pump 17. Alkali 18, preferably sodium hydroxide (NaOH) , is first supplied to this device 17, and then immediately thereafter oxygen 19 (02) , after which the oxygen is allowed to act on the pulp in a reaction tower 20. The oxygen is separated off in a separating device 21 and conducted away at the top 22. Hydrogen peroxide 23 is supplied at the bottom of the separating device, after which the pulp is pumped into a reaction vessel 24 which at the top is fitted with a KAMYR ® diffuser washing device 25. It is the filtrate from this washing device 25 which is conveyed back to the previously mentioned washing device 14. Washing liquid for the washing device 25 after the P stage is supplied via a conduit 26 which is connected to a later stage in the process. The pH of the pulp can be adjusted to the desired value via a separate conduit 27. After the P stage there follows that stage which earlier in the description was called the R stage 28, which in the present instance can be designated a pulp mixing vessel. The metal-containing filtrate 5 is consequently conveyed to this vessel 28 and, due to the pH being appropriate, the fibres in the vessel 28 adsorb the metals. After this, the pulp is conveyed to a washing press 29 in which the pulp is thickened and pumped out to a chute 30 for further transport to subsequent treatment. The filtrate from this washing press 29, which filtrate is free of metals, is supplied to the first washing device 4 via conduit 7 and/or the washing device 25 of the P stage via conduit 26. It will be evident to the person skilled in the art that the invention is not limited by that which has been described above, but can be varied within the scope of the subsequent patent claims. Thus, it is evident to the person skilled in the art that the invention can be applied to all types of pulp (the manufacture of) which may conceivably contain some component stage where metals have a negative influence, such as, for example, recovery fibre pulp, mechanical wood pulp (TMP, CTMP, RMP, etc.) . Furthermore, it will be evident to the person skilled in the art that with a multiplicity of parallel lines there is a large number of combinations which can be selected using the invention in which connection different characteristics can be optimised. For example, a factory comprising three flow lines can have two lines for bleached pulp without metals in the final product and one line for mechanical wood pulp, such as CTMP, in which all the metals are collected, for example for the manufacture of 3-layered composite paperboard comprising two outer bleached layers and a middle metal-containing CTMP layer. This avoids the presence of metal in any outer layer, which can be advantageous for certain uses of the composite paperboard. In addition, it is evident that the equipment which is shown in Figure 3 is only exemplifying and that the person skilled in the art can easily find alternatives to the equipment shown, such as, for example, a pressure diffuser or a drum washer as a washing apparatus, etc. In order, as in a preferred case, to utilise to the full the environmental advantages of the process according to the invention, it is desirable to arrange the subsequent paper manufacture so that the retention of the metals at the wet end of the paper-machine is as great as possible, i.e. so that the metals are chiefly present in the finished paper and not in the waste water from the paper manufacture. In certain cases it can be preferred to treat the un¬ bleached pulp according to Fig. 2, for example by acidi¬ fying it, before supplying the metal-containing filtrate 2 from the parallel line, in order, if possible, to increase, in this way, the ability of the pulp to adsorb metals and/or confer on it selective adsorption capa¬ bility. In addition, it is perfectly possible to influence the selectivity in the adsorption by the use of supplementary chemicals.

Claims: PATENT CLAIMS 1. Process for bleaching pulp in which, after defibration, a first filtrate containing metals, princi¬ pally in ionic form, is separated off, preferably by means of acidification, from pulp, characterised in that the separated metals are supplied to a certain fibre quantity which adsorbs the metals, which fibre quantity, during its further treatment, is at least not significantly negatively affected by the presence of the metals. 2. Process according to Patent Claim 1, characterised in that the filtrate, after being freed of the metals, is supplied to the process, preferably at an earlier stage, so that the possibility is realised of completely closing the bleaching plant. 3. Process according to Patent Claim 1, characterised in that the said fibre quantity is assigned to one and the same flow line, and that the metals which have been separated out of the flow line are supplied to the same flow line at a later stage, after the sequence in which the metals are not wanted. 4. Process according to Patent Claim 1, characterised in that there are several parallel flow lines for fibres and that the metals are selectively supplied to one or more of the said lines. 5. Process according to Patent Claim 4, characterised in that at least one flow line with bleached pulp is not supplied with the metals. 6. Process according to Patent Claim 5, characterised in that the metals are supplied to a flow line containing unbleached fibres. 7. Process according to Patent Claim 3, characterised in that the bleaching sequence during which the metals are not wanted contains at least one EO stage. 8. Process according to Patent Claim 3 or 7, characterised in that the bleaching sequence during which the metals are not wanted contains at least one hydrogen peroxide bleaching stage (P) . 9. Process according to Patent Claim 3, 7 or 8, characterised in that the bleaching sequence during which the metals are not wanted contains at least one ozone bleaching stage (Z) . 10. Process according to Patent Claim 6, characterised in that pulp components with different metal contents are employed in the manufacture of multi-layered paper so that metals are not present in at least one of the outside layers and in which the metals are preferably present in an intermediate layer.

Inventors: BERGQVIST ANDERS, DAHLLOEF HAAKAN, BERGQVIST, ANDERS, DAHLLOEF, HAAKAN
Applicants: BERGQVIST ANDERS, DAHLLOEF HAAKAN, KAMYR AB, KAMYR AKTIEBOLAG
Application year: 1993
Main classification: D21C9
IPC codes: D21C9