METHOD OF TREATING PROCESS WATER

Abstract: Treatment of process water containing metal ions, in connection with the bleaching of pulp of lignocellulose-containing material. The process water is brought into contact with pulp at pH above 7 and a temperature of 50-120 C. The process thereby is relieved entirely or partially from the metal ion content.

Description: Method of treating process water This invention relates to a method of treating process water, which contains metal ions, in connection with the bleaching of pulp of lignocellulose-containing material. In order to minimize the effect by various emissions, such as BOD, COD, paint etc. on the environment, there is a strong interest to restrict the emissions by closing the wastewater system of the bleach plants. This implies, that the wastewater should be recycled to the process instead of being emitted. As a result of closing the system, however, the content of metal ions, such as manganese, copper, iron etc. in the system can increase to a level giving rise to problems at bleaching with hydroperoxide and, respectively, ozone. The metal ions enter the system via the wood raw material and therefore, the affluent of metal ions cannot be eliminated as would be desirable from a process point of view. The content of these metal ions in the pulp, however, is very low and, therefore, they do not cause any environ¬ mental problem, not in the local working environment and not in the recipient, either. In future, the bleaching of papermaking pulp will be based to a high degree on hydroperoxide bleaching and ozone bleaching. For achieving high ISO-brigthness, this type of bleaching reguires the unbleached pulp and process water to be free, or substantially free, of certain metal ions, because otherwise a considerable disintegration of charged hydroperoxide to water and oxygen and, respectively, of charged ozone to oxygen will take place, without any simultaneous bleaching effect. In order to remove the metal ions prior to bleaching, the pulp must be pretreated with chemicals, for example acid or complex formers such as EDTA or DTPA, which selectively bind the undesired metal ions and thereby render it possible to wash them out prior to the bleaching. For process reasons, the resulting washing water must be eliminated from the system, and at the same time environmental reasons require that the washing water is not emitted. SUBSTITUTESHEET The present invention offers a solution for the aforesaid problems. The invention, thus, has the object to remove metal ions from process water enriched in metals and thereby render it possible to re-use the process water and thereby to close the wastewater system of the bleach plant, which is desirable from an environmental point of view. The invention is described in greater detail in the following, with reference to the accompanying Figure 1 showing by way of a flow sheet an application of the method according to the invention. Figure 2 is a diagram showing the manganese content as a function of the reaction time at one embodiment. Unbleached pulp is supplied through a duct 1 to a pre-treatment step 2 where the pulps is treated with acid or complex former of the type EDTA or DTPA. Metals such as manganese are here solved out of the fiber material. The pulp is directed from here to a first washing step 3, for example in the form of a washing press or washing filter, where wastewater containing the critical metal ions is removed through a duct 4. The pulp free of critical metal ions is directed to a bleaching step 5 for being bleached with hydroperoxide or ozone. After the bleaching step 5, the pulp is directed to a second washing step 6 where the wastewater from the first washing step 3 via the duct 4 again is brought into contact with the pulp, and where the pulp again binds the original metal ions. The wastewater from the second washing step is discharged through a duct 7, and the washed pulp is taken out through a duct 8. The wastewater substantially free of metal ions thereby can be used as washing water in positions where a low content of metals is of importance, for example in a bleach plant entirely free of chlorine. The wastewater can be used in other steps of the process. For achieving an improved washing effect relating, for example, to COD etc., it can also be of interest to use an additional washing liquid 9 in the washing step 6 after the bleaching. SUBSTITUTE SHEET The pulp, thus, will transport the metals out of the bleach plant, and the accumulation effect in the bleach plant thereby can be avoided at the closing of the drain system. The pulp somewhat enriched in metal according to the invention can be used in a conventional manner directly in the paper aking, or first be purified from metal ions, for example by treatment with washing water containing complex former at a lower pH, or be destructed. It was found that suitable conditions for achieving good elimination of the metal ions in the second washing step 6 shall be as follows: pH above 7, suitably above 10.5 and preferably pH 11 or higher. Of particular interest is the pH- interval 11.5-12. The temperature should be 50-l20°C, preferably 70-100°C. The contact time between process water and pulp can be very short. At optimum conditions, the time can be shorter than 1 second, but slightly longer time, 1-6 seconds, generally is better. Longer reaction times 0.1-6 min, can be applied, but do not appreciably affect the result. A time exceeding 1-5 min, therefore, normally is of no interest. The reaction time normally is determined by the washing equipment used. Very short stay times can be achieved both at the usage of a washing press and washing filter in the second washing step 6. At an alternative method, only a partial flow of the pulp is utilized for separating the metal ions from the wastewater from the first washing step. It is, of course, also possible to use an unbleached pulp, which is not intended to be bleached, or a reject pulp flow for separating the metal ions. It is further imaginable to utilize the metal separation according to the invention as a complement to another type of metal separation. EXAMPLE 1 The manganese content in an aqueous solution free of lignin, which was treated with pulp bleached with hydroperoxide showed a substantial decrease when the treatment was carried out at pH 12 compared to pH 10-11. Treatment for 10 minutes at 80°C. Liquid Initial content (mg/1) 6.5 Treatment at pH 10 6.4 Treatment at pH 11 6.5 Treatment at pH 12 1.2 EXAMPLE 2 The manganese content was measured in a laboratory- made bleach plant drain after treatment with pulp bleached with hydroperoxide. The treatment was carried out at different pH values. At pH 12 as well as in two cases even at pH 11, a strong reduction of the manganese content was observed. Liquid 2 Liquid 3 Liquid 4 Initial content (mg/1) 3.3 3.5 3.6 Treatment at pH 10 3.4 Treatment at pH 11 0.7 and, 2.7 0.3 resp. 0.4 Treatment at pH 12 1.3 0.03 SUBSTITUTE SHEET EXAMPLE 3 The manganese content was measured in a laboratory- made bleach plant drain after treatment with pulp bleached with hydroperoxide, where the treatment was carried out at different pH and temperatures. The results show that reaction times exceeding two minutes do not appreciably affect the result. Increase in pH also shows to be more important than increase in temperature. PH 11/80°C pH 12/80°C pH 11/90°C Initial content (mg/1) 3.5 3.5 3.5 Reaction time 2 min 2.3 1.9 2.6 Reaction time 10 min 2.6 1.3 2.5 Reaction time 60 min 2.3 EXAMPLE 4 The manganese content was measured in laboratory-made bleach plant drains from hydroperoxide bleaching of pulps with different lignin contents. Treatment for 10 minutes at 80°C. The lignin content of the pulp measured as kappa number has no decisive effect on the manganese content obtained. Initial content (mg/1) Treatment at pH 10 Treatment at pH 11 Treatment at pH 12 SUBSTITUTESHEET EXAMPLE 5 The manganese content in an aqueous solution free of lignin which was treated with pulp bleached with hydroperoxide at different pH values was measured after different reaction times. The result is shown in Figure 2. It appears here that a 99% reduction of manganese can be achieved at reaction times of less than about 2 seconds when pH is 11.8-11.9 at 80°C. It also appears that reaction times exceeding about 2 seconds do not further reduce the manganese content. The invention, of course, is not restricted to these embodiments, but can be varied within the scope of the invention idea. SUBSTITUTE SHEET

Claims: Claims 1. A method of treating process water containing metal ions which are washed out from pulp of lignocellulose- containing material, prior to bleaching of the pulp with hydroperoxide or ozone, c h a r a c t e r i z e d in that the process water is brought into contact with pulp which is not intended to be bleached by hydroperoxide or ozone at pH above 7 and at a temperature of 50-120°C, whereby the metal ion content entirely or partially is removed from the process water. 2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the process water is brought into contact with pulp at pH 11 or higher. 3. A method as defined in claim 1 or 2, c h a r a c t ¬ e r i z e d in that the process water is brought into contact with pulp at pH 11-12. 4. A method as defined in claims 1-3, c h a r a c t ¬ e r i z e d in that the process water is wastewater from the washing of pulp after treatment with complex former. 5. A method as defined in any one of the claims 1-4, c h a r a c t e r i z e d in that the metal ions are separated at the washing of the pulp prior to bleaching with hydroperoxide or ozone and are returned entirely or partially to a least a partial flow of the pulp after said bleaching. 6. A method as defined in any one of the claims 1-4, c h a r a c t e r i z e d in that the pulp used for separating the metal ions from the process water consists of bleached pulp, unbleached pulp or reject pulp. SUBSTITUTE SHEET

Inventors: GERMGAARD ULF, NORDEN SOLVEIG, GERMGAARD, ULF, NORDEN, SOLVEIG
Applicants: GERMGAARD ULF, NORDEN SOLVEIG, SUNDS DEFIBRATOR IND AB, SUNDS DEFIBRATOR INDUSTRIES AKTIEBOLAG
Application year: 1993
Main classification: D21C9
IPC codes: C02F9, D21C9, D21C11