METHOD FOR PURIFYING PROCESS WATER OR WASTEWATER CONTAINING WOOD RESIN

Abstract: Wood-resin-containing process water or wastewater from the pulp industry is purified by adding, as flocculants, a high-molecular-weight polyethylene oxide and a water-soluble, non-ionic cellulose ether, suitably having a flocculation temperature of 35-80 DEG C. The added amount per litre of water is preferably 2-20 mg of the high-molecular-weight polyethylene oxide and preferably 5-30 mg of the non-ionic cellulose ether.

Description: METHOD FOR PURIFYING PROCESS WATER OR WASTEWATER CONTAINING WOOD RESIN The present invention relates to a method for puri- fying process water or wastewater containing wood resin, by adding a high-molecular-weight polyethylene oxide and a water-soluble, non-ionic cellulose ether as flocculants. In the making of pulp, the pulp washing step yields process water and wastewater containing a number of unde- sired substances and products, such as resin, hemicellu- lose, lignin and fines. Such process water and wastewater give rise, among other things, to discoloration of the pulp as well as high oxygen consumption upon emission into a water recipient. There is thus a manifest need to be able to reduce, as far as possible, the contents of these substances and products. It is generally known, e.g. from Canadian Patent 1,004,782, to add a high-molecular-weight polyethylene oxide and a phenolformaldehyde resin to wastewater from the pulp industry with a view to removing such undesired products by flocculation. A similar process is known from Swedish Patent Publication 454,507, where, in addition to a high-molecular-weight polyethylene oxide and a phenol- formaldehyde resin, a cationic starch or cellulose deri- vative is added. It has however been found that it is difficult to bring about effective purification of wood-resin-contain¬ ing process water or wastewater without using high con¬ tents of high-molecular-weight polyethylene oxide. It is however desirable to reduce the amount of high-molecular- weight polyethylene oxide, since it is difficult to pro¬ duce and hence is very expensive, as well as to further improve the degree of purification of the resin-rich pro¬ cess water or wastewater. It has now been found that surprisingly effective purification of wood-resin-containing process water or wastewater from the pulp industry can be achieved by adding a water-soluble, non-ionic cellulose ether, prefer¬ ably having a flocculation temperature of 35-80°C, and a high-molecular-weight polyethylene oxide as flocculants. The content of wood resin in the process water or waste- water may vary within wide limits. Even at as low con¬ tents as 0.1 g/1 water, significant effects are achieved, although this content generally amounts to at least 0.4 g/1 water, measured according to the method described in SE 8403587-2. Normally, the content does not exceed 50 g/1 water. For process water or wastewater originating from the making of mechanical pulps, the resin content preferably is 0.7-2 g/1 water, while the resin content of process water or wastewater from the production of chemi¬ cal pulps, such as sulphite pulp, preferably is 10-20 g/1 water..The amount of the added high polymer polyethylene oxide and the non-ionic cellulose ether should suitably be conformed to the amount and the type of the undesired sub¬ stances and products. Generally, the amount of high-mole¬ cular-weight polyethylene oxide is at least 1 mg/1 water, preferably 2-20 mg/1 water, and the amount of non-ionic cellulose ether generally is at least 3 mg/1 water, pre¬ ferably 5-30 mg/1 water. The weight ratio of high-polymer polyethylene oxide to non-iononic cellulose ether general¬ ly is 2:1-1:5, preferably 1:1-1:4. The method according to the invention yields excel¬ lent purification, especially when the treated process water or wastewater contains considerable amounts of wood resin, e.g. above 10 g/1. Solids in the water, e.g. in the form of fines, do not interfere with the process. The process water or wastewater to be treated should have a pH within the range of 1-9, preferably 5-6 for chemical pulp, such as sulphite pulp or sulphate pulp, and a pH of 7-9 for mechanical pulps, such as CTMP and TMP. The result of the purification is promoted if the process water or wastewater in the -purification process has a temperature above room temperature, preferably a temperature of 35-70°C. The high-molecular-weight polyethylene oxide suited for use in the method according to the invention should have a molecular weight above 500,000, and most preferred above 2,000,000. No critical upper limit has been observ- ed, and good results have thus been obtained by using polyethylene oxide having a molecular weight of about 6,000,000. Essentially higher molecular weights have proved difficult to obtain because of chain-shortening in the production. The polyethylene oxide according to the invention is mainly made up of oxyethylene groups, but a small amount of other groups may also be included, such as oxypropylene. The amount of groups other than oxyethylene should only be so high as to allow the polyethylene oxide to be water-soluble. The water-soluble, non-ionic cellulose ether, which is soluble in an amount of at least 1 g/1 water at 20°C, preferably has a turbidity point of 35-80°C in a 1% aqueous solution. Such cellulose ethers may contain alkyl groups, such as methyl, ethyl, propyl and butyl groups; as well as hydroxyalkyl groups, such as hydroxyethyl, hydroxypropyl and hydroxybutyl groups. Especially pre¬ ferred cellulose ethers are alkylhydroxyalkyl cellulose, such as ethylhydroxyethyl cellulose, propylhydroxyethyl cellulose, methylhydroxyethyl cellulose and methylethyl- hydroxyethyl cellulose. Other suitable cellulose ethers are methyl cellulose and cellulose ethers having more than one type of hydroxyalkyl group, such as hydroxyethyl- hydroxypropyl, hydroxyethylhydroxybutyl, hydroxypropyl- hydroxybutyl and their equivalents containing methyl and ethyl groups as well. In the application of the invention, the high-poly¬ mer polyethylene oxide and the non-ionic cellulose ether are suitably dissolved in water and added in dissolved form to the process water or wastewater. The additions can be made continuously or batchwise. Good results are achieved if the additions are made simultaneously, although it is preferred to first add the non-ionic eel- lulose ether and then the high-molecular-weight polyethy¬ lene oxide. The difference in time between the additions is dependent on the design of the technical equipment, but generally amounts to 4-20 sec. If so desired, both the cellulose ether and the high-molecular-weight poly¬ ethylene oxide can be added in more than one place. More¬ over, it should be ensured that the two flocculation che¬ micals are properly admixed. The separation of the flocculated resin-containing products is preferably done be flotation by means of air, e.g. by froth flotation or microflotation, where small air bubbles lift the flocculated material to the surface. The resulting froth is then removed in conventional manner. It is also possible to remove the floes from the process water or wastewater by sedimentation or filtration. The invention will be further illustrated by the fol¬ lowing Examples. Example 1 To a process water (pH 5.0, temperature 70°C) origi- nating from a sulphite pulp and containing 15 g wood resin per litre process water were added, under vigorous agita¬ tion, a cellulose ether and/or a high-molecular-weight polyethylene oxide as flocculants according to the Table below. When the cellulose ether as well as the high-mole- cular-weight polyethylene oxide were used as flocculants, the polyethylene oxide was added about 10 sec after the cellulose ether. After the addition of the cellulose ether and/or the polyethylene oxide, only gentle agitation was performed, whereupon the mixture was left for 5 min to allow the flocculated material to settle. Samples were drawn from the supernatant and analysed for turbidity by means of a turbidimeter. Low turbidity means a good reduc¬ tion of colloidal and particulate substances. The follow¬ ing results were obtained. 1) A = low-viscosity ethyl hydroxyethyl cellulose, turbi¬ dity point about 69°C, viscosity about 1000 cP (1% solution, temperature 20°C) 2)B = methyl hydroxyethyl cellulose, turbidity point about 70°C. The results show that the use of flocculants accord¬ ing to the invention gives a very high reduction of col¬ loidal and particulate substances for a low content of added high-molecular-weight polyethylene oxide. Example 2 The same procedure was adopted as in Example 1, but with the exception that the water was wastewater having a temperature of 35°C and a pH of 7.2 and originating from the making of bleached CTMP pulp. The amount of suspended substances, TSS-GF/A, was determined at 870 mg/1 by filter¬ ing the wastewater through a Whatman filtering paper designated GF/A. The amount of wood resin was 0.9 g/1 water. After sedimentation, samples were drawn from the supernatant which was analysed in respect of suspended substances according to TSS-GF/A above. The following results were obtained. The results show that a combination of flocculants according to the invention provides surprisingly good purification. Example 3 The same procedure was adopted as in Example 1, but with the exception that process water having a wood resin content of 17 g/1 and originating from the making of sul¬ phite pulp was purified. The pH of the process water was 5.6, temperature 60°C and turbidity > 1000 NTU. The fol¬ lowing results were obtained. 1) C = high-viscosity ethyl hydroxy ethyl cellulose, turbi¬ dity point about 69°C, viscosity 5000 cP (1% solu¬ tion, 20°C). The results show that by using both a cellulose ether and a high-molecular-weight polyethylene oxide, it is pos¬ sible to achieve excellent purification with a relatively low content of the high-molecular-weight polyethylene oxide. Example 4 To process water having a pH of 11.2 and a resin con¬ tent of 490 mg/1 from the making of birch pulp was added sulphuric acid in an amount to bring its pH to 7.8. Then, a purification process was carried out at 70°C according to Example 1, ethyl hydroxyethyl cellulose C according to Example 3 and/or polyethylene oxide according to Example 3 were added as flocculants. After purification, turbidity was determined and the resin content was measured accord¬ ing to the method disclosed in SE 8403587-2. The following results were obtained. The results show that a combination of flocculants according to the invention has a very clear synergistic effect.

Claims: CLAIMS 1. A method for purifying process water or wastewater containing wood resin by adding a high-molecular-weight polyethylene oxide as flocculant, c h a r a c t e r i s e by also adding, in addition to the polyethylene oxide, a water-soluble, nonionic cellulose ether. 2. A method as claimed in claim 1, c h a r a c - t e r i s e d in that the turbidity point of the cellu¬ lose ether is 35-80°C. 3. A method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the cellulose ether is alkyl hydroxyalkyl cellulose. 4. A method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the cellulose ether is alkyl cellulose. 5. A method as claimed in claims 1-4, c h a r a c ¬ t e r i s e d in -that the content of wood resin of the process water or wastewater is at least 0.4 g/1 water. 6. A method as claimed in claim 5, c h a r a c ¬ t e r i s e d in that the content of wood resin is 10- 20 g/1 water. 7. A method as claimed in claims 1-6, c h a r a c - t e r i s e d in that the high-molecular-weight polyethy¬ lene oxide is added in an amount of 2-20 mg/1 and the non¬ ionic cellulose ether in an amount of 5-30 mg/1. 8. A method as claimed in claims 1-7, c h a r a c ¬ t e r i s e d in that the process water and wastewater have a temperature above room temperature, preferably 35-60°C. SUBSTITUTE SHEET

Inventors: ANDERSSON ARNE OLOV ROLAND, GOERANSSON GERT GUNNAR MAGNUS, WRETBORN MAGNUS JOHAN, ANDERSSON, ARNE, OLOV, ROLAND, GOERANSSON, GERT, GUNNAR, MAGNUS, WRETBORN, MAGNUS, JOHAN
Applicants: EKA NOBEL AB
Application year: 1993
Main classification: C02F1
IPC codes: C02F1, C02F9, D21C11