A METHOD OF REMOVING ORGANIC SUBSTANCES FROM EFFLUENTS FROM PULP AND PAPER MANUFACTURING PROCESSES

Abstract: A method of removing dissolved and colloidal macro-molecular organic substances from effluents from processes used in the manufacture of pulp and paper, wherein an acid sludge containing cellulose fibres and being acidified to a pH value of 1.6-2.5, is added to an effluent containing said organic substances, and the pH value is adjusted to 4.5-6.0 before the mixture is subjected to sedimentation to achieve precipitation of the fibres with said organic substances adhered thereto. According to the invention the fibres used as precipitant are subjected to mechanical treatment prior to said acidification, so that the fibres are broken and split open, increasing their specific fibre surface and thereby their ability to catch the organic substances in the effluent.

Description: A method of removing organic substances from effluents from pulp and paper manufacturing processes The present invention relates to a method of removing dissolved and colloidal macro-molecular organic substances from effluents from processes used in the manufacture of pulp and paper, wherein an acid sludge containing cellulose fibres and being acidified to a pH value of 1.6-2.5, is added to an effluent containing said organic substances, and the pH value is adjusted to 4.5-6.0 before the mixture is subjected to sedimentation to achieve precipitation of the fibres with said organic substances adhered thereto. The method described above is known through WO 86/03236 and WO 86/04888 and is particularly effective for reducing the lignin content in bleach plant effluents. The results obtained by using this method are equivalent to those obtained through chemical precipitation using chemicals such as aluminium sulphate. Para¬ meters such as COD (chemical oxygen demand), colour and BOD (bio- logical oxygen demand) are taken into consideration in this compa¬ rison. However, claims are being made for even more efficient cleaning of the effluents, particularly with respect to the lignin content. It is therefore an object of the invention to achieve an improved method enabl ng still greater quantities of macro-molecular orga¬ nic substances to be removed from the effluents. By the expression "macromolecular organic substances" it is meant organic substances having a molecular size mainly above 500. The novelty of the invention lies substantially in that the fibres used as precipitant are subjected to mechanical treatment prior to said acidification so that the fibres are broken and split open, increasing their specific fibre surface and thereby their ability to catch the organic substances in the effluent. Depending on the type of fibres, the mechanical treatment is performed so that the specific surface of the fibres is then generally greater than 1.3 m 2 /g. The invention will be described in more detail with reference to the accompanying drawing and some examples, wherein Figure 1 shows schematically the flow schedule of an installation for removing dissolved and colloidal macro-molecular organic substances accord¬ ing to the invention, from effluents from a displacement bleaching process. Referring now to the drawing, 1 denotes a mixing vessel for an acid phase of the process. An acid effluent 2 from the chlorine phase and dioxide phase of a displacement bleaching process is supplied to the mixing vessel 1, together with a sludge 3 of cellulose fibres obtained from a treated effluent from a pulp or paper mill. Before being supplied to the mixing vessel 1 the fibre sludge is subjected to a beating operation or some other mechani¬ cal treatment giving the fibres greater specific surface. After remaining in the mixer 1 for 2-4 minutes, the acid fibre sludge 4 is transferred to a mixer 5 for an alkaline phase of the process. An alkaline effluent 6 from said d splacement bleaching process is added to the mixer 5 and after a period of 6-10 minutes the mix¬ ture 7 of fibre sludge and effluent is transferred to a mixing through 8. Here CaO is added and treatment is carried out for 3-5 minutes. A cationic polymer 11 is added before the mixture 9 con¬ taining CaO is introduced to a sedimentation tank 10. The fibres with the organic material adhering to them are precipitated in the sedimentation tank. A portion 12 of this precipitate is recircula¬ ted to the mixing vessel 1 for reuse of the fibres while the re- mainder 13, to which a polymer 14 may be added, is suppl ed to a centrifuge 15 for dewatering. The water 16 separated out is recir¬ culated to the mixing vessel 1 while the filtrate 17 (fibres and organic material ) is removed from the pl ant. The effl uent 18 at the overfl ow 19 i s conveyed to an equipment for biol ogical treat¬ ment. The fol l owing exampl es further il lustrate the invention . Exampl e 1 In this example bleached and unbleached kraft pulps were used as precipitant. The fibres therein were subjected to a beating opera¬ tion to increase their surface area. The beating process broke and split open the fibres, giving them an irregular shape and split parts and thus considerably increasing their specific surface. Each suspension of beaten fibres was acidified with acid effluent obtained from a bleaching process, the acid effluent constituting 2/3 of the total volume of added effluents. The final pH value was between 1.8 and 2.1, the object of this pH adjustment being to increase the zeta-potential of the sludge to 0. 3 gram fibres, calculated as solid material, was used for every l tre of the total quantity of effluents. The quantity of fibres shall be from approximately half up to twice the quantity COD in the effluent to be treated. The acid fibre suspension was mixed with an alkaline effluent from the bleaching plant, which constituted 1/3 of the total volume of added effluents. Calcium oxide was added to adjust the pH value of the mixture to between 4.8 and 5.3. 2 milligrams per liter total effluent of a cationic polymer under the designation Allied Colloids Magnafloc 352 were also added. The mixture was then allowed to settle for two hours, after which samples of the pre¬ cipitate of fibres and organic substances adhered thereto were taken in order to determine the reduction in COD (chemical oxygen demand) as a function of the beating degree (freenes) expressed as °SR (Shopper-Riegler). Two test series were carried out with the difference that a in the mixture was removed in test No. 2 by means of an evacuating system whereas such an extra step was not carried out in test No. 1. Such air will be entrapped in the fibre sludge during the beat¬ ing operation and may reduce the settling ability of the fibres. An increased settling velocity was noted in test No. 2 compared with test No. 1. The results obtained by the tests with respect to the reduction of COD (chemical oxygen demand) are shown in Table 1. A higher freenes will give a greater fibre surface. It is evi¬ dent from these results that the COD reduction increased in rela¬ tion to the reference sample and the unbeaten sample in each serie in an unexpected way when the freenes and thereby the specific fibre surface increased. The influence of the beating on the zeta-potential, mV/cm, at various pH values was measured in the fibre sludges used and was found to have altered negligibly. Table I Example 2 In this example beaten and unbeaten (control) fibres from pulps were used as precipitant. Sludges of these fibres were prepared in tap water and acidified with sulphuric acid to a pH value between 1.8 and 2.0. The zeta-potent al of the suspensions was 0. 1 gram of fibres, calculated as solid material, was used for each litre of effluent to be treated. The acid fibre sludge was mixed with an ultra-filtered alkaline effluent from a displacement bleaching process. Calcium oxide was added to adjust the pH value to between 4.8 and 5.3. 2 milligrams of a cationic polymer under the designation Allied Colloids Magnafloc 352 were also added per litre effluent. The mixture was then allowed to settle for one hour, after which samples of the precipitate of fibres and organic substances adhered thereto were taken. The precipitate was filtered through a coarse filter paper. The specific surface of the fibres and the COD were determined stating the reduction in COD in comparison with the reference samples. The results are shown in the following Table II. The results thus show that beating the pulp to increase the fibre surface gives a substantial and unexpectedly improved precipita¬ tion of lignin from the effluent. During the mechanical treatment the fibres are broken and split open, thus becoming irregular in shape and opened up so that their specific surface is considerably increased. Microscopic examina¬ tion of the precipitate obtained by the method according to the invention showed that the macro-molecular organic material adhered to the fibres particularly at the break points or other surface irregularities in the fibres forming an irregular network. These break points and surface irregularities appear to function as growth centres for the macro-molecular organic material. Examina¬ tion also showed that the macro-molecular material was caught more efficiently by a tight network of the irregular fibres than by a thin network.

Claims: C L A I M S 1. A method of removing dissolved and colloidal macro-molecular organic substances from effluents from processes used in the manu¬ facture of pulp and paper, wherein an acid sludge containing cel¬ lulose fibres and being acidified to a pH value of 1.6-2.5, is added to an effluent containing said organic substances, and the pH value is adjusted to 4.5-6.0 before the mixture is subjected to sedimentation to achieve precipitation of the fibres with said organic substances adhered thereto, characterized in that the fibres used as precipitant are subjected to mechanical treatment prior to said aci ification, so that the fibres are broken and split open, increasing their specific fibre surface and thereby their ability to catch the organic substances in the effluent. 2. A method as claimed in claim 1, characterized in that the fibres are subjected to a beating operation or similar mechanical treatment. 3. A method as claimed in claim 1 or 2, characterized in that the fibres are obtained from a pulp or paper mill. 4. A method as claimed in any of the preceding claims, charac¬ terized in that after the mechanical treatment the fibres have a specific surface greater than 1.3 m 2 /g. 5. A method as claimed in any of the preceding claims, charac¬ terized in that the quantity of fibres added, calculated as solid material, is 1-20 grams per liter of the total quantity of treated effluent and constitutes 0.5-2 times the quantity of COD in the effluent to be treated. 6. A method as claimed in any of the preceding claims, charac¬ terized in that a strong acid is used to acidify the fibrous sludge. 7. A method as claimed in any of claims 1-5, characterized in that the fibrous sludge is acidified by means of an acid effluent from a bleaching process. 8. A method as claimed in claim 6 or 7, characterized in that acidification is performed so that the zeta-potential is 0 or as close to 0 as possible. 9. A method as claimed in any of the preceding claims, charac¬ terized in that adjustment of the pH value of the mixture to 4.5-6.0 is achived by the addition of an alkaline agent such as lime and/or by the effluent consisting of an alkaline effluent from a bleaching process. 10. A method as claimed in any of the preceding claims, charac¬ terized in that air entrapped during the mechanical treatment is removed from said mixture prior to sedimentation.

Inventors: HYNNINEN PERTTI, HYNNINEN, PERTTI
Applicants: KAMYR AB, KAMYR AKTIEBOLAG
Application year: 1988
Main classification: D21C11
IPC codes: C02F1, D21C11, D21F1