Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1990-10-29
1991-05-07
Hruskoci, Peter
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210725, 210728, 210730, 210928, 162 29, 162 38, 162 39, C02F 154
Patent
active
050134540
DESCRIPTION:
BRIEF SUMMARY
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. Parameters such as COD (chemical oxygen demand), colour and BOD (biological oxygen demand) are taken into consideration in this comparison. 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 enabling still greater quantities of macro-molecular organic 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.sup.2 /g.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in more detail with reference to the accompanying drawing and some examples, wherein FIG. 1 shows schematically the flow schedule of an installation for removing dissolved and colloidal macro-molecular organic substances according 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 mechanical 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 displacement bleaching process is added to the mixer 5 and after a period of 6-10 minutes the mixture 7 of fibre sludge and effluent is transferred to a mixing trough 8. Here CaO is added and treatment is carried out for 3-5 minutes. A cationic polymer 11 is added before the mixture 9 containing 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 recirculated to the mixing vessel 1 for reuse of the fibres while the remainder 13, to which a polymer 14 may be added, is supplied to a centrifuge 15 for dewatering. The water 16 separated out is recirculated to the mixing vessel 1 while the filtrate 17 (fibres and organic material) is removed from the plant. The effluent 18 at the overflow 19 is conveyed to an equipment for biological treatment.
The following examples further illustrate the invention.
EXAMPLE 1
In this example bleached and unbleached kraft pulps were used as precipitant. The fibres therein were subjected to a beating operation to increase their surface area. The beating process broke and split op
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patent: 4812207 (1989-03-01), Gullichsen et al.
patent: 4842688 (1989-06-01), Gullichsen et al.
A. Ahlstrom Corporation
Hruskoci Peter
Shideler Krisanne
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