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Method for washing pulp in a bleaching line and a bleaching lineRelated Patent Categories: Paper Making And Fiber Liberation, Processes Of Chemical Liberation, Recovery Or Purification Of Natural Cellulose Or Fibrous Material, Treatment With Particular ChemicalMethod for washing pulp in a bleaching line and a bleaching line description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060113048, Method for washing pulp in a bleaching line and a bleaching line. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention relates to a method of bleaching cellulose pulp and a bleach line for the method. PRIOR ART [0002] It is a desire in multi-stage bleaching of cellulose pulp to reduce the water requirement and the quantity of outlet contaminated process water, which contaminated process water is either pumped to sewage, possibly via sedimentation basins and/or to costly destruction/deposition. [0003] With the object of reducing the liquor quantities, the bleaching department is ever more closed and most often the process water is lead in counter-current to the direction of flow of the cellulose pulp in the process steps of the bleaching line. Accordingly, the fresh water or the clean process water is used in the wash of the last step and the wash filtrate obtained there from is led as wash liquor to the wash of the preceding step and further up through the bleaching line. [0004] Commonly, a number of filtrate tanks are used between each bleaching step wash for this leading of wash filtrate through the process, with the object of guaranteeing supply of wash liquor and securing that the wash liquor is lead in counter-current to the flow of cellulose pulp. [0005] In addition to such filtrate tanks, expensive control and regulation systems with valves are required for this handling of the wash liquor in filtrate tanks, to monitor the levels in the filtrate tanks since the risk can not be taken that a filtrate tank is emptied whereby it may cause a stoppage in the wash of the bleaching step in question. [0006] The number or filtrate tanks also results in a risk of an increased outlet of odorous gases as all filtrate tanks require ventilation in order to level out changes in the volume in the filtrate tanks. Often, special degassing systems are required to handle and destruct such odorous gases. [0007] Accordingly, big advantages could be attained if the number of filtrate tanks between the process steps could be minimised. THE OBJECT AND PURPOSE OF THE INVENTION [0008] One object of the invention is to reduce the need of, and in some cases completely eliminate, such expensive filtrate tanks, control systems and valves in the wash liquor systems, whereby the investment costs for the bleaching line may be strongly reduced. The reduced number of necessary filtrate tanks also results in the possibility of a more compact and more optimal design of the bleaching line, without consideration of such filtrate tanks that conventionally numbers to at least the same number as the number of bleaching steps, with a more efficient layout of the bleaching steps of the bleaching line. [0009] Yet another object is to increase the runnability/accessability of the system as several control valves may be removed, that otherwise are always potential risks for plugging/stoppage of the liquor distribution system. [0010] Yet another object is to improve the runnability as the risk of mixing-in of air in the filtrate system is considerably reduced when the number of filtrate tanks can be considerably reduced. At the same time, accumulation of floating pulp is avoided, which floating pulp usually accumulates after a certain running time, by surface flotation in filtrate tanks. Principally in alkaline steps, such accumulated floating pulp may rise to a level of a few metres above the surface in the filtrate tanks and it must be continuously taken care of or recycled to the bleaching line in order not to risk plugging of the filtrate systems. [0011] Yet another object is that the bleaching plant can be rendered more environmentally friendly as occasional overloads in certain positions, so called over-runs, need not result in outlet of gas or liquor. [0012] Yet another object is to minimise the water consumption. [0013] By the system, the system itself may compensate for occasional changes in wash liquor requirements in the various bleaching steps and secure that a required wash liquor quantity is always guaranteed the bleaching steps. [0014] It is yet another object to minimise the energy consumption in pumps in the filtrate distribution system, where instead a pressurised filtrate main conduit is maintained and any required liquor quantity is drawn off form the main conduit, as needed. [0015] Yet another object is to decrease the length of the tube system, which reduces the costs of installation and the complexity of the system, whereby in the latter case the lucidity is also increased for the operators. [0016] A cost reduction of between 1 and 2 millions USD can be obtained for a 4-step bleaching line D.sub.0-EOP-D.sub.1-D.sub.2 with intermediate wash steps, if the invention is fully applied. LIST OF DRAWINGS [0017] FIG. 1 shows a conventional prior art bleaching sequence D.sub.0-EOP-D.sub.1-D.sub.2 in which the filtrate is led in counter-current, via filtrate tanks; [0018] FIG. 2 shows the same bleaching sequence D0-EOP-D1-D2 in which the filtrate is lead between the steps in accordance with the invention, according to an embodiment using a high pressure mode, [0019] FIG. 3 shows a part of the bleaching sequence as also shown in FIG. 2, but according to another embodiment of the invention where a low pressure mode is used, and, [0020] FIG. 4 shows a section of a further embodiment according to any of the bleaching sequences according to FIG. 2 or FIG. 3, wherein minor modifications have been performed. 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