METHOD OF DRAINING WATER FROM LOW-CONSISTENCY WATER-FIBRE SUSPENSIONS

Abstract: A method of draining water from low-consistency water-fibre suspensions. In the method, the water-fibre suspension is fed to a filtering surface comprising holes. In the method, the rougher fibres are first separated from the water-fibre suspension and fed to the filtering surface to form a separate filtering layer and the water-fibre suspension is led to flow through said filtering layer formed by the rougher fibres.

Description: METHOD OF DRAINING WATER FROM LOW-CONSISTENCY WATER-FIBRE SUSPENSIONS The invention relates to a method of draining water from low- consistency water-fibre suspensions, in which method the water-fibre suspension is led to a filtering surface comprising holes, through which holes water, short fibre material and possible particle-like solids can flow. In paper-making processes, there are situations in which a fibre- containing water suspension is processed, whose consistency is low, i.e. the quantity of fibre stock is small, the suspension also contains a great deal of short fibres and possibly also particle-like solids derived from filling agents. Concentrating this kind of water suspension is quite difficult and in practice, not possible to achieve efficiently by means of the present thickeners or filters. This type of water suspension is obtained for instance by thermo-mechanical processes, whereby the freeness level of the obtained stock is approximately 70-80 ml and its consistency is low, typically approximately 1 %. Further, the same situation occurs in processing process water recycled in paper-making processes where when dewatering the web during web formation, short fibres in particular and particle-like solids used in paper-making are left in the water. It is an object of the present invention to provide a method by which short fibres and/or filling agents can more efficiently than before be filtered from the water-fibre suspension, thus producing a cleaner than before filtrate and circulating water for the process. The method of the invention is characterized in that first the rougher fibre material is separated from said water-fibre suspension, said separated rougher fibre material is led to a filtering surface to form a separate filtering layer, and the water-fibre suspension is fed through said filtering layer, whereby at least a part of the short fibre material and possible particle-like solids remains in said filtering layer formed by the rougher fibres. The essential idea of the invention is that the rougher fibre material is first separated from the water suspension and fed to a thickener as a filtering layer and the suspension to be thickened is fed to the thickener or filter in such a manner that it has to flow through the filtering layer thus formed, whereby the fines and the particle-like solids remain in the filtering layer without substantially passing through the filtering surface. This type of rougher fibre material preferably has a freeness value of 200-400 ml, in which case it forms a sufficiently dense layer but still has a good water permeability. The essential idea of a preferred embodiment of the invention is that two filtering layers are formed of the rougher fibre material, and the suspension is fed between them. The invention provides the advantage that with a simple supplementary apparatus, it is possible to form of one and the same water- fibre suspension being processed a filtering layer which aids thickening by retaining fine fibres and possible particle-like solids very efficiently, making it thus possible to obtain from the thickener a filtrate whose solids content is even below 30 mg/litre. This way, sufficiently clean filtrate water, i.e. one containing only a small amount of fibre material and particle-like solids, is obtained and can be used at various stages of the paper-making process. The invention is described in greater detail in the appended drawing in which Figure 1 shows a schematic of the method of the invention, Figure 2 shows a detailed schematic of filtering, and Figure 3 shows a detailed schematic of filtering according to another embodiment of the invention. Figure 1 shows an inlet channel 1 along which the low-consistency water-fibre suspension being processed is fed into the equipment and led to a filter, such as a thickener 2. Part of the water-fibre suspension, in this case approximately 10%, is led from the inlet channel 1 along a channel 3 to a separate washer 4 where rougher fibres having a freeness value of approximately 300-400 ml, for example, is separated. Depending on the used equipment or the required investments, the rougher fibres can also be separated directly from the inlet water-fibre suspension, in which case only a required quantity of the rougher fibres is separated and the excess can be returned to the water-fibre suspension. The filtrate, the fibres possibly remaining in it and the particle-like materials separated in the washer 4 are led back to the inlet channel 1 , and the rough fibre material separated in the washer 4 is led to a filter, such as the thickener 2, along a channel 5. The rough fibre material is fed into the thickener 2 in such a manner that it forms a filtering layer on the filtering surfaces of the thickener 2, through which layer the water-fibre suspension coming from the channel 1 must flow to pass through the filtering surfaces of the thickener 2. The fine, i.e. short, fibre material and particle-like solids in the circulating water then remain in the rough fibre layer being substantially unable to pass through it, and the filtrate water obtained from the thickener 2 is quite clean. The main part of the filtrate water is led along a channel 6 back to the paper-making process, but a part of it can be returned from the washer 4 along a channel 7 to the thickener 2 to facilitate the feeding-in of material to the channel 5 in which the rough fibre material flows. The filtrate material accumulated on the filtering surfaces of the thickener 2 as well as the fines and particle-like solids filtered to it are removed from the process or led to further processing. Figure 2 shows a schematic of a detail of the filtering performed using the method of the invention. The figure shows a filtering surface 8 which is known per se from different filters and thickeners and whose details need, therefore, not be explained herein. The filtering surface 8 is made of a material which has holes so that a fluid, such as water, and part of the material in the fluid can flow through the holes of the filtering surface from one side of the filtering surface 8 to the other. According to the invention, the filtering surface 8 has a filtering layer 9 formed by the rougher fibre material, the filtering layer forming on top of the filtering surface 8 a denser layer having considerably smaller holes. The water-fibre suspension being filtered or thickened is fed to a filter or thickener to the side of the filtering layer 9 formed by the rougher fibre material, which means that the fibre material in it, including the fine, i.e. short, fibres and possible particle-like solids, cannot pass through the filtering layer, but remain on it while water flows through both the filtering layer 9 and filtering surface 8. This way, the filtrate water obtained from the filtering or thickening contains only small quantities of fibres or particle-like solids, and is thus quite clean for use in different stages of the paper-making process. Correspondingly, the fibre material and possible particle-like solids in the water-fibre suspension can be retained and forwarded to further processing depending on the situation or, if required, removed from the process. Figure 3 shows a schematic of another filtering used with the method of the invention. In this case, two filtering surfaces 8 are used to form a gap-like structure. This kind of gap filter formed by means of two filtering surfaces, such as continuous wires, is generally known and obvious to a person skilled in the art, which is why they need not be described in more detail herein. In this embodiment, a filtering layer 9 of the rougher fibre material is formed on both filtering surfaces 9 in such a manner that the filtering layers 9 are between the filtering surfaces 8 facing each other. The water-fibre suspension is then fed into the gap between the filtering layers 9 in such a manner that water flows through both filtering layers to both directions passing through the filtering surfaces while fibre material and possible particlelike solids accumulate on both filtering layers. This produces an efficient filtering/thickening. In the equipment shown in the figure, the filter/thickener 2 and the washer 4 can be any apparatus suited for the purpose as long as the desired rough-fibre separation is achieved by them and a sufficiently clean filtrate is correspondingly produced for use in the paper-making process by leading it through the filtering layer formed by the rough fibres.

Claims: CLAIMS 1. A method of draining water from low-consistency water-fibre suspensions, in which method the water-fibre suspension is led to a filtering surface (8) comprising holes, through which holes water, short fibre material and possible particle-like solids can flow, and a separate filtering layer (9) of the rougher fibre material is formed on the filtering surface and the water-fibre suspension is fed through said filtering layer (9), whereby at least part of the short fibre material and possible solids is retained in the filtering layer (9) formed by the rougher fibres, characterized in that first the rougher fibre material is separated from said water-fibre suspension and said separated rougher fibre material is led to the filtering surface (8) to form said separate filtering layer (9). 2. A method as claimed in claim ^characterized in that for forming said filtering layer (9), fibre material having a freeness value of approximately 200^400 ml is separated from the water-fibre suspension. 3. A method as claimed in claim 1 or 2, characterized in that for draining water, a thickener (2) having a gap-like structure formed by two filtering surfaces (8) is used, said filtering layer (9) formed by the rougher fibres is formed on both filtering surfaces (8), the filtering layers face each other, and the water-fibre suspension is fed between the formed filtering layers (9) in such a manner that water drains through both filtering layers (9) and filtering surfaces (8). 4. A method as claimed in any one of claims 1 to 3, characterized in that to separate the rougher fibres, only part of the water-fibre suspension is led to a separate separating apparatus and after the separation of the rougher fibres, the remaining water-fibre suspension is returned to the process for draining water from it along with the rest of the water-fibre suspension.

Inventors: JUUTINEN VESA, JUUTINEN, VESA
Applicants: JUUTINEN VESA, METSO PAPER INC, JUUTINEN, VESA, METSO PAPER INC.
Application year: 2000
Main classification: D21F1
IPC codes: B01D37, D21C9, D21D5, D21F1