METHOD IN A PAPER MACHINE FOR ARRANGEMENT OF ITS WATER CIRCULATION

Abstract: The invention concerns a method in a paper machine for arrangement of its water circulation. In view of full or partial closing of the water circulation in a paper machine, the wash waters from the fabrics and from other devices in the paper machine as well as the waters drained from the paper web to be produced are recovered selectively based on the place of origin of the waters. At least a part of said recovered different waters are cleaned, and the cleaned waters are recirculated to applications of reuse suitable in view of their washing potential in the paper-making process.

Description: Method in a paper machine for arrangement of its water circulation The invention concerns a method in a paper machine for arrangement of its water circulation. Paper and board grades are produced in a paper machine by draining water out of the mixture of water and wood fibers prepared in a stock preparation plant and by using constantly moving fabrics in the different parts of the paper machine, which parts are mainly called the wire part, the press section, and the dryer section. The fabrics in a paper machine are permeable members which form a closed loop and which have been made of a plastic and/or of a metal material and/or which are felts consisting of natural and/or synthetic fibers. The fabric loops are rotated constantly by means of drive rolls or by means of some other equipment. During the draining of water the fabrics are contaminated by materials which come from the paper web and from the different process waters. In order that the fabrics and the elements in the paper machine, such as rolls, doctors, forming ribs, suction boxes, etc. , should operate well, they must be washed constantly by means of water jets, and the wash water must be removed. The wash water from the fabrics is contaminated, but it can, however, be used as circulation water in the paper machine. In the present-day paper mills, an abundance of fresh water is needed for cooling and, after that, among other things, for the above washing requirements in the wire part and in the press section and for dilution in the stock preparation plant. After the wire part and the press, these waters are passed mainly to among the fibrous circulation waters. Any excess amount of circulation water is disposed of as waste water. The net amount of fresh water that is needed for the washing jets in a paper machine is of an order of 10 cubic metres per ton of paper produced. Thus, from a paper mill, an abundance of warm waste water is obtained, which must be cleaned, for example biologically, and, if necessary, cooled before the cleaning. As is known from the prior art, the wash jet waters in a paper machine are collected by means of various basins and troughs and passed into the circulation water system. Besides fresh water, circulation water of the paper machine is also employed as the jet water in the wire part and the press section. The circulation water is, as a rule, cleaned by means of filters, whose screen measure is about 150 μ (corresponds to 100 mesh). Such a screen measure, however, permits the passage of fine particles and dissolved material. A clear filtrate obtained from such a filtering device still contains finer particles and dissolved material. These impurities cause blocking of jet nozzles and their structures and contamination of said devices and of other equip- ment out of the disturbing materials in the water system in a paper machine, which results in negative effects in the quality and production of paper. The use of such water as additional substitute for fresh water would risk the operation of the equip¬ ment and the production. Therefore, for more demanding washing of the fabrics and parts of a paper machine, fresh water is used, which is mostly chemically cleaned. Fresh water is cold, and it must be heated to a considerable extent to the operating temperature that is required in the papermaking. The temperature of new fresh cold water must be raised, for example, from about 7 °C to about 50 °C, and usually it is treated chemically in order to remove humus materials and colour, in compliance with the quality requirements, and its use involves high costs of processing. The high cost of cleaning of fresh water and waste water arises from the fact that an abundance of fresh water must be introduced into the process constantly. Fresh water which is used in the jets in a paper machine and which has been treated chemically also increases the concentrations of inorganic materials in the system. The primary object of the present invention is lowering of the consumption of fresh water in a paper machine. More specifically, the object of the present invention is to improve the papermaking process from the points of view of burdening of the environment and of the require¬ ment of fresh water. In particular, the object is optimal cleaning of the various contaminated waters present in a paper machine by interconnecting different cleaning devices in a novel way and by using cleaned waters and the concentrates coming from the cleaning devices in an optimal way in consideration of their degree of purity and their washing potential. An object of the invention is to provide novel overall solutions of technology, which solutions, at the same time as they reduce the burdening of the environment in the form of lower consumption of fresh water and lower quantities of waste water, also provide economies of energy and reduce the consumption of chemicals in the different parts in a paper mill. The invention is characterized in that in view of full or partial closing of the water circulations in a paper machine, the wash waters from the fabrics and from other devices in the paper machine as well as the waters drained from the paper web to be produced are recovered selectively based on the place of origin of the waters, and that at least a part of said recovered different waters are cleaned, and the cleaned waters are recirculated to applications of reuse suitable in view of their washing potential in the papermaking process. Preferred embodiments of the invention have the characteristic features as claimed in the sub-claims. It has been estimated that the cleaning and evaporation of process waters optimally in a plant integrated in accordance with the present invention lowers the burdening of the environment considerably and is more advantageous compared with abundant use of fresh water with the resulting high requirement of cleaning. The trough waters from the wash jets in a paper machine are, on the average, cleaner than the waters of the short cycle in a paper machine. In the prior art, all of these wash waters were mixed with fibrous circulation waters after their use, but in a preferred embodiment of the present invention the semi-clean fabric conditioning water coming from the formers and presses is cleaned and used in a novel way. The waters that clean the wires and felts have not been contaminated to the level of contamination of wire water, so that these waters still have a washing potential etc. potential of use, which are utilized in the present invention. In the present invention, this washing potential still possessed by said relatively clean waters is utilized. In the present invention, the waters that are collected selectively in accordance with the place of origin can also be cleaned more readily. In respect of the waste waters, in the present invention a what is called sorting based on the place of origin is applied. For example, water from conditioning of fabrics in a paper machine can be collected and taken for useful use. In this way the use of chemically pure water can be reduced. It is an advantage of the invention that chemically purified fresh water is not needed as equally large quantities as in the prior art for jet water in a paper machine. For example, the jet waters can be cleaned by means of cleaning systems of their own so that a paper machine is obtained which requires a smaller amount of fresh water. In a preferred embodiment of the invention, the waters recovered by means of selective collecting of wash waters can be cleaned within the limits that are set by the washing potential either by cleaning the waters that were collected selectively or even by using said waters without cleaning. Disturbing materials arrive along with the stock, and the papermaking process produces more such materials. These disturbing materials must be washed along with the water out of the water circulation. In selective collecting in accordance with a preferred embodiment of the invention, the water departing from a paper machine is divided into at least two parts, of which one part is cleaner than the other. In such a case the short cycle in the paper machine can be operated less clean than in the prior art. Paper mills in accordance with the present invention have a possibility to choose the amount of fresh water that is used. In the present invention, the levels of disturbing materials in a paper machine depend on how large a proportion of the waters in fresh water jets is replaced and on the sort of the replacement water used. In the present invention, there is a novel process arrangement for selective collecting of jet waters, which jet waters are used for cleaning of elements and fabrics, and the collected mixture is cleaned for different sorts of reuse in the production of pulp and paper by using novel combinations of the following separation technologies: 1) Flotation followed by microfiltration in an area in which the pore size of the filtering medium is, for example, 50 μ ... 10 μ. This cleaning combination can be used alone or as a pre-processing stage for subsequent separation technologies. The separated solid matter can be passed to reuse in the production of pulp and paper, or it can be passed to waste water treatment. 2) Membrane technology in the levels of ultrafiltration osmosis, nanofiltration osmosis or inverse osmosis. The concentrate from the membrane separation can pass to various reuses in the production of pulp and paper or to evapor¬ ation in the processing of waste water. 3) Evaporation technology by means of vacuum and/or compressor evaporation. The steam in the vacuum evaporation can be low-pressure steam from the power plant or recovered waste heat from the manufacture of paper or mechanical pulp. To the evaporation, at the same time, various other waters can be fed from the production of paper and pulp. After the jet water has done its job, it can be collected selectively by means of novel constructions and arrangements in the process and be passed into the cleaning system, which consists of the combination of separation technologies mentioned above. Since selectively collected water contains a smaller amount of solid matter and dissolved organic and inorganic material than the more contaminated circulation water of a paper machine, the water can be cleaned more readily and with a higher capacity directly after the washing duty than if it were fully mixed with the circula¬ tion water in the paper machine. The cleaned water can be passed to the jets or different other uses of a paper machine as of a better quality than the circulation waters. The method of the invention does not result in the prior-art problems mentioned on page 2, in which circulation waters of the paper machine are used as jet waters. A higher proportion of the jets can be connected to the cleaned waters obtained from the separation technologies and from their said combinations. Thus, the amount of fresh water needed in the paper machine is reduced. The flotation filtrate is then cleaned by means of microfi Itration. Since the flotation removes most of the solid matter, the flow resistance for microfiltration is lowered to such an extent that the pore size in the filtering medium can be as little as 10 μ and, nevertheless, a satisfactory hydraulic capacity is achieved. In the following, the invention will be described in more detail with the aid of the drawings, to whose details the invention is not supposed to be confined. Figure 1 shows a prior-art paper machine. Figure 2 shows a paper machine in accordance with the invention, which is an improvement over the paper machine shown in Fig. 1. Figure 3 shows a second prior-art paper machine. Figure 4 shows a paper machine in accordance with the invention, which is an improvement over the paper machine shown in Fig. 3. Figure 5 shows a preferred embodiment of the invention related to the paper machine shown in Fig. 4. Figure 6 is a more detailed illustration of selective collecting of the waters from the wire part. Figure 7 is a more detailed illustration of selective collecting of the waters from the press section. In the prior-art solution shown in Fig. 1 the jet water that is used is mainly fresh water, which has been passed to the paper machine along the ducts 1 , 1',! " , 13, 13' , 13". Jet water is needed both in the wire part 4, to which it is passed along the duct 7" , and in the press section 5, to which it is passed along the ducts 7,7'. Fresh water is heated to the required temperature by means of the heat exchangers 88,88' and passed further into the warm fresh water tank 2 along the duct 1 " . In the prior art, a certain amount of cold fresh water is also used as make-up water for the cooling tower 100. This water is passed along the duct 1 ' to the water circu¬ lation of the cooling tower. Water to be cooled in the cooling tower 100 is lost in the form of humid air, and some water passes then into the sewer along the duct F. The circulation waters from the press section 5 are passed into the circulation water tank 31 along the ducts 9,9' . The wash waters and the conditioning waters pass into the sewers V. Wash and circulation waters enter into the circulation water tank 31 along the ducts 30,30' also directly from the wire part 4. Circulation waters 4 from the wire part are also passed into the wire pit 7, from which they are passed into the circulation water tank 31 along the duct 30' and as circulation water along the duct 38. Stock broke is passed through the couch chest C into the broke system along the duct D. The water from the circulation water tank 31 is passed along the duct 38' as circulation water and/or into the broke system D. This circulation water is passed, after various treatments (which are not shown), along the duct 6 to constitute jet water in the wire part 4 and in the press section 5. Fig. 2 shows an embodiment in accordance with the present invention as applied to an environment as shown in Fig. 1. In Fig. 2, conditioning waters and circulation waters are collected from the press section 5 and passed along the ducts 3,3' into the conditioning water tank 82, and not into the circulation water tank 31 , which is the case in Fig. 1. Also from the wire part 4, conditioning waters are collected along the duct 3" and passed through the wash water tank 82 to flotation cleaning 57. In the flotation cleaning 57, first a coarse cleaning takes place in the screens 83, after which the water is cleaned in the flotator 57. The clean water is then passed through the screen 96 into the clarification tank 84 and further to microfiltration 57' along the duct 85. After this the clean water is passed through the microfiltration tank 86 to the ultra-membrane filter 57" along the duct 59 and/or to nano-membrane filtration 57'" along the duct 59' and from there into the clean- water tank 87. An overflow from the microfiltration tank 86 is passed to among the outlet waters of the clarification tank 84 along the duct 86'. Fresh water enters into the clean- water tank 87 along the duct 13 from the clean-water tank 2. The regulators 103,104 operate together so that the supply of water into the tank 87 in the paper machine is secured. The excess amount of warm water is passed to other use (not shown). Part of the fresh water is, however, also passed directly into the warm water tank 87 along the duct 1. Into the warm clean-water tank 87, water also arrives from the evaporator 76 along the duct 80. From the warm water tank 87 water is passed through the heat exchanger 88 into the hot water tank 89 along the duct 13' to constitute jet water for the press section 5 and for the wire part 4. Part of the cleaned waters are passed from the microfiltrations and ultra-membrane filtrations etc. away along the ducts 90 to constitute conditioning and wash waters and later along the duct 6 to the wire part and the press section. After the ultra- membrane filtration the concentrate passes along the duct 62, and after nano- membrane filtration along the duct 62' , to the concentrate tank 91 and from there to among the process waters along the duct 92. The process water passes through a fine screen 93, and after the fine screen 93 the reject passes into the sludge tank 94, from which the sludge passes further along the duct 95 to sludge treatment. To the sludge tank 94 the sludges are also collected from the flotator 57 and from the fine screen 96 and from the microfiltration 57' along the duct 94'. From the fine screen 93 the filtrate is passed through the feed tank 97 to the evaporator 76. The concentrates from the evaporator or evaporators are passed away along the duct 98. The clean condensate is passed along the duct 80 into the warm water tank 87. The vapour from the evaporator passes to the condenser 99, to which cooling water arrives from the cooling tower 100, and the temperature of the cooling water is regulated by means of a regulation device 59. The water that has become warm in the condenser is passed to the top portion of the cooling tower 100 along the duct 101. It is one embodiment and idea of the invention that, after the cooling tower 100, fresh water is passed to the warm- water tank 2 for fresh water along the duct 102 through the condenser 99. Fig. 3 shows a second prior-art paper machine. In it corresponding parts are denoted with the same reference numerals as in Fig. 1. In the paper machine shown in Fig. 3 the raw-material comes from the stock tank M first to the wire part 4 and after that to the press section 5 for dewatering. In the wire part 4 and in the press section 5 the wires and the machine parts are cleaned by means of water jets, to which water is passed along the ducts 6,7. In this prior-art paper mill the conditioning and wash waters are passed into a common wire pit along the duct 8, and from there a smaller proportion to the sewer for further processing, for example biological purification B, along the duct 10. Mainly, these conditioning waters have been combined with the circulation waters, which is shown to take place along the duct 9 in Fig. 1. From the tank 34 the waste waters are passed along the duct 10 to treatment, e.g. biological treatment B, and further away along the duct 11, and the sludge is passed along the duct 12. Fresh water is passed to the paper machine along the duct 13 to constitute jet water for the wire part and for the press section along the duct 7. Further, the supply of fresh water is also shown for other needs besides for the wet end of the paper machine, for example for dilution of chemicals etc. , to which uses fresh water is passed in this figure along the duct 16. The circulation waters of the wire part enter along the duct 30 into the tank 31 and are passed along the duct 32 to the disk filter 33, and from there the filtrates pass into the tank 34 along the ducts 35 and 36. The cleanest filtrate from the disk filter 33 is used as jet water in the wire part 4, to which it is passed along the duct 6. From the wire pit 31 dilution water is taken into the stock after the mixing tank 37 along the duct 38. The raw-material runs through the processing stages 37,39,40,41 (stock dilution, screening, rotary cleaning, removal of gases) before the raw-material enters into the wire part 4. From the tank 34, part of the waters is passed along the duct 42 as dilution water for the stock arriving from the stock preparation plant. Generally speaking, depending on the particular paper mill, the fabric conditioning waters from a paper machine are, in the prior art, either passed back into the circulation water system or passed into the sewer. If the fresh-water jets in a paper machine are not sufficient to keep the water circulations in the machine clean enough, additional fresh water can, as a rule, be supplied directly into the short cycle in the paper machine. Fig. 4 shows an embodiment of the invention as applied to a prior-art environment as shown in Fig. 3. In the embodiment shown in Fig. 4, the waters used for conditioning of the fabrics are passed along the duct 56 for cleaning to the flotator 57, after which the water is cleaned further by means of microfiltration 57', to which it is passed along the duct 85, in which cleaning stages suspended solids and colloidal agents are removed from these waters, and a part of the waters thus cleaned is passed as jet waters for the paper machine along the duct 58, which duct is connected to the duct 6, along which circulation waters are passed to the jets in the wire part 4. After the microfiltration, water can be favourably cleaned further by means of ultrafi Itration and/or nanofiltration 57" , to which it is passed along the duct 59. The clean fraction from the ultrafi Itration and/or nanofiltration is passed along the duct 60 to substitute for fresh water, and the concentrate is passed to the waste waters along the duct 62 and from there further, for example, to biological purifica¬ tion B. The duct 62 communicates with the duct 10. The rejects from the cleaning stages 57,57' are passed along the ducts 49" and 49"' , which communicate with the duct 49 passing to the sludge press L. Fig. 5 shows an embodiment in which the sludges from the cleaning stages 57,57' and 57" and the rejects coming from the short circulation are passed into the sludge press L, from which the sludge is passed to the sludge treatment, and the filtrate is cleaned further by means of a second flotator 74 and by means of subsequent microfiltration 74', to which it is passed along the duct 75. The sludge from the flotation and from the microfiltration 74' is passed again into the sludge press L along the duct 78 and along the duct 49, and the filtrate is passed to evaporation 76. A substantial proportion of the fresh water passed to the paper machine is substituted for by the cleaned water obtained from the evaporation device 76, and said cleaned water is passed along the duct 80 to the fresh-water tank 81. Clean water obtained from the evaporation plant 76 is also passed to other use along the duct 16. The concentrate of the evaporator 76 is passed along the duct 77 away, and the contami¬ nated condensate is passed along the duct 77' further to cleaning. In Fig. 5 the duct 77" is also shown, along which duct any other process waters whatsoever can also be passed to the evaporator. Fig. 6 is a detailed illustration of selective collecting of the wash waters from the wire part 4. Circulation water which has been cleaned arrives as jet water in the wire part along the duct 16, which is branched into the ducts 16' and 16". Along the duct 16' the circulation water passes into the jet pipe 105 to constitute wire condi¬ tioning and wash water. Along the duct 16" the circulation water passes preferably into the other two jet pipes 106 and 107 to constitute wash and conditioning water. The paper web is denoted with the reference R. After washing and conditioning the waters are collected in the trough 108. In the figure the doctor 109 is shown, and the wash water used for its lubrication drains into the trough 108. Fresh water is also passed as conditioning water along the duct 110 and further into the jet pipe 111 and into the jet pipe 112, in which it is used for wash and conditioning requirements. The wash waters coming from the fresh water jets are collected by means of the water collecting equipment 113 and 114 and passed further into the wash water tank along the duct 3" . The wash waters coming from the circulation water jets pass along the duct 30 into the circulation water tank (not shown). Thus, Fig. 6 is a detailed illustration of the collecting of waters in the wire parts in Figs. 2, 4 and 5. Fig. 7 illustrates selective collecting of the press waters. Circulation water flows along the duct 6' into the jet pipe 115 and 116 in the press section. In the figure, the press felt is denoted with the reference numeral 117. The conditioning waters coming from circulation water jets are collected by means of the troughs 118 and 119 of the doctors 120 and 121 and further along the ducts 3' and the duct 3 into the circulation water tank. Fresh water is also used in the press section as conditioning water, which is passed along the duct 7. The water passes along the ducts 123 into the jet pipes 124 and is collected by means of the trough 125 and passed further into the wash water tank along the ducts 126. The figure also shows felt conditioning devices 127. Thus, Fig. 7 is a detailed illustration of the collecting of wash waters in the press section in Figs. 2, 4 and 5.

Claims: Claims 1. A method in a paper machine for arrangement of its water circulation, charac¬ terized in that in view of full or partial closing of the water circulations in a paper machine, the wash waters from the fabrics and from other devices in the paper machine as well as the waters drained from the paper web to be produced are recovered selectively based on the place of origin of the waters, and that at least a part of said recovered different waters are cleaned, and the cleaned waters are recirculated to applications of reuse suitable in view of their washing potential in the papermaking process. 2. A method as claimed in claim 1 , characterized in that, in the wet end of the paper machine, the circulation waters removed out of the paper web and the waters used for conditioning of the fabrics and for washing of other devices and/or for lubrication and/or for sealing are collected separately, and that said circulation waters are passed, in a way in itself known, back to the short cycle of the paper machine, and that the waters used for conditioning of the fabrics and for washing of other devices are passed to cleaning, where any solid matter and colloidal materials suspended in these waters are removed from them by for the cleaning using flotation and/or microfiltration (Figs. 4 and 5). 3. A method as claimed in claim 1 or 2, characterized in that, from said cleaning stage, part of the waters is passed to circulation water jets and/or to other applica¬ tions of use in the paper machine, such as for dilution water for papermaking chemicals, and possibly part of said waters is passed through membrane separation, preferably through an ultra/nano filtration stage, to constitute jet waters for the paper machine, which jet waters are in the prior art exclusively composed of circulation water and/or fresh water supplied to the paper machine, which fresh water is chemically purified water and/or water taken from a waterway. 4. A method as claimed in any of the claims 1 to 3, characterized in that circula¬ tion waters removed from the paper web are also cleaned. 08 14 5. A method as claimed in any of the claims 1 to 4, characterized in that the concentrate obtained from said membrane separation stage is passed into an evapor¬ ator, the clean water obtained from said evaporator being passed to the fresh water jets in the paper machine and the contaminated fractions being passed to further processing (Figs. 2 and 3). 6. A method as claimed in any of the claims 1 to 4, characterized in that the concentrate obtained from said membrane separation is passed to among the waste waters (Fig. 4). 7. A method as claimed in any of the claims 1 to 4, characterized in that the sludge obtained from said flotation/micro-filtrations is passed into a sludge press, whose filtrate is cleaned by means of flotation and by means of subsequent micro¬ filtration (Fig. 5). 8. A method as claimed in claim 7, characterized in that the filtrate from micro¬ filtration is passed into an evaporator, the clean water obtained from said evaporator being passed to the fresh water jets in the paper machine and the contaminated fractions being passed to further processing (Figs. 2 and 5) and the sludge from the flotation/ micro-filtration being again passed into the sludge press. 9. A method as claimed in claim 2, characterized in that the screen measure of the microfiltration is preferably 50...10 μ. 10. A method as claimed in any of the claims 1 to 9, characterized in that cold fresh water is used as cooling water in the condenser and, after it has become warm, is recirculated to the warm- water tank for fresh water, from where it is passed, as required, to other applications of use of fresh water in the paper machine. 11. A method as claimed in claim 10, characterized in that the amount of water passing from the tank (2) into the sewer is kept at the minimum by means of regulation devices (54,61,61 ') so that the quantities of fresh water passed into the process and condensing in the paper machine become smaller. 12. A method as claimed in claim 11 , characterized in that cleaned water is passed from the evaporator into the warm- water tank. 13. A method as claimed in any of the claims 1 to 12, characterized in that the collecting of the wash waters from the wire parts has been arranged so that therein cleaned circulation water (clear filtrate) is used as the jet water, and this water is collected by means of troughs of its own and passed into the circulation water tank. 14. A method as claimed in any of the claims 1 to 13, characterized in that cleaned fabric conditioning water and warm fresh water are also used as jet water in the wire part, and these conditioning and wash waters are collected separately by means of troughs of their own and passed into the wash water tank, after which the water is cleaned in accordance with the invention by means of flotation and subsequent microfiltration and possibly by means of membrane technology. 15. A method as claimed in any of the claims 1 to 12, characterized in that in the press section cleaned circulation water (clear filtrate) is used as wash and condition¬ ing water, which is collected into its own troughs to be passed to circulation water. 16. A method as claimed in any of the claims 1 to 15, characterized in that cleaned fabric conditioning water and warm fresh water are also used as jet water in the press section, which waters are collected by means of troughs of their own and passed into the wash water tank, after which the water is cleaned by means of flotation/micro-filtration and possibly by means of subsequent membrane technology.

Inventors: GARTZ RAINER, KAIJALUOTO SAKARI, KARLSSON MARKKU, SUONPERAE ANTTI, GARTZ, RAINER, KAIJALUOTO, SAKARI, KARLSSON, MARKKU, SUONPERAE, ANTTI
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1997
Main classification: D21F1
IPC codes: D21F1