Abstract:
A method in a paper machine for arrangement of water circulation therefor and arrangement of a water circulation system in a paper machine. In view of full or partial closing of the water circulations in a paper machine, the wash water flows from the fabrics and from other devices in the paper machine as well as the water drained from the paper web to be produced are recovered selectively based on the place of origin thereof in the paper machine. At least a part of the recovered different water flows are cleaned, and the cleaned water flows are recirculated to applications of reuse suitable in view of their washing potential in the papermaking process.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method in a paper machine for arranging its water circulation and an arrangement for the water circulation in the paper machine. 
     BACKGROUND OF THE INVENTION 
     Paper and board grades are produced in a paper machine by draining water out of a mixture of water and wood fibers prepared in a stock preparation plant and by using constantly moving fabrics in the different principal parts of the paper machine, which parts are typically called the wire part, the press section, and the dryer section. The fabrics in a paper machine are usually permeable members which form a closed loop, i.e., are endless, and which are made of a plastic and/or 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 some other comparable equipment. During the draining of water from the mixture, 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., may operate satisfactorily, they must be washed constantly or at least periodically by means of water jets, and the resultant 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 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 section, these waters are passed mainly to mix with 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 about 10 cubic meters 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 known in 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 usually cleaned by means of filters having screens whose measure is about 150μ (corresponding to about 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 may cause blocking of jet nozzles and their structures and contamination of these devices and other equipment 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 an additional substitute for fresh water would risk damage to the efficient operation of the equipment and the production of paper. Therefore, for more demanding washing of the fabrics and parts of a paper machine, fresh water is used, which is often chemically cleaned prior to actual use in the paper machine. 
     Fresh water is generally cold, and it must be heated to a considerable extent, often to the operating temperature that is required in the papermaking process. 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 provide a proper color, in compliance with the quality requirements of the paper machine, and its use involves high processing costs. 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. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an important object of the present invention to lower the consumption of fresh water in a paper machine. 
     More specifically, it is an object of the present invention to improve the papermaking process from the point of view of decreasing the burden on the environment (in the form of less water products) and the requirement of fresh water (in the form of lowering the amount of fresh water needed). 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 manner and by using cleaned waters and the concentrates coming from the cleaning devices in an optimal manner in consideration of their degree of purity and their washing potential. 
     Another object of the present invention is to provide novel overall solutions of technology, which solutions, at the same time as they reduce the burden on 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. 
     In order to achieve these objects and others, in accordance with the invention, 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 at least a part of the 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. 
     It has been estimated that the cleaning and evaporation of process waters optimally in a plant integrated in accordance with the present invention considerably lowers the burden to the environment, in the form of a reduction in waste water and a lower requirement of fresh water, and is more advantageous compared with the unlimited and abundant use of fresh water with the resulting high requirement of cleaning in most present-day paper mills. 
     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 are 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 manner. 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 the relatively clean waters is utilized. Moreover, 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 so-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 manner, the use of chemically pure fresh water can be reduced. It is an advantage of the invention that chemically purified fresh water is not needed in equally large quantities as in prior art systems 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 one preferred embodiment of the invention, the water flows 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 these water flows without cleaning. 
     Disturbing materials (impurities and contaminants) arrive along with the stock, and the papermaking process produces more such materials. These disturbing materials must be removed from the water and taken out of the water circulation system. In the selective water collecting method in accordance with one 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, from about 50μ to about 10μ. This cleaning combination can be used alone or as a pre-processing stage for subsequent stage employing various 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 evaporation in the processing of waste water. 
     3) Evaporation technology by means of vacuum and/or compressor evaporation systems. 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 system, 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 any one of the various combinations 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 easily and readily and with a higher capacity directly after the washing duty than if it were fully mixed with the circulation water in the paper machine. The cleaned water can be passed to the jets or different other uses of a paper machine having a better quality than the circulation waters. The method of the invention does not result in the prior art problems mentioned above, in which circulation waters of the paper machine are used as jet waters, e.g., damage to the equipment. A higher proportion of the jets can be connected to the cleaned waters obtained from the stages employing separation technologies and from combinations of such stages. Thus, the amount of fresh water needed in the paper machine is reduced. 
     The flotation filtrate may be cleaned by means of microfiltration. 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 small as 10μ and, nevertheless, a satisfactory hydraulic capacity is achieved. 
     The arrangement for circulating water in a paper machine in accordance with the invention comprises first means for recovering separate flows of wash water from fabrics and other devices in the paper machine, each flows originating from a different location in the paper machine, second means for recovering water drained from a paper web to be produced in the paper machine, the second recovering means being separate from the first recovering means, cleaning means for cleaning flows of at least some of the recovered water to produce a clean water flow, and recirculating means for recirculating the clean water flow to applications of reuse in the paper machine suitable in view of their washing potential in the papermaking process. The first recovering means may be arranged to recover separate flows of water used for conditioning the fabrics, water used for washing devices in the paper machine, water used for lubricating components in the paper machine, and water used for sealing components in the paper machine. The cleaning means may comprise a flotation cleaning stage and a microfiltration cleaning stage for separating suspended solid matter and colloids, and an optional membrane separation stage. If the cleaned water flows are the fabric conditioning water and the washing device flows, and the paper machine includes a first set of water jets which operate on circulation water and a second set of water jets which operate on water cleaner than the circulation water, the recirculating means may include first passage means for directing a first part of the clean water flow from the flotation/microfiltration stages to the first set of water jets, and second passage means for directing a second part of the clean water flow into and through the membrane separation stage for further cleaning and at least a part of the products from the membrane separation stage to the second set of water jets. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects of the invention will be apparent from the following description of the preferred embodiment thereof taken in conjunction with the accompanying non-limiting drawings, in which: 
     FIG. 1 shows a prior art paper machine; 
     FIG. 2 shows a paper machine in accordance with the invention, which is an improvement over the paper machine shown in FIG. 1; 
     FIG. 3 shows a second prior art paper machine; 
     FIG. 4 shows a paper machine in accordance with the invention, which is an improvement over the paper machine shown in FIG. 3; 
     FIG. 5 shows a preferred embodiment of the invention related to the paper machine shown in FIG. 4; 
     FIG. 6 is a more detailed illustration of selective collecting of the waters from the wire part; and 
     FIG. 7 is a more detailed illustration of selective collecting of the waters from the press section. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1-7 wherein like reference numerals refer to the same or similar elements, in the prior art paper machine construction shown in FIG. 1, the jet water that is used is mainly fresh water which is passed to the paper machine along ducts  1 , 1 ′, 1 ″, 13 , 13 ′, 13 ″. Jet water is needed both in a wire part  4  of the paper machine, to which it is passed along a duct  7 ″, and in a press section  5  of the paper machine, to which it is passed along ducts  7 , 7 ′. Fresh water is heated to the required temperature by means of heat exchangers  88 , 88 ′ and initially passed into a 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 a cooling tower  100 . This water is passed along the duct  1 ′ to the water circulation system 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 a sewer along a duct F. 
     The circulation water flows from the press section  5  are passed into a circulation water tank  31  along ducts  9 , 9 ′. The wash water flows and the conditioning water flows pass into sewers V. Wash and circulation water flows enter into the circulation water tank  31  along ducts  30 , 30 ′ also directly from the wire part  4 . Circulation water flows from the wire part  4  are also passed into a wire pit  7 , from which they are passed into the circulation water tank  31  along the duct  30 ′ and as circulation water along a duct  38 . Stock broke is passed through a couch chest C into the broke system along a duct D. The water from the circulation water tank  31  is passed along a 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 a 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 of the paper machine shown in FIG.  1 . In FIG. 2, conditioning water flows or waters and circulation water flows or waters are collected from the press section  5  and passed along ducts  3 , 3 ′ into a conditioning and wash 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 a duct  3 ″ and passed through the conditioning and wash water tank  82  to a flotation cleaning system. In the flotation cleaning system, first a coarse cleaning takes place in screens  83 , after which the water is cleaned in a flotator  57 . The clean water is then passed through a screen  96  into a clarification tank  84  and further to microfiltration system  57 ′ along a duct  85 . After this, the clean water is passed through a microfiltration tank  86  to an ultra-membrane filter  57 ″ along a duct  59  and/or to a nanomembrane filtration system  57 ′″ along a duct  59 ′ and from there into a clean-water tank  87 . An overflow from the microfiltration tank  86  is passed to mix with the outlet waters of the clarification tank  84  along a duct  86 ′. Fresh water enters into the clean-water tank  87  along a duct  13  from the clean-water tank  2 . Regulators  103 , 104  operate together so that the supply of water into the tank  87  in the paper machine is secured and maintained as desired. The excess amount of warm water is passed to other uses (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 an evaporator  76  along a duct  80 . From the warm water tank  87 , water is passed through a heat exchanger  88  into a hot water tank  89  along a 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 stages etc., away along ducts  90  to constitute conditioning and wash waters and later along the duct  6  to the wire part and the press section. After the ultramembrane filtration, the concentrate passes along the duct  62 , and after nanomembrane filtration along the duct  62 ′, to a concentrate tank  91  and from there to mix with the process water flow along the duct  92 . The process water passes through a fine screen  93 , and after the fine screen  93  the reject passes into a sludge tank  94 , from which the sludge passes further along a duct  95  to sludge treatment. To the sludge tank  94 , the sludges are also collected from the flotator  57 , from the fine screen  96  and from the microfiltration  57 ′ along the duct  94 ′. From the fine screen  93 , the filtrate is passed through a feed tank  97  to the evaporator  76 . The concentrates from the evaporator or evaporators are passed away along a duct  98 . The clean condensate is passed along a duct  80  into the warm water tank  87 . The vapor from the evaporator passes to a 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 a duct  101 . It is one embodiment of the invention that, after the cooling tower  100 , fresh water is passed to the warm-water tank  2  for fresh water along a duct  102  through the condenser  99 . 
     FIG. 3 shows a second prior art paper machine. In FIG. 3, corresponding parts are denoted by the same reference numerals as in the paper machine shown in FIG.  1 . 
     In the paper machine shown in FIG. 3, the raw material comes from a 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 water flows or 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 a duct  11 , and the sludge is passed along a duct  12 . 
     Fresh water is passed to the paper machine along a 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 embodiment along a duct  16 . 
     The circulation waters of the wire part enter along the duct  30  into the tank  31  and are passed along a duct  32  to a disk filter  33 , and from there the filtrates pass into a tank  34  along 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 or tank  31 , dilution water is taken into the stock after a mixing tank  37  along a duct  38 , and a filtrate from the disk filter  33  is also passed to the mixing tank. The raw material runs through processing stages  37 , 39 , 40 , 41  (e.g., 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 a 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 usually be supplied directly into the short cycle in the paper machine. 
     FIG. 4 shows an embodiment of the invention as applied to the prior art environment of a paper machine shown in FIG.  3 . In the embodiment shown in FIG. 4, the waters used for conditioning of the fabrics are passed along a duct  56  for cleaning to the flotator  57 , after which the water is cleaned further by means of microfiltration system  57 , to which it is passed along a 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 favorably cleaned further by means of ultrafiltration and/or nanofiltration systems  57 ″, to which it is passed along a duct  59 . The clean fraction from the ultrafiltration and/or nanofiltration is passed along a duct  60  to substitute for fresh water, and the concentrate is passed to the waste waters along a duct  62  and from there further, for example, to biological purification and treatment B. The duct  62  communicates with the duct  10 . The rejects from the cleaning stages  57 , 57 ′ are passed along ducts  49 ″ and  49 ′″, which communicate with a duct  49  passing to a sludge press L. From the sludge press, the waste water and sludge may be passed either to the biological treatment B, along duct  71  leading into duct  10 , or directly to the waste stream. 
     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 a subsequent microfiltration system  74 ′, to which it is passed along a duct  75 . The sludge from the flotation and from the microfiltration system  74 ′ is passed again into the sludge press L along a duct  78  and along the duct  49 , and the filtrate is passed to evaporation unit  76  along duct  79 . 
     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 this cleaned water is passed along a duct  80  to a 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 evaporation device or evaporator  76  is passed along a duct  77  away, and the contaminated condensate is passed along a duct  77 ′ further to cleaning. In FIG. 5, the duct  77 ″ is also shown, along which any other process waters whatsoever can also be passed to the evaporator. A portion of the cleanest filtrate from the disk filter  33  may also be passed to the duct  79  leading into the evaporator unit  76 . 
     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  6 , which is branched into the ducts  6 ′ and  6 ″. Along the duct  6 ′, the circulation water passes into a jet pipe  105  to constitute wire conditioning and wash water. Along the duct  16 ″, the circulation water passes preferably into another two jet pipes  106  and  107  to constitute wash and conditioning water. The paper web formed in the paper machine is denoted by reference R. After washing and conditioning, the waters are collected in a trough  108 . FIG. 6 also shows a doctor  109  operative against roll, and the wash water used for its lubrication drains into the trough  108 . Fresh water is also passed as conditioning water along a duct  110  and further into a jet pipe  111  and a 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 water collecting equipment  113  and  114  situated in opposed relationship to the jet pipes  111 , 112 , respectively, and passed through ducts  3 P into a duct  3 ″ into a wash water tank. The wash waters coming from the circulation water jets pass from trough  108  along the duct  41  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 in accordance with the invention. Circulation water flows along the duct  6 ′ into jet pipes  115  and  116  in the press section. The press felt is denoted by reference numeral  117 . The conditioning waters coming from circulation water jets are collected by means of troughs  118  and  119  associated with doctors  120  and  121 , respectively, and further passed 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 a ducts  123  into jet pipes  124  and is collected by means of a trough  125  and passed further into the wash water tank along ducts  126 . FIG. 7 also shows felt conditioning devices  127  for conditioning the press felt. Thus, FIG. 7 is a detailed illustration of the collecting of wash waters in the press section in FIGS. 2,  4  and  5 . 
     In the following, the patent claims will be given, and the various details of the invention can show variation within the scope of the inventive idea defined in the claims and differ even to a considerable extent from the details stated above by way of example only. As such, the examples provided above are not meant to be exclusive and many other variations of the present invention would be obvious to those skilled in the art, and are contemplated to be within the scope of the appended claims.