AU672026B2 - Reducing pulp mill discharge - Google Patents

Description

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Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): A. AHLSTROM CORPORATION Invention Title: REDUCING PULP MILL DISCHARGE a. S if a *5 r S S I S Os 0

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The following statement is a full description of this invention, including the best method of performing it known to me/us:

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I 4 W T 1 REDUCING PULP MILL LIQUID DISCHARGE BACKGROUND AND SUMMARY OF THE INVENTION It has long been a desire of those working in the O" ~paper pulp art to produce a pulp mill that does not in any way significantly pollute the environment. A number of proposals have been made for such a pulp mill in the past, but the desired goal has yet to be achieved. For example, a "closed mill" was constructed at Great Lakes Forest *0 Products, Thunde Bay, Ontario, in the 1970s, but it was difficult to run the mill closed for extended periods of time as a result of corrosion problems in the recovery S"*o boiler, and elsewhere, due to chloride buildup. See "Bleaching in the Closed Cycle Mill at Great Lakes Forest Products Ltd." by Pattyson et al, Pulp Paper Canada, S" Vol. 82, No. 6, pp. 113-122 (1981). In the Great Lakes mill, bleaching plant effluents were introduced directly into the chemical recovery loop, as shown schematically in U.S. patent 4,039,372.

More recently, it has been proposed by HPD and Jaakko eo Poyry that closing of a pulp mill can be accomplished by evaporating acid effluent and then returning the E bleach plant effluent to the brown stock washers. Howe-er that approach has yet to be successful, despite the utilization of inexpensive thin plastic film, falling film evaporators which allow effective evaporation of the bleaching chemicals, and it is believed unlikely that it will ultimately be successful because of the buildup of undesired chemicals due to the introduction of the flofrom the E stage back to the brown stock washing stage.

According to the present invention, a method and apparatus are provided which utilize only existing -I d I I 2 technology, so that future development of sophisticated additional equipment or processes is not necessary, which essentially can reduce the liquid polluting effluents from a pulp mill to zero, provide only a minimum amount of .solid waste for disposal (and provide the high probability that such solid waste can be used in an environmentally acceptable manner), and minimize the production of gaseous NO and SO products, so that the only significant gaseous x x pollutant from the pulp mgTll is carbon dioxide.

One of the basic aspects of the present invention *000 that makes it possible to achieve these beneficial results is to treat the bleaching effluents completely separately from the chemical recovery loop until the effluents are in a particularly desirable form, and to then introduce the chemicals in that desirable form into the recovery loop.

*6 *o Another significant aspect of the present invention is the essentially complete oxidation of white liquor produced in the chemical recovery loop, which is then returned to the bleaching stage so that the proper balance between the various chemical treatment sequences is provided. Another significant aspect of the present invention that allows the desired results to be achieved are the production on site at the pulp mill, directly from the effluent streams and gaseous waste streams themselves, of essentially all of the sulfur dioxide, sulfuric acid, caustic or caustic 0*Se substitute, and (if utilized) chlorine dioxide necessary to effect treatment of the pulp and recovery of the chemicals. Another factor which minimizes the amount of bleach plant effluents so as to make a proper treatment thereof practical, is advanced digesting techniques where delignification can be extended so that the pulp without significant strength loss discharged from the digesting stages has a low Kappa No. 24 or below) and then the pulp is subjected to oxygen delignification to reduce the Kappa No. still further to 14 or below, typically 10 or below) before bleaching is effected, allowing the production of prime market pulp 88-90 ISO).

3 In order to achieve substantially zero discharge from a pulp mill, it will be necessary to process about to 40 tons of liquid per ton of pulp produced over and above the normal liquid processing requirements from the chemical recovery system. Also, it will be necessary to produce bleaching chemicals directly on site, which is a highly electricity intensive endeavour. Therefore in order to achieve the desired goals of the invention, it is highly desirable to utilise a condensing type steam turbine which is fed with super heated steam from the burning of organic wastes (concentrated black liquor, bark, etc) from the pulp mill. Different steam exhausts are provided from the steam turbine, each exhaust having steam of different composite pressure and temperature values, and each different value being utilised in the manner most suited for it within the pulp mill. In particular, the substantially lowest heat and pressure composite value of the steam is used with multiple effect evaporators (such as metal-plastic film laminated evaporator elements) to evaporate the collected 20 liquid wastes (such as bleach plant effluents, spills, and the like) within the pulp mill.

The ability to produce prime market pulp with minimal adverse effect on the environment, according to the invention is a quantum leap forward in pulping technology, 25 and allows fulfilment of a long felt need to accomplish 0* •this desirable result.

i: According to one aspect of the present invention there is provided a method of operating a cellulose pulp mill, having a bleach plant, so as to allow substantially 30 zero discharge of pollutants therefrom, utilising a condens.Lng type steam turbine with at least one exhaust, said method comprising the steps of: producing super heated steam by burning organics containing waste products from the pulp mill; feeding the super heated steam to the condensing type steam turbine to generate a maximum amount staffliridlkeeppalretypest474A93 15i 796 4 of electricity from a given amount, temperature, and pressure of steam; exhausting steam from the at least one exhaust of the steam turbine; collecting liquid wastes from the cellulose pulp mill, including liquid effluents from the bleach plant; and evaporating the collected liquid wastes using the exhausted steam from the at least one exhaust of the steam turbine as heating fluid for effecting evaporation to produce clean condensate and a concentrated liquid waste.

Step may be practised utilising a multiple effect evaporator having 6-10 falling film effects) with a temperature differential of about 1-40C (preferably 2-3 0 C) between effects. Step may be practised to exhaust steam having a pressure of about 1-3 psia and a ;oe" temperature of about 120-140 0

F.

Step is preferably further practised utilising steam at a predetermined first composite pressure and temperature value, and comprises the further step of withdrawing a second stream of steam from the steam turbine at a predetermined second composite temperature and pressure value greater than the first value, and a third 25 exhaust stream having a third temperature and pressure composite valute greater than the second value. The second and third streams are used where appropriate within the pulp mill, while the first stream is used for the evaporation of the collected liquid wastes.

30 According to another aspect of the present invention there is provided a method of evaporating an industrial waste liquid stream to produce clean condensate and a concentrated waste stream, using a condensing type steam turbine, said method comprising the steps of: producing super heated steam by burning organic fuel; st..i/irid/keop/pat/retyp.,I4744O 93 15 796 4a feeding the super heated steam to the condensing type steam turbine to generate electricity; exhausting steam from the exhaust of the steam turbine utilising steam at a predetermined first composite pressure and temperature value; evaporating the industrial waste liguid stream using the exhausted steam from the steam turbine as heating fluid for effecting evaporation; withdrawing a second stream of steam from the steam turbine at a predetermined second composite pressure and temperature value greater than the first value, and a third stream of steam at a predetermined third composite pressure and temperature value, greater than the second value; wherein step is practised with the steam having the first composite pressure and temperature.

According to yet another aspect of the invention, .e a method of minimising effluents from a cellulose pulp mill having a digester, bleach plant, condensing type steam turbine with at least one exhaust, and a recovery boiler, and a chemical recovery loop, is provided. The method comprises the following eo eo stalfiridlkeeplpatretyposl47440 93 1579G steps: ka) Producing super heated steam by burning organics-containing waste products from the pulp mill.

Feeding the super heated steam to the condensing type steam turbine to generate electricity. Exhausting steam from the at least one exhaust of the steam turbine.

Evaporating liquid effluents from the bleach plant to a concentration level high enough for incineration, by using the exhausted steam from the at least onekturbine as heating fluid, to produce a concentrated effluent. (e) .as Incinerating the concentrated effluent to produce a eoo residue containing sodium, sulfate and carbonate. (f) Leaching the residue to produce a leachate. And, (g) feeding at least a substantial portion of the leachate to the chemical recovery loop associated with the recovery o boiler.

The invention also contemplates, in the method of producing cellulose chemical pulp in a pulp mill which requires sulfur dioxide, sulfuric acid, electricity for the production of bleaching chemicals and other uses, and caustic, and which has process effluents, the following 9 step: Directly at the pulp mill producing substantially all of the sulfuric acid, sulfur dioxide, electricity, and caustic, or caustic substitute, necessary to effectively produce chemical pulp at the pulp mill from the pulp mill process effluents, organic waste, and waste gas streams, S. so that no substantial amount of additional sulfuric acid, sulfur dioxide, or caustic, nor any electricity, are necessary from external sources. The electricity is produced by burning concentrated black liquor and organic cellulose solid wastes to produce super heated steam.

Then the super heated steam is fed to a condensing type steam turbine, and spent steam from the steam turbine is exhausted at at least three different temperature and pressure composite values, each different exhaust used in a suitable mill process.

The invention also comprises a method of operating a ^-,cellulosic pulp mill so as to allow substantiall-' zero Y, d.scharge of pollutants therefrom, util zin a condensing L. type steam turbine with at least three exhausts, comprising the steps of: Producing super heated steam by burning organics containing waste products from the pulp mill. Feeding the super heated steam to the condensing type steam turbine to generate electricity.

Exhausting steam from each of the at least three exhausts of the steam turbine, the pressure and temperature of the steam at each exhaust being different.

And, utilizing the di-fferent exhausted steam streamns from the condensing turbine in different mill processes, the mill process in which each different steam stream is utilized and suited to utilization of steam of that particular pressure and temperature.

.o The evaporator means utilized preferably comprise a plurality of stages of metal-plastic laminate, falling film evaporators. Such evaporators are available from A.

Ahlstrom Corporation of Helsinki, Finland, and Ahlstrom Recovery Inc. of Roswell, Georgia under the trademark .o D. "Zedivap", and described in Finnish Application 915424 Although other evaporators, such as desalination evaporators, als' are feasible, the "Zedivap"' evaporators are particularly advantageous and make the evaporating process for bleach plant effluents, and collected spills, practical.

00000. It is the primary object of the present invention to provide for the production of cellulose chemical pulp with essentially zero discharge of liquid pollutants to the environment, with a minimum amount of gaseous pollution, and with the minimum amount of solid waste products, including by using a condensing type steam turbine with multiple exhausts. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.

BRIEF DESCRIPTION OF TEE DRAWINGS FIGURE 1 is a schematic view of the most basic components of one exemplary system according to the .present invention, and for practicing exemplary methods according to the present invention; FIGURE 2 is a flow sheet similar to that of FIGURE 1, only showing a number of the particular processes involved in more detail; I .S FIGURE 3 is a schematic of an alternative system according to the present invention based upon the same concepts.-as the systems of FIGURES 1 and 2 only showing different details of the handling of bleach plant effluents, the particular bleach plant stages involved, and the like; and FIGURE 4 is a schematic view of a system according to the present invention practicing a method of evaporating 0 large volumes of liquid, and generating sufficient electricity to produce all of the bleaching chemical requirements of a -pulp mill, utilizing a condensing type steam turbine.

ft* DETAILED DESCRIPTION OF Tl'EDRAWINGS The exemplary system illustrated in FIGURE 1 incluides a conventional digester 10, such as a Kamyr® continuous digester, to which hard wood or soft wood chips, or other comminuted cellulosic material, is fed. In the digester the wood chips are acted upon by the cooking chemicals at conventional temnperature and pressure conditions so as to produce chemical cellulose pulp, such as kraft pulp, which then is preferably subjected to oxygen delignification at stage 11. According to the present invention it is desirable to delignify the pulp so that it has a minimum Kappa No. when discharged from hhe digester 8 such as by using a Kamyr EMCC® digester and process, which produces a Kappa No. of about 24 or below. The oxygen delignification stage 11 reduces the Kappa No. to about 14 or below, preferably to about 10 or beloi.

After oxygen delignification, the pulp proceeds to the bleach plant 12 where it is subjected to bleaching in a plurality of different bleaching stages. The particular bleaching stages that are utilised can be varied, and are also dependent upon the particular cellulose material being treated. After the bleaching stages 12, the pulp may proceed on to storage or further treatment stages 13. For example the pulp may be dried and then shipped to a paper mill.

As is conventional, black liquor is withdrawn from the digester 10 (or brown stock washer associated therewith), and is passed to evaporators 14. The black liquor also is preferably subjected to heat treatment such as shown in U.S. patent 4,929,307 (the disclosure cf which is hereby incorporated by reference herein). Sulfur- 20 containing gases driven off by the heat treatment 15 may be handled as shown in European Patent Application No.

92890125.5, for example to produce high sulfidity liquor at stage 16, where the production of fuel gas primarily methane) as indicated schematically at 17, makes possible 25 generation of power as indicated generally at 18.

After treatment at stage 15 the black liquor is ultimately passed (there may be intervening evaporation stages if desired) to a conventional recovery boiler 19.

S. Steam produced from the recovery boiler 19, as indicated 30 generally at 20 in FIGURE 1, is used for various processes within the pulp mill. The gases discharged from the recovery boiler 19 include sulfur dioxide which can be used as the feed material for the production of sulfuric acid according to conventional techniques. As indicated at 21 in FIGURE 1, sulfur dioxide and sulfuric acid (produced from the SO2) can be used wherever necessary in the mill.

stalifriddkeepioaltretypes/4744 93 15796 9 For example the sulfur dioxide is used as an anti-chlor for the last stage of chlorine dioxide bleaching (if utilised), and for the tall oil plant. According to the invention, sufficient sulfur dioxide and sulfuric acid are available from block 21 to fulfil the needs of the pulp mill without requiring those chemicals from an external source. While of course one cannot expect the chemical recoveries and consumptions to balance exactly, according to the invention they may be expected to be within a few percent of each other. Of course any small amount of excess chemical can be sold, and any deficiency made up by purchase.

The melt from the recovery boiler 19, as is conventional, is used to from green liquor as indicated by reference numeral 22 in Figure 1, and the green liquor is then preferably ultimately used to make white liquor, as indicated generally by reference numeral 23 in Figure 1.

Alternatively, or in addition, the green liquor may be crystallised and otherwise acted upon to produce essentially sulfur free sodium hydroxide, as disclosed in Canadian Patent Application No. 2101005, the disclosure of which is incorporated by reference herein.

The sulfur content of the melt may be adjusted by bringing a portion of the melt discharged from the recovery boiler 19 into contact with a sulphurous gas of the pulp mill. Also, one can thermally split the methyl mercaptan and dimethyl sulphide of the sulphurous gas into ethene and hydrogen sulphide before it is brought into contact with the melt, or into contact with ash from the recovery boiler 19. Any white liquor produced from this melt will have controlled and/or enhanced sulfidity. These techniques are disclosed in Finnish Applications 914585 and 914586, both filed September 27, 1991.

Some of the white liquor is fed via line 24 back to the digester 10, and according to the present intention, in order to balance the chemical flows, it is highly sta'l'lrtd/koopipateotypos/4744093 15 7

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^z 10 desirable that a portion of the white liquor from 23 be oxidised at stage 25 in a conventional or known manner, and then used in the oxygen delignification stage 11. One known manner of oxidation termed "bubbleless membrane aeration" is described in an article by Michael Semmens in the April, 1991 edition of "WATER/Engneering Management", pp 18 19. Also, a portion of the oxidised white liquor from 25 is preferably subjected to a second oxidation stage 26 such as shown in International Publication No. WO 94/01616 (the disclosure of which is hereby incorporated by reference herein) in order to oxidise all of the sulfur forms within the white liquor to sulfates. The resulting essentially completely oxidised white liquor is then returned to the bleaching plant 12 and used in place of caustic in the bleach plant 12.

Sufficient oxidised white liquor can be produced in 26 according to the invention so that all of the caustic needs for the bleach plant 12 are taken care of, without the necessity of requiring caustic from an external source.

S 20 Also according to the present invention, the liquid effluents from the bleach plant 12 such as the Sacid effluent in line 27 from the first bleaching stage, and the alkali effluent in line 28 from the second bleaching stage are concentrated, e.g. by passage to 25 evaporator stages 29, 30, respectively. The evaporators which comprise the stages 29, 30 preferably are low cost metal-plastic laminate, falling film evaporators, such as sold by A. Ahlstrom Corporation of Helsinki, Finland and Ahlstrom Recovery Inc. under the trademark "Zedivap". Such 30 laminates are typically of aluminum (or brass or copper) and plastic polyethylene, polypropylene, or polyester), each layer having a thickness of less than 100 pm. For example an aluminum layer may be 9-18 tm thick, and a polyester layer 12-25 tm thick. A plastic film may be extruded on a metal foil to produce a laminate. A heat exchanger is formed by attaching two rectangular laminated staffirid/ko.epipaoretyper47440 93 15 7 96 10a strips to each other, for example by a glued joint. The laminated strips may also be connected to each other by dot-like junction points between the joints at the edges.

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a a a a a staff/inikoop/paUretypes/474AG 93 15 7 96 u _L I The pulp mill liquids may flow down the plastic layer, or the metal layer. Such an evaporator surface is disclosed in Finnish Application 915424. However, conventional desalination evaporators may be used instead.

Where both acid and alkali liquid effluent lines 27, 28 are provided, it is desirable not to mix them until the effluents have been concentrated in the evaporators 29, otherwise a severe foaming problem may ensue. If the foaming problem can be overcome, then the lines 27, 28 may be combined before the evaporators 29, ge* 496 After the stages 29, 30, the more concentrated *:as effluent passes to the concentrator 31, which comprises a V1 series of high-efficiency evaporator stages which rue concentrate the effluent to a sufficient level so that it can be incinerated. For example, the concentration of the effluent in lines 27 and 28 may be 0.2-0.5% solids, which is concentrated to a solids content of about 10-30% by the evaporators 29, 30, and then concentrated to a r concentration of about 50-60% by the concentrator 31.

Concentration of the bleach plant effluents may be complished by other techniques aside from evaporation.

For example, conventional ultra-filtration, reverse eq osmosis, freeze crystallization, or a combination of these techniques with each other and/or with evaporation, may be utilized to produce effluent with a sufficiently high 4*&C concentration.

The concentrated effluent from the concentrator 31 or the like is fed to an incinerator 32 where it is burned to produce a residue. Incineration may be practiced according to a number of conventional or known techniques, such as slagging combustion or gasification (as by means of a circulating fluidized bed gasifier).

Valuable chemical components of the residue from incinerator 32 are ultimately returned to the recovery loop components 14, 15, 19, 22, 23, etc.). In order -e I I to effectively return valuable components of the residue, such as sodium, sulfate, and carbonate, the residue is preferably leached by a conventional leaching apparatus, as indicated at 33 in FIGURE 1. Preferably, the leachate .from the leaching stage 33 is crystallized freeze crystallized; see U. S. Patents 4,420,318, 4,505,728, and 4,654,064) and washed as indicated at 34. Leaching and crystallizing per se (although in a recovery loop) are known as indicated by TAPBI Journal Volume 66, No. 7, July, 1983 "Recovering Chemicals in a Closed Sulfite Mill" by Davies et al.

The crystallized and washed leachate from stage 34 (or at least a portion thereof) is fed via line 35 to the recovery loop, such as just before the recovery boiler 19. In that way the valuable chemicals from the bleach plant effluent in lines 27, 28 are returned to the recovery loop. The washing separates out metals above monovalent, such as calcium and magnesium, which may be land-filled or treated as indicated at 36 in FIGURE 1.

The solid material at 36 is essentially the only solid waste material from the pulp mill of FIGURE 1, and only comprises about 5% of the chemicals from the residue of incinerator 32, the other 95% being used elsewhere (e.g.

in the recovery loop).

The residue from the incinerator 32 also typically includes sodium chloride, and the chlorine content thereof can be used as indicated by dotted line 37 and box 38 in FIGURE 1 to produce chlorine dioxide and sodium chloride. In this circumstance, some of the leachate from stage 34 flows to the chlorine dioxide production stage 38, while the rest is returned to the recovery loop via line In many pulp mills, regardless of age, the amount of spill liquid can be a significant percentage of the total liquid effluents. Spill liquids as high as 33% of a mill total liquid effluents (including the bleach plant liquid effluents in lines 27, 28) are not unusual. Of course if such spills are allowed to leak into the environment, then

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the goal of a low or zeio discharge mill will not be realized. Therefore according to the present invention, the liquid spills preferably from the entire pulp mill are collected utilizing conventional drainage and collection systems, as indicated schematically at 39 in FIGURE 1. Those spills are then clarified in the clarifier 40, and passed to spill storage 40' and then to the evaporator stages 41. The evaporators in stages 41 are preferably Zedivap

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M evaporators. The concentrated spills from the evaporators 41 are then combined with the concentrated effluents from evaporators 29 and 30, and passed to concentrator 31.

Of course all of the evaporator stages 29, 30, and 41 will produce water, which has been removed from the bleach plants effluents during the concentrating action thereof.

The water from each of the evaporator stages 29, 30, and 41 is passed to a water treatment facility 42 which treats it so that it does not have any components which are 0 harmful if the water is used for other purposes. This "recovery" of water is also a big advantage of the method and apparatus according to the invention. Part of the water is then returned, via line 43, to the bleach plant 12 to serve as wash liquid flowing countercurrently to the pulp from one stage to another in the bleach plant 12, o while another part of the water passes ir line 44, which goes ti the recovery boiler 19 as feed cer, for the production of process steam at FIGURE 2 provides an illustration of the same basic system, for practicing the same basic method, as in FIGURE 1, only shows a number of the components in more detail.

In the illustration of FIGURE 2 components comparable to those in FIGURE 1 are shown by the same reference numeral.

In the illustration in FIGURE 2, a wood yard 45 is shown connected to the digester 10, and also to a conventional hog fuel boiler 46. A brown stock washing stage 47 is disclosed after the digester 10, as well as a screen room 48 cooperating with a press 49, the press 49 L I Lalso connected to the clarifier 40. Downstream of the oxygen delignification stage 11 is a further washing stage which is then connected to the first stage 51 of the bleach plant 12. In the embodiment illustrated in FIGURE the first bleaching stage 51 is a 100% chlorine dioxide stage. The second stage 52 is an E stage, a source of caustic being provided by the oxidized white liquor from 26. A third bleach stage 53 is a neutral chlorine dioxide stage. That is a portion of the oxidized white liquor from source 26 (or caustic) is added to the top of the tower of stage 53 in order to neutralize the pulp acidity. The fourth stage 54 is a last chlorine dioxide stage. Chlorine dioxide from the production stage 38 is fed to each of the stages 51, 53, and 54, while a portion of the wash water from the water treatment plant 42 enters the fourth stage 54.

The further treatment stages 13 in the FIGURE 2 illustration include the "wet end" 55 and dryer 56, which may be connected to a storage facility 57'.

As part of the recovery system, other conventional components are illustrated in FIGURE 2, such as the green ""liquor clarifier 57, the slaker 58 for causticizing the green liquor, and the lime mud handling components including the mud filter 59, pre-coat filter 60, lime kiln o* 61, etc.

.00 Associated with the components acting upon the bleach plant effluents is the dregs stage 63, which may be supplied with the higher than monovalent metals from the crystallizing and wash stage 34, as well as fly ash from the hog fuel boiler 46. The materials from the dreg stage 63 may be passed to a land-fill 64, or treated to recover the chemicals therefrom, or the chemicals therein can be utilized in an environmentally acceptable manner.

Also illustrated in FIGURE 2 is an optional ozone treatment stage 65 for treating water from the water treatment plant 42. The water from plant 42 is ozonated before flowing to the feed water source 66 which supplies the recovery boiler 19, and which also receives water from

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the dryer 56. Water from the wet end 55 may pass to the water treatment plant 42, or to the interface between the second and third bleaching stages 52, 53.

FIGURE 3 illustrates another alternative system according to the present invention. One of the major differences between the system of FIGURE 3 and that of FIGURES 1 and 2 is in the particular bleach sequence which is provided, namely an A2E PZP bleach sequence. In FIGURE 3 components comparable tO those in the FIGURES 1 and 2 embodiments are shown by the same reference numeral only preceded by a Also FIGURE 3 schematically illustrates a number of the components used in the system rather than merely showing them in block diagram, as in FIGURES 1 and 2.

•6 .The digester 110 may be part of a two vessel hydraulic system, including an impregnation vessel 68, such as an EMCC® digester sold by Kamyr, Inc. of Glens Falls, New York. A pressure diffuser, 69, or similar brown stock washer may be downstream of the digester 110, which in turn is connected to high-density storage tank, Il, and then the brown stock screen room 148. The oxygen 40:0 delignification reactors 111 are connected to the post oxygen washing stage 150, which is then connected to the first bleach stage 70, in this case an acid, stage.

ont* 1 The second stage of the bleach plant 112 is the first ozone stage 71, and after a wash 72 the E stage 152 is provided. Following the E stage 152 is a first peroxide stage 73, then the second ozone stage 74, and the second peroxide stage 75, connected up to the high density storage tank 157' In the embodiment of FIGURE 3, the acid bleach plant effluent line 127 is connected to the Zedivap T m evaporator stages 129, just like in the FIGURES 1 and 2 embodiment, which in turn are connected to the concentrator 131, incinerator 132, leach stage 133, and crystallizing and wash stage 134. However the alkaline effluent line 128 is not connected up to evaporators, but instead is connected up to the recovery loop, typically to the green liquor I I I dissolving tank 122. Also a part of the alkali effluent in line 128 may be used for causticizing, e.g. connected to stage 158; however, much of the alkali effluent would be added to the post-oxygen washing stage.

The pulp mills of FIGURES 1 through 3, in addition to producing essentially zero liquid effluent discharges, produce little air pollution. Sulfur dioxide and other sulfur compounds are recovered from the recovery boilers 19, 119 stacks, and electrostatic precipitators are also provided in the stacks. Also, the recovery boilers 19, 119 and all the other components, such as incinerators 32, 132, are operated so as to have minimal NO discharge.

9 0 4 x The major gaseous pollutant, then, from the pulp mill will only be carbon dioxide.

FIGURE 4 schematically illustrates a system that is particularly useful in generating sufficient electricity to provide all of the needs for the pulp mill, including for the production of bleaching chemicals such as chlorine dioxide (from sodium chloride), and/or ozone. The system ~of FIGURE 4 also facilitates handling the large amount of waste liquids thlat must be handled in the closed mill of e FIGURES 1 through 3. In the FIGURE 4 embodiment, components comparable to those in the FIGURES 1 and 2 embodiments are shown by the same reference numeral only D* preceded by a "2" In the FIGURE 4 embodiment, the recovery boiler 219 and the bark boiler 246 (which burns solid organic, cellulose wastes associated with the mill) are used to produce super heated steam, being discharged in lines 71, respectively, and combined in line 72 to be fed to a condensing type steam turbine 73, which is part of the power generation facility 218, 220. The turbine 73 drives a generator 74. Substantially all of the super heated steam from the boilers 219, 246 is fed to the turbine 73 (obviously multiple turbines can be provided) to generate substantially the maximum amount of electricity possible since large amounts of electricity will be necessary for producing bleaching chemicals, water treatment, and the

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powering of conventional electrical equipment in a pulp mill.

According to the present invention, a plurality (typically three or four) of exhaust streams of spent steam are taken off of the turbine 73. The conduits 76, 77, and the optional conduit 78, illustrate these different streams. The stream taken off in line 75 has the highest composite temperature and pressure value, while that in line 76 has the second highest composite temperature and pressure value, and that in line 77 the next highest composite temperature and pressure value. If the exhaust conduit 78 is utilized, it has the lowest composite pressure and temperature value.

The highest pressure/temperature steam 75 is used in those portions of the pulp mill where primary steam is necessary, such as in those portions of the digester 210, oxygen delignification 211, or bleaching stages 212 that need primary steam, and also in the pulp/paper machine 213. The next highest grade steam in line 76 is used in the multiple effect evaporators 214 for concentrating of black liquor from the digester 210, while other portions of steam from the line 76 may be used for steaming of wood chips, oi in other places within the mill where secondary steam is useful.

O* The steam withdrawn in exhaust line 77 is used in association with the multiple effect evaporator or evaporators 229, 230, 241 for concentrating various bleaching waste liquids, and spills that are collected from throughout the plant (see source 239, 212 in FIGURE As described earlier, the evaporators 229, 230, 241 preferably are metal/plastic film laminates, and may be operated with extremely low temperature differentials about 1-4 0 C, preferably about 2-3 0 Typically 6-10 evaporator effects of the falling film type of metal/plastic film laminates are utilized.

The steam in line 77 provides a total temperature differential across the evaporator 229, 230, 241 of about 0 C. The steam in conduit 77 needs to have a high enougi L composite pressure and temperature value so as to achieve this temperature differential, but should not have a higher value than that, which would be luLed (and thereby detract from the power produced by generator 74).

STypically the steam in conduit 77 has a pressure range of about 1-3 psia and a temperature range of about 120-140F; one specific exemplary value would be a temperature of about 130 0 F and a pressure of about 1 psia (14.7 psia 1 atmosphere).

In the evaporator 229, 230, 241, clean condensate is produced in line 79, and is utilized as process liquid elsewhere in the pulp mill. The steam that does not condense passes in line 80 to a condenser 81, to which the e9• .steam in line 78 may be fed also (to condense). Cool water 82 is preferably used as the condensing fluid, and the heated water in line 83 is used in the pulp mill where preheated water is necessary or desirable.

The waste liquids from the pulp mill 212 and from the collection of spill source 239) evaporated in the evaporator 229, 230, 241 pass in line 228 to the incinerator 232, for subsequent treatment as described in FIGURES 1 through 3.

The use of low pressure turbine exhaust steam (77) as a source for the bleach effluent evaporators (229, 230, 241) according to the invention is a comprehensive utilization of the available thermal energy in the most energy-efficient fashion. The nLture (low consistency) of the bleach effluent allows it to be much more easily evaporated (zhat is requires less thermal energy) than the viscous black liquor. However a large volume of water must be evaporated 25-30 ton/hr. compared to 6-10 ton/hr. for black liquor). The low viscosity of the bleach plant effluent permits the use of low pressure sub-atmospheric) steam at a high flow rate ton/hr. for 1-3 psia steam in line 77). The other two, conventional, steam flows exiting the turbine 76) are typically 65k steam for black liquor evaporation in 214 at 1-2 ton/hr., and 165# steam for the digester 210 1

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19 at 2-3 ton/hr. The adverse effect on power transmitted to generator 74 by removal at 77 is minimal, whereas if steam from line 75, 76 were utilized for evaporators 229, 230, 241, the adverse effect on power generation by generator .74 would be dramatic.

The system illustrated in FIGURE 4, with the multiple steam exhausts 75-78 from the steam turbine 73, is specifically advantageous in a pulp mill, particularly one in which it is desirable to eliminate or g. atly minimize liquid discharges. However, it is also applicable to other industrial processes in which large amounts of liquid must be processed, and there is a ready fuel source available for production of super heated steam.

It will thus be seen that according to the present invention an effective method and apparatus have been provided for absolutely minimizing effluents from a cellulose pulp mill. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and apparatus.

Claims (16)

1. A method of evaporating an industrial waste liquid stream to produce clean condensate and a concentrated waste stream, using a condensing type steam turbine, said method comprising the steps of: producing super heated steam by burning organic fuel; feeding the super heated steam to the condensing type steam turbine to generate electricity; exhausting steam from the exhaust of the steam turbine utilising steam at a predetermined first composite pressure and temperature value; evaporating the industrial waste liquid stream using the exhausted steam from the steam turbine as heating fluid for effecting evaporation; withdrawing a second stream of steam from the steam turbine at a predetermined second composite pressure and temperature value greater than the .eoo first value, and a third stream of steam at a S 20 predetermined third composite pressure and temperature value, greater than the second value; wherein step is practised with the steam having the first composite pressure and temperature. 9* f

2. A method as recited in claim 1, wherein step (d) is practised utilising a multiple effect evaporator with a temperature differential of about 1-4 0 C between effects.

3. A method as recited in claim 1 or 2, wherein step is practised to exhaust steam having a pressure of about 1-3 psia and a temperature of about 120-140F which is used in step stallfirid:k9vpaitetypos!474O 93 15796 21

4. A method as recited in any one of the preceding claims, wherein step is practised utilising a multiple effect evaporator having a temperature differential between the first and last effects of about 20 0 C.

5. A method as recited in any one of claims 2 to 4 wherein step is practised utilising a multiple effect evaporator with a temperature differential of 2-3 0 °C between effects.

6. A method of operating a cellulose pulp mill, having a bleach plant, so as to allow substantially zero discharge of pollutants therefrom, utilising a condensing type steam turbine with at least one exhaust, said method comprising the steps of: producing super heated steam by burning organics containing waste products from the pulp mill; feeding the super heated steam to the condensing type steam turbine to generate a maximum amount of electricity from a given amount, temperature, and pressure of steam; 20 exhausting steam from the at least one exhaust of the steam turbine; collecting liquid wastes from the cellulose pulp k. mill, including liquid effl:-ents from the bleach plant; and 25 evaporating the collec-.ed liquid wastes using the exhausted steam from the at least one exhaust of the steam turbine as heating fluid for effecting :evaporation to produce clean condensate and a concentrated liquid waste.

7. A method as recited in claim 6, wherein step (e) is practised utilising a multiple effect evaporator with a temperature differential of about 1-4 0 C between effects. statfmildkeop~paiLroypo 40 93 15796 22

8. A method as recited in claim 6 or 7, wherein step is practised utilising an evaporator having 6-10 falling film effects with a temperature differential of about 2-3 0 C between effects. 9 A method as recited in claim 6, 7 or 9, wherein step is practised to exhaust steam having a pressure of about 1-3 psia and a temperature of about 120-140 0 F. A method as recited in any one of claims 6 to 9, wherein step is practised utilising a multiple effect eva:orator having a tevperature differential between the first and last effects of about 200C.

11. A method as recited in any one of claims 6 to wherein step is practised by burning concentrated black liquor from digestion of cellulose, and burning organic cellulosic solid waste.

12. A method as recited in any one of claims 6 to 11, wherein step is practised by collecting liquid spills from throughout the pulp mill.

13. A method as recited in any one of claims 6 to 12, 20 wherein step is practised using a plurality of different multiple effect evaporators.

14. A method as recited in any one of claims 6 to 13, S: wherein step is practised utilising steam at a predetermined first com:osite pressure and temperature value; and further comprising the step of withdrawing a second stream of steam from the steam turbine at a predetermined second composite pressure and temperature value greater than the first value. staffliridlkoepipa/retypes4744(193 15 796 /V AI 23 A method as recited in claim 14 comprising the further steps of withdrawing a third stream of steam from the steam turbine at a predetermined composite third pressure and temperature value, greater than the second value.

16. A method as recited in claim 14 or 15, wherein by utilising the different exhausted steam streams from the condensing turbine in different mill processes, the mill process in which each different steam stream is utilised particularly suited to utilisation of steam of that particular pressure and temperature, such that a steam stream is selected for a mill process based on its pressure and temperature.

17. A method as recited in claim 16, wherein the second stream of steam is utilised to evaporate black licqor produced during cellulose digestion in the pulp mill.

18. A method as recited in claim 16 or 17, wherein S, the third stream of steam is utilised as primary steam to 20 facilitate the production of cellulose pulp in the pulp mill. 0 :o 19. A method as recited in any one of claims 16 to 18, wherein the first exhausted steam stream has pressure of about 1-3 psia and a temperature of about 120-140 0 F, and 25 is utilised to practise step 0 A method of operating a cellulose pulp mill substantially as hereinbefore described with reference to any one of the accompanying drawings.

21. A method of evaporating an industrial waste liquid stream to produce clean condensate and a staffliridIkeop/parotypes4744093 15 7.96 24 concentrat'ed waste stree. substantially as hereinbefore described with reference to any one of the accompanying drawings. DATED: 15 July 1996 A AHLSTROM CORPORATION By its Patent Attorneys: GRIFFITH HACK CO Fellows Institute of Patent Attorneys of Australia V .a staff/irid/keoplpatiretypos/47440.93 151,796 I ABSTRACT OF THE DISCLOSURE A pulp mill has zero or minimum liquid discharges. The large amounts of liquid that must be processed to achieve this result are handled utilizing a multiple effect evaporator provided with heat from a steam exhaust of a condensing type steam turbine. Super heated steam from the recovery and bark boilers is fed to the turbine to generate electricity for the production of bleaching chemical and other uses in the mill. Three or four steam exhausts having different composite temperature and pressure values are taken from the turbine and used where most suitable in the pulp mill. For evaporation of bleach &S plant effluent the steam taken from the turbine preferably has a pressure of about 1-3 psia and a temperature of about 120-1400F, and has no significant adverse affect on power generation. 0 S 0t S rC j