WO1980001291A1

WO1980001291A1 - Liquor recovery

Info

Publication number: WO1980001291A1
Application number: PCT/SE1979/000250
Authority: WO
Inventors: R Lindroos
Original assignee: R Lindroos
Priority date: 1978-12-12
Filing date: 1979-12-12
Publication date: 1980-06-26
Also published as: FI68687B; FI793894A

Abstract

Method of the liquor recovery process and more precisely after the melt dissolver and causticizing in a continuously operating mixing department. From the melt dissolver the waste gases are led through the vapour pipe and to the vapour condenser and condensate separator via a pre-connected scrubber, which uses first water as wash liquor and after the washing with the first water discharges the wash liquor with the substances washed out from the waste gases included therein directly to the suction side of the first water pump for feed to the melt dissolver. The processing tanks comprised in the mixing department, the limeslaker with sorter, causticizing tanks and intermediate tank, are provided with scrubbers in the vapour pipes, in which scrubbers the waste gases flowing out are washed and cooled by atomized cooling water. Due to the pressure-drop thereby effected, vacuum is created in the respective tank. The heat in the waste gases can be recovered in the form of hot water.

Description

LIQUOR RECOVERY

This invention relates to a method of recovering sulphate or liquor after the soda recovery boiler in sulphate proc¬ esses, which method is a further development over the prior art.

The melt of liquor obtained from the soda recovery boiler is discharged into the dissolver, in which the melt is stirred in first water from the mixing department and dissolved to a density of about 22° Be. The resulting green liquor is pumped to the mixing department for calcination. The first water formed at the washing of the lime sludge obtained in the mixing department is passed to a first water clarifier, from which the first water is pumped to the melt dissolver. The emissions therefrom are discharged to the atmosphere via a vapour pipe, in which a heat exchange unit and a condensate separator are installed in series. A damper renders it possible to cause the emissions to flow either directly to the atmosphere or via the heat exchange unit and separator. In said heat exchange unit and separator the heat from the waste gases is to be recovered, and vapours and dust included therein are to be condensed and, respectively, separated. The emission to the atmosphere shall hereby be as clean as possible. The said emissions from the melt dissolver consist a.o. of dust, SO_? and H-S.

It^'has proved impossible in practice to purify the waste gases from the melt dissolver in this way, because the heat exchange unit and the condensate separator rapidly get clogged and be¬ come ineffective. The waste gases, therefore, are passed via the damper through the vapour pipe and unpurified enter the atmosphere. The environment in and about a paper mill is there¬ by heavily affected.

At a partial process within the sulphate process. for the pulp industry, the melt from the soda recovery boiler at phate or liquor recovery is dissolved in first water from the mixing department. The dissolving takes place in a melt dissolver and the solution, so-called green liquor, is pumpe to a green liquor clarifier and further to the mixing depart ment for continued treatment and conversion into white liquo In a lime-slaker an adjusted amount of quick lime is added to the green liquor purified by clarification, whereby the green liquor is converted to white liquor. This process is called causticizing.

This type of method, to which the present invention relates, heretofore has been regarded operating satisfactorily. As the energy costs rise, however, and the demands for a cleane environment increase, the said known processes and associate plants cannot longer be justified economically and be deemed to meet the environmental requirements of the community.

The green liquor from the melt dissolver has a temperature of about 90 C, and at the addition of lime in the lime-slake the temperature increases by about 10-15 C. High temperature in the lime-slaker has been a disadvantage, due to the heavy steam formation, which in its turn implies that there is a substantial gas flow through the vapour chimney, lime bin an from sorters as well as from subsequent causticizing tanks (large polluted emissions), and that the heat losses are hig because^"there has been no possibility of recovering the heat in the waste gases. It was, therefore, tried heretofore to maintain the temperature in the process on a low level.

The method according to the present invention, as it is de¬ fined in the attached claims, has rendered it possible to keep a high temperature, about 106-108 C, i.e. close to cooki temperature, in the mixing department, but yet to limit the gas leakage. The gas emitted is purified efficiently and at the same time the heat energy therein is recovered. Due to the high temperature of the liquor in the mixing department and to the recovery of the heat in the emitted vapours, a substantially higher efficiency degree is achieved when the method according to the present invention is applied instead of the known ones. As the vapours emitted from the chimneys or vapour pipes are cooled efficiently,, vacuum is created in the respective processing tanks (lime-slaker with sorter, causticizing tank and intermediate tank) , whereby steam and gas leakage in the plant is eliminated. Said vacuum can be increased by the possibility of directing the cooling water jets in the outflow direction of the waste gases, which here¬ by are imparted a forced flow, as a result of which the vacuum in the tank increases.

It is possible by the present invention to efficiently puri¬ fy the waste gases in the vapour pipe from SO-, and to recover the dust (sodium sulphate) in the gases. The invention renders it possible to continuously pass^' the waste gases through the heat exchange unit and separator, whereby the heat content in the gas can be utilized efficiently, and the thermal ef¬ ficiency degree of the installation is increased substantially. By returning the wash liquor (first water) from the scrubber to the suction side of the first water pump for feeding the melt dissolver, the control of first water in the first water clarifier is not disturbed. As the wash liguor increases the temperature of the first water fed to the melt dissolver by about 4 C, the temperature of the green liquor in the melt dissolver is increased, which is of advantage for the further processing, viz. calcinizing, in the mixing department.

The invention is described in greater detail in the follow¬ ing by way of an example and with reference to the accomp¬ anying draw^ring, in which

Fig. 1 is a flow sheet for the liquor recovery and soda recov¬ ery boiler,

Fig. 2 is a schematic section from the side of the scrubber according to the invention,

Fig. 3 is a cross-section of the scrubber along -III-III in Fig. 2 , and Fig. 4 shows an embodiment of a mixing department installation.

The installation after the soda recovery boiler (not shown) comprises a melt dissolver 1 , into which the melt of sulphate coming from the boiler is discharged. The melt is dissolved by stirring by means of stirrers 2 in first water, which is pumped by the pump 3 from a first water clarifier 4 via a conduit 5 with a four-way valve 6, from which the conduit 7 extends. The green liquor obtained in the melt dissolver 1 is pumped by the pump 8 via the conduit 9 to the -four-way valve 6 and from there through the conduit 10 to a green liquor clarifier 11. From the green liguor clarifier 11 the green liquor is pumped via the pump 12 and conduit 13 to the mixing department (not shown) . By adjusting the four-way val 6, and in order to maintain the conduits 7 and 9 clean, the flow in these conduits can be reversed so that the first wat flows through the conduit 9 to the melt dissolver 1 and is pumped therefrom through the conduit 7 by the pump 14.

From the melt dissolver 1 a vapour pipe 15 extends for cond¬ ucting away the emissions from the melt dissolver 1. These emissions, as mentioned, substantially consist of dust, S0~ and H_S. In parallel with said vapour pipe 15 a heat exchange unit or vapour condenser 16 and a cendensate separator 17 are connected in series, which separator 17 in principle is a cy clone. A fan 18 blows the waste gases again further outward in the vapour pipe 15. The heat exchange unit is cooled by water, which flows in at 19, for example at a temperature of about 45 -C, and flows out at 20, for example at a temperature of about 75 C. The damper 21 is used for controlling the flow path of the emissions, either through said unit and vapour condenser or directly out to the atmosphere.

Between the melt dissolver 1 and the heat exchange unit a scrubber 22 is provided in the vapour pipe. The scrubber, which will be described in greater detail below, has the object ot washing-out and condensing the emissions from the melt dissolver 1. As wash liquor first water is used, which is drained through the conduit 23 from the conduit 5. The pressure of the first water is increased by means of a pump 24 and the first water is sprayed out into the scrubber via a riser pipe and -branch lines through nozzles 25-30 arranged in a certain way. The liquid collecting at the bottom of the scrubber and consisting of substances washed out of the waste gases and of the wash liquor, is passed via the conduit 31 to the suction side of the pump 3. To said conduit 31 also are connected the drains from the heat exchange unit 16 and the condensate separator 17 as well as the fan 18. Due to the fact that the return liquid is pumped from the conduit 31 directly via the pump 3 to the melt dissolver, stoft (soot and the like) included in the liquid is pumped from the melt dis¬ solver to the green liquor clarifier 11, from which the dust at regular intervals can be removed in the form of sludge.

The scrubber 22 is shown in greater detail in Figs. 2 and 3. It comprises, as can be seen, a cylindric shell 32 with a bottom 33 and a concentrically disposed inlet 34 for the vapour pipe 15 in said bottom. The inlet extends a distance upward above the bottom 33, as shown. The shell 32 terminates upwardly in a cone transforming to an outlet 35 as a continu¬ ation of the vapour pipe 15. Within the scrubber, above the inlet 34 a downwardly open screen 36 is located, the diameter of which is slightly greater than the inlet. A corresponding screen 37 is located beneath the outlet 35. The aforesaid nozzles 25-29 are located along the longitudinal axis of the scrubber in equally spaced relationship and directed upward and spray the wash liquor (first water) cone-shaped upward, as indicated in the drawing. A nozzle 30 is located in the inlet and also faces upxvard, so that the spray moves in the flow direction of the waste gases all the way to the mouth of the inlet. The nozzles 25-30 are supported in the scrubber by means of the feed conduits (indicated dash-dotted in the drawing) . Owing to the kinetic energy and direction of the jets, a thorough washing of the waste gases and a forced draft in the vapour pipe are obtained. The screen 37 provents liquid from being taken along outward through the outlet, and the screen 36 prevents liquid from penetrating down into the inlet. Due to the substances included in the waste gases, deposits can accumulate on parts in the scrubber, particularly on the screen 36. In order to prevent this, a pair of tangentially directed conduits 38 open on the inside of the screen 36, and a pair of nozzles 39 are directed obliquely upward to the same side of the screen. A pair of conduits 40 further may be arranged to open tangentially in the inlet 34 for the same purpose. The said conduits and nozzles are fed wit hot water under pressure for flushing the parts concerned when deemed necessary. This conduits 38, 40 and the nozzles 39 are supplied with hot water via a collecting pipe 43 surrounding the scrubber. The conduits preferably are pro¬ vided with valves, as indicated.

A conduit 41 is provided (see Fig. 1) for intermittently flushing the scrubber bottom, which conduit is connected to the first water conduit 23. Said conduit 41 is not shown in Fig. 2 ( for reason of clearness) , but it can be provided in a suitable way with one or more nozzles directed oblique to the bottom 33.

The wash liquor with washed-out emissions is conducted away through the drain 42 (Fig. 2) via the aforesaid conduit 31 back to the suction side of the pump 3.

44 designates a nozzle located in the outlet 35 and fed wit water. Its object is at interruption of the first water sup to be able to wash the waste gases with water. This is bett than no washing at all of the waste gases. Several water no zles, of course, may be provided suitably placed.

A downward inclined flange 45 is attached to the inside of the shell in such a way, that the jets from the upper noz¬ zle 25 arrive at the upper surface of the flange, and the jets from the nozzle 26 arrive at the lower surface of said flange. Said flange guides the liquid flowing along the inside of the shell to the centre of the scrubber.

According to another embodiment of the invention according to Fig. 4, is green liquor pumped to the green liquor clari fier 11. The green liquor clarified therein is pumped throu the conduit 46 and distributor 47 to the lime-slaker 48. Through a lime bin 49 and via a conveyor-typ weigher 50 a uniform^' flow of lime is batched to the lime-slaker 48, in

^■'- O.V. which the lime is mixed with the green liquor. Said green liquor is allowed to have a temperature of about 94 C when it enters the lime-slaker and increases the temperature there¬ in to about 106-108°C at the supply of the lime, partly due to the temperature of the lime and partly due to the occur¬ ring reaction. The lime-slaker is provided with a sorter 51 supplied also with green liquor from the distributor 47, to which sorter the content in the lime-slaker flows out. The sorter includes in conventional manner a scraping mech¬ anism, which continuously removes sand etc. The mixture of liquor and lime is passed via the sorter 51 through the two causticizing tanks 52, 53 where the causticizing reaction takes place at heavy stirring. From the causticizing tank 53 the mixture flows over to the intermediate tank 54, from which the misture is pumped to white liquor clarifiers, strainers or filters, in which the lime sludge is separated from the liquor. The clear liquor and the lime sludge are passed from the white liquor clarifier to continued proces¬ sing.

In such a system a scrubber 57 and, respectively, 58 have been attached in the vapour pipe 55 to the lime-slaker 48 and in the vapour pipe 56 to the sorter 51. These scrubbers are identical in principle and of the type disclosed before. The nozzles 59 provided in the scrubber and located after its central axis bring about a disc-like spray pattern (indicated by dashed lines), which, for example by nozzle exchange, can be varied from the indicated upward directed cone-shape to a plane or even downward directed cone-shape. The- ozzles 59 are supplied with cold water from the conduit 60. Above the scrubber and in the vapour pipe also one or several noz¬ zles 61 supplied with cold water may be provided. The scrub¬ ber 57, besides, is provided downwardly with a system of noz¬ zles for intermittently flushing the scrubber bottom with hot water from a hot water tank 62, and a nozzle 63 is pro¬ vided in the mouth of the vapour pipe in the scrubber. The said nozzle 63 like the other nozzles is variable with respect to its spray pattern. The nozzle 63, however is supplied continuously either with hot water or with green liquor fro the green liquor clarifier 11 vis the conduit 46 and 64. The reason of this is to maintain the deposits in the scru ber in solution. It is possible by means of suitable valves also in this latter case to flush hot water intermittently through the nozzle 63 in order to clean it from deposits in the form of lime sludge etc.

In the vapour pipe 65, 66, 67 to the causticizing tanks 52, 53 and the intermediate tank 54 also scrubbers 68, 69 and respectively, 70 are provided, which are of the same type as in the vapour pipe for the sorter 51. The scrubber 76 for the intermediate tank, however, at the embodiment shown is provided with four nozzles. The spray and cooling liquid for these scrubbers is also cold water from the condiut 60.

The wash liquor collecting on the bottom of the scrubber 57 and including large amounts of lime sludge is led via a con uit 71 to a collecting tank 72, in which the sludge is sepa ated and the liquid is passed to a recausticizing tank 73, from which the lime sludge is pumped to the sludge washer. The wash liquor from the scrubbers 58, 68, 69 which substan ally consists of the cooling water now heated, is led via the conduit 74 to a second collecting tank 75, in which exi ing sludge amounts are separated and the water is led to a hot water tank or accumulator 62, from which a.o. the hot water for the nozzles and the mouthpiece 63 in the scrubber 57 can be drained. The wash liquor from the scrubber 70 to the intermediate tank 54, which liquid is relatively "clean^' is passed directly via the conduit 76 to the tank 62. The water in said tank holds about 70°C and can be used for different purposes in the process.

The system comprises, in.addition to the pumps designated - by P, also a plurality of valves in connection to mouth¬ pieces and nozzles.Said valves are not designated especiall but only indicated by conventional symbols in the drawing. 0 designates stirrers provided in the tanks concerned. It should become apparent through the present invention as described above, that it has become possible for the first time to recover the heat in the waste gases from the compon¬ ents comprised in a mixing department, especially the li e- -slaker. The waste gases or vapours therefrom contain, as mentioned, a.o. large amounts of dust, lime. By cooling the waste gases a vacuum is created in the vapour pipe (pipes) and thereby in the lime-slaker or respective tank, whereby gas leakage through the lime bin and other openings and gates as well as from the sorter is eliminated. As a vacuum is maintained in the lime-slaker, it is automatically also pos¬ sible to increase the temperature therein without increas¬ ing the gas leakage. The temperature of the green liquor and added lime thereby is not so critical, and it is pos¬ sible to hold an operation temperature as close as possible to the cooking temperature. The said vacuum can be increased by directing the cooling water sprayed from the nozzles in the scrubbers in the outflow direction of the vapours whereby a forced drive of the waste gases is brought about.

The number of nozzles in the respective scrubber is determin¬ ed by the temperature of the available cooling water and, of course, on the operation data of the installation in general. The .collecting tanks 72 and 75 can also be imagined be replaced by a single tank for separating the sludge, from which tank the liquid is led to the hot water tank 33.

As mentioned in the introductory portion, a substantially better and more efficient purification of the waste gases from the melt dissolver is achieved by the present invention compared with prior art, at the same time as the thermal efficiency degree of the installation is improved. This is possible, because the heat exchange unit or vapour condenser, which according^' to^' the invention must condense only water vapour and very small residues of the emissions, can be operated continuously and, thus, without being subjected to a high degree of pollution, and because the temperature of the ^"firts water supplied to the melt dissolver can be held higher by the addition of heated wash liquor scrubber.

Claims

10CLAIMS

1. A method at the recovery of liquor after the soda recov¬ ery boiler and at causticizing in a continuously operating mixing department in sulphate processes, in which are com¬ prised the lime-slaker supplied with green liquor from a green liquor clarifier and fed with lime and associated with a sorter, one or several causticizing tanks and inter¬ mediate tank, where the liquor received from the soda recov¬ ery boiler is supplied into a melt dissolver, to which first water from a first water clarifier is pumped and from which green liquor is led to a green liquor clarifier, and from which a vapour chimney extends with a heat exchange system coupled thereto, c h a r a c t e r i z e d i n that the vapour emitted from the melt dissolver and consisting sub¬ stantially of dust, S0₂ and H₂S is passed prior to the heat exchange system through a scrubber, in which the vapour is washed by first water, which is atomized in the scrub.ber, and that substances washed out of the vapour in the scrubber together with the added first water is collected in the scrubber and led to the suction side of the pump for pumping the first water to the melt dissolver.

2. A method as defined in claim 1, c h a r a c t e r i z¬ e d^' i n that from the first water clarifier the first water is pumped by the same pump both to the melt dissolver and to the scrubber, and that the first water is injected into the same by means of a pressure-increasing pump.

3. A method as defined in claim 1 o 2, c h a r a c t e r¬ i z e d i n that substances possibly condensed and coll¬ ected in the heat exchange system and condensate separator are collected and conducted away together with substances collected from the scrubber.

4. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the vapour after the heat exchange system is passed through a condensate separa¬ tor, preferably in the form of a cyclone. 1 1

5. An apparatus for carrying out the method according to claim 1 , at liquor recovery after the soda recovery boiler in sulphate processes, in which the liquor obtained from the soda recovery boiler is supplied into a melt dissolver, to which by means of a first pump first water is pumped- from a first water clarifier, and from which by means of a second pump green liquor is pumped to a green liquor clari¬ fier, and from which a vapour chimney extends with heat exchange system coupled thereto, c h a r a c t e r i z e d i n that, in the vapour chimney between the melt dissolver and the heat exchange system a scruber is located for washing the vapour, which substantially consists of dust, SO- and H₂S, which scrubber comprises a cylindric shell, which down¬ wardly is provided with an inlet concentric with the shell for the vapour chimney and upwardly is provided with an outlet concentric with the inlet as a continuation of the vapour chimney to the heat exchange system, that a row of spray nozzles are provided in the scrubber along the shell axis and directed each to atomize the first water in a disc- -like spray pattern, and that a first screen in the form of a cone with the tip directed

is located between the row of spray nozzles and the inlet.

6. A method as defined in claim 1, c h a r a c t e r i z¬ e d i n that the vapour emitted from the lime-slaker and the vapour emitted from the sorter are cooled and washed by cold water jets, and that the substance in the wash liquor is recovered.

7. A method as defined in claim 6, c h a r a c t e r i z¬ e d i n that the vapour emitted from the lime-slaker is passed through a first scrubber and washed and cooled therein by cold water jets, and the vapour emitted from the sorter is passed through a second scrubber and cooled and washed therein by cold water jets, and that wash liquor collecting in the second scrubber is led to a hot water system, from which hot water is led to the first scrubber for inter¬ mittently washing the same. •_

12

8. A method as defined in claim 7, c h a r a c t e r i z- e d i n that wash liquor collecting in the first scrubber is led to a recausticizing tank.

9. A method as defined in any one of the claims 6-8, c h a r a c t e r i z e d i n that the vapour from the causticizing tank is passed through a third scrubber and cooled and washed therein by cold water jets, and that wash liquor collecting in the third scrubber is led to the hot water system.

10. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the vapour from the intermediate tank is passed through a fourth scrubber and cooled and washed therein by cold water jets, and that wash liquor collecting in the fourth scrubber is led to the hot water system.

11. A method as defined in any one of the claims 6-10, c h a r a c t e r i z e d i n that the cold water jets can be directed by a movement component in the flow directi of the vapour through the respective scrubber.

12. A method as defined in any one of the claims 6-11, c h a r a c t e r i z e d i n that the cooling and wash¬ ing cold water jets are sprayed out in a disc-like spray pattern-from nozzles located centrally in the scrubbers along the flow path of the vapour.

13. A method as defined in claim 7, c h a r a c t e r i z e d i n that the spray pappern is conical, the tip direct ed against the flow direction of the vapour.

14. A method as defined in any one of the claims 6-13, c h a r a c t e r i z e d i n that green liquor from the green liquor clarifier is injected into the first scrubber in connection to its inlet.

15. A method as defined in any one of the claims 6-14, c h a r a c t e r i z e d i n that hos water is injected into the first scrubber in connection to its inlet