WO2009153404A1

WO2009153404A1 - Method and apparatus for condensing the water in process gas and scrubbing the gas

Info

Publication number: WO2009153404A1
Application number: PCT/FI2009/050532
Authority: WO
Inventors: Tero Kolhinen; Launo Lilja; Maija-Leena Metsärinta; Pasi MÄKELÄ
Original assignee: Outotec Oyj
Priority date: 2008-06-19
Filing date: 2009-06-17
Publication date: 2009-12-23
Also published as: FI20080409A0; FI121409B; FI20080409A

Abstract

The invention relates to a method and apparatus for condensing the abundant water vapour in process exhaust gases and for scrubbing the gas of impurities in gas, solid and liquid droplet form. According to the method, scrubbing liquid is fed into the gas flow in order to cool the gas i.e. to condense the water vapour, in at least two different steps, the later of which occurs as the velocity of the gas is accelerated. After this the gas is subjected to wet scrubbing by routing the gas thus reduced in volume into a scrubbing chamber containing scrubbing liquid, from where the purified gas is discharged after droplet separation into the outside air. The scrubbing liquid is circulated so that some of the liquid is routed from the scrubbing chamber to cooling and solids separation, after which it is recirculated via high-pressure pumps to be fed to the cooling and scrubbing of the gas flow.

Description

METHOD AND APPARATUS FOR CONDENSING THE WATER IN PROCESS GAS AND SCRUBBING THE GAS

FIELD OF THE INVENTION The invention relates to a method and apparatus for condensing the abundant water vapour in process exhaust gases and for scrubbing the gas of impurities in gas, solid and liquid droplet form. According to the method, scrubbing liquid is fed into the gas flow in order to cool the gas i.e. to condense the water vapour in at least two different steps, the later of which occurs as the velocity of the gas is accelerated. After this the gas is subjected to wet scrubbing by routing the gas thus reduced in volume into a scrubbing chamber containing scrubbing liquid, from where the purified gas is discharged after droplet separation into the outside air. The scrubbing liquid is circulated so that some of the liquid is routed from the scrubbing chamber to cooling and solids separation, after which it is recirculated via high-pressure pumps to be fed to cooling and scrubbing of the gas flow.

BACKGROUND OF THE INVENTION

The multiphase scrubbing of hot gas (800 - 1200⁰C) has been described earlier, for instance in US patent 6,149,715. The method and apparatus consist of the possible prewetting of the gas, the actual cooling of the gas in an ejector-venturi scrubber with a high-pressure water jet, a second scrubbing step, in which the gas collides with the surface of the liquid in the water tank below the venturi. In the third scrubbing step the venturi scrubber proper is used, where the gas velocity is greatly accelerated (40 - 150 m/s) and a smaller amount of scrubbing water is used than in the first venturi. There is also a water tank below the second venturi scrubber, where gas scrubbing continues. The gas from the water tank is routed to a horizontal ejector, where the speed of the water jet is greater than that of the gas and from there it is routed on to droplet separation. The gas may be for instance mainly carbon monoxide, in which case the purified gas is fed to combustion. A method is known from US patent 4,643,742 for the cooling and scrubbing of metallurgical industry gases of dust and water vapour. The temperature of the gas is high (over 100° C) and the amount of dust large. The purpose is to carry out cooling and scrubbing using as little water as possible and low water and gas velocities in order to save energy etc. In the first stage, the gas is routed from the top down inside a gas inlet pipe. Water is fed into the pipe via nozzles, the water vaporises and the gas is saturated with water vapour at the desired temperature, which is normally below 100⁰C. The gas inlet pipe is shaped after the first set of water nozzles into a venturi. Water droplets are injected into the venturi throat radially with regard to the cross- section of the venturi throat via one or more jets in order to maximise the difference in velocity between the gas and the water among other things. The problem is the wear and clogging (accretions) caused by the solid particles in the throat of the venturi. Attempts are made to avoid this by feeding water tangentially along the inner walls of the inlet pipe. When the temperature of the sprayed water is lower than that of the gas, the gas is simultaneously cooled.

There is a water tank below the venturi outlet pipe, and as the gas loaded with water droplets and dust particles flows above the water surface to the second scrubbing step, upon changing the direction of the gas flow, the majority of the water droplets and dust fall into the water tank. In the second scrubbing step the direction of the gas is diverted to flow from the bottom upwards through a bed filled with moulded forms, onto which water is sprayed via nozzles. There is also a water tank below the second scrubbing step, into which the water droplets fall. The water tanks are connected to each other and the second tank is always higher than the first one. A discharge pipe for the purified gas with a droplet separator is located in the centre of the second scrubbing stage. The water to be sprayed via nozzles is recirculated from the water tanks of the apparatus. In order to achieve sufficient cooling, if required the temperature of the water to be fed into the second scrubbing step in particular is adjusted by routing cold water to the nozzles from outside the circuit.

In the method accordant with US patent 4,643,742 the cooling water fed in above the venturi and the washing water fed into the venturi are fed radially or crosswise (at an angle of 90 degrees) to the gas flow. In practice at higher water speeds this causes what is termed a pressure lock in the gas flow, whereupon the gas flow stops momentarily and is then discharged again from time to time. The pressure lock effect causes vibrations in the apparatus, which are harmful for the equipment. Another drawback of the method is the fact that the gas pipe is flushed with tangential water jets only just above the venturi. Where dust-containing gases are concerned, a film of water should form on the inner surface of the gas pipe immediately in the upper end of the pipe and not only halfway along the pipe, so that accretions are unable to form. In addition, tangential water feed causes rotation in the gas flow and the function concept of the venturi below is broken. The use of a bed of moulded forms is not beneficial when handling gases containing a lot of dust, since the dust can easily block the bed of filler pieces. Additionally, the bed of filler pieces needs constant flushing.

US patent 6,805,734 describes a wet scrubber for exhaust gas, in which the outer edge has numerous cascade tubes for scrubbing the gas and the central section of the scrubber is shaped as a droplet separator, through which the purified gas is discharged into the environment. It is typical of the cascade scrubber that the gas to be purified is routed inside the scrubbing liquid, whereupon the separation of the solid particles from the gas occurs with more certainty. The scrubber is constructed to function at negative pressure, whereby the gas is given suitable speed by the blower situated after the scrubber. PURPOSE OF THE INVENTION

The purpose of the method and apparatus accordant with the invention is to eliminate the shortcomings of the methods above and to introduce a simple method and apparatus for removing water vapour from gas from a hydrometallurgical process or some other gas containing a great deal of moisture and at the same time for scrubbing it of droplets of liquid and other impurities, such as solid particles.

SUMMARY OF THE INVENTION The essential features of the invention will be made apparent in the attached claims.

The invention relates to a method for condensing the water vapour in process exhaust gas and for scrubbing the gas of impurities in the form of gas, solids or liquid droplets. It is typical of the method that the gas is made to flow downwards in the feed pipe and is cooled by means of pressurised jets of scrubbing liquid, which are fed obliquely downwards towards the gas flow. The gas is routed on to a venturi scrubber, in which the gas is further cooled and scrubbed preferably in two stages by means of jets of scrubbing liquid directed obliquely downwards, after which the gas flow is made to flow to a cascade scrubber located below the venturi scrubber, comprising at least one cascade tube. By means of the cascade scrubber, the majority of the cooled water from the gas as well as the droplets and solid particles that have been in the gas are separated into the cooling liquid of the scrubbing chamber and the purified gas is routed via a droplet separator into the outside air.

It is typical of the method accordant with the invention that the temperature of the exhaust gas entering cooling and scrubbing is a maximum of 100⁰C. The exhaust gas is hydrometallurgical process exhaust gas or granulation exhaust gas. According to the method the gas is cooled in two stages before the venturi. Typically over half of the scrubbing liquid is fed into the gas before the venturi.

According to the method, the jets of scrubbing liquid are fed into the gas at a 25 - 35 degree angle to the horizontal.

In the venturi the gases are scrubbed and cooled in two stages. In order to obtain a good scrubbing effect the velocity of the gas is accelerated by means of the venturi at least fivefold compared to the velocity of the gas in the feed pipe.

According to one embodiment of the invention the scrubbing liquid used is a lye solution. According to another embodiment of the invention the scrubbing liquid used is an acid solution. According to a third embodiment the scrubbing liquid is a permanganate solution or a mixture of lye solution and permanganate solution.

According to one embodiment of the invention the gas feed pipe is flushed by means of a flushing arrangement situated at the top end of the pipe, whereby the cut feed pipe is surrounded by a gastight water lock, into which the flushing liquid is fed; at the cut-off point the upper edge of the pipe is serrated, so that the flushing liquid flows evenly along the inner surface of the pipe.

The invention also relates to the apparatus for condensing the water vapour in process exhaust gas and for scrubbing the gas of impurities in the form of gas, solids and liquid droplets. It is typical of the apparatus that it includes a feed pipe, to the upper part of which is attached at least one set of nozzles for feeding pressurised scrubbing liquid obliquely down into the feed pipe. The lower part of the feed pipe is connected to a venturi scrubber, which is equipped with at least two sets of nozzles for feeding pressurised scrubbing liquid obliquely down into the venturi. The lower part of the venturi scrubber is connected to an intermediate pipe and this is connected to a cascade scrubber, which is surrounded by a scrubbing chamber. One end of the gas discharge pipe is connected to the scrubbing chamber and the other end to a droplet evaporator.

According to another embodiment of the invention, the number of sets of nozzles attached to the upper part of the feed pipe is two.

It is typical of the apparatus that the set of nozzles attached to the upper part of the feed pipe is made up of nozzles located symmetrically crosswise to the feed pipe, numbering from 3 to 8 and connected to a distribution device for feeding the scrubbing liquid.

Preferably the lower nozzles of the nozzle groups attached to the upper part of the feed pipe are phase shift with regard to the upper nozzles.

In the apparatus the sets of nozzles attached to the tapered cone of the venturi and the venturi throat are made up of nozzles located symmetrically crosswise to the venturi, numbering from 2 to 6 and connected to a distribution device for feeding the scrubbing liquid. The nozzles of the nozzle group attached to the venturi cone are typically phase shift with regard to the nozzles in the venturi throat.

Preferably the nozzles of the nozzle groups attached to the upper part of the feed pipe and the venturi are directed obliquely downwards at an angle of 25 - 35 ° to the horizontal.

According to one embodiment of the apparatus, a flushing arrangement is placed in the feed pipe above the nozzle groups, formed of a cut feed pipe and a gas-tight liquid lock surrounding it, whereby the upper edge of the feed pipe is serrated at the cut-off point. LIST OF DRAWINGS

Figure 1 is a diagram of the apparatus according to the invention, where the capacity is small, Figure 2A presents a more detailed longitudinal section of the upper part of the apparatus,

Figures 2B and 2C are cross-sections of points A-A and B-B in Figure 2A, Figure 3A is a longitudinal section of the venturi section of the apparatus, Figures 3B and 3C are cross-sections of points A-A and B-B in Figure 3A, Figure 4 is a longitudinal section of the lower section of the apparatus,

Figure 5A presents a more detailed longitudinal section of the lower section of the apparatus, into which several cascade tubes are placed,

Figure 5B is a cross-section of Figure 5A, and

Figure 6 is a diagram of the way the water film is formed in the gas pipe.

DETAILED DESCRIPTION OF THE INVENTION

The gas exiting a hydrometallurgical process related to the production of metals may contain a great deal of water vapour, as much as over half its volume. The gas generally originates from many, even many tens of reactors, from which the gas can be combined for treatment with the method and equipment accordant with the invention. The gases to be scrubbed are made up of for instance the excess of air used in leaching in leaching reactors, the nitrogen from reacted air and the water vapour brought along with them. The gas may also contain drops of solution transported from the reactors for example with the ventilation gases. Solids are also transported with the gas as small particles. The solids may be the feed material of the process, such as concentrate, or the end product, such as sulphur for instance. In addition to hydrometallurgical process gas, the invention also relates to gases from other treatments, which also contain a lot of moisture. Such is for example the gas from granulation, the temperature of which is typically around 100° C or below, because as is well known, the outside air brought with the gas cools the gas down. The purpose of gas treatment is thus to reduce the amount of gas significantly by removing water vapour from it and simultaneously scrubbing the gas clean from impurities. The gas is scrubbed with a scrubbing liquid, which according to the impurities in the gas is either alkaline, acid or the liquid may be for instance a solution of permanganate. Alkaline scrubbing liquids are used for the scrubbing of acid components, acidic liquids are used for scrubbing away metal residues, and when the gas contains for example arsines, they can be removed with a scrubbing liquid containing permanganate. The scrubbing solution may also be a mixture of alkaline and permanganate solutions. The permanganate is preferably potassium permanganate.

Figure 1 reveals the apparatus used in scrubbing. The temperature of gas entering gas treatment is usually around 90 - 100⁰C, but the apparatus is also suitable for treating gases exiting an autoclave. The gas is routed to flow downwards in the essentially vertical feed pipe 1 , where it is subjected in the first stage to precooling. In the precooling phase over half and typically ³A of the amount of liquid used for cooling is sprayed onto the gas to be cooled via nozzle groups 2 and 3. Preferably the number of nozzle groups is two, but however at least one. The scrubbing liquid used for cooling is at around room temperature. The amount of scrubbing liquid used for cooling and scrubbing is regulated to be such that the temperature of the gas exiting the equipment into the outside air is as low as possible, so that the amount of water vapour remaining in it is also small. Preferably the temperature is a maximum of 4O⁰C. The velocity in the pipe of the gas to be treated is around 10 - 15 m/s, and the pressure of the scrubbing liquid to be sprayed on it is adjusted to around 2 - 15 bar.

The precooled gas is routed next to the second cooling stage, which takes place in venturi scrubber 4, in which the rest of the scrubbing liquid used in cooling is sprayed onto the gas via nozzle groups 5 and 6. The upper nozzle group 5 is located at the point of the tapered cone 7 of the venturi 4 and the lower nozzle group 6 at the point of the venturi throat 8. The pressure of the scrubbing liquid is of the same order as that in the first cooling stage. By means of the venturi and the nozzle groups located in it, the gas is cooled further and also prescrubbing of the gas is carried out. It is advantageous to dimension the venturi so that the gas velocity rises in the venturi throat to at least five times the gas velocity in the feed pipe.

After the venturi scrubber 4, the gas is fed via an intermediate pipe 9 to the actual gas scrubbing stage, whereby the gas is fed via a cascade scrubber

10 into a chamber 1 1 containing scrubbing liquid. Generally there are several cascade tubes in a cascade scrubber, but the case shown in Figure 1 presents an apparatus intended for a small capacity, in which there is only one cascade tube through which the whole amount of gas is fed into the scrubbing chamber. The number of cascade tubes depends on the amount of gas, i.e. the capacity, so that when the capacity increases, the number of cascade tubes is increased. The principle is that the discharge opening of the nozzle is a maximum of 300 mm, and when this is not sufficient for the amount of gas to be treated, the number of tubes is increased. A cascade tube is usually made up of an inner nozzle 12 and an outer gas deflection tube 13 as well as a dish-like collision plate 14 fixed to the inner tube.

In the case accordant with the invention, the cascade tube is dimensioned so that the velocity of the gas and the scrubbing liquid brought with it is around 30 - 50% of the gas velocity in the throat of the venturi 8. The lower section of the scrubbing chamber or liquid section 15 is dimensioned so that the temperature of the liquid remains almost constant or, if necessary, the lower section of the chamber is equipped with cooling devices. The upper section 16 of the chamber i.e. the gas space is made wide enough that the gas can achieve a slow discharge velocity, for example, less than 1 m/s. The gas space is also made to be high enough to prevent the splashing of scrubbing liquid when the gas rises from the scrubbing bath. The cascade scrubber is especially effective in the removal of dust from gas.

The clean gas, from which the majority of the liquids have been removed, is routed through discharge pipe 17 to the droplet separator 18, in which separation of droplets takes place for instance using a technique known in the prior art. After droplet separation the gas is discharged into the atmosphere. The droplets of liquid separated from the gas in droplet separation are routed back to the liquid circuit (not shown in detail in the drawing).

The scrubbing liquid is taken for recirculation from the scrubbing chamber 1 1 , so it is routed either completely or partially to purification. The scrubbing liquid is cooled and fed via pumps to the nozzle groups described above for gas cooling.

Figure 2A presents in more detail the gas feed pipe 1 and the pre-cooling nozzle groups 2 and 3 mounted in it, into which the scrubbing liquid is fed from a distribution device, for example ring-shaped distribution pipes 19 and 20 encircling the feed pipe. As shown in the drawing, the upper nozzle group 2 is made up of four nozzles 21 , but when the capacity of the scrubber is greater than that of the case in the drawing, obviously the number of nozzles is greater. The nozzles are directed downward, preferably at an angle of 25 - 35° to the horizontal. When the nozzles are directed to some extent in the direction of the flow, the jets of liquid do not form a "pressure lock" resisting the gas flow, which occurs in equipment according to the prior art. As shown in the cross-section in Figure 2B, the nozzles are at 90° to each other. The number of nozzles in the drawing is four, but it could also be anything else between 3 and 8, so that the angle to each other is between 120 - 45°. In every case the nozzles are located symmetrically with regard to the cross- section of the feed pipe. The jets of liquid that come out of the nozzles are aimed at the centre of the feed pipe, i.e. the feed does not take place tangentially.

The lower pre-cooling nozzle group 3 is also made up of four nozzles 22, which are also directed downward, preferably at an angle of 25 - 35°, and as shown in cross-section 2C, they are also at an angle of 90° to each other. A comparison of cross-sections 2B and 2C also shows that the nozzles 22 of the lower nozzle group are not at the same point of the gas feed pipe as the nozzles 21 of the upper nozzle group, but located between the upper nozzles, i.e. in this case they are phase shift by 45° to the horizontal compared to the upper nozzles. What was stated above also applies to the nozzles of the lower nozzle group, i.e. that the number of nozzles may vary correspondingly to the upper nozzle group, so that the amount of nozzles in each group is the same.

Gas cooling continues in the venturi scrubber 4, to which the gas feed pipe 1 is connected. In Figure 3A there is a downward tapered cone 7 in the upper section of the venturi before the actual throat 8 of the venturi, where the cross-sectional area of the venturi is at its smallest. Also second stage cooling, which occurs in the venturi with scrubbing liquid, is advantageously performed in two parts from the ring-like distribution pipes 23 and 24 surrounding the venturi. In this case the first part of the scrubbing liquid is fed into the tapered cone 7 above the throat of the venturi via the upper nozzle group 5, which is formed of several nozzles 25. In the case of Figure 3B the number of nozzles is 3, but naturally it could be otherwise, for example between 2 and 6. The nozzles in this case too are distributed evenly on the cross-section of the venturi cone so that when there are three of them, the angle between them is 120°. The nozzles are directed downward, preferably at an angle of 25 - 35° to the horizontal. The nozzles are also located symmetrically with regard to the cross-section of the venturi. The lower nozzle group 6 of the second cooling stage, which is simultaneously the first gas scrubbing stage, is located in the throat 8 of the venturi. The lower nozzle group 6 is also made up of several nozzles 26, numbering preferably between 2 and 6. In Figure 3C the number of nozzles is three. The nozzles are also directed downward here, as in all the previous cases, preferably at an angle of 25 - 35°, and as shown in the cross-section 3C, they are also at a degree of 120° to each other. A comparison of cross- sections 3B and 3C reveals that the nozzles 26 of the lower nozzle group are not located at the same point in the gas feed pipe as the nozzles 25 of the upper nozzle group, but are situated between the upper nozzle groups, i.e. they are phase shift with regard to the venturi cone nozzles.

Normally, the kind of venturi described above operates at high gas velocities, whereby the gas causes a suction effect in the scrubbing water gaps at the throttling point and atomises the water into small droplets, which is why high water pressure is not required. In the method and apparatus accordant with this invention, however, a fairly high pressure is usedfor the scrubbing liquid, so that a jet of liquid containing a light mist is formed, which has the power to penetrate into the centre of the gas flow and even further. In this kind of case, the liquid jets must not be completely crosswise to the gas flow, because in that case what is known as a pressure lock is generated, which causes vibration as mentioned earlier.

After the throat, the venturi widens gradually as a conical tube 27 below the throat so that the lower end of the tube, which is connected to the cascade scrubber nozzle tube 12 via an intermediate pipe 9, has a diameter that is 50

- 100% that of the feed pipe. Figure 4 presents the second gas scrubbing stage in more detail. It consists of a cascade scrubber 10 and the scrubbing chamber 1 1 surrounding it. The intermediate pipe 9 of the gas is connected to the inner nozzle tube 12, to which is also fixed the collision plate 14 that causes droplet formation. The nozzle tube narrows conically and is connected at its lower section, below the collision plate, to the outer tube 13 surrounding the nozzle tube. The cascade tube is positioned in the scrubbing chamber so that the liquid surface 28 of the scrubbing chamber is more or less at the same height as the lower end of the nozzle tube. The gas and the scrubbing liquid entrained with it are discharged into the lower section 15 of the chamber.

The scrubbing liquid discharged with the gas into the scrubbing chamber and the water separated from the gas plus the droplets of liquid and solid particles that had been in the gas remain in the scrubbing chamber and the purified gas flows into the upper part of the chamber i.e. the gas space 16. The gas is removed from the gas space via a discharge pipe 17 into the droplet separator 18 and from there further into the outside air. The pressure regulation of the gas flowing through the cooling and scrubbing apparatus is taken care of with a fan situated after the droplet separator (not shown in detail in the drawing), so that the apparatus operates at reduced pressure.

Figures 5A and 5B show an example of the lower section of the apparatus accordant with the invention, in which the number of cascade nozzles has been increased with a rise in capacity.

The method and apparatus accordant with the invention comprise several different stages, whereby the cooling of the gas takes place mainly by means of jets of scrubbing liquid directed into the gas and gas scrubbing in both a venturi scrubber and a cascade scrubber, whereby both scrubbing stages complement each other to achieve a good final result. It is clear that when scrubbing liquid, which may be alkaline or acid, is used for cooling and scrubbing, cooling and scrubbing are not only physical phenomena, but the scrubbing liquid is selected so that desired chemical reactions also occur in them all the time in order to purify the gas.

Figure 6 presents one preferred way to flush the inner surface of the gas feed pipe 1 , in order to prevent solid particles from adhering to the surface of the pipe. The inner surface flushing arrangement 29 is positioned above the upper cooling nozzle group (not shown in the drawing). The gas pipe is cut and the cut-off point is encircled by a water- and gas-tight water lock 30, into which the flushing liquid 31 is fed. If the amount of dust contained in the gas is small, the scrubbing liquid in the circuit may be used as flushing liquid, but if the amount of dust is large, it is preferable to use clean water. The upper edge 32 of the pipe is notched, i.e. serrated, and the flushing liquid flows from it evenly downwards along the edges of the pipe.

EXAMPLE

Copper concentrate (Cu 28.6 %, Fe 30.1 %, Zn 2.4 % and Pb 0.35 %) and air were fed into a chloride leaching reactor with mixing. Exhaust gas was removed from the reactor at a temperature of 95 °C.

The exhaust gas was cooled and scrubbed with a solution of lye in a condensing venturi scrubber accordant with the invention. The pH of the scrubbing solution was adjusted by the lye solution to a value of 6 - 8 and the temperature of the discharged gas was adjusted by the circulation of solution. The scrubbing solution was circulated via a heat exchanger. Energy was recovered from the heat exchanger. On cooling, 200 kg/h of water was condensed.

Gas removed from scrubbing: Flow 914 m³/h Temperature 40 °C

Moisture content 7.2 %

Impurities content:

Cl 0.26 mg/m³ Cu <0.02 mg/m³

Zn <0.02 mg/m³

Pb <0.02 mg/m³ The impurities content of the gas to be discharged is so low that the gas can be discharged freely into the outside air.

Claims

PATENT CLAIMS

1. A method for condensing the water vapour in process exhaust gas and scrubbing the gas of impurities in the form of gas, solids and droplets of liquid, characterised in that the gas is routed in a feed pipe to flow downwards and is cooled by means of pressurised jets of scrubbing liquid, which are fed obliquely downwards toward the flow of gas, after which the gas is routed to a venturi scrubber, in which the gas is further cooled and scrubbed preferably in two stages by means of jets of scrubbing liquid directed obliquely downwards, after which the flow of gas is routed to flow into a cascade scrubber comprising at least one cascade tube and located below the venturi scrubber, by means of which the majority of the cooled water of the gas and the liquid droplets and solid particles in the gas are separated into the cooling liquid in the scrubbing chamber and the purified gas is routed via a droplet separator into the outside air.

2. A method according to claim 1 , characterised in that the temperature of the exhaust gas entering cooling and scrubbing is a maximum of 100° C.

3. A method according to claims 1 and 2, characterised in that the exhaust gas is the exhaust gas from a hydrometallurgical process.

4. A method according to claims 1 and 2, characterised in that the exhaust gas is granulation exhaust gas.

5. A method according to claim 1 , characterised in that the gas is cooled in two stages before the venturi.

6. A method according to claim 1 , characterised in that the jets of scrubbing liquid are fed into the gas at an angle of 25 - 35 degrees to the horizontal.

7. A method according to claim 1 , characterised in that over half of the scrubbing liquid is fed into the gas before the venturi.

8. A method according to claim 1 , characterised in that the gases in the venturi are scrubbed and cooled in two stages.

9. A method according to claim 1 , characterised in that in order to obtain the scrubbing effect the velocity of the gas is raised by means of the venturi to at least five times the gas velocity in the feed pipe.

10. A method according to claim 1 , characterised in that the scrubbing liquid used is a lye solution.

1 1. A method according to claim 1 , characterised in that the scrubbing liquid used is an acid solution.

12. A method according to claim 1 , characterised in that the scrubbing liquid used is a permanganate solution or a mixture of lye solution and permanganate solution.

13. A method according to claim 1 , characterised in that the gas feed pipe is flushed by means of a flushing arrangement located in the upper end of the pipe, whereby the cut feed pipe is surrounded by a gas-tight water lock, into which the flushing liquid is fed; at the cut-off point the upper edge of the pipe is serrated, so that the flushing liquid flows evenly along the inner surface of the pipe.

14. An apparatus for condensing the water vapour in process exhaust gas and for scrubbing the gas of impurities in the form of gas, solids and droplets of liquid, characterised in that the apparatus comprises a feed pipe (1 ), to the upper section of which at least one nozzle group (2,3) is attached for feeding pressurised scrubbing liquid obliquely downwards into the feed pipe; the feed pipe is connected at the lower section to a venturi scrubber (4), which is equipped with at least two nozzle groups (5,6) for feeding pressurised scrubbing liquid obliquely downwards into the venturi; the venturi scrubber is connected at the lower section to an intermediate pipe (9) and this in turn to a cascade scrubber (10), which is surrounded by a scrubbing chamber (1 1 ); the gas discharge pipe (17) is connected at one end to the scrubbing chamber and at the other end to a droplet separator (18).

15. An apparatus according to claim 14, characterised in that the number of nozzle groups (2,3) attached to the upper part of the feed pipe is two.

16. An apparatus according to claim 14, characterised in that the nozzle group (2,3) attached to the upper part of the feed pipe is made up of nozzles (21 ,22) placed symmetrically with regard to the cross-section of the feed pipe, numbering 3-8 and connected to a distribution device (19, 20) for feeding the scrubbing liquid.

17. An apparatus according to claim 16, characterised in that the lower nozzles (22) of the nozzle groups (2,3) attached to the upper part of the feed pipe are phase shift with regard to the upper nozzles (21 ).

18. An apparatus according to claim 14, characterised in that the nozzle groups (5,6) attached to the tapered cone (7) of the venturi and to the venturi throat (8) are made up of nozzles (25,26) placed symmetrically with regard to the cross-section of the venturi, numbering 2-6 and connected to a distribution device (23,24) for feeding the scrubbing liquid.

19. An apparatus according to claim 18, characterised in that the nozzles (25) of the nozzle group (5) attached to the venturi cone are phase sift with regard to the nozzles (26) of the venturi throat.

20. An apparatus according to claim 14, characterised in that the nozzles (21 ,22,25,26) of the nozzle groups attached to the upper part of the feed pipe and to the venturi are directed obliquely downwards at an angle of 25 - 35° to the horizontal.

21. An apparatus according to claim 14, characterised in that a flushing arrangement (29) is located above the nozzle groups (2,3) of the feed pipe (1 ), comprising a cut feed pipe and a gas-tight fluid lock (30) encircling it, whereby the upper edge (32) of the feed pipe is serrated at the cut-off point.