MXPA99000954A - A method and apparatus for removing gaseous elementary mercury from a gas - Google Patents

Abstract

A method for removing gaseous elementary mercury from a gas that can be permitted to contain sulphur dioxide. The gas is treated in a washing tower with washing liquid that circulates in a closed system and that contains 0.01-300 mmol/l mercury (II) ions and at least twice this amount of chloride ions. The elementary mercury present in the gas is oxidised and solidmercury (I) chloride is formed. A part of said formed solid mercury (I) chloride is passed to a reaction unit for oxidation to mercury (II) chloride. Chlorine intended for said oxidation and liquid intended for taking-up mercury (II) ions formed by said oxidation process is caused to pass up through a reaction zone in the bottom part of the reaction unit and said zone is kept substantially full of solid mercury (I) chloride. The quantity of chlorine is adapted so that said chlorine will be essentially consumed during its passage up through the reaction zone. The liquid and its content of formed mercury (II) ions is united and mixed with the washing liquid present above said reaction zone and thereafter the so united washing liquid is passed with its increased mercury (II) content to the closed circulating system to be recycled to said washing tower as a compensation for the mercury (II) ions consumed for the oxidation of elementary mercury. An apparatus for effecting the method has a washing tower (11) and a reaction unit (26) with a smaller cross section than the tower (11). The reaction unit (26) is connected to the bottom of the tower (11) and has conduits and means for maintaining a closed circuit system of washing liquid from a liquid outlet (41) in the lower part of the tower (11) to a liquid inlet (40) in the upper part of the tower (11). The lower part (42) of the washing tower (11) below the level of the liquid outlet (41) has a tapering cross section for adaptation to the reaction unit (26).

Description

METHOD AND APPARATUS TO REMOVE GASEOUS ELEMENTAL MERCURY OF A GAS DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for removing gaseous elemental mercury from a gas, wherein the gas is treated in a washing plant with a washing liquid that is circulated in a closed system and that it includes 0.01-300 mmoles / 1 of mercury ions (II) and at least double this amount of chlorine ions that are capable of complexing with mercury (II) ions, and in that method the elemental mercury contained in the gas it is oxidized and solid mercury chloride (I) is formed. The method is particularly suitable for removing mercury from gases that are generated when sulfur minerals containing mercury are burned. However, the method can also be advantageously used to remove mercury from other gases having lower sulfur dioxide contents or containing no sulfur dioxide at all. Most countries have extremely severe requirements with respect to the emission of mercury from industrial processes. Gases containing elemental mercury have been one of the largest sources of industrial mercury emissions into the environment, and many new gas cleaning procedures have been proposed over the past twenty-five years to remove elemental mercury from these gases. However, most of these proposed gas cleaning procedures, and particularly those that are more efficient, are technically much more complicated and require the use of a special expensive device, or sophisticated reagents and additives in order to obtain a satisfactory result. . One of the few processes that has gained worldwide use in practice and that also belongs to the most effective processes and that has so far dominated the market with respect to its application in the metallurgical field is the so-called "Boliden Process". Norzin ", also referred to as the" chlorine process ". The process, of which several embodiments are described in more detail in US-A 3,849,537, US-A 4,233,274 and US-A 4,640,751, is carried out in a washing plant that includes a separate absorption tower where a washing solution, the which in addition to the content of mercury chloride (II) will also contain any sulfur dioxide extracted from the gas, and mercury extracted and separated in the form of solid mercury chloride (I) (calomel Hg2Cl2), is sprayed through nozzles on bodies of packaging and the solution is then collected at the bottom of the tower. The mercury vapor, ie the elemental mercury Hg present in the gas, is quickly and effectively oxidized in the absorption tower with the help of mercury chloride (II) in the wash solution, to form mercury chloride ( I) solid. The wash solution leaving the absorption tower is circulated in an essentially closed system from which a subflow is removed and released from the precipitated calomelles and subsequently subjected to a "necessary purification process, part This subflow is released into a vessel The subflow of wash solution that will be released from calomel is passed to a sedimentation tank or the like for the physical separation of solid calomel extracted in the form of a mud, sometimes referred to as The slurry is passed to a mud edge and then to a regeneration plant at different intervals as required, where the calomel are oxidized to mercury chloride (II) with chlorine gas. ) resulting is transferred to the closed main wash liquid system in order to keep the mercury content (II) of the washing liquid circulating within a Default scale The content of mercury (II) in the wash liquid continues inuame.nt e is consumed according to the reaction: Hg ° + HgCl2? Hg2Cl2 (calomelos) This way, the consumption of mercury (II) in the washing liquid will increase with increasing mercury contents of the gas. Consequently, the mercury (II) content of the gas will relatively easily fall when the mercury content of the gas varies to any great degree, representing that additional mercury (II) chloride should be added to the system as quickly as possible. It will be seen that 'this development of the washing liquid constitutes a very essential aspect of this process. If the mercury (II) ion content of the washing liquid drops rapidly due to the high temporary contents of Hg ° in the gas, it can be difficult to constantly maintain the process at optimum conditions with respect to efficiency and the degree of cleanliness desired . The oxidation of the calomelos results in problems that are associated with the occurrence of dissolved sulfur dioxide in the washing liquid among other things, since the sulfur dioxide is oxidized through the chlorine resulting in the additional consumption of chlorine by one side, and on the other hand forms sulfuric acid and hydrochloric acid, which can only be accepted in the washing liquid in limited concentrations. Consequently, oxidation of calomel with chlorine has hitherto been carried out in a circuit which is generally separated from the circuit of the main washing liquid and, as indicated above, has hitherto been carried out in periodic campaigns. Although the amounts of the calomel slurry treated in the oxidation process are relatively moderate, this oxidation process however has so far presented several disadvantages with respect to the "chlorine process". The mercury (II) chloride losses that occur have been found to be partially dependent on the total amount of liquid in the system and the mercury (II) chloride content of the liquid.

With admission, this chlorine content can, as a rule, be maintained within predetermined limits through the intermittent supply of mercury (II) ions formed by the oxidation process, optionally in combination with an external supply of mercury chloride (II). ). However, in view of the increasing demands on the effectiveness and efficiency of the gas purification process, it could be more beneficial if the mercury ion (II) content of the circulating washing liquid can be maintained more evenly within more limits. narrows so that the time variations in the mercury ion content (II) can be kept as small as possible and preferably independent of any variation in the mercury content of the inlet gas, to the best possible degree. In this way, it would be possible to control the average content of mercury (II) ions in the washing liquid more quickly and more effectively without needing the introduction of external mercury (II) chloride manually into the system or if a Regulated volume of oxidized calomelles, the entire process could provide considerable technical advantages over the previously known chlorine process. As a whole, it will be seen that if it were possible to improve the oxidation of calomel while taking into account the actual requirement of mercury (II) ions in the washing liquid, the whole "chlorine process" could be more beneficial under of the most uniform and most effective removal of mercury from gas, even with respect to temporary variations in the mercury content of the incoming gas. As mentioned above, one of the most serious problems in this regard is the presence of sulfur dioxide in the washing liquid, since it can react with the oxidizing agent, chlorine, to form hydrochloric acid and sulfur dioxide, which in turn, it forms sulfuric acid, and makes the oxidation of calomel to mercury (II) chloride difficult to obtain. The washing liquid also becomes more and more acidic as a result of the formation of hydrochloric acid and sulfuric acid, and, therefore, must be neutralized, at least before it is released into a container. The presence of mercury compounds also complicates the situation since it is not always possible to exclude the formation of complex chlorine and sulfur compounds, which, of course, also upset the process. If it were possible to carry out the oxidation process in order to eliminate at least essentially the above problems, the chlorine process can be improved (in spite of its twenty-five years of age) in many important aspects and thus be able to compete with the processes more advanced and more modern to treat gases that contain mercury, for years to come. The object of the present invention is to provide an improved "cooling process" which eliminates essentially all the problems and disadvantages discussed above and which can satisfy any of the future industrial requirements with respect to the control of the improved process and simplified process operation and apparatus. without adding any cost. The invention is characterized up to this point by the process steps and apparatus features set forth in the following claims. In this way, according to the invention, the solid mercury chloride (I) formed from the oxidation of elemental mercury in the gas is brought into motion to a reaction unit for the oxidation of mercury chloride (I). The chlorine and liquid required for this oxidation process to take the mercury ions (II) formed by said oxidation process are passed upwards through a reaction zone in the lower part of the reaction unit. This zone is maintained substantially filled with solid mercury chloride (I) and the amount of added chlorine is adapted so that essentially all of the chlorine is consumed during this ascending passage through the reaction zone. The liquid containing the mercury ions (II) thus formed is then united with the washing liquid present above in the reaction zone, and the washing liquid thus bound with its increased content of mercury (II) is then brought to the closed circulation system to be recirculated to the washing tower as a compensation of the content of mercury (II) ion consumed for the oxidation of elemental mercury. Said formed mercury chloride (I) can be brought to the reaction unit using two different forms in principle, either being transported through the washing liquid or, in those cases where the reaction unit is directly connected to the lower part. or bottom of the wash tower, through the action of gravity forces or in other words through sedimentation. Chlorine and liquid are supplied to the reaction unit either from the bottom or from the top of such unit, through pipes and / or hoses that discharge close to the bottom of the reaction zone. The calomel content of the reaction unit is verified essential and continuously, in order to allow the process to be controlled. This allows the supply of washing liquid to the reaction unit that is easily controlled based on the variation of the calomel content, which can be achieved by observing the changes in the weight of the reaction unit, although other methods are also possible which can be easily obtained in practice, for example by optically reading the height of the calomel column. The amount of chlorine added to the system per unit of time can be controlled with respect to the content of mercury ion (II) of the washing liquid in the closed circulation system. The oxidation process in the reaction unit is ideally performed and essential continuously, although it is also possible to interrupt the oxidation process for short periods wherein the liquid can be subjected to some other treatment processes, for example, a reduction process. In the latter case, part of the washing liquid is removed from the liquid circulation system for the reduction of mercury (II) ions with a reducing agent in order to form mercury chloride (I) (calomel), which it is brought, where the washing liquid can be separated as a purified bleeding subsequent to possible additional washing steps. The reaction unit may consist of a single container or two separate containers, in the latter case the upper part of the lower container will be connected to the lower part of the upper container, preferably through a generally vertical pipe or hose. The upper reaction vessel may be a hydrocyclone or some similar effective drainage apparatus, in order to promote sedimentation. The reaction unit also as indicated above, preferably can be directly connected below the washing tower and *, in this way, with respect to the apparatus to be developed with the latter, where also the bottom of the washing tower it can be provided and configured to facilitate and carry out the sedimentation partially above the inlet of the reaction unit and also to facilitate the transport of the sediment materials towards the upper part of the reaction unit. The invention is based, inter alia, on two specific properties of mercury chloride (I) formed, one of which is that it has extremely good sedimentation properties due to its particle shape and very high density, and the other is which is quickly oxidized by chlorine (Cl?). These properties are effectively used in combination according to the invention, which allows the oxidation of calomels with chlorine to take place essentially in the absence of those problems caused by the presence of sulfur dioxide in the washing liquid, as mentioned above. The oxidation process is carried out in this way in a reaction unit whose lower part, called the reaction zone, is adapted to use the high density of the calomel and also the relatively heavy liquid phase to expel or displace the liquid from the sludge. calomel The upper part of the reaction unit may be constructed or adapted to promote sedimentation or other physical separation of the calomel mud from the washing liquid. A suitable reaction unit, therefore, will be relatively wide at the top and narrow downward, that is, at the reaction zone, and can be composed of two parts as mentioned above, these two parts together being referred to herein as the reaction vessel. In those cases, in which the reaction unit is directly connected to the lower part of the wash tower, the main part of the sedimentation will be carried out from above. give the reaction unit and, in this way, the upper part of the reaction unit does not necessarily have to be wider in the upper part than in the lower part, so that in some cases the whole reaction unit can have the same cross section and elongated shape. • The solid particles of calomel in the washing liquid will settle, through sedimentation, successively in the upper part of the reaction unit and the calomel mud. formed will be "drained" or progressively enriched to greater degrees as it moves toward it. reaction unit. The weight and density of the calomel present in the reaction zone will "compress" the liquid from the calomel mud and force the liquid upwards towards the top of the reaction unit, so that essentially only the calomel particles in the The steady state of the process will be present in the reaction zones, except for any additions and errors. The reaction zone in this manner will remain constantly filled with solid mercury (I) chloride when the size of the reaction zone has been adapted for a supply of normal washing liquid to the reaction unit. If the predetermined reaction zone in the reaction unit is not completely filled with calomel for some reason or another, it may be necessary to add calomel from an external source. During operation- the chlorine gas and water or some other suitable liquid, for example, washing liquid of the closed system of circulation, are passed to the elongated reaction zone essentially full of calomelles, from the bottom of such zone and upwards , where the calomelos are oxidized in the reaction zone and the mercury ions (II) that are formed are taken up by the liquid and taken to the upper parts of the reaction unit; where they join with the washing liquid. It is necessary that the amount of. Introduced chlorine gas is adapted so that it will be consumed during its passage up through the column of calomelles in the reaction zone and so that no chlorine gas arrives and comes into contact with the washing liquid containing sulfur dioxide in the upper part of the reaction unit. Since the calomelites present will be oxidized rapidly by chlorine gas, the relative size of the reaction zone can be restricted to very reasonable proportions. Depending on local requirements, chlorine gas and liquid can be supplied either from below, through an opening in the unit or into the reaction zone, or from above, with the help of pipes or hoses that discharge close to the reaction zone. Providing the reaction zone with an installation that allows the amount of calomel in the reaction zone to be verified, for example, by checking the weight of the reaction unit and its calomel content, or in some other way, as mentioned above , it can be ensured that the reaction zone is constantly maintained substantially full of calomelles, by adapting the subflow of the supplied washing liquid to the reaction unit from the liquid circulation system. This can be done totally and automatically in response to signals that are derived from the aforementioned measurements of the amount of calomelles in the reaction unit and with the help of suitable automatically controlled valves. The invention will now be described in greater detail with reference to the preferred illustrative embodiments thereof and also with reference to the accompanying drawings, wherein Figures 1-3 each illustrate a plant for carrying out the invention. Figure 1 shows a plant 10 for carrying out the invention. The illustrated plant includes an absorption tower 11 having an inlet 12 for gas containing mercury and an outlet 13 for the treated mercury-free gas. The washing liquid is sprayed onto the packing bodies 15 in the absorption tower 11 through nozzles 14, and is collected in the bottom 16 of the tower. Although not shown, a droplet separator is provided at the top of the tower, in order to prevent the liquid from being retained by the gas. The washing liquid is passed from the absorption tower 11 to a pump tank 19 through a conduit 18, and from the tank 19 through a pump 19A for recirculation to the absorption tower 11 via a conduit 20. A subflow of washing liquid is taken from the duct 20 and passed to a sludge separator 23 through a duct 22. The sludge-free liquid is then passed from the top of the sludge separator 23 back to the tank of pump 19, through a conduit 24. The washing liquid containing calomelles is passed from the lower part of the separator 23 through a conduit 25 to a reaction unit 26, where the lime is oxidized. As illustrated, the reaction unit 26 can consist of a single container or two containers, in the latter case, the lower container is directly connected to the lower part, of the upper container, for example, through at least one pipe or hose connection generally vertical and straight. The lower part of the single container or the bottom container of the two container unit 26 forms a reaction zone 26B, which is long and narrow relative to the upper part of the individual container or the upper container 26A. Chlorine gas and water or some other suitable transport liquid are supplied to the reaction zone 26B through a conduit 27 that opens. near the bottom of the reaction zone 26B, and is forced to pass through the reaction zone 26B. The mercury ions (II) formed by the oxidation of the calomelles in the reaction zone 26B are transported upwards towards the upper part of the reaction unit or towards the upper vessel 26A through the supplied transport liquid and are passed through from there to the pump tank 19 together with the liquid separated from the calomel mud, through a conduit 28. When it is desired to remove the calomel and / or the purified bleeding from the system, a subflow of washing liquid is taken from the conduit 20 and passed through a conduit 29 to a reduction reactor. to which zinc powder is also supplied in order to reduce all the mercury ions (II) present in the solution and with them form a precipitate of calomel, which is separated in a collection vessel 31 together with the calomel are already present, while the purified bleeding can be further purified with respect to any of the environmentally hazardous components and then discarded from the system through a conduit 32, or returned to the pump tank 19 through a conduit 32A for delivery of clean liquid, if desired. Subsequent to the extraction of calomelles in the reactor 30 the washing liquid can optionally be returned from the reactor 30 to the pump tank 19 through a conduit 30A without first being purified. The plant illustrated in Figure 2 is similar to that shown in Figure 1, although in the embodiment of Figure 2, the reduction of the washing liquid with zinc is performed intermittently in the mud separator 23 together with the reaction unit. The purified bleeding is taken from the separator 23 through a conduit 23A. The conduit 25 includes a conduit 25A co-ordinately an opening-closing valve, which is used to close the conduit 25 when the cleaning need is acute or when the reaction unit 26 contains an excessive amount of calomel, while stopping the chlorine supply to the reaction zone 26B at the same time. The supply of zinc powder to the sludge separator 23 is started and a stirrer 23 is activated in order to accelerate the reduction reaction. The zinc supply and agitator 23 are stopped after a predetermined period, so that valve 25A is opened and the oxidation process is continued again supplying chlorine to reaction zone 26B. The calomelles are extracted from the system into a container 34 through a conduit 34A. Alternatively, the calomels can be taken from the lower part of the reaction zone 26B and passed to a container 35 through a conduit 36. An apparatus according to Claims 8 and 9 is schematically shown in Figure 3. the shape of a plant 10 and a highly preferred embodiment of the invention is illustrated. The plant 10 comprises an absorption tower 11 (wash tower) provided with an inlet 12 for the gas containing mercury and an outlet 13 for the treated mercury-free gas released. The absorption tower 11 has nozzles 14 and packing bodies 15 provided above the level of the gas outlet 13. Connected to the lower part 42 of the absorption tower 11, whose lower part 42 is shown here with a tapered shape towards below conical, there is the reaction unit 26, which has connections for the conduits 22, 27 and 37 in its lower part and a connection for the conduit 43 for water supply. The flushing liquid withdrawn from the closed loop system can be supplied to the lower part 42 of the flushing head 11 near the reaction unit 26 and / or be supplied to the upper part 26A of the reaction unit near the lower part of the tower 11. Below the lower part of the reaction unit 26, there is a container 35, which through a conduit and / or container 36 is connected to the reaction unit 26. The liquid The wash extracted from the closed circulation system can also be supplied to the container 36 for washing the calomel and for the absorption of any elemental mercury present in the container 36. In the lower part 42 of the tower 11 suitable means 45 are arranged, for example, in the form of laminar units or the like, in order to avoid any movement of horizontal liquid flow as well as any rotational flow. The chlorine supply line 37 is shown connected to a chlorine container 38. During operation the solid mercury (I) chloride (calomel) formed by the oxidation of the elemental mercury will follow the washing liquid in its circuit through of the ducts 18, 20 and the pumping means 19 back to the upper part of the tower 11, but a part of the formed calomelles will move, through sedimentation, downwards through the washing liquid present in the lower part, said liquid only slightly flowing upwards, and in this way, the calomelles will move towards and in the reaction unit 26. In the lower reaction unit 26, the calomelos, as described in relation to Figure 1 they will be oxidized by chlorine 37, which can be supplied together with the compressed air 27. Through the oxidation of the calomels in the lower reaction zone 26B, mercury (II) ions are formed which they will follow the upward moving liquid that is displaced by the calomelles and this can be facilitated by the liquid supplied to the lower part 26B of the reaction unit 26. This liquid, which can be the wash liquid recirculated through the conduit 22 and / or additional water supplied through the conduit 43, will contribute to forcing the calomelles upwards through the reaction unit 26 and towards the lower part 42 of the tower 11. In this lower part 42, the liquid with its The mercury content (II) will be found and mixed with the upstream washing liquid present only slightly, to which the additional washing liquid can also be supplied from the closed system through the conduit 22. The washing liquid thus bound and mixed having an increased content of mercury ions (II) is carried through the outlet of liquid 41 and conduits 18, 20 to be returned through s enerada of liquid 40 towards the top of the absorption tower 11 and further to the nozzles 14 to resume using as oxidizing agents in the elemental mercury in the gas inlet. As in the plant illustrated in Figure 1, a wash liquid undercurrent can be requested through a conduit 29 and be removed for the removal of calomel and / or purified bleeding from the washing system and be reduced with zinc powder in the reaction reactor 30, when the purified bleeding is extracted through line 32 and 4 the calomelles through the exits in the bottom of the reactor 30, from one of the exits, the calomelles can be recirculated towards the closed washing liquid system (18, 20, 14) and from the other exit, the calomelles They can be extracted for sale or storage. In summary, it will be noted that the invention provides a number of advantages with respect to the known "chlorine washing process", these advantages being beneficial for: • the surrounding environment • the working environment • plant and operating costs • yields of process • space requirements, since: 1. no separation of any of the pH regulator volumes of calomel sludge or mercury chloride solution (II) nor the need for the sludge or the solution is required be treated separately; 2. the contents of the circulating washing liquid leaving the absorption tower can be used to control the addition of chlorine to the system, so that a suitably adapted amount of mercury (II) can be constantly satisfied by the oxidation of calomel integrated in the system; 3. the losses of mercury (II) chloride can be essentially reduced due to the fact that the total liquid volume in the system is smaller than in the previous case; 4. Peripheral equipment can be serviced more easily, since the equipment can have smaller dimensions, so that maintenance costs are also reduced; 5. the peripheral equipment may be of smaller dimensions, because: • the oxidation process is essentially continuous (not based on regulated supplies), • the process steps and the parts of the apparatus can be coordinated and combined to a higher than what was previously possible, • the risk of oxidation of the washing liquid containing sulfur dioxide with subsequent process problems has been eliminated probably and completely. 6. Environmental problems can be avoided to a greater degree, because: the stages of the process that require the handling of chlorine can be carried out with such small dimensions to allow these stages to be easily installed in isolated, close spaces, and to the absence of pH regulator vessels with associated duct networks for handling calomel sludge and mercury chloride (II) solution.

Claims (9)

1. A method to remove gaseous elemental mercury from a gas that can be allowed to contain sulfur dioxide, in that method, the gas is treated in a washing tower with washing liquid circulating in a closed system and containing 0.01-300 mmole / 1 of mercury ions (II) and at least twice this amount of chlorine ions, and where the elemental mercury present in the gas is oxidized and solid mercury chloride (I) is formed, characterized by passing a part of the solid mercury chloride (I) formed to a reaction unit for oxidation to mercury (II) chloride; make the chlorine intended for such oxidation and the liquid destined to collect the mercury ions (II) formed by the oxidation process pass through a reaction zone in the lower part of the reaction unit, such area being kept substantially full of solid mercury (I) chloride; adapting such an amount of chlorine so that the chlorine will be essentially consumed during its upward stage through the reaction zone; joining the liquid and its content of mercury ions (II) formed with the washing liquid present above said reaction zone; and then passing the washing liquid thus bound with its increased content of mercury (II) to the closed circulation system so that it is recycled to the washing tower as a compensation for the mercury (II) ions consumed by the oxidation of the elemental mercury

2. The method according to claim. 1, characterized by the supply of chlorine and liquid to the reaction zone from the bottom of the zone.

3. The method according to claim 1, characterized by the supply of chlorine and liquid to the reaction zone from above such area, through pipes and / or hoses that open near the bottom of the reaction zone.

4. The method according to claims 1-3, characterized by checking the calomel content of the reaction unit essentially continuously.

5. The method according to claims 1-4, characterized by controlling the amount of chlorine delivered per unit time based on the concentration of the mercury ion (II) in the rinse liquid of the closed system.

6. The method according to claims 1-5, characterized by effecting the oxidation in the reaction unit essentially continuously.

7. The method according to claims 1-6, characterized in that a subflow of washing liquid is taken from the liquid circulation system for the reduction of mercury (II) ions, preferably with zinc powder, so that Mercury chloride form (I) (calomelos), which is separated from the system, where purified bleeding can be taken.

8. The apparatus for carrying out the method according to the preceding claims, characterized in a washing tower and a reaction unit having a smaller cross section than the tower connected to the bottom of the tower and having conduits and means for maintaining a closed loop system of washing liquid from a liquid outlet at the bottom of the tower to a liquid inlet at the top of the tower, where the bottom of the wash tower below the level of the exit The liquid has a tapered section to adapt to the reaction unit, and wherein the reaction unit in its lower part is arranged to supply the oxidation agent and the liquid.

9. The apparatus according to claim 8, characterized in that the washing tower below the level of the outlet of the washing liquid has means to prevent any horizontal or rotational flow of liquid. SUMMARY A method to remove gaseous elemental mercury from a gas that can be allowed to contain sulfur dioxide. He Gas is treated in a washing tower with washing liquid circulating in a closed system and containing 0.01-300 mmoles / l of mercury (II) ions and at least twice this amount of chlorine ions. The elemental mercury present in the gas is oxidized and solid mercury (II) chloride is formed. A portion of said solid mercury chloride (I) formed is passed to a reaction unit to be oxidized to mercury (II) chloride. The chlorine intended for oxidation and the liquid intended to take the mercury ions (II) formed by the oxidation process are passed through a reaction zone in the lower part of the reaction unit and the zone is substantially maintained filled with solid mercury chloride (I). The amount of chlorine is adapted so that the chlorine will be essentially consumed during its passage through the reaction zone. The liquid and its content of mercury ions (II) formed is joined and mixed with the washing liquid present above the reaction zone and then the washing liquid thus bound is passed with its content of mercury (II) increased towards the closed circulation system so that it is recirculated towards the washing tower as a compensation to the mercury ions (II) consumed by the oxidation of the elemental mercury. An apparatus for carrying out the method has a wash tower (11) and a reaction unit (26) with a smaller cross section than the tower (11). The reaction unit (26) is connected to the bottom of the tower (11) and has conduits and means for maintaining a closed loop system of washing liquid from a liquid outlet (41) in the lower part of the tower (11) towards a liquid inlet (40) in the upper part of the tower (11). The lower part (42) of the washing tower (11) below the level of the liquid outlet (41) has a tapered cross-section to adapt to the reaction unit (26).