GB1582232A

GB1582232A - Gas cleaning

Info

Publication number: GB1582232A
Application number: GB33926/76A
Authority: GB
Original assignee: Lodge Cottrell Ltd
Current assignee: Lodge Cottrell Ltd
Priority date: 1976-08-14
Filing date: 1976-08-14
Publication date: 1981-01-07
Also published as: ZA774767B

Description

(54) IMPROVEMENTS IN OR RELATING TO GAS CLEANING (71) We, LODGE-COTTRELL LIMITED, a British Company, of George Street Parade, Birmingham B3 I QQ, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is concerned with improvements in or relating to gas cleaning.

In certain processes, for example some steelmaking processes, the process is carried out in an enclosed environment and generates a dirty particle-laden gas. It is usual to make arrangements to collect the dirty gas and clean it be fore, for example, discharge to the atmosphere, and the problem also arises of ventilating the environment of escaped dirty gas and cleaning this before discharge.

It is an object of the present invention to provide an improved method and apparatus for cleaning a dirty particle-laden gas emanating from a process carried out in an enclosed en environment and ventilating the environment of escaped dirty gas.

The invention provides a method of cleaning a hot dirty particle-laden gas emanating from a process carried out in an enclosed environment and ventilating the environment of escaped dirty gas: comprising the steps of (a) collecting hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) allowing hot dirty gas escaping into the enclosed environment to rise by convection to an upper portion of the enclosed environment and collecting the dirty gas at said upper portion to provide a second dirty gas stream diluted with air from the environment, (c) cooling the first dirty gas stream; (d) mixing the cooled first stream with the second stream; and (e) subjecting the mixed streams to a gas cleaning operation.

The invention also provides a method of cleaning a hot dirty particle-laden gas emanating from a process carried out in an enclosed environment and ventilating the environment of escaped dirty gas: comprising the steps of (a) collecting hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) allowing hot dirty gas escaping into the enclosed environment to rise by convection to an upper portion of the enclosed environment and collecting the dirty gas at said upper portion to provide a second dirty gas stream diluted with air from the environment; (c) subjecting the first dirty gas stream to a preliminary gas cleaning operation; (d) mixing the first stream from the preliminary gas cleaning operation with the second stream; and (e) subjecting the mixed streams to a second gas cleaning operation.

The invention also provides a method of cleaning a hot dirty particle-laden gas emanating from a process carried out in an enclosed environment and ventilating the environment of escaped dirty gas; comprising the steps of (a) collecting hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) allowing hot dirty gas escaping into the enclosed environment to rise by convection to an upper portion of the enclosed environment and collecting the dirty gas at said upper portion to provide a second dirty gas stream diluted with air from the environment, (c) conditioning, for electro-precipitation, the first gas stream with water; (d) mixing the conditioned first stream with the second stream; and (e) subjecting the mixed streams to a gas cleaning operation by electro-precipitation.

The invention also provides gas when cleaned by a method according to the invention.

The invention also provides apparatus adapted for use in cleaning a hot dirty particle-laden gas emanating from a process carried out in a building and ventilating the building of escaped dirty gas: comprising (a) a primary gas collection system arranged to collect hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) a secondary gas collection system comprising inlet means in a roof portion of the building arranged to collect dirty gas escaping into the building and rising by con vection to the roof portion, said collected gas providing a second dirty gas stream diluted with ambient air; (c) means for cooling the first dirty gas stream, the arrangement being such that the first stream from the cooling means is mixed with the second stream when the apparatus is in use; and (d) means for subjecting the mixed streams to a gas cleaning operation.

The invention also provides apparatus adapted for use in cleaning a hot dirty particle-laden gas emanating from a process carried out in a building and ventilating the building of escaped dirty gas; comprising (a) a primary gas collection system arranged to collect hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) a secondary gas collection system comprising inlet means in a roof portion of the building arranged to collect dirty gas escaping into the building and rising by convection to the roof portion, said collected gas providing a second dirty gas stream diluted with ambient air; (c) means for subjecting the first dirty gas stream to a preliminary gas cleaning operation, the arrangement being such that the first stream from the gas cleaning operation is infixed with the second stream when the apparatus is in use, and (d) means for subjecting the mixed streams to a second gas cleaning opera- tion.

The invention also provides apparatus adapted for use in cleaning a hot dirty particle-laden gas emanating from a process carried out in a building and ventilating the building of escaped dirty gas; comprising (a) a primary gas collection system arranged to collect hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) a secondary gas collection system comprising inlet means in a roof portion of the building arranged to collect dirty gas escaping into the building, said collected gas providing a second dirty gas stream diluted with ambient air; (c) means for conditioning, for electro-precipitation, the first stream with moisture, the arrangement being such that the first stream from the condition ing means is mixed with the second stream when the apparatus is in use; and (d) an electro-precipitator arranged to subject the mixed streams to a gas cleaning operation.

The invention also provides in a steelmaking plant, apparatus adapted for use in cleaning hot dirty particle-laden gas emanating from a steelmaking process carried out in a building and ventilating the building of escaped dirty gas; comprising (a) a primary gas collection system arranged to collect dirty gas substantially direct from the process to provide a first dirty gas stream of relatively high particle con centration; (b) a secondary gas collection system comprising inlet means in a roof portion of the building arranged to collect dirty gas escaping into the building, said collected gas providing a second dirty gas stream diluted with air from the environment to a relatively low particle concentration; (c) a gas scrubber arranged to subject the first gas stream to a pre liminary gas cleaning and cooling operation, the arrangement being such that the first stream from the preliminary gas cleaning operation is mixed with the second stream when the apparatus is in use; and (d) an electro-precipitator arranged to subject the mixed streams to a second gas cleaning operation.

Examples of the processes emanating a dirty particle-laden gas include oxygen steelmaking, electric arc steelmaking, nonrferrous production metallurgy, ferrous and non-ferrous foundries and cast houses.

In a typical oxygen or electric arc steelmaking plant, the gas entering the primary gas collection system, for example, is at a temperature between 700 C and 1500 C, and has a particle concentration between 10 and 60 grams per N cubic metre, e.g. 30 grams per N cubic metre. The gas entering the secondary gas collection system for example, is at a temperature between 50 and 180 C, e.g. 90 C, and has a particle concentration up to 1 gram per N cubic metre, e.g. 0.5 grams per N cubic metre. Where the two gas streams mix, for example the first stream is at a temperature between 80 C and 300 SS, with a high moisture content, and the second stream is still substantially between 50 C and 180 C. The mixed gas stream has for example a temperature between 55 C and 175 C. The gas discharged from the primary gas cleaning step for example has a solid particle concentration up to 5 grams per N cubic metre. The dilution ratio between the volumetric flow rates (NTP) of the second gas stream and the first gas stream where they mix is for example between 20:1 and 5: 1, and the particle concentration of the mixed streams prior to the secondary gas cleaning step is for example between 0.5 and 1.5 grams per N cubic metre, e.g. 1.0 grams per N cubic metre. The dew point of the mixed gas streams is for example between 30 C and 60 C. The gas from the secondary gas cleaning step for example has a particle concentration no greater than 0.05 grams per N cubic metre, and is at a temperature between 55 C and 160 C.

Tllere now follows a description, to be read witll reference to the accompanying drawings, of apparatus embodying the invention by way of example. This description, which is illustrative of apparatus, method, and product aspects of the invention, is given by way of example only and not by way of limitation thereof.

In the accompanying drawings: Figure 1 shows a flow diagram of the apparatus embodying the invention; and Figure 2 is a pictorial representation of the apparatus embodying the invention.

The apparatus embodying the invention is adapted for use in cleaning a hot dirty solid particle-laden gas emanating from an oxygen steel.-making converter 10 and ancillary equipment all located in an enclosed building 12, and ventilating the building 12 of escaped dirty gas.

In addition to the steel-making converter 10 which is pivotally mounted, there are located in the building 12, a pivotally mounted vessel 11 for charging the converter 10, and various ladle and storage vessels for molten iron or steel generally represented by the reference numeral 13. Dirty particle-laden gas escapes from the vessels 13 into the building when they are in use. The vessel 11 is pivotally mounted on a bifurcated bracket assembly 15, which is itself supported by a beam structure 17;one of the vessels 13 is similarly pivotally mounted on a bracket assembly 15a supported by a beam structure 17a.

The apparatus comprises a primary gas collection system 14 comprising a hood 16 which is arranged to extend over a mouth portion 18 of the converter 10 to collect dirty gas substantially direct therefrom and provide a first, hot, dirty gas stream of relatively high particle concentration; the collection system 14 also comprises a conduit 20 leading from the hood 16. There is no seal between the mouth 18 and the hood 16, and sufficient air reaches the converter 10 for complete combustion of the gas before entry into the conduit 20. The hood 16 is fully effective only when the converter 10 is in an upright operative position (not shown in Figure 2); and when the converter 10 is pivoted to the position shown in Figure 2 for pouring or charging, there will be an increased amount of dirty gas escaping the hood 16.

The conduit 20 leads to a gas scrubber cooler unit 22 where the first dirty gas stream is scrubbed with water as a preliminary gas cleaning operation, and cooled. The unit 22 also serves to condition for electro-precipitation the first gas stream with water. The unit 22 comprises a venturi device 23 which corresponds substantially to the one described in our U.K. Patent Specification No. 940,930, to which reference may be made. Scrubbing water enters the venturi device 23 at 25; and spent water leaves the device 23 via a void collection chamber 27 and a line 24. The pressure drop across the unit 23 is low, being between 2 and 6 inches of water gauge. A horizontal gas outlet 29 leads from a lower portion of the venturi device 23 into a cyclone separator 26, where entrained moisture is removed from the gas, the separated water leaving the separator 26 at 28.

From the cyclone separator 26, a gas conduit 30 leads via a centrifugal fan 32 to a gas conduit 34 mounted on a roof portion 36 of the building 12; the conduit 30 joins the conduit 34 where indicated by the arrow 60 on Figure 2. The conduit 30 terminates in an ejector 37 centrally located within the conduit 34. The conduit 34 leads to an electro-precipitator 38 generally adjacent the level of the roof portion 36. The electro-precipitator 38 is shown located to one side of the building 12, being mounted on a supporting structure 40, alternatively the electro-precipitator 30 may be mounted on the roof portion 36.

The apparatus embodying the invention also comprises a secondary gas collection system 42 comprising a plurality of inlets 44 spaced along the roof portion 36 and leading into the conduit 34 upstream of the ejector 37. Hot dirty solid particle-laden gas escaping the hood 16 and from the vessels 13 is allowed to rise by convection and passes through the inlets 44 into the conduit 34 to provide a second dirty gas stream diluted with ambient air from the building to a relatively low particle concentration; it will be realised that the volumetric flow rate of the second gas stream is much greater than that of the first gas stream. When the apparatus is in use, the gas stream from the conduit 30 mixes with the gas stream from the inlets 44 in the conduit 34, and the mixed gas streams are cleaned in the precipitator 38, as a second gas cleaning operation. The cleaned gas is discharged to the atmosphere at 46. The effect of the ejector 37 is to create a high linear gas velocity in the ejected gas, which not only promotes thorough gas mixing, but also utilises some of the energy of the fan 32 to increase the flow rate of the mixed gas streams. The gas stream from the conduit 30 is saturated with water at about 80"C and when mixed with the second gas stream having an ambient dew point, gives a mixed gas condition below saturation, but at a satisfactory humidity for efficient electro-precipitation, even when the solid particles comprise predominantly lead or zinc, well known as difficult materials in this connection.

One inlet 44 of the secondary gas collection system (Figure 2) is located generally above the converter 10 and comprises a horizontal elongated opening 53 in the roof portion 36 leading into correspondingly elongated trunking 50.

The trunking 50 communicates with the conduit 34 via vertical louvres 52 which can be closed. The other inlet 44 also comprises an elongated horizontal opening 54 leading into corresponding elongated trunking 56, and in this case the dirty gas enters the trunking 56 via closable horizontal louvres 58 provided in the opening 54. The louvres 52 or 58 may be closed when it is desired to use only one of the inlets 44, and this type of arrangement is of special interest in cases where there are multiple converters 10 in a single building, all of which may not be operating at the same time.

The conduit 34 leads into horizontal trunking 62 which provides a horizontally elongated inlet for the precipitator 38. The gas flows generally horizontally through the precipitator 38 in the gas cleaning operation, and is discharged from the precipitator 38 to atmosphere via a plurality of vertically directed outlets 64.

The precipitator 38 comprises, in known manner, a plurality of vertical parallel planar collector electrodes 70 (only one of which is shown) between which are located a plurality of rows of vertical discharge electrodes 72 (only one of which is shown). In the operation of the precipitator 38 the discharge electrodes 72 are charged to a high voltage,the collector electrodes 70 being earthed, and corona discharge results causing particles from the gas to deposit on the collector electrodes 70 from which the deposited dust is removed by the action of rapping gear, (not shown).

Relatively low gas pressures are involved in the precipitator 38, and thus its shell may be of light fabricated sheet metal construction. The spacing between each pair of adjacent collector electrodes is not less than 400 mm., preferably between 450 mm. and 700 mm. The voltage to which the discharge electrodes are charged is between 80 and 100 Kv. Alternatively a narrower spacing may be used between 250 and 350 mm., in which case the voltage is between 40 and 50 Kv. Since the gases exiting from the precipitator are warm but unsaturated, there should be no steam plume arising. It is believed that U.K. Clean Air Regulations will be met if the precipitator operates at 90 to 98% efficiency.

Various modifications are possible. For example, the electro-precipitator 38 may be replaced by a bag filter unit; the venturi device 23 may be replaced by a simple spray tower of uniform cross-section;the outlets 64 may be replaced by a conduit leading to a chimney stack.

Again, a nitrogen seal may be provided between the mouth portion 18 and the hood 16 so that the gas from the converter 10 is not completely combusted having for example a carbon monoxide content as high as 80% by volume; the high levels of dilution with the second gas stream can reduce the carbon monoxide content of the incompletely burned gases to a level where the explosion hazard is minimal. The dilution ratio (NTP) is increased when there is incomplete combustion; this is because with complete combustion excess air is drawn into the gas stream at the hood 16, and also the first gas stream is likely to contain more water vapour following the unit 22, since it is essentially hotter as a result of the complete combustion. For example, a ratio of 5:1 may apply for complete combustion and 15:1 for 80% CO; it is believed there would be no explosion hazard at this degree of dilution. With incomplete combustion also the dew point of the mixed gas streams is decreased; for example, a dew point of 35"C may apply for 80% CO, and 50"C for complete combustion.

In further modifications, the humidity of the mixed gas streams is reduced to minimise the possibility of saturated gas entering the electro-precipitator 38. For example in the case of incomplete gas combustion at the convertor 10, the gas may be burned at the ejector 37; this raises the temperature of the mixed gas streams entering the electro-precipitator, thus reducing the humidity; for example the temperature of the mixed gas streams could then be at least 200"C. Alternatively in either of the cases of complete or incomplete combustion at the convertor 10, only part of the gas leaving the scrubber/cooler unit 22 may pass into the conduit 34, the remainder being treated by conventional gas cleaning methods, e.g. a further gas scrubber as described in U.K. Specification No. 940,930, but arranged for high energy operation. In another alternative which again may be used for either case of complete or incomplete combustion, some of the gas from the convertor 10 bypasses the unit 22 via a bypass conduit 80 (borken line in Figure 1) and rejoins between the unit 22 and the ejector 37 (e.g. at 82); in this way by varying the extent of bypass the temperature and humidity of the mixed gas streams entering the precipitator can be controlled. A valve 83 is provided to vary the extent of bypass.

WHAT WE CLAIM IS: 1. A method of cleaning a hot dirty particle laden gas emanating from a process carried out in an enclosed environment and ventilating the environment of escaped dirty gas: comprising the steps of (a) collecting hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) allowing hot dirty gas escaping into the enclosed environment to rise by convection to an upper portion of the enclosed environment and collecting the dirty gas at said upper portion to provide a second dirty gas stream diluted with air from the environment; (c) cooling the first dirty gas stream; (d) mixing the cooled first stream with the second stream; and (e) subjecting the mixed streams to a gas cleaning operation.

2. A method according to Claim 1 ,wherein the first dirty gas stream is cooled by scrubbing the gas with water which also serves as a preliminary gas cleaning operation.

3. A method according to Claim 2, wherein the pressure drop across the scrubbing operation is between 2 and 6 inches of water gauge.

4. A method according to any one of Claims 1,2 and 3, wherein the process is carried out in a building, and the first-mentioned gas cleaning operation is carried out in a gas cleaning unit mounted adjacent the level of a roof portion of the building.

5. A method according to any one of the preceding Claims, wherein the first-mentioned gas cleaning operation is carried out by electroprecipitation.

6. A method according to any one of the preceding Claims, wherein the first dirty gas stream is collected at a temperature between 700"C and 1500"C.

7. A method according to any one of the preceding Claims, wherein the first dirty gas stream has a particle concentration between 10 and 60 grams per N cubic metre.

8. A method according to any one of the preceding Claims, wherein the second dirty gas stream is collected at a temperature between 500Cand 180"C.

9. A method according to any one of the preceding Claims, wherein the second dirty gas

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. precipitator 38 the discharge electrodes 72 are charged to a high voltage,the collector electrodes 70 being earthed, and corona discharge results causing particles from the gas to deposit on the collector electrodes 70 from which the deposited dust is removed by the action of rapping gear, (not shown). Relatively low gas pressures are involved in the precipitator 38, and thus its shell may be of light fabricated sheet metal construction. The spacing between each pair of adjacent collector electrodes is not less than 400 mm., preferably between 450 mm. and 700 mm. The voltage to which the discharge electrodes are charged is between 80 and 100 Kv. Alternatively a narrower spacing may be used between 250 and 350 mm., in which case the voltage is between 40 and 50 Kv. Since the gases exiting from the precipitator are warm but unsaturated, there should be no steam plume arising. It is believed that U.K. Clean Air Regulations will be met if the precipitator operates at 90 to 98% efficiency. Various modifications are possible. For example, the electro-precipitator 38 may be replaced by a bag filter unit; the venturi device 23 may be replaced by a simple spray tower of uniform cross-section;the outlets 64 may be replaced by a conduit leading to a chimney stack. Again, a nitrogen seal may be provided between the mouth portion 18 and the hood 16 so that the gas from the converter 10 is not completely combusted having for example a carbon monoxide content as high as 80% by volume; the high levels of dilution with the second gas stream can reduce the carbon monoxide content of the incompletely burned gases to a level where the explosion hazard is minimal. The dilution ratio (NTP) is increased when there is incomplete combustion; this is because with complete combustion excess air is drawn into the gas stream at the hood 16, and also the first gas stream is likely to contain more water vapour following the unit 22, since it is essentially hotter as a result of the complete combustion. For example, a ratio of 5:1 may apply for complete combustion and 15:1 for 80% CO; it is believed there would be no explosion hazard at this degree of dilution. With incomplete combustion also the dew point of the mixed gas streams is decreased; for example, a dew point of 35"C may apply for 80% CO, and 50"C for complete combustion. In further modifications, the humidity of the mixed gas streams is reduced to minimise the possibility of saturated gas entering the electro-precipitator 38. For example in the case of incomplete gas combustion at the convertor 10, the gas may be burned at the ejector 37; this raises the temperature of the mixed gas streams entering the electro-precipitator, thus reducing the humidity; for example the temperature of the mixed gas streams could then be at least 200"C. Alternatively in either of the cases of complete or incomplete combustion at the convertor 10, only part of the gas leaving the scrubber/cooler unit 22 may pass into the conduit 34, the remainder being treated by conventional gas cleaning methods, e.g. a further gas scrubber as described in U.K. Specification No. 940,930, but arranged for high energy operation. In another alternative which again may be used for either case of complete or incomplete combustion, some of the gas from the convertor 10 bypasses the unit 22 via a bypass conduit 80 (borken line in Figure 1) and rejoins between the unit 22 and the ejector 37 (e.g. at 82); in this way by varying the extent of bypass the temperature and humidity of the mixed gas streams entering the precipitator can be controlled. A valve 83 is provided to vary the extent of bypass. WHAT WE CLAIM IS:

1. A method of cleaning a hot dirty particle laden gas emanating from a process carried out in an enclosed environment and ventilating the environment of escaped dirty gas: comprising the steps of (a) collecting hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) allowing hot dirty gas escaping into the enclosed environment to rise by convection to an upper portion of the enclosed environment and collecting the dirty gas at said upper portion to provide a second dirty gas stream diluted with air from the environment; (c) cooling the first dirty gas stream; (d) mixing the cooled first stream with the second stream; and (e) subjecting the mixed streams to a gas cleaning operation.

stream has a particle concentration up to 1 gram per N cubic metre.

10. A method according to any one of the preceding Claims, wherein the dilution ratio between the volumetric flow rates (N.T.P.) of the second gas stream and the first gas stream where they mix is between 20:1 and 5:1.

11. A method according to any one of the preceding Claims, wherein the gas from the first-mentioned gas cleaning operation has a particle concentration no greater than 0.05 grams per N cubic metre.

12. A method according to any one of the preceding Claims, wherein the gas from the first-mentioned gas cleaning operation is discharged to atmosphere at a temperature between 55"C and 160"C and in an unsaturated condition.

13. A method according to any one of the preceding Claims, wherein the dirty gas from the process is combustible and the gas is substantially completely burned where it is collected from the process.

14. A method according to any one of Claims 1 to 12, wherein the dirty gas from the process is combustible and the gas is not completely burned where it is collected from the process.

15. A method according to any one of Claims 1 to 12, wherein the first gas stream is combustible and is burned where it is mixed with the second gas stream.

16. A method according to any one of the preceding Claims, in which a portion of the first gas stream bypasses the gas cooling operation and rejoins prior to the first-mentioned gas cleaning operation, the extent of bypass being varied to control conditions at said gas cleaning operation.

17. A method of cleaning a hot dirty particle-laden gas emanating from a process carried out in an enclosed environment and ventilating the environment of escaped dirty gas: comprising the steps of (a) collecting hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) allowing hot dirty gas escaping into the enclosed environment to rise by convection to an upper portion of the enclosed environment and collecting the dirty gas at said upper portion to provide a second dirty gas stream diluted with air from the environment; (c) subjecting the first dirty gas stream to a preliminary gas cleaning operation; (d) mixing the first stream from the preliminary gas cleaning operation with the second stream; and (e) subjecting the mixed streams to a second gas cleaning operation.

18. A method according to Claim 17, wherein the second gas cleaning operation is carried out by electro-precipitation.

19. A method of cleaning a hot dirty particle-laden gas emanating from a process carried out in an enclosed environment and ventilating the environment of escaped dirty gas: comprising the steps of (a) collecting hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) allowing hot dirty gas escaping into the enclosed environment to rise by convection to an upper portion of the enclosed environment and collecting the dirty gas at said upper portion to provide a second dirty gas stream diluted with air from the environment; (c) conditioning, for electro-precipitation, the first gas stream with water; (d) mixing the conditioned first stream with the second stream; and (e) subjecting the mixed streams to a gas cleaning operation by electro-precipitation.

20. A method according to any one of the preceding Claims, wherein said process is steelmaking.

21. A method of cleaning a dirty particleladen gas substantially as hereinbefore described with reference to the accompanying drawings.

22. Gas when cleaned by a method according to any one of the preceding Claims.

23. Apparatus adapted for use in cleaning a hot dirty particle-laden gas emanating from a process carried out in a building and ventilating the building of escaped dirty gas: comprising (a) a primary gas collection system arranged to collect hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) a secondary gas collection system comprising inlet means in a roof portion of the building arranged to collect dirty gas escaping into the building and rising by convection to the roof portion, said collected gas providing a second dirty gas stream diluted with ambient air; (c) means for cooling the first dirty gas stream, the arrangement being such that the first stream from the cooling means is mixed with the second stream when the apparatus is in use; and (d) means for subjecting the mixed streams to a gas cleaning operation.

24. Apparatus according to Claim 23, wherein the cooling means comprises a gas scrubber, which also serves to perform a preliminary gas cleaning operation.

25. Apparatus according to Claim 23 or Claim 24, wherein the means for subjecting the mixed streams to a gas cleaning operation is mounted adjacent the level of the roof portion of the building.

26. Apparatus according to any one of Claims 23,24 and 25, wherein the means for subjecting the mixed streams to a gas cleaning operation comprises an electro-precipitator.

27. Apparatus according to Claim 26, wherein the electro-precipitator comprises a plurality of parallel planar collector electrodes between which are located a plurality of discharge electrodes, and the spacing between each pair of adjacent collector electrodes is not less than 400 mm.

28. Apparatus according to Claim 26 or Claim 27, wherein the electro-precipitator comprises a shell of light fabricated sheet metal construction.

29. Apparatus according to any one of Claims 23 to 28, comprising means for burning the first gas stream where it is mixed with the second gas stream.

30. Apparatus according to any one of Claims 23 to 29, comprising a bypass conduit, whereby in the operation of the apparatus a portion of the first gas stream bypasses the cooling means and rejoins prior to the first-mentioned gas cleaning means and means for varying the extent of bypass to control conditions at the secondary gas cleaning operation.

31. Apparatus adapted for use in cleaning a hot dirty particle-laded gas emanations from a process carried out in a building and ventilating the building of escaped dirty gas: comprising (a) a primary gas collection system arranged to collect hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) a secondary gas collection system comprising inlet means in a roof portion of the building arranged to collect dirty gas escaping into the building and rising by convection to the roof portion, said collected gas providing a second dirty gas stream diluted with ambient air; (c) means for subjecting the first dirty gas stream to a preliminary gas cleaning operation, the rarangement bieng such that the first stream from the gas cleaning operation is mixed with the second stream when the apparatus is in use; and (d) means for subjecting the mixed streams to a second gas cleaning operation.

32. Apparatus adapted for use in cleaning a hot dirty particle-laden gas emanating from a process carried out in a building and ventilating the building of escaped dirty gas: comprising (a) a primary gas collection system arranged to collect hot dirty gas substantially direct from the process to provide a first dirty gas stream; (b) a secondary gas collection system comprising inlet means in a roof portion of the building arranged to collect dirty gas escaping into the building, said collected gas providing a second dirty gas stream diluted with ambient air; (c) means for conditioning, for electro-precipitation, the first stream with moisuret, the arrangement being such that the first stream from the conditioning means is mixed with the second stream when the apparatus is in use; and (d) an electroprecipitator arranged to subject the mixed streams to a gas cleaning operation.

33. In a steelmaking plan, apparatus adapted for use in cleaning hot dirty particle-laden gas emanati g from a steelmaking process carried out out in a building and ventilating the building of escaped dirty gas: comprising (a) a primary gas collection system arranged to collect dirty gas substantially direct from the process to provide a first dirty gas stream of relatively high prrticle concentration; (b) a secondary gas collection system comprising inlet means in a roof portion of the building arranged to collect dirty gas escaping into the building, said collected gas providing a second dirty gas stream diluted with air from the environment to a relatively low particle concentration; (c) a gas scrubber arranged to subject the first gas stream to a preliminary gas cleaning and cooling operation, the arrangement being such that the first stream from the preliminary gas cleaning operation is mixed with the second stream when the apparatus is in use; and (d) an eiectro- precipitator arranged to subject the mixed streams to a second gas cleaning operation.

34. Apparatus adapted for use in cleaning a dirty particle-laden gas constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.