Abstract:
A method comprises providing a first gas stream containing a first concentration of sulfur dioxide; passing the first gas stream through at least a first catalyst volume whereby at least a portion of the sulfur dioxide is reacted to produce sulfur trioxide and removing from the at least a first catalyst volume a first treated gas stream containing sulfur trioxide and unreacted sulfur dioxide wherein the reaction of sulfur dioxide to sulfur trioxide is not limited by catalyst volume; providing a second gas stream containing a second and higher concentration of sulfur dioxide; and, passing the first treated gas stream and the second gas stream through at least at least a second catalyst volume to produce a second treated gas stream containing sulfur trioxide and unreacted sulfur dioxide.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the processing of sulfur dioxide containing gases. In one embodiment, this invention relates to the processing of waste gases from metallurgical processes, sulfur combustors or the like to remove sulfur dioxide therefrom and obtain sulfur dioxide, sulfur trioxide and/or sulfuric acid therefrom.  
         BACKGROUND OF THE INVENTION  
         [0002]    The conventional sulfuric acid contact process is based on once through treatment of dry process gases containing sulfur dioxide and enough excess oxygen to drive the catalytic conversion process to a high degree of completion. Where very stringent degrees of completion are needed, the process is often interrupted to remove the sulfur trioxide present so that an improved equilibrium can be approached with a significantly lower sulfur dioxide content in the unreacted process gasses. There are a number of limitations on the process that include having enough oxygen for both conversion and generating an acceptable equilibrium conversion and also managing the process in such a way that the temperatures which are obtained in the conversion process do not damage the catalyst.  
           [0003]    Classic design of sulfuric acid contact plants has been based on experience with gases formed by burning sulfur in which the sum of the oxygen and sulfur dioxide contents add to 21%. The contact process as now typically practiced uses dry sulfur dioxide containing gases to which enough air has been added to furnish the oxygen for conversion of the sulfur dioxide to sulfur trioxide together with enough extra oxygen to create a final equilibrium conversion in which a very low concentration of sulfur dioxide is achieved. For plants in which sulfur is burned with air and there is a single stage of sulfur trioxide removal, [the single absorption process], the normal gas feed strength is 8 percent sulfur dioxide and 13 percent oxygen and the tail gas will contain less than 0.2 percent sulfur dioxide.  
           [0004]    [0004]FIG. 1 is a typical gas flow diagram for a single stage absorption plant using metallurgical gas [i.e. gas obtained from, e.g., a smelting process]. The smelting operation may generate a plurality of waste gas streams  12  from roasters, furnaces or converters, each of which contains sulfur dioxide. Gas streams  12  are combined to form a combined waste gas stream  14 , which is fed it to pre-treater  16 . At pre-treater  16 , combined waste gas stream  14  is subjected to dry gas cleaning techniques known in the art and then scrubbed to form a saturated sulfur dioxide containing gas which is mixed, if necessary, with air to form a wet process gas. This wet process gas is then dried using sulfuric acid as a desiccant. Typically, the gas cleaning and drying equipment of pre-treater  16  is operated under vacuum. Accordingly, pretreated waste gas  18  is fed to blower  20 , which is located downstream of pretreater  16 , which provides enough pressure to force the gas through the converter system comprising, in this case, the exchangers  28 ,  30 ,  32  and the converter  22  and through the acid plant absorber which is not shown.  
           [0005]    From blower  20 , the waste gas flows through several heat exchangers [e.g. heat exchangers  28 ,  30  and  32 ] to preheat the gas to reaction temperature prior to the first catalyst bed  26 . Typically, first catalyst bed  26  inlet gas temperatures range from 380 to 430 degrees centigrade and the temperature of exit gas stream  34  ranges from 550 to 625 degrees centigrade. At temperatures above about 625 degrees centigrade, catalyst, converter and heat exchanger life is significantly shortened. Accordingly, the gas strength of waste gas stream  18  is normally kept below 12% sulfur dioxide. Exit gas stream  34  is passed through first heat exchanger  28  to produce cooled reacted gas stream  36  which is fed to the second catalyst bed  26 . Second exit gas stream  38  is obtained from second catalyst bed  26  and passed through second heat exchanger  30  to obtain second cooled reacted gas stream  40 . Second cooled reacted gas stream  40  is fed to the third catalyst bed  26  to obtain third exit gas stream  42 . Third exit gas stream  42  is fed through the third heat exchanger  32  to obtain cooled product gas stream  44 . While FIG. 1 shows three catalyst beds  26 , four or five beds are more common. In addition, as shown in FIG. 2, many plants now absorb sulfur trioxide between the third and fourth catalyst beds  26  so that a new equilibrium can be obtained between the residual oxygen and sulfur dioxide in the last catalyst beds  26  thus decreasing sulfur dioxide emissions.  
           [0006]    In new plants sulfur trioxide is removed in two stages [the double absorption process]. Sulfur trioxide builds up in the gas stream as the gas stream passes through the catalyst beds. The first stage absorption process to remove some of the build up sulfur trioxide occurs after the second or third catalyst bed and removes in excess of 90 percent of the sulfur trioxide. This allows the gas to approach a new equilibrium unaffected by the previously removed sulfur trioxide during the passage of the gas stream through subsequent catalyst beds. In such a case, the overall gas strength entering the plant is commonly increased to around 11.5% sulfur dioxide with 9.5% oxygen and the tail gas will have a sulfur dioxide content of less than 350 ppm of sulfur dioxide, corresponding to an overall efficiency of 99.7% which compares to 98% for the single absorption process. Under the conditions in these two processes, there is a significant flow of nitrogen through the process due to the air used. This nitrogen must be heated in each catalyst bed and accordingly moderates the temperature rise. The predominant use of the double absorption process with a sulfur feed and air has resulted in catalysts being developed with these temperature constraints as limits.  
           [0007]    Several other processes also generate sulfur dioxide containing gases and the stoichiometry may be very different. One major source is the incineration of waste acids. In such processes, waste acids are thoroughly decomposed by hot gases obtained from hydrocarbon combustion and the mixed gases are often quite dilute, well below the gas strengths obtained from the combustion of sulfur. In some cases, these plants may use a significant amount to sulfur as fuel and gas strengths may rise beyond the levels of sulfur burner gas but such operations are not common. A further version that can be found is the use of tonnage oxygen to combust the hydrocarbons in the waste acid furnaces in which case the gas strength rises significantly and the gas volume decreases. These last two cases can result in gases that are somewhat stronger than sulfur burner gas but are currently typically treated in a similar manner to sulfur burner gas.  
           [0008]    A more important source of strong gases is the smelting of ores using oxygen flash smelting techniques instead of air. Typically, in the classic smelting of sulfide ores, sulfur dioxide gas and an oxide slag/product are formed. If air is used, typically two-thirds of the oxygen ends up in the exhaust gas and one-third is combined in the metal oxide. In order to recover the sulfur dioxide as sulfuric acid, more air then has to be added to provide not only the oxygen for the reaction but also the extra oxygen to create the proper conversion of sulfur dioxide to sulfur trioxide. Such plants typically operate with gas strengths below 8% sulfur dioxide, which is below the level found in sulfur burner plants.  
           [0009]    With the increasing costs of fuel, and pressures to removed sulfur oxides from the atmosphere, the use of oxygen as a substitute for air or for enriching air, has become common in smelters. For the smelter operator, the heat previously carried out of the smelting furnaces by hot nitrogen in the furnace off-gases is no longer being lost and, accordingly, more of the heat produced by the oxidation of sulfide to sulfur dioxide is directed to melting the concentrate. The concentrate can also be reacted to a higher degree than with previous techniques, reducing the load on pyrometallurgical operations, which now include oxygen as the raw material.  
           [0010]    With tonnage oxygen, the smelter off-gases are now often much more concentrated than the gases obtained by burning sulfur in the air and stronger gases are available for use in acid plants. Several approaches have been used to handle stronger gases. However, the primary problem is the very high temperature rise in the initial catalyst bed and the damage it does to catalyst and converter vessels and the downstream gas exchanger.  
           [0011]    For example, one approach is to use a pre-converter in which a part of the gas stream is contacted with a limited catalyst bed to create a partially converted gas which is then mixed with the remainder of the gas stream and fed to the first catalyst bed with sulfur trioxide already present to limit the temperature rise in the first catalyst bed. Referring to FIG. 2, a plant schematic is shown which utilizes a pre-converter. In this embodiment, pretreated gas stream  18  is fed through blower  20  and through passage  24  to pre-converter  52 . The exit gas from pre-converter  52  is combined with the remaining unreacted gas in feed gas stream  54  to produce first treated gas stream  56 . While the concentration of sulfur oxides in gas stream  56  may be relatively high bracket [e.g. 14 to 15% by volume], the presence of sulfur trioxide that was produced in pre-converter  52  limits the temperature rise in the downstream catalyst beds. First treated gas stream  56  is fed through first heat exchanger  58  to produce first cooled gas stream  60  which is fed to the first catalyst bed  26  to produce second exit gas stream  62 . Second exit gas stream  62  is fed through second heat exchanger  64  to produce second cooled gas stream  66 , which is fed to the second catalyst bed  26  to produce third exit gas stream  68 . Third exit gas stream  68  passes through third heat exchanger  70  to obtain third cooled gas stream  72  which is fed to the third catalyst bed  26  to obtain fourth exit gas stream  74 . Fourth exit gas stream  74  may be subjected to further treatment (e.g. a sulfur trioxide removal step) before passing through one or both of fourth heat exchanger  76  and third heat exchanger  70  to obtain fourth cooled gas stream  78  which is fed to the fourth catalyst bed  26  to obtain fifth exit gas stream  80 . Fifth exit gas stream  80  is fed through fifth heat exchanger  82  to produce cooled product gas stream  84 .  
           [0012]    The difficulty with this approach is that the catalyst in the pre-converter is sacrificial, as the catalyst in it will normally approach relatively closely to a very high temperature equilibrium. If it is proposed to end the conversion of this pre-converter well away from equilibrium, then control becomes difficult, as the strength of the feed gas is already variable as a result of the metallurgical operation.  
           [0013]    In an alternate approach, the gas strength in a plant has simply been allowed to rise and the higher temperatures accepted. This plant has been in operation for over five years and has experienced significant operating and maintenance problems in and downstream of the pre-converter catalyst bed. Although the gas in this plant is strong, there is still more than enough oxygen for conversion and occasional rises in gas strength due to fluctuations in flow add to the difficulty of the operations.  
           [0014]    In a further alternate approach, an older plant using strong gas sources has now been in operation for over 10 years and has succeeded by adding sufficient dilution air to dilute the sulfur dioxide content to 12% sulfur dioxide, the limit in sulfur burning plants. In this plant, unlike the sulfur burning plant, the oxygen content entering the first bed is over 15% compared with 9% for a sulfur burner plant, and the catalyst is more reactive and rises to higher equilibrium temperature.  
         SUMMARY OF THE INVENTION  
         [0015]    In accordance with the instant invention, it has been surprisingly found that the converter bed temperature limitation and the overall gas strength limitation set by the initial catalyst step or steps can be overcome by utilizing feed gas streams, namely a first gas stream consisting of gas containing sulfur dioxide and oxygen and compatible with the simple use of the gas in the first catalyst bed without overheating the catalyst, and a second more concentrated stream which can be added after the first catalyst bed and before the second catalyst bed. In this way, an initial conversion of sulfur dioxide to sulfur trioxide can be achieved without risking the catalyst in a pre-converter. The feed gas stream for the second catalyst bed [namely the second more concentrated stream and the gas stream from the initial conversion step] has sufficient sulfur trioxide to limit the temperature rise in the second catalyst bed to acceptable levels even with much higher sulfur dioxide concentrations, up to the limits of gas composition which would be set by a the overall conversion constraints.  
           [0016]    By operating a process in accordance with this invention, some or all of the strong gas [e.g., with the sulfur dioxide concentration of from about 13 to about 24%, based on volume, and with the oxygen concentration of from about 11 to about 18%, based on volume] may be used to produce sulfuric acid without dilution. By reducing or eliminating the amount of dilution air that must be added, the size of acid plant equipment, and its attendant cost, may be reduced. In addition, the operating cost of acid plants may be reduced as smaller volumes of gas travel through the equipment. For example, modern large smelter acid plants may consume 10 to 15 MW of power simply to pump fluids through the plant.  
           [0017]    A further advantage of the instant process is that a greater amount of heat may be recovered from the system. In a typical acid plant, the amount of dilution air that must be added results in the temperature of the waste gas being sufficiently low so that efficient and reliable energy recovery is not practical. By avoiding the use of such large volumes of dilution air, the operating temperature of the process may be higher, thus permitting heat recovery and, therefore, an improvement in the overall economics of the process.  
           [0018]    A further advantage of the instant process is that the amount of sulfur dioxide released to the atmosphere may be reduced. One of the regulatory requirements for acid plant emissions is the stack gas composition [i.e. the concentration of pollutants in the waste gas] and not the total emission of pollutants from the plant. The low gas strength needed in typical acid plants requires a large quantity of dilution air. Thus the total volume of stack gas is relatively large and carries a substantial amount of sulfur dioxide into the atmosphere even despite low levels of sulfur dioxide in the stack gas. The process of the instant invention emits lower amounts of stack gasses [since less or no dilution air may be required] and, in addition, may result in the stack gas have an even lower quantity of sulfur dioxide.  
           [0019]    In accordance with the instant invention, there is provided a method comprising:  
           [0020]    (a) providing a first gas stream containing a first concentration of sulfur dioxide;  
           [0021]    (b) passing the first gas stream through at least a first catalyst volume whereby at least a portion of the sulfur dioxide is reacted to produce sulfur trioxide and removing from the at least a first catalyst volume a first treated gas stream containing sulfur trioxide and unreacted sulfur dioxide wherein the reaction of sulfur dioxide to sulfur trioxide is not limited by catalyst volume;  
           [0022]    (c) providing a second gas stream containing a second concentration of sulfur dioxide; and,  
           [0023]    (d) passing the first treated gas stream and the second gas stream through at least a second catalyst volume to produce a second treated gas stream containing sulfur trioxide and unreacted sulfur dioxide.  
           [0024]    In one embodiment, the second concentration is higher than the first concentration.  
           [0025]    In another embodiment, the second concentration is greater than about 12% sulfur dioxide.  
           [0026]    In another embodiment, the first and second gas streams are obtained from different sources.  
           [0027]    In another embodiment, the first and second gas streams are obtained from different stages of a smelting operation.  
           [0028]    In another embodiment, the first gas stream is diluted to reduce the concentration of sulfur dioxide to a concentration at which the first catalyst volume will not overheat.  
           [0029]    In another embodiment, a feed stream containing sulfur dioxide is split to produce the first and second gas streams wherein sulfur dioxide concentration of the first and second streams is varied by a step selected from the group consisting of diluting the first stream to reduce the concentration of sulfur dioxide, increasing the concentration of sulfur dioxide in the second stream and a combination of both.  
           [0030]    In another embodiment, the first and second gas streams are separately dried before contacting a catalyst volume.  
           [0031]    In accordance with the instant invention, there is also provided a method comprising:  
           [0032]    (a) obtaining a first gas stream containing a first concentration of sulfur dioxide from a first source of sulfur dioxide;  
           [0033]    (b) subjecting the first gas stream to catalysis to produce a first treated gas stream containing sulfur trioxide and sulfur dioxide;  
           [0034]    (c) obtaining a second gas stream containing a second concentration of sulfur dioxide from a second source of sulfur dioxide; and,  
           [0035]    (d) subjecting the first treated gas stream and the second gas stream together to catalysis to produce a second treated gas stream containing sulfur trioxide and sulfur dioxide.  
           [0036]    In one embodiment, each of the first and second gas streams is separately pretreated before being subjected to catalysis. Preferably, the pretreatment step includes a drying step.  
           [0037]    In accordance with the instant invention, there is also provided an apparatus comprising:  
           [0038]    (a) a first catalyst bed;  
           [0039]    (b) a first gas stream passage positioned upstream from the first catalyst bed and connecting in fluid flow communication a first source of sulfur dioxide with the first catalyst bed;  
           [0040]    (c) a second catalyst bed positioned downstream from the first catalyst bed;  
           [0041]    (d) a treated gas passage connecting the first and second catalyst beds in fluid flow communication; and,  
           [0042]    (e) a second gas stream passage positioned upstream from the second catalyst bed and connecting in fluid flow communication a second source of sulfur dioxide with the second catalyst bed.  
           [0043]    In one embodiment, the second gas stream passage communicates with the second catalyst bed via the treated gas passage.  
           [0044]    In another embodiment, the concentration of sulfur dioxide in the second source of sulfur dioxide is higher than the concentration of sulfur dioxide in the first source of sulfur dioxide.  
           [0045]    In another embodiment, the concentration of sulfur dioxide in the second source of sulfur dioxide is greater than about 12% sulfur dioxide.  
           [0046]    In another embodiment, the first and second sources of sulfur dioxide are obtained from different stages of a smelting operation.  
           [0047]    In another embodiment, gas from the first source of sulfur dioxide is diluted to reduce the concentration of sulfur dioxide to a concentration at which the first catalyst bed will not overheat.  
           [0048]    In another embodiment, the apparatus further comprises a first dryer upstream from the first catalyst bed for drying gas from the first source of sulfur dioxide and a second dryer for drying gas from the second source of sulfur dioxide.  
           [0049]    In accordance with the instant invention, there is also provided an apparatus comprising:  
           [0050]    (a) a first catalyst bed;  
           [0051]    (b) a first gas stream passage positioned upstream from the first catalyst bed and in fluid flow communication with the first catalyst bed, the first gas stream passage providing sulfur dioxide to the first catalyst bed, the first catalyst bed having a volume of catalyst which does not limit conversion of sulfur dioxide to sulfur trioxide;  
           [0052]    (c) a second catalyst bed positioned downstream from the first catalyst bed;  
           [0053]    (d) a treated gas passage connecting the first and second catalyst beds in fluid flow communication; and,  
           [0054]    (e) a second gas stream passage positioned upstream from the second catalyst bed and in fluid flow communication with the second catalyst bed, the second gas stream passage providing sulfur dioxide, which has not passed through the first catalyst bed, to the second catalyst bed.  
           [0055]    In one embodiment, the second gas stream passage communicates with the second catalyst bed via the treated gas passage.  
           [0056]    In another embodiment, gas having a first concentration of sulfur dioxide is provided to the first catalyst bed and gas having a higher concentration of sulfur dioxide is provided to the second catalyst bed.  
           [0057]    In another embodiment, the first gas stream passage is in fluid flow communication with a different source of sulfur dioxide then the second gas stream passage.  
           [0058]    In another embodiment, the apparatus further comprises a first dryer upstream from the first catalyst bed and a second dryer upstream from the second catalyst bed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0059]    For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying drawings, of the preferred embodiments of the present invention, in which:  
         [0060]    [0060]FIG. 1 is a schematic drawing of a typical gas flow diagram of a single stage absorption plant using metallurgical gas as is known in the art;  
         [0061]    [0061]FIG. 2 is a schematic drawings of a plant that uses a pre-converter as is known in the art;  
         [0062]    [0062]FIG. 3 is a schematic drawing of a plant according to the one embodiment of the instant invention;  
         [0063]    [0063]FIG. 4 is a schematic drawing of a plant according to another embodiment of the instant invention;  
         [0064]    [0064]FIG. 5 is a schematic drawing of a process to obtain the feed gas for the process of FIGS. 3 and 4; and,  
         [0065]    [0065]FIG. 6 is a schematic drawing of a metallurgical process to obtain the feed gas for the process of FIGS. 3 and 4. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0066]    [0066]FIG. 3 shows a schematic drawing of the sulfur trioxide production portion of a sulfuric acid plant incorporating the method and apparatus of the instant invention. Pursuant to the instant invention, at least two feed streams of gas containing sulfur dioxide are obtained and optionally separately pretreated in pretreaters to obtain pretreated streams which are then fed through a plurality of catalyst beds or volumes of catalyst such that at least a portion of the sulfur dioxide, and preferably essentially all of the sulfur dioxide, is converted to sulfur trioxide. The number and sequence of catalyst beds through which the pretreated streams are passed may be varied as is known in the art, provided that the first catalyst bed does not receive all of the at least two feed streams of gas containing sulfur dioxide. It will be appreciated that the use of additional catalyst beds, as well as the interim removal of sulfur trioxide, will increase the amount of sulfur dioxide that is converted to sulfur trioxide and, accordingly, reduce the amount of sulfur dioxide which is emitted from the plant.  
         [0067]    In the preferred embodiment of FIG. 3, two sulfur dioxide feed streams  102 ,  104  are utilized. Each feed stream  102 ,  104  is optionally separately pretreated in a pretreater  106 ,  108  to obtain pretreatment streams  110 ,  112 . The pretreatment steps may be any that are known in the sulfuric acid production of art. Preferably, the pretreatment includes cleaning and drying the sulfur dioxide containing gas in preparation for its catalytic conversion to sulfur trioxide. Two separate pretreaters  106  and  108  are used so that streams  102  and  104  are not commingled. It will be appreciated that only one pretreater may be used if streams  102  and  104  are fed separately through the pretreater in which case storage vessels will be required if a continuous operation is to be maintained. It will be appreciated that each of streams  102  and  104  may be obtained by combining a plurality of streams, which may be obtained from different sources.  
         [0068]    Each feed stream  102 ,  104  contains a different concentration of sulfur dioxide. The concentration of the feed stream fed to the first catalyst bed  122  is set such that the catalyst in the first catalyst bed  122  will not overheat. Accordingly, the feed stream fed to the first catalyst bed  122  may contain from about 8 to about 13, preferably from about 10 to about 12 and more preferably from about 11 to about 12% by volume sulfur dioxide and from about 9 to about 18, and preferably from about 9 to about 15 by volume oxygen. The volume of catalyst in the first catalyst bed  112  is preferable set so that the reaction of sulfur dioxide to sulfur trioxide is not limited by the amount of catalyst in the first catalyst bed  122 .  
         [0069]    Feed streams  102 ,  104  of may be obtained in a variety of manners provided they have different concentrations of sulfur dioxide. For example, as shown in the embodiment of FIG. 5, a single sulfur dioxide containing stream  160  may be initially obtained. This stream may be pretreated and then split into two portions  162  and  164 . The concentration of at least one of the divided out streams  162  and  164  is adjusted to obtain feed streams  102  and  104 . For example, the initial sulfur dioxide containing stream may be concentrated [e.g. above about 8 volume percent sulfur dioxide], in which case one of the divided out streams may be diluted in dilution step  168  with air, oxygen, oxygen enriched air or any other suitable dilution gas  170  to obtain first feed stream  102 . Alternately, if the initial sulfur dioxide containing stream is weak, then divided out stream  162  may be concentrated by, e.g., air separation techniques, in concentration step  166  to produce second feed stream  104 . In a still further alternate embodiment, one of the divided out streams may be diluted to obtain first feed stream  102  and the other divided out stream may be concentrated to obtain second feed stream  104 .  
         [0070]    In the alternate preferred embodiment shown in FIG. 6, feed streams  102  and  104  are obtained from a metallurgical process and, more preferably, from a smelter handling sulfide ores. For example, the smelter may contain one or more flash smelting furnaces  176  in which sulfide concentrate  172  is contacted with oxygen  174  to form a slag stream  178  containing essentially iron oxide, a matte stream  180  comprising metal sulfides, some remaining iron and sulfur, and gaseous effluent  104  consisting mainly of sulfur dioxide with some nitrogen, oxygen and miscellaneous contaminants. In such a case, the molten matte is then further reacted in a reactor  184  with air, enriched air, or oxygen  182  to form the molten metal or other the final product  186  and a gas stream  102  containing the remaining sulfur values as sulfur dioxide with other common gases. The second stream will typically contain about one-third of the sulfur dioxide emissions with significantly more dilutants than the stream from the flash smelting furnace.  
         [0071]    The gas streams from reactors  176  and  184  may be fed directly into the process of the instant invention. Alternately, one or both of the streams  102  and  104  may be subjected to a pretreatment step. Gasses from both of the effluent streams from reactors  176  and  184  are preferably cleaned and dried separately [e.g., in pretreaters  106  and  108 ]. However, part of the effluent from the flash furnace  176  may be combined with the effluent stream from reactor  184  if the stoichiometry permits. For example, a portion of the effluent from flash furnace  176  may be combined with the effluent from reactor  184  so as to increase the concentration of stream  102 .  
         [0072]    If dilution air is required for the overall process, then the dilution air may be added to the effluent stream from reactor  184 , the effluent from flash furnace  176  or both. Preferably, the dilution air, if required, is added to the effluent from reactor  184  so as not to dilute the strong gas that is obtained from flash furnace  176 . For example, the effluent gas stream from  184  may contain up to 12% by volume sulfur dioxide and significantly more oxygen by volume then in the effluent from flash furnace  176 .  
         [0073]    Referring again to FIG. 3, subsequent to the preferred cleaning step, the strong and dilute gases [pretreated streams  112  and  110  respectively] are fed, such as by passing them through blowers  116  and  114  respectively, to the acid plant via streams  120  and  118 . Dilute pretreated gas stream  110  is preferably preheated as is known in the art to a suitable temperature for catalytic conversion in the first catalyst bed  122 . As shown in FIG. 3, a portion of pretreated strong gas stream  112  may also be fed to the first catalyst bed  122  via stream  126  [which may be combined with stream  118  or fed separately into first catalyst bed  122 ].  
         [0074]    In first catalyst bed  122 , from about 50 to about 75, and preferably about two-thirds of the sulfur dioxide in the gas fed to the first catalyst bed  122  is converted into sulfur trioxide. Preferably, the gas that is fed to the first catalyst bed  122  has as much sulfur dioxide and oxygen as is compatible with conversion in a first catalyst bed within the temperature limits normally observed in sulfuric acid plants. The temperature of the exit gas from the first catalyst bed  122  may vary from about 550 to about 650 degrees centigrade and, preferably, is about 600 degrees centigrade.  
         [0075]    First exit gas stream  124 , which is the first treated stream, is passed through first heat exchanger  130  to obtain first cooled stream  132 . First cooled stream  132  is fed to the second catalyst bed to produce second exit gas stream  134 , which is the second treated stream. Subsequent to first catalyst bed  122 , all of the remaining strong gas stream is fed to one more of the subsequent catalyst beds is  122 . As shown in FIG. 3, all of the remaining strong gas stream is fed by stream  128  to a point up stream of first heat exchanger  130  where it is combined with exit gas stream  124 . Alternately, as shown in FIG. 4, all of the remaining strong gas stream is fed by stream  128  directly into the second catalyst bed  122 . It will be appreciated by those skilled in the art that none or only a portion of the remaining strong gas stream  112  may be fed directly [e.g., as shown in FIG. 4] or indirectly [e.g., as shown in FIG. 3] to the second catalyst bed  122  and the remaining portion may be fed to one a more of the catalyst beds  122  down stream from the second catalyst bed  122 . In a most preferred embodiment, all of strong gas stream  112  not fed to the first catalyst bed  122  is fed to the process upstream of second catalyst bed  122 . In a further alternate embodiment, a portion of the dilute gas stream may be fed to the catalyst beds after bypassing the first catalyst bed  122 . This alternate embodiment may be utilized if the volume of dilute gas stream  110  is greater than the capacity of first catalyst bed  122 . Pursuant to this alternate embodiment, a portion of dilute stream  110  may be fed to the second catalyst bed  122  by bypass stream  127  (see FIGS. 3 and 4). The addition of a portion of the dilute gas stream  110  will reduce the overall concentration of strong gas stream  112 . However, even in this case, the concentration of the feed gas which has not been catalytically treated (i.e., strong gas stream  112  either alone or in combination with a portion of the dilute gas stream  110  that by passed the first catalyst bed  122 ), which is mixed with the treated dilute gas stream provided to the second catalyst bed  122  (i.e. gas stream  132 ), is greater than the concentration of dilute gas stream  110 .  
         [0076]    The feed stream fed to the second catalyst bed  122  may contain from about 12 to about 24, preferably from about 14 to about 18 and more preferably from about 14 to about 16% by volume sulfur dioxide and from about 13 to about 18, preferably from about 12 to about 15 and more preferably from about 9 to about 13% by volume oxygen. It will be appreciated that depending upon the concentration of strong gas stream  112 , strong gas stream  112  may be diluted if additional gas volume is required or metered at a sufficient rate to obtain a desirable conversion level in the second catalyst bed  122 . As first cooled gas stream  132  already contains sulfur trioxide, the effect of the stronger gas on the temperature rise in second catalyst bed  122  is limited by the sulfur trioxide present in the feed gas to the second catalyst bed  122 . Preferably, the overall gas composition of the gas fed to the second catalyst bed  122  is limited by the need to have enough oxygen in the last catalyst bed  122  to reduce the sulfur dioxide content to levels compatible with permitted emission levels.  
         [0077]    Second exit gas stream  134  may then be treated as is known in the art. For example, it may pass through a series of conventional catalyst, exchanger and absorption steps until an appropriate gas is generated for venting to the atmosphere. For example, as shown in FIGS. 3 and 4, second exit gas stream  134  is passed through second heat exchanger  136  to produce second cooled stream  138 , which is fed to third catalyst bed  122  to produce third exit gas stream  140 . Third exit gas stream  140  is passed through third heat exchanger  142  and is then preferably subjected to a sulfur trioxide absorption process  144 . The effluent from sulfur trioxide absorption process  144  is passed optionally through third heat exchanger  142  and second heat exchanger  136  to obtain third cooled gas stream  146  which is fed to fourth catalyst bed  122 . Fourth exit gas stream  148  is removed from fourth catalyst bed  122  and passed through fourth heat exchanger  150  to obtain cooled product gas stream  152 .  
         [0078]    Assuming the sulfur dioxide gas content is reduced to conventional levels of 250 to 350 ppm, there will be an overall reduction in sulfur dioxide emissions by using the method and apparatus of the instant invention as the total quantity of stack gas may be around 70 percent of conventional emissions, and overall efficiency as well may be significantly higher, and a preferably it may be up to about 99.9% compared with present targets of 99.7%.  
         [0079]    Conventional acid plants are limited in the gas strengths they can process by the need on the one hand to have enough excess oxygen to drive the conversion process to the extent needed to meet environmental emission limits and the need to avoid overheating the first catalyst bed. Until recently, the common limits were set to based on the combustion of sulfur in air and 12 percent sulfur dioxide with 9 percent oxygen resulted, giving an oxygen content in the tail gas of 4%. With most metallurgical processes using air to provide the oxygen, the gases were more dilute as part of the oxygen ended up as metal oxides in slag or metal product. The introduction of flash smelting dramatically changed the stoichiometric relationships in the gas feed to acid plants. With essentially pure oxygen as a feed, the gas streams from metallurgical processes may contain as high as 80 percent sulfur dioxide with some oxygen. If dilution air is added to produce the same exit oxygen levels from the converter system as in sulfur burning plants, then the overall gas composition of the feed gas to the converter would be in the range of 19 to 20% by volume sulfur dioxide which would result in an extremely hot catalyst if used directly in an acid plant. To date, the solution that has been utilized is to add enough air to dilute the feed gas to about 12%. The instant invention permits the gas feed to the first catalyst bed to be provided at standard strengths [e.g. up to 12%] and then it adds the remaining strong gas, which has been generated as a separate stream, resulting in a gas feed to the second catalyst bed which is essentially full strength. In the second catalyst bed, the unconverted sulfur dioxide is diluted by the sulfur trioxide formed in the first catalyst bed thus preventing the second bed from overheating. Beyond the second bed, gas handling can proceed using normal practice.  
         [0080]    One advantage of the instant invention is that the catalyst bed temperatures in the first and second catalyst beds can be limited to safe long-term values independent of the overall gas strength used. Longer equipment life and more reliable operation are a direct result of easing the severity of conditions in the first catalyst bed. A second advantage is that the overall gas strength used in the process may be higher and the volume of gas handled is significantly decreased with savings in both capital and operating costs. A third advantage is that the overall process may operate on an overall basis at higher temperatures and facilitates energy recovery from the gas streams. A fourth advantage is that a higher sulfur trioxide gas composition may be obtained which allows higher strength oleum to be produced. A fifth advantage is that a separate high strength sulfur dioxide gas stream is required which could lead easily to producing a liquid sulfur dioxide stream if needed to trim the acid plant or for other purposes.  
         [0081]    It will be appreciated by one skilled in the art that various additions and modifications may be made to the formation treating fluid disclosed herein and each is within the scope of this invention.