Process and plant for producing sulfuric acid

Abstract

The invention is directed to a process to produce sulfuric acid and a plant for carrying out this process. At least one source is producing a sulfur dioxide-containing gas in an amount a, such that at least a part of the sulfur dioxide-containing gas and added oxygen are introduced as a feed gas into a pre-conversion stage to produce a sulfur trioxide-containing gas stream. The amount of inert gas added together with the oxygen is between 0 and 78 vol.-%. A partial stream of the sulfur trioxide-containing gas stream is branched off and is recycled directly into the pre- conversion stage or into the feed gas of the pre-conversion stage. The remaining sulfur dioxide-containing gas is passed through a conventional sulfuric acid plant, which comprises at least one contact stage arranged in a main converter to react sulfur dioxide with oxygen to produce sulfur trioxide. Therein, the generated sulfur trioxide-containing gas is fed to at least one absorber, where the produced sulfur trioxide is absorbed with sulfuric acid as absorption medium to form sulfuric acid. The conventional sulfuric acid plant has a capacity to convert an amount of sulfur dioxide below the amount a.

Description

 Process and plant for producing sulfuric acid The present invention relates to a process and its corresponding plant for producing sulfuric acid, wherein at least one source is producing a sulfur dioxide-containing gas in an amount a, wherein at least a part of the sulfur dioxide-containing gas and added oxygen are introduced as a feed gas into a pre-conversion stage to produce a sulfur trioxide-containing gas stream, wherein a partial stream of the sulfur trioxide-containing gas stream is branched off and is recycled directly into the pre-conversion stage or into the feed gas of the pre-conversion stage while a remaining part of the sulfur trioxide-containing gas stream is passed through a pre-converter absorber wherein it is absorbed with sulfuric acid as an absorption medium to produce a liquid sulfuric acid stream and a remaining sulfur dioxide-containing gas, wherein the remaining sulfur dioxide-containing gas is passed through a conventional sulfuric acid plant, comprising at least two contact stages of main converters arranged in series to react sulfur dioxide with oxygen to produce sulfur trioxide, and wherein the generated sulfur trioxide-containing gas is fed to at least one absorber, wherein the produced sulfur trioxide is absorbed with sulfuric acid as absorption medium to form sulfuric acid.  Sulfuric acid production is typically achieved using the double absorption process, as explained e.g., in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A 25, pp. 635-700. Sulfur dioxide (SO2) is acquired via sulfur combustion or as waste gas from metallurgical plants and is then transformed into sulfur trioxide (SO3) in a typically four- or five-stage converter using a solid catalyst, such as vanadium pentoxide as an active ingredient. The produced sulfur trioxide is withdrawn following the contact stages of the converter and conveyed to an intermediate absorber. Alternatively, it may be supplied to a final absorber after the last converter stage. In the final absorber, the sulfur trioxide-containing gas is supplied counter-currently to concentrated sulfuric acid and absorbed in the same.  The reaction of SO2 to SO3 in the converter stages is exothermic and proceeds according to the following equation: 2 SO2(g) + O2(g) ⇌ 2 SO3(g) : ΔH = -197 kJ This highly exothermic reaction leads to increasing temperatures. Particularly the forming of so-called hot-spots, which are spatially limited areas at which the catalyst is exposed at very high local temperature, which causes irreversible damages of the catalyst. This problem occurs especially in the first converter stage, because here the entering sulfur dioxide concentrations are particularly high and no sulfur trioxide is yet present in the gas stream, which allows the reaction from sulfur dioxide to sulfur trioxide to achieve a high conversion until equilibrium is reached. This high conversion comes with a high enthalpy release, which depending on the initial SO2 content might increase the temperature of the catalyst beyond its stability limits.  Therefore, the converter is divided in stages which gives some possibilities to control the rebuff of energy. Typically, heat exchangers are foreseen between each stage such that the gas leaving a converter stage is cooled down before it is passed into the next stage. Moreover, catalyst concentration can be varied, whereby the overall turnover rate can be controlled. However, all these measures are not sufficient to avoid an over-heating reliably for high sulfur dioxide concentrations. Thus, feed gases are usually applied with a maximum sulfur dioxide content between 11 and 13 vol.-% to avoid the development of excessive temperatures in the converter stages.  However, processes such as the above-mentioned sulfur burning as well as metallurgical processes generate sulfur dioxide containing gases typically having a much higher sulfur dioxide content between 18 and 50 vol.-%. Therefore, these gases have to be diluted prior to being fed to conventional sulfuric acid plants, which leads to large volumes of gas and consequently high dimensions of all apparatuses. Therefore, the so-called LUREC® (described in WO 2004 / 037719 A1) has been developed which allows processing of contact gases featuring a sulfur dioxide content of more than 13 vol-%. Thereby, a partial gas stream comprising sulfur dioxide and sulfur trioxide is withdrawn from the product stream of a contact stage and recycled to the feed gas of the first contact stage. The increased proportion of sulfur trioxides influences the equilibrium between sulfur dioxide and sulfur trioxides shown in the above reaction equation in such a way that the conversion is reduced. This lower turnover also produces less energy, which prevents the catalyst from overheating.  However, this process cannot be implemented into existing, so-called brown field sulfuric acid plants without substantial, costly modifications to handle the occurring high-temperature and high-pressure gas streams. In cases where metallurgical processes produce increased quantities of sulfur dioxide due to a decrease in the quality of ores or where the plant capacity is to be increased by burning more sulfur, the capacity c of such existing plants cannot be increased beyond their original design for an amount a of sulfur dioxide. Therefore, it is the object of the present invention to provide a process and a corresponding plant for producing sulfuric acid which allows operating a conventional sulfuric acid plant featuring a capacity cin combination with a source generating sulfur dioxide-containing gas in an amount exceeding the plant's capacity to convert sulfur dioxide.  This object is solved by a process with the features of claim 1. In this process, a conventional plant with a capacity c is joined with at least one source is producing a sulfur dioxide-containing gas in an amount a. At least a part of the sulfur dioxide-containing gas and added oxygen are introduced as a feed gas into a first pre-conversion stage of a pre-converter to produce a sulfur trioxide-containing gas stream. The added oxygen stream can be added at every position, however, in each case it features an amount of inert gas / and or impurities in a range between 80 and 0,0001 vol.-%.  After the first pre-converter stage or another subsequent pre-converter stage of the pre-converter, a partial stream of the sulfur trioxide-containing outlet gas stream is branched off and recycled directly into the first pre-conversion stage or into the feed gas of the pre-conversion stage while a remaining part of the sulfur trioxide-containing gas stream is passed through downward stages of the pre-converter and afterwards a pre-converter absorber or directly in the pre-converter absorber. Therein, the gas is absorbed with sulfuric acid as an absorption medium to produce a liquid sulfuric acid stream and a remaining sulfur dioxide-containing gas in an amount of a or less. The remaining sulfur dioxide-containing gas in an amount of a or less is then passed through a conventional sulfuric acid plant, comprising at least two contact stages of main converters arranged in series to react sulfur dioxide with oxygen to produce sulfur trioxide, and wherein the generated sulfur trioxide-containing gas is fed to at least one absorber, wherein the produced sulfur trioxide is absorbed with sulfuric acid as absorption medium to form sulfuric acid. Thereby, no changes at the existing plant are necessary, however, capacity can be increased. Therefore, the basic idea underlying the invention an interaction of the addition of oxygen with a concentration significantly higher than the oxygen concentration in air. As a result, the volume flows through the conventional plant can be greatly reduced. At the same time, the pre-converter in combination with the recirculation of sulfur trioxide ensures that sulfur dioxide load on the first stage of the converter in the conventional plant does not exceed the specific maximum concentration of sulfur dioxide which can be handled in the converter. In this context it is also important that the addition of relatively pure oxygen leads also for the pre-converter to a desirable reduction in gas volume, which relieves components such as, e.g., heat exchangers and blowers. However, these lower gas volumes generally lead to higher temperatures in sulfur dioxide conversion stages since an equal amount of sulfur dioxide in lower total gas volume of the feed gas corresponds to a higher concentration, which shifts the reaction equilibrium towards sulfur trioxide. Further an equal amount of released reaction heat will lead to a higher gas temperature if the volume is lower. Therefore, the pre-converter according to the invention features a recycling of a partial stream of the sulfur trioxide-containing gas stream into the first pre-conversion stage or into the feed gas of this first pre-conversion stage allows keeping the temperature within the working range of the catalyst of typically 370 to 440 °C. Simultaneously it reduces the remaining sulfur dioxide concentration to a level suitable for the downstream conventional sulfuric acid plant despite having capacity to convert an amount of sulfur dioxide below the amount a generated by the at least one source. For this effect to be partially great, the added oxygen is introduced in a concentration of between 90 and 99 vol.-% and / or the concentration of the sulfur dioxide in the sulfur dioxide-containing gas is at least 14 vol.-%, particularly preferably between 18 and 65 vol.-% before mixing these streams. In other word, the amount of inert gases added together with the oxygen is between 0 and 78 vol.-%, preferably between 0 and 50 vol.-% in the oxygen feed stream. Consequently, the ratio of sulfur dioxide to oxygen entering the preconversion stage, at constant volume flows of sulfurdioxide containing gas and oxygen, is between 0,14 and 2,95, preferably 0,18 and 1,3, most preferably 0,45 to 0,6. Whereas a higher purity of oxygen further reduces the gas volume, oxygen with a concentration between 95 and 99.5 vol.-%, is a readily available and cheap oxygen source. This holds particularly true for technically pure oxygen, i.e., oxygen grade 2.5 (99.5 vol-%). Thereby, the problem typically associated with the use of lower grade oxygen gases such as technically pure oxygen and below, i.e., the required removal of impurities contained therein by an exhaust gas flow in a later stage is mitigated. While normally impurities and a high concentration in the purge stream with its sulfur dioxide content leads to too high cost to meet today's ecological standards, the passing through the conventional sulfuric acid plant eliminates the need for a separate cleaning of the purge stream.  In a preferred embodiment, the oxygen for the pre-conversion stage is added into the at least one source for producing a sulfur dioxide-containing gas. This allows implementing the inventive process with a minimum of structural changes, since the generation of sulfur dioxide requires the addition of an oxygen-containing gas and thus, means for adding oxygen to the source are already present. Furthermore, smaller blowers may be used for introducing the oxygen-containing gas into the source for producing a sulfur dioxide-containing gas since a smaller volume of gas suffices to supply enough oxygen in contrast to, e.g., air comprising only approximately 20 vol.-% of oxygen. In addition, all downstream components can then be designed to be smaller. It is further preferred, that the at least one source for the sulfur dioxide-containing feed gas is a burning of elemental sulfur with oxygen. With elemental sulfur burners particularly concentrated and highly pure sulfur dioxide-containing gases may be generated with concentrations of up to 66 vol.-%. In the sense of the invention burning of elemental sulfur with oxygen covers all catalytic and non-catalytic processes in which sulfur and oxygen are reacted to sulphur dioxide, irrespective of whether or not flame formation occurs. Alternatively or additionally, off-gases of metallurgical processes as mentioned in the introduction may be used as the at least one source for the sulfur dioxide-containing feed gas.  According to a preferred embodiment, the oxygen introduced in the above-described burning of the elemental sulfur is introduced in a gas stream with at least 25 vol-%, preferably more than 95 vol.-% oxygen. This has also a positive impact in form of an increased efficiency of the burning. It is further preferred,that the at least a part of the sulfur dioxide-containing feed gas is cooled to temperature of between 350 and 450 °C, preferably 380 and 430 °C, and most preferably of between 390 and 410 °C before entering the pre-converter. Thereby, excessively high temperatures in the pre-converter stage may be avoided and the heat contained in the at least a part of the sulfur dioxide-containing feed gas may be used to (pre)heat other process streams such as, e.g., water for the production of steam or the oxygen for the pre-conversion stage. According to a further embodiment, the partial stream of the sulfur trioxide-containing gas stream which is branched off from the sulfur trioxide-containing gas stream generated in the pre-conversion stage has a volume fraction of between 10 and 80 %, preferably between 12 and 50 % of the sulfur trioxide-containing gas stream. Thus, a sufficient concentration of sulfur trioxide in the feed gas for the pre-conversion stage is assured. Correspondingly, the remaining part of the sulfur trioxide-containing gas stream, which is fed to the pre-converter absorber, may account for between 90 and 20 vol%, preferably between 50 and 88 vol.-% of the sulfur trioxide-containing gas stream. The preferred split fractions depend on the content of sulfur trioxide after the contact, which depends on the amount of contact stages. The more contact stages are employed the higher the sulfur trioxide content is and the lower the recycle stream can be to achieve the required modification of the equilibrium. According to another embodiment, sulfur dioxide-containing gas is branched off prior to being introduced into the pre-conversion stage and is passed directly into the conventional sulfuric acid plant. So, the at least a part of the sulfur dioxide-containing gas for the pre-converter stage is less than 100 vol.-% of the sulfur dioxide-containing gas produced by the at least one source, preferably the amount is between 12 and 90 % of the sulfur dioxide-containing gas generated in the at least one source. Normally, the sulfur dioxide-containing feed gas is branched off prior to adding the oxygen for the conversion stage. The partially bypassing of the pre-converter has the advantage that the and the capacity of the existing plant is fully utilised. Also, the sulfur dioxide load may be distributed between the pre-conversion stage and the conventional sulfuric acid plant allowing to, e.g., react to variations in the sulfur dioxide concentration. In a further embodiment, a further sulfur trioxide-containing gas stream is branched off downwards of a contact stage of main converters in the conventional sulfuric acid plant and is recycled directly into the pre-converter or into the feed gas of the first pre-conversion stage. This not only generates a redundancy for the recirculation of the partial stream of the sulfur trioxide-containing gas stream at the pre-conversion stage, but also allows a better control of the sulfur trioxide content, which is fed to the pre-conversion stage as part of the feed gas. Also a remaining sulfur dioxide stream from the final absorber of the conventional plant can be admixed in any position prior the pre-converter to increase the overall turnover. In a further modification, the pre-converter absorber and the at least one absorber of the conventional sulfuric acid plant feature a common pump tank for the sulfuric acid used as the absorption medium. Consequently, a pump tank already present in a conventional sulfuric acid plant can be utilized, which lowers the costs for retrofitting existing plants and further provides the opportunity for a central further processing of the generated sulfuric acid. In a particularly preferred embodiment, the partial stream of the sulfur trioxide-containing gas stream is mixed into the feed gas for the pre-conversion stage by means of a gas ejector. Thereby, the sulfur dioxide containing gas and added oxygen are introduced as a motive medium in an ejector in a motive medium inlet such that it sucks in and accelerates the recycling stream working in a sucking medium inlet as a sucking medium to produce a feed gas streaming through a discharge outlet for the converter stage. In contrast to this solution, the hot gas blowers are usually utilized to increase the pressure of the partial stream of the sulfur trioxide-containing gas stream, which is branched off and recycled into the pre-conversion stage or into the feed gas of the pre-conversion stage. However, the fans and seals of these blowers are prone to wear due to being in contact with the hot and highly corrosive sulfur trioxide-containing gas stream. In addition to the described pressurization a homogeneously mixed feed gas is obtained, which renders an additional gas mixer obsolete and reduces the chance for locally excessive temperatures, i.e., hot spots, in the pre-conversion stage. Considering further that the ejector lacks sensitive parts such as fans or the necessary seals in hot gas blowers, the recycled partial stream may neither be actively heated nor cooled. The pressure of the feed gas leaving the ejector preferably has a pressure between 130 and 150 kPa, to ensure a sufficient pressure in the system. The invention further relates to a plant according to claim 13, particular a plant for performing a process according to claims 1 to 12. Such a plant for the production of sulfuric acid, comprises at least one source for producing a sulfur dioxide-containing gas in an amount a. Further, it features a pre-converter with at least one pre-conversion stage for reacting a feed gas comprising at least a part of the sulfur dioxide-containing gas and added oxygen to produce a sulfur trioxide-containing gas stream. Thereby, the line for adding oxygen and the dimensions of the pre-converter and related apparatuses like heat-exchangers, fans etc. are designed such that the oxygen concentration in this gas stream is at least 11,6 vol.-%. In this context the plant also features a line for branching off a partial stream of the sulfur trioxide-containing gas stream and for recycling it into the first pre-converter stage or into the feed gas of the first pre-conversion stage. Downwards of the pre-converter, an absorber for absorbing a remaining part of the sulfur trioxide-containing gas stream with sulfuric acid to produce a liquid sulfuric acid stream and a remaining sulfur dioxide-containing gas is foreseen. Afterwards a conventional sulfuric acid plant with a capacity c is positioned. Said conventional sulfuric acid plant comprises at least two contact stages of main converters arranged in series to react sulfur dioxide with oxygen to produce sulfur trioxide and at least one absorber for absorbing the produced sulfur trioxide in sulfuric acid and has a capacity to convert an amount of sulfur dioxide below the amount a. This plant has all the advantages which are previously described for the process which can be carried out in such a plant. All preferred embodiments also apply to such a preferred design of the plant. As described above, it is preferred that the at least one source for producing a sulfur dioxide-containing feed gas comprises an elemental sulfur burner. It is preferred that the line introducing oxygen into this burner is designed such that oxygen in an amount of at least 22 vol.-% are introduced. In one embodiment, the plant comprises the elemental sulfur burner in combination with a firetube boiler to utilize the heat generated in the burner to produce superheated steam.  Alternatively, a roaster, such as a fluidized bed reactor used for the roasting of sulfidic ores, or a smelter which are configured to provide a sulfur-dioxide containing off-gas may be used as the at least one source for the sulfur dioxide-containing gas. In particular, these sources have in common that the amount a of the generated sulfur dioxide-containing gas readily surpasses the conventional sulfuric acid plant's capacity to convert sulfur dioxide in a continuous process. In a one embodiment, the plant comprises at least one heat exchanger configured to cool the at least part of the sulfur dioxide-containing gas to a temperature of between 380 and 430 °C, preferably of between 390 and 410 °C before entering the pre-conversion stage. Additionally or alternatively, a heat exchanger configured to cool the remaining sulfur dioxide-containing gas to a temperature of between 250 and 180 °C, may be foreseen. In a further embodiment the plant comprises a gas ejector for mixing of the partial stream of the sulfur trioxide-containing gas stream with the feed gas of the pre-conversion stage and simultaneously adjusting the pressure of the feed gas for the pre-converter. Thereby, the ejector is configured such that the sulfur dioxide containing gas and added oxygen, typically having a pressure of 130 and 150 kPa, enter the ejector as motive gas via the ejector nozzle. This leads to an acceleration creating a low-pressure region around the nozzle tip. The lower pressure causes the recycled partial stream of the sulfur trioxide-containing gas stream to be sucked into the ejector as suction medium, upon which the gases are mixed and the feed gas with a set pressure, typically between 130 and 150 kPa, , is generated in the ejector's diffuser. In another embodiment, a line, i.e., a conduit, is provided which is configured to branch off a further sulfur trioxide-containing gas stream downwards of at least one of the at least two contact stages of main converters in the conventional sulfuric acid plant and to recycle said further stream directly into the pre-conversion stage or into the feed gas of the pre-conversion stage. Preferably, the further sulfur trioxide-containing gas stream works as suction medium in the above-described gas ejector. Further developments, advantages and possible applications can also be taken from the following description of exemplary embodiments and the drawings. All features described and / or illustrated from the subject matter of the invention per se or in any combination, independent of their inclusion in the claims or their back reference. In the drawings:  Fig. 1 shows schematically a conventional sulfuric acid plant, Fig. 2 shows schematically a first embodiment of a source for producing a sulfur dioxide-containing gas coupled with a conventional sulfuric acid plant via a pre-converter as according to the invention, Fig. 3 shows schematically a second embodiment of the invention featuring a pre-converter with two conversion stages, Fig. 4 shows schematically a third embodiment of the invention comprising a bypass line for sulfur dioxide-containing gas, Fig. 5 shows schematically a fourth embodiment of the invention comprising two sources for producing the sulfur dioxide-containing gas, Fig. 6 shows schematically a detail view of a pre-conversion stage according to a third embodiment of the invention comprising an ejector. Figure 1 shows a conventional sulfuric acid plant according to the state of the art. However, the invention is not limited to this specific plant but can be applied to any type of sulfuric acid plant featuring a converter with at least one contact stage and at least one absorber. Therein, a sulfur dioxide-containing gas stream is introduced into a main converter 20, featuring four contact stages of main converters 21' to 21'''' arranged in series to react sulfur dioxide with oxygen to produce sulfur trioxide. Between each contact stage 21' to 21'''', not-shown heat exchangers are foreseen to cool the sulfur trioxide-containing gas stream obtained from the respective contact stage to a temperature sufficiently low to avoid excessive temperatures in the subsequent contact stage 21'' to 21'''' and / or absorber 30, 30'. The sulfur trioxide-containing gas obtained from contact stage 21''' is then withdrawn via conduit 24 and fed to the intermediate absorber 30. Concentrated sulfuric acid, preferably having a concentration between 93 and 99,5 %, is supplied as absorption medium for the produced sulfur trioxide to the intermediate absorber 30, preferably countercurrent, to the sulfur trioxide-containing gas, via conduit 31 from pump tank 34. The obtained remaining sulfur-dioxide containing gas is withdrawn via conduit 32 and supplied to any o the contact stages 21' to 21''', preferably the latest, via conduit 32, while the formed sulfuric acid is recirculated into pump tank 34. From there, a product stream of sulfuric acid is withdrawn via conduit 35 for further processing. For an improved absorption sulfur trioxide from at least one of the latest contact stages 21 '' to 21''' is fed to the final absorber 30'. It is preferred that all absorbers 30 and 30' feature a common pump tank for supplying sulfuric acid. A heat exchanger is typically be positioned such that the concentrated sulfuric acid supplied to the absorbers 30, 30' has essentially the same temperature, i.e., preferably between 60 and 230 °C, preferably 80+ / - 5 °C. Thereby, optimum conditions for the absorption of sulfur trioxide may be ensured in a simple manner. The remaining tail gas is subjected to a gas cleaning section (not shown) via conduit 36 to remove any remaining sulfur dioxide and / or other impurities, while the sulfuric acid formed in the final absorber 30' is recirculated into pump tank 34 via conduit 33'.  It should be noted hat the conventional sulfuric acid plant 20 may also have a different setup than the shown 3+1 arrangement of the contact stages of main converters 21' to 21''''. For example, a 3+2 setup of the contact stages is possible. Furthermore, the number and arrangement of absorbers 30 and 30' may vary. Moreover, it is possible to branch off a sulfur trioxide containing gas stream at any position and to recycle it into a converter stage, preferably the first converter stage 21' as it is described in WO 2004 / 037719 A1. Any conventional sulfuric acid plant has a distinct capacity c, which is readily exceeded by the amount a of the sulfur dioxide-containing gas, since any increase of the amount a results in even larger volumes of sulfur dioxide containing feed gases supplied to the conventional sulfuric acid plant 20.  Typically. sulfur-containing material, such as elemental sulfur in case of an elemental sulfur burner or sulfur containing ores in case of a pyrometallurgical reactor, are introduced in a reactor, wherein they are converted with oxygen. One possible oxygen source is dry air obtained from a drying tower in a downwards sulfuric acid plant. However, the amount of converted sulfur is the determining factor for the produced amount of sulfur dioxide, which is why the amount of converted sulfur has to be controlled such that the amount a is not exceeded. Moreover, not even the amount a but also the concentration of the sulfur dioxide has to be controlled due to the reasons already described, namely to avoid catalyst destroying hot-spots. Opposite to these limiting factors, the current invention gives a possibility to increase plant capacities of existing plants. Sulfur dioxide-containing gas is produced by at least one source 10 and is fed together with added oxygen to a pre-converter 40 comprising at least one, i.e., at least a first pre-conversion stage 40' as feed gas via conduits 14, 42 and 43. In principle, the added oxygen stream can be added at every position. In the present embodiment, conduit 41 is configured to add the added oxygen stream to the sulfur-dioxide containing gas, whereby the added oxygen stream features a total amount of inert gas and / or impurities in the range between 0 and 78 vol.-%. That means the oxygen is added with in a high concentration of preferably at least 25 vol.-% oxygen. Preferably, conduit 41 is arranged such that the oxygen for the pre-converter is added after the produced sulfur dioxide-containing gas is cooled to a temperature between 350 and 430 °C by means of a not-shown heat exchanger). So, in contrast to the dilution with air comprising only around 20 vol.-% oxygen, a lower volume of feed gas is generated. Thus, the capacity c of conventional sulfuric acid plant 20 in terms of maximum processible gas volume is not exceeded.  Having a closer look at the sulfur dioxide concentration which can be handled in the sulfuric acid plant 20, the pre-converter 40 plays an important role: In the pre-conversion stage 40', a sulfur trioxide-containing gas stream is produced, which is withdrawn via conduit 44. To avoid excessive temperatures in the pre-conversion stage 40', a partial stream of the sulfur trioxide-containing gas stream is branched off and recycled into the feed gas of the pre-conversion stage 40' by means of recycling conduit 45. Alternatively, the partial stream may directly be recycled into the pre-conversion stage 40'. The partial stream of the sulfur trioxide-containing gas stream accounts for a volume portion of between 7 and 90 % of the sulfur trioxide-containing gas stream. Consequently, the reaction equilibrium is shifted such that the reaction rate for producing sulfur trioxide is reduced and less heat is generated in pre-conversion stage 40'. Naturally, it is possible to have further pre-converting stages. However, the easiest arrangement is a pre-converter featuring only one stage. To increase the gas pressure of the partial stream of the sulfur trioxide-containing gas stream, preferably to a pressure between 110 and 160 kPa, a hot gas blower 47 is foreseen. However, for such a hot gas blower it is necessary that the partial stream of the sulfur trioxide-containing gas stream has to be cooled to a temperature of between 250 and 300 °C by means of a heat exchanger 46 prior to passing through hot gas blower 47 and subsequently re-heated to a temperature of between 380 and 450 °C by means of heat exchanger 46'. The remaining sulfur trioxide-containing gas stream, i.e., the fraction not branched of and recycled, is supplied to pre-converter absorber 50 via conduit 51. The remaining sulfur trioxide containing gas stream is cooled to a temperature between 150 and 250 °C by means of heat exchanger 52 prior to entering pre-converter absorber 50. In the pre-converter absorber 50, concentrated sulfuric acid as absorption medium is introduced via conduit 53 from a pump tank to produce a liquid sulfuric acid stream and a remaining sulfur dioxide-containing gas in an amount of a or less. Thereby, the pre-converter is able to reduce the sulfur dioxide concentration entering the conventional sulfuric acid plant 20 to a concentration which can be handled in this plant. A second embodiment of the invention is shown in Figure 3. In comparison to the embodiment shown in Figure 2, conduit 12 is configured to add the oxygen for the feed gas to reactor 10, in particular in form of an oxygen-containing gas with an oxygen content of 50 to 95 vol.-% or technically pure oxygen. Therefore, a separate conduit 41 for introducing the required oxygen is not required. If the reactor 10 is designed for a roasting of sulfur containing gases, e.g. it can be built as a fluidized bas which means that the amount of oxygen in the fluidizing gas in increased. In a more preferred embodiment, sulfur is burnt in reactor 10. In the sense of the invention, the term "is burnt" covers any reaction of sulfur with oxygen, independent whether a flame occurs or not. In particular, it covers spray guns, nozzles, lances and all other possibilities taking place in a hot furnace wherein droplets of molten sulfur are introduced. Furthermore, pre-converter 40 preferably comprises a first pre-conversion stage 40' and optionally also an additional second pre-conversion stage 40'' for increasing the total sulfur dioxide conversion in the pre-converter 40. The first sulfur trioxide-containing gas stream obtained from the first pre-conversion stage 40' is fed to the second pre-conversion stage via conduit 48, which may feature a heat exchanger (not shown) to reduce the temperature of the first sulfur trioxide-containing gas stream from 650 °C to a temperature of between 400 and 450 °C to avoid excessive temperatures in the second pre-conversion stage 40''. Also the second pre-conversion stage 40'' can ensure a higher all-over turnover rate, a design with only one pre-conversion stage 40' has the benefit of a very easy and reasonably priced design. In the present embodiment, the partial stream of the sulfur trioxide containing gas stream obtained from the second pre-conversion stage 40'' is recycled to the feed gas for the first pre-conversion stage 40' via recycling conduit 45 in the same manner as described above. Alternatively, a partial stream of the sulfur trioxide containing gas stream may be withdrawn and recycled from conduit 48. Furthermore, a partial stream of a sulfur trioxide-containing gas stream obtained from a contact stage 21' to 21'''' of main converter 21 in the conventional sulfuric acid plant 20 may be recycled to the first pre-conversion stage 40' or to the feed gas of the pre-converter 40. This has the advantage of recycling a gas stream still comprising a considerable amount of remaining sulfur dioxide. It is particularly preferred if pump tank 34 constitutes the common reservoir for the concentrated sulfuric acid used as absorption medium in the pre-converter absorber 50 as well as the absorbers 30, 30' of the conventional sulfuric acid plant 20. Thereby, concentrated sulfuric acid can be supplied to the pre-converter absorber 50 and the generated is liquid sulfuric acid stream is returned to pump tank 34 via conduit 54. Consequently, a central sulfuric acid supply as well as a further processing of the sulfuric acid product stream is realized requiring only minor adaptions of already existing structures. A third embodiment of the invention is shown in Figure 4. Therein, only a part of the produced sulfur dioxide-containing gas is supplied to the pre-converter 40 via conduits 42 and 43. The remaining fraction is branched off upstream of the pre-converter via conduit 19. Preferably, conduit 19 is configured such that the branched-off sulfur dioxide-containing gas has a volume fraction of between 13 and 90 % of the sulfur dioxide-containing gas. The branched-off gas is subsequently mixed with the remaining sulfur dioxide-containing gas withdrawn from the pre-converter absorber 50 via conduit 55 and supplied to the conventional sulfuric acid plant 20 via conduit 56. In this case the concentration of sulfur dioxide in conduit 56 is below 14 vol.-% for conventional plant operation. As one option amongst others, the produced sulfur dioxide-containing gas is passed from reactor 10 through a watertube boiler 18 via conduit 14. Independent from this design a further heat exchanger 49 for cooling the at least part of the sulfur dioxide containing gas in conduit 42 can be foreseen. This allows a better temperature control of the feed gas for the pre-converter 40, in particular in case of varying volumes of sulfur dioxide-containing gas branched-off via conduit 19. A further embodiment is shown in Figure 6. As described with respect to Figure 2, main blower 17 can be operated at less than full capacity, e.g., only at 66 %, in the inventive embodiments. The freed capacity may be utilized to supply the oxygen-containing gas such as air or technically pure oxygen to a further reactor 11 via conduit 13'. Preferably, the further reactor 11 is an elemental oxygen burner. Sulfur-containing material, such as elemental sulfur in case of an elemental sulfur burner or for example sulfidic ores in case of a pyrometallurgical reactor, is introduced into reactor 11 via conduit 12'. Similar to source 10, the further sulfur dioxide containing gas produced in reactor 11 is fed the further firetube boiler 18' via conduit 14'. The further sulfur dioxide-containing gas is then added to the branched-off sulfur dioxide containing gas in conduit 19 via conduit 19'. Alternatively, the further sulfur dioxide-containing gas may also be admixed to the at least part of for the sulfur dioxide-containing gas as part of the feed gas for pre-converter 40 (not shown).  In the present example, the total amount of sulfur containing material fed to reactors 10 and 11 may be increased by up to 37 % in comparison to the state of the art described above. Fig. 6 gives a more detailed view for the pre-converter 40. Therein, a gas ejector 49 is foreseen which is configured to mix the partial stream of the sulfur trioxide-containing gas stream recycled via recycling conduit 45 with the at least a part of the sulfur dioxide-containing gas and added oxygen supplied via conduit 42. In other words, the sulfur dioxide containing gas and added oxygen are introduced as a motive medium such that it sucks in and accelerates the recycling stream in conduit 45 working as a sucking medium to produce a feed gas streaming through a discharge outlet for the converter stage at a set pressure range. Thereby, both streams are perfectly mixed and not further mixing element is necessary. Moreover, no blower, fan or the like is necessary in recycling conduit 45. This also eliminates the need to cool the stream to a temperature at which such a blower can be used. Reheating is also no longer necessary. Simultaneously, the pressure of the feed gas supplied to the at least one (first) pre-conversion stage 40' of pre-converter 40 is adjusted. Thereby, the sulfur dioxide containing gas and added oxygen, having a pressure of 130 kPa to 160 kPa, enter the ejector 49. The lower pressure causes the recycled partial stream of the sulfur trioxide-containing gas stream to be sucked into the ejector 49 as suction medium generating the feed gas with the set pressure, typically between 130 kPa and 150 kPa. This modification may be used in any of the previous embodiments.  Reference numbers 10 reactor11 reactor12-16 conduit17 main blower18, 18' firetube boiler19, 19' conduit20 conventional sulfuric acid plant21 main converter21'-21'''' contact stages 22-25 conduit30 intermediate absorber30' final absorber 31-33'' conduit34 pump tank35,36 conduit40 pre-converter40', 40'' pre-conversion stages41-44 conduit45 recycling conduit46, 46' heat exchanger47 hot gas blower48 conduit49 ejector50 pre-converter absorber51 conduit52 heat exchanger53-56 conduit

Claims

1. A process for the production of sulfuric acid, wherein at least one source is producing a sulfur dioxide-containing gas in an amount a, wherein at least a part of the sulfur dioxide-containing gas and added oxygen are introduced as a feed gas into a pre-conversion stage to produce a sulfur trioxide-containing gas stream, whereby an amount of inert gas added together with the oxygen is between 0 and 78 vol.-%, wherein a partial stream of the sulfur trioxide-containing gas stream is branched off and is recycled directly into the pre-conversion stage or into the feed gas of the pre-conversion stage, wherein the remaining, not-recycled sulfur dioxide-containing gas is passed through a conventional sulfuric acid plant, comprising at least one contact stage arranged in a main converter to react sulfur dioxide with oxygen to produce sulfur trioxide, and wherein the generated sulfur trioxide-containing gas is fed to at least one absorber, wherein the produced sulfur trioxide is absorbed with sulfuric acid as absorption medium to form sulfuric acid, whereby the conventional sulfuric acid plant has a capacity to convert an amount of sulfur dioxide below the amount a.

2. A process according to claim 1, characterized in that the oxygen for the pre-conversion stage is introduced in a concentration of between 22 and 100 vol.-% and / or the concentration of the sulfur dioxide in the sulfur dioxide-containing gas is at least 13 vol.-%.

3. A process according to claim 1 or 2, characterized in that the oxygen for the pre-conversion stage is added into the at least one source for producing a sulfur dioxide-containing gas.

4. A process according to any of the previous claims, characterized in that the at least one source for the sulfur dioxide-containing feed gas is a reaction of elemental sulfur with oxygen.

5. A process according to claim 4, characterized in that the oxygen introduced in the reaction of the elemental sulfur is introduced in a gas stream with at least 22 vol-% oxygen.

6. A process according to any of the previous claims, characterized in that the at least a part of the sulfur dioxide-containing feed gas is cooled to temperature, so that the gas entering the catalyst bed has a temperature of technically feasible catalyst inlet temperature, between 350 and 450 °C before entering the pre-conversion stage.

7. A process according to any of the preceding claims, characterized in that the partial stream of the sulfur trioxide-containing gas stream which is branched off has a volume fraction of between 7 and 90 % of the complete sulfur trioxide-containing gas stream.

8. A process according to any of the preceding claims, characterized in that sulfur dioxide-containing feed gas is branched off prior to being introduced into the pre-conversion stage and is passed directly into the conventional sulfuric acid plant.

9. A process according to claim 8, characterized in that volume fraction of the branched off sulfur dioxide-containing feed gas is used for adjusting the sulfur dioxide content of the conventional downstream acid plant.  10. A process according to any of the preceding claims, characterized in that while a remaining part of the sulfur trioxide-containing gas stream of the pre-converter stage is passed through a pre-converter absorber.

11. A process according to claim 10, characterized in that sulfur trioxide-containing gas is branched off prior to being introduced into the pre-conversion absorber and is passed directly into the conventional sulfuric acid plant.

12. A process according to any of the preceding claims, characterized in that a further sulfur trioxide-containing gas stream is branched off downstream of at least one of the at least two contact stages in the conventional sulfuric acid plant and is recycled directly into the pre-conversion stage or into the feed gas of the pre-conversion stage.

13. A process according to any of the preceding claims, characterized in < / b>that the pre-converter absorber and the at least one absorber of the conventional sulfuric acid plant feature a common pump tank for the sulfuric acid used as the absorption medium.

14. A process according to any of the preceding claims, characterized in that the partial stream of the sulfur trioxide-containing gas stream is mixed into the feed gas for the pre-conversion stage by means of a gas ejector whereby the pressure of the feed gas for the pre-conversion stage is simultaneously adjusted.

15. A plant for the production of sulfuric acid, in particular, for carrying out the process according to any one of claims 1 through 14, comprising at least one reactor (10) for producing a sulfur dioxide-containing gas in an amount a, a pre-converter (40) for reacting a feed gas comprising at least a part of the sulfur dioxide-containing gas and added oxygen to produce a sulfur trioxide-containing gas stream, a recycling conduit (45) for branching off a partial stream of the sulfur trioxide-containing gas stream and for recycling it into the pre-converter (40) or into the feed gas of the pre-converter (40), an pre-converter absorber (50) for absorbing a remaining part of the sulfur trioxide-containing gas stream with sulfuric acid to produce a liquid sulfuric acid stream and a remaining sulfur dioxide-containing gas and a conventional sulfuric acid plant (20), comprising at least one contact stages (21) of a main converter to react sulfur dioxide with oxygen to produce sulfur trioxide and at least one absorber (30, 30') for absorbing the produced sulfur trioxide in sulfuric acid, whereby the conventional sulfuric acid plant (20) has a capacity to convert an amount of sulfur dioxide below the amount a.

16. A plant according to claim 15, characterized in that the at least one reactor (10) for producing a sulfur dioxide-containing feed gas comprises an elemental sulfur burner.

17. A plant according to any of claims 15 or 16, <b>characterized in that the plant comprises a gas ejector (49) for mixing of the partial stream of the sulfur trioxide-containing gas stream with the feed gas of the pre-converter (40) and simultaneously adjusting the pressure of the feed gas for the pre-converter (40).  

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