WO2021249628A1

WO2021249628A1 - Plant and process for producing sulfuric acid from an off-gas with low sulfur dioxide content

Info

Publication number: WO2021249628A1
Application number: PCT/EP2020/065979
Authority: WO
Inventors: Karl-Heinz Daum; Ralf Neumann; Dana NURGALIYEVA; Jörg HAMMERSCHMIDT; Jochen Güntner
Original assignee: Outotec (Finland) Oy
Priority date: 2020-06-09
Filing date: 2020-06-09
Publication date: 2021-12-16

Abstract

The invention is directed to a plant and its relating method for producing sulfuric acid. It comprises a source (10), wherein ore and/or a gas is thermally treated to produce amongst others a raw gas containing sulfur dioxide with a concentration below 3.5 vol.-%. Further, it contains a blower (122) for the cleaned gas and a converter (160) with at least one stage filled with a catalyst to convert the contained sulfur dioxide into sulfur trioxide. At least one cooling stage (40) is foreseen, which is arranged upstream the blower (122) to cool the gas to a temperature below 30 °C to reduce its water content, as well as a at least one first heat exchanger (150) arranged between blower (122) and converter (160) or inside the converter (160) to heat the gas such that it enters the converter (160) at elevated temperature, preferably with a temperature above 400 °C. Moreover, it contains a second heat exchanger (20), whereby the heat transfer medium of the second heat exchanger (20) is used to heat the first heat exchanger (150).

Description

Plant and process for producing sulfuric acid from an off-gas with low sulfur dioxide content

The invention relates to a plant and a relating process for producing sulfuric acid, comprising a source wherein ore and/or gas is thermally treated to produce amongst others a raw gas containing sulfur dioxide with a concentration between 0,5 and 3.5 vol.-%, a blower for the cleaned gas and a converter with at least one stage filled with a catalyst converter containing sulfur dioxide into sulfur trioxide.

In recent years, ore with very low sulfur content has been employed more and more. Moreover, processes with a sulfur dioxide content with a concentration above ~4 vol.-% of SO2 are well established. Therefore, the demand for plants for processing gas with lower SO2 concentrations converting that SO2 into sulfuric acid has increased.

There are mainly two processes, which would lead to SO2 concentrations around 1 to 4 vol.-%:

- off-gases of metallurgical processes, primarily originating from roasting, e.g. sul- fatizing roasting or precious metals roasting, which require a wet gas cleaning

- off-gases from combustion of lean H2S gas or sour gas, which contain significant amounts of water moisture.

The difference between the off-gas from metallurgical processes and the off-gas from lean gas or sour gas is that the metallurgical off-gas requires a full scale wet gas cleaning plant prior to the SO2 conversion due to the very large number of contained impurities, while the off-gas from combustion of lean H2S gas or sour gas can be fed directly to a wet catalyst plant without the necessity of prior gas cooling and cleaning. Very low SO2 concentrations, particular concentrations below 1 vol.-% SO2 can be treated and scrubbed with alkaline neutralization agents such as Ca(OH)2, CaO, CaCC , NaOH, Nhh, Na₂C03, dual-alkali and the like. This obviously require related chemicals and produce byproducts that are mostly difficult to market or have to be deposited as significant costs. Using higher SO2 concentrated gas, particular with a concentration above 1 vol.-% SO2, these technologies can only be used with enormous technical challenges, which is why these technologies are normally not or very rarely be used in practice.

For this problematic concentration range, particular for concentrations between 1 and 3.5 vol.-%, it is not possible or straight forward to use any e.g. calcium-based agent, as a solubility of Ca²⁺ compounds is very low and the system does not offer any buffering capacity, which would be required to cope with fluctuating gas flows and SO2 concentrations. Therefore, crystallization and encrustation of e.g. gyp sum is inevitable. Moreover, it is common knowledge that a gas can be exposed to activated carbon, which generates typically 20 to 30 mass.-% H2SO4 what, however, leads not to a product, which can be sold profitably.

The last two decades have created other scrubbing technologies for low concen trated gas, mostly based on amine solution absorption followed by desorption and thus providing an almost 100 % SO2 gas, which either be recycled to an acid plant or else captured as e.g. liquid SO2. Examples hereof are Cansolv® or SolvR®. High energy demand (steam stripping and costly absorbens regeneration) have prevented wide spread use of these technologies.

Summing up, all the discussed technologies are very expensive, but offer very little revenue. As it turns out when comparing the available technologies, each of the explained processes will lead to increased capex and opex without any com pensation by a saleable product. As a result, it is the aim of the invention to provide a plant and its relating process, to treat gases with SO2 concentrations below 3.5 vol.-% but above 0,5 vol.-%, by converting these into sulphuric acid

This task is solved with a plant according to claim 1.

Such a plant features sources, wherein ore and/or a gas is thermally treated to produce amongst others a raw gas containing sulfur dioxide with a concentration between 0,5 and 3.5 vol.-%. The plant further features a blower for the cleaned gas as well as a catalytic converter with at least one stage. The stage of the con verter is filled with a catalyst to convert this contained sulfur dioxide into sulfur trioxide. It is essential for the invention that at least one cooling stage is arranged upstream the blower whereby the gas containing sulfur dioxide is cooled, prefer ably to a temperature below 30 °C, - in accordance with the water balance of the sulphuric acid plant. By cooling the gas, water moisture is condensed and, there fore, it is possible to receive a gas, which can be fed into the converter without the negative impact of contained water in form of a highly corrosive sulfuric acid with a concentration below 93 vol.-%

However, in previous processes, a cooling and condensing seems to be possible only for gases with a much higher SO₂ concentration since otherwise the cooled gas temperature is much too low for the significant turnover in the downward con verter, which is why according to the invention between blower and converter a heat exchanger is foreseen. Alternatively, this heat exchanger is positioned inside the converter. Independent from the position of that heat exchanger, the cooled gas is re-heated to a temperature enabling a conversion to SO₃, particular to a temperature well above 400 °C in the heat exchanger. It is also part of the idea underlying the invention, that this re-heating uses energy generated at any other device of the plant and, therefore, increasing the overall energy efficiency. Only the combination of cooling and heating with heat gener ated in the process itself provides an economical basis.

Moreover, the entry temperature in the converter is preferably above 400 °C, which is a temperature leading to a conversion rate next to the theoretical rate, but on the other side preventing hot or cold spots disturbing the catalytic reaction in the converter. The first heat exchanger is heated by a heat transfer medium, which comes from a second heat exchanger, which is positioned at any place of the process. This enables a good energy efficiency of the overall process, even in the case that the off-gas containing SO2 is first cooled and then re-heated.

Preferably, the first cooling stage within the gas cleaning section features two zones, wherein the first zone is removing heat using cooling water and the second zone is cooled with another heat exchange medium. Thereby, it is possible to cool the off-gas well below 30 °C, which is not possible with normal cooling water due to the very little temperature difference. This will further reduce energy costs.

In addition, already existing cooling equipment can be used and only the second stage has to be retrofitted. Moreover, a lower amount of the other heat exchange medium is necessary. Also, as the second exchange medium also cooling water can be used which is further cooled by a chiller, which is a particular easy embod iment.

As an alternative, the cooling stage is a chiller which reduces the number of plant components.

A particular embodiment for the cooling stage with two zones is a cooling tower with a first packed bed section in the tower and a second packed bed section on top of the tower. Therein, in the first packed bed section cooling is done indirectly with cooling water, whereby the temperature difference is too small to cool the gas to temperatures significantly below 30 °C. Therefore, in the second packed bed used as a second stage, the remaining heat is also indirectly removed with colder water, which is cooled by a chiller. That embodiment is particularly pre ferred with regard to the ratio of capex and opex.

Moreover, it is favored that a cleaning stage is foreseen, which is positioned up stream of the first heat exchanger in a particularly preferred embodiments. Pref- erably, this cleaning stage comprises at least one electrostatic precipitator and/or a scrubber. This cleaning stage is mainly necessary for the use of metallurgical gases so that these gases can be converted without impurities negatively affect ing the catalyst and converter. Moreover, it is preferred that the plant according to the invention comprises a control unit to adjust the temperature of the cooled gas at the gas cleaning section depending on the sulfur dioxide content in the raw gas. Thereby, it is always pos sible to ensure an adequate cooling temperature depending on the sulfur dioxide content in the off-gas. Therefore, an adequate water moisture removing can be ensured, while an unnecessary cooling to too low temperatures is avoided. As a result, the energy efficiency is further maximized.

In addition, the source of the raw gas can be a roaster or an off-gas combustion of lean hhS gas or sour gas. With regard to the roaster it is typically a sulfatizing roaster or a whole ore roaster. Reactor types are mostly rotary kilns, fluidized bed reactors and hearth furnaces. It is one of the advantages of the inventive plant that it can be used for such a broad range of off-gases independent from the used reactor types. However, the amount of off-gases can be particularly high in a fluidized bed reac tor due to the fluidizing gas, which is why these type of reactor is of special interest for a plant according to the invention.

If a roaster is used as a source of the SO₂ gas, it is possible that the first off-gas line passes at least parts of the off-gas from the roaster into the second heat exchanger to produce heated air and/or steam and/or to heat a heat transfer me dium on mineral oil basis, which is then passed into the first heat exchanger.

So, it is possible to use the high gas temperatures of the roaster indirectly for the first heat exchanger. In cases, wherein heated air is used as a heat transfer me dium, the second heat exchanger is a gas-gas heat exchanger to produce the heated air. In another option, the second heat exchanger is a waste heat boiler, whereby it is possible to produce steam.

In addition, it is possible to use cooling bundles in the roaster in case of exother mic processes therein.

Moreover, it is possible to use only parts of the produced hot air and/or the steam and/or the heat transfer medium to heat the gas in the first heat exchanger. Thereby, an excess of heat not needed in the first heat exchanger can be used on other parts of the plant.

In particular, parts of the produced hot air can be used as a stripping gas in a stripping of produced sulfuric acid from dissolved SO₂ or in a further gas cleaning of the off-gases from the sources, whereby the energy efficiency of the process is further increased. This holds also true for cases, wherein at least parts of the produced air and/or the steam and/or the heat transfer medium are used as an energy source in the chiller. It is also possible to use parts of any of the named heat transfer mediums, partic ularly air, steam and/or a medium on mineral oil basis to recirculate it into the source and to use it in a process, which is preferably endothermic or autothermal.

However, it is the most preferred option, that heat transfer medium from a heat exchanger coupled to the catalytic converter is used as a second heat exchanger. Naturally, heat generated in the converter can also be used in other energy -con suming parts of the plants partly or as the whole.

The invention moreover covers also a process according to the features of claim 12. Naturally, this process can feature all options described with regard to the invented plant.

The process is based on the production of sulfuric acid from a raw gas. That raw gas originates from a source and comprises sulfur dioxide with a concentration above ~3.5 vol.-%, which makes it possible to apply a conventional single cataly sis technology for the production of sulphuric acid. The invention process enables the acid production also with gas concentrations below said ~3.5 vol.-%, particu lar, when the SO2 concentration is below 2, mostly preferably below 1.5 vol.-%, but in every case above 0.5 vol.-%, preferably 1 vol.-%.

The gas is passed to a blower from which it is passed into a converter, wherein the sulfur dioxide is catalytically converted into sulfur trioxide. It is essential that the gas is cooled to a temperature below 30 °C to reduce its water content before passing the drying tower and blower. After passing the blower and before entering the converter, the gas is heated, preferably to a temperature above 400 °C. Alter natively, the heating is performed with an enclosed heat exchanger inside the converter Most preferably, the entry temperature into the converter section is above 400 °C. The heat for the heat exchanger leading off the gas is gained in another part of the process, particular in a heat exchanger coupled. The S0₂ concentration of a gas originating from e.g. a sulfatizing roaster is 1 ,5 vol.- % (like all other given values on a dry basis). Following the cleaning and drying of the gas, the S02-blower exit temperature is around 60 °C. As a result, the exit temperature of the gas passed to the downward absorber is theoretically calculated as 60 + 1 .5 ^* 30.5 = ~105 °C. The resulting temperature of about 105 °C is far too low for sustainable operation. This holds particularly true for the downwards ab sorbing steps wherein SO₃ is absorbed in sulfuric acid to produce sulfuric acid. A typical target gas inlet temperature for an absorber is between 145 and 170 °C.

To achieve a suitable gas temperature of at least 150 °C, the process gas does require additional energy import, e.g. 150-105 = 45 °C (no heat loss considered). So, the gas fed into the catalytic section of the converter has to be heated prior to entering the converter. According to the invention, this can be realized by a sepa rate preheater where the equivalent energy is generated through combustion of fuel, - or by utilizing another source of thermal energy in the system.

Such source can be e.g. the off-gas originating from the roaster. In this example, sulfatizing roasting results in an off-gas stream of typically 550 to 700 °C. That dust laden off-gas is fed to the downstream wet gas cleaning plant quench tower. In stalling a gas-gas heat exchanger (e.g. air heater) in-between, enables the recovery of an adequate amount of energy from this source by producing e.g. in this example hot air of around 500 °C, which in turn is fed to a heat exchanger upstream of the converter. Thus, the energy balance of the sulfuric acid plant can be easily balanced or even over-compensated.

Table 1 presents the relevant temperatures as function of the S0₂-concentration with the aim to keep the gas temperature to the absorber at or above 150 °C. The main focus is on the fact that the conversion of SO₂ and SO₃ leads to an increase in temperature, but this may not be sufficient to reach the inlet temperature required in the absorber.

Table 1 : Gas temperature after conversion and required further heating in correla- tion to different SO2 concentrations

As it is obvious from table 1 , at concentrations significantly above ~3.0 vol.-% S0₂ no external heating is needed. Any concentration above does not require additional energy input, but rather results in higher (above 150 °C) gas temperature to the absorber. E.g. at 3.5 vol.-% S0₂, said gas temperature will be 166 °C, well suited for the purpose.

In this example, parts of the generated hot air can also be used for other purposes at the arrangement, namely:

• Partially recirculation to the roaster feed blower and thus supply addi tional energy input that may be required by the roasting process

• Energy supply to operate the chiller (see below), as an alternative to con ventional electric drive • Stripping air for the effluent stripper at the gas cleaning section. Thus, one can volatilize dissolved HF and/or HCI and feed the off-gas to a separate treatment section

• Stripping air for the product acid SC -stripper at the sulfuric acid plant. Such hot air amount is substantially smaller as compared to ambient air and thus enables the minimization of the size of said stripper

Alternatively, instead of generating hot air at the said air heater, a waste heat boiler may be installed at said location, whereas the generated steam can be used as carrier for the energy to the above applications.

As a further alternative, it is proposed to install a suitable heat exchanger at said location and transfer heat / energy to a heat transfer media on mineral oil basis and thus use this as an energy transport medium for the same applications.

The water balance of the acid plant is equally sensitive. When producing concen trated sulfuric acid of say 96% H₂S0₄, this can tolerate only a certain amount of moisture / water entering the acid plant’s drying tower, e.g. carried by the gas stream from the wet gas cleaning plant. Hence, this gas temperature and its satu ration with moisture has to be carefully controlled to avoid dilution of the desired product acid concentration.

Based on the understanding of the process, it is most preferred to control the cooling temperature such that T (cooling) has a value between T ( cooling ) = 8,7 * c S02) - 0,3 and T ( cooling ) = 8,7 * c S02 ) - 1,5. Thereby, a sufficient cooling without unnecessary effort is ensured.

Additional features, advantages and possible applications of the invention are found in the following description of exemplary embodiments and the drawings. All the features described and/or illustrated graphically form the subject matter of the invention, either alone or in any desired combination, regardless of how they are combined in the claims or in their references back to preceding claims.

It shows

Fig. 1 roaster section according to the invention,

Fig. 2 a wet gas cleaning section according to option 1 of the invention,

Fig. 3 a wet gas cleaning section according to option 2 of the invention,

Fig. 4 sulfuric acid section and

Fig. 5 different embodiments of a converter

Fig. 1 shows a roaster section as one possibility for a source of an off-gas with an SO2 content below 3.5 vol.-% and more than 0.5 vol.-%.

In the shown embodiment, a metal sulfide (MeS_x) is passed via line 11 into a roaster 10, wherein it is thermally treated. In a particular case, the roaster is built as a fluidized-bed reactor, so, fluidizing gas for the roaster is injected via line 18 and blower 19. However, also other reactor types like rotary kiln or hearth fur naces are possible.

The oxidized metal is withdrawn via line 12 as an oxide (MeO) or as a sulfate (MeSC ), while the off-gases are passed via conduit 13. Parts of the off-gases are recirculated via line 15, cyclone 16 and line 17 back into the reactor 10, while the other part is passed into heat exchanger 20. In the heat exchanger 20, the heat transfer medium is injected via line 21 and fan 22 and is withdrawn via line 25 while the cooled off-gases is withdrawn via line 23. That heat exchanger is one possible source of heat for any heat-consuming device in the plant, e.g. also a pre-heating of fluidizing gases.

Fig. 2 shows a first embodiment of a gas cleaning section. The withdrawn off gases are passed from roaster (one example for the roaster is shown in fig. 1 ) via line 25 into a quench 30, which may optionally show two quench stages 30a and 30b. From the bottom of the quench stage, quench medium is withdrawn via line 31 and pump 32. Parts of the quench medium are recirculated into the two quench stages 30a and 30b via lines 33, 34 and 35. Other parts of the quench medium are passed via line 36 into pump tank 90.

The quenched gas is withdrawn via line 37 and 38 into a gas cooling tower 40. Therein, the gas is cooled indirectly with cooling water, which is withdrawn via line 42 and pump 43 and passed into a heat exchanger 44 and is then recirculated via line 45. The cooled gas is passed via line 46 into an electrostatic precipitator (ESP), which is built in this particular embodiment as a first and a second stage 60a and 60b. Naturally, all other number of stages are possible. With line 61 , the gas is passed from the first stage 60a to the second stage 60b.

The further cleaned gas is then passed via line 64 to a further cooling tower 70, where the cooling liquid is withdrawn via line 72 and pump 73. Parts of the cooling medium are recirculated via line 74 and 76 into the chiller 80a and 80b, whereby it is cooled in the chiller heat exchangers 80a and 80b. The other part of the cool ing medium of this cooling tower is withdrawn via line 77 and repassed in the sump of gas cooling tower 40. That chiller is one of the heat-consuming parts of plant. Removed condensate from the electrostatic precipitator stages 60a and 60b is passed in the pump tank 90 via lines 62 and 63 from which it is removed via line 91 and pump 92. The removed liquid from the pump tank is passed into a stripper 50. In this stripper, heated air from the roaster is used so that parts of this gas are recirculated via line 53 such that it can enter gas cooling tower 40. The stripped liquid is passed via line 51 and pump 52 to an effluent treatment.

Fig. 3 is showing another embodiment of a wet gas cleaning section, wherein again the off-gas from the roaster is passed via line 25 into the quench 30 with two quench stages 30a and 30b. As described above for Fig. 2, quench liquid is withdrawn via lines 31 , pump 32 and is recycled via lines 33, 34 and 35, while the other part of the quench water is passed via line 36 into pump tank 90.

The removed quenched gas is passed via lines 37 and 38 into the gas cooling tower, showing two stages 40a and 40b. The removed cooling liquid from the first stage 40a is recirculated via line 42, pump 43, heat exchanger 44 and line 45 back into the first stage 40a.

The cooling liquid used in the second stage positioned above, preferably designed as a packed bed section, is removed via line 46 and passed into a pump tank 47, where it can also be mixed with fresh water from line 48. Liquid removed from that pump tank 47 is passed via line 87 and pump 88 into the heat exchangers 80a and 80b and recycled back via line 86 into the second stage 40b.

A cleaned gas from the second stage of the gas cooling tower is again passed via line 46 into at least one electrostatic precipitator, which is here presented as a two stage ESP with the stage 60a and 60b, being connected via line 61.

Liquid removed in these two stages 60a and 60b is also passed via line 62 and 63 into pump tank 90 and from there passed via line 91 and pump 92 in the stripper 50. From there, it can be passed back to the cooling tower via line 53. Cleaned gas is removed from the second ESP 60b via line 64. Moreover, a line 26 is branched off from the hot gas coming from the heat exchanger 20 of roasting stages and used as a heat transfer medium for the chiller system 80a, 80b.

Fig. 4 finally shows the sulfuric acid section, wherein the gas from the gas clean ing is further prepared for the conversion stage turning SO2 into SO3. Therefore, line 64 leads the SO2 containing gas into a drying tower 120. From there, the gas is removed via line 121 and blower 122 into the first heat exchanger which is divided into heat exchanger 140 and heat exchanger 150.

In the primary first heat exchanger 140, the gas is heated with a heat transfer medium previously used in the heat exchangers of the downward converter 160,

Preferably, the produced gas itself is used a heat transfer medium. In this case the gas is removed from heat exchanger 140 and is injected via line 138 into an SO3 absorber 130. From this SO3 absorber, acid is recirculated via lines 132 and pump 133 into heat exchanger 134 and further recycled via line 136. Gases for this stage are removed via exhaust line 131.

From the first heat exchanger 140, the gas is passed via line 144 into a second heat exchanger 150. In this second heat exchanger, the gas is further heated so it can be transferred via line 152 into a converter, preferably with a temperature above 400 °C, so a theoretically possible conversion of SO2 can be assumed in converter 160. Thereby, a heat transfer medium is used, which is e.g. branched- off from the heat exchanger after the roaster 10.

A control unit 153 is controlling the temperature in said first heat exchanger, pref erably using the S02 concentration. A stored correlation for setting the relating value of the temperature is used to set this value. Control unit 47 is operated in the same way.

The converter 160 is cooled with heat exchangers inside the converter. As de scribed, the used heat transfer medium is used in heat exchanger 150.

Further, it is necessary to withdraw acid from acid cooler 134 via line 137 and pass it into drying tower 120. It is essential to understand that acid withdrawn from the sump of the drying tower and not recirculated via line 125 is passed via line 126 into the stripper 110. There it is stripped such that part of the sump is passed via line 117 into the SO3 absorber 130, while the other part as product acid is withdrawn via line 111 and pump 112 into product heat exchanger 113.

Fig. 5 shows two different designs for converters wherein the internal heat ex changer is a hot inter heat exchanger and another including also a cold inter ex changer. This enable the second position of the first heat exchanger, namely a heating inside the converter instead of a heat exchanger between blower and converter. That second alternative is space saving, but more difficult in retrofitting.

While the above description refers to a single or double catalyst stage, said stages can be increased to 3 or even 4 layers, subject to the required degree of overall conversion of the SO2 to SO3. The Fig.5 refers to these options.

Example 1

One normal cubic meter of gas with 1.5% S0₂ will be transferred into sulfuric acid of 96% H2SO4. The amount of resulting acid can be calculated as

0.015 * 98.08 / 21.89 / 0.96 = 0.070 kg H₂S0₄ of 96%. Therein, the total amount of water amounts to

0.070 * (1 - 80.064 / 98.08 * 0.96) = 0.015 kg of H₂0.

This is the maximum tolerable amount of water moisture added to the plant. Making allowance for some liquid water addition for the purpose of acid concentration con trol at the absorber of about 10 % of the total amount, this reduces the tolerable water moisture of the gas originating from the wet gas cleaning plant to ~0.0136 kg H₂0 / Nm³ dry gas.

Table 2 presents the relevant moisture contents and temperatures as function of the S0₂-concentration for the production of 96 % H₂S0₄.

Table 2: Water content in a gas in correlation to its SO2 concentration and its temperature

*: For S0₂ concentrations above 2,0 vol.-% cooling to the exact temperature is of minor importance. Keeping this gas temperature at or below 18.6°C will in turn avoid installing an acid cooler at the drying tower. Coming back to the example's values given above, the corresponding gas temper ature is 13,8 °C. Hence, the water moisture saturated gas originating from the gas cleaning plant (GCP) must be cooled to or below said 13.8°C.

As a result, the gas entering the wet gas cleaning plant must be cleaned, washed and cooled down to the required temperature by transferring an adequate amount of heat to typically cooling water. Assuming the latter is available at 22 °C, the achievable gas temperature at a conventional gas cooling tower can thus be about 30 °C, it is too high to comply with the above requirements. So, further cooling has to be provided by the utilization of a water chiller with ad equate auxiliary equipment. The conventional gas cooling tower can therefore e.g. be modified for this purpose and equipped with a second packed bed section on top of the tower as shown in Fig.2. Alternatively, the chiller-based cooling tower can be located downstream the wet electrostatic precipitators, as shown in Fig. 3.

The required chiller (conventional, electric driven) capacity for a typical dry gas flow of 50,000 Nm³/h, can be taken from the following Table 3 depending on the SO2 concentration.

Table 3: Energy demand of a chiller for cooling a gas depending on the SO2 con centration

*: For S0₂ concentrations above 2,0 vol.-% cooling to the exact temperature is of minor importance. This will in turn avoid installing an acid cooler at the drying tower. Reference signs

10 roaster

11 to 15 line 16 cyclone

17, 18 line

19 pump

20 heat exchanger

21 to 29 line

30 quench

30a, 30b quench stage

31 line

32 pump

33 to 39 line 40 gas cooling tower

41, 42 line

43 pump

44 heat exchanger

45, 46 line 47 control unit

50 stripper

51 line

52 pump

53 to 55 line 60 electrostatic precipitator

60a, 60b stages of the electrostatic precipitator 61 to 64 line 70 chiller-based cooling tower

71, 72 line 73 pump

74 to 76 line 80a, 80b heat exchanger 81 to 83 line 84 pump

90 pump tank

91 line

92 pump 110 stripper 111 line 112 pump

113 heat exchanger

114 to 117 line 120 drying tower

121 line

122 blower

123 line

124 pump

125, 126 line 130 SO3 absorber

131 , 132 line

133 pump

134 heat exchanger

135 to 138 line

140 heat exchanger

141 line 150 heat exchanger

151 , 152 line 153 control unit 160 converter

Claims

1. A plant for producing sulfuric acid, comprising a source (10), wherein ore and/or a gas is thermally treated to produce amongst others a raw gas containing sulfur dioxide with a concentration below 3.5 vol.-%, a blower (122) for the cleaned gas and a converter (160) with at least one stage filled with a catalyst to convert the contained sulfur dioxide into sulfur triox ide, characterized by at least one cooling stage (40) which is arranged upstream the blower (122) to cool the gas to a temperature below 30 °C to reduce its water content, a at least one first heat exchanger (150) arranged between blower (122) and converter (160) or inside the converter (160) to heat the gas such that it enters the converter (160) at elevated tempera ture, preferably with a temperature above 400 °C, and a second heat ex changer (20), whereby the heat transfer medium of the second heat ex changer (20) is used to heat the first heat exchanger (150).

2. A plant according to claim 1, characterized in that the cooling stage (40) features two zones, wherein the first zone (40a) is cooled with cooling wa ter and the second zone (40b) is cooled with another heat exchange me dium.

3. A plant according to claim 1 or 2, characterized in that the cooling stage (40b) is made by means of a chiller.

4. A plant according to any of the previous claims, characterized in that a cleaning stage is foreseen which comprises preferably at least one wet electrostatic precipitator (60) and/or another cooling tower (70).

5. A plant according to any of the previous claims, characterized in that the plant comprises a control unit (47, 153) to adjust the cooling temperature in the cooling stage (40) and/or the first heat exchanger (150) depending on the sulfur dioxide content in the raw gas.

6. A plant according to any of the previous claims, characterized in that the source (10) of the raw gas is a roaster or an off-gas combustion of lean hhS gas or sour gas.

7. A plant according to claim 6, characterized in that a first off-gas line (26) passes at least parts of the off-gases from the source (10) into a further heat exchanger (20) to produce heated air and/or to produce steam and/or to heat a heat transfer medium on mineral oil basis.

8. A plant according to claim 7, characterized in that parts of the produced hot air and/or the steam and/or the heat transfer medium on mineral oil basis are used to heat the gas in the first heat exchanger (150) to heat the sulfur dioxide containing gas.

9. A plant according to claim 7 or 8, characterized in that at least parts of the produced hot air is used as a stripping gas in a stripper (110) for stripping of produced sulfuric acid from dissolved SO2 and/or is used in a further gas cleaning of the off-gases from the source (10).

10. A plant according to any of claims 7 to 9, characterized in that at least parts of the produced hot air and/or the steam and/or the heat transfer me dium on mineral oil basis are used as an energy source in a chiller.

11. A plant according to any of the previous claims, characterized in that the second heat exchanger (140) cools gas withdrawn from a stage of the con verter (160).

12. A process for producing sulfuric acid from a raw gas originating from a source comprising sulfur dioxide with a concentration below 3,5 vol.-%, whereby the gas is passed to a blower from which it is fed into a converter wherein the sulfur dioxide is catalytically converted into sulfur trioxide, characterized in that the gas is cooled to a temperature below 30 °C to reduce its water content before passing the drying tower (120), that the gas is heated after the blower with a heat transfer medium and before or after entering the converter, preferably to a temperature above 400 °C, at that the heat transfer medium is heated in any stage of the process.

13. The process according to claim 12, characterized in that the temperature T (cooling) to which the cleaned gas is defined between T ( cooling ) = 8,7 * c S02 ) — 0,3 and T ( cooling ) = 8,7 * c S02 ) — 1,5.