A PROCESS FOR CONVERSION OF AQUEOUS HYDROGEN SULFIDE TO SULFURIC ACID

Abstract

The present disclosure relates to a process for purification of an aqueous solution comprising hydrogen sulfide comprising the steps of a. directing an amount of recycle gas to contact the aqueous solution comprising hydrogen sulfide, to separate a gas comprising hydrogen sulfide from the aqueous solution, b. heating said gas comprising hydrogen sulfide optionally after addition of a source of oxygen to provide a process feed gas, c. in a hydrogen sulfide oxidation step directing said process feed gas to oxidation of hydrogen sulfide to sulfur dioxide, d. in a sulfur dioxide oxidation step directing said sulfur dioxide rich gas to contact a material catalytically active in oxidation of sulfur dioxide to sulfur trioxide, to provide a sulfur trioxide rich gas e. in a condensation step cooling said sulfur trioxide rich gas, to enable hydration of sulfur trioxide and condensation of sulfuric acid to provide a stream of concentration sulfuric acid and a purified process gas, and in a recycling step, directing at least a part of the purified process gas as said recycle gas.

Claims

1. A process for purification of an aqueous solution comprising hydrogen sulfide comprising the steps of a. directing an amount of recycle gas to contact the aqueous solution comprising hydrogen sulfide, to separate a gas comprising hydrogen sulfide from the aqueous solution comprising hydrogen sulfide, b. optionally heating said gas comprising hydrogen sulfide optionally after addition of a source of oxygen to provide a process feed gas, c. in a hydrogen sulfide oxidation step directing said process feed gas optionally after addition of a source of oxygen under conditions efficient in oxidation of hydrogen sulfide to sulfur dioxide, to provide a sulfur dioxide rich gas, d. in a sulfur dioxide oxidation step directing said sulfur dioxide rich gas optionally after addition of a source of oxygen to contact a material catalytically active in oxidation of sulfur dioxide to sulfur trioxide under conditions efficient in catalytic oxidation of sulfur dioxide to sulfur trioxide, to provide a sulfur trioxide rich gas e. in a condensation step cooling said sulfur trioxide rich gas by heat exchange with a condenser heat exchange medium, to enable hydration of sulfur trioxide and condensation of sulfuric acid to provide a stream of concentrated sulfuric acid and a purified process gas, and f. in a recycling step, directing at least a part of the purified process gas as said recycle gas.

2. A process according to claim 1 wherein said conditions efficient in oxidation of hydrogen sulfide to sulfur dioxide involve an elevated temperature.

3. A process according to claim 1 wherein said conditions efficient in oxidation of hydrogen sulfide to sulfur dioxide involve contact with a material catalytically active in oxidation of hydrogen sulfide to sulfur dioxide.

4. A process according to claim 1, wherein the amount of hydrogen sulfide in the process feed gas is at least 0.1 vol % or 0.5 vol % and less than 2 vol % or 3 vol %.

5. A process according to claim 1, wherein the amount of dioxygen in the purified process gas is at least 0.1 vol % or 0.5 vol % and less than 3 vol % or 5 vol %.

6. A process according to claim 1, wherein said material catalytic active in oxidation of hydrogen sulfide to sulfur dioxide comprises one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper on a support comprising one or more oxides of metals taken from the group of aluminum, silicon and titanium and a temperature being at least 200 C. or 220 C. and less than 500 C. or 550 C.

7. A process according to claim 1, wherein conditions efficient in catalytic oxidation of sulfur dioxide to sulfur trioxide involve a catalytically active material comprising vanadium pentoxide, sulfur in the form of sulfate, pyrosulfate, tri- or tetrasulfate and alkali metals, on a porous carrier and a temperature being at least 380 C. or 400 C. and less than 700 C. or 650 C.

8. A process according to claim 1, wherein heating said process gas comprising hydrogen sulfide involves one or both of heat exchange in a heat exchanger with a first hot process fluid and addition of a second hot process gas.

9. A process according to claim 1, wherein said first hot process fluid and second hot process fluid may be the same or different and may be taken from the group of a heat exchange medium including said condenser heat exchange medium, said sulfur dioxide rich gas, said sulfur trioxide rich gas and said purified process gas.

10. A process according to claim 1, wherein said process feed gas has a temperature such that the temperature of the sulfur dioxide rich gas is at least 370 C. and less than 420 C.

11. A process according to claim 1, wherein said aqueous solution comprising hydrogen sulfide is provided by microbiological reduction of sulfate.

12. A process according to claim 11, wherein at least an amount of the sulfuric acid produced is directed to be used in an upstream process producing sulfate.

13. A process according to claim 1, wherein at least an amount of the sulfuric acid produced is directed to be used for leaching of metal ore, to provide an aqueous solution comprising metal sulfate.

14. A process according to claim 1, wherein at least an amount of the sulfuric acid produced is directed to be used for hydrolysis of ligneous compounds.

15. A process plant comprising a vessel for contacting a liquid stream and a gas stream, having a liquid stream inlet and outlet and a gaseous stream inlet and outlet, a means for hydrogen sulfide oxidation and a sulfur dioxide reactor containing a material catalytically active in sulfur dioxide oxidation, each having an inlet and an outlet, and a condenser, having a cooling medium inlet and a cooling medium outlet, a gas inlet, a liquid outlet and a gas outlet, wherein the gas outlet of the vessel for contacting a liquid stream and a gas stream is in fluid communication with the inlet of the hydrogen sulfide oxidation reactor, the outlet of the hydrogen sulfide oxidation reactor is in fluid communication with the inlet of the sulfur dioxide oxidation reactor, the outlet of the sulfur dioxide oxidation reactor is in fluid communication with the gas inlet of the condenser and the gas outlet of the condenser is in fluid communication with the gaseous stream inlet of the vessel for contacting a liquid stream and a gas stream.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0063] FIG. 1 shows a process integrating stripping of hydrogen sulfide with a wet gas sulfuric acid plant.

[0064] FIG. 1 shows a process plant where an aqueous solution containing hydrogen sulfide (2) is directed to be contacted by a recycle gas (42) in a gas/liquid contacting device (4), such as a stripping column. The contacting device releases a liquid outlet stream (6), with a reduced amount of sulfide and a H.sub.2S containing gaseous stream (8), which typically would contain 0.5-2% H2S. The H.sub.2S containing gaseous stream (8) is optionally heated and directed as process feed gas to a material catalytically active in oxidation of H.sub.2S (10), which provides an SO.sub.2 rich gas (11). The SO.sub.2 rich gas (11) is combined with an amount of oxygen rich gas (12), such as atmospheric air which may be the heated cooling medium (34) of the condenser (30) and directed to an SO.sub.2 converter (18), where it contacts a first bed of material catalytically active in oxidation of SO.sub.2 to SO.sub.3 (20), which releases heat, to be recuperated in an interbed heat exchanger (22), before being directed to a second bed of material catalytically active in oxidation of SO.sub.2 to SO.sub.3 (24) which may be similar to or different from the first bed of catalytically active material (20). The oxidized process gas out of the second bed of catalytically active material (24) is directed to a further heat exchanger (26) before being directed as oxidized process gas (28) to a condenser (30). The heat recuperated in the two heat exchangers (22 and 26) may beneficially directed to heat the H.sub.2S containing gaseous stream (8) and/or the SO.sub.2 rich stream (16) to enable sufficient temperatures for initiating reaction on the catalysts.

[0065] The condenser (30) receives a stream of cooling medium (32), typically atmospheric air, which is heated in the condenser to form heated cooling medium (34). When this is atmospheric air, it may conveniently be directed as the oxygen rich gas (12), to increase the temperature of the inlet gas to the material catalytically active in SO.sub.2 oxidation (16). In the condenser, SO.sub.3 is hydrated and condensed as sulfuric acid (36) and desulfurized process gas is released (38). The desulfurized process gas (38) is split in purge gas (40) and recycle gas (42).

[0066] In a specific embodiment the aqueous solution comprising sulfide (2) may be a process stream from a metal sulfide precipitation stream. In this case, it may be beneficial to add an amount of CO.sub.2 with the recycle gas (42), to control pH in the aqueous solution comprising sulfide, to provide optimal solutions for the sulfate reducing bacteria.

[0067] In a number of positions heat integration may be beneficial, although not illustrated in the Figure. Thermal energy obtained in the two heat exchangers (22 and 26) and in the heated cooling medium (34) of the condenser may be used to heat up the process gas from the stripper column anywhere between the outlet of the stripper column to the inlet of the SO.sub.2 converter (18). Heating may be required prior to the catalytically active materials active in H.sub.2S oxidation (10) and SO.sub.2 oxidation (20), to increase the temperature above the catalyst ignition point. As the typical materials catalytically active in oxidation of hydrogen sulfide are thermally robust, it may be beneficial to heat the process feed gas (8) to a higher temperature than required, to avoid heating between the material catalytically active in oxidation of hydrogen sulfide (10) and the material catalytically active in oxidation of sulfur dioxide (18). Such heat integration is not shown in detail, but it may be obtained by heat exchange or addition of a hot stream.

[0068] Depending on the specific situation It may also be less costly or necessary to add thermal energy from an extraneous source, such as an electrical or a fired heater.

FIG. 2

[0069] FIG. 2 shows a similar process plant with thermal incineration of H.sub.2S. Here an aqueous solution containing hydrogen sulfide (2) is directed to be contacted by a recycle gas (42) in a gas/liquid contacting device (4), such as a stripping column. The contacting device releases a liquid outlet stream (6), with a reduced amount of sulfide and a H.sub.2S containing gaseous stream (8), which typically would contain 0.5-2% H2S. The H.sub.2S containing gaseous stream (8) is directed as process feed gas (8) to an incinerator (13), receiving an amount of oxygen rich gas (12), such as atmospheric air which may be the heated cooling medium (34) of the condenser (30), and a fuel such as natural gas (14) to provide a SO.sub.2 rich gas (16). The SO.sub.2 rich gas (16) is directed to an SO.sub.2 converter (18), where it contacts a first bed of material catalytically active in oxidation of SO.sub.2 to SO.sub.3 (20), which releases heat, to be recuperated in an interbed heat exchanger (22), before being directed to a second bed of material catalytically active in oxidation of SO.sub.2 to SO.sub.3 (24) which may be similar to or different from the first bed of catalytically active material (20). The oxidized process gas out of the second bed of catalytically active material (24) is directed to a further heat exchanger (26) before being directed as oxidized process gas (28) to a condenser (30). The heat recuperated in the two coolers (22 and 26) is beneficially directed to heat the H.sub.2S containing gaseous stream (8) to reduce the required amount of support fuel, but contrary to FIG. 1, the recuperated heat is not of value in other positions of the process.

[0070] The condenser (30) receives a stream of cooling medium (32), typically atmospheric air, which is heated in the condenser (30) to form heated cooling medium (34). When this is atmospheric air, it may conveniently be directed as the oxygen rich gas (12), to increase the temperature of the inlet gas to the incinerator (13). In the condenser, SO.sub.3 is hydrated and condensed as sulfuric acid (36) and desulfurized process gas is released (38). The desulfurized process gas (38) may be split in purge gas (40) and recycle gas (42).

EXAMPLES

[0071] To illustrate the benefit of the integrated process producing sulfuric acid, possibly for use on site, an example of the process illustrated in FIG. 1 is provided, as well as a process with thermal oxidation of H.sub.2S by combustion of a support fuel in accordance with FIG. 2.

[0072] A specific example is not provided for the current practice of selective microbiological oxidation of H.sub.2S to elemental sulfur. While it may appear beneficial, the microbiological processes available unfortunately generate sulfur of a quality, which without further purification is insufficient for use, e.g. to generate sulfuric acid to be used for ore leaching. This sulfur may be further purified and sold for the purpose of use as e.g. fertilizer, but there is no immediate use of the sulfur on site.

[0073] In the two specific examples the process feed gas corresponds to a gas which could be obtained by stripping H.sub.2S from an aqueous solution such as microbiologically reduced sulfate and adding a minimal viable amount of oxygen for stable operation. The oxygen was added downstream the material catalytically active in H.sub.2S oxidation as atmospheric air, from the cooling side of a sulfuric acid condenser, which generates a process feed gas at a temperature of 200-230 C. which is sufficient for ignition in the material catalytically active in H.sub.2S oxidation.

[0074] The product of catalytic H.sub.2S oxidation can be heated further by heat exchange with the heat exchange medium of the SO.sub.2 converter or the condenser, recuperating heat of the SO.sub.2 oxidation to SO.sub.3. Alternatively, the inlet temperature to the H.sub.2S oxidation catalyst can be chosen, such that the outlet temperature from the H.sub.2S oxidation catalyst fits the inlet temperature of the SO.sub.2 oxidation catalyst, which simplifies the design of the sulfuric acid plant. Such optimal inlet temperature can be obtained by proper heating of the feed gas from the stripper column, optionally combined with a recycle of converted process gas from outlet of H.sub.2S oxidation catalyst to inlet of H.sub.2S oxidation catalyst. The overall process is exothermal, with export of 11 t/h steam at 244 C.

[0075] Commonly for abatement of H.sub.2S, it is oxidized thermally by incineration aided by a support fuel. For the present process this is illustrated in FIG. 2, and an alternative example is provided in Table 2. Here the material catalytically active in H.sub.2S oxidation is replaced by an incinerator, which requires addition of 3.9 t/h of natural gas as support fuel to achieve the required temperature for thermal oxidation of H.sub.2S. In addition, combustion air for the oxidation of the natural gas must be added. As a result, the total volume of process gas (and thus equipment) is 58% higher in the process shown in Table 2 compared to Table 1. The addition of support fuel results in an increased export of steam, which in this example is 86 t/h at 249 C.

[0076] A comparison of the two examples, clearly demonstrates the benefit of the process illustrated in Table 1, in that the equipment size is significantly smaller. In addition, a consumption of 3.9 t/h natural gas as support fuel is avoided. This reduced use of support fuel of course has the consequence that steam export is reduced by 75 t/h. When the process is implemented in a metallurgy process, the produced sulfuric acid will in addition provide the benefit that sulfuric acid is produced locally, without a need for importing.

TABLE-US-00001 TABLE 1 Stream: 8 12 16 28 38 36 CH.sub.4 [% vol] CO.sub.2 [% vol] 10 8.71 8.82 8.99 H.sub.2S [% vol] 1.4 N.sub.2 [% vol] 80.6 78 80.55 81.56 83.14 O.sub.2 [% vol] 3 21 3.58 3.03 3.09 SO.sub.2 [% vol] 0 1.22 480 ppm.sub.v 489 ppm.sub.v SO.sub.3 [% vol] 0 0 0.52 0 ppm.sub.v H.sub.2SO.sub.4 [% vol] 0 0 0.66 10 ppm.sub.v 97 wt % H.sub.2O [% vol] 5 1 5.83 5.24 4.6 3 wt % T [ C.] 35 213 400 265 100 40 Flow [T/h] 131 20 151 151 145 6.1

TABLE-US-00002 TABLE 2 Stream: 8 12 14 16 28 38 36 CH.sub.4 [% vol] 100 CO.sub.2 [% vol] 10 8.39 8.46 8.56 H.sub.2S [% vol] 1.4 N.sub.2 [% vol] 80.6 78 76.39 77.08 78 O.sub.2 [% vol] 3 21 3.38 3.04 3.08 SO.sub.2 [% vol] 0.75 231 ppm.sub.v 234 ppm.sub.v SO.sub.3 [% vol] 229 ppm.sub.v 0.22 0 ppm.sub.v H.sub.2SO.sub.4 [% vol] 8.5 ppm.sub.v 0.54 10 ppm.sub.v 97 wt % H.sub.2O [% vol] 5 1 10.68 10.24 9.93 3 wt % T [ C.] 35 220 30 400 265 100 40 Flow [T/h] 131 98.5 3.9 233 233 227 6.1