Process for converting aqueous hydrogen sulfide to sulfuric acid

Abstract

The present disclosure includes the following steps: a. directing a quantity of recycle gas into contact with an aqueous solution containing hydrogen sulfide and separating the hydrogen sulfide-containing gas from the aqueous solution; b. heating the gas, optionally including hydrogen sulfide, after addition of the oxygen source to provide a process feed gas; c. Inducing the oxidation of hydrogen sulfide to sulfur dioxide in a hydrogen sulfide oxidation step; d. directing the sulfur dioxide rich gas into contact with a material catalytically active in the oxidation of sulfur dioxide to sulfur trioxide in a sulfur dioxide oxidation step to provide a sulfur trioxide rich gas; e. cooling the sulfur trioxide rich gas in a condensing step to allow for hydration of sulfur trioxide and condensation of sulfuric acid to provide a concentrated sulfuric acid stream and a purified process gas; The present invention relates to a method for purifying an aqueous solution containing hydrogen sulfide, comprising: In a recycling step, at least a portion of the purified process gas is directed as a recycled gas.

Description

[Technical field]

[0001] The present invention relates to the field of purification of wastewaters containing sulfides, and in particular to the production of concentrated sulfuric acid from aqueous solutions contaminated with hydrogen sulfide. [Background technology]

[0002] The mining and metallurgical industries aim to maximize the yield of metals. This involves leaching metals from their ores by dissolving them in sulfuric acid to provide aqueous solutions of metal sulfates.

[0003] Precipitation of metal sulfides from metalworking waters and microbial systems is performed in combination with the recovery of excess sulfur by a further microbiological step of reducing sulfates to form sulfides and oxidizing the excess sulfide to sulfur. Although this serves the purpose of removing sulfur from an aqueous solution, the recovered sulfur is not suitable for immediate use because it contains a lot of metal impurities.

[0004] Pure elemental sulfur can be converted to sulfuric acid by the combustion of sulfur to sulfur dioxide, by the catalytic oxidation of sulfur dioxide to sulfur trioxide, or by the hydration of sulfur trioxide and condensation with sulfuric acid or by absorption of sulfur trioxide into concentrated sulfuric acid to provide concentrated sulfuric acid. Summary of the Invention [Problem to be solved by the invention]

[0005] Contaminated sulfur emissions from wastewater generation processes produce large amounts of particulate matter during sulfur combustion and can cause catalyst blockage and shorten catalyst life in catalytic processes.

[0006] Similar related technologies for the conversion of sulfates or sulfides to sulfuric acid are found in pulp and paper industry processes, pharmaceutical waste streams, and sulfuric acid hydrolysis of lignocellulose to form fermentable compounds, and the present method can also be used to treat wastewater from such processes. [Means for solving the problem]

[0007] Summary of the Invention This disclosure provides a more efficient method of recovering sulfides to provide sulfuric acid for use in leaching processes or for commercial sale, and hydrogen sulfide can also be recovered from aqueous solutions originating from other processes.

[0008] In the present process, there is no need to microbiologically convert hydrogen sulfide to elemental sulfur, as it is instead catalytically oxidized to sulfur dioxide and further processed to provide concentrated sulfuric acid in a wet gas sulfuric acid (WSA) plant.

[0009] The process outlined is suitable for the conversion of aqueous hydrogen sulfide to sulfuric acid in a wide range of feedstocks, although further synergies can be obtained through the use of sulfuric acid for ore leaching and / or the use of purified process gas as a stripping medium to drive off sulfides from the microbial reaction.

[0010] definition In this application, the unit "mass %" means mass / mass %, the unit "volume %" means volume / volume %, and the unit "ppmv" means parts per million by volume.

[0011] For the purposes of this application, when concentrations in the gas phase are given, they are given as volume / volume (i.e., molar) concentrations, unless otherwise specified.

[0012] For the purposes of this application, when concentrations in the liquid or solid phase are given, they are given as mass / mass concentrations, unless otherwise stated.

[0013] A broad aspect of the present disclosure discloses an invention relating to a method for purifying an aqueous solution containing hydrogen sulfide, the method comprising the steps of: directing a quantity of the recycle gas into contact with an aqueous solution comprising hydrogen sulfide and separating the hydrogen sulfide-containing gas from the aqueous solution comprising hydrogen sulfide; heating the gas comprising hydrogen sulfide, optionally after addition of an oxygen source, to provide a process feed gas; directing said process feed gas, optionally after the addition of a source of oxygen, under conditions effective for the oxidation of hydrogen sulfide to sulfur dioxide in a hydrogen sulfide oxidation step to provide a sulfur dioxide rich gas; directing the sulfur dioxide rich gas, optionally after adding a source of oxygen, to contact a material catalytically active in the oxidation of sulfur dioxide to sulfur trioxide under conditions effective for the catalytic oxidation of sulfur dioxide to sulfur trioxide to provide a sulfur trioxide rich gas; in a condensation step, cooling the sulfur trioxide rich gas (28) by heat exchange with a condenser heat exchange medium, e.g., process gas or air, to allow hydration of sulfur trioxide and condensation of sulfuric acid to provide a concentrated sulfuric acid stream and a purified process gas; and Directing at least a portion of the purified process gas as a recycle gas in a recycle step.

[0014] This has the associated advantage that even if the hydrogen sulfide is in low concentration, it is effective in transferring some amount of hydrogen sulfide into the gas phase and converting it to sulfur dioxide and thus sulfuric acid.

[0015] In a second embodiment, the conditions effective for the oxidation of hydrogen sulfide to sulfur dioxide include elevated temperatures obtained, for example, by combustion, optionally in combination with heat exchange.

[0016] Such a process has the associated advantage of allowing the conversion of hydrogen sulfide to sulfur dioxide in an energy efficient manner at low temperatures.

[0017] In a third embodiment, the conditions effective for the oxidation of hydrogen sulfide to sulfur dioxide include contact with a material catalytically active for the oxidation of hydrogen sulfide to sulfur dioxide.

[0018] Such a process has the advantage of being able to convert hydrogen sulfide to sulfur dioxide in an energy efficient manner at low temperatures.

[0019] In a fourth embodiment, the amount of hydrogen sulfide in the process feed gas is at least 0.1% by volume or at least 0.5% by volume and less than 2% by volume or less than 3% by volume.

[0020] This process also has the associated advantage of being able to receive low concentrations of hydrogen sulfide and convert it to sulfuric acid.

[0021] In a fifth embodiment, the amount of dioxygen in the purified process gas is at least 0.1% by volume or at least 0.5% by volume and less than 3% by volume or less than 5% by volume.

[0022] This has the associated advantage of ensuring at least 0.5%, 1% or even 3% by volume of oxygen in the process gas in contact with said material catalytically active for the oxidation of sulphur dioxide, whilst such suitable concentrations of oxygen do not significantly interfere with the sulphate reducing microorganisms.

[0023] In a sixth embodiment, the catalytically active material in the oxidation of hydrogen sulfide to sulfur dioxide comprises an oxide of one or more metals selected from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper on a support comprising an oxide of one or more metals selected from the group consisting of aluminium, silicon and titanium, and the temperature is at least 200°C or 220°C and less than 500°C or less than 550°C.

[0024] This has the added advantage that a material capable of oxidizing hydrogen sulfide at a suitable temperature can be provided at a reasonable cost.

[0025] In a seventh embodiment, conditions effective for the catalytic oxidation of sulfur dioxide to sulfur trioxide include a catalytically active material comprising vanadium pentoxide (VO), sulfur in the form of a sulfate, pyrosulfate, trisulfate or tetrasulfate, and an alkali metal, e.g., Li, Na, K, Rb or Cs, on a porous support, and a temperature of at least 380°C or at least 400°C and less than 700°C or less than 650°C.

[0026] This has the advantage of providing a cost-effective and stable material that allows for the oxidation of sulfur dioxide at suitable temperatures.

[0027] In an eighth embodiment, heating the process gas containing hydrogen sulfide includes one or both of: (a) heat exchange with a first hot process fluid in a heat exchanger; and (b) the addition of a second hot process gas.

[0028] This has the added benefit of allowing the process energy to operate above the required catalyst light-off temperature.

[0029] In a ninth embodiment, the first hot process fluid and the second hot process fluid may be the same or different and may be selected from the group of a heat exchange medium including a condenser heat exchange medium, a sulfur dioxide rich gas, a sulfur trioxide rich gas, and a purified process gas.

[0030] This has the associated advantage that these streams can recover heat released from exothermic processes.

[0031] In a tenth embodiment, the 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.

[0032] This has the associated advantage that heating of the process gas between the material catalytically active for hydrogen sulphide oxidation and the material catalytically active for sulphur dioxide oxidation can be avoided, simplifying the process layout and improving the energy efficiency of plant operation, made possible by the range of applications of materials catalytically active for hydrogen sulphide oxidation.

[0033] In an eleventh embodiment, the aqueous solution containing hydrogen sulfide is provided by microbial reduction of sulfate.

[0034] This has the associated advantage of providing a source of sulfuric acid while at the same time providing a solution to the environmental problems associated with sulfuric acid.

[0035] In a twelfth embodiment, at least a portion of the produced sulfuric acid is used to dissolve a metal ore to provide an aqueous solution containing a metal sulfate.

[0036] This has the associated benefit of providing a solution to the metallurgical industry while also providing a source of sulphuric acid for industry processes.

[0037] A further aspect of the disclosure is a vessel for contacting a liquid stream with a gas stream, the vessel having a liquid inlet and outlet and a gas inlet and outlet; a hydrogen sulfide oxidation means and a sulfur dioxide reactor comprising a material catalytically active for sulfur dioxide oxidation, each having an inlet and an outlet; 1. A process plant comprising a condenser having a cooling medium inlet and a cooling medium outlet, a gas inlet, a liquid outlet and a gas outlet, wherein a gas outlet of the vessel for contacting the liquid and gas streams is in fluid communication with an inlet of a hydrogen sulfide oxidation reactor, an outlet of the hydrogen sulfide oxidation reactor is in fluid communication with an inlet of a sulfur dioxide oxidation reactor, an outlet of the sulfur dioxide oxidation reactor is in fluid communication with a gas inlet of a condenser, and a gas outlet of the condenser is in fluid communication with a gas inlet of the vessel for contacting the liquid and gas streams.

[0038] This has the associated advantage of being an efficient plant for converting aqueous sulfide streams to sulfuric acid. The means for hydrogen sulfide oxidation is either a thermal means such as an incinerator, optionally with an additional inlet for fuel, or a reactor containing a material catalytically active for hydrogen sulfide oxidation.

[0039] technical challenges Because aqueous solutions containing sulfates (and sulfides) are environmentally undesirable and regulated, it is desirable to recover the sulfates / sulfides and convert them into more attractive sulfur compounds such as sulfuric acid.

[0040] This includes aqueous wastes and intermediate streams from metallurgical processes, including mining, which may contain valuable metals and environmentally undesirable sulfur compounds, such as sulfates. Cost-effective and environmentally friendly methods for removing such metals are desirable.

[0041] In the extraction of metals from the mining process, a common method is the leaching of ores with sulfuric acid, which releases metal sulfates into a water stream. The metals in this stream can then be precipitated and reduced to obtain the pure metal. Such methods are used for a wide range of metals, especially Ni, Cu, U and rare earth metals.

[0042] Sulfur in aqueous wastes and intermediate fluids resulting from such metallurgical processes, including mining, is environmentally undesirable, and therefore cost-effective and environmentally friendly methods for removing sulfur are desirable.

[0043] Similar related technologies for converting sulfate or sulfide to sulfuric acid are found in processes in the paper and pulp industry, pharmaceutical waste streams, and waste streams from the sulfuric acid hydrolysis of lignocellulose to form fermentable compounds, and the present method can be applied to the treatment of wastewater from such processes.

[0044] A common technique in this regard is microbial sulfide production, in which sulfates are reduced to sulfides by cultures of sulfate-reducing microorganisms and a metal sulfide sludge is precipitated from a solution containing excess sulfide. Because sulfides (including H2S) are environmentally undesirable, a common method involves inducing sulfide-containing wastewater to be oxidized to elemental sulfur by sulfide-oxidizing microorganisms, and recovering the sulfur. This microbiologically produced sulfur contains metals and is therefore not directly suitable for further processing unless purified.

[0045] The present disclosure relates to an alternative to this process in which aqueous formed hydrogen sulfide is introduced into the gas phase, and further relates to a wet gas sulfuric acid process plant in which a process gas containing hydrogen sulfide is catalytically oxidized to form a process gas containing sulfur dioxide. The process gas containing sulfur dioxide can then be introduced into contact with a material catalytically active for sulfur dioxide oxidation to sulfur trioxide, which is hydrated to form sulfuric acid and can be condensed in an air-cooled condenser from which liquid concentrated sulfuric acid and purified process gas are extracted.

[0046] The step of directing H2S into the gas phase can be carried out with the aid of a stripping medium, advantageously in a stripper column that ensures good contact between gas and liquid. The reduction of sulfates to sulfides requires reducing conditions, so the absence or low presence of oxygen is desirable.

[0047] Optimization of the integrated process involves limiting the amount of oxygen in the stripping medium (recycle gas). However, the sulfuric acid process requires a constant excess of O2 to maintain catalytic oxidation, and a good compromise between the need for anaerobic conditions for the bacteria and oxidizing conditions for the sulfuric acid process is in the range of 1-5 vol% O2 in the off-gas from the sulfuric acid process.

[0048] Because sulfate-reducing microorganisms have a limited range of optimum operating conditions, including a limited pH range, an amount of CO2 can be added to the substrate to control the pH. If this amount of CO2 is added along with an oxygen source, it can be recycled and CO2 consumption is also limited.

[0049] The CO2 and O2 concentrations of the recycled purified process gas can be modified, if necessary, to more optimal values, which can be achieved, for example, by adding to the purified gas stream another gas stream characterized by having a higher CO2 concentration and a lower O2 concentration than the purified gas stream.

[0050] The O2 concentration in the purified process gas can also be reduced by adding a reducing agent to the purified process gas and reacting the O2 with the reducing agent, optionally with a suitable catalyst or microorganism, such as H2, which reacts with O2 to produce water, or methanol, which reacts with CO2 to produce water.

[0051] The catalytic oxidation of hydrogen sulfide to sulfur dioxide has typically been carried out by combustion, with hydrogen sulfide as the sole fuel or with the addition of a supporting fuel to ensure the necessary combustion temperatures of 700° C. or higher. However, when high concentrations of hydrogen sulfide, for example greater than 25% by volume, are not available, the process may instead be beneficially directed to contacting the process feed gas with a catalytically active material comprising an oxide of one or more active metals selected from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper on a refractive support comprising an oxide of one or more metals selected from the group consisting of aluminum, silicon and titanium. Examples of materials include 1% by weight, 2% by weight, or 3% by weight, 4% by weight, 5% by weight, 10% by weight, 25% by weight, or 50% by weight of V2O5, a stabilizing component, preferably 2% by weight, or 3% by weight, 5% by weight, 10% by weight, or 50% by weight of WO3, and one or more supports selected from the group consisting of Al2O3, SiO2, SiC, and TiO2, and may further include 1% to 5% by weight of a metal selected from the group consisting of chromium, molybdenum, cerium, niobium, manganese, and copper. Optionally, when the porous support comprises TiO2, this is preferably in the form of anatase, which has the associated advantage that TiO2, and especially anatase, is highly porous and therefore active as a catalyst support.

[0052] The catalytically active material in the oxidation of hydrogen sulfide may be in the form of pellets or extrudates in the reaction bed, or in the form of a monolith catalyst, preferably comprising a structural substrate and a catalyst layer made from one of metal, high silicon glass fiber, glass paper, cordierite and silicon carbide, with the associated advantage of providing a stable and well-defined physical shape. The monolith catalyst may have a void volume of 65% or 70% to 70% or 85% and has the associated advantage of a good balance between the amount of catalytic material and an open monolith with low pressure drop. The catalyst layer of the monolith catalyst may have a thickness of 10 to 150 μm and has the associated advantage of providing a catalytically active material with a high pore volume.

[0053] Depending on the type of material, the light-off temperature of the oxidation reaction is from 200°C or 220°C to 300°C or 320°C higher than the temperature of the hydrogen sulfide feed gas, so that heating of this gas may be required before contacting it with a material catalytically active for the oxidation of hydrogen sulfide. Furthermore, since it is desirable to introduce only a small amount of oxygen into the aqueous sulfide solution, it may be desirable to add an oxygen source, for example, atmospheric air or oxygen-enriched air. If the oxygen source is provided at a sufficiently high temperature, heating and addition of the oxygen source can be done by a single means. Since the oxidation reaction of H2S is exothermic, the temperature of the process gas containing a large amount of sulfur dioxide will increase if operated adiabatically, as is commonly done.

[0054] Ideally, the outlet temperature from the H2S oxidation step corresponds to the optimal inlet temperature to the SO2 oxidation step, so that no temperature adjustment is required between the two oxidation steps, easing process design.

[0055] If the temperature or heating value of the feed to the H2S oxidation step is insufficient, the optimum inlet temperature to the SO2 oxidation step can be established by increasing the inlet temperature to the H2S oxidation step or the heating value of the feed to the H2S oxidation step, for example, if the temperature rise in the H2S oxidation step is 50°C, the inlet temperature to the H2S oxidation step will be 350°C, providing 400°C at the inlet to the SO2 oxidation step.

[0056] For feed gases with higher heating values, it may not be possible to limit the outlet temperature from the H2S oxidation step by adjusting the inlet temperature to provide an adequate inlet temperature to the SO2 oxidation step. If the temperature rise in the H2S oxidation step is, for example, 250 °C, the minimum inlet temperature to the SO2 oxidation step will be 450-470 °C, i.e. higher than the optimum inlet temperature. This temperature can be lowered by installing a simple heat exchanger between the two oxidation steps or, more energy efficient, by adding a recycle loop around the H2S oxidation step, allowing the inlet and outlet temperatures of the H2S oxidation step to be easily controlled.

[0057] The amount of O2 required for the oxidation of H2S and the subsequent oxidation of SO2 must be added to the process. Depending on availability and temperature, it may be beneficial to add it either upstream or downstream of the material catalytically active for the oxidation of H2S. In the step of catalytically oxidizing sulfur dioxide to sulfur trioxide in the so-called SO2 converter, catalytically active vanadium sulfate melt materials consisting of vanadium pentoxide (VO5), sulfur in the form of sulfates, pyrosulfates, trisulfates, tetrasulfates, and alkali metals such as Li, Na, K, Rb, Cs, on a porous support such as silica or alumina, may be advantageously included. The porous support of the material catalytically active for the oxidation of sulfur dioxide is silica, for example, diatomaceous earth with less than 2% by weight, preferably less than 1% by weight of alumina. The alkali metal content is at least 2%, at least 4% by weight or at least 8% by weight and is less than 16%, less than 20% by weight or less than 24% by weight. The V2O5 content is at least 1%, at least 2%, or at least 4% by weight and less than 10%, 12%, or 15% by weight. The sulfur content is at least 1%, at least 2%, or at least 3% by weight and less than 10%, 18%, or 20% by weight in the form of sulfate, pyrosulfate, trisulfate, or tetrasulfate.

[0058] The light-off temperature of this exothermic oxidation process is usually around 370°C to 400°C, with the maximum temperature usually being 650°C or 700°C. However, since at higher temperatures the equilibrium shifts towards SO2, multiple beds with interbedded cooling are often implemented, but since the concentration of SO2 is very low in the present invention, one or two beds are generally the most efficient. Oxygen is consumed in this step, and the light-off temperature at which the reaction activates on the catalytically active material may not be met here either. Typically, sufficient oxygen is added upstream of the catalytically active material in the oxidation of hydrogen sulfide, but it may be beneficial to heat the sulfur dioxide-rich gas. Due to the required temperatures of 370°C to over 400°C, the heat source may be beneficial from the heat exchange medium used between the process beds or at the outlet of the SO2 converter.

[0059] The final conversion step of the process is the condensation of the hydrated SO3 as sulfuric acid. As it is the oxidation of hydrogen sulfide removed from the aqueous solution, the process gas stream contains a large amount of water and therefore the SO3 is hydrated to form H2SO4, which can be condensed as concentrated sulfuric acid in the condenser, provided that an adequate amount of nucleation seeds and cooling are provided, as known in the wet gas sulfuric acid process. The cooling medium is typically atmospheric air, which is heated in the condenser from ambient temperature to about 180-270 °C, while the process gas is typically cooled from 290 °C to 100 °C. The heated cooling medium is suitable to provide at least part of the thermal energy and / or oxygen required for the combustion and oxidation of hydrogen sulfide.

[0060] The process gas outlet from the condenser contains small amounts of unconverted sulfur dioxide and unused oxygen, as well as nitrogen and other inert compounds. A certain amount of this purified gas can be recycled for use as a stripping medium to release gaseous hydrogen sulfide from aqueous solutions containing hydrogen sulfide. To avoid excessive accumulation of inert gases in the gas loop, it is necessary to withdraw a certain amount of gas from the process as a purge stream.

[0061] The concentrated sulfuric acid produced in the wet gas sulfuric acid process plant can be used in the upstream leaching step.

[0062] The refinery gas from the wet gas sulfuric acid process contains O2, H2O, N2, SO2, SO3, and, if CO2 is added or supporting fuels are burned, CO2. Recycling this refinery gas can significantly affect the amount of inert components in the tail gas and can also affect the amount of sulfur etc. released into the environment, since in addition to the sulfur recovered as sulfuric acid, some sulfur is recovered during recycling. [Brief description of the drawings]

[0063] Figure 1 shows the integrated hydrogen sulfide stripping and wet gas sulfuric acid plant process. Figure 2 shows a similar process plant with thermal incineration of hydrogen sulfide. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] Figure 1 FIG. 1 shows a process plant in which an aqueous solution containing hydrogen sulfide (2) is directed to contact with a recycle gas (42) in a gas / liquid contactor (4), such as a stripping tower. The contactor releases a liquid outlet stream (6) having a reduced amount of sulfide and an H2S-containing gas stream (8), which typically contains 0.5-2% H2S. The H2S-containing gas stream (8) is optionally heated and directed as process feed gas to a material (10) catalytically active for the oxidation of H2S to provide a SO2-rich gas (11). The SO2-rich gas (11) is combined with a quantity of oxygen-rich gas (12), e.g. atmospheric air, which may be the heated cooling medium (34) of the condenser (30), and directed to the SO2 converter (18), where it contacts a first bed of catalytically active material (20) in the oxidation of SO2 to SO3 releasing heat, is recovered in an interlayer heat exchanger (22), and is then directed to a second bed of catalytically active material (24) for the oxidation of SO2 to SO3, which may be the same or different from the first bed of catalytically active material (20) for the oxidation. The oxidized process gas leaving the second bed of catalytically active material (24) is directed to a further heat exchanger (26) before being directed to the condenser (30) as oxidized process gas (28). The heat recuperated in the two heat exchangers (22 and 26) is useful for heating the H2S containing gas stream (8) and / or the SO2 rich stream (16) to a temperature sufficient to initiate a reaction on the catalyst.

[0065] The condenser (30) receives a flow of cooling medium (32), typically atmospheric air, which is heated in the condenser to form a heating cooling medium (34). If this is atmospheric air, it can be suitably directed as oxygen-rich gas (12) to raise the temperature of the inlet gas to the catalytically active material in SO2 oxidation (16). In the condenser, SO3 is hydrated and condensed as sulfuric acid (36), and a desulfurization process gas is released (38). The desulfurization process gas (38) is split into a purge gas (40) and a recycle gas (42).

[0066] In a specific embodiment, the aqueous solution containing sulfides (2) may be a process stream from a metal sulfide precipitation stream, in which case it may be beneficial to add CO2 with the recycle gas (42) to control the pH of the aqueous solution containing sulfides and provide an optimal solution for sulfate-reducing bacteria.

[0067] Although not shown in the figure, heat integration may be beneficial in many locations. The thermal energy available in the two heat exchangers (22 and 26) and the condenser heating and cooling medium (34) can be used to heat the process gas from the stripper column anywhere between the outlet of the stripper column and the inlet of the SO2 converter (18). Heating may be required to bring the temperature above the catalytic ignition point before the materials catalytically active in the H2S oxidation (10) and SO2 oxidation (20). Since typical materials catalytically active in the oxidation of hydrogen sulfide are thermally robust, it may be beneficial to heat the process feed gas (8) to a higher temperature than necessary to avoid heating between the material catalytically active in the oxidation of hydrogen sulfide (10) and the material catalytically active in the oxidation of sulfur dioxide (18). Such heat integration may be obtained by heat exchange or the addition of hot streams, not shown in detail.

[0068] Also, in certain circumstances, it may be less costly or necessary to add heat energy from an additional heat source, such as an electric heater or furnace.

[0069] Figure 2 Figure 2 shows a similar process plant with thermal incineration of hydrogen sulfide. Here, an aqueous solution (2) containing hydrogen sulfide is directed to contact with a recycle gas (42) in a gas / liquid contactor (4), such as a stripping tower. The contactor releases a liquid outlet stream (6) with a reduced amount of sulfides and an H2S-containing gas stream (8), which typically contains 0.5-2% H2S. The H2S-containing gas stream (8) is directed as process feed gas (8) to an incinerator (13), which receives a quantity of oxygen-rich gas (12), e.g. atmospheric air, which may be the heated cooling medium (34) of the condenser (30), and a fuel, e.g. natural gas (14), to provide a SO2-rich gas (16). The SO2-rich gas (16) is directed to the SO2 converter (18) where it contacts a first bed (20) of material catalytically active for the oxidation of SO2 to SO3, releases heat, is recuperated in an interbed heat exchanger (22), and is then directed to a second bed (24) of material catalytically active for the oxidation of SO2 to SO3, which may be the same as or different from the first bed of material catalytically active for the oxidation (20). The oxidized process gas leaving the second bed (24) of catalytically active material is directed to a further heat exchanger (26) before being directed to a condenser (30) as the oxidized process gas (28). The heat recuperated in the two coolers (22 and 26) is useful for heating the H2S-containing gas stream (8) to reduce the amount of support fuel required, but contrary to FIG. 1, the recuperated heat is of no value elsewhere in the process.

[0070] The condenser (30) receives a flow of cooling medium (32), typically atmospheric air, which is heated in the condenser (30) to form a heated cooling medium (34). If this is atmospheric air, it may be suitably introduced as oxygen-rich gas (12) to increase the temperature of the inlet gas to the incinerator (13). In the condenser, the SO3 is hydrated and condensed as sulfuric acid (36), and a desulfurized process gas (38) is released. The desulfurized process gas (38) is split into a purge gas (40) and a recycle gas (42). EXAMPLES

[0071] example To illustrate the advantages of an integrated process for producing sulfuric acid, an example of the method is shown in Figure 1 and the thermal oxidation process of H2S by combustion of a supporting fuel is shown in Figure 2.

[0072] No examples are given of current practices for the selective microbial oxidation of H2S to elemental sulfur. While seemingly beneficial, the available microbial methods unfortunately produce sulfur of insufficient quality for use, such as sulfuric acid for ore leaching, without further purification. This sulfur may be further purified and sold for use as fertilizer, etc., but is not immediately utilized on-site.

[0073] In two specific examples, the process feed gas corresponds to a gas obtained by stripping H2S from an aqueous solution, such as microbiologically reduced sulfate, and adding the minimum feasible amount of oxygen for stable operation. Oxygen was added downstream of the material catalytically active for H2S oxidation as atmospheric air from the cooled side of the sulfuric acid condenser, producing a process feed gas at a temperature of 200-230 °C sufficient for the combustion of the material catalytically active for H2S oxidation.

[0074] The product of catalytic H2S oxidation can be further heated by heat exchange with the heat exchange medium of the SO2 converter or condenser, transferring the heat of SO2 oxidation back to SO3. Alternatively, the inlet temperature to the H2S oxidation catalyst can be selected so that the outlet temperature from the H2S oxidation catalyst matches the inlet temperature of the SO2 oxidation catalyst, which simplifies the design of the sulfuric acid plant. Such an optimal inlet temperature can be obtained by appropriate heating of the feed gas from the stripper column, optionally combined with a recycle of the conversion process gas from the outlet of the H2S oxidation catalyst to the inlet of the H2S oxidation catalyst. The whole process is exothermic, with an exhaust of 11 tonnes / h of steam at 244 °C.

[0075] Typically, H2S emissions are thermally oxidized by incineration with a supporting fuel. In this process, this is shown in Figure 2, and alternative examples are given in Table 2. Substitute materials that are catalytically active in H2S oxidation for the incinerator. 3.9 t / h of natural gas must be added as support fuel to achieve the temperature required for thermal oxidation of H2S. In addition, combustion air must be added to oxidize the natural gas. As a result, the total amount of process gas (and therefore equipment) is 58% higher in the process shown in Table 2 compared to Table 1. The addition of support fuel increases the steam emissions, which in this example is 86 t / h at 249°C.

[0076] Comparing the two examples, the clear advantage of the method shown in Table 1 is the significantly smaller equipment size. In addition, the consumption of 3.9 tonnes / h of natural gas as support fuel is avoided. Such a reduction in the use of support fuel naturally results in a reduction in steam emissions of 75 tonnes / h. If this method is implemented in a metallurgical production process, it also has the advantage that the sulfuric acid produced is produced locally without the need for imports.

[0077] [Table 1]

[0078] [Table 2]

Claims

1. Steps below a. directing a quantity of recycle gas (42) into contact with an aqueous solution containing hydrogen sulfide (2) and separating a gas containing hydrogen sulfide (8) from the aqueous solution containing hydrogen sulfide (2); b. heating the gas (8) optionally containing hydrogen sulfide, optionally after addition of a source of oxygen (12), to provide a process feed gas; c. inducing said process feed gas under conditions effective for the oxidation of hydrogen sulfide to sulfur dioxide, optionally after the addition of a source of oxygen, in a hydrogen sulfide oxidation step to provide a sulfur dioxide rich gas (11, 16); d. in a sulfur dioxide oxidation step, optionally after the addition of an oxygen source (12), directing said sulfur dioxide rich gas (11, 16) to contact a material (20, 24) catalytically active in the oxidation of sulfur dioxide to sulfur trioxide under conditions effective for the catalytic oxidation of sulfur dioxide to sulfur trioxide to provide a sulfur trioxide rich gas (28); e. In a condensation step, cooling said sulfur trioxide rich gas (28) by heat exchange with a condenser heat exchange medium, e.g., process gas or air, to allow hydration of sulfur trioxide and condensation of sulfuric acid to provide a stream of concentrated sulfuric acid (36) and a purified process gas (38); f. directing at least a portion of the purified process gas (38) as a recycle gas (42) in a recycle step; A method for purifying an aqueous solution (2) containing hydrogen sulfide, comprising:

2. 2. The method of claim 1, wherein the conditions effective for the oxidation of hydrogen sulfide to sulfur dioxide include elevated temperatures obtained, for example, by combustion, optionally in combination with heat exchange.

3. 2. The method of claim 1, wherein the conditions effective for the oxidation of hydrogen sulfide to sulfur dioxide include contact with a material (10) that is catalytically active for the oxidation of hydrogen sulfide to sulfur dioxide.

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

5. 5. The method of claim 1, 2, 4 or 4, wherein the amount of dioxygen in the purified process gas (38) is at least 0.1 vol.% or at least 0.5 vol.% and less than 3 vol.% or less than 5 vol.%.

6. 6. A process according to claim 1, 3, 4 or 5, wherein the material catalytically active in the oxidation of hydrogen sulphide to sulphur dioxide comprises an oxide of one or more metals selected from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper on a support comprising an oxide of one or more metals selected from the group consisting of aluminium, silicon and titanium, and the temperature is at least 200°C or at least 220°C and less than 500°C or less than 550°C.

7. Conditions effective for the catalytic oxidation of sulfur dioxide to sulfur trioxide are found to be ２ O ５ 7. The method of claim 1, 3, 4, 5 or 6, comprising a catalytically active material comprising sulfur in the form of sulfate, pyrosulfate, trisulfate or tetrasulfate, and an alkali metal, e.g., Li, Na, K, Rb or Cs, and a temperature of at least 380° C. or at least 400° C. and less than 700° C. or less than 650° C.

8. 8. The method of claim 1, 2, 3, 4, 5, 6 or 7, wherein heating the process gas containing hydrogen sulfide comprises one or both of: (a) heat exchange in a heat exchanger with a first hot process fluid, and (b) the addition of a second hot process gas.

9. 9. The method of claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the first hot process fluid and the second hot process fluid may be the same or different and may be selected from the group of a heat exchange medium including the condenser heat exchange medium, the sulfur dioxide rich gas, the sulfur trioxide rich gas, and the purified process gas.

10. 10. The method of claim 1, 3, 4, 5, 6, 7, 8 or 9, wherein the 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. 11. The method of claim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, wherein the aqueous solution containing hydrogen sulfide is provided by microbial reduction of sulfate.

12. 12. The method of claim 11, wherein at least a portion of the sulfuric acid produced is directed for use in an upstream process that produces sulfate salts.

13. 13. The method of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12, wherein at least a portion of the produced sulfuric acid is directed to be used for leaching metal ores to provide an aqueous solution containing metal sulfates.

14. 13. The method of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12, wherein at least a portion of the sulfuric acid produced is directed for use in the hydrolysis of lignin compounds.

15. A vessel (4) for contacting a liquid stream with a gas stream, the vessel having a liquid inlet and outlet and a gas inlet and outlet; a sulfur dioxide reactor including a hydrogen sulfide oxidation means and a material catalytically active for sulfur dioxide oxidation, each having an inlet and an outlet; and 1. A process plant comprising a condenser (30) having a cooling medium inlet (32) and a cooling medium outlet (34), a gas inlet (28), a liquid outlet (36) and a gas outlet (38), a gas outlet of the vessel for contacting the liquid stream with the gas stream in fluid communication with the inlet of the hydrogen sulfide oxidation reactor; an outlet of the hydrogen sulfide oxidation reactor in fluid communication with an inlet of the sulfur dioxide oxidation reactor; an outlet of the sulfur dioxide oxidation reactor in fluid communication with the gas inlet of the condenser; and a gas outlet of the condenser in fluid communication with a gas inlet of a vessel for contacting the liquid stream with the gas stream; The process plant.

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