Concentrated sulfuric acid recovery and recycling process

Abstract

The invention relates to a method for producing sulfuric acid from a process gas comprising 1-10 vol% SO3 and 10-70 vol% H2O, and 1-5000 mg / Nm3 of particulate solids having a diameter of 1 [mu] m or more, comprising the following steps: a) condensing sulfuric acid from the process gas by indirect heat exchange with a cooling medium, the method comprises the following steps: a) generating a concentrated sulfuric acid stream, a desulfurized process gas containing a sulfuric acid mist and an amount of heated cooling medium, b) removing the sulfuric acid mist from the desulfurized process gas in an acid mist removal step to generate a dilute sulfuric acid stream and a clean gas, c) recycling at least an amount of concentrated sulfuric acid to an upstream process. This has a related benefit of providing an economical and efficient process for recycling SO3 in the form of sulfuric acid.

Description

[0001] The present invention relates to a method for recovering concentrated sulfuric acid from wet waste gas and recycling the concentrated sulfuric acid to an upstream process requiring concentrated sulfuric acid.

[0002] In the production of precious metals such as nickel and copper, when the metal of interest is present in the form of an oxide (e.g. NiO), so-called sulfuric acid leaching processes are often used. In these processes, the oxides of the desired metal in the ore are converted into sulfates, which are soluble in water and can therefore be easily separated from the rest of the ore material and further refined into pure metal.

[0003] The acid leaching process can take several forms, depending on the ratio of sulfuric acid to ore, temperature, pressure, time and recovery of the metal sulfate.

[0004] In one such process of nickel production, concentrated sulfuric acid is mixed with ore powder to form a slurry. During the digestion step and subsequent drying and heating steps, the formation of nickel sulfate occurs in the slurry.

[0005] We have now found a cost-effective method for recycling sulfuric acid by condensing concentrated sulfuric acid.

[0006] For the purpose of this application, the unit wt% shall mean weight / weight, the unit vol% shall mean volume / volume, and the unit ppm shall mean volume / volume. v Should be expressed as volume / volume parts per million.

[0007] For the purposes of this application, when concentrations in the gas phase are given, they are given as volume / volume concentrations unless otherwise specified.

[0008] For the purposes of this application, it is assumed that all hexavalent sulfur is present in the form of SO3, and the concentration of SO3 in gaseous form is expressed in volume %, which therefore includes SO3 as well as hydrated SO3 in the form of gaseous H2SO4.

[0009] For the purposes of this application, vertical and horizontal should be interpreted as allowing for moderate deviations, such as less than 5° or 10° from vertical or horizontal, respectively.

[0010] Liquid sulfuric acid H2SO4 exists in the form of a mixture with water in a concentration of 0-100 wt% H2SO4. The properties of sulfuric acid solutions depend on the concentration and are related both to chemical reactions and to corrosion of materials used for storage and transportation of sulfuric acid.

[0011] For the purpose of this application, the concentration of concentrated sulfuric acid is 85-100 wt% H2SO4. The concentration of dilute sulfuric acid is 0-10 wt% H2SO4. The concentration of sulfuric acid on the international market is 93-98.5 wt% H2SO4.

[0012] A first aspect of the present disclosure relates to a method for producing sulfuric acid from a process gas comprising 1-10 vol% SO3 and 10-70 vol% H2O on an unhydrated basis, and 0.1 mg / Nm 3 、1mg / Nm 3 or 10mg / Nm 3 Up to 5000mg / Nm 3 The method comprises the following steps:

[0013] a) condensing sulfuric acid from the process gas by indirect heat exchange with a cooling medium to produce a concentrated sulfuric acid stream, a desulfurized process gas containing sulfuric acid mist and a certain amount of heated cooling medium,

[0014] b) removing sulfuric acid mist from the desulfurized process gas in an acid mist removal step to produce a dilute sulfuric acid stream and a clean gas,

[0015] c) recycling at least an amount of concentrated sulphuric acid, optionally together with an amount of said dilute sulphuric acid, to an upstream process, such as acid leaching of an ore.

[0016] This has the associated benefit of providing a cost-effective method of recycling SO3 in the form of sulfuric acid. For a particular process, the solids level may be 100 mg / Nm 3 、500mg / Nm 3 or 1000mg / Nm 3 The diameter of granular solids is usually less than 1 mm.

[0017] The second aspect of the present disclosure relates to the method according to the first aspect, the method further comprising the steps of: 3 The dust loaded process gas of particulate solids is passed through a dust removal unit, such as an electrostatic precipitator, candle filter or cyclone separator, to produce a solids stream and the process gas. This has the associated benefit of providing a process gas that allows very high amounts of dust (e.g. 1-50 g / Nm 3 or 5-30g / Nm 3 ) of a dust laden process gas stream.

[0018] A third aspect of the present disclosure relates to a method according to any of the above aspects, wherein the cooling medium is air or a process gas, the method further comprising the step of diluting the process gas or dust laden process gas with a heated cooling medium stream having a temperature of 160° C. to 300° C. This has the associated benefit of providing a combined heated dilution stream while reducing the risk of H2SO4 condensation.

[0019] A fourth aspect of the present disclosure relates to a method according to any of the above aspects, wherein the process gas comprises 2-6 vol% SO3 and 20-50 vol% H2O on a non-hydrated basis. This has the associated benefit of providing a process gas suitable for condensing H2SO4 in concentrated form.

[0020] A fifth aspect of the present disclosure relates to a method according to any of the above aspects, wherein the condensation of sulfuric acid is carried out in a falling film condenser in which a process gas flows inside a substantially vertical glass tube and a cooling medium flows outside the substantially vertical glass tube to separate the process gas and the cooling medium, wherein the inlet temperature of the process gas is 250-300° C. and the outlet temperature of the process gas is 70-120° C. This has the associated benefit of providing a cost and thermally efficient condenser.

[0021] A sixth aspect of the present disclosure relates to a method according to any one of the first four aspects above, wherein the condensation of sulfuric acid is carried out in an acid condenser in which the process gas flows outside a substantially horizontal glass tube and a cooling medium flows inside a substantially horizontal glass tube to separate the process gas and the cooling medium, wherein the inlet temperature of the process gas is 220-300° C. and the outlet temperature of the process gas is 70-120° C. This has the associated benefit of providing a condenser that is robust with respect to clogging by particulate matter.

[0022] A seventh aspect of the present disclosure relates to a method according to any of the above aspects, wherein the concentrated sulfuric acid produced is 90-98.5 wt% H2SO4. This has the associated benefit of providing a method that produces high quality concentrated acid for recycling to an upstream process, such as mineral leaching, such as leaching of nickel or copper ores. Advantageously, the sulfuric acid is at least 92 wt% H2SO4 or 94 wt% H2SO4.

[0023] An eighth aspect of the present disclosure relates to a method according to any of the above aspects, wherein the sulfuric acid condensation unit is equipped with an anti-clogging demister device, such as an electrostatic precipitator or a knitted, woven or non-woven mist filter. This has the associated benefit of avoiding the release of sulfuric acid aerosols or droplets from the process. Anti-clogging properties can be established by a range of measures, such as dust blowing, leaching and mechanical shaking.

[0024] A ninth aspect of the present disclosure relates to a method according to any of the above aspects, wherein the cooling medium is air or process gas, wherein the heated cooling medium from the sulfuric acid condensation unit is 160-260° C. and is recycled to one or more upstream processes requiring combustion air and / or dilution air. This has the associated benefit of energy-saving processes, where the thermal energy can reduce, for example, the demand for fuel in the upstream processes.

[0025] A tenth aspect of the present disclosure relates to a process according to any of the above aspects, wherein seed particles are added to the process gas so that the total amount of the dust particles and the added seed particles reaches 10 based on the non-hydrated state. 10 Up to 10 13 Particles / Nm 3 Process gas / 1 vol% SO3. This has the associated benefit of replenishing particulate matter in the process gas so that nucleation seeds are available, resulting in the appropriate number and size of sulfuric acid droplets.

[0026] An eleventh aspect of the present disclosure relates to a process according to any of the above aspects, wherein the concentration of particulate solids in the sulphuric acid product from the acid condensation unit is 0.01-2.0 wt%. This has the associated benefits of recycling an amount of unleached material with the sulphuric acid to an upstream process and reducing the amount of particulate matter in the clean gas stream.

[0027] The twelfth aspect of the present disclosure relates to a method according to any of the above aspects, wherein the sulfuric acid streams produced in step a) and step b) are recycled to an upstream process, such as ore acid leaching. This has the associated benefit of providing a process with no or little waste sulfuric acid stream and maximizing the use of sulfuric acid.

[0028] A thirteenth aspect of the present disclosure relates to a method according to any of the above aspects, wherein at least 20wt%, 50wt%, 80wt% or 90wt% of the particulate solids having a diameter of 1 μm or more are salts of one or more metals, such as sulfates, selected from Ni, Zn, U, Cu, Mo, Fe, Li, Au and rare earth metals. This has the associated benefit that sulfuric acid contamination from such particulate solids is compatible with sulfuric acid leaching processes for ores. In a particular embodiment, the metal with the highest concentration in the particulate solids having a diameter of 1 μm or more will also be the metal with the highest concentration in the ore subjected to acid leaching.

[0029] A fourteenth aspect of the present disclosure relates to a process equipment for implementing the method according to any one of the above claims.

[0030] In wet gases, i.e. gases containing water vapor, there is an equilibrium between gaseous SO3 and H2SO4 according to the hydration reaction:

[0031]

[0032] At high temperatures, i.e. >400°C, only SO3 is actually present, while at temperatures below 200°C, H2SO4 will predominate. In the intermediate temperature range, both SO3 and H2SO4 are present. For convenience, the acid is assumed to be in the non-hydrated state, i.e. SO3, thereby combining the sum of the SO3 and H2SO4 concentrations into a single value.

[0033] The drying and heating steps of the acid leaching process require heat input to dry the slurry and heat the solids to the desired temperature, ie >400°C and up to over 1,000°C, depending on the desired chemical reactions in this step.

[0034] Heat is usually provided by burning fuel with air as a source of O2. The drying and heating steps can be carried out, for example, in a rotary kiln, where combustion gases sweep through the kiln to dry and heat the slurry in the kiln. The drying step transfers water from the slurry to the combustion gases, and further heating will result in evaporation of excess sulfuric acid and decomposition of metal sulfates. When the combustion gases leave the drying and heating step, the water concentration may be above 20 vol%, and the sulfuric acid vapor concentration may be above 2 vol%.

[0035] In another configuration of the rotary kiln, the kiln can be indirectly fired / heated, meaning that the slurry in the rotary kiln is in contact only with a carrier gas, which can be air, partially burned gases from indirect combustion, or other gases. Indirect heating can also be provided by electrical heating.

[0036] Dust concentrations in the exhaust air from drying and heating units can be as high as 50 g / Nm 3 The exact concentration depends on the powder properties and treatment in the drying and heating steps and the rate of the combustion gas / carrier gas. The dust is mainly mineral, for example containing at least 20wt%, 50wt%, 80wt% or 90wt% of the metal to be recovered, such as sulfates or oxides of Ni, Zn, U, Cu, Mo, Fe, Li, Au and rare earth metals. The metal in the dust usually corresponds to the metal in the ore, and preferably corresponds to the metal to be produced from the ore. The upper limit of the metal content can be from 2 times the lower limit to 95wt%, 99wt% or 100wt%.

[0037] The process gas temperature at the outlet of the drying and heating step is 250-400°C for the acid leaching process, but may be higher if chemical conversion of the slurry is desired. In this case, a heat exchanger can be installed to recover part of the high temperature energy and cool the process gas at the same time. The pressure of the process gas can be slightly below or slightly above atmospheric pressure, depending on the process layout.

[0038] The dried slurry will fall out of the rotary kiln for further processing, but a portion of the dried slurry will be entrained in the combustion / carrier gas in the form of dust and leave the kiln through the drying and heating exhaust gases.

[0039] It is undesirable for such waste gases to be discharged into the atmosphere and therefore they are treated to remove the sulphuric acid vapour, preferably by recovering the sulphuric acid vapour in the form of concentrated sulphuric acid which can then be recycled to the acid leaching process.

[0040] Sulfuric acid is hygroscopic in nature and combines with high concentrations of water in the exhaust gas to produce concentrated sulphuric acid, which requires careful process design.

[0041] The exhaust gas is then usually quenched to cool the exhaust gas and remove soluble gases such as SO3 / H2SO4, HCl, HF, residual dust and gaseous metal compounds. The quench is usually a direct contact unit in which the hot exhaust gas is directly contacted with a cold liquid to cool the exhaust gas by a combination of sensible heat transfer and evaporation of water. The cold liquid can be water, but is usually a dilute sulfuric acid solution that circulates in the quench tower. The type of quench can be a venturi type contactor, a packed bed, a nozzle device, etc. To reduce the risk of clogging, dust can be removed from the gas by hot electrostatic precipitators, candle filters, cyclone separators or other methods.

[0042] The quench off-gas typically leaves the quench unit at a temperature of 50-70°C and contains some sulphuric acid mist which is typically separated from the quench off-gas in a wet electrostatic precipitator.

[0043] The clean off-gas is then typically sent to a sulfuric acid plant to convert the SO2 in the clean off-gas into concentrated sulfuric acid, which can then be recycled to the acid leaching process.

[0044] CA 1,338,815 and WO 2008 / 064698 describe processes for the catalytic production of sulfuric acid from SO2. These processes involve the catalytic oxidation of SO2 to SO3 in multiple catalytic steps with intermediate cooling, followed by hydration of SO3 and condensation of H2SO4. In order to achieve efficient condensation, for example, a 1% H2SO4 vapor per Nm2 of H2SO4 vapor is added to the H2SO4-containing gas by combustion of silicone oil. 3 Add 10 10 to 10 13 In CA 1,338,815, the size of the seed particles is estimated to be less than 0.6 μm.

[0045] For such a process for the catalytic production of sulfuric acid from SO2, high concentrations of mineral dust are detrimental to the uninterrupted operation of the process, since the catalyst bed can become clogged, since the catalyst commonly used is a viscous vanadium alkali melt on a porous oxide. This has the added effect that typically 95% of the mineral dust will be captured by the catalyst particles, so the amount of dust entering the condenser will be very limited. Furthermore, in the production of sulfuric acid by catalytic oxidation of SO2-rich gases (e.g. coal combustion flue gases, Claus process tail gases or carbon black production tail gases), some dust may be present in the SO2-rich gas, but most of this dust will be carbon particles, which will be oxidized when they come into contact with the vanadium alkali melt catalyst. Therefore, the amount of dust directed to the downstream condensation is typically 1 mg / Nm 3 the following.

[0046] For the acid leaching process, the drying and heating steps are carried out at low to medium temperatures, and almost all of the sulfur is in the form of SO3 or H2SO4, so there is no need for a dedicated sulfuric acid plant to oxidize the SO2; all of the sulfuric acid will be recovered in the quench tower and wet electrostatic precipitator. Depending on the conditions, a mixture of SO2 and SO3 may also be present in the leaching process off-gas, in which case the condensation process of the hydrated SO3 can be followed by any known SO2 removal process, including its catalytic oxidation to SO3 and a scrubber process.

[0047] Furthermore, if the acid is to be recycled into the dust forming acid leaching process, dust contamination from the acid leaching process will not be a problem and the tolerance for contaminants will be higher.

[0048] Quenching towers are not suitable for the production of concentrated sulfuric acid because of the high water concentration in the drying and heating offgases and the low operating temperatures of the quenching towers. In addition to the low acid concentration, the quenching operation requires a large amount of cooling capacity, either as cooling water and / or cooling air for the circulating acid in the quench, or by injecting water directly into the gas. The latter will increase the water concentration in the drying and heating offgases, resulting in even lower sulfuric acid that is not suitable for recycling to the acid leaching process.

[0049] In principle, the sulfuric acid from the quench tower can be concentrated in a separate unit by evaporation of water, but this is a complex process and the concentration unit requires a large energy input.

[0050] The present invention describes a process in which concentrated sulphuric acid suitable for recycle to the acid leaching process is recovered from hot off-gases while minimising energy requirements by a sulphuric acid condensation principle similar to that used in the production of sulphuric acid by catalytic oxidation of SO2-rich gases in a wet gas sulphuric acid plant.

[0051] The hot exhaust gas from the drying and heating process can be conditioned by adding hot gas to dilute the exhaust gas to control the sulfuric acid dew point temperature and process gas temperature, ensuring that the process gas has a temperature suitable for the sulfuric acid condenser and the sulfuric acid dew point temperature.

[0052] The offgas from the drying and heating steps can also optionally be passed through a dust removal unit, such as a thermal ESP, a cyclone separator or a candle filter, to remove at least a portion of the dust from the process gas before it enters the sulfuric acid condenser, thereby achieving a moderate dust load of, for example, 3000 mg / Nm 3 or 5000mg / Nm 3 the following.

[0053] The sulfuric acid condenser is usually a falling film condenser, in which the process gas containing sulfuric acid vapor is indirectly cooled as it flows upward inside a vertical glass tube. When cooled, the sulfuric acid vapor condenses and moves downward under the action of gravity and is stripped of water by the upward moving process gas. The sulfuric acid taken from the bottom of the acid condenser is concentrated and suitable for the acid leaching process. The cooled gas usually leaves the condenser at 60-120°C, which allows a large amount of water vapor to leave with the cooled exhaust gas while ensuring complete condensation of the sulfuric acid vapor.

[0054] Indirect cooling of the process gases is usually carried out by air or other gas flowing outside the vertical glass tubes, and the hot, cooled air from the sulfuric acid condenser can, for example, be used as combustion air in the drying and heating step, thereby saving fuel in this step, and / or as conditioning air for drying and heating the exhaust gases.

[0055] In a specific layout of a vertical falling film condenser, a dense demister will be located at the top of the condenser tubes to collect the acid droplets and return them to the falling film condenser. In situations where the process gas contains a lot of dust, such a dense demister runs the risk of becoming blocked by solids, although the liquid flow in the demister should flush the solids away. In this case, a loose demister will be used to minimize the risk of dust accumulation in the demister.

[0056] Typically, mist eliminators are made of woven metal or plastic wire with a void fraction of more than 95%. For high-efficiency mist eliminators, the void fraction is lower to allow for a larger collection area, but at the expense of a higher pressure drop and a higher risk of eliminator plugging (if there are solids in the gas).

[0057] In principle, the demister can be omitted, but in this case the acid droplet emission from the falling film condenser may be significant.

[0058] In a special arrangement of the sulfuric acid condenser, nozzles can be installed above the mist eliminator to provide intermittent flushing of the mist eliminator as solids accumulate.

[0059] Falling film condensers will usually have so-called nucleation control to minimize the formation of sulfuric acid mist (i.e., submicron sulfuric acid droplets), which are very difficult to collect in a mist eliminator. Nucleation control provides particles to the process gas, for example by the combustion of silicone oil, so that the sulfuric acid vapor has an available condensation surface, thereby effectively suppressing the formation of acid mist. Since dust particles are already present in the hot exhaust gas, nucleation control may be redundant and can be omitted, thus simplifying the condensation process. Typically, the amount of particles is preferably between 10 10 Up to 10 13 Particles / Nm 3 In the range of process gas / 1 vol% H2SO4, this will ensure a balance between the number of particles and the amount of condensable H2SO4, forming droplets of appropriate size for collection by the demister, and a small number of seed particles leading to a similar result of spontaneous formation of many small droplets (which can only be collected in the demister with low efficiency). In order to minimize the consumption of silicone oil and contamination of the product acid, it is desirable that the size of the seed particles is less than 0.1 μm. However, if it is intended to recycle the acid in a dust-forming acid leaching process, dust contamination from the acid leaching process will not cause problems and the tolerance for contamination will be higher.

[0060] It is inevitable that there will be some sulphuric acid mist in the cooled off-gas leaving the sulphuric acid condenser and it is preferred to install suitable acid mist removal equipment to ensure that the clean off-gas discharged to the atmosphere complies with environmental regulations. Mist eliminators, Brinker-type candle demisters and wet electrostatic precipitators of varying arrangements can be used to remove the acid mist. For the present application where the off-gas contains dust, wet electrostatic precipitators will be the most robust in resisting clogging by dust particles and are therefore the preferred technology, but Brinker-type candle demisters may also be used, especially if a cleaning mechanism is configured.

[0061] Downstream of the acid mist removal device, the cleaned and cooled gas can be sent to other gas cleaning units, such as SO2 scrubbers and / or blowers, and then the cleaned gas is discharged to the atmosphere through a chimney. The gas cleaning unit can also be located between the acid condensation unit and the acid mist removal device.

[0062] The sulphuric acid condenser can also be equipped with horizontal tubes with the process gas stream flowing outside the tubes and the cooling medium stream flowing inside the tubes. Since nucleation control does not work well under such conditions, the off-gas from such condensers may require an acid mist removal device and usually it is an integrated part of such sulphuric acid condenser design. This design is robust in preventing dust particles in the process gas. The produced sulphuric acid concentration will be lower than that produced in the vertical falling film condenser described above but still high enough to be recycled to the acid leaching process.

[0063] The sulphuric acid condenser may also be a packed bed type condenser in which the process gas is cooled and the sulphuric acid is condensed in countercurrent to the relatively hot concentrated sulphuric acid. Under these conditions, acid mist control is ineffective and the packed bed condenser may be configured to provide a sulphuric acid concentration suitable for recycle to the acid leaching process. The choice of materials of construction is very limited as the hot concentrated acid is very corrosive to metals so a brick lining and / or fluoropolymer lining may be required. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 A flow chart of a process plant having a quench condenser according to the prior art is shown.

[0065] Figure 2 A flow chart of a process plant according to a preferred embodiment of the present disclosure is shown.

[0066] exist Figure 1 In the process, metal ore (1) is fed into a digestion unit (3) and mixed with a combination of fresh concentrated acid (2) and sulfuric acid (38) recycled from a quench unit (34).

[0067] The resulting acidic ore slurry (4) is conveyed to a drying and heating unit (5), such as a rotary kiln, where the slurry is dried and heated to produce a solid product of sulfuric acid treated metal ore (8). Drying and heating are preferably carried out by burning a fuel (6) with combustion air (9).

[0068] The exhaust gas (10) from the drying and heating unit contains sulfuric acid vapor, water vapor and a large amount of dust, such as up to 50g / Nm 3 , which is sent to an optional dust removal unit (15), such as an electrostatic separator, a cyclone separator or a candle filter, thereby separating most of the dust in the exhaust gas into a solid stream (16) and a partially dust-removed exhaust gas (17).

[0069] The partially dedusted waste gas (17) enters a quench unit (34) where the gas is cooled and the sulfuric acid and water vapor are partially condensed to form sulfuric acid product (36). The cooling can be carried out by adding water (42) for evaporative cooling directly to the circulation system of the quench unit and / or a combination of cooling water / air (43 inlet and 44 outlet).

[0070] The sulfuric acid product (36) is split into two parts, one part (38) is recycled back to the acid leaching process and the other part (40) can be used in other processes and / or sent to a sulfuric acid concentration unit.

[0071] The off-gas (46) from the quench unit will contain some sulfuric acid mist which is separated in a wet electrostatic precipitator (48) to form a dilute sulfuric acid stream (50) and clean off-gas (52) which can be sent to the atmosphere or other gas cleaning units.

[0072] exist Figure 2 In the process, metal ore (1) is fed to a digestion unit (3) and mixed with a combination of fresh concentrated acid (2), concentrated sulfuric acid recycled from a sulfuric acid condensation unit (22) and optionally dilute sulfuric acid from an acid mist removal unit (28).

[0073] The acidic ore slurry (4) formed is conveyed to a drying and heating unit (5) where the slurry is dried and heated to produce a solid product of sulfuric acid treated metal ore (8). Drying is preferably carried out by burning a fuel (6) with combustion air, preferably hot air (7) from an acid condensation unit.

[0074] The exhaust gas (10) from the drying and heating unit contains sulfuric acid vapor, water vapor and a large amount of dust, for example up to 50 g / Nm 3 , optionally mixed with hot air (12), and the optionally diluted dried and heated exhaust gas (14) is sent to an optional dust removal unit (15), such as an electrostatic separator, a cyclone separator or a candle filter, to separate most of the dust in the exhaust gas into a solid stream (16) and a partially dedusted exhaust gas (17), the partially dedusted exhaust gas containing less than 5000 mg / Nm 3 of dust.

[0075] To maintain the temperature above the dew point of sulfuric acid, the dedusted off-gas (17) may optionally be diluted with hot air via line 18, and the diluted dedusted off-gas (20) is then directed to the sulfuric acid condensation unit (22). Concentrated sulfuric acid from the acid condensation unit is withdrawn via line 24, cooled (not shown) and recycled to the digestion unit (3).

[0076] The cooling of the dedusted off-gas in the sulfuric acid condensation unit is carried out by cooling air supplied via line 32, and the heated cooling air leaves the acid condensation unit via line 34. A portion of the heated cooling air is directed back to the upstream process via line 36, and the unused heated cooling air is discharged via line 35. The recycled heated cooling air (36) is optionally heated (or cooled) in a heat exchanger 38, and the temperature-regulated cooling air is then directed to one or more air streams 7, 12 and 18. Depending on the process requirements, the heat exchanger can be located on a single air stream. If it is found to be more advantageous, the heat exchanger can be replaced by other heating means (such as a fuel-fired heater or an electrically supported heater).

[0077] The off-gas (26) from the condensation unit is sent to a sulfuric acid demisting unit (28), where sulfuric acid mist is separated from the off-gas as dilute sulfuric acid in stream 30 and optionally recycled to the digestion unit (3). The mist-free off-gas (32) is discharged to the atmosphere, optionally passing through further gas cleaning units and / or a process gas blower (not shown). Example

[0078] This example compares the operating conditions of a basic quench-type solution with the operating conditions of a sulfuric acid concentration process according to the present invention.

[0079] In this example, Table 1 shows the operating data of the basic quench design according to the prior art and the proposed design with a sulfuric acid condensing unit. The process gas composition is 4.1 vol% SO3, 42 vol% H2O, 300°C, atmospheric pressure, the exhaust gas from the quench and condensing unit contains 1,000 ppmv sulfuric acid, and the flow rate of the exhaust gas from the drying and heating unit is 26,000 Nm 3 / h. The sulfuric acid dew point temperature of the process gas is about 270°C.

[0080] In the arrangement design of the present invention, adding 5000Nm 3 / h of heated dilution air to reduce the sulfuric acid dew point temperature and thus protect the building materials within the sulfuric acid condensation unit.

[0081] In the basic quench type solution of Example 1, the exhaust gases from the drying and heating unit are cooled by direct injection of water for evaporative cooling, and in Example 2, the exhaust gases from the drying and heating unit are cooled by cooling water in a circulation loop through circulating sulfuric acid. The basic operating parameters of the different process layouts are given in Table 1.

[0082] It is clear from the data that the quench type operation will produce a lower concentration of sulfuric acid than that produced by the sulfuric acid condensation unit, with the amount of medium strength sulfuric acid produced in Examples 1 and 2 more than doubling compared to the concentrated sulfuric acid of Example 3. H2SO4 concentrations in the 40-50 wt% range are generally too weak to provide an effective leaching process, so concentrated acid addition is required, which makes it impossible to recycle all of the acid to the upstream process. Furthermore, this low concentration sulfuric acid is not useful in the commercial acid market, which requires a minimum of 93 wt% H2SO4.

[0083] A conservative estimate is that 1,000 ppmv of acid mist (100 wt% H2SO4) will be found in the offgas from the main acid production unit. The very dilute acid from the wet ESP will significantly reduce the combined acid concentration from the two production units. However, the 69 wt% H2SO4 from the layout of the present invention is still high enough to be used in the acid leaching process, and the entire acid production can be recycled, eliminating the problems that can result from the generation of "spent acid".

[0084] For process gas cooling, basic quenching with evaporative cooling is very efficient as no external cooling is required during the quenching process. Water consumption is 2,050 kg / h, most of which will be present in the sulfuric acid product, thereby reducing the sulfuric acid concentration and increasing the mass flow rate.

[0085] Only cooling water is used for quenching, and assuming that the cooling water temperature rises by 10°C, the cooling water flow rate will be 687,000 kg / h, which is a very large flow rate. The sulfuric acid concentration will be slightly higher than that of direct water quenching.

[0086] The sulfuric acid condensation unit of Example 3 requires atmospheric air as a cooling medium; about half of the cooling air flow can be used as dilution air before the condensation step, and as combustion air in the drying and heating steps. The latter will reduce the fuel demand of the drying and heating units.

[0087] Table 1

[0088] describe unit Example 1 Example 2 Example 3 Exhaust air from drying / heating unit <![CDATA[Nm 3 / h]]> 26,000 26,000 26,000 Added dilution air <![CDATA[Nm 3 / h]]> 0 0 5,000 Sulfuric acid product concentration wt% 41 46 96 Sulfuric acid production Kg / h 11,250 9,970 4,725 Process gas cooling duty in the condenser unit kW - 7,980 3,610 Quench water flow Kg / h 2,050 - - Cooling water flow Kg / h - 687,000 - Cooling air flow <![CDATA[Nm 3 / h]]> - - 53,400 Wet ESP acid concentration wt% 6.3 6.3 6.2 Wet ESP Acid Production Kg / h 1,610 1,530 2,020 Cooling water flow kg / h 100,000 101,000 133,000 Water in the exhaust gas from a wet ESP Vol% 11 7 28 Total acid production Kg 12,870 11,500 6,745 Mixed acid concentration wt% 36 41 69

Claims

1. A method for producing sulfuric acid from a process gas, the process gas comprising 1-10 vol% SO3 and 10-70 vol% H2O on a non-hydrated basis, and 1-5000 mg / Nm 3 The method comprises the following steps: a) condensing sulfuric acid from the process gas by indirect heat exchange with a cooling medium to produce a concentrated sulfuric acid stream, a desulfurized process gas containing sulfuric acid mist and a certain amount of heated cooling medium, b) removing sulfuric acid mist from the desulfurized process gas in an acid mist removal step to produce a dilute sulfuric acid stream and a clean gas, c) recycling at least an amount of concentrated sulphuric acid, optionally together with an amount of said dilute sulphuric acid, to an upstream process, such as acid leaching of an ore.

2. The method according to claim 1, further comprising the steps of: Contains 1-50g / Nm 3 The dust-laden process gas of particulate solids is passed through a dust removal unit, such as an electrostatic precipitator, candle filter or cyclone separator, which generates a solids stream and the process gas.

3. The method according to claim 1 or 2, wherein the cooling medium is air or a process gas, the method further comprising the step of diluting the process gas or the dust-laden process gas with a heated cooling medium stream having a temperature of 160°C to 300°C.

4. A method according to claim 1, 2 or 3, wherein the process gas contains 2-6 vol% SO3 and 20-50 vol% H2O on a non-hydrated basis.

5. The method according to claim 1, 2, 3 or 4, wherein the condensation of sulfuric acid is carried out in a falling film condenser in which the process gas flows inside a substantially vertical glass tube and the cooling medium flows outside the substantially vertical glass tube to separate the process gas and the cooling medium, wherein the inlet temperature of the process gas is 250-300°C and the outlet temperature of the process gas is 70-120°C.

6. A method according to claim 1, 2, 3 or 4, wherein the condensation of sulfuric acid is carried out in an acid condenser in which the process gas flows outside a substantially horizontal glass tube and a cooling medium flows inside a substantially horizontal glass tube to separate the process gas and the cooling medium, wherein the inlet temperature of the process gas is 220-300°C and the outlet temperature of the process gas is 70-120°C.

7. A method according to claim 1, 2, 3, 4, 5 or 6, wherein the concentrated sulfuric acid produced is 90-98.5 wt% H2SO4.

8. The process of claim 1, 2, 3, 4, 5, 6 or 7, wherein the sulfuric acid condensation unit is equipped with an anti-clogging demister device, such as an electrostatic precipitator or a knitted, woven or non-woven mist filter.

9. A process according to claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the cooling medium is air or process gas, wherein the heated cooling medium from the sulphuric acid condensation unit is 160-260°C and is recycled to one or more upstream processes requiring combustion air and / or dilution air.

10. The method according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein seed particles are added to the process gas so that the total amount of dust particles and the added seed particles reaches 10% based on the non-hydrated state. 10 Up to 10 13 Particles / Nm 3 Process gas / 1vol%SO3.

11. The process of claim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 wherein the concentration of particulate solids in the sulfuric acid product from the acid condensation unit is 0.01 - 2.0 wt%.

12. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11, wherein the sulphuric acid streams produced in step a) and step b) are both recycled to an upstream process, such as acid leaching of an ore.

13. A method according to claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12, wherein at least 20 wt%, 50 wt%, 80 wt% or 90 wt% of the particulate solids having a diameter of 1 μm or more are compounds, such as sulfates, comprising one or more metals selected from Ni, Zn, U, Cu, Mo, Fe, Li, Au and rare earth metals.

14. A process plant for carrying out the method according to any one of the preceding claims.

Images