WO2023213961A1

WO2023213961A1 - Process for recovery and recirculation of concentrated sulfuric acid

Info

Publication number: WO2023213961A1
Application number: PCT/EP2023/061833
Authority: WO
Inventors: Morten Thellefsen; Mårten Nils Rickard GRANROTH; Samuel Wiktor Scherman JOHANSSON; Mads Lykke
Original assignee: Topsoe A/S
Priority date: 2022-05-06
Filing date: 2023-05-04
Publication date: 2023-11-09

Abstract

The present invention relates to a process for the produc- tion of sulfuric acid from a process gas comprising 1-10 vol% SO₃ and 10-70 vol% H₂O, as well as 1-5000 mg/Nm₃ of particulate solids having a diameter above 1 µm, comprising the following steps: a) condensing sulfuric acid from the process gas by indi- rect heat exchange with a cooling medium, producing a stream of concentrated sulfuric acid, a desulfurized process gas containing sulfuric acid mist and an amount of heated cooling medium, b) removing sulfuric acid mist from the desulfurized pro- cess gas in an acid mist removal step, in order to pro- duce a stream of dilute sulfuric acid and a cleaned gas, c) recycling at least an amount of concentrated sulfuric acid to an upstream process. This has the associated benefit of providing a process for cost effective recycle of SO₃ as sulfuric acid.

Description

Title: Process for recovery and recirculation of concen- trated sulfuric acid. The invention relates to a process for recovery of concen- trated sulfuric acid from a wet off gas and recirculation of the concentrated sulfuric acid to the upstream process, requiring concentrated sulfuric acid. In the production of valuable metals such and nickel and copper, so-called sulfuric acid leaching processes are com- monly used when the metals of interest exist as oxides, e.g. NiO. In these processes the oxides of the desired met- als in the ore are converted into sulfates, which are water soluble and thus can easily be separated from the rest of the ore material and further refined into the pure metal. The acid leaching process can take several forms with re- gard to the ratio of sulfuric acid to ore, temperature, pressure, time and recovery of metal-sulfates. In one such process for nickel production, concentrated sulfuric acid is mixed with ore powder to form a slurry. The formation of nickel sulfate takes place in the slurry in a digestion step and in a later drying and heating step. We have now identified a cost effective process for recir- culation of sulfuric acid by condensation of concentrated sulfuric acid. For the purpose of the present application, the unit wt% shall designate weight/weight %, the unit vol% shall desig- nate volume/volume % and the unit ppm_v shall designate vol- ume/volume parts per million. For the purpose of the present application, where concen- trations in the gas phase are given, they are, unless oth- erwise specified, given as volume/volume concentration. For the purpose of the present application, concentrations of SO₃ in gas form are stated as vol % under the assumption that all hexavalent sulfur is present as SO₃, and therefore it includes SO₃ as well as hydrated SO₃ present as gaseous H₂SO₄. For the purpose of the present application, vertical and horizontal shall be construed as allowing moderate devia- tion such as less than 5° or 10° from vertical or horizon- tal respectively. Liquid sulfuric acid, H₂SO₄, exist in mixtures with water in concentrations in the range 0-100 wt% H₂SO₄. The proper- ties of sulfuric acid solutions depend on the concentra- tion, both with regard to chemical reactions and corrosion of the material for the sulfuric acid storage and transpor- tation. For the purpose of the present application, concentrated sulfuric acid has a concentration of 85-100 wt% H₂SO₄. Di- lute sulfuric acid has a concentration of 0-10 wt% H₂SO₄. The concentration of sulfuric acid on the world market is 93-98.5 wt% H₂SO₄. A first aspect of the present disclosure relates to a pro- cess for the production of sulfuric acid from a process gas comprising 1-10 vol% SO₃ and 10-70 vol% H₂O on unhydrated basis, as well as 0.1 mg/Nm³, 1 mg/Nm³ orb 10 mg/Nm³ to 5000 mg/Nm³ of particulate solids having a diameter above 1 µm, said process comprising the following steps: a) condensing sulfuric acid from the process gas by indi- rect heat exchange with a cooling medium, producing a stream of concentrated sulfuric acid, a desulfurized process gas containing sulfuric acid mist and an amount of heated cooling medium, b) removing sulfuric acid mist from the desulfurized pro- cess gas in an acid mist removal step, in order to pro- duce a stream of dilute sulfuric acid and a cleaned gas, c) recycling at least an amount of concentrated sulfuric acid to an upstream process, such as mineral ore acid leaching, optionally together with an amount of said di- lute sulfuric acid. This has the associated benefit of providing a process for cost effective recycle of SO₃ as sulfuric acid. For spe- cific processes the amount of solids may be from 100 mg/Nm³, 500 mg/Nm³ or 1000 mg/Nm³. The particulate solids may typically have a diameter of less than 1 mm. A second aspect of the present disclosure relates to a pro- cess according to the first aspect further comprising the step of passing a dust loaded process gas comprising 1-50 g/Nm³ of particulate solids through a dedusting unit, such as an electrostatic precipitator, a candle filter or a cy- clone, producing a stream of solids and said process gas. This has the associated benefit of providing a process al- lowing a dust loaded process gas stream comprising a very high amount of dust, such as 1-50 g/Nm³ or 5-30 g/Nm³. A third aspect of the present disclosure relates to a pro- cess according to any aspect above, wherein the cooling me- dium is air or process gas further comprising the step of diluting the process gas or the dust loaded process gas with a stream of heated cooling medium, having a tempera- ture of 160°C to 300°C. This has the associated benefit of providing a combined heated diluted stream, with reduced risk of condensation of H₂SO₄. A fourth aspect of the present disclosure relates to a pro- cess according to any aspect above in which the process gas contains 2-6 vol% SO₃ and 20-50 vol% H₂O on unhydrated ba- sis. This has the associated benefit of providing a process gas suited for condensation of H₂SO₄ in concentrated form. A fifth aspect of the present disclosure relates to a pro- cess according to any aspect above in which condensing the sulfuric acid is carried out in a falling film condenser, where process gas flow inside substantially vertical glass tubes and cooling medium outside the substantially vertical glass tubes to separate process gas and cooling medium wherein the process gas inlet temperature is 250-300 °C and the process gas outlet temperature is 70-120 °C. This has the associated benefit of providing a cost and thermal ef- fective condenser. A sixth aspect of the present disclosure relates to a pro- cess according to any of the first four aspects above in which condensing the sulfuric acid is carried out in an acid condenser where process gas flow outside substantially horizontal glass tubes and cooling medium inside the sub- stantially horizontal glass tubes to separate process gas and cooling medium wherein the process gas inlet tempera- ture is 220-300 °C and the process gas outlet temperature is 70-120 °C. This has the associated benefit of providing a condenser which is robust with respect to plugging by particulate matter. A seventh aspect of the present disclosure relates to a process according to any aspect above in which the concen- trated sulfuric acid produced is 90-98.5 wt% H₂SO₄. This has the associated benefit of providing a process, which produces high quality concentrated acid, for recycle to up- stream processes, such as mineralogical leaching, e.g. of nickel or cupper ore. Beneficially the sulfuric acid is at least 92 wt% H₂SO₄ or 94 wt% H₂SO₄. An eighth aspect of the present disclosure relates to a process according to any aspect above in which the sulfuric acid condensation unit is equipped with a plugging re- sistant demister device, such as an electrostatic precipi- tator or a knitted, woven or non-woven, mist filter. This has the associated benefit of avoiding release of and sul- furic acid aerosol or droplets from the process. Plugging resistance may be established by a range of means, such as dust blowing, rinsing and mechaniscal shaking. A ninth aspect of the present disclosure relates to a pro- cess according to any aspect above, wherein the cooling me- dium is air or process gas in which the heated cooling me- dium 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 an energy efficient process, in which thermal energy can reduce e.g. the requirementefor fuel in upstream processes. A tenth aspect of the present disclosure relates to a pro- cess according to any aspect above in which seed particles are added to the process gas to achieve a total amount of dust particles and added seed particles in the range 10¹⁰ to 10¹³ particles/Nm³ process gas/1 vol% SO₃ on unhydrated basis. This has the associated benefit of supplementing the particulate matter in the process gas, such that nucleation seeds are available, leading to an appropriate number and size of sulfuric acid droplets. An eleventh aspect of the present disclosure relates to a process according to any aspect above in which the concen- tration of particulate solids in the sulfuric acid product from the acid condensation unit is 0.01 – 2.0 wt%. This has the associated benefit of recycling an amount of unleached material with the sulfuric acid to the upstream process, and reducing the amount of particulates in the cleaned gas stream. A twelfth aspect of the present disclosure relates to a process according to any aspect above in which both of the sulfuric acid streams produced in step a) and step b) are recycled to the upstream process, such as mineral ore acid leaching. This has the associated benefit of providing a process with none or little waste sulfuric acid stream, and maximum use of sulfuric acid. A thirteenth aspect of the present disclosure relates to a process according to any aspect above in which at least 20 wt%, 50 wt%, 80 wt% or 90 wt% of said particulate solids having a diameter above 1 µm are salts of one or more met- als taken from the group of Ni, Zn, U, Cu, Mo, Fe, Li, Au and rare earth metals, such as sulfate salts. This has the associated benefit of sulfuric acid contamination from such particulate solids being compatible with a process for sul- furic acid leaching of ore. In a specific embodiment the metal having the highest concentration in said particulate solids having a diameter above 1 µm will also be the metal having the highest concentration in the mineral ore under- going acid leaching. A fourteenth aspect of the present disclosure relates to a process plant for carrying out the process according to any claim above. In wet gases, i.e. gases containing water vapor, there will be an equilibrium between gaseous SO₃ and H₂SO₄ according the hydration reaction: SO₃(g) + H₂O(g) ↔ H₂SO₄(g) At high temperature, i.e. > 400 °C, practically only SO₃ will exist whereas H₂SO₄ will be dominant at temperatures below 200 °C. In the intermediate temperature range, both SO₃ and H₂SO₄ exist. For convenience the sum of SO₃ and H₂SO₄ concentrations are combined into a single value, assuming that the acid is in unhydrated state i.e. SO₃. The drying and heating step of the acid leaching process requires heat input to dry out the slurry and heat up the solids to the desired temperature, i.e. > 400 °C and up to more than 1,000 °C, depending on which chemical reactions are desired in this step. The heat is typically supplied by combusting a fuel with air as O2 source. The drying and heating step may e.g. be carried out in a rotary kiln, where the combustion gas sweeps through the kiln to dry and heat up the slurry in the kiln. The drying step transfers water from the slurry to the combustion gas and further heating will result in evaporation of excess sulfuric acid and decomposition of metal-sulfates. When the combustion gas leaves the drying and heating step, the water concentration can be higher than 20 vol% and the sulfuric acid vapor concentration can be higher than 2 vol%. In another layout of the rotary kiln, the kiln can be indi- rectly fired/heated, which means that the slurry inside the rotary kiln is only in contact with a carrier gas, which can be air, a fraction of the combustion gas from the indi- rect firing or another gas. Indirect heating may also be provided by electrical heating. The dust concentration in the off gas from the drying and heating unit can be as high as 50 g/Nm³, the exact concen- tration depend on the powder properties and handling in the drying and heating step and the velocity of the combus- tion/carrier gas. The dust will mainly be mineral, such as comprising at least 20 wt%, 50 wt%. 80 wt% or 90 wt% metals to be recovered, such as sulfates or oxides of Ni, Zn, U, Cu, Mo, Fe, Li, Au and rare earth metals. The metals of the dust will commonly correspond to the metals of the mineral ore, and preferable the metals to be produced from the mineral ore. The upper limit of the metal content may be from 2 times the lower limit, to 95 wt%, 99 wt% or 100 wt%. The process gas temperature at the outlet of the drying and heating step will be range 250-400 °C for an acid leaching process but could be higher if chemical conversion of the slurry is desired. In that case, heat exchangers may be in- stalled to recover some of the high temperature energy while cooling the process gas. The pressure of the process gas can be either slightly below or slightly above atmos- pheric pressure, depending on the process layout. The dried slurry will fall out of the rotary kiln to be further processed, however a fraction of the dried slurry will be entrained by the combustion/carrier gas as dust and leave the kiln via the drying and heating off gas. It is undesired that such an off gas is emitted to the at- mosphere and thus the off gas is treated to remove the sul- furic acid vapor, preferably by recovering the sulfuric acid vapor as concentrated sulfuric acid which then can be recycled to the acid leaching process. Sulfuric acid is by nature hygroscopic and in combination with the high water concentration in the off gas, producing concentrated sulfuric acid requires careful process design. The off gas is then typically quench cooled to cool the off gas, withdraw soluble gases such as SO₃/H₂SO₄, HCl, HF, re- maining dust and gaseous metal-compounds. The quench is typically a direct contact unit in which the hot off gas is contacted directly with a cold liquid, cooling the off gas by a combination of sensible heat transfer and evaporation of water. The cold liquid can be water, but will typically be a dilute sulfuric acid solution, circulating in the quench tower. Types of quenches can be e.g. venturi-type contactors, packed beds, spray nozzle arrangements and so on. To reduce the risk of plugging, dust in the gas may be removed by means of a hot electrostatic precipitator, can- dle filters, cyclones or by other methods. The quenched off gas typically leaves the quench unit at 50-70 °C and will contain some sulfuric acid mist, which is typically separated from the quench off gas in a wet elec- trostatic precipitator. This cleaned off gas will then typically be send to a sul- furic acid plant for converting the SO₂ in the cleaned off gas into concentrated sulfuric acid, which can be recycled to the acid leaching process. In CA 1,338,815 and WO 2008/064698 processes for catalytic production of sulfuric acid from SO₂ are presented. The processes involve catalytic oxidation of SO₂ to SO₃ in mul- tiple catalytic steps with intermediate cooling, followed by hydration of SO₃ and condensation of H₂SO₄. To obtain ef- ficient condensation 10¹⁰ to 10¹³ solid particles per Nm³ per 1% H₂SO₄ vapor are incorporated in the H₂SO₄ containing gas, e.g. by combustion of silicon oil. In CA 1,338,815 it is estimated that the size of the seed particles is below 0.6 µm. For such catalytic sulfuric acid production from SO₂ a high concentration of mineral dust would be detrimental for uninterrupted operation of the process, since the catalyst bed would be plugged, as the commonly used catalyst is a sticky vanadium alkali melt on a porous oxide. The effect of this will also be that typically 95% of the mineral dust will be captured on the catalyst particles, such that the amount of dust to the condenser will be very limited. In addition, in sulfuric acid production by catalytic oxida- tion of SO2 rich gases, such as flue gas from coal combus- tion, Claus process tail gas or carbon black production tail gas, some dust may be present in the SO₂ rich gases, but a majority of this dust will be carbon particulates, which are oxidized when contacting the vanadium alkali melt catalyst. Therefore, the amount of dust directed to down- stream condensation will typically be below 1 mg/Nm³. For an acid leaching process, where the drying and heating step takes place at moderately low temperatures, almost all the sulfur is found in the form of SO₃ or H₂SO₄ and there is no need to have a dedicated sulfuric acid plant to oxidize SO₂; all sulfuric acid will be recovered in the quench tower and the wet electrostatic precipitator. Depending on conditions a mixture of SO₂ and SO₃ may also be present in the leaching process off gas, and in this case a process for condensation of hydrated SO₃ may be followed by any known SO₂ removal process, including catalytic oxidation to SO₃ and scrubber processes. Furthermore, if the acid is intended to be recycled to the acid leaching process in which the dust was formed, a con- tamination with dust originating from the acid leaching process will not pose a problem, and the tolerance for con- tamination will be higher. The quench tower is not suited for production of concen- trated sulfuric acid as the water concentration in the dry- ing and heating off gas is high and the operating tempera- ture of the quench is low. Besides the low acid concentra- tion, the quench operation requires substantial amounts of cooling power, either as cooling water and/or cooling air on the circulating acid in the quench or direct water in- jection into the gas. The latter will increase the water concentration in the drying and heating off gas, thus pro- ducing even lower concentration of sulfuric acid, unsuita- ble to recycle to the acid leaching process. In principle, the sulfuric acid from the quench tower can be concentrated in a separate unit by means of water evapo- ration, but it is a complication and the concentration unit require a substantial energy input. The present invention describes a process in which concen- trated sulfuric acid, suitable for recycling to the acid leaching process, is recovered from the hot off gas while minimizing the energy requirements, by a sulfuric acid con- densation principle similar to what is used in the produc- tion of sulfuric acid by catalytic oxidation of SO₂ rich gases in a wet gas sulfuric acid plants. The hot off gas from the drying and heating process can be conditioned by the addition of hot gas to dilute the off gas for control of the sulfuric acid dew point temperature and the process gas temperature, ensuring that the process gas has a temperature and sulfuric acid dew point tempera- ture, suitable for the sulfuric acid condenser. The off gas from the drying and heating step can optionally also pass through a dedusting unit, such as a hot ESP, cy- clone or candle filter, to take out at least a fraction of the dust from the process gas before the process gas enter the sulfuric acid condenser to arrive at a moderate dust load such as below 3000 mg/Nm³ or 5000 mg/Nm³. The sulfuric acid condenser is typically a falling film type condenser in which the process gas containing sulfuric acid vapor is indirectly cooled as it flows upwards on the inside of vertical glass tubes. When cooled, the sulfuric acid vapor condenses and by gravity move downwards and be- come stripped from water by the upward moving process gas and the sulfuric acid withdrawn from the bottom of the acid condenser become concentrated and suitable for use in the acid leaching process. The cooled off gas typically leaves the condenser at 60-120 °C, which allows for a substantial amount of water vapor to leave with the cooled off gas while ensuring complete condensation of sulfuric acid va- por. The indirect cooling of the process gas is typically car- ried out by air or other gases flowing on the outside of the vertical glass tubes and the hot cooling air from the sulfuric acid condenser can e.g. be used as combustion air in the drying and heating step, saving fuel in that step, and/or as conditioning air to the drying and heating off gas. In a specific layout of the vertical falling film conden- ser, a dense demister will be located at the top of the condenser tube to collect and return acid droplets to the falling film condenser. In the case with substantial amounts of dust in the process gas, there is a risk that such dense demisters will plug with solids, although the liquid flow in the demisters should wash away the solids. In such a case, a loose demister will be used to minimize the risk of dust collecting in the demister. Typically, the demisters are made of knitted metal or plas- tic wire with voids in excess of 95%. For high efficiency demisters, the void is lower to allow for higher collection area, with the cost of higher pressure drop and higher risk of plugging of the demister, should there be solids in the gas. In principle, the demisters can be omitted but in that case the acid droplet emission from the falling film condenser can be substantial. In a special layout of a sulfuric acid condenser, spray nozzles can be installed above the demister(s) to provide for intermittent washing of the demisters, should solids build up. The falling film condenser will typically have so-called nucleation control to minimize formation of sulfuric acid mist, i.e. submicron sulfuric acid droplets, which are very difficult to collect in the demisters. The nucleation con- trol provides particles to the process gas, e.g. by combus- tion of silicon oil, such that the sulfuric acid vapor has available surfaces for condensation, efficiently suppress- ing acid mist formation. With dust particles already present in the hot off gas, the nucleation control may be superfluous and thus it can be omitted, simplifying the condensation process. Typically the amount of particles are preferably in the range 10¹⁰ to 10¹³ particles/Nm³ process gas/1 vol% H₂SO₄, which will ensure a balance between the number of particles and the amount of condensable H₂SO₄, forming droplets with an appropriate size for collection in demisters, and the similar consequence of few seed parti- cles causing spontaneous formation of many small droplets which are only collected in demisters with low efficiency. To minimize the consumption of silicon oil and the contami- nation of product acid, the size of seed particles is de- sired to be less than 0.1 µm. However, if the acid is in- tended to be recycled in the acid leaching process forming the dust, a contamination with dust originating from the acid leaching process will not pose a problem, and the tol- erance for contamination will be higher. Inevitably, there will be some sulfuric acid mist in the cooled off gas leaving the sulfuric acid condenser and a suitable acid mist removal device is preferably installed to ensure that the cleaned off gas emitted to the atmos- phere is in compliance with environmental legislation. De- misters of different layouts, Brink type demister candles and wet electrostatic precipitators may be used for acid mist removal. For the present application with dust in the off gas, a wet electrostatic precipitator will be most ro- bust against plugging with dust particles and is the pre- ferred technology, but Brink type demister candles, espe- cially if configured with a cleaning mechanism, can also be used. Downstream the acid mist removal device, the cleaned and cooled off gas can be send to other gas cleaning units, such as a SO₂ scrubber, and/or a blower before the cleaned gas is emitted to the atmosphere through a stack. The gas cleaning unit can also be located between the acid condensation unit and the acid mist removal device. The sulfuric acid condenser can also be provided with hori- zontal tubes in which the process gas flow is on the out- side of the tubes and the cooling media flow is inside the tubes. As nucleation control does not work very well under such conditions, the off gas from such a condenser may re- quire an acid mist removal device and typically it is an integrated part of such a sulfuric acid condenser design. This design is robust against dust particles in the process gas. The produced sulfuric acid concentration will be lower than from the above described vertical falling film conden- ser, but still sufficiently high for recirculation to the acid leaching process. The sulfuric acid condenser can also be a packed bed type condenser, where the process gas is cooled and the sulfuric acid is condensed in a counter current flow with relatively hot concentrated sulfuric acid. Acid mist control does not work under these conditions and it will be possible to con- figure the packed bed condenser to provide a sulfuric acid concentration suitable for the recirculation to the acid leaching process. The choice of construction materials is very limited as the hot concentrated acid will be very cor- rosive to metals and thus brick lining and/or fluoropolymer lining may be needed. Figures Figure 1 show a flow sheet of a process plant with the quench type condenser according to the established technol- ogy. Figure 2 shows a flow sheet of a process plant according to a preferred embodiment of the present disclosure. In Figure 1, metal ore (1) is fed to the digestion unit (3) and mixed with sulfuric acid from a combination of fresh concentrated acid (2) and sulfuric acid (38) recycled from the quench unit (34). The formed acidic ore slurry (4) is transferred 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). The drying and heating is preferably carried out by combusting a fuel (6) with combustion air (9). The off gas from the drying and heating unit (10), contain- ing sulfuric acid vapor, water vapor and a high amount of dust, such as up to 50 g/Nm³ is passed to an optional de- dusting unit (15), such as an electrostatic separator, cy- clone, or a candle filter, separating the majority of the dust in the off gas into a solid stream (16) and a par- tially dedusted off gas (17). The partially dedusted off gas (17) enter the quench unit (34), where the gas is cooled and the sulfuric acid and wa- ter vapor is partially condensed to form a sulfuric acid product (36). The cooling can be carried out by a combination of direct addition of water for evaporative cooling (42) and/or cooling water/air (43 inlet and 44 out- let) in the circulation system of the quench unit. The sulfuric acid product (36) is split up in a fraction, which is recycled back to the acid leaching process (38) and a fraction (40), which can be used in other processes and/or will be send to a sulfuric acid concentration unit. The off gas from the quench unit (46) will contain some sulfuric acid mist, which is separated in the wet electro- static precipitator (48), forming a dilute sulfuric acid stream (50) and a clean off gas (52), which can be send to the atmosphere or other gas cleaning units. In Figure 2, metal ore (1) is fed to the digestion unit (3) and mixed with sulfuric acid from a combination of fresh concentrated acid (2), concentrated sulfuric acid recycled from the sulfuric acid condensation unit (22) and option- ally dilute sulfuric acid from the acid mist removal unit (28). The formed acidic ore slurry (4) is transferred 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). The drying is preferably carried out by combusting a fuel (6) with combustion air, preferably hot air from the acid condensation unit (7). The off gas from the drying and heating unit (10), contain- ing sulfuric acid vapor, water vapor and a high amount of dust, such as up to 50 g/Nm³, is optionally mixed with hot air (12) and the optionally diluted drying and heating off gas (14) is passed to an optional dedusting unit (15), such as an electrostatic separator, cyclone or a candle filter, separating the majority of the dust in the off gas into a solid stream (16) and a partially dedusted off gas (17) comprising less than 5000 mg/Nm³ dust. To maintain a temperature above the sulfuric acid dew point, the dedusted off gas (17) is optionally diluted with hot air via line 18 and the diluted dedusted off gas (20) is then directed to a sulfuric acid condensation unit (22). Concentrated sulfuric acid from the acid condensation unit is withdrawn through line 24, cooled (not shown) and recy- cled to the digestion unit (3). 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 is leaving the acid condensation unit via line 34. A fraction of the heated cooling air is directed back to upstream processes via line 36 and the unused heated cooling air is vented via line 35. The recycled heated cooling air (36) is optionally heated (or cooled) in heat exchanger 38, before the temperature conditioned cooling air is directed to one or more of air streams 7, 12 and 18. Depending on the process require- ments, the heat exchanger may be positioned on the individ- ual air streams. If found more beneficial, the heat ex- changer can be replaced with other means for heating, e.g. a fuel-fired or electrical support heater. The off gas from the condensation unit (26) is send to a sulfuric acid demisting unit (28), in which 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 emitted to the atmos- phere, optionally by passing through other gas cleaning units and/or a process gas blower (not shown). Examples This example compares the operating conditions for the basic quench type solution with the operating conditions for the sulfuric acid condensation process according to the invention. In this example, Table 1 shows operating data for basic quench designs according to the prior art and a proposed design with a sulfuric acid condensation unit. The process gas composition is 4.1 vol% SO₃, 42 vol% H₂O, 300 °C, at- mospheric pressure and 1,000 ppmv sulfuric acid in the off gas from the quench and condensation unit, where the off gas from the drying and heating unit has a flow of 26,000 Nm³/h. The sulfuric acid dew point temperature of this pro- cess gas is around 270°C. In the design of the inventive layout, 5,000 Nm³/h heated dilution air is added to lower the sulfuric acid dew point temperature to protect construction materials in the sulfu- ric acid condensation unit. In Example 1, the basic quench type solution, the off gas from the drying and heating unit is cooled by direct injec- tion of water for evaporative cooling and in Example 2, by means of circulating sulfuric acid, cooled by cooling water in the circulation loop. In Table 1, the basic operating parameters for the different process layouts are given. It is clear from the data, that the quench type operation will produce sulfuric acid with a lower concentration than with the sulfuric acid condensation unit, more than dou- bling the produced volume of intermediate strength sulfuric acid in Examples 1 and 2 compared to the concentrated sul- furic acid of Example 3. The concentration range 40-50 wt% H₂SO₄ is commonly too weak to provide an efficient leaching process and thus addition of concentrated acid is required, such that it will not be possible to recycle the entire amount of acid to the upstream process. Furthermore, this low concentration has no use on the market for commercial acid, where minimum 93 wt% H₂SO₄ is required. It is conservatively assumed that 1,000 ppm_v acid mist (100wt% H₂SO₄) in the off gas will be found from the pri- mary acid production units. The very dilute acid from the wet ESP will significantly decrease the mixed acid concen- tration from the two production units. However, the 69 wt% H₂SO₄ from the inventive layout will still be sufficiently high for the acid leaching process and the entire acid pro- duction can be recycled, thus eliminating a possible issue with a “waste acid” production. With regard to process gas cooling, the basic quench with evaporative cooling is quite efficient as no external cool- ing is required in the quench. The water consumption is 2,050 kg/h and most of this water will be found in the sul- furic acid product, thus lowering the sulfuric acid concen- tration and increasing the mass flow. Using only cooling water in the quench and assuming a 10 °C temperature increase of the cooling water the flow of cool- ing water will be 687,000 kg/h, i.e. a very large flow. The sulfuric acid concentration will be slightly higher than for the direct water quench. The sulfuric acid condensation unit of Example 3 requires atmospheric air as cooling media; around half of the cool- ing air flow can be used as dilution air before the conden- sation step and combustion air in the drying and heating step. The latter will decrease the need for fuel in the drying and heating unit.

Claims

Claims: 1. A process for the production of sulfuric acid from a process gas comprising 1-10 vol% SO₃ and 10-70 vol% H₂O on unhydrated basis, as well as 1-5000 mg/Nm³ of partic- ulate solids having a diameter above 1 µm, said process comprising the following steps: a) condensing sulfuric acid from the process gas by indi- rect heat exchange with a cooling medium, producing a stream of concentrated sulfuric acid, a desulfurized process gas containing sulfuric acid mist and an amount of heated cooling medium, b) removing sulfuric acid mist from the desulfurized pro- cess gas in an acid mist removal step, in order to pro- duce a stream of dilute sulfuric acid and a cleaned gas, c) recycling at least an amount of concentrated sulfuric acid to an upstream process, such as mineral ore acid leaching, optionally together with an amount of said di- lute sulfuric acid.

2. A process according to claim 1 further comprising the step of passing a dust loaded process gas comprising 1-50 g/Nm³ of particulate solids through a dedusting unit, such as an electrostatic precipitator, a candle filter or a cyclone, producing a stream of solids and said process gas.

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

4. A process according to claim 1, 2 or 3 in which the process gas contains 2-6 vol% SO₃ and 20-50 vol% H₂O on unhydrated basis.

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

6. A process according to claim 1, 2, 3 or 4 in which condensing the sulfuric acid is carried out in an acid condenser where process gas flow outside sub- stantially horizontal glass tubes and cooling medium inside the substantially horizontal glass tubes to separate process gas and cooling medium wherein the process gas inlet temperature is 220-300 °C and the process gas outlet temperature is 70-120 °C.

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

8. A process according to claim 1, 2, 3, 4, 5, 6 or 7 in which the sulfuric acid condensation unit is equipped with a plugging resistant 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 in which the heated cooling medium from the sulfuric acid condensation unit is 160-260 °C and is recycled to one or more upstream processes requiring combus- tion air and/or dilution air.

10. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9 in which seed particles are added to the process gas to achieve a total amount of dust particles and added seed particles in the range 10¹⁰ to 10¹³ parti- cles/Nm³ process gas/1 vol% SO₃ on unhydrated basis.

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

13. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 in which at least 20 wt%, 50 wt%, 80 wt% or 90 wt% of said particulate solids having a di- ameter above 1 µm are compounds comprising one or more metals taken from the group of Ni, Zn, U, Cu, Mo, Fe, Li, Au and rare earth metals, such as sulfate salts.

14. A process plant for carrying out the process accord- ing to any claim above.