Use of forced circulation reboiler for hydrofluoric acid generation

Abstract

The present disclosure relates to an improved method to produce hydrofluoric acid from an aqueous sulfuric acid–hydrofluoric acid solution. Aspects of the disclosure are directed to a method where forced circulation under pressure is used to generate a hot liquid stream that is vaporized to produce a hydrofluoric acid vapor–water vapor stream comprising essentially only water and HF. Additionally, there may be a liquid recycle.

Description

USE OF FORCED CIRCULATION REBOILER FOR HYDROFLUORIC ACID GENERATIONTechnical Field

[0001] The present invention generally relates to an improved method to produce hydrofluoric acid from a fluorosilicic acid mixture. In particular, and without limitation, the present invention relates to a method where forced circulation under pressure can be used to vaporize a mixture / solution comprising hydrofluoric acid, sulfuric acid and water, which is produced from a fluorosilicic acid mixture. The present disclosure seeks to reduce the extent to which physical damage, corrosion and / or erosion which may occur in the heat exchangers and / or other vessels / piping / equipment used in the hydrofluoric acid reaction process.Background

[0002] Previous methods for producing hydrofluoric acid from a fluorosilicic acid mixture used heat exchangers to heat and vaporize a mixture / solution comprising hydrofluoric acid, sulfuric acid and water, which is produced from a fluorosilicic acid mixture as part of a reaction process, as distinct from a standalone distillation and / or purification process. Additionally, small amounts of hydrochloric acid, silicon tetrafluoride and other low boilers may be present. In these prior methods, the heat exchangers used were costly and had a high rate of failure because the reactant stream was unstable, hot and highly corrosive. Specifically, excessive and unintended boiling is known to cause short equipment life in the heat exchangers. A focus of the present disclosure is forced circulation under pressure vaporizing the hydrofluoric acid, sulfuric acid, and water solution across a pressure let-down device or pressure drop. As a result, boiling in the heat exchanger system is suppressed by evaporation under pressure, allowing increased lifetime for the heat exchanger components.Summary

[0003] The inventors have discovered an improved method of producing hydrofluoric acid from a solution comprising hydrofluoric acid, sulfuric acid, and water, which is produced from a fluorosilicic acid mixture. In one embodiment, the present disclosure is directed to a method for producing (separating out) hydrofluoric acid from a solution comprising hydrofluoric acid, sulfuric acid, and water, which is produced from a fluorosilicic acid mixture, the method comprising: providing an aqueous stream comprising hydrofluoric acid and sulfuric acid to a flash tank comprising a gas outlet and a liquid outlet; before reaching the flash tank, heating thehydrofluoric acid-sulfuric acid mixture to a temperature ranging from 140°C to 220°C under enough pressure to maintain the mixture in an essentially all liquid form; reducing the pressure of the aqueous hydrofluoric acid-sulfuric acid stream across a pressure reducer with the resulting stream going tothe flash tank; vaporizing at least a portion of the hydrofluoric acid-sulfuric acid in the flash tank; and sending the hydrofluoric acid vapor-water vapor stream to an absorber.

[0004] As used herein, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise.Brief Description of the Drawings

[0005] The accompanying drawings, which are incorporated in and comprise a part of this specification, illustrate several disclosed embodiments and, together with the description, may help to explain the principles disclosed herein.

[0006] FIGURE 1 shows a diagram of an HF generation process.

[0007] FIGURE 2 shows a diagram of an HF generation process with recycle.

[0008] The drawings are not necessarily to scale. Certain dimensions, for example, may be exaggerated for purposes of clearer illustration.Detailed Description of the Illustrative Embodiments

[0009] Generally to begin the process, an aqueous fluorosilicic acid mixture, for example, a solution or an at least partially miscible mixture, enters a first reactor and is mixed with sulfuric acid. The sulfuric acid-fluorosilicic acid mixture can occur at a temperature ranging from 70°C to 150°C, such as, for example, from 80°C to 140°C, from 80°C to 130°C, from 90°C to 130°C, from 90°C to 120°C, or from 90°C to 110°C. In particular, the mixture can be at a temperature ranging from 80°C to 130°C, or at a temperature ranging from 90°C to 110°C. In further embodiments, the mixture occurs at about 100°C-105°C.

[0010] In the first reactor the reaction: H2SiF62HF + SiF4 can occur. A gas outlet from the first reactor can be sent to an absorption tower wherein water and hydrofluoric acid (HF) are scrubbed by sulfuric acid, which go back to the first reactor, and the gas phase containing SiF4 (silicon tetrafluoride or STF) can be sent back to hydrolysis reactors, wherein the SiF4 is converted to fluorosilicate acid by the reaction: 3SiF42H2SiF6 + SiO2. The SiO2 can be removed by filtration and the fluorosilicate mixture can be sent back to the first reactor. In the hydrolysis reactors, fresh fluorosilicate mixture can be fed as a feed stock.

[0011] The first reactor (which can additionally function as a separator) also comprises a liquid outlet, which can pump the resulting liquid H2SO4, H2O and HF mixture to a reboiler system. At this point the liquid is essentially all liquid, as it is the liquid stream from the reactor. It may contain small amounts of low boiling components, for example but not limited to STF, HC1 or air.

[0012] In the reboiler system, steam or other heating media can be applied using one or more heat exchangers to raise the temperature of the aqueous H2SO4 and HF mixture above its atmospheric pressure vaporization temperature. In one embodiment the stream is heated in one heat exchanger to a temperature ranging from 100°C to 250°C, such as, for example, from 100°C to 220°C, from 110°C to 200°C, from 110°C to 190°C, from 115°C to 180°C, from 120°C to 180°C, from 120°C to 170°C, from 120°C to 160°C, from 125°C to 180°C, from 125°C to 170°C, from 125°C to 160°C, from 130°C to 180°C, or from 130°C to 175°C. The temperature at which the operation occurs can depend on the FSA concentration that was employed to the first reactor, among other things.

[0013] In certain embodiments, the aqueous H2SO4 and HF mixture can be heated in one or more heat exchangers. The first heat exchanger can heat to a temperature ranging from 100°C to 250°C, such as, for example, from 100°C to 220°C, from 110°C to 200°C, from 110°C to 190°C, from 115°C to 180°C, from 120°C to 180°C, from 120°C to 170°C, from 120°C to 160°C, from 125°C to 180°C, from 125°C to 170°C, from 125°C to 160°C, from 130°C to 180°C, or from 130°C to 175°C. If present, a second heat exchanger can heat to a temperature ranging from 120°C to 250°C, such as, for example, from 120°C to 200°C, from 120°C to 140°C to 180°C, from 150°C to 250°C, from 150°C to 220°C or from 150°C to 200°C.

[0014] Included herein is the use of a plurality of heat exchangers each one getting subsequently hotter. In certain embodiments, the heat exchangers collectively heat to a temperature ranging from 150°C to 250°C, such as from 150°C to 230°C, from 150°C to 220°C, from 150°C to 200°C, from 150°C to 180°C, from 170°C to 250°C, from 170°C to 230°C, from 170°C to 220°C, from 170°C to 200°C, from 190°C to 250°C, from 190°C to 230°C, from I 9O°C to 220°C, from 200°C to 250°C, or from 200°C to 230°C. The required temperature can depend on the concentration of FSA sent to the first reactor and the pressure of the process stream in the heat exchangers, among other things.

[0015] In certain embodiments of the reboiler system, the heating can be done under pressure, such as, for example, a pressure ranging from 0 bar to 10 bar. Nonlimiting examples include pressure ranging from 0 barg to 10 barg, from 0 barg to 9 barg, from 0 barg to 8 barg, from 0 barg to 6 barg, from 0 barg to 4 barg, from 0.5 barg to 10 barg, from 0.5 barg to 9 barg, from 0.5 barg to 8 barg, from 0.5 barg to 6 barg, from 0.5 barg to 4 barg, from 1 barg to 10 barg, from 1 barg to 9 barg, from 1 barg to 8 barg, from 1 barg to 6 barg, from 1 barg to 4 barg, from 1.5 barg to 10 barg, from 1.5 barg to 9 barg, from 1.5 barg to 8 barg, from 1.5 barg to 6 barg, from 1.5 barg to 4 barg, from 2 barg to 10 barg, from 2 barg to 9 barg, from 2 barg to 8 barg, from 2 barg to 6 barg, from 2 barg to 4 barg, from 3 barg to 10 barg, from 3 barg to 9 barg, from 3 barg to 8 barg, from 3 barg to 6 barg, from 4 barg to 10 barg, from 4 barg to 9 barg, from 4 barg to 8 barg, or from 4 bar to 6 bar. In particular, the pressure can range from 0 barg to 6 barg, from 0.5 barg to 10 barg, from 1 barg to 9 barg, or from 1.5 barg to 8 barg. In all cases the solution is heated under pressure to ensure the liquid stream is (remains) essentially all liquid, meaning the stream is at least 97 vol% liquid, such as, for example, at least 98 vol% liquid, at least 99.0 vol% at least 99.5 vol%, at least 99.7 vol% or at least 99.9 vol% liquid (and thus up to 100 vol% liquid), based on the mass of the stream.

[0016] The heated aqueous H2SO4 and HF mixture can then be flashed to a downstream pressure across one or more of the following: a flashing valve, an orifice plate, a restriction in piping, a spray nozzle, and / or other such device to cause a decrease in pressure in order to release pressure, thus vaporizing a portion of the mixture. Although it can be preferred that the flashed (pressure-reduced, and typically also temperature-reduced) stream is piped to a vessel (such as a flash tank), it is not required.

[0017] The pressure differential between the pressure in the heating system and the downstream pressure can range from 0.5 to 50 psi, such as, for example, from 0.5 to 40 psi, from 0.5 to 30 psi, from 0.5 to 20 psi, from 0.5 to 15 psi, from 0.5 to 10 psi, from 0.5 to 5 psi, from 1 to 50 psi, from 1 to 40 psi, from 1 to 30 psi, from 1 to 20 psi, from 1 to 15 psi, from 1 to 10 psi, from 1 to 5 psi, from 2 to 50 psi, from 2 to 40 psi, from 2 to 30 psi, from 2 to 20 psi, from 2 to 15 psi, from 2 to 10 psi, from 2 to 5 psi, from 3 to 50 psi, from 3 to 40 psi, from 3 to 30 psi, from 3 to 20 psi, from 3 to 15 psi, from 3 to 10 psi, from 5 to 20 psi, from 5 to 15 psi, from 5 to 10 psi, from 10 to 50 psi, from 10 to 40 psi, from 10 to 30 psi, from 15 to 50 psi, from 15 to 40 psi, or from 15 to 30 psi.

[0018] Additionally or alternatively, the temperature of the stream after flashing can range from 125°C to 200°C, such as from 130°C to 190°C, from 130°C to 180°C, from 130°C to 170°C, from 130°C to 160°C, from 135°C to 190°C, from 135°C to 180°C, from 135°C to 170°C, from 135°C to 160°C, from 140°C to 190°C, from I 4O°C to 180°C, from 140°C to 170°C, from 140°C to 160°C, from 145°C to 180°C, from 145°C to 170°C, from 145°C to 160°C, from 150°C to 180°C, from 150°C to 170°C, or from 150°C to 160°C. In particular, the temperature differential between the heat exchanged (heated) stream and the flashed stream can be at least 5°C or at least 10°C, and typically not more than 90°C or not more than 70°C. Again, while the disclosure refers to gas and liquid “streams” post-flashing, it should be understood that could equally refer to a composition in a vessel, such as a flash tank, part of which is in a gas phase and part of which is in a liquid phase.

[0019] The gas stream from the flash can be sent to a flash vessel and the resulting hydrofluoric acid vapor-water vapor stream is sent to an absorber, wherein sulfuric acid can be used to remove the water, and some HF, which flows or is pumped back to the first reactor. The gaseous HF resulting from the absorber can be sent for final purification. The amount of sulfuric acid in the hydrofluoric acid vapor-water vapor stream is less than 0.1% by weight, such as, for example, less than 0.01% by weight.

[0020] The liquid stream staying liquid after the pressure drop can be sent to a stripper column to remove the HF from this stream. The tails of the stripper column can be sent through a cooler, and then can be sent to optionally three different places: back to the stripper column to cool the stripper column receiver, send to the drying column to remove water from the HF rich gas stream, and sent for disposal or to another process that can use the diluted sulfuric acid, e.g., a phosphoric acid production unit.

[0021] The stripper column can use both steam and air (or alternatively either one) or nitrogen or other inert gas to remove residual HF from the liquid stream that came from the second reactor (flash tank). The gaseous stream from the stripper, containing HF, water and air, can be sent to a drying column which used sulfuric acid, optionally sent from the cooled tails of the stripper column, to remove water. The water-sulfuric acid stream can be sent back to the first reactor.

[0022] The gaseous stream from the drying column can be sent to an absorber wherein sulfuric acid can absorb the HF from a stream of mostly air and other noncondensables resulting from theheads of the absorber. This stream can be sent directly or indirectly to the scrubber. The sulfuric acid stream, containing HF, can be sent to the HF generator absorber as the sulfuric acid inlet stream.

[0023] FIGURE 1 depicts a process in accordance with the present disclosure. The aqueous H2SO4 and HF mixture (solution) can originate from one or more reactors / separators (not shown) and can be pumped (also not shown) to the heat exchangers. Despite the plural reference in FIGURE 1, this may include one or a plurality of heat exchangers (if plurality, in series and / or in parallel). The stream is pumped through this heat exchanger, which uses steam to heat the aqueous H2SO4 and HF mixture stream. The exiting aqueous H2SO4 and HF mixture stream is essentially all liquid and is sent to a pressure reducing device. In this descriptive figure, the pressure reducing device is a pressure control valve. The pressure and / or temperature of the stream is reduced across this valve and the stream is sent to the flash tank. Also not shown are the draws of gas / vapor stream and liquid stream from the flash tank, nor the subsequent treatment of the components / intermediate products to form final products.

[0024] Additionally or alternatively, it may be desired to increase control over the HF-forming capacity of the reaction system and / or the heating (and pressure) duty(ies) throughout the system, either or both which can occur by increasing the flow rate / volume of aqueous H2SO4 and HF mixture coming from the first reactor. Such increased flow can generally result in the heat exchanger(s) heating to a lower temperature than in FIGURE 1, which can also affect the pressure (and / or temperature) drop(s) at flash, in order to create the gas and liquid portions.

[0025] With such additional flow, most or essentially all of which remains liquid, also would come increased load / duty on the stripping column. However, if the stripping column is at optimal or desired capacity with the liquid stream and / or if it is desired to maintain an advantageous mass balance throughout the reaction system, it is possible for a portion of the liquid stream to be recycled, such as to the first reactor and / or directly to the flow (being pumped) into the heat exchanger(s), with the remaining portion of the liquid stream still being sent to the stripping column.

[0026] Therefore, FIGURE 2 depicts a process in accordance with the present disclosure. Similarly to FIGURE 1, the aqueous H2SO4 and HF mixture (solution) can originate from one or more reactors / separators and can be pumped to the heat exchanger(s), which use(s) steam to heat the aqueous H2SO4 and HF mixture / stream. The heated aqueous H2SO4 and HF mixture / stream,which is essentially all liquid is sent to a pressure-reducing device (pressure control valve). The pressure and / or temperature is dropped as entering the flash tank, which causes at least part of the flashed stream to become gaseous, with the remainder to remain essentially liquid, which can be passed to the stripping column.

[0027] However, in FIGURE 2, different from FIGURE 1, the increased flow (from about 110% to about 250% of that provided in FIGURE 1) can cause additional essentially liquid phase material, in this case more than can be, or is desired to be, handled by the stripping column. That additional essentially liquid phase material can be recycled to the first (STF) reactor and / or directly to the flow being pumped into the heat exchanged s), optionally but preferably with an advantaged material balance for the overall reaction system.

[0028] Without being bound by theory, this increased flow is believed to reduce the temperature to which the aqueous H2SO4 and HF mixture / stream is heated in the heat exchanger(s), which can, over time, significantly increase lifetime, such as due to reduced corrosion, in said heat exchange^ s). Additionally or alternatively, again without being bound by theory, the increased flow and / or the recycle may advantageously reduce the heat and / or pressure differentials necessary within the system, thereby improving efficiency.

[0029] The following examples are directed to potential embodiments of the present disclosure. The flows and concentrations are not limiting and are representative of the general concepts of the present disclosure. All percentages given in these examples are weight percents, unless otherwise specified.Example 1 (baseline flow)

[0030] Example 1 represents a simulation. In this Example, a stream containing ~42wt% fluorosilicic acid in water flowed to a first reactor. Sulfuric acid was also flowed to the first reactor by the tails of the STF generation column (also contained the water that is vaporized in the first reactor), and the tails of the HF generator column (also contains HF and water). The streams were mixed in the first reactor at about 100°C, which allowed the following reaction to take place: H2SiF6 -> 2HF + SiF4. The vapor from the first reactor was sent to the STF generation column, which removed water from the gaseous stream, allowing essentially pure STF gas to be sent to the concentration section of the reactor for hydrolysis to convert the STF recycle to H2SiF6.

[0031] The liquid outlet of the first reactor / separator, with optionally a sulfuric acid stream resulting from the drying column, containing some or all of the liquid within the first reactor, was pumped to the reboiler system. This stream was indirectly heated using one or more heat exchangers. The aqueous H2SO4 and HF mixture was heated above its atmospheric vaporization temperature. Backpressure was applied via a flashing valve to ensure that essentially no boiling occurred in the reboiler system.

[0032] Steam increased the temperature of the aqueous sulfuric acid- hydrofluoric acid system to about 220°C (although this could be alternatively accomplished with another heat transfer fluid). This liquid stream was flashed across a flashing valve and was sent to a flash vessel for separation and the vapor phase to an HF generation absorption column. In the HF generation column sulfuric acid absorbed water from the HF allowing dry HF to be produced, which then went to the purification operations.

[0033] The liquid stream from the reboiler vessel contains sulfuric acid, water and -1-5% HF. This HF was stripped by steam and air in a stripper column. The tails of the stripper were cooled, some sent to the stripper receiver to cool it, some sent to the drying column and an amount equal to the sulfuric acid and water, from the fluorosilicic acid solution and steam, flows which came into the process were sent as waste to either waste treatment or to a customer, e.g., a phosphoric acid producer, who can use it.

[0034] The gaseous stream from the stripper was dried by sulfuric acid. Optionally, depending on the size and temperature of the purification operation, HF may be recycled to the drying column. The water in the gaseous stream was removed by the cooled sulfuric acid-water mix from the tails of the stripper and sent back to the reboiler system by the pumping system. The HF from the HF-air stream from the heads of the drying column was sent to a final absorber which had sulfuric acid to recover the HF and allowed essentially HF-free air and other noncondensables, to pass through the vent system to the scrubber. The HF-sulfuric acid stream was sent to the HF generator column to recover the HF and to allow the sulfuric acid to flow to the first reactor.Example 2 (effect of extra flow)

[0035] Example 2 represents a simulation. In this Example, a stream containing ~42wt% fluorosilicic acid in water flowed to a first reactor. Sulfuric acid was also flowed to the first reactor by the tails of the STF generation column (also containing the water that was vaporizedin the first reactor), and the tails of the HF generator column (also containing HF and water). The streams were mixed in the first reactor at about 100°C to 105°C, which allowed the following reaction to take place: H2SiF6 -> 2HF + SiF4. The vapor from the first reactor was sent to the STF generation column, which removed any water from the gaseous stream, allowing essentially pure STF gas to be sent to the concentration section of the reactor for hydrolysis to convert the STF recycle to H2SiF6.

[0036] The liquid outlet of the first reactor / separator, with optionally a sulfuric acid stream resulting from the drying column, containing some or all of the liquid within the first reactor, was pumped to the reboiler system. In the Example 2 simulation, the amount (flow rate / volume) of liquid was increased between 110% and 250% over that in Prophetic Example 1 (representing a 10% to 150% increase). By way of the example, the stream flowrate was increased by 150%. This stream was indirectly heated using one or more heat exchangers, the aqueous H2SO4 and HF mixture was heated above its atmospheric vaporization temperature and backpressure applied via a flashing valve to ensure that essentially no boiling occurred in the reboiler system.

[0037] Steam increased the temperature of the aqueous sulfuric acid- hydrofluoric acid system to a temperature from about 160°C (although this could be alternatively accomplished with another heat transfer fluid). This reduced temperature can be due specifically to the additional liquid flow through the heat exchangers, relative to simulated Example 1. This gaseous stream was flashed across a flashing valve and was sent to a flash vessel for separation and the vapor phase to an HF generation absorption column. In the HF generation column sulfuric acid absorbed water from the HF allowing dry HF to be produced, which then went to the purification operations.

[0038] The liquid stream from the reboiler vessel contains sulfuric acid, water and -1-5% HF. A portion of this liquid stream had its HF stripped by steam and air in a stripper column. The tails of the stripper were cooled, some sent to the stripper receiver to cool it, some sent to the drying column and an amount equal to the sulfuric acid and water, from the fluorosilicic acid solution and steam, flows which came into the process were sent as waste to either waste treatment or to a customer, e.g., a phosphoric acid producer, who can use it. The excess portion of the liquid stream was recycled directly back to the first reactor and / or to directly to the stream being pumped to the heat exchangers.

[0039] The gaseous stream from the stripper was dried by sulfuric acid. Optionally, depending on the size and temperature of the purification operation, HF may be recycled to the drying column. The water in the gaseous stream was removed by the cooled sulfuric acid-water mix from the tails of the stripper and sent back to the reboiler system by the pumping system. The HF from the HF-air stream from the heads of the drying column was sent to a final absorber which had sulfuric acid to recover the HF and allowed essentially HF-free air and other noncondensables, to pass through the vent system to the scrubber. The HF-sulfuric acid stream was sent to the HF generator column to recover the HF and to allow the sulfuric acid to flow to the first reactor.

Claims

CLAIMS:What is claimed is:

1. A method for producing hydrofluoric acid comprising: providing an aqueous stream comprising hydrofluoric acid and sulfuric acid to a flash tank comprising a gas outlet and a liquid outlet; heating the hydrofluoric acid-sulfuric acid mixture to a temperature ranging from110°C to 220°C under enough pressure to maintain the mixture in an essentially all liquid form; reducing the pressure of the aqueous hydrofluoric acid-sulfuric acid stream across a pressure reducer with the resulting stream going to a flash tank; vaporizing a portion of the hydrofluoric acid-sulfuric acid in the flash tank; and sending the hydrofluoric acid vapor-water vapor stream to an absorber.

2. The method according to claim 1, wherein the temperature of the aqueous sulfuric acid-hydrofluoric acid stream is heated under pressure from 140°C to 220°C.

3. The method according to claim 1, wherein the pressure of the aqueous sulfuric acid- hydrofluoric acid stream is maintained to ensure that the stream is essentially all liquid.

4. The method according to claim 3, wherein the pressure of the aqueous sulfuric acid- hydrofluoric acid stream is between 0.5 and 10 barg.

5. The method according to claim 1 wherein the stream is at least 99% liquid based on volume.

6. The method according to claim 1, wherein the pressure drop between the pressurized liquid stream and the flash tank is greater than 1 bar.

7. The method according to claim 6, wherein the pressure drop between the pressurized liquid stream and the flash tank is greater than 1.5 bar.

8. The method according to claim 1, wherein pressure drop is taken by one or more of a flashing valve, an orifice plate, a restriction in piping and a spray nozzle.

9. The method according to claim 1, wherein the hydrofluoric acid vapor-water vapor stream comprises essentially all water and HF.

10. The method according to claim 9, wherein the hydrofluoric acid vapor-water vapor stream comprises less than 0.1% sulfuric acid based on weight.

11. The method according to claim 10, wherein hydrofluoric acid vapor-water vapor stream comprises less than 0.01% sulfuric acid based on weight.

12. The method according to claim 1 , wherein the hydrofluoric acid vapor-water vapor stream comprises between 37% and 100% HF, with the balance being water by mass percentage.

13. A method for producing hydrofluoric acid comprising: providing an aqueous stream comprising hydrofluoric acid and sulfuric acid from a reaction vessel to a separation vessel comprising a gas outlet and a liquid outlet; heating the hydrofluoric acid-sulfuric acid mixture to a temperature ranging from110°C to 220°C under enough pressure to maintain the mixture in an essentially all liquid form; reducing the pressure of the aqueous hydrofluoric acid-sulfuric acid stream across a pressure reducer with the resulting stream going to a separation vessel; vaporizing a portion of the hydrofluoric acid-sulfuric acid in the flash tank to form a gaseous stream comprising hydrofluoric acid vapor and water vapor and a liquid stream comprising hydrofluoric acid, sulfuric acid, and water; sending the hydrofluoric acid vapor-water vapor stream to an absorber; sending a portion of the liquid stream to a stripper; and recycling the remaining portion of the liquid stream to the reaction vessel and / or to the hydrofluoric acid-sulfuric acid mixture before it is heated.

14. The method according to claim 13, wherein the temperature of the aqueous sulfuric acid-hydrofluoric acid stream is heated under pressure from 115°C to 180°C.

15. The method according to claim 13, wherein the pressure of the aqueous sulfuric acid- hydrofluoric acid stream is maintained to ensure that the stream is essentially all liquid.

16. The method according to claim 15, wherein the pressure of the aqueous sulfuric acid- hydrofluoric acid stream is between 0.5 and 10 barg.

17. The method according to claim 13, wherein the heated hydrofluoric acid-sulfuric acid is at least 99% liquid based on the volume.

18. The method according to claim 13, wherein the pressure drop between the pressurized liquid stream and the flash tank is greater than 0.5 bar.

19. The method according to claim 13, wherein pressure drop is taken by one or more of a flashing valve, an orifice plate, a restriction in piping and a spray nozzle.

20. The method according to claim 13, wherein the hydrofluoric acid vapor-water vapor stream comprises less than 0.01 sulfuric acid based on weight.

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