Process for acid regeneration

Abstract

The invention relates to a method for obtaining or recovering acids from a metal-containing solution (13) by subjecting the solution to a thermal hydrolysis treatment and then absorbing and / or condensing the gaseous acids thus formed. The metal-containing solution (13) is fed to a heated reaction chamber (1) in which the metal-containing solution (13) is evaporated and pyrolyzed into acids and metal oxides (12). According to the invention, a heat transfer medium (11), which consists partially or entirely of water vapor, is supplied to the reaction chamber (1) for heating, by means of which heat transfer medium the metal-containing solution (13) is heated to the temperature required for thermal hydrolysis.

Description

Field of the Invention

[0001] The subject matter of the present invention is to provide a method for obtaining or recovering an acid from a metal-containing acid solution by subjecting the solution to a pyrohydrolysis treatment and then absorbing and / or condensing the gaseous acid thus formed, wherein the metal-containing solution is added to a heated reaction chamber in which the metal-containing solution is evaporated and pyrolyzed into an acid and a metal oxide. Background Art

[0002] Metal salt solutions, such as hydrochloric acid or hydrofluoric acid, are produced in different processes, including pickling in the steel industry, where scale is removed by a chemical reaction with hydrochloric acid or with a mixture of nitric acid and hydrofluoric acid, or when obtaining non-ferrous metals from ore slurries. For economic and ecological reasons, the acids used in these processes are regenerated and returned to the process, such that a closed acid cycle is formed. When selecting a suitable regeneration method, metal oxides can be obtained from the metals contained in the solution as valuable materials.

[0003] ANDRITZ AG provides acid regeneration equipment for the regeneration of pickling solutions for carbon steel and stainless steel treatment, as well as for the regeneration of solutions in leaching processes and for the production of metal oxides from metal salt solutions. These devices operate according to the pyrohydrolysis method, in which the metal salt solution (metal-containing acid solution) is evaporated in a spray roaster or a fluidized bed reactor, and the metal salt remaining after evaporation is converted into a metal oxide in the presence of high temperature and water vapor. The amount of energy used herein is quite considerable and is invariably provided by the combustion of fossil fuels mainly. Spray roasting and fluidized bed devices have by far the largest market share in the regeneration processes of industrially used acids and are provided by several enterprises, especially for applications in the field of steel production.

[0004] Alternative methods for acid regeneration are known and commercially available, but are not more popular than the pyrohydrolysis method because either the acid bound to the salt cannot be regenerated by the alternative methods and thus parts of the metal salt and the acid must be discarded and treated (e.g., distillation method or retardation method), or because the alternative methods have not been used in practice, mainly due to complex process control and material technology reasons (e.g., hydrothermal method, crystallization method).

[0005] The proven pyrohydrolysis process is generally carried out in a spray roaster (spray roasting reactor).

[0006] For example, AT 395 312B describes a method in which the acid is recovered by spray roasting a metal-containing solution and then absorbing and / or condensing the gas thus formed in an aqueous absorption solution. The metal oxide produced during the pyrohydrolysis process is removed at the bottom of the spray roaster.

[0007] EP 0 775 760 A1 describes a similar method for recovering acids by means of a hydrothermal treatment, in which the pickling residues are subjected to a pre-evaporation before the hydrothermal treatment.

[0008] The structures of these apparatuses are known in the specialist fields from AT 395 312 B, EP 0 635 586 A1 and EP 0 775 760 A1 and are therefore not described in detail.

[0009] DE 30 21 589 describes a method and an apparatus for obtaining hydrofluoric acid by means of an indirectly heated rotary furnace. The energy required for the hydrothermal treatment is provided by means of a gas burner.

[0010] US 3658483 A describes a method for recovering acids using a spray roasting reactor or a fluidized bed reactor.

[0011] In the spray roasting process, the metal salt solution is atomized by a nozzle mounted on the head of the spray roasting furnace and brought into contact with hot gas, such that the solution evaporates. The remaining metal salt particles fall to the bottom and pass through a hotter zone there, where the metal salt particles reach the temperature required for thermal conversion and are converted to metal oxides in the presence of water vapor and possibly oxygen. The metal oxide particles are removed at the bottom of the spray roasting furnace, while the hot gas and the acid vapor and water vapor leave the spray roasting furnace through the top and are sent to the next process step.

[0012] In a conventional spray roasting furnace, the flue gas from the combustion of fossil fuels is used as the hot gas, which is generated by a burner burning directly in the furnace chamber. This flue gas requires complex post-treatment to reduce emissions and is a source of greenhouse gases.

[0013] Conventional spray roasting furnaces can also be operated using hydrogen or carbon dioxide-neutral fuels (such as biomethane) without major changes to the process. However, providing corresponding amounts of such fuels usually requires significant investment in the corresponding production and distribution infrastructure. Summary of the Invention

[0014] The technical problem to be solved by the present invention is to provide an acid regeneration method which avoids flue gas as much as possible in the acid regeneration cycle. The acid can be, for example, waste acid from pickling equipment or leaching equipment and similar applications.

[0015] This technical problem is solved by the method according to claim 1.

[0016] According to the present invention, a heat transfer medium is supplied to the reaction chamber for heating, and the metal-containing solution is heated to the temperature required for hydrolysis by the heat transfer medium. This heat transfer medium consists partly or entirely of water vapor here. Therefore, the hydrolysis reactor is mainly heated by the supplied water vapor. This heating should be understood here as meaning that the reaction chamber is not mainly heated by a burner arranged in the reaction chamber, but mainly by the heat transfer medium, which is heated structurally separately from the reaction chamber before being supplied to the reaction chamber. Therefore, heating by a burner arranged in the reaction chamber is no longer required. The thermal energy required for evaporation of the metal salt solution and hydrolysis of the metal salt can be provided to the heat transfer medium, for example, by means of a heat exchanger, by means of electrical heating, or by means of partly electrical heating and supplementary combustion of fuel in the heat transfer medium.

[0017] Preferably, the hydrolysis of the metal-containing solution is carried out in a spray roasting reactor.

[0018] By supplying the heated heat transfer medium to provide the required thermal energy, combustion of fossil fuels in the spray roasting reactor, which is common in conventional spray roasting equipment, is completely avoided. Thereby, formation of combustion products discharged together with the process gas is largely prevented.

[0019] Of course, it is also conceivable that the hydrolysis of the metal-containing solution is carried out in a fluidized bed reactor or in a rotary kiln, wherein the heating of the fluidized bed reactor or the rotary kiln is also carried out by supplying hot water vapor as the heat transfer medium.

[0020] For example, the heat transfer medium can be heated by means of a heat exchanger before being supplied to the reaction chamber.

[0021] It is particularly advantageous for the heat transfer medium to be heated electrically because there is usually an infrastructure for providing electrical energy for electrical heating.

[0022] When the heat transfer medium is heated electrically, a sufficiently high gas temperature (water vapor temperature) should be provided. The outlet temperature of commercially available resistive process gas heaters is usually limited to between 750 °C and 800 °C, and higher temperatures cannot be achieved using conventional heat conductor materials. Plasma torches, which also exist on the market, can reach significantly higher temperatures, but due to the low thermal power output per device and the complexity of the power electronics, they cannot be used economically in the above applications.

[0023] Therefore, it is advantageous to provide the required thermal power in the range of several megawatts by directly heating the heat transfer medium in an arc plasma outside the hydrolysis reactor. Due to the high energy density of the arc plasma, high thermal energy can be introduced into the heat transfer medium at a high outlet temperature with a relatively small equipment size. The arc can be generated between electrodes supplied with an alternating voltage, similar to the case of a Birkeland–Eyde reactor, whereby complex and expensive power electronics can be dispensed with.

[0024] However, the heating of the heat transfer medium is not limited to electric heating. For example, in a hot gas generator using the combustion of traditional fuels (hydrogen, natural gas or other fuel gases), the required temperature can be generated through heat exchange between the combustion products and the heat transfer medium without introducing the combustion products into the heat transfer medium (hot gas). Although emissions from combustion cannot be avoided in this way, the cleaning of the undiluted combustion gas can be simply and in an industrially proven and commercially available method, while the mixture of process gas and combustion products is more complex due to the dilution of the combustion products.

[0025] It is advantageous that the heat transfer medium is at least partially recycled and reheated before being supplied to the reaction chamber.

[0026] Here, the energy required for evaporation and hydrolysis in the hot hydrolysis reactor is introduced through the recycled heat transfer medium, which mainly consists of water vapor. The amount of the recycled heat transfer medium required for this depends on the temperature to which the heat transfer medium can be heated. By increasing the temperature of the heat transfer medium, the amount of the recycled heat transfer medium can be reduced, thereby reducing the hydraulic load on the hot hydrolysis reactor and downstream equipment parts. A decrease in the temperature of the heat transfer medium leads to an increase in the process gas flow rate, which results in larger and more expensive equipment in the case of new equipment. In particular, when converting existing equipment to electric heating equipment, this increase in the process gas flow rate is particularly disadvantageous because the size of at least the main equipment cannot be easily determined.

[0027] It is advantageous that the recycled heat transfer medium is guided through the arc plasma.

[0028] Of course, it is also conceivable that the water vapor used for heating comes from another thermal process, such as strongly superheated steam from a gas turbine or blast furnace, or saturated low-temperature steam from geothermal energy.

[0029] The heat transfer medium can also be heated by electric heating or with the aid of a heat exchanger. Here, an electric heater can be used to heat the water vapor to, for example, 650 °C and further heated by burning fuel gas. Then, the heat transfer medium can be further heated with the aid of a heat exchanger, or fuel gas, such as hydrogen, can be directly burned in the heat transfer medium. Although the combustion products may enter the heat transfer medium in this case, the amount has been significantly reduced compared to traditional equipment.

[0030] Preferably, the metal-containing solution is evaporated before the hot hydrolysis treatment.

[0031] When using hydrochloric acid solution as the metal-containing solution and using this method to recover hydrochloric acid, the effect of this method is particularly good. It is also conceivable to use, for example, nitric acid and hydrofluoric acid solutions as the metal-containing solution and use this method to recover nitric acid and hydrofluoric acid. Description of the Drawings

[0032] The present invention will be described below in conjunction with the accompanying drawings.

[0033] Figure 1 Showing an electrically heated variant with an open exhaust gas system;

[0034] Figure 2 Showing the possibility of a spray roasting apparatus that uses electric heating and operates in a completely exhaust-free manner;

[0035] Figure 3 Showing an embodiment where the heat transfer medium (steam) comes from another process;

[0036] Figure 4 Showing an apparatus for electrically heating the heat transfer medium by means of arc plasma. Detailed Description

[0037] Figure 1 and Figure 2 Examples of electrically heated variants are shown, but do not limit the applicability of the other heating methods mentioned. The same reference numerals in the figures denote the same equipment parts or material flows.

[0038] The process technology and equipment implementation of the presented embodiments largely correspond to traditional acid regeneration equipment according to the spray roasting method that has been verified in industrial use.

[0039] Figure 1 An electrically heated variant with an open exhaust gas system is shown. At the head of the spray roasting furnace 1, the circulating pre-concentrated metal salt solution (= metal-containing acid solution) 13 from the pre-evaporator 3 (evaporator) is atomized. At the lower end of the spray roasting furnace 1, the heat transfer medium 11 heated by the electric heater 7 is introduced into the spray roasting furnace 1, such that the sprayed metal salt solution 13 and the heat transfer medium 11 move in a countercurrent manner. Thereby, the metal salt solution 13 evaporates in the upper region of the spray roasting furnace 1, and metal salts crystallize out, while the heat transfer medium 11 is cooled.

[0040] The thus formed metal salt particles reach the lower region of the spray roasting furnace 1, where they reach a certain temperature by contact with the heat transfer medium 11. At this temperature, in the presence of water vapor and possibly oxygen, the metal salts undergo hot hydrolysis to produce metal oxides 12. Hydrochloric acid pickling solutions commonly used for pickling carbon steel contain, for example, iron chloride, which is converted to iron oxide according to the following reaction equation:

[0041] 4FeCl2 + 4H2O + O2 → 2Fe2O3 + 8HCl

[0042] The metal oxide particles 12 are discharged through the bottom of the spray calcination furnace 1, while the heat transfer medium 11 and the acid vapors resulting from the evaporation and pyrolysis of the metal salt solution 13 leave the furnace 1 through the furnace head (flow 18). To separate the particles entrained in the gas stream, the gas is directed through a cyclone separator 2, and the separated particles 19 are directed back to the spray calcination furnace 1. The gas - essentially water vapor and acid vapors - is further washed in a Venturi scrubber 3, and the metal salt solution 13 is circulated through the Venturi scrubber 3. Thereby the gas is cooled, and a part of the circulated metal salt solution 13 evaporates and is thus pre-concentrated. In a subsequent absorption tower 4, the gas comes into contact with water 16 through a mass exchange packing, such that the hydrogen chloride contained in the gas is absorbed. The resulting hydrochloric acid is withdrawn as regenerated acid 14 for further use at the bottom of the tower. The exhaust gas ventilator 5 conveys the heat transfer medium 11 back to the spray calcination furnace 1, where the heat transfer medium 11 is superheated by an electric heater 7 to a temperature suitable for spray calcining the respective metal salt. In all embodiments, a heat exchanger 22 for heating the heat transfer medium 11 can also be provided as an alternative or supplement to the electric heater 7. The heat transfer medium 11 can also be heated by the combustion of hydrogen, wherein the combustion of hydrogen can also be carried out directly in the heat transfer medium 11.

[0043] To balance the water being exported and imported, a partial flow of the process gas 17 is discharged before the electric heater 7 and thus a certain amount of water vapor is discharged, this amount being approximately corresponding to the amount of water 16 fed into the absorption tower 4. For this purpose, a control valve 6 is provided. Although at startup the device is filled with ambient air and the circulated gas mainly consists of air components, due to the continuous discharge this component decreases and is replaced by the water vapor generated by spray calcination. After a short running time, the gas mainly consists of water vapor, which offers the advantage of a significantly higher heat capacity compared to, for example, heated ambient air. This water vapor is subsequently used as the heat transfer medium 11 for indirectly heating the reaction chamber 1.

[0044] The discharged gas 17 contains impurities in addition to water vapor, and as is usually the case in conventional spray calcination equipment, the impurities must be cleaned in subsequent scrubber stages to comply with, for example, the emission limits for hydrogen chloride, chlorine, and dust. If the pyrolysis reaction requires oxygen 15, oxygen 15 is mixed into the heat transfer medium 11 in the form of pure oxygen or air oxygen before the heat transfer medium 11 returns to the spray calcination furnace 1.

[0045] The described embodiments are advantageous when there is sufficient process wastewater from other processes that can be used as an absorbent in the absorption stage 4. This is for example the case in pickling processes, where usually water comes from the rinsing stage. If the use of additional water should be avoided, the regenerated acid 14 can alternatively be produced by a condensation step. As in Figure 2As shown. Here, the acid vapor 18 is guided through the condenser 8, where the acid vapor 18 is cooled below the dew point, such that diluted regenerated acid 14 is produced in the separator 9. The condenser 8 has a cooling water inflow 20 and a cooling water return 21 here. The heat transfer medium 11 is fed back to the electric heater 7 via the ventilator 10. Here, as an alternative or supplement to the electric heater 7, heating can also be provided by means of the heat exchanger 22 or by directly burning hydrogen in the heat transfer medium 11.

[0046] In addition to the advantages of the electric heater 7, the fully enclosed process control shown according to Figure 2 also allows the plant to operate without waste gas and thus without emissions. However, contrary to the embodiment shown in Figure 1 , the inert gas components present in the system are not discharged, which results in a significantly higher required flow rate of the heat transfer medium 11 to account for the lower specific heat capacity of the water vapor and inert gas mixture. This means a corresponding adaptation of the plant size and results in higher investment costs. Alternatively, the inert gas components can be avoided by inerting the system as much as possible by injecting water vapor before the actual acidic process stage , although this makes the process significantly more complex. In both cases, the start-up and shut-down of the plant are associated with the significant production of strongly diluted regenerates, which may have to be discarded.

[0047] In Figure 3 yet another embodiment of the invention is shown. Here, the hot heat transfer medium 11 (i.e., water vapor) comes from other thermal processes, such as strongly superheated steam from a gas turbine or blast furnace, or saturated low-temperature steam from geothermal energy.

[0048] Figure 4 The principle structure of an arc reactor 32 for heating the heat transfer medium 11 is shown. The structure of the arc reactor 32 is similar to that of a Birkeland-Eyde reactor. However, different from the Birkeland-Eyde reactor, of course, the ambient air is not supplied here, but the heat transfer medium 11 mainly composed of water vapor.

[0049] An electric arc is ignited between two water-cooled electrodes 23 and is deflected semicircularly outwards by a static magnetic field applied between the magnetic poles 24. The electrodes 23 are supplied with an alternating voltage such that the electric arc reignites at each zero crossing and the direction of deflection of the electric arc changes alternately. Thereby, an optical image of a plasma disk burning in the disk-shaped combustion chamber 25 is generated. The heat transfer medium 11 is supplied to the arc reactor 32 through a nozzle 26 under a slight overpressure. The heat transfer medium 11 is introduced into the combustion chamber 25 through a perforated refractory lining 27. Here, the heat transfer medium 11 cools the refractory lining 27 heated by the thermal radiation from the plasma. Inside the combustion chamber 25, the heat transfer medium 11 flows radially in the direction of the surrounding collecting channel 28 existing on the outside. Here, a mass exchange and a heat exchange take place between the plasma and the heat transfer medium 11, which results in heating the heat transfer medium 11 to 950 °C to 1200 °C. The heat transfer medium 11 leaves the arc reactor 32 through a nozzle 29.

[0050] The magnetic field required to form the plasma disk is generated by an electromagnet 30 directly mounted on the magnetic poles 24. The magnetic circuit is closed through the soft magnetic jacket 31 of the arc reactor 32.

[0051] List of reference numerals

[0052] 1 Spray roasting furnace or reaction chamber

[0053] 2 Cyclone separator

[0054] 3 Pre-evaporator / evaporator / Venturi scrubber

[0055] 4 Absorption tower

[0056] 5 Exhaust gas ventilator

[0057] 6 Control valve

[0058] 7 Electric heater

[0059] 8 Condenser

[0060] 9 Separator

[0061] 10 Ventilator

[0062] 11 Heat transfer medium

[0063] 12 Metal oxide

[0064] 13 Metal salt solution (metal-containing acid solution)

[0065] 14 Regenerated acid

[0066] 15 Oxygen / air

[0067] 16 Water

[0068] 17 Exhausted gas

[0069] 18 Acid vapor and heat transfer medium

[0070] 19 Separated particles

[0071] 20 Cooling water inflow

[0072] 21 Cooling water return flow

[0073] 22 Heat exchanger

[0074] 23 Water-cooled electrode

[0075] 24 Magnetic pole

[0076] 25 Combustion chamber

[0077] 26 Nozzle

[0078] 27 Refractory lining

[0079] 28 Collection channel

[0080] 29 Nozzle

[0081] 30 Electromagnet

[0082] 31 Soft magnetic jacket

[0083] 32 Arc reactor

Claims

1. A method for obtaining or recovering an acid from a metal-containing solution (13), which method is carried out by subjecting the solution to a hydrothermal treatment and subsequently absorbing and / or condensing the gaseous acid formed thereby, wherein, The metal-containing solution (13) is supplied to a heated reaction chamber (1), in which the metal-containing solution (13) evaporates and pyrolyzes into an acid and a metal oxide (12), characterized in that a heat transfer medium (11) is supplied to the reaction chamber (1) for heating, by means of which the metal-containing solution (13) evaporates and is heated to the temperature required for hydrothermal decomposition, wherein the heat transfer medium (11) consists partly or entirely of water vapor.

2. The method according to claim 1, wherein The hydrothermal decomposition of the metal-containing solution (13) is carried out in a spray roasting reactor (1).

3. The method according to claim 1, wherein The hydrothermal decomposition of the metal-containing solution (13) is carried out in a fluidized bed reactor.

4. The method according to claim 1, wherein The hydrothermal decomposition of the metal-containing solution (13) is carried out in a rotary kiln.

5. The method according to any one of claims 1 to 4, characterized in that, The heat transfer medium (11) is heated by a heat exchanger (22) before being supplied to the reaction chamber (1).

6. The method according to any one of claims 1 to 5, characterized in that, The heat transfer medium (11) is electrically heated.

7. The method according to claim 6, characterized in that, The heating of the heat transfer medium (11) is carried out in an arc plasma outside the reaction chamber (1).

8. The method according to any one of claims 1 to 7, characterized in that, The heat transfer medium (11) is at least partly recycled and reheated before being supplied to the reaction chamber (1).

9. The method according to any one of claims 1 to 4, characterized in that The water vapor (11) used for heating is sourced from another thermal process.

10. The method according to any one of claims 1 to 8, characterized in that, The heat transfer medium (11) is both electrically heated and heated by means of the heat exchanger (22).

11. The method according to any one of claims 1 to 4, characterized in that, In a first step, the heat transfer medium (11) is electrically heated, and in a next step, the heat transfer medium (11) is further heated by directly burning a fuel gas, such as hydrogen, in the heat transfer medium (11).

12. The method according to claim 5 or 9, characterized in that, The heat transfer medium (11) is heated by the combustion of a fuel gas, in particular hydrogen or natural gas, by means of the heat exchanger (22).

13. The method according to any one of claims 1 to 12, characterized in that, The metal-containing solution (13) is evaporated (3) before the hydrothermal treatment.

14. The method according to any one of claims 1 to 13, characterized in that, The metal-containing acid solution (13) is sourced from a pickling process, and the regenerated acid (14) is returned to the pickling process.

15. The method according to any one of claims 1 to 13, characterized in that, The metal-containing acid solution (13) is sourced from a leaching process, and the regenerated acid (14) is returned to the leaching process.

16. The method according to any one of claims 1 to 15, characterized in that A hydrochloric acid solution is used as the metal-containing solution (13), and hydrochloric acid is recovered by the method.

17. The method according to any one of claims 1 to 15, characterized in that, A nitric acid and hydrofluoric acid solution is used as the metal-containing solution (13), and nitric acid and hydrofluoric acid are recovered by the method.

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