Method for the regeneration of acid

Abstract

The invention relates to a method for obtaining or recovering acid from metal-containing solutions (13) of the acid through treatment by pyrohydrolysis of the solution and subsequent absorption and / or condensation of the gaseous acid formed in the process. The metal-containing solution (13) is fed to a heated reactor chamber (1) in which the metal-containing solution (13) is evaporated and in which the solution is split by pyrolysis into acids and metal oxides (12). According to the invention, for heating, a heat transfer medium (11), which partially or completely consists of water vapour, is fed to the reactor chamber (1), by means of which the metal-containing solution (13) is heated to the temperature required for pyrohydrolysis.

Description

[0001] ACID REGENERATION PROCESS

[0002] The subject of this invention is a process for the extraction or recovery of acids from metal-containing solutions of the acid by pyrohydrolytic treatment of the solution and subsequent absorption and / or condensation of the gaseous acid formed in the process, wherein the metal-containing solution is fed to a heated reactor chamber in which the metal-containing solution is evaporated and in which a pyrolytic decomposition into acids and metal oxides takes place.

[0003] Solutions of metal salts, such as hydrochloric or hydrofluoric acid, are created in various processes. These include pickling in the steel industry, where scale is removed by chemical reaction with hydrochloric acid or mixtures of nitric acid and hydrofluoric acid, and the extraction of non-ferrous metals from ore liquors. For economic and ecological reasons, the acids used in these processes are regenerated and returned to the process, creating a closed acid cycle. With a suitable choice of regeneration process, metal oxides can be extracted as valuable materials from the metals contained in the solution.

[0004] ANDRITZ AG offers acid regeneration plants for the regeneration of pickling solutions from the treatment of carbon and stainless steels, as well as for the regeneration of solutions from leaching processes and the production of metal oxides from metal salt solutions. These plants operate according to the pyrohydrolysis process, in which metal salt solutions (metal-containing solutions of the acid) are evaporated in spray roasters or fluidized bed reactors and the metal salts remaining after evaporation are converted into metal oxides at high temperatures and in the presence of steam. The amount of energy used is considerable and is always provided by the combustion of predominantly fossil fuels. Spray roasting and fluidized bed plants account for by far the largest market share of industrially used acid regeneration processes and are offered by several companies, primarily for applications in the steel production sector.

[0005] Alternative processes for acid regeneration are known and commercially available, but have not prevailed over pyrohydrolysis processes because they are either not able to regenerate acid in the form of salt-bound acids and therefore the metal salts and part of the acid have to be discarded and disposed of (e.g. distillation processes or retardation processes), or because they have not proven themselves in practical use, mainly due to complex process controls and material-related reasons (e.g. hydrothermal processes, crystallization processes).

[0006] The proven pyrohydrolysis process is often carried out in spray roasting ovens (spray roasting reactors).

[0007] For example, AT 395 312 B describes a process in which the acid is recovered by spray roasting the metal-containing solution and subsequent absorption and / or condensation of the resulting gases in an aqueous absorption solution. The metal oxides formed during pyrohydrolysis are removed at the bottom of the spray roaster. EP 0 775 760 A1 describes a similar process for recovering acid by pyrohydrolytic treatment, in which the pickling residue is subjected to pre-evaporation prior to pyrohydrolysis.

[0008] The structure of these systems is known to experts or from AT 395 312 B, EP 0 635 586 A1 and EP 0 775 760 A1 and is therefore not described in more detail.

[0009] DE 30 21 589 describes a process and a plant for producing hydrofluoric acid using an indirectly heated rotary kiln. The energy required for pyrohydrolysis is supplied by a gas burner.

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

[0011] In the spray roasting process, the metal salt solution is atomized via nozzle lances attached to the top of a spray roaster and brought into contact with hot gas, causing the solution to evaporate. The remaining metal salt particles fall to the bottom and pass through a hotter zone where they reach the temperature required for thermal conversion and are converted into metal oxides in the presence of steam and, if present, oxygen. The metal oxide particles are drawn off at the bottom of the spray roaster, while the hot gas, acid vapor, and steam leave the spray roaster through the top and are fed to further process steps.

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

[0013] Conventional spray roasters can also be operated with hydrogen or CO2-neutral fuels such as biomethane without significant changes to the process. However, the provision of sufficient quantities of such fuels will generally require significant investments in the appropriate production and distribution infrastructure.

[0014] The object of the invention is to provide an acid regeneration process with the substantial avoidance of flue gases in the acid regeneration cycle. The acid can be, for example, waste acids from leaching or leaching plants and similar applications.

[0015] This object is achieved by a method according to patent claim 1 .

[0016] According to the invention, a heat transfer medium is supplied to the reactor chamber for heating, by means of which the metal-containing solution is heated to the temperature required for pyrohydrolysis. This heat transfer medium consists partly or entirely of steam. The pyrohydrolysis reactor is thus primarily heated by the supplied steam. This heating is to be understood in such a way that the reactor chamber is not primarily heated by burners arranged in the reactor chamber, but primarily by the heat transfer medium, which is heated structurally separately from the reactor chamber before it is fed to the reactor chamber. Heating by burners arranged in the reactor chamber is therefore no longer necessary.The thermal energy required for the evaporation of the metal salt solution and pyrohydrolysis of the metal salts can be supplied to the heat transfer medium, for example, via a heat exchanger, by electrical heating or by partial electrical heating and additional combustion of fuels in the heat transfer medium.

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

[0018] By providing the required thermal energy through the supply of the heated heat transfer medium, the combustion of fossil fuels in the spray roasting reactor, which is common in conventional spray roasting plants, is completely avoided. This largely prevents the formation of combustion products, which are emitted along with process gases.

[0019] It is of course also conceivable that the pyrohydrolysis of the metal-containing solution is carried out in a fluidized bed reactor or in a rotary kiln, in which case the heating of the fluidized bed reactor or the rotary kiln is also carried out by the supply of hot steam as a heat transfer medium.

[0020] The heat transfer medium can, for example, be heated using a heat exchanger before it is fed into the reactor chamber.

[0021] It is particularly advantageous if the heat transfer medium is heated electrically, as the infrastructure for providing electrical energy for electrical heating is usually available. When heating the heat transfer medium electrically, a sufficiently high gas temperature (steam temperature) should be provided. Commercially available resistive process gas heaters are usually limited to outlet temperatures of 750 - 800 °C and cannot be designed for significantly higher temperatures using conventional heating element materials. Plasma torches also available on the market can reach significantly higher temperatures, but are not economically viable for this application due to their low thermal output per unit and complex power electronics.

[0022] It is therefore advantageous to provide the required thermal power in the range of several MW by directly heating the heat transfer medium in an arc plasma outside the pyrohydrolysis reactor. Due to the high energy densities in the arc plasma, high thermal power can be introduced into the heat transfer medium at high outlet temperatures using comparatively small apparatus. The arc can be generated between electrodes fed with alternating current, similar to the Birkeland-Eyde reactor, thus eliminating the need for complex and costly power electronics.

[0023] However, heating of the heat transfer medium is not limited to electrical heating. For example, in hot gas generators fired with conventional fuels (hydrogen, natural gas, or other fuel gases), the required temperatures can be generated by heat exchange between the combustion products and the heat transfer medium, without introducing combustion products into the heat transfer medium (hot gas). Emissions from combustion cannot be avoided in this way, but cleaning the undiluted combustion gases is easier and possible using industrially proven and commercially available processes. A mixture of process gases and combustion products is more complex, simply because of the dilution of the combustion products.

[0024] It is advantageous if the heat transfer medium is at least partially recirculated and reheated before it is fed into the reactor chamber.

[0025] The energy required for evaporation and hydrolysis in the pyrohydrolysis reactor is then introduced via a recirculated heat transfer medium, which essentially consists of steam. The amount of recirculated heat transfer medium required for this purpose is determined by the temperature to which the heat transfer medium can be heated. By increasing the heat transfer temperature, the amount of recirculated heat transfer medium can be reduced, thereby reducing the hydraulic load on the pyrohydrolysis reactor and downstream plant components. Reducing the heat transfer temperature increases the process gas flow, which requires larger and more expensive equipment in new plants. Such an increase in the process gas flow is particularly disadvantageous when converting existing plants to electric heating, because dimensioning the main equipment, at least, is not easily possible.

[0026] Advantageously, the recirculated heat transfer medium is passed through an arc plasma. However, it is also conceivable that the steam used for heating comes from another thermal process, for example, highly superheated steam from gas turbines or blast furnaces, or saturated low-temperature steam from geothermal energy.

[0027] The heat transfer medium could also be heated electrically or with the aid of a heat exchanger. The steam could be heated with electric heaters to, for example, 650 °C, and further heating could be achieved by burning a fuel gas. This further heating of the heat transfer medium could then be achieved with the aid of heat exchangers, or the fuel gas, such as hydrogen, could be burned directly in the heat transfer medium. Although combustion products can enter the heat transfer medium, the amount is significantly reduced compared to conventional systems.

[0028] Preferably, the metal-containing solution is subjected to evaporation before the pyrohydrolytic treatment.

[0029] The process works particularly well when hydrochloric acid solutions are used as the metal-containing solution, and hydrochloric acid is recovered using the process. It is also conceivable that solutions of nitric acid and hydrofluoric acid, for example, are used as the metal-containing solution, and that nitric acid and hydrofluoric acid are recovered using the process.

[0030] The invention is described below with reference to drawings.

[0031] Fig. 1 shows an electrically heated variant with an open exhaust system; Fig. 2 shows a possibility of operating spray roasting systems with electrical heating completely exhaust-free;

[0032] Fig. 3 shows an embodiment in which the heat transfer medium (water vapor) comes from another process;

[0033] Fig. 4 shows a device for electrically heating the heat transfer medium by means of an arc plasma;

[0034] Figures 1 and 2 show examples of electrically heated variants without limiting their applicability to other heating methods mentioned. Identical reference numerals in the figures indicate identical system components or material flows.

[0035] The process engineering and equipment design of the presented examples corresponds to a large extent to the conventional acid regeneration plants based on the spray roasting process that have proven successful in industrial use.

[0036] Figure 1 shows an electrically heated variant with an open exhaust system. At the top of the spray roasting furnace 1, pre-concentrated metal salt solution (= metal-containing solution of the acid) 13 from the circulation of the pre-evaporator 3 (evaporation) is atomized. At the lower end of the spray roasting furnace 1, the heated heat transfer medium 11 from the electrical heating system 7 is introduced into the spray roasting furnace 1, so that the sprayed metal salt solution 13 moves countercurrently to the heat transfer medium 11. As a result, the metal salt solution 13 is evaporated in the upper region of the spray roasting furnace 1, and the metal salts crystallize while the heat transfer medium 11 cools.

[0037] The metal salt particles thus formed reach the lower area of ​​the spray roasting furnace 1, where they are brought to a temperature by contact with the heat transfer medium 11, at which, in the presence of steam and optionally oxygen, pyrohydrolysis of the metal salts to metal oxides 12 takes place. Hydrochloric acid pickling solutions, as they are generally used in the pickling of carbon steels, contain, for example, iron chloride, which, according to the reaction equation

[0038] 4 FeCl2+ 4 H2O + O22 Fe2O3+ 8 HCl is converted into iron oxide.

[0039] The metal oxide particles 12 are discharged via the bottom of the spray roasting furnace 1, while the heat transfer medium 11 and acid vapor from the evaporation and pyrolysis of the metal salt solution 13 leave the furnace 1 via the furnace head (stream 18). In order to separate particles entrained in the gas stream, the gas is passed through a cyclone separator 2 and the separated particles 19 are returned to the spray roasting furnace 1. The gas, essentially water vapor and acid vapor, is further scrubbed in a Venturi scrubber 3, through which the metal salt solution 13 is circulated. This cools the gas, while a portion of the circulated metal salt solution 13 is evaporated and thus pre-concentrated. In the downstream absorption column 4, the gas is brought into contact with water 16 via a mass transfer packing, so that hydrogen chloride contained in the gas is absorbed.The hydrochloric acid thus produced is withdrawn as regenerated acid 14 for further use at the bottom of the column. The exhaust gas fan 5 conveys the heat transfer medium 11 back to the spray roasting furnace 1, where it is superheated by the electrical heater 7 to a temperature suitable for spray roasting the respective metal salt. Instead of the electrical heater 7 or in addition to the electrical heater 7, a heat exchanger 22 can also be provided in all embodiments, by means of which the heat transfer medium 11 is heated. The heat transfer medium 11 can also be heated by the combustion of hydrogen, wherein the combustion of hydrogen can also take place directly in the heat transfer medium 11.

[0040] In order to equalize the balance of water supplied and removed, a partial flow of the process gas 17 and thus a quantity of water vapor is discharged upstream of the electrical heater 7, which approximately corresponds to the quantity of water 16 that is fed to the absorption column 4. A control valve 6 is provided for this purpose. While the plant is filled with ambient air during start-up and the circulated gas consists predominantly of air components, this proportion decreases due to the continuous discharge and is replaced by water vapor from the spray roasting. After a short operating time, the gas consists predominantly of water vapor, which offers advantages in terms of the higher heat capacity compared to operation with, for example, heated ambient air. This water vapor then serves as the heat transfer medium 11 for the indirect heating of the reactor chamber 1.

[0041] The discharged gas 17 contains not only water vapor but also impurities and, as is usual in conventional spray roasting systems, must be cleaned in subsequent scrubber stages to comply with emission limits, e.g., for hydrogen chloride, chlorine, and dust. If oxygen 15 is required for the pyrolysis reaction, it is added to the heat transfer medium 11 in pure form or as atmospheric oxygen before being returned to the spray roasting furnace 1.

[0042] The described embodiment is advantageous when sufficient process wastewater from other processes is available which can be used as an absorbent in the absorption stage 4. This is the case, for example, in pickling processes where water usually arises from rinsing stages. If the use of additional water is to be avoided, the regenerated acid 14 can alternatively be generated from a condensation step. This is shown in Figure 2. The acid vapor 18 is passed through a condenser 8 where it is cooled below the dew point so that a diluted regenerated acid 14 is produced in the separator 9. The condenser 8 has a cooling water supply 20 and a cooling water return 21. The heat transfer medium 11 is fed back to the electric heater 7 via the fan 10.Here too, alternatively or in addition to the electrical heating 7, heating can be provided via heat exchanger 22 or via the direct combustion of hydrogen in the heat transfer medium 11.

[0043] In addition to the advantages of electrical heating 7, this completely closed process according to Figure 2 allows the plant to be operated without exhaust gases and therefore emissions. In contrast to the embodiment shown in Figure 1, however, any inert gas content in the system is not discharged, which means that the required heat transfer medium flow 11 is significantly higher in order to account for the lower specific heat capacity of the mixture of steam and inert gas. This requires a corresponding adjustment of the apparatus dimensions and entails higher investment costs. Alternatively, the inert gas content can be avoided by largely inerting the system by purging it with steam before the actual acid operation, although this would result in a significantly more complex process.In both cases, starting up and shutting down the plant involves a considerable amount of highly diluted regenerated material, which may have to be discarded.

[0044] Figure 3 shows yet another exemplary embodiment of the invention. Here, the hot heat transfer medium 11, i.e., steam, originates from other thermal processes, for example, highly superheated steam from gas turbines or blast furnaces, or saturated low-temperature steam from geothermal energy.

[0045] Figure 4 shows the basic structure of an arc reactor 32 for heating the heat transfer medium 11. The structure of the arc reactor 32 is similar to that of the Birkeland-Eyde reactor. However, unlike the Birkeland-Eyde reactor, it is not supplied with ambient air, but rather with a heat transfer medium 11, which essentially consists of steam.

[0046] An arc is ignited between two water-cooled electrodes 23, which is deflected outward in a semicircle by a static magnetic field applied between the poles 24. The electrodes 23 are supplied with alternating voltage, so that at each zero crossing, the arc is reignited and the direction in which the arc is deflected changes. This creates the optical image of a plasma disk burning in the disk-shaped combustion chamber 25.

[0047] The heat transfer medium 11 is fed to the arc reactor 32 via a nozzle 26 under slight excess pressure. The heat transfer medium 11 is introduced into the combustion chamber 25 via a perforated refractory lining 27. The heat transfer medium 11 cools the refractory lining 27, which is heated by thermal radiation from the plasma. In the combustion chamber 25, the heat transfer medium 11 flows radially in the direction of an external, circumferential collecting duct 28. This results in a mass and heat exchange between the plasma and the heat transfer medium 11, which leads to a heating of the heat transfer medium 11 to 950 - 1200 °C. The heat transfer medium 11 leaves the arc reactor 32 via the nozzle 29.

[0048] The magnetic field required to form the plasma disk is generated by electromagnets 30, which are attached directly to the poles 24. The magnetic circuit is closed by the soft magnetic jacket 31 of the arc reactor 32.

[0049] Reference sign

[0050] 1 spray roasting oven or reactor room

[0051] 2 cyclone separators

[0052] 3 Pre-evaporator / evaporation / venturi rather

[0053] 4 Absorption column

[0054] 5 exhaust fan

[0055] 6 Control valve

[0056] 7 Electric heating 8 Condenser

[0057] 9 separators

[0058] 10 fan

[0059] 11 Heat transfer medium

[0060] 12 Metal oxide

[0061] 13 Metal salt solution (metal-containing solution of the acid)

[0062] 14 regenerated acid

[0063] 15 Oxygen / Air

[0064] 16 Water

[0065] 17 gas discharged

[0066] 18 Acid vapor and heat transfer medium

[0067] 19 separated particles

[0068] 20 Cooling water supply

[0069] 21 Cooling water return

[0070] 22 heat exchangers

[0071] 23 water-cooled electrodes

[0072] 24 poles

[0073] 25 Combustion chamber

[0074] 26 nozzles

[0075] 27 Refractory lining

[0076] 28 Collecting channel

[0077] 29 nozzles

[0078] 30 electromagnets

[0079] 31 soft magnetic sheath

[0080] 32 arc reactor

Claims

Patent claims 1. A process for the recovery of acid from metal-containing solutions (13) of the acid by pyrohydrolytic treatment of the solution and subsequent absorption and / or condensation of the gaseous acid formed thereby, wherein the metal-containing solution (13) is introduced into a heated reactor chamber (I) in which the metal-containing solution (13) is evaporated and in which a pyrolytic Decomposition into acids and metal oxides (12), characterized in that a heat transfer medium (11) is supplied to the reactor space (1) for heating, by means of which the metal-containing solution (13) is evaporated and heated to the temperature required for the pyrohydrolysis, wherein the heat transfer medium (II) consists partly or wholly of water vapor.

2. Process according to claim 1, characterized in that the pyrohydrolysis of the metal-containing solution (13) is carried out in a spray roasting reactor (1).

3. Process according to claim 1, characterized in that the pyrohydrolysis of the metal-containing solution (13) is carried out in a fluidized bed reactor.

4. Process according to claim 1, characterized in that the pyrohydrolysis of the metal-containing solution (13) is carried out in a rotary kiln.

5. Method according to one of claims 1 to 4, characterized in that the heat transfer medium (11) is heated by means of a heat exchanger (22) before it is fed to the reactor chamber (1).

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

7. The method according to claim 6, characterized in that the heating of the heat transfer medium (11) takes place in an arc plasma outside the reactor chamber (1).

8. Method according to one of claims 1 to 7, characterized in that the heat transfer medium (11) is at least partially recirculated and reheated before it is fed to the reactor chamber (1).

9. Method according to one of claims 1 to 4, characterized in that the steam (11) used for heating originates from another thermal process.

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

11. Method according to one of claims 1 to 4, characterized in that the heat transfer medium (11) is heated electrically in a first step and is further heated in a further step by the direct combustion of a fuel gas, for example hydrogen, in the heat transfer medium (11).

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

13. Method according to one of claims 1 to 12, characterized in that the metal-containing solution (13) is subjected to evaporation (3) before the pyrohydrolytic treatment.

14. Method according to one of claims 1 to 13, characterized in that the metal-containing solution (13) of the acid originates from a pickling process and the regenerated acid (14) is fed back into the pickling process.

15. Method according to one of claims 1 to 13, characterized in that the metal-containing solution (13) of the acid originates from a leaching process and the regenerated acid (14) is fed back into the leaching process.

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

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

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