Method and apparatus for producing hydrochloric acid

Abstract

The present invention provides a method of producing hydrochloric acid, comprising reacting high-temperature gaseous chlorine and water vapour in a carbon-free reverse Deacon reaction (RDR) at a temperature of from about 450 to about 750 °Celsius to produce a mixture of gases comprising hydrogen chloride and oxygen. The chlorine is produced by fusing and electrolysing sodium or magnesium chloride in solid phase. The water vapour comes from thermally decomposing a hydroxide of at least one of calcium, magnesium and iron in the absence of oxygen to produce the water vapour without any admixture of oxygen (e.g., from atmospheric air). The mixture of gases produced by the RDR is immediately contacted with liquid water to dissolve the hydrogen chloride therein, thereby producing hydrochloric acid and a stream of tail gases. Heat is extracted from the hydrochloric acid to maintain its temperature substantially constant until the stream of tail gases is no longer in contact therewith. Thus the chlorine and water vapour are both supplied to the RDR at high purity and require little, if any, preheating. Heat extracted from contacting the mixture of gases with liquid water may be used to keep the RDR within its range of operating temperatures. Oxygen can be separated from the stream of tail gases, and the oxygen-depleted stream can be recycled back to the RDR to give a high percentage conversion of chlorine to hydrogen chloride. The RDR may be uncatalysed and kept within its operating temperature range by radio-frequency volumetric heating in at least one of the IEEE Ku-, K- and Ka-bands. The invention also provides a corresponding apparatus (1h) for producing hydrochloric acid, comprising an electrolytic cell (10) for the electrolysis, a gas-tight kiln (20) for the thermal decomposition, a gas-phase reactor (30) for the RDR, and an absorber (40) for dissolving the hydrogen chloride in the liquid water. The apparatus (1h) may further comprise a separator (80) for separating the stream of tail gases into the oxygen-depleted stream and a stream of oxygen, which may be fed to a steelmaking apparatus (90), for example. The liquid water for dissolving the hydrogen chloride can come from condensing water vapour obtained by drying an aqueous solution of sodium or magnesium chloride or of sodium hydroxide. The apparatus (1h) may therefore also comprise a dryer (60) for drying this aqueous solution, and a condenser (70) for condensing the water vapour thus obtained. Sodium chloride or magnesium chloride in solid phase derived from drying the aqueous solution may be recycled back to the electrolytic cell (10). Electricity for the electrolysis can come from a renewable resource. At least some of the heat for the thermal decomposition may come from the liquid metal which is a co-product of the electrolysis. The invention is therefore highly energy efficient and requires little, if any, extra energy, apart from the electricity required for the electrolysis. The invention also uses commonly available ingredients and produces no greenhouse gases.

Description

[0001] Method and Apparatus for Producing Hydrochloric Acid

[0002] Field of the Invention

[0003] The present invention concerns a method and apparatus for producing hydrochloric acid.

[0004] Background of the Invention

[0005] Hydrochloric acid ( / '. e., an aqueous solution of hydrogen chloride) is generally produced industrially by one of two main processes. The first is by combusting gaseous chlorine (C ) with gaseous hydrogen (H2) to produce hydrogen chloride gas (HCI). The hydrogen chloride gas is then dissolved in deionised water to produce hydrochloric acid. This has the advantage that the hydrochloric acid thus produced is generally very pure. However, it has the disadvantage that it requires a source of hydrogen, in addition to one of chlorine. Hydrogen and chlorine can both be produced by the chlor-alkali process, in which brine ( / '. e., an aqueous solution of sodium chloride) is electrolysed to produce the hydrogen and chlorine as gases, with an aqueous solution of sodium hydroxide being produced as a co-product. However, since electrolytic cells in which the chlor-alkali process is carried out traditionally comprise a membrane, the brine which is used must be of high purity for the process to work without damaging the membrane.

[0006] The second main process for the industrial production of hydrochloric acid is as a co-product from the industrial production of chlorinated organic compounds. In a chlorination reaction of one or more organic compounds with chlorine gas, a chlorine atom from a diatomic molecule of the gaseous chlorine is substituted for a hydrogen atom from a molecule of an organic compound. The hydrogen atom removed from the organic compound combines with the remaining chlorine atom from the chlorine gas molecule to produce a molecule of hydrogen chloride gas. Once again, the hydrogen chloride gas is then dissolved in deionised water to produce hydrochloric acid. Whereas this second process does not require a source of hydrogen in addition to one of chlorine, it has the disadvantage that the hydrochloric acid thus produced generally contains impurities derived from the chlorinated organic compounds.

[0007] By way of further background, several alkali and alkaline earth metals are produced industrially by fusing and electrolysing a chloride salt of the metal in question. These metals include lithium, sodium, magnesium and calcium. Application of an electrical current to the chloride salt of the selected metal in solid phase fuses ( / '.e., melts) the chloride salt by ohmic heating. Subsequent electrolysis of the fused salt produces the selected metal, either in liquid phase (in the cases of lithium, sodium and magnesium) or in solid phase (in the case of calcium), with chlorine gas produced as a co-product. In order to lower the temperature at which the electrolysis takes place, the chloride salt of the selected metal may be mixed with one or more other salts to form a eutectic mixture. For example, electrolysis of sodium chloride to produce elemental sodium typically occurs at about 600 to 625 “Celsius, and electrolysis of magnesium chloride to produce elemental magnesium at about 700 to 750 “Celsius. The high- temperature chlorine gas produced as a co-product is usually cooled, condensed and bottled. The chlorine produced in this way generally has a high purity of better than 99%. The bottled chlorine may therefore subsequently be used, for example, in the industrial production of chlorinated organic compounds, as described above. However, the thermal energy which the high-temperature chlorine has immediately after it has been produced in this way is generally lost to the environment.

[0008] Gaseous chlorine can also be produced by reacting hydrogen chloride gas with gaseous oxygen (either from a pure source or in air), which also produces water vapour as a by-product. This reaction, which is known as the (forward) Deacon reaction, proceeds according to the equation:

[0009] 4 HCI(g) + O2 <g) — > 2 CI2 (g) + 2 H2O(g) [Eqn. 1]

[0010] The equilibrium constant for this homogeneous gas-phase reaction decreases with increasing temperature, until above about 600 “Celsius, the reaction of Eqn. 1 reverses ( / '.e., the theoretical maximum percentage conversion of hydrogen chloride to chlorine drops below 50%) and instead becomes the reverse Deacon reaction (hereinafter, RDR). The RDR proceeds according to the opposite equation:

[0011] 2 CI2 (g) + 2 H2O(g) — > 4 HCI(g) + O2 <g) [Eqn. 2]

[0012] In other words, at atmospheric pressure, the Gibbs free energies of Eqns. 1 and 2 both equal 0 at about 600 “Celsius and the reactions of Eqns. 1 and 2 are in thermodynamic equilibrium with each other. Below about 600 “Celsius, the Gibbs free energy of Eqn. 1 is negative, so the (forward) Deacon reaction is thermodynamically favoured. Above about 600 “Celsius, on the other hand, the Gibbs free energy of Eqn. 2 is negative, so the RDR is favoured instead. However, since the reaction kinetics of Eqn. 1 also increase the rate of reaction with increasing temperature, the (forward) Deacon reaction is generally carried out industrially at a temperature of from about 400 to about 450 “Celsius, to ensure a sufficient rate of reaction. In this range of temperatures, the (forward) Deacon reaction can in principle achieve a percentage conversion of hydrogen chloride to chlorine of about 80%. The rate of the (forward) Deacon reaction can also be increased by conducting it in the presence of one of several different catalysts, such as copper (II) chloride (CuCh).

[0013] It has previously been proposed that the RDR of Eqn. 2 could be used to produce hydrogen chloride gas, which may then be dissolved in water to produce hydrochloric acid. However, this process is not currently practised industrially. For example, US patent nos. 1 229 509, 1 420 209, 1 485 816, 1 695 552 and 1 847 196, French patent no. 992 928 and GB patent no. 189 723, all describe the production of hydrogen chloride gas by conducting the RDR in the presence of a reducing agent, such as heated activated carbon, charcoal or coke. The carbon reacts with the oxygen produced by the reaction of Eqn. 2 to produce either carbon monoxide gas (CO), carbon dioxide gas (CO2), or a mixture of both, according to the reaction conditions. This pulls the reaction of Eqn. 2 to the right according to Le Chatelier’s principle, which reduces the temperature at which the RDR is thermodynamically favoured to temperatures as low as about 200 “Celsius. When the above-mentioned patents were issued, the carbon monoxide and / or carbon dioxide thus produced were seen as acceptable waste products, and in the case of carbon monoxide, even as a useful co-product from the production of hydrochloric acid. However, the production of such greenhouse gases is instead seen today as entirely undesirable.

[0014] US patent no. 1 874 225 instead proposes the production of hydrogen chloride gas by the RDR without using a reducing agent like activated carbon to remove the oxygen produced by the reaction of Eqn. 2, but at an elevated temperature of at least 1 000 “Celsius up to about 1 600 “Celsius, in order to reverse the (forward) Deacon reaction of Eqn. 1 sufficiently to achieve a high percentage conversion of hydrogen chloride. However, operating the RDR at such a high temperature is impractical and uneconomic, not only in view of the large amount of energy required to achieve such high temperatures, but particularly considering the highly corrosive nature at such elevated temperatures of the hydrogen chloride thus produced.

[0015] It is also known that calcium hydroxide, magnesium hydroxide and iron hydroxides can all be thermally decomposed into calcium oxide, magnesium oxide and iron oxides, respectively, and water vapour. At atmospheric pressure, calcium hydroxide decomposes into calcium oxide and water vapour at about 512 “Celsius. Also at atmospheric pressure, magnesium hydroxide decomposes into magnesium oxide and water vapour at about 350 “Celsius. At atmospheric pressure, the thermal decomposition of, for example, pure ferric hydroxide (Fe(OH)s) into ferric oxide (Fe2O3) and water vapour occurs at about 475 “Celsius, whereas ferrous hydroxide (Fe(OH)2) may be thermally decomposed in the absence of oxygen into ferrous oxide (FeO) and water vapour at a lower temperature of from about 150 to about 200 “Celsius (depending on the rate of removal of the water vapour). The water vapour thus produced is generally free of contaminants and, being harmless, is released into the environment. However, the thermal energy which the water vapour carries with it is therefore lost to the environment as well.

[0016] Object of the Invention

[0017] It is therefore an object of the invention to provide a method and apparatus for producing hydrochloric acid.

[0018] Description of the Invention

[0019] Accordingly, in one aspect, the present invention provides a method of producing hydrochloric acid. The method comprises fusing and electrolysing a solid chloride of a metal selected from the group consisting of sodium and magnesium, to produce the selected metal in liquid phase and chlorine gas. The method also comprises thermally decomposing at least one of calcium hydroxide, magnesium hydroxide and an iron hydroxide to produce water vapour and at least one of calcium oxide, magnesium oxide and an iron oxide, respectively. The thermal decomposition is performed in the absence of oxygen, and at least some of the water vapour produced by the thermal decomposition is captured. The method then comprises reacting at least some of the chlorine gas with at least some of the captured water vapour in a carbon-free reverse Deacon reaction at a temperature of from about 450 to about 750 “Celsius, inclusive, to produce a mixture of gases at least comprising hydrogen chloride and oxygen, and immediately contacting at least some of this mixture of gases with liquid water to dissolve the hydrogen chloride therein, thereby producing hydrochloric acid and a stream of tail gases. Since dissolving hydrogen chloride in liquid water is strongly exothermic, the hydrogen chloride is dissolved in the liquid water isothermally or substantially isothermally by extracting heat from the hydrochloric acid thus produced, until the stream of tail gases is no longer in contact with it.

[0020] This method has at least the following advantages.

[0021] Compared to some of the prior art methods of producing hydrogen chloride gas which use an RDR, the method described herein does not use carbon as a reducing agent to remove oxygen from the mixture of gases resulting from the RDR, and therefore does not produce any carbon monoxide or carbon dioxide, which can act as a source of greenhouse gas emissions. However, the method described herein is also considerably more energy efficient than prior art methods of producing hydrogen chloride gas via an RDR which do not use carbon as a reducing agent, for the following reasons. Firstly, according to the invention, the RDR is supplied with gaseous chlorine which, apart from being of high purity, also has a temperature which is already either about the same as or greater than the temperature at which the Deacon reaction reverses. Thus the chlorine gas requires no preheating to reach the range of operating temperatures for the RDR. Moreover, the thermal energy which the high-temperature chlorine has immediately after it has been produced is re-used, and is not lost to the environment. Similarly, since the thermal decomposition of the respective hydroxide(s) is performed in the absence of oxygen, the RDR is supplied with water vapour uncontaminated by reactive oxygen and which also has a temperature that in some cases is as much as about 512 “Celsius. Thus the thermal energy which the captured water vapour carries is not lost to the environment either, and since it is already at an elevated temperature, the captured water vapour requires much less, if any, preheating to reach the operating temperature range for the RDR than boiling liquid water from ambient temperature and then heating the resulting steam up from 100 “Celsius to the operating temperature range for the RDR.

[0022] Once the chlorine gas and the water vapour have been produced, therefore, the method described herein has a very low energy consumption, partly because the RDR uses reagents which are both supplied at sufficiently high temperatures for the reaction to occur and partly because they are also both co-products of other processes ( / '. e., producing the selected metal in liquid phase and producing the respective oxide(s)). Indeed, once the chlorine gas and water vapour have been produced, in some circumstances, the method of the invention can even have a negative energy consumption, as detailed below.

[0023] The thermal decomposition of the respective hydroxide(s) may be performed in the absence of oxygen by performing it under vacuum or in an atmosphere of deoxygenated air, for example. If the water vapour is supplied to the RDR from the thermal decomposition of ferric hydroxide or calcium hydroxide, it will already have a temperature of from about 475 to about 512 “Celsius. When mixed with the higher temperature chlorine gas, the mixture of these two gases will therefore have an equilibrium temperature within the desired range of operating temperatures for the RDR. Moreover, even if the water vapour is supplied to this reaction from the thermal decomposition of ferrous hydroxide, magnesium hydroxide or from the thermal decomposition of a combination of calcium hydroxide and magnesium hydroxide, the temperature of the water vapour may still be brought within the desired range of operating temperatures for the RDR by heating the water vapour up further before supplying it to the RDR, as described further below.

[0024] Fusing and electrolysing the solid chloride of the selected metal may be performed using one or more of several known techniques. For example, if the selected metal is sodium, the electrolysis may be carried out in an electrolytic cell based on the design of Downs, originally described in US patent no. 1 501 756. If, on the other hand, the selected metal is magnesium, the electrolysis may instead be carried out using any of the techniques described in "The Chemistry and Electrochemistry of Magnesium Production” by G.J. Kipouros & D.R. Sadoway in Advances in Molten Salt Chemistry, Vol. 6, edited by G. Mamantov, C.B. Mamantov & J. Braunstein, Elsevier, Amsterdam, pp. 127-209 (1987). Some embodiments may comprise fusing and electrolysing both solid sodium chloride and solid magnesium chloride separately from each other, and then combining gaseous chlorine from these two different sources. Electricity for the electrolysis may in any case be provided by a source of renewable energy, such as wind or solar, or come from nuclear power, and therefore need not create any greenhouse gas emissions.

[0025] The calcium hydroxide, magnesium hydroxide and / or iron hydroxide(s) may themselves be supplied to the thermal decomposition at above ambient temperature, for example from another process. This has the advantage that it reduces the amount of heat required for the thermal decomposition. At least some of the heat required to thermally decompose the at least one of the calcium hydroxide, magnesium hydroxide and iron hydroxide(s) may be supplied to the respective hydroxide(s) by transferring heat from one of the other products in the method of the invention which is at a higher temperature, such as from the selected metal in liquid phase. Since the selected metal in liquid phase generally has to be cooled significantly before it can be used in another process, this has the advantage that the heat which the liquid metal carries immediately after it has been produced is not wasted, but is re-used, making the overall process more energy efficient.

[0026] If an iron hydroxide is thermally decomposed to produce an iron oxide, the iron oxide may comprise Fe2O3, FeO and / or Fe3O4, depending on the conditions for the thermal decomposition. For example, ferrous hydroxide (Fe(OH)2) may be firstly oxidised in the presence of atmospheric air to ferric hydroxide (Fe(OH)3), which is then thermally decomposed in the absence of oxygen into ferric oxide (Fe2O3) and water vapour. In another example, the ferrous hydroxide may be thermally decomposed in the absence of oxygen into water vapour and ferrous oxide (FeO), which may then at least partially disproportionate into FesO4 and metallic iron. The latter example may be preferable if the end-product is to be subsequently used as a feedstock for ironmaking, as it is then in a lower oxidation state, requiring less energy for its reduction to elemental iron.

[0027] In some embodiments, the captured water vapour may be heated by at least one of adiabatic or substantially adiabatic mechanical compression, radio-frequency ( / .e., microwave) volumetric heating and transferring heat to the captured water vapour from one of the other products in the method of the invention which is at a higher temperature, such as from the selected metal in liquid phase. If the captured water vapour is heated using one of the last two techniques, it is preferably heated isochorically ( / '.e., at constant volume) or substantially isochorically, thereby simultaneously pressurizing the water vapour as well. Only a mild pressurization may be sufficient to achieve the desired increase in temperature. For example, compressing water vapour derived from the thermal decomposition of calcium hydroxide to an absolute pressure of about 1.75 atmospheres is sufficient to heat the water vapour up from an initial temperature of 500 “Celsius to 600 “Celsius. In another example, compressing water vapour derived from the thermal decomposition of magnesium hydroxide to an absolute pressure of about 5 atmospheres is sufficient to heat the water vapour up from an initial temperature of 340 “Celsius to 600 “Celsius. Compressing the captured water vapour may be used, for example, to heat the captured water vapour up enough to bring it within the desired range of operating temperatures for the RDR and possibly also to raise the temperature of the captured water vapour until it reaches or exceeds the temperature at which the Deacon reaction reverses.

[0028] Compressing the captured water vapour has several advantages, as follows. Firstly, apart from raising the temperature of the captured water vapour, it also increases the flow rate of the water vapour supplied to the RDR in comparison, mol for mol, to the flow rate of the unpressurized chlorine. In turn, this can be used to increase the percentage conversion of chlorine into hydrogen chloride by the reaction. Secondly, it can simultaneously be used to reduce the vapour pressure of water above the calcium hydroxide, magnesium hydroxide and / or iron hydroxide(s) which are being thermally decomposed. Since this also reduces the energy consumption of the thermal decomposition, and since only a mild pressurization is generally required to reach the desired temperature range for the RDR, compressing the captured water vapour in this manner requires the input of very little additional energy.

[0029] The RDR may be conducted at any temperature in a range of from about 450 “Celsius up to about 750 “Celsius. At temperatures below about 450 “Celsius, the percentage conversion of chlorine to hydrogen chloride drops below an acceptable level as the (forward) Deacon reaction is increasingly favoured. Preferably, the reaction is carried out at a temperature above about 500 “Celsius, more preferably above about 550 “Celsius, and most preferably above about 600 “Celsius, in order to increase both the percentage conversion of chlorine to hydrogen chloride and the rate of reaction through the reaction kinetics. Whereas a higher operating temperature increases both the percentage conversion and the reaction rate, carrying out the RDR at a temperature below about 750 “Celsius has the advantages that no additional heating of the chlorine gas is required and that the corrosive effects of the hydrogen chloride produced by the reaction are reduced, in comparison to conducting the RDR at more elevated temperatures. Preferably, the reaction is carried out at a temperature below about 700 “Celsius, and more preferably below about 650 “Celsius, because unlike the (forward) Deacon reaction, which is exothermic, the RDR is increasingly endothermic at higher temperatures. This therefore also reduces the amount of thermal energy required to maintain the reverse reaction.

[0030] If the reaction were performed adiabatically, or substantially adiabatically, it would therefore cool as it proceeded. This would tend to reduce both the percentage conversion of chlorine to hydrogen chloride by adjusting the thermodynamic equilibrium, as well as the rate of reaction through the reaction kinetics. The RDR is therefore preferably heated to counteract this cooling effect and to maintain the temperature of the reaction within its desired range of operating temperatures. For example, the reaction may be maintained within its desired range of from about 450 to about 750 “Celsius by transferring heat to it from one of the other products in the method of the invention which is at a higher temperature, such as from the selected metal in liquid phase.

[0031] Although the molecular mechanism of both the forward and reverse Deacon reactions is not at present fully understood, the overall reactions of Eqns. 1 and 2 undoubtedly involve several sub-steps. In the absence of a catalyst, the RDR of Eqn. 2 probably involves the substitution of a hydrogen atom in a water molecule by a chlorine atom from a Ch molecule to create a molecule of HCI and a HOCI molecule as a first intermediate species, followed by the further substitution of the remaining hydrogen atom in the HOCI molecule by a chlorine atom from another CI2 molecule to create another HCI molecule and OCI2 as another intermediate species, the latter of which then decomposes into OCI • and Cl • radicals. Decomposition of the OCI2 intermediate species into these radicals is thought to be the rate-determining step. These radicals may then attack two further water molecules to produce two more HCI molecules, as well as an OH • radical and an HO2 * radical ( / '.e., HO-O • ), which recombine to produce an O2 molecule and another molecule of water, thereby producing a total of four HCI molecules and one molecule of O2 for the consumption of two water molecules and two CI2 molecules.

[0032] The water molecule is an asymmetric top and water vapour has a strong absorption peak at 22.24 GHz in the IEEE K-band of the microwave spectrum. The HOCI and OCI2 intermediate species, both of which are isostructural to water, also have microwave absorption peaks as follows: HOCI at 29.3 and 29.8 GHz in the IEEE Ka-band (with resonances at intervals every 29-30 GHz above that) and OCI2 over a range of frequencies from about 9.4 to about 40 GHz, which are so numerous as to form a continuum in the IEEE Ku-, K- and Ka-bands. However, since CI2 and O2 are both homonuclear diatomic molecules, neither of them has an electric dipole which is able to interact with an applied electromagnetic field (although O2 has a magnetic dipole which gives molecular oxygen an absorption peak at 50.9-61.2 GHz in the IEEE V-band). Moreover, HCI, which has a strong electric dipole, only has an absorption peak at the much higher frequency of 625 GHz in the IEEE THF band. In some embodiments, therefore, the method may comprise radio-frequency volumetric heating of the RDR in the absence of a catalyst in at least one of the IEEE Ku-, K- and Ka-bands. This has the advantage that it directly heats not just one of the reagents in the reaction ( / '. e., water), but also both of the intermediate species HOCI and OCI2, in preference to heating either of the reaction products, HCI or O2. Moreover, since the rotational modes of the intermediate species OCI2 are excited by electromagnetic radiation at these frequencies, this may also help to contribute to the decomposition of OCI2 molecules into OCI • and Cl • radicals. In particular, a frequency band at 24 to 24.25 GHz, which lies within the continuum for OCI2 and between the absorption peaks for water and HOCI, is designated by the International Telecommunications Union (ITU) as available for industrial, scientific, and medical (ISM) use without requiring a licence (a Type B band). The enthalpy of reaction of Eqn. 2 would be about + 119 kJ at 600 “Celsius and an absolute pressure of one atmosphere if the conversion rate of chlorine to hydrogen chloride were 100%. However, since the reaction of Eqn. 2 is in thermodynamic equilibrium with the reaction of Eqn. 1 at this temperature, the theoretical conversion rate at 600 “Celsius is instead 50%. At or around this temperature, therefore, the RDR is better represented by:

[0033] 2 Cl2 (g) + 2 H2O (g) — > 2 HCI (g) + CI2 (g) + V2 O2 (g) + H2O(g) [Eqn. 3] for which the enthalpy of reaction is about + 119 / 2 = + 59.5 kJ. On the other hand, dissolving the hydrogen chloride gas produced by the RDR in liquid water is strongly exothermic. For example, if the RDR has a 50% conversion rate of chlorine to hydrogen chloride, the total quantity of heat, Qtotai, produced by dissolving the 2 mol of hydrogen chloride gas from Eqn. 3 at one atmosphere pressure is given by:

[0034] Qtotai = 2 [ Hsoiution(HCI) - Hdiiution(HCI) + Qout(HCI) - Qin(H2O)] [Eqn. 4a]

[0035] = 2 [ AHSOiution(HCI) - AHdiiution(HCI) + CP(HCI) AT(HCI) - n(H2O) CP(H2O) AT(H2O)] [Eqn. 4b] where AHSOiution(HCI) is the standard enthalpy of solution of hydrogen chloride gas (= - 74.85 kJ mol’1), AHdiiution(HCI) accounts for the fact that the hydrochloric acid thus produced is not at infinite dilution, Q0Ut(HCI) is the heat produced by cooling the hydrogen chloride gas from the temperature of the RDR to the temperature of the liquid water, and Qin(H2O) accounts for the fact that the liquid water is not at standard temperature. AHdiiution(HCI), which depends on the molality of the hydrochloric acid produced, typically has a value of from about + 3 to about + 7 kJ mol1at molalities of from about 2.5 m to about 7 m. Q0Ut(HCI) = CP(HCI) AT(HCI), wherein CP(HCI) is the molar heat capacity at constant pressure of the hydrogen chloride gas and AT(HCI) is the temperature drop of the hydrogen chloride gas, « -17 kJ mol’1if AT(HCI) = - 550 K, for example. Qin(H2O) = n(H2O) CP(H2O) AT(H2O), wherein n(H2O) is the number of mols of liquid water present per mol of hydrogen chloride, CP(H2O) is the molar heat capacity at constant pressure of liquid water and AT(H2O) is the temperature difference of the liquid water above standard temperature, « + 26 kJ mol’1(HCI) if n(H2O) = 10 and AT(H2O) = + 35 K, for example. Thus, in this example, the total quantity of heat produced by dissolving the hydrogen chloride gas in the liquid water, Qtotai = 2 (- 74.85 + c.5 - 17 + 26) ~ - 122 kJ, which is more than double the quantity of heat (+ 59.5 kJ) required to maintain the RDR within its desired range of operating temperatures. However, as mentioned above, heat is extracted when contacting the mixture of gases produced by the RDR with liquid water, in order to keep this process substantially isothermal, where in this context, "substantially” means that the hydrochloric acid is kept preferably within about 30 “Celsius, more preferably within about 20 “Celsius and most preferably within about 10 “Celsius of the initial temperature of the liquid water. Therefore, at least some of this extracted heat may be used to maintain the RDR within its intended range of operating temperatures of from about 450 to about 750 “Celsius. For example, heat recovered from contacting the mixture of gases with liquid water can be used to generate electricity for radiofrequency volumetric heating of the RDR as described above and / or for at least one of adiabatic mechanical compression and radio-frequency volumetric heating of the captured water vapour, even if the heat recovery and electricity generation are less than 100% efficient. The method of the invention is therefore considerably more economic than, for example, the method described in US patent no. 1 874 225, in spite of the lower operating temperatures at which the RDR is carried out, because it is more energy efficient, for the reasons explained above.

[0036] The residence time of the chlorine gas and the captured water vapour in the reaction chamber in which the RDR is carried out should preferably be long enough for their complete mixing, in order to increase the percentage conversion of chlorine to hydrogen chloride. However, since the chlorine gas may have a higher temperature than the captured water vapour, the residence time can be reduced by transferring heat from the chlorine gas to the captured water vapour, without bringing the chlorine gas into contact with the captured water vapour, before the chlorine and the water vapour are reacted with each other in the RDR. Thus the two gases required for the RDR can be brought into thermal equilibrium with each other before they are reacted together. The reaction rate may also be enhanced significantly, and therefore the residence time reduced, either by employing a catalyst or by radio-frequency volumetric heating as described above.

[0037] Since catalysts enhance the reaction rate of reversible reactions in both directions, any of the same catalysts as are already known and / or used to enhance the reaction rate of the (forward) Deacon reaction may also be used to enhance the reaction rate of the RDR, to the extent that such catalysts can withstand the higher range of operating temperatures at which the RDR is carried out according to the invention. For example, some suitable catalysts for the (forward) Deacon reaction are disclosed in M. W. H. Hisham & S. W. Benson: "Thermochemistry of the Deacon Process”, J. Phys. Chem., Vol. 99, No. 16, pp. 6194-6198 (1995), H. Over & R. Schomacker: "What Makes a Good Catalyst for the Deacon Process?”, ACS Catal., Vol. 3, No. 5, pp. 1034-1046 (2013), and European patent appln. no. EP 2 098 290 A1 . Examples of catalysts known to be suitable for the (forward) Deacon reaction which can also withstand the higher range of operating temperatures of the present invention include lanthanum (III) oxide (La2O3) and ruthenium (IV) oxide (RUO2).

[0038] The relative rates of supply of the chlorine gas and the captured water vapour to the RDR are preferably such that the flow rate of water vapour is in excess of the stoichiometric amount of water vapour required for the reaction, and more preferably, at least twice as great, mol for mol, as the flow rate of chlorine gas, as is known from the prior art. US patent no. 1 874 225 teaches supplying the water vapour to the RDR at a flow rate which is in excess of the stoichiometric amount thereof required for the reaction of Eqn. 2. US patent no. 1 229 509 suggests that even in the presence of a reducing agent such as carbon, the yield of hydrogen chloride from the RDR can be increased by supplying the water vapour at a flow rate which is significantly higher than the flow rate of chlorine gas.

[0039] Since the chlorine gas and the captured water vapour both have very high purities, the mixture of gases produced by the RDR has a very high purity as well, and just comprises the hydrogen chloride and oxygen produced by the reaction, as well as any unreacted chlorine and water vapour. For example, if the conversion rate of chlorine to hydrogen chloride is 50%, the mixture of gases will comprise 1 mol of unreacted chlorine, 0.5 mol of oxygen and 1 mol of water vapour for every 2 mol of hydrogen chloride produced by the reaction, as well as however much excess water vapour is supplied to the reaction. Moreover, since the RDR is carried out at a temperature above that at which the (forward) Deacon reaction is usually performed, it does not require the use of a reducing agent, such as carbon, to pull the reaction of Eqn. 2 to the right, in order to increase the percentage conversion of chlorine to hydrogen chloride at lower temperatures. The mixture of gases produced by the reaction is therefore not contaminated by any other gases like carbon monoxide or carbon dioxide, for example.

[0040] Immediately contacting this mixture of gases with liquid water in the present context means contacting the mixture of gases with liquid water before the mixture of gases is able to cool to below the stated range of operating temperatures for the RDR. This has several advantages, as follows. Firstly, the corrosive effects of the high- temperature hydrogen chloride in the mixture of gases are minimized because it dissolves in the liquid water as soon as it is produced. Secondly, it pulls the reactions of Eqns. 2 and 3 to the right according to Le Chatelier's principle, by removing the hydrogen chloride gas from the reaction mixture. Thirdly, since liquid water has a high specific heat capacity in comparison to the mixture of gases, not only is the RDR immediately terminated, but the (forward) Deacon reaction of Eqn. 1 is quenched as well, because the temperature of the liquid water is so much lower than the temperature at which the (forward) Deacon reaction can proceed. Any back-conversion of the products of the RDR into its starting materials is therefore substantially prevented, in comparison to a slower gasphase cooling of the reaction products.

[0041] Preferably, the mixture of gases produced by the RDR is contacted with liquid water having a temperature of from about 50 to about 70 “Celsius, inclusive. This has the advantage that in this temperature range, only the hydrogen chloride produced by the RDR dissolves in the liquid water to any appreciable extent. The graphs of Figs. 1 and 2, both of which represent the situation at atmospheric pressure, demonstrate why. As shown in Fig. 1 , above about 50 “Celsius, the solubility of hydrogen chloride gas in liquid water is always greater than about 420 g kg-1(H2O). In contrast, as Fig. 2 shows, above about 50 “Celsius, the solubility of chlorine gas in liquid water is at most about 4.2 g kg FkO) and the solubility of oxygen in liquid water is negligible. Moreover, at temperatures of 50 “Celsius or more, the solubility of chlorine gas in hydrochloric acid is less than the solubility of chlorine gas in liquid water. The hydrochloric acid produced by contacting the mixture of gases with liquid water above this temperature will therefore have a purity of at least 99%. On the other hand, since the liquid water has a temperature not greater than about 70 “Celsius, and more preferably not greater than about 60 “Celsius, evaporation of water vapour from the liquid water into the mixture of gases is substantially avoided, even after the liquid water has absorbed some of the heat from the mixture of gases.

[0042] The mixture of gases does not have to reach thermal equilibrium with the liquid water. Thus the stream of tail gases produced by contacting the mixture of gases with the water may still have a temperature greater than that of the hydrochloric acid which is also produced. Nonetheless, since contacting the mixture of gases with liquid water can reduce the temperature of the stream of tail gases significantly in comparison to the temperature of the mixture of gases produced by the RDR, the isochoric ( / . e. , constant-volume) pressure of the stream of tail gases may also be considerably less than that of the mixture of gases produced by the RDR. Dissolving the hydrogen chloride gas in the liquid water also reduces the isochoric pressure of the stream of tail gases significantly in comparison to that of the mixture of gases from the RDR, by removing substantially all of one of the products from the reaction of Eqn. 3. Thus, the RDR may be performed at or around atmospheric pressure, and if so, the pressure of the stream of tail gases may be maintained at a similar level by proportionally reducing the cross-sectional area of any pipework carrying away the stream of tail gases. Alternatively, the RDR may be performed with a mild pressurization above atmospheric pressure, of up to about 2, 3 or even 5 atmospheres, for example. In such a case, the pressure of the stream of tail gases may be maintained at or around atmospheric pressure by using pipework to carry away the stream of tail gases which has a cross-sectional area not significantly less than that of any pipework carrying the gases to the RDR.

[0043] The mixture of gases produced by the RDR may be contacted with the liquid water in one of several different ways. In some embodiments, for example, contacting the mixture of gases with liquid water may comprise bubbling the mixture of gases through a body of liquid water. In other embodiments, for example, contacting the mixture of gases with liquid water may comprise passing the mixture of gases through a mist or stream of droplets of liquid water, as in a scrubber. Preferably, however, contacting the mixture of gases with liquid water comprises using a falling film absorber. In any event, any unreacted water vapour which remains from the RDR will form a dynamic equilibrium with the liquid water at the partial pressure of water vapour which prevails at the temperature of the liquid-vapour system. Thus the stream of tail gases will only comprise oxygen produced by the RDR, water vapour and unreacted chlorine.

[0044] The relative rates of supply of the mixture of gases and the liquid water, as well as the rates of outflow of the hydrochloric acid thus produced and of the stream of tail gases can be determined according to the desired concentration of the hydrochloric acid. However, the concentration of the hydrochloric acid should preferably not be allowed to rise above about 27% by weight or 8.4 M, in order to avoid its undesirable evaporation or boiling, which would tend to obstruct the dissolution of the hydrogen chloride gas therein. At this and lower concentrations, an aqueous solution of hydrogen chloride always boils at more than 100 “Celsius under one atmosphere of pressure. This boiling point is far enough above the preferred maximum temperature of 70 “Celsius for the liquid water before the hydrogen chloride gas is dissolved therein to avoid any substantial evaporation or risk of boiling. However, the concentration of the hydrochloric acid should preferably be at least about 0.5 M because at lower concentrations than this, the solubility of chlorine gas in hydrochloric acid starts to rise significantly. More preferably still, the concentration of the hydrochloric acid should be at least about 3 M. This has the advantage that by reducing the consumption of liquid water, which has a high specific heat capacity, the amount of thermal energy required to raise the temperature of the liquid water to within its preferred temperature range (= Qin(H2O) in the example given above) is thereby also reduced.

[0045] In some embodiments, the liquid water used to dissolve the hydrogen chloride from the RDR in order to produce the hydrochloric acid may be produced in the following way. Firstly, an aqueous solution of the chloride of the selected metal or an aqueous solution of sodium hydroxide is dried to produce the chloride of the selected metal or sodium hydroxide in solid phase and water vapour. The water vapour is then captured and condensed, for example by thermal contact with a cooler surface in a heat exchanger, to produce the liquid water. This method has the advantage that the liquid water thus produced is of high purity.

[0046] The aqueous solution of the chloride of the selected metal or of sodium hydroxide may be supplied at above ambient temperature, for example from another process. This has the advantage that it reduces the amount of energy required to dry the aqueous solution. Drying the aqueous solution may be carried out, for example, by evaporation and / or by boiling the aqueous solution until the chloride of the selected metal or the sodium hydroxide reaches saturation in the solution and crystallizes out. In some embodiments, a so-called "multi-effect” evaporator may be used. If the aqueous solution is supplied at a temperature which is close to its boiling point, very little additional energy is required to dry it out, other than the enthalpy of vaporization of the water of solution. Moreover, if, for example, the aqueous solution is heated at less than atmospheric pressure, the aqueous solution may be caused to boil at a temperature of less than 100 “Celsius, and even at a temperature which is within the desired temperature range for the liquid water used to dissolve the hydrogen chloride from the RDR. This has the advantage that the liquid water used to dissolve the hydrogen chloride from the RDR can then be supplied in its preferred temperature range of from about 50 up to about 70 “Celsius at the same time as the chloride of the selected metal in solid phase is produced, and at least some of the energy used to dry the aqueous solution can simultaneously be recovered as enthalpy of condensation when the water vapour captured from the aqueous solution is condensed. This makes the production of the liquid water in the desired temperature range more energy efficient than heating liquid water from ambient temperature, and also reduces the consumption of water. Furthermore, at least some of the chloride of the selected metal in solid phase produced by drying the aqueous solution may then be used as a feedstock for fusing and electrolysing the solid chloride of the selected metal.

[0047] At this stage, the hydrochloric acid has been produced, so in principle, it would be possible to treat the stream of tail gases as a waste product. However, it is desirable, both from an economic and from an environmental point of view, to process the stream of tail gases. In particular, the stream of tail gases may be processed to provide a stream of oxygen and an oxygen-depleted stream. This allows the stream of tail gases from which at least some of the oxygen has been removed to be recycled back to the RDR. This has the advantage of greatly reducing the overall consumption of reagents by the RDR. In some embodiments, therefore, at least some of the oxygen produced by the RDR may be separated from any unreacted chlorine and water vapour in the stream of tail gases, for further use of one or both of these products of such a separation process. For example, at least some of the oxygen separated from the stream of tail gases may be used in a steelmaking process by injecting the oxygen into molten iron. In another example, at least some of the oxygen-depleted stream may be recycled back to the RDR. In such a case, at or around the temperature at which the (forward) Deacon reaction reverses, Eqn. 3 may be replaced by Eqn. 5a shown below. As can be seen by comparing Eqn. 5a with Eqn. 3, recycling at least some of the oxygen-depleted stream in this manner effectively doubles the overall percentage conversion of chlorine into hydrogen chloride by the RDR, and therefore the overall atom economy and efficiency of the process. Water vapour captured from thermal decomposition: H2O <g)

[0048] > I Unreacted chlorine

[0049] I 1 t

[0050] 2 CI2 (g) + 2 H2O (g) — > 2 HCI (g) + CI2 (g) +1 / 2 O2 (g) + H2O(g) [Eqn. 5a]

[0051] Chlorine from f electrolysis: Chfg) Unreacted water vapour

[0052] Moreover, recycling at least some of the oxygen-depleted stream also has the advantage that if the water vapour is supplied to the RDR in excess of the stoichiometric amount thereof which is required, since the reaction only actually consumes the stoichiometric amount, the excess water vapour is also recycled, rather than being wasted. In such a case, at or around the temperature at which the (forward) Deacon reaction reverses, Eqn. 5a then becomes:

[0053] Water vapour captured from thermal decomposition: H2O <g) | Unreacted chlorine

[0054] 2 CI2 (g> + (2 + e) H2O(g)— > 2 HCI(g)+ CI2 (g) +1 / 2 O2 (g) + (1 + e) H2O(g)[Eqn. 5b]

[0055] Chlorine from f electrolysis: Cl2 (g) Unreacted water vapour wherein e represents the excess number of mols of unreacted water vapour recycled to the reaction. This therefore also maintains the overall consumption of water vapour by the reaction at or below the level supplied by thermally decomposing the calcium hydroxide, magnesium hydroxide and / or iron hydroxide(s).

[0056] The oxygen can be separated from the stream of tail gases using one or more known techniques, including distillation, condensation and flash evaporation, a membrane separation technique such as gas permeation according to the molecular sieve principle, for example using a carbon membrane, and / or gas permeation according to the solution diffusion principle, for example using a polymer membrane, including any combination of the techniques just mentioned. The separation of a stream of tail gases from the (forward) Deacon reaction into a stream of oxygen and an oxygen-depleted stream is a well-known and understood process. Since the stream of tail gases from the RDR contains the same component gases, any of the same techniques may also be used to separate a stream of tail gases from the RDR into a stream of oxygen and an oxygen-depleted stream. For example, some suitable separation techniques are described in US patent no. US 5 861 049 A and US patent appln. nos. US 2007 / 0 286 793 A1 and US 2007 / 0 277 551 A1. With any of these techniques, or a combination thereof, separation of the stream of tail gases into a stream of oxygen and an oxygen-depleted stream can be achieved to a very high degree of purity, better than 99%.

[0057] If required, the stream of oxygen can be dried using a known technique to remove any remaining water vapour, and subjected to repeated separation as above to remove any residual chlorine or hydrogen chloride, before the oxygen is used. In contrast, if the oxygen-depleted stream is going to be recycled back to the RDR, there is no need to dry the oxygen-depleted stream to remove any remaining water vapour, since water vapour is, of course, one of the reagents of the RDR. Similarly, a small amount of residual oxygen may be left in the oxygen-depleted stream if it is to be recycled back to the RDR. However, any more than just a small amount of residual oxygen is generally undesirable, as it will tend to reduce the percentage conversion of chlorine into hydrogen chloride, by pushing the reaction of Eqns. 2, 3, 5a and 5b to the left.

[0058] In some cases, separating at least some of the oxygen from the stream of tail gases may comprise cooling the stream of tail gases, for example as part of a distillation process. At atmospheric pressure, water boils at 100 “Celsius and chlorine at -34 “Celsius, whereas oxygen boils at -183 “Celsius. Separating at least some of the oxygen from the stream of tail gases may therefore comprise, for example, cooling the stream of tail gases at atmospheric pressure, firstly to less than 100 “Celsius to condense out the water vapour, then to a temperature below -34 “Celsius but still considerably above -183 “Celsius to condense out the chlorine and leave the oxygen uncondensed. If so, heat extracted from the stream of tail gases may be transferred to at least one of the products of separating the oxygen from the stream of tail gases. This has the advantage that at least some of the heat which the stream of tail gases has after the hydrochloric acid has been produced is not wasted, but is re-used to at least partially reheat a product of the separation process, making the overall process more energy efficient.

[0059] If separating at least some of the oxygen from the stream of tail gases comprises cooling the stream of tail gases, condensing the water vapour captured from drying the aqueous solution may comprise cooling the captured water vapour by bringing it into thermal but not physical contact with either the cooled stream of tail gases or a product of separating the oxygen from the stream of tail gases, or both, thereby at least partially reheating the cooled stream of tail gases and / or the separation product, respectively. This has the advantage that heat can be recovered from the water vapour when it condenses and be reused. For example, the recovered heat may be used to contribute to reheating the stream of oxygen before it is used in a steelmaking process. In another example, the recovered heat may be used to contribute to reheating the oxygen-depleted stream before it is recycled back to the RDR. In the latter case, since the molar enthalpy of condensation of water is almost exactly double that of chlorine, the recovered heat is more than enough to revaporise the chlorine in the oxygen-depleted stream, if it has been condensed out during separation, as well to raise the temperature of the oxygen-depleted stream thereafter.

[0060] In some embodiments, fusing and electrolysing the solid chloride of the selected metal may produce an amount of chlorine gas which is in excess of a stoichiometric amount thereof required for the RDR. If so, the excess amount of chlorine can be branched off from the stoichiometric amount of chlorine before the stoichiometric amount is allowed to react with at least some of the captured water vapour in the RDR. In such a case, heat may also be transferred from at least some of the excess chlorine which is branched off to at least one of the stream of oxygen and the oxygen-depleted stream, which are produced by separating the oxygen from the stream of tail gases. This has the effect of at least partially reheating at least one of these products of separating the oxygen from the stream of tail gases, respectively. For example, electrolysing the fused chloride of the selected metal may produce double the stoichiometric amount of chlorine gas required for the RDR. In such a case, half the total amount of chlorine produced by electrolysis may be branched off and the other half may be reacted with the captured water vapour in the RDR. The half which is branched off may then be used to reheat at least one of the products of separating the oxygen from the stream of tail gases. In the example just given, wherein the electrolysis produces double the stoichiometric amount of chlorine required for the RDR, the amount of chlorine branched off will also be at least double the amount of oxygen produced by the reaction of Eqn. 3. Since the molar heat capacity at constant pressure of diatomic chlorine is somewhat greater than that of diatomic oxygen anyway, the excess chlorine is therefore able to raise the temperature of the stream of oxygen and / or of the oxygen-depleted stream significantly.

[0061] This has several advantages, as follows. It avoids supplying an excess amount of chlorine to the RDR. This is undesirable because it would leave the excess chlorine unconverted into hydrogen chloride by the reaction. On the other hand, since chlorine is also a valuable commercial product, the excess chlorine should not be wasted. However, the excess amount of chlorine is at the same temperature immediately after it has been produced as the stoichiometric amount of chlorine which is required for the RDR. It therefore carries away some of the thermal energy from the electrolysis. T ransferring heat from the excess chlorine to one or both of the products of separating the oxygen from the stream of tail gases simultaneously reduces the temperature of the excess chlorine, thereby preparing it to be bottled and transported, whilst also raising the temperature of the stream of oxygen and / or of the oxygen-depleted stream produced by separating the oxygen from the stream of tail gases.

[0062] The method of the invention may be carried out as a continuous, semi-batch or batch process. However, it is preferably carried out as a continuous process for reasons of economy and efficiency.

[0063] In a second aspect, the present invention also provides an apparatus for producing hydrochloric acid. The apparatus at least comprises an electrolytic cell for fusing and electrolysing a solid chloride of a metal selected from the group consisting of sodium and magnesium, to produce the selected metal in liquid phase and chlorine gas, a gas-tight kiln for thermally decomposing at least one of calcium hydroxide, magnesium hydroxide and an iron hydroxide in the absence of oxygen to produce water vapour and at least one of calcium oxide, magnesium oxide and an iron oxide, respectively, a gas-phase reactor for reacting at least some of the chlorine gas with at least some of the water vapour in a carbon-free reverse Deacon reaction at a temperature of from 450 to 750 “Celsius to produce a mixture of gases at least comprising hydrogen chloride and oxygen, and an absorber for contacting the mixture of gases with liquid water to produce hydrochloric acid and a stream of tail gases.

[0064] The electrolytic cell comprises an inlet for the chloride of the selected metal in solid phase, a first outlet for the selected metal in liquid phase and a second outlet for the chlorine gas. The kiln comprises an inlet for receiving the respective hydroxide(s), a first outlet for the water vapour, a second outlet for the respective oxide(s), and a heater for injecting heat into the respective hydroxide(s). Since the kiln is gas-tight, the respective hydroxide(s) can be thermally decomposed therein in the absence of oxygen. The kiln may be purged of atmospheric air before commencing operation. This may be done, for example, by evacuating the kiln or by flushing the kiln with deoxygenated air or with steam.

[0065] The gas-phase reactor comprises a first inlet for receiving the chlorine gas from the second outlet of the electrolytic cell, a second inlet for receiving the water vapour from the first outlet of the kiln, and an outlet for the mixture of gases. In some embodiments, the gas-phase reactor may contain a catalyst, such as one of those mentioned above, to enhance the reaction rate of the RDR. However, the gas-phase reactor does not contain any carbon to act as a reducing agent for removing oxygen from the mixture of gases produced by the RDR. The absorber comprises a first inlet for receiving the mixture of gases from the outlet of the gas-phase reactor, a second inlet for receiving the liquid water, a first outlet for the hydrochloric acid, a second outlet for the stream of tail gases, and means for extracting heat from the hydrochloric acid before it leaves via the first outlet.

[0066] Components of the apparatus which come into contact with chlorine gas, hydrogen chloride gas or hydrochloric acid should preferably be made of materials resistant to the corrosive effects of these substances. Such materials as are already used to resist their corrosive effects may therefore be employed, as described, for example, in the article "Corrosion by Hydrogen Chloride and Hydrochloric Acid” by J.R. Crum in Volume 13C of the ASM Handbook, Corrosion: Environments and Industries, edited by S. D. Cramer and B.S. Covino, Jnr. ASM International (2006), ISBN: 978 1 62708 184 9. For example, the gas-phase reactor and the absorber may both be made of borosilicate glass. The absorber may, for example, comprise a QVF® Supra shell-and-tube heat exchanger available from De Dietrich Process Systems GmbH of Mainz, Germany.

[0067] Such an apparatus may be used to carry out a method according to the first aspect of the invention.

[0068] In some embodiments, the apparatus may comprise a separator for separating at least some of the oxygen produced by the RDR from any unreacted chlorine and water vapour in the stream of tail gases. The separator comprises an inlet for receiving the stream of tail gases from the second outlet of the absorber, a first outlet for an oxygen-depleted stream and a second outlet for a stream of oxygen. Such a separator may therefore be used to process the stream of tail gases into usable products. Preferably, the first outlet of the separator is connected upstream of at least one of the first and second inlets of the gas-phase reactor, so that the oxygen-depleted stream can be recycled back to the gas-phase reactor as reagents for the RDR, thereby increasing the atom economy and efficiency of the process.

[0069] In some embodiments, the second outlet of the separator may be connected upstream of an inlet of a steelmaking apparatus, thereby allowing the stream of oxygen to be used in a steelmaking process by injecting the stream of oxygen into molten iron.

[0070] In some embodiments, the apparatus may comprise an electric generator for generating electricity from the heat extracted from the hydrochloric acid produced in the absorber by the means for extracting heat therefrom, and a compressor, at least partially powered by the electric generator and located downstream of the first outlet of the kiln and upstream of the second inlet of the gas-phase reactor. For example, the electric generator may comprise an organic Rankine cycle generator connected to a heat exchanger which circulates an organic heat transfer fluid through and / or around the absorber in a loop, allowing the electric generator to use the heat thus extracted to generate electricity via the Rankine cycle. The compressor may be used to compress the water vapour from the kiln adiabatically or substantially adiabatically, and / or to depressurise the kiln, as described above.

[0071] In some embodiments, the apparatus may comprise an electric generator for generating electricity from the heat extracted from the hydrochloric acid produced in the absorber by the means for extracting heat therefrom, and a dielectric heater, at least partially powered by the electric generator, for radio-frequency volumetric heating of the contents of at least one of: a point downstream of the first outlet of the kiln and upstream of the second inlet of the gas-phase reactor, and the gas-phase reactor itself. For example, the electric generator may comprise an organic Rankine cycle generator which operates as just described above. The dielectric heater may comprise a radiofrequency generator and a waveguide, for radio-frequency volumetric heating of water vapour from the kiln, either downstream of the first outlet of the kiln and upstream of the second inlet of the gas-phase reactor, within the gasphase reactor itself, or both.

[0072] In some embodiments, the apparatus may comprise a first heat exchanger for transferring heat from downstream of the first outlet of the electrolytic cell to at least one of: the kiln, a point downstream of the first outlet of the kiln and upstream of the second inlet of the gas-phase reactor, the gas-phase reactor itself, and a point downstream of at least one of the first and second outlets of the separator. Such a heat exchanger may be used to transfer heat from the liquid sodium to the respective hydroxide(s) and / or to the water vapour produced by thermal decomposition of the respective hydroxide(s) and / or to at least one of the products of separating the oxygen from the stream of tail gases. For example, the stream of oxygen may be reheated in this manner before being injected into molten iron as part of a steelmaking process, and / or the oxygen-depleted stream may be similarly reheated before it is recycled back to the RDR. In general, however, the oxygen-depleted stream should only be reheated to a temperature well below the bottom of the range of operating temperatures for the RDR (for example to less than about 350 “Celsius), to prevent any premature reaction of the constituents of the oxygen-depleted stream before it reaches the gas-phase reactor.

[0073] In some embodiments, the apparatus may comprise a second heat exchanger for transferring heat from downstream of the second outlet of the electrolytic cell and upstream of the first inlet of the gas-phase reactor to downstream of the first outlet of the kiln and upstream of the second inlet of the gas-phase reactor. This heat exchanger may be used to transfer heat from the chlorine gas to the water vapour produced by thermal decomposition of the respective hydroxide(s).

[0074] In some embodiments, the apparatus may comprise a dryer for drying an aqueous solution of the chloride of the selected metal or of sodium hydroxide to produce the chloride of the selected metal or sodium hydroxide, respectively, in solid phase and water vapour, and a condenser for condensing at least some of the water vapour produced by drying the aqueous solution. The dryer comprises an inlet for receiving the aqueous solution, a first outlet for the water vapour and a second outlet for the chloride of the selected metal or sodium hydroxide in solid phase. The condenser comprises an inlet for receiving the water vapour from the first outlet of the dryer and an outlet for liquid water. The outlet of the condenser is connected upstream of the second inlet of the absorber. Such a dryer and condenser may be used to produce at least some of the liquid water in which the hydrogen chloride is dissolved to produce the hydrochloric acid.

[0075] The dryer may be gas-tight, firstly to facilitate capture of the water vapour from the aqueous solution, and secondly to allow evaporation and / or boiling of the aqueous solution at a reduced temperature by depressurising the dryer to less than atmospheric pressure. The dryer and the condenser may be parts of a so-called "multi-effect” evaporator, for example. However, unlike the kiln, there is no need to purge the dryer of atmospheric air before commencing operation, because the subsequent condensation of the captured water vapour ensures that it is separated from any atmospheric air entrained with the water vapour.

[0076] In some embodiments, the condenser may comprise a cooler of the water vapour, and the separator may comprise a cooler of the stream of tail gases. If so, the cooler of the water vapour may be adapted and arranged to transfer heat to downstream of the cooler of the stream of tail gases. For example, downstream of the cooler of the stream of tail gases may be downstream of at least one of the first and second outlets of the separator. Thus heat extracted by cooling the water vapour may be reused to at least partially reheat a product of the separation process, rather than being wasted.

[0077] In some embodiments, the separator may comprise at least two coolers of the stream of tail gases. Each of these coolers may comprise a heat exchanger and / or a cold side of a heat pump, such as a cold side of a vapourcompression cycle refrigerator or of a thermoelectric heat pump, for example. In such embodiments, at least one of the coolers may be adapted and arranged to transfer heat to a point downstream of another one of the coolers. Thus heat extracted by cooling the stream of tail gases may be reused to at least partially reheat a product of the separation process, rather than being wasted.

[0078] In some embodiments, the apparatus may comprise an outlet for chlorine gas produced by the electrolytic cell in excess of the stoichiometric amount thereof required for the RDR, wherein the outlet for excess chlorine gas branches off from the second outlet of the electrolytic cell. If so and if the separator also comprises a cooler of the stream of tail gases, the apparatus may also comprise a third heat exchanger for transferring heat from downstream of the outlet for excess chlorine gas to at least one of: the kiln, a point downstream of the first outlet of the kiln and upstream of the second inlet of the gas-phase reactor, the gas-phase reactor itself, and a point downstream of the cooler of the stream of tail gases. For example, downstream of the cooler of the stream of tail gases may be downstream of at least one of the first and second outlets of the separator. Thus at least some of the heat carried by the excess chlorine gas may be used to heat water vapour from the kiln and / or to at least partially reheat one or both products of the separation process, rather than being wasted.

[0079] In some embodiments, the second outlet of the dryer may be connected upstream of the inlet of the electrolytic cell. Thus the chloride of the selected metal in solid phase produced in the dryer may be recycled back into the electrolytic cell for electrolysis. In some embodiments, the gas-phase reactor may have a cross-sectional area which increases by at least about one-eighth in a direction towards the outlet of the gas-phase reactor. This has the following advantages. Since the reaction of Eqns. 3, 5a and 5b produces a total of 4.5 mols of gaseous products for every 4 mols of gaseous chlorine and water vapour which are consumed, the volume of gas produced by the reaction can increase by at least one-eighth, if the percentage conversion of chlorine into hydrogen chloride is at least 50%. Increasing the cross-sectional area of the gas-phase reactor by at least one-eighth in a direction towards its outlet therefore prevents a gradient of increasing pressure from developing within the gas-phase reactor in the same direction, caused by the RDR. Such a gradient of increasing pressure would be undesirable, as it would tend to reduce the percentage conversion of chlorine into hydrogen chloride by pushing the reaction of Eqns. 3, 5a and 5b to the left. On the other hand, increasing the cross-sectional area of the gas-phase reactor by more than about one-eighth in the same direction creates a gradient of decreasing pressure towards the outlet, which therefore favours the conversion of chlorine into hydrogen chloride, by helping to pull the reaction of Eqn. 2 to the right. However, the cross-sectional area of the gas-phase reactor should not increase by more than about 50% in the direction towards its outlet, as this would tend to reduce the reaction rate, by reducing the frequency of collisions between the reacting species. In some embodiments, the percentage increase in the cross-sectional area of the gas-phase reactor in the direction towards its outlet may be selected in dependence on the temperature of the RDR inside the gasphase reactor, because a higher operating temperature for the RDR produces a higher percentage conversion of chlorine to hydrogen chloride, which, therefore, would otherwise result in an increased pressure gradient towards the outlet of the gas-phase reactor.

[0080] It should, of course, be understood that the apparatus of the invention is not open to the environment and is therefore operated as a closed system, into which reagents are introduced and from which products are removed. The apparatus of the invention should, of course, also be properly thermally insulated from its environment for greater thermal efficiency.

[0081] In another aspect not forming part of the present invention, there is also provided a method of producing hydrochloric acid, comprising reacting chlorine gas with water vapour in an uncatalysed reverse Deacon reaction to produce a mixture of gases at least comprising hydrogen chloride and oxygen, heating the reverse Deacon reaction by radio-frequency volumetric heating in at least one of the IEEE Ku-, K- and Ka-bands to maintain its temperature at from about 450 to about 750 “Celsius, inclusive, immediately contacting at least some of the mixture of gases with liquid water to dissolve the hydrogen chloride therein, thereby producing hydrochloric acid and a stream of tail gases, and extracting heat from the hydrochloric acid thus produced to maintain its temperature substantially constant, until the stream of tail gases is no longer in contact therewith.

[0082] Brief Description of the Drawings

[0083] Further features and advantages of the present invention will become apparent from the following detailed description, which is given by way of example and in association with the accompanying drawings, in which: Fig. 1 is a graph of the solubility of hydrogen chloride gas in liquid water at atmospheric pressure and as a function of temperature;

[0084] Fig. 2 is a graph of the solubility of chlorine gas and oxygen gas in liquid water at atmospheric pressure and as a function of temperature;

[0085] Fig. 3 is a flow diagram of a first embodiment of a method of producing hydrochloric acid;

[0086] Fig. 4 is a flow diagram of a second embodiment of such a method;

[0087] Fig. 5 is a flow diagram of a third embodiment of such a method;

[0088] Fig. 6 is a flow diagram of a fourth embodiment of such a method;

[0089] Fig. 7 is a flow diagram of a fifth embodiment of such a method;

[0090] Fig. 8 is a flow diagram of a sixth embodiment of such a method;

[0091] Fig. 9 is a schematic diagram of a first embodiment of an apparatus for producing hydrochloric acid;

[0092] Figs. 10A, 10B and 10C are schematic diagrams of first, second and third embodiments, respectively, of a first heat exchanger;

[0093] Fig. 11A is a schematic diagram of an embodiment of a second heat exchanger and an embodiment of a gasphase reactor;

[0094] Fig. 11 B is a cross-section through the embodiment of the gas-phase reactor shown in Fig. 11 A, looking in the direction of the arrows labelled B-B' in Fig. 11 A;

[0095] Fig. 11C is a cross-section through the embodiment of the gas-phase reactor shown in Fig. 11 A, looking in the direction of the arrows labelled C-C in Fig. 11 A;

[0096] Fig. 12 is a schematic diagram of a second embodiment of an apparatus for producing hydrochloric acid;

[0097] Figs. 13A and 13B are schematic diagrams of third and fourth embodiments, respectively, of an apparatus for producing hydrochloric acid;

[0098] Figs. 14A, 14B and 14C are schematic diagrams of fifth, sixth and seventh embodiments, respectively, of an apparatus for producing hydrochloric acid;

[0099] Fig. 15 is a schematic diagram of an eighth embodiment of an apparatus for producing hydrochloric acid;

[0100] Fig. 16 is a schematic diagram of an embodiment of a separator and an embodiment of a condenser; and Fig. 17 is a schematic diagram of first and second embodiments of a third heat exchanger.

[0101] Detailed Description

[0102] Fig. 3 shows a first embodiment of a method 100a of producing hydrochloric acid. The method 100a comprises fusing and electrolysing a solid chloride of a metal selected from the group consisting of sodium and magnesium. In this embodiment, the selected metal is magnesium, so that the chloride which is fused and electrolysed 101 a is MgCk. The electrolysis 101 a therefore produces liquid magnesium and chlorine gas, both at a temperature of about 725 “Celsius. The method 100a also comprises thermally decomposing at least one of calcium hydroxide, magnesium hydroxide and an iron hydroxide. In this case, calcium hydroxide derived from limestone is subjected to thermal decomposition 102a. This produces calcium oxide and water vapour, both at a temperature of about 512 “Celsius. The thermal decomposition 102 is performed in the absence of oxygen, which allows the water vapour thus produced to be captured 103 without any admixture of oxygen from atmospheric air. Since in the present embodiment, both the chlorine gas and the water vapour already each have a temperature within the desired range of operating temperatures for the RDR of from about 450 to about 750 “Celsius, neither reagent needs heating or cooling. Therefore, the method 100a then comprises reacting 104 at least some of the chlorine gas with at least some of the captured water vapour in an RDR. This reaction 104 produces a mixture of gases comprising hydrogen chloride and oxygen, as well as some unreacted chlorine and water vapour. The RDR is performed without using carbon as a reducing agent to remove oxygen from the mixture of gases produced by the RDR. This mixture of gases therefore does not contain any carbon monoxide or carbon dioxide. The method 100a then comprises immediately contacting 105 at least some of the mixture of gases with liquid water. The hydrogen chloride from the mixture of gases dissolves in the liquid water to produce hydrochloric acid, leaving a stream of tail gases. Heat is extracted 106 from the hydrochloric acid thus produced to maintain its temperature substantially constant until the stream of tail gases is no longer in contact with the hydrochloric acid. In this embodiment, the method 100a also comprises controlling 107 the molarity of the hydrochloric acid to lie in a range of from 3 M to 8.4 M, inclusive. This is done by adjusting the relative flow rates of the liquid water and the mixture of gases.

[0103] Fig. 4 shows a second embodiment of a method 100b of producing hydrochloric acid. Parts of the method 100b which are the same as parts of the method 100a described above in relation to Fig. 3 are labelled in Fig. 4 by the same reference numerals as in Fig. 3 and will not be described again for the sake of brevity. In this embodiment, however, the selected metal is sodium, so that the chloride which is fused and electrolysed 101 b is NaCI. The electrolysis 101 b therefore produces liquid sodium and chlorine gas, both at a temperature of about 615 “Celsius. The method 100b in this embodiment also comprises thermally decomposing a mixture of calcium hydroxide and magnesium hydroxide, derived from dolomitic limestone. This therefore produces a mixture of calcium oxide and magnesium oxide and water vapour at a temperature of about 420 “Celsius. Since in this case, this is below the desired range of operating temperatures for the RDR, the water vapour is heated before it is introduced to react with the chlorine gas. However, since the water vapour only requires a small amount of heating to reach the desired range of operating temperatures, the method 100b comprises heating 108 the captured water vapour by a mild adiabatic compression, before the water vapour is reacted 104a with the chlorine gas in the RDR. In this case, the reaction is uncatalysed 104a because it is heated 123 by radio-frequency volumetric heating in a frequency band of from 24 to 24.25 GHz, to maintain its temperature within the desired range of operating temperatures of from about 450 to about 750 “Celsius. Moreover, unlike the method 100a of Fig. 3, the method 100b does not comprise controlling 107 the molarity of the hydrochloric acid produced by contacting the mixture of gases with liquid water to lie in a range of from 3 M to 8.4 M, inclusive. Instead, in this embodiment, the hydrochloric acid thus produced is controlled to have a lower concentration, by increasing the flow rate of the liquid water relative to the flow rate of the mixture of gases. Furthermore, the method 100b also comprises using 122 heat extracted 106 from contacting the mixture of gases with liquid water to generate electricity. The electricity thus generated contributes to powering both the heating 108 of the captured water vapour by adiabatic compression and the heating 123 of the RDR by radio-frequency volumetric heating.

[0104] Fig. 5 shows a third embodiment of a method 100c of producing hydrochloric acid. Again, parts of the method 100c which are the same as parts of the method 100b described above in relation to Fig. 4 are labelled in Fig. 5 by the same reference numerals as in Fig. 4 and will not be described again for the sake of brevity. However, in this embodiment, the selected metal is magnesium, so that the chloride which is fused and electrolysed 101c is MgCk. The electrolysis 101c therefore produces liquid magnesium and chlorine gas, both at a temperature of about 725 “Celsius. On the other hand, the method 100c in this embodiment also comprises thermally decomposing magnesium hydroxide, derived from magnesite. This therefore produces magnesium oxide and water vapour at a temperature of about 350 “Celsius. In this case, the temperature of the chlorine gas is undesirably high, making the chlorine gas harder to control and process, whereas the temperature of the water vapour is significantly below the desired range of operating temperatures for the RDR. Thus the method 100c of Fig. 5 instead comprises transferring 109 heat from the chlorine gas to the captured water vapour, without bringing the chlorine gas and the water vapour into contact with each other, until the chlorine gas and the water vapour come into thermal equilibrium with each other. This is carried out before reacting 104 the chlorine gas with the captured water vapour in the RDR. The chlorine gas is therefore cooled to a temperature at which it can be handled more easily, and the water vapour is also heated to within the desired range of operating temperatures for the RDR. Furthermore, following the RDR, the mixture of gases produced by the RDR is immediately contacted 105a with liquid water having a temperature of from 50 to 70 “Celsius, inclusive. The lower bound of this temperature range limits the absorption of unreacted chlorine from the mixture of gases into the liquid water, whilst its upper bound also suppresses vaporization of the liquid water into the stream of tail gases.

[0105] Fig. 6 shows a fourth embodiment of a method 10Od of producing hydrochloric acid. Once again, parts of the method 100d which are the same as parts of the methods 100a and 100b described above in relation to Figs. 3 and 4 are labelled in Fig. 6 by the same reference numerals as in Figs. 3 and 4 and will therefore not be described again. In this embodiment, the chloride which is fused and electrolysed 101 b is again NaCI, which therefore produces liquid sodium and chlorine gas, both at a temperature of about 615 “Celsius. However, the method 10Od of Fig. 6 differs from the method 100b of Fig. 4 as follows. In this embodiment, ferrous hydroxide (Fe(OH)2) derived from siderite is thermally decomposed 102d in the absence of oxygen to produce ferrous oxide (FeO) and water vapour. Since this thermal decomposition 102d occurs at a lower temperature of from about 150 to about 200 “Celsius, the captured water vapour is then heated 108a by radio-frequency volumetric heating to bring it within the range of operating temperatures for the RDR 104. As in the embodiment of Fig. 4, the heating 108a of the captured water vapour is at least partially powered by electricity generated using 122 heat extracted 106 from contacting the mixture of gases with liquid water. Unlike the embodiment of Fig. 4, however, in the present embodiment, the captured water vapour is heated 108a isochorically, thereby simultaneously pressurizing the captured water vapour. Moreover, the present embodiment further comprises drying 110 an aqueous solution of sodium chloride to produce solid sodium chloride and water vapour. At least some of this water vapour is then captured 111 and condensed 112 to produce liquid water. At least some of this liquid water is then used 113 as the liquid water in which the hydrogen chloride gas is dissolved to produce the hydrochloric acid. Meanwhile, at least some of the solid sodium chloride thus produced is recycled 114 as a feedstock for the electrolysis 101 b.

[0106] Fig. 7 shows a fifth embodiment of a method 100e of producing hydrochloric acid. Like the method 10Od of Fig. 6, the method 100e comprises electrolysing 101 b fused NaCI to produce liquid sodium and chlorine gas. In this embodiment, however, ferrous hydroxide derived from siderite is firstly oxidised in the presence of atmospheric oxygen to ferric hydroxide (Fe(OH)3), which is then subjected to thermal decomposition 102e in the absence of oxygen, which therefore produces ferric oxide (Fe2C>3) and water vapour, both at a temperature of about 475 “Celsius. Since the chlorine gas and the water vapour both have temperatures which are within the desired range of operating temperatures for the RDR, neither reagent needs heating or cooling. Moreover, unlike the method 10Od of Fig. 6, the method 100e does not comprise also drying an aqueous solution of sodium chloride to produce solid sodium chloride and water vapour, capturing and condensing the water vapour to produce liquid water, and then using the liquid water to dissolve the hydrogen chloride gas, whilst recycling the solid sodium chloride as a feedstock for the electrolysis. Other parts of the method 100e which are the same as parts of the method 10Od are labelled in Fig. 7 by the same reference numerals as in Fig. 6 and will not be described again for the sake of brevity. In the present embodiment, however, the method 100e further comprises transferring 115 some of the heat from the liquid sodium to the ferric hydroxide, which contributes to its thermal decomposition 102e. Moreover, the method 100e further comprises separating 116 at least some of the oxygen from the stream of tail gases, to produce a stream of oxygen and an oxygen-depleted stream. The oxygen-depleted stream is recycled 117 back to the RDR. On the other hand, some more of the heat from the liquid sodium is also transferred 115a to the stream of oxygen, which is then used 118 in a steelmaking process by injecting the oxygen into molten iron.

[0107] Whereas Figs. 3 to 7 show various different embodiments of a method of producing hydrochloric acid respectively having some features in common and some which are different, these features may be combined in other possible ways than those described above, whilst still remaining within the scope of the appended claims. Thus, for example, Fig. 8 shows a sixth embodiment of a method 100f of producing hydrochloric acid, which comprises features from the first, fourth and fifth embodiments respectively described above in relation to Figs. 3, 6 and 7. Boxes in Fig. 8 representing parts of the method 100f which are the same as parts of the methods 100a, 100d and 100e are labelled by the same reference numerals in Fig. 8 as were used in Figs. 3, 6 and 7 to denote the corresponding parts of the methods 100a, 100d and 100e, respectively, which will therefore not be described again for the sake of brevity. However, the method 10Of of Fig. 8 further comprises the following features.

[0108] Firstly, separating 116 at least some of the oxygen from the stream of tail gases comprises cooling 116c the stream of tail gases. The method 10Of then comprises transferring 119 heat extracted from the stream of tail gases to the oxygen separated from the stream of tail gases, before the oxygen is used 118 in a steelmaking process by injecting the oxygen into molten iron. The oxygen is therefore reheated by this heat transfer 119. Secondly, condensing 112 the water vapour captured 111 from drying 110 the aqueous solution comprises cooling 112c the captured water vapour. In the method 100f, this is done by bringing the captured water vapour into thermal but not physical contact with the oxygen-depleted stream produced by separating 116 the oxygen from the stream of tail gases, before the oxygen-depleted stream is recycled 117 back to the RDR. The enthalpy of condensation of the captured water vapour is therefore transferred to the oxygen-depleted stream. Since the molar enthalpy of condensation of water is almost exactly double that of chlorine, this heat is more than enough to revaporise the chlorine in the oxygen-depleted stream, which has been condensed during its separation 116 by cooling 116c, with enough heat left over to raise the temperature of the oxygen-depleted stream significantly as well.

[0109] Thirdly, in the method 10Of, electrolysing 101 b fused sodium chloride comprises producing an amount of chlorine gas which is in excess of the stoichiometric amount thereof required for the RDR. The method 100f therefore further comprises branching off 120 this excess chlorine from the stoichiometric amount of chlorine gas, before the stoichiometric amount is reacted 104 in the RDR with at least some of the water vapour captured 103 from the thermal decomposition 102a. In the method 10Of, heat carried by the excess chlorine from the electrolysis 101 b is then transferred 121 to the oxygen-depleted stream which is being recycled 117 back to the RDR. This therefore brings the temperature of the recycled oxygen-depleted stream back up to within the range of desired temperatures for the RDR.

[0110] Finally, like the method 100b of Fig. 4, the method 100f of Fig. 8 also comprises using 122 heat extracted 106 from contacting the mixture of gases with liquid water to generate electricity, which is then used to heat 123 the RDR by radio-frequency volumetric heating in a frequency band of from 24 to 24.25 GHz, in order to maintain the temperature of the RDR within the desired range of operating temperatures of from about 450 to about 750 “Celsius. Because the RDR is heated 123 in this manner, in this case, the reaction is uncatalysed 104a.

[0111] Whereas in the embodiments respectively described above in relation to Figs. 3 to 8, all the gaseous chlorine supplied to the RDR was produced by fusing and electrolysing a solid chloride either of sodium or of magnesium, this was for ease of illustration and explanatory purposes only. Thus in alternative possible embodiments, some of the chlorine supplied to the RDR could be produced by fusing and electrolysing solid sodium chloride, some other of the chlorine supplied to the RDR could instead be produced by separately fusing and electrolysing solid magnesium chloride, and the gaseous chlorine from these two sources could be combined and mixed with each other, before being supplied to the RDR.

[0112] Fig. 9 schematically shows a first embodiment of an apparatus 1a for producing hydrochloric acid. The apparatus 1 a comprises an electrolytic cell 10, a kiln 20, a gas-phase reactor 30, and an absorber 40. The electrolytic cell 10 is for fusing and electrolysing a solid chloride of a metal selected from the group consisting of sodium and magnesium, to produce the selected metal in liquid phase and chlorine gas. The electrolytic cell 10 may be of a conventional type. For example, it may be similar, therefore, to a Downs cell or to a cell of a type described in the document by Kipouros & Sadoway (1987) referred to above. The electrolytic cell 10 comprises an inlet 11 for the chloride of the selected metal in solid phase, a first outlet 17 for the selected metal in liquid phase and a second outlet 15 for the chlorine gas. The kiln 20 is gas-tight and is for thermally decomposing at least one of calcium hydroxide, magnesium hydroxide and an iron hydroxide to produce water vapour and at least one of calcium oxide, magnesium oxide and an iron oxide, respectively. The kiln 20 comprises an inlet 21 for receiving the respective hydroxide(s), a first outlet 24 for water vapour, a second outlet 25 for the respective oxide(s), and a heater 23 for injecting heat into the respective hydroxide(s). Since the kiln 20 is gas-tight, the respective hydroxide(s) can be thermally decomposed inside the kiln 20 in the absence of oxygen.

[0113] The gas-phase reactor 30 is for reacting at least some of the chlorine gas with at least some of the water vapour in an RDR at a temperature of from 450 to 750 “Celsius, to produce a mixture of gases at least comprising hydrogen chloride and oxygen. The gas-phase reactor 30 comprises a first inlet 31 for receiving the chlorine gas from the second outlet 15 of the electrolytic cell 10, a second inlet 32 for receiving the water vapour from the first outlet 24 of the kiln 20, and an outlet 34 for the mixture of gases produced by the RDR. The absorber 40 is for contacting this mixture of gases with liquid water to produce hydrochloric acid and a stream of tail gases. The absorber 40 comprises a first inlet 41 for receiving the mixture of gases from the outlet 34 of the gas-phase reactor, a second inlet 42 for receiving the liquid water, a first outlet 44 for the hydrochloric acid and a second outlet 45 for the stream of tail gases. The first inlet 41 of the absorber 40 is connected directly to the outlet 34 of the gas-phase reactor, so that the mixture of gases produced by the RDR passes straight into the absorber 40 and immediately contacts the liquid water in the absorber 40. Although not represented in Fig. 9, the absorber 40 also comprises means for extracting heat from the hydrochloric acid before it leaves via the first outlet 44, as described in greater detail below in relation to Fig. 11C.

[0114] Chlorine gas produced in the electrolytic cell 10 and water vapour produced in the kiln 20 do not need to be pumped to the gas-phase reactor 30 because the absorber 40 consumes the hydrogen chloride gas produced by the RDR taking place in the gas-phase reactor 30. The chlorine gas and the water vapour are therefore propelled from the electrolytic cell 10 and the kiln 20 to the gas-phase reactor 30 to replace the hydrogen chloride gas which is consumed when it dissolves in the liquid water in the absorber 40. This flow of gases out of the gas-phase reactor 30 into the absorber 40 also prevents the liquid water in the absorber 40 from flowing back into the gas-phase reactor 30, even though the inlet 41 of the absorber 40 is directly connected to the outlet 34 of the gas-phase reactor 30.

[0115] Energy for the thermal decomposition performed in the kiln 20 is supplied by the heater 23. At least some of this thermal energy may come from the liquid metal produced at the first outlet 17 of the electrolytic cell 10, via a first heat exchanger HE1 , as schematically shown in Fig. 10A. Figs. 10B and 10C respectively schematically show other embodiments of the first heat exchanger HE1 , in which heat carried by the liquid metal is transferred to the water vapour produced by the thermal decomposition and to the gas-phase reactor 30, respectively. Heat may also be transferred to the water vapour produced by the thermal decomposition from the chlorine gas produced at the second outlet 15 of the electrolytic cell 10, via a second heat exchanger HE2, as shown schematically in Fig.

[0116] 11 A.

[0117] In the first embodiment of an apparatus 1 a shown in Fig. 9, the gas-phase reactor 30 has a cross-sectional area which increases by at least one-eighth but not more than 50% in a direction towards its outlet 34. This may be better understood by referring to Fig. 11 A, wherein the direction in which the cross-sectional area increases is represented by an arrow labelled D. In the embodiment shown in Fig. 11 A, this increasing cross-sectional area gives the body 33 of the gas-phase reactor 30 a flared shape, like the bell of a trumpet. However, in other alternative possible embodiments, the body 33 of the gas-phase reactor 30 may have a frustoconical or frustopyramidal shape instead. As may also be seen in Fig. 11 A, the gas-phase reactor 30 contains a catalyst 37 of the type described above, borne on a suitable, gas-porous carrier material. In other possible alternative embodiments, however, the catalyst 37 may be absent from the gas-phase reactor 30, for example if the contents of the gas-phase reactor 30 are to be subjected to radio-frequency volumetric heating instead.

[0118] Figs. 11 B and 11C respectively schematically show the inlets 31 and 32 and the outlet 34 of the gas-phase reactor 30. In Fig 1 1 B, the inlets 32 for water vapour are shaded grey and the inlets 31 for chlorine gas are not. It may be seen in Fig 11 B that the chlorine inlets 31 are distributed amongst the water vapour inlets 32 in such a way as to promote mixing of these two gases for the RDR. It may also be seen that the number of inlets 32 for water vapour is approximately double the number of chlorine inlets 31 , so that the water vapour can be supplied at a rate which is well in excess of the stoichiometric amount thereof required for the reaction.

[0119] Fig. 11C shows a view through the outlet 34 of the gas-phase reactor 30 into the first inlet 41 of the absorber 40. It may be seen that the inlet 41 exposes a plurality of hollow borosilicate glass tubes 47 arranged substantially vertically within the absorber 40. These hollow glass tubes 47 are large in number and small in diameter. Liquid water from the second inlet 42 to the absorber 40 flows down the exterior surfaces of these tubes 47 under gravity and adheres to the tubes 47 under its own surface tension, to form a film on the outside of each tube. This provides a large surface area for the liquid water to absorb the hydrogen chloride gas from the gas-phase reactor 30, whilst still allowing oxygen produced by the RDR to flow through gaps 48 between the tubes 47, along with other components of the stream of tail gases. A counterflow of heat transfer fluid within the interior of each of the hollow tubes 47 extracts the heat generated by absorbing the hydrogen chloride gas in the liquid water on the exterior surface of the tubes 47, in order to maintain the hydrochloric acid thus produced within its desired temperature range. This arrangement provides a larger surface area and is more easily penetrated by the mixture of gases produced by the RDR than a falling film of liquid water formed on the interior surface of a single tube which is surrounded on its exterior surface by a shell of heat transfer fluid. It is also more easily controllable than a mist or spray of water, as in a scrubber. Further layers of such glass tubes 47 are also arranged behind those which can be seen in Fig. 11C and are offset from the front layer of tubes visible through the outlet 34, to provide a circuitous path for the mixture of gases therebetween. The hydrochloric acid produced by dissolution of the hydrogen chloride gas in the liquid water then leaves the absorber 40 by the first outlet 44 of the absorber 40. The stream of tail gases, minus the absorbed hydrogen chloride, leaves by the second outlet 45 thereof.

[0120] Fig. 12 schematically shows a second embodiment of an apparatus 1 b for producing hydrochloric acid. Like the apparatus 1 a of Fig. 9, the apparatus 1 b comprises an electrolytic cell 10, a kiln 20, a gas-phase reactor 30, and an absorber 40. These, therefore, will not be described again for the sake of brevity. However, the apparatus 1 b of Fig. 12 differs from the apparatus 1 a in that it also comprises the following three elements.

[0121] Firstly, the apparatus 1 b comprises an outlet 16 for chlorine gas produced by the electrolytic cell 10 in excess of the stoichiometric amount thereof required for the RDR. The outlet 16 for this excess chlorine gas branches off from the second outlet 15 of the electrolytic cell 10.

[0122] Secondly, the kiln 20 of the apparatus 1b comprises a second inlet 22 for water vapour. The second inlet 22 is opened and closed by a valve V1. The second inlet 22 allows water vapour to be injected into the kiln 20 by opening valve V1. Thus the kiln 20 may be purged of atmospheric air before commencing operation, by flushing it with steam. The steam injected into the second inlet 22 drives the atmospheric air from the kiln 20 through to the gas-phase reactor 30 and thence to the absorber 40, where it exits via the second outlet 45 thereof. Once the atmospheric air has been purged, the valve V1 may then be closed again.

[0123] Thirdly, the apparatus 1 b also comprises a compressor 50 for adiabatic compression of the water vapour captured from the thermal decomposition. The compressor 50 is located downstream of the first outlet 24 of the kiln 20 and upstream of the second inlet 32 of the gas-phase reactor 30. After flushing the kiln 20 with steam and before commencing the thermal decomposition, the compressor 50 is run to reduce the pressure of water vapour within the kiln 20, thereby aiding the thermal decomposition.

[0124] Finally, the apparatus 1 b further comprises an arrangement for using the heat extracted from the hydrochloric acid being produced inside the absorber 40 to heat the RDR in the gas-phase reactor 30. Since the operating temperature of the RDR is far greater than the operating temperature of the hydrochloric acid production, heat cannot just be transferred from the absorber 40 to the gas-phase reactor 30 via a heat exchanger between the two. As can be seen in Fig. 12, therefore, this arrangement instead comprises an electric generator 94 and a dielectric heater 96. The electric generator 94 is connected to a heat exchanger 92, which extracts heat from the absorber 40 by circulating an organic heat transfer fluid HTF through the absorber 40 within the tubes 47 thereof, as described above in relation to Fig. 11C. The heat transfer fluid HTF transfers the heat thus extracted to the electric generator 94, which uses this heat to generate electricity via the Rankine cycle. The electricity thus produced powers the dielectric heater 96. In this embodiment, the dielectric heater 96 comprises a radio-frequency generator and a waveguide. The radio-frequency generator generates radio waves in a frequency band of from 24 to 24.25 GHz and the waveguide directs these radio waves into the gas-phase reactor 30 for radio-frequency volumetric heating of the RDR, whilst avoiding contact with the corrosive contents of the gas-phase reactor 30. Fig. 13A schematically shows a third embodiment of an apparatus 1c for producing hydrochloric acid. The apparatus 1c of Fig 13A differs from the apparatus 1 b of Fig. 12 in that it does not comprise a compressor 50 or the heat transfer arrangement 92, 94, 96. However, the apparatus 1c comprises all the other same features as the apparatus 1 b, which will therefore not be described again for the sake of brevity. In addition, in comparison to the apparatus 1 b, the apparatus 1c further comprises a dryer 60 and a condenser 70.

[0125] The dryer 60 is for drying an aqueous solution of sodium hydroxide to produce water vapour and solid sodium hydroxide. It comprises an inlet 61 for receiving the aqueous solution of sodium hydroxide, a first outlet 64 for the water vapour and a second outlet 65 for the solid sodium hydroxide. The condenser 70 is for condensing at least some of the water vapour produced by drying the aqueous solution. It comprises an inlet 71 for receiving the water vapour from the first outlet 64 of the dryer 60 and an outlet 74 for liquid water. The outlet 74 of the condenser 70 is connected to the second inlet 42 of the absorber 40. The solid sodium hydroxide may be fed, for example, to an electrolytic cell for electrolysis by the Castner process.

[0126] Fig. 13B schematically shows a fourth embodiment of an apparatus 1d for producing hydrochloric acid. The apparatus 1d of Fig 13B is the same as the apparatus 1c of Fig 13A, except that the dryer 60 is instead for drying an aqueous solution of the chloride of the selected metal to produce water vapour and the chloride of the selected metal in solid phase. The inlet 61 of dryer 60 is for receiving the aqueous solution, the first outlet 64 thereof is for the water vapour and the second outlet 65 is for the chloride of the selected metal in solid phase. The second outlet 65 of the dryer 60 is connected upstream of the inlet 11 of the electrolytic cell 10. Thus the chloride of the selected metal in solid phase can be recycled back into the electrolytic cell 10 for electrolysis.

[0127] Fig. 14A schematically shows a fifth embodiment of an apparatus 1e for producing hydrochloric acid. The apparatus 1e of Fig 14A differs from the apparatus 1b of Fig. 12 in that it does not comprise a compressor 50. However, the apparatus 1e comprises all the other same features as the apparatus 1b, which will therefore not be described again for the sake of brevity. However, in comparison to the apparatus 1 b, the apparatus 1e further comprises a separator 80 and a second valve V2.

[0128] The separator 80 comprises an inlet 81 for receiving the stream of tail gases from the second outlet 45 of the absorber 40, a first outlet 84 for an oxygen-depleted stream and a second outlet 85 for a stream of oxygen. The second valve V2 is located downstream of the second outlet 45 of the absorber 40 and upstream of the inlet 81 of the separator 80. Valve V2 is a two-way valve, having a single inlet from the outlet 45 of the absorber 40 and two outlets, one of which is the inlet 81 to the separator 80 and the other of which is to the environment. Thus when the apparatus 1e is purged of atmospheric air by opening valve V1 before commencing operation, as described above in relation to Fig. 12, valve V2 can be opened to divert the purged atmospheric air from the separator 80 to the environment. Once the atmospheric air has been purged, valve V2 can be closed again to redirect the stream of tail gases from the absorber 40 to the inlet 81 of the separator 80.

[0129] Fig. 14B schematically shows a sixth embodiment of an apparatus 1f for producing hydrochloric acid. The apparatus 1f of Fig 14B is the same as the apparatus 1e of Fig 14A, except that the first outlet 84 of the separator 80 is connected upstream of the first inlet 31 of the gas-phase reactor 30. Thus the stream of tail gases from which at least some of the oxygen has been separated can be recycled back into the gas-phase reactor 30 as reagents for the RDR. In addition, the apparatus 1f further comprises a first pump 18 downstream of the second outlet 15 of the electrolytic cell 10 and upstream of the first inlet 31 of the gas-phase reactor 30, and a second pump 28 downstream of the first outlet 24 of the kiln 20 and upstream of the second inlet 32 of the gas-phase reactor 30. These pumps 18, 28 prevent the recycled stream of tail gases from backing up into the electrolytic cell 10 and the kiln 20, respectively.

[0130] Fig. 14C schematically shows a seventh embodiment of an apparatus 1g for producing hydrochloric acid. The apparatus 1g of Fig 14B is the same as the apparatus 1e of Fig 14A, except that the second outlet 85 of the separator 80 is connected upstream of an inlet of a steelmaking apparatus 90. Thus the stream of oxygen separated from the stream of tail gases can be used in a steelmaking process by injecting the oxygen into molten iron.

[0131] Whereas Figs. 9 to 14C show various different embodiments of an apparatus for producing hydrochloric acid respectively having some features in common and some which are different, these features may be combined in other possible ways than those described above, whilst still remaining within the scope of the appended claims. Thus, for example, Fig. 15 schematically shows an eighth embodiment of an apparatus 1h for producing hydrochloric acid, which comprises all the features of the apparatuses 1 b, 1d, 1f and 1g described above in relation to Figs. 12, 13B, 14B and 14C, respectively. Moreover, whereas the apparatuses 1 a to 1 h described above and respectively shown in Figs. 9 and 12 to 15 each comprise only a single electrolytic cell 10 for fusing and electrolysing a solid chloride either of sodium or of magnesium, this was for ease of illustration and explanatory purposes only. Thus in alternative possible embodiments, an apparatus of the invention may comprise both an electrolytic cell for fusing and electrolysing solid sodium chloride and an electrolytic cell for fusing and electrolysing solid magnesium chloride, each having respective outlets for gaseous chlorine, which are connected upstream of the first inlet 31 of the gas-phase reactor 30.

[0132] Fig. 16 schematically shows an embodiment of a condenser 70 and an embodiment of a separator 80, which may both form part of an apparatus for producing hydrochloric acid as described herein. The condenser 70 comprises a heat exchanger 72 as shown in Fig. 16. The separator 80 comprises a heat exchanger 88 and a heat pump, which in this embodiment is a vapour-compression cycle refrigerator, around which a working fluid WF circulates in a closed loop as a refrigerant. The vapour-compression cycle refrigerator comprises an evaporator 82 of the working fluid on a cold side thereof, a working fluid compressor 83, a working fluid condenser 86 on a hot side thereof and an expansion valve 87, which are connected to each other as shown in Fig. 16. The cold side of the refrigerator has a working temperature Ti < the boiling point, Tb (Ch) but also » Tb (O2), so that T1 is typically in a range of from about -90 to about -50 “Celsius, for example. Thus if the stream of tail gases from the absorber 40 is introduced to the cold side of the refrigerator, unreacted chlorine in the stream of tail gases condenses out as a liquid, but oxygen produced by the RDR does not, thereby producing a stream of gaseous oxygen and an oxygen- depleted liquid stream, both at Ti. However, the stream of tail gases is not introduced to the cold side of the refrigerator directly. Instead, the stream of tail gases, which enters the inlet 81 of the separator 80 at a hot temperature T4, is first introduced to the heat exchanger 88, where it transfers heat to one of the products of separating the oxygen from the stream of tail gases, which is at the very cold temperature T1 and which in this embodiment, is the stream of gaseous oxygen. Both therefore leave the heat exchanger 88 at an intermediate temperature T2, where Tb (H2O) » T2 > the freezing point, Tm(H2O), so that T2 might typically be in a range of from about +5 to about +20 “Celsius, for example. This is for several reasons, as follows.

[0133] Firstly, it significantly improves the thermodynamic efficiency of the refrigerator, and therefore reduces its power consumption, by chilling the hot stream of tail gases before they are introduced to the cold side of the refrigerator. Secondly, it condenses out water vapour remaining in the stream of tail gases. Although as mentioned previously, it does not matter if the oxygen-depleted stream contains some water vapour if it is recycled back to the RDR, if a significant quantity of water vapour remains in the stream of tail gases when they are introduced to the cold side of the refrigerator, this water vapour would freeze at T1 « Tm(H2O), which would therefore risk plugging the refrigerator with ice. It is also desirable for the stream of gaseous oxygen to be dried if it is subsequently to be used in steelmaking, for example. A third reason for bringing the hot stream of tail gases into thermal contact with the very cold stream of gaseous oxygen is that it also reheats the stream of oxygen. This stream of oxygen therefore leaves the outlet 85 of the separator 80 at the intermediate temperature T2, which, being not much different from ambient temperatures, ensures the overall thermodynamic efficiency of the separator 80. Finally, by condensing out water vapour from the stream of tail gases, any gaseous hydrogen chloride remaining in the stream of tail gases tends to dissolve in this chilled stream of water, in accordance with the graph of Fig. 1 . This helps to remove such residual hydrogen chloride from the stream of oxygen produced in the separator 80.

[0134] The chilled stream of liquid water condensed out from the heat exchanger 88 at T2 is then circulated past the working fluid condenser 86 on the hot side of the refrigerator, which therefore transfers heat from the hot working fluid WF to the chilled stream of water. This is for several reasons, as follows. Firstly, it again improves the thermodynamic efficiency of the refrigerator, and therefore reduces its power consumption, by reducing the temperature difference between the hot side and the cold side of the refrigerator. For example, if T1 = -63 “Celsius = 210 K and T2 = +7 “Celsius = 280 K, the coefficient of performance of the refrigerator = Tcoid side I (Thot side - Tcoid side) = T1 1 (T2 - T1) = 210 / (280 - 210) = 3. Secondly, it reheats the stream of liquid water to a warmer temperature T3 > T2, making it more ready for reuse as water in which the hydrogen chloride can dissolve inside the absorber 40. Thirdly, by heating this stream of water, the solubility of chlorine therein is reduced in accordance with the graph of Fig. 2. Any gaseous chlorine evolved from this warm stream of water as a result is then redirected to combine with the oxygen-depleted stream, as shown in Fig. 16.

[0135] Meanwhile, the very cold oxygen-depleted stream at T1, which therefore now chiefly comprises liquid chlorine, enters the heat exchanger 72 of the condenser 70. Here, hot water vapour from the dryer 60 entering the inlet 71 of the condenser 70 at a temperature Te comes into contact with the heat exchanger 72 and condenses into liquid water at a lower temperature T5 < Te. The enthalpy of condensation of this water is sufficient to reheat and revaporise the chlorine in the oxygen-depleted stream, which therefore leaves the heat exchanger 72 as a gas at T5 » T1, after which it is rejoined by the gaseous chlorine redirected from the warm stream of water from condenser 86. The liquid water condensed out by the heat exchanger 72 leaves the condenser 70 via its outlet 74 and is combined with the warm stream of water at T3 < T5. This considerably dilutes any residual chlorine or hydrogen chloride which might remain in the warm stream of water from condenser 86 and brings the temperature of the combined stream of water to an intermediate temperature, T7, where T3 < T7 < T5. T7 may be chosen to be within the preferred temperature range of from about 50 to about 70 “Celsius for the liquid water in which the hydrogen chloride is dissolved within the absorber 40.

[0136] Whereas in the embodiments just described in relation to Fig. 16, the oxygen-depleted stream at T1 is used to condense out water vapour from the dryer 60, and the stream of oxygen also at T1 is used to condense out water vapour from the stream of tail gases, in other possible embodiments, these two roles may be reversed. However, the arrangement illustrated in Fig. 16 is preferred to this because of the different phases of the chlorine (liquid) and oxygen (gaseous) in these two streams, their relative proportions as determined by the RDR, and the amount of water vapour from the dryer 60 compared to the amount of the stream of tail gases, as determined by the molarity of the hydrochloric acid.

[0137] Fig. 17 schematically shows an embodiment of two heat exchangers HE3, HE4, whereby excess gaseous chlorine branched off from the outlet 16 of the electrolytic cell 10 may be used to heat both the gaseous oxygen-depleted stream at T5 before this stream is recycled to the gas-phase reactor 30 and to heat the stream of gaseous oxygen at T2, if this other stream is intended subsequently to be used in steelmaking, for example. The excess gaseous chlorine leaves the electrolytic cell 10 at a high temperature, T , which as mentioned previously, may be at least about 600 “Celsius if produced by electrolysing sodium chloride and at least about 700 “Celsius if produced by electrolysing magnesium chloride. In the embodiment of Fig. 17, this hot excess chlorine is therefore used to heat the oxygen-depleted stream to a temperature T9, where Tg > 300 “Celsius for example, by transferring heat to it via the heat exchanger HE3. Thereafter, the stream of oxygen is heated by this excess chlorine as well, which is now also at Tg, to a temperature Te, where Ts > 150 “Celsius for example, by transferring heat to it via the heat exchanger HE4. These two, sequential heat transfers also significantly reduce the temperature of the excess chlorine to Ts « Tw, thereby preparing it to be bottled and transported. In principle, heat remaining in the excess chlorine at Ts before it cools down to ambient temperature may be used to generate electricity for powering the working fluid compressor 83. The oxygen-depleted stream and / or the stream of oxygen may also be heated further by transferring heat to them from the liquid metal produced in the electrolytic cell 10 as well, in a manner similar to that described above in relation to Figs. 10A to 10C.

[0138] Whereas in Fig. 17, the excess chlorine branched off from the electrolytic cell 10 has been used to heat both the oxygen-depleted stream and the stream of oxygen, in other possible embodiments, this excess chlorine may be used to heat only one of them, or alternatively or additionally, to heat the water vapour produced in the kiln 20, since the molar heat capacities of chlorine and water vapour are about the same as each other. This may be done by using a similar type of heat exchanger to those shown in Fig. 17 to transfer heat from the excess chlorine to at least one of the kiln 20, a point downstream of the first outlet 24 of the kiln 20 and upstream of the second inlet 32 of the gas-phase reactor 30, and the gas phase reactor 30 itself.

[0139] In summary, therefore, the present invention provides a method and apparatus for producing hydrochloric acid via a reverse Deacon reaction, which does not use any carbon as a reductant to remove oxygen from the products of the RDR. High-temperature chlorine is supplied to the RDR by fusing and electrolysing a solid chloride of at least one of sodium and magnesium. For example, sodium chloride may be fused and electrolysed using the Downs process and / or magnesium chloride may be fused and electrolysed using the US Magnesium / VAMI process. High- temperature water vapour is also supplied to the RDR from the thermal decomposition of a hydroxide of at least one of calcium, magnesium and iron, which is conducted in the absence of oxygen. Heat required for this thermal decomposition may be supplied from one or more of the electrolysis products, for example. Thus both reagents for the RDR are supplied to it at high purity and at a sufficiently high temperature for them to be able to react with each other without requiring the application of any heat. Hydrogen chloride produced by the RDR is then dissolved in liquid water and heat is extracted from this exothermic process to keep it substantially isothermal. However, since the RDR is itself endothermic, in some embodiments, it may be supplied with heat to keep it substantially isothermal as well. This heat may be directly provided by one of the electrolysis products, or, since the quantity of heat produced by the exothermic dissolution of the hydrogen chloride in liquid water is considerably greater than the quantity of heat consumed by the RDR, it may alternatively or additionally be provided indirectly from at least some of the heat extracted from dissolving the hydrogen chloride in liquid water. Thus no heat needs to be provided to the RDR from another, external source, making the method of the invention a highly energy-efficient technique for the manufacture of hydrochloric acid.

[0140] Whereas the present invention has been described above by reference to particular examples and embodiments, the scope of the invention should not be taken to be limited thereby and is instead defined by the appended claims.

Claims

Claims1. A method (100a - 100f) of producing hydrochloric acid, comprising: fusing and electrolysing (101 a, 101 b, 101c) a solid chloride of a metal selected from the group consisting of sodium and magnesium, to produce the selected metal in liquid phase and chlorine gas; thermally decomposing (102a, 102b, 102c, 102d) at least one of calcium hydroxide, magnesium hydroxide and an iron hydroxide in the absence of oxygen to produce water vapour and at least one of calcium oxide, magnesium oxide and an iron oxide, respectively; capturing (103) at least some of the water vapour produced by the thermal decomposition; reacting (104, 104a) at least some of the chlorine gas with at least some of the captured water vapour in a carbon-free reverse Deacon reaction at a temperature of from 450 to 750 “Celsius, inclusive, to produce a mixture of gases at least comprising hydrogen chloride and oxygen; immediately contacting (105, 105a) at least some of the mixture of gases with liquid water to dissolve the hydrogen chloride therein, thereby producing hydrochloric acid and a stream of tail gases; and extracting (106) heat from the hydrochloric acid thus produced to maintain its temperature substantially constant until the stream of tail gases is no longer in contact therewith.

2. A method (100b, 10Od) according to claim 1 , further comprising heating (108) the captured water vapour by at least one of adiabatic mechanical compression and radio-frequency volumetric heating, wherein heating (108) the captured water vapour uses (122) electricity generated by extracting (106) heat from the hydrochloric acid.

3. A method (100b, 100f) according to claim 1 or claim 2, further comprising heating (123) the reverse Deacon reaction in the absence of a catalyst (104a) by radio-frequency volumetric heating in at least one of the IEEE Ku-, K- and Ka-bands.

4. A method (100b, 100f) according to claim 3, wherein heating (123) the reverse Deacon reaction uses (122) electricity generated by extracting (106) heat from the hydrochloric acid.

5. A method (100e) according to any one of the preceding claims, further comprising transferring (115) heat from the selected metal in liquid phase to at least one of: the respective hydroxide(s); the captured water vapour; and the reverse Deacon reaction.

6. A method (100e, 100f) according to any one of the preceding claims, further comprising: separating (116) at least some of the oxygen from the stream of tail gases to produce an oxygen-depleted stream and a stream of oxygen; and recycling (117) at least some of the oxygen-depleted stream back to the reverse Deacon reaction.

7. A method (100e) according to claim 6, further comprising transferring (115a) heat from the selected metal in liquid phase to at least one of the products of separating (116) the oxygen from the stream of tail gases.

8. A method (100c) according to any one of the preceding claims, further comprising transferring (109) heat from the chlorine gas to the captured water vapour, without bringing the chlorine gas into contact with the captured water vapour, before reacting (105, 105a) at least some of the chlorine gas with at least some of the captured water vapour in the reverse Deacon reaction.

9. A method (100c) according to any one of the preceding claims, wherein the mixture of gases produced by the reverse Deacon reaction is contacted (105a) with liquid water having a temperature of from 50 to 70 “Celsius, inclusive.

10. A method (1 OOd, 10Of) according to any one of the preceding claims, further comprising: drying (110) an aqueous solution of the chloride of the selected metal or an aqueous solution of sodium hydroxide to produce water vapour and the chloride of the selected metal or sodium hydroxide, respectively, in solid phase; capturing (111) at least some of the water vapour produced by drying the aqueous solution; condensing (112) at least some of the water vapour captured from drying the aqueous solution, to produce liquid water; and using (113) at least some of the liquid water thus produced as the liquid water in which the hydrogen chloride is dissolved to produce the hydrochloric acid.

11. A method (100f) according to claim 10 when dependent on claim 6, wherein: condensing (112) the captured water vapour comprises cooling (112c) the captured water vapour; separating (116) at least some of the oxygen from the stream of tail gases comprises cooling (116c) the stream of tail gases; and the captured water vapour is cooled (112c) by bringing it into thermal but not physical contact with at least one of the cooled stream of tail gases and at least one of the products of separating (116) the oxygen from the stream of tail gases, thereby at least partially reheating at least one of the cooled stream of tail gases and said at least one of the products of separating the oxygen from the stream of tail gases, respectively.

12. A method (1 OOf) according to any one of claims 6 to 1 1 , wherein separating (116) at least some of the oxygen from the stream of tail gases comprises cooling (116c) the stream of tail gases, and the method further comprises transferring (119) heat extracted from the stream of tail gases to at least one of the products of separating (116) the oxygen from the stream of tail gases.

13. A method (100f) according to any one of claims 6 to 12, wherein fusing and electrolysing (101 a, 101 b, 101c) the solid chloride of the selected metal comprises producing (101 b) an amount of chlorine gas in excess of a stoichiometric amount thereof required for the reverse Deacon reaction, and the method further comprises: branching off (120) the excess amount of chlorine gas from the stoichiometric amount of chlorine gas before reacting (104) the stoichiometric amount of chlorine gas with at least some of the captured water vapour in the reverse Deacon reaction; and transferring (121) heat from at least some of the excess amount of chlorine gas to at least one of: the respective hydroxide(s); the water vapour captured (103) from the thermal decomposition; the reverse Deacon reaction; and at least one of the products of separating (116) the oxygen from the stream of tail gases.

14. A method (1 OOd) according to any one of claims 10 to 13, further comprising using (114) at least some of the chloride of the selected metal in solid phase produced by drying (1 10) the aqueous solution of the chloride of the selected metal as a feedstock for fusing and electrolysing (101) the solid chloride of the selected metal.

15. A method (1 OOd) according to any one of claims 2 to 14, wherein the water vapour captured (103) from the thermal decomposition is heated isochorically (108a), thereby simultaneously pressurizing the captured water vapour.

16. An apparatus (1 a - 1 h) for producing hydrochloric acid, comprising: an electrolytic cell (10) for fusing and electrolysing a solid chloride of a metal selected from the group consisting of sodium and magnesium, to produce the selected metal in liquid phase and chlorine gas, wherein the electrolytic cell (10) comprises an inlet (11) for the chloride of the selected metal in solid phase, a first outlet (17) for the selected metal in liquid phase and a second outlet (15) for the chlorine gas; a gas-tight kiln (20) for thermally decomposing at least one of calcium hydroxide, magnesium hydroxide and an iron hydroxide in the absence of oxygen to produce water vapour and at least one of calcium oxide, magnesium oxide and an iron oxide, respectively, wherein the kiln (20) comprises an inlet (21) for receiving the respective hydroxide(s), a first outlet (24) for the water vapour, a second outlet (25) for the respective oxide(s), and a heater (23) for injecting heat into the respective hydroxide(s); a gas-phase reactor (30) for reacting at least some of the chlorine gas with at least some of the water vapour in a carbon-free reverse Deacon reaction at a temperature of from 450 to 750 “Celsius to produce a mixtureof gases at least comprising hydrogen chloride and oxygen, wherein the gas-phase reactor (30) comprises a first inlet (31) for receiving the chlorine gas from the second outlet (15) of the electrolytic cell (10), a second inlet (32) for receiving the water vapour from the first outlet (24) of the kiln (20), and an outlet (34) for the mixture of gases; and an absorber (40) for contacting the mixture of gases with liquid water to produce hydrochloric acid and a stream of tail gases, wherein the absorber (40) comprises a first inlet (41) for receiving the mixture of gases from the outlet (34) of the gas-phase reactor (30), a second inlet (42) for receiving the liquid water, a first outlet (44) for the hydrochloric acid, a second outlet (45) for the stream of tail gases, and means (HTF, 47, 92) for extracting heat from the hydrochloric acid before it leaves via the first outlet (44).

17. An apparatus (1f, 1 h) according to claim 16, further comprising a separator (80) for separating at least some of the oxygen from the stream of tail gases, wherein: the separator (80) comprises an inlet (81) for receiving the stream of tail gases from the second outlet (45) of the absorber (40), a first outlet (84) for an oxygen-depleted stream and a second outlet (85) for a stream of oxygen; and the first outlet (84) of the separator (80) is connected upstream of at least one of the first and second inlets (31 , 32) of the gas-phase reactor (30).

18. An apparatus (1 b, 1 h) according to claim 16 or claim 17, further comprising: an electric generator (94) for generating electricity from heat extracted from the hydrochloric acid in the absorber (40) by the means (HTF, 47, 92) for extracting heat therefrom; and a compressor (50), at least partially powered by the electric generator (94), for adiabatically compressing the water vapour downstream of the first outlet (24) of the kiln (20) and upstream of the second inlet (32) of the gas-phase reactor (30).

19. An apparatus according to claim 16 or claim 17, further comprising: an electric generator (94) for generating electricity from heat extracted from the hydrochloric acid in the absorber (40) by the means (HTF, 47, 92) for extracting heat therefrom; and a dielectric heater (96), at least partially powered by the electric generator (94), for radio-frequency volumetric heating of the contents of at least one of: a point downstream of the first outlet (24) of the kiln (20) and upstream of the second inlet (32) of the gasphase reactor (30); and the gas-phase reactor (30).

20. An apparatus according to any one of claims 16 to 19, further comprising a first heat exchanger (HE1) for transferring heat from downstream of the first outlet (17) of the electrolytic cell (10) to at least one of: the kiln (20);downstream of the first outlet (24) of the kiln (20) and upstream of the second inlet (32) of the gas-phase reactor (30); the gas-phase reactor (30); and downstream of at least one of the first and second outlets (84, 85) of the separator (80).21 . An apparatus according to any one of claims 16 to 20, further comprising a second heat exchanger (HE2) for transferring heat from downstream of the second outlet (15) of the electrolytic cell (10) and upstream of the first inlet (31) of the gas-phase reactor (30) to downstream of the first outlet (24) of the kiln (20) and upstream of the second inlet (32) of the gas-phase reactor (30).

22. An apparatus (1c, 1d, 1 h) according to any one of claims 16 to 21 , further comprising: a dryer (60) for drying an aqueous solution of the chloride of the selected metal or an aqueous solution of sodium hydroxide to produce water vapour and the chloride of the selected metal or sodium hydroxide, respectively, in solid phase, wherein the dryer (60) comprises an inlet (61) for receiving the aqueous solution, a first outlet (64) for the water vapour and a second outlet (65) for the chloride of the selected metal or for the sodium hydroxide, respectively, in solid phase; and a condenser (70) for condensing at least some of the water vapour produced by drying the aqueous solution, wherein the condenser (70) comprises an inlet (71) for receiving the water vapour from the first outlet (64) of the dryer (60) and an outlet (74) for liquid water, wherein the outlet (74) of the condenser (70) is connected upstream of the second inlet (42) of the absorber (40).

23. An apparatus according to claim 22 when dependent on claim 17, wherein: the condenser (70) comprises a cooler (72) of the water vapour; the separator (80) comprises a cooler (82) of the stream of tail gases; and the cooler (72) of the water vapour is adapted and arranged to transfer heat to downstream (84) of the cooler (82) of the stream of tail gases.

24. An apparatus according to any one of claims 17 to 23, wherein the separator (80) comprises at least two coolers (88, 82) of the stream of tail gases, at least one of which is adapted and arranged to transfer heat to downstream (85, 86) of another one of the coolers (82, 88) of the stream of tail gases.

25. An apparatus according to claim 23 or claim 24, further comprising: an outlet (16) for chlorine gas produced by the electrolytic cell (10) in excess of the stoichiometric amount thereof required for the reverse Deacon reaction, wherein the outlet (16) for excess chlorine gas branches off from the second outlet (15) of the electrolytic cell (10); and a third heat exchanger (HE3, HE4) for transferring heat from downstream of the outlet (16) for excess chlorine gas to at least one of:the kiln (20); downstream of the first outlet (24) of the kiln (20) and upstream of the second inlet (32) of the gas-phase reactor (30); the gas-phase reactor (30); and downstream (84, 85) of the cooler (82) of the stream of tail gases.

26. An apparatus (1d, 1h) according to any one of claims 22 to 25, wherein the second outlet (65) of the dryer (60) is connected upstream of the inlet (11) of the electrolytic cell (10).

27. An apparatus according to any one of claims 16 to 26, wherein the gas-phase reactor (30) has a cross- sectional area which increases by at least one-eighth but not more than 50% in a direction (D) towards the outlet (34) of the gas-phase reactor (30).

28. An apparatus according to claim 27, wherein the cross-sectional area of the gas-phase reactor (30) in the direction (D) increases by a percentage which is selected in dependence on the temperature of the reverse Deacon reaction inside the gas-phase reactor (30).

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