Recovery of Chlorine from Hydrogen Chloride Produced by the Carbon Chlorination Method

Abstract

The present invention relates to a method for recovering chlorine from hydrogen chloride produced by a carbon chlorination method. Further, the present invention relates to the use of this method for recovering chlorine from hydrogen chloride produced by a carbon chlorination method.

Description

Technical Field

[0001] The present invention relates to a method for recovering chlorine from hydrogen chloride produced by the carbochlorination process. Further, the present invention relates to the use of this method for recovering chlorine from hydrogen chloride produced by the carbochlorination process.

[0002] Technical Background of the Invention The carbochlorination process is a process for converting a metal oxide into its corresponding metal chloride in the presence of chlorine gas and a carbonaceous material. The most common carbochlorination processes involve the treatment of feedstocks such as ores and slags containing oxides of refractory metals, especially niobium, tantalum, tungsten, molybdenum and rhenium, or oxides of rare earth metals such as cerium, neodymium, samarium, or oxides of light metals such as aluminum, silicon, vanadium or titanium, or oxides of other metals such as zirconium. Metal chlorides often exhibit relatively low vapor pressures and can be removed from the original solid matrix by sublimation or purified by fractional sublimation or distillation using the various boiling points of the metal chlorides, and solvents may or may not be used. Further, the metal chlorides can also be purified by extraction or other separation methods.

[0003] Typically, the metal chlorides are further processed into metals, metal alloys, or purified oxides or hydroxides, all of which are economically attractive. Further, the carbochlorination processes and the issues associated with the use of chlorine in these processes will be discussed in more detail using titanium dioxide as an example.

[0004] Titanium dioxide is produced by either the well-established sulfate process or the chloride process. The latter uses a titanium-containing feedstock that is subjected to the carbochlorination process. The chloride thus obtained is then separated by re-sublimation or distillation. Titanium tetrachloride is finally converted to titanium dioxide, and the chlorine liberated from the reaction is separated and reused in the reaction with the titanium-containing feedstock. Among other reasons, the chloride process is more economically attractive than the sulfate process in terms of chlorine reuse.

[0005] When the titanium-containing feedstock contains, in addition to titanium, a large amount of components, especially iron, it is very burdensome to reuse chlorine in the reaction with the titanium-containing feedstock. Since chlorides other than titanium tetrachloride are separated and removed from the process, the chlorine from these components cannot be easily recovered, and thus is lost for internal recycling, reducing the economic attractiveness of the method.

[0006] To compensate for the loss of chlorine, new chlorine has to be added to the system, which leads to an increase in cost. The higher the content of components other than titanium, the more necessary it is to increase the compensation for the loss. Therefore, feedstocks with a titanium dioxide content of at least 90% by weight and a low content of other elements, especially iron, which may produce by-products with a substantially low commercial benefit as a result of conversion to chlorides, are substantially preferred. These beneficial criteria are met by titanium-containing slag and natural rutile. Unfortunately, the production of titanium-containing slag requires a high energy input, and natural rutile resources are becoming increasingly limited worldwide. Due to the attention focused on the chloride process, the demand for natural rutile is also continuously increasing. The increasing demand is driving up the prices of these rutiles. Furthermore, titanium sources such as ilmenite are more readily available but contain a significant amount of iron, making the chloride process significantly more expensive for the reasons stated above.

[0007] In European Patent Application No. 21172411.7, a titanium-containing feedstock containing iron is used, and it also faces the above problems. The feedstock is contacted with chlorine in a carbchlorination reactor to react the metals contained in the feedstock with chlorine. After separating ferrous chloride thus obtained from titanium tetrachloride, sulfuric acid is reacted with ferrous chloride to obtain hydrogen chloride and iron sulfate through anion exchange. Then, hydrogen chloride can be converted to chlorine, for example, by the Deacon process and reintroduced into the carbchlorination reactor. However, iron sulfate is contaminated with heavy metals and other impurities derived from the carbchlorination reactor, such as coke, residues of ores, etc. The commercial value of the contaminated monohydrate is low and it needs to be discarded or purified, which of course involves additional undesirable costs.

[0008] To sum up the above, depending on the composition of the slag or ore used as the feedstock, isolating metals from the ore or slag using the carbchlorination method may lead to significant losses of chlorine due to the presence of additional metals, especially iron, in the slag or ore used. For this reason, throughout the carbchlorination method, the cost becomes high, the sustainability decreases, and thus the attractiveness decreases.

[0009] Therefore, in this technical field, there is a need for a method for recovering chlorine from hydrogen chloride in a carbchlorination method using a metal-containing feedstock containing iron, which has substantially few by-products with low commercial value and is economically attractive.

[0010] Object and Summary of the Invention An object of the present invention is to provide an improved method for recovering chlorine from hydrogen chloride in a carbchlorination method, which has substantially few by-products with low commercial value and is economically attractive.

[0011] This object is achieved by the method and use of the present invention.

[0012] The present invention provides a method for recovering chlorine from hydrogen chloride produced by a carbon chlorination process, wherein the metal-containing feedstock contains iron. The resulting metal chloride can be isolated and further processed into the desired final product, such as the respective metal, metal alloy, or purified oxide or hydroxide. This method advantageously enables the recovery of chlorine from iron chloride, i.e., ferrous chloride and / or ferric chloride, by isolating iron chloride and then subjecting the iron chloride to a hydrolysis step to obtain iron oxide and hydrogen chloride. The hydrogen chloride can be subsequently converted to chlorine, for example, by using the Deacon process, and reused in the carbon chlorination reaction. As a result, natural ores and produced expensive slags with a high metal content are not the only suitable sources as feedstocks in the carbon chlorination process, whereby the method and use according to the present invention become economically beneficial.

[0013] Accordingly, in a first aspect, the present invention relates to a method for recovering chlorine from hydrogen chloride produced by a carbon chlorination process, comprising: a) contacting a carbonaceous material and chlorine with a metal-containing feedstock containing iron to obtain a chloride mixture comprising ferrous chloride and / or ferric chloride and metal chlorides; b) separating ferrous chloride and / or ferric chloride from the chloride mixture; c) subjecting ferrous chloride and / or ferric chloride to a hydrolysis step to obtain hydrogen chloride; d) converting at least a portion of the resulting hydrogen chloride to chlorine; and optionally, e) using at least a portion of the chlorine obtained in step d) in step a).

[0014] In a second aspect, the present invention relates to the use of the method disclosed herein for recovering chlorine from hydrogen chloride produced by a carbon chlorination process.

[0015] Further advantageous embodiments of the present invention are described in the dependent claims.

[0016] Description of the Invention These and other aspects, configurations, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and consideration of the claims. Each feature from one aspect of the present invention can be employed in any other aspect of the present invention. A numerical range described in the form of "x to y" includes the recited values and values within the respective measurement accuracies known to those skilled in the art. When several preferred numerical ranges are described in this form, all ranges formed by various combinations of the endpoints are of course included.

[0017] In step a) of the method according to the present invention, a carbonaceous material and chlorine are brought into contact with a metal-containing feedstock containing iron to obtain a chloride mixture containing ferrous chloride and / or ferric chloride and metal chloride. Preferably, the metal-containing feedstock contains 70% by weight or less, preferably 60% by weight or less, more preferably 50% by weight or less of iron based on the total weight of the metal-containing feedstock. In connection with the configuration of "metal-containing feedstock containing iron", the metal means any metal other than iron. The metal is preferably selected from the group consisting of refractory metals, rare earth metals, and light metals. More preferably, the metal is selected from the group consisting of niobium, tantalum, tungsten, molybdenum, rhenium, zirconium, cerium, neodymium, samarium, aluminum, silicon, vanadium, or titanium, and even more preferably, titanium. The metal may be present in the feedstock as its respective salt and / or oxide, and generally may be present as its respective oxide. The iron in the metal-containing feedstock may be present as its respective salt and / or oxide, and generally may be present as its respective oxide in the oxidation state (II) or (III). Any carbonaceous material containing sufficient carbon to generate a sufficient reduction potential and temperature required for the carbon chlorination process can be used. The material can be selected from the group consisting of petroleum coke, carbon produced by hydrothermal action, charcoal produced from various raw materials such as wood and seeds, carbon black, and coal. Further metals and metalloids may be present in the feedstock. As a result of step a), a chloride mixture containing ferrous chloride and / or ferric chloride and metal chloride is obtained.

[0018] From the chloride mixture consisting of the metal chloride obtained in step a) and ferrous chloride and / or ferric chloride, ferrous chloride and / or ferric chloride is separated from the chloride mixture in the following step b). This can be achieved by common techniques such as re-sublimation or distillation, but can also be achieved by extraction or any other separation method. Ferrous chloride and / or ferric chloride is preferably purified before step c) to remove impurities including unreacted metal-containing feedstock and unreacted carbonaceous material to which the aforementioned chlorides are attached.

[0019] In step c), ferrous chloride and / or ferric chloride is subjected to a hydrolysis process to obtain hydrogen chloride. Ferrous chloride and / or ferric chloride is obtained in step b). Step c) is preferably carried out at an elevated temperature in the range of 120°C to 350°C, preferably 160°C to 260°C, more preferably 180°C to 240°C, in the presence of a mediator. The mediator is a medium for carrying out the hydrolysis. This can consist of an inert or reactive molten salt, an ionic liquid, a deep eutectic solvent, a non-aqueous solvent, with or without the addition of an auxiliary solvent. In one embodiment of the present invention, a molten salt consisting of zinc chloride has been found to be particularly preferred for carrying out the hydrolysis reaction that can be implemented in both cases, in a slurry when the dominant species in the metal chloride phase is ferrous chloride or in a homogeneous melt when the dominant species in the metal chloride phase is ferric chloride. Alternatively, the hydrolysis reaction can be further carried out without a mediator when ferric chloride is present during step c). An initial amount of water can be added to the selected mediator to achieve a high viscosity that is preferred for operating the process. During the hydrolysis reaction, the amount of water in the mediator is kept constant at the initial level, and this level is selected such that the vapor pressure of water above the mixture is always lower than the vapor pressure of hydrogen chloride. Thereby, gaseous hydrogen chloride can be recovered from the hydrolysis reaction in a state substantially free of water.

[0020] Regarding the "water-free" hydrogen chloride used in this specification, "water-free" refers to the concentration of water in gaseous hydrogen chloride that does not exceed 15% by volume and is not less than 0.01% by volume of the total weight of hydrogen chloride and water. The hydrolysis reaction can be maintained by the introduction of oxygen or other oxidizing agents required when ferrous iron is the dominant iron chloride species. Usually, iron oxide in the form of hematite is obtained as a by-product.

[0021] In another preferred embodiment of the present invention, the hydrolysis in step c) is carried out as thermal hydrolysis at a temperature of 600°C to 1200°C, preferably 700°C to 1000°C, more preferably 800°C to 900°C. For this purpose, a spray roasting apparatus such as that used in the so-called Ruthner process can be used, in which case natural gas is oxidized in the reactor in the presence of oxygen. Alternatively, the Lurgi process can be carried out in a fluidized bed to carry out the thermal hydrolysis.

[0022] In a preferred embodiment, the water generated during the hydrolysis method or the thermal hydrolysis method is removed to the level required for carrying out the gas-phase oxidation of hydrogen chloride on the Deacon catalyst. Therefore, in a preferred embodiment of the present invention, before step d), water is removed from the hydrogen chloride such that the hydrogen chloride contains water not exceeding 25% by volume, preferably not exceeding 15% by volume, more preferably not exceeding 5% by volume, and even more preferably not exceeding 1% by volume, based on the total volume of gaseous hydrogen chloride and gaseous water. This is achieved by general techniques and apparatuses for obtaining gaseous hydrogen chloride.

[0023] Next, in step d), at least a part of the obtained hydrogen chloride is converted to chlorine. The hydrogen chloride is obtained in step c). In a preferred embodiment of the present invention, the conversion is typically carried out in the presence of a catalyst at an elevated temperature in the range of 400°C to 450°C, although certain catalysts may be able to significantly lower this temperature range. For example, European Patent Application No. 0980340.5 discloses a catalyst, preferably a supported ruthenium oxide catalyst, which enables the lower limit temperature of the temperature range to be lowered to 100°C. As is known to those skilled in the art, the oxidation of hydrogen chloride is an equilibrium reaction. When the reaction is carried out at a high temperature, the equilibrium conversion rate decreases. Therefore, it is preferred to carry out the oxidation at a low temperature in the range of 100°C to 300°C. The catalyst and method disclosed in European Patent Application No. 0980340.5 are incorporated herein by reference. Catalysts that can also be used in this step are those based on copper chloride, i.e., a known mixture of copper(I) chloride and copper(II) chloride, zinc chloride, or ferric chloride. Other catalysts include ruthenium oxide, cerium oxide, or chromium(III) oxide supported on tin oxide, silicon dioxide, titanium dioxide, aluminum oxide, or lanthanum oxide. The oxidation of hydrochloric acid to chlorine based on a catalyst is also known as the so-called Deacon process. A method using a so-called fixed-bed reaction system in which hydrogen chloride and oxygen are passed through a catalyst bed filled with a catalyst in a reaction tube for use in chlorine production. When hydrogen chloride is oxidized on an industrial scale by a fixed-bed reaction system, a fixed-bed multitubular reactor is generally used. In this oxidation reaction, a large amount of catalyst for use in the production of chlorine is required to form a catalyst bed in all of these reaction tubes.

[0024] In another preferred embodiment, the hydrogen chloride generated in step c) is subjected to an absorption step in water to produce hydrochloric acid water having a concentration of 28% to 34% by weight before the conversion in step d), and the conversion in step d) is carried out by electrolysis. Electrolysis using an implemented oxygen evolution cathode (ODC) is particularly suitable when the water content of the gaseous hydrogen chloride before absorption exceeds 25% by volume based on the total weight of water and gaseous hydrogen chloride. Both conversion techniques are established in the art.

[0025] The chlorine obtained can be used as a valuable raw material in the production of vinyl chloride and phosgene. Optionally, in step e), at least a part of the chlorine obtained can be used in step a) and thus recycled. Before the reuse of chlorine in step a), water is removed from the chlorine to the level required for carbon chlorination.

[0026] The method steps described herein are carried out in the order of a), b), c), d) and then e).

[0027] When the metal of the metal-containing feedstock is titanium, the feedstock is preferably selected from ilmenite, perovskite, rutile, titanite or mixtures thereof. Techniques and apparatuses commonly used in the chloride process, such as reactors for carrying out the carbon chlorination reaction (a fluidized bed can be used in this step).

[0028] In a further aspect of the invention, the method disclosed herein is used to recover chlorine from hydrogen chloride produced by the carbon chlorination process. Preferably, the carbon chlorination process is used in the chloride process to obtain titanium dioxide. Titanium dioxide is particularly suitable as a white pigment in applications such as lacquers, paints, and decorative papers.

Claims

1. A method for recovering chlorine from hydrogen chloride produced by the carbon chlorination method, comprising: a) contacting a carbonaceous material and chlorine with a metal-containing feedstock containing iron to obtain a chloride mixture containing ferrous chloride and / or ferric chloride and a metal chloride; b) separating the ferrous chloride and / or ferric chloride from the chloride mixture; c) subjecting the ferrous chloride and / or ferric chloride to a hydrolysis step to obtain hydrogen chloride; d) converting at least a part of the obtained hydrogen chloride into chlorine, and optionally e) using at least a part of the chlorine obtained in step d) in step a). A method comprising the above steps.

2. The method according to claim 1, wherein step c) is carried out in the presence of a mediator at a temperature of 120°C to 350°C, preferably 160°C to 260°C, more preferably 180°C to 240°C.

3. The method according to claim 1, wherein the hydrolysis in step c) is carried out as thermal hydrolysis at a temperature of 600°C to 1200°C, preferably 700°C to 1000°C, more preferably 800°C to 900°C.

4. The method according to claim 1, wherein the hydrogen chloride obtained in step c) contains water, and the water is removed from the hydrogen chloride before step d) such that the hydrogen chloride contains water in an amount of 25% by volume or less, preferably 15% by volume or less, more preferably 5% by volume or less, even more preferably 1% by volume or less, based on the total volume of the gaseous hydrogen chloride and the gaseous water.

5. The method according to claim 1, wherein the metal is selected from the group consisting of refractory metals, rare earth metals and light metals, and is preferably titanium.

6. The method according to claim 1, wherein the metal in the metal-containing feedstock is titanium, and the feedstock contains iron in an amount of 70% by weight or less, preferably 60% by weight or less, more preferably 50% by weight or less, based on the total weight of the titanium-containing feedstock.

7. The method according to claim 6, wherein the titanium-containing feedstock is selected from the group consisting of ilmenite, perovskite, rutile, titanite or a mixture thereof.

8. The method according to claim 1, wherein step d) is carried out at an elevated temperature in the presence of a catalyst and oxygen.

9. The method according to claim 8, characterized in that the catalyst is selected from the group consisting of ruthenium oxide, cerium oxide, chromium(III) oxide, copper chloride, ferric chloride and zinc chloride.

10. The method according to claim 9, characterized in that the catalyst is supported on tin oxide, silicon dioxide, titanium dioxide, aluminum oxide, lanthanum oxide or a mixture thereof.

11. The method according to claim 1, characterized in that the hydrogen chloride produced in step c) is subjected to an absorption step in water before the conversion in step d), and at least a part of the obtained hydrogen chloride is converted to chlorine by electrolysis.

12. Use of the method according to any one of claims 1 to 10 for recovering chlorine from hydrogen chloride produced by the carbon chlorination method.

13. Use of the method according to claim 11, characterized in that the carbon chlorination method is used in the chloride method to obtain titanium dioxide.