METAL RECOVERY PROCESS

Abstract

A process is described for recovering a metal in the form of a metal chloride from a metal-containing source, the process comprising the steps of: (i) forming a solid metal chloride-containing product by contacting the metal-containing source with a chloride gas in an oxidizing environment and at a temperature below the vaporization temperature of the metal chloride of interest; (ii) heating the metal chloride-containing product formed in step (i) to a temperature equal to or above the vaporization temperature of the metal chloride to form a metal chloride-containing gaseous product; and (iii) condensing the metal chloride-containing gaseous product from step (ii) to recover the metal chloride of interest.

Description

METAL RECOVERY PROCESS MA / I ¿l ¿ó The present invention relates to a process for recovering a metal of interest from a metal-containing source. More particularly, the present invention relates to a process for recovering a metal of interest in the form of a metal halide from a metal-containing source, such as a metallic ore or metalliferous waste material, using a halogenation process. Background of the invention Metalliferous mineral deposits formed in the Earth's crust typically contain more than one metalliferous element, with certain combinations of metals commonly occurring together. Lead deposits, for example, are commonly associated with zinc-lead concentrates formed in processing plants, and therefore often contain zinc as an impurity. Methods for extracting metals from ores that are known in the art include cyanide leaching and autoclave processes. Extraction methods based on chlorination, which involve the release of the metal in the form of its chloride, have also been described, as outlined in WO 2006 / 043065A. Metalliferous waste materials, such as process slag waste produced during smelting and refining processes, also contain valuable metal-bearing constituents. Due to the high-energy nature of their formation, metalliferous waste tends to be very stable, and therefore these metal-bearing constituents cannot be easily recovered in an economically viable manner. The metallic impurities trapped within the slag waste have not been considered commodities until now, and little attention has been paid to developing methods to capture their value. Depending on the origin of the metal waste, a wide variety of valuable metals of interest are potentially recoverable. Lead smelting waste typically comprises zinc-containing components, for example, as well as other valuable metals such as copper, silver, indium, gold, molybdenum, tin, gallium, and indium, while copper smelting waste is a potential source of nickel-containing components. Methods for extracting and recovering metals from waste materials that have found application in the field include hydrometallurgical processes. However, these processes are not selective, and the indiscriminate dissolution of the waste complicates the recovery of the metal of interest from the slag waste material. In more recent years, interest has been expressed in chlorination-based extraction processes in which a metal is recovered from a metal-containing source by converting it into a volatile metal chloride that can then be recovered and further treated to extract the metal itself. In the aforementioned document WO 2006 / 043065A, for example, a method for extracting precious metals from precious metal-bearing sources is described. This method involves forming a metal-bearing chloride compound in the vapor phase, condensing the precious metal-bearing vapor-phase product to extract the metal from its source as a metal chloride, and then recovering the precious metal from the condensed product. Although described as applicable to the extraction of metals from a variety of sources, metal-bearing ores are preferred sources for use in the method, and the extraction of gold from a complex gold ore is exemplified. WO 2010 / 082065A describes the recovery of metal-containing constituents from slag residues using a chlorination-based extraction process. As described therein, the metal-containing constituents are converted into volatile metal chlorides, which are then recovered and further treated to extract the metal of interest. The metal-containing residues are heated, and the heated material is ML / I is brought into contact with a chloride gas, preferably hydrogen chloride, to form a product containing gaseous metal chloride from which the metal chloride is recovered and the metal extracted. Conveniently, the product containing gaseous metal chloride is treated to recover the metal chloride by slaking with water to form an aqueous solution of the metal chloride. In the method described in WO 2010 / 082065A, the starting solid waste material is contacted with gaseous chloride at a temperature equal to or higher than the vaporization temperature of the metal chloride to be recovered. In this way, the target metal chloride is formed and volatilized in a single step. The continuous formation and separation of the metal chlorides takes place within a fluidized bed reactor heated to a temperature in the region of 700°C to 1600°C, with temperatures generally in the range of 700°C to 900°C. One disadvantage of the method described in WO 2010 / 082065A is its inefficient use of hydrogen chloride. Hydrogen chloride gas is an expensive reagent, with current estimated reagent and energy costs for hydrogen chloride gas generation from the chlor-alkali plant amounting to approximately US$327 per tonne, making the optimal use of hydrogen chloride crucial for the commercial viability of the process. To maximize the concentration of metal chlorides in the gas, the extraction gas must be recirculated before the metal chlorides are recovered. However, this requires that the recirculation loop be maintained at the same temperature as the fluidized bed reactor, as otherwise the metal chlorides will condense within the recirculation loop. This presents considerable technical difficulties; gas recirculation requires a recirculation pump that must not only withstand the attack of the hydrochloric acid gas at the extremely high temperatures employed, but also cope with vaporized metal chlorides, superheated steam (present in the water, a byproduct of metal chloride formation), and fine metallurgical waste dust mobilized by the gas flow velocity.To date, no commercially viable solution has been developed for this problem. Another problem related to the use of extraction gas arises during the quenching stage. Hydrogen chloride gas is highly soluble in water and will form concentrated hydrochloric acid during the quenching stage, which must then be neutralized to release the dissolved metal chloride. The loss of hydrogen chloride gas to the quenching solution during this stage, along with the cost of the alkaline reagent required for neutralization, represents a significant additional processing cost. Further inefficiencies arise in the utilization of hydrogen chloride gas in the method described in WO 2010 / 082065A due to losses within the calcination residues of the fluidized bed reactor. These residues, which come directly from the hydrogen chloride-rich environment of the fluidized bed reactor, contain some residual hydrochloric acid gas (and potentially also some residual metal chlorides), which again require neutralization. Therefore, there remains a need to develop more commercially viable methods based on halogenation, particularly chlorination, to recover metals of interest from metal-bearing sources such as metallic ores and metalliferous waste materials, avoiding the disadvantages associated with methods known in the art. Summary of the invention According to a first aspect, the present invention provides a process for recovering a metal in the form of a metal halide from a metal-containing source, the process comprising the steps of: (i) forming a solid product containing a metal halide by contacting the source containing the metal with a gaseous halide in an oxidizing environment and at a temperature below the vaporization temperature of the metal halide of interest; (ii) heating the metal halide-containing product formed in step (i) to a temperature equal to or higher than the vaporization temperature of said metal halide to form a gaseous metal halide-containing product; and (iii) condensing the gaseous metal halide-containing product from step (ii) to recover the metal halide of interest. A process is also provided for recovering a metal of interest from a metal-containing source comprising recovering the metal in the form of a metal halide from the metal-containing source in accordance with the first aspect of the invention and then extracting the metal from the metal halide. The present invention is based on the finding that metals can be extracted and recovered from a source containing metals in the form of a metal halide in a cost-effective manner, with savings in operating and capital costs compared to, for example, existing chlorination methods. The present invention involves forming a metal halide containing a halide in the solid phase and heating the solid metal halide product to a temperature equal to or above the vaporization temperature. MA / I ¿l ¿ó of the metal halide to produce a gaseous metal halide and then condense the gaseous metal halide compound to recover the metal halide of interest. In particular, the inventors of the present invention have found that carrying out the formation and volatilization steps of metal chloride in separate steps, rather than forming and volatilizing the metal chloride in a single step as in the method of WO 2010 / 082065A, offers significant commercial advantages. This not only leads to better utilization of the gaseous chloride reagent but also minimizes losses of gaseous chloride. This, in turn, leads to savings in operating costs. Capital costs are also reduced due to the corresponding reduction in the size of the industrial plant required to produce the gaseous chloride for the process. ML / t / ZUZZ / UIZ l¿ó Detailed description of the invention The process of the invention is applicable to the recovery of metals in the form of their halides from a variety of sources, including metalliferous ores and waste materials produced by mining processes. Suitable waste materials include slag waste from smelting or refining processes of lead, copper, or zinc, or other waste materials containing sufficiently high levels of constituents with recoverable metals of interest to be economically viable. The nature and levels of components containing potentially recoverable metals in metalliferous waste materials will vary depending on the origin of the slag waste. Slag waste from lead smelting processes commonly comprises constituents containing zinc, generally in the form of oxides and silicates, for example. Other valuable metals commonly found in lead smelting slags include copper, silver, indium, gold, molybdenum, tin, gallium, and germanium. Since copper deposits are commonly associated with nickel, copper smelting slags are a potentially useful source of nickel compounds. Therefore, the method of the present invention offers the possibility of recovering a wide variety of valuable metals of interest. The gaseous halide for use in the process according to the invention can be any halide that is normally present in gaseous form or one that readily volatilizes. Any conventional halide, such as a chloride, fluoride, bromide, or iodide, can be used. In one embodiment, the gaseous halide for use in the process according to the present invention is a gaseous chloride, properly a chloride that normally occurs in gaseous form as hydrogen chloride or a chloride that readily volatilizes such as ammonium chloride or iron(III) chloride. In one particular form, gaseous chloride is hydrogen chloride gas. The hydrogen chloride gas for use in the process of the invention can be conveniently prepared by conventional methods, for example, by electrolysis of a brine solution to produce hydrogen and chlorine gases, which are then combined to produce hydrogen chloride gas. A commercially useful byproduct of this process is sodium hydroxide, the preparation of which represents another aspect of the invention. In one embodiment, hydrogen and chlorine gases are combined to produce hydrogen chloride gas before being brought into contact with the metal-bearing waste material. In another embodiment, hydrogen chloride gas is formed through the degradation of chemicals and is subsequently brought into contact with the metal-containing source. Conveniently, the solid metal halide-containing product of step (i) of the process of the invention is formed by bringing the metal-containing source with a stream of the gaseous halide, for example, a gaseous chloride, into a suitable reaction vessel such as a fluidized bed reactor or rotary furnace. In one embodiment, the reaction vessel is a fluidized bed reactor. Conveniently, the bed ML / I ¿l ¿ó fluidized will be maintained with compressed air to ensure a homogeneous bed. To optimize the amount of metal halide-containing product formed per unit of gaseous halide consumed, it will be appreciated that the gaseous halide stream can be properly recirculated within the reaction vessel. The process for forming the metal halide-containing product can be carried out in batch mode, by intermittently contacting the gaseous chloride with the metalliferous waste material. In one embodiment, the process to form the metal halide-containing product can be carried out continuously, for example, by continuously replenishing the supply of gaseous halide and the supply of metalliferous waste material in the reaction vessel. Forming the metal halide by contacting the metal-containing source with the gaseous halide at a temperature below the vaporization temperature of the metal halide of interest, as in step (1) of the process of the present invention, means that the metal halide remains in the solid phase instead of volatilizing as in the prior art processes described above. This offers considerable processing advantages since the metal halides thus formed remain stable and can subsequently be extracted from an environment enriched with gaseous halides, thereby reducing operating costs by improving the utilization of the gaseous halide reagent and minimizing gas losses. It will be noted that when there is more than one recoverable metal of interest present in the metal-containing source material, the temperature at which the metal-containing source material and the gaseous halide are brought into contact must be below the vaporization temperature of the most volatile of the metal halides formed. Generally, when the gaseous halide is a gaseous chloride, temperatures in the range of 100°C to 350°C are used, preferably from 300°C to 350°C. In one embodiment, the source material containing metal and the gaseous chloride are brought into contact at a temperature in the range of 320°C to 350°C. When the source material containing metal comprises iron and other metals of greater interest, the inventors hereof have discovered that the selectivity of the recovery of the metals of greater interest can be improved when a metal chloride is formed at a temperature above 316°C. Iron(III) chloride, formed by the reaction of iron present in the metalliferous source material with chloride gas, is not stable above its boiling point of 316°C, decomposing above this temperature to form iron(III) oxide, which condenses and releases hydrogen chloride. By carrying out the metal chloride formation step of the process of the present invention at a temperature above 316°C, hydrogen chloride gas is not lost from the iron, making the metal chloride formation step more selective. In another modality, therefore, the metalliferous material and the gaseous chloride are brought into contact at a temperature above 316°C, appropriately at a temperature in the range above 316°C but not above 350°C. It will be noted that if the gaseous halide is other than a chloride, the boiling point of the iron(III) halide, and correspondingly the lower limit of the temperature range over which the loss of volatilized iron(III) halide is avoided, would be different. The boiling points of other halides are conventionally known in the art and readily available in the literature. In principle, the metal halide formation step could be carried out at a temperature as low as room temperature, but in this case, the water formed as a byproduct of metal halide formation will remain with the metal halides. The presence of water has a detrimental effect on the efficiency of the MA / t / ZUZZ / UIZ / Zó use of gaseous halides, since gaseous halides react with water to form the halogen acid (e.g., hydrochloric acid in the case of gaseous chloride), resulting in the loss of gaseous halides. Furthermore, dry recovery of the metal halide is not an option in the presence of water. Conveniently, therefore, the gaseous halide and the source material containing metal are brought into contact with each other at a temperature above 100°C. The advantage of contacting the gaseous halide and the metal-containing source material at a temperature above 100°C can be easily understood by referring to the situation where gaseous chloride is used as the metal halide. Under these temperature conditions, the water formed as a byproduct of metal chloride formation will not condense (with the consequent risk of it subsequently forming hydrochloric acid with the gaseous chloride), but will remain in the gaseous phase with the excess gaseous chloride. This gaseous water (vapor) can be easily removed by passing it through a sulfuric acid scrubber, leaving dry gaseous chloride that can be recirculated. In this way, the loss of gaseous chloride reagent is minimized, and its utilization is maximized since it is used solely for the formation of metal chloride. When the metal halide formation step is carried out in an oxidizing environment, for example, in a fluidized bed reactor maintained by compressed air, any iron present in the source metal is converted to iron(III) oxide. As described earlier in relation to the use of chloride gas as the halide gas, any iron(III) chloride formed is unstable above its relatively low boiling point, decomposing to form iron(III) oxide and releasing hydrogen chloride. This minimizes the waste of hydrogen chloride through the loss of iron. In one modality, the oxidizing environment is provided by bringing the metal-containing source material and the gaseous halide into contact in a continuous flow of air. The metal halide-enriched feed produced in step (i) of the process of the present invention can be appropriately treated to recover any residual gaseous halide trapped within the feed, as well as any wastewater. In one embodiment, this is achieved by transferring the metal halide-enriched feed to a rotary kiln operating at the same temperature as that used in the metal halide formation step. Any recovered gaseous halide can then be returned to the reaction vessel used in the metal halide formation step and recirculated. In this way, calcination losses can be significantly reduced. The metal halide-containing product (without gaseous halide) from step (i) is then heated in step (ii) of the process of the present invention to a temperature equal to or higher than its vaporization temperature to form a gaseous metal halide-containing product. In one embodiment, the metal halide-containing product (e.g., chloride) from step (i) is heated in a fluidized bed reactor maintained at a temperature in the range of 700°C to 1500°C, typically 1000°C. The gaseous metal halide product thus produced can be condensed using conventional methods in the art. Since the gaseous metal halide stream produced in step (ii) is dry and contains no excess gaseous halide, the recovery of the metal chloride of interest is a straightforward matter. In one embodiment, the recovered metal halide can optionally be further refined to extract the metal of interest. Conveniently, the metal of interest can be recovered from the metal halide produced by the process of the invention using conventional hydrometallurgical processing techniques. Figure 1 schematically illustrates an apparatus for use in accordance with one modality of the process of the invention. MA / I ¿l ¿ó Figure 1 shows a fluidized bed reactor (2) into which a smelter slag feed material can be introduced through an inlet (1). Gaseous halide (e.g., gaseous chloride) is introduced into the fluidized bed through a gas and air inlet (7) at the base of the fluidized bed reactor, and the gas is recirculated through the fluidized bed reactor in a gas recirculation circuit (4) by means of a circulation pump (6). The recirculating gases (in the case of gaseous chlorides, these will be water formed as a byproduct of metal chloride formation and excess gaseous chlorides) are passed through a sulfuric acid scrubber (5), leaving dry gaseous chloride for recirculation.The metal chloride-enriched material is passed through outlet (3) to an intermediate rotary kiln (8), and the residual chloride gas trapped within the treated feed is recovered, along with wastewater, and returned to the gas recirculation circuit (4) via a recovered gas recirculation loop (9). The metal chloride-enriched feed, now free of chloride gas, is passed to a second rotary kiln (recovery) (10) where the temperature is raised, and the metal chloride-rich vapors pass through outlet (11) and are collected. Process residues are expelled from the recovery rotary kiln through outlet (12). Throughout the description and claims of this specification, the words "comprises" and "contains," and variations of these words, e.g., "comprising" and "comprising," mean that it includes, but is not limited to, and does not exclude other portions, additives, components, wholes, or steps. The singular encompasses the plural unless the context requires otherwise. In particular, when the indefinite article is used, the specification should be understood to encompass both plurals and singulars, unless the context requires otherwise. The preferred features of each aspect of the invention may be those described in relation to any of the other aspects. In general terms, the invention extends to any novel feature or novel combination of features described in this specification (including the claims and accompanying drawings). Therefore, it should be understood that the features, whole numbers, characteristics, compounds, chemical fractions, or groups described in conjunction with a particular aspect, embodiment, or example of the invention are applicable to any other aspect, embodiment, or example described herein unless incompatible with it. The process of the invention is further illustrated by the following non-limiting example: Twenty grams of lead smelting slag were heated to 350°C in a fluidized bed reactor in the presence of a continuous mixed gas flow of air and hydrogen chloride gas. This was maintained for 3 hours. After 3 hours, the hydrogen chloride gas was removed, but the air flow was maintained. The sample was held at 350°C for a further 1.5 hours and then subjected to a temperature of 1000°C for 1.5 hours under a constant air flow. After cooling, the sample was analyzed and compared with the head grade of the feed material and the treated material according to the process described in WO 2010 / 082065A. Five samples were evaluated in duplicate. The results obtained, which are presented below, demonstrate very high extractions of the three target metals of copper, lead and zinc, while iron remained with the calcination residues. Analyte Fe Cu Pb Zn Raw Ore Quality 19.74% 1.45% 0.99% 10.81% New Process Conditions % Extraction <0.1% 99.1% 99.5% 96.4% <0.1% 98.3% 99.8% 95.1% <0.1% 98.0% 99.7% 97.5% <0.1% 98.6% 99.8% 96.4% <0.1% 99.1% 99.8% 97.8% Average <0.1% 98.6% 9 9.7% 9 6.6% Comparing these results with those of the samples processed in WO 2010 / 082065A, it can be seen that similar results were obtained for copper and lead, but there is a notable increase in zinc extraction using the process of the present invention, and a significant difference in the iron extracted. The process of the present invention is more selective with respect to iron and eliminates any problems of iron contamination in downstream product recovery processes. MA / I ¿l ¿ó Analyte Fe Cu Pb Zn Raw Ore Quality 19.74% 1.45% 0.99% 10.81% WO Conditions 2010 / 082065A Extraction % 11.4% 98.6% 99.4% 88.1% 10.1% 99.2% 99.8% 89.5% 9.2% 99.0% 99.7% 89.0% 9.8% 99.2% 99.8% 91.0% 8.2% 99.2% 99.8% 89.1% Average 9.7% 99.0% 99.7% 89.3%

Claims

1. A process for recovering a metal in the form of a metal halide from a metal-containing source, the process comprising the steps of: (i) forming a solid product containing a metal halide by contacting the metal-containing source with a gaseous halide in an oxidizing environment and at a temperature below the vaporization temperature of the metal halide of interest; (ii) heating the metal halide-containing product formed in step (i) to a temperature equal to or above the vaporization temperature of the metal halide to form a gaseous metal halide-containing product; and (iii) condensing the gaseous metal halide-containing product from step (ii) to recover the metal halide of interest.

2. The process according to claim 1, wherein the metal-containing source is a metallic ore or metalliferous waste material.

3. The process according to claim 1 or claim 2, wherein the gaseous halide is a gaseous chloride. MA / I ¿l ¿ó 4. The process according to claim 3, wherein the gaseous chloride is gaseous ammonium chloride or gaseous iron(III) chloride.

5. The process according to claim 3, wherein the gaseous chloride is hydrogen chloride gas.

6. The process according to any of claims 3 to 5, wherein, in step (i), the metal-containing source and the chloride gas are brought into contact at a temperature above 316°C.

7. The process according to any of claims 3 to 6, wherein in step (i), the metal-containing source and the aqueous chloride are brought into contact at a temperature of up to 350°C.

8. The process according to any of the preceding claims, wherein in step (i) the metal-containing source and the gaseous halide are brought into contact in a fluidized bed reactor.

9. The process according to claim 8, wherein the fluidized bed is maintained in the presence of air.

10. The process in accordance with any of the preceding claims, wherein in step (i), the metal-containing source and the gaseous halide are brought into contact at a temperature above 100°C 11. The process in accordance with any of the preceding claims, wherein in step (ii), the metal halide-containing product from step (i) is heated to a temperature of 700°C to 1500°C.

12. The process in accordance with any of the preceding claims, wherein the metal halide 5 recovered from step (iii) is further treated to extract the metal of interest.