PROCESS AND APPARATUS FOR EXTRACTING METAL

Abstract: A process for extracting one or more metals from a metal containing material, as herein defined, including the steps of: (a) treating said metal containing material with an aqueous, acidic solution containing an acid and one or more Transition Element containing species; (b) introducing an oxidising agent into said solution in order to oxidise the Transition Element ion in said Transition Element containing species to a higher oxidation state; and (c) oxidising said metal containing material with the oxidised Transition Element containing species to effect release of said one or more metals from said metal containing material. Also described is an apparatus for use in a process for extracting metal from a metal containing material. The apparatus includes an agitating means, a conduit for introducing an oxidising agent, and an operating means for operating the agitating means.

Description: PROCESS AND APPARATUS FOR EXTRACTING METAL FIELD OF THE INVENTION This invention relates to a process for extracting metals from metal containing materials, and an apparatus for use in such a process. The process is particularly useful for extracting metals by dissolution of a metal mixture, metal alloy or metal compound in an acidic solution, thereby releasing metal or metals for recovery in such forms as metallic particles, as ions in solution or by precipitation as metal compounds. The process can enable extraction of metals from materials via an oxidation step, by using a non oxidising acid, where an oxidising acid would normally be required. The process finds particular application in extraction of metals from metal sulphides, such as from mineral concentrates. The process can further extend to economical extraction of metals from metal containing waste or co-products from mineral, metal refining, chemical or manufacturing processes, such as tailings, metal precipitates, dross and metal scrap. BACKGROUND OF THE INVENTION The chemical extraction of metal values from materials using a process requiring an oxidising step, but which does not involve the specific use of an oxidising acid, such as nitric acid, is of great commercial interest, particularly in the case of metal extraction from metal sulphides and specific transition metals and their compounds. Due to the high cost of processing by oxidising acids, or by use of oxidation agents such as hydrogen peroxide, much interest has been focused on the use of non oxidising acids, incorporating some less expensive oxidising agent, such as air, oxygen, other chemical oxidants, or microorganisms such as bacteria. Previous work has concerned either reaction of the non oxidising acid with the metal containing material in the presence of oxidising gases or bacteria, or by reaction of the non oxidising acid with the metal containing material in the presence of the stoichiometric, or near stoichiometric, quantity of an oxidising agent, often combined with a subsequent step for the regeneration of the oxidising agent. Examples are the manufacture of copper sulphate from copper metal, using sulphuric acid, where air/oxygen is blown into the acid, while in contact with the metal, causing the reaction:- Cu + 1/20 2 → Cu++ and the reaction of metal sulphides with sulphuric acid and ferric sulphate, causing the reaction :- MeS + 2Fe + → Me ++ + 2Fe ++ + S° where, for example, Me is a divalent metal. In this case the ferrous ion is usually reoxidised to ferric ion for reuse, either chemically or by air/oxygen, often incoφorating bacteria as an oxidising aid. In another example cupric copper ions are used:- MeS + 2Cu ++ → Me ++ + 2Cu + + S° The cuprous ions generated, then need to be reoxidised to the cupric state. These processes are limited by the speed of the processes controlling the redox reaction, which is often very slow, stoichiometrically inefficient, and frequently requires a different pH range to that required for effective metal extraction thus requiring expensive pH adjustment. The use of particular oxidising agents also is limited or disadvantaged by the introduction of an often non-compatible contaminant into the acidic solution, which then needs a recovery or removal step to separate it from the desired metal product. This is particularly the case with iron, which is often used because it is cheap and readily available, often from the material to be leached. It is an object of the invention to provide a process and apparatus for extracting metals which overcome or at least alleviate one or more disadvantages of the prior art. SUMMARY OF THE INVENTION According to the present invention, there is provided a process for extracting one or more metals from a metal containing material, as herein defined, including the steps of: treating said metal containing material with an aqueous, acidic solution containing an acid and one or more Transition Element containing species; introducing an oxidising agent into said solution in order to oxidise the Transition Element ion in said Transition Element containing species to a higher oxidation state; and oxidising said metal containing material with the oxidised Transition Element containing species to effect release of said one or more metals from said metal containing material. The present invention also provides a process for extracting one or more metals from a metal containing material, as herein defined, including the steps of: (a) treating said metal containing material with an aqueous, acidic solution containing an acid and one or more Transition Element containing species; (b) introducing an oxidising agent into said solution in order to oxidise the Transition Element ion in said Transition Element containing species to a higher oxidation state; (c) oxidising said metal containing material with the oxidised Transition Element containing species to effect release of said one or more metals from said metal containing material and thereby effecting reduction of said oxidised Transition Element containing species; (d) reoxidising said Transition Element ion to said higher oxidation state with said oxidising agent; and (e) repeating steps (c) and (d) until the desired amount of said one or more metals are extracted. As used throughout the specification, the term ' metal containing material" is intended to mean any material containing one or more metals and includes metal, metal oxide or metal sulphide mixtures, such as by-products of metal refining, metal alloys or metal compounds such as ore minerals, mineral concentrates and tailings. The invention is particularly suitable for material containing metal in the sulphide form. The acid is preferably a non-oxidising acid, although oxidising acids, such as nitric acid, may be used if desired. The acid is more preferably a mineral acid. A suitable non-oxidising acid is sulphuric acid which is generally the most economical acid to use. However, other acids which can be used include, but are not limited to HCl and H 3 PO 4 . An embodiment of the invention utilises waste acid solution containing Transition Metal ions, such as iron ions, such as those from metal pickling processes, or from the production of TiO 2 pigments. The concentration of the acid in the acidic solution may be as high as that for the pure acid. However, typically the concentration of acid is 500 g/l or less, such as a maximum of 350 g/l. In some embodiments, the acid concentration is a minimum of 1 g/l, whereas in other embodiments, the minimum concentration is 10 g/l. In a preferred embodiment, the concentration of acid is in the range of from 150 g/l to 350 g/l. The Transition Element containing species may be any aqueous species containing one or more Transition Elements capable of having variable valence. As used herein, the term Transition Elements is intended to include the main transition elements, or first transition series, from Sc to Cu; the lanthanide elements, or second transition series, from Y to Ag; and the actinide elements, or third transition series, from Hf to Au. Preferably, however, the Transition Element is selected from the main transition elements. Examples of preferred main transition elements are iron and copper. For economic reasons, the more preferred transition element is iron. It is preferred that the aqueous acidic solution is an Fe + /H 2 S0 4 solution. Such solutions are low in cost and often readily available, for example as a waste by¬ product of other chemical processes. Alternatively, the Fe 2+ /H 2 S0 4 solution may be formed in situ, such as where the metal containing material contains iron (eg. chaicopyrite) and Fe 2+ ions are released into solution during leaching thereof. The concentration of the Transition Element in the aqueous acidic solution may be as high as 250g/l. Preferably, the concentration is around 80g/l or less. However, in one preferred embodiment, the maximum concentration is 20g/l. For some embodiments, the concentration may be a minimum of 0.1 g/l whereas in other embodiments the minimum concentration is 5g/l. Preferably, the minimum concentration is 1 g/l. It is a feature of this invention that metal extraction can be successfully conducted at relatively low concentrations of acid and Transition Element containing species. An example of such a solution is one having an acid concentration of 80g/l or less, and a concentration of Transition Element containing species of 5g/l or less (expressed as equivalent concentration of Fe in solution). The process of the invention includes the step of oxidising the one or more Transition Elements to a higher oxidation state. Preferably, both before and after oxidation, the Transition Element is present in solution in an ionic state, such as in a simple or complex ion. The invention enables use of inexpensive oxidising agents, such as air or oxygen gas. However, other oxidising agents, such as hydrogen peroxide or ozone, may be used instead, if desired. The use of oxidising agents such as oxygen gas or air has the further advantage of avoiding the introduction of contaminants into the reaction mixture, as might be the case if other oxidising agents were used, which therefore avoids the need for a subsequent step of removing the contaminant. Where the oxidising agent is gaseous, such as air or oxygen, it is preferably introduced into solution by bubbling the gas through the acidic solution. A preferred means of introducing the oxidising gas into the solution is by using an aeration tube or a glass frit type aeration tube. In general, the finer the bubbles of oxidising gas introduced into the solution, the faster the oxidation reaction. The bubbles of oxidising gas may be as high as 3 millimetres in diameter. Preferably the bubble diameter is less than 0.8 millimetres. More preferably, the bubble diameter is in the range of from 0.01 to 0.1 millimetres. The amount of oxidising agent introduced to the aqueous acidic solution is preferably 1 to 2 times the stoichiometric amount needed to achieve the desired reaction rate. Where oxygen gas is the oxidising agent, it is preferably introduced into the solution at a rate of from 0.001 to 2.0 grams of oxygen per litre of leaching solution per minute. For example, in the case of material containing 5% Zn and 5% Pb, it may be necessary to feed oxygen at a rate of between 0.05 and 0.10 grams of oxygen/litre/minute into a leaching solution containing 20% w/v of metal containing material in order to achieve 90% or above reaction in 60 minutes. It is further preferred to bubble the gaseous oxidising agent through the reaction mixture in a continuous or semi-continuous stream. The process may be conducted over a wide pressure range. Preferably, however, the process is conducted at atmospheric pressure. The process of the invention may be conducted over a wide temperature range from, for example, 0 to 300°C, with the higher end of the temperature range covering embodiments of the process conducted under pressure, such as in pressure leaching. However, it is preferred that the acidic solution is reacted with the metal containing material at an elevated solution temperature. For most process conditions, reaction rate increases with increasing temperature. Preferably, the solution temperature is at least ambient temperature. More preferably, the solution temperature is at least 60°C. For some process conditions, reaction rate increases sharply at 70°C and above. Reaction rates of some ore tailings are optimised between 70°C and 90°C. However, for other embodiments, the solution temperature is 90°C or higher. The pH of the aqueous acidic solution is of course acidic. Preferably, the pH of the solution is no higher than 6.5. More preferably, solution pH is in the range of 0 to 5. Oxidation rate can be improved by agitating the acidic solution during introduction of the oxidising agent. Agitation is effected to ensure the metal containing material is adequately suspended in the solution and the oxidising species is adequately dispersed in the acidic solution. Agitation may be effected by a rotating impeller or the like within the acidic solution. The following agitation rates are for an impeller having a diameter of 10.3cm. The "tip speed" of a rotating agitator is the speed of a point on the periphery of the agitator and is independent of the diameter of the agitator. To convert revolution rate ' to tip speed (in metres/minute) it is necessary to multiply by approximately 0.32. Tip speeds are given in brackets after each agitation rate. Preferably the impeller rotates at a speed greater than 200rpm (64.74m/min). More preferably, the rate of rotation of the impeller is 400 rpm (129.49m/min) or higher. While agitation rates may be as high as 1700 rpm (550.31 m/min) for many applications, optimum results are achieved at rates no higher than 750rpm (242.79 m/min). Agitation during the process can have the disadvantage of causing foaming or frothing of the solution. Foaming can entrain solids and physically separate them from the acidic solution, thereby making the handling of the solution difficult. An effective amount of a foam control agent may therefore be advantageously added to the acidic solution. One such foam control agent is calcium lignosulphonate. It may be present at a concentration of up to 1% w/v. However, for most applications, the calcium lignosulphonate has a maximum concentration of 0.05% w/v, such as around 0.025% w/v. The minimum concentration of calcium lignosulphonate is typically around 0.0001% w/v. Where the metal containing material contains metal sulphides, the leaching process can result in formation of free sulphur and/or sulphur compounds which may coat unreacted metal containing particles. This phenomenon can prevent or reduce reaction of the coated particles with the leaching solution, thereby adversely affecting the leaching rate. Coating by sulphur-containing material is particularly problematic where the material being leached contains chalcopyrite. This problem can be alleviated by including the step of attrition of the metal containing material. This may be effected by addition of an attriting agent to the leaching solution during agitation thereof. The attriting agent assists to physically remove the sulphur containing coating by attrition, thereby exposing the surface of the unreacted particles to the leaching solution. A suitable attriting agent is particulate SiO 2 , such as sand. Where an attriting agent is used, it is preferably present in an amount which is approximately equal to the amount of metal containing material. Thus the ratio of sand to metal containing material is preferably 0.2:1 to 1.5:1. Attrition can also be effected by increasing the volume of solids in the reaction mixture, either in the absence or presence of a separate attriting agent. Calcium ligonsulphonate, in addition to its defoaming properties, also acts as a dispersant of free sulphur and/or sulphur compounds. Thus, the addition of both sand and calcium lignosulphonate to the solution further enhances leaching rate. The acidic solution may additionally contain one or more reaction promoters. Such promoters include copper ions and/or ions derived from carboxyllic acids, such as acetic acid. Copper is principally used as a promoter when it is not the transition element containing species. The copper may be added to solution such as by adding copper sulphate, CuS0 4 .5H 2 O. Alternatively, the copper may be already present in the acidic solution, such as where copper has been released into the solution as a result of leaching copper containing materials, eg. tailings, mineral concentrates etc. Acetate ions may be added as acetic acid. The preferred concentration of copper ions is about 0.6 g/l. Acetate ions, if present, are preferably present at a concentration of about 1.25 g/l, expressed as equivalent amount of acetic acid. The aqueous acidic solution may also include chloride species. Chloride may be present at a concentration of up to 20 g/l. However, in some embodiments, it is present at a concentration of 10 g/l or less. Typically, the minimum chloride concentration is around 0.5 g/l. Reaction rate is also dependent on the chemical and physical form of metal in the metal containing material, such as particle size, chemistry and percentage of constituent particles and overall metal content. Where the metal containing material is mine tailings containing extremely fine grained and intermixed ore minerals, reaction rate can be relatively slow. Grinding of such material prior to and/or during treatment with the acidic solution can assist in releasing ore minerals, resulting in an improvement of reaction rate. In a preferred embodiment of the invention, the aqueous acidic solution contains sulphuric acid and iron-containing species. The solution is used to extract a metal from a metal sulphide, such as an ore mineral. Oxygen gas is introduced into the solution as a continuous stream of fine bubbles while the reaction mixture is agitated by means of a rotating impeller or a circulating pump. The reactions believed to be occurring in this embodiment are: 0 2 (q) 2 Fe 2+ (aq) → 2 Fe 3+ (aq) 2 Fe 3+ (aq) + MeS → 2 Fe 2+ (aq) + Me 2+ + S° 0 2 (g) 2 Fe 2+ (aq) → 2 Fe 3+ (aq) where MeS is a sulphide of a divalent metal ion denoted as Me. Thus, in the above mentioned preferred embodiment, the Transition Element is iron, present in solution as Fe + (aq) ions. The Fe + (aq) ions react with the oxidising agent, oxygen gas, as it is bubbled through the solution to produce Fe 3+ (aq) ions. The oxidation of iron ions is enhanced by agitation of the reaction mixture which assists in dispersing the oxygen gas and suspending the metal containing material in the solution. The ferric ions thus produced themselves become an oxidising agent for the metal containing material, in this case a metal sulphide. Accordingly, the ferric ions react with the metal sulphide MeS to give Me 2+ (aq) ions and to oxidise S 2' in the sulphide to elemental sulphur. In the process, the ferric ions are reduced to ferrous ions which then become available to commence the cycle again. In this manner, a continuous, cyclic oxidation process can be effected. It is to be noted that the above preferred embodiment of the process of the invention has as a by-product elemental sulphur. This is advantageous in that the production of sulphate is minimised which avoids the need for expensive sulphate removal treatment. Moreover, the sulphur is recoverable in solid form which can be easily stored or transported. The present invention also provides an apparatus for use in a process for extracting metal from a metal containing material, said apparatus including: agitating means for agitating an aqueous acidic solution to which has been added a metal containing material; a conduit for introducing an oxidising agent to said aqueous acidic solution, said conduit having an outlet located close to, preferably adjacent or below, said agitating means; and operating means for operating said agitating means. The agitating means may comprise a blade, paddle or impeller, or the like, rotatable about a drive shaft. Preferably, the agitating means comprises a rotating disk having teeth extending downwardly from the periphery thereof. The agitating means is operated by an operating means which may comprise a motor driving the drive shaft. In use, the agitating means is preferably vertically positioned in the aqueous solution so as to achieve maximum agitation with minimal frothing of the solution. The apparatus further includes a conduit through which an oxidising agent, preferably an oxidising gas, is introduced to the aqueous solution. The conduit may comprise a tube made from any suitable material compatible with the aqueous acidic solution, such as glass, rubber, polythene, etc. The outlet of the conduit is located in close proximity to, such as adjacent, below or within, the agitating means. Such an arrangement assists in minimising the bubble size of oxidising gas fed into solution and maximising the dispersion of the oxidising gas through solution. In order to further maximise dispersion of the oxidising gas through the solution, the outlet of the conduit may comprise a plurality of fine holes or perforations. Such an outlet may comprise a glass frit or a "weep hose". The apparatus may further include a second agitating means located above the first agitating means in a position suitable for minimising froth formation. Typically, the second agitation means, if present, is positioned so that in use, it is in the upper part of the reaction mixture. The second agitating means also may comprise a blade, paddle, disk or impeller. Preferably, the second agitating means is an impeller rotatable about the same shaft as for the first agitating means. The apparatus may further comprise a vessel for containing the reaction mixture of aqueous acidic solution and metal containing material. Preferably, the vessel is made from corrosion resistant or other compatible material. The vessel further preferably includes within its interior, means for increasing the turbulence and shear of the solution during agitation thereof, thereby increasing dispersion of the oxidising agent in solution. Such means preferably comprises projecting structures within the interior of the vessel, such as baffles. The projecting structures prevent the reaction mixture from merely "circulating" the vessel during agitation which can prevent adequate mixing of the reactants. Accordingly, the projecting structures enhance turbulence and dispersion of the oxidising gas in solution. In one form of the apparatus of the invention, the apparatus includes two vessels in fluid communication with each other. The first vessel contains the agitating means and the conduit for introducing an oxidising agent, as described above. In the second vessel, attrition of the reaction mixture takes place, optionally together with further oxidation of the reaction mixture. Attrition may be effected by the presence in the second vessel of an attriting agent, such as sand, or of the solid components of the reaction mixture, particularly the metal containing material. The reaction mixture may then be pumped continuously through a circuit between the two vessels. The apparatus may further include heating means for heating the reaction mixture, if required, to the appropriate temperature. The heating means may comprise a gas burner, resistance heater, or direct or indirect steam. DESCRIPTION OF THE DRAWINGS The invention will become more readily apparent from the following exemplary description in connection with the accompanying drawings and Examples. FIGURE 1 is a graph plotting the amount of copper leached (percent) versus time (min) for Example 1 (triangles) and Comparative Examples 1 (closed squares), 2 (diamonds) and 3 (open squares). FIGURE 2 is a graph plotting the amount of copper leached (percent) versus Time (min) for Example 2 (squares) and Comparative Example 4 (diamonds). FIGURE 3 is a graph plotting the amount of zinc leached (percent) versus time (min) for Example 3 (squares) and comparative Example 5 (triangles). FIGURE 4 is a graph plotting the amount of zinc recovered (percent) versus time (min) for Examples 4 (triangles), 5 (squares) and 6 (diamonds). FIGURE 5 is a graph plotting the amount of zinc leached (percent) versus time (min) for Examples 7 (open squares), 8 (triangles), 9 (diamonds) and 10 (closed squares). FIGURE 6 is a schematic diagram of a first embodiment of the apparatus of the present invention. FIGURE 7 is a schematic diagram of a second embodiment of the apparatus of the present invention. FIGURE 8 is schematic diagram of a third embodiment of the apparatus of the present invention. FIGURE 9 is a graph plotting the amount of copper recovered (percent) versus time (min) for Examples 11 (diamonds) and 12 (triangles) and Comparative Example 6 (circles). FIGURE 10 is a graph plotting the recovery (percent) of copper (squares) and zinc (triangles) versus time (min) for Example 13. FIGURE 11 is a graph plotting the amount of copper reacted (percent) versus time (minutes) for fine (diamonds) and coarse (squares) sized bubbles of oxygen gas. FIGURE 12 is a graph plotting the amount of copper leached (percent) versus time (minutes) for different concentrations of H 2 SO 4 : 20 g/l (diamonds), 50 g/l (small squares), 70 g/l (triangles), 100 g/l (large squares) and 250 g/l (asterisks). FIGURE 13 is a graph plotting the amount of copper leached (percent) versus time (minutes) for different concentrations of iron in solution: 0 g/l (triangles), 4 g/l (squares) and 16 g/l (diamonds). FIGURE 14 is a graph plotting the rate of production of Fe 3* ions (g/l) versus time (minutes) in an acidic solution into which air is being introduced. EXAMPLE 1 and COMPARATIVE EXAMPLES 1 to 3 The following Example 1 and Comparative Examples 1 to 3 relate to leaching of the ore mineral chalcopyrite (CuFeS 2 ) and are presented in Figure 1. Example 1 (triangles) shows the leaching rate of Cu, using the process of the invention, in which 320 gram chalcopyrite is reacted with 1600 ml of an acidic solution including 16 g/l Fe 2+ and 260 g/l H 2 S0 4 . The reaction mixture also included a feed rate of oxygen of 50 litre/hour, and a temperature of 90°C. The reaction mixture was agitated by an impeller rotating at 750 rpm (corresponding to a tip speed of 242.79m/min). Comparative Example 1 (closed squares) shows the results of leaching chalcopyrite using a leaching solution comprising a mixture of 0.25 M Fe 2 (SO 4 ) 3 and 0.5 M H 2 SO 4 in a conventional acid leaching process. The concentration of chalcopyrite in the leaching solution is 0.2 g/l. Comparative Example 2 (diamonds) shows the best reported results of leaching chalcopyrite using a typical pressure leaching process. Comparative Example 3 (open squares) shows the leaching rate of chalcopyrite predicted theoretically by thermodynamics. The leaching rates achievable by using the process of the invention are significantly higher than those achieved by a conventional acid leach process or by pressure leaching. Further, after 60 minutes of leaching, the slope of the leaching curve for Example 1 more closely approximates the slope of Comparative Example 3 than either of Comparative Examples 1 or 2. For example, at a reaction time of approximately 180 minutes, the amount of copper leached is approximately 50% of the theoretically achievable amount, compared with approximately 10%, for Comparative Example 1 , and approximately 27%, for Comparative Example 2. EXAMPLE 2 and COMPARATIVE EXAMPLE 4 Figure 2 shows the results of leaching a dross containing 58% lead as sulphide and/or oxide and 23% copper present substantially as cuprous sulphide (matte). Diamonds represent the results of Comparative Example 4, in which 400g of the dross was leached with 1600ml of 250 g/l H 2 SO 4 using a conventional acid leaching process. Squares represent the results of Example 2, in which 400g of the dross was leached with 1600ml of an acidic solution including 16 g/l Fe 2+ and 260 g/l H 2 S0 4 at 85°C with agitation by an impeller rotating at 700rpm (corresponding to a tip speed of 226.60m/mn) and an oxygen feed rate of 1.5 l/minute. Figure 2 illustrates the considerably higher leaching rate of copper from the dross using the process of the invention compared with the results using the conventional acid leach process. In fact, for a reaction time of 120 mins. essentially all the copper in the dross had been leached using the process of the invention, whereas only approximately 40% of the dross had been leached using the conventional process. The following Examples 3 to 10 and Comparative Example 5 describe the results of leaching ore tailings having a particle size of less than 38 microns and an average composition of 6 to 9% Zn, 7-8% Pb, 0.5% Cu, 250-300 ppm Ag and 2 to 4 ppm Au. The tailings are produced as a 60% pulp density slurry having the following mineralogy: principally pyrite (FeS 2 ), with some sphalerite (ZnS) and Galena (PbS) and minor amounts of Tetrahedrite (4Cu 2 S. Sb 2 S 3 ), Chalcopyrite (CuFeS 2 ), Arsenopyrite (FeAsS 2 ), Barytes (BaSO 4 ), Pyrrhotite (Fe 1-x S), Argentite (Ag 2 S) and gold. These minerals are very fine grained and intermixed, making metal recovery by conventional means very difficult. Results of the leaching tests on the ore tailings are presented in terms of % Zn leached vs time. However, it should be noted that leaching rates of other metals, such as Cu and Pb, follow approximately the same path as for Zn. EXAMPLE 3 AND COMPARATIVE EXAMPLE 5 320g of ore tailings were treated with a solution containing 1.6 I of an acidic solution including 250g/l H 2 S0 4 , 16g/l Fe 2+ , 1 g Cu ions; 2ml acetic acid; 320g sand; and 0.5g calcium lignosulphate (Example 3); and with a solution containing 250 g/l H 2 SO (Comparative Example 5). Each solution was agitated and heated to a temperature of 90°C and oxygen gas was introduced into the solution at 60 l/hour. Figure 3 shows % Zn leached over time of Example 3 (squares) compared with zinc recovery from Comparative Example 5 (triangles). The results indicate that, at least for the conditions of Figure 3, the leaching rate for the process of the invention is between approximately 150 and 250% higher than those rates for H 2 SO 4 . EXAMPLES 4 to 6 Figure 4 shows the results of leaching zinc from ore tailings using three different apparatus. Example 4 (triangles) represents the results of leaching ore tailings in an apparatus similar to the one shown in Figure 8. Example 5 (squares) represents leaching results in an apparatus similar to the one shown in Figure 7. Example 6 (diamonds) represents the results of leaching ore tailings using another apparatus having less effective agitation and dispersion features and where the oxidising gas (in this case O 2 ) bubble size is not minimised. For example, it has been demonstrated that for the apparatus shown in Figure 8 a bubble size range of 0.05 - 1.00mm is achieved. Clearly, Examples 4 and 5 exhibit greater leaching rates than Example 6. For example, at approximately 80 minutes of reaction time, the leaching rates of Examples 4 and 5 were more than twice the rate of Example 6. EXAMPLES 7 to 10 Examples 7 to 10 illustrate the effect on leaching rate of agitation speed and the presence of sand in the reaction mixture, as shown in Figure 5. In each Example, 320 grams of unground dry tailings equivalent were reacted with 1600ml of an aqueous acidic solution including 250g/l H 2 SO and 16g/l Fe 2+ at a temperature of 90°C and feed rate of air/oxygen of 50 litre/hour. Examples 7 and 8 were each agitated at 400 rpm (corresponding to a tip speed of 129.49m/min) with Example 8 further including sand at a ratio of 1 :1 sand to tailings. At leaching times above approximately 15 to 20 minutes, Example 8 (triangles) exhibited a higher leaching rate than Example 7 (open squares). The difference between the respective leaching rates of Examples 7 and 8 increased over time, such that at approximately 90 minutes, the leaching rate of Example 8 was approximately 140% of that of Example 7. Example 9 (diamonds) was conducted under the same conditions as Example 8, except that the agitation rate was increased to 750 rpm (corresponding to a tip speed of 242.79m/min). It is evident that the increase in agitation speed from 400 φm to 750 rpm results in an increase in leaching rate of about 150% at 70 minutes. However, Example 10 (closed squares) indicates that a further increase in agitation rate to 1100 rpm (corresponding to a tip speed of 356.09m/min) has little effect on leaching rate. This suggests that the maximum, cost beneficial, agitation rate, at least for the conditions of Examples 7 to 10 is likely to be around 750 rpm, ie. a tip speed of around 242.79m/min. Figures 6 to 8 Figures 6 to 8 of the accompanying drawings illustrate three embodiments of the apparatus of the present invention. Figure 6 illustrates a first embodiment of the apparatus of the invention. Apparatus 10 includes glass vessel 12 having baffles 14. A conduit 16 feeds air from a compressor (not shown) to a rubber weep hose 18 including a number of fine perforations 20. An agitating means comprising an impeller 22 mounted on rotatable shaft 24 is operable by drive motor 26. Impeller 22 comprises a disk 28 having peripheral teeth 30 extending from the base thereof. Weep hose 18 is positioned below impeller 22. During operation of apparatus 10, air fed into conduit 16 is expelled from weep hose 18 via the perforations 20 as fine bubbles. These are dispersed through the reaction mixture by impeller 22. Figure 7 illustrates a second embodiment of the apparatus of the invention, in which like reference numerals refer to like parts of the embodiment of Figure 6. One difference between the first and second embodiments is that the outlet of the conduit 116 is a glass frit 118 instead of a weep hose. A second difference is that the agitating means comprises triangular stirrer 122 mounted on rotatable shaft 124 instead a disk-shaped impeller with peripheral teeth. Figure 8 illustrates a third embodiment of the apparatus of the invention in which, like reference numerals again refer to like parts of the first and second embodiments. One difference between the third embodiment and each of the first and second embodiments is that the outlet of conduit 216 comprises an open ended polythene tube 218 positioned so that air is expelled under the centre of impeller 222 just above teeth 230. A second difference is that mounted on rotatable shaft 224 above impeller 22 is a second impeller 232. Impeller 232 includes vanes 234 extending therefrom. The action of the second impeller 232 agitating the upper part of the reaction mixture assists to reduce foam formation. EXAMPLES 11 and 12 and COMPARATIVE EXAMPLE 6 Figure 9 illustrates leaching rates of embodiments of the process of the invention, using three different solutions. Example 11 (diamonds) used a waste acid solution from the manufacture of TiO 2 including 250 g/l H 2 S0 4 and 16 g/l Fe 2+ . Example 12 (triangles) used an aqueous acidic solution containing 250 g/l H 2 S0 4 and 16 g/l Fe 2+ only and Comparative Example 6 (circles) used a 250 g/l H 2 SO 4 solution. The results show that significantly higher recovery of copper is achieved using either the waste acid solution or H 2 SO 4 /Fe 2+ solution, compared with a solution comprising only H 2 S0 4 . Moreover, at shorter reaction times, the waste acid solution outperforms the H 2 SO 4 /Fe 2+ solution, although this difference in performance decreases with higher reaction times. EXAMPLE 13 Example 13 illustrates the effectiveness of the process of the invention at relatively low concentrations of H 2 SO 4 and Fe 2+ in the acidic solution. In Example 13, 320g of a complex sulphide concentrate containing 1.6% zinc and 8.6% copper was mixed with 1600ml of an acidic solution containing 30g/l sulphuric acid and 8g/l iron (as Fe ++ ) in the apparatus of Figure 8. Oxygen was fed to the mixture at a rate of 50l/h and the mixture was maintained at a temperature of 90°C. The mixture was agitated by an impeller rotating at 750 φm (corresponding to a tip speed of 242.79m/min). The results are shown in Figure 10 from which it can be seen that 90% of the zinc (triangles) was recovered in 150 min.. After this time, 10% of the copper (squares) was also recovered. EXAMPLE 14 and 15 and COMPARATIVE EXAMPLE 7 The advantage of achieving a dispersion of finely sized bubbles of oxidising gas is demonstrated by Example 14 and Comparative Example 7, where an acid solution of 1.60L containing 250 g/L H 2 S0 4 and 16 g/L Fe ++ and 320g or ore tailings, as described for Examples 3-10, was agitated in the apparatus at 750 rpm at a temperature of 90°C. In Example 14 oxygen gas was introduced into the apparatus at a rate of 60 L/hr for dispersion and the dissolution of zinc at 90 minutes was 67.4%. In Comparative Example 7 using identical reaction conditions, dissolved oxygen in the form of hydrogen peroxide was added instead of oxygen gas. The dissolution of zinc at 90 minutes was 38.5%. This is further illustrated by Example 15, where under identical reaction conditions to Example 14, and Comparative Example 8 using different apparatus to change bubble size, copper is leached from chalcopyrite concentrate using oxygen gas as the oxygenating agent. Figure 11 shows the effect of varying bubble size on the rate of dissolution of copper. EXAMPLE 16 Example 16 demonstrates the ability of the process to operate effectively over a range of concentrations of H 2 SO 4 as depicted in Figure 12. In this example, 100 gm of chalcopyrite mineral ore concentrate was reacted in the reactor with 16 g/L Fe ++ ion concentration in a volume of 1.600L, at 90°C and agitated at 750 φm. Oxygen gas was introduced into the apparatus at a feed rate of 70 L/hr and the acid concentration varied between 20 and 250 g/L H 2 SO 4 . EXAMPLE 17 Example 17 shows that the process has the ability to operate at the Fe ++ ion concentrations close to those at which the maximum concentration of the oxidised Fe +++ is achieved, ie., 1.5 - 3.0 g/L. This is an added advantage in that the cyclic oxidation of Fe ions in solution can be achieved at low soluble Fe concentrations. In this example 100 gm of chalcopyrite mineral ore concentrate was reacted in the reactor with 250 g/L H 2 SO 4 concentration in a volume of 1 ,600L, at 90°C and agitated at 750 rpm with oxygen gas introduced into the apparatus at a feed rate of 70 LJhr and with acid concentrations varying between 20 and 250 g/L H 2 SO 4 As shown in Figure 13 the reaction proceeds at an acceptable rate at low concentrations of soluble Fe ++ . EXAMPLE 18 In Example 18, 1.60L of an acid solution containing 250 g/L H 2 SO 4 and 16 g/L Fe ++ is agitated in the apparatus and oxygen is introduced to the solution in the form of air at a rate of 120 L/hr. The results are illustrated in Figure 14. Support for the reaction of cyclic oxidation/reduction of iron in this system is given by this example, where it is shown that in the absence of material to be leached, there is a limiting concentration of ferric iron reached in this apparatus under the acid reaction conditions in this system. Subsequent analysis of a range of leaching examples has shown that there is a maximum concentration of oxidised iron, Fe +++ of 1.5 - 3.0 g/L at any one time in these systems. It is noted, however that there is a rapid oxidation rate achieved over the early stages of the oxidation curve. This rate is consistent with the rate of the metal dissolution reaction described above. EXAMPLE 19 Example 19 provides information about the reaction mechanism, by way of analysis of the products of reaction. Example 18 relates to leaching 50 gm of the ore mineral chalcopyrite in the form of a concentrate containing approximately 27% copper, produced by conventional flotation separation processes, in an acidic solution containing 16 g/L Fe" "** and 250 g/l H 2 SO 4 at 95°C in the presence of 320 gm of sand. The reaction mixture was agitated in the apparatus by an impeller rotating at 750 rpm (corresponding to a tip speed of 242.79m/min) and fed with oxygen at a rate of 70 LJhr. The leach was allowed to proceed to completion and the following analysis results were obtained: 0, Solid Sulphur total 46.7 Sulphur elemental Solution Copper recovered 99.57 The process and apparatus of the invention are capable of providing effective extraction of metals from a range of metal containing materials at atmospheric pressure and at temperatures which are often much lower than those required in conventional metal extraction processes. The process is capable of satisfactorily recovering metal values at atmospheric pressure from materials which do not normally respond to acid attack at atmospheric pressure. Advantages of at least preferred embodiments of the process and apparatus include rapid reaction rates, leading to high recoveries of metal values, very low levels of sulphur oxidation (in the case of reaction with metal sulphides), very low levels of product solution contamination from conventional oxidant, much higher content of extracted metal in solution at much lower residual acid content after reaction compared with conventional metal extraction processes the potential for a much smaller and compact plant to achieve equivalent metal recovery rates, for existing techniques. This in turn leads lower capital and operating costs. Finally, it is to be understood that various alterations, modifications and/or addition may be introduced into the constructions and arrangements of parts and/or steps previously described without departing from the spirit or ambit of the invention.

Claims: CLAIMS: 1. A process for extracting one or more metals from a metal containing material, as herein defined, including the steps of: (a) treating said metal containing material with an aqueous, acidic solution containing an acid and one or more Transition Element containing species; (b) introducing an oxidising agent into said solution in order to oxidise the Transition Element ion in said Transition Element containing species to a higher oxidation state; and (c) oxidising said metal containing material with the oxidised Transition Element containing species to effect release of said one or more metals from said metal containing material. 2. The process of claim 1 , wherein said acidic solution is agitated during the step of introducing the oxidising agent. 3. The process of claim 2, wherein the agitation is effected by a rotating agitating means having a tip speed of between 64.74 and 242.79 m/min. 4. The process of any one of claims 1 to 3, wherein said oxidising agent is gaseous and is introduced by bubbling the oxidising gas through the acidic solution. 5. The process of claim 4, wherein the oxidising gas is bubbled through the acidic solution in a continuous or semi-continuous stream. 6. The process of claim 4 or claim 5 wherein the bubbles of oxidising gas have a diameter less than 0.8mm. 7. The process of claim 4 or claim 5, wherein the bubbles of oxidising gas have a diameter between 0.01 and 0.1mm. 8. The process of any one of claims 1 to 7, wherein the oxidising agent is gaseous oxygen. 9. The process of claim 8, wherein the gaseous oxygen is introduced into the solution at a rate of from 0.001 to 2 grams of oxygen per litre of acidic solution per minute. 10. A process for extracting one or more metals from a metal containing material, as herein defined, containing said one or more metals in the sulphide form, including the steps of: treating said metal containing material with an aqueous, acidic solution containing an acid and one or more Transition Element containing species; introducing an oxidising agent into said solution in order to oxidise the Transition Element ion in said Transition Element containing species to a higher oxidation state; and oxidising said metal containing material with the oxidised Transition Element containing species to effect release of said one or more metals from said metal containing material. 11. A process for extracting one or more metals from a metal containing material, as herein defined, including the steps of: treating said metal containing material with an aqueous, acidic solution containing an acid and one or more Transition Element containing species; introducing an oxidising agent into said solution in order to oxidise the Transition Element ion in said Transition Element containing species to a higher oxidation state; and oxidising said metal containing material with the oxidised Transition Element containing species to effect release of said one or more metals from said metal containing material; wherein the treatment of said metal containing material is conducted at atmospheric pressure. 12. The process of any one of claims 1 to 11 , wherein the acidic solution has a concentration of acid of 80g/l or lower, expressed as equivalent H 2 S0 4 concentration, and a concentration of Transition Element containing species of 5g/l or lower, expressed as equivalent Fe concentration. 13. The process of any one of claims 1 to 12, wherein the acidic solution also includes an effective amount of a foam control agent. 14. The process of any one of claims 2 to 13 wherein the process further includes the step of attriting the metal-containing material during agitation of the acidic solution. 15. The process of claim 14, wherein the step of attriting comprises adding sand to the acidic solution at a ratio of sand to metal containing material of between 0.2:1 and 1.5:1. 16. The process of any one of claims 1 to 15, wherein the acidic solution further includes one or more reaction promotors. 17. The process of claim 16, wherein the reaction promotors are selected from copper ions and ions derived from carboxyllic acids. 18. The process of any one of claims 1 to 17, further including the step of grinding said metal-containing material prior to treatment with said aqueous acidic solution. 19. The process of any one of claims 1 to 18, wherein the acidic solution further includes a chloride concentration of between 0.5 to 10g/l. 20. The process of any one of claims 1 to 19, wherein said acid is sulphuric acid and said Transition Element is iron. 21. The process of any one of claims 1 to 20, wherein the temperature of the acidic solution is from ambient to 90°C. 22. The process of any one of claims 1 to 20 wherein the temperature of the acidic solution is from 70°C to 90°C. 23. The process of any one of claims 1 to 22, wherein the pH of the acidic solution is from 0 to 5. 24. An apparatus for use in a process for extracting metal from a metal containing material, said apparatus including: first agitating means for agitating an aqueous acidic solution to which has been added a metal containing material; a conduit for introducing an oxidising agent to said aqueous acidic solution, said conduit having an outlet located close to, preferably adjacent or below, said agitating means; and operating means for operating said agitating means. 25. The apparatus of claim 24, wherein said agitating means comprises a blade, paddle or impeller or the like, rotatable about a drive shaft. 26. The apparatus of claim 24, wherein said agitating means comprises a rotatable disk having teeth extending from the periphery thereof. 27. The apparatus of any one of claims 24 to 26, wherein said agitating means is vertically positioned such that in use of the apparatus, it achieves maximum agitation with minimal frothing of the acidic solution. 28. The apparatus of any one of claims 24 to 27, wherein said outlet of said conduit comprises a plurality of fine holes or perforations. 29. The apparatus of any one of claims 24 to 28 further including a second agitating means located above said first agitating means in a position suitable for minimising frothing of the acidic solution. 30. The apparatus of claim 29, wherein said second agitating means comprises a blade, paddle, disk or impeller. 31. The apparatus of claim 29 or 30, wherein said first and second agitating means are rotatable about a common shaft. 32. The apparatus of any one of claims 24 to 31 , further including a vessel for containing said aqueous acidic solution and said metal containing material, said vessel including means for increasing turbulence and shear of the acidic solution during agitation thereof. 33. The apparatus of claim 32, wherein said means for increasing turbulence and shear comprises projecting structures, such as baffles, within the interior of said vessel. 34. The apparatus of any one of claims 24 to 33, further including heating means for heating said acidic solution. 35. A process for extracting one or more metals from a metal containing material, substantially as herein described with reference to any one of the Examples 1 to 13. 36. An apparatus for use in a process for extracting metal from a metal containing material, substantially as herein described with reference to any one of Figures 6 to 8 of the accompanying drawings. 37. A process for extracting one or more metals from a metal containing material, as herein defined, including the steps of: (a) treating said metal containing material with an aqueous, acidic solution containing an acid and one or more Transition Element containing species; (b) introducing an oxidising agent into said solution in order to oxidise the Transition Element ion in said Transition Element containing species to a higher oxidation state; (c) oxidising said metal containing material with the oxidised Transition Element containing species to effect release of said one or more metals from said metal containing material and thereby effecting reduction of said oxidised Transition Element containing species; (d) reoxidising said Transition Element ion to said higher oxidation state with said oxidising agent; and (e) repeating steps (c) and (d) until the desired amount of said one or more metals are extracted.

Inventors: BROWNE CHRISTOPHER JOHN, HUTCHISON PETER ROBERT, BROWNE, CHRISTOPHER, JOHN, HUTCHISON, PETER, ROBERT
Applicants: BROWNE CHRISTOPHER JOHN, HUTCHISON PETER ROBERT, TIOXIDE AUSTRALIA PTY LTD, BROWNE, CHRISTOPHER, JOHN, HUTCHISON, PETER, ROBERT, TIOXIDE AUSTRALIA PTY. LTD.
Application year: 1996
Main classification: C22B3
IPC codes: C22B3, C22B15