[0001 ] The present invention relates to a method for leaching zinc from zinc silicate ores. More particularly, the present invention relates to a method for leaching zinc from zinc silicate ores with an ammoniacal media.

Background Art

[0002] The ammonia-based Schnabel process for the recovery of zinc from oxide ores was used for a number of years before being superseded by the acid-based roast-leach-electrowin process which could directly treat sulphides. The Schnabel process feed was typically roasted sphalerite flotation concentrate, but selective mining also allowed processing of zinc oxide ores. The Schnabel process is complex, as is evident from the summary by Harvey (Mineral Processing & Extractive Metallurgy Review, volume: 27, pages: 231-279, 2006), and it is perhaps unsurprising that there are few, if any Schnabel process plants in operation. In particular, the Schnabel process has several undesirable features:

• elevated leaching temperature, typically in excess of 30°C;

• leaching solution concentrations of >50g/L ammonia;

• use of steam to strip; and

• economic necessity to recover both ammonia and carbon dioxide.

[0003] Although methods for ammoniacal leaching of various metals are described in the literature, including the patent literature, all require one or more of multiple leaching stages, energy-intensive pre-treatment of the ore (for example roasting or grinding) and/or elevated leach temperatures and/or pressures. These requirements necessitate complicated engineering and / or significant energy consumption, so it is little surprise that none have been widely adopted.

[0004] Further, many ammoniacal leaching methods either suffer from significant ammonia loss, or require complicated chemistry or engineering to overcome such, providing a further impediment to the widespread adoption of ammoniacal leaching methods. [0005] An alternate approach is acid leaching. Acid leaching of zinc silicate ores is carried out on an industrial scale at only one plant, Skorpion in Namibia. This plant requires a head grade of more than 10% zinc to be economic as the tailings are typically around 4%. The ore at Skorpion consists of a mixture of zinc minerals, smithsonite (ZnCOs), hemimorphite (Zn ₄ Si ₂ 0 ₇ (OH) ₂ 'H ₂ 0) and the clay mineral sauconite (Nao.3Zn3(Si,AI) ₄ Oi ₀ (OH) ₂ '4H ₂ 0). The gangue minerals are primarily quartz, muscovite and orthoclase. The ore is ground and then leached in sulphuric acid at 60°C, the high temperature is essential to maximise the solubility of the silicon dissolved from the silicate minerals. If the leaching were at a lower temperature the silica would re-precipitate and form a gelatinous polymer which would cause major problems in the plant. The higher temperature allows the silica time to form discrete particles which can be separated from the solution. The final solution undergoes a very complex solvent extraction process where it is purified to a level which is sufficient for electrowinning. Clearly, heating the solution and ore to 60°C is an expensive undertaking and must be essential to prevent problems due to silica dissolution. A process which circumvents this problem and / or simplifies the complex solution purification prior to electrowinning is clearly of commercial value.

[0006] The method of leaching of the present invention has as one object thereof to overcome the abovementioned problems associated with the prior art, or to at least provide a useful alternative thereto.

[0007] Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0008] The preceding discussion of the background art is included exclusively for the purpose of providing a context for the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was common general knowledge in the field relevant to the present invention in Australia or elsewhere before the priority date.

Disclosure of the Invention

[0009] In accordance with the present invention there is provided a method for leaching zinc from a zinc silicate ore, the method comprising the method steps of: curing the zinc silicate ore to be leached through the application of a low volume of an aqueous acid solution having a pH of 0 or above, producing a cured ore more amenable to leaching through the application of an ammonium carbonate solution containing free ammonia; leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia, producing a pregnant leach solution; and passing the pregnant leach solution to a means for zinc recovery.

[0010] As would be understood by a person skilled in the art, rendering the zinc silicate ore more amenable to leaching through the application of an ammonium carbonate solution containing free ammonia means effecting one or more of improved zinc recovery, in a shorter time, under milder leach conditions. Milder leach conditions may include, but are not limited to, lower temperatures, lower pressures, lower concentrations of leaching agents or combinations thereof. For example, a given zinc recovery may be effected in a shorter time, or by using milder leach conditions, or both, as a result of the curing step of the method of the present invention.

[001 1] In one form of the present invention, rendering the zinc silicate ore more amenable to leaching through the application of an ammonium carbonate solution containing free ammonia provides one or more of the following leach conditions: lower temperatures, lower pressures and lower concentrations of leaching agents. The term lower is used to describe a lower leach temperature, pressure or concentration relative to that required for leaching ores that had not undergone the curing step of the present invention.

[0012] Ammoniacal leaching is particularly attractive for high carbonate ores, which are generally not amenable to acid leaching-based processes for economic reasons, as the carbonate component consumes significant amounts of acid. However, the scope of the present invention is not limited to high carbonate ores.

[0013] Although the identity of the acid, the volume of the aqueous acid solution having a pH of 0 or above, the concentration of the aqueous acid solution having a pH of 0 or above, and the conditions under which the curing step takes place will differ for different zinc silicate ores, the present invention identifies a principle enabling the economical recovery of zinc from a wide range of zinc silicate ores, in that conventional, energy-intensive physical pre-treatment techniques such as grinding or roasting, used successfully or otherwise in ammoniacal leaching, can be replaced or enhanced by using chemical techniques, where application of the aqueous acid solution having a pH of 0 or above renders the zinc silicate ore amenable to the subsequent ammoniacal leaching.

[0014] Although the methods of the present invention allow economic recovery of zinc without the need for conventional physical p re-treatments such as roasting or grinding, thereby offering significant energy savings, the method of the present invention should not be understood to exclude such physical pre-treatments.

[0015] In one form of the invention, the pH of the aqueous acid solution is 0 or above. In one form of the invention, the pH of the aqueous acid solution is 0.25 or above. In one form of the invention, the pH of the aqueous acid solution is 0.5 or above. In one form of the invention, the pH of the aqueous acid solution is 0.75 or above. In one form of the invention, the pH of the aqueous acid solution is 1 or above. In one form of the invention, the pH of the aqueous acid solution is 1.5 or above. In one form of the invention, the pH of the aqueous solution is 2 or above. In one form of the invention, the pH of the aqueous acid solution is 2.5 or above. ln one form of the invention, the pH of the aqueous acid solution is 3 or above. In one form of the invention, the pH of the aqueous solution is 3.5 or above. In one form of the invention, the pH of the aqueous acid solution is 4 or above. In one form of the invention, the pH of the aqueous solution is 4.5 or above. In one form of the invention, the pH of the aqueous acid solution is 5 or above. In one form of the invention, the pH of the aqueous solution is 5.5 or above.

[0016] In one form of the invention, the pH of the aqueous acid solution is 6 or below. In one form of the invention, the pH of the aqueous solution is 5.5 or below. In one form of the invention, the pH of the aqueous solution is 5 or below. In one form of the invention, the pH of the aqueous solution is 4.5 or below. In one form of the invention, the pH of the aqueous solution is 4 or below. In one form of the invention, the pH of the aqueous solution is 3.5 or below. In one form of the invention, the pH of the aqueous solution is 3 or below. In one form of the invention, the pH of the aqueous solution is 2.5 or below. In one form of the invention, the pH of the aqueous solution is 2 or below. In one form of the invention, the pH of the aqueous solution is 1.5 or below. In one form of the invention, the pH of the aqueous solution is 1 or below. In one form of the invention, the pH of the aqueous solution is 0.75 or below. In one form of the invention, the pH of the aqueous solution is 0.5 or below. In one form of the invention, the pH of the aqueous solution is 0.25 or below.

[0017] In one form of the invention, the pH of the aqueous acid solution has a range with an upper limit selected from one of the following: 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7 and a lower limit selected from one of the following: 0, 0.5, 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5 and 5, where the upper limit is greater than the lower limit.

[0018] In one form of the invention, the pH of the aqueous acid solution has a range with an upper limit selected from one of the following: 2, 2.5, 3, 3.5, 4, 4.5, 5, and a lower limit selected from one of the following: 0, 0.5, 1 , 1.5, 2, 2.5, 3, where the upper limit is greater than the lower limit. [0019] In one form of the invention, the pH of the aqueous acid solution has a range with an upper limit selected from one of the following: 2, 2.5, 3, 3.5, 4, and a lower limit selected from one of the following: 0, 0.5, 1 , 1.5, 2, where the upper limit is greater than the lower limit.

[0020] The aqueous acid solution may be an aqueous solution of a weak acid or a strong acid, or a mixture of both. As would be understood by a person skilled in the art, for a given pH, an aqueous solution of a weak acid will be more concentrated than an aqueous solution of a strong acid.

[0021] As would be readily understood by persons skilled in the art, aqueous acid solutions with pH below zero, being outside the scope of the claimed invention, are stronger and/or more concentrated than the aqueous acid solutions of the methods of the present invention. It is generally understood that more concentrated solutions of stronger acids will have a greater chemical effect within a given time than more dilute solutions of weaker acids. Aqueous acid solutions having a pH below zero, particularly aqueous solutions of strong acids, are widely available and routinely used in the field of hydrometallurgy. Accordingly, the present invention represents a counter-intuitive discovery that better results may be achieved through the use of more dilute or weaker acid solutions than those conventionally used in the art.

[0022] Without wishing to be bound by theory, this discovery suggests an unknown and unexpected mechanism by which the acid effects solubilisation of the zinc silicate ore. The existence of such a mechanism is supported by the experimental observations, reported herein, that quantities of acid well below stoichiometric quantities (based on the zinc content of the zinc silicate ore) are capable of effecting curing in accordance with the methods of the invention. For example, and as described in more detail in the Examples below with reference to Figure 8, a sample containing 45 kg of zinc was effectively cured by only approximately 5 g of acid. [0023] Again without wishing to be bound by theory, it is believed that the relatively low proton concentration of the aqueous acid solutions of the invention produces a chemical environment in which the zinc silicate minerals are intrinsically unstable and decompose without consuming either protons or acid anions. That is, the partially dissociated weak acids (or more dilute fully dissociated strong acids) produce a localised pH where zinc silicate minerals such as hemimorphite, willemite and sauconite are all intrinsically unstable and therefore dissolve in the solution until it is saturated whereupon the zinc then precipitates in another, more thermodynamically stable form. The more stable form is the phase which, again without wishing to be bound by theory, is subsequently dissolved in the ammoniacal ammonium carbonate leach solution. The foregoing discussion should not be construed as limiting the method of the present invention is not limited to any particular mechanism of action by which the aqueous acidic solution cures the zinc silicate ore rendering such more amenable to leaching through the application of an ammonium carbonate solution containing free ammonia.

[0024] A further advantage of the invention is that by avoiding high concentrations of strong acids, dissolution of gangue is decreased thereby substantially simplifying subsequent purification of the leach solution.

[0025] A further advantage of the invention is that by using ammonia as the leaching agent acid soluble elements, such as Fe, Ca, Al, Mg, Si, etc are not dissolved thereby substantially simplifying subsequent purification of the leach solution.

Zinc silicate ores

[0026] Throughout this specification, unless the context requires otherwise, the term "zinc silicate ore" or variations thereof, will be understood to include, for example, the product of one or more pre-treatment steps, such as a roast or calcination steps, and / or one or more concentration steps, but is not limited thereto. [0027] As is evident from the preceding definition of ore, the scope of the present invention should not be understood to exclude ores that have been pre-treated by conventional methods, and in such circumstances, the method of the present invention may afford better recoveries than conventional methods. In many cases, the improved recoveries afforded by the method of the invention may render an ore body commercially viable.

[0028] The scope of the present invention includes methods where an aqueous acid solution having a pH of 0 or above is applied to a mixture of ores; at least one component of said mixture is a zinc silicate ore.

[0029] Throughout this specification, unless the context requires otherwise, the phrase "zinc silicate ore", or variations thereof, will be understood to include ores comprising one or more of the following minerals: hemimorphite, sauconite and willemite.

[0030] The method of the present invention is particularly effective where the zinc silicate ore contains a significant quantity of hemimorphite. In one form of the invention, the zinc silicate ore comprises hemimorphite.

[0031] In one form of the invention, the zinc content of the zinc silicate ore is predominantly in the form of hemimorphite. In one form of the invention, at least 10% of the zinc content of the zinc silicate ore is in the form of hemimorphite. In one form of the invention, at least 20% of the zinc content of the zinc silicate ore is in the form of hemimorphite. In one form of the invention, at least 30% of the zinc content of the zinc silicate ore is in the form of hemimorphite. In one form of the invention, at least 40% of the zinc content of the zinc silicate ore is in the form of hemimorphite. In one form of the invention, at least 50% of the zinc content of the zinc silicate ore is in the form of hemimorphite. In one form of the invention, at least 60% of the zinc content of the zinc silicate ore is in the form of hemimorphite. In one form of the invention, at least 70% of the zinc content of the zinc silicate ore is in the form of hemimorphite. In one form of the invention, at least 80% of the zinc content of the zinc silicate ore is in the form of hemimorphite. In one form of the invention, at least 90% of the zinc content of the zinc silicate ore is in the form of hemimorphite.

[0032] The inventors have discovered that the method of the present invention is particularly effective on sauconite where the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid, In one form of the invention, at least 10% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid. In one form of the invention, at least 20% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid. In one form of the invention, at least 30% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid. In one form of the invention, at least 40% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid. In one form of the invention, at least 50% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid. In one form of the invention, at least 60% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid. In one form of the invention, at least 70% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid. In one form of the invention, at least 80% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid. In one form of the invention, at least 90% of the zinc content of the zinc silicate ore is in the form of sauconite, and the aqueous acid solution having a pH of 0 or above is a concentrated solution of a weak acid.

[0033] As would be recognised by those skilled in the art, the minimum fraction of zinc present as a specific mineral required for the deposit to be economically treated will decrease as the zinc head grade increases. Similarly, those skilled in the art will recognise that such minerals can occur together and the economics will be a function of the combined fraction of zinc present as hemimorphite and / or willemite and /or sauconite.

Acids

[0034] As understood by those versed in the art, a weak acid is an acid that dissociates incompletely in aqueous solution. That is, it does not release all of its protons when in solution, donating only a partial amount of its protons to the solution. These acids have higher pKa than strong acids, which release all of their protons when dissolved in aqueous solution. Accordingly, weak acid solutions exhibit higher pH than solutions containing an equal concentrations of a strong acid.

[0035] In one form of the invention, the aqueous acid solution is an aqueous solution of an acid with a K _a less than 1.8x10 ^~16 .

[0036] In one form of the invention the aqueous acid solution is an aqueous solution of a weak acid having a pKai of from 0 to 7.

[0037] In one form of the invention the aqueous acid solution is an aqueous solution of a weak acid having a pKai of from 0 to 7 is selected from the list within the following lUPAC compilations:

Dissociation Constants of Organic Acids in Aqueous Solution, by G. Kortum, W. Vogel, and K. Andrussow; lonisation Constants of Organic Acids in Aqueous Solution, by E. P. Serjeant and Boyd Dempsey; and/or D. D. Perrin, Ionization Constants of Inorganic Acids and Bases in Aqueous Solution.

[0038] In one form of the invention, the aqueous acid solution is an aqueous solution of an acid selected from the group: aliphatic mono- and di-carboxylic acids, phenyl substituted alkanoic acids, hydroxyalkanoic and hydroxyalkandoic acids, aromatic acids, aliphatic and aromatic sulfonic acids, [0039] In one form of the invention, the aqueous acid solution is an aqueous solution of an acid selected from the group: acetic acid, phenylacetic acid, trifluoroacetic acid, acrylic acid, ascorbic acid, benzoic acid, chlorobenzoic acid, dinitrobenzoic acid, hydroxybenzoic acid, methoxybenzoic acid, methylbenzoic acid, o-acetoxybenzoic acid, napthalene-2-benzoic acid, hydrofluoric acid, hydrobromic acid, hydroiodic acid isobutyric acid, phenylbutyric acid, β- hydroxybutyric acid, butyne-1 ,4-dioic acid, hexyne-1 ,4-dioc acid, capric (decanoic) acid, cinnamic acid, citric acid, formic acid, fumaric acid, glycolic acid, heptanoic acid, hippuric acid, lactic acid, malic acid, maleic acid, hydroxymaleic acid, malonic acid, mandelic acid, methanesulfonic acid, nicotinic acid, isonicotinic acid, oxalic acid, phthalic acid, teraphthalic acid, propionic acid, phenylpropionic acid, salicylic acid, sebacic acid, succinic acid, suberic acid, sulfonic acid, benzene- sulfonic acid, p-bromophenylsulfonic acid, chlorobenzosulfonic acid, ethanesulfonic acid, 2-hydroxyethanesulfonic acid, p-toluenesulfonic acid, xylenesulfonic acid, boric acid, hydrazidic acid, hypochlorous acid, nitrous acid, phenol acid, propanic acid, succinic acid, a-hydroxybutyric acid, ethylenediamine- Ν,Ν-diacetic acid, isobutyric acid, sulfurous acid, tartaric acid, uric acid, tartaric acid hydrocyanic acid, hydrogen sulphide,

[0040] In one form of the invention, the aqueous acid solution is an aqueous solution of an acid selected from the group: citric acid, oxalic acid, tartaric acid, acetic acid.

[0041 ] In one form of the invention, the aqueous acid solution is an aqueous solution of an acid selected from the group: sulphuric acid, phosphoric acid, hydrochloric acid, nitric acid, perchloric acid.

Curing

[0042] As would be understood by a person skilled in the art, the term curing is fundamentally distinct from leaching. Leaching describes a process by which a solution containing a leaching agent is contacted with an ore, the solution recovered and valuable metals extracted therefrom. The curing step of the present invention renders the zinc silicate ore to be leached more amenable to the leaching process. The process is one where there is dissolution of the ammonia insoluble mineral and precipitation of an ammonia soluble phase. No metal recovery is achieved during curing.

[0043] The scope of the present invention encompasses methods where the aqueous acid solution is collected after the step of curing the zinc silicate ore to be leached, and metal values recovered therefrom. However, conventional aqueous leaching solutions do not fall within the meaning of aqueous acid solution , as they do not render the ore to be leached more amenable to the subsequent leaching process. For example, two stage ammoniacal leaching processes differ markedly from the method of the present invention as there is no enhancement of the second leaching stage by performance of the first.

[0044] However, in a preferred form of the invention, the step of curing the ore to be leached through the application of aqueous acid solution more specifically comprises substantially retaining the aqueous acid solution in contact with the zinc silicate ore to be leached when the ammonium carbonate solution containing free ammonia is added. As would be understood by a person skilled in the art, in many applications, it is virtually impossible to completely retain a solution in contact with the ore to be leached. For example, in a heap leaching context, it is virtually impossible to stop drainage from the ore.

Curing conditions: temperature and pressure

[0045] The most desirable conditions under which the zinc silicate ore is cured vary as the composition, mineralogy and texture of the ore varies. For example, the temperature at which the curing step occurs, the pH at which the curing step occurs and the time for which the zinc silicate ore is exposed to the aqueous acid solution may all be varied in response to the composition, mineralogy, texture, particle size and pore volume of the zinc silicate ore. [0046] In a highly preferred form of the invention, the step of curing the zinc silicate ore to be leached takes place at atmospheric pressure.

[0047] In a highly preferred form of the invention, the step of curing the zinc silicate ore to be leached takes place at ambient temperature.

Curing conditions: curing time

[0048] As would be understood by a person skilled in the art, the curing time will be a product of many factors, not least of which is the particle size of the zinc silicate ore, with smaller particle sizes enabling shorter curing time (at the expense, of course, of the energy associated with reducing the particle size of the ore).

[0049] In one form of the invention, the step of:

curing the zinc silicate ore to be leached through the application of a low volume of an aqueous acid solution having a pH of 0 or above, producing a cured ore more amenable to leaching through the application of an ammonium carbonate solution containing free ammonia;

is undertaken for a period of less than 28 days prior to the step of;

leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia, producing a pregnant leach solution.

[0050] The inventors have discovered that while the method of the present invention is effective at rendering the zinc silicate mineral willemite amenable to ammoniacal ammonium carbonate leaching, the rate at which curing takes place is slower than for ores predominantly composed of hemimorphite and/or sauconite.

[0051] Where the zinc silicate ore is predominantly willemite, the period preferably is within a range having a lower limit of 2 hours, and an upper limit selected from the group: 28 days, 20 days, 14 days, 7 days, 3 days, 2 days, 1 day, 18 hours, 12 hours, 8 hours, 4 hours and 3 hours.

[0052] Where the zinc silicate ore is predominantly sauconite, hemimorphite or predominantly a combination of sauconite and hemimorphite the period preferably is within a range having a lower limit of 30 minutes, and an upper limit selected from the following group: 14 days, 7 days, 6 days, 5 days, 4 days, 3 days 2 days, 1 day, 18 hours, 12 hours, 8 hours, 4 hours, 3 hours, 2 hours and 1 hour.

[0053] Low volume (of the aqueous acid solution having a pH of O or above)

[0054] Throughout this specification, unless the context requires otherwise, the phrases "pore space" and "pore volume" refer to the space comprising the pores within the ore particles, as opposed to inter-particle pores created by any stacking process.

[0055] The volume of the aqueous acid solution having a pH of 0 or above is a function of a number of parameters including, but not limited to, the texture of the zinc silicate ore, the residence time (the time for which the zinc silicate ore is exposed to the aqueous acid solution having a pH of 0 or above prior to the leaching step), the concentration of the aqueous acid solution having a pH of 0 or above, desired zinc recovery and the leach conditions.

[0056] However, the volumes of cure solution exposed to the ore are as low as practicable. That is, preferred forms of the invention utilise low volumes of aqueous acid solution having a pH of 0 or above, and preferred methods for curing the zinc silicate ore to be leached through the application of aqueous acid solution having a pH of 0 or above are those adapted to utilise low volumes of aqueous solution of aqueous acid solution having a pH of 0 or above.

[0057] The ideal extent of saturation of the pore space of the zinc silicate ore with the aqueous acid solution having a pH of 0 or above, will depend largely on the texture of the zinc silicate ore. [0058] Preferably the step of curing the zinc silicate ore to be leached through the application of aqueous acid solution having a pH of 0 or above, saturates at least 20% of the pore space with solution. Preferably still, the step of curing the zinc silicate ore to be leached through the application of aqueous acid solution having a pH of 0 or above, saturates at least 30% of the pore space with solution. Preferably still, the step of curing the zinc silicate ore to be leached through the application of aqueous acid solution having a pH of 0 or above, saturates at least 40% of the pore space with solution. Preferably still, the step of curing the zinc silicate ore to be leached through the application of aqueous acid solution having a pH of 0 or above, saturates at least 50% of the pore space with solution. Preferably still, the step of curing the zinc silicate ore to be leached through the application of aqueous acid solution having a pH of 0 or above, saturates at least 60% of the pore space with solution. Preferably still, the step of curing the zinc silicate ore to be leached through the application of the aqueous acid solution having a pH of 0 or above, saturates at least 70% of the pore space with solution. Preferably still, the step of curing the zinc silicate ore to be leached through the application of the aqueous acid solution having a pH of 0 or above, saturates at least 80% of the pore space with solution. Preferably still, the step of curing the zinc silicate ore to be leached through the application of the aqueous acid solution having a pH of 0 or above or above, saturates at least 90% of the pore space with solution.

[0059] Curing conditions: application of the aqueous acid solution having a pH of O or above to the zinc silicate ore

[0060] In one form of the invention, the step of curing the zinc silicate ore to be leached through the application of aqueous acid solution having a pH of 0 or above more specifically comprises:

spraying the aqueous acid solution having a pH of 0 or above onto the zinc silicate ore prior to the step of leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia. [0061] The method of the present invention may include the step of:

reducing the size of the zinc silicate ore to be leached by crushing and / or grinding.

[0062] In a particular form of the invention, the method of the present invention includes the step of:

[0063] reducing the size of the zinc silicate ore to be leached by wet grinding, wherein the zinc silicate ore is ground in contact with water or an aqueous grinding solution.

[0064] Where the invention comprises reducing the size of the zinc silicate ore to be treated by wet grinding, wherein the zinc silicate ore is ground in contact with water or a grinding aqueous solution, the aqueous grinding solution may be provided in the form of the aqueous acid solution having a pH of 0 or above.

[0065] In one form of the invention, the method comprises the steps of:

grinding the zinc silicate ore in an aqueous acid solution having a pH of 0 or above , thereby curing the zinc silicate ore to be leached through the application of aqueous acid solution having a pH of 0 or above ;

resting the zinc silicate ore for a predetermined period; then

leaching the cured and rested zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia to produce a pregnant leach solution; and

passing the pregnant leach solution to a means for metals recovery.

[0066] The method of the present invention may include a step of:

reducing the size of the zinc silicate ore to be leached by crushing.

[0067] In a particular form of the invention, the method of the present invention includes the step of:

reducing the size of the zinc silicate ore to be leached by wet crushing, wherein the zinc silicate ore is crushed in contact with water or an aqueous crushing solution. [0068] Where the invention comprises reducing the size of the zinc silicate ore to be treated by wet crushing, wherein the zinc silicate ore is crushed in contact with water or an aqueous crushing solution, the aqueous crushing solution may be provided in the form of the aqueous acid solution having a pH of 0 or above .

[0069] In one form of the invention, the method comprises the steps of:

crushing the zinc silicate ore in an aqueous acid solution having a pH of 0 or above thereby curing the ore to be leached through the application of an aqueous acid solution having a pH of 0 or above ;

resting the zinc silicate ore for a predetermined period; then

leaching the cured and rested zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia to produce a pregnant leach solution; and

passing the pregnant leach solution to a means for metals recovery.

[0070] In one form of the invention, the step of curing the zinc silicate ore to be leached through the application of an aqueous solution of aqueous acid solution having a pH of 0 or above more specifically comprises:

stacking the zinc silicate ore to form a heap;

irrigating the surface of the heap with the aqueous acid solution having a pH of 0 or above such that the aqueous acid solution having a pH of 0 or above percolates down through the heap;

resting the zinc silicate ore for a predetermined period, prior to the step of leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia.

[0071] In one form of the invention, the step of curing the zinc silicate ore to be leached through the application of an aqueous acid solution having a pH of 0 or above more specifically comprises:

immersing the zinc silicate ore in the aqueous acid solution having a pH of 0 or above for a sufficient time such that the aqueous acid solution having a pH of 0 or above infiltrates a desired fraction of pore volume; resting the zinc silicate ore for a predetermined period prior to the step of leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia.

[0072] The method of the present invention may include a step of:

agglomerating the zinc silicate ore to be leached.

[0073] In a particular form of the invention, the method of the present invention includes the step of:

agglomerating the zinc silicate ore to be leached by contacting the zinc silicate ore with water or an aqueous solution of an agglomerating agent.

[0074] In one form of the invention, the aqueous acid solution having a pH of 0 or above is also the aqueous solution of the agglomerating agent. That is, the aqueous solution contains both a curing agent and an agglomerating agent. In one form of the invention, the curing agent is an agglomerating agent, such that the step of curing the ore to be leached through the application of an aqueous solution of a curing agent more specifically comprises:

agglomerating the zinc silicate ore with the aqueous acid solution having a pH of 0 or above ;

resting the zinc silicate ore for a predetermined period prior to the step of leaching the cured ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia.

[0075] In one form of the invention, the step of curing the zinc silicate ore to be leached through the application of an aqueous acid solution having a pH of 0 or above more specifically comprises:

spraying the aqueous acid solution having a pH of 0 or above onto the zinc silicate ore; and

resting the zinc silicate ore for a predetermined residence time, prior to the step of leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia. [0076] The predetermined time for which the zinc silicate ore is rested prior to the step of leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia will be a function of a number of parameters including, but not limited to the particle size of the ore, the concentration of the curing agent and the texture of the ore.

[0077] In preferred forms of the invention, the predetermined period is between 5 minutes and twenty eight days. Preferably still, the predetermined period is between 2 hours and 14 days. Preferably still, the predetermined period is between 1 day and 7 days. A person skilled in the art will realise that the curing time will be a function of particle size, small particles requiring substantially less curing time than large particles.

[0078] In a preferred form of the invention, the cured ore- aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 100 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 200 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 400 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 700 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 1000 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 2000 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 4000 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 7000 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 10000 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 20000 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 40000 g/L. In a preferred form of the invention, the cured ore-aqueous acid solution having a pH of 0 or above mixture has a solids content not less than about 50000 g/L.

[0079] In one form of the invention, the solids content of the cured ore-aqueous acid solution having a pH of 0 or above mixture falls within a range of contents having a lower limit of 100 g/L In a preferred form of the invention, the range of contents has a lower limit of 200 g/L. In a preferred form of the invention, the range of contents has a lower limit of 400 g/L. In a preferred form of the invention, the range of contents has a lower limit of 700 g/L. In a preferred form of the invention, the range of contents has a lower limit of 1000 g/L. In a preferred form of the invention, the range of contents has a lower limit of 2000 g/L. In a preferred form of the invention, the range of contents has a lower limit of 4000 g/L. In a preferred form of the invention, the range of contents has a lower limit of 7000 g/L. In a preferred form of the invention, the range of contents has a lower limit of 10000 g/L. In a preferred form of the invention, the range of contents has a lower limit of 20000 g/L. In a preferred form of the invention, the range of contents has a lower limit of 40000 g/L. In a preferred form of the invention, the range of contents has a lower limit of 50000 g/L.

[0080] In one form of the invention, the solids content of the cured ore-aqueous acid solution having a pH of 0 or above mixture falls within a range of contents having an upper limit of 100000 g/L. In one form of the invention, the solids content of the mixture falls within a range of contents having an upper limit of 50000 g/L. In one form of the invention, the solids content of the mixture falls within a range of contents having an upper limit of 40000 g/L. In one form of the invention, the solids content of the mixture falls within a range of contents having an upper limit of 20000 g/L. [0081] As would be obvious to those skilled in the art, the volume and concentration of the reducing solution will vary according to several factors. The product of concentration and volume is the dose of reductant applied. The specific dose required by an ore can be applied using a low volume of a high concentration or a high volume of low concentration. The volume to be used is a function of the surface area of the ore. A large particle size will require a lower volume than a fine particle size as the surface area is smaller. An ore with a high cobalt headgrade will require more solution and / or higher concentration than an ore with a low cobalt headgrade.

Leaching: pressure and temperature

[0082] The most desirable conditions under which the cured zinc silicate ore is leached will vary as the conditions under which the zinc silicate ore is cured vary.

[0083] Methods for leaching ore generally, although not zinc silicate ore specifically, at atmospheric pressure are well known to persons skilled in the art, and include heap leaching, vat leaching, tank leaching and dump leaching. In preferred forms of the invention, the step of leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia, producing a pregnant leach solution takes place at ambient temperatures. Atmospheric leaching, particularly at ambient temperatures, is one of the least energy-intensive leaching techniques available. A curing step that is not energy intensive and that renders a zinc silicate ore amenable to an ammoniacal leaching step that is also not energy intensive has clear advantages over prior art methods.

[0084] Leaching: ammonium carbonate solution containing free ammonia

Ammonium carbonate

[0085] Ammonium carbonate fixes the operating pH to a relatively narrow range and is, to some extent, self-regulating as the ammonium carbonate acts as a buffer. Importantly, the pH range buffered by the ammonium carbonate is a range in which zinc is soluble. A second advantage of carbonate systems is that there is less prospect of gypsum scaling as the sulphate level is always too low for precipitation to occur. The calcium level will also be extremely low as the precipitation of CaC0 ₃ will occur whenever calcium ions are released into solution. A third advantage is that at the operating pH many undesirable metals, including iron, calcium, magnesium, aluminium, silicon and manganese, have very low solubilities. This simplifies the overall process by eliminating or reducing the need to include unit operations designed to remove these metals from the zinc solution.

[0086] Preferably, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is sufficient to prevent the pH decreasing below 8 during the step of leaching the cured zinc silicate ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia, producing a pregnant leach solution. Preferably, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is at least 0.1 g/L. Preferably, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is at least 1 g/L Preferably still, the concentration of ammonium carbonate is at least 5 g/L. Preferably still, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is at least 8 g/L. Preferably still, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is at least 10 g/L. Preferably still, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is at least 20 g/L. Preferably still, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is at least 30 g/L. Preferably, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is between 0.1 g/L and 500 g/L. Preferably still, the concentration of ammonium carbonate is between 1 g/L and 500g/L. Preferably still, the concentration of ammonium carbonate is between 5 g/L and 500g/L. Preferably still, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is between 8 g/L and 500g/L. Preferably still, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is between 10 g/L and 500g/L. Preferably still, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is between 20 g/L and 500g/L. Preferably still, the ammonium carbonate concentration of the ammonium carbonate solution containing free ammonia is between 30 g/L and 500g/L

[0087] Preferably, the ammonium carbonate concentration of the solution is about 10 g/L ammonium carbonate.

Ammonia

[0088] The ammonia of the ammonium carbonate solution containing free ammonia may be generated in situ, such as by hydrolysis of urea.

[0089] The free ammonia concentration of the ammonium carbonate solution containing free ammonia may be tailored to the rate at which the zinc is leached from the cured zinc silicate ore, thereby minimising excess free ammonia and thus minimising ammonia losses due to evaporation. Specifically, the resulting pregnant leach solution preferably contains only a slight excess of free ammonia over that necessary to retain the zinc in solution. As there is little free ammonia in the pregnant leach solution, ammonia losses due to evaporation are low.

[0090] A person skilled in the art will readily be able to calculate the free ammonia concentration required to retain zinc in solution at a desired concentration. The conditions under which ammoniacal complexes of various target metals, including zinc, form are readily calculable based on data contained in NIST Standard Reference Database 46, NIST Critically Selected Stability Constants of Metal Complexes: Version 6.0, the contents of which are incorporated by reference.

[0091 ] For example, where the target metal is zinc and the ore contained 18.2% zinc in a rapidly leaching form the ammonium carbonate solution containing free ammonia comprises about 30-70 g/L ammonia. [0092] As would be realised by a person skilled in the art the level of ammonia in the solution applied in step (b) would be matched to the level of zinc in the ore and the rate at which it leaches. A low grade ore where the zinc leaches slowly would require a lower concentration of ammonia than a high grade ore where the leaching is rapid.

[0093] Concentration of acid in the aqueous acid solution having a pH of 0 or above

[0094] As would be understood by a person skilled in the art, for strong acids, the concentration of acid is substantially dictated by the pH of the solution. For weak acids, the relationship between the concentration of the acid and the pH is substantially determined by the K _a (or K _a s or Ka-i) of the acid.

[0095] For most zinc silicate ores rich in hemimorphite and/or willemite, extremely low (relative to stoichiometric) doses of acid are possible, and from an economic perspective, highly desirable. In one form of the invention, the molar ratio of acid in the aqueous acid solution having a pH of 0 or above to zinc in the zinc silicate ore is less than 1 . In one form of the invention, the molar ratio of acid in the aqueous acid solution having a pH of 0 or above to zinc in the zinc silicate ore is less than 0.1. In one form of the invention, the molar ratio of acid in the aqueous acid solution having a pH of 0 or above to zinc in the zinc silicate ore is less than 0.01. In one form of the invention, the molar ratio of acid in the aqueous acid solution having a pH of 0 or above to zinc in the zinc silicate ore is less than 0.001. In one form of the invention, the molar ratio of acid in the aqueous acid solution having a pH of 0 or above to zinc in the zinc silicate ore is less than 0.0001.

[0096] For zinc silicate ores rich in sauconite, doses of acid approximating double the stoichiometric ratio are desirable. In one form of the invention, the molar ratio of acid to zinc for ores rich in sauconite is between 1 :1 and 4:1. In one form of the invention, the molar ratio of acid to zinc for ores rich in sauconite is between 1 :1 and 3:1. In one form of the invention, the molar ratio of acid to zinc for ores rich in sauconite is between 1.5:1. and 2.5:1 .

[0097] Without wishing to be bound by theory, it is believed that sauconite exchanges protons with the acid. As would be understood by a person skilled in the art, for a stoichiometric quantity of acid to fall within the pH range that the inventors have found to be effective, the acid must be a weak acid.

Means for metal recovery

[0098] The means for metal recovery of the present invention may comprise one or more of the following: solvent extraction, ion exchange, precipitation and cementation.

Brief Description of the Drawings

[0099] The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawing, in which:-

Figure 1 is a schematic flow sheet of a method for leaching one or more target metals, at least one of which is zinc, from a zinc silicate ore in accordance with the present invention;

Figure 2 shows the head grades of the ore samples in the examples Figure 3 shows the relative abundance of the zinc minerals present in the samples of Figure 2;

Figure 4 shows the zinc dissolution for all of the samples after the leaching treatment of Example 1 ;

Figure 5 shows the recoveries as a function of dosage for the samples of Example 2;

Figure 6 shows the data of Figure 5 as function of citric acid dosage; Figure 7 shows the zinc dissolution for all of the samples after the treatment of Example 3;

Figure 8 shows the zinc dissolution for all the samples after being cured with strong acids at different doses;

Figure 9 is a graph showing the extent of Direct leaching of the ores of example 5 ores in ammoniacal - ammonium carbonate solutions (AAC and SAC) and sulphuric acid;

Figure 10 is a graph showing the extent of Direct leaching of the ores of example 5 ores in ammoniacal - ammonium carbonate solutions (AAC and SAC) and sulphuric acid after a pretreatment step; and

Figure 1 1 is a graph showing the extent of Direct leaching of the ores of example 5 ores in ammoniacal - ammonium carbonate solution after the ores were pretreated using AAC and two different concentrations of citric acid.

Best Mode(s) for Carrying Out the Invention

[00100] The zinc silicate ore 1 is sprayed with a weakly acidic solution 2 sufficient to wet the pore volume of the ore. The cured zinc silicate ore is then stacked 3 into a heap. The heap is irrigated with an ammoniacal - ammonium carbonate solution 4, which is a combination of solution recycled from the solvent extraction and fresh solution. The run off is collected and sent to solvent extraction 5, the zinc-depleted solids 6 are sent to tailings. The zinc-rich ammoniacal solution 5 is contacted with a suitable solvent extraction reagent and zinc extracted. The zinc-depleted ammoniacal solution 4 is recycled back to the heap leach stage. The zinc-rich solution from the solvent extraction stage 7 is sent to a process for zinc recovery e.g. electrowinning. After recovery, the zinc- depleted solution 8 is recycled back to solvent extraction for reuse. The zinc product 9 is sent for sale.

[00101 ]

Examples

[00102] A suite of different samples were obtained from a zinc oxide deposit. The head grades of the samples are shown in Figure 2. Quantitative X-ray diffraction was used to determine the relative abundance of the zinc minerals present, this data is shown in Figure 3. From this it is evident that the mineralogy of the zinc changed with sample, samples 1-7 contain predominantly zinc silicate in the form of hemimorphite (Zn Si ₂ O ₇ (OH) ₂ H ₂ O), samples 8-11 were mainly zinc silicate in the form of willemite (Zn2Si0 ₄ ) and samples 12-20 were predominantly sphalerite (ZnS). Also present was smithsonite (ZnC0 ₃ ). The other major minerals present in the ore were calcite (CaC0 ₃ ), dolomite (CaMg(C0 ₃ ) ₂ ) and garnet On the basis of the mineralogy, the acid neutralisation capacity of the ores were calculated to range from 10 to 850 kg H ₂ SO ₄ / 1 ore, the average being 440 kg H ₂ SO / 1 ore. Economically, such high acid consumptions effectively discounts the use of acid as a leaching agent for these ores.

EXAMPLE 1

[00103] The samples were cured using a strong acid (sulphuric acid), and a weak acid, (citric acid), at two different dosages, by forming a paste. After curing, the paste was slurried using 20 g/L ammonia + 20 g/L ammonium carbonate solution. After 24 h of leaching, the solution was sampled and analysed for zinc. Figure 4 shows the zinc dissolution for all of the samples. For comparison, the uncured sample is also shown after 24 h leaching in 20 g/L ammonia + 20 g/L ammonium carbonate solution.

[00104] Clearly, to achieve increased zinc dissolution in the leach stage it is necessary to have a curing stage. However, not all of the cures used were effective. Surprisingly, curing using a high dosage of strong acid (400 kg H ₂ SO / t ore) was actually counterproductive. In fact, the lower dose (40 kg/t) of sulphuric acid was more effective than the higher dosage in all cases. .

[00105] The greatest increases in solubility were achieved with the weak acid, citric acid. Both dosages were more effective than sulphuric acid and the lower citric acid dosage was generally more effective. As noted previously, greater leaching is expected using higher reagent dosages so this outcome is counter intuitive.

EXAMPLE 2

[00106] A further, otherwise identical, series of experiments were performed with varying citric acid dosage on a subset of samples covering the different mineralogical types of ore. The recoveries as a function of dosage are shown in Figure 5. The zero dose run is also shown for comparison. As can be seen there is a relatively small range of recoveries from the ore despite the fiftyfold change in reagent dosage applied. Clearly, it is not the quantity of citric acid present that determines the extent of enhanced leaching.

[00107] The same data was plotted showing the zinc dissolution as a function of citric acid dosage, this is shown in Figure 6. Note the x-axis is logarithmic and the zero dosage sample is shown at a dosage of 0.1 kg/t for comparative purposes only. This more clearly shows that citric acid dosage is of comparatively little importance with only relatively minor increases in zinc recovery at higher dosages. From an economic view this is an excellent result as high doses will inevitably cost more. Clearly, there will be an economic balance between the cost of citric acid and the enhancement in zinc recovery, the capacity to use extremely low doses to achieve significant increases in zinc solubility is economically invaluable.

[00108] The effect of curing in weak acid is clearly higher in the non-sulphide samples 1 -1 1. [00109] Without wishing to be bound by theory, it is believed that the weak acid produces a chemical environment in which the zinc minerals are intrinsically unstable and therefore decompose without consuming either protons or citrate ions. This would explain the effect even at the lowest dosage. Consider curing a 5 % zinc headgrade using 0.5 kg / 1 of citric acid, e.g. sample 1. In each tonne of ore there is 50 kg of zinc, zinc has a molar mass of 65.4 g / mol, so each tonne of ore contains 50000/65.35 = 764.5 mol of Zn. For each tonne of ore 0.5 kg of citric acid is used, citric acid has a molar mass of 192 g / mol giving 500/192 = 2.60 moles of citric acid present. The molar ratio of citric acid to zinc is 2.60/764.5 = 0.0034.

[00110] The poor effect on the ores predominant in sphalerite is believed to be due to this phase being stable in the citric acid solutions used.

[0011 1] Anyone skilled in the art will realise that there is a very substantial deficit of citrate ions to form a 1 :1 molar ratio zinc citrate complex ion. Were such a complex formed the citrate ions present would have been depleted with very little increase in solubility. A similar calculation can be made for protons, citric acid has three available protons so the molar ratio of protons to zinc would be 0.01 , had all protons been consumed in solubilising the zinc then little extra solubility would have been observed.

EXAMPLE 3

[00112] A sample from a different deposit was analysed and found to contain 4.54% Zn, quantitative X-ray diffraction showed that ~98% of the zinc was present as the silicate hemimorphite. Four different weak acids were used to cure this sample using 5 or 50kg of acid per tonne. After 24h resting, the sample was leached in ammoniacal ammonium carbonate for 24h and then the solution analysed to assess the extent of dissolution. Figure 7 shows the extent of zinc leached. The horizontal line shows the extent of leaching in the absence of any curing was 35%. Clearly, all of the weak acids trialled were effective with >73% of the zinc dissolving. Closer examination of the data shows that the lower acid dose of 5kg/t was actually more effective than the higher dose.

EXAMPLE 4

[001 13] The same sample was then cured with strong acids at different doses. Figure 8 shows that sulphuric acid is far more effective at low doses than at high doses. Phosphoric acid is also as effective as citric acid at low doses. This indicates that the identity of the acid used is not as important as the dose used. The horizontal line shows the extent of leaching in the absence of any curing was 35%.

[001 14] It would be reasonably expected by those skilled in the art that a longer curing time than the 24 h used would lead to greater enhancements in solubility during the subsequent leach stage.

EXAMPLE 5

[001 15] Fifteen ore samples were obtained from a European zinc oxide deposit. The headgrades ranged from 14.7 to 29.9% zinc and the zinc mineralogy was a mixture of smithsonite and hemimorphite. Direct leaching of these ores in ammoniacal - ammonium carbonate solutions (AAC and SAC) and sulphuric acid are shown in Figure 9. As is clear, the acid is by far the most effective with high recoveries achieved. However, the acid consumed in the process is also extremely high with the ores consuming 200->500kg of H2S04 per tonne of ore. Even at the high headgrades this consumption is unlikely to be economic. The other disadvantage of acid leaching is that the leach solution will also contain other dissolved elements, notably iron, aluminium, silicon, calcium and magnesium, all of which will need to be removed and disposed of in an environmentally acceptable manner. None of these elements are soluble in the ammoniacal ammonium carbonate solution thereby greatly simplifying the overall process flowsheet and reducing the environmental impact. [001 16] The same ores were then pretreated by making up a paste by mixing a small volume of the same solutions as used for leaching with ore. The mixture was allowed to rest for 24h and then leached by adding the paste to a larger volume of AAC. The extent of leaching is shown in Figure 10, As is clear, resting in AAC or SAC prior to leaching was more effective at enhancing leaching in AAC than resting in sulphuric acid. This is counter-intuitive on the basis of the direct leaching result shown in Figure 9 where acid was a more effective leachant than ammoniacal solution.

[00117] The same ores were pretreated using AAC and two different concentrations of citric acid and then leached in a more concentrated ammoniacal - ammonium carbonate solution (SAC). The results are shown in Figure 1 1. As is clear, the use of a more concentrated leaching solution greatly enhances the recovery after pretreatment in AAC compared with leaching in AAC. The use of 10g/L citric acid in the pretreatment is also clearly far more effective than 1g/L citric acid with significantly higher recoveries achieved from all ore samples.

[001 18] Without wishing to be bound by theory, it is believed that the higher concentration of citric acid for efficacy is due to the high proportion of hemimorphite in the ore and the high head grade compared to previous ores. The same effect as 10g/L may have been achieved using a lower citric acid concentration as the average recovery increased by a factor of around 4 (from ~23% to >90%). The solution zinc concentration for the 10g/L citric acid pretreatment option varied from 8-20g/L. As would be recognised by those skilled in the art further optimisation of the pretreatment and leaching conditions can be expected to lead to higher recoveries and higher leach solution concentrations.

[001 19] Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of this invention.