AU2010217184A1

AU2010217184A1 - Zinc oxide purification

Info

Publication number: AU2010217184A1
Application number: AU2010217184A
Authority: AU
Inventors: Philip Owen; Michael Perry; Stephen A. Short
Original assignee: Auszinc Metals and Alloys
Current assignee: Auszinc Metals and Alloys
Priority date: 2009-02-24
Filing date: 2010-02-24
Publication date: 2011-08-25
Also published as: WO2010096862A1

Abstract

The present invention is directed to a method of purifying a zinc sulphate solution in the production of zinc oxide. In the preferred methodology there is a series of processes ranging from converting a zinc oxide/zinc metal residue material to zinc sulphate solution by leaching within acid, for example sulphuric acid, reacting the zinc sulphate solution with an alkaline metal carbonate to produce zinc carbonate, and then heating the zinc carbonate, for example, by calcining to produce zinc oxide in a highly pure form.

Description

WO 2010/096862 PCT/AU2010/000206 1 ZINC OXIDE PURIFICATION FIELD OF THE INVENTION The present invention relates broadly to a method of purifying a zinc sulfate solution in the production of zinc oxide. 5 BACKGROUND OF THE INVENTION Zinc oxide is widely used as an additive in numerous materials and products including, for example, plastics, ceramics, glass, cement, rubber (e.g. car tyres), lubricants, paints, ornaments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants. For industrial use, zinc oxide is generally produced by one of three main processes: 10 (i) the French Process, where metallic zinc is melted in a graphite crucible and vaporized to react with the ambient oxygen to give zinc oxide; (ii) the Direct Method, in which zinc ores or roasted sulfide concentrates are mixed with coal in a reduction furnace, wherein the ore is reduced to metallic zinc and the vaporized zinc is allowed to react with oxygen to form zinc oxide; and 15 (iii) the American Process, in which zinc ore (zinc ash) is dissolved in, for example, hydrochloric acid (as ZnCI 2 ) and precipitated with alkali to give "active" zinc oxide. SUMMARY OF THE INVENTION According to one aspect of the invention there is provided a method of purifying a zinc 20 sulfate solution comprising mixing the zinc sulfate solution together with an aqueous metal carbonate to produce a zinc carbonate slurry. According to another aspect of the present invention there is provided a method of purifying a zinc sulfate solution in the production of zinc oxide, said method comprising the steps of: 25 mixing the zinc sulfate solution together with an aqueous metal carbonate to produce a zinc carbonate slurry; separating zinc carbonate from the zinc carbonate slurry; and heat treating the zinc carbonate to produce the zinc oxide. Preferably the step of mixing the zinc sulfate solution together with an aqueous metal 30 carbonate comprises controlling the rate of addition of the aqueous metal carbonate. More WO 2010/096862 PCT/AU2010/000206 2 preferably the step of mixing comprises controlling the rate of addition of the aqueous metal carbonate so that the mixing occurs at a pH of between 5 to 9. Even more preferably the step of mixing comprises controlling the rate of addition of the aqueous metal carbonate so that the mixing occurs at a pH of between 6 to 8. Still more preferably, 5 the step of mixing comprises controlling the rate of addition of the aqueous metal carbonate so that the mixing occurs at a pH of between 7 to 7.2. Preferably the step of mixing the zinc sulfate solution together with an aqueous metal carbonate comprises controlling the rate of addition of the aqueous metal carbonate so that the mixing occurs at a temperature of between 509C to 609C and at a pH of between 10 7 to 7.2. Preferably the aqueous metal carbonate comprises one or more types of alkali metal carbonates. More preferably, the one or more types of alkali metal carbonates include those of sodium, potassium. Preferably the method of purifying a zinc sulfate solution further comprises a preliminary 15 step of removing one or more contaminants from the zinc sulfate solution. More preferably the step of removing one or more contaminants from the zinc sulfate solution includes the addition of an alkaline solution and an oxidant in a temperature range of between 509C to 1009C. Even more preferably the alkaline solution com prises one or more types of alkali metal cations, and the oxidant is selected from the group consisting of: hydrogen 20 peroxide, sodium hypochlorite, potassium permanganate, oxygen. Still more preferably, the one or more types of alkali metal cations include those of sodium or potassium and the oxidant is potassium permanganate. Preferably the one or more contaminants includes nickel, aluminium, iron, calcium or manganese. More preferably the step of removing nickel from the zinc sulfate solution 25 comprises adding a chelating agent material. Even more preferably the chelating agent material is dimethyl glyoxime. Preferably the step of separating zinc carbonate from the zinc carbonate slurry involves centrifuging the zinc carbonate slurry. Preferably the step of heat treating the zinc carbonate to produce the zinc oxide involves 30 calcining the zinc carbonate at a temperature of between 4009C to 4409C. More preferably the step of calcining the zinc carbonate occurs at a temperature of 4209C.

WO 2010/096862 PCT/AU2010/000206 3 BRIEF DESCRIPTION OF THE FIGURES In order to achieve a better understanding of the nature of the invention a preferred embodiment of a method of purifying a zinc sulfate solution in the production of zinc oxide will now be described, by way of example only, with reference to the accompanying 5 figures in which: Figure 1 is a schematic illustration of one embodiment of a method of purifying zinc sulfate solution in the production of zinc oxide in accordance with the present invention. Figure 2 is a schematic flow diagram of an exemplary plant for the production of zinc oxide in accordance with the present invention. 10 DETAILED DESCRIPTION OF THE INVENTION The present invention in one embodiment contemplates a method of purifying a zinc sulfate solution in the production of zinc oxide. Briefly, the preferred method involves a series of processes ranging from converting a zinc oxide/zinc metal residue material to zinc sulfate solution by leaching with an acid, for example sulfuric acid, reacting the zinc 15 sulfate solution with an alkali metal carbonate to produce zinc carbonate, and then heating the zinc carbonate, for example, by calcining, to produce zinc oxide in a highly pure form. The zinc sulfate solution may be obtained by processing a zinc bearing material such as a zinc oxide/zinc metal residue material sourced as a primary feed from industrial or other 20 processes. A preferred method for recovering and purifying zinc oxide disclosed herein is carried out using zinc oxide/zinc metal residue material sourced as a primary feed from a galvanizing plant. It will be appreciated by persons skilled in the art that using zinc oxide/zinc metal residue material sourced as a primary feed from other sources may require the process circuit to be modified depending on the contaminants present. In the 25 preferred method, prior to forming the zinc sulfate solution, the primary feed may undergo one or more pre-treatment stages to reduce the size of the fragments in the feed and to remove certain contaminants that may hinder the purification of zinc oxide. Such pre treatment stages may include but are not limited to any one or more of the following: crushing, milling, roasting, size- and density-selective separation, filtering, washing. 30 Leaching Stage After processing, the zinc may be leached out of the zinc oxide fraction using any suitable leachant. The leaching stage may be performed using wet or dry zinc oxide fractions. In a WO 2010/096862 PCT/AU2010/000206 4 preferred embodiment, the leaching stage is performed using a zinc oxide/water slurry. Any suitable amount of water may be envisaged to produce a zinc oxide slurry appropriate for use in the leaching stage. Good results have been obtained using a slurry formed from a 50% zinc oxide/water mixture. 5 Figure 1 outlines a preferred embodiment of the zinc oxide purification process using a zinc oxide slurry feed produced by any suitable process. The zinc is preferably leached from the zinc oxide slurry using any suitable acid. Good results have been obtained using sulfuric acid as the leachant. Dilute sulfuric acid digestion of the zinc oxide in the slurry produces a zinc sulfate solution. 10 In a preferred embodiment as shown in Figure 2, the zinc oxide slurry is continuously fed to a leach mixing tank 1, to which sulfuric acid is added continuously to achieve a leach solution with a measured finishing pH of 1.8. Good results have also been obtained using leach solutions with a pH value in the range of 1 to 3.5. However, it will be appreciated by persons skilled in the art that a finishing pH value greater than 1.8 (a more basic pH) may 15 lengthen the reaction time and may reduce the leaching efficiency. In addition, the finishing pH also affects the dosage of alkali that may need to be added to the zinc sulfate solution in the later purification stage. The concentration of acid used in the leaching stage may depend on the amount of zinc in the slurry. Good results have been obtained using a leach concentration of 80 g/L zinc (corresponding to 9% v/v sulfuric acid). 20 The continuous addition of sulfuric acid to the leach solution may cause an initial rise in temperature due to exothermic reaction. Preferably, the temperature for the leaching stage may be maintained using any suitable heating means. Such heating means may include but are not limited to any one or more of the following: a hot plate, a heat exchanger, direct steam injection, insulation, gas burner, electrical coil. In the preferred 25 embodiment, the temperature of the leaching stage is maintained around 809C via steam heating using an external heat exchanger (not shown). Good results have also been obtained using a temperature in the range of between 50 to 1 009C. It will be appreciated by persons skilled in the art that the leach time may vary with temperature. To ensure maximum leaching efficiency (i.e. to extract the highest possible amount of zinc from the 30 zinc oxide slurry), the leach solution may be transferred from the main leach mixing tank 1 to one or more smaller leach tanks 2. The use of the one or more smaller leach tanks 2, preferably maintained at a temperature of around 809C, may reduce the leach time.

WO 2010/096862 PCT/AU2010/000206 5 Desirable leach conditions are as set out in Table 1 for a 1 tonne feed. Table 1 Leach Conditions Feed dry 1.000 tonne Water in 50% slurry 1.000 tonne Feed slurry (50%) 1.250 m 3 Sulfuric acid (98%) 1.093 tonne Water for acid dilution 4.877 tonne Temperature 809C Potassium permanganate (KMnO 4 ) addition to achieve target ORP 300 mV Leach time 0.5 hours Finishing pH 1.8 Leach efficiency >95% Target zinc concentration 80 g/L Metallic zinc in the zinc oxide slurry may be converted to zinc oxide during the leaching stage by adding an oxidant to the leach solution. The presence of an oxidant in the leach 5 solution may increase the leach efficiency. Any suitable oxidant may be used. Suitable oxidants may include but are not limited to one or more of the following: hydrogen peroxide, sodium hypochlorite, potassium permanganate, oxygen. Good results have been obtained using potassium permanganate (KMnO 4 ) as an oxidant. The oxidative state of the leach solution may be monitored using any suitable measuring means, for example, 10 an oxidation/reduction potential (ORP) meter. Good results have been obtained where a sufficient amount of oxidant is added to the leach solution to maintain an oxidative state of around 200 to 900 mV. In a preferred embodiment, good zinc dissolution may be achieved in (0.5 hours) by maintaining the temperature and oxidative state of the leach solution around 80'C and 15 300 mV, respectively.

WO 2010/096862 PCT/AU2010/000206 6 Once the leaching stage is complete, most or all of the zinc oxide in the leach solution will have been digested by the sulfuric acid to produce the desired zinc sulfate solution. Purification Stage Following the leaching stage, it may be necessary to remove a number of low level 5 contaminants from the zinc sulfate solution. For example, contaminants may include but are not limited to those containing one or more of the following metals: aluminium, iron, manganese, calcium, nickel. Raising the pH of the zinc sulfate solution and increasing the oxidation/reduction potential (ORP) may create conditions within the solution that result in the formation of insoluble 10 compounds of the unwanted contaminants, which may subsequently be removed from solution by any suitable means including but not limited to any one or more of the following: filtration, decantation, centrifugation. As such, these one or more contaminants may be removed to acceptable levels through controlled addition of an alkali metal cation solution containing a suitable oxidant. Suitable solutions may be prepared comprising: (i) 15 an alkali metal cation including but not limited to any one or more of the following: sodium, potassium, .and (ii) an oxidant, including but not limited to one or more of the following: sodium hypochlorite, potassium permanganate. From at least a cost and availability perspective, sodium hydroxide solution may be preferable over alkaline solutions prepared from other alkali metals. 20 In a preferred embodiment, a solution comprising sodium hydroxide and potassium permanganate (KMnO 4 ) may be added to the zinc sulfate solution in the precipitation tank 3 to achieve a pH of 3.5 and an ORP of >850 mV. Any suitable means of heating may be used to maintain the temperature of the zinc sulfate solution in the precipitation tank 3 around 80'C. Preferable forms of heating may include but are not limited to any one or 25 more of the following: steam heating, heat exchanger, direct steam injection, insulation, gas burner, electrical coil. Desirable purification conditions are shown in Table 2.

WO 2010/096862 PCT/AU2010/000206 7 Table 2 Purification Conditions Temperature 801C pH 3.5 Reagent for pH change Sodium hydroxide (NaOH) Sodium hydroxide concentration 50 g/L Oxidation/Reduction Potential; (ORP) Peak >850mV Reagent for oxidation Dry potassium permanganate (KMnO 4 ) Approximate Time 0.5 hours It will be apparent to persons skilled in the art that solution pH values above 3.7 may cause zinc to precipitate from solution in the form of zinc diiron (III) tetraoxide. Hence, maintenance of the pH during the purification stage may be important to maximize zinc 5 oxide recovery. Diluting the concentration of the sodium hydroxide solution may allow any precipitated zinc hydroxide to re-dissolve into solution. Aluminum as a contaminant may be removed from the zinc sulfate solution by forming a potassium-alunite compound which precipitates from solution. The maximum conversion to this insoluble compound occurs as the pH is raised to a finishing pH of 3.7. It will be 10 appreciated that adding a suitable flocculent to the zinc sulfate solution or filtering the solution at temperatures of >601C may prevent the alu minium compound from re dissolving into solution. In a preferred embodiment, a concentration of 0.5g.L of K* in solution may ensure a high degree of aluminium removal from solution. Iron as a contaminant may be removed from the zinc sulfate solution via oxidation of the 15 soluble ferrous (Fe 2 +) to give the more insoluble ferric (Fe 3 +) form. In a preferred embodiment, oxidation coupled with a solution pH of >3.0 may ensure a high degree of iron removal from solution. Manganese as a contaminant may be removed from the zinc sulfate solution via oxidation of the Mn 2 + to Mn 4 * and the subsequent formation of MnO 2 . It will be appreciated that 20 sufficient concentrations of either KMnO 4 or sodium hypochlorite (NaCIO 3 ) may achieve an ORP >850 mV to yield Mn02.

WO 2010/096862 PCT/AU2010/000206 8 Calcium as a contaminant may be present in the zinc sulfate solution in the form of calcium sulfate (CaSO 4 ) or gypsum. Gypsum may be removed from the zinc sulfate solution by virtue of exploiting the solubility character of gypsum in the solution, which is around 0.650 g/L calcium. In a preferred embodiment, the leaching stage may be modified 5 to increase the concentration of the leach solution from 80 g/L to 300 g/L zinc. This may be achieved by increasing the rate of feed and acid, and reducing the volume of make-up water added to the leach solution. This approach has the desirable effect of increasing the equivalent calcium concentration for example, to around 10.7 g/L, while the gypsum solubility remains the same (around 0.650 g/L calcium). In this example, the balance of 10 gypsum (around 0.05 g/L) in the leach solution may be removed by filtration. The filtered leach solution may then be diluted to a desirable concentration of, for example 80 g/L zinc, with a lower calcium concentration. Nickel as a contaminant may be removed from the zinc sulfate solution through the addition of any one or more ligands that have a good affinity towards the metal or metal 15 ion. Such ligands may include, for example, chelating agents. Chelating or sequestering agents form chelate complexes with metals or metal ions through the formation of multiple bonds with the metal or metal ion. Preferably, the chelating agent is dimethyl glyoxime (DMG). As is known from gravimetric analysis, DMG is widely used as a chelating agent in the gravimetric determination of 20 nickel, for example, in ores. The complex formed between nickel and DMG (Ni-DMG) is poorly soluble in water at pH 5-8 and so precipitates from solution, thereby simplifying the removal of the nickel contaminants. Solid DMG has low solubility in water. The solubility of DMG in water may be increased by, for example, dissolving DMG in an alkali metal cation solution. The alkali metal cation may be selected from but is not limited to any one or 25 more of the following alkali metals: sodium, potassium. From at least a cost and availability perspective, sodium hydroxide solution may be preferable over alkaline solutions prepared from other alkali metals. DMG may be dissolved in sodium hydroxide solution to form a soluble sodium-DMG compound, which may be dosed directly into the zinc sulfate solution. Good levels of nickel removal from the zinc sulfate solution may be 30 achieved using the sodium-DMG compound at a solution pH of >5.0. Advantageously, DMG has little or no affinity for zinc; hence, addition of sodium-DMG to a zinc sulfate solution preferentially forms a water-insoluble complex with the nickel contaminant. The insoluble contaminants may be removed from the treated zinc sulfate solution using any suitable means. For example, the insoluble contaminants may be removed from WO 2010/096862 PCT/AU2010/000206 9 solution by filtration, decantation, centrifugation. The removal of insoluble contaminants, particularly, contaminants that are suspended in solution, may be enhanced through the use of flocculation. Flocculents may be anionic or cationic in character and may cause particles suspended in solution to aggregate into clumps or floc, which may then float to 5 the top of the solution, or settle to the bottom leaving a clear solution. For example, to remove one or more of the abovementioned insoluble low level contaminants from the zinc sulfate solution, it may be necessary to introduce a suitable flocculent into the solution. Suitable flocculents may include but are not limited to any one or more of the following anionic flocculents: Nalco Core@ Shell 71301, Nalco 82205, Ciba@ Magnafloc@ 10 338 It will be appreciated by persons skilled in the art that in the embodiments of the present invention, the flocculent concentration and dosage required to flocculate the zinc sulfate solution may depend on the zinc concentration of the solution to which the flocculent is added. For example, flocculation of a zinc sulfate solution with a high zinc concentration of say 300 g/L may form aggregates that settle more slowly than say a 15 solution containing 80 g/L zinc. Desirable flocculents are shown in Table 3 and Table 4. Table 3 Flocculent type Nalco Core@ Shell 71301 Working flocculent concentration 0.1% Dosage required 30 mL/L Delivery method Add the total flocculent volume to the zinc sulfate solution and stir gently at 150 rpm to allow the flocculent to disperse through the solution. Lower the stirring rate to 50-80 rpm until aggregates are well formed (15-20 seconds). Allow to settle.

WO 2010/096862 PCT/AU2010/000206 10 Table 4 Flocculent type Nalco 82205 Working flocculent concentration 0.3% Dosage required 70 mL/L Delivery method Add 25% of the total flocculent volume to the zinc sulfate solution and stir gently at 250 rpm to allow the flocculent to disperse through the solution. Allow to stir for 15 seconds and then add the remaining 75% volume and stir for another 10 seconds. Lower the stirring rate to 50-80 rpm until aggregates are well formed (0.75-1 minute). Allow to settle. Removal of the suspended floc from the zinc sulfate solution produces a clean zinc sulfate solution, which may pass to the next stage in the zinc oxide purification process for further treatment, and a settled residue. The suspended floc may be removed from solution by 5 decanting or filtering the solution using any suitable means. Preferably, the suspended floc may be separated from the zinc sulfate solution by decanting, filtering or by a combination of decanting and filtering of the solution at a temperature above 60'C to avoid undesirable crystallization of zinc sulfate compounds from solution. In a preferred embodiment (see Figure 2), the zinc sulfate solution may be flocculated 10 through a filter feed tank 4 and filtered through one or more pressure leaf filters 5 that have been selected to achieve a relatively low moisture level of, for example, around 10%. Optionally, the zinc sulfate solution may be flocculated through a thickener (not shown) equipped with an in-line flocculent addition (not shown). The resulting slurry is then transferred into a second thickener (not shown) where the solids may form aggregates 15 and settle. Any overflow from the second thickener may be fine filtered through any suitable process filter (not shown) to remove any suspended floc, and then possibly diluted to an appropriate zinc concentration desirable for processing in the subsequent zinc carbonate precipitation stage. Further processing of the settled residue may be beneficial if it is deemed to still contain 20 zinc sulfate solution or if it contains materials that may be extracted using other processes, for example, lead/zinc smelting.

WO 2010/096862 PCT/AU2010/000206 11 Optionally, it may be desirable to extract any remaining zinc sulfate solution from the settled residue by transferring the residue to a plate and frame filter (not shown) to form a filter cake, which may be air dried and furnaced to reduce the cake moisture to, for example, <10%. The zinc sulfate solution filtrate, on the other hand, may be treated to 5 remove any unwanted contaminants, fine filtered, for example, using a polishing filter (not shown), and then diluted to an appropriate zinc concentration desirable for further processing in the subsequent zinc carbonate precipitation stage. In a preferred embodiment, as shown in Figure 2, nickel as a contaminant may be removed from the zinc sulfate solution by adding sodium-DMG compound to the solution 10 in tank 6, where the mixture is stirred at 80'C. A high concentration of sodium hydroxide solution in the zinc sulfate solution may result in the undesirable formation of a basic zinc sulfate compound. To avoid the formation of this basic zinc sulfate compound, the stirred solution is passed through a heat exchanger 7 to raise the solution temperature to 950C. The solution is then transferred to a further tank 8, where more sodium hydroxide is added 15 to raise the solution pH to 3.8. The solution is then passed through a second heat exchanger 9 to reduce the temperature to <40'C. The pH is then increased to >5.0 through addition of more sodium hydroxide to precipitate the insoluble Ni-DMG complex. The zinc sulfate solution is then filtered using a plate and frame filter 10 and the filtrate transferred to a reactor feed tank 11. 20 Zinc Carbonate Precipitation A preferred embodiment of this aspect of the zinc oxide purification process involves reacting the filtered zinc sulfate solution with an aqueous metal carbonate to form zinc carbonate. Any suitable aqueous metal carbonate may be used to react with the zinc sulfate solution. Suitable aqueous metal carbonate solutions may be prepared comprising 25 an alkali metal carbonate, including but not limited to any one or more of the following alkali metals: sodium, potassium. In a preferred embodiment, the aqueous metal carbonate solution is a sodium-containing carbonate, more preferably, sodium carbonate. The chemical reaction between zinc sulfate solution and aqueous sodium carbonate produces solid zinc carbonate and aqueous sodium sulfate. The precipitation of zinc 30 carbonate from solution may be accompanied by the undesirable co-precipitation of one or more contaminants, including but not limited to contaminants containing one or more of the following: sulfates, sodium, magnesium, calcium, lead, nickel. Any sulfate contaminants present in solution originate mainly from the aqueous sodium sulfate byproduct formed during the reaction between zinc sulfate solution and aqueous sodium 35 carbonate, as well as from other zinc sulfate compounds that may have been introduced, WO 2010/096862 PCT/AU2010/000206 12 for example, as sulfuric acid, during the leaching stage or earlier. The amount of sulfate that co-precipitates with the zinc carbonate product may depend on the pH and temperature of the reaction conditions. In a preferred embodiment, co-precipitation of sulfate contaminants may be reduced by maintaining a solution pH of > 7.0 at low 5 temperature, for example, around 251C. Any sodium contaminants present in solution may originate from any one or more sodium containing compounds that have been introduced as, for example, sodium carbonate, during the precipitation stage or earlier. The amount of sodium that co-precipitates with the zinc carbonate product may depend on the pH and temperature of the reaction 10 conditions. In a preferred embodiment, co-precipitation of sodium contaminants may be reduced by maintaining a solution pH of < 7.0 at a raised temperature, for example, >601C. Any calcium contaminants present in solution may originate from the formation of two compounds: (i) calcite (calcium carbonate); and (i) disordered dolomite (calcium 15 magnesium carbonate), both of which may have been introduced during the feed stage or earlier. The amount of calcium and magnesium contaminants that co-precipitates with the zinc carbonate product may depend on the pH and temperature of the reaction conditions. In a preferred embodiment, calcium and magnesium co-precipitation may be reduced by maintaining a solution pH of < 7.0 at low temperature, for example, around 25'C. 20 Any lead contaminants present in solution may originate from any one or more lead containing compounds that have been introduced during the feed stage or earlier. The amount of lead that co-precipitates with the zinc carbonate product may depend on the pH and temperature of the reaction conditions. In a preferred embodiment, co-precipitation of lead contaminants may be reduced by maintaining a solution pH of < 7.0 at low 25 temperature, for example, around 25C. Any nickel contaminants present in solution may originate from any one or more nickel containing compounds that have been introduced during the feed stage or earlier. The amount of nickel that co-precipitates with the zinc carbonate product may depend on the pH of the reaction conditions. The amount of nickel contaminant in solution may be 30 reduced by manipulating the feed pre-treatment and leaching conditions, and by introducing DMG as a chelating or sequestering agent during the leaching stage. In a preferred embodiment, co-precipitation of nickel contaminants may be reduced by maintaining a solution pH of < 7.0 at low temperature, for example, around 25C.

WO 2010/096862 PCT/AU2010/000206 13 In a preferred embodiment of the zinc oxide purification process, sodium carbonate solution is added to the zinc sulfate solution in the reactor feed tank 11 at a constant temperature of 550C. The rate at which the sodium carb onate is added into the reactor feed tank 11 is ideally controlled to maintain a solution pH of 7.15. This allows the solid 5 form of zinc carbonate to precipitate, from solution to afford a zinc carbonate slurry. Maintaining a constant pH of 7.15 and a constant temperature of 559C may minimise the inclusion of small amounts of impurities contained within the resulting zinc carbonate slurry and is essential to producing a high purity zinc oxide product. Tests have shown that this process should produce crystals of sufficiently large size to obtain good recovery 10 using a suitable separating process, for example, centrifugation. Optionally, it may be desirable to incorporate a seed recycle step (not shown) at a later stage if a larger particle size is preferred. Since the co-precipitation of sulfate contaminants from the zinc carbonate slurry may have been reduced by maintaining the solution pH of > 7.0 at low temperature, for example, around 259C, it will be a ppreciated that a large amount of 15 sulfate contaminants may be present in the zinc carbonate slurry. Zinc Carbonate Separation and Washing Following the zinc carbonate precipitation reaction, the next stage in the zinc oxide purification process is to separate the zinc carbonate from the zinc carbonate slurry. In particular, it is desirable to separate the zinc carbonate product from the sodium sulfate 20 solution. The zinc carbonate product may be separated from the sodium sulfate solution using any suitable method. For example, separation may be achieved using a vacuum belt filter (not shown) or a Peeler centrifuge 13. As described in the zinc carbonate precipitation stage, maintaining the temperature around 559C may minimise the inclusion of small amounts of 25 impurities contained within the resulting zinc carbonate slurry. Similarly, maintaining the temperature in this range during the separation stage may reduce the amount of impurities in the separated zinc carbonate product and also aid settling. In a preferred embodiment, the zinc carbonate slurry from the zinc carbonate precipitation stage is collected in a slurry tank 12, where it may be stirred at a constant temperature of 30 559C prior to separation.The zinc carbonate slurry is th en fed in batches to a Peeler centrifuge 13 where the zinc carbonate crystals may be separated from the zinc carbonate slurry.

WO 2010/096862 PCT/AU2010/000206 14 Desirable separation conditions are shown in Table 5. Table 5 Achievable cake moisture: Vacuum belt filtration 32% Peeler centrifuge <32% Particle size retention Vacuum belt filtration 20jm Peeler centrifuge 5pm Zinc carbonate washing media Water Water temperature (C) 55 Wash volume (M 3 ) / tonne zinc carbonate cake 1.4 - 6.7 The isolated zinc carbonate crystals are ideally washed with, for example, with water and then fed to the zinc carbonate drying stage. 5 Following centrifugation of the zinc carbonate slurry, the remaining centrifugate, or mother liquor, is preferably collected in a precipitation tank 14 for calcium and magnesium removal. In a preferred embodiment, the solution is heated to around 100C and a suitable alkali metal carbonate solution, for example, sodium carbonate solution, is added to achieve a pH of 9.1. The solution may then be filtered on a plate and frame filter (15) to 10 recover the calcium magnesium carbonate contaminants for disposal. Zinc Carbonate Drying and Calcining The zinc carbonate crystals may be dried and bagged for storage, or they may be converted to the zinc oxide product directly. As shown in the preferred embodiment in Figure 2, the zinc carbonate crystals isolated 15 using the Peeler centrifuge 13 may be collected in any suitable receptacle, for example, a feed hopper 17 and fed to a suitable drying device, for example a flash drier 18 to dry the crystals. The dried zinc carbonate crystals may be then collected in a bag filter 19 for storage. The dried zinc carbonate crystals may be converted to zinc oxide by heating the crystals 20 to a suitable temperature over an appropriate length of time, where the main byproduct is carbon dioxide. Heating the zinc carbonate crystals at too high a temperature may WO 2010/096862 PCT/AU2010/000206 15 produce an adversely coloured zinc oxide product if certain contaminants are present in the crystals. Preferably, the zinc carbonate crystals are heated to a temperature in the range between 400 to 4409C to avoid undesirable discol ouration of the zinc oxide product. Good results have been obtained by heating the crystals at 420M for a period of 60 5 minutes. Heating the zinc carbonate crystals may be performed using any suitable heating method, including but not limited to any one of the following heating methods: fluidized bed reactor, directly heated calciner, flash dryer, batch electric furnace, rotary furnace. Preferably the zinc carbonate crystals are heated in a rotary calciner 20. The rotary action of the calciner 10 20 ensures that the zinc carbonate crystals may be heated uniformly. Desirable calcination conditions are shown in Table 6. Table 6 Sample size (kg) Time (min) Temperature (C) Calcinati on (%) 0.438 90 460-490 99.7 0.600 90 460-480 99.7 1.00 180 460-490 99.3 12.00 240 430-450 98.5 As shown in the preferred embodiment in Figure 2, the zinc carbonate crystals stored in the bag filter 19 are continuously fed to an indirectly heated rotary calciner 20 where the 15 zinc carbonate crystals are calcined at 4209C for a perio d of 60 minutes to form the zinc oxide product. The zinc oxide product isolated from the rotary calciner 20 may be collected through a rotary valve (not shown) and bagged in any suitable container, for example a bulk product storage bag 21. The ventilation gas comprising the carbon dioxide byproduct, is removed 20 from the rotary calciner passes through a bag filter (not shown) prior to discharge to the atmosphere. Sodium Sulfate Crystallization and Recovery As an aside to the zinc oxide purification process, it may be desirable to purify the sodium sulfate solution isolated during the zinc carbonate precipitation stage.

WO 2010/096862 PCT/AU2010/000206 16 In a preferred embodiment, the sodium sulfate solution may be transferred from the Peeler centrifuge 13 to a precipitation tank 14, where sulfuric acid is added to achieve a slightly acidic solution pH of, for example pH 6.5. A slightly acidic solution pH may help to prevent scaling of any impurities remaining in solution. Preferably, the precipitation tank 5 14 is maintained at a temperature of around 100'C to m aximize the removal of unwanted contaminants. The acidified sodium sulfate solution is then evaporated in a Mechanical Vapour Recompression (MVR) unit 16, where around 80% of the water is desirably removed. Evaporated water from the solution is desirably collected and recycled and the more concentrated solution exits this stage close to the saturation point of the sodium 10 sulfate. The concentrated solution may then be fed to an atmospheric crystallizer (not shown) where sodium sulfate is crystallized. . It will be appreciated by persons skilled in the art that the large magnitude of solution evaporation may raise the concentration of any contaminants present to significant levels. Accordingly, removal of calcium and magnesium contaminants prior to the sodium sulfate crystallization and recovery stage 15 may prevent fouling of the evaporation equipment. There may also be a quantity of chloride contaminants within the liquor, which may have originated from the feed and/or make-up water. Removal of any chloride contaminants may be achieved by operating the crystallizer at the saturation point of sodium chloride (265 g/L CI). To maintain and control the reaction, a purge stream is desirably removed from the system at a rate equal to the 20 mass of chlorides being entered into the reactor from the MVR unit 16. The chloride concentration within the reactor therefore remains constant. A stream from the crystallizer (not shown) may then be fed into a suitable separation device, for example, a batch operated pusher centrifuge (not shown) to dewater the sodium sulfate crystals present in the slurry, or may be first recovered using a thickener (not shown) before being fed to the 25 pusher centrifuge (not shown). The remaining filtrate may then be returned to the MVR unit 16 and the recovered sodium sulfate crystals transferred into a storage device, for example a feed hopper (not shown). It may be desirable to scrub the off-gas from the drier using a wet scrubber (not shown) prior to venting it to the atmosphere. Now that a preferred embodiment for a method of purifying a zinc sulfate solution in the 30 production of zinc oxide has been described it will be apparent to those skilled in the art that it has the following advantages: 1. The zinc oxide purified by the process of the present invention is of an exceptionally high grade compared to zinc oxide produced by known hydrometallurgical processes; 35 2. All major contaminants within the feed are removed during the purification stage and hence are not included within the final zinc oxide product.

WO 2010/096862 PCT/AU2010/000206 17 3. The purification stage occurs over two simple steps and uses cost effective reagents adding to the commercial viability of the invention. 4. The use of a zinc residue to reprocess into a high grade zinc oxide. It will be appreciated by persons skilled in the art that numerous variations and/or 5 modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. A method of purifying a zinc sulfate solution comprising mixing the zinc sulfate solution together with an aqueous metal carbonate to produce a zinc carbonate slurry. 5

2. A method of purifying a zinc sulfate solution in the production of zinc oxide, said method comprising the steps of: mixing the zinc sulfate solution together with an aqueous metal carbonate to produce a zinc carbonate slurry; separating zinc carbonate from the zinc carbonate slurry; and 10 heat treating the zinc carbonate to produce the zinc oxide.

3. A method of purifying a zinc sulfate solution as defined in either of claims 1 or 2 wherein the step of mixing the zinc sulfate solution together with an aqueous metal carbonate comprises controlling the rate of addition of the aqueous metal carbonate. 15

4. A method of purifying a zinc sulfate solution as defined in claim 3 wherein the step of mixing comprises controlling the rate of addition of the aqueous metal carbonate so that the mixing occurs at a pH of between 5 to 9.

5. A method of purifying a zinc sulfate solution as defined in claim 3 wherein the step of mixing comprises controlling the rate of addition of the aqueous metal carbonate 20 so that the mixing occurs at a pH of between 6 to 8.

6. A method of purifying a zinc sulfate solution as defined in claim 3 wherein the step of mixing comprises controlling the rate of addition of the aqueous metal carbonate so that the mixing occurs at a pH of between 7 to 7.2.

7. A method of purifying a zinc sulfate solution as defined in claim 6 wherein the step 25 of mixing the zinc sulfate solution together with an aqueous metal carbonate comprises controlling the rate of addition of the aqueous metal carbonate so that the mixing occurs at a temperature of between 509C to 609C and at a pH of between 7 to 7.2.

8. A method of purifying a zinc sulfate solution as defined in any one of the preceding 30 claims wherein the aqueous metal carbonate comprises one or more types of alkali metal carbonates. WO 2010/096862 PCT/AU2010/000206 19

9. A method of purifying a zinc sulfate solution as defined in claim 8 wherein the one or more types of alkali metal carbonates include those of sodium, potassium.

10. A method of purifying a zinc sulfate solution as defined in any one of the preceding claims further comprising a preliminary step of removing one or more 5 contaminants from the zinc sulfate solution.

11. A method of purifying a zinc sulfate solution as defined in claim 10 wherein the step of removing one or more contaminants from the zinc sulfate solution includes the addition of an alkaline solution and an oxidant in a temperature range of between 50'C to 100'C. 10

12. A method of purifying a zinc sulfate solution as defined in claim 11 wherein the alkaline solution comprises one or more types of alkali metal cations, and the oxidant is selected from the group consisting of: hydrogen peroxide, sodium hypochlorite, potassium permanganate, oxygen.

13. A method of purifying a zinc sulfate solution as defined in claim 12 wherein the one 15 or more types of alkali metal cations include those of sodium or potassium and the oxidant is potassium permanganate.

14. A method of purifying a zinc sulfate solution as defined in any one of claims 10 to 13 wherein the one or more contaminants includes nickel, aluminium, iron, calcium or manganese. 20

15. A method of purifying a zinc sulfate solution as defined in claim 14 wherein the step of removing nickel from the zinc sulfate solution comprises adding a chelating agent material.

16. A method of purifying a zinc sulfate solution as defined in claim 15 wherein the chelating agent material is dimethyl glyoxime. 25

17. A method of purifying a zinc sulfate solution as defined in claim 2 wherein the step of separating zinc carbonate from the zinc carbonate slurry involves centrifuging the zinc carbonate slurry.

18. A method of purifying a zinc sulfate solution as defined in claim 2 wherein the step of heat treating the zinc carbonate to produce the zinc oxide involves calcining the 30 zinc carbonate at a temperature of between 4009C to 4 409C. WO 2010/096862 PCT/AU2010/000206 20

19. A method of purifying a zinc sulfate solution as defined in claim 18 wherein the step of calcining the zinc carbonate occurs at a temperature of 420'C.