[0001]This invention relates to an improved process for recovering zinc from a wide range of oxidized zinc materials using a novel hydrometallurgical process to recover the zinc as high purity controlled surface area zinc oxide.

[0002]The hydrometallurgical process includes liquor purification and a carefully controlled crystallization step such that the zinc reports as high purity zinc oxide suitable for use directly in commercial applications such as rubber production and/or agriculture or for zinc metal production via dissolving in acid and electrowinning.

[0003]This invention can be used for almost all oxidized zinc materials including oxidized ores such as those containing significant contents of smithsonite and/or hemimorphite and/or

Willemite and is particularly suited to treating zinc containing by product streams such as those arising from Electric Arc Furnace steelmaking, zinc galvanizing and zinc electroplating. For materials where some zinc is present in difficult to leach refractory compounds a

pyrometallurgical step can be incorporated into the process to free up this zinc such that it can also be recovered.

Background of the Invention.

[0004]The current dominant process for producing zinc oxide is to volatilize zinc from zinc metal and oxidise it in the gas stream to give fine particulate zinc oxide. The economics of the process are therefore largely driven by the cost and availability of acceptable quality zinc metal. The most common method of producing the zinc metal needed is to mine zinc sulphide mineral ores, produce a zinc sulphide concentrate (ZnS), which typically contains some iron and lead as impurities, and then treat this concentrate to make zinc metal.

[0005]Another important source of zinc metal is to recover zinc from EAF Dust arising during the processing of galvanized steel scrap. This is achieved by treating the EAF Dust pyrometallurgically, most commonly in Waelz Kilns, to fume off the zinc which reports as a crude zinc oxide (CZO). This CZO typically contains around 60% Zinc but is contaminated by metallic impurities such as lead, cadmium, calcium, sodium and potassium and by non metals such as chlorine and Sulphur and cannot be used directly but has to be converted to zinc metal via electrowinning. This conversion is also complex because of the impurities and the CZO is therefore usually less favoured than conventional zinc sulphide concentrate for zinc metal production.

[0006]A third source of zinc metal is from acid leaching of zinc silicate ore with the pregnant liquor being upgraded through a solvent extraction step into an electrolyte from which the zinc is electrowon. This acid leach process has proven economic for one large operation where the ore does not consume excessive acid but has not been viable for smaller deposits and where the ore contains significant levels of acid consuming gangue minerals such as calcium and/or magnesium carbonates. A recent variation of this process is to use this acid leach - solvent extraction - electrowinning technology to process CZO from Waelz kilns to produce zinc metal.

[0007]The need to use these zinc metal production processes to provide zinc metal for zinc oxide production means that the overall zinc oxide production is complex, energy intensive and high cost. The process which is the subject of this invention provides a much more energy efficient and lower cost means of producing zinc oxide with a range of purities and surface areas which can be tailored to suit the specific commercial application of the zinc oxide. It also enables processing of zinc feed materials that are unable to be economically processed using the current technology.

Summary of the Invention

[0008]This invention uses a novel crystallisation process to enable production of zinc oxide powder with controlled purity and surface area with these being tailored to suit the end use of the powder. In most but not all cases the zinc oxide will be high purity high surface area powder suited for use in vulcanizing rubber for use in car tyres or similar applications and/or for latex for use in rubber gloves and other similar applications.

[0009]The crystallization process is carried out from high purity zinc containing liquor which is used as the source of the zinc crystals. This liquor is a purified zinc-ammonia-ammonium chloride-water pregnant liquor typically containing from 0.3 to 1M/1 Zn, from 1 to 5M/1 H3 and from 0.5 to 2.5 M/1 NH4C1. The liquor contains only very low levels of metallic ions such as copper, cadmium and lead which can precipitate with the zinc but can tolerate higher levels of soluble cations such as calcium, sodium and potassium. The liquor can also contain some soluble anions such as carbonate and sulphate but these need to be at such a level that they do not affect the efficiency of the process nor precipitate with the product and/or cause build ups in the process equipment.

[OOlOjThe zinc containing crystals are precipitated from this liquor by stripping off the ammonia. The change in solubility of the zinc in the liquor with changes in ammonia concentration is shown in Figure 1 for the ammonia concentration range under which this invention operates. The ammonia is typically stripped off by heating the liquor sufficiently to volatilize off the ammonia whilst controlling the liquor temperature within 10C around a set point which is within the overall range from as low as 60C to as high as 120C. The set point is largely controlled by maintaining the liquor at boiling point where the boiling point predominantly depends upon the operating pressure of the system.

[001 l Even lower temperatures can be used for ammonia stripping such as using air stripping such as suggested by Frada and which is used in processing waste streams but this introduces difficulties in achieving high rates of ammonia removal and/or with recovering the stripped ammonia. Higher temperatures above 120C are also possible through operating at above atmospheric pressure but this introduces complexity and cost through needing specialized materials of construction and pressure vessels.

[00l2]In carrying out this invention the ammonia stripping can be carried out in any one or a combination of different systems ranging from vacuum assisted stripping in bubble cap columns to stirred reactors operating under pressure with direct steam injection through to membrane distillation. The common features are that some heat input is required, either directly by steam injection or indirectly via heat exchanges, to provide the heat required to evaporate off ammonia and associated water, and that the crystallization temperature is largely set by the operating pressure with a lesser contribution from the liquor composition.

[00l3]The form of the zinc crystals obtained depends mainly upon the settings used for three parameters which are the crystallization temperature, the total amount of chloride present and the ratio of NH3 to Zn in the liquor. The chemistry of the Zn-NH3-Cl-H20 is quite complex as there are a number of possible crystal types with quite similar solubility limits and hence the crystal type can change with only fairly minor changes in conditions. The individual effects of these are:

[OOl4]Zn(OH)2 is less stable than ZnO at higher temperatures with the transformation point being dependent upon the exact conditions but is typically in the temperature range from 60-80C. Lower temperature favours the formation of crystals of zinc hydroxide (Zn(OH)2) and/or zinc chloride hydroxide monohydrate [Zn5(0H]8C12.H20] which is also often described as zinc hydroxychloride (ZnOHCl) or tetra basic zinc chloride (TBCZ). Lower chloride levels

(<2.5M/L) favour the formation of ammonia free crystals such as zinc hydroxid and/or zinc hydroxychloride and/or Zinc oxide over zinc diammine chloride (ZDC).

The formation of the zinc hydroxychloride in preference to the chloride free zinc hydroxide and/or zinc oxide depends upon both the temperature and the ratio of H3 to Zn in the liquor. The total amount of chloride in the liquor in the form of H4C1 also has a significant affect.

[00l5]In one embodiment of the invention the liquor with <2.5mol/l chloride present has the ammonia stripped off at temperatures above 85C and preferably above 95C with zinc levels of above 0.5 in the pregnant liquor such that the zinc is precipitated directly as zinc oxide. This zinc oxide is characterized by having relatively low surface area (typically <2m2/g) a high bulk density (>l.5g/cc) and low tendency to generate dust during handling. This zinc oxide is particularly suited to use in applications such as ceramics where the high bulk density and low dust generation are advantageous. This production method is much more economic than the current method of making high bulk density surface area zinc oxide where zinc oxide produced by zinc metal volatilization (The French Processes then calcined to reduce the surface area and increase the bulk density.

[00l6]In a further embodiment the precipitation is carried out at low temperatures (<80C and preferably <65C) by stripping the ammonia out by boiling the liquor whilst it is held under vacuum. Under these conditions the crystals formed contain little or no zinc oxide and are primarily either zinchydroxychloride or zinc hydroxide or a mix of the two depending on the concentrations of zinc, ammonia and chloride in the solution.

[00l7]The main chemical factors governing the type of crystal formed at any set temperature are the concentrations of zinc and chloride in the system. In general higher chloride levels favour zinchydroxychloride formation whilst higher Zinc levels favour Zn(OH)2 and/or ZnO formation. The ammonia level is also important as it controls the amount of zinc in the solution for any given chloride level when the chloride is present predominantly as ammonium chloride.

[00l8]The ammonia stripping can be carried out under a range of conditions dependent upon the desired properties of the zinc containing crystals. The preferred strip system is the use of a steam heated stripping column operating under vacuum at a temperature of <80C as shown in Figure 7. This has been found to produce crystals of a preferred composition and morphology for converting to high purity high surface area zinc oxide. The ammonia is stripped down to <lM/l and more preferably <0.3M/l to maximize the zinc yield without using excessive energy.

[OOl9]Other ammonia stripping systems can be used such as air stripping and stripping in stirred reactors but these are more energy intensive and are only preferred if there is low cost energy available such as through low cost coal or natural gas, or waste heat from a smelting operation or an electricity power plant and/or solar heating.

[0020]The crystals can then converted to high purity high surface area ZnO by hydrolysis and or thermally during a drying step. Where the crystals contain significant amounts of zinchydroxychloride the hydrolysis is preferably carried out using NaOH at ambient temperatures at a sufficient concentration that the OH ions displace the Cl ions in the crystal lattice. The concentration is related to the total chloride level in the crystals and is typically of the order of 1.0-2.0 times the stoichiometric required for the hydroxide ions to replace the chloride ions in the crystals..

[0021]An alternative hydrolysis route is to slurry the crystals in hot water at temperatures high enough (typically>80C) that the chloride ions are displaced by oxygen from the water. This procedure has the advantage of not requiring the use of NaOH but does give a lower surface area product and commonly leaves a product with some residual chloride. This hydrolysis system is particularly suited to producing ZnO for use in agricultural applications where the lower surface area and the presence of some residual chloride (typically 0.1-0.75%) is not detrimental to the application.

[0022]A further alternative hydrolysis method is to reslurry the crystals and add Ca(OH)2 to drive the substitution of the chloride ions by the hydroxides. This method has significant cost advantages over the use of NaOH and can give high surface area ZnO but can result in some contamination of the product with Calcium. This calcium can be removed by subsequent treatment and/or the product can be sold for use in applications where the presence of some Calcium impurity is not of concern.

[0023] Where the crystals are directly produced as Zn(OH)2 and/or converted into Zn(OH)2 by the hydrolysis with NaOH the Zn(OH)2 is readily converted to ZnO by drying at temperatures above 80C such as is commonly achieved within a spray dryer. [0024]In the embodiment where the crystals precipitate as zinchydroxychloride the crystals are plate like with a chloride content of around 12-14%. These crystals can then be separated, washed and sold for use in animal feed where they have been found to have excellent release properties and give improved performance compared to other sources of zinc. In this

embodiment it is not necessary to carry out an hydrolysis step.

[0025]Where the desired product is zinc oxide these zinchydroxychloride crystals can be hydrolysed using a hydroxide such as sodium hydroxide or calcium hydroxide or just using hot water to replace the chloride ions in the lattice by oxide ions. This hydrolysis also disrupts the structure of the crystals and markedly changes the morphology and hence properties such as particle size, surface area and bulk density. The use of hydroxide such as sodium hydroxide to drive the hydrolysis gives a quite different product to that obtained when hot water is used.

[0026]Without being bound by the theory it is believed that the hydroxide acts by displacing the chloride ions by a hydroxide ion. Hydrolysing the zinchydroxychloride using the hydroxide at low temperatures (<50C) gives a zinc hydroxide product. This can then be converted to zinc oxide by heating to >80C either in water, or the hydrolysis solution, or by separating from the liquor and converted during drying in air at temperatures above around 80C. This low

temperature hydrolysis gives a very high surface area zinc oxide (typically >12m2/g) with low levels of residual chloride.

[0027]Hydrolysing the zinchydroxychloride with hydroxide at higher temperature (>70C) gives crystals with a different morphology characterized by having a surface area of between 4m2/g and l2m2/g depending on both the exact temperature and the concentration of hydroxide ions present.

[0028]Without being bound by the theory it is believed that under these conditions the hydroxide ions present replace the chloride ions and then convert to oxide while still in contact with the hot hydroxide solution. In parallel with this reaction the temperature is high enough to drive the replacement of some of the chloride ions by oxide ions from the water. The two mechanisms effectively compete to replace the chloride ions with the hydroxide reaction being favoured by higher concentrations of hydroxide and the water reaction being favoured by higher temperatures. [0029]The balance between the two reactions is important in determining the properties of the zinc oxide product. The caustic driven substitution has been found to give higher surface area zinc oxide than the high temperature water substitution. By balancing the reactions through controlling the hydroxide level and temperature in the range 60-100C zinc oxide products with controlled surface areas in the range 4-l2m2/g can be produced.

[0030]In a further embodiment the hydrolysis is carried out just using hot water at >80C which is sufficient to drive substitution of the chloride by oxide ions to give zinc oxide. Under these conditions the crystals produced typically have a surface area in the range l .5-4m2/g and most commonly around 2-2.5m2/g. This product is characterized by having a high bulk density and a low propensity to forming dust during handling and is therefore particularly suited to

applications in the ceramic industry.

[003 l]In a further embodiment the crystal product from precipitation is a mix of

zinchydroxychloride and Zn(OH)2 and ZnO with a chloride content typically in the range 2- lO%Cl. This product can then be sold as is for some applications or more commonly is hydrolysed to give ZnO. This mixed product can occur in situations where the preferred liquor composition required for other parts of the process is such that it is not possible to control the crystallization sufficiently that a single crystal type can be produced.

[0032]With this mixed product it can be advantageous to minimize the amount of

zinchydroxychloride present in the case where hydrolysis is to be carried out using a base such as sodium hydroxide as the amount of hydroxide needed depends upon the amount of chloride to be removed. Having the lower chloride level improves the economics of the process especially in configurations where sodium hydroxide is used and the liquor from the hydroxide hydrolysis is not recycled within the process. This results in a loss of both the sodium and chloride from the process.

[0033]The chloride level may not be so important where calcium hydroxide is used to drive the hydrolysis as the calcium concentration in solution can be controlled through precipitation as the carbonate or the sulphate which is not possible with sodium. This can enable recycle of the chloride and in cases where the zinc feed contains carbonate can be advantageous in controlling carbonate levels in the process liquors. [0034]The characteristics of the different crystals are important in establishing their value and their target end use. ZDC is primarily used as a flux in galvanizing and can also be used as an additive in fertilizer as a source of zinc zinchydroxychloride is primarily used in agriculture as an animal feed supplement and/or as a fertilizer additive. ZnO has a wide range of applications with the major one being in rubber as a vulcanization agent but there is significant use in agriculture, ceramics and the production of a range of other chemicals

[0035]The pregnant liquor which is provided to the crystallization step can come from direct leaching of suitable oxidized zinc feeds and/or from feed materials where part or all of the feed has undergone pyrometallurgical treatment to free up any refractory, non leachable, zinc that may be present.

[0036]The most suitable feed materials for direct leaching are high grade Electric Arc Furnace Dusts (EAFD), where the majority of the zinc is present as zincite, oxidic wastes from galvanizing operations and zinc ores where the majority of the zinc is present as smithsonite and/or hemimorphite. The zinc in these materials readily dissolves into the ammonia - ammonium chloride liquor. For these the leaching can be carried out with the material in powder form in stirred reactors or even as coarser material in static reactors and/or in heaps and/or dumps of the types commonly used for leaching gold and other metals.

[0037]For material with significant amounts of soluble carbonate present such as for smithsonite ore the carbonate that enters the liquor this can readily be removed by precipitation with calcium either during the leach step and/or by an additional purification step carried out on the zinc rich liquor. The direct ore leach is favoured for low grade ore where the leaching can be carried out in situ and/or in dumps and/or in heaps such as are commonly used in the copper industry.

[0038]For higher grade ores where the zinc is mainly present as zinc carbonate (smithsonite) a low temperature roast may be used to decompose the zinc carbonate to zinc oxide before carrying out the leach step. The zinc can be leached without this thermal pretreatment but carbonate also enters the liquor and this can decrease the efficiency of subsequent processing. A liquor with reasonably low (<0.03M/1) carbonate is preferred for feeding into the ammonia stripping - crystallization stage.

[0039]Most other attempts at developing a commercial process have included a roasting step to roast the ore prior to leaching to decompose the metal carbonates into oxides. The decision on whether or not to include the thermal pretreatment step or to control carbonate level by precipitation is driven by economics for any particular ore and location. The roasting step is both CAPEX and OPEX expensive especially for small tonnage operations and/or low grade ores where significant amounts of ore need to be processed for each tonne of ZnO product.

[0040]For refractory materials containing significant amounts of franklinite and/or Willemite the pyrometallurgical step most commonly used involves heating a mix of the zinc containing material and a carbon source to temperatures in excess of 1000C to fume off the zinc as zinc metal which then oxidises in the gas stream to produce a crude zinc oxide which typically contains from 55% to 65% zinc.. This is most commonly done in a rotary kiln such as a Waelz Kiln but other furnace types such as Rotary Hearth Furnaces can also be used.

[0041]This pyrometallurgical process is particularly suited for treating Electric Arc Furnace Dust (EAFD) directly or for treating the residue from EAFD which has been leached using the ammonia-ammonium chloride liquor as described in this invention. The CZO from the Waelz Kiln and/or Rotary Heart Furnace is an ideal zinc feed for this invention.

[0042]For feed material such as Electric Arc Furnace Dust and mixed smithsonite - willemite ores where the zinc content is quite high being typically over 15% and is present in a mix of readily leached and refractory compounds the preferred approach is to initially leach the feed to extract the readily leachable zinc and then pass the residue to a pyrometallurgical step to convert the majority of the remaining zinc to a more leachable form.

[0043]In most cases the pyrometallurgical step used is a Waelz Kiln or Rotary Hearth Furnace where the zinc is volatilized off and recovered as leachable CZO. In selected cases where the feed material contains significant quantities of iron such as with leach residue from direct leaching EAF Dust the pyrometallurgical step can be an Electric Arc Furnace such as the one where the zinc originated from. This arrangement using the electric arc furnace for the pyrometallurgical treatment of the residue is particularly beneficial as much of the iron present in the EAF Dust is recovered further improving the economics and reducing the amount of waste generated. In this case the zinc reports back as EAF Dust rather than CZO and is directly leached again to recover much of the zinc. This in effect gives a circular largely closed loop process for the zinc. [0044]For EAF Dust containing significant amounts of lead the residue can be further processed to recover the majority of this lead before recycling to the electric arc furnace. This lead removal both gives a marketable lead product and also decreases potential environmental problems through building up lead concentrations in the EAF Slag. This lead removal is done by leaching the lead using a known lead solvent such as acetic or citric acid or their salts.

[0045]The leach and purification steps used for the zinc feed material depends upon what elements besides zinc that are present in the material. For materials that have water soluble impurities such as the chlorides of sodium, potassium it is preferable to wash out the majority of these compounds prior to the leaching with the ammonia-ammonium chloride lixivant. Without this prewash these elements build up in the liquor and ultimately decrease the efficiency of the process.

[0046]The sodium and potassium are particularly important impurities as these are very difficult to remove from the process liquor given their extreme solubility. The water wash may also remove a substantial amount of the calcium if it present as the chloride. The removal of the calcium and chloride in the water wash is less critical as these can be controlled by removal in other precipitation steps.

[0047]The oxidic zinc feed material is leached in the ammonia-ammonium chloride liquor to dissolve out much of the zinc. The fraction of the zinc extracted is related to the zinc concentration in the feed and the zinc compounds present. The leach system used readily dissolves zinc oxides, zinc chlorides and zinc carbonates with very high extraction levels with physical factors being the main limitation which can prevent full dissolution.

[0048]Hydrated zinc silicate (hemimorphite) is also leachable but the total extraction can vary depending upon the exact form of the compound and the amount of the more refractory zinc silicate willemite which is incorporated in the structure.

[0049]The more refractory compounds zinc silicate (usually as the mineral willemite) and zinc ferrite ( as found in EAF Dust and in ore as the mineral franklinite) are much more difficult to leach in the ammonia-ammonium chloride system and typically give <10% zinc extraction and need more aggressive leaching conditions such as using hydrochloric acid and/or pyrometallurgical treatment to become leachable. [0050]The ammonia - ammonium chloride leach is conducted with solid/liquid ratios that are high enough that the liquor reaches close to saturation point with respect to zinc such as to maximize the yield of zinc during subsequent stripping of the ammonia. This saturation can be achieved directly either from the leach or through subsequent addition in a separate leach step of high grade leachable zinc materials such as high grade smithsonite ore and/or secondary materials such as CZO from a pyrometallurgical step and/or galvanizing byproducts.

[0051]The leach slurry undergoes solid-liquid separation to give a pregnant liquor and a zinc depleted leach residue. The leach residue is washed to recover reagents and make it environmentally acceptable and then either disposed of in a suitable facility such as a tailings dam or if there are economic levels of zinc and/or iron present transferred to a suitable pyrometallurgical process where it is treated thermally to release the zinc present in a leachable form. This is most commonly through the use of a Waelz kiln or similar system to generate a crude zinc oxide which is then returned to the hydrometallurgical circuit where the zinc is leached either directly with the fresh EAF Dust or more commonly added as a sweetener to increase the zinc concentration in the pregnant liquor.

[0052]The pregnant leach liquor requires purification to remove other metals and unwanted soluble calcium and/or carbonate and/or sulphate before stripping off the ammonia to precipitate out the zinc crystals. The two main techniques used for this are cementation with zinc metal to remove cadmium, copper and lead, and using control of the calcium level to also control the level of carbonate and sulphate within the liquor.

[0053]The cementation step is quite well known in the industry with zinc metal displacing other metals in accord with their relative electrochemical potentials. There are a range of methods for carrying out this step but for liquor such as is generated by the ammonia - ammonium chloride leach the simplest procedure is to add zinc powder in a stirred reactor and then filter off the solid residue which contains the unwanted impurities.

[0054]The control of the calcium, carbonate and sulphate levels is somewhat more complex and is dependent upon the zinc feed material being used and on the availability and cost of reagents especially lime and/or calcium chloride.

[0055]In some cases where the feed contains soluble calcium and/or carbonate and/or sulphates additional purification is carried out to avoid contamination and also to maintain a high efficiency process. This purification is achieved by maintaining the calcium concentration at a level where it is not so high as to interfere with the zinc solubility or to contaminate the product but is sufficient that the majority of any carbonate or sulphate present precipitates from the liquor before it passes to the ammonia stripping step. This calcium level is preferably maintained in the range of 0.01 - 0.1M/1 with the calcium either coming directly from the feed material or through addition of calcium compounds such as calcium chloride and/or lime. Where there is excess calcium present in the feed the concentration is reduced by additions of a carbonate source such as carbon dioxide or zinc carbonate preferably in the form of smithsonite ore.

[0056]A mixed ammonia - ammonium chloride liquor can dissolve metal carbonates such as the zinc carbonate found as the mineral smithsonite but this leaching adds carbonate into the liquor as part of the leach reaction. This carbonate then ties up part of the free ammonia and reduces the solution pH and the metal solubility is likely to be lower than compared with what is obtained when leaching oxides. This carbonate leaching causes a build up of carbonate in the system which as it builds up further reduces the leaching efficiency

[0057]The carbonate entering the solution is also a major disadvantage for processes which use stripping of the ammonia to precipitate the target metals by reducing their solubility. The lower concentration of free ammonia in these liquors due to the carbonate present, as reflected in their having a lower pH, makes stripping the ammonia more difficult. The main affect of this is the stripping requires either far more air flow, when air stripping, and/or far more energy when steam stripping or boiling. The amount of water reporting with the ammonia also increases making the subsequent recovery of the ammonia from the gas stream for recycling more complex and costly.

[0058]The presence of carbonate in the leach pregnant liquor precludes the use of lime to increase the liquor pH and free up ammonia to improve the ammonia stripping efficiency as the added lime will react with the carbonate present to precipitate calcium carbonate and

contaminate the zinc crystals which form due to both the lime addition and due to the removal of the ammonia.

[0059]This need to roast the ore prior to processing has major disadvantages both in the need for expensive equipment and energy for the roasting but also because it precludes the use of a preferred simple heap leach system for coarser ore. It also precludes leaching coarsely crushed ore using vat leaching (either continuous or batch) and the use of in situ leaching such as is commonly carried out for metals such as gold, copper and uranium.

[0060]This invention enables these simpler leach systems to be used by providing an economic means of controlling the amount of carbonate leached without incurring the major cost penalties associated with mining and presenting large tonnages of ore to a roasting step and of the roasting itself.

[0061]In this invention the leaching step is carried out at ambient, or slightly higher, temperatures with only low to moderate levels of ammonia present, preferably <4.4M/1 and more preferably <3.5M/1 such that there is limited volatility of that ammonia. With heap leaching the heap is arranged with a cover system such that minimal ammonia escapes as volatiles to minimize both any environmental affects and also costs of requiring replacement ammonia additions. For all other leach systems sealed vats and/or reactors are used to minimize any escape of ammonia.

[0062]The calcium additions are made at strategic points within an overall flowsheet such that when possible the calcium used gives additional benefits beyond just precipitating carbonate as calcium carbonate to control the carbonate concentration. Typically but not essentially this calcium would be added as CaO as either quicklime or slaked lime or as calcium chloride or as some combination of these.

[0063 ]In one embodiment the carbonate control is achieved by having some calcium chloride present in the leach liquor as it comes into contact with the ore in either heap leaching or reactor leaching and then adding some further calcium to the leach liquor either after it has been in contact with the ore such as would usually be the case with heap leaching and may be the case after reactor leaching or added directly to the leach reactor. In heap leaching it may be disadvantageous to precipitate too much calcium carbonate by reactions within the heap as the precipitate may fill up voids within the heap and decrease the permeability and hence the flow of the leach liquor.

[0064]The addition of calcium to the system precipitates the carbonate as calcium carbonate because of the calcium carbonate’s low solubility. This calcium carbonate is then separated from the liquor typically by filtration. Where the addition is to leach liquor the calcium carbonate may be sold or roasted to regenerate lime or where it is to the reactor it can be disposed of as part of the residue stream.

[0065]The amount of calcium added is matched to the amount of teachable zinc carbonate in the ore such that the desired amount of carbonate is precipitated during the leach step. For the heap leach arrangement the target is to precipitate sufficient carbonate that the solution is still capable of dissolving all of the available zinc but not so much that the precipitate formed fills all of the voids in the heap and prevents acceptable flow of the leach liquor.

[0066]For the reactor leach system where the precipitate formed remains with the leach residue the situation is quite different and a small excess of calcium is preferred such that the level of carbonate remaining in the liquor is very low and little or preferably no further carbonate precipitation occurs within the process.

[0067]After the leach step further calcium can be added in a separate carbonate removal step to ensure that there is only a very small amount of carbonate remaining in the liquor which is fed to the ammonia stripping section. Any carbonate remaining in that liquor is likely to precipitate out during the ammonia stripping, especially if further calcium is added, and this will contaminate the final product.

[0068]It is also important not to add too much calcium into the leach liquor as excess calcium present is likely to interfere with the zinc leaching as it can compete with zinc for the reactant species and ultimately reduce the amount of zinc in the solution. As the residue from the calcium treatment of the leach liquor is filtered off any unreacted or precipitated zinc in these solids is a loss from the circuit. In practice the preferred system is to add sufficient calcium that there is a small amount preferably <0. l3M/l and even more preferably <0.05M/1 remaining in the pregnant liquor from the leach step.

[0069]This level of calcium in the liquor also serves to control the sulphate levels where any excess sulphate present precipitates out of solution as calcium sulphate. This sulphate precipitation ideally occurs within the leach stage such that the sulphate reports with the residue.

[0070]In another embodiment this invention makes use of calcium carbonate precipitation to control the amount of calcium in solution to allow leaching of any materials containing soluble calcium and/or to enable the use of calcium to assist in stripping ammonia and precipitating zinc, and also in hydrolysis of any chloride containing zinc compounds within the precipitated zinc crystals.

[0071]In the case where excess calcium is present in the liquor after the leach step either through the feed having soluble calcium compounds and/or due to the additions needed in subsequent process steps then carbonate is used to control the calcium levels control by adding a source of carbonate or by adding carbon dioxide directly to the liquor. The carbonate source is preferably a metal carbonate, such as zinc carbonate contained in smithsonite ore, and the carbon dioxide source is preferably from burning calcium carbonate and/or zinc carbonate such that value is achieved from both the carbon dioxide and the residual solids.

[0072]Preferably at least part of the calcium required for carbonate control is also utilized to carry out other functions in the process. The leach liquor is therefore not the only possible primary addition point for the calcium but it can receive secondary calcium which has been added elsewhere in the process which is then recycled within other process streams. At least three other primary addition points within the process can be used to add calcium to the carbonate removal step, if one is included, and to the ammonia stripping circuit and in the hydrolysis step. In some cases all of the calcium needed may be added at any one of these points but more typically additions will be made into all of these steps.

[0073]The use of lime additions to provide free ammonia for stripping from the liquor is well established technology both in treating waste waters and for process liquors. The lime increases the solution pH and releases NH3 from NH4CL and/or other ammonium compounds and this markedly increases the amount of free H3 present in the liquor. This in turn greatly increases the ease of stripping the NH3 allowing lower levels to be achieved with much less energy than would otherwise be the case. This stripping of ammonia reduces the solubility of zinc in solution causing it to precipitate out of solution. The calcium additions themselves can also reduce the zinc solubility and further help with the precipitation of the zinc. This precipitated zinc is desired to give the final zinc product from the process.

[0074]The lime can be added directly as solid quicklime (CaO) but more typically is slaked with either water or process liquor prior to addition to improve the efficiency of its utilization. The use of process liquor is particularly beneficial as this minimizes the amount of water added to the process and helps in maintaining the overall water balance. [0075]The lime can be added either at the commencement of the stripping step or stagewise throughout the stripping depending on the system configuration. Where the ammonia stripping is carried out in a column this may utilize a reservoir system part way down the column to enable the lime to be mixed in and the combined solution is then returned to the column. In some cases use may be made of more than one column with the lime being added to the liquor at the pumping stages between the columns.

[0076]Figure 1 shows the effect of the NH3 concentration on the zinc solubility for a range of different chloride levels varying from 0.56M/1 to 1.4M/1. Under these conditions the zinc concentration varies from as low as 0.12M/1 to as high as 0.7M/1. When operating at 0.93M/1 Chloride and stripping the ammonia by boiling it off at atmospheric pressure and temperatures >80C the crystals formed for zinc concentrations below around 0.38M/1 are predominantly zinchydroxychloride whereas for zinc concentrations above 0.42M/1 the crystals are

predominantly ZnO.

[0077]Figure 2 shows how the solubility of zinc changes with temperature for aqueous solutions of Zn-NH3-NH4Cl. The very small temperature dependence confirms that cooling these liquors is not sufficient to precipitate zinc compounds and that the NH3 stripping is the preferred method. They also show that the leaching temperature is unimportant in respect of solubility and is only important if the temperature changes the leach kinetics.

[0078]Figure 3 shows an overall process for producing controlled surface area high purity ZnO (BET>2-l5M2/g, Purity >99.5% ZnO) from an oxidic zinc feed such as

crude zinc oxide (CZO) as is produced in Waelz Kilns. This processed to give high surface area high purity ZnO for use in the rubber industry. The CZO may be either as produced from the Waelz Kiln or washed material where the majority of water soluble impurities such as the chloride compounds have been removed by washing with water and/or a sodium carbonate- water mix. The CZO typically contains around 55-65% zinc as the oxide with lesser amounts of metallic impurities such as lead and cadmium. The oxidic zinc solids (1) are fed to a stirred reactor (2) such as is commonly used in the minerals industry for leaching. A liquor containing around 3M/1 ammonia and 1M/1 ammonium chloride (26) is also fed to the reactor. In the leach reactor the zinc passes into solution to give a solution containing around 0.5M/1 Zinc as do small amounts of the metallic impurities. The slurry (14) from the reactor containing unleached materials from the CZO is passed to a filter (4) to separate the solids from the liquor. The solids (17) which are predominantly lead compounds are removed from the filter and sold to a suitable lead smelting operation. The zinc containing liquor (16) is then passed to another reactor (4) where zinc powder(l9) is added to cement out the small amounts of metallic impurities such as cadmium, copper and lead which are in the liquor. The slurry from this reactor (18) is then passed to a filter (5) to separate the solids(21) from the purified liquor (22). The solids (21) are disposed of preferably by sale to a metal recycler. The purified zinc containing liquor (22) is then fed to the ammonia stripping system (6) which is the critical part of this invention. The zinc containing liquor is heated by injecting steam (15) such that the liquor is maintained at its boiling point which is controlled by controlling the pressure of the system. To produce high surface area material the system is preferably maintained under vacuum such that the boiling point of the system is kept below 85C and commonly below 75C. In this situation the ammonia is stripped off and crystals that are predominantly Zn(OH)2 and/or zinchydroxychloride with possible minor amounts of ZnO are precipitated. To produce low surface area material the temperature of the system is kept at around 100C by operating at ambient or slightly elevated pressures. In this situation the ammonia is stripped off and crystals which are predominantly ZnO with possible minor amounts of zinchydroxychloride are precipitated. The ammonia - water gas stream (24) from the stripping is contacted with the spent liquor (25) from the strip section (6) in a suitable ammonia capture column(8) to recover the ammonia and regenerate the leach liquor (26) for recycle to the leach step(2). Part or all of this regenerated leach liquor may be treated in a reverse osmosis step (32) to remove some of the water added as steam and the permeate (33 Containing this excess water used elsewhere in the process such as in Hydrolysis or for washing the leach residue or else discarded. The concentrate from the RO system (34) is recycled to the leach stage. This RO step may be inserted to treat other streams such as the liquor from the hydrolysis (30) or the low NH3 spent liquor from the NH3 stripping (24) to maintain the overall process water balance. The ammonia depleted stream containing zinc compound crystals (23) is filtered(7) with the liquor(25) which typically contains <lM/l ammonia and <0.25M/1 zinc is then cooled and recycled to the ammonia capture column(8). The solids (27) are passed to the hydrolysis stage (9) where the zinc containing crystals are hydrolysed to force the substitution of any residual chloride in the crystals by hydroxide and/or oxide. After the hydrolysis reaction is completed the slurry stream(29) is fdtered (10) with the chloride containing liquor (31) being further processed for either recycle or discard and the high purity high zinc oxide crystals (30) being passed to a dryer (11). The dryer typically operates at above 100C such that all oif the crystals are converted to ZnO and after they are dried they are packaged for sale.(12).

[0079]Description of Figure 4. In this flowsheet a zinc carbonate containing ore (1) is mined with the low grade material (1A) being coarse crushed to around 20mm and placed in a heap (2) for leaching whilst the high grade material(lB) is crushed and ground to around l50um particle size and placed into a stirred reactor (3) such as is commonly used in the minerals industry for leaching. The partially zinc saturated liquor from the heap leaching step(l4) containing ammonia and ammonium chloride and some carbonate is also fed to the reactor along with a calcium containing liquor (15). This calcium containing liquor is prepared from calcium oxide (13) and recycled streams from the ammonia stripping step (26) and from the hydrolysis step (30). In the leach reactor the zinc from the high grade ore passes into solution but the associated carbonate precipitates as calcium carbonate due to the addition of the calcium containing liquor(l5). The slurry from the reactor containing unleached gangue materials from the ore and the precipitated calcium carbonate (16) is passed to a filter (4) to separate the solids from the liquor. The solids (17) are removed from the filter disposed of preferably into a suitable tailings facility. The zinc containing liquor (18) is then passed to another reactor (5) where zinc powder(l9) is added to cement out metallic impurities such as cadmium, copper and lead. The slurry from this reactor (20) is then passed to a filter (6) to separate the solids(2l) from the purified liquor (22). The solids (21) are disposed of preferably by sale to a metal recycler. The purified zinc containing liquor (22) is then fed to a suitable ammonia stripper(7) where some calcium oxide (13) may also be added to increase the pH of the liquor and improve the efficiency of the ammonia removal. The ammonia - water gas stream (25) from the stripping is contacted with the spent liquor (24) from the strip section (7) to recover the ammonia and regenerate the leach liquor (26) for recycle to the heap leach step(2). The ammonia depleted stream containing zinc compound crystals (23) is filtered(8) with the liquor(24) being cooled and recycled to the ammonia capture column(l3) and the solids (27) being passed to the hydrolysis stage(9). The zinc containing crystals are reslurried and mixed with a calcium oxide containing slurry (13) to force the substitution of the chloride in the crystals by oxygen and/or hydroxide with the chloride dissolving in the liquor as calcium chloride. After the hydrolysis reaction is completed the slurry stream(28) is filtered (10) with the calcium chloride containing liquor (29) being recycled to the leach reactor (3) and the high purity zinc oxide/hydroxide crystals (30) being passed to a dryer (11) and then being packaged for sale as high purity zinc oxide(l2).

[0080]Description of Figure 5. A process for the treatment of an ore containing a mixture of sulphides and carbonates is shown in Figure 5 where an ore containing both lead and zinc is processed. The ore containing lead and zinc sulphides and zinc carbonate (100) is fed to a crushing circuit (101) and crushed (110) then subject to gravity separation(l09) such as by using jigging to separate out ore with little or no sulphide present (121). The sulphide rich ore is then ground(l02) to a P80 of - lOOum before being fed(l 12) to a lead sulphide flotation stage(l03). A lead concentrate^ 13) for sale is separated out from this stage with the lead sulphide depleted stream (114) then being fed to a zinc sulphide flotation stage(104). A zinc sulphide concentrate (115) is separated out for sale and the tailings stream rich in zinc carbonate (116) is filtered (105) and then fed (117) to an ammonia - ammonium chloride leach circuit. The sulphide depleted stream (121) is either discarded or if it contains significant amounts of leachable oxidic zinc zinc is fed to a grinding circuit and then into the ammonia-ammonium chloride leach circuit. In cases where there this material contains significant amounts of calcium carbonate and there is a need for calcium to control carbonate levels in the liquor part or all of this material may be fed to a roasting stage where the material is roasted (122) at above 600C to decompose both the calcium carbonate and the zinc carbonate and after roasting is then fed (123) to a grinding stage (124). This ground roasted ore (125) is then utilized in the ammonia-ammonium chloride zinc leach circuit as a source of calcium for carbonate control and to provide valuable extra zinc.

[0081 description of Figure 6. For feed material containing a mixture of zinc oxide and calcium oxide but little or no carbonate a modified process flowsheet is used as shown in figure 5. The feed material (41) is fed directly to a leach step(42) or can be crushed and ground as described in other figures. After leaching the slurry (14) is fed to a calcium precipitation stage(43) where a carbonate source such as zinc carbonate or carbon dioxide (50) is added to precipitate calcium in the liquor as calcium carbonate. The slurry from this (16) is then filtered and continues through purification, ammonia stipping and hydrolysis as per Figure 1. The residue from the

filtration(17) is normally disposed of as waste.

[0082]Figure 7 shows the extraction of zinc from a range of EAF Dust materials and from CZO using the ammonia - ammonium chloride leach solution. The extraction varies over the range of around 50% to around 85% for the EAF Dust samples tested. This is lower than achievable with CZO where the zinc extraction is typically over 95%.

[0083]Figure 8 shows the major steps in an efficient process for treating EAF Dust where a Waelz Kiln is available to treat the residue from the initial leach . The EAF Dust (1) is first fed to a stirred water wash reactor (2) to solubilize the chloride compounds. The brine solution from this step (3) is treated and discarded. The washed EAF Dust (4) is then fed to a stirred leach reactor (5) where the zinc and other soluble metals are dissolved into an ammonia - ammonium chloride solution. The proportions of leach liquor and EAF Dust are generally managed such that the pregnant leach liquor (7) from this reactor is unsaturated with respect to zinc. The leach residue (6) is separated and then fed into a Waelz Kiln (8) along with a suitable carbon source to volatilize the zinc and produce a low chloride crude zinc oxide. (9). The crude zinc oxide (9) is fed to another stirred leach reactor (12) where the zinc is mixed with the unsaturated pregnant liquor (7) from the EAF Dust leach reactor (5) and the zinc is dissolved into the ammonia - ammonium chloride liquor. The solid residue (14) from this leach reactor is high in lead and is sold to a lead smelter for recovery of the lead. The zinc rich liquor (13) from the CZO leach is fed into a cementation reactor (11) where metallic zinc is added to cement out metallic impurities such as copper and cadmium. The purified pregnant liquor (15) is then pumped to the Ammonia Strip and Crystallisation Step (16) where the ammonia is stripped off and recovered and the zinc crystallises out as any one of or any combination of Zn(OH)2, ZnO and zinchydroxychloride. The zinc containing crystals (17) are then transferred to the Hydrolysis and Drying stage where all of the crystals are converted to Zinc Oxide (19) for sale. This arrangement can be beneficial in giving a higher zinc content liquor to feed into the Ammonia Strip and Crystallisation Step as the CZO is more reactive than the original EAF Dust and provides sweetening of the liquor.

[0084]Figure 9 shows the process for treating zinc ores where the large majority of the zinc is in a refractory form that is not readily extracted using ammonia - ammonium chloride leach liquor. The zinc ore (1) is fed into a Waelz Kiln (2) along with a suitable carbon source to volatilize the zinc and produce a slag (3) and crude zinc oxide. (4). This crude zinc oxide (4) is fed to a stirred reactor (5) where water is added to dissolve out soluble impurities such as chlorides such that they do not enter the process liquor in the subsequent zinc leach step. The brine liquor (6) from this step is normally further treated and discarded. The washed crude zinc oxide (7) is then fed into a stirred leach reactor (8)and zinc is dissolved into an ammonia - ammonium chloride liquor. The solid residue (9) from this leach reactor is washed to recover the reagents and then either sold for recovery of other metals such as lead if that is high or is discarded. The zinc rich liquor (10) from the CZO leach is fed into a cementation reactor (11) where metallic zinc is added to cement out metallic impurities such as copper and cadmium. The purified pregnant liquor (12) is then pumped to the Ammonia Strip and Crystallisation Step (13) where the ammonia is stripped off and recovered and the zinc crystallises out as any one of or any combination of Zn(OH)2, ZnO and zinchydroxychloride. The zinc containing crystals (14) are then transferred to the Hydrolysis and Drying stage(15) where all of the crystals are converted to Zinc Oxide (16) for sale.

[0085]Figure 10 shows a typical ammonia stripping column for stripping ammonia from the liquor. These type of bubble cap columns are particularly suited for the situation where crystals are forming as the ammonia is stripped. Zinc rich pregnant liquor (1) is fed to a column (2) which is of the bubble cap type (6) as these are often preferred to minimize the chance of the solids forming blocking the column. In this arrangement the column (2) has a barometric leg (4) attached to allow the zinc crystal containing slurry (7) to be removed with the columns operating under vacuum. Within the column NH3 and H20 are boiled off (9) and transferred to NH3 recovery columns under vacuum. The slurry containing zinc crystals which can then be pumped to either another column or to further processing such as fdtration using conventional pumps. Where desired lime can be added to the slurry within the column or at interstage points in cases where there is more than one column to increase the pH and assist in both ammonia stripping and precipitation of the zinc.

[0086]Figurel 1 shows a 2 stage column arrangement for stripping ammonia from the liquor which is particularly suited for vacuum assisted low temperature ammonia removal in the situation where large amounts of crystals are forming as the ammonia is stripped. Zinc rich pregnant liquor (1) is fed to a first column (2) which is of the bubble cap type as these are preferred to minimize the chance of the solids forming blocking the column. In this arrangement both columns (2 & 3) have barometric legs (4 & 5) attached to allow the zinc crystal containing slurries (7 & 8) to be removed with the columns operating under vacuum. Within the columns ammonia and water are boiled off (9 & 10) and passed through ammonia absorption recovery columns (not shown) which also operate under vacuum. The zinc containing slurry (8) is pumped to further processing such as filtration using conventional pumps. pH and assist in both ammonia stripping and precipitation of the zinc.

[0087]Figure 12 shows a typical NH3 stripping system where steam is directly injected to provide the heat for stripping the ammonia. The rich zinc pregnant liquor (1) is fed into a crystallizer tank (3) and steam (2) is injected to maintain the liquor at boiling point. This releases an ammonia-water gas (4) which passes to an Ammonia Absorption Packed Column(6) where the vapour is contacted with spent ammonium chloride liquor(5) to recover the ammonia. The ammonia rich liquor (7) is returned to the leach step to dissolve more Zinc. The ammonia stripping causes zinc containing crystals to precipitate and the slurry containing these and zinc depleted liquor(8) is removed from the crystallizer and filtered for subsequent processing.

[0088]Figure 13 shows an alternative ammonia stripping arrangement where a 2 stage configuration is used. The rich zinc pregnant liquor (1) is fed into a crystallizer tank (3) and steam (2) is injected to maintain the liquor at boiling point. This releases an ammonia- water gas (4) which passes to an Ammonia Absorption Packed Column(6) where the vapour is contacted with spent ammonium chloride liquor(5) to recover the ammonia. The ammonia rich liquor (7) is returned to the leach step to dissolve more Zinc. The ammonia stripping causes zinc containing crystals to precipitate The slurry(8) containing these crystals and zinc depleted liquor is removed from the crystallizer and either passed to a filter (9)to separate the solids(lO) for further processing with the liquor (11) being passed to a second crystallizer. An alternative is to not filter the slurry (8) but to pass this directly to the second crystallizer (12). In the second crystallizer (12) stream(2) is injected to maintain the contained liquor at boiling point and more ammonia is stripped off to precipitate more zinc containing crystals. The ammonia-water gas (13) from the crystallizer is combined with the gas from the first crystallizer and passed to the absorption column(6). The slurry (14) from the second crystallizer (12) is pumped to a filter (15) and the crystasls (16) separated out for further processing with the liquor (5) being cooled and pumped to the absorption column to recover the ammonia.

[0089]These examples describe some means of carrying out the invention but many more configurations are possible using the invention with the exact configuration depending on the composition and mineralogy of the metal feed and on local economic especially regarding the supply of energy, water and calcium compounds.