FOR RECOVERING ZINC AND/OR ZINC OXIDE I

Field of the Invention

[0001 ] The present invention generally relates to a process for the recovery of zinc preferably in the form of zinc or zinc oxide. The invention is particularly applicable for recovering zinc from Electric Arc Furnace dust ("EAF") and it will be convenient to hereinafter disclose the invention in relation to that exemplary application. However, it is to be appreciated that the invention is not limited to that application and could be used to recover zinc from a variety of sources including materials containing zinc oxide and other metal oxides, such as galvanisers' ash, oxidised ores, mineral processing residues, water treatment precipitates, contaminated soils, waste stockpiles and/or, solid waste streams, materials containing mixed-metal oxides including zinc where a "mixed-metal" oxide is a compound composed of zinc, oxygen and at least one other metal (e.g. zinc ferrite, or zinc ferrate) such as oxidised ores, or mineral processing residues.

Background of the Invention

[0002] The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

[0003] Electric Arc Furnace dust ("EAF dust") is produced as fume from operating an electric arc furnace during steel making. This dust has to be collected and treated or disposed of in some way. Disposal of EAF dust by stabilization and burial is widely practiced. In many markets this is costly. Economic treatments to recover contained zinc in EAF dust has been a challenge to industry for many years.

[0004] The zinc in EAF dust is typically in the form of zinc oxide and zinc ferrite. The zinc oxide in EAF dust can be leached by a variety of lixiviants. However the associated zinc ferrite has generally proven more difficult. Such zinc ferrite has not been easy to attack by purely hydrometallurgical means. Usually a thermal reduction is performed before leaching to further convert zinc ferrite to zinc oxide to make more zinc amenable to aqueous leaching agents.

[0005] A further problem with EAF dust is that the high iron content in EAF dust is leached by many acidic lixiviants. The removal of this solubilised iron from leach solutions complicates hydrometallurgical processes and flow charts.

[0006] It would therefore be desirable to provide an improved or at least alternative process which leaches and solubilises zinc from a variety of sources of zinc containing materials associated with other metal oxide(s) and/or "mixed-metal" oxides.

[0007] It should be understood that any metal such as zinc, manganese, lead etc should be understood to include any chemical form (i.e. metal, salts, complexes, chelates, etc) or ionic form.

Summary of the Invention

[0008] According to a first aspect of the present invention, there is provided a process for recovering zinc from a zinc containing material, the process including the steps of: leaching the zinc containing material with an alkaline lixiviant which includes an ammonium species, a hydroxide species and a halide species to produce a zinc containing leachate;

stripping ammonia from the leachate to produce a stripped liquor containing a zinc containing precipitate; and

recovering the zinc from the stripped liquor.

[0009] The process of the present invention therefore uses an alkaline ammonium based lixiviant to selectively leach the zinc from the zinc containing material. The overall process predominantly leaves zinc in solution in the resultant leachate. By selection of lixiviant composition, a zinc solution can be produced that can have the zinc solubility modified by ammonia stripping which can facilitate recovery of a substantial portion of the contained zinc as basic zinc compounds leaving a zinc depleted ammonium chloride liquor for recycle.

[0010] The lixiviant composition can comprise any suitable alkaline solution which includes an ammonium species, a hydroxide species and a halide species, and in particular ionic forms of those species. In a preferred embodiment, the lixiviant comprises a mixture of an ammonium halide and ammonium hydroxide. One example of suitable lixiviant is an aqueous mixture of ammonium chloride and ammonia. In one form, the lixiviant comprises an aqueous solution of 10 to 1000g/L of ammonium chloride, preferably between 100 and 500 g/L, more preferably between 200 and 300 g/L, and most preferably about 280 g/L, and between 5 to 300 g/L total ammonia, preferably between 100 and 200 g/L, most preferably about 150 g/L. However, an ammonium chloride content of between 200 to 500g/L is preferred when the lixiviant has a temperature of between ambient to 50°C.

[001 1 ] The mixture of ammonia into water will produce a lixiviant mixture of ammonium chloride, ammonium hydroxide and water (included in the ammonium hydroxide). It should be appreciated that any ammonium halide can be substituted ammonium chloride, for example ammonium bromide. This substitution would require a molar equivalence of that ammonium halide to be used in place of ammonium chloride.

[0012] The lixiviant has a pH > 7. An alkaline lixiviant is less conducive to iron solubilisation and minimizes lead and manganese solubilisation. The addition of ammonia, ammonium hydroxide should be sufficient to make the lixiviant alkaline in terms of normal aqueous chemistry.

[0013] The lixiviant mixture need not be fully in a solution form at the commencement of its use. For example, the ammonium chloride does not necessarily have to be dissolved at the start of leaching. Heating the lixiviant to above ambient temperatures, preferably greater than 50°C, would assist in dissolution of the ammonium chloride into solution and thereby optimise the ammonium chloride and ammonium hydroxide concentrations. When ammonia/ammonium hydroxide is used, it is preferably for the leaching step is preferably performed in a gas tight environment to prevent loss of ammonia gas to the atmosphere.

[0014] The amount of lixiviant added to the zinc containing material can be varied depending on the composition of that material because the lixiviant/solids ratio is sensitive to the zinc content and nature of the material to be leached. In some embodiments, the lixiviant/solids ratio is 20 to 1500 g of zinc containing material per L of lixiviant is leached in the leaching step.

[0015] The leach contact time is variable depending on the composition of the zinc containing material being leached in the leaching step. The contact time can therefore range from minutes to several days. It should however be appreciated that high temperature favours shorter leach times. Where the leaching step is conducted in temperatures above ambient, the leaching step should be conducted in a pressure vessel designed to withstand at least the vapour pressure of ammonia and water at that operational temperature. Low temperature (ambient temperature(s) and below) leaching steps can be performed in a closed vessel. The leaching step is preferably conducted at a temperature of between -10°C to 150°C, more preferably between 20 to 70 °C.

[0016] The overall process is preferably set up to predominantly leave zinc in solution. The selected lixiviant is alkaline through the use of ammonia/ammonium hydroxide. A zinc rich leachate can be produced that can have its zinc solubility modified by ammonia stripping. This can recover a substantial portion of the contained zinc as basic zinc compounds leaving a zinc depleted ammonium chloride liquor for recycle.

[0017] The process of the present invention includes the step of stripping ammonia from the leachate. This stripping step precipitates out basic zinc compounds which can then be recovered. Advantageously, the stripped ammonia can be captured for recycle to the leaching step. This stripping step preferably provides a stripped liquid containing solid zinc product, and the process further includes the step of removing solids from the stripped liquid. Though, this stripped liquor could also be used as an electrolyte for electrowinning. [0018] Ammonia stripping can be performed by any number of processes. In one embodiment, the step of stripping ammonia from the leachate includes a gas sparging step. Waste heat from a co-located process is preferably used for the gas sparging step. For example, in the case of EAF dust, waste heat from the steel plant can be used to heat air for gas sparging. In the case of a hot dip galvanising plant treating ash, waste heat from the hot dip galvanising furnace can be used in the ammonia stripping step.

[0019] The zinc can be extracted from the process in various forms including zinc hydroxide, zinc hydroxychloride, zinc oxide, or zinc dichlorodiammine or combinations thereof. The composition of the lixiviant can be modified to control the composition of the final stripped liquor composition, which in turn assists in the selection of the dominant product.

[0020] Zinc oxide is favoured by stripping followed by neutralization by caustic soda. The process of the present invention can therefore further include a neutralisation step following the ammonia stripping step including adding sodium hydroxide to the stripped liquid.

[0021 ] The zinc can be extracted from the stripped liquor substantially as zinc dichlorodiammine. Zinc dichlorodiammine crystallization is favoured from hot leach liquors derived from the preferred ammonium chloride lixiviant loadings. The process then preferably further includes a hydrolysis step. The hydrolysis step typically includes a step of reacting the solid zinc containing material product with water to convert the zinc dichlorodiammine to Zn(OH) ₂ . The step of stripping ammonia can promote the precipitation of large amounts of Zinc dichlorodiammine without the need for excessive heating and cooling to drive solubility. Subsequent hydrolysis of Zinc dichlorodiammine avoids the need to dilute the leach liquor prior to crystallization. Zinc oxide can then be produced by heating the hydrolysis product (substantially Zn(OH) ₂ ) to drive off water.

[0022] The "zinc containing material" can be any material including material containing zinc species are such as: [0023] Materials containing zinc oxide and other metal oxides such as galvanisers' ash, EAF dust, oxidised ores, mineral processing residues, water treatment precipitates, contaminated soils, waste stock-piles, or solid waste streams.

[0024] Materials containing mixed-metal oxides including zinc where a "mixed- metal" oxide is a compound composed of zinc oxygen and at least one other metal (e.g. zinc ferrite, or zinc ferrate, such as EAF dust, oxidised ores or the like; or

[0025] Mineral processing residues.

[0026] In preferred embodiments, the zinc containing material is substantially particulate.

[0027] Recovery of the zinc from the leachate preferably includes one or more process steps which separate solids from the leachate, removal procedures for other metal species which may be present in the leachate such as lead, manganese, copper and cadmium and/or process steps to separate the zinc from the leachate.

[0028] The separation of solid and liquid can be performed using any suitable method. However, the process may include at least one filtering step to remove solids from the leachate.

[0029] The zinc containing material may also include at least one of manganese, lead, copper or cadmium. Other trace elements, species or impurities may also be present. The process of the present invention therefore can include steps of removing any lead, manganese, copper or cadmium from the leachate.

[0030] Where the zinc containing material includes lead, these can be removed using selective removal processes. Firstly, it should be appreciated that lead(ll) is leached in the leaching step while lead(IV) is not leached. Those lead leached from the zinc containing material are preferably returned to the leached solids by oxidation to an insoluble form.

[0031 ] Controlled oxidation preferentially oxidizes lead before any manganese which may be present in the zinc containing material. Thus, any lead can be isolated before manganese. The lead removal step is preferably conducted prior to any solids/liquid separation of leachate after the leaching step. However, in some embodiments, lead can be removed and separated after a liquid solids separation step and collected separately.

[0032] Where the zinc containing material includes manganese, these can be removed using selective removal processes. Firstly, it should be appreciated that manganese in oxidation states less than +4 is leached in the leaching process but manganese(IV) is not leached. The manganese leached from the zinc containing material is preferably returned to the leached solids by oxidation to an insoluble form.

[0033] The manganese removal step is preferably conducted prior to any solids/liquid separation of leachate after the leaching step. However, in some embodiments, manganese can be removed and separated after a liquid solids separation step and collected separately. Again, it is preferable to conduct the controlled oxidation of lead prior to manganese so lead can be optionally isolated before manganese then removed and manganese subsequently oxidized to manganese dioxide and separated.

[0034] Each of the lead and manganese removal steps can comprise chemical oxidation using one of peroxide or chlorine and/or chloride based oxidants such as hypochlorite or chloramine. Air oxidation may also be used to remove these metals. Alternatively, electrochemical oxidation may also be used.

[0035] Where the zinc containing material includes copper and cadmium, these can be removed by the well established process of cementation on zinc. The manganese leached from the zinc containing material is preferably removed using a cementation process which includes the step of adding zinc metal to the leachate.

[0036] However it should be appreciated that lead, manganese, copper and/or cadmium can be removed by electrolytic means. Such electrolytic processes can be combined or consecutive or a combination of combined and consecutive. [0037] In a preferred embodiment, the zinc containing material also includes manganese, lead, copper and cadmium. In this embodiment the process preferably includes the steps of:

leaching the zinc containing material with an alkaline lixiviant which includes an ammonium species, a hydroxide species and a halide species to produce a leachate substantially including the zinc and leached solids content;

removing the lead, manganese, copper and cadmium from the leachate to produce a modified leachate and cementation solids;

separating the modified leachate and cementation solids;

stripping ammonia from the modified leachate to precipitate zinc containing compounds.

[0038] As discussed previously, the lead, manganese, copper and cadmium can be removed using various processes.

[0039] In a preferred embodiment, the steps of removing the lead, manganese, copper and cadmium from the leachate to produce a modified leachate and cementation solids; and separating the modified leachate and cementation solids include the steps of

removing the lead and manganese from the leachate by chemical oxidation; separating the leachate and leached solids in a filtration step;

removal of the copper and cadmium from the leachate by zinc cementation to produce a modified leachate and cementation solids;

separating the modified leachate and cementation solids in a filtration step.

[0040] In a second aspect of the present invention, there is provided a plant which includes a process according to the first aspect of the present invention. This plant preferably includes a pressure vessel able to present the lixiviant solution and particulate material(s) to ammonia saving confinement for the purpose of the leaching out of the zinc.

[0041 ] The present invention also provides a zinc containing material produced from a process according to the first aspect of the present invention. [0042] According to a further aspect of the present invention, there is provided a process for recovering zinc from a zinc containing material, the process including the steps of:

leaching the zinc containing material with an alkaline lixiviant which includes an ammonium species, a hydroxide species and a chloride species to produce a zinc containing leachate;

stripping ammonia from the leachate to modify the solubility of the zinc to produce a stripped liquor containing a zinc rich precipitate substantially comprising zinc dichlorodiammine; and

recovering the zinc from the stripped liquor.

Brief Description of the Drawings

[0043] The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

[0044] Figure 1 is a flow diagram showing the process steps for one preferred embodiment of the process according to the present invention.

[0045] Figure 2 shows flow diagram of one preferred embodiment of the process according to the present invention for producing zinc oxide from EAF dust.

Detailed Description

[0046] The process of the present invention relates to the recovery of zinc from a zinc containing material. The "zinc containing material" can be any material including material containing zinc species are such as galvanisers' ash, EAF dust, oxidised ores, mineral processing residues, water treatment precipitates, contaminated soils, waste stock-piles, or solid waste streams. Such zinc containing material typically also includes manganese, lead, copper and cadmium which can also be solubilised in a lixiviant applied during a leaching step.

[0047] Without wishing to be bound by any one theory, the applicant has designed the leaching and step of recovering the zincs of the process of the present invention to take advantage of the equilibrium which is established between the soluble and insoluble metal complexes of the oxides and mixed metal oxides in such a zinc containing material when leached by a lixiviant. The particular lixiviant of the present invention provides a mix of ligands which facilitate this equilibrium. Ammonium, ammonia chloride, hydroxide and water are all available for metal complex formation.

[0048] Using this mix of ligands, the resultant zinc complexes under the conditions of the leach are predominantly soluble. Iron(lll) is not solubilised to any great extent as its solubility is controlled by the solubility product of iron(lll) hydroxide which is an equilibrium species in the leach mixture. This is effective in keeping iron out of solution. The major risk of iron contamination of leach liquors is the formation of colloidal iron hydroxide. Lead(IV) and manganese(IV) are either not leached or leached and returned to the solid phase in a similar manner to iron(lll). Both manganese(ll) and lead(ll) are soluble in the lixiviant but are easily oxidized to the less soluble manganese(IV) and lead(IV) oxides. They can be returned to the solids in the leach by oxidation or subsequently removed after primary solids separation by oxidation and filtration. Cadmium and copper, being generally soluble in the lixiviant mix can be removed to low levels by cementation on zinc.

[0049] The overall process thus predominantly leaves zinc in solution. By selection of lixiviant composition, a zinc solution can be produced that can have the zinc solubility modified by ammonia stripping in order to recover a substantial portion of the contained zinc as basic zinc compounds leaving a zinc depleted ammonium chloride liquor for recycle. This zinc in this zinc solution can be recovered using a variety of recovery processes. In some embodiments, this zinc solution may be suitable for zinc electrowin in a divided electrowin cell.

[0050] Referring now to Figure 1 , there is shown a flow diagram showing the basic steps in recovering the zinc for a zinc containing material using a leaching step in accordance with a preferred embodiment of the present invention. The zinc containing material treated in the process includes zinc, manganese, lead, copper and cadmium.

[0051 ] In the first step, the zinc containing material is leached with an alkaline lixiviant. The lixiviant composition comprises an aqueous mixture of ammonium chloride and ammonia mixed to provide a pH > 7. An alkaline lixiviant is less conducive to iron solubilisation and minimizes lead and manganese solubilisation. The lixiviant mixture need not be fully in a solution form at the commencement of its use. Heating the lixiviant to above ambient temperatures assists in dissolution of the ammonium chloride into solution and thereby optimises the ammonium chloride and ammonium hydroxide concentrations. The lixiviant/solids ratio for this leaching step is between 20 to 1500 g of zinc containing material per L of lixiviant. The leach contact time is variable depending on the composition of the zinc containing material being leached in the leaching step. The leaching step produces a leachate substantially including the zinc. However, the leachate also includes solubilised manganese, lead, copper and cadmium. A leached solid content is also present.

[0052] As shown by the broken arrows, this mixture can either be filtered to remove the leached solids, or undergo a process step where the lead and manganese are removed from the leachate. The lead and manganese removal step is however preferably conducted prior to any solids/liquid separation of leachate after the leaching step.

[0053] Firstly, it should be appreciated that lead(ll) is leached in the leaching step while lead(IV) is not leached and that manganese in oxidation states less than +4 is leached in the leaching process but manganese(IV) is not leached. Each of the lead and manganese removal steps can comprise a controlled oxidation step. This can be chemical oxidation using one of peroxide or chlorine and/or chloride based oxidants such as hypochlorite or chloramine. Air oxidation may also be used to remove these metals. Alternatively, electrochemical oxidation may also be used. Controlled oxidation preferentially oxidizes lead before any manganese which may be present in the zinc containing material. Thus, any lead can be isolated before manganese.

[0054] The resulting liquor is then passed through a separation step, typically a filtration step to separate the leachate and leached solids. The resulting liquor then undergoes a process step where the copper and cadmium are removed from that liquor. This step is typically conducted by the well established process of cementation on zinc to produce a modified leachate and cementation solids. However, it should be appreciated that other processes could be used, for example electrolytic separation process.

[0055] The resulting liquor is then passed through a separation step, typically a filtration step to separate the liquor and cementation solids.

[0056] The resultant filtered liquor predominantly includes the zinc in solution. This filtered liquor can optionally be treated with alkali such as sodium hydroxide, diluted and ammonia stripped to precipitate zinc compounds and produce a disposal salt solution of low zinc content. This is useful for ammonia recycle when a lixiviant has reached the end of its recycle life due to build up of contaminants such as calcium and magnesium.

[0057] Following the process flow chart shown in Figure 1 , the zinc rich leachate is normally passed directly into an ammonia stripping step in order to modify the solubility of zinc in that solution. This stripping step precipitates out basic zinc compounds producing a stripped liquid containing solid zinc product. The stripped ammonia is captured for recycle to the leaching step.

[0058] Ammonia stripping can be performed by any number of processes, for example gas sparging. Waste heat from a co-located process can be used for the gas sparging step.

[0059] The zinc can be extracted from the process in various forms including zinc hydroxide, zinc hydroxychloride, zinc oxide, or zinc dichlorodiammine or combinations thereof. The composition of the lixiviant can be modified to control the composition of the final stripped liquor composition, which in turn assists in the selection of the dominant product. For example, zinc oxide is favoured by stripping followed by neutralization by caustic soda. Zinc dichlorodiammine crystallization is favoured from hot leach liquors derived from high ammonium chloride lixiviant loadings. Zinc hydroxychloride and Zn(OH) ₂ are favoured by dilution of the leach liquor with water prior to the precipitation. [0060] The isolated zinc containing solid product can be treated by any suitable means to produce zinc compounds or zinc metal. For example:

Zinc hydroxide can be heated at temperatures above 150°C to produce zinc oxide.

Zinc dichlorodiammine can be hydrolysed in water to form zinc oxide and/or Zn(OH) ₂ and recycle liquor bearing ammonium chloride.

Zinc oxide or hydroxide can be fed to a conventional sulphate electrowin cell for production of zinc metal.

Zinc dichlorodiammine and zinc hydroxychloride are suitable feeds for zinc ammonium chloride based chemicals.

Zinc dichlorodiammine and zinc hydroxychloride are suitable feeds for zinc metal electrowin in divided chloride electrowin cells.

[0061 ] After treatment, the final solution undergoes a filtration step to recover the solid product. The liquor can be recycled.

[0062] Figure 2 shows flow diagram of one preferred embodiment of the process according to the present invention for producing zinc oxide from EAF dust. EAF dust typically includes zinc, manganese, lead, copper and cadmium. Other trace elements may also be present. The process therefore follows the same basic steps as illustrated in Figure 1 .

[0063] In the first step, the EAF dust is leached with an alkaline lixiviant comprising an aqueous mixture of ammonium chloride and ammonia. The leaching step produces a leachate substantially including the zinc with solubilised manganese, lead, copper and cadmium. A leached solids content is also present.

[0064] The leached mixture is then filtered to remove the leached solids. The resulting leach liquor then passed through a controlled oxidation step to remove the lead and manganese from the liquor, followed by a cementation step where the copper and cadmium are removed by cementation on zinc. Further filtration steps may be used after the controlled oxidation step and/or cementation step to separate out the solid content from the liquor. [0065] The resultant liquor predominantly includes the zinc in solution. This zinc rich leachate is then passed into a hot ammonia stripping step which results in basic zinc compounds precipitating out into the stripped solution. In this case, zinc dichlorodiammine crystals are predominantly formed. This stripping step produces a stripped liquid containing solid zinc product. Again, the stripped ammonia is captured for recycle to the leaching step.

[0066] The stripped liquid is then passed through a separation stage to separate the zinc compound precipitate from the liquor. The resulting separated liquor is then recycled for use in the leaching step. The separation stage also includes a solids wash cycle.

[0067] The separated the zinc dichlorodiammine precipitate is then hydrolysed with hot water to form a hydrolysis product (typically Zn(OH) ₂ ), which is then heated to drive off water to form a zinc oxide product. The hydrolysis step preferably includes a hot dilution step, however this could be conducted at any temperature, hot or cold. The liquor bearing ammonium chloride from this step is also recycled for use in the leaching step. The zinc oxide product is then dried.

[0068] Examples of leaching solutions and outcomes will now be described.

The Particulate Material(s) of the Examples

[0069] The leach examples hereafter have treated EAF dust with the content of Table 1 .

Table 1

Wt%

Zn 43.32

Cr 13.32

Fe 12.80

Cd 1.59

Pb 1.55

Ni 0.69

Cu 0.13

Mn 0.02 The Leaching Examples

Example 1

[0070] 175 g of the above EAF dust was well mixed with 1 litre lixiviant of composition 280 g/L NH CI and 150 g/L NH ₃ . The mixture was stored in a closed plastic bottle at 23°C over 2 days. The mixture was then vacuum filtered through a 2 μιτι filter pad, the filter cake was washed with lixiviant then water. The volume of the filtrate and the washings was measured, and the concentration of zinc was determined by atomic absorption spectroscopy (AA). The zinc solubilised was calculated as 69.86 g representing 95 % of the zinc contained in the sample of EAF dust.

Example 2

[0071 ] 500 g of the above EAF dust was well mixed with 1 litre lixiviant of composition 280 g/L NH CI and 150 g/L NH ₃ . The mixture was stored in a closed plastic bottle at 25°C over 3 days. The mixture was then vacuum filtered through a 2 μιτι filter pad, the filter cake was rinsed with lixiviant then water. The volume of the filtrate and the washings was measured, and the concentration of zinc was determined by AA. The zinc solubilised was calculated as 161 .01 g representing 76.7 % of the zinc contained in the sample of EAF dust. The wet cake was taken out and mixed with 1 litre lixiviant of the same composition by stirring. The mixture was then kept in the closed plastic bottle for 2 days at 23°C. The same procedures of filtering, rinsing and measurements as described above were followed. The zinc solubilised was calculated as 0.71 g representing 0.34 % of the zinc contained in the sample of EAF dust. Thus, the total zinc extraction was 77 %.

Example 3

[0072] 17.5 g of the above EAF dust and 20 g NH ₄ CI were mixed with 100 ml 32% NH OH. The mixture was stored in a capped plastic bottle at 23°C for 2 hours, during which a number of shakes were given. The mixture was then filtered through a 2 μιτι filter pad. The filtrate was diluted and analysed by AA. The zinc solubilised was calculated as 5.28 g representing 71 .87 % of the zinc contained in the sample of EAF dust. Example 4

[0073] 175 g of the above EAF dust was mixed with 1 litre lixiviant of composition 200 g/L NH CI, 150 g/L NH ₃ and distilled water in a closed plastic bottle. The bottle was kept in a 25°C water bath for 24 hours including half an hour stirring time. The mixture was then vacuum filtered through a 2 μιτι filter pad, the filter cake was rinsed with lixiviant, ammonium then water. The volume of the filtrate and the washings was measured, and the concentration of zinc was determined by AA. The zinc solubilised was calculated as 60.94 g representing 82.9 % of the zinc contained in the sample of EAF dust.

Example 5

[0074] 100 g of the above EAF dust was mixed with 1 litre lixiviant of composition 120 g/L NH ₄ CI, 100 g/L NH ₃ and distilled water in a closed plastic bottle. The bottle was kept at 23°C water bath for 18 hours. The mixture was then vacuum filtered through a 2 μιτι filter pad, the filter cake was rinsed with lixiviant then water. The volume of the filtrate and the washings was measured, and the concentration of zinc was determined by AA. The zinc solubilised was calculated as 35.96 g representing 85.6% of the zinc contained in the sample of EAF dust.

Example 6

[0075] 17.5 g of the above EAF dust was leached by the lixiviant of composition 36.62 g NH ₄ Br (same molar amount as 200 g/L NH ₄ CI in 100 ml lixiviant) and one-tenth the volumes of the ammonium solution as well as the distilled water of Example 4. The mixture was stored in a closed glass bottle at 23°C. After 24-hour leach, the mixture was vacuum filtered through a 2 μιτι filter pad. The filter cake was rinsed with lixiviant then water. The volume of filtrate and washings was measured, and the concentration of zinc was determined by AA. The zinc solubilised was calculated as 8.84 g representing 79.5 % of the zinc contained in the sample of EAF dust.

Example 7

[0076] 8.75 g of the above EAF dust was leached by the lixiviant of composition 27.09g NH CI (same molar amount as 200 g/L NH CI in 50 ml lixiviant) and one twentieth the volume of the ammonium solution as well as the distilled water of Example 4. The mixture was stored in a closed glass bottle at 23°C. After 24-hour leach, the mixture was vacuum filtered through a 2 μιτι filter pad. The filter cake was rinsed with lixiviant then water. The volume of filtrate and washings was measured, and the concentration of zinc was determined by AA. The zinc solubilised was calculated as 2.08 g representing 56.6 % of the zinc contained in the sample of EAF dust.

Example 8

[0077] 20 g of the above EAF dust was mixed with lixiviant of composition 54 g NH CI and 125 ml 32% NH ₄ OH in a closed glass bottle. The mixture was kept under room temperature for 6 hours. After that, it was vacuum filtered through a 2 μιτι glass filter pad. The filter cake formed was rinsed with lixiviant and hot water. The volume of filtrate and washings was measured, and the concentration of zinc was determined by ROTA titration. The zinc solubilised was calculated as 8.34 g representing 99.3% of the zinc contained in the sample of EAF dust.

Example 9

[0078] 40 g of the above EAF dust was leached by the lixiviant of composition 93.75 g NH ₄ C1 , 28.4 ml 32% NH ₄ OH and 96.6 ml H ₂ 0 in a 250 ml glass bottle. The bottle was kept in a 50 °C for 2 hours and cool for 10 minutes. The bottle was enclosed by clamping a rubber stop to avoid ammonia escape. After that, it was vacuum filtered through a 2 μιτι filter pad. The filter cake was washed with lixiviant and hot water. The volume of the filtrate and the washings was measured, and the concentration of zinc was determined by AA. The zinc solubilised was calculated as 12.44 g representing 74.02 % of the zinc contained in the sample of EAF dust.

Example 10

[0079] 40 g of the above EAF dust was leached by the lixiviant of composition 54 g NH ₄ CI and 125 ml 32% NH ₄ OH in a 250 ml glass bottle. The bottle was kept at 4°C for 17 hours. The lid was wrapped by paraflim to avoid ammonia escape. After that it was vacuum filtered through a 2 μιτι filter pad, the filter cake was washed with lixiviant and hot water. The volume of the filtrate and the washings was measured, and the concentration of zinc was determined by AA. The zinc solubilised was calculated as 12.74 g representing 75.85 % of the zinc contained in the sample of EAF dust. Example 11

[0080] 40 g of the above EAF dust was leached by the lixiviant of composition 54 g NH ₄ CI and 125 ml 32 c NH ₄ OH in a 250 ml glass bottle. The bottle was kept in a 50 °C water bath for 2 hours. After that, it was vacuum filtered through a 2 μιτι filter pad, the filter cake was washed with lixiviant and hot water. The volume of the filtrate and the washings was measured, and the concentration of zinc was determined by AA. The zinc solubilised was calculated as 14.15 g representing 84.23 % of the zinc contained in the sample of EAF dust.

Example 12

[0081 ] 1162 mis of combined leach liquor and cake washings from a 1 L of 200g/L ammonium chloride and 150 g/L ammonia recovery of 175g of EAF dust was treated with zinc turnings to remove contaminant metals. The clear solution was then heated to strip free ammonia and the volume topped up as required to maintain approximately 1 litre of solution.

[0082] At the end of ammonia stripping the volume was diluted with water and made up to 2 litres with hot water and allowed to cool to ambient temperature.

[0083] The resulting precipitate was filtered off, washed and dried at 80°C giving 34.53 g of white zinc compound

[0084] Approximately 5 g of white product, weighed accurately, was dissolved in 20 mis of 1 :1 nitric acid and diluted to 100 mis. This solution was subjected to atomic absorption analysis and the metals content of the product solid was calculated. Chloride was determined by silver nitrate titration with silver chloride ion selective electrode end point determination.

[0085] The solid contained:

Zinc - 61 .9%

Lead ^" Not detected <1 ppm

Manganese - 37 ppm

Copper - 1 .3 ppm

Iron - 133 ppm Nickel - Not detected <1 ppm

Cadmium Not detected <1 ppm

Chloride-13.49%

[0086] The filtrate and washings from the above filtration were evaporated to 0.85 litres giving a recycle lixiviant solution of composition:

Zinc - 45 g/L

Ammonia and ammonium expressed as ammonia - 64.03 g/L

Chloride - 178.56 g/L

pH - 6.6 (after cooling and standing)

[0087] Example 13

[0088] A portion (~ 500 mis) of leach solution of composition:

Zinc - 33.8 g/L,

Lead - 340 pg/ml,

Manganese - 147 g/ml

Copper - <489 g/ml

Iron - nd g/ml ^* ,

Nickel <0.5 pg/ml,

Cadmium - <1 pg/ml,

( ^* nd - not detected)

[0089] was treated with zinc to remove copper, cadmium and lead. The resulting clear solution was treated with sodium hydroxide and ammonia removed by ammonia stripping. The precipitated solid was dried at 80°C, weighed as 22.25 g, and analysed.

[0090] Analysis of the solid gave the following results:

Zinc - 79.2%

Lead - 59 ppm

Manganese - 3 ppm

Copper -<1 ppm

Iron -1 5.1 ppm

Nickel - <1 ppm

Cadmium - < 1 ppm Chloride - 0.56% [0091] Example 14

[0092] 175 g of EAF dust of composition by analysis as in table I was leached at 70°C for 30 minutes in a 1 L of lixiviant of composition:

120 g/L ammonium chloride.

150 g/L ammonia.

[0093] The mixture was filtered. Filtrate and washings combined gave 2.03 L of solution with a zinc concentration of 31 .1 g/L.

[0094] The solution was aerated to remove lead and manganese and treated with zinc to remove dissolved metals other than zinc and produced a solution of composition:

Zinc - 34.3 g/L

Lead - nd

Manganese - 1 .6 pg/ml,

Copper - 0.98 μ/ml,

Iron - 1 .32 pg/ml ^* ,

Cadmium <1 g/ml.

[0095] -1 -5L of this solution was heated to strip ammonia and the final mixture diluted with water to give a total volume of 3 L.

[0096] The solid was separated, washed with water and dried for 18 hours at 80°C. The product solid weighed 52.3 g and analysis was as follows.

Zinc - 60.24% by AA, 60.26% by EDTA titration.

Lead - nd

Manganese - 35 ppm

Copper - 8 ppm

Iron - 8 ppm

Nickel - nd

Cadmium - 2.8 ppm

Chloride - 12.51 % [0097] 3.6 L of combined filtrate and washings with a zinc concentration of 5.16 g/L were treated with NaOH and the released ammonia stripped. The solution was boiled down to 1 .5 L cooled and the precipitate formed was filtered off and washed with water.

[0098] The product white solid was dried for 18 hours at 80°C and weighed, 20.33 g.

[0099] The solid analysed as follows:

Zinc - 68.3%

Lead - 250 ppm

Manganese - 4 ppm

Copper - 24 ppm

Iron - 17 ppm

Cadmium - 35ppm

Chloride - 1 .00%

Example 15

[00100] 400g of EAF dust as in table 1 was mixed with lixiviant of composition 540 g NH CI and 1250 ml 32% NH OH in a closed plastic bottle. The mixture was kept under room temperature for 20 hours. After that, it was vacuum filtered through a 2 μιτι glass filter pad. The filter cake formed was rinsed with lixiviant and hot water.

[00101] 3.3 L of filtrate and washings were collected and the solution treated with 3 mis of 30% hydrogen peroxide then 10 g zinc turnings. The concentrations of manganese, lead, copper and cadmium as determined by AA are reported in table 2.

Table 2

Mn- g/ml Pb - Mg/ml Cu - Mg/ml Cd - Mg/ml

Leach solution + filter 148.2 21 1.62 46.06 20

cake washings

After oxidisation 10.6 nd 46.32 19.2

After zinc 21 nd nd nd cementation

"nd " - not detected. [00102] The cleaned solution was evaporated to strip ammonia and to 1 .3L giving 1 .3L of hot solution with a zinc concentration of ~ 125 g/L suitable for cycle to electrowin system or for galvaniser's pre-flux pH adjustment, galvaniser's pre-flux balance additive or crystallisation of galviniser's flux.

[00103] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

[00104] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.