BACKGROUND TO THE INVENTION The treatment of industrial dusts and residues containing metal contaminants such as zinc, lead, cadmium, mercury, silver etc. presents certain problems. Typically such dusts have fine particle size making their handling difficult. The cost for disposal of such dusts has increased dramatically in recent years because the metal contaminants represent an environmental hazard. Hence, there is some imperative to treat the dusts to make them suitable for disposal and thus less costly to dispose of.

Recent attempts to treat EAF dusts include the forming of dusts into briquettes to enable their feeding to smelters and furnaces ; rotary hearth reduction of dusts to pelletise, in combination with other feed stock, for subsequent processing; and solution based approaches such as leaching in sulfuric acid to produce zinc sulfate.

However, the existing solution processes do not leach lead, silver or mercury.

The present applicant has previously developed a number of combined leaching/electrolysis processes for the electro-recovery of metals including copper, zinc, nickel and lead. Examples of such processes are shown in the

applicant's Australian patent 669906 and the applicant's Australian patent application 2002328671 (WO 03/023077).

The applicant has surprisingly discovered that industrial dusts such as EAF dusts can be treated in a modified leaching process as part of a combined leaching/electrolysis circuit, enabling the subsequent electro-recovery of various metals such as lead and zinc, and the separation out of other metals such as cadmium, mercury, silver etc. In other words, metals which might otherwise be considered contaminants can advantageously be turned into valuable products.

SUMMARY OF THE INVENTION Accordingly, in a first aspect the present invention provides a process for leaching metal contaminant (s) from an EAF dust, the process replacing a leaching stage in a combined leaching/electrolysis circuit wherein, in the leaching stage, a metal-containing material is leached in a solution and, in an electrolysis stage, one or more metals are recovered from the solution and an oxidising agent is produced, with the resultant solution and oxidising agent being recycled to the leaching stage for further leaching, the process comprising the steps of: feeding the EAF dust to a first leaching stage to form a mixture of EAF solids with a solution of relatively low acidity such that at least some of the metal contaminant (s) are leached into the solution; passing solids from the first leaching stage to a second leaching stage to form a mixture of the solids with a solution of relatively high acidity such that at least some of any remaining metal contaminant (s) are leached into the solution;

passing at least a portion of the solution from the first leaching stage to the electrolysis stage for metal recovery; and recycling the solution from the second leaching stage to the first leaching stage; wherein the solution of relatively high acidity is produced by reacting the oxidising agent with an acid generating substance.

Advantageously, the leaching process of the present invention provides two leaching stages, one of relatively low acidity and one of relatively high acidity. This enables metals that are leached in the first leaching stage to be withdrawn from the process, typically in a clarified solution, and fed to the electrolysis stage.

More difficult to leach components of the EAF dust can then be passed with the solids to the second leaching stage having a higher acidity, and can be leached in that stage without affecting the leaching conditions in the first leaching stage.

Preferably the solution employed in the combined leaching/electrolysis circuit, incorporating the leaching process of the present invention, includes a halide or a mixture of different halides. Thus, when this solution is fed to the electrolysis circuit, an oxidising agent is produced in an anode compartment of the or each electrolysis cell employed in that circuit. Typically the oxidising agent produced is a halogen gas (when the solution includes a single halide) or is a soluble complex halide species (when the solution includes two or more different halides-eg. BrCl2-when the solution includes both chloride and bromide). Other complex halide species may also be produced, including dissolved metal halide complexes. Halide based oxidising agents have the

advantage that they have a high oxidising potential, and thus can be used to generate strong acids for feeding to the second leaching stage.

In this regard, typically the acid generating substance reacts with the halogen gas or the soluble halide species to produce acid in the solution that is recycled to the second leaching stage of the leaching process. Typically the acid produced is a halide-based acid such as hydrochloric acid, although other halide- based acids may be produced depending on the solution chemistry (eg. hydrobromic acid etc.).

In one advantageous embodiment, the acid generating substance employed is sulphur so that two acids are formed, eg. hydrochloric acid (when the solution halide is chloride) and sulphuric acid. A number of process advantages arise from using sulphur as the acid generating substance, including the fact that sulphiric acid is produced, and the resultant sulphate ion can be used to precipitate out undesirable metals (eg. calcium) in the first leaching stage, for subsequent removal and disposal (calcium sulphate being environmentaly friendly and readily disposable). However, the acid generating substance can be for example, a gas (eg. hydrogen gas) which can be reacted with a halogen gas produced in the electrolysis circuit (eg. chlorine or bromine gas) to then produce an acid for feeding into the second leaching stage.

Preferably the solution of relatively high acidity has a sufficiently high acidity (sufficiently low pH) to leach and dissolve difficult to leach/react substances in the dust and/or residue such as spinnels (ie. ZnO. Fie203).

Thus, a low pH can be employed in the second leaching stage and a relatively high pH can be employed in the

first leaching stage. By having two different low and high acidity leaching stages, those metal contaminants which are leached at relatively high pH can be leached in one stage in optimal conditions. Then, the more refractory (ie. difficult to leach) compounds can be leached in the higher acidity, lower pH stage.

Because EAF dust often includes a difficult to leach zinc oxide ferric oxide spinnel, by leaching this, ferric ion can advantageously be produced which can then be recycled from the second leaching stage to the first leaching stage to participate in further leaching (oxidation) of the EAF dust fed to the process.

Advantageously, because the first leaching stage has a relatively higher pH, (typically greater than pH 1.5) hematite is not preferentially formed with the ferric ion recycled from the second to the first leaching stage, thus freeing the ferric ion to participate in EAF solids leaching.

Preferably the solution recycled to the leaching process is reacted with the acid generating substance in a separate reaction stage to thereby separately generate the acid. Preferably the resultant acidic solution is then recycled to the leaching process. Producing the acid in a separate stage enables the solution parameters to be controlled and optimised, typically the pH and Eh (oxidising potential). However, a separate reaction stage may not necessarily be required and, for example, the acid can be formed in the solution as it is recycled to the second leaching stage, or may even be formed in the second leaching stage itself.

Preferably a portion of the mixture from the first leaching stage is passed to a thickening stage. Typically this portion represents the entire out flow from the first

leaching stage to match the mass inflow into the first leaching stage (ie. feed of EAF dust together with recycled solution from the second leaching stage).

Preferably in the thickening stage an overflow therefrom defines the solution passed to the electrolysis stage and an underflow therefrom defines the solids passed to the second leaching stage. The underflow is typically a slurry incorporating a small portion of the solution withdrawn from the first leaching stage (but in an idealised model would be entirely solids).

Preferably the solution recycled from the second to the first leaching stage is first passed through a solid- liquid separation stage. The separation stage is used to separate out solids present in that solution prior to recycling a clarified (typically filtered) solution to the first leaching stage. Typically a filtration process comprises the solid-liquid separation stage (eg. using one or a multiple number of plate filters). The solids removed from the separation stage can then be disposed of or re-used elsewhere.

Preferably at least one of the metal contaminants in the EAF dust is electrolytically recovered in the electrolysis stage. For example, EAF dust usually comprises zinc oxide so that at least zinc is electrolytically recovered in the electrolysis stage.

Where the EAF dust includes high enough levels of lead, lead may also be electrolytically recovered. Typically other"contaminants"such as cadmium, copper, manganese, silver and mercury are converted to by-products. For example, these contaminants may be precipitated out of solution in a separate purification stage, prior to electrolytic recovery, for subsequent processing or disposal. However, where they are present in sufficient

quantities, a number of them may be electrolytically or otherwise recovered in a parallel or series processing circuit.

Most EAF dusts include high levels of iron and, to a lesser extent, calcium. When the EAF dust includes iron, typically a ferric oxide precipitate (eg. hematite) is formed in the first leaching stage, which is passed with the solids fed to the second leaching stage for subsequent removal therefrom (typically in the solid-liquid separation stage). When the EAF dust includes calcium, typically the acid generating substance employed is sulphur, so that a calcium sulphate precipitate is formed in the first leaching stage (ie. as a result of sulphate ion present in the solution recycled from the second to the first leaching stage). Again, this calcium sulphate precipitate can be passed with the solids fed to the second leaching stage for subsequent removal therefrom (typically in the solid-liquid separation stage).

In a second aspect, the present invention provides a combined leaching/electrolysis process for the recovery of one or more metals from an EAF dust, wherein the leaching process of the combined process is as defined in the first aspect.

In a third aspect the present invention provides apparatus for leaching metal contaminant (s) from an EAF dust, the apparatus for use in a leaching stage in a combined leaching/electrolysis circuit wherein, in the leaching stage, a metal-containing material is leached in a solution and, in an electrolysis stage, one or more metals are recovered from the solution and an oxidising agent is produced, with the resultant solution and oxidising agent being recyclable to the leaching stage for further leaching, the apparatus comprising:

a first leaching vessel into which the EAF dust is fed to form a mixture of EAF solids with a solution of relatively low acidity so that at least some of the metal contaminant (s) can be leached into the solution; a second leaching vessel into which solids-from the first leaching stage are passed to form a mixture of the solids with a solution of relatively high acidity so that at least some of any remaining metal contaminant (s) can be leached into the solution; and acid generating means for producing the solution of relatively high acidity by enabling the reaction of the oxidising agent with an acid generating substance.

In the apparatus, preferably the acid generating means is a reactor, typically a continuously stirred tank reactor, into which the recycled solution with oxidising agent and acid generating substance are fed and reacted.

The reactor is typically operated such that the solution of relatively high acidity is optimally produced which is then passed to the second leaching vessel.

Preferably each of the first and second leaching vessels is a continuously stirred tank reactor and typically the vessels are operated in a countercurrent leaching configuration. Typically a single vessel is employed in each stage, however, multiple vessels may also constitute each of the first and second leaching stages.

Preferably the apparatus further comprises a thickener into which is fed a mixture of EAF solids and the solution of relatively low acidity withdrawn from the first leaching vessel. Preferably a clarified overflow solution is withdrawn from the thickener for passing to the electrolysis stage whilst a solids underflow is

withdrawn from the thickener for passing to the second leaching vessel.

Preferably the apparatus further comprises a solid- liquid separator, typically a filtration apparatus, into which is passed a solution to be recycled from the second to the first leaching vessel. The separator separates and removes solids present in that recycled solution so that a filtered (clarified) solution is returned to the first leaching vessel. The solids can then be disposed of or re-used elsewhere.

A fourth aspect the present invention relates to any metal produced by the processes of the first and second aspects or recovered in the apparatus of the third aspect.

BRIEF DESCRIPTION OF THE DRAWINGS Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a process flow diagram for a known zinc recovery process ; and Figure 2 shows a process flow diagram for a preferred leaching process in accordance with the present invention.

MODES FOR CARRYING OUT THE INVENTION The present invention will first be described with reference to its incorporation into a known zinc electrolytic recovery process, however, it should be appreciated that it is not in any way limited to use in this process.

In this regard, the applicant has developed a zinc recovery process (ZRP) for the production of high purity zinc ingot from sulfide concentrates. This process is set

forth in detail in WO 03/023077 (Australian patent application number 2002328671), the entire contents of which are incorporated herein by reference. This zinc recovery process is schematically depicted in Figure 1.

In its simplest preferred form, the present invention was advantageously and surprisingly able to be combined with the applicant's ZRP to allow the processing of secondary zinc-bearing materials such as electric arc furnace (EAF) dusts. From the EAF dust, high purity zinc ingot was able to be produced with minor metals such as lead, cadmium, copper, manganese, silver and mercury also being recoverable or convertible to by-products. In addition, all leachable iron was advantageously able to be precipitated into the leach residue as haematite along with calcium as gypsum (for disposal or re-use).

Non-limiting examples according to the present invention will now be described.

Examples Processing of EAF Dust In this example a modification was made to the applicant's ZRP to enable processing of EAF dusts. This modification related to the leach circuit only. The purification and electrowinning circuits of the ZRP remained unaltered and thus were able to be used for the recovery of zinc from the EAF dust.

As stated above, Figure 1 schematically depicts the applicants ZRP. In the existing ZRP as depicted, the leach circuit is shown bounded by dotted outline.

Figure 2 schematically depicts modifications made to the leach circuit of Figure 1 in accordance with the present invention. The modified leach circuit had three primary reaction stages (or zones) labelled in Figure 2 as

(1), (2) and (3). The chemical reactions which took place in these three primary stages were as follows: The modified leach circuit was based on a two-stage counter-current configuration with the EAF dust added to stage (1) and the leach liquor added to stage (2). The third stage (3) was included for the purpose of generating acid used in the leaching stages, making use of an oxidant generated in and supplied by the zinc electrowinning circuit.

A description of each stage will now be made based on the passage of leach liquor from stage (3) through to stage (1).

Stage (3) The electrowinning step in the applicant's ZRP concurrently deposited zinc metal at the cathode and regenerated an oxidant in the leach liquor (anolyte), typically a multi-halide species known by the generic name of Halex (trade mark of the applicant). This regeneration took place according to the reaction below: Br + 2C1- BrCl2~ (Halex) + 2e-

The Halex oxidising agent was typically recycled to the leaching stage in the ZRP to oxidise sulfide mineral concentrates fed to the process.

The applicants noted that EAF dust was a fully oxidised material and could only be leached with acid.

The applicants were able to transform Halex into acid by reacting it with elemental sulfur according to reaction (3) as outlined above. This reaction took place in Stage (3) as shown in Figure 2, and is repeated as follows: 3BrCl2~ (Halex) + S + 4H2O < H2SO4 + 6HC1 + 3Br- (3) When this reaction was used in conjunction with the ZRP, the sulfur consumed was one third the quantity of zinc produced on a mole basis in the ZRP, or 160kg of sulfur consumed per tonne of zinc produced.

In the ZRP it was noted that for each mole of zinc deposited at the cathode, one mole of Halex was generated at the anode. However, during the oxidation of the sulfur shown above, each mole of HalexTM generated 2 moles of HC1 and 1/3 of a mole of H2SO4. The 2 moles of HC1 were then capable of leaching one mole of zinc from the EAF dust feed, leaving the H2SO4 unutilised. This was observed to be very beneficial in maximising zinc extraction in leach Stage (2) and compensated for lime and magnesia in the EAF dust feed, as described below.

Stage (2) The highly acid liquor from Stage (3) was fed to Stage (2), along with thickened solids from a thickener (T) located in stage (1) (the thickener was used to separate the Stage (1) slurry into clear liquor and solids (described below)).

In Stage (1), the EAF dust had been depleted of all easily leachable components leaving only the relatively refractory components, such as zinc ferrite (ZnO. Fe203), to be leached in Stage (2). In Stage (2) the relatively refractory components were contacted with the highly acidic Stage (3) liquor. Leaching occurred according to reaction (2), repeated as follows: ZnO. Fe203 + 8H+ Zn2+ + 2Fe + 4H20 (2) Each mole of zinc ferrite was observed to consume 8 moles of acid during leaching, whereas in Stage (1) the zinc oxide only consumed 2 moles of acid. Consequently, the extra 6 moles of acid consumed were not available to directly leach further zinc. To compensate for this, solubilized ferric ions (Fe3+) in Stage (2) and residual acid were passed to Stage (1) after filtration of the Stage (2) slurry (in Figure 2 this filtration is indicated by S/L-a solid-liquid separator). In the different conditions (higher pH) present in Stage (1), the ferric ions and residual acid were able to leach more zinc from the fresh EAF dust feed (described below).

Stage (1) As described above, the acidic ferric liquor from Stage (2) was fed to Stage (1) along with fresh EAF dust where the residual acid was utilised to directly to leach zinc and other minor metals such as lead, copper, cadmium, silver, mercury, etc. This leaching is represented by reactions 1 (a), (b) and (d) above.

These metals were then fed in the pregnant liquor to Purification (Figure 2) where zinc, lead, etc were electrolytically or otherwise recovered.

As the residual acid was consumed in Stage (1), the liquor pH rose to the point where the ferric ion became unstable (at about pH 1.5) causing the ferric to precipitate as haematite according to reaction l (c), repeated as follows: 2Fe + 3H2O < Fe203 + 6H+ l (c) As can be seen from this reaction, the 6 moles of acid consumed in Stage (2) in the leaching of each mole of zinc ferrite were regenerated by the precipitation of hematite and were therefore available to leach more zinc.

Thus, the leaching of zinc in Stage (1) took place at a pH above that at which hematite was formed (i. e. pH 1.5). In fact zinc oxide was observed to leach in the pH range of 5-6.

The applicants also observed that EAF dust often contained considerable quantities of calcium and magnesium oxides. These oxides were highly leachable and consumed acid which was otherwise required to leach zinc. As described in Stage 3, each 2 moles of HC1 generated in Stage (3) was accompanied by 1/3 of a mole of H2SOg that was surplus to acid requirements. Thus, this sulfuric acid was available to leach the calcium and magnesium oxides according to reaction l (d), repeated as follows: CaO + 2H+ # Ca2+ + H20 l (d) The solubilised calcium was than precipitated with the sulfate from the sulfuric acid as gypsum according to reaction l (e), repeated as follows: Ca2+ + SO42- # CaSO4 1 (e)

Magnesium did not precipitate under the conditions existing in the leach circuit and so was periodically removed from a circuit bleed stream.

Whilst the leaching solution included a mix of different halides, the process was able to be operated with eg. a single halide, such as chloride or bromide.

Thus, a halogen gas was able to be produced in the electrolysis stage (eg. chlorine or bromine gas). This was then able to be recycled to the acid generation stage, where the gas was reacted with eg. hydrogen gas, to produce an acid for dissolution in the solution recycled to the second leaching stage.

Also, depending on the constituency of the EAF dust, additives to the process could include additional acid.

Where, for example, sulphuric acid was not generated, sulphate ion was also able to be added to the process to precipitate out eg. calcium sulphate.

The following advantages were noted with the present invention : o a high recovery of zinc and all minor metals in EAF dusts was attainable ; g the use of smelting and/or roasting was avoided ; * difficult to leach substances such as spinnels were able to be leached from the EAF dust; * iron present in the EAF dust was converted to stable crystalline hematite; a high purity zinc metal was produced; the process produced a non-toxic residue (eg. hematite and calcium sulphate) suitable for land fill.

Whilst the invention has been described with reference to a number of preferred embodiments, it will be appreciated that it can be embodied in many other forms.