WO 2005/093107 PCT/AU2005/000426 1 Recovery of Metals from Oxidised Metalliferous Materials TECHNICAL FIELD A process for the recovery of metals from oxidised metalliferous materials is disclosed. The term "oxidised metalliferous material" includes lateritic materials, electric arc furnace (EAF) dusts & residues, electrolytic sine plant residues, zinc oxides and zinc ferrites, goethite, arsenic trioxide, etc. The lateritic materials are typically laterite ores, such as nickel, cobalt and optionally other metal laterites. BACKGROUND ART Oxidised metalliferous materials such as laterites can have a refractory characteristic. For this reason smelting processes have been used to recover metals such as nickel and cobalt from such materials. However, the severe environmental repercussions of smelting processes have lead to the development of hydrometallurgical recovery processes for some oxidised metalliferous materials. Known hydrometallurgical processes for the recovery of metals such as nickel and/or cobalt from laterite ores have primarily involved pressure acid leaching, typically at high pressures and employing sulfuric! acid. Sulfuric acid is employed because of its abundance, cost and well-known chemistry. US 6,261,527 does discloses a hydrometallurgical process for the recovery of nickel and/or cobalt from laterite ores involving atmospheric pressures, however, it still employs sulfuric acid leaching. PCT/AU2005/000426 Received 28 February 2006 2 Recently, a process has been proposed which is based on a chloride acid leach ae opposed to a sulfuric acid leach. Whilst a chloride leaching medium is a powerful lixiviant, it is corrosive and requires apparatus to be chloride resistant:. Chloride media have also been avoided because they have a high acid consumption and can present difficulties with fche Control of iron and magnesium leaching, both metals typically present :in lateriteS. The company Chesbar Resources (now known as Jaguar Nickel Inc.) presented a paper at the AI/TA 2002 Conference (Nickel/Cobalt-9 session} in Perth, Western Australia on May 18-20, 2003. The paper entitled "Beyond PAX;: The Chesbar Option, TiKL" outlined a process for thfc atmospheric chl.oride acid leaching of nickel laterifc& ores. The process is now disclosed-in—&?Q-.-2-Q-£4V-i-Oi5-3->r The Cheebar process requires the -use of a pyrohydrolysis stage to regenerate from the process liquor HC1 as a gas for recycle to leaching, and to regenerate magnesium oxide £or use in. a. nickel/cobalt precipitation stage. However, HC1 gas ia difficult to handle, being highly corrosive. In addition, a pyrohydrolysis stage is endothermic, requiring the input of significant energy, and hence has both high capital and operating costs. It would be advantageous if a halide based process could be provided for the recovery from oxidised metalliferous materials of metals such as nickel, cobalt, copperf precious metals, magnesium etc, which does not require a pyrohydrolysis stage and which does' not require recycle of a corrosive gas for the acid leaching stage. PCT/AU2005/d0042 Received 28 February 20C 3 SUMMARY OP THE DISCLOSURE! In a first aspect there is provided a. process for recovering a target metal from' an oxidised metalliferous material, the process comprising fche ateps of: - in an acid generation stage, adding sulfuric acid to a solution comprising a metal halide to 'generate an acidic aqueous halide solution/ in a leaching stage that is separate to the 'acid generation stage, leaching the oxidised metalliferous material with the acidic aqueous halide solution to leach the target metal into solution/ - passing the solution from the leaching stage to a target metal recovery stage in which the target metal is recovered from the solution whilst the metal halide is retained in polution; and - returning the solution with the metal halide therein from the target metal recovery stage to the acid generation stage. The process thus generates an acidic aqueous halide solution, rather than employing sulfuric acid leaching. The resultant halide based leaching process is typically operated at atmospheric pressures. Whilst elevated leaching pressures can be employed (eg.. using autoclave leaching) this will depend on the oxidised metalliferoua material to be leached axtd whether more rapid target metal extraction is required. Further, the addition of sulfuric acid to generate the acidic aqueous halide solution is exothermic and hence allows the pyrohydrolysis ¦ stage of the prior art to be eliminated. This part of the process also avoids the recycling of hydrogen chloride gas as per the prior art. Thus, sulfuric acid addition and acidic, aqueous halide solution generation can provide substantial savings in capital and operating casts, typically well in excess ot the cost of producing eulfuric acid. PCT/AU2005/000426 Received 28 Februan?2006 4 However, in applications such &s the treatment of electrolytic zinc plant residues, suliruric acid is a byproduct o£ the electrolytic zinc plant process, so the acid can then be utilised economically' in the process for treatment of such residues. Where a sulfuric acici plant ie employed to produce the sulfuric acid for addition to the present process, such plants produce massive amounts of excess heat (ie. as a result of exothermic reactions) which can then he used to heat the process ablution, to provide further savings an capital and operating costs. Depending on the oxidised metalliferous material to be treated, the target matal can include nickel, cobalt, zine, copper, arsenic, iron, magnesium, precious metale such as gold, silver, platinum etc. Usually the metal selected for the metal halide solution is one that does not interfere with leaching of the target metal or its recovery as a precipitate, A metal may be selected that forms & precipitate with the eulfate anion of the acid such that, with the generation of the leaching solution, a hydrohalous acid forms together with a precipitate of the metal gulfate. In this regard, the metal m^y be calcium so that the metal suifate precipitate is calcium eulfate, which can then form a saleable byproduct. However, sodium can be present as a solution metal where the halide is derived from a sodium halide salt, in addition, magnesium chloride may also be used when the material contains high levels of magnesium (eg. to suppress Kg extraction), Usually and expediently the halide of the metal halide solution is chloride, again because of the abundance of. low cost chloride salts such as Nad- Hence, hydrochloric acid will continuously be formed as the PCT/AU2005/000426 Received 28 February 200'6 s solution metal precipitates with the sulfate, without the need to form and add or recycle hydrogen chloride) gas, ae in the prior art. This avoids the handling difficulties and hasards associated with hydrogen chloride gaa. However, other halides such as bromide or iodide can be employed, for example, where the metalliferous material includes precious metals. In this regard NaBr may then also be added to the solution, as bromide complexes more strongly than chloride and hence can stabilise the precious metals in solution. In one form the acid generation stage is defined by a second leaching stage in which both acid generation, and secondary leaching of the oxidised metalliferous material takes place. In one form the sulfuric acid can be added directly to the second leaching stage. Again, the metal of the metal halide solution can ba one that forms a precipitate with the sulfate anion of the oulfuric acid such that the acidic aqueous halide solution generated is a hydrohalous acid with a precipitate of the metal sulfate oimultaneously forming and being removed with the second leached solids to t>e discarded &e jresidue. Usually the target metal recovery 'stage comprises a precipitation stage in which a precipitate of the target metal is formed by adding a precipitation agent to the solution. This is a simple and expedient way of removing the or each target metal. When the oxidised metalliferous material comprises more than one target metal, a respective precipitation stage can thus be provided for each target metal. These stages may be arranged in the process in series. Depending on the type of oxidised metalliferous material, the leaching stage may comprise first and second PGT/AU2OO5/0OO426 Received 28 February 2006 6 leaching stages that operate in a counter-current configuration, whereby; - the oxidised metalliferous material is added to the first leaching stage to contact the solution and leach target metal into solution,- and - the solution from the first leaching stage is separated from first leached solida and passed to the target metal recovery stage? and - the first leached solids are passed to the second leaching stage to be mixed with the acidic aqueous halide solution generated in the acid generation stage,- and - the solution from the second leaching stage is separated from second leached solids an& passed to the first leaching stage, and the efecond leached solids are discarded as residue. By employing a two-stage leaching process, target metal (s) can be partially leached from the oxidised metalliferous material in the first leaching stage, and can be further leached into solution in the second leaching stage by contacting the first solids with the hydrohalous acid, Then,, leached target metal(s) from the aecond leaching stage can toe returned wilih the solution i to the first leaching stage, and thereafter lean pass with the separated solution from the first leaching stage to the target metaJ ZGcav&xy stage. Further, when the solids from the first leaching stage are passed to the second leaching stage they are, in effect, contacted with a solution of relatively high acidity (ie. compared to the solution acidity in the first leaching stage) such that a proportion of remaining target metal in the solids is then leached into the solution for subsequent recovery. in one form a portion of the solution from the second PCT/AU2005/000426 Received 28 February '2006 7 leaching stage is not passed to the first leaching stage but ie diverted to the acid generation stage, whereby after the sulfuric acid ifi added to this solution the acidic aqueous halide solution is generated for feeding to the second leaching stage to mix with the first leached solids. This stage allows for a metal sulfate precipitate to be formed and easily separated out £rom the acidic aqueous halide solution/ which metal eulfatc may be of a relative high purity. For example, the jmetal of the metal halide solution can be one that forms ia precipitate with the sulfate anion of the sulfuric acid such that, the acidic aqueous halide solution generated in the acid generation stage is a hydrohaloue acid, and such that a precipitate o£ the metal with sulfate anion forme, In addition, prior to passing the acidic aqueous halide solution to the second leaching stage, any precipitate of the metal formed with the sulfate anion can be removed, in addition, whilst an anion of the precipitation agent can cause target metal precipitation, the agent may also be used to introduce the metal of the metal halide solution (eg. the precipitation agent cation may be calcium) . Thus, addition of the precipitation agent ca.n maintain a desired concentration of that metal in solution, and to balance ,fchs process the metal may later be removed as a metal sul£ate precipitate when the hydrohaloue acid is formed. In addition, when the metal of the metal halide solution forms a precipitate with the sulf&te anion and is removed in the e generated in the second leaching stage. The process of the second aspect finds particular application where the oxidised metalliferous material includes iron, particularly at high levals- Thus a WO 2005/093107 PCT/AV2QOS/000426 11 proportion of the iron can be leached into solution in the first leaching stage and a proportion of the leached iron can later be removed (eg. precipitated as ferric oxide, typically haematite). The ferric oxide (eg haematite) precipitate can for example pass with the solids to the second leaching stage, to thereafter pass With leaching residue from the second leaching stage to disposal. The process of the second aspect can1otherwise be as defined for the first aspect. This disclosure also extends to any metal produced by the process of the first and second aspects. BRIEF DESCRIPTION OF THE DRAWINGS Notwithstanding any other forms which may fall within the scope of the process as defined in the Summary, specific forms of the process will now be described, by way of examp3 e only, with reference to the accompanying drawing in which: Figure 1 shows a flow diagram for a first process for recovering metals from a laterite ore; Figure 2 shows a flow diagram for a second process for recovering metals from a laterite ore; and Figure 3 is an X-ray diffraction plot for a bassanite precipitate. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Process of Figure 1 Figure 1 depicts a first recovery process for a lateritic ore and comprises a LEACH process coupled to a PURIFICATION (eg. precipitation) process, together with solution recycle. The recovery process of Figure 1 can be generalised to the recovery of one or more target metals from other oxidised metalliferous materials. PCT/AU20G5/000426 Received 28 February' 2006 12 In the LEACH process, a lateritic ore 10 (which can also be a pre-prepared laterite concentrate) is crushed and ground at 12 ajid is then fed to an optional two stage counter-current leaching process having a first leach stage 14 and an acid generation stag© 16, which may also function as a second leach stage, both of which operate at atmospheric pressure. The counter-current two stage leach benefits H2SO4 consumption as compared to a single stage leach but is more complex. In addition/ either leach stage hac the option of being run at higher pressures (and thus higher temperatures) using ail autoclave (eg, to minimise H2£Q4 consumption and increase target metal extraction efficiencies, although increasing capital and operating costs). whilst a single stage leach can be used for simplicity and cost, the actual configuration used depends on the metalliferous material feed composition. "With feed variation, the degree of benefit of foSOt consumption will vary and the two stage configuration may or may not be required- An acidic aqueous calcxum chloride solution is passed through stage 16, the solution having a pH in the range of 0-1. This pH is achieved through the addition of sulfuric acid at levels sufficient to leach, into solution target metals such as nickel, cobalt and precious metale. The solution leaches the already partially leached laterite residual solids that are received from 'leach stage 14 via a thickening stage 18. In stage 16 the solution has a temperature in the range of 35 to 11O°C, an Eh controlled at ~60QmV {versus Ag/Agcl), and a total chloride concentration in the range of 6 to 8M. However, the required solution for leaching depends on the mineralogy of ' the metalliferous material, in particular the types and quantities of acid WO 2005/093107 PCT/AU2OO5/O00426 13 consumers contained in the ore. For example, for lateritic ores, a minimum of 30g/l of CaCl^ is maintained in the leach to suppress jarosite formation, in turn optimising iron precipitation as haematite. The solution residence time is typically greater than 10 hours, to achieve full iron oxidation, and to release target metals into solution. Optionally, air can be sparged into the solution to maximise iron conversion through to haematite, and copper can be added to further assist oxidation as described below. When higher leach solution temperatures are required (eg. up to and in the vicinity of 200°C) then leaching in an autoclave can be employed. The slurry of leached solids and solution is then passed from stage 16 to a filtration stage 20 where residual solids are filtered and separated, the solids being washed by a CaCl2 recycle stream 22 from the PURIFICATION process (with an optional additional water wash being employed) to recover interstitial target metals. The washed solids are disposed of whereas the solution and stream 22 (and any wash water) are combined and recycled to the first leach stage 14. The acid depleted recycle now has a higher pH (the differential may be 1 or greater) and is used for a preliminary leach of the ore 10. The acid depleted recycle typically leaches a proportion of the iron into solution, from goethite (oc-FeOOH) and akagenite (/3-FeOOH) through to haematite (Fe2Oa) as described below. Again the residence time can be 10 hours or greater and, aside from pH and Eh, the solution parameters in stage 14 are similar to stage 16 described above. The partially leached solids and solution are then passed to thickening stage IS where a PCT/AU2005/00042;6 Received 28 February 2Q(f<$ 14 clear liquor overflow is paaaed to the PURIFICATION process and the solids underflow ie passed to stage 16* In the PURIFICATION process the cleat liquor overflow is first passed to an iron removal stage 24, where calcium carbonate is added to cause haematite (Fe2O3) to form and precipitate (described below). The haematite is filtered out at separation, stage 26. Optionally, where copper has been used in the leaching process to enhance oxidation, the copper is next removed from the liquor at copper precipitation stage 28 by the addition of calcium carbonate, tand is filtered out at separation stage 30. The copper residue can be reclaimed, or recycled back to the leaching process for re-use, as recycle 32. The liguor is now passed to a precious metal recovery stage 34 where NaSH is added to precipitate out ,the precious metale (described belaw), The previous metals are filtered out at separation stage 36 and recovered by smelting etc. Next, Che liquor is passed to. a nickel /cobalt recovery stage 38 where slaked lime (Ce(OH) %) is added to precipitate out the nickel and cobalt. The nickel and cobalt are filtered out at separation stage 4 0 ana" are then recovered. Any lead (Pb) in the ore can also be recovered at this stage. Finally/ where magnesium is present in the ore the liquor can be passed to a magnesium recovery stage 42, again where slaked lime (Ca(OHJa) is added to precipitate out the magnesium (described below). The' magnesium is then filtered out at separation stage 44 and recovered, optionally with any other metals still preoent in the liquor. The resulting purified liquor (CaCl2 recycle solution 22) PCT/AU2005/0:00426 Received 28 February 2006 15 is now returned to the LEACHING process. Process of Figure 2 Figure 2 depicts a second alternative ^recovery process, where like reference numerals are used to denote similar or like process stages to those of Figure 1- The recovery process of Figure 2 is again depicted for a laterite feed taut can be generalised to the recovery of one or more target metals from other oxidised metalliferous materials. The recovery process of Figure 2 again comprises a two-stage leach process, wxth separated, solution from the first leaching stage again being passed to target metal recovery stages (eg. precipitation and/or electrolytic recovery), but with solution recycle from the target metal recovery stages direct to the first leaching stage. In the two-stage leach process, a crushed and ground laterite feed 10 (having the metal concentration listed) is fed to the first leach stage 14 and then, to second leach stage 16, both of which again operate at atmospheric pressure. Again, either leach stage has the option of being run at higher pressures (and temperatures) using an autoclave. As distinct from the process of Figure 1, the process of Figure 2 comprises a separate acid generation stage 17, in which the acidic chloride solution is generated, and a second leaching stage. In this regard, like Figure 1, an HsSCu solution is added to the acid generation stage 17, A diverted stream 19, being a portion of the leach recycle ("stream 9") ie also added to the acid generation stage 17. The stream 9 comprises aqueous calcium chloride so that the diverted stream 13, which when contacted with the H2SO< solution, generates the acidic chloride - (HCl) solution and a. calcium sulfate PCT/AU2005/000426 Received 28 February 2006 16 precipitate (separated in stage 17 as a saleable byproduct) . The acidic chloride (HCl) solution is paBsed to leach stage 16, the solution having a pH in the range of 0-1, sufficient co leach into solution target mstals such as nickel, cobalt, iron, magnesium/ precious metals etc. The acidic solution leaches the partially leached residual aolids ("stream B") that are Hreceived from leach stage 14 via solid/liquid separation stage IS. in leach stage 16 the solution has a temperature in the range of 85 up to 110°C, with other parameters being similar to those described for the process of Figure 1. The slurry of leached solids and' solution is th2) is added to precipitate out the magnesium. The magnesium is then separated out (eg. filtered) at separation stage 44, for- subsequent recovery. The separated liquor from stage 44 ("stream 1") is recycled to the Eirst leach stage 14, (combining with streams 2 and 3 as stream 4). Examples Non-limiting examples of the processes described above will now be provided. Example 1 A first process; hereafter referred to as the Intec Oxidised Metalliferous Materials Process (IOMMP) was developed as a halide-based alternative- for the recovery of nickel and associated by-products from laterxtic deposits. Previously the development of such deposits W&B generally by way of pressure acid leach (PAL) or high pressure acid leaching (HPAL). The IOMMP employed a chloride medium, as opposed to the conventional sulfate medium in PAL and HPAL. The main advantage of the chloride medium is the ability to operate the leach at atmospheric pressure, without reliance on pyrohydrolysis to recover HC1 for leaching and MgO for liquor purification. WO 2005/093107 PCT/AU20 05/000426 18 The IOMMP was based on the input of H2S04 for leaching and a calcium based alkali for purification, which eliminated the need for pyrohydrolysis. The IOMMP process was also not constrained by the types of halide salts employed. In this respect NaCl was a more cost effective source of chloride ion, whilst NaBr was able to be used to enhance the completing of precious metals (Au, Ag, Pt, etc) . Conditions in the IOMMP leach, were conducive to haematite precipitation. In particular, the temperature was in the range of 85 to X1O°C, pH 0-1, residence time >10hours. Eh was controlled at -SOOmV (versus Ag/AgCl) , and total chloride was in the range of 6 to 8M. Chemistry Reference will now be made to the two main circuits of leaching and target metal recovery of Figures 1 and 2. Leaching The leach configuration and conditions depended on: • the mineralogy of the feed material; and • the relationship between acid consumption and metal extraction. The counter-current two stage leach of Figures 1 and 2 was observed to benefit H2SO4 consumption as compared to a single stage leach. The solution for leaching depended on the mineralogy of the material, in particular the types and quantities of contained acid consumers. A minimum of 30g/l of CaCl2 was maintained in the leach to suppress jarosite formation, in turn optimising iron precipitation as haematite. The preferred source of chloride was NaCl due to its low cost, but when the feed material contained high levels of Mg, then MgCl2 was used to suppress Mg extraction in order to minimise H2SO4 demand. The actual H2SO4 demand was WO 2005/093107 PCT/AU2005/000426 19 a compromise between its cost and the value of extracted metals. The level of CaCl2 in the incoming leach liquor was equivalent to the HaSO4 demand according to the following mechanism: H2S04 + CaCl3 ¦> CaSO4 + 2HC1 (acid addition to leach) (1) 2HC1 + MO ¦$> H2O 4- MCl2 (metal oxide leaching) (2) MC12 + CaO -> CaCl2 + MO (purification) (3) Thus the higher the overall metal leached the greater was the background of" CaCl2/ due to the increased addition of acid to the leach and alkali in purification. The option existed to, add NeiBr to the background solution where high levels of precious metals existed in the feed. Bromide was observed to be a stronger complex than chloride in terms of its ability to stabilise the precious metal ions in solution. An important aspect of the leach was to maximise the rejection of iron as haematite (Fe2O3) . The formation of goethite (a-FeOOH) and akagenite (p-FeOOH) in the chloride medium was noted/ and over time the FeOOH underwent dehydration to haematite according to the reaction; 2FeOOH -^ Fe2O3 + H20 (4) Haematite was the main form of iron oxide generated and this was attributed to the high residence time at > 10 hours; the relatively high temperature at >85°C, the desiccating effect of the chloride medium and the availability of seed particles in the continuous leach. Another important aspect of the process was to minimize the consumption of H^SOi and consequently CaCO^. This was achieved through the dehydration reaction. (4) described above. Haematite (Fe2Q3) was a significantly more stable form of iron oxide than goethite or the various forms found in laterite and other feed materials. "WO 2005/093107 PCT/AU20 05/0004 26 20 The transformation of iron minerals through goethite to haematite showed no net consumption of acid ae demonstrated by the reactions below: (iron minerals) + 6H* "> 2Fe3"t" + 3H3O (5) 2Fe3+ + 3H20 ¦> Fe2O3 + 6lT (6) (iron minerals) •>¦ Fe2O3 (7) Acid consumption was very significantly reduced by this route, and consequently, process economics were significantly improved. Air was able to be added to the leach to maximise iron precipitation by ensuring any reduced species such as FeO were oxidised to Fe2C>3 as per the reaction: 4FeO + O2 * 2Fe2O3 " (B) The rate of air addition was controlled to maintain the Bh at -iSOOtnV (versus Ag/AgCl) . Further a possibility existed for the addition of copper into the solution, as the Cu+/Cu2+ oxidation couple was more effective than the Fe2+/Fe3+ couple in the uptake of oxygen. The residue generated from the leach is firstly washed with brine from the purification circuit to displace valuable metal ions from the interstitial liquor. Subsequently a countercurrent washing regime is used to minimise wash water, which ultimately must be evaporated from the liquor through the input of heat. Target Metal Recovery Metal recovery in Figure 1 was usually based on precipitation using the calcium based alkalis of CaCO3 and Ca(OH)2. An alternative to precipitation was electrolytic recovery or even solvent extraction or ion exchange, where the various metal cations could be extracted in the process, replenishing the solution with acid (H+) , The choice of metal recovery step was a trade-off between the cost of the process step and its increased complexity, WO 2005/093107 PCT/AU2005/000426 21 versus the reduced acid demand and the possibly higher value of products generated. In the alkali precipitation route iron was the first to be precipitated at pH 2 with the addition of limestone according to the reaction; 2Fe3+ + 3CaCO3 -^ Fe3O3 + 3C02 + 3Ca2+ (9) Subsequently; copper {where added or present) was precipitated at pH 3 to 4 with the addition of limestone according to the reaction; 4CuCl2 + 3CaCO3 + 3H2O & 2Cu2(OH)3Cl+ 3CO2 + 3CaCl2 (10) Precious metal extraction where required was via KTaSH addition and was followed by precipitation of the remaining base metal ions with slaked lime addition according to the reaction: M2+ -f- Ca(0H)2 ¦* MO + Ca3+ + H2O (11) In the nickel and cobalt removal stage, nickel and- cobalt were precipitated by slaked lime addition. In the final removal stage magnesium was precipitated by slaked lime addition according to the reaction: Mg3+ + Ca(OH)2 ¦* HgO + Ca2+ + H2O (12) The CaCl2 rich licjuor remaining was returned to the leach. Example 2 ginc Ferrite Leach Trial A leach trial was carried out on a sample of zinc ferrite residue from an Electrolytic Zinc Plant to determine metal extraction efficiencies. A 50kg (wet) sample of the zinc ferrite was slurried at a density of £00 g/L in a brine formulated to match the process conditions for the first leach stage. The brine had the following major components: WO 2005/093107 PCT/AU2005/000426 22 Component , ebriceiitratdCon, g/L v>- -V, ir Cacl2 280 NaCl 50 FeCl3 50 ZnCl2 75 The solution metal concentrations were monitored over time with the results shown in the table below. \ t ' i&'•'£> $J (Ho,u!rsr «JCi/j.); fffprr '* 2 ¦ $ -t PH ;. ,. \. >-<, wsr ,'% )W WeigSt 0 20.5 1.0 S.2 36.4. 20 105 0.05 46.0 1 21.0 - - 40.4 - 106 0.30 _ 3 21.3 - 42.5 - 106 0.55 - 5 22.5 - &3.7 105 0.56t - 7 21.4 - - 44.3 - 106 0.4B - 9 23.4 - - - - 106 0.50 - 11 24.6 1.6 7.6 45.4 88 106 0.49 32.9 First leach stage The leach residue from the first leach stage simulation was filtered and washed with water and analysed. The results are shown in the table below. Ziy Eirsfc". FirsV.i ; Element emit1 Ferrite Feed Stage' Leach -¦ Stkge beach Residue Extract. Ag ppm 444 15 97.6% Cu 4960 2295 66-9% WO 2005/093107 PCT/AU2OO5/O0O426 23 Pe % 22 .2 27 .5 11. 3a% Pb % 9. 22 1. 3 90. 0% Zn % 15 .0 14 .6 30. 6% Mass * kg 32 .9 As can be seen from the table above, essentially all of the silver and lead v/ere extracted, while only 30% of the zinc was extracted. The leach residue from the first leach stage simulation was then used as the feed to a second leach stage simulation according to the process flowsheets (Figures 1 and 2) . The residue was slurried in 250 litres of brine prepared according to the process, flowsheet and sulfuric acid was added over time. The results are shown in the table below. Second leach stage A total of 30 kg of sulfuric acid was added over a period of 44 hours and three separate doses equivalent to addition rates of 33Okg/t, 490kg/t and 650kg/t of zinc WO 2005/093107 PCT/AU2005/000426 24 ferrate residue (dry) were made, A small sample of the leach residue was collected just before each acid addition and analysed for zinc with the results shown in the table below, along with the zinc extraction for the second leach 5 stage test, and the cumulative extraction for both first and second leach stages. From these results it can be seen that high zinc D extraction can be achieved at an acid addition rate of 490 kg/t. The whole of the leach" residue from the second leach stage test was filtered and washed with water; and then analysed for a range of metals, with the results shown in S the table below (and including the first leach stage results for clarity and comparison), * " 1' «Element Ohit SSff ' • «¦, Perrite Peed " First, Stage Leach Residue First';- T Sfcag<£- ' Loach'-; < Extrab11 S'eaond'u > ¦ Stkge-EieaciL v Residue Second StageLeach Extract, Cwcaila.£'xv Co Cppm) Fe (ppm) Mg (%) Mn (ppm) Ni (ppm) S C%) Cl(ppm) Residue27.3 <0.1 0.'07 <0.0X 20 5 21 .1 100 The high purity of the bassanite precipitate (as confirmed by X-ray diffraction shown in Figure 3) confirmed that the generation of HC1 by the addition of H2SO4 to a CaCl2 rich solution occurred without loss of metals to the precipitate. This! was highly significant, indicating that the precipitate could be sold or sent for disposal without incurring penalties for metal impurities. Furthermore, the water content of the mineral could be controlled by the temperature of the reaction. Example 5 Arsenic trioxide from smelter and roaster waste stockpiles .was converted to a safe-to-dispoee of ferric arsenate using a first stage type leach similar to that described in Examples 2 to 4. A slurry containing the arsenic trioxide was prepared in an acidified (HCl acid prepared as described in Example 4) chloride-based brine comprising dissolved ferrous ions (eg. from a leachable source of iron, such as a lateritef pyrrhotite etc) . The slurry was agitated and sparged with air for two hours at 90-95°C and a crystalline ferric arsenate precipitate was recovered. The relevant equations were: As2O3 + 6H* -> 2As3* + 3H2O (1) 2As3+ + O2 + 6H2O -* 2H3As04 (2) 2H3As04 + 2Fe3+ -* 2FeAsO4 + 6H* (3) WO 2005/093107 PCI7AU20O5/O00426 32 2Fe + 1.50a + 6H4 -* 2Pe3+ + 3HaO (4) As2O3 + 2Fe + 2.5O2 -*- 2FaAsO4 (5) Whilst a number of process embodiments have been described, it will be appreciated that the processes described herein can be embodied in many other forms. PCT/AU2OO5/O00426 Received 28 February'2006 33 CLAIMS 1. A process for recovering a target metal from an oxidised metalliferous material comprising the steps of; - in an acid generation stage, adding- sulfuric acid to a solution comprising a metal halide to generate an acidic aqueous halide solution; - in a leaching stag© that is separate to the acid generation stage, Leaching the oxidised metalliferous material with the acidic aqueous halide solution to leach the target metal into solution; - passing the solution from the leaching stage to a target metal recovery stage in which the target metal is recovered from the solution whilst the metal iialide is retained in solution; and - returning the solution with the metal halide therein from the target metal recovery stage to the acid generation stage. 2. A process as claimed in, claim l wherein the metal of the metal halide solution is one that forms a precipitate with the eulfate anlon of the sulfuric acid such that, with the generation of the acidic aqueous halide solution in the acid generation stage, a hydrohaloug acid forms together with a precipitate of the metal sulfate. 3. A process as claimed in claim 2 wherein the metal of the metal ha'lide solution is calcium so that the metal sulfate precipitate-is calcium sulfafce. 4. A process as claimed in any one of the preceding claima wherein the halide of Che metal halide solution is chloride. 5. A process as claimed in a.ny one of the preceding claims wherein, when the oxidised metalliferous material includes precious metal(s) r the halide of the metal halide solution then comprises chloride and bromide. PCT/AU20057000426 Received 28 February; 2006 34 6. A process as claimed in any one of the preceding Claims wherein the acid generation stage is defined by a second leaching stage in which both acid generation, and secondary leaching of the oxidised metalliferous material taJce place. 7. A process as claimed in claim 6 wherein the eulfuric acid is added directly to the second leaching stage. 8. A process as claimed in claim 1 wherein Che metal of the metal halide solution is one that forme a precipitate with the sulfate anion of the sulfuric acid such that the acidic aqueous halide solution generated is a hydrohalous acid, with a precipitate of the metal sulfate simultaneously forming and being removed with the second leached solids to be discarded as residue. 9. A process as claimed in any one of claims 1 to 5 wherein the leaching stage comprises ¦ first and second leaching stages that operate in a counter-current configuration, whereby: the oxidised metalliferous material is added to the first leaching stag© to contact the solution and leach target metal into solution? and - the solution from the first lsaching £tage is separated from first leached solids and passed to the target metal recovery stage; and the first leached aollds are passed to the second leaching stage to be mixed with the acidic aqueous halide solution generated in the acid generation stage? and - the solution from the second leaching stage is separated from second leached solids and passed to the first leaching stage, and the second leached solids are discarded as residue, 10. A process ae claimed in claim 9 wherein a portion of the solution from the second leaching stage is not passed PCT/AU2005/000426 Received 2% February P2OOi5 35 to the first leaching stage but is diverted to the acid generation stage whereby, after the sulfuric acid is added to this solution, the acidic aqueous halide solution is then generated for feeding to the second leaching stage to mix. with the first leached solids. 11. A. process as claimed in claim 9 or 10 wherein the metal of the metal halide solution is one that forms a. precipitate with the sul£ate anion of the sulfuric acid such that, the acidic aqueous halide solution generated in the acid generation stage is a hydrohalous acid, and such that a precipitate o£ the metal with sulfate anion forms. 12* A process as claimed in claim 11 wherein, prior to passing the acidic aqueous halide solution to the second leaching stage, any precipitate of the metal formed with the sulfafc© anion is removed. 13. A process as claimed in any one of tha preceding claims wherein the target metal recovery stage comprises a precipitation atage in which a precipitate of the target metal is formed by adding a ¦precipitation agent to the solution. ±4. A process as claimed in claim 13 wharein the precipitation agent can include a metal that is the metal of the metal halxde solution, . such that addition of the precipitation agent can maintain a desired concentration of that metal in solution, IS. A process as claimed in claim 14 wherein, when the metal of the metal halide solution forms a precipitate with the sulfate anion and is removed in the acid generation stage, a corresponding amount of that metal is added in the target metal recovery stage to maintain the desired concentration. is. A process as claimed xn any one of claims 13 to 15 wherein tne oxidised metalliferous material comprises more PCT/AU2005/OQ04?6 Received 28 February 2006 36 than one target metal, and a respective precipitation stage is provided for each target metal. 17. A process as claimed in any one of claims 13 to 16 wherein the oxidised metalliferous material includea iron, whereby- a proportion of that iron is leached into solution in the leaching stage, with at least a proportion of the leached iron then being precipitated in an iron precipitation stage as ferric oxide through the addition of calcium carbonate as the precipitation agent. 18. A proceed as claimed in claim 17 wherein the oxidised metalliferous material has sufficient 'residence time in the leaching stage such that leached iron can be oxidised through to haematite, 19. A process as claimed in any one of claims 13 to 18 wherein, when the target metal includes copper, the previous metal is precipitated in a copper precipitation stage by adding calcium carbonate as > the precipitation. agent, 20. A process as claimed in, any one of claims 13 Co 19 wherein, when the target metal includes' a precious metal, the precious metal is precipitated in' a precious metal precipitation stage by adding NaSH as the precipitation agent. 21. A proeesa as claimed in a-ny one of claims 13 to 20 wherein, when Che target metal includes nickel and/or cobalt, the nickel and/or cobalt is precipitated in a nickel/cobalt precipitation stage by adding calcium hydroxide as the precipitation agent. 22. A process as claimed in any one of claims 13 to 21 wherein, when the target metal includes magnesium, the magnesium is precipitated in a magnesium precipitation Gtage by adding calcium hydroxide as the precipitation agent. PCT/AU2005/00042S Received 28 February 2006 37 23, A process &a claimed in claim 21, or 22 wherein the calcium hydroxide is slaked lime. 24, A process as claimed in any one of claim© I to 12 wherein the target metal recovery stage comprises an electrolytic recovery stage, whereby the solution from the leaching stage is passed to one or wore electrolysis cells for metal recovery by electro-deposition. 25, A process as claimed in claim 24 -wherein the oxidised. metalliferous material comprises more than one target metal, and a respective electrolytic recovery stage is provided £QV each target metal, 26, A process as claimed ia any one of the preceding claims wherein the sulfiuric acid is added to the acid generation stage to achieve a pH in the range 0 to 1 and a solution Eh of -6O0mV. 27, A. process as claimed in any one of the preceding claims wherein the temperature of the solution ia the leaching stage is in the range 85 - ,95°C. 28, A process as claimed, in any one of the preceding claims wherein, when the halide is chloride, total chloride concentration is in the range of 6 to fl M. 29, A process as claimed in any one of the preceding claims wherein, when the halide is chloride and the solution metal ia calcium, at least 30g/l of CaCl3 is maintained in the process solution. 30, A process for leaching a target metal from an oxidiaed metalliferous material, the process ooTnprising first and second leaching stages in •which an aoidits aqueous halide solution generated from sulfuric acid and used for leaching the target metal into solution paeeee counter-currently therethrough, wherein the acid generated from sulfuric acid is added to or formed in the second Leaching stage, and the oxidised metalliferous piaterial is PCT/AU2005/006426 Received 28 February 2006 38 fed to the first leaching stage and contacted with a recycle of residual acid in solution from the second leaching stage to leach tire material and produce first: leached solids, and wherein the solution is separated from the first leached solids and may be pa.eeed to target metal recovery, whereas the first leached solids are passed to the second leaching atags for contact with the acidic agueous halide solution for further leaching of the solids whilst producing the residual acid recycle aolution. 31. A process as claimed in claim 30 wherein the acid generated from sulfurxc acid ie generated in a separate stage from the second leaching stage prior to being added thereto/ or is generated in bhe second leaching stage. 321 A pr-ocece as claimed in claim 30 or 31 wherein the oxidised metalliferous material includes iron such that a proportion of iron is leached into solution in the first leaching stage and precipitated as ferric oxide, with ' the ferric oxide precipitate passing with the solids to' the second leaching stage. 33. A process as claimed in any one of claims 30 to 32 wherein the leaching process is otherwise as defined, in any one of claims 1 to 29. 34. A target metal recovered by the process of any one of the preceding claims. A prooeee for recovering a target metal from an Qxidised metalliferous material comprises the steps of: - in an acid generation stage adding sulfuric acid to a solution comprising a metal halide to generate an acidic aqueous halide solution; in a leaching atage that is separate to the acid generation atage, leaching the oxidised metalliferous material with, the acidic aqueous halide solution to leach the target metal into solution; - passing the solution from the leaching stage to a target metal recovery stage in which the target metal ifc recovered from the solution whilst the metal halide ira retained in solution; and - returning the solution with the metal halide therein from thgj target metal recovery st]age to the acid generation stage.