TECHNICAL FIELD

[0001] The present invention relates to a method for the recovery of vanadium from leach residues using a combined hydrometallurgical and pyrometallurgical process. The present invention further relates to a method for the recovery of vanadium from alkaline feedstocks using a combined hydrometallurgical and pyrometallurgical process. More specifically, the method of the present invention is adapted to recover vanadium from secondary materials such as steel slags

BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] Vanadium is most prominently found within magnetite iron ore deposits and is typically present in slags generated during iron recovery processes. To extract or recover vanadium, the concentrates or slags are typically processed with the so-called ‘salt roast process’. In the salt roast process, the vanadium slag is mixed with one or more alkali salts and subjected to a roast typically at 800 - 900 °C, to produce sodium metavanadate. These vanadium values are subsequently and selectively leached with water. Vanadium values are then recovered in a refining process that includes precipitation from the leach solution as ammonium metavanadate or ammonium polyvanadate, both of which can be treated at high temperature to de-ammoniate and convert to product vanadium pentoxide. The process and particularly the initial high temperature salt roast step is highly energy intensive and so the vanadium tenor in the feed needs to be at a particular level to make the process economical.

[0004] A number of alternative hydrometallurgical processes have been employed to process the slags for the recovery of vanadium. Such processes typically comprise an acid leach step in order to extract vanadium into solution. The main issue faced with the recovery of vanadium by hydrometallurgical means is that other metals species, such as iron, titanium, calcium, magnesium and silica, are typically co-extracted with the vanadium during the acid leach step. The presence of these species in the leach solution must be accounted for when recovering vanadium from the leach solution. The separation of vanadium from a leach solution that also contains dissolved iron species poses a significant challenge. Most processes by which this can be achieved are economically challenging. Both vanadium and iron can be found in multiple oxidation states and degrees of coordination with varying leach systems and the mixture of species containing these elements alone can be quite complex. As a consequence, many traditional separation techniques and established reagents are unable to efficiently separate vanadium from iron. In order to address this problem, most processes require that the leach solution is first treated to remove these impurities, particularly iron and titanium, before vanadium can be recovered. This adds complexity and overall cost to processes.

[0005] CaO and other alkaline materials are also commonly found in slag materials. Consequently, a further problem with the use of an acid leach on these materials is the high consumption of acid as a result of the high alkaline content of the feed. Furthermore, when sulphuric acid is used as the leachate, a byproduct of the leach is solid CaSC .xFteO which forms at large volumes. Along with solids generated during final effluent neutralization solids, this product must be adequately disposed of.

[0006] Basic or alkaline leach systems for the recovery of vanadium from such feedstocks has not been widely applied to industry. The main complications of such systems appear to be the recovery being limited by the poor liberation of vanadium from the feedstock. Undissolved vanadium is subsequently lost to tailings with the leach residue.

[0007] Throughout this specification, unless the context requires otherwise, the term "alkaline carbonate leach solution" or similar variations, will be understood to refer to an aqueous solution comprising a carbonate or bicarbonate of an alkali metal or a carbonate or bicarbonate of an alkaline earth metal.

[0008] Throughout this specification, unless the context requires otherwise, the term “alkaline feedstock” will be understood to refer to feedstocks that comprise one or more alkali metal compounds and alkaline earth metal compounds or form an alkaline solution or slurry when mixed with water.

[0009] Throughout this specification, unless the context requires otherwise, the term “steel slag” will be understood to refer to the slag byproduct of a steel manufacturing process. As would be appreciated by a person skilled in the art, when an iron containing material is exposed to high temperatures, at least some impurities or gangue material are separated from the molten metal and are removed as a slag. This slag is subsequently cooled, and a solid material is formed.

[0010] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

SUMMARY OF INVENTION

[0011] In accordance with a first aspect of the present invention, there is provided a method for the recovery of vanadium from a vanadium containing leach residue, the method comprising the steps of: subjecting the leach residue to a calcination step to produce a calcined residue; subjecting the calcined residue to a leach step to form a calcine leach slurry; passing the calcine leach slurry to a solid liquid separation step to produce a calcine leach solution containing vanadium and a calcine leach residue; and recovering a vanadium product from the calcine leach solution.

[0012] Preferably, the leach step comprises contacting the calcined residue with an alkaline carbonate leach solution.

[0013] In one form of the present invention, the leach residue is obtained from a leach process comprising the contact of a vanadium containing feed stream with an alkaline carbonate leach solution. [0014] In one form of the present invention, the vanadium containing feed stream is an alkaline feedstock. It is envisaged that suitable feedstocks include slags, residues and other by-products of industrial processes. In one form of the present invention, the vanadium containing feed stream comprises a steel slag.

[0015] The inventors have found that vanadium can be selectively leached from alkaline feedstocks with the use of an alkaline carbonate leach solution with leach efficiencies of >75%. Without wishing to be bound by theory, the inventors believe that residual vanadium is distributed through the leach residue in original minerals and alkali and alkaline earth carbonate mineral phases that form during the leach. The inventors have found that the calcination of the leach residue will allow for further vanadium to be recovered from the calcine in a subsequent leach step. Without wishing to be bound by theory, it is understood that the calcination of the leach residue will thermally decompose the alkali and alkaline earth carbonate mineral phases, assisting to free up any vanadium distributed through these minerals. The calcined materials can then be hydrometallurgical processed to recover this vanadium into solution.

[0016] In one form of the present invention, the leach residue is passed to a drying step prior to the calcination step. Preferably, the drying step is operated at a temperature lower than the calcination step.

[0017] In one form of the present invention, the leach residue undergoes a preparation step. In one form of the present invention, the leach residue is subjected to a comminution step. The comminution step should be understood to refer to any physical process which breaks up the leach residue. This step may be avoided if the leach residue is subjected to sufficient physical forces during transport through to the calcination step.

[0018] In one form of the present invention, the calcination step comprises heating the leach residue to a temperature of at least 500 °C. In one form of the present invention, the calcination step comprises heating the leach residue to a temperature of at least 600 °C. In one form of the present invention, the calcination step comprises heating the leach residue to a temperature of at least 700 °C. In one form of the present invention, the calcination step comprises heating the leach residue to a temperature of at least 800 °C. In one form of the present invention, the calcination step comprises heating the leach residue to a temperature of at least 900 °C. In one form of the present invention, the calcination step comprises heating the leach residue to a temperature of at least 1000°C.

[0019] In one form of the present invention, the residence time of the calcination step is between 15 minutes and 4 hours. Preferably, the residence time of the calcination step is between 2 hours and 3 hours.

[0020] In one form of the present invention, the residence time of the calcination step is at least 15 minutes. Preferably, the residence time of the calcination step is at least 30 minutes. More preferably, the residence time of the calcination step is at least 1 hour. More preferably, the residence time of the calcination step is at least 2 hours.

[0021] In one form of the present invention, evolved carbon dioxide from the calcination step is recovered. Preferably, the recovered carbon dioxide is recycled to the leach process.

[0022] In one form of the present invention, the alkaline carbonate leach solution comprises one or more of sodium carbonate (Na2C03), sodium bicarbonate (NaHCOs) and sodium hydroxide (NaOH). In one form of the present invention, the alkaline carbonate leach solution comprises one or more of potassium carbonate (K2CO3), potassium bicarbonate (KHCO3) and potassium hydroxide (KOH). Any reference to sodium salts or species throughout the specification should be understood to be analogous to the use of potassium salts or species and any other alkali or alkaline earth carbonates and bicarbonates or mixtures thereof. As would be appreciated by a person skilled in the art, carbonates, bicarbonates and hydroxides exist together in aqueous solutions in a dynamic equilibrium in in the leach solution during the leach step. In strongly basic conditions, the hydroxide and carbonate ions predominate, while in weakly basic conditions the bicarbonate ion is more prevalent.

[0023] In one form of the present invention, the alkaline carbonate leach solution comprises ammonium carbonate.

[0024] In one form of the present invention, the leach step is conducted under oxidative conditions. [0025] In one form of the present invention, a carbon dioxide stream is injected into the leach step. Preferably, the carbon dioxide stream is used to control the pH of the leach step. Alternatively, carbonic acid may be added to the leach step.

[0026] In one form of the present invention, the step of: subjecting the calcined residue to a leach step comprising contacting the calcined residue with an alkaline carbonate leach solution to form a leach slurry, more specifically comprises subjecting the calcined residue to a leach step in one or more leach reactors. Preferably, the step comprises subjecting the calcined residue to a leach step in two or more leach reactors. More preferably, the step comprises subjecting the calcined residue to a leach step in three or more leach reactors. More preferably, the step comprises subjecting the calcined residue to a leach step in four or more leach reactors. More preferably, the step comprises subjecting the calcined residue to a leach step in five or more leach reactors.

[0027] In one form of the present invention, leach step is conducted at atmospheric pressure. In one form of the present invention, the leach step is conducted at elevated pressure.

[0028] In one form of the present invention, the leach step is conducted at ambient temperature. In one form of the present invention, the leach step is conducted at elevated temperature. Preferably, the leach step is conducted at a temperature of at least 50 °C. More preferably, the leach step is conducted at a temperature of at least 60 °C. Still preferably, the leach step is conducted at a temperature of at least 70 °C.

[0029] In one form of the present invention, the leach step is conducted at pH above 7.5.

[0030] In one form of the present invention, the pH of the leach step in controlled. Preferably, the leach step is maintained at a pH between 7.5 and 14. More preferably, the leach step is maintained at a pH between 9 and 10.

[0031] In one form of the present invention, the solid liquid separation step comprises the treatment of the leach slurry in a filtration apparatus. In one embodiment, the solid liquid separation step comprises a thickening apparatus upstream of the filtration apparatus.

[0032] In one form of the present invention, the step of recovering a vanadium product from the calcine leach solution is conducted following the solid liquid separation step.

[0033] In an alternative form of the present invention, the step of recovering a vanadium product from the calcine leach solution is conducted prior to the solid liquid separation step. In this form of the present invention, the calcine leach slurry is preferably contacted with an ion exchange resin. Preferably, the loaded resin is recovered from the calcine leach slurry prior to the solid liquid separation step.

[0034] In one form of the present invention, the calcine leach residue is directed to a repulp circuit to recover residual vanadium. Preferably the repulp circuit comprises a pulping step, a solid liquid separation step and an ion exchange step. More preferably, the pulping step comprises the contact of the calcine leach residue with an aqueous solution.

[0035] In one form of the present invention, the step of recovering a vanadium product from the calcine leach solution comprises precipitating a vanadium rich solid and separating the vanadium rich solid from the barren leach solution. Throughout the specification, the term “barren leach solution” will be understood to refer to a leach solution to which at least a portion of the vanadium has been recovered. It should be understood to include a solution that contains vanadium.

[0036] In an alternative form of the present invention, the step of recovering a vanadium product from the calcine leach solution comprises contacting the calcine leach solution with an ion exchange medium to selectively recover vanadium and separate it from the barren leach solution. Preferably, the loaded ion exchange medium is contacted with an eluent to recover vanadium into a vanadium eluate solution. More preferably, the eluent is sodium hydroxide or potassium hydroxide.

[0037] In an alternative form of the present invention, the step of recovering a vanadium product from the calcine leach solution comprises contacting the calcine leach solution with an organic solution comprising a vanadium extractant and subsequently separating the loaded organic solution from the barren leach solution. Preferably, the loaded organic solution is contacted with a scrub solution. In one form of the present invention, vanadium is recovered from the loaded organic solution with a strip solution. Preferably, the strip solution is sodium hydroxide or potassium hydroxide.

[0038] In embodiments where an organic solution comprising a vanadium extractant is used to recover vanadium from the calcine leach solution, the present invention further comprises the step of recovering vanadium products from the strip solution. In one embodiment, the recovery of vanadium products comprises precipitating a vanadium rich solid and separating the vanadium rich solid from the barren strip solution.

[0039] In an alternative form of the present invention, the step of recovering a vanadium product from the calcine leach solution comprises passing the calcine leach solution through a nanofiltration system in which the bicarbonate ion is preferentially removed from the vanadium species. Preferably, the bicarbonate stream is returned to the leach step.

[0040] In an alternative form of the present invention, the step of recovering a vanadium product from the calcine leach solution comprises passing the calcine leach solution to a crystallisation circuit where the sodium carbonate and / or bicarbonate salts are selectively crystalised over vanadium species. Preferably, the crystallisation circuit comprises one or more crystallisation stages. In one form of the present invention, the crystalised solids are removed and recycled to the leach step.

[0041] In accordance with a second aspect of the present invention, there is provided a method for the recovery of vanadium from a vanadium containing feed stream, the method comprising the steps of: subjecting the vanadium feed stream to a leach process, the leach process comprising contacting the vanadium feed stream with an alkaline carbonate leach solution to form a leach slurry; passing the leach slurry to a solid liquid separation step to produce a pregnant leach solution containing vanadium and a leach residue; subjecting the leach residue to a calcination step to produce a calcined residue; subjecting the calcined residue to a leach step comprising contacting the calcined residue with an alkaline carbonate leach solution to form a calcine leach slurry; passing the calcine leach slurry to a solid liquid separation step to produce a calcine leach solution containing vanadium and a calcine leach residue; and recovering a vanadium product from the pregnant leach solution and/or the calcine leach solution.

[0042] The method of present invention has been found to be suitable for use on alkaline feedstocks. It is envisaged that the present invention may be used to recover vanadium from a range of different sources, including slags, residues and other by products of industrial processes.

[0043] The method of the present invention is preferably adapted to recover vanadium products from slag materials that result from the steel industry. In addition to vanadium, such materials will contain iron, along with other species such as manganese titanium and chromium. The method of the present invention allows for vanadium to be leached from such materials with high selectivity over other impurity metals. This has been found to simplify the subsequent recovery of vanadium from the calcine leach solution.

[0044] In one form of the present invention, the pregnant leach solution and the calcine leach solution are combined prior to the recovery of vanadium. In one form of the present invention, the calcine leach solution is treated to increase the vanadium concentration prior to mixing with the pregnant leach solution.

[0045] In an alternative form of this invention the pregnant leach solution and the calcine leach solution are treated separately for vanadium recovery. [0046] In one form of the present invention, at least a portion of the leach slurry is subjected to a size reduction step. In one form of the present invention, the size reduction step is conducted during the leach process. In an alternative form of the present invention, at least a portion of the leach slurry is transferred to a size reduction step to produce a process stream with a reduced particle size. In one form of the present invention, the process stream is returned to the leach process.

[0047] In one form of the present invention, the slag leach step more specifically comprises one or more separate leach stages, with each stage comprising the contact of the feed stream with an alkaline carbonate leach solution. In one form of the present invention, the leach process more specifically comprises two or more separate leach stages. In one form of the present invention, the leach process more specifically comprises three or more separate leach stages.

[0048] In forms of the present invention where the leach process comprises two or more separate leach stages, the leach stages are arranged in series. Preferably, the leach slurry from each upstream leach stage is directed to the downstream leach stage.

[0049] In forms of the present invention where the leach process comprises two or more separate leach stages, the leach slurry is subjected to a size reduction step between successive leach stages.

[0050] In one form of the present invention, each leach stage is conducted in one or more leach reactors. In one form of the present invention, each leach stage is conducted in two or more leach reactors. In one form of the present invention, each leach stage is conducted in three or more leach reactors. In one form of the present invention, each leach stage is conducted in four or more leach reactors.

[0051] In one form of the present invention, the alkaline carbonate leach solution comprises one or more of sodium carbonate (Na2C03), sodium bicarbonate (NaHCOs) and sodium hydroxide (NaOH). In one form of the present invention, the alkaline carbonate leach solution comprises one or more of potassium carbonate (K2CO3), potassium bicarbonate (KHCO3) and potassium hydroxide (KOH). Any reference to sodium salts or species throughout the specification should be understood to be analogous to the use of potassium salts or species and any other alkali or alkaline earth carbonates and bicarbonates or mixtures thereof. As would be appreciated by a person skilled in the art, carbonates, bicarbonates and hydroxides exist together in aqueous solutions in a dynamic equilibrium in in the leach solution during the leach step. In strongly basic conditions, the hydroxide and carbonate ion predominates, while in weakly basic conditions the bicarbonate ion is more prevalent.

[0052] In one form of the present invention, the alkaline carbonate leach solution comprises ammonium carbonate.

[0053] In one form of the present invention, the leach process is conducted under oxidative conditions. Preferably, the leach process is conducted in the presence of an oxidant. More preferably, the oxidant is selected from oxygen, air, and hydrogen peroxide. Alternatively, the feedstock is combined with an oxidant prior to the leach process. Suitable oxidants include MnC>2. In an alternative form of the present invention, no oxidant is added to the leach process.

[0054] In one form of the present invention, an oxidant is added to the leach process. Preferably, the addition of the oxidant will target a solution Eh of > -100 mV against a Ag/AgCI reference electrode.

[0055] In one form of the present invention, the step of subjecting the feed stream to a leach process is conducted at atmospheric pressure. In one form of the present invention, the step of subjecting the feed stream to a leach process is conducted at elevated pressure.

[0056] In one form of the present invention, the step of subjecting the feed stream to a leach process is conducted at ambient temperature. In one form of the present invention, the step of subjecting the feed stream to a leach process is conducted at elevated temperature.

[0057] In one form of the present invention, the leach process is conducted at pH above 7.5. [0058] In one form of the present invention, the pH of the leach process in controlled. Preferably, the leach process is maintained at a pH between 7.5 and 14. More preferably, the leach process is maintained at a pH between 9 and 10.

[0059] In one form of the present invention, a carbon dioxide stream is injected into the leach process. Preferably, the carbon dioxide stream is used to control the pH of the leach process. Alternatively, carbonic acid may be added to the leach process.

[0060] In an alternative form of the present invention, the leach process is conducted until the carbonate concentration is reduced to a target concentration. Preferably, the target concentration is below 5 g/L.

[0061] In one embodiment of the present invention, the method further comprises the step of: subjecting the feed stream to a pretreatment process, prior to the step of subjecting the feed stream to the leach step.

[0062] Preferably, the pre-treatment process comprises one or more size reduction steps. More preferably, the one or more size reduction steps comprise one or more of a crushing step, a grinding step and a milling step.

[0063] In one form of the present invention, the pre-treatment process comprises one or more beneficiation steps. Preferably, the one or more beneficiation steps include one or more of a gravity classification step, a magnetic classification step and a flotation step.

[0064] In one form of the present invention, the feed stream is subjected to a pre leach step, prior to the leach process. Preferably, the pre-leach step comprises the contact of the feed stream with an alkaline liquor to produce a pre-leach slurry. In one form of the present invention, the alkaline liquor is an alkaline carbonate leach solution. Preferably, the alkaline liquor comprises sodium carbonate and/or sodium bicarbonate. In one form of the present invention, the alkaline liquor comprises sodium hydroxide. In one form of the present invention the alkaline liquor comprises potassium carbonate and/or potassium bicarbonate. In one form of the present invention, the alkaline liquor comprises potassium hydroxide. In one form of the present invention, at least a portion of the alkaline liquor may be supplemented by recycle streams. As discussed above, carbonates, bicarbonates and hydroxides exist together in aqueous solutions in a dynamic equilibrium. It is envisaged that all three will be present in varying proportions in the alkaline leach solution.

[0065] In one form of the present invention, the pre-leach step is conducted after the feedstock has been subjected to one or more size reduction steps. In one form of the present invention, the pre-leach slurry is subjected to one or more size reduction steps.

[0066] In one form of the present invention, a carbon dioxide stream is injected into the pre-leach step.

[0067] In one form of the present invention, the leach slurry is directed to a classification apparatus with the overflow being directed to the solid liquid separation step and the underflow being recycled back to the leach step, the pre-leach step or the size reduction step.

[0068] In one form of the present invention, the solid liquid separation step comprises the treatment of the slurry in a filtration apparatus. In one embodiment, the solid liquid separation step comprises a thickening apparatus upstream of the filtration apparatus.

[0069] In an alternative form of the present invention, the solid liquid separation step comprises the treatment of the slurry in a counter current decantation (CCD) circuit. In one embodiment, the CCD circuit comprises two or more thickeners arranged in series.

[0070] In one form of the present invention, the step of recovering a vanadium product from the pregnant leach solution and/or the calcine leach solution comprises precipitating a vanadium rich solid and separating the vanadium rich solid from the barren leach solution. Throughout the specification, the term “barren leach solution” will be understood to refer to a leach solution to which at least a portion of the vanadium has been recovered. It should be understood to include a solution that contains vanadium. [0071] In an alternative form of the present invention, the step of recovering a vanadium product from the pregnant leach solution and/or the calcine leach solution comprises contacting the pregnant leach solution and/or the calcine leach solution with an ion exchange medium to selectively recover vanadium and separate it from the barren leach solution.

[0072] In an alternative form of the present invention, the step of recovering a vanadium product from the pregnant leach solution and/or the calcine leach solution comprises contacting the pregnant leach solution and/or the calcine leach solution with an organic solution comprising a vanadium extractant and subsequently separating the loaded organic solution from the barren leach solution. Preferably, the loaded organic solution is contacted with a scrub solution. In one form of the present invention, vanadium is recovered from the loaded organic solution with a strip solution. Preferably, the strip solution is sodium hydroxide or potassium hydroxide.

[0073] In embodiments where an organic solution comprising a vanadium extractant is used to recover vanadium from the pregnant leach solution and/or the calcine leach solution, the present invention further comprises the step of recovering vanadium products from the strip solution. In one embodiment, the recovery of vanadium products comprises precipitating a vanadium rich solid and separating the vanadium rich solid from the barren strip solution.

[0074] In an alternative form of the present invention, the step of recovering a vanadium product from the pregnant leach solution and/or the calcine leach solution comprises passing the pregnant leach solution and/or the calcine leach solution through a nanofiltration system in which the bicarbonate ion is preferentially removed from the vanadium species. Preferably, the bicarbonate stream is returned to the leach step.

[0075] In an alternative form of the present invention, the step of recovering a vanadium product from the pregnant leach solution and/or the calcine leach solution comprises passing the pregnant leach solution and/or the calcine leach solution to a crystallisation circuit where the sodium carbonate and / or bicarbonate salts are selectively crystalised over vanadium species. Preferably, the crystallisation circuit comprises one or more crystallisation stages. In one form of the present invention, the crystalised solids are removed and recycled to the leach process. [0076] A barren leach solution results from the step of recovering a vanadium product from the pregnant leach solution and/or the calcine leach solution. In one form of the present invention, at least a portion of the barren leach solution is recycled to the leach process. In one form of the present invention, the barren leach solution is carbonated prior to being recycled to the leach process. In embodiments where the leach process is conducted in the presence of a carbon dioxide stream, the barren leach solution may alternatively be directly recycled to the leach process.

[0077] Preferably, the barren leach solution is used to supplement at least a portion of the alkaline carbonate leach solution. In one form of the present invention, the barren leach solution is carbonated prior to being recycled to the pre-leach step.

[0078] In one form of the present invention, the barren leach solution is used as a wash water in a solid liquid separation step.

[0079] In one form of the present invention, the barren leach solution is used in the feed or intermediate size reductions step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a flowsheet of the method according to a first embodiment of the present invention; and

Figure 2 is a flowsheet of the method according to a second embodiment of the present invention. DESCRIPTION OF EMBODIMENTS

[0081] A first aspect of the present invention relates to a method for the recovery of vanadium from a vanadium containing leach residue. In a broad sense, the method comprises the steps of: subjecting the leach residue to a calcination step to produce a calcined residue; subjecting the calcined residue to a leach step comprising contacting the calcined residue with an alkaline carbonate leach solution to form a leach slurry; passing the leach slurry to a solid liquid separation step to produce a calcine leach solution containing vanadium and a calcine leach residue; and recovering a vanadium product from the calcine leach solution.

[0082] The method of the present invention is intended to recover residual vanadium contained within leach residues that result from hydrometallurgical processes. As would be appreciated by a person skilled in the art, hydrometallurgical processes involve a leach step in which an aqueous leachant is used to extract soluble metals from a feedstock into solution. Following the leach step, the calcine leach solution is separated from the undissolved material, referred to as the leach residue. Separation typically involves the use of a filter to retain the solid residue on the filter as a filter cake. Whilst the majority of the soluble metals will pass into solution, a certain percentage of the soluble materials will remain entrained in the leach residue. The loss of metals in the leach residue reduces overall metal recovery/production and revenue.

[0083] As would be appreciated by a person skilled in the art, certain leach systems will precipitate salts throughout the leach step. This is typically the result of simultaneous redox reactions that occur during the leach process. The inventors have identified that at least some of the dissolved vanadium species can be retained within the crystal structure of the precipitated solids. This prevents the dissolved vanadium passing through the filter and into the filtrate. The inventors have found that the method of the present invention can be used to recover vanadium retained in such crystal structures. Without wishing to be bound by theory, it is understood that the calcination of the leach residue will thermally decompose precipitated carbonate mineral phases, thereby freeing any vanadium distributed through these minerals. The calcined materials can then be hydrometallurgical processed to recover this vanadium into solution.

[0084] The inventors have identified that the method of the present invention is particularly useful for the recovery of soluble metal species from the leach residue resulting from a leach step comprising the contact of a feed stream with an alkaline carbonate leach solution. As would be appreciated by a person skilled in the art, alkaline carbonate leach solutions comprise alkali metal or alkaline earth metal carbonate/bicarbonate species. In preferred embodiments of the present invention, the alkaline carbonate leach solution comprises sodium species.

[0085] The method of the present invention has been found to be suitable for use on leach residues that result from the leaching of alkaline feedstocks. Suitable alkaline feedstocks include slags, residues and/or other by-products of industrial processes. In a preferred embodiment, the alkaline feedstock is a slag material that results from the steel industry.

[0086] In a preferred embodiment of the present invention, the leach reside is obtained from the leaching of an alkaline feedstock with an alkaline carbonate leach solution. The inventors have found that an alkaline carbonate leach solution demonstrates good selectivity for vanadium over other metals that may be found in the feedstock. The method of the present invention has been found to be particularly useful in the recovery of vanadium from the leach residues that result from such leach processes. Without wishing to be bound by theory, the inventors believe that residual vanadium is distributed through the leach residue in original minerals and calcium carbonate mineral phases that form during the leach. It is understood that such feedstocks typically comprise a high CaO and Ca(OH)2 content. These species will react with carbonates in the leach solution to precipitate one or more mineral form of calcium carbonate. It is believed that at least some of the dissolved vanadium will be entrained within the crystal structure of these precipitated species. The inventors have found that the calcination of leach residue will allow for further vanadium to be recovered from the leach residue in a subsequent leach step. Without wishing to be bound by theory, it is understood that the calcination of the leach residue will thermally decompose the calcium carbonate mineral phases, thereby assisting to free up a significant portion of the vanadium distributed through these minerals. The calcined materials can then be hydrometallurgical processed to recover this vanadium into solution.

[0087] In one embodiment of the present invention, the method further comprises a leach step, the leach step comprising the contact of an alkaline feedstock with an alkaline carbonate leach solution to produce a pregnant leach solution and the leach residue.

[0088] In Figure 1 , there is shown a method for the recovery of vanadium from a leach residue 10 in accordance with an embodiment of the present invention. In the embodiment shown in Figure 1 , a raw feedstock 14 is subjected to a processing circuit 16. It is envisaged that the processing circuit 16 may comprises one of more size reduction steps (not shown). It is envisaged that conventional crushing and grinding apparatus available to those skilled in the art can be used to reduce the particle size of the feedstock 14. It is envisaged that the processing circuit 16 may one or more beneficiation steps (not shown) to remove excess low value bearing components of the feedstock 14. The one or more beneficiation steps can include one or more of a gravity classification step, a magnetic classification step and a flotation step. It is envisaged that both wet and dry particle size reduction apparatus may be utilised.

[0089] The processed feed 18 is directed to a leach process, for example leach circuit 20 where it is contacted with a leach solution 22 in order to produce leach slurry 24. It is envisaged that a range of different feedstocks and leach solutions may be utilised in the leach circuit 20 in order to target one or more metals from the feedstock. The inventors have found that the method of the present invention is particularly useful for leach steps that target vanadium in steel slag feedstocks using an alkaline carbonate leach solution. A suitable leaching method is described in the Applicant’s co-pending PCT/AU2021/050094, the contents of which are hereby incorporated herein by reference in their entirety. The following discussion is made in reference to a process for the leaching of vanadium from such feedstocks using a leach solution comprising sodium carbonate.

[0090] The leach slurry 24 is directed to a solid liquid separation step 26 to separate out a leach residue 28 from a pregnant leach solution 30. The solid liquid separation step 26 may comprise one or more washing steps in which wash water is flushed through the leach residue 28 whilst on the filter in order to recover some of the soluble metals that are retained in the leach residue 28. The wash filtrate may then be recycled back to the leach circuit 20 to prevent metal losses. As discussed above, the inventors of the present invention have found that even after washing, the leach residue 28 can still contain entrained vanadium. Disposal of the leach residue 28 will result in the loss of this vanadium. The method of the present invention seeks to recover at least some of the vanadium in the leach residue 28.

[0091] In the embodiment shown in Figure 1 , the leach residue 28 is directed to a drying step 32 to remove the majority of moisture present in the leach residue 28. The dried residue 34 is then directed to calcination step 36 where is treated at high temperature to produce a calcined residue 38. It is understood by the inventors that calcination units may incorporate a drying stage. In such apparatus, both the drying step and the calcination step may be conducted in a single unit.

[0092] In one embodiment, the calcination step is conducted at a temperature of over 500°C. Preferably, the calcination step is conducted at a temperature of at least 1000°C.

[0093] In one embodiment, the residence time of the calcination step is between 15 minutes and 4 hours. Preferably, the residence time of the calcination step is between 2 hours and 3 hours.

[0094] In one embodiment, the residence time of the calcination step is at least 15 minutes. Preferably, the residence time of the calcination step is at least 30 minutes. More preferably, the residence time of the calcination step is at least 1 hour. More preferably, the residence time of the calcination step is at least 2 hours.

[0095] As would be appreciated by a person skilled in the art, the residence time of the calcination step will depend on the apparatus used and the calcination temperature. As discussed above, the calcination of the leach residue will thermally decompose the calcium carbonate mineral phases. The decomposition rate will be initially high, but will slow down throughout the calcination step until a theoretical maximum amount of material has been decomposed. At this point, the continued calcination of the solid provides marginal additional material decomposition. As calcination processes are energy intensive, the operating costs eventually outweigh the additional benefit. Preferably, the weight loss of the leach residue is monitored throughout the calcination step. Preferably, the calcination step is conducted until at least 90% of the maximum weight loss is achieved. More preferably, the calcination step is conducted until at least 95% of the maximum weight loss is achieved. Still preferably, the calcination step is conducted until at least 98% of the maximum weight loss is achieved.

[0096] Any suitable calciner apparatus known to those skilled in the art my be used in the calcination step. In a preferred embodiment, the calciner apparatus includes a means of agitation. The inventors have found that agitation of the solids assists in breaking up the leach residue filter cake and increases the degree of carbonate decomposition. In one embodiment, a small amount of air or other ingress gas may be allowed to pass through the calciner apparatus to assist in decomposition off-gas removal from the calciner apparatus.

[0097] The calcined residue 38 is directed to a leach step 40. In leach step 40, the calcined residue is contacted with an alkaline carbonate leach solution 42 to form a calcine leach slurry 44. The calcine leach slurry 44 is directed to a solid liquid separation step 46 to remove a solid calcine residue 48 from a calcine leach solution 50. Wash water (not shown) is used in the solid liquid separation step 46 to ensure the maximum entrained liquids and soluble species are fully separated from the calcine residue 48. The calcine residue 48 is preferably directed to tailings or otherwise removed from the circuit. It is envisaged that the solid liquid separation step 46 will be conducted in a filtration apparatus, such as belt filter. Alternative solid liquid separation devices may be utilised in solid liquid separation step 46. It is envisaged that a pre-filter thickener may also be used. In one embodiment, a counter-current washing process is used in the solid liquid separation step 46 in order to maximise vanadium recovery.

[0098] In the embodiment shown in Figure 1 , leach step 40 is separate from leach circuit 20. In an alternative form of the present invention, the calcined residue 38 may be returned to any leach stage of leach circuit 20. If the calcined solids are returned to the leach circuit, it is envisaged that additional process controls would need to be implemented to provide a route in which solids are able to be removed from the overall circuit, for example a solids bleed or in a batch operation. In a further alternative form of the present invention, the calcination step 36 may be incorporated between successive stages of the leach circuit 20. In one form of the present invention, the calcine residue 38 is directed to a repulp circuit to recover residual vanadium. Preferably the repulp circuit comprises a pulping step, a solid liquid separation step and an ion exchange step. More preferably, the pulping step comprises the contact of the calcine residue 38 with an aqueous solution.

[0099] The calcine leach solution 50 comprises dissolved vanadium and it is directed to a vanadium recovery circuit 52 to recover vanadium. In the embodiment shown in Figure 1 , the calcine leach solution 50 is combined with the pregnant leach solution 30 to produce a combined leach solution 54 prior to vanadium recovery. It is envisaged that the combination of these streams will allow for vanadium recovery in a single circuit. In an alternative embodiment of the present invention, vanadium may be recovered from the calcine leach solution 50 is a separate recovery circuit.

[00100] It is envisaged that vanadium may be recovered from the calcine leach solution 50 (or the combined leach solution 54) by any suitable means known in the art. In the embodiment shown in Figure 1 , the combined leach solution 54 is directed to a solvent extraction circuit 56. In the solvent extraction circuit 56, the combined leach solution 54 is contacted with an organic extractant to extract vanadium ions from the aqueous phase into the organic phase. The loaded organic phase may be separated from the barren leach solution. The loaded organic can then be contacted with a scrub solution to displace entrained aqueous phase or weakly extracted impurities from the loaded organic. The loaded organic will then be contacted with an aqueous strip solution to recover vanadium from the loaded organic into a vanadium strip solution 58. Vanadium may then be recovered from the vanadium strip solution 58 by conventional means, such as for example precipitation, crystallisation or electrolysis.

[00101] In the embodiment shown in Figure 1 , the vanadium strip solution 58 is directed to a desilication step 60 where it is contacted with an aluminium salt, for example aluminium sulphate to precipitate aluminium silica compounds. Silica removal may also require pH adjustment at this stage, and this is most readily achieved with a small quantity of sulphuric acid. The desilication stage is also best performed at higher temperatures, over 50 °C and preferably over 70 °C. The precipitated solids and other insoluble materials are removed in a solid liquid separation step. The filtrate 62 is cooled and directed to a precipitation step 64 where it is contacted with ammonium sulphate to precipitate ammonium metavanadate. If needed, sulphuric acid may be added to precipitation step 64 to control solution pH for optimal vanadium recovery. A target pH of between 8 - 9 is preferred. The resulting slurry is directed to a filtration step. The filtered solids are washed with dilute ammonium sulphate solution to remove any entrained liquors and further purify the filter cake. The recovered solids 66 are directed to calcination step 68 for deammoniation and subsequent powder melting and production of solid V2O5 flakes 70. Barren liquor 72 from the precipitation step 64 is directed to a crystallisation step 74 to recover sodium sulphate crystals.

[00102] In an alternative embodiment of the present invention, the calcine leach solution 50 is directed to a precipitation step where vanadium solids are precipitated from the calcine leach solution 50. The precipitated solids are subsequently separated from the barren leach solution and directed for further processing. It is envisaged that sodium vanadate may be directly crystallized from the calcine leach solution 50. Alternatively, the calcine leach solution 50 may be contacted with an ammonium species to precipitate NH4VO3. The resulting precipitate can be recovered and directed to a calcination step to produce a V2O5 product.

[00103] In an alternative embodiment of the present invention, the calcine leach solution 50 is directed to an ion exchange step to extract and subsequently recover a vanadium rich eluent from the barren leach solution. The vanadium rich eluent is directed to a precipitation step where is it contacted with ammonia and/or ammonium sulphate, preferably in the presence of sulfuric acid to precipitate ammonium metavanadate. The resulting slurry is directed to a solid liquid separation step. The resulting solid stream is directed to a calcination step to produce V2O5 product.

[00104] In accordance with a second embodiment of the present invention, there is provided a method for the recovery of vanadium from a vanadium containing feed stream. In a broad sense, the method comprises the steps of: subjecting the vanadium feed stream to a leach process, the leach step comprising contacting the vanadium feed stream with an alkaline carbonate leach solution to form a leach slurry; passing the leach slurry to a solid liquid separation step to produce a calcine leach solution containing vanadium and a leach residue; subjecting the leach residue to a calcination step to produce a calcined residue; subjecting the calcined residue to a leach step comprising contacting the calcine stream with an alkaline carbonate leach solution to form a calcine leach slurry; passing the calcine leach slurry to a solid liquid separation step to produce a calcine leach solution containing vanadium and a calcine leach residue; and recovering a vanadium product from the pregnant leach solution and/or the calcine leach solution.

[00105] The method of the second embodiment of the invention is intended to maximise the recovery of vanadium from the feed stream. The inventors have found that whilst the majority of the vanadium in the feed stream is recovered in the leach process, that the leach residues resulting from the leach process still contains vanadium. The present invention seeks to process these leach residues to allow for further extraction of vanadium.

[00106] As discussed above, the inventors have identified that the method of the second embodiment of the present invention is particularly useful for the recovery of vanadium from alkaline feedstocks. It is envisaged that the present invention may be used to recover vanadium from a range of different sources, including slags, residues and other by-products of industrial processes. The method of the present invention is preferably adapted to recover vanadium products from slag materials that result from the steel industry. In addition to vanadium, such materials will contain iron, along with other species such as manganese titanium and chromium. The method of the present invention allows for vanadium to be leached from such materials with high selectivity over other impurity metals. This has been found to simplify the subsequent recovery of vanadium from the pregnant leach solution. [00107] In Figure 2 there is shown a method for the recovery of vanadium 100 from a vanadium containing feed stream 102 in accordance with an embodiment of the present invention.

[00108] Feed stream 102 is directed to a primary mill step 104 where the particle size of the feed stream is reduced to the target size. Process water is added to the mill with the material feed in the primary mill step 104. The primary mill discharge is directed to a cyclone (not shown) to separate a primary feed stream 106 (cyclone overflow). The oversized underflow is directed back to primary mill step 104 for further processing. The primary feed stream 106 should have a target particle size of <200 pm, with a target particle size of <100 pm being preferred and < 75 pm being more preferable. It is envisaged that conventional crushing and grinding apparatus available to those skilled in the art can be used to reduce the particle size of the feed stream 102. Suitable milling apparatus include ball mills, rod mills, autogenous grinding (AG) mills, and semi- autogenous grinding (SAG) mills. The feed stream 102 may be subjected to one or more beneficiation steps (not shown) to remove excess low value bearing components of the feedstock 12. As discussed previously, the one or more beneficiation steps can include one or more of a gravity classification step, a magnetic classification step and a flotation step. It is envisaged that both wet and dry particle size reduction apparatus may be utilised. Whilst a wet size reduction process is discussed above, it is envisaged that a dry size reduction process could also be used. Suitable dry size reduction means include high pressure grinding rolls (HPGR) or vertical roller mill (VROM) with air cyclones for classification

[00109] The primary feed stream 106 is directed to a leach process, for example leach circuit 108 where it is contacted with an alkaline carbonate leach solution.

[00110] Depending on the precise feed material being used, the vanadium species in the feed may exist in a number of different forms. For example, calcium vanadate species may include Ca(V0 ₃ )2, CaV2O6.3H20, CaV206.4H20, CaV60i6.9H20, Ca2V207.9H20, Ca ₃ Vio028.16H20, amongst others. By way of illustration, the calcium vanadates in the feedstock may react with the alkaline carbonate leach solution in the leaching process according to the following reactions:

Ca(V0 ₃ )2 + M2CO3 = CaCOs + 2MV0 ₃ Ca2V207 + 2M2CO3 = CaC03 + M4V2O7 (where M is an alkaline metal )

It would be appreciated by a person skilled in the art the above reactions are included for illustration purposes only and are not an exhaustive list of the reactions occurring during the leach circuit 108.

[00111] Additional alkaline liquor or alkaline carbonate leach solution may be directed into leach circuit 108 to increase the CO3 ² / HCO3 concentration in the leach solution. Alternatively, CO3 ² / HCOs imay be regenerated in the solution.

[00112] An oxidant stream may be injected into the leach process 108 to oxidise at least a portion of the components of the pre-leach slurry and improve vanadium recovery. Without wishing to be bound by theory, it is believed that at least some of the vanadium in the feedstock may be encapsulated in various Fe(ll) compounds found in the feedstock. The inventors have found that the addition of the oxidant will oxidise Fe(ll) to Fe(lll) which can assist in the liberation of vanadium from within these compounds. Furthermore, any dissolved Fe(lll) will likely re-precipitate as FeO(OFI) (also written as Fe203.Fl20). In one embodiment, the oxidant is selected from hydrogen peroxide and potassium permanganate.

[00113] A carbon dioxide stream (not shown) may also be directed into the leach circuit 108. As would be appreciated by a person skilled in the art, alkaline feedstocks such as steel slags contain a high CaO and Ca(OFI)2 content. The Na2C03/NaFIC03 in the leach liquor will react with CaO/Ca(OFI)2 to produce solid CaCC>3 and NaOFI. The reactions can be summarized by the following simplified reactions:

CaO + Na ₂ C0 ₃ + H2O -» CaCOs + 2NaOH CaO + NaHCOs -» CaCOs + NaOH

Ca(OH) ₂ + Na ₂ COs - 2NaOH + CaCOs [00114] The inventors have found that carbon dioxide will react with sodium hydroxide and other species in the leach solution to form a reactive carbonate system as represented by the following simplified reactions:

H2O + CO2 ®· H2CO3

NaOH + CO2 -> NaHCOs

2NaOH + CO2 - Na ₂ C03 + H ₂ 0

[00115] The addition of carbon dioxide has therefore been found to regenerate Na2C0 ₃ / NaHC0 ₃ in the leach solution from the NaOH produced during the reaction of CaO with carbonate ions, allowing for further extraction of vanadium from the feedstock.

[00116] In one embodiment, the pH of the leach solution is controlled throughout the leach step. In one embodiment, the leach solution is maintained at a pH above 7.5. In one embodiment, the leach solution is maintained at a pH above 8. In one embodiment, the leach solution is maintained at a pH above 8.5. In one embodiment, the leach solution is maintained at a pH above 9.

[00117] In one embodiment, the leach solution is maintained at a pH between 7.5 and 14. In one embodiment the pH is maintained between 8 and 11. In one embodiment, the pH is maintained between 9 and 10. The inventors have found that the pH of the leach solution will naturally increase throughout the leach process as a result of the formation of NaOH during the reaction of the alkaline carbonates and CaO/Ca(OH)2. However, at pH above 14 silica is increasingly soluble and will be leached into solution. It has also been found that at pH below 9, small quantities of impurities such as manganese, magnesium, iron and titanium begin to dissolve. This will increase the complexity and may impact the subsequent recovery of vanadium form the leach solution. By maintaining a pH between 9 and 11 , vanadium extraction can be achieved with minimal silica being leached into solution. [00118] In one embodiment, the pH is maintained by the addition of carbon dioxide into the leach solution. As discussed above, carbon dioxide will convert NaOH to Na2C03/NaHC03. Alternatively, the pH is maintained by the addition of acid.

[00119] In a preferred embodiment, the leach process 108 is conduct at a pH of approximately 10 to prevent the above discussed impurities leaching into solution. Following separation of undissolved solids, the pH of the calcine leach solution may then be lowered to between 9-9.5 in order to precipitate at least some silica out of the solution. A coagulant may be added to assist in the silica removal. The precipitated solids may then be filtered out of the solution.

Leach Conditions

[00120] In one embodiment, the feedstock has a particle size of Pso 106 pm. In one embodiment, the feedstock has a particle size of Pso 75 pm. In one embodiment, the feedstock has a particle size of Pso 53 pm. In one embodiment, the feedstock has a particle size of Pioo 25 pm.

[00121] In one embodiment, the leach is conducted at ambient temperature. In one embodiment, the leach is conducted at elevated temperature. Preferably, the leach step is conducted at a temperature up to boiling point. In one embodiment, the leach step is conducted at a temperature above 50°C. In one embodiment, the leach step is conducted at a temperature above 60°C. In one embodiment, the leach step is conducted at a temperature above 70°C. In one embodiment, the leach step is conducted at a temperature above 80°C. In one embodiment, the leach step is conducted at a temperature above 90°C.

[00122] In one embodiment, the leach step is conducted at ambient pressure. In one embodiment, the leach step is conducted at elevated pressure. It is envisaged that carbon dioxide may be injected into the headspace of the leach reactor.

[00123] In one embodiment, the pulp density of the leach step is 10-50%. In one embodiment, the pulp density is 20 - 40 % [00124] In one embodiment, the Na2C03/NaHC03 concentration in the leach solution is between 2-35%. In one embodiment, the Na2C03/NaHCC>3 concentration in the leach solution is at least 50 g/L. In one embodiment, the Na2C03/NaHCC>3 concentration in the leach solution is at least 75 g/L. In one embodiment, the Na2C03/NaHC03 concentration in the leach solution is at least 125 g/L.

[00125] In embodiments where carbon dioxide is sparged through the leach vessel, the Na concentration in the leach solution is at least 40 g/L.

[00126] In one embodiment, the residence time of the leach step is greater than 1 hour. In one embodiment, the residence time of the leach step is greater than 2 hours. In one embodiment, the residence time of the leach step is greater than 3 hours. In one embodiment, the residence time of the leach step is greater than 4 hours. In one embodiment, the residence time of the leach step is greater than 5 hours. In one embodiment, the residence time of the leach step is greater than 6 hours. In one embodiment, the residence time of the leach step is greater than 7 hours. In one embodiment, the residence time of the leach step is greater than 8 hours. In one embodiment, the residence time of the leach step is greater than 9 hours. In one embodiment, the residence time of the leach step is greater than 10 hours. In one embodiment, the residence time of the leach step is greater than 11 hours. In one embodiment, the residence time of the leach step is greater than 12 hours.

[00127] In one embodiment, a solution Eh of > -100 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -90 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -80 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -70 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -60 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -50 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -40 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -30 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -20 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > -10 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of > 0 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of >50 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of >100 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of >200 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of >300 mV against a Ag/AgCI reference electrode is maintained. In one embodiment, a solution Eh of >400 mV against a Ag/AgCI reference electrode is maintained.

Leach Stages

[00128] The leach circuit 108 may comprise one or more separate leach stages. In the embodiment shown in Figure 1 , the leach process 108 comprises a first leach stage 110 and a second leach stage 112. The leach slurry 114 resulting from the first leach stage 110 is directed to a size reduction step 116. In size reduction step 116, the leach slurry 114 is treated to reduce the particle size of the solids in the leach slurry 114. The presence of CaO or Ca(OH)2 in the feed material will react with Na2C03/NaHCC>3 in the leach solution to produce solid CaC03 and NaOH. Without wishing to be bound by theory, the inventors understand that these species will react with CaO at the exposed surfaces of the feedstock particles or with Ca ²⁺ in solution to form solid CaC03. At least some of the precipitated CaC03 will form as a coating on the feedstock, thereby hindering further leaching of the vanadium. The inventors have found that by subjecting the coated (partially leached) feedstock to a size reduction step 116, at least a portion of the CaC03 may be removed, exposing the surface of the feedstock and allowing further leaching of the vanadium.

[00129] It is envisaged that any suitable milling apparatus may be used in size reduction step 28. Examples of suitable apparatus include ball mills, rod mills, autogenous grinding (AG) mills, semi-autogenous grinding (SAG) mills, stirred media mills and stirred media detritors.

[00130] In one embodiment, the size reduction step reduces the particle size to Pso 38 pm. In one embodiment, the size reduction step reduces the particle size to P9538 pm. In one embodiment, the size reduction step reduces the particle size to Pso 10 pm. [00131] The treated stream 118 exiting the size reduction step 116 is directed to the second leach stage 112 where it is contacted with an alkaline carbonate leach solution to further extract vanadium. Whilst additional alkaline carbonate leach solution may be directed into the second leach stage 112, it is preferred that a carbon dioxide stream is injected into the second leach stage 112 to regenerate Na2C03/NaHCC>3 in the leach solution from NaOH and maintain the desired pH as described earlier.

[00132] Each of the first leach stage 110 and the second leach stage 112 are operated at similar conditions to primary leach circuit 108 as discussed above. It is envisaged that each leach stage may comprise two or more leach vessels operated in series.

[00133] Whilst the embodiment shown in Figure 2 comprises two leach stages, it is envisaged that additional leach stages may be included in the leach process 108. In embodiments where further leach stages are included, a further size reduction step may also be included between each stage.

[00134] The leach slurry 120 is directed to a solid liquid separation step 122 to separate a leach residue 124 from a pregnant leach solution 126. The leach residue 124 is washed on the filter with water.

[00135] The leach residue 124 may still contain a significant vanadium content as a result of the limitations of the leach (including particle size, vanadium liberation and calcium carbonate precipitation) and may be further processed to recover further vanadium. In the embodiment shown in Figure 2, the leach residue 124 is directed to a drying step 128 to remove the majority of moisture present in the leach residue 124. The dried residue 130 is then directed to calcination step 132 where is treated at high temperature to produce a calcined residue 133

[00136] The calcined residue 133 is directed to a leach step 134 where it is contacted with an alkaline carbonate leach solution to extract vanadium from the calcined residue 133. Leach step 134 is operated under substantially the same conditions as leach circuit 108 and the discussion above applies equally. The solution temperature and pH are maintained at target values during the leach step 134 in a manner similar to that used in the slag leach circuit 108. In the embodiment shown in Figure 2, leach step 134 is separate from the leach circuit 108.

[00137] The resulting calcine leach slurry 136 is directed to a solid liquid separation step 138 to separate a solid calcine residue stream 140 from a calcine leach solution 142. It is envisaged that the solid liquid separation step 138 will be conducted in a filtration apparatus, such as belt filter. Alternative solid liquid separation devices may be utilised in solid liquid separation step 138. It is envisaged that a pre-filter thickener may also be used. In one embodiment, a counter-current washing process is used in the solid liquid separation step 138 in order to maximise vanadium recovery.

[00138] In one form of the present invention, the solid residue 140 is directed to a repulp circuit to recover residual vanadium. Preferably the repulp circuit comprises a pulping step, a solid liquid separation step and an ion exchange step. More preferably, the repulping step comprises the contact of the solid residue 140 with an aqueous solution.

[00139] Calcine leach solution 142 is combined with pregnant leach solution 126 to produce combined leach solution 144. It is envisaged that the calcine leach solution 142 may be directly combined with the pregnant leach solution 126 or that it may be concentrated by appropriate means prior to combination. If no additional concentration is conducted, the pregnant leach solution 126 will be diluted.

[00140] In the embodiment shown in Figure 2, the combined leach solution 144 is directed to a solvent extraction circuit 56. In the solvent extraction circuit 56, the combined leach solution 144 is contacted with an organic extractant to extract vanadium ions from the aqueous phase into the loaded organic phase. The loaded organic phase may be separated from the barren leach solution. The loaded organic can then be contacted with a scrub solution to displace entrained aqueous phase or weakly extracted impurities from the loaded organic. The scrubbed loaded organic will then be contacted with an aqueous strip solution to recover vanadium from the loaded organic into a vanadium strip solution 58. Vanadium may then be recovered from the vanadium strip solution 58 by conventional means, such as for example precipitation, crystallisation or electrolysis. [00141] In one embodiment of the present invention, the solvent extraction circuit 56 comprises an extraction stage, a scrubbing stage and a stripping stage to selectively recover vanadium from the combined leach solution 144 into the vanadium strip solution 58.

[00142] In one embodiment of the present invention, the extraction stage comprises one or more solvent extraction contactors such as mixer settlers or column contactors. More preferably, the extraction stage comprises two or more solvent extraction mixer settlers. Still preferably, the extraction stage comprises three or more solvent extraction mixer settlers.

[00143] In one embodiment, where the extraction stage comprises two or more solvent extraction mixer settlers, the two or more solvent extraction mixer settlers are arranged in series.

[00144] In one embodiment, where the extraction stage comprises two or more solvent extraction mixer settlers arranged in series, the mixer settlers are arranged for counter-current operation.

[00145] The combined leach solution 144 is directed to the first mixer settler where it is contacted with an organic solution comprising a vanadium extractant to selectively extract vanadium from the combined leach solution 144. The combined leach solution 144 and the organic solution comprising a vanadium extractant are contacted in a counter-current arrangement to maximize extraction efficiency.

[00146] In a preferred form of the present invention, the vanadium extractant is a quaternary ammonium extractant. Preferably, the vanadium extractant is Aliquat 336.

[00147] In one form of the present invention, the organic solution comprises a modifier. In one form of the present invention, the organic solution comprises between 0% and 50% modifier. In one embodiment, the organic solution comprises between 25% and 35% modifier. In one embodiment, the organic solution comprises between 5% and 10% modifier. As would be appreciated by a person skilled in the art, modifiers can be used to improve the chemical or physical performance of the solvent extraction system. In one form of the present invention, the modifier is selected from tridecanol, isodecanol or isotridecanol.

[00148] In one form of the present invention, the organic solution is not diluted. In one form of the present invention, the organic extractant is diluted to a target concentration with a diluent. Preferably, the target concentration is between 20% and 40% on a volume basis. More preferably, the target concentration is about 25% on a volume basis. In one embodiment, the target concentration is between 5% and 10%. As would be appreciated by a person skilled in the art the dilution of the organic solution is used to control viscosity and improve phase separation. In one form of the invention, where the organic solution is diluted with a diluent, the diluent is kerosene, Shellsol 2046, Vivasol D80 or Neste Renewable Isoalkane.

[00149] In one form of the present invention, the ratio of organic to aqueous in the extraction stage is between 12:1 and 1 :12. Preferably, the ratio of organic to aqueous in the extraction stage is between 4:1 and 1 :4. More preferably, the ratio of organic to aqueous in the extraction stage is between 1 :1 and 1 :2. As would be appreciated by a person skilled in the art, the ratio of organic to aqueous in the extraction stage is dependent on the vanadium tenor in the leach solution as well as the loading of vanadium on the organic. One method for calculation of equilibrium concentration of extractant is using the material balance, i.e. it is equal to the difference between total (analytical) concentration of extractant and the sum of all solvated species in the solvent phase. As will be familiar to someone with experience in solvent extraction, manipulation of the 0:A ratio in both the vanadium extraction and stripping stages can be used to simultaneously increase the concentration of vanadium in the final strip solution whilst improving the purity of the product vanadium solution.

[00150] In one form of the present invention, the target pH of the combined leach solution in the extraction stage is between 8 and 11. In one form of the present invention the target pH of the combined leach solution in the extraction stage is between 9 and 10. In an alternative form of the present invention, the pH is not controlled.

[00151] In one form of the invention pH adjustment of the aqueous phase between extraction stages is adjusted closer to the target incoming pH. In one form of this invention, carbon dioxide can be sparged into the aqueous phase in an inter-stage tank to convert some contained hydroxide ions to carbonate ions and hence reduce the pH. This may be most impactfully done between the penultimate and last extraction stages to ensure the pH of the aqueous entering the last extraction mixer settler is closer to the target pH for best vanadium extraction.

[00152] The loaded organic solution is directed to a scrubbing stage. In scrubbing stage, the vanadium loaded organic solution is contacted with a portion of a scrub solution. The scrub solution displaces any weakly loaded or entrained impurity elements loaded onto the loaded organic solution in the extraction stage. In a preferred embodiment, the scrub solution is prepared from a diluted loaded strip liquor. 0:A ratio is managed in the scrubbing section with an internal recycle stream to ensure a reasonable 0:A ratio in the contactor whilst only having a relatively small quantity of loaded scrub solution progressing forward to the extraction stage. In one embodiment of the present invention, the ratio of the vanadium loaded organic solution to the aqueous scrub solution is between 1 :1 and 1 :2 (organic:aqueous) on a volume basis.

[00153] In one embodiment of the present invention, the scrubbing stage comprises one or more mixer settlers.

[00154] The aqueous phase from the scrubbing stage is directed back to the first extraction mixer-settler. Loaded organic solution from the scrubbing stage advances to a stripping stage. In the stripping stage, the loaded organic solution is contacted with a strip solution designed to displace the majority of vanadium ions in the organic into the aqueous phase, producing the vanadium strip solution 58.

[00155] In one embodiment of the present invention, the strip solution is selected from the group comprising: sodium carbonate, ammonium carbonate, ammonium chloride, , ammonium hydroxide, sulfuric acid, sodium hydroxide and potassium hydroxide. In a preferred embodiment, the strip solution is a sodium hydroxide solution. In one embodiment, the stripping stage comprises one or more mixer settlers. Preferably, the stripping stage comprises two or more mixer settlers. More preferably, the stripping stage comprises three or more mixer settlers. In an embodiment where the stripping stage comprises two or more mixer settlers, the two or more mixer settlers are arranged in series. [00156] In one form of the present invention, the ratio of the vanadium loaded organic solution to the aqueous strip solution is between 2:1 and 10:1 (organic:aqueous) on a volume basis. Preferably the ratio is between 5:1 and 7:1 (organic:aqueous) on a volume basis.

[00157] The organic phase exiting the stripping stage is recycled to the extraction stage where it again loads with vanadium. In this way, the organic phase is kept in a closed circuit within the vanadium solvent extraction circuit. The vanadium strip liquor from the stripping stage is directed to vanadium recovery.

[00158] In the embodiment shown in Figure 2, the vanadium strip solution 58 is directed to a desilication step 60 where it is contacted with an aluminium salt, for example aluminium sulphate to precipitate aluminium silicon compounds. Silicon removal may require pH adjustment and this is most readily achieved with a small quantity of sulphuric acid. The precipitated solids and other insoluble materials are removed in a solid liquid separation step. The filtrate 62 is directed to a precipitation step 64 where it is contacted with ammonium sulphate to precipitate ammonium metavanadate (AMV). Sulphuric acid may be added to precipitation step 64 to control solution pH for optimal vanadium recovery. A target pH of between 8 - 9 is preferred. The resulting slurry is directed to a filtration step. The filtered solids are washed with dilute ammonium sulphate solution to remove any entrained liquors and further purify the filter cake. The recovered solids 66 are directed to calcination step 68 for deammoniation and subsequent powder melting and production of solid V2O5 flakes 70. Barren liquor 72 from the precipitation step 64 is directed to a crystallisation step 74 to recover sodium sulphate crystals.

EXAMPLE 1

[00159] A sample of leach residue resulting from the leaching of a steel slag sample in sodium carbonate was heated to 1 ,000 °C for a period of 2 hours (without any additive), which achieved a 27 % mass loss, due to moisture and CO2 loss from the decomposition of the sodium and calcium carbonates. An assay was conducted on the residue prior to and following the calcination and the results are shown in Table 1 . Table 1 : Experimental record for thermal decomposition of slag leach residue (SSM) at 1,000 °C

[00160] The calcined residue was then subjected to leach using a 120 g/L sodium carbonate solution with the addition of CC o target a solution pH 10. Solids contented was 30% and the solution temperature was 70°C. The leach was conducted for 4 hrs with periodic sampling. The results of the trial are shown in Table 2.

Table 2: Experimental record for standard single stage alkaline carbonate leach on the thermally decomposed SSM

[00161] The results showed that calcination at 1 ,000 °C achieves high degree of decomposition of the carbonates (> 95% carbon loss). Carbonate leaching of the calcined residue led to - 74 % leaching of vanadium, with the initial ~ 0.5% V in primary leach residue (i.e. pre-calcination feed) being reduced to ~ 0.16% V in the calcine leach residue. This may represent a significant potential increase in overall vanadium recovery for the alkaline carbonate flowsheet from 75 - 80% to over 90%.

EXAMPLE 2

[00162] A sample of Lulea slag (milled to P80 of 75 pm) was prepared for leaching using a three-stage alkaline carbonate leach circuit. The initial feed was milled to P80 of 75 microns. The processed feed was then subjected to a primary leach at 30% pulp density, 125 g/L Na2C03, held at 70 °C for up to 10 hours, with CO2 addition to target pulp pH of 10. The primary leach discharge was subjected to a regrind step to reduce the particle size of the solids to P80 of ~ 20 microns. The resulting discharge was subjected to a secondary leach at ~ 70 °C for up to 10 hours with CO2 addition as needed for a target of pH 10. The discharge of the secondary leach. The secondary leach discharge was subjected to a regrind step to reduce the particle size of the solids to P80 of ~ 10 microns. The resulting discharge was subjected to a tertiary leach at ~ 70 °C for up to 10 hours with CO2 addition as needed for a target of pH 10. The extraction of the various metals in the initial feed throughout the three-stage alkaline carbonate leach circuit is shown in Table 3. Table 3: Experimental record for standard three-stage alkaline carbonate leach on a slag sample

Extraction (%) (residues)

[00163] Undissolved solids from the leach solution were recovered as a leach residue. A portion of the leach reside was at 1 ,000 °C for 2 hrs. Calcination achieved a 17.4% mass loss, due to moisture and CO2 loss from the decomposition of the sodium and calcium carbonates. An assay was conducted on the residue prior to and following the calcination and the results are shown in Table 4.

Table 4: Experimental record for thermal decomposition of slag leach residue (SSM) at 1,000 °C

[00164] The calcined residue was then subjected to a standard single-stage alkaline carbonate leach using a 125 g/L sodium carbonate solution with the addition of CC o target a solution pH 10. Solids contented was 30% and the solution temperature was 70°C. The leach was conducted for 4 hrs with periodic sampling. The results of the trial are shown in Table 5.

Table 5: Experimental record for re-leach of calcined SSM

[00165] Undissolved solids were separated and were repulped in water at 50°C at 10% solid content for 4 hrs to remove any vanadium that may be entrained in the solids. The extent of the extraction is detailed in Table 6, together with the overall extraction.

Table 6: Experimental record for repulp wash of re-leached calcined SSM

[00166] The anticipated vanadium leaching profile was observed across the three leaching stages with > 50% of the vanadium leaching occurring in the primary leach and another 10 - 15 % vanadium leaching being achieved in the regrind/releach stages. From an incoming vanadium level of 2.37% in the slag the leach residue appeared to contain around 0.6% V. Repulp wash of the tertiary leach residue effectively removed entrained liquors and/or redissolved soluble species that had precipitated during leaching. The repulp wash reduced the vanadium in the washed solids down to 0.54% and dissolved about 1/3 of the contained sodium without changing the concentration of other key elements.

[00167] The thermal decomposition of the washed leach residue achieved a 99% removal of the carbon indicating a high degree of carbonate decomposition. Due to the mass loss the vanadium concentration was increased from 0.54% in primary leach residue to 0.74% in the calcined residue. The single-stage alkaline carbonate re-leach of the calcined residue reduced the vanadium grade in the residue to 0.16% due in part to the mass increase but largely due to an additional recovery of ~ 75% of the vanadium contained in the calcined residue. The repulp wash of the re-leached calcined SSM further removed entrained and soluble vanadium and left the washed residue with only 0.135 % vanadium. This repulp wash also reduced the contained sodium by just over 1/3.

[00168] The combination of the current leaching circuit (three-stage alkaline carbonate with interstage regrinds) combined with repulp washing of the leach residue, thermal decomposition of the leach residue and re-leach with repulp washing of this secondary leach residue can result in a nearly 94% extraction of vanadium from the initial slag.

EXAMPLE 3

[00169] A grab sample from the Lulea pilot plant feed was first taken sequentially through three leach stages with interstage regrinding and based on solids assay the following degrees of vanadium extraction are achieved at the discharge point of each stage of leaching and final repulp washing. Head grade of vanadium in this test was 23,680 ppm. The first leach stage reduced the vanadium level to 10,270 ppm in the discharge solids representing 57.6% vanadium extraction. The second leach stage reduced the vanadium level to 6,750 ppm in the discharge solids representing a cumulative 66.7% vanadium extraction from slag. The third leach stage reduced the vanadium level to 6,065 ppm in the discharge solids representing a cumulative 68.9% vanadium extraction from slag. The repulp washing of the residue resulting from the third leach stage reduced the vanadium level to 5,380 ppm in the discharge solids representing a cumulative 73.2% vanadium extraction.

[00170] The well washed leach residue prepared using a base case three stage leach process described above was dried and subjected to calcination at 1 ,000°C. The calcination reduced the solids mass by 24.6% and the contained carbon from 6.75% to 0.12% (essentially removing all the contained carbonate). This solid product was cooled and subjected to a single stage secondary leach in sodium carbonate solution. Due to the mass loss during calcination the vanadium grade of the feed for the calcine leach stage was raised to 7,360 ppm. The calcine leach step reduced the vanadium level to 1 ,590 ppm in the discharge solids representing a cumulative 92.6% vanadium extraction from slag. The filtered solids from above were repulp washed using standard conditions and the final filtered solids had a vanadium level of 1 ,345 ppm representing a cumulative 93.8% vanadium extraction from slag.

EXAMPLE 4

[00171] Further trials were undertaken on a second sample of composite Lulea slag. The vanadium concentration in this sample was found to be slightly lower at 21 ,940 ppm (equivalent to 3.917 % V2O5) and all recoveries given below are based on this head grade.

[00172] The following tests and determinations were made to consider the impact of insertion of a drying / calcination process into the flowsheet with three, two or one leaching stages prior to calcination. These are described generally below:

I. Three pre-calcination leaching stages: A. Bench scale base case flowsheet involving three stage comminution/leach circuit with on filter washing only (JR051).

B. Repulp wash of final leach residue (SSM) from A above (JR051 ) with filtration and on filter washing (JR054)

C. Final washed SSM from B above, calcined at 1 ,000 °C, 2 hours, (JR054) and re-leached in one stage under standard leach conditions (JR061)

D. Repulp wash (JR064) of leach residue from above.

II. Two pre-calcination leaching stages:

E. Base case flowsheet stopping at secondary leach discharge (JR050)

F. Repulp wash of final leach residue (SSM) from E above (JR050) with filtration and on filter washing (JR053)

G. Final washed SSM from F above, calcined at 1 ,000 °C, 2 hours (JR050 ^* ) and re-leached in one stage under standard leach conditions (JR060)

H. Repulp wash (JR063) of leach residue.

III. One pre-calcination leaching stage:

I. Base case flowsheet stopping at primary leach discharge (JR049)

J. Repulp wash of primary leach residue (SSM) from G above (JR049) with filtration and on filter washing (JR052)

K. Final washed SSM from FI above, calcined at 1 ,000 °C, 2 hours (JR049 ^* ) and re-leached in one stage under standard leach conditions (JR058).

L. Repulp wash (JR062) of leach residue from above.

[00173] The vanadium recovery numbers are given for these tests in Table 7.

Table 7. Summary of residual vanadium levels and overall vanadium recovery for the tests A - L.

[00174] The results indicate that sequential 1 , 2 and 3 stages of leaching (with interstage regrind) achieve 50 - 55%, 70 - 72% and 73 - 75% vanadium recovery, respectively. Repulp washing of leach residues at any of these stages recovers an additional 2 - 3% vanadium. Calcination of each of these solids achieves > 95% calcium carbonate decomposition. Re-leaching of calcined solids achieves a significant increase in vanadium extraction from slag of 20 - 30%. Repulp washing of the final leach residue (post calcination and re-leaching) extracts a small further quantity of vanadium but the value of this repulp wash diminishes due to the very low residual levels of vanadium.

[00175] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.