Systems and processes for treating aluminium-containing materials

Abstract

Disclosed is a process of treating aluminium-based waste (12) that includes aluminium or aluminium-containing materials. The process includes dissolving aluminium-based waste (12) in an alkali solution, such as in a digestor (14), to form a solution that includes an aluminate species. The process also includes precipitating the aluminate species, such as in a precipitator (16), to form an alumina species.

Description

[0001] Systems and processes for treating aluminium-containinq materials

[0002] Technical Field

[0003] The present disclosure relates to systems and processes for treating aluminium-containing materials.

[0004] Background

[0005] Aluminium is often touted as a more environmentally friendly metal given the relative ease in which it can be recycled. Aluminium is formed by smelting alumina, with alumina being formed by processing bauxite using the Bayer process. The Bayer process itself generates bauxite residue from the digestion of bauxite to extract alumina. Although aluminium has its benefits, the overall lifecycle of processing bauxite to form aluminium needs to be improved to reduce its energy use and environmental impact.

[0006] Summary

[0007] An embodiment provides a process of treating aluminium-based waste that includes aluminium or aluminium-containing materials, the process comprising: dissolving aluminium-based waste in an alkali solution to form a solution that includes an aluminate species; and precipitating the aluminate species to form an alumina species.

[0008] The process may further comprise collecting hydrogen gas generated during dissolving the aluminium-based waste in the alkali solution. The hydrogen gas may be collected as a mixture of vapour and hydrogen gas. The vapour and hydrogen gas may be separated by condensation of the vapour. The process may further comprise washing the aluminium- based waste to remove at least some contaminates associated with a surface of the aluminium-based waste prior to dissolution in the alkali solution. Washing the aluminium- based waste may include washing the aluminium-based waste with an alkali solution to remove a passivation layer from the aluminium-based waste. The process may further comprise thermally treating the aluminium-based waste to remove at least some nonaluminium materials (such as paper, plastic and the like) associated with a surface of the aluminium-based waste prior to dissolution in the alkali solution. A combination of washing and thermal treatment may be used to clean the aluminium-based waste.

[0009] The aluminium-based waste may be conveyed to the digestor as a slurry formed from water and the aluminium-based waste. The slurry may reduce or eliminate the aluminium-based waste from contacting oxygen gas to prevent unwanted hydrogen evolution before digestion and to minimise other possible oxidative loses.

[0010] The process may further comprise decreasing a particle size of the aluminium-based waste prior to dissolution in the alkali solution. The particle size of the aluminium-based waste may be >10 pm, such as >50 pm, >100 pm or >500 pm. The particle size of the aluminium-based waste may range from about 0.1 mm to about 100 mm. The aluminium-based waste may be melted to form a molten material. The aluminium-based waste may be heated to a plastic solid or semi-solid state to allow for extrusion, forging, wire drawing, and the like. The molten material may be pelletised in a molten state to form pellets of aluminium-based waste. The molten material may be cast and then pelletised to form pellets of aluminium-based waste.

[0011] The process may further comprise maintaining a temperature of the alkali solution within a predefined condition. The process may further comprise recovering heat generated during dissolving the aluminium-based waste in the alkali solution. Precipitating the aluminate species to form the alumina species may include adding a seed crystal of the alumina species. Accordingly, a seed crystal is not required in all embodiments. Precipitating the aluminate species to form the alumina species may include decreasing a temperature of the solution that includes the aluminate species.

[0012] The process may further comprise removing insoluble and / or soluble material from the solution that includes the aluminate species prior to precipitating the aluminate species. The insoluble material may include insoluble metals, metal hydroxides and / or metal oxyhydroxides. The soluble material may include Zn, Mg, Mn, Fe and / or Si species. The insoluble material may include non-metal species such as paper and / or plastic and / or insoluble metals such as noble metals.

[0013] The process may further comprise recycling spent alkali solution following precipitation of the alumina species back to digesting aluminium-based waste. The recycled spent alkali may be processed such as by evaporation, oxalate removal, organics removal, soluble metal or anion impurity removal, and the like, prior to being passed back to the digestor. This processing of spent liquor may be considered as a polishing step. A pH of the alkali solution may range from 8 to 15. A pH of the alkali solution may range from 8 to 11 . The alumina species may be alumina hydrate. The process may further comprise calcining the alumina hydrate to form alumina. The alumina species may be alumina. The alkali solution may include NaOH and / or KOH. It should be appreciated that the alkali solution may also include sodium carbonate, for example due to reaction of alkali with any air ingress in the pure system and / or from organic material in the bauxite. An embodiment provides alumina produced from a process that includes the process as set forth above.

[0014] An embodiment provides a system for digesting aluminium-based waste to form an alumina species, the system comprising: a digestor for dissolving aluminium-based waste in an alkali solution to form a solution that includes an aluminate species; a precipitator for precipitating an alumina species from the solution that includes the aluminate species and forming spent liquor, the precipitator being in communication with the digestor such that spent liquor can be recycled back to the digestor.

[0015] The system may further comprise a calciner for receiving and calcining the alumina species. The system may further comprise a polisher upstream of the precipitator. The polisher may be configured to remove insoluble material from the solution that includes the aluminate species prior to precipitating the aluminate species in the precipitator. The polisher may include a solids filter. The solids filter may remove insoluble metals, undigested paper and / or plastic, and so on. The digestor may include a hydrogen gas off-take configured to collect hydrogen gas generated from dissolving the aluminium-based waste in the digestor.

[0016] The system may further comprise comprising an energy conversion device that can generate electricity and / or heat from the hydrogen gas collected from the hydrogen gas off-take. The system may comprise a combustor, such as to generate heat, that uses as a combustion fuel hydrogen gas generated from dissolving the aluminium-based waste. The system may further comprise a reduction reactor configured to utilise hydrogen as a reducing agent to reduce a metal value.

[0017] The system may further comprise a heat exchange system in thermal communication with the digestor for maintaining a temperature of the alkali solution in the digestor during dissolution of the aluminium-based waste. The heat exchange system may be configured to cool the digestor to be within a predefined condition. The heat exchange system may be configured to heat the digestor to be within a predefined condition. The heat exchange system may be configured to maintain the digestor to be within a predefined condition. The digestor may be a tank. The digestor may be a continuous flow reactor. The digestor may include a counter-current reactor. The system may further comprise a feeding system configured to control a flow of heat regulating fluid to control a temperature in the digestor. The system may further comprise a cleaning system positioned upstream of the digestor for cleaning the aluminium-based waste prior to dissolution in the digestor. The cleaning system may be configured to receive spent liquor, fresh alkali and / or a combination of spent liquor and fresh alkali to wash the aluminium-based waste in spent liquor to remove a passivation layer on the aluminium-based waste. Fresh alkali used in the cleaning system may have a lower concentration than alkali used in digestion. For example, the fresh alkali used in the cleaning system may have a pH >11 . The cleaning system may include a washing system and / or a thermal processing system.

[0018] The system may further comprise a particleiser configured to reduce a particle size of the aluminium-based waste. The particleiser may be positioned upstream of the digestor such that the digestor can receive aluminium-based waste with a reduced particle size.

[0019] The system may further comprise a slurry pump configured to pump a slurry of the aluminium-based waste to the digestor.

[0020] The system may include a separator unit configured to separate the aluminium-based waste from non-aluminium materials

[0021] An embodiment provides alumina produced from a process that includes the process as set forth above.

[0022] An embodiment provides alumina hydrate produced from a process that includes the process as set forth above. The alumina hydrate may include gibbsite and boehmite.

[0023] An embodiment provides a hydrogen generator comprising the system as set forth above.

[0024] An embodiment provides a process for treating an aluminium-containing material, the process comprising: digesting bauxite in a digestor to form a first source of aluminate; dissolving aluminium-based waste in an alkali solution to form a second source of aluminate; blending the first source of aluminate with the second source of aluminate to form a combined source of aluminate; and precipitating the combined source of aluminate to form an alumina species.

[0025] The first source of aluminate may be clarified before blending with the second source of aluminate. Insoluble material in the alkali solution having the second source of aluminate may be removed before blending with the first source of aluminate. The insoluble material may be collected and processed to extract metal values from the insoluble material. The combined source of aluminate may have a ratio of [first source of aluminate]:[second source of aluminate] ranging from 1000:0.1 to 0.1 :1000. The combined source of aluminate may have a ratio of [first source of aluminate]:[second source of aluminate] ranging from 1000:1 to 1 :1000. The combined source of aluminate may have a ratio of [first source of aluminate]:[second source of aluminate] ranging from 100:1 to 1 :100, such as about 75:25 to 10:90. The ratio of [first source of aluminate]:[second source of aluminate] may be selected to control a purity of the alumina species and / or condition of the pregnant liquor. The ratio of [first source of aluminate]:[second source of aluminate] may be selected such that a concentration of silica content of the combined source of aluminate is below a desilication product precipitation threshold to minimise or prevent precipitation of desilication products in or from the combined source of aluminate. The ratio of [first source of aluminate]:[second source of aluminate] may be selected to elevate and stabilise silica levels in liquor to facilitate solid liquid separation and isolation of Si-rich liquor with reduced Si precipitation as desilication product. This may allow the pregnant liquor to be desilicated to isolate desilication product from bauxite residue and scrap residue. The alumina species may be alumina hydrate or alumina.

[0026] The ratio of [first source of aluminate]:[second source of aluminate] may be selected to provide silica levels in liquor to facilitate desilication of the combined source of aluminate. This may allow the pregnant liquor to be desilicated to isolate desilication product from bauxite residue and scrap residue where the bauxite residue and scrap residue was separated, or predominantly separated prior to desilication. The alumina species may be alumina hydrate or alumina.

[0027] The desilication product may be processed for re-use in the process, for example by recovering sodium hydroxide, or may be converted into a saleable product.

[0028] An embodiment provides a process for treating an aluminium-containing material, the process comprising: digesting bauxite in a digestorto form a first source of aluminate; dissolving aluminium-based waste in an alkali solution to form a second source of aluminate; precipitating the first source of aluminate to form a first source of alumina species and precipitating the second source of aluminate to form a second source of alumina species; simultaneously passing the first source of aluminate and the second source of aluminate through a single calciner system to form alumina.

[0029] The first source of aluminate may be clarified before precipitation to form the first source of alumina species. Insoluble material present in the alkali solution having the second source of aluminate may be removed before precipitation to form the second source of alumina species. The insoluble material may be collected and processed to extract metal values from the insoluble material. The first source of aluminate and the second source of aluminate may be blended to form a combined source of aluminate. The first source of alumina species and the second source of alumina species may be precipitated simultaneously from the combined source of aluminate. Heat generated by dissolving the aluminium-based waste may be recovered and utilised by a component of a Bayer plant. The heat generated by dissolving the aluminium-based waste may be recovered and utilised by the digestor during digestion of the bauxite. A temperature of the alkali solution during dissolution of the aluminium-based waste may be lower than a temperature of a solution in the digestor during digesting bauxite. The alumina species may be alumina hydrate or alumina.

[0030] The process may further comprise extracting hydrogen gas generated during dissolution of the aluminium-based waste in the alkali solution. The extracted hydrogen gas may be within the process to produce heat, electricity and / or as a reactant.

[0031] An embodiment provides a process for treating an aluminium-containing material, the process comprising: adding aluminium-based waste and bauxite to a digestor having an alkali solution to generate a pregnant liquor having aluminate formed from dissolution of the aluminium-based waste and digestion of bauxite; and precipitating the aluminate to form an alumina species.

[0032] The process may further comprise extracting hydrogen gas generated during the dissolution of the aluminium-based waste. The extracted hydrogen gas may be utilised within the process to produce heat, electricity and / or as a reactant. The process may further comprise collecting a residue formed from dissolving the aluminium-based waste and digestion of bauxite and treating the residue with the hydrogen gas under reducing conditions to reduce metal values in the residue. The process may further comprise adding red mud to the residue prior to treatment with hydrogen under reducing conditions such that metal values from the red mud can be recovered. The red mud may be derived from a plant that processes bauxite such as red mud generated from a Bayer plant that adds the aluminium-based waste to the digestor. The red mud may be generated off-site and transported to the plant that digests the aluminium-based waste in the digestor. The red mud may include material considered as being a sub-par grade iron ore, such as mine tailings. The red mud may be blended with other iron ore-containing materials such as high-quality iron ore to increase an iron content of the red mud. The red mud may be pre-treated prior to treatment with hydrogen under reducing conditions. The red mud may be pre-treated prior to addition to the residue.

[0033] The process may further comprise forming a slurry of bauxite and aluminium-based waste and introducing the slurry into the digestor. The process may further comprise reducing a particle size of the aluminium-based waste prior to adding the aluminium-based waste to the digestor of the alkali solution and bauxite.

[0034] A ratio of [bauxite]:[aluminium-based waste] may range from 0.01 :100 to 100:0.01. A ratio of [bauxite]:[aluminium-based waste] may range from 0.1 :100 to 100:0.1 A ratio of [bauxite]:[aluminium-based waste] may range from 1 :100 to 100:1 A ratio of [bauxite]:[aluminium-based waste] may be selected to control a purity of the alumina species and / or condition of the pregnant liquor. For example, the condition of the pregnant or even spent liquor may be controlled by selecting a ratio of [bauxite]:[aluminium-based waste] such that a concentration of silica content of the pregnant liquor and / or spent liquor is below a desilication product precipitation threshold to minimise or prevent precipitation of desilication products in the pregnant liquor and / or spent liquor. In addition or alternatively, a ratio of [bauxite]:[aluminium-based waste] may be selected to reduce a concentration of impurity elements in the pregnant liquor and / or in the alumina species below a threshold value. The impurity elements may include calcium, phosphorous, iron, vanadium, titanium, gallium and / or beryllium. Reducing a concentration of impurity elements below a threshold value may help to improve a purity of the alumina species and products made subsequently from the alumina species, for example, by calcination to smelting grade alumina.

[0035] The process may further comprise heating the alkali solution in the digestor with an external heat source. Heat generated by the dissolution of the aluminium-based waste is used to heat the alkali solution to aid in digestion of the bauxite.

[0036] An embodiment provides a process of digesting bauxite, the process comprising: digesting bauxite in an alkali solution to form a pregnant liquor; adding to the pregnant liquor a source of aluminate formed from dissolving aluminium-based waste in an alkali solution; wherein an amount of the source of aluminate added to the pregnant liquor is selected to decrease a relative concentration of silica content in the pregnant liquor below a desilication product precipitation threshold to minimise or prevent precipitation of desilication products in the in the pregnant liquor and / or spent liquor following precipitation of an alumina species.

[0037] The aluminium-based waste may be added directly to the alkali solution used to digest bauxite. The aluminium-based waste may be added to the pregnant liquor after digestion. The aluminium-based waste may be added to the pregnant liquor after clarification of the pregnant liquor. An aluminium-based waste digestor may be used to dissolve the aluminium- based waste such that an aluminate species formed therefrom can be added to the pregnant liquor. The aluminium-based waste digestor may have a system to capture any hydrogen gas generated therein. The aluminium-based waste may be dissolved in a second alkali solution to form the source of aluminate. The source of aluminate may then be added to the pregnant liquor. The source of aluminate may be added to the pregnant liquor after the pregnant liquor has been clarified. The process may further comprise removing insoluble and / or soluble material present in the source of aluminate before the source of aluminate is added to the pregnant liquor.

[0038] An embodiment provides a plant for treating an aluminium-containing material, the plant comprising: a first digestor for digesting bauxite; a second digestor for dissolving aluminium-based waste; and a precipitator for precipitating an alumina species from aluminate generated in the first digestor and / or the second digestor.

[0039] The second digestor may include a hydrogen gas off-take for collecting hydrogen gas generated from dissolving the aluminium-based waste. The plant may further comprise a hydrogen gas combustor for combusting the hydrogen gas collected by the hydrogen gas offtake to generate heat. The plant may further comprise a calciner. The hydrogen gas combustor may be operable with the calciner to generate heat for calcination. The plant may further comprise a reactor for treating residue formed from the first digestor and / or the second digestor. The reactor may utilise hydrogen gas collected by the hydrogen gas takeoff as a reducing agent to reduce metal values in the residue. The plant may further comprise a fuel cell for converting the hydrogen gas collected by the hydrogen gas off-take. The plant may further comprise a hydrogen storage system in fluid communication with the hydrogen gas off-take for storing hydrogen gas.

[0040] An embodiment provides a plant for treating an aluminium-containing material, the plant comprising: a mill for milling bauxite to form milled bauxite; a particleiser configured to decrease a particle size of aluminium-based waste to produce particleised aluminium-based waste; and a digestor system for digesting the milled bauxite and / or dissolving the particleised aluminium-based waste in alkali to form a source of aluminate.

[0041] The mill for milling bauxite and the particleiser may be a same device such that bauxite and aluminium-based waste are both passed through the same device to form the milled bauxite and the particleised aluminium-based waste. The digestor system may include a first digestor for digesting milled bauxite and a second digestor for dissolving the particleised aluminium- based waste. The plant may further comprise a heat exchanger configured to be in thermal communication between the first digestor and the second digestor such that heat generated in the second digestor can be transferred to the first digestor. For example, the second digestor may be in thermal communication with spent liquor being returned to the first digestor. The digestor system may include a single digestor that is used to digest milled bauxite and dissolve the particleised aluminium-based waste. The plant may further comprise a hydrogen gas off-take configured to collect hydrogen gas generated during dissolution of the particleised aluminium-based waste. The plant may further comprise a hydrogen reactor configured to treat a residue formed from digestion of bauxite and / or dissolution of the particleised aluminium-based waste to reduce metal values in the residue. The hydrogen reactor may be in fluid communication with the hydrogen gas off-take.

[0042] An embodiment provides a process of treating residue having one or more metal oxides, such as residue formed from extraction of alumina from bauxite, the process of treating residue comprising:

[0043] (i) dissolving aluminium-based waste in an alkali solution to generate hydrogen gas and a solution containing aluminate; and

[0044] (ii) treating the residue with the hydrogen gas generated at step (i) under reductive conditions to reduce one or more metal values present in the residue.

[0045] The residue may include digested bauxite, insoluble material formed from dissolution of the aluminium-based waste and / or red mud. The process may further comprise adding red mud from a tailing facility to the residue. The one or more metal values may include iron, titanium, manganese, magnesium, zinc, silicon , calcium and / or rare earth elements. Reducing one or more metal values present in the residue may include reducing an ion form of a metal into its corresponding metallic form. In an embodiment, step (i) is performed in a reactor separate to a digestor used to digest bauxite. In an embodiment, step (i) is performed in a digestor that is used to digest bauxite. The aluminium-based waste may be particleised prior to step (i). The aluminium-based waste may be washed prior to step (i). The aluminium-based waste may be washed with an alkali solution to remove a passivation layer. The process may further comprise removing insoluble material present in the solution containing aluminate. The insoluble material may be added to the residue for treatment at step (ii). The process may further comprise precipitating the aluminate in the solution containing aluminate to form an alumina species.

[0046] An embodiment provides Bayer residue formed from the process as set forth above /

[0047] An embodiment provides a plant for treating aluminium-containing material, the plant comprising a digestor for digesting bauxite; a digestor for dissolving aluminium-based waste and generating hydrogen gas; a hydrogen gas reactor in fluid communication with the digestor for dissolving aluminium-based waste and configured to utilise hydrogen gas generated by the digestor for dissolving aluminium-based waste to reduce metal values present in residue formed by digestion of bauxite.

[0048] An embodiment provides a process of smelting alumina to form aluminium, the process comprising: dissolving aluminium-based waste in an alkali solution to generate hydrogen gas and aluminate; utilising the hydrogen gas as an energy source to at least partially power one or more components of an aluminium smelter system.

[0049] The aluminium-based waste may be particleised prior to dissolution in the alkali solution. Heat generated during dissolution of the aluminium-based waste may be used during alumina smelting. The aluminium-based waste may include aluminium dross.

[0050] An embodiment provides an aluminium smelter system, comprising: a digestor for dissolving aluminium-based waste in an alkali solution to generate hydrogen gas, wherein the hydrogen gas produced by digestor is utilised as an energy source to at least partially power one or more components of the aluminium smelter system.

[0051] The aluminium smelter system typically includes an electrochemical reactor and a cast house. The hydrogen gas produced may be utilised as an energy source to generate electricity to at least partially power the electrochemical reactor and / or cast house. For example, the electricity generated from the hydrogen gas may be used to displace purchased power. The digestor may include a heat exchange system configured to maintain a temperature of the alkali solution in the digestor during dissolution of the aluminium-based waste within a preset temperature range. The heat exchange system may be in thermal communication with one or more components of the aluminium smelter system. The heat exchange system may be configured to cool the alkali solution in the digestor to be within a predefined condition. The digestor may be a tank. The digestor may be a continuous flow reactor. The smelter system may further comprise a particleiser configured to reduce a particle size of the aluminium-based waste. The particleiser may be positioned upstream of the digestor such that the digestor can receive aluminium-based waste with a reduced particle size. The hydrogen gas produced by the digestor may be combusted to generate thermal energy or converted into electricity with a device including a fuel cell.

[0052] Brief Description of the Drawings

[0053] Embodiments will now be described, by way of example only, with reference to the accompanying non-limiting drawings, in which:

[0054] Figure 1 shows an embodiment of a system used to digest aluminium to form an alumina species.

[0055] Figure 2 shows another embodiment of a system used to digest aluminium to form an alumina species.

[0056] Figure 3 shows an embodiment of an energy generation device.

[0057] Figure 4 shows a schematic system of a Bayer plant.

[0058] Figure 5 shows an embodiment of a plant used to treat an aluminium-containing material. Figure 6 shows another embodiment of a plant used to treat an aluminium-containing material.

[0059] Figure 7 shows another embodiment of a plant used to treat an aluminium-containing material.

[0060] Figure 8 shows another embodiment of a plant used to treat an aluminium-containing material.

[0061] Figure 9 shows an embodiment of an aluminium smelter plant.

[0062] Figure 10 shows another embodiment of an aluminium smelter plant.

[0063] Detailed Description

[0064] To reduce the carbon footprint of aluminium, many different options are being explored to use renewable energy sources such as hydrogen and renewably generated electricity to replace components of the refining of bauxite to form alumina and smelting of alumina to form aluminium. Although these efforts are necessary, issues remain in addressing large scale reduction of carbon impacts of the alumina / aluminium industry.

[0065] Aluminium is one of the easier metals to recycle. For example, during production of specific aluminium products, such as those formed from specific aluminium alloys as used in the aerospace industry, filings and millings formed during milling and production of these specific aluminium products can be collected to recycle the specific aluminium alloys. This type of scrap is generally termed “primary aluminium scrap”. However, not all aluminium products can be recycled straightforwardly, such as automobile engine blocks and those used in consumable / consumer products including soft drink (soda) cans that have a defined lifetime whereby once used they become a post-consumer form of aluminium, which is generally referred to as secondary aluminium scrap. These post-consumer aluminium products cannot always be economically recycled using current methods and processes because the recycling smelters used to melt and form new ingots of aluminium do not tend to mix different types of aluminium products (e.g., wrought and cast allows, pure aluminium vs aluminium alloys, and so on). Accordingly, approximately 30% of all aluminium products can only be subject to downgraded recycling (i.e. downcycling) where the products are utilised in “lesser” forms. Currently aluminium that cannot be easily smelted into new ingots tends to be sent to landfill. However, the unique conditions of landfill sites can result in aluminium-induced fires such as from the dissolution of aluminium to form hydrogen which can then combust. A similar problem exists for aluminium dross produced during smelting of alumina to form aluminium, especially when aluminium dross contaminates such as cryolite are considered.

[0066] Aluminium is inherently an energy store in that the energy required to convert alumina into aluminium can be released upon conversion of aluminium into an aluminate specie(s), aluminium hydrate or alumina. Dissolution of aluminium in alkali is exothermic and releases hydrogen. The generated heat and hydrogen are therefore types of energy that can be used as work in industrial processes. In this way, aluminium can act as an energy store or battery. Despite this, to date there has been little interest in utilising aluminium as an energy source on an industrial scale. Two reasons for this include the fact that recycling of aluminium by smelting does not consider dissolution of aluminium into aluminates and alumina refineries tend to only concentrate on forming alumina from bauxite. However, over the past few decades, the amount of waste aluminium that cannot be recycled and that is sent to landfill has increased.

[0067] One or more embodiments of the disclosure utilises aluminium-based waste as an energy source in alumina refineries and / or alumina smelters. For example, an embodiment may allow for the processing of not just low impurity bauxite, but low aluminium grade bauxite such as below what would usually be economical to mine, an increase in bauxite reserve for an existing resource, identification of new previously non-viable resources, extended mine life and / or greater resource utilisation.

[0068] Throughout this disclosure, the term “aluminium-based waste” as used herein includes aluminium waste such as low-quality aluminium scrap, post-consumer aluminium scrap, high-quality aluminium scrap, and aluminium-containing materials such as aluminium alloys, aluminium oxides including alumina hydrate and alumina, aluminium dross where aluminium in either metallic or oxide form is the primary component that cannot be economically recycled or that may be discarded for quality reasons, or any other source of aluminium scrap (also known as home scrap) such as packaging, aluminium foil, coffee pods, cartons and containers, and so on. For example, the term “aluminium-based waste” would not include alloys where aluminium is in a minor component. The term “aluminium-based waste” may also include scrap and may be used interchangeably with the term “aluminium-based scrap”. The use of the term “waste” or “scrap” does not limit the scope of the disclosure to a specific type of aluminium source or type of aluminium-containing material. The aluminium- based waste may have an aluminium content of >40%.

[0069] In an embodiment, the aluminium-based waste includes one or more waste types, such as post-consumer aluminium waste and aluminium dross, that are solubilised in same reaction conditions, such as in an alkali solution.

[0070] An embodiment of a system 10 for digesting aluminium-based waste to form an alumina species is shown in Figure 1. The system 10 includes a source of aluminium-based waste 12 and a digestor 14 for dissolving the aluminium-based waste 12 in an alkali solution to form a solution that includes an aluminate species. The digestor 14 may also directly form or produce hydrate or alumina directly. The aluminium-based waste 12 could be added to the digestor 14 as a dry solid. Alternatively, the aluminium-based waste 12 could be added to the digestor 14 as a water slurry. A slurry may allow the aluminium-based waste 12 to be pumped to the digestor 14. A slurry may also help to eliminate the presence of air (e.g. oxygen gas) which may be important should the aluminium-based waste 12 contact any alkali before the digestor and begin to produce hydrogen. Reducing or eliminating the presence of oxygen surrounding the aluminium-based waste 12 may help to reduce the chance of forming a combustible mix of hydrogen and oxygen. Similarly, the digestor 14 may be configured to reduce the presence of oxygen.

[0071] The term “aluminate” as used herein covers soluble species, such as those soluble in alkali solutions, that contain aluminium. In an embodiment, the aluminate species includes sodium or potassium aluminate species, for example (Na[AI(OH)4] or K[AI(OH)4]). The term “aluminate” may also include soluble aluminosilicates and aluminate species that include species in addition to aluminium-based species.

[0072] The system may also include a solids filter downstream of the digestor 14 to remove nondissolved waste or residue (not shown). The non-dissolved waste or residue may include plastic, metals and / or paper that was associated with the aluminium-based waste 12. The metals may be elemental metal including alloys, noble metal(s), metal hydroxides and / or metal oxyhydroxides. The non-dissolved waste or residue may be further separated, for example to extract value and recycle them such as plastics.

[0073] The system 10 also includes a precipitator 16 for precipitating the aluminate species into an alumina species. In an embodiment, the precipitator 16 is in fluid communication with the digestor 14 such that spent liquor (e.g. spent alkali solution) formed in the precipitator 16 can be recycled back to the digestor 14. Recycling the spent liquor helps to reduce the amount of alkali needed to dissolve the aluminium-base waste. Recycling the spent liquor from the precipitator 16 back to the digestor 14 may include liquor treatment to remove for example impurities. Accordingly, the spent liquor from the precipitator 16 may not be directly returned to the digestor 14. If the solids filter is included, it is typically positioned upstream of the precipitator 16.

[0074] The alumina species formed by the precipitator 16 can include alumina hydrate (AI(OH)s), boehmite (AIO(OH)) and / or alumina (AI2O3). The formation of alumina hydrate or alumina depends on the precipitation conditions in the precipitator 16. For example, high pressure and high temperature precipitation can allow sodium aluminate to precipitate directly as alumina. In instances when the precipitator 16 forms alumina hydrate, the system 10 includes a calciner 18 that can calcine the alumina hydrate to form alumina. In an embodiment, precipitating the aluminate species to form the alumina species may include adding a seed crystal of the alumina species to the alkali solution in the precipitator 16. In an embodiment, precipitating the aluminate species to form the alumina species in the precipitator 16 includes decreasing a temperature of the alkali solution that includes the aluminate species. For example, cooling may result in the alkali solution that includes the aluminate species reaching a saturation point to initiate precipitation.

[0075] System 10 also includes a hydrogen gas off-take 26 that is in fluid communication with the digestor 14. Hydrogen gas generated during dissolution of aluminium-based waste in the digestor 14 can be received by the hydrogen gas off-take 26. The hydrogen gas developed by system 10 can be used by an energy source such as a boiler, turbine generator, and fuel cell. The hydrogen gas may also be used as a reactant such as in treatment of ore or residue to reduce of metal values. The hydrogen gas may be used as a combustion fuel in whole or supplementing other combustion fuels such as fuel enrichment for example as required in calcination, boilers and so the like.

[0076] In an embodiment, the digestor 14 is a tank where the aluminium-based waste 12 is dissolved in alkali solution in a batch or semi-batch process. Alternatively, the digestor 14 may be a continuous flow reactor.

[0077] In an embodiment, the alkali solution used in the digestor 14 has a pH ranging from about pH 8 up to about pH 15, such as pH 8 to pH 11 . A pH of the alkali solution in the digestor 14 depends on the type of aluminium-based waste 12 and the desired reaction conditions in the digestor 14. For example, if high rates of dissolution are required, a higher pH may be used. Conversely, if slower rates of dissolution are required, a lower pH may be used. Similarly, a concentration of the alkali solution may vary. Generally, the alkali solution has a concentration up to 6N. The rate of dissolution of the aluminium-based waste 12 also depends on a particle size of the aluminium-based waste 12 and a temperature of the alkali solution in the digestor 14. Typically, a lower particle size of the aluminium-based waste 12 and / or a higher temperature of the alkali solution will increase a rate of dissolution. The alkali solution may be formed from NaOH, KOH or mixtures thereof. In an embodiment, the digestor 14 is configured to maintain a temperature of the alkali solution within a predefined condition. For example, the predefined condition may be dependent on a rate of hydrogen and / or heat production. In an embodiment, the predefined condition is a temperature up to 500°C, up to 450°C, up to 400°C, up to 350°C, up to 300°C, up to 250°C, up to 240°C, up to 230°C, up to 220°C, up to 210°C, up to 200°C, up to 190°C, up to 180°C, up to 170°C, up to 160°C, up to 150°C, up to 140°C, up to 130°C, up to 120°C, up to 110°C, up to 100°C, up to 90°C, up to 80°C, up to 70°C, up to 60°C, up to 50°C, or up to 40°C. In an embodiment, a temperature of the alkali solution in the digestor 14 is maintained at a temperature <45°C. Heating up >250°C such as 500°C, may help to produce a mixture of high-temperature steam and hydrogen gas (plus the precipitated hydroxide). Such a mixture can be vented to separate the steam from hydrogen gas. The separated steam may then be used to perform mechanical work or be cooled down through heat interchange. The separated hydrogen gas may be combusted or used in a fuel cell (or, in our case, a reducing agent for residue smelting).

[0078] In an embodiment, the aluminium-based waste may also include aluminium oxide such as alumina hydrate and alumina. For example, hydrate produced by precipitation of aluminate species from aluminium-based waste may not have a purity sufficient to allow calcining to produce smelter-grade alumina. In system 10, this “impure” alumina hydrate can be recycled back to digestor 14. Waste aluminium oxides may include impure alumina species produced by precipitation of aluminate-based species, commercial alumina products such as aluminabased abrasives, and calcined alumina hydrate that does not smelter-grade alumina standards. For example, fume captured from calcination of alumina hydrate may be recovered and recycled back to the digestor 14. The addition of impure alumina hydrate and / or waste alumina may displace a required amount of bauxite in a Bayer plant. Accordingly, in an embodiment, a feedstock in a Bayer plant may include alumina hydrate and / or alumina. The hydrate and / or alumina may be combined or blended with bauxite prior to addition in the digestor 14 or may optionally be combined in the digestor 14. Unlike dissolution of aluminium which produces hydrogen, dissolution of hydrate and / or alumina does not produce hydrogen and is endothermic.

[0079] In an embodiment, a combination of different waste streams may be combined or blended to form the aluminium-based waste 12. For example, the aluminium-based waste 12 may be formed from a blend of scrap aluminium metal and aluminium dross. In an embodiment, the aluminium-based waste 12 is melted and then molten metal is pelletised or cast into strips for cutting or milling or grinding into particles in melting system 17. The term “melting” when describing melting the aluminium-based waste 12 includes forming a melt mixture that may include non-melted particulate matter. For example, a melt mixture can include molten scrap aluminium metal having particulate or non-melted aluminium and optionally alumina distributed in the molten scrap aluminium metal. Melting the aluminium-based waste 12, especially if the aluminium-based waste 12 is formed from a blend of different waste streams, in melting system 17 prior to digestion in the digestor 14 may help to eliminate the need to clean the aluminium-base waste 12 and potentially reduce the risk of downstream contamination such as with organics from plastics. The melting system 17 may help to provide particleised waste aluminium having a size and / or surface area profile selected to promote favourable dissolution kinetics in the digestor 14. For example, the evolution of hydrogen vs the generation of heat in the digestor 14 by dissolution of the aluminium-based waste 12 may be determined by a particle size of the aluminium- based waste 12 formed in the melting system 17.

[0080] The melting system 17 is not required in all embodiments. The melting system 17 may be replaced with a shredder 24 as described below.

[0081] Another embodiment of a system 10a for digesting aluminium-based waste to form an alumina species is shown in Figure 2 and Figure 3. System 10a is similar to system 10 and like features are referred to with the same numerical references. System 10a builds upon system 10 by including additional features. In system 10a, a spent liquor line 21 is shown as extending from the precipitator 16 to the digestor 14. The spent liquor line 21 also applies to system 10. The spent liquor may be processed to remove insoluble (solid) and / or insoluble impurities prior to use in the digestor 14 and / or cleaner 22. For example, a polisher may be provided on spent liquor line 17 and / or spent liquor line 19 (not shown). Impurities in the spent liquor may include organics and oxalates.

[0082] System 10a includes a polisher 15 configured to receive the solution that includes an aluminate species from the digestor 14. The polisher 15 is provided upstream of the precipitator 16 such that insoluble material in the solution that includes an aluminate species can be removed prior to precipitating the aluminate species in the precipitator 16. In an embodiment, the insoluble material includes alloying elements and impurities in the aluminium-based waste 12, insoluble metal hydroxides and metal oxyhydroxide such as FeO and FeOOH, Mg(OH)2, Mn(OH)2, silicon oxo-hydroxides, and impurities and salts from aluminium dross including F-based salts. Although not shown in Figure 2, the soluble material collected or separated from the solution that includes an aluminate species in the polisher 15 may be collected and further processed. The polisher 15 may also remove soluble impurities dissolved in the solution that includes the aluminate species. For example, the soluble impurities may include Ca, P, V, Ga, Be Fe, Zn, Mn, Mg, Zn and / or Ti. Removal of soluble impurities may help to ensure the hydrate or alumina formed from system 10a meet product quality for the intended markets. The polisher 15 may include both solid (insoluble) phase and soluble phase polishers. Removal of solid phase and soluble phase may be performed by the same polisher.

[0083] System 10a may also include a cleaning system in the form of cleaner 22 for cleaning aluminium-based waste 12 prior to dissolution in the digestor 14. The cleaning system 22 is typically used as an alternative to the melting system 17 may in some instances may be used to clean aluminium-based waste 12 being fed into the melting system 17. In an embodiment, the cleaner 22 is positioned upstream of the digestor 14 such that the aluminium-based waste 12 is cleaned remove at least some contaminants associated with a surface of the aluminium-based waste 12 such as oils and other surface-bound material.

[0084] The cleaner 22 may include a washing system that uses a washing solution to wash the aluminium-based waste 12. The washing solution may be used for thermal decoating, for example to remove plastic materials sandwiched to the aluminium. The cleaner may include a thermal treatment system configured to thermally treat and remove non-aluminium material associated with the aluminium-based waste 12, such as dyes, paints, plastics and / or paper. The cleaner 22 may use a combination of washing and thermal treatment to clean the aluminium-based waste 12. The cleaner 22 may also be configured for chemical and / or mechanical decoating of non-aluminium material or species from the aluminium in the aluminium-based waste 12. Thermal decoating and / or thermal treatment is typically conducted at a temperature below the melting point of aluminium e.g. <630°C. The cleaner 22 may also help to physically separate different forms of aluminium-based waste or product from one another. This may help with cleaning in the cleaner. For example, the cleaning requirements for coffee pods may differ from aluminium alloys, so separating out these different types of waste may help to improve the cleaning process.

[0085] In an embodiment, the system 10 includes a separator unit upstream of the cleaner 22, shredder 24 and / or melting system 17 (not shown in Figures). The separator unit may separate the aluminium-based waste 12 into different stream that can be cleaned and processed as required before being combined to form a processed aluminium-based waste stream. The separator unit may also separate aluminium-containing from non-aluminium- containing materials. The separator unit may alternatively be position immediately upstream of the digestor 14, for example to separate out waste that is outside a predefined size range. The rejected waste may be sent back to the shredder 24 for further processing.

[0086] The use of the cleaner 22 and / or the separator may help to eliminate the need for the solids filter downstream of the digestor 12. However, even if the cleaner 22 and / or the separator are used, the solids filter may be used to polish the solution that includes an aluminate species (e.g. green liquor) prior to precipitation in the precipitator 16.

[0087] The cleaner 22 may also help to remove a passivation layer associated with the aluminium- based waste 12. In an embodiment, the washing solution is an alkali solution. An advantage of using an alkali solution to clean the aluminium-based waste 12 is that the aluminium oxide layer on the surface of the aluminium-based waste 12 can be removed. Removing the aluminium oxide layer in the cleaner 22 can act as a pre-dissolution stage such that, once the aluminium-based waste 12 that has been cleaned and is received in the digestor 14, the dwell time that would otherwise be required in the digestor 14 to remove the aluminium oxide layer is minimised or eliminated. Simply put, removing the aluminium oxide layer in the cleaner 22 using alkali can help to start dissolution of the aluminium-based waste 12. In this way, dissolution of the aluminium-based waste 12 can be broken into two stages, a predissolution or conditioning stage in the cleaner 22 where the passivation layer is removed, and a dissolution stage in the digestor 14 where the aluminium-based waste 12 is dissolved.

[0088] A headspace or interior volume of the cleaner 22 may be in fluid communication with the digestor 14 such that any hydrogen generated in the cleaner 22, if any, can be transferred to the digestor 14 and then onto the hydrogen gas off-take 26. Typically, a residence time of the aluminium-based waste 12 in the cleaner 22 is such that no hydrogen is generated in the cleaner 22.

[0089] In an embodiment, the cleaner 22 is configured to receive spent liquor from the precipitator 16 via spent liquor line 19. Generally, as the cleaner 22 only requires enough alkali solution for cleaning, only a small proportion of the spent liquor is sent to the cleaner 22 from the precipitator 16. The solution used to clean the aluminium-based waste 12 in cleaner 22 is generally passed into the digestor 14 to help reduce an amount of the alkali solution used to dissolve the aluminium-based waste 12. However, in an embodiment, the washing solution (e.g. alkali solution) used in the cleaner 22 is collected and not sent to the digestor 14. For example, if the aluminium-based waste 12 is particularly dirty, it may be beneficial to not allow the washing solution to be mixed in the alkali solution used in the digestor 14.

[0090] Although the cleaner 22 is shown in system 10a, it is not required in all embodiments. For example, the cleaner 22 may only be required or be utilised depending on the properties of the aluminium-based waste 12. If the aluminium-based waste 12 is dirty, then cleaning using the cleaner 22 may be warranted. However, if the aluminium-based waste 12 is clean or has a contaminate level below a threshold value the cleaner 22 may not be required. Alternatively, if the aluminium-based waste 12 is in the form of remelted and pelletised waste, as described above, the cleaner 22 may not be required.

[0091] System 10 also includes a particleiser configured to reduce a particle size of the aluminium- based waste 12. The particleiser is shown as shredder 24. The shredder 24 is configured to receive the aluminium-based waste 12 and is positioned upstream of the digestor 14 such that the digestor 14 can receive aluminium-based waste with a reduced particle size. The term “shredder” as used herein can also include cutting, crushing, milling, grinding and attiring to reduce a particle size of the aluminium-based waste 12. The type of aluminium- based waste 12 may determine whether cutting, milling grinding or shredding is required to reduce a particle size of the aluminium-based waste 12. In an embodiment, the shredder 24 reduces a particle size of the aluminium-based waste 12 to be >100 pm, such as >500 pm. In an embodiment, the shredder 24 reduces a particle size of the aluminium-based waste 12 to be <100mm, such as <50 mm, <10 mm or <5mm. In an embodiment, the particle size of the aluminium-based waste 12 is >10 pm, such as >50 pm, >100 pm or >500 pm. In an embodiment, the particle size of the aluminium-based waste 12 processed in the shredder 24 ranges from about 0.01 mm to about 10 mm, 0.01 mm to about 10 mm, 0.1 mm to about 10 mm, or 0.1 mm to about 5 mm.

[0092] A particle size <10 pm may lead to a rate of dissolution in the digestor 14 to be too high, such as causing explosive dissolution where the amount of heat and / or hydrogen gas generated in the digestor 14 cannot be controlled. However, a rate of addition of the aluminium-based waste to the digestor 14 or any other source of alkali may be controlled to control a rate of hydrogen gas and / or heat generation.

[0093] A feedstock with a higher proportion of smaller particles may help to increase a rate of aluminium dissolution and hydrogen and / or heat product, while a feedstock with a high proportion of larger particles may decrease a rate of aluminium dissolution and hydrogen and / or heat production. The ratio of smaller to larger particles can be adjusted to control the reaction rate of aluminium dissolution. For example, the system 10 may include a feed controller (not shown) that is configured to adjust a particle size distribution of the aluminium- based waste 12. In some embodiments, the aluminium-based waste 12 or processed aluminium-based waste stream may be a blend of aluminium-based waste having different particle sizes. For example, the aluminium-based waste 12 or processed aluminium-based waste stream could be formed from different blends or sources of aluminium-based waste that are mixed to form a homogenous scrap feed. These different sources may be from different types of crap for example consumer waste vs alloys. Accordingly, the form of the aluminium-based waste can range from powders (low pm) to large (e.g. >50mm) particulate matter.

[0094] In an embodiment, the aluminium-based waste 12 does not include powdered forms (i.e. <10 pm) of aluminium-based waste. Conversely, a particle size that is too high may not provide favourable rates of dissolution in the digestor 14, such as leading to insufficient heat and / or hydrogen generation. The required reduction in size of the particle of the aluminium-based waste 12 may also depend on the type and form or composition of the aluminium-based waste 12. For example, solid aluminium may require a finer particle size compared to e.g. aluminium dross. The shredder 24 may be used as an alternative to the melting system 17. However, in an embodiment, solidified material existing the melting system 17 may be shredded in shredder 24. The shredder 24 may perform wet and / or dry shredding. Wet shredding may be performed with water and / or an alkali solution.

[0095] In the Figures, the melting system 17 and combination of the cleaner 22 and shredder 24 are shown as alternatives, but it should be appreciated that the melting system 17, cleaner 22 and shredder 24 may be utilised in different combinations. For example, in an embodiment, the aluminium-based waste is thermomechanical processed (e.g. melted) and extruded, followed by chopping into 1-5 mm size pieces.

[0096] In an embodiment, and as shown in Figure 2, the shredded aluminium-based waste output from the shredder 24 is received by the cleaner 22 such that the shredded aluminium-based waste is cleaned or washed in the cleaner 22 prior to dissolution in the digestor 14. However, in an embodiment (not shown) the output from the shredder 24 is received directly by the digestor 14.

[0097] The system 10 also includes a heat exchange system 20. The heat exchange system 20 is in thermal communication with the digestor 14 and is configured for maintaining a temperature of the alkali solution in the digestor 14 during dissolution of the aluminium-based waste. Depending on the environmental conditions in which the digestor 14 is operated, the heat exchange system 20 may heat the digestor 14. However, given dissolution of aluminium in alkali is exothermic, in many cases the heat exchange system 20 is configured to cool the digestor 14 to be within a predefined condition. For example, the predefined condition may be maintaining a temperature of the alkali solution to be up to 250°C, up to 240°C, up to 230°C, up to 220°C, up to 210°C, up to 200°C, up to 190°C, up to 180°C, up to 170°C, up to 160°C, up to 150°C, up to 140°C, up to 130°C, up to 120°C, up to 110°C, up to 100°C, up to 90°C, up to 80°C, up to 70°C, up to 60°C, up to 50°C, or up to 40°C. When the digester 14 is a net heat generator, the heat exchange system 20 is used to recover heat generated during dissolving the aluminium-based waste in the alkali solution in the digester 14. The recovered heat may be utilised elsewhere within the system 10a. For example, heat recovered from the digester 14 by the heat exchange system 20 may be used to heat the aluminium-based waste 12 in the cleaner 22 or, if the cleaner 22 is not required, heat the shredder 24 or to heat the aluminium-based waste 12 prior to digestion in the digestor 14. Heat recovered by the heat exchange system 20 may also be utilised by the calciner 18 such as preheating alumina hydrate prior to calcination. The heat recovered by the heat exchange system 20 may be utilised by a separate system outside of the system 10a. Heat recovered by the heat exchange system 20 may be used to dry bauxite, heat liquor to mill and / or heat pre-desilication tanks (not shown).

[0098] In an embodiment, the digester 14 may be used as an evaporator whereby a mixture of generated hydrogen gas and water vapor (and other non-combustible or combustible gasses) is evaporated. The water vapor and other condensable gases may then be removed from the mixture to isolate hydrogen gas and non-condensable gasses. Isolation of the hydrogen gas from the water vapour may be performed in the hydrogen gas off-take 26. Isolation of the hydrogen gas from the water vapour may be performed separate to the hydrogen gas off-take 26, such as in a condenser. In such an embodiment, the digestor 14 may replace in part or fully conventional Bayer evaporators. Exothermic heat generated by dissolution of the aluminium-based waste may be used for evaporation in the digestor 14. The mixture of hydrogen gas and water vapour may be high-temperature steam.

[0099] System 10a also includes a hydrogen gas off-take 26 similar to system 10. Hydrogen gas collected by the hydrogen gas off-take 26 from the digestor 14 can be utilised by an energy conversion device 30. The energy conversion device 30 is configured to generate electricity and / or heat from the hydrogen gas collected from the hydrogen gas off-take 26. In an embodiment, the energy conversion device 30 includes one or more of a combustor 32, a fuel cell 34 and hydrogen gas turbine 36. A hydrogen storage system (not shown) may be associated with the energy conversion device 30. For example, hydrogen gas may be stored in the hydrogen storage system and later utilised by the energy conversion device 30. The hydrogen storage system may store hydrogen gas by compression, by sorption into / onto a sorption material, or forming hydrides. The energy conversion device 30 may be used to power the shredder 24, cleaner 22, digestor 14, polisher 15, precipitator 16 and / or calciner 18. The hydrogen gas may be used as a replacement fuel for carbon-based combustion or reductant fuels. System 10a also includes a reduction reactor 28. The reduction reactor 28 is configured to utilise hydrogen gas collected by the hydrogen gas off-take 26 to reduce metal ions in a feedstock to a lower oxidation state. In an embodiment, the reduction reactor 28 is used to reduce metal values in the feedstock into their metallic form. The feedstock may be residue formed from processing bauxite. An embodiment provides a reduction reactor system comprising system 10. For example, the reduction reactor system includes the source of aluminium-based waste 12, digestor 14, and hydrogen gas off-take 26. The reduction reactor system may also include the precipitator 16 and optionally calciner 18.

[0100] Typically, the amount of hydrogen generated by system 10a (and system 10) is more than an energy required of the system 10a (and system 10). Therefore, there tends to be an excess generation of energy from system 10 and system 10a. For example, 756 kg of scrap aluminium will generate about 15,688 MJ of heat energy, about 59kg of hydrogen gas (equivalent to about 7,080 MJ of energy), and 1 ,530 kg of alumina hydrate. However, calcination of 1 ,530kg of alumina hydrate only requires approximately 3,200 MJ of energy, leaving a net energy production of 19,586 MJ and 1 ,000 kg of smelter grade alumina. Even if the energy requirements of the reduction reactor 28 is included when the feedstock of the reduction reactor 28 includes Bayer residue (such as red mud), the system 10a is still a net energy producer. For example, approximately 1000 kg of residue having an iron content of approximately 35%, only requires about 10 kg of hydrogen (about 2.765MJ) to covert the iron to its metallic form by hydrogen reduction at a 70% metallization ratio. The amount of hydrogen gas generated by the system 10 and system 10a may be control to produce a desired amount of energy for use in one or more adjacent processes.

[0101] The system 10 and / or system 10a may include a delivery system to control a rate at which aluminium-based waste 12 is added to the digestor 14 to help control a temperature of the alkali solution in the digestor 14.

[0102] In an embodiment, use of the system 10 and / or system 10a can result in a process of treating aluminium-based waste that includes aluminium or aluminium-containing materials, the process comprising: dissolving aluminium-based waste in an alkali solution to form a solution that includes an aluminate species; and precipitating the aluminate species to form an alumina species.

[0103] Given the system 10 and system 10a generates heat and hydrogen gas, in an embodiment the system 10 and / or system 10a can be considered as acting as a heat generator and / or hydrogen gas generator. Further, utilising both the heat and hydrogen generated by dissolving the aluminium-based waste 12 can help to extract or recover energy in the aluminium-based waste 12 initially instilled during smelting to form aluminium. An embodiment provides a hydrogen generator comprising the system 10 or system 10a. An embodiment provides a heat generator comprising the system 10 or system 10a.

[0104] An embodiment provides alumina produced from the system 10 and / or system 10a.

[0105] Embodiments of systems and processes used to treat an aluminium-containing material will now be described with reference to Figure 4 to Figure 8.

[0106] By way of background, the Bayer process focuses on processing bauxite ore as an aluminium source to form alumina. The alumina is then smeltered to form aluminium. Digestion of bauxite requires an alkali solution at high temperatures (>100°C) at high pH and typically the use of steam as a heating fluid. Following digestion, alumina hydrate is precipitated which is then calcined to form alumina such as smelter grade alumina.

[0107] Calcination requires considerable energy inputs. The two main outputs of the Bayer process are alumina (following calcination) and digestion residue such as red mud. Red mud has typically been stored in tailings facilities which can present long-term obligations and costs. Not only this, but red mud and other forms of digestion residue contain other metal values such as iron and rare earths depending on the type and grade of bauxite.

[0108] Now referring to Figure 4, system 100 is used to treat an aluminium-containing material and incorporates the features and principles of heat and hydrogen generation from system 10 and / or system 10a. Like features from system 10 and / or system 10a are referred to with the same references when describing system 100. The system 100 includes a digestor system 112 used to digest the bauxite 110, and a precipitator 116 for precipitating an alumina species such as alumina hydrate or alumina from a pregnant liquor formed in the digestor system 112. The precipitator 116 is connected to the digestor system 112 via return line 117 for returning spent liquor formed in the precipitator 116 back to the digestor system 112. Typically, the precipitator 116 precipitates alumina hydrate. The digestor system 112 and precipitator 116 can be embodied and utilised in different forms, which is explained in more detail with reference to Figure 5 to Figure 7.

[0109] System 100 also includes aluminium-based waste 12 which is dissolved in the digestor system 112 either separately to or in the presence of bauxite as is explained in more detail with reference to Figure 5 to Figure 7. The digestor 112 may be a multistage digestor. For example, a first stage may be directed to dissolving the aluminium-based waste 12 and hydrogen gas generation, a second stage may focus on predesilication, and a third stage may focus on digesting bauxite. Accordingly, through this disclosure, the term “digestor” may include a digestion system having two or more digestion vessels / chambers / reactors. The alumina species resulting from dissolution of the aluminium-based waste 12 can be calcined in calciner 120 separately to or together with alumina species resulting from digestion of bauxite. Hydrogen gas generated by the dissolution of the aluminium-based waste 12 is collected by hydrogen gas off-take 26 (not shown in Figure 4) and utilised by the reduction reactor 28. The reduction reactor 28 typically treats residue produced by the digestor system 112 and optionally may also treat previously formed residue such as red mud. In an embodiment, the reduction reactor 28 may release water as vapour e.g. as evaporate. The treated residue 118 may include metallic forms of metal values present in the residue such as iron. In an embodiment, red mud 114 or previously generated residue may also be added to the residue or separately processed by the reduction reactor 28 to extract metal values from the red mud. In an embodiment, 2 Mt / yr of alumina produced by system 100 can produce enough hydrogen gas to treat 11 .5 Mt of old residue from tailings (e.g. red mud) using an electric arc furnace (as the reduction reactor 28) to generate 1 .75 Mt of high purity (>99 wt%) iron. Generally, the amount of hydrogen gas produced from dissolution of aluminium-based waste 12 will be greater than amount of residue generated by processing the bauxite 110 so it tends to be possible to utilise red mud or other previously formed tailings or residues processed by the reduction reactor 28 in addition to the residue formed by the digestor system 112.

[0110] The reduction reactor 28 may be take other forms such as a blast furnace, gas suspension or fluidised bed calciner or furnace with pelletised feed that utilises hydrogen gas as a reductant. The reduction process performed in the reduction reactor 28 may be applied to ores containing iron or other metallic oxides and other industrial wastes.

[0111] The fact that system 100 processes bauxite using similar processes to existing Bayer plants and Bayer processes, system 100 may be considered as forming a modified-Bayer plant.

[0112] System 100 may also be considered as plant or system to treat aluminium-containing material, the plant or system comprising: a first digestor for digesting bauxite a source of aluminium-based waste for dissolving in an alkali solution for generating hydrogen gas; a hydrogen gas reactor (e.g. reduction reactor 28) that utilises hydrogen gas generated by dissolution of the aluminium-based waste for reducing metal values present in residue formed by at least the first digestor. The aluminium-based waste may be dissolved in alkali in the first digestor.

[0113] In system 100, the heat generated by the dissolution of the aluminium-based waste 12 can be utilised by the digestor system 112 helping to reduce the energy required from an external heat source. Similarly, hydrogen gas generated by the dissolution of the aluminium-based waste 12 can be utilised by the reduction reactor 28, calciner 120, as a combustion fuel, a reductant, or an energy source used to convert hydrogen gas into heat energy or electricity that can be utilised by the system 100. Therefore, the use of aluminium-based waste 12 can:

[0114] • recover energy stored in aluminium to help decrease the amount of energy required to digest bauxite and form alumina;

[0115] • recycle aluminium waste that would otherwise not be able to be recycled; and

[0116] • treat residue and optionally previously formed red mud to extract further metal values using hydrogen without an additional hydrogen generator.

[0117] Further, the use of aluminium-based waste 12 means that there is no carbon, energy or environmental penalty associated with using aluminium-based waste 12 as a feedstock since its carbon and energy impacts were previously produced and accounted for during its original manufacture, apart from those used to transport the aluminium-based waste 12 to site. Accordingly, system 100 may help to significantly reduce the energy usage and environmental impact of processes of processing bauxite to form alumina. Additionally, system 100 may allow low grade or uneconomical bauxite ores to be mined since the potential to form alumina can be increased by a higher amount of aluminium-based waste 12 relative to the bauxite 110.

[0118] Another embodiment of a system 100a to treat an aluminium-containing material will now be described with reference to Figure 5. System 100a builds upon system 100 by including additional features, and like features are described with like references. System 100a has a mill 122 for milling bauxite down to a predetermined particle size or property before being passed into a digestor 112a to form a pregnant liquor and digestion residue. Typically, milled bauxite is added to the digestor 112a as a slurry. The digestor 112a forms a first digestor of system 100a. The pregnant liquor provides a first source of aluminate 123. The pregnant liquor is isolated from digestion residue (not shown) and the pregnant liquor may optionally be clarified in clarifier 115. The clarified pregnant liquor (i.e. first source of aluminate 123) is then subjected to precipitation, typically by seeding, in precipitator 116 to precipitated alumina hydrate and form spent liquor. The spent liquor is returned to the digestor 112a by the return line 117. Spent liquor in the return line 117 may be polished as described for spent liquor return line 21 in system 10a. The alumina hydrate is then calcined in calciner 120. The steps performed in the mill 122, digestor 112a, precipitator 116 and calciner 120 form the Bayer process, as shown generally by box 101 .

[0119] System 100a also includes system 10 or system 10a. The aluminium-based waste 12 is sent to a particleiser e.g. the shredder 24 (and optionally cleaner 22 which is not shown in Figure 5) before being dissolved in digestor 14 to form a second source of aluminate 40. The digestor 14 forms a second digestor of system 100a. The system 100a has a mixer 124 that receives and combines or blends the first source of aluminate 123 and the second source of aluminate to form a combined source of aluminate 126. The second source of aluminate 40 may optionally be polished in polisher 15, as outlined previously for system 10 and system 10a, before combining or blending with the first source of aluminate 123 in mixer 124 to form the combined source of aluminate 126. The combined source of aluminate 126 is then precipitated in the precipitator 116 to form aluminate hydrate 128. Put another way, the first source of alumina hydrate and the second source of alumina hydrate are precipitated simultaneously to form the aluminate hydrate 128 (e.g. a combined source of aluminate). In an alternative embodiment, the mixer 124 is omitted and the second source of aluminate 40 is passed directly to the precipitator 116 where it mixes therein with the first source of aluminate 123. Put another way, the precipitator 116 is configured to precipitate alumina hydrate from aluminate generated in the first digestor and / or the second digestor. Whatever way the aluminate hydrate 128 is formed, it is then calcined in calciner 120.

[0120] The mixer 124 is shown as being downstream of the clarifier 115. However, the mixer 124 may alternatively be positioned upstream of the clarifier 115 such that the combined source of aluminate 126 is polished or clarified by the clarifier 115. Such an arrangement may eliminate the need for polisher 15.

[0121] In an embodiment, the combined source of aluminate has a ratio of [first source of aluminate 123]:[second source of aluminate 40] ranging from 100:0.01 to 0.01 :100 In an embodiment, the combined source of aluminate has a ratio of [first source of aluminate 123]:[second source of aluminate 40] ranging from 100:0.1 to 0.1 :100 In an embodiment, the combined source of aluminate has a ratio of [first source of aluminate 123]:[second source of aluminate 40] ranging from 100:1 to 1 :100 such as about 75:25 to 10:90. The ratio of [first source of aluminate 123]:[second source of aluminate 40] may be selected such that a concentration of silica content of the combined source of aluminate 126 is below a desilication product precipitation threshold to minimise or prevent precipitation of desilication products in or from the combined source of aluminate 126. Reducing or eliminating the precipitation of formation of desilication products can help to eliminate or reduce fouling and scale buildup of the components of the system such as the mixer 124, precipitator 116, spent liquor circuit in areas such as evaporation, and so on, plus also help to improve the purity and / or recovery of alumina hydrate from the precipitator 116.

[0122] The mixer 124 may be controllable to control a blend ratio of the [first source of aluminate 123]:[second source of aluminate 40], Being able to alter ratios of the [first source of aluminate 123]:[second source of aluminate 40] can help to provide flexibility of the system 100a to respond to changing properties of the bauxite 110 and / or aluminium-based waste 12, market conditions and so on. The use of aluminium-based waste 12 may be purely to provide heat and / or hydrogen gas. In another example, the price of aluminium-based waste 12 may be above an economically viable level at which point the amount of aluminium-based waste 12 used may be reduced. Alternatively, the rate of hydrogen gas generated by the digestor 14 may determine the ratio of the [first source of aluminate 123]:[second source of aluminate 40], The ratio of the [first source of aluminate 123]:[second source of aluminate 40] may be adjusted such that a concentration or level of organics, oxalate and / or carbonates in the combined source of aluminate 126 is below a threshold value. The ratio of the [first source of aluminate 123]:[second source of aluminate 40] may be adjusted such that a concentration of aluminate in the combined source of aluminate 126 is above a threshold value, for example when bauxite ores that were previously considered uneconomic are used.

[0123] The use of the second source of aluminate 40 to minimise or prevent precipitation of desilication products means the disclosed provides in an embodiment a method for digesting bauxite, the method comprising: digesting bauxite in an alkali solution to form a pregnant liquor; adding to the pregnant liquor a source of aluminate formed from dissolving aluminium-based waste in an alkali solution; wherein an amount of the source of aluminate added to the pregnant liquor is selected to decrease a relative concentration of silica content in the pregnant liquor below a desilication product precipitation threshold to minimise or prevent precipitation of desilication products in the pregnant liquor.

[0124] In system 100a, heat generated by dissolving the aluminium-based waste in the digestor 14 is recovered by heat exchange system 20 and utilised by a component of the Bayer plant / process as illustrated in box 101 . In the embodiment shown in Figure 5, the heat exchange system 20 is in thermal communication with the digestor 112a such that heat generated by the aluminium-based waste 12 is utilised by the digestor 112a during digestion of the bauxite. Given dissolution of the aluminium-based waste 12 is exothermic and digestion of bauxite 110 in the digestor 112a is endothermic, the energy provided in the aluminium-based waste 12 can be released in the digestor 14 and passed to the digestor 112a as heat thereby reducing the external heating requirements of the digestor 112a such as with boilers and heat exchange systems used to heat e.g. spent liquor recirculated back to the digestor 112a.

[0125] Depending on how the heat exchange system 20 is configured, a temperature of the alkali solution during dissolution of the aluminium-based waste 12 in the digestor 14 may be lower than a temperature of a solution in the digestor 112a during digestion of bauxite 110. For example, a rate of dissolution of the aluminium-based waste 12 in the digestor 14 may be determined by a hydrogen output for treatment of residue in the reduction reactor 28. Accordingly, the heat exchange system 20 may be operable to control a temperature of the alkali solution in the digestor 14 to control a dissolution rate of the aluminium-based waste 12.

[0126] As already discussed, the digestor 14 generates hydrogen gas which is collected by hydrogen gas off-take 26, where the hydrogen gas can be utilised by the reduction reactor 28 or the energy conversion device 30. System 100a also utilises hydrogen gas generated by the digestor 14. A residue line 130 connects the digestor 112a with the reduction reactor 28. Accordingly, residue generated in or by the digestor 112a, which typically is in the form of digested bauxite, is passed to the reduction reactor 28 via residue line 130. The reduction reactor 28 is used to reduce metal values in the residue, such as to their metallic form. For example, bauxite typically has around 35-55% iron oxide depending on the bauxite characteristics, and the reduction reactor 28 can convert this iron oxide into iron. The residue generated in of by the digestor 112a may be processed before entering the reduction reactor 28. For example, the residue may be concentrated, non-target metal values removed, a concentration of target metal values is increased, and so on. The residue may be clarified and / or subject to filtration prior to entering the reduction reactor 28. An iron content of the residue treated in the reduction reactor 28 may be at least 20%. An iron content of the residue treated in the reduction reactor 28 may be at least 25%. An iron content of the residue treated in the reduction reactor 28 may be at least 30%. An iron content of the residue treated in the reduction reactor 28 may range from 20% up to about 80%, such as 25% to 55%.

[0127] In an embodiment, the reduction reactor 28 is an electric arc reactor that uses hydrogen gas from the hydrogen gas off-take 26 as a reducing agent. In an embodiment, a second residue line 132 connects the digestor 14 or optionally the polisher 15 (not shown) with the reduction reactor 28. Accordingly, residue generated in or by the digestor 14 and / or the polisher 15, such as metal hydroxides and / or metal oxyhydroxides, is passed to the reduction reactor 28 via second residue line 132. In an embodiment, the reduction reactor 28 comprises a plurality of reactors. For example, a first reactor may treat bauxite residue from digestor 112a and a second reactor may treat residue formed in the digestor 14 and / or polisher 15.

[0128] The hydrogen gas collected by the hydrogen gas off-take 26 can be utilised by the digestor 112a for heating liquor for digestion of bauxite. For example, a heating source that uses hydrogen gas as an input, such as a hydrogen gas combustor, a boiler or similar, can be used to the digestor 112a to heat the liquor used for digestion. In an embodiment, a combination of heat generated from the digestor 14 and conversion of hydrogen gas collected from the hydrogen gas off-take 26 into heat is used to heat the digestor 112a such that no other external heat source is required. Therefore, in an embodiment, the digestor 112a is heated using energy liberated from dissolution of aluminium-based waste 12 in the digestor 14.

[0129] When the system 100a includes the calciner 120, hydrogen gas collected by the hydrogen gas off-take 26 can be utilised by the calciner 120 to calcine the aluminate hydrate 128. However, even if the calciner 120 is included in system 100a, the generated hydrogen gas may also be utilised to generate power. The system 100a may include a hydrogen gas controller to control the utilisation of hydrogen gas with the system 100a, such as utilisation by the reduction reactor 28 and calciner 120.

[0130] Another embodiment of a system 100b t to treat an aluminium-containing material will now be described with reference to Figure 6. System 100b is similar to system 100a and like features are described with like references numerals. In system 100b, the first source of aluminate 123 is precipitated in precipitator 116 to form a first source of alumina hydrate 136 and spent liquor. The spent liquor is returned to the digestor 112a by the return line 117. The second source of aluminate 40 is precipitated in precipitator 16 to form a second source of alumina hydrate 42. The first source of alumina hydrate 136 and the second source of alumina hydrate 42 are combined at mixing point 134 before being simultaneously passed through the calciner 120 to form alumina. The calciner 120 in this embodiment may represent a single calciner system having a single alumina outlet. In an embodiment the mixing point 134 is external to the calciner 120 such that the mixing point 134 forms a combined stream of alumina hydrate 138 that is then calcined in the calciner 120. However, in an embodiment, the first source of alumina hydrate 136 and the second source of alumina hydrate 42 are fed separately to the calciner 120 such that the first source of alumina hydrate 136 and the second source of alumina hydrate 42 are simultaneously passed through the calciner 120. In such an arrangement, the combined stream of alumina hydrate is formed within the calciner 120. The first source of aluminate 123 may be clarified by clarifier 115 prior to precipitation in the precipitator 116.

[0131] Another embodiment of a system 100c to treat an aluminium-containing material will now be described with reference to Figure 7. Similar to system 100a and system 100b, in system 100c the bauxite 110 is milled in mill 122 before being digested in a digestor 112b to form pregnant liquor 140 that may optionally be clarified in clarifier 115 before being precipitated in precipitator 116 to form alumina hydrate that is then calcined in calciner 120 and spent liquor. The spent liquor is returned to the digestor 112b by the return line 117. In an embodiment, aluminium-based waste 12 is shredded in shredder 24 (and optionally cleaned as described for system 10 and system 10a) before being added to the digestor 112b. Therefore, aluminate formed from bauxite and aluminate formed from the aluminium-based waste 12 are formed together in situ. This means the digestor 112b acts a single digestor for digesting bauxite and dissolving aluminium-based waste 12. Accordingly, use of the system 100c requires adding the aluminium-based waste 12 to the digestor 112b having an alkali solution and bauxite to generate a pregnant liquor having aluminate formed from dissolution of the aluminium-based waste 12 and digestion of bauxite 110. The pregnant liquor having aluminate formed from dissolution of the aluminium-based waste 12 and digestion of bauxite 110 is then precipitated in precipitator 116 to form the alumina hydrate that is then calcined in calciner 120. Hydrogen gas generated in the digestor 112b from dissolution of the aluminium-based waste 12 is captured with hydrogen gas off-take 26a. The hydrogen gas captured by the hydrogen gas off-take 26a is then utilised in the reduction reactor 28 to process residue from the digestor 112b as described for system 100, system 100a and system 100b.

[0132] An advantage of system 100c is that heat generated by the exothermic dissolution of aluminium-based waste 12 in the alkali solution in the digestor 112a is used directly to heat the alkali solution rather than relying on heat exchange system 20. Therefore, system 100c may be simpler to operate compared to system 100, system 100a and / or system 100b. In an embodiment, the system 100c requires no external heat source to provide thermal energy to heat the alkali solution in the digestor 112a to digest the bauxite. However, in one or more embodiments, the system 100c is provided with an external heating system to heat the alkali solution in the digestor 112a. The heating system may use hydrogen gas generated from the digestor 112a as a fuel source for either combustion or electrical heating. In system 100c, the aluminium-based waste 12 is added directly to the digestor 112b. The aluminium-based waste 12 may be added to the digestor 112b separately to the bauxite 110 or may optionally be added at the same time as bauxite 110. For example, the aluminium- based waste 12 may be added to the digestor 112b using a dedicated supply port / inlet. Alternatively, the aluminium-based waste 12 may be added to the digestor 112b with the bauxite 110. In an embodiment, a slurry of bauxite 110 and aluminium-based waste 12 is first formed, and this slurry is then introduced into the digestor 112b. Accordingly, in an embodiment, the system 100c may include a slurry generator positioned upstream of the digestor 112b that mixes the aluminium-based waste 12 and bauxite 110 to form the slurry (not shown).

[0133] Similar to system 100, system 100a and system 100b, where a ratio of the aluminate formed from the aluminium-based waste 12 and the aluminate formed from bauxite can be adjusted, the relative ratio of the aluminium-based waste 12 and bauxite 110 can be varied in system 100c. In an embodiment, a ratio of [bauxite]:[aluminium-based waste] ranges from 1 :100 to 100:1 . The ratio of [bauxite]:[aluminium-based waste] may be adjusted for a variety of reasons. For example, if the bauxite is low grade, the relative amount of aluminium-based waste 12 may be increased to help maintain a sufficient alumina output. The ratio of [bauxite]:[aluminium-based waste] may be adjusted to ensure a predetermined flow of hydrogen gas is generated by the dissolution of the aluminium-based waste 12 and / or a predetermined or required heat output from the dissolution of the aluminium-based waste 12. In an embodiment, the ratio of [bauxite]:[aluminium-based waste] is selected, such that a concentration of silica content of the pregnant liquor 140 is below a desilication product precipitation threshold to minimise or prevent precipitation of desilication products in the pregnant liquor.

[0134] In system 100c, the reduction reactor 28 is connect to the digestor 112b via residue line 142. Residue formed in the digestor 112b, such as digested bauxite residue, can be removed or isolated from the digestor 112b and passed to the reduction reactor 28, such that the residue can be processed under hydrogen reduction conditions, such as to convert iron oxide into iron.

[0135] Another embodiment of a system 100d to treat an aluminium-containing material will now be described with reference to Figure 8. System 100d is similar to system 100c, but instead of passing aluminium-based waste 12 into the digestor 14, the aluminium-based waste 12 is mixed or blended with the bauxite 110 prior to digestion. In Figure 7, the aluminium-based waste 12 is shown as being mixed with bauxite 110 prior to milling in mill 122. However, the aluminium-based waste 12 may be mixed with the bauxite 110 after the mill 122. Adding aluminium-based waste 12 to the bauxite 110 may help to increase an aluminium content of the bauxite 110. Mixing the aluminium-based waste 12 with the bauxite 110 prior to digestion in the aluminium-based waste 12 may allow for a higher solubility of silica in pregnant (and spent) liquor thereby allowing solid / liquor separation to separate slurry and disilicate liquor to isolate desilication product.

[0136] The system 100a, system 100b, system 100c and system 100d each has mill 122 for milling the bauxite 110 and a shredder 24 for reducing a particle size of the aluminium-based waste 12. Accordingly, an embodiment provides a plant for treating an aluminium-containing material, the plant comprising: a mill for milling bauxite to form milled bauxite; a particleiser for decreasing a particle size of aluminium-based waste, such as shredder 24; and a digestor system for digesting the milled bauxite and / or dissolving the particleised aluminium-based waste in alkali to form a source of aluminate. The digestor may be digestor 112a, digestor 112b and / or digestor 14. Although not shown in the Figures, the mill for milling bauxite to form milled bauxite and the particleiser for decreasing a particle size of aluminium-based waste may be the same device. For example, the shredder 24 may omitted and the aluminium-based waste 12 is instead passed through mill 122 such that the bauxite 110 and aluminium-based waste 12 are milled together.

[0137] Although the shredder 24 is shown in each of system 100a, system 100b and system 100c, it is not required in all embodiments. For example, if the aluminium-based waste 12 has a desired particle size, there may be no need for the shredder 24 to further reduce or alter the particle size of the aluminium-based waste 12 prior to dissolution in the aluminium-based waste 12.

[0138] Each of system 100, system 100a, system 100b and system 100c can be considered as forming a plant for treating aluminium-containing material. In an embodiment, the plant comprises: a digestor (e.g. digestor system 112, digestor 112a or digestor 112b) for digesting bauxite; a digestor (e.g. digestor 14 or digestor 112b) for dissolving aluminium-based waste 12 and generating hydrogen gas; a hydrogen gas reactor (e.g. reduction reactor 28) in fluid communication with the digestor for dissolving aluminium-based waste and configured to utilise hydrogen gas generated by the digestor for dissolving aluminium-based waste to reduce metal values present in residue formed by the digestion of bauxite.

[0139] Each of system 100, system 100a, system 100b and system 100c can be used to perform a process of treating residue formed from extraction of alumina from bauxite. In an embodiment, the process of treating residue comprises:

[0140] (i) dissolving aluminium-based waste 12 in an alkali solution (e.g. in digestor 14 or digestor 112b) to generate hydrogen gas and a solution containing aluminate; and

[0141] (ii) treating the residue with the hydrogen gas generated at step (i) under reductive conditions (e.g. in reduction reactor 28) to reduce one or more metal values present in the residue.

[0142] The residue may include digested bauxite, insoluble material formed from dissolution of the aluminium-based waste and / or red mud. The process may further comprise adding red mud from a tailing facility to the residue. The metal values include iron titanium, manganese, magnesium, zinc, and rare earths. The metal values may also include other site- and wastespecific elements depending on the original source of the bauxite and / or the waste products. Reducing one or more metal values present in the residue may include reducing an ion form of a metal into its corresponding metallic form. As already described for system 10 and system 10a, the process may include removing insoluble material present in the solution containing aluminate such as with using polisher 15. The insoluble material may be added to the residue for treatment at step (ii). The residue may be from industry other than the alumina / aluminium industry. The reductive processes used in reduction reactor 28 may support metal or other extraction or purification / concentration of residue.

[0143] Referring now to Figure 9 and Figure 10, embodiments of systems and processes related to aluminium smelter system 200 used to smelt alumina to form aluminium will now be described.

[0144] Figure 9 relates to an embodiment of an aluminium smelter system 200 that has smelting system 300 used to form aluminium from alumina. The smelting system 300 has an electrochemical reactor 310 for electrochemically converting alumina to aluminium. Aluminium smelter system 200 also includes system 10 or system 10a, and as shown in Figure 9 uses the digestor 14 for dissolving aluminium-based waste 12 in an alkali solution to generate hydrogen gas. The aluminium smelter system 200 may also include system one of 100a-100c.The aluminium smelter system 200 includes energy conversion device 30 to convert hydrogen gas generated by the digestor 14 into an energy form usable by the electrochemical reactor 310. Accordingly, the hydrogen gas produced by digestor 14 is utilised as an energy source to at least partially power the electrochemical reactor 310.

[0145] The hydrogen gas produced by the digestor 14 may be combusted to generate thermal energy. In addition to or alternatively, the hydrogen gas produced by the digestor 14 may be converted into electricity with a device including a fuel cell.

[0146] Now referring to Figure 10, an embodiment of an aluminium smelter system 200a that has smelting system 300 used to form aluminium from alumina will now be described. Aluminium smelter system 200a incorporates system 10a. In this way, the system 10a acts as a hydrogen generator for generating hydrogen gas that can be utilised by the smelting system 300.

[0147] Similar to system 10a, aluminium smelter system 200a includes heat exchange system 210 configured to maintain a temperature of the alkali solution in the digestor 14 during dissolution of the aluminium-based waste 12 within a preset temperature range. The heat exchange system 210 is similar or the same as heat exchange system 20. In an embodiment, the heat exchange system 210 is in thermal communication with one or more components of the smelting system 300 that requires heat, such as a furnace where one or more inputs of the furnace require preheating.

[0148] Since the primary purpose of the digestor 14 is to generate hydrogen gas, the aluminium smelter system 200a may be provided with the shredder 24 to reduce a particle size of the aluminium-based waste 12 prior to dissolution in the digestor 14. The particle size of the aluminium-based waste 12 may be selected to optimise hydrogen production rather than, for example, heat produced during dissolution.

[0149] The aluminium smelter system 200a may be configured to process aluminium dross as the aluminium-based waste 12. Aluminium dross is one of the main waste products of aluminium smelting. For context, approximately 65 million tonnes of aluminium is produced per annum, which results in approximately 1 million tonnes of aluminium dross.

[0150] Use of the aluminium smelter system 200a may result in a process of smelting alumina to form aluminium. The process may comprise: dissolving aluminium-based waste 12 in an alkali solution (for example in digestor 14) to generate hydrogen gas and source of aluminate; utilising the hydrogen gas as an energy source to at least partially power one or more components of an alumina smelter.

[0151] Heat generated by the electrochemical reactor 310 may be utilised by the melting system 17 in system 200a.

[0152] Reference numeral list

[0153] 10 system 110 bauxite 10a system 112 digestor system

[0154] 12 aluminium-based waste 112a digestor

[0155] 14 digestor 112b digestor

[0156] 15 polisher 114 red mud

[0157] 16 precipitator 115 clarifier

[0158] 17 melting system 116 precipitator

[0159] 18 calciner 117 return line

[0160] 19 spent liquor line 118 treated residue

[0161] 20 heat exchange system 120 calciner

[0162] 21 spent liquor line 122 mill

[0163] 22 cleaner 123 aluminate

[0164] 24 shredder 124 mixer

[0165] 26 hydrogen gas off-take 126 aluminate

[0166] 26a hydrogen gas off-take 128 aluminate hydrate

[0167] 28 reduction reactor 130 residue line

[0168] 30 energy conversion device 132 second residue line

[0169] 32 combustor 134 mixing point

[0170] 34 fuel cell 136 alumina hydrate

[0171] 36 hydrogen gas turbine 138 alumina hydrate

[0172] 40 aluminate 140 pregnant liquor

[0173] 42 alumina hydrate 142 residue line

[0174] 100 system 200 aluminium smelter system

[0175] 100a system 200a aluminium smelter system

[0176] 100b system 210 heat exchange system

[0177] 100c system 300 smelting system

[0178] 101 box 310 electrochemical reactor Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present disclosure.

[0179] In the claims that follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the disclosure. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

Claims

Claims1 . A process of treating aluminium-based waste that includes aluminium or aluminium- containing materials, the process comprising: dissolving aluminium-based waste in an alkali solution to form a solution that includes an aluminate species; and precipitating the aluminate species to form an alumina species.

2. A process according to claim 1 , further comprising collecting hydrogen gas generated during dissolving the aluminium-based waste in the alkali solution.

3. A process according to claim 1 or 2, further comprising washing the aluminium-based waste to remove at least some contaminates associated with a surface of the aluminium-based waste prior to dissolution in the alkali solution.

4. A process according to claim 3, wherein washing the aluminium-based waste includes washing the aluminium-based waste with an alkali solution to remove a passivation layer from the aluminium-based waste.

5. A process according to any one of claims 1 to 4, further comprising decreasing a particle size of the aluminium-based waste prior to dissolution in the alkali solution.

6. A process according to claim 5, wherein the particle size of the aluminium-based waste is >10 pm, such as >50 pm, >100 pm or >500 pm.

7. A process according to claim 5 or 6, wherein the particle size of the aluminium-based waste ranges from about 0.1 mm to about 5 mm.

8. A process according to any one of claims 1 to 7, further comprising maintaining a temperature of the alkali solution within a predefined condition.

9. A process according to any one of claims 1 to 8, further comprising recovering heat generated during dissolving the aluminium-based waste in the alkali solution.

10. A process according to any one of claims 1 to 9, wherein precipitating the aluminate species to form the alumina species includes adding a seed crystal of the alumina species.

11. A process according to any one of claims 1 to 10, wherein precipitating the aluminate species to form the alumina species includes decreasing a temperature of the solution that includes the aluminate species.

12. A process according to any one of claims 1 to 11 , further comprising removing insoluble and / or soluble material from the solution that includes the aluminate species prior to precipitating the aluminate species.

13. A process according to claim 12, wherein the insoluble material includes insoluble metals, metal hydroxides and / or metal oxyhydroxides.

14. A process according to any one of claims 1 to 13, further comprising recycling spent alkali solution following precipitation of the alumina species back to digesting the aluminium-based waste.

15. A process according to any one of claims 1 to 14, wherein a pH of the alkali solution ranges from 8 to 14.

16. A process according to any one of claims 1 to 15, wherein the alumina species is alumina hydrate, and wherein the process further comprises calcining the alumina hydrate to form alumina.

17. A process according to any one of claims 1 to 15, wherein the alumina species is alumina.

18. A process according to any one of claims 1 to 17, wherein the alkali solution includes NaOH and / or KOH.

19. Alumina produced from a process that includes the process according to any one of claims 1 to 18.

20. A system for digesting aluminium-based waste to form an alumina species, the system comprising: a digestor for dissolving aluminium-based waste in an alkali solution to form a solution that includes an aluminate species; a precipitator for precipitating an alumina species from the solution thatincludes the aluminate species and forming spent liquor, the precipitator being in communication with the digestor such that spent liquor can be recycled back to the digestor.

21. A system according to claim 20, further comprising a calciner for receiving a calcining the alumina species.

22. A system according to claim 20 or 21 , further comprising a polisher upstream of the precipitator, the polisher being configured to remove insoluble material from the solution that includes the aluminate species prior to precipitating the aluminate species in the precipitator.

23. A system according to any one of claims 20 to 22, wherein the digestor includes a hydrogen gas off-take configured to collect hydrogen gas generated from dissolving the aluminium-based waste in the digestor.

24. A system according to claim 23, further comprising an energy conversion device that can generate electricity and / or heat from the hydrogen gas collected from the hydrogen gas off-take.

25. A system according to claim 23 or 24, further comprising a reduction reactor configured to utilise hydrogen as a reducing agent to reduce a metal value.

26. A system according to any one of claims 20 to 25, further comprising a heat exchange system in thermal communication with the digestor for maintaining a temperature of the alkali solution in the digestor during dissolution of the aluminium- based waste.

27. A system according to claim 26, wherein the heat exchange system is configured to cool the digestor to be within a predefined condition.

28. A system according to any one of claims 20 to 27, wherein the digestor is a tank.

29. A system according to any one of claims 20 to 27, wherein the digestor is a continuous flow reactor.

30. A system according to any one of claims 20 to 29, further comprising a cleaningsystem positioned upstream of the digestor for cleaning the aluminium-based waste prior to dissolution in the digestor.

31. A system according to claim 30, wherein the cleaning system is configured to receive spent liquor to wash the aluminium-based waste in spent liquor to remove a passivation layer on the aluminium-based waste.

32. A system according to any one of claims 20 to 31 , further comprising a particleiser configured to reduce a particle size of the aluminium-based waste, the particleiser being positioned upstream of the digestor such that the digestor can receive aluminium-based waste with a reduced particle size.

33. Alumina produced from the system according to any one of claims 20 to 32.

34. A hydrogen generator comprising the system according to any one of claims 20 to 33.

35. A process for treating an aluminium-containing material, the process comprising: digesting bauxite in a digestor to form a first source of aluminate; dissolving aluminium-based waste in an alkali solution to form a second source of aluminate; blending the first source of aluminate with the second source of aluminate to form a combined source of aluminate; and precipitating the combined source of aluminate to form alumina species.

36. A process according to claim 35, wherein the first source of aluminate is clarified before blending with the second source of aluminate.

37. A process according to claim 35 or 36, wherein insoluble material present in the alkali solution having the second source of aluminate is removed before blending with the first source of aluminate.

38. A process according to claim 37, wherein the insoluble material is collected and processed to extract metal values from the insoluble material.

39. A process according to any one of claims 35 to 38, wherein the combined source of aluminate has a ratio of [first source of aluminate]:[second source of aluminate] ranging from 1000:0.1 to 0.1 :1000.

40. A process according to claim 39, wherein the ratio of [first source of aluminate]:[second source of aluminate] is selected to control a purity of the alumina species and / or condition of the pregnant liquor.

41. A process for treating an aluminium-containing material, the process comprising: digesting bauxite in a digestorto form a first source of aluminate; dissolving aluminium-based waste in an alkali solution to form a second source of aluminate; precipitating the first source of aluminate to form a first source of alumina species and precipitating the second source of aluminate to form a second source of alumina species; simultaneously passing the first source of aluminate and the second source of aluminate through a single calciner system to form alumina.

42. A process according to claim 41 , wherein the first source of aluminate is clarified before precipitation to form the first source of alumina species.

43. A process according to claim 41 or 42, wherein insoluble material present in the alkali solution having the second source of aluminate is removed before precipitation to form the second source of alumina species.

44. A process according to claim 43, wherein the insoluble material is collected and processed to extract metal values from the insoluble material.

45. A process according to any one of claims 41 to 44, wherein the first source of aluminate and the second source of aluminate are blended to form a combined source of aluminate, and the first source of alumina species and the second source of alumina species are precipitated simultaneously from the combined source of aluminate.

46. A process according to any one of claims 35 to 45, wherein heat generated by dissolving the aluminium-based waste is recovered and utilised by a component of a Bayer plant.

47. A process according to claim 46, wherein the heat generated by dissolving the aluminium-based waste is recovered and utilised by the digestor during digestion ofthe bauxite.

48. A process according to any one of claims 35 to 47, wherein a temperature of the alkali solution during dissolution of the aluminium-based waste is lower than a temperature of a solution in the digestor during digesting bauxite.

49. A process according to any one of claims 35 to 48, further comprising extracting hydrogen gas generated during dissolution of the aluminium-based waste in the alkali solution.

50. A process according to claim 49, wherein the extracted hydrogen gas is utilised within the process to produce heat, electricity and / or as a reactant.

51. A process for treating an aluminium-containing material, the process comprising: adding aluminium-based waste and bauxite to a digestor having an alkali solution and bauxite to generate a pregnant liquor having aluminate formed from dissolution of the aluminium-based waste and digestion of bauxite; and precipitating the aluminate to form an alumina species.

52. A process according to claim 51 , comprising extracting hydrogen gas generated during the dissolution of the aluminium-based waste.

53. A process according to claim 52, wherein the extracted hydrogen gas is utilised within the process to produce heat, electricity and / or as a reactant.

54. A process according to claim 52 or 53, further comprising collecting a residue formed from dissolving the aluminium-based waste and digestion of bauxite and treating the residue with the hydrogen gas under reducing conditions to reduce metal values in the residue.

55. A process according to claim 54, further comprising adding red mud to the residue prior to treatment with hydrogen under reducing conditions such that metal values from the red mud can be recovered.

56. A process according to any one of claims 51 to 55, further comprising forming a slurry of bauxite and aluminium-based waste and introducing the slurry into the digestor.

57. A process according to any one of claims 51 to 56, further comprising reducing a particle size of the aluminium-based waste prior to adding the aluminium-based waste to the digestor of the alkali solution and bauxite.

58. A process according to any one of claims 51 to 57, wherein a ratio of [bauxite]:[aluminium-based waste] ranges from 1 :100 to 100:1.

59. A process according to claim 58, wherein a ratio of [bauxite]:[aluminium-based waste] is selected to control a purity of the alumina species and / or condition of the pregnant liquor.

60. A process according to any one of claims 51 to 59, further comprising heating the alkali solution in the digestor with an external heat source.

61. A process according to any one of claims 51 to 60, wherein heat generated by the dissolution of the aluminium-based waste is used to heat the alkali solution to aid in digestion of the bauxite.

62. A process of digesting bauxite, the process comprising: digesting bauxite in an alkali solution to form a pregnant liquor; adding to the pregnant liquor a source of aluminate formed from dissolving aluminium-based waste in an alkali solution; wherein an amount of the source of aluminate added to the pregnant liquor is selected to decrease a relative concentration of silica content in the pregnant and / or spent liquor below a desilication product precipitation threshold to minimise or prevent precipitation of desilication products in the pregnant liquor and / or spent liquor following precipitation of an alumina species.

63. A process according to claim 62, wherein the aluminium-based waste is added directly to the alkali solution used to digest bauxite.

64. A process according to claim 62, wherein the aluminium-based waste is dissolved in a second alkali solution to form the source of aluminate, and wherein the source of aluminate is then added to the pregnant liquor.

65. A process according to claim 64, wherein the source of aluminate is added to the pregnant liquor after the pregnant liquor has been clarified.

66. A process according to claim 64 or 65, further comprising removing insoluble and / or insoluble material present in the source of aluminate before the source of aluminate is added to the pregnant liquor.

67. A plant for treating an aluminium-containing material, the plant comprising: a first digestor for digesting bauxite; a second digestor for dissolving aluminium-based waste; and a precipitator for precipitating an alumina species from aluminate generated in the first digestor and / or the second digestor.

68. A plant according to claim 67, wherein the second digestor includes a hydrogen gas off-take for collecting hydrogen gas generated from dissolving the aluminium-based waste.

69. A plant according to claim 68, further comprising a hydrogen gas combustor for combusting the hydrogen gas collected by the hydrogen gas off-take to generate heat.

70. A plant according to claim 69, further comprising a calciner, wherein the hydrogen gas combustor is operable with the calciner to generate heat for calcination.

71. A plant according to any one of claims 68 to 70, further comprising a reactor for treating residue formed from the first digestor and / or the second digestor, wherein the reactor utilises hydrogen gas collected by the hydrogen gas take-off as a reducing agent to reduce metal values in the residue.

72. A plant according to any one of claims 68 to 71 , further comprising a fuel cell for converting the hydrogen gas collected by the hydrogen gas off-take.

73. A plant according to any one of claims 68 to 72, further comprising a hydrogen storage system in fluid communication with the hydrogen gas off-take for storing hydrogen gas.

74. A plant for treating an aluminium-containing material, the plant comprising: a mill for milling bauxite to form milled bauxite; a particleiser configured to decrease a particle size of aluminium-based wasteto produce particleised aluminium-based waste; and a digestor system for digesting the milled bauxite and / or dissolving the particleised aluminium-based waste in alkali to form a source of aluminate.

75. A plant according to claim 74, wherein the mill for milling bauxite and the particleiser are a same device such that bauxite and aluminium-based waste are both passed through the same device to form the milled bauxite and the particleised aluminium- based waste.

76. A plant according to claim 74 or 75, wherein the digestor system includes a first digestor for digesting milled bauxite and a second digestor for dissolving the particleised aluminium-based waste.

77. A plant according to claim 76, further comprising a heat exchanger configured to be in thermal communication between the first digestor and the second digestor such that heat generated in the second digestor can be transferred to the first digestor.

78. A plant according to claim 74 or 75, wherein the digestor system includes a single digestor that is used to digest milled bauxite and dissolve the particleised aluminium- based waste.

79. A plant according to any one of claims 74 to 78, further comprising a hydrogen gas off-take configured to collect hydrogen gas generated during dissolution of the particleised aluminium-based waste.

80. A plant according to claim 79, further comprising a hydrogen reactor configured to treat a residue formed from digestion of bauxite and / or dissolution of the particleised aluminium-based waste to reduce metal values in the residue, wherein the hydrogen reactor is in fluid communication with the hydrogen gas off-take.

81. A process of treating residue having one or more metal oxides, such as residue formed from extraction of alumina from bauxite, the process of treating residue comprising:(i) dissolving aluminium-based waste in an alkali solution to generate hydrogen gas and a solution containing aluminate; and(ii) treating the residue with the hydrogen gas generated at step (i) under reductive conditions to reduce one or more metal values present in the residue.- M -82. A process according to claim 81 , wherein the residue includes digested bauxite, insoluble material formed from dissolution of the aluminium-based waste and / or red mud.

83. A process according to claim 81 or 82, further comprising adding red mud from a tailing facility to the residue.

84. A process according to any one of claims 81 to 83, wherein the one or more metal values includes iron, titanium, manganese, magnesium, zinc, silicon and / or rare earths.

85. A process according to any one of claims 81 to 84, wherein reducing one or more metal values present in the residue includes reducing an ion form of a metal into its corresponding metallic form.

86. A process according to any one of claims 82 to 85, wherein step (i) is performed in a reactor separate to a digestor used to digest bauxite.

87. A process according to any one of claims 81 to 85, wherein step (i) is performed in a digestor that is used to digest bauxite.

88. A process according to any one of claims 80 to 87, wherein the aluminium-based waste is particleised prior to step (i).

89. A process according to any one of claims 81 to 88, wherein the aluminium-based waste is washed prior to step (i).

90. A process according to claim 89, wherein the aluminium-based waste is washed with an alkali solution to remove a passivation layer.

91. A process according to any one of claims 81 to 90, further comprising removing insoluble material present in the solution containing aluminate.

92. A process according to claim 91 , wherein the insoluble material is added to the residue for treatment at step (ii).

93. A process according to any one of claims 81 to 92, further comprising precipitating the aluminate in the solution containing aluminate to form an alumina species.

94. Bayer residue formed from the process according to any one of claims 81 to 93.

95. A plant for treating aluminium-containing material, the plant comprising a digestor for digesting bauxite; a digestor for dissolving aluminium-based waste and generating hydrogen gas; a hydrogen gas reactor in fluid communication with the digestor for dissolving aluminium-based waste and configured to utilise hydrogen gas generated by the digestor for dissolving aluminium-based waste to reduce metal values present in residue formed by digestion of bauxite.

96. A process of smelting alumina to form aluminium, the process comprising: dissolving aluminium-based waste in an alkali solution to generate hydrogen gas and aluminate; utilising the hydrogen gas as an energy source to at least partially power one or more components of an aluminium smelter system.

97. A process according to claim 96, wherein the aluminium-based waste is particleised prior to dissolution in the alkali solution.

98. A process according to claim 96 or 97, wherein heat generated during dissolution of the aluminium-based waste is used during alumina smelting.

99. A process according to any one of claims 96 to 98, wherein the aluminium-based waste includes aluminium dross.

100. An aluminium smelter system, comprising: a digestor for dissolving aluminium-based waste in an alkali solution to generate hydrogen gas, wherein the hydrogen gas produced by digestor is utilised as an energy source to at least partially power one or more components of the aluminium smelter system.101 . An aluminium smelter system according to claim 100, wherein the digestor includes aheat exchange system configured to maintain a temperature of the alkali solution in the digestor during dissolution of the aluminium-based waste within a preset temperature range.

102. An aluminium smelter according to claim 101 , wherein the heat exchange system is in thermal communication with one or more components of the aluminium smelter system that requires heat such as a furnace.

103. An aluminium smelter system according to claim 101 or 102, wherein the heat exchange system is configured to cool the alkali solution in the digestor to be within a predefined condition.

104. An aluminium smelter system according to any one of claims 100 to 103, wherein the digestor is a tank.

105. An aluminium smelter system according to any one of claims 100 to 104, wherein the digestor is a continuous flow reactor.

106. An aluminium smelter system according to any one of claims 100 to 105, further comprising a particleiser configured to reduce a particle size of the aluminium-based waste, the particleiser being positioned upstream of the digestor such that the digestor can receive aluminium-based waste with a reduced particle size.

107. An aluminium smelter system according to any one of claims 100 to 106, the hydrogen gas produced by the digestor is combusted to generate thermal energy or converted into electricity with a device including a fuel cell.

108. An aluminium smelter system according to any one of claims 100 to 107, wherein the one or more components of the aluminium smelter system include an electrochemical reactor and / or a cast house, wherein the hydrogen gas is used to generate electricity that is used to at least partially power the electrochemical reactor and / or a cast house.

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