Cross-Reference to Related Application

[0001] This application claims priority of Provisional Patent Application 63/149,999 filed on February 16, 2021, the contents of which are incorporated herein by reference in their entirety and for all purposes.

Background

[0002] This disclosure relates to a process for recovering metals and oxides from iron- containing tailings.

[0003] Iron-containing tailings can present challenges for safe, effective and cost-efficient disposal or content recovery. For example, bauxite residue (a/k/a “red mud”) has been stored in containment ponds. Mine tailings are also in some cases stored in containment ponds. Containment ponds present serious environmental, health and economic challenges. Their contents will in some cases need to be processed, or in some cases held indefinitely - posing unrealized liabilities for companies. Also, spent mineral oxides that have been used to treat water are disposed of as hazardous waste in landfills, which is both expensive and environmentally problematic.

Summary

[0004] The present disclosure relates in part to a process for the ultimate recovery of metals and oxides bound in metal oxide tailings. In some but not all examples the tailings contain at least 15% iron oxide. In one example the high percentage removal of the iron oxide in the form of metallic iron is a first step, utilizing a carbon source (for example graphite susceptors) in an electric induction furnace. The carbon source reduces the oxides. The carbon can be in any form. Even wood chips could be used, for example. A second optional step, for accessing the remaining metals and oxides of the slag from the first step, utilizes a microwave heated acid treatment.

[0005] The iron content in the tailings should be (but need not be) sufficient such that the process recovers enough relatively pure iron to be economical, given the current market conditions for iron and the electricity costs to operate the furnace. The other constituents of the tailings that can be recovered in the second step will also have an effect on the economics; these can allow for variations in the amount of iron in the tailings while still remaining economically viable.

[0006] In a more specific embodiment, the high percentage of metallic iron recovery from tailings containing metal oxides in a furnace reduction technique is achieved by using recycled graphite in a solid form acting both as a susceptor and as a reductant for the oxides from the input material. A susceptor allows melting without a starting heel of iron. Since graphite acts as a susceptor, a starting heel of liquid iron is not needed when graphite is used. If another source of carbon is used instead of graphite (e.g., powdered carbon) then the starting heel of liquid iron is used. Furnace types that can be used include an electric induction furnace and an electric arc furnace. In the case of an electric arc furnace, the carbon needed for the reaction of reduction may come from the graphite electrodes utilized in the process. In an induction furnace the solid graphite when used acts as both a susceptor and a source of carbon for the iron reduction reactions. Other source(s) of carbon can be used.

[0007] The input tailings include, but are not limited to: copper smelter slag and dust, bauxite residue (also known as “red mud”), and other ore tailings.

[0008] In some examples the recovery of a high percentage of metallic iron is in part achieved by having a starting heel of metallic iron in the furnace and utilizing solid recycled graphite pieces placed into the liquid iron (vs. powdered carbon). These together allow the reduction reaction needing the carbon to occur below the surface of the melt bath, allowing the melt bath to maintain the appropriate temperature for the reduction reactions to occur. During the reduction reaction carbon monoxide gas is generated and bubbles up through the liquid iron. Because of the reduced buoyancy due to the carbon monoxide gas bubbles entrained in the liquid iron, the graphite susceptors will stay submerged in the iron bath until all possible reduction of oxides is complete. At which time, there are no bubbles of gas entrained in the melt bath and the susceptors will float on top of the liquid iron. The floating of the graphite susceptors becomes a good indicator that reduction has been completed. [0009] Certain advantages of the present process include but are not limited to: better control of the reduction reaction temperature, a more complete reduction reaction below the surface of the melt bath, allows for the recovery of over 90% of the metallic iron from the input material, and the ability to control and reduce any residual iron oxides that do not react in the melt and migrate to the slag.

[0010] The slag from the reduction furnace typically contains residual oxides. The particular oxides are dependent on the source of the input tailings. In some examples the residual oxides include one or more of the oxides of Sc, Ti, Si, Al, Ca, and rare earth elements (REE). In an example of the present process, the slag is subjected to a microwave heated acid treatment, followed by a one or more water-based leaching steps.

[0011] Microwave heated acid treatment uses microwave energy for heating a mixture of slag and sulfuric acid. Sulfuric acid is an excellent microwave absorber due to having a high dielectric constant value of about 100. Materials with high dielectric constants will more easily absorb microwave energy. The microwave energy causes rapid internal heating of the mixture, reducing reaction time to minutes. The primary reaction involves the conversion of oxides present in slag into water soluble sulfates, which is achieved by heating the mixture. The reaction is carried out in a modified microwave reactor with exhaust; the sulfation reaction is achieved in a short duration compared to conventional, external heating.

[0012] Knowing the dielectric constant for the components of the slag, left over after the removal of iron from the tailings, can be beneficial. If the material has a high dielectric constant, the reaction with sulfuric acid when heated in microwave energy will be even faster than expected. This factor makes the microwave heated acid treatment even more energy efficient.

[0013] Some metal oxides also exhibit high dielectric constants. These are shown below in

Table 1.

TABLE 1

Material Dielectric Constant

Titanium Dioxide 110

Sulfuric Acid 100 Iron Oxide 14.2

Calcium Oxide 11.8 Magnesium Oxide 9.7 Silicon Dioxide 4.5

Aluminum Oxide 4.5

Oxygen 1

Carbon Dioxide 1

[0014] All examples and features mentioned below can be combined in any technically possible way.

[0015] In one aspect, a method of recovering iron and other species from tailings that contain at least 15% iron oxide includes charging an electric induction furnace or electric arc furnace with: tailings that contain at least 15% iron oxide; a solid graphitic portion that comprises recycled graphite, wherein the solid graphitic portion functions at least as an iron oxide reductant; and a starting heel of liquid iron, operating the furnace so as to reduce and melt iron from the tailings and thereby create an iron-rich component and a residual slag, and separating the iron-rich component and the slag.

[0016] Some examples include one of the above and/or below features, or any combination thereof. In some examples the solid graphitic portion comprises recycled graphitic anodes, cathodes or electrodes, which in an example, are from the internal lining of aluminum electrolytic pots used in aluminum smelting. In an example the solid graphitic portion also functions as an electromagnetic field susceptor. In an example the tailings that contain at least 15% iron oxide comprise at least one of bauxite residue, smelter slag or dust, and ore tailings. In an example over 90% of metallic iron in the iron oxide containing tailings is recovered. In some examples the process further includes recovering residual oxides in the slag utilizing a microwave heated acid treatment that utilizes microwave energy. The microwave heated acid treatment is effective to convert oxides into water soluble sulfates. In an example the process further includes recovering metals after the microwave heated acid treatment through one or more water based leaching steps. In an example the microwave heated acid treatment utilizes sulfuric acid added to the slag and that is effective to absorb microwave energy and cause rapid, internal heating of the mixture, to reduce reaction time.

Brief Description of the Drawings

[0017] Various aspects of at least one example are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and examples, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the inventions. For purposes of clarity, not every component may be labeled in every figure. In the figures:

[0018] Fig. 1 is a high-level process diagram of the described furnace process.

[0019] Fig. 2 is a more detailed process diagram of the described furnace process.

[0020] Fig. 3 is a process diagram of the described slag acid treatment process.

[0021] Figs. 4-6 detail processes for recovery of alumina, titania, and REE from the furnace slag, respectively.

Detailed Description

[0022] Examples discussed herein are not limited in application to the details set forth in the following description or illustrated in the accompanying drawings. The methods are capable of implementation in other examples and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, functions, components, elements, and features discussed in connection with any one or more examples are not intended to be excluded from a similar role in any other examples.

[0023] Examples disclosed herein may be combined with other examples in any manner consistent with at least one of the principles disclosed herein, and references to “an example,” “some examples,” “an alternate example,” “various examples,” “one example” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one example. The appearances of such terms herein are not necessarily all referring to the same example.

[0024] Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Any references to examples, components, elements, acts, or functions herein referred to in the singular may also embrace embodiments including a plurality, and any references in plural may also embrace examples including only a singularity. Accordingly, references in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms.

[0025] Figures 1-6 include process flow diagrams that detail examples of the two steps that are individually or in combination involved in the process. Figure l is a high-level overview of a process that involves induction smelting of iron-containing oxides, with outputs of pig iron and slag that can be further processed, and all as described in more detail elsewhere herein.

[0026] In examples of the process that takes place in an electric induction furnace as detailed in Fig. 2, a slag conditioner (fluxing agent) can be added to the furnace. Fluxes are used to reduce the melting points of non-reducible metal oxides, such as Aluminum Oxide, Titanium Dioxide, and Silicon Dioxide. It is beneficial to melt all of these non-reducible oxides to a liquid slag. This will ensure that all the reduced metallic particles (like Fe) can be removed from the molten slag by simple gravity separation. Common fluxing agents include: Borax (Sodium Borate) Na ₂ B ₄ 0 ₇ , Soda Ash (Sodium Carbonate) Na ₂ C0 ₃ , Quicklime (Calcium Oxide) CaO, Fluorspar (Calcium Fluoride) CaF2, Magnesia (Magnesium Oxide) MgO, Lye (Sodium Hydroxide) NaOH, Silica (Silicon Dioxide) S1O2 _. The output material can be cooled in a controlled manner. Outputs include metallic iron and slag that includes metal oxides.

[0027] In some examples reactions that take in the furnace include but are not necessarily limited to:

2Fe ₂ 0 ₃ + 3C ® 3C02 + 4Fe Fe ₂ 0 ₃ + CO — > C0 ₂ + 2FeO Fe ₂ 0 ₃ + C — > CO + 2FeO FeO + C ® CO + Fe Ti0 ₂ + C ® Ti ₂ 0 ₃ + CO Ti ₂ 0 ₃ + FeO ® 2Ti02 + Fe 2A1(0H) ₃ ® A1 ₂ 0 ₃ + 3H20 2A10(0H) ® A1 ₂ 0 ₃ + H ₂ 0

[0028] Regarding the use of an induction furnace to reduce iron oxide bearing tailings of a minimum content to make metallic pig iron. The process can utilize a broad range of tailing inputs (e.g., mine tailings from different mining operations) and specifically processing (or disposal) of bauxite residue (also known as “red mud”).

[0029] It can also be used to remove iron preferentially to other elements that don’t follow the iron into metallic solution and thus remain in the slag. This is a manner to isolate these other elements from the tailings, for further isolation or recovery using different processes.

[0030] In some examples this furnace-based process is relevant in the example of processing bauxite residue (red mud). Bauxite residue is a mixed mineral compound containing a matrix of metal oxides (typically but not necessarily FeiCb 30%-50%, AI2O3 15%-30%, T1O2 5%-8%, REE, and others). In the furnace-based process the Al, Ti and other elements will generally not follow the Fe into metallic iron solution. They will instead remain in the slag. Since they remain in the slag, they are now isolated, and a candidate for further extraction and recovery using different processes. Illustrative, non-limiting processes for recovery of alumina, titania, and FEE are detailed in Figures 4-6 respectively.

[0031] Also, because the bauxite residue input material has consistent known chemistries, the output pig iron will also have consistent chemistry values. Below in Table 2 are chemistry examples of elements in the pig iron generated from a bauxite residue containing roughly 31% iron oxide in the subject furnace process. The aluminum, titanium and other elements did not follow the iron into solution, and so they predominantly remained behind in the furnace slag. Table 2

[0032] In another example of the furnace process the input material includes or comprises calcined based mineral oxide compounds that can be but need not be high in iron oxide content, and alternatively are of a mixed (diverse) content such as bauxite residue, and that have been used to treat industrial waste water for the removal of contaminants such as heavy metals, phosphates, fluorides, and Per- and polyfluoroaikyl substances (PFASs). The mineral oxide compounds with the absorbed the contaminants can be inputted to the subject furnace process. The furnace process in this case is effective to collect iron to make pig iron, avoid putting the spent mineral compound used to absorb the heavy metals into a landfill for disposal (typically a ‘hazardous landfill’ is used for disposal of mineral compounds used to remove heavy metals from water) and/or isolate the remaining elements in the furnace slag for potential recovery.

[0033] In the case of heavy metal content in the input furnace material, some of the heavy metals (e.g., Cd, Cr, As, Hg, Pb) may follow the Fe into solution, but none in amounts that would affect the chemistry or use of the metallic iron recovered when used as pig iron. [0034] In some examples a second step is conducted to extract elements from the slag material. This second step in some examples contemplates a microwave heated acid treatment. See the process diagram of Fig. 3. The microwave heated acid treatment uses microwave energy for heating the input material (a slag and sulfuric acid mixture). Sulfuric acid is an excellent microwave absorber and causes rapid internal heating of the mixture, reducing reaction time to minutes. The primary reaction involves the conversion of oxides present in slag into water soluble sulfates, which is achieved by heating the mixture. The reaction is carried out in a modified microwave reactor with exhaust; the sulfation reaction is achieved in a short duration compared to conventional heating.

[0035] In some examples a goal of this second step acid treatment is the recovery of alumina, titania, and/or REE from the slag generated in the reduction of bauxite residue. See the process diagrams of Figs. 4-6, respectively.

[0036] In some examples the acid treatment is detailed in Fig. 3. In an example the water leaching involves mechanical stirring/mixing of sulfated material and water to dissolve metal sulfates. In an example the pH of the solution is raised to a value where oxide(s) precipitate out. In an example oxalic acid is added to the filtered solution to precipitate and recover rare earth element (REE) oxalates from the solution.

[0037] In some examples the process flow diagrams of Figures 4-6 are sequential. They work by process of elimination and concentration by removal. The base input material is the iron oxide compound (e.g., bauxite residue). The iron oxide is first removed with the induction process in the form of metallic pig iron. The residue is the furnace slag. The slag contains the elements that don’t follow the iron into metallic solution. The slag will have concentrated levels of Al, Ti,

REE, etc. in the form of oxides.

[0038] Figure 4 relates to the processing of the slag to recover alumina. After the filtration step, part of the material is a residue that is an input to the process detailed in Figures 5 and 6, for further processing to recover the Ti and REE. The remaining part gets pH adjusted and processed as outlined for recovery of the Alumina. [0039] In Figure 5 the input material is the Residue from Figure 4. That material goes through the acid bake and water leaching, creating two flows of material. One flow goes on to be used for REE recovery as detailed in Figure 6. The other flow goes through a hydrolysis process for Ti02 recovery. Figure 6 details its input material from the residue in Figure 5 (“Further REE recovery”) that gets processed in another acid bake and water leaching and then precipitated out. Recovery of REE (and use of the process) will depend on the content of REE in the starting material (bauxite residue vs. another) and economics from its recovery.

[0040] In examples where the tailings include little iron, the first step (iron reduction in a furnace) can potentially be omitted, with the tailings treated only by the second step (microwave heated acid treatment followed by leaching).

[0041] Having described above several aspects of at least one example, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.