Clean hydrometallurgy process to produce alumina from low grade materials

Abstract

The subject disclosure is directed to improved methods for processing starting materials in the form of bauxite residues, low grade or off-spec ores, process tailings, and fly ash. More specifically, these starting materials include anything that cannot be processed by other methods, yet which still include valuable alumina and other metals, so as to recover the metal compounds contained therein. The improved methods also enables the recovery of most of the nitric acid required by the process, to reduce acid waste generated by previous processes.

Description

CLEAN HYDROMETALLURGY PROCESS TO PRODUCE ALUMINA FROM LOW GRADE MATERIALSTECHNICAL FIELD

[0001] The subject disclosure is directed to improved methods for processing starting materials that are low grade or off-spec. These starting materials include process tailings, off-spec left over ore, fly ash, and waste materials such as bauxite residues. More specifically, these starting materials include anything that cannot be processed by other methods, yet which still include valuable alumina and other metals, so as to recover the metal compounds contained therein. The improved method also enables the recovery of most of the nitric acid required by the process, to reduce acid waste generated by previous processes.BACKGROUND ART

[0002] Raw metal-bearing ores, such as aluminum-bearing ores, represent a rich source of valuable metal compounds. Traditionally, only ores that have sufficient metal content and meet other detailed criteria have been useful for extracting the metals therein. However, there are many other sources of valuable metal compounds that have been neglected as waste material. The process disclosed herein uses these low grade sources as starting materials to extract the valuable metal compounds therein. The term “ores” as used herein means oxide and sulfide ores, and other metal-bearing materials, it being understood that metals can be combined with other elements. The compositions of ores vary tremendously, even taken from the same mining site. Accordingly, processes that can extract valuable metal compounds from a full range of aluminum bearing ores, including transitional ores, are desirable. Even more desirable are processes that can extract valuable metal compounds from low grade materials that have previously been regarded as useless. Most bauxite ores that have been deemed useful for metal extraction are between 30-60% AI2O3 by weight. It would be beneficial to have available metal extraction processes that can successfully extract valuable metals from sources that have less than 30% AI2O3 by weight.

[0003] Metal and metal oxide processes generate significant waste products, and especially acidic waste products, that can be harmful to the environment. Accordingly, there is a need for more efficient and environmentally friendly techniques to extract valuable metal compounds from starting materials while reducing the generation of acidic wastes.Docket No. 15082-012DISCLOSURE OF INVENTION

[0004] In various implementations, a method for obtaining recovered metal compounds from starting materials is provided. The disclosed methods are useful for obtaining recovered metal compounds from low grade starting materials that have previously been overlooked and / or discarded as waste. The term “starting materials” as used herein refers to bauxite residues, low grade or off-spec ores, process tailings, and fly ash, wherein these starting materials have less than 30% AI2O3 by weight. The starting materials are leached using a nitric acid solution containing a predetermined nitric acid concentration, at a predetermined temperature, and at a predetermined atmospheric pressure to produce a leached solution and a residue. The residue is washed with a neutralizing solution. The leached solution is crystallized to obtain a mixture of nitrate crystals and a post-crystallization solution. Aluminum nitrate crystals are purified from the mixture of nitrate crystals. The post-crystallization solution is subjected to iron hydrolysis at about 150-165°C, which results in iron in the form of hematite, a solution free from aluminum and iron, and nitric acid in the resulting vapor stream. Throughout this process, at least 80%, and up to 98% of nitric acid can be recovered (95-98% range) from within the process. The recovered nitric acid is suitable for reuse in leaching starting materials.

[0005] After the aluminum and iron are extracted by this process, the resulting solutions can be used to extract further metals therefrom. Ion exchange and solvent extraction can be used to recover valuable metals such as scandium, gallium, and rare earth metals, and further processing of the solution after these processes can precipitate other metals such as nickel, cobalt, and manganese.

[0006] Throughout the process steps of the present invention, it is readily possible to recover nitrates and NOx gases created therein, and reconvert these nitrates and NOx gases to nitric acid, which can then be recovered and used again for the leaching of starting materials.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a flow diagram of an embodiment of a hydrometallurgy process in accordance with the disclosed subject matter.

[0008] FIG. 2 is another flow diagram illustrating a portion of the process shown in FIG. 1.Docket No. 15082-012MODES FOR CARRYING OUT THE INVENTION

[0009] The subject disclosure is directed to improved methods for processing starting materials that are low grade or off-spec. These starting materials include process tailings, off-spec left over ore, fly ash, and waste materials such as bauxite residues. More specifically, these starting materials include anything that cannot be processed by other methods, yet which still include valuable alumina and other metals, so as to recover the metal compounds contained therein. The process is able to recycle the majority of the acid used therein.

[0010] The detailed description provided below in connection with the appended drawings is intended as a description of examples and is not intended to represent the only forms in which the present examples can be constructed or utilized. The description sets forth functions of the examples and sequences of steps for constructing and operating the examples. However, the same or equivalent functions and sequences can be accomplished by different examples.

[0011] References to “one embodiment,” “an embodiment,” “an example embodiment,” “one implementation,” “an implementation,” “one example,” “an example” and the like, indicate that the described embodiment, implementation or example can include a particular feature, structure or characteristic, but every embodiment, implementation or example can not necessarily include the particular feature, structure or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment, implementation or example. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, implementation or example, it is to be appreciated that such feature, structure or characteristic can be implemented in connection with other embodiments, implementations or examples whether or not explicitly described.

[0012] Numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments of the described subject matter. It is to be appreciated, however, that such embodiments can be practiced without these specific details.

[0013] Various features of the subject disclosure are now described in more detail with reference to the drawings, wherein like numerals generally refer to like or corresponding elements throughout. The drawings and detailed description are not intended to limit the claimed subject matter to the particular form described. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed subject matter. Unless otherwise indicated, percentages are expressed by weight.Docket No. 15082-012

[0014] The starting materials for the disclosed process are sources of valuable metals that have previously been considered unusable for economical extraction of the metals therein. Examples include process tailings, off-spec left over ore, fly ash, and waste materials such as bauxite residues from which much of the metal content has already been extracted. Previously, such materials were not cost-effective for metal extraction, as they were relatively poor in metal content, or difficult to process. Using the present process, it is now possible to extract the valuable metals from these materials. This results in less waste material in total than that from previous processes.

[0015] Some starting materials, especially the residue from extraction of bauxite ore, may require pre-treatment prior to their use in the disclosed process. Such materials have a high alkaline content, and as such would require significantly extra nitric acid for the leaching step. To avoid the need for extra nitric acid, these materials can be washed with sea water and / or sparged with CO2 to reduce their pH to close to neutral. After this pre-treatment, these materials can be used as appropriate starting materials in the disclosed process.

[0016] The disclosed process includes leaching starting materials using nitric acid within a concentration range of about 10% to about 95% at a temperature range of about 90°C to about 115°C and at atmospheric pressure to leach valuables such as aluminum, iron, nickel, cobalt, manganese, magnesium, rare earth elements, scandium, gallium, and others into the solution. In preferred embodiments, the concentration of the nitic acid is about 60-90%. In more preferred embodiments, the concentration of the nitric acid is about 65-75%.

[0017] The lower and upper limit of temperature range is about 90-110°C. In a preferred embodiment, the temperature range is between about 100-110°C. However, the lower limit of the temperature range will change with the concentration of the nitric acid. As the nitric acid concentration increases, the boiling point of the nitric acid solution decreases, so that the lower limit of the temperature range can decrease.

[0018] The residue is washed and neutralized to produce a product that is rich in silicates and titanium, and can be used for further recovery of titanium as well as other metals. The final residue can be used as a by-product for mine rehabilitation and other purposes known to those of skill in the art.

[0019] In a preferred embodiment, aluminum is crystallized as nitrates from the leached solution by natural cooling of the leached solution. This natural cooling can also be done inDocket No. 15082-012combination with seeding of the leached solution with aluminum nitrate, or seeding can be used without significant natural cooling.

[0020] It is advantageous to purify the aluminum nitrate crystals produced by the crystallization step before further actions are performed. In a preferred embodiment, aluminum nitrate crystals are purified by washing them with nitric acid. In another preferred embodiment, aluminum nitrate crystals are purified by re-dissolution and recrystallization while rejecting iron and other metals from the crystals into the solution.

[0021] Once purified, the aluminum nitrate crystals can be treated in various ways. One preferred method is to thermally decompose the aluminum nitrate crystals by heating to about 150°C up to 1250°C, which results in the production of various forms of alumina and NOx gases. The NOx gases can be recovered and reconstituted into nitric acid for reuse within the process. The forms of alumina can include AI2O3, alpha alumina, beta alumina, and gamma alumina.

[0022] In another preferred embodiment, the purified aluminum nitrate crystals are dissolved and combined with an alkali such as MgO or NaOH to selectively remove iron as a hydroxide. After the iron is removed, the aluminum hydroxide can then be precipitated, and is now largely free of the iron.

[0023] The post-crystallization solution from the crystallization step can be treated in a variety of ways. It can be recycled back into the leaching step with the starting material. This allows aluminum that is still dissolved in the post-crystallization solution to be recovered as crystallized aluminum nitrate in the crystallization step.

[0024] In addition, or alternatively, the post-crystallization solution can be sent to an iron hydrolysis reactor to recover iron as hematite at about 150°C to about 195°C. In some embodiments, the upper end of the range can be as low as about 165 °C. Other products of this iron hydrolysis step include nitric acid in the vapor stream, and a solution that is free from aluminum and iron. Preferably, this nitric acid is recovered and reused in this process to leach starting materials.

[0025] The solution that is free from aluminum and iron is preferably processed to recover scandium, gallium, rare earth elements and other metals via ion exchange and / or solvent extraction. The raffinate solution that remains after these metals are removed is preferably treated by ion exchange and / or solvent extraction, followed by precipitation of metals using an alkali to precipitate individual elements or groups of elements as hydroxides or other compounds.Docket No. 15082-012

[0026] In some embodiments, the raffinate solution is treated with an alkali to produce MHP (mixed hydroxide product) that includes at least one of a nickel, cobalt, or manganese hydroxide product, or a mixture thereof.

[0027] After the raffinate solution has been treated to remove more metals, a barren solution remains. In a preferred embodiment, the barren solution is concentrated with magnesium nitrate and thermally decomposed at about 500°C and up to about 900°C into magnesia and NOx gases. As in other steps of this process, the NOx gases are preferably converted into nitric acid, and recycled back to the initial leaching step of this process.

[0028] The disclosed systems and methods represent improvements over the systems and methods disclosed within U.S. Patent Nos. 8,016,913 and 8,038,767 to Drinkard, Jr., the entire disclosures of which, except for any definitions, disclaimers, disavowals, and inconsistencies, are incorporated herein by reference.

[0029] Referring now to the drawings and, in particular, to FIGS. 1-2, there is shown an improved hydrometallurgy process, generally designated with the numeral 100, for recovering valuable metal compounds through the crystallization of aluminum and other processing steps. The process 100 can be used to process starting materials in the form of raw ores. Exemplary raw ores include aluminum-bearing ores such as bauxite ores, and more specifically ores that cannot be processed by other methods.

[0030] The process 100 can be used to recover metals and metal compounds. Exemplary metal compounds include compounds that include aluminum, iron, nickel, cobalt, manganese, magnesium, rare earth elements, scandium, and gallium.

[0031] The process 100 begins by combining a nitric acid solution 110 with starting materials 112 for leaching at 114. The nitric acid solution 110 contains a predetermined concentration of nitric acid, which can range from about 10% to about 95%. In this exemplary embodiment, the predetermined concentration is about 70%. The starting materials 112 are any of a variety of aluminum bearing ores.

[0032] At 114, the precursor materials 112 are leached with the nitric acid solution 110 at a predetermined temperature and predetermined atmospheric pressure. In this exemplary embodiment, the predetermined temperature is within the range of about 90°C to about 115°C. The predetermined atmospheric pressure is within the range of about 0.95 atmospheres to aboutDocket No. 15082-0121.05 atmospheres. In this exemplary embodiment, the predetermined atmospheric pressure is about 1.0 atmosphere.

[0033] The leaching step at 114 produces a leached solution and a residue 113. The leached solution is transported for crystallization at 116. The crystallization step at 116 produces a mixture of nitrate crystals and a post-crystallization solution.

[0034] Crystallization can be accomplished through any suitable means. In this exemplary embodiment, crystallization is accomplished by naturally cooling the leached solution and / or by seeding the leached solution with recycled aluminum nitrate crystals.

[0035] The residue 113 from step 114 is washed with a neutralizing solution at 118 to produce a byproduct therefrom. Doing this will also recover some of the nitrates therein, which can then be recycled as nitric acid for reuse in the process. The residue 113 is rich in silicates and titanium, which can be recovered as the byproduct at 118.

[0036] The crystallization step at 116 produces a post-crystallization solution 120 which is split into separate streams. The post-crystallization solution 120 can be recycled back into the leaching step 114 with the starting material 112. This allows aluminum that is still dissolved in the post-crystallization solution 120 to be recovered as crystallized aluminum nitrate in the crystallization step 116.

[0037] The post-crystallization solution stream 120 can be treated by process 122 as shown in FIG. 2. The post-crystallization solution 120 is treated in an iron hydrolysis reactor 140 to recover iron as hematite 142. Other products of this iron hydrolysis step 140 include nitric acid in the vapor stream 144, and a solution that is free from aluminum and iron. Preferably, this nitric acid is recovered, and reused in this process to leach starting materials at 114. The solution that is free from aluminum and iron is preferably processed to recover scandium, rare earth elements and other metals via ion exchange and / or solvent extraction at 146. This results in a rich strip liquor 150 that is rich in scandium, gallium, rare earth elements, and other elements and from which those metals can be purified by additional processes. The raffinate solution that remains after these metals are removed at 146 is subjected to thermal decomposition at 148. In the same process, MgO and nitric acid can be recovered.

[0038] The crystals formed at 116 are subject to purification at 126. At 126, the crystals from 116 can be subject to a nitric acid wash and / or redissolution followed by recrystallization. Redissolution and recrystallization can be accomplished by rejecting iron and other metals fromDocket No. 15082-012the crystals into the solution. The result of step 126 are washed crystals 128 and a second postcrystallization solution 130 which will be recycled back to leaching 114.

[0039] The washed crystals 128 can be thermally decomposed at 132. The thermal decomposition will produce a high-grade alumina product and NOx gases at 134. In this exemplary embodiment, the predetermined temperature range is from about 150°C up to about 1250°C. The NOx can be recovered and converted back to nitric acid for use in leaching at 114.

[0040] Alternatively, the washed crystals 128 from step 126 are dissolved in water and combined with MgO in a reactor 136 to create an aluminum precipitate suitable 137 for drying and / or calcination.Supported Features and Embodiments

[0041] The detailed description provided above in connection with the appended drawings explicitly describes and supports various features of systems, apparatus, and methods for processing starting materials, such as raw ores, bauxite, and other aluminum bearing ores to recover metal compounds therein through hydrometallurgy. By way of illustration and not limitation, supported embodiments include a method for obtaining recovered metal compounds from starting materials comprising leaching said starting materials using a nitric acid solution containing a predetermined nitric acid concentration, at a predetermined temperature, and at a predetermined atmospheric pressure to produce a leached solution and a residue; crystallizing said leached solution to obtain a mixture of nitrate crystals and a post-crystallization solution; purifying aluminum nitrate crystals from said mixture of nitrate crystals; washing and neutralizing said residue to recover nitric acid therefrom; treating said post-crystallization solution by iron hydrolysis at about 150-165oC, which yields iron in the form of hematite, a solution free from aluminum and iron, and nitric acid in the resulting vapor stream; wherein through this process, at least 80% of the nitric acid is recoverable and suitable for reuse in leaching starting materials. In more preferred embodiments, at least 90% of the nitric acid is recoverable, and in the most preferred embodiments, at least 95% (and up to 98%) of the nitric acid is recoverable and suitable for reuse in leaching starting materials.

[0042] Supported embodiments include the foregoing method, wherein the precursor materials are selected from the group consisting of waste materials such as bauxite residues, process tailings, off-spec or left over ore, and fly ash.Docket No. 15082-012

[0043] Supported embodiments include any of the foregoing methods, wherein the predetermined nitric acid concentration is within the range of about 10% to about 90%, the predetermined temperature is within the range of about 100°C to about 115°C, and the predetermined atmospheric pressure is within the range of about 0.95 atmospheres to about 1.05 atmospheres.

[0044] Supported embodiments include any of the foregoing methods, wherein said crystallizing comprises naturally cooling said leached solution, and / or wherein said crystallization comprises seeding with aluminum oxide.

[0045] Supported embodiments include any of the foregoing methods, wherein said step of purifying aluminum nitrate crystals comprises washing said aluminum nitrate crystals with nitric acid.

[0046] Supported embodiments include any of the foregoing methods, wherein said purified aluminum nitrate crystals are heated to about 150-200°C to thermally decompose them into alumina and NOx gases, and wherein said NOx gases are converted to nitric acid suitable for reuse in leaching starting materials in future cycles of this method.

[0047] Supported embodiments include any of the foregoing methods, wherein said solution free from aluminum and iron is treated by ion exchange extraction to selectively recover other metals therefrom, and produce a raffinate solution. In a preferred embodiment, said other metals comprise scandium, gallium and rare earth elements.

[0048] Supported embodiments include any of the foregoing methods, wherein said solution free from aluminum and iron is treated by solvent extraction to selectively recover other metals therefrom. In a preferred embodiment, said other metals comprise scandium, gallium and rare earth elements.

[0049] Supported embodiments include any of the foregoing methods, wherein said raffinate solution is treated with MgO to precipitate a hydroxide product that includes nickel, cobalt, and manganese.

[0050] Supported embodiments include any of the foregoing methods, wherein the solution resulting from any step in the method, that has no further use for extracting metals therefrom, is concentrated with magnesium nitrate and heated to between 500-900°C to thermally decompose the solution into magnesia and NOx gases, and wherein said NOx gases are converted to nitric acid suitable for use in leaching starting materials.Docket No. 15082-012

[0051] Supported embodiments include a system, a kit, an apparatus, and / or means for implementing any of the foregoing methods or a portion thereof.

[0052] Supported embodiments can provide various attendant and / or technical advantages in terms of an energy efficient hydrometallurgical processing route of producing critical minerals / metals such as aluminum, nickel, cobalt, iron, rear Earth elements, scandium, gallium, magnesium and others from the leached solution and titanium product from the residue.

[0053] Supported embodiments include a hydrometallurgical process that uses nitric acid recycle and recovery technology. The technology can be used to recycle the majority of the acid used in the process.

[0054] Supported embodiments include a hydrometallurgical process that utilizes leaching, crystallization, hydrolysis, ion exchange, solvent extraction, precipitation, thermal decomposition, solid liquid separation, and gravity separation / magnetic separation.

[0055] The detailed description provided above in connection with the appended drawings is intended as a description of examples and is not intended to represent the only forms in which the present examples can be constructed or utilized. It is to be understood that the configurations and / or approaches described herein are exemplary in nature, and that the described embodiments, implementations and / or examples are not to be considered in a limiting sense, because numerous variations are possible.

[0056] Although the subject matter has been described in language specific to structural features and / or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are presented as example forms of implementing the claims.

Claims

Docket No. 15082-012CLAIMSWhat is claimed is:

1. A method for obtaining recovered metal compounds from starting materials comprising:leaching said starting materials using a nitric acid solution containing a predetermined nitric acid concentration, at a predetermined temperature, and at a predetermined atmospheric pressure to produce a leached solution and a residue;crystallizing said leached solution to obtain a mixture of nitrate crystals and a postcrystallization solution;purifying aluminum nitrate crystals from said mixture of nitrate crystals;washing and neutralizing said residue to recover nitric acid therefrom;treating said post-crystallization solution by iron hydrolysis at about 150-165°C, which yields iron in the form of hematite, a solution free from aluminum and iron, and nitric acid in the resulting vapor stream;wherein through this process, at least 80% of the nitric acid is recoverable and suitable for reuse in leaching starting materials.

2. The method of claim 1, wherein said starting materials are selected from the group consisting of waste materials such as bauxite residues, process tailings, off-spec or left over ore, and fly ash.

3. The method of claim 1, wherein the predetermined nitric acid concentration is within the range of about 10% to about 90%, the predetermined temperature is within the range of about 100°C to about 115°C, and the predetermined atmospheric pressure is within the range of about 0.95 atmospheres to about 1.05 atmospheres.

4. The method of claim 1, wherein said crystallizing comprises naturally cooling said leached solution.

5. The method of claim 4, wherein said crystallization comprises seeding with aluminum oxide.Docket No. 15082-0126. The method of claim 1, wherein said purifying aluminum nitrate crystals comprises washing said aluminum nitrate crystals with nitric acid.

7. The method of claim 6, wherein said purified aluminum nitrate crystals are heated to about 150-200C to thermally decompose them into alumina and NOx gases, and wherein said NOx gases are converted to nitric acid suitable for reuse in leaching starting materials in future cycles of this method.

8. The method of claim 1, wherein said solution free from aluminum and iron is treated by ion exchange extraction to selectively recover other metals therefrom, and produce a raffinate solution.

9. The method of claim 8, wherein said other metals comprise scandium, gallium and rare earth elements.

10. The method of claim 1, wherein said solution free from aluminum and iron is treated by solvent extraction to selectively recover other metals therefrom.

11. The method of claim 10, wherein said other metals comprise scandium, gallium and rare earth elements.

12. The method of claim 8, wherein said raffinate solution is treated with MgO to precipitate a hydroxide product that includes nickel, cobalt, and manganese.

13. The method of claim 1, wherein the solution resulting from any step in the method, that has no further use for extracting metals therefrom, is concentrated with magnesium nitrate and heated to between 500-900°C to thermally decompose the solution into magnesia and NOx gases, and wherein said NOx gases are converted to nitric acid suitable for use in leaching starting materials.Docket No. 15082-01214. The method of claim 1, wherein at least 90% of the nitric acid is recoverable and suitable for reuse in leaching starting materials.

15. The method of claim 1, wherein at least 95% of the nitric acid is recoverable and suitable for reuse in leaching starting materials.

16. The method of claim 1, wherein said starting materials have comprise less than 25% AI2O3 by weight.

17. The method of claim 1, wherein said starting materials have comprise less than 20% AI2O3 by weight.

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