Inorganic polymer production from bauxite residues

Abstract

There is provided herein a process for producing inorganic polymers from bauxite residues. The present process mixes under vacuum the bauxite residue with water and an alkali salt. The vacuum applied results in a pressure of less than 70 kPa which drives the polymerization. The application of vacuum advantageously allows to maintain a high content of bauxite residues while still maintaining good compressive strength properties for the inorganic polymers obtained.

Description

TECHNICAL FIELD

[0001] This disclosure relates to the production of inorganic polymers from bauxite residues. BACKGROUND OF THE ART

[0002] The by-products of the Bayer process often generate disposal problems, as they cannot be easily recycled or reused, and it becomes necessary to store the by-products, for example, in holding ponds, taking up valuable land space and impacting the environment. Red mud, also known as bauxite tailings, red slurry, bauxite residue or alumina refinery residues, is a residual by-product of the production of alumina from bauxite by the Bayer process. Red mud comprises a mixture of solid and metallic oxides, including large amounts of iron oxide which provide the red color. It is estimated that for every ton of alumina produced using the Bayer process, approximately 1 to 2 tons of red mud are produced. Revalorization of the bauxite residues has been traditionally performed by using the bauxite residues as an ingredient in alkali activation for production of inorganic polymers, also known as alkali activated materials and geopolymers. These traditional methods require high shear mixing and / or heating in order to be able to produce the inorganic polymers. Furthermore, the traditional methods are limited in the amount of bauxite residues that can be included while maintaining appropriate strength properties for the inorganic polymers to have useful applications. Accordingly, improvements in the production of inorganic polymers from bauxite residue are desired. SUMMARY

[0003] Embodiments of the present disclosure present a process for producing an inorganic polymer that can be shaped or formed into construction components such as bricks or other building blocks. The process generally includes a step of mixing bauxite residue, an alkali salt and water in a closed environment which is under vacuum. It was found that the vacuum can improve the solubility of aluminum oxides and silicon oxides in a way which makes the bauxite residue more reactive. This in turn, favors the creation of strong inorganic polymers for use in construction components.

[0004] In a first aspect of the present disclosure, there is provided a process for producing an inorganic polymer, comprising: providing bauxite residue, an alkali salt, and water in a closed environment to obtain a mixture; and mixing the mixture under a pressure of less than 70 kPa to obtain the inorganic polymer.

[0005] In one embodiment of the first aspect, the process can for example further comprise curing the inorganic polymer.

[0006] In one embodiment of the first aspect, the process can for example further comprise extruding the inorganic polymer into a predetermined shape.

[0007] In one embodiment of the first aspect, the process can for example further comprise molding the inorganic polymer into a predetermined shape, and optionally further drying the inorganic polymer.

[0008] In one embodiment of the first aspect, the bauxite residue can for example be dry bauxite residue and the water is provided in a concentration of from 10 to 30 wt. % of the mixture.

[0009] In one embodiment of the first aspect, the bauxite residue can for example be wet bauxite residue which comprises water, and wherein up to 15 wt. % of additional water is provided such that the concentration of water in the mixture is from 10 to 30 wt. %.

[0010] In one embodiment of the first aspect, the bauxite residue can for example be provided in a concentration of from 50 to 90 wt. % of the mixture.

[0011] In one embodiment of the first aspect, the dry bauxite residue can for example comprise in weight percent 25 - 55 % of iron oxides, 10-25 % of aluminum oxides, 5-25 % of silicon oxides, 2 -10 % of sodium oxides, 2-20 % of calcium oxides, 5 -15 % of titanium oxides, 10 - 15 % of carbon and organic species, and less than 1 % of water.

[0012] In one embodiment of the first aspect, the wet bauxite residue can for example comprise in weight percent 25 - 55 % of iron oxides, 10-25 % of aluminum oxides, 5-25 % of silicon oxides, 2 -10 % of sodium oxides, 2-20 % of calcium oxides, 5 -15 % of titanium oxides, 10 - 15 % of carbon and organic species, and 15-30 % of water.

[0013] In one embodiment of the first aspect, the alkali salt can for example be provided in a concentration of from 2 to 20 wt. % of the mixture.

[0014] In one embodiment of the first aspect, the alkali salt can for example be one or more of sodium hydroxide, potassium hydroxide, sodium silicate, potassium silicate, sodium carbonate and sodium aluminate.

[0015] In one embodiment of the first aspect, the step of providing can for example further comprise providing an aluminosilicate supplement, which comprises SiO2 and AI2O3.

[0016] In one embodiment of the first aspect, the aluminosilicate supplement can for example be one or more of fly ash, clay, sand and metallurgical slag. In some embodiments, the aluminosilicate supplement is sand.

[0017] In one embodiment of the first aspect, the aluminosilicate supplement can for example be provided in a concentration of up to 40 wt. % of the mixture.

[0018] In one embodiment of the first aspect, the step of providing can for example further comprise providing a superplasticizing agent.

[0019] In one embodiment of the first aspect, the superplasticizing agent can for example be provided in a concentration of up to 3 wt. % of the mixture.

[0020] In one embodiment of the first aspect, the step of providing can for example further comprise providing a calcium and / or magnesium salt.

[0021] In one embodiment of the first aspect, the calcium and / or magnesium salt can for example be provided in a concentration of up to 20 wt. %.

[0022] In one embodiment of the first aspect, the step of providing can for example further comprise providing an aggregate.

[0023] All technical implementation details and advantages described with respect to a particular aspect of the present invention are self-evidently mutatis mutandis applicable for all other aspects of the present invention.

[0024] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure. DESCRIPTION OF THE DRAWINGS

[0025] Fig. 1 is a schematic of a system for producing inorganic polymers according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0026] The Bayer process is a process in which alumina hydroxide is precipitated from a saturated sodium aluminate solution, which results from the hydrothermal digestion of bauxite with caustic soda. The essentially non-soluble fraction of bauxite forms a residue termed referred to as bauxite residue, which can be separated from the sodium aluminate solution, known as Bayer liquor, using thickeners, or in some cases filters or pressure decanters. The composition of the bauxite residue is mainly influenced by the composition of the bauxite ore initially provided to the Bayer process. The composition of the bauxite residue is also influenced by the processing parameters of the Bayer process, and the lime additions for digestion and other additives. The mineralogy of bauxite residue is generally made of iron-bearing phases, such as hematite and goethite, aluminium hydroxides, desilication products (such as cancrinite and hydrogarnet), calcite and Ti-minerals, such as rutile.

[0027] The term bauxite residue, as used herein, refers to the by-product obtained from bauxite by the Bayer process as explained above. This by-product is the slurry residue generated during the digestion of bauxite. Bauxite residue slurries are strongly alkaline, and have a reasonably high electrical conductivity and ionic strength. It is contemplated herein that the bauxite residue is used in its wet form or in its dry form (i.e. the slurry is further processed to reduce the water content). The bauxite residue, in some embodiments, is also referred to as red mud or bauxite tailings.

[0028] As explained above, the composition of the bauxite residue is dependent on the bauxite ore composition (among other things). Accordingly, in a non-limitative embodiment, the bauxite residue has a composition, in weight percent, of 25-55 % iron oxides, 10-25 % aluminum oxides, 5-25 % silicon oxides, 2-10 % sodium oxides, 2-20 % calcium oxides, 5-15% titanium oxides, 10-15% carbon and / or residual organic materials, and either 15-30 % water content if wet or less than 1% water if the bauxite residue is dried. The source of the bauxite (i.e. the geographic location from which the bauxite was obtained) affects its composition.

[0029] The present disclosure provides a process for producing inorganic polymers using the bauxite residues and therefore revalorize the bauxite residues that would otherwise go into landfills. One advantage of the present process is achieving a high compressive strength in the inorganic polymer (for example above 30 MPa, above 40 MPa, or above 50 MPa) while still including a significant proportion of bauxite residues in the composition of the inorganic polymers. Indeed, in preferred embodiments, the bauxite residues are provided in a concentration of at least 50 wt. %, at least 55 wt. %, at least 60 wt. % or at least 65 wt. % of the mixture that reacts to form the inorganic polymers.

[0030] The present process for producing inorganic polymers is performed under vacuum. In some embodiments, the vacuum is defined as a pressure of less than 70 kPa, less than 65 kPa, less than 60 kPa, from 70 to 1 kPa, from 65 to 1 kPa, from 60 to 1 kPa, from 70 to 5 kPa, from 65 to 5 kPa, from 60 to 5 kPa, from 70 to 10 kPa, from 65 to 10 kPa, or from 60 to 10 kPa. In other words, in some embodiments, the vacuum applied is of from 30 to 100 kPa to reduce the atmospheric pressure accordingly. The presence of a vacuum improves the solubility of the aluminum oxides and silicon oxides in the bauxite residues and makes it more reactive. More specifically, the presence of negative pressure pulls residual Bayer liquor out of the pores of the bauxite residue. This creates localized environments of high alkalinity within the mixture which rapidly solubilize the aluminum oxides and silicon oxides within the bauxite residue. The enhanced solubility promotes the formation of aluminosilicate oligomers and a more complete reaction of the aluminum oxides and silicon oxides to form an inorganic polymer. The vacuum can be applied by any suitable means such as a vacuum pump and the like.

[0031] Accordingly, to produce the inorganic polymer, the bauxite residue, an alkali salt and water are mixed in a closed environment under vacuum. It should be noted that water can be provided with the bauxite residue, in other words, the bauxite residue is a wet bauxite residue that still contains water. Alternatively or in addition, an addition of water is provided to the mixture to arrive at the desired water content. In some embodiments, the total water in the mixture is from 5 to 30 wt. %, from 10 to 30 wt. %, or from 15 to 30 wt. % (regardless whether it is an addition of water or water part of the wet bauxite residues). As explained above, the present process has the advantage of utilizing a higher concentration of bauxite residue while still obtaining adequate compressive strength properties. In some embodiments, the concentration of the bauxite residues in the mixture is from 50 to 90 wt. %, from 55 to 85 wt. %, from 60 to 80 wt. %, from 65 to 75 wt. %, from 50 to 85 wt. %, from 50 to 80 wt. %, from 50 to 75 wt. %, from 55 to 90 wt. %, from 60 to 90 wt. %, from 65 to 90 wt. %, from 70 to 90 wt. %, from 50 to 85 wt. %, from 50 to 80 wt. %, from 50 to 75 wt. %, or from 50 to 70 wt. %.

[0032] The term alkali salt as used herein refers to metal hydroxides, metal silicates, metal acid salts, metal aluminates, metal aluminosilicates, and the like. In some embodiments, the alkali activator is a metal silicate such as lithium silicate, sodium silicate, potassium silicates, rubidium silicate, cesium silicate and combinations thereof. In preferred embodiments, the alkali salt is one or more of sodium hydroxide, potassium hydroxide, sodium silicate, potassium silicate, sodium carbonate and sodium aluminate. The alkali salt renders the water in the mixture alkaline and plays a synergistic role with the application of a vacuum to drive the polymerization and formation of inorganic polymers. The alkali salt can be provided in a concentration of from 2 to 20 wt. %, from 2 to 17.5 wt. %, from 2 to 15 wt. %, from 2 to 12.5 wt. %, from 2 to 10 wt. %, from 2 to 7.5 wt. %, from 2 to 5 wt. %, from 3 to 20 wt. %, from 4 to 20 wt. %, from 5 to 20 wt. %, from 3 to 18 wt. %, from 4 to 16 wt. %, or from 5 to 15 wt. %.

[0033] Optionally, an aluminosilicate supplement is added to the mixture. The aluminosilicate supplement can be added depending on the concentration of Al and Si in the bauxite to increase their content. Indeed, as explained above, the contents of the bauxite residues will depend on the bauxite ores and they may have to be adjusted with the aluminosilicate supplement. The aluminosilicate generally contains SiO2 and AI2O3 and is for example fly ash, bottom ash, sand, clay, metallurgical dross and / or metallurgical slag. In the embodiments where fly ash, bottom ash, metallurgical dross and / or metallurgical slag are included in the mixture, it should be noted that the present process further revalorizes another waste source which renders the present process even more environmentally friendly. The aluminosilicate supplement can be provided in the mixture in a concentration of up to 40 wt. %, up to 35 wt. %, up to 30 wt. %, up to 25 wt. %, up to 20 wt. %, up to 15 wt. %, up to 10 wt. %, up to 5 wt. %, from 5 to 40 wt. %, from 5 to 35 wt. %, from 5 to 30 wt. %, or from 5 to 20 wt. %.

[0034] In at least some embodiments, the aluminosilicate supplement is sand and / or aggregates in order to produce mortar or concrete products. The sand can have any fineness, for example coarse sand, fine sand, very fine sand, manufactured sand, or river sand can all be used. The aggregate can also have any suitable coarseness based on the desired product to be manufactured. For example, the aggregate is a coarse aggregate, fine aggregate, graded aggregate, natural aggregate, synthetic aggregate, recycled aggregate, or gravel.

[0035] Another optional additive is a calcium or magnesium salt. The addition of these salts allows to increase the Ca and Mg content and provide improved compressive strength properties to the resulting inorganic polymer. In some embodiments, the calcium and / or the magnesium salt are provided in concentrations of up to 20 wt. %, up to 15 wt. %, up to 10 wt. %, up to 5 wt. %, from 5 to 20 wt. %, from 5 to 15 wt. %, or from 5 to 10 wt. %. The calcium and / or magnesium salts can be in the form of alkali earth oxide, hydroxide, sulfate, or silicate, or as Portland cement.

[0036] In optional embodiments, a superplasticizing agent is further provided in the mixture. Superplasticizing agents are for example synthetic polymers that include sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers. Superplasticizing agents exhibit surfactant properties and are well known in the art. The list provided herein is in no means limitative or exhaustive. The superplasticizing agent can be provided in the mixture in a concentration of up to 3 wt. %, up to 2 wt. %, up to 1.5 wt. %, or up to 1 wt. %.

[0037] The mixture can be mixed by any means known in the art, such as mechanical mixing with an impeller. The viscosity of the mixing can optionally be monitored to determine the progress of the reaction and the formation of the inorganic polymer. The application of vacuum affects the viscosity. It is estimated that when mixing under vacuum, in under 2 minutes the mixture can become softer and more pliable which helps obtain a more uniform mixing and a better reactivity and efficacy. Instead of the viscosity, the shear stress of the mixture could also be measured.

[0038] The mixing can advantageously be performed in the absence of heating, for example at temperature of from 10 to 40 °C. However, it is also contemplated herein that heating could be applied and therefore the temperature may be in the range of from 10 to 80 °C. It is more costly to perform heating and therefore it is preferable to avoid heating the mixture.

[0039] Once the inorganic polymer has been formed, further processing can be performed outside of the vacuum to for example drying the inorganic polymer and then shaping or molding the inorganic polymer. Alternatively, a shaping step can be performed before drying. Indeed, in some embodiments, making reference to Fig. 1, a system 1 for producing the inorganic polymers has a reactor 10 in fluid communication with a vacuum pump 11. An extrusion apparatus 12 can be directly connected to the reactor 10 in order to extrude the inorganic polymer (while still wet) into a desired shape which can then be dried. The polymer can also be left to cure for long periods of time as needed. For example, a curing step can be performed over 7 to 28 days or 1 to 3 days at a temperature of more than 40 °C.

[0040] The inorganic polymer obtained herein by the present process is characterized in that it comprises anatase, rutile, hematite, goethite, kaolinite and / or quartz. The inorganic polymer has desirable applications in the construction industry including as part of concrete or mortar products or intermediates. For example, the inorganic polymer of the present disclosure can be shaped into construction bricks, masonry units, landscaping materials, pipes, poles, siding panels, shingles or other building blocks. The inorganic polymer can also be used in the construction of bridges or other infrastructures or as a component in asphalt. To be suitable for these applications, the inorganic polymers produced by the present method preferably have a compressive strength of 30 MPa or more, preferably 50 MPa or more.

[0041] As can be seen from the above, with the application of vacuum, the present process allows to obtain a more cost effective process compared to traditional methods, which also results in energy savings and improved mechanical properties of the inorganic polymer.

[0042] Examples of different possible mixture composition:

[0043] Embodiment 1 of a mixture for the production of inorganic polymers in weight percent: wet bauxite residue 70%, metakaolin (calcined clay): 20%, potassium hydroxide: 3%, sodium hydroxide: 2%, and sodium silicate: 5%. To produce the inorganic polymers, the mixture was mixed for 10 minutes at 20°C and a pressure of around 60 kPa, molded into shape and cured at 20°C for 14 days.

[0044] Embodiment 2 of a mixture for the production of inorganic polymers in weight percent : dry bauxite residue: 70%, coal fly ash: 5%, sodium silicate: 4%, calcium sulfate: 1%, and water: 20%. To produce the inorganic polymers, the mixture was mixed for 30 minutes at 20°C and a pressure of around 10 kPa, extruded into shape and cured at 90°C for 7 days.

[0045] Embodiment 3 of a mixture for the production of inorganic polymers in weight percent: dry bauxite residue: 60%, ground, granulated blast furnace slag: 10%, sodium hydroxide: 7%, sodium silicate: 4%, calcium hydroxide: 2%, superplasticizer: 1.5%, and water: 15.5%. To produce the inorganic polymers, the mixture is mixed for 25 minutes at 20°C and a pressure of around 20 kPa, extruded into shape and cured at 60°C for 3 days.

[0046] Embodiment 4 of a mixture for the production of inorganic polymers in weight percent: wet bauxite residue: 55%, rice husk bottom ash: 30%, potassium hydroxide: 1%, sodium aluminate: 2%, sodium silicate: 4%, superplasticizer: 1%, and water: 7%. To produce the inorganic polymers, the mixture was mixed for 20 minutes at 20°C and a pressure of around 40 kPa, molded into shape and cured at 90°C for 1 day.

[0047] Embodiment 5 of a mixture for the production of inorganic polymers in weight percent: wet bauxite residue: 75%, metakaolin (calcined clay): 10%, rice husk bottom ash: 5%, sodium hydroxide: 2%, sodium silicate: 5%, superplasticizer: 1%, and water: 2%. The mixture was mixed for 30 minutes at 20°C and a pressure of around 10 kPa, molded into shape and cured at 50°C for 3 days.

[0048] As can be understood, the examples described above and illustrated are intended to be exemplary only. The scope is indicated by the appended claims.

Claims

1. A process for producing an inorganic polymer, comprising:providing bauxite residue, an alkali salt, and water in a closed environment to obtain a mixture; andmixing the mixture under a pressure of less than 70 kPa to obtain the inorganic polymer.

2. The process of claim 1, further comprising curing the inorganic polymer.

3. The process of claim 1, further comprising extruding the inorganic polymer into a predetermined shape.

4. The process of claim 1, further comprising molding the inorganic polymer into a predetermined shape.

5. The process of claim 4, further comprising curing the inorganic polymer.

6. The process of any one of claims 1 to 5, wherein the bauxite residue is dry bauxite residueand the water is provided in a concentration of from 10 to 30 wt. % of the mixture.

7. The process of any one of claims 1 to 5, wherein the bauxite residue is wet bauxite residuewhich comprises water, and wherein up to 15 wt. % of additional water is provided such that the concentration of water in the mixture is from 10 to 30 wt. %.

8. The process of any one of claims 1 to 7, wherein the bauxite residue is provided in a concentration of from 50 to 90 wt. % of the mixture.

9. The process of claim 6 or 8, wherein the dry bauxite residue comprises in weight percent 25 - 55 % of iron oxides, 10-25 % of aluminum oxides, 5-25 % of silicon oxides, 2 -10 % of sodium oxides, 2-20 % of calcium oxides, 5 - 15 % of titanium oxides, 10-15 % of carbon and organic species, and less than 1 % of water.

10. The process of claim 7 or 8, wherein the wet bauxite residue comprises in weight percent 25 - 55 % of iron oxides, 10-25 % of aluminum oxides, 5-25 % of silicon oxides, 2 -10 % of sodium oxides, 2-20 % of calcium oxides, 5 - 15 % of titanium oxides, 10-15 % of carbon and organic species, and 15-30 % of water.

11. The process of any one of claims 1 to 10, wherein the alkali salt is provided in a concentration of from 2 to 20 wt. % of the mixture.

12. The process of any one of claims 1 to 11, wherein the alkali salt is one or more of sodium hydroxide, potassium hydroxide, sodium silicate, potassium silicate, sodium carbonate and sodium aluminate.

13. The process of any one of claims 1 to 12, wherein the step of providing further comprises providing an aluminosilicate supplement.

14. The process of claim 13, wherein the aluminosilicate supplement comprises SiO2 and AI2O3.

15. The process of claim 13 or 14, wherein the aluminosilicate supplement is one or more of fly ash, bottom ash, clay, sand, metallurgical dross and metallurgical slag.

16. The process of any one of claims 13 to 15, wherein the aluminosilicate supplement is provided in a concentration of up to 40 wt. % of the mixture.

17. The process of any one of claims 13 to 16, wherein the aluminosilicate is sand and the step of providing further comprises providing an aggregate.

18. The process of claim 17, further comprising producing a concrete or mortar.

19. The process of any one of claims 1 to 18, wherein the step of providing further comprisesproviding a superplasticizing agent.

20. The process of claim 19, wherein the superplasticizing agent is provided in a concentration of up to 3 wt. % of the mixture.

21. The process of any one of claims 1 to 18, wherein the step of providing further comprises providing a calcium and / or magnesium salt.

22. The process of claim 21, wherein the calcium and / or magnesium salt is provided in a concentration of up to 20 wt. %.

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