Electrolytic cells containing fused cast refractories and lining components

Abstract

It is disclosed an electrolytic cell for the production of aluminium by reaction of alumina, the electrolytic cell being configured to contain an electrolyte bath comprising alumina, the sidewalls of the cell being protected by fused cast refractory blocks comprising: from about 55 to 95 % of Al2O3 having alpha and beta alumina, wherein a beta alumina concentration is of about 40 to 60%, and about 5 to 45% of other oxides. The refractory blocks, object of the disclosed invention, are substantially free of cracks. Advantageously, the fused cast refractory blocks are impervious to the molten electrolyte bath and metals, are resistant to dissolution into said molten bath electrolyte and metals; and have a high electrical resistivity. A method for manufacturing the fused cast refractory blocks is also disclosed. The invention preferably concerns electrolytic cells working with inert or oxygen-evolving anodes for an Eco-friendly production of aluminum.

Description

 Cross-Reference to Related Applications

[0001] The present patent application claims the benefits of priority of U.S. Provisional Patent Application No. 63 / 554,553 entitled “electrolytic CellS CONTAINING Fused Cast REFRACTORIES AND Lining Component”and filed at the United States Patent and Trademark Office on February 16, 2024, the content of which is incorporated herein by reference.Field

[0002] The present invention generally relates to compositions of fused cast refractories for the making of lining components of an electrolytic cell, for instance for the production of a metal, such as aluminum.Background

[0003] Sidewall linings in conventional Hall-Héroult (H-H) electrolytic cells normally consist of a combination of carbonaceous materials or these materials used in conjunction with silicon carbide blocks contained in a steel shell. The carbonaceous materials can be anthracite based, graphitic carbon, graphitized carbon, or graphite. The cryolite-based electrolytic bath used in the process is very corrosive to virtually all materials, and therefore the solution to containing liquid bath in H-H technology is to freeze a layer of solid bath on the walls to contain the liquid bath. This frozen layer of cryolite is achieved by the sidewall design to allow a high amount of heat loss as well as choosing a bath composition with a liquidus temperature just below the operating temperature of the cell. M. Sørlie and H. A. Øye, in “Cathodes in Aluminium Electrolysis”, Aluminium-Verlag, 3rd Edition 2010, teaches a typical arrangement for the sidewall of a conventional H-H cell in which the frozen layer protects the sidewall. The requirement to maintain a frozen sidewall necessitates a certain amount of heat transferred through the sidewall. If insufficient heat is transferred through the sidewall, the frozen layer becomes thin and eventually disappears. This unacceptably exposes the carbonaceous / SiC lining to corrosion by liquid bath. The heat transferred through the sidewall to maintain the frozen ledge is a loss to the process and increases the energy requirements of the process.

[0004] To reduce the energy required for the process, the heat transferred through the sidewall should be reduced, for example, by adding insulation materials to the lining. To have a meaningful impact, the frozen ledge will be lost, and the cell would need to be operated with a bare sidewall.

[0005] There is thus a need for a new electrolytic cell having lining components that allow the electrolytic cell to be run without a frozen layer as the sidewalls, and that are relatively resistant to corrosion by bath. This is true for electrolytic cells using carbon anodes as well as electrolytic cells using inert or oxygen evolving electrodes, in particular for the production of aluminum.Summary

[0006] The shortcomings of the prior art are generally mitigated by a material that has not been previously used in aluminum electrolysis cells, either in conventional carbon anode cells or in recently developed inert anode cells, and mitigated in the particular way in the materials are installed.

[0007] It is first disclosed an electrolytic cell for the production of aluminum by reaction of alumina, wherein the cell comprises a cell reservoir having a bottom wall and sidewalls extending therefrom and configured for retaining an electrolytic bath containing molten alumina, at least one anode and one cathode configured to be immersed into the electrolytic bath; and a plurality of fused cast refractory blocks for lining the sidewalls of the cell reservoir, wherein each of the plurality of fused cast refractory blocks comprises, in weight %:about 55 % to about 95 % of Al2O3 comprising alpha and beta alumina, wherein a concentration of the beta alumina in the fused cast refractory block is of about 40 to about 60%, andabout 5 % to about 45 % of other oxides,wherein each of the plurality of fused cast refractory blocks is impervious to the molten electrolyte bath and metals, is resistant to dissolution into said molten bath electrolyte and metals, and has a high electrical resistivity.

[0008] According to a preferred embodiment, the other oxides comprise at least one oxide selected from the group consisting of Na2O, SiO2, Cr2O3, Fe2O3, and MgO.

[0009] According to a preferred embodiment, each of the plurality of fused cast refractory blocks is substantially free of cracks.

[0010] According to a preferred embodiment, each of the plurality of fused cast refractory blocks is obtained by casting a fused castable composition in a casting mold, the fused castable composition comprising, in weight %, about 55 % to about 95 % of said Al2O3 having the beta alumina concentration of about 40 to about 60%, and about 5 % to about 45 % of the other oxides. Preferably, casting the fused castable composition in the casting mold comprises cooling the fused castable composition in the casting mold at a given temperature cooling ramp. More preferably, each of the plurality of fused cast refractory blocks has a higher concentration of alpha alumina at a surface of the block that is adjacent the casting mold during casting compared to the concentration of alpha alumina inside the block below its surface.

[0011] According to another preferred embodiment, each of the sidewalls comprises a bottom side block with a first surface directly exposed to aluminum, the electrolytic bath or both, and an upper side block with a second surface directly or indirectly exposed to the bath.

[0012] According to a preferred embodiment, each of said sidewalls comprises a bottom side block with a first surface directly exposed, at least in part, to aluminum, the electrolytic bath or both, and an upper side block with a second surface directly or indirectly exposed to the bath.

[0013] According to a preferred embodiment, the bottom side block is configured to support a lining material extending along the second surface of the upper side block for protecting the upper side block.

[0014] According to a preferred embodiment, the bottom side block has a rectangular shape with a longitudinal side defining the first surface directly exposed, at least in part, to aluminum.

[0015] According to a preferred embodiment, the bottom side block has a top surface declining inwardly from the bath toward the upper side block, the upper side block comprising a bottom section having a rectangular shape for leaning against the adjacent rectangular shaped bottom side block and a top section having a triangular shape defining a width decreasing from the bottom section to an upper end of the top section, and wherein the top surface of the bottom side block and the the second surface of the upper side block form an angle of about 90° for matching and supporting the lining material extending along the second surface of the upper side block.

[0016] According to a preferred embodiment, the bottom side block and the upper side block form a monolithic block, the monolithic block having a stepped shape with a bottom section advancing into the bath towards aluminum.

[0017] According to a preferred embodiment, the bottom side block is configured to support a lining material for protecting the second surface of the upper section of the monolithic block.

[0018] According to another preferred embodiment, the bottom side block has a shape of a right triangle with its hypotenuse side forming the first surface directly exposed, at least in part, to aluminum, and the vertical side adjacent the right angle of the right triangle extending, at least in part, from a bottom section of the upper side block. Preferably, the bottom side block and the upper side block form a monolithic block, the monolithic block having a triangular shape of a right triangle with a surface of its hypotenuse advancing towards the bath and aluminum.

[0019] According to another preferred embodiment, the cell further comprises a lining material covering, at least in part, the surface of the monolithic block exposed to the bath.

[0020] According to another preferred embodiment, the at least one anodes of the electrolytic cell are inert or oxygen evolving anodes for an ecofriendly production of aluminum.

[0021] According to a preferred embodiment, for each of said sidewalls, the cell comprises a first side block having a first surface directly exposed to both aluminum and electrolytic bath, and a second side block outwardly extending from and along the first side block, the first side block being optionally free of additional lining material for facing the aluminum and bath. Preferably, a concentration of the alpha alumina is maximized at the first surface of the first side block exposed to the aluminum and bath.

[0022] According to another aspect, the invention is also directed to a method for manufacturing a fused cast refractory block used for lining a sidewall of a cell reservoir of an electrolytic cell used for the production of aluminum by reaction of alumina, the method comprising:casting a fused castable composition in a casting mold by cooling the fused castable composition in the casting mold at a given temperature cooling ramp, wherein the fused castable composition comprises, in weight %:about 55 % to about 95 % of Al2O3 comprising alpha and beta alumina, wherein a concentration of the beta alumina in the fused cast refractory block is of about 40 to about 60%, andabout 5 % to about 45 % of other oxides,wherein the fused cast refractory block is impervious to molten electrolyte bath and aluminum, is resistant to dissolution into said molten bath electrolyte and aluminum, and has a high electrical resistivity.

[0023] According to a preferred embodiment, the fused cast refractory block obtained by the method is substantially free of cracks.

[0024] According to a preferred embodiment, the other oxides used in the method comprise at least one oxide selected from the group consisting of Na2O, SiO2, Cr2O3, Fe2O3, and MgO.

[0025] According to a preferred embodiment, the obtained fused cast refractory blocks has a higher concentration of alpha alumina at a surface of the block that is adjacent the casting mold during casting compared to the concentration of alpha alumina inside the block below its surface.

[0026] According to another aspect, the invention is also directed to fused cast refractory blocks as defined herein, or manufactured by the method as defined herein, wherein the refractory blocks are impervious to molten electrolyte bath and aluminum, are resistant to dissolution into said molten bath electrolyte and aluminum, and has a high electrical resistivity. Preferably, the refractory blocks are substantially free of cracks.

[0027] According to a preferred embodiment, each of the refractory blocks has a higher concentration of alpha alumina at a surface of the block that is adjacent the casting mold during casting compared to the concentration of alpha alumina inside the refractory block below its surface.

[0028] According to another aspect, the invention is also directed to use of at least one fused cast refractory block as defined herein, or manufactured by the method as claimed herein, for lining sidewalls of a cell reservoir of an electrolytic cell used for the production of aluminum by reaction of alumina.

[0029] The invention is can be used for either electrolytic cells using carboneous anodes or using inert or oxygen evolving electrodes, for a green or ecofriendly production of a metal, such as aluminum.

[0030] The new technology as disclosed herein allows cells to be run without a frozen layer at the sidewalls.

[0031] Additionally, the new technology as disclosed herein is preferably using inert anodes will benefit from being able to operate without a frozen ledge. Benefits may include improved energy efficiency, improved current distribution, increased productive use of the internal volume of the pot, increase temperature operating window, a greater ability to throttle back aluminum production during periods of peak electrical demand on the electric power grid.

[0032] Other and further aspects and advantages of the present invention will be better understood upon the reading of the illustrative embodiments about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice. Brief Description of the Drawings

[0033] The above and other aspects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying drawings in which:

[0034] Figure 1 is a schematic illustration of an electrolytic cell showing the effect on the a) “steady state” side ledge when b) less and c) more energy is supplied to the cell (redrawn from Kvande et al.);

[0035] Figure 2A shows a partial cut side view of an electrolytic cell with rectangular shaped bottom FCA block, according to a preferred embodiment;

[0036] Figure 2B shows a partial cut side view of an electrolytic cell with fused rectangular shaped bottom and upper side FCA blocks, according to different preferred embodiments;

[0037] Figure 2C shows a partial cut side view of an electrolytic cell with a first side block having its surface directly exposed to both metal and electrolytic bath, and a second side block outwardly extending from and along the first side block, according to a preferred embodiment;

[0038] Figure 2D shows a partial cut side view of an electrolytic cell with fused triangular shaped bottom and upper side FCA blocks, according to a preferred embodiment;

[0039] Figure 2E shows a partial cut side view of an electrolytic cell with fused triangular shaped bottom FCA blocks, according to a preferred embodiment;

[0040] Figure 2F shows a partial cut side view of an electrolytic cell with a rectangular shaped bottom FCA block with a declining top surface, and an upper triangular shaped side FCA block, according to different preferred embodiments; and

[0041] Figure 3 is a top view of an electrolytic cell according to a preferred embodiment. Detailed Description of the Preferred Embodiment

[0042] A novel electrolytic cell will be described hereinafter. Although the invention is described in terms of specific illustrative embodiments, it is to be understood that the embodiments described herein are by way of example only and that the scope of the invention is not intended to be limited thereby.

[0043] The terminology used herein is in accordance with definitions set out below.

[0044] As used herein % or wt.% means weight % unless otherwise indicated. When used herein % refers to weight % as compared to the total weight percent of the phase or composition that is being discussed.

[0045] By "about", it is meant that the value or data can vary within a certain range depending on the margin of error of the method or device used to measure or evaluate such value or data. A margin of error of 10% is generally accepted.

[0046] As “electrolytic cell” or “cell” mentioned herein, it is understood a cell comprising at least one cathode or a cathode assembly having at least one cathode, such as, but not limited to vertical cathodes, and configured for receiving or interacting with at least one corresponding anode or an anode assembly having at least one anode, such as, but not limited to vertical anodes. The cell is also configured to receive an electrolytic bath of a molten electrolyte (such as cryolite) for the electrolytic production of metals, such as aluminum.

[0047] The description which follows, and the embodiments described therein are provided by way of illustration of an example of particular embodiments of principles and aspects of the present invention. These examples are provided for the purposes of explanation and not of limitation, of those principles of the invention. In the description that follows, like parts and / or steps are marked throughout the specification and the drawing with the same respective reference numerals.

[0048] Figure 1 shows a typical arrangement for the sidewall (7’) of a conventional Hall- Héroult (H-H) electrolytic cell (1’) for the production of aluminum (15’). The H-H cell (1’) comprises a cell reservoir (3’) having a bottom wall (5’) and sidewalls (7’) extending therefrom and configured for retaining an electrolytic bath (9’) of molten AlF3 and the produced aluminum (15’). The cell (1’) is configured to comprises at least one anode (4’), and the cell bottom (5’) serves as the cathode. Figure 1 shows the frozen layer (2’) that protects the sidewall (7’) and the effect on the a) “steady state” side ledge, when the energy supply is stable, when b) less and c) more energy is supplied to the cell (cited in M. Sørlie and H. A. Øye, in “Cathodes in Aluminium Electrolysis”, Aluminium-Verlag, 3rd Edition 2010, redrawn from Kvande et al.). In Figure 1(b), the energy supply is reduced, the temperature of the bath is reduced, the side ledge (2’) is increased and the liquid bath’s (9’) volume is decreased. On the contrary, in Figure 1(c), the energy supply is increased, the temperature of the liquid bath is increased, the side ledge (2’) is reduces and the liquid bath’s (9’) volume is increased.

[0049] Advantageously, the electrolytic cells as disclosed herein after can be run without a frozen ledge at the sidewall. Indeed, the requirement to maintain a frozen ledge at the cell’s sidewalls necessitates a certain amount of heat transferred outwardly through the sidewall (7’). If insufficient heat is transferred through the sidewall, the frozen ledge becomes thin (see Figure 1(c)) and eventually disappears. This unacceptably exposes the carbonaceous / SiC lining to corrosion by the molten liquid bath (9’). The heat transferred through the sidewall to maintain the frozen ledge (2’) is a loss to the process and increases the energy requirements of the process.

[0050] As aforesaid, disclosed herein are electrolytic cells comprising lining systems of fused cast alumina (FCA) compositions. Fused cast refractories are part of a class of ceramic materials that are produced by melting the oxides and casting in a mold, using for instance graphite as the mold material. This contrasts with most refractory ceramic materials in which oxide powders are mixed, pressed into bricks or other shapes, and sintered to achieve strength. The fused cast process is in many ways more difficult to produce since the melting temperatures of oxides are generally high, and the cast parts needs to be cooled extremely slowly to avoid cracking from thermal shock. For example, alumina fusion point is at 2072°C, and fused cast alumina parts need to be slow-cooled over a period of days after casting to avoid cracking. The process to produce fused cast refractories is analogous to the way metals are melted and cast into products.

[0051] The making process of fused cast refractories produces a denser product than can be achieved by the press and sinter method used to produce most refractories. For example, the typical bulk density of a fused cast alumina (FCA) material is 3.4 g / cm3. Density demonstrates an inherent advantage of a fused cast refractory as compared to a pressed and sintered material. Corundum bricks are the pressed and sintered analogue to FCA.

[0052] ) The density of sintered corundum bricks is known to range from 3.22 to 3.26 g / cm3. This gives FCA a significant advantage in using it an electrolysis cell since by nature of the process, the electrolyte has significant solubility of alumina and a denser sidewall material will dissolve slower, yielding increased longevity of the sidewall.

[0053] Fused cast materials available in the marketplace today fall in three categories, based on either aluminum oxide, zirconium oxide, and chromium oxide. These materials have historically been used in certain portions of the linings of glass melting furnaces.

[0054] According to a preferred embodiment, the refractory blocks comprise alpha and beta phases, for instance preferably about 40% of alpha phase and 60% of beta phase. Alpha alumina is essentially pure Al2O3, and the beta alumina phase is Na2O.xAl2O3 where x can range from approximately 9 to 11. Although alpha alumina is more resistant to corrosion (dissolution) by bath than beta alumina, a higher concentration of beta alumina is used in order to produce large blocks required for the cell.

[0055] According to the present invention, each of the fused cast refractory blocks disclosed herein comprises, in weight %:about 55% to 95% of Al2O3 having a beta alumina concentration of about 40% to about 60%, andabout 5 to 45% of other oxides.

[0056] The fused cast refractory blocks used in the cell is impervious to the molten electrolyte bath and metals, is resistant to dissolution into said molten bath electrolyte and metals; and has a high electrical resistivity.

[0057] According to a preferred embodiment, the other oxides comprise at least one oxide selected from the group consisting of Na2O, SiO2, Cr2O3, Fe2O3, and MgO.

[0058] According to a preferred embodiment, the obtained fused cast refractory blocks are substantially free, or free, of cracks, which make them more resistant to wear and to dissolving in the bath, extending as such their useful life.

[0059] According to a preferred embodiment, the refractory blocks used in the cell has a higher concentration of alpha alumina at a surface of the blocks (that is adjacent the casting mold used for the production of the refractory block), compared to the concentration of alpha alumina inside the blocks below the surface.

[0060] Figures 2A to 2F show different embodiments according to the present invention illustrated by different schematic cross sections of arrangements of fused cast refractory blocks used in an electrolytic cell (1). Figure 3 shows a top plan view of a cell (1) with fused cast refractory blocks disposed around the bath for lining the sidewalls (7) of the cell (1), according to a preferred embodiment. The electrolytic cell (1) may comprise an external steel shell (7a) and other lining materials (7b) sandwich between the refractory FCA blocks as describer herein below, and the external steel shell (7a).

[0061] According to a preferred embodiment, the electrolytic cell (1) for the production of aluminium by reaction of alumina is provided. The cell comprises a cell reservoir (3) having a bottom wall (5) and sidewalls (7) extending therefrom and configured for retaining an electrolytic bath (9) containing dissolved alumina. The bath (9) defines an upper bath-vapor interface or bath level (8). The bottom wall (5) may comprise an external steel shell (5a) and cell bottom lining and / or protective material (5b).

[0062] The electrolytic cell (1) is configured to comprises at least one anode and one cathode (not illustrated) configured to be immersed into the electrolytic bath (9) for the production of a metal, such as aluminum. WO 2018 / 009862 (Xinghua Liu), the content of which is enclosed herewith by reference, describes a cell with vertically oriented anodes and cathodes that can be used in the cell (1) as described herein. Once produced by the electrolytic process, the metal accumulates at the bottom (5) of cell to form a metal pad (15). The metal pad defines a metal-bath interface (19), the level of which can vary between a minimum level (19a) and a maximum level (19b).

[0063] As aforesaid, the electrolytic cell (1) may comprise a plurality of refractory blocks (11, 12, 13, 14, 16, 18, 21, or 23) for lining the sidewalls (7) of the cell reservoir (3). The plurality of refractory blocks comprises the fused cast refractory blocks as disclosed herein and illustrated in Figures 2A to 2F or 3.

[0064] Some of the fused cast refractory blocks (11, 12, 13, 14, 16, or 21) are preferably used in regions of the cell (1) that can be exposed to the bath (9), metal (15), or both. The fused cast refractory can be used in direct contact with bath and metal, or alternatively, they can be used as a backup behind other materials (17) that would be directly exposed to the bath (9).

[0065] Figures 2A, 2B, 2F and 3 show the upper side block (11, 18) used as a backup behind another optional material (17), such as lining material, while a lower or bottom side block (13, 16) is directly exposed to bath (9) and metal (15). The interface (19) between the bath and metal moves up (19b) and down (19a) as metal is produced and subsequently tapped from the cell.

[0066] According to the preferred embodiments illustrated on Figures 2A and 2B, each of said sidewalls may comprise a bottom side block (13) with a first surface (S1) directly exposed to the metal (15), such as aluminum, the electrolytic bath (9) or both, and an upper side block (11) with a second surface (S2) directly or indirectly exposed to the bath.

[0067] According to the preferred embodiment illustrated on Figure 2A, the bottom side block (13) may have a rectangular shape with a longitudinal side (13d) having a surface (S1) directly exposed, at least in part, to the metal. According to the preferred embodiment illustrated on Figures 2B, the bottom and upper side blocks may form one monolithic block (12). The monolithic block (12) may have a stepped shape with a bottom section (12a) advancing into the bath towards the metal, and optionally supporting a lining material (17) protecting the second surface (S2) of the upper section (12b) of the monolithic block (12).

[0068] Indeed, according to the preferred embodiments illustrated on Figures 2A, 2B or 2F, the electrolytic cell (1) may further comprise a lining material (17) for covering the second surface (S2) of the upper side block (11, 18) or the upper section (12b) of the monolithic block (12) when the upper side block or section (11, 12b) is or should be indirectly exposed to the bath. Preferably, the concentration or percentage of alpha alumina is maximized at the second surface (S2) of the upper block or section (11, 12b, or 18).

[0069] According to the preferred embodiment illustrated on Figure 2E, the bottom side block (13) may have a shape of a right triangle with the hypotenuse side (13c) of the right triangle forming the first surface (S1) directly exposed the metal (15), and the vertical side (13a) adjacent the right angle of the right triangle extending, at least in part, from a bottom section (11b) of the upper side block (11). In other words, the bottom section (11b) of the upper block (11), is covered, at least in part, by the vertical side (13a) of the lower triangular side block (13).

[0070] According to the preferred embodiment illustrated on Figure 2D, the bottom triangular side block and the upper side block, such as the ones illustrated on Figure 2E, may also form a monolithic block (14). The monolithic block (14) may have a shape of a right triangle with the hypotenuse side (14c) forming one surface (S) facing the metal (15) and bath (9). One advantage of the monolithic block (14) illustrated on Figure 2D is to have less FCA material at the top section of the cell (1) over the bath level (8), and more FCA material at the bottom section below the bath level (8).

[0071] According to another preferred embodiment illustrated on Figure 2C, the electrolytic (1) cell comprises, for each of said sidewalls, a first side block (21) having a surface (S) directly exposed to both metal and electrolytic bath, and a second side block (23) outwardly extending from and along the first side block (21), the first side block being optionally free of additional lining material for facing the aluminum and bath. Preferably, a concentration or a percentage of alpha alumina is maximized at the surface (S) of the first side block exposed to the metal and bath.

[0072] According to another preferred embodiment illustrated on Figure 2F shows, the electrolytic cell (1) comprises a rectangular shaped bottom block (16), preferably defining a top surface (S4) declining inwardly from the bath (9) toward an upper triangular shaped side block (18). More particularly, the upper triangular shaped side block (18) comprises a bottom section (18a) having a rectangular shape for leaning against the adjacent rectangular shaped bottom block (16), and a top section (18b) having a triangular shape with a width (ω) decreasing from the bottom section (18a) to the upper end (18c) of the top section (18b). As for the monolithic block (14) disclosed above and illustrated on Figure 2D, the upper triangular shaped side block (18) having a decreasing width (ω) advantageously allows having less FCA material at the top section of the cell (1) over the liquid bath’s level (8), and more FCA material at the bottom section below the liquid bath’s level (8).

[0073] According to the preferred embodiment illustrated on Figure 2F, the top surface (S4) of the rectangular shaped bottom side block (16) is declining inwardly from the bath (9) toward the surface (S2) of the upper triangular shaped side block (18), forming as such an angle α of about 90° between the surfaces S4 and S2 in order to better match with and support a lining material (17) extending from the surface S2 of the upper side block (18). Although not illustrated in the Figures, it has to be understood that the side refractory blocks (16, 18) illustrated on Figure 2F may form a monolithic refractory block, as the ones illustrated on Figure 2B or 2D, the monolithic block having the shapes and angles disclosed herein above.

[0074] The invention as disclosed herein is also directed to fused cast refractory blocks as defined herein, in particular having the shapes and functions disclosed in the present description in relation to the Figures 2A to 2F and 3, or manufactured by the method as defined herein. The refractory blocks object to the present invention are impervious to molten electrolyte bath and aluminum, resistant to dissolution into said molten bath electrolyte and aluminum, and has a high electrical resistivity. Preferably, the refractory blocks are free of cracks or substantially free of cracks, which make them more resistant to wear and to dissolving in the bath, extending as such their useful life.

[0075] According to a preferred embodiment, each of the refractory blocks object to the present invention has a higher concentration of alpha alumina at a surface of the block that is adjacent the casting mold during casting compared to the concentration of alpha alumina inside the refractory block below its surface.

[0076] According to another aspect, the invention is also directed to the use of at least one fused cast refractory block as defined herein, in particular having the shapes and functions disclosed in the present description in relation to the Figures 2A to 2F and 3, or manufactured by the method as disclosed herein, for lining sidewalls of a cell reservoir of an electrolytic cell used for the production of aluminum by reaction of alumina.

[0077] As mentioned above, alpha alumina is more resistant to bath than beta alumina. Due to the thermochemistry of the Al2O3 – Na2O system (the phase diagram) and the specific composition of the FCA composition used to cast the lining blocks, upon cooling the molten refractory the first solid to form is alpha alumina. Therefore, cast FCA blocks will advantageously contain a high concentration of alpha at the surface of the block that was in direct contact with the casting mold. When installed in the cell, the FCA surfaces that are in direct contact with the bath are as-cast surfaces to maximize the resistance to bath by maximizing the percentage of the alpha phase at these surfaces. The goal is that after casting of the FCA blocks, there is no cutting or grinding of surfaces that will be in direct contact with the bath. While there is a need for cutting and grinding of some surfaces of the blocks to meet dimensional criteria, it is desirable that this is done on surfaces that are not in direct contact with the bath.

[0078] As aforesaid, the obtained fused cast refractory blocks used in the cell are impervious to the molten electrolyte bath and metals, are resistant to dissolution into a molten electrolyte bath and metals; and have a high electrical resistivity. Preferably, the obtained fused cast refractory block is substantially free of cracks.

[0079] According to another aspect, the invention is also directed to a method for manufacturing a fused cast refractory block used for lining a sidewall of a cell reservoir of an electrolytic cell used for the production of aluminum by reaction of alumina. The method comprises casting a fused castable composition in a casting mold by cooling the fused castable composition in the casting mold at a given temperature cooling ramp. The fused castable composition comprises, in weight %, about 55 % to about 95 % of Al2O3 comprising alpha and beta alumina, wherein a concentration of the beta alumina in the fused cast refractory block is of about 40 to about 60%, and about 5 % to about 45 % of other oxides. Preferably, the other oxides used in the method comprise at least one oxide selected from the group consisting of Na2O, SiO2, Cr2O3, Fe2O3, and MgO.

[0080] As aforesaid, the resulting fused cast refractory block is impervious to molten electrolyte bath and aluminum, is resistant to dissolution into said molten bath electrolyte and aluminum, and has a high electrical resistivity. Preferably, the fused cast refractory block obtained by the method is substantially free of cracks.

[0081] As aforesaid, casting the composition in the casting mold comprises cooling the castable composition in the casting mold at a given temperature cooling ramp. The obtained refractory block has therefore a higher concentration of alpha alumina at a surface of the block that is adjacent the casting mold compared to the concentration of alpha alumina inside the block below the surface.

[0082] A first key feature of the fused cast refractories for the cell lining disclosed herein is that they are impervious to molten bath and metal. The objective is to contain the liquids without the requirement of maintaining a layer of frozen bath on the sidewall. As compared to their pressed and sintered counterparts, fused cast oxides or refractories have the advantage of higher bulk density and lower interconnected porosity, and therefore are better at containing the liquids.

[0083] A second key feature is that fused cast oxides, particularly aluminum oxide and chromium oxide, are highly resistant to dissolution into molten bath and metals. Taking fused cast alumina as an example, the solubility of aluminum oxide in bath is relatively low, about 4% to about 10%. This low solubility contributes to its resistance to bath. Finally, the relatively high density and low porosity of fused cast alumina further contributes to its resistance to bath.

[0084] Advantageously, the fused cast materials, particularly fused cast alumina, as disclosed herein have high electrical resistivity. This is critical since it can be partially in contact with molten aluminum. If the side wall material was partially conductive and in contact with aluminum, it would serve as a cathode and possibly have aluminum and other trace metals depositing on its surface. Near the aluminum metal pad, the deposit would likely be aluminum and would not be problematic. However, because of voltage drop in the material, at some distance away from the metal pad the deposit could be other metals that are more electronegative than aluminum, or lower than aluminum in the electrochemical series of metals. Based on experimental data by the Applicant, the non-aluminum metallic growth could be dendritic. This metallic growth on the sidewall would cause a major problem because over time, these dendrites would grow quite large and eventually break off and fall into the metal pad, degrading the metal quality.

[0085] The electrolytic cell as disclosed herein uses a block material with a beta alumina content of preferably about 40 to about 60% and a target Na2O content of preferably about 4.05%. A beta alumina concentration of at least about 40 to 60% allows producing blocks without cracking.

[0086] Example : Electrolytic cells containing fused cast alumina linings as disclosed herein have been confidentially operated by the Applicant over 3 years, thus demonstrating the resistance of the FCA material disclosed herein to bath and metal.

[0087] In summary, the electrolytic cell as disclosed herein has acceptable resistance to bath corrosion and acceptable low electrical conductivity as well as acceptable manufacturability.

[0088] This electrolytic cell as disclosed herein solves the problem of containment of molten bath and metal without the need to maintain a layer of frozen bath.

[0089] The electrolytic cell as disclosed herein is one of the key enablers to the new clean-technology by the Applicant, which has the overall advantage of smelting aluminum without producing greenhouse gas by-products in the process.

[0090] While illustrative and presently preferred embodiments of the invention have been described in detail hereinabove, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art. 

Claims

1. An electrolytic cell for the production of aluminum by reaction of alumina, wherein the cell comprises a cell reservoir having a bottom wall and sidewalls extending therefrom and configured for retaining an electrolytic bath containing molten alumina, at least one anode and one cathode configured to be immersed into the electrolytic bath; and a plurality of fused cast refractory blocks for lining the sidewalls of the cell reservoir, wherein each of the plurality of fused cast refractory blocks comprises, in weight %:about 55 % to about 95 % of Al2O3 comprising alpha and beta alumina, wherein a concentration of the beta alumina in the fused cast refractory block is of about 40 to about 60%, and about 5 % to about 45 % of other oxides,wherein each of the plurality of fused cast refractory blocks is impervious to the molten electrolyte bath and metals, is resistant to dissolution into said molten bath electrolyte and metals, and has a high electrical resistivity.

2. The electrolytic cell as claimed in claim 1, wherein the other oxides comprise at least one oxide selected from the group consisting of Na2O, SiO2, Cr2O3, Fe2O3, and MgO.

3. The electrolytic cell as claimed in claim 1 or 2, wherein each of the plurality of fused cast refractory blocks is substantially free of cracks.

4. The electrolytic cell as claimed in any one of claims 1 to 3, wherein each of the plurality of fused cast refractory blocks is obtained by casting a fused castable composition in a casting mold, the fused castable composition comprising, in weight %, about 55 % to about 95 % of said Al2O3 having the beta alumina concentration of about 40 to about 60%, and about 5 % to about 45 % of the other oxides.

5. The electrolytic cell as claimed in claim 4, wherein casting the fused castable composition in the casting mold comprises cooling the fused castable composition in the casting mold at a given temperature cooling ramp.

6. The electrolytic cell as claimed in claim 5, wherein each of the plurality of fused cast refractory blocks has a higher concentration of alpha alumina at a surface of the block that is adjacent the casting mold during casting compared to the concentration of alpha alumina inside the block below its surface.

7. The electrolytic cell as claimed in any one of claims 1 to 6, wherein each of said sidewalls comprises a bottom side block with a first surface directly exposed, at least in part, to aluminum, the electrolytic bath or both, and an upper side block with a second surface directly or indirectly exposed to the bath.

8. The electrolytic cell as claimed in claim 7, wherein the bottom side block is configured to support a lining material extending along the second surface of the upper side block for protecting the upper side block.

9. The electrolytic cell as claimed in claim 7 or 8, wherein the bottom side block has a rectangular shape with a longitudinal side defining the first surface directly exposed, at least in part, to aluminum.

10. The electrolytic cell as claimed in claim 9 when depending on claim 8, wherein the bottom side block has a top surface declining inwardly from the bath toward the upper side block, the upper side block comprising a bottom section having a rectangular shape for leaning against the adjacent rectangular shaped bottom side block and a top section having a triangular shape defining a width decreasing from the bottom section to an upper end of the top section, and wherein the top surface of the bottom side block and the the second surface of the upper side block form an angle of about 90° for matching and supporting the lining material extending along the second surface of the upper side block.

11. The electrolytic cell as claimed in any one of claims 7 to 10, wherein the bottom side block and the upper side block form a monolithic block, the monolithic block having a stepped shape with a bottom section advancing into the bath towards aluminum.

12. The electrolytic cell as claimed in claim 7 or 8, wherein the bottom side block has a shape of a right triangle with its hypotenuse side forming the first surface directly exposed, at least in part, to aluminum, and the vertical side adjacent the right angle of the right triangle extending, at least in part, from a bottom section of the upper side block.

13. The electrolytic cell as claimed in claim 12, wherein the bottom side block and the upper side block form a monolithic block, the monolithic block having a triangular shape of a right triangle with a surface of its hypotenuse advancing towards the bath and aluminum.

14. The electrolytic cell as claimed in claim 13, wherein the cell further comprises a lining material covering, at least in part, the surface of the monolithic block exposed to the bath.

15. The electrolytic cell as claimed in any one of claims 7 to 14, wherein a concentration of the alpha alumina is maximized at the second surface of the upper side block.

16. The electrolytic cell as claimed in any one of claims 1 to 6, wherein for each of said sidewalls, the cell comprises a first side block having a first surface directly exposed to both aluminum and electrolytic bath, and a second side block outwardly extending from and along the first side block.

17. The electrolytic cell as claimed in claim 16, wherein a concentration of the alpha alumina is maximized at the first surface of the first side block exposed to the aluminum and bath.

18. The electrolytic cell as claimed in claim 16 or 17, wherein the first side block is free of additional lining material for facing the aluminum and bath.

19. The electrolytic cell as claimed in any one of claims 1 to 18, wherein the at least one anodes of the electrolytic cell are inert or oxygen evolving anodes for an ecofriendly production of aluminum.

20. A method for manufacturing a fused cast refractory block for lining a sidewall of a cell reservoir of an electrolytic cell used for the production of aluminum by reaction of alumina, the method comprising:casting a fused castable composition in a casting mold by cooling the fused castable composition in the casting mold at a given temperature cooling ramp, wherein the fused castable composition comprises, in weight %:about 55 % to about 95 % of Al2O3 comprising alpha and beta alumina, wherein a concentration of the beta alumina in the fused cast refractory block is of about 40 to about 60%, and about 5 % to about 45 % of other oxides,wherein the fused cast refractory block is impervious to molten electrolyte bath and aluminum, is resistant to dissolution into said molten bath electrolyte and aluminum, and has a high electrical resistivity.

21. The method as claimed in claim 20, wherein the other oxides comprise at least one oxide selected from the group consisting of Na2O, SiO2, Cr2O3, Fe2O3, and MgO.

22. The method as claimed in claim 20 or 21, wherein the fused cast refractory block obtained by the method is substantially free of cracks.

23. The method as claimed in any one of claims 20 to 22, wherein the obtained fused cast refractory block has a higher concentration of alpha alumina at a surface of the block that is adjacent the casting mold during casting compared to the concentration of alpha alumina inside the block below its surface.

24. Fused cast refractory blocks as defined in any one of claims 1 to 17, or manufactured by the method as claimed in any one of claims 20 or 21, wherein the refractory blocks are impervious to molten electrolyte bath and aluminum, are resistant to dissolution into said molten bath electrolyte and aluminum, and has a high electrical resistivity.

25. Fused cast refractory blocks as claimed in claim 24, wherein the refractory blocks are substantially free of cracks.

26. Fused cast refractory blocks as claimed in claim 24 or 25, wherein each of the refractory blocks has a higher concentration of alpha alumina at a surface of the block that is adjacent the casting mold during casting compared to the concentration of alpha alumina inside the refractory block below its surface.

27. Use of at least one fused cast refractory block as defined in any one of claims 1 to 17, or manufactured by the method as claimed in any one of claims 20 to 23, for lining sidewalls of a cell reservoir of an electrolytic cell used for the production of aluminum by reaction of alumina.  

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