Metal inert anode for aluminum production of by the electrolysis of a melt

Abstract

The design of a metal inert anode is proposed, it is made in the form of a perforated structure with through-openings, in particular formed by longitudinal and transverse anode elements intersecting each other and limited by the lateral sides of the intersecting anode elements, and contains vertical or inclined fins that protrude from the bath and are integrated with the anode elements or a current conductor. As a result, it ensures a reduction in the voltage drop in the anode and in the bubble layer under the anode, a reduction in the anode overvoltage and anode consumption, an increase in current efficiency and the reliability of the cryolite-alumina crust, which leads to an increase in the anode service life and promotes the formation of a reliable and durable cryolite-alumina crust above the melt surface, which improves process efficiency.

Claims

1. A metal inert anode for producing aluminium by the electrolysis of a melt which has multiple electrochemically active anode elements, current distributors and a current conductor, wherein: the anode contains no less than two vertical or inclined fins protruding from the bath, the anode is designed to be positioned horizontally, and the anode is made in a form of a perforated structure with through-openings distributed across the anode, the degree of anode perforation being 15-35% such that an area of a through-opening is 10-100 cm.sup.2.

2. The anode of claim 1, wherein the degree of anode perforation is about 20%.

3. The anode of claim 1, wherein the no less than two vertical or inclined fins are integrated with the current conductor.

4. The anode of claim 1, wherein the no less than two vertical or inclined fins support formation of a reliable and durable cryolite-alumina crust above a surface of the melt, wherein a height of the no less than two vertical or inclined fins is such that they protrude from the bath by about 5-20 cm.

5. The anode of claim 4, wherein an integrity of the reliable and durable cryolite-alumina crust is supported by the no less than two vertical or inclined fins and the current conductor located above the melt surface.

6. The anode of claim 1, wherein the anode contains longitudinal and transverse anode elements intersecting each other and forming a perforated anode structure with through-openings limited by lateral sides of the intersecting anode elements.

7. The anode of claim 6, wherein the no less than two vertical or inclined fins protruding from the bath are integrated with the anode elements.

8. The anode of claim 6, wherein the anode elements are made in a form of straight or curved rods, bars or plates with a cross section in a form of a polygon with rounded corners, an ellipse, or a circle and are located in the same plane.

9. The anode of claim 6, wherein the longitudinal and transverse anode elements intersect at a right angle.

10. The anode of claim 6, wherein the longitudinal and transverse anode elements intersect at an angle different from a right angle.

11. The anode of claim 6, wherein the anode has no less than one current distributor connected to the anode elements.

12. The anode of claim 11, wherein the anode has no less than one current conductor connected to the current distributors.

13. The anode of claim 6, wherein the distances between the longitudinal anode elements and between the transverse anode elements are the same.

14. The anode of claim 6, wherein the distances between the longitudinal anode elements and between the transverse anode elements are different.

15. The anode of claim 6, wherein the anode elements have rounding at the points of intersection.

16. The anode of claim 1, wherein the anode is manufactured by metal or sand mould casting.

17. A metal inert anode for producing aluminium by the electrolysis of a melt, which has multiple electrochemically active anode elements, current distributors and a current conductor, wherein: the anode is made in the form of a perforated structure formed by longitudinal and transverse anode elements intersecting each other and limited by lateral sides of the intersecting anode elements, the anode contains vertical or inclined fins that protrude from the bath and are integrated with the anode elements or the current conductor, the anode is designed to be positioned horizontally, the degree of anode perforation being about 15-35% such that an area of the anode perforation is about 10-100 cm.sup.2.

18. The anode of claim 17, wherein: the degree of anode perforation is preferably 20% with the area of an opening being about 0.001 m.sup.2 or the degree of anode perforation is preferably about 20%, with the area of an opening being preferably about 50 cm.sup.2.

19. A cell for producing aluminium by the electrolysis of a melt, which contains a metal inert anode, wherein the metal inert anode is an anode made according to any one of claims 1-18.

Description

BRIEF DRAWING DESCRIPTION

(1) FIG. 1—Example of an embodiment of the perforated anode under the proposed invention; FIG. 2—Example of the installation of the proposed perforated anode in a cell.

(2) FIG. 1 shows a metal inert anode that, according to the invention, has an optimal design, including longitudinal (1) and transverse (2) anode elements, vertical fins (3) and a current conductor (4). The intersecting longitudinal (1) and transverse (2) anode elements in the form of rectangular-sectioned bars form a perforated anode structure with through-openings (5) limited by the lateral sides of the intersecting anode elements. The vertical fins (3) are integrated with the current conductor (4) and the anode elements (1) and (2), which allows improving the strength of the anode structure and the current distribution across the anode.

(3) FIG. 2 shows the metal inert anode of the optimal design installed in a cell for aluminium production. During cell operation, aluminium (7) is deposited and accumulated on its carbon bottom (6), and oxygen bubbles are released on the lower anode surface. Aluminium is deposited and oxygen is released when the direct current passes through the cell as a result of the decomposition of alumina (aluminium oxide) dissolved in the bath melt (8). A cryolite-alumina crust consisting of the crystallized components of the bath and alumina is located above the bath surface (8) with a small gap (9). During the operation of the cell, oxygen bubbles, which release on the lower surface of the elements (2) of the perforated anode, pass via the through-openings (5) and escape into the gap (9) between the bath and the crust (10). In case there are no openings (5), bubbles will accumulate under the anode, which will result in the growth of voltage in the cell and the oxidation of aluminium (7). The elements (2) of the rectangular-sectioned anode are fully submerged into the bath melt, and the fins (3) and the current conductor (4) protrude from the bath. Therefore, the integrity of the crust is supported by the fins (3) and the current conductor (4) above the melt surface. If there are no fins, the crust will collapse downwards on the anode elements (2) submerged into the bath. As a consequence, the openings (5) will be blocked preventing the escape of oxygen bubbles, melt circulation will be disrupted, the cell voltage and the temperature of the molten bath will increase, i.e. there will be a significant disruption of the aluminium production process. The fins (3) are structurally integrated with the anode elements (2) and the current conductor (4). Therefore, the electric current passes from the current conductor (4) via the fins (3) and is uniformly distributed across the anode elements (2) allowing reducing the voltage drop in the anode and the overvoltage of oxygen evolution.

(4) The essence of the invention is that it is proposed to optimize the perforation of the anode to improve the escape of oxygen bubbles from under the anode in order to reduce the cell voltage, thus, to reduce the voltage drop in the bubble layer and, simultaneously, achieve the minimum possible increase in the anode current density (in order to ensure low anode overvoltage, a low voltage drop in the anode and a low anode consumption). The higher is the degree of anode perforation, i.e. the higher is the share of the area occupied by openings, the easier escape gas bubbles from under the anode (from the inter-electrode space), the smaller is the thickness of the bubble layer and the less is the voltage drop therein. Besides, the smaller is the thickness of the bubble layer under the anode, the smaller is the oxygen oxidation of the aluminium metal produced, which is accumulated on the cell bottom and is the cathode. Hence, a decrease in the thickness of the bubble layer increases current efficiency and reduces specific energy consumption.

(5) On the other hand, the higher is the perforation of the anode, the less is the anode surface area and the higher is the anode current density.

(6) It is known that an increase in the anode current density results in an increase in the anode overvoltage and anode consumption.

(7) Besides, an increase in the perforation of the anode leads to an increase in the current density in the anode itself and, consequently, the voltage drop in the anode decreases. This is also accompanied by current distribution deterioration across the anode resulting in changes to the current density in various anode areas and, as a result, non-uniform anode consumption.

(8) Thus, when increasing the degree of perforation, the reduction in the cell voltage will continue until a certain optimal value of the degree of perforation has been achieved. To achieve the technical result, it is necessary to solve the problem of optimizing the degree of anode perforation and the size of openings.

(9) A similar problem was being solved for anodes in cells for the production of chlorine and caustic soda [L. M. Yakimenko. Electrode materials in applied electrochemistry. M., ‘Chemistry’, 1977, page 264]. In case low-wearing anodes are used in cells with a mercury cathode, and electrodes are positioned horizontally, it is necessary to provide for the drainage of chlorine released on the anode from the zone where the current passes. To this end, various designs of plate electrodes, as well as electrodes made of perforated sheets have been developed. The issue of finding an optimal degree of perforation for a horizontal sheet anode was studied based on a model of a cell with a mercury cathode operating with the use of a NaOH aqueous solution. The dependence of the degree of perforation on voltage was determined for an anode with openings 6-8 mm in diameter. The minimum voltage values were observed at a degree of perforation of 35-40% (at all current density values). Besides, it was established that the slope of the dependence of the cell voltage on the current density also decreases with an increase in the degree of perforation.

(10) At the same degree of anode perforation, as the diameter of perforation openings decreases, the total anode surface (including the internal surface of openings) increases, and the path of gas bubbles from the point of generation to the edge of an opening becomes shorter. Besides, at a smaller diameter of an opening, the electric field between the electrodes is of a more uniform nature, and the effective resistance of the bath is smaller in this case than in the case of a larger diameter of an opening. However, the voltage decreases (as the diameter of openings decreases) only down to a certain point. At small diameters of perforation openings, the hindered gas' ability to escape is explained by the fact that gas bubbles stay in openings due to the surface tension forces forming plugs.

(11) To determine the influence of the diameter of perforation openings on the conditions of gas extraction, the following anodes were studied: anodes with the same degree of perforation (35%) and various diameters of perforation openings, anodes 3 mm thick perforated with openings with a diameter of 2, 4, 6, 8 and 12 mm, and anodes 10 mm thick perforated with openings with a diameter of 4, 6, 8 and 12 mm. The centres of perforation openings are located at the corners of a regular triangular grid (<60°). At an electrode thickness of 3 mm, the smallest voltage values were obtained for a diameter of perforation openings of 4 and 6 mm, and at an anode thickness of 10 mm, such voltage values were obtained for a cell with an anode perforated with openings 6 mm in diameter; the voltage is higher (by 20-40 mV) in cells with anodes perforated with 4- and 8-mm openings. If electrodes have approximately the same voltage, then electrodes perforated with larger diameter openings should be recommended for industrial application (since they are easier to manufacture). For industrial electrodes about 10 mm thick, perforation with openings 6-8 mm in diameter can be recommended; anodes 3-5 mm thick should be perforated with openings about 6 mm in diameter.

(12) A formula was derived to calculate the limit value of the opening diameter, at which the gas might still stay in openings:

(13) $d_{\mathrm{np}}=\sqrt{2.25b^2+\frac{12\sigma }{\gamma }}-1.5b$

(14) where d— the diameter of openings; σ— surface tension of a solution, b — thickness of an anode sheet; γ— solution density.

(15) The σ/γ value for sodium chloride solutions with a concentration of 250-300 g/l varies within 6.0-6.7 mm.sup.2 at 60-100° C. The limit values of the diameter of perforation openings for a sheet anode 3 mm thick will be 5.1-5.5 mm under these conditions, and for an anode 10 mm thick they will be 2.2-2.5 mm.

(16) It is apparent that for anodes 10 mm thick the calculated values of the opening diameter significantly differ from the optimal values found in real life. The reason for this is that as the diameter of an opening decreases (and the degree of perforation is the same), there is an increase in the hydrodynamic resistance to the flow of gas bubbles (together with the fluid they are entraining) from under the anode via perforation openings.

(17) The reason for the non-obviousness of the proposed solution is that it is impossible to determine any optimal degree of perforation and the size of openings in an inert anode for producing aluminium by the electrolysis of melts based on the known data since the bath properties (electrical conductivity, viscosity, density, surface tension), the size of bubbles and the hydrodynamics of two-phase flows are very much different.

(18) Besides, it is necessary to consider that the size of openings in an inert anode for aluminium production significantly changes over time since a protective oxide layer is formed and growing on the surface of metal anodes.

(19) To correctly calculate an optimal degree and diameter of perforation, it is also necessary to calculate gas-hydrodynamic circulation flows for a two-phase gas-fluid flow, which is a difficult task related to developing a proper mathematical model and verifying it based on measurement results obtained from real systems and physical models.

(20) Since it is problematic to carry out large-scale experiments in the melt to determine an optimal degree of perforation and the size of openings, simulations (mathematical and physical) are the most rational way of solving the problem related to the reduction of the voltage drop in the anode and in the bubble layer under the anode, as well as to the reduction of anode overvoltage.

(21) Modelling included the development of two- and three-dimensional two-phase models of bubble flows to describe the electric field and the hydrodynamic processes in the inter-electrode space of the cell, including gas evolution on the anode: electrochemical processes of gas formation on the surface of an inert anode, two-phase models of a bubble flow, models of the electric field in the working zone of the cell with consideration of the amount of gas in the bath.

(22) Thus, the developed mathematical model is based on a system of two coupled elliptical equations for the electric potential and GVF (gas volume fraction) and hydrodynamics equations (equations for velocity components and a continuity equation). The system of equations is coupled. In particular, the electric field depends on the gas content; the gas content depends of the flow of the gas-filled bath, etc. The system of equations is non-linear.

(23) For implementing the said mathematical model of gas evolution on the anode in an aluminium reduction cell, a computational algorithm was developed for solving a two- and three-dimensional stationary problem related to the gas content, which is based on finite element approximation in space and the iterative solution of a nonlinear coupled system of equations by the Newton method.

(24) Calculations were made with the use of the application software developed. The model was verified based on the results of measuring the gas content and the size of bubbles in an experimental electrolytic cell. The perforation of an inert anode was optimized based on the results of multiple-parameter three-dimensional calculations (which were based on grid models, considering the real geometry of an inert anode) made with the help of the application software developed. For the calculations, the anode size was set to be 1×1 m.sub.2 and 0.06 m thick. The anode is uniformly perforated in two directions with circular-sectioned openings. The calculations were made at the distance between the anodes equal to 0.1 m in transverse direction and equal to 0.2 m in longitudinal direction. The distance between the anodes and the cell sidewall was taken to be equal to 0.2 m. The inter-electrode distance was 0.06 m.

(25) The optimality criterion was the current (amperage) passing through the anode at the fixed voltage drop, in fractions, with respect to the anode without perforation and without considering the gas content of the bath (I.sub.0). The smaller is this parameter, the worse, since the cell voltage will be higher at the same amperage.

(26) The following parameters were selected as variable: the number of openings 36-100, the degree of perforation 0-30% and the diameter of circular-shaped openings 0.04-0.10 m:

(27) TABLE-US-00001 Degree of Number of perforation I/Io (w/o the gas I/Io (with the gas openings % (d, m) content in the bath) content in the bath)  0 0% 1.000 0.246 36 4.5% (0.04)  0.999 0.926 6 × 6 10% (0.06) 0.994 0.944 18% (0.08) 0.985 0.960 28% (0.10) 0.970 0.956 64  8% (0.04) 0.998 0.962 8 × 8 20% (0.06) 0.990 0.973 32% (0.08) 0.973 0.965 100  12.6% (0.04)   0.996 0.979 10 × 10 19.6% (0.05)   0.991 0.980 28.3% (0.06)   0.984 0.976

(28) It is evident from the table above that if the gas content in the bath is not taken into account, then the anode perforation leads to a decrease in the amperage, which is due to the growth of the voltage drop in the anode because of a decrease in the area of the anode. However, under conditions of gas evolution, the current (amperage) on a non-perforated anode is reduced about 4 times (due to an increase in the voltage drop in the gas-filled layer of the bath under the anode). Under conditions of gas evolution under the anode (for increasing the amperage passing through an anode), the optimal degree of perforation is 20%, and the optimal diameter of an opening is about 0.04 m, which corresponds to an opening area of about 0.001 m.sup.2, taking into account that the shape of an opening may differ from a round one.

(29) Despite the fact that the optimization, for the sake of simplicity, was carried out for round openings, the shape of openings may be in the form of a polygon with rounded corners, whose area and dimensions approximately correspond to the area and diameter of round openings.

(30) An additional feature of the invention is that vertical or inclined fins are provisioned in the design of the perforated metal anode (whose main part is submerged into the bath) in order to form a reliable and durable cryolite-alumina crust. The optimal height of these fins is such that they protrude from the bath to a height of about 5-20 cm. Thus, when using the proposed anodes, not only the anode current conductors protrude from the bath but also the fins. This allows shortening the distance between the anode elements protruding from the bath. Thus, the fins divide the crust into small areas, which reduces the risk of its collapse. Besides, the protruding fins promote crust strengthening since they carry the heat away and reduce the crust temperature near the fins. Therefore, the risk of crust melting and collapsing is reduced.

(31) The anode fins are integrated with the anode current conductor and/or the perforated part of the anode. When the fins are simultaneously integrated with both the anode current conductor and the perforated part of the anode, it leads to a better structural integrity of the anode and a better current distribution across the anode, but, in the meantime, the anode weight and material consumption become higher. When increasing the number of fins, crust reliability will also be increased since the distance between the fins is shorter and, accordingly, the crust extension is less, as a result, the crust is better cooled and, hence, stronger.

(32) The fins can also serve to control bath circulation flows caused by the movement of oxygen bubbles upwards (for the purpose of improving the dissolution of alumina at the bath surface and delivering the alumina-enriched bath to the lower working surface of the anode). To this end, the angle of inclination and the location of the fins may vary to ensure a directed movement of the bath to the alumina feeding points.

(33) Industrial tests of the perforated anodes with the fins, compared to the anodes under the prior-art, have showed that the proposed design solutions are efficient in terms of reducing the cell voltage and ensuring a reliable crust above the bath.