Formation of Lining Layers in the Cathode Shells of Aluminium Electrolytic Reduction Cells

Abstract

The invention relates to non-ferrous metallurgy and the electrolytic production of aluminium, and can be used for lining the cathode assembly of an electrolytic cell. The present method consists in laying materials while simultaneously distributing same over the surface of a base and levelling them at a height measured from the plane of the top edge of the shell of the cathode assembly of the electrolytic cell by gradually moving a device for installing unformed lining materials along the longitudinal axis of the cathode of the aluminium electrolytic cell. Said device is configured in the form of a bridge equipped with a mechanical drive for movement. The bridge has guides on which a frame is mounted for vertical movement, said frame having cassettes provided with gates with a mechanical drive. The technical result is reduced labour costs, healthier working conditions for operatives, and better quality installation of the base of an electrolytic cell.

Claims

1. A method of forming one or more lining layers in the cathode shell of an aluminum electrolysis cell, wherein one or more layers of at least one lining material is poured onto the bottom of the cathode shell, each layer is spread and leveled over the surface of the cathode shell, characterized in that a layer of the lining material is poured and simultaneously spread and leveled over the surface of the cathode shell by means of a belt-roller sector gate, wherein leveling is carried out at a preset level determined by the plane of the upper edge of the cathode shell of the aluminum electrolysis cell, wherein one or more lining layers are formed in succession with similar or different physical and performance properties specified according to the process.

2. The method of claim 1, characterized in that the lining material layer is poured, spread over the surface of the cathode shell and leveled at a rate of 0.2-0.9 m/min.

3. The method of claim 1, characterized in that the rate of pouring the layer, as well as the parameters of spreading and leveling the layer, are additionally controlled, and the operating conditions are adjusted as necessary.

4. A device for forming lining layers in the cathode shell of an aluminum electrolysis cell configured as a supporting metal structure fixable on the longitudinal sides of the cathode shell and sequentially moved along the longitudinal axis of the cathode shell, comprising longitudinal and transverse beams as well as vertical guides whereon a frame is mounted and configured for vertical movement, at least one cassette with a lining material is fixed on the frame and provided with a belt-roller gate in the lower part with a mechanical actuator designed to be controlled for pouring the lining layer onto the surface of the cathode shell while simultaneously spreading and leveling the layer.

5. The device of claim 4, characterized in that the mechanical actuator consists of two drive wheels receiving rotation from a gear motor mounted between the drive wheels by means of chain gears equipped with tensioners designed to ensure the reverse motion.

6. The device of claim 4, characterized in that discretely adjustable thrust rollers are fixed on the metal structure.

7. The device of claim 4, characterized in that smoothly adjustable guide rollers are installed at the fixing points of the frame with vertical guides.

8. The device of claim 4, characterized in that traction screws are pivotally suspended on the guides and engage with nuts pivotally mounted to the frame.

9. The device of claim 4, characterized in that the gate driven by a mechanical actuator is configured on the lateral surface in the lower part of the cassette.

10. The device of claim 4, characterized in that it includes a control panel designed to control the movement and pouring of lining layers from the cassettes.

11. The device of claim 4, characterized in that the cassette is configured as a bin.

12. The device of claim 4, characterized in that the gap between the cassette gate and the bottom of the cathode shell is equal to the thickness of the lining material layer being laid.

13. The device of claim 9, characterized in that the cassette gate is configured as belt-roller sections.

14. The device of claim 4, characterized in that the edge of the gate is the outermost roller, whereon a circular elastic belt is installed having a width equal to the roller length, wherein the rollers covered with the circular elastic belt block the outlet window of the cassette with the material, and the elastic belt ensures tightness.

15. The device of claim 4, characterized in that the gate consists of a pivoting shaft with sector plates rigidly fixed on its ends, whereon the rollers in the circular rubber belt are fixed such that when the shaft rotates, the rollers roll over the surface of the cassette to open or close the outlet orifice.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The essence of the proposed method of forming the lining layers in the cathode shells of aluminum electrolysis cells and the device used for its embodiment are illustrated by the specific exemplary embodiments of the method and the device design (FIG. 1-8).

[0037] FIG. 1 shows the device for forming lining material layers—Section A-A.

[0038] FIG. 2 shows the device in its operating position.

[0039] FIG. 3 shows the general view of the device.

[0040] FIG. 4 shows the view along arrow B.

[0041] FIG. 5 shows the view of thrust rollers along arrow C.

[0042] FIG. 6 shows detail section I indicated in FIG. 2.

[0043] FIG. 7 shows detail section II indicated in FIG. 1.

[0044] FIG. 8 shows the general view of the cassette.

[0045] The device for forming lining material layers (hereinafter referred to as the “device”) comprises a supporting metal structure configured as a bridge 1, which is a spatial metal structure, whereon two longitudinal beams 2 and two transverse beams 3 are mounted. The bridge 1 is mounted on the transverse beams 3 with mechanical actuators 4 for moving the device along the longitudinal axis of the cathode of the aluminum electrolysis cell and forming layers of lining materials. Scaffolds 5 and 6 with railings 7 and 8, respectively, are installed along the perimeter of the bridge 1. The bridge 1 comprises guides 9, whereon a frame 10 is arranged for vertical movement with cassettes 11, each equipped with a gate 12.

[0046] Mechanical actuators 4 mounted on the transverse beams 3 of the bridge 1 consist of two-stage gear motors 13, chain gears 14, tensioners 15 for reverse gears, and wide drive wheels 16 for the translational movement of the device. The drive wheels 16 are designed wide with limiting flanges to enable their use on electrolysis cells of different widths. For the device alignment during movement with respect to the longitudinal sides 17 of the cathode shell of the electrolysis cell, discretely adjustable thrust rollers 18 are provided in the bridge 1, which are pressed against and roll over the sides of the cathode shell, and are set by levers 19 with retainers 20 depending on the type of the electrolysis cell.

[0047] The frame 10 is equipped with two sets of smoothly adjustable guide rollers 21 to enable its clearance-free vertical movement along the guides 9 of the bridge 1. A mechanism for raising and lowering the cassettes is arranged on the guides of the metal structure 9. It consists of pivotally suspended traction screws 22 that engage with nuts 23, pivotally mounted on the frame 10. The screws 22 are rotated using flywheels 24.

[0048] Each cassette 11 is provided with a belt-roller sector gate 12 (FIG. 2, 8) equipped with a mechanical actuator 25.

[0049] The cassette 11 is a bin configured as a prism in the upper part and as a truncated wedge reinforced with stiffeners in the lower part.

[0050] The belt-roller sector gate 12 with the mechanical actuator 25 comprises a gear motor 27, a drive sprocket (not shown in the figures) arranged on the output shaft of the gear motor, a chain gear 28, a driven sprocket 29 and a pivoting shaft 30, and provides an outlet for unformed lining materials through a window in the lower part of the cassette. The belt-roller sector gate 12 is also equipped with a bank of rollers 31 covered with a circular conveyor belt 32 that bears against the outlet window of the cassette 11, thus preventing the material from spilling and reducing the force required to open and close the window.

[0051] Slide gates, disk valves or simple valves may be an alternative, but the belt-roller sector gate is more reliable and has a simpler design.

[0052] The gate is preferably composed of a pivoting shaft with sector plates rigidly fixed on its ends. Rollers that are in a circular rubber belt are fixed on the plates. When the shaft rotates, the rollers roll over the surface of the cassette and open or close the outlet orifice.

[0053] The device is controlled by buttons and switches arranged on a panel 33, which may be secured to the outer side of the railing 8 of the bridge 1.

EMBODIMENTS OF THE INVENTION

[0054] The method comprises laying the materials while simultaneously spreading them over the surface of the base, and leveling them at a level measured from the plane of the upper edge of the cathode assembly shell of the electrolysis cell by successively moving the device for installing unformed lining materials along the longitudinal axis of the cathode of the aluminum electrolysis cell. Two and/or more lining layers with variable physical and performance properties are formed in succession. The device is configured as a bridge equipped with a mechanical actuator for movement and provided with scaffolds and railings along the perimeter. The bridge has guides on which a frame is arranged for vertical movement with cassettes, each equipped with a gate. The mechanical actuator of the bridge is mounted on both ends, each having two wide drive wheels receiving movement from the gear motor using chain gears equipped with tensioners for reverse gears. The bridge is equipped with discretely adjustable thrust rollers. The frame is equipped with two sets of smoothly adjustable guide rollers. Traction screws are pivotally suspended on the frame guides and engage with nuts pivotally mounted to the frame. Each cassette is provided with a belt-roller sector gate equipped with a mechanical actuator.

[0055] The proposed method of forming lining layers in the cathode shells of aluminum electrolysis cells using unformed lining materials is implemented with a device designed for the same purpose as follows.

[0056] The device, comprising two longitudinal beams 2, two transverse beams 3, and the railing 8, is arranged on the longitudinal sides 17 of the cathode shell of the electrolysis cell. The bridge 1 is aligned by pressing the rollers 18 against the inner surface of the longitudinal sides 17 by turning the levers 19 and setting the retainers 20 in the closest slots (not marked by a reference number in FIG. 5). The guide rollers 21 are set on the frame 10 by moving them on inclined planes until their contact with the guides 9 of the bridge 1 and by fixing them. This allows for free and clearance-free vertical movement of the frame 10. A gap equal to the thickness of the first layer 34 being laid is set between the belt-roller sector gate 12 of the cassette 11 and the bottom of the cathode shell of the electrolysis cell. The gap is set by rotating the traction screw 22 with the flywheel 24 by raising or lowering the frame 10 with the cassettes 11. The cassettes 11 are removed from the frame 10 with the shop crane and positioned in the place (not shown in the figures) where the cassettes are filled with a relevant unformed lining material required to form the first lining layer 34. After filling, the cassettes are reinstalled into the frame using the shop crane.

[0057] The cable connectors (not shown in the figures) of the cassettes 11 are connected to the appropriate receptacles on the control panel 33, and the control panel is connected to the 50 Hz 380 V three-phase AC power supply. The gear motors 13 of the mechanical actuators 4 of the bridge 1 are started on the control panel 33. The torque from the output shafts of the gear motors 13 is transmitted via the chain gears 14 to the driven sprockets arranged on the shafts of the wide drive wheels 16. The device is moved along the longitudinal sides 17 of the cathode shell of the electrolysis cell. During movement, slight slippage of the wide drive wheels 16 of the bridge 1 may occur, skewing the device. By using a converter to change the frequency of the alternating current feeding the electric motors of the gear motors 13 of the mechanical actuators 4 of the bridge 1, the device aligned with the thrust rollers 18 is steered to ensure that it moves strictly along the longitudinal sides 17 of the cathode shell of the electrolysis cell.

[0058] The device is installed at one end of the cathode shell of the electrolysis cell. The control panel 33 is then used to start the gear motors 27 of the mechanical actuator 25, which drives the driven sprocket 29 and the pivoting shaft 30, which moves the belt-roller sector gate on free-wheeling rollers on which the circular conveyor belt 32 is installed. For convenience of filling the end zones of the cathode assembly, the belt-roller sector gate can be opened by the actuator in any direction. When the gate opens, the unformed lining material pours out and fills the space between the shell bottom and the gate surface.

[0059] The gear motors 13 of the mechanical actuators 4 of the bridge 1 are started on the control panel 33 so that the device moves to the opposite end of the cathode shell of the electrolysis cell and the first lining material layer 34 can be formed. A lining material layer is formed by two processes progressing simultaneously: pouring out the material and leveling the material with the gate surface.

[0060] When the first layer 34 is completed, the belt-roller sector gates 12 of the cassettes 11 are closed. The cassettes 11 are removed from the frame 10 with the shop crane and positioned in the place (not shown in the figures) where the unformed lining material used to lay the first layer is removed from the cassettes. When the cassettes 11 are filled with an unformed lining material 26 having other physical and performance properties (porosity, thermal conductivity, heat insulation) specified according to the technology and caused by the design features of the electrolysis cell, the cassettes with the material are reinstalled into the frame 1.

[0061] Note that barrier materials and heat-insulating materials have few similar properties and many differing properties. The table below lists examples of properties.

TABLE-US-00001 Thermal Operating Por- Conductivity Tem- Density, osity, Coefficient, Cryolite perature Materials kg/m3 % W/mK Resistance ° C. Refractory ~2,000 15-20 0.65 Good 1,350 Heat- 300-600 75-90 0.08-0.1 Bad 800-1,000 insulating

[0062] The main purpose of the lining of cathode assemblies of electrolysis cells is to provide the required temperature conditions in the inter-electrode space. This is achieved by installing the required heat insulation. However, bottom blocks are heterogeneous substances with a solid constituent that is well wetted with fluoride salts penetrating through open pores. This allows for the ingress of molten fluoride salts and aggressive fluorine-containing gases into underlying zones. Various barrier materials are used to protect the heat insulation. The requirements to barrier and heat-insulating materials are diverse and somewhat contradictory.

[0063] Traditionally, shaped products in the form of bricks of various sizes, primarily with aluminosilicate composition, are used in the structures of cathode assemblies of electrolysis cells as barrier materials to protect the underlying heat-insulating materials. This is due, primarily, to their relatively low cost and the properties of the resultant products of interaction with fluoride salts and sodium vapors. Modern high-quality barrier bricks for cathodes of aluminum electrolysis cells have a low apparent porosity (up to 13%) and small pore sizes to reduce the ingress of aggressive gaseous and liquid components into the heat-insulating layers. However, the gas permeability of the barrier masonry as a whole is determined not by the properties of individual bricks, but mostly by the condition of joints between them. Therefore, an alternative to masonry are unformed materials compacted directly in the cathode assembly.

[0064] The amount of fluoride salts penetrating a bather depends on the particle size distribution of the initial powdered mixture, the compaction method, and the conditions of the subsequent thermal and chemical exposure.

[0065] According to Darcy's law, the driving force of the process of penetration of molten fluoride salts is the pressure gradient along the height of the barrier material.

[00001]$\begin{array}{cc}q=-\frac{k}{\mu }.Math.\frac{\mathrm{dP}}{\mathrm{dx}},& \left(1\right)\end{array}$

[0066] where: q is the volumetric flow of fluoride salts through the cross-section S, m3/(m2s); k is the permeability coefficient, m.sup.2; dP/dx is the pressure gradient along the barrier material height, Pa; μ is dynamic viscosity, Pa*s.

[0067] The permeability coefficient included in equation (1) depends on the size and number of pores and can be estimated based on structural parameters: open porosity amount, pore size distribution, and the sinuosity coefficient of pores:

[00002]$\begin{array}{cc}\kappa =\frac{\mathrm{.Math.}.Math.{\stackrel{\_}{D}}^2}{32.Math.\tau },& \left(2\right)\end{array}$

[0068] where: ε is open porosity; D is the average pore radius; τ is the sinuosity coefficient of pores.

[0069] For polydisperse materials, if the following relationship is satisfied:

[0070] d.sub.min/d.sub.max≥3, D.sup.2 is calculated using the formula:

[00003]$\begin{array}{cc}{\stackrel{\_}{D}}^2=\frac{1}{\mathrm{.Math.}}.Math.\underset{d_{\min }}{\overset{d_{\max }}{\int }}.Math.D^2.Math.\varphi \left(D\right).Math.\mathrm{dD},& \left(3\right)\end{array}$

[0071] where: d.sub.min, d.sub.max is the minimum and maximum radii of pores, respectively; φ(D) is the size distribution of pores.

[0072] For large pores (more than 100 μm), the pressure gradient is mainly caused by hydrostatic and gravitational forces. For channel pores (5-25 μm in size), the pressure gradient is much higher than for large pores due to the potential energy of the field of capillary forces; such capillaries can actively absorb molten fluoride salts. If the pore sizes are smaller than the critical value determined using the relationship:

[00004]$\begin{array}{cc}{d}_{\mathrm{cr}}=0.2.Math.86.Math.\sigma .Math.\frac{\cos .Math..Math.\Theta }{\rho .Math..Math.\mathrm{gl}},& \left(4\right)\end{array}$

[0073] where: dcr is the critical pore size, m; σ is surface tension, N/m2; ⊖ is the wetting angle; ρ is density, kg/m3; g is gravitational acceleration, m/s2,

[0074] then the action of gravitational and hydrostatic forces on fluoride salts in capillaries can be neglected, and the pressure can be calculated using the formula:

[00005]$\begin{array}{cc}P=4.Math.\sigma .Math.\frac{\cos .Math..Math.\Theta }{d}.& \left(5\right)\end{array}$

[0075] For such channel pores in the form of thin cylindrical tubes wherein laminar flow conditions are realized with the predominance of viscous forces over inertial forces (Re<<1) in accordance with Hagen-Poiseuille's law, the volumetric flow rate per second is proportional to the capillary diameter to the fourth power:

[00006]$\begin{array}{cc}q=\frac{\pi .Math..Math.d^4}{8.Math.\mu }.Math.\frac{\Delta .Math..Math.P}{l},& \left(6\right)\end{array}$

[0076] where q is the volume of liquid flowing through the capillary cross-section per second; 1 and d are the capillary length and diameter, respectively; ΔP is the differential pressure, Pa.

[0077] Therefore, the hydraulic resistance to the flow of liquid is very high for such pores, and they are filled not by the capillary flow of the melt, but by the evaporation and condensation of vapors on pore walls.

[0078] For porous materials with evenly distributed and mutually disjointed pores in the form of cylindrical channels with a small cross-section, the permeability coefficient can be determined using the relationship:

[00007]$\begin{array}{cc}k=P.Math.\frac{d^2}{3.Math.2},& \left(7\right)\end{array}$

[0079] where: P is porosity; d is the pore size, m.

[0080] With a decrease in pore size, the amount of penetrating electrolyte components is reduced and the difference in the permeability coefficients caused by the different porosity values drops out of the equation. Therefore, barrier materials should have the densest structure possible and minimal porosity.

[0081] Heat-insulating materials, on the contrary, should have the highest porosity possible because the gases in pores have the best heat-insulating properties. Note that the thermal conductivity coefficient depends not only on the total porosity of a material, but also on the pore size and shape, the nature of the structure and the mineralogical composition. With a decrease in pore size, free convection in the pores of a heat-insulating material decreases, while its heat resistance and mechanical strength increase. That is why modern microporous heat-insulating materials with pores smaller than 0.1 μm have the lowest thermal conductivity under normal technical conditions.

[0082] As the temperature increases, the thermal conductivity coefficient of microporous materials becomes higher than that of materials with larger pores due to the increased fraction of energy transferred through the heat insulation structure by radiation. Therefore, there is an optimum pore size distribution depending on the temperature. For this reason, the number of heat insulation layers along the height of the sub-cathode space may be more than one. However, an excessive number of heat-insulating layers is undesirable due to the reduced workability. The formation of 2 or 3 heat-insulating layers is the most reasonable solution.

[0083] Inaccurate installation of lining layers can adversely affect the service life of electrolysis cells. It is important that the design of the cathode assembly and the lining materials provide a steeply dipping isotherm of the liquidus temperature of penetrating fluoride salts in the periphery, and it must be positioned horizontally in the center of the cathode assembly bath. The isotherm should be located outside the cathode block (to avoid sodium condensation, which destroys the cathode block structure), without entering the heat insulation layer.

[0084] Excessive heat insulation shortens the service life of electrolysis cells. “Overinsulation” causes higher temperatures of barrier materials and deeper penetration of fluoride salts down to the heat insulation. The impregnation of barrier materials with electrolyte components at early stages of electrolysis cell service increases their thermal conductivity coefficient and causes the restructuring of temperature fields, resulting in downward movement of the isotherm.

[0085] The less dense the barrier layer material, the deeper the isotherm moves down and the greater amount of the barrier material is found in the high temperature zone, being exposed to chemical action throughout its volume, resulting in volumetric changes that produce a vertical effect on the bottom blocks.

[0086] In view of the above, the amount of molten fluoride salts and aggressive fluorine-containing gases penetrating the barrier layers can be reduced by creating a mostly finely porous structure of barrier materials with pore sizes smaller than 3-5 μm to exclude the dangerous interval of capillary pores from the structure, by adding silicon-containing components to the unformed barrier material, and by selecting heat-insulating materials that provide optimum heat resistance of the base and the preset isotherm position. In each specific case, the dimensions of the functional layers may vary as determined by the electrolysis cell design and the type of lining materials used.

[0087] The operation cycle of the device is then repeated for each layer: a subsequent layer 35 having the process-specific thickness is formed with the unformed lining material 26.

[0088] When dispersed carbon materials are used as lining materials, reaction (1) can occur:

0.5N.sub.2+3Na+3C=3NaCN  (1).

[0089] Cyanides are environmentally hazardous substances, which can be suppressed by adding various substances to the lining materials. For example, boron trioxide can be used, which interacts with cyanides according to reaction (2):

3NaCN+6B.sub.2O.sub.3=2NaBO.sub.2+2Na.sub.2B.sub.4O.sub.7+2BN+6C  (2).

[0090] Another substance that destroys cyanides is aluminum oxides, which react with cyanides according to reaction (3):

1.5NaCN+3Al.sub.2O.sub.3+3Na=4.5NaAl.sub.2O.sub.3+1.5AlN+1.5C  (3).

[0091] Therefore, the composition of unformed materials can include materials that perform barrier functions both with respect to penetrating liquid and gaseous components, and with respect to temperature, as well as heat-insulating layers with different structures and chemical and mineralogical compositions.

[0092] This method of forming lining layers in the cathode shells of aluminum electrolysis cells and the device for its embodiment allow the combined functional-gradient structure of the electrolysis cell cathode assembly lining to be obtained. At temperatures of up to 400° C., materials having the lowest apparent density are the most effective, while denser heat-insulating materials with pores smaller than 10 μm have an advantage at temperatures above 600° C. Therefore, the method of forming lining material layers will be more efficient when two or more heat-insulating layers with variable thermophysical properties are formed in succession, as described above.

[0093] The optimum speed of the device for forming lining layers is 0.1-0.9 m/min. At a speed of less than 0.1 m/min, the device productivity decreases unreasonably, and when the speed is above 0.9 m/min, the quality of laying the lining material deteriorates and dusting of the lining material occurs.

[0094] The principle of leveling the material using the “tail” of the machine is well known from other arts, but in the proposed technical solution, the device is unique in its ability to change the “tail” to the “head” and vice versa. This is particularly important when the work is performed in the constrained environment of the cathode assembly. For example, given the operating position of the gate with the unit moving from left to right: in the initial state (rightmost), the gate is in the mirror position and the material is poured into the space between the gate and the cathode end; the gate is then set into its operating position and movement of the unit to the left is started. This allows movement in different directions.

[0095] In addition, with this gate, the height of the resultant layer may be increased or decreased.

[0096] The above method of forming the cathode shells of aluminum electrolysis cells with unformed lining materials and the device for its embodiment will produce a total economic effect of at least $4.14 thousand per 1 electrolysis cell annually by reducing the downtime of electrolysis cells in overhauls, extending the service life of electrolysis cells, and reducing labor inputs required to spread the material over the base surface. In addition, the method improves the sanitary working conditions for the personnel due to reduced dusting of the material.