The cathode of the electrolytic cell for primary aluminum production consists of electrically conductive cathode blocks that are thermally insulated from below. There is a layer of barrier refractory materials between the cathode blocks and the thermal insulation; these materials are designed to prevent penetration of fluoride salts and sodium vapors into the thermal insulation layers. The process of infiltration of the liquid phase of components of the bath from the bottom blocks into the refractory materials, as well as their interaction, is a complex phenomenon, which involves both physical and chemical interactions at the liquid melt interface between NaF/Na3AlF6 and refractory materials. The structure of the refractory material is the primary factor in the indicated interaction.
According to Darcy's law, the driving force for penetration of molten fluoride salts into the barrier materials is a pressure gradient along the height of a barrier material.
                    q        =                              -                          k              μ                                ⁢                      dP            dx                                              (        1        )            
where: q—volumetric flow rate of molten fluoride salts through the cross section (S), m3/(m2s); k—permeability coefficient, m2; dP/dx—pressure gradient along the height of the barrier material, Pa; μ—dynamic viscosity, Pa*s.
Since barrier materials are heterogeneous structures with different pore-size distributions, then, the range of pore sizes can be conventionally divided into three areas. For large pores (greater than 100 microns) the pressure gradient is primarily determined by hydrostatic and gravitational forces. For smaller channel pores, along with the aforementioned forces, capillary forces begin to appear. Due to the potential capillary action energy, the pressure gradient is much higher than that for large pores, and such capillaries are able to rapidly absorb melted fluoride salts. The depth of penetration of molten fluoride salts may be determined by the ratio arising from Poiseuille's law:
                    h        =                                            d              ⁢                                                          ⁢              σ              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θτ                                      4              ⁢              η                                                          (        2        )            
where: h—depth of penetration; d—diameter of pores; σ—surface tension; η—melt viscosity.
With a further reduction of pore sizes, there is an increase in the pressure gradient (caused by capillary action), but, on the other hand, the hydraulic resistance to fluid flow is growing much faster; therefore, penetration of fluoride salts through such pores can be neglected.
As it follows from equation (2), the depth of penetration of the fluorinated melt decreases with an increase in melt viscosity, a decrease in surface tension and a decrease in the contact (wetting) angle. The physical and chemical characteristics of the melt, which are part of equation (2), depend on the temperature and composition of the melt.
At the initial stage of the penetration process, the main component in the area under the cathode is NaF. It can be explained by the following reaction taking place within the body of the cathode block during cryolite infiltration:4Na3AlF6+12Na+3C=Al4C3+24NaF  (3)
Interaction between pure alumina refractories and sodium fluoride is taking place as per the β-alumina formation reaction:12NaF+34Al2O3=3(Na2O*113Al2O3)+2Na3AlF6  (4)
Thus, due to a significantly lower density of the β-alumina reaction product, volumetric changes occur in the lining, causing vertical stresses in the bottom and its possible destruction. When a relatively small amount of SiO2 (˜25%) appears in the refractory, in addition to reaction (4), the following formation reaction for nepheline will occur (5):6NaF+2Al2O3+3SiO2=3NaAlSiO4+Na3AlF6  (5)
If there is an excess of the refractory material and a small amount of NaF, nepheline reacts with silicon dioxide to form albite, NaAlSi3O8, which will be in the glassy viscous molten state to prevent further movement of the interaction front down to the lower part of the cathode in the electrolytic cell:NaAlSiO4+2SiO2=NaAlSi3O8  (6)
An increase in melt viscosity due to the presence of albite in the reaction zone between the aluminosilicate refractory lining and molten cryolite reduces the likelihood of the penetration of fluoride salts into the lower insulating layers of the pit.
As a result of the further increase in the SiO2 content in the aluminosilicate refractory material (above 47%) β-alumina is not present in the reaction zone, and albite and nepheline are formed by the combination of reactions (5) and (6). At a very high SiO2 content (72%), due to insufficient Al2O3, nepheline formation will be difficult.
Therefore, among a significant number of refractories used in the pit, the most widely used materials are aluminosilicate-containing materials with 28%<Al2O3<34%, their relatively low cost being one of the important factors.
The above shows that barrier materials with thin and serpentine channels, having a dense (particle-to-particle) packing of small-sized particles, are characterized by low gas permeability and, obviously, by slow penetration of molten fluoride salts or products of their reaction into barrier materials. In addition, the presence of a temperature gradient in the direction of the penetration along with the increase in melt viscosity due to the formation of albite, will also slow down the penetration process.
Traditionally, shaped materials, in the form of bricks of different size, are used for lining the cell's cathode; preferably, these are aluminosilicate bricks having low porosity and low gas permeability. However, the permeability of the barrier brickwork is generally defined not by the properties of individual bricks, but mostly by the condition of seams between them. The refractory mortar used for sealing seams (on which brickwork mortar is based) is vulnerable to fluoride salts and aggressive gases due to its high porosity. In addition, water used for preparing brickwork mortar causes, at low temperatures, problems with the assembly of the electrolytic cell and has a negative impact on the durability of thermal insulation materials in the cell's cathode.
Along with shaped barrier materials, there has been considerable experience with using loose powders with different particle size distribution and mineralogical composition; they help produce seamless layers. The process of using unshaped materials, during the process of lining the cell's cathode, compares favorably with the process of using brickwork in terms of lining time and less labor.
A lining method is known, comprising filling the cell's cathode shell with powder material and leveling the material with a rack, wherein the unshaped fill material is used, which reacts with fluoride salts to form a compound which is solid at the operation temperature in the cathode (Seltveit A., Diffusion Barrier for Aluminium Electrolysis Furnaces, U.S. Pat. No. 4,536,273, 1985). Test results, however, did not confirm the viability of this lining method because a high porosity of the un-compacted layer led to a continuous supply of gaseous and liquid components to the thermal insulation.
A lining method is known, comprising filling the cell's cathode shell with powder material, leveling the material with a rack, wherein compaction is performed by regular rollers (L. Forrssblad, Vibratory Compaction of Soil and Foundations. Translated from English under editorship of M. P. Kostelov, Transport, 1987, 191 pages.) However, an evaluation of static formation results shows that it does not provide for the desired structure of a lining material: low porosity and small-sized pores.
A method is known for lining, including filling the cell's cathode shell with powder material, leveling the material with a rack, wherein compaction was performed by compactors equipped with a vibratory mechanism (U.S. Pat. No. 4,184,787; E01C 19/38). This leads to a certain increase in packing density but the resulting barrier layer still has a relatively high porosity (up to 25%) and, moreover, it has wave-like defects on the surface.
A lining method is known, comprising filling the cell's cathode shell with powder material, leveling the material with a rack, wherein the compaction of unshaped materials is performed by external vibration of the railway platform, on which the cathode is installed (O. Siljan, O. Junge, B. Trygve, T. Svendsen, K. Thovsen Experiences with Dry Barrier Powder Materials in Aluminium Electrolysis Cells—Light Metals, 1998, p. 573-581). The disadvantage of this method is material segregation and particle separation along the layer's height; hence, there is a low degree of resistance to penetration of fluoride salts. This leads to high rates of chemical reactions, which reduces the operation life of the cell.
A method for lining the cell's cathode is known, comprising filling the cell's cathode shell with powder material, leveling the material with a rack, wherein compaction is performed by air ramming from above through hot ramming paste (R. Weibel, Advantages and Disadvantages of Application of Various Refractory Materials for Cathodes. Proceedings: Aluminum of Siberia. Krasnoyarsk, 2002, p. 14-24). However, the use of hot ramming paste is environmentally hazardous, and the transition to cold ramming paste and a decrease in cryolite ratio reduces the operation life of the cell.
A lining method is known (Refractories for Cathodes of Electrolytic cells/S. G. Sennikov et al. —Ogneupory I Technicheskaya Keramika, 2003, No. 10, p. 22-31), comprising filling the cell's cathode shell with powder material, leveling the material with a rack, sequentially laying of layers of polyethylene film, glass fiber laminate sheets or MDF on the fill material, and compacting the material by the dynamic method (using sleds with a vibrator.) However, when using such a device, both compaction and de-compaction of the mix occur at the same time; as a result, dusting of the material being compacted is observed.
A lining method is known, comprising filling the cell's cathode shell with powder material, leveling the material with a rack, wherein compaction is performed by compactors equipped with a vibratory mechanism (U.S. Pat. No. 4,184,787; E01C 19/38). This leads to a certain increase in packing density but the resulting barrier layer still has a relatively high porosity (up to 25%) and, moreover, it has wave-like defects on the surface.
A lining method is known, comprising filling the cell's cathode shell with powder material, leveling the material with a rack, wherein the process of compaction begins in a corner of the cathode shell, and is performed spirally (from the outside toward the center of the cathode.) When moving the vibrator, overlapping of the previously compacted area (by several centimeters) takes place. To finish the process of compacting barrier mixes, it is required to make several passes (trips) of the vibrator.
The main disadvantage of this method is multiple passes (trips) of the vibratory platform over the surface of the barrier material (due to a small size of the platform.) The parameters of the resulting barrier layer depend on the skills and scrupulosity of the operator. However, the most significant disadvantage is that the operation of the vibratory platform is primarily based on the dynamic method of formation (under non-optimum frequency and weight characteristics.) At a low bulk density of the lining material, it leads to that both compaction and de-compaction processes take place at the same time. As a result, dusting of the material being compacted is observed. The use of relatively thin glass fiber laminate sheets or MDF, not having sufficient hardness, results in an un-even surface; the surface of the barrier material after lining, as in the case of using vibratory compactors, is wave-like. Attempts to increase the hardness of the covering material lead to a decrease in the efficiency of the process of compaction (EP 1127983; E01C 19/38; E02D 3/046).
A method for forming seamless lining layers in electrolytic cells is known, comprising filling the cell's cathode shell with powder material, leveling the material with a rack, covering the fill material with dust-proof film, and compaction wherein material compaction is performed in two stages: preliminary static and final dynamic impact (compaction), by consequent movement of static and dynamic work tools of compaction along the longitudinal axis of the cathode of the electrolytic cell over the whole width of the lining layer being formed through a cushion; the dynamic material compaction is carried out by under-consonant-static-load vibratory units.
Based on its purpose and similar characteristics, this solution has been chosen as a prototype.
According to this solution, compaction is carried out in two stages: preliminary static and final dynamic impact (compaction), by consequent movement of static and dynamic work tools of compaction along the longitudinal axis of the cathode of the electrolytic cell over the whole width of the lining layer being formed through a cushion; the dynamic material compaction is carried out by under-consonant-static-load vibratory units.
This lining method does not meet the requirements regarding producing a high-quality, large depth and low bulk density barrier layer.
The technical device, through which the above lining process becomes possible, is an apparatus for forming seamless lining layers in electrolytic cells (RF Patent 2296819 Int. Cl. C25C 3/06, C25C 3/08, published in Bulletin of Inventions No. 10, 2007).
Based on its purpose and similar characteristics, this solution has been chosen as a prototype.
The apparatus for forming seamless lining layers in the electrolytic cell comprises a drive, a compacting device consisting of a unit for static treatment and a unit for dynamic treatment; the unit for static treatment is designed as a roller with a drive connected to the roller by means of a rocker arm and a pull-rod of the unit for dynamic treatment designed as a vibratory unit, including a vibratory exciter (with a directional driving force) mounted in a way so it is possible to move it around the horizontal axis of the roller.
The main disadvantage of the prototype apparatus is that the compacted material is pushed out right before the unit for static treatment, when forming a barrier layer of great depth and low bulk density. Moreover, the lack of such design elements that damp the horizontal component of vibration causes technical problems, when using, as a source of oscillations, vibratory exciters with a circular driving force or vibratory exciters with a directional driving force mounted on the vibratory unit at an acute angle to the treated surface (due to the transmission of vibration of the whole structure.) When using such oscillation sources, the electric motors of the unit for static treatment and other elements of the apparatus undergo vibration, which can lead to their failure, and, hence, reduce operational reliability.