Lining of a cathode assembly of a reduction cell for production of aluminum, method for installation thereof and reduction cell having such lining

Abstract

The present invention relates to nonferrous metallurgy, in particular to the electrolytic production of aluminum, more particularly to a structure of a cathode assembly of a reduction cell for production of aluminum. A lining of a cathode assembly of an aluminum reduction cell is provided which comprises a thermal insulation layer and a fire-resistant layer consisting of no less than two sub-layers, wherein the porosity of the thermal insulation layer and the fire-resistant layer increases from an upper sub-layer to a bottom sub-layer and the thickness ratio of the fire-resistant layer and the thermal insulation layer is no less than . Also, the present invention provides a method for lining a cathode assembly of a reduction cell and a reduction cell having the claimed cathode assembly lining. The invention is aimed at the reduction of the cyanide content in upper thermal insulation layers and to provision of conditions for material reuse in the thermal insulation layer, waste reduction and improvement of the environmental situation on aluminum production facilities.

Claims

1. A lining of a cathode assembly of a reduction cell for production of aluminum which comprises bottom and side blocks interconnected with a cold ramming paste, a fire-resistant layer and a thermal insulation layer made of non-shaped materials, wherein the fire-resistant layer consists of an alumino-silicate material and the thermal insulation layer consists of non-graphitic carbon or a mixture thereof with an alumino-silicate or alumina powder, characterized in that the thermal insulation layer and the fire-resistant layer consist of no less than two sub-layers, wherein the porosity of the thermal insulation and fire-resistant layers increases from an upper sub-layer to a bottom sub-layer and the thickness ratio of the fire-resistant layer and the thermal insulation layer is no less than .

2. The lining of claim 1, characterized in that the thickness ratio of the fire-resistant layer and the thermal insulation layer is 1: (1-3).

3. The lining of claim 1, characterized in that the growth rate of the fire-resistant layer porosity from the upper sub-layer to the bottom sub-layer is between 17 and 40% and the porosity growth rate of the thermal insulation layer from the upper sub-layer to the bottom sub-layer is between 60 to 90%.

4. The lining of claim 1, characterized in that as one of the sub-layers of the fire-resistant layer a natural material is used, in particular, porcellanite.

5. The lining of claim 1, characterized in that a graphite foil is placed between sub-layers of the fire-resistant layer.

6. The lining of claim 1, characterized in that products of lignite pyrolysis produced at 600-800 C. are used as non-graphitic carbon.

7. A method for lining a cathode assembly of a reduction cell for production of aluminum which comprises filling a cathode assembly shell with a thermal insulation layer consisting of non-graphitic carbon, forming a fire-resistant layer, installing bottom and side blocks followed by sealing joints therebetween with a cold ramming paste, characterized in that an upper sub-layer of a thermal insulation layer is advantageously filled with non-graphitic carbon previously removed from a lower sub-layer of a thermal insulation layer of an earlier used cathode assembly of the reduction cell or a mixture thereof with porcellanite and having a thermal conductivity coefficient and packed density not exceeding the initial ones, wherein the thermal insulation layer and the fire-resistant layer consist of no less than two sub-layers, wherein the porosity of the thermal insulation and fire-resistant layers increases from the upper sub-layer to the bottom sub-layer and the thickness ratio of the fire-resistant layer and the thermal insulation layer is no less than .

8. The method of claim 7, characterized in that the thickness ratio of the fire-resistant layer and the thermal insulation layer is advantageously 1: (1-3).

9. The method of claim 7, characterized in that the growth rate of the fire-resistant layer porosity from the upper sub-layer to the bottom sub-layer is between 17 and 40% and the porosity growth rate of the thermal insulation layer from the upper sub-layer to the bottom sub-layer is between 60 to 90%.

10. The method of claim 7, characterized in that as one of the sub-layers of the fire-resistant layer a natural material is used, in particular, porcellanite.

11. The method of claim 7, characterized in that a graphite foil is placed between the sub-layers of the fire-resistant layer.

12. A reduction cell for production of aluminum which comprises a cathode assembly comprising a bath with a carbon bottom made of angular blocks having cathode conductors embedded therein and enclosed inside a metal shell, wherein fire-resistant and thermal insulation materials are placed between the metal shell and the angular blocks; an anode device comprising one or more angular anodes connected to an anode bus and arranged at the top of the bath and immersed in a molten electrolyte, characterized in that the lining of the cathode assembly is made in accordance with claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The essence of the invention will be better understood upon studying following drawings:

(2) FIG. 1 is a representation of a cathode lining of a reduction cell,

(3) FIG. 2 is a graph of the computed distribution of temperatures over the height of the lining base, where the X-axis represents a distance in depth of the base passing vertically from a floor of a bottom unit, and the Y-axis represents temperature estimated values,

(4) FIG. 3 is a representation of the permeability vs pore sizes,

(5) FIG. 4 is a representation of sodium cyanide content in different materials vs temperature,

(6) FIG. 5 is a representation of sodium cyanide content in different materials vs temperature,

EMBODIMENTS OF THE INVENTION

(7) In FIG. 1 a lining consists from a lower sub-layer of a thermal insulation layer comprised of non-graphitic carbon material 1 with the porosity to 90%, an overlying upper sub-layer of a thermal insulation layer 2 with the porosity to 60% over which is arranged a lower sublayer 3 of an alumino-silicate fire-resistant layer (porcellanite) with the porosity up to 40% covered with an upper sub-layer of a fire-resistant layer 4 with the porosity up to 17% and highly resistant to permeation of electrolytic components through a bottom consisted of carbon blocks 5. The periphery of an inner side of a metal shell is laid with brick lip 6. A bottom mass 7 fills the space between carbon blocks 5 and a side block 8. A collector bar 9 is connected to the carbon block 5. A graphite foil 10 is placed under the upper sub-layer of the fire-resistant layer. A peripheral joint 11 passes between the carbon blocks 5 and the brick lip 6.

(8) The calculation results for three embodiments of cathode lining of the reduction cell for production of primary aluminum are shown in FIG. 2.

(9) In accordance with the first embodiment, for the total height of the space under a cathode of 425 mm, the thickness of the fire-resistant layer was 100 mm and the thickness of the thermal insulation layer was 325 mm. Thickness ratio of the fire-resistant layer and the thermal insulation layer was (1:3.25).

(10) In accordance with the second embodiment, the thickness of the fire-resistant layer was 155 mm and the thickness of the thermal insulation layer was 280 mm. Thickness ratio of the fire-resistant layer and the thermal insulation layer was (1:1.8).

(11) In accordance with the third embodiment, the thickness of the fire-resistant layer was 200 mm and the thickness of the thermal insulation layer was 215 mm. Thickness ratio of the fire-resistant layer and the thermal insulation layer was (1:1.1).

(12) The Y-axis represents two temperature values. The first value 852 C. is the melt temperature of sodium carbonate, the second value 542 C. is the sodium crystallization temperature under the cathode.

(13) As can be seen from the data for the first embodiment, sodium carbonate is formed at the depth of 120-125 mm. The thickness of the alumino-silicate fire-resistant layer (the barrier mix) for the given mixture was 100 mm. That is why at the depth of 20-25 mm inside the thermal insulation layer a rich in cyanide powder material is formed. In the lower layer, cyanides are located in monolithic sodium carbonate and the ecological threat is minimal since bottom blocks are a typical place for sodium cyanides to form.

(14) In accordance with the third embodiment where the maximum thickness of the fire-resistant layer is 200 mm, sodium carbonate in the thermal insulation is formed below the layer and there is no risk of cyanide dispersion in the form of dust. However, at the same time thermal- and cost-effectiveness of the cathode assembly is at the lowest because of the high thermal conductivity coefficient and the high price of the fire-resistant layer comparing to the carbon material.

(15) That is why the embodiment 2 where the thickness of the dry barrier mix is 155 mm is preferable compared to the embodiments 1 and 3, since in the first embodiment, in the upper sub-layers of the thermal insulation layer unacceptably high amount of sodium cyanides is formed which is confirmed by results of the autopsy of a test reduction cell. The third embodiment is not optimal because of the heat loss through the shell bottom, and some sub-layers of the thermal insulation layer are replaced by sub-layers of the fire-resistant layer which have the higher thermal conductivity coefficient. Besides, since the fire-resistant material is more expensive, the lining cost is also increased.

(16) The cathode lining of the reduction cell for production of primary aluminum is implemented using the same method as follows.

(17) A used cathode assembly having non-shaped materials is pre-disassembled. In use, non-graphitic carbon from a thermal insulation layer is transformed into a two-layer material. From below it preserves its powder state and from above it has a bound monolithic structure with a dark-greasy shade. The material is arranged in the space between isotherm 850 C. that corresponds to the liquidus temperature of sodium carbonate and the condensation temperature 540 C. of sodium under a condition of operation of materials under the cathode.

(18) The material from the lower sub-layer of the thermal insulation layer placed below isotherm 540 C. preserves its initial characteristics and advantageously consists of carbon 95% (Table 1).

(19) TABLE-US-00001 TABLE 1 Results of X-ray phase analysis of the material composition of the lower sub-layer of the thermal insulation layer of the lining Substance Material Center Periphery C Carbon 88.7 76.6 C Graphite 6.25 5.13 CaO Lime 1.13 3.04 Na.sub.2CO.sub.3 Gregoryite, syn 0 1.15 Na.sub.2CO.sub.3 0 10.3 CaCO.sub.3 Calcite 2.06 2.57 CaMg.sub.0.7Fe.sub.0.3(CO.sub.3).sub.2 Dolomite 0 0.28 NaCN 0 0.76 SiO.sub.2 Quartz 1.75 0

(20) Cyanide concentration in this area found by the photometric technique was 0.12 and 0.43%, respectively.

(21) The monolithic area arranged above advantageously consists of sodium carbonate and carbon (Table 2). Cyanide concentration in this area found by the photometric technique was 4.3%. The thermal conductivity coefficient of lower layers of lining materials doesn't change its initial value: 0.09 W/(K). That is why non-graphitic carbon or a mixture thereof with an alumina-silicate or alumina powder can be re-used to shape the upper sublayer of the thermal insulation layer without additional treatment.

(22) TABLE-US-00002 TABLE 2 Results of X-ray phase analysis of the material composition of the upper sub-layer of the thermal insulation layer of the lining Substance Material Center Periphery C Carbon 33.1 31.5 C Graphite 0.96 1.96 CaO Lime 4.41 6.32 Na2CO3 Gregoryite, syn 3.48 5.4 Na.sub.2CO.sub.3 25.9 0 Na.sub.2CO.sub.3 Natrite 30.1 54 CaMg.sub.0.7Fe.sub.0.3(CO.sub.3).sub.2 Dolomite 1.85 0.67

(23) At the same time, non-graphitic carbon mixed with an alumino-silicate material (porcellaniteo.sub.M) can be used. The lower thermal conductivity coefficient of this mixture is lower than for the single porcellanite and the cyanide content therein is lower than in the non-graphitic carbon. It is confirmed by the results obtained based on the operation of a test reduction cell where a mixture of non-graphitic carbon and an alumino-silicate powder was arranged directly beneath bottom blocks. The content of sodium cyanides in the mixed material removed from the reduction cell after more than 2300 days of operation was 0.4%.

(24) For the upper sublayer of the thermal insulation layer, a thermal conductivity coefficient is much higher 0.5 W/(K). Taking into account the higher content of cyanides and the presence of lumps, it is impossible to reuse the material from the upper sub-layer of the thermal insulation layer for a direct purpose. The most efficient way to dispose of the material of the upper sub-layer of the thermal insulation layer is the direct incineration accompanying with heat energy generation. According to the results of the derivatographic analysis (FIG. 3), this needs sufficient temperatures above 600 C.

(25) As a non-graphitic carbon, it is desired to use products of lignite pyrolysis produced at 600-800 C. At lower temperatures, there is no explosion security because the content of volatile substances is high, and at a higher temperature the carbon residue is reduced as well as the process performance.

(26) The use of abovementioned cathode lining and the method for lining allows to reduce the cyanide content in the upper thermal insulation layers and to provide conditions for reuse of the material for the thermal insulation layer and to reduce wastes and improve the environmental situation in places of aluminum production facilities.