Method for providing a cathode lining barrier layer in an electrolysis cell and a material for same

Abstract

The present invention relates to a method and a material for establishing a cathode barrier layer in electrolysis cells for production of aluminum of Hall-Heroult type, the barrier layer can comprise minerals combined with a compound that lowers the melting temperature of the minerals, such as fluorides.

Claims

1. A method for providing a cathode lining barrier layer in a Hall-Héroult electrolysis cell for production of aluminum, wherein the cell comprises an electrolytic bath having components of NaF, AlF.sub.3, and Na.sub.3AlF.sub.6 in addition to other components, wherein the cathode lining comprises a cathode panel of cathode blocks supported by at least one layer of a refractory material, and wherein during lining of the pot, a mixed material that comprises a mixture of mineral(s) and a chemical compound(s) that effectively lowers the melting temperature of the mineral(s) is arranged between the at least one layer of refractory material and the cathode blocks, wherein the mixture of mineral(s) comprises sodium feldspar and/or nepheline syenite, and wherein the chemical compound(s) comprise fluorides, where during start-up of the cell and as a certain temperature is achieved in the mixed material, and further before penetration of the bath, the mixed material forms a viscous liquid, and where the viscous liquid has high viscosity, and a density higher than that of the bath components eligible to penetrate the cathode blocks and forms a barrier that is effectively immiscible with the penetrating bath components.

2. The method according to claim 1, wherein the minerals of the mixed material comprise sodium feldspar.

3. The method according to claim 1, wherein the minerals of the mixed material comprise nepheline syenite.

4. The method according to claim 1, wherein the mixed material comprises 2.sup.nd cut spent pot lining having mineral components and fluoride.

5. The method according to claim 1, wherein the mixed material can be in the state of a powder material, as pre-shaped bricks or a slurry.

6. The method according to claim 1, wherein the fluoride concentration is >0-30% wt of the mixed material.

7. The method according to claim 1, wherein the fluoride concentration is 10-20% wt of the mixed material.

8. The method according to claim 1, wherein the fluoride concentration is 15% wt of the mixed material.

9. The method according to claim 1, wherein the barrier layer has a thickness of 1-300 mm.

10. The method according to claim 1, wherein the barrier layer has a thickness of 50-100 mm.

11. The method according to claim 1, wherein the viscosity of the material can be tuned by additives comprising SiO.sub.2.

12. A material to be used as a cathode lining barrier layer in a Hall-Héroult electrolysis cell for aluminiuim production, wherein, the material comprises a mixture of mineral(s) comprising sodium feldspar and/or nepheline syenite and a chemical compound(s) comprising fluorides that effectively lower the melting point of the mineral(s) and forms a viscous liquid at a predefined lower temperature than the melting temperature of the mineral(s) where the viscous liquid has a density higher than bath components eligible for penetration through the cathode, thus preventing such penetration.

13. The material according to claim 12, wherein the lower limit for the melting temperature of the material is in the range 600-970° C.

14. The material according to claim 12, wherein the material comprises a homogenous mixture of minerals and fluoride compounds.

Description

(1) The invention shall be further described by examples and figures where:

(2) FIG. 1 discloses a sketch of the typical cross section of a prior art reduction cell showing thermal insulation, typical layers of barrier bricks and/or steel/glass plates and optional levelling powder below the cathode block,

(3) FIG. 2 discloses in more detail how a barrier layer according to the present invention can be arranged beneath a cathode block above the thermal insulation,

(4) FIG. 3 discloses two immiscible phases with the barrier layer at the bottom in a solid piece of albite and nepheline based brick,

(5) FIG. 4 discloses a cup-test with a conventional chamotte barrier brick.

DESCRIPTION

(6) FIG. 1 discloses a sketch of the typical cross section of a prior art reduction cell showing the thermal insulation, typical layers of barrier bricks and/or steel/glass plates and levelling powder below the cathode block. The main parts from top to bottom are: anode, bath, liquid metal, cathode block, optional levelling layer of powder and/or barrier, barrier brick layer, optional layer of additional barrier e.g. steel plate or glass plate, thermal insulation and steel shell.

(7) FIG. 2 discloses in more detail how a barrier layer can be arranged beneath a cathode block above the thermal insulation, according to the present invention. The main components from top to bottom are: Anode, bath, liquid metal, cathode block, barrier layer mixed material as powder, brick or a combination of both powder and brick, or possibly a slurry, alternatively re-used material, optional barrier brick layer, thermal insulation and steel shell.

(8) The mixed material (minerals and compound(s) that lowers the melting temperature of the minerals, such as fluorides) that results in the above-mentioned cathode barrier layer should be installed during lining of the cell. Typically, the thermal insulation in the lining will be installed in the steel shell. The barrier layer mixed material should be installed directly on top of the thermal insulation bricks. The barrier layer mixed material is installed as a dry powder, as pre-shaped bricks (fired or non-fired) or as a slurry. If pre-shaped bricks are used, a layer of dry powder or slurry with the barrier mixed material can be used for leveling purposes before installing the carbon cathode blocks. If the barrier layer mixed material is installed as a powder or slurry an optional compaction step is possible using a vibrating plate compactor or similar before the carbon cathode blocks are installed.

(9) In one embodiment of the invention, the grain size distribution of the minerals can be 1-2.8 mm (24%), 0.25-1 mm (15%), 0.25-0.5 mm (15%), 0.125-0.25 mm (16%), 0.063-0.125 mm (16%), <0.063 mm (14%). However, the grain size distribution can be adjusted to adapt to economical and practical considerations during installation and utilization of the current invention.

(10) After the barrier layer mixed material is installed, the rest of the cathode is lined and the cell is prepared for conventional pre-heating. No active action is required with respect to the barrier layer mixed material after the lining is finished.

(11) During pre-heating of the cell, the center of the cathode surface will show a higher temperature than the cathode edges. Liquid electrolysis bath will be added to the cell when the temperature of the cathode center and the near surroundings are sufficiently high. The electrolysis bath has a freezing temperature relatively close to the normal operating temperature of the cell and it is not uncommon that the added bath freezes on the cathode surface, at least towards the edges of the cathode surface. This frozen bath will melt gradually as the temperature of the cell homogenizes during the first hours after bath addition.

(12) In the case where a mixed material comprising minerals together with the melting point lowering compounds, such as fluoride, are installed during lining of the cell, a viscous liquid will form during pre-heating of the cell. The barrier functionality with a viscous liquid will have formed when the cell reaches operating temperature. A full barrier functionality will hence have been obtained before bath starts penetrating through the cathode after bath addition.

(13) It is not crucial that the viscous liquid forms under the whole cathode at the same time because bath penetration will be prohibited by low cathode temperatures during bath addition in locations where the viscous liquid has not yet formed due to low temperature.

(14) The viscous liquid formed under the cathode by the barrier layer material mixture is not miscible with the bath penetrating the cathode after bath addition to the cell and the protective nature of the barrier layer is hence assured.

(15) According to the present invention, it is noted that no action is required after the cell lining is finished. The cell start-up can proceed as normal with pre-heating times of e.g. 48-72 hours. The viscosity of the viscous liquid cathode barrier layer can be predefined before being installed in a cell by adjusting the mineral's melting temperature by adding the chemical compounds for that, such as fluoride and also with addition of SiO.sub.2.

(16) The source of fluoride in this context can be, but is not limited to, cryolite, electrolytic bath, NaF, AlF and spent pot lining with fluoride content, either as single fluoride sources or a combination of the above mentioned.

(17) On top of the thermal insulation and below the invented barrier layer it is possible, but not required for the functionality of the barrier, to install additional conventional barrier bricks (for example based on chamotte).

(18) The compositional window for the cathode barrier layer is 0.1-30 wt % cryolite (or its fluoride equivalent if other fluorine sources are used) mixed with 99.9-70 wt % minerals. The resulting cathode barrier layer should have a density higher than the bath components penetrating the cathode. The bath penetrating the cathode is typically enriched with NaF relative to the bath where the aluminium reduction occurs.

(19) FIG. 3 discloses two immiscible phases with the barrier layer at the bottom in a solid piece of albite and nepheline based brick, the brick was heated to 950° C. and kept at 950° C. for 24 hours. The fluoride melt consists of 60 wt % cryolite and 40 wt % NaF. The top left bulge is due to porosity and not reaction between the brick and the melt. The white lower part is the viscous liquid phase and the upper grey part is the fluoride.

(20) FIG. 4 discloses a cup-test with a conventional chamotte barrier brick. The pictures of the species M2 and S2 show the low reactivity of the barrier mixed material in the bottom of the cups (white) based on sodium feldspar (left, Sibelco Germany) and nepheline syenite (right, Sibelco Canada). The fluoride melt is floating at the top. The cups were heated to 950° C. and kept at this temperature for 24 hours. The barrier material mixture contains 85 wt % feldspar or nepheline together with 15 wt % cryolite.

(21) The tests clearly show the capability of the viscous barrier to hinder the fluoride to pass it.

(22) In one embodiment, the barrier material comprises a powder mix of fluoride and feldspar applied as a layer below the carbon cathode block.

(23) In a second embodiment, feldspar powder with some melting temperature lowering compound is applied as a layer below the carbon cathode blocks. Additional Fluoride may be added over time by operating the pot, due to penetration of bath through the carbon cathode material.

(24) In a third embodiment, the barrier material comprises a powder mix of fluoride and nepheline applied as a layer below the cathode.

(25) In a fourth embodiment, nepheline powder with some melting temperature lowering compound is applied as a layer below the cathode. Additional Fluoride may be added over time by operating the pot, due to penetration of fluoride through the cathode material.

(26) Alternatively, the powder in the embodiments above may be substituted by pre-shaped bricks, or a combination of bricks and powder may be applied. The mixed material may also be installed as a slurry.

(27) The minerals applied in accordance with the invention can be either natural or synthetic or a mix of same.

(28) A powder containing both minerals and fluoride components from re-used lining materials (spent pot lining) can also be applied.

(29) The thickness of the barrier layer is 1-300 mm, preferably 50-100 mm, but thinner and thicker layers are also possible.