GB2568246A

GB2568246A - Process for protecting carbon anodes for use in the Hall-Heroult process

Info

Publication number: GB2568246A
Application number: GB1718435.9A
Authority: GB
Inventors: Morfor Edouard; Jassim Banjab Ali; Jamal Eddin Jamil
Original assignee: Dubai Aluminium PJSC
Current assignee: Dubai Aluminium PJSC
Priority date: 2017-11-08
Filing date: 2017-11-08
Publication date: 2019-05-15
Also published as: GB201718435D0; GB2568246A8

Abstract

Process for protecting carbon anodes 13 for use in the Hall-Heroult process and the anodes themselves are described. A method for coating an anode 13 intended to be used in molten salt electrolysis for refining aluminium (figure 1), wherein a coating 53 is applied onto at least part of the upper and/or side surfaces of said anode by thermal spray coating. The coating may comprise an aluminium or alumina coating. The anode can be prebaked before coating and the coating applied using a spray gun 55. The spray gun may be mounted on a robotic arm 60, controlled by a robot 61. The aluminum feedstock may be in the form of aluminium rod or wire 56; the coating can be in the range 0.2-3.5mm thick.

Description

Process for protecting carbon anodes for use in the Hall-Heroult process

Technical field of the invention

The invention relates to the field of fused salt electrolysis, and more precisely to anodes for use in the Hall-Heroult process for making aluminium by fused salt electrolysis. Carbon anodes are consumed in the course of the electrolysis process, but their thermal oxidation in ambient air is undesirable. The invention relates in particular to a method for coating the sides of anode blocks by a layer of aluminium or alumina in order to protect them against oxidation in the ambient air.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (AI₂O₃) is dissolved in molten cryolite (Na₃AIF₆), and the resulting mixture (typically at a temperature comprised between 940 °C and 970 °C) acts as a liquid electrolyte in an electrolytic cell. An electrolytic cell (also called “pot”) used for the Hall-Heroult process typically comprises a steel shell (so-called potshell), a lining (comprising refractory bricks protecting said steel potshell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), a superstructure and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte contained in the volume defined by the cathode bottom and a side lining made from carbonaceous material. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.5 V and 5 V) which electrochemically reduces the aluminium oxide, split by the electrolyte into aluminium and oxygen ions, into aluminium at the cathode and oxygen at the anode; said oxygen reacting with the carbon of the anode to form carbon dioxide. The resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as a liquid metal pad on the cathode surface from where it needs to be removed from time to time, usually by suction into a crucible.

Anodes in the Hall-Heroult process are usually prebaked cuboids made from a carbonaceous material. The anodes axed fixedly connected to so-called anode hangers. The latter serve two different purposes, namely to keep the carbon anodes at a predetermined distance from the cathode, and to carry the electrical current form an anode busbar (also called “anode beam”) down to the carbon anodes. Anode hangers are fixed to the overhanging anode beam in a detachable manner using clamps. Anode hangers comprise an upper part called “anode rod”, which is connected to the anode beam, and a lower part, called “anode yoke”. The anode yoke has a number of legs each of which terminates in a cylindrical stub that is embedded in pre-formed stubholes of the anodes and fixed with cast iron acting as temperature-resistant, electrically conductive glue; this process is called “rodding”. The assembly formed by an anode fixed to its anode hanger is called an “anode assembly”.

Anodes are consumed during the electrolysis process because the carbon reacts with the oxygen ions generated by electrolysis of alumina according to the chemical reaction

C + 2 O² CO₂ + 4 e ; the carbon dioxide is bubbling out of the liquid electrolyte.

The present invention relates to the protection of the anode surface that is not in contact with the electrolytic bath. Anodes protrude out of the liquid electrolyte and can chemically burn in contact with air. This reaction is undesirable, and several methods have been proposed to limit or avoid it by protecting the anode surface against the atmosphere. Partial protection can be reached by covering the upper anode surface with alumina powder, but this is not efficient with tall anodes used in modern Hall-Heroult cells. US 3,053,748 (assigned to Pechiney) proposes to protect the anodes by projecting liquid aluminium against its lateral faces. US 3,060,115 (assigned to Aluminum Company of America) proposes to wrap the upper part of the anode block with aluminium foil bonded to the anode surfaces. This is rather labor intensive, and the foil and adhesive represents a cost.

US 3,442,786 (assigned to Kaiser Aluminum & Chemical Corp.) proposes another method to protect the anode surface by applying a coating that uses liquid aluminium produced by the smelting plant itself. This method uses a nozzle connected to an air line that projects liquid aluminium onto the anode surface; partial oxidation of the aluminium coating is critical to the protective qualities of the coating, and the aluminium coating should have an oxide content between 1 and 10 weight %.

Aluminium coatings on anodes are widely used in smelting plants. Typically, the spray station comprises a holding furnace containing liquid aluminum, and means to hold and rotate an anode assembly in front of said nozzles. For applying the protective coating an anode assembly is suspended in front of the furnace, and liquid aluminium is withdrawn through a tap hole and introduced into one or more nozzles operated with compressed air. The anode is then rotated in front of said nozzles, so that the liquid aluminium is sprayed over the desired areas of the anode.

This process has several shortcomings. First of all, it requires a significant investment cost, due to the requirement of a holding furnace for liquid aluminium. Secondly, it is rather difficult to automate this process while meeting all the requirements of the precise, spatially controlled application of a coating of constant thickness. Indeed, the aluminium coating is required only over part of the anode surface, that is to say especially over the upper part of the side surfaces (typically over a height of about one third of anode height) and possibly also over at least part of the upper surface (if this surface is not covered by alumina powder). A plurality of fixed nozzles is needed to cover the whole target surface of the anode. The positioning and rotation of the anode, suspended by its anode hanger, is rather unreliable due to possible swinging movement. In current industrial practice, a significant fraction (typically about one half) of the aluminium sprayed on the anode is wasted.

The present inventors have tried to find a method for applying a protective coating onto a carbon anode that avoids these shortcoming of prior art.

Object of the invention

The present inventors believe that prior persons skilled in the art who had tried to improve the methods for applying protective aluminium coating on the anodes had been misled by the availability of liquid aluminium in the smelting plant: prior inventors have focussed on methods to spray liquid aluminium onto the anode surfaces. However, this process is intrinsically rather difficult to optimise, and is also rather difficult to automatize because liquid aluminium cannot be sprayed through nozzles that are readily movable.

According to the invention, the problem is solved by coating the anode by a thermal spray process, preferably using a moveable nozzle. The thermal spray process according to the present invention does not use liquid aluminium as a starting material supplied to the nozzle, but uses solid feedstock, i.e. solid aluminium if an aluminium coating is intended to be applied, or alumina powder if an alumina coating is intended to be applied; said solid aluminium feedstock is used preferably in the form of a rod or a wire. Cast aluminium wire is readily available on the market, albeit at a cost higher than that of liquid aluminium supplied from a melting plant. Moreover, in the cast house of many aluminium smelting plants wire is manufactured by solidification of liquid aluminium: this would avoid to purchase raw material for the coating process on the market.

The inventors have recognized that this method can also be used for other coating materials that are capable of protection the anode material against oxidation and that are chemically compatible with the Hall-Heroult process (that is to say, in particular, that do not introduce unwanted impurities into the that electrolysis cell). Such a coating material is alumina.

A first subject-matter of the invention is a method for coating an anode intended to be used in molten salt electrolysis for making aluminium, wherein a coating is applied onto at least part of the upper and/or side surfaces of said anode by thermal spray coating. Advantageously said anode is a prebaked carbonaceous anode. No specific treatment of said upper and side surfaces of the anode is required prior to thermal spray coating. Said spray coating can be applied through one or more nozzles. Said nozzle can be moved with respect to said anode.

In an embodiment, said method comprises the steps of: (a) providing an anode, and (b) applying a coating onto at least part of the upper and/or side surfaces of said anode by thermal spray coating. Said thermal spray can be generated by the steps of: (i) melting a feedstock material by a heat source to generate melted spray material, and (ii) propelling, by using a process gas, said molten spray material through a nozzle onto at least part of the upper and/or side surfaces of said anode, where said spray material solidifies. Said heat source and said nozzle can be parts of a spray gun. Said feedstock material can be aluminium rod or aluminium wire for an aluminium coating, or alumina for an alumina coating.

The anode block to be coated can be part of the anode assembly (i.e. it can be fixed to the anode rod, preferably at the upper surface of said anode). Alternatively the anode block is coated prior to being fixed to the anode rod; in this case the interior surface of the stub holes should be protected from the metallization jet. In an advantageous embodiment said anode assembly is suspended on its anode rod during spray coating. In this case, in an advantageous embodiment the method according to the invention can comprise the steps of: (a) Providing an anode assembly; (b) Transporting said anode assembly to a spray coating station; (c) Spray coating at least part of the upper and/or side surfaces of said anode. Said anode assembly can be suspended on its anode rod during steps (a) and (b) at least, but also during step (c).

Another subject-matter of the present invention is an anode intended to be used in molten salt electrolysis for making aluminium, said anode being at least partially coated by a coating, such as an aluminium coating or an alumina coating, characterised in that said anode is obtainable by the method according to any of the embodiments of the present invention. This coating has a peculiar structure that can be identified by microscopic analysis, and that can be distinguished from the coating obtained according to prior art processes.

Still another subject-matter of the invention is a process for making aluminium by using the Hall-Heroult process, wherein at least one anode according to the invention is plunged into the molten electrolyte of an electrolysis cell.

Figures

Figure 1 refers to prior art, figure 5 to the invention, and figures 2, 3 and 4 refer to the examples.

Figure 1 shows a schematic transverse cross-sectional view of a prior art electrolytic cell for aluminium production according to the Hall-Heroult process.

Figure 2 shows a photograph of an aluminium coating according to prior art (figure 2(a)) and according to the invention (figure 2(b)).

Figure 3 shows a cross sectional micrograph of an aluminium coating according to prior art (figure 3(a)) and according to the invention (figure 3(b)).

Figure 4 shows an enlarged view of a cross sectional micrograph (scanning electron microscopy) of an aluminium coating according to prior art (figure 4(a)) and according to the invention (figure 4(b)).

Figure 5 shows a schematic side view of a piece of equipment for spray coating when used for coating an anode.

Detailed description

An aluminium electrolysis plant (also called “smelting plant” or “aluminium smelter”) comprises a plurality of aluminium electrolysis cells (also called “pots”) arranged in series. The Hall-Heroult process as such, the way to operate the latter, as well as the general structure of the cell arrangement are known to a person skilled in the art and will not be described here in more detail. It is sufficient to explain, in particular in relation with Figure 1_, that a typical cell 1 includes a potshell comprising first 16 and second 17 longitudinal sidewalls, first and second transversal end walls (not visible on figure 1) and a bottom 12. The cell walls define a space lined on its bottom and sides with refractory materials 11 (protecting the potshell against heat) along with the cathode blocks 12, thereby defining a volume containing the molten metal 15 and electrolyte 14. The lining 11 comprises a layer of carbonaceous material (not shown on the figures) in contact with molten liquid material.

Said cathode blocks 12 comprise one or more cathode collector bars 10 protruding out of the potshell. Electrical current enters the cell through anodes 13 (suspended above the cell by anode rods 3 attached to an anode frame 4), passes through the molten electrolytic bath 14 and the molten aluminium pad 15, and then enters the carbon cathode blocks 12. The current is carried out of the cell by the cathode collector bar 10 connected to a cathode busbar system (not shown on the figures). The cell 1 is closed by a set of hoods 5. The present invention relates to the anodes 13 which are made from a carbonaceous material.

The subject matter of the invention is a process in which anodes or anode assemblies are coated at least in part with a layer of solid material such as aluminium or alumina, using a thermal spray process. Thermal spray processes are known in prior art, but have never been applied to this specific purpose. They use a spay gun comprising a localized heat source and a nozzle. The heat source is able to melt a solid feedstock material. Aluminium rod or aluminium wire can be used here for an aluminium coating, or alumina powder for an alumina coating. The melted material is then propelled out of the nozzle by a high speed flux of process gas. Said process gas can advantageously be selected from nitrogen, hydrogen, argon and helium, and mixtures of these elements.

Figure 5 schematically shows a device 54 used for coating an anode 13. Said equipment comprises a thermal spray gun 55. This is a piece of equipment that is rather small in size and very easy to handle. Its supply lines (for example electric energy 58, feed gas 59, aluminium wire 56, the latter typically supplied from a coil 57) can be made very flexible, so that the spray gun 55 can be mounted on a robotic arm 60 controlled by an industrial robot 61. The thermal spray 56 is directed against the anode surface to be coated, here a side surface. Reference number 53 refers to the aluminium coating already made, reference number 52 to the surface that is to remain uncoated.

In an advantageous embodiment the anode block 13 is at rest and the nozzle (as a part of the spray gun 55) is moving with respect to said anode 13. The anode 13 can be part of an anode assembly 9 which comprises the anode rod 3; the anode assembly 9 can be suspended and transported with its anode rod. Alternatively, the anode block 13 can be moving in front of the nozzle(s) while being coated. In any case a plurality of nozzles or spray guns can be used, for instance simultaneously or sequentially. The process according to the invention lends itself to automatization, in particular when using an industrial robot 61.

The applicant has observed that the coating obtained by the inventive process is denser than coatings obtained by prior art processes. In particular the coatings do not include large voids but only small porosity, reflecting the difference between a coating obtained by solidification of coarse droplets of liquid metal (prior art process) and a coating obtained by a fine spray of small particles. This applies in particular to aluminium coatings. As a consequence, the inventive process allows to use thinner coatings, the thickness of which is easier to control, and which provide excellent barrier properties with respect to hot air (which is the environment against which the anode surface is to be protected and .

As the thermal spray 62 coming out of the nozzle is rather focussed, only a negligible amount of feedstock material will be lost in the operation, whereas waste of feedstock material is significant in the prior art process. Furthermore, the control of the coating thickness is easier than in the prior art process. In an advantageous embodiment the coating has an average thickness comprised between 0.2 mm and 3.5 mm, and preferably between 0.2 and 2.5 mm, and still more preferably between 0.3 and 1.7 mm. The coating should have a thickness of at least about 0.1 mm. If the coating is too thin it will not provide adequate protection against oxidation of the carbonaceous anode material against the environment comprising hot air. If the coating is too thick it may crack; a coating that is too thick will also consume too much feedstock material.

The coating can be applied in one run in several runs, thereby superimposing individual coating layers. The inventors have observed that when the first layer is sufficiently dense, there is no need to apply more than one layer. As an example, anodes running in industrial Hall-Heroult cells did exhibit similar behaviour when coated with one aluminium layer of 0.2 mm thickness as anodes coated with two to four superimposed aluminium layers of 0.2 mm thickness each.

The process according to the invention has several advantages.

First of all, the process according to the invention results in savings on equipment because it does not use liquid aluminium. Prior art processes using liquid aluminium metal require dedicated equipment to handle liquid metal, and particular a furnace in fluid connection with the nozzle. A workshop for implementing the prior art process also requires more ground space than a workshop for implementing the process according to the invention. As a consequence, equipment for the prior art process represents a significant capital cost compared to the inventive process; it also represents a higher maintenance cost.

Secondly, the process according to the invention avoids the handling of liquid aluminium which always includes a safety hazard; spraying liquid aluminium in finely divided form as droplets implies even more risks.

Thirdly, the process according to the invention allows a further simplification of the equipment because a thermal spray can be applied through a moveable nozzle or set of nozzles: there is no more need to move the anode in front of the nozzle.

A further advantage is the operational flexibility of the thermal spray process with respect to the liquid spray process; in particular, the movement of the spray gun or nozzle can be automatized.

Another advantage is the possibility to form coatings (such as aluminium or alumina layers) onto the anodes that are denser than those obtained by spraying liquid metal; this improves their barrier effect against ambient hot air. A further improvement of the barrier effect is due to the better adhesion of thermally sprayed layers onto the anode surface compared to the prior art process.

The process according to the invention allows forming coatings (and in particular aluminium and alumina coatings) that are thinner than those obtained by spraying liquid metal. Thermal spraying is also a process that is intrinsically more precise than liquid metal spraying: it allows coating the anode exactly where a coating is needed. For these various reasons the process according to the invention consumes less aluminium per anode. Further savings of metal are due to the absence of spilling of liquid metal and formation of dross. This decrease in aluminium consumption per anode can offset, fully or in part, the higher price of aluminium wire (whether manufactured in the smelting plant or purchased externally) compared to the liquid aluminium used in the prior art process.

Example

The process according to the invention has been used for coating anodes used in industrial electrolysis cells. Comparative tests with both prior art coating process and inventive coating process were carried out on anodes of the same size running in the same cells. The inventive thermal spray process was run on commercially available spraying equipment using 2 mm aluminium wire. Figure 2 shows a typical photographic picture of aluminium coatings according to prior art (figure 1(a)) and according to the invention (figure 1(b)), broken off from an anode coated by the process according to prior art (figure 1(a)) and according to the invention (figure 1(b)).

The prior art anode coating process based on spraying of liquid aluminium leads to a coating with a thickness comprised between 3 mm and 10 mm. The inventive process using thermal spraying leads to a coating with a thickness that does not exceed 2 mm. When for a given anode size the prior art process consumes about 4 to 9 kg of liquid aluminium per anode, consumption of the inventive process consumes was less than 2 kg of aluminium wire. Coating adhesion and protective effect were excellent for coatings obtained by thermal spraying.

Figure 3 shows a cross sectional micrograph of a coating sample according to prior art (figure 3(a)) and according to the invention (figure 3(b)). The solid bar on the right of each micrograph is representing a length of 1 000 pm. It can be seen that the sample according to the invention is denser than the prior art sample; the latter includes large voids that are absent in the former. Figure 4 shows typical micrographs obtained by scanning electron microscopy on a coating sample according to prior art (figure 4(a)) and according to the invention (figure 4(b)); the solid bar at the bottom of each picture is representing a length of 400 pm.

The average thickness of the prior art coating sample was about 2.65 mm (based on 20 individual measurements, minimum value 2.09 mm, maximum value 3.13 mm) whereas the average thickness of the coating sample according to the invention was 1.01 mm (based on 20 individual measurements, minimum value 0.89 mm, maximum value 1.33 mm). Vickers microhardness measurement were carried at several locations of each of the polished transverse sections; with an average value of 36 HV for the coating according to the invention, and 29 HV for the prior art coating (based on 6 individual measurements).

Claims

1. Method for coating an anode (13) intended to be used in molten salt electrolysis for making aluminium, wherein a coating (53) is applied onto at least part of the upper and/or side surfaces of said anode by thermal spray coating.

2. Method according to claim 1, wherein said anode (13) is a prebaked carbonaceous anode.

3. Method according to any of claims 1 or 2, wherein said anode (13) is part of an anode assembly (9), said anode assembly further comprising an anode rod (3) fixed to said anode, preferably at the upper surface of said anode.

4. Method according to any of claims 1 to 3, comprising the steps of:

(a) Providing an anode (13), (b) Applying a coating (53) onto at least part of the upper and/or side surfaces of said anode by thermal spray coating.

5. Method according to any of claims 1 to 4, wherein said spray coating is applied through one or more nozzles.

6. Method according to claim 5, wherein said nozzle is moved with respect to said anode (13).

7. Method according to any of claims 3 to 6, wherein said anode assembly (9) is suspended on its anode rod (3) during spray coating.

8. Method according to any of claims 1 to 8, wherein said thermal spray (62) is generated by the steps of:

- melting a feedstock material (56) to generate melted spray material,

- propelling, by using a process gas, said molten spray material through a nozzle onto at least part of the upper and/or side surfaces of said anode, where said spray material solidifies.

9. Method according to claim 8, wherein said heat source and said nozzle are parts of a spray gun (55).

10. Method according to any of claims 1 to 9, wherein said coating is selected from an aluminium coating or an alumina coating.

11. Method according to any of claims 8 to 10, wherein said feedstock material is aluminium rod or aluminium wire (56).

12. Method according to any of claims 8 to 10, wherein said feedstock material is alumina.

13. Method according to any of claims 8 to 12, wherein said process gas is selected from nitrogen, hydrogen, argon and helium, and mixtures of these elements.

14. Method according to any of claims 1 to 13, wherein said coating (53) has an average thickness comprised between 0.2 mm and 3.5 mm, and preferably between 0.2 and

2.5 mm, and still more preferably between 0.3 and 1.7 mm.

15. Method according to any of claims 3 to 14, comprising the steps of:

(a) Providing an anode assembly (9), (b) Transporting said anode assembly to a spray coating station comprising a spray gun (55) capable of generating a thermal spray (62);

(c) Spray coating at least part of the upper and/or side surfaces of said anode.

16. Method according to claim 15, wherein said anode assembly is suspended on its anode rod (3) during steps (a) and (b).

17. Method according to any of claims 1 to 16, wherein said upper and side surfaces of the anode do not undergo any specific treatment prior to thermal spray coating.

18. Anode intended to be used in molten salt electrolysis for making aluminium, said anode being at least partially coated by acoating, characterised in that said anode is obtainable by the method according to any of claims 1 to 17.

19. Process for making aluminium by using the Hall-Heroult process, wherein at least one anode according to claim 17 is plunged into the molten electrolyte of an electrolysis cell.