Anode Servicing Assembly for an Aluminium Electrolysis Plant, and Methods for Operating the Same

The invention relates to the field of fused salt electrolysis, and more precisely to anodes for use in the Hall-Héroult process for making aluminium by fused salt electrolysis and to the methods for replacing anode assemblies. Carbon anodes are consumed in the course of the electrolysis process; they need to be replaced regularly. The invention relates to a device for replacing anode assemblies in electrolytic cells that is used in conjunction with a dedicated overhead crane, forming with the latter a system for replacing anode assemblies and for carrying out certain maintenance operations related to the replacement of anode assemblies.

The Hall-Héroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (AlO) is dissolved in a molten cryolite (NaAlF) based electrolyte. The dissolved alumina is electrochemically decomposed with the aid of a consumable anode at a temperature that is typically comprised between 950° C. and 970° C. in an electrolytic cell. The typically rectangular electrolytic cell (also called “pot”) used for all modern Hall-Héroult cells typically comprises a steel shell (so-called potshell), the base of which contains a thermally insulating lining that includes refractory bricks and supports heat, and an array of transversely positioned cathode blocks usually made from graphite, anthracite or a mixture of both. These blocks are jointed to each other and each one has an embedded metal bar or bars for connection to the negative side of a DC power source. The sidewall is also sealed to carbonaceous lining base with carbonaceous material making a leakproof container for the liquid aluminium produced, and the less dense electrolyte containing the dissolved alumina. An electrically isolated superstructure is fixed on the top of the steel pot shell, and this is designed to contain consumable materials (alumina and aluminium fluoride) hoppers, breakers and feeders. The superstructure supports a height adjustable aluminium busbar (also called “anode beam”) to which the anodes are clamped, and enable enclosing the atmosphere so that emissions can be collected and appropriately treated.

Anodes in the Hall-Héroult process are usually prebaked cuboids made from a carbonaceous material. The anodes are 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 from the anode beam down to the carbon anodes. Anode hangers are fixed to the overhanging anode beam in a detachable manner using a system comprising hooks and 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 stub holes of the carbon anodes and is fixed with cast iron acting as temperature-resistant, electrically conductive contact element. This process is called “rodding”. The assembly formed by a carbon anode fixed to its anode hanger is called an “anode assembly”.

The plurality of anodes supported by the adjustable anode beam, and which are usually arranged in two rows are immersed into the molten salt electrolyte to the required height above the liquid aluminium cathode. Anodes and cathodes are connected to external busbar circuitry to enable electrical current through the cell at a controller determined voltage that is invariably somewhere between 3.8 V and 5 V. This arrangement combined with the electrical energy provided enables electrochemical decomposition of the ionic species arising from dissolving the aluminium oxide in the molten fluoride-based solvent. The aluminium containing cationic species formed from the alumina dissolution are electrochemically reduced at the metal pad surface to liquid aluminium. Simultaneously the oxy-anions formed from alumina are reactively oxidised to carbon monoxide and more dominantly to carbon dioxide at the carbon-anode interface. 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 (so-called “tapping” procedure), said crucible being typically carried by a vehicle moving on the ground floor or by an overhead crane.

Because of the electrochemical consumption of the carbon, each anode assembly has a finite life. Furthermore, in order to maintain metal quality, and thus ensure no metallic part of the anode assembly comes into contact with the electrolyte, only approximately 80% of each anode is consumed electrochemically and chemically before it is considered to be a “spent anode assembly”, thus needing replacement. Depending on the design and dimensions of the carbon block, the anode assembly, usually has an operating lifetime between 18 to 30 days. Knowing that a modern electrolysis cell typically comprises between 16 and 40 anodes, the replacement of an anode or a pair of anodes occurs between 24 and 48 hours in each pot and more often every 32 hours. In order to minimize the disturbance to the operating cell condition the replacement of all spent anode assemblies are scheduled at a regular interval such that each is changed once in the operating life cycle. Hence depending on the number of anodes and their size, typically, the replacement of an anode or anode pair will occur at intervals somewhere between once every approximately 24 to 48 hours for each cell.

Together with liquid metal tapping, anode change is the most frequent periodic maintenance operation in the Hall-Héroult process, which is, as such, a continuous process. Anode change is carried out by replacing a spent anode assembly by a new anode assembly. The spent anode assemblies are transported to a dedicated workshop in the plant, where the remains of the spent carbon anode are removed from the anode yokes, and the anode hangers of the spent anode assemblies, are cleaned and reused in new anode assemblies; new carbon anodes are fixed to the anode yoke, thereby creating a “new anode assembly”.

The present technology and work practices associated with removing a spent anode assembly and replacing it with a new anode assembly has numerous manual labour-intensive steps that not only lead to excessive harmful emissions, but are also wasteful of the stored energy as well as introducing performance harming disturbances, and presenting a health hazard to workers.

The said cathode blocks that form the bottom of the electrolytic cell undergo a series of very slow degradation steps, limiting their lifetime to typically between five to eight years. On failure the superstructure is removed and the spent potshell is lifted using a heavy overhead crane (so-called “Cathode Transport Crane”, abridged CTC), enabling a replacement pre-lined pot shell to be positioned, ready for commissioning. Said cathode transport crane needs to have a very high lifting power, typically of more than 200 tonnes.

Electrical energy is a major operational cost in the Hall-Héroult process, and two of the contributors to the energy wastage, cell heat loss and lowering of current efficiency, are magnified by the present procedure and equipment used to enable anode changing. Unnecessary heat loss occurs through wastage of the energy content of the anode cover material used for the spent anode, while a high radiant heat loss occurs during the extended duration of cavity cleaning that becomes necessary from secondary effects of the Jack hammering of the anode cover sealing crust. There are inevitable spillages of anode cover material lumps into the electrolyte arising from the jack-hammering which leads to metal pad turbulence that harms current efficiency.

Besides carbon dioxide, gaseous emissions of the Hall-Héroult electrolytic process also comprise fine particulate condensed fluoride arising from the electrolyte as well as hydrogen fluoride, originating from impurities in the smelter grade alumina that also dissolve in the electrolytic bath, and minor amounts of SOoriginating from impurities in the anode carbon. These are noxious to workers and to the environment. Therefore, since the 1970s, gaseous emissions of Hall-Héroult cells are no longer released into the environment but undergo purification. More precisely, gaseous emissions from electrolytic pots are collected using protection devices such as hooding systems and are treated in gas treatment stations to remove fluorine compounds and other noxious compounds before releasing the collected air into the environment. The prior art work practice for replacing spent anodes results in some of these harmful chemicals escaping into the working environment.

In practice, the hooding system at the level of the electrolytic cell comprises an arrangement of movable hood panels, which are typically disposed on an elongated frame which extends substantially parallel to each of the two parallel lines of anodes. This frame supports two fixed end walls, located at the opposite extremities of the anodes line, and facing the front side of the anodes, several panels (so-called “hood panels”) are provided between these end walls. These hood panels are typically shaped such as to ensure an optimal protection and tightness. They also act as heat shields, thereby limiting thermal losses of the electrolytic cell and protecting the workers. For maintenance, the hood panels can be removed, as needed, to gain access to the anode hangers.

Nowadays anode assembly replacement (this operation is more conventionally called “anode change”, and this expression will be used in the following) is usually carried out using a so-called pot tending machine and a floor operator. During anode change, a number of individual operations need to be carried out, starting with the removal of at least one of the pot hood panels (and more frequently, of two adjacent hood panels) in order to access to the anode assembly to be removed. The removal of the anode assembly is also done by the pot tending machine which is controlled by an operator. The preparatory operation prior to this is to break the solidified electrolyte/vapors/alumina crust used to conserve heat and prevent air access to the anode carbon surface that is not immersed in the electrolyte. The actual removal operation of the spent anode assembly involves unfastening the clamps by which the anode rod is fixed to the height adjustable anode beam, vertical lifting of the spent anode assembly, and positioning of the removed anode assembly on the floor level (usually in a pallet). Good work practice usually then requires cleaning the cavity of the crust of solidified electrolyte that has entered the electrolyte by using a cavity scoop. Then the new anode assembly is inserted into the cavity, fixed to the anode beam and precisely positioned; in particular, the anode bottom needs to be positioned at a given height for accurate control of the anode-cathode distance. Then the hood panel/s is/are put in place (possibly after having been inspected, or another hood panel in good condition is used).

This procedure invariably takes between ten and twenty minutes. As noxious gases may escape from the pot when the hood is open, it is desirable to minimize both the time during which the hood is open and the presence of a floor operator.

Pot tending machines (PTM) have been available for many years, manufactured by ECL (Fives group) or Noell-NKM (REEL group). Their basic function is to carry the anode assembly during an operation known as anode beam raising. Since the carbon anodes are gradually consumed during the electrolysis process, the anode beam, which is holding all the anodes in position, has to be lowered gradually in order to maintain a constant anode-cathode distance. Eventually, the position of the anode beam reaches a physical lower limit. At this point, the anode beam has to be raised by use of a special anode-beam raising equipment (ABRE) carried by the pot-tending machine. During this process, the anode rods remain in electrical contact with the anode-beam although they are mechanically disconnected (anode connectors loosened) from the anode beam while the anode beam is raised sliding against the anode rods to maintain the electrical contact. The anodes are hold in position by said ABRE, which presses the anode rods against the anode beam to maintain the electrical contact.

Modern pot tending machines have several additional functions. They can be combined with the overhead travelling crane (also called bridge crane), and a cabin for the PTM operator. They can also be used for carrying the crucible during the tapping procedure. The lifting power of modern pot tending machines is usually in excess of 25 metric tons, and more frequently in excess of 35 or 40 metric tons, especially when they are designed for tapping.

Anode changing devices also exist mounted on dedicated ground vehicles; these are usually specifically designed trucks. Such anode changing vehicles are offered for sale by GLAMA company, as well as by HENCON and TECHMO. They may be too large to be usable in potlines having a very small distance between two neighboring electrolysis cells. Furthermore and more generally, it may be undesirable to use vehicles at the floor level between the electrolysis cells, and as a matter of policy, it may be deemed even more undesirable to use long metallic tools extending horizontally at the floor level between the electrolysis cells and the potroom building, as this implies electrical hazards.

Pot tending machines integrated on an overhead crane are designed to avoid such electrical hazards. Pot tending machines that carry out specific tasks in the anode assembly replacement process in Hall-Héroult cells have been described in a large number of patent publications. These machines may have a variable degree of automatization. WO 2004/079046 and U.S. Pat. No. 8,888,156 describe handling grippers for gripping anode rods prior to their transportation by a crane. WO 2004/101853 describes a tool for unlocking and locking the clamps by which the anode rod is fixed to the anode beam. U.S. Pat. No. 8,066,856 describes a pneumatic impact generator that may improve the electrical contact between the anode rod and the anode beam when mounting the new anode assembly. WO 2005/095676 describes a service module for anode changing equipped with different tools that can be mounted on an overhead crane; this movable service module comprises a cabin for the operator. WO 2006/010816 and US 2007/0205104 describe further tools for such service modules. U.S. Pat. No. 8,273,223 describes tools for handling hood panels. WO 2010/079266 describes a service module equipped with a crust shovel for cleaning the cavity during anode change; WO 2011/130892 and US 2012/0234690 disclose other designs for crust shovels. US 2008/0251392 describes a method for changing an anode, comprising a step of positioning the anode assembly replacing a spent anode assembly that can be automatized. WO 2006/030092 describes another such method that includes a device for measuring the vertical distances travelled by the tool using electromagnetic or acoustic waves. WO 2016/128631 and FR 3 032 457 disclose a pot tending machine comprising two separate service modules that can carry out different functions in parallel, which speeds up the anode changing process. All the patents mentioned in this paragraph are assigned to E.C.L.

These prior art pot tending machines aim at simplifying the work of the floor operator by reducing the number of individual tasks he has to carry out, such as handling the hood panels, assisting in the accurate positioning of the anode, cleaning the cavity. Some of these machines may be able to replace the floor operator and are only controlled by the PTM operator from the cabin. Such machines have been described for example in the publication “Electrolysis pots anode changing automation: impact on process and safety performance” by N. Dupas, Light Metals 2009 (TMS), p. 515-518, and in WO 2015/132479 (E.C.L.) that describes an automatic pot tending machine for anode change including multiple tools and a hood panel storage element. The publication “” by J. Guérin and A. G. Hequet, published in 2015 in Light Metals (TMS), p. 695-697 announces even a fully automatic anode change robot that no longer requires a crane operator. This new robot comprises a telescopic mast fixed to the overhead travelling crane, said mast bearing a plurality of individual tools, each being dedicated to an individual task.

This system has a number of shortcomings. First of all, whether or not there is a floor operator, it is desirable to shorten the time interval during which the hood is open, in order to minimize the release of noxious gases into the atmosphere which would otherwise be sucked into the exhaust gas purification system. Secondly, the existing automatic or semi-automatic anode changing machines, while they do simplify the work of operators, take longer than conventional methods for changing an anode. And thirdly, all existing automatic or semi-automatic anode changing machines depend on the availability of an overhead travelling crane. Knowing that these cranes may be required to carry out other tasks unrelated to anode changing, it is particularly undesirable to increase the total period of time that is required for changing an anode, even if the floor operator is no longer necessary: it is the crane time available for changing anodes that may be a limiting factor if the crane is needed elsewhere in the potline.

The present inventors, while acknowledging that it is desirable to move towards a fully automatized anode changing process, have tried to overcome these shortcomings of prior art anode changing robots.

The present invention presents a new anode servicing assembly for an aluminium electrolysis plant capable of changing anodes. Changing anodes is a rather complex procedure, referring to a certain number of individual operations, such as removing (and replacing) hood panels, unlocking (and locking) anode hooks, removing (and replacing) the anode assembly, vacuum cleaning, and so on. The term “servicing” as used herein refers to general servicing operations, and according to the invention, the anode servicing assembly can carry out one or more task related to anode change, and possibly additional task not related to anode change. Said anode servicing assembly comprises a crane beam and a specific servicing machine; said servicing machine comprises movable operating devices. The latter are capable of carrying out one or more individual operations carried out on individual anode assemblies (such as locking and unlocking anode hooks, and lifting used anode assemblies), and/or one or more individual operations carried out in relation with the change of individual anode assemblies (such as opening and closing anode hoods, and vacuum cleaning in the vicinity of the anode in the pot).

According to the invention, the problem is solved by two independent means that can be combined.

According to a first aspect of the invention, the anode servicing assembly is based on an additional, independent crane beam, adapted to move on the rails of the existing overhead travelling crane and/or on the rails of the existing pot tending machine (which may be all the same rails), but that is much lighter than conventional pot tending machines, because of dedicated, independent tools instead of complex multifunction service modules. As a consequence, a lifting force of less than about 20 tons, and preferably less than about 10 tons is sufficient for an anode servicing assembly according to the invention.

Furthermore, the inventors have recognized that prior art anode change robots tend to be rather slow, because their tools are all mounted on one or two service modules, each of which service modules can carry out only one function at a given time. As a consequence, these individual tools cannot operate fully independently one from the other.

According to a second aspect of the invention, the anode servicing assembly according to the invention comprises a plurality of independent tools, each of which is mounted on a separate service module. This allows a total independence of the different tools, each of which is dedicated to a given function. This total independence may allow, in certain cases, that two or more tools are carrying out movements simultaneously, and may allow, in certain cases, that two or more tools are carrying out different functions simultaneously.

A first object of the invention is an anode servicing assembly for an aluminium electrolysis plant, said aluminium electrolysis plant comprising at least one line (L, L) of electrolysis cells (C-Cn, C′-C′n) connected in series, each cell being connected to a cathodic busbar and each cell having a plurality of anode assemblies connected to an anode beam,

said anode servicing assembly being intended to be provided on at least one line of said plant,said anode servicing assembly comprising:

According to other aspects of the invention, at least one support assembly comprises at least one carrier movable with respect to said elongated body along said longitudinal axis (L) of said body, said respective operating device being fixed with respect to said carrier, at least in translation.

According to another aspect, at least one support assembly comprises at least one carrier movable with respect to said elongated body along said longitudinal axis of said elongated body, as well as at least one drive member provided with driving means, adapted to move said operating device with respect to said carrier along said vertical axis (ZZ).

Furthermore, at least one support assembly can comprise two carriers as well as two drive members, said driving means being adapted to move one single operating device with respect to said carriers along said vertical axis.

Said elongated body can comprise at least one, and in particular at least two parallel rails, at least one rail being provided with at least one track for translation motion of at least one carrier.

Said specific anode servicing function(s) is (are) advantageously chosen amongst: lifting a new anode, vacuum cleaning anode cover material surrounding and/or covering an anode, sawing solid crust material surrounding an anode, lifting of the spent anode, placing the new anode at specific height, adding anode cover material on and around an anode, recording images representative of the position and the shape of an anode, moving a hood panel giving access to an anode, and moving back the hood panels after completing the anode change.

It is possible, and advantageous, to design the devices such that the lifting power of each operating device is inferior to 20 metric tons, preferably less than 15 metric tons, and even more preferably less than 10 metric tons.

According to an embodiment, an anode servicing assembly according to the invention comprises at least two so-called lifting operating devices, each being adapted for lifting a respective anode assembly.

According to another embodiment, it may comprise (or further comprise) a so-called multi use operating device, said multi use operating device being provided with a head, said head being adapted to cooperate with

A second object of the invention is an aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, said plant further comprising means for electrically connecting said cells in series and means for connecting the cathodic busbar of a cell to the anode beam of a downstream cell, said plant comprising at least one heavy lifting assembly, and/or at least one pot tending assembly (),

This plant may further comprise at least one common running path usable by running means of said anode servicing assembly, adapted to cooperate with running means of the heavy lifting assembly and/or with the running means of said pot tending assembly, in particular two common running paths, provided on either side of said cells, with reference to transversal axis of said line.

A third object of the invention is a method of operating an anode servicing assembly according to the present invention, said anode servicing assembly being in particular part of an aluminium electrolysis plant according to the invention, said operating method comprising:

Said operating method can further comprise:

Said operating method advantageously further comprises:

Yet another object of the present invention is a method for making aluminium by the Hall-Héroult electrolysis process, characterized in that said method is carried out in an aluminium electrolysis plant according to the invention.

The following reference numbers are used in the figures:

The present invention is directed to the arrangement of a plant, also called aluminium smelting plant or aluminium smelter, using the Hall-Héroult process. This plant comprises a plurality of electrolysis cells (potline) connected in series. The Hall-Héroult process as such, the way to operate the latter, as well as the general structure of above electrolysis cells are known to a person skilled in the art and will not be described here. In the present description, the terms “upper” and “lower” refer to mechanical elements in use, with respect to a horizontal working surface. Moreover, unless otherwise specifically mentioned, “conductive” means “electrically conductive”.

As schematically shown on, the aluminium smelter of the invention comprises a plurality of electrolytic cells, typically arranged along two parallel lines Land L, each of which comprises n cells, i.e., Cto Cn and C′to C′n. The electrolysis current therefore passes in a cascade fashion from one cell to the next cell, along arrow DC. The number of cells in a series is typically comprised between 50 and over 250, even over 400 in the most recent smelters, but this figure is not substantial for the present invention.

The electrolytic cells are rectangular shaped and are arranged transversally (side by side), in reference to the line they constitute. In other words, the main dimension, or length, of each cell is substantially orthogonal to the main direction of the line, i.e., the circulation direction of current. The large sides of two adjacent cells are parallel. The electrolytic cells, or pots, can implement various technological variants that do not form a part of the present invention; such pots are known to a person skilled in the art. On the, only the contour of the external metal (steel) shell, or “potshell”, of the cells is shown.

The general structure of a Hall-Héroult electrolysis pot is known per se and will not be explained here in detail. It is sufficient to explain, in particular in relation with, that a typical cellincludes a potshell comprising a first longitudinal sidewall, a second longitudinal sidewall′, first and second transversal end walls (not visible on) and a bottom. The potshell walls define a space lined on its bottom and sides with refractory materials(maintaining the thermal balance during the electrolysis process) along with the cathode blocks, thereby defining a volume containing the molten metal and electrolyte. The side liningcomprises a layer of carbonaceous material (not shown on the figures) protected in steady state operation by solid electrolyte in contact with molten liquid material.

Said cathode blockscomprise one or more cathode collector bars. They protrude out of the potshell. Several anode assembliesare also provided, each comprising an anode rodand an anode. Electrical current enters the cell through anodes(suspended above the cell by anode rodsattached to an aluminium beam called anode beam, which is supported by the cell superstructure), passes through the molten electrolytic bathand the molten aluminium pad, and then enters the carbon cathode block. The current is carried out of the cell by the cathode collector barconnected to the cathode busbar not shown on. The cellis closed by a set of hood panels. In a way known as such, clamps, which are schematically represented, ensure removable fixing of anode rods to anode beam.

Typically, all the cells of the plant have the same structure. Two sheds,′ are also provided, each covering a respective line. On, these sheds are illustrated in phantom lines. As shown on, said shed,′ comprises sidewalls,as well as top walls. The shed,′ can be open or closed, at one or both longitudinal ends. With reference to these figures, each cell is arranged so as to leave running paths,on internal faces of side walls,of said shed,′. These paths allow the displacement of anode servicing assemblies along main axis of each line, as will be explained here after. Each path is provided with appropriate means, such as rails, for cooperating with complementary means of anode servicing assembly, such as wheels.

The combination, according to the invention, between a heavy lifting assembly (often designated as a cathode transport crane, abridged CTC) and/or a pot tending assembly (abridged PTM) on the one hand, and at least one anode servicing assembly on the other hand, will now be described. In particular, the anode servicing assembly advantageously uses the same running paths as the heavy lifting assembly and/or the pot tending assembly. When the heavy lifting machine is not needed, it does not stay in the line but (usually) in the pot repair area (not shown on the figures), from where it is moved into the pot line with a transboarding machine (not shown on the figures) generally located in the central passage.