Patent Application
20240240342
The invention relates to the production of Aluminium and relates in particular to a cathode current collector bar in contact with a carbonaceous cathode of a Hall-Héroult Aluminium production cell.
Aluminium is produced by the Hall-Héroult process, by electrolysis of alumina dissolved in cryolite based electrolytes at temperature up to 1000° C. A typical Hall-Héroult cell is composed of a steel shell, an insulating lining of refractory materials and a carbon cathode holding the liquid metal. The cathode is composed of a number of cathode blocks in which collector bars are embedded at their bottom to extract the current flowing through the cell.
WO 01/63014 describes a collector bar construction for use in a Hall-Héroult electric reduction cell to produce Aluminium. Each collector bar includes a core of relatively high electrical conductivity material (copper or a copper alloy) and an outer housing of a more chemically resistant material than the core material, typically of steel. Preferably the collector bar is cylindrical, with the diameter of the core being 60-80%, preferably 70%, of the diameter of the collector bar. This means the diameter and hence the thickness of the steel housing is at least 20% of the diameter of the copper or copper alloy core. In the given example, a 70 mm diameter copper rod is fitted into a steel tube 100 mm outer diameter and 70 mm inner diameter, ie. the steel tube has a wall thickness of 15 mm. This corresponds to a relative volume of copper to steel of 96% or steel to copper of 104%.
Each collector bar includes a section that is cast or glued in a channel of the cathode block. This typically involves rodding the end surface that fits against the cathode and its side surfaces with cast iron when the cell is put into service.
US 2010/00258434 provides a composite conductor bar of simpler construction than that of WO2001/063014, with a massive body of lower conductivity (steel) that is bonded to and supports a smaller body of greater conductivity (copper) that contacts the carbon cathode, and where the relative cross-sectional areas of the two conductors of the composite collector bar optimize electrical current and heat flux through the composite.
An object of the invention is to provide a cathode current collector assembly for a Hall-Héroult Aluminium production cell, the assembly comprising an elongated current collector bar made of highly electrically conductive copper or a copper alloy coated on its surface facing the cathode or all around with a protective layer that is more mechanically and chemically resistant than the copper or copper alloy, which assembly:
The invention relates to an Aluminium production cell comprising an elongated cathode current collector bar in contact with a carbonaceous cathode, of the type where the elongated cathode current collector is made of highly electrically conductive copper or a copper alloy is coated on its surface facing the cathode or all around with a thin steel protective layer that is more mechanically and chemically resistant than the copper or copper alloy.
According to the invention, the thickness of the thin protective steel layer corresponds to a minimum thickness of the layer that is sufficient to form an effective diffusion barrier to protect the copper or copper alloy of the current collector bar from diffusion of reaction products produced on the carbonaceous cathode during operation, wherein:
Preferably, the protective thin steel layer or an optional pre-applied thinner conductive non-ferrous under or overcoat on the protective steel layer is in direct contact with walls of a slot in the carbonaceous cathode.
Alternatively, but less preferred, the protective thin steel layer—possibly including an optional pre-applied thinner conductive non-ferrous under or overcoat—is in contact with the carbonaceous cathode through a conductive layer of ramming paste, cast iron or glue.
The protective thin steel layer can be made of standard steel or alloy steel. Standard steel is an alloy of iron with typically a few tenths of a percent of carbon to improve its strength compared to iron. Alloy steels are made of iron, carbon and other elements such as vanadium, silicon, nickel, manganese, copper and chromium. Preferably the protective thin layer is made of low-carbon steel, chromium-based steel, nickel-based steel or chromium-nickel based steel. Other examples are low carbon manganese based steels with various impurities.
In preferred embodiments, the thickness of the protective thin steel layer is preferably from 1.5 mm to 3 mm. The volume ratio between the conductive copper or copper alloy and the protective thin steel layer is higher than 200% and preferably higher than 300% or more preferably higher than 400%, for example in the range 300%-950% or 400%-500%.
The cathode collector bars of the RuC cathode, with the thin steel protective layer on a copper or copper alloy core or bar, can be produced by hot or cold extrusion processes, hot or cold rolling processes, hot or cold drawing processes, hot or cold hammering processes, wrapping and/or welding processes and shrink fitting process.
One example of this production process is that the cathode collector bar can have a cylindrical core of copper or copper alloy bar and the protective thin steel layer can be a tube that is pressed against the copper or copper alloy bar in such a way that the copper or copper alloy core is in full contact with the protective layer to achieve a homogeneous pressure of the cathode current collector bar towards the carbon cathode once in operation.
Initially there can be a gap between the copper or copper alloy and the protective thin steel layer, which gap is smaller than the thermal expansion of the copper or copper alloy, to achieve a contact pressure between the copper or copper alloy and the protective thin steel layer and as well between the protective thin steel layer and the carbon cathode.
In another embodiment, the copper or copper alloy is in the form of a bar of rectangular or square cross-section that is protected on one side facing the cathode with the protective thin steel layer.
In some embodiments, the protective thin steel layer is coated with an additional top layer and/or under layer of copper, nickel and/or chromium and/or a graphite paint or foil layer wherein the additional top layer and/or underlayer preferably has a thickness of from 1 μm to 1 mm. The copper, nickel and chromium layers can be electrodeposited or otherwise applied.
When the copper or copper alloy is in the form of a rectangular bar, the protective thin steel layer is coated on all sides of the rectangular bar, or on one side of the rectangular bar and, from adjacent to the coated side, at least partly along the two other sides of the rectangular bar.
In the cell according to the invention, the external end of the collector bar of copper or copper alloy is connected to the external current bus preferably by a massive steel bar as described in WO 2016/07905 and U.S. Pat. No. 11,136,682. Alternative connections to the external current bus are described in WO 2018/019888 and WO 2018/019910.
Also, in the cell according to the invention, the collector bar of copper or copper alloy with a thin protective steel layer is normally disposed horizontally as in conventional cells, but in a variation the collector bar could include an inclined part as described in WO 2018/019910 to consolidate the feature that the cell does not require rodding with cast iron.
As illustrated, the collector bar 7 can be split in its center to leave a gap 7′ mainly to compensate for thermal expansion, but such a gap is not essential.
Zone 10 of the collector bar is for example insulated by being wrapped in a sheet of alumina or by being encased in electrically insulating ceramic material or simply electrically insulating material.
The collector bar 7 is made of a copper or copper alloy core or bar with a thin protective steel layer that can be applied along its entire length for manufacturing convenience. However, this thin protective steel layer is not required in the insulated zone 10 but is essential in the central zone 11 where it contacts the carbon cathode for transfer of electric current and protects the copper or copper alloy.
The RuC cathode uses a thin protective steel layer on the copper bars and differs from the conventional carbon cathode from many points of view:
The current collector bars of the RuC cathode can be assembled by thermal fit. Thermal fit means a way of simply inserting the current collector bars into a precisely machined cavity into the carbon block, that is sufficient to hold it in place without any intermediate holding material such as used in all traditional rodding steps. An electrical contact is made through the different thermal expansion behavior of the collector bar and the carbon cathode block during heat up from room temperature to operating temperature in the cell start-up phase. Traditional rodding, glue or ramming paste is advantageously avoided with the use of the copper bar surrounded by the thin protective steel layer as described. The current collector bar is simply inserted into the precision-machined graphite slot with an interference that is sufficient to hold it in place without any intermediate holding material such as used in all traditional rodding steps. The thin protective steel layer allows the thermal fit that makes the cathode ready to use without any further process or materials.
In traditional technology on cell start-up, the cathode collector bar has to be rodded by a cast iron process that is time consuming and involves safety hazards and technical risks for performance and integrity of the carbon cathode blocks. Also, the use of cast iron involves large contact areas between the bar and the carbon block due to the cast iron shrinkage at rodding time. Moreover, pouring of cast iron needs large gaps in between the current collector bars and cathode. All these disadvantages are advantageously obviated with the RuC cathode which however can be implemented in a less preferred way in contact with the carbonaceous cathode through a conductive layer of ramming paste, cast iron or glue.
The specific conductivity of copper is much higher compared to that of steel, cast iron, carbon pastes, graphite pastes and carbonaceous glues used inside cathode blocks.
At an operation temperature of 1000° C., the ratio of specific electrical conductivities for Cu to steel is from 8-15. For example, Cu is 10 times more conductive than steel. To achieve the same or an equivalent electrical resistance of a steel bar by a Cu bar of the same length, with Cu bars a 10 times less cross section and volume is needed.
As an example, a typical cross section of a conventional steel bar used inside cathodes is 122×122 mm2 (14,884 mm2). To replace this by a Cu bar (conductivity ratio 10) and have the same electrical resistance for the same bar length, a Cu bar with a cross section of 1,488 mm2 would be enough, which means a Cu bar with 70 mm height and a thickness of 21.3 mm. The Cu bar height is only 57% of a steel bar and the Cu bar width is only 17% that of the steel bar.
The thermal expansion of the Cu bars from room temperature up to 1000° C. is 0.3-0.4 mm while a steel bar of 122 mm width would expand 1.4-1.5 mm, which means 4-5 times more. This expansion of more than 1 mm for steel bars, leads to enormous stress in the slot radius and leads to wing cracks. To prevent this with steel bars, initial air gaps must be provided at room temperature. The overlapping thermal expansion of a steel bar at operating temperature has to be typically on a level of 0.1-0.3 mm. This range is achieved with Cu bars by thermal expansion without any initial air gap or with a tight mechanical fit (i.e. initial air gap measured of the order of several μm) in the slot. A tight mechanical fit is good enough, and is advantageous to ensure a high contact pressure at operation temperature but at the same time not overstressing the wings of cathode materials. Lower wing height also supports this low stressing of cathode material.
Measurements of CVD (Cathode Voltage Drop) and cathodic resistance of the RuC cathode have shown and demonstrated a low contact resistance and low contact voltage, even with 30-50% of the contact area compared to steel bars.
With the inventive RuC cathode, by providing a very thin steel protective layer on the copper/copper alloy, the problem of differential thermal expansion is minimized. The problem of initial air gap and poor electrical contact is suppressed, and the contact pressure is assured at all time.
Cu Melting Temperature Decreases with Increased Alloying with Al and Si or Other Elements
The inventive thin steel barrier will prevent the alloying of Cu with elements like Al and Si during operation in the electrolysis cell. This will prevent any melting that can happen without protection: Electrical conductivity, as well as the melting temperature of Cu (1083° C.), are lowered by alloying, as can be shown by a phase diagram.
In conventional Cu bar designs, the Cu is protected solely by cast iron, but in most cases the Cu parts are protected by two thick layers. A first layer is cast iron coming from cast iron rodding with a typical thickness of 10-30 mm. A second layer is a thick steel layer around the Cu insert whose thickness depends on the shape and design and is typically in the range 10-200 mm. The overall thickness of the 2 layers is 20-200 mm.
With the new RuC solution the barrier is reduced to one thin layer of steel (advantageously with no cast iron), which is enough for the protection over the cell lifetime. For the RuC the thickness of the thin steel layer is reduced by 5-20 times less compared to conventional designs.
A rectangular slot with 27 mm width and 105 mm depth was machined into conventional carbonaceous cathode feedstock blocks with dimensions 400×450×3300 mm3 using a conventional end mill that was moved through the bottom face of the feedstock block. Two copper cathode collector bars steel clad according to the invention as indicated below, sized 27×85×1670 mm3 (width×height×length), were inserted symmetrically into the formed slot of each block, leaving a gap of 150 mm in the middle of the block, which is filled with conventional refractory material. If required due to slight deformations of the collector bar, a mechanical or hydraulic press was used to push the collector bars into the slot. Different variants of steel clad were produced by cold rolling on rectangular copper bars: steel clad thicknesses of 1.0 mm, 1.7 mm, 2.0 mm, and 2.5 mm, with corresponding copper bar cross sections (width×height) of 25.0×83.0 mm2, 23.6×81.6 mm2, 23.0×81.0 mm2, and 22.0×80.0 mm2, with approximate volume ratios copper to steel of 9.4, 5.2, 4.5, and 3.3. The collector bars are then connected at their outer ends through a steel block of larger cross-section which is then connected to the current source of the electrolysis cell.
Copper cathode collector bars as in Example 1 were each coated with a layer of low carbon steel 2 mm thick. The cross section of the cathode collector bar, including a 2 mm thick layer of low carbon steel, applied by cold rolling, was 30×75×1670 mm3 (width×height×length). The collector bar consists of a copper core with rectangular cross section of 26×71 mm2 (width×height). The steel layer has on all four corners outside over the length of the rectangular shape a radius of 3.4 mm. A steel plate of 30×75 mm2 (width×height) with a thickness of 3 mm was applied at the end of the cathode collector bar for connection to an external current supply. The tolerance of width of the cathode collector bars was +/−30 μm over the full length.
A rectangular slot with 30.07 mm width (+/−30 μm over the full length) and radius 4.0 mm and 105 mm depth was machined into a conventional carbonaceous cathode feedstock block with dimensions 400×450×3300 mm3 using a conventional end mill that was moved through the bottom face of the feedstock block. An initial nominal air gap of 0.07 mm was applied in between the cathode collector bar and the machined slot. Two cathode collector bars according to the invention, sized 30×75×1670 mm3 (width×height×length), were inserted symmetrically into the formed slot, leaving a gap of 150 mm in the middle of the block, which is filled with conventional refractory material. If required due to slight deformations of the collector bar, a mechanical or hydraulic press was used to push the collector bars into the slot.
The collector bars are then connected at their outer ends through a steel block of larger cross-section which is then connected to the current source of the electrolysis cell.
A rectangular slot with 27 mm width and 105 mm depth was machined into a conventional carbonaceous cathode feedstock block with dimensions 400×450×3300 mm3 using a conventional end mill that was moved through the bottom face of the feedstock block. Two cathode collector bars steel clad according to the invention as indicated below, sized 27×85×1670 mm3 (width×height×length), were inserted symmetrically into the formed slot, leaving a gap of 150 mm in the middle of the block, which is filled with conventional refractory material. If required due to slight deformations of the collector bar, a mechanical or hydraulic press was used to push the collector bars into the slot. The collector bar consists of a copper core with rectangular cross section of 21.4×79.4 mm2 (width×height) which is surrounded by a double layer of graphite foil having a thickness of 0.1 mm each. This intermediate is then covered with a 1.7 mm thick layer of low-carbon steel through cold rolling. Finally, the steel layer is coated with layers of nickel (0.4 mm thick) and chromium (0.4 mm thick), followed by another layer of graphite foil (0.1 mm thick), giving the aforementioned overall dimensions. The collector bars are then connected at their outer ends through a steel block of larger cross-section which is then connected to the current source of the electrolysis cell.
Aluminium production cells were fitted with the cathode blocks and cathode current collectors of Examples 1 and 2, without any rodding with cast iron or glue or ramming paste, and were subjected to long term testing for a period of at least 20 months. The cells were started with an electrical preheat using full current load of 11.0 kA per cathode (no shunts available). The average cathode current density was 0.83 A/cm2. During operation, the cells operated with an average current of 5.5 kA per copper bar on each side of the cathodes. The operating bath temperature was in the range of 955° C. to 975° C. To test the robustness of the copper bars at higher temperature, the cell was brought to 1100° C. for 10 hours. No impact of the high temperature could be observed at autopsy time. The cathodes were graphitized with a thermal conductivity close to 100 W/mK at 1000° C. The voltage was measured between the liquid metal and the end of the collector bar periodically together with the current. As shown in
Aluminium 42 formed at the protective layer on the carbon cathode side 44 after operation in a Hall-Héroult cell for 18 months. The 100% pure copper 40 is protected and shows no Aluminium concentration after 18 months. The Aluminium layer 42 may vary depending on the carbon cathode grade. It is 400 microns thick in our example. On the copper core side, a layer 45 of 50 microns contains some copper diffused in the protective layer. The protective thin steel layer 41 shows a network of carbides 46.
The conditions of the above Examples can be varied as indicated below without compromising the performance in terms of the cell operating voltage, lifetime, and protection of the copper layer from unwanted alloying by aluminium.
Instead of being of rectangular cross-section, the copper collector bars can have a square cross-section or a round cross-section.
The thickness of the steel layer can be varied from 0.15 mm to 4 mm. Below a thickness of 0.15 mm the steel layer provides an insufficient protective effect. A steel layer thicker than 4 mm leads to an increase in operating potential and problems of recuperation of copper at the end of the cell lifetime. Within these extremes, the steel layer is preferably from 1.5 to 3 mm thick.
If an intermediate under or over layer of graphite and/or nickel and/or chromium and/or copper is applied its thickness is preferably from 1 μm to 1 mm and should normally be less than the thickness of the steel layer.
The gap between the facing ends of the collector bars can be varied notably as a function of the length of the copper collector bars to account for their thermal expansion at the cell operating temperature.
| Number | Date | Country | Kind |
|---|---|---|---|
| 00522/21 | May 2021 | CH | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2022/050595 | 1/24/2022 | WO |
