WO2016088001A1

WO2016088001A1 - Electrolytic pot for hall-heroult process, having a cathode formed of high and low cathode blocks

Info

Publication number: WO2016088001A1
Application number: PCT/IB2015/059142
Authority: WO
Inventors: Ibrahim BAGGASH; Sergey AKHMETOV; Alexander Arkhipov
Original assignee: Dubai Aluminium Pjsc
Priority date: 2014-12-02
Filing date: 2015-11-26
Publication date: 2016-06-09
Also published as: GB201421335D0; GB2532949A

Abstract

A cathode structure of length x and width y (with x > y) for an electrolytic cell comprising a plurality of parallel cathode blocks of length a and width b (with a > b) extending over substantially the whole length x of said cell (with y ~ a), said cathode structure comprising a majority of so-called standard cathodes of height c_S and a minority of so-called tall cathodes of height c_T, the flat upper surface of said standard cathodes forming at least one so-called high portion at a height c_S, and the flat upper surface of said tall cathodes forming at least one so-called high portion at a height c_T, wherein c_T > c_S, and wherein each high portion has a width y_H ~ y, and each low portion has a width y_L ~ y, and wherein each low portion extends over at least two adjacent standard cathode blocks such x_L ≥ 2 b(x_L being the length of the low portion).

Description

Electrolytic pot for Hall-Heroult process, having a cathode formed of high and low cathode blocks

Technical field of the invention

The invention relates to a cathode for an electrolytic cell for producing aluminium by fused salt electrolysis using the Hall-Heroult-process.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium form aluminium oxide. Aluminium oxide (Al₂0₃) 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 "potsheH"), a lining (comprising refractory bricks protecting said potshell against heat, and carbon blocks usually covering the whole bottom of the pot (and which are usually made from graphite, anthracite or a mixture of both), said carbon blocks forming the cathode), a superstructure and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.8 V to 5 V) which electrochemical ly reduces the aluminium oxide into aluminium ions and oxygen ions. The oxide ions are reduced to oxygen at the anode, said oxygen reacting with the carbon of the anode. The aluminium ions move to the cathode where they accept electrons supplied by the cathode; the resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as liquid metal pad on the cathode surface from where it needs to be removed from time to time, usually by suction into a crucible.

The electrical energy is a major operational cost in the Hall-Heroult process. Capital cost is an issue, too. Ever since the invention of the process at the end of the 19^th century much effort has been undertaken to improve the energy efficiency (expressed in kWh per kg or ton of aluminium), and there has been a trend to increase the size of the pots and the current intensity at which they are operated in order to increase the plant productivity and bring down the capital cost per unit of aluminium produced in the plant. Industrial electrolytic cells used for the Hall-Heroult process are generally rectangular in shape and have a length usually comprised between 8 and 25 meters and a width usually comprised between 3 and 5 meters. Most newly installed pots operate at a current intensity comprised between about 400 kA and 600 kA. They are always operated in series of several tens (up to more than a hundred) pots (such a series being called a "potline"); within each series DC currents flow from one cell to the neighbouring cell.

The enormous currents at which these pots are operated induce high magnetic fields; they induce movement of the liquid metal pad and deformation of the interface between the liquid metal and the liquid electrolyte; these effects are so-called magneto-hydrodynamic effects. As a consequence the metal - electrolyte surface is not flat, and lateral movement of the liquid metal occurs. This has two disadvantages. The first disadvantage relates to the lack of flatness of the liquid metal - liquid electrolyte interface. As a rule it is desirable to keep the anode-cathode distance (also called "interelectrode gap") at a minimum level because this will decrease the electrical resistance of the electrolysis cell and improve its energy efficiency, knowing that the electrical resistivity of the liquid electrolyte is high. The interelectrode gap can be decreased by lowering the anodes plunging into the liquid electrolyte. However, a direct contact between the anode and the liquid metal must be avoided, as this leads to a short circuit in the cell. As a consequence, knowing that the metal-electrolyte interface is moving due to magneto-hydrodynamic effects and bubble formation, the interelectrode gap is usually higher than it could be if the metal-electrolyte interface was perfectly flat.

The second disadvantage relates to the lateral movement of electrical current in the metal pad which increases local current density on the cathode surface at the sides of the cathode that may lead to local wear of the cathode surface, which is one of the factors limiting the lifetime of the cathode. While in the Hall-Heroult process the carbon anodes are consumables that can be replaced easily each few weeks without disrupting the operation of the pot, the cathode is not a consumable, and its replacement requires a shutdown of the pot for several days, and the removal of the pot lining. Knowing that the normal lifetime of cathodes is five to eight years, early cathode failure due to local wear can seriously impair the economic efficiency of the plant.

Traditionally the cathode surface in contact with the liquid metal is essentially flat. The cathode is usually assembled from individual cathode blocks having a rectangular, flat upper surface, the length of which is close to the width of the pot, and the width of which is usually comprised between 40 and 70 cm. Usually all cathode blocks of a pot have the same width and are identical.

Non-flat cathode surfaces have been developed in order to improve the magneto- hydrodynamic stability of the cell. As an example, WO 2011/148346 (KANAK S.A.) describes a cell with the top of the carbon cathode cell bottom has a recessed central area.

In another approach, cathode structures with non-flat surfaces in contact with the liquid metal have been developed in which protrusions of various shape protrude out of the flat bottom (liner) of the pot. They are intended to act as a barrier to metal flow. This is expected to improve stability of the cell, to decrease energy consumption (by increasing current efficiency due to decreased inter electrode distance which is possible if the metal surface is flatter), and to improve the lifetime of the cathodes (by reducing wear due to reduced lateral current flow).

The publication « Development and application of an energy saving technology for aluminum reduction cells » by Peng Juanping et al., Light Metals 2011 , p. 1023-1027, shows various cathode structures with structured surfaces: longitudinal ridges, crisscross ridges and smaller blocks (mostly containing anthracite) inserted into the graphitized carbon cathode blocks. Such cathode structures are also described in various patent documents, especially WO 201 1/057483, WO 2011/079548, US 201 1/0056826, US 2013/01 12549, US 8,206,560. Similar cathode structures have been described in IN2011 DE0131 1 , CA 2 737 182, US 2013/0277212. They are all based on cathode blocks of standard form that are submitted to complex machining operations; these machining operations either generate directly the complex surface structure (ridges, bosses, protrusions), or machined pieces are inlaid into pot holes or grooves that have been drilled or machined into the cathode block. The manufacture of these structured cathodes thus implies expensive machining operations and a loss of carbon material due to cutting and machining. All these documents show electrolytic cells in which each cathode block is structured.

In particular, US 2011/0056826 (Shenyang Beiye Metallurgical Technology Co., Ltd.) proposes a shaped cathode structure providing various regular patterns based on alternate high and low sections of the carbon blocks. Channels or chess-board patterns can be formed. The protruding sections of the carbon blocks can have different machined cross sections. WO 201 1/057483 and WO 201 1/079548 filed by the same applicant show other embodiments of the same principle. Furthermore, CA 2 737 182 (Guangxi Qiangqiang Carbon Co. Ltd.) and IN 201 1 DE0131 1 describe a cathode structure in which graphitized blocks are inlaid at a junction seam between two cathode body blocks in a manner bridging over two adjacent cathode blocks. US 2013/0233704 assigned to China Aluminum International Engineering Corporation Ltd. describes displaceable cathode choking devices placed at the bottom of a Hall-Heroult cell; these choking devices are made of mullite, spinel or zirconite. US 2012/0279054 (China Aluminum International Engineering Corporation Limited) discloses a cathode structure in which high (tall) cathode blocks alternate with low cathode blocks; rectangular grooves are "disposed to ensure normal flow of molten aluminum during production". The description discloses three embodiments which are distinguished only by the chamfers between two adjacent blocks.

EP 2 133 446 A1 (Shenyang Beiye Metallurgical Technology Co. et al.) discloses a pot in which all cathodes have at least one protruding portion: embodiments with one, two and six protruding portions are shown. The height of the protruding portion may range from 50 to 200 mm. The cross section of the protruding portions may be shaped in rectangle or in steps.

WO 2012/159839 (SGL Carbon) proposes a shaped cathode structure which reflects a simplified outer contour if the respective peak of the distribution of the reference wave formation potential is viewed vertically from above. This complex shape is calculated using electrical, magnetic and magneto-hydrodynamic modelling. It implies complex machining.

US 2013/0112549 assigned to Shenyang Beiye Metallurgical Technology Co., Ltd. (China) proposes a shaped cathode structure providing columnar protrusions embedded on its upper surface. This surface is obtained by machining internally threaded pot holes on which the columns with an externally threaded section are screwed. These potholes can also be obtained at during the molding or compression stage of the green compact. Both pathways to these threaded pot holes are expensive processes. WO 2013/07845 filed by Sheneyang Beiye Metallurgical and Technological Co. Ltd. proposes a V-shaped cathode structure which may contain protruding portions, possibly coated with a TiB₂/C composite material.

CN 101 899677 A discloses a cathode structure with V-shaped machined cathode blocks. CN 201 908137 U discloses another example of laterally machined cathode blocks.

WO 2013/068412 and WO 2013/068485 (SGL Carbon SE) propose modifications of known shaped cathode structures, the modification referring to the rim of the cathode rather than to its central section. The first document provides a domed or rounded surface rather than angular surfaces. The second document provides a cathode structure in which the width (taken along the x-axis) of at least one of the bottom region and the edge region varies over the length of the cathode, and/or the height (taken along the z-axis) of the top side of the top side of the cathode varies over the length (taken along the y-axis) of the cathode. WO 2012/107397 (SGL Carbon SE) discloses a cathode with longitudinal recesses machined in its upper surface; the cross-section of the recess can be rectangular, V-shaped or undulated. WO 2012/038426 (SGL Carbon SE) proposes modifications of known shaped cathode structures, the modification being based on projections added to the surface of the cathode block. More precisely, the document proposes flat plates fixed to lateral or bottom surface of the cathode block; these plates need to be machined in order to include a threaded section. WO 2012/107397 and WO 2012/107396 (SGE Carbon SE) propose a cathode structure in which the cathode block comprises a base portion made from graphite, and a shaped top portion made of a carbon composite material containing between 15 and 50% by mass of a hard material with a melting point of at least 1000°C.

US 2013/0277212 assigned to China Aluminum International Engineering Corporation Ltd. proposes a cathode structure showing a protrusion (called "boss" in this document) on the top of each cathode, either in the centre of each cathode block or bridging the gap between two adjacent cathode blocks. The boss extends over the whole length of the cathode block (i.e. over the whole width of the cathode); it is not fully clear from the description how these bosses are fixed onto the cathode blocks. US 2013/0233704 assigned to the same applicant describes a similar cathode structure as one of the embodiment of the previous one (bosses in the centre of the width of the cathode block), but the "bosses" are divided in two sections over the length of a cathode block.

Similar periodic patterns of the cathode surface in contact with the liquid metal are described in the publication "Shaped cathode for the minimisation of the Hall-Heroult process specific energy consumption" by R. von Kaenel & J. Antille in Aluminium 1- 2/2012, p. 28 - 32, in the publication "Energy Reduction Technology for Aluminum Electrolysis: Choice of the Cell Voltage" by Feng Naixiang et al., Light Metals 2103, p. 549 - 552, and in the publication "Magnetohydrodynamic model with coupling multiphase flow in aluminum reduction cell with innovative cathode protrusion" by Wang Qiang et al. in Light Metals 2013, p. 615 - 619. All these periodic patterns show longitudinal ridges, crisscross ridges or grooves. The publication" -/D of aluminium cells with the effect of channels and cathode perturbation elements" by V. Bojarevics in Light Metals 2013, p. 609 - 614 describes computer modelling of certain ridge patterns of the cathode surface in contact with the liquid metal.

All these cathode structures have in common that they require rather complex machining operations that increase the cost of the cathode lining. Moreover, machining of intricate shapes in carbon blocks always presents the risk of crack formation or of propagation of existing cracks, which may eventually lead to the loss of the whole carbon block. Furthermore, the rather intricate protrusions and ridges intended to act as barriers to lateral metal flow being themselves subject to wear, the efficiency of this effect will decrease over the lifetime of the cathode lining, which may render the cell operation more complex. Another problem related to cathode structures showing intricate shapes and protrusions is preheating during the start-up phase of the pot that may require specific procedures and equipment.

CN 2015 45919 U, CN 2013 90784 Y, CN 101 775621 A, CN 101 775621 , CN 2014 16035 Y, CN 102 534668 A (as well as US 2012/0279054, cited above) show a cathode structure with staggered high and low cathode blocks. Electric preheating of such a cathode can be expected to be rather complicated.

It is the objective of the present invention to come up with a cathode structure that acts as an efficient barrier to metal flow, that is simple and inexpensive to manufacture, and that is resistant against local wear, and that can be preheated and operated using standard procedures and standard equipment.

Object of the invention

According to the invention the problem is solved by a cathode structure of length x and width y (with x > y) for an electrolytic cell comprising a plurality of parallel cathode blocks of length a and width b (with a > b) extending over substantially the whole length x of said cell (with y ~ a), said cathode structure comprising a majority of so-called standard cathodes of height c_s and a minority of so-called tall cathodes of height c_T, the flat upper surface of said standard cathodes forming at least one so-called high portion at a height c_s, and the flat upper surface of said tall cathodes forming at least one so-called high portion at a height c_T, wherein c_T > c_s, and wherein each high portion has a width y_H ~ y, and each low portion has a width y_L ~ y, and wherein each low portion extends over at least two adjacent standard cathode blocks such x_L≥ 2 b (x_L being the length of the low portion). This cathode structure is the first object of the invention.

According to an embodiment, said parallel cathode blocks have a uniform width b and a uniform length a.

According to another embodiment, each low portion extends over at least three adjacent standard cathode blocks such x_L≥ 3 b (x_L being the length of the low portion). According to another embodiment each high portion is formed by one single tall cathode block.

According to another embodiment at least one high portion has a length x_T greater than b, and is preferably formed by two or more adjacent tall cathode blocks.

According ot another embodiment at least one high portion has a length smaller than the width of the cathode blocks.

According to another embodiment at least one high portion has a bevelled or chamfered ridge.

Accordng to another embodiment at least one of the tall cathodes has an intermediate height between c_T and c_s. According to another embodiment not more than 40% (and preferably not more than 35%) of the upper cathode surface is a high portion.

According to another embodiment (c_T- c_s) = 50 mm to 150 mm, and preferably 60 mm to 100 mm.

According to another embodiment said cathode structure is substantially symmetrical with respect to a plane perpendicular to the cathode surface and parallel to the width y of said cathode structure. According to another embodiment everywhere in the pot the condition x_L≥ 2 b applies, and preferably everywhere in the pot the condition x_L≥ 3 b applies. According to another embodiment the high portions are machined in a way as to allow an electrical contact surface between the lower surface of the anodes and at least part of the upper surfaces of each cathode block during electrical preheating. All these embodiments can be combined with each other.

Another objet of the invention is an electrolysis cell comprising a cathode structure according to the invention and a plurality of anodes above said cathode structure. A last object of the invention is a process for producing aluminium by the Hall-Heroult process, wherein an electrolysis cell according to the invention is used.

Figures

Figure 1 illustrates the prior art. Figures 2, 3, 4 and 5 illustrate embodiments according to the invention.

Figure 1a shows a schematic perspective view of the cathode structure of an electrolysis cell for the Hall-Heroult process, the cathode structure comprising a plurality of parallel and substantially identical cathode blocks. Figure 1 b shows a schematic cross section of such cathode structure.

Figure 2 shows a schematic perspective view of a cathode structure according to the present invention, comprising a plurality of substantially standard cathode blocks forming two low surface portions and a plurality of substantially identical tall cathode blocks forming a high surface portion.

Figure 3 shows schematic cross sections of five different embodiments (figures 3a to 3e) of a cathode structure according to the present invention, each showing a majority of substantially identical standard cathode blocks and a minority of substantially identical tall cathode blocks, forming various low surface portions and high surface portions.

Figure 4 shows a schematic cross section of a cathode structure according to the present invention in which the high surface portion has bevelled or chamfered ridges.

Figure 5 shows a schematic cross section of a cathode structure according to the present invention in which the length x_H of the high surface portion is smaller than the width b of the cathode block. The following reference numbers are used in the figures: 1 Cathode (prior art) 1 1 Cathode (invention)

2 (Standard) cathode block 12 Tall cathode block (C_T)

3 Upper surface of the cathode 30 Low portion (P_L)

4 Bottom surface of the cathode 31 High portion (P_H)

5 Cathode bar 32 Chamfered or bevelled ridge

6 Joint between two cathode blocks

Description

Figure 1 a schematically shows a cathode structure 1 according to prior art that is used in an electrolytic pot for producing aluminium according to the Hall-Heroult process. Said cathode structure 1 according to prior art has a flat upper surface 3; in the schematic example of figure 1 a the cathode 1 is composed of 10 identical cathode blocks 2 of essentially rectangular cross section. In reality the number of individual cathode blocks that form a cathode is somewhere between 12 and 30. Figure 1 b shows an example with 17 individual cathode blocks 2.

As can be seen on figure 1 a, the coordinates x and y refer to the length and width of the cathode 1 (i.e. of the bottom of the pot), and the coordinates a, b and c refer to the length, width and height of the individual cathode blocks 2a, 2b knowing that the axis of cathode block length a coincides with the axis of cathode width y and the axis of cathode block width b coincides with the axis of cathode length x. The width y of the cathode 1 corresponds to the length a of the cathode blocks 2 (y = a). The heights c of all cathode blocks 2 is identical, which means that the upper surface 3 of the cathode 1 is flat. Each cathode block has at least one cathode bar 5a, 5b allowing to connect the cathode block to the cathode bus bar; these cathode bars 5a, 5b are made from steel and are inserted into a groove machined in the lower surface 4 of the cathode blocks. The small gap 6 between two adjacent cathode blocks 2a, 2b is filled sealed with a carbonaceous paste (so-called "cathode paste" or "ram paste") during the lining operation. The expression "essentially rectangular cross section" as used here for the cathode blocks does not take into account small deviations from the rectangular shape that are usual, such as said grooves on the lower side 4 of the cathode blocks 2 that are necessary for inserting the cathode bars 5a, 5b, and ribs (not shown on the figures) on one or both lateral surfaces of the cathode blocks 2 that are sealed with cathode paste during the lining operation.

According to the invention the problem is solved by replacing in a cathode 1 having a flat upper surface 3 and that is formed by a plurality of cathode blocks 2 (called here "standard cathode blocks", C_s), a limited number of standard cathode blocks 2 by so- called "tall cathode blocks" (C_T) 12. Tall cathode blocks 12 differ from standard cathode blocks 2 by their height c: tall cathode blocks 12 are higher than standard cathode blocks 2 and protrude into the pot volume. The length, width and height of standard 2 and tall cathode blocks 12 are designated by a_s, b_s, c_s for standard cathode blocks 2, and by a 7, b_T and c₇ for tall cathode blocks 12, the latter being defined by c_T > c_s. Preferably a_T = a_s and b_T = b_s, which means that a cathode 11 according to the invention can be assembled from a set of standard cathode blocks C_s 1 in which a given number of standard cathode blocks 2 C_s has been replaced by the same number of tall cathode blocks C_T 12.

As a consequence, the cathode 11 according to the invention comprises so-called "high" portions 31 P_H and so-called "low" portions 30 P_L, the length and width of which is designated by x_L and y_L for P_L and x_H and y_H for P_H, as can be seen from figure 2. The width y of each high 31 and low portion 30 preferably corresponds to the length a of the cathode blocks, and preferably a_s = a_T.

In one embodiment, shown on figures 3a, 3b and 3d, each high portion P_H is formed by one single tall cathode block 12a, 12b C_T : in this embodiment the length x_H of each high portion 31 P_H corresponds to the width b_T of the tall cathodes 12 C_T.

In another embodiment, shown on figures 3c, 3e and 2, at least one high portion P_H is formed by two or more adjacent tall cathode blocks 12 C_T. In still another embodiment; shown on figure 5, at least one high portion P_H has a length x_H smaller than the width b of the cathode blocks.

Figure 3c shows a variant of the second embodiment having two high sections with a length x_H(1) and x_H(2) and three low sections with a length Xi_(1), (2) and (3), respectively. As can be seen in this example, x_H(1) > b_T and x_H(2) > b_T and x_L(1) > b_L and x_L(2) > b_{L and} x_L(3) > b_L. Furthermore, b_T = b_L.

According to an essential feature of the invention, the cathode 11 comprises at least one low portion 30 P_L that extends over more than one cathode block C_s. Preferably all low portions 30a, 30b P_L of the cathode 11 extend over more than one cathode block C_s, that is to say all low portions 30 P_L have a length x_L larger than the width b of the cathode blocks 2, 30. Preferably everywhere in the pot the condition x_L≥ 2b_s applies; this is shown on figures 3a, 3b, 3c, 3d and 3e. Even more preferably x_L≥ 3b_s applies everywhere in the pot, as shown in figures 3a, 3b and 3c. In a variant of this most preferred embodiment the low portions 30a, 30c at the ends of the pot can extend over less than three cathode blocks, and the condition x_L≥ 2b_s can apply there; this is shown on figures 3d and 3e.

It is preferred that the geometry of the cathode 11 as modified by the substitution of standard cathode blocks 2 by tall cathode blocks 12 be symmetrical over the length of the cathode 11 (i.e. symmetrical with respect to a plane perpendicular to the cathode surface and parallel to the width y of the cathode structure) this is the case in all of the embodiments shown on figures 2 and 3. The cathode 11 according to the invention comprises at least one high portion with a length x_H≥ b_s. In an advantageous embodiment all high portions have a length x_H≥ b_s. In a variant, the length of at least one of the high portions is an integer multiple of the width of the cathode blocks C_s. In another variant shown on figure 5, at least one high portion can have a length smaller than the width of a cathode block C_s; this is typically achieved by machining the tall cathode block.

The upper surface of the high portions P_L preferably protrudes by 50 mm to 150 mm above the upper surface of the low portions P_L (that is to say c_T = (c_s + 50 mm to 150 mm)), and event more preferably by 60 mm to 100 mm.

In one embodiment the tall cathodes are not profiled. In another embodiment their ridges are profiled. In a variant of this embodiment that can be combined with any of the previous variants and embodiments, at least one of the high portions has a bevelled or chamfered ridge 32 as shown on figure 4; this profiling of edges avoids angular ridges and may slightly increase the stability of the cell. In another embodiment (not shown on the figures) at least one high portion can have a portion of intermediate height; this can be combined with bevelled or chamfered ridges.

According to the invention, only a small number of cathode blocks in the pot are different form the others, compared to prior art embodiments where most or all cathodes in a pot are profiled or structured. In other words, the cathode geometry according to the invention is not based on alternating (staggered) high and low cathode blocks, rather there are a few higher (tall) cathode blocks among a majority of low (standard) cathode blocks. Advantageously, not more than 40% of the upper cathode surface is a high portion P_H, and preferably not more than 35%.

It may be necessary to adapt the geometry of the cathode structure according to the present invention to the presence of anodes and alumina feeders above the cathode structure. In general it is preferred not to have high portions P_H beneath the alumina inlets (not shown on the figures).

In an embodiment of the invention which particularly advantageous if the preheating process of the electrolytic cell during its start-up (prior to the addition of liquid bath and/or liquid metal) includes electrical preheating (i.e. by Joule effect), the high portions are machined in a way as to allow an electrical contact surface between the lower surface of the anodes and at least part of the upper surfaces of each cathode block. This embodiment is not necessary if all anodes are centred with respect to the cathode blocks (as it is the case in electrolytic cells that form part of the so-called Pechiney AP30™ technology) and may not be necessary if three adjacent anodes are centred over two adjacent cathode blocks (as it is the case in electrolytic cells that form part of the so-called Pechiney AP18™ technology). In other cases it may be advantageous to reduce the width y_H or the length x_H of at least one of the high portions, and/or to machine a notch into said high at least one portion, so as to allow an electrical contact between the lower surface of the anode and the upper surface of each cathode block during the electrical preheating. This cathode structure allows a preheating process leading to uniform heating of the cell; uniform preheating has been found to be an important factor for ensuring a long operation lifetime of the electrolytic cell.

The invention has many advantages over prior art.

With respect to prior art embodiments with a flat cathode surface 3 the cathode structure 11 according to the invention can lead to a decrease in voltage drop by 40 to 60 mV, at constant current intensity: this saves electric energy. Furthermore, the inventors have found that the use of a cathode structure according to the invention increases the stability of the pot at low voltage, that is to say when the anode-cathode distance is lowered. In other words compared to standard cell operation conditions at 4.20 to 4.40 V, a Hall-Heroult electrolysis cell comprising a cathode structure according to the invention can be operated at a lower voltage without reaching the instability limit: the inventors have been able to reduce voltage to 3.95 to 4.00 V, and the cell did not cross the instability limit until 3.80 V. The location of the higher cathode blocks is linked to the metal velocity pattern in order to reduce the highest velocities. , The erosion of the cathode surface is decreased by using the cathode structure according to the invention.

With respect to prior art embodiments having a cathode structure with intricate protrusions and ridges, such as longitudinal ridges, crisscross ridges, or grooves or inserts, tall cathode blocks of essentially rectangular cross sections are much simpler to manufacture and to implement than structured cathodes according to the prior art because they are not specifically profiled: tall cathode blocks are conventional cathode blocks of rectangular cross section, just higher than standard cathode blocks. Any additional machining can be totally avoided, which implies both a significant cost reduction and a reduced risk of cracks being induced or revealed during machining operations. In those cases where machining is necessary (especially for creating beveled ridges) the machining operation is rather simple and inexpensive compared to prior art embodiments.

With respect to prior art embodiment having a cathode structure with alternating tall and standard cathode blocks, the cathode structure according to the present invention uses less tall cathodes, thereby reducing the investment cost, and facilitates electric preheating when starting the operation of a pot.

Examples

In a first pot comprising nineteen standard cathode blocks of identical width and length forming a flat bottom surface in the pot, cathode blocks n° 5 and 15 were replaced by tall cathode blocks. Cathode block width was 420 mm, cathode block height was 425 mm for standard cathode blocks and 550 mm for tall cathode blocks. As a consequence tall cathode blocks protruded by about 125 mm into the liquid metal pad.

In a second pot of the same series, cathode blocks n° 4, 7, 13 and 16 were replaced by tall cathode blocks as described above.

Both pots were operated at about 270 kA current intensity over more than 730 days. Their cell voltage was decreased by 60 mV with respect to identical pots in the same series using only standard cathode blocks. No loss in aluminium production was noticed. This lower cell voltage illustrates decreased energy consumption.

In a third pot of the same series, cathode blocks n° 3,4,5,6 and 14, 15, 16 and 17 were replaced by tall cathode blocks as described above. These tall cathode blocks were machined such that a rib of rectangular cross sections, 200 mm wide and 125 mm high, was centred in the middle of the width of the tall cathode blocks. This was necessary in order to accommodate the position of the anodes. In another example, pots using the DUBAL DX technology comprising twenty-eight standard cathode blocks forming a flat bottom surface in the pot, cathode blocks n° 3, 6, 9, 12, 17, 20, 23 and 26 were replaced by tall cathode blocks. Cathode block width was 423 mm, cathode block height was 460 mm for standard cathode blocks and 550 mm for tall cathode blocks. As a consequence tall cathode blocks protruded by about 90 mm into the liquid metal pad. The cell voltage was decreased by 50 mV with respect to identical pots in the same series using only standard cathode blocks. No loss in aluminium production was noticed. This lower cell voltage illustrates decreased energy consumption.

Claims

1. A cathode structure of length x and width y (with x > y) for an electrolytic cell comprising a plurality of parallel cathode blocks of length a and width b (with a > b) extending over substantially the whole length x of said cell (with y ~ a), said cathode structure comprising a majority of so-called standard cathodes of height c_s and a minority of so-called tall cathodes of height c_T, the flat upper surface of said standard cathodes forming at least one so-called high portion at a height c_s, and the flat upper surface of said tall cathodes forming at least one so-called high portion at a height c_T, wherein c_T > c_s, and wherein each high portion has a width y_H ~ y, and each low portion has a width y_L ~ y, and wherein each low portion extends over at least two adjacent standard cathode blocks such x_L≥ 2 b (x_L being the length of the low portion).

2. A cathode structure according to claim 1 , wherein said parallel cathode blocks have a uniform width b and a uniform length a.

3. A cathode structure according to claim 1 or 2, wherein each low portion extends over at least three adjacent standard cathode blocks such x_L≥ 3 b (x_L being the length of the low portion).

4. A cathode structure according to any of claims 1 to 3, wherein each high portion is formed by one single tall cathode block.

5. A cathode structure according to any of claims 1 to 3, wherein at least one high portion has a length x_T greater than b, and is preferably formed by two or more adjacent tall cathode blocks.

6. A cathode structure according to any of claims 1 to 5, wherein at least one high portion has a length smaller than the width of the cathode blocks.

7. A cathode structure according to any of claims 1 to 6, where at least one high portion has a bevelled or chamfered ridge.

8. A cathode structure according to any of claims 1 to 7, wherein at least one of the tall cathodes has an intermediate height between c₇ and c_s.

9. A cathode structure according to any of claims 1 to 8, wherein not more than 40% (and preferably not more than 35%) of the upper cathode surface is a high portion.

10. A cathode structure according to any of claims 1 to 9, wherein (c_T- c_s) = 50 mm to 150 mm, and preferably 60 mm to 100 mm.

1 1. A cathode structure according to any of claims 1 to 10, wherein said cathode structure is substantially symmetrical with respect to a plane perpendicular to the cathode surface and parallel to the width y of said cathode structure.

12. A cathode structure according to any of claims 1 to 1 1 , wherein everywhere in the pot the condition x_L≥ 2 b applies.

13. A cathode structure according to any of claims 1 to 12, wherein everywhere in the pot the condition x_L≥3 b applies.

14. A cathode structure according to any of claims 1 to 13, wherein the high portions are machined in a way as to allow an electrical contact surface between the lower surface of the anodes and at least part of the upper surfaces of each cathode block during electrical preheating.

15. An electrolysis cell comprising a cathode structure according to any of claims 1 to 14 and a plurality of anodes above said cathode structure.

16. A process for producing aluminium by the Hall-Heroult process, wherein an electrolysis cell according to claim 15 is used.