FIELD OF THE INVENTION

The present invention relates to a cathode bar for use in electrolysis cells for aluminium electrowinning and its method of manufacturing.

BACKGROUND OF THE INVENTION

Production of aluminium is conventionally carried out according to the well-known Hall-Heroult process, which is based on the electrolysis of alumin- ium salts.

This process uses electrolytic cells (or pots) with a steel shell lined with carbonaceous material. The hearth of the cell, i.e. the bottom of the carbonaceous lining together with a layer of electrolytically produced molten aluminium — which collects thereon during operation— serves as the cathode.

Consumable carbon electrodes are disposed from the top of the cell and partially immersed into the molten electrolyte in the cell. The carbon electrodes are connected to an anode bus bar, which in turn is connected to a current supply. Cathode bars are arranged in the bottom of the cell lining and are connected to the source of current, completing the circuit. The anode and cathode bars are typically hot-rolled steel bars.

In operation, the electrolyte (a mixture of alumina and cryolite) is charged into the cell and the current is passed through the cell from the anode through the cathode via the electrolyte. The alumina is dissociated by the current so that aluminium is deposited at the bottom and oxygen is liberated at the carbon anode, forming CO and CO2.

Referring more specifically to the design of the lining at the cell bottom, it conventionally consists of joined carbon blocks, in which the cathode bars are embedded. Since the lining undergoes wearing off, the cathode bars are located away from the upper side, which is in contact with the molten aluminium. The conventional preparation of cathode blocks 10 is illustrated in Fig.1. An open groove 12 is milled in the underside 14 of the cathode block 10. A cathode bar 16 of rectangular section is then placed in the groove 12 and secured therein by casting of liquid pig iron 18 (or other electrically conductive binder).

Depending on the cell design at the production site, cathode bars may have a rectangular, square or circular cross-section.

Fig.2 shows a cathode block 30 with a rounded cathode bar 32 (or rod) secured in a groove 34 in its underside 36. Here also, the bar 32 is secured by cast pig iron 38, but the entrance region of the groove is filled with refractory material 40. As can be seen, the bar cross-section is not entirely circular, but it has a flattened side that is flush with the underside 36 of the cathode block 30. Such shape is obtained by a machining/milling operation performed over the length of the bar 32 having an entirely circular cross-section from the hot-rolling process.

The milling step is typically performed at the aluminium plant and involves additional costs in terms of handling and tools as compared to a square or rectangular-section cathode bar, not to mention the loss of material.

Production of round bars also requires specific equipment. Typically, a heated billet is hot rolled to size in a rolling mill comprising roll stands with 3 cylinders arranged at 120°. The Conveying of the hot-rolled bar then requires specific equipment to avoid slipping of the bar that is able to roll over itself.

OBJECT OF THE INVENTION

The object of the present invention is to provide an improved method of manufacturing such rounded cathode bars for electrolytic cells that is simple to implement and less expensive. SUMMARY OF THE INVENTION

The present invention proposes a method for producing a cathode bar for an electrolytic cell by hot rolling, wherein the rolling operation is carried out in such a way that the as-rolled bar has a rounded cross-section with a flattened portion.

Hence, the flattened portion is rolled-in so that the rounded cathode bar does not require additional machining to provide such flattened portion, thereby saving costs related to handling and equipment. Furthermore, there is no loss of material from the bar. When, according to the traditional process, the flattened portion is milled along the bar, the excised material is lost; and this represents a direct loss of money since the price of a steel section is typically proportional to its weight.

Contrary to known circular cathode bars or rods, the present rounded cathode bar can be produced in a traditional rolling mill and does not require a rolling stand for circular bars nor conveying equipment specialized for circular objects. Since the present rounded cathode bar comprises a rolled-in flattened portion, it may be laid on this flattened section in the rolling stand, conveyors and in the cooling bed, which avoids overturning (rolling of the bar over itself) as it would occur with an entirely circular bar or rod.

Hence, the present method provides an efficient way of manufacturing cathode bars, that significantly simplifies the handling and manufacturing process of such bars and reduces the costs for the user since the as-rolled bar is ready to use. One operation that may remain with the user is the cutting to the desired length, depending on the cathode block design.

For an improved stability of the present cathode bar when resting on its flattened portion, the flattened portion advantageously has a width that corresponds to at least 55% of the largest cross-sectional dimension of the hot rolled cathode bar. The width of flattened portion may namely correspond to at least 60%, 65%, 70%, 72.5% or 75% of the largest cross-sectional dimension of the hot rolled cathode bar. In one embodiment, the as-rolled cross-sectional shape of the cathode bar is composed of a circular portion extending over at least 150°, the flattened portion, and two flanks joining each side of said circular portion to said flattened portion. Preferably, the circular portion extends over 180°, i.e. the bar com- prises a semi-circular portion.

Again, for stability reasons, the flattened portion advantageously has a width that at least 1.1 times the radius of curvature of the circular portion. The width may namely be at least 1.2, 1.3, 1.4 or 1.5 times the radius of curvature of the circular portion.

While a wide flattened portion is appreciable in terms of stability, a too wide flattened section may hinder the casting of metal into the cathode block groove, since the flattened side will be located at the entrance of the groove. Therefore, the width of the flattened section preferably does not exceed 1.8 times the radius of curvature of the circular portion, respectively 0.9 times the largest dimension of the bar in cross-section.

Still considering the issue of pig iron casting when the cathode bar is laid in a cathode block, it is preferable that the flanks joining the flattened section to the circular portion do not hinder the flow of pig iron. The flanks may be generally designed as curvilinear portions that provide a predetermined taper- ing shape from the maximum width of bar at the circular portion to the flattened portion. The flanks may for example comprise a first, flattened section that forms a rectangular section at the base of circular portion and a second, curved section that connects the first section to the flattened portion.

As already mentioned, the shape of the flanks should preferably take into account the need for allowing a convenient flow of pig iron thereabout when the bar is installed in the carbon block; this however allows a variety of designs.

In a preferred embodiment, the shape of the flanks is such that they extend on each side within a virtual envelope that is defined by an inner line corresponding to an arc of circle of same radius as the circular portion that joins the flattened portion and by an outer line of predetermined shape. This external shape of the virtual envelope preferably is given by a line

7 -0,1·1η(2)·^

corresponding to the formula: AC · e , which gives the abscissa in function of the elevation y and where the origin of the axis (x,y) is at the height of the flattened portion.

In a preferred practical embodiment, the virtual envelope has its inner and outer lines defined by the above formula, with k=3/4*Ri-12 (Ri being the radius of the semi-circular portion) and k=15, respectively (k, Ri and y in mm).

As it has been understood, the present method allows manufacturing a cathode bar in a conventional rolling mill starting from a conventional steel semi-product such as e.g. a billet or bloom, or any appropriate starting shape. The shape of the present cathode bar can be obtained in three passes through a 2 high rolling stand (also called duo type).

The cathode bar is advantageously made from a low resistivity steel grade, preferably having a conductivity of no more than 13 μΩ.αη ² /αη at 0°C.

The present invention also concerns a hot rolled cathode bar for an electrolytic cell obtained according to the above-described method. Accordingly, the cathode bar exhibits one or more of the above-described features.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawing, in which:

FIG. 1 : is a principle view of the conventional sealing process of a cathode bar in a cathode block;

FIG. 2: is a cross-sectional view through a cathode block with an embedded, conventional cathode bar;

FIG. 3: is a cross-sectional view through a preferred embodiment of the present cathode bar; and

FIG. 4: is a cross-sectional view through a preferred embodiment of the present cathode bar mounted in a cathode block. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred embodiment of a cathode bar 50 manufactured in accordance with the present method is shown in Fig.3. This cathode bar is manufactured by hot rolling in the longitudinal direction, whereby the bar's cross-section as illustrated in Fig.3 is the same over its whole length. As it can be seen, the bar 50 exhibits a generally rounded shape that is however interrupted by a flattened portion 52. According to the present method, this shape is obtained through the hot rolling process. In other words, the present cathode bar 50 includes a rolled-in flattened section 52. The obtained bar thus has a cross sectional shape that is directly suitable for use at the aluminium plant and can advantageously be substituted to a circular cathode bar with a milled flat portion as shown in Fig.2.

More specifically, in the embodiment of Fig.3, the as-rolled cross-sectional shape of the bar 50 advantageously consists of a circular portion 54, the flattened portion 52 and of two flanks 56, each joining a respective side of the flattened portion 52 to the circular portion 54. The circular portion 54 here extends over 180°, but shall generally extend over at least 150°. Preferably, the flattened portion 52 is parallel to the diameter of the semi-circular portion 54 and centrally aligned with respect to the centre of the semi-circular portion 54, indicated C. As it can be seen, the bar 50 is symmetrical with respect to an axis of symmetry A passing through the centre C of the circular portion 54 and perpendicular to the flattened portion 52 (also passing through its centre).

The bar 50 has a width W that here corresponds to the diameter of the semi-circular portion 54 and a height H; the distance of the flattened portion to the centre C is indicated h.

For an improved stability of the bar during its manufacturing and handling, the flattened portion 52 generally has a width of at least 0.55 times the largest cross-section dimension of the bar, which can be expressed in function of the radius of the semi-circular portion: WF > 1 .1 *Ri . The stability of the bar when resting on its flattened portion is further improved by increasing the width of the flattened section to 1.2, 1.3, 1.4, 1.5 or 1.6 times R1. In the embodiment of Fig.3, WF = 1.5«R1.

The flanks 56 each consist of a first and second section 58 and 60. The first section 58 is also flattened. It may be a large diameter arc or preferably a straight line, possibly slightly concave in part. Whatever the shape of this first section 56, it is made to form a large section at the base of the semi-circular portion 54, but it shall not have a dimension greater than the width W. This portion defined by the first section may extend over a maximum height corre- sponding to about 0.5 times the height h. Besides, the distance h is preferably lower than Ri. In practice, the distance h from the centre C to the flattened portion 52 is preferably not greater than Ri and may correspond to 0.7 to 0.9 times the radius R-i .

The second, curved section 60 joins the end of the first section 58 oppo- site the circular portion 54 to a respective end of the flattened portion 52.

The bar 50 has been designed so that it can be substituted to a circular bar of same diameter W with a milled/machine flat at same distance h from the centre C. However, since the present bar 50 is obtained by the hot rolling process directly with a rolled-in flat, there is no loss of matter. Also, here the weight of steel of bar 50 may be the same as that of a conventional circular bar, but before the milling of the flattened portion.

It is to be noted that in the present embodiment, the shape of the flanks 56 is a compromise between the amount of steel, the required shape of the bar 50 for stability and rolling capability on a conventional mill, and the need that the entrance section of the cathode block groove in which the bar 50 is arranged, in use, should allow an easy flow of pig iron around the bar.

Preferably, the second section 60 describes a convex line so that the bar's cross-section progressively tapers from the first section 58 towards the flattened portion 52.

In the embodiment shown in Fig.3, the second section is constructed us- ing two consecutive curvatures of appropriate radii, noted R ₂ and R3.

In order to further illustrate the present invention, table 1 below summarizes the possible dimensions for three cathode bars of different widths (W). This value W being the width of the bar at the basis of the semi-circular portion, it can be considered as the equivalent diameter for a conventional circular bar.

Table 1 (all dimensions in mm)

In one embodiment, the shape of the flanks 56 is designed so that they remain on each side of the bar 50 within a given virtual envelope. In its broad- est definition, this virtual envelope is defined by an inner line corresponding to an arc of circle of same radius Ri as the circular portion 54 that joins the flattened portion 52, and by an outer line of predetermined shape.

The shape of the outer line of the virtual envelope preferably is given by the following formula: x -— - k - e ^' °' ^l - ^2>y

~ 2 t ^ec l- H which indicates the abscissa x (in mm) in function of the elevation y (in mm) from the flattened portion, the origin of the axis (x,y) being at the centre of the flattened portion, increasing abscissa to the right. This equation 1 thus gives the shape of the outer line of the envelope, starting from the flattened portion 52 until it joins the substantially vertical flank portion 58. In the broadest envelope definition k = 6.

The bar being symmetrical, an inversion of sign is of course to be made between the right hand flank and the left hand flank.

In a preferred embodiment, both the inner and outer lines of the virtual envelope are defined according to equation 1. This is shown in Fig.3, where the dashed lines indicate the outer line 60a and inner line 60b of the virtual envelope. The outer line 60a corresponds to a k value of 15. The k value of the inner line 60b depends on the radius Ri of the bar, and is defined as 3/4*Ri-12 (here 48). As can be seen, these inner and outer lines asymptotically join the vertical section 58.

The shape of curved flank section 60 may have any shape within this envelope defines between lines 60a and 60 b. In the variant of Fig.3, flank section 60 that is built with the double radii R2 and R3 can actually be approximated with a k value of 24.

Turning now to Fig.4, there is shown the present cathode bar as arranged in a square or rectangular cathode block 70 (shown only in part). The bar 50 is arranged in a groove 72 with its flat portion 52 laying flat with the cathode block underside 74. The cathode bar 50 is sealed in place by cast pig iron 76 and the entrance of the groove 72 is filled with refractory material 78.

The dashed line 80 indicates, for comparison purposes, the shape of a conventional circular bar of diameter Ri . The flat 52 of the present cathode bar is substantially greater than that of the conventional bar 80. However the shape of the flanks is advantageously designed as described above to allow an easy flow of pig iron about the bar 50. It may be noted that the cross-section area of the bar 50 corresponds to that of a circle of radius Ri . Hence, there is more matter in groove 72 than for a conventional bar 80, since the flat portion 52 is directly obtained through the hot rolling operation. This may also be favourable from the electrical point of view.

It remains to be note that while the flattened portion is illustrated on the Figs, as a straight line, it may not be as regular in practice and may include a cavity over part of its width, by design or due to rolling constraints.