NZ198976A

NZ198976A - Electrolytic cell cathode bar anchored in carbon

Info

Publication number: NZ198976A
Application number: NZ198976A
Authority: NZ
Inventors: R Jemec
Original assignee: Alusuisse
Priority date: 1980-11-19
Filing date: 1981-11-16
Publication date: 1985-04-30
Also published as: EP0052577B1; NO154433B; AU7708981A; YU272781A; AU545284B2; CA1190515A; ATE6273T1; NO154433C; EP0052577A1; NO813888L

Abstract

1. Cathode element for a fusion electrolysis cell, especially for manufacture of aluminium, with a carbon block, in which a cathode bar is anchored in a groove, while in each side wall of this groove there is formed a recess extending over the entire length of a block, for reception of a cast iron sheath filling the space between bar and groove, characterised in that in the working condition the recess in the upper part of the groove (12) has oblique surfaces (14) diverging in the direction of the groove opening, which at their lower ends run into approximately horizontal abutment surfaces (16) terminating at the corresponding side surfaces (15) of the groove.

Description

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Publication Date: .. . 3.0.APB.Wtt P.O. Journal, No: N.-Z. No, NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION "flEANS OF ANCHORING A CATHODE BAR IN PLACE." We, SWISS ALUMINIUM LTD., a corporation organised and existing under the laws of Switzerland, of CH-396 5 CHIPPIS, Switzerland do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- 1 ~ (Followed by 1A.) 198976 Means of anchoring a cathode bar in place The invention relates to a means of anchoring a cathode bar in a channel in a carbon block, in particular in a carbon block for an electrolytic cell for producing aluminum by 5 the fused salt electrolytic process, and such that each side-wall of the said channel features a recess which runs the full length of the block and accommodates a cast iron mantle which fills the space between the bar and the groove.

In the electrolytic production of aluminum from aluminum 10 oxide, the latter is dissolved in a fluoride melt which is comprised.for the main part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the aluminum itself forming the cathode. Dipping into the melt from above 15 are anodes which, in conventional processes, are made of amor phous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is formed at the carbon anodes This oxygen combines with the carbon of the anodes to form CO^ and CO. The electrolytic process takes place in a temp-20 erature range of about 940-970°C.

The carbon floor of the electrolytic cell is made up of cathode elements in which there is a continuous iron bar, or iron bar separated at the middle. In order to'contribute - 1*- f 198976 to optimum current efficiency of the cell, the electrical contact resistance between the iron bar and the carbon block must be as small as possible.

The connection between the carbon block and the iron bar 5 can be achieved in a number of different ways, for example, by a) stamping-in a conductive paste, b) pouring in cast iron, c) adhesion.

The carbon blocks and iron bars in conventional electrolytic cells can vary considerably with respect to width, height, length and groove shape.

The method of casting-in is widely used today for making carbon floor elements and cathode elements. The iron bars 15 in the groove or channel in the carbon block are joined to the carbon by pouring cast iron around them after they are placed in the groove or channel. The iron bars in'the channels are heated up together with the carbon and then, after being cast-in, are cooled to ambient temperature. As the 20 thermal expansion and contraction of iron is approximately four times that of the carbon, a gap forms between the carbon and the cast iron on cooling down. If the cathode element with iron bar is built into an electrolytic cell, this gap 198976 is not bridged until heating the cell up for operation -which then produces an electrical and mechanical contact between the iron and the carbon.

If the gap formed by contraction is closed before the oper-5 ating temperature is reached, the more rapidly expanding iron bar can act on the carbon of the cathode element to such an extent that cracks are formed in the cathode in the longitudinal direction.

The closing of the gap i.e. the pressing of the cathode bar 10 on the carbon as the cell is put into service depends on -various parameters, for example, on the shape of the carbon block (channel) and the iron bar, on the temperature used for pre-heating the iron and carbon, the manner of preheating the iron and carbon, the manner of pre-heating, the 15 composition of and the temperature at which the cast iron is cast. Often the channel in the carbon block is dovetailed in cross-section. An iron bar is placed in this channel and anchored in the block with the help of the cast-in cast iron. This method of anchorage has however been shown 20 to suffer from the disadvantage that the gap, formed between the cast iron and the walls of the dove-tailed channel due to the more rapid contraction of the cast iron during cooling, is sufficient to allow a slight displacement of the cast iron clad bar to take place in the channel, when 198976 for example the cathode element is turned from the casting position into the working position, and/or the electrode block suffers mechanical shock during transport or during the ramming of the paste in the interfaces and at the edge. This causes the gap between cast iron and the walls of the dove-tailed channel to become too small i.e. it becomes wedge shaped, and the iron causes cracks on heating up the carbon which expands by only a quarter as much as the iron.

The iron wedged in the dove-tailed channel can hardly be returned to the initial position because of the high coefficient of friction between iron and carbon. The gap remains between the base of the channel and the iron which has slipped down and leads to poor electrical contact and thus to energy losses. These losses are increased by longitudinal cracks or even broken off edges, and the danger of damage due to the penetration of molten aluminum during operation of the cell increases markedly.

T.he U.S. patent No. 3 86 7 56 2 proposes a channel-shape which holds the cast-in bar in position throughout all these operations and therefore does not permit wedging to take place Provided in each side-wall of the channel is at least one recess which serves to anchor into place at least one projec tion in the cast iron mantle around the bar. The ability of the bar to slip in the longitudinal direction (especially in longer blocks) is however not completely satisfactory 198976 although the forces remain clearly below that needed to form cracks in.the carbon, and therefore prevent fracture occurring .

It is therefore an object of the invention to develop a 5 means of anchoring a cathode bar in a channel in a carbon block such that the said means of anchoring does not suffer damage during and after the casting-in, features an iron-carbon contact interface with low voltage drop across it, is economical to manufacture and exhibits a relatively 10 good ability to slide in the longitudinal direction.

This object is achieved by way of the invention in that in the working position the recess in the upper region of the channel forms inclined surfaces which open outwards towards the opening in the channel, and which at their lower 15 end change over to approximately horizontal supporting faces ending at the corresponding side faces in the channel.

The correctly cast-in iron bar, which is rectangular or square in cross section, can sag in the casting-in position until after cooling, at most by an amount corresponding to 20 the degree of shrinkage of the iron. As a result a gap forms in the vicinity of the inclined faces, the supporting faces and the vertical side faces in the channel. On positioning the inverted cathode element in the working position, or at 198976 the latest on ramming in the paste into the interfaces and the sides, the cast-in bar slides back again into the same position as that for casting-in. On putting the cell into operation the cathode elements are heated to the operating 5 temperature, whereupon both the iron bar and the iron cast ing around the bar expand to a greater extent than the carbon. Due to the thermal expansion, the iron is pressed in an optimal manner into the conical shape of the upper part (working position) and produces good electrical contact between 10 the iron and the carbon. The approximately horizontal supporting surface acts as a restraint.

The - in the working position - upper part of the channel -is cut away in such a manner that the cast-in bar is fully capable of sliding in the longitudinal direction.

Usefully, the approximately horizontal supporting surface for the cast-in cathode bar runs parallel to the base or top surface of the block provided with the channel.

The height of the conical, inclined faces which open outwards towards the opening in the channel amounts preferably 20 to 40-70% of the total depth of the channel. If the height of these inclined faces is too small, the effect according to the invention may not be fully realised. If the height is too great, there is the danger that on heating the cell to the operating temperature cracks may form or even that 25 the part of the carbon block below the supporting surface - fi - 198976 will be broken. For these reasons of stability, the conical side faces join up directly to the bottom face of the channel, preferably after a curvature.

The distance between the approximately horizontal supporting 5 surface and the bottom face of the carbon block provided with the channel is equal to at least 30% of the depth of the channel.

The angle of inclination of the conical faces from the vertical is - both in the casting-in position and in the 10 working position - preferably between 3 and 15°.

If this angle is too large, the carbon block is weakened too much, the contraction of the cast-in iron would be too great, and the bar - as a result of too pronounced heating -could be distorted in the longitudinal direction on casting-15 in the cast iron. If, on the other hand, the angle is too small, the supporting surface would be too small. The contact between iron and carbon could break off a small ledge.

The cathode elements inverted to bring them into the working position are fitted together in the normal manner to 20 form a carbon floor for the cell; the cast-in bars can in the process not become wedged in the channel.

The invention will now be explained in greater detail with the help of exemplified embodiments shown in the drawings viz. , 19897G Figure 1: A cathode element which is in the casting-in position and which has cooled down.

Figure 2: A cold cathode element inverted into the working position.

Figure 1 shows a cathode bar 11 which has been cast-in to a carbon block 10, still in the casting position but having already cooled down. The channel 12 cut into the carbon block 10 has contracted less than the cathode bar 11 and the cast iron layer IS around it. A gap 17 forms between the conical, inclined faces 16, the vertical side faces 15 and supporting faces 16 on the one hand and the cast iron layer 13 on the other hand. The cast-in cathode bar 11 lies below the level of face 18 of the carbon block 10. This face 18, unlike face 20 of the block 10,.does not come into contact with the liquid metal when in operation in the cell. botfefW Suyfcca.

The baoe 24 of the channel 12 of depth t continues at the sides as a curvature 22 and then as the conical, inclined faces 14 of height h. The faces 14 are inclined at an angle dL to the vertical.

The iron bar 11, shown in the inverted, working position in figure 2, has slid down to such an extent that the lower face of the cast iron - as before the contraction of the iron and the carbon - lies in the region of the plane in 198976 which the bottom face 18 of the carbon block lies. The gap 17 extends now over the vicinity .of the vertical side-walls 15/ the inclined faces 14, the curvatures 22 and face 24 of channel 12. The cast iron 13 lies on the supporting sur-5 faces 16 and prevents the cast iron jamming on the side- walls of the channel.

On heating up the cathode element to operating temperature the supporting surface 16 acts as a restraint and the cast iron 13 is pressed against the carbon in such a way that 10 good electrical contact is achieved. 198976

Claims

WHAT WE CLAIM IS:

1. Means of anchoring a cathode bar in a channel in a carbon block, for a cell for producing aluminum via fused salt electrolysis, comprising in each side-wall of this channel a recess along the whole length of the block, in which each recess is formed by an inclined face which widens outwards from the bottom surface of the channel and joins up with an approximately horizontal inwardly extending supporting face ending at the corresponding side-wall of the channel.

2. Means of anchoring a cathode bar according to claim 1, in which the supporting faces run parallel to the base and top surface of the carbon block.

3. Means of anchoring a cathode bar according to claim 1 or 2, in which the height of the inclined faces amounts to 40-70% of the depth of the channel.

4. Means of anchoring a cathode bar according to any one of the claims 1 to 3, in which the inclined faces extend from a curved portion between the side-walls and the bottom surface of the channel. j 198976

5. Means of anchoring a cathode bar according to any one of the claims 1 to 4, in which the distance of the supporting surfaces from the bottom face of the channel amounts to at least 30% of the depth of the channel.

6. Means of anchoring a cathode bar according to any one of the claims 1 to 5, in which the inclination of the inclined faces to the vertical lies between 3° and 15°.

7. Means of anchoring a cathode bar in a channel in a carbon block substantially as herein described with reference to the accompanying drawings. SWISS ALUMINIUM LIMITED By Their Attorneys