NZ201580A - Reduction pot for fused salt production of aluminium and process for making cathodes - Google Patents

Description

6. S- 201580 ,, Priority Date(s): " 13-&-82 Complete Specification Filed: C-2>[&& Class: 1 C2$C7/o& oc'fraM' Publication Date: .. .v. f •••••••••■" • ... .0.1.

N.Z.No.

NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION "REDUCTION POT FOR FUSED SALT ELECTROLYTIC PRODUCTION OF ALUMINUM AND PROCESS FOR INSTALLING THE IRON CATHODE BARS" We, SWISS ALUMINIUM LTD, a corporation organised and existing under the laws of Switzerland, of CH-3965 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 : - 2 0 15 8 0 !i j! Reduction pot for fused salt electrolytic production of .. aluminum and process for installing the iron cathode bars 11 u i; ■ !i ;; The present invention relates to a reduction pot for fused ■' salt electrolytic production of aluminum where the said 5 . pot comprises a steel shell, a thermally insulating layer and an electrically conductive inner lining which is resist ant to molten materials and comprises carbon blocks running j! in the transverse direction with solid iron bars embedded ij in them and projecting out of the ends, and relates too to 10 | a process for installing the iron bars. ji : The electrolytic process for producing aluminum from alum- l : inum oxide involves dissolving the latter in a fluoride j; melt which is comprised for the greater part of cryolite, ji The cathodically precipitated aluminum collects on the 15 carbon floor of the cell under the fluoride melt, the sur- 'i face of the liquid aluminum itself acting as the cathode. , Dipping into the melt from above are anodes which in conven 'l tional processes are made of amorphous carbon. Oxygen is formed as a result of the electrolytic decomposition of the 20 aluminum oxide; this oxygen reacts with the carbon of the anodes to form CO2 and CO. The electrolytic process takes 'l place in the temperature range ca. 940-970°C. ,i The inner lining of the reduction pot is made of carbon 201580 ' blocks which have embedded in them at least one iron bar which is either continuous or separated in the middle. In order to achieve the smallest possible voltage drop accross i: ;! the cell, the contact resistance between the iron bar and ij i! the carbon block must be as small as possible.

Various methods for joining the carbon block and the iron , bar are known to the expert in the field e.g. |i - ramming in with a conductive paste ;i ii : - casting in with cast iron 10 ' - adhesive fixing. i, . In conventional reduction pots the carbon blocks and the iron bars may be of various dimensions viz., length, breadth ; and height and shape. :i i; Today the method of casting the bars into the carbon blocks 15 : to attain good electrical contact is widely practised. The i; ; iron bars, laid .into the channel in the carbon block f are ; held in place by pouring cast iron in around the bars. Both the iron bars and the carbon block are preheated together ; and then cooled to the temperature of the surroundings after 20 : the cast iron has been poured in. As the thermal expansion and contraction of the iron xs four times greater than that 201580 of the carbon, a gap results between the carbon and the cast ; iron when they are cooled. When the carbon block containing n { the iron bars is installed in a reduction cell, this gap i! : | • does not close until the temperature of the cell is raised j ii for putting it into service; the closing of this gap improves the electrical and mechanical contact between the iron and f ' the carbon. If the gap formed by contraction closes before j reaching the operating temperature, the faster expanding J i iron bar can act upon the carbon of the cathode element in j J ' i such a manner that cracks are produced in the cathode element '! i } ;j A disadvantage of cast iron is that it has a relatively low ; , t electrical conductivity. Furthermore, with the normal cast-i in iron bars the compressive force in the uppermost region of the bar in its working position is often insufficient to j i produce the desired low contact resistance between the ; ;j carbon block and the iron. This means that the electric curr- i: ent does not then flow via the shortest route through the ; i carbon floor of the cell, but makes a detour into the side- ; i '! i ! walls of the iron bar instead of flowing directly through ! : the uppermost face. Both factors can result in a voltage j drop of e.g. up to 0.1 volt which is detrimental in terms I : of the energy balance of the cell. Some years ago therefore , . attempts were made, see e.g. J. Electrochemical Technology ;l j i! Vol. 5, No. 3-4, (1967), pp. 152-154, to achieve direct 25 contact between the iron bars and the carbon blocks. 2015 00 i A hole which corresponds to the shape and size of the iron : ; bar is made in the carbon block and the iron bar placed in . the hole without ramming paste, cast iron or adhesive. How- '! j ;! ever, as no aluminum reduction cell has been built using i '! this principle in the relatively long intervening period t ; p ! this process has apparently not proved itself.

: According to the above mentioned article the iron bar must lie flush against the carbon at the working temperature of : the cell. In practice this is hardly possible to achieve. ,i i : During the electrolysis process ,the smallest dimensional • ! (exists,either\ « I inaccuracy?tne electrical contact resistance is too i; j great or cracks are produced in the graphite block, the latter leading to a much reduced service life of the pot.

; It is therefore an object of the invention to avoid the above 15 !! mentioned disadvantages of casting-in the iron bars in the i; ji carbon blocks, without reducing the service life of the ; reduction pot, yet lowering the contact resistance between ! J : the carbon block and the iron bar. ,, i 'i ' ) This object is achieved by way of the invention in that ; l : - channels - which open downwards when in the working position - are provided running in the longitudinal direc- i tion in each carbon block over at least 2 0% of its length, ' 201580 starting from both end faces, and such that the cross section of the channels at 500-850°C corresponds exactly to - i that of the iron bar heated to the same temperature, !! i: ! - the iron bars set in the channels and extending over at 5 j least 20% of the length of the carbon block, starting i: from the end faces, project out of the whole of the lower i: face of the carbon block during the reduction process, and ii ■ - a part of the weight of the carbon blocks is borne, approx •; imately uniformly, by all of the iron bars. sj ii Trials have shown that the bottom opening in the channel -with normal dimensions of carbon block and iron bar - can be stretched approximately 1 mm before the carbon cracks or ii chips. A certain degree of elasticity in the carbon blocks i is very important if anchoring the iron bars in the carbon 15 without the use of cast iron.

This can be illustrated for the preferred temperature of around 700°C: At this temperature the channel and the iron bar in it have exactly the same cross section i.e. the iron ; bar lies intimately against the carbon along its whole 20 length but without exerting any pressure against it. At O O !: higher temperatures, above 700 C, for example at 94 0-970 C at which the reduction process takes place, the iron bar i •\ ■; - 6 - l 2 015 0 0 presses against the carbon. Thanks to the elasticity of the carbon block, however, no crack is formed as would in i ' the case of a hole rather than a channel. i 't A part of the weight of the carbon lining is distributed 5 approximately uniformly on the iron bars projecting out of the carbon blocks. When the iron bars are in the working position therefore, they press against the correspondingly : shaped part of the channel, even if the cross section of the :< iron bar is smaller than that of the channel. This pressure 10 results in the smallest possible contact resistance between carbon and iron in that area closest to the liquid aluminum forming the cathode of the cell. The voltage drop is therefore minimised with respect to two factors: I; - contact resistance between carbon and iron 15 - voltage drop in the carbon lining of the pot.

: The iron bars can, in the conventional manner, extend over i: : the whole length of the carbon blocks or be separated by i • a short or longer distance in the middle. It is known that, during the fused salt electrolytic production of aluminum, 20 most of the electric current flows in the outer part of the ;; iron bars in the reduction pot. It suffices therefore if ii : the iron bars extend from both end faces of the carbon blocks into at least 20% of "the length of the carbon ; blocks towards the centre of the cell. In the centre of ii - ;; - 7 - 2015*8 ; J \ ; the carbon blocks the iron bars can therefore be separated by up to 60% of the length of the block. The channels can • j extend the whole length of the carbon block or only over : a length corresponding to that required by the iron bar; in 5 i the latter case, however, there is preferably a gap of 0.5 - 1 cm provided between the end faces of the iron bar and '! the channel.

: The iron bars projecting preferably about 0.5 - 1.5 cm out ■j of the bottom face of the carbon blocks and the correspond-10 ingly shaped channel can be of any suitable geometric form. Preferably, however, the iron bars and the corresponding | channel have a rounded cross section at least in the upper-; most part as viewed in the working position. This provides ; the great advantage that, on prizing the sidewalls of the i| j channel in the carbon block apart, the notch effect is «i : I 1 minimised i.e. in channels with a rounded-off upper section crack formation starts at higher pressures from the iron " bars than in channels with angular cross sections there.

, Iron bars and the corresponding channels of circular cross 20 section are particularly advantageous as - round bars have the smallest surface area for a constant cross section - which means the same electrical conductivity with smaller thermal losses, I I 2 015 8 0 - the strength of the block is increased considerably because of the absence of a notch effect, - because of the higher maximum contact pressure, the con- ii tact resistance between the carbon and the iron can be 5 reduced, - simple, round windows for the cathode bars can be made in the steel shell of the pot, and these can be easily (( ;; sealed with pre-fabricated materials, and :i - it is particularly easy to manufacture the bars.

The essential feature of the process according to the in- vention for installing the iron bars in a reduction pot with channels which are open at the bottom in the working position if , is that the iron bars , at the temperature of the surroundings, are pushed, through the windows in the steel shell into 15 the interior of the pot. 'j The windows are usefully of the same geometric shape as I the cross section of the iron bars. These windows are preferably only slightly, in particular 0.5 - 2 cm larger than the linear dimension of the bar cross section i.e.

•J there is only little play to allow the bar to pass through the window. In this case the gaps are easy to seal off. 2 015 8 0 ! If the iron bars are, at least in the lower part, rectangular or tapering upwards, then they can - in keeping with the corresponding channel - be laid on the layer of insulation. The carbon block can then be lowered onto it.

However, with all bar cross sections - and for reasons of geometry usually a necessity with bars of round cross section - the carbon blocks are lowered until e.g. about !:0.5 - 2.5 cm above the layer of insulation; the iron bars j 3 -are then pushed into the channels, and finally the carbon » 10 block lowered completely. These then rest on the iron bars i '! 1 ,!which project out of the channels. ; The invention explained in greater detail in the following with the help of a drawing illustrating an exemplified j | embodiment. The figure shows a schematic cross section through 15 a carbon block with two iron bars in the operating position j but before completely lowering the carbon block. ; i 1 1 : i The carbon block 10 which is rectangular in cross section contains parallel, round iron bars 12 lying in the longitudinal direction in correspondingly shaped channels 14 which 20 are partly open at the bottom. Each iron bar 12 is surrounded ,by inner sidewalls 16 of the carbon block 10. When the -'iron bars 12 exert pressure on the carbon block 10 at the operating temperature, the sidewalls 16 are prized apart Z01580 slightly. The edges 18 of the sidewalls 16 are preferably rounded or cut away.

- When the carbon block 10 has been fully lowered, the lower extremity 20 of the iron bar 12 rests on the insulation -5 . not shown here. This causes the uppermost surface of the iron bar 12 to be pressed against the corresponding part of the channel 14; the contact resistance between the carbon and the iron is then reduced to a minimum. The direct electr-: ic current can then flow via the most direct route from the ' 10 ; top surface of the carbon block 10 in the direction of the i arrows and into the iron bar 12 with a minimum of contact s ' resistance to overcome.

!; The essential advantages of the invention can be summarised :j as follows: - The iron bars no longer have to be cast, stuck or rammed , ; in but simply pushed through a window into a correspond- j ing, downwards open, channel in the carbon block. This ! permits savings in energy, material, time, equipment i and workshop facilities. 1' : j - The carbon block is no longer exposed to the risks associ- i ated with casting-in i.e. during transport, pre-heating, casting-in and handling. 201580 - The voltage drop in the electrolytic process can be lowered in the cathode part of the cell by up to 0.1 volt. 201SB0

Claims (12)

WHAT WE CLAIM IS:

1. A reduction pot for the production of aluminium by means of fusion electrolysis, consisting of an outer steel shell, a thermally insulating layer and an electrically conductive inner lining which is resistant to the molten charge and is made up of carbon blocks running in the transverse direction and having running in their longitudinal direction grooves which are open downwards and extend over at least 20% of the length - starting from the two endfaces - and in which are inserted solid iron bars which project downwards out of the bottom face and on both sides out of the endfaces of the carbon blocks, where part of the weight of the carbon blocks is borne approximately uniformly by all of the iron bars projecting out of their bottom faces, characterized in that the grooves in the carbon blocks have cross-sections which at a temperature between 500 and 850°C correspond exactly with the cross-section of the iron bars heated to the same temperature.

2. A reduction pot as claimed in claim 1, characterized in that the cross-section of the grooves in the carbon blocks heated to substantially 7 00°C corresponds exactly with the cross-section of the iron bars heated to the same temperature.

3. A reduction pot as claimed in claim 1 or 2, characterized in that during the electrolytic process the iron bars project 0.5-1.5 cm out of the bottom face of the carbon blocks.

4. A reduction pot as claimed in any one of the claims 1-3, characterized in that the grooves in the carbon blocks and the corresponding iron bars have at least in their uppear-,.- with respect to the working position - region rounded cross-sections.

5. A reduction pot as claimed in claim 4, characterized in that the cross-section of the grooves and of the iron bars is circular.

6. A reduction pot as claimed in any one of the claims 1-5, characterized in that ift steel shell windows are provided for the bars, which have the same geometric shape as the cross-section of the iron bars.

7. A reduction pot as claimed in claim 6, characterized in that the windows for the bars are only a littlo,—preferably 0.5-2cm larger than the linear dimensions of the cross-sections of the iron bars.

8. A process for installing the iron bars in a reduction pot according to any one of claims 1-7, in which the iron bars at the temperature of the surroundings are pushed through windows in the steel shell into the interior of the reduction pot.

9. A process according to claim 8, in which, when the iron bars are rectangular at the bottom or taper inwards towards the top, the carbon blocks with appropriately shaped channels are lowered onto the iron bars.

10. A process according to claim 8, in which the carbon blocks are first lowered until only slightly,—preferably cubctantially 0.5-2.5 cm above the insulating layer, the iron bars inserted into the channels and the blocks finally lowered completely.

11. A reduction pot as claimed in claim 1 substantially as herein described with reference to the accompanying drawing. 14 201580

12. A process according to any one of claims 8-10 substantially as herein described. SWISS ALUMINIUM LIMITED By Their Attorneys HENRY HUGHES LIMITED