METHOD FOR OBTAINING ALUMINIUM AND OTHER METALS

Description

The field of application.

[0001] The invention pertains to the technique of producing metals by electrolysis. More specifically, it relates to the methods of producing metals by electrolysis while using the liquid-metallic cathode.

[0002] In molten electrolytes, such a technique is used for producing aluminium, alloys of alcali and alcaline-earth metals with lead and copper. The amalgam of sodium is obtained on the mercury cathode in the water solutions of sodium chloride.

The current state of the technique.

[0003] The use of the liquid-metallic cathode in electrolytic cells is combined with the use of the anodes, the working surface of which is horizontal and faces downwards. Under such a surface there forms a powerful gas-liquid flow and the efficiency of modern electrolytic cells with the liquid-metallic cathode depends on the solution of the problem of controlling the gas fluid dynamics of the electrolysis.

[0004] One of the known suggestions to solve this problem is the removal of anodic gases. This can be realised by providing tubes in the anodes ( DE-Patent No 817529, US-Patent No 2917441) or by perforating the anodes (US Patent No 3714002).

[0005] Another known suggestion to control the gas fluid dynamics of the electrolysis is the reduction of the anodic effect occurence by applying oscillations or small amplitudes to the anode (France Patent No 2083362). The reduction of the gas content in an electrolytic cell can be achieved, for exemple, by applying pendulum-like oscillations to the anode (US Patent No 3501386).

[0006] However, the solutions cited above as well as a number of other solutions have not come into use mainly because of problems in designing due to aggressive media and high temperatures. Those suggestions, which were employed on an industrial scale, proved to be little efficient in controlling the gas fluid dynamics of electrolysis.

[0007] Another important shortcoming inherent in an electrolytic cell with the liquid-metallic cathode and the horizontal anode is feed problems in the electrolysis of aluminium, where the cryolite-alumina melt is used as an electrolyte. Thus, in the known technique (J.Thonstad, Aluminium electrolysis, electrolyte electrochemistry Advances in molten salt chemistry. Amsterdam-Oxford-New-York-Tokio, 1987, p.73-126) the electrolyte crust with alumina deposited on it has to be broken by special devices 10 and more times a day. This causes pieces of the crust and alumina to get into the liquid electrolyte. Some other solutions envisage different kinds of devices for the continuous feed of alumina to electrolytic cells without the breakage of the electrolyte crust or only with the partial one. The technique comprising the frequent breakage of the electrolyte crust is accompanied by a periodic loss of the tightness of a cell and an additional escape of fluoric hydrogen, aluminium and sodium fluorides.

[0008] The continuous feed to electrolytic cells by means of mounting special feeding devices on the bath envolves great capital and maintenance costs and reduces the total economic efficiency of the electrolysis.

The description of the invention.

[0009] The aim of this invention is to raise the efficiency of the electrolysis.

[0010] The authors came upon the fact that, if the level of the electrolyte is kept possibly minimal and equal to one or two-fold value of the interpolar distance (IPD), the metal loss decreases and the cathode current efficiency increases while the unit escape of toxic substances, including cancer-causing polyaromatic hydrocarbons, per a ton of produced metal goes down. Thus, both the efficiency of electrolytic cells and the ecological acceptability of the process are improved.

[0011] The investigations on the gas fluid dynamics of the electrolysis, carried out by the authors, showed that the metal loss occur mainly through the dispersion in the electrolyte from the crest of the standing wave which is situated at the anode and the side carbon near the anodic edge. The lowering of the electrolyte level to one or two-fold value of the IPD reduces the velocities of electrolyte flows that cause the metal dispersion.

[0012] Another problem being solved by this invention is the simplification of the feeding process and feeding devices. It is suggested to feed a cell by means of periodic vertical movements of the anode. The alumina is poured down into the electrolyte through the slot along the perimeter of the anode.

[0013] When the alumina stays on the unbroken crust for a long time, it may loose some of its absorptivity. To avoid this, the whole crust may by periodically, say once every 5-10 days, broken by the conventional method and large quantities of fresh alumina may by fed to a cell.

[0014] The examples of the realisation of the invention.

Example 1.

[0015] The fields of the electrolyte velocities were studied on the physical model of a vertical cross-cut of a H.Z. aluminium electrolytic cell when the anode width is 2m, the anodic current density is 6920 A/m², the levels of the electrolyte are 20 and 8cm for the IPD of 6cm.

[0016] The data obtained is shown in Table 1.

Table 1

Solutions compared	Electrolyte level		Maximum averaged velocity mm/sec	Horizontal dimension on the vortex mm
	cm	fractions of the IPD
Conventional	20	3.33	200	350-400
Suggested	8	1.33	100	100-150

[0017] Thus, the characteristics of the vortex that determines the gas fluid dynamic share of the metal loss become more favourable.

Example 2.

[0018] The data given in Table 2 was obtained in the electrolysis of the system, comprising lead and potassium chlorides, with the lead cathode and the graphite anode at a temperature of 700°C, a cathode current density of 0.5A/cm², the IPD of 2cm and the anode width of 160mm.

Table 2

Solutions compared	Electrolyte level		Lead output current efficiency %
	Cm	fractions of the IPD
Conventional	12	6.0	68 ± 3
Suggested	3	1.5	97 ± 3

[0019] As can be seen, the use of the minimum levels of the electrolyte allows to minimize the metal loss and raise the current efficiency to the maximum values. The data quite agrees with the correlation, well known in the aluminium industry, between the current efficiency and the quantity of metal in a cell. With a given depth of the bath, the increase in the level of metal leads to the decrease of the level of electrolyte. It is due to the latter factor that the metal loss is reduced. The greatest effect can be achieved by employing the suggested solution.

Example 3.

[0020] On the group of 5 industrial aluminium V.Z. cells at a current of 155kA the feed of raw materials was performed via the vertical oscillations of the anode under the following conditions:

1. The lift of the anode was effected to 450mv of the ohmic resistance of the interpolar gap.

2. The anode was lowered to the initial position.

3. The lift was repeated.

4. The lowering was repeated.

[0021] Such a vertical displacement of the anode was performed twice a shift, i.e.,once every 3 hours. The alumina was, thus, fed into a cell through the slot being formed between the bell-shaped gas collector and the crust and through a few number of local cavings of the electrolyte crust. The slot and cavings were made tight by pouring the alumina over them. The frequency of anodic effects on the tested baths during a week was 0.25 per day while on the control ones it was 1.7, which proves a high efficiency of the suggested solution. There were no cases of accumulation and formation of the sediments on the cell bottom. The redistribution of the alumina feeding over the whole perimeter of the anode allows to maintain the minimum levels of the electrolyte.

The industrial applicability.

[0022] The industrial applicability of the invention to be patented is determined by the fact that its introduction does not require any essential design changes in the cells being used now. No additional devices are needed either to maintain the electrolyte level or to feed cells. The vertical movements of the anode for feeding raw materials into a cell can be realised through the conventional devices for regulating the anode position.

[0023] The employment of the invention to be patented possesses the following advantages as compared to the conventional methods:

1. The cathode current efficiency rises to 96-97 per cent and more.

2. The consumption of materials and power is reduced.

3. The full automatization of the procces becomes a possibility.

4. The escape of toxic substances into the atmosphere from the anodes of H.Z. and V.Z. aluminium cells is reduced.

5. It becomes possible to modernize the outdated aluminium plants equipped with Soderberg cells.

[0024] The H.Z. and V.Z. baths can reach a 92-95 per cent current efficiency, i.e., the values obtained on the most recent P.A. cells. If the invention is introduced in the latter type of cells, the current efficiency can be raised to 96-98 per cent.

Claims

1. A method of producing aluminium and other metals by electrolysis with the use of the liquid -metallic cathode and the horizontal downwards facing anode, having the electrolyte level kept to one or two-fold value of the interpolar distance.

2. A method according to claim 1, having the raw materials fed into the electrolyte by the periodic vertical movements of the anode.