DE2120888C3 - Aluminum electrolytic cell - Google Patents

Description

'5

is determined, whereby the calculated cross-section may deviate upwards or downwards by 10%.

For the production of aluminum by electrolysis of aluminum oxide (Al ₂ O ₃ , alumina) this is dissolved in a fluoride melt. The electrolysis takes place in a temperature range of about 940 to 975 C. The cathodically deposited aluminum collects under the fluoride melt on the bottom of the cell. Anodes made of amorphous carbon are immersed in the melt from above. At the anodes, the electrolytic decomposition of the aluminum oxide creates oxygen, which combines _{with the carbon of the anodes to form CO and CO 2.}

The principle of an aluminum electrolysis cell is based on FIG. 1 showing a section in the longitudinal direction. The fluoride melt 10 (the electrolyte) is located in a steel tub 12 lined with carbon 11, which is provided with thermal insulation 13 made of heat-resistant material. The cathodically deposited aluminum 14 lies on the floor 15 of the cell. The surface 16 of the liquid aluminum represents the cathode. Iron cathode bars 17 are embedded in the carbon lining 11, which conduct the current from the bottom of the cell to the outside. Anodes 18 made of amorphous carbon, which feed the direct current to the electrolyte, are immersed in the fluoride melt 10 from above. They are firmly connected to the anode bar 21 via conductor rods 19 and locks 20. The electrolyte 10 is covered with a crust 22 of solidified melt and an alumina layer 23 located above it. The distance d between the anode underside 24 and the aluminum surface 16, also called the interpolar distance, can be changed by raising or lowering the anode bar 21 with the aid of the lifting mechanisms 25 which are mounted on columns 26. As a result of the attack by the oxygen released during electrolysis, the anodes on their underside are consumed by about 1.5 to 2 cm per day, depending on the 6c cell type.

The iron cathode bars in the coal floor are good heat conductors. Accordingly, they carry amounts of heat from inside the cell to the outside; these dissipated amounts of heat represent losses which have to be compensated for by supplying electrical energy. In addition, the Cathode bar Joule heat. This reduces the heat flow that occurs without the Joule heat through the bars to the outside would flow.

Fiü. 2 schematically indicates the cross section of a Electrolysis cell on.

The cathode bars 17 serve two purposes. They collect the current from the active part 27 of the carbon base below the anodes 18 and lead it out of the cell, so serve as a pure conductor outside the active part of the carbon base. They are labeled 28 there. Where they collect the current and are labeled 29, the current strength in the cathode bar increases to the outside. Efforts are being made to build a large cross-section of the Kaihoden bar within the active part of the coal soil. in order to increase the contact surface of the iron bar and the carbon base and thus the contact resistance. ^; r> aen "i.

In the Pwixis, the same cross-section is used for outside the active part of the coal floor the cathode bar as inside the active part. However, this puts you in a position to relate to the heat losses unfavorable due to the bars. A lot of heat is dissipated from inside the cell.

According to the invention, the cross section of the iron cathode bar is now used to reduce the heat losses outside the active part of the coal floor. This turns off the heat flow the melt is reduced to the outside through the ingot. However, the generation of Joules increases Heat in the part of the cathode bar with reduced cross-section, since there the current density compared to that is increased in the active part of the carbon soil.

The increase in the generation of Joule heat continues the tapering of the cathode bar cross-section a limit, since from a certain point more Joule heat is generated than by reduction the heat losses due to thermal conduction of the cathode bar can be saved. This limit can best be determined by practical experiments with the in Determine the relevant aluminum electrolysis cell.

In economic considerations, however, the material savings in the cathode bars are also important to consider.

A further development of the invention relates to the optimal tapering of the cathode bars outside the active part of the coal floor without taking into account the saving of material. (This plays a subordinate role compared to energy saving.)

This optimal taper (ratio of the cross section outside the active part of the coal soil to the cross section within the active part) is described by the following equation:

1 / / ² ■ »-" 0.86 · L ²

Ratio of the cathode bar cross section outside the active part of the carbon base to the cathode bar cross section inside the active part of the carbon base (%).
Amperage in the cathode bar (A).
Specific electrical contradiction of the cathode bars at a temperature corresponding to 1/2 (T, + T ₂ ) (Um).

Length of the cathude bars outside the active part of the cooling floor (m).
Share of Joule heat from the cathode bar that does not flow out to the outside. Specific thermal conductivity of the cathode bar at a temperature _{corresponding to 1 2 (T 1} + T,) (~ * ~

\ mn L J

T _x = temperature of the cathode tubes at the exit point of the bars from the active part of the carbon base (C).

7 ", = temperature of the cathode bars at their outer ends (C).

A, ² , = cross section of the cathode bar within the active part of the carbon bottom (rrrj.

The calculated outer cross-section may deviate upwards or downwards by 10%.

example

100 kA electrolysis cell with 19 iron cathode bars (= 38 cathode bar ends that lead to the outside).

/ = Current intensity per cathode

bar end 2630A

ο = more specific electrical

Resistance 0.4K) ¹¹ LIm

L = length 0.6 m

u - (usually) 0.2

Specific thermal
conductivity

45

kcal
mh C

A ,, temperature of the cathode bars at their exit point from the active part

of the coal floor 700 C

Temperature of the cathode bars at the outer end .. 200 C
Cross section of the cathode bar within the
active part of the coal soil Π, Ο- lCT-'nr

The result is: V - 50% (values between 45 and 55% would be permissible here).

An electrolytic cell of the specified size loses through the 19 cathode bars with a non-tapered cross-section, an amount of heat of 4 (X) OOO to 500,000 kcal. By tapering to the proposed extent, 250,000 to Save 300,000 kcal in 24 hours, which means a reduction in the specific electrical energy consumption " corresponds to up to 0.5 kWh / kg Al.

In addition, by reducing the heat dissipation from the cathode, the thermal gradient is reduced in size in the cell floor, which has a positive effect on the durability of the carbon tub lining affects. There are also material and weight savings that amount to a ton per cell can trust more.

1 sheet of drawings

Claims (2)

Patent claims: 1. Aluminum electrolytic cell with iron cathode bars in the carbon lining of the Soil, characterized by a lesser Cross section of the cathode bar (17) outside the active part (27) of the carbon base. 2. Aluminum electrolytic cell according to claim 1, characterized in that the outer cross section according to the equation