WO1995006145A1

WO1995006145A1 - Procedure and device for electrical drying and preheating of cells for electrolysis

Info

Publication number: WO1995006145A1
Application number: PCT/NO1994/000141
Authority: WO
Inventors: Asbjørn MOEN
Original assignee: Moen Asbjoern
Priority date: 1993-08-27
Filing date: 1994-08-29
Publication date: 1995-03-02
Also published as: AU7625594A

Abstract

When starting up new or repaired pots for the electrolysis of for example aluminium, it is necessary to dry out and preheat the pot before it is restarted and put into operation. Up until now, this has normally been done by using open heat from various sources, gas or oil. Another method has been to place a layer of coke in the pot, lowering the anode down to the coke and let the main current pass the layer of coke which is thereby heated. Some producers heat the pot by pouring liquid aluminium into the cell. All these methods have their limitations and entail grave disadvantages. The main negative factor is poor quality of the cathode lining and thereby a shorter working life. The invention is to use electrical heating with elements (4) which are shaped as 90° angles. Furthermore these elements are fastened to a frame (3) by means of vertical bearings. This frame is attached - vertically movable - to another frame (2), which again is horizontally movable in the main frame (1). The main frame is placed alongside the anode. The elements may be moved horizontally along the anode, lowered through the slit and turned 90° forward under the anode, as shown in the drawings. During the heating period, frames (2 and 3) and the elements may be moved horizontally backwards and forwards under the anode, thereby ensuring a uniform temperature in the cathode.

Description

PROCEDURE AND DEVICE FOR ELECTRICAL DRYING AND PREHEATING OF CELLS FOR ELECTROLYSIS.

Description

When starting up new or rebuilt electrolytic cells (pots) for the production of aluminium, the pots must be preheated to about 800-900°C before they are applied for normal production.

Until now, this has normally been done by using open heat from various sources such as gas or oil. Another method has been to place a layer of coke in the pot> lowering the anode down to the coke and let the main current pass through the layer of coke which is thereby heated. Some producers heat the pot by pouring liquid aluminium into the cell.

All these methods have their limitations and entail serious disadvantages. The main negative factor is poor quality of the cathode lining and thereby a shorter working life. The present invention aims at applying electrical elements of a very special shape of for this preheating process. Pots having a slit between the cathode well and the anode are presupposed.

Normally, elements sufficient for such heating are difficult to enter into the pot because of the narrow slit. The invention includes shaping the electrical elements (4) to angles of 90°. These elements are then attached to a frame (3) by means of vertical bearings. This frame (3) is attached so as to move vertically on another frame (2), which again is horizontally movable on the main frame (1).

The main frame is placed alongside the anode. The elements may be moved horizontally along the anode, lowered through the slit and turned 90° forward under the anode, as shown in the attached drawing fig. 1-5.

The elements are turned by means of a mechanical device, which may be driven manually, pneumatically or by motor.

Raising or lowering may be achieved by using manual devices in the form of a screw or something similar, or electro¬ mechanical, pneumatic or hydraulic devices. Or else the crane in the pot-room may be used for this movement.

There is an insulated lid (3) attached to the element-frame (5) to preserve the heat in the cell, and thereby minimize loss of energy.

To achieve a uniform preheating of the cathode, a slow movement of the elements backwards and forwards in the pot si adviseable. This is done by having the frame (2) move in its horizontal guidings alongside the anode.

A complete system for preheating an electrolytic cell (pot) will consist of two identical sections placed one on each side of the anode.

For a 4 by 9 meter pot, the power needed for preheating will be approximately 300-400 kW.

Power may be supplied from adistribution network especially designed for this system, or direct current can be taken from the Line. In the latter case, current can be selected as outlet from a certain amount of pots. A maximum, voltage of 200 V is assumed.

The procedure for using the pot-heating system is as follows: The installation consisting of two identical sections is driven on a trailer or carriage to the pot which is to be preheated. Each section is hooked and lifted in to place alongside the anode by means of the potroom crane, or the section may be equipped with wheels so that it may be driven in to place.

During transportation of the section, frame (3) is mechanically locked and placed in its upper position. The elements are turned in position alongside the anode.

■While it is still hanging from a crane or other lifting equipment, the frame (3) must be released and moved to one side. This move must be accomplished before the elements are lowered, so as to ensure free passage through the slit. In the described position, the frame (3) with elements and cover may be lowered, and the elements are placed into the cell and thereafter turned under the anode.

The same procedure is carried out for both sections. The sections are connected electrically and pneumatically. (If desired, an air compressor may be installed with the equipment. )

An air cylinder or a motor operation is used for the backward and forward horizontal movement of the elements.

The end slits are closed with covers (6), the power is connected and the installation is put into operation.

The equipment is mobile and can be connected to the Line current over a certain number of pots. Or it may be connected to the plant's electrical mains. In principle, the equipment can be used for all pots in a smelting plant.

As stated previously the raising/lowering movements may be done by using the pot-room crane. The turning of the elements may be done manually by the use of for instance a screw device, or by using electromechanical, pneumatic or hydraulic devices.

An insulated lid is attached to the element frame. This lid keeps the heat inside the pot and prevents unnecessary energy loss.

For a 4 by 9 meter pot, the necessary power is approximately 300-400 kW.

The elements (4) may be heat-resistant pipes which are formed in a 90° angle. The heating elements will be placed in the horizontal (hot) part of the pipe. The vertical part of the pipe will be cold and will help to lead the current down to the hot part. At the same time, it acts as the turning axis for the hot, horizontal part which is to be turned under the anode.

The elements may also be designed in such a way that the horizontal part consists of a conductive material which makes a circuit of its own. In this case, the heating element will have a self-supporting function.

To adjust and control the temperature, thermoelements and adjusting instruments would be used.

Claims

CLAIMS:

1. A process for preheating of cathode and cells for the electrolysis of, for example, aluminium, characterized in that a number of electrical heating elements being shaped as angles of 90°, being vertically movable in a frame (2) and being attached revolvingly to a frame construction (3) in such a way that they may be lowered through the slit between anode and cathode and thereafter turned under the anode.

2. A process as stated in claim 1, characterized in that the elements with suspension and insulated cover being movable with slow movements backwards and forwards in a frame (1), thereby securing a uniform temperature in the cathode and the anode.

3. A process as stated in claims 1 and 2, characterized in that element suspension frames which may be fastened to a corresponding number of carriages, making the preheating equipment mobile inside the plant.

4. Apparatus for carrying out the process according to the preceding claims, characterized in that an insulating cover (5), being attached to the suspension frame (3), thereby preventing heat loss from the pot.

5. Apparatus as stated in the preceding claims, characterized in that raising/lowering movements as well as turning movements being manually or electrically executed by the use of screw devices, or being executed by the use of pneumatic or hydraulic cylinders.

6. Apparatus as stated in all the preceding claims, characterized in that the preheating equipment preferably consisting of two identical units which are used simultaneously, one on each side of the cell.

7. Apparatus as stated in all the preceding claims, characterized in that the heating elements being made from heat-resistant pipes welded or fitted together at an angle of 90° and the heating elements themselves being placed in the horizontal part of the tube; or the horizontal part of the element is made from self- supported, conductive, heat-resistant material which in itself leads the current and acts as a heating element.