EP0025418B1 - Chisel for a crust-breaking device and method of operating it - Google Patents

Abstract

Device and method for breaking open the solidified crust of electrolyte on an electrolytic cell, in particular a cell for producing aluminum. At least one projection is provided on the lower part of the shaft of a chisel used on a crust breaker. After breaking through the crust, the chisel is lowered further at least until the lowest projection or projections reaches the lower half of the crust.

Description

The present invention relates to a chisel for an impact device for breaking the solidified crust of an electrolysis furnace, in particular for the production of aluminum, and to a method for operating the chisel.

For the production of aluminum by electrolysis of aluminum oxide, this is dissolved in a fluoride melt, which largely consists of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon bottom of the cell, the surface of the liquid aluminum forming the cathode. Anodes, which consist of amorphous carbon in conventional processes, are immersed in the melt. At the carbon anodes, the electrolytic decomposition of the aluminum oxide produces oxygen, which combines with the carbon of the anodes to form CO ₂ and CO. The electrolysis takes place in a temperature range of approximately 940-970 ° C.

In the course of electrolysis, the electrolyte becomes poor in aluminum oxide. At a lower concentration of 1-2% by weight aluminum oxide in the electrolyte, there is an anode effect, which results in a voltage increase of, for example, 4-4.5 V to 30 V and above. At the latest then the crust must be hammered in and the aluminum oxide concentration increased by adding new aluminum oxide (alumina).

The cell is usually operated periodically in normal operation, even if there is no anode effect. In addition, with each anode effect, the bath crust has to be hammered in and the alumina concentration increased by adding new aluminum oxide, which corresponds to cell operation.

To operate the cell, the crust of solidified melt was hammered in between the anodes and the side board of the electrolysis cell and new aluminum oxide was then added. This practice, which is still widely used today, is met with increasing criticism for pollution of the air in the electrolysis hall and the outside atmosphere. The demand for encapsulation of the electrolysis furnaces and the treatment of the exhaust gases has become an imperative in recent years. However, maximum containment of the electrolysis gases by encapsulation cannot be guaranteed if classic long side operation takes place between the anodes and the side board of the furnaces.

In recent times, aluminum manufacturers have therefore increasingly switched to operating in the longitudinal axis of the furnace. After the crust has been hammered in, the alumina is added either locally and continuously according to the «point feeder principle» or not continuously distributed over the entire longitudinal axis of the furnace. In both cases, a storage hopper for the alumina is arranged on the electrolysis cell. The same applies to the transverse operation of the electrolysis furnaces recently proposed by the applicant (DE patent 27 31 908).

In known devices, rectangular or round chisels are used in cross-section to strike the solidified flow.

The lower region of the chisel, which directly engages with the crust when the crust is hammered in, in these known devices is designed, for example, as an end face arranged vertically to the side faces, a conical tip or a truncated cone-shaped constriction. The European patent application 0 025 417 describes chisel shapes suitable for punching or shearing off.

When using stationary installation impact devices, a fit is formed in the crust due to the repeated cell operation and previous actuation of the chisel, in relatively short intervals. H. there is only a very small space between the crust and the chisel. Depending on the nature of the impact device, in particular the chisel, there is a smaller or greater latent risk that the chisel will get stuck in the punched hole.

AT-B-280 626 describes a chisel with a hole for introducing compressed air into the melt flow of the electrolytic cell. According to a preferred embodiment, a large amount of alumina should be able to be injected into the bath of the electrolytic cell. For this purpose, the chisel according to this embodiment has widenings in the form of side crushing vanes attached near the tip. However, these relatively large crushing wings require a high impact force and also have a disadvantageous effect on other cell manipulations because of their bulkiness.

The inventors have therefore set themselves the task of creating a device for hammering in the solidified crust of an electrolysis furnace and a method for its operation, which ensure continuous, continuous operation without jamming.

The object is achieved according to the invention in that at least one cam is arranged in the lower region of the vertical chisel side surfaces, which comprises at least part of the chisel circumference, is rectangular in cross section, preferably square, and 5 to 15 mm from the chisel side surfaces protrudes.

Such a cam is expediently of elongated shape and extends, arranged horizontally, over at least part of the circumference of the chisel. However, two or more cams can also be arranged in superimposed levels to be orderly; their distance from the bottom of the chisel and their distance from one another can be varied depending on the cell geometry.

The cams are preferably made of the same material as the chisel, in particular a hard and weldable steel, e.g. B. St 45-50. Using a suitable fastening method, such as welding or screwing, preformed cams can be fixed on the vertical side surfaces of the chisel. The cams can also be applied in the form of welding beads and brought into the final shape using a suitable machining process. Furthermore, chisel and cam can be formed in one piece by z. B. be milled out. The cam cross sections are generally rectangular, preferably square and often have a slight undercut on the underside.

The dimensioning of the cams is essential: cams that protrude too far from the chisel are exposed to the risk of deformation, cams that protrude too little are not effective enough. Therefore the cams protrude 5-15 mm from the side of the chisel.

The solution according to the invention, based on the method, is characterized in that the chisel according to the invention with at least one cam arranged in the lower region of the vertical side surfaces after the crust has been pierced, at least until the lower half is reached in relation to the thickness of the crust the lowest cam is further lowered.

• - der Meissel klemmt nicht
• - der Meissel kann problemlos ausgezogen werden
• - die Kräfte auf die Kolbenstangenführung können vermindert werden.

When the cams are pressed into the solidified melt flow, they break out a gap, thereby avoiding that the chisel creates a precisely fitting fit in the crust. If it is desired that the cams penetrate the crust completely when the chisel is lowered, they are fastened far below, ie near the face of the chisel. If, on the other hand, it is desired that the cams only break through the upper half of the crust, the cams are fastened accordingly further up. It is possible, but not very sensible, to attach the cams even further away from the end face, ie further up, because then the following advantages will not be achieved or will only be achieved in part when the chisel is lowered again:

• - the chisel is not jammed
• - The chisel can be pulled out easily
• - The forces on the piston rod guide can be reduced.

The impact device, which consists in principle of a pressure cylinder, piston rod and chisel, is attached directly or indirectly to the cell structure or is part of an operating vehicle or furnace manipulator.

• Figur 1 einen Längsschnitt durch einen im Querschnitt rechteckigen Meissel mit Nocken auf den Schmalseiten, in der untersten Position der Arbeitsstellung.
• Figur 2 eine Seitenansicht des Meissels von Fig. 1.
• Figur 3 eine Ansicht eines im Querschnitt runden Meissels mit zwei übereinander angeordneten Nockenpaaren.
• Figur4 einen Querschnitt durch IV-IV in Fig. 3.
• Figur 5 einen teilweisen Längsschnitt durch einen Meissel mit verschieden grossen Nocken.

Examples of embodiments of the invention are shown schematically with the aid of the drawing. Show it :

• 1 shows a longitudinal section through a chisel with a rectangular cross section with cams on the narrow sides, in the lowest position of the working position.
• FIG. 2 shows a side view of the chisel from FIG. 1.
• Figure 3 is a view of a round chisel in cross section with two pairs of cams arranged one above the other.
• Figure 4 shows a cross section through IV-IV in Fig. 3rd
• Figure 5 is a partial longitudinal section through a chisel with different sized cams.

In Fig. 1, a chisel formed in cross section as an elongated rectangle, in the present case 150 x 40 mm, is shown. The lower area of the chisel 10 is immersed in the liquid melt 14, that is to say it has completely penetrated the solidified melt 16. This lower area is shown in the shape of a fishtail. Although this shape is used to advantage, any other suitable shape of the lower portion of the bit can be used.

The lower pair of cams 12 arranged on the narrow side has almost completely penetrated the crust 16. An almost continuous space 18 has been created between the chisel 10 and the solidified melt 16. As indicated in FIG. 1, the alumina 20 lying on the crust 16 trickles into this gap. In this way, the chisel 10 does not jam and can easily be pulled out again after being pushed in. The next time the furnace is operated, which takes place after a short time interval in automatic systems, the chisel can be inserted into the hole widened by the clearing function of the cams without difficulty. If the chisel is not exactly centered, it pushes away the lugs 17 of the solidified melt flow 16 left over from the last operating process without any problems and without great force.

In embodiments not shown, additional cams can be arranged on the wider side surfaces of the chisel.

Furthermore, the chisel can also be pressed down more, then the lower pair of cams 12 completely pierce the crust.

The side surface of the cams facing the bath or the chisel side, which have a cross-sectional dimension of approximately 1 cm ² , is undercut, preferably with an undercut angle of up to 20 °. Through this end face, which is inclined upwards in the direction of the chisel, the cams act in the sense of clearing teeth.

For the sake of simplicity, crust fragments and alumina pushed into the molten electrolyte from the underside of the chisel are not shown.

3 and 4, a chisel 22 which is round in cross section is shown. In this case, too, the cone-shaped lower area of the chisel can assume any other suitable shape.

A lower pair of cams 24 encompasses the largest Most part of the chisel circumference, as can be seen in particular from FIG. 4, a horizontal section through FIG. In contrast, a pair of cams 26 arranged in an overlying plane only comprises a relatively small part of the circumference of the chisel.

1-4 cams are shown, which are characterized by their elongated shape and their horizontal course by the same width, Fig. Shows a section of a longitudinal section through a chisel with cams of different widths. The lowest cam 30, which first acts on the solidified melt flow, is narrow, the cam 32 arranged above it is wider and the cam 34 arranged uppermost is widest. As a result, the space created between the chisel and the crust is gradually expanded from bottom to top when the impact device is actuated.

It goes without saying that the cams according to the invention can still be fastened in the lower region of the chisel in many design variants which have the same function and achieve the same result. The lowermost part of the chisel which carries or forms the cams can also be designed as an interchangeable unit which is detachably attached to the chisel shaft. This embodiment has the advantage that after a certain amount of wear or in the event of a repair, not the entire chisel, but only its lowermost part, has to be replaced.

Claims (9)

1. Chisel for a crust-breaking device for breaking through the solidified crust of an electrolytic cell, in particular for production of aluminium, characterised in that at least one projection (12, 24, 26) is arranged in the lower region of the vertical side surfaces of the chisel, which extends round at least a part of the chisel periphery, is rectangular, preferably square, in cross section, and projects out 5-15 mm from the side surfaces of the chisel.

2. Chisel according to claim 1, characterised in that the pre-shaped projections (12, 24, 26) are secured in a horizontal position to the side surfaces of the chisel (10, 22, 28) by means of welding or screwing.

3. Chisel according to claim 1, characterised in that the projections (12, 24, 26) are raised weld beads.

4. Chisel according to claim 1, characterised in that the projections (12, 24, 26) and the chisel (10, 22) are made unitary.

5. Chisel according to one of claims 1-4, characterised in that the face of the projection (12, 24, 26) facing towards the lower end of the chisel is undercut, preferably at an angle of up to 20°.

6. Chisel according to one of claims 1-5, characterised in that a plurality of projections, (12, 24, 26), arranged in superimposed planes, project out equally far from the side surfaces of the chisel.

7. Chisel according to one of claims 1-5, characterised in that a plurality of projections (30, 32, 34), arranged in superimposed planes, project out from the side surfaces of the chisel increasingly from below upwards.

8. Method of breaking the solidified crust of an electrolytic cell, in particular for production of aluminium, by operating a crust-breaking device fitted with a chisel according to claim 1, characterised in that the chisel (10, 22, 28) with at least one projection arranged in the lower region of the vertical side surfaces, after the penetration of the crust (16), is lowered further at least until the lowest projection or projections (24, 30) reach the lower half of the crust in respect of thickness.

9. Method according to claim 8, characterised in that the chisel (10, 22, 28) is lowered further until complete penetration of the crust (16) by the lowest projection or projections (24, 30).

Images