Field of the Invention

The present invention relates to a refractory insulating ring. In particular, the present invention relates to a refractory insulating ring for electrically insulating an alumina point feeder in an aluminium smelter.

Background of the Invention

The chemical composition of alumina consists of aluminium and oxygen. The process of smelting alumina to create aluminium involves separating the aluminium and the oxygen, and is known as the Hall-Heroult process. In this process a carbon lined furnace, or reduction cell is filled with a molten bath of flux, such as cryolite and aluminium fluoride. The alumina dissolves in the molten flux bath and electricity is introduced into the reduction cell through carbon anodes.

The electric current flows from the anode into the alumina flux mixture and into a carbon cathode lining of the reduction cell. The electricity causes the alumina to react with the carbon anode, resulting in the production of carbon dioxide and aluminium. During the smelting process, liquid aluminium is tapped from the base of the reduction cell, and new alumina is introduced into the molten flux.

During the reaction, if the alumina level in the bath falls below a critical level, perfluorocarbon green house gas emissions are generated by fluorine reacting with the carbon anode which is environmentally unfavourable. In order to maintain the alumina content in the cryolite at an optimum level, point feeders are generally used to deliver the alumina into the cryolite bath from above. The point feeder includes an elongate tube through which the alumina flows, and the point feeder can control the flow rate of alumina into the flux bath.

The elongate feeder pipe is located in part within a larger outer tube. An insulating ring is located around part of the feeder pipe. The insulating ring must prevent electricity from passing from the feeder tube into the surrounding machinery, which would short circuit the system. In addition, the insulating ring limits the transfer of heat from the feeder pipe to the outer tube. A problem with existing insulator rings is that they are manufactured during a casting process around the feeder tube. Accordingly, the replacement of a given insulating ring requires that the feeder tube is removed from the reduction cell, and taken to a refractory casting location. This process can be time consuming, and hence costly to the reduction cell operator.

Object of the Invention

It is an object of the present invention to substantially overcome, or at least ameliorate one of the above disadvantages, or to provide a useful alternative.

Summary of the Invention

In a first aspect, the present invention provides an insulator for an alumina point feeder, said insulator comprising: an annular ring of a refractory material, said ring being defined by first and second generally semi-annular portions, said annular ring having a radially inner region locatable adjacent to a generally cylindrical portion of said point feeder, and a radially outer region; and a band extending circumferentially around the radially outer region, said band securing the first and second generally semi-annular portions to each other.

The band is preferably formed from two generally semi-circular strips of mild steel which are connected to each other by welding.

A radially inner portion of the band preferably includes one or more circumferentially extending grooves.

The refractory material is preferably a castable refractory cement.

The radially inner surface of the ring has a circumferentially extending channel adapted to receive a circumferentially extending rib mounted to the point feeder.

In a second aspect, the present invention provides a method of forming an insulator for an alumina point feeder, said method including the steps of: placing a generally semi-circular steel strip in a mould; pouring a castable refractory cement into the mould, and allowing the cement to harden and form a first semi-annular portion having a radially inner region, and a radially outer region, said strip defining part of the radially outer region; removing the mould; forming a second similar semi-annular portion; locating the radially inner regions of the first and second semi-annular portions around a generally cylindrical portion of said point feeder, to define an insulator ring; and connecting the semi-circular steel strip of the first semi-annular portion to the steel strip of the second semi-annular portion to define a continuous steel band.

The step of connecting the steel strips preferably comprises welding.

Brief Description of the Drawings

A preferred embodiment of the invention will now be described by way of specific example with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view showing two halves of an insulator;

Fig. 2 is an end view of a mould for making the insulator of Fig. 1;

Fig. 3 is a side view of the mould of Fig. 2;

Fig. 4 is a perspective view of the mould of Fig. 2;

Fig. 5 is a cross-sectional view of a band of the insulator of Fig. 1;

Fig. 6 is a side view of the band of Fig. 5;

Fig. 7 is a front view of the band of Fig. 5;

Fig. 8 is a partial schematic diagram of a portion of a reduction cell and point feeder; and

Fig. 9 is a side view of a portion of a point feeder during installation of a new insulator.

Detailed Description of the Preferred Embodiments

An insulator 10 for electrically insulating an alumina point feeder 12 in a reduction cell is shown in the drawings. The insulator 10 is shown in isolation on Fig. 1 and is made from a suitable low cement castable refractory. In the reduction cell, the insulator 10 operates at temperatures up to 500 degrees Celsius. The insulator has properties which insulate both electricity and heat.

During manufacture, the cast insulator is fired to a temperature of around 500 degrees to remove water from the refractory material, including chemically bonded water. An example of a suitable refractory material has the following composition:

83% AI2O3

9.8% Siθ2

1.5% Fe2θ3

2.6% TiOz

2.3% CaO

0.2% Alkalies

An alternative suitable refractory material has the following composition:

51% AI2O3

44% Siθ2

0.9% Fe2θ3

1.8% Tiθ2

1.7% CaO

0.3% Alkalies

As shown in Fig. 1, the insulator 10 is formed by casting in a mould 20 which forms two separate, semi-annular portions 14, 16, which when placed side by side define an annular ring 15. The mould 20 is shown in detail in Figs. 2 to 4. The mould 20 has a first plate 22, and a second plate 24 which are mounted at 90 degrees relative to each other. The mould 20 also includes an inner core 26 mounted to the first plate 24 and an outer shell 28. The refractory material is poured into the mould 20, and fills the space between the core 26 and the shell 28.

As shown in Fig. 3, the core 26 has a projection 29, which creates a corresponding circumferentially channel 17 in the insulator 10.

When the semi-annular insulator halves 14, 16, are arranged as a ring 15, the insulator 10 has a radially inner region 18 and a radially outer region 19. The radially inner region 18 is locatable around a cylindrical portion of the point feeder 12, as shown in Figs. 8 and 9.

Figs. 5 to 7 show a band 30 which is located adjacent to the radially outer region 19. The band 30 is formed from mild steel and during formation of the insulator 10, is initially placed into the mould 20, abutting against the shell 28. Accordingly, when each semi-annular half 14, 16 is removed from the mould, the band 30 forms part of the radially outer region 19.

The operation of the insulator 10 will now be described. When an existing insulator requires replacement, that insulator 10 is removed from the point feeder 12. Two new, semi-annular portions 14, 16 are placed on radially opposing sides of the point feeder 12.

The ends of each band 30 of the two semi-annular portions 14, 16 are connected to each, other by welding or another suitable means. Accordingly, the bands 30 are used to hold the insulator 10 together around the point feeder 12. In addition, the groove 17 is located on a flange 34 formed on the point feeder 12. This prevents the insulator 10 from moving axially relative to the point feeder 12. The point feeder 12 operates within a tube 40, and during operation of the reduction cell, the point feeder 12 may move axially within the tube 40.

The insulator 10 electrically insulates the point feeder 12, thereby preventing an electric current from running up the point feeder 12 and passing into the wall of the tube 40. In addition, the insulator 10 also limits the amount of heat which is transmitted to the tube 40, and hence the surrounding parts of the reduction cell.

Referring to Hg. 5, the band 30 has a profile which is generally flat on a radially outer side 32, and the band 30 has circumferentially extending grooves 34, on a radially inner side 36. The grooves 34 assist with bonding of the band 30 to the semi annular portions 14, 16 of refractory material. The band 30 has rounded edges which assist in limiting the amount of shock which is transmitted to the refractory material in the event that the band 30 is impacted by the tube 40 for example.

As shown in Figs. 1 and 9, the cross-sectional profile of the insulator 10 axially tapers away through an angle of about 20 degrees in both directions from the central band 30. Accordingly, the band 30 forms the radially, outer most part of the insulator 10. This limits the chance of the refractory material being damaged during use, as the band protects the refractory material from being damaged by the tube 40 within which the insulator 10 is located during operation. Advantageously, a replacement insulator 10 can be fitted to the point feeder 12 relatively quickly, by simply welding the steel bands 30 to each other around the point feeder 12.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.