GB2046412A - Anode baking ovens - Google Patents

Abstract

An anode baking oven includes a chamber which has at least one pair of heating walls (1') with a compartment (2') therebetween for receiving (11') to be baked, each heating wall comprising two spaced-apart outer walls (4'), means (8', 9') to inject a flame downwards into the space between the outer walls, an air inlet (6') through which compressed air can be fed into the space and through the flame, a plurality of spaced-apart vertical baffles (5') extending across the space from one outer wall to the other and staggered in the vertical direction to provide a serpentine path for the air and for flue gases from the flame, and a plurality of heat-resisting tie rods (13) extending across the space to support the outer walls, the tie rods being positioned between the baffles and within the serpentine path. The presence of the tie rods (13) allows a considerable reduction in the thickness of the outer walls and the baffles. <IMAGE>

Description

SPECIFICATION Anode baking ovens This invention relates to ovens for baking anodes which are to be used in the electrolytic reduction of alumina for the production of aluminium.

The anodes are made of pure carbon, in the form of ground coke, which after heating is mixed with coal tar or pitch which bonds it together. The mixture is forced into moulds to form compressed blocks, known as "green" anodes. These anodes then have to be baked slowly in ovens at approximately 1200"C before they are ready for use in electrolytic smelting cells.

The anodes are rapidly destroyed in the course of the aluminium production process, mainly due to the action of the oxygen which is produced during the electrolysis of the alumina. Large quantities of the pre-baked anodes must, therefore, be continuously available.

Ovens for pre-baking the anodes conventionally comprise a large number of chambers arranged in two interconnected rows, each chamber comprising a number of compartments separated by heating walls. There may, for example, be twenty chambers in a row, each comprising six compartments and seven heating walls.

The green anodes are loaded into the compartments in layers. The anodes are usually placed lengthwise across the width of the compartments in order to achieve optimum use of the space available.

Each heating wall comprises two spaced-apart outer walls of fire bricks, the outer walls also forming the side walls of the adjacent compartments. Jets are provided for projecting a flame of liquid or gaseous fuel downwards into the space between the outer walls. Hot air is pumped into the space from one end of the heating wall and passes round a serpentine path between vertical baffles located across the space. The air and flue gases are extracted from the opposite end of the heating wall, and pass into the inlet of the corresponding heating wall of the next chamber in the row.

The bricks of the outer walls of each heating wall are laid in bonded courses, but the vertical joints are not cemented, so that gases which are baked out of the anodes can be sucked out of the compartments into the heating walls and thence extracted with the flue gases.

Further constructional details of the conventional ovens will be described later with reference to the accompanying drawings.

It will be realised that in order to withstand the great weight of the anodes and the other forces produced in the baking process, the heating walls have been of massive construction. In particular, the outer walls and the baffles have been thick. This has produced two results. Firstly, the thickness of the outer walls has resulted in relatively poor heat transfer to the anodes, and the baking process has therefore taken many hours. The output of the oven is therefore limited. Furthermore, the protracted initial baking has a detrimental effect on the strength of the anodes, which tend to collapse.

Secondly, since the width of the heating walls must be relatively large, only a limited number of heating walls and compartments can be accommodated in a given overall width of chamber. The compartments have therefore been made relatively wide, and in order to fill the compartments adequately to make satisfactory use of the compartment space, the anodes have been placed lengthwise across the compartments as mentioned above. If the anodes were placed widthwise across the compartments, a considerable amount of lateral space would be wasted. This has had the effect of limiting the lengths of the anodes, because they have had to fit across the compartments.

It is an object of the present invention to allow the use of narrower heating walls and compartments, so that better heat transfer to the anodes is accomplished, and so that more compartments and heating walls are accommodated in a given chamber width.

According to the present invention, an anode baking oven includes a chamber including a pair of heating walls with a compartmenttherebetween for receiving anodes to be baked, each heating wall comprising two spaced-apart outer walls; means to inject a flame downwards into the space between the outer walls; an air inlet through which compressed air can be fed into the space and through the flame; a plurality of spaced-apart vertical baffles extending across the space from one outer wall to the other and staggered in the vertical direction to provide a serpentine path for the air and for flue gases from the flame; and a plurality of heatresisting tie rods extending across the space to support the outer walls, the tie rods being positioned between the baffles and within the serpentine path.

The tie rods are preferably formed of bricks of a material having a high alumina content.

Preferably, the outer walls include bricks containing carborundum.

Due to the use of the tie rods, the thickness of the outer walls and the baffles can be appreciably reduced.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1(a) and 1(b) are plan views of a chamber of a conventional oven, and an oven according to the present invention, respectively, Figures 2(a) and 2(b) are sectional views on lines ll-ll of Figures 1 (a) and 1(b), respectively, Figures 3(a) and 3(b) are enlarged sections on lines Ill-Ill of Figures 1(a) and 1(b), respectively, Figure 4 is a schematic plan view of a conventional complete oven installation including the two rows of chambers, and Figure 5 is a schematic plan view of an installation including two rows of chambers in accordance with the present invention.

Referring to Figure 1 (a) of the drawings, a typical conventional oven chamber comprises seven heating walls 1 with six compartments 2 therebetween for holding the anodes. Typically, the heating walls have been approximately 0.45 metre wide and the compartments 0.85 metre wide. Each heating wall comprises two spaced-apart outer walls 4 (Figure 3a) with baffles 5 (Figure 2a) built into the outer walls and extending across the space from one outer wall to the other. The baffles are staggered so that they are alternately low and high within the space. An air inlet 6 is provided at the top of the left-hand end-wall of the heating wall, and an outlet 7 is provided at the top of the right-hand end-wall of the heating wall.

Two jets 8 and 9 inject burning fuel into the space between the baffles.

Pre-heated air from the cooling chambers is pumped into the inlet 6 and helps to complete the combustion of the burning fuel from the jets 8 and 9.

The heating air follows a serpentine path up and down between the baffles. The air and flue gases leave the heating wall via the outlet 7, and may pass to the inlet of an adjacent heating wall, as will be explained later. Cross members 10 are located near the base of the heating wall.

Figure 3(a) shows how anodes 11 are loaded into the compartments 2, the dimension 12 being the length of the anode. Typically, six iayers of anodes are contained in the height of the compartment.

Referring now to Figure 1 (b), 2(b) and 3(b) of the drawings, a chamber of an oven according to the present invention has basically the same configuration of components as the conventional chamber, those of the present invention having the same reference numerals but marked with primes (e.g. 1', 2' etc.).

However, the outer walls 4', the baffles 5' and the cross members 10' are all approximately 50% thinnew than the corresponding components in the conventional chamber. This is made possible by the use of tie rods 13 arranged between the baffles. The ends of the tie rods 13 are built into the outer walls 4' so that the rods support the walls, and make up for the decreased thickness of the walls 4', the baffles 5' and the cross members 10'.

It will be seen that some of these tie rods are directly in line with the flames from the jets 8' and 9', whilst the others, although not actually subjected to the flames, are in the path of the extremely hot air and flue gases. All of the tie rods must, therefore, be made of a material which is capable of withstanding the high temperatures. Preferable the tie rods comprise firebricks made of a material with a high alumina content.

Because the heating walls 1' and the compartments 2' are narrower overall, nine heating walls and eight compartments can be accommodated in a chamber of the same overall width as the conventional chamber. The compartments 2' are made narrower than the conventional compartments 2 so that the width 14 of the anodes 11' just fits the width of the compartments, leaving the required clearance.

Itwillbeapparentthatthethinnerouterwalland the smaller distance D' between the centre of the anode 11' and the inner surface of the wall 4' (say 70% of the corresponding distance in Figure 3(a)), results in much faster heat transfer to the anodes in the oven of the present invention, and must faster subsequent cooling of the baked anodes.

The positioning of the anodes lengthwise along the length of the compartments means that longer anodes can be accommodated than could previously. In fact, the anodes can be of any desired length up to the full length of the compartments. This is clearly advantageous as regards their use in the electrolysis process.

A further novel feature of the oven which is described herein is the presence of a number of courses 15 of carborundum bricks in the outer walls 4'. These bricks have a high thermal conductivity, and so improve still furtherthe rate of heat transfer to and from the anodes.

The relatively small size of the compartments of the present invention makes it practicable to fit air-tight covers 24 (Figure 3(b), perhaps in two halves in view of their weight, over the open tops of these compartments. These covers, used at a parti cularstage of the manufacturing cycle, preventfresh air from entering the compartments from above, passing through the outer walls 4' into the spaces of the adjacent heating walls, and thereby cooling the gas/air mixture. The covers therefore contribute to the faster heating of the anodes past the temperature at which they soften, improved thermal efficiency, and a shorter manufacturing cycle.

The baking cycle using the oven of the present invention can be made 24 hours long, so that the same work schedule is repeated each day, with the work area advancing by one chamber per day (i.e.

the next chamber in the sequence being fired each day). The cycle fits in very conveniently with shift work. This is to be compared with the very inconvenient cycle time of approximately 40 hours which is obtained with conventional ovens.

A plan view of a complete illustration of a conventional oven is illustrated schematically in Figure 4. Two rows of chambers 25 and 26, each row approximately 85 metres long and 9 metres wide, are positioned side-by-side, with a corridor 27 approximately 4 metres wide therebetween. A double girder travelling overhead crane 28 is used to load the green anodes into the compartments and to unload the baked anodes. All of the activity of the workmen takes place within the corridor 27. Each row typically comprises 20 chambers.

Figure 5 shows the re-arrangement which is possible using an oven of the present invention. The rows of chambers 29 and 30 can be positioned close together so that the chambers may be made wider than the conventional chambers. Each row contains 15 chambers and the overall length is only about 69 metres. The decreased cycle time and the increased chamber width can result in up to 22 times the production rate of the conventional ovens per square metre of oven.

In this case, two overhead cranes 28' are used to service the respective rows of chambers.

Claims (10)

1. An anode baking oven, including a chamber which includes a pair of heating walls with a compartment therebetween for receiving anodes to be baked, each heating wall comprising two spacedapart outer wal Is; means to inject a flame downwards into the space between the outer walls; an air inlet through which compressed air can be fed into the space and through the flame; a plurality of spaced-apart vertical baffles extending across the space from one outer wall to the other and staggered in the vertical direction to provide a serpentine path for the air and for flue gases from the flame; and a plurality of heat-resisting tie rods extending across the space to support the outer walls, the tie rods being positioned between the baffles and within the serpentine path.

2. An oven as claimed in Claim 1, wherein the tie rods are formed of a material having a high alumina content.

3. An oven as claimed in Claim 1 or Claim 2, wherein the outer walls include bricks containing carborundum.

4. An oven as claimed in any preceding claim, comprising a plurality of said chambers arranged end to end; wherein each heating wall includes an outletforthe air and flue gases; wherein corresponding heating walls of adjacent chambers are aligned end-to-end, with the air inlets and the outlets connected in series.

5. An oven as claimed in any preceding claim, wherein the or each chamber includes a plurality of said compartments and a plurality of said heating walls, the compartments and the heating walls being arranged alternately side by side across the width of the chamber.

6. An oven as claimed in Claim 5, wherein the or each chamber includes eight of said compartments and nine of said heating walls.

7. An oven as claimed in Claim 2, including means to feed air under pressure into the heating wall inlets of one chamber; means to operate the flame injection means of a later chamber in the end-to-end arrangement; and means to extract the air and flue gases from the outlets of a still later chamber in the arrangement.

8. An oven as claimed in Claim 7, including means to seal the tops of the compartments of said still later chamber.

9. An oven as claimed in Claim 4, including two of said end-to-end chamber arrangements, the two arrangements being positioned side-by-side and substantially abutting each other.

10. An oven as claimed in Claim 1 and substantially as hereinbefore described with reference to Figures 1(b), 2(b), 3(b) and 5 of the accompanying drawings.