DE2630396A1 - Baking carbon blocks e.g. aluminium cell cathodes - using rapid heating rate and fluid coke packing material - Google Patents

Abstract

A method of baking elongated green carbon blocks, prepd. from a mixt. of carbonaceous aggregate and 28-35 wt. % (of mixt.) of pitch, comprises supporting the upright block in a packing of calcined fluid coke in a baking furnace and heating to 950-1250 degrees C, the heating rate in the range 250-550 degrees C being >100 degrees C/24 hrs. A method of producing cathode blocks for an aluminium reduction cell is also claimed, the method being as above but the aggregate is of calcined anthracite coal and the heating is carried out in a ring furnace for is not >8-10 days. More than double the heating rate normally employed can be used when using a fluid coke packing. The fluid coke provides adequate support and has better heat transfer characteristics than prior packing materials.

Description

Method of burning coal blocks

The invention relates to methods of burning coal, in particular on the production of burned coal bodies from mixtures of coal-containing batches, such as coke, pitch or some other material as a binding agent.

These mixtures can be made, for example, by molding or extrusion brought into a certain shape and then fired to make the binder to coke, under temperature conditions that make an electrically conductive Let structure arise.

There are different forms of coal in the aluminum industry been used as the anode and cathode electrodes of the electrolytic cells. Some Cells are self-baking Anodes while others Cells work with pre-burned anodes. Both types of cells work together either with a rammed earth lining or with pre-burned coal blocks, the one form the inner cathode lining of the cell.

The burning of cathode blocks was considered to be particularly problematic in the Compared to prefired anodes, mainly because of the differences in dimensions and in the content of binder. For example, pre-burned anodes are usually used from mixtures with a relatively low pitch content and with a graded Amount of calcined coke produced, the pitch content being about 15 percent by weight of the mixture. The shaped green anode bodies are in rows in a furnace shaft, and packing material is placed around the parts in each row attached before the following row is used, etc. When burning such Parts can be worked with a very large increase in temperature. On the other hand included Cathode block mixtures generally have more pitch (for example 30% by weight) -, and it calcined anthracite is used for at least the greater part of the mixture, often for 75 to 100% of the mixture. Cathode blocks usually become upright brought standing on their ends, and the rates of temperature increase are generally considerably lower, probably based on the assumption that that the higher pitch content and other characteristics of cathode blocks make it necessary make those blocks burn slower.

As for the shape differences, prefired anodes are proportionate compact structures compared to cathode blocks, which tend to be larger, one have greater elongation and are thinner, making them more difficult in the oven can be held. Furthermore, cathode blocks usually have a longitudinal slot on, which receives an electric collector rod made of steel.

Because of their dimensions and more complicated shape associated with The additional difficulty due to the higher pitch content is when burning cathode blocks numerous difficulties encountered. There were bends, subsidence and tearing, in addition, compression of the slot receiving the collector rod due to the expansion of the adjacent parts of the block during heating. From these and more Reasons were a must when burning cathode blocks in a conventional ring shaft furnace very slow temperature rise required (usually limited to around 500 C per 24 hours), so that when operating such a furnace only 14 to 18 cycles could be driven in the year. Mantell mentions in "Industrial Carbon" that a average work cycle looks like this: Loading the furnace (including routine maintenance) 2 to 4 days; Heating for 12 to 20 days; cooling down 20 days; Discharge 3 to 5 days. This gives the average cycle time from (at least) 37 days to (at most) 49 days.

According to the known method, it was expressly customary to Burning the cathodes to operate at a rate of temperature increase that was much lower than the usual burning of anodes, despite the limitation, which experiences the efficiency of the furnace through it. According to the invention it is however possible, with a temperature increase of at least 0 125 C every 24 hours, i.e. at more than the rate of temperature increase usually used to work when burning cathode blocks that are wrapped in flowing coke.

The use of flowing coke as a packing material when burning anodes has been known for some time, and left it join there are some benefits to this procedure.

One such advantage is better flow behavior when filling the Space around the anode shape. In this way the anodes can be arranged in numerous rows without first filling packing material around and between the anodes of a row before the next row is used. Another benefit is that Elimination of dust problems that used to arise with other packaging materials.

Manufacturers of carbon anodes have already recognized that flowing coke as Packing material is also appropriate for other reasons. So it was found that the anode blocks after firing relatively low cleaning costs compared to other substances required like on the finished products got stuck. It was also shown that the burning went on more evenly, so that there was a more uniform electrical resistance throughout the anode.

Despite these successes with flowing coke in the manufacture of anodes, the professional world obviously has the possibilities of using flowing coke the manufacture of cathodes is not taken into account. It is true that in the literature about some early attempts with flowing coke as cathode packing material have been reported but only used as a replacement material while maintaining the usual firing cycles, and conventional delayed coke and mixtures of sand and coke the material currently used in industry. There are various explanations for this.

Unlike anodes, cathode blocks are usually, as above mentioned standing upright on their ends and not vertically separated from each other Rows burned, so that it is relatively less troublesome to use the packing material to feed. As a result, any benefit that derives from the cheaper Filling properties of flowing coke results, in fact, of less importance when they are cathodes.

Probably the main reason for this is that the cathodes are burning Industry has not considered the use of flowing coke in the at Cathode blocks tend to bend and settle on heating when they are heated plastic range between about 250 and 550 C to be seen. Because maintaining the shape or geometry of the block is very important and a very difficult one to achieve The aim is, there was a tendency in the professional world, rather the mounting properties of the packaging material as its other properties. The thermal conductivity properties of slow "coke and of sand-coke mixtures were considered to be suitable for slow heating appropriately considered as a necessary concomitant for the burning of Cathodes were accepted, and flowing coke made no apparent in this regard Advantage.

According to the invention it was found that flowing coke in comparison not only has suitable support properties compared to conventional packaging materials, but also has better thermal conductivity, so that it is surprisingly well suited for use in burning cathodes, but particularly suitable in burning of cathode blocks at significantly higher rates of temperature increase.

Fluid coke of the type suitable within the scope of the invention normally has 3 has a bulk density of about 1.2 g / cm (75 lbs./ cu.ft.) compared to 1.8 g / cm3 - (110 lbs./cu.ft.) For a typical mixture of 70% sand and 30 8 slow coke. Production, use and properties of flowing coke are detailed below entitled The Use of Calcined Fluid Coke As packing material in Anode Baking Pits "(use of calcined flowing coke as packing material in anode kilns) from to. Stuart and B.H. Pippin, in the AlME publication Licht Metals 1974 ", Vol. 3, described.

The dimensions of characteristic fired blocks following manufactured using the process of the invention are: 39 cm (15 1/2 ") wide, 33 cm (13) high and 165 cm (65 n) long, with a standard longitudinal slot for the Acceptance of a collector rod with a 10 cm (4 ") square cross-section.

Such blocks each weigh more than 270 kg (600 lbs). They consist of mixtures of pitch and a suitable coal mixture with about 75 to 100 percent anthracite with the pitch about 28 to 35 percent by weight of the total mixture. If necessary, a suitable H shrink rod " 5 x 15 cm (2 "x 6") wood to be inserted into the slot of each block, as is customary to prevent jumping. Good products could in some Cases can also be produced without the use of shrink rods.

According to the invention, a common burning process becomes a common one annular furnace used with four sections of shafts for heating in one 42 hours Cycle per section, i.e. a total heating time of 168 hours (that is 7 days) for heating to about 1150 ° C, which is a 0 mean rate of temperature rise of more than 150 C in 24 hours. The elongated green blocks of coal are in adjacent shafts of the furnace with or without shrink rods, provided that the Blocks, as if pale in themselves, are provided with slots, inserted. Flow coke was packed around the inserted blocks so that the blocks are supported and against burning are protected. The furnace can be heated with oil or gas or also electrically. In the usual furnace designs, a heating line carries natural gas as fuel to the Generating the required temperature. The line is in consecutive Steps, and causes the preheating in some shafts, the burning in others and the subsequent cooling.

All of the processes mentioned can be carried out in the usual way, however, is the heating rate used for the purposes of the present invention very high and reaches an average of more than 1000 C per 24 hours up to one suitable final temperature between 950 and 1250 C, generally around 0 1150 C. After the burning process is finished, the fluid coke packing material can be reused easily removed using, for example, a conventional suction device.

In contrast, a common earlier practice was to a system divided into 5 or 6 sections (including 3 or 4 preheating stages) to use with a residence time of about 6 days in each firing stage, resulting in a total heating cycle of 30 days resulted. By the method according to the invention becomes the efficiency of an existing method using the current method Facility practically doubled. For example, 60 blocks can be saved in just the Burns half the time it used to take to make 72 blocks was. A burn time of 8 to 10 days is the rule.

Typical ring furnace constructions and the common working methods are described in U.S. Patent 2,699,931 (Buhler et al.), and others Information on the subject is in Chapter XVI of "Industrial Carbon" (Mantell), 1946 (see in particular Figures 85 and 87 on pages 253 and 254. The inventive measures described above are based on the use of a furnace system, which is described in Fig. 87 by Mantell, However is in each focal section only a single middle baffle instead of the two surfaces shown there, alternating upward and downward.

Patent claims:

Claims (5)

P a t e n t a n s p r ü c h e: Method of burning greens Coal blocks, which consist of a mixture of a coal-containing mixture with about 28 to 35 wt.% Pitch content of the mixture, characterized in that the green Blocks standing upright on their ends in a pack of calcined flowing coke kept in a kiln and heated to a temperature of 950 to 12500 C, the heating in the range between 250 and 5500 C with an increase in temperature of more than 1000 C every 24 hours. 2. The method according to claim 1, characterized in that said Heating in the range between 250 and 5500 C with a practically uniform Temperature increase of at least 1250 C per 24 hours progresses. 3. The method according to claim 1, characterized in that the batch content of the mixture consists of 75 to 100% calcined anthracite. 4. The method according to claim 1, characterized in that the pitch content of the mixture is about 30% by weight. 5. The method according to claims 1 to 4 for the production of elongated, electrically conductive blocks for the cathode lining of an aluminum reduction cell, characterized in that the green blocks are placed in a ring oven, in which the pack of calcined flowing coke holds and protects the blocks.