GB2185559A - Process and apparatus for continuously graphitizing carbon bodies - Google Patents

Abstract

In a process of continuously graphitizing carbon bodies by electric resistance heating, the carbon bodies 1,2 move in columnar formation vertically through a graphitizing furnace surrounded by counterflow of granular carbon material 3 and are heated by an electric current flowing over electrodes 6,7 through the granular material and through the carbon bodies. The column of carbon bodies 1,2 is preheated by the granular carbon in a lower heat exchange zone III, then moves through an intermediate graphitizing zone II in which the carbon bodies 1,2 are heated by electric resistance and finally through an upper heat-exchange zone I, in which it is cooled by the granular carbon. The electrodes 6,7 may be inclined to the axis of the furnace. <IMAGE>

Description

SPECIFICATION Process and apparatus for continuously graphitizing carbon bodies The invention relates to a process and a device for continuously graphitizing, i.e. converting into graphite, carbon bodies by electric resistance heating, in which the carbon bodies move in columnar formation vertically through a graphitizing furnace surrounded by granular carbon material and are heated by an electric current flowing over electrodes through the granular material and through the carbon bodies.

From German Patent Specification 487373 is known a processforgraphitizing carbon bodies in which the carbon bodies, which are in the form of plates, move end-to-end in columnarformation vertically downwards through the graphitizing furnace.

In its downward movement the column of carbon plates is surrounded by carbon material which is itself stationary. A disadvantage of thins arrangement isthatthefurnacewalls are soon damagedandthe furnace has a short life.

This disadvantage is alleviated in the process and furnace described in the German Patent Specification 2311467 for continuously graphitizing hardbaked carbon bodies. The carbon bodies move in columnarformationthrougha hollowgraphiteelectrode and travel downwards through the graphitiz ing furnacetogetherwith a surrounding body of granular carbon material. The electric currentflows from the hollow electrode through the granular carbon material and is then led through and away over the column of carbon bodies.

As described there, this arrangement not only lengthens the life ofthefurnace but also has the following advantages: - Recovery of the effluent gases isfacilitated.

- Dust problems are reduced and can be avoided.

- Electric power consumption remains almost con- stant. The electric power can therefore be bought at a favourable rate.

- Power consumption is comparatively low.

-Wasteheatcan be conveniently utilised.

- The graphitizing of the carbon body is very homogeneous.

- As a byproductthere is obtained a valuable, highly calcined and partly graphitized coke material.

- The graphitizing is highly mechanised and automated.

An object of the present invention is to improve still further on these advantages, in particular by reducing electric power consumption still further, by increasing the output of product from the furnace and by improving the degree of graphitizing ob tained in the product.

These objects are achieved by a process as defined in claim 1. Apparatus suitable for performing such a process is defined in claim 6.

The invention is based on the factthat the column of carbon bodies moves upwardsthroughthe furnace while the granular carbon material surrounding the column of carbon bodies moves downwards in counter flow.

Additional advantageous butoptionalfurtherfea- tures of the process and apparatus of the invention are defined respectively in claims 2 to 5 and 7 to 16.

Compared to the process described by Acheson and Castner, and the process of the German Patent Specification 2311 467, the principle of continuous operation and counterflow used in the present invention has considerable advantages. Energy consumption can be less than 2 kWh per kg of graphitized product, and as a valuable by-product one obtains a highly calcined granular carbon material.

Heat losses may be greatly reduced and the process is environmentallyfavourable without requiring special precautions.

Afurther advantage of the process of the invention is that the carbon bodies fed to the graphitizing furnace can be baked (carbonised) and, if desired, impregnated with pitch, in a particularly convenient and energy-saving manner before feeding to the graphitizing furnace. This is made possible by the counterflow graphitizing process defined in claim 1.

In the previously known graphitizing processes, for example as described by Acheson and Castner, the carbon bodies are baked, before feeding to the graphitizing furnace, at 9000Cto 1 0000C during a period of 14to 21 days. The resulting carbonised carbon bodies are then very carefully stacked in the cold state in the graphitizing furnace. Electric current is then applied to the cold carbon bodies. A prerequisite in thatthe carbon bodies have been largely de-gassed, that they have sufficient electric conductance and that they do not shrink excessively in the graphitizing furnace. It is to satisfy these require mentsthatthecarbon bodieswerepreviouslyhea- ted to at least 900"C.

In contrast to this, in the process of the present invention the carbon bodies, in moving upwards from the bottom of the graphitizing furnace, are first warmed by heat exchange in the lower zone ofthe furnace, for example up to temperatures between 1 200 C and 1 600 C, the heat being taken from the descending granular carbon material such as coke or anthracite. It is only when the carbon bodies reach the current supply region IV2 that they are subjected to electric resistance heating.Due to the preheating ofthe carbon bodies in the lower zone Liy, they only need to be baked, before feeding to the graphitizing furnace,to a maximum temperature between 550 and 650 Crfor example 600 C. Consequentlythe carbon bodies fed to the graphitizing furnace can exhibit a much higher specific resistance, by about a factor often, compared to carbon bodies which have been baked at 900"C or more. This saves at least 25% of the energy and time consumed in the preliminary baking, or carbonisation, of the carbon bodies before they are fed to the graphitizing furnace. And afterthe carbonisation,the cooling period is shorter. Con sequently the output of product from the plant is increased by at least 25%.Moreover, with fully automated handling of the carbon bodies, in the process ofthe present invention, it becomes possible to synchronise the carbonising and graphitizing in such a way that the carbon bodies are fed to the g raphitizing furnace while they are still hot from the carbonising, for example at 300 to 400 C, no intermediate cooling taking place.

Baked carbon electrodes, particularly those which will ultimately be used as highly stressed graphite electrodes in electric arc steel-melting furnaces are often impregnated with pitch to make them mechanically stronger and to increase electrical conductance. After impregnation the carbon body is usually heated in a furnace to 800"C, for converting the pitch in the pores of the carbon body into coke. The graphitizing furnace of the present invention, by its enclosed construction and its environment4avourable method of operation, particularly by the preheating ofthe carbon bodies in the lower zone Ill ofthe furnace, makes it possible to do without a preliminary baking ofthe pitch-impregnated carbon bodies.The volatile pyrolysis products ofthe impregnation pitch are partly cracked in the graphitiz ingfurnace and the resulting gases, togetherwith the other gases produced by the graphitizing process, can be extracted from the furnace and can be burnt as an energy-rich fuel.

Thus the graphitizing process ofthe present invention of itself produces savings in capital investment, intimeand in electrical energy consumed in the manufacturing processes which precede the feeding ofthe carbon bodies to the graphitizing furnace.

Two embodiments of the invention will now be described in further detail by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is an axial section of an embodiment of graphitizing furnace,to illustrate its method offunctioning.

Figure2 is a cross section taken in the plane A-A in Figure 1.

Figure 3 shows a second embodiment of graphitiz ing furnace.

The vertical shaft-like graphitizing furnace shown in Figure 1 has three long heat-treatmentzones 1,11 and Ill and, between them, two short regions IV1 and lV2for electric current supply. The upper and lower zones land Ill serve for heat transfer. In the inter mediate zone II, between the two current supply re gions IV1 and IV2, heat is generated by electric resist ance heating. The intermediate zone II may be, and preferably is, the central zone of the furnace.

The upper and lower zones land Ill have com parativelysmall shaft diameters. The lower end of the upper zone I, and the upper end ofthe lower zone III, are both conically expanded so thatthe two cur rent-supply regions IV1 and IV2, as well as the inter mediatezonell, havelargershaftdiameters.Thetwo current supply regions IV1 and IV2 are equipped with electrodes 6,7 inserted horizontally, at right angles to the path of travel of the moulded carbon bodies 1, 2, through the vertical side-walls of the furnace shaft, the electrodes receiving current over electricter minals 8.Referring now to Figure 2, it will be seen thatthe upper current supply region IV1 has two op posite pairs of electrodes 6 spaced equally apart over the circumference of the shaft. The lower current supply region lV2 is arranged similarly. Each elec trode 6,7 is rectangular in cross section. Alternati vely, if desired,the electrodes can be different in number and arranged differently, and they can have other cross sections, for example they may be circular in cross section, and it is often sufficientto have, in each current supply region IV1, IV2, only one pair of opposite electrodes 6, 7.

Moreover the arrangement and construction ofthe electrodes can, if desired, be varied in other ways.

For example, current can be fed in and led away in the sametransverse plan. There need beonlyone transverse current supply place arranged in this way, or several such planes distributed at regular or irregular intervals axially. The heights ofthe electrodes, measured axially with respect to the shaft of the furnace, can varyfromtransverse planetotrans- verse plane.

Nevertheless it is advantageous to feed current in at one ofthe transverse planes IV1 or IV2. After passing through the column of moulded carbon bodies the current is led away atthe othertransverse plane IV1 or IV2. The cross sections of the electrodes are chosen to suitthe requirements, particularly in regard to their heights, measured axially with respect to thefurnace shaft. As an alternative there can be used, instead of an even number of electrodes in each transverse plane, an odd number. Furthermore current can be fed in at one transverse plane, led out at the next one and this arrangement repeated several times down the shaft, the electrical conditions varying between the one step and the other.Finally, 3-phase current can, if desired, be used, the star point being located in the column of moulded carbon bodies.

The graphitizing furnace has a refractory lining 11 all the wayfrom top to bottom and made, for example, of a material with a high content of Awl203. The cylindrical portion of the upper zone I has an outer steel jacket 9. Belowthatthe refractory lining 11 is surrounded by a double-walled cooling jacket 10 through which a cooling medium, preferably water, circulates. If desired the cooling jacket 10 can extend all the way down to the bottom of the furnace. In certain cases a double-walled cooling jacket is not nec essay.

The graphitizing furnace need not necessarily be of circular cross section. The cross section can, if desired, be square or rectangular, particularly if the graphitized product is intended to be used as carbon blocks for the bottom walls of aluminium molten salt electrolytic reduction cells. For carbon bodies of other cross sections furnaces with corresponding cross sections can be used.

The carbon bodies 1,2 are fed automatically and continuously, by means which are not shown, into the lower end of the furnace and, after passing upwards through the three zones 111,11 and I, are delivered from the top, as indicated by the arrows in the drawing.

In counterflow to the upwards movementofthe carbon bodies 1,2 a stream of granular carbon material 3, such as petroleum coke or anthracite, moves downwards through the furnace, surrounding the column of carbon bodies 1,2, as indicated bythe arrows in the drawing. The granular carbon material is fed continuously into the top of the furnace,for examplefrom a silo, and leaves through lower outlet channels 5, whence it is conveyed away.

The moulded carbon bodies of compressed carbon can be fed in the cold state to the lower end of the furnace. In zone Ill they are preheated by heat ex changewiththehotdescending granularcarbon material. In zone II, and inthetwo current supply regions IV1 and lV2 they are strongly heated by the electric current and converted entirely into graphite. In zone I the carbon bodies preheat the granular material by heat exchange. The granular material passing down the furnace is preheated in zone I, strongly heated in zone II and in regions IV1 and IV2, and finally cooled in zone Ill.

In moving upwardsthroughthe lower zone Ill the moulded carbon bodies are preheated by the descending granular material 3 which surrounds them, up to temperatures of 1 200"C to 1 600"C before enter ing the lower current supply region IV2. In passing downwards through the furnace the granular material is heated sufficiently to change it largeiy into graphite.

The granular material 3 is fed to the furnace in the cold state. In the upper zone lit is preheated to a high temperature by the rising column of baked carbon bodies 2 before reaching the upper current supply region IV1,and in passing downwards through zone Ill the granular material is cooled by the rising carbon bodies 1. Both the carbon bodies 1,2 and the granularmaterial 3therefore leave the furnace in acom paratively cool state.

In orderto utilise the electric powerto the best ef- fectthe mass-rate-of-flow of the descending granular material should, in principle, be approximately equal to the mass-rate-of-flow of the rising carbon bodies. But due to the unavoidabie heat losses it has been found that the mass-rate-of-flow ofthe descending granular material should, in practice, be somewhat greater than the mass-rate-of-flow of the rising carbon bodies.

In moving upwardsthroughthegraphitizing furnace the carbon bodies 1,2 are heated very smoothly. This is due not only to the gradual heattransfer from the hot descending granular material, but also duets the heattransferreddownwards through the column of carbon bodies from the hottest zone lltothe preheating zone Ill. By using a long preheating zone Ill, and by suitably adjusting the speed at which the carbon bodies travel upwards through the graphitizing furnace, it can be ensured that the temperature of each carbon body 1,2 rises at a rate of no more than 1 500 C/h until graphitizing be gins at about 2000 C.

In the lowercurrentsupply region IV2thegranular material 3, which is largely graphite at this location, forms the electric bridge for the flow of current between the electrodes 6,7 and between them and the column of carbon bodies 1,2. The constantdown- ward flow of granular material 3 ensures a very con stantflow of well distributed electric current in the region IV2. The same applies to the region IV1. If the electrodes 6,7 are equally spaced from the clolumn of carbon bodies 1,2 the voltage drops are the same.

Up to 50% of the heat can be generated in this gap between the electrodes and the carbon bodies.

In the intermediate zone lIthe column of carbon bodies 1,2 is heated to the high temperature of 2500to 3000 C required to complete conversion into graphite. In this zone the electric currentflows mainly through the carbon bodies 1, 2. Current den sities there are determined bythe quality ofthe carbon body matrial and by the cross-sectional area of the carbon body. Current densities are usually around 30 to 90 A/cm2. The residence time ofthe carbon bodies 1,2 in the graphitizing zone II depends on the length ofthatzone and on the speed of upwards movement of the column of carbon bodies 1, 2.If the carbon bodies 1,2 are easily converted into graphite, residence time in the intermediate zone II can be less than one hour, giving a high manufacturing outputandcomparativelylittleenergyconsumption per unit of product.

The top of the graphitizing furnace, atthe upper end of zone I, is equipped with automatic devices (not shown) for removal of the graphitized carbon bodies 1,2 and for feeding the granular carbon material 3, and has a hood, for example made of steel,for removing the waste gases. The graphitizing furnace operates with minimal pollution of the site and environmental atmosphere.

Referring now to Figure 3, in this version ofthe invention the current supply electrodes 12, 13 penetrate inwards through the conically expanded ends of zones I and Ill, the electrodes 12, 13 being inclined to the path of movement of the column of carbon bodies 1,2 Each electrode 12, can, for example, be circular in cross section and can consist of a water-cooled metal shaft 15 projecting outwards from the furnace and connected to a current lead, and a graphite tip 14projecting inwards into the furnace. Apart from that the furnace of Figure 3 is similarto that of Figures 1 and 2.

The greater furnace cross section in the intermediate zone II, as shown in Figures 1 and 3, isto protect the furnace walls from the high temperatures prevailing here. Nevertheless under certain circumstances the fu rnace can, if desi red, be constructed with the same cross section all the way along. Furthermore, the relative lengths of the three zones need not be as shown in Figures 1 and 3. The relative lengths of the zones are chosen in dependence on the overall height of the furnace, the speed of travel of the carbon bodies, the nature of the carbon body material and on other considerations. Nevertheless zones I and Ill usually have approximately the same lengths.

The gaps between the electrodes and the column of carbon bodies, which determine electric resistances and therefore rate of heat generation, are chosen to suit the requirements in each particular case.

Claims (16)

1. A process of continuously graphitizing carbon bodies by electric resistance heating, in which the carbon bodies move in columnarformation vertically through a graphitizing furnace surrounded by granular carbon material and are heated by an electric current flowing over electrodes through the granular material and through the carbon bodies, characterised in that the column of carbon bodies moves upwards through the furnace while the granularcarbon material surrounding the column of carbon bodies moves downwards in counterflow.

2. A process according to claim 1, wherein the column of carbon bodies moves upwards from the lower end ofthefurnacefirstlythrough a lower heat exchange zone, then through an intermediate graphitizing zone in which the carbon bodies are heated by electric resistance and finally through an upper heatexchange zone, the granular carbon material moving downwards in counterflowthrough these three zones but in the reverse order.

3. A process according to claim 2, wherein electric current is supplied to orconducted from the graphitizing zone by electrodes located in current supply regions at upper and lower borders of that zone.

4. A process according to any of claims 1 to 3, wherein before being fed to the graphitizing furnace each carbon body is heat-treated to a maximum temperature of between 550 and 650 C.

5. A process according to any of claims 1 to 4, wherein each heat-treated carbon body is impregnated with pitch butthen notfurtherheat-treated before being fed to the g raphitizing furnace.

6. Apparatusforcontinuouslygraphitizing carbon bodies by electric resistance heating comprising a graphitizingfurnace having a vertically extending shaft and means for moving a column of carbon bodies along a path through said shaft while surrounded by granular carbon material, thefurnace being provided with electrodesforthesupplyofelec- tric current to flowthrough said granular carbon material and through the carbon bodies to heat them, characterised in that said means for moving the carbon bodies is arranged to move them in the upward direction, and inthatthereisfurtherprov- ided means for continuously supplying said granular carbon material to the top of said shaft and means for continuously conducting said granular carbon material away from the bottom of said shaft.

7. Apparatus according to claim 6, wherein the graphitizing furnace shaft has an intermediate zone of greater diameter than upper and lower zones.

8. Apparatus according to claim 7, wherein the lower end ofthe upper zone and the upper end ofthe lower zone ofthefurnace are expanded conically where theyjoin the intermediate zone.

9. Apparatus according to claim 7 or8,wherein the upper zone and the lower zone of the furnace are constructed to function as heat-exchange zones.

10. Apparatus according to claim 9, wherein the upperzone and the lowerzone have approximately equal lengths.

11. Apparatus according to any of claims 6 to 10, wherein electrodes project into said shaft at current supply regions at upper and lower ends of an inter- mediate graphitizing zone of the shaft.

12. Apparatus according to claim 11 ,wherein el- ectrodes of each said current supply region form opposite pairs penetrating inwards from either side into the shaft but leaving clearance to the path of the carbon bodies.

13. Apparatus according to claim 11 or 12, wherein the electrodes penetrate approximately horizontally into the shaft.

14. Apparatus according to claim 11 or 12, wherein the electrode(s) of the upper current supply region is or are inclined downwardly, and the electrode(s) of the lower current supply region is or are inclined upwardly.

15. Apparatus according to any of claims 11 to 1 4,wherein the electrodes are circular or rectangular in cross section.

16. Apparatus according to any of claims 6to 15, wherein the furnace has a refractory lining extending over its entire length, an outer steel jacket around the upper zone, and a double-walled cooling jacket around the intermediate and lowerzones.