Field of Invention The present invention relates to an electric calcing furnace for calcining of particulate materials, particularly carbon materials.

Background Art Electric calcining of carbon-containing materials such as anthracite, metallurgical coke and petrol coke is carried out by conducting electric current through the material to be calcined. The carbon-containing materials are thereby heated and during heating, water and other volatile matters in the materials will be driven off as the temperature is increasing. Depending on the temperature to which the material is heated, a part of the material will be transferred into graphite.

Electric calcining furnaces for calcining of the above mentioned carbon materials generally comprise a vertical elongated cylindershaped furnace equipped with a bottom electrode and a top electrode. The material to be calcined is supplied at the top of the furnace and calcined material is withdrawn at the bottom of the furnace. An example of such furnace is disclosed in WO 98/46954. In electric calcining furnaces of this kind the temperature between the electrodes in the center of the furnace becomes very high, above 2500°C, while the temperature at the periphery of the furnace is substantially lower, between 800 and 1200 °C. This uneven temperature distribution over the cross-section of the furnace results in that the calcined material that is withdrawn from the furnace has been subjected to an uneven temperature during the calcining process. This gives a product having inhomogeneous properties which is a disadvantage during subsequent use of the calcined material.

According to WO 98/46954 this problem is solved by withdrawing two separate fractions from the furnace; one fraction from the center of the

furnace and one fraction from the periphery of the furnace. The fraction that is withdrawn from the center of the furnace has been heated to a high temperature and is very well calcined, while the fraction withdrawn from the periphery has been heated to a substantially lower temperature and thus still contains volatile matters. This fraction thus has a substantial lower quality and thus a lower value then the fraction withdrawn from the center of the furnace.

When a high degree of graphite is aimed at by calcining of anthracite, the size of the center fraction must be reduced in order to ensure that the outer part of the center fraction has been heated to a sufficiently high temperature for a sufficiently long time in order that the aimed degree and graphitization is obtained. By production of calcined anthracite having a high degree of graphitization, the productivity of the calcining furnace according to WO 98/4695 is low.

There is thus a need for an electric calcining furnace which makes it possible to heat all material supplied to the furnace to an approximately even temperature whereby a homogeneous calcined material can be obtained.

Description of the Invention By the present invention it is now provided an electric calcining furnace where the supply of energy is substantially constant across the cross-section of the calcining furnace. All material which passes through the calcining furnace is thus heated to a substantial same temperature.

The present invention thus relates to an electric calcining furnace for calcining of solid particulate materials, particularly carbon materials, which furnace comprises a substantially cylinder-shaped, vertical furnace having means for supply of material to be calcined to the top of the furnace and means for withdrawing calcined material from the bottom of the furnace, the furnace being characterized in that the furnace has a horizontally arranged heating zone with a plurality of substantially horizontal electrodes arranged about the periphery of the cylinder-shaped furnace, which electrodes are connected to an electric power source and a control unit for control of the power supply

where the control unit is such arranged that electric current alternates between to two and two electrodes while the other electrodes at the same time carries no electric current.

The control unit for power supply is preferably such that electric current is supplied to two and two electrodes which are defined as pairs and arranged about the circumference of the heating zone.

According to another embodiment the control unit for power supply to the electrodes is a thyristortype regulator. Alternatively the control unit can comprise switches which according to a predefined program only supplies electric current to one pair of electrodes at the same time.

The number of electrodes is at least four and it is preferred to use eight electrodes.

As electric current is only supplied between two and two electrodes at the same time and the supply of current by means of the control unit more or less continuously is shifted from one pair of electrodes to another pair of electrodes, the current in the furnace will be substantially evenly distributed t over the furnace cross-section and the materials which are to be calcined will thus be heated to the substantially same temperature over the cross-section of the furnace.

The electrodes are preferably graphite electrodes, where each electrode is arranged in a gas-light electrode sealing in the wall of the calcining furnace, and where means are arranged for moving the electrode inwardly into the furnace in order to compensate for electrode consumption. In order to reduce the electrode consumption it is preferred to use hollow electrodes where a heat pipe is inserted in the hollow electrode for cooling of the electrode. A heat pipe is a closed pipe containing a liquid, evaporizable cooling medium and where a wick may be arranged for transport of condensed cooling medium back to the hot end of the pipe.

According to a preferred embodiment the heating zone of the calcining furnace has a greater diameter than the diameter of the furnace above and below the heating zone, where the section between the heating zone and the furnace wall above the heating zone is slightly inclined, outwardly and downwardly, while the lower end of the heating zone is strongly inclined inwardly in such a way that the lower end of the heating zone forms a ring- shaped area. The electrode tips will thereby be protected from damage and abrasion from the materials flowing downwards in the calcining furnace as the rate of the downward flow of the materials in the heating zone will be low due to the ring-shaped area below the electrodes.

The materials to be calcined will be heated to very high temperatures of above 2000°C also in the peripheral part of the heating zone. The furnace wall in the heating zone is therefore preferably made from liquid cooled or evaporation cooled panels. In order to further increase the cooling in the heating zone, vertical extending radially, sheets can be arranged between the electrodes, which sheets have an inner reservoir intended to contain a liquid cooling medium which has a boiling point at or near the operating temperature of the sheets. Each of the sheets is equipped with a pipe where the inside of the pipe is connected to the reservoir in the sheet for condensing of vapor from the cooling medium, which pipes extend from the top of each sheet to above the top of the calcining furnace. The pipes preferably have outer radial, vertically extending ribs in order to increase the outer surface area of the pipes. In the area above the top of the calcining furnace, each of the pipes is equipped with a liquid cooled condenser.

By operation of the calcining furnace a part of the cooling medium in the reservoirs of the sheets will evaporate when the temperature in the cooling medium reaches the boiling point. The vapour will flow upwardly into the pipes and a part of the vapour will be condensed due to contact between the pipes and the cold material which is supplied to the top of the calcining furnace. The heat of condensation will thereby be transferred to the material to be caleined

and preheat the material. The condensed cooling medium will flow downwardly into the reservoirs in the sheets. The heat of condensation is thus utilized for preheating of the material to be calcined. If all vapour is not condensed inside the pipes, any remaining vapour will be condensed in the condensers above the top of the calcining furnace. The evaporation cooling ensures that the temperature in the sheets is kept close to the boiling point of the cooling medium.

The calcined material is cooled in the lower part of the calcining furnace and is withdrawn from the bottom of the furnace by means of a conventional discharging means such as a rotatable scraper which is rotated horizontally on the bottom of the calcining furnace.

By the calcining furnace according to the present invention it is achieved an even heating of the material to be calcined over the cross-section of the furnace. It is thereby obtained a calcined material with a homogeneous quality as the complete column at material flowing through the furnace is heated to approximately the same temperature. By regulating the ratio between supplied energy and the rate of discharging of calcined material from the furnace, it can be obtained a preset degree of calcination for all material flowing through the furnace. By calcining of carbon-containing materials such as anthracite, the degree of graphitization can be controlled.

Short Description of the Drawings Figure 1 is a vertical cut through a calcining furnace according to the invention, Figure 2 is a cut taken along line I : I in Figure 1, and Figure 3 is an enlarged vertical cut through one of the electrodes in the calcining furnace.

Detailed Description of the Invention On Figures 1 and 2 it is shown a calcining furnace 1. The calcining furnace is cylinder-shaped and comprises an upper supply and preheating part 2, a heating zone 3 and a cooling and discharging part 4. The calcining furnace 1 is affixed to a building by means of beams 5. Raw material, such as uncalcined anthracite, is supplied to the top of the supply and preheating part 2 by means of a dobble-bell arrangement 6, or by means of another conventional supply means, for instance such as shown in W098/46954.

Calcined material is discharged at the bottom of the calcining furnace 1 by means of a discharging means 7. Off-gases from the calcining are withdraw through a gas-outlet opening 8.

The calcining furnace has an outer shell 9 made from steel. In the supply-and preheating part 2, a layer of heat insulating material 10 can be arranged on the outside of the steel shell 9 in order to prevent heat losses from this part of the calcining furnace. In the cooling-and discharging part 4, a refractory lining 11 in arranged on the inside of the steel shell 9. If necessary the steel shell 9 can be equipped with means (not shown) for cooling, such as air-or watercooling in the cooling-and discharging part 4.

According to the invention the calcining furnace 1 is quipped with a plurality of horizontal electrodes 12 about the circumference of the heating zone 3.

In the area of the heating zone 3, the calcining furnace 1 has a greater diameter than the supply-and preheating zone 2. In the area between the supply-and preheating part 2 and the heating zone 3, the inner surface of the calcining furnace slopes slightly outwardly as shown by reference numeral 13, while the lower end of heating zone 3 is strongly inclined inwardly and forms a ring-shaped area 14. By this design of the heating zone 3, the electrodes 12 are shielded from the materials flowing downwardly in the calcining furnace as the flow rate of the materials in the peripheral part of the heating zone 3 is strongly reduced due the ring-shaped area 14.

The electrodes 12 are arranged at equal circumferential distance about the heating zone 3 as shown in Figure 2. In the embodiment shown in the figures it is shown eight electrodes 12, but it is within the scope of the present invention to use four, six and more than eight electrodes.

The electrodes 12 are made from graphite and are inserted into the wall of the heating zone 3 in a gas-sealed way as shown in Figure 3. In figure 3 one of the electrodes are shown in an enlarged view. The electrode 12 is inserted through the layer of thermal insulation 10 and through the steel sheet 9 of the calcining furnace. An electrode seal 15 is arranged in order to obtain a gas- sealed insertion of the electrodes 12. The electrode seal 15 shown in Figure 3 comprises a ring-shaped cylinder 16 and a wedge-shaped cylinder 17 which forces the cylinder 16 against the outer surface of the electrode 12. The wedge-shaped cylinder 17 has locking pins or bolts 18. Other conventional gas-tight electrode seals can also be used.

A heat pipe 19 is inserted in the hollow electrode 12. The heat pipe 19 is partly filled with a liquid, evaporizable cooling medium, and has a condenser 20 arranged outside the electrode 12 in order to condense vapour from the cooling medium. The condenser has a cooling loop 21 (see Figure 2) for circulation of a liquid cooling medium for condensing of vapour from the heat pipe 19. If necessary, a wick can be arranged inside the heat pipe 19 for transport of condensed cooling medium from the condenser 20 to the hot end of the heat pipe 19.

As shown in Figure 1, electric current is conducted to the electrode via terminals 22 from an electric power source (not shown). The supply of current to the electrodes is controlled by means of a control unit 23. The control unit 23 operate in such a way that at the same time electric current is only supplied between two of the electrodes, and hence the remaining electrodes do not carry any electric current during this interval. The supply of electric current is more or less continuously shifted from one pair of electrodes to another pair of electrodes by means of the control unit 23. In this way it is

ensured that the current paths cover most of the cross-section of the calcining furnace in the heating zone 3. It is thereby ensured that the materials that are calcined in the furnace are heated to the approximately same temperature over the cross-section of the heating zone 3. The material calcined in the furnace will thereby have a homogeneous quality. Calcining of anthracite to a predefined degree of graphitization can thereby be achieved for the calcined material by regulating the ratio between supply of energy and rate of discharging of calcined material from the calcining furnace.

In order to cool the space about the electrodes 12, vertical, extending evaporation cooled sheets 24 are radially arranged between each of the electrodes as shown in Figures 1 and 2. Each of the sheets 24 has an internal reservoir intended to contain a cooling medium that is liquid at the operating temperature in the heating zone 3 and has a boiling point at the operating temperature in the heating zone 3. Each of the sheets 24 has a condenser pipe 25 for condensing of vapour from the cooling medium. Each of the sheets 24 further has a plurality of outer radial vertical extending ribs for increasing the outer surface area of the pipes 25. The pipes 25 extend from the sheets 24 and up to a level above the top of the calcining furnace. Above the top of the calcing furnace the pipes 25 are surrounded by a coolers 27 for circulation of a cooling liquid. By operation of the calcining furnace a part of the cooling medium in the reservoir in the sheets 24 will evaporate when the temperature reaches the boiling point. The vapour will flow upwardly in the condenser pipes 25 where the vapour will be cooled and will condense due to contact between the condenser pipes 25 and the material supplied at the top of the calcining furnace 1. The heat of condensation will be transferred to the material to be calcined and thereby preheat the materials to be calcined. The condensed cooling medium will flow back into the reservoir in the sheets 24.

The heat of condensation is thus utilized to preheat the material to be calcined. Any vapor that is not condensed in the pipes 25 inside the calcining furnace, will be condensed in the area of the coolers 27 above the top of the calcining furnace.