WO1991015727A1

WO1991015727A1 - Method and apparatus for controlling the raw material supply to an electrical melting furnace

Info

Publication number: WO1991015727A1
Application number: PCT/FI1991/000106
Authority: WO
Inventors: Peter Arnold Henrik Solin; Pertti Kalevi Hjerppe; Carl-Gustav NYGÅRDAS
Original assignee: Paroc Oy AB
Current assignee: Paroc Hold Oy AB
Priority date: 1990-04-11
Filing date: 1991-04-10
Publication date: 1991-10-17
Anticipated expiration: 1992-10-11
Also published as: FI85912B; AU7566591A; FI901843A0; FI901843L; FI85912C

Abstract

Method and apparatus for controlling the raw material supply to a continuously operated electrical melting furnace provided with electrodes (4) extending from above into the melt, a plurality of supply means (5) for the raw material as well as means (8) for sensing the temperature in a plurality of areas above the melt surface in the furnace. The raw material supply is controlled on the basis of the temperature measurements so as to keep the layer of unmelted raw material on the melt substantially even.

Description

Method and apparatus for controlling the raw material supply to an electrical melting furnace

The present invention relates to a method for controlling the raw material supply to an electrical melting furnace over a plurality of supply means for the raw material. The invention relates also to an apparatus for controlling the said raw material supply to an electrical melting furnace provided with electrodes extending into the melt from above, a plurality of supply means for the raw material, as well as means for sensing the temperature in the furnace.

The space above the melt surface in an electrical furnace of the said kind should be maintained as free as possible from oxygen, as the electrodes, which preferably are of graphite, ignite very easily due to the high temperature in the furnace, when oxygen is present. It can also be advantageous, depending on the type of lining in the furnace, that the oxygen content in the furnace is low. It is thus of advantage to use, in a known manner, a slight overpressure in the furnace above the melt by means of a suitable inert gas.

A melting furnace for continuous operation naturally requires that the raw material supply takes place substan¬ tially continuously so that the quantity of melt withdrawn corresponds to the quantity of raw material introduced. The raw material introduced forms a layer of solid material on the surface of the melt, wherefrom it gradually melts. The solid material on the surface forms a heat insulating layer, which promotes the protection of especially the inner roof and the supply means of the furnace from excess heat stresses. It is therefore desirable that the introdu¬ ced raw material is distributed relatively evenly over the whole melt surface, and does not form local heaps of raw material thereon. In order to achieve this objective it is known to introduce the raw material over a plurality of supply tubes arranged between the electrodes (e.g. FI 85 3458).

The melting process does not, however, take place in the furnace evenly over the whole surface, but the material has a tendency to melt faster in the areas around the electro¬ des. This in turn leads to the fact that the heat ra¬ diation around the electrodes is greater than from other surface areas, which in turn leads on the one hand to uneconomical heat losses and on the other hand to excess stress, especially on the roof of the furnace and the lower parts of the supply tubes. In addition, these free local melt surfaces result in an uneven temperature in the melt and consequently in temperature variations in the melt which is withdrawn through the melt opening.

There thus exists a need for a method and an apparatus by means of which it is possible, without viewing into the furnace, to achieve an even distribution of raw material on the surface of the melt, and in this manner reduce or even eliminate the above mentioned disadvantages.

This is obtainable with the method according to the invention which allows for a controlled charging of raw material to the part of the melt surface which exhibits the greatest heat emission. According to the invention it is not only possible to direct the raw material to the parts of the melt surface from which the heat emission is the greatest, but it is also possible to regulate the quantity or the proportion of the raw material which is charged to these parts. The method according to the invention is characterized in that the temperature is sensed at a plurality of areas above the melt surface in the furnace, whereby at every sensing area a deviation of the temperatu¬ re from a set value or from a temperature value sensed at another sensing area, is used to regulate the quantity of raw material which is fed to the furnace over that or those supply means which supply the said sensing area.

Thus the method according to the invention makes it possible to divide or proportion the quantity of raw material over the different supply means in relation to the temperatures sensed by the sensing means. Hereby the temperature deviation for the different sensing areas is determined either in relation to a "normal value" esta¬ blished for the process conditions in question, or then the temperature deviations between the different sensing areas are established. Thereby the quantities or proportions of raw material supplied by the different supply means are regulated in relation to the values obtained. Thus prefera¬ bly the greatest raw material quantity is fed through that supply tube which is situated in the area where the highest temperature is measured, etc.

For the control of the raw material supply it is appro¬ priate to use a control device, in which the temperature differences are established and which produces signals in response to the differences. These are in turn used for regulating the raw material supply by actuating in a suitable manner e.g. conventional valve means in the supply devices, raw material conveyors etc. It is thereby possible and easier from a process technical viewpoint to introduce raw material to the furnace over one supply means at a time, whereby the total amount of raw material to be introduced per time unit is divided over the separate supply means in accordance with the temperature differences measured for these by the sensing means.

Alternatively it is possible that the raw material supply to the furnace takes place simultaneously over a plurality of supply means in proportions which correspond to the temperature differences measured for the said supply means by the sensing means. The sensing means can be of any known type provided that they withstand the high furnace temperatures. The use of sensing means in melting furnaces is in itself known (e.g. NO 150 460).

When carrying out the process, the quantity of raw material introduced per time unit should correspond to the quantity of melt withdrawn. This can be monitored in different ways, e.g. by continuously or intermittently measuring the weight of the furnace. According to the invention the quantity of raw material which is to be introduced into the furnace is divided between the separate supply means in a manner which results in optimal conditions in the furnace with regard to heat losses from the surface and heat stresses on linings and cover.

The invention concerns also an apparatus for carrying out the above defined method. The apparatus is characterized in that each supply means over a control unit is connected with at least one temperature sensing means for an asso¬ ciated area and arranged above the melt, that the control unit comprises means for comparing a temperature sensed by one or more means with a temperature value set for the process conditions in question, or with a temperature sensed by one or more other sensing means, that the apparatus comprises means for regulating the raw material supply to the supply means based on the temperature differences established by the control device.

In the furnace the electrodes are preferably arranged symmetrically substantially along the periphery of a circle but also other possibilities exist. It is of importance that the electrodes are so arranged that as even as possible heating of the melt is obtained. The supply means, which preferably are supply tubes, should be arranged so as to result in as good a coverage of the melt surface as possible. This is suitably achieved so that rotating supply tubes provided with suitable spreading means are placed between the electrodes substantially along the said circle periphery and in the center of the furnace, which thus can cover large areas of the melt with raw material. Further it is of advantage to place directional supply tubes between the electrodes and the furnace wall in order to direct the raw material towards the electrodes where the melting of the raw material is most intensive. The raw material should be supplied to the furnace so as not to mechanically wear on the furnace surfaces or the electrodes.

Thereby the end of the tube can be designed in a suitable manner, e.g. it can be bevelled or bent and/or provided with a shield against radiation from the melting raw material and made of a suitable heat resistant material, e.g. a heat resistant ceramic material (e.g. a Al₂0₃-based material). It is also possible, by a suitable design of the tube end, e.g. by providing a tube end bent to an arc/angle with suitably designed extensions or "fingers", to obtain a relatively even distribution of raw material over the whole area of influence of the supply tube and not only along a circular line on the surface of the melt.

The invention is described in the following in the form of an example of an embodiment referring to the appended drawing, wherein:

Fig. 1 shows a cross-section through a melting furnace included in the apparatus according to the invention and

Fig. 2 shows the rotating supply means in more detail.

The melting furnace as a whole is designated with the reference numeral 1, and it contains a mineral melt 2 with a layer 3 of unmelted raw material. In the Fig. 1 only two electrodes 4 and three supply tubes 5 are shown, which all extend into the furnace through the furnace cover. The number of electrodes 4 and supply tubes 5 can naturally be arbitrary. The end 6 of the supply tube 5 is bevelled in order to obtain a spreading of the raw material when the tube 5 rotates around its longitudinal axis. The tubes 5 further comprise means 7, e.g. suitable valve means, for the raw material which are activated by a control signal produced by a control device P.

According to the invention the furnace 1 is provided with temperature sensing means 8 arranged in the furnace cover, preferably heat resistant thermocouples, which extend to some degree into the furnace space. In the embodiment shown in the Fig. 1, every supply tube has a corresponding sensing means. The sensing means produce a signal corres¬ ponding to the temperature at the sensing point to the control device P, which can be e.g. a processor. The control device P compares the temperatures sensed by the different sensing means 8 with each other or with a temperature value set for the process conditions in question, and provides signals corresponding to the established differences for regulating the raw material supply to the associated supply tube.

The temperature sensing means 8 can to their number be greater or smaller than the supply tubes 5, whereby a direct association between a specific supply tube and a temperature sensing means need not exist.

The regulation of the raw material introduction suitably takes place over valve means 7 not shown in detail. These can be activated to feed any quantity of raw material of the total amount to be supplied over a certain time period. According to one embodiment, one tube at a time is activa¬ ted in an order and in a quantity corresponding to the greatest temperature differences, but it is also possible to activate several tubes simultaneously and to supply through these quantities of raw material corresponding to the sensed temperature differences. When a raw material charge intended for a specific time unit has been used and distributed in the above mentioned manner, the procedure is repeated.

When the raw material is supplied continuously the regula¬ tion takes place by increasing the raw material supply in that or those tubes which supply raw material to that or those areas where there is a temperature increase and by decreasing the same correspondingly in the other tubes.

The rotating supply tubes 5 shown in the Fig. 2 comprise finger-like spreading means 9 which are advantageous in view of obtaining an even spreading of the raw material over the whole area of influence of the supply tube. It is also possible to provide the lower ends of the supply means with shielding elements 10 of a heat resistant material, preferably a ceramic material.

In the said manner an optimal distribution of the raw material with respect to heating economics and heat resistancy in the furnace is obtained contrary to prior known systems.

Claims

1. Method for controlling the raw material supply to an electrical melting furnace having a plurality of supply means for the raw material, characterized in that the temperature is sensed at a plurality of areas above the melt surface in the furnace, whereby at every sensing area a deviation of the temperature from a set value or from a temperature value sensed at a another sensing area is used to regulate the proportion of the raw material which is fed to the furnace over that or those supply means which supply the said sensing area.

2. Method according to the Claim 1, characterized in that the magnitude of the said deviation is determined in a control device, which produces a signal corresponding to the difference for regulating the raw material supply to that or those supply means which supply the area in question.

3. Method according to the Claim 1 or 2, characterized in that the raw material is supplied to the furnace over one supply means at a time, whereby the total amount of raw material to be introduced per time unit is divided in accordance with the temperature differences established by the associated sensing means for the separate supply means.

4. Method according to the Claim 1 or 2, characterized in that the raw material is supplied to the furnace con¬ tinuously over a plurality of supply means in accordance to the temperature differences established for these by the associated sensing means.

5. Apparatus for controlling the supply of raw material to an electrical melting furnace comprising electrodes (4) extending from above into the melt, a plurality of supply means (5) for the raw material, and means (8) for sensing the temperature in the furnace, characterized in that each supply means (5) over a control unit (P) is connected with at least one temperature sensing means (8) for an asso¬ ciated area and arranged above the melt, that the control unit (P) comprises means for comparing a temperature sensed by one or more means (8) with a temperature value set for the process conditions in question or with a temperature sensed by one or more other sensing means (8), that the apparatus comprises means (7) for regulating the raw material supply to the supply means (5) based on the temperature difference established by the control device (P).

6. Apparatus according to the Claim 5, characterized in that the electrodes (4) which preferably are three to their number, are arranged symmetrically in the furnace substan¬ tially along a circular periphery, whereby the supply means (5) preferably are arranged on the one hand between the electrodes (4) substantially along the said circular periphery, in the center of the furnace and between each electrode (4) and the side wall situated closest to respective electrode.

7. Apparatus according to the Claim 6, characerized in that supply means (5) arranged between the electrodes (4) and in the center of the furnace are rotatable supply tubes.

8. Apparatus according to the Claim 7, characterized in that the ends of the rotatable supply tubes (5) comprise fingerlike spreading means (9) for spreading of the raw material.

9. Apparatus according to any one of the preceeding Claims, characterized in that the lower ends of the supply means (5) comprise shielding elements (10) of a heat resistant material, such as a heat resistant ceramic material.