DE4111385A1 - Filter dust smelting with reduced electrode erosion - in which electrodes are inserted into glass from above into mixt. of dust and additives - Google Patents

Abstract

To melt filter dust, in smelting furnace fitted with glass troughs, the dust is mixed with other materials, e.g. lime, quartz sand or flue ash to be melted by electrodes in the melting bath to give a glassy material when cool. The electrodes, using AC and pref. at mains frequency are inserted into the bath from above and pref. vertically, to heat the material through direct current passage. The depth of the electrode insertion is adjustable according to the bath level and deg. of electrode wear. The bath can be cooled, pref. by water, at a level below the surface. The zone over the bath is shrouded against the ambient atmosphere by a lid-type cover. Perf. three electrodes are used, inserted into the molten bath with a small penetration depth. The parameters to control the electrode penetration depth are the electrode voltages, the electrode currents or a combination of voltage and current. ADVANTAGE - The method reduces the problems of material in glass bath furnaces, reduces the strong electrode erosion and dust development, and also cuts heat loss. The additives can also be mixed at a higher speed

Description

The invention relates to a method for melting Filter dust according to claim 1, wherein the melt solidified as a glassy good.

In incineration plants, for example for waste and sewage treatment Sludge combustion creates highly toxic filter dusts strongly fluctuating chemical composition. Such fil Dust is classified in a high hazard class, so that these dusts are untreated only in special landfills (Mines) may be stored. The transportation of this Dusts must be secured accordingly and used The costs for transport and storage are considerable. One way to reduce the potential hazard is, this dust in a known manner in a body-fixed To put state in which they z. B. of water not more drained and thus ent in a normal landfill can be taken care of.

For this glazing process, it is not enough only to melt the dust that accumulates, so that after the Melt and solidify to create a glass. To have to other substances such as B. lime, quartz sand, alumina, field  late, .. are added so that the melt amount of filter dust around, for safe glazing required aggregates increased.

According to the state of the art, these mixtures are melted down Technology glass melting tanks used, which by means of electrical continuity are heated. Usually the current is supplied via electrodes, either from the Floor or from the side surfaces through the refractory reach into the weld pool.

However, since it differs when filter dust is melted from conventional glass production not with one predetermined chemical analysis of the melt ge The following disadvantages can be expected: The refractory materials of the furnace pan are made by the Melt heavily attacked and the electrodes over which the Melt the electricity is supplied, erode very strongly. The strongly fluctuating analysis and the existence almost all element combinations means for the glass tubs technology is a fundamental, hardly manageable problem. It it is not to be expected that the material of glass tub ovens problems can be solved.

The object of the present invention is now a method to develop what the material problems in the glass wall NEN oven technology solves the strong erosion of the electrodes and avoids dust formation, reduced heat loss and a higher mixing speed of the additives enables.  

This object is achieved according to the invention as in the patent Say 1 described by the electrodes from above into the Immerse the melt, as is the case with the ESU (Electro-slag remelting) process is common.

The melting tank is said to be water-cooled overall. The necessary thermal barrier between the liquid Slag and the enamel can be solidified by a Glass layer are generated. At the beginning it is advantageous line this tub with a refractory material. But after a long period of operation, this refractory layer becomes partially removed there and then results in dynamic balance between solidified melt the water-cooled wall and the glass melt. (A dynamic equilibrium means that with increasing Melt temperature the melt becomes more fluid and thus also the melt solidified on the water-cooled wall is getting thinner; when the melt temperature drops, due to the tougher melt, the solidified layer thicker). Because the melting tank in the proposed facility can become significantly smaller than a conventional, large-volume glass melt, the heat loss through the tub floor can be kept within tolerable limits. The furnace should be largely closed, so that no gases can escape uncontrolled.

To make this possible, the liquid spout Melt proposed over a siphon. Then you can this melt can be shed in ingots or in large ones  Blocks solidify, which then correspond to the application can be further processed. Depending on the melting egg The casting can be continuous or intermittent respectively. With a continuous discharge, it is not necessary nimble to tilt the oven. With intermittent casting, partial quantities by tilting or opening one Wall or floor outlet take place.

A closed lid is suggested as the upper part of the furnace similar to the heating part of a VAD or VHD system (Vacuum Arc Degassing or Vacuum Heating Degassing). At combined these used in ladle metallurgy Heating and vacuum degassing equipment is turned electric arcs heated the melt in the pan from above. The electrode tips are often up to the immersed in the molten metal floating slag layer. The arc burns in a self-generated gas bubble between electrode and molten metal. The conventional Graphite electrodes are attached in vacuum-tight telescopes arranges so that in VAD systems also heated at negative pressure can be.

In glass melting technology, it is advantageous from three-phase current starting to work with three electrodes, but two, four or six electrodes are also conceivable. For the Melting of the filter dust should not, however, as with the VAD systems are operated with arcs, but it is a pure resistance operation like the electric slag Remelting provided. Arc operation would be too a strong evaporation of melting material, which  then again a great deal of effort in removing the dust would require exercise. To avoid arcing, must smaller current densities can be worked in the electrodes.

As electrode material for the power supply in the melt are graphite electrodes like those used in arc furnace melting operation are usual, Söderberg electrodes, as in Reduk tion ovens are common, and electrodes from other elec tric conductive materials conceivable.

To the powdery or possibly pre-granulated material If possible, the electrodes should only be mixed in quickly Immerse slightly under the surface of the melt. This driving style creates high flow velocities indirectly under the electrodes. The continuous and quasi continuous addition (e.g. in small portions) of Melted material is carried out via a lock (cellular wheel lock, Bell lock or the like) and one or more Sliding on the surface of the weld pool. The free badober surface can be covered with the material to be melted will. This has the advantage that evaporating substances on the Cover layer can condense and thus the dust becomes very small. Only those at low temperatures gaseous substances must pass through filter devices and poss Licher removed a reaction purification line will.

Covering the weld pool surface offers the following advantages:
reduced heat loss,
less dust formation,
greater mixing speed.

The cover layer should be overall, but especially in the Near the electrodes, not too high, otherwise that Material can bake with the warm electrode.

About the electrode erosion and possibly something to compensate for fluctuating melt pool heights Electrodes movable through the melting chamber leads. For a continuous smelting operation it is advantageous if the electrodes are withdrawn without which need to be renewable. This is at Söderberg Electrodes are common by placing a new pipe segment piece on top welded on and then filled with coal and pitch becomes.

When using conventional graphite electrodes such as the one used e.g. B. are common in arc furnace technology, the The top of the electrode. (Nippling means the electrode by means of a connecting part, for example as a screw-in double-sided cone piece extend.) For an industrial plant with as much as possible short power interruption time, it is advantageous if that Re-sip the electrodes with a semi-automatic pre direction. For repositioning the current contact jaw relative to the electrode, the electrode can then be used with it semi-automatic post-nipple device can be held. Otherwise a crane or other stop must be used device can be used.  

The electrodes are only replaced when used one pair of contact pliers per electrode when the current is interrupted flow. When using two movable against each other Contact pliers do not need to interrupt the current the.

The seal between the electrode and the chamber he follows via a telescopic tube enveloping the electrode Sliding seal. The seal between the telescopic tube and the electrode can e.g. B. by an inflatable seal above half of the power connection. For repositioning the elec trode, the inflation seal is only opened for a short time and closed again immediately after the repositioning process was completed sen. So it can be avoided that a larger amount of Oven atmosphere can escape or, if the oven with Vacuum is operated that a larger amount is wrong air gets into the oven, which is then additionally with the Exhaust gas flow must be carried away.

The cold cargo has a very low electrical charge Conductivity. This fact requires special measures when starting a cold furnace. To start the new furnace or after a long shutdown, while the old melt has solidified, he is on top of it the melt or the tub lining stared at an electrically conductive plate (e.g. made of graphite or metallic Material), which is the contact points of the electrodes sharp connects. The actual starting process can be done in different ways:

• - Char. Is placed around the electrodes placed on the plate greedy good heaped up; the power turned on and that Molten material liquefied by means of an arc. This ver driving has the disadvantage of frequent false starts.
• - Between the electrode tips and the starter plate becomes a mixture of melting material and metal chips stratified by the electrical heat loss liquefied in the relatively high-resistance chip pile becomes.
• - A is between the electrode tip and the starter plate exothermic powder (e.g. thermite) laid what by its aluminum or magnesium content is conductive. Ignited after switching on the voltage source the exothermic powder. A first is formed Melting lake, in which the electrode filled melted material.

A power source is sufficient for this melting process regulated high current transformer. The current strength is thereby checks that the respective electrode is deep enough is immersed in the melt, such as this is known from electro-slag remelting. Each of the stream feed electrodes are separated in their immersion depth regulated. The electrode is used as the measured variable for the controller current of the phase voltage or a combination of both used with a corresponding setpoint.  

As with changes in the chemical composition of the melt changes in bath resistance will be expected surrender. To compensate for this influence, an over stored control loop are provided, the fluctuation keeps the width of the electrical measured variables constant:

• - Immerses the electrode insufficiently in the Melt, so the melt can be due to the evaporate local current density increase and Arcing occurs under the electrode. Arcing should always be avoided otherwise a lot of dust is expected got to.
• - Immersing the electrode too deep in the Melt reduces the ability to intervene and it increases the heating of the melt near the bottom.

To adapt the power supply to very different ones Melt pool resistances in the case of strongly differing chemical additives the furnace transformer with a voltage tap changer must be provided. A no-load switchover is sufficient the process requirements.

The solidification of the poured melt can, depending on the outcome Leach resistance in small pieces (e.g. plates, the broken, or granules by solidification of the pour in water) or in large format. The lump size is preferable, but that requires low-crack solidification of large blocks a small solidification speed. By choosing suitable Zu  the permissible rate of solidification be raised. The increased cost of these raw materials and the possibly increased electricity costs stand lower operating costs in the solidification process step across from.

A medium piece size can be done using a pig casting machine can be achieved. The tilted pigs are in Trans port containers collected, the slow further crack-free Allow the ingots to cool down.

Claims (9)

1.Method for melting filter dusts in melting furnaces equipped with glass tubs, the filter dusts being mixed with other substances, such as lime, quartz sand or fly ash, the filter dusts and the additives being melted in the melting tank by means of current supply electrodes and when the melt solidifies A glassy material is formed, characterized in that the electrodes , which can be operated with alternating current, preferably with mains frequency, preferably plunge vertically into the melt bath surface from above and heat the melt by direct current passage, the depth of immersion of the electrodes can be adjusted and this adjustability can be regulated according to the bath level and electrode burn-off is, the melting tank in a region below the melting bath surface, preferably can be cooled with water and that the space of the melting furnace, which is located above the surface of the melting bath, compared to the surrounding atmosphere ch a lid-like seal is separated. 2. The method according to claim 1, characterized in that preferably three electrodes are used and these electrodes compared to the depth of the molten bath only have a small immersion depth.   3. The method according to claim 1 or 2, characterized net that as a control parameter for the electrodes immersion the electrode voltages, the electrodes currents or a combination of voltages and Stream is usable. 4. The method according to one or more of claims 1 to 3, characterized in that the power supply elec Tread into the room using gas-tight bushings into the furnace above the melt surface can be introduced. 5. The method according to one or more of claims 1 to 4, characterized in that the melting material about Conveying devices can be introduced into the furnace chamber, without gas exchange between the furnace interior and the atmosphere surrounding the furnace. 6. The method according to one or more of claims 1 to 5, characterized in that the melting material is preferred in powdery or granular form from above the melt pool surface can be applied and over the ge entire area is distributable. 7. The method according to one or more of claims 1 to 6. characterized in that the melted good preferably via a bottom hole, side holes or a siphon arrangement can be brought out of the melting tank is.   8. The method according to one or more of claims 1 to 7, characterized in that the cooling of the applied melt first in containers for example in watering boxes or ingot molds he follows. 9. The method according to claim 8, characterized in that prefer the further cooling of the melted goods done in warming pits or containers.