GB2055029A - Method and apparatus for melting material - Google Patents

Abstract

A material of poor thermal conductivity such as a polymer or other thermoplastic is melted by feeding it downwardly under gravity from a hopper 10 through a tube 13 and applying a high frequency electric field to the material within the tube. The rate of flow is controlled by a valve 17 at the lower end of the tube 13. The tube is surrounded by a temperature-controlled jacket 25 through which a fluid is circulated. The electric field may be applied by a coil or electrodes 20 outside the tube or inside the tube. The outlet valve may be automatically controlled according to the temperature of the material. <IMAGE>

Description

SPECIFICATION Methods of and apparatus for melting substances forming a viscous liquid when molten This invention relates to methods of an apparatus for melting substances of poor thermal conductivity which form a viscous liquid when molten.

There are many materials which have to be melted in large quantities or reduced in viscosity for subsequent processing. Such materials are commonly in particulate form, e.g. granular solids or powders. Conventional heat transfer to the whole mass of material is difficult not only for the solid material but also when it is molten in the form of a viscous liquid. The materials are poor conductors of heat and heat transfer by convection in the body of the liquid is slow and inefficient because of the viscosity. Many such materials also are liable to suffer from thermal degradation if subjected to prolonged heating.

According to one aspect of the present invention, a method of melting a material of poor thermal conductivity which forms a viscous liquid when molten comprises the steps of feeding the material by gravity into the top of a tube, applying a high frequency electric field to the material within at least a part of the tube and controlling the flow outwardly from the bottom of the tube to draw off molten material. By controlling the output flow from the tube, it is possible to regulate the residence time of the material in the tube to give the required amount of heating. By this technique, the material can be melted or softened in a continuous flow operation. The heat is effectively applied throughout the body of material within the heating zone in the tube and thus heat transfer is no longer a limiting factor.The temperature of the melt can be controlled both by controlling the power input and by controlling the rate of flow past the valve.

The molten material may be taken into a receiving vessel, e.g. a transportable container, or may be conducted away through a duct or the like for subsequent use.

This method has particular application to material where it is required that melting should take place in a closed container. The material may be fed into the top of the tube from a hopper which may be closed. It becomes possible to carry out the process under vacuum or under pressure.

Preferably provision is made for stirring or agitating or force feeding the material in the hopper to avoid bridging.

The high frequency electric field may be applied to the material by means of plate electrodes which may be inside the tube or, if the tube is of nonconducting material, may be outside the tube.

Another way is to form the tube of non-ducting material and to provide an energising coil or coils externally of the tube, e.g. around the tube. In many cases however it is convenient to form the tube of metal and to use this as one electrode, the other electrode being constituted by a concentric cylindrical member within the tube. This cylindrical member may be conveniently a metal rod or tube. In such an arrangement, the outer tube forming the containment may conveniently by earthed and the inner electrode energised with a high frequency alternating potential.

For such a melting process, it will in general be preferable to employ a radio frequency field, for example within the range of 1 to 100 MHz but in some cases it may be preferred to use higher frequencies, for example microwave frequencies.

The above-described technique may be used for melting a wide range of materials. As one example it may be employed for the melting of polymers and other thermoplastic materials such as polyamides, polyvinyl-chloride and acrylics prior to mechanical working such as spinning into filament or extrusion. It may be used for the softening or melting of materials such as gels or sugar products.

According to another aspect of the present invention, an apparatus for the melting of a material of poor thermal conductivity which when molten forms a viscous liquid comprises an upright tube, means for feeding the material into the top of the tube, a flow control valve for controlling the flow of molten material outwardly from the bottom of the tube and means for applying a high frequency field across the material in at least part of the tube.

The means for applying a high frequency field may comprise electrodes outside the tube, in which case the tube must be formed of nonconducting material. In another arrangement, the electrodes are arranged inside the tube. In a particularly convenient arrangement, a metal tube is provided and arranged to constitute one electrode co-operating with a concentric inner electrode.

In yet another construction, the tube is formed of non-conducting material and the high frequency field is applied by means of a coil or coils around the tube or located externally of the tube.

For some materials, it may be desirable to apply temperature control of the surface to the tube to maintain the tube at a predetermined minimum temperature. This may arise for example where the material contains a vaporisable liquid such as water which might, on heating, vaporise, and then condense on the incoming cold material or where edge effects may cause localised overheating of the material. In such a case, a temperaturecontrolled jacket may be provided around the melting tube.

The control valve is conveniently a mushroom or a plunger valve for closing or throttling the lower end of the tube.

The feed means may comprise a hopper which may be closed so that the material may if necessary be maintained under pressure or under vacuum.

Provision may be made for automatic control of the outlet valve, for example in accordance with the temperature of the material in the neighbourhood of the valve. Provision may also be made for automatic control of the power applied to the heating means.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawing which illustrates diagrammatically one form of apparatus for the melting of a solid material such as poiyamide which, when molten forms a viscous liquid.

Referring to the drawing there is shown a hopper 10 containing material to be melted. In this particular apparatus, the hopper is closed by a cap 11 and the material is maintained under vacuum by means of a vacuum inlet pipe 12 to prevent the inclusion of air bubbles in the melt.

The material from the hopper is fed by gravity into the top end of an upright melting tube 13 formed of dielectric material. To avoid bridging, the material in the top of the tube 1 3 and in the hopper immediately above the top of the tube is stirred by a paddle 14 driven by a rotary shaft 1 5 passing through a seal 1 6 in the cap ii.

The flow out from the lower end of the melting tube 1 3 is controlled by means of a plunger valve 1 7 having an operating drive means 1 8. This valve controls the flow from the tube 13 into a collection chamber 19.

A radio frequency field is applied to the material in the tube 13 by means of electrodes 20, 21 each partially embracing the tube, the electrodes being connected to a radio frequency power supply source 22 so that a radio frequency field is established across the material within the tube.

This radio frequency field effects heating of the material in the tube. The materials would in general be predominantly non-conducting and would be heated by dielectric heating. As the material melts, it flows downwardly. The valve 1 7 controls the rate of flow out of the tube and hence enables the temperature of the outflowing material to be controlled. Control means 23 are provided for controlling the operation of the valve 17. These may be automatic control means responsive for example to the temperature of the oufflowing material. This outflowing material passes into the collection chamber 1 9 and can be drawn off via a valve (not shown) into a closed transportable receiver (not shown). A jacket 25 is provided around the melting tube 13 and a temperature-controlled fluid, e.g. air is circulated through this jacket, as indicated at 26, via a heat exchanger 27 to maintain the melting tube 1 3 at a suitable temperature to prevent localised overheating of the material in contact with it. This feature of a temperature-controlled heating jacket is commonly desirable in melting materials which have a tendency to degrade with excess heat or which may have a very critical temperature/viscosity relationship.

Control means 28 are provided for regulating the radio frequency power input to the material.

These control means may also be automatic control means responsive to the temperature of the material.

Claims (14)

1. A method of melting a material of poor thermal conductivity which forms a viscous liquid when molten comprising the steps of feeding the material by gravity into the top of a tube, applying a high frequency electric field to the material within at least a part of the tube and controlling the flow outwardly from the bottom of the tube to draw off molten material.

2. A method as claimed in claim 1 wherein the high frequency electric power input is controlled to control the temperature of the melt.

3. A method as claimed in either claim 1 or claim 2 wherein the rate of flow past the valve is controlled to control the temperature of the melt.

4. A method as claimed in any of the preceding claims wherein the material is fed into the top of the tube from a closed hopper.

5. A method as claimed in any of the preceding claims wherein the material is stirred or agitated or force fed in the hopper to avoid bridging.

6. An apparatus for the melting of a material of poor thermal conductivity which when molten forms a viscous liquid comprising an upright tube, means for feeding the material into the top of the tube, a flow control valve for controlling the flow of molten material outwardly from the bottom of the tube and means for applying a high frequency field across the material in at least part of the tube.

7. Apparatus as claimed in claim 6 wherein said tube is formed of non-conducting material and wherein the means for applying the high frequency field comprises a coil or coils around the tube or located externally of the tube.

8. Apparatus as claimed in claim 6 wherein said tube is formed of non-conducting material and wherein the means for applying a high frequency field comprise electrodes outside the tube.

9. Apparatus as claimed in claim 6 wherein the means for applying the high frequency field comprise electrodes arranged inside the tube.

10. Apparatus as claimed in claim 6 wherein said tube is one electrode of the means of applying the high frequency field, and wherein there is provided a concentric inner electrode.

11. Apparatus as claimed in any of claims 6 to 10 wherein a temperature-controlled jacket is provided around the tube.

12. Apparatus as claimed in any of the claims 6 to 11 wherein the control valve is a mushroom valve or plunger valve for closing the lower end of the tube.

13. Apparatus as claimed in any of claims 6 to 1 2 wherein the feed means comprise a closed hopper.

14. Apparatus as claimed in any of the claims 6 to 13 wherein means are provided for automatic control of the outlet valve in accordance with the temperature of the material.

1 5. Apparatus as claimed in any of claims 6 to 14 wherein means are provided for automatic control of the power applied to the heating means.

1 6. A method of melting a material substantially as hereinbefore described.

1 7. Apparatus for melting a material substantially as hereinbefore described with reference to the accompanying drawing.