GB2085134A

GB2085134A - High temperature electric crucible furnace

Info

Publication number: GB2085134A
Application number: GB8126701A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-09-08
Filing date: 1981-09-03
Publication date: 1982-04-21
Also published as: FI812613L; SU1243639A3; FR2489948B1; IT8112619A0; IT8115204V0; DE3033738C2; FI66486C; AT380564B; GB2085134B; IT1145931B; CH653433A5; DE3033738A1; FR2489948A1; ATA380481A; FI66486B

Abstract

A furnace 20, particularly for copper or copper alloys is bounded by sidewall 1, roof wall 2 and floor wall 3, comprising heat insulating material 4, mainly of aluminium oxide and silicon oxide fibres, and an outer steel coat. A crucible is fitted into the furnace on a stand 6. High temperature silicon carbide heating elements 8 are located between two opposite side walls of the furnace and the outer surface of the crucible 7 free from contact with any other article in the furnace. The heating elements are supported by shock absorbers and each element has a clearance between its central axis and the outer surface of the crucible of two to four times the outer diameter of one heating element. <IMAGE>

Description

SPECIFICATION Apparatus for setting up and casting molten minerals or metals This invention relates to apparatus for setting up and casting molten minerals or metals, particularly high smelting metals or metal alloys, such as copper or copper alloys, said apparatus comprising a furnace with side walls, a roof wall and a floor wall, which are, as a whole, heat insulated by a material consisting essentially of aluminium oxide and silicon oxide and which are surrounded by a steel coat, a crucible stand located inside the furnace and a crucible fitted onto said stand, and at least three silicon carbide heating elements provided with electric supply cables and located within the furnace between the furnace walls and the crucible, said heating elements being free from contact with any other article in the furnace.

Smelting and casting apparatus of this type, in which the heat insulation is formed of a fibrous material consisting essentially of aluminium oxide and silicon oxide is described in German Patent 2 725 884. This material has, for many reasons, proved to be most effective when used in a gas or oil-fired smelting and casting oven, or in an induction oven. Apart from comparatively low operating costs, such known electrically-operated apparatus allows extraordinary economy in connection with its further advantage in preventing oxidation ofthe molten material, since no movement is required as would be in, for example, an induction oven and also there is no possibility of chemical reaction with fuel gases as, for example, in gas or oil-fired ovens.A further advantage is the avoidance of any environmental pollution with practically noiseless working and the possibility of being able to keep the temperature of the furnace and the molten material constant at almost any required value.

In apparatus of this type, it is known to form the insulation between the masonry, the heat storage material and the outer steel coat as a one-piece bowl of insulating material of the type described, in order to decrease still further the heat energy lost by conduction compared to apparatus in which the side walls and the floor of the furnace are individually thermally insulated by this insulating material. It should be appreciated that in apparatus of this type a sufficient mass of heat storage material must befit- ted around the furnace in order to achieve economy compared to induction ovens or gas or oii-fired ovens.In orderto achieve this, a sufficiently thick insulating layer must be fitted between the heat storage material and the outer steel coat since otherwise, either the loss of heat is too high or the outside temperature of the apparatus at the outer surface of its steel coat is unbearably high for satisfactory handling.

This heat storage material also necessarily possesses a relatively high mass density and is therefore extremely heavy, so that the apparatus becomes not only costly with regard to the heat storage material itself, but also with regard to the necessary supporting construction and the not inconsiderable space required, and particularly when the apparatus is designed as a tip-oven it is relatively difficult to manipulate.

Furthermore, in this type of known apparatus, each heating element must be reinforced at both ends by insulating tubes of ceramic material in the region of the heat storage material. Thus the heating elements are unavoidably subject to certain stresses caused by knocks or shocks, particularly when such apparatus is designed to be tipped, due to vibration of the storage material during the casting process.

Experience shows that these mechanical stresses combined with the high thermal stressing of the heating elements very soon lead to the failure of one or more of the heating elements due to the formation of cracks therein. Such cracks cause excessive local electrical loading leading to total breakdown of the whole heating element concerned. In order to obtain a useful length of life for such heating elements, an upper limit of 13500C has been set for the charge temperature of such heating elements.

Thus, if the operator does not wish to accept premature failure of the heating elements with the associated increased cost in time and money for the time lost in exchanging heating elements and the consequent production losses, the types of charge for such apparatus must be limited to charge requiring a working temperature of 13500C or less.

Enlargement of the crucible charge, i.e. the reception space in the crucible has hitherto not been considered practical. Firstly, a certain limit to the size of the crucible charge is set by the handlability of the whole apparatus. If the outside measurements of the apparatus were retained and the crucible size increased, the clearance between the heating elements on the one hand and the outer surface of the crucible and/or the inner surface of the heat storage material on the other hand would have to be decreased.The former would, however, lead to overheating of the heating elements because of the unavoidable back-radiation of heat from the crucible, andthe latter would, without additional insulating measures, which would be unfavourable for the weight and volume and thus the handlability of the apparatus as well as for the cost of production, result in the surface temperature of the outer surface of the steel coat of the apparatus being considerably raised. Thus, forthese reasons, an increase in capacity, which is desirable in itself, is not regarded as possible.

An object of the present invention is to improve apparatus of the type described by simple and costeffective measures in such a way that, without the disadvantageous results described above, a considerably increase in capacity is achieved so that, on the one hand smelting processes can be carried out which require a working temperature in the furnace of up to 1 6500C and is necessary for short periods even higher and, on the other hand, without the necessity of enlarging the outer measurements of the device, an enlargement of the crucible charge is achieved without a substantial rise in the temperature of the outer surface of the steel coat of the apparatus and without any decrease in the working life of the heating elements, in fact an increase in their working life may be possible.The overall result is increased cost-effectiveness of such apparatus by heat saving and the lower cost of production due to simpler construction and the choice of suitable construction materials.

This object is achieved by the present invention in a surprisingly simple and economical way by provision of apparatus of the aforesaid type in which the furnace has side walls, a roof wall and a floor wall which are all made of a material consisting essentially of aluminium oxide and silicon oxide and that the silicon carbide heating elements are located adjacent at least one furnace wall, are supported by shock absorbers and each have a clearance between their respective central axes and the outer surface of the crucible in the furnace, which corresponds to two to four times the outer diameter of one heating element.

The side walls, the roof wall and the floor wall of the furnace may be formed of fibrous material, con venientlythefibrous material already inserted in known apparatus as heat insulation. Alternatively, the side walls, the roof wall and the floor wall of the furnace are formed at least partly from moulded parts of castable heat insulating material. Consider ably simplifications of lay-out and/or of manufacturing technique with corresponding influence on the total cost of production can be achieved by construction ofthe apparatus in accordance with the present invention.

In the present invention the advantages of known types of apparatus have been retained while their economy has been improved. For instance, in accordance with the invention it is quite possible to construct a sufficiently strong furnace simply from heat insulating material of the type described. Moreover, it is recognised that the economy of the apparatus depends far less on the presence of sufficient mass of heat storage material of great thickness, by which the heat energy radiated from the heating elements in the direction of the furnace is stored and substantially uniformly radiated back onto the crucible, than was previously understood.It is recognised in the present invention that sufficient back radiation of heat from the furnace walls to the crucible can also be attained by the comparatively small heat storage capacity of the heat insulating materials, and the main factors which might reduce the life of the crucible and/or heating elements are effectively cut out.

Such factors include in particular the mechanical sensitivity of the silicon-carbide heating elements and the material of the crucible. The susceptability of the crucible material to cracking increases with rising temperature such that at high temperatures small local differences in thermal capacity of the outer surface of the crucible combined with mechanical stresses occurring during the smelting and casting process often lead to the formation of cracks.

Experience shows that after a short period of operation leakage of the material being smelted into the furnace often necessitates an interruption of the work, (in order to cool the apparatus and remove said material from the furnace) which rapidly lowers the economy of the whole production process. In a further aspect of the invention, it is recognised that it is quite possible to find an optimal arrangement of the heating elements in relation to the crucible, which makes possible the optimal supply of heat energy to the crucible and avoids thermal overstressing of both the crucible walls and the heat elements.In order to allow an operating temperature of up to 1 6500C, the invention also provides specific supportforthe heating elements, which support prevents the transmission of knocking and shock stresses, to which the furnace is unavoidably subjected during the casting process, to the heating elements and thus makes possible the use of silicon carbide heating elements, which can withstand high working temperatures of at least 16500C.

A further aspect of the present invention with regard to enlargement of the crucible charge and increased economy of working of the apparatus comprises locating the heating elements each with a clearance between their respective central axes and the inner surface of the adjacent furnace wall which corresponds to two to four times the outer diameter of one heating element. Furthermore the heating elements are preferably supported at two opposite side walls ofthe furnace and preferably extend horizontally in known manner parallel to an adjacent furnace wall. It is also convenient within the framework of the invention to group the heating elements vertically in known manner around the crucible.

A preferred embodiment ofthe invention is characterised in that the furnace is substantially in the form of an elongated prism of rectangular cross-section with the crucible located substantially centrally within said furnace, and in that the heating elements extend horizontally and project through the longer opposing side walls of the furnace and are arranged in two parallel vertical rows at opposite sides of the crucible between said crucible and the shorter side walls of the furnace, such that the clearance between the respective central axes of the heating elements and the outer surface of the crucible corresponds to 2.3 to 3.5 times the outer diameter of one heating element. In this embodiment it is preferably that the heating elements are respectively located with a clearance between their central axes and the shorter side walls of the furnace which corresponds to two to three times the outer diameter of one heating element. A second embodiment of the invention is characterised in that the furnace is substantially in the form of a prism of hexagonal crosssection with the crucible located substantially centrally within said furnace and in that the heating elements are arranged in vertical rows of, in each case, at least two heating elements parallel to each side wall, each heating element extending horizontally and projecting obliquely through two adjacent side walls of the furnace such that corresponding heating elements of the rows lying parallel to each other at opposite sides of the crucible have, in each case, the same clearance between their central axes and the floor of the furnace, which clearance is however different for each opposing pair of rows.

In these two embodiments of the invention, it is particularly advantageous if the lowest heating element has a clearance between its central axis and the floor of the furnace which corresponds to three to five times the outer diameter of the heating element. This provides a sufficiently large space for the reception of leaks beneath the heating elements so that an appropriate quantity of leaked material can collect without there being any liklihood of damage to the lowest heating elements.

In accordance with a further aspect of the present invention, the heating elements are preferably arranged with a mutual clearance between the central axes of adjacent heating elements which corresponds to two to four times the outer diameter of one heating element. in this way, the radiation of heat to the crucible is optimised not only with regard to its amount but also with regard to the uniformity of its distribution. A further increase in the life expectancy of the heating elements and the resultant troublefree running of the device even after prolonged operation in the high temperature region can be achieved if the heating elements comprise known types of helical silicon carbide heating elements.

Afurther aspect of the present invention provides that the shock absorbing support of at least one heating element comprises an elastic mounting device which supports the respective heating element in the region of the heating element which projects through any furnace wall. It is simple and cost effective to construct the shock absorbing support of at least one heating element by directly laying its region which projects through the furnace wall onto the elastic heat insulating material. Thus each heating element may conveniently, in its region which projects through the furnace wall, be surrounded by an insulating casing.

In some instances it is advantageous if a heat radiation reflector is located between the heating elements and the furnace walls so that heat radiated by the heating elements in the direction of the furnace walls is reflected back in the direction of the crucible.

Furthermore, at least the roof wall and/or the floor wall of the furnace may be partly reinforced. Such reinforcements may conveniently comprise insertions in the material of any of the furnace walls and they are preferably formed at least partly of heat storage material.

It may also be convenient to provide a seal for preventing entry of metal vapours to the furnace and/or a seal for preventing entry of oxidising gases, particularly air, into the space above the crucible.

Such a seal is preferably located between the opening of the crucible and the roof wall of the furnace so that both sealing functions can be carried out by a single suitably designed seal.

It has proved advantageous that the heat insulating fibrous material has an outer layer heat conductivity of 0.1 Kcal/m h "C at 400"C and an inner layer heat conductivity of 0.3 kcal/m h "C at 1 200 C.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a vertical section of a first embodiment of a tippable smelting and casting apparatus of the present invention.

Fig. 2 is a vertical section of the same embodiment as in Fig. 1, the plane of this section being substantially perpendicular to the plane of the section in Fig. 1 Fig. 3 is a side view of the same embodiment of the smelting and casting apparatus as shown in Figs.

1 and 2, including the tipping mechanism, Fig. 4 is a vertical section of a second embodiment of the invention, Fig. 5 is a plan view of the same embodiment as in Fig. 4 with the raised lid partially broken away, and Fig. 6 is a vertical section of a third embodiment of the invention.

The embodiments of smelting and casting apparatus according to the present invention, as shown in Figs. 1 to 6, have in each case a furnace 20, which is bounded laterally, above and below respectively by side wall 1, roof wall 2 and floor wall 3. The walls 1,2 and 3 are enveloped in a steel coat. A crucible stand 6 is fitted into the floor of the furnace 20 and a crucible 7 with an outer surface 9 is fastened onto the said stand 6. At least three heating elements 8, each having an electric supply cable 19, are located at opposite sides of the furnace 20 between two opposite side walls 1 of the furnace and the crucible outer surface 9 such that any contact between said elements 8 and the crucible is excluded.

The heating elements 8 are high temperature long lasting silicon carbide heating elements which facilitate maintenance of the temperature of the crucible 7 at between 11 000C and 1650or in order to be able to melt and cast copper and copper alloys. The walls 1, 2 and 3 of the furnace 20 comprise heat insulating material 4, which consists mainly of aluminium oxide and silicon oxide fibres. The heating elements 8 are located in the furnace 20 in such a way that the clearance between the central axis of each heating element 8 and the outer surface 9 of the crucible 7 corresponds to two to four times the outer diameter of one heating element 8.The heating elements 8 are each supported elastically over their end portions, where they project through the furnace side wall 1 which supports them, so that the stresses of knocking and shock, to which the furnace side wall 1 is unavoidably subject particularly during casting, are not transmitted to the heating elements 8. An appropriate shock-absorbing support for the ends of the heating elements 8 adjacent the furnace side wall is provided by the insulating material 4. Each heating element 8, over its end portions where it projects through the furnace sidewall 1, is surrounded by a suitable insulating casing.

The smelting and casting apparatus shown in Figs.

1 to 3 is substantially in the form of a prism with a rectangular cross-section with the outer surface of the roof wall 2 sloping upwards and the lid 15 located substantially at the centre of the roof wall 2 directly above the crucible 7. The side wall 1 defines an elongated space also substantially in the form of a prism with a rectangular cross-section, inside which the crucible 7 is fitted onto the crucible stand 6. The crucible 7 extends into a shallow recess in the roof wall 2 so that a secure connection is formed between the crucible and the roof wall 2 and metal vapours cannot penetrate into the furnace 20. A seal 21 is fitted between the crucible mouth, which opens upwards, and the roof wall 2. This seal 21 may, for example, be composed of the same elastic fibrous material as the heat insulating material 4.

The furnace 20 is also in the form of a prism with a rectangular cross-section and its length is substantially 1.6 to 1.7 times its breadth. In this embodiment there are six heating elements 8 at opposite sides of the crucible. The clearance between the central axes of the four uppermost heating elements 8 on each side of the crucible 7 and the outer surface 9 of the crucible 7 is substantially the same on each side and is about 2.7 to 2.8 times the outer diameter of one heating element 8, and the same clearance of the two lower heating elements 8 on each side of the crucible 7 is substantially 3.1 times their outer diameter.All twelve heating elements 8, and indeed the six on each side of the crucible 7, are located such that a clearance corresponding to 2.8 times the outer diameter of one of the heating elements 8 exists between their central axes and the adjacent short side walls of the furnace 20.

The heating elements 8 are round in cross-section and are arranged parallel to each other in two parallel rows each said row being located adjacent one of the shorter side walls 1 of the furnace 20 and at right angles to the longer side walls of the furnace 20. The maximum wattage of each heating element 8 is approximately 8.4 KW, so that the maximum wattage of the whole apparatus is approximately 100 KW. The volume capacity of the apparatus is approximately 500 KG copper or 150 KG aluminium. The furnace 20, as shown in Figs. 1 to 3, is bounded by the walls t, 2 and 3 is, with its crucible and its heating elements 8 connected to the supporting frame 11 by a horizontal axle 13, which is parallel to the longer side walls af the furnace 20, so that said furnace 20 can be tilted.The apparatus includes a hydraulic cylinder 14 which is connected in such a way that it is capable of swinging the furnace 20 with respect to the frame 11 in order that the furnace 20 and the crucible 7 therein which are both mounted upon the frame 1 7, can tilt or tip in the direction of the casting opening 16.

When the apparatus is being used, the lid 15 is opened, the crucible 7 is partially filled with the metal to be smelted, the lid 15 is closed again and the electric current for the resistance-heating elements 8 is switched on so that said elements 8 heat up. The heat from these elements 8 is transmitted, mainly by radiation and partly by convection, to the crucible 7 and the metal located therein and also to the inner surface of the side walls 1 of the furnace 20.

Because the heating elements 8 are arranged with a certain clearance between themselves and respectively the crucible 7 and the side walls 1 the heat can be transmitted uniformly to the crucible 7, without any temperature peaks andlor troughs, which could cause thermal stresses in the crucible material. Once the molten metal has reached the required temperature, pressurised fluid is supplied to the hydraulic cylinder 14 which thereby causes the whole apparatus to be tipped or tilted in the direction of the casting opening 16in order, for example, to pourthe molten metal into a mould. After this casting process is completed the pressure in the hydraulic cylinder 14 is reduced by means of a control valve (not shown) so that the furnace 20 with its crucible 7 is returned to its original upright position.

The end portions of the heating elements 8 project through the longer side walls of the furnace 20.

Within said walls, each end portion is supported by a surrounding casing which also projects through the wall. In Fig. 1 these casings are shown as circles around the obliquely shaded cross-sections of the heating elements 8. Due to the aforesaid support of the heating elements overtheirend portions, the various knocks and shocks to which the apparatus is inevitably subject cause minimal mechanical stressing of the heating elements 8. The casings are formed of fibrous heat insulating material 4 which also absorbs general knocks and the like which the heating elements 8 might be subject to during the casting process when the apparatus usually vibrates.

In the embodiment shown in Figs. 1 to 3, the heating elements 8 extend horizontally at a predetermined distance above the floor of the furnace 20.

The clearance between the central axis of the lowest heating element and the floor is substantially five times the outer diameter of one heating element. It is thus ensured that none of the heating elements 8, and indeed not even the lowest ones, could be damaged by metal leaking from a partly broken crucible 7.

In the embodiment of the invention illustrated in Figs. 4 and 5, the length of the furnace 20 is approximately 1.6 times its breadth. Inside the furnace 20 there are three heating elements 8 in each row at opposing sides of the crucible 7. The heating elements 8 extend horizontally parallel to the shorter side walls of the furnace 20. The clearance between the central axes of the uppermost heating elements of each of the two rows and the outer su rface 9 of the crucible 7 is approximately 2.7 times the outer diameter of one heating element. The corresponding clearance for the middle heating elements is approximately 2.9 times the outer diameter of one heating element 8 and for the lowest heating elements approximately 3.4 times the outer diameter of one element 8. The clearance between the central axes of the heating elements and the inner surface oftheir respective adjacent shorter side walls 1 of the fur nace 20 corresponds approximately to 2.3 times the outer diameter of one heating element 8.

The clearance between adjacent heating elements in each of the two rows corresponds to approxi mately 3.3 times the outer diameter of one heating element, measured from central axis to central axis.

The clearance between the lowest heating element of each row and the furnace floor 3 is approximately 4.5. times the outer diameter of one heating element.

The maximum wattage of each heating element in the apparatus shown in Figs. 4 and 5 is approxi mately 8.3 KW, which means that the whole apparatus has a maximum wattage of approximately 50 KW. The volume capacity of this apparatus is approximately 300 KG copper or 90 KG aluminium.

The melting and casting apparatus shown in Figs.

4 and 5 is intended for horizontal casting processes, for example, floor casting. The casting opening 16 of the crucible 7 is located near the floor of the crucible 7 and it is intended to be equipped with a casting bowl and/or a cooling device. The crucible 7 ofthe melting and casting apparatus as shown in Figs. 4 and 5 has another casting opening 16' to facilitate the emptying of the crucible. The floor wall 3 and the roof wall 2 of the furnace 20 are partly set up with the insertion of a sealing compound connection in order to achieve sufficient mechanical firmness. The opening of the crucible 7 is sealed adjacent the roof wall 2 by a seal 21.

A third embodiment of the present invention is illustrated in Fig. 6. The furnace 20 is again in the form of a prism with a rectangular cross-section and its longer sides are approximately 1.6 times its shorter sides. This apparatus has six heating elements 8 which extend horizontally in two parallel rows. Each row comprises three heating elements one above the other adjacent one of the shorter side walls of the furnace, on opposite sides of the crucible 7, as shown.The clearance between the central axes of the heating elements and the outer surface 9 of the crucible 7 is approximately 2.4 times the outer diameter of one heating element 8 as far as the uppermost heating elements are concerned, approximately 2.8 times the outer diameter of one heating element 8 as far as the middle heating elements are concerned, and approximately 3.3. times the outer diameter of one heating element 8 as far as the lowest heating elements are concerned. The clearance between the central axes of the heating elements and the inner surface of the respective adjacent shorter furnace wall is approximately 2.1 times the outer diameter of one heating element 8. The clearance between the central axes of adjacent heating elements in each row is approximately 2.8 times the outer diameter of one heating element 8.The clearance between the central axes of the lowest heating elements and the floor of the furnace is approximately 3.2 times the outer diameter of one heating element 8. The maximum wattage of each heating element 8 is approximately 14 KW, so the total wattage of the whole apparatus is approximately 85 KW.

The volume capacity of the apparatus approximately 1,000KG copper or 300 KF aluminium. The working of the apparatus illustrated in Fig. 6 is, so far as the heating up of the apparatus and the melting of the metal are concerned, similar two the working described in connection with the embodiment ofthe apparatus as shown in Figs. 1 to 3. However, the apparatus as shown in Fig. 6 remains motionless, while in use. During the costing process, the crucible 7 is simply lifted out of the interior of the furnace 20 through the furnace wall 5. The lifting apparatus is of the usual type and is not shown in Fig. 6.

In modifications of the aforementioned embodiments, the crucible dimensions, and the number and wattage of heating elements may vary. In respect of the first embodiment as shown in Figs. 1 to 3, the modified types or apparatus as indicated below in Table I have been constructed.

Tablet No. of Volume Dimensions heating capacity Overall Type in mm elements in KG wattage Weight D d H Cu Al KW KG T-50 305 265 400 9 60 220 28 950 T-200 380 355 580 19 200 60 45 1500 T-500 445 385 990 12 500 150 100 2600 T-1000 615560945 12 1000 300 170 3300 T-2000 670 650 1200 18 2000 600 250 4000 D = Diameter of crucible in upper region d = Diameter of crucible in regionoffloor H = Heightof crucible In respect of the third embodiment as shown in Fig. 6, the modified types of apparatus as indicated below in Table II have been constructed.

Table II No. of Volume Dimensions heating capacity Overall Type in mm elements in KG capacity Weight D d H Cu Al KW KG S-50 305 205 375 9 80 25 28 700 S-200 380 265 480 9 200 60 45 1250 S-300 440 295 540 6 300 90 50 1400 S-500 510 325 650 9 500 150 75 1700 S-1000 615355 700 6 1000 300 85 2000 D = Diameter of crucible in the upper region d = Diameter of crucible in the region of the floor H = Height of the crucible The mutual clearances between the heating elements 8, that is the distance between the central axes of adjacent heating elements in each row is for example between two and four times the outer diameter of one heating element 8. In the embodiment shown in Figs. 1 to 3 this clearance amounts to 2.6 times the outer diameter of one heating element 8. On the other hand, in the apparatus shown in Figs.

4 and 5 this clearance is 3.4 times the outer diameter of one heating element 8 and in the embodiment shown in Fig. 6 it is about 2.8 times the outer diameter of one heating element, and in Type T-2000 indicated in Table lit is 2.2 times the outer diameter of one heating element. Because of these particular mutual clearances between heating elements 8 and because of the predetermined clearance between the heating elements 8 and the outer surface 9 of the crucible 7 and between the said elements 8 and the inner surface of the shorter side walls of the furnace 20 heat is radiated uniformly from the heating ele ments 8 onto the surface 9 of the crucible 7 so that the surface temperature of the crucible 7 is practically uniform.Furthermore the heat radiated from the heating elements onto the shorter side walls of the furnace has also, on the whole, uniform distribution over the side wall surface. Thus, temperature differences between different points on the outer surface of the crucible and also between different points on the inner surface of the appropriate furnace wall are held at the lowest possible value ana the thermal stresses acting on these surfaces are minimised. If the clearances between the heating elements and the crucible were smaller than specified by the invention and explained above, the surface temperature of the crucible would become higher due to the heat transferred by convection and radiation being greater and this would cause greater thermal stressing of the crucible. The unfavourable consequence ofthis would be a reduced life of such a crucible.A greater clearance between the heating elements and the outer surface of the crucible than specified by the invention would result in decreased heat convection and radiation as a result so that the heat-economy efficiency of the apparatus would be adversely affected.

If the clearance between the heating elements and the respective adjacent shorter side walls of the respective furnaces were selected to be shorter than those specified by the invention and explained above, the amount of heat transmitted by convection and radiation to these side walling surfaces would increase, whereupon the temperature of these wall surfaces would rise. The unfavourable consequence of this would be thermal and mechanical stressing of the side wall which would mean higher running costs and a shorter life for the furnace walls. If the clearance between the heating elements and these side walls were greaterthan those specified by with the invention, the size of the furnace would be unnecessarily large if the clearance between the heating elements and the crucible was maintained.

Such increased size would increase heattransmis- sion surfaces exposed to the surrounding space and thus increase the loss of heat. The result of this would again adversely affect heat economy of the apparatus and lead to higher costs.

The proportions of the components in the mineral fibrous material which is used to make the heat storing furnace walls vary considerably. The amount of Al203 and SiO2 is 50%-100% of the weight of the material; the amount of Al203 may be between 1% and 100% by weight, for example over 40%, and preferably 40% to 60% by weight. The amount of SiO2 may also be between 0% and 100% by weight, for example over 40%, and conveniently between 40% and 60% by weight. Other additives and impurities may be present in the fibres, provided that these impurities are not likely to cause corrosion or other damage to the crucible, the heating elements or the other components of the apparatus. For example, it must betaken into account, that iron compounds are often damaging to the heating elements and other components of the furnace.

The heat conductivity of the heat insulating mater ial 4 is preferably 0.1 kcal/m h "C at 400"C in its outer layer and 0.3 kcal/m h "C in its inner layer which is laid next to the crucible.

The heat insulating material 4 consists of mineral fibrous material, such as materials available on the market under such names as "Triton Kaowool" or "Fiberfax" which contain the following components: Awl203 43 to 70% by weight SiO2 20 to 54% by weight Fe203 Traces TiO2 Traces MgO Traces CaO 0.1 to 1.0% by weight Na2O 0.1 to 2.0% by weight The efficiency of the apparatus may be improved and the necessary quantity of heat insulating material may be reduced if a heat radiation reflector (not shown in the embodiments illustrated in the drawings) is located between and parallel to the shorter side walls of the furnace and the heating elements in such a way that it reflects the heat in the direction of the crucible.

Further known supplementary pieces of equipment may be incorporated into the smelting and casting apparatus of the invention such as, for example, a thermostat with temperature sensor (shown in Fig. 4 above the opening 16 and above upper heating element), casting devices, locking devices for the openings etc., To summarise, it must be established that the invention primarily provides smelting and casting apparatus by means of which minerals and/or metals or metal alloys, which for melting require a working temperature of up to 16500C and occasionally for short periods a higher temperature, may be melted and cast. The invention relies on the use of mineral heat insulating material with a high resistance to the conduction of heat, which material may be such fibrous material as is already known for heat insulating of furnace walls.In this way, the disadvantages of known embodiments of furnaces, in which considerably heavier building materials, such as ceramic materials, fire resistant tiles or other heat storage material, are used, are overcome. Furthermore the invention makes possible an increase in the life of the heating elements used, despite their higherthermal stressing, and at least a maintenance, if not an increase, of the life of the crucible in comparison with known apparatus. This is achieved by reducing the transmission of knock and shock stresses from the furnace walls to the heating elements by inserting an elastic support or insulating case preferably of heat insulating mineral fibre material around the ends of the heating elements.The proposed arrangement of the heating elements with respect to each other, the outer surface of the crucible and/orthe inner surface of the furnace walls provides for operation of the furnace at the named higher temperature and also improves the heat economy efficiency of the apparatus as maximum radiated energy is transmitted from the heating elements to the crucible without the liklihood of thermal stressing of the said elements. The operating life of all I h high-temperature stressed component parts of the apparatus, particularly the heating elements, the crucible and the furnace walls, is increased. Thus the waste of time due to premature fatigue and failure of these component parts is obviated. All the above results in an increased total economy of the apparatus of the invention, compared to known apparatus.

Although the invention is described with reference to some preferred embodiments, it is obviously not confined to these. Within the scope of the invention, many varied modifications may be made, for example by combining appropriate features of the invention with each other or with other known devices or manufacturing techniques, depending on the particular requirements of any individual case.

Claims

1. Apparatus for setting up and casting of molten minerals or metals, particularly high-smelting metals or metal alloys such as, copper or copper alloys, said apparatus comprising a furnace with side walls, a roof wall and a floor wall, which are, as a whole, heat insulated by a material consisting essentially of aluminium oxide and silicon oxide and which are surrounded by a steel coat, a crucible stand located inside the furnace and a crucible fitted onto said stand, and at least three silicon carbide heating elements provided with electric supply cables and located within the furnace between the furnace walls and the crucible, said heating elements being free from contact with any other article in the furnace, characterised in that the furnace has side walls, a roof wall and a floor wall are all made of a material consisting essentially of aluminium oxide and silicon oxide and that the silicon carbide heating elements are located adjacent at least one furnace wall, are supported by shock absorbers and each have a clearance between their respective central axes and the outer surface of the crucible in the furnace, which corresponds to two to four times the outer diameter of one heating element.

2. Apparatus as claimed in Claim 1 characterised in that the side walls, the roof wall and the floor wall of the furnace are formed of a fibrous material.

3. Apparatus as claimed in Claim 1 or 2 characterised in that the side walls, the roof wall and the floor wall of the furnace are formed at least partly from moulded parts of castable heat insulating material.

4. Apparatus as claimed in any preceding claim characterised in that the heating elements are respectively located with a clearance between their central axes and the inner surface of the adjacent furnace wall which corresponds to two to four times the outer diameter of one heating element.

5. Apparatus as claimed in any preceding claim characterised in that the heating elements are located horizontally in a known manner parallel to an adjacent furnace wall.

6. Apparatus as claimed in any preceding claim characterised in that the heating elements are supported between two opposite walls of the furnace.

7. Apparatus as claimed in any preceding claim characterised in that the heating elements are vertically grouped in a known manner around the crucible.

8. Apparatus as claimed in any of claims 1 to 6 characterised in that the furnace is substantially in the form of an elongated prism of rectangular cross-section with the crucible located substantially centrally within said furnace, and in that the heating elements extend horizontally and project through the longer opposing side walls of the furnace and are arranged in two parallel vertical rows at opposite sides of the crucible between said crucible and the shorter side walls of the furnace such that the clearance between the respective centre axes of the heating elements and the outer surface of the crucible corresponds to 2.3 to 3.5 times the outer diameter (1) of one heating element.

9. Apparatus as claimed in Claim 8 characterised in that the heating elements are respectively located with a clearance between their central axes and the shorter side walls of the furnace which corresponds to two to three times the outer diameter of one heating element.

10. Apparatus as claimed in any of claims 1 to 6 characterised in that the furnace is substantially in the form of a prism of hexagonal cross-section with the crucible located substantially centrally within said furnace, and in that the heating elements are arranged in vertical rows of, in each case, at least two heating elements parallel to each side wall, each heating element extending horizontally and projecting obliquely th rough two adjacent side walls of the furnace such that corresponding heating elements of the rows parallel to each other at opposite sides of the crucible have, in each case, the same clearance between their central axes and the floor of the furnace, which clearance is however different for each opposing pair of rows.

11. Apparatus as claimed in any of claims 8 to 10 characterised in that the lowest heating element has a clearance between its central axis and the floor of the furnace which corresponds to three to five times its outer diameter.

12. Apparatus as claimed in any preceding claim characterised in that the mutual clearance between the central axes of two adjacent heating elements corresponds to two to four times the outer diameter of one heating element.

13. Apparatus as claimed in any preceding claim characterised in that the heating elements comprise known types of helical silicon carbide heating elements.

14. Apparatus as claimed in any preceding claim characterised in that the shock absorbing support of at least one heating element comprises an elastic mounting device, which supports the respective heating element in the region of the heating element which projects through the furnace side wall, roof wall or floor wall.

15. Apparatus as claimed in any preceding claim characterised in that the shock absorbing support of at least one heating element is constructed by directly laying its region which projects through the furnace side wall onto the elastic heat insulating material.

16. Apparatus as claimed in Claim 14 or 15 characterised in that each heating element, in its regions which project through the furnace wall, is surrounded by an insulating casing.

17. Apparatus as claimed in any preceding claim characterised in that a heat radiation reflector is located between the heating elements and the furnace walls so that heat radiated by the heating elements in the direction of the furnace walls is reflected back by means of said reflector in the direction of the crucible.

18. Apparatus as claimed in any preceding claim characterised in that at least the roof wall and/or the floor wall of the furnace is partly reinforced.

19. Apparatus as claimed in Claim 18 characterised in that the reinforcements comprise insertions in the material of any of the furnace walls.

20. Apparatus as claimed in Claim 18 or 19 characterised in that the reinforcements are formed at least partly of heat storage material.

21. Apparatus as claimed in any preceding claim characterised in that a seal for preventing entry of metal vapours to the furnace is located between the crucible opening and the roof wall of the furnace.

23. Apparatus as claimed in Claim 21 characterised in that the seal for preventing entry of metal vapours to the furnace is also a seal for preventing entry of oxidising gases into the space above the crucible.

24. Apparatus as claimed in any preceding claim characterised in that the heat insulating fibrous material has an outer layer heat conductivity of 0.1 kcal/m h "C at 4000C and an inner layer heat conductivity of 0.3 keal/m h "C at 1200"C.

25. Apparatus for setting up and casting molten minerals or metals substantially as hereinbefore described with reference to and as illustrated by Figs. 1 to 3 or Figs. 4 and 5 or Fig. 6.