US1998084A - Electrical heating device - Google Patents

Description

April 16, 1935. R. F. JAMES .ET AL 1,998,084

ELEGTRI CAL HEATING DEVICE Filed Dec. 23, 1929 INVENTORS ATTORNEYS Patented Apr. 16, 1935 q UNITED STATES PATENT OFFICE I 1,99s,os4 ELECTRICAL HEATING DEVICE Application December 23, 1929, Serial No. 416,136

4 Claims.

The invention relates to electrically heated heat treating apparatus and more particularly to electrically heated cyanide pots and similar devices utilizing chrome nickel resistance units for heating purposes.

The prime object of our invention is to increase the effective life of a resistance unit when employed in such apparatus and to this end we have employed a modified design of heat treating apparatus as hereinafter more fully set forth.

It has been observed that in certain types of heat treating apparatus using chrome nickel heating elements the units have a much shorter length of life than do similar units in other types of heat treating apparatus. This is particularly true in electrically heated cyanide pots, that is, apparatus in which a molten cyanide salt is maintained in a heated condition by electric resistors.

We have discovered that in apparatus of the type above mentioned the electrical resistors may be maintained in operation for a longer length of time if means is provided for preventing certain chemical actions on the resistance elements. We have discovered that one of the factors contributing to the failure of the heating elements is apparently the alternate oxidizing and reducing atmospheric conditions surrounding the heating elements. When the apparatus is in operation a reducing atmosphere surrounds the element by reason of the introduction of carbon monoxide or soduim oxide into the chamber containing the resistance element by seepage through the top of the apparatus or gaseous penetration of the walls of the metallic container for the cyanide or both. The oxidizing atmosphere is caused by the oxygen of atmospheric air seeping into the heating or element chamber during the cooling or nonworking periods. Because of temperature diiferential a slight vacuum is created in the element chamber during the heating or working periods. Cooling of the pot following a working period causes an equilibrium condition with the consequent infiltration of atmospheric air.

The primary cause of element failure may be attributed to this cyclic change of atmospheres. The reducing atmosphere reduces the oxide scale which covers the element surfaces. This scale, upon reduction, does not recombine with the metal of the elements but scales or sloughs off. Subsequent cooling and creation of an oxidizing atmosphere causes a fresh oxide coating to form which in due turn is reduced and lost. This cycle tends to eventually reduce the cross section area of the element and causes hot spots and burning with consequent failure.

. To overcome the objectionableconditions as outlined above, we have designed a heat treating apparatus in which means is provided for maintaining around the heating element an atmosphere which does not alternate from oxidizing to reducing characteristics.v Since it is easier to maintain an oxidizing atmosphere than a reducing atmosphere, the preferred embodiment of our invention consists in a construction designed to permit ventilation of atmospheric air around the resistor at all times.

We have also discovered that when the temperature of a chrome nickel heating element is above 1700 F., the reaction between a reducing atmosphere and the oxide scale on the heating element is greatly accelerated and we therefore find it preferable to design the apparatus so that the temperature of the heating element is not above 1700 F. .under operating conditions. This may be carried out by providing sufficient area 8 of the heating element to permit maintaining the proper bath temperature without exceed-ing the desired element temperature.

Our invention may be embodied in various constructions, one of which is illustrated in the accompanying drawing, wherein Figure 1 is a sectional elevation of the apparatus;

Figure 2 is a sectional plan view thereof.

In the drawing, the apparatus consists of a base i and a tubular refractory lining l l extending :upwardly therefrom. The lining preferably consists of a series of refractory rings I2, each containing inwardly projecting annular flanges 13 forming recesses M therebetween. At the top of the apparatus there is an inwardly projecting refractory ring [5. Surrounding the lining II is an outer metallic casing it which supports a cover I! arranged over the top refractory ring IS. A plate I8 is supported by the cover and the top refractory ring and in turn supports the bath container Hi. This container is preferably in the form of a cast metal pot extending within the lining l l and having a top annular flange 20 supported on the plate 18. The arrangement is such that the bath container I9 may be readily removed from the furnace and replaced either by a pot of the same size or in some cases by another ,of greater depth. Within the base In there is arranged a tubular conduit 2! forming a drain for the interior of the furnace, one of the functions of this drain being to remove the molten material from the furnace in case of breakage of the bath container while the furnace is in operation. The

construction, as thus far described, is similar to conventional apparatus as used prior to the instant invention, except that our improved device has a greater space between the pot I9 and the outer refractory lining H than is the customary practice.

In order to provide means for maintaining a substantially constant condition of oxidation or reduction within the interior of the apparatus, we provide the apparatus with means for inswing adequate ventilation of the same during the heating periods. For this purpose we make use of the drain 2! allowing air to enter'the .apparatus through the drain. Atrthe top 22 of the drain there is preferably arranged a bafile 23 in the form of an invertedtruncated cone having downwardly extending sides 24. These'sides serve to deflect the air downwardly andoutwarldly thereby causing the air to come in contact with the refractory lining II. To complete the natural draft of the apparatus, a series of openings is provided in the upper part of the furnace and as shown, the top refractory ring if: is provided With a series :of radial passageways 25 from which the tubes 26 project and lead into an ammlar header duct. This duct is arranged within the outer casing 16 and communicates with :the outside atmosphere'ibymeans of a flue passage 28. r I The heating elements are preferably arranged within the recesses M and .as shown they are represented by the numeral 29. The terminals from the heating elements lead outwardly through the tubes 30 and Si extending from the refractory lining H to a point outside of the outer casing 16.

The effect of the ventilating arrangement the apparatus as above described is to cause a continual change of atmosphere within the reiractory lining, thereby preventing the formation of alternate oxidizing and reducing conditions in the practica'l operation of the furnace, Since the ventilation is caused by introduction of air into proximity to the electrical resistor, our apparatus mai-nta'ins oxidizing conditions surrounding the heating element both when'the furnace is being heated and when it not being heated but since the chrome nickel heatingmaterial is capable of standing up for long periods of time under ordinary atmospheric conditions at incandescent temperatures, it has Foeen found that the'ventilationof the furnace does not cause any further oxidation .or deterioration of the element but on the contrary it actually prolongs the life of the element. Further it has been found that the continual addition of fresh air while it causes a certain heat loss due to the convexing currents, nevertheless more than makes up for this loss in the added heating efiiciency due to the presence of fresh air instead of the partial vacuum which heretofore existed in the older furnace types.

What We claim as our invention is:

1. A heating apparatus comprising a metallic container for molten cyanide, a refractory lining surrounding said container, electrical heating means between said container and said lining and means for circulating air through said'lining to preventdeterioration of said electrical heating means by chemical action of the molten cyanide.

2; A heating apparatus comprising a base, a refractory cylindrical lining extending upwardly from said base, an electrical resistor associated with said refractory lininga'metal pot within said lining, passageways at the upper end of the apparatus permitting the escape of air from said lining, an inclined duct within said base and a bafile for directing the air from said inclined duct.

.3. 1n a-heating apparatus comprising an outer casing, an anmfiar refractory lining Within said casing, an? electrical resistor within said lining, a metallic container within said lining, means for introducing air into the space between said lini-ng and'said container, a series of radial pass'agewaysat the top of saidrefractory lining commnnicating with the interior of said lining, an

annular duct connected to said radial passageways and arranged between the refractory lining and said outer casing and a vent extending through said outer casing from said annular duct.

4. A heating :aparatus comprising a metallic container for a cyanide compound, a casing sur-' rounding said container, .a metallic resistor within said casing and spaced from said container, said resistor being adapted to be electrically heated to incandescence thereby melting said cyanide compound through radiation and means for circulating an oxidizing gas into contact with said resistor to prevent deterioration of said resistor dueto the chemical action on the metal of said resistor by said cyanide compound.

ROBERT JAMES. GLENN COL'EY.

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