US3924098A - Heating element, method and composition - Google Patents

Abstract

A heating element is cast, extruded or formed from a composition comprising finely comminuted ferrous metal in a matrix essentially consisting of a base composition consisting essentially of tailings muds from lithium ore extraction, comprising a major percentage of silicon, lesser percentages of aluminum and calcium, and to which some alkali metals or lead may be added in order to ease working and depress the melting point. An electrically conductive composition of controlled resistance is prepared in which the ferrous particles contact each other so as to give the resultant element resistivity of 10 3 to 1,000 Ohm centimeters. The tailings mud is a by-product from ore extraction, which is particularly suitable for this purpose because of its fine state of comminution, and its workability.

Description

United States Patent Dunn Dec. 2, 1975 HEATlNG ELEMENT, METHOD AND COMPOSITION [751 lnventor: Stanley Austin Dunn. Madison. Wis,

[73] Assignee: Bjorksten Research Laboratories,

Inc, Madison, Wis

[22] Filed: Dec. 3, I973 [211 Applt No: 421,073

Related U.S. Application Data [63] Continuation-impart of Ser. No. 242.436. April 10.

1972 abandoned [521 U.S. Cl. .1 219/345; 29/611129/621; 219/553; 252/513; 252/518; 252/512 [51] Int. Cl. a. F24H 9/02 [581 Field of Search 252/513 518; 29/611 345 [56] References Cited UNITED STATES PATENTS 2.855.491 10/1958 Navias 252/513 X 2,924,540 2/1960 D'Andrea 252/514 X 2,946 937 7/1960 Herbert 252/521 2,950,995 8/1960 Place ct a1. 252/518 X Primary E.t'amincrBenjamin R. Padgett Ari/stunt ExaminerE. Suzanne Parr Armrney. Age-m or FirmJohn M. Diehl; Johan Bjorksten [57] ABSTRACT A heating element is cast extruded or formed from a composition comprising finely comminuted ferrous metal in a matrix essentially consisting of a base composition consisting essentially of tailings muds from lithium ore extraction comprising a major percentage of silicon lesser percentages of aluminum and calcium, and to which some alkali metals or lead may he added in order to ease working and depress the melting point.

An electrically conductive composition of controlied resistance is prepared in which the ferrous particles contact each other so as to give the resultant clement resistivity of 10*" to 1,000 Ohm centimeters. The tailings mud is a by-product from ore extraction. which is particularly suitable for this purpose because of its fine state of comminution and its workability.

4 Claims. 6 Drawing Figures US. Patent Dec.2, 1975 HEATING ELEMENT, METHOD AND COMPOSITION BACKGROUND AND PRIOR ART withstanding ceramic bodies can be made by impreg- I nating a porous metal matrix with a ceramic compound which has a lower melting point than the metal. The importance of wetting the metal by the ceramic has been stressed (Grubel et al. U.S. Pat. Nos. 2,671,955 and 2,672,426). Furthermore, metals have been sintered together with lower melting conductive oxides to produce resistant contact surfaces (Ruben, U.S. Pat. No. 2,200,854). This last mentioned patent stresses the importance of electrical conductivity in the oxide, and limits claims to the conductive vanadium pentoxide.

In a prior patent application (Ser. No. 376,721), Applicant disclosed the production of electrically conductive thermal shock-resistant metal-ceramic compounds by the use of certain heat treatment cycles to cause cocrystallization of metal fragments at temperatures close to the sintering temperatures of said metals.

OBJECTS OF THE INVENTION An object of the invention is a superior composition for electrical heating, utilizing a waste material.

Another object is a method for preparing said composition, and heating elements made therefrom.

Further objects will appear from the following description.

BRIEF STATEMENT OF THE INVENTION A low cost product, heatable by electrical resistance to temperatures suitable for radiant heating of human dwellings and other applications calling for moderate temperature (from slightly elevated to about 150C) is prepared by mixing a finely comminuted electrically conductive particulate substance with a flotation mud from an ore refining process said mud comprising silica-alumina-calcia type tailings after milling, roasting and extracting; and compacting the resultant product so as to secure the desired electrical resistance for resistance heating, and stabilizing it, for example, by one or more of the following operations: further drying, compacting, sintering, or fusion and enameling steps.

THE DRAWINGS Reference is made to the drawings, of which:

FIG. 1, FIG. 2, and FIG. are enlarged cross-sectional views.

FIG. 3 and FIG. 4 are cross-sectional views.

FIG. 6 is a perspective view.

DETAILED STATEMENT OF THE INVENTION Referring to the drawings, FIG. 1 illustrates a magnified (about X) section through a dispersion of conductive ore or metal fragments or particles in a matrix of fused flotation mud taken from the continuous vacuum (Oliver") filters, removing flotation mud from a process resulting in flotation tailings being formed following ore treating steps including milling, roasting, extracting and flotation.

This composite is referred to in the drawings as l0.

The essential points are that the tailings used as raw material be finely comminuted, roasted, to remove volatiles and converted, as much as possible based on the raw material, to an oxide form suitable for glass formation. Subsequent leaching has removed the wateror acid-solubles. After the final flotation process, vacuum filtration removes the major percentage of water, thus leaving a pasty product in a form easily and economically handled and excellently suitable for blending with iron filings, crushed conductive metal ores, or oxides of ferrous or other ores reducible to conductive metal by treatment with reducing agents of which hydrogen, car bon monoxide, and hydrocarbon gases are the most convenient and inexpensive.

The blending can be done on conventional blending equipment, such as, for example, continuous ribbon mixers such as Day double spiral ribbon mixers, or in sigma blade mixers, but most economically by multiple pass through differential speed rollers on a paint mill.

Subsequent drying and sintering and particularly fu sion at about l,000 1,200 C leaves the conductive particles 2 scattered, enclosed in matrix I, to such extent that they are protected from oxidation by atmospheric oxygen when resistance heated at normal operating temperatures for residential heating panels, say at about to l50 C. The density and form of the suspended conductive particles are such that these frequently touch each other.

In FIG. 2 the melt has been kept 3 hours at 1,200, with very slow stirring (stirrer at about 5 revolutions per hour) so that some coalescence or co-crystallization of the particles has taken place, augmenting thereby the contact between particles and resulting in a more feathery structure, 3.

FIG. 3 shows an embodiment in which the matrix is very thick, for example, like a brick or a concrete block 5, or other conventional building elements and the particles have been introduced only in a layer of about a couple mils to 36 inch, so that the building elements can be assembled and then heated by electric current conducted only through the thin layer. The particles can be introduced, for example, by projecting them into the still soft mass at high velocity by spraying or centrifugal throwing, with optional subsequent application of an enamel, or a sealing coat, for example, of soluble silica or silicate solution such as sodium silicate, or colloidal silicic acid solutions.

FIG. 4 shows such a building element in which the main body is free from metal and has been foamed, for example, by inclusion of a gas forming sulfide or carbonate in the mix as well known in the art. 4 indicates bubbles, l0 the metal carrying conductive layer.

FIG. 5 is a magnified detail view, showing a surface sealing layer 6 of enamel, silicate, or the like, to close such cracks as might form and enhance the resistance of the element to oxidation. It may also serve decorative purposes.

FIG. 6 shows a perspective view of a heating element in panel form. 7 is the heating surface, 8 the electrodes attached thereto in any of the many manners known to the art. Preferably the electrode surface is applied before any sealant coat. The electrodes may be simply clamped firmly over the surface, or the surface may be first specially prepared by abrading and fusing over it a solder with appropriate flux to better bind the electrode. Since the art in this field is voluminous and throughly known, I believe further discussion of this aspect to be superfluous.

EXAMPLE 1 A heating rod was made from the tailings mud from spodumene, a lithium aluminum silicate mineral, LiAl(SiO belonging to the pyroxene group, from which the lithium had been extracted by the steps of crushing, ballmilling 4 hours, roasting in a rotary kiln. cooling, extracting with dilute sulfuric acid. washing, froth flotation, and separation of the finely divided (over 50 percent through 325 mesh screen) wet mud on a rotary vacuum filter (Oliver filter). The tailings mud used in this experiment was a composite of four samples, and had the following analysis:

This tailings mud was dried. and the resulting powder was mixed with volume percent of iron filings. The mixed powders were ballmilled together over night, and 17.5 grams were placed in a graphite mold 6 inches long by 56 inch wide and as inch deep of U-shaped cross section. The mold and powders were placed in a vacuum furnace, the pressure was reduced to below 10 Torricelli and the temperature raised.

This was repeated four times, the temperature being raised to successively higher values each time. The first time a temperature of 450 C was reached over 6 hours; the sample lost 0.25 gm. weight. The second heat was to 660 C in 5 hours; the third to 890 C in 5 hours. In the latter instance the sample was fused over most ofits length, a little of its initial powdery nature being discernible at one end. The final heat to 940 C lasted for 7 hours. The sample was fused over its entire length.

A half inch of each end of one 3 inch length of the above was painted with conductive silver paint, the remainder with Na siO solution and the coatings dried. Electrodes were attached to the silvered ends and the bar was subjected to several cycles of electrical heating to temperatures between 100 and 140 C.

Temperatures were measured with thermally insulated chromel alumel thermocouples made of fine wires (0.005 inch dia.) electrically insulated from the heater bar by l/32 inch thick piece of boron nitride and recorded on a chart recorder. Current through. and potential across the bar were measured to an accuracy of 1 3 percent.

The heating was cyclic during three working days averaging about 8 hours/cycle. Power was held to 4.7 1' .5 watts and resistance measured 0.065 :t 0.003 ohms. The fluctuations in the latter appear to be solely a function of the power and not of time. insofar is discernible with the accuracy of measurements.

The left end of the bar consistently read 20 30 C higher than the right end. The bar is oblong shaped in cross section. Major axis inch; minor 4 inch. Hence cross sectional area is -3/32 sq. in. or 0.60 cm. The length between contacts is 2 inches or 5.1 cm. Thus, the resistivity is approximately:

0.6 cm R Z (.005 I .003) Ohm 5.1 cm .0077 t .0004 ohm cm (7.7 1.41) i 10" ohm cm under the above conditions.

EXAMPLE 2 Tailings mud from the Oliver filters in a production operation is moved continuously to differential speed rollers on two paint mills in tandem, and mixed there with finely pulverized electrically conductive metal or metal ore, approximately 8 percent by volume on the total dry weight present, so as to achieve a resistivity of the final product of about 1 to 25 ohm cm. Prior to this step the material of the tailing mud has passed through the steps of crushing, comminuting to a fineness where a major percentage thereof passes through a 200 mesh screen, roasting, most conveniently in a rotary kiln, to remove volatiles and convert the less stable metal compounds to oxides, leaching with sulfuric acid to remove the monovalent metal constituent of the pyroxene, and separating tailings by flotation as discussed above.

The composition of tailings mud and conductive metal is then heated by any suitable means, preferably in vacuum, to a temperature between the melting point of the matrix material and of the metal. Temperatures of about 1,000 C l,200 C appear suitable. Addition of some leador alkali metal compounds in about 5 15 percent by weight further depress the fusion temperatures by up to several hundred degrees.

Prior to the final fusion, the composition may be pressed, cut, shaped or otherwise formed to the desired physical shape.

If the end product is to be a heater, the electrodes are applied prior to the final melting so that they will be intimately contacted with the metal network formed by mutually contacting metal particles within the product. Alternatively, electrodes can be applied subsequently, for example, by applying a solder layer with appropriate flux to the ends of the article, and soldering conductors to these. In some cases abrasion of the ends and subsequent compression application of electrodes may suffice.

[n this manner large surface heating panels can be produced at a very low cost, using as the principal raw material tailings which would otherwise represent a difficult disposal problem.

As pointed out above, it is possible to render only the surface of a heavy body conductive in the manner indicated and to confine the conductivity to any desired part thereof. Thus, building blocks such as tiles or concrete blocks can be made suitable for resistance heating of a wall, or or even the entire inner walls, floors and ceilings of a structure. The compositions described herein can also be applied as pastes to ordinary concrete blocks or tiles before these are burned" in the kiln, so as to render them electrically heatable on the surface. A metal mixed mortar may be made to bridge the gap between bricks or blocks, so as to provide continuous conductivity. This may also serve as outside conductor to protect buildings from lightning.

Generally, the tailings employed for the purposes of this invention have a percentage analysis on dry basis within the following limits:

so, 55 mp, 15 :5 C210 15 :5

If it is desired to further lower the melting point of the matrix employed, we find it convenient to lower this by the addition of 4 to 35 percent by weight of a substance comprising at least one member of the class consisting of leadand alkali metalcarbonates, oxides and hydroxides.

While the particular flotation mud employed in Example l was derived from a Spodumene, from which lithium had been extracted with dilute sulfuric acid, it is understood that other finely subdivided flotation muds can be equally employed, provided that they have been comminuted to a similar extent by any suitable means, such as ball milling, milling at hyper critical speeds by the process of Hukki, hammermilling, or the like. While this normally results in a fineness of the major portion of the product passing through a ZOO-mesh screen, and usually even a 325-mesh screen, this is not too critical, so long as the resultant flotation mud is easily intermixed with finely comminuted electrically conducting material to give a fairly uniform composition. The precise distribution of the particle size in the drilling mud used in not critical, because the finer particles will in any case coalesce in subsequent sintering or/and fusing steps.

The roasting temperature is not critical so long as it adequately prepares the material for the action of the extraction medium to be used for removal of soluble constituents. Generally, addition of 4 to 30 percent by volume of the electrically conductive ingredient will be found suitable. When iron or steel filings are employed,

we usually find the preferred range of electrically conductive ingredient to be about 8 percent by volume. The resistivity is then usually about 0.1 to 25 ohm cm.

In adding the conductive component, I find it advantageous to work roughly with the folloqing percentage composition lron l0 20% by weight Silica 35 60% by weight Alumina 5 l5% by weight Calcia l0 15% by weight The balance to make I00 percent, where indicated, will consist of secondary percentages of compounds of bound iron, magnesia, sulfur, carbon, phosphorus, manganese, titanium, sodium, potassium, and lead. The three last mentioned, in particular, may be added to depress the melting point of the matrix, in quantities of about 5 15 percent.

The flotation mud described in Example 1 has a melting range below 1,200C. it may be convenient to work at still lower temperatures, and this becomes possible when additional melting point depressant substances are added, such as a substance selected from the following class: oxides, hydroxides and carbonates of lead and of alkali metals added in amounts of 4 35 percent by weight to the said flotation muds.

In some instances, particularly when the desired form of an article renders fusing undesirable, we may employ drying with subsequent compression to achieve form stability. Binding agents such as silica dispersions may be employed to this end.

Having thus described my invention, 1 claim:

1. A method of making an electrical heating element which comprises the combination of the steps of:

providing a tailings mud comprising by weight on a dry basis 40 to percent of silica, 10 to 20 percent of alumina and ID to 20 percent of calcia, and mixing said tailings mud with from 4 to 30 percent by volume of particles selected from iron and steel, forming said mixture into a pre-selected shape,

firing said pre-sel'ected shape at from about 940 C.

to about l,200 C. to cause said mixture to coalesce,

cooling and resultant article,

abrading portions of the article to which electrodes are to be attached, and

attaching electrodes to said abraded areas to provide said electrical heating element.

2. The electrical element of claim 1, said element having the form of a flat panel.

3. The product of the process of claim I.

4. The product of the process of claim 2.

i 1 I i i

Claims (4)

1. A METHOD OF MAKING AN ELECTRICAL HEATING ELEMENT WHICH COMPRISES THE COMBINATION OF THE STEPS OF: PROVIDING A TAILINGS MUD COMPRISING BY WEIGHT ON A DRY BASIS 40 TO 70 PERCENT OF SILICA, 10 TO 20 PERCENT OF ALUMINA AND 10 TO 20 PERCENT OF CALCIA; AND MIXING SAID TAILINGS MUD WITH FROM 4 TO 30 PERCENT BY VOLUME OF PARTICLES SELECTED FROM IRON AND STEEL, FORMING SAID MIXTURE INTO A PHE-SELECTED SHAPE, FIRING SAID PHR-SELECTED SHAPE AT FROM ABOUT940*C. TO ABOUT 1,200*C. TO CAUSE SAID MIXTURE TO COALESCE, COOLING AND RESULTANT ARTICLE, ABRADING PORTIONS OF THE ARTICLE TO WHICH ELECTRODES ARE TO BE ATTACHED, AND ATTACHING ELECTRODES TO SAID ABRADED AREAS TO PROVIDE SAID ELECTRICAL HEATING ELEMENT.

2. The electrical element of claim 1, said element having the form of a flat panel.

3. The product of the process of claim 1.

4. The product of the process of claim 2.

Images