Method op treating magnesia and electrical insulating
US 2280516 A
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P" 1942- R. R. RIDGWAY' EIAL 2,280,516
METHOD OF TREATING. MAGNESIA AND ELECTRlCAL INSULATING MATERIAL DERIVED THEREFROM Filed Oct. 27, 1939 3nvento: and Raymond R.R'Ldgwag BB AYch1baIdH.Ba1lard witness (Ittomeg Havbexc E. Covey' Patented Apr. 21, 1942 METHOD OF TREATING MAGNESIA AND ELECTRICAL INSULATING MATERIAL DE- RIVED THEREFROM Raymond R. Ridgway, Niagara Falls, N. Y., and Archibald H. Ballard, Niagara Falls, Ontario, Canada, assignors to Norton Company, Worcester, Mass, a corporation of Massachusetts Application October 27, 1939,-Serial No. 301,643
This invention relates to a method of improving the electrical insulating properties of commercially available magnesia and to an insulating material produced thereby.
Owing to its highly refractive nature and its property of being a non-conductor at temperatures in excess of 1000 C., crystallized magnesia, or periclase,,is well adapted for use as an electrical insulation in high temperature electrical apparatus. This material is particularly useful in electrical resistance units comprising-a resistance wire embedded in and'separated from a metal sheath by the magnesia, such as is used in electric stoves. Owing to the requirement that the insulating material possess a high specific resistance so as to minimize the thickness of the insulation and the attendant difference in temperature between the resistor wire and sheath, various attempts have been made to improve its electrical resistivity, such as by heating the material to a high temperature and slowly cooling it. Nevertheless, it has been found that the heating units deteriorate with use due to a grad ual loss of insulating properties of the magnesia.
The commercial crystalline magnesia obtained from the available ores contains various im-v purities including iron oxide, lime and silica. We have discovered that this decrease of resistivity is due to the presence of iron oxide, and that an amount in excess of 0.2% by weight of iron oxide in magnesia causes a marked and rapid deterioration of the unit during use. We believe that this is due to the reduction of an external phase of iron oxide to the metallic and conductive form by the hydrogen developed by electrolysis of abnesia which has such physical and chemical characteristics that its resistivity will not deteriorate to a material and serious extent before it has given a long life of useful service.
A further object of the invention is to provide a method Pf making crystalline magnesia from h commercially available materials and so treating the same that contaminating impurities will not be highly, detrimental and pariicularly so that the iron oxide will be rendered substantially innocuous when the magnesia is used as an electrical insulation. Further objects will be apparent in the following disclosure.
In accordance with this invention we have provided an electrical insulating material for various types of electric heating units. The accompanying drawing is a view, partly broken away, of one form of heating unit embodying this invention. The structure comprises a helically coiled metal resistance wire I0 located centrally within a metal sheath or casing l l and supported by the special insulating material l2 as herein described. An electric terminal I3 may be suitably secured to the resistance wire at each end thereof and similarly insulated from the casing by the material I2. Any suitable structure may, however, be employed.
We have discovered that if the impure magnesia is roasted under oxidizing conditions to insure that the iron content is present as ferric oxide, and if the material is allowed to cool slowly in mufiles or kilns then the final product will show a more or less yellow to brown coloration which is related to the oxidized iron content of v the product.
this product shows that the magnesia is coated Careful microscopic inspection of with tiny crystals of iron oxide which occur along the fissures and is distributed over the surfaces of themagnesia grains. These almost cryptocrystalline iron oxide crystals, therefore, exist as a free phase which is largely external, and it is this iron-containing phase characteristically distributed over the surfaces of the magnesia crystals which imparts a uniform coloration to the product. The extent of the brown coloration which shows in a grain containing definite but small amounts of iron is proportional to the slowness with which the product has cooled from the high temperature.
We found that ifthis product was cooled rapidly, this brown color wholly disappeared or the material became lighter in color, and that the higher the temperature to which the grain was heated and the quicker it was cooled from this high temperature, the lighter became the color of the resultant refractory powder of magnesia. If a product containing as much as 0.5% by weight of iron oxide was heated to 1400 C. for 24 hours and then suddenly quenched, the resultant grain appeared to be free from iron as judged from the coloring, since it was practically white. A chemical and microscopic investigation showed that the material which had been quenched from 1400? C. contained all of the original ferric oxide, but that this oxide was now absorbed into and uniformly distributed throughout the magnesia crystals. There were no longer the characteristic well developed crystals of a phase rich in iron oxide appearing on the surface and throughout the grain boundaries We have assumed, without proof, that by holding the magnesia grain at a high temperature for a long time we have caused the ferric oxide to combine with the magnesia crystals in some form. A possible explanation is that a spinel of the composition of MgO.FezO3 is formed by the prolonged heating, and that this spinel is isomorphous with the magnesia crystals and so enters into solid solution therewith and results in a uniform distribution of iron throughout the crystalline mag. nesia phase.
When these two types of material, the slowly cooled and the quenched magnesia, were subjected to electrical resistance tests, we found'that this difference in physical distribution of the iron resulted in altogether different resistance characteristics. That is, the heated and quenched material showed a higher initial resistance than the other. Also, this procedure greatly retarded the rate at which such iron rich products lost their insulating qualities and especially in the presence of absorbed moisture.
Hence,- it is not necessary to purify the magnesia to the extent of eliminating all of its iron content, since the insulating properties of the material may be greatly enhanced and the useful life thereof prolonged by so treating the material as to insure that the iron content is present in the oxidized condition but is absorbed or dissolved in or dispersed throughout the ma nesia crystal and does not exist as a free phase external of the individual magnesia crystals where it may be reduced to iron or otherwise develop conducting paths between the resistance wire and the surrounding metallic sheath of a heating unit. Thus, we may use a lower grade of magnesia, i. e., which has a higher iron content, and still produce a product having the characteristics of a substantially pure magnesia.
In accordance with this discovery We, therefore,
so treat crystallined magnesia as to insure that the iron content thereof is in the oxide form, and preferably ferric oxide, and We heat the material to a proper temperature, such as from 1200 to 1400 0., and for a suitable length of time, preferl ably in excess of 24 hours, and ordinarily from 24 to 72 hours, so as to insure that the iron oxide becomes gradually absorbed into the magnesia crystal grain. This term absorbedis used broadly to define the location and condition of the iron oxide, without reference to any particular theory as to what happens during the heating stage. When this condition has been attained, then the product is cooled rapidly from its high temperature to a comparatively low temperature, so as to insure that the iron oxide will not sepa-. rate out as a separate, distinct and external crystalline phase but will remain absorbed within the magnesia crystal, as by combination with the magnesia or by a physical dispersion throughout the magnesia. During this heating stage, the fine sizes of magnesia are recrystallized with the development of larger crystals, and the electrical properties of the material are improved.
This rapid cooling or quenching operation may be accomplished by dumping the heated material into a bath of cold water; or the quenching may be accomplished by quickly spreading out the heated material as a shallow layer on the floor or in a large metal pan where it is exposed to the electric resistance unit, and so it cannot develop a metallic phase which would readily conduct the current and short circuit the resistance element with the grounded metal sheath that is normally used to cover the same,
Various sources of raw material may be uti-- lized in the preparation of this insulating magnesia, and the percentages and nature of the various impurities may vary widely. It, however, is desirable to select a material which is not highly contaminated and which preferably contains at least 95% of magnesia and not more than 0.5% of iron oxide, and preferably less than 0.2%. to 3% of silica and from 0 to 2% of calcium oxide. A magnesite ore having these characteristics is readily available. However, an orehaving a higher iron content will be beneficiated by this procedure as will be readily apparent.
The selected magnesite ore may be calcined to remove CO2 and then fused in an open top Higgins type are furnace having graphite elec-- trodes and under temperature conditions which result in the complete fusion of the material, after which it may be cooled and crystallized. The power input may be so controlled as to obtain a temperature in excess of 2600 C. which results in the bath being highly fluid and at which the iron tends to boil off, and much of it is thus removed from the bath. Standard or desired procedure may be adopted for this furnacing operation. After the material has beenpartially cooled and solidified in the furnace, the shell of the furnace is removed and the ingot is allowed to stand on the floor and further cool and crystallize. The resultant product is crystalline magnesia of the general type of the mineral periclase. Since the present-procedure renders the iron oxide harmless, it is feasible to utilize substantially all or the major portion of the magnesia ingot for the production of electrical insulation material. Accordingly, the cooled and crystallized ingot is broken up and crushed in a crusher which is preferably of the type that will not contaminate the mass seriously with iron. The material may be crushed to a suitable powdered size, such as one which will pass through a screen of 40 meshes and ,be retained on a screen of 325 meshes per linear inch, The finer the powder, the more complete will be the separationpf magnetic conless. The roasting operation is preferably carried on at a temperature above 1250" C. for at least 48 hours, During this roasting process, air
is admitted freely and oxidizing conditions are The material may also contain from 0 maintained so that any metallic iron or lower oxide of iron that may be present is thoroughly oxidized to ferric oxide. If the grain is heated in saggers, care should be taken to avoid the presence of reducing agents which might hinder or prevent the oxidizing action, Hence a reduc ing flame should not contact with the material, but a kiln heated internally with an oxidizing gas or oil flame which provides free access of air is well adapted for the process. The kiln may be lined with magnesia bricks for the protection of the product. The material is to be held at the required high temperature for a sufficient time, ordinarily over 24 hours, to insure that the iron becomes fully oxidized and then absorbed in or otherwise distributed within the magnesia crystal, whether as a solution or combined with the magnesia. The length of this soaking period depends upon the temperature maintained and the final results desired.
After the material has been sumciently heated, it is quenched so as to cause a quick cooling of the'crystal and prevent the iron oxide from separating out as a phase which is external of the magnesia crystal. This may be done, as above explained, by dumping the heated product directly from the rotary kiln onto a stone floor and quickly spreading the same into a thin layer where it is exposed fully to the external atmosphere. It is preferable to dump the material into a vat or pan carrying a suflicient quantity of cooling water to cause a rapid lowering of temperature and a stabilization of the iron oxide in its absorbed condition. It is of primary importance that the iron oxide remain absorbed in solid solution or be otherwise distributed in the magnesia crystal. It is easy to control the process of quenching, since the best results are attained when the product is in its lightest colored condition. Too slow a cooling gives a tan or light brown color which is an indication of the formation of the objectionable free iron oxide phase to even a slight extent.
The material may contain appreciable amounts of lime and silica, but preferably within the limits above defined, and without any noticeable detrimental effects because of such impurities being present. Hence, in accordance with this invention, a magnesia which contains a comparatively high content of iron, but preferably below 0.5% by weight, and which contains lime and silica may be used satisfactorily as an electrical insulation material.
For certain purposes, we prefer that the magnesia have a very low content of iron oxide and we may, therefore, use the process which is set forth and claimed in our copending application Serial No. 301,642 filed on even date herewith. In accordance with that procedure, we preferably select a magnesite ore which contains less than 0.2% of iron in the form of iron oxide and we so treat the material throughout its fusion and other procedure as to reduce that iron content as much as possible and preferably to make sure that the final product contains less than 0.1% by weight of iron oxide. We also prefer that this iron oxide be present in the highly oxidized condition of ferric oxide or that it be not readily reducible by hydrogen in the metallic form under the conditionsof usage, as above indicated. Hence, the carefully selected magnesite ore is sohandled and treated as to avoid further contamination with iron. It is fused in accordance with the procedure above specified, and in that case we preferably select only that amount of iron oxide.
portion of the ingot which contains the lesser The other part of the ingot is discarded because it is found to contain more iron than the intermediate portion, while the extreme center or pipe of the ingot should also be discarded because of the tendency for the lower melting impurities to concentrate in this zone which is the last to cool. This selected material is crushed in a way to avoid contamination with iron and then passed through a magnetic separator as above defined to eliminate further iron content. Owing to the boiling ofi of metallic iron in the electric melting operation and the oxidatiomof the iron to the higher oxide, the iron content is not only very low but it is present only in the absorbed condition within the magnesia crystal and the product has substantially no detrimental external phase of iron oxide.
The following table shows the effect on the electrical resistance of the material as caused by the quenching operation:
commercial electrical grade of periclase, or crystalline magnesia, obtainable on the market and which has not been treated in accordance with this invention. The first column gives the values of the resistance in megohms per inch cube obtained in a standardized method of testing specific resistance. The second column, Example B, gives the megohms of the resistance of material A when employed in a standard commercial electric stove heating element. The materials of Example C, in column 3 were different grades of purified crystalline magnesia, or periclase, which had been purified to reduce the iron content to less than 0.2% by weight, and the measurements of the resistance were made by the same type of apparatus as that required for Example A. As shown in column 1, the specific resistance of the standard commercial material is nearly doubled by the quenching operation. The second column, Example B, shows that the resistance of the material assembled in the heat ing unit is more than doubled by the quenching step. The materials of Example C had higher resistances in the untreated form than had the magnesia of Example A, because of the lesser iron content. It will, however, be noted that these better grades of magnesia were also materially improved by the quenching operation. It was also found in a further test that the stove heating element of Example B, when subjectedto a destructive life test, failed in approximately 1300 hours; while a stove element having the improved insulating powder of Example C has lasted for more than 3500 hours at the same temperature.
The benefits of this process are therefore ap- In the above table, Example A is a standard 0 parent, and particularly since the cost of production is lowered and the technical diificulties of producing such a metallurgical product on a large scale are materially lessened by our beneficiation of either a low grade or a purified periclase by the above method. The rate at which the electrical resistivity of the magnesia drops is materially reduced with a great increase in utility and life of service of a heating un't or other apparatus containing an insulation of magnesia subjected to this heating and quenching procedure. Owing to its being a good conductor of heat and a poor conductor of electricity at the high temperatures of 850 C. or more, it is well adapted for use in stove units and the like at higher temperatures than heretofore found feasible. The magnesia conducts radiant heat readily, and since the insulation does not deteriorate rapidly, as heretofore, it may be used in a very thin layer, such as 1 to inch, around a standard resistance wire,
which is much thinner than has been the past practice. This reduces the difference in temperature between the wire and the surrounding metal sheathing and so makes it possible to operate a given heating unit at a higher temperature than is the case where the untreated magnesia is used. Such a unit formerly limited to about 850 C. may now be used at a temperature as high as 1000 C. with good results.
While we have attempted to explain the phenomena attending this invention in the light of our present knowledge, it is to be understood that the claims are not to be considered as limited to any particular theories and that the terms thereof are to be interpreted broadly as covering the general process above set forth andthe product produced thereby. as well as such equivalent steps and compositions as will now be apparent to one skilled in the art in the light of the above disclosure. Also, since many variations may be made in the methods of procedure and in the composition of the final product, the above disclosure is to be considered as illustrating the general principles and certain preferred features of the invention and not as limitations thereon, except as the invention is defined by the claims appended hereto.
1. The method of making an electrical insulating material from crystalline magnesia contaminated with iron oxide comprising the steps of heating the crystalline material to a temperature above 1200 C. and forming a product having its iron content present substantially wholly as an oxide absorbed within the magnesia crystals and then quickly cooling the mass and thereby preventing the formation of a material amount of a separate phase of iron oxide crystals external of the magnesia crystals.
2. The method of making an insulating material from crystalline magnesia contaminated with iron oxide comprising the steps of heating the material under oxidizing conditions to a temperature above 1200 C. and forming a product having its iron content present largely as ferric oxide absorbed within the magnesia crystals and then quickly cooling the same and preventing the formation of a material amount of a separate external phase of iron oxide crystals.
3. The method of making an electrical insulating material comprising the steps of selecting a material containing at least 95%, by weight of crystalline magnesia, from 0 to 3% of silica,
from 0 to 2% of CaO and not over 0.5% by weight of iron oxide, heating the material in a crystalline granular condition under oxidizing conditions to a temperature above 1200 C. and forming a product having its iron oxide content largely absorbed within the magnesia crystals and thereafter cooling the mass rapidly and preventing the iron oxide from crystallizing materially as a separate external phase.
4. The method of making an electrical lnsu lating material from an impure magnesia comprising the steps of selecting a grade of magnesia containing not over 0.5% by weight of iron oxide, melting and crystallizing the mass, crushing the crystalline material to a powder which will pass through a screen having 40 meshes to the linear inch, heating the crystalline material to a temperature between 1200 and 1400 C. for a period of 24 to 72 hours and then quenching the heated material at a rapid rate at which iron oxide will not crystallize materially as a separate phase external of the magnesia crystals.
5. The method of making an electrical insulating material comprising the step of providing crystalline magnesia containing not over 0.2% by weight of iron oxide, not over 3% of silica and not over 2% of calcium oxide, heating the material for several hours under oxidizing conditions to a temperature as high as 1250 C. and forming a product having the iron content present as ferric oxide substantially wholly absorbed Within the magnesia crystals, and thereafter cooling the mass rapidly and preventing the iron oxide from separating out materially as a separate external crystalline phase.
6. A refractory, heat conductive, electrical resistance heating element embedding and insulating material comprising primarily granular crystalline magnesia containing an iron compound in a substantial amount of not over 0.5%
by weight, calculated as ferric oxide, wherein anyiron compound therein is present substantially wholly as an internal phase within the magnesia crystals.
7. A refractory, heat conductiv electrical resistance heating element embedding and insulating material composed chiefly of partially recrystallized, granular, crystalline magnesia containing appreciable amounts of not over 3% by weight of SiOz and not over 2% of CaO and from about 0.1% to 0.5% of an iron compound, calculated as Fezos, the total iron content being present substantially wholly as an internal phase of an iron compound absorbed within the magnesia crystals.
8. A refractory, heat conductive, electrical heating element embedding and electrically insulating granular material comprising as its major ingredient crystalline magnesia containing a substantial amount of not over 0.5% by weight of an iron compound, calculated as FezOs, and wherein the total iron content is present substantially wholly as an internal phase of an iron compound in the highest state of oxidation absorbed within the magnesia crystals.
9. A granular, refractory, heat conductive, electrical resistance heating element embedding and electrically insulating material which comprises primarily crystalline magnesia containing substantial amounts of not over 3% by weight of SiO2 and not over 2% of CaO and containing from about 0.1 to 0.5% of iron compounds, calculated as F6203, the total iron content existing substantially wholly as an internal phase formed of an iron compound in the highest state of oxidation absorbed within the magnesia crystals.
10. A granular, refractory, heat conductive, of from about 0.1 to 0.5% by weight, calculated electrical resistance heating element embedding as i'erric oxide, the iron content existing suband electrically insulating material which is comstantially wholly as an internal phase of a ferric posed chiefly of partially recrystallized, granucompound absorbed within the magnesia cryslar, crystalline magnesia containing substantial 5 tals. amounts of not over 3% of B10: and not over RAYMOND R. RIDGWAY. 2% oi CaO and containing a total iron content ARCHIBAID H. BALLARD.