The iron content of asbestos affects important properties such as the electrical resistance of electrical insulation; because the iron content of asbestos varies by type of asbestos selected, this property is controlled for special applications.
This article discusses the iron content of various forms of asbestos. This article series describes the physical properties of asbestos including its mechanical, chemical, electrical and related properties both in pure asbestos form and when asbestos is mixed with other materials like cement or rubber.
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The iron content of chrysotile asbestos may be expressed a number of different ways, some of which are factual, and as such serve to reveal the over-all characteristics of an asbestos fiber so contaminated; however, other expres- sions are mere indications of possible chemical relationships. They offer a convenient means of representing the presence of some compounds of iron but which do not necessarily give any indication as to the true identity of the compound or the properties which may be imparted as a result of its presence.
The varieties of iron oxide which occur most frequently in nature are ferric oxide (Fe203) and ferroso-ferric oxide (Fe304) , the latter being mineralogically identified as mag- netite. The element, Fe, is rarely found in natural form, be- cause of its readily oxidizable character; similarly, FeO is a rarity in nature; usually it is the product of man's efforts to produce such a compound, or it occurs as a by-product of a metallurgical process.
The only iron compounds that directly concern the manu- facturers of asbestos products are Fe203, Fe304, the silicates of iron and perhaps some varieties of the hydrated iron oxide minerals. Of these, only Fe3O4, has any marked influ- ence on the electrical properties of products containing such impurities.
* Shaw, M. C. Asbestos Textile Institute Phila., Penn.
It is important to point out the significance of a proper and accurate understanding with regard to the state of the iron which actually exists in the materials under investiga- tion. An analysis report in which the Fe is mentioned as total iron, without respect to the state or valence, means . nothing. Similarly, to present the same information in terms of FeO or Fe203 is equally useless, if these figures are mere calculations based upon the total Fe content without respect to the state of the Fe.
In such asbestos textile products as cloth, lap, roving and paper, which are intended for use as electrical insulators, the presence of Fe3O4 is objectionable. Such particles are electrical conductors and may establish points of electrical weakness in materials in which they are found. The remaining varieties of the aforë-mentionediron bearing compounds exert only a minor influence insofar as electrical properties are concerned; however, occasionally they do influence the color differences which may occur between the several vaneties of chrysotile found throughout the world.
Perhaps the most striking failure of a chemical analysis ; to portray accurately the presence of, and the qualities to be imparted by, Fe3O4 in particular, and the other varieties in general, is its inability to take into account tho factor of grain size. The grain size and distribution of Fe3O4 are as important as the amount present. A minority of largegrains of these impurities, well dispersed throughout an asbestos product, can prove to be much more troublesome than the same amount of extremely fine grained impurities.
Various methods and devices for determining the relative magnetic iron content of asbestos have been investigated. The results indicated quite clearly that the Mapes Analyzer ASTM method provides the most significant and reproducible information, provided that the proper techniques I are adopted in making such determinations (ASTM D1118- 50T). However, it has also been established that the results of such tests must be given proper interpretations if the in-
formation is to serve any useful purpose. The grain sizes of the magnetite inclusions and the orientation of the grains influence the results to a degree that tends to defy correlation.
The Mapes Analyzer does, however, serve a useful purpose in providing rough approximations of the magnetic iron content of a given material; when the results so obtained are judiciously evaluated, in thelight of other contributing factors, information of significant value can be ascertained.
In order to substantiate the belief that grain size does markedly influence magnetic properties, a sample of known magnetic material was fractionated into four divisions of grain size; 0.5 gram of each fraction was thoroughly distributed in equal amounts of a known inert material, ZnO. Magnetic rating determinations were made on each sample; the results of this series of tests are given in Table 2.4.
TABLE 2.4. MAGNETIC RATINGS OF ASBESTOS SAMPLES
On 35 mesh 4.40 MR * Through 35 mesh on 80 mesh 3.70 MR Through 80 mesh on 200 mesh 3.50 MR Through 200 mesh 3.05 M}
* MR = Mapes Ratings
Magnetite is an oxide of iron which, under the proper conditions, can be further oxidized and thus relieved of its magnetic properties. Elevated temperatures, with an excess of air, effectively provide the necessary conditions for such oxidation. For example, magnetite heated to 420°F retains only 80 per cent of the specific magnetism 'held at room temperature, at 1,000°F less than 50 per cent is retained, and at 1,080°F the magnetic properties are eliminated' through complete oxidation and the conversion of Fe3O4 to Fe2O3.
To demonstrate this property, samples of National Bu-' of Standards' normal magnetite were heated to three
different temperatures and the MR value for the result- ing materials determined. These results are indicated in Table 2.5.
TABLE 2.5. MAGNETIC RATING VS. TEMPERATURE
Temperature MR Room 7.8 500F 7.8 800°F 42 1,2OOF 0.1
Electrical Properties of Asbestos
The electrical characteristics and properties of asbestos are important when these particular types of products are used to manufacture such electrical units as asbestos wire insulation.
The low magnetic iron content chrysotile fibers are used in these type applications.
Also see the practical examples discussed at ASBESTOS PLASTIC ELECTRICAL PROPERTIES
Continue reading at ASBESTOS MECHANICAL PROPERTIES or select a topic from the More Reading links shown below.
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