What is Mica ?
Mica is a natural valuable mineral product applied to a group of a complex aluminosilicate minerals having a sheet or plate like structure with different chemical compositions and physical properties. All micas form flat six sided monoclinical crystals with a remarkable cleavage in the direction of the large surfaces, which permits them to split easily into optically flat films, as thin as one micron in thickness. When split into thin films, they remain tough and elastic even at high temperature.
What makes micas so important to command such a unique position ? Because it had been gifted with exceptional values. It possesses some of the most outstanding combination of chemical, physical, electrical, thermal and mechanical properties which are not found in any other product.
The ordinary mica crystals as they come out of a mine is in form of rough books or lumps of irregular shape, size and thickness, associated with impurities and structural imperfections. They have to undergo a long operation of cutting, sorting and processing from crude to commercial quality. The waste resulting from the production of crude to commercially useful sheet mica forms about 90 per cent Some times even the total output of mica from a mine is sold as scrap mica and it can be used as Mica Scrap or can be used for Grinding Mica Powder.
No machine has yet been designed to process mica mechanically. The processing technique, therefore, involves only a pair of hands, eyes and a simple knife. The operations performed consists of cobbing or cleaning the crude crystals as extracted from the mine with its associated impurities; rifting or splitting the cobbed mica into usable sheets by sickle and then by sharp knife removing major flaws and structural imperfections, such as cracks, holes, reeves, crossgrains and other physical defects to obtain the maximum usable area with minimum wastage. The trimming of mica is a skilled hand-operation in which cracks and imperfections are removed at the edges of the sheets using a knife while saving the best area of usable sheet. The knife-dressed sheet mica of irregular polygonal shapes is finally graded into different standard sizes and commercial qualities before they are offered for marketing.
For commercial and industrial purpose natural mica are mainly divided into three catagories,viz :
1. Processed Mica
2. Fabricated Mica
3. Manufactured Mica.
Quality grading of mica
The quality classification of mica, is based visual tests, depends on individual opinion. Also products of different mines vary in physical characteristics to such an extent that the development of a single standard with reasonable limits of tolerance becomes an acutely dificult task. Added to these difficulties are the facts that quality clasification has to be carried out by a large number of individual workers, and that it is highly influenced by such circumstantial factors as lighting conditions, mood of the inspector, etc. Under these circumstances any standard for quality classification of mica, such as the one described in this standard can give at best an approximate idea of a particular quality and its relative position with respect to other qualities. As such, the quality of mica depends upon the and the seller who have to work out a basis on mutual understanding and peculier requirements of the industrial applications. The application of common sense to the task of procurement is therefore always helpful especially so in the matter of choosing a right supplier as only an experienced supplier knowing the individual buyer’s specific requirements, is able to deliver to the near correct material. In fact, it is an expert’s job to process and assort mica to suit to individual buyers requirements and maintain uniform and satisfactory supplies on a regular basis.
Technical data of mica
Physically Mica is transparent, optically flat, easily splittable into thin films along its cleavage, colourless in thin sheets, resillient and incompressible.
Chemically Mica is a complex hydrous silicate of aluminium containing potassium, magnesium, iron, sodium fluorine and/or lithium and also traces of several other elements, It is stable and completely inert to the action of water, acids (except hydro-fluoric and concentrated sulphuric), alkalies, conventional solvents, oils and is virtually unaffected by atmospheric action.
Electrically Mica has the unique combination of great dielectric strength, uniform dielectric constant and capacitance stability, low power loss (high Q factor), high electrical resistivity and low temperature co-efficient and capacitance. It is noted for its resistance to arc and corona discharge with no permanent injury, and has little or no effect when exposed to electronic radiation dosages upto 1018 nvt. Thus maintaining its a-c dielectronic material when exposed to the oxidising action of electrical discharges either on the surface or within internal gas spaces in built-up mica insulation.
Thermally Mica is fireproof, infusable, incombustible and non-flammable and can resist temperatures of 600° C to 900° C (1112° F to 1652° F) depending on the type of mica. It has low heat conductivity, excellent thermal stability and may be exposed to high temperatures without noticeable effect.
Mechanically Mica is relatibility soft and can be hand-cut, machined or die-punched. It is flexible, elastic and tough, having high tensile strength. It can withstand great mechanical pressure perpendicular to plane but the lamination have weak cleavage and can be easily split into very thin leaves.