Mica production methods

The general pieces of equipment used in grinding flake mica or mica concentrate into saleable mica products are hammer mills of various types, fluid energy mills, Chaser or Muller mills for wet grinding, and Raymond or WiUiams high side roUer mills. Another method is being developed, called a Duncan mill (f. M. Huber, Inc.), that is similar in many respects to an attrition mill. AH of these mills are used in conjunction with sieves, and all but some types of hammer mills incorporate air classifiers as a part of the circuit. 

TABLE 2.10 ASTM Specifications and Methods of Test for Mica and Mica Products... 

The bomb method for sulfur determination (ASTM D129) uses sample combustion in oxygen and conversion of the sulfur to barium sulfate, which is determined by mass. This method is suitable for samples containing 0.1 to 5.0% w/w sulfur and can be used for most low-volatility petroleum products. Elements that produce residues insoluble in hydrochloric acid interfere with this method this includes aluminum, calcium, iron, lead, and silicon, plus minerals such as asbestos, mica, and silica, and an alternative method (ASTM D1552) is preferred. This method describes three procedures the sample is first pyrolyzed in either an induction furnace or a resistance furnace the sulfur is then converted to sulfur dioxide, and the sulfur dioxide is either titrated with potassium iodate-starch reagent or is analyzed by infrared spectroscopy. This method is generally suitable for samples containing from 0.06 to 8.0% w/w sulfur that distill at temperatures above 177°C (351°F). 

These methods are the principal ones used in the production of fillers based on calcium carbonate and dolomite, clays, talcs, micas and wollastonite. These fillers, on a volume basis, dominate the industry. 

The production of mica for polymer applications has been reviewed by Hawley [89]. The aim of the processing is to purify the deposit and to produce particles of relatively small diameter with an aspect ratio of 50-200. The natural minerals are generally of much larger size than required and so the milling has both to delaminate and fracture the particles. The milling is the key process and a variety of methods, both wet and dry, are used, accompanied by various classification methods. Surface modification is important in many mica applications and a variety of treatments are used, especially organo-silanes. The methods of treatment are generally not disclosed. 

Dry ground mica concentrate is processed into usable products hy several dillerent grinding methods. Relatively coarse particle sizes (1.651-0.147 mm (10-100 mesh)) are used in oil-well drilling muds. 

In most techniques for studying adsorption on metals, xmiform, clean, and reproducible metal surfaces are difficult to prepare and the adsorption process cannot be followed continuously [2, 3,4,7,10,11,16, 18,2l]. Clean and reproducible metal surfaces are also difficult to prepare and maintain in methods that measure adsorption continuously and directly on a metal-coated window of a Geiger tube [l, 6,7,13]. A recently developed apparatus and technique provide controlled conditions for the production and maintenance of relatively clean metal films and the precise measurement of adsorption [20j. Metal is evaporated onto a mica window supported within a high-vacuum apparatus adsorption onto the metal film is measured directly and continuously by a counter tube below the window. 

Clay and mineral fillers have been used for reducing production costs and improving the comprehensive water absorbing properties of superabsorbent materials For example, a poly(acrylic acid)/mica superabsorbent has been synthesized with water absorbency higher than 1100 g H20/g In a typical method of preparation, acrybc acid monomer is neutralized at ambient temperature with an amount of aqueous sodium hydroxide solution to achieve 65% neutralization (optimum) Dry ultrafine (<0.2 tm) mica powder (10 wt%) is added, followed by cross-linker N,N-methylene-bisacrylamide (0.10 wt%) and radical initiator, potassium persulfate The mixture is heated to 60-70°C in a water bath for 4 h. The product is washed, dried under vacuum at 50°C, and screened. 

Quantitative predictions of the effects of fillers on the properties of the final product are difficult to make, considering that they also depend on the method of manufacture, which controls the dispersion and orientation of the filler and its distribution in the final part. Short-fiber- and flake-filled thermoplastics are usually anisotropic products with variable aspect ratio distribution and orientation varying across the thickness of a molded part. The situation becomes more complex if one considers anisotropy, not only in the macroscopic composite but also in the matrix (as a result of molecular orientation) and in the filler itself (e.g., graphite and aramid fibers and mica fiakes have directional properties). Thus, thermoplastic composites are not always amenable to rigorous analytical treatments, in contrast to continuous thermoset composites, which usually have controlled macrostructures and reinforcement orientation [8, 17]. 

Mica particle size has significant effects on most composite properties. Table 14.23 shows these effects for a 30 wt% filled polypropylene copolymer composite. It is not possible to get good trends from this table based on particle size only. These products were prepared by different methods including wet grinding, dry grinding, and screening of spiral mica. These production processes provide different degrees of and different aspect ratios for particles of similar diameter. 

The aspect ratio of the final flake products depends upon the origin of the mica and the processing method. Large flake pegmatite mica and wet milled mica can have aspect ratios in excess of 100. By contrast, processed mica flakes from schist and kaolin deposits tend to have aspect ratios in the area of 30 to 60. 

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