The size of the ball mill employed may vary in accordance with the commercial requirements and the amount of film material to be produced at one time. Recent literature (see briefs of papers presented at the Atlantic City meeting, fall, 1941, Division of Paint, Varnish and Plastics Chemistry, published by the American Chemical Society,<a href="http://www.sellercrusher.com/project/iron-ore-mining-process.html">iron ore mining process</a> relating to ball mill grinding) indicate that a ball mill larger than 1'A gallon mill described above may be used, and if the conditions surrounding the charging such as size of ball and ratio of the ball and total charge to mill volume, and speed of rotation be kept such that the ball motion is the same (slipping, cascading, cataracting), then for all practical purposes about the same result will be obtained regardless of the size of mill utilized.
A satisfactory colloidal dispersion or slurry can also be formed without dry grinding tobacco to a fine powder although the product is not as uniform as when the two steps above described are employed. If this method be resorted to, water or other suitable fluid is added to a quantity of dry tobacco and the mixture placed in a ball mill 4 for grinding. We have found that when this mixture is ground for a period of about eight hours good results are obtained. This is greater than the minimum time necessary for complete dry grinding but less than the total time usually consumed in both dry and wet grinding. The resulting product is not so uniform, however,<a href="http://www.sellercrusher.com/project/crushing-plant-layout.html">crushing plant layout</a> due to the veins of leaves and parts of stems which resist grinding, especially when they contain any moisture in excess of five per cent and hence it is possible that the resulting product may contain minute fibrous particles which are recognizable as such and add to the strength of the film.
We have also found that a good sheet may be obtained by wet grinding whole tobacco leaves for one or more hours and then filtering the grind through a coarse filter. When such a grind has been prepared and filtered, the unground portions, consisting principally of stems and veins, are removed and the filtrate consisting of the better portions of the leaves can then be wet ground in a ball mill as above described for a suitable time. This forms a good slurry which can be extruded, spread out or centrifugally applied to suitable sheet forming surfaces or formed into filaments. The proportions of tobacco and water used in the above case runs substantially the same as when a charge of powdered tobacco and water are introduced into a ball mill.