Many plants depend largely upon a local market, and there has not been the incentive to improvement that comes from sharp competition in a broader field, and consequently the rule of thumb methods, handed down from the early operation of the plant, are still employed. To select the rock with great care and burn each stratum at the temperature required to develop the best results, as determined by experiments on a practical scale, would undoubtedly improve the product. As this would call for the installation of rotary kilns and a higher Cost of production throughout, it is usually considered that it would not be profitable, but this has not been established.
The calcined rock is conveyed first to some sort of stone crusher; a common form is known as a "pot-cracker/' and consists of a corrugated conical shell in which works a cast iron core, also corrugated. The elements of the shell and core have a smaller angle to the vertical in the lower part of the mill, providing for gradual reduction and greater power on the smaller particles. The Berthelet and McEntre crackers are mills of this general type. A small cracker, about 15 inches diameter and 18 inches high, working as a coarse grinder only, will pass 250 to 300 barrels per day.
The ordinary buhr millstones have been used from the inception of the natural cement industry and are still very generally employed. The " rock-emery " mill, a stone in which a layer of harder composition is set about the exterior face, has replaced the older buhr mills in some plants, as it requires less frequent dressing. While ball mills, Williams crushers, Stedman disintegrators, and dry pans have been introduced to some extent in natural cement plants, tube mills have, perhaps, proved the most efficient for this work of any of the modern types of reducers. These are described in Article 6 under Portland cement manufacture. When, as is now common practice, the reduction is accomplished in several stages, the fine material or finished product is separated after each stage, the power consumption for a given output being reduced thereby, although the proportion of extremely fine dust is probably also diminished. Mr. Berthelet of the Milwaukee Cement Company has perfected a screening system for this purpose that has proved very successful, and has been installed in other cement plants.
After passing the grinding mills the product is sometimes conveyed to "mixers" by means of which the cement is thoroughly mixed to promote uniformity. It is now ready for packing, and may be placed in bins or sent direct to the chute from which the bags or barrels are filled. Most natural cement is packed in paper or cloth sacks, as barrels add too large a proportionate Cost to the product.
It is seen that the manufacture of natural cement is very similar to that portion of Portland cement manufacture succeeding the preparation of the raw material for burning. In general, less care is requisite with natural cement, the burning is carried on at a lower temperature, and the calcined rock is softer, so that less expense is incurred in grinding.