Marl and clay.................. 14 "
Soft limestone and clay.............. 3 "
Hard limestone and clay.............. 28 "
The essentials of any method of Portland cement manufacture are that the materials shall be correctly proportioned, very finely comminuted and thoroughly mixed; that the mixture shall be carefully burned to just the proper degree of calcination, and the resulting clinker ground to extreme fineness. How these essentials can be best accomplished depends upon the character of the raw materials and the Cost of fuel and labor, so that the details of the method vary with the materials used and with the local conditions.
In order that the proportions may be accurately determined, it is usually necessary to dry one or both of the raw materials. The ingredients may be ground separately and afterward mixed, though with certain materials the grinding and mixing may be done at the same time. In this mixing, a large amount of water may be used, giving a thin slurry, as in the wet process; a moderate amount of water may be employed, as in the semi-dry process, giving a mix of the consistency of puddled clay; or the dry process may be employed, in which after the materials are once dried no water is added before the burning of the clinker. The burning may be accomplished in any one of several styles of kilns, although the advantages of the rotary kiln are so great that it has nearly superseded all other styles in this country. The grinding is a simple mechanical problem, to secure the required degree of fineness with least Cost.
This method of manufacture is especially adapted to such materials as cement rock, limestone, and shale, that must be ground before they can be mixed, although it is occasionally employed for marl and clay. In order that the proper chemical combinations shall take place in the kiln it is essential that the material shall be finely ground as well as thoroughly mixed. The rock is obtained by the ordinary quarry methods and is usually conveyed to the plant in cars which dump into the hoppers of large rock crushers. From these the crushed rock is passed to driers, usually of the rotary type. The drying of the raw materials serves two purposes — it prepares the rock for further reduction in mills in which wet material would tend to clog, and it facilitates the proper weighing of the two ingredients to form the mixture.
1 "Cements, Limes and Plasters," Wiley and Sons, New York, 1905.
After drying, the two raw materials may be combined, and the continuation of the process is on the mix so formed. This, however, is not the universal practice; some manufacturers combine the two ingredients even before crushing, others mix them before they pass to the drier, while others keep them separate until after the second or third stage of the preliminary grinding. The second stage of the grinding is frequently accomplished in ball mills or in kominuters, although rolls and grinders of the impact type are also employed. The final mixing and grinding preliminary to burning is usually done in tube mills, this type being considered most efficient for fine materials.
From the tube mills the mix passes directly to the rotary kilns for burning. As the clinker falls from the lower end of the rotary, a small stream of water is allowed to trickle over it. This partially cools the clinker, renders it somewhat easier of reduction, and probably makes the cement somewhat slower setting. After further cooling, either in coolers or in sheds, the clinker is ground, usually by two or more stages, ball mills or rolls being frequently used for the first reduction, and tube mills or Griffin mills for the final grinding. Before the grinding is completed a proportion of calcium sulphate is added to regulate the rate of setting of the cement. The ground cement is stored in large bins and packed as desired in cloth or paper sacks, or in barrels, for shipment.
Although water may be used to facilitate the mixing of materials that require previous grinding, the wet process is particularly adapted to such raw materials as are easily acted upon by water. The present types of grinding machinery are such that it is no longer necessary to add water to assist in the mixing and grinding, and the use of the wet process is now confined almost entirely to the treatment of the softer raw materials, marl and clay, which, as they occur in nature, contain a considerable percentage of water. This contained water constitutes, from an economic standpoint, one of the serious problems in treating this class of materials, because of the expense involved in evaporating it at some stage of the process; it is, however, not without its advantages. Aside from the fact that the mixing is more readily accomplished in the wet state, the grinding is comparatively easy, the slurry may be pumped from point to point instead of being carried and elevated, and the correction of the mix, by adding the desired amount of over-clayed or over-limed slurry, is more readily accomplished than is the case with the dry process.
The marl and clay are commonly loaded in cars by dredges or steam shovels and drawn to the works by locomotive, cable or horses. The stones and sticks are removed from the marl by separators if necessary, and as a first step the clay is usually dried and ground to facilitate the calculation of the mix. The marl is sometimes passed through a pug mill before mixing with the clay, though more frequently the weighed amount of dried clay is added during the first pugging. Here sufficient water is added so that the slurry may be pumped to tube mills in which the final grinding of the raw materials is usually accomplished. Either just before or just after passing the tube mill the slurry is caught in vats where it is kept agitated either by compressed air or mechanical agitators. Here the mix is analyzed and corrected if need be by adding the proper amount of an over-clayed or over-limed slurry kept in separate tanks for this purpose. As the slurry is pumped to the rotaries it usually contains as much as fifty per cent, water, which must be evaporated in the upper end of the kiln by the gases from the burning which is taking place in the lower half.