While until recently the ordinary length of a rotary kiln was 60 feet, it was realized that, especially for wet mixtures, a greater length was economical, and recently a much advertised plant has installed, among several other innovations, kilns 150 feet long, and these have shown such results that just now a period of evolution to kilns of greater length seems to be fairly inaugurated. While the results at different mills vary greatly, and it is not easy to give a correct estimate of output and fuel consumption, since these vary with the character of the raw materials and fuel as well as with the details of the kiln, it may be said that with the dry process using cement rock and limestone a 60-foot kiln will give an output of about 200 barrels per day with a fuel consumption of 100 pounds of coal per barrel, while a kiln 150 feet long may give 300 barrels per day with only about 75 pounds of coal per barrel. With wet mixtures an output of 100 barrels per day may require as much as 200 pounds of coal per barrel with a 60-foot kiln, more than half of the coal being required to evaporate the water in the mix. The advantage of the longer kiln in this case would probably be still greater. Dr. Richards found from tests of a 60-foot kiln in a dry process plant that of the 110 pounds of coal required to burn a barrel of cement the heat from about 80 pounds was carried in the waste gases, half of which was due to excess of air introduced, while the heat from 12 pounds remained in the hot clinker issuing from the kiln, and the equivalent of 14 pounds of fuel was lost in radiation. The keen competition in cement making, and the occasional low prices prevailing, have led to efforts to utilize the heat contained in the waste gases. With a longer kiln the waste gases are expelled at a much lower temperature.

26. Advantages Of The Rotary Kiln

A comparison of the average output of the several styles of kilns described above, and the approximate fuel consumption, are given in the following table. Where it is necessary to dry the materials before introducing them into the burning kiln, the fuel required in drying is not included.

Fuel Required For Various Kilns

Although, as seen from the above table, the burning of cement in a rotary kiln of the ordinary length requires a larger fuel consumption than with some other types, the ability to use a cheaper form of fuel and the saving in the amount of labor required, much more than offset this disadvantage. Since either wet or dry materials may be fed into the kiln, the necessity of forming the slurry into bricks, drying and stacking them in the kiln, is eliminated, and it is possible to so arrange a plant that the material is handled entirely by machinery from the raw state to the finished product. In fact, the conveying and elevating devices in a cement plant form a most important part of its equipment, and the mechanical problems in cement manufacture are given, in one sense, more careful attention than the chemical questions. The control possible in burning with the rotary is much better than with fixed kilns, since the clinker may be watched while burning and the temperature may be gaged according to its appearance. The fact that the pieces of clinker are much smaller with the rotary process also contributes to uniform burning as well as to greater ease in grinding. The remarkable development of the Portland cement industry in the United States is due in no small measure to the adoption and perfection of the rotary kiln, for the labor expense in manufacture is so reduced thereby that we are able to successfully compete with cements made abroad where lower wages prevail.

27. Grinding

In grinding it is not sufficient that the cement be so reduced that a certain percentage of it will pass a sieve having, say, 10,000 holes per square inch; but it is desired that as large a proportion as possible shall be of the finest floury nature. To accomplish this result it has been claimed that French buhr millstones are the best, but their great consumption of power has led to the introduction of other forms of grinding machinery, so that at present millstones find their chief use in natural cement manufacture.

It is usually considered that the greatest economy results from a gradual reduction of the clinker as it passes from one form of grinder to another, each machine being supplied with the size of pieces it is best adapted to handle. Large pieces of clinker are first passed through an ordinary rock crusher, such as the Gates or Blake. Where rotary kilns are in use, this step in the process may be omitted, as the clinker comes from the kiln in small, nut-like pieces, and the grinding is completed in two stages. The first reduction may be accomplished in ball mills, Kominuters. Kent mills, Disintegrators or rolls, while the final grinding may be done in tube mills, Griffin mills, rolls or Huntington mills.