Although rolls have not as yet been extensively employed for cement grinding, one mill has recently been equipped entirely with rolls for both coarse and fine grinding, and in other mills rolls are employed for a first reduction of the clinker. The rolls are 24 to 30 inches in diameter with 16 to 18 inch face. The shells may be of forged steel, chrome steel, or chilled cast iron, with beveled edges to prevent breaking the corners. The distance between the rolls varies with the fineness of the clinker and of the finished product; the feed is regulated by an automatic device. The rolls are sometimes mounted in sets of three for double passage of the material, only one roll being actuated, the others being driven by friction, a peripheric speed of about six feet per second being maintained. The three-roll fine grinders in the Edison Portland Cement Mill at New Village, N. J., have rolls 28 inches diameter and 8-inch face. The rolls are held together with a pressure of about 9 tons per square inch, this pressure being obtained by an endless rope passing over loose sheaves on the ends of the outer rolls at each side of the machine; a sheave in the bight of this endless rope is attached to a piston working in a cylinder to which compressed air may be admitted. This relieves the bearings of all pressure except the weight of the rolls. These machines have a large power consumption, but give a correspondingly large output.
This class, including all the machines which crush the material by impact, may be briefly described here, although seldom employed in the manufacture of Portland cement. Under this head may be mentioned the Williams Crusher, the Raymond Pulverizers, and the Stedman Disintegrator. The Williams Mill consists of a series of hammers hinged to a horizontal rotating shaft, the whole being inclosed in a case. The grinding is accomplished by the rapid blows of the hammers upon the material fed into the case.
In the Raymond Automatic Pulverizer large square blades are attached to discs keyed to a horizontal shaft that is revolved rapidly. The blades are set at such an angle with the shaft that the material is thrown alternately to right and left, the pulverizing being the result of the impact of the particles against each other and against the walls of the inclosing chamber.
The Stedman Disintegrator consists of a series of circular cages composed of steel bars, and fitting one within another. The cages are mounted on a horizontal shaft, and are arranged to revolve, two right-handed and two left-handed. The material being fed to the inside of the central cage is thrown out to the bars of the second cage moving in the opposite direction, and so on through the series. This mill is especially adapted to materials not so hard as Portland cement clinker.
32. The grinding of the clinker, being an important and expensive part of the process of cement making, has been the subject of much experimentation on the part of the manufacturers. That they have not all arrived at the same conclusion is partly due to variations in the character of the material. A high-limed hard-burned clinker is harder to grind than one of higher silica and alumina content which is burned at a somewhat lower temperature. The fineness requirements in cement specifications have been constantly increasing, and manufacturers are vying with each other to produce a finely ground product without incurring prohibitory expenses in the grinding. While the reduction from clinker to finished cement was formerly sometimes accomplished in one machine, the one-stage reduction process, nearly all plants now pass the material through two mills, usually of different styles. Ball mills for first reduction and tube mills for final reduction probably now form the most common combination. The user of cement is interested chiefly in having a large percentage of the finest material present, and it has been found that in this regard there is little to choose between several of the best types.
This product, which is also called silica cement, is composed of Portland cement and silicious sand mixed in any desired proportion and then ground to extreme fineness» This product is placed on the market by dealers, but rights to use the process may be purchased. In the construction of Lock and Dam No. 2, Mississippi River, between Minneapolis and St. Paul, Major F. V. Abbot 1 used the process; grinding with a tube mill one part of Portland cement with one part fine sand. The Cost, exclusive of plant, is estimated as follows: —
1/2 barrel of Portland cement at $2.85........$1.42
1/2 " " sand at .05...............03
Cost of grinding................. .50
Cost of royalty.................. .05
Cost of one barrel Silica cement.........$2.00