CHAPTER VIII.

COOLING AND GRINDING THE CLINKER, STORING AND PACKING THE CEMEMT, ETC.

Cooling the Clinker.

The clinker leaves the kiln at a temperature of about 2100° F. It is, of course, entirely too hot to grind and must be cooled. It has generally been found preferable to do this mechanically instead of letting the clinker lie in heaps and cool of itself. In some mills this has been done in pits, in others in rotary coolers, but the majority of the mills use the upright cooler shown in Fig. 33. This consists of an upright steel cylinder about 8 feet in diameter and 35 feet high provided as shown with baffle plates and shelves. As the clinker falls over these it meets a current of air blown in through a perforated pipe running up through the centre of the cylinder and is thus cooled. There is usually one cooler to each pair of kilns. The clinker is lead from these latter by chutes to a common bucket elevator which carries it to the top of the cooler. Usually water is added to the clinker in a steady stream as it falls into the elevator pit. This helps to cool the clinker, makes it more brittle and easier to grind in the ball mill and saves the elevator from handling such very hot material. There is probably nothing in the curing of the cement or the hydration of the free lime, since any of the former present is usually locked up in the interior of the clinker. It may, however, prevent the crystallization of the more soluble di-calcium aluminate from the magma of the clinker and so help the quality of the cement. The writer has frequently cooled clinker suddenly by plunging it, red hot from the mouth of the kilns, into water. The only perceptible effect is to bleach the color from dark greenish black to nearly white. If this clinker is dried and ground, it will be found to have pretty much the same properties as clinker caught at the same time and allowed to cool slowly in air. The writer has never observed that unsound cement could be

made sound by this process. It does take up some water (probably on the outside of the lumps only, however), as a loss on ignition test will show. Such clinker is easily ground and the resulting cement trowels nicely. This greenish white color of

Fig. 33. Upright Clinker Cooler (Mosser & Son.)

water cooled clinker is probably the normal color of the cement clinker when hot, as in this state all compounds are in solution in the fusible magma of the clinker. On cooling slowly the iron

separates out as black magnetic oxide of iron, Fe,O1, but on suddenly cooling it does not have time to do this.

The clinker is usually drawn from the bottom of the cooler on to belt conveyors, or else into barrows and carried to an elevator, which carries it up to the bins above the ball mills or rolls, which ever are used to grind the clinker.

Some mills use a rotary cooler which consists of a steel shell similar to that of the kiln and mounted on friction rollers just as the kiln is. Usually this is lined with fire brick. One cooler is used for two kilns and air is blown up through the cylinder to cool the clinker. Such coolers act as pre-heaters to the air entering the kiln. A form of cooler used by one of the New York State mills recently came to the writer's notice. It consisted of a water jacketed revolving cooler. The water entering and leaving the jacket through pipes leading from a specially designed feeder placed in the centre of the discharge end of the cooler. The water after leaving the cooler was used in the boilers, the cooler simply acting as a preheater. These coolers are said to have worked well, and to have cooled the clinker perfectly.

Cooling clinker in pits has been tried at a number of places, but does not seem to have worked very well anywhere. Some few mills convey their clinker red hot out into the fields and allow it to cool naturally. Such clinker is easily ground and has usually absorbed some moisture so that for every 100 barrels of clinker burned the manufacturer should grind from 101 to 104 barrels. A sort of case of watered stock, except that in this case the water probably does some good. The great difficulty with this system of storage is the conveying of the red hot clinkers out into the fields. Getting them back to the mill is, of course, easy as any of the coal handling devices, such as ærial cables and orange peel buckets, which will do this satisfactorily, or tunnels. provided with belt conveyors under the piles may be used.

Grinding the Clinker.

The clinker is ground in Griffin mills or ball and tube mills, all of which have been described in the section on grinding the raw materials. At the mills of the Atlas Portland Cement Co. the clinker is ground in Huntington mills and at one or two mills the

Kent mill is being used for this purpose. It is usual for the clinker to be ground by the same type of machinery as is used to grind the raw material. The principal reason for this is that only one set of repair parts have to be carried in stock. A number of mills, however, use ball and tube mills to grind the raw materials and Griffin mills to grind the clinker, the idea being that the former is the better of the two for soft materials, and the latter the more suited to the hard clinker. The tube mill is also a splendid mixer for hard and soft materials, such as clay and limestone. The tube mill is also well suited to wet grinding, and hence to marl and clay.

Kent Mill.

This is a new mill which has so far been used in connection with some form of separator, both to prepare material for the tube mill and also to do the final grinding. This mill, the prin

ciple of which is illustrated in Fig. 34, consists of a vertical revolving ring, having three grinding rolls which are mounted on horizontal shafts and which press against the inner surface of the ring. The material to be ground is fed on the inner surface of the ring and is ground between this and the rolls. The inner surface of the ring is slightly concave and the cement is kept in

the center of this and between the ring and rolls by centrifugal force.

The Kent mill requires the use of an outside separator; that is, the material as delivered by the mill itself consists of both coarse and fine particles, and the latter must be separated from the mixture, and the former returned to the mill for further grinding. At the plant of the Newago Portland Cement Co., where the Kent mill is used, a shaking screen of wire cloth has been devised for this purpose, but the majority of the manufacturers who are experimenting with the Kent mill are using the Emerick separator or the Pfeiffer separator. Both these machines are similar in principle, and make use of a current of air to separate the fine from the coarse particles.

Air Separators.

The Pfeiffer separator is much used in Germany and Europe to take out the fine material from the product of the ball mill, and so relieve the tube mill of some of its work. The separator is shown (in section) in Fig. 35. It consists of an outer and an inner cone of sheet metal as shown in the drawing. The material to be separated is fed into the mill through the hopper on to a plate, which is connected to a vertical shaft, and is revolved at a speed of about 200 revolutions per minute. The material is thrown off this plate in a thin spray, by centrifugal force, and is met by a current of air, going in the direction shown by the arrows. The coarse particles fall through this current into the inner case, and the finer ones are carried into the outer space, between the inner and outer cones. The air currents are maintained by the fans as shown.

At the plant of the Hudson Cement Co. the separators are used after the tube mills. The grit passes directly from the ball mills through the tube mills and from the latter to the separators. The fine material is sent from the separators to the stock house, and coarse particles are returned to the tube mill. The separators increased the capacity of the tube mills from 8 barrels per hour, of a fineness of from 93 to 94 per cent. through a No. 100 sieve, to 13 to 15 barrels per hour, of a fineness of 96-98 per cent. through a No. 100 sieve.