It is a matter of common knowledge that the Portland cement industry in this country has been passing through seasons of great vicissitude, and that during the past few years it has become necessary for much of the old plant to be consigned to the scrap-heap. It is scarcely too much to say that in the short space of a decade the whole process of manufacture has been revolutionised, and that many works thought to be efficient and up-to-date at the close of last century have been proved to be completely obsolete and incapable of being conducted at a profit.
A change of this character has taken place at West Thurrock in the works of the Tunnel Cement Company, originally erected in 1874, which have now been reconstructed and equipped with entirely new plant by Messrs. F. L. Smidth and Co., of Copenhagen. The original works, kilns and all, have disappeared and the only trace of the old buildings is a part of the former ware-house. The works may be now looked upon as being typical of the modern methods introduced into this branch of manufacture. The most remarkable feature is the great length of the rotary kiln (Note 1) and the extreme uniformity of the quality of the cement, consequent upon the employment of single units of the largest size.
The chalk, brought in trucks from a neighbouring quarry, and the clay, also loaded in trucks are conveyed to the edge of the wash mill, and after being weighed are tipped by hydraulic pressure, in truckloads at a time, into the wash mill (shown in Fig. 47), capable of dealing with 50 tons of raw material an hour. The chalk and clay are reduced to slip or "slurry" by revolving stirrers of the drag-harrow type, and the slip is pumped up to the separators, where the coarse particles are removed. This is effected by centrifugal wringers (Note 2), the sides of which are lined with wire gauze that permits the fine particles in suspension to pass through, but retains the coarser matters for return to the wash mill. The roughly screened slurry, after being further ground in a tube mill, is run into a series of tanks, provided with stirrers in order to prevent deposition, after which it is lifted by a pump to the top of the kiln building. This is a fine hall, about 240 ft. in length by 55 ft. in width, containing two rotary kilns with clinker-cooling cylinders and coal-grinding plant. A photograph is given (Fig. 48).
Each rotary kiln, into the upper end of which the slurry is introduced at the rate of about 45 gallons a minute (Note 3), is inclined at an angle of 1 in 25 with the horizontal and is supported on five sets of friction rollers. Each kiln, which is 210 ft. in length by 8 ft. in diameter, increased to 9 ft. near the firing end (Note 4), is driven by powerful gearing at the centre and revolves at the rate of one complete turn in 65 sec. It consists of a tube, formed of mild steel plates riveted together, and lined throughout with firebrick. The fuel, which is Newcastle coal ground to a fine powder (about 15 % residuum on the 180-mesh sieve), is blown into the kiln by a high-pressure fan, and the air needed for combustion, previously heated by being passed over the red-hot clinker quitting the kiln, is also blown in by a fan (Note 5). This kiln, when in normal work, is capable of yielding 7½ tons of clinker an hour, with a fuel consumption equal to 28 % of the weight of the cement. The coal is ground in a 'kominor', or ball mill, and is finished in a tube mill, whence it is raised into a feeding hopper.
The clinker, as it issues from the cooling cylinder, placed beneath the rotary kiln, is lifted by a conveyor into a storage-hopper, whence it is removed in iron trucks and wheeled by hand on a narrow-gauge railway to the mill house (Note 6). The kominor into which it is tipped along with a measured quantity of gypsum, to delay the setting time of the cement, will turn out 10 tons of finely crushed clinker an hour, and the ground material from the kominor is conveyed to the tube mill, 6 ft. 6 in. in diameter and 24 ft. in length, in which the cement is ground to an extremely fine powder. The specified fineness of grinding is 12 % residue on the 180-mesh sieve. The ground cement issuing from the tube mill is transferred by a spiral screw conveyor to an endless rubber belt, 170 ft. in length, leading to the silos, six in number, 30 ft. in diameter and 50 ft. in height, with a united capacity of 10,000 tons of cement.
AUTOMATIC SACK FILLING
The use of the "Exilor" machines for sack filling constitutes a marked improvement on the old-fashioned plan of working. The empty sack, attached by clips to the orifice of a tube leading from the silo, is shut into an airtight chamber, formed in two sections, and in the act of closing the door an air-exhaust is started. Owing to the vacuum created a stream of cement from the silo is drawn through the tube into the sack. The sack is hung at one extremity of the beam of a weighing machine, in the scale-pan of which is the exact weight of a filled sack. In 10 or 12 seconds the requisite quantity of cement has been drawn into the sack, which then tilts the beam, thereby opening the air-valve : this breaks the vacuum and stops the flow of cement. The opening of the door to remove the sack enables a small fan to carry away the dust from the mouth of the sack and the door is ready to be swung back and closed against the other chamber of the Exilor, in which another sack has in the meantime been fixed in readiness for filling. The door is hinged on the centre line, so as to be capable of being closed against each section of the chamber in turn. A gang of three expert work-men fill and wheel away 120 sacks of cement in the hour, with the precise weight required in every sack. A small air pump, driven by motor, is all the power needed to convey the cement from the silo into the sack, and the vacuum created is about 15 in. of mercury.
The power throughout the works is generated electrically and is conveyed by cables to the various machines. The power-house is a handsome building (Note 7), containing ample space for additional plant. At present a steam turbine driving a powerful dynamo, furnishing three-phase current at 50 periods per second, is the main source of supply, but there is a 250 h.p. gas-engine to serve as a stand-by. The total weight of the turbine and dynamo is 27 tons, whereas the fly-wheel alone of a reciprocating engine to furnish the same amount of power was estimated to weigh over 50 tons. Steam is supplied by two Steinmüller boilers, each of 1000 h.p., with chain grate stokers.