On the evening of Monday 7/2/1916, two months before the Easter Rising, A C Davis read a paper at the general meeting of the Institution of Civil Engineers of Ireland at 35 Dawson Street, Dublin. It was entitled “Portland Cement” and was largely written by S G S Panisset, but was credited to Davis. It outlined the usual definitions, history, nature and manufacturing processes of Portland cement – the historical parts containing many errors that were prevalent at the time. As an addendum, it also contained a description of the recently (1914) opened Magheramorne cement plant. Having been written by its promoters, it is a fairly accurate description of the original one-kiln plant as commissioned. The text appeared in The Dublin Builder, 8/4/1916, pp 9 & 10. The Magheramorne plant, although Davis, in his application for ICE membership, claimed its design as his own, was essentially an off-the-peg F L Smidth plant of typical design paralleled by several others constructed in the immediate pre-WWI period. It was designed, as were others, as a one-kiln plant, immediately upgradeable to two kilns. Because the war intervened, the second kiln was not installed until 1921. The technical details can be confirmed and augmented in the 1924 plant schedule. A/BPCM were also pursuing developments elsewhere in Ireland, at Skerries in Co. Dublin and at Drinagh in Co. Wexford, but these fell foul of the politics of the Free State in the 1920s.
New Works at Maghermorne
The Larne Lough Portland Cement Works have been built near the Magheramorne (Note 1) station of the Northern Counties Railway upon an alluvial deposit which juts out above the clay of Larne Lough. The site has been quarried for its limestone (indurated chalk) deposits for upwards of a century, the quarried rock having been manufactured into lime and also exported into the Clyde districts for use in smelting processes. The limestone in reality belongs to the same geological formation as the familiar chalk-with-flints which lies on the upper greensand beds (Note 2), and is contemporary with the chalk deposits on the Thames and Medway, in which districts Portland cement has been manufactured from its earliest days.
Originally the chalk was quarried wherever it outcropped from the beds of basalt which overlie the deposit practically throughout Co. Antrim. The further use of the Magheramorne quarries now necessitates working the chalk at a point where it is overlain by from 10 to 50 ft (Note 3) of basalt. The removal of this overburden has been facilitated by the erection of a basalt crushing plant which enables the company to manufacture and export road metal.
The second important constituent of the Portland cement manufacture here (the clay) is obtained by means of dredging from the bed of Larne Lough. This bed consists of alluvium found near the shore with occasional boulders from the glacial deposits of clay which overlie the basalt.
The process of manufacture is carried out as follows:— The clay having been dredged and brought inshore is lifted by means of a double-chain grab into the washmill provided with a grating to keep back the boulders. At this washmill an additional quantity of water is added, and the clay is washed until it becomes a thin slip which is pumped into a continuous running mixer for storage purposes. The chalk, on being quarried, is first deprived of its flints (Note 4) and thrown into a jaw crusher from which it is elevated into a steel storage bin which holds about 250 tons of chalk crushed from 3-in downwards. At the bottom of the chalk bin the material is extracted in measured quantities by means of a revolving feed table. This is a horizontal table, the centre of which is directly below the mouth of the storage bin. The chalk falls on to the table at its own angle of repose, and as the table slowly rotates an adjustable knife scrapes off from the table the outer edge of the heap of chalk. By shifting the knife nearer or further from the centre the greater or less amount of chalk is withdrawn from the table and replaced by gravity from the storage bin. From this table the chalk falls into a ball-mill or Kominor for its first reduction down to the size of grains of sand.
The ball-mill is a revolving cylinder with horizontal axis, charged with about 7 tons of steel bans varying from 4½-in diameter downwards. Water is added to the raw material in this mill (Note 5). The rotation of the mill causes the balls constantly to fall over each other and through the chalk which is mixed with them. By the time the stone has passed from one end of the ball-mill to the other it has been reduced from 3-inch pieces down to 1-mm grains. From the ball-mill the material is elevated into a sieving machine (Note 6) which projects the material now called slurry by centrifugal force against a stationary screen. The rejections from this screen pass back into the ball-mill for further grinding with water, where the finer portions from 1 mm downward, pass through a pipe to the tube-mill below.
The tube-mill is of the usual pattern in which pebbles, by continually rolling over the slurry, further reduce its fineness, until only a very small percentage of the finished slurry is held up in a sieve containing 32,400 meshes per square inch (Note 7). Before allowing the finer ground slurry to pass forward to another stage in the manufacturing process, the material is held up in small mixers for analysis. It is kept in a state of continuous motion until a calcimeter test is taken to determine whether its calcium carbonate content is within the limits set down as the standard of manufacture.
These limits are between 74.5 and 75% (Note 8). Having passed the laboratory test, which also includes a test for fineness of grinding, the slurry is admitted into the main mixer (Note 9), where it is kept in a state of continuous movement ready to be pumped into the kiln storage mixers at a distant part of the works (Note 10).
The two kiln storage mixers, containing slurry ready for burning in the kiln have a capacity sufficient to keep the kiln burning for three days (Note 11). From the mixer, the slurry is pumped into a controlling tank, from which measured quantities can be allowed to fall into the kiln (Note 12). This kiln is 164 ft long and 8 ft diameter (Note 13) and rotates at a speed of about one revolution per minute. The heat is introduced into the kiln in the shape of a blast of coal dust and air mixed together, which with continuous ignition gradually heats up the slurry to its fusing point by the time it has reached the enlarged burning zone. The firebrick lining of the kiln varies from 7 in thick at the cold end to 14 in thick at the burning zone. It has a capacity of 800 tons per week, but can be speeded up to 900 tons per week (Note 14). From the kiln the burned clinker falls into a rotating cooling cylinder (Note 15) where it meets a forced draught of cold air which, after cooling the clinker, and itself becoming hot, is used as a medium of the combustion of the coal dust used in the firing of the kiln. There is thus a regenerative action in which heat is taken out of the hot clinker, and put back again into the kiln as hot air.
Part of the hot air, however, is used for drying the wet coal, which, when received, contains from 10 to 12% of moisture. One of the essential conditions of grinding this coal into a fine powder is that it shall contain not more than one or two % of moisture. The coal is, therefore, elevated into another rotating cylinder through which a portion of the air heated by the hot clinker is forced. Coming into contact with the air as it passes through this rotating cylinder, the excess of moisture is evaporated, and the coal is delivered to the mill sufficiently dry to admit of grinding to a fine powder.
The grinding plant of the kiln consists of a ball-mill and a tube-mill precisely similar, but of smaller capacity, to the raw material grinding plant already described. After grinding, the coal is blown into the kiln in measured quantities by means of a screw feed and fan. The screw feed is driven from a friction disc arranged so that the speed of the acres, and, therefore, the amount of coal supplied in a given time, can be varied at will, to suit the burning conditions required. After the clinker has left the cooler, it is stored and subsequently ground to a fineness of from 3 to 5 % on a sieve containing 32,400 meshes per square inch. The grinding unit for the clinker again consists of ball and tube mills as already described in connection with the grinding of the raw material. From the clinker grinding tube-mill the finished cement passes into an automatic weighing scale. and is then conveyed into the storage bins.
Two main storage bins, each having a capacity of 3,000 tons (Note 16), have been built in reinforced concrete on piles running down into the clay, a distance of from 40 to 50 ft. The storage silos are of annular section, having 42 ft diameter throughout. From the bottom of these silos the cement is drawn by vacuum in a somewhat novel manner (Note 17). In addition to these silos, further small bins, five in number, are provided to hold 1,000 tons, making the total cement storage capacity 7,000 tons.
The power plant at the Magheramorne Works consists of an up-to-date equipment with electric drive throughout (Note 18). The coal, both for the power and kiln, comes by sea, and it is discharged from the vessels by means of a crane grab which is capable of dealing with 30 tons per hour. This crane deposits the coal into a hopper, and from thence the coal is conveyed on a belt-conveyor into the reinforced concrete coal hopper standing immediately above the boilers, or into the kiln coal bins. The combined storage capacity in the coal bins is about 1,200 tons. In the case of boiler coal, rotating feed tables already described in connection with the raw material mills automatically feed the coal into the boiler hoppers, and by means of revolution counters the exact quantity of coal consumed in each boiler can be determined at any time.
Steam production is provided for by three Babcock and Wilcox boilers, each having a heating surface of 3,580 square ft, which provide steam at a pressure of 200 lb per square inch superheated to 620°F; mechanical chain grate stokers of 70 square ft active grate area are provided, and each boiler is capable of evaporating 12,000 lb of water per hour from the usual temperature supplied by a Green’s economiser. Power is generated in an up-to-date 1,250 kW Curtis type turbo-alternator, which provides three-phase current at 500 V. The turbine has three stages of the impulse type and runs at a speed of 3,000 revolutions per minute. The steam consumption at full load is 13½ lb per kWh.
The generator coupled to the turbine has a rotating field and carries on its shaft a separate exciter. Condensing water is obtained from the lough at a rate of nearly 3,000 gallons per minute. A Lea recorder is provided which gives in diagram form the rate of flow of the condensed water. This provides a suitable record of steam consumed and admits of the efficiency of the boiler and power plant being ascertained from time to time. In addition to the turbine set, an additional small stand-by generator of 250-kW capacity direct-coupled to an Allen's high-speed engine is provided for light weekend loads. The economiser contains 360 pipes arranged in three nests.