Cement Kilns

Holborough in 1928

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The remaining pictures for this article are in preparation.

The Holborough plant was set up as a private venture in 1923 by William Lee Henry Roberts - High Sheriff of Kent, Rochester Bridge Commissioner, native of Snodland, and previously manager of Lee's plant - and Percy Girouard of Armstrong Whitworth. The latter, no doubt, brought about the association of the plant with Vickers, at a very early stage in their development as plant suppliers. After the plant had been commissioned (December 1924), the Holborough Cement Company, Ltd. was set up in October, 1925, with Roberts as chairman, and on the board Percy Girouard, Ralph Montague Cook (also local, Roberts' brother-in-law), Henry Horne and Richard Lakin, so there was an incipient link with the Red Triangle group from the outset. Roberts retired almost immediately, and the company was formally merged into the Red Triangle group in August 1928. Horne was in the chair at the second AGM on 24/5/1928. Roberts died 18/10/1928.

The Red Triangle group finally crashed in 1931, and was nearly all acquired by Blue Circle. Holborough was selected as one of the four plants that were to be kept running, and as the best of the Blue Circle plants in the Medway area, with by far the best raw material reserves, it continued in operation as late as October 1984, by which time it was among the least efficient plants in Britain. Planning permission for a new cement plant (to be called Medway Works) was granted by the Secretary of State for the Environment on 26 November 2001, the intention being that it should replace the Northfleet plant. However, Northfleet closed in 2008, and after construction of a rail spur, no further development has taken place at Holborough (as of 2017). The chalk at Holborough is alleged to be very wet. Manufacturers have in general concentrated on installing new plant where dry raw materials are available, in the interests of energy conservation and CO2 reduction.

The following rather terse description of the Holborough plant appeared in an anonymous article in Cement and Cement Manufacture (II, Jan 1929, pp 7-13) in 1929, a few years after start-up. The article is believed to be out of copyright. No mention is made of the Red Triangle organisation. A training film made at Holborough in the 1940s can be viewed at British Pathé.

Values of imperial units (as of 1926) used in the text (alphabetical order): 1 acre = 0.40468424 Ha: 1 ft = 0.30479947 m: 1 gallon = 4.5460756 dm3: 1 HP (horse-power) = 0.7456998 kW: 1 inch = 25.399956 mm: 1 psi (pound-force per square inch) = 6.89478 kPa: 1 ton = 1.01604684 tonne: 1 yard = 0.91439841 m.

A Modern Portland Cement Factory


The "Holborough" works of the Holborough Cement Company, Ltd. was erected four years ago in an area renowned for the quality of the cement produced, i.e. the Medway Valley. The plant is one of the most up-to-date in Great Britain, and is equipped entirely with machinery of British manufacture by Messrs. Vickers, Ltd. The output capacity is over a quarter of a million tons per annum. The works are situated on the left bank of the river Medway, at a point midway between Rochester and the village of Aylesford. Labour-saving devices of the most modern type are everywhere installed with a view to reducing costs to a minimum (Note 1).

Fig 1
Aerial view of works
The image used in the journal article is one of a number produced by Aerofilms, under contract to Allied Cement Manufacturers. For clarity, I have used the print of a similar image once kept at Blue Circle Southern Area Engineering Office.

An asset of the greatest importance is the nature and quality of the raw material deposit, a deposit of natural marl (Note 2) some 300 acres in extent being adjacent to the plant. The marl deposit is, as quarried from the pit, of such a chemical constitution and physical condition as to be eminently suitable for the production of Portland cement. The marl is excavated by four steam navvies, each capable of a 3-ton bite at each operation.

The quarried material is dumped into rail cars, railed to the opposite side of the quarry, and tipped into a battery of rough or primary washmills of 30-ft. diameter and capable of washing 150 tons of marl per hour. These primary washmills are fitted with slotted sieves having apertures of 5/64 inch (Note 3). The slurry is forced through these grids at the sides of the mills, and is then pumped to a second battery of washmills, which have sieves with openings of 1/50-inch diameter (Note 4).

After passing though the sieves of the secondary or finishing washmills, the slurry is pumped to a number of correction and mixing tanks. These tanks, constructed of concrete, are 50 ft. in diameter and have a total capacity of 4,000 tons (Note 5). The slurry feed presents no unusual feature in the most modern practice, being of the usual spoon-feed type.

The kilns are 200 ft. in length by 9-ft. diameter through the drying zone and enlarged to 10 ft. in the burning zone (Note 6). Induced-draught fans, driven by 40 H.P. Crompton motors, increase the kiln output in addition to enabling a smoother control of the burning to be maintained. The kilns are driven 60-H.P. slip-ring motors and have a total weight of well over 500 tons. All are direct-driven through gearing from the motor to the girth ring. A pulverising plant consisting of Raymond mills (Note 7) (supplied by Messrs. International Combustion, Ltd.), including magnetic sets for the elimination of any tramp metal in the coal, prepares the coal for burning. The pulverising plant and transference apparatus are situated immediately in front of the kiln hood.

The clinker, after passing through rotary coolers of 90 ft. length (Note 8) by 6 ft. 6 in. diameter, is conveyed by belt conveyors to stock or direct to the grinding mills. There is a clinker storage for 10,000 tons of clinker, these storages being constructed entirely of concrete (Note 9).

The grinding mills are of the compound tube-mill type, over 30 ft. long by 7 ft. in diameter, and are driven by synchronous motors, each of 600 H.P., driven through machine-cut helical gearing (Note 10). The mills are divided by slotted diaphragms into three compartments. They are loaded with approximately 45 tons of grinding media in the form of steel balls ranging from 4 in. to ¾ in. in size.

The cement transference machinery including screws, elevators, and inclines are all electrically driven. There is a cement storage capacity of 15,000 tons (Note 11); these storages, as in the case of the clinker storages, are constructed entirely in reinforced concrete, as also are all elevator casings, stages, etc.

The cement storage consists of several batteries of the modern silo type, with packing and loading equipment. The Bates' packers have a capacity of 250 tons per shift. Excellent facilities exist for packing the cement on rail, road or water-side loading stages; 400 tons a day can be despatched from the road loading stage alone. Railway sidings connect direct to the main line of the Southern Railway.

A wharf with 700-ft. frontage enables ten barges to be loaded at the same time. The wharf, and the permanent way to it from the factory, are electrically lighted, and cement loading goes on night and day (Note 12).

Coal is delivered direct from the Southern Railway to the coal plant, thus keeping handling costs to a minimum. The coal conveyor is of somewhat unusual design, as far as its use in cement works is concerned; the coal plant as a whole is claimed to be of the most efficient in the industry.

A total of 3,000,000 bags of cement are handled at this plant in the course of a year, and improvements are now being effected or are planned with a view to still further reducing production costs. The whole of the plant is electrically driven, the power being transmitted from Barking at a pressure of 33,000 volts, by means of underground cable (Note 13). This is transformed down to 440 volts at the plant sub-station. The social side of the lives of the workers is not neglected, and arrangements are in hand for welfare and social care, including sports grounds, etc.

- o - O - o -


Note 1. They were currently in the midst of a price war with Blue Circle, and were keen to show that they could survive this. In the event, they didn't.

Note 2. The plant lay on the lower edge of the Chalk Marl outcrop, so the early quarry extracted various layers towards the base: the material on the far side of the quarry provided higher carbonate material to meet chemistry targets. The aerial view shows a new bench being opened up on the west side to provide reliable high-grade.

Note 3. This is 2 mm: maybe the supplier was continental.

Note 4. The small original washmills were inadequate after the fourth kiln was installed in 1938, and after WWII, a new system was installed further into the quarry, with two 350 HP rough mills and four 100 HP screening mills.

Note 5. The sun-and-planet mixer tanks were 58' ID and 9' deep, giving a capacity of 1020 wet tonnes at a moisture content of 44% (591 tonnes dry) each. There were two at the washmills in the quarry, and two more at the kiln site. Two 66' mixers were installed at the new washmills after the war, giving a total capacity of 4628 dry tonnes at 41% moisture. By pumping through three successive storage systems, cascade blending could be employed.

Note 6. As was usually the case, Vickers diameters were external - the internal dimensions of the kilns were 200’0” × 9’10½” / 8’10½”, the shell being ¾" thoughout. Kilns 2 and 3 maintained these dimensions throughout their life. Kiln 4, installed in 1938, had an early desiccator, and Kiln 1 was modified at the same time to more-or-less match it.

Note 7. The use of air-swept Raymond mills in closed circuit may be unique in Britain.

This drawing was given in an article on Raymond mills in Cement and Cement Manufacture (I, Nov 1928, p 85), and although not identified, it is clearly Holborough. The ducts and cyclone enclosure can clearly be seen on the aerial photograph.

Although these mills could be used for direct firing, a complex indirect system was chosen, as with most installations of the time. The system was later replaced with direct-firing Atritors. The coal was dried through a vertical drying cylinder, using co-current secondary air as a heat source, pulled through by the firing fans. The mill circuit, swept with ambient air, used a fan on the coal-laden side of the mill to keep the mill under suction. Note the little pressure-relief cowl on top of the U-bend. One wonders if the process engineering of the time, with virtually no instrumentation, was up to dealing with such a fiery system. The drying system evidently proved inadequate, because a separate, conventional rotary drier upstream was subsequently installed in 1928.

Note 8. The coolers were 86'3" in length, with a 2'0" outlet grid.

Note 9. Previously, all the clinker had been stored in the open. As part of the Red Triangle investment, four 1200 ton clinker silos had recently been completed. The rest of the storage was in open bunkers.

Note 10. There were three mills at this stage, with total capacity 46 t/h on OPC or 28 t/h on RHPC. The OPC was probably being made at a fineness of about 280 m2/kg, and the RHPC at 390 m2/kg. Three more mills were subsequently added.

Note 11. Here, as with the clinker storage, they are being somewhat economical with the truth. The plant originally had old-fashioned bins in the packing area. Red Triangle added a block of ten 300-ton octagonal silos, with four 45-ton interstices, giving total silo storage of only 3180 tons. Because of the cramped nature of the site, much more substantial silos for bulk loading and packing were built 200 m to the north in the 1950s, with cement transferred by FK pump.

Note 12. This gives the impression that a large proportion of the output went by water. Aerial photographs of the time show no more than one or two sailing barges at the wharf. It is unlikely that very much went by water. In view of the fact that Roberts was head of the Medway Conservancy and the Port of Rochester, it's understandable that he had a dogged commitment to barge transport, although, this far up the river, it was hopelessly inefficient.

Note 13. The construction of the plant - like Dunstable - was predicated on the availability of purchased power. The Barking power station started up in 1925, and for the few months before the plant was hooked up, electricity was improvised using second-hand marine generator sets.

Note 13.

Note 14.

Note 15.

Original content © Dylan Moore 2017: commenced 26/04/2017: last edit 25/05/2017.

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