Castle Cement"
Castle Cement Works, Padeswood, Buckley
1985
Reprinted from the January 1968 issue of "Cement, Lime & Gravel"
CEMENT MANUFACTURE IN NORTH WALES
Operations of Tunnel Cement Ltd at Padeswood, where three cements are made
The Tunnel Portland Cement Co's Padeswood works is situated some eight miles west of Chester. At the time it was conceived just after the last war cement was delivered mainly by rail; therefore the works was sited near a railway junction.
The site, however, is not underlain by raw materials, limestone and shale being brought in by road. The limestone comes from the company's own quarry at Cefn Mawr to the west of Mold and the shale from the National Coal Board's colliery at Gresford. Almost all Padeswood's output now travels by road. The siting of the works is proving advantageous: only limestone has to travel through the congested streets of Mold instead of most of the works' output.
Initially the works had two wet-process kilns. These were commissioned in 1949 and 1950 and were coal-fired at first, being converted to oil firing in 1960. However oil-price rises have prompted a return to coal-firing.
In a recently-completed expansion programme a new cement mill and additional silos were installed. These were followed by a new dry-process long kiln which was lit up in April 1966, having taken two years to build. It was installed by Tunnel and F. L. Smidth & Company Ltd acted as consultants and supplied most of the machinery. Like the original kilns the new kiln is coal-fired, but it could be converted rapidly to oil-firing, most of the necessary equipment for which was installed when the kiln was built.
LIMESTONE QUARRYING
The limestone used at Padeswood belongs to the Corallian series of the carboniferous system and is obtained from the company's Cefn Mawr quarry about 74 miles from the works.
The beds dip about 20° to the east and some shale partings separate them. The original quarry, which was worked up the dip, was abandoned because slips of limestone occurred at a face height of l00 ft after some 2 million tons had been extracted.
Limestone was next obtained by removing the top of the hill on which the quarry is sited. The present face is below this excavation and is held at a height of 6Oft. It is being worked along the strike to avoid the slips that were a problem in the original quarry.
Blasting is necessary at Cefn Mawr. A Holman Vole Trac drill working at 150 lb/in2 is used to put down 4in diameter holes on a 12ft x l2ft pattern. These are drilled at 50 off the vertical and a penetration rate of 37ft/h is achieved. For drilling toe holes a Halco Steinuick drill is available. The holes are charged with Opencast Gelignite, Quarry Dynamite and Quarrex, all initiated by Cordtex. The amounts and kinds of explosives used are carefully calculated to provide the minimum of vibration, and all blasts are monitored on a Cambridge vibrograph owned by the company.. Some 400 blasts have been checked and the data accumulated provide useful information if complaints are received.
About 5% of the limestone has to be broken by secondary blasting, for which pop holing is used. The overall yield of rock is about 4 tons/lb of explosive used in both primary and secondary blasting.
There are two excavators for loading broken limestone, of which only one is normally in use, the second machine acting as a standby. Both machines are Ward-Leonard l00-RBs rigged as face shovels. One has a 3½yd3 bucket; the other unit, which has a longer reach, is equipped with a 3yd3 bucket. These machines load into dumpers which run between the face and the primary crusher, a distance of about 400yd. Two shifts are worked in the quarry. On the day shift an Aveling Barford SN 35 dumper and a Euclid R15 are used; on the afternoon shift the SN 35 is used alone.
All the dumpers tip into an intake hopper positioned above an F. L Smidth variable-speed pan feeder having a width of 6ft. Stone drops from this feeder into the primary crusher, a 601n x 48in Hadfield unit. Driven through a gearbox by a 250hp motor this machine has a throughput amounting to 300 tons/h.
Stone from the crusher drops on to a reciprocating feeder which transfers it to a conveyor belt, 48m wide, leading to the secondary crusher. An electronic metal detector is affixed to this belt to protect the secondary crusher, a 1,500mm x 1,500mm F. L. Smidth double-hammer mill driven through torsion shafts by two 150hp motors. Stone from the secondary crusher falls on to a conveyor belt which delivers to a reversible shuttle belt running above four 1,000-ton capacity concrete silos. These fiat-bottomed silos each have a 'live' capacity of about 600 tons, and are lined with engineering brickwork to prevent wear.
Crushed limestone is drawn from the silos into the special hopper vehicles that run between the quarry and the works. Having a payload of 22 tons these are articulated vehicles with an AEC Mandator tractor unit and a twin-axle bottom-dump semi-trailer; seven of them are in use at the moment. To supply the works with the 13,000 tons/week of limestone needed the articulated vehicles are worked on a two-shift basis from Monday to Friday, maintenance being carried out at weekends.
LIMESTONE HANDLING IN THE WORKS
On arrival at Padeswood the loaded vehicles drive over an intake hopper into which they drop their loads through bottom doors opened by air cylinders powered by the vehicles' own compressor.
Discharge into the intake hopper takes about 30s, the driver judging the moment to drop his load and clearing the spillage if he judges wrongly.
In the bottom of the hopper there is an F. L. Smidth laminated feeder which delivers the stone to a conveyor belt whence the limestone travels either to a stockpile or tertiary crusher. An electronic metal detector is fitted to the conveyor to protect the crusher against tramp metal.
If the limestone is to be stockpiled it is transferred to a second conveyor which in turn feeds a radial conveyor mounted on top of a concrete tower. Stone falling from the radial conveyor forms a ground stockpile round the tower. The 'live' capacity of the stockpile is about 80,000 tons, but if the stone has been in the pile for any length of time a bulldozer is needed to aid reclamation.
The stock is recovered through two vibrating feeders in the base of the tower and fed back on to a conveyor leading to the tertiary crusher. A second metal detector is fitted to this belt to ensure that tramp iron coming back from the stockpile will not enter the crusher.
Before entering the tertiary crusher the stone from the belt passes over an F. L. Smidth single-deck vibrating screen; oversize goes into the crusher and the undersize (- ½ in) joins the crusher product. The crusher is a 900mm x 1,250mm single hammer mill direct-driven at 9I5rev/min by a 235hp motor. All stone leaving the hammer mill has a size of - ½ in.
From the crusher a conveyor belt carries stone to the central crane store where all raw materials, as well as coal, clinker and gypsum, are handled. The store measures 750ft x 85ft and is served by two 12-ton gantry cranes.
SHALE TREATMENT
Shale, the other main raw material, contains about 30% of coal which is recovered and used to augment the feed of the kilos. The shale is brought to the works by road tipper trucks. These vehicles are tipped by a gallows which engages with hooks on the bodies. The shale is first crushed in a hammer mill after which it is delivered into the feed hopper of the shale-cleaning plant. Shale which cannot be held in the hopper is delivered to the store.
The shale is drawn out of the hopper by a laminated feeder; then it is transferred by conveyor to a washing screen where shale and coat are screened at 3/8 in and water is added. The coarse material is crushed in a roller mill and then all the flow, including the material that passed through the washing screen, is screened again - this time at BS 14 mesh.
The coarse fraction flows on to four Wilfley shaking tables which separate the coal from the shale, the coal passing over the side of the table and the shale coming over the end of the table. The fines from the screens in front of the tables are added to the shale product. Both coal and shale are pumped to separate dewatering screens.
Dewatered coal is loaded on to a railway wagon and returned to the crane store, and the shale is pumped to a hydrocyclone adjoining the store. The partly-dewatered shale is then passed over a Linatex dewatering screen and falls straight into the crane store.
COAL PREPARATION
At Padeswood, coal to be burnt is first moved to a combined blending and stocking plant, being sampled as it leaves. It is stockpiled on a concrete floor so that when it is recovered its properties are known and its behaviour in the kiln can be predicted. It is also possible to adjust the composition of the kiln feed to ensure optimum burning.
The three kilns are positioned parallel to each other: there are two coal-milling circuits, one for the two kilns and one for the new dry kiln. Since hot air from the kilns is not used for coal drying, the older coal mill can be used to feed the new kiln and the new kiln's mill to feed the older kilns.
Coal is brought in by rail and unloaded by one of the overhead cranes in the crane store. This crane also transfers coal to two hoppers connecting to a conveyor-system leading to the stocking area. There are separate hoppers for bought coal and the coal recovered from the shale, so that the two may be accurately blended.
On one of the conveyors there is a belt weigher to record the amount of coal passing; coal is also automatically sampled every 30 minutes. A full batch of coal consists of 3,000 tons and the sampler will take a 400 lb sample which is broken down to a more manageable size for analysis.
The stocking ground is a long concrete pad divided in two down its length by a concrete wall on top of which runs the final conveyor from the crane store. On the wall there is a travelling tripper carriage equipped with two wing conveyors. Thus a continuous stockpile can be built for the length of the stocking ground either side of the wall. Down each side of the stocking ground there is a recovery conveyor with a rail-borne movable feed hopper. Coal is loaded on to these conveyors from the stockpile by a Chaseside SL 900 loading shovel.
From the recovery conveyor the coal flows on to a cross conveyor and then to an inclined conveyor from which it can either be fed to the raw coal bin on the older kilns, or transferred to another conveyor discharging into the raw coal bin connected to the new kiln coal-milling plant.
The coal mill on the older kilos is a Tirax ball mill 24m in diameter and 59m long with a drying chamber and two grinding compartments. The mill is driven by a 260hp motor, and can grind 8~ tons/h of coal. Heat for drying the coal comes from an auxiliary furnace fired by heavy oil. Air for sweeping the mill is drawn through it mainly by the firing fan with assistance if necessary from a circulating fan.
Ground coal is taken out of the air stream by a cyclone arid passes to a fine coal hopper from which it is withdrawn by variable speed Fuller feeders and fed into the firing tubes on the kilns. Alternatively, from the cyclone ground coal can be passed through a screw conveyor and fed to the new kiln fine coal hopper. By reversing this screw conveyor ground coal can be brought back from the new kiln.
On the new dry-process kiln the coal-milling circuit is very similar: a coal mill of the same size is used with a 300hp motor and an auxiliary furnace utilizing heavy oil as fuel. This furnace has three air sources: primary air is used for atomizing the oil fuel, secondary air is introduced round the burner and tertiary air is introduced into the furnace itself.
The operation of this unit is automatic, the amount of heat supplied being governed by the outlet temperature of the coal mill. Should the coal mill block and the temperature start to rise, an automatic alarm is given on one of the kiln fault annunciator panels and a bell rings to draw attention to it. Feeding from the fine coal hopper into the firing tube of the kiln is done by means of two variable screw conveyors.
WET-PROCESS CLINKER PRODUCI'ION
At Padeswood, sand is used in small quantities to correct the silica ratio. It is delivered from tipper trucks direct into the crane store.
The three raw materials (limestone, cleaned shale and sand) are transferred from the store to the slurry mill feed hoppers by one of the cranes. There are two slurry mills each capable of producing 40 tons/h of slurry. The mills are two compartment Unidan units 2'4m in dia. x 1 lm long. Each is driven by a 900hp motor through an F. L. Smidth Symetro gearbox.
The mills are run in closed circuit. Slurry coming from them is first pumped to a small trommel screen where very coarse material is removed. The coarse material then passes to a magnetic separator which removes fragments of grinding media, the remainder returning to the slurry mill. Fines from the trommel screen pass to a centrifugal screen. Material that does not pass through this screen returns to the mill for regrinding, whilst the fines passing through it are accepted.
Finished slurry is distributed among six blending silos and is passed from them to two 2.5m dia. slurry basins 5m deep. From the basins the slurry is pumped to the two wet-process kilns, each of which is fitted with standard FLS bucket feeders operating in a constant-level tank. Overflow from the tank returns by gravity to the basin. The presence of this overflow is indicated on the kiln floor to show that the feeders are working satisfactorily.
The speed of the bucket feeders is linked electrically with the kiln rotation speed. To provide a further check on the operation of the feeders automatic sampling buckets are fitted into which the flow of slurry can be timed.
The sampling buckets, which can be remote-controlled from the kiln floor, are also used for checking kiln output. The two kilns in this section of the plant were originally identical units l00m long with a diameter of 3.00m and an enlarged chain zone of 345m diameter.
However since the introduction of the new kiln the oldest kiln has been rebuilt. The burning zone has been enlarged, increasing output by some 20 tons/day to 370 tons, compared with the 350 tons/day produced by the other kiln. Both kilns are fitted with Lodge-Cottrelt electrostatic precipitators. The dust recovered in them is returned by a system of screw conveyors to a hopper mounted between the two kilns.
More dust is recovered from the smoke chamber at the back end of the kilns and this too is added to the supply in the hopper. From the hopper two separate screw conveyors return the dust to the kilns at a point about half way down their length.
The fuel supply to both kilns is automatically controlled, the amount of coal supplied being linked to the excess oxygen available at the back end of the kiln. Clinker flows from the kiln into the Unax Integral coolers arranged round the firing end of the unit. A small jaw crusher is fitted at the discharge end of the coolers to break oversize clinker. Two drag chains are provided to convey clinker from the coolers to the crane store - only one is in use at a time, the other being a standby unit.
DRY-PROCESS RAW-MEAL PREPARATION
The dry-process kiln is independent of the two wet-process kilns, but it is linked to the crane store, as the raw materials conic from there and the clinker produced returns to it. In the crane store there are three feed hoppers into which are placed limestone, shale and sand.
These materials are withdrawn by three Pendan constant weight feeders and placed on a single 24in conveyor belt, which is 390ft long and takes the feed through the original works, delivering it on to the intake conveyor of the dry process plant. From the second conveyor the feed is passed through a sluice valve into the raw mill, a Tirax Unidan mill, 3.66m long with a dia. of 6.8m, driven by a 1,500hp motor. The function of the Tirax mill is to reduce the feed to a satisfactory fineness and also to dry it. The drying is carried out by kiln exhaust gases.
Drying circuit -- Hot exhaust gases are taken from the ducting between the kiln and the precipitators and drawn through the mill, drying the feed as they pass through. From the mill the gases pass through a cyclone. If any solids are present, some of them are removed at this point via the 135hp circulating fan and are discharged into the main precipitators for final cleaning. The gases then pass through the precipitators, together with the remaining kiln exhaust which has not been through the raw mill, and are discharged to atmosphere via the chimney by the 200hp kiln exhaust fan.
Drying air can be provided from an auxiliary furnace if the kiln is not running and it is desired to run the raw mill.
Raw material from the crane store passes through the mill and is discharged on to a screw conveyor which feeds it into the boot of an elevator. At this point the solids removed from the gas stream by the cyclone are added to the raw meal. The elevator, which, like all elevators incorporated in the new plant, has a concrete casing, lifts the raw meal into an F. L. Smidth CV 6300/5600 air separator.
Finished raw meal which has 1% retained on 72 mesh and 13% on 170 mesh is separated from the coarser material in this machine. The coarse material returns to the mill, which has a throughput of 70 tons/h and a circulating load of about twice this. Finished raw meal goes along a screw conveyor, up an elevator and into a rotary distributor mounted over four 500-ton capacity silos. The raw meal is continuously distributed between the four silos.
Ideally one silo should be a quarter full, another half full, the third three quarters full and the fourth empty, or full and just about to be emptied. When a silo is full it is emptied. by means of a worm conveyor that runs beneath the silos into one of a pair of elevators (the other being a stand-by unit).
Meal is transferred from the head of the elevator via a screw conveyor into one of two 2,800-ton capacity concrete raw-meal storage silos. The actual kiln feed is drawn from these silos. Each silo has 16 outlets beneath it arranged in two banks of eight.
At all times one bank of eight outlets is in use, every two hours the bank of outlets in use is changed and every eight hours the silo being drawn is changed. Thus a stream of raw meal is being circulated continuously; it is drawn from beneath the storage silos, passes along screw conveyors up an elevator (with a second available as a stand-by) and returns by another screw conveyor to the storage silos.
From the circulating stream of raw meal the kiln feed is drawn off. This is done from the final screw conveyor just beyond the elevators. The basic feed rate is controlled by a Pendan feeder whose speed is synchronized with the speed of the kiln. The feeder in its turn controls a pair of screw conveyors drawing from the return screw conveyor between the elevators and the silos.
To check whether the raw meal is circulating satisfactorily some of it is fed back to a third small screw conveyor beyond the kiln feed take-off point and the performance of this screw conveyor is indicated on the kiln floor. It is also possible to by-pass the Pendan feeder in the event of a breakdown and feed the kiln volumetrically instead of by weight.
As the foregoing indicates, the raw milling is a continuous process. Blending off of silos containing high and low carbonate materials is not necessary due to the method of controlling the raw mill feed. The ratio between the shale and sand feeds are kept constant to avoid interference with the silica ratio.
The mill product is sampled and assayed for carbonate content at hourly intervals. The result is then compared with. an 'aim' laid down by the chief chemist and a note is made of the difference - whether + or -. The data so collected is added up, but alterations are not made to the mill feed until the sum of the from the aim has gone beyond a prescribed amount. When this has happened the limestone feed is altered by a set amount to bring the carbonate analysis back towards the aim. By this means a much steadier run is obtained on the mill and the effect of freak samples is eliminated.
Usually only four or five changes are necessary in the raw-mill-feed rate during a 24-hour day.
Raw meal leaving the Pendan feeder drops on to worm conveyors which deliver it into the back end of the kiln. Dust caught in the twin Lodge-Cottrell electrostatic precipitators is added to this flow as is dust from the smoke chamber at the back end of the kiln. To allow the dust-transport from the electrostatic precipitators to continue to operate when the kiln is shut down one of the worm conveyors can be reversed so that the dust can be transferred to the four 500-ton blending silos in order to avoid dust build-ups in the precipitator hoppers.
In the event of the raw mill not being required the milling section is isolated from the kiln by means of a hoisting damper.
THE NEW DRY-PROCESS KILN
The new kiln is built parallel to the other two kilns, but is separated from them by a road, beneath which there is a concrete service tunnel enclosing all the electrical cables and cooling water pipes for the new kiln. A branch tunnel leads off to the new cement mill. The dry-process kiln is a Unax unit. l28m long x 4.15m in diameter, with an enlarged chain zone of 4.55m diameter. It has a throughput of 800 tons/day of clinker and is coal-fired at present, although it could be converted to oil-firing at very short notice.
The kiln is supported on four tyres; a hydraulic thrust gear acts in the two upper tyres to even out the wear on them. Drive is by a 40bhp induction motor through a Ward-Leonard system and two d.c. motors developing a maximum of 170hp each. Electrical barring drives and a stand by diesel generator are fitted. Air is drawn through the kiln and the two precipitators by a 200hp exhaust fan.
The exhaust fan is on full load at the moment; it is intended to replace the motor by a 400hp machine to give the flexibility required for automatic control. The stack is of reinforced concrete and is designed to take the gases from two kilns. To maintain the discharge velocities whilst there is only the one kiln the tip of the stack is fitted with a stainless steel nozzle.
Raw meal entering the kiln passes first through the chain zone and then through a six-compartment cross 14.4m long.
The cross is of cast heat-resistant steel and a pyrometer connected to slip rings on the kiln shell is fitted to monitor its temperature. Stainless steel water injection tubes fitted in the smoke chamber are used to bring the exhaust gas temperatures down when the raw mill is not running and exhaust gases are flowing straight to the precipitators.
The back-end temperature is about 500°C and this is reduced to about 300°C before the gases enter the precipitators. This temperature is affected by the combustion of the coal remaining in the shale even after it has been cleaned.
Burnt clinker leaves the kiln and is cooled to about 150°C in 10 integral Unax coolers. There is a Pegson l4in x 211n jaw crusher for dealing with oversize material, and the cooled clinker is transferred to the crane store by one of two drag chains, the other acting as a standby. These two chains are the same as that serving the older kilns. An additional concrete trough with a drag chain in the bottom is provided to collect any spillage from the Unax coolers.
KILN CONTROL
The whole kiln and coal mill system, as well as the raw mill, is operated from a central control room containing a large console, the centrepiece of which is an illuminated mimic diagram of the complete plant. On the diagram coloured lights indicate which items arc running and faults are shown on annunciator panels. Some ninety-six possible failures are covered.
The kiln is fully instrumented and has a large number of recorders, including six for the voltages on the electrostatic precipitators. Automatic control is employed as far as possible.
The feed rate of raw meal from the Pendan feeder is linked to the kiln's rotation speed and the fuel is coupled with the O2 content of the exhaust gases, whose amount is regulated by a louvre damper on the exhaust fan. The firing fan has a variable-speed motor to provide the right amount of primary air.
Raw-mill throughput is controlled by signals from a microphone installed alongside it and the air temperature in it is also controlled automatically.
The kiln is lit up by a special gas-oil fired torch, which consists of a pump for the oil with a compressed air line for atomization and enables the light up and preheating of the brickwork to be carefully controlled.
CEMENT MILLING
Clinker from the three kilos travels to the crane store by a drag-chain conveyor whence it is transferred to the feed hoppers of the cement mills. Gypsum brought in by road is also handled in the crane store.
In the original works there were two Unidan cement mills 24m in diameter x 11m long (the same dimensions as those of the slurry mills). Powered by 900hp motors, these mills have three compartments; cooling is effected by injecting water into the last of these. The throughput of each mill is 18 tons/h when ordinary Portland cement is being ground. The mills are kept under suction and are fitted with bag filters for cleaning the air drawn through them.
When the new kiln was installed a third cement mill was put in. This Unidan unit is much bigger; it is 3.66m in diameter x 11.4m long and has three compartments, two charged with balls and the third finishing compartment with Cylpebs. Once again this mill is cooled internally by means of an injection of water into the third compartment.
The third mill is driven by a 2,700hp English Electric motor through a Symetro gearbox and on ordinary Portland cement has a throughput of 55 tons/h. Raw materials are fed to the mill by Pendan feeders acting on signals from a microphone monitoring the sound of the mill. The mill is operated from a panel incorporating a mimic diagram in a central control room which also houses the panel for the two older mills.
CEMENT STORAGE AND PACKING
Three cements are manufactured at Padeswood: Ordinary Portland, Rapid Hardening and Masonry. These are transferred to the silos pneumatically. Cement is stored in six concrete silos with capacities varying from 1,000-4,000 tons, the largest being used for ordinary Portland cement to be bulk-loaded. In addition there is a small 100-ton silo used for Sulphate-Resisting cement imported from Tunnel's works at Pit stone.
Bagged cement is handled on two 12-spout Fluxo packers, one of which is reserved exclusively for Masonry cement to avoid contamination problems. A load cell weighing device is being developed for loading ordinary Portland cement in bulk. The empty bulk tanker drives beneath the silo and is tarred off by load cells in the floor.
The driver sets on a dial the amount of cement required in his load and the delivery chutes are automatically lowered by our cylinders. The preset quantity of cement is dropped into the tanker and the delivery chute is automatically retracted. The load is then check weighed on a conventional weigh-bridge before the lorry leaves the works.
ACKNOWLEDGEMENTS
We wish to thank the directors of the Tunnel Portland Cement Co. Ltd for arranging for us to visit Padeswood and the staff there for providing the information on which this article is based.
Author: Tunnel Cement
Tags
Year = 1985
Building = Industrial
Work = Heavy Industry
Extra = 1980s
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