A MODERN CEMENT WORKS

By A. Grant (Visitor)

1.   INTRODUCTION

2.   PLANNING

3.   THE PLANT

4.   ACKNOWLEDGEMENT

ABSTRACT

The latest developments in the cement manufacturing industry in the Republic of South Africa are illustrated in a hypothetical dry process plant located near the raw material quarry and designed to produce 600 000 tons of clinker per annum from two kilns.

Rubber-tyred vehicles are used to convey limestone and shale to the plant consisting of primary crushers, raw material stores, secondary crushers and two 1 000 bp closed' circuit mills, with appropriate storage and blending units. Two 2500 h.p, finished milling unit are provided, one on open circuit and the other on closed-circuit milling. Facilities are provided for despatching bagged and bulk cement by rail and road transport.

Details of the various mechanical and pneumatic conveying systems and the means for providing automatic and manual control are described.

All sections of the plant are automated with supervision from a central control room.

Allowance is made in the design and layout of the plant for the addition of more sophisticated controls such as computers at a later stage.

1. INTRODUCTION

Cement is a basic construction material needs no introduction, and its part in the commercial, industrial and communal development of civilised and developing countries is an accepted fact. Various attempts have been made to provide substitutes for this basic commodity, and although these have been met with limited successes and many failures, there is no reason for complacency by the cement producers. On the contrary, every effort should be made at all times by the producers and the manufacturers of cement plant and equipment to develop and expand the uses of cement, and to produce equipment which will ensure the maintenance and improvement of the quality of cement produced at stable and competitive prices. It is pleasing, however, that this approach is followed by the cement producing fraternity and is particularly actively pursued in this country.

There has been a growing demand for cement throughout the world over the years, with a substantial rate of increase in demand during the post World War II years in which this country had its share.

This growth in demand is both quantitative as well as qualitative, and the challenge had to be met by both producers of cement and manufacturers of cement plant and equipment. Research and investigations into improved process and control technology are and will have to be continually and actively pursued and to share experiences and theories and to ensure cross-fertilisation between the cement producers, symposia on cement chemistry are held every few years in the various major centres of the world. This principle is highly commendable and ensures that all producers share in the latest developments m cement chemistry and have an equal opportunity of applying these developments.

Fundamentally, the selection of plant capacity is Governed by the scope of the market, but to meet the growing demand, keeping cognisance of the unit cost of production, whilst making due allowance for the increased capital cost of plant and equipment, increased cost of fuel, transport, labour, etc., larger units of advanced and reliable design are resorted to. The equipment manufacturers had to revise, modernize and adapt the manufacturing techniques and constantly had to improve on these in a highly competitive field. Some of the aspects that the designers have to bear in mind are highest possible efficiency at lowest maintenance and operating cost; reliability of equipment, most of which operate continuously; accessibility and hence ease of maintenance; selected and special materials to meet the arduous operating conditions present in sections of the plant; advanced mechanisation and labour saving techniques by utilizing centralized process control; reproducibility of the required quality standards of the end products; and the reduction of material and labour wastage to a minimum.

Through operational experience, it is not uncommon to find that the cement producer can and does materially contribute to these developments in the design and manufacturing fields. In the execution of the various operations and quality controls, the latest techniques developed within and applied from outside the industry are employed to meet the goal of the lowest unit cost for the quality standard end product. These vary from automatic weighing and proportioning units used for material feeds to mills, automatic sampling and batching devices of materials in the various stages of the process, to computer-controlled plant operations and computer processing procedures of the various cost centres throughout the process to maintain the most efficient production and economic balance. This is further applied in the marketing field.

2. PLANNING

The planning stage of a cement plant is of utmost importance, as the ultimate efficiency obtainable is heavily dependent upon efficient planning.

This approach is naturally influenced as to whether an existing plant is to be extended or whether a new plant is to be erected. In both cases, the need for expansion had to be substantiated by the existing and future potential market. In the case of a new plant, the selection of a suitable site could be influenced by a combination of factors, such as the centre of gravity of potential market, raw material sources, a geographic advantage over competitors, transport facilities and transport policies, power, labour market, etc.

In the planning of a new plant or the extension of an existing plant, the equipment layout calls for detailed study and investigation to ensure the most efficient and least costly materials flow, handling, and storage. The layout must provide for adequate access and maintenance space around all units and allow for future expansion.

During the planning stage, it is most essential to investigate the latest and best equipment available for the particular environment and application and to provide for the installation-if not installed initially with the plant-of the most advanced operational and control techniques and processes. An initial oversight during this stage could later result in embarrassing and costly modifications. The plant has often to meet the requirements of national regulations or local authority bylaws, and it must be selected to meet these standards in addition to any set by the producer.

In this country, as in many others, the availability of sufficient water for the proposed and future plants is often a deciding factor on the type of process to be selected. The comparative economics on the choice of process to be adopted may thus have to be disregarded.

When the output of the plant has been decided upon and the sources of raw materials established, chemical and physical tests must be carried out on the raw materials to establish the type and size of the plant most suited to meet the operational and economic requirements of the venture. Most manufacturers of the plant are fully equipped with testing laboratories to establish the plant requirements, and the plant recommended is usually selected from standard equipment and auxiliaries sized and designed with the processing system in full perspective.

As a design aid, the use of a mathematical simulator employing a digital computer has been developed to do the many calculations required to determine the optimum system rapidly and accurately and this will play an important part in the design of plant where a large number of operational and process variables are present, as in a cement works. Any number of systems could be analysed using the computer to determine the components which will guarantee consistency of performance and the set of components which will operate at the least cost per unit of production.

3. THE PLANT

1. General

The fundamental raw materials required for the manufacture of normal Portland Cement are calcium-carbonate bearing materials such as limestone, marl, etc., and a siliceous material in the form of shale, clay or boiler ash. Small amounts of iron oxide are also at times added to meet the chemical requirements of the raw mix to be burnt. The other raw materials required are coal and gypsum. The latter is used as a retarder in the cement and is added during the finishing milling process.

There are two types of production processes for the preparation of raw materials in cement manufacture, viz., the wet process and the dry process. In the wet process, the raw materials are wet milled in the form of a slurry with the water content usually of the order of 34 per cent to 40 per cent, depending upon the characteristics of the raw material used and to a lesser degree on the transport system employed. In the dry process, the raw materials are ground in the dry state, and depending upon the moisture content of the materials, pre-mill drying has often to be applied. The milled product is generally referred to as taw meal and its fineness is usually more than 4 000 cm2/gm Blaine. Fundamentally, the blending of the slurry is simple in comparison with a dry powder, and for this reason, together with lower initial capital outlay, the wet process has been most commonly adopted until recent years. More heat is required to produce clinker from slurry than from raw meal by the large percentage of water to be evaporated. With the rise in fuel costs which accounts for a substantial part of the production of cement-the dry process, where practicable, must receive serious consideration. As a result of researches carried out on improving the efficiency of blending the raw meal, and the fact that such efficiency now approaches that of the wet process, this operational difficulty is successfully met in practice and dry process plants are gaining popularity. When the average moisture in the raw materials approaches 17 per cent, the economic advantage of the dry process becomes marginal and depending upon fuel costs, the two methods can be economically equal.

The three main types of cement produced in this country are Ordinary Portland Cement, Rapid Hardening Cement and Portland Blast Furnace Cement.

Because it contains blast furnace slag, the manufacture of Portland Blast Furnace Cement for economic reasons is done by factories situated in the vicinity of steel producers.

To describe some of the more salient features of a modern cement plant, a hypothetical case will be considered where quarried limestone and shale are used as the basic raw materials. A two kiln, dry process plant with a total annual clinker productive capacity of 600 000 short tons, situated at the raw material source and producing Ordinary and Rapid Hardening cement, will be assumed. The general layout of the plant is shown in Fig. 1.

2. Quarrying

The open cast mining method is employed in the winning of both limestone and shale. Due to the hard nature of the deposits, explosives are used for the primary breaking of the rock faces and secondary breaking is by drop-ball. The blast holes are drilled by self-propelled drilling units, each of which is an independent entity with its mobile compressor and dust collector unit, thus ensuring the required flexibility of drilling operations.

The material is loaded at the quarry faces by 3 cubic yard electrically powered excavators with Ward-Leonard control. Given the fast preceding working faces, and the distances of the quarries from the factory, heavy-duty diesel-powered quarry vehicles of the 25-ton payload were decided upon in preference to either rail or belt-conveyor transportation. In certain circumstances where undulating terrain has to be negotiated over a distance, aerial ropeways can be economically and successfully employed.

Overburden stripping and face cleaning functions are catered for by a track-type bulldozer, whilst the quarry roads are maintained with a road-grader.

The quarries are operated on a single shift basis for 5 days per week.

3. Primary crushing

The primary breaking is using a large gyratory crusher which reduces the material to minus 5 in. Its capacity meets the requirements of the factory on a single shift basis, and ample capacity is therefore in hand for future increased production when required.

The material from the quarry is dumped into the primary crusher receiving hopper from where the plus 3 in the material is fed into the primary crusher by an elliptical-bar feeder and the minus 3 in the material is discharged directly with the crusher product onto the take-off belt. To maintain the optimum output of the crusher, the rate of feed to the crusher is automatically regulated by a sensing device operated by the power drawn by the crusher motor. To obviate overfeeding when the crusher is choked, an overriding control through a load-cell on the off-take belt is provided. An overriding manual control is also provided for emergency and maintenance purposes. Sequence starting is provided between the off-take belt, automatic crusher lubricator, dust collecting plant, crusher and feeder. Here again, overriding switches are provided for maintenance and emergency purposes.

Because it is desirable to protect the feeder by having a layer of material over it at all times whilst dumping, three-point gamma-ray level control is provided through the sides of the receiving hopper. When the system has to be cleaned off the on material, this control is manually overridden.

A parabolic pan deck vibrating feeder is provided at the loading point onto the off-take belt to protect the belt from cuts and bruises from the falling rock.

4. Raw materials store

The primary crushed materials are conveyed directly to a store between the primary and secondary crushing stages. This store ensures the required flexibility in the production process by combining the quarrying and primary crushing complex into a single production entity.

The off-take belt from the primary crusher is provided with a weighing device that indicates and records at the control panel the rate of material transportation and the total tonnage passed through the crusher. An automatic material sampler is installed and is equipped for automatic quartering, crushing and grinding of the raw material samples for analysis.

Provision is made at this point for the subsequent installation of an on-line X-Ray Spectrometer for automatic quality grading and storage of high and low-quality limestone. At present this grading is done manually at statistically predetermined intervals, and the discharge of the respective streams of materials in their quality graded bays is manually controlled from the crusher operator's panel.

The materials are distributed in the store by a conveyor belt and tripper running in the apex of the store roof structure. The store is divided into sections for the storage of the high and the low-grade limestone. A third section is for the storage of shale.

A longitudinal tunnel with an extraction slot is provided underneath the store to accommodate the scraper feeder extraction units and the belt conveyors. Variable speed scraper units are used, two being for limestone and one being for shale extraction. All of them can operate simultaneously or independently as required. Three weighers are installed under the tunnel extraction belt conveyors. One serves the shale bay, one the low-grade limestone bay and the third measures the total quantity of both the high and the low-grade limestone. The initial blending possible with this combination is a decided advantage in the quality control of the raw meal. Material extraction is controlled by the Centralized Control Room in the kiln building.

The main object for adopting the scheme outlined is to obviate the use of overhead gantry cranes.

Provision has been made for future extension to the raw material store.

5. Secondary crushing

As the silica content of the raw materials is low, impact breakers are used for the secondary crushing to produce a final product of minus 1 in the material. Should the silica content be such as to produce ex-cessive abrasive wear, gyratory crushers would have been selected for this duty.

Given the difficulties associated with extraction from bins when coarse and fine materials are associated, the final crushing has been arranged immediately ahead of the raw mills. One breaker each for limestone and shale provide the requirements for two raw mills. The breakers discharge into feed hoppers serving the gravimetric proportioning unit regulating the feed to the individual raw mills.

The rate of feed to the impact breakers is regulated by the rate of extraction by the scraper feeders which are interlocked with the extraction belts, breakers, proportioning units, mill feed belts and mills. The rate of extraction is firstly governed by the speed of movement of the scraper feeder remotely controlled from the central control room. It is set as closely as possible to the required rate of feed to the mills. When the feed hopper serving the proportioning units become full, the scraper feeders and extraction belt are automatically stopped but are automatically restarted as the level in the hoppers becomes low enough. Under normal operating conditions a balance is struck and only occasional stops and starts of the extraction systems are necessary.

6. Raw milling

As mentioned before, gravimetric units are used for proportioning the raw material constituents before they are discharged onto the common feed belt to the raw mill. Individual controls are provided on the centralized control panel for each proportioning unit and one master control adjusts the total material feed while maintaining the pre-set raw material proportions. Warning devices are incorporated to draw attention when the required amount of any one of the materials is not met and should corrective action not be taken after a pre-set time limit, the entire mill and feed circuits are stopped.

The raw mills are two-compartment ball mill, the trunnion is driven through heavy-duty gearboxes and torsion shafts by 1 000 hp 6.6 kV auto-synchronous motors. Rolled steel liners with clamping bars are used in the first compartment in which the grinding media varies from 2? into 4 in diameter balls and cast alloy steel liners-are used in the second compartment with grinding media of 1 in to 2?  in diameter balls. The mill drives are equipped with slow turning devices or barring gears, and the water-cooled white metal trunnion bearings are provided with high-pressure lubricating oil for 'inching' and starting-up purposes.

The materials enter the mill through a drying chamber, where initial drying takes place by hot gases supplied from a furnace with an oscillating grate stoker, the feed rate of which is thermostatically controlled by the mill inlet gas temperature. Air from the mill is discharged into the atmosphere after passing through a dust collector.

The product from the mill discharges into a sluicing screw and is elevated by a bucket elevator to the separator where the coarse and fine materials are separated. The fine material or raw meal together with the dust collector product are conveyed to the blending silos, whilst the coarse materials are returned to the feed end of the mill. To maintain automatically a constant total feed to the mill, the coarse return material is weighed. The weigher is electrically linked with the master controller of the gravimetric raw material feeders, which compensates for any variations in the circulating load.

An added refinement to maintain optimum mill performance for any given requirements of the end product is a sensing device actuated by the load on the mill elevator motor, in conjunction with a sonic controller operating on the noise level adjacent to the mill shell of the first compartment, and these collectively control the maximum feed to the mill.

Two independent transport systems are installed from the mills to the blending plant when two types of clinker are to be produced simultaneously. When one type of clinker is made, either system may be used.

An automatic sampler is installed in each system and samples are tested at regular intervals for quality control purposes. Provision has been made for the installation of X-Ray Spectrometer equipment at a later stage when raw material proportioning adjustments would be effected automatically through computerised control.

7. Raw meal blending & Storage

Two sets of four blending bins are installed, each bin having a live material capacity of 800 tons.

The raw meal from the mills is elevated by either or both of the two bucket elevators from where it is conveyed by air slides to each of the two rotary distributors serving the two sets of blending bins. The stream of material entering the rotary distributor is divided equally between the four bins, the material levels of which are arranged so that when the one is empty, the second is approximately a quarter full, the third half full and the fourth three-quarters full. This is to facilitate one bin per set being discharged at a time without interrupting the milling process, in addition to the homogenising advantage obtained. Extraction from the bins is by aeration with pneumatically operated control valves into a screw conveyor with sufficient capacity to handle two bins simultaneously. Two screw conveyors are provided for handling either type of raw meal when required.

Some mention may be made of the extraction control and the screw conveyors. The rate of extraction from each bin is governed by the capacity of the screw conveyor, and to prevent the latter from flooding, pivoted paddles set at pre-determined levels are mounted in the screw conveyor casing. When the material reaches the paddle level, the paddle is displaced about the pivot point. This causes an ail valve to operate which directs the air supply to the one side of the air cylinder linked to the extraction valve and reduces the rate of extraction. When the material level in the screw conveyor falls below the paddle, the opposite action takes place. The blending bins are discharged automatically using a signal from a load cell which, through a relay system, start the conveying system in sequence and after a pre-set time discharge commences. A necessary safety device is built into the circuit to prevent more than two bins from the two sets being discharged simultaneously into one screw conveyor. The load cell detects when the bin is empty and stops the extraction system. If another bin is not being discharged, the conveying system is automatically stopped.

Continuous level indicators operated from the load-cells are installed for each bin and the levels are indicated on the control panel given the raw mill operator and are also indicated on the panel in the central control room. Should the stepped filling of the bins become out of phase, for example, due to discharge difficulties, the necessary overriding controls to take corrective action are provided.

From the blending bins, the raw meal is elevated and distributed to the storage silos using air slides. Three storage silos are provided, each with a full capacity of 4 000 tons of raw meal. High-level alarms are provided and the filling system is stopped after a predetermined period after the alarm has sounded.

Extraction from the storage silos is by aeration control and two screw conveyors to the two kiln feed elevators. The same principle of paddle control as described earlier is also employed here. Each silo is provided with sixteen extraction points, eight on either side of the extraction tunnel, with every alternate one connected to the same screw.

The entire blending and storage system is maintained under suction. A suitably sized bag type dust collector is provided. The dust collector as well as other moving machinery have a centralized lubrication system. To facilitate maintenance, inspection and supervision, a man lift is installed with stops at each intermediate level up to the top of the silos and blending bins.

8. Burning and coal milling

The two kilns have a combined output of 1 800 short tons of clinker per day and require some 2 800 short tons of raw meal. Each kiln is supported on six piers and is set at a gradient of 3 per cent to the horizontal. Hydraulically operated thrust rollers counteract the downward thrust and also ensure a positive float of the kiln to obviate grooving of the support rollers. The use of these thrust rollers obviates the undesirable practice of setting the support rollers at a slight angle to the longitudinal axis of the kiln to counteract the downward thrust. As all riding ring and support roller faces are set parallel, grooving is eliminated. Graphite blocks between the riding rings and rollers lubricate them with great success.

Integral planetary coolers, attached to the periphery of the kiln shell at the discharge end of the kiln, are used and were preferred for this installation due to their simplicity, effectiveness, reliability, low maintenance costs and efficient heat transfer to the secondary air. The discharge temperature of the clinker is not as low as could be obtained from some other types of coolers, but this was more than outweighed by the factors mentioned above. Both lifters and chains are employed inside the cooler tubes and heat resistant steel inlet cooler castings and liners are used.

A star type heat exchanger is mounted on the inside and at the feed end of the kiln. It is made up in sections and bolted to the kiln shell, with sufficient space provided for refractory lining between the edges of the plates and shell. Heat resistant materials are used for the half of the heat exchanger on the discharge side with mild steel for the remainder. Provision has been made to replace a section of the heat exchanger with chains at a later stage if economics warrant the saving in heat consumption expected from this modification.

The units are refractory lined throughout and insulation bricks are used under the refractory lining in the burning and calcining zones. Several dam rings are provided between the discharge end of the heat exchanger and the clinker discharge ports to reduce the flushing tendency of the material inside the kiln. The weight of the rotating mass, excluding the live load of material inside the kiln, is of the order of 1 900 short tons.

The kiln is driven by an enclosed A.C. commutator motor with variable speed, variable horsepower, constant torque characteristics, supplied with a speed matching control to within 1 per cent variation, irrespective of load. A two speed barring gear drive is provided and drives onto the main motor shaft through a freewheel coupling. In the event of an electric power failure, a standby diesel-generating set starts up automatically and the kiln is turned slowly at will through this auxiliary drive. It is also used for maintenance and refractory relining purposes. The drive from the motor consists of a heavy-duty gearbox, torsion shaft and a further spur gear reduction onto the girth gear which is fixed to the kiln shell through spring steel spider blades.

Retractable kiln hoods with motorised drives for ease of operation are used to facilitate internal maintenance and refractory repairs. Double-walled burner pipes are used with induced air passing through the annular space for cooling purposes.

The burner tips, which are about six feet in length, are made of high-quality heat resistant steel. The inner tube is fitted with longitudinal fins both to centralize it in the outer tube and also to counteract distortion due to the high surrounding temperature.

Motorised drives are provided for longitudinal and circular adjustment of the burner pipe during operation A portable liquid-fuel-fired burner pipe is provided for firing up the kilns.

The raw meal feed to the kiln is controlled by a continuous weighing device, from which the instantaneous rate of feed, as well as the total feed, are remotely indicated on the kiln control panel. This unit is electrically linked to the kiln drive whereby the feed rate is controlled in harmony with the kiln speed. When the kiln slows down, the feed rate is reduced in proportion The excess raw meal supplied by the kiln feed elevators is returned to the storage silos, thereby further enhancing the blending process.

As can be expected, an appreciable amount of dust is entrained in the kiln gases and a mechanical collector of the cyclone type is used immediately after the gases leave the smoke chamber. This is followed by a high-efficiency electrostatic precipitator. The filtered gas is ejected by an induced draft fan through the chimney stack into the atmosphere. The induced draft fan is driven by a variable speed A.C. commutator motor similar to the one for the kiln drive. The electrostatic precipitator is equipped with an automatic voltage regulator control which ensures optimum operating efficiency at all times.

To ensure efficient and economical burning, the amount of free oxygen in the kiln gases is continuously indicated and recorded by an oxygen analyser of the paramagnetic type. To protect the electrostatic precipitator from an explosion or fire damage, which could result from an excess of carbon monoxide or unburnt coal in the gases, safety devices are provided whereby the pulverized coal feed-screws are tripped, the precipitator power supply isolated and the induced draft fan stopped when the temperature of the kiln exit gases exceeds a predetermined maximum, and/or when the temperature gradient across the electrostatic precipitator is positive, i.e. when the exit temperature exceeds the inlet temperature. The temperature of the waste gas is controlled by injecting water into the gas stream, the rate of injection being thermostatically regulated.

The precipitated dust is returned to the system at the feed end of the kiln through the raw meal feed pipe. The precipitated dust is collected in a surge hopper mounted on load cells, and from there is fed to the kiln through the raw meal feed pipe at a pre-set rate controlled by a continuous weigher which operates in harmony with the kiln speed similarly to the kiln feed unit. The rate, as well as the total dust returned, are indicated and recorded on the centralised control panel. The rate of dust precipitation can be checked at will by utilizing the load cells under the surge hopper. These also serve to indicate on the control panel the weight of the dust in this hopper, which is required for determining the dust return rate setting into the kiln.

Thermo-couple operated thermometers are installed to record and indicate the temperatures in the smoke chamber, at the material discharge end of the heat exchanger, inside the kiln hood, at the clinker discharge from the coolers and the temperature of the primary air. A scanning device, which indicates and records the instantaneous average shell temperature progressively along the length of the burning zone, is also provided. This is a most useful operational tool because its intelligent use not only extends the life of the refractory lining in the burning zone and protects the kiln shell from damage due to overheating, but also indicates the extent of material build-up or coating in the burning zone.

A hammer mill type of crusher is used to reduce the oversize clinker leaving the kiln, thus ensuring that all the clinker sent to the store is of minus 1 in size. The clinker, when discharged from the cooler and crusher, is transported to the store using bucket type conveyors. A bag type dust collector is used to suppress the clinker dust at the loading end of this conveyor. An automatic clinker sampler is installed for density determination.

The coal mills have two grinding compartments preceded by, by a drying chamber. Spherical grinding media are used in the first compartment, whereas cylindrical media or 'cylpebs' are used in the second. Lifters are mounted inside the drying chamber to impart a tumbling action to the duff coal, thus ensuring more effective drying by the hot air drawn from the kiln hood and passed through the coal mill. The hot air supply is thermostatically controlled by a motorised damper in the circuit. These mills operate on the air-swept principle. A static separator is used for separating the coarse particles in the mill product and these are returned to the mill for further grinding. The fine coal is separated from the air stream by a cyclone and is stored in the fine coal bin. The air from the cyclone is led through ducting to the inlet of the primary air fan and provision is made using remotely controlled motorised dampers to supply atmospheric air as well as hot air from the duct leading to the coal mill inlet, to control the amount and the temperature of the primary air used. The primary air fan is also driven by a variable speed A.C. Commutator Motor.

The rate of pulverized coal fed via the burner pipe to the kiln is remotely controlled by a continuous weigher installed below the fine coal bin. This bin is mounted on load cells which remotely indicate on the control panel the level of coal in the bin. Operational alarms are installed to draw attention when the material in the fine coal bin and the raw coal storage bin has reached high or low-level settings. In the case of the fine coal bin, the mill is manually stopped and started as required, but provision has been made for the automatic stopping and starting of the coal mill circuit at a later stage. Automatic stopping and starting of the raw coal extraction and conveying systems has been provided and is actuated through relays by the high and low-level controls on the raw bin bins.

Automatic sampling units are installed for the collection of both raw and fine coal samples. The raw coal sampler is electrically linked with the raw coal extraction circuit and is thus stopped and started with the latter. In design and layout of the burning and coal milling installations, as well as their instrumentation, provision has been made for the eventual link-up for centralized computer control. 

9. Clinker, coal and gypsum store

The construction of this store is similar to that of the raw material store. The incoming materials, coal and gypsum, are received in rail trucks which are off-loaded using a tippler provided with a weigh-bridge. The material is distributed into the respective stores using a belt conveyor and tripper running along and under the apex of the store roof. A second belt conveyor and tripper similarly mounted is used for distributing the clinker along with the clinker store.

The nuisance of clinker dust is overcome with a marked degree of success by the low sloping roof and enclosed ends of the store. As in the case of the raw material store, the materials are moved towards the extraction tunnel by a rubber tyred front end loader. The longitudinal extraction tunnel is provided with extraction belts and scraper feeders for reclaiming materials from this store.

10. Finishing milling

Two mills, each powered by a 2 500 hp 6.6 kV auto-synchronous motor, are installed. The one is an open circuit unit, whilst there can be used both on open and on closed-circuit grinding and is used for the production of high, as well as low, Blaine products. Each mill is provided with three feed bins located along with the mill building. The third bin, in each case, is provided for supplying the extra constituents used for the production of special cement as and when required.

These bins are automatically filled by the scraper feeders, the extraction belt conveyors and feed belt conveyors from the clinker, coal and gypsum store. The constituents from each bin are continuously weighed and their proportions maintained by gravimetric proportioning units, which are individually and collectively controlled, as for the raw mills.

Each of the mills is equipped with internal water injection for product temperature control, and are automatically governed by thermostat controls in the air circuit to the dust collector. A bag filter is used for each mill, one bag filter serving the feeder chambers and feed belts of both units.

The closed-circuit mill has two milling compartments, the first with rolled steel shell liners and lifter bars and the second with white iron cast liners. Spherical grinding media of 2 1/2 into 4 in diameter are used in the first compartment, and cylindrical media or 'cylpebs' are used in the second compartment. The open-circuit mill has three compartments, of which the first two are charged with spherical media and have rolled steel liners and lifter bars. The third compartment has liners and a charge similar to the second compartment of the closed-circuit unit.

Central or trunnion drives through torsion shafts and heavy-duty gearboxes from the 6.6 kV auto-synchronous motors are installed. Each unit is also provided with a barring gear for inching and maintenance purposes. The common motor room for all three mills is separate from the mill rooms and is pressurised with filtered air. Both the motor and mill rooms are equipped with overhead gantry cranes for maintenance and mill charge purposes. A charging bucket is used for adding media to the mills. When discharging a compartment the media is dropped onto the floor to gravitate to a central point and is deposited onto a common belt conveyor running below the floor level at right angles to the longitudinal axis of the mill. The conveyor discharges into the hopper serving the automatic media grading unit.

The closed-circuit mill is served by an elevator which handles the mill product to the separator, where the coarse material is separated and returned to the mill feed end again by an air slide. The finished product is conveyed by an air slide to the cement storage elevator. The product from the open circuit mill is conveyed by screw conveyor to the storage elevator. In both cases, provisions have been made for the installation of cement coolers if required at a later stage.

Apart from the pre-heating of the bag filters by thermostatically controlled heating elements before starting up, the cement mills are on automatic operation. Each mill circuit is started by a pushbutton, all units in the circuit being interlocked in the correct sequence. The following safety or protective devices are provided: on the feeders; mill trunnion oil temperature and coolant water flow; water injection air and water circuits; gearbox oil level, oil temperature and float on the output shaft; motor bearing temperatures; cement discharge temperature; air temperature at the filter inlet and filter fan; and where applicable, material levels in the boots of the elevators. Should any of these be outside the pre-set limits, the mill circuit is automatically stopped and both audible and visual signals become operative. The feed rates are automatically controlled by sonic and load sensing devices for closed circuit milling, whereas only a sonic device is employed for open circuit milling.

Automatic sampling units are installed for sampling each mill product, before and after the respective dust collector products have been added to the conveying circuit. Surface area determinations are done manually on the samples collected at pre-determined intervals, and provision has been made for automatic determination when a reliable and accurate device has been developed.

The control panels for these mills are located in the centralized control room at the end of the common mill motor room, under the control of the master operator.

From the storage elevators, the different types of cement are distributed to various storage silos using air slides. The cement is stored in 10 silos with a total capacity of 30 000 short tons. Bag type dust collectors, mounted on top of the silos, are used for dedusting the cement circuits and the storage silos.

11. Packing and despatch

Facilities are provided for bag and bulk loading by road vehicles and by rail. For record purposes, all cement is continuously sampled before despatch. When certificates are required by the customer, the samples are tested.

Fluidised extraction, as for the raw meal, is employed and. screw conveyors are used to conveying the cement to the elevators serving the rotary packers and bulk despatching silos. Two rotary packers, each with an output of 2 200 pockets per hour, are installed. Each packer is served by a surge bin followed by the packer hopper. The surge bin is fed by an elevator and when the cement in the bin has reached the high level, a pneumatic valve is operated and the fluidised air supply to the silo extraction is cut and extraction stopped with the conveying systems still running. This has an overriding effect over the paddle control in the extraction screw.

The empty bags are manually attached to the filling spouts of the rotary packer; thereafter, the bags are automatically filled and weighed and when the correct weight is attained, the filled bags are automatically dropped on to an off-take conveyor which takes them to the loading point. A check weighing system is installed whereby bag weights are periodically checked on a statistically predetermined pattern of time interval and quantity from each spout. The weighing unit is mounted in line with the conveyor belt, and the bags are not manually handled at all. The individual bag weights, spout number, date and type of cement, are automatically recorded and kept for future reference.

Automatically operated bag deflectors are installed at the various off-take points for both rail and road loading, by utilising photo-electric cells and decatron units in the control circuits. The number of successive bags deflected and the number allowed to pass to the next loading chute, are remotely controlled from the control panel by pre-selection of the deflection controller. Photo-cells are also used to operate counters on the control panel; these counters are pre-set to the number of bags to be loaded by a particular chute and when the pre-set quantity has passed, the deflector is retained in the bye-pass position until the signal for the next load is switched on.

A degree of flexibility exists whereby any of the two main commodities could be packed by anyone of the packers. Rail loading of bagged cement can be done simultaneously on four tracks with two types of cement, and a similar facility exists for road loading. Two road lanes are provided under the two rows of three bulk loading silos, and weighbridges are provided in each lane which, through remote-controlled extraction, fill the bulk tankers to the pre-set loads ordered. The rail tankers are also filled on a weighbridge installed in the track adjacent to the three silos, and here again, the bulk cement is automatically cut off when the pre-set load has been reached. A dust collector is mounted on top of the silos serving both the dedusting of the silos and the air slides to these, as well as the road and rail tankers when these are being filled.

12. Centralized control

A central control or programming room is provided at the one end of the common mill motor room, being approximately the centre of gravity of operations. Consoles, with mimic diagrams, indicating and recording instruments, and control switches are installed in this room for each of the following departments: raw materials extraction; final crushing and proportioning; raw milling and blending; burning and coal milling, including raw meal extraction, kiln feed and dust return; finishing milling; and cement storage.

The primary crusher, being intimately associated with quarrying operations, and the packing and despatch plant are independently controlled from control panels in these sections and are not duplicated in the central control room. Intercommunication is, however, provided between all operating centres and the central control room.

Use is made of industrial television cameras, some of which are equipped with remotely controlled zoom lenses and/or pan and tilt motors. Cameras are installed in the extraction tunnels of both stores at the material transfer points from the extraction belts and at each kiln to observe the burning zone and flame characteristics. Receivers or screens are provided in each appropriate console in the central control room. A receiver provided with three channels from three scanning cameras on a timer circuit is also provided on the packing plant control panel for observing road, rail and bulk loading.

A teleprinter is provided for communication between the chief chemist's office and the master operator's desk so that quality control adjustments can be effected to the weigh feeders from the central control room. Provision has been made in an adjoining room for a computer centre to a later stage. Both rooms are sound and vibration proof and are air-conditioned.

Solid-state equipment, with standard modules, is used in the control systems and designed for future adaptation to computer control. The consoles are designed with standard sections to facilitate easy modifications and additions as and when required.

13. Quality control

The main laboratory has been placed as close to the centre of gravity of quality control points as practicable. A. pneumatic conveyor network has been provided for the conveyance of material samples from all the sampling points to a central receiving point in the laboratory. Samples taken from the automatic samplers are placed in the pneumatic conveyance containers by the operators at predetermined intervals laid down by the chief chemist.

As quality control is as effective as the equipment used for analysis and testing, it is essential that reliable and accurate equipment that requires a minimum of human judgment be provided with a far as possible. The laboratory is equipped, amongst others, with an X-ray spectrometer for speedy analysis of the major control constituents of the materials, an air-conditioned controlled curing room, and physical and chemical testing and analysing sections with their appropriate equipment, to meet requirements. A vibration-proofed, air-conditioned balance room forms part of the laboratory complex.

The basis of quality control in the cement plant is the achieving of compliance at each stage of the process with predetermined chemical and physical standards, with the greatest practicable degree of uniformity.

The functions of the laboratory include routine control, continuously, of the following items:

Quality of all incoming raw materials.

Proportioning, raw milling and blending processes.

Kiln feed quality.

Fuel as fired in the rotary kilns.

Clinker as produced in the kilns.

Cement as milled, and cement as despatched to customers.

Tests on cement milled and despatched ensure that the finished product conforms not only to the manufacturer's internal standard of quality but also to a recognised standard specification.

14. Maintenance

A high standard of the maintenance organization, with the required facilities and trained labour, is essential to ensure continuity of production and uninterrupted operation of the plant and equipment.

The workshop facilities provided are a fitting and turning shop, an electrical repair shop, a boilermaker and welding shop, a blacksmith shop, a vehicle repair shop, and an instrument repair and testing shop. Also, a mobile service unit is provided for the field servicing of the quarry equipment. Each of the shops is provided with the necessary tools and equipment, including overhead cranes, to facilitate repair work to be done as expeditiously as possible. The instrument shop is air-conditioned and adequately fitted out with testing and service equipment. All the workshops except the blacksmith shop are equipped with under-floor heating, thermostatically controlled for use during the cold season.

A planned maintenance office is provided and is under the direct control of the planning officer. The inspection, maintenance and repair schedules, which are revised at regular intervals, for all items of plant and equipment for both the quarry and factory, are issued, processed and recorded in this office. With maintenance stores and factory costs being computer-processed, maintenance records are designed to make the maximum use of this facility.

Where possible, and economically feasible, maintenance aids have been incorporated in the plant. These include a V.H.F. two-way radio installation between the various sections of the plant and the foremen and stores offices; fixed electric welding cable extensions to high elevations enabling the welding unit to be plugged in at floor level and the welding leads connected to the appropriate sockets at the various levels; central lubrication dispensing station; and lifting units to serve the various sections of plant as required.

Due to the isolation of the plant about industrial service facilities, use is made of service replacement spares, and a complete overhaul of diesel power units, motors, recording instruments, etc., are undertaken by the agents.

15. Labour

The estimated labour requirements for the management, operation and maintenance of the plant described, are 81 Europeans and 150 non-Europeans. This does not include labour employed in extraneous services such as employees' welfare, village and compounding facilities.

The labour breakdown is estimated as follows:-

In conclusion, I would like to thank my employers, Messrs Pretoria Portland Cement Company, for permission to read this paper and to use the photographs of some of the factory installations.

I also wish to thank Messrs F. L. Smidth, of Denmark, for the use of some of their ideas and photographs, and Mr Bob Riegel, of the Analytical Division of the Fuller Company of America, for some of the ideas incorporated in this paper.

DISCUSSION

Mr A. Simpson (Member): To those not directly connected with the cement industry it may seem odd that the establishment of such a factory in the Republic should be any different from similar establishments anywhere else in the world. On this aspect, I would draw attention to a few points where special consideration is necessary by comparison with other parts of the world, especially the more densely populated parts.

The Republic is a sparsely populated country. Distances between our densely populated areas are considerable, and long hauls have to be faced either for incoming raw materials or for the marketing of the finished product depending on whether the factory has been sited on the raw materials deposit or the markets. In the case of the hypothetical works, it has been assumed with good reason that. it should be sited on the raw material deposit.

In the Republic, we enjoy relatively cheap coal. It has often been said during the planning stage of a cement factory that it is wrong to think that this will always be so. I do not agree with this reasoning because I believe that coal will always be relatively cheap by comparison with its cost in other parts of the world. The price of coal will without doubt increase but I am quite sure that whatever the reason for these increases, the same reason will be experienced throughout the world so that we will continue to enjoy the advantage of lower-priced coal. Fuel economy in the Republic is important but not quite as important as it is in countries where the cost of coal is much higher.

In many parts of the world fuel economy is possibly the most important factor in deciding the processes to be adopted. This is a decision between wet or dry or between semi-wet and semi-dry processes. In these cases, the installation of an expensive plant may be justified because of the fuel economies which are achieved. In the Republic, however, other important factors to be considered together with fuel economy in the choice of the system are the availability of water and the complexity of the plant. In many instances in this country, factories have to survive on borehole water and this fact introduces the risk that the supply may fail or the quantity may reduce. As this. aspect applies to all of the larger factories in the Republic, there is no doubt that this point had a strong influence on the author's choice of a dry process. The type of dry process chosen by the author indicates that simplicity was also in his mind.

The author has also pointed out another important factor in the choice between the wet or the dry process. This is the moisture content in the raw materials and has established the dividing line at 17 per cent. Even considering our cheaper fuel I believe the 17 per cent dividing line is a reasonable one.

Regarding the hypothetical dry process plant out-

lined by the author, I have these few comments to make. Although the author has chosen rubber-tyred vehicles as the means of conveyance of limestone and shale from the quarry to the plant, I think that if the raw material deposit is uniform in its composition economics are strongly in favour of rail transport. Rubber-tyred vehicles are usually only resorted to where the raw material is not uniform in composition and selective quarrying becomes necessary.

The installation of a three-point gamma-ray level control at the primary crusher feeder is an excellent idea. Although most people take steps to ensure a protective layer of material at all times on the feeder, it is usually achieved manually and a gamma-ray level controller would appear to be a worthwhile labour-saving device.

The installation of an automatic material sampler after the primary crusher is a step in the right direction, but I wonder if it would not be better to see that the required grade of material reaches the works. However, this may have already been taken care of in the quarries and the automatic material sampler at the primary crusher is used purely as a check. When the X-ray spectrometer for automatic quality grading to assist in the separation of high and low-grade limestone has been installed it would be interesting to know how well this operates as it would be an advancement over the present methods.

The author has decided on an extraction slot design for the material store. With free-flowing materials, I believe the percentage of material recovered without any mechanical assistance is quite good in this design but I wonder if a general acceptance of this principle for all materials other than free-flowing is the right thing. It would appear from the author's paper that the more difficult materials do require the assistance of a front-end loader to achieve a reasonable percentage recovery of the material stored.

In the selection of plant for secondary crushing, I note the author's preference for impact breakers where the silica content of the raw materials is low. He mentions, however, that his choice would be gyratory crushers if the silica content gives rise to excessive wear in the impact breakers.

Some 19 years ago, I left an industry that made extensive use of gyratory crushers. The working life of the wearing parts of these crushers in that industry was short and their renewal was a major task. When I joined the cement industry, I was given the task of supervising the erection of large works and given my experience, I made quite sure that adequate means would be provided to make the task of the renewal of the wearing parts of the five gyratories crushers an easy one. These same crushers have now been in operation for fifteen years without ever requiring the renewal of any wearing parts. All the elaborate arrangements I made for their speedy replacement still stand as a monument to my failure to appreciate the difference between the materials handled. Given this experience, I disagree with the author insofar that I recommend that only gyratory crushers be considered whether the silica content is favourable or otherwise.

In the raw mills, the author shows a preference for a rolled steel liner in the first compartment and a cast steel alloy liner in the second compartment. With the advancement that has been made with cast steel alloy liners, I am wondering what the reason is for the preference for the rolled steel liners, and I would be pleased to have the author's comments.

The extraction of materials from bunkers or hoppers containing a large quantity of fine is undoubtedly a problem experienced by most manufacturers, and I notice in this paper that to minimize the number of fines in the bunkers, final crushing is carried out just before the entrance to the mill feed hopper. However, several advances have been made in dealing with this problem of extraction from bunkers and I wonder if it would not be easier to in tall these better extraction methods rather than to resort to crushing after extraction.

It is noticed that two independent systems of transport are installed from the raw mills to the blending plant when two types of clinker have to be produced simultaneously. This would appear to be an unusual feature in the hypothetical works.

In the kiln section, the device installed for scanning the firing zone area of the shell has brought forth some praise, and it is claimed that its intelligent use will extend the life of the refractory lining in the burning zone. I feel that the author should give more information as to the exact use that is being made of this device. Is the kiln automatically slowed down when the hot spot dips into the hot clinker to facilitate an increased build-up of coating, or does he envisage the use of an air blower which tends to cool the hot spot and 0 facilitate the build-up of a thicker coating?

The author mentions that the exit gas temperature is regulated by water injection. As the exit gas temperature would be of the order of 9 000P I wonder why this heat is not being used for drying the raw materials either before or during milling.

The use of an automatic sampling device for the collection of both raw and fine coal samples is a commendable one and the eventual expansion and link-up of this scheme with the proposed computer control is something I will follow with interest.

In the finish milling department, it is recommended that each of the mills is equipped with internal water injection for control of the temperature of the cement.

 I wonder if the author would advise what reduction in temperature can be achieved by this method by comparison with a mill not fitted with an internal injection system.

I now deal with a few points connected with quality control. From the author's choice of rubber-tyred vehicles, it appears that he has in mind a raw material deposit which is variable in composition and requires selective quarrying. This could only be done intelligently if the quarry manager knows exactly what type of material he is going to encounter in his quarry. This means that extensive sampling and analysis of the raw materials ahead of the faces being worked is essential. In other words, quality control starts in the quarry. But even this will not prevent appreciable variations in the composition of the limestone arriving at the works.

As it is much easier to get an accurate analysis of a thoroughly mixed, fine powder I wonder why the high and low-grade limestone are not first ground and stored separately in silos and then analysed. The proportions of each material required for a constant kiln feed could then be extracted from the silos and mixed in a blending tank. Would not this system be more positive than the one outlined by the author? I would appreciate his comments.

Mr R. Sandler (Visitor): I am reminded of the comment of a visitor to a cement works not many years ago. He exclaimed after being taken through the works: 'But I thought that cement was made by digging up a grey stone and grinding it to a fine powder: Mr Grant's paper has shown what goes into the manufacture of cement from the time the stone is dug up.

In the planning of a hypothetical dry process plant, Mr Grant described a long rotary kiln with an internal heat exchanger towards the feed end of the kiln. I assume that Mr Grant considered the other types of heat exchangers available for the dry process. I am thinking of the Humboldt suspension pre-heater, the Krupp pre-heater and the Lepol grate pre-heater. All of these types of pre-heaters are in use in Europe, although they have not found great favour in the United States.

Most of these pre-heaters are sensitive to certain physical and chemical properties of the raw materials, e.g. the presence of alkalis and chlorides in the raw materials. Perhaps Mr Grant would care to comment on these types of heat exchangers and the possibility of their application in future cement works in South Africa.

In his description of the rotary kilns, Mr Grant mentioned that these kilns would have a capacity of 900 ton/day each and that they would have a planetary type of cooler attached to the kiln shell. The capital and other costs of new cement factories are increasing, with the result that manufacturers are installing larger and larger units. This has in some circumstances required a revision of the type of equipment. We were considering planetary coolers for our new factory being built near Lichtenburg. We were advised, however, that the maximum available size for this type of cooler world be for an output of 900 ton/day. As the single kiln at our new factory is designed for about 1 350 ton/day, it was decided to use a grate cooler of the air-quenching type built as a separate unit independent of the kiln.

In this connection, the function of the cooler is not merely to cool the clinker to the required temperature. The rate of cooling is important because the chemical compounds in the Portland cement clinker are sensitive to the rate of cooling. To obtain good quality in the finished cement there must be a certain amount of glass in the clinker. This effects achieved by rapid cooling from the material burning temperature of about 1 450°C to about 800°C. Thereafter, the cooling to ambient temperature does not affect the quality of the clinker. The design of the cooler must be as thoroughly planned as the rest of the plant.

The layout of the new factory is similar to that described by Mr Grant. Most of the operations are controlled from a central control room from which the controller operates the raw mill, kiln and cement mill. These are arranged in much the same way as the layout shown by the author. Sensing devices actuated by the load on the mill elevator motor in conjunction. with sonic controllers actuated by the noise level m the compartments of the mills as well as other controls similar to those described in the paper, are used in our design.

These sophisticated methods of automatic control are a feature of most new cement works. The planning of these controls is usually done so that allowance is made for future computer control.

At this stage, I would like to sound a warning regarding too quick a step to complete automation. Computerised control has developed mostly in the United States. In the highly competitive market in that country, fully computerised automation has become a sales gimmick with the result that there have been some costly failures due to insufficient study of the problems and variables which occur in the operation of the cement rotary kiln and the chemical reactions taking place as the raw material passes through it. To illustrate the complexity of the rotary kiln operation, there is about thirty to forty variable which should be controlled. There may be an interaction between these variables, some of these interactions being random, with the result that the number of combinations of these variables may amount to many thousands.

A computer may be able to handle this, but you have to feed reliable information to a computer before it can give the correct answer. We have not, as yet, been able to measure accurately many of the variables mentioned. Perhaps Mr Grant would care to comment on computer control?

To give an idea of the magnitude of the work involved in planning a new cement works, the number of drawings required for a single unit plant, i.e. 1 raw mill, 1 kiln and 1 cement mill with their ancillaries amount to between 2 000 and 3 000.

I agree with Mr Grant's remarks on the importance of quality control during all the stages of manufacture. From the time the raw materials are taken from the quarries, through the various milling and burning operations, frequent tests are carried out by day and by night. Samples are taken hourly and tested chemically and physically. The results are transmitted to the controller or operator for immediate corrective action, if necessary. The finished cement is continuously tested to see that the quality is always to specification. After all this testing and control, it is most depressing for the cement manufacturer to be told by a customer who is having trouble with his concrete: 'There must be something wrong with your cement.’

Me P. W. Moss (Visitor): Mr Grant has delivered an illuminating paper on a hypothetical cement works and certain aspects associated with the production of the important commodity, cement. I will elaborate further on the variable speed equipment features on the kiln, fans and certain auxiliaries which give optimum output on this hypothetical plant.

The variable-speed motors are of the A.C. stator-fed induction regulator speed control type (a) which because of their robust design and reliability are used extensively for this and other applications requiring a variable speed drive. The development of this machine by the late Dr B. Schwarz and in particular the technique devised to give spark-free commutation has rendered it possible to manufacture motors of this design of 3 000 h.p. and above for supply systems up to 11 kV. For the works under consideration, Mr Grant has nominated a machine enclosed provided with air to air heat exchanger unit and suitable for a 2.2 kV supply.

The 'N-S' AC. commutator type variable speed machine consists essentially of a stator similar to that of a squirrel cage machine and a rotor with commutator similar to that of a D.C. armature. Speed control is effected using a variable voltage induction regulator connected in the rotor circuit, and the function of the commutator is to enable the rotor, irrespective of its speed, to commutate this regulator voltage which is at the same frequency as the supply to the stator. By lowering the regulator voltage the rotor speeds up whereas an increase in regulator voltage causes the rotor to slow down. It is also therefore possible to obtain speeds over synchronous speed.

The induction regulator affects exchange of energy between rotor and stator supply in that at sub-synchronous speeds energy is returned from the rotor to the supply and at hyper-synchronous speeds, the converse is the case which makes for an efficient method of speed control. The electrical and mechanical losses in the N-S commutator motor are no more than that associated with any rotating electrical machine of the same size. Furthermore, the N-S machine lends itself to dynamic braking.

Reference: Laurence Scott and Electromotors Limited, Publication No. 31/1.

Mr H. J. Yelland (Associate Member): It is noted that on the large mill drives (1 000 h.p. and 2 500 h.p.) auto-synchronous motors are employed, no doubt to improve the plant power factor. Could the author give any typical kVA demand figures applicable to the size of the plant he mentions and the power factor without power factor correction and with synchronous motor power factor correction only? Are static power factor capacitors used to supplement the synchronous motor power factor correction?

The author mentions that variable speed scraper feeder extraction units draw off material from under the shale and lime storage bins. Is this the type of mobile feeder sometimes termed a rotary plough, consisting of a unit which automatically traverses to and from on rails between adjustable limits? The variable speed feature here is interesting. Could the author give further details? Is it the speed of traverse that can be varied or the speed of rotation of the plough? What type of variable speed motor is used? The feed to these mobile ploughs is of interest and modern practice tends to the use of multi-core metal-clad safety conductor feeds in a single enclosure with a suitable composite pick-up device.

Could the author give some details on the voltages to be found for motor drives within the cement works described? 6.6kV seems ideal for the 1 000 h.p. drive and more especially for the 1 500 h.p. drive. Is 3.3 kV or 2.2 kV used for motors in the range of 200 h.p. to 1 000 h.p. and 500 V for motors of lower horsepower?

The modern tendency appears to be to use high voltage air break contactors up to 6.6 kV where any sort of frequent duty, or inching, plugging, reversing and complex interlocking is involved and where ease of inspection and maintenance and a reduction of outage time is important. A kiln drive is a typical example of where the use of such control gear is advantageous, and Fig. 1 shows a high tension air-break contactor control panel controlling a variable speed AC. type NS motor, for a kiln drive being currently supplied to a modern cement works in the Republic.

The author mentions the use of gamma-ray level control. This is known to be expensive but has the advantage of long life and there are no probes to be damaged by the material. What is the author's experience of this type of equipment compared with, say, the capacitance probe method?

The author has mentioned pressurised rooms for housing main mill drives and control equipment. Is it considered essential that nearly all motor control equipment and relay gear should be housed in such centralized pressurized rooms from where the feed is taken to the motors or other apparatus? This is one solution where bad environmental conditions prevail. The use of control panels with heavily gasket doors (often double doors) and other special constructional features, has been reported to in the past, but it is amazing how fine dust gets in even in these cases. In any case, you have to open up the doors for maintenance purposes.

The use of static switching devices has been mentioned. Would the author say that the desirable aim for the modern cement works is to eliminate all control equipment auxiliary contacts and relays and to use static components instead? Most static systems appear to employ plug-in units or modules. There is good reason to believe that the plugs and sockets themselves form the weak link in the chain and that modules, as shown in the photograph (Fig. 2), are the better proposition, to give maximum reliability. Fig. 3 illustrates an application of this type of gear.

AUTHOR'S REPLY TO DISCUSSION

Mr A. Grant (Visitor):

In reply to Mr A. Simpson:

As mentioned, rubber tyred units were selected by the shallow deposits and the consequent fast receding faces. Furthermore, due to the variation in the quality of the deposits, two faces are usually worked simultaneously to affect the initial blending of materials at the quarry, which, in effect, is the selective quarrying referred to by Mr Simpson.

The quality analysis of materials by conventional methods is cumbersome and slow, and tests conducted with the X-ray spectrometer have been most encouraging, both about accuracy and speed.

Primary crushed materials, i.e. minus 5 in, which proved less prone to arching when damp than the finer crushed materials, are extracted from the r.aw material store. The improved methods of extraction developed for fine materials may have merit, but from the author's experience these become too complicated to be entirely effective, and the simplified method described in the hypothetical case is regarded as more trouble-free and reliable in practice.

The comparison of the two different duties of the gyratory crushers described by Mr Simpson, and the reference to wearing and replacements, was most interesting. The selection of the impact breakers for the final crushing of non-abrasive materials was three-fold. Firstly, the wear in the breakers is small, and when replacements are required, it is comparatively easy and inexpensive. Secondly, the breaker gives a better size reduction, and thirdly, less headroom and floor area are required for this installation.

With the advent of the larger diameter mills, the duties imposed on the primary milling compartments with their large-sized grinding media, become demanding indeed, and only the best and consistent quality liners can meet the economic life expectancy. It is the author's experience that in the larger mills the physical properties of cast alloy steel liners must be consistent. Given the difficulty in ensuring the consistency of the quality of the liners required, rolled steel liners were found to be more reliable in practice.

The necessity of producing two types of clinker for ordinary and special cement is greatly influenced by the quality of raw materials available, and for special cement, it may be necessary to add materials which are not necessary for ordinary cement clinker.

With the relative cheapness of our coals, the emphasis is more on centralization of equipment and simplicity, rather than on heat conservation, hence the decision not to use waste kiln gases for drying of raw materials before milling. With the average moisture content of the raw materials only being of the order of 2? per cent to 3 per cent, the complications of using waste gases are not justified.

The scanning device described has proven its usefulness. The warning temperature could be selected to suit the plant, which will vary- between different types of installations. In all cases, however, it will be in the black heat range, and to detect deterioration of coating and/or lining before it is visible on the shell. is already a decided advantage. It is, of course. possible to link the kiln drive with the scanner and to vary kiln speed to assist coating build-up in the affected area. In the same way, air valves in a common duct with a series of outlets along the length of the burning zone could be actuated by signals from the scanner and thereby cooling the sections of the shell reaching high temperatures. Portable air fans are being used in conjunction with scanners by the author's company, and although probably premature, kiln shell temperature control has been most encouraging with the desired effect on refractory life expectancy.

Water injection is finished milling is automatically and thermostatically regulated by the product discharge temperature. Should the regulation temperature be such that by feeding cold clinker to the mill, no cooling is required, no water will be injected. Water injection into the mill is much neater and can be better controlled than shell cooling. Experiments have shown that internal water cooling is an effective means of suppressing temperature peaks in the temperature gradient of materials through the mill, and excessive dehydration of the gypsum constituent is averted.

With sophisticated analytical and proportioning equipment used in the raw material preparation, the separate grinding of raw materials of different qualities and subsequent proportioning and blending is eliminated. The latter is a common practice in wet plants, particularly where materials proportioning is not strictly controlled before milling, but in a dry plant, this method is not usually followed by the additional expense involved and the availability of the sophisticated controlling units.

In reply to Mr R. Sandler:

The heat savings claimed, and actually attained by the manufacturers of suspension pre-heaters, are impressive and consequently are in high demand in the countries where the price of fuel is high. It is known that this type of pre-heater has been sensitive to certain chemical and physical properties of raw materials and given the consequent operational difficulties experienced, it has not found the same degree of appeal in countries with relatively cheap fuels. Recent results on some new designs are most encouraging, and this type of pre-heater will deserve careful consideration in future kiln installations in the Republic of South Africa.

The maximum size of planetary coolers available is for units producing some 900 tons of clinker per day. In this connection, the author saw an installation in Finland, where a riding ring has been provided on the discharge end of the planetary coolers to obviate the overhanging load caused by the conventional planetary coolers. This was requested by the purchaser. It is not considered a physical impossibility to design and construct coolers to handle appreciably higher outputs than 900 tons per day, and it would behove the manufacturers to give serious consideration to this requirement. About the rate of cooling the clinker from the burning temperature to approximately 800°C, it is my opinion that the planetary cooler meets this requirement adequately, provided the lifters and cooling chains are properly maintained, and the units are not pushed too far beyond the designed capacities.

It would be foolhardy and short-sighted not to provide for computer control in future new installations. I concur that there are many variables which have not as yet been accurately determined, and it is considered fundamental to ensure that as reliable information as possible should be made available to the computer to ensure that the corrective measures computed are correct in operation. Computers are essentially labour-saving devices, and in the burning operation, the labour that could be saved is negligible in a modern plant. A computer in a cement plant must therefore justify itself by increasing output and refractory life and possibly by maintaining units at optimum performance level. One must guard against having sound reasoning and economic approach clouded by fashionable tendencies.