The purpose of this report is to provide a description of the period of vacation work spent at Exxaro Resources’ Grootegeluk mine. The requirement was set to the student to spend time with and work with artisans in their everyday environment to gain valuable practical training and experience. The training provided a broad overview of the environment that an engineer can be exposed to and must be able to handle during his or her practice. A comprehensive understanding of different disciplines was attained by working with diesel mechanics, millwrights, electricians and fitters. Tasks completed range from the manufacturing and assembly of components to the maintenance of existing systems and troubleshooting of systems that don’t function as expected. The incorporation of a study of personnel management added another dimension to the training period by informing the student about the enabling, employee lifecycle and supportive services offered to the employee to ensure a satisfying and advancing working environment.
INTRODUCTION
Exxaro Resources Ltd is a South African mining company that owns coal, mineral sands, base metals as well as industrial metal related assets. The majority of its revenue is however generated by its coal mining business units. Exxaro’s Coal Mining Division is established as the fourth largest producer of coal in South Africa with eight different coal mines throughout the Limpopo and Mpumalanga provinces under their control. Exxaro’s Grootegeluk Coal mine in Ellisras in the Limpopo province is an open-cast coal mine with the largest coal beneficiation facility in the world. It supplies power station coal to the Matimba Power Station and also produces coking coal for use in the production of steel, as well as high quality metallurgical coal. It is also the home of the Medupi power station expansion project which, after completion will lead to Grootegeluk being the largest open-cast mining facility in the world.
CONTENT
2.1. PRACTICAL TRAINING
2.1.1. SECONDARY MINING MAINTENANCE
2.1.1.1. INTRODUCTION
The central workshop on the Grootegeluk site is responsible for tasks such as repair, maintenance, assembly and fabrication. The workshop is divided into three sections, namely Mining Maintenance, Plating and Refurbishment.
The secondary mining maintenance section situated in the central workshop on the Grootegeluk site is primarily responsible for the maintenance of the water trucks, tippers, low-beds and smaller trackless mobile machinery. Time was spent working in this section, in cooperation with diesel mechanics, to acquire knowledge of the maintenance and operation of the trucks.
2.1.1.2. TRUCK INVENTORY
3 water trucks and 3 tippers are required to operate at any time. The trucks used are supplied by Caterpillar and are classified as the CAT 777 models. The models that operate on the mine currently are the CAT 777D and CAT 777F models. The plan at the moment is to phase out the CAT 777D trucks and incorporate the newer CAT 777F models.
2.1.1.3. LAYOUT OF CHASSIS AND POWER TRAINs
A major advantage of the fact that the CAT 777 models are being used is the fact that the chassis and power train can be used interchangeably between trucks that are of the same model. Depending on what is required, either a water tank or bucket can be mounted on the chassis making it a versatile alternative to the purchasing of trucks manufactured for specific purposes only.
The trucks are powered by an 870 horsepower, 27.9 litre engine. The power from the engine is sent through a torque converter, which is followed by a prop shaft leading to a semi-automatic transmission, differential and the left and right rear final drives.
The basic layout of the truck can be shown as follows:
2
2.1.1.4. WATER TRUCKS
Responsibility of water trucks:
Spray water on the mine roads to reduce the amount of dust caused by the passing of trucks and mine machinery
In the event of a fire on the mine, the water trucks may be used to extinguish the fire by use of a nozzle mounted on the front
Capacity:
The total load carrying capacity of the water trucks depends on the model. The capacity of the tank on the older CAT 777D trucks is 80000 litres while the tank on the newer CAT 777F has a slightly larger capacity of 82000 litres.
Pump and spray system:
A centrifugal pump system is used at the back of the truck to pump water from the tank to the spray nozzles. Water flows down from the tank into the pump where it is then rerouted upwards into the piping system leading to the spray nozzles. The spray nozzles then project a jet of water at a small obstruction which again changes the direction of travel causing the water to disperse
Diagram of piping:
Diagram of side view of spray nozzles
2.1.1.5. TIPPERS
Responsibility of tippers:
Waste and overburden in the mine is defined as rock and soil that cannot be used for the extraction of product. It also includes the waste produced during the extraction of the product. Because of the small amount of coal content that might still be present in the waste there is always a risk of spontaneous combustion. The tippers are responsible for moving and dumping red sand and topsoil on the waste dumps and the areas around the pit to shield the area from intense direct sunlight and reduce the probability of spontaneous combustion.
The tippers also operate occasionally in the pit together with front-end loaders to clean out small amounts of material that the shovels and larger trucks are unable to collect because of the lack of space for operation.
Capacity:
The load carrying capacity of the tippers is dependent upon the condition of the hydraulic system used to lift the bucket. Two hydraulic cylinders are used to lift the bucket and tip the load. In a brand new condition, the tippers are able to handle a load of 120 tons while a tipper that has been in service is typically only loaded up to 100 tons to ensure that the system will be able to dump the load.
2.1.1.6. SERVICING THE TRUCKS
In the coal mining environment, the trucks are required to be serviced after every 300 hours of operation. During the training period, both the CAT 777F and CAT 777D models were serviced.
The 300 hour service on the CAT 777D truck entails the following:
Sampling
3 different samples of fluids were taken while the engine was still running
Engine oil
Transmission fluid
Hydraulic fluid
6 more samples were taken after the engine was switched off
Left front hub oil
Right front hub oil
Left final drive oil
Right final drive oil
Steering fluid
Differential oil
These samples were then sent to the mine laboratory. The samples are then analysed to check for the presence of iron filings or debris that could indicate the presence of wear on the components.
Replacement of filters
The sump plugs and used oil filters were removed to drain the engine oil
Although the primary sump is the most important to be drained, the engine also has a small secondary sump that was also drained
Used oil was caught in an oil trolley
New oil filters were installed
Diesel filters were removed
New diesel filters were installed
New steering filter was installed
Checking the fluid levels
The level of the oil in the final drives, front hubs, differential, hydraulic system and transmission were checked
Transmission fluid was filled up
Engine oil was filled up
The servicing of the CAT 777F trucks was handled by certified Barloworld technicians, since the trucks have only been operating on the mine for a short time. Mechanics present in the workshop had to thoroughly observe the tasks that the technicians were performing in order to learn what needs to be done. Eventually the task of servicing will be handed over to the diesel mechanics.
2.1.1.7. DRIVING THE TRUCKS
The trucks operate using as semi-automatic transmission. This transmission eliminates the need for a clutch pedal, leaving only a brake pedal and an accelerator pedal. A torque converter is however incorporated with the gearbox to fulfil the purpose of the clutch.
Three different braking systems are available on the trucks. Operators seldom use the foot brake which exerts a braking force on the all four wheels of the truck because of the heat generated. The steering column is fitted with levers to operate two other braking systems. The retarder lever is primarily used to slow the truck down by exerting a braking force on the rear wheels. The secondary lever is a last resort for operators and, when pulled, exerts a braking force on all four of the truck’s wheels simultaneously. When the trucks are stationary and the engine switched off, the park brake has to be engaged. Additional stop blocks are placed behind the wheels of the trucks to prevent them from rolling.
When the truck needs to be driven the following steps are taken:
Engage the park brake
Engage the retarder lever
Select the number of gears required from the transmission
Disengage the park brake
Disengage the retarder lever
Press the accelerator pedal
The transmission will shift through the number of gears selected
Steer the truck
2.1.2. CENTRAL WORKSHOP: PLATING
2.1.2.1. INTRODUCTION
The central plating workshop at Grootegeluk mine is responsible for the majority of the sheet metal work that needs to be done on the mine. Artisans in this section have the task of manufacturing and assembling components of structures or machinery in and around the mine. The plating workshop also has facilities to sandblast and spray the components manufactured. The majority of the workload of the workshop finds application in the production and beneficiation plants. Time was spent with boilermakers in order to gain an understanding of sheet metal work.
2.1.2.2. MATERIAL
Mild steel and stainless steel are the dominating types of steel used in the workshop. The type of material used for an application is in some instances limited by the capabilities of the available machinery.
2.1.2.3. PREHEATING
Preheating of materials is an advantageous process when welding has to be performed and aids in the assurance of a strong, quality weld. Four main reasons for preheating exist.
Preheating of the metal reduces the rate at which the welded component cools down. Rapid cooling of the welded joint could cause shrinkage of the metal in the vicinity of the weld which eventually leads to the formation of cracks and reduces the strength of the weld.
In materials with low ductility, the shrinkage stress in the weld area could cause extensive deformation of the component after welding. Preheating lessens the effect of distortion by giving the welder the opportunity to utilise a momentary increase in ductility during the welding process.
When the temperature of the parent material that is being used is too low, it can cause the deposited electrode metal to cool rapidly, leading to the prevention of the fusion of the metals. Preheating lowers the risk of this situation occurring. The amount of preheating required is dependent upon the thickness and configuration of the plates to be welded.
The final reason for preheating relates to the presence of moisture on the surface of the metal. If the surface of the metal is wet during the welding process it could lead to the rejection of the weld or an accelerated tempo of surface crack formation in the welding region.
2.1.2.4. TIPS FOR FLUX-CORED ARC WELDING
The flux-cored arc welding used in the workshop uses a wire electrode which is shielded by an appropriate gas. In general, flux-cored wires are manufactured to function with either carbon dioxide or a mixture of argon and carbon dioxide as shielding gas. The shielding gas prevents the spark from causing the uncontrolled dispersion or oxidation of the electrode metal.
Flux-cored arc welding is generally performed by dragging the welding gun along the joint that needs to be welded. When welding t-joints it is important to maintain the welding gun at a 45á´¼ angle to ensure that the electrode metal is evenly deposited in both pieces of metal.
When completing butt welds the torch needs to remain in an upright position and should not deviate from the upright position by more than 15á´¼. This will also ensure even distribution of the electrode metal during the welding process.
75á´¼
2.1.2.5. EQUIPMENT
Welding
Lincoln electric Idealarc DC-600 power source
Direct current welding power source with a maximum current output of 850 A and maximum voltage output of 44 V
Lincoln electric LN-25 PRO semi-automatic wire feeder
Wire feeders are connected to the power source to feed electrode wire through the welding gun
Tri-mark TM-791
Flux-cored electrode wire used in conjunction with CO2 as shielding gas
Matweld Anti-spatter Silicone mat 0810
Spray canister that is used to prevent the spatter of electrode metal during the welding process
The spray is applied to the welding gun
Cutting torches:
Two combinations of gases are used in the cutting torches. LP gas and oxygen are used together, or acetylene and oxygen
Lighting the torch
The operator opens the LPG or acetylene feed and lights the gas
After the LPG or acetylene has been lit, the oxygen supply is opened to enhance the flame
An optimal flame to cut metal with is a quiet flame of blue colour with no visible or distortions
Application of torches
Torches are primarily used to cut mild steel in the workshop. Torches cannot be used to cut stainless steel. Technically speaking torches do not cut, they burn the metal. Burning involves oxidation of the metal. The high temperature of the flame accelerates this oxidation process. Stainless steel has low iron content and will not rust in the presence of the flame.
Automated cutting torch
This machine has the ability to follow a shape by means of a proximity eye on a table at the left side of the machine, while simultaneously moving the torch in the exact same pattern to cut a component from metal plates on the right side of the machine. Shapes are drawn and cut out on a yellow plastic sheet. The edges of the shape are then painted white. The shape is then placed under the eye which follows the outline of the shape while cutting the metal in the same way.
Plasma cutter:
Cebora Plasma Prof 80 art 947
Uses only compressed air to make accurate cuts in metal up to a thickness of 20 mm and rough severance cuts up to 30 mm
Operation:
The plasma cutter used in the workshop utilises high pressure gas which is sent through a small tubular gun. The small tubular gun contains a negative electrode that creates a circuit when the gun is brought close to the metal. This electric spark caused by the circuit causes the gas to be heated into the plasma state of matter and reaches a temperature of about 16000á´¼C. This extremely hot plasma then melts the metal that is being cut. The plasma cutter can be used to cut any metal. In the workshop it is used to cut stainless steel plates.
Sandblasting equipment:
Spartan engineering 800M pressure vessel
200 L capacity
The purpose of the sandblasting equipment is to clean and remove paint from the surface of metal components which then prepares the surface to be spray painted.
2.1.2.6. TASKS OBSERVED
Due to the nature of the precision and accuracy required to complete the jobs, most of the time in the workshop was spent observing and assisting.
The following jobs were in progress:
Manufacturing and assembly of waste buckets
Waste buckets are used around the mine for different waste material. These waste buckets are made by the mine boilermakers. Three waste buckets were being built for use around the mine. Plates for the structure had to be cut, bent and welded together using a cutting torch, bending machine and welding machine.
Basic side layout
Basic front layout
Wear plates
Two sets of wear plates are used as sections of a vibrating beam in one of the assemblies in the plants
One set of plates have six holes through which it fastens the beam
The second set of plates are rectangular and support two springs
Because of the magnitude of the forces acting on these wear plates, bearing failure of the plates occurs during operation. This wear is however allowed and monitored for a period of time before the beams are then removed and the worn out plates cut off
New wear plates were manufactured according to specification
Clamps for pipes
Clamps were manufactured to fasten the pipes used in the plants
Haulpak truck operators cab
Boilermakers assembled the frame of an operators cab for one of the Haulpak trucks. Drawings were supplied giving detail of the cover plates that had to be fabricated, as well as assembly drawings to show the final required layout. The welds utilised were to be either 3 or 6 mm one-sided fillet welds. The M12 nuts that were used to assemble the frame also had to be tag welded.
Extractor fan piping system
Maintenance on the plants requires occasional replacement of the piping on the extractor fan system. A 6 pipe assembly was manufactured to replace the old system. Flanges were cut and holes for bolts were punched. Pipes were cut to the appropriate lengths and shapes after which flanges were welded onto the pipes.
2.1.3. GG 3/4/5 AND WASTE MANAGEMENT WORKSHOP
2.1.3.1. INTRODUCTION
The GG 3/4/5 and waste management workshop at Grootegeluk mine is responsible for the mechanical and electrical maintenance of the GG 3,4 and 5 plants as well as the system set up to convey waste to the dumps. A wide variety of systems, from substations to conveyor belts, are the responsibility of the artisans in this workshop. Time was spent with fitters and electricians to gain an understanding of the tasks required.
2.1.3.2. TASKS COMPLETED
Replacement of motors in GG 5 tunnel:
After completing the necessary safety protocol the first task was to replace two 380 V electric motors in the GG 5 feeder tunnel. The job required both electricians and fitters to complete and was completed by these steps:
Since the motors operate on a 380 V control voltage, the first task was to cut the electricity supply to the motors by isolating the breaker in the substation
The new motors, weighing in at 118 kg each were carried down into the tunnel using a sling wound around the motors
The electrical supply wires in the cable box were disconnected
After disconnection, the bolts on the old motors were loosened and the old motors were removed
The new motors were then hoisted into place by a small handheld portable crane
The bolts were fastened to keep the motors in place
The electrical supply wires in the cable box were reconnected
The supply to the motors in the substation was switched on
The final step was to check if the motors were in fact turning in the right direction
Proximity sensors:
The job relating to the proximity sensors required the attention of electricians and required thorough investigation into and troubleshooting of the wiring circuit leading to the sensor on the feeder motors. The proximity sensors kept burning when they switch after connection.
The function of a proximity sensor is to detect the presence of a metal component within 5 mm of its periphery and takes the form of a small circular cylinder with a threaded outer casing and electronic components inside. These sensors are used to indicate to the operator whether the lever at the electric motors has been engaged. This prevents the motor from running without being engaged to the feeders.
The troubleshooting followed a sequential path:
First the cable leading from the junction box at the motors was followed back to the PLC in the substation
The basic function of a PLC (Programmable Logic Controller) is to provide an electronic interface between the supply and the components
The PLC can be set up to perform certain tasks during certain time intervals and can also receive and respond to inputs from other electronic components
From the PLC the total length of the cable was divided into 4 sections
PLC to distribution panel
Distribution panel to junction box
Junction box to cable box
Cable box to proximity sensor
The connection at the proximity sensor requires the presence of a live and neutral wire
At first glance the suspicion was that both the wires available were live wires, thereby causing a short circuit when the proximity sensor switches
A Meggar insulation tester was then used to test each length of cable
The basic function of a Meggar is to test the magnitude of insulation between the conductor and the earth
A low reading on the Meggar indicates the possible presence of a short circuit or damage to the wire insulation
After the use of the Meggar the proximity sensor was sequentially wired into the circuit at each section and tested
Eventually the short circuit was found between the cable box and the proximity sensor
Servicing of slip ring motors;
The high voltage electric motors used to drive the waste conveyor belt system are 6.6 kV slip ring motors. These motors need to be serviced regularly to ensure efficient functioning of the system and to prevent the motors from being damaged.
Equipment needed:
Meggar insulation tester
Blower
Cleaning solvent
Extension cord
In order to complete the service of the slip ring motor, the following steps had to be taken:
The electrical supply to the motor was cut off by isolating the breaker in the substation
The side cover panels of the electric motor were removed
Twelve brushes inside the motor were then removed from the brush holders
Special care had to be taken to make sure that the brushes don’t touch each other
The Meggar insulation tester was then connected to the slip rings
Negative terminal connects to the body of the motor
Positive terminal connects to the slip rings
An initial reading of 640 MΩ was recorded
The control voltage on the motor is an indication of the reading required from the Meggar
For a 6.6 kV motor the reading from the Meggar should at least be 6.6 kΩ
The Meggar was removed and the inside of the slip ring and brush casing was blown out with the blower
After blowing out dust and fine copper, the slip rings and the inside of the casing were wiped with a cloth and solvent to remove the last bit of fine copper
The Meggar was reconnected and a reading of 3.22 GΩ was obtained which was adequate
The side cover panels were replaced and the electrical supply to the motor was switched back on
Replacement of a 6.6 kV slip ring motor:
The 6.6 kV electric motors used for the conveyor system eventually deteriorate in such a manner that they cannot be brought back to an acceptable operating state by means of a service only. These motors then need to be replaced and are sent away to be properly refurbished if it is possible.
The following steps were followed:
The electric supply to the motor was cut off by isolating the breaker in the substation
The electromagnetic drum brake was released and moved out of the way
The shaft coupling (jaw coupling) between the shaft on the motor and the shaft on the gearbox was disengaged
The panels covering the electrical phase terminals and winding terminals were removed
and the supply cables were disconnected
The next step was to loosen the bolts at the bottom that fasten the motor to the structural frame
The old motor was then hoisted by means of a forklift and removed
The new motor was then put in place
The electric supply was reconnected to the phase and winding terminals
The bolts at the bottom of the motor were fastened slightly and the shaft coupling replaced to prevent excessive movement of the motor
Alignment of the shafts was then done
Shaft coupling was properly engaged
The bolts on the structural frame were fastened
2.1.4. CONVEYOR BELTS
2.1.4.1. INTRODUCTION
A brief introduction was given to a typical engineering problem to provide insight into what is often required from engineers. The belt on the waste management system leading to the dump needs to be extended. This is causing problems in terms of the power required from the slip ring motors. The extension of the belt causes additional load to be hauled by the motors. The motors that are currently installed trip when started up on full load. Time was spent studying and applying calculation to determine the power required from the motors to drive the conveyor belt system.
2.1.4.2. POWER REQUIREMENTS OF CONVEYOR BELT SYSTEMS
The governing factors relating to the power requirements of a conveyor belt system concerns the provision of the necessary force to overcome the resistances posed by the entire system.
These resistances can be divided into 5 subsections:
Main resistances FH
Secondary resistances FN
Special main resistances Fs1
Special secondary resistances Fs2
Slope resistance Fst
Main resistances:
The main resistances that the motors have to overcome relate to the resistance of the rotating idlers, the resistance by the movement of the empty belt, the resistance of the material to horizontal movement as well as the resistance of the belt due to a slope along its conveying length.
The resistance posed by the rotation of the idlers is manifested in the form of the frictional resistance of the idler bearings and seals. Rotational inertia of the idlers also contributes to the resistance posed.
The belt creates resistance by means of the indentation resistance of the belt on the rollers. The flexure of the belt and the material that the belt is made of also resists the movement of the belt.
An overall calculation to determine the resistance due to rotation of idlers and movement of the empty belt takes the following form:
FH1 = (qro + qru + 2qb cos α) x f x L x g
qro – Mass per unit length of rotating idler parts on the carrying side [kg/m]
qru – Mass per unit length of rotating idler parts on the return side [kg/m]
qb – Mass per unit length of the belt [kg/m]
α – Angle of inclination [degrees/radians]
L – Centre to centre conveyor length [m]
g – Gravitational acceleration [m/s2]
f – Friction factor due to idlers
The mass of rotating idler parts and the mass per unit length of the belt itself is determined by means of the tables of specifications given in the design catalogue or procedure followed. This requires the belt width which can be decided upon or determined mathematically.
These masses are then converted into mass per unit length by means of the following formulas:
qro = mro/ao [kg/m] qru = mru/au [kg/m]
ao – carry side idler spacing
au – return side idler spacing
The decision then needs to be made with regard to the selection of a friction factor. This is also specified by the design catalogue. The Phoenix Conveyor Belt Design Fundamentals catalogue provides the following guidelines for selection:
f = 0.017 for well aligned belts with smooth running idlers and low friction
f = 0.02 for normal applications
f = 0.023 to 0.027 for harsh operating environments, high frictional forces and the occasional overloading of the belt
The centre to centre distance can be described as the distance from the head to the tail of the conveying system and encompasses the total possible length over which material can be conveyed. This is usually a parameter that is pre-determined by the specific situation.
Finally the calculation of the resistance due to the rotation of the idlers and the empty belt force can be done.
The next calculation that needs to be done relates to the resistance of the material to being conveyed horizontally.
FH2 = qm x L x g x f x cos α [N]
qm – mass of the material per unit length that is being conveyed
To calculate the value of qm needed in the above formula, the total capacity or throughput of the belt needs to known.
qm = Qm/v [kg/m]
Qm – The capacity or thoughput of the conveyor system [kg/s]
v – velocity of the belt [m/s]
The calculation for the resistance of the material to horizontal movement can now be done.
The resistance posed by gravity due to a slope/gradient along the conveying length also needs to be taken into consideration.
Fst = qm x H x g [N]
H – change in the elevation of the belt along the length [m]
After this calculation, all the major primary resistances needed have been calculated
Special main resistances:
The friction caused by the movement of the belt past the chute flaps is regarded as an important factor that needs to be calculated as part of the power requirements.
Secondary resistances:
Secondary resistance to the movement of the belt takes into consideration the detail relating to the operation and design of the belt.
When material is deposited onto the conveyor belt system, a force is required to accelerate the material in the direction of conveying. This force is manifested in the form of the change of momentum of the material when dropped onto the belt and. Additional resistance to movement is then imposed on the drive system.
The presence of skirt plates in the vicinity of the chute to keep material from dropping off the belt causes more resistance to the movement of the belt. This resistance depends on the magnitude of the friction force between the belt and the plate as well as the length of belt in contact with the plate. Occasionally the skirt plates also cause the material to exert a force on the belt which leads to additional frictional resistance.
Other secondary resistances also include the resistance caused by the pulley bearings and the wrap of the belt around the pulleys.
Special secondary resistances:
Additional systems installed on the belt can also cause resistance to the movement of the belt. The basic operation of belt cleaners leads to friction forces being present between the belt and the material as well as between the material and cleaners. These friction forces, combined with the forces of the discharge ploughs, impose an additional load that need to be overcome.
The inversion of the belt at the head and tail causes resistance to movement due to the combined effect of the flexure of the belt material and the friction of the pulleys.
When designing a long conveyor belt system, the magnitude of the primary resistances generally exceeds the magnitude of the secondary resistances. To simplify the resistance calculations, the secondary resistances are simply accounted for by means of a correction factor on the primary resistances.
C = 0.85 + 13.31L-0.576 for 10 < L < 1500
C = 1.025 for 1500 < L < 5000
L – Conveying length [m]
When this factor has bee
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