Mining Methods and Machinery

Surface mining equipment

Surface mining equipment is selected according to the cutting action it performs. The bucket wheel excavator is the machine that carries out both the drop cut and the terrace cut, because its boom can be lowered to sweep the wheel progressively down the face and it can also remove material in successive benches. A surface miner is a tracked continuous milling machine that cuts a uniform layer with a rotating drum fitted with picks. It has a conveyor and a scraper plate behind the drum to load the cut material, a tensioning arrangement for its crawlers, and water sprays for pick cooling and dust control. Steering is obtained by varying the speeds of the two tracks, so a differential gear for turning is not a standard feature. In truck and shovel operations the dumper cycle must match the shovel service. If the loading time per dumper is T_{load} and the dumper cycle time is T_c, the minimum number of dumpers that keeps the shovel busy is N \ge T_c / T_{load}. When fewer dumpers are present, each dumper waits for the shovel for N T_{load} - T_c minutes per cycle. For a surface miner, the in situ production rate is found from drum width, cutting depth and cutting speed, and any turning time and truck exchange time occurring in each pass must be added to the pass cycle. Dragline operation has its own vocabulary. The first cut that opens the working face is called the key cut, a tandem method uses two machines working together in the same cut, and a dragline digs below its own level and travels by walking on its walking pads. The hourly output of a dragline is the bucket volume multiplied by the fill factor, multiplied by 3600/T_c, and multiplied by the utilization factor, where T_c is the sum of digging and filling, swinging and dumping times.

Longwall face production rate

The production of a longwall face depends on the volume of coal cut per unit time. For a shearer with seam height h, web depth w and cutting speed v, the in situ volume cut per unit time is Q = h \times w \times v, and multiplying by the in situ density gives the mass output. A double-ended ranging drum shearer removes the web in two passes, one in each direction, so the production per pass is half of the production for the full web. The armoured face conveyor must carry this coal away from the face. Its required speed is the coal flow rate divided by the effective cross-sectional area of coal in the trough, and if a fraction of the cut coal is lost as spillage, the flow to be carried is reduced by that fraction. The number of webs in a panel is the panel length divided by the web depth. If uni-directional cutting is used, each web needs a cutting pass, a non-cutting return and an operational delay, and the number of days follows from the total time divided by the working hours per day. The drum diameter does not enter this calculation.

Hydraulic sand stowing pipelines

Hydraulic sand stowing transports sand as a slurry through a pipeline that is normally driven by gravity. The pipeline must therefore be laid so that its profile stays beneath the hydraulic gradient line, which is the line of pressure head plus elevation. Wherever the pipe rises above this line the pressure becomes negative, which causes vacuum, air release and blockage. The concentration of a slurry can be expressed by volume or by weight. For a volumetric sand concentration C_v and a sand specific gravity G_s, with water of unit specific gravity, the weight concentration is C_w = \dfrac{C_v G_s}{C_v G_s + (1 - C_v)}. The conversion is needed whenever a design quotes one basis and the process uses the other. In hydraulic stowing, the sand-to-slurry ratio and the hydraulic fill factor together determine how much water accompanies the solids. The hourly water consumption therefore follows from the slurry volume handled per hour and the proportion of that volume that is water.

Shuttle car haulage capacity

Shuttle car haulage is limited by the time each car needs for a round trip. The hourly capacity of a fleet is the number of cars multiplied by the average payload per trip and by the number of round trips per hour. The average payload is often less than the rated capacity, so the rated figure must be reduced by the average load factor. A haulage system is only as productive as its slowest link. The loading machine sets its own limit: a continuous miner cannot load more loads per hour than 60/t_{load}, where t_{load} is its loading time per car, regardless of how many cars are available. When one car of a fleet is out of service, the output is reduced, and the reduction is bounded by the loading machine's own limit, so the hauling capacity lost is not always the full share of the fleet. The capacity of a belt conveyor is its effective cross-sectional area multiplied by belt speed and by the bulk density of the broken material, expressed per hour. The ratio of production to belt capacity indicates how fully the belt is loaded.

Laubscher mining method selection

Laubscher's modified stability graph classifies a stope by its hydraulic radius, HR = A/P, where A is the area and P the perimeter of the exposed surface, and by the modified rock mass rating. The plotted point falls in a stable zone, a supportable zone or a caving zone, and each zone recommends a method. Open stoping is used in stable ground, supported methods such as cut and fill are used in the supportable zone, and block caving is used in the caving zone. For an erratic, high-grade orebody in competent ground, breast stoping is suitable because it takes slices across the orebody and allows selective extraction of the rich zones. Matching geometry, strength, support and method is a useful habit for these questions. Tabular, flat, strong ore is mined by room and pillar and is self-supported by pillars. Tabular, moderately steep ore of moderate strength is mined by cut and fill with artificial support. Massive, steep, weak ore is mined by block caving and is left unsupported.

Constant deceleration kinematics

Winding and hoisting problems assume constant acceleration and constant deceleration, with a steady cruise between them. For a ramp that reaches speed v at acceleration a, the ramp time is v/a and the ramp distance is v^2/(2a). The remaining distance is covered at constant speed, so the one-way hoisting time is the sum of the three phases. For a single-skip winder, the cycle time is twice the one-way hoisting time plus the loading and unloading times. The number of cycles per day is the number of seconds in a day divided by the cycle time, and the daily capacity follows from the payload multiplied by the overall utilization. In a friction winder, the motor torque must overcome the static load torque during steady running. During acceleration and deceleration the net torque accelerates the same inertia, so the angular acceleration is proportional to the net torque. If the inertia is the same in both phases, the times are related by t_a T_{a,net} = t_d T_{d,net}, which gives the deceleration time directly.

Locomotive haulage tractive effort

The design of a mine locomotive uses the total resistance per unit weight of the train. This includes rolling resistance, given by the coefficient of rolling friction, the grade resistance, which is approximately 1/n for a gradient of 1 in n, and the inertia term a/g during acceleration. The locomotive must also be heavy enough that its tractive effort does not exceed the adhesion between its driving wheels and the rails. The limiting tractive force is the coefficient of adhesion multiplied by the locomotive weight, and this limit is different from the coefficient of friction that governs rolling resistance. Requiring the adhesion limit to be at least the total resistance gives the minimum locomotive weight. The rolling resistance of a hauled train can be found by applying Newton's second law to the whole mass. The tractive force minus the grade component, the inertial force and the rolling resistance must balance the net force, and dividing the remaining resistance by the total mass gives the rolling resistance per tonne in kN per tonne.

Mine pump power requirement

The power of a mine pump is calculated from the total head, which is the static lift plus the losses due to pipe friction and shock, so H = H_{static} + h_{loss}. The hydraulic power is P_w = \rho g Q H, where Q must be converted from the hourly rate to \text{m}^3/\text{s}, and the motor input power is P_{in} = P_w / \eta_{overall}. When the specific weight of the water is given in \text{kN/m}^3, the hydraulic power is \gamma Q H, so the density is not needed. The annual electrical energy is the input power multiplied by the operating hours. Where the operating hours differ between periods of the year, the hours of each period are summed before converting the energy into \text{GWh}.

Centrifugal pump manometric head and impellers

The tip speed of a centrifugal pump impeller is u_2 = \pi D N/60, where D is the diameter in metres and N is the speed in rpm. For a blade whose outlet angle is measured from the tangent, the whirl component of the absolute velocity at outlet is V_{w2} = u_2 - V_{r2}\cot\beta_2, and the Euler head per impeller is H_{th} = u_2 V_{w2}/g. The manometric head is the manometric efficiency multiplied by this theoretical head. When the blade curvature is neglected, the whirl velocity at outlet equals the tip speed, so the theoretical head reduces to u_2^2/g. The impeller diameter then follows directly from the required head and the speed. The number of impellers needed in series is the required lift divided by the manometric head of one impeller, rounded up to the next integer.

Ore recovery in stoping

Ore recovery in stoping is the ratio of the ore actually extracted to the ore contained in the block. For a stope of uniform thickness, the thickness cancels, so the calculation can be carried out on plan or section areas. The outline of a stope may include fillets and rounded corners, and a corner of radius R removes an area of (1 - \pi/4)R^2 from a square corner. These losses reduce the extracted area, and the area of the shaded block must be taken from the block boundaries. In a sublevel or ring hole stope, the design tonnage is multiplied by the fraction actually broken. Overbreak of wall rock adds barren tonnage and dilutes the mined ore, and the metal content of the blasted muck is found from the metal grades of the ore and the diluting rock. The total metal recovered is the metal content of the blasted muck multiplied by the mucking recovery, which accounts for ore left behind in the stope.

Narrow vein mining methods

Resuing is a supported stoping method in which timber sets are removed from the mined-out area and reused as the working face advances. It is practised in narrow vein deposits, where the small excavation width allows the ground to be held by timber supports, and the recovery and reuse of the timber make the method economical. Thick deposits, massive shallow deposits and massive deep-seated deposits are normally mined by other methods, such as open stoping, open-pit or room and pillar methods, sublevel stoping or caving, so resuing is not selected for them.

Block caving

Block caving relies on undercutting a large ore block so that the unsupported block fractures and caves under gravity. Induced blasting is used to start and accelerate the caving of the undercut block, and this enhances the rate of ore production. Open stoping and supported stoping methods do not depend on induced caving, so induced blasting is associated with block caving rather than with them.

Haul truck tractive effort

For a truck moving up an incline, the forces acting along the slope are the tractive effort, the rolling resistance and the grade resistance mg\sin\theta. The net accelerating force is the tractive effort minus the sum of these resistances, and dividing the net force by the gross mass gives the acceleration. For constant acceleration, the time required to change speed from u to v is t = (v - u)/a. A small net accelerating force therefore gives a low acceleration and a long time to reach a higher speed.

Ram pump volumetric efficiency

A single-acting reciprocating ram pump delivers water during only one stroke per crank revolution, so the number of strokes per second is N/60, where N is the speed in rpm. The theoretical discharge is Q_{th} = \dfrac{\pi}{4} D^2 L N_s, where D is the ram diameter, L is the stroke length and N_s is the number of strokes per second. The volumetric efficiency is the ratio of the actual discharge to this theoretical discharge, \eta_v = Q_{act}/Q_{th}, and it is expressed as a percentage.

Underground mining machine components

Each underground mining machine has a characteristic component that identifies it. The continuous miner gathers and loads broken material over a loading apron. An armoured face conveyor moves coal along the face by means of flight bars carried on its chain. A jack hammer uses a rifle bar to transmit rotation to the drill steel through its ratchet mechanism. A powered shield support has a lemniscate link in its canopy linkage. Learning the component together with its machine is the efficient way to answer matching questions of this type.

Haul road pavement layers

A haul road is built as a layered structure so that the wheel loads of heavy haul trucks spread over a wider area and the stress reaching the weak ground beneath stays within safe limits. Each layer has a defined role, and the layers must be placed in a fixed order from the surface downward: wearing course, base course, sub-base, and subgrade. Each layer is designed to receive load that has already been reduced by the layer above it, so the order is not interchangeable.

• Wearing course: the top surface layer that resists abrasion, tyre cutting and weathering. It is usually a well-graded crushed material kept free of dust. e.g. A grader re-blades the wearing course after heavy rain to keep the surface smooth.
• Base course: the load-bearing layer directly under the wearing course, made of stronger and coarser crushed stone. e.g. Most of the vertical load is spread out in the base course before it reaches the layers below.
• Sub-base: the layer between the base and the subgrade. It adds thickness, improves drainage and acts as a filter. e.g. A granular sub-base stops fines from a soft subgrade from rising into the base.
• Subgrade: the prepared natural or compacted soil that forms the foundation at the bottom of the structure. e.g. The subgrade must be compacted and trimmed to design level before any aggregate is placed.