Ground sluicing utilises the erosive power of flowing streams of water in open channels to process material broken by hand and is one of the oldest methods of mining. Conventional practice is to construct a dam across the watercourse above the section to be mined and to channel the water along flumes cut into the pay gravels. Material shovelled in from the sides is broken up and slurried manually to release the values. The gold is recovered behind riffles in wooden sluice boxes that arc given gradients of 1:12 to 1:10 or steeper.
In larger-scale operations where much fine gold is present, a ground sluice may be sectionalised with the downstream sections acting as scavengers. The slurry flow is stopped and the water is diverted back into the main stream when gold first appears in the final sluice section. The boxes arc then cleaned out and the gold is recovered by panning. Periodically, when shovelling distances become excessive, a fresh sluice is dug closer to the foot of the receding bank. The procedures arc repeated as necessary until all of the gold-bearing wash has been mined.
Ground textures vary widely and the slopes and dimensions of ditches and other earth channels must be designed accordingly. Channels are usually trapezoidal in section with sides sloping at some angle less than the angle of repose to avoid slumping. This angle may be around 45° for soft ground up to 60° for hard compact ground: wooden flumes may be used when steeper slopes
cannot be avoided. The best hydraulic section has width greater than height; a common W.H. ratio is 2:1. Smirnov (1962) lists critical channel flow velocities for different sized materials.
A discharge route (tailrace) is common to all sluicing methods. Normally this comprises a channel cut into the soil at a gradient sufficient to carry away all of the waste material. A gradient between 1 and 2 degrees is usually adequate to prevent settling, but it may have to be steeper depending upon the gravel size and the depth of water flowing through the race. If necessary, the race must be cut progressively deeper into the natural ground surface with increasing distance from the face. The slope of the ground is a limiting factor and. at some stage, the spoil may have to be elevated and disposed of by hydraulic elevation or by pumping.
Ground sluicing was practised widely in early Roman times. Army engineers of the day recognised that a natural head of water could be utilised to supply energy at the working face, so streams of water were channelled for great distances in mountain areas to gold mines on which much of the prosperity of Rome depended. The method, first described by Pliny the Elder in relation to gold mining in Spain during the first century AD employs a dam which fills slowly and is periodically breached when full. The water is then directed through flumes to the pay gravels.
The same method, referred to as ‘booming’, was used in the early days of some North American goldfields in areas of less intense precipitation, i.e.. where run-off and stream flow provides only a small trickle of water. Dams were fitted with lightweight gates (counter-balanced) to which a long lever was attached. A large container was hung from the end of the lever. When the dam filled, water overflowed and filled the container. This activated the lever allowing the water to rush out and scour the channel bottom. In its lowest position the bucket tilted, spilling out the water, thus allowing the gate to reposition itself under its own weight. The gold was trapped behind riffles or stones laid along the floor of the sluice while the light materials were washed away.
Early miners used a different form of ground sluicing to mine surface exposures of gold-bearing conglomerates. The ground surface in this area was traversed by herringbone patterns of channels radiating out from single channels located in the lowest parts of the terrace. These channels acted as tributaries to collect large volumes of water running off from higher ground during heavy monsoon rain periods. The flow from these channels was directed into a central channel, which cut back into the sluicing face dislodging material for treatment in ground sluices.
DEROCKING & SLUICING
The first recorded use of pipes to convey high-pressure water to the face was in the USSR in 1830. The method then emerged in the California goldfields in 1840 and soon spread to alluvial goldfields in other parts of the world. Monitors, otherwise called hydraulic giants (Fig. 7.10), were developed to enable high-pressure jets of water to be directed against the face as required. The resulting slurry was washed into a pump sump through races cut into the bedrock. Hydraulic elevators (Fig. 7.11) used to elevate the slurry to a sluice box were very inefficient, and the subsequent introduction of centrifugal gravel pumps extended the availability of gravel pump mining, to any area having an adequate supply of water, regardless of head.
Suitable ground conditions for hydraulic sluicing arc provided by small gravelly wash that is easily slurried and soft weathered bedrock in which races can be cut to direct the slurry from the face to a head feed pump sump. A natural slope of about five degrees from the horizontal is an optimal gradient but slopes may be 30-40% flatter or steeper without seriously affecting the operation. At any such gradients, most of the slurried material gravitates from the face to the sump without excessive surging or settling out of the finer gravels.
The monitor unit, or hydraulic giant as it is sometimes called, is a nozzle for directing a stream of high-pressure water against the working face. Some larger units incorporate deflectors to give a better control of jet direction. Various degrees of sophistication arc applied to balancing the re-active thrusts developed by the jet, the simplest being counterweights attached to the arm.
Monitors arc used to undermine a pit face and so encourage slumping. Material broken by the monitor jet is slurried by the jet and washed down through races (channels) into a gravel pump sump in the pit floor. Riffle boxes may be placed in the ground races to effect an initial recovery of coarse gold. The larger stones are forked out and stacked along the sides and back of the excavation. A gravel pump elevates the remaining slurry to a gold-saving plant, which may either be a riffled sluice box or a more sophisticated jigging plant. Nozzle diameters range from around 25 mm up to 125 mm and provide jet velocities of the order of 20-50 m/sec. Pressure heads are given by the equation:
V = C(2gh)0.5…………………………………………………………………7.1
In consistent units: V is the velocity at the nozzle outlet, h is the head of water at the nozzle, g is the acceleration due to gravity, and C is the nozzle coefficient. Values for C can be obtained from the supplier: C = 0.95 is a general average.
As an example, to find the required head for a jet velocity of 40 m/sec. From eqn 7.1:
h = V²/C² x 2G = 1600/0.95² x 2 x 9.81 = 90.4 m
Sufficient additional head is added to compensate for line friction and other hydraulic losses. The total required head might be of the order of 100 m or more, depending upon the length and diameter of the pipe. It is generally wise to add 20% to the calculated value to allow a safe degree of flexibility to deal with puggy clays and partly cemented gravels that might require additional energy- for dispersal. The work done by a jet of water varies according to the distance of the nozzle outlet from the point of impact. The jet loses power from the moment it leaves the nozzle. Energy is expended progressively in overcoming air friction and gravity, and the further the jet has to travel the less energy is available to do useful work. Approximate performance figures for jets of water at varying distances from the working face are given in Table 7.6. Distance from the face is a critical factor for operator safety. Because of slumping, the monitor should not be located less than bank height from the face in average ground conditions. This distance may have to be increased if there is any danger of mudflow or of dislodged boulders rolling down into the workings.
An inherent disadvantage of monitoring is the unconfined nature of the slurrying action. The method makes poor use of the available energy because the jet momentum is utilised for only part of the time in breaking down the face. There are practical difficulties in being able to direct the jet continually against the unbroken face and excessive amounts of water brought into the pit may have to be elevated out and away from it, thus increasing pumping costs. Large amounts of energy are also wasted in trying to disperse lumps of clayey wash which are moved backwards and forwards by the jet and in having to wash the resulting slurry down to a gravel pump sump for elevation to the plant. The inefficient use of hydraulic power is not critical when an adequate natural head of water is available, however, useful energy usage is often only a fraction of that generated in mechanical operations, some of which face crippling costs for power.
Gravel pumps were originally single stage, open impeller, centrifugal types, belt-driven from a diesel engine or slip ring electric motor to give a range of working speed. Pump layouts were cumbersome, difficult to prime and were usually operated close to the limit of their suction lifts. Vertical, submersible types that could be raised or lowered in the sump casing using a simple tripod and pulley arrangement, or block and tackle replaccd this pump type. Raising or lowering the pump in the sump regulated flow from the pump to the treatment plant.
Gravel pumps with enhanced priming facilities now operate from rafts floated in the sumps. This arrangement has eliminated most of the pump suction problems attendant upon high suction lifts but new maintenance problems have developed associated with submergence of the electric motor. The main problems are due to electrical breakdowns. Because of the low demand for pumps of this type, there has been little research in trying to develop better insulating qualities for the motors and shutdowns for maintenance add significantly to running costs.
Each sluicing plan is different, but a common denominator is the need to synchronise all of the pit activities. Combinations of wet and dry mining methods of mining often give the best results. Figure 7.12 shows dry feed materials being dumped into a central sluicing paddock for sluicing in the New England District of NSW where a series of small deposits are mined by dry-methods over a comparatively wide area. Many difficult materials respond better to jetting if they are stockpiled and fragmented initially by mechanical means (e.g. by bulldozing). Large earth-moving equipment may also be essential within the pit for the systematic exploitation of ground containing numerous large stones and boulders. Monitoring of such activities calls for close co-operation between the head box operator at the treatment plant and the monitor operators in the pit. Since the head box operator alone has a full overview of the working area, there is a clear requirement for him to direct all of the pit activities including the earth-moving functions.
A typical sluicing operation (as illustrated in Fig. 7.13 for Yakatabari Creek in Papua New Guinea ) commences with the development of a working paddock using mechanical earth-moving equipment to move the overburden and open up a face for monitoring. The length of the paddock will probably be about 75 mm from face to tailings disposal at the back of the excavation. Slurry monitoring from the working face is directed downslope to gravel pump sumps through races cut into the floor of the paddock as shown in the illustration. The width of the cut is held to a practical minimum, according to the variable stability of the sides and face of the channel. In unstable ground there is always the possibility of block flow and monitors will be positioned for sluicing in two or more parallel strips across the full width of the deposit. A bulldozer is used to break down the face of wash ahead of the monitors large stones and small boulders are stacked along the sides using traxcavators. The ongoing sequence will involve:
monitoring the broken ground and washing the slurry into the sluices
bulldozing the washed gravels to the sides of the excavation and stacking the small to medium sized boulders and large stones along the back and sides of the pit using a traxcavator for the purpose
advancing the face in one strip of the paddock for a distance of 30 to 50 m while slurrying and washing the broken gravels in the adjacent strip into the sluices
maintaining a sluice along the side of the paddock to channel excess water away
pumping the sluice box tailings to the top of piled up stone to refill the channel at the back of the excavation
levelling and resoiling to form a finished surface for replanting.
Increasingly high maintenance and energy costs tend to restrict the gravel pump method to deposits having a natural head of water available at the face. Sluicing is mainly disadvantaged by its very large water requirement, particularly in ground that does not slurry easily. This problem can be alleviated to some degree if the overburden can be removed by stripping. Mechanical stripping of overburden and the handling of heavy stones and boulders in the sluicing paddock can usually be done at less cost than by hydraulic methods. Very tough clays and partly cemented gravels respond better to jetting if broken initially by some mechanical means.