Hydraulic mining in the clay industry is accomplished by the use of a jet of water from a monitor which breaks up the decomposed granite and converts it to a slurry; this is the beginning of what is essentially a steady flow process, carried through the slurry and gravel handling system, leading to refined and blended clays ready for shipment.
In some cases the rock may be too hard to break up with a monitor jet alone, such that pre-loosening will be accomplished by other methods, such as blasting and mechanical excavation.
Monitors may operate at various levels within an open-cast pit, which entails certain requirements for siting and proximity to the working face (stope) on any given level or bench, and are capable or being progressively moved to new sites depending on the requirements of the pit.
Similarly, this equipment can be used in other slurrification applications, such as tailings recovery from mining deposits.
Monitors are powered by water at relatively high pressure (25 bar, 360 psi max.), converting this into a high velocity jet (for which there are various sizes) with sufficient force to accomplish the breaking down of the working face. The monitor can be set up to move through both horizontal and vertical arcs of limited sweep, or operated remotely, usually at a minimum of 15 metres from the jetting operation with hydraulic actuation.
The quality of a water jet in air depends on the Reynolds number of the flow, which varies with pressure, nozzle diameter and shape, surface roughness and turbulence. Flow straightening vanes are used to reduce turbulence and vortices. Stone grids may be employed to trap larger solids which contaminate the water supply. Corrosion and erosion are also factors which determine the operating quality and periodic checking is necessary to ensure safe working of equipment.
As a general guide the water jet retains its energy efficiency when the standoff distance between the nozzle and the working face (stope or mineral deposit) is within 100 times nozzle diameter. This is rarely achieved because of other factors which affect safe working, eg. stability of stope, impacted debris or mineral endangering operators, access for loading shovels, and the length of the operating arc. Extra measures, which are site specific and subject to local rules, must be in place to enable safe working.