Several mechanisms are used by mining companies (large and small scale) to obtain gold from extracted rocks. Some of the techniques employed for gold liberation from ore are dangerous to the health and wellbeing of workers, their families and immediate community. While some of the techniques are able to ensure that the ore contained in the ore are totally liberated with good quality, others are unable to do so.
It is the objective of every mining company that they do not run at a loss but maximise profit. To achieve this, they have to ensure that all the methods they employ result in maximum gold extraction output.
Gravity concentration techniques of obtaining gold in mines are able to generate high quality gold output in large volumes that guarantees good market price and maximum returns on investment.
Concentration refers to the act of increasing the amount of minerals output from ore or sediment by selectively removing lighter particles. When effectively handled, concentration can guarantee a high amount of good quality Gold recovery from ore.
For any form of concentration to take place, ore must first be crushed or milled to free gold particles from the rock and to reduce the size of grains. In order to make concentration work out effectively , the grain sizes of the ore must be relatively consistent with each other.
To achieve an appropriate grain size, screens or sieves are used. Once the rock containing gold has been transformed into appropriate grain sizes, concentration techniques are then used to liberate gold.
Gravity concentration techniques for gold liberation rely on the density of gold relative to other minerals in the ore.
Gravity concentration methods commonly used to liberate gold from ore include,
Using a medium size pan, water is used to separate heavy gold particles from other lighter particles in ore. The Panning process requires that the milled ore containing gold is placed in a wide, curved pan along with water. The worker uses specialised motion movements to eject lighter particles from the pan along with water as the density of gold keeps gold at the bottom of the pan. The gold content is left completely exposed at the bottom of the pan after a successful series of motion movements.
In sluicing, a series of angled platforms with water are used to wash the ore. Gold particles sink as water washes down the sediments in the sluice and are captured by a material layer covering the bottom of the sluice. At an inclination of about 5 – 15 degrees for which sluice is positioned, it allows water to travel down the sluice at greater force which keeps the gold particles from sinking easily. This system allows a large amount of gold to be captured at the beginning of the sluice. The underlying material covering the bottom of the sluice is then removed and washed in a bucket or bowl to remove the captured gold particles.
These are elevated tables tilted to one side with raised ridges spanning horizontally down the length of the ridges. Sediments and water are released at one end of the table and the water washes down the sediments. As the sediment is washed down the table, specially designed grooves capture the gold particles and moves them to collection points on the side of the table as lighter particles are washed away. The elevated tables during the process are shaken continuously by a motor to agitate the material and facilitate the liberation of gold particles.
Spiral concentration involves the use of specially designed pans that are tilted on an angle with special grooves. The grooves are designed to lead to the center of the pan where a hole leading to a collector exists. The collector serves as the collection point for gold.
A specially designed mortar is used to rotate the pan continuously as the sediments are moved onto the pan by an operator. A running pipe extending horizontally across the pan sprays water along the surface of the pan as the concentrator spins. Lighter particles are washed down the spiral pan into a bucket or bowl by water, while denser particles of gold are transported by the grooves to the hole where they finally dislodge into the collector.
The spiral concentration process requires that the process should be repeated several times by the worker for maximum liberation of gold contents from the ore.
The centrifuging concentration technique involves the use of a vessel that rotates around a central point. The centrifuge vessel is fed with sediment through a pipe located at the top of the machine in a slurry of about 60 – 75% water and 35 – 40% solids. The materials come together in the center of the centrifuge where high speed rotation creates a force that moves the material up the sides of the vessel’s walls. The denser particles (gold) are arrested in the ridges while other lighter particles are moved out of the vessel as the sediment is pushed out up the sides of the bowl’s walls.
The technique of vortex concentration involves the use of a circular tub with water input on the side of the tub and a raised drain in the center. The tub is filled with water until it reaches the level of the hole. Water is then pumped into the side-input when sediment is poured onto a thin layer around the bottom of the bowl. The vortex process then pulls lighter particles up from the bottom of the bowl which is then pushed out of the drain hole. When this happens, the heavy particles of gold remain at the bottom of the tub. When gold is finally seen to be left at the bottom of the bowl, the water source is then turned off and the gold material is removed from the bowl.
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