1 Principle of Gold Gravity Beneficiation
If you want to know how to extract gold by gravity beneficiation from alluvial gold deposits, you must first understand what gravity beneficiation is.
Gravity beneficiation is a method of sorting under the condition of monomer dissociation according to the difference in density and size of ore particles. In addition to various gravity beneficiation equipment, a medium (air, water/heavy liquid or heavy suspension liquid) must be used for gravity beneficiation.
In the process of gravity beneficiation, the ore particles are subject to gravity (centrifugal force if in the centrifugal force field), the mechanical force exerted by the equipment and the force of the medium. The ingenious combination of these forces makes the ore particles with different densities produce different moving speeds and the trajectory of motion can eventually separate them from each other. Gravity beneficiation is mainly used to separate useful minerals in ore that have high density and large particle size, which can dissociate the monomer (generally greater than 0.04mm).
The essence of gravity beneficiation can be summed up as the process of “loose-layered” and “transport-separated”. The bulk materials placed in the sorting equipment are loosened by the buoyancy of the fluid, power or other mechanical forces in the moving medium. Due to the difference in the movement state of the loosened ore particles, particles of different densities (or particle sizes) undergo layered transfer. In other words, gravity beneficiation is to achieve stratification according to density and achieve separation through the action of moving media.
The basic law can be summarized as: loose-layered-separated. The problem of gravity beneficiation theory research is simply to explore the relationship between looseness and stratification. Loosening and handling separation occur almost simultaneously, but loosening is a condition of stratification. Stratification is the purpose, and separation is the result.
The effect of gravity beneficiation depends not only on the density and size of the ore particles, but also on the density of the medium. The following formula can approximate the difficulty of sorting by density:
In the formula:
- δ2——the density of useful minerals.
- δ1——The density of gangue minerals.
- Δ——The density of the medium.
According to the ratio of the above formula, the ore particles can be divided into five grades according to the difficulty of density beneficiation:
- e > 2.5: very easy to extracted
- e in 2.5 ~ 1.75: easy to be extracted
- e in 1.75 ~ 1.5: can be extracted
- e in 1.5 ~ 1.25: hard to be extracted
- e < 1.25: extremely hard to be extracted
Because the density of natural gold is higher, which is much higher than that of common gangue minerals and common metal minerals, the recovery of free gold by gravity beneficiation method is very effective.
In the process of gravity beneficiation, ore particles move in a flowing medium (such as water). The flow modes of the medium are: continuous upflow, intermittent upflow, up and down alternating flow, and nearly inclined horizontal flow. Each gravity beneficiation method is not one kind of medium flow, but several medium flows and a certain mechanical action cooperate to complete the separation process.
For example, in the jig beneficiation process, the upper and lower alternating medium flows play the role of ore particle separation, and the horizontal medium flow plays the role of tailings discharge. In the beneficiation process of shaking table and sluice box, the main medium flow is horizontal flow and inclined flow, but the rising water flow formed between the baffles plays an important role in ore separation.
In addition to the gravity beneficiation process carried out in the gravity field, some beneficiation processes can also be carried out in the centrifugal force field. The use of centrifugal force can greatly strengthen the sorting process. Centrifugal concentrators have been used in the industry to recover particulate materials.
In addition, the use of the principle of centrifugal force to improve hydraulic classification equipment (such as hydrocyclones) and heavy-medium beneficiation equipment has also been put into use in production practice.
2 Gold Gravity Beneficiation Method and Equipment
(1) Jig Concentrator for Gold Gravity Beneficiation
Jig concentrator beneficiation is a process in which ore particles are sorted according to density in a vertical variable-speed medium flow (water flow). There are three basic forms of vertical medium flow: intermittently rising medium flow, intermittently falling medium flow, and rising and falling alternating medium flow. Modern jigging machines mainly use alternating ascending and descending medium flows.
(2) Centrifugal Concentrator for Gold Gravity Beneficiation
The Nielsen concentrator is a new generation of centrifugal concentrator. It increases the centrifugal force of the rotating drum centrifuge and at the same time solves the problem of fluidization of the enrichment layer, thereby broadening the operating performance and collecting particularly fine-grained gold . The enrichment ratio of gold is increased from 20 to 100 times to 1000 to 5000 times than that of conventional gravity separation equipment.
(3) Sluice Box for Gold Gravity Beneficiation
Sluice box beneficiation is a method of beneficiation by using the difference in the movement state of the ore particles in the inclined medium flow.
(4) Shaking table for Gold Gravity Beneficiation
Shaker is a device for beneficiation in a horizontal medium flow. The mineral processing process takes place on an inclined bed with a wide surface. Through the stratification of the water flow on the bed surface and the stratification when the bed surface shakes, the mineral particles are stratified and move horizontally and longitudinally, so that the mineral particles with different densities can be separated from different sections of the bed are discharged to achieve the purpose of beneficiation.
(5) Spiral Concentrator for Gold Gravity Beneficiation
Spiral concentrator is a kind of chute beneficiation equipment that uses the combined effects of gravity, friction, centrifugal force and water flow to separate the ore particles according to density, size and shape.
Its characteristic is that the entire chute is bent into a spiral in the vertical direction.
3 Features of Gold Gravity Beneficiation
Gravity separation is mainly used to process ores with a large difference in density between gold minerals and gangue. It is a traditional method for extracting gold and coarse-grained gold and silver from placer gold and vein gold ore.
For placer gold mines, gold has generally been dissociated in the ore deposit, so placer gold mines can directly use gravity separation. In vein gold mines, the size of the gold particles embedded in the ore may be very small. The disadvantage of gravity beneficiation is its low processing capacity for micro-fine particle ore and low separation efficiency. Therefore, in vein gold mines, gravity beneficiation method is mainly used to recover medium and coarse-grained gold in advance, and it is used in conjunction with flotation method to ensure the stability of recovery index.
4 Summary
The gravity beneficiation method of gold mine has many different methods according to different equipment. These equipments include jig concentrators, sluice boxes, centrifugal concentrators, shakers, etc. We can choose according to the nature of the ore, the beneficiation process, and the overall budget, etc.
References
- Swapan Kumar Haldar.(2018).Mineral Exploration.Second Edition
- 2020 Mining Mill Operator Training – AMIT 145: Introduction to Mineral Benefication
- Gold Sluice Box Design
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