Gravity separation is one of the oldest and most widely used methods of mineral concentration. It relies on the different densities and sizes of minerals to separate them from each other and from the gangue. In this article, you will learn about the benefits and drawbacks of using gravity separation in mineral concentration methods.
Gravity separation works by applying a force that is greater than the gravitational force on a mineral particle. This force can be generated by various devices, such as shaking tables, jigs, spirals, centrifugal concentrators, and dense media separators. The force causes the heavier and coarser particles to move towards the lower or outer part of the device, while the lighter and finer particles move towards the upper or inner part. The result is a separation of the mineral particles based on their specific gravity and size.
Small-Scale (Limited Resources) Pans/Tube: Inexpensive and manually operated, they allow the separation of heavier particles by tilting, rotating, and gently shaking the pan. The backbone of this operation is that it is suitable for minimal resources. Small-Scale Shaking Tables or Jigs: Affordable, mechanical devices that use shaking and vibrations to separate that can be fabricated easily, requiring minimal infrastructure but feasible for limited resource setups. Large-Scale Centrifugal Concentrators and Dense Media Separators: Mechanized and efficient, these are suitable for large-scale operations as they need more resources and initial investment, with high-capacity gravity separation.
Gravity separation is a cost-effective and uncomplicated method that can be applied to many ores and minerals. It does not require any chemicals or water, thus reducing the environmental impact and operational costs. Furthermore, it can produce high-grade concentrates with low mass and volume, which decreases downstream processing and transportation costs. Additionally, gravity separation can recover coarse and fine particles, as well as free gold and other heavy minerals, enhancing the overall recovery and value of the ore. Moreover, it can pre-concentrate low-grade ores and reject waste, improving the efficiency and capacity of the following concentration stages.
Gravity separation is a relatively simple process, based on SG differences between valuable and gangue minerals. Most technologies are water based, with some dry applications as well. Generally these processes are low energy consumers and for coarser particles >0.5 mm, high capacity. Fine particles separation is also possible down <45 microns. Process control is generally straightforward.
Small-Scale (Limited Resources) - Low Cost: Manual methods like panning are economically viable - Environmentally Friendly: No need for chemicals, minimizing environmental impact - Versatility: Suitable for various ores or (alluvial) without complex equipment Small-Scale - Cost-Effective: Mechanized methods (e.g., shaking tables) are budget-friendly, self-fabricated - Minimal Infrastructure: Requires less complex setup Large-Scale - Operational Efficiency: Scalable for efficient separation in specific large-scale alluvial operations - Reduced Environmental Impact: Minimal chemical usage promotes eco-friendly practices - Resource Recovery: Efficiently recovers particles, maximizing mineral value
Gravity separation can have some drawbacks that need to be taken into account. Variations in feed characteristics, such as particle size, shape, density, and moisture content can affect the efficiency and performance of the process. Additionally, gravity separation may not be effective for certain minerals with similar specific gravities or that are closely associated with the gangue. It may require multiple stages or devices to achieve the desired separation and concentration, which can increase complexity and capital costs. Furthermore, tailings containing residual valuable minerals may be generated, reducing overall recovery and requiring further treatment or disposal.
As a rule of thumb you need a SG differential of at least 1 for effective gravity separation. Liberation of valuable minerals is critical. The degree of liberation establishes the required SG differential. Near density particles are the curse of gravity separation. This happens when you have ineffective liberation, resulting in the production of composite particles which have a SG close to that of the valuable minerals. This impacts concentrate grade. Fine particles recovery is a major issue. Technologies are available but there will be a sacrifice in capacity.
Optimizing the gravity separation process requires understanding the characteristics of the ore and the separation device, as well as monitoring and controlling the operating parameters. The feed rate and distribution should be uniform and consistent to prevent overloading or underloading, while the feed size and shape should be within the optimal range, preferably spherical, to improve separation efficiency. Additionally, adjusting the feed density and moisture content can minimize viscosity and surface tension of the pulp. Furthermore, optimizing device angle, speed, stroke, and amplitude can create the optimal force and motion for separation. Lastly, device riffles, slots, ridges, or grooves should be designed and maintained to provide adequate retention and stratification of particles.
Particle size- keep a narrow PSD and remove fines for disposal or separate treatment Feed slurry density- control the density. High density will interfere with separation and low density will impact capacity. Each technology will have an optimum feed slurry density
Gravity separation is a widely used technique in various mineral processing industries and applications, such as gold processing to recover free gold and coarse nuggets from alluvial or hard rock deposits; coal processing to separate coal from shale, pyrite, and other impurities; iron ore processing to upgrade hematite and magnetite ores; diamond processing to recover diamonds and other heavy minerals from kimberlite and alluvial deposits; and mineral sands processing to separate ilmenite, rutile, zircon, and other heavy minerals from quartz, feldspar, and other light minerals. This technique is also used to produce different grades of concentrates for various industrial purposes.
Spirals for coal , iron ore, mineral sands Jigging for coal, iron ore, manganese Shaking tables for mineral sands, gold, cassiterite Centrifugal separators for gold Hindered settling classifiers for coal, iron ore
I Snacked Well That Month I had a partner whose I felt trommel was revolving too fast, and the feed of materials was too quick. He felt that it wasn't. He bet me a month's worth of any snacks of choice that the waste (tailings) would not have a loss of more than .03 grams per ton. I walked over and, within two handfuls, picked out two thumb-tack-size nuggets. The re-washed yield just as much as the first wash. Gravity doesn't only take place in the trommel. It depends on the feed of materials, the speed and grid spacing of the trommel, the nugget trap fill time, the sluice tilt, the type of carpet (if), and the shaking tables or jigs. Consider adjusting angles, tilt, vibrations, speed, density of materials, etc. I snacked well that month.