Gravity separation is an effective method for separating minerals of significantly different densities. It is an industrial method of separating two components from a suspension or any other homogeneous mixture where separating the components with gravity is sufficiently practical. It is a unit process in which gravity removes settleable solids and associated pollutants, floatables and dispersed petroleum products. The effectiveness of separation is also dependent on particle size, not just density. In some cases separation may be effective down to 50µm. There are two opposing forces which are always present in gravity separation are, Gravity ( i.e. dependent on specific gravity) and Resistance to movement. For effective separation, there must be a marked difference in densities. The concentration criteria will be given as an idea as to the effectiveness of the separation. Concentration criterion = (DH – DF) / (DL – DF). Where DH is the density of the heavy material, DF is the density of fluid and DL is the density of the light material. When the concentration is more than ±2.5, gravity separation should relatively easy.
Use of gravity separation:
Gravity separation is an accepted technique of concentrating minerals and has been used as a primary form of mineral concentration for centuries. Due to its high efficiency and low cost, gravity separation is always the first consideration in any flow sheet where there are sufficient differences between the specific gravity of the valuable and gangue minerals. Gravity plants means less capital outlay and better security for the small volume high value concentrates. Its use is recommended as it permits the recovery of values at as coarse size range as possible. Gravity separation reduces the costs involved in crushing, grinding and reduces slime losses which usually result from size reduction operations.
Different categories of gravity separation:
Gravity separation methods fall into three broad categories:
- Dense medium separation, in which particles are immersed in a bath containing a medium of intermediate density so that some particles float and another sinks.
- Vertical current separation, in which advantage is taken of differing rates of settlement, as typified by jigging.
- Stream current separation. Example: Spiral separator.
Conditions for effective gravity separation:
• Proper water balance.
o Gravity concentrators have an optimum feed pulp density.
• Feed must be prepared carefully
o primary grinding, where possible, in open circuit rod mills.
o closed circuit fine grinding in ball mill for fine grinding.
• No slimes. Gravity separation is negatively impacted by slimes.
o Slimes increase viscosity of fluid.
o –10 micron particles should be removed from feed.
• Particle sizes should be relatively close.
o Screening, then separation of the screened fractions will improve results.
• If slurry pumps are used, keep the flow rate as low as possible while keeping particles in suspension. Friable materials may be ground finer in the pump, otherwise.
Reasons to Assess Gravity Separation:
• To reject barren waste as an initial pre-concentration step.
• To recover malleable and/or friable coarse heavy minerals from grinding circuit circulating loads. Such minerals are otherwise hard to recover after regrinding.
• To pre-concentrate heavy minerals, to minimize downstream processing costs.
• To concentrate heavy minerals.
• To clean low weight yield bulk concentrates.
• To scavenging plant tailings.
• To generate a precious metal concentrate that can go direct to a refinery rather than a smelter.
Applicability of gravity separation:
Precious Metals (Au and PGE)
• Creation of a direct refining vs. to-smelter concentrate.
• Gravity recovery from grinding circuits.
• Separation from base metals and gangue.
• Separation from gangue.
• Recovery from tailings.
Base Metals (Zn, Pb, Cu)
• Separation of base metals from gangue, especially carbonate rocks and marble.
• Very appropriate for cassiterite, tantalite/columbite and tungsten minerals.
• Dense media separation is the preferred processing route for diamond bulk samples Mineral Sands.
• Separation of ilmenite, rutile and monazite from beach sand.
• Separation of metallic zinc and lead from recycled batteries.
• Separation of steel from waste.
• Slag re-processing.
• Precious metal recovery from shredded circuit boards.
• Copper recovery from shredded copper cable.
• Precious metal recovery from dusts.
• Separation of garnet from biotite, magnetite and quartz.
• Separation of eudialyte and monzonite from gangue.
• Separation of graphite from mica.
• Separation of muscovite from gangue.
•Separation of kaolinite from muscovite.
•Silica sand cleaning.
Jigging is an old technique that can achieve good separation , even within close specific gravities, if the particle sizes are fairly close. With larger specific gravity differences, separation improves.
• Jigging can achieve good results down to ~150 micrometers.
• Operates by dilating a bed and allowing heavier particles to fall through.
• Frequency: 55-330 min-1.
• Larger particles require longer, slower strokes.
Types of fixed jigs:
On the suction stroke, the bed may be compacted too much to fully dilate in the next pulsion stroke. Hutch water may be added at a continuous rate to reduce the suction effect. A continuous flow of water from beneath the bed will reduce the effect of the suction stroke.
Many types of jigs have been deviced, for different purposes and with improved performance. One of the oldest is the Hartz jig. It contained a plunger connected to crank, just like a reciprocating engine. This design produces a suction-pulsion pattern that follows the shape of a sine wave. A sine wave pattern is not the most efficient., However. Later jigs were designed to produce a constant, fast pulsion, followed by a slower, constant suction. This pattern has two advantages:
• Maximize dilation across the bed during pulsion.
• Minimize compaction during suction.
Spirals are a cheap and effective method of gravity separation. They have found use in treating sand deposits carrying rutile, zircon and ilmenite. They are also used for coal cleaning. In coal cleaning, they are typically used in the size range of 0.2-1 mm, between cyclones and flotation.
Advantages of spirals include:
• Low capitol cost.
• Low maintenance.
• Low floor space requirements.
Gravity Separation Equipment:
• Knelson and Falcon Superbowl centrifugal bowl concentrators in many sizes (3”, 7.5” and 9”).
• Gekko and Acacia protocols.
• Holman, Denver, Deister, Wilfley, Mozley, Gemini tables ranging in size from full size to “1/8”.
• Humphreys, Vickers, Reichert, Carpco, and Mineral Technologies, Single and double spiral ranging from full size to “pee-wee”.
• Reichert & various trays and sluices.
• Denver, Hazin-Quinn, Kelsey,In-Line Pressure jigs ranging in size from 12” to 1”.
• Bateman 1 tonne/hr and 15 tonne/hr dense media separation plants.
- Handout prepared by Michael Anderson based on book by BA Wills.
- Gaudin AM, Principles of Mineral Dressing, Tata Mc Graw – Hill Publishing Company Ltd. 2001, New Delhi.
Question and Answers:
1. What is meant by gravity separation?
A. Gravity separation is an effective method for separating minerals of significantly different densities.
2. What is the formula for concentration criterion with respect to the gravity separation?
A. Concentration criterion = (DH – DF) / (DL – DF). Where DH is the density of the heavy material, DF is the density of fluid and DL is the density of the light material.
3. What are the minimum conditions for effective gravity separation?
A. Proper water balance, feed must be prepared carefully, particle size should be relatively close.
4.Give some reasons to assess gravity separation?
A. To concentrate heavy minerals to clean low weight yield bulk concentrates, to reject barren waste as an initial free concentration step, for scavenging plant tailing, etc.
5. What are the different gravity separation equipments?
A. 1. Denver, Hazin-Quinn, Kelsey,In-Line Pressure jigs ranging in size from 12” to 1”.
2. Knelson and Falcon Superbowl centrifugal bowl concentrators in many sizes (3”, 7.5” and 9”).
3. Holman, Denver, Deister, Wilfley, Mozley, Gemini tables ranging in size from full size to “1/8”.