Summary: A complete guide to gravity separation in mineral processing. Learn about multi-stage physical mineral processing layouts including shaking tables, jigs and spiral....
What is the Gravity Separation Method?
Gravity separation (also named gravity concentration) is one of the oldest, most widely applied mineral beneficiation technologies. It remains a cornerstone of the mining industry, particularly for processing tungsten, tin, placer gold, and other heavy minerals. Its enduring relevance stems from its simplicity, cost-effectiveness, and environmental friendliness.
Core Physical Principle
It separates valuable target minerals and useless gangue rock based on the obvious difference in mineral specific gravity (density) under gravity and fluid dynamic force, using water, air or heavy suspension as separation medium.
Different mineral particles sink at different speeds in flowing medium: high-density heavy minerals settle fast, while low-density gangue floats or drifts away with medium flow. Effective stratification only happens in continuously moving fluid.
Effective Particle Size Range Boundary
- Coarse ore: >25 mm (heavy medium separation, large jig)
- Medium ore: 2–25 mm (conventional jig, spiral chute)
- Fine ore: 0.1–2 mm (shaking table, spiral concentrator)
- Slime: <0.1 mm (poor separation efficiency, usually matched with centrifugal gravity concentrators)
Key Conditions for Effective Separation
For gravity separation to be economically and technically viable, the following conditions must be met:
| Condition | Explanation |
|---|---|
| Sufficient Density Difference | There must be a significant difference in specific gravity between the valuable minerals and the gangue (waste rock). A higher density difference results in a more efficient and easier separation. |
| Liberation | The valuable minerals must be liberated (freed) from the gangue minerals through crushing and grinding. The particle size must be small enough so that individual particles are either predominantly valuable mineral or predominantly gangue. |
| Appropriate Particle Size Range | Gravity separation is most effective for processing coarse-grained, medium-grained, and fine-grained ores (approximately >0.1 mm to 25 mm). It becomes progressively less efficient for very fine particles (slimes, <0.1 mm) due to their low mass and high surface area. |
Full 3-Stage Gravity Separation Process
The complete gravity separation process typically involves three main stages: Preparation, Beneficiation (Separation), and Product Treatment.
1. Preparation Stage
The goal of the preparation stage is to condition the ore to maximize the efficiency of the separation process. Key steps include:
- Crushing and Grinding: Size reduction is essential to achieve liberation of the valuable minerals from the gangue. The optimal grind size depends on the ore's characteristics and the grain size of the target minerals.
- Washing and Desliming: For ores containing high levels of clay or colloidal materials, washing is performed to remove these fine contaminants, which can interfere with the separation process. This is often done in a scrubber or log washer.
- Screening and Classification: The crushed ore is separated into different size fractions (e.g., coarse, medium, fine) by screening or hydraulic classification. This allows for the use of different equipment or operating conditions optimized for each specific size range.
2. Beneficiation Stage
This is the core stage where the actual separation of valuable minerals from gangue occurs. The ore passes through one or more gravity separation units, such as jigs, shaking tables, spirals, or centrifugal concentrators.
Common Configurations:
- Simple Circuit: For coarse, easily liberated ores, a single pass through a gravity device (e.g., a jig) may be sufficient.
- Complex Circuit: For ores with unevenly disseminated valuable minerals, a multi-stage “stage separation” process is often used. This involves:
- Roughing: An initial pass to discard a large portion of the tailings.
- Cleaning: A second pass to upgrade the concentrate from the rougher stage.
- Scavenging: A third pass to recover any remaining valuable minerals from the rougher tailings.
3. Concentrate & Tailings Post-Treatment Stage
This stage deals with the handling of the final products (concentrate and tailings).
Gravity Concentrate Dewatering
Coarse/medium concentrate only needs natural percolation drainage on ore slope; fine micro concentrate requires filter press dehydration. For packaged smelting delivery, drying equipment is necessary.
Tailings Disposal
Coarse tailings can be directly transported by trucks for mine backfill; fine tailings are pumped into tailing ponds or processed by dry stacking systems to meet environmental rules.
Types of Gravity Separation Methods
Based on the movement form of the medium and the purpose of the operation, gravity separation methods can be classified into several categories, including classification, heavy medium separation, jigging, shaking table, spiral, centrifugal separation, air separation, and washing. Among these, heavy medium separation, jigging, shaking table, and spiral are the four most widely used technologies in industrial mineral processing.
1. Heavy Medium Separation (HMS) / Dense Medium Separation (DMS)
Heavy Medium Separation is a static separation method that utilizes a fluid medium with a density directly between the densities of the two minerals to be separated.
The Process: The raw ore is submerged into a dense fluid suspension (typically water mixed with fine ferrosilicon or magnetite). The lighter gangue rock floats to the surface, while the heavier valuable mineral sinks to the bottom.
Best Used For: Coarse particle pre-concentration (coal washing, diamond recovery, and rejecting waste rock before expensive grinding circuits to save CapEx).
2. Jig Concentration (Jigging)
Jigging is a dynamic process that separates minerals of different densities in a vertical, pulsating fluid bed.
The Process: The ore sits on a perforated screen while a mechanism forces water to pulse vertically up and down through the bed. The upward water stroke expands the particle bed (stratification), and the downward stroke allows particles to settle. Heavy particles settle faster and accumulate at the bottom layer, while light particles stay at the top and are washed away.
Best Used For: Coarse to medium-grained materials (10 mm down to 0.5 mm), commonly used in iron ore, manganese, barite, and tungsten concentration.
3. Shaking Table Concentration
The shaking table (or Wilfley table) is a film-sizing concentrator that utilizes a sloped, ribbed deck that shakes asymmetrical longitudinally.
The Process: A thin film of water flows across the tilted table surface while the deck shakes back and forth. The high-density particles settle into the grooves (rifles) and are driven horizontally by the shaking motion into the concentrate launder. The light gangue particles remain suspended in the water film and are washed over the lower edge as tailings.
Best Used For: Fine-grained materials (0.5 mm down to 0.037 mm). It delivers an exceptionally high enrichment ratio, making it the universal standard for producing final concentrates in artisanal and large-scale gold mining alike.
4. Spiral Concentration
Spiral concentrators are continuous devices utilizing a vertical helical sluice channel that leverages centrifugal force alongside gravity.
The Process: Slurry is fed at the top of the spiral. As it flows downwards in a swirling motion, the heavier particles migrate toward the low-velocity inside channel of the spiral due to friction and drag forces. The lighter particles are flung by centrifugal force toward the outer edge. Adjustable splitters at the bottom cut the stream into concentrate, middlings, and tailings.
Best Used For: Fine-grained ores (2 mm down to 0.074 mm) requiring massive throughputs, such as silica sand purification, chromite ore, zircon, and iron ore fines.
Complete Gravity Separation Equipment Comparison
| Equipment Name | Effective Feeding Particle Size | Core Advantage | Main Limitation | Best Suitable Ore |
|---|---|---|---|---|
| Jig Machine | 20 ~ 0.5 mm | Huge hourly throughput, low operation cost, simple maintenance | Low concentrate grade, cannot process ultra-fine slime | Placer gold, hematite, chrome ore, coal washing |
| Shaking Table | 2 ~ 0.038 mm | Ultra-high enrichment ratio, visible mineral bands, high-purity final concentrate | Small single machine capacity, large floor space | Tungsten, tin, fine gold cleaning, rare heavy mineral sand |
| Spiral Chute / Spiral Concentrator | 1 ~ 0.075 mm | No moving wearing parts, low power & water consumption, continuous running | Low concentrate enrichment multiple | Iron sand, beach zircon/rutile, rough pre-concentration |
| Centrifugal Concentrator | 0.074 ~ 0.01 mm | Recover ultra-fine heavy gold/tin slime lost by traditional gravity devices | Regular automatic cleaning cycle required | Fine vein gold, disseminated tin ore tailings recovery |
| Heavy Medium Separator (DMS) | 100 ~ 6 mm lump ore | Discard massive waste rock before grinding, cut overall power cost | Heavy medium slurry recycling system needed | Coal, low-grade iron lump ore pre-separation |
| Dry Air Gravity Separator | 10 ~ 0.1 mm dry ore | Zero water consumption, no tailing slurry pollution | Dust control system mandatory, lower recovery than wet process | Water-shortage desert mine, dry coal separation |
Key Factors Influencing Gravity Separation Performance
Three core decisive factor categories: ore mineral property, separation medium, equipment model & operation parameters.
Ore Property (Primary Factor)
- 1. Density difference between valuable mineral and gangue: Larger gap = better stratification and higher recovery rate.
- 2. Ore particle size distribution: Uniform classified feeding avoids coarse/fine cross-interference; over-grinded slime causes metal loss.
- 3. Clay & mud content: Excess slime increases medium viscosity, weakens density stratification, must be deslimed in advance.
- 4. Mineral dissemination form: Coarse monomer disseminated ore achieves far better gravity effect than fine intergrowth ore
Separation Medium Performance
The medium (usually water) affects the effective density of the system. While water is standard, the density of the medium can be altered.
- Air (Dry Separation): Least efficient due to low medium density. Used in arid regions or for specific ores.
- Water: The standard medium; efficient and cheap.
- Heavy Liquids / Suspensions (HMS): Most efficient, but expensive and requires careful reagent handling.
Equipment Type and Operating Conditions
Each piece of equipment has an optimal operating range for particle size, capacity, and feed density. Key factors to control include:
- Feed Rate: Too high can cause overload and loss of separation; too low reduces equipment efficiency.
- Feed Solids Density: Affects the viscosity of the pulp; must be maintained within the equipment’s optimal range.
- Water Flow Rate (for spirals, tables): Critical for creating the correct fluidizing action and particle stratification.
- Pulsation Frequency and Amplitude (for jigs): Affects how the particles settle and stratify.
- Centrifugal Force (for concentrators): Determines the “effective gravity” and the recovery of fine particles.
Practical Applications by Ore Type
| Ore Type | Target Mineral(s) | Preferred Gravity Equipment | Notes |
|---|---|---|---|
| Placer Gold | Native Gold | Jigs, Spirals, Centrifugal Concentrators, Sluices | Often a primary processing method |
| Hard Rock Gold | Native Gold, Tellurides | Centrifugal Concentrators (in grinding circuit), Shaking Tables | Used to recover gravity-recoverable gold |
| Tin | Cassiterite | Shaking Tables, Jigs, Spirals | Classic application for shaking tables |
| Tungsten | Scheelite, Wolframite | Jigs, Shaking Tables, Spirals | Often combined with other beneficiation methods |
| Mineral Sands | Ilmenite, Zircon, Rutile | Spirals, Shaking Tables, HMS | High-tonnage applications |
| Chromite | Chromite | Spirals, Shaking Tables, Jigs | Concentration based on density difference |
| Iron Ore | Hematite, Magnetite | Jigs, Spirals, HMS | Used for coarse-grained iron ore |
| Manganese | Pyrolusite, Psilomelane | Jigs, Spirals | Often used for coarse, high-grade ores |
Typical Ore & Industry Applications of Gravity Separation
1. Precious Metal Ore (Placer Gold & Vein Gold)
Natural gold has extreme density difference with quartz gangue. Jig + centrifugal concentrator + shaking table combined flow recovers over 93% coarse free gold, widely used in African, Southeast Asian placer gold mines. For vein lode gold, gravity pre-concentration reduces grinding load before cyanidation or flotation.
2. Tungsten & Tin Vein Ore
Wolframite and cassiterite are typical heavy density oxide minerals. Multi-stage jig roughing + shaking table cleaning is the standard mainstream process for tungsten-tin separation in Fujian, Jiangxi China and Southeast Asian tin mines.
3. Ferrous & Weak Magnetic Ore
Hematite, limonite, chrome ore with weak magnetism adopt jig and spiral gravity separation when magnetic separation indexes are poor; heavy medium separation pre-discards waste rock for lump iron ore.
4. Beach & River Heavy Mineral Sand
Zircon, rutile, monazite placer sand use spiral chute roughing + shaking table gravity cleaning to separate valuable rare heavy minerals from quartz sand.
5. Coal Washing Industry
Heavy medium separator and jig machine are the core coal preparation equipment, separate clean coal from gangue rock by density difference.
6. Construction Waste & Mine Tailings Secondary Recovery
Gravity separation recovers residual heavy metal aggregates from demolition waste and old mine tailings, realizing circular mineral resource utilization.
Real Global Gravity Separation Mine Project Cases
Case 1: East Africa 500 TPD Placer Gold Gravity Plant
- Ore feature: Coarse native gold 0.1–1 mm, low clay content, local water shortage
- Process flow: Two-stage jig roughing + centrifugal concentrator + shaking table final cleaning + circulating water system
- Operation index: Gold recovery rate 93.38%, final concentrate gold grade 85%, water recycling rate 86%, 40% water consumption cut vs traditional process
Case 2: 2000 TPD Wolframite Vein Ore Gravity Plant
- Ore feature: Feldspar-quartz vein type, uneven disseminated wolframite
- Process flow: Stage grinding + multi-jig roughing + multi-group shaking table cleaning
- Operation index: Tungsten concentrate grade 66%, total tungsten recovery 82%, discard 65% barren tailings in rough stage to save grinding power
Case 3: 1200 TPH Beach Heavy Mineral Sand Separation Line
- Ore feature: Mixed zircon, rutile and quartz sand
- Process flow: Spiral concentrator bulk roughing + shaking table high-purity cleaning
- Operation index: Heavy mineral total recovery 88%, zero chemical reagent consumption, low daily operation cost
Best Practices for Plant Design
Gravity separation remains a fundamental and highly effective technology in modern mineral processing. Its strengths lie in its simplicity, low operating cost, and environmental compatibility. However, success depends on applying the right equipment to the right ore type and particle size.
For plant designers, engineers, and operators, the following principles are essential:
- 1. Thorough Ore Characterization is Non-Negotiable: Test for mineralogy, liberation size, and density differences.
- 2. Match Equipment to Particle Size: Use jigs for coarse material, spirals for intermediate, and tables for fine material. Use centrifugal concentrators for fine free gold.
- 3. Consider Stage Separation: For ores with unevenly disseminated minerals, a multiple-stage circuit is often far more effective than a single pass.
- 4. Control Operating Variables: Feed rate, water flow, and medium density must be carefully monitored and controlled.
- 5. Address Slimes: Fine slimes (<0.1 mm) are detrimental to gravity separation. They should be removed through desliming and treated by other methods (e.g., flotation) if needed.
Frequently Asked Questions about Gravity Separation
Q: Can gravity separation be used to process ultra-fine slimes?
A: Traditional gravity equipment like jigs and spirals lose efficiency below 74 microns (200 mesh) because the surface drag forces of water overcome gravity. However, advanced Centrifugal Gravity Concentrators spin the slurry to generate forces up to 60G or more, allowing efficient recovery of heavy minerals down to 10 microns.
Q: Can gravity separation replace flotation completely?
A: Only if the Concentration Criterion is high enough. For complex, finely disseminated sulfide ores where valuable minerals are tightly bound to gangue at microscopic sizes, flotation remains mandatory. However, modern plants frequently use a hybrid circuit: gravity separation to recover coarse minerals early, followed by flotation to scavenge the ultra-fine remnants.
Q: What are the primary factors that affect the efficiency of gravity separation?
A: The efficiency of gravity separation depends on three critical parameters: the specific gravity difference between the target mineral and gangue (a larger difference ensures easier separation), the feed particle size distribution (strict classification prevents overlapping stratification), and fluid dynamics, including water flow velocity and slurry density control on the equipment surface.
Q:Why is gravity separation highly recommended for small to medium-scale eco-friendly mines?
A:Gravity separation is a purely physical beneficiation method that requires zero chemical reagents. This completely eliminates the need for expensive chemical inputs, avoids environmental tailing contamination, lowers overall operational costs (Opex), and ensures quick investment recovery by catching coarse free-milling minerals instantly.
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