Gravity Separation

Gravity methods separate minerals by density — cheap, chemical-free and ideal for gold, tin, tungsten, coal, iron ore and mineral sands. Learn the concentration criterion, jigs, spirals, shaking tables, centrifugal concentrators and dense medium separation.

The oldest mineral processing method — gold panning — uses gravity. Modern gravity circuits recover coarse free gold before leaching, clean coal from shale in dense-medium cyclones, and separate heavy mineral sands on spirals.

An analogy

Gravity separation is like shaking a jar of mixed nuts: the heavy ones sink to the bottom, the light ones rise.

Equipment

Jigs (pulsating water bed), spirals (flowing film on a helical trough), shaking tables (riffled deck), centrifugal concentrators (Knelson, Falcon — 60–200 G for fine gold), dense medium separation (DMS) using magnetite or ferrosilicon suspensions.

Concentration criterion

The larger CC, the easier the separation and the finer the particles that can be treated. Gold (19.3) from quartz (2.65) in water: CC ≈ 11 — very easy. Centrifugal devices extend separation to finer sizes by increasing effective gravity.

Dense medium separation

A suspension of fine ferrosilicon/magnetite creates a fluid of chosen density (e.g. 1.6 for coal, 2.9 for diamonds/ores): lighter particles float, heavier sink. DMS cyclones treat 0.5–50 mm particles with sharp cuts.

Performance evaluation

Washability (float–sink) curves predict yield and quality at each cut density. Partition curves and the Ecart probable (Ep) measure DMS efficiency. Gravity recoverable gold (GRG) tests predict gold recovery.

Formulas

• Concentration criterion: CC = (ρ_h − ρ_f) / (ρ_l − ρ_f)
• Interpretation: CC > 2.5 easy (down to ~75 µm); 1.75–2.5 possible (coarser); < 1.25 impractical
• Centrifugal G: G = ω² r / g

Worked example: Gold from quartz

Given: Gold 19.3, quartz 2.65, water 1.0

Find: CC.

• CC = 18.3/1.65 = 11.1

Answer: Very easy — even fine gold can be recovered

Worked example: Centrifugal G

Given: Bowl radius 0.25 m at 600 rpm

Find: G-force.

• ω = 600 × 2π/60 = 62.8 rad/s
• G = 62.8² × 0.25/9.81 = 101

Answer: ≈ 100 G

Common mistakes

• Wrong: Using gravity for very fine, low-CC ores. — Right: Check CC and particle size — use flotation instead.
• Wrong: Letting medium density drift in DMS. — Right: Control medium density continuously.

Applications

• Gold plants: Centrifugal concentrators in grinding circuits.
• Coal preparation: DMS cyclones reject shale.
• Mineral sands: Spirals concentrate ilmenite, rutile, zircon.

Key takeaways

• Separate by density.
• CC = (ρ_h − ρ_f)/(ρ_l − ρ_f).
• Jigs, spirals, tables, centrifugal, DMS.
• Cheap and chemical-free when CC is high.

Going further

• Washability curves: Float–sink data predicting coal yield.
• GRG testing: Gravity recoverable gold characterisation.
• Ep and imperfection: DMS efficiency measures.