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Qwestrum Engineering360 · Mining & Metallurgy · Mineral Processing

Gravity Separation

Gravity separation exploits density differences — Stokes settling for fines, heavy medium baths for coal and iron ore, spirals and tables for minerals. Concentration ratio CR = feed grade/concentrate grade measures enrichment.

Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

Jig pulsating bed stratification
Spiral concentrator for fines
Shaking table for precious metals

Topic details

Introduction

Indian coal washeries use dense medium cyclones at ρ_m 1.4–1.8 for ash reduction. Iron ore spirals concentrate hematite from silica gangue in Goa and Karnataka operations.

Scope in B.Tech and GATE syllabus

Stokes valid for fine particles low Re — Newton regime for coarse fast settling. Exam questions compare terminal velocity formulae crossover at ~0.1 mm for quartz in water.

Why this topic matters in practice

Hartman mineral processing covers jigging theory for metalliferous ores.

Key relations & formulas

Formulas (Indian textbook notation)

$settling velocity Stokes vs Newton regime$

Formulas (Indian textbook notation)

$heavy medium \rho_{m} between float and sink$

Formulas (Indian textbook notation)

$concentration ratio CR = feed \frac{grade}{concentrate} grade$

Notation and sign conventions

Relation 1 —

settling velocity Stokes vs Newton regime

Formulas (Indian textbook notation)

$settling velocity Stokes vs Newton regime$

Write this relation with symbols exactly as in Wills Mineral Processing — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

heavy medium \rho_{m} between float and sink

Formulas (Indian textbook notation)

$heavy medium \rho_{m} between float and sink$

Write this relation with symbols exactly as in Wills Mineral Processing — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

concentration ratio CR = feed \frac{grade}{concentrate} grade

Formulas (Indian textbook notation)

$concentration ratio CR = feed \frac{grade}{concentrate} grade$

Write this relation with symbols exactly as in Wills Mineral Processing — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

Stokes: v_t = g(ρ_s − ρ_f)d²/(18μ) — valid Re < 1. Newton: v_t = √[3gd(ρ_s − ρ_f)/(2ρ_f)] — high Re. Intermediate needs drag coefficient curve.

Governing relations in practice

Heavy medium: particles ρ > ρ_m sink; ρ < ρ_m float — sharp separation if medium stable. Ferrosilicon or magnetite suspension for coal; cyclone DM separator for fines.

Design and analysis considerations

CR = feed grade / concentrate grade — higher better enrichment. Recovery separate metric — high CR with low recovery means tailing losses.

Advanced theory and extensions

Jig: pulsating water stratifies bed by density. Spiral: centrifugal and gravity on flowing film. Table: asymmetric stroke and riffles — gold, tin, chromite in Indian placer operations.

Assumptions and validity limits

State assumptions explicitly before using any relation for gravity separation — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Mineral Processing viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Mineral Processing papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to gravity separation.
4. Use equation 1:

settling velocity Stokes vs Newton regime

.
5. Use equation 2:

heavy medium \rho_{m} between float and sink

.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Gravity Separation appears in beneficiation plants. In Indian mining curricula this topic is tested because it connects theory to comminution and separation.

GATE and semester exams often combine gravity separation with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use gravity separation?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Stokes formula for coarse sand particles (wrong regime)
• ρ_m outside float-sink range of minerals
• CR inverted (concentrate/feed)
• Ignoring near-density material misplacement in DM bath

Quick revision checklist

Before attempting gravity separation problems, confirm you can:

1. Jig pulsating bed stratification
2. Spiral concentrator for fines
3. Shaking table for precious metals

Revise the solved examples in Wills Mineral Processing — Standard reference and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Guided practice — Gravity Separation

Problem

A standard Mineral Processing numerical on gravity separation supplies given data in SI units. Using settling velocity Stokes vs Newton regime and heavy medium ρ_m between float and sink, find the unknown quantity and state whether the result is physically reasonable.

Solution

1. List all given quantities with units (convert to SI if needed).
2. Draw a neat labelled diagram — diagram marks are common in Indian B.Tech papers.
3. Select

settling velocity Stokes vs Newton regime

and write it symbolically before substitution.
4. Substitute values, compute, and attach correct units.
5. Sanity-check: magnitude, sign, and direction must match comminution and separation.

Cross-check with solved examples in your Mineral Processing textbook.

Conceptual check — Gravity Separation

Problem

In a Mineral Processing semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of gravity separation." What should a complete answer include?

📖 Standard books (India)

Wills Mineral Processing — Standard reference
Read: Syllabus unit
Referenced in Indian B.Tech syllabus

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