AMIT 145: Lesson 4 Gravity Separation



Gravity Concentration Ratio (CC)

CC = SG of heavy mineral – SG of fluid/SG of light material – SG of fluid

Concentration CriteriaSuitability for Gravity Separation
CC > 2.5Easy down to 75 microns
1.75 < CC < 2.5Possible down to 150 microns
1.5 < CC < 1.75Possible down to 1.7mm
1.25 < CC < 1.5Possible down to 6.35mm
CC < 1.25Impossible at any size
  • Reflects the difficulty in separating particles of near density and ultra-fine particles.
  • New technologies have enabled density-based separations at finer particle sizes.

Concentration Ratio Examples

CC = SG of heavy mineral – SG of fluid/SG of light material – SG of fluid

*Gangue material SG = 2.65

Separation by Differential Setting Velocity

A graph showing Separation By Differential Setting Velocity
Separation By Differential Setting Velocity
[image 145-4-0]
Note that the low CC value results in a narrow particle size range in which density separation can be achieved in H2O.

A graph showing Separation By Differential Setting Velocity
Separation By Differential Setting Velocity
[image 145-4-1]

Daniels Dense Separators

CC = SG of heavy mineral – SG of fluid/SG of light material – SG of fluid

Separation graph
Daniels Dense Separators
[image 145-4-2]

Viscosity Limits on Medium Type

A graph showing Viscosity Limits on Medium Type
Viscosity Limits on Medium Type
[image 145-4-3]

Heavy Medium Separation Mineral Applications

Operator/Plant LocationMineral ProcessedSize RangePlant Feed tphHMS UnitsSink/Float RatioSeparation Density
Aluminum Co. od Canada, Ltd.
St. Lawrence, Newfoundland
-3/4" + 20M802-15"40/602.72
Barton Mines
North Creek, NY
Garnets-1/4" + 45M601-12"
Basics Inc.
Gabbs, NV
Magnesite-3/8" + 20M301-9"75/252.9
Bethlehem Steel Corp.
Icomi Mine, Amapa, Brazil
Manganese-1/4" + 20M1302-15"80/202.9
Cia. Minera de Autlan S.A.
Autlan, Mine
Universe Tankships Inc.
Para, Brazil (Jari, Project)
Companhia Mineira do Lobito
Jamba Mine, Angola, Africa
Iron-1/4" + 20M4006-15"80/202.7
Dresser Minerals
Ryder Point Plant
Fundy Gypsum Co. Ltd
Windsor, Nova Scotia
-3/4" + 20M802-15"30/702.5
International Mining Co.
Enramada Mine, Bolivia
Tin/Tungsten-1" + 20M301-12"25/752.95
Lithium Corp. of America
Bessemer City, NC
Lithium-1/4" + 65M651-15"30/702.8
NL Industries Inc
Scheelite-1/2" + 14M101-9"60/402.7
Renison Ltd.
Zeehan, Tasmania
Tin-1/2" + 28M802-15"80/203.0
Southern Peru Copper Corp.
Ilo, Peru
-1/4" + 30M501-15"50/502.7
Turk Maadin Sirketi
Beyoglu, Turkey
Chrome-1/4" + 20M101-9"60/402.9

Emeralds Processing by Heavy Medium

A diagram of emerald processing circuit by heavy medium
Emerald Processing By Heavy Medium
[image 145-4-4]

Jigging Principles

Jigging uses a pulsation of a fluid at a given frequency and amplitude to induce a separation based on differential acceleration, hindered settling and consolidated trickling.

For small particles, short pulsations are preferred to emphasize separation based on differential acceleration.

  1. Particles in a mixed pile before laying
  2. Rising water level lifts the bed layer
  3. Particle sedimentation stratification in the water
  4. The water level drops, the bed layer is dense, and the heavy mineral settles at the bottom
A diagram showing the phases of jigging
Jigging Principles
[image 145-4-5]

An animated gif showing particle behavior during jigging.
Jigging action [140-4-06]

Jig Types

A photo of a Baum jig
A Baum Jig

A diagram of a Batac jig
A Batac Jig

Industrial Jig

A photo of an industrial jig in situ.
Industrial Jigs
[image 145-4-8]

Teeter Bed Separators

Utilizes an upward current of water that has a velocity equal to the high-ash content particles thereby creating a fluidized particle bed.

The bed level is monitored and control by measuring the bed pressure and manipulating the underflow discharge valve.

To report to the underflow stream, the particles must have a density or total mass that can overcome the hindered settling conditions in the fluidized particle bed.

As such, Teeter-Bed units are commonly referred to as autogenous dense-medium devices.

3:1 particle size range.

Diagram of a CMI Stokes TBS Separator
CMI Stokes TBS Separator
[image 145-4-10]
  • Typical throughput capacity of around 1.0 — 2.0 tons/hr/ft2.
  • Previous studies have indicated the ability to achieve efficient density-based separations over a range of medium densities.
  • Concern is the bypass of coarse, light particles to the underflow.
  • Benefits include a large feed capacity which would eliminate distribution needs.
  • Commercial units:
    • Stokes Hydrosizer
    • Crossflow Separator
    • Reflux Classifier
    • Floatex Classifier
A labeled diagram of a teeter bed separator
A teeter bed separator

Iron Ore Processing Circuit

A diagram of flotation benefication of iron ore
Iron Ore Processing

Iron Ore Processing Using Teeter-beds

First Stage SeparationSecond Stage SeparationThird Stage Separation
Capacity (t/h)Nom. 216
Max. 246
Min. 186
Degree of Enrichment≥64.5% Fe≥68% Fe
Or 68% in total over the two stages
≤0.15% P
≤1.0% SiO2
Iron Recovery
(Each stage)
Iron Recovery
(Compared to incoming Fe-content)
Solids concentration in underflow (% by weight)≥79Average ≥76

Spiral Concentrators (Flowing Film)

  • Separation by density occurs as a result of primary flow and circulating secondary flow patterns that are created as the feed slurry travels along an elongated helical trough surface that spirals downward around a central axis.
  • The primary flow is responsible for carrying the particles in the downward direction toward the discharge point.
  • The secondary flow caused by retardation of the fluid flow near the trough surface provides the density separation by carrying light particles to the outer trough.
A Spiral Concentrator diagram
A diagram of a Spiral Concentrator
[image 145-4-14]
A photo of a spiral concentrator
Spiral Concentrator

Iron Ore Gravity Circuit

A diagram of an iron ore gravity circuit
Iron Ore Gravity Circuit
[image 145-4-15]

Chromite Processing

A diagram of a chromite proccessing circuit
Chromite Processing
[image 145-4-16]

Centrifugal Force, Fc

When the particle size falls below 1 mm, the rate of separation significantly impacts efficiency.

To allow density’based separations, a centrifugal field is applied by either a mechanical action or by accelerating the particles around a rotational axis.

Particle acceleration is given by:

ac = rw2

r = radial distance from center of rotation

ω = angular velocity, (rad/s)

The centrifugal force, Fc , is given by:

Fc= mac= mrw2

The angular velocity ω can be expressed as a function of the tangential velocity of the particle (νT) and the radial distance (r):

w = νr/r

The centrifugal force, Fc is given by:

Fc = mr(νr/r)2 = w = mνT2/r

The angular velocity (ω) is often expressed in terms of the rotational speed, N

w = 2πN/60 where w = 60νr/2πr

The centrifugal force, Fc, is given by:

Fc = mf(2Ï€N/60)2 = 0.01097 mrN2

Centrifugal force is often expressed in the magnitude of the force with respect to the gravitation force (g’s):

Fc/Fg = mrw2/mg = r/g(2Ï€N/60)2 = 0.00118 rN2

Centrifugal Particle Setting Rate

Msap = Fc – Fb – Fd

Fc – centrifugal force

Fb = buoyancy force

Fd = drag force

The terminal settling velocity of a particle in a centrifugal field and under Stokes laminar flow conditions can be expressed as:

νt = rw2d2(ρs-ρf)/18μ

Diagram of the centrifugal particle setting rate equations
Diagram of the centrifugal particle setting rate equations
[image 145-4-17]

Centrifugal Force Effect

A graph showing the centrifugal force effect
Centrifugal force effect
[image 145-4-18]

Enhanced Gravity Separators

Examples of enhanced gravity separators
Enhanced gravity separators
[image 145-4-19]

A diagram depicting fluidized bed separation
Fluidized bed separation
[image 145-4-20]

Falcon Concentrator

Flowing film separation principles.

Bed is controlled by the width of the overflow lip.

High density particles removed by a series of valves placed along the circumference of the bow.

Centrifugal forces up to 300 g’s.

Capacities up to 100 tph and around 2200 gpm.

A diagram of a falcon concentrator
Falcon Concentrator
[image 145-4-21]

Knelson Concentrator

Fluidized bed separation principles.

Fluidization water is injected through two rings located at the top of a bowl.

High density particles settle against the fluidization water and removed by valves placed along the circumference.

Around 60 G’s of force applied.

Capacities up to 100 tph.

A diagram of a Knelson consentrator
Knelson concentrator

Altair Centrifugal Jig

Rotating bowl contains a cylindrical screen with a lip, whose height can be adjusted to vary the natural depth of the ragging bed.

Pressurized water is injected under the bed periodically through the four pulse-blocks to cause alternating dilation and contraction of the ragging and feed bed

A diagram and detail of an Altair centrifugal jig
Altair centrifugal jig
[image 145-4-23]
Two photo details of an Altair centrifugal jig
Altair centrifugal jig
[image 145-4-24]
An example gold circuit
Source: Ausenco Services Pty Ltd, Report 2010

Circuit Boards

A circuit board diagram
Circuit boards
[image 145-4-26]

Gold Processing

A diagram of a gold processing circuit
Gold processing circuit

A diagram of a gold proccessing circuit
Gold processing circuit [image 145-4-28]

Ft Knox Gold Processing Circuit

A diagram of the Ft. Knox gold processing circuit
Ft. Knox processing circuit
[image 145-4-29]