8.3 Gravity Concentration & Magnetic/Electrostatic Separation

Gravity Concentration & Magnetic/Electrostatic Separation

Physical mineral separation techniques exploit intrinsic physical property differences between valuable minerals and barren gangue without altering chemical structures. The primary physical property differentials exploited in mineral dressing are specific gravity (density), magnetic susceptibility, and electrical conductivity. These methods can provide low-reagent preconcentration or recovery alternatives to flotation, but water, energy, dust, medium loss, tailings, and equipment impacts still require management.

Concentration Criterion & Gravity Concentration Theory

Gravity concentration separates mineral particles based on differential movement in response to gravity and fluid drag forces. The feasibility of wet gravity separation is predicted by the empirical Concentration Criterion ($CC$):

$CC = \frac{\rho_h - \rho_f}{\rho_l - \rho_f}$

Where:

• $\rho_h$ = Specific gravity of the heavy mineral ($\text{g/cm}^3$)
• $\rho_l$ = Specific gravity of the light gangue mineral ($\text{g/cm}^3$)
• $\rho_f$ = Density of the fluid medium (Water = $1.0\ \text{g/cm}^3$)

Operational Rules for Gravity Separation Feasibility

Published CC-to-size tables are empirical heuristics, not physical guarantees. Use representative partition or recovery tests at the actual size distribution and solids conditions before declaring technical or commercial feasibility.

Gravity Separation Equipment & Mechanisms

1. Jigs (Harz, Denver, Pan-American Jigs): Utilize vertical pulsation of water through a fixed screen basket, stratifying particles by differential acceleration, hindered settling, and consolidation trickling. Used for coarse lead, zinc, and alluvial tin ores ($1\text{--}25\ \text{mm}$).
2. Wilfley Shaking Tables: Consist of a gently sloped wooden deck covered with linoleum or rubber and fitted with longitudinal riffles. An asymmetrical reciprocating motion (slow forward stroke, rapid return stroke) pushes heavy mineral particles along the riffle troughs, while a cross-flowing water film washes light gangue down over the riffles. Achieves sharp separation for fine feeds ($0.05\text{--}2.0\ \text{mm}$).
3. Spiral Concentrators (Humphreys Spirals): Open helical vertical sluices with a modified semicircular cross-section. As pulp flows downward, high-velocity fluid carries light gangue toward the outer rim under centrifugal force, while low-velocity fluid allows heavy minerals to concentrate along the inner radius, where splitter gates collect concentrate.
4. Centrifugal High-G Concentrators (Knelson & Falcon Concentrators): Use a rapidly rotating bowl to multiply the settling force on dense particles. Depending on the model, fluidization and bowl geometry retain a dense bed and can recover fine liberated free gold. No universal lower size or recovery follows from nominal g-force; test liberation, feed rate, fluidization, cycle time, and concentrate mass pull.
5. Heavy Media Separation (HMS / DMS): A Float-Sink process utilizing an artificial dense liquid suspension (milled ferrosilicon, $\text{SG} = 6.8$, or magnetite, $\text{SG} = 5.0$) diluted in water to establish an intermediate medium density $\rho_m$. Heavy ore sinks while light waste floats. Used extensively for pre-concentrating coal, iron ore, and diamonds.

Magnetic Separation Principles & Machinery

Magnetic separation isolates minerals based on differences in magnetic susceptibility ($\chi$), which determines the magnetic force ($F_m$) exerted in an inhomogeneous magnetic field gradient:

$F_m = V \cdot \chi \cdot H \cdot \frac{dH}{dx}$

Where $V$ is particle volume, $H$ is magnetic field intensity, and $\frac{dH}{dx}$ is field gradient.

Mineral Magnetic Classification

• Ferromagnetic: Strongly attracted by weak magnetic fields (e.g., Magnetite $\text{Fe}_3\text{O}4$, Pyrrhotite $\text{Fe}{1-x}\text{S}$). Recovered using Low-Intensity Magnetic Separators (LIMS) operating below $0.3\ \text{Tesla}$ ($3,000\ \text{Gauss}$).
• Paramagnetic: Weakly attracted in strong magnetic fields (e.g., Hematite $\text{Fe}_2\text{O}_3$, Ilmenite $\text{FeTiO}_3$, Chromite $\text{FeCr}_2\text{O}_4$, Wolframite). Recovered using High-Intensity Magnetic Separators (HIMS) operating between $1.0$ and $2.5\ \text{Tesla}$ ($10,000\text{--}25,000\ \text{Gauss}$).
• Diamagnetic: Repelled by magnetic fields (e.g., Quartz $\text{SiO}_2$, Calcite $\text{CaCO}_3$, Feldspar, Zircon). Report to non-magnetic tailings.

Magnetic Separator Configurations

• Wet Drum LIMS: Revolving non-magnetic stainless steel drum containing internal permanent magnet arcs. Used in Concurrent (coarse feed), Counter-Rotation (heavy magnetite recovery), and Counter-Current (fine slurry) configurations.
• High-Gradient Magnetic Separators (HGMS): Utilize magnetized stainless steel wool matrix filters inside high-field solenoids to trap fine weakly paramagnetic slimes.

Electrostatic & High-Tension Separation

Electrostatic separation differentiates minerals based on electrical conductivity and surface charge retention.

High-Tension Corona Discharge Separator

Ore particles are fed as a thin dry layer onto a grounded rotating metal drum exposed to an ionizing corona wire charged at $20\text{--}50\ \text{kV}$.

• Conductive Particles (e.g., Rutile, Ilmenite, Magnetite): Rapidly transfer their acquired charge to the grounded metal drum upon contact. Uncharged, they are thrown off the drum by centrifugal force into the conductor collector hopper.
• Non-Conductive Particles (e.g., Zircon, Monazite, Quartz): Retain their surface charge and become electrostatically pinned to the drum by image charges. Pinned particles travel to the underside of the drum, where a rotating brush sweeps them into the non-conductor hopper.

                          Dry Feed Hopper
                                |
                                v
                     +---------------------+  <-- Corona Electrode
                     |   High-Voltage      |     (20-50 kV Ionizing)
                     |   Corona Discharge  |
                     +----------+----------+
                                |
                                v
        +-----------------------------------------------+  <-- Grounded
        |         Rotating Grounded Metal Drum          |      Drum
        +-------+-------------------------------+-------+
                |                               |
                v                               v
       (Centrifugal Throw)              (Pinned / Brushed)
  +---------------------------+   +----------------------------+
  | Conductors (Rutile, FeTiO3)|   | Non-Conductors (Zircon,SiO2)|
  +---------------------------+   +----------------------------+

Philippine Mining Industrial Context

Physical mineral separation plays a vital role in Philippine mining operations:

1. Northern Luzon Magnetite Sands (Cagayan, Ilocos): Exploits heavy beach sand placer deposits using wet dredge pumps, spirals for pre-concentration, and drum LIMS to produce high-grade magnetite concentrates ($

62%\ \text{Fe}$).

1. Zambales & Dinagat Chromite Dressing: Utilizes shaking tables and spiral concentrators to separate high-density chromite ($\text{SG} = 4.5\text{--}4.8$) from serpentine gangue ($\text{SG} = 2.5\text{--}2.7$), followed by dry high-tension separators to upgrade metallurgical-grade lump chromite.

Comparative Summary of Physical Separation Methods