A Review of Placer Gold Concentrate
Recovery & Upgrade Options
Yukon Geological Survey
Miscellaneous Report - 5
(YGS MR-5)
Preamble: In 2010-11, Yukon Geological Survey awarded a contract to NEW
ERA Engineering Corporation of Whitehorse to undertake a study of recent
developments in gravity gold recovery techniques. In partial fulfillment of the
contract, Randy Clarkson attended the Gravity Gold 2010 Optimizing Recovery
Conference in Ballarat, Australia, and presented the following report and
recommendations at the Yukon Placer Workshop in November 2010.
A Review of Placer Gold Concentrate
Recovery & Upgrade Options
Yukon Placer Forum
By: Randy Clarkson, P.Eng. - November, 2010
NEW ERA Engineering Corporation
About the Author
Professional mining engineer working out of
Whitehorse since 1980 on placer, lode mining and
small hydro projects
Developed innovative technique using
Radiotracers to assess the real efficiency of
various gold recovery and sampling systems
World Authority regarding Placer Gold Mining
and Gravity Gold Recovery
Author of several publications on placer
exploration, sampling, gold recovery and alluvial
mining technology
NEW ERA Engineering Corporation
Gravity Recovery in Lode Gold Operations
Gravity pre-concentration
in lode gold ores was
common prior to 1960’s
Used mineral jigs, pinched
sluices, riffled drums and
tables
Generally located at
grinding mill discharge
Fell out of favor with
improvements in froth
flotation and cyanide
leaching
Considered to be out-of-
date technology
Concerns over security of
concentrates
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Rebirth of Gravity in Lode Gold 1980’s
High losses of coarse gold in traps
in milling circuits – security
concerns
Limited residence time in CN- leach
circuits to dissolve coarse particles
Losses of coarse gold in flotation circuits
Faster pay back on gold dore bar
Less expensive than most other methods
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Rebirth of Gravity in Lode Gold 1980’s
Facilitated by the invention and promotion of the
Knelson and Falcon centrifugal batch concentrators
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Typical 1980 Installation
Knelson or Falcon
centrifugal concentrators
installed in one of the
many cyclone underflow
streams
Issues with addition of
clean water to water jacket
– upset water balance
Reduced but continuing
issues with gold lock up
As only part of cyclone U/F
is treated at one time
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Typical 1980 Installation
Need to upgrade these
concentrates
Generally with shaking tables
Then smelting to dore bar
Table tailings to CN or
flotation
Generally only 20-30% of
gross (free) gold recovered
by gravity
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Gravity Recoverable Gold
Test process developed by late Professor Andre
Laplante at McGill University
Samples were concentrated on lab scale Knelson
concentrator and resulting concentrates were fire
assayed
Allowed the determination of gross gravity recoverable
gold
Often some of this gold was locked in sulphides
Could not be upgradeable sufficiently with gravity
methods
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Continuous Discharge Concentrators
Knelson and Falcon adapt/develop continuous
discharge centrifugal concentrators
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Other Continuous Discharge
Kelsey centrifugal jig
Pulsing action, water injection
and ragging
2 models 10 & 60 tph
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Kelsey Centrifugal Jig
Expensive to buy and complex to operate
All feed must be finer than internal screen
All tailings screened to recover ragging as oversize
Regular lubrication and tuning of internal parts
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Gekko InLine Pressure Jig
Sealed unit operates under pressure
Requires pressure locks on feed and
on discharge of concentrate and
tailings
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Gekko InLine Pressure Jig
10:1 concentration ratio 90%
Similar recoveries as
conventional jigs
Much more expensive than
conventional jigs
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Continuous Gravity Concentrators
Originally designed for other applications
than gold
All have low concentration ratios 10:1
Created large volumes of concentrates
with locked gold in sulphide minerals
Development of intensive leach reactors
to treat the locked gold concentrates
Move away from gravity upgrading
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Intensive Leach Reactors
Fast Leach Kinetics due to continuous contact with high
concentrations of CN and oxidant
Used for free and sulphide locked gold
Coupled with electro-winning or carbon absorption to
remove gold from pregnant solution
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Consep ACACIA Intensive Leach Reactors
Consep (Knelson) ACACIA - Fluid Bed Version
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Gekko InLine Reactor
Rotary Reactor Drum Lined with Baffles
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Modular (Underground) Plants
Crushing, grinding,
cyclone sizing,
continuous discharge
gravity recovery devices,
intensive leach reactors,
and electro-winning cells
on portable skids
Gekko Python
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Placer Gravity Recovery & Upgrading
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Placer Gold Recovery & Upgrading
Separate into Stages :
Primary recovery - sluices
or jigs
Secondary upgrading –
long toms, jigs, spiral
drums & centrifugal
concentrators
Final cleaning – shaking
tables, gold wheels,
panning & other methods
Smelting
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Sluiceboxes
Screening improves recovery
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Common Shapes of Jigs
Rectangular – 1/3 yd3 per
square feet of jigging area
Circular – simpler central
feeder – diminishing radial
velocity acts like a
scavenger – higher
capacity ~0.5 yd3 per
square foot of jigging area
high density slurry
Jigs require at least three
stages of concentration
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Methods for Secondary Upgrade
Long Tom (sluices)
Hydraulic Jigs
Mechanical Jigs
Reverse Spirals
Centrifugal Concentration
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Secondary Concentration with Sluices
Feed Hopper with screen to control feed rate;
Long narrow sluicebox fitted with expanded
metal riffles/ Nomad matting;
2/12 or 10 degree slope
Use clear water where available;
Wash down sluice when gold migrate or mats
harden
Watch for coarse gold loss
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Concentration with Mechanical Jigs
Alternating compaction and dilation of
bed (ragging)
Cause by diaphram or mechanical
movement of screen
Requires constant -6 mm (prefer -1 mm)
feed gravels & high density slurries
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Secondary Concentration with Hydraulic Jigs
Water pressure activated jigging action
Requires a constant pressure volume of water
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Secondary Concentration with Hydraulic Jigs
Clean ragging periodically to
recover coarse gold;
Not great at fine (-150 micron) gold
recovery;
Locate sluice downstream to catch
fine gold
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Reverse Spiral Drums
Inclined long cylinder with smaller diameter trommel screen
inside a barrel with reverse spirals
Concentrate is carried up behind spirals - tailings flow over
the lower end
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Centrifugal Secondary Concentration
Vertical and Horizontal
axis
Riffled and Shaped
With and without water
jacket
Use rotation for
centrifugal force to
concentrate heavy
minerals in riffles / or
annulus of cone (Falcon)
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Centrifugals w/o Water Jacket
Horizontal and
Vertical Axis
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Centrifugals w/o Water Jacket
Guyanese
Version
Riffled without water
jacket
Riffles pack easily
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Centrifugals with Water Jacket
Vertical
High radial “g” forces
Use high pressure water
to loosen riffles
Helps avoid riffle packing
Need to balance rotation
and back pressure
Easy to loose
concentrate on shut down
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Final Cleaning Methods
Can be the Most Labor Intensive, Time Consuming and Potentially
Frustrating Job at Placer Mine
Magnetic
Electromagnetic
Gold Wheels
Shaking Table
Wave Table
Mozeley Concentrators
Spirals
Froth Flotation
Mercury Amalgamation
CN leach methods
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Enhancements to Final Cleaning
Pre-screening- vibrating
and rotating trommel
screens
Conditioning to remove
particle coatings – lime or
other reagents
Regrinding to polish
surfaces or grind softer /
less malleable minerals to
a finer size
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HI Magnetic Wet Drum Separator
Magnetics in slurry attach to rotating
drum and are discharged over the
edge (HIWMS)
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Magnetic Belt Separation
Vibrating pan underneath spreads out the concentrate
Permanent magnet above pulls magnetics to belt
Belt conveys magnetics to separate discharge port
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Electromagnetic Separation
Applies a surface electric
charge to mineral particles
before entering an
electrostatic field
Particles will be repelled
from one of the electrodes
and attracted to the other
depending on the charge
on the particle
Can direct the particles to
fall into separate chutes
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Gold Wheel
Relatively cheap
Low capacity for cleaning
concentrates
Available in large
diameters
Some models are portable
with 12 volt motors
Easy to fabricate
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Shaking Tables (Flowing Film)
Conventional Deister Type Table
Operates with flowing film and elliptical throw
Waste flows with water, gold to end of table
Need fine screening – 2 mm screened feed
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Shaking Tables
Homemade conventional and Gemini
tables – higher grade – lower recovery
Models with rotating magnet
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Wave Table
Micron Mill Wave Table
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Mozeley Gravity Separator
Centrifugal and shaking forces cause
heavy particles to move up the drum and
light particles to move down slope and
discharge as tailings
Discharge of heavy particles is assisted
by internal scrapers which rotate slightly
faster than the drum
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Spiral Concentrators
Reichert and Humphrey spirals have a
capacity of about 2 tph (40% density
slurry by weight)
Humphrey requires wash water at
several points and has several
concentrate withdrawal ports
Reichert spirals – no wash water –
concentrate withdrawal at bottom only
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Spiral Concentrators
As the slurry flows down the spiral it
is subject to centrifugal forces which
place water near the outside of the rim
and heavy concentrate along the inner
part of the spiral
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Riechert Cones
Form of pinched sluice used in
beach sand industry
Capy of 50-75 tph for double
cone
Requires screening to minus 2
mm (wedge wire screen) and
hydrocycloning to dewater the
screen undersize
Constant feed and density
Affected by tramp oversize
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Froth Flotation
Limited to fine (-0.3 mm) and/or flat particles
High density slurry (25 to 40% by weight)
Surfaces of economic minerals are rendered hydrophobic (water hating) with
the use of chemical surfactants – need to clean surfaces
Frothers are used to create a stable froth
Agitation and air bubbles introduced – attach to hydrophobic particles
Rise to surface and are skimmed off as a concentrate
Need to depress other hydrophobic minerals
Higher cost – potentially toxic effluents
NEW ERA Engineering Corporation
Froth Flotation
Walsh reported low 24-43% recoveries on
Alaskan alluvial concentrates due to size and
surface characteristics
South African research>95% recovery of gold
smaller than 0.25 mm and 70% at smaller than
0.003 mm
Berry – Saskatchewan River – recoveries similar
to gravity 97% - no magnetic minerals in
concentrate – need to recycle process water
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Mercury Amalgamation
1) Uniquely Effective at cleaning
fine gold concentrates
provided the gold particle
surface is clean;
2) Mercury is cheap relative to
price of gold;
3) Inexpensive simple equipment;
4) Very Portable
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Poor Practices
Improper handling/ equipment
Use in open circuit
Open burning of amalgam
pastes
Chronic health effects to
workers, village and camp
residents;
Losses to tailings/ waters
Mercury accumulation in
humans and environment
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How to Improve Mercury Usage
Personal protection – gloves,
respirators;
Locate away from camp /
village facilities;
Use in closed circuit as final
concentrate cleaning only;
Recycle process water;
Recover mercury from
concentrate tailings; and
Proper disposal of final
concentrate tailings.
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Smelting Dore
Mix dirty concentrates with flux
Melt to slag
Cool and separate slag
Safer to ship dore bar than
particles of gold
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Smelting
Concentrate and Flux (Soda Ash & Borax) are melted in furnace
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Smelting
Clean Placer Gold – Flux = 50% Borax, 50% soda
ash
Dirty Placer Gold = 100 % Soda Ash
Use caution when using high soda ash fluxes, as
gas escapes the level will rise in the crucible
Mix 50% concentrate and 50% flux
Always warm the mold before use
Move back and forth when pouring
Regrind and pan flux as it has some gold
NEW ERA Engineering Corporation
Future Research on Upgrading
Placer Industry is on its own – New Hard rock mines are
using CN reactors to upgrade
Literature Search for other concentrate upgrade methods –
(Russia and China)
Select equipment and methods to evaluate
Conduct laboratory testing to optimize equipment and
methods for upgrading concentrates
Requires access to highly specialized high-cost process
equipment and laboratory
Requires experience and familiarity with the placer industry
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Suggestion
Use the facilities and equipment at the University
of B.C.
Sponsor a post graduate student to conduct the
experimental design and test work
Laboratory Supervision by professors
Assistance and field supervision from KPMA?
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UBC Mining Faculty
Dr. Bernhard Klein: Head of the UBC Department of Mining
Engineering, Faculty Supervisor, Center for Coal and Mineral
Processing (CMP) Laboratory Facility
Director, Center for Industrial Minerals Innovation (CIMI)
Research Interests: Ultrafine grinding; High pressure grinding rolls;
Hydraulic transport of non-Newtonian mineral slurries; Industrial minerals;
Mine-mill integration; Continuous centrifugal gravity concentration;
Improved technologies for artisanal and small scale gold miners; and
Metal leaching from waste rock
Dr. Marcello Veigo Associate Professor
Research Interests: Biogeochemical cycle of heavy metals in the environment;
Bioaccumulation and adverse health effects of metals in the environment, specially
mercury
Artisanal and small-scale gold mining; Mercury pollution from gold mining and
hydroelectric reservoirs
Acid Rock Drainage; Process mineralogy applied to mining and mineral processing;
Sustainable development in mining
Mining communities and social issues related to mining; Mine closure and reclamation
planning;
NEW ERA Engineering Corporation
Recommendations for Further Research*
*The recommendations presented below are excerpted from the Final project report submitted to YGS from NEW
ERA Engineering Corporation.
Most of the lode gold mines are phasing out conventional gravity concentrate upgrading methods such as jigs,
mineral tables and batch operated centrifugal concentrators. Therefore at the Ballarat conference there was no
discussion of innovations in these conventional technologies which are generally applied to upgrade placer gold
concentrates.
This signals a fundamental change in the methodology of gravity concentration in lode gold mines. The
mines are less likely to attempt to recover the coarser free gold particles in a high grade batch concentrate and are
switching to recovering larger volumes of low grade concentrates of auriferous sulphides. These larger volumes of
auriferous sulphides are upgraded with intensive leach reactors. These are reactor vessels with very high
concentrations of cyanide (3,000 to 50,000 ppm) and oxygen (>15 ppm).
Intensive leach reactors are less likely to find an application in most of our Yukon placer mines due to
their high cost, complexity and toxicity. It is also unlikely that we will see many innovations in the upgrading of
gravity concentrates from placer mines at similar lode gold technical forums as the market for these innovations will
be further restricted to placer mine applications. Innovations in gravity recovery of gold and for the upgrading of
gravity concentrates will have to be developed specifically for placer mining applications.
I am recommending that the Klondike Placer Miners’ Association spearhead further research into
upgrading of gravity gold concentrates by developing a joint post graduate industry research program. This would
possibly involved post graduate students and their advisors from the Mining Engineering Departments of the
University of British Columbia in Vancouver, and the University of Alaska in Fairbanks.
A laboratory and field program would be designed to allow the testing of various gravity concentrates
in the field during the operating season and in the university laboratories during the academic season. Technologies
tested should include gravity, magnetic, electromagnetic, flotation as well as intensive cyanide. From this test work
a guideline could be developed to assist placer miners in selecting and operating concentrate upgrading equipment.
Tara Christie and I have already been in discussions with the head of Mining Engineering at UBC and
intend to meet with the U of Alaska shortly.
NEW ERA Engineering Corporation
