fine coal cleaning by froth...
TRANSCRIPT
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•Fine Coal Cleaning by •Froth Flotation
• Rodrigo Araya, Process Engineer • XT Canada • March 2014
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• Coal Preparation
• Froth Flotation
• Flotation Machine
• Bubble Size
• Froth Phase
• Jameson Cell
Outline
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General
Coal Preparation
Desliming screen 1.4 mm
Dense medium circuit
Clasifying cyclones
2 Stage spirals
Flotation
Dewatering
Co-disposal
Dewatering
Dewatering
75 – 0 mm 75 – 1.4 mm
1.4 - 0 mm
1.4 – 0.25 mm
0.25 - 0 mm
Product
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General
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• Surface chemistry based separation technique.
– Pulp consists of hydrophobic and hydrophilic species
– Air bubbles are introduced into an agitated pulp
– Hydrophobic particles attach to air bubbles and are carried to the surface
– Laden bubbles overflow the flotation cell into the collecting launder
– Hydrophilic particles remain in continuous phase
Froth Flotation
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Hydrophobic and hydrophilic species
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Froth Flotation
Drop of water Air
Hydrophobic Hydrophilic
Drop of water
Solid surface
Air
Coal is hydrophobic and ash is hydrophilic
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Hydrophobic and hydrophilic species
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Froth Flotation
Coal particles attach to air bubbles
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Background
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Froth Flotation
Dynamic Fixed Features of the technology
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Bubble generation
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There exist many different techniques and devices to disperse air
- Shear action of a metallic surface moving at high speed in a relatively stationary liquid (rotor in mechanical cells)
- Jetting of air through small holes in a porous material (laboratory flotation columns)
- Jetting of air through a single hole (jetting spargers)
- Jetting of liquid into a liquid surface (Jameson cell)
- Shear action of a high velocity liquid striking a stationary metallic surface (in-line mixers and contact cells)
Flotation Machine
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Ultimate goal
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Froth flotation needs to be performed on machines that are designed for:
- Generation and distribution of bubbles
- Suspension and distribution of particles
- Generation of a “clean” froth phase
Flotation Machine
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Ultimate goal
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Flotation Machine
Coal is collected from the concentrate stream
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Fine bubbles are better for the whole size range
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Bubble Size
(Diaz-Penafiel & Dobby, 1994)
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Fine bubbles are better for the whole size range
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Bubble Size
Bubble size depends on the flotation technology
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Bubb
le S
ize
(mm
)
Superficial Gas Velocity - Jg (cm/s)
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Fine bubbles are better for the whole size range
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• Why do we need small bubbles?
Bubble Size
Total Vol, mm3
Bubble Diam, mm
NoBubbles
Bubblesurface area
mm2
Particle Diam, mm
Particle surface area,
mm2
No particles
3 0.5 46 36 0.25 0.20 180
3 1.5 2 12 0.25 0.20 60
Small bubbles means higher throughput
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Water entrainment
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Froth Phase
Air bubble
wake: water carried by bubble
Coal Ash
Non-Selective recovery of coal and ash
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Water entrainment
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Froth Phase
Coal Ash
Froth phase
Liquid phase (slurry)
Low ash
Medium Ash
Same ash as feed
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Water entrainment
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Froth Phase
Drainage
Low liquid content
Medium liquid content
High liquid content
Ash particles prefer to stay in water (hydrohilic)
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Removal of entrained ash by wash water
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Froth Phase
froth
pulp
start
contaminated froth
after a few minutes
water sprayed in via this ring
with wash water on
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Jameson Cell
• Pressurized slurry enters downcomer through a nozzle at high velocity
• The jet entrains air from the atmosphere
• Jet plunges into slurry surface causing the air to shear into fine bubbles
• High intensity mixing leads to high probably of bubble-particle collision and contact
• Slurry and collected particles exit downcomer
• Particle laden bubbles are separated from the pulp
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Jameson Cell
• To account for fresh feed flow fluctuations, a portion of tailings are re-circulated
• Downcomer always operates at constant feed pressure and flow rate which provides consistent:
• Jet velocity
• Air entrainment
• Mixing intensity
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Jameson Cell
Split slurry-air distributor
Separation tank
External launder outlet
Downcomer feed inlet
Tailings outlet
Cross lauder
Washwater tray
Air intake
Internal launder outlet
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Jameson Cell Operation
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Jameson Cell
Three 2.5 m x 16 m columns
Two 1.9 m Jameson Cells Each Jameson Cell has the same capacity as one column
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Coal Flotation
• Need to characterize the flotation response of each coal seam: Standard tree test, release analysis or another procedure
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Plant Performance
0
20
40
60
80
100
4 6 8 10 12 14
Com
bust
ible
s Rec
over
y (%
)
Ash (%)
Jameson CellWemco Cells
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Plant Performance
• Performance depend on the selected operating variables
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Questions?