fine coal cleaning by froth...

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•Fine Coal Cleaning by •Froth Flotation

• Rodrigo Araya, Process Engineer • XT Canada • March 2014

• Coal Preparation

• Froth Flotation

• Flotation Machine

• Bubble Size

• Froth Phase

• Jameson Cell

Outline

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

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

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

Hydrophobic and hydrophilic species

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Froth Flotation

Coal particles attach to air bubbles

Background

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Froth Flotation

Dynamic Fixed Features of the technology

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

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

Ultimate goal

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Flotation Machine

Coal is collected from the concentrate stream

Fine bubbles are better for the whole size range

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Bubble Size

(Diaz-Penafiel & Dobby, 1994)


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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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• 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

Water entrainment

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Froth Phase

Air bubble

wake: water carried by bubble

Coal Ash

Non-Selective recovery of coal and ash

Water entrainment

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Froth Phase

Coal Ash

Froth phase

Liquid phase (slurry)

Low ash

Medium Ash

Same ash as feed

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)

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

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

Presenter

Presentation Notes

Slurry is pumped through a flow restriction and creates a high velocity jet. The velocity is in the range of 15 to 17 m/s. Vacuum is generated inside the downcomer which causes slurry to rise inside the downcomer. The jet of slurry plunges onto the slurry surface. Here is where bubbles are created, the jet shears the air into fine bubbles. At the same time a high intensity mixing condition is also created. This is the heart of the technology, here is where flotation happens. Hydrophobic particles attach to air bubbles.

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

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

Presenter

Presentation Notes

This slide shows one of the large models. Slurry enters from the bottom and gets to the slurry distributor. From the distributor slurry feeds the “downcomers”. The air is aspirated from the atmosphere, it accumulates in the lower part of the distributor and finally reach the downcomer through these hoses.

Jameson Cell Operation

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

Coal Flotation

• Need to characterize the flotation response of each coal seam: Standard tree test, release analysis or another procedure

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

Plant Performance

• Performance depend on the selected operating variables

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Questions?

fine coal cleaning by froth...

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