Column Flotation Cell. Operating and Maintenance Manual - Canadian Process Technologies Inc. (2002)

8/18/2019 Column Flotation Cell. Operating and Maintenance Manual - Canadian Process Technologies Inc. (2002)

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o l um n F l o t a t i o n e l l

Operating and

Maintenance Manual

Prepared By:

Canadian Process Technologies Inc. 1636 West 75

th Avenue

Vancouver, B.C.

Canada V6P 6G2Tel: +1 604 264 5610

Fax: +1 604 264 5555

Email: [email protected]

URL: http://www.cpti.bc.ca

May 2002


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CPT Inc. Column Cell Operating & Maintenance Manual

The purpose of this manual is to provide an overall guide to column flotation technology along with

detailed operating instructions for the CPT Flotation Column and the SlamJet® Gas Sparging

System. This manual has been divided into seven sections as follows;

1. Introduction

2. Column Operation

3. Instrumentation and Control

4. System Maintenance

5. Collection Zone Theory

6. Froth Zone Theory

7. Production Column Testwork

Companion Manual

SlamJet® Operating and Maintenance Manual

including the following sections;

1. Introduction

2. Components

3. Installation

4. Operation

5. Maintenance

6. Control System

7. Cracking Pressure Adjustment

8. SlamJet® Repair Procedures


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TABLE OF CONTENTS

1.0 INTRODUCTION...........................................................................................................1

1.1 DESCRIPTION OF A FLOTATION COLUMN...................................................................................... 1

1.2 SlamJet® DESCRIPTION............................................................ ERROR! BOOKMARK NOT DEFINED.

1.3 GLOSSARY OF COLUMN FLOTATION TERMINOLOGY ..................................................................... 4

1.3.1 Bias ................................................................................................................................... 4 1.3.2 Carrying Capacity ............................................................................................................. 5 1.3.3 Difference Wash ...............................................................................................................5 1.3.4 Displacement Wash.......................................................................................................... 5 1.3.5 Entrainment....................................................................................................................... 5 1.3.6 Flow Conventions ............................................................................................................. 5 1.3.7 Gas Holdup.......................................................................................................................5 1.3.8 Air Sparger........................................................................................................................ 6 1.3.9 Superficial Velocities.........................................................................................................6

1.4 TERMS ....................................................................................................................................... 6

1.4 W ASH W ATER ............................................................................................................................ 7

2.0 COLUMN OPERATION ................................................................................................8

2.1 INTRODUCTION........................................................................................................................... 8

2.2 COLUMN KEY ............................................................................................................................. 8

2.3 COLUMN START-UP .................................................................................................................... 8 2.3.1 Start-up Options................................................................................................................ 8 2.3.2 Set-up ............................................................................................................................... 9 2.3.3 Sparger Check.................................................................................................................. 9 2.3.4 Pressurise the Flowmeters ............................................................................................... 9 2.3.5 Air Line Checks............................................................................................................... 10 2.3.6 Sparger Air Pressurisation.............................................................................................. 10

2.3.7 Sparger Hose Connections............................................................................................. 10 2.3.8 Wash Water Check......................................................................................................... 10 2.3.9 Gas Holdup Calibration................................................................................................... 11 2.3.10 Empty Column Start-up .................................................................................................. 11 2.3.11 Check Launder Sprays ................................................................................................... 12 2.3.12 Starting of Slurry Flows................................................................................................... 12 2.3.13 Oil in Sparger Air Supply ................................................................................................ 13

2.4 COLUMN SHUTDOWN......................................................................................................... 13 2.4.1 Emergency Shutdown..................................................................................................... 13 2.4.2 Initial Procedure..............................................................................................................13 2.4.3 Final Procedure............................................................................................................... 13 2.4.4 Short Term Shutdown.....................................................................................................13 2.4.5 Long Term Shutdown .....................................................................................................13 2.4.6 Power Failure Shutdown................................................................................................. 14

2.5 COLUMN RE-START FOLLOWING SHUTDOWN ............................................................................ 14 2.5.1 Re-Pulping ......................................................................................................................14 2.5.2 Re-Starting Underflow ....................................................................................................14


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2.6 OPERATING GUIDES ................................................................................................................. 15

2.6.1 Sparger Air...................................................................................................................... 15 2.6.2 Interface Level ................................................................................................................15 2.6.3 Wash Water .................................................................................................................... 16 2.6.4 Wash Water Bias ............................................................................................................ 16 2.6.5 Reagent Dosage............................................................................................................. 16

2.7 TROUBLESHOOTING .......................................................................................................... 17 2.7.1 Is Froth Purity Low?........................................................................................................ 17 2.7.2 Is Flotation Recovery Low? ............................................................................................17

3.0 INSTRUMENTATION AND CONTROL ......................................................................19

3.1 INTRODUCTION......................................................................................................................... 19

3.2 SPARGER PRESSURE ............................................................................................................... 19

3.3 SPARGER AIR FLOW................................................................................................................. 19

3.4 COLUMN INTERFACE LEVEL ...................................................................................................... 20

3.5 W ASH W ATER .......................................................................................................................... 20

4.0 SYSTEM MAINTENANCE ..........................................................................................21

4.1 INTRODUCTION......................................................................................................................... 21

4.2 COLUMN M AINTENANCE............................................................................................................ 21

4.3 SPARGER M AINTENANCE .......................................................................................................... 22

4.3.1 Orifice Wear .................................................................................................................... 22 4.3.2 Tip Blockage ................................................................................................................... 22 4.3.3 External Scaling ..............................................................................................................22 4.3.4 Sparger Removal............................................................................................................ 23 4.3.5 Poor Air Distribution........................................................................................................ 24

4.4 COMPRESSOR AND AIR LINES ................................................................................................... 24

5.0 COLLECTION ZONE THEORY.................................................................................. 25

5.1 INTRODUCTION......................................................................................................................... 25

5.2 RESIDENCE TIME...................................................................................................................... 25

5.2.1 Column Volume ..............................................................................................................26 5.2.2 Gas Holdup.....................................................................................................................26

5.3 AIR R ATE................................................................................................................................. 26

5.3.1 Maximum ........................................................................................................................26 5.3.2 Optimum ......................................................................................................................... 27

5.4 PARTICLE AND BUBBLE SIZE ...................................................................................................... 28 5.4.1 Gas Velocity....................................................................................................................28 5.4.2 Probability of Collection .................................................................................................. 28 5.4.3 Solids Carrying Capacity ................................................................................................ 29


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6.0 FROTH ZONE THEORY............................................................................................. 30

6.1 INTRODUCTION......................................................................................................................... 30

6.2 FROTH CLEANING ..................................................................................................................... 30

6.3 G AS VELOCITY ......................................................................................................................... 31

6.4 FROTHER CONCENTRATION ...................................................................................................... 33 6.5 SUPERFICIAL BIAS R ATE .......................................................................................................... 33

6.6 SELECTIVITY ............................................................................................................................ 35

6.7 REFERENCES ........................................................................................................................... 35

7.0 PRODUCTION COLUMN TESTING........................................................................... 36

7.1 INTRODUCTION......................................................................................................................... 36

7.2 AIR R ATE................................................................................................................................. 36

7.3 W ASH W ATER BIAS.................................................................................................................. 37

7.4 OPERATING LEVEL ................................................................................................................... 37

7.5 BUBBLE SIZE ........................................................................................................................... 38

7.6 RESIDENCE TIME...................................................................................................................... 38 7.7 W ASH W ATER DISTRIBUTOR HEIGHT......................................................................................... 39

7.8 S AMPLING................................................................................................................................ 39

7.9 COLLECTION ZONE DENSITY ESTIMATION..................................................................................39

7.10 FROTH ZONE DENSITY ESTIMATION ....................................................................................... 40

7.11 COLUMN TEST D ATA SHEET - S AMPLE...................................................................................41


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1.0 INTRODUCTION

1.1 DESCRIPTION OF A FLOTATION COLUMN

A Flotation Column is a type of flotation machine that incorporates some design features to enhance

metallurgical performance. Some of these features include:

• Reduced surface area to cell volume ratio to promote froth stability

• Froth washing system to minimise the entrainment of impurities

• Quiescent flotation conditions to promote selectivity

• Air sparging system to generate a supply of uniform bubbles

1.2 UNIQUE DESIGN FEATURES OF THE CPT COLUMN

The CPT Column Flotation System incorporates a number of unique, advanced features that

significantly enhance performance, such as . . .

• QuickLift adjustable wash water manifold that provides overhead or submerged (in-froth) wash

water addition to allow adjustment of cleaning and froth density as required.

• Internal feed distribution with lateral dispersion plates to promote uniform, low pressure

distribution of the incoming slurry.

• Circular internal launders to reduce the travel distance for loaded bubbles, thus enhancing

recovery of coarse particles.

• Launder baffles extending slightly into the slurry, to provide froth stabilization and minimize lateral

mixing during froth drainage.

• The patented SlamJet® Gas Sparging System providing air only or air + water injection through a

single ceramic wear-protected nozzle, and featuring a self-adjusting mechanism that provides

automatic fail closed operation on loss of air.

• Internal conical tank bottom to promote underflow removal and minimize the potential for sanding

at the bottom of the column.


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Flotation Columns derive their name from the

geometric shape of the vessel. Unlike conventional

mechanically agitated flotation machines which tend

to use relatively shallow rectangular tanks, column

cells are tall vessels with heights typically ranging

from 8 m to 15 m. The tank cross-section may be

either round, square or rectangular depending on the

specific application. For an equivalent volumetric

capacity, the surface area of the column cell is much

smaller than a conventional cell. This reduced area

is beneficial for promoting froth stability and allowing

very deep froth beds to be formed.

Figure 1 - Typical Column Cell

An important feature of flotation columns is the froth washing system. Froth washing provides an

additional means for removing unwanted impurities from the flotation froth. Wash water, added at the

top of the column, filters through the froth zone displacing process water and entrained particles

trapped between the bubbles. In addition, froth wash water serves to stabilise the froth by separating

bubbles into a “packed bed” of spherical, and therefore very strong, bubbles.

Unlike conventional flotation machines, columns do not use mechanical agitation. The absence of

intense agitation promotes selectivity and aids in the recovery of very fine particles. The basic flow

streams in a column are illustrated in Figure 1. Feed slurry enters the column at one or more feed

points located in the upper third of the column body and descends against a rising swarm of fine

bubbles generated by the air sparging system. Particles which collide with, and attach to the bubbles,

rise to the top of the column, eventually reaching the interface between the pulp (collection zone) and

the froth (cleaning zone). The location of the interface, which can be adjusted by the operator, is held


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constant by means of an automatic control loop which regulates a valve on the column tailings line.

Varying the location of the interface will increase or decrease the height of the froth zone.

Flotation air is introduced into an external manifold and is injected through a series of air lances

(sparger tubes) located near the bottom of the column. The air rate used in the column is selected

according to the feed rate and concentrate production requirements and will determine, in part, the

point on the grade / recovery curve at which the column operates.

CPT’s patented SlamJet® air sparging system, shown schematically in Figure 2, comprises an air

manifold that surrounds the column cell and supplies air to a series of SlamJet® sparger tubes. The

top of the manifold is fitted with connections for air as well as connections for a pressure gauge and a

pressure transmitter. The side of the manifold is equipped with a series of couplings for connecting to

the sparger tubes and is also fitted with one or more drains located at the bottom of the manifold.

Inser t ion Port As sembly

S l a m J e t

®

S l a m J e t

®

Advanced Gas

Sparging SystemColumn BodyVessel Wall

SLAMJET

Gas

Mani fo ld

Figure 2 - SlamJet® System


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Air enters the manifold through the connections located on the top of the manifold and exits through

the series of threaded couplings, (Figure 2) located on the side of the header. The manifold is sized

to act as a buffer against turbulence and to provide sufficient reservoir capacity to ensure even air

distribution to all spargers. The volume of the header also provides some opportunity for foreign

material to settle out.

The air then flows through the connecting hose to the sparger tube and is injected into the column

through the single ceramic lined orifices of the SlamJet®s. The number and length of SlamJet

®s and

the pattern of insertion is designed so as to ensure an even distribution of fine bubbles.

The exact number and size of SlamJet®s included for each column is specifically designed to provide

for a maximum superficial gas velocity of 2.5 cm/s. Each SlamJet® is attached to the header by a

single flexible hose fitted with a quick-disconnect swing coupling to allow for easy removal for

inspection or maintenance.

The sparger elements have been designed to allow easy removal from the column. A full port ball

valve and liquid-tight seal assembly comprises the sparger insertion port, and prevents process

slurries from exiting the column when the sparger is removed. Each SlamJet® is also fitted with a self-

adjusting mechanism that actuates a nozzle-mounted needle valve to control air flow and also allows

the SlamJet® to fail closed on loss of air, thus eliminating any possibility of slurry back-flow into the air

delivery system..

A quick-disconnect coupling is used to isolate the sparger air flow during removal.

For the first few months of operation, SlamJet® nozzles should be checked monthly for fouling and

wear. Worn nozzles can be quickly and easily replaced while the column is in full operation. The

inspection frequency can then be adjusted on an experience basis.

The SlamJet® Model SLJ-75 has a larger (7.62 mm Ø) bore and incorporates a small flow of water to

promote generation of small bubbles. The sparger water flow, about 8 L/min, is administered by

means of an additional distribution manifold located just below the main air manifold.

1.3 GLOSSARY OF COLUMN FLOTATION TERMINOLOGY

The study of column flotation systems has resulted in new ways of examining flotation and somespecific terminology has evolved to describe the process. Some of the common terms are:

1.3.1 Bias

The term Bias (Superficial bias - Jb) is used to describe the flow of water (magnitude and direction)

through the froth zone into the collection zone. A positive bias is a net downward flow. It is often


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estimated as the difference between the volumetric flow of the column underflow slurry and the

volumetric flow of the feed slurry rates divided by the cross-sectional area of the column.

1.3.2 Carry ing Capacity

Three different carrying capacities have evolved;

Ca - The maximum overflow mass that a column can produce. This is commonly quoted in terms

of tonnes of solids per hour per square meter of cross sectional column area - t/h•m2. A

theoretical value can be estimated from Ca = 0.03 D80 ρs.

Cg - The maximum solids floated per unit of air, commonly expressed in units of kilograms of

concentrate solids per cubic meter of air - kg/m3.

Cl - The maximum mass of pulp that can be transported over the lip of the column, normally

expressed as tonnes per hour per meter of lip length - t/h•m. Note that lip length must include

the length of all internal launders. This figure is critical in the design of large columns.

1.3.3 Difference Wash

The volume of water in the feed subtracted from the volume of water in the underflow is one

difference wash. Any wash water added in excess of this amount is assumed to report to the

overflow.

1.3.4 Disp lacement Wash

Displacement wash is the ratio of wash water to overflow water. A displacement wash of 1.0 means

that all the wash water reports to the overflow, while a displacement wash of 2.0 indicates an equal

amount of the water flow in the bias and overflow streams (assuming no feed water in the overflow).

1.3.5 Entrainment

Non-selective flotation caused by particles riding in a bubble's wake thereby passing into the column

overflow. Entrainment is common in mechanical cells, particularly with small particles, but is virtually

eliminated in column flotation by the use of wash water.

1.3.6 Flow Conventions

Downward flow of slurry or liquid, and upward flow of air are defined as positive.

1.3.7 Gas Holdup

There are three types of holdup in a column reflecting it's three phase nature - solids (mineral), liquid

(water) and gas (air).


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Gas Holdup (f g, or εg) is the fractional volume of gas. A typical value of gas holdup is 0.15 (or 15%)

but this value may range between 0.05 and 0.25 (5% to 25%).

Solids Holdup (f s, or εs) is the solids fractional volume.

Liquid Holdup (f l, or εl) is the fractional volume of liquid.

1.3.8 Air Sparger

Any device used to create the bubbles in a flotation column. The common types are the CPT

SparJet® System, the CPT SlamJet

® System and various constructions of porous media.

1.3.9 Superfic ial Velocities

Superficial velocity is the volumetric flow rate of the material in question (slurry, water or air) dividedby the cross sectional area of the column, normally expressed in cm/s. This normalised variable

allows evaluation of column performance characteristics independent of column diameter.

1.4 TERMS

(Units may vary according to local use)

Cx Carrying capacity where x = :

a = theoretical bubble loading (t/h•m2)

g = loading per gas unit (kg/m3)

l = removal capacity per unit lip length (t/h•m)

x Holdup (all either fractional or %) where x = :

g = gas

l = liquid

s = solids

Jx Superficial Velocity where x = :

b = bias

o = overflow

f = feed

g = gas


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sl = slurry

u = underflow

w = wash water

spa = sparger air

Qx Mass flow rate, with x values the same as for superficial velocities

Vx Volumetric flow rate

x Density of x = :

col = bulk, collection zone

fro = bulk, froth zone

s = average concentrate solids

sl = column slurry estimated by using tails density

u = underflow density

(l) = liquids fraction

(s) = solids fraction

Hc = height of column (m)

Hf = height of interface (m)

Hspa = height of spargers (m)

P = pressure (kPa)

L = distance from column lip to pressure transducer (m)

1.4 W ASH W ATER

Wash water (Jw) is the water added to clean the froth zone. In positive bias operation this J w forms

both the overflow liquid (Jo) and the bias (Jb).


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CPT Column Column Operation Page 8

2.0 COLUMN OPERATION

2.1 INTRODUCTION

This section covers column start-up and shut-down, and gives estimates of column parameters and

guidelines for operation. It is intended to familiarise operators with column operation as a reference

and as a start to determining the column's operational characteristics. The values it contains are

estimates which must be confirmed in plant practice.

2.2 COLUMN KEY

(Start-up and Shut-Down)

A key is a written flow chart of questions and directions. These keys are meant to serve only as a

guide. Not all steps are needed all of the time. Warnings and notes draw attention to important

features. Advance in order unless instructed otherwise. Each key describes steps performed on one

column.

End indicates completion of instructions. This may be followed for each column.

2.3 COLUMN START-UP

2.3.1 Start-up Options

First start-up, go to 2.3.2

Normal operation, column empty, go to 2.3.10; column full, go to 2.3.11

CONTROL WARNING:

For mineral columns operating at higher densities (20% to 30% solids or higher) a minimum flow must

be set on the underflow valve to prevent sanding. This is done by setting a minimum position on the

underflow valve. Whenever possible this minimum should be maintained.


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2.3.2 Check All Auto Valves

The automatic valves should all be “stroked” using the control system.

Ensure that all air and water lines are de-pressurized and the column is empty.

Ensure that the instrument air supply to all automatic valves (for valve actuation) is fully pressurized.

Check all auto valves for air leaks and correct if necessary.

Apply power to all automatic valve controllers and ensure that all controllers are set for MANUAL

operation.

One at a time, manually change the controller valve position set-points from fully closed to fully open.

Visually confirm that all valves move in the desired direction and to the desired limits.

Change the controller set-points back to fully closed and again verify valve movement.

Close all column drain valves and water lance.1

Activate the level control system.

2.3.3 Sparger Check

Check that all SlamJet® spargers are installed in the correct location as per Sparger General

Arrangement drawings:

The SlamJet® tubes should be pushed fully into the insertion port assembly.

Note that the self-adjusting mechanism will ensure that all SlamJet®s are closed when not pressurised.

Turn all the individual sparger isolation header valves OFF (see Fig. 2) to isolate the header from the

spargers.

2.3.4 Pressurise the Flowmeters

Manual shutoff valves at the battery limits on the air and wash water supply must be confirmed

CLOSED.

Leave all control valves CLOSED.

SLOWLY OPEN the air supply manual control valve. Check for leaks. Do the same for the wash water

and sparger water supply control valves.

1 The water lances are located opposite the underflow exit pipe and can be used to unplug the

column if either the underflow line or bottom of the column sand out.


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2.3.5 Air Line Checks

The air pressure should be between 500 kPa (70 psi) and 600 kPa (90 psi).

Make sure that the air filter is clear.

Check for air line leaks and correct if necessary.

2.3.6 Sparger Air Pressurisation

Open the air manifold drain valves, then gently open the manual air flow control valve to approximately

10% of full flow.

Slowly increase to full flow to purge the air system.

Purge the piping at full flow for several minutes, then check for oil entrained in the air.

If oil persists, go to 2.3.13.

NOTE:

It is important that as little oil as possible is present in the airlines. Although small amounts of oil will not

adversely affect operation of the SlamJet®s, the presence of oil may interfere with metallurgical

performance of the column by causing non-selective flotation. An oil filter may be required.

Check that the air flowmeter indicates a flow. Record this flow.

Close the drain valve, then close the air flow control valve to ensure stoppage of flow.

2.3.7 Sparger Hose Connections

Close all sparger isolation valves. (done in 2.3.3)

Pressurise the air manifold to approximately 500 kPa (70 psi) and check for leaks.

One at a time, slowly open each SlamJet® isolation valve to check sparger hose connections and to

verify that the SlamJet® auto-close mechanism opens and allows air flow to commence.

Correct any leaks.

Close all air manifold valves.

2.3.8 Wash Water Check

Purge the wash water line so that no tramp material reaches the wash water distributor.

NOTE:


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Air, sparger water and wash water supply lines should be flushed and verified clean PRIOR to

installation of flowmeters and control valves.

Make sure that the manual wash water flow control valve is closed, then pressurize the wash water

piping system.

Slowly open the wash water isolation valve. Check for leaks.

Open the wash water flow control valve to ensure it is operative with water flow.

Clear any blocked distributor holes and check for even wash water distribution at low flow.

Close the control valve.

Check for tramp material in the distributor.

2.3.9 Gas Holdup Calibration

Check the holdup reading with the column filled with water (εg=0).2

Set the air rate to approximately 10% of full flow and open all ½" header sparger valves except the drain.

2.3.10 Empty Column Start-up

If the column is empty, fill it with water using the wash water distributor, the feed pump or a simple water

hose.

CALIBRATION NOTE:

From time to time, confirm the actual level with a hand held float probe. This simple confirmation should

be conducted on a regular basis, particularly if the slurry or pulp density changes.

Check the interface calibration by filling the column with water. The underflow pinch valve should open

to maintain column level.

Check the gas holdup reading with the column filled with water - ie., εg = 0.

2 Pressure transducer readings at εg = 0 should be equal to ρgh where ρ = 1 (water), g = 9.81,

h = distance from transducer to water level (m).


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NOTE:

Due to density variations within the froth zone, locating the interface may require detection of the

point at which a distinct difference in the buoyancy force is felt.

WARNING:

To avoid unnecessary start-up problems it is recommended that the column be filled with water prior to

feeding pulp. The column can be shut down by stopping the feed, air and wash water. The column may

sand out, but can be re-started using the repulping lances (see 2.4.4 & 2.4.5). A recycle line (not

installed) between the underflow and feed would allow the column to be filled with slurry and continue to

operate during periods when no feed is available.

2.3.11 Check Launder Sprays

Check for proper water flow from the launder spray system, if installed. Leave the launder spray system

OFF if not required.

2.3.12 Starting of Slurry Flows

Make sure that all valves are positioned to send flow through the desired pumps and pipes.

Adjust the sparger air and water to normal operational levels.

Set desired wash water flow.

Open the underflow isolation valve.

Start the column feed pump. End


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OPERATING NOTE:

The column should now be running. It will take some time before enough solids accumulate in the

column to permit formation of a stable froth.

2.3.13 Oil in Sparger Air Supply

Close the header drain and clean the air line filters.

Repeat Step 2.3.6. If oil continues to flow, shut off the air supply and check for the cause of the oil.

2.4 COLUMN SHUTDOWN

2.4.1 Emergency Shutdown

If the shut down is caused by a power failure go to 2.4.6.

2.4.2 Initial Procedure

When the feed supply is discontinued, adjust the wash water so that the underflow pinch valve can

maintain the interface level without sanding out lines. This may involve increasing the wash water flow.

2.4.3 Final Procedure

The column will continue to float the contained material and any material being sent to it from existing

recycle flows. When this flotation ceases, assess the type of shut down expected. If the shut down is to

be for longer than one to two hours go to 2.4.5.

2.4.4 Short Term Shutdown

When the column is to be shut down for a short period of time it need not be emptied. Sparger air flow

can be shut off and the SlamJet®s will automatically fail closed to prevent backflow of slurry. Wash water

flow can be shut off, and the underflow isolation valve can be closed (see Operations Note below).When feed is reintroduced, simply open the underflow isolation valve, and reset the wash water rate and

sparger settings to their previous levels. End

2.4.5 Long Term Shutdown

When the column is not to be used for extended periods it can be drained. Reduce the wash water

addition rate and allow the column to slowly drain by opening the underflow valve manually. When the


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interface level is below the spargers, the wash water and air may be shut off. If an underflow pump is

present it may be shut off. End

OPERATING NOTE:

In a shutdown situation, the column underflow line between the underflow isolation valve and the flow

control pinch valve should be drained to prevent sanding.

2.4.6 Power Failure Shutdown

Turn off all sparger header isolation valves. Note that all SlamJet®s should automatically fail closed on

loss of air. Immediately shut off all streams in and out of the column. This includes pump gland water

and sparger air. The underflow valve may continue to bypass material due to the minimum setpoint (if

used), so the underflow isolation valve must also be closed. The underflow line downstream of the

isolation valve should be drained to aid start up. When power is restored, return the sparger air flows,

then the underflow followed by the remaining streams, to normal operating levels. End

2.5 COLUMN RE-START FOLLOWING SHUTDOWN

If the column is shutdown during operation, it is possible that the column will sand out. If the column

is operating at higher density and there is a probability of sanding, the following re-start procedure

should be observed;

2.5.1 Re-Pulping

To repulp the column, connect water lines to the repulp lances and apply a water flow. After a short

period of time (15 minutes), the air should be re-started. The combination of lance water and sparger

air will serve to repulp the sanded contents at the bottom of the column.

See Warning Below – Possibility of SEVERE Damage to Spargers

2.5.2 Re-Starting Underflow

After the repulp lances and sparger air flows have been running for a period of time (15 to 30

minutes), the underflow isolation valve should be opened SLOWLY to initiate flow.

WARNING:

If, during the shutdown period, material settles to a level ABOVE the spargers, it is IMPERATIVE to

thoroughly repulp the sanded contents and to open the underflow valve SLOWLY. Failure to observe


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this procedure can cause a large “slug” of sanded material to move suddenly downward, resulting in

SEVERE damage to the spargers by bending them down with the moving slug of material.

2.6 OPERATING GUIDES

This section suggests approximate settings for some of the more common variables which should be

optimised during operation. Also given here are the general effects that changing one variable will

have on a column at steady state. Compensation for variations can be made for gradual changes, but

quick swings in flow rates or pulses of high or low grade material may cause poorer performance

than would otherwise be expected. Therefore the feed volumetric slurry rate and total flux of floatable

material should be kept as constant as possible. Make sure that all the control loops are tuned to

prevent unwanted oscillations.

2.6.1 Sparger Air

The column air rate is the most commonly adjusted control variable. The response to changes in air rate

will be very rapid. The normal operating levels for air addition rates for column cells range from 0.5 cm/s

to about 2.0 cm/s depending on the application. The optimum rate will vary depending on bubble size,

bubble loading and slurry velocities and must be determined during the commissioning period.

Increasing the air flow will generally have the following effects:

• reduce the grade of the froth product

• increase the recovery of solids to the column overflow

• reduce the percent solids in the overflow

These effects are only valid within a specific region of flow. As the air rate is increased, the bubble size

will eventually change producing a different flow regime that may be detrimental to performance. Some

indications of excessive air rates are the loss of a well defined interface or excessive turbulence in the

froth zone. Increases in air will lead to increases in overflow production and will require a corresponding

increase in wash water rates to maintain a positive bias. Air volumetric flows below 0.5 cm/s may cause

froth bed collapse. In this case the froth zone depth will have to be decreased. (i.e. pulp level raised).

2.6.2 Interface Level

The interface level between the froth and pulp zones can influence both the concentrate grade and

mineral recovery to overflow. A deeper froth will increase the purity of the froth by providing more time

for the entrained impurities to drain from the froth. It will also, however, result in a decrease in the

recovery of the minerals being floated due to an increase in "drop-back".


21/46


The column level should normally be controlled in a range from 500 mm to 1,000 mm but can vary

depending on the stability of the froth. Little benefit is expected at depths greater than 1,500 mm. Much

shallower froths are possible and may be desirable if the column is being used in a reverse flotation

application or as a scavenger. Tests can be performed to predict performance at various interface

levels.

2.6.3 Wash Water

Wash water is increased or decreased to control the bias (see section 2.6.4).

2.6.4 Wash Water Bias

The non-selective entrainment of hydrophilic minerals can be reduced by preventing feed water from

entering the overflow. This is done by operating with a positive bias (+Jb). Jb should be operated within

the range of 0.0 to 0.3 cm/s. A Jb of 0.05 cm/s is suggested at startup.

Increased bias flow will:

• increase the displacement wash and underflow/feed ratio3

• increase the grade of the froth

• decrease the recovery of the minerals being floated.

These effects do not continue indefinitely with increased amounts of bias. Eventually channelling of the

water and breaking of the froth occurs that negates all the benefits of extra wash water. To ensure a

positive bias, maintain an underflow liquid flow greater than the feed liquid flow by altering the wash

water rate.

2.6.5 Reagent Dosage

In order for a column to function properly, it is imperative that the chemical conditions of the feed are

correct prior to feeding the column. If the process is sensitive to fluctuations in pH, measurements

should be made at the column underflow to account for dilution effects caused by wash water addition.

Increasing the frother concentration will:

• reduce the bias rate

• reduce the percent solids in the overflow.

• increase the gas holdup of the collection zone

• reduce froth zone gas holdup

3 Displacement wash, differential wash, bias ratio and superficial bias rates are all different ways of

describing the liquid flux through the column froth zone.


22/46


• reduce the maximum air rate

• reduce bubble size

Frother acts to stabilise the froth zone. Increased frother produces smaller bubbles which may or may

not improve collection of particles.

2.7 TROUBLESHOOTING

This section gives possible causes of poor performance and suggests improvements. Two conditions

are dealt with: poor froth grades and poor mineral recovery. Each is then subdivided into two categories:

problems originating with the column and those that originate in the rest of the circuit. Possible solutions

are given in order from most to least likely.

2.7.1 Is Froth Purity Low?

2.7.1.1 Column Problems

For direct flotation applications, the column froth product is the concentrate and hence the objective is to

minimise the entrainment of impurities. For reverse flotation applications, the overflow is considered

tailings and therefore entrainment can result in recovery losses. High air rates may cause an increase in

material entrained in the overflow by increasing the amount of feed water that is carried by the bubbles.

To counteract this try, one at a time:

Reduce the air rate.

If the wash water bias rate Jb is less than 0.25 cm/s, increase the wash water flow QW

The interface level may be too high which reduces the froth zone cleaning action. Increase the froth

depth.

2.7.1.2 Circuit Problems

High reagent dosages or incorrect feed pH could cause excessive amounts of feed water to report to the

overflow causing increased entrainment and increased wash water rates. Check reagent addition rates.

Grind size may be too large creating middling particles that are recovered to the froth. Check the size

distribution of the feed, concentrate and tailings streams. Analyse the various size fractions to determine

if there is a possible liberation problem.

2.7.2 Is Flotation Recovery Low?

2.7.2.1 Column Problems

Low recoveries of the minerals being floated are quite often caused by low air rates. Increase the air

rate, but watch for loss of interface, and approach air rates greater than 2.0 cm/s with caution. If the

column is operating with a deep froth (greater than 1,000 mm) try reducing the froth depth.


23/46


High air rates may cause a turbulent flow in the column which is not conducive to good flotation

separation. The interface may also be lost. This may appear as volcanoes or geysers in the froth. (The

volcanoes can also be caused by very high froth density). Reduce the air rate.

Excessive wash water may cause froth breakage which reduces the production capacity. This will be

seen as an unstable froth. Reduce the wash water rate.

Poor air distribution causes eddies within the collection and froth zones that decrease the effective

column residence time. This may be seen as an unstable froth. Ensure that all spargers are clear, and

accelerate the sparger tube inspection and cleaning schedule.

2.7.2.2 Circuit Problems

Insufficient collector addition or incorrect pH may contribute to poor flotation performance. Check

reagent addition rates and adjust as required. If frother is being used, low frother dosages may cause

froth instability and can result in an increase in bubble size. Increase frother dosage or increase sparger

air flow rate.

Grind size may be too large creating middling particles that are difficult to float. This can be determined

by microscopic examination for locked particles.


24/46

CPT Column Instrumentation & Control Page 19

3.0 INSTRUMENTATION AND CONTROL

3.1 INTRODUCTION

Columns can be instrumented with up to three automatic control loops: Column Interface Level,

Sparger Air Flow and Wash Water Flow. In many applications the wash water addition rate is

manually controlled and only two automatic control loops are required. For small diameter columns, it

is common to control both the air and water with manual control valves, and to employ automatic

control only for column level.

The description of the operation of the control loops should be read in conjunction with the Process

and Instrumentation Diagram (P&ID). This drawing lists the normal, maximum and minimum flows for

each process stream.

3.2 SPARGER PRESSURE

The pressure in the sparger manifold must be maintained between approximately 400 kPa and 700

kPa. Sparger manifold pressure is a function of

• SlamJet® Control Setting (factory pre-set cracking pressure)

• Sparger Air Flow Rate

• Battery Limit Air Supply Pressure

An initial operating pressure of 550 kPa is recommended. During the course of commissioning,

different operating pressures should be tested to determine the optimum value for each application.

The maximum pressure of the system is the point at which air flow becomes limited which is a

function of the supply pressures.

For systems requiring remote sensing of the header pressure, a pressure transmitter can be mounted

on the gas header.

3.3 SPARGER AIR FLOW

Air flow control is the most critical parameter for maintaining proper column operation. The air flow

rate is measured by a flowmeter (rotameter, orifice plate or vortex flowmeter) and is controlled

automatically with a ball valve.

The recovery of solids to the column overflow is dependant on the air rate. Air flows corresponding to

a superficial gas rate below 0.5 cm/s are not recommended but can be run with appropriate care.

The estimated maximum air rate should correspond to a superficial gas rate of approximately 2.0 to

2.5 cm/s. This rate could be lower or higher depending on column conditions. The air flow rate should


25/46

CPT Column Instrumentation & Control Page 20

be changed in a stepwise fashion using small set-point changes in order to minimise process upsets.

A maximum change of 10% of total flow at a time is recommended.

The maximum and minimum air flow rates are only estimates. The exact values should be

determined under operating conditions. Below the minimum gas rate the froth becomes unstable.

Above the maximum gas rate recovery of solids to the overflow will decrease as bubble coalescence

begins to occur. Both of these values will vary with changes in interface level, reagent dosages, feed

tonnages and grades, and percent solids.

The amount of air needed to meet grade / recovery requirements depends on the particle size and

amount of material to be floated but should fall within the previously mentioned ranges.

3.4 COLUMN INTERFACE LEVEL

The position of the froth / pulp interface is measured using either a ball float and ultrasonic detector or

single or dual pressure transducers. Both systems deliver a 4 to 20 mA signal proportional to the

position of the froth / pulp interface. The interface level is controlled by a PID Controller (or DCS or

PLC) which adjusts an automatic pinch valve on the column underflow line.

Slight variations in column level are not critical to performance as long as the level remains within a

certain range. Variations of ± 100 mm are acceptable over a time span of 10 minutes as long as the

variations are gentle. Column performance will improve with more stable control especially when

operating with a bias close to zero.

It is suggested that the level be operated at depths greater than 300 mm and less than 1,500 mm

with normal operation at 750 mm.

3.5 W ASH W ATER

The wash water flow is measured by a flowmeter (rotameter, orifice plate or magnetic flowmeter) and

is controlled either manually or automatically with a flow control valve. The expected range of flow will

be approximately 0.15 to 0.40 cm/s. Slow variations in flow are acceptable but better performance will

be achieved with smooth control. The wash water rates quoted are estimates and should be adjusted

to produce the proper bias rate. Actual rates should be determined through column tests during

operation as they will depend on the feed rate, feed grade and expected mass recovery to the column

overflow. The minimum bias occurs when no wash water is added to the column. As bias is

increased, the purity of the froth will increase due to the displacement of entrained particles. An initial

bias setpoint of approximately 0.1 cm/s is suggested.


26/46

CPT Column System Maintenance Page 21

4.0 SYSTEM MAINTENANCE

4.1 INTRODUCTION

All maintenance should be done on a scheduled basis. Operators should regularly inspect all parts of

the circuit. Operational and performance trends from the column should be noted for indications of

instrument problems including faulty calibration and tuning.

4.2 COLUMN M AINTENANCE

The overflow launders, both external and internal, should be kept clear of scale and accumulated

solids. This will ensure that the overflow of froth is not hindered. Scale and/or solids can have a

tendency to build up on the lip of the launder and on the column walls at a point just below the lip.

This material should be routinely scraped or washed off by the operators.

The wash water system must be maintained so that the flow of water is evenly dispersed within the

froth. Periodically inspect the distributor for build-up of solids or for blocked holes. The distributor

holes must remain clear.

MAINTENANCE NOTE:

Pinch valve sleeves can wear out quickly if they are required to operate at low opening percentages

or with high pressure drops across the valve. Careful attention must be paid to the wear of this valve

as indicated by the valve percent opening Vs flow rate relationship and regular inspections.

If pressure transducers are installed on the column, regular inspections should be made to ensure

that they are functioning properly. The calibration should be checked from time to time by noting the

readings when the column is full of water. Any scaling should be carefully removed according to

manufacture's maintenance procedures.

Level control systems that use a ball float should be cleaned regularly to prevent a build-up of solids

on the float or target.

Level calibration can be checked by using a manual float. At times the density of the froth is high

enough to “float” the ball float but little resistance will be encountered when the ball float is pushed

further down. This condition is commonly called a double interface: the true interface will be the lower

one.


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4.3 SPARGER M AINTENANCE

The spargers have been designed for reliability and durability, but some maintenance is required.

Regular inspections should be made both to maintain performance and to prevent a build-up of scale

on the tube surfaces.

4.3.1 Orif ice Wear

No rapid deterioration is expected at the tip orifice, but they are expected to enlarge slowly over a

number of years. However, in the unlikely event that the sparger orifices become enlarged they must

be replaced to avoid a decrease in the metallurgical performance.

Care should be taken to limit the SlamJet®’s exposure to highly acid environments. Hydrochloric acid will

corrode both the stainless steel tubes and the adhesive compound which holds the ceramic inserts in

place.

WARNING:

Cleaning spargers with a strong acidic solution could result in corrosion to the stainless steel tubes as

well as damage to the retaining compound used to adhere ceramic inserts in the sparger nozzles.

4.3.2 Tip Blockage

A maintenance program should be devised based on inspections for blocked nozzle orifices and poor air

distribution. Each sparger tube should be pulled on a regular basis. The interval of inspections depends

on the scale forming tendency of the slurry and sparger water. Note that the SlamJet® orifice diameter is

7.62 mm, and blockage is not expected to be a problem.

Initially CPT recommends that spargers be checked monthly. This interval can then be extended if no

plugging or serious scale formation problems are encountered.

In the case of blocked orifices the blockage should be removed. If tip blockage becomes a recurring

problem, consideration should be given to the installation of additional filters for the air lines.

4.3.3 External Scaling

In the presence of highly scale-forming slurries, the external surfaces of sparger tubes will become

scaled, making them difficult to remove for inspection. In severe cases, the external scale can cause

damage to the rubber seal in the liquid-tight insertion port seal assembly.


28/46


If scale formation is a problem then different spargers should be pulled on a rotating basis, as often as

required, and the external surfaces cleaned with an emery cloth. If scaling is a persistent problem,

please contact CPT.

4.3.4 Sparger Removal

Close the isolation valve at the manifold. The SlamJet® will then automatically fail closed.

Remove the air hose by disconnecting the coupling.

Slightly loosen the compression fitting so that the sparger can be slowly withdrawn from the column.

WARNING:

Slurry will leak from the compression fitting if it is loosened too much.

Each sparger tube has been marked with a groove located approximately 300 mm from the end. This

groove provides a visual indication of when the end of the sparger tube has cleared the insertion port

ball valve.

When the groove appears, close the ball valve. If this is not done before fully removing the sparger, a

stream of slurry will emerge from the column at high velocity through the sparger nipple.

Assess the condition of the sparger nozzle by direct visual observation and clear any blockages. If the

blockages are serious, remove the plug at the end of the tube to allow loosened material to flow out ofthe sparger.

Removal of exterior scaling of the sparger tube will ensure that the tubes are always easy to remove.

To re-install the SlamJet®, insert the tube until the end comes into contact with the closed ball valve in

the insertion port. Tighten the compression fitting enough to stop leaks. Open the ball valve and push

the sparger tube into the column, then tighten the compression fitting by hand.

WARNING:

If SlamJet®s are pressurised while outside the column, the resulting high velocity air jet is extremely

hazardous. As a precautionary measure, always wear gloves when handling spargers that are

operating outside the column and DO NOT direct the air jet at personnel.


29/46


4.3.5 Poor Air Distribution

Uneven swelling of froth in the column may be a sign of poor air distribution caused by one or more

partially or fully blocked SlamJet®s.

This may be caused by insufficient air pressure in the system, blocked hoses which prevent air entry

into the sparger or one or more spargers being turned off.

In the unlikely event that the sparger holes become enlarged, the sparger nozzles must be replaced to

avoid a loss in metallurgical performance.

4.4 COMPRESSOR AND AIR LINES

Oil from the compressor should not be allowed to enter the column. This oil may cause flotation

problems, such as a excessive frothing and poor selectivity.


30/46

CPT Column Collection Zone Theory Page 25

5.0 COLLECTION ZONE THEORY

5.1 INTRODUCTION

The collection zone of the column is located between the froth interface (Hf ) and the point of maximum

descent of the bubbles below the spargers (Hspa). In this zone floatable material from the feed stream

and material returned after rejection from the froth zone (dropback) are collected by rising bubbles.

Recovery of material within the collection zone is dependent on the zone's residence time and mixing

characteristics.

5.2 RESIDENCE TIME

Collection zone residence time, as mentioned in the testwork section, is estimated for round columns by

the following equation;

sl

c2

c spa f col sl

tail

( )=15 d ( H - H - H )(1 - / )

V τ

π ρ ρ min

Where

τ sl = collection zone residence time (min)

dc = column diameter (m)

Hc = total column height (m)

Hspa = sparger level (m)

Hf = interface level (m)

ρcol = collection zone density (t/m3)

ρsl = concentrate slurry density (t/m3)

Vtail = tailing (underflow) volumetric flowrate (m3/hr)

The equation has three parts: column volume, gas holdup and slurry velocity.


31/46


G a s H o l d u

p - E

g ( % )

Superficial Gas Velocity - Jg (cm/s)

Figure 3 - Gas Holdup Vs Gas Velocityfrom Dobby & Finch - 1988

10

30

20

Frother Concentration (ppm)15

10

5

0

1 2 3 4

Figure 4 - Gas Holdup Vs Slurry Velocityfrom Dobby & Finch - 1988

G a s H o l d u

p - E g

( % )

Superficial Slurry Velocity - Jsl (cm/s)

0.5 1.0 1.5

10

30

20

Liquid Velocity JL ( cm/s)

0.38

1.00

1.26

5.2.1 Column Volume

The volume of the collection zone is essentially fixed. Hf , the interface level, is the only variable which

can be easily changed to increase collection zone volume but the impact on residence time will be

small. The only significant way to influence residence time is to vary the column feed rate.

5.2.2 Gas Holdup

Gas holdup is the volume fraction of gas in the column. It is a parameter dependant on other variables

such as volumetric air rate, size of the bubbles, slurry density, solids bubble loading, and slurry velocity.

Increased gas holdup reduces collection zone residence time.

Gas holdup may increase due to three factors:

An increase in gas flow will increase the number of bubbles present in the column as more bubbles are

being generated in any time period.

A bubble size decrease caused by sparger operation or frother dosage will cause each bubble to rise

more slowly in the slurry, again causing an increased amount of air in the column.

An increase in downward slurry velocity will decrease bubble rise velocity relative to the column, also

resulting in an increase in the quantity of air “held up” in the column.

5.3 AIR R ATE

5.3.1 Maximum

The maximum air rate which a column is capable of handling is determined by three limits: (i) the

superficial feed rate must be less than the bubble rise velocity, (ii) the collection zone density must be

greater than the froth zone density and (iii) bubble coalescence must not form air slugs.


32/46


S u p e r f i c i a l G a s V e l o c

i t y - J

g ( c m / s )

Superficial Slurry Velocity - Jsl (cm/s)

Figure 5 - Column Flood ingfrom Dobby & Finch - 1986

1

3

2

-2.0 -1.5 -1.0 -0.5 0.0 +0.5

Countercurrent Cocurrent

0.020.04

0.07

0.10

db = 0.13 cm

Particle Size ( )祄

Figure 6 - Bubb le & Particle Size Effects

from Dobby & Finch - 1988

E c

* E a

( % )

Bubble Diameter (mm)

1.3

0.5

0.7

1.0

20 40 60

0.5

3.0

1.0

(i) Bubble rise velocity depends on bubble size, the difference between the apparent collection

zone and bubble densities, and downward slurry velocity. A distribution of bubbles sizes is produced by

any sparging system. Therefore, when the slurry feed rate exceeds the rise velocity of the smallest

bubble, a percentage of the air is lost to the underflow.

(ii) As the gas rate to the column increases the three phase density of the collection zone

decreases. At the same time the density of the froth zone will increase until the two are equal. At this

point the gas holdup will suddenly increase from about 15% to over 50%. When this occurs, the column

is said to be “frothed up”. Recoveries drop significantly under these conditions.

(iii) The increased turbulence and larger bubbles formed at higher gas rates cause an increase in

bubble coalescence which results in a decrease in the incremental gas holdup. Eventually, severe

coalescence will result in the formation of large “slugs” of air. This effect changes the column mixing

characteristics and reduces both the surface area available for flotation and the collection of particles on

the bubble surface. One of these factors will probably limit the maximum column gas velocity to between

1.8 and 3.5 cm/s.

5.3.2 Optimum

The optimum gas rate usually occurs at the point at which solids loading per gas volume is maximised

(Cg) rather than at the maximum gas holdup. Maximum gas holdup usually occurs at a gas rate larger

than optimum.


33/46


5.4 PARTICLE AND BUBBLE SIZE

Both the average size of bubbles and distribution of bubble sizes are important to column flotation. They

affect maximum gas rate, probability of particle collection on the bubble, and solids gas carrying capacity

(Cg). Bubbles used are typically between 0.8 and 1.6 mm in diameter with the actual size depending on

reagent conditions, sparger design, and operating pressure.

5.4.1 Gas Veloci ty

Smaller bubbles have a reduced rise velocity, therefore, the maximum gas rate will be less than that

possible with larger bubbles.

5.4.2 Probabil ity of Collection

The flotation rate constant of a column has been related to the probability of particle - bubble collision

and attachment by the following equation:

c

g a c

b

k =1.5 J

d

ε ε c

g a c

b

K =1.5 J E E

D

Where

kc is the flotation rate constant (min-1

).

εc is defined as the fraction of particles contained in the cylindrical volume travelled by

the bubble which collide with that bubble.

εa is defined by the fraction of particles which have collided with the bubble that attach

to that bubble.

db is defined as the bubble diameter in µm.

Bubble size should be adjusted for changing particle size to fully optimise a column. This can be

achieved in two ways: (i) by adding a frother to the feed or (ii) by increasing the sparger pressure.


34/46


5.4.3 Solids Carrying Capacity

0.05 0.10 0.15

B u b b l e D i a m e t e r ( m m )

Frother Consumption (mg/s m昪 2)

Figure 7 - Effect of Frother on Holdupfrom Dobby & Finch - 1988

Slurry

Frother

Sparger

Frother 1.0

2.5

2.0

1.5

The amount of solids which can be carried by a bubble are influenced by the following factors: surface

area to volume ratio, solids density, and particle size, hydrophobicity, and particle shape.

Smaller bubbles have larger surface areas per volume and therefore can carry more solids per air

volume. (The slower rise velocity of the smaller bubbles reduces the air volume which can be added to

the column.) Very small bubbles (micro bubbles) may cause loss of collected solids to the underflow

stream when bubble density (collected solids plus air) reduces bubble rise velocity to values smaller

than the slurry velocity. More hydrophobic particles attach to the bubble faster therefore have a larger

Ea. Particles which are highly angular tend to attach more quickly.


35/46

CPT Column Froth Zone Theory Page 30

6.0 FROTH ZONE THEORY

6.1 INTRODUCTION

The froth zone extends from the collection or pulp zone interface to the column lip. In this zone solids

and water are carried between bubbles. The three phase density of this zone usually ranges from 0.2

to 1.0 g/cm3, depending on the solids floated, location of measurement, froth depth, wash water, and

frother dosage.

Wash water stabilises the bubbles and reduces coalescence. Figure 8 illustrates this effect by showing

the change in holdup with level of the column (with wash water) and a mechanical cell (without wash

water).

20 40 60 80

Gas Holdup - Eg (%)

F r o t h D e p t h ( a p p r o x . 1 . 0 m )

Figure 8 - Conv. Vs Column Holdupfrom Dobby & Finch - 1988

Overflow

Interface

ConventionalFroth

Collection

Zone

Froth Zone

Column Froth

Wash Water Concentrate

Negative

Bias

Positive

Bias

Interface

Level

Draining

Froth Bed

Eg > 0.80

Packed

Bubble Bed

Eg > 0.74

Expanded

Bubble Bed

Eg < 0.74

Bubbling

Zone

Eg < 0.20

Figure 9 - Froth Zone Profilefrom Yianatos - 1985

As the bubbles ascend the froth zone a certain amount of coalescence occurs. This reduces the

bubble surface area available for solids and also decreases the volume of the voids between bubbles

occupied by water. This property appears as an increase in gas holdup with height and is illustrated

in Figure 9.

6.2 FROTH CLEANING

Since there is a net downward flow of water in the froth zone (assuming a positive bias) particles not

attached to bubbles will be carried back into the collection zone. This includes particles that are


36/46


carried with, but not attached, to bubbles (non-selective entrainment) and particles that become

detached from the bubble due to coalescence. These particles, returned to the collection zone via the

wash water, form a partial internal solids recycle. This is commonly termed "drop-back", and is the

reason behind the increased grade and lower recovery at deeper interface levels (Figures 10 and

11).

C o p p e

r R e c o v e r y ( % )

Froth Depth (m)

Figure 10 - Cu Recovery Vs Froth Depthfrom Huls - 1989

50

80

70

60

0.8 1.0 1.2 1.4

C o n c e n t r a t e G r a d e ( % N i )

Froth Depth (m)

Figure 11 - Ni Grade Vs Froth Depthfrom Huls - 1989

0.8 1.0 1.2 1.4

0.4

1.0

0.8

0.6

6.3 G AS VELOCITY

An increase in gas velocity results in an increased collection zone holdup and a decreased froth zone

holdup (Figure 12). Less coalescence occurs in the froth zone as gas rate is increased, therefore,

froth grade deteriorates. Very high air rates may result in a loss of the interface.

This is apparent when the holdup in the froth and collection zones are equal. This condition can

contribute to the rapid drop in grade found at high recoveries on typical column grade / recovery

curves (Figure 13).


37/46


Overflow

Interface

Collection

Zone

Froth Zone

Gas Holdup - Eg (%)


Figure 12 - Effect of Jg on Gas Holdup


20 40 60 80

Jg

Jg

G r a d e ( % )

Recovery (%)

Figure 13 - Rghr Column Grade/Recovery

48

54

52

50

92 94 96 98

20 40 60 80

Gas Holdup - Eg (%)

F r o t h D e p t h ( a p p r o x . 1 . 0 m

)

Overflow

Interface

Collection

Zone

Froth Zone

Figure 14 - Effect o f Frother on Gas Holdupfrom Dobby & Finch - 1988

Frother

Frother

20 40 60 80

Gas Holdup - Eg (%)

F r o t h D e p t h ( a p p r o x . 1 . 0 m

)

Overflow

Interface

Collection

Zone

Froth

Zone

Figure 15 - Effect of Bias on Gas Holdupfrom Dobby & Finch - 1988

Jb

Increased gas velocity also results in a higher possibility of feed water reporting to the overflow (Figure

14) which lowers the purity of the froth product.


38/46


6.4 FROTHER CONCENTRATION

For applications where frothers are used, an increase in frother concentration has an effect similar to

that of high air addition rates. The froth zone gas holdup (Figure 15) decreases causing lower froth

percent solids and a decrease in froth grade. High frother dosages, especially when combined with

high air rates, can be detrimental to column performance.

20 40 60 80

Feed Water in Froth (%)


Overflow

Interface

Collection

Zone

Froth

Zone

Figure 16 - Effect of Jg on Entrainmentfrom Dobby & Finch - 1988

Feed Water Bias

Water

Jg

>2.5 cm/s


39/46


20 40 60 80

Feed Water in Froth (%)


Overflow

Interface

Collection

Zone

Froth Zone

Figure 18 - Effect o f Jb on Feed Water in Conc


Jb ~0.5 cm/s

Jb ~0.3 cm/s

Jb ~0.1 cm/s

S o l i d s R e c o v e r y ( % )

Water Recovery (%)

Figure 19 - Recovery of Soli ds Vs Water

40

5祄

10祄

20祄

20 40 60

10

30

20

C o n c e n t r a t e G r a d e ( % )

Distance Below Column Lip (cm)

Figure 20 - Grade & Selectivity with Depthfrom Dobby & Finch - 1988

20

60

30

10

70

50 1000 150

Column Size 45 x 45 x 1,200 cm

Molybdenum

Chalcopyrite

Silica

Pyrite W a s h W

a t e r I n p u t

I n t e r f a c e


40/46


6.6 SELECTIVITY

Selectivity occurs in the froth zone; the less floatable particles drop back into the collection zone as

indicated in Fig 20. In this diagram Silica, Chalcopyrite and Pyrite grade reductions are traced.

There is also a size class selection. Larger particles are usually the ones that remain locked and are

therefore "middling" in nature and subject to washing. Larger particles project further from the bubble

surface and are subject to more force from the passing wash water stream.

6.7 REFERENCES

Amelunxen, R.L., (1985). The Mechanics of Operation of Column Flotation Machines.

Proceedings of the 17th Annual CMP Meeting, Ottawa, January 1985Egan, J.R., Fairweather, M.J., and Meekel, W.A., (1988). Applications of Column Flotation to Lead and

Zinc Beneficiation at Cominco.

1st International Symposium on Column Flotation, AIME, Phoenix

del Villar, R., Gomez, C.O., Finch, J.A., and Espinosa-Gomez, R.,(1989) . Flotation column

amenability and scale-up parameter estimation tests. C.I.M. International Symposium on the Processing

of Complex Ores, Halifax.

Dobby, G.S., and Finch, J.A., (1988). Column Short Course, Cambourne School of Mines, Cornwall,

England.

Huls, B.J., Lachance, C.D., and Dobby, G.S., (1989). Gas rate and froth depth effects on performance

of a Cu-Ni separation flotation column. C.I.M. International Symposium on the Processing of Complex

Ores, Halifax.

Wheeler, D.A., (1988). Historical View of Column Cell Development. Presented at 1st Int. Symp. on

Column Flotation, A.I.M.E., Phoenix

Wilson, S.W., (1987). The Study of Flotation Column Rate Constants as a Function of Particle Size.

B.A.Sc. Thesis, Dept. of Geological Engineering, University of Toronto.


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CPT Column Production Column Testing Page 36

7.0 PRODUCTION COLUMN TESTING

7.1 INTRODUCTION

The purpose of conducting testwork is to determine the optimum parameter settings to maximise

column performance under varying plant operating conditions. Generally, a column operates best

under steady-state conditions; the fewer disturbances in the system the better. Considering this,

operation should be as smooth as possible, and any changes in interface level, wash water, air or

reagent additions should be made slowly.

When performing testwork, a minimum number of variables should be changed at one time -

ideally only one - so that the fewest number of effects in teract.

It is important to remember that a column has a significant residence time and therefore an ability to

accumulate floatable solids through internal recycle. If solids are building up due to insufficient air rate

or other froth transport problems, excellent grades can be achieved without immediately sacrificing

recovery. These solids, however, will eventually report to the underflow resulting in a significant

deterioration of metallurgical performance. High solids loading may cause excessive coalescence in

the froth zone and appear as "burping" or "volcanoes" in the froth. Therefore, whenever a grade /

recovery curve is being constructed, values should not be recorded until a steady state is reached -

which may take as long as three column residence times after the last change.

7.2 AIR R ATE

The amount of material floated is very sensitive to the rate of air introduced through the spargers. In

the first few days of operation with a new type of feed material, the reagent dosages and air flow rate

should be adjusted to give optimal grade/recovery performance. Record the air loading (amount of

solids carried per unit of air) as kg/m3. When the column feed rate changes, repeat the test. The

purpose of these tests is to create a chart of air requirement Vs overflow tonnage production that can

be used by operators. The amount of air needed to float a specific amount of solids is dependant on

both the particle size and the bubble size. A decrease in particle size will increase the required air

while an decrease in bubble size will decrease the required air.


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7.3 W ASH W ATER BIAS

Jb values should be determined for each test in order to optimise the wash water addition rate. During

these tests the feed tonnage and all operating parameters should remain constant while the wash

water is altered.

As wash water addition is reduced from the optimum, a slight, possibly unnoticeable, decrease in

froth purity should occur. This will be accompanied by a decrease in the overflow percent solids. This

relationship between the bias flow and decreasing percent solids continues until a critical point is

reached at which time some water from the pulp zone moves upward and overflows with the froth.

When this happens, the column is said to be operating at negative bias and entrainment of fine

impurities in the froth will increase.

At high gas rates the froth will be much more turbulent causing both a higher froth density and

entrainment at higher bias rates.

As the water addition is increased, the recovery of gangue to overflow will begin to decrease and the

froth density will increase. As the water is further increased, larger particles and middling particles will

begin to detach from the bubbles causing a decrease in the recovery of the floated minerals. This will

continue until channelling of the wash water occurs in the froth, or the wash water beats the froth too

severely causing collapse. These conditions will cause a rapid deterioration of performance.

Under normal production rates the wash water should be increased until performance deteriorates.

This value is the maximum wash water rate. This upper flowrate should be consistent at different

production rates.

The minimum wash water rate can be determined by reducing the wash water flow until froth impurity

levels become too high. This lower limit will depend on the production rate.

7.4 OPERATING LEVEL

Both the maximum and minimum column froth levels should be determined.

The maximum froth depth or lowest pulp level is the point at which the froth collapses. This level will

depend on the gas rate, reagent concentration, and the amount of solids being floated. Generally,

there is little benefit to operating at depths greater tha

Documents

Column Flotation Cell. Operating and Maintenance Manual - Canadian Process Technologies Inc. (2002)