CoalPro Manual (Column Flotation Cell)

CoalPro Column Flotation Cell

Operating and Maintenance Manual

Prepared By:

Canadian Process Technologies Inc. 1636 West 75th Avenue

Vancouver, B.C.

Canada V6P 6G2

Tel: +1 604 264 5610

Fax: +1 604 264 5555

Email: [email protected]

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

October 2001

CPT Inc. CoalPro Operating & Maintenance Manual

The purpose of this manual is to provide an overall guide to column flotation technology and operation,

with emphasis on the CoalPro Coal Flotation Column and the SlamJet® Gas Sparging System.

This manual has been divided into seven sections, each dealing with a different aspect of the column

flotation process, as follows;

1. Introduction

2. Column Commissioning

3. Column Operation

4. Instrumentation and Control

5. Troubleshooting

6. System Maintenance

7. Collection Zone Theory

8. Froth Zone Theory

9. Production Column Testwork

Companion Manual

SlamJet® Operating and Maintenance Manual

1. Introduction 2. Components 3. Installation 4. Operation 5. Maintenance 6. Control System 7. Cracking Pressure Adjustment 8. Repair Procedures


TABLE OF CONTENTS

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

1.1 DESCRIPTION OF A FLOTATION COLUMN................................................................................1 1.2 SlamJet®

DESCRIPTION .....................................................................................................3 1.3 GLOSSARY OF COLUMN FLOTATION TERMINOLOGY .................................................................5

1.3.1 Bias .........................................................................................................................5 1.3.2 Carrying Capacity......................................................................................................5 1.3.3 Difference Wash........................................................................................................6 1.3.4 Displacement Wash ..................................................................................................6 1.3.5 Entrainment ..............................................................................................................6 1.3.6 Flow Conventions ......................................................................................................6 1.3.7 Gas Holdup...............................................................................................................6 1.3.8 Air Sparger ...............................................................................................................7 1.3.9 Superficial Velocities .................................................................................................7

1.4 TERMS.............................................................................................................................7 1.4 WASH WATER...................................................................................................................8

2.0 COLUMN COMMISSIONING ........................................................................................9

2.1 INTRODUCTION ..................................................................................................................9 2.2 COMMISSIONING................................................................................................................9 2.3 AIR SPARGING SYSTEM ......................................................................................................9

2.3.1 Installation Check......................................................................................................9 2.3.2 Pressurization Check.................................................................................................9 2.3.3 Air System Purge.................................................................................................... 10 2.3.4 Sparger Hose Connections ....................................................................................... 10

2.4 SPARGER WATER SYSTEM ................................................................................................ 11 2.4.1 Installation Check.................................................................................................... 11 2.4.2 Booster Pump Startup ............................................................................................. 11 2.4.3 Pressure Relief Valve Check..................................................................................... 11

2.5 WASH WATER SYSTEM .................................................................................................... 12 2.5.1 Installation Check.................................................................................................... 12 2.5.2 Wash Water System Purge...................................................................................... 12 2.5.3 Wash Water System Start-up................................................................................... 12

2.6 LEVEL CONTROL SYSTEM ................................................................................................. 13 2.7 COLUMN STARTUP........................................................................................................... 14 2.8 NORMAL COLUMN SHUTDOWN........................................................................................... 15

2.8.1 Initial Procedure ...................................................................................................... 15 2.8.2 Final Procedure....................................................................................................... 15

2.9 SHORT TERM SHUTDOWN – HOURS ..................................................................................... 15 2.10 LONG TERM SHUTDOWN – DAYS ..................................................................................... 16 2.11 EMERGENCY SHUTDOWN................................................................................................ 16


2.12 COLUMN RE-START ...................................................................................................... 16 2.12.1 Re-Pulping.............................................................................................................. 17 2.12.2 Re-Starting Underflow .............................................................................................. 17

3.0 COLUMN OPERATION................................................................................................18

3.1 SPARGER AIR.................................................................................................................. 18 3.2 INTERFACE LEVEL ............................................................................................................ 19 3.3 WASH WATER BIAS ......................................................................................................... 19 3.4 REAGENT DOSAGE ........................................................................................................... 20

3.4.1 Frother ................................................................................................................... 20 3.4.2 Collector................................................................................................................. 21

4.0 INSTRUMENTATION AND CONTROL ......................................................................22

4.1 INTRODUCTION ................................................................................................................ 22 4.2 SPARGER PRESSURE......................................................................................................... 22 4.3 SPARGER AIR FLOW......................................................................................................... 22 4.4 COLUMN INTERFACE LEVEL ............................................................................................... 23 4.5 WASH WATER................................................................................................................. 23

5.0 TROUBLESHOOTING.................................................................................................25

5.1 PROBLEM – HIGH ASH...................................................................................................... 25 5.1.1 Column Problems.................................................................................................... 25 5.1.2 Circuit Problems...................................................................................................... 25

5.2 PROBLEM – LOW YIELD .................................................................................................... 25 5.2.1 Column Problems.................................................................................................... 25 5.2.2 Circuit Problems...................................................................................................... 26

6.0 SYSTEM MAINTENANCE............................................................................................27

6.1 INTRODUCTION ................................................................................................................ 27 6.2 COLUMN MAINTENANCE.................................................................................................... 27 6.3 SPARGER MAINTENANCE................................................................................................... 28

6.3.1 Orifice Wear............................................................................................................ 28 6.3.2 Tip Blockage........................................................................................................... 28 6.3.3 External Scaling...................................................................................................... 28 6.3.4 Sparger Removal for Inspection and Cleaning: ............................................................ 29 6.3.5 Poor Air Distribution................................................................................................. 30

6.4 COMPRESSOR AND AIR LINES ............................................................................................. 30

7.0 COLLECTION ZONE THEORY...................................................................................31

7.1 INTRODUCTION ................................................................................................................ 31 7.2 RESIDENCE TIME.............................................................................................................. 31

7.2.1 Column Volume....................................................................................................... 32 7.2.2 Gas Holdup............................................................................................................. 32

7.3 AIR RATE ....................................................................................................................... 33


7.3.1 Maximum ............................................................................................................... 33 7.3.2 Optimum ................................................................................................................ 34

7.4 PARTICLE AND BUBBLE SIZE ............................................................................................... 34 7.4.1 Gas Velocity........................................................................................................... 34 7.4.2 Probability of Collection............................................................................................ 34 7.4.3 Solids Carrying Capacity.......................................................................................... 35

8.0 FROTH ZONE THEORY ..............................................................................................36

8.1 INTRODUCTION ................................................................................................................ 36 8.2 FROTH CLEANING............................................................................................................. 37 8.3 GAS VELOCITY................................................................................................................ 37 8.4 FROTHER CONCENTRATION ................................................................................................ 39 8.5 SUPERFICIAL BIAS RATE................................................................................................... 39 8.6 SELECTIVITY................................................................................................................... 41 8.7 REFERENCES ................................................................................................................... 42

9.0 PRODUCTION COLUMN TESTING ..........................................................................43

9.1 INTRODUCTION ................................................................................................................ 43 9.2 AIR RATE ....................................................................................................................... 43 9.3 WASH WATER BIAS ......................................................................................................... 44 9.4 OPERATING LEVEL ........................................................................................................... 44 9.5 BUBBLE SIZE................................................................................................................... 45 9.6 RESIDENCE TIME.............................................................................................................. 45 9.7 WASH WATER DISTRIBUTOR HEIGHT ................................................................................... 46 9.8 SAMPLING...................................................................................................................... 46 9.9 COLLECTION ZONE DENSITY ESTIMATION ............................................................................. 46 9.10 FROTH ZONE DENSITY ESTIMATION.................................................................................. 47

CPT CoalPro Introduction Page 1

Operating & Maintenance Manual

1.0 INTRODUCTION

1.1 DESCRIPTION OF A FLOTATION COLUMN

The CPT CoalPro Flotation Column is a type of flotation machine that incorporates some unique 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 stabilize the froth and to minimize the entrainment of impurities

• Quiescent flotation conditions to promote selectivity and enhance collection

• Adjustable air sparging system to allow control of bubble size

• Circular internal launders to enhance froth stability and minimize loaded bubble travel

distances, thus increasing recovery, especially of coarse particles.



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 25 ft

to 50 ft. 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 – CoalPro Schematic

Feed

Air &Water

CleanCoal

Tailing

CoalPro

SlamJet SlamJet

Wash Water



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 stabilize 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 coarse 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

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 ash / yield curve at which the column operates.

1.2 SlamJet® DESCRIPTION

CPT’s patented SlamJet® Model SLJ-75 Air Sparging System, shown schematically in Figure 2,

comprises air and water manifolds that surround the column cell and supply a mixture of air with a small

amount of water to a series of SlamJet® spargers. 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 the water manifold to the air manifold. The bottom of

the manifold is fitted with a series of recessed couplings for connecting to the sparger tubes and is also

fitted with one or more drains.

A small amount of water – approximately 2 USgpm per SlamJet® – is supplied to the water manifold

using a positive displacement pump. The water manifold is then connected in (typically) four places to

the air manifold to provide even distribution of water within the air manifold.



Vessel

SlamJet

Check ValveGasManifold

Optional

Manifold Support Bracket

Air Distribution Manifold

Isolation Valve

Sparger Water Manifold

Flexible Hose

QuickDisconnectCoupling

Air-ActuatedSelf-AdjustingAuto Shut-off Assembly

Liquid-Tight Seal

Full Port Ball Valve

Replaceable Ceramic WearProtected Injection Nozzle

Figure 2 – SlamJet® Model SLJ-75 System

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

series of couplings, (Figure 2) located on the bottom of the header. The manifold is sized to act as a

buffer against turbulence and to provide sufficient reservoir capacity to ensure even distribution of the air

/ water mixture to all spargers. The volume of the header also provides some opportunity for foreign

material to settle out.



The air / water mixture 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 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 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 nozzle-mounted

needle valve which provides air flow control and also provides automatic sealing of the SlamJet® nozzle

prior to removal.

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

SlamJet® nozzles initially 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 schedule can later be modified

on an experience basis.

In some applications it can be advantageous to administer flotation reagents, especially frother, into the

column along with the sparging air. This technique can result in tighter control of bubble size and can

also result in a reduction in frother consumption. In such cases, the frother (or other reagent) is added

with the sparger water, and is thus evenly distributed within the column.

1.3 GLOSSARY OF COLUMN FLOTATION TERMINOLOGY

The study of column flotation systems has resulted in new ways of examining flotation and some

specific 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

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 Carrying 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 – ST/h•ft2 or 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 – lb/ft3 or 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 – ST/h•ft or 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 Displacement 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 and 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).

Gas Holdup (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 (εs) is the solids fractional volume.



Liquid Holdup (ε l) 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 Superficial Velocities

Superficial velocity is the volumetric flow rate of the material in question (slurry, water or air) divided by

the cross sectional area of the column, normally expressed in cm/s. This normalized 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 (ST/h•ft2 or t/h•m2)

g = loading per gas unit (lb/ft3 or kg/m3)

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

ε 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

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

ρ 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 (ft or m)

Hf = height of interface (ft or m)

Hspa = height of spargers (ft or m)

P = pressure (psi or kPa)

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

1.4 WASH WATER

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

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

CPT CoalPro Column Commissioning Page 9


2.0 COLUMN COMMISSIONING

2.1 INTRODUCTION

This section covers column start-up and shut-down procedures and provides estimates of column

parameters and guidelines for operation. It is intended to familiarize 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 COMMISSIONING

All air and water lines should be carefully and completely purged of tramp material and construction

trash BEFORE installing instrumentation or connecting to distribution manifolds.

Perform a “walk around” to visually check that all piping for slurries, sparger air, sparger dilution water

and column wash water are in place and properly connected.

Ensure that all manual and automatic valves are closed.

COMMISSIONING NOTE:

Air and water supply lines should be flushed and verified clean PRIOR to the installation of flowmeters

and control valves.

2.3 AIR SPARGING SYSTEM

2.3.1 Installation Check

Perform a “walk around” to visually check that all SlamJet® spargers are installed in the proper location

as per Sparger General Arrangement drawings;

All SlamJet® tubes should be pushed fully into the insertion port assemblies.

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

2.3.2 Pressurization Check

Turn all individual sparger air manifold and sparger water manifold isolation valves OFF (see Figure 2) to

isolate the header from the spargers.

Ensure that manual shut-off valves at battery limits of the air supply system are CLOSED.



If automatic air flow control valves are supplied, stroke the valves in manual and check for full travel and

correct opening, then return to fully CLOSED position.

Ensure that air filter, if installed, is clear.

SLOWLY OPEN the manual air supply shut-off valve. Check for leaks and repair if necessary.

Ensure that and air supply pressure of 70 psi to 90 psi is available.

CLOSE the manual air supply shut-off valve.

2.3.3 Air System Purge

OPEN the sparger manifold drain valve(s).

SLOWLY OPEN the manual air supply shut-off valve to purge the air system through the manifold drain

line(s). Gradually increase the air flow to 100% and purge for several minutes.

While the system is purging, check the air flow meter to ensure that the flowmeter is operating

correctly.

Check emerging air flow for oil contamination.

OPERATING NOTE:

It is important that as little oil as possible is present in the air lines. Although small amounts of oil will

not adversely affect operation of the SlamJet®s, the presence of oil may interfere with the metallurgical

performance of the column by causing non-selective flotation. If this is determined to be the case, an oil

filter will be required.

When purging is complete, close the manifold drain valve(s) but leave the sparger manifold pressurized.

2.3.4 Sparger Hose Connections

With the sparger manifold pressurized to at least 70 psi, SLOWLY OPEN individual sparger manifold

valves, one at a time. Check for air leaks and ensure that each SlamJet® “auto-close” mechanism opens

and allows air flow to begin.

Correct any air leaks and tag any SlamJet®s that do not appear to be operating correctly.

Note that commissioning of the column can continue with a few spargers not working.

Leave all individual sparger manifold isolation valves OPEN, but CLOSE the manual air supply shut-off

valve. Slowly open a manifold drain valve to de-pressurize the manifold.

Sparger system commissioning is now complete.



2.4 SPARGER WATER SYSTEM


Perform a “walk around” to visually check that the sparger water system is correctly installed and piped.

2.4.2 Booster Pump Startup

OPEN the sparger water supply shut-off valve and ensure that water is available to the booster pump.

Note that the booster pump is a progressive cavity pump and the rubber stator will be very quickly

damaged if the pump runs dry.

Note that the sparger water system can ONLY be operated when the sparger air system is operating.

OPEN the sparger air supply shut-off valve and set air flow to provide a manifold pressure of 70 to 80 psi

using either the automatic flow control valve in manual mode (if supplied) or the manual valve.

Verify that all spargers are operating and delivering air to the column.

OPEN all sparger water manual flow control valves, one at a time. Check for system leaks and repair if

necessary.

Using the adjustable frequency controller, start the pump at very low speed, and immediately check for

pump rotation and positive water discharge.

Note – The booster pump is protected by a pressure switch on the pump suction line that will not allow

the pump to start unless the required minimum water supply pressure is available.

SLOWLY INCREASE booster pump speed until the sparger water flowmeters indicate a flow equivalent

to 2 USgpm per SlamJet®, and visually confirm that water mist is being expelled from the spargers.

Note – individual SlamJet® manifold connections are mounted vertically in the manifold using recessed

couplings. Water will not reach the spargers until the bottom of the manifold has filled with water to the

level of the recessed couplings (refer to Figure 2).

2.4.3 Pressure Relief Valve Check

The sparger water booster pump is a positive displacement pump, and the sparger system is therefore

fitted with an emergency pressure relief valve to prevent over-pressure on the system. The relief valve

should be set to about 10 psi higher than the maximum expected air supply pressure.

Two people are required to safely perform the pressure relief check. One person should be stationed at

the local start/stop switch for the booster pump, and the other person is required to close the sparger

water manual flow control valves to initiate the test. One of these two people should be able to SEE the

relief valve.



Ensure that the sparger air system is running and the booster pump is running and delivering water to

the spargers. With one person stationed at the booster pump local stop, the other person must close

the flow control valves one at a time. As the last valve is SLOWLY CLOSED, the sparger water system

should begin to go into an over-pressure condition, and the pressure relief valve SHOULD OPEN to let

down the pressure.

If the system pressure begins to exceed the relief valve setting, IMMEDIATELY STOP the booster pump

or open the flow control valve, then shut down the sparger water system and correct the problem with

the relief valve.

2.5 WASH WATER SYSTEM


Perform a “walk around” to visually check that the wash water system is correctly installed and piped.

2.5.2 Wash Water System Purge

Purge the wash water supply piping so that no tramp material or construction trash reaches the

perforated wash water distribution header(s).

2.5.3 Wash Water System Start-up

Verify that the wash water flow control valve, either automatic or manual, is CLOSED.

SLOWLY OPEN the wash water system isolation valve. Check for leaks and repair if necessary.

SLOWLY OPEN the wash water flow control valve(s) to verify water flow.

Clear any blocked distributor holes.

Check for even wash water distribution at low flow. If flow is not even, check that the wash water

distributor is installed level, and check for partial blockages in the wash water distributor piping.

CALIBRATION NOTE:

Calibrate the wash water flow meter (if installed) by timing the filling of the column. If one or more

pressure transducers are being used for level control, these can be calibrated at the same time.



2.6 LEVEL CONTROL SYSTEM

Level sensing is performed either by a mechanical ball float and ultrasonic detector system, or by single

or dual pressure transducers. Level control is performed either by a local PID controller or a plant DCS.

The controller receives a level sensor input (4 to 20 mA signal), compares this signal to a setpoint

established by the flotation operator, and sends a control output (4 to 20 mA signal) to modulate the

opening of the underflow pinch valve. If, for example, the sensed level rises above the operator’s

setpoint, the underflow valve should open slightly to bring column level back down to the setpoint.

For either the ultrasonic or pressure transducer system, check that the field sensor(s) is reporting a

level signal, and also insure that the PID controller (or plant DCS) actuates the underflow valve in the

appropriate manner – opens when it should open, closes when it should close.



CALIBRATION NOTE:

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

should be performed regularly, particularly if the slurry pulp density changes.

Column gas holdup – the air fraction in an aerated slurry – can be checked with the column filled with

water. With the air turned off, (εg = 0) the pressure transducer reading should be equal to ρgh, where ρ

= 1 (water), g = 9.81 m/s2 (gravity) and h = distance from transducer to water level (meters).

If column sanding is a significant concern, the level controller should be programmed so that it will close

the underflow pinch valve only to a preset minimum closure – say 5% open. This will prevent the valve

from closing fully and will help to alleviate sanding in the column.

2.7 COLUMN STARTUP

Perform a visual “walk around” to confirm that all process piping is correctly installed and that all manual

and automatic valves are positioned to send slurry flow in the proper direction through the proper pumps

and pipelines.

Visually confirm that the column underflow isolation valve is OPEN and the flow control pinch valve is

CLOSED.

If launder sprays are installed, turn these ON and check for adequate flow and distribution of launder

water. The launder spray system should be turned OFF until concentrate flow begins and the sprays

become needed.

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

hose. In preparation for normal operation, enter a medium setpoint on the level controller. The level

setpoint will be optimized for metallurgical performance during normal operation.

Once the water level in the column rises about three feet (one meter) above the SlamJet® spargers, the

air sparging system (sparger air and sparger water) can be activated. Visually check that all spargers

are discharging air.

As the water level reaches the setpoint, visually confirm that the underflow pinch valve begins to open

and that the level control system is functioning correctly.



At this time, the level control system should be checked by inputting new setpoints above and below

the initial setpoint, and visually confirming that the control system adequately maintains a stable level in

the column.

Once the column is operating in a stable manner, slurry flow can be started.

2.8 NORMAL COLUMN SHUTDOWN

2.8.1 Initial Procedure

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

maintain the interface level without sanding out lines. This may require an INCREASE in wash water

flow.

2.8.2 Final Procedure

The column will continue to recovery and discharge to the overflow launder any contained floatable

material as well as any floatable material entering the column from existing recycle flows. When this

flotation ceases, assess the type of shutdown expected. A short term shutdown is a few hours. A long

term shutdown is many hours or days.

2.9 SHORT TERM SHUTDOWN – HOURS

When the column is to be taken off line for a short period of time, it should NOT be necessary to empty

the column. The following procedures should be followed;

The sparger air and water flows 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.

OPERATIONS NOTE:

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

valve should be drained to prevent sanding.

When feed is reintroduced, simply open the underflow isolation valve and reset the wash water and

sparger air and water flow controls to their previous levels.



2.10 LONG TERM SHUTDOWN – DAYS

If the column is to be taken off line for an extended period of time it should be drained. Confirm that the

column has been “floated out” as described in Section 2.8.2. When all flotation ceases, stop all feed to

the column, reduce wash water flow and allow the column to drain by opening the underflow pinch valve.

This is done by placing the level controller in manual mode and setting the valve position to full open.

When the level in the column drops below the spargers, sparger air and water flows and the wash water

flow can all be shut off.

The underflow pump, if installed, can now be shut down and the remaining material in the column can be

drained to waste through the column drain valve.

2.11 EMERGENCY SHUTDOWN

In the event of a power failure or other emergency situation that results in a “crash shutdown” of the

column, the following procedures should be followed;

All streams entering or leaving the column should be IMMEDIATELY STOPPED, including sparger air

and water, as well as any pump gland water flows.

All SlamJet®s are designed to automatically fail closed. Visually check that this has occurred.

The automatic air flow control valve, if installed, is designed to fail closed. If automatic air flow control is

not installed, manually close the sparger air shut-off valve.

If the sparger water booster pump is still running, it should be shut down.

If the level controller has been programmed with a minimum closing for the underflow valve, then material

will continue to drain from the column. In this case, the manual underflow isolation valve should also be

closed, and care must be taken to drain the underflow line between the isolation valve and the pinch

valve to prevent sanding. Note that the short term shut-down procedure does NOT apply here since, in

an emergency situation, the column floatables will not have been removed.

2.12 COLUMN RE-START

If the column was shut down while under full operational load, it is possible that the column will be

sanded out. For coal processing columns which typically operate at fairly low densities, the probability

of sanding is low. If the column is operating at higher densities and there is a possibility of sanding, the

following re-start procedure should be followed;



2.12.1 Re-Pulping

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

period of time (10 to 20 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.

2.12.2 Re-Starting Underflow

After the repulp lances and sparger air flows have been running for a period of time (20 to 30 minutes),

the underflow isolation valve should be SLOWLY opened 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 this

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

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

CPT CoalPro Column Operation Page 18


3.0 COLUMN OPERATION

This section suggests approximate initial settings for some of the more common variables. These

variables should later be optimised during operation. Also given here are the general effects that

changing one variable will have on a column at steady state. The control system can compensate for

gradual variations, 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 control loops are

tuned to prevent unwanted oscillations.

3.1 SPARGER AIR

The column air rate is the most commonly adjusted and most effective control variable. The response to

changes in air rate will be very rapid (seconds to minutes). 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 normal operation.

Superficial gas velocities (cm/s) can be converted to free air flows as follows;

Example for 2.0 cm/s air velocity in a 14 ft. diameter column

Column Area = π Ø2 / 4 = 3.142 x 14 x 14 / 4 = 153.9 ft2.

Gas Velocity = 2.0 cm/s x 1.969 = 3.937 ft / min

Gas Flow (free gas) = 153.9 ft2 x 3.937 ft/min = 606 cfm (ft3/min)

Increasing the air flow will generally have the following effects:

• grade of the froth product will be reduced (more ash entrainment)

• density (percent solids) in the overflow will be reduced (more water recovery)

• recovery of solids to the column overflow will increase

These effects are only valid within a specific range of flow. Continued increase in the air rate will lead to

the onset of bubble coalescence which will have a severely detrimental effect on 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 reduced. (i.e. pulp level raised).



3.2 INTERFACE LEVEL

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

quality (ash) and clean coal recovery to overflow (yield). 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 clean coal due to an increase in "drop-back".

The column level should normally be controlled in a range from 20 inches to 40 inches but can vary

depending on the stability of the froth. Little benefit is expected at depths greater than 60 inches. Much

shallower froths are possible and may be desirable if high yield is required and high ash can be

tolerated. Tests should be performed to predict performance at various interface levels.

3.3 WASH WATER BIAS

The non-selective entrainment of hydrophilic minerals (ash) can be reduced by preventing feed water

from entering the overflow. This is done by operating with a positive wash water bias. In general, this

means that the volumetric flow rate of underflow should be at least slightly greater than the volumetric

flow rate of feed to the column. The excess flow (tails minus feed) divided by column cross-sectional

area is defined as the bias velocity, or Jb.

The column should be operated with Jb in the range of 0.0 to 0.3 cm/s. A Jb of 0.05 cm/s is suggested at

start-up.

Increased bias flow will:

• increase the displacement wash and underflow/feed ratio

• increase the grade of the froth

• reduce the recovery of clean coal

These effects do not continue indefinitely with increasing bias. Eventually channeling of the wash water

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

bias, maintain an underflow volumetric flow rate at least slightly greater than the feed volumetric flow rate

by adjusting the wash water flow rate.



3.4 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.

3.4.1 Frother

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

not improve collection of particles.

Frothers for coal flotation generally fall into two categories – glycols and alcohols.

Glycols are much more effective than alcohols, but also produce a more persistent froth that can cause

material handling problems in other parts of the plant.

Alcohols are less effective but alcohol froths break and de-aerate more quickly than glycol froths.

In general, increasing the frother dosage will:

• reduce bubble size

• reduce the bias rate

• reduce the percent solids in the overflow.

• increase gas holdup in the collection zone

• reduce the maximum air rate



3.4.2 Collector

The most common coal collectors are fuel oil, kerosene or Diesel.

The presence of collector enhances the attachment of coal particles to air bubbles and thus directly

influences clean coal recovery.

In general, increasing the collector dosage will:

• increase mass recovery

• increase the percent solids in the overflow

• increase ash content of froth

• reduce gas holdup in the collection zone

• increase the maximum air rate

• reduce bubble size

Note that frother and collector often work “against” each other, and there is a tendency to overcome

problems caused by too much of one reagent by adding more of the other. This will almost certainly

lead to reagent overdosing, and must be avoided.

For example, low recovery typically indicates a need for more collector. Increasing the collector dosage

may cause the froth to dry out and collapse. This can be compensated for by increasing the frother

dosage. More frother can lead to a runny froth and increased ash entrainment in the froth. This can be

compensated for by adding more collector – and so on.

The flotation operator’s first move to solve a flotation problem should always be to check dosing levels

and compare with “normal” levels. If reagent levels are already high, then selective reduction of reagents

may solve the problem at hand without causing any other problems.

CPT CoalPro Instrumentation & Control Page 22


4.0 INSTRUMENTATION AND CONTROL

4.1 INTRODUCTION

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

Sparger Air Flow, Sparger Water Flow and Wash Water Flow . For most CoalPro applications,

sparger air, sparger water and wash water addition rates are manually controlled and only one automatic

control loop is required – for column level. The description of the operation of the control loop(s) 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.

4.2 SPARGER PRESSURE

The pressure in the sparger manifold should be maintained between approximately 60 psi and 100 psi.

Sparger manifold pressure is a function of

• SlamJet® control setting (factory pre-set cracking pressure)

• Sparger air flow rate

• Sparger water flow rate

• Plant air supply pressure

• Hydrostatic head in the column

An initial operating pressure of 80 psi is recommended. During commissioning, different operating

pressures should be tested to determine the optimum value for each application. For systems requiring

remote sensing of the header pressure, a pressure transmitter can be mounted on the air manifold.

4.3 SPARGER AIR FLOW

Air flow control is the most effective parameter available to the operator for maintaining proper column

operation. The air flow rate is optionally measured with a vortex flow meter and is controlled manually

with a simple valve, or automatically using a PID controller (or DCS or PLC) to adjust the position of a

pneumatically actuated flow control valve.

The recovery of solids to the column overflow (yield) and the quality of product (ash) are directly

dependant on the air rate. Air flows corresponding to a superficial gas rate below 0.5 cm/s are not

recommended but can be used 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 may be lower or higher depending on column conditions. The air flow rate should be



changed in a stepwise fashion using small incremental changes in order to minimize process upsets. A

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

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

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

Above the maximum gas rate recovery of solids to the overflow will decrease with the onset of bubble

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

tonnages and grades, particle size and slurry density.

The amount of air needed to meet ash / yield requirements depends on the particle size and amount of

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

For systems with automatic air flow control, operators should check periodically to verify that the air flow

has reached the flow required by the set-point when operating with high sparger pressures.

4.4 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 the position of the 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 ± 5 inches are generally 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 or with shallow froth depths.

It is suggested that the level be operated at depths greater than 10 inches and less than 60 inches with

normal operation at 20 to 40 inches.

4.5 WASH WATER

The wash water flow is optionally measured with a magnetic flow meter and is controlled by a local

manually operated valve. The expected range of flow of the wash water will correspond to a superficial

wash water flow of approximately 0.15 to 0.40 cm/s. Slow variations in flow are acceptable but better

performance will be achieved with smooth control. The suggested wash water rates are estimates only

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.

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 of approximately 0.1

cm/s is suggested.



The volumetric flow rate required to achieve the desired bias (Jb in cm/s) can be calculated as follows;

Example for 0.1 cm/s wash water bias in a 14 ft. diameter column

Column Area = π Ø2 / 4 = 3.142 x 14 x 14 / 4 = 153.9 ft2.

Bias Velocity = 0.1 cm/s x 1.969 = 0.197 ft / min

Wash Water Flow = 153.9 ft2 x 0.197 ft/min = 30.3 ft3/min x 7.48 = 226 USgpm.

In other words, the tails volumetric flow rate should exceed the feed volumetric flow rate by about 200

USgpm.

CPT CoalPro Troubleshooting Page 25


5.0 TROUBLESHOOTING

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

are dealt with: poor grades (high ash) and poor recovery (low yield). 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.

5.1 PROBLEM – HIGH ASH

5.1.1 Column Problems

High air rates may cause an increase in ash entrainment 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.

5.1.2 Circuit Problems

High reagent dosages 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, resulting in a higher than normal concentration of middling particles in the

froth. Check the size distribution of the feed, concentrate and tailings streams. Analyze the various size

fractions to determine if there is a possible liberation problem.

5.2 PROBLEM – LOW YIELD

5.2.1 Column Problems

Low yield (recovery of clean coal) is 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.

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.

CPT CoalPro Troubleshooting Page 26


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

appear 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, if

necessary, accelerate the sparger tube inspection and cleaning schedule.

5.2.2 Circuit Problems

Insufficient collector addition may contribute to poor flotation performance. Check reagent addition rates

and adjust as required. If frothers are 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.

CPT CoalPro System Maintenance Page 27


6.0 SYSTEM MAINTENANCE

6.1 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.

6.2 COLUMN MAINTENANCE

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.

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.

The 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.



6.3 SPARGER MAINTENANCE

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.

6.3.1 Orifice Wear

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

of time. However, in the unlikely event that the sparger orifices become enlarged they must be replaced

to avoid a reduction in 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.

6.3.2 Tip Blockage

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

distribution. Each sparger tube should be removed and inspected 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 0.30 inches (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.

6.3.3 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.



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.

6.3.4 Sparger Removal for Inspection and Cleaning:

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

Remove the air hose by disconnecting the “quick disconnect” 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 12 in (300 mm) from the end,

which provides a visual indication of when the end of the sparger tube has cleared the full 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 through the sparger nipple.

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

blockages are serious, it may be necessary to disassemble the SlamJet® to clear it.

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, the tighten the compression fitting by hand and attach the air line.

WARNING:

If SlamJet®s are pressurized 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.



6.3.5 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.

6.4 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.

CPT CoalPro Collection Zone Theory Page 31


7.0 COLLECTION ZONE THEORY

7.1 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 (drop-back) are collected by rising bubbles.

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

characteristics.

7.2 RESIDENCE TIME

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

by the following equation;

slc2

c spa f col sl

tail( )=

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

τπ ρ ρ

min

Where

τsl = 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.



Gas

Hol

dup

- Eg (%

)

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

Gas

Hol

dup

- Eg (%

)

Superficial Slurry Velocity - J sl (cm/s)0.5 1.0 1.5

10

30

20

Liquid Velocity JL ( cm/s)

0.38

1.00

1.26

7.2.1 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.

7.2.2 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.



7.3 AIR RATE

7.3.1 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.

Sup

erfic

ial G

as V

eloc

ity -

J g (c

m/s

)

Superficial Slurry Velocity - J sl (cm/s)

Figure 5 - Column Floodingfrom 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 (µm)

Figure 6 - Bubble & Particle Size Effectsfrom Dobby & Finch - 1988

Ec *

Ea (

%)

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.



7.3.2 Optimum

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

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

than optimum.

7.4 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.

7.4.1 Gas Velocity

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

possible with larger bubbles.

7.4.2 Probability 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:

cg a c

bk =

1.5 Jd

ε ε

Where

kc is the flotation rate constant (min-1).

εc is defined as the fraction of particles contained in the cylindrical volume traveled 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 optimize a column. This can be

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



7.4.3 Solids Carrying Capacity

0.05 0.10 0.15

Bub

ble

Dia

met

er (m

m)

Frother Consumption (mg/s•cm2)

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

SlurryFrother

SpargerFrother1.0

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.

CPT CoalPro Froth Zone Theory Page 36


8.0 FROTH ZONE THEORY

8.1 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 stabilizes 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 (%)

Fro

th D

epth

(app

rox.

1.0

m)

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

Overflow

Interface

ConventionalFroth

CollectionZone

Froth Zone

Column Froth

Wash Water Concentrate

NegativeBias

PositiveBias

InterfaceLevel

DrainingFroth BedEg > 0.80

PackedBubble Bed

Eg > 0.74

ExpandedBubble Bed

Eg < 0.74

BubblingZone

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.



8.2 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 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).

Cop

per R

ecov

ery

(%)

Froth Depth (m)

Figure 10 - Cu Recovery Vs Froth Depthfrom Huls - 1989

50

80

70

60

0.8 1.0 1.2 1.4

Con

cent

rate

Gra

de (%

Ni)

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

8.3 GAS 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).



Overflow

Interface

CollectionZone

Froth Zone

Gas Holdup - Eg (%)

Fro

th D

epth

(app

rox.

1.0

m)

Figure 12 - Effect of Jg on Gas Holdupfrom Dobby & Finch - 1988

20 40 60 80

Jg

Jg

Gra

de (%

)

Recovery (%)

Figure 13 - Rghr Column Grade/Recovery

48

54

52

50

92 94 96 98

20 40 60 80Gas Holdup - Eg (%)

Fro

th D

epth

(app

rox.

1.0

m)

Overflow

Interface

CollectionZone

Froth Zone

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

Frother

Frother

20 40 60 80Gas Holdup - Eg (%)

Fro

th D

epth

(app

rox.

1.0

m)

Overflow

Interface

CollectionZone

FrothZone

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.

8.4 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 (%)

Frot

h D

epth

(ap

prox

. 1.0

m)

Overflow

Interface

CollectionZone

FrothZone

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

Feed Water BiasWater

Jg

>2.5 cm/s

<1.5 cm/s

Pb

Rec

over

y (%

)

Superficial Wash Velocity - Jw (cm/s)

Figure 17 - Wash Water Grade/Recoveryfrom Kosick & Kuehn - 1987

Con

cent

rate

Gra

de (

%P

b)

72

84

80

76

20

80

60

40

0.08 0.16 0.24 0.32 0.40

PolarisGrade

Recovery

8.5 SUPERFICIAL BIAS RATE

A change in the superficial bias flow can also influence the gas holdup (Figure 16). Increased wash

water flow rates reduce the amount of coalescence that occurs by minimizing inter-bubble contact. At

very high addition rates, bubble collapse due to impaction and washing (depends on both flow and wash

water distribution design) may occur. Any improvement in performance resulting from an increased posi-

tive bias (Figure 17) suddenly disappears when channeling of the wash water begins.

The drop is illustrated in Figure 17 and Figure 18. As the superficial bias velocity (Jb) is increased from

0.1 cm/s to 0.3 cm/s, in Figure 11, less feed water is found in the overflowing froth. This changes with a

sudden transition somewhere between 0.3 and 0.5 cm/s (the exact flow will depend on the design of the

wash water header). Figure 19 illustrates the entrainment relationship, in a mechanical cell, with feed



water that reports to the flotation product. Three different sized particles are shown: 5, 10 and 20

micrometers. Manual changes in the wash water rate will alter the superficial bias rate when using level,

or displacement bias control method.

20 40 60 80Feed Water in Froth (%)

Fro

th D

epth

(app

rox.

1.0

m)

Overflow

Interface

CollectionZone

Froth Zone

Figure 18 - Effect of Jb on Feed Water in Concfrom Dobby & Finch - 1988

Jb ~0.5 cm/s

Jb ~0.3 cm/s

Jb ~0.1 cm/s

Sol

ids

Rec

over

y (%

)

Water Recovery (%)

Figure 19 - Recovery of Solids Vs Water

40

5 µm

10 µm

20 µm

20 40 60

10

30

20

Con

cent

rate

Gra

de (%

)

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

PyriteWas

h W

ater

Inpu

t

Inte

rfac

e



8.6 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.



8.7 REFERENCES

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

Proceedings of the 17th Annual CMP Meeting, Ottawa, January 1985

Egan, 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.

CPT CoalPro Production Column Testing Page 43


9.0 PRODUCTION COLUMN TESTING

9.1 INTRODUCTION

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

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 interact.

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.

9.2 AIR RATE

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.



9.3 WASH WATER BIAS

Jb values should be determined for each test in order to optimize 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 channeling 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.

9.4 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 than 1,000 mm.

The minimum froth depth or highest froth pulp level is the point where insufficient froth cleaning occurs.

Significant entrainment can occur when operating with very shallow froths.

The column should be able to maintain performance at interface levels as high as 400 mm below the lip,

possibly higher, and also retain a stable froth while operating as low as 2,000 mm.



While operating under steady conditions, slowly alter the level and note any changes in grade and

recovery. Use this information to construct a grade / recovery curve Vs interface level response plot at

varied air rates.

9.5 BUBBLE SIZE

Bubble size is a column parameter that is not monitored very frequently. It will affect the carrying

capacities of the column and possibly the selectivity of different size fractions.

Bubble size can be altered somewhat by changing the sparger system pressure. This pressure should

be altered to determine the grade / recovery response. For finer particle sizes, recovery is usually better

when smaller bubbles are generated. Bubble size can also be significantly affected by frother addition.

Note, however, that use of frother addition solely to control bubble size, especially in circuits

incorporating recirculating streams, can lead to serious frother overdoses.

9.6 RESIDENCE TIME

The following equation is used to determine the column slurry residence time:

slc c spa f col sl

conc( )=

60 A ( H - H - H )(1 - / )V

τρ ρ

min

Where1

Ac = column area (m2)

Hspa = sparger height from base (m)

Hc = total column height (m)

Hf = interface level (m)

ρcol = collection zone density (t/m3)

ρslu2 = concentrate slurry density (t/m3)

Vconc = concentrate volumetric flowrate (m3/hr)

1 Collection zone volume = π dc2(Hc - Hspa - Hf)

Gas holdup = 1- (ρ col)

ρsl

2 As determined by sampling the underflow line density or on-line density meter.



9.7 WASH WATER DISTRIBUTOR HEIGHT

For columns using a submerged “in-froth” style wash water distribution system, the position of the wash

water distribution pipes can influence the froth characteristics. When the pipes are set deep into the

froth bed, the efficiency of cleaning will improve. There will less short circuiting of wash water to the

column overflow and this will result in an increase in the froth density. Froth zone recovery may

decrease slightly.

If the distributor is positioned right at the froth surface it may interfere with transport of froth to the

launders. When the distributor is located above the froth, froth density will be lower due to some short

circuiting of water to the column overflow. This will result in a slight decrease in cleaning efficiency.

Tests should be conducted with the distributor at different positions to determine the performance

characteristics of the system. Maintain as many variables constant as possible and vary the distributor

height. Record the recovery and concentrate density.

9.8 SAMPLING

All sampling should be standardized and samples should easy to obtain.

Sampling should commence at t = 3 x NRT (nominal residence time) after the last parameter change.

Samples should consist of a composite of at least three (3) separate cuts taken at timed intervals. For

example, one cut every 5 minutes over a fifteen minute period will generate a “four cut” composite.

Sample all streams and obtain the wet and dry weights. Assay, then calculate the mass balances.

Sample sizes should be large enough to allow future screen analyses, possible including chemical

screen analyses or “size-by-assay”.

Note - it is also advisable to use a stopwatch to record the exact collection period for each sample, and

to also record the wet sample volume. These data can then be used to determine volumetric flow rates,

which can, in turn, be used as a check against material balances calculated from assays.

Use a results table similar to Table I for each test. If possible, O.S.A. results and feed, concentrate and

wash water flow charts should be included covering a period of two column residence times.

9.9 COLLECTION ZONE DENSITY ESTIMATION

If the column has been equipped with pressure transducers, the collection zone density can be

estimated by raising the interface level to the a point just below the overflow lip and reading the pressure

transducer signal. This estimation will only be good at one point on a density contour which is a function

of air rate, slurry velocity, and bubble size.



9.10 FROTH ZONE DENSITY ESTIMATION

Density can be estimated by lowering the level of the interface from one point to another while noting the

pressure readings of both levels. The pressure difference will be a function of the difference between the

densities and level. This represents only one point on a continual plane which is a function of feed

grade, feed rate, reagent concentration, air rate, and wash water rate.

A typical column cell test data sheet is shown below. This sheet is intended as an example. Data

sheets suitable to each specific application should be prepared on site to reflect actual conditions,

analyses, etc.

Prior to column cell testing, the geometry of the cell should be accurately recorded to allow calculation

of the several key column performance parameters, such as carrying rate, bias rate, gas holdup, etc.

Documents

CoalPro Manual (Column Flotation Cell)