FURTHER DEVELOPMENTS IN THE CONTROL OF LEVEL IN … · 2010. 9. 23. · Moys and Finch (1988a,...

FURTHER DEVELOPMENTS IN THE CONTROL OF LEVEL IN FLOTATION COLUMNS

M H MOYS

DEPARTMENT OF CHEMICAL ENGINEERING UNIVERSITY OF THE WITWATERSRAND

Further Developments in the Control of Level

Abstract

in Flotation Columns

M. H. Moys,

Consultant in Column Flotation,

Multotec Cyclones (Pty) Ltd.

and

Department of Chemical Engineering

University of the Witwatersrand

Johannesburg

Interface level in flotation columns is generally measured with the

aid of a pressure-sensitive instrument ego a differential pressure

cell or Metritape. The use of this type of measurement is critisised,

and other methods which are claimed to give more accurate measures of

interface level are discussed. The paper focusses on the development

of methods based on the measurement of conductivity across the froth-

-slurry interface. These are generally sabotaged by variations in the

conductivity of the feed slurry, particularly when the pH of the

slurry is being controlled, ego in sulphide flotation. Methods of

improving the accuracy and reliability of this technique are devel-

oped. The result is a level control scheme which successfully rejects

large disturbances in plant operation (including a 5:1 change in

slurry conductivity) while controlling level to an accuracy of ± 4cm.

1. Introduction

For effective operation of a flotation column the level must be

controlled near a pre-specified level Lset ' so that the froth height

h f is (a) not so low that the significant mixing that occurs in the

froth phase results in a loss in grade, Hnd (b) not too high, n~stll

ting both in an excessive residence time of bubbles in the froth phase

and a loss of residence time in the pulp phase (both of whi~h effects

result in a loss of recovery). Yianatos et. al. (1987) recommend a

froth height of approximately 1 m. Current methods for measuring

interface level L are reviewed below, and research work aimed at

developing a reliable method of measuring L using conductivity

measurements is described.

Level Control in Column Flotation Moys Page 2

2. Review of methods for measuring interface level

The method most widely used is that based on a measurement of hydro-

static pressure P at a distance between 1,5 and 2 m below the overflow

lip. P is however not an accurate indicator of interface level because

it is a strong function of other variables, such as slurry density,

hold-up of liquid in the froth, bubble loading, etc. Moys and Finch

(l988a) showed that errors in L exceeding 0,5 m are easily possible

when high gas rates, small bubbles and high bubble loadings occur,

particularly when the flotable mineral is dense, e.g. pyrite or

galena. Moys and Finch (1989) measured an error of 0,4 m in galena

flotation in a limited test programme at the Polaris Mine, N.W.T.,

Canada and the variation of level with pneumatic pressure measurement

in pyrite testwork in South Africa is shown in Figure 1. This lack of

precision means that frequent checks on level must be made by the

operator in situations where a sight window on the side of the column

exists; in the absence of a sight window the poor performance of the

measurement cannot be checked, so the operator is forced to set the

froth depth at a conservatively high level to avoid disaster, thereby

incurring losses associated with unnecessarily deep froths most of the

time.

Several papers have reported the use of Metritape, a patented device

which consists of a wire coil wound round a contact strip and insula-

ted from the slurry by a flexible, corrosion-resistant sheath. A strip

of Metritape 2m long is inserted in the top of the column. The design

is such that the wire does not make contact with the contact strip

until the pressure in the fluid outside the tape exceeds l50mm water.

This is sufficient to collapse the sheath and wire coil so that

contact is made. The resistance of the wire which is not in contact

with the contact strip is measured and provides and accurate (± 1 cm) measure of the level at which the first contact is made. ~~ile Metri-

tape is widely used for the measurement of fluid level in tanks, its

use in colu~n flotation must be seriously questioned, since it

provides a measure of the depth in the column at which the pressure is

150 mm of water above atmospheric pressure. This can only provide

relatively accurate measurements of interface level for columns where

the pressure at the interface level is less than this critical value -

a condition which can only occur for froths which are lightly laden,

have little interstitial fluid and/or where froth height is low. The

measurement is in addition subject to all the disadvantages of dip

cells discussed above.

Accurate measurement of interface level relies on the measurement of

intensive properties of the fluids near the interface (such as densi-

ty, temperature or conductivity) rather than an extensive property

such as pneumatic pressure which is a function of the total mass of


material obove the measurement point. Some of these are discussed

below.

Techniques based on the measurement of the position of a float at the

interface (this is essentially a density technique and is used

successfully

the density

with conventional flotation) cannot be relied upon since

of the froth near the froth-slurry interface varies over

too wide a range. It is not generally possible to select the density

of the float which will apply to all possible conditions.

Moys and Finch (1988a, 1988b and 1989) have reported the use of

measurements of temperature profile across the froth-slurry interface.

The technique relies on the existence of a significant and reliable

temperature differential between the wash water and the feed slurry.

Provided that the column is operated under positive bias conditions

(i.e. with wash water flowing downwards through the froth phase, so

that the froth phase temperature is close to the wash water tempera-

ture) there is a sharp change in temperature at the interface.

Measurements obtained in an industrial galena cleaner are shown in

Figure 2 (Moys and Finch, 1989). Detection of the level at which this

change occurs provides an estimate of interface level accurate to

± 7 cm. The method suffers the disadvantage of requiring intelligent analysis of 10-15 temperature measurements, so will be expensive.

Significant other advantages (see Moys and Finch, 1989) to be obtained

from such measurements (such as online measurements of froth washing

efficiency) may make this technique an attractive option, particularly

in an environment where computer control resources are already avail-

able.

The use of conductivity

is reliable, simple to

discussed initial work

measurements may provide a measurement which

implement and cheap. Moys and Finch (1988a)

in this area, using the conductivity sensor

arrangements shown in Figures 3 and 4. The first involves the use of

two paralllel, vertical probes which provide a fairly linear correla-

tion between level and conductivity. The second involves measurement

of conductivity between two probes, one above and one below the inter-

face. The correlation is non-linear but the simplicity of the design

(the two probes can be mounted on a vertical rod passing through the

froth phase) makes the exploitation of this approach attractive.

Multotec Cyclones (Pty) Ltd (a South African supplier of flotation

columns) have used this method successfully on pilot column tests at a

local fluorspar mine, and it has also been used in coal column flota-

tion tests (sponsored by the National Energy Council) at the Universi-

ty of the Witwatersrand. The conductivity signal is connected to a PID

controller with a conductivity setpoint which keeps the interface just

below the lower edge of the upper conductivity probe. Level was


controlled to an accuracy of ± 1 cm.

While the method worked well in the above applications it failed when

applied to pyrite flotation, because the conductivity of the slurry

varied widely as a result of the necessity to control the pH of the

slurry at a setpoint of 3,8 pH units. Small variations in pH resulted

in large variations in conductivity, so it was impossible to select a

reliable setpoint for the conductivity (i.e. level) controller. It was

necessary in this situation to revert to the use of pneumatic measure-

ments for level control. The data given in Figure 1 was collected

during this experimental programme: clearly a more reliable estimate

of interface level is required.

We decided to invetigate further the use of conductivity measurements.

It was evident that

the variations in

scheme is described

a control scheme was required which would adapt to

slurry conductivity. The development of such a

below.

3. Theoretical Developments

3.1 The variation of conductivity across the interface.

This is discussed by Moys and Finch (1988a) in some detail; the

salient points are summarised here.

The conductivity of a gas-liquid dispersion typical of column flota-

tion froths (Yianatos, et al, 1985) is given by

- 0,432 kl,fr

\vhere kl fr , respectively

and

of €l,fr are

the liquid

the conductivity and fractional holdup

between the bubbles in the froth phase.

Typically ~l,fr is leGs than 0,4 at the base of the froth and drops rapidly to approximately 0,2 at the top of the froth phase. kfr 1 , will be similar to the conductivity of the slurry ksl at the base of

the froth but in columns where washing of the froth phase by the wash

water is efficient it will change rapidly towards the conductivity of

the wash water.

In the pulp phase the conductivity is given by

k pulp 1,55 - 0,55€sl

where slurry hold-up €sl is generally greater than 0,7. For kl fr ,


k sl ' €l,fr 0,4 and €sl 0,7 the ratio between froth and

pulp conductivities is 0,48. In cases where ~ash < ksl this ratio will be decreased, giving ample variation in k across the interface,

while if ~ash > ksl (e.g. this can occur in cases where the pH of wash water must be controlled) the variation of k across the interface

will not provide a reliable basis for interface detection.

3.2 A model for conductivity across the interface

Conduction in fluids between two small probes a significant distance

apart is a complex phenomenon. In an infinite uniform medium the

conduction paths follow lines through the medium that constitute the

lines of steepest descent in the voltage field set up between the two

probes as shown in Figure 5(a) (please note that the diagrams in

Figure 5 are not accurate; they were created for illustrative purposes

only). While most of the current follows a relatively short, direct

path between the probes, a significant proportion of it follows much

longer paths. The conductivity between the two probes is a stong

function of the specific conductivity of the liquid, the distance

between the probes and the surface area and the geometrical arrange-

ment of the probes. The very simple model that assumes that the

current is conducted through two resistances set up by the froth and

the pulp provides the following model (formulated in terms of resis-

tances; see Figure 5(b»:

R tot +

where R is resistance per cm, L

Zbot are the probe positions.

between Rtot (=l/ktot ) and L.

distance from the top probe, as

is the interface level and Ztop and This implies a linear relationship

This is true only for L some

shown in Figure 4. A more comprehen-

sive model must account for the geometrical design of the sensors and

must in particular account for the different conduction paths followed

by the current when the interface level is rising past the probe

faces. This is illustrated in Figures 5(c) and (d), where the presence

of an interface at the top probe introduces a distortion in the paths

followed by the current between the probes.

3.3 The measurement of conductivity

Relatively cheap conductivity meters rely on the (simplified) circuit

diagram shown in Figure 6. A high-frequency alternating voltage E is

applied to a range resistor RR connected in series with the conducti-

vity sensor (which has a resistance Rk ). The voltage drop VR across

RR is easily related to Rk :


In th~ conductivity meters we used, this voltage was converted into a

4-20 mA signal i k corresponding to a two-decade range on k (=l/Rk )

and a 0-100% guage output p as follows:

i k (mA)

k (0- 1 )

p(l)

Thus the meter

the relationship

change in p or

4

RR/10

0

12

RR 50

20

10RR

100

output is strongly and non-linearly related to k;

is such that a 10% change in k results in a large

i k , relatively independent of the value of k. This

means that the signal is sensitive to changes in k over a twodecade

range of k, something which could not be achieved with a linear

instrument. The design of this instrument lends itself ideally to the

purpose of this investigation as will be revealed below at the appro-

priate dramatic moment!

4. Equipment

Experiments were performed in a square (12 cm x 12 cm) perspex labora-tory column 1,8 m tall. The pilot plant is illustrated in Figure 7. Two sources of feed material were available (water only; one source

was dosed with HCl to raise its conductivity substantially above that

in the other source, which was ordinary tap water). Turbine meters

provided measurements of these flowrates. Wash water dosed with

Dowfroth 250 was added at the top of the column. Gas was added through

a canvas sparger; both wash water and gas rate were measured using

rotameters. Two LTH DCM-l conductivity meters and an analog PID

controller were available. All measurements were interfaced to an

IBM-compatible PC which was programmed to display, print and store all data collected.

5. Experimental Results and Discussion

The objective of the work was to develop a level control method which

would adapt to changes in feed conductivity. Several approaches were

investigated.

5.1 Setpoint obtained from measurement of pulp conductivity.

2 Conductivity sensors were used. Each sensor consisted of 2 brass

strips 1 cm long clamped around a 6 mm diameter perspex rod 30 cm


apart. The second conductivity meter was connected to a sensor approx-

imately a,6m below the level sensor. This was used to obtain a

measurement of pulp conductivity which was then passed through a vari-

able resistor to obtain a remote setpoint for the level controller. It /

was found that there was excessive interference between the two

conductivity meters; this was reduced by making sure that the top

probe of the bottom sensor and the bottom probe of the top sensor were

connected to the earth connection of their respective meters, and

finally overcome by increasing the size of these probes with a wire

grid that spanned the crossectional area of the col~~n. This ensured

that no current could bypass these probes and travel between the two

non-earth probes of the sensors. Secondly it was found that matching

the sensors (both in terms of the large scale geometry of the sensor

design and in terms of the size of the probes used) was critical to

the performance of the control system. This arrangement was used

successfully to control level for a wide range of operating variables

(feedrate, gas rate and wash water rate) but failed when large changes

(e.g. 3:1) in feed conductivity were imposed.

5.2 Use of a modified single conductivity meter

The problems found in 5.1 above were probably caused by the need to

use two conductivity meters plus associated sensors which interfered

with each other and were difficult to match accurately for the whole

range of conductivities used in the these tests. An examination of the

circuit diagram for the meters led to a dramatically simplified solu-

tion to the problem. This consisted of using the resistance across the

second probe discussed above (used for measuring the pulp conductivi-

ty) as the range resistor for the first conductivity meter, as shown

in Figure 8. The governing equation for this meter then becomes

where 'Y

This arrangement eliminates problems associated with both unmatched

sensors and unmatched meters and obviously simplifies the whole

control system.

One of the existing sensors was modified to provide the sensor shown

in Figure 9. This contained only three probe surfaces, with the

centre probe being used to provide the VR signal rather than the more

general probe illustrated in Figure 8. Existing terminal connections

on the conductivity meter.were used. The measurement was now a 4-20 mA signal denoted iq (to distinguish it from i k ) which was recorded as

a variable q (0-100% of span). This signal was connected to the

controller which was given a manual setpoint qsP = 15-20% as shown

in Figures 10 and 11 below.


To characterise the measurement the signal q was recorded as the

interface was allowed to drop over the full height spanned by the

level sensor. The variation of interface level L with q (and 1) is shown in Figure 9. Clearly the sensor design was not symmetrical with

respect to the middle probe surface; if this had been the case, then

we would have obtained q - 50% when the sensor was completely submer-

ged (L> 50 cm.), because in this case we would have had equal conduc-tivities (1 - 1) between the two probe pairs. No adjustments were made

so that the necessity for probe matching could be assessed.

The

in

ability

the face

of this arrangement to provide adequate control of level

of operating changes is shown in Figure 10. Clearly

changes in the gas superficial

disturbances than changes in

control scheme controlled level

4 cm.

velocity J G provide much more serious

the wash water J W. Nevertheless the near 38 cm with maximum deviations of

The effect of a setpoint change and a major disturbance in feed

conductivity is shown in Figure 11. The setpoint response overshoots

slightly but is nevertheless completely satisfactory. The conductivity

disturbance produces very little effect on the level, in spite of the

fact that conductivity changed by a factor of approximately 5:l!

Extensive tests of various combinations of these disturbances showed

that the control scheme was able to control level successfully in the

face of disturbances which are generally much larger than those to be

encountered during normal operation of flotation columns.

The main problem that needs a solution before a rugged industrial

instrument becomes available is the selection of materials of

construction for the probe which will resist the corrosive environment

found in different applications while maintaining a repeatable

response over long periods of time. Reliability is more important than

accuracy where level control is concerned, and the differential

pressure cell is a tough competitor.

6. Summary and Conlusions

Various methods for level measurement have been reviewed. The work-

horse of industry, the differential pressure cell is reliable but

inaccurate. An instrument based on the measurement of conductivity has

been developed which can provide accurate control of level under a

wide range of operating conditions and changes in feed conductivity.

The method must now be tested in an industrial environment to provide

an instrument which will eliminate the need for the frequent operator

attention which is typical of existing techniques.


7, Acknowledgements

This work has been funded by a grant from the Chamber of Mines Block

Grant made available in 1988, and by Mu1totec Cyclones (Pty) Ltd. The

assistance provided by Mr G Gloag in setting up the experimental

equipment and running the experiments is gratefully acknowledged.

8. References

Moys, M.H. and Finch, J.A., 1988a. The Measurement and Control of

Level in Flotation Columns. Presented at the Int. Symp. on COlU~l

Flotation, SME Annual Meeting, Phoenix, Arizona, January 1988.

Moys, M.H. and Finch, J.A., 1988b. Developments in the Control of

Flotation Columns. Int. J. Hiner. Process., 23 : 265-278.

Moys, M.H and Finch, J.A., 1989. The use of Temperature Measurement in

the Analysis and Control of Flotation Columns. Presented at the "Role

of the Practical Metallurgist" Symposium, Mine Metallurgical Managers

. Association of South Africa, Johannesburg, June 1989.

Yianatos, J.B., Finch, J.A. and Laplante, A.R., 1987. The Cleaning

Action in Column Flotation Froths, Trans. I.H.H., 96: C199-C205

Yianatos, J.B., Laplante, A.R. and Finch, J.A., 1985. Estimation of

Local Hold-up in the Bubbling and Froth Zones of a Gas-liquid Column,

Chem. Eng. Sci., vol 40, no la, pp 1965-1968.

BO

90

100

E ~11 :r: f-n. W 0

~ 0 0:: "-

/' /'

/'

III /' /'

/'

MULTOTEC CYCLONES A COLUMN FLOTATIOI~ V

/1Il /'

/Ill

III III III

/' /'

III III

ill

III

III 111111

III

/' /'

III

III

/' /'

/'

E u

N ...

/1i!I /'

/' /'

/' / III III fill

fill I!I

III III

III 1/1

III III III III

III

III

III

III III III III III

III

L-___________________________ ~ __ ~ __

460 470 4BO A90 500 510

PRESSURE READING (mV)

Figure 1. Variation of true interface level with pneumatic pressure

signal.

145

E ~ -'

v > .. --'

i \

Frolh Surface \-----~----

\ \ \ \

\

\ ~

"-'2 '-.

\ \

\

"-

\ 20 '\

\ \

E:

Tesl No. 36

J G = 1.6 J w =0.12 J F =0.64 J o = 0.08

(I) O.L-_______ . ______________ ~L__

Frolh Surface \,\\\---\~------

\\ \ \ \ \ \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

\ \ \ \ \ \ ~ '{io

\ \ , \ 20 \ \\"10\\

\ " . \ '\ " ~ \ \ " "Z~'" \ \ ,1 .,. \ \

E: "" ,,\\ " ">\ \

-.., -" '- ~\~\ ~ "'..,,~

'- ,,~

'- --"'~'. ------------- ;:t. resl 110. 169

h= 14 Jw=0.12 J, =0.64 J o =0.01

(11)

0.0 1.0 0.0 1.0 Normalised Temoeraturo t{J

+

+* ...

... + ... ... ...

...

... ~ T "mp. Probp

Locallons

F

Figure 2. Measurement of interface level by analysis of temperature

measurements obtained from probes located at the positions indicated

in the Figure.

E

~

-J

~

MULTOTEC CYCLONES. COLUMN FLOTATION

r Terminal Connections Recorder t-------{)--------, or Co m ut 0 r 1----------0---,

Cond Probe

Vollage lo Curr 0fll Conversion

O--~--l

E ('lolls I\C)

FIgure 6. Simplified circuit for a conductivity meter.

LEV EL PROBE

Figure 7. Pilot flotation test ri' g.

Level ~ Probe ~

Insulators --~

Conductors --~

MULTOTEC CYCLONES. COLUMN FLOTATION

r~ecorder

or Cornputor

v voltage to Current Conversion

--1::r-----'--fO se i" a to rI E (Volts AC)

Figure 8. Conductivity cicuit adapted for the measurement of

conductivity across the interface relative to slurry conductivity.

60

w co 0

E 50 0: D... ~ -1 -1 W W > > w W -1 -1

40 w ~ 0 Lt :> f= 0: 0 W - - - => l- Q ~ 30 z

0 0

20

I . 10 15 20 25

METER OUTPUT (('0) 0,2 0,3 0.4 0,5

CONDUCTIVITY RATIO, ll=kLIkSf

Figure 9. Variation of relative conductivity with interface level.

rr u c

'" E ~ ....J

::0-~ rr (3

...: C 0 0

40

30

20

10

100

50

0

TT J w : 0,1 -,0,3 cmls

I I J . 3 ~ 1 clllls 0'

Jo : 1-43 cmls

I I~ r----L--~-~~ ~

I

I I I

~'-J~~~ 700 800 I T 900

I TIME (5) I

Coni roller Output

--'------"-- - _____ -LJ, ____ L __ L ________ ~ 800 900 1000

TIME (s)

Figure 10. Level control subject to disturbances 1.n gas rate and WilSI! water rate.

;;g ~

" to .. E rr

E ~ ....J

z ~ :J W

t1: a: $: 0 ....J LL

,-----------------------~---~-~------

.-.-.

40-

30

20

10

0 100

5 Fl (Iow k)

0 I 0 100

Figure 11. ,Level control conductivity.

I I

/

/ /

/

------

I / .-

200

I

200

subj ect

" / k -1>0-SL

I

I

" "

I~ /

TtME (s) ....--- - ---

TIME (s)

to a large

---.-

300 400

---------["2 (high k)

-, 300 400

vClriation in feed

30

;;>0

15

10

FURTHER DEVELOPMENTS IN THE CONTROL OF LEVEL IN … · 2010. 9. 23. · Moys and Finch (1988a,...

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