Analytical Instrumentation for Control of the Leaching Process

T6C; ANALYTICAL

INSTF~W~TATION

FOR CONTROL OF

THE LEACHING PROCESS

G.T.W. Ormrod, and G. Sornmer National Institute for Metallurgy,

Johannesbur~,

South Africa

On-line measuring probes are only semi-analytical, not possessing the accuracy of instruments for the off-line analysis of samples. However, where time is important and where the composition of the material to be measured does not vary too much (both of which conditions apply in local leaching), the probes are useful accessories to automatic systems. The National Institute for Metallurgy (NIM) has investigated the use of probes that measure the following parameters of leach slurry: acid content, oxidation state, concentration of cyanide, and efficiency of uranium extraction. It should be borne in mind that the results obtained from the various systems are compared with plant titrations, which are, in themselves, of limited accuracy.

ABSTRACT The on-line measurement of pulp streams by electrochemical probes provides a continuous check that can lead of facilities for the control of reagents. The probes are semi-analytical in nature and thus do not possess the absolute accuracy of instruments for the off-line analysis of samples. However, where time is an important factor and where the composition of the pulp stream is a limited variable, the probes have been shown to be a viable and useful accessory to the automation of hydrometallurgical plants. The emphasis in this paper is on the application of these probes under plant conditions. The basic theory of the probe's operation is considered, together with deviations likely to be encountered in pulps.

ACID CONTENT The sulphuric acid in a slurry reaches an optimum leaching efficiency at a particular concentration, beyond which acid is wasted(l). Any excess acid has later to be neutralized by lime, which increases tue material costs and often causes sulphate to precipitate and block pipes. Direct pH control of the acid concentration has been tried but without success, the pH range in leaching being in a cramped-scale region (from 1 to 2). Further, the glass electrodes used in pH control are very fragile and do not tolerate coatings or abrasion, no matter how slight.

The pulp parameters measured by these probes are acid content, oxidation state, concentration of cyanide, and efficiency of uranium extraction. INTRODUCTION The classical approach to process control in the metallurgical industry involves sampling, wetchemical analysis, and manual adjustment and, although this system is dependable, it is unwieldy and the time lags between changes in the process and the operator's awareness of them can result in an excessive consumption of reagents.

On the other hand, the measurement of conductance provides a sensitive method for the determination of acid concentration. The specific conductivity of pure sulphuric acid solution is directly related to the concentration of the solution up to levels of 10 per cent by mass and, above a concentration of 30 per cent, the specific conductivity actually decreases. For weaker solutions in the desired range of measurement, the relationship is linear. Figure 1 demonstrates this for the concentration range 0 to 5 g/l. This simple relationship is modified in acidic pulps, and the effects of other parameters are discussed later.

Probes containing suitable electrodes can provide a continuous check on hydrometallurgical processes and, although these systems are not specific to the parameter they measure, an equilibrium develops on a plant so that interferences to the system become constant. Manual checks are still made, but the control is largely left to the machine and many of the large process swings are nullified. Automatic control generally provides for a faster start-up with less human effort, and obviates most of the art of the process operator so that the less-skilled operator matches his skilled counterpart. As a result, operator time is used more economically enabling other tasks to be done and resulting in a reduction in the labour force. One other benefit of automatic control is that a variable parameter can be held to closer limits, thus providing the metallurgist with more opportunities to experiment with the proc~ss.

Method of Measurement Because of contamination, the use of direct electrode conductivity probes is unpractical in pulps, but the electrodeless conductivity probe used by NIM provides a viable measurement with little maintenance. The system was first described by Ruben(2) in 1926 but was not applied to extractive metallurgy until 1960, when

521

Eicholz and Bettens (3) reported on the measurement of acid concentrations in pulps. NI~ developed this application further and tested the system on local nlants(4). The principal advantage of the electrode less system is that it can be clad in any non-conducting material and it is therefore robust. Electrically, the system is reliable and stable, and the absence of a direct connection with the measured medium precludes contamination and subsequent drift. Electrodeless Conductivity

The attenuation factor of y = Yo/Ys, where vp and Ys are the specific conductivities of a pulp· and of a pure solution respectively containing equal dilute additions of sulphuric acid. Pulps derived from ~itwatersr~nd ores have a sp~cific gravity of about 1,6 and their solids co~tent represents 35 per cent by volume. Experimental determinations of y by different mines were between 0,3 and 0,4. The factor y is related to the degree of occupancy by non-conducting sand particles in the measuring cell, and thus to the concentration (percentage by volume) of the solids in a pulp. Other factors, including the shape and distribution of the component particles, contribute to the complexity of this relationship.

~eter

The sensor consists of two toroidal coils that are mounted on a plastic inner tube together with a thermistor (for temperature compensation), and the whole is surrounded hy a larger cylinder, the space between being filled with epoxy resin. One coil induces high-frequency alternating current into a loop of slurry with a magnitude dependent on the conductance of the slurry, and a second toroidal coil, mutually screened from the first, receives this induced current. In a commercially available instrument, this current is backed-off by a servo-amplifier system, nroducing excellent electrical stability through phasesensitive detection.

An approximation can be made by consideration of n spherical particles of radius r in a cube of side 1 cm. The total volume of particles V = 4/3nnr3. If these particles· art touching in a rectangular lattice, the number of availab1 conduction paths approaches zero and n= 1/2r 3. Thus, the maximum volume in this configuration (I'm) can be derived as Vm

4nr 3 ---3

or

3 0,5236 cm , i.e. ,

3.8 r The cell constant (kJ of the probe depends approximately upon the ratio ~/a, where ~ is the length of the inner cylinder and a is a crosssectional area of the inner cylinder. The ratio assumes that a large vOlume of electrolyte surrounds the probe, completing the liquid loop with a relatively high conductance path. A typical cell constant for a probe operating in a leach slurry is 0,75.

52,34 per cent solids by volume. A relationship between y and the percentage volume of solids in a pulp can be deduced if the limits y = 1 are taken for acid solution and y ~ 0 for an acidic pulp with a theoretical solids content of 52,4 per cent by volume. By interpOlating the values for the typical pulp of 35 per cent (by volume) one derives y = 0,33, a figure in agreement with the experimental determination.

The following considerations were borne in mind in the application of the electrodeless conductivity probe.

Effect of Temperature (a)

Conductivity is non-specific, i.e., any ion species can contribute. However, in local leaching plants, the interfering ions are low in concentration and conductance, and their effect remains relatively constant.

The conductivity (Y) of dilute sulphuric acid solutions varies with temperature, according to the simplified relationship

yt (b)

(c)

(d)

Non-conducting sand particles partially occlude the measuring cell, reducing the effective conductivity as described below. If the pulp density remains tolerably constant, the meter gain can be proportionately increased to a level determined by local calibration.

Yr [1

+

o(t - t r )],

where t and t are the reference and measurement temperalures, and is the temperature coefficient. In practice, this relationship is non-linear and is therefore not a constant.

°

°

Typically,

o o

The variation of conductivity with temperature can be overcome by temperature compensation, provided by the thermistor in the head of the probe. Accurate compensation is difficult to apply when there are larger variations in temperature, but fortunately the local working range of temperature fluctuations is small.

1,2% °C- l 0,6% °C- l

The temperature coefficient 0tcan be determined for a limited temperature range (t ± 6t), and linear compensation becomes adequate. It is therefore desirable to have an adjustable temperature-coefficient control on the instrument. Calibration

The conductance signals are noisy because the air bubbles normally encountered in pachucas cause variations in the conductance. However, time constants can be effectively applied to the amplifiers and read-out systems.

Dilute solutions of sulphuric acid (expressed in grams per litre) have a conductance coefficient of concentration of 4,8 mS/cm at 25 0 C, but this figure is reduced to approximately 1,6 mS/cm when a typical pulp (35 per cent by volume) is measured. If the conductance of the pulp before the addition of acid is known (a local example is 8 mS/cm), the conductivity meter can be ranged and provisionally calibrated by dummy-test loop resistances. (This feature can be provided in the instrument.) The calibration is done empirically in situ by the correlation of the meter readings with appropriate acid concentrations obtained by volumetric analyses. The

Effect of Solids Content As the density of a pulp increases, so its effective conductivity decreases. The effect of solids content can be calculated as follows.

522

instrument settings may require trimming, but these new settings (for intercept and slope) are evaluated by use of the test loop resistances. This practice helps during later maintenance periods, when the whole instrument can be tested without the need for liquid samples representing a range of conductances. A typical calibration in pulp for varying acid concentrations (0 to 8 g/l) is shown in Figure 2.

The probe head is constructed of solid polypropylene and contains two chambers accommodating the active electrode and a reference electrode. The electrodes are sealed into the head chambers of the probe by screwed glands and O-rings. The electrodes are protected by a cone, which shields them from the direct impingement of particles and precludes a build-up of mud on the sloping surface of the cone. The orobe head is supported by a 2 m stainless-steel pipe, which also carries the shielded connecting cable. The probe is slightly air-pressurized to preclude moisture, which could cause electrical leakage.

Control of Acid A simplified loop to control the addition of acid to a leach is shown in Figure 3.

The active electrode is in the form of a capsule for ease of replacement. The body of the capsule, also fabricated from polypropylene, carries a sealing ridge and is cone-shaped at one end, the platinum pellet being thermally sealed into the cone so that the tip protrudes at the apex. This shape was found to minimize the problems resulting from the coating of the electrode by deposits.

Standard methods of automatic control were employed so that the addition rate of acid was related to the conductivity of the slurry. The times for the system to respond to a change in the addition of acid were less than expected, ranging from 10 to 20 minutes. Thus, adequate control was obtained in three applications with controllers having two control terms, proportional and integral. Tn general, it was possible to contain the mean deviation in acid level to within 0,5 g/l of the required level as shown by plant titrations, when compared with a mean deviation of 4,0 g/l previously found on two plants with manual control. Blockages could occur in the electrodeless probes originally used but, since the aperture has been increased to approximately 50 mm, only slight deposits occur, which require weekly cleaning with a bottle hrush. The probes have operated for over two years without failure.

Use has been made of the sealed 'No Flow' reference electrode(5). This is of the silversilver chloride type with an inert plastic casing (fluorcarbon), which is thinned and made porous at the tip forming the junction area. Plant experience with this electrode has been favourable because of its freedom from maintenance and its resistance to coatings. Unlike the flowing reference electrode, the sealed electrode presents a high liquid junction impedance, so that the reference element requires an individual amplifier. This requirement is provided in the dual highimoedance input amplifier, which, together with the 'No Flow' reference electrode, is commercially available. The differential potential measurement (reference to active electrode) is held relative to the solution ground. As this connection is made at the support pipe of the probe, external interfering currents from the plant are rejected - an important aspect when, as generally experienced, the signal potentials are very low.

OXIDATION STATE Iron in the form of Fe 3+ plays an important role in the leaching of uranium, and its concentration must be controlled(l). During the reaction, the Fe 3 + is normally reduced but the addition of an oxidant (usually manganese dioxide) reverses this process. In plants on manual control, the quantity of oxidant required is determined by set-chemical methods. In automatic control, the ratio of Fe 3 + to Fe 2 + (ferric to ferrous) is determined by measurement of the redox potential. A predetermined redox potential is used as the setpoint for a controller, which, in turn, controls the admission of oxidant to the pachuca.

To prevent the pick-up of line noise, a preamplifier head is connected in the vicinity of the probe and the connection to the main amplifier is of low-impedance cable.

A platinum electrode immersed in a solution of Fe 3 + and Fe 2 + assumes a half-cell potential, gaining or losing electrons according to the state of the redox reaction: +

Extra provision was made for increased sensitivity and back-off facilities. The order of the gross signal is 500 mV, and the order of the wanted (recorded) signal is 100 mV but, in this sensitive range, electrical noise is also generated, originating mainly from the agitation of the pulp. Hence, the signal was electrically damped by the addition of time constants.

e.

The principle and measurement of redox potential is well documented and is derived from the Nernst equation. However, this potential varies with the concentration of other ion species present, e.g., H+ ,S04 r ,an d P0 3 j - ' b ut t h"1S 1nterference was minimized w1th the advent of acid control by conductivity.

Performance on a Plant Initially, the output of the redox electrode system was ~orrelated with plant analyses of Fe 3 + and Fe + and the logarithmic relationship verified, despite some p~int scatter. As the variations of H+ and S04 - decreased with improved conductivity control, so this scatter lessened. A slope of 66 mV per decade of Fe 3 + to Fe 2 + ratio was found, agreeing well with the Nernstian value. Figure 5 shows the logarithmic relation of this ratio to the potentials generated by the system.

The plant measurement of redox potential introduces mechanical and electrical problems, which were met by the development of a special probe and the use of a stable amplification system. The NIM Probe and Electrical System A robust probe (4) was specially developed to withstand the rigorous conditions of agitated pulp. (See Figure 4.)

A redox potential (Em) of 530 mV was found to represent an approximate Fe 3 + to Fe 2 + ratio (Cm) of 7. This potential was used as the

523

under carefully controlled conditions.

control setting from which the amount of oxidant (manganese dioxide) to be added to the pachuca was determined. The control loop comprised a two-term nneumatic controller and a pneumatically driven splitter hox that apportioned a slurry of the oxidant to the pachuca or recycle, as dictated by the control signal. (The control loop is incorporated in Figure 3.) Under controlled conditions, routine chemical analyses of the Fe 3+ and Fe 2 + concentrations showed a marked increase in stability over the previous manual methods. Contamination of the electrodes

A response curve for the system in cyanide concentrations ranging from 0,014 to 0,030 per cent was developed (Figure 6) from almost 300 titration analyses. Accuracies at confidence level (2c) for two common points were (0,020)

FFFTCIF:\CY OF I'RA!' 11"1 EYTRACTIO!': The mixed potential developed in a leaching medium by a uranium dioxide reference electrode system was found to be related to the dissolution rate of the \'02 crystal(7). The overall reaction for the leaching of U02 in the presence of an oxidant, ~ln+, can be represented by

OF CYAKIDf

The potentials from a conditioned silver electrode and a reference cell have been used successfully for the estimation of the cyanide concentration of gold pulns in a leaching pachuca (6). The response is non-linear and non-~ernstian and is thought to be a mixed potential from the dissolution of silver hy cyanide and from a redox reaction.

+

This can he separated into two half-cell reactions, which are considered to occur

siw.ultaneously at the surface of the U0 : 2 +

The electrochemical half-reactions proposed for silver in dilute alkaline cyanide solutions are 11

as ro110",.75:

Cathodic

( (

°2

+ 2H+ + 2e

(H 20 2 + 2H+ + 2e

0,003 per cent.

Because of its rapid response to changes in cyanide concentration, the system has also found application in process-evaluation tests on the plant as a monitor for additions of cyanide.

the system has operated over two years.

[Ag (CN) 2]

+

off-limit cyanide concentrations. Confidence and exneriencE increased to a level where the system became the primary element in the control loop admitting cyanide to the pulp stream.

As vith the acid nrohe, a slight deposit occurred on the electrode but weekly cleaning nroved to hc the onlv maintenance required. The electrode stands un well to the leaching environment, and

Ag + 2(CN)

0,002

and

a year as a 'pOliceman', providing an alarm for

three weeks showed the system to deviate in its output by only 5 mY. Before automatic control was employed, the ratio of Fe 3+ to Fe 2 + could vary betveen 1 and 20, and a ratio of 7 was seldom obtained. These extreme values meant either that the extraction was very low or the wastage very high. After the application of control, plant analyses showed a ratio snread tvnically hctween 5 and )0.

Anodic

0,002 per cent

The system was applied on a particular plant for

was very small, and re-standardization after

CO~CENTRATION

±

(0,025)

+

+

e

I"

2e

(n-l)+

.

Under defined conditions this potential provides an immediate measure of the leaching power of the medium. The electrode responds to changes in the concentration of Fe 3 + and is relatively insensitive to changes in the Fe 2 +. It is pH-sensitive and responds to changes in the phosphate concentration.

+ e

H2 0 2 2 H2O

The potential generated by the electrode system was found to be modified after 24 hours in the pachuca, hut after that the system stabilized and the response was then related to the cyanide concentration. It is interesting to note that, despite the favourable leaching conditions in the pachuca, the conditioned silver tip was not fully consumed after six months. It was apparent that a protective coating had developed on the surface of the silver, and analysis of the coating showed the presence of minor quantities of acanthite (Ag25) in finely divided form. Changes in the response of the system are assumed to occur through modifications to this coating. Observations of the changes show there is a dependence on pulp flow, stagnation or batch leaching producing an unstable response. Further, high concentrations of cyanide cause the electrode to become unresponsive but, after a period of leaching under normal conditions, the response is again stabilized.

Fxperiments on Plants In trials of the system on two plants, a [102 crystal was thermally embedded in plastic and shaped to fit the standard NIM probe. One application was found in a ferric type of leach at elevated temperatures, where the measurement of the redox potential was valueless because of its relation to the ratio of Fe 3+ to Fe 2 +. The U02 electrode, whilst reflecting the total content of Fe 3+, gave readings that were subject to variations in acid concentration because there was no automatic control of this concentration. The correlation between [Fe3+] and the mixed U02/reference electrode potential was consequently poor. The physical dissolution of the U02 crystal was rapid (two weeks), although crystals of different manufacture were found to have longer lives.

Experiments on a Plant In the other application, which was a standard leach at ambient temperature, the U02 crystal lasted three months. Here, the electrode system was found to have general application as a fault detector in the leaching system, in that the alarm could be given when there was acid

The standard NI~ probe and electrical system was used for plant trials. The active electrode capsule was fitted with a silver pellet. The probe was directly immersed in the flowing pulp in a pachuca and an evaluation was undertaken

524

failure, or the total iron content was lower than that required for an economic leach. This electrode system is still in the experimental stage, and further trials are to be conducted.

J.S. AFR. IN~T. MIN. ME TALL , Vol. 76, No. 4, Nov. 1975. (8)

GENERAL EXAMPLES OF ECONOMIC SAVINGS According to early(8) and subsequent in-depth study, the automatic control of sulphuric acid and manganese dioxide in a leaching section of a uranium-extraction plant resulted in the following savings: Consumotion of:

Reduced by:

Sulphuric acid

15,6 per cent per cent

Manganese dioxide

32

Calcium oxide

48,6 per cent

The savings were obtained without a reduction in the leaching efficiency. Furthermore, 4 skilled and 3 unskilled operators were transferred to other duties. In addition, the plant personnel claimed that start-up after the weekly shut-down was faster and that the operation of the plant was much easier. Because of tighter control, the contact time was minimized, resulting in subsequent power savings. The extra maintenance that had to be carried out by the instrumentation personnel was negligible, and the installation of the instrumentation paid for itself in a few weeks. ACKNOWLEDGEMENT This paper is presented by permission of the Director General, National Institute for Metallurgy, Johannesburg. The cooperation of the plant personnel at Western Deep Levels Limited is greatly appreciated. REFERENCES (1)

Pinkney, E.T., The Chemistry of the Uranium Leaching Process in South Africa. S. AFR. IND. CHEM, Vol 10, Nov. 1956, pp. 264-272.

(2)

Ruben, S., Electrochemical Testing Device. U.S. PATENT 1610971, 14th Dec., 1927.

(3)

Eicholz, G.G. and Bettens, A.H., Conductimetric Measurement and Control of Acid Concentrations in Leach Pulps. CAN. MIN. METALL. BULL, Vol 53, No. 1960, pp. 901-907.

(4)

Sommer, G., Ormrod, G.T.W. and Chaix, R.P., Recent Developments in the Instrumentation and Automation of Uranium-processing Plants. J.S. AFR. INST. MIN. ME TALL , Vol 73, No. 12, 1973.

(5)

Murti Neti, R. and Jones, R.H, Performance and Applications of a New Reference Electrode for Process Potentiometric Measurements. I.S.A. TRANS, Vol 10, No. I, 1971.

(6)

Ormrod, G.T.W., Lombaard, S.L. and Sommer, G. The Industrial Application of KEGOLD Electrodes. Johannesburg, National Institute for Metallurgy, REPORT NO. 1647, 1974.

(7)

Needes, C.R.S., Nicol, M.J., Finkelstein, N.P. and Ormrod, G.T.W. The Industrial Application of a Uranium Dioxide Electrode.

525

McLeod, D.S. and Taylor, S.H.J. Developments in Instrument Control Systems at Western Deep Levels Limited, Metallurgical Plants. PRIVATE COMMUNICATION, Nov. 1972.

Figure 1

Conductivity of dilute sulphuric acid solutions at saoc.

SPECIFIC H2 S04

CONDUCTIVITY SOLUTIONS at 50° C

40 ~ t-

30

>

t; 20

c=>

""en

E

z

8

E

(,)

10

O----..I--......I..--.....L....--..I------I 2 3 H2 S04t g/ I

526

4

5

Figure 2

Conductivity calibration for acid in a typical pulp.

22,5

20

~ ~

> ~ ()

:J

17,5 0

z

0

()

15

PULP SOLIDS 60% m/v. Fe 2 + 1·5 g/I 3 Fe + 1·5 g/I NoCI 4-0 g/I TEMP. 50° C

E

u

12,5

,/

"'-

/

U)

/

E

/ /

10

,/ ,/

/

/

/

/

7fJ~-....,....----r-----,.---r----'---~--r-----, o 2 3 4 5 6 7 8 ACID

CONCENTRATION,

527

g/ I

">1 ~.

()Q

c:

'1

ro

w

(p + I)

From

Mn02

Pulp in

Slurry make-up

...---------,

I~

b

~

I~

~.

~ r-------~~•

.

ff)

.a......

t-tI ~.

rt>

C.

I

C.

Slurry return

L-"'out~

~.

cu

()Q

'1

PUlP!

H2S04 (conc)

~

_

o

t-tI

(Ptl)

'1

rt>

cu

()Q

rt> ::3

rt

KEY: S Pneumatically c.n

(')

o

::3

controlled splitter box

V Sulphuric acid control valve A Redox probe C Electrodeless conductivity probe B Baffle plate

N

co

rt '1

o

...... ......

o o

"0 CIl ~.

::3

cu ...... rt> cu (')

;:T

"0

cu

(')

;:T

c:(')

cu

•

A ir

Figure 4

Section through the NIM probe.

~

529

SUPPORT

PIPE

2m

Figure 5

Redox potentials generated in pulp for various ratios of ferric to ferrous ions.

600

>

E

-

E;;;------

c

c:

Q)

500

o

Q.

E co

)(

o

"0 Q)

a::

400,"'---------=..0......-'-----------o Cm 1,0 2,0

log

Figure 6

Relation between the potentials generated by the silver system and the cyanide

concentration of plant pulps.

S50

sao >

E

-c

c:

4S0

Q)

0

a..

400 0,010

0,015

0,020

Cyanide,

530

%

0,025

(by mass)

0,030