The effect of pH on the recovery of uranium and gold by high-gradient magnetic separation

International Journal of Mineral Processing, 17 (1986) 83--98 83 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

T H E E F F E C T OF p H ON T H E R E C O V E R Y OF U R A N I U M A N D G O L D BY H I G H - G R A D I E N T M A G N E T I C S E P A R A T I O N

J. SVOBODA, I.J. CORRANS*, and M.H.E. SPITZE ÷

Council for Mineral Technology, Private Bag X 3015, Randburg 2125, South Africa

(Received November 7, 1984; revised and accepted August 2, 1985)

ABSTRACT

Svoboda, J., Corrans, I.J. and Spitze, M.H.E., 1986. The effect of pH on the recovery of uranium and gold by high-gradient magnetic separation. Int. J. Miner. Process., 17: 83--98.

Results of the experimental investigation of the effect of pH on the recovery of uranium and gold by high-gradient magnetic separation are presented. It has been demonstrated that, by alteration of the surface potential of the minerals present in the uranium- gold tailings, the quality of the magnetic concentrate can be enhanced and the recovery of uranium increased. Gold is observed to follow uranium closely in the magnetic concentrate as a function of pH. The experimental results imply that the highest metallurgical performance of a magnetic separator can be expected at a pH value at which the surface potentials of the valuable and gangue minerals have the same sign, and the pH value is not too far removed from the point of zero charge of the valuable mineral.

INTRODUCTION

The c o n c e n t r a t i o n o f gold and u r a n i u m in Wi twate r s rand c y a n i d a t i o n railings b y h igh-gradien t magne t i c separa t ion (HGMS) is a wel l -es tabl ished process t h a t has been tes ted in t he l a b o r a t o r y and on an industr ia l scale (Corrans and Levin, 1979 ; Corrans , 1980 ; Levin, 1980 ; Watson e t al., 1983 ; Corrans e t al, 1984}. I t has been shown tha t , wi th m o d e r a t e magne t i c f ields and at eco- nomica l ly accep tab le f lowrates , recoveries o f u r an ium ranging f r o m 50 to 75% can be achieved a t a U308 grade in the magne t i c c o n c e n t r a t e o f up to 1200 g/ t w i th the c o n c e n t r a t i o n ra t io in excess of 10.

Since the c y a n i d a t i o n tailings t r e a t ed by HGMS are very f ine, usual ly 50% smal ler t h a n 25 ~m, it is c lear tha t , fo r m o s t par t ic les , the vo lume t r i c forces p resen t in magne t i c separa t ion , viz. magne t i c force , force o f gravi ty, and h y d r o d y n a m i c drag have to c o m p e t e wi th surface forces. Surface in terac- t ions b e t w e e n par t ic les and those b e t w e e n the par t ic les and the m a t r i x are rare ly t a k e n in to a c c o u n t in magne t i c separa t ion , a l though S v o b o d a (1982 ,

*Present address: Department of Extractive Metallurgy, Western Australian School of Mines, Kalgoorlie, W.A. 6430, Australia.

* Present address: TSUMEB Corporation Ltd., Tsumeb 9000, Namibia.

0301-7516/86/$03.50 © 1986 Elsevier Science Publishers B.V.

84

1983) and AreUano (1984) have established that these forces may have a substantial effect on the performance of a magnetic separator. The question of whether a particle will be captured by a matrix element covered by particles deposited previously is determined, inter alia, by the surface character- istics of the particles. In practice, therefore, the particles to be separated should be colloidally unstable, i.e. they should not repel each other and thus prevent contact. On the other hand, the particles of the gangue minerals must not coagulate with those of the valuable mineral or they may become entrained in the magnetic concentrate.

The object of this work was an investigation on the effect of surface interactions on the magnetic separation of uranium-gold tailings and the determination of the conditions under which the metallurgical efficiency is max- imized.

THEORY

The interaction of the solid particles in a suspension that is placed in an external magnetic field is determined by the interaction of the electric double layers, Van der Waals interaction, magnetic dipolar energy, and hydrodynamic shear stress. The relative magnitudes and range of these terms determine the degree of colloidal stability of a suspension. Electric double- layer energy is repulsive for the interaction of identical particles, but can be either repulsive or attractive for the interaction of the particles with the matrix and for the interaction of the particles of valuable minerals with the particles of gangue minerals. The Van der Waals interaction is usually attractive but, in a s tudy of a three-component system, Visser (1972, 1981) has shown that, when two materials 1 and 2 are suspended in a liquid medium 3, and an inequality A,1 < A33 < A22 is satisfied (where Aii is the Hamakar constant of material i), the overall Hamaker constant is negative and the Van der Waals interaction becomes repulsive. The values of the Hamaker constant are not easily measured and they are rarely mentioned in the literature. It is therefore usually difficult for one to ascertain whether the above-mentioned inequality is satisfied or not. Svoboda and Corrans (1985) have found that, when a system of hematite and steel particles is suspended in an aqueous medium, the condition for the existence of a negative Hamaker constant is satisfied. A similar situation may arise for uraninite-steel, uraninite-gangue, and gangue-steel systems.

If the repulsive term in the total interaction energy is dominant, the activation energy prevents the particles from contacting one another. The particles cannot coagulate, and their capture on a matrix element covered by particles deposited previously, will also be hindered. If the repulsive energy is negligible compared to the attractive interactions, either because the magnetic field is sufficiently high or because the pH value of a slurry is suitable, the particles will flocculate in the intermatrix space and the probabili ty of their capture on the matrix will be enhanced.

85

The Derjaguin-Landau-Verwey-Overbeek theory of colloids (Verwey and Overbeek, 1948; Hogg et al., 1966) shows that, by the adoption of certain assumptions, the interaction energy between electric double layers surround- ing any two similar or dissimilar colloidal particles can be given by:

eaaa2(d/]4(al+a2)+¢]) r 2ff1~: 1 - e - K h l + e -Kh )] V . = L ( ~ +-~i) In + l n ( 1 - e -2~h (1)

where al, a2 are the radii and ~ 1, ~a are the surface potentials (zeta potentials) of dissimilar particles, e is the dielectric constant of the medium, r is the Debye-Htickel reciprocal-length factor, and h is the distance between the surfaces of the interacting particles, as shown in Fig. 1.

R - I ~

Fig. 1. Geometrical arrangement of the interaction of two dissimilar particles.

The Van der Waals interaction for dissimilar spheres can be expressed as:

A [ 2a~a2 2a~a2 VA = 6 h 2 + 2 ( a l + a 2 ) h +h 2+2(al+a2)h+4ala2 L

h 2 + 2(al + a2)h "1 (2)

+ In h2 + 2(a~ + aa)h + 4ala2 J It can be seen that the Van der Waals interaction is fully determined by

the magnitude and sign of the Hamaker constant A, and there are no means to modify this interaction. On the other hand, the electric double-layer energy is determined, inter alia, by the magnitude of the surface potential and by the thickness of the double layer ~-I. The magnitude of the Debye- Hiickel factor K is given by the relation:

= ( 87re2NA ) V~ r \l-~()e-~ 11/2 (3)

where e is the elementary charge, NA is the Avogadro number, kT is the thermal energy, and I is the ionic strength of the electrolyte, defined as I = 1/2 ~ cizi 2, where ci is the concentration of ions of type i, and zi is their valency. The surface potential ~ of the oxide minerals can be determined from the relationship:

86

k T - (PHezc - pHI (4~

2.303 e

where PHpzc denotes the pH value of the point of zero charge. Clearly, therefore, the electric double-layer interaction can be altered, for instance by varying the pH value of the slurry to control the surface potential and the thickness of the double layer.

The interplay of the surface potentials and the double-layer thicknesses of various minerals influences the nature of the electric interaction term, its magnitude, and its sign.

Electrostatic, Van der Waals, and magnetic dipolar interactions are of comparatively short range and come into play when the particles are separated by a distance much smaller than the particle size. They cannot, therefore, be used to account for the transport of particles, which can occur, for instance by sedimentation or diffusion, or by a magnetic traction force generated in a magnetic separator by magnetization of the matrix with a sufficiently strong magnetic field. Nevertheless, these surface forces are of vital importance in the determination of whether a t tachment occurs and of the strength of adhesion. A combination of the flocculation (ionic or magnetic, or both) of the particles, of the transport of the particles to the matrix, and of the promoted or hindered adhesion of the particles on the matrix (de- pending upon a balance of attractive and repulsive forces) will determine the performance of a magnetic separator.

Svoboda (1982, 1983) showed experimentally that, to achieve high recoveries and excellent magnetic separations for iron ores, the pH value of a slurry must be adjusted in such a way as to correspond with the point of zero charge (PZC) of the oxide mineral. The probability that magnetic particles will be captured on the matrix increases and the volume of the adhered particles rises. Simultaneously, the concentration of the oxides and hydro- oxides of the metals in the railings increases, provided that they are diamag- netic and their PZCs are far removed from the working pH value.

However, in the present investigation, the reasoning was as follows. For a high degree of selectivity in flocculation and magnetic sepm:ation to be achieved, a mixture of oxides must be dispersed (e.g. of iron oxide and quartz), which requires that, at a given pH value, the signs of both surface potentials must be the same; simultaneously, the magnitude of the surface potential of the undesired oxide must be much larger than the surface potential of the valuable mineral. Under such conditions, both types of particles will repel one another, and flocculation of the particles of the desired mineral will be selective in the vicinity of the PZC. Where the signs of the two surface potentials are different, different kinds of minerals can coagulate, and the quality of a concentrate obtained by magnetic separation will decrease. This situation is depicted in Fig. 2. If the valuable mineral is denoted by A, it will be necessary for a pH value to be used that is higher than pH(A) but as close as possible to the point pH(A), because in this region the particles

87

¢o

Mineral B Mineral A

0 H

Fig. 2. Genera l curves for the ze ta po ten t i a l s of two minerals .

of dissimilar mineral will repel one another. In the region where pH(B) < pH < pH(A), the separation will be non-selective. At pH < pH(B), the process will be selective, but the efficiency of separation of mineral A will decrease since the working pH value will be too far removed from the PZC, pH(A), and mutual coagulation of the particles of mineral A will be hindered (Svo- boda and Cibulka, 1982).

In summary, then, a high recovery can be achieved at pH = pH(A) at the expense, possibly, of the grade of the concentrate, which will be lower owing to heterocoagulation of the particles of the mineral with those of the gangue mineral. On the other hand, if quality is of importance, it will be more fa- vourable for the separation to be carried out at a pH value at which the signs of the surface potentials of different minerals are certain to be the same. However, this may result in reduced recovery because the pH value employed is not exactly at the PZC.

EXPERIMENTAL WORK

Tests were done so that the effect of the surface forces on the magnetic separation of uranium-gold tailings from the Witwatersrand-Klerksdorp area could be established. The batch high-intensity magnetic separator employed the transverse configuration in which the directions of magnetic induction and the slurry flowmte were mutually perpendicular. The volume of the separation chamber was 300 cm 3 and the height of the matrix 20 cm. The tailings were mixed with distilled water at 10% by mass and fed via a funnel through the matrix, the velocity of the flow being controlled by the size of a nozzle at the discharge end. The pH value of the slurry was adjusted in the range 1.2 to 13 by the addition of solutions of HC1 and NaOH. All tests were carried out a few minutes after the samples had been prepared so that possible variation in their magnetic susceptibility (×) due to chemical changes could be avoided. The magnetic susceptibility of the slurry was monitored for 8 hours, and the dependence of the variation in magnetic susceptibility on the time that elapsed after the preparation of the samples is shown in

88

~,1- ~ ~ ~ . A ~3 J ~

:~6_ ~ - o ~ ~'~'~"---.--..,.~ ~ ~

32-

E. 28-

24-

~ 20-

. . . . . . . . i . . . . . . . . i . . . . If) IO0

r ime (rain)

Fig. 3. The dependence of the magnetic susceptibility of the sample, adjusted to various values of pH, on time measured from the moment of preparation of the sample.

Fig. 3. It was observed that the magnetic susceptibility was almost independent of time except at very low values of pH. At a pH value of 1.2, × decreased to 50% of its original value after 400 minutes. At a pH value of 3, the decrease was substantially lower since, alter 400 minutes, × dropped by only 20%. At very alkaline values of pH, the decrease in magnetic susceptibility was even lower, being 7% at the most after 8 hours. It is therefore clear that, a few minutes after the preparation of the samples, a slight reduction in magnetic susceptibility will not affect the results, even under the most severe conditions (a pH value of 1.2). The same observations have been made for iron ores by Svoboda (1982).

The histograms for particle-size distribution are shown in Fig. 4, and the cumulative results are given in Table 1. It can be seen that 55% of the sample is smaller than 22/am, and that 20% is smaller than 5.5 gm.

The cyanide leaching residue contains the following minerals: quartz, chlorite, mica, pyrophyUite, chloritoid, pyrite, iron oxides, chromite, ilme- nite, garnet, thucholite, uranium-containing zircon, uraninite, brannerite and gold. Concentrates produced by HGMS are characterized by the presence of

I d = 16.0+Lm 12-

10'

8 -

,~ 4-,

2=

I0 I 00

Particle diameter (k~m)

Fig. 4. Particle-size distribution.

89

TABLE 1

Cumulative particle-size distribution of the Witwatersrand cyanidation residues

Size (~m) Cure. distn. (%)

<176 100 <125 94.4

<88 85.4 <62 79.9 <44 72.1 <31 63.9 <22 55.0 <16 49.4 <11 36.7

<7.8 28.1 <5.5 20.0 <3.9 9.9 <2.8 4.4

Mean particle diameter 16.0 .m .

a large proport ion of magnetic matrix minerals, chiefly chlorite and chlori- told, but quartz and other apparently non-magnetic minerals are frequently also present probably because of iron-staining and of entrainment. Only moderate proport ions of the sulphides are recovered. The most valuable components of the HGMS concentrate are uraninite, thucholite, brannerite and other leucoxene-type uraniferous minerals.

Steel balls of 6 mm diameter and woven-wire mesh were used as a matrix. The loading L of the matrix, defined as:

mass of magnetic fraction L = (g/cm 3) (5)

matrix volume

was equal to 0.11 f o r a ball matrix and to 0.14 for a mesh matrix on the assumption that 8% of the feed reported to the magnetic fraction. Since the mass yield of a mesh matrix is slightly higher than 8% and that of a steel-ball matrix slightly lower than 8%, it may be assumed that the loading of both types of matrices was the same. The experiments were carried out at two values of the background magnetic induction, namely 0.8 T and 1.1 T. The interstitial velocity of the slurry was 12.3 and 4.3 cm/s for a ball matrix and 7.8 and 2.6 cm/s for a mesh matrix. The products of separation were weighed and analysed for uranium and gold. Each test was repeated at least three times to ensure reproducibility. The arithmetic mean values of the mass, grade, and recovery of U3Os and gold were used in the analysis of the results.

90

DISCUSSION

The results of the selected magnetic separation tests are given in Table 2. The dependence of uranium recovery on the pH value of the slurry

for a mesh matrix is shown in Fig. 5 for various values of magnetic induction and flowrates. It can be seen that maximum recovery is achieved at a pH value from 1.2 to 1.8; also, with rising pH, recovery decreases, and the differences between the maximum recovery and that corresponding to the natural pH (pH 9) of the tailings amounts to about 8% for a mesh matrix and to 5% for a ball matrix, irrespective of the magnitude of the magnetic field and the flowrate.

The influence of pH on the grade of the magnetic concentrate is shown in Fig. 6. It can be seen that maximum grade is achieved at a pH value of 1.2, and that the grade decreases with rising pH, although the curves exhibit a secondary maximum at a pH value of approximately 10. The difference between the maximum grade at a pH value of 1.2 and that at a pH value of 9 amounts to at least 20% for both types of matrices and for all flowrates and magnetic fields. The opposite trend is observed for the grade of the non- magnetic fraction, as is shown in Fig. 7. The lowest grade of the tailings is found at a pH value of 1.2, and it rises to a value at least 20% higher at the natural and alkaline values of pH.

A similar dependence on pH can be observed for the mass yield into the magnetic concentrate. The minimum is found at a pH value of 1.2, and a secondary minimum appears at a pH value of about 10, as can be seen in Fig. 8.

In order to evaluate the effect of a possible dissolution of uranium from uraninite at very low values of pH the concentration of uranium in the solution at pH 1.2 was followed as a function of time after the preparation of a

T A B L E 2

Resul ts of se lected HGMS tests (mat r ix : balls of 6 mm, mesh, B = 1.1 T, V = 7.8 cm]s for mesh and 12.3 cm]s for balls)

pH Mass yield in to mags U~O~ grade of mags Recovery of U30 s (%) (g/ t ) (%)

Balls Mesh Balls Mesh Balls Mesh

1.2 6.0 8.0 997 952 47.2 60.3 1.8 6.6 9.3 973 857 50.1 62.3 3.0 7.1 10.2 963 791 46.6 56.4 5.0 7.5 9.9 936 835 47.3 55.9 8.9 8.2 11.1 821 765 49.7 55.7

11.5 8.3 11.4 840 732 46.0 56.7 13.0 7.9 12.6 842 683 45.1 57.0

Mags = magne t i c f ract ion.

91

80- Matrix: Mesh • 0.8 T, V :- 7.8cm/s o 1.1 T, V :- 7.8cm/s a 0.8 T, V :- 2.6cm/s O l . lY, V -: 2.6cm/s

~ 70-

i ~ 60- ~

50- ~ ~ ~

i ; ; i ; ; ~ ~ ; l'o ,'l ¢2 ;3 p H o f s l u r r y

Fig. 5. The d e p e n d e n c e o f U308 recovery on the pH value o f the slurry in magnet ic separ- at ion on a w o v e n wire-mesh matrix.

M a t r i x : Mesh • 0,8 T, V = 7.8cm/s o 1.1 T, V = 7.8cm/s

~ zx 0.8 T, V = 2 . 6 c m / s 900. ~ l ~ O 1.1 T, V = 2,6cm/s

i t i i i | r i i I i i ! 1 2 3 4 5 6 7 8 9 10 11 12 13

p H o f s l u r r y

Fig. 6. The e f f ec t o f the pH value o f the slurry on the grade o f the magnet ic concentrate for a mesh matrix.

suspension. It was found that after 2 min the concentration of uranium in the solution amounted to 0.4 g/t and after 30 min 0.6 g/t. We can thus assume that the uraninite dissolution and a subsequent precipitation does not seriously affect the results.

In summary, the highest grade and recovery for the magnetic fraction are obtained at a pH value of 1.2, but the mass yield is at its lowest at this pH value. Therefore, the increase in recovery at a pH value of 1.2 is clearly due to a substantial increase in the grade of the magnetic fraction and a decrease in the grade of the tailings. Magnetic separation is therefore highly selective

"(5L6I 'IlOA°"I pu~ ot.zuo~a~18 Jo~JV) '(0) a~o aood-mn!uBan pu~ '([I) al!u!uBan '(V) oao qo!a-mn.tu~an ao$ H d snsao^ i~Duo~od u~o Z "6 "~t.~I

-0~ -

, 0 [

g -Og- ~

-o l - g.

H d ,, "

Ol

8) [-Og

• x!alem qsom ~ aoj £aanls aq~ Jo onlR^ H d oq~ uo o~,~J~,uoauoa al.$eu~RoJ oq] osu[ plo!$ s ~ t u aq$ jo oauopuodop aqJ. "9 "J]!~[

(aanls JO H d ~1 [I I I O[ 6 8 L 9 ~ t' E [ l

i ~ i i t I t t i t i

S/tu39"E = A '~ 17 0

SlUa39"~ = A ' 1 8"0 V ~ ~- s1~38"1 = A 'l |H O -L I ~ .6 E

- I I ~

g -Ll N

? 4S~IN : xIJIEIN -6I

• x ! a ~ m qsam

aoj uo.Ba~aJ o!~ou~em-uou jo op~a$ oq~ uo £aanl s aql j o onl¢A H d oql jo ~ojgo aq,L 'L '$.[~I

?~JJH[S JO H d

~l ~1 II 0[ 6 8 L 9 ~ t7 E i i i i i i i i i i i i

o . . . . _ . . . . _ ~ -0~' ) ~

-09 ~

-09

~ - - ~ _ oL 8 ~

• J~ 9 " I ' [ i I ( 06 U 1 3 9 ~ ] ' l ~0V

LISOIN : X!JlUI~. u13 8 L I '1 [ I

g6

93

Fe 1 40-

20-

~" 20. \ 40-

60 -

Fig. 10. The zeta potent ial of the uranium-rieh ore in the presence of l ] and Fe cations. (After Mackenzie and Lovel], 1972)

at this pH value, and the entrainment of the gangue particles diminishes, as can be seen from the curves for the mass yield. This effect is independent of magnetic induction, flowrate, and the type of matrix. These conclusions re- semble those for the magnetic separation of hematite (Svoboda, 1982, 1983) and an a t tempt will be made to explain the experimental behaviour accord- ing to the reasoning outlined earlier in the theoretical section.

The surfaces properties of particles of uranium-rich and uranium-poor ore fractions of Witwatersrand ores were studied by Mackenzie and Lovell (1972). Their curves for the zeta potential versus pH are shown in Fig. 9, from which it can be seen that the curve for the zeta potential of the uranium-rich ore (A) is similar to the results for uraninite (B), for which the PZC is at the pH value of 3.0 to 3.2. On the other hand, the curve for the uranium-poor ore (C) is similar to that for quartz with the PZC in the pH region 2 to 2.3. Mackenzie and Lovell (1972) also measured the zeta potential for uranium ore with absorbed metal cations. (These results are of interest in the present investigation since the pulps to be treated contain some metal cations, commonly as a result of usual t reatment of the ore.) Their curves for the effect of pH on the zeta potential of uranium ore in the presence of A1 and Fe ions are shown in Fig. 10. Examination of these curves shows that the PZC of the complexes moves to the pH region 8 to 10. The most im- portant regions of pH for a complex system o f uranium Witwatersrand ores are therefore approximately 3 and 9.

In order to explain the experimental results for the magnetic separation of the uranium-gold tailings, we shall assume that the gangue mineral is quartz with a PZC equal to a pH value of 2 and with the zeta-potential curve (C) in Fig. 9. Figure 2 shows that it would be preferable for the work to be done at a pH value lower than 2 since, in this region, the potentials of quartz and uraninite would have the same sign, the particles of SiO2 and U308 would repel one another, and the separation would be selective. Unfortunately, we cannot work close to the PZC of uraninite (a pH value of 3.2), which would correspond to a high recovery, owing to the coagulation of the uraninite particles since, in this region of pH, the signs of the surface potentials of quartz and uraninite would be opposite, the minerals would coagulate, and the

9 4

separation would not be selective. Furthermore, the pH values used should be as low as possible iless than 2) since only in this region would the surface potential of quartz be sufficiently high (~20 mV) to ensure the stability of the suspension. For too low a value of zeta potential (e.g. at the PZC of quartz), the potential barrier for quartz particles would be so low that it could be overcome easily by the Brownian thermal motion, and heterocoagn- lation might occur.

On the other hand, at a pH value higher than 4, we would expect a selective separation, since the signs of the potentials are the same, and we should get a reasonably high recovery. At high values of pH that are too far removed from 4, however, the recovery should decrease, since this would be too far from the PZC of uraninite.

An examination of the correlation of these predictions with the results of the experiments shows the following. At a pH value of 1.2, a high-quality concentrate with low entrainment of the quartz was obtained; this agrees very well with the theory outlined above. The entrainment of the quartz was low because high activation energy prevented the quartz particles from heretocoagulation with the uraninite particles. This is confirmed by the low mass yield that was obtained at this value of pH. In the pH region of 2 to 4, the grade decreased very rapidly; this also agrees with the theory. The uraninite particles coagulated with the quartz particles because the signs of their potentials were opposite, as was demonstrated by an abrupt increase in the mass yield; which implies a decrease in the grade of the magnetic concentrate.

On the other hand, at pH values higher than 4 the grade and the recovery decreased and the mass yield increased; this contradicts the assumptions of the theory. An explanation for this might be found in the association of at least a part of the uraninite with the metal cations, which cause a substantial shift of the PZC towards the alkaline region, as can be seen in Fig. 10. If the presence of these ions were taken into account, it could be predicted that, in the pH region of 2 to 8 the separation would be non-selective because the signs of the surface potentials would be opposite, which would lead to a rapid decrease in the grade and an increase in the mass yield into the magnetic fraction. The experimental results confirm this speculation. For pH values higher than 8, the separation should be selective; this agrees with some of the experimental curves shown in Figs. 5 and 6, which exhibit a sudden increase in the grade.

As was mentioned earlier, magnetic separation recovers uranium as well as gold from the cyanidation residues. Since the assays for uranium are cheaper, the usual practice is for one to rely on them, but the correlation between the magnetic recoveries of gold and uranium is fairly good (Levin, 1980). The mineralogical composition of these residues is very complex, but it has been established that a large portion of the gold recovered by magnetic separation is associated with the uranium minerals (Corrans and Levin, 1979). We would therefore expect the colloidal interactions of the uranium oxides to have a direct effect on the behaviour of the gold.

95

T A B L E 3

Results of additional HGMS tests {matrix: mesh, B = 1.1 T, V = 7.8 cm/s)

pH Mass yield Grade of mags (g/t) Recovery (%) of mags (%)

Au U30 s Au U308

13.0 12.6 1.16 629 60.7 51.3 11.5 11.4 1.28 759 58.3 54.0 8.5 11.9 1.31 691 59.7 54.4 5.0 10.2 1.44 731 ~31.0 51.5 3.0 10.3 1.42 760 58.5 50.2 1.8 9.2 1.57 831 60.0 54.8 1.2 8.8 1.68 873 59.2 57.6

Mags = magnetic fraction.

It was therefore deemed of interest for gold recovery to be examined as a function of the pH value of the slurry. A series of tests similar to those de- scribed above was performed, and uranium and gold assays were done simultaneously. The results of these tests are summarized in Table 3, and Fig. 11 depicts the grade of the magnetic concentrate as a function of pH. It can be seen that the gold follows the same pattern as the uranium, exhibiting the highest grade at a pH value of 1.2 and then decreasing continuously to the alkaline region. The secondary maximum observed for uranium at pH values from 10 to 11 (Figs. 6 and 11) seems to be absent for gold. The increment in the gold content of the concentrate, which results from the adjustment of pH from the value corresponding to the usual operating conditions, viz. 9 to 1.2 is about 25%, which is even higher than that for uranium (20%). The very high selectivity of magnetic separation in the acid region is confirmed by the strong dependence of the mass yield on pH. On the other hand, the

Matrix : Mesh I,l T -900

1.9- ~ V =7,8cm/s

1.7- ~ ~ , , ~ ~ -800

~: , 5 - ~:

1.3- -700

1.1- Au" %

oH Fig. 11. Magnetic concentrate grade as a function of oH.

96

I ~ x , f

62 - \ .

sO

I ! l l

Fig. 12. The recovery of uranium and gold as a funct ion of pH.

recovery of gold does not exhibit the same pattern as uranium, as can be seen in Fig. 12. The recovery of gold is practically independent of pH, whereas the recovery of uranium follows the same course as before {Fig. 5). A maximum at a pH value of 1.2 is followed b y a n abrupt decrease at higher pH values, and the secondary maximum is observed in the alkaline region. The insensitivity of gold recovery to pH can be unders tood in terms of a combination of the following factors. The high-quality magnetic concentrate corresponds to a much lower mass yield, and the grade of the non-magnetic fraction is not dependent on pH, whereas the mass of this fraction rises with rising pH. The independence of gold recovery on pH also implies that a portion of the gold is not associated with the uranium and cannot be recovered by magnetic separation; also, it does not interact colloidally with the uranium oxides.

CONCLUSIONS

It has been demonstrated experimentally that, by alteration of the surface potential of the minerals present in uranium-gold tailings from Witwaters- rand ore, the quality of the magnetic concentrate can be enhanced substantially (by at least 20% for uranium and 25% for gold) and the recovery of uranium increased (by 6 to 8%). As a function of pH, the gold grade in the magnetic concentrate follows a pattern similar to that of the uranium grade, confirming the assumption that gold and uranium are intimately associated in the cyanidation residue. However, a certain portion of the gold is associated with minerals that do no t respond to magnetic separation and to colloidal interactions with the magnetic species, as can be inferred from the finding that gold recovery is not dependent on pH in the acid region.

The proposed theory indicates that the highest metallurgical performance of a magnetic separator can be expected at a pH value at which the surface potentials of the valuable and gangne minerals have the same sign, but which

97

is not too far removed from the PZC of the valuable mineral. Equations (1) and (2) therefore describe well the conditions leading to optimum separation. Quantitative comparison of the experimental results with the theory is, however, impossible.

The ore is very complex, and the valuable mineral is present in a variety of forms that have different surface properties, so all the aspects of the surface interactions need to be taken into account. Owing to the overall complexity of the process, further experiments on ores with well-defined particle systems are needed to confirm the theory proposed. The observed experimental results agree very well in most instances with the theoretical predictions.

We consider that improved recovery of the valuable minerals in uranium- gold railings cannot be achieved only by increase of the magnitude of the magnetic field, since this approach results in only a marginal improvement, if any (Watson et al., 1983). On the other hand, alteration of the colloidal properties of the slurries may bring about a substantial improvement in the metallurgical performance of a magnetic separator.

ACKNOWLEDGEMENT

This paper is published by permission of the Council for Mineral Tech- nology.

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The effect of pH on the recovery of uranium and gold by high-gradient magnetic separation