Potential control in the flotation of sulphide minerals and precious metals

Minerals Engineering, Vol. 8, No. 10, pp. 1151-1158, 1995

Pergamon 0892--6875(95)00080--1

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892-6875/95 $9.50+0.00

POTENTIAL CONTROL IN THE FLOTATION OF SULPHIDE MINERALS AND PRECIOUS METALS

V.V. HINTIKKA and J.O. LEPPINEN Technical Research Centre of Finland, VTT Chemical Technology, Mineral Processing, FIN-83500 Outokumpu, Finland (Received 16 June 1995; accepted 4 July 1995)

ABSTRACT In this work a method based on the use of flotation gases (air, nitrogen, etc.) is presented for potential controlled flotation. Examples of the flotation of a complex sulphide ore, and two gold ores are presented. With this control method, the potential could be kept constant within 2 to 5 mV throughout the flotation stage. The selectivity in the flotation of copper minerals from the complex ore was significantly improved under conditions of controlled potential compared to open potential conditions. At low potentials the recovery of sulphide minerals was very low whereas at high potentials created by excessive aeration the selectivity was poor. The maximum selectivity was achieved at about 0 mV vs. SHE. In the flotation of gold ores, the recovery of iron and arsenic minerals such as pyrite and arsenopyrite could be minimized by using potential control. The optimum potential for gold flotation was +250 to +300 mV vs. SHE.

Keywords Flotation; sulphide minerals; complex ores; Eh; potential control; gold ores

INTRODUCTION Convincing evidence exists today in regard to the significance of E h to the flotation of sulphide minerals and precious metals [ 1,2]. It is well established that the development of hydrophobicity on sulphide minerals and metals arises from an anodic process of collector which is coupled with a cathodic process such as reduction of oxygen [3]. Since the anodic reactions give rise to the hydrophobic character of the mineral surface [4], a clear relationship between the potential and flotation recovery exists in various flotation systems. Monitoring of potential with noble metal electrodes [5], such as platinum, is now almost routinely carried out in laboratory testwork and full-scale processes. A more sophisticated way of potential measurement can be carried out with sulphide mineral electrodes prepared of pure minerals or minerals from the particular orebody [6]. Although the monitoring of E h in the flotation is well established a major practical problem is still the actual method of potential control. Different reagents can be used for E h control, such as sodium sulphide, hydrazine, sodium dithionite, sulfur dioxide, hydrogen peroxide, etc. The disadvantages of potential controlling chemicals are high consumption of reagents and chemical side-reactions which modify mineral surfaces in the pulp or cause decomposition of flotation collectors. High consumption of reducing agents is generally due to the fact that they are excessively consumed by the oxygen in the flotation gas. Presented at Minerals Engineering '95, St. Ives, Cornwall,England,June 1995

1151

1152

V . V . HINTIKKA and J. O. LEPPINEN

Air is the most common gas used in flotation and the minerals in the flotation pulp are subjected to free aeration. Most of the sulphide flotation plants operate at 'air set' potentials which are usually +100 to +300 mV vs. SHE. The potential level in aerated systems is often too oxidising for maximum selectivity. This is based on the fact that each mineral has a characteristic potential where the flotation starts [7]. At too anodic potentials the potential ranges for individual minerals overlap causing simultaneous flotation of several minerals and consequently poor selectivity. Reduced selectivity due to excessive oxidation may also result from oxidation products of easily oxidative minerals, such as pyrrhotite and arsenopyrite. These products can attach unselectively on several minerals in the pulp enhancing their floatability. Reduced selectivity can also arise from the formation of elemental sulphur, thiosulphate, metal hydroxides and other surface layers [8] frequently detected in aerated mineral slurries. In flotation processes where excessive oxidation causes problems, reducing agents are sometimes used to restore potential to the optimum level. The adverse effect of the oxidation products already formed at anodic potentials, however, cannot be completely eliminated in this manner because of the irreversibility of the reactions. To create the optimum conditions, the formation of undesired oxidation products should be completely eliminated e.g. by using nonoxidising gas such as nitrogen. This paper presents a technique for accurate potential control based on the use of flotation gases. Examples of the performance of the control system are given in the flotation of a complex Cu-Zn-Pb sulphide ore and two gold ores.

EXPERIMENTAL Method of potential control The 'advanced flotation system' developed by Mineral Processing of the Technical Research Centre of Finland is presented in Figure 1. The principle of this method is that at least one other gas besides the oxidative gas is used in the machine. The selection of the gas mixture is based on the potential level in the pulp which is monitored either by mineral electrodes prepared from the individual ore or by metal electrodes in the case of precious metal flotation.

WATER TACHOMETER

~ t I

I 7 POT. METER I . . . . . TITRATOR ~ 1 BURETTE r-~

TITRATOR

- ;i-

[ f= I t

pH METER FLOTATION I MACHINE ) NITROGEN

FLOWMETERS

L

, ~

'l

l---I

....

I

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

AIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

MICROCOMPUTER

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..........

IH II II

ill II ~I

FLOTATION

',"" ~'

---7

INLET O F pH-POTENTIAL

7~

pH ELECTRODE REF. ELECTRODE POT. ELECTRODE

Fig. 1 Schematic diagram of the advanced flotation system applying potential control with gases.

Potential control in flotation

1153

In grinding, the potential is usually low due to the reducing conditions created in the mill. The redox conditions in grinding can also be controlled with potential modifying reagents added in the mill. Usually after grinding in a normal steel mill, the potential level is too low for significant flotation recovery of sulphide minerals or precious metals. In this flotation system, air is first added in the flotation gas so that the desired potential level is reached. As soon as the desired potential level for the flotation of an individual mineral is obtained, the aeration is stopped and an inert flotation gas, usually nitrogen, is used. After this stage air is fed to the flotation gas through a titrator unit only to maintain the desired potential level. With this control system potential can be adjusted within 2 to 5 millivolts during the flotation period. Materials The complex ore sample was provided by Pirites Alentejanas S/A, Aljustrel Mine, Portugal. The lump sample was jaw crushed and dry screened to -1 mm and stored in a freezer under nitrogen. One kilogram of the crushed ore was ground in a normal steel mill or in a stainless steel mill. For flotation, the grinding time was 45 min resulting in 80% passing size of about 30 Wn. The complex ore sample contained 88% pyrite. The main copper mineral was chalcopyrite which contained over 90% of the total copper. Other copper-bearing minerals were tetrahedrite and bournonite. The zinc occurred as sphalerite and lead as galena and bournonite. The chemical composition of the sample is presented in Table 1. TABLE 1 Chemical composition of the complex C u - Z n - P b sulphide ore sample

Cu %

Zn %

Pb %

Fe %

S %

As %

Au g/t

Ag g/t

0.62

3.16

1.13

37.8

48.0

0.74

0.4

28

The Saattopora gold ore sample was from Kittil~i, Finland and the Piril~i gold ore sample was from Rantasalmi, Finland. The gold ore samples were crushed to 100% -1 m m and stored in a freezer. The Saattopora ore sample was ground for 20 min resulting in a fineness of 84% - 7 4 ~rn. The grinding time for the Piril~i ore sample was 30 rain and the corresponding fineness 67% - 7 4 prn. The main sulphide minerals in the Saattopora ore sample were pyrrhotite and pyrite whereas the Piril~i ore contained 1611ingite (FeAs2), arsenopyrite, pyrrhotite and pyrite. The Saattopora sample contained 18 % dolomite, 4% calcite, 40% plagioclase and 9% quartz. The predominant gangue minerals in the Piril~i ore sample were quartz accounting to about 70%. The chemical composition of the ore samples is presented in Table 2. TABLE 2 Chemical composition of the gold ore sam des Ore

Au g/t

Ag g/t

Cu %

Fe %

Pb %

S %

As %

Saattopora

3.7

0.5

0.23

8.38

<0.001

3.80

0.009

Piril~i

8.4

16.5

0.17

5.46

0.27

3.03

1.39

After 20 rain grinding, gold occurred predominantly as free particles in the Saattopora ore sample. Part of the gold was locked in sulphide minerals such as pyrrhotite. In the Piril~i ore sample, most of the gold occurred as 32 to 90 pm free particles. Minor portion of the gold occurred locked in arsenopyrite and lOllingite. The gold in the Pirilii ore sample was almost totally electrum with a wide range of silver content. The lowest silver content was 6% and the highest about 80%.

V.V. HINTIKKA and J. O. LEPPINEN

1154

Flotation experiments

The flotation experiments were carried out with 1 kg samples in a 4 liter cell using the control system described in this paper. Potassium amyl xanthate was used for the complex ore flotation and a Aerophine 3418A (iso-propyl dithiophosphinate) for the gold ores. Natural pH was used in the flotation of gold ores whereas the flotation of complex ore was carried out at pH 11.5. The natural pH of the Pirilii and Saattopora gold ore was 7.0 and 8.2, respectively. The products from the flotation experiments of the complex ore sample were analysed for Cu, Zn, Pb, Fe, As and S. In the flotation of the gold ore samples Au, Ag, Cu, Pb, As, Fe and S were analyzed. In the flotation experiments of gold ores, potentials of gold, gold-silver alloys, pyrite, pyrrhotite, chalcopyrite, arsenopyrite and 1611ingite electrodes were monitored. In the flotation of the complex ore sample chalcopyrite, pyrite, galena, arsenopyrite, platinum and gold electrodes were monitored. A calomel reference electrode was used. All potentials reported are vs the standard hydrogen electrode assuming that the standard electrode potential of the calomel electrode is 0.244 V.

RESULTS AND DISCUSSION Flotation of the complex C u - Z n - P b ore sample

When grinding of the Aljustrel Cu-Zn-Pb complex ore was carried out in a normal steel mill [9] the potential was low (Figure 2). Without control, the potential of chalcopyrite electrode was about -350 mV during conditioning with lime at pH 11.5. At this stage the respective value for pyrite electrode was approximately the same as the potential of chalcopyrite. Due to aeration in the flotation stage, the potential level shifted from -350 mV to more anodic values ending at about +100 to +150 mV. At the end of the flotation stage the potential of pyrite electrode was 30 to 90 mV higher than that of the chalcopyrite electrode.

200 C O M P L E X Cu-Zn-Pb ORE .-w -r oo

100 o

E

"---100 ...I

CONDITIONING /

j A J

/

//

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\\

-200

/

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-300 -400

L

i

i

I

i

i

~

i

2

3

4

5

6

7

8

9

10

STAGE Fig.2 Potential of chalcopyrite electrode in the flotation experiment of a complex ore sample at open potential conditions. Stage 1: thick slurry after grinding, Stage 2: dilution to 25 w-%, Stage 3: SO 2 conditioning, Stage 4: pH adjustment; Stage 5: KAX conditioning, Stages 6 to 10: flotation 1 to 15 min.

Potential control in flotation

1155

Under conditions of open potential, a pyrite-rich pre-float was obtained at low potential in the beginning of the flotation stage. The reason for pyrite flotation at abnormally low potentials was not studied in this work, but it can be due to oxidation products present in the ore sample or specific conditions for incipient oxidation of pyrite leading to collectorless flotation mechanism. The flotation behaviour of pyrite at very low potential is in line with the findings of Trahar et al. [10] indicating an overlap with the potential range of collectorless flotation of pyrite. In flotation tests under controlled potential, where the redox level was elevated directly to a more anodic value the pre-float effect was not distinct. In these conditions, the flotation rate of copper sulphides was obviously much higher than the flotation rate pyrite and the pre-float effect was not dominating. At -170 mV the recovery of chalcopyrite was low whereas the recovery of galena was relatively high (Figure 3). At more positive potentials (-40 mV on chalcopyrite electrode) the recovery of copper minerals was significantly higher that at -170 mV and the selectivity against lead and zinc minerals was high. At +40 mV the selectivity of copper flotation against zinc sulphides was markedly decreased whereas at +110 mV the selectivity decreased also against lead sulphides. An EDTA analysis [11] of the ore slurry at different potential conditions indicated that high recovery of lead and zinc corresponded to high EDTA soluble lead and zinc. Consequently, at high potentials, oxidation products of lead and zinc were formed and the interaction with amyl xanthate was enhanced. This effect may also be due to formation of elemental sulphur or a sulphur-rich surface on galena and sphalerite.

80

f--------A

70 60

Cu

V

>.. n," t.LI

> O 0

ILl n,"

5O

-o-

Zn

40

-0-

Pb

>< S

3O 20 10 0

-200

I

L

l

I

L

I

-150

-100

-50

0

50

100

150

POTENTIAL (mV vs SHE) Fig.3 Effect of potential on the recovery of elements from a complex Cu-Zn-Pb ore. KAX 150 g/t, SO 2 400 g/t, pH 11.5, flotation time 10 min.

Flotation of the gold ore samples On the Saattopora ore sample the recovery of gold increased as a function of potential measured with a gold electrode (Figure 4). At potentials lower than +50 mV the recovery was below 20% whereas at +270 mV, the uppermost level achieved with this control method, the recovery was over 80%. It was surprising to note that the recovery of gold decreased at +230 mV. The origin of this negative hump has not been characterised but it may be due to adsorption of metal hydroxides on the surface of gold or changes in the collector layer. The highest Au grade in the rougher concentrate, 40 g/t, was obtained at +270 mV while the Au grade was only 20 g/t at +200 mV. This is due to the dilution effect of iron sulphides in the

1156

v . v . HINTIKKAand J. O. LEPPINEN

concentrate at +200 mV. The recovery of copper minerals from the Saattopora ore increased up to +200 mV and levelled off at +230 to + 270 mV range. The recovery of iron sulphides was highest at +50 mV while the recovery started to decrease at higher potentials.

100 --A- AU

CU

80

o~ >n~ uJ

> O 0

-~-S

jJ~ j J r

60

Oj

40

ILl r~

20 SAATTOPORA ORE 0

I

L

I

I,

I

50

100

150

200

250

300

POTENTIAL (mV vs SHE) Fig.4 Effect of potential on recovery of elements from the Saattopora gold ore sample. Aerophine 3418A 40 g/t, pH 8.2, flotation time 12 min. The recovery of gold also increased as a function of potential on the Piril~i ore sample (Figure 5), but unlike the Saattopora ore, no drop in the recovery was observed at about +230 mV. The recovery of copper minerals reached the maximum values at +150 to +200 inV. The behaviour of iron sulphides was essentially the same as in the flotation of the Saattopora ore sample. The recovery of arsenic was highest at ca. +150 mV followed by a light drop at +200 mV and rapid decrease at higher potentials. The high silver content of the gold in the Pirila ore was reflected as higher recovery of gold at low potentials compared to the Saattopora ore sample with very low silver content. It is well established that collectors react with silver and gold-silver alloys at lower potentials than with pure gold [12] which is also seen in the potentials of various electrodes in the pulp. When the potential of gold electrode was +200 mV the potential of the gold-silver (50:50) electrode gold was +1 I0 inV. The potential drop of arsenic at +200 mV can be explained by the two arsenic minerals 1611ingite and arsenopyrite which are expected to have different flotation behaviours. According to electrochemical studies, 1611ingite is likely to float at lower potential than arsenopyrite resulting in a recovery curve having two separate parts which explain its peculiar shape. Even though not shown in this paper, the flotation of the Pirilii ore sample with ethyl xanthate as the collector resulted in very low recovery of gold and other minerals below +50 mV while the recovery was about 75% at +250 mV. The recovery of arsenic minerals was highest at +150 mV and levelled off at higher potentials. With ethyl xanthate, the recovery of iron minerals increased as a function of potential and no decrease was observed at +250 inV.

Potential control in flotation

1157

100 -A-

80

Au

- e - Cu -4)- As

_~

60

×

Fe

20

0

0

I

I

I

I

I

50

100

150

200

250

300

POTENTIAL (mV vs SHE) Fig.5 Effect of potential on recovery of elements from the Piril~i gold ore sample. Aerophine 3418A 40 g/t, pH 7.0, flotation time 12 rain.

CONCLUSIONS An advanced flotation system with potential control using flotation gases was presented. Examples of the performance of the system were presented on a complex sulphide ore sample and two different gold ore samples. The following conclusions can be drawn: --

potential can be accurately controlled using flotation gases optimum selectivity is achieved in the copper flotation of complex sulphide ore at about 0 mV vs. SHE measured with a chalcopyrite electrode the recovery of iron sulphide minerals (pyrite and pyrrhotite) arsenic minerals) 1611ingite and arsenopyrite) can be minimised in the flotation of gold ores

ACKNOWLEDGEMENTS The authors thank Outokumpu Finnmines Oy and Outokumpu Mining Services for permission to publish the flotation results of the ore samples.

REFERENCES .

RE 8-10-F

Heimala, S., Jounela, S., Rantapuska, S. & Saari M., New potential controlled flotation methods developed by Outokumpu Oy. Proc. XV Int. Min. Process. Congr., Cannes, France, III, 85-98 (1985).

1158 .

3.

,

5. 6.

.

.

9.

10. 11.

12.

v.v. HINTIKKAand J. O. LEPPINEN Ralston, J., Eh and its consequences in sulphide mineral flotation, Minerals Engng., 2, 859-878 (1994). Woods, R. & Richardson, P.E., The flotation of sulfide minerals - - electrochemical aspects. Chapter 9 in Advances in Mineral Processing, Somasundaran, P. (editor), SME, Littleton, Colorado, 154-170 (1986). Leppinen, J., Basilio, C. & Yoon R., In situ FTIR study of ethylxanthate adsorption on sulfide minerals under controlled potential. Int. J. Min. Process., 26, 259-274 (1989). Labonte, G. & Finch, J.A., Behavior of redox electrodes during flotation and relationship to mineral flotabilities. Minerals & Metallurgical Processing, 106-109 (May 1990). Heimala, S., Jounela, S. & Saari M., Flotation control with mineral electrodes. Proc. XVI International Mineral Processing Congress, Stockholm, Forssberg, K.S. (editor), Part. B., Elsevier, 1713-1718 (1988). Richardson, P.E. & Walker, G.W., The flotation of chalcocite, bornite, chalcopyrite and pyrite in an electrochemical flotation cell. Proc. XV Int. Min. Process. Congr., Cannes, France, 198-210 (1985). Smart, R., Surface layers in base metal sulphide flotation. Minerals Engng, 4, 891-909 (1991). Leppinen, J.O., Laukkanen, J.M. & Palosaari, V., Modern mineralogical and flotation control methods in selective flotation of complex sulphide ores. First Symposium on the Polymetallic Sulphides of the Iberian Pyrite Belt, Evora, Portugal, (3-6 Oct., 1993). Trahar, W.J., Senior, G.D. and Shannon, L.K., Interaction between sulphide minerals - - the collectorless flotation of pyrite. Int. J. Min. Process., 40, 287-321 (1994). Senior, G.D. & Trahar, J.W., The influence of metal hydroxides and collector on the flotation of chalcopyrite. In Flotation of Sulphide Minerals, Forssberg, K.S.E. (editor), Elsevier, 321-341 (1991). Leppinen, J.O., Mielczarski, J.A. & Yoon, R.-H., FT-IR studies of ethyl xanthate adsorption on gold, silver and gold-silver alloys. Colloids and Surfaces, 61, 189-203 (1991).