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Redox control in a pilot flotation column
Minerals Engineering, Vol. 9, No. 1, pp. 73-83, 1996
Pergamon 0892--6875(95)00132-8
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892-6875/96 $15.00+0.00
REDOX CONTROL IN A PILOT FLOTATION COLUMN
B.I. PALSSON and H. PERSSON* Department of Chemical and Metallurgical Engineering, Lule~ University of Technology, S-971 87 LuleL Sweden Present address: H6gan~is AB, S-263 83 H6gan~is, Sweden (Received 26 July 1995; accepted 30 September 1995)
ABSTRACT Results from pilot-column flotation tests on a lead-zinc ore show that it is possible to control the redox potential of the column pulp by varying the oxygen activity of the flotation gas fed to the spargers. As a result, the process changes so that the fine zinc minerals are more actively floating in an oxidising environment, at the same time xanthate and frother consumptions increase. The placement of electrodes in the column does change the redox potential readings but only as a result of a change in the oxygen concentration of the pulp. However, the values from a Glassy-Carbon (GC) electrode are consistently lower than those from a Pt-foil electrode at the same location. Actual positions of operating points as measured by GCelectrodes fall within the theoretical area for precipitation of lead xanthate.
Keywords Sulphide ores; column flotation; redox control; process control
INTRODUCTION Over the last decades, considerable attention has been focused on pulp potentials and their influence on the results of sulphide mineral flotation [ 1]. Several investigators have dealt with potential-controlled laboratory flotation tests, among them Heyes and Trahar [2,3] and Guy and Trahar [4]. These tests generally used oxidising or reducing agents such as hypochlorite and dithionite respectively. Another way to control the pulp potential of a flotation system is to use gases with varying oxygen activity, achieving a pure redox effect in the pulp. This has been reported for mechanical cells at laboratory [5] and pilot-plant scale [6] by Berglund and Forssberg. It is not easy, however, to precisely introduce and recover gases (e.g., nitrogen) from mechanical cells. With the flotation column such a device emerged. In a chemical engineering sense, it can be regarded as a chemical reactor. It has been used as such in copper-moly separations [7], although the analogy to the chemical reactor seems not to have been drawn. Otherwise, very little appears to have been done on deliberately controlling the oxidising environment of the column. The only exception may be the work on nitrogen flotation in pyrite/sphalerite flotation by Martin et al. [8].
Presented at Minerals Engineering "95, St. Ives. Cornwall, England, June 1995
73
74
B. 1. P~lsson and H. Persson
Given this background, the intention o f the reported project was to study the concentration chemistry of the flotation column, especially methods for controlling the oxidation environment within the column. The project should to try to answer if - - and in what way - - does the pulp chemistry in a column differ from that in a conventional mechanical cell.
EXPERIMENTAL
Grinding and Flotation The test material was a lead-zinc ore from the Laisvall mine of Boliden Mineral AB, Sweden. This ore contains 3.6% Pb and 0.8% Zn as impregnations o f galena and sphalerite in a sandstone hoist rock. An ore sample o f 2.5 tonnes of ore pebbles (40-80 mm) was taken from the pebble screen in the crushing station by the concentrator personnel. The reason for selecting ore pebbles as the test material was that it is a dry material stream and the risk of inadvertent oxidation is small, due to the low amount o f fine particles in the sample. On arrival to LuleA, the bulk ore sample was crushed dry in three stages to d90=2 mm, and stored in the drums it was delivered in. The column installed at the Division of Mineral Processing, L u l d t University of Technology, is a Deister type, delivered by S A L A International AB. It is 3.5 m in height and has a diameter of 7.5 cm. On two levels, it has external bubble generators by Deister. It was modified to our specifications, with four one-inch sampling ports (arranged in a cross) installed for every half a meter, starting at the one meter level. Circuit layout is given in Figure 1. Besides sampling, the one-inch ports are used for inserting electrodes into the column. This is done in a very simple way. A hole for an electrode is drilled through a rubber stop cork, which is then squeezed into a sampling port.
4
4.0m
5 Feed
D
2
3
0
©
3.5 m
Fig. 1. Pilot-column flotation circuit. 1) flotation column 75 m m diameter x 3500 m m with two acrylic sections, float-sensor pipe and sampling ports; 2) peristaltic pump - feed; 3) speed-controlled peristaltic pump - tailings; 4) speed-controlled mixer 70 litres; 5) mild steel rod mill 600 m m diam. x 900 ram; 6 - - - -location of gas spargers.
Redox control in a pilot flotation colunm
75
Otherwise, the circuit consists of: a mild steel rod mill 600 m diameter x 900 mm; a 70 litres mixing tank with speed-controlled impeller; a controlled-speed peristaltic pump for feeding the column; the same type of pump for pumping the tailings. The latter is controlled not only by a manually pre-set value, but also through a float in a communicating side-pipe to the column. In this way the interface between pulp and froth zone is kept constant. The mixer and the pumps are driven by DC motors, which are controlled by rheostats. Pulp flows are checked by point sampling and through measuring the decrease of the pulp level in the mixing tank. Gas and liquid flows are manually controlled by needle valves and the flows are read by rotameters. Frother is added to the water for the external bubble generators and the amount of frother is regulated by a membrane pump with variable stroke frequency. Collector is added to the mixing tank before each test. Batch-wise rod milling is done in the 600 mm diameter x 900 mm mild steel rod mill. It is modified to have a discharge slot on the mill end. The slot is covered by a movable sliding steel lid. At discharge time a frequency transformer is coupled between the net and the AC motor, the mill is rotated slowly into discharge position and the material is flushed out into the mixing tank through an 850 pm chip screen. For 50 kilograms of Laisvall ore, pre-crushed to <3 mm, a grinding time of 20 minutes gives d80=125 pm and 33% <44 pm. For mixing and homogenising a small barrel is used. Any coarse particles from the mill are trapped by the 850 pm chip screen mounted on the barrel. The correct feed pulp density, 48-50 weight-%, is set in the mixing tank. From the bottom of the barrel the pulp is delivered by a peristaltic pump to the column. The hose dimension was selected as 6 mm i.d. to as far as possible prevent sedimentation in the hose. Preliminary tests with the column showed that the flotable amount was approx. 5% of the feed rate, therefore, it was possible to run the column with only the lower bubble generator. This also gave the positive side effect that the dilution ratio in the column decreased. Since the first tests showed that it was not possible to use ordinary tap water as wash water, a small amount of frother was added in the wash to stabilise the froth. To avoid frothing in the float-pipe, a small amounts of water was kept running down the float-pipe. The tests were run at natural pH. The only reagents added were Na-isopropyl xanthate and the frother DowFroth 250. Both reagents were of technical grade and delivered from Laisvall. Collector addition rate was kept at 25 g/tonne, except in the oxygen tests where it had to be raised to 40 g/tonne. Gases were commercial high-purity nitrogen and oxygen (AGA Gas AB), synthetic mixtures with 5 and 50 mole-% oxygen in nitrogen (AGA Specialgas AB), and compressed air from the laboratory net. In the trials, the primary interest was in comparing the extreme gas atmospheres with ordinary air flotation as a reference. Therefore, several tests were run with oxygen and nitrogen as the flotation gas. Tests with 5% and 50% oxygen are extra tests to check the general tendencies of the oxygen-nitrogen-air experiment. The column was operated visually as far as the froth appearance with collector and frother adjusted accordingly.
Pulp chemistry Battery operated pH-meters (PHM80, Radiometer) were used to measure: pH with a combination electrode (GK2501B, Radiometer), pulp potentials with Pt-foil (P101, Radiometer) and Glassy-Carbon (Metrohm) electrodes against a common calomel electrode (K401. Radiometer). Pulp potentials are reported on the hydrogen scale. Electrode readings were taken on two levels, a lower at 1.0 m, and a higher one just above the teed point at 2.5 m. During a test, the lower level was used for the initial measurements until a pulp sample was taken, after that the pH-meters were moved to the higher level and the measurements continued with the electrodes installed there. The oxygen electrode (Bibby SM01) was installed at the lower level only.
76
B.I. P~lsson and H. Persson
In every test, a small pulp liquid sample (2 cm3) was withdrawn and filtered on 0.22 [am membrane filter following a standard procedure [9]. This sample was then injected into an ion chromatograph (Dionex 2010i with HPIC-AS4 column) for analysis of chloride, nitrate, sulphite, sulphate, thiosulphate, perxanthate and xanthate [10]. An additional larger pulp sample (50 cm 3) was collected and filtered on 0.22 [am. It was acidified with nitric acid (suprapur) and sent to an outside laboratory for analysis of metals in solution.
RESULTS AND DISCUSSION General Typical material, pulp, water and gas flows from a test with air as the flotation gas are given in Table 1. It is clear that the water added through the external bubble generator has the greatest influence on the water balance of the column.
TABLE 1 Measured flow data for test Air-2. Stream
Water to float-pipe Wash water Sparger water
Solids flow (kg/min) ----
Water flow (1/min) 0.068 0.230 1.13
Dilution ratio ----
Froth product Tailings Feed
0.031 0.45 0.47
0.065 1.42 0.53
2.10 3.16 1.13
%Zn
%Pb
Comment
m
_
_
m
Gas flow: 2.83 l/rain 50.7 0.37
6.34 0.26
In Table 2, key data from tests with different gases are shown. The residence time is calculated as the pulp zone volume divided by the tailings flow rate. The amount of floated material per minute and surface unit is the "froth capacity". Since the tests were not always mn to the flotation limit of the column it is not the "carrying capacity" used by other investigators. The net downward water flow is the bias and it is expressed as a velocity. In all cases the gas flow rate was kept constant at 1.07 cm/s. In the tests with 5% and 50% oxygen problems with the feed rate, occured shown in Table 2 as longer residence times. Due to the feeding problem gangue material started to float and the bias became negative for the 5% oxygen test.
TABLE 2 Key data (averages) Test Nitrogen 5% Oxygen Air 50% Oxygen Oxygen Literature values
Feed (kg/min) 0.53 0.36 0.48 0.33 0.50
Residence time (min) 6.8 10.3 7.6 8.6 7.0
Froth capacity (g flot/(min.cm2)) 0.45 1.15 0.84 0.82 0.86 1-5
Bias (cm/s) O.O56 -0.013 0.044 0.032 0.046 0--0.3
Redox control in a pilot flotation column
77
Process results The tests have been run as rougher flotations giving concentrates with 40-50% Pb at approx. 90% recovery in the tests where the feed rate was kept constant, cf. Figure 2. The lower concentrate grade in the test for 5% and 50% oxygen depends on the lower feed rate in these cases. It is not possible to declare any flotation gas better than the other. 60
I
I
'
I
I
[]
50 []
40 ¸
[]
v
O
(1)
-o 3 0 -
@
-Q 20 I:L
•
Nitrogen
•
Air
o
50% Oxygen
[]
Oxygen
5% Oxygen 10
I
50
'
60
I
I
I
70
80
90
00
Pb Recovery (%) Fig.2 Grade-recovery for lead. However, the selectivity diagram in Figure 3 shows a difference, that a pure nitrogen atmosphere gives better selectivity between lead and zinc. Tests with 5% and 50% oxygen are inferior, probably because of the lower feed rate. From Figures 2 and 3, a general observation can be inferred that the points represent different operating positions only on the grade-recovery and selectivity curves.
100
'
I
'
!
'
~
80
v
co
60 ¸
.-~
.Q ..--
........ -.... 40 ¸
.....,. .........
.,,..'
o_
....
a
c-
N 20 ¸
J o
[3
o 50
5% Oxygen Air 50% Oxygen Oxygen "
610
l ................... 710
I
80
Pb Recovery (%)
Fig.3 Selectivity between lead and zinc.
,i0
100
78
B.I. P&lsson and H. Persson
Recoveries by size fraction for lead and zinc are shown in Figures 4 and 5 respectively. It is not possible to read any tendency from the lead diagram. The zinc diagram shows that the 5% and 50% oxygen tests differ for reasons given earlier. Otherwise the pattern is to be expected from a flotation chemistry point of view. A nitrogen environment gives the lowest zinc activation, and the oxygen flotation the highest zinc content in the concentrate. Air flotation gives approximately the same results as for the oxygen environment. The second sample for oxygen was taken a little early and so the zinc recovery is lower. It is included here to give an estimate of how much the results can differ if the conditions are changed slightly. 100
80
v
60
o
o o
40
rr c~ n 20
0 <38
38-53
53-75
75-106
>106
Particle Size (pm) Fig.4 Lead recoveries by size. 100
80 s'-",-.
v
60
(D
> o o o
40
rr c N 2O
<38
38-53
53-75
Particle Size (urn) Fig.5 Zinc recoveries by size.
75-106
>106
Redox control in a pilot flotation column
79
The reason for the more active zinc mineral might rise from two different mechanisms: activation of sphalerite by lead ions released into the pulp liquid as a result of galena oxidation, or an indiscriminate flotation due to dixanthogen formation on sulphide mineral surfaces. Flotation tests were carried out at low collector dosage, giving an initial xanthate concentration of 5.10 -5 M in the pulp liquid. This should be compared with reported [ 11 ] dixanthogen solubilities of 1.14.10 -5 M (ethyl) and 1.5.10 -6 M (n-propyl). The actual solubility of the dixanthogen formed from isopropyl xanthate should fall between these two values. It is not conceivable, therefore, that dixanthogen induced flotation of sphalerite is responsible for the increased zinc appearance in the froth product. Furthermore, the sphalerite from Laisvall is colourless to pale yellow, implying a low iron content. Such a sphalerite has been found electrochemically inactive [12]. This means that dixanthogen formation as an effect of oxygen reduction on the mineral surface is unlikely. Therefore, the increase in zinc flotation activity is considered to be a case of autogenous lead ion activation.
Pulp chemistry In all tests, samples of the pulp liquid were taken for analysis of dissolved metals and anions. Anions were analysed by an ion chromatograph and a summary of the analyses is shown in Table 3. Cation analyses were found to reflect changes in pH mainly, and are therefore not included here.
TABLE 3 Anions in pulp liquid. Test Nitrogen 5% Oxygen Air 50% Oxygen Oxygen
Cl-
SO~-
SO~-
$20~-
(mg/1)
(mg/1)
(mg/l)
(mg/l)
Xanthate (laM)
Perxanthate (pM)
28 19 19 18 26
34 26 28 15 35
-1.0 1.1 0.8 --
-0.3 0.3 0.5 0.4
8.2 2.2 3.6 2.1 6.5
--2.7 1.4 7.4
It can be seen that the different flotation gases give different pulp environments. This is mirrored by the variation in anion contents. In the least oxidising environment, i.e., nitrogen, there is a higher residual xanthate ion level in the pulp liquid, but no perxanthate, sulphite or thiosulphate. For the most oxidising environment xanthate is found as perxanthate, and there is no sulphite. The other tests fall between. In the tests, pH and redox potentials were measured at two levels, while oxygen was only measured at the lower level, cf. Table 4. The pH measured at the higher level is always a little higher and the redox level lower, except in the nitrogen test where the redox level is about the same if the pH difference is compensated for. A probable explanation is that the oxygen content of the sparger water itself is not nil. This is supported by the oxygen level never falling below 2 mg/1, not even for the nitrogen test. The Ptelectrode always shows a higher reading than the Glassy-Carbon at the same location, except in the nitrogen test where the readings are close. Both observations point to the same explanation, that the sensitivity of the Pt-electrode for the oxygen reduction on the electrode surface causes abnormally high readings in the low ionic strength, lead-zinc pulp environment. The same phenomena have been noticed in laboratory flotation tests of copper-lead separation [13], where it was also observed that the variations in potential levels as measured by the Glassy-Carbon electrode correlated better to variations in readings from oxygen and galena electrodes, than the ordinary Pt-foil electrode. If the redox potentials are plotted on a pe-pH-diagram (cf. Figure 6), the values for the Glassy-Carbonelectrode fall within the theoretical precipitation area for the stoichiometric lead xanthate, in this case Pb(iPX)2(s). Here, pe is a quantity directly related to the potential of the solution through the equation pe = Eh/g
(1)
80
B.I. P~lsson and H. Persson
where E h is a redox potential, g = R T / F . I n l 0 (=59.16 mV at 25o C and 1 atm), with R=the gas constant, T=temperature in Kelvin, F=Faraday constant.
TABLE 4 Electrode data (at time of sampling). Level
Low
High
Test
pH
Nitrogen 5% Oxygen Air 50% Oxygen Oxygen Nitrogen 5% Oxygen Air 50% Oxygen Oxygen
Eh(Pt) (mV) -34 289 348 391 437 6 201 284 326 373
7.55 8.42 7.71 8.30 7.90 8.31 8.39 7.93 8.39 8.07
02
Eh(GC) (mV) -42 128 182 234 251 -4 86 133 223 222
(mg/1) 2.8 2.2 2.8 4.7 19.4 ------
Temp (°C) 14 18 18 18 14 ------
The diagram was calculated with the computer program S O L G A S W A T E R [14] and thermodynamic data were kinetically adapted to the Laisvall ore from previously published galena [15] and sphalerite [16] systems. Total concentrations (Tc) of each component were calculated to approximate an average condition in the flotation column. They are: Tc(Pb)=0.26 mM, Tc(Zn)=0.09 mM, Tc(S)=0.35 mM, and Tc(iPX)=0.05 mM. The pulp systems were considered essentially carbonate-free, since the tests were run in a closed vessel with a controlled gas atmosphere.
108-
6-
pb 2+
Pb304
PbOH +
4Pb(iPX) 2
•
2Q..
t Pb(OH)iPX
o-2-4-
PbS
-6 -
-8-10-
•r
5
T
i
|
7
r
T
'I"--
8
"'-
9
-
10
pH Fig.6 Predominance diagram for lead - non-carbonate system. --- maximum extension area of lead xanthate species; o operational points measured with Pt-foil electrode; • operational points measured with Glassy-Carbon-electrode.
11
-
12
Redox control in a pilot flotation column
81
Flotation reagents The distribution of the total amount of isopropyl xanthate added, expressed in molar units, on residual levels in pulp liquid and what is assumed to have been adsorbed or consumed is shown in Figure 7. Interestingly, perxanthate only appears in tests with an oxygen activity equal to air or higher. As shown by Jones and Woodcock [17] perxanthate is formed from xanthate by a reaction with peroxide, accordingly (2)
R - O - CS.S- + H202 .-, R - O - CS.SO- + H20
where R denotes an alkyl chain. Since we have not found any perxanthate for the nitrogen flotation, we may assume that a small amount of hydrogen peroxide is produced at the mineral surfaces by the reduction of oxygen from the sparger gas.
Ads.Cons. Perxanthate Xanthate
Nitrogen
5% Oxygen
Air
50% Oxygen
Oxygen 0
10
20
30
40
50
60
70
Content (pM) Fig.7 Distribution of xanthate in different tests. The higher rest concentration of xanthate in the nitrogen tests suggests that there are fewer side-reactions here. For an inert atmosphere, this is hardly surprising. However, the recovery is also a little lower for nitrogen flotation, and this may suggest that some step in the hydrophobation process be hindered by the low oxygen activity. Xanthate consumption increases with the oxygen content of the flotation gas. For the tests with 100% oxygen the adsorption/consumption is approx. 15 prnole/litre higher than in the other gas tests. It is likely that this extra xanthate is consumed by an oxidation process, most probable the formation of dixanthogen in the pulp liquid. This could also explain the great difficulties with the froth stability for this condition, since an excess of xanthate collector produces a sticky froth. For the frother we were forced to greatly increase the addition to achieve stable froth layers. In combination with the feed problems it gave unrealistically high levels in the tests with 5% and 50% oxygen, as is shown in Table 5. The frother dosages in grams/tonne become disturbingly high. Moreover, the high frother consumption for the tests with 50% and 100% oxygen points to some degradation process of the frother - - a polypropylene glycol methyl ether. Although, ethers normally are very stable they may react to produce small amounts of peroxides when in contact with air [18]. A similar reaction might occur here.
82
B.I. P5lsson and H. Persson
TABLE 5 Frother addition (0.4 weight-% solution Dowfroth 250). Test
Feed (kg/min)
Dilution ratio in column
Nitrogen 5% Oxygen Air 50% Oxygen Oxygen
0.53 0.36 0.48 0.33 0.50
3.35 3.37 3.42 3.86 3.64
Frother addition (ml/min) 11.6 12.6 12.5 15.7 16.5
Theoretical frother conc. in column (mg/1) 26.4 41.8 30.4 48.9 36.4
Dosage (g/ton) 88 141 104 189 133
CONCLUSIONS Flotation columns are sensitive to changes in the reagent dosages. Too much frother creates a running froth, while frother deficiency - - or xanthate surplus - - gives a sticky froth. This makes the column hard to run. The tests have shown that it is possible to control the redox environment of the pulp by changing the oxygen activity of the flotation gas. These changes bring about more active zinc minerals in an oxidising environment, with a loss of selectivity between lead and zinc. Both xanthate and frother consumption increase in very oxidising conditions. The pulp chemistry of the column otherwise distinguishes itself from the mechanical cell in that it is run with a higher dilution ratio. From a practical point of view, it seems that varying the oxygen activity of the flotation gas only moves the operational point on the grade-recovery curve. It has not been possible, with gases only, to move outside the theoretical precipitation area for the metal-collector compound. Any economical benefit of changing the oxygen activity of the flotation gas must be sought in lower reagent consumption, or enhanced selectivity. Another observation is that while introducing redox control, one must be sure to move the operational point of the flotation process well outside the metal-collector precipitation area. Such an area can be theoretically predicted by computations based on bulk thermodynamic data. Also, the relevance of the Pt-foil electrode for measurements of redox levels in low ionic strength pulp liquids is questionable.
ACKNOWLEDGEMENT NUTEK (Swedish National Board for Industrial and Technical Development) provided the financing for this project. The authors would like to thank the personnel at the Laisvall concentrator for all their help with the ore and reagent samples.
REFERENCES .
2. 3. 4. 5.
Ralston, J., E h and its consequences in sulphide mineral flotation. Minerals Engineering, 4, 859-878 (1991). Heyes, G.W. & Trahar, W.J., The natural flotability of chalcopyrite. Inter. J. Mineral Proc. 4, 317-344 (1977). Heyes, G.W. & Trahar, W.J., Oxidation - reduction effects in the flotation of chalcocite and cuprite. Inter. J. Mineral Proc., 6, 229-252 (1979). Guy, P.J. & Trahar, W.J., The influence of grinding and flotation environments on the laboratory batch flotation of galena. Inter. J. Mineral Proc., 12, 15-38 (1984). Berglund, G. & Forssberg, E., Influence of different gases in flotation of sulphide minerals. In: S. Chander and R.R. Klimpel (Editors), Advances in Coal and Mineral Processing Using Flotation. Proc. Eng. Found. Conf., Palm Coast, Florida, 3-8 December, 1989, pp. 71-76. Soc. Min., Met. and Exploration, Inc., Littleton, Colorado, (1989).
Redox control in a pilot flotationcolumn .
.
8.
.
10. II. 12. 13. 14. 15. 16. 17. 18.
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Berglund, G. & Forssberg, E., Redox and oxygen influence on the flotation behaviour of sulphide ores. In: P.E. Richardson and R. Woods (Editors), Electrochemistry in Mineral and Metal Processing II, Proc. Vol. 88-21, pp. 183-197. Electrochemical Society, Pennington, NJ. (1988). Finch, J.A. & Dobby, G.S., Colunm Flotation. Pergamon Press, Oxford, UK. (1990). Martin, C.J., Finch, J.A. & Rao, S.R., Pilot flotation-colunm test of nitrogen flotation of pyrite in pyrite/sphalerite separation. Trans. Inst. Min. Met. Sect. C, 99, Cl15--Cl16 (1990). Forssberg, K.S.E., J6nsson, H.R. & P~lsson, B.I., Full scale test of process water reuse in a complex sulphide ore circuit. In: K.S.E. Forssberg (Editor), Flotation of Sulphide Minerals, Elsevier, Amsterdam, Holland, 197-217 (1985). P~dsson, B.I., Analysis of xanthate and its degradation products by ion chromatography. Trans. Inst. Min. Met. Sect. C, 98, C132-C140 (1989). Hamilton, I.C. & Woods, R., The effect of alkyl chain length on the aqueous solubility and redox properties of symmetrical dixanthogens. Aust. J. Chem., 32, 2171-2179 (1979). Ahlberg, E. & AsbjOrnsson, J., Carbon paste electrodes in mineral processing: an electrochemical study of sphalerite. HydrometalIurgy, 36, 19-37 (1994). P~sson, B.I. & Oberg, E. Interpretation of electrode responses with the help of a multivariate technique. J. Electrochent. Soc., 140, 2519-2525 (1993). Eriksson, G.A., An algorithm for the computation of aqueous multicomponent, multiphase equilibria. Anal Chim. Acta, 112, 375-383 (1979). PS.lsson, B.I. & Forssberg, K.S.E. Computer-assisted calculations of thermodynamic equilibria in the galena - ethyl xanthate system. Inter. J. Mineral Process., 23, 93-121 (1988). Pfilsson, B.I. & Forssberg, K.S.E., Computer-assisted calculations of thermodynamic equilibria in sphalerite - xanthate systems. Inter. J. Mineral Process. 26, 223-258 (1989). Jones, M.H. & Woodcock, J.T., Perxanthates - - a new factor in the theory and practice of flotation. Inter. J. Mineral Proc., 5, 285-296 (1978). Linstromberg, W.W., Organisk kemi, Svenska UtbildningsfOrlaget Liber AB, Stockholm, 431 (1969).
Pergamon 0892--6875(95)00132-8
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892-6875/96 $15.00+0.00
REDOX CONTROL IN A PILOT FLOTATION COLUMN
B.I. PALSSON and H. PERSSON* Department of Chemical and Metallurgical Engineering, Lule~ University of Technology, S-971 87 LuleL Sweden Present address: H6gan~is AB, S-263 83 H6gan~is, Sweden (Received 26 July 1995; accepted 30 September 1995)
ABSTRACT Results from pilot-column flotation tests on a lead-zinc ore show that it is possible to control the redox potential of the column pulp by varying the oxygen activity of the flotation gas fed to the spargers. As a result, the process changes so that the fine zinc minerals are more actively floating in an oxidising environment, at the same time xanthate and frother consumptions increase. The placement of electrodes in the column does change the redox potential readings but only as a result of a change in the oxygen concentration of the pulp. However, the values from a Glassy-Carbon (GC) electrode are consistently lower than those from a Pt-foil electrode at the same location. Actual positions of operating points as measured by GCelectrodes fall within the theoretical area for precipitation of lead xanthate.
Keywords Sulphide ores; column flotation; redox control; process control
INTRODUCTION Over the last decades, considerable attention has been focused on pulp potentials and their influence on the results of sulphide mineral flotation [ 1]. Several investigators have dealt with potential-controlled laboratory flotation tests, among them Heyes and Trahar [2,3] and Guy and Trahar [4]. These tests generally used oxidising or reducing agents such as hypochlorite and dithionite respectively. Another way to control the pulp potential of a flotation system is to use gases with varying oxygen activity, achieving a pure redox effect in the pulp. This has been reported for mechanical cells at laboratory [5] and pilot-plant scale [6] by Berglund and Forssberg. It is not easy, however, to precisely introduce and recover gases (e.g., nitrogen) from mechanical cells. With the flotation column such a device emerged. In a chemical engineering sense, it can be regarded as a chemical reactor. It has been used as such in copper-moly separations [7], although the analogy to the chemical reactor seems not to have been drawn. Otherwise, very little appears to have been done on deliberately controlling the oxidising environment of the column. The only exception may be the work on nitrogen flotation in pyrite/sphalerite flotation by Martin et al. [8].
Presented at Minerals Engineering "95, St. Ives. Cornwall, England, June 1995
73
74
B. 1. P~lsson and H. Persson
Given this background, the intention o f the reported project was to study the concentration chemistry of the flotation column, especially methods for controlling the oxidation environment within the column. The project should to try to answer if - - and in what way - - does the pulp chemistry in a column differ from that in a conventional mechanical cell.
EXPERIMENTAL
Grinding and Flotation The test material was a lead-zinc ore from the Laisvall mine of Boliden Mineral AB, Sweden. This ore contains 3.6% Pb and 0.8% Zn as impregnations o f galena and sphalerite in a sandstone hoist rock. An ore sample o f 2.5 tonnes of ore pebbles (40-80 mm) was taken from the pebble screen in the crushing station by the concentrator personnel. The reason for selecting ore pebbles as the test material was that it is a dry material stream and the risk of inadvertent oxidation is small, due to the low amount o f fine particles in the sample. On arrival to LuleA, the bulk ore sample was crushed dry in three stages to d90=2 mm, and stored in the drums it was delivered in. The column installed at the Division of Mineral Processing, L u l d t University of Technology, is a Deister type, delivered by S A L A International AB. It is 3.5 m in height and has a diameter of 7.5 cm. On two levels, it has external bubble generators by Deister. It was modified to our specifications, with four one-inch sampling ports (arranged in a cross) installed for every half a meter, starting at the one meter level. Circuit layout is given in Figure 1. Besides sampling, the one-inch ports are used for inserting electrodes into the column. This is done in a very simple way. A hole for an electrode is drilled through a rubber stop cork, which is then squeezed into a sampling port.
4
4.0m
5 Feed
D
2
3
0
©
3.5 m
Fig. 1. Pilot-column flotation circuit. 1) flotation column 75 m m diameter x 3500 m m with two acrylic sections, float-sensor pipe and sampling ports; 2) peristaltic pump - feed; 3) speed-controlled peristaltic pump - tailings; 4) speed-controlled mixer 70 litres; 5) mild steel rod mill 600 m m diam. x 900 ram; 6 - - - -location of gas spargers.
Redox control in a pilot flotation colunm
75
Otherwise, the circuit consists of: a mild steel rod mill 600 m diameter x 900 mm; a 70 litres mixing tank with speed-controlled impeller; a controlled-speed peristaltic pump for feeding the column; the same type of pump for pumping the tailings. The latter is controlled not only by a manually pre-set value, but also through a float in a communicating side-pipe to the column. In this way the interface between pulp and froth zone is kept constant. The mixer and the pumps are driven by DC motors, which are controlled by rheostats. Pulp flows are checked by point sampling and through measuring the decrease of the pulp level in the mixing tank. Gas and liquid flows are manually controlled by needle valves and the flows are read by rotameters. Frother is added to the water for the external bubble generators and the amount of frother is regulated by a membrane pump with variable stroke frequency. Collector is added to the mixing tank before each test. Batch-wise rod milling is done in the 600 mm diameter x 900 mm mild steel rod mill. It is modified to have a discharge slot on the mill end. The slot is covered by a movable sliding steel lid. At discharge time a frequency transformer is coupled between the net and the AC motor, the mill is rotated slowly into discharge position and the material is flushed out into the mixing tank through an 850 pm chip screen. For 50 kilograms of Laisvall ore, pre-crushed to <3 mm, a grinding time of 20 minutes gives d80=125 pm and 33% <44 pm. For mixing and homogenising a small barrel is used. Any coarse particles from the mill are trapped by the 850 pm chip screen mounted on the barrel. The correct feed pulp density, 48-50 weight-%, is set in the mixing tank. From the bottom of the barrel the pulp is delivered by a peristaltic pump to the column. The hose dimension was selected as 6 mm i.d. to as far as possible prevent sedimentation in the hose. Preliminary tests with the column showed that the flotable amount was approx. 5% of the feed rate, therefore, it was possible to run the column with only the lower bubble generator. This also gave the positive side effect that the dilution ratio in the column decreased. Since the first tests showed that it was not possible to use ordinary tap water as wash water, a small amount of frother was added in the wash to stabilise the froth. To avoid frothing in the float-pipe, a small amounts of water was kept running down the float-pipe. The tests were run at natural pH. The only reagents added were Na-isopropyl xanthate and the frother DowFroth 250. Both reagents were of technical grade and delivered from Laisvall. Collector addition rate was kept at 25 g/tonne, except in the oxygen tests where it had to be raised to 40 g/tonne. Gases were commercial high-purity nitrogen and oxygen (AGA Gas AB), synthetic mixtures with 5 and 50 mole-% oxygen in nitrogen (AGA Specialgas AB), and compressed air from the laboratory net. In the trials, the primary interest was in comparing the extreme gas atmospheres with ordinary air flotation as a reference. Therefore, several tests were run with oxygen and nitrogen as the flotation gas. Tests with 5% and 50% oxygen are extra tests to check the general tendencies of the oxygen-nitrogen-air experiment. The column was operated visually as far as the froth appearance with collector and frother adjusted accordingly.
Pulp chemistry Battery operated pH-meters (PHM80, Radiometer) were used to measure: pH with a combination electrode (GK2501B, Radiometer), pulp potentials with Pt-foil (P101, Radiometer) and Glassy-Carbon (Metrohm) electrodes against a common calomel electrode (K401. Radiometer). Pulp potentials are reported on the hydrogen scale. Electrode readings were taken on two levels, a lower at 1.0 m, and a higher one just above the teed point at 2.5 m. During a test, the lower level was used for the initial measurements until a pulp sample was taken, after that the pH-meters were moved to the higher level and the measurements continued with the electrodes installed there. The oxygen electrode (Bibby SM01) was installed at the lower level only.
76
B.I. P~lsson and H. Persson
In every test, a small pulp liquid sample (2 cm3) was withdrawn and filtered on 0.22 [am membrane filter following a standard procedure [9]. This sample was then injected into an ion chromatograph (Dionex 2010i with HPIC-AS4 column) for analysis of chloride, nitrate, sulphite, sulphate, thiosulphate, perxanthate and xanthate [10]. An additional larger pulp sample (50 cm 3) was collected and filtered on 0.22 [am. It was acidified with nitric acid (suprapur) and sent to an outside laboratory for analysis of metals in solution.
RESULTS AND DISCUSSION General Typical material, pulp, water and gas flows from a test with air as the flotation gas are given in Table 1. It is clear that the water added through the external bubble generator has the greatest influence on the water balance of the column.
TABLE 1 Measured flow data for test Air-2. Stream
Water to float-pipe Wash water Sparger water
Solids flow (kg/min) ----
Water flow (1/min) 0.068 0.230 1.13
Dilution ratio ----
Froth product Tailings Feed
0.031 0.45 0.47
0.065 1.42 0.53
2.10 3.16 1.13
%Zn
%Pb
Comment
m
_
_
m
Gas flow: 2.83 l/rain 50.7 0.37
6.34 0.26
In Table 2, key data from tests with different gases are shown. The residence time is calculated as the pulp zone volume divided by the tailings flow rate. The amount of floated material per minute and surface unit is the "froth capacity". Since the tests were not always mn to the flotation limit of the column it is not the "carrying capacity" used by other investigators. The net downward water flow is the bias and it is expressed as a velocity. In all cases the gas flow rate was kept constant at 1.07 cm/s. In the tests with 5% and 50% oxygen problems with the feed rate, occured shown in Table 2 as longer residence times. Due to the feeding problem gangue material started to float and the bias became negative for the 5% oxygen test.
TABLE 2 Key data (averages) Test Nitrogen 5% Oxygen Air 50% Oxygen Oxygen Literature values
Feed (kg/min) 0.53 0.36 0.48 0.33 0.50
Residence time (min) 6.8 10.3 7.6 8.6 7.0
Froth capacity (g flot/(min.cm2)) 0.45 1.15 0.84 0.82 0.86 1-5
Bias (cm/s) O.O56 -0.013 0.044 0.032 0.046 0--0.3
Redox control in a pilot flotation column
77
Process results The tests have been run as rougher flotations giving concentrates with 40-50% Pb at approx. 90% recovery in the tests where the feed rate was kept constant, cf. Figure 2. The lower concentrate grade in the test for 5% and 50% oxygen depends on the lower feed rate in these cases. It is not possible to declare any flotation gas better than the other. 60
I
I
'
I
I
[]
50 []
40 ¸
[]
v
O
(1)
-o 3 0 -
@
-Q 20 I:L
•
Nitrogen
•
Air
o
50% Oxygen
[]
Oxygen
5% Oxygen 10
I
50
'
60
I
I
I
70
80
90
00
Pb Recovery (%) Fig.2 Grade-recovery for lead. However, the selectivity diagram in Figure 3 shows a difference, that a pure nitrogen atmosphere gives better selectivity between lead and zinc. Tests with 5% and 50% oxygen are inferior, probably because of the lower feed rate. From Figures 2 and 3, a general observation can be inferred that the points represent different operating positions only on the grade-recovery and selectivity curves.
100
'
I
'
!
'
~
80
v
co
60 ¸
.-~
.Q ..--
........ -.... 40 ¸
.....,. .........
.,,..'
o_
....
a
c-
N 20 ¸
J o
[3
o 50
5% Oxygen Air 50% Oxygen Oxygen "
610
l ................... 710
I
80
Pb Recovery (%)
Fig.3 Selectivity between lead and zinc.
,i0
100
78
B.I. P&lsson and H. Persson
Recoveries by size fraction for lead and zinc are shown in Figures 4 and 5 respectively. It is not possible to read any tendency from the lead diagram. The zinc diagram shows that the 5% and 50% oxygen tests differ for reasons given earlier. Otherwise the pattern is to be expected from a flotation chemistry point of view. A nitrogen environment gives the lowest zinc activation, and the oxygen flotation the highest zinc content in the concentrate. Air flotation gives approximately the same results as for the oxygen environment. The second sample for oxygen was taken a little early and so the zinc recovery is lower. It is included here to give an estimate of how much the results can differ if the conditions are changed slightly. 100
80
v
60
o
o o
40
rr c~ n 20
0 <38
38-53
53-75
75-106
>106
Particle Size (pm) Fig.4 Lead recoveries by size. 100
80 s'-",-.
v
60
(D
> o o o
40
rr c N 2O
<38
38-53
53-75
Particle Size (urn) Fig.5 Zinc recoveries by size.
75-106
>106
Redox control in a pilot flotation column
79
The reason for the more active zinc mineral might rise from two different mechanisms: activation of sphalerite by lead ions released into the pulp liquid as a result of galena oxidation, or an indiscriminate flotation due to dixanthogen formation on sulphide mineral surfaces. Flotation tests were carried out at low collector dosage, giving an initial xanthate concentration of 5.10 -5 M in the pulp liquid. This should be compared with reported [ 11 ] dixanthogen solubilities of 1.14.10 -5 M (ethyl) and 1.5.10 -6 M (n-propyl). The actual solubility of the dixanthogen formed from isopropyl xanthate should fall between these two values. It is not conceivable, therefore, that dixanthogen induced flotation of sphalerite is responsible for the increased zinc appearance in the froth product. Furthermore, the sphalerite from Laisvall is colourless to pale yellow, implying a low iron content. Such a sphalerite has been found electrochemically inactive [12]. This means that dixanthogen formation as an effect of oxygen reduction on the mineral surface is unlikely. Therefore, the increase in zinc flotation activity is considered to be a case of autogenous lead ion activation.
Pulp chemistry In all tests, samples of the pulp liquid were taken for analysis of dissolved metals and anions. Anions were analysed by an ion chromatograph and a summary of the analyses is shown in Table 3. Cation analyses were found to reflect changes in pH mainly, and are therefore not included here.
TABLE 3 Anions in pulp liquid. Test Nitrogen 5% Oxygen Air 50% Oxygen Oxygen
Cl-
SO~-
SO~-
$20~-
(mg/1)
(mg/1)
(mg/l)
(mg/l)
Xanthate (laM)
Perxanthate (pM)
28 19 19 18 26
34 26 28 15 35
-1.0 1.1 0.8 --
-0.3 0.3 0.5 0.4
8.2 2.2 3.6 2.1 6.5
--2.7 1.4 7.4
It can be seen that the different flotation gases give different pulp environments. This is mirrored by the variation in anion contents. In the least oxidising environment, i.e., nitrogen, there is a higher residual xanthate ion level in the pulp liquid, but no perxanthate, sulphite or thiosulphate. For the most oxidising environment xanthate is found as perxanthate, and there is no sulphite. The other tests fall between. In the tests, pH and redox potentials were measured at two levels, while oxygen was only measured at the lower level, cf. Table 4. The pH measured at the higher level is always a little higher and the redox level lower, except in the nitrogen test where the redox level is about the same if the pH difference is compensated for. A probable explanation is that the oxygen content of the sparger water itself is not nil. This is supported by the oxygen level never falling below 2 mg/1, not even for the nitrogen test. The Ptelectrode always shows a higher reading than the Glassy-Carbon at the same location, except in the nitrogen test where the readings are close. Both observations point to the same explanation, that the sensitivity of the Pt-electrode for the oxygen reduction on the electrode surface causes abnormally high readings in the low ionic strength, lead-zinc pulp environment. The same phenomena have been noticed in laboratory flotation tests of copper-lead separation [13], where it was also observed that the variations in potential levels as measured by the Glassy-Carbon electrode correlated better to variations in readings from oxygen and galena electrodes, than the ordinary Pt-foil electrode. If the redox potentials are plotted on a pe-pH-diagram (cf. Figure 6), the values for the Glassy-Carbonelectrode fall within the theoretical precipitation area for the stoichiometric lead xanthate, in this case Pb(iPX)2(s). Here, pe is a quantity directly related to the potential of the solution through the equation pe = Eh/g
(1)
80
B.I. P~lsson and H. Persson
where E h is a redox potential, g = R T / F . I n l 0 (=59.16 mV at 25o C and 1 atm), with R=the gas constant, T=temperature in Kelvin, F=Faraday constant.
TABLE 4 Electrode data (at time of sampling). Level
Low
High
Test
pH
Nitrogen 5% Oxygen Air 50% Oxygen Oxygen Nitrogen 5% Oxygen Air 50% Oxygen Oxygen
Eh(Pt) (mV) -34 289 348 391 437 6 201 284 326 373
7.55 8.42 7.71 8.30 7.90 8.31 8.39 7.93 8.39 8.07
02
Eh(GC) (mV) -42 128 182 234 251 -4 86 133 223 222
(mg/1) 2.8 2.2 2.8 4.7 19.4 ------
Temp (°C) 14 18 18 18 14 ------
The diagram was calculated with the computer program S O L G A S W A T E R [14] and thermodynamic data were kinetically adapted to the Laisvall ore from previously published galena [15] and sphalerite [16] systems. Total concentrations (Tc) of each component were calculated to approximate an average condition in the flotation column. They are: Tc(Pb)=0.26 mM, Tc(Zn)=0.09 mM, Tc(S)=0.35 mM, and Tc(iPX)=0.05 mM. The pulp systems were considered essentially carbonate-free, since the tests were run in a closed vessel with a controlled gas atmosphere.
108-
6-
pb 2+
Pb304
PbOH +
4Pb(iPX) 2
•
2Q..
t Pb(OH)iPX
o-2-4-
PbS
-6 -
-8-10-
•r
5
T
i
|
7
r
T
'I"--
8
"'-
9
-
10
pH Fig.6 Predominance diagram for lead - non-carbonate system. --- maximum extension area of lead xanthate species; o operational points measured with Pt-foil electrode; • operational points measured with Glassy-Carbon-electrode.
11
-
12
Redox control in a pilot flotation column
81
Flotation reagents The distribution of the total amount of isopropyl xanthate added, expressed in molar units, on residual levels in pulp liquid and what is assumed to have been adsorbed or consumed is shown in Figure 7. Interestingly, perxanthate only appears in tests with an oxygen activity equal to air or higher. As shown by Jones and Woodcock [17] perxanthate is formed from xanthate by a reaction with peroxide, accordingly (2)
R - O - CS.S- + H202 .-, R - O - CS.SO- + H20
where R denotes an alkyl chain. Since we have not found any perxanthate for the nitrogen flotation, we may assume that a small amount of hydrogen peroxide is produced at the mineral surfaces by the reduction of oxygen from the sparger gas.
Ads.Cons. Perxanthate Xanthate
Nitrogen
5% Oxygen
Air
50% Oxygen
Oxygen 0
10
20
30
40
50
60
70
Content (pM) Fig.7 Distribution of xanthate in different tests. The higher rest concentration of xanthate in the nitrogen tests suggests that there are fewer side-reactions here. For an inert atmosphere, this is hardly surprising. However, the recovery is also a little lower for nitrogen flotation, and this may suggest that some step in the hydrophobation process be hindered by the low oxygen activity. Xanthate consumption increases with the oxygen content of the flotation gas. For the tests with 100% oxygen the adsorption/consumption is approx. 15 prnole/litre higher than in the other gas tests. It is likely that this extra xanthate is consumed by an oxidation process, most probable the formation of dixanthogen in the pulp liquid. This could also explain the great difficulties with the froth stability for this condition, since an excess of xanthate collector produces a sticky froth. For the frother we were forced to greatly increase the addition to achieve stable froth layers. In combination with the feed problems it gave unrealistically high levels in the tests with 5% and 50% oxygen, as is shown in Table 5. The frother dosages in grams/tonne become disturbingly high. Moreover, the high frother consumption for the tests with 50% and 100% oxygen points to some degradation process of the frother - - a polypropylene glycol methyl ether. Although, ethers normally are very stable they may react to produce small amounts of peroxides when in contact with air [18]. A similar reaction might occur here.
82
B.I. P5lsson and H. Persson
TABLE 5 Frother addition (0.4 weight-% solution Dowfroth 250). Test
Feed (kg/min)
Dilution ratio in column
Nitrogen 5% Oxygen Air 50% Oxygen Oxygen
0.53 0.36 0.48 0.33 0.50
3.35 3.37 3.42 3.86 3.64
Frother addition (ml/min) 11.6 12.6 12.5 15.7 16.5
Theoretical frother conc. in column (mg/1) 26.4 41.8 30.4 48.9 36.4
Dosage (g/ton) 88 141 104 189 133
CONCLUSIONS Flotation columns are sensitive to changes in the reagent dosages. Too much frother creates a running froth, while frother deficiency - - or xanthate surplus - - gives a sticky froth. This makes the column hard to run. The tests have shown that it is possible to control the redox environment of the pulp by changing the oxygen activity of the flotation gas. These changes bring about more active zinc minerals in an oxidising environment, with a loss of selectivity between lead and zinc. Both xanthate and frother consumption increase in very oxidising conditions. The pulp chemistry of the column otherwise distinguishes itself from the mechanical cell in that it is run with a higher dilution ratio. From a practical point of view, it seems that varying the oxygen activity of the flotation gas only moves the operational point on the grade-recovery curve. It has not been possible, with gases only, to move outside the theoretical precipitation area for the metal-collector compound. Any economical benefit of changing the oxygen activity of the flotation gas must be sought in lower reagent consumption, or enhanced selectivity. Another observation is that while introducing redox control, one must be sure to move the operational point of the flotation process well outside the metal-collector precipitation area. Such an area can be theoretically predicted by computations based on bulk thermodynamic data. Also, the relevance of the Pt-foil electrode for measurements of redox levels in low ionic strength pulp liquids is questionable.
ACKNOWLEDGEMENT NUTEK (Swedish National Board for Industrial and Technical Development) provided the financing for this project. The authors would like to thank the personnel at the Laisvall concentrator for all their help with the ore and reagent samples.
REFERENCES .
2. 3. 4. 5.
Ralston, J., E h and its consequences in sulphide mineral flotation. Minerals Engineering, 4, 859-878 (1991). Heyes, G.W. & Trahar, W.J., The natural flotability of chalcopyrite. Inter. J. Mineral Proc. 4, 317-344 (1977). Heyes, G.W. & Trahar, W.J., Oxidation - reduction effects in the flotation of chalcocite and cuprite. Inter. J. Mineral Proc., 6, 229-252 (1979). Guy, P.J. & Trahar, W.J., The influence of grinding and flotation environments on the laboratory batch flotation of galena. Inter. J. Mineral Proc., 12, 15-38 (1984). Berglund, G. & Forssberg, E., Influence of different gases in flotation of sulphide minerals. In: S. Chander and R.R. Klimpel (Editors), Advances in Coal and Mineral Processing Using Flotation. Proc. Eng. Found. Conf., Palm Coast, Florida, 3-8 December, 1989, pp. 71-76. Soc. Min., Met. and Exploration, Inc., Littleton, Colorado, (1989).
Redox control in a pilot flotationcolumn .
.
8.
.
10. II. 12. 13. 14. 15. 16. 17. 18.
83
Berglund, G. & Forssberg, E., Redox and oxygen influence on the flotation behaviour of sulphide ores. In: P.E. Richardson and R. Woods (Editors), Electrochemistry in Mineral and Metal Processing II, Proc. Vol. 88-21, pp. 183-197. Electrochemical Society, Pennington, NJ. (1988). Finch, J.A. & Dobby, G.S., Colunm Flotation. Pergamon Press, Oxford, UK. (1990). Martin, C.J., Finch, J.A. & Rao, S.R., Pilot flotation-colunm test of nitrogen flotation of pyrite in pyrite/sphalerite separation. Trans. Inst. Min. Met. Sect. C, 99, Cl15--Cl16 (1990). Forssberg, K.S.E., J6nsson, H.R. & P~lsson, B.I., Full scale test of process water reuse in a complex sulphide ore circuit. In: K.S.E. Forssberg (Editor), Flotation of Sulphide Minerals, Elsevier, Amsterdam, Holland, 197-217 (1985). P~dsson, B.I., Analysis of xanthate and its degradation products by ion chromatography. Trans. Inst. Min. Met. Sect. C, 98, C132-C140 (1989). Hamilton, I.C. & Woods, R., The effect of alkyl chain length on the aqueous solubility and redox properties of symmetrical dixanthogens. Aust. J. Chem., 32, 2171-2179 (1979). Ahlberg, E. & AsbjOrnsson, J., Carbon paste electrodes in mineral processing: an electrochemical study of sphalerite. HydrometalIurgy, 36, 19-37 (1994). P~sson, B.I. & Oberg, E. Interpretation of electrode responses with the help of a multivariate technique. J. Electrochent. Soc., 140, 2519-2525 (1993). Eriksson, G.A., An algorithm for the computation of aqueous multicomponent, multiphase equilibria. Anal Chim. Acta, 112, 375-383 (1979). PS.lsson, B.I. & Forssberg, K.S.E. Computer-assisted calculations of thermodynamic equilibria in the galena - ethyl xanthate system. Inter. J. Mineral Process., 23, 93-121 (1988). Pfilsson, B.I. & Forssberg, K.S.E., Computer-assisted calculations of thermodynamic equilibria in sphalerite - xanthate systems. Inter. J. Mineral Process. 26, 223-258 (1989). Jones, M.H. & Woodcock, J.T., Perxanthates - - a new factor in the theory and practice of flotation. Inter. J. Mineral Proc., 5, 285-296 (1978). Linstromberg, W.W., Organisk kemi, Svenska UtbildningsfOrlaget Liber AB, Stockholm, 431 (1969).