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An electrochemical study of the effect of potential on the selective dissolution of base metal sulphides in sulphuric acid
Minerals Engineering, Vol. 5, Nos. 3-5, pp. 535-545, 1992
0892-6875/92 $5.00+0.00 © 1992 P e ~ Press ptc
Printed in Great Britain
AN ELECTROCHEMICAL STUDY OF THE EFFECT OF POTENTIAL ON THE SELECTIVE DISSOLUTION OF BASE METAL SULPHIDES IN SULPHURIC ACID L. LORENZEN Department of Metallurgical Engineering, University of Stellenbosch, Stellenbosch, 7600, South Africa
ABSTRACT
Diagnostic leaching is an analytical tool which was developed at Anglo American Research Laboratories to examine the deportment of gold in an ore, or any type of plant product or intermediary. In this study we make use of the same principle to study the effect of potential and pH on the dissolution of base metal sulphides in a sulphate medium (sulphuric acid) at high temperatures (80°C). This technique was developed to preferentially liberate a particular base metal from a complex ore. In this way, existing base metal sulphide extraction processes, especially where other unwanted base metal sulphides are present, can be optimised so as to yield substantial savings in operating costs. An oxidizing sulphuric acid leach at high temperatures will destroy pyrrhotite, uraninite, labile base metal sulphides (chalcopyrite, sphalerite, orpiment, marcasite), and to a lesser degree pyrite and arsenopyrite. From the results of this testwork it is possible to find an optimum between reagent consumption ( MnO 2 and H 2 g 0 4), temperature, leaching time, potential (Eh), costs and to a lesser degree pH, for the optimal extraction of the selective base metal, and/or gold. This can be achieved without the dissolution of less desirable base metal sulphides associated with the preferred mineral. Keywords Base Metal Sulphides; potential; dissolution; sulphuric acid INTRODUCTION The first step in the design of a process to treat a possible economic precious metal deposit should be an examination of the mineralogy of the refractory ore. Using this and other initial tests, a process to treat the ore can be designed. In the field of gold and base metal metallurgy, the reliance placed on the mineralogical information available to the researcher is very little. If more accurate mineralogical data were available, the first question might be "With which minerals is the desired base metal/precious metal associated, and how will this affect the extraction route I use" [1,2]. Diagnostic leaching (for gold), and the selective dissolution of base metal sulphides (base metals) were designed to answer this question [3].
To find which minerals, base metals/precious metals are associated with, a specific mineral or base metal is first extracted using a selective acid leach. Moreover, the residue from this first stage can be subjected to another selective acid leach, and the process repeated. The procedure can be varied to suit the mineralogy of the matrix material. At the end of the sequential leach, the researcher is left with a complex record of the refractory ore. He can now use this information to formulate a flowsheet to treat the ore for optimal extraction of the preferred mineral, without the dissolution of less desirable base metal sulphides. 535
536
L. LORENZEN
As various ores each have their own characteristics and mineralogies, it is recommended that a mineralogical study and/or a diagnostic leach be conducted first on a sample, to determine the feasibility of such a recommended sequential leach for the selected sample. At the end of the sequential leach it is recommended that an economic evaluation of the desired leach sequence be conducted, to determine if the proposed dissolution is economical viable with relation to reagents. A number of samples were treated. The results were all similar, and in this paper the dissolution of base metals and uranium from a representative sample of a refractory ore, with a high sulphide and uraninite content (+ 50%), will be discussed systematically. EXPERIMENTAL The aim of the project was to generate data on the leaching of base metal sulphides in a sulphate medium. This would establish leaching conditions (of Eh and pH) to preferentially liberate a particular base metal from a complex ore. Every element in nature develops an electrochemical potential in an electrolyte. Internationally, hydrogen's potential has been chosen as the standard potential. Metals with a dissolution potential greater than the dissolution potential of hydrogen (0.0 V), and lower than the dissolution potential of oxygen (1.23 V), are not corroded with the evolution of hydrogen. However, they may be corroded by solutions containing dissolved oxygen [4]. The Nernst equation [5] provides a convenient way for determining potential, knowing the standard reduction potential, E o, pH and the concentration of oxidant or reductant added. Nernst found that Eh = E ° - (RT/nF) In k where Eh is the total change in potential k the equilibrium constant, and n the number of electrons involved in the half reaction. This equation may be simplified to Eh = E ° - 0.0591 pH + k log [oxidant] where k is a constant determined by n, and [ ] denotes concentration in mol/l. Therefore, it can be seen that by adding an oxidant, the Eh value is increased, and likewise by adding a reductant, the Eh is reduced. This principle was used extensively in the project described here. Sulphuric acid (H2SO4) was used as leaching medium [6]. The effects of Eh (100 mV to 700 mV with the selective addition of manganese dioxide, MnO 2, or hydrogen peroxide, H202), pH (0.5 to 2 with the selective addition of H~O4), temperature (ambient to 80°C) and leaching times (16 to 48 hours) on the dissolution of base metal sulphides and uraninite, were determined. The solutions and solids were analysed for the various base metals in question after each test. Testwork was performed on 70% - 200 mesh refractory ore sample. The mineralogical breakdown of the ore are presented in Tables 1 and 2. The general characteristics of the sulphide minerals are, that the pyrite and marcasite are mainly represented by very fine grained crystals which seldom exceed 5 micrometers in diameter. The pyrite and marcasite tend to cluster together and are always finely intergrown with the gangue sphalerite. Chalcopyrite and orpiment on the other hand are present as discrete compact crystals [7].
537
Selective dissolution of base metal sulphides
TABLE 1 Estimated bulk mineral composition
Mineral
Mass (%)
Sulphide minerals Gypsum (CaSO2; 2H20 ) Quartz (SiO2) Uraninite Calcite and minor siderite (CaCO3; FeCO3) Feldspar (sanicline) (Na, K, Ca, alumina silicates) Garnet (andradite) 3CaO Fe203 . 3SiO 2 Alunite and minor jarosite Chlorite, mica and clay minerals Graphite (C)
40 14 12 10 9 9 4 2 1 __1 100
TABLE 2 Sulphide mineral composition
Mass % Ore Minerals
Mineral
Total sample
Pyrite (4FeS2) Marcasite (2FeS2) Arsenopyrite (8FeAsS) Orpiment (4As2S3) Sphalarite (4ZnS) Chalcopyrite (4CuFeS2) Tetra hedrite, molybdenite and other rare sulphide minerals
18,0 13,3 1,8 2,8 2,0 2,0
45,0 33,3 4,5 7,0 5,0 5,0
0.1 40,0
0.2 100
RESULTS AND DISCUSSIONS The results are presented in Figures 1 to 9. From the results and graphs of percentage extraction vs potential for each sulphurie acid leach plotted, several important observations can be made which will be discussed in detail. Note that the lines in the graphs do not represent actual points, but are there to show variations in values. Testwork, was done only on the base metals Cu, As, Fe, Zn and uranium, as they were seen from the head assay to be the primary base metals in this ore. The effect of pH and potential (Eh) on base metal extraction
From the results presented in Figures 1 to 5, it is evident that an increase in potential leads to increased extractions in all five minerals analysed. The increase varies with potential, and ME 5/3--5--R
538
L. LORENZEN
is particularly noticeable in the cases of uranium, zinc and iron. The extraction of arsenic is relatively independent of potential and pH, although average extractions of up to 66% were achieved. In the case of copper the extractions were low and never increased above 32%, irrespective of the potential, pH, leaching time and temperature.
i00
% [
80
I' 0 Ft
60
E X
l r
40
Cl C
I t
20
0 12
0
I
I
I
I
I
200
300
400
800
600
I
0
I00
700
~.h CrnV) pH = 0.5
4-
p H ffi 1.0
~
DH ffi [,5
Fig.l The effect of pH and potential on iron extraction (80°C, 24 hrs)
%
35
C o 13 p
3O 25
e
r
20
E x !
15
r
ca c 1 l
IO
5
0
n
0 0
I
I
I
I
|
I
I00
200
300
400
800
600
700
r.h (mY) pH ~ 0.5
4- pH ffi 1.0
~
pH ~ 1.5
Fig.2 The effect of pH and potential on copper extraction (80°C, 24 hrs) Up to 88% of the iron in the pyrite and marcasite can be extracted at potentials in excess of 550 inV. Zinc behaves very favourably towards redox potential, and extraction values of 80% and higher were achieved at potentials in excess of 400 inV. Almost all the uranium can be selectively leached at low potentials (< 400 mV). An increase in potential (Eh) results in an increase in manganese dioxide or hydrogen peroxide addition which in turn could result in an uneconomical leach, as MnO 2 and H20 2 are very expensive. The amount of oxidant used during the leaches at Eh values o f up to
Selective dissolution of base metal sulphides
539
400 mV were negligible in terms of costs. Basically no HzO z were used during the leaching of uranium and only a small amount of HzO z were consumed during the leaching of zinc at Eh values less than 400 inV. From 400 to 500 mV there is an increase in oxidant consumption and this increase become increasingly prominent at Eh values in excess o f 500 inV. As pyrite and marcasite are the major base metal sulphides present in this ore, it can be calculated that selective iron extraction would be more costly as higher potentials and thus more oxidant are needed to extract the iron.
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7O
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50 40 3O 20 l0 0 0
I
I
I
[
I
I
100
200
300
400
500
600
700
Eh (mY) DH ffi 0,5
~- pH = 1.0
~
DH = 1.5
Fig.3 The effect of pH and potential on arsenic extraction (80°C, 24 hrs)
120
% Z
I00
l Ft C
80
E x
60
1 r a G
t 1
40
20
o 0 0
I
I
I
I
I
I
Leo
200
30o
4eo
500
6oo
700
Eh (mY) pH - 0 . 5
÷
pH = [ . 0
~,pI{
•l.5
Fig.4 The effect of pH and potential on zinc extraction (80°C, 24 hrs) It can also be seen from the results presented in Figures 1 to 5, that the effect of pH is greatly reduced due to the overwhelming effect of potential. Results show that leaching at pH values of 0.5 to 1.0 is no more beneficial than leaching at pH values o f 1.0 to 1.5. From the results it is evident that due to the high consumption of sulphuric acid, it is not economically viable to leach at pH values less than 1.0.
540
L. LORENZEN
%
t20
U r
100
t
80
E
60
CX D_ U m
X
t
40
t t
20
r c~ c
0
n
0 0
I
I
too
200
DH-0.6
i
i
I
aoo 4oo Eh (mV) 4- p H - L 0
c~oo ~
I
6oo
700
pH-[.6
Fig.5 The effect of pH and potential on uranium extraction (80°C, 24 hrs) The results of the above mentioned extraction tests are summarized in Figure 6. From the figure it can be seen that at a pH of 1.0, temperature of 80°12 and leaching time of 24 hours, it is possible: to leach uranium (U308) partially selective from pyrite and marcasite at potentials of 200 to 400 mV to leach sphalerite (4ZnS) from the other sulphide minerals at potentials of 400 to 500 mV to leach iron or pyrite (FeS~) at potentials of 550 to 650 mV to partially leach arsenic minerals at potentials of 200 to 600 mV to selectively leach arsenic, iron and zinc sulphides as well as uranium from chalcopyrite (4 CuFeS2) at any potential between 200 and 600 mV. As base metals and uraninite are either associated with the pyrite and marcasite or sometimes locked in the pyrite, a hundred percent selective dissolution of a desired base metal cannot be achieved at potentials less than 400 mV. The effect of temperature and leaching time on base metal extraction
Comparing Figure 6 and Figure 7 and as presented in Table 3, it is evident that a decrease in leaching temperature (80°C to 30°), has only a small negative affect on extractions at a potential value less than 400 mV. At potentials in excess of 400 mV this negative affect is even smaller, and extractions similar to that at high temperatures were achieved. There was however, a substantial increase in H202 consumption at lower temperatures, which could be uneconomical at high Eh values. An increase in leaching time at 80°(2 (from 24 hours to 48 hours) have a small favourable influence on arsenic and copper extractions at all Eh values (Table 4 and Figures 6 and 8). An increase in leaching time results in an increase of 15% in uranium extraction at potentials less than 300 mV (Figure 8), an increase of about 10% in iron extractions at potentials higher than 400 mV and an increase of about 8% in zinc extractions at a potential of 400 mV and less. The effect of selective base metal extraction
Ninety eight percent of the uranium, eighty six percent of the zinc and in the region of sixty percent of the arsenic was selectively destroyed by selective sulphuric acid leaches.
Selective dissolution of base metal sulphides
541
The remainder of the sample which contained about 81.8 percent of the original iron and 96.5 percent of the original copper were then evaluated at potentials of 200 to 600 mV at temperatures of 80°C and a leaching time of 24 hours. The results are presented in Figure 9. ( p H - 1,0. T e m p - BO C. 24 hrz)
100 90-
8070 // i/ l/ // // // // /1 Ii // // // /i // // // // //
60-
4O
1
30-
// .-/
•
/J
20-
// // f/ //
10-
f/ // // fL
0
!
20O
400
600
Pot.enUai Eh (mV)
I=
Iron / ~,mnlo ~
Copper ~ Uranium
Zlno
Fig.6 Metal extraction vs Potential (pH 1.0, 80°C, 24 hrs)
TABLE 3 The effect of temperature (24 hrs)
Potential (mV) Temperature (°C)
200 30 80
400 30 80
600 30 80
Metal/Mineral Extraction (%) Iron Copper Zinc Arsenic Uranium
11 2.1 17 32 44
22 14 80 38 83
75 23 95 44 98
12 2.3 25 48 68
22 19 89 52 98
87 28 97 66 98
542
L. LO~NZEN (pH - 1.0. Temp - 30 12. 24 hr's)
100
90 8070E 60-
30" 2010" 0
II
j I
200
)
400
600
PoLentlal Igh (mV) I lmml Iron ~ ::::::::: Arsenlo ~
Copper ~ Uranium
glno
[
Fig.7 The extraction of base metals and uranium (pH 1.0, Temp. 30°C, 24 hrs)
TABLE 4 The effect of leaching time (80°(3)
Potential (mV) Leaching Time (hrs)
200 24 48
400 24 48
600 24 48
Copper Zinc Arsenic
12 2.3 25 48
18 4.2 30 53
22 19 89 52
Uranium
68
85
98
87 28 97 66 98
Metal/Mineral Extraction (%) Iron
43 27 93 56 98
94 34 97 67 98
No significant increase in copper extractions was achieved. There was however an increase of about 25% in iron extraction at potentials lower than 500 inV. From these results it is evident that there is a negative effect on the extraction of the various metals due to the galvanic interactions that exist between uraninite, arsenic, pyrite and sphalerite minerals. Preferential corrosion occurs when all these minerals are present in the same sample in a sulphurie acid solution where an oxidant is present at various potentials.
Selective dissolution of base metal sulphides
543
( p H - 1.0. T e m p - 80 C 4B hrs)
lO0 908070~ 6050-
i 4030, 20" 10" i
2OO
400
600
Potential Eh (mY)
[
[umm Iron £rsenlo
Copper ~ [7"f'f~Uranium
~
Zlno
Fig.8 The extraction of base metals and uranium (pH 1.0, Temp. 80°C, 48 hrs) (pH -
1.0,
Yemp - BO C. 2 4 brawl
100"
90" 8070-
8 sol 5o40" 30" 20" 10" 0
m
200
400
600
PoLentlal ][h (mV)
Fig.9 The effect of selective base metal extraction on iron and copper extractions
544
L. LOI~NZEN
Costs
Reagents are expensive and an increase in reagent consumption would obviously result in an increase in costs. The testwork however, showed that at high temperatures the amount of oxidant used at low Eh values is negligible and substantial savings can thus be achieved in the cases of uranium and zinc. It is recommended that after a sequential leach is designed, an economic evaluation of the selected leach sequence be conducted, to determine the economical viability of the selected leach, with relation to reagents and metal extraction. CONCLUSIONS Almost all the uranium can be selectively leached at potentials of 200 to 300 mV at 80°C, and a leaching time of 24 to 48 hours. Sphalerite can be selectively leached at 400 mV at 80°C and 24 hours leaching time. Iron (pyrite and marcasite) is found to be best extracted at potential values of about 600 mV at 80°*C and 48 hours leaching time. Arsenic minerals tend to leach partially (60%) at potentials of 200 to 600 V. Copper extractions were low (< 32%) at the potential range of 200 to 600 V, which makes it possible to leach iron, zinc, uranium and partially arsenic bearing minerals from chalcopyrite at Eh values of 600 V and less. The effect of pH is greatly reduced due to the overwhelming effect of potential. Results show that leaching at pH values of 0.5 to 1.0 is no more beneficial than leaching at pH values of 1.0 to 1.5. It is however, not really economically viable to leach at potentials in excess of 600 V, and pH values less than 1.0. Residence time during leaching markedly increases the extraction of iron, zinc and uranium. By leaching at lower potentials, i.e. 200 to 400 V, selective liberation of uranium and zinc can also be achieved at various leaching times. Higher temperature increases leaching rates and extractions of all minerals evaluated. Preferential corrosion occurs when pyrite, sphalerite, uraninite and arsenic minerals are present in the same oxidizing solution. From the results of this testwork it is possible to find an optimum between reagent consumption, temperature, leaching time, potential, costs, and to a lesser degree pH for optimum extraction of the selective base metal, uranium and/or gold. This can be achieved without the significant dissolution of less desirable base metal sulphides or uranium associated with the preferred mineral. As various ores each have their own characteristics and mineralogies, it is recommended that a mineralogical study and/or a diagnostic leach be conducted on a sample first, to determine the feasibility of such a recommended sequential leach for the selected sample. REFERENCES
.
Tumilty, J.A. & Schmidt, C.G. Deportment of Gold in the Witwatersrand system. Gold 100. SAIMM. Proceedings of the International Conference on Gold. Volume 2: Extractive Metallurgy of Gold. Johannesburg. 541-554 (1986).
.
Tumilty, J.A., Sweeney, A.G. & Lorenzen, L. Diagnostic leaching in the development of flowsheets for new ore deposits. Proceedings of the international symposium on Gold Metallurgy, Volume 1: Proceedings of the Metallurgical Society of the Canadian Institute of Mining and Metallurgy, Winnipeg, Canada, CIM, Pergamon Press, 157-169 (1987).
Selective dissolution of base metal sulphides
.
.
545
Lorenzen, L., Francis, P.A. & Sweeney, A.G. A Guide to the practice of diagnostic leaching for process control and optimization. AARL Project No. R/85/201, Report No. 4. (1986). Jackson, E., Hydrometallurgical Extraction and Reclamation. Ellis Horwood Series in Industrial Metals. (1986).
.
Picket, D.J., Electrochemical Reactor Design, Chemical Engineering Monographs 9. Elsevier. 2nd Edition. (1979).
.
Smits, G. Uranium-bearing minerals in Witwatersrand Rocks, and their behaviour during leaching. Mintek 50. Proceedings of the International Conference of Mineral Science and Technology. Ed. L.F. Haughton. 2, 527-538 (1984).
.
Noguera, E.D. Recent advances in the development of hydrometallurgical processes for the treatment of base metal sulphides. Mintek 50. Proceedings of the International Conference of Mineral Science and Technology. Ed. L.F. Haughton. 2, 677-693. (1984).
0892-6875/92 $5.00+0.00 © 1992 P e ~ Press ptc
Printed in Great Britain
AN ELECTROCHEMICAL STUDY OF THE EFFECT OF POTENTIAL ON THE SELECTIVE DISSOLUTION OF BASE METAL SULPHIDES IN SULPHURIC ACID L. LORENZEN Department of Metallurgical Engineering, University of Stellenbosch, Stellenbosch, 7600, South Africa
ABSTRACT
Diagnostic leaching is an analytical tool which was developed at Anglo American Research Laboratories to examine the deportment of gold in an ore, or any type of plant product or intermediary. In this study we make use of the same principle to study the effect of potential and pH on the dissolution of base metal sulphides in a sulphate medium (sulphuric acid) at high temperatures (80°C). This technique was developed to preferentially liberate a particular base metal from a complex ore. In this way, existing base metal sulphide extraction processes, especially where other unwanted base metal sulphides are present, can be optimised so as to yield substantial savings in operating costs. An oxidizing sulphuric acid leach at high temperatures will destroy pyrrhotite, uraninite, labile base metal sulphides (chalcopyrite, sphalerite, orpiment, marcasite), and to a lesser degree pyrite and arsenopyrite. From the results of this testwork it is possible to find an optimum between reagent consumption ( MnO 2 and H 2 g 0 4), temperature, leaching time, potential (Eh), costs and to a lesser degree pH, for the optimal extraction of the selective base metal, and/or gold. This can be achieved without the dissolution of less desirable base metal sulphides associated with the preferred mineral. Keywords Base Metal Sulphides; potential; dissolution; sulphuric acid INTRODUCTION The first step in the design of a process to treat a possible economic precious metal deposit should be an examination of the mineralogy of the refractory ore. Using this and other initial tests, a process to treat the ore can be designed. In the field of gold and base metal metallurgy, the reliance placed on the mineralogical information available to the researcher is very little. If more accurate mineralogical data were available, the first question might be "With which minerals is the desired base metal/precious metal associated, and how will this affect the extraction route I use" [1,2]. Diagnostic leaching (for gold), and the selective dissolution of base metal sulphides (base metals) were designed to answer this question [3].
To find which minerals, base metals/precious metals are associated with, a specific mineral or base metal is first extracted using a selective acid leach. Moreover, the residue from this first stage can be subjected to another selective acid leach, and the process repeated. The procedure can be varied to suit the mineralogy of the matrix material. At the end of the sequential leach, the researcher is left with a complex record of the refractory ore. He can now use this information to formulate a flowsheet to treat the ore for optimal extraction of the preferred mineral, without the dissolution of less desirable base metal sulphides. 535
536
L. LORENZEN
As various ores each have their own characteristics and mineralogies, it is recommended that a mineralogical study and/or a diagnostic leach be conducted first on a sample, to determine the feasibility of such a recommended sequential leach for the selected sample. At the end of the sequential leach it is recommended that an economic evaluation of the desired leach sequence be conducted, to determine if the proposed dissolution is economical viable with relation to reagents. A number of samples were treated. The results were all similar, and in this paper the dissolution of base metals and uranium from a representative sample of a refractory ore, with a high sulphide and uraninite content (+ 50%), will be discussed systematically. EXPERIMENTAL The aim of the project was to generate data on the leaching of base metal sulphides in a sulphate medium. This would establish leaching conditions (of Eh and pH) to preferentially liberate a particular base metal from a complex ore. Every element in nature develops an electrochemical potential in an electrolyte. Internationally, hydrogen's potential has been chosen as the standard potential. Metals with a dissolution potential greater than the dissolution potential of hydrogen (0.0 V), and lower than the dissolution potential of oxygen (1.23 V), are not corroded with the evolution of hydrogen. However, they may be corroded by solutions containing dissolved oxygen [4]. The Nernst equation [5] provides a convenient way for determining potential, knowing the standard reduction potential, E o, pH and the concentration of oxidant or reductant added. Nernst found that Eh = E ° - (RT/nF) In k where Eh is the total change in potential k the equilibrium constant, and n the number of electrons involved in the half reaction. This equation may be simplified to Eh = E ° - 0.0591 pH + k log [oxidant] where k is a constant determined by n, and [ ] denotes concentration in mol/l. Therefore, it can be seen that by adding an oxidant, the Eh value is increased, and likewise by adding a reductant, the Eh is reduced. This principle was used extensively in the project described here. Sulphuric acid (H2SO4) was used as leaching medium [6]. The effects of Eh (100 mV to 700 mV with the selective addition of manganese dioxide, MnO 2, or hydrogen peroxide, H202), pH (0.5 to 2 with the selective addition of H~O4), temperature (ambient to 80°C) and leaching times (16 to 48 hours) on the dissolution of base metal sulphides and uraninite, were determined. The solutions and solids were analysed for the various base metals in question after each test. Testwork was performed on 70% - 200 mesh refractory ore sample. The mineralogical breakdown of the ore are presented in Tables 1 and 2. The general characteristics of the sulphide minerals are, that the pyrite and marcasite are mainly represented by very fine grained crystals which seldom exceed 5 micrometers in diameter. The pyrite and marcasite tend to cluster together and are always finely intergrown with the gangue sphalerite. Chalcopyrite and orpiment on the other hand are present as discrete compact crystals [7].
537
Selective dissolution of base metal sulphides
TABLE 1 Estimated bulk mineral composition
Mineral
Mass (%)
Sulphide minerals Gypsum (CaSO2; 2H20 ) Quartz (SiO2) Uraninite Calcite and minor siderite (CaCO3; FeCO3) Feldspar (sanicline) (Na, K, Ca, alumina silicates) Garnet (andradite) 3CaO Fe203 . 3SiO 2 Alunite and minor jarosite Chlorite, mica and clay minerals Graphite (C)
40 14 12 10 9 9 4 2 1 __1 100
TABLE 2 Sulphide mineral composition
Mass % Ore Minerals
Mineral
Total sample
Pyrite (4FeS2) Marcasite (2FeS2) Arsenopyrite (8FeAsS) Orpiment (4As2S3) Sphalarite (4ZnS) Chalcopyrite (4CuFeS2) Tetra hedrite, molybdenite and other rare sulphide minerals
18,0 13,3 1,8 2,8 2,0 2,0
45,0 33,3 4,5 7,0 5,0 5,0
0.1 40,0
0.2 100
RESULTS AND DISCUSSIONS The results are presented in Figures 1 to 9. From the results and graphs of percentage extraction vs potential for each sulphurie acid leach plotted, several important observations can be made which will be discussed in detail. Note that the lines in the graphs do not represent actual points, but are there to show variations in values. Testwork, was done only on the base metals Cu, As, Fe, Zn and uranium, as they were seen from the head assay to be the primary base metals in this ore. The effect of pH and potential (Eh) on base metal extraction
From the results presented in Figures 1 to 5, it is evident that an increase in potential leads to increased extractions in all five minerals analysed. The increase varies with potential, and ME 5/3--5--R
538
L. LORENZEN
is particularly noticeable in the cases of uranium, zinc and iron. The extraction of arsenic is relatively independent of potential and pH, although average extractions of up to 66% were achieved. In the case of copper the extractions were low and never increased above 32%, irrespective of the potential, pH, leaching time and temperature.
i00
% [
80
I' 0 Ft
60
E X
l r
40
Cl C
I t
20
0 12
0
I
I
I
I
I
200
300
400
800
600
I
0
I00
700
~.h CrnV) pH = 0.5
4-
p H ffi 1.0
~
DH ffi [,5
Fig.l The effect of pH and potential on iron extraction (80°C, 24 hrs)
%
35
C o 13 p
3O 25
e
r
20
E x !
15
r
ca c 1 l
IO
5
0
n
0 0
I
I
I
I
|
I
I00
200
300
400
800
600
700
r.h (mY) pH ~ 0.5
4- pH ffi 1.0
~
pH ~ 1.5
Fig.2 The effect of pH and potential on copper extraction (80°C, 24 hrs) Up to 88% of the iron in the pyrite and marcasite can be extracted at potentials in excess of 550 inV. Zinc behaves very favourably towards redox potential, and extraction values of 80% and higher were achieved at potentials in excess of 400 inV. Almost all the uranium can be selectively leached at low potentials (< 400 mV). An increase in potential (Eh) results in an increase in manganese dioxide or hydrogen peroxide addition which in turn could result in an uneconomical leach, as MnO 2 and H20 2 are very expensive. The amount of oxidant used during the leaches at Eh values o f up to
Selective dissolution of base metal sulphides
539
400 mV were negligible in terms of costs. Basically no HzO z were used during the leaching of uranium and only a small amount of HzO z were consumed during the leaching of zinc at Eh values less than 400 inV. From 400 to 500 mV there is an increase in oxidant consumption and this increase become increasingly prominent at Eh values in excess o f 500 inV. As pyrite and marcasite are the major base metal sulphides present in this ore, it can be calculated that selective iron extraction would be more costly as higher potentials and thus more oxidant are needed to extract the iron.
%
7O
A r $
60
e n 1 c E x t r rl c t 1 o n
J
50 40 3O 20 l0 0 0
I
I
I
[
I
I
100
200
300
400
500
600
700
Eh (mY) DH ffi 0,5
~- pH = 1.0
~
DH = 1.5
Fig.3 The effect of pH and potential on arsenic extraction (80°C, 24 hrs)
120
% Z
I00
l Ft C
80
E x
60
1 r a G
t 1
40
20
o 0 0
I
I
I
I
I
I
Leo
200
30o
4eo
500
6oo
700
Eh (mY) pH - 0 . 5
÷
pH = [ . 0
~,pI{
•l.5
Fig.4 The effect of pH and potential on zinc extraction (80°C, 24 hrs) It can also be seen from the results presented in Figures 1 to 5, that the effect of pH is greatly reduced due to the overwhelming effect of potential. Results show that leaching at pH values of 0.5 to 1.0 is no more beneficial than leaching at pH values o f 1.0 to 1.5. From the results it is evident that due to the high consumption of sulphuric acid, it is not economically viable to leach at pH values less than 1.0.
540
L. LORENZEN
%
t20
U r
100
t
80
E
60
CX D_ U m
X
t
40
t t
20
r c~ c
0
n
0 0
I
I
too
200
DH-0.6
i
i
I
aoo 4oo Eh (mV) 4- p H - L 0
c~oo ~
I
6oo
700
pH-[.6
Fig.5 The effect of pH and potential on uranium extraction (80°C, 24 hrs) The results of the above mentioned extraction tests are summarized in Figure 6. From the figure it can be seen that at a pH of 1.0, temperature of 80°12 and leaching time of 24 hours, it is possible: to leach uranium (U308) partially selective from pyrite and marcasite at potentials of 200 to 400 mV to leach sphalerite (4ZnS) from the other sulphide minerals at potentials of 400 to 500 mV to leach iron or pyrite (FeS~) at potentials of 550 to 650 mV to partially leach arsenic minerals at potentials of 200 to 600 mV to selectively leach arsenic, iron and zinc sulphides as well as uranium from chalcopyrite (4 CuFeS2) at any potential between 200 and 600 mV. As base metals and uraninite are either associated with the pyrite and marcasite or sometimes locked in the pyrite, a hundred percent selective dissolution of a desired base metal cannot be achieved at potentials less than 400 mV. The effect of temperature and leaching time on base metal extraction
Comparing Figure 6 and Figure 7 and as presented in Table 3, it is evident that a decrease in leaching temperature (80°C to 30°), has only a small negative affect on extractions at a potential value less than 400 mV. At potentials in excess of 400 mV this negative affect is even smaller, and extractions similar to that at high temperatures were achieved. There was however, a substantial increase in H202 consumption at lower temperatures, which could be uneconomical at high Eh values. An increase in leaching time at 80°(2 (from 24 hours to 48 hours) have a small favourable influence on arsenic and copper extractions at all Eh values (Table 4 and Figures 6 and 8). An increase in leaching time results in an increase of 15% in uranium extraction at potentials less than 300 mV (Figure 8), an increase of about 10% in iron extractions at potentials higher than 400 mV and an increase of about 8% in zinc extractions at a potential of 400 mV and less. The effect of selective base metal extraction
Ninety eight percent of the uranium, eighty six percent of the zinc and in the region of sixty percent of the arsenic was selectively destroyed by selective sulphuric acid leaches.
Selective dissolution of base metal sulphides
541
The remainder of the sample which contained about 81.8 percent of the original iron and 96.5 percent of the original copper were then evaluated at potentials of 200 to 600 mV at temperatures of 80°C and a leaching time of 24 hours. The results are presented in Figure 9. ( p H - 1,0. T e m p - BO C. 24 hrz)
100 90-
8070 // i/ l/ // // // // /1 Ii // // // /i // // // // //
60-
4O
1
30-
// .-/
•
/J
20-
// // f/ //
10-
f/ // // fL
0
!
20O
400
600
Pot.enUai Eh (mV)
I=
Iron / ~,mnlo ~
Copper ~ Uranium
Zlno
Fig.6 Metal extraction vs Potential (pH 1.0, 80°C, 24 hrs)
TABLE 3 The effect of temperature (24 hrs)
Potential (mV) Temperature (°C)
200 30 80
400 30 80
600 30 80
Metal/Mineral Extraction (%) Iron Copper Zinc Arsenic Uranium
11 2.1 17 32 44
22 14 80 38 83
75 23 95 44 98
12 2.3 25 48 68
22 19 89 52 98
87 28 97 66 98
542
L. LO~NZEN (pH - 1.0. Temp - 30 12. 24 hr's)
100
90 8070E 60-
30" 2010" 0
II
j I
200
)
400
600
PoLentlal Igh (mV) I lmml Iron ~ ::::::::: Arsenlo ~
Copper ~ Uranium
glno
[
Fig.7 The extraction of base metals and uranium (pH 1.0, Temp. 30°C, 24 hrs)
TABLE 4 The effect of leaching time (80°(3)
Potential (mV) Leaching Time (hrs)
200 24 48
400 24 48
600 24 48
Copper Zinc Arsenic
12 2.3 25 48
18 4.2 30 53
22 19 89 52
Uranium
68
85
98
87 28 97 66 98
Metal/Mineral Extraction (%) Iron
43 27 93 56 98
94 34 97 67 98
No significant increase in copper extractions was achieved. There was however an increase of about 25% in iron extraction at potentials lower than 500 inV. From these results it is evident that there is a negative effect on the extraction of the various metals due to the galvanic interactions that exist between uraninite, arsenic, pyrite and sphalerite minerals. Preferential corrosion occurs when all these minerals are present in the same sample in a sulphurie acid solution where an oxidant is present at various potentials.
Selective dissolution of base metal sulphides
543
( p H - 1.0. T e m p - 80 C 4B hrs)
lO0 908070~ 6050-
i 4030, 20" 10" i
2OO
400
600
Potential Eh (mY)
[
[umm Iron £rsenlo
Copper ~ [7"f'f~Uranium
~
Zlno
Fig.8 The extraction of base metals and uranium (pH 1.0, Temp. 80°C, 48 hrs) (pH -
1.0,
Yemp - BO C. 2 4 brawl
100"
90" 8070-
8 sol 5o40" 30" 20" 10" 0
m
200
400
600
PoLentlal ][h (mV)
Fig.9 The effect of selective base metal extraction on iron and copper extractions
544
L. LOI~NZEN
Costs
Reagents are expensive and an increase in reagent consumption would obviously result in an increase in costs. The testwork however, showed that at high temperatures the amount of oxidant used at low Eh values is negligible and substantial savings can thus be achieved in the cases of uranium and zinc. It is recommended that after a sequential leach is designed, an economic evaluation of the selected leach sequence be conducted, to determine the economical viability of the selected leach, with relation to reagents and metal extraction. CONCLUSIONS Almost all the uranium can be selectively leached at potentials of 200 to 300 mV at 80°C, and a leaching time of 24 to 48 hours. Sphalerite can be selectively leached at 400 mV at 80°C and 24 hours leaching time. Iron (pyrite and marcasite) is found to be best extracted at potential values of about 600 mV at 80°*C and 48 hours leaching time. Arsenic minerals tend to leach partially (60%) at potentials of 200 to 600 V. Copper extractions were low (< 32%) at the potential range of 200 to 600 V, which makes it possible to leach iron, zinc, uranium and partially arsenic bearing minerals from chalcopyrite at Eh values of 600 V and less. The effect of pH is greatly reduced due to the overwhelming effect of potential. Results show that leaching at pH values of 0.5 to 1.0 is no more beneficial than leaching at pH values of 1.0 to 1.5. It is however, not really economically viable to leach at potentials in excess of 600 V, and pH values less than 1.0. Residence time during leaching markedly increases the extraction of iron, zinc and uranium. By leaching at lower potentials, i.e. 200 to 400 V, selective liberation of uranium and zinc can also be achieved at various leaching times. Higher temperature increases leaching rates and extractions of all minerals evaluated. Preferential corrosion occurs when pyrite, sphalerite, uraninite and arsenic minerals are present in the same oxidizing solution. From the results of this testwork it is possible to find an optimum between reagent consumption, temperature, leaching time, potential, costs, and to a lesser degree pH for optimum extraction of the selective base metal, uranium and/or gold. This can be achieved without the significant dissolution of less desirable base metal sulphides or uranium associated with the preferred mineral. As various ores each have their own characteristics and mineralogies, it is recommended that a mineralogical study and/or a diagnostic leach be conducted on a sample first, to determine the feasibility of such a recommended sequential leach for the selected sample. REFERENCES
.
Tumilty, J.A. & Schmidt, C.G. Deportment of Gold in the Witwatersrand system. Gold 100. SAIMM. Proceedings of the International Conference on Gold. Volume 2: Extractive Metallurgy of Gold. Johannesburg. 541-554 (1986).
.
Tumilty, J.A., Sweeney, A.G. & Lorenzen, L. Diagnostic leaching in the development of flowsheets for new ore deposits. Proceedings of the international symposium on Gold Metallurgy, Volume 1: Proceedings of the Metallurgical Society of the Canadian Institute of Mining and Metallurgy, Winnipeg, Canada, CIM, Pergamon Press, 157-169 (1987).
Selective dissolution of base metal sulphides
.
.
545
Lorenzen, L., Francis, P.A. & Sweeney, A.G. A Guide to the practice of diagnostic leaching for process control and optimization. AARL Project No. R/85/201, Report No. 4. (1986). Jackson, E., Hydrometallurgical Extraction and Reclamation. Ellis Horwood Series in Industrial Metals. (1986).
.
Picket, D.J., Electrochemical Reactor Design, Chemical Engineering Monographs 9. Elsevier. 2nd Edition. (1979).
.
Smits, G. Uranium-bearing minerals in Witwatersrand Rocks, and their behaviour during leaching. Mintek 50. Proceedings of the International Conference of Mineral Science and Technology. Ed. L.F. Haughton. 2, 527-538 (1984).
.
Noguera, E.D. Recent advances in the development of hydrometallurgical processes for the treatment of base metal sulphides. Mintek 50. Proceedings of the International Conference of Mineral Science and Technology. Ed. L.F. Haughton. 2, 677-693. (1984).