B. Mwakila,1 R. Silwamba,1 G. Miller,2 and GC Chisakuta3 1Nkana

IMPROVEMENT IN COPPER AND COBALT PRODUCTION DUE TO
INCLUSION OF SOLVENT EXTRACTION OF COPPER AT MOPANI COPPER
MINES PLC’S NKANA COBALT RLE PLANT.
B. Mwakila,1 R. Silwamba,1 G. Miller,2 and G. C. Chisakuta3
1
Nkana RLE Cobalt Plant, Mopani Copper Mines Plc, Box 22000, Kitwe,
Zambia. 2Miller Metallurgical Services, Australia. 3Cognis Corporation,
Chingola, Zambia. Corresponding author email address:
[email protected]
ABSTRACT
Nkana Cobalt Plant Roast-Leach-Electrowin produces cobalt from cobalt concentrates
from Nkana Concentrator by roasting to convert insoluble metal sulphides into soluble
sulphates and oxides. The soluble sulphates and oxides (calcine) are then leached, getting
the cobalt into solution along with copper and other impurities. Up until December 2006,
advance electrolyte was routed to the copper tankhouse for electrowinning and
electrostripping of copper as a first step in impurity removal. Stripped liquor was then
sent to a cobalt purification circuit for classical precipitation by automated pH control.
Theis process has now been modified to include copper solvent extraction to first remove
copper from calcine leach solution. This paper discusses how inclusion of Solvent
Extraction has resulted in increased productivity.
INTRODUCTION
Nkana Cobalt Plant Roast-Leach-Electrowin (RLE) was commissioned in 1982 to
produce cobalt as a by-product of a copper producing metallurgical complex, receiving
its cobalt concentrates from Nkana Concentrator [1]. The cobalt is associated with a
copper ore as a carrolite. Resultant cobalt concentrate has the following typical assay
grades: 1.6 % Cobalt, 9.0% Copper, and 21.0% Sulphur
When the plant was designed in the 1980’s, solvent extraction technology was not
immediately adopted as a means of copper impurity separation because most of the
copper solvent extraction reagents at that time were mainly meant for very dilute
solutions (max 0.5-7 gpl Cu in PLS) and were quite sensitive to pH [2]. Separation of
copper from the cobalt bearing solution thus continued to be via electrochemical means.
Copper bearing cobalt solution was initially sent to an electrowinning copper tank
house where copper was subjected to electrowoninning, then electrostrippeding. Copper
produced from these processes was treated as scrap copper suitable for doping in
converters at the smelter. Resultant stripped solution, relatively free of copper, was
routed to the purification circuit, where classical pH precipitation methods are, up to
today, used to remove other impurities from the cobalt solution.
The process of electrowinning and electrostripping had posed serious production
constraints, as it required meticulous blends to be prepared with a Copper to Cobalt ratio
of 4:1 in the roaster feed. There was also a limit placed on the resultant copper advance
electrolyte to the electrowinning tank house. Average acceptable concentrations were 20
– 25 gpl Cu. Concentrations above these were a serious process constraint.
To remedy this, solvent extraction was reexaminedlooked at. With development
of solvent extraction technology, stronger and more powerful reagents capable of
handling very high copper concentrations in the region of 25-30 gpl Cu in pregnant
liquor solution had become available. After pilot plant trials, the circuit was successfully
modified to include copper solvent extraction as a first step in impurity removal, and a
solvent extraction plant was commissioned in January 2007.
PROCESS DESCRIPTION OF MODIFIED CIRCUIT
Copper SX was included between calcine leach and iron removal. To remove
copper to the desirable level, the copper solvent extraction plant was designed with a two
stage extraction circuit, namely Primary and Secondary extraction [3]. The configuration
used is shown in Figure 1, whileand Table 1 shows typical design parameters.
Ferric Thickener
Overflow
Final
Raffinate
Primary
Raffinate
PLS
E4
E1
E2
E5
E3
Stripped Organic
Loaded Organic
Advance
S1
S2
Spent
Formatted: Justified, Indent: First
line: 1.27 cm
Figure 1. Configuration of the SX circuit at Nkana RLE Cobalt Plant
Formatted: Left
Table 1. Design parameters of the SX at Nkana RLE Cobalt Plant
Solution
PLS
Primary Raffinate
Ferric Thickener
O/F
Loaded Organic
Parameter
Cu tenor (g/L)
Co tenor (g/L)
Flow (m3/h)
pH
Recovery (%)
Cu tenor (g/L)
Co tenor (g/L)
Flow (m3/h)
pH
Recovery (%)
LIX 984N conc. (%v/v)
Flow (m3/h)
Value
25.0
10.0
127
0.7
89
2.7
9.5
60
3.5
97
23
350
Primary Solvent Extraction
Primary solvent extraction comprises three extraction mixers-settlers (E1, E2, and
E3) that lower the concentration of copper in calcine leach solution (CLS) from 25 gpl
Cu down to 1 gpl Cu. The resulting primary raffinate is pumped back to the leaching
circuit, and about 60 m3/hr, is bled off to Ferric cascade for iron removal; by lime
precipitation method. The resulting ferric thickener overflow from iron removal is
pumped back to the secondary solvent extraction.
Secondary Solvent Extraction
There are two smaller mixer-settlers (E4 and E5) that make up the secondary
extraction circuit. Ferric thickener overflow from iron precipitation enters E4 and exits
E5 after residue copper from the primary raffinate has been recovered. Final raffinate
from E5 is pumped to the cobalt purification circuit where classical pH precipitation
techniques are employed to remove other impurities such as zinc in the clean-up cascade
circuit. The cobalt purification circuit produces an advance electrolyte for cobalt
electrowinning.
Stripping of Loaded Organic
After loading copper from cobalt solution in secondary extraction, and from
calcine leach solution in primary extraction, the organic is stripped in two stages (S1 and
S2) to produce an advance electrolyte that feeds the copper electrowinning tankhouse. In
Comment [G1]: 1.0 g/L in the text
total, the solvent extraction plant has a design capacity to transfer 20,000 tonnes of
copper per annum.
The process modification is shown in Figure 2 along with the original circuit.
ORIGINAL CIRCUIT
MODIFIED CIRCUIT
Roaster
Roaster
Leaching
Copper
Electrowinning
Scrap
Cu
Leaching
LME
Grade A
Copper
Solvent
Extraction
Cu
Tank
House
Copper
Electrostripping
Ferric
Cascade
Ferric
Cascade
Clean-up II
Cascade
Clean-up II
Cascade
Hydroxide
Cascade
Hydroxide
Cascade
Resolution
Cascade
Resolution
Cascade
Solution
Classification
Cobalt
Electrowinning
pH
based
precipitation
Cobalt
Metal
Solution
Classification
Cobalt
Electrowinning
Figure 2. Comparison of the original and the modified circuits
DISCUSSION
Nkana Cobalt Plant has derived significant improvements in productivity
following this circuit modification. The constraint placed on roaster blend to limit copper
input to the circuit has been removed, as copper tank house now operates a separate
entity that picks up copper, not directly from CLS, but from solvent extraction to produce
LME grade A copper. Figure 3 shows copper input, copper production as well as cobalt
metal production, both before and after the modification.
2000
500
Cu Production
1600
Tonnes Cu
450
Cu Input
400
Co Production
1400
350
1200
300
1000
250
800
200
600
150
400
100
200
50
'07
ay
M
ar
'07
M
'07
Ja
n
'06
v
06
No
Se
p'
'06
Ju
l
'06
ay
M
ar
'06
0
M
Ja
n
'06
0
Tonnes Co
1800
Month
Figure 3. Historic Trends of Copper/Cobalt input and production
Copper input into the circuit and corresponding production has gone up. Copper
production has increase from an average 800 tonnes of copper per month to 1,300 tonnes.
Similarly, cobalt production at an average of 160 tonnes per month is above the previous
average of 140 tonnes.
Production of LME grade copper from electrowinning has reduced costs and has
freed up smelter capacity.
CONCLUSION
Use of solvent extraction of copper to remove copper as an impurity from cobalt
solution while producing LME grade copper has been successfully implemented at
Nkana Cobalt plant. Major benefits of this have been (i) reduction of roaster constraint,
which means higher copper input than before can now be handleds than before, (ii)
double handling of scrap copper from electrowinning and electrostripping has been
eliminated, and (iii) a consistent production of cobalt can be sustained.
ACKNOWLEDGEMENTS
The authors would like to thank Mopani Copper Mines Plc, especially
Management and Staff at Nkana Cobalt Plant, for their permission and support in writing
this paper. Our special thanks also go to Shankar Aburi Rao for the proof reading the
manuscript and offering invaluable critique to the technical content of the presentation.
REFERENCES
1.
2.
3.
J. Aird, R. S. Celmer and A. V. May, “New Cobalt Production from R.C.M.’s Chambishi
Roast-Leach-Electrowin Process,” Mining Magazine, 320-323 (October 1980).
Teh C. Lo, Malcom H I Baird and Carl Hanson, Handbook of Solvent Extraction, John
Wiley & Sons, New York, 1985; pp. 650-657.
G. M. Miller and A. Nisbett, “Decreasing Operating Costs and Soluble Loss in Copper
Hydrometallurgy With Use of Innovative Solvent Extraction Circuits,” First Extractive
Metallurgy Operator’s Conference, Brisbane, QLD, Australia, 2005.