REWAS 2008
Dr. B. Mishra, Dr. C. Ludwig, and Dr. S. Das
TMS (The Minerals, Metals & Materials Society), 2008
EXTRACTION OF GOLD, SILVER AND COPPER FROM THE
COPPER ELECTROREFINING ANODE SLIME: SEPARATION OF
THE METALS.
Gabrielle N. Bard1 and Luis G. S. Sobral1
1
Centre for Mineral Technology-CETEM
Key-words: Electrorefine, anode slime, copper.
Abstract
Copper sulphides Concentrates, from the flotation process of copper primary ore; contain,
commonly, small amounts of precious metals, such as gold and silver. During the smelting
process of those concentrate all gold and silver are practically in the blister copper, which is
cast into anodes. Those copper anodes are electrorefined in an electrolytic system, where
the electrolyte is an acid copper sulphate solution, where a high purity electrolytic copper is
produced (>999,9/1000). The slime when submitted to an oxidative leaching process, using
sodium hypochlorite and hydrogen peroxide as oxidizing agents, turn those metals into
solution. This research practical work, apart from concentrating effort on the digestion of
the anode slime, aims at evaluating the efficiency of the cementation process of the metals
of interest for the separation of them to take place.
141
Introduction
The Flash Smelting process, from Outokumpu/Finland, is the pyrometallurgical that
transform directly copper sulphides, comprising the flotation concentrate, in impure
metallic copper, also known as blister copper [1 - 4].
The pyrometallurgical copper extraction from copper sulphides consists, traditionally, of
the following basic steps: a) roasting of the concentrate; b) smelting of the matte; c)
Conversion, and d) fire Refining [5].
After fire refining, the copper purity is around 99.5% being later purified by electrolytic
refining it to produce high pure copper (>99.99%). Once obtained the blister copper,
properly fire refined, this is cast into anodes so as to feed the electrorefining cells [4, 5] .
The copper electrorefining is accomplished in big cells where a series of cathodes, made
out of pure copper (starting sheets), are positioned face-to-face to impure copper anodes
inside appropriate tanks (steel covered with a high density polypropylene lining) with a
copper sulphate acid solution (H2SO4), using average current intensities around 200A.
During the electrorefining process, metallic copper is deposited in the cathodes, through the
reduction of cupric ions from the electrolyte, while those ions are restored in solution by the
main anodic reaction, which means the dissolution of the impure copper anodes
[6]
. The
copper electrorefining process can be described by the reactions 1 and 2, as follows:
In the anode surface one has:
Cu o ⇔ Cu 2+ + 2e −
(1)
Cu 2+ + 2e − ⇔ Cu o
(2)
In the cathode one has:
The anode slime is collected in the bottom of the electrolytic cells during the copper
electrorefining. This slime generated during the copper refining process contains, among
other base metals, costly precious metals. As previously mentioned, the blister copper, from
the pyrometallurgical processing of copper sulphides (such as chalcopyrite - CuFeS2 and
Bornite - Cu5FeS4) flotation concentrate, is electrorefined generating an anode slime that
accumulates in the bottom of the electrolytic cells [6]. The Flowchart of Figure 1, as follows,
shows, in a concise way, how that slime is produced.
142
Primary Ore
Copper Sulphides
(Chalcopyrite and Bornite)
Mineral Processing
(Crushing, Grinding and Flotation)
o
t > 1000 C
(CuFeS2 , Cu5FeS 4 )
(Flotation Concentrate)
Flash Smelting
Blister Copper
Casting into Anodes
Anode: Blister Copper
Cathode: Copper
(Stating Sheets)
Electrorefining
FTRI
Anode Slime
XRF
(Precious metals +
Base metals)
Cu
Electrolytic
FTIR: Fourier Transformer Infrared
XRF: X-rays Fluorescence
Precious
Metals
Recovery
Figure 1- Anode slime generation from the primary ore.
The main objective of this work was to evaluate and identify what are the best experimental
conditions for extracting gold, silver and copper of anode slime generated during the copper
electrorefining through oxidative hydrometallurgical processes. Such processes consist in
suspending the slime in hydrochloric acid solution in the first place adding drop wise
hypochlorite or hydrogen peroxide solutions, which generate chlorine gas that react with
the aqueous phase to produce strong oxidizing agents , such as hypochlorite (ClO-) and
further chlorate (ClO3-) ions as the reaction goes on. Such ions are strong enough to
dissolve copper, silver and gold.
143
Experimental
In the elaboration of the experiments planning, a factorial design at 2 levels was
used so as to study the best experimental conditions for a better efficiency for extracting the
metals of interest, which in this case Au, Ag and Cu.
Four parameters were defined: solid:liquid ratio, hydrochloric acid concentration,
oxidizing agents and reaction time, and their levels. The stirring speed and the temperature
were maintained, respectively, in 200rpm and 25ºC. The Table 1, below, shows those
parameters and their levels.
A
B
C
D
Table 1- Parameters and their levels considered in the factorial design.
Levels
Parameters:
(-)
(+)
Solid:liquid ratio
1:5
1:2
Oxidizing agent used
H2O2
NaClO
Reaction time
2h
4h
Hydrochloric acid concentration
10% de C=37%w/v 50% de C=37% w/v
The Table 2 and 3, as follows, show the planning and experiments matrixes from the
factorial design.
Table 2- Planning Matrix
Exp.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
A
+
+
+
+
+
+
+
+
B
+
+
+
+
+
+
+
+
C
+
+
+
+
+
+
+
+
Table 3- Experiments Matrix
D
+
+
+
+
+
+
+
+
Exp.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
144
A
1:5
1:2
1:5
1:2
1:5
1:2
1:5
1:2
1:5
1:2
1:5
1:2
1:5
1:2
1:5
1:2
B
H2O2
H2O2
NaClO
NaClO
H2O2
H2O2
NaClO
NaClO
H2O2
H2O2
NaClO
NaClO
H2O2
H2O2
NaClO
NaClO
C
2h
2h
2h
2h
4h
4h
4h
4h
2h
2h
2h
2h
4h
4h
4h
4h
D
10%
10%
10%
10%
10%
10%
10%
10%
50%
50%
50%
50%
50%
50%
50%
50%
The reaction system used is constituted of a magnetic stirrer and a 500mL glass
reactor. In that reactor 50 grams of sludge was suspended in 250mL of 10% or 50% (v/v)
hydrochloric acid solution, where, a funnel was coupled for adding slowly 100 mL of a
30% v/v hydrogen peroxide (H2O2) or 250 mL of 4-6% P.A sodium hypochlorite (NaClO).
(oxidizing agents used for generating chlorine gas (Cl2), in charge of generating the
oxidizing agents to dissolve the metals of the anode slime under study), and, still, an exit
for the gases washing flask containing 250 mL of 20% w/v sodium hydroxide (NaOH)
solution for neutralizing chlorine gas leaving the reaction system. The Figure 2, as follows,
shows an outline of such reaction system:
Figure 2- Reaction system used for treating the anode slime.
The chloride ions oxidation either using hydrogen peroxide or hypochlorite ions is
shown through reactions 3 and 4, as follows:
2 HCl + H 2 O2 ↔ Cl2 + 2 H 2O
(3)
NaClO + 2 HCl ↔ Cl 2 + NaCl + 2 H 2 O
(4)
145
The oxidation of those metals, Au, Ag and Cu, by reacting with chorine is
represented by the following reactions:
2 Au + 3Cl 2 → 2 AuCl3
−
(5)
−
−
Cl
Cl
Cl
Ag 2 SO4 + 
→ AgCl 
→ AgCl 2− 
→ AgCl32−
(6)
Cu 0 + Cl 2 → CuCl 2
(7)
The Table 4, as follows, shows the gold, silver and copper extraction bearing in
mind the aforementioned established experimental conditions.
Table 4- Gold, silver and copper extraction.
Experiment
Au
Ag
1
2.20
3.04
2
----------*
0.86
3
----------*
8.62
4
----------*
3.45
5
2.53
2.32
6
----------*
0.55
7
----------*
5.75
8
----------*
2.89
9
17.80
72.51
10
15.32
27.21
11
10.87
86.15
12
3.62
22.08
13
18.55
57.95
14
21.78
26.35
15
9.40
75.00
16
2.03
61.08
* Not detected
Cu
73.61
81.10
85.86
75.97
69.07
85.10
92.19
67.90
53.51
43.10
99.56
93.58
54.43
58.55
99.56
45.14
As can be observed, analyzing the Table 4, the copper extraction in most of the
accomplished tests was high no matter what oxidizing agent was used as the
reaction of chlorine with the aqueous phase generated oxidizing agents (ClO- and
ClO3-) strong enough to dissolve it despite of any experimental conditions used. In
the case of gold and silver the results were not that efficient either due to the low
HCl concentration, which is the case of tests 1 to 8, or higher solid:liquid ratio
associated with different reaction time, as pointed out in the tests 9 to 16, even with
higher HCl concentration.
146
After precipitating Cu, Ag and Au, the mixture of metals goes through a series of
unit processes and operations so as to separate and further recover them. The flow
diagram of Figure 3, as follows, shows how that separation was accomplished.
o
o
Au , Ag & Cu
o
HNO3
Ag +, Cu 2+
Au
-
o
NO 3
NaCl or HCl
Electrorefining
AgCl
Ag
o
HCOH
+ NaOH
Cu
2+
Cu
o
Fe o
Electrorefining
Electrorefining
Figure 3- Unit processes and operations for separating and recovering Au, Ag and
Cu.
Conclusions
As could be observed, analysing the experimental results, one can conclude that:
The reaction system was quite effective on extracting the metallic values of the anode slime
under study.
The chlorine generation, either using the hydrogen peroxide or sodium hypochlorite, as
oxidizing agents for the chloride ions from the HCl used for suspending the anode slime,
was enough to provide the right atmosphere for dissolving the metals of interest.
The use o H2O2 was more effective in the metallic powder dissolution as its reduction
product is water, which contributes to the ionic strength to rise slowly dissolving,
consequently, more chlorine in the aqueous phase producing more soluble oxidizing
species.
Once dissolved those metals, their separation is straightforward getting them high purity.
147
References
[1] AMER, A.M., Physicochemical Problems of Mineral Processing. vol.36, 2002.
p.123-134.
[2] LANDSBERG, R., Elektrochemische Reaktionen und Prozesse. VEB Deutscher
Verlag der Wissenschaften, Berlin, 1977.
[3] SUBBAIAH, T.; DAS, S.C.; DAS, R.P., Electrowinning of copper under forced
convection. Erzmetall 34, Nr. 10, 1986. p.501-506.
[4] HABASHI, F., The Future of Extractive Metallurgy. ed. Roberto C. Villas Bôas. Rio
de Janeiro: LAVAL/CETEM, 1996.
[5] RIEKKOLA-VANHANEN, M., Finnish expert report on best available techniques
in copper production and by-production of precious metals. Helsinki: Finnish
Environment Institute, 1999. 72p.
[6] BARD, G.N.; SOBRAL, L.G.S.; LIMA, R.B., Recuperação de Metais Preciosos de
Lamas Anódicas de Processos de Eletrorrefino de Ouro, Prata e Cobre. Série de
Tecnologia Ambiental, vol.38, CETEM/MCT, 2006.
148