International Conference on Mining, Mineral Processing and Metallurgical Engineering (ICMMME'2013) April 15-16, 2013 Johannesburg (South Africa)
An investigation of aggressive leaching of
Uitkomst complex (South Africa) Ore and
tailings
M.S. Madiba, and A. F. Mulaba- Bafubiandi
I. INTRODUCTION
Abstract— Nickel in South Africa is mainly produced as
by-product in the production of PGMS from the Uitkomst
Complex deposits. Pentlandite as the main nickel-bearing
mineral is disseminated within a sulphide matrix of pyrrhotite,
pyrite and chalcopyrite while accessories are chromite and
platinum in solid solution with the sulphides. The sulphides
are hosted mainly in ultramafic rocks. Head grade in the
decline, from earlier 0.7 to more recently 0.3% or even lower;
standard froth flotation yields a concentrate of 7 – 9% nickel
at a recovery of up to 70%.
A higher nickel recovery from such low – grade feed stock
is desired. To achieve this aggressive leaching as an
alternative to flotation was employed on run of mine as well
as on tailings materials. The usage of strong hydrochloric and
sulphuric acid and also aqueous ammonia solutions at various
liquid-to-solid ratios was carried out at room temperature.
The tests were conducted on crushed run of mine ore from
MMZ zone in the Uitkomst complex of South Africa, with a
head grade of 0.25% nickel and fine flotation tailings retrieved
from dump with a head grade of 0.15% nickel obtained from a
nickel mine. The materials were ground to 65% < 75 µm in a
laboratory rod mill before being leached.
Leaching of tailings in strong acids at room temperature
resulted in high nickel dissolutions, for example, close to
100% after digestion for 3hours with 1-molar hydrochloric or
sulphuric acid, but only at high liquid/solids ratios of L/S
~100; 80% dissolution can be achieved after only 30 minutes
under these circumstances.
Leaching ore with acids was slower and less complete:
hydrochloric was superior to sulphuric acid (~30% vs. 15%
nickel dissolution) and surprisingly lower liquid/solids ratios
were beneficial. Longer leaching times would yield higher
dissolutions.
Leaching with aqueous ammonia solutions resulted in only
poor nickel dissolutions for both tailings and ore (~1%).
T
HE roughly boat shaped Uikomst complex in South
Africa with a trough at the base is related to the Bushveld
Complex in both composition and age except that metal
sulfides include pyrrhotite, pentlandite, chalcopyrite and
pyrite predominate more than the platinum group elements.
South Africa nickel output was expected to probably
increase from 2008 to 2015 (Yager, et al., 2008). Particularly,
by 2011 it was projected to nearly double because of
increased capacity at local nickel mines and the PGM mines
(Yager, et al., 2007). Out of all South African mines
producing nickel, there is only one which is a primary and
only nickel concentrator while the rest produce nickel as coproduct or by product. As a result of the latter statement this
research was narrowed to investigate only materials from the
Uitkomst complex particularly the main mineralized zone
(MMZ).
The concentrator process comprises of crushing, milling
and flotation. The flow sheet consists of two stage crushing
and one stage milling. Average run of mill is 0.58% grade
while concentrate grade is 9%. The concentrate is toll smelted
and refined while the tailings are presently discarded.
Challenges to remove nickel from the feed due to
complexity of mineralogy of South African uitkomst complex
and chromite content coupled with consideration by mining
companies to exploit low grade ore and tailings, have
motivated the need to investigate in this study alternative
process for the recovery of nickel from sulphide ore and
tailings. So far, current operation of South African mine
consists of floating directly after milling which gives
recoveries of about 60 to 75%. The following is a proposed
concentration processes which was under investigation to
attempt to improve recoveries (1) replacing flotation step by
leaching process using sulphuric acid, hydrochloric acid and
ammonium hydroxide, independently.
Leaching process – The two materials which were run-ofmine ore (ROM), sulphide-nickel ore, and its tailings were
examined. What is interesting is, most of operations utilize
sulphuric acid leaching on oxidised ores rather than sulphides
as it is generally known and proven that oxidised materials
leach reasonably easily compared to sulphides.
Keywords— hydrochloric acid; sulphuric acid; aqueous ammonia
solutions; pentlandite; grade and recovery.
M.S. Madiba, and A. F. Mulaba- Bafubiandi are with Mineral Processing
and Technology Research Group, Metallurgy Department, Faculty of
Engineering and the Built Environment, University of Johannesburg, PO Box
5216, Wits 2050, South Africa, [email protected].
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International Conference on Mining, Mineral Processing and Metallurgical Engineering (ICMMME'2013) April 15-16, 2013 Johannesburg (South Africa)
II. EXPERIMENTAL
III. RESULTS
A. Communition process
Nickel ore and flotation tailings were received, crushed and
milled before being pulverized. The milled sample was
pulverized using a Siebtechnik pulveriser to make materials
finer for production of effective and reliable results. The
pulverized 1 kg’s samples of nickel sulfide ore were further
split into 250 g which were further divided into 10
representative portions using small spinning riffle of 250 ml
cups to give 25 g each. The 25 g samples were collected and
labeled for characterization tests and leaching testwork.
A. Characterization of two samples:
Representative portions of the sample were analyzed by
qualitative, backscattered SEM to determine the major gangue
and ore minerals and their relative abundances. This analysis
revealed that the samples consisted of various proportions of
silicate minerals which were found to be particularly abundant
on both run of mine ore and tailings; pyrite was also detected
as one of the primary mineral contained in both samples and
consisted generally of pentlandite and pyrrhotite within the
sample (approximately 0.15% in flotation tailings and 0.25%
in run of mine samples); the presence of iron oxide was
confirmed with traces of magnesium, aluminum and chloride
in the flotation tailings sample. Pentlandite and pyrrhotite
minerals often occurred as discrete, liberated grains (often <50
µm in float tailings and less than 75 µm in run of mine
samples) and to a lesser extent as intergrowths with
transparent gangue. EDAX examination of the silicates
minerals confirmed that these contained predominantly iron,
calcium, magnesium and aluminum; being present in smaller
amounts were sodium, manganese, titanium, potassium,
chromium and chloride traces. The principal nickel bearing
sulphide minerals contained mainly pyrhotite and pentlandite,
and small traces of chromium.
B. Particle size analyzer
Microtrac, S3500, particle size analyzer with Tri- laser
technology was utilized on two materials to determine the
contained percentage of particles of less than 75 μm for
leaching process as it has been established that the finer the
particle size the better the leaching behavior, resulting in
improved recovery (Ntengwe, 2010; David, 2007).
The pulverised 25 g’s samples were leached by;
• Hydrochloric acid leaching at different leaching duration
• Sulphuric acid leaching at different leaching duration
• Ammonium hydroxide leaching at different leaching
duration.
C. Hydrochloric acid leaching
Two materials were leached with HCl independently for 30,
45, 60, 90, 120, and 180 minutes to determine the effect of
leaching time on nickel recovery at three lixiviant
concentrations of 1, 3 and 5M HCl acid. Solid/liquid ratio
used was 1: 20. 300 ml was added to 15 g of solid sample. For
each lixiviant concentration, the results of Ni Recovery versus
leaching time at constant solid ratio of 1:20 were plotted.
Then three graph lines of each lixiviant concentration were
compared to determine the lixiviant concentration which gives
improved Ni recovery.
S/L ratio of 1: 40 and 1: 100 were investigated, using the
same procedure as for 1:20 solid ratio above. Then S/L ratios
of each lixiviant concentrations were also compared to
examine the one that renders better results.
B. Particle size analyser:
The particle size used was found to be approximately at
70% passing 75 μm for both two materials when manual wet
screening was employed and particle size analyzer used,
identified the amount of particles to be at approximately 80%
passing 75 μm which implies that efficient leaching might be
expected since it is established that the finer the material the
more efficient is the leaching process.
C. Results of leaching of the tailings at different leaching
duration
Results of hydrochloric acid leaching – The following
results were obtained when hydrochloric acid leach was
employed at different leaching duration;
100
100
80
Ni Rec.
Ni Rec.
D. Sulphuric acid leaching
Methods used were the same as hydrochloric leaching
except that in this case sulphuric acid was used instead of
hydrochloric acid as lixiviant.
80
70
60
50
40
40
30
0
1
2
3
4
0
5
2
3
4
1:20
5M HCl
90
1
Time(hr)
Time(hr)
100
1:40
80
Ni Rec.
70
60
50
30
E. Ammonium hydroxide leaching
Same leaching method as sulphuric acid and hydrochloric
acid leaching was applied except that ammonium hydroxide
was used as lixiviant.
After all mentioned leaching tests conducted, the results
that rendered better performance for each lixiviant were
identified and eventually compared. In all cases, AAS was
used to analyse the amount of nickel dissolved.
3M HCl
90
1M HCl
90
1:100
70
60
50
40
30
0
1
2
3
4
5
Time(hr)
Fig. 1: 1M HCl, 3M HCl and 5M HCl leaching of the tailings at
different S/L ratio
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5
International Conference on Mining, Mineral Processing and Metallurgical Engineering (ICMMME'2013) April 15-16, 2013 Johannesburg (South Africa)
The results presented in the figure above also confirm that as
the leaching time increases the recovery also increases. The
recovery appears to be sensitive to changes in solid liquid
ratio. In all the trends in the above figure, solid liquid ratio of
1:100 is showing recoveries of 75%, 95% and 95% with
lixiviant concentrations of 5M, 1M and 3M, respectively,
compared to the less diluted pulps. These results are the
inverse of the HCl acid leach of the ore. When the solid
concentration increases the recovery increases. Nickel
recoveries of the tailings substantially increased from 39 –
100 % as compared to the HCl acid leach of the ore which
recovered 9 – 50%.
When all the solid liquid ratios of 1:100 of 1M HCl, 3M
HCl and 5M HCl are compared, 5M HCl had proven to give
more than 80% recovery at 30 min leach time as compared to
1M and 3M HCl, see Figure 4.2 below.
100
80
Ni Rec.
Ni Rec.
3M H2SO4
90
70
60
80
70
60
50
50
40
40
30
30
2
1
0
5
4
3
0
1
2
Time (hr)
4
5
1:20
90
Ni Rec.
3
Time (hr)
100
5M H2SO4
1:40
80
1:100
70
60
50
40
30
0
1
2
3
4
5
Time (hr)
Fig. 3: 1M H 2 SO 4 , 3M H 2 SO 4 and 5M H 2 SO 4 leaching
of the tailings at different S/L ratio
100
3M
80
100
1M H2SO4
90
1M
90
5M
1M
3M
90
70
Ni Rec.
Ni Rec.
100
60
5M
80
50
70
40
30
60
0
1
2
3
4
5
5
4
3
2
1
0
Time (hr)
Time(hr)
Fig. 2: Comparison of maximum recoveries of 1, 3 and 5M
HCl leaching of the tailings at the S/L ratio of 1:100
Fig. 4: Comparison of maximum recoveries of 1, 3 and 5M
H 2 SO 4 leaching of the tailings at the S/L ratio of 1:100
In this case, 3M HCl leach produced approximately 58%
nickel which was slightly lower than its counterpart, 1M HCl,
with 65% nickel. Since 3M HCl leach is more concentrated
than 1M HCl, it was expected to dissolve more of nickel than
1M HCl leach. Sulphuric acid leach of the tailings was also
carried out to check which acid leach will give improved
recoveries. The results are as follow;
Results of sulphuric acid leach – Better recovery (98%) is
achieved using 1M H 2 SO 4 at 1hr leach time as compared to
other higher lixiviant concentrations used at the same leach
time. However, the results of 3M H 2 SO 4 are acceptable as it
has recovered 80% at 1hr. According to the findings above,
the less the diluted lixiviant is, the better the recovery.
The highest recoveries obtained from HCl and H 2 SO 4
leaching was compared to check the lixiviant that gives
improved results, see figure 5 below. 1M H 2 SO 4 leach
rendered improved results as compared to 5M HCl.
100
5M HCl
Ni Rec.
1M H2SO4
90
80
70
0
1
2
3
4
5
Time(hr)
Fig. 5: Comparison of maximum recoveries obtained by 5M HCl
and 1M H 2 SO 4 leaching of the tailings at the S/L ratio of 1:100
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International Conference on Mining, Mineral Processing and Metallurgical Engineering (ICMMME'2013) April 15-16, 2013 Johannesburg (South Africa)
Ni Rec.
The results observed are contrary to HCl leach kinetics which
is known to be more aggressive than H 2 SO 4 . The result
confirms that for HCl to have high dissolution rates it depends
on the mineralogy of the material.
Results of NH 4 OH leaching - Ammonium leaching was
also conducted using different concentration at solid liquid
ratio of 1:20, see figure 6 for the results.
3.0
2.7
2.4
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0.0
and environmental benefits (Han, 1996) however in this case
the expected results were not reached for nickel sulphide ore.
Recovery from tailings was substantially higher when
compared to run of mine ore.
As it was expected increase in leaching time increased the
recovery except H2SO4 leaching of the ore and NH4OH
leaching of both materials. It was expected that when the
lixiviant concentration increases the recovery will also
escalate but in this study the testwork showed the opposite of
what was expected which could have been caused by the
variation of the amount of grains of value mineral contained in
each batch collected.
As the solid liquid ratio increased the recovery also
increased for both HCl and H2SO4 leaching of tailings.
However, run of mine ore decreased in value recovery as the
solid liquid ratio increased.
When three types of lixiviant were compared in terms of
their dissolution strength; for the leaching of tailings,
sulphuric acid recovered more than hydrochloric acid while in
the ore leaching solution hydrochloric acid kinetics were
faster than sulphuric acid. As a consequence, it can be
concluded that leaching efficiencies of nickel from the tailings
were favored by an increase in the concentration of sulphuric
acid as well as contact time. The leaching efficiencies of
nickel from the ore were favored by an increase in the
concentration of hydrochloric acid.
1M
3M
5M
0
1
2
3
4
5
Time (hr)
Fig. 6: NH 4 OH leaching of the tailings at different
concentration at the S/L ratio of 1:20
It can be seen that ammonium leach far less than HCl and
H 2 SO 4 . Recoveries are unacceptable as a consequence they
are excluded.
Comparing the effectiveness of the all types of lixiviant, it
was observed that HCl and H 2 SO 4 achieved a maximum
recovery around 100% than in general HCl performed better
than H 2 SO 4 , this finding correlate with other findings as it is
stipulated that HCl is more aggressive than H 2 SO 4 . Although
HCl has been reported to be more aggressive than H 2 SO 4 it
should be noticed that most of the works conducted on the
ore, which had different characteristics to the tailings used in
this study. Ammonium leach recovered almost nothing.
Overall summary of the leach results – Sulphuric acid
leach of the ore showed slightly low recoveries than
hydrochloric acid leaching. As for the ore, hydrochloric acid
was more aggressive than sulphuric acid leaching. Dissimilar
results were obtained for the tailings. Sulphuric acid leaching
was more aggressive than hydrochloric acid. HCl acid leach of
the ore, the recovery increased as the solid concentration
decreased. Similar results were observed in H 2 SO 4 acid leach
of the ore.
ACKNOWLEDGMENT
The authors wish to acknowledge the financial contribution
from the University Research Committee (UC) and the ore
and tailing materials received from the South African only
primary concentrator of nickel minerals.
REFERENCES
[1]
[2]
[3]
[4]
[5]
IV. CONCLUSION
It was found that ore mineralogy plays a major role in the
dissolution process. Although most of these studies showed
promising potential for high metal recovery except for
ammonium hydroxide leaching. All ammonium hydroxide
leaching testwork of two materials faced a problem of
extremely slow process kinetics. For both materials
ammonium hydroxide leaching recovered far less than 1.5%.
As much as it is known that hydrometallurgical metal
extraction is highly attractive due to its potential for economic
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