American Journal of Engineering, Technology and Society 2015; 2(6): 162-166
Published online November 10, 2015 (http://www.openscienceonline.com/journal/ajets)
ISSN: 2381-6171 (Print); ISSN: 2381-618X (Online)
Gold Recovery by Cyanide Leaching: A Case Study of Small Scale Miners in Tanzania
Justin William Ntalikwa
Department of Mining and Mineral Processing Engineering, School of Mines and Petroleum Engineering, College of Earth Sciences, the
University of Dodoma, Dodoma, Tanzania
Email address
[email protected], [email protected]
To cite this article Justin William Ntalikwa. Gold Recovery by Cyanide Leaching: A Case Study of Small Scale Miners in Tanzania. American Journal of
Engineering, Technology and Society. Vol. 2, No. 6, 2015, pp. 162-166.
Abstract
In this study, sodium cyanide leaching technology has been used to recover gold from tailings that are used by Mawelo small
scale miners, located in Chunya district, Mbeya region, Tanzania. The sample collected was sent for analysis of mineralogical
composition and average particle size. The fractions retained on each sieve, which ranged 180 – 500 µm were used in the
leaching experiments. The leaching was implemented using sodium cyanide with concentration in the range of 500 – 1200
ppm, the pH of the reaction mixture was maintained in the range of 10.2 to 10.5 by addition of 5 g of lime (CaO). The retention
time spanned the range of 24 to 96 hours. It was observed that the average particle size, P80 (80% of material passing) of the
sample was 480 µm this was not equal to the liberation size of the sample. In order to increase the gold recovery, grinding of
the sample to 180 µm is required. The mineralogical composition of the sample revealed: gold: 5.85 g/t, copper: 150 ppm,
sulphur: < 0.01 ppm, arsenic: 1.82 ppm, cobalt: 18.25 ppm and nickel: 23. 89 ppm. With 180 µm particle size, the cyanide
dosage in the range of 700-1000 ppm, retention time of 72 hrs, gave a gold recovery of 2.45 ppm which was better than all
parameters studied but represented 42% of the gold recovery in the sample. From this study it is evident that analysis of the
mineralogical composition of the ore and attaining its liberation size are mandatory requirements to effective and efficient
cyanide leaching process.
Keywords
Mawelo, Small Scale Miners, Cyanide Leaching, Gold Recovery, Retention Time
1. Introduction
In Tanzania, extraction of gold from ores by small scale
miners takes place in various areas; these include, the Lake
Victoria, Lupa, Mpanda and Chunya goldfields. Currently,
small scale gold reserves have also been discovered and
exploited by artisanal miners in areas of Tanga, Morogoro
and Iringa regions [1]. The recovery of gold in these areas is
commonly done by using local technology of amalgamation
using mercury. Due to environmental and health concerns
associated with mercury, a good number of artisanal miners
have now started using the technology of leaching by using
sodium cyanide. This technology is more efficient in terms of
gold recovery as compared to the previous one. However,
there are many draw backs associated to the use of this
technology by artisanal miners, these include, among others,
excessive dosage of cyanide subsequently leading to low
profit, and health effects associated with toxicity of sodium
cyanide.
The chemical element gold, symbol Au, is classified as a
noble metal due to its inertness to chemical reactions in non-
complex media like aqueous bases. It does, however, react
with numerous reagents like a mixture of hydrochloric acid
and nitric acid (aqua-regia) and also gold can react with
halogens example solution of chlorine to form gold chloride
(AuCl3). It belongs to the same group as copper and silver in
the periodic table and it is commonly found to be associated
with these elements in rocks. In nature, gold occurs
predominantly in the native state or as a major constituent of
various alloys containing mainly silver, copper, or platinum
metals. Several gold and gold-silver tellurides are known, of
which the most common are sylvanite (AuAgTe3), calaverite
(AuTe3), montbroyite (Au2Te3) petzite, krennerite, and
nagyagite. The antimonide, aurostibite, (AuSb3), occurs in
some auriferous deposits, and there are also argentiferous
American Journal of Engineering, Technology and Society 2015; 2(6): 162-166 163
gold selenide, fischesserite, (Ag3AuSe2), an argentiferous
gold sulfide (Ag3AuS2), and a bismuthide, maldonite,
(Au2Bi), which is fairly well differentiated [2].
There are many possible methods to recover gold from
ores; these include amalgamation, gravity concentration,
leaching and flotation. This work focuses only cyanide
leaching, a method that is on its onset application by artisanal
miners in Tanzania. This method has been the main
metallurgical process for gold extraction for more than one
century [2]. Cyanide is universally used because of its
relatively low cost and great effectiveness for gold
dissolution [3].
The chemistry of gold dissolution in cyanide solution has
been reported by many researchers [2, 4, 5, 6, 7]. Cyanide
salts, e.g. sodium cyanide (NaCN) and potassium cyanide
(KCN) have been widely used with sodium cyanide more
preferable than potassium cyanide because potassium
cyanide form toxic at low concentration compared to sodium
cyanide and the solubility of sodium cyanide is 37mg/100ml
while that of potassium cyanide is 41mg/100ml. Sodium
cyanide salt ionize and dissolve in water to form their
respective metal cation (Na+) and free cyanide ions (CN
-).
Cyanide ions hydrolyze in water to form molecular hydrogen
cyanide (HCN) and hydroxyl ions (OH-) with a
corresponding increase in acidic pH. Hydrogen cyanide is a
weak acid which incompletely dissociates in water to form
H3O+ and CN
-.
At approximately pH 9.3, half of the total cyanide exists as
hydrogen cyanide and half as a free cyanide ion. At pH 10.2
more than 90% of the total cyanide is free cyanide (CN-),
while at pH 8.4 over 90% exists as hydrogen cyanide. This is
important because hydrogen cyanide has a relatively high
vapor pressure (100kPa at 260C) and consequently it
volatilizes readily at the liquid surface under ambient
condition, causing a loss of cyanide from solution. The rate of
volatilization depends on the hydrogen cyanide concentration,
surface area and depth of the liquid, temperature and transport
phenomena associated with mixing. As a result most cyanide
leaching system operates at pH which minimizes cyanide loss,
typically above pH 10 [2]. The dissolution can typically be
represented as shown in equation 1
4Au + + 8CN− + O2 + 2H2O → 4Au (CN−)2 + 4OH− (1)
The factors that affect the gold dissolution reaction include
particle size of the ore, slurry pH, concentration of oxygen,
mixing or agitation, temperature, residence time, slurry
density, presence of cyanocides and oxygen consumers,
presence of sulphide minerals, pyrite, chalcopyrite and
pyrrhotite, presence of galena and asenopyrite, copper ions,
iron ions and carbonaceous materials. All of these parameters
have various effects on the gold dissolution reaction and
substantive discussion has been presented in literature [8, 9,
10, 11, 12]. It has been noted that gold extraction increases
with increasing concentration of cyanide but it it becomes
independent of cyanide concentration when it exceeds 750
ppm [13, 14]. Also a ratio of cyanide to copper ions of 3:1 is
considered to be sufficient to obtain high gold leaching rate
[2, 15].
At Mawelo scale miners the leaching is normally carried
out in concrete vats whereby tailing are normally contacted
with sodium cyanide at a concentration of up to 1500 ppm at
ambient temperatures with practically no agitation. The
slurry pH is maintained between10-11 by addition of lime
(CaO). The retation times are normally in the range of 3 -4
days. These parameters have led to most of the miners to
complain that they are not making profit out of this process.
This work therefore aims at evaluating the extent of gold
recovery achieved by the parameters maintained by the
miners at the site and recommending appropriate practice that
must be adhered to. The tailings were collected at Mawelo
small scale miners and sent for laboratory analyses. The
specific objectives of the work included determination of:
mineralogical composition of the ore, particle size analysis,
concentration of cyanide required for leaching the ore and
required retention time for effective leaching.
2. Materials and Methods
A sample of tailings of about 30 kg used by one of the
Mawelo small scale miners was collected from the site and
sent for laboratory investigations at the Geological Survey of
Tanzania (GST). The laboratory analysis conducted on the
sample included: Mineralogical composition, particle size
analysis and cyanide leaching. The sieve size used ranged
between 180-500 µm, and one kilogramme (1.0 kg) of the
sample was used in each sieve analysis experiment and the
material retained in each sieve was sent for cyanide leaching.
A 6.0 litre bottle was used for leaching experiments, whereby
250g of ore sample were contacted with sodium cyanide
solution at ambient temperature. The concentration of
cyanide dosage was varied in the range of 500 to 1000 ppm,
pH of the materials was maintained in the range of 10.2 to
10.5 by addition of 5.0g of lime. The retention time for
leaching experiments was varied between 24 to 96 hours. The
mineralogical composition and gold recovery were
determined using Atomic Absorption Spectrometer (AAS)
and X ray fluorescent (XRF). No attempt was made to grind
the sample so as to mimic what is done at Mawelo small
scale miners.
3. Results and Discussion
Table 1 provides the results for mineralogical composition
of the ore, suggesting that the sample has a grade of 5.85 ppm
(g/t) gold which is good enough for artisanal gold recovery.
The ore has also high percentages of SiO2 and other elements
such as Cr, V, Sr, Zr, Pb and Ni are also present in considerable
amounts. It can also be noted that the potential cyanide
consumers such as Cu, Ni, Fe, S and As are present in this
sample. Thus for efficient leaching to proceed the ratio of 3:1
of cyanide with copper and other cyanide consumers should be
considered [2, 15]. For sample, cyanide concentration equal
to/greater than 500 ppm is suitable for efficient leaching of this
sample so as to counteract the effect of cyanocides.
164 Justin William Ntalikwa: Gold Recovery by Cyanide Leaching: A Case Study of Small Scale Miners in Tanzania
Table 1. Results for mineralogical composition of the sample.
Component SiO2 AL2O3 Fe2O3 MnO MgO CaO Na2O TiO2 TiO2
% w/w 89.19 0.0 4.70 0.07 0.84 1.62 1.43 0.31 0.31
component Au Cu As Ce Co Cr S Cs Ga Nb
ppm 5.85 150 1.82 3.87 18.25 178.44 < 0.01 6.61 6.98 9.12
component Ni Pb Rb Sc Ta Sr V Zn Zr
ppm 23.89 27 .63 17.47 4.42 0.52 54.36 94.06 15.03 60.83
Figure 1. Results of particle size analysis of the sample.
The average particle size (P80) of the ore sample collected
was 480 µm (Figure 1) suggesting that most of the particles
for this material were in the course range. The gold recovery
decreases with increase in particle size and also increases
with increase in cyanide concentration (Figure 2). For this
material, it is apparent that the liberation size has not been
attained and grinding of the sample is needed to further
reduce the particle size. For the sample collected, the gold
recovery appears to be much better (0.9-1.3 ppm) for particle
size of 180 µm (Figure 2). The gold recovery also increases
with increase of cyanide concentration; however, for cyanide
concentration in the range of 1000 to 1200 ppm, the gold
recovery appears to have no significant difference between
the two concentration levels for the particle sizes studied.
This indicates that addition of cyanide greater than 1000 ppm
is unnecessary and imposes unnecessary costs.
The trends depicted in Figure 3, 4, and 5 are similar to that
in Figure 2, however, there is a significant improvement of
gold recovery. For instance at 180 µm, 1000 ppm, the gold
recoveries are 1.58, 2.45 and 2.46 ppm for Figure 3, Figure 4
and Figure 5 and retention times of 48, 72, and 96 hours
respectively. This suggests that gold recoveries obtained at
retention times of 72 and 96 hours have no significant
difference and hence the retention time of 72 hours appears
to be sufficient for leaching the ore collected.
It can be noted that the recovery of 2.46 ppm represents
about 42% of the total amount of gold in the sample and thus
the rest (58%) was unrecovered by this process. This means
that the status quo of operations of small scale miners at
Mawelo leaves a significant amount of gold which is
unrecovered. This calls for significant improvement of the
processes they use and this could possibly be done through
training on appropriate practice, grinding the sample to
required liberation size. The grinding imposes an additional
cost to the process through the energy and grinding
equipment requirements in which the artisanal miner have to
incur in order to have effective and efficient leaching
process.
Figure 2. Gold recovery as a function of particle size and cyanide
concentration at a retention time of 24 hrs.
Figure 3. Gold recovery as a function of particle size and cyanide
concentration at a retention time of 48 hrs.
Figure 4. Gold recovery as a function of particle size and cyanide
concentration at a retention time of 72 hrs.
The gold recovery increases with increase cyanide
concentration and it generally increases with decrease in
particle size (Figure 6 and 7). The particle size of 180 µm
produces higher gold recovery compared to that of larger
particle sizes. This reflects that small particles have higher
specific surface area and subsequently large area for
interaction between the cyanide and the particle and hence
leading to more dissolution of gold particles as compared to
particles with large particle size.
American Journal of Engineering, Technology and Society 2015; 2(6): 162-166 165
Figure 5. Gold recovery as a function of particle size and cyanide
concentration at a retention time of 96 hrs.
Figure 6. Gold recovery as a function of cyanide concentration and particle
size at a retention time of 72 hrs.
When the particle size was 180 µm the recovery of gold
was observed to be high (2.34 ppm) with cyanide
concentration of 700 ppm and when the cyanide
concentration increased to 1000 ppm the recovery of gold
was 2.33 ppm (Figure 6). The recovery of gold does not
increase when cyanide concentration was increased to 1200
ppm (Figure 7). This observation suggests that increasing the
cyanide concentration from 700 ppm to 1200 ppm does not
considerably favour the recovery of gold; instead, it is an
additional unnecessary cost to the leaching of this ore.
The increase of retention time from 72 to 96 hrs does not
significantly change the gold recovery (Figure 8), which
suggests that the ore can be reached to sufficient gold recovery
at 72 hrs. This study also suggests that cyanide concentration
in the range of 700 - 1000 ppm provides maximum gold
recovery (2.45 ppm). This concentration of cyanide is in good
agreement with the recommendation from literature that
requires a ratio of copper to cyanide to be 1:3 respectively.
The study also reveals that, it is very important to know the
mineralogical composition of the sample at hand before
subjecting it to leaching. This will ascertain the presence of
cyanide or oxygen consumers in the sample and hence
appropriate dosage of cyanide can be worked out. The problem
that normally faces small scale miners is that they are normally
very far to access laboratory facilities to analyze the samples
and in addition the costs of sample analysis are normally high.
In this regard, the leaching process is normally carried out
without knowing the sample composition, and the cyanide
dosage is normally worked out using experience and rule of
thumb. These challenges have led to the misuse of the
technology and subsequent complaints of not getting profits. It
is therefore very important to analyze the composition of the
ore and to attain its liberation size in order to get maximum
outputs from the cyanide leaching technology.
Figure 7. Gold recovery as a function of cyanide concentration and particle
size at a retention time of 96 hrs.
Figure 8. Gold recovery ass a function of cyanide concentration and
retention time for a sample with particle size of 180 µm.
4. Conclusion
In this study, cyanide leaching technology was applied for
leaching an ore sample collected from Mawelo Small Scale
miners with the aim of maximizing the recovery of gold from
the sample using the parameters adopted at the site. From this
study the following conclusions can be drawn:
(1) The average particle size, P80 (80% of material passing)
of the sample was 480 µm. this was not equal to the
liberation size of the sample. In order to increase the
gold recovery, grinding of the sample to 180 µm is
required. .
(2) The mineralogical composition of the sample is as
follows: gold: 5.85 g/t, copper: 150 ppm, sulphur: <
0.01 ppm, arsenic: 1.82 ppm, cobalt: 18.25 ppm and
nickel: 23. 89 ppm.
(3) With 180 µm particle size, the cyanide dosage in the
range of 700-1000ppm, retention time of 72 hrs, gave a
gold recovery of 2.45 ppm which was much better than
all parameters studied.
(4) Analysis of the mineralogical composition of the ore
and attaining its liberation size are mandatory
requirements for effective and efficient cyanide
leaching process.
166 Justin William Ntalikwa: Gold Recovery by Cyanide Leaching: A Case Study of Small Scale Miners in Tanzania
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