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

jntalikwa@udom.ac.tz, jntalikwa@yahoo.com

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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