Engineering, 2011, 3, 555-560
doi:10.4236/eng.2011.34065 Published Online May 2011 (http://www.SciRP.org/journal/eng)
Copyright © 2011 SciRes. ENG
Gold Recovery from Gold Bearing Materials Using
Bio-Diesel, Vegetable Oils and Coal
Anderson Mlaki, Jamidu Katima, Henry Kimweri
Department of C hemi c al an d Mi ni n g Engineering , College of Engineering and Technology,
University of Dar es Salaam, Dar es Salaam, Tanzania
E-mail: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
Received March 14, 2011; revised March 30, 2011; accepted April 25, 2011
The present work is focused on the performance of three types of coal-oil agglomerates on recovery of
liberated gold from gold bearing materials. Pre-formed agglomerates developed using bio-diesel, castor oil
and mineral diesel as liquid hydrophobic phases were used in the study. The influence of the type of liquid
hydrophobic phase on the degree of gold recovery and the effect of factors such as gold particle
concentration, viscosity, agglomerate size, agglomerate/ore ratios and particle penetration into agglomerates
have been studied as a function of time. It is shown that gold recovery rate can be increased by an increase in
agglomerate loading, surface area and the viscosity of the hydrophobic phase and high recoveries are at-
tained up to 98.5%. Increase in concentration of gold particles per unit volume of slurry increased attachment
rate but did not change the final recoveries attained. It is shown that gold particle penetration occurs mainly
in the coarse agglomerates if contact is prolonged beyond 60 minutes. Examination of sections of
gold-loaded agglomerates under reflected light microscope showed gold and some silicate particles pene-
trated in few cracked agglomerates and only gold particles were observed inside the uncracked agglomerates
suggesting the possibility of gold selectivity during particles penetration. It was shown that the increase in
gold recovery attained at prolonged contact time is due to both gold penetration and oleophilic attachment.
Keywords: Pre-Formed Agglomerates, Selective Gold Penetration, Oleophilic Attachment
Many researchers in the area of extractive metallurgy
and chemical engineering have reported the possibility
of gold recovery by coal-gold-oil agglomeration
method, which is a potential alternative to the mercury
amalgamation. It is widely accepted that gold surface
covered by a layer of hydrocarbon molecule is hydro-
phobic and hence capable of migrating from gold
bearing slurry to agglomerates .
Although there are several methods commonly used
to extract gold from its ores, the selection of a particular
method highly depend on the mineralogical characteris-
tics, commercial viability  and environmental friend-
liness. The hydrophobic recovery of gold using ag-
glomerates is environmentally seen as the best alterna-
tive to mercury amalgamation in small scale mining .
The positive results from previous research works on
coal-oil-gold agglomeration [4-11] and the promising
environmental and technical viabilities of the process
 have raised the need for adop tion of this method in
small scale mining in Tanzania where the hazardous use
of mercury is still dominant , and was the motiva-
tion for doing this research.
This work addresses some areas that have hindered
the development of this method in small scale mining in
Tanzania. The sophistication of the operation in terms
of types of hydrophobic materials used and the handling
of the effective variables [8,9,10] compared to mercury
amalgamation has been addressed.
As an attempt to bring the gold recovery method to
small scale miners levels in Tanzania, this research
focused on the use of pre-formed agglomerates with
parameters such as oil/coal ratio and coal particle size
already optimized during agglomerate formation.
Using the standard pre-formed agglomerates it was
possible to achieve high gold recoveries.
Three types of preformed agglomerates developed
using bio-diesel, castor oil, petroleum diesel and
bituminous coal from Kiwira coal deposit were avai-
A. MLAKI ET AL.
lable for the contact experiments. The performance of
the agglomerates is presented in terms of gold re-
The selection of the liquid hydrophobic phases used
in formation of the agglomerates was based mainly on
availability and environmental friendliness. Bio-diesel
was considered environmentally friendly and could be
produced using locally available oils, the particular
biodiesel which was used in the experiments was pro-
duced from palm oil and methanol at controlled condi-
tions at the Department of Chemical and Mining Engi-
neering (CME), College of Engineering and Technol-
ogy UDSM. Castor o il was sele cted for the experime nts
as non edible vegetable oil available locally. Small
scale miners could be encouraged to plant castor plants
for easy production of the oil. Petroleum diesel is
mentioned in a number of previous reports and was
used as a reference oil phase. Bio-diesel and castor oil
agglomerates were tested here for the first time and
proved very effective compared to previously tested
liquid hydrophobic phases in the ongoing researches on
recovery of gold from ores using hydrophobic and
a) Agglomerate samples
Three sets of samples of pre-formed agglomerates,
developed from three different types of liquid hydro-
phobic phases, castor oil, bio-diesel and mineral/ pe-
troleum diesel using bituminous coal from Kiwira
were used. The agglomerates were prepared at oil/coal
ratio 0.3, coal particle size of -110 microns and agita-
tion time of 20 minute at 1000 rpm as established from
the previous work done and published by the author
b) Gold bearing materials
The gold bearing materials used in the study con-
tained classified fine quartz (100% - 150% microns)
and metallic gold powder minus 150 microns originat-
ing from small scale mining areas in Chunya, Mbeya
Potassium Amyl Xanthate (KAX) commercial yellow
pellet form-water soluble, Hydrochloric acid (HCl) and
Nitric ac id (HNO 3).
Gold at ta chm ent tests
The Gold attachment tests were carried out in a 1000
ml beaker equipped with two baffles 180˚ apart. The
contents stirred at 700 rpm by a four-bladed stainless
steel impeller and 20% solids. The procedure involved
wetting 100g of gold bearing material with de-ionized
water, adding potassium amyl xanthate collector and
agitating for 5 minutes, adding specific amount of ag-
glomerate sample and stirring for a given time. The
mixture was screened of on a 0.325 mm sieve with
wash water to separate gold-loaded agglomerates from
the pulp. Drying loaded agglomerates in open air for 6
hrs and weighing them followed by assessing the re-
covery of gold by analyzing the amount of gold at-
tached to agglomerates. Gold analysis was done by
roasting the dry agglomerates to ash at 500˚C in a muf-
fle furnace then treating the ash with aquaregia to dis-
solve g old . The amoun t of go ld in the d ilu ted g old solu-
tion was determined by atomic absorption spectropho-
All the tests were carried out on the basis of liquid
hydrophobic phase used in forming the agglomerates
(Bio-diesel, Castor oils, Petrol-diesel). Optimum re-
coveries, agitation time and agglomerate loadings were
3. Results and Discussion
3.1. Effect of oil type on the Performance of
Three types of preformed agglomerates made from
castor oil, bio-diesel and petroleum-diesel respectively
were brought in contact with gold bearing material
containing 1% gold for 60 minutes as described in
section 2.2. The results were evaluated in terms of
percentage gold recoveries.
At agglomerate loading of 0.4, the castor oil ag-
glomerates attained gold recovery of 98.5% as shown in
Figure 1. The performances of bio-diesel and petro-
leum diesel agglomerates were similar and both at-
tained highest recovery of 93.5%.
Higher gold recoveries were attained by castor oil
agglomerates with viscosity of about 891 cP at 25˚C as
Figure 1. Effect of oil type on the performance of agglom-
erates at different agglomarate/ore ratios. (Contact time of
60 minutes, agitation speed of 700 rpm).
Copyright © 2011 SciRes. ENG
A. MLAKI ET AL.557
compared to bio-diesel (77.0 cP), and petroleum-diesel
(76.2 cP). This was attributed to the formation of more
stable agglomerates which stabilized the available ad-
sorption area for gold particles suggesting increased
adhesion due to higher viscosity. This is consistent with
literature according to Elblbesy .
3.2. Effect of Agglomerate Loading Using
Figures 2(a)-(c) show gold recoveries plotted against
contact time at different agglomerate loading. The ex-
periments were conducted in order to determine com-
pletion time for gold adsorption and optimum recover-
ies. Three samples of different agg lomerate/ore ratios of
0.25, 0.3, and 0.4 were each divided into five equal
parts making a total of 15 sub-samples. Each set of five
were treated for gold recovery at contact times of; 30
minutes; 60 minutes; 90 minutes; 120 minutes and 150
minutes respectively to compare performance between
short and long contact time.
The results showed that low agglomerate/ore ratios
required prolonged contact time for completion of gold
recovery. High agglomerate loading lowered the time
taken to attain optimum gold recovery as shown in
Figures 2(a)-(c). It is also shown that, high agglomer-
ate-ore ratios increase the rate at which gold particles
are recovered per unit time. The available surface area
for particle attachment which varied according to ag-
glomerate loading was considered as the main factor
affecting completion time, optimum recoveries and re-
3.3. Effect of Viscosity of the Liquid
Hydrophobic Phase on the Rate of Gold
In order to show the effect of viscosity on the rate of
gold recovery, the recoveries were plotted against time
Figure 2. Recovery-Time plots showing completion of gold
recovery for castor oil, bio-diesel and petroleum diesel
agglomerates (700 rpm, 20% solids pulp density)”.
Figure 3. Recovery/Time Curves showing the effect of vis-
cosity on recovery rate at constant aggl/ore ratio of 0.4.
at constant agglomerate loading as shown in Figure 3.
It was noted that at any point on the curves, the higher
viscosity agglomerates had higher recoveries than the
lower viscosity agglomerates, indicating an increase in
recovery rate with increase in viscosity of hydrophobic
phase. After 120 minutes the gradients of the curves
were the same indicating possible completion of gold
removal. It was suggested that higher viscosity reduced
shear on agglomerates and increased stability of ag-
glomerate surface area available for adsorption of gold
Gener ally, the resu lts showed that increase in vis cos-
ity of the hydrophobic phase increases not only the
Copyright © 2011 SciRes. ENG
A. MLAKI ET AL.
amount of gold recovered, but also the gold recovery
rates as shown in Figure 3.
3.4. Effect of Gold Particle Concentration on
Gold Attachment Rate
High gold recovery rates were observed at the initial
stages of contact and decreased with incr ease in contact
time regardless of the type of liquid hydrophobic phase
used. This was explained as due to higher chances of
gold-agglomerate contact at high concentration of free
gold particles per unit volume in the slurry. Concentra-
tion was high during the initial stages of adsorption and
decreas ed with in crea se in con tact time.
According to the recovery-time plots, the gradients of
recovery/time curves decreased gradually to hori- zon-
tal after 90 minutes (Figures 2(a)-(c)) indicating com-
pletion of gold transfer from the slurry to agglomerates.
The gold transfer process suggested a first order reac-
tion with high concentration indicating high driving
force in agreement with the mass balance equation at
completion of gold removal that; Cin = Cout where ‘Cin’
represent gold in slurry and ‘Cout’ is the gold at tach ed to
Images of the ore samples taken under reflected light
microscope were used to show the particles structure at
different contact time and showing the gradual changes
in gold conc entr at ion in the sl urr y ‘Cin’.
The images showed a decrease in gold particles
available in the slurry with increase in contact time as
shown in Figures 4-6 when the ore samples were taken
from the slurry at different contact times, filtered, air
dried and observed under a microscope.
It is noted in Figures 4-6, that after 60 minutes con-
tact time, gold particles could still be observed in the
ore sample indicating that the transfer of gold particles
to agglomerates could not be completed due to high
number of particles available. The results have shown
that the concentr ation of go ld particle s in the slurr y has
Figure 4. Gold bearing material showing gold particles be-
fore the start of gold contact with agglomerates. Magnifica-
tion × 10.
Figure 5. Image showing gold particles in after 60 minutes
contact with agglomerates. Magnification × 10.
Figure 6. Image, showing absence of gold particles after 90
minutes contact time due to completion of gold removal.
Magnification × 15.
an influence on the completion time for gold transfer
from slurry to agglomerates and hence affects gold re-
3.5. Effect of Agglomerate Size on the Amount
of Recoverable Gold
The chart in Figure 5 shows the performance of classi-
fied agglomerates comprising coarse and fine size frac-
tions. Basing on the results it was possible to draw cer-
tain inferences on the effect of agglomerate size on gold
recovery at prolonged contact time.
Coarse agglomerates of size (–2 mm + 1.5 mm) and
(–1.5 + 1 mm) and fine agglomerates of size (–1 mm +
0.5 mm) and (–0.5 mm + 0.325 mm), were used in the
tests. Gold recoveries were attained from each agglom-
erate size at different contact time.
At 60 minutes contact time, the fine agglomerate
fractions (+0.5 – 1.0 mm) and (–0.5 mm) attained re-
spective recoveries of 96.10% and 96.25% while the
coarser fractions (+1 – 1.5) and (+1.5 – 2.0) attained
lower recoveries 92.26% and 92.56% respectively. It is
clear that the differences in recovery between the fine
and the coarse were due to greater surface area per unit
weight of the finer agglomerates.
At 90 minutes contact time, all agglomerate sizes had
Copyright © 2011 SciRes. ENG
A. MLAKI ET AL.559
Figure 7. A chart showing the recoveries attained by dif-
ferent agglomerate sizes.
Figure 8. Images of loaded agglomerates showing gold ad-
sorption on the surface of agglomerate. (a) Magnification ×
10 and (b) Magnification × 25. Note: The gold particles
used originated from burning of gold-mercury amalgam
and had variable shapes
attained additional recoveries though at different mag-
nitudes. It was noted that the coarsest agglomerates had
higher additional recovery of 5.24% compared to the
increase in the finest agglomerates of only 1%. This
difference was explained as due to interstitial penetra-
tion of fine gold particles into the coarse agglomerates
caused by probably capillarity and/or external forces
resulting from inter-particle collision and shearing
forces caused by impellers and baffles .
3.6. Gold Loaded Agglomerates Examined
Using Reflected Light Microscope
Figure 9. Image of loaded sliced coarse agglomerate of size
+1.5 – 2 mm at magnification × 15.
Figure 10. Gold inside sliced coarse agglomerate withou
fter 60 minutes contact time showing gold particles
gold particles were
cracks × 50 magnification.
attachment on the surface of fine and coarse agglomer-
ate respectively. According to the images, mainly gold
particles were observed on the surface of agglomerates
indicating high selectivity during attachment at 60 min-
utes contact time. Washing the loaded agglomerates
during screening improved visibility of the attached
gold under reflected light as shown in Figure 10(b).
Figures 9 and 10 show images of sliced sections
aded agglomerates after 90 minutes contact. It was
noted that fine gold particles were penetrated and some
of them were completely locked in the agglomerate,
indicating interstitial penetration.
In Figures 9 some penetrated
tuated very close to the crack, indicating the possibil-
ity of gold penetration through the cracks. Some quartz
particles could be observed in the cracked agglomerates
indicating recovery dilution due to presence of cracks.
Figures 10 shows the section of uncracked agglo
erate at high magnification. Mainly gold particles
were observed inside the agglomerate. It appears there
was limited penetration of gangue particles like quartz
due to absence of cracks. The presence of gold particle
domination was probably due to its oleophilic proper-
ties and higher inertia due to superior density (19.5
Figure 8(a) and (b), are image s o f lo aded agglomer a te
Copyright © 2011 SciRes. ENG
A. MLAKI ET AL.
Copyright © 2011 SciRes. ENG
enerally the results from this work showed that
be increased by increase in
ollowing the good performance of the agglomerates
] A. Y. Ngenya, “Some Chemical Aspects of th
of Extractive metallurg
g/cm3) compared to other metals [17, 18].
pre-formed agglomerates developed from bio-diesel,
castor oil and petroleum diesel as liquid hydrophobic
phases can be used as effectiv e material for recover y of
liberated gold using bituminous coal from Kiwira as the
solid hydrophobic phase.
Gold recovery rate can
glomerate loading, surface area and the viscosity of
the hydrophobic phase. Increase in gold particle con-
centration in th e slurr y incr eas es recov ery ra te.
Prolonging contact time increased the capacity
g of agglomerates with possibility of selective gold
penetration into coarse agglomerates making it possible
to attain completion of gold recovery at lower agglom-
used in the experiments it is recommended to carry out
further work to test their performance using ores that
contain variable gangue especially high sulphides ores
rich in pyrites and arsenal pyrites, common in Tanza-
nian gold ores.
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