STUDY ON EFFECT OF TEMPER.\TURE ON GOLD LEACHING FROM<br />
ELECTRONIC SCRAPS USING THIOSULPHATE WITH COPPER(II) CATALYST<br />
IN AMMONIA MEDIA<br />
NGHIEN CUU ANH HUONG CUA NHIET DO DEN QUA TRJNH HOA TACH VANG TU" RAC<br />
THAI DIEN TU" SU" DUNG THIOSULFATE TRONG MOI TRUONG AMMONIAC VCDl XUC<br />
TAC D6NG(I1)<br />
<br />
<br />
Ha Vinh Hung ', Huynh Trung Hai', Ngo Thi Nga ', Jae-chun Lee \ Jinki Jeong'<br />
Hanoi University of Set 'nee and Technology (HUST)<br />
Korea Institute ofGeoscienc - and Mineral Resources (KJGAM)<br />
<br />
ABSTRACT<br />
In order to develop an environmentally friendly technique for recovery of precious metals from<br />
electronic scrap, thiosulfate leaching of gold has focused in recent years. This study investigates the<br />
effect of temperature on the gold leaching kinetics and the undesirable homogeneous copper(ll)<br />
reduction by thiosulfate under experimental conditions of 10 mM CUSO4, 0.1 M Na2S203 and 0.45M<br />
NH3/NH4* The leaching rate of gold Is determined at essentially constant solution concentrations<br />
using the method of initial rates, which measures concentration changes in real time within minutes.<br />
The activation energy of the leaching reaction in the temperature range 20 - 50°C was found to be<br />
78.6 kJ.mof' The gold leaching kinetics were found to be chemically controlled.<br />
<br />
TOM T.\T<br />
Nhim thuc diy phit triin cdng nghe thin thien mdi trwding di tii chi thu hdi klm loai quy tir<br />
chit thii dien tir, qui trinh hda tich ving bing thiosulfate da dwac quan tim trong vil nam gin day<br />
Nghien ciru niy khio sit sw inh hwdng cua nhiet do din ddng hoc cua qua trinh hda tich ving vi<br />
qui trinh khir khdng mong mudn ciia ddng (II) bdi thiosulfate trong diiu kien thi nghiem 10 mM<br />
CuSOi, 0 1M /VajSsOs vi 0.45M NH3/NH4* Tdc do hda tich ciia ving dwac xic dinh trong diiu kien<br />
ndng dd dung dich khdng ddi sir diing phwong phip tdc dd ban diu, bing cich do sw thay ddi ndng<br />
dd theo th&i gian trong vdng vii phut Nang lwang hoat hda cua phin irng trong khoing nhiet do tir 20<br />
- 50°C dwac tinh la 78.6 kJmof\ Ddng hoc qui trinh hda tich ving do phin irng hda hoc chi phdi.<br />
<br />
<br />
I. INTRODUCTION The leaching of gold with thiosulfate<br />
Nowadays, waste electric and electronic solutions has been extensively studied as an<br />
equipment, or electronic waste, has been taken altemative method for the traditional<br />
into consideration not only bv the government cvanidation technology [2]. It is known that in<br />
but also by the public due to their hazardous the presence of ammonia and copper ions, gold<br />
nialerial contents. CurrentK. the main options can be readily brought into solution by the<br />
for the treatment of electronic waste are formation of a gold thiosulfate complex, as<br />
involved in reuse, remanufacturing, and shown in Eq. (1).<br />
recycling, as well as incineration & landfilling.<br />
(iold metal has a wide application in the Au - Cu(NH,)4 '" + 5S2O,-" -^ Au(S,O.0:"'"<br />
manufacture of electronic appliances, serving as -Cu(S:05),' +4NH, (1)<br />
contact material due to their high chemical<br />
The thiosulphate leaching system is<br />
stabilitv and their good conducting properties.<br />
complicated by the homogeneous reduction of<br />
Ihe detailed literatuie siirvev of J irang Cui et<br />
Cu(ll) by thiosulphate according to the<br />
al. on value distributions for different electronic<br />
simplified overall reaction, Eq. (2) [3].<br />
waste samples showed that the major economic<br />
driver for recvcling of electronic waste is from Cu(NH04"'-8S:O/ - ^ 2CU(S203)3''<br />
the recovery of gold metal [I].<br />
- S4O,," 8NH-, (2)<br />
JOI RN \ L OF SCIENCE & TECHNOLOGV<br />
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In the absence of oxvgen the products titration, a certain amount of acetic acid (10%<br />
from this reaction are cuprous complex and solution) was added prior to the titration uiih<br />
tetrathionate [4]; therefore, the reaction not only the indicator \ i t e x . The concentration of the<br />
reduces the cupric ion concentration required cupric ammonia complex was detennined ai<br />
for leaching but also destrovs thiosulfate. 608 nm wavelength using L"V-\is spectro-<br />
photometry (Shimadzu). The solution<br />
The aim of this studv is application the<br />
absorbance was recorded at regular time<br />
technologv for thiosulphate leaching of gold to<br />
intervals via a computer. The temperature. pH<br />
recover gold in scrap from electronic industry<br />
and redox potential of the solution were<br />
in condition of optimal temperature. This is a<br />
monitored during leaching all time. Surface of<br />
new approach. important from both<br />
scrap samples before and after leaching were<br />
environmental and economical points of view.<br />
analv'zed using a Hitachi Jeol scanning electron<br />
II. MATERIAL AND METHODS microscope, combined with an X-ray analyzer<br />
[scanning electron microscopv-energ)'<br />
Material. \\ aste scrap samples were<br />
dispersive spectroscopy (SEM-EDS)].<br />
obtained from mobile phone factorv in Korea,<br />
which the average content of gold of 440 g't. III. RESUUTS AND DISSCUTION<br />
Contact surface of gold metal of each scrap<br />
3.1 SEM-EDS analysis: formation of sample<br />
plate is 486 mm"" Analv tical grade ammonium<br />
layers<br />
thiosulphate. ammonium sulphate, ammonia<br />
solution (25%) and cupric sulphate were used in Morphologv studies of the dilTerenl<br />
the experiments. The leach solution was composition of the sample section were carried<br />
prepared bv dissolving the proper chemicals in out bv means of SEM. Its image is showed in<br />
deionized water to the required concentration. Fig. 1 shows that there are four lavers from<br />
In each instance, the aqueous ammonia was outside to inside, including: gold, nickel, copper<br />
mixed with the eopper(ll) sulphate prior to and plastic. Therefore, other metals in sample<br />
addition to the ammonium thiosulfate (nickel, copper) were not contacted with the<br />
containing solution, and finallv the pH of the solution so did not affect gold leaching.<br />
solution was adjusted 9 - 1 0 also with aqueous<br />
ammonia.<br />
Methods Leaching experiments were<br />
carried out in a 0.5-L Pvrex glass reactor using<br />
a magnetic stirrer at a rotating speed of 400<br />
rpm. Each test was used 400 mL leaching<br />
solution and 10 plates of scrap sample. The<br />
scrap samples were suspended in the upper part<br />
of the leaching reactor with a nvlon thread,<br />
ensuring no contact w ith the reactor wall during<br />
leaching. 1 he temperature was controlled with a<br />
thennoslalie water bath at _-0.5 C of the set<br />
values. During the experiment, the solution was<br />
deaerated with high-puritv nitrogen. Samples<br />
were taken continuouslv at certain intervals<br />
during a total retention lime of 10 minutes.<br />
Fig. 1. SEM image of .sample section helnrc<br />
fhe gold concentration in the leach leaching.<br />
solutions was detennined by ..\.\S (Perkin Elmer<br />
The EDS analysis resuls also indicated<br />
\ \4iiiii. In each lest, the final residue was dried that the gold layer outside was of over 90°o<br />
and leached bv aqua regia to analvze for gold. puritv.<br />
The thiosulphate concentration was determined<br />
by iodomelrie method. In order lo eliminate the<br />
cllcci ol the cupric ammonia complex on iodine<br />
jwi,iv>.ALur SCIENCE & TECHNOLOGY * No.82A-201l<br />
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3.2 Dissolution of gold In thiosulphate reduction rate is negligible reduced with 10<br />
system solution minutes required for the copper(ll)<br />
concentration to decrease from 10 to 9 mM; in<br />
The changes in leaching performance of<br />
comparison, the copper(Il) concentration is<br />
gold and cupric ammonia complex<br />
reduced to 5 mM in 1 minute in temperature at<br />
concentration with time under various<br />
50°C. This explains the cause of the leaching<br />
experimental conditions are illustrated in Fig. 2<br />
performance of gold is 91% only in this case.<br />
and Fig. 3, respectively. It is evident from Fig.<br />
2 that temperature has a significant effect on the The energy dispersive X-ray analysis<br />
reaction rate of gold leaching (which is (EDX) was cartied out using a SEM-EDS for<br />
expected for a chemically controlled reaction). qualitative analysis of surfaces before and after<br />
At 20°C, the reaction rate is very low, less than leaching. Its spectrum (Fig. 4) shows that gold<br />
half of gold is leached in 5 minutes. While is not detected in sample affter leaching.<br />
almost all the gold being extracted in 2 minutes<br />
at above 40°C. It is worth noting that the 3.3 The activation energy of gold leaching<br />
mixed potential of gold leaching at steady reaction<br />
state was found to decrease with increasing The general form of the initial rate equation<br />
temperature. for reaction (1) can be expressed as:<br />
Beside, the temperature also has r = i[NH3r[S203'T[Cu(NH3)4' (3)<br />
dramatically affected the kinetics of the<br />
where k is the rate constant, the k value is<br />
copper(II) reduction by thiosulfate during the<br />
dependent on temperature and pressure; and a.<br />
thiosulfate leaching of gold. The results in Fig.<br />
b and c are reaction orders with respect to<br />
3 shows that the rate of copper(ll) reduction<br />
ammonia, thiosulfate and cupric ammine<br />
by thiosulphate is increased with increasing<br />
complexes, respectively. At chemical initial<br />
temperature. For instance, the initial reaction<br />
concentrations of constant, Eq. (3) can be<br />
rate at 40°C is 3.2 times higher than the initial<br />
written as:<br />
rate at SCC and 3.9 times lower than the<br />
innitial rate at 50°C. However, it can be seen<br />
from Fig. 3 that at 20°C, the copper(Il)<br />
<br />
<br />
<br />
<br />
4 6<br />
<br />
0 1 2 3 4 5 6 Time (min)<br />
<br />
Time (min)<br />
<br />
Fig. 2. The effect of temperature on gold Fig. 3. Changes in gold concentration with time<br />
leaching. Experimental conditions: 10 mM during its leaching under various condition of<br />
CiiSOj. OlMXa.S.'O.i, 0.45.M Mf. XH4 temperature. Experimental condition: 10 mM<br />
CiiSOe. O.l.MXayS^Oe. 045MNHeNH4<br />
JOL RNAL OF SCIENCE & TECHNOLOGV *<br />
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