sustainability

Article
Chlorination Treatment for Gold Extraction from Refractory
Gold-Copper-Arsenic-Bearing Concentrates
Nurlan Dosmukhamedov 1 , Valery Kaplan 2 , Erzhan Zholdasbay 1 , Aidar Argyn 1, *, Erzhan Kuldeyev 1 ,
Gulzada Koishina 1 and Yeleussiz Tazhiev 1

1 Department of Metallurgy and Mineral Processing, Satbayev University, Almaty 050000, Kazakhstan
2 Weizmann Institute of Science, Rehovot 76000-76878, Israel
* Correspondence: aidarargyn@gmail.com

Abstract: New experimental results have been obtained on the behavior of arsenic and other associ-
ated metals (Re and others) under conditions of oxidative and reductive sintering. It has been estab-
lished that the extraction of arsenic strongly depends on the process temperature during oxidative
sintering. The extraction of arsenic into dust media at 873 K is 50% and rhenium is 88–90%. The effect
of excess air on the extraction of arsenic and rhenium into dust was studied: the higher the excess air
coefficient, the more complete the extraction of arsenic and rhenium into the dust. The obtained data
indicate that achieving a high level of arsenic extraction from the initial product is not possible during
oxidative sintering. The best arsenic removal results were reached under the conditions of reductive
sintering of initial material by natural gas. The extraction of arsenic into dust at 823 K was 88%, and
at 1373 K arsenic is almost completely converted into dust. Obtained new experimental results have
a fundamental importance for the selection and organization of a comprehensive technology for the
processing complex in composition refractory gold-copper-arsenic-bearing products.
Citation: Dosmukhamedov, N.;
Kaplan, V.; Zholdasbay, E.; Argyn, A.;
Keywords: arsenic; gold; chlorine treatment; preliminary sintering; refractory gold-copper-arsenic-bearing
Kuldeyev, E.; Koishina, G.; Tazhiev, Y.
products
Chlorination Treatment for Gold
Extraction from Refractory
Gold-Copper-Arsenic-Bearing
Concentrates. Sustainability 2022, 14,
11019. https://doi.org/10.3390/ 1. Introduction
su141711019 In refractory gold-bearing concentrates, the gold metal is generally contaminated and
Academic Editors: Gujie Qian
encapsulated either in a sulfide matrix or in quartz, while the efficiency of the extraction
and Tara Hosseini
of gold is low due to the structural/chemical characteristics of the concentrates itself [1].
Refractory concentrates, which have gold in the form of fine inclusions in sulfides, contain
Received: 25 May 2022 30% of the global reserves of pristine gold metal [2]. For obvious reasons, the development
Accepted: 31 August 2022 of effective technologies for leaching gold from refractory concentrates has a great prac-
Published: 3 September 2022 tical and economical importance. During the processing of gold-containing concentrates,
Publisher’s Note: MDPI stays neutral attention should be focused on the particular form (or forms) of gold contained in—free
with regard to jurisdictional claims in metal or metal bound in a sulfide matrix (e.g., pyrite, arsenopyrite, chalcopyrite, galena,
published maps and institutional affil- antimonite), in quartz, or in carbonaceous materials [2,3].
iations. The main problems of the mineral resource base of gold in Kazakhstan are the lack of
large reserve gold deposits that could be considered as basic facilities for the sustainable
development of the industry in the long term.
Currently, more than half of the active gold reserves in Kazakhstan are “refractory”
Copyright: © 2022 by the authors. ores that are difficult to enrich and contain a significant content of arsenic. The process
Licensee MDPI, Basel, Switzerland.
of removing arsenic not only complicates the technology of mineral processing but also
This article is an open access article
requires additional costs for their storage in compliance with the conditions for preserving
distributed under the terms and
the ecology of the environment [4].
conditions of the Creative Commons
The gold mining industry in Kazakhstan faces the challenging problem of treating
Attribution (CC BY) license (https://
refractory gold-copper-arsenic-bearing concentrates [5].
creativecommons.org/licenses/by/
4.0/).

Sustainability 2022, 14, 11019. https://doi.org/10.3390/su141711019 https://www.mdpi.com/journal/sustainability

Sustainability 2022, 14, 11019 2 of 14

The amount of gold in different phases of refractory gold-copper-arsenic-bearing

concentrates has been described in ref. [1] (% from total gold quantity): metallic gold—
23.32; gold as carbonate—0.14; gold as oxide—4.11; gold as sulfides (arsenopyrite and
pyrite)—67.42; gold in gangue—5.01. Therefore, it is difficult to extract gold from such
minerals directly by traditional processes without pretreatment; the extraction yield of
gold from concentrates with a high content of arsenic and sulfur is much lower than from
materials in which arsenic and/or sulfur are absent [6]. In the mining sector of Kazakhstan,
rhenium (Re) is also present in copper concentrates. Kazakhstan is currently one of the
largest producers of rhenium in the world [7,8]; Cu-Au-As concentrates contains 20–50 ppm
of rhenium [9]. Since rhenium is used in the construction of jet engines and as a catalyst,
the extraction of rhenium from gold-copper-containing concentrates is economically very
important. Rhenium can be extracted to dust by oxidation sintering of the Cu-Au-As
concentrates [10,11].
In this century, it is planned to ensure the main increase in gold production in the world
through a wider involvement in the operation of refractory and complex gold-bearing
ores. With the development of the planned general trend, one of the most important
problems facing the modern gold mining industry should be questions of increasing the
completeness of gold extraction from refractory (difficult-to-enrich) raw materials, as well
as other gold-bearing sources (polymetallic concentrates, industrial waste), which requires
comprehensive research in order to develop reasonable solutions.
Nowadays, in world practice, there is a depletion of the raw material base of non-
ferrous metal ores with a low content of arsenic, containing gold. More and more materials
of complex composition are supplied to metallurgical plants, and one of the poisoning
harmful impurities there is arsenic.
Involvement in the processing of polymetallic concentrates accumulated in the East
Kazakhstan Region, containing non-ferrous metals, gold, and other valuable metals, could
become a significant reserve for increasing the volume of raw materials. For example,
Berezovsky polymetallic concentrate can be a potential source of raw materials for Kazzinc
LLP for the complex extraction of non-ferrous metals and gold from them. Today, their
high arsenic content restricts their processing.
The ores of the Berezovsky deposit are the gold-bearing sulfide-polymetallic type.
The ores are characterized by a wide range of elements presented in the sulfide form:
lead, zinc, copper, cobalt, nickel, molybdenum, etc. The high content of arsenic (from 1 to
1.5%) is noteworthy. Acicular arsenopyrite and lautite are gold-bearing. Gold is present
in microscopic and submicroscopic forms. The distribution of gold in ores is uneven, the
coefficients of variation for these ore bodies range from 56.5 to 86%. Most of the gold
(95–97%) is associated with sulfides—arsenopyrite, lautite—the amount of which varies
from 1 to 8%, i.e., in composition the ores are low-sulfide [12,13].
For Kazakhstan, as well as for Russia and Ukraine, gold-containing wastes, which are
part of the structure of accumulated industrial wastes, have a particular concern in terms
of the degree of impact on the environment and human life. Because of the activities of the
industrial enterprises of the East Kazakhstan Region in 2007, about 1,401,130 thousand tons
of industrial waste were accumulated.
Wastes of metallurgical production are placed in dumps, waste heaps, sludge col-
lectors, and occupy an area of more than 430 hectares. Such disposal of waste leads to
environmental pollution due to dust dispersion and erosion by rain and melt water and
also pollutes surface and ground water.
With the accumulated waste generated in the production chain “mining–mineral
processing–metallurgy”, more than a third of the non-ferrous and precious metals mined
with ores are lost, huge reserves of various construction raw materials are frozen. The loss
of lead is 33%, zinc—28%, copper—21%, gold—41%, silver—35%. The total losses during
mining amount to 15–20% of non-ferrous and 8–12% of precious metals, during mineral
processing—43–48% and 74–75%, respectively, in metallurgy 35–42% of non-ferrous and
14–17% of precious metals. The excess of losses of valuable components during mineral
Sustainability 2022, 14, 11019 3 of 14

processing by 2–3 times, compared with losses in mining and metallurgical processing, led
to the emergence of numerous man-made deposits [14].
The processing of gold-bearing ore and technogenic raw materials, due to their com-
plex composition and the presence of a high content of arsenic, on the one hand, and
the lack of an alternative effective technology for their processing, on the other hand,
restricts an increase in their processing volumes with a complex high extraction of valuable
components into targeted products.
Nowadays, the existing technologies for processing gold-bearing raw materials by
known methods do not provide a high complex extraction of valuable metals, including
gold. There are many ways to remove arsenic from raw materials and intermediate products
of lead and copper production. However, active work on improving the flowsheet for
obtaining non-ferrous metals leads to a change in the movement of impurity components
and the need to develop a new flowsheet for the utilization of arsenic.
One of the promising areas used for the processing of arsenic-, gold-containing poly-
metallic raw materials is the use of modern bubbling processes for melting to matte [15,16].
The use of autogenous processes (Outokumpu, Isasmelt) [17] makes possible to minimize
the loss of non-ferrous metals with slags and ensure a high extraction of As, Sb into dust.
However, the obtained arsenic dusts are characterized by a complex composition and
require special treatment for the subsequent extraction of non-ferrous metals from them.
According to the flowsheet that existed at Kazzinc LLP until 2012, the output of arsenic
from the lead plant as part of arsenate cakes was 43.65%.
Copper-lead mattes contained, %: Cu—10–20, Pb—12–25, Zn—8–12, As—up to 1.5,
Sb—up to 1.0; copper slips, %: Cu—15–20, Pb—18–30, Zn—2–4, As—4.0, Sb—up to 2.5;
converter slag contained, % (wt.): Cu—3–5, Pb—up to 34, Zn—up to 4.5, As—2–2.5, Sb—
up to 2.0, which accounted for the main part (up to 30%) of the structure of the initial
feed [18,19].
During smelting, significant volumes of non-ferrous, precious, rare, and rare-earth, as
well as accompanying impurity metals (As, Sb) circulate in the “melting-converting” pro-
cess chain. Valuable metals are smeared over the smelting products, and the accompanying
impurity metals (As, Sb) accumulate in the process chain. Their negative impact on the envi-
ronment and life safety of workers increase; the quality of the products obtained decreases.
Recovery of copper to matte is low ~83% and lead to rough lead reaches 60%. The
increased content of lead in slags (up to 2%) led to a high content of precious metals in
them. According to practice, up to 4 g/t of gold and ~800 g/t of silver are concentrated in
the resulting copper-lead mattes sent for further processing.
The process was accompanied by a high consumption of expensive coke and slag
output (up to 60% of the weight of the loaded charge), high energy, and material costs. The
dust yield was ~15% of the weight of the loaded mixture and is characterized by a high
content of non-ferrous metals and gold, %: Pb—20; Cu—6–7; Zn—up to 10. The extraction
of As, Sb into dust is low and amounts to 70% and 57%, respectively [18,19].
Modernization of the technological scheme for the production of lead and the launch
of a copper plant at Kazzinc LLP in 2011–2012 solved the problem of removing arsenic
from the technological scheme with arsenate cake, however, led to the need to solve a
number of new tasks. Against the background of the modernization of the technological
flowsheet, there was an increase in the distribution of arsenic in copper scraps of lead
production. If earlier, according to the classical scheme “agglomerating roasting—blast
smelting—refining”, 27.65% of arsenic from the total load to the lead plant was transferred
to copper removals, then according to the new scheme—83.72%.
With an increased content of arsenic in the feedstock, the circulating load of arsenic
between the lead and copper plants increased due to the processing of copper scrap. This
led to the risk of obtaining low-quality commercial products, wear of the lining of py-
rometallurgical units when interacting with aggressive copper arsenide, and environmental
pollution with volatile arsenic compounds.
Sustainability 2022, 14, 11019 4 of 14

Today, at Kazzinc LLP the copper scraps are sent to the stage of electric smelting, where
they are separated into lead- and copper-containing phases. Arsenic is then distributed
between the smelting products: copper-lead matte, rough lead, and slag.
With the rough dehydrogenation of rough lead from mine smelting, copper strips
are formed, containing up to 84% of arsenic from loading to the lead plant. This is
three times higher than the distribution of arsenic in copper strips (27.65%) obtained
by classical technology.
The general technological scheme, including the integration of copper and lead plants,
is very complex, overloaded with the need to process copper, lead, arsenic, and gold
intermediate products and is characterized by a high content of arsenic in the targeted
products: copper-lead mattes (up to 3%) and draft lead (up to 6%). At the same time, the
redistribution of gold and silver between the smelting products leads to their significant
losses, which affects their total extraction from polymetallic raw materials into a commercial
product [20].
Due to the lack of a rational technology for processing copper, lead, arsenic, gold-
bearing concentrates, and intermediate products, the existing flowsheet is a forced measure
with all the ensuing shortcomings, both technologically and from an environmental point
of view.
The use of our own polymetallic raw materials containing copper, lead, gold, and
high arsenic content is a significant reserve for increasing the volume of raw materials for
the extraction of valuable metals. It is obvious that their processing by known-melting
technologies to matte is currently not possible due to the high content of arsenic. This
requires finding alternative solutions.
While processing the concentrates containing gold, copper, and arsenic, which are
undoubtedly complex, chlorination is considered as a very effective method for fractionat-
ing the raw materials in order to obtain high purity metal. The advantageous features of
the chlorination process are fast chemical reactions and the high level of chlorination of
all components. For example, refractory gold-containing concentrates may be successfully
processed to produce the sublimation of gold chlorides. Either solid sodium, calcium
chloride, or gaseous chlorine can be used as chlorinating agents [3,21–24]. The chlorination
process is very versatile and can extract gold from concentrates of almost any composition.
An important advantage of this process is the possibility of the complex processing of con-
centrates, which result in the extraction not only of gold and silver, but also other valuable
metals, such as rhenium [2]. However, when arsenic is present in refractory gold-containing
concentrates, pretreatment (oxidation or reduction sintering) is required.
Therefore, in order to develop a method for extracting gold from gold-bearing concen-
trates, this work was carried out to select a method for removing arsenic. Two technology
variants were tested: preliminary oxidation or preliminary reduction sintering of the initial
material prior to chlorination.

2. Materials and Method

2.1. Materials
Cu-Au-As concentrate was obtained from mine fields located in East Kazakhstan
(Table 1). The forms of presence of copper, arsenic, and gold are studied in detail in [25].
The main copper minerals in the concentrate are present in the form of sulfide and the
arsenic is in the form of arsenopyrite. Gold is embedded in copper sulfide minerals.
The initial materials were crushed and milled to produce powders with particle size
of <200 mesh (74 microns). Natural gas received from the Bukhara gas field (Uzbekistan)
had the following composition (volume%): 92.6 CH4 ; 4.1 C2 H6 ; 1.0 C3 H8 .

2.2. Sample Preparation

Sample preparation for all analytical measurements included milling and averaging.
Material composition, prior to and following gold extraction, was characterized by solution
inductively coupled plasma mass spectroscopy (ICP-MS, Agilent Technologies, Santa Clara,
Sustainability 2022, 14, 11019 5 of 14

CA, USA). The ICP-MS measurements required preliminary aqua regia leaching of the
sample at boiling temperature. For ICP-MS samples containing compounds, which are
insoluble in aqua regia, the following protocol was used: a 250 mg sample was mixed
with 1.5 g lithium tetraborate (Li2 B4 O7 ) and fused at 1373 K in a platinum crucible. After
cooling to room temperature, the melt was dissolved in 10 vol.% hydrochloric acid and
then analyzed.

Table 1. Chemical analysis of the initial material.

Cu-Au-As Concentrate
(Berezovsky Field, East Kazakhstan Region)
Elements Unit of Measurements Value
Cu % 20.9
Fe % 20.4
Pb % 2.5
Zn % 2.6
S % 10.8
Au ppm 11
Ag ppm 85
As % 6.3
Re ppm 35
Sb % 0.30
Al % 0.3
Ba % 0.004
Ca % 0.1
Si % 2.0
K % 0.1
Mg % 0.02
Mn % 0.01
Na % 0.2
P % 0.02
Sn % 0.28

2.3. Thermodynamic Calculations

Calculations of Gibbs free energy were carried out using a computer program devel-
oped by some of the authors and based on standard values for the pure substances [26].

2.4. Oxidation Sintering

The mixture of gases consisting of water vapor, carbon dioxide, nitrogen, and oxygen,
corresponding to the products of natural gas combustion, was used as an initial gas phase.
The composition of the initial gas mixture was calculated and depended on the excess air
coefficient (Table 2). The excess air ratio varied from 100% to 170%.

Table 2. Composition of the initial gas mixture for preliminary oxidation sintering.

Excess Air Coefficient Composition of the Initial Gas Mixture

(% from Stoichiometric Air) (Volume %)
CO2 H2 O N2 O2
100 9.7 18.8 71.7 0
150 6.4 12.5 71.6 9.5
170 5.6 10.9 70.4 13.1
 Excess Air Coefficient Composition of the Initial Gas Mixture
(% from Stoichiometric Air) (Volume %)
CO2 H2O N2 O2
100 9.7 18.8 71.7 0
Sustainability 2022, 14, 11019 150 6.4 12.5 71.6 9.5 6 of 14
170 5.6 10.9 70.4 13.1

The flow rate of the initial gas mixture was varied in the range from 150–300 cc/min.
OnceThe flow rate
the desired of the temperature
sintering initial gas mixture was varied
was reached, in the
it was held forrange from
30 min. 150–300 cc/min.
Experiments
Once the desired
were performed at sintering
623–873 K.temperature was in
Material quantity reached, it was
each test was50held fortests
g. The 30 min.
wereExperiments
car-
were performed
ried out at 623–873
in a standard K. Material
horizontal quantity
muffle furnace in each
with test(FeCrAl)
Kanthal was 50 g. The tests
heating were carried
element.
A diagram
out of the furnace
in a standard is shown
horizontal in Figure
muffle 1. with Kanthal (FeCrAl) heating element. A
furnace
diagram of the furnace is shown in Figure 1.

Figure Laboratorysetup
Figure 1. Laboratory setupfor
foroxidation
oxidation sintering:
sintering: 1—furnace
1—furnace withwith temperature
temperature controller;
controller; 2—re-2—reactor;
actor; 3—sample;
3—sample; 4—CO 4—CO
2 2 cylinder;
cylinder; 5—gas
5—gas cleaning
cleaning bottle;
bottle; 6—gas
6—gas sampler
sampler for
forGCGCanalysis; 7—ni-
analysis; 7—nitrogen
trogen cylinder;
cylinder; 8—additional
8—additional temperature
temperature monitor;
monitor; 9—flow9—flow meter;
meter; 10—valve;
10—valve; 11—air
11—air cylinder;12—water
cylinder;
12—water vapor generator; 13—oxygen cylinder.
vapor generator; 13—oxygen cylinder.
2.5. Reduction
2.5. Reduction Sintering
Sintering
Experiments were performed at 723–1373 K and all experiments repeated three times.
Experiments were performed at 723–1373 K and all experiments repeated three times.
Once the desired sintering temperature was reached, it was held for 60 min. Natural gas
Once the desired sintering temperature was reached, it was held for 60 min. Natural gas
consumption was 10 cc/min, and air consumption was 2.5 cc/min. The gas atmosphere in
consumption
the reactor waswas
Sustainability 2022, 14, x FOR PEER REVIEW 10 cc/min,
reducing and air
significantly; theconsumption was 2.5 cc/min.
incomplete combustion Thewith
coefficient, gasrespect
atmosphere
7 of 15 in
the reactor was reducing significantly; the incomplete combustion coefficient,
to stoichiometric, was around 75%. A diagram of the reactor is shown in Figure 2. with respect
to stoichiometric, was around 75%. A diagram of the reactor is shown in Figure 2.

Figure 2. Laboratory setup for preliminary reduction sintering:1—furnace with temperature con-
Figure 2. Laboratory setup for preliminary reduction sintering:1—furnace with temperature con-
troller; 2—reactor;
troller; 2—reactor; 3—sample;
3—sample; 4—CH4—CH 4 cylinder;
4 cylinder; 5—gas bottle;
5—gas cleaning cleaning bottle;
6—gas 6—gas
sampler sampler
for GC anal- for GC
ysis;7—thermocouple type K;type
analysis;7—thermocouple 8—additional temperature
K; 8—additional monitor; 9—flow
temperature meter;
monitor; 10—valve;
9—flow 11—
meter; 10—valve;
air cylinder.
11—air cylinder.

2.6. Chlorination
Three alternative chloride sublimation-based gold extraction techniques were used:
two with elemental chlorine and one with calcium chloride. The tests were carried out in
a standard vertical muffle furnace with Kanthal (FeCrAl) heating element (length 250 mm,
 Sustainability 2022, 14, 11019 7 of 14

2.6. Chlorination
Three alternative chloride sublimation-based gold extraction techniques were used:
two with elemental chlorine and one with calcium chloride. The tests were carried out in a
standard vertical muffle furnace with Kanthal (FeCrAl) heating element (length 250 mm,
width 250 mm, height 300 mm). The temperature range was 473–1373 K. The furnace
atmosphere was chlorine gas, when chlorine was used for extraction, or air, when calcium
chloride was used. A diagram of the furnace, including placement of the crucible with the
powdered sample, is shown in Figure 3. Two protocols were tested: (i) a powdered sample
(Cu-Au-As ore) was treated in a stream of chlorine gas; or (ii) a dry mixture of the powdered
sample (Cu-Au-As ore/calcium chloride/SiO2 at various weight ratios), was sintered in
an air stream. Alumina crucibles were used for sintering 30–60 g powder. Chlorine flow
was 400 cc/min for tests with chlorine gas; air flow was 400 cc/min for tests with calcium
chloride. The presence of a fine powder of SiO2 in addition to the calcium chloride [22]
improved process efficiency. Silica binds to calcium oxide to form calcium-silicate, which
is then removed from the reaction zone. In both protocols (i) and (ii), once the desired
temperature was reached, it was held for two hours. During this time, sublimation products
were removed with the gas flow. Nitrogen was fed into the reactor during the heating
Sustainability 2022, 14, x FOR PEER REVIEW
and cooling stages (600 cc/min). Each experiment was performed at least twice in order8 of 15
to verify reproducibility of results. After cooling, the clinker was analyzed for elemental
composition by ICP-MS spectroscopy.

Figure 3. Laboratory setup for chlorination: 1—air cylinder; 2—chlorine cylinder; 3—valve; 4—flow
Figure
meter;3.5—chlorine
Laboratoryor setup fortube;
air inlet chlorination: 1—air
6—gas stop; cylinder;
7—reactor; 2—chlorine
8—furnace with cylinder; 3—valve;
temperature 4—flow
controller;
meter; 5—chlorine or air inlet tube;
9—crucible; 10—gas absorption bottle. 6—gas stop; 7—reactor; 8—furnace with temperature controller;
9—crucible; 10—gas absorption bottle.
3. Results and Discussion
3.3.1. Thermodynamic
Results Calculations
and Discussion
The calculated Calculations
3.1. Thermodynamic values of the Gibbs energy for reactions of the pure substances within
the temperature range 473–1473 K are shown in Table 3.
The calculated values of the Gibbs energy for reactions of the pure substances within
the temperature range 473–1473 K are shown in Table 3.

Table 3. Calculated values of the Gibbs energy (ΔG).

Gibbs Energy (ΔG), kJ/mol

No. Reactions Temperature, K
473 673 873 1073 1273 1473
Sustainability 2022, 14, 11019 8 of 14

Table 3. Calculated values of the Gibbs energy (∆G).

Gibbs Energy (∆G), kJ/mol

No. Reactions Temperature, K
473 673 873 1073 1273 1473
Group 1
FeAsS(s) + 2.5 O2 (g) = 0.5 Fe2 O3 (s)
1 −753 −721 −688 −654 −620 −585
+ 0.5 As2 O3 (g) + SO2 (g)
2 As2 S3 + 4.5O2 (g) = As2 O3 (g) + 3SO2 (g) −1133 −1128 −1113 −1095 −1072 −1056
3 As2 S3 + 5.5O2 (g) = As2 O5 (g) + 3SO2 (g) −1428 −1335 −1240 −1147 −1056 −968
4 ReS2 (s) + 3.75 O2 (g) = 0.5 Re2 O7 (g) + 2SO2 (g) −926 −898 −871 −843 −816 −788
5 Cu2 S(s) + 1.5O2 (g) = Cu2 O(s) + SO2 (g) −343 −321 −298 −276 −254 −231
Group 2
FeAsS(s) + 0.5 CH4 (g) + 3.25 O2 (g)
6 = FeO (s) + 0.5 As2 O3 (g) + SO2 (g) −1040 −1020 −1000 −978 −956 −932
+ 0.5 CO2 (g) + H2 O(g)
FeAsS (s) + 0.5 CH4 (g) + 3O2 (g) = FeO + 0.5
7 −219 −217 −214 −211 −207 −204
As2 O3 (g) + SO2 (g) + 0.5 CO(g) + H2 O(g)
As2 S3 + 1.5CH4 (g) + 3O2 (g)
8 −888 −931 −963 −991 −1016 −1032
= As2 O3 (g) + 1.5CO2 (g) + 3H2 S(g)
As2 S3 + 1.5CH4 (g) + 2.25O2 (g) = As2 O3 (g) +
9 −125 −142 −156 −169 −181 −192
1.5CO(g) + 3H2 S(g)
ReS2 (s) + 0.5CH4 (g) + 4.75O2 (g)
10 −1326 −1299 −1271 −1244 −1216 −1187
= 0.5Re2 O7 (g) + 0.5CO2 (g) + 2SO2 (g) + H2 O(g)
ReS2 + 0.5 CH4 (g) + 4.5O2 (g) = 0.5 Re2 O7 (g) +
11 −287 −283 −279 −274 −270 −265
0.5 CO(g) + 2SO2 (g) + H2 O(g)
Cu2 S(s) + 0.5 CH4 (g) + 0.5 O2 (g) = 2Cu(s) +
12 −128 −136 −143 −149 −155 −162
H2 S(g) + 0.5 CO2 (g)
Group 3
FeAsS(s) + 3Cl2 (g) + O2 (g)
13 −784 −738 −701 −665 −629 −594
= FeCl3 (s) + AsCl3 (g) + SO2 (g)
As2 S3 + 3Cl2 (g) + 3O2 (g)
14 −1296 −1278 −1250 −1220 −1186 −1150
= 2AsCl3 (g) + 3SO2 (g)
ReS2 (s) + 1.5Cl2 (g) + 2O2 (g)
15 −591 −556 −521 −488 −456 −425
= ReCl3 (s) + 2SO2 (g)
16 Au (s) + 1.5Cl2 (g) = AuCl3 (g) −16 23 61 97 130 163
17 Au (s) + 3 Cl(g) = AuCl3 (g) −304 −230 −157 −86 −17 52
18 Cu2 S(s) + 2Cl2 (g) = 2CuCl2 (s) + 0.5 S2 (g) −179 −131 −84 −45 −9 26
Group 4
FeAsS(s) + 3CaCl2 (s) + 2.5O2 (g) + 3SiO2 (s)
19 −643 −625 −614 −600 −568 −535
= FeCl3 (s) + AsCl3 (g) + 3CaSiO3 (s) + SO2 (g)
As2 S3 (s) + 3CaCl2 (s) + 4.5O2 (g) + 3SiO2 (s)
20 −1156 −1165 −1164 −1154 −1125 −1091
= 2AsCl3 (g) + 3CaSiO3 (s) + 3SO2 (g)
ReS2 (s) + 1.5CaCl2 (s) + 2.75O2 (g)
21 −386 −363 −341 −319 −289 −260
= ReCl3 (s) + 1.5CaO (s) + 2SO2 (g)
Au (s) + 1.5 CaCl2 (s) + 0.75 O2 (g)
22 189 216 241 266 297 329
= AuCl3 (g) + 1.5CaO
Au (s) + 1.5CaCl2 (s) + 1.5SiO2 (s)
23 54 80 104 129 161 192
+ 0.75O2 (g) = AuCl3 (g) + 1.5CaSiO3 (s)
Cu2 S(s) + 2CaCl2 (s) + 2SiO2 (s) + 2O2 (g) =
24 −413 −369 −324 −286 −237 −189
2CuCl2 (g) + 2CaSiO3 (g) + SO2 (g)
(s)—solid, (g)—gas.
Sustainability 2022, 14, 11019 9 of 14

In the temperature range of interest, 473–1473 K, the Gibbs energy (∆G) for four
groups of reactions is shown in Table 3: group 1—oxidation reactions with oxygen; group
2—reduction reactions with natural gas; group 3—chlorination reactions with chlorine
gas; group 4—chlorination reactions with calcium chloride. Under sintering conditions,
the Gibbs energy of the reactions in group 1 is strongly negative (600–1400 kJ/mol). The
thermodynamic calculations predict that the oxidation reactions of arsenopyrite, arsenic,
copper, and rhenium sulfides can result in the formation of sublimated As2 O3 , As2 O5 ,
Cu2 O, and Re2 O7 within a wide temperature range. Reactions in the group 2 with the
sublimated As2 O3 and Re2 O7 have strongly negative Gibbs energies (900–1300 kJ/mol).
Thermodynamic calculations predict that reduction reactions of arsenopyrite and rhenium
sulfide can result in the formation of sublimated As2 O3 and Re2 O7 within a wide tem-
perature range, including the range of interest 473–1473 K. In this case, the reactions of
interaction of components with natural gas in the presence of oxygen can proceed both
with the formation of CO2 and with the formation of CO. A comparative analysis of the
loss of the Gibbs free energy of these reactions shows that reactions with the formation of
CO2 are much more thermodynamically probable. This is due to the presence of oxygen
in the gas phase, which oxidizes CO to CO2 . The obtained results show the fundamental
possibility of the decomposition of arsenic compounds (arsenopyrite, lautite, etc.) and
their removal during reduction sintering of the material with natural gas. Groups 3 and
4 present chlorination reactions between remaining arsenopyrite, Cu2 S, As2 S3 , ReS2 , and
gold with chlorine gas, pure CaCl2 , and CaCl2 in the presence of SiO2 . The values of the
Gibbs energy of the chlorination reactions for arsenopyrite (13, 19) are strongly negative
(500–800 kJ/mol). The Gibbs energy of the chlorination reactions for As2 S3 and ReS2 (14,
15, 20, 21) is strongly negative (300–1200 kJ/mol) as well. Reactions (18, 24) with Cu2 S
is negative (10–400 kJ/mol) as well. Reactions (13–15 and 20–21) with AsCl3 and ReCl3
production are thermodynamically more probable than chlorination reactions for gold.
Obviously, for a more complete chlorination of gold, it is necessary to first remove arsenic
and rhenium from the mineral ore to the possible extent. Results of experiments that
were previously reported [3,15–19], as well as those reported below, demonstrate that the
sublimation of gold metal following chlorination goes practically to completion. This was
not anticipated based on thermodynamic calculations (Table 3, Groups 3, 4): only weak
gold chlorination is predicted at high temperature. This discrepancy should disappear if
atomic chlorine participates in the gold chlorination reaction (reaction (17)). The authors
of the works [3,24] came to the same conclusion. Atomic chlorine can be formed during
thermal decomposition of CaCl2 [3,24]. Therefore, the chlorination of gold may take place
in the temperature range of interest.

3.2. Oxidation Sintering

Preliminary oxidation sintering of Au- and Re-containing concentrate with high arsenic
content was performed. Experimental results are shown in Figure 4.
We found that the extraction yield of arsenic depends strongly on sintering tempera-
ture. At 873 K, the extraction yield reached 50%. In this case, the extraction yield of rhenium
sublimation was 88–90% and desulfurization during sintering was 35%. An effect of the
excess air on the recovery yield of arsenic and rhenium, as well as the desulfurization of
the ore, was also observed. The higher the excess air ratio the more complete the extraction
of arsenic and rhenium and the desulfurization process.

3.3. Reduction Sintering

Preliminary reduction sintering of concentrate with a high arsenic content was also
performed. Experimental results are shown in Figure 5.
Figure 5 shows the effect of temperature and test duration of the reduction sintering
process on arsenic recovery. At 823 K, the arsenic extraction yield is 88%, and at 1373 K it
reaches 100%. Extraction of rhenium is not successful: this failure is apparently due to the
fact that volatile rhenium oxide (Re2 O7 ) in a reducing atmosphere at temperatures >700 K
 dicted at high temperature. This discrepancy should disappear if atomic chlorine partici-
pates in the gold chlorination reaction (reaction (17)). The authors of the works [3,24] came
to the same conclusion. Atomic chlorine can be formed during thermal decomposition of
CaCl2 [3,24]. Therefore, the chlorination of gold may take place in the temperature range
of interest.
Sustainability 2022, 14, 11019 10 of 14
3.2. Oxidation Sintering
Preliminary oxidation sintering of Au- and Re-containing concentrate with high ar-
transforms into nonvolatile
senic content rhenium
was performed. oxides, ReO
Experimental 2 and ReO
results 3 [27]. in
are shown Desulfurization
Figure 4. also does
not occur in this case.
50

40
80
3
3 2
As extraction (%)

Re extraction (%)
30 1
60

20 2
40
1
10

20
0
600 650 700 750 800 850 900 600 650 700 750 800 850 900
Temperature (K) Temperature (K)

(a) (b)

30
Desulfurization (%)

Sustainability 2022, 14, x FOR PEER REVIEW 11 of 15

20 3

2
We found 10that the extraction yield of arsenic depends strongly on sintering temper-
ature. At 873 K, the extraction yield reached 50%. In this case, the extraction yield of rhe-
1
nium sublimation was 88–90% and desulfurization during sintering was 35%. An effect of
the excess air on0the recovery yield of arsenic and rhenium, as well as the desulfurization
of the ore, was also
600observed.
650 The higher
700 750 the
800excess
850 air ratio
900 the more complete the extrac-
tion of arsenic and rhenium andTemperature
the desulfurization
(K) process.
(c)
3.3. Reduction Sintering
Figure
Figure 4. Effect
4. Effect of of temperature
temperature on:(a)
on: (a)arsenic
arsenic extraction
extraction yield;
yield; (b)
(b)rhenium
rheniumextraction
extraction yield; (c)
yield;
Preliminary reduction
desulfurization. Curves sintering
are labeled:of1—excess
concentrate with a high
air coefficient 100%,arsenic content
2—excess was also
air coefficient 150%,
(c) desulfurization. Curves are labeled: 1—excess air coefficient 100%, 2—excess air coefficient
performed. Experimental
3—excess results
air coefficient 170%. are shown in Figure 5.
150%, 3—excess air coefficient 170%.

100
3
100 4
2
Arsenic extraction yield (%)

80
98 1
Arsenic extraction yield (%)

96
60

94
40
92

20
90

88
0
800 900 1000 1100 1200 1300 1400 0 10 20 30 40 50 60
Temperature (K)
Duration (min)

(a) (b)
Figure 5. Dependence
Figure of arsenic
5. Dependence extraction
of arsenic on on
extraction (a)—temperature andand
(a)—temperature (b)—time: 1–773
(b)—time: K, 2—973
1–773 K, K,
K, 2—973
3—1173 K, 4—1373 K.
3—1173 K, 4—1373 K.

Figure 5 shows the effect of temperature and test duration of the reduction sintering
process on arsenic recovery. At 823 K, the arsenic extraction yield is 88%, and at 1373 K it
reaches 100%. Extraction of rhenium is not successful: this failure is apparently due to the
fact that volatile rhenium oxide (Re2O7) in a reducing atmosphere at temperatures >700 K
Sustainability 2022, 14, 11019 11 of 14

3.4. Chlorination Processing of the Cu-Au-As Concentrate

Experiments with the Cu-Au-As concentrate without preliminary oxidation or reduc-
tion treatment demonstrated that the proposed chlorination process does not allow efficient
gold extraction from this material.
Gold sublimation was 30–50 wt.% for low-temperature chlorination of the concen-
trate with elemental chlorine. In all experiments, including low-temperature chlorination,
considerable sublimation of other metals was observed. Chlorination at these low tempera-
tures (523 K) does not cause noticeable sublimation of other metals (Fe, Cu, Zn, and Ag).
However, 60–70% of the present tin does undergo sublimation, an effect which may be
suppressed by sintering in air at 973 K, presumably due to the heavy oxidation of tin.
Gold extraction yield of 99% was reached only for high-temperature chlorination of
the material with elemental chlorine. However, poor selectivity was the price paid. At
higher temperatures, the sublimation of other metals also increased dramatically: extraction
yield of Ag, Zn, Fe, As, and Sb was within the range of 50–70%, Cu ~10%, Sn ~75–85%.
Calcium chloride as an alternative chlorinating agent was mixed with the nanocrys-
talline SiO2 under dry conditions. For high-temperature chlorination of the concentrate
with calcium chloride gold extraction, yield reached 50–65% after 2 h at 1223 K. The subli-
mation of Fe and Cu was not observed; sublimation of Ag reached 30–40%. Sublimation of
Zn, As, Sb, Sn was nearly complete: 80–90%. Gold extraction yield following sintering at
1273 K is lower than at 1223 K, presumably because at 1273 K the clinker begins to melt
and agglomerate; therefore, successful chlorination and sublimation into the gas phase of
gold-containing materials is not achieved.
Following preliminary oxidation, sintering at 873 K with 170% excess of air of Cu-Au-
As concentrate, arsenic was removed from the concentrate to the extent of 50%; desulfur-
ization was 35% complete. Rhenium sublimation yield was equal to 80%. The resulting
clinker was sintered with chlorine gas at 523 K or with a mixture of CaCl2 + SiO2 at 1223 K.
Gold recovery in both cases was only 75–80%. The recovery of the remaining non-ferrous
metals was not higher than as without preliminary oxidation sintering. Tin sublimation is
significantly reduced.
With preliminary reduction sintering of Cu-Au-As ore at 1123 K arsenic was completely
removed. The resulting arsenic-free clinker was sintered with chlorine at a temperature
of 523 K and with a mixture of CaCl2 + SiO2 at a temperature of 1223 K. Gold recovery
was 87–93%. The recovery of the remaining non-ferrous metals was not higher than that
obtained without preliminary reduction sintering.
Experiments with the dry mixture from Cu-Au-As concentrate + calcium chloride +
nanocrystalline SiO2 were carried out with slow heating: the crucible was heated in the
furnace from room temperature to 1223 K at a heating rate of 8 K/min; or they were carried
out with rapid heating: the crucible was inserted directly into the hot furnace at 1223 K,
heating rate—100 K/min. Slow heating simulated the sintering process in a standard
rotary kiln, while fast heating was similar to that obtaining in a fluidized bed furnace. Our
previous experience with other products (coal fly ash, Cu-Zn-Pb polymetallic concentrates)
has shown that the sublimation of some metals under fast heating with CaCl2 increasing in
comparison with slow heating (our unpublished data). We believe that this is due to the fact
that, upon fast heating, thermal decomposition of calcium chloride occurs immediately in
the chlorination reaction zone. Slow heating causes calcium chloride thermal decomposition
to occur before the material reaches the temperature required for chlorination reactions. As
can be seen in Table 4, our attempts at efficient gold sublimation from Cu-Au-As ore did
not confirm the results of our previous investigations for other products.
It is known that chloride fumes are hygroscopic dust. The significant disadvantages of
dry capture in the chlorination of gold-bearing materials are mechanical losses of the gold
and difficulties with their storage and transportation [21]. It is known that gold chlorides
are metastable and external factors also affect the stability of gold chlorides. So, in the
case of contact of chloride fumes with the surface of dry capture devices, gold chlorides
decompose with the formation of metallic gold, which settles on the surface of the device,
Sustainability 2022, 14, 11019 12 of 14

and it is practically impossible to remove them, especially when metal steel devices are
used. As a result, there are significant losses and unbalances in gold in the process. When
capturing sublimates in steel apparatus, the unbalance of gold is 30–60%. The recovery of
gold from hydrochloric acid solution by these methods is 98–99% [21].
Table 4. Results of laboratory scale chlorination of the Cu-Au-As concentrate.

CaCl2 , SiO2 , Gold

% from Cu-Au-As % from Cu-Au-As Extraction
Procedure T, K Heating Reagent
Concentrate Concentrate Yield,
Quantity Quantity %
Cu-Au-As concentrate 523 Slow Chlorine - - 51.3
Cu-Au-As concentrate 1223 Slow CaCl2 + SiO2 37.5 6.4 58.7
Cu-Au-As concentrate 1223 Slow Chlorine - - 99.4
Cu-Au-As concentrate 1223 Fast CaCl2 + SiO2 43.2 10.1 56.5
Cu/Au/As
concentrate following
523 Slow Chlorine - - 74.3
oxidation sintering at
873 K
Cu/Au/As
concentrate following
1223 Slow CaCl2 + SiO2 40.0 10.5 79.1
oxidation sintering at
873 K
Cu/Au/As
concentrate following
1223 Fast CaCl2 + SiO2 40.0 10.5 78.3
oxidation sintering at
873 K
Cu-Au-As concentrate
following reduction 523 Slow Chlorine - - 85.7
sintering at 873 K
Cu-Au-As concentrate
following reduction 1223 Slow CaCl2 + SiO2 40.0 10.5 88.3
sintering at 873 K
Cu-Au-As concentrate
following reduction 1223 Fast CaCl2 + SiO2 40.0 10.5 88.4
sintering at 873 K

4. Conclusions
The problem of processing gold-bearing copper materials in Kazakhstan is becoming
relevant due to their large, accumulated volumes. This is due to the lack of a rational
processing technology, due to the significant content of arsenic in them. This paper presents
the results of a new process for extracting gold from refractory gold-copper-arsenic-bearing
products by chlorinating sintering.
The results of thermodynamic calculations showed the possibility of extracting gold
in the form of its chloride from complex refractory gold-copper-arsenic-bearing products
under the conditions of chlorinating sintering using chlorine gas and calcium chloride as
chlorinating reagents. However, as shown by the experimental results, it is not possible to
achieve a high extraction of gold during chlorinating sintering in the temperature range
of 800–1000 K, where the recovery of gold is 35–60%. The high extraction of gold during
sintering is restrained by the presence of arsenic and other impurities which stop the
mechanism of gold reduction by chlorine-containing reagents. This requires preliminary
operations to remove arsenic. According to this, the new experimental results on the
behavior of arsenic and other associated metals (Re and others) under the conditions of
oxidative and reductive sintering were obtained in the work.
Sustainability 2022, 14, 11019 13 of 14

We found that during oxidative sintering, the extraction of arsenic strongly depends
on the process temperature: at 873 K, the extraction of arsenic into dust was 50%, rhenium—
88–90%. The effect of excess air on the extraction of arsenic and rhenium into dust has been
established. It is shown that the higher the excess air coefficient, the more complete the
extraction of arsenic and rhenium into dust. The obtained data indicate that it is not possible
to achieve a high removal of arsenic from the initial product during oxidative sintering.
The best results in arsenic removal are achieved under conditions of reductive sintering
of the source material with natural gas: at 823 K, the extraction of arsenic into dust is 88%,
and at 1373 K, arsenic is almost completely removed into dust from the initial product.
The obtained new experimental results have fundamental importance for the choice
and organization of technology for the comprehensive processing of complex refractory
gold-copper-arsenic-bearing products. For processing these products, we consider starting
with reductive sintering with natural gas in order to completely remove arsenic from them,
and then chlorinating sintering of the resulting cinder with chlorine or calcium chloride
with a maximum extraction of gold. The residual condensed product after sintering is quite
suitable and without much effort can be processed by existing methods of the extraction
of copper.

Author Contributions: Conceptualization, N.D. and V.K.; methodology, E.Z. and A.A.; investigation,
E.K. and Y.T.; writing—review and editing, G.K. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.

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