ADSORPTION OF GOLD FROM Au(III)-CHLORIDE SOLUTIONS ON ALTERNATIVE MATERIALS TO ACTIVATED CARBON INORGANIC MATERIALS

11 Pages·00.875 MB·English

by  T. Feldmann ,G.P. Demopoulos

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ADSORPTION OF GOLD FROM Au(III)-CHLORIDE SOLUTIONS ON ALTERNATIVE
MATERIALS TO ACTIVATED CARBON INORGANIC MATERIALS

*T. Feldmann1 and *G.P. Demopoulos1

1McGill University
3610 University Street
Montreal, QC, Canada, H3A 0C5
(*Corresponding authors: [email protected] and [email protected])

ABSTRACT

The adsorption of gold complexes on inorganic metal oxides has been mainly discussed in
geochemical literature, but it is also of interest to the hydrometallurgical industry. This is so because it could
be responsible for valuable metal losses or because it could potentially serve as alternative adsorbent material
to activated carbon in certain cases such as from hot and acidic process slurries. This paper constitutes an
early investigation into this subject by focusing on the effectiveness of FeOOH, Fe O and Al O as sorbent
3 4 2 3
materials. All tests were done in a batch stirred-tank reactor under atmospheric conditions with synthetic
gold(III)-chloride solutions at 23°C and 80°C, in the pH range of 1.6 to 9.0. Detailed kinetic tests were
performed at different pH values (3 and 6) on FeOOH and Fe O with solutions having gold concentrations
3 4
from 5 mg/L to 130 mg/L. The adsorption of gold was found to be best both in terms of kinetics and loading
characteristics at 80°C and pH=6. This was explained by a change in Au(III)-Cl speciation from a gold(III)-
chloride complex to a gold(III)-chloride-hydroxy complex. By far, magnetite (reagent grade chemical)
yielded the best gold adsorption characteristics making it an interesting sorbent material for further
development considering that it can be advantageously recovered from slurry streams via magnetic
separation. On the other hand, the tendency of gold(III)-chloro complexes to adsorb on metal (hydro) oxides
needs to be taken into account to minimize undesirable metal losses in chloride-containing leaching media.

KEYWORDS

Gold, chloride, adsorption, magnetite, goethite, alumina, titanium dioxide, hydrometallurgy

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INTRODUCTION

The most common process for the extraction of gold from ores or secondary sources is cyanide
leaching in combination with adsorption on activated carbon. Typically, the adsorption process is done
directly in the pulp, i.e., activated carbon is directly added to slurry (Fleming, 1992; La Brooy, Linge, &
Walker, 1994). The use of cyanide, however, is increasingly under the threat of severe restriction due to
associated social and health-related issues. Hence, alternative leaching systems have been investigated in the
past, for example, systems based on thiosulfate (Aylmore & Muir, 2001; Grosse, Dicinoski, Shaw, &
Haddad, 2003; Senanayake, 2012), thiourea, bromine, iodine (Prasad, Mensah-Biney, & Pizarro, 1991) and
chloride (Ferron, Fleming, Dreisinger, & O’Kane, 2003). The chloride system is of interest due to its ability
to achieve near complete gold extraction and the option to integrate it into existing acid pressure oxidation
processes such as those practiced in the case of refractory gold ores or advocated for copper concentrates.
During the leaching step a gold(III)-chloride complex is formed, which is subsequently recovered by
adsorption on activated carbon (Ferron et al., 2003).

The use of activated carbon has been studied to a great extent in connection with chloride based
gold extraction. The kinetics of this process are very fast (the adsorption is finished within several minutes)
(Avraamides, Hefter, & Budiselic, 1985) and the process follows first order reaction kinetics (Hughes &
Linge, 1989; Pacławski & Wojnicki, 2009). The “adsorption” of gold onto activated carbon is actually a
reduction process where metallic gold is formed (Pacławski & Wojnicki, 2009; Sun & Yen, 1993; Tarasenko,
Lapko, Kopyl, Kuts’, & Gerasimyuk, 2003). Even though activated carbon seems to be a good material, it is
interesting to investigate alternative adsorbents, especially in the case of “in-leach” applications involving
strongly agitated hot slurries that lead to gold losses due to carbon fragmentation. Another complication is
the rather difficult stripping of the metallic gold from the loaded carbon.

According to geochemical literature it is known that dissolved gold(III)-complexes adsorb on iron
oxides such as hematite (Karasyova, Ivanova, Lakshtanov, Lövgren, & Sjöberg, 1998; Nechaev &
Nikolenko, 1986), goethite (Machesky, Andrade, & Rose, 1991; Schoonen, Fisher, & Wente, 1992), iron(III)
hydroxide (Uchida et al., 2002) and magnetite (Alorro, Hiroyoshi, Kijitani, Ito, & Tsunekawa, 2010; Odio,
Lartundo, Santiago-Jacinto, Martinez, & Reguera, 2014). This can present a problem of losses in the pressure
oxidation/leaching processes (Rusanen, Aromaa, & Forsen, 2013) often used to liberate refractory gold or to
treat copper concentrates, as in the CESL process(Robinson, Mayhew, Jones, & Murray, 2011). The
mobilization of gold during oxidative pressure leaching in the presence of chloride ions and its subsequent
co-precipitation or adsorption was the subject of a recent McGill HydroMET paper (Demopoulos, Parisien-
La Salle, & Blais, 2012). Furthermore, the gold complexes are known to adsorb on the surface of aluminum
(oxy)-hydroxides (Berrodier et al., 2004; Uchida et al., 2002). Interestingly, magnetite was embedded into
activated carbon in order to make it magnetic for easy gold recovery (Kahani, Hamadanian, & Vandadi,
2007). In general, the adsorption of gold(III) proceeds via the interaction of gold(III)-chloride hydroxide
complexes with surface –OH groups of the solid (Cohen & Waite, 2004; Nechaev & Nikolenko, 1986;
Uchida et al., 2002), with the exception of magnetite where the adsorption seems to happen via the reduction
of the gold(III)-complex to metallic gold (Alorro et al., 2010; Odio et al., 2014). According to Cances et
al.(2007) the adsorption occurs via the formation of a bidentate surface complex with the solid. Furthermore,
this means that the adsorption from chloride solutions by these solids depends primarily on pH and chloride
content. This was discussed by Cohen and Waite (2004) and Karasyova et al. (1998), who showed that
gold(III) exists only at pH<4 in the form of a [AuCl ]– complex, with increasing pH the Cl– ligands are
4
stepwise replaced by OH– ions (Murphy & LaGrange, 1998).

Even though the mechanisms of adsorption seem to be understood to some degree, there has been
no systematic study that actually compares these inorganic adsorbents. This paper follows an earlier work
by our group (Demopoulos et al., 2012) dealing with gold(III) chloride complex adsorption on carbon, and
investigates and compares the kinetics of gold adsorption on a number inorganic oxides, namely ferric oxide,
ferric oxide hydroxide, ferrous ferric oxide, aluminum oxide and titanium oxide. Special consideration is
given to materials that are easy and cost-effective to prepare, non-toxic and have favorable solid-liquid
separation properties such as a large uniform particle size or magnetic properties.
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EXPERIMENTAL

The experiments were carried out in a 2 L temperature-controlled reactor (Syrris Atlas Reactor)
system, with the ability to monitor and record temperature and pH. The tests were done with 500 mL
gold(III)-chloride containing solution, which was adjusted to a certain target pH in some cases, as discussed
later. The pH was adjusted with HCl, and NaOH solution, respectively. Furthermore, the solution was stirred
with a 4-blade, 45°-pitched impeller. All internal parts of the reactor were made from glass or PTFE. A
typical experiment was conducted for 2 h; some tests were run for 3 h or 6 h. The adsorbent material was
added as a slurry made with a small amount of water, which helped with the addition of the solid to the
reactor. During the scoping tests the solid sorbent loading was 15 g/L, but it was reduced to 7.5 g/L during
the more detailed tests.

Samples of 10 mL were taken from the reactor periodically and filtered with 0.22 µm syringe filters
and in the case of nano-size materials, such as the tested TiO , a Millipore Amicon centrifuge filtration tube
2
was used and centrifuged for 10 min at 5000 rpm. A list of the solid adsorbent materials used is given in
Table 1.The gold-containing solutions were prepared by diluting an appropriate amount of 1000 mg/L
analytical grade AAS standard solution obtained from Sigma Aldrich. In these solutions gold is present in
the form of a H+[AuCl ]– complex.
4

Table1 – List of used adsorbent materials and their surface area

Substance Supplier Surface area
(BET)
m2/g
Aluminum oxide, fused Sigma Aldrich 0.1
Aluminum oxide, activated Sigma Aldrich 136.2
Ferric oxide Sigma Aldrich 1.9
Ferric oxide hydroxide, catalyst grade Sigma Aldrich 158.0
Ferrous ferric oxide, black Fisher 7.5
Titanium dioxide, nano powder Alfa Aesar 139.8

The concentration of gold was analyzed by means of atomic adsorption spectroscopy (AAS) with a
Varian AA240FS instrument. The surface area of the samples was determined with a Micromeritics Tristar
Surface Area Analyzer. The results are given based on the Brunauer-Emmett-Teller (BET) method of surface
area determination. X-ray powder diffraction (XRD) was performed on a Bruker D8 Discovery instrument
with a Co-Kα radiation source. The characterization via scanning electron microscopy (SEM) was done with
a Philips XL30 field emission gun microscope. Particle size distribution measurements were made with a
Horiba LA 920 Laser Scattering Particle Size Analyzer. The medium for dispersion was water with 0.1%
sodium hexametaphosphate, which acts as a dispersion agent. Each sample was ultrasonicated in the
instrument for 2.5 min. The measurement of the zeta-potential of solid particles (0.04 wt.%) in 10-3 mol/L
KCl electrolyte was done with a Brookhaven ZetaPlus instrument. The pH of the samples was adjusted with
0.05 mol/L and 0.5 mol/L HCl/NaOH solutions. X-ray photoelectron spectroscopy (XPS) measurements
were conducted on a Thermo Scientific KAlpha instrument, using an Al Kα X-ray source at 1486.6 eV.
Spectra were generated at a perpendicular takeoff angle, using a pass energy of 20 eV and steps of 0.1 eV.
During analysis, the pressure was in the order of ≈1.33×10−9 Pa. As an internal reference for the absolute
binding energies, the Au (4f ) peak was used. The experimental spectra were deconvoluted after subtraction
7/2
of the Shirley background using the VG Avantage program.

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RESULTS AND DISCUSSION

Material screening

In order to understand the behavior of the materials tested it is necessary to know their properties.
Especially the available surface area is considered to be of importance for the adsorption process. By
comparing the surface areas, given in Table 1, it can be seen that activated carbon has by far the highest
surface area, which makes it a preferred adsorption material in the gold industry (Staunton, 2005). In
comparison to that FeOOH, activated Al O and TiO had surface areas in the range of 130-150 m2/g. Lastly,
2 3 2
Fe O , Fe O , fused Al O with comparatively low surface areas of <10 m2/g were tested as well. The high
3 4 2 3 2 3
surface area of the materials went hand in hand with a low crystallinity as was verified by XRD
measurements.

In order to understand the general behavior of the tested materials, scoping tests were conducted in
25 mg/L gold(III)-chloride solutions at 25°C and 80°C with different initial pH values, namely 1.5, 4 and 8.
After the start of the experiment the pH was not further adjusted, i.e., it could change freely. It remained
similar to the initial pH in all cases. From these tests it was found that, in the case of the iron-oxide based
adsorbents, dissolution of the solid took place at pH≤1.5. No dissolution of the aluminum oxides and titanium
oxide was observed at this pH. At pH≥2 all adsorbents were found to be stable and did not dissolve during
the experiment.

From concentration-time profiles it was seen that some materials did not remove gold from solution
regardless of the pH or temperature. On the other hand, some materials removed gold quite effectively from
solution. Generally, it was found that a higher temperature caused a faster and more complete gold removal.
It was also seen that less in acidic conditions the amount of gold removed from solution increased.

Figure 1 gives some examples of the concentration-time trends obtained from the experiments. It
shows the results of tests done with ferric oxide hydroxide (FeOOH) and ferrous ferric oxide (Fe O ). It
3 4
clearly shows the effect of temperature and pH on the adsorption of gold. Generally, if gold was removed
from solution, it was more completely removed at higher pH values and at high temperature (80°C).
Therefore, all further tests described in the following sections were done at 80°C. It was seen that on a
mg /g basis, especially activated aluminum oxide (activated Al O ), ferric oxide hydroxide, ferrous
Au solid 2 3
ferric oxide and titanium dioxide were capable to remove gold nearly completely from solution. However,
for a further study, only FeOOH, Fe O and activated Al O were considered, since the titanium dioxide
3 4 2 3
tested was a nano-material that posed operational difficulties in separating it from the solution. The FeOOH
and activated Al O had a relatively uniform particle size of well above 100 µm, while Fe O could be
2 3 3 4
easily recovered due to its magnetic property.

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(a) Ferric oxide hydroxide (b) Ferrous ferric oxide

Figure1 – Examples of concentration-time curves showing the influence of temperature and pH for a solid
loading of 15 g/L (given as average;during the experiment the initial pH was 1.5, 5.0 and 8.0 at 25°C and
8.0 at 80°C)

Effect of pH

From the preliminary tests done, it was seen that pH had an influence on the amount of the gold
adsorbed on the inorganic material’s surface, or removed from solution. Hence, experiments under pH-
controlled conditions for ferric oxide hydroxide, ferrous ferric oxide and activated aluminum oxide were
performed. Figure 2 shows a graphical summary of the residual gold concentration after 120 min of contact-
adsorption experiment. In general it can be said that an increase of pH from 2 to 6 resulted in faster and more
complete adsorption in all cases. However, there are also differences among the materials. In the case of
FeOOH, an increased pH resulted in a gradually higher gold removal up to the complete removal at pH≈6.
In the case of FeOOH and activated Al O , the gold concentration typically plateaued after 30 min.
2 3
Interestingly, there was no gradual decrease of final gold concentration seen for the latter material. It was
rather low at pH=2, 3 and 4 and then increased at pH=5 and 6. This may be explained by a change in
speciation gold chloride complex that occurs around pH=3.5-4 (Murphy & LaGrange, 1998), according to
equation 1:

[AuCl ]– + xH O → [AuCl (OH) ]– + xCl– + xH+ with x=0...4 (1)
4 2 4-x x

The first hydrolysis step forming [AuCl OH]– occurs at pH≈3.5, the next hydrolysis steps have been
3
established to occur at pH≈5.2, pH≈7-8.2 and at pH=11.

Figure 2 –Residual concentration of Au in solution after 120 min of contact with solid at different pH at
80°C with 15 g/L solid material loading
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Ferrous ferric oxide showed a different concentration-time profile with change of pH in comparison
to the two other materials – it was linear, while the decline in gold concentration was rather steep for FeOOH
and Al O within the first minutes of the experiment. This behavior was observed under all pH conditions.
2 3
Furthermore, in the case of Fe O it was seen that an increase in pH resulted in a gradual increase in speed
3 4
of gold removal from solution, e.g., while it took 90 min to remove all gold present at pH=2 and 3 it took
about 60 min at pH=4 and had finished after 5 min at pH=5 and 6, which can also be explained with the
mentioned speciation effect in connection with the associated different complex stabilities.

Mechanism of adsorption on FeOOH and Al(OH)
3

Adsorption of gold(III) on metal oxide/hydroxide surfaces has been studied in the past. Karasyova
et al.(1998) concluded from their work with hematite that the observed adsorption behavior is based on inner-
sphere complexation of Au(III). Cohen and Waite (2004) studied the adsorption of gold chloride on the
surface of synthetic goethite. It was found that the positively charged surface functional group of goethite
(Fe-OH +) interacts with the negatively charged gold(III) complex. In the acidic pH range, where [AlCl ]–
2 4
and [AlCl OH]– exist, one proton of the goethite surface group is removed during the adsorption process,
3
either together with one Cl– ion or by formation of H O, in the case of chloride-hydroxy complex.
2
Furthermore, the adsorbed amount of gold(III) was found to be a function of pH. Around 90% adsorption of
0.5 µM gold took place in the pH range of 6 to 8 after 24 h onto 0.02 g of synthetic goethite. This confirms
the results presented above.

It is plausible that a similar mechanism of adsorption occurs on the surface of activated aluminum
oxide, which will also form surface hydroxide groups upon contact with an aqueous solution. For example,
Uchida et al. (2002) performed gold adsorption experiments on in-situ precipitated, amorphous aluminum
hydroxide by raising the pH of an aluminum chloride solution. It was found that the adsorption of gold
increased linearly with the amount of precipitated Al(OH) , while it declined with increasing chloride
3
concentration (adjusted by NaCl addition). The highest adsorption values were seen in the range of pH=6-8.
The later corresponds well to the results of this work, where the highest adsorption occurred at pH=6 and
80°C. Unfortunately, Uchida et al.(2002) did not perform experiments at temperatures other than 30°C. They
explain the adsorption by electrostatic attraction between the negatively charged gold(III)-chloride hydroxy
complex and the particles, which had a positive zeta-potential at pH<8.

Mechanism of adsorption on Fe O
3 4

The concentration-time trend for gold(III) adsorption on Fe O was different from that of
3 4
activated Al O and FeOOH at all pH values investigated. While the latter two solids contain only fully
2 3
oxidized metal ions in their lattice, Fe O has a mixed +2/+3 oxidation state. This means in addition to the
3 4
potential “pure adsorption” on the surface described above, gold can also be removed from solution by
reduction, through the oxidation of FeII to FeIII. This explains why the relatively low surface area solid
Fe O (see Table 1) was found to be equally effective in removing gold from solution. Since adsorption
3 4
capacity of a material is generally proportional to its specific surface area, Fe O would be expected to be
3 4
at least one order of magnitude less efficient compared to activated Al O and FeOOH. In work done by
2 3
Alorro et al. (2010) it was found that adsorbed gold exists in its reduced, elemental form on the surface of
magnetite. This was confirmed in the present work through XPS measurements and SEM examination.

Figure 3 shows an energy dispersive X-ray spectroscopy map of gold-loaded Fe O and the
3 4
corresponding SEM image. It can be seen that reduced (elemental) gold particles are present on the surface
of the solid (bright yellow dots).

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(a) Secondary electron image of EDX spectroscopy site (b) EDX map for Au

Figure 3 – Secondary electron image and EDX spectroscopy map showing the presence of elemental gold
on the surface of Fe O
3 4

The effect of pH found in the study by Alorro et al.(2010) as well as in the present work can be
explained by the influence of pH on the zeta-potential. According to these authors the magnetite particles
have a positive zeta-potential at pH<3-4. This is supposed to lead to high adsorption since the gold complex
is negatively charged and it should be lower at higher pH values, where the zeta-potential is negative. This
however, was not the case in the current experiments as well as in those of Alorro et al.(2010), who give the
explanation that the recovery of [AuCl ]– is not due to electrostatic adsorption, in contrast to the work by
4
Uchida et al.(2002). These authors found that gold adsorption happens in the region of a positive zeta-
potential (in their case for aluminum hydroxide and iron hydroxide this was at pH<8), however, the presence
of the Au(III) chloride-hydroxy complex is a condition for adsorption to occur as discussed above.
Unfortunately, no other mechanism for the gold(III) removal was offered by Alorro et al. (2010) for the high
adsorption on magnetite at pH values that had a negative zeta-potential. Our own measurements of the zeta-
potential gave an isoelectric point around pH 6.7, which is a typical value for synthetic Fe O (Carlson &
3 4
Kawatra, 2013; Salazar-Camacho et al., 2013). Below this pH the particles are positively charged. In this
context our results agree in general with the work of Uchida et al.(2002). Furthermore, this explains the
observed lower adsorption of gold out of solution at 25°C pH=9.0 compared with pH=6.1 (see Figure 1 (b)).

Adsorption kinetics

After establishing the temperature and pH effects, further experiments were conducted in order to
understand the influence of initial gold concentration on the rate of adsorption and to define adsorption
isotherms. Iron oxide hydroxide and ferrous ferric oxide were chosen for these tests, as were the materials
exhibiting the most interesting effects/properties. Since true equilibrium may take a very long time to attain,
it was decided to construct “practical adsorption isotherms” corresponding to 2-3 h contact time, which is
realistic considering the kinetic curves of Figure 1.

From Figure 4 (a) and (b) it is seen that especially the initial rate of adsorption (adsorption within
the first 5 min of the experiment) as well as the adsorption isotherm at pH=6 have a near linear behavior.
The situation is less clear at pH=3, where the initial rate increases much slower with initial gold concentration
and the adsorption isotherm shows non-linear behavior. This shows furthermore, that pH=6 is more favorable
for the adsorption of more gold from solution.

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(a) (b)

Figure 4 – The influence of initial gold(III) complex concentration on (a) rate of adsorption and (b)
“practical” adsorption isotherms

CONCLUSION

In this paper the extraction of gold chlorocomplexes on alternative metal oxide (FeOOH, Fe O
3 4
and Al O ) sorbents was investigated. The investigations reported in this paper can be summarized as
2 3
follows:
• In the case of FeOOH and Al O , gold chlorocomplex adsorption was found to be surface area-
2 3
dependent. By contrast, in the case of Fe O gold chlorocomplexes were found to be reduced to the
3 4,
metallic state and thus be less dependent on surface area.

• The amount of gold removed from solution was a function of pH, with near neutral pH resulting in
more complete gold removal from solution in comparison to lower pH values. This behavior is
caused by speciation changes associated with the successive replacement of a Cl– ligand with a
OH– ion upon pH increase.

• The adsorption of gold on FeOOH at pH=6 and at 80°C was usually accomplished up to 85% of the
total gold content for initial concentrations ranging from 6 to 124 mg/L at a solid loading of 7.5 g/L
in solution within 20-30 min. The initial rate of adsorption appeared to be of first order nature.

• The adsorption on Fe O appeared to follow the concentration-time relationship of FeOOH
3 4
initially, however, after around 30-40 min it changed into a linear dependency, which was explained
by a reductive adsorption mechanism.

• The tendency of gold(III) chlorocomplexes to adsorb on iron (hydrous) oxides that tend to form via
in situ precipitation in acidic pressure leaching operations needs to be controlled to avoid
undesirable metal losses.

• Magnetite deserves further investigation as robust inorganic sorbent material.

ACKNOWLEDGMENTS

This work was supported through the NSERC Discovery research program.

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