M. ^ARNOGURSKÁ et al.: PYROMETALLURGICAL TREATMENT OF SILVER-CONTAINING CATALYSTS
133–138
PYROMETALLURGICAL TREATMENT OF SILVER-CONTAINING
CATALYSTS
PIROMETALUR[KA OBDELAVA KATALIZATORJA Z
VSEBNOSTJO SREBRA
Mária ^arnogurská1, Miroslav Pøíhoda2, Marián Lázár1, Peter Kurilla1
1Technical University of Ko{ice, Faculty of Mechanical Engineering, Ko{ice, Slovakia
2V[B – Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Ostrava – Poruba, Czech Republic
maria.carnogurska@tuke.sk
Prejem rokopisa – received: 2017-05-03; sprejem za objavo – accepted for publication: 2017-08-11
doi:10.17222/mit.2017.050
The following article describes the thermal process for the recovery of used silver from Ag catalysts using an 80-kVA plasma
reactor, along with an appropriate flux and reducing agent. An Ag catalyst was melted in three separate experiments with
different weights. The overall recovery of silver from the melted Ag catalysts was high (93.8–96.4) %. The byproducts of the
melting of the Ag catalysts were the inert vitrified slag, synthesis gas and fly ash. The chemical analysis of fly ash confirmed
that, in addition to mechanically stripped oxides that were present in the batch (CaO, SiO2, MgO, and Al2O3), fly ash also
contained a high amount of condensed silver. Indeed, silver evaporated at a high temperature during the melting process. The
silver condensed in fly ash was at a level of 37.08–48.37 % of the total weight of fly ash. Therefore, fly ash had to be recycled.
The synthesis gas from the process had a relatively low heating value (0.6335 MJ m–3).
Keywords: hazardous waste, Ag catalyst, plasma technology
^lanek opisuje termi~ni na~in pridobivanja srebra iz rabljenih Ag katalizatorjev v 80 kVA plazemskem reaktorju v prisotnosti
ustreznih talil in redukcijskega sredstva. Ag katalizator se je talil v treh poskusih z razli~no maso. Skupi iznos srebra pri procesu
taljenja Ag katalizatorjev je bil visok (od 93,8 % do 96,4 %). Pri taljenju katalizatorja je nastajala inertna vitrificirana `lindra,
sintezni plin in ostale ube`ne emisije. Kemi~na analiza ube`nih emisij je potrdila, da so poleg mehansko odtrganih oksidov iz
vhodnega materiala (CaO, SiO2, MgO, Al2O3) vsebovale tudi vi{jo vsebnost kondenziranega srebra. Srebro je v procesu taljenja
izparelo pri visoki temperaturi. V ube`nih emisijah kondenzirano srebro je bilo na nivoju od 37,08 % do 48,37 %. Ube`ne
emisije je bilo treba zato reciklirati. S procesom pridobljeni sintezni plin je bil razmeroma nizke kalori~ne vrednosti
(0.6335 MJ m–3).
Klju~ne besede: nevarni odpadek, Ag katalizator, plazemska tehnologija
1 INTRODUCTION
Separation and subsequent recovery of waste is
currently of major society-wide importance. Waste often
contains secondary raw materials with a relatively high
energy and material potential. In many cases, a portion
of waste that ends up in landfills or in waste-incineration
plants contains valuable components such as non-ferrous
and precious metals. With landfilling or conventional
waste incineration without an energy recovery,1
irreplaceable losses of metals and energy occur, which
are accompanied by multiple environmental problems,
since there are many different dangerous chemical
compounds released, including heavy metals such as
mercury, lead, cadmium and others.
Precious metals and also the metals of the platinum
group (Pt, Pd, Rh, Ir, etc.) are widely used in different
industries ranging from the electronics sector (hard
disks, multilayer ceramic capacitors, or integrated
circuits) to the production of liquid-crystal displays or
the chemical industry (catalysts).2
2 DESCRIPTION OF CATALYTIC CONVERTERS
The development of new catalysts of all types3 brings
the need for their subsequent recovery, reprocessing and
a permanent disposal. Heterogeneous catalysts contain-
ing silver are generally used in research and industry as a
part of the oxidation, reduction and polymerization pro-
cesses. A typical example of an application of silver-
based catalysts is the ethylene oxide production process.
During the catalytic reaction, the oxidation of ethylene
into ethylene oxide occurs. This change occurs in the
presence of a catalytic Ag layer deposited on an Al2O3
carrier at a temperature of 220–280 °C.4–5 The com-
position of the catalyst layer depends on the production
method and on the catalyst generator. The silver amount
in the majority of these types of catalysts is estimated to
be in a range of 8–20 % of mass fractions. The resto-
ration of the properties related to a particular catalyst
layer after their depletion is carried out through a
regeneration process. The number of regeneration cycles
is usually limited by the manufacturer. After the catalyst
is exhausted, the material the catalyst was made of is
recovered.
Materiali in tehnologije / Materials and technology 52 (2018) 2, 133–138 133
UDK 669.053:621.745.4:669.22 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(2)133(2018)
Recycling of silver and other metals constituting a
catalytic layer occurs mainly during a pyrometallurgical,
hydrometallurgical or a combined process.6–9 A key
aspect of the proposal of the recycling process is an
accurate determination of the type of the precious metals
in the waste, which is subject to a thorough pre-treatment
and homogenization.
Specifically, a pyrometallurgical recovery of waste
containing precious metals is a waste-reprocessing
technology utilizing plasma. High-temperature melting
of electronic waste in a plasma reactor is described
elsewhere.10 In addition to the synthesis gas that is pro-
duced during the melting and gasification of electronic
waste, and which can be energetically used, there are
additional byproducts like alloy metal, inert slag and fly
ash. The reduced metal portion and fly ash are used as
secondary raw materials for the metallurgical industry,
while inert slag, after a modification of its composition,
is used in the building sector, for example.
3 MATERIALS, TECHNOLOGY AND
THERMODYNAMIC REFLECTION
The catalyst used in the chemical industry for the
production of ethylene oxide (CH2(O)CH2) was sub-
jected to the experimental research of melting and gasifi-
cation. This oxide is an intermediate in the production of
ethylene glycol (C2H4(OH)2) and other chemicals.
Ethylene oxide finds its use in the health care as an
effective sterilizing agent, especially for those materials
that could be otherwise damaged due to a high tempe-
rature. A relatively small percentage is used for gaseous
sterilization of food and cosmetics.
A typical feature of an Ag catalyst is its coloring
(gray to gray-green). The catalyst is a non-sticky and
powdery product with a grain size below 8 mm. Its com-
position, indicated by the manufacturer, is represented by
two components: Ag and Al2O3. One pattern of a used
catalyst relating to the quality involves the percentages
of the respective oxides, metals, carbon deposits and the
moisture. Typical chemical composition of the used Ag
catalyst that undergoes an examination and is specified
by the manufacturer is described in Table 1.
Table 1: Typical chemical composition of a catalyst as stated by the
waste generator
Quality characteristics Value (%)
Amount of Al2O3 60.0–85.0
Amount of SiO2 0.50–1.50
Amount of MgO 0.00–0.05
Amount of CaO 0.20–1.00
Amount of Fetotal 0.10–1.50
Amount of carbon sediments 0.05–1.00
Amount of Ag 15.0–35.0
Moisture 0.05–1.50
Before the experiment with the plasma melting of the
used Ag catalyst was conducted, a chemical analysis was
carried out and the results are as follows:
57.86 % Al2O3 0.22 % C 0.19 % CaO
39.15 % Ag2O
(36.45 % Ag) 0.12 % moisture 0.011 % MgO
0.95 % SiO2 0.12 % FeO
The total sum of the majority of the components con-
stituting the waste was 98.6 % of mass fractions.
The recovery of precious metals by means of a
high-temperature melting process depends on various
operating factors, but mainly on the operational pro-
cessing temperature. If this melting temperature is low,
the viscosity of slag is high; moreover, the metals are not
able to sediment, and the recovery of precious metals is
low.
If the melting point is high, the viscosity of the waste
slag is low and the process can result in a high mutual
solubility of the liquid phases – the slag and metal. In
addition, the slag and metal can leak into the lining of
the reactor, increasing the corrosion of the lining itself.
At the same time, the volatilization of metals with a low
boiling point increases. Nevertheless, the recovery of
precious metals can be high.
In order to ensure the maximum recovery of precious
metals, it is necessary not only to establish a suitable
melting temperature of the catalyst, but also to ensure a
composition of the slag that will not cause any corrosion
of the lining of the plasma reactor and will have a
relatively low viscosity. Table 2 provides the information
regarding the melting point, the boiling point and the
density of the catalyst component of the Ag catalyst used
in this work.
Table 2: Melting point, boiling point and density of Ag and Al2O313
Component
Melting
point
Boiling
point Density (kg dm
–3)
(°C) (°C) 20 °C(s) 960.8 °C(l)
Ag 960.8 2161 10.49 9.32
Al2O3 2050.8 2999 3.95 –
During the melting of the Ag catalyst, there is a need
to take into consideration the dependence of the
saturated pressure p on the temperature T. The reference
value for this dependence can be determined in Equation
(1) according to the expression from:11
lg p = –14400·T–1 – 0.85·lg T + 13.8249
<(1233 – 2434) K> (1)
A previously published paper12 states that silver oxide
is formed due to chemical reaction in Equation (2):
4 Ag(s) + O2(g) = 2Ag2O(s) (2)
During the reaction, the standard change in the Gibbs
free energy is expressed as follows in Equation (3):
ΔG T
0 = –56260 + 121.31·T <(298 – 1000) K> (3)
M. ^ARNOGURSKÁ et al.: PYROMETALLURGICAL TREATMENT OF SILVER-CONTAINING CATALYSTS
134 Materiali in tehnologije / Materials and technology 52 (2018) 2, 133–138
Up to a temperature of 191°C, the silver oxide is
stable. The solubility of oxygen in molten silver is
described13 with the following reaction in Equation (4):
1/2 O2 = [O]Ag(1) (4)
The standard change in the Gibbs free energy in this
case is
ΔG T
0 = –10460 + 18.03·T (5)
For the solubility of hydrogen in molten silver, the
Sieverts law applies. There is an interaction between the
gas and the liquid phase according to the following
reactions:
H2 = 2[H]Ag(1) (6)
[H]Ag(1) = A· pH 2 (7)
The dependence of the solubility of silver atoms on
the temperature at a hydrogen pressure of 101.325 kPa is
shown in Figure 1.13
During the melting experiment, an appropriate
melting agent should be added, so that it forms, with the
aluminum oxide, a slag with a low melting point and low
viscosity. The choice of the additive is based on the
understanding that there is a significant difference
between the melting temperature of the Ag catalyst and
the melting temperature of silver alumina. It is requested
as well that the amount of SiO2 in the melted slag does
not exceed the limit of 3–5 % so that the slag can be
eventually reused, if possible, in the steel industry, for
example. The melting agent that meets these require-
ments is CaO.
The amount of recovered metal from the melting
process depends on the chemical reactions taking place
in the reaction chamber of a plasma reactor. At constant
pressure and temperature, the reactions can be described
with the standard change in the Gibbs free energy for the
reduction of metal oxides. Based on the diagram of the
stability of the phases in the system Cu-Fe-Ni-Sn-Zn-
Cd-Pb-O, the reaction is discussed in detail elsewhere.8
The optimal melting and gasification temperature of the
given waste was found to be 1400 °C. The melting
temperature of silver is 960.8 °C and that of alumina is
2050.8 °C. In order to reduce the melting point, an
additional melting agent is needed in the batch. Suitable
fluxing agents are fluorspar (CaF2) and anhydrous borax
(Na2B4O7). To provide a reducing atmosphere in the
reactor, it is necessary to add a reducing agent to the
batch. This agent also serves to reduce the amount of
water that is present in the reactor in the form of
humidity. The reduction of the water vapor takes place
according to reaction in Equation (8). The Equation
shows that, for 1 kg of moisture, 0.67 kg of C should be
added.
C + H2O  CO + H2 (8)
The quantity of the reducing agent added to the batch
is usually about 20 % higher than the amount that is
calculated theoretically. This amount, added to the
plasma reactor, provides a sufficient quantity of the
reducing agent for an eventual oxygen reduction. Indeed,
oxygen enters the melting process in the reactor and can
cause an oxidation of the graphite hearth. A sufficient
amount of the reducing agent in a plasma reactor is
needed to protect the graphite hearth against the
oxidation. At the same time, strong reducing conditions
occur in the reactor. These particular conditions induce
an increase of carbon monoxide in the synthesis gas and
reduce the proportion of carbon dioxide.
4 MELTING OF SILVER-BASED CATALYSTS IN
A PLASMA REACTOR
The composition of the waste was selected, prior to
the melting, in such a way that the emerging slag had a
relatively low melting point (below 1400 °C). At the
same time, and at a given temperature, the final slag had
to be saturated with a compound of the type
xCaO·yAl2O3, or pure alumina, thus avoiding the
corrosion of the lining that contains a high amount of
Al2O3. In addition, after a partial corrosion of the lining,
the temperature in the plasma reactor stabilized and, at
the level of this stabilized temperature, a solidified slag
layer was created on the inner surface of the lining. This
layer protected it from further corrosion.
The experiment was carried out in an 80-kVA plasma
reactor with a hollow graphite electrode. The plasma gas
was nitrogen. In Table 3, there is a description of the
starting materials, which constituted the charge, along
with the other parameters characterizing the melting
process. The Ag catalyst was melted in three separate
experiments with various weights.
M. ^ARNOGURSKÁ et al.: PYROMETALLURGICAL TREATMENT OF SILVER-CONTAINING CATALYSTS
Materiali in tehnologije / Materials and technology 52 (2018) 2, 133–138 135
Figure 1: Solubility dependence of hydrogen in silver
The melted alloy that resulted from each experiment
was subjected to a chemical analysis. The results of the
analysis show that, in addition to silver, the alloy con-
tains iron particles. The resulting alloy was further
refined with electrolysis. The amount of the alloy ob-
tained during the melting of 100–123 kg of the catalyst
ranged from 34.94 kg to 42.15 kg (Table 4). The total
recovery of silver from the melting of the Ag catalyst in
the plasma reactor was high (ranging from 93.8 % to
96.4 %) (Table 4). The chemical composition of the slag
from the melting and gasification of the Ag catalyst is
described in Table 5. All the elements with a high
affinity to oxygen, including, in particular Al, Si, Mg and
Ca became part of the slag along with a small portion of
iron in the form of FeO. The slag contained only stripped
and non-sedimented droplets of Ag as documented on
the macro-picture of the fractured surface of the slag in
Figure 2. The quantity of silver in the slag determines
the type of slag (dump or returned slag) that must be
treated again. Based on the requirements for the recovery
of silver, the silver content in the dump slag should not
exceed the limit of 0.5 ‰ of Ag. Both types of slag were
subjected to a leaching test. The chemical composition
of the fly ash, generated during the gasification and
melting of the Ag catalyst, which was collected around
the area of the synthesis-gas purification, is shown in
Table 6.
Table 3: Basic data of the raw material
E
xp
er
im
en
t
C
at
al
ys
t
(k
g)
S
la
gg
in
g
ag
en
t
(k
g)
R
ed
uc
in
g
ag
en
t
(k
g)
D
ur
at
io
n
of
th
e
ex
pe
ri
m
en
t
(m
in
)
E
ne
rg
y
us
e
(k
W
h)
U
se
of
th
e
pl
as
m
a
ga
s
(d
m
3
m
in
–1
)
1 100.0 68.45 0.50 405 74.15 8.5
2 100.0 68.80 1.00 445 70.31 8.5
3 123.0 84.20 1.50 510 66.30 8.5
dm3 – volume of gas media at the pressure of p = 101.325 kPa and
temperature of 25 °C
Table 4: Alloy – mass and chemical composition
Experi-
ment
Alloy Chemical composition (%)
Ag alloy
(kg) Ag Fe Al Si Cu
1 34.94 99.40 0,72 0.04 0.02 0.00
2 35.21 99.78 0.20 0.02 0.00 0.00
3 42.15 99.80 0.15 0.05 0.00 0.00
Table 5: Slag – mass and chemical composition
Experi
ment
Dump
slag Chemical composition (%)
Slag
(kg) CaO MgO SiO2 Al2O3 Agtotal Fetotal
1 129.50 46.72 0.05 1.05 52.46 0.017 0.11
2 126.00 45.05 0.05 0.93 53.10 0.021 0.07
3 159.50 47.84 0.04 1.08 51.02 0.038 0.10
The energy of the synthesis gas, which was produced
during the melting of the Ag catalyst and subsequently
purified, was used in a cogeneration unit with a micro-
turbine CAPSTONE C65. The rates of the average
concentrations of the main constituents of synthesis-gas
components (CO, CO2, H2, N2 and CH4) obtained with
continuous chromatographic measurements, are shown
in Table 7.
Table 6: Fly ash – mass and chemical composition of the main com-
ponents
Experi-
ment
Fly ash
(kg) CaO MgO SiO2 Al2O3 Agtotal Fetotal
1 2.5 15.15 0.02 0.30 23.74 43.45 1.12
2 2.0 15.77 0.02 0.51 25.99 37.08 0.90
3 4.1 17.60 0.01 0.22 22.17 48.37 0.84
Table 7: Syngas and average values of the components
Experi-
ment
Syngas
(m3 h–1)
(vol.%)
H2 CO CO2 CH4 N2
1 10.70 3.03 2.30 1.16 0.00 93.51
2 12.45 1.87 1.27 1.07 0.00 95.79
3 12.30 2.68 2.12 1.31 0.00 93.89
m3 – volume of gas media at the pressure of p = 101.325 kPa and
temperature of 25 °C
5 DISCUSSION
The plasma melting and gasification of the used Ag
catalyst in the 80-kVA plasma reactor can be evaluated
as follows in terms of material and energy use of the
waste.
Silver losses occurred due to the evaporation and the
subsequent condensation in the fly ash as well as due to
the tapping of the metal alloy with the slag phase, i.e., a
deficient sedimentation of silver at the bottom of the
reactor. Throughout the continuous tapping of the slag
during the waste-melting process, very fine particles of
silver were pulled in the dump slag due to the flow of the
M. ^ARNOGURSKÁ et al.: PYROMETALLURGICAL TREATMENT OF SILVER-CONTAINING CATALYSTS
136 Materiali in tehnologije / Materials and technology 52 (2018) 2, 133–138
Figure 2: Macro photography of a mechanically trapped silver ball in
the slag matrix
slag itself. This loss of metal can be considered as the
total loss of Ag during the plasma-melting process
because the other loss of silver from the fly ash and from
the reusable slag can be eliminated with repeated
melting. A reversible Ag loss is the loss into the lining of
the reactor that becomes saturated with silver during the
first melting processes due to the diffusion of silver and
the leaking between the segments of the lining.
The thermodynamic analysis of the melting and
gasification of the Ag catalyst and fluxes show that
throughout the treatment, besides the vitrified slag, there
were also synthesis gas and fly ash. In a strong reducing
atmosphere, all the compounds with a high affinity to
oxygen, which mainly include Al2O3, SiO2, MgO and
CaO, form an inert slag. Inert slag with a silicate
structure concentrates as a separate liquid phase above
the surface of the metal. The part of the iron that is found
in the slag in the form of FeO creates, together with
silver, a metal phase – an alloy, concentrated at the
bottom of the reactor. The liquid phases – the slag and
alloy – are periodically tapped from the reactor during
the melting. The leaching tests show that calcium is
extracted from the slag. However, this does not interfere
with a further effective use of the slag in the process of
ferrous metallurgy.
The chemical analysis of the obtained fly ash shows
that, in addition to the oxides that were mechanically
stripped from the batch (CaO, SiO2, MgO, and Al2O3),
the ash contained a high quantity of the condensed silver
that evaporated at a high temperature during the melting
and gasification of the Ag catalyst. The amount of silver
in the fly ash ranges from 37.08 % to 48.37 %. There-
fore, the fly ash needs to be recycled.
The synthesis gas, arising from a strongly reducing
condition, has a higher quantity of CO than CO2. The
gas, generated throughout the melting of the Ag catalyst
with a high-temperature pyrolysis decomposition of the
organic raw material, was discharged from the plasma
reactor through a cyclone separator. The majority of the
mechanically stripped fly ash particles were collected in
the cyclone. Subsequently, the synthesis gas was rapidly
cooled from a temperature of 1500–1600 °C to
160–170 °C in a hydrocyclone. In such a way, the rest of
the Ag microparticles were captured in a bag filter. In the
next step, the acidic components of the synthesis gas
were neutralized in a countercurrent absorption column
using NaOH with pH = 9. From the absorption column,
the synthesis gas was led into the cooling system, where,
in the first step, it was cooled below the dew point of
60 °C. The moisture, which entered the gas during the
cooling in the hydrocyclone and neutralization in the
countercurrent absorption column, was condensed. After
the cooling, the purified synthesis gas was heated above
the dew point temperature and then burned in a cogene-
ration unit with a micro-turbine CAPSTONE C65.
Figure 3 indicates the composition of the synthesis
gas produced during experiment no. 1 at the steady-state
batch feeding. The syngas was then used in an energy
co-generation unit. The synthesis gas had a very low
heating value. The mean value was 0.6335 MJ m–3. For
an amount of the combusted syngas equal to 10.7 m3 h–1
(Table 7) and the given heating value of 0.6335 MJ m-3,
the energy contribution of the syngas was negligible
(Figure 4). The energy contained in the synthesis gas
was on average only 0.076 % of the total energy con-
tained in the consumed natural gas. Nevertheless, it is
useful to use syngas in the process.
6 CONCLUSION
Negative environmental impact of used Ag catalysts
can be avoided with their melting and gasification in a
plasma reactor. Indeed, three main products can be
obtained in the melting process: a silver alloy, syngas
and synthetic slag that is useable also in the metallurgy
of iron and steel. A sign accompanying the melting of an
Ag catalyst is the formation of fly ash in the synthesis
gas, which, in addition to the stripped oxides of calcium,
magnesium, silicon and aluminum, contains condensed
silver. Fly ash must be therefore recycled and the amount
of silver obtained with the melting of the catalysts
M. ^ARNOGURSKÁ et al.: PYROMETALLURGICAL TREATMENT OF SILVER-CONTAINING CATALYSTS
Materiali in tehnologije / Materials and technology 52 (2018) 2, 133–138 137
Figure 4: Proportion of natural gas in the mixture with syngas during
the first experiment
Figure 3: Change in the syngas composition during the first experi-
ment
increases. The synthesis gas has energy parameters that
allow its use in the production of heat and electricity.14–15
Based on the results presented in this study, concern-
ing the melting of an Ag catalyst, it can be concluded
that such a hazardous waste can be transformed into an
inert product using the plasma technology, and the
products of the melting can be subsequently used.
Therefore, the process of liquefaction of the analyzed
waste can be considered as the BAT (best available
technology).
Acknowledgments
The paper was supported by grant agencies KEGA,
no. 003TUKE-4/2016, and SP2017/37-FMMI V[B TUO.
Nomenclature
A – constant /1
ΔG T
0 – standard change in Gibbs free energy /J mol–1
[H]Ag(l) – hydrogen concentration in melted Ag
/cm3/100 g
[O]Ag(l) – oxygen concentration in melted Ag /cm3/100 g
pH 2 – partial pressure of oxygen in the atmosphere /Pa
T – temperature /K
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M. ^ARNOGURSKÁ et al.: PYROMETALLURGICAL TREATMENT OF SILVER-CONTAINING CATALYSTS
138 Materiali in tehnologije / Materials and technology 52 (2018) 2, 133–138