Recovery of precious metals from spent catalyst

Restoration of precious metals from spent catalyst has benefits, including the easy process,
quick production period, and less investment and environmental influence compared to
mining production. Nevertheless, the concentration of PMs in spent catalysts ranges from 100
ppm to 100%, including the catalyst carriers also different in various applications. Many
researchers have made attempts on precious metals recycling from spent catalysts. Among
the recycling methods, the most effective ones are concentrating PMs by the pyrometallurgy
or by the selective leaching process.

Pyrometallurgy process

The constituents of PMs in spent catalysts are always fewer than 1%. To improve the
recycling yield and decrease reactants consumption, concentrating PMs by the
pyrometallurgical process is a reliable alternative. PMs can be enhanced in the metal or metal
alloy phase by pyrometallurgy and then collected by refining technologies.
Copper, iron, lead, nickel, and matte are great agents of PMs. In the pyrometallurgical
process, pounded spent catalysts combined with fluxes (Al2O3, CaO, or SiO2), collector and
reductant were refined. Mutual solubility, chemical characteristics, melting point, and
environmental influence among collector materials and PMs should be taken into
attention.

Metal trapping methods

Lead is a popular collector which has been employed to treat secondary resources before
1980s. Employing blast furnace or electric furnace, lead oxides reduced to lead bullion, while
PMs set into alloy and stayed in the metal form. The supporting materials were oxidized or
instantly entered into the slag form. The PMs in lead bullion were enhanced by selective
oxidation in a blast furnace or converter. Lead collection has the advantages of simple
procedure, low working temperature, the mature consequent refining process. Still, the
limitations are the long processing time, low recovery rate of Rh (about 70%–80%), and
discharge of volatile leads to the operators and ecosystem.
With a point towards a greater recovery rate and less contamination, copper and iron trapping
technologies have been considered and implemented in the industry. They suggested
“wetting” and “settling” mechanisms on metal recovery in this process. The wetting
mechanism reveals that micro-dispersed particulates of PGMs in the slag were wetted by
molten copper and make copper droplets. The settling mechanism suggests that solid PGMs
micro-particles settle through the molten slag and create a firm solution with copper.
Outcomes showed that the heavier Pt was obtained primarily (almost 88%) through “settling”
mechanism. Meantime, Pd and Rh were recovered following by both
mechanisms. 
An industrial purpose of copper concentration was at Tanaka Precious Metals. Spent
automobile catalysts, copper oxide, flux, and reducing agent were combined and served into
closed-door electric furnace. PGMs were obtained in the copper phase during the smelting
stage. After separation of slag and metal form, the total content of PGMs (Pt, Pd, Rh)
was less than 1.0 g/t.
Iron is an important promising collector due to its low-costing and great chemical affinity
with PGMs to build a solid solution. Plasma melting technology is the most extensively used
method for recycling PGMs from spent catalysts, concentrating PGMs in the iron phase. The
Fe-PGMs alloy and slag can be separated easily because of their large densities difference
(6.0∼7.0 g/cm3 versus 3.0∼3.5 g/cm3). Through industrial test on plasma melting iron
capture technology, the recovery rates of Pt, Pd and Rh were over 98%, 98% and 97%,
respectively.

Matte trapping methods

Due to the close association of matte for PGMs, matte is a great collector. We can adopt the
high nickel matte (Ni2S3-CuS) trapping-aluminothermic activation method to retrieve
rhodium from spent organic rhodium catalysts. The materials have a low melting point (about
1000 °C) and enough liquidity. The recovery rate of Rh reached 94.65% in this enhancement
process. Nickel and sulfur were used as collectors to smelt spent catalysts in the proximity of
Na2CO3 and Na2B4O7 at 1050 °C for 30 min. The renewal rates of Pt, Pd, and Rh were
90%, 93%, and 88%, respectively. Due to the novel chemical composition of copper and
nickel ore , PGMs were concentrated in FeS-Ni2S3-CuS matter. The smelting environment is
essential in this operation since oxidation of nickel will create refining difficulty after the
division of PGMs-containing matte from slag. In general, the slag was composed of FeO-
CaO-MgO-SiO2.

Hydrometallurgical Process

In the hydrometallurgical process, valuable metals are suspended in cyanide, aqua regia and
strong acids, such as HCl, HNO3 and H2SO4. The oxidizing agents (such as H2O2, O2, Cl2
and I2) are also attached to improve the leaching performance. Pd was most suitable to be
leached, while Rh was difficult to achieve a high recovery rate (90%). The recovery rate of
PGMs was high by cyanide leaching. Still, it needed high temperature, and the cyanide was
highly lethal. The position is comparable to aqua regia leaching. High leaching performance
can also be achieved in environmentally friendly leaching agents (HCl/H2SO4 +
H2O2/Cu2+) in optimal circumstances.

Cyanide Leaching 
Sodium cyanide has been extensively applied to obtain PGMs. The dissolution of PGMs in
cyanide suspension was given as follows.
2 Pt + 8NaCN+O2+ 2H2O → 2Na2[Pt(CN)4] + 4NaOH (3-1)
2Pd + 8NaCN+O2+ 2H2O → 2Na2[Pd(CN)4] + 4NaOH (3-2)
Rh + 24NaCN + 3O2+ 6H2O → 4Na3[Rh(CN)6] + 12NaOH (3-3)
A surface chemical reaction of precious mainly controlled the cyanide reaction mechanism,
and the leaching rate was the function of cyanide and oxygen concentration. The cyanide
leaching of PGMs needs higher temperatures than gold because the metallic bonding strength
of PGMs is higher. With an oxygen pressure 1.5 MPa, NaCN 6.25 g/L, solid: liquid=1:4, and
160 °C, the recovery of Pt, Pd and Rh were 96%, 98% and 92%, respectively. Due to the
metal bonding strength of their complexes, the leaching order in cyanide solution was Pt > Pd
> Rh.
Greater than 95% and 90% of PGMs were diffused from virgin bulky and used pellet
catalysts by leaching doubly with 1 wt.% NaCN and 0.1 mol/L NaOH solution at 160 °C. The
dissolution distinctions among virgin and used monolith catalyst were due to contamination
from combustion and sintering of substrate substances, reducing the reaction surface and
action.

HCL + oxidant leaching

To lessen the environmental influence caused by cyanide, precious metals are leached in HCl
medium and oxidizing agents. PGMs are often oxidized and make soluble complexes (e.g
[PtCl6]2−, [PdCl4]2−, [RhCl6]3-) with chloride. Over 95% of PGMs were obtained from
roasted concentrates in 6.0 mol/L HCl solution using chlorine gas. Still, chlorine gas is highly
corrosive and poisonous, which is the major barrier to overcome. Excess chlorine gas must be
converted into harmless chloride for release—chlorine leaching method for leaching PGMs
from used automotive catalysts. The produced Cl2 was fed into HCl solution, forming
Cl2(aq), Cl3 −, HClO. Cl3- seemed to be the most efficient chlorine species for dissolving Pt,
which improved Pt dissolution by increasing Cl3- at a particular HCl concentration. Leaching
rate of Pt, Pd, and Rh were 71%, 68%, and 60%, respectively, under the optimized situations.
The activation energies for Pt, Pd, and Rh were 29.6, 26.4, 20.6 kJ/mol. The films of carbon
or sulfur-containing materials on the surface of spent catalysts may prevent the dissolution of
PGMs.

Industrial Process for extraction of Precious metal-

1.0 Direct Sampling 

The spent catalyst is received as a wet filter cake with a PSD within the range of 5 and 500
µm. This is added to water and surfactant in a container and stirred by agitation and by
utilizing a pumped recirculation loop to create a homogeneous distribution. A range of small
representative units of the mixture is then extracted from the recirculation loop by
mechanized extraction. These are assembled together to form a sample that is sub-sampled in
the laboratory and examined for its precious metal content employing conventional analytical
methods. The sampling method was widely approved on a pilot scale before being sized up
for usage in production. 

1.0 Supercritical Water Oxidation

The water-based slurry is later tapped into the feed tank for the subsequent step in the
method: supercritical water oxidation (SCWO). Water becomes supercritical at a temperature
over 374ºC and pressure over 221 bar, when its viscosity is near the gas phase only with
higher density. In this method, organic substances get soluble in the supercritical water,
which is therefore used as a solvent for the oxidation of inorganic substances.

Processing Heterogeneous Catalyst

The slurry comprising ~ 5% of the spent catalyst is drawn up to a pressure of ~ 240 bar and
later moved through the heat-exchanger into the heater to elevate its temperature to 385ºC.
The substance then enters the reactor, where adequate high-pressure oxygen is introduced to
provide the heat of reaction to raise the temperature to ~ 600ºC. Next, quench water is
introduced to reduce the temperature before a second oxygen injection, which enables the
reaction to go to completion. The organic components are transformed to carbon dioxide,
water, and nitrogen, and the metals form their oxides.
The output from the reactor subsequently moves through the heat-exchanger to preheat the
incoming feed and next to a steam boiler (where up to one tonne per hour of steam is
produced and supplied into the site's steam main). The product is reduced to ambient
temperature and pressure before the combustion gases being divided off. The precious metal
oxides and any other trace elements, like base metal oxides and silica, are carried on for
additional refining.

Environmental Influence from the recovery of precious metals from spent catalyst

The recycling technologies for valuable metals from used catalysts have been enhanced to
improve the regeneration rate and minimize
environmental contamination. Hydrometallurgical leaching of PGMs from used catalysts was
done in several situations. Cyanide leaching displays decreased performance at room
temperature and 1 atm. The corrosivity of cyanide needs to be understood seriously. Aqua
regia leaching was practiced on an industrial range to gain PGMs. The probable flow of toxic
cyanide may create critical environmental difficulties and endanger the well-being of
workers. H2O2, too has the potential to be utilized for the leaching of PGMs.
Many thoughts have been proposed to produce environment-friendly
and sustainable recycling for valuable metals with an excellent recovery rate. First, several
technologies should be produced with the purpose of
commercialization in terms of investment and value. Next, recycling
 technologies should be diverse and combined based on numerous materials' physical and
chemical properties. Three, with the intention of higher leaching capacity and more limited
pollution, few pre-treatments are essential.

Method Metals Yield Advantages Disadvantages Reference

extracted

Pyrometa Copper, Rh- 80- Easy Low recovery https://www.sciencedirec

llurgy iron, lead, 94.65% operation, low rate, biohazards t.com/science/article/pii/
nickel, Pt ~ 90% working S0921344918304166
Pt,Rh Pd ~ 93% temperature

Cyanide Pt,Rh,Pd Pt 95–97%  High efficiency High https://www.sciencedirec

leaching Pd temperatures t.com/science/article/pii/
94∼95% needed, cyanide S0921344918304166
Rh highly toxic
64∼81%

Hydromet Co, Mo For High recovery Greater reagent https://www.researchgat

allurgy Pd(easie industrial consumption e.net/publication/263988
st) catalysts 265_Metal_Recovery_fr
Rh(harde Pt 99% om_Hydroprocessing_S
st) to Rh<90% pent_Catalyst_A_Green
leach Co~ 72.7%  _Chemical_Engineering
Mo~ 76.5% _Approach

HCl(aq) All Pt 95% of eco-friendly Chlorine gas https://www.sciencedirec

+oxidants group PGMs corrosive and t.com/science/article/pii/
leaching metals toxic S0921344918304166
 Hydrometallurgical Recovery of Cobalt(II) from Spent Catalysts

INTRODUCTION:-

Because of their excellent corrosion resistance, good electrical conductivity, and high
catalytic activity, precious metals are commonly used in a variety of industries. However,
precious metals stocks are insufficient to meet global demand. The rapid production of end-
of-life goods has turned precious metals into significant resources.

The quest for modern, alternate metal sources is no longer just a matter of scientific curiosity;
it is also a matter of industrial necessity. Economic and financial need, as well as a transition
from the so-called linear economy to the world circular economy, are all factors that
influence the duty. Electronic waste (also known as waste electrical and electronic
appliances, or e-was) is one such metal source.including waste printed circuit boards
(WPCBs)), spent automotive catalysts, electroplating wastes, or a wide range of catalysts,
especially from the chemical and petrochemical industry.

It's worth noting that catalysts are used in the production of over 60% of all chemicals and in
90% of all industrial chemical processes. Whereas solid catalysts account for 80% of all
catalysts, homogeneous and biocatalysts account for just 17% and 3% of all catalysts,
respectively. The issue of catalyst use is widened due to the widespread use of catalysts, as
well as their restricted lifespan and ability to be regenerated.discussed in the literature.

Especially, that according to the United State Environmental Protection

Agency (EPA) catalysts are classified as “hazardous waste as one posing a substantial or
potential hazard to human health and the environment”. Besides, many studies are carried out
on the recovery of valuable elements from spent catalyst, especially the platinum-group
metals (PGMs) , the rare-earth elements (REEs) , and the variety of transition metals.

.
Bioleaching is a modern process that complements traditional metal reclamation methods
such as pyrometallurgy and hydrometallurgy (leaching, ion exchange, and extraction). As
previously mentioned, such approaches are also used in commercial solutions. Despite the
fact that metal recovery from spent catalysts has been established for many years, new
recovery methods are still being pursued, taking into account a variety of factors.various
industrial catalysts, as well as operating conditions. Therefore, taking into account the
advantages and disadvantages of the available recovery methods, the work aims in the
investigation of the possibility of the hydrometallurgical recovery of cobalt(II) from spent
industrial hydrodesulfurization catalyst, with the optimal selection of leaching
and extraction processes applied.
PROCESS:-

CoMo catalysts are made by impregnating the alumina help with metal solutions in multiple
stages. Co occurs as a thin layer between the Mo layer and the Al help as a result of the
impregnation. Cobalt is typically deposited on an alumina support first., and then
molybdenum (in the oxidized form of Mo(VI)) is impregnated on the Al-Co material. Spent
hydrodesulfurization (HDS) catalysts can be treated as a valuable source of cobalt. Thus,
hydrometallurgical recovery of Co(II) from
CoMo catalyst was proposed in this work in a four-step process

1) Leaching of Co(II):-

Metals were leached from a ground CoMo catalyst with sulfuric acid solutions or mixtures of
sulfuric acid and hydrogen peroxide in the first stage of spent catalyst treatment. Although
some authors suggest leaching from HDS catalysts with mixtures of mineral acids (e.g.,
HNO3/H2SO4/HCl or HNO3/H2SO4, in this case sulfuric acid was chosen as the leaching
agent. a leaching agent to provide appropriate solution for the third step of the planned
process, i.e., extraction with acidic extractant . It should be noted that most data in the
literature refer to sulfate solutions, while the application of in chloride and nitrate solutions is
possible but less effective.The results of leaching efficiency (LE), for different sulfuric acid
concentrations, are shown in Figure 2
The leaching efficiency (LE) of cobalt(II) and molybdenum(VI) was measured, with
m0,mleach representing the mass of metal ions in the spent catalyst before and after leaching,
respectively.. Initial mass of metal in the catalyst was estimated on the base ofmetal content
(uM) according to XRF analysis (uCo=0.124, uMo=0.359) and mass of the sample takento
leaching (mcat) as follows

m0= mcat * uM

The efficiency of Co(II) leaching with the solutions of sulfuric acid (in the studied range of
concentrations) without and with the addition of H2O2reached as maximum 20% (Figure
2).The leaching agents were more effective for Mo(VI) leading to the dissolution of 30–40%
of this metal (Figure 2). An increase in acid concentration (between 0.5 and 5 M) did not
affect significantly the amount of the leached Co(II), while it increased the amount of Mo(VI)
leached .

2)Precipitation of Impurities:-

The solid residue was isolated from the leach solution after leaching. The aqueous
phase, which contained ions of various metals (Al(III), Fe(III), Cu(II), Ni(II), Co(II),
Mo(VI)), was treated with NaOH solution to precipitate metal hydroxides (II)

Along with Mo(VI) and Fe(III), also Co(II) is precipitated which is disadvantageous
for Co(II)recovery due to loss of this metal. The main aim of the precipitation step is the
removal of Al(III)before the extraction of Co(II). The smallest loss of Co(II) in the deposit
(0.100 mg) is noted from the leachate of 0.5 M H2SO4, probably because the final pH of
precipitation was equal to 3.5, while Co(II)precipitation increases with increasing pH. As
Huang et al. indicated, major cobalt precipitation occurred within a pH range of 5–7.

However, the pH increase from 4 to 5 enhances substantially also molybdenum(VI)

precipitation from the solutions. Consequently, various ratios Co(II)and Mo(VI) in the
precipitate can result from changes in composition of the deposit, for example, from cobalt
hydroxide (Co(OH)2), through cobalt molybdenum oxide hydrate (CoMoO6·0.9H2O) to
sodium cobalt molybdenum oxide (NaCo2.31(MoO4)3). Thus, as Co(II) loss was the lowest,
0.5 MH2SO4was selected as the most appropriate for the four-step process.

3)Extraction of Co(II):-

Various organic compounds were reported as extractants for Co(II) or Mo(VI) separation
from leachates. For example, basic tertiary amine was proposed to extract Mo(VI)
fromH2SO4at pH 1.8 . Also, two-step extraction with neutral tributyl phosphate (TBP) or
basictris-2-ethylhexylamine (TEHA) was shown to be effective for removal of Mo(VI) or
Co(II), respectively, from HCl. Another combination of extractants was proposed .
It was shown in our previous work that the best conditions for Co(II) extraction are at pH5.2–
5.5. Also, manufacturer of reports that at pH 5.5 maximum extraction of cobalt(II)from
sulfate solution can be obtained, while ions of other metals present in our system extract
efficiently at different pH values.

4)Stripping of Co(II):-

After selective extraction of Co(II) to the organic phase, an effective stripping of Co(II) from
the loaded to 1 M sulfuric acid solution was accomplished to form sulfate electrolyte
solution. The final concentrations of Co(II) and Mo(VI) in the stripping aqueous phases in the
selected experiments
In almost all solutions after stripping no Mo(VI) was detected. Very low concentration of
Mo(VI)was noted after the four-stage process in which the temperature was changed during
the first step, i.e., leaching. Based on the results presented. it can be concluded that in most
cases Co(II) was efficiently separated from Mo(VI) by extraction and stripping.

Generally, the efficiency of Co(II) separation from solution after leaching did not
exceed 20%.The efficiency of Mo(VI) stripping did not exceed 1%, thus, it was proven that
Co(II) can be effectively separated from Mo(VI). However, further research should be carried
out to increase the total yield of Co(II) recovery. As Al(III) is leached in a great amount from
the catalyst, also the content of Al(III) in various solutions of the four-step process .

Methods and materials:-

A Polish waste treatment company produced a sample of spent industrial CoMo

catalyst. The percentage content of elements calculated by XRF analysis is shown in the table
below. The extractant (0.4 M) was acidicextractant, phosphinic acid supplied by Cytec
IndustriesInc., and was dissolved with the addition of modifier. After the previous extraction
with sulfuric acida, the organic phase was regenerated.

After leaching, a 30 percent sodium hydroxide solution was used to precipitate

metalhydroxides from the solution, and was then used to change pH during extraction with.
As a leaching agent, sulfuric acid in the range of 0.5 to 5 M was used, with the addition of
30% H2O2 in some cases. Since sulfuric acid is the most convenient electrolyte, it was often
used as a stripping step (1 M) in all situations.

1. Leaching Procedure:-
The spent catalyst was ground, sieved, and a fraction of particle size less than 45m
was isolated during the leaching procedure. 1.5 g of ground catalyst was put in a reactor,
along with 30 cm3 of H2SO4 or 25 cm3 of H2SO4 plus 5 cm3 of H2O2. The solid-to-liquid
ratio (S/L) was held constant at 1/20 g/cm3.. The mixture of catalyst and the leaching agent
was heated and mixed at 300 rpm, and the leaching was carried out in the range of
temperature between 23 and 95±2◦C. Leaching was investigated for 3 h and 1 cm3sample
was taken after 5, 15, 30, 60, 90, 120, 150, 180 min from the beginning of the leaching.

2. Precipitation Procedure:-
Precipitation of hydroxide deposit was carried out by slow addition of 30% NaOH to the
leach solution up to pH 5. The precipitation was separated by centrifugation, and the liquid
phase was taken to the next step, i.e., liquid-liquid extraction.

3. Extraction-Stripping Procedure:-

Extraction from model solutions was done in a batch manner: aqueous feeds
containing metal ions were mechanically shaken with the organic phase 0.4 M (volume ratio
w/o=2) in glass separation funnels for 15 minutes at 232°C, and then allowed to stand for
phase separation..The pH of the aqueous phase was controlled by the addition of drops of
30% NaOH solution and kept equal to 5–5.2. Stripping of metal ions from the organic phases
loaded was carried out for 5 min with1MH2SO4at o/w=3

Atomic absorption spectroscopy was applied to determine Co(II), Cu(II), Fe

ions, Ni(II) concentrations in the aqueous solutions, microwave plasma-atomic emission
spectroscopy was applied for Al(III), and Mo(VI). The content of elements in the spent
catalyst was determined using XRF analysis .Standard deviations calculated for four
repetitions of each step of the process did not exceed 5%for leaching, 8% for precipitation
and extraction, and 8.5% for stripping.

Conclusions:-
Studies on the recovery of Co(II) from leaching solutions show that the proposed four-step
hydrometallurgical process, which includes metal leaching, metal hydroxide precipitation,
Co(II) extraction with bis(2,4,4-trimethylpentyl) phosphinic acid, and Co(II) stripping from
the organic phase, can be used to isolate cobalt ions from molybdenum in sulphate solutions..
During purification of the leaching solution by precipitation of Al(III) and Mo(VI)
hydroxides, cobalt(II) losses are noted, as Co(II).

Catalysts, such as Co(SO4)i(OH)jkH2O or CoMoO60.9H2O, co-

precipitate as hydroxide or mixed compounds and pass into the sludge. This phenomenon is
detrimental to future cobalt(II) recovery levels.. Generally, the yield of Co(II) recovery from
solution after leaching did not exceed 20%. The yield of Mo(VI) recovery did not exceed 1%,
thus, it is proven that Co(II) can be effectively separated from Mo(VI). However, further
research should be carried out to increase the total yield of Co(II) recovery.

References:-

1. Wieszczycka, K.; Tylkowski, B.; Staszak, K. Metals in Waste, 1st ed.

2.Friedrich, B. Sustainable utilization of metals-processing, recovery and recycling. Metals

2019

3. Regel-Rosocka, M. Electronic wastes. Phys. Sci. Rev. 2018, 20180020.

4.Rzelewska, M.; Regel-Rosocka, M. Wastes generated by automotive industry—Spent

automotive catalysts.Phys.

5.Shiju, N.R.; Guliants, V.V. Recent developments in catalysis using nanostructured

materials. Appl. Catal.

6.Misono, M. Basis of Heterogeneous Catalysis. In Studies in Surface Science and Catalysis;

7. Cimino, S.; Lisi, L. Catalyst deactivation, poisoning and regeneration. Catalysts

Metal recovery process from E-waste

First, thorough literature review was undertaken exploring different databases like Science
Direct, Google Scholar, Emerald Insight etc. Several keywords such as ‘E-waste Recycling’,
‘Metal recovery from E-waste’, ‘Printed Circuit Board Recycling’, ‘Leaching of E-waste’,
‘Gold recovery from E-waste’, ‘Sustainability of E-waste recycling’ etc. were used in this
case. Thereafter, case study organizations were visited and their process operations were
understood and reviewed thoroughly. Based on the findings, two conceptual frameworks
were developed for integrated MREW. Then a generalized comparative analysis of the two
scenarios was carried out from the perspectives of three pillars of sustainability.

Metal recovery from E-waste is a lucrative business opportunity for the small and medium
enterprises (SMEs). Many conventional thermo-chemical and bio-chemical processes have
been tested both in pilot and laboratory scale for metal recovery from E-waste. Various
researchers have used hydro-metallurgical , pyrometallurgical and bio-metallurgical methods
for this purpose.
However, physical processes for metal recovery are quite in practice for easier operation and
lower carbon footprint.
Typical composition of basic metals in PCB (E-waste guide info)

Name Cu AL Pb Zn NI Fe Sn
Concentrationin wt 6.9287 14.172 6.2988 2.2046 0.8503 20.4712 1.0078
% 3

1.1 Physical recycling methods

Physical recycling methods are generally used for upgradation which helps in liberation of
metals and nonmetals contained in E-waste. To the best of the authors’ experience in visiting
several E-waste recycling facilities around the globe, physical recycling is the commonly
practiced method for recycling E-waste all over the world. When it comes to the metal
recovery, this is one of the efficient methods for metal recovery. Often it is considered as one
of the pre-treatment step before further processing. Physical recycling includes
disassembling, dismantling, chopping, shredding, crushing etc. These steps are achieved by
using machineries like shredder, pre-granulator, granulator etc. After this, the separations of
metal from the non-metals are achieved. Different methods such as magnetic separation, eddy
current separation and density separation are used. It is possible to recover metal fraction
containing more than 50% of copper, 24% of tin, and 8% of lead by implementing a
combination of electrostatic and magnetic separation which separates the metal part from the
nonmetal ones. There are other methods reported in literature such as corona discharge
method(suitable for separation of metallic and non-metallic fractions), density based
separatiotn , milling, froth floatation etc.

1.2 Thermo-chemical methods

When it comes to thermo-chemical processes, pyrolysis is an important process. Pyrolysis is
a thermo-chemical process which ensures thermal degradation of a targeted material in
absence of air. A significant number of studies have been carried out on pyrolysis of E-waste.
Vacuum pyrolysis, microwave induced pyrolysis, catalytic pyrolysis and co-pyrolysis (with
biomass) are different types of pyrolysis that has been reported. Though pyrolysis of E-waste
is mostly limited within the laboratory, Jectec (a company from Japan) has already
implemented pyrolysis in their facility. Another process which has gained attention these
days is the plasma process that is being implemented for E-waste treatment and metal
recovery. Plasma Technology is a high temperature and environment friendly technology.
This is applied in MSW and biomedical waste disposal. Ruj and Chang (2013) reported
plasma treatment of mobile phone waste and it showed that the process helps in recovery of
metals. High enthalpy plasma jet has been explored for processing of E-waste. Despite very
small number of available literature, it is already being used industrially in Progenesis
Canada Inc.
1.3 Pyro-metallurgical methods
Pyro-metallurgical processes are widely employed for MREW around the world. Generally,
when the metals are present in a complex matrix with other non-metals and ceramics etc, it is
often difficult to recover them implementing the physical recycling processes. In that case,
pyro-metallurgical method is the option. Printed circuit boards (PCB) are complex and is
easier to recycle using these methods. The PCB’s are first shredded or chopped into suitable
pieces and then they are subjected to pyro-metallurgical processing. Smelting, refining,
incineration, combustion are the common processes in this route. The state-of-art facilities
available in the smelting and refining facilities are capable of extracting valuable metals from
the complex matrix and are quite efficient. A typical pyro-metallurgical treatment process is
smelting followed by electro-chemical refining. First E-waste or metals recovered by physical
recycling are fed into the furnace. The metals are collected in a molten bath and the oxides
are obtained from the slag phase. Umicore, Outotec TSL, Aurubis recycling are to name a
few that employs pyrometallurgical processes for metal recovery from E-waste.

1.4 Hydro-metallurgical methods

Hydro-metallurgical methods implemented for metal recovery from E-waste are the modified
version of the traditional hydro-metallurgical methods used for metal extraction from primary
ores. Leaching is carried out by means of acid, alkali or other solvents to leach out metals in
form of soluble salts. Impurities are removed with the gangue materials and the isolation of
metals from the solution is achieved by processes such as adsorption, solvent extraction etc.
Final forms of metals are achieved through electro-refining or chemical reduction processes.
Four types of common leaching processes, namely—cyanide leaching, thiourea leaching and
thiosulfate leaching are there. Copper and other precious metals such as gold, silver etc can
be recovered via hydrometallurgical route and a detailed discussion can be found in the study
by Wu and group (2017). Rare Earth Elements (REE) can also be recovered via this route and
consolidated studies have been reported. Aqua regia was used as leaching agent for recovery
of gold from printed circuit board . Metals such as nickel , tin, copper, silver , palladium has
been reported to be recovered from E-waste. It was found that nitric acid; sulphuric acid and
muriatic acid based solutions are majorly implemented for primary leaching of precious
metals from E-waste. Recent focus on tin recycling from E-waste has been found among the
researchers . A green hydrometallurgical process has been developed for recovery of tin from
PCBs via co-processing of waste PCBs and spent tin stripping solution which is generated
during production of PCBs. Umicore uses hydro-metallurgical processes for metal recovery.
Industrial applications of such green processes are essential for sustainability.
1.5 Bio-metallurgical processes
Bio-metallurgical processing of E-waste is an emerging and a very promising area. There
exist plenty of opportunities for research and development as well as business. Bio-
metallurgical processes can be classified into two sections – a) Biosorption and b)
Bioleaching. Biosorption means adsorption of metals by means of adsorbents prepared from
waste biomass or abundant biomass. Metal recovery from E-waste by biosorption has been
achieved by using algae, fungi, bacteria, hen eggshell membrane, ovalbumin, alfalfa etc. The
mechanism associated with biosorption is complex and no clear picture is available. There are
certain factors that affect the process —a) Type of biological ligands accessible for metal
binding, b) Type of the biosorbent (living, non-living), c) Chemical, stereochemical and co-
ordination characteristics of the targeted metals and d) Characteristics of the metal solution
such as pH and the competing ions. According to Ilyas and Lee (2014), the mobilization of
metal cations from often almost insoluble materials by biological oxidation and complexation
processes is referred to as bioleaching. There are three major group of bacteria associated
with in bioleaching of E-waste are – a) Autotrophic bacteria (e.g. Thiobacilli sp.), b)
Heterotrophic bacteria (e.g. Pseudomonas sp., Bacillus sp.) and c) Heterotrophic fungi (e.g.
Aspergillus sp., Penicillium sp.). Typically bioleaching occurs in four steps—a) Acidolysis,
b) Complexolysis, c) Redoxolysis and d) Bioaccumulation . Bioleaching has been explored
by researchers for recovery of Gold, Aluminum, Copper, Nickel, Zinc and Lead from E-
waste. There is no example of any industrialized process in the bio-metallurgical route.