hydrometallugy

ELSEVIER Hydrometallurgy 45 (1997) 333-344

Separation technologies for metals recovery from

industrial wastes
A.G. Chmielewski *, T.S. Urbtiski, W. Migdal
Institute ofNucleur Chemistry und Technology, Dorodna 1603-195 Wursuw, Poland

Received 21 June 1996; accepted 2 December 1996

Abstract

Three hydrometallurgical processes for industrial wastes treatment are presented. The main
separation techniques are: solvent extraction, leaching-precipitation, electro-oxidation, and ion
exchange. Recovery of gold from solid wastes generated in the electronic and jewellery industries
consists of thermal degradation, two-stage leaching with nitric acid solution to remove silver and
other metals and then with aqua regia to dissolve gold, selective solvent extraction of gold with
diethyl malonate, and reduction of gold from the organic phase.
Vanadium recovery from residue ashes after burning heavy oil fractions consists of alkaline
leaching of vanadium, filtration, neutralization of sodium vanadate solution, precipitation of
ammonium metavanadate, drying of the precipitate, and adsorption of the remaining vanadium
from the filtrate on an anionite. From the remaining ashes nickel is recovered using acidic
leaching, filtration, precipitation of ammonium-nickelous sulphate, filtration, and drying.
The third process concerns processing of electroplating sludges and waste waters containing
chromium and copper. The waste waters are electro-oxidized to transform C&II> into chromate.
Then metal cations are separated on a cationite. The purified electroplating baths are recycled
directly to electroplating; other solutions are first concentrated using anionite, followed by sodium
chromate eluate conversion into concentrated chromic acid solution. The sludges accumulated
from waste water processing by hydroxide precipitation are re-dissolved in chromic acid solution
generated progressively by circulation between the dissolving and electro-oxidation steps. The
concentrated chromic acid solution obtained is purified on the cationite and recycled.

1. Introduction

The processing of industrial wastes containing substantial quantities of toxic and/or

valuable components for their recovery or separation often becomes an absolute

” Corresponding author. Fax: + 48 22 I I 1532. E-mail: achmiele@orange.ichtj.waw.pl

0304-386X/97/$1 7.00 0 1997 Elsevier Science B.V. All rights reserved.

PII SO304-386X(96)00090-4
334 A.G. Chmielewski et al./Hydrometallurgy 45 (1997) 333-344

necessity. There are two important aspects of the problem: one is economy and to save
raw materials, and the second is the protection of the environment from dispersed toxic
compounds, especially compounds of heavy metals. Therefore, studies are being carried
out aimed at developing new or modified hydrometallurgical processes for the separa-
tion of metals, mainly from industrial waste by-products. As a result of our investiga-
tions, three technologies have been elaborated for metals recovery and recycling.

2. Recovery of gold from jewellery waste

A process of gold reclamation from jewellery production wastes has been developed.
The problem is important because of significant losses of valuable gold in the produc-
tion wastes. It is, however, difficult to solve because the physical form and chemical
composition of the wastes vary considerably. A process including solvent extraction
seemed to be promising from bibliographic studies (e.g. [l-4]) and our previous
investigation [5]. On the basis of extensive laboratory studies a process that is especially
suited to the technical conditions of jewellery manufacturing and a production line for
processing of gold-containing wastes has been designed and built. The process consists
of the following steps (Fig. 1):
1. low-temperature carbonization and roasting of the wastes;
2. a first step of leaching with nitric acid solution in order to remove silver and other
metals;
3. a second step of leaching with nitro-hydrochloric acid (aqua regia);
4. selective solvent extraction of gold with diethyl malonate;
5. separation of metallic gold from the organic phase by reduction.
The process of thermal degradation was investigated using DTA and other methods.
The influence of temperature, mixing of the batch, and time of heating was studied.
Optimum conditions were established as follows: the wastes are introduced into a cold
furnace, then the temperature is elevated continuously up to about 750°C within 4-5 h.
The wastes are then digested for 3 h at 750-850°C while being periodically mixed (3-4
times). The concentration of gold in the residue was between 3 and 10 wt%. In the first
leaching step the impact of nitric acid concentration, mixing time, temperature and
1iquid:solid ratio was investigated. As a result, the following parameters of the process
were chosen: HNO, concentration about 8 M (1: 1 dilution of concentrated HNO,),
temperature 40-50°C time of mixing about 7 h, using a 1iquid:solid ratio of 5: 1.
In the second step of leaching with aqua regia the influence of temperature and
mixing time was studied. Optimum results were obtained at 40-60°C for 7 h mixing. In
the solid residue the concentration of gold did not exceed 0.03-0.08 wt%. Besides gold,
the solution obtained contained other elements: B, Si, P, Fe, Mn, Pb, Bi, Sn, Ti, Ag, Al,
Ca, Cu, Na and Zn.
For purification of gold the liquid-liquid extraction process was selected. Various
extractants were investigated: hexanol, methyl-iso-butyl ketone, di-n-butyl ketone, di-
ethyl malonate, dibutyl ether, ethylene glycol, n-amyl ether, iso-amyl ether, 2,2-dichlo-
roethyl ether, TBP, and several natural oil fractions. The main properties taken into
account were: selectivity, possible loading with gold, resistance to contact with aqua
 A.G. Chmielewski et al./ Hydrometallurgy 45 (1997) 333-344 335

Jewelry waste

Thermal degradation of waste

------j=Yz~1

7~l~ermally treated
waste material

Desilverued waste

Ati+ Solid waste disposal

Diethyl malonate

Au extract

OOt ML1l.;.. Waste_water

H,SO, cone +
H~O~+(COOH)z

Au reduction reextraet& ’
1 L Waste-water
Metallic gold

Fig. 1.Block diagram of hydrometallurgical Aureclamation process.

regia, organic-aqueous phase disengagement, and price of the extractant. After gold was
dissolved in nitro-hydrochloric acid, it was essential to establish correct extraction
parameters, such as phase contact time and proper acidity of the aqueous solution
(degree of aqua regia dilution) to obtain high extraction coefficient values, and the
influence of gold concentration in the feed (extraction isotherm). As it is very expensive
and time consuming to follow changes in extraction efficiency while changing these
parameters using conventional analytical methods, the radiotracer lg8Au, having the
half-hfe T,,, = 2.5 d and decay energy y = 411 keV, was used to determine extraction
336 A.G. Chmielewski et al./Hydrometallurgy 4.5 (1997) 333-344

coefficient values. Gold-containing solutions of aqua regia were labelled with the
radiotracer obtained by exposing high-purity metallic gold to radiation in a nuclear
reactor and then dissolving it in a small amount of nitro-hydrochloric acid. The
extraction experiments were carried out in separatory funnels. After phase disengage-
ment both phases, organic and aqueous, were sampled, and their radiation intensity was
measured using a semiconductor detector Ge(Li) coupled with a multichannel pulse
amplitude analyzer. Then the extraction coefficient (as the radiation intensities ratio
instead of gold concentration ratio in the phases) and extraction yield values were
calculated.
On the basis of results obtained, diethyl malonate was selected as the best extractant
among those investigated for industrial applications. It shows very high extraction
coefficient values under proper conditions (only one extraction stage is sufficient to
ensure practically quantitative separation of gold) and the highest saturation capacity
with gold of 140 g/l, as well as a good selectivity. The proper conditions are: phase
contact time not less than IO-15 min, dilution of the aqua regia solution at least 1: 1,
phase volume ratio O/A = l/2, disengagement time about 30 min.
The structure of the strong gold complex with the extractant was also investigated
using IR, UV, NMR, and mass spectrometry methods. It has been determined as:
CH,(COOC,H,),Au(H,0),C13 independently of gold concentration in the extractant.
The malonate complexing takes place symmetrically with two oxygen atoms, probably
from the ester group.
In order to achieve gold purification, the organic extract was washed by mixing with
0.01 M HNO, for 15 min at a phase volume ratio of l:l, which removed practically
quantitatively all impurities.
Two methods of gold separation from the extract by reduction were developed and
patented. In the first, successive introduction of concentrated sulphuric acid, 30%
hydrogen peroxide and oxalic acid was applied at 80-90°C which resulted in extractant
degradation [6]. In the second, saturated ferrous sulphate solution was used as a reducing
agent at 80°C without extractant degradation, which could be used many times [7]. In
both methods, metallic gold precipitated after several minutes. The gold obtained is of
99.99% purity. The total efficiency of gold recovery is about 97%. The economic
effectiveness factor of this technology calculated as a ratio of the gold recovered value
to a sum of operation and investment costs, assuming 10 years amortization, was
estimated at 27.
Particular elements of the gold recovery methods presented above have been adapted
for gold recovery from various wastes arising in the electronics industry.

3. Recovery of vanadium and nickel from soft asphalt fuel ashes

The soft asphalt fractions obtained from crude oil from the former Soviet Union
(Russia) contain about 0.013% vanadium. They are commonly used as a fuel in heat and
electricity generating plants at the rate of about 1 million t/yr. At the average vanadium
content of 0.013%, the total produced is equal to 130 t of vanadium annually. After
 A.G. Chmielewski et al./Hydrometallurgy 45 (1997) 333-344 337

burning the soft asphalt the residue ashes (about 100 t/yr) contain on average 15%
vanadium.
Its recovery is an important problem because not only does this allows reclamation of
vanadium, thus limiting the amounts to be imported, but it also lowers the possible
environmental damage resulting from very toxic vanadium compounds in waste waters.
Therefore, investigations have been carried out to develop a method of vanadium
recovery from this ash. The starting point was earlier work on this problem (e.g. [8-lo]).
The ash contains l-20% vanadium, several per cent Fe and Ni and, sometimes, several
per cent Mg. A hydrometallurgical method was chosen for their processing. After
preliminary tests, acid leaching was rejected because of inefficient vanadium dissolution
(only 74%). An alk a 1ine leaching method proved to be more promising. The impact of
such parameters as the quantities of NaOH and NaNO, (as oxidant) and water used,
mixing time, leaching temperature, and particle size of the ashes was investigated. It has
been established that the influence of the particle size is insignificant and that NaNO,
was unnecessary, if a sufficiently high NaOH concentration was used. When the
NaOH:ashes ratio attained a certain value, dependent on the vanadium content in the
ash, after 2 h mixing at 100-l 10°C the leaching efficiency was 94%, if 30% NaOH
solution was used. The following reactions probably occurred:
2VOl + 4NaOH + Na,V,O, + H,O + H+

VO: + 3NaOH -+ Na,VO, + H,O + H+

The sodium vanadate solution that was obtained may contain small amounts of V4+,
which can be quickly oxidized to V5+ by 30% H *OZ. The solution was neutralized from
pH 12 to about 8 with 30% H,SO, in order to precipitate impurities. Then ammonium
salt was added (sulphate or chloride) for ammonium metavanadate precipitation accord-
ing to the reaction:
Na,V,O, + 4NH: + 2NH,VO, + 2NH, + 4Na++ H,O

Ammonium salt consumption increased when the solution was more dilute. The
precipitated NH,VO, was easily filtered and contained about 30% H,O. It was dried
and could be decomposed to V,O,, NH,, and H,O by roasting at 200-440°C.
The filtrate contained 0.2-l g/l V. Precipitation of vanadium compounds from such
dilute solution was ineffective, so anion exchangers were used for recovery of vanadium
residue at a pH of about 5, lowering the vanadium concentration in the filtrate to less
than 1 mg/l.
As a result of our studies, the vanadium recovery method developed consists of the
following operations (Fig. 2):
1. alkaline leaching of vanadium from ground ash; process yields approximately 94%;
2. filtration of the suspension;
3. neutralization of the sodium vanadate solution to pH 7-9;
4. precipitation of ammonium metavanadate; process yields approximately 85%;
5. filtration of the suspension;
6. drying of the ammonium metavanadate sediment;
7. adsorption of the vanadium residue in the filtrate on an anionite.
338 A.G. Chmielewski et al./Hydrometallurgy 45 (1997) 333-344

Ashes Water i’rom sediment washing

1
1 1
30% NaOH Alkallne leaching
of vanadium from ash
I

NH Cl L
L Preclpltatlon of
ammonlu.. metavanadate

I
Flltratlon
I

I
Drylng of the ammonium
metavanadate sediment
I

Fig. 2. Technological diagram of the process for obtaining NH4V03.

The total output of the process of vanadium recovery from ashes amounts to 94%
(vanadium remaining in the solution after the NH,VO, precipitation is recycled back to
the process by the ion-exchange installation).
A similar method was developed for vanadium recovery in the form of pure (99.5%)
V,O, from washing waters with yield of 95% [l I].
After vanadium recovery, the remaining ashes (a quantity of about 50 t) still contain
some 15% nickel. The latter can also be recovered by leaching with 30% H,SO, using
about 6 kg H2S0, per 1 kg of ash. This is done using a method consisting of the
following steps (Fig. 3):
1. acidic leaching of nickel from the residual ashes; process yields about 96%;
2. filtration of the suspension;
3. precipitation of ammonium-nickelous sulphate; process yields about 90%;
4. filtration of the suspension;
5. drying of the ammonium-nickelous sulphate sediment;
6. precipitation of Fe(OH), from the filtrate together with almost the total remaining
nickel still in the form of ammonium-nickelous sulphate.
Total yield of the nickel recovery process amounts to approximately 86%. The
method has been patented [ 121.
 A.C. Chmielewski et al./Hydrometallurgy 45 (1997) 333-344 339

4. Recovery of chromium from electroplating industry wastes

The utilization of electroplating wastes is a serious world-wide problem, because of

the high content of heavy metals. Chromium compounds are particularly hazardous to
the environment as they may pollute water courses and reservoirs as well as soils. On
the other hand, they contain substantial amounts of valuable metals which ,- in
conventional waste-water treatment methods - are precipitated as hydroxides together
with other sparingly soluble compounds and are thus usually irreversibly wasted,
causing the formation of waste sludges and secondary saline waste waters [13]. The
chemical processing of these sludges for recovery of valuable metals is difficult and it is
economical only under favourable conditions [ 14- 161.
Good results in the treatment of electroplating waste water can be attained using
hydrometallurgical methods such as ion exchange, liquid-liquid extraction, electrolysis
and reverse osmosis. There are a number publications concerned with use of ion
exchange for this aim [13,17-311.

Sludge

‘Sediment
I I

25% NH3

1 ??

Precipitation of ammonium-
nickelous sulfate

nickelous sulfate sediment

25% NH,
I
1 1 1
Precipitation of Fe(OH)I
from the filtrate

1
Filtration

Drain
I
Fe(OH)S with remaming
nickel compounds

Fig. 3. Technological diagram of the process for obtaining NiS0,(NH,)2S0,~6H,0.

340 A.G. Chmielewski et al./ Hydrometallurgy 4.5 (1997) 333-344

The possibility of using an ion-exchange method for processing highly and intermedi-
ately concentrated electroplating waste waters containing chromium has been experi-
mentally investigated, including previous electro-oxidation of Cr(II1) to Cr(V1). This
method leads to the recovery of chromium, copper, and water and eliminates the
formation of chromium-containing sludge. The possibility of processing successively
large heaps of such sludge already accumulated over many years, consisting of dissolv-
ing them in chromic acid solution, formed as a result of simultaneous electro-oxidation
of chromium, has also been studied. Concentrated impure chromic acid solution
obtained in this way can thereafter be purified in an ion-exchange unit as highly
concentrated waste water.
Experiments were carried out using materials from a large Polish electroplating plant
at the Factory of Mechanized Longwall Lining, FAZOS, in Tarnowskie Gory. Their
purpose was to determine the optimum treatment process conditions (e.g., liquid flow
rates, kind and concentration of eluents being used in ion-exchange process and
parameters of chromium electro-oxidation processes) as well as the overall dimensions
of apparatus corresponding to the expected quantities of effluents and sludge to be
processed, including the possibilities of monitoring and automatically controlling all
processes involved. In these experiments, radiotracer methods were widely used to study
on-line ion-exchange processes with 64Cu, 59Fe, and 5’Cr radiotracers [32]. They made it
possible to determine the influence of electrode areas, current density, and temperature
on the efficiency and duration of the chromium electro-oxidation processes, as well as
the impact of sorption, elution, and washing operation procedures and parameters (flow
rates, liquid volumes, etc.) in the ion-exchange processes on their efficiencies, ion-ex-
change column saturation, and breakthrough points. On the basis of results obtained,
technologies for the treatment of electroplating chromium-containing waste waters and
sludge have been developed. In order to improve process economy it is anticipated that
highly concentrated waste waters, formed by spent electroplating baths should be treated
separately from moderately concentrated ones originating from pump, valve, and tank
leakages as well as from spent rinsing baths and washing processes (excluding very
dilute rinse waste waters).
An ideogram of the treatment of highly concentrated waste waters (HCW) is shown
in Fig. 4. These waste waters contain 60-130 g/l Cr (with about 15% of CNIII)), up to
40 g/l Cu, and less than 2 g/l Fe. After quantitative electro-oxidation of C&II) at 70°C
using lead covered by PbO, as the anode and a steel cathode, at an anode to cathode
area ratio of 3O:l or more, using an anodic current density of 200 A/m2 and after
filtration and (if necessary) dilution to a maximum proportion of 1:l (in the case when
there is a hazard of exceeding the cation exchanger resistance), the waste waters pass
through cation-exchange columns (Wofatit KS10 in the Hf form> in order to catch all
metal cations (mainly copper and iron and minute quantities of other metals). The
effluent takes the form of a sufficiently pure and concentrated solution of chromic acid
which can be directly recycled to electroplating baths. Metal cations are eluted from the
cation exchanger with 10% H 2SO, solution at a specific flow rate of 1: 1- 1.2 m/h. By
an electrodeposition process copper is removed from the eluate which can be then used
several times for elution, until the concentration of iron and other metals becomes
excessive. The solution containing excessive amounts of the metals then passes to
 A.G. Chmielewski er ul./Hydrometallurgy 45 (1997) 333-344 341

Dilution Elution
(if neceswy) HzSOp aq

t
1 I
I
I I
HCW Electrooxidation Cation H2Crz07a
exchanger (H’)
(Wofatit KS 10)
I
I
I
I CP

+
Solid waste Cu metal

Fig. 4. Highly concentrated waste water (HCW) treatment. HCW = spent electroplating baths; Cr = 60- 130
g/l (15% C&II)); Cu = < 40 g/l; Fe = < 2 g/l.

conventional treatment by precipitation to form a small amount of non-toxic sludge

containing mainly Fe(OH),, which can be either metallurgically processed or safely kept
in a dumping yard. Purified secondary waste water contains only sodium sulphate.
In Fig. 5, an ideogram of the treatment process of intermediate concentrated waste
waters (ICW) is presented. The electro-oxidation of chromium, filtration, and cation
removal are similar to the case of highly concentrated waste waters. However, chromic

Elut ion
flut Ion
NaOH *q
t$SO,aq

Reclaimed

I----’
water
T

Elutlon
H*SO, w NazCr20,

etc.

L
I
cat&ion exchanger
(tl 1 azSO,aq
(Uofatlt KSIO)

pfi-meter
tlzCr20, aq

Fig. 5. Intermediately concentrated waste water (ICW) treatment. ICW= pump, valve, and tanks leakage,
waste water from washing and rinsing processes; Cr = l-15 g/l; Cu = < 1 g/l; Fe = < 0.2 g/l.
342 A.G. Chmielewski et al./ Hydrometallurgy 45 (1997) 333-344

Water

c c
Washing Mixing Electrooxldatlon
(heated) 200 mm, 99.4%

&
20%soluble Recirculation stleam
matter

Fig. 6. Solid waste (SW) treatment. SW = precipitate of hydroxides from waste water treatment plant; Cr
10.5-10.8 wt% (Cr (III)); Cu 1.2-2 wt%; Fe 0.5-0.6 wt%; traces of Sn and Zn.

acid concentration in the effluent from the cation exchanger is low, so it is then passed
through anion-exchange columns (Wofatit AD41) where chromates are quantitatively
sorbed (along with minute amounts of other anions). The effluent of these columns
constitutes pure water (up to breakthrough) which is returned to the process. Chromates
are eluted with 15% sodium hydroxide solution from the anion exchanger. Concentrated
sodium chromate eluate is passed directly through another system of cation-exchange
columns (in the H+ form) where the transformation into free chromic acid takes place.
Effluent from these columns can be also recycled to the electroplating bath. The removal
of sodium from these columns to regenerate the cation exchanger is done by elution with
10% H 2SO, solution, partially recycled.
An ideogram of the treatment process of accumulated electroplating sludges (solid
wastes, SW> is shown in Fig. 6. The sludges contain about 10 wt% Cr, l-2 wt% Cu,
and about 0.5 wt% Fe as well as minute amounts of other heavy metals, such as Sn and
Zn. Their pre-treatment is washing with hot water to remove soluble matter (up to about
20%), mainly sodium compounds from precipitation process with NaOH, which is
necessary to provide efficient and sufficiently fast electro-oxidation of C&II). The
sludge is then dissolved in a heated tank provided with an agitator and sub-divided into
mixing and sedimentation zones, adding appropriate amounts of ICW. The solution
obtained is transferred into the vat where electro-oxidation of Cr@I) takes place and is
recirculated continuously to the previous tank until sufficient chromic acid concentration
is attained. Then it passes in part into another vat where the final oxidation of C&II)
occurs. The two-step electro-oxidation ensures complete transformation of C&II) into
Cr(V1). The solution is then passed through the filter and processed in the same way as
highly concentrated waste waters. This method of chromium electroplating sludge
processing has been patented 1331. The economic effectiveness factors of the above
technologies (the ratio of the values of recovered chromic acid for recycling to
electroplating, copper, saved chemicals, etc., to the sum of operation and investment
costs) calculated independently for both processes were estimated as amounting at least
about 1.4 for the chromium waste water treatment and 2.8 for the sludge treatment.
However, as investment costs are joint for a great part of both the technologies, these
factors should be higher. Moreover, the significant reduction in environmental contami-
nation with this very toxic metal another cost factor which is very important.
These processes are in the implementation phase in the FAZOS Factory for recovery
of about 25 Mt/yr of chromium and about 3 Mt/yr of copper. Other Polish factories are
also interested in these processes.
 A.G. Chmielewski et al./Hydrometallurgy 45 (1997) 333-344 343

5. Conclusions

The material presented here shows we have developed several verified hydrometallur-
gical methods for the efficient recovery of some heavy metals, among them gold, silver,
vanadium, chromium, nickel, and copper, from various industrial solid wastes or waste
waters. These methods lead to economic recovery of valuable metals while simultane-
ously reducing the environmental impact.

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