Hydrometallurgy, 7 (1981) 77--97 77

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

HYDROMETALLURGICAL PROCESS FOR RECOVERY OF SILVER

FROM SILVER BEARING MATERIALS*

WASYL KUNDA
Research Consultant, formerly with the Sherritt Research Centre of Sherritt Gordon
Mines Limited, Fort Saskatchewan, Alberta (Canada)
(Received June 23rd, 1980; accepted in revised form September 30th, 1980)

ABSTRACT

Kunda, W., 1981. Hydrometallurgical process for recovery of silver from silver bearing
materials. Hydrometallurgy, 7: 77--97.

A laboratory study was carried out on the treatment of silver bearing materials in a
sulphuric acid system to recover the silver as a high purity product.
The study resulted in the development of a hydrometallurgical process comprising the
following steps: (1) dissolution of silver in concentrated sulphuric acid, (2) precipitation
of silver sulphate salt by dilution with water, (3) dissolution of silver sulphate and (4) re-
duction of silver by hydrogen.
Impurities present in the feed materials were effectively rejected during the proposed
processing steps, so that the silver product was free of contaminants.
Application of this hydrometallurgical process to the treatment of coinage webbing,
jewelry scrap and silver metal containing most common impurities, resulted in 99% recov-
ery of 99.9 grade silver as a fine powder.

INTRODUCTION

The U.S. consumption of silver in 1977 was 154 million ounces with
domestic production accounting for 86 million ounces, of which 44% was
mine production and 56% was from secondary sources.
H.J. Drake [1], reviewing the consumption and production in the last ten
years (see Table 1) shows that the major users of silver are in the manufac-
ture of silverware, photography, electrical equipment, electronic componenets,
appliances, refrigeration, jewelry and commemorative coins and medallions.
The bulk of these manufactured articles will ultimately be recycled. At the
present time, recycling of secondary materials constitutes more than 50% of
the total production and it is reasonable to expect that due to the great dis-
crepancy between domestic production and consumption, the recycling of
silver will increase in the future.

*This paper has been presented at the Third International Precious Metals Conference,
held May 8--10, 1979 in Chicago.

0304-386X/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

TABLE 1
O0

Uses and production of silver in the U.S. for the period 1968--1978 (Herold J. Drake, Silver, Mineral C o m m o d i t y Profiles,
MCP-24, September 1978, United States Department of the Interior, p. 9)

Million Troy Ounces of Silver

Category
1968 1969 1970 1971 1972 1973 1974 1975 1976 1977

US Uses:

Silverware 43.6 33.0 30.6 32.6 36.4 45.1 35.3 32.4 29.3 23.5
Jewelry and Arts 4.5 3.0 5.1 3.4 5.9 7.3 5.2 12.7 Ii.0 8.1
Photography 41.6 41.4 38.0 36.1 38.2 52.0 49.5 46.1 55.5 53.7
Refrigeration 8.0 i0.0 8.6 9.0 i0.i 12.3 8.7 11.2 9.0 8.0
Coinage 36.8 19.4 0.7 2.5 2.3 0.9 i+I 2.7 1.3 0.I
Appliances 9.5 11.2 9.4 9.0 11.3 14.0 9.9 11.2 ii.0 i0.0
Batteries 5.8 3.8 6.3 5.6 6.0 4.2 4.2 4.3 3.5 5.8
Electrical Equipment 15.0 19.0 14.5 15.2 19.7 25.6 17.2 16.2 19.4 18.8
Electronic Components 8.0 9.3 6.8 9.0 11.2 14.2 8.5 ll.0 12.9 12.5
Coins, medallions I) . . . . . 22.3 7.2 8.2 4.3
Others 9.3 10.9 9.1 9.2 12.2 21.3 16.1 5.4 10.8 8.9
Total 182.1 161.0 129.1 131.6 153.3 196.9 178.0 160.4 171.9 153.7

US Production:

Mines 32.7 41.9 45.0 41.6 37.2 37.8 33.8 34.9 34.3 38.2
Secondary Refined 64.2 79.8 56.0 30.1 31.1 34.6 54.1 49.6 50.2 47.9
Metal
Total 96.9 121.7 i01.0 71.7 68.3 72.4 87.9 84.5 84.5 86.1
i) Data of this category for the period 1968-1972 were reported partly in silverware and partly
in miscellaneous categories.
 79

The technology of silver production has only undergone minor changes in

the last few decades. Hydrometallurgical methods were recently introduced
at the beginning of the process by adopting brine leaching, cyanidation or
thiosulphate leaching for the extraction of silver from ores or from leach
residues from non-ferrous metal processes. However, in the refining stage the
pyrometallurgy is used as a universal method for processing the silver to bul-
lion or dor~. The final purification is accomplished by electrowinning.
The present process for treatment of secondary materials and in particular
the silver scrap described by C.H. Schack and B.H. Clemmons [2] involves
the following steps: (1) segregation of scrap into high and low grade products;
(2) mixing the low grade scrap with recycled metallic lead; (3) reverberatory
smelting to produce impure lead bullion, matte and slag; (4) remelting the
slag in a blast furnace to produce a barren slag for discard and additional bul-
lion and matte; (5) combining the high grade scrap from step (1) with bullion
for melting and cupellation; (6) reduction of litharge formed in cupellation
for recycling to reverberatory smelter; (7) electrolytic refining the dor~ from
cupellation and recovery of high purity silver; (8) crushing, roasting and acid
leaching the combined mattes from reverberatory and blast furnace smelt-
ing; (9) recovery of silver from leach solution (8) by electroplating; and (10)
recovery of copper as copper sulphate from silver free solution.
The above process involves a large number of processing steps and many
operations have to be carried out at high temperatures. The application of a
hydrometallurgical method as an alternative for the presently used smelting
process was investigated independently at O u t o k u m p u Oy [3] and at the
Sherritt Research Centre. The results of the study at the Sherritt Research
Centre are presented in this paper.
A new process for the treatment of silver bearing material, especially scrap
silver, was investigated by the following route: (1) Sulphuric acid leaching;
(2) Recovery of silver from leach solution by dilution with water; (3) Dissolu-
tion of silver sulphate in ammonia solution; (4) Reduction of silver ammine
sulphate by hydrogen to metallic form.
The impurities present in various feed materials were followed during the
processing.

EXPERIMENTAL

Materials

The feed materials used in the tests were:

(1) Silver powder, Johnson Mattey & Mallory
(2) Silver shots, Johnson Mattey & Mallory
(3) Silver coins webbing
(4) Jewelery silver scrap
(5) Mixture of metals: Ag, As, Cd, Te, Sb, Se and Sn.
80

The reagents employed in the process were:

(6) Sulphuric acid, commercial grade
(7) Ammonia solution, commercial grade
(8) Hydrogen sulphide
(9) Sodium borohydride
(10) Sodium chloride
(11) Hydrogen
A systematic study was first carried o u t with pure silver metal and the
process was confirmed with (a) coinage webbing, (b) jewelry silver scrap, (c)
a mixture of metals containing silver and most c o m m o n impurities.

Procedure and equipment

Leaching
Leaching was carried o u t in a glass container using concentrated sulphuric
acid. The acid was heated to a predetermined temperature and then the feed
material was added. Samples of solution were taken during the test and ana-
lysed for silver and sulphur. On completion of the leach, the solution was
separated from the leach residue and was used as the head material for silver
precipitation.

Precipitation o f silver
The precipitation of silver was carried o u t in a glass container b y diluting
the leach solution with water. Silver precipitated as silver sulphate. The
precipitate was separated b y filtration on a sintered glass filter.

Reduction o f silver solu tion with hydrogen

The silver sulphate salt was dissolved in dilute ammonia solution and re-
duced to silver metal in a stainless steel autoclave, of 4 1 capacity, using the
following procedure: The autoclave was charged with silver solution of pre-
determined composition and sealed. Air in the autoclave was replaced first by
nitrogen and then with hydrogen. The charge was heated to the reaction
temperature and at this temperature the predetermined pressure of hydrogen
was added and kept constant for the duration of the test. Solution samples
taken during the run were analysed for silver. The pH was also recorded. Upon
completion of the test, the autoclave was cooled. The contents of the auto-
clave, comprising a silver p o w d e r and silver depleted solution, were filtered.
The washed silver product and filtrate were analysed.

R E S U L T S A N D DISCUSSION

1. Dissolution o f silver in sulphuric acid

The reaction of metallic silver with sulphuric acid has been known for a
long time [4]. However, no a t t e m p t was made to use it as a solvent for the
 81

refining of silver bearing feed material. Due to insufficient information on

the kinetics of dissolution of silver in sulphuric acid, experiments were carried
out studying the effect of temperature and particle size of silver on the dis-
solution rate.
An exploratory test showed that reaction takes place at a reasonable rate
above 150 ° C. The initial dissolution rate was very fast, b u t later it slowed
down considerably. The passivation was revived b y increasing the temperature
to about 200 °C.
Following the preliminary test, a systematic study was carried o u t to
establish the effect of temperature and particle size of the silver feed materials
on the dissolution rate. Charges comprising 500 g silver metal of 5 or 5,000
#m diameter and 1 1 sulphuric acid were heated to 160°C or 200°C and the
dissolution rate was followed b y analysing the solution samples for silver. The
results presented in Fig. 1 show that the leaching rate increased with an in-
crease in temperature and a decrease of particle size of feed material.

600

500
/ o/ o s

2 3 /I o

4OO

0
300
g
LEGEND
Ag POWDER TEMR
,urn oC
200 I 5 160
2 5 200
3 5,000 200

I00

0
0 tOO 200 300 400 500 600
TIME ( MIN, )

Fig. 1. D i s s o l u t i o n o f silver in sulphuric acid; e f f e c t o f t e m p e r a t u r e and particle size o f

silver feed m a t e r i a l o n d i s s o l u t i o n rate.
82

After a short initial period, characterized b y a rapid dissolution of silver,

the reaction stabilized and proceeded with constant rate. The summary of
dissolution rates for various stages and conditions is given in Table 2.

TABLE2

Effect of temperature and particle size of silver on dissolution rate

Conditions Leaching Rate (g/l h)

Test
No. Temp Particle Size Initial Later
°C ~rn Stage Stage

160 5 588 47

200 5 612 118

200 5,000 150 71

The recommended leach temperature is 200 ° C, which is 130°C below the

boiling point of sulphuric acid. The dissolution rate at this temperature is ex-
pected to be 70--120 g/1 depending on the physical form of the feed material.
During dissolution, the evolved gas was collected. Analysis showed that
this gas consisted o f sulphur dioxide (98%) and hydrogen (2%). The presence
of sulphur dioxide and hydrogen in the gaseous p r o d u c t suggests that the fol-
lowing reactions are taking place:
2Ag + 2H2SO4 -~ Ag2SO4 + SO2 + 2H20 (1)
2Ag + H2SO4 -" Ag2804 + H2 (2)
Reaction (1) is predominant.
According to reaction (1), one mol of sulphuric acid is required to dissolve
1 mol of silver; half a mole of sulphuric acid forms Ag2SO4 salt and the other
half evolves as sulphur dioxide gas.
The leach solution contained 500--600 g/1 Ag as silver sulphate and a b o u t
1300 g/1 H2SO4. This solution crystallized when cooled below 170 ° C.

2. Recovery of silver from leach solution

The solubility of silver in concentrated sulphuric acid is temperature

dependent; silver sulphate leach solution starts to crystallize at a tempera-
ture of 170°C and at 25°C only 52 g/1 Ag remains in solution. However, the
crystallized silver salt contains entrained sulphuric acid and is difficult to
handle. The alternate method for silver recovery is by dilution of the leach
solution with water.
 83

The extent of silver precipitation as a function of (a) dilution and (b)

temperature was studied on leach solutions containing (a) 504 g/1 Ag and 422
gfl S and (b) 647 g/1 Ag and 575 g/1 S, respectively. Solution (a) was diluted
25% to 100% with water and filtered. The filtrates were analysed for silver.
The results presented in Fig. 2 showed that by simple addition of 25, 50, 75
and 100 ml water per 100 ml silver solution, the silver concentration decreased
from 504 to 95, 25, 12 and 8 g/1 respectively.

600

500

400

.~. 3OO
q:D

200

I00

0
0 :>5 50 75 I00
ml WATER A D D E D / I O O m l SILVER SOLUTION

Fig. 2. Precipitation of silver from Ag~SO4--H~SO 4 system by dilution with water.

The effect of temperature was studied on solution (b) diluted 34% and
66% with water. Both solutions were heated to almost 100°C and slowly cooled
to 25°C. Solution samples were taken during cooling and were analysed for
silver. The solubility of silver in both solutions as a function of temperature
is presented in Fig. 3. The results showed that the solubility of silver in the
34% diluted solution is very much temperature dependent and varied from
26 g/1 to 90 g/1 Ag at 25°C and 100 ° , respectively. The solubility of silver in
84
I00
HEAD SOLUTION~Ag=647g/I
S=575g/l /
80

60 /
40

20

66%

0 ~ I [ I [ I I I

20 40 60 80 I00
oC

Fig. 3. Effect o f t e m p e r a t u r e on solubility o f silver in a typical silver leach solution diluted

34% and 66% with water.

66% diluted solution was 8 g/1 at 25°C and increased only to 12 g/1 Ag at
95°C.
On the basis o f the obtained results it was concluded that precipitation of
silver from the leach solution should be carried out at a minimum 66% dilu-
tion and preferably higher. At this dilution the effect of temperature is less
pronounced and there will be no need for cooling the system to low tempera-
ture.

Silver sulphate precipitate

The white silver sulphate salts precipitated from the diluted sulphuric acid
solutions were characterized by rapid settling and very fast filtration proper-
ties. The typical chemical composition was (%): Ag = 69, S = 10.4, correspond-
ing to Ag2SO4.

3. Reduction of silver to metallic form by hydrogen

The feasibility of precipitating metals from aqueous solution in metallic

form using hydrogen has been known since 1869. However, the application
of hydrogen reduction to a commercial process was not considered until
1950 when Sherritt Gordon decided to use this technique for recovery of
nickel and cobalt from ammonia--ammonium sulphate solutions. Principles
 85

established in the laboratory for the precipitation of nickel [5] and cobalt
[6] were confirmed in Pilot Plant tests and then in commercial operation.
Interest in the hydrogen reduction technique has been growing rapidly and
numerous papers have been published. Examples of a few describing the
reduction of nickel, copper or m o l y b d e n u m in various systems are: nickel in
ammonium carbonate system [ 7 ] , copper in ammonium sulphate system [ 8 ] ,
copper in ammonium carbonate system [ 9 ] , copper in sulphuric acid system
[ 10 ], m o l y b d e n u m in ammonia--ammonium sulphate system [ 11 ]. All these
studies show that the important parameters controlling the kinetics of metal
precipitation are the composition of the feed solution, temperature and hy-
drogen partial pressure. Reduction of these metals requires 150°C to 200°C
and 20--30 kg/cm 2 hydrogen partial pressure.
The reduction of silver sulphate by hydrogen was never systematically
studied before. Therefore, an extensive investigation was carried o u t in an
ammonium sulphate system using two types of feed solution: (1) silver sul-
phate salt dissolved in ammonia solution, and (2) acidic silver leach solution
neutralized with ammonia and diluted with water. Both solutions were used
as feed materials in the study. The main difference between these solutions
was the ammonium sulphate concentration. The first solution contained no
ammonium sulphate and the second one had 185 g/1.

Solubility o f silver sulphate in ammonia solution

Silver sulphate is only slightly soluble in water b u t its solubility is enhanced
by ammonia addition. Silver sulphate forms soluble ammines in aqueous am-
monia according to the following reaction:
Ag2SO4 + xNH3 -* [Ag(NH3)x] 2SO4 (3)
where x = 0.5 to 2.0.
The highest solubility of silver (245 g/l) was attained at NH3/Ag molar ratio
of 0.5/1.0 to 1.5/1.0. A further increase in ammonia concentration lowered
the silver concentration to about 100 g/l, as shown in Fig. 4. The solubility
of silver was also a function of temperature and by increasing the tempera-
ture from 25°C to 50°C, the silver concentration was increased from 245 to
338 g/1.
The second feed solution was prepared by mixing 100 ml of a typical silver
leach solution with 200 ml of water containing 0 , 1 4 7 or 156 g/1 NH3. Am-
monia in the system was in the form of (NH4)2SO4 and Ag2 (NH3}xSO4 ; the
ammonia b o u n d with ammonium sulphate and silver ammine sulphate was
designated as ammonia total (NH3T), and ammonia b o u n d with silver as am-
monia free (NH3F). The resulting mixtures were slurries containing 193 g/1 Ag,
and 513 g/1 H2SO4 or 692 g/1 (NH4)2SO4, depending on the quantity of am-
monia added. All three slurries were heated to 95°C and then cooled slowly
to 25 ° C. Solution samples were taken at various temperatures and analysed
for silver. The results given in Fig. 5 show that the solubility of silver increased
with the addition of ammonia and with increase of temperature.
86

:550
I

50oc

300

FEED: Ag 2 SO4 = IOOg

SOLUTION = 20Oral

Ag PRECIPITATE: ~Ag(NH3)21 2 S 0 4

\
20° /
om

~25 o C

I00

0 2 4 6 8 I0
N H3F//A g MOLAR RATIO

Fig. 4. Solubility o f Ag2SO 4 in H20--NH 3 at 25°C and 50°C at various NH3F/Ag molar
ratios.

This system (with 156 g/1 NH3) contained 40--65 g/1 of soluble silver which
was much less than in feed solution (1) containing only silver sulphate and a m
monia (see Fig. 4). The silver salt precipitated from the system containing am-
monia, had the following analysis (%): Ag = 59, S = 9, NH3 = 16, suggesting
the following compound: [Ag(NH3)I.7 ] 2SO4.

Hydrogen reduction o f silver solution; kinetic studies

Reduction tests were carried out on two feed solutions containing about
 87

70

60

50
i/
g/lN H 3 /
15S

~//~~sF/Ag M.R.=3.2
40
or""

:50

Z
20
,47 g/, N . 3 ~ ~

/ - -----0
I0

.._.-.---'°'NO NH3

0 I I I I
0 20 40 60 80 100
TEMPERATURE ( °C )

Fig. 5. Effect of temperature on solubility of silver in Ag~SO4--(NH4)2 SO4--H20--NH 3

system (typical silver leach solution diluted with 2 parts water containing 0--156 g/l NH3).

56 g/1 Ag and no (NH4)2SO4 (solution 1) or 185 g/1 (NH4)2SO4 (solution 2).

The objective of these tests was to study the kinetics of silver reduction under
various conditions. The investigated variables were: (1) temperature, (2) hy-
drogen pressure, (3) NH3F/Ag molar ratio and (4) (NH4)2904 concentration.
The results on silver reduction from silver feed solution (1) are given in
Fig. 6 and from feed solution (2) in Fig. 7.
These results can be summarized as follows: the most important parameters
affecting the kinetics are (1) temperature, (2) hydrogen partial pressure and
(3) composition of the feed solution with respect to ammonia. The presence
of up to 200 g/l (NH4)2SO4 in the system has no effect on the rate of silver
reduction. The chemical reactions taking place during reduction are:
Ag2SO4 + H2 -+ 2Ag + H2SO4 (4)
88
IA 1 EFFECT OF TEMPERATURE IB) EFFECT OF TEMPERATURE
NH3 / A w M.R.:2.0 NPI3/Ag M.R.=0.5
60 i i , l so

50 50
~o
40- \ 40
v 30-
o 0" Z
30
20- 5oC 20"

I0- x IO
~50oC !~'~o o I°°°c

0 00 i ! i
4~)
o ,~ I0 20 30 50

TIME (rain) TIME (rain)

2. EFFECT OF NH3F/Ag 3. EFFECT OF (NH4)2SO 4 CONCENTRATION

60 , , , SO i

50, ~~, 50-

40, I
40.
13D
o, 50- LEGEND ( NH4)2SOA(¢/I)
0--0 0
2O
30 ~ ~ H iiiii~ii 20" X- X 200

I0- I0'

0
0 iO 20 30 40 50 0 I0 20 3,0 4'0 50

TIME (rain) TIME (rain)

Fig. 6. Reduction of silver with gaseous hydrogen from feed solution (1) under various con-
ditions. Conditions, unless otherwise stated: 125°C, 25 kg/cm ~ H2; no (NH4)~SO 4.

[Ag(NH3)] 2SO4 + H2 ~ 2Ag + (NH4)2SO4 (5)

[Ag(NH~)2] 2SO4 + H2 -~ 2Ag + (NH4)~SO4 + 2NH3 (6)
The above equations illustrate the importance of the NH3F/Ag molar ratio.
At molar ratio of 1.0, the reaction products are metallic silver and ammonium
sulphate; the pH of the reduced solution is about 5.4 and this appears to be
the optimum for the reduction of silver. A NH3F/Ag molar ratio lower than
1.0 produces sulphuric acid, and a higher ratio produces free ammonia. The
extreme variations of NH3F/Ag molar ratio affect the rate of silver reduction
and should be avoided.
 89

I. EFFECT OF Nil 3 ADDITION 2. EFFECT OF H2 PRESSURE

60 r , INH3 ! PH 60 , i
LEGEND I , / i I . . . . . . I . . . .
• - - e s4.0 5.4 1.50
50 x~x GI.0 5.4 2.8 " 50
0 - - o ss.2 s.4 e.o

40- 40

v 30- 30

20. 20'

~
I0- e I0

0 i 0
0 I0 20 30 40 80 0 I0 20 30 40 50
TIME (rain) TiME (min)

3. EFFECT OF TEMPERATURE
60 ~ , ,

50-

40-

30-

20-
\
I0-
x

0
0 I0 20 30 ¢0 50
TIME (mist)

Fig. 7. Reduction of silver with gaseous hydrogen from feed solution (2) under various
conditions. Conditions, unless otherwise stated: 1 1 0 ° C, 25 k g / c m 2 H 2 ; feed solution
neutralized with NH 3 to p H 5.4, (NH4)=SO 4 = 1 8 5 g/l.

The heating of silver solution without hydrogen resulted in partial precipi-

tation of silver as Ag2SO4.
On the basis of these results, the recommended conditions for silver reduc-
tion are: 1 1 0 ° C - - 1 2 5 ° C, 5--10 kg/cm 2 hydrogen partial pressure and NH3F/Ag
molar ratio 0.5--2.0.

Silver produc t
The silver precipitates as a very fine powder with a tendency to agglomerate
into a larger spherical ball. This agglomeration can be prevented by the addi-
tion o f a small quantity {0.05 g/l) of Acrysol to the feed solution. The effect
of Acrysol addition on screen size distribution is shown in Fig. 8.
90

80

70 --

60 --

%
~-0 059/I ACR~SOL NO ADDITIVE

50 --

40 --

30 --

!
20 --

,°

o ~o 200 300 400 500 600

PARTICLE SIZE (~rn)

Fig. 8. Effect of aerysol addition to the reduction feed solution on screen size distribution
of silver product.

T y p i c a l physical p r o p e r t i e s and chemical analyses o f the silver p r o d u c t

were:
Physical Properties Chemical analysis (%)
A.D. = 1.8 g / c m 3 S = 0.002
Fisher N u m b e r = 3.3 C = 0.01
Screen (Tyler): mesh % 02 = 0.1
Ag = balance
150 -- 1.2
150/200 -- 6.8
200/325 -- 22.0
--325 -- 70.0
L o w e r r e d u c t i o n t e m p e r a t u r e a n d / o r higher a m m o n i a c o n c e n t r a t i o n pro-
d u c e d silver with a o x y g e n c o n t e n t u p to 5%. The s c a n n i n g - e l e c t r o n m i c r o -
graphs o f the silver p r o d u c t are s h o w n in Fig. 9.
~J

c~

o~

C)

a~
~o

el,

0
0

0
0
c~

p~L
92

Flowsheet o f silver process

The results obtained in this study suggest the following processing steps
for the treatment of silver bearing materials for the recovery of silver: (Fig. 10)
(1) Leaching of silver in concentrated sulphuric acid at 200 ° C.
(2) Precipitation of Ag2SO4 by dilution of the leach solution with water.
(3) Dissolution o f Ag2SO4 in aqueous ammonia.
(4) Reduction of Ag+ with hydrogen.
In the presence of same impurities, it is preferable to combine steps (2) and
(3).

Silver feed
material ~ Silver
H2 SO4 1 dissolution

Filtration [I Dissolution residue

for recovery of

Water 7
1
J Agso4
precipitation
I
impurities

1
I Filtration II Filtrate for recovery
of impurities

Water
NH3
"J
:J Ag2 S04
t
7 dissolution
g2 SO4

[
I

J Hydrogen
1
H~ "1 reduetion ~ ' - ~ ( N H 4 ) : SO4 by-product

Silver
product

Fig. 10. Flowsheet for treatment of silver bearing materials by hydrometallurgical method.

Behaviour o f impurities in silver process

Silver bearing feed materials such as silver scrap, refinery bullion or dor~
which can be treated by this process usually contain many impurities. The
most c o m m o n are: copper, nickel, cobalt, zinc. tin, lead, cadmium, antimony,
TABLE 3

Behaviour of impurities during processing of various silver-bearing feed materials in sul-

phuric acid (Head material No. 1: coinage webbing (sterling) and tableware; head material
No. 2: jewelry silver scrap; head material No. 3: mixture of metals, Ag, As, Cd, Te, Sb,
Sn, Se)

Analyses: Chemical I) or Spectrographic (emission) 2)

Feed I) Material
Material Ag As Cd Co Cr Cu Fe Ni Pb Sb Si Sn Te Ti Se Zn

i Head Material #1 (77.9) - -

- (22.1) . . . . . .- (0-001) -
Ag Product M <0.01 X X X 0.002 0.001 X X X i0.04 X <0.005i X . X
i

2 Head Material #2 (65.5) <0.01 (0.05) M M (27.6) M (1.2) M X M M 0.003 ! X - (2.0)

Ag Product M <0.01 X X 0.02 0.i 0.02 0.002 0.003i X 10.02 0.005 <0.005[0.001 <0.005

3 Head Material #3 [ 79.0 3.5 3.5 - . . . . 35 3.5 3.5 3.5

Ag Product bal. <0.01 X X - 0.004 X - - X 0 04 X <0.005 0.001 (0.008) -

Notes: i) Chemical Anal ses in brackets

2) Emission spectrographic analyses without brackets: X = not detected, M = major (>0.i)

¢O
5o
94

chromium, arsenic, tellurium and selenium. The behaviour of these impurities

during processing of silver-bearing materials Was investigated using three types
of feed:
(1) Coinage webbing (sterling) and silver plated table ware
(2) Jewelry silver scrap
(3) Mixture of metals: Ag, As, Cd, Te, Se, Sb and Sn.
All these feed materials were processed using a somewhat modified pro-
cedure. Feed materials were treated with sulphuric acid and the leach slurry
was diluted with water to precipitate Ag~SO4 and filtered. The soluble
impurities were removed with the filtrate. The residue containing silver sul-
phate and insoluble impurities was treated with weak ammonia solution and
filtered. Silver went into solution as soluble silver ammine sulphate essentially
free from impurities and was reduced by hydrogen to the metallic form.
Using this procedure, all four basic processing steps were implemented: (1)
leaching, (2) precipitation of silver sulphate, (3) dissolution of silver sulphate
in a weak ammonia solution and (4) recovery of silver from solution by hy-
drogen reduction. All these steps were very effective in separation of impuri-
ties and resulted in the recovery of pure silver as shown in Table 3.
During leaching, some impurities such as various alloys in jewelry scrap
were unreactive; the other impurities reacted with sulphuric acid and formed
either the insoluble salts as such metals as lead, copper, nickel, antimony, tin
and tellurium or went into solution as arsenic, cadmium and selenium. It
should be mentioned that the separation into insoluble and soluble fractions
refers to the bulk of impurities.
The deportment of impurities from silver in various stages was followed on
head material #3 and the results are given in Table 4.
These results showed that almost all impurities were removed with the leach
residue and with the filtrate in the silver sulphate precipitation step; silver sul-

TABLE 4

Behaviour of impurities during processing of head material No. 3 (Synthetic mixture)

Removal of Impurities (Cumulative) from

Silver Stream (%)
Processing Step
As Cd Te Sb Sn Se

Leaching 6.5 1.2 74.5 96. i 75.4 7.2

Silver sulphate precipitation 93.2 98.6 98.0 99.6 97.8 96.2
Silver sulphate dissolution 98.6 99.7 99.6 >99.9 >99.9 99.8
Hydrogen reduction >99.9 >99.9 >99.9 >99.9

Head Material #3 3.5 3.5 3.5 3.5 3.5 3.5 79% Ag

Silver Product <0. 01 ×i) <0.005 Xl) Xl) 0.008

Note: i) not detected by emission spectrograph

 95

phate dissolution and hydrogen reduction were only polishing steps. However,
the hydrogen reduction method is very effective for the separation of larger
quantities of Cu, Ni, Co and Zn from silver. The separation of these impurities
is based on preferential reduction of silver under conditions at which copper,
nickel and cobalt are not reduced to the metallic state. Zinc can not be pre-
cipitated by hydrogen.

Stripping of silver from spent solutions

The solutions from various processing streams contain 0.1 (hydrogen reduc-
tion) to 5.0 g/l Ag (silver sulphate precipitation). Tests were carried o u t to
strip the silver with (l) sodium borohydride, (2) hydrogen sulphide or (3)
sodium chloride. All three methods have been found to be effective, and
silver was removed to a level of 0.015--0.001 g/1 (see Table 5).

TABLE 5

Stripping of silver from dilute silver sulphate solution

7 Solution (g/L) Residue
Reag./Ag
Method] Reagent M.R. Medium Ag Cu Analyses (%)

i) Initial Final Initial Ag S 02

i Sodium Borohydride 0.25 Acid 5.55 2.50 - 99.8 0 0.1~

(Na B H4) 0.23 Alkaline 4.85 0,015 - 86.3 0 4.94

2 Hydrogen Sulphide excess Acid 4.62 0,001 - 8 4 . 3 , 1 3 . 3 0.52'

(H2s) excess Alkaline 4.62 0,001 - 78.1 13.1 0.58

Cl Cu ~
3 NaCI i.i Alkaline 4.92 0,003 2.3 66.712-i7.9 0.02
i.i Alkaline 5.47 0.006 23.4 62.8 20.6 0.69

Conditions: 25°C, atmospheric pressure, but autoclave was used for NaBH 4

Note: i) mol reagent per mol Ag

Silver was recovered in these tests as (1) silver metal, (2) silver sulphide or
(3) silver chloride. The reactions taking place during stripping were:
4Ag2SO4 + NaBH4 + 3H20 ~ 8Ag + B(OH)3 + 0.5Na2SO4 + 3.5H2SO4 (7)
Ag2SO4 + H2 S -~ Ag2S + H~SO4 (8)
Ag2SO4 + 2NaC1 -+ 2AgC1 + Na2SO4 (9)
Except for reaction (7) these reactions can be carried o u t at room tempera-
ture and atmospheric pressure. In method (1) the NaBH4 decomposed very
rapidly and the evolved hydrogen escaped before being able to react with
silver. However, the efficiency of NaBH4 utilization was improved by carry-
ing o u t the reaction in a closed vessel in an alkaline medium.
96

CONCLUSIONS

Laboratory investigations were carried out on the application of a hydro-

metallurgical method for the refining of silver. This study showed that hydro-
metallurgy is viable for the processing of silver bearing material. The process
is comprised of four steps: (1) leaching in sulphuric acid; (2) precipitation of
silver sulphate from leach solution by dilution with water; (3) dissolution of
silver sulphate with ammonia and (4) reduction of silver from solution to
metallic form by gaseous hydrogen.
The conditions for each of the processing steps are:
(1) Leaching: 200°C, concentrated H2SO4, leaching rate 70--120 g/1 h Ag.
(2) Precipitation of Ag2SO4: room temperature, dilution of I or 2 parts water
per part of leach solution
(3) Dissolution of silver sulphate in diluted ammonia (NH3F/Ag molar ratio
0.5/1.0 to 2.0/1.0).
(4) Reduction by gaseous hydrogen at 110-125°C using 2--10 kg/cm 2 hy-
drogen pressure; retention time of 10--30 min.
The silver product is in the form of silver powder.
The process is very flexible and can be applied to the treatment of various
feed materials such as silver scrap, bullion, dor~ and refinery slimes. The most
common impurities in feed materials such as Cu, Ni, Co, Zn, As, Cd, Pb, Sb,
Sn, Te and Se are removed during processing, and silver is recovered as a high
purity product. The impurities can be recovered separately as a by-product.
Reagents used in the process are hydrogen, sulphuric acid and ammonia,
all readily available and relatively inexpensive. A portion of the value of
those reagents may be recovered as an ammonium sulphate by-product.
Hydrogen reduction of silver can be advantageous in the preparation of
silver alloys, silver catalysts and electrical contacts.

ACKNOWLEDGEMENT

This work was carried out in the Research Centre of Sherritt Gordon
Mines Limited, Fort Saskatchewan, Alberta. The author wishes to thank the
Management of Sherritt Gordon Mines Limited for their permission to publish
this paper.

REFERENCES
1. Drake, Herold J., 1978. Silver, Mineral Commodity Profiles MCP-24. US Department
of the Interior.
2. Schack, C.H. and Clemmons, B.H., 1967. In: Butts, A. (Ed.), Silver: Economics,
Metallurgy and Use. D. Van Nostrand Company, Inc., Princeton, New Jersey, p.
57--77.
3. Heimala, S.O., Hyvarinen, O.V.J. and Kinnunen, J.P.I., 1977. Hydrometallurgical
Process for the Recovery of Valuable Components from Anode Slime Produced in
Electrolytical Refining of Copper. U.S. Patent 4,002,544.
 97

4. Mellor, J.W., 1946. Comprehensive Treatise on Inorganic and Theoretical Chemistry,

Vol. III. Longmans, Green and Co., London--New York--Toronto, New Impression,
p. 350.
5. Mackiw, V.N., Lin, W.C. and Kunda, W., 1957. Reduction of nickel by hydrogen from
ammoniacal nickel sulphate solutions. J. Metals 9, AIME Trans. Vol. 209: 786--793.
6. Kunda, W., Warner, J.P. and Mackiw, V.N., 1962. The hydrometallurgical production
of cobalt. Can. Min. Metall. Bull. Vol. 55 (597): 25--29.
7. Kunda, W., Evans, D.J.I. and Mackiw, V.N., 1964. Low density nickel powder by
hydrogen reduction from the aqueous ammonium carbonate system. Planseeber.
Pulvermetall., 12: 153.
8. Evans, D.J.I., Romanchuk, S. and Mackiw, V.N., 1961. Copper powder by hydrogen
reduction techniques. Can. Min. Metall. Bull. Vol. 54 (591): 523.
9. Kunda, W. and Evans, D.J.I., 1971. Hydrogen Reduction of Copper to Powder from
Ammine Carbonate System. In: Hausner, H.H. (Ed.) Modern Development in Powder
Metallurgy, Vol. 4. Plenum Press, p. 49--61.
10. Genik-Sas-Berezowsky, R.M., 1977. Copper by Gaseous Reduction. U.S. Patent
4,018,595.
11. Kunda, W. and Rudyk, B., 1965. Recovery of molybdenum from cupriferrous molyb-
denite. Planseeber. Pulvermetall., 13: 157--177.