The examination of the stability of hydrochloric acid – tin chloride electrolyte
applied for tin electrorefining
Gergo Rimaszeki+, Tamas Kekesi++,
+PhD student, ++professor,
Department of Metallurgical and Foundry Engineering, University of Miskolc
Abstract
The growing demand for tin and its increasing price make it necessary to extract this valuable
metal from secondary raw materials with higher efficiency and purity. Electrorefining in
aqueous solutions may be applied to tin if suitable operating conditions are found for a
cathodic deposition of the desired efficiency and quality. Application of pure hydrochloric
acid solutions may offer a new and straightforward possibility. During the long electrolytic
refining process the electrolyte solution loses its stability. This means a significant problem.
Tin can be present in both the Sn(II) and the Sn(IV) forms in hydrochloric solutions. The
metallic form can be oxidized by the Sn(IV) ions, which may cause serious cathode corrosion.
As a result, tin concentration and the Sn(IV)/Sn(II) ratio are increasing, leading to the
precipitation tetravalent tin compounds. Therefore, solutions containing aqua regia are less
stable than pure hydrochloric acid solutions. The presence of Sn(IV) can be demonstrated by
spectrophotometry using phenyl fluorine as the indicator. For the purpose of testing, Sn(IV)
ions were reduced effectively with zinc powder. The oxidation of the Sn(II) ions to Sn(IV)
can be avoided during the electrolysis procedure by covering the surface of the electrolyte
solutions with oil or using nitrogen atmosphere.
1. Introduction
With the development of the electronic industry, the processing of secondary row materials
and scarp - especially tin containing scarp - have gained significant importance. The copper
concentration in tin or the Sn-Ag alloy increase continually during soldering and the
purification of this alloy means a serious technological challenge. Pyrometallurgical and
hydro/electrometallurgical methods are known for the processing of tin scarp. The main
disadvantages of the pyrometallurgical way are the multi stage process and the high primary
costs. On the other hand, electrolytic tin refining is able to refine the tin containing scarp in
one operational step. The anode is cast of the impure metal and the purified metal is deposited
on a cathode sheet. Alkaline and acid electrolyte solutions are used to the electrolytic tin
refining. The main disadvantages of the alkaline baths are the high working temperature and
the significant electric charge required for the reduction of the dominantly tetravalent species.
The use of the conventional sulphuric acid-cresilic-phenylic acid solutions implies relatively
high costs caused by the expensive additives. A novel way is the use of pure hydrochloric acid
solutions. This can ensure high purity metal produced, as chlorine residues are eliminated
during the final melting of the cathodic product. The cost of the ingredients can be lower and
the productivity can be higher.[2,3] However, pure acid solutions of tin may lose their stability
during longer standing in contact with air. The causes standing behind the loss of dissolved tin
chloride stability are unknown. The need to refresh the solution periodically causes significant
deficit. The main aim of this work is to investigate the chemical reaction and equilibrium in
the solution and to find a suitable method for the stability examination of the tin in aqueous
chloride media. It may help finding the practical methods to avoid the harmful effects of
instability.
2. The equilibrium in the solution and the typical chemical reactions
Tin can be present in both Sn(II) and Sn(IV) forms in hydrochloric acid solutions. The two
ions can be transformed into each other according to the equation (1).
Sn 2+ + 4OH − ↔ Sn ( OH ) 2 + 2OH − ↔ [ Sn ( OH ) 4 ]
2−
(1)
The hydroxo-complex form is stabilized in alkaline media. The Sn(II) form can be oxidized to
Sn(IV) very easily, therefore it is a very aggressive reductant. The following equilibria are
present in aqueous solutions [4]:
Sn 4+ + 6OH − ↔ Sn ( OH ) 4 + 2OH − ↔ [ Sn ( OH ) 6 ]
2−
(2)
Tin is able to form anionic complexes in chloride media. This feature can be very favorable
for the cathodic deposition, because it can inhibit the charge transfer step and allow the tin ion
supply by diffusion to catch up. The stability of the different complexes depends on the
chloride ion concentration and the ionic strength. The re-dissolution of metal deposited at the
cathode decreases the current efficiency. The presence of Sn(IV) means the danger in this
respect because it can oxidize metallic tin. In turn, tetravalent tin ions are formed from Sn(II)
by oxidation.[5,6,7]
2.1 The equilibrium of tin ions and their redox transformation in the solution.
The following redox and chloro-complex formation reactions effect the oxidation states of tin
in the solution:
Sn(IV) + Sn = 2Sn(II),
(3)
or in chloride medium:
[SnCl ] (
y
4− y )
+ Sn = 2[ SnCl x ]
( 2− x )
+ ( y − x)Cl −
(4)
The activity of the [ SnCl x ] ( ni − x ) ions can be determined from the cumulative stability
constans ( β ni , x ) and from the activity of the corresponding aquo-ion:
(
a[ SnCl ] ( ni − x ) = β ni , x γ ± ⋅ cCl −
x
) x aSn
ni +
,
(5)
where cCl − is the concentration of the chloride ions and γ± is the activity coefficient in the
hydrochloric acid solution. The activity of the examined complex element can be expressed
with the activity of the Sn2+ .
(
)
a[ SnCl ] ( 4− x ) = f aSn 2+ =
x
(
= β 4, x ⋅ γ ± ⋅ cCl −
)
(
2 E− Eo
x
⋅ aSn 2+ ⋅ 10
Sn 4+ / Sn 2 +
0.059
),
(i > 1)
(6)
The concentration of all the dissolved tin species ( [ SnCl x ] ( ni − x ) , i ≥ 1, x ≥ 0) is equal with the
concentration of the dissolved tin [8-12]:
[
]
 a ni +

c[ SnCl ] ( ni − x ) = ∑  ∑ Sn
1 + β ni , x γ ± ⋅ cCl − x  = cSn
(9)
x
i x
i  x γ±

Modeling the conditions in the electrolyte solution, the distribution of tin among the ions in
solutions of varied chloride ion concentrations in contact with metallic tin or with air can be
represented by the curves of Fig. 1. The Sn(II) solution has a good stability in contact with
metallic tin, but it may be easily oxidized if the solution is equilibrated with air, or the redox
potential is determined by an oxidizing agent.
∑∑
1
1
Σ Sn(II) ~ 1
[SnCl] +
(
Σ Sn(IV) ~ 1
[SnCl]3+
1E-5
[SnCl 2 ] 0
[SnCl 2 ]2+
[SnCl 4 ] 2-
Sn2+
Relative concentration
Relative concentration
[SnCl 3 ] -
1E-5
Dissolved Sn = 0,1 M
Reagent: Sn-powder
γ +2 = γ HCl
a
-
1E-10
Σ Sn(IV)
Sn4+
[SnCl 3 ] +
1E-10
-
[SnCl 5]
[SnCl 4]0
1E-15
1E-20
Dissolved Sn = 0,1 M
Reagent: air
γ +2 = γ HCl
-
b
1E-25
1E-30
[SnCl]3+
[SnCl] +
[SnCl2]0
[SnCl 2 ] 2+
[SnCl 3 ] +
1E-15
10-4
)
10-3
10-2
[SnCl 4]0
[SnCl 5]-
10-1
1E-35
4+
Sn
100
Concentration of free Cl- -ions, mol dm-3
101
10-4
ΣS
2-
-
[SnCl 3 ]
l ]
[SnC 4
10-3
10-2
2+
Sn
10-1
n(
II)
100
101
Concentration of free Cl- -ions, mol dm-3
Fig 1. Equilibrium distribution of Sn among different species dissolved in HCl solutions in
contact with (a) tin metal (b) air [12]
3. Different methods for the examination of the tin stability
In this part of the study, there are two different ways applied for the examination of tin
stability. These are the spectrophotometric and polarographic methods. In the first case we
can get information on the stability characteristics of tin, in the second case the rate of the
Sn(II)-Sn(IV) form can be examined in the presence of stabilizing agents.
3.1 The spectrophotometric investigation of chloro-complex stability in aqueous solution.
The measurements were made at room temperature with 0,0001 M Sn, 0,02 M HCl and
different NaCl concentrations. The hydrolysis of Sn(IV) results in the formation of
compounds like SnCl(OH)62- and SnCl4(OH)22-. It is important to avoid the formation of
Sn(IV) because their complexes may absorb the UV radiation. Effective prevention can be
argon flushing. Figure 2. shows the effect of the temperature on the light absorbance spectra
between 25 oC and 300 oC [13]
Absorbance
Wavelenght 1/cm
Figure 2.The characteristic spectrums of the solutions with 0,0001 m tin and 0,101m
chloride ion concentration between 25 oC and 300 oC temperatures (T). [13]
3.2 The polarographic investigation of Sn(II) solutions in the presence of stabilizing
agents.
We need a weak oxidant for the polarographic investigation of the Sn(II) containing solutions.
Pyrogallol, hydrazin and phenol-sulfon acid are suitable to this examination. The reaction
between the Sn(II) and the oxygen is a chain reaction, which is blocked by the stabilizing
agents. The Sn(II) containing solutions can be oxidized easily by air and no appropriate
solution have been found so far to increase stability. A common method is the storage of the
solution in nitrogen atmosphere. Table 1 shows the effect of the stabilizing agents in different
concentrations to the stability of Sn(II).[14]
Table 1. The effect of the different stabilizing agents on the stability of Sn(II) .[14]
concentration,
mol/l
0,1
0,01
0,001
0,0001
0,00001
The amount of the stable remained Sn(II), %
24 hours
1week
pyrogallol
84
60
92
90
90
86
98
75
98
72
24 hours
1 week
phenol-sulfon acid
96
62
96
60
92
54
80
26
58
5
24 hours
1 week
hydrazin
100
90
96
68
96
74
96
76
18
0
4. Experimental
During the long electrolysis the electrolyte solution loses its stability, this is the main problem
of the electrolytic tin refining in pure acid solutions. Two methods have been tested to
produce the electrolyte solutions. The first method is the use of aqua regia to the dissolution
of tin powder, the second way is using concentrated (6M) hydrochloride acid and dissolve the
tin powder during boiling. These solutions were subsequently diluted with to the desired HCl
concentration. During a long time of electrolysis or the open solution standing in contact with
metallic tin, a yellow precipitation is formed. The viscosity is increasing in contact with air
according to the decreasing water content. According to literature, SnO nH2O compound
forms at the anode which during the storage gradually loses its water content and is oxidized
to tin-oxide (SnO2) through the following states: orto-(H 4SnO4), a pyro- (H6Sn2O7) and metastannic acid (H2SnO3)n. [15]. The determination of the Sn(IV) - which is responsible for the loss
of stability - and the examination of the redox reaction constituted the main elements of this
work. UV-Vis spectrometry was used to this investigation. Electrolyte solutions were made
containing 10 and 20 g/l tin and 1 M hydrochloride acid. Phenyl-fluoron was used as an
analytical reagent to indicate the Sn(IV) ions in the solution. The Sn4+ together with the
phenyl-fluoron gives a typical spectral peak at 490 nm.[16]
4.1 The examination of the redox conditions using UV-Vis spectrometry
In the first series of the experiments, we tried to determine the Sn(IV) ions and Figure 3.
demonstrates the main results. The red and blue curves show the spectra of the pure
hydrochloride acid solution containing phenyl-fluoron and hydrogen-peroxide. The green
curve shows the spectra of the fresh electrolyte solution containing phenyl-fluoron, the light
blue curve shows just the same solution but after 5 minutes storage. The purple curve
indicates the spectra of the electrolyte solution after 5 minutes storage containing phenylfluoron and hydrogen-peroxide. The determination of the Sn(IV) ions was calculated by
difference. The brown dashed line indicate the Sn(IV) ions in the electrolyte solution (as the
difference of the light blue and black lines). The black dashed line indicate the Sn(IV) ions
after adding hydrogen-peroxide (the difference of purple and dark blue lines).
2
Sn2-4ff10
HCl+phenil-fluoron
HCl+phenil-fluoron+4 drop H2O2
electrolyte solution+phenil-fluoron
electrolyte solution+phenil-fluoron after 5 minutes storage
HCl+phenil-fluoron+4drop H2O2 after 5 minutes storage
electrolyte solution+phenil-fluoron+ 4 drop H2O2 after 5 minutes storage
Sn4+ in the electrolyte solution
Sn4+ in the electrolite solution after H2O2
Abs
1.5
1
0.5
0
400
450
500
550
Wavelenght, nm
600
650
Figure 3.The detection of the Sn4+ ion in the electrolyte solution with and without hydrogenperoxide (with 20g/l Sn and 1 M HCl concentration)
The electrolyte solution contains some Sn(IV) ions and the hydrogen-peroxide increases its
amount. During the electrolytic tin refining a strong reducing agent have to be used to ensure
the Sn4+-Sn2+ reduction and avoid the loss of stability. Therefore, the reduction of the Sn(IV)
ions was examined with the application of zinc powder. Figure 4 shows the results.
2
electrolyte solution+phenil-fluoron
electrolyte solution+phenil-fluoron after 5 minutes storage
electrolyte solution+ Zn powder+phenil-fluoron
Sn2+
Abs
1.5
1
0.5
0
400
450
500
550
Wavelanght, nm
600
650
Figure 4. The detection of the Sn2+ ion in the electrolyte solutions with reduction using zinc
powder (with 20g/l Sn, 1 M HCl concentration).
The amount of the Sn(IV) ions stabilizes after 5 minutes according to the phenyl-fluoron
addition (green curve). The spectral peak of Sn(IV) was diminished by zinc powder. The
determination of Sn(II) was made by difference. The red dashed line - which shows the
presence of the Sn(II) ions - was given by the difference of the green and light blue curves.
The zinc powder proved to be a suitable reducing agent
.
4.2 Comparative experiment to the examining of solution stability
An important comparative experiment can be the examination of electrolyte solutions, which
was made by dissolving tin powder in aqua regia and hot hydrochloric acid, resp. The
electrolyte solutions - containing 20 g/l tin and 1 mol/l HCl – proved adequate to the stability
examinations. The samples of 50 ml electrolyte solutions were contacted with tin of measured
weight. This represented the stored electrolyte solution. The aim of the experiment was to
examine the solution stability. The experiment lasted about 9 days and in the first series
samples were taken from the solutions and were analyzed by UV-Vis spectrometry for Sn(IV)
ions. Figure 5 shows that the spectra of the electrolyte solution - made by aqua regia – ran out
of scale after 3 days. Figure 6 expresses these results with photograps. The solution made with
the aqua regia assisted dissolution method has become thick and white precipitation occurred.
Electrolyte solution , made by hot acid
after 1 day
after 2 day
after 3 day
Electrolyte solution, made by aqua regia
after 1 day
after 2 day
after 3 day
Abs
3
2
1
0
400
450
500
550
600
Wavelenght, nm
650
Figure 5 The characteristic spectra of the electrolyte solutions, made by aqua regia and hot
hydrochloride acid, as a function of the time (with 20 g/l Sn and 1M HCl concentration in the
electrolyte solution).
2
2
1
1
a
b
Figure 6.Pictures of the stable and instable electrolyte solutions (1-in case of aqua regia, 2in case of hot acid); a) first day, b) seventh day.
4.3 The examination of the redox circumstances during the electrolytic tin refining
Further examinations were made to clarify the electrode and redox reactions during the
electrolytic tin refining. Changes - around the anode and cathode - were examined with UVVis spectrometry. The concentration of the electrolyte solution was 20 g/l tin and 1 M HCl.
The purpose of the experimental work was to compare and contrast the anodic and cathodic
spectra of the electrolyte solutions. In the first series of the experiments we applied oil
covering on the electrolyte solution to avoid the oxidation. In the second series of the
experiments the electrolyte solution was not covered. In both cases, a porous diaphragm was
used to separate the anodic and cathodic compartments. Three samples were taken from the
vicinities of the anode and cathode. The experimental results in the electrolyte solution
without oil covering are shown in Figure 7.
anode part (start)
anode part (after 30 minutes)
anode part ( after 60 minutes)
cathode part (start)
cathode part ( after 30 minutes)
cathode part (after 60 minutes
Abs
1.2
0.8
0.4
0
400
450
500
550
Wavelenght, nm
600
650
Figure 7.The characteristic spectrums of the electrolyte solution without oil covering (with 20
g/l Sn and 1 M HCl concentration in the electrolyte solution).
The peaks of the Sn(IV) increase during the electrolytic tin refining, so the oxidizing effect of
the anode and the ambient air predominates. In the second series of the experiments oil
covering was used to avoid the oxidation in the electrolyte solution. Preliminary nitrogen
flushing was used to remove the oxygen form the solution. The experimental results are
shown in Figure 8.
anode part (start)
anode part (after 30 minutes)
anode part (after 60 minutes)
cathode part (start)
cathode part (after 30 minutes)
cathode part (after 60 minutes)
Abs
0.8
0.4
0
400
450
500
550
Wavelenght,nm
600
650
Figure 8.The characteristic spectra of the electrolyte solution with oil covering (with 20 g/l
Sn and 1 M HCl concentration)
During the one hour long electrolytic tin refining the Sn(IV) peaks are almost unchanged. The
electrolyte solution covered with oil is suitable to protect the solution from the oxidation. It
also implies that the main source of harmful oxidation of the tin species in the electrolyte
solution is the ambient air.
5. Conclusion
According to examinations the electrolyte solution, made by hot acid, proved more beneficial
against the solution, made by aqua regia. The presence of the Sn 4+ ions in the electrolyte was
determined successfully with UV-Vis spectrometer and using phenil-fluoron reagent. The
strong oxidizing circumstances and the storage in the air increase the amount of Sn 4+ ions in
the electrolyte solutions. The Sn4+ ions can be reduced by zinc powder, and the presence of
the Sn2+ ions can manifest. During the electrolytic tin refining the oxidation cause harmful
cathodic corrosion, on the other hand the oil covering on the electrolyte solution prevent the
Sn2+ ions form the oxidation. Primarily the oxidation effect of the air can be harmful for the
process.
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