D. Atanasova,
N. Chemical
Lyakov, V. Technology
Vassilev, G. and
Angelov,
G. Haralampiev
Journal of the
University of
Metallurgy,
43, 2, 2008, 267-272
DESULPHURIZATION OF LEAD CAKE BY SODIUM CARBONATE
AND SODIUM HYDROXIDE
D. Atanasova1, N. Lyakov, V. Vassilev1, G. Angelov2, G. Haralampiev
1
University of Chemical Technology and Metallurgy
8 Kl. Ohridski, 1756 Sofia, Bulgaria
E-mail: [email protected]
2
KCM 2000 S. A. Plovdiv,
Assenovgradsko shosse, Bulgaria
Received 11 March 2008
Accepted 12 May 2008
ABSTRACT
Investigations are carried out related to desulphurization of Pb-cake by Na2CO3 and NaOH and the optimum
conditions for process running are defined in two installations: laboratory agitator and drum type rotary reactor. The
degree of desulphurization by Na2CO3 reaches 93-94 %, while by NaOH - 95-96 %, values which satisfy the technological
and ecological requirements for processing of Pb-cake. The content of impurities in the solutions is also examined with a
view to produce sufficiently pure crystalline Na2SO4. The obtained solutions desulphurized by Na2CO3 are purer concerning the impurities Pb, Zn, Cd and Fe compared to those desulphurized by NaOH. The produced crystalline Na2SO4 is
suitable to be used in other productions.
Keywords: lead sulphate, desulphurization of Pb-cake, processing of Pb waste.
INTRODUCTION
Some secondary lead containing products contain considerable quantity of sulphur in the form of SO4
and the following ones refer to them:
• damped battery paste containing: Pb ( ≈ 68-76
%) in the form of PbSO4 (50-60 %), PbO2 (30-35 %),
PbO (10-15 %) and Sb (0,2-0,7 %) [1, 2];
• Pb-cake from the hydrometallurgical cycle of
Zn-production which contains: Pb ( ≈ 35-45 %), Zn (810 %) and S (9-11 %), in the form of PbSO4, ZnSO4 and
SO4, respectively.
As a rule PbSO4 is desulfurized before melting [3-6]. The most often used reagents for that purpose are Na 2CO3 and NaOH [1-8]. The interaction
between PbSO 4 with Na2CO 3 and NaOH goes according the reactions:
PbSO4 (solid) + Na2CO3 (liquid) →
→ PbCO3 (solid) + Na2SO4 (liquid)
(1)
PbSO4 (solid) + NaOH (liquid) →
→ Pb(OH)2 (solid) + Na2SO4 (liquid)
(2)
The produced saturated solution is subjected to
evaporation, centrifuging and precipitation of the crystalline Na2SO4 [7-9].
The following values are obtained for Gibbs’
energy and the equilibrium constant of reaction (1) at
T=298 and T=343 Ê: G o = - 41,91 kJ mol-1, Kc =
298
2,213.107 (T=298 K) and Kc = 2,174.106 (T=343 K).
The equilibrium constant values are high and it
can be considered that in this temperature range the
reaction continues to PbSO4 exhaustion (if there are
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Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008
present the necessary quantity of Na2CO3 and possibility for reagents contacts).
Another ground is the difference between the
solubility products of PbSO4 and PbCO3. At T=298 K
they are: L(PbSO4) = 1,6.10-8 and L(PbCO3) = 7,5.10-14.
These values provide the basis for the conclusion that if
the reaction goes in solution then PbCO3 shall pass in
the solid phase, i.å. the CO 32 − ions will replace
SO 24 − ions because of the lower PbCO3 solubility [10].
The above said refers also to equation (2), where for
desulphurization of Pb-containing products NaOH is
used (at T=298 Ê, L(Pb(OH)2)=1,1.10-20).
Pb- cake contains, besides Pb (30-45 %) and S
(9-11 %), also Zn (8-10 %), Cu ( ≈ 0,5 %), Cd, Fe, As,
Sb, Bi, etc. The solubility products of some of these
metals are as follows [10]:
When Na 2 CO 3 is used: L(PbCO 3 )=7,5.10 -14;
L(ZnCO3)=1,45.10-11; L(CdCO3)=5,2.10-12; L(FeCO3)=
3,47.10-11; L(CuCO3)=2,5.10-10;
When NaOH is used:
L(Pb(OH) 2 )=1,1.10 -20 ;
L(Zn(OH) 2 )=7,1.10 -18 ;
-14
L(Cd(OH) 2 )=2,2.10 ;
L(Cu(OH) 2 )=2,2.10 -20 ;
L(Fe(OH) 3 )=3,2.10 -38 ;
L(Fe(OH) 2 )=1,0.10 -15 ;
-32
L(Bi(OH)3)=3,2.10 .
The desulphurization of lead containing products
gives pure Na2SO4 solutions, which contain insignificant quantities of heavy metals and iron [11].
In connection to the above said and the fact
that no data are found in the publications in relation to
Pb- cake desulphurization from different authors, the
aim of the present study is to:
(i) determine the effects of the factors initial
concentration of (Na2CO3 or NaOH) in the solution,
liquid to solid phase ratio, process duration and temperature on the process characteristics - degree of Pbcake desulphurization and degree of reagent utilization;
(ii) specify the optimum conditions for carrying out Pb-cake desulphurization by Na2CO3 or NaOH
in dependence of the type of the reactors used;
(iii) examine the content of impurities in the
solutions with the purpose to produce sufficiently pure
crystalline Na2SO4.
EXPERIMENTAL
Before desulphurization the Pb-cake is dried
to constant weight. The experiments are performed in
standard reactor with laboratory agitator (ER 10) or in
268
drum type rotary reactor VEB Elmo Hartha (DDR) and
for every experiment 200 g of Pb-cake is used. There is
studied Pb-cake from the hydrometallurgical cycle of
Zn-production of the following composition [%]: Pb 38,35; sulphate sulphur ( S SO4 ) - 9,9; sulphide sulphur
(SS) - 0,5; Zn - 8,34.
The paste composition is determined by chemical analysis and the content of sulphate sulphur S 2−
SO 4
by gravimetric analysis.
The lead cake is treated by Na2CO3 or NaOH
solution of preset initial concentration and liquid to
solid phase ratio at defined temperature and duration
of the desulphurization process.
After completion of the chemical treatment the
pulp is filtered and the solid phase washed by warm water on the filter, dried to constant weight and analyzed
for SO42 - content. The SO42- content in the solid phase is
determined by gravimetric method. The liquid phases
are analyzed for SO42 -, Na2CO3 or NaOH contents.
The content of free Na2CO3 and NaOH in the
liquid phases (residual concentration) is determined by
titration with 0,1 Ì HCl and indicator methyl orange.
The desulphurization degree á [%] is calculated
by the formula:
where: Ds (initial) and Ds(final) are initial and residual SO42concentration.
The reagent utilization degree â [%] is calculated
by the formula:
where: C(initial) and C(final) - initial and residual reagent
concentration, g L-1.
RESULTS AND DISCUSSION
Investigation of the desulphurization process in the
lead cake treatment by Na2CO3
Effect of Na2CO3 concentration
The effect of the Na2CO3 initial concentration
(Cinitial) in the solution during the Pb-cake treatment by
D. Atanasova, N. Lyakov, V. Vassilev, G. Angelov, G. Haralampiev
Fig. 1. Effect of Na2CO3 initial concentration on the degree of
Pb-cake desulphurization (α) and degree of Na 2CO 3 utilization
(β) for m=2,5; t=40; τ=90 min and Cinitial=140 g L-1 Na 2CO3 (6,7
% excess), 155 g L-1 (18,2 % excess), 170 g L-1 (29,6 % excess)
and 185 g L -1 (41,0 % excess): - laboratory agitator (α1, β1); rotary reactor (α2, β 2).
Fig. 2. Effect of the process duration (τ) of Pb- cake
desulphurization by Na 2 CO 3 on the degree of Pb-cake
desulphurization (α) and degree of Na 2CO3 utilization (β) for
m=2,5 , Cinitial=155 g L-1 Na2CO3 and τ= 5, 15, 30 and 60 min : laboratory agitator (α1, β1); -rotary reactor (α2, β2).
laboratory agitator and drum type rotary reactor (Fig.1)
is studied for liquid to solid phase ratio = 2,5:1 (m=2,5),
temperature t=40oC, process duration ô= 90 min and
Cinitial=140 g L-1 (6,7 % excess), 155 g L-1 (18,2 % excess),
170 g L-1 (29,6 % excess) and 185 g L-1 (41,0 % excess).
The results obtained clearly display the above
described kinetic dependences, i.e. with increase of the
concentration the desulphurization degree grows but the
degree of reagent utilization decreases. For optimum
concentration Cinitial=155 g L-1 Na2CO3 could be accepted.
It should be noted that the results of the cake treatment in a rotary reactor are higher (á2= 94,8 %) compared to those of the treatment by standard reactor with
laboratory agitator (á1 = 92,4 %). The inconvenience
when using rotary reactor is the maintenance of a constant temperature as the heating of the pulp is difficult.
Our previous investigations [11] reveal that when
the ratio m is increased from 2,0 to 3,5 the degree of
Na2CO3 utilization (â) decreases from 99,04 to 13,92 %,
while the degree of Pb-cake desulphurization (á) increases
from 80,41 to 95,93 %. This rule is more strongly expressed in the range m=2,0-2,5. At greater values of m
the desulphurization degree grows slightly but on the account of this the degree of Na2CO3 utilization decreases
and the total volume of the system increases. As a result
of this the reagent consumption is increased which in its
turn results in the formation of insoluble double salts
Na2Pb2(CO3)2OH and NaHCO3 [5]. The value of the relation m=2,5 is recommended as optimum.
Effect of the duration of the desulphurization process by Na2CO3
The effect of the duration of the cake
desulphurization by Na2CO3 (ô= 5, 15, 30 and 60 min)
is studied in standard reactor with laboratory agitator
and in drum type rotary reactor (Fig.2) under the following conditions: Cinitial=155 g L-1 Na2CO3 (18,2 % excess), m= 2,5, t= 40 oC.
In case of stirring by laboratory agitator optimum desulphurization degree is obtained for ô= 30 (min
- 92,4 %). Optimum values for the desulphurization
degree in rotary reactor are obtained in the range 15 –
30 min (92,8 % - 93,6 %). This fact suggests that the
process may be carried out in continuous mode of operation.
The concentrations of impurities in the solutions are as follows [mg L-1]:
30 min: Pb < 0,1; Zn < 0,5; Cd - 0,04 - 0,28;
Fe < 1,00 - 1,38;
45 min: Pb < 0,1; Zn < 4,0 - 16,0;
Cd - 1,3 - 10,0; Fe - 1,2 - 1,7.
By experiments performed in advance it is found
out that the temperature affects slightly the process char-
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Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008
a)
Fig. 3. Effect of the NaOH initial concentration on the degree of
Pb-cake desulphurization (α) and the degree of NaOH utilization
(β) for m=2,5; t= 40; τ=30 min and Cinitial=104 g L-1 NaOH (5 %
excess), 118 g L-1 (19 % excess), 135 g L-1 (36 % excess) and 150
g L-1 (51 % excess): - laboratory agitator (α1, β1); - rotary reactor
(α2, β2).
b)
acteristics. If there is such influence it is related to the
properties of Na2SO4 and Na2CO3 [10,12,13]. The best
results are obtained in the temperature range of 30-40 oC,
where the values of the desulphurization degree are approximately equal.
Investigation of the desulphurization process in the
lead cake treatment by NaOÍ
Effect of NaOH concentration
The studies are performed under the following
conditions: Cinitial=104 g L-1 NaOH (5% excess), 118 g
L-1 (19 % excess), 135 g L-1 (36 % excess) è 150 g L-1
(51 % excess); m=2,5, ô=30 min, t= 40 oC (Fig.3).
With concentration of 104 g L-1 (5 % excess)
very good results are obtained concerning (á), but NaOH
utilization (â) is high which imposes increase of Cinitial
NaOH for practical assurance of the process run. The
optimum NaOH concentration in relation to the
desulphurization degree in reactor with laboratory agitator and in rotary reactor is 118 g L-1 (19 % excess).
The Pb concentration in the solution for 118 g L-1
NaOH is 1724,6 mg L-1 and for 104 g L-1 it is 2,18 mg L-1.
270
Fig. 4. Effect of the liquid to solid phase ratio (m) on the Pbcake desulphurization degree (α) and the degree of NaOH
utilization (β) for Cinitial=104 g L-1 NaOH, respectively - m=2 (16
% insufficiency), m=2,5 (5 % excess), m=3 (26 % excess) and
m=3,5 (47 % excess); t=40oC and τ=30 min: a) laboratory agitator
(α1, β1); b) rotary reactor (α2, β2).
Effect of the liquid to solid phase ratio
The investigations are performed in both types of the
reactors under the following conditions: Cinitial=104 g L-1 NaOH,
at liquid to solid phase ratio - m=2 (16 % insufficiency),
m=2,5 (5 % excess), m=3 (26 % excess), m=3,5 (47 %
excess); at t=40oC and ô=30 min, respectively (Fig.4).
The optimum value of the liquid to solid phase
ratio is m=2,5 (á=92,8 % - in reactor with laboratory
agitator and á=93,2 % - in rotary reactor).
D. Atanasova, N. Lyakov, V. Vassilev, G. Angelov, G. Haralampiev
a)
b)
desulphurization degree has high values 95,7-96,3 %
and 96,5-96,8 %, respectively.
In case of desulphurization in reactor with laboratory agitator and 118 g L-1 NaOH, the impurities concentrations in the solution mg L-1 are: Pb-1724,6; Zn154,4, Cd<0,01 and Fe-1,07.
In case of desulphurization in rotary reactor the
impurities concentrations in the solution mg L-1 are:
For concentration 104 g L-1 NaOH: Pb - 49,28;
Zn - 20,85; Cd < 0,01; Fe - 0,71;
For concentration 118 g L-1 NaOH: Pb - 104,35;
Zn - 57,12; Cd < 0,01; Fe - 0,54.
When using NaOH, it should be operated with
excess of not more than 10-20 %, because it is found
out that when the reagent excess is greater the Pb concentration in the solution grows.
CONCLUSIONS
Fig. 5. Effect of the process duration (τ) of Pb-cake desulphurization
by NaOH on the Pb-cake desulphurization degree (α) and degree of
NaOH utilization (β) at m=2,5, t=40 oC. Samples are examined at
τ= 5, 15, 30, 45 and 60 min: a) laboratory agitator Cinitial=104 g L1
(α1, β1) and 118 g L-1 NaOH, (α2, β2); b) rotary reactor Cinitial=104
g L-1 (α3, β3) and 118 g L-1 NaOH, (α4, β4).
On the basis of the experiments performed the
following conclusions could be made:
• it is found out that the process goes more favorably in rotary reactor (for 15-30 min), which provides precondition for continuous mode of operation;
• the degree of desulphurization by Na2CO3
reaches 93-94 % and by NaOH - 95-96 %, values that
meet the technological and environmental requirements
for Pb-cake processing;
• the solutions obtained after desulphurization
by Na2CO3 are more pure regarding the impurities Pb,
Zn, Cd and Fe, compared to those by NaOH. In both
cases the solutions could be used for production of pure
crystalline Na2SO4.
REFERENCES
Effect of the duration of the desulphurization
process by NaOH
The investigations are performed in both types
of the reactors under the following conditions: Cinitial=104
g L-1 and 118 g L-1 NaOH, m=2,5, t=40 oC and process
duration ô=5, 15, 30, 45 and 60 min (Fig.5).
In case of stirring by laboratory agitator and concentration of 104 g L-1 NaOH the optimum process duration is ô=30 min (á=92,8 %), and for concentration
118 g L-1 NaOH - ô=15 min (á=96,4 %).
The optimum process duration in rotary reactor for both concentrations is 5-15 min, and the
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