Reduction of PbSO4 by (CO+CO2) mixture

Article abstract of DOI:10.1016/j.tca.2004.03.014

This paper includes experimental results of PbSO₄ reduction. The experiments were carried out within temperature range 923-1073 K and for varying CO/CO₂ ratio. The obtained results allow to determine the influence of the temperature range and gas composition on the quantity and constituents of the resulting products. It shows the distribution of lead sulphate on two reactions, concerned with the direct transformation of lead sulphate (PbSO₄) to lead sulphide (PbS) and with the reaction of PbSO₄ with PbS which takes place at a phase PbSO₄-PbS boundary.

Full text of DOI:10.1016/j.tca.2004.03.014

Thermochimica Acta 423 (2004) 143–148
Reduction of PbSO4 by (CO + CO2) mixture
∗
˙
Czesław Malinowski, Stanisław Małecki , Daniel Zołnierczyk
Faculty of Non-Ferrous Metals, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Cracow, Poland
Received 20 August 2003; received in revised form 23 March 2004; accepted 23 March 2004
Available online 28 July 2004
Abstract
This paper includes experimental results of PbSO4 reduction. The experiments were carried out within temperature range 923–1073 K and
for varying CO/CO2 ratio. The obtained results allow to determine the influence of the temperature range and gas composition on the quantity
and constituents of the resulting products. It shows the distribution of lead sulphate on two reactions, concerned with the direct transformation
of lead sulphate (PbSO4) to lead sulphide (PbS) and with the reaction of PbSO4 with PbS which takes place at a phase PbSO4–PbS boundary.
© 2004 Published by Elsevier B.V.
Keywords: Reduction; Lead sulphate; Lead sulphide; Reaction in solid state
1. Introduction
reduction runs in a more complex way including converting
MSO4 to MS as well as the reaction at the phase boundary
between MSO4 and MS which produces metal oxide. The
direct reduction of MO to M is also possible in this case.
These reactions can be described according to the following
equations:
The reduction of metal sulphates as well as the reduc-
tion of metal oxides is an important problem in chemistry
and metallurgy. In carried out industrial processes metal sul-
phates are converted into metal oxides by means of thermal
decomposition or metal sulphides in case of reducer atten-
dance. Owing to the effect produced by reducers on metal
sulphates, there are formed phases that depend on a temper-
ature and types of metals. In this case such phases can be
formed as: metallic phases (Pb, Cu, Ag); sulphidic phases
(ZnS, PbS, K2S, Na2S) as well as oxidic phases, e.g. MgO
[1]. It is shared view that the reduction of metal sulphates by
means of hydrogen (H2) or carbon monoxide (CO) runs in
lower temperatures than their thermal decomposition. The
work [2] assumed that the reaction of MSO4 reduction by
means of H2 runs according to the following scheme:
MSO4 + 4CO(H2) = MS + 4CO2(H2O)
MS + 3MSO4 = 4MO + 4SO2
(3)
(4)
(5)
MO + CO(H2) = M + CO2(H2O)
The quantity and type of the resulting products depend on
the differences of the reaction (Eqs. (3)–(5)) rate. If in the
conditions of carried out MSO4 reduction processes, reac-
tions (Eqs. (4) and (5)) do not occur, just MS will only be
obtained. Whereas, in case when the reaction (Eq. (5)) does
not occur, the products of the MSO4 reduction will only
contain MS and MO, but their participation depends on the
differences of reactions (Eqs. (3) and (4)) rate. In case of
the high rate of the reaction (Eq. (4)) the only product of
the process is MO. The occurrence of a metallic phase as a
result of the reaction (Eq. (5)) is possible only in the appro-
priate conditions.
2MSO4 + 5H2 = MO + MS + 5H2O + SO2
MO + H2 = M + H2O
(1)
(2)
If we assume that the equation (Eq. (1)) is possible the
obtained products should always contain two phases MO and
MS (in various mole ratio) or M and MS in case when the
reaction (Eq. (2)) occurs. It seems that the reaction of MSO4
The hypothesis about the presented above courses of the
reactions can be confirmed thanks to the following investi-
gation and the experimental results:
∗
Corresponding author. Tel.: +48-12-617-2672;
• The reduction of SrSO4 by means of gaseous (CO +
CO2) mixtures in the range of temperatures 1121–1240 K
fax: +48-12-633-2316.
E-mail address: stanmal@uci.agh.edu.pl (S. Małecki).
0040-6031/$ – see front matter © 2004 Published by Elsevier B.V.
doi:10.1016/j.tca.2004.03.014

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C. Malinowski et al. / Thermochimica Acta 423 (2004) 143–148
directly to SrS as the only product of this reaction [3–5].
furnace tube with the regulation of temperature by means
of Pt–PtRh thermocouple from below of the furnace and
kept immediately under the examined sample. The gaseous
phase of the constant amount and of predetermined com-
position which was controlled by means of the rotameter,
flowed from below of the furnace to its top. The gaseous
phase with the flow rate of 60 dm3/h was passed through
the furnace. This rate was adjusted in order to prevent its
influence on the speed of the process. The experiments
were carried out within the temperature range 893–1073 K
and for varying CO/CO2 ratio ranging from 20 to 100%
of CO. The measurements were conducted to an amount in
which samples reached the constant mass. On the basis of
registered mass changes of the samples during the reduction
process relative mass changes were calculated, which as a
function of time for different conditions were presented in
Figs. 1–4. The obtained products were subjected to X-ray
phase analysis, which showed only Pb and PbS presence.
Due to this fact on the presented figures the relative mass
changes were indicated. They correspond to Pb or possibly
PbS occurrence in the sample after the end of the process.
The sigmoidal character of the obtained relations points to
the autocatalytic course of process; such a process course
can be either a result of the reaction surface developing
or of an increased participation of secondary reactions in
the process in question whereas those secondary reactions
proceed among substrates and transitory reaction products
being formed and it is possible that they can increase the
summary rate of the entire process.
• CaSO4 reduction by means of hydrogen in the range of
temperatures 1023–1088 K to CaS [6,7].
• Whereas MgSO4 reduction by means of CO in the range
of temperatures 873–1048 K resulting in MgO in the final
products [8].
The conducted thermodynamic analysis of the reaction
between MSO4 and MS showed that especially for stron-
tium and calcium mentioned reaction does not occur in the
considered range of temperature, therefore the only product
in this case is a metal sulphide. In the event of magnesium
occurrence coursing of these reaction is possible, therefore
MgO occurs in the final products. In the considered condi-
tions the reduction of MgO does not occur, therefore metallic
magnesium does not appear in the final products. In order to
confirm this hypothesis we can refer to the work [9] in which
the reduction of ZnSO4 was conducted. The temperature of
this process was 948 K. The obtained results contained in
this work showed the occurrence of ZnO and ZnS in the
final products. It results from the fact that reactions at the
phase ZnSO4–ZnS boundary run in considered conditions.
This work is aimed to determine the influence of the tem-
perature range and the gaseous (CO+CO2) phase composi-
tion on the quantity and composition of obtained products.
2. Experimental results
The investigation of PbSO4 reduction was conducted by
thermogravimetric method. The investigated samples in the
form of powder were placed on a special pan of Al2O3,
which was suspended on the electronic balance. This made
possible the continuous registration of mass changes during
the process with the accuracy of 1 × 10−3 g. The appara-
tus used for investigations consisted of the vertical electric
It should be mentioned that the total mass decrease de-
pends on both: the process temperature and the gaseous
phase composition. Making a comparison between relative
mass decreases in temperature equal to 973 and 1048 K it
should be noticed that the time of the process is considerably
influenced by the CO concentration in the gaseous phase.
Also simultaneously with the increasing CO concentration,
Reduction of PbSO4 for 973 K
35
30
25
20
15
10
5
31,66%
21,10%
Pb
70% CO
30% CO2
100% CO
20% CO
80% CO2
50% CO
50% CO2
PbS
0
0
10
20
30
40
50
60
Time of process/min
Fig. 1. The influence of gaseous phase composition on the relative mass changes for temperature 973 K.

C. Malinowski et al. / Thermochimica Acta 423 (2004) 143–148
145
Reduction of PbSO4 for 1048 K
35
30
25
20
15
10
5
31,66%
21,10%
Pb
50% CO
50% CO2
20% CO
80% CO2
70% CO
30% CO2
100% CO
PbS
0
0
5
10
15
20
Time of process/min
Fig. 2. The influence of gaseous phase composition on the relative mass changes for temperature 1048 K.
the total mass changes decrease, which could suggest the
rising quantity of PbS in the final products. The influence of
temperature on the relative mass changes for constant CO
concentration in the gaseous mixture is presented in Figs. 3
and 4. For CO concentration equal to 50% (Fig. 3) simul-
taneously with the increasing temperature, the total mass
change also increases and reaches the value of mass decrease
which corresponds to the formation of metallic lead. In case
when the process is carried out in pure CO, the quantity of
PbS in the final products increases.
lead and lead sulphide in the final products were performed
according to the following relations:
(ꢀmt/m0)PbSO →Pb − (ꢀmp/m0)
%PbS =
(ꢀmt/m )
4− (ꢀmt/m )
0 PbSO4→Pb
0 PbSO4→PbS
×100
where %PbS is the contents of PbS in preparation after
the process (%); (ꢀmt/m0)PbSO →Pb the relative PbSO4
4
mass change in case when the only product is Pb, is equal
to 31.66%; (ꢀmt/m0)PbSO →PbS the relative PbSO4 mass
4
change in case when the only product is PbS, is equal to
21.10%; (ꢀmp/m0) the registered relative change of the mass
of the sample during the process.
3. Results and discussion
The results of the PbS and Pb quantity calculations in the
preparation after the process are presented in Table 1.
Having the mass of the preparation after the process and
the relative mass change, the mass calculations of metallic
Reduction of PbSO4 for 50 % CO, 50 % CO2
35
30
25
20
15
10
5
31,66%
1073K
Pb
1048K
1023K
998K
973K
948K
923K
21,10%
PbS
0
0
10
20
30
40
50
60
70
Time of process/min
Fig. 3. The influence of temperature on the relative mass changes for gaseous mixture composition 50% CO, 50% CO2.

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C. Malinowski et al. / Thermochimica Acta 423 (2004) 143–148
Reduction of PbSO4 for 100 % CO
35
30
25
20
15
10
5
31,66%
1073K
Pb
1048K
1023K
998K
973K
893K
923K
948K
PbS
0
0
10
20
30
40
50
60
70
80
90
100 110 120
Time of process/min
Fig. 4. The influence of temperature on the relative mass changes for gaseous mixture composition 100% CO.
Making the analysis of obtained results we can claim that
taking into consideration the influence of temperature on the
constitution of obtained products at constant CO concentra-
tion in the gaseous phase, the received data of the PbSO4
reduction show that simultaneously with the increasing
in the constant temperature of the process, simultaneously
with increasing CO concentration in the gaseous phase the
participation of PbS in the final products increases. Whereas,
Table 1
Mass fraction of PbS and Pb in the end products for different process conditions
Temperature (K)
923
Concentration of CO (vol.%)
Mass fraction in the end product (%)
ꢀm/m0
PbS
Pb
50
70
100
58.33
54.83
65.53
41.67
45.17
34.47
25.50
25.87
24.74
948
973
50
70
100
40.72
46.40
57.01
59.28
53.60
42.99
27.36
26.76
25.64
20
50
70
33.90
42.71
44.32
48.48
66.10
57.29
55.68
51.52
28.08
27.15
26.98
26.54
100
998
1023
1048
1073
20
50
70
15.34
23.77
33.62
35.13
84.66
76.23
66.38
64.87
30.04
29.15
28.11
27.95
100
20
50
70
12.50
19.98
34.38
36.84
87.50
80.02
65.63
63.16
30.34
29.55
28.03
27.77
100
20
50
70
0
100
31.66
30.90
30.07
29.43
7.20
15.06
21.12
92.80
84.94
78.88
100
20
50
70
0
0
100
100
93.47
81.16
31.66
31.66
30.97
29.67
6.53
18.84
100

C. Malinowski et al. / Thermochimica Acta 423 (2004) 143–148
147
Table 2
Distribution of PbSO4 to the particular reactions
Temperature (K)
Concentration
of CO (vol.%)
Distribution of PbSO4 (%)
PbSO4 + 4CO = PbS + 4CO2
62.29
3PbSO4 + PbS = 4PbO + 4SO2
PbSO4 not reacted
1.09
893
923
100
36.62
50
70
100
66.16
63.46
71.71
32.78
35.46
27.30
1.06
1.09
0.99
948
973
50
70
100
52.89
57.09
65.14
46.05
41.81
33.78
1.07
1.10
1.07
20
50
70
47.95
54.38
55.58
58.67
51.06
44.54
43.33
40.23
0.99
1.08
1.09
1.10
100
998
1023
1048
1073
20
50
70
35.09
40.83
47.80
48.87
64.33
58.36
51.21
50.12
0.58
0.81
0.99
1.01
100
20
50
70
33.17
38.26
48.35
50.07
66.34
61.03
50.66
48.90
0.49
0.71
1.00
1.03
100
20
50
70
25.26
29.68
34.87
37.88
74.71
70.01
64.56
61.42
0.03
0.30
0.57
0.70
100
20
50
25
25
75
75
0
0
70
100
29.25
37.46
70.47
61.86
0.28
0.68
temperature, the contamination of PbS decreases and metal-
lic Pb increases. Thermodynamic accounts testify that the
occurrence of metallic lead is caused by the reduction of
PbO by means of CO, whereas PbO was formed by the
reaction at PbSO4–PbS phase boundary. According to the
literature data [10] the reaction between PbSO4 and PbS
occurs, forming basic sulphates which in reducer atten-
dance are subjected to the reduction resulting in Pb and PbS
as products. Obtained PbS reacts again with PbSO4 at the
PbS–PbSO4 phase boundary. Due to this fact, the following
reactions of PbSO4 reduction have been assumed:
The data obtained by the distribution calculations indicate
that participation of the reaction (Eq. (6)) in the reduction
progress increases for the preset temperature and increas-
ing CO concentration in the gaseous phase, whereas this
participation decreases with the increasing temperature for
constant CO concentration. Differences of the reactions rate
determine the quantity of PbS in the final products. In case
when the rate of the reactions (Eq. (6)) is higher in compari-
son to the reaction (Eq. (7)) a considerable part of PbSO4 is
subjected to the reduction to PbS (it is if we assume that the
reaction (Eq. (8)) runs with an appropriate rate the whole
quantity of PbO is reduced to PbS). The increasing of the
process temperature influences on the growth of the reaction
rate at the PbSO4–PbS phase boundary to a higher degree
than on the reaction reduction. In this case reaction has a
considerable participation (Eq. (7)), it also springs from the
final products composition. If the rate of this reaction is ef-
ficiently high in proportion to the reaction (Eq. (6)) rate, re-
sulting PbS will immediately react with not reacted PbSO4
resulting in Pb presence. The last column in Table 2 presents
the quantity of not reacted PbSO4 which springs from the
performed mass balance.
PbSO4 + 4CO = PbS + 4CO2
PbS + 3PbSO4 = 4PbO + 4SO2
PbO + CO = Pb + CO2
(6)
(7)
(8)
On the basis of coursing of the reactions (Eqs. (6) and (7))
the distribution of PbSO4 to the particular reactions was
calculated. Results are presented in Table 2. The formation
of metallic lead according to the reaction: PbS + 2PbO =
3Pb + SO2 was left out.

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C. Malinowski et al. / Thermochimica Acta 423 (2004) 143–148
4. Conclusion
Acknowledgements
The obtained experimental results of the PbSO4 reduction
by means of gaseous (CO+CO2) mixture and the performed
calculations allow to formulate the following conclusions:
Financial support from the Committee for Scientific Re-
search (KBN) is acknowledged.
1. In the reduction process simultaneously three reactions
take place; the direct reduction of PbSO4 to PbS; reaction
at a phase boundary of PbSO4–PbS in which PbO is
formed; PbO reduction to metallic lead.
2. The constituents of obtained products including only lead
sulphide (PbS) and metallic lead (Pb) depend on differ-
ences of the reaction (Eqs. (6) and (7)) rate.
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a constant temperature causes an increase in the PbSO4
to PbS reduction rate, and the result is the increased PbS
fraction in the reaction products. Provided the CO content
in the system is constant, an increase in a temperature
influences the rate of reaction at a phase boundary of
PbSO4–PbS to a higher degree than the rate of the direct
reduction of PbO4 to PbS, and the result is that a metallic
Pb content in the reaction products grows.
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the reaction. This refers especially to a lower temperature
range and a lower CO concentration in the gaseous phase.
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