Sonoleaching: Development of a rapid determination of Pb ... - SER

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4. Preparation of the Stabilized Slag
The waste sample used in this study is a slag from
a Pb recovery smelting plant. The slag was pulverized
to pass sieve #35 (0.5 mm). Ordinary Portland cement
was used to stabilize the slag. A 1:1 ratio of slag to
cement was used. 750 g of slag and 750 g of ordinary
Portland cement were weighed and thoroughly mixed.
Deionized water was added until a paste like consistency
was obtained. The mixture was then poured into
a mould and allowed to cure for 3 wk. After the curing
period, the stabilized waste was then pulverized again
to pass sieve #35.
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5. Design of Experiment
The Design-Expert 7 [30] consists of three major
parts, the actual design, the analysis process and the
optimization. In the actual design, the factors, parameters
and expected responses considered in the study
are used to determine the minimum number of runs.
The response data are then used in the analysis process
which includes full analysis of the variance. The optimization
part determines the combination of factors
and responses that simultaneously satisfy the requirements
of the factors and responses.
.
RESULTS AND DISCUSSION
A second order equation based on dissolution [31]
can be expressed in the form:
(1)
Where k = rate constant of second order dissolu-
tion, S = maximum dissolution (g), S = solubility (g),
max
and T = sonication time, min
The equation is then linearized to:
(2)
Where intercept = r = initial dissolution rate =
1/(k S ) and slope = reciprocal of the dissolution
max
equilibrium = 1/S .
max
The straight lines in Figs. 2a and 2b are the graphs
of the linearized form of the second order dissolution
equation (Eq. 2) which shows that dissolution of Pb
with ultrasound follows a second-order kinetics. Table
2 shows the values of S , r and k for the slag and
max
stabilized slag as determined from the lines in Figs. 2a
and 2b.
.
The optimization feature of the Design-Expert 7
can generate a combination of solutions for optimal
conditions. These are ranked in the order of desirability.
Table 3 shows the optimization criteria for the
determination of optimal conditions. For factors [Pb]
and temperature, the criteria are the values used in the
study. 30 min was chosen as the sonication time as
Bellotindos et al., Sustain. Environ. Res., 21(6), 375-380 (2011)
.
.
.
.
.
Table 2. Dissolution rate and rate constant for extraction
of Pb in slag and stabilized slag
-1
S max (% Pb g sample)
Dissolution Rate (r)
-1 -1
(% Pb min g sample)
t / S
Rate constant (k)
12
10
8
6
4
2
Slag Stabilized Slag
31.5 12.1
0.16
0.0064
0.05
0.13
0
0 20 40 60 80 100 120 140
Time (min)
Fig. 2. Second order dissolution of Pb (a) original slag (b)
stabilized slag.
this was the time that equilibrium was attained during
sonication. For the % Pb extraction, the value 80 was
chosen as this was the average of the TCLP extractions.
Of the several combinations that were generated, the
conditions with comparable extraction to TLCP were
sonication time of 30 min and temperature of 30 °C. .
Table 3. Optimization criteria for the determination of
optimal conditions
Factor
[Pb], ppm
Temperature, °C
Sonication Time, min
% Pb extraction
CONCLUSIONS
(b)
(a)
Criteria
500-5000
25-30
30
80
Using the optimization feature of the Design-
Expert 7 Software, the optimal conditions for an extraction
comparable to TCLP are: sonication time = 30
min and the ultrasonic cleaner bath temperature =
30 °C. The 30 min sonication time as compared to the
TCLP extraction of 18 h, means that the extraction
time has been reduced by about 97% and a bath temperature
of 30 °C means that the temperature can easily
be obtained as it is about the ambient temperature
and there is no need to monitor the ultrasonic bath until
the end of the sonication. The sonoleaching of lead
from slag and stabilized slag is a second order dissolution
process. Extraction with the use of ultrasound did
not result in chemical changes but rather physical
changes occurred. The weight of the waste and the