XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1669
Leaching Kinetics
To understand the kinetics of leaching zinc from CaO-
treated EAFD, the Shrinking Core Model (SCM) with
constant particle siz was employed. Based on the morphol-
ogy and element distribution of the CaO-treated EAFD,
ZnO and Ca2Fe2O5 appeared uniformly and finely distrib-
uted, suggesting that an unreacted product layer composed
largely of Ca2Fe2O5 remains as zinc dissolves into solution.
The agitation employed during leaching ensured all samples
remained suspended and in motion, significantly reducing
the effect of liquid film diffusion. In this case, kinetics of
the leaching process can be described using the following
expressions (Levenspiel, 1999):
For solid product layer diffusion control:
x xh k t 1 3^1 2^1 /3 2 #--+-=h (3)
And for chemical reaction control:
x k t 1 1 /3 1 #--=^h (4)
where × is the fractional conversion of zinc, k is the appar-
ent rate constant (per min) and t is the leaching time (min).
Using experimental zinc dissolution values, the left-hand
side of the above equations can be plotted versus leaching
time to find the best linear fit.
At 25°C, it was found that the best linear fit can
be obtained for chemical reaction control, indicating
that chemical reaction on the unreacted surface was the
rate-controlling step. The significant improvement in zinc
extraction from 25°C to 50°C also suggests that leaching
temperature strongly affected the dissolution rate of zinc,
confirming a chemical reaction-controlled process. At
50°C, excellent linearity was obtained for both product
layer diffusion and chemical reaction control, indicating
that either one of these steps, or a combination of both,
was rate-controlling.
To distinguish between these two steps, leaching of
pure ZnO was carried out using 2M NAOH solution at
50°C. It was found that 92% of zinc from ZnO readily
dissolved in the leachate after only 15 minutes, while only
27% of zinc from CaO-treated EAFD was extracted under
the same conditions. The significantly higher leaching effi-
ciency of zinc from ZnO compared to CaO-treated EAFD
suggests that product layer formation during leaching lim-
its the extraction at 50°C since there is no intraparticle dif-
fusion in pure ZnO leaching. These results confirm that
diffusion through this product layer is the rate-limiting step
at 50°C. Finally, at 70°C, the best linear fit was achieved
for product layer diffusion control, indicating that diffu-
sion through the unreacted product layer was still the rate-
controlling step.
The above analysis suggests that the rate-controlling
mechanism shifted from chemical reaction control to prod-
uct layer diffusion control with change in temperature.
To verify this shift, apparent activation energies (Ea) of
Figure 5. Arrhenius plots of the alkaline leaching of zinc from CaO-treated EAFD
Leaching Kinetics
To understand the kinetics of leaching zinc from CaO-
treated EAFD, the Shrinking Core Model (SCM) with
constant particle siz was employed. Based on the morphol-
ogy and element distribution of the CaO-treated EAFD,
ZnO and Ca2Fe2O5 appeared uniformly and finely distrib-
uted, suggesting that an unreacted product layer composed
largely of Ca2Fe2O5 remains as zinc dissolves into solution.
The agitation employed during leaching ensured all samples
remained suspended and in motion, significantly reducing
the effect of liquid film diffusion. In this case, kinetics of
the leaching process can be described using the following
expressions (Levenspiel, 1999):
For solid product layer diffusion control:
x xh k t 1 3^1 2^1 /3 2 #--+-=h (3)
And for chemical reaction control:
x k t 1 1 /3 1 #--=^h (4)
where × is the fractional conversion of zinc, k is the appar-
ent rate constant (per min) and t is the leaching time (min).
Using experimental zinc dissolution values, the left-hand
side of the above equations can be plotted versus leaching
time to find the best linear fit.
At 25°C, it was found that the best linear fit can
be obtained for chemical reaction control, indicating
that chemical reaction on the unreacted surface was the
rate-controlling step. The significant improvement in zinc
extraction from 25°C to 50°C also suggests that leaching
temperature strongly affected the dissolution rate of zinc,
confirming a chemical reaction-controlled process. At
50°C, excellent linearity was obtained for both product
layer diffusion and chemical reaction control, indicating
that either one of these steps, or a combination of both,
was rate-controlling.
To distinguish between these two steps, leaching of
pure ZnO was carried out using 2M NAOH solution at
50°C. It was found that 92% of zinc from ZnO readily
dissolved in the leachate after only 15 minutes, while only
27% of zinc from CaO-treated EAFD was extracted under
the same conditions. The significantly higher leaching effi-
ciency of zinc from ZnO compared to CaO-treated EAFD
suggests that product layer formation during leaching lim-
its the extraction at 50°C since there is no intraparticle dif-
fusion in pure ZnO leaching. These results confirm that
diffusion through this product layer is the rate-limiting step
at 50°C. Finally, at 70°C, the best linear fit was achieved
for product layer diffusion control, indicating that diffu-
sion through the unreacted product layer was still the rate-
controlling step.
The above analysis suggests that the rate-controlling
mechanism shifted from chemical reaction control to prod-
uct layer diffusion control with change in temperature.
To verify this shift, apparent activation energies (Ea) of
Figure 5. Arrhenius plots of the alkaline leaching of zinc from CaO-treated EAFD