The
calculation
of
Ca
2+
crystallization kinetics by the formula
(5) and its comparison with the results
of typical experiments of CaCO
3
crystallization carried out at PTLAS
of NUWEE, see [1,12], are shown in
fig. 9.
We can see that calculated and
measured values are close except
time period i < 65. The reasons for the
discrepancy may be the sensors inertia [11]
or inadequate reflection of fast gas exchange
by Henry’s model within this time range, see
(6) and fig. 10, where current values of a
2
parameter are shown. From fig. 10 we can see
that for i < 65 a
2
parameters
fluctuations are
beyond the mean-root square error and
Henry’s approximation is not adequate at
modeling CO
2
desorption process.
In general, on the basis of data in fig.9 we can come to the conclusion that equation
(2) reflects real processes in CHCS except the initial area of kinetic curve where
crystallization still does not begin and where Henry’s model is considered to give
significant error. It may be used at the analysis of the processes in CHCS under the
controlled conditions (HCO
3
-
CO
2
and CO
3
2-
concentrations measurement and the
amount of removed CO
2
).
It follows from equation (5) that the change of Ca
2+
concentration is
accompanied by the changes of all CHCS parameters and CO
2
exchange with the
environment.
Analyze it in detail. From the expression (5) we can see that three regimes of Ca
2+
ions concentration change in an open CHCS are possible depending upon pH.
Equation of Ca
2+
dynamics for pH < 5.5. At pH < 5.5 concentration of Ca
2+
ions
is not changed since in the absence of the initial solid calcium carbonate in the
system practically all carbonates are in the form of dissolved CO
2
, that is Sc ≈ X
2
and
Figure 10. Current desorption parameter
value a
2
depending upon the number of
measurement i. Average of a within 50 < i
<300 in terms of indices and equal to 0.2,
in time 0.2/27 = 7.41∙10
-3
c
.
і
65
0.34
Figure 9. The rates of Ca
2+
active concentrations
change: measured, thick curve, and calculated with
X
1
determined from the condition of electrical
neutrality and on the basis of formula (2).
і
- 1379 -