68 2 Joint Growth of Crystals of Different Phases
in our experiments we used the reactant obtained from natural ores. According to
published data, growth of isolated crystalline seeds of KCl is diffusion-controlled
(Kleber 1964; Panov et al. 1969; Punin and Petrov 1972; König et al. 1987).
According to the data obtained by Treivus et al. (1985), the absolute values of
growth rates for NaCl isolated cubic crystals (chemically pure reactant) were within
the range of 0.0–3.5 μm/min (ranges: for saturation temperature T
sat
= 33–48°C, for
supercooling ΔT ≈ 12–25°C), and at ΔT = 1.0–2.0°C the “dead area” was observed.
We found a direct correlation between the growth rates of NaCl cubes in solution
having saturation temperature of 40°C and supercoolings ΔT = 1.0 and 2.0°C; the
growth rates were 2.38 and 2.98 μm/min, correspondingly. Growth rates of NaCl
determined in mass crystallization process (Rumford and Bain 1960; Panov et al.
1969) and specific conditions of formation of fibrous crystals (“whiskers”) (Hayashi
and Shichiri 1974) are not to be compared with the growth rates of isolated seeds.
The growth rates of cube facets of NaCl and KCl crystals in binary solutions can
vary in wide ranges, being 0–45 μm/min (ΔT = 1.0°C) and 0.5–6 μm/min (ΔT =
2.0°C) for KCl, and 0–5 μm/min (ΔT = 1.0°C) and 0–7.5 μm/min (ΔT = 2.0°C) for
NaCl. It is to be noted that within the temperature range of 5–85°C the crystals of
KCl can show several anomalous bursts of the growth rates (up to 30%). These
anomalies are accounted for by structural rearrangements occurring in the solutions
(Punin and Petrov 1972). This phenomenon can be discovered only in high preci-
sion experiments. Random selection of the process variables that inevitably involve
hitting the maxima or minima of the growth rate domain can create a wrong impres-
sion of the extended dispersion of the data. At the same time, measurements made
in profoundly anomalous regions can result in real extension of data dispersion due
to temperature variations within the margin of the temperature error.
Crystallization of individual seeds of KCl and NaCl in ternary eutonic solutions is
not covered in literature except for some indications of the growth rate anomalies
observed for KCl crystals in industrial solutions, which have compositions close to
those of eutonic solutions (Punin 1969). We observed (Glikin et al. 2001) that the aver-
age growth rates of cubic faces of both investigated substances tend to increase with
enhancing the supercooling. Thus, for KCl crystals at saturation temperature of 40°C
and ΔT = 1.0 and 2.0°C the observed rates were 1.3 ± 0.01 and 1.6 ± 0.01 μm/min,
correspondingly; for NaCl crystals these values were 0.8 ± 0.01 and 0.9 ± 0.02 μm/min.
The values were averaged on the results of 20 experiments, which provided narrow
confidence intervals. It can be seen that in ternary systems the growth of KCl crystals at
ΔT = 1.0°C accelerates significantly, while growth rates of NaCl decrease substantially
in comparison with growth rates obtained in their binary solutions; at ΔT = 2.0°C the
growth rates for KCl are close to their values obtained for binary solutions, while
growth of NaCl crystals becomes considerably slower. It should be noted that introduc-
tion of 7.9 wt% of NaCl into a KCl solution (approximately one third of NaCl content
in eutonic solution at 40°C) also increases growth rate of KCl, which can reach up to
1.91 and 3.76 μm/min for ΔT = 1.0 and 2.0°C, respectively. The above figures exceed
KCl growth rates in eutonic solution (1.3 and 1.6 μm/min at ΔT = 1.0 and 2.0°C,
respectively). In other words, function of KCl growth rate versus NaCl content in the
solution has a maximum (see Fig. 2.5, curves 1 and 2).