Chemical Engineering March 2022 - 30

of impurity
* A typical feed has 1,000 mg/kg
of impurity
FIGURE 3. The ratio of mass of impurity in the inclusion to the mass of the crystal is plotted versus the ratio
of the diameter of the inclusion to the diameter of the crystal
steady state. Therefore, our focus
is on the mechanisms that are directly
controlled by the crystallization
process - agglomeration,
surface deposition and mother
liquor inclusions.
Agglomeration. When particles
aggregate during a crystallization
process to form larger agglomerates,
pockets of impurity-rich
mother liquor can become trapped
between the particles. Agglomeration
can be prevented by operating
with a low degree of supersaturation
and paying careful attention to the
circulation patterns of liquor within
the crystallizer.
Surface deposition. The amount
of mother liquor that is entrained
on the crystal surface decreases
relative to the crystal mass as the
diameter of the crystal increases.
A high-efficiency wash can be
used on a pusher centrifuge to
dilute
the
concentration
of
the
mother liquor by a factor of five or
more, thereby further reducing the
amount of impurity entrained on the
crystal product.
Inclusions. If crystals grow too
fast, then impurity-rich mother liquor
can become trapped within
the growing crystal. Measures
can be taken to restrict the rate
of growth, but sometimes it is inevitable.
It is possible to determine
the impact of these inclusions
based on the ratio of the
diameter of the inclusion to that of
the crystal. The case of nickel sulfate
hexahydrate, a common battery
chemical, is considered as an
illustrative example.
A nickel
sulfate
crystallizer
will
typically operate with a mother liquor
concentration up to 20 times
that of the feed. It is convenient to
express the concentration of impurity
in the feed relative to nickel
as the equivalent amount of nickel
sulfate hexahydrate, which is the
desired product. The concentration
can range from as low as 50 mg/kg
to as high as 2,000 mg/kg, depending
on the upstream process to
remove impurity. General purity
considerations are detailed below:
* A poor feed has 2,000 mg/kg
Operability
Crystal size
Energy requirements
Typical operating solids in
suspension
30
TABLE 1. COMPARISON OF CRYSTALLIZER DESIGNS [2 ]
Forced circulation (FC)
Excellent
Poor
Good
Oslo (Growth )
Satisfactory
Satisfactory
Good
20 to 25%
50% (In settling zone)
* A good feed has 100 mg/kg
* An excellent feed has 50 mg/kg
of impurity
In Figure 3, the ratio of mass of
impurity in the inclusion to the mass
of the crystal is plotted versus the
ratio of the diameter of the inclusion
to the diameter of the crystal.
If the desired product purity is four
nines, then a ratio of 10 mg of impurity
per kg of product is significant,
because this represents 10%
of the maximum allowance. For the
typical feed, this is achieved once
the ratio of diameters exceeds
0.1. We typically grow nickel sulfate
hexahydrate crystals with an
average size of 1,200 μm using a
DTB crystallizer. Thus, an inclusion
would have to exceed a 120-μm
dia. to have a significant influence
on the desired purity. In practice,
inclusions of this size are rarely observed.
The impact of inclusions
upon nickel sulfate hexahydrate
purity is therefore not considered
to be significant at the four nines
level for feeds that are typical or
better. For the poor feed, the effect
of an inclusion becomes significant
if the ratio of diameters exceeds
0.08, equivalent to a 96-μm
inclusion for the product from a
DTB crystallizer. Inclusions smaller
than 100-μm are sometimes observed
with consequent impact
on purity.
If the desired product purity is
five nines, then a ratio of 1 mg of
impurity per kg of product is significant,
because this represents 10%
of the maximum allowance. For the
typical feed, this is achieved once
the ratio of diameters exceeds
0.05, which is equivalent to a diameter
of 60 μm. Whereas for the
good feed, this is achieved once
the ratio of diameters exceeds
0.1,
which
Draft-tube baffle (DTB)
Good
Excellent
Better
30 to 40%
is
equivalent
to a diameter of 120 μm.
Consequently, the impact
of inclusions upon product
purity may become
significant at five nines
purity for feeds containing
more than 100 mg/kg
of impurity.
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