che_may-2024 - 26

TABLE 1. RELATIVE SELECTIVITY COEFFICIENTS
(H+ = 1) OF SAC RESINS AS A FUNCTION OF THE
DEGREE OF CROSSLINKING*
DVB
H+
Na+
NH4
+
Mg2+
Ca2+
Sr2+
Ba2+
4%
1
1.3
1.6
2.4
3.4
3.85
6.15
8%
1
1.5
1.95
2.5
3.9
4.95
8.7
12%
1
1.7
2.3
2.6
4.6
6.25
11.6
* Fewer fine and fewer coarse
beads leading to less ion leakage
and better regeneration performance
* Higher operating capacity due
to more uniform flow over the surface
and less tendency for channel
formation
* Lower pressure drop due to the
existence of evenly wide, unblocked
channels between the beads to enable
high flowrates
* Higher mechanical stability
due to homogeneous, optimized
functionalization - longer service
life, less generation of fines, which
would increase the pressure drop
* Higher osmotic shock stability
- especially important for
macroporous chelating resins, for
example when a chelating resin
loaded with calcium as obtained
from brine polishing for chlor-alkali
electrolysis (see above) is regenerated
with hydrochloric acid and
afterwards conditioned with caustic
soda solution. The latter causes osmotic
stress resulting from a 60%
increase in volume.
Today, most of the monodispersed
resins are based on polystyrene as
polymeric backbone. Polyacrylate
resins are mostly heterodispersed
due to the lack of an economically
feasible production process. However,
membrane emulsification processes
were developed in
recent
years for this purpose [23]. Currently,
most acrylate resins belong
to the WAC class of resins, where
this is not of crucial importance.
Morphology. Basically, two types of
resin can be distinguished in terms
of morphology (Figure 6). In gel-type
or microporous resins, on one hand,
26
16%
1
1.9
2.5
2.8
5.8
8.1
16.5
Note: DVB = divinylbenzene; SAC = strong acid cation exchange
the bead surface is covered
by a gel layer containing
the functional groups
that are easily available for
ion exchange. The micropores
are usually less than
2 nm in diameter. Gel-type
resins exhibit high operative
capacities. Typical
applications include acidcatalyzed
reactions
with
gel-type
SAC resins such
as dehydrations, (co-)condensations,
esterifications
and Friedel-Crafts-type
alkylations [24]. However, their surface
is sensitive to fouling, induced,
for example, by natural organic matter
(NOM), which makes access to
the functional groups more difficult.
In macroporous resins, on the
other hand, not only the bead surface,
but also wide channels of
more than 50 nm in diameter within
the bead are equipped with functional
groups. This leads to a markedly
increased active surface and
enhanced mechanical stability of
the beads. Because of their high
mechanical and osmotic stability,
they are employed in a variety of
processes, including those in nonaqueous
solvents, such as for binding
heavy metal ions. Whenever a
high total capacity and therefore a
high degree of cross-linking is required
and at the same time stable
resins are needed, there is no way
around macroporous resins. Even
relatively large contaminant molecules,
such as NOM,
can be adsorbed in
the pores and are
subsequently liberated
during regeneration.
Macr opor ous,
strongly basic anion-exchange
resins
based
on a crosslinked
polyacrylate can
be tailored to exhibit
a special pore structure
and resin matrix.
They are then ideally
suited for the capture
of high-molecularweight
compounds,
for example, for the
treatment and purification
of products
derived from biomass. This means
that liquid sugar syrups or complex
process solutions, such as fermentation
broths, can be purified and
treated. As an example, the naturally
occurring
glycosaminoglycan
polymer
heparin, which is used to
prevent blood coagulation, can be
extracted and thus
purified
with
such a resin [25, 26].
Bead size. The particle size of
monodispersed resins can be
adjusted with high precision by
means of continuous bead formation
through a perforated plate. In
aqueous
suspensions
containing
monomer droplets of uniform size,
the resin beads are then formed by
means of polymerization (Figure 7).
This method allows beads of different
sizes to be created in a flexible
and reproducible manner [27, 28].
To obtain ultrapure metals - ultimately
through electrolytic separation
- by means of hydrometallurgy,
interfering foreign ions have
to be removed right down to the
trace level. This presents special
requirements regarding selectivity,
capacity and exchange speed. The
size of the resin beads plays a key
role here.
Macroporous resins with a
sponge-like structure and a large
inner surface area are usually employed
here. The polymer beads in
standard resin types measure between
0.5 and 0.7 mm in diameter.
In addition to the type of functional
groups in the polymer, their suitabilFIGURE
7. Monodispersed ion-exchange resins are produced by
a two-stage process: continuous bead formation and subsequent
batch polymerization
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