1 Introduction
Solid–liquid separations are an important part of many industrial processes such as
papermaking, water treatment, or mineral processing.
For the separation of particles or unwanted components from a dispersion, it is
necessary to add flocculants. Salts, such as ferrous (III) or aluminum salts, which
were used as flocculants in the past, have many drawbacks such as high demand for
salt, the formation of small, unstable flocs, and a large volume of sludge. Therefore,
they were replaced by water-soluble polymers (or were used in combination with
them). Small polymer concentrations can produce large aggregates that can be
separated easily. Numerous flocculating agents with different chemical properties
are commercially available. Their flocculation mechanism as well as the results of
the separation process are influenced by the properties of the polymers, such as their
charge and molecular weight, and of the dispersed material. Nearly every
solid–liquid system is different from every other. Therefore, there is no general
rule on how to treat them. In different fields of application, the solid–liquid systems
can be extremely different. So, the particle size can range from a few nanometers up
to micrometers, or the solid content from parts per million up to 20%. The removal of
solids of nanometer-size range from the dispersion is a crucial stage in many
environmental technologies. Such colloidal particles are too small to be effectively
separated by filtration, flotation, or sedimentation. Therefore, the most effective way
to remove them is to cause the particles to flocculate so that larger units are formed.
As already mentioned, an effective separation can be realized by using only one
polymer (monoflocculation). But, because water is becoming an increasingly scarce
and limiting resource, the demand for treatment technologies has grown and, in
recent years, there has been considerable interest in cases where more than one
polymer is used. Such combinations of polymers can have significant benefits over
the use of single polymers. Moreover, we will show that some of the new challenges
in the industry can only be solved by using new types of flocculants.
We will discuss the possible interactions of such polymer mixtures from
polycations (PC) and polyanions (PA), which can form polyelectrolyte complexes
(PECs) or can be applied as “dual systems”.
There are some different possibilities for the appearance of PECs in flocculation
applications. The most important options are presented in Fig. 1.
The first option is the application of two-component flocculants of opposite
charge, which are added step by step (Fig. 1, top). During the flocculation process
an interaction can occur between the two flocculants PA and PC, resulting in the
formation of PECs, as well as between the polymer (mostly PC) and the suspension
(inorganic particles or fibers). A summary of former results and recent developments
will be presented in Sect. 2.
The complex formation between a (mostly negative) charged suspension and PC
(Fig. 1, center) will be described in Sect. 3. The “basic” type of flocculation, i.e., the
interaction between a negatively charged particle suspension and PC has often been
studied and is not the topic of this review. But, instead of a particle suspension, the PC
Polyelectrolyte Complexes in Flocculation Applications
29
Solid–liquid separations are an important part of many industrial processes such as
papermaking, water treatment, or mineral processing.
For the separation of particles or unwanted components from a dispersion, it is
necessary to add flocculants. Salts, such as ferrous (III) or aluminum salts, which
were used as flocculants in the past, have many drawbacks such as high demand for
salt, the formation of small, unstable flocs, and a large volume of sludge. Therefore,
they were replaced by water-soluble polymers (or were used in combination with
them). Small polymer concentrations can produce large aggregates that can be
separated easily. Numerous flocculating agents with different chemical properties
are commercially available. Their flocculation mechanism as well as the results of
the separation process are influenced by the properties of the polymers, such as their
charge and molecular weight, and of the dispersed material. Nearly every
solid–liquid system is different from every other. Therefore, there is no general
rule on how to treat them. In different fields of application, the solid–liquid systems
can be extremely different. So, the particle size can range from a few nanometers up
to micrometers, or the solid content from parts per million up to 20%. The removal of
solids of nanometer-size range from the dispersion is a crucial stage in many
environmental technologies. Such colloidal particles are too small to be effectively
separated by filtration, flotation, or sedimentation. Therefore, the most effective way
to remove them is to cause the particles to flocculate so that larger units are formed.
As already mentioned, an effective separation can be realized by using only one
polymer (monoflocculation). But, because water is becoming an increasingly scarce
and limiting resource, the demand for treatment technologies has grown and, in
recent years, there has been considerable interest in cases where more than one
polymer is used. Such combinations of polymers can have significant benefits over
the use of single polymers. Moreover, we will show that some of the new challenges
in the industry can only be solved by using new types of flocculants.
We will discuss the possible interactions of such polymer mixtures from
polycations (PC) and polyanions (PA), which can form polyelectrolyte complexes
(PECs) or can be applied as “dual systems”.
There are some different possibilities for the appearance of PECs in flocculation
applications. The most important options are presented in Fig. 1.
The first option is the application of two-component flocculants of opposite
charge, which are added step by step (Fig. 1, top). During the flocculation process
an interaction can occur between the two flocculants PA and PC, resulting in the
formation of PECs, as well as between the polymer (mostly PC) and the suspension
(inorganic particles or fibers). A summary of former results and recent developments
will be presented in Sect. 2.
The complex formation between a (mostly negative) charged suspension and PC
(Fig. 1, center) will be described in Sect. 3. The “basic” type of flocculation, i.e., the
interaction between a negatively charged particle suspension and PC has often been
studied and is not the topic of this review. But, instead of a particle suspension, the PC
Polyelectrolyte Complexes in Flocculation Applications
29