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E. Lukas et al.
1 Introduction
The treatment of raw materials, intermediates, products, and waste is one of the
most important processes for many industrial applications. Usually, the final process
outcome depends strongly on the quality of separation. Separation by particle size is
mostly done by sieving. For lower cut points (for a particle diameter in the order of
mm), air classifying performs better because fine particles often adhere to and block
the openings of the sieves. One apparatus for air classifying is the zigzag air classifier.
This system has been known for a long time [1–3]. It is widely used for a variety of
industrial applications, e.g. for classifying shredded PET bottles [4], municipal solid
waste [5], scrap cables, or stalks and leaves for the tea and tobacco industry [6].
The main advantage of multi-stage air classifying is the wide range of possible
cut sizes, in the range of micrometers to several millimeters. Sorting (separation by
density) can be done within a wide density range as well. Separation is done based
on the differences in settling velocity, which is the main characteristic parameter and
is determined by particle properties as size, density, and shape [6].
Obviously, the air velocity in any separation chamber impacts the particle dynamics, in particular with respect to the flow direction. Due to this fact, the zigzag air
classifier has a wide field of possible applications. The mass flux of one stage in practical applications is typically between 5 and 15 t/(m h); an even higher throughput
can be reached by using several channels in parallel. The number of stages affects the
separation performance since, at every stage, separation of fine and coarse (or light
and heavy) particles occurs as the particles flow across the air stream. Therefore,
every particle that leaves the channel has been separated repeatedly, which leads to
a high operational efficiency. Additionally, the process can be linked to pneumatic
conveying without the requirements for any additional device [6].
However, problems are also encountered when using a zigzag air classifier (ZAC).
In particular, variations in properties of the feed (in size, density, or shape) e.g. due
to segregation in the silo, eventually lead to local and temporal fluctuations of the
mass loading of air. This affects the efficiency of the separation in a negative way.
Due to these fluctuations, pulsations of the air stream may also be observed. All in
all, the unknown dynamics of the process lead to insufficient reliability of the ZAC
operation, reducing separation efficiency. Though the efficiency might be increased
again by employing a larger number of stages, those cause additional pressure loss,
increasing energy consumption. As a consequence of this trade-off, it is expected
that an optimal number of separation stages exists for a given process.
Since ZACs have been used for many decades, they have been the topic of several
investigations. Selected publications are discussed in what follows. For instance,
Worrell and Vesilind [7] investigated the separation performance of different air
classifiers based on various throat configurations. They used municipal solid waste
to separate light (paper and plastics) and heavy (aluminum and steel) materials and
introduced a new concept to evaluate the operational efficiency. The total efficiency
was defined as the product of the fractional recoveries of light and heavy material in
the overflow and underflow. Therefore, the highest efficiency of 100% can only be
E. Lukas et al.
1 Introduction
The treatment of raw materials, intermediates, products, and waste is one of the
most important processes for many industrial applications. Usually, the final process
outcome depends strongly on the quality of separation. Separation by particle size is
mostly done by sieving. For lower cut points (for a particle diameter in the order of
mm), air classifying performs better because fine particles often adhere to and block
the openings of the sieves. One apparatus for air classifying is the zigzag air classifier.
This system has been known for a long time [1–3]. It is widely used for a variety of
industrial applications, e.g. for classifying shredded PET bottles [4], municipal solid
waste [5], scrap cables, or stalks and leaves for the tea and tobacco industry [6].
The main advantage of multi-stage air classifying is the wide range of possible
cut sizes, in the range of micrometers to several millimeters. Sorting (separation by
density) can be done within a wide density range as well. Separation is done based
on the differences in settling velocity, which is the main characteristic parameter and
is determined by particle properties as size, density, and shape [6].
Obviously, the air velocity in any separation chamber impacts the particle dynamics, in particular with respect to the flow direction. Due to this fact, the zigzag air
classifier has a wide field of possible applications. The mass flux of one stage in practical applications is typically between 5 and 15 t/(m h); an even higher throughput
can be reached by using several channels in parallel. The number of stages affects the
separation performance since, at every stage, separation of fine and coarse (or light
and heavy) particles occurs as the particles flow across the air stream. Therefore,
every particle that leaves the channel has been separated repeatedly, which leads to
a high operational efficiency. Additionally, the process can be linked to pneumatic
conveying without the requirements for any additional device [6].
However, problems are also encountered when using a zigzag air classifier (ZAC).
In particular, variations in properties of the feed (in size, density, or shape) e.g. due
to segregation in the silo, eventually lead to local and temporal fluctuations of the
mass loading of air. This affects the efficiency of the separation in a negative way.
Due to these fluctuations, pulsations of the air stream may also be observed. All in
all, the unknown dynamics of the process lead to insufficient reliability of the ZAC
operation, reducing separation efficiency. Though the efficiency might be increased
again by employing a larger number of stages, those cause additional pressure loss,
increasing energy consumption. As a consequence of this trade-off, it is expected
that an optimal number of separation stages exists for a given process.
Since ZACs have been used for many decades, they have been the topic of several
investigations. Selected publications are discussed in what follows. For instance,
Worrell and Vesilind [7] investigated the separation performance of different air
classifiers based on various throat configurations. They used municipal solid waste
to separate light (paper and plastics) and heavy (aluminum and steel) materials and
introduced a new concept to evaluate the operational efficiency. The total efficiency
was defined as the product of the fractional recoveries of light and heavy material in
the overflow and underflow. Therefore, the highest efficiency of 100% can only be