29-11-2012, 04:21 PM
Cryogenic grinding: an independent voice
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P robing the fundamentals
Conventional cryogenic grinding uses
liquid nitrogen to cool the material to be
ground, making it brittle and so reducing
the amount of energy required by the
mill (see box on page 20). It also
increases throughput – often by 100% or
more for the same particle size
distribution – or enables finer grinding at
the same throughput, and it tends to
narrow the product particle size
d i s t r i b u t i o n .
Liquid nitrogen consumption varies
from around 0.7 kg for every kg of
material ground, for low-value products
such as recycled rubber, right up to
2 0 kg/kg for difficult, high-value products
that cannot be ground by any other
method. Figure 1 shows some of the
factors affecting liquid nitrogen use.
One reason liquid nitrogen
consumption figures are so hard to pin
down, says Bertling, is that different users
get different performance from similar
equipment: “One plant may use 1 kg of
liquid nitrogen per kg of product, while
another may use three or five times as
much for the same product in an identical
mill. At the moment we are working on
different ideas for optimising the process.
Up to now there is not very much knowhow
in this area; mills are an old
technology, but at the theoretical level
they are quite poorly understood,
especially in cryogenic service.”
Grinding quickly, grinding cool
GRINDING is a pretty inefficient process: up to 99% of the
mechanical energy entering the mill ends up as heat. With
industrial mills typically in the size range 20–100 kW, cooling is
a significant issue – and for materials that deform or melt when
they are warmed, temperature rise can also be a problem.
Water or other liquids provide effective heat transfer, but not
all materials are suitable for wet grinding. Indirect water cooling
of the mill is of limited effectiveness because of the lack of heat
transfer area.
Most dry mills therefore rely on a large flow of air or
nitrogen to both cool and transport the product. But even with
a large airflow, some particles can reach temperatures of up to
300°C, which is often high enough to cause a significant loss of
q u a l i t y .
A clean and effective way to boost cooling is to inject liquid
nitrogen at a temperature of –196°C into the product upstream
of the grinding process. An example is the Cryo-grind system
offered by industrial gas supplier Air Products.
How low can you go?
Grinding processes using liquid nitrogen fall into two types:
temperature-controlled grinding and true cryogenic grinding. In
the first type, the liquid nitrogen is injected directly into the mill,
where it acts as a heat transfer medium rather than a refrigerant.
Liquid nitrogen injection rate is controlled by a sensor that
measures the temperature of the air and nitrogen leaving the
mill. The ground product leaves the mill at typically 10–30°C,
and at no stage does its temperature fall low enough to cause
e m b r i t t l e m e n t .
True cryogenic grinding is a different process designed to
exploit the tendency of many materials to become brittle at low
temperatures. In grinding plastics and rubbers, for example, the
stresses needed to break up the material are dissipated by
relaxation. As a result, plastics need 10–100 times as much
energy to grind as inorganic materials, or typically
1 0 0 – 1 0 0 0 kWh/t at room temperature for particles in the size
range 100–1000 μ m .