Retsch MM 500 Vario, Manual

Page: 41 / 87

Operating the Device
41
WARNING
W9.0000
Risk of injury caused by liquid nitrogen and dry ice
Use of liquid nitrogen and dry ice in closed grinding jars
- Liquid nitrogen and dry ice expand and generate high pressure in closed
receptacles. This high pressure bursts grinding jars and leads to serious
injuries.
•
Under no circumstances should you put liquid nitrogen or dry ice
into the grinding jar and then close the jar.
•
Only undertake indirect embrittlement for cryogenic grinding.
Materials that can only be ground with difficulty or not at all under normal temperatures must be
ground cold. Indirect embrittlement with liquid nitrogen ( - 196 °C) improves the fracture
behaviour of thermoplastics, rubber products, greasy foodstuffs, pharmaceuticals etc.

For cryogenic grinding, Retsch GmbH offers the Cryo Kit (Order Number: 22.354.0003) for
cooling the grinding jars with liquid nitrogen.
Carry out the embrittlement of elastic and tough sample material as follows:

The embrittlement of sample material for grinding must be performed indirectly.

Only use grinding balls and grinding jars made of stainless steel or hardened steel for
cryogenic grinding.
Grinding balls and grinding jars made of zirconium oxide or tungsten carbide are not
suitable for grinding at very low temperatures.

The sample material must be filled together with the grinding balls (steel) in a grinding jar
(steel), and the grinding jar (steel) firmly sealed.

Using the Cryo Kit tongs, the firmly sealed grinding jar (steel) is then dipped into a bath of
liquid nitrogen until this stops bubbling.

The sample material inside the grinding jar (steel) is then likewise cooled and ready for
grinding.

Under no circumstances should you fill liquid nitrogen or dry ice into the grinding jar and
then close it. The high pressure generated in the grinding jar would burst it.
6.4.2 Wet Grinding with Highly Flammable Materials
Wet grinding using highly flammable materials is permitted with this device if certain
precautionary measures are complied with.
When using highly flammable materials such as hexane, isopropyl, ethanol, benzine etc. as a
grinding aid , the inside of the grinding jars should be classed as Zone 0, i.e. as a permanent
explosive mixture.
It is therefore necessary to prevent potentially explosive vapours escaping from the clamped
grinding jars during a grinding process or being able to reach places which have the necessary
ignition energy. These vapours are in particular also pressed outwards by the temperature rise
that takes place and by the consequent increase in pressure inside the grinding jar.
For this reason we urgently recommend that the user of the device (the employer) assesses the
existing hazards within a coherent explosion protection concept according to local conditions
before using such solvents and, where necessary, records supplementary organisational
measures in an explosion protection document.