Producing nano particles by colloidal grinding

So-called colloidal or nano grindings are usually carried out as wet grindings. To reduce already small particles further in size by using mechanical force, a high energy input is required. This input is provided by RETSCH’s Emax and planetary ball mills. Further important criteria for nano grindings are suitable grinding tools and optimum grinding ball fillings.

 

Preliminary grinding

 

Depending on the size of the initial sample material and desired final fineness, a preliminary size reduction step can be useful. A dry grinding process with grinding balls of >3 mm Ø is usually carried out by filling one third of the jar with grinding balls and one third with sample material. The obtained sample is then used for the actual colloidal process.

With the planetary ball mills and the new Emax, RETSCH offers two types of ball mills which provide the required energy input for colloidal grinding down to the nanometer range. Grinding jars and balls made of an abrasion-resistant material such as zirconium oxide are best suited for this type of application. 60 % of the grinding jar volume is filled with grinding balls of 0.5 to 3 mm Ø, providing a large number of frictional points. The actual sample fills about one third of the jar volume. By adding a suitable dispersant (e. g. water, isopropanol, buffer), the consistency of the sample should become pasty thus providing ideal preconditions for colloidal grinding. If a very high final fineness is required, it is recommended to proceed with a second colloidal grinding with 0.1 to 0.5 mm Ø grinding balls, particularly if 2 to 3 mm balls were used in the first process (the balls need to be 3 x bigger than the particle size of the initial material). To separate the sample from the grinding balls, both are put on a sieve (with aperture sizes 20 to 50 % smaller than the balls) with a collecting pan. For the subsequent colloidal grinding 60 % of the jar is filled with small beads. The suspension from the previous grinding is carefully mixed with the grinding beads until a pasty consistency is obtained.

If a sample tends to swell during wet grinding, the consistency of the sample-grinding ball mixture should be checked during the process to add more dispersant if required. If the material is known to swell easily, it should be diluted more strongly before the grinding is started.

Care must be taken when removing the grinding jar from the planetary ball mill as it can have a temperature of up to 150 °C due to the heat generated during the grinding process. Moreover, pressure builds up inside the grinding jar. Therefore, it is recommendable to use the optional safety closure for the “comfort” grinding jars of the PM series which allows for safe removal of the jar. After the grinding process the jar should cool down for a while. The Emax jar already has an integrated safety closure. Moreover, the effective cooling system of the Emax prevents the jars from heating up too much. Both jars can be equipped with optional aeration covers which allow working under inert atmosphere.

We are pleased to present RETSCH, a provider of Colloidal Grindring Equipment:

 

RETSCH is active in the fields of homogenizing laboratory samples for analysis as well as particle size analysis of solid substances. On the basis of more than 95 years' experience we have developed into the leading instrument manufacturer in these technologies.