Ultrafine Grinding vertical roller mill

Nanotechnology is one of the most innovative developments of our time which revolutionises industries such as materials science, pharmaceutics, food, pigments or semi-conductor technology. Nanotechnology deals with particles in a range from 1 to 100 nm. These particles possess special properties due to their size, as their surface is greatly enlarged in relation to their volume (so-called ‘size-induced functionalities’). Ultrafine particles are, for example, harder and more break-resistant than larger particles. Nanotechnology brings effects which occur in nature to a commercial scale, such as, for example, the lotus effect: nanocoated fabrics or paints are water- and dirt-repellent just like the lotus flower.

How are Nano Particles Produced? The ‘Bottom-Up’ method synthesises particles from atoms or molecules. The ‘Top-Down’ method involves reducing the size of larger particles to nanoscale, for example with laboratory mills. Due to their significantly enlarged surface in relation to the volume, small particles are drawn to each other by their electrostatic charges. Nano particles are produced by colloidal grinding which involves dispersion of the particles in liquid to neutralise the surface charges. Both water and alcohol can be used as dispersion medium, depending on the sample material. In some cases the neutralisation of surface charges is only possible by adding a buffer such as sodium phosphate or molecules with longer uncharged tails such as diaminopimelic acid (electrostatic or steric stabilisation).

Figure 1. Neutralisation of charged particles by adding a buffer (electrostatic stabilisation, left) or by adding long-chained molecules (steric stabilisation, right) Factors such as energy input and size reduction principle make vertical roller mill the best choice for the production of nanoparticles. The most important criteria for selecting a mill and appropriate accessories are:

• • Material of the grinding tools
• • Grinding ball size
• • Grinding balls/sample/dispersant ratio
• • Grinding time • Energy input

Top-Down Method: Production of Nanoparticles with vertical roller mill Nanoparticles are created with the Top-Down method by colloidal grinding using a suitable dispersant to keep the particles from agglomerating. To reduce small particles with mechanical force to even smaller sizes, a high energy input is required. The choice of suitable grinding tools and the correct grinding jar filling are further aspects to be considered.

Figure 2. The steps of colloidal grinding Preliminary Grinding Depending on the size of the raw material and the desired end fineness it is advisable to prepare the sample first. In dry grinding, grinding balls Ø > 3 mm are usually used. A third of the total volume of the grinding jar is balls, a third the sample and the final third free space to allow movement of the balls. The crushed sample material (approx. < 10 μm) is then used for actual colloid grinding

Colloidal Grinding With the planetary vertical roller mill and the new high energy ball mill Emax, Retsch offers two types of vertical roller mill which provide the required energy input for colloidal grinding down to the nanometre 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.

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Created on Dec 31st 1969 18:00. Viewed 0 times.

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