Steel Slag

By optical microscopy, we still found a few particles about 250 µm length. For such a start size of particles, we recommend using 5 mm Ø grinding balls to be on the safe side. With smaller grinding balls, eventually largest fragments might stay intact.

Usually about 5-10 g of sample can be ground into lower nano range within one 80 ml grinding bowl. The finer the particles will be ground, the more of solvent needs to be added to maintain a motor oil like viscosity (optimum for grinding, too high viscosity and grinding balls will slowing down; too low viscosity and particles might slide off the grinding balls).

We started our trial with 15 grams of sample and the mentioned 5 mm Ø grinding balls. The mentioned water cannot been used to comminute the sample. Metal fragments (which sample might contain with its low magnetic properties), or abrasion of tungsten carbide might react with water during the grinding process, forming a constant gas pressure.

We recommend solvents with higher boiling points and lower viscosity for comminution. Test wise, we added 10 ml of ethanol to maintain a slurry for grinding.

To avoid over pressure, we ground the sample in steps of 2 minutes, followed by a programmed pausing time of 6 minutes. After several cycles, the outside temperature of the bowl should be checked (remain below 80°C); grinding time or programmed pausing time might be readjusted afterwards.

After 10 minutes, present fineness got checked with our Laser Particle Sizer ANALYSETTE 22 NanoTec plus. The present fineness is fulfilling to use smallest grinding WC balls of 0,6 mm Ø (see result two).

After sample was ground < 30 µm in result one, now the grinding balls were exchanged against 0,6 mm Ø hardmetal tungsten carbide grinding balls. Those balls should deliver finest possible grinding results.

It was observed that heating of the grinding bowl is significant faster and we set grinding time of just 1 minute with 7 minutes of programmed pausing time to proceed. It also has been observed that the total volume of sample is not rising as it uses to show typically. This way, even higher amounts of sample might be grindable per bowl.

Fineness got checked after a total of 30 minutes of grinding (+ 20 min with 0,6 mm dia. WC balls), a d50 < 200 nm was determined. The presented grinding result has been prepared without knowing the Mie parameters of sample (are required to achieve most reliable measurements). If the Mie parameters can be handed out, a more precise particles size distribution can be recalculated.

We proceeded comminution to the full hour and tried to check the particle size distribution again. Now it has shown that the nanoparticles flocculated instantly, forming much larger particles which have been sticking together (also possible when particles show magnetism again). We tried several additives to prepare a measurable suspension without success. A precise measurement couldn’t been performed. Most likely, also the packed sample will be flocculated when sample arrives at customer side.

To avoid flocculation, it will be beneficial to use a proper additive which keeps nano sized particles separated. A huge number of surfactants, also acids, salts or basis might come into question to optimize and stabilize grinding results. We aborted the trial at this point and packed the sample best possible.