NiMH batteries

In our first trial, the complete sample has been pre ground with our Universal Cutting Mill PULVERISETTE 19 to < 1,5 mm. The smaller the sample will be pre ground, the better the fine grinding will be done.

For a fast output, the 1,5 mm sieve cassette has been chosen. After grinding, only a few fibers use to be left inside of the grinding chamber.

The sieve cassettes openings still remain free of residues. For this, a grinding of higher amounts should still be possible. Also the usage of a small sieve cassette is plausible.

Before grinding, we tested the pH-value of small black fragments and found, that the sample uses to be very alkaline. To prevent corrosion on grinding parts, we always recommend washing and drying alkaline samples until the pH-value will be neutral before feeding. In this trial, we wiped off all grinding parts with a damped cloth and finally with alcohol.

Sample which has been pre ground in this trail will be used for fine grinding tests in results 2-4.

Sample should be brittle to get ground with a planetary mill. We are expecting that ductile samples (like nickel) will form thin flakes which will only be flattened out. So the thickness of particles might become < 100 µm as desired, but a total length of flakes up to 2-3 mm use to be typical for metal samples.

With a planetary mill, several grinding materials (ceramics, hardmetal tungsten carbide, steel, etc.) are available. In reference to abrasion, a grinding with a planetary mill should be considered. To maintain enough impact force for a proper comminution of biggest particles, we recommend choosing materials with higher density (like zirconium oxide, steel or hardmetal tungsten carbide).

After the first minute of dry grinding, sample use to stick strongly the grinding balls and bowl. Probably moisture content of sample is still too high. To proceed grinding, we have added 15 ml of ethanol (EtOH) to eliminate sticking effects and proceeded grinding. Fineness after a total grinding time of 10 minutes appears to be equal to the 5 minute result (checked by optical microscopy). For this, the grinding trial has been aborted.

As expected, still thin nickel flakes with a length about 2-3 mm can be found after 10 minutes of grinding. Grinding balls surface appears to be polished after this trial.

Used solvent can be removed rapidly by using a rotary vacuum evaporator (be aware of reagglomeration of smallest particles when drying up).

With a Variable Speed Rotor Mill PULVERISETTE 14, typically > 50% of the ground particles will be smaller than half of the openings size. With a 0,25 mm sieve ring, d50 < 125 µm is expected to be found.

After a total of 50 seconds, all sample has been fed to the feeding funnel.

Grinding sound indicates that mainly shearing force might comminute the sample (not the impact force of the rotor).

High ratio of sample can be found inside of the collecting pan. A few lightweight fibers are laying in front of the impact rotor ribs. These not ground particles can be fed a second time for a proper comminution.

By feeding of metal particles, we expect that higher wear off should be found compared to standard applications.

After a total of 50 seconds, all sample has been fed to the feeding funnel. Grinding sound indicates that mainly shearing force might comminute the sample (not the impact force of the rotor).

High ratio of sample can be found inside of the collecting pan. A few lightweight fibers are laying in front of the impact rotors ribs and have been found a second time (+ 20s).

We checked the weight of rotor and sieve rings (of result 3 & 4) and found a loss of only 0,01 g on each sieve ring. That could be caused by the impreciseness of the used balance.

Nevertheless, a grinding of metal particles will cause abrasion which will be found sooner or later. We cannot give a forecast of the durability of rotor, sieve ring or labyrinth disk {which is made of aluminum} – it might probably last for several month by a daily grinding of only 10 g of sample). The smaller the metal particles can be pre ground before feeding, the longer the durability of grinding parts should be.