Straw sample

Presented sample was lightweight and very large. An active cyclone separator is required to speed up comminution immensely. Without the active air flow, fragments would need to be pushed manually into the grinding chamber. Also for higher amounts of sample which should be ground, the sample exhauster with > 2800 liters of air per minute, will chill sample material and grinding parts. Ground samples will be blown out of the grinding chamber within the moment they are capable to pass the sieve perforations of the used cassette.

By standard, we recommend to comminute a sample only as fine as required, not as fine as possible. This way, processing of the sample and the durability of grinding tool will be improved immensely.

Typically, a Universal Cutting Mill PULVERISETTE 19 comminutes samples to < 0,5 mm fast; a result < 0,25 mm requires a much longer grinding time, because the gap between static knives and rotor knives is about 0,2-0,25 mm.

With our first trial, we used the 0,5 mm sieve cassette to demonstrate a relatively fine grinding result with much higher throughput. Continuously, we fed sample into the standard funnel for one minute. After one minute, the grinding sound vanished rapidly. After 80 seconds, the mill and sample exhauster were stopped and the grinding chamber was inspected. Only a few particles remained on top of the 0,5 mm sieve cassette; few straws stuck inside some perforations of the sieve cassettes.

For this trial, we used one of our finest sieve cassettes for the Universal Cutting Mill. Usually sieve cassette perforations = 0,25 mm are only recommend for brittle materials like coal or coke.

We fed the sample for a total grinding time of 3,5 minutes. After the first minute of grinding, the air flow started to collapse. This happens when lightweight sample remained attached to the sieve cassette by the vacuum of the sample exhauster. It was required to take off and on the hose of the vacuum cleaner during the grinding process.

After 3,5 minutes, we stopped feeding of sample. It took relatively long until the grinding sound vanished. After 4 minutes, the machine was stopped. High residues of stronger attached straw dust remained inside the grinding chamber. Cleaning effort should be much higher, compared to result one. It would be required to wipe off the sticking dust with damped piece of cloth.

The output which was found inside the cyclones collecting glass was much finer, compared to result one, because the sample remained much longer inside the grinding chamber.

We don’t recommend grinding higher amounts this way. Sample should only be ground < 0,5 mm when up to 30 liters of sample should be ground per batch.

A Variable Speed Rotor Mill PULVERISETTE 14 premium line can be converted into a cutting mill. With up to 18.000 rpm (instead of 3000 rpm), samples can often be ground to < 0,2 mm. For this mill, a precutting of the sample is required to pass the feeding funnel are. The max. feed size should not be > 1 cm.

For precutting, we used the Universal Cutting Mill PULVERISETTE 19 and equipped the mill with a sieve cassette with 0,75 mm trapezoidal perforations. We did not payed attention to the maximum throughput, the sample was used to remove traces of sticking dust from the grinding chamber (from result 02). Usually we would recommend feeding of soft wood to bind fatty or creasy residues and minimize cleaning effort.

We fed sample continuously for 7 minutes when ~ 4 liters of bulk output were found inside the cyclones collecting glass. The mill was stopped after ~ 7,5 minutes (no grinding sound recognized at that moment). The grinding chamber was almost clean. Only a few traces of sticking fine dust were left, mainly located on the feeding funnel side. We vacuumed the grinding chamber and used a simple moistened piece of cloth to wipe off the present residues.

Surely much higher amounts could be ground this way, too. A batch of this kind of sample will be used for the fine grinding trial with Variable Speed Rotor Mill PULVERISETTE 14 premium line (see result 04).

After precutting the straw sample in result 03, the sample could be fed directly into the Variable Speed Rotor Mill PULVERISETTE 14 premium line which was converted into a cutting mill. We used a sieve shell set with 0,2 mm perforations directly. Even coarser precutting results of straw should be possible to feed.

We prepared a batch of 130 grams and slowly fed the sample into the feeding funnel. It took exactly 2,5 minutes until the complete sample was transferred into the grinding chamber. The sound of grinding was typical and vanished directly after the last fragments were fed.

The grinding chamber was not significantly clogged, also the sieve shell set was only lightly polluted with fine sample dust. Surly much higher amounts of the sample could be ground this way, too.

For higher amounts, it is possible to use a Vibratory Feeder LABORETTE 24 next to the mill to automatize the grinding process. Compared to the result 02, much more sample passed the sieve cassette longitudinal and the fineness seemed larger.