Gypsum Cardboard

A dry grinding of samples was limited in fineness in the past. With only a few expectations (e.g. sulfides or carbon samples with dry lubricant properties), all samples started sticking when a certain level of fineness was reached.

Typically, when a d50 value < 20-30 µm was reached, dry ground samples would become compressed. This happened when the surface-active forces become bigger as the particles own g-force. They stay attached to available surfaces and would not fall back to the bottom of the vessel. Often, compressed sample also contained particles > 100 µm which would become embedded into those clusters and would not be ground any further. Crystalline water inside the gypsum could promote a rapid sticking of sample additionally.

Before the sample could be fed into the desired Mortar Grinder PULVERISETTE 2, a pre-crushing of the ~ 11 cm long and wide pieces was required. We used a gardening scissors to pre-cut the sample to mainly < 15 mm.

For dry grinding, too much of sample could build up in front of the scraper and might become pushed above the edge of the grinding set. We started our trial with 58 grams which included gypsum and cardboard fragments as well. Surely it would be beneficial to separate the paper; if paper would be red, we estimate that the gypsum minerals can help to rupture the paper fibers.

To feed the largest fragments, the scraper was lifted and turned by 90° to allow a passing of the ~ 15 mm long and wide pieces. The pestle was placed into the center of the grinding set with 0 daN downforce. The fragments were fed by the window-opening inside the lid. It took about 60 seconds until all sample was transferred into the mill. Now we slowly started to move the pestle in its position. Afterwards, the scraper was placed into the radius of the mortar. After 3 minutes, we packed about 8 grams of ground sample for demonstration.

Within the third minute, the scraper started “jumping” above lightly compressed sample. We modified the angle and position several times. The trial was aborted after a total dry grinding time of 10 minutes.

A few paper fragments were still visible, such a sample would not become ground sufficiently when the sample would become compressed already.

We checked the particle size distribution by static light scattering and still found > 100 µm particles as expected.

To eliminate sticking effects, it is possible to grind a sample in suspension. With enough of a liquid phase to cover all particles (only water should be used with our PULVERISETTE 2), even finest particles would not attach to the available surfaces.

We used the “sample 2” marked piece of cardboard and placed it in a basin of water for about 10 minutes. Afterwards, the top layers of paper could be peeled off (still paper was attached to the front and back of the mill).

The piece was broken into smaller fragments relatively easy. Now we also spotted fine, very thin fibers inside the sample (we expect that those are glass fibers to enforce the gypsum).

The grinding was performed with the same pestle and scraper settings as shown in result 01. After we fed the < 15 mm fragments within 60 seconds, we added about 100 grams of water to the sample to maintain a slurry with ~ motor oil like viscosity.

We ground the sample for a total grinding time of 30 minutes and still spotted some larger fragments inside the paste. Even after 60 minutes of grinding time, some larger fragments (most likely made of paper) were spotted. We checked a small amount of sample by optical microscopy.

As expected, a fragment of ~ 500 µm length of pulp was spotted. The majority of gypsum was comminuted < 40 µm as desired and finer. Only a few fragment > 50 µm were spotted.

Surely a planetary mill or a Vibrating Cup Mill PULVERISETTE 9 would be much faster in comminution. Both types of sample require a pre-crushing and a grinding in suspension to reach the desired level of fineness.