# A Systematic Study of the Cryogenic Milling of Chrysotile Asbestos
## 2 Materials and Methods  
### Sample Preparation and Characterisation  
A commercial chrysotile sample from the Russian Federation, extracted by Orenburg Minerals from an open pit mine near the town Yasniy was used in this work. This particular sample was extensively characterised in our previous work wherefore only a short description will be given here [21]. The sample is composed of bundles of fibres up to 1 mm in length that easily split lengthwise into fine fibres and thin bundles. The length of a single fibre and thin bundles is in the rage of 1.36 to 188.1 um (mean 33.9 um), whereas the fibre width is in the 0.05-2.79 um range (average 0.7 um). Clino-chrysotile is the major phase, accompanied by minor amounts of ortho-chrysotile, lizardite, magnetite, hydromagnesite and calcite. The composition, determined by electron probe microanalysis (EPMA), was [(Mg\({}_{2.870}\)Fe\({}^{2+}\)\({}_{0.027}\)Al\({}_{0.034}\)Cr\({}_{0.005}\)Ni\({}_{0.006}\))\({}_{2.986}\)(OH)\({}_{4}\)Si\({}_{1.92}\)O\({}_{5}\)] (Fe\({}^{2+}\)/Fe\({}^{3+}\) ratio from Mossbauer spectroscopy).  
The milling experiments were conducted, using a Retsch mixer mill MM 400 (Dussendorf, Germany), equipped with two processing stations. This is the very same equipment that was used to prepare powder batches for various analyses in our previous work dealing  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**References** & **Milling Methods** & **Environmental Conditions** & **Advantages/Disadvantages** \\ \hline
[23] &
\begin{tabular}{c} Cryogenic vibro-milling with \\ steel balls \\ \end{tabular} & \(-150\)\({}^{\circ}\)C and wet condition & Preservation of chrysotile \\ \hline
[32] &
\begin{tabular}{c} Mechanical friction with \\ cylindrical milling elements \\ \end{tabular} & Room temperature and wet condition & Reduction in chrysotile crystallinity \\ \hline
[32] &
\begin{tabular}{c} Agate mortar \\ \end{tabular} & Room temperature and dry condition & Reduction in chrysotile crystallinity \\ \hline
[33; 35] & Ball milling & Room temperature and dry condition & Reduction in chrysotile crystallinity \\ \hline
[34; 35] & Ball milling & Room temperature and wet condition & Water-wet conditions preserves the chrysotile crystallinity \\ \hline
[36] &
\begin{tabular}{c} Ring mill equipped with a \\ tungsten carbide bowl \\ \end{tabular} & Room temperature and dry condition & Reduction in chrysotile crystallinity \\ \hline
[37] & Rotary mill & Room temperature and dry condition & Reduction in chrysotile crystallinity \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of different methods used for milling chrysotile.  
with this Russian sample [21]. The milling jars (35 mL) are made of steel and each one is accompanied with a steel milling ball (diameter of 20 mm, weight 14.24 g). The lining of the jar and balls is made of polytetrafluoroethylene (PTFE), which serves to prevent contamination of the sample with metals from the jar and balls during milling. The loaded jar is submerged in liquid nitrogen until acquiring the temperature of the surrounding medium and subsequently mounted in the mixer mill. During operation, the jar oscillates from side to side with a pre-set frequency (3-30 Hz). The amplitude of the oscillations is fixed. The repeated impaction of the ball against the embrittled material leads to size reduction. In the experiments performed in our laboratory, the jars were loaded with 1 g of raw chrysotile and an oscillation frequency of 30 Hz was applied for various durations, i.e., 5, 10, 20, 30, and 40 min. The investigated range of milling durations were chosen based on preliminary tests, evidencing too-low efficiency following durations of less than 5 min and a non-significant improvement of the size reduction for duration longer than 40 min. From now on, samples will be denoted by CHR (pristine chrysotile) followed by the milling duration if processed.