# A Systematic Study of the Cryogenic Milling of Chrysotile Asbestos
## 1 Introduction
In our recent work [21], a commercial Russian chrysotile extracted by Orenburg minerals (Yasniy) was extensively characterized and found to resemble the Italian Balingero chrysotile that has caused, and continues to cause, asbestos-related diseases in the exposed population [22]. This chrysotile is currently used for a myriad of products found in everyday life (e.g., textiles, paddings, sealing materials, frictional production, cardboard, and asbestos-cement products), and claimed to be safe by the mining industry due to the lack of conclusive scientific evidence linking exposure to chrysotile with MM. Hence, the continuous exploitation of Russian chrysotile may be an important health concern that, unfortunately, today, is in the shadow of financial gain. Although much progress has been made in enlightening the mechanism by which chrysotile induces cytod and genotoxic damage, work is still needed and surely also complicated by structural and chemical variabilities that are observed among fibres of different geological origin [21; 23]. Considering the major role played by fibre length in determining the toxicological action of asbestos, controlling this parameter is of paramount importance in targeted studies. Methods need to be developed that allow the preparation of sets of fibres that only differ in size. Many more or less sophisticated routes have been suggested for chrysotile, generally involving communition of pristine fibres followed by separation by size [24; 25; 26], although direct separation of pristine samples through sedimentation in water was also attempted [27]. However, the latter approach was shown to be rather inflexible, as only a fine fraction with controlled length was obtained (90% < 2 \(\upmu\)m) [27]. In fact, considering that the chrysotile fibres are very long (>10 \(\upmu\)m), reduction in fibre length before any separation process is preferred. The choice of communition method is of paramount importance, as the crystal-chemical properties of the fibres must not be altered. Table 1 lists the methods currently available for mechanically milling chrysotile fibres. Mechanical milling is one of the most common size reduction methods in general and has been extensively evaluatedas a potential route for the preparation of fibres with controlled lengths. Size reduction by mechanical milling occurs due to a combination of impact, compression and shear forces [28]. Both the type of equipment and experimental parameters highly influence the imposed forces and the consequent material's response. Many different types of instrumental setups have been used to systematically investigate the effect of mechanical action on chrysotile. These include dry milling in planetary ball mills [29; 30; 31; 32], milling in vibration ball mills in dry conditions as well as in the presence of hydrocarbons or water [33; 34; 35], dry milling in a vibration disc mill [36], and rotary mill (also called ring and puck mill or shutter box) [37]. Generally speaking, size reduction was accompanied by plastic deformation of the chrysotile structure and prolonged mechanical action eventually led to complete amorphization. Hence, alternative methods have been proposed that were shown to avoid structural-chemical modifications with respect to the pristine fibres: prolonged ultrasonification [34; 38], gentle hand-milling [26] and chopping [24]. A promising communication method that leaves intact the chemical-structural properties of chrysotile is cryogenic milling [23; 39], which exploits the brittle fracture of materials cooled down to cryogenic temperatures [40; 41; 42]. In the present study, the effect of cryogenic milling on the chemical-structural properties of a commercial Russian chrysotile (Orenburg minerals, Yasniy) was investigated for various milling times in order to prepare different length fractions of fibres for toxicological studies.