The various changes that occur in the crushing process of the crushed materials are insignificant compared to the coarse crushing process, but for the ultrafine crushing process, due to the high crushing strength, long crushing time, and large changes in material properties, It seems very important. This change in the crystal structure and physicochemical properties of the crushed material caused by mechanical ultrafine crushing is called the mechanochemical effect of crushing.
1. Changes in granularity
After superfine pulverization
, the most obvious change of the powder material is the finer particle size. According to different particle sizes, ultrafine powders are usually divided into: micron level (particle size 1-30μm), sub-micron level (particle size 1~0.1μm) and nanometer level (particle size 0.001~0.1μm).
2. Changes in crystal structure
In the process of ultra-fine grinding, due to the strong and persistent mechanical force, the powder material has different degrees of lattice distortion, the crystal grain size becomes smaller, the structure becomes disordered, the surface forms an amorphous or amorphous substance, and even occurs Polycrystalline conversion. These changes can be detected by X-ray diffraction, infrared spectroscopy, nuclear magnetic resonance, electron paramagnetic resonance, and differential calorimetry.
Taking kaolin and calcite for example here:
Kaolin layered silicate minerals (kaolin, mica, talc, bentonite, illite, etc.) lose their ordered crystal structure to varying degrees and become amorphous under mechanical activation during the ultrafine grinding process. Because the amorphous in these minerals is generally related to the dehydroxylation in the crystal structure and the decrease of bond energy. For example, the crystal structure of the well-crystallized kaolin, after 60 seconds of crushing, the crystal structure of the kaolin has undergone significant changes; after 120 seconds of crushing, the crystal structure of the well-crystallized kaolin is similar to ball clay; after 400 seconds, it is similar to the crystal structure of dickite.
The polycrystalline transformation of calcite is a structural change induced by mechanical force during the ultrafine grinding process that does not change the chemical composition of the material being ground. There are generally two forms: double denaturation transformation, which is usually reversible and endothermic; monodenaturation transformation, large Most are irreversible and exothermic. Calcite is converted into diamond-shaped aragonite during grinding. This transformation is unstable at room temperature and normal pressure, that is, the transformation between calcite and aragonite is reversible. After long time grinding of calcite or aragonite, the ratio of the two products is basically equal.
3. The change of chemical composition. Due to the strong mechanical activation, the material will directly undergo a chemical reaction in some cases during the ultrafine grinding process. Reaction types include decomposition, gas-solid, liquid-solid, solid-solid reaction and so on. Carbon dioxide is decomposed when grinding calcite, magnesite, iron dolomite, nepheline and iron spar in a vacuum mill; sodium carbonate, alkaline earth metals, nickel, pot, manganese, zinc and other carbonates also occur during grinding Decomposition; Zinc oxide and aluminum oxide are mixed in a vibrating ball mill to produce some spinel and amorphous zinc oxide powder.