Relationship between structure and hydration process in heat-activated serpentine-group minerals (antigorite, chrysotile, and lizardite)

Metal ore mining and beneficiation led to the formation of mine waters and artificial effluents with high metal concentrations, which need cleaning to prevent vertical and lateral metal propagation in the environment. Metal precipitation in the form of hydroxides is the most common method of wastewater purification with high metal concentrations. Serpentines are common in the Earth’s crust and often are by-products of overburden and enclosing rocks. Unlimited reserves of serpentines have stimulated the search for new technology for processing these raw materials. Serpentine structure and properties allow the production of materials used in environmental management. Serpentines heat treatment increases the ability to neutralize acids and precipitate metals from water solutions. The ability of heat-activated serpentines to form a binder through water mixing allows producing granular material. It can be used as an alkaline reagent in a bulk filter to purify highly concentrated solutions with the possibility of separate precipitation of metals.

The influence of the type of serpentine mineral (antigorite, chrysotile, and lizardite) on the hydration of thermally activated materials and the formation of magnesium silicate binder was studied. The serpentine samples were studied using X-ray diffraction analysis, differential scanning calorimetry, and surface texture analysis. The hydration of heat-activated serpentines through their interaction with water vapor and the strength characteristics of the resulting binder agents were investigated. The results show an essential role of serpentine structure in destroying mineral crystal lattices during heat treatment. The lower the activation energy of dehydroxylation, the higher the transformation of serpentines into the active metastable phase. It was found that thermo-antigorite does not sorb water, in contrast to thermally activated chrysotile and lizardite. The acid-neutralizing ability of latter minerals significantly differed with the higher values for thermo-chrysotile. The weight loss of hydrated samples at the temperature of 350-600℃ decreased in the same sequence – from chrysotile to antigorite. Therefore, this characteristic could be considered an indirect indicator of the total content and precursor of binder formed during the thermo-serpentine hydration.

In contrast, the strength of the samples based on heat-activated serpentines decreased in the row chrysotile–antigorite–lizardite. The structural features of chrysotile determined the greatest strength of serpentine binder samples compared with antigorite and lizardite. Lizardite acid-neutralizing ability (activity) was noticeably higher than antigorite, but its strength was lower due to the layered mineral structure and the presence of impurities reducing the strength of the resulting material.

Thus, the structural features of serpentines played a crucial role in the mineral hydration and, as a result, in selecting a material for producing a granular magnesium silicate reagent. Chrysotile is a promising mineral for obtaining granular materials, whereas lizardite is advisable to use in fractionated powders. Antigorite differs from the other two serpentines because it has a less acid-neutralizing ability and can be used for magnesia and silicate products.

The work was supported by the Russian Science Foundation project #21-77-10111.