The quality of the mineral material is determined by the degree of dispersion and crystal-chemical characteristics (topochemical) of the surface of its particles. The crushing of coarse-grained mineral materials ensures the production of particles of different sizes and shapes, which allows them to be divided into fractions. When grinding fine-grained materials, the specific surface area increases and its physico-chemical and chemical activity increases. The consequent decrease in particle size in the process of grinding minerals and rocks is accompanied by a rapid increase in their total and specific surface (see Figure 2.2). With the increase in the specific surface area of ​​materials, its potential energies and the ability to transfer to a different phase, for example, by dissolving particles, increase. During mechanical grinding, some chemical bonds are broken with the formation of groups of free radicals and free ions with uncompensated charges on the surface of the particles, for example, Ca * cations and complex anions (COS) upon breaking bonds between them in the crystal lattice of calcite, or by the appearance of unsaturated cations Ca * and anionic tetrahedral groups SO 4 with the crushing of gypsum. The resulting particles - fragments of crystal lattices - become complex spatial systems interacting with the external environment as complex electric fields whose sign and magnitude depend on the chemical composition of the substance, the nature of the structure and the size of the particles. Freshly formed surface of mineral particles has an increased reactivity, and it can be charged predominantly positively, as, for example, in calcite, or negatively, as in quartz, or be neutral, as in graphite.

Powders with different crystallochemical features can be obtained by dry grinding of mineral materials: a) with a high potential of a positive sign and a large number of adsorption centers in the form of Ca * and Mg * cations on the surface of particles - from calcite, dolomite, limestone; b) with a high potential of a negative sign and a significant number of adsorption centers in the form of O- ions from quartz, silicon, kaolinite, granite, trachyte, volcanic tuff; c) with a reduced negative sign potential due to the presence of

on the surface of their cation particles with different valences of K, Na, Cu, Mg Fe Fe, etc. - in the grinding of feldspar, mica, hornblende, augite, asbestos, hydromica, gypsum, gabbro , diabase, etc .; d) with a predominantly neutral particle surface - obtained from talc and graphite. Partial breaking of chemical bonds, caused by the grinding of materials, contributes to the appearance of chemical centers with increased activity on the surface of particles, the effectiveness of which is determined by the nature and composition of the latter when interacting with environmental reagents (water, alkaline and acid solutions, etc.). Usually, favorable conditions are created for the occurrence of physicochemical processes at the interface of phases in the form of wetting, adsorption, dissolution, etc.

One of the prerequisites for a reasonable choice of source material is the prediction of the energy properties of its surface in a highly dispersed powdered state, including a change in the sign of the potential at the interface. An example of preservation or modification of crystallochemical features of highly disperse mineral materials can be the use of dry freshly prepared chrysotile-asbestos powder in two-component bituminous mineral mixtures or in complex systems - in the production of asbestos-cement products by the wet method. In the first case, when preparing an asphalt substance based on bitumen, the dry chrysotile-asbestos powder, when combined with the latter, retains its slightly reduced negative surface potential of the particles, which does not provide sufficient enough interaction with the surfactant (free asphaltenic acids, asphaltenes) of bitumen at the interface is somewhat compensated by the mechanical reinforcement of the bituminous mineral mixture with the finest (up to 0.1 p and less) elastic fibers of asbestos.

The subsequent contact of bitumen-asbestos material with water, i.e., the appearance of a new (aqueous) phase in this system, is accompanied by a sharp decrease in its structural and mechanical properties. A significant part of the film bitumen, weakly bound to the surface of the asbestos particles, is again transferred to the free state by polar water molecules, which simultaneously hydrate the released surface of the asbestos particles. Surface active agents of bitumen, which lost adsorption bond with asbestos particles, become active hydrophilic centers in the system, especially strongly hydrating and deteriorating its properties with increasing dispersity and quantity of asbestos powder. It is possible to change the negative sign of the potential of the surface of chrysotile-asbestos particles to positive as a result of its recharging. The latter can be caused by the preferential cleavage of ions (OH) - from their surface by water molecules and the resulting excess of positive ions Mg arising on it.

In the production of asbestos-cement products by wet method, chrysotile-asbestos is an integral part of a complex asbestos-cement-water solution of gypsum, lime and alkalis. The properties of its surface begin to change rapidly on the boundary with water, alkaline or other media, acquiring a positive potential, about 100 mv, due to the recharge of the surface layer of asbestos particles (fibers), which consist of hydroxyl groups (OH) -, connected to the adjacent inner layer of Mg ions. The significant surface activity of these alkali groups, when they partially dissolve or attract ions of the opposite sign, gives rise to a pronounced positive charge of chrysotile asbestos. These phenomena are related to the efficiency of the technological process for obtaining asbestos-cement products and its quality, which are determined by the nature of the reactions taking place at the interface of the components in the system and depend on the filtering capacity of the asbestos-cement suspension, i.e., on the surface properties of solid asbestos particles and the solution in which they are dispersed. The dimensions of the sedimentation volume during the filtration of the asbestos-cement suspension are related by the inverse relationship to the value of the surface charges of the particles. In the case of a chrysotile-asbestos component with its relatively large charge, there is a significant repulsion of the particles, preventing their coalescence, and small sedimentation volumes with dense packing of solid particles, reduced water permeability and an increased tendency to self-compacting occur that prevent the adjustment of product density.

When using chrysotile asbestos from the upper horizons of deposits, its external brucite layer Mg (OH ) 2 may be disrupted due to weathering. The deeper layer of silicon-oxygen tetrahedra then enters into the interaction with the liquid medium, which causes the formation of the thinnest layer of silicic acid HjSiCb, which dissociates with the elimination of predominantly H ions. The remaining Si 03 ions on the particle surface inform them of a negative charge, and the exposed The weathering of chrysotile asbestos along the potential sign resembles am phybol asbestos. This last type of asbestos is characterized by a small negative charge and the ability to form randomly oriented mesh asbestos-cement structures with good filtration properties (very important in the production of asbestos cement).

When grinding mineral materials, the rational limit of the degree of dispersion is established experimentally. By this excess, the energy activity of the surface increases so much that spontaneous aggregation of particles occurs with the appearance of lumpiness, a decrease in the specific surface and homogeneity. There is a growing danger of loss of surface activity of powdered material in the period of its long storage, which reduces the strength of adhesion of particles to binders. It will be necessary to introduce a surfactant into the grinding unit to shield with their help the emerging new surface with increased energy activity and, possibly, with a different electric charge.

Increase in the energy activity of mineral natural substances occurs not only as a result of forced dispersion during mechanical grinding of minerals and rocks. Studies carried out on the by-products of the Kursk magnetic anomaly, obtained during the development of minerals from great depths of quarries (deeper than 500 m), showed their increased energy and chemical activity. In natural conditions, it arises from the corrosion of rocks, especially quartz-bearing rocks, the formation of metamorphosed structures under the influence of high pressures and temperatures, the appearance of defects and a significant reduction in the structural ordering of crystals with complete or partial destruction of the crystal lattices of rock-forming minerals. Synthesis of new chemical compounds in the solid phase involving fine-grained quartz with a corroded surface in the thickness of the weathering crust was also noted.

thematic pictures

Also We Can Offer!

Other services that we offer

If you don’t see the necessary subject, paper type, or topic in our list of available services and examples, don’t worry! We have a number of other academic disciplines to suit the needs of anyone who visits this website looking for help.

How to ...

We made your life easier with putting together a big number of articles and guidelines on how to plan and write different types of assignments (Essay, Research Paper, Dissertation etc)