Concentrating rare earth elements in brown coal humic acids by mechanochemical treatment.

Coals are now viewed as a promising source of rare earth elements increasingly often. Rare earth elements (REE) are known to occur both in the organic and mineral components of brown coals. This study aims at investigating the applicability of mechanochemical activation for concentrating rare earth elements (including Sc, Y, La and lanthanides) in different brown coal fractions. Mechanochemical activation of brown coal in the absence of reagents, as well as additives of sodium percarbonate, monosodium phosphate, and sodium chloride, was carried out. Mechanochemical activation does not cause degradation of humic acid-REE complexes contained in pristine coal. The REE concentration process in the samples of mechanochemically activated coal can be attributed both to formation of new oxygen-containing groups in humic acids (HA) and to binding of REEs to oxygen-containing groups already contained in coal due to vigorous solid-phase mechanical mixing. A method for mechanochemical activation of coal, which allows one to transfer up to 93 ± 7% REEs into the organic alkali-soluble fraction - the HA fraction (while HA in the pristine coal contain only 38 ± 3% REE) - has been developed. The estimated total concentration of REEs in pristine coal ash is 8000 ppm. The estimated REE content in the ash of the product (HA fraction) is as high as 17 300 ppm. Concentrations of Ce, Nd and Y in the ash of the product are 6000 ppm, 4200 ppm and 2500 ppm, respectively.

Mechanochemical and Size Reduction Machines for Biorefining.

In recent years, we have witnessed an increasing interest in the application of mechanochemical methods for processing materials in biomass refining techniques. Grinding and mechanical pretreatment are very popular methods utilized to enhance the reactivity of polymers and plant raw materials; however, the choice of devices and their modes of action is often performed through trial and error. An inadequate choice of equipment often results in inefficient grinding, low reactivity of the product, excess energy expenditure, and significant wear of the equipment. In the present review, modern equipment employing various types of mechanical impacts, which show the highest promise for mechanochemical pretreatment of plant raw materials, is examined and compared-disc mills, attritors and bead mills, ball mills, planetary mills, vibration and vibrocentrifugal mills, roller and centrifugal roller mills, extruders, hammer mills, knife mills, pin mills, disintegrators, and jet mills. The properly chosen type of mechanochemical activation (and equipment) allows an energetically and economically sound enhancement of the reactivity of solid-phase polymers by increasing the effective surface area accessible to reagents, reducing the amount of crystalline regions and the diffusion coefficient, disordering the supramolecular structure of the material, and mechanochemically reacting with the target substances.

Unbound water in mechanochemical reactions of brown coal.

Mechanochemical activation of coal is commonly employed in industry. However, even the simplest solid-phase reactions, such as neutralization of humic acids in brown coal, remain insufficiently studied. The hypothesis regarding the occurrence of mechanochemical neutralization under local hydrothermal conditions for humic acids in brown coal has been tested in this study. 3D modelling of the "block-interlayer" system (where coal particles are separated by air interlayers saturated with water vapor) was used. The 3D model showed that the permittivity is expected to rise from 14 to 16% as the moisture content in the system increases from 12 to 15%. The actual permittivities of coal with different moisture contents have been measured by dielectric spectroscopy. In the real system, the permittivity increases more than threefold as the moisture content rises from 12 to 15%. This increase is much greater than the calculated one, demonstrating that the phase containing unbound water appears in the system at a moisture content of ∼12-13% and may exert various effects on the solid-phase reaction. There is a correlation between the moisture content, permittivity, and predominant mechanisms of the reaction between the organic matter in brown coal and sodium percarbonate (a reagent simultaneously containing the alkaline and peroxidic components). The reactions between brown coal and alkaline reagents proceed under local hydrothermal conditions. Both the alkaline and peroxidic components of sodium percarbonate participate in the solid-phase reaction between brown coal and sodium percarbonate. The emergence of unbound water in coal significantly inhibits the oxidation reaction.