The primary nucleation mechanism of the gypsum in a bulk aqueous medium was identified as a heterogeneous one for 0.05 and 0.03 mol·L−1 CaSO4·2H2O solutions and 25 °C. By means of a particle counter and dynamic light scattering (DLS) technique, solid nano/microimpurities were found, and controlled in stock brines for gypsum supersaturated solutions preparation. It is demonstrated that the common procedure of reagent grade 0.10 mol·L‒1 CaCl2 and Na2SO4 aqueous solutions filtration via 200 nm membranes is capable to reduce the foreign solid microimpurities content (size > 100 nm) from 106 to 103 units in 1 mL, but fails to affect the more numerous nanofraction (size < 100 nm). Thus, the gypsum nucleation takes place in presence of a significant amount of “nano/microcodust” templates, and has a heterogenous character. The induction time, measured by conductivity for the similar supersaturation levels, reveals a well detectable dependence on nano/microdust concentent: an increasing background particle concentration substantially decreases the induction period at a constant saturation state and temperature, and thus increases the nucleation rate. Therefore, the gypsum nucleation reaction starts tentatively through the fast heterogeneous formation of well-defined, primary nuclei via [Ca2+], [SO42‒], and [CaSO4]o species sorption on the surface of “nano/microdust” particles. Thus, the “nano/microdust”, naturally occurring in any high purity chemical, plays a key role in sparingly soluble salts nucleation in the bulk aqueous medium.

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The article presents the results of studies of the process of obtaining NPK fertilizer from low-grade phosphate raw materials with P2O5 of about 18%. Phosphate raw materials were leached with a mixture of nitric-phosphoric acids with the addition of potassium carbonate, which serves as a source of potassium in the final product. The main parameters determined were the content of the main nutrients P2O5:N:K2O, temperature and time of the leaching process. According to the graphical method, the “apparent” activation energy of the heterogeneous process is found, which is equal to 3.8 kJ/mol indicates the intradiffusion nature of the process. Methods of chemical analysis, scanning electron microscopy and XRD analysis were used for a comprehensive study of raw materials and final products.

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