Vacuum-sublimation method of producing ultradisperse powders of inorganic salts

Abstract

Designs and operating principles of vacuum-sublimation sprayers, results of investigations of process kinetics and the dispersitvity of the organic-salt powders produced, the formation mechanism of the solid-phase structure, and the process of evaporative freezing in a vacuum are discussed. The size distribution of KNO 3 crystals in porous granules is indicated as a function of cooling rate.Technologies of nanomaterials and ultradisperse powders (UDP) of various nature and purpose are of interest for the production of new materials with enhanced qualitative characteristics [1][2][3][4].Chemical methods (precipitation in liquid and gaseous media, heterogeneous synthesis, cryochemical method), electrochemical methods of synthesis using a plasma or laser beam, electroerosion, shock-wave, and electroexplosive methods, method of self-propagating high-temperature synthesis, and mechano-chemical synthesis are currently employed for the production of UDP [5].Of the existing methods of converting solid macroparticles to the ultradisperse state, cryochemical methods [6, 7], which have been successfully implemented in the production of such materials as chemical reagents, catalysts, pigments, composites, pharmaceutical preparations, etc., exhibit potential for process and ecological safety with a high productivity and low net processing cost.Theoretical and experimental investigations conducted at the MGUIÉ have indicated that the cryochemical method of producing UDP of inorganic salts (solid oxidizers) is implemented most effectively when vacuum-sublimation processes are carried out. The following sequence of basic production operations is employed for practical production of UDP by the vacuum-sublimation method: preparation of a homogeneous solution; dispersion of the solution; freezing of drops of the disperse medium; sublimation removal of solvent from the cryogranulate obtained in the previous step; passivation and preservation of the granules; utilization (desublimation) of the solvent; and, dispersion of the granules into ultradisperse fragments.Selection of the solvent in accordance with the following conditions plays a major role in the step where a solution of initial substance is obtained:• sufficiently high solubility of the initial substance;• a triple-point interval ranging from -5 to + 20°C; • a crystallization temperature (eutectic point) of the solution no lower than -(15-20)°C; and • fire resistance, nontoxic nature, chemical inertness, and economic effectiveness. In executing the freezing step, the salt solution or suspension is preliminarily dispersed into drops to increase the freezing rate and improve the engineering properties of the product.

Nanomaterials in the chemical industry

“…Growth of the feeding zone of previously developed crystals and reduction in the free volume of the solution occur simultaneously. As the feeding zone propagates over the entire volume of the system, nuclei formation will cease, and in subsequent moments, each crystal will "complete its growth" to the dimensions defined by the feeding zone itself [9].…”