Krystaldur

As part of a project initiated by the company, a technology was developed for the application of protective coatings on graphite work surfaces in industrial crystallizers.
Coatings on work surfaces in crystallizers are based on Al₂O₃ in the form of powder of low grain size, deposited using the thermal plasma spraying technology. The deposition procedure uses inert process gases. The temperature in the plasma burner is about 15,000°C, and the outlet particle velocity is of the order of the speed of sound. Under the above conditions, nearly all engineering materials are melted and deposited in the form of particles supplied via a stream of exhaust gases on a pre-processed surface.The resulting coatings have porosity of up to several percent.

Work surfaces in crystallizers are provided with coatings based on aluminium oxide to prevent the thermal degradation of graphite layers. When applied, coatings based on Al₂O₃ can be left untreated or they can be modified (smoothed). Before the coating is applied, the surface is properly cleaned and slightly extended. Before applying the top coating, a metallic bonding interlayer is deposited to improve adhesion. A top coating varying in thickness from 150 to 200 μm is considered sufficient. Powder is supplied to the burner via automatic disc feeders with an accuracy of ± 0.08% or vibratory disc feeders with an accuracy of ± 2%. Argon or nitrogen is used as a powder carrier gas. The burner is guided via an advanced manipulator which guarantees that individual movements are reproducible with an accuracy of up to 0.2 mm.