Basic requirements are formulated for starting materials for manufacturing porous ceramic diaphragms, and alumina-containing raw materials of domestic and overseas production are studied. Porous ceramic is manufactured with 35% open porosity and ultimate strength in compression of 72 MPa.Keywords: alumina, porous ceramic, diaphragm.Porous ceramic diaphragms exhibit a number of unique properties, and due to this have a broad range of applications. The main advantages of ceramic diaphragms compared with those manufactured from other materials are an increase in structural mechanical strength and stiffness, corrosion and heat resistance, operating property stability with prolonged use, the possibility of object repeated regeneration, and resistance to action of bacteria. Ceramic diaphragms are readily cleaned by calcining, reverse flow of water or other liquid. These properties make it possible to use ceramic filter elements during separation of solutions over a wide pH range (from 0 to 14), and with high temperature and pressure (1 -10 MPa). Ceramic diaphragms are reliable and long lasting in operation [1,2]. However, in order to manufacture ceramic materials high-temperature firing is required, during which there is formation of the structure required. The sintering temperature is reduced by introducing sintering additions into a charge or use of special materials in order to obtain a porous structure [3].During mass production of microporous diaphragms it is interesting to prepare them on the basis of aluminum oxide, as the most widespread and available material. Some of the powders offered on the market have been analyzed in this work.In selecting aluminum oxide Al 2 O 3 powder suitable for manufacturing porous diaphragms, one is guided by the following criteria established by experiment.1. Aluminum oxide should be in the form of corundum.
The processes of obtaining ceramic microporous diaphragms based on aluminum and zirconium oxides have been studied. The diaphragm test results for long-term corrosion resistance are presented. It was shown that a ceramic material based on zirconium and aluminum oxides obtained at 1350 °C with linear shrinkage of 7,5 % has an open porosity of 49 % and a flexural strength of 48 MPa. The diaphragms obtained from this material can withstand pressure up to 1.0 MPa and can work under the conditions of electrolysis of aqueous solutions of chlorides for at least 40,000 hours.
Slip casting is one of the most effective methods for obtaining zirconium dioxide products [i, 2]. It ensures high density of the unfinished (raw) products and uniform particle stacking. However, in this case, the porosity of the formed products exceeds 45-50% and, consequently, during the firing process of large products, stresses are generated in the body of the products because of nonuniform shrinkage which, in turn, lead to distortion and crack formation.We made an attempt to decrease nhe shrinkage during the firing process of the slip cast (in gypsum molds) products of tetragonal zirconium dioxide by subjecting the shaped products to additional hydrostatic treatment at a pressure of 0.I-I.0 GN/m 2.Stabilized zirconium dioxide (TU 48-0502-63/0-86) containing 6% (weight content) yttrium oxide additive (a product of the Verkhnedneprovsk Mining and Metallurgical Combine) was used as the raw material. Specimens measuring 8 x 8 x 80 mm were slip cast in gypsum molds according to the previously established technology [3]. The ultimate bend strength of the specimens was determined using the three-point bending scheme according to GOST 5458-75, and the apparent density of the unfinished (raw) products and the ceramics was determined from the ratio of the mass and the volume of the specimens. The formed specimens had an apparent density of 3.2 g/cm 3.Hydrostatic treatment of the specimens and the products was carried out using rubber molds (bags) in hydrostats [4]. It was established (see Fig. la) that the apparent density increases almost linearly with increasing pressure.This fact appears to contradict ~he vast majority of the results previously recorded in similar experiments [5]. According to us, this owes to the fact that the data presented in Pig. la correspond to a narrow segment of the density vs pressure curve and an approximately linear relationship exists within this range. In this case, the initial period of densification of powders is absent since the cast specimens correspond to an intermediate density and the observed results differ from the compaction curve of loosely (freely) stacked powders. It is also evident that at pressures exceeding I GN/m 2, the linear relationship would be disturbed again (otherwise, the theoretical density can be attained at a pressure of approximately 2 GN/m2~No variation of the shrinkage of the specimens along their length and thickness was observed within the error of measurement; i.e., during slip casting, the differences in the particle stacking in the directions parallel and perpendicular to the direction of moisture removal by the gypsum mold are insignificant.Hydrostatic treatment eliminates the defects of slip casting (such as air bubbles and cavities). Large cavities (developed due to improper design of the gypsum molds) migrate to the surface of the products after the treatment and this permits one to reject such defective products before firing. Evidently, in this case, the particle stacking obtained during the casting process is disturbed.In order to study this p...
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