Silicates

*University of Chemical Technology and Metallurgy,
8 Kl. Ohridski, 1756 Sofia, Bulgaria svetozar@sf.icn.bg
**University of Minerals & Geology
***University of Sofia-department General & industrial
microbiology and virology

Received 20 November r 2000;
Accepted 30 November 2000

Aim: Comparable investigations on two Bulgarian kaolins and porcelain body before and after treatment with “silicate” bacteria of the species Bacillus mucilaginosus and Bacillus circulans, extracted from sandy ground.

Microbial treatment conditions: bacterial cultures- about 10⁸ cells/ml; temperature 30^oC; treatment time 24, 48 and 72 h.

Results after the microbial treatment: The chemical composition is unchanged. The granulometric content shows insignificant change. The exothermoeffect at a temperature of 400^oC is increased twice, due to combustion of the organic substance. The strengths of the kaolin in dry condition increases by 40% after 24h treatment. After treatment time of 24 h the change is insignificant. The strength of the porcelain body increases up to 185% after 48-h time of treatment. The changes of the fired porcelain structure are indirect indication of the possibility to decrease the firing temperature.

Conclusion: This study does not verify the leaching of Al, Si and Fe, as well as the destruction of the crystalline aluminosilicate phases. The increase of bending strengths can be explained by the action of exopolysacharides in the sense that the substances stick. The microbial treatment of the ceramics raw materials and bodies intensifies the firing processes and increases the mechanical strengths.

Keywords: “silicate” bacteria, kaolin, granulometric content, mechanical strengths.

N. Daskalova, Chr. Bojadjieva, T. Datzkova

University of Chemical Technology and Metallurgy,
8 Kl. Ohridski, 1756 Sofia, Bulgaria
neda@uctm.edu

Received 20 November 2000 Accepted 30 November 2000

Low temperature frits with a melting point of about 850-1000^oC and thermal – expansion coefficient a=10-14.10^-6K^-1 are synthesized.

SiO₂ is used as a main glass former. PbO and Na₂O are used as modifiers. The compounds in the system SiO₂-Na₂O-PbO change within 50± 5 mol % SiO₂, 18-36 mol % Na₂O, 13-30 mol % PbO.

In order to lower the melting point in some frits other modifiers are added and B₂O₃ is introduced as a second glass former. Al₂O₃ is brought in for glass stabilization.

The oxide composition of the frits in mol % is within the following limits: Li₂O 0-5,8; K₂O 0-11,8; Na₂O 12-28; CaO 0-5; ZnO 0-17,5; PbO 10-30; Al₂O₃ 0-6; TiO₂ 0-14,8; B₂O₃ 0-26,5; SiO₂ 22-52.

Preliminary planning for the main properties of frits was made by calculating the thermal-expansion coefficient (Appen’s method); the melting properties by empirical formulas for low- and high-melting glass and by Zeger’s molecular formulas.

The used raw materials are: kaolin, quartz sand, borax, boric acid, minium, soda ash, potash, anatase, Li₂CO₃, ZnO. The frits were formed in a temperature interval between 850-1000^oC with water-cooling.

There have been defined experimentally thermal-expansion coefficient, the melting point of frits by quartz dilatometer, chemical resistance, phase composition by X-ray analysis and optical microscopy.

It is established that all frits except one are fully glassy. The chemical resistance of frits is depends the content of alkali oxides and is changed from I go V hydrolitical class. The transformation temperature of the investigated frits is about 340-460^oC. With most of the frits at a temperature higher than 350^oC there is a sharp expansion because of getting into the transformation interval. An attempt has been made to find the correlation of melting with the content of mol % Al₂O₃ and the different ratio of glassformer and modifiers.

As a result, there has been assessed the possibility to approximately prognosticate the melting properties and thermal-expansion coefficient of low-temperature glass in the system Na₂O-PbO-SiO₂.