Αξιοποίηση μεταλλευτικών βιομηχανικών παραπροϊόντων στην παραγωγή δομικών κεραμικών υλικών

Abstract

In the present thesis, the valorisation of mining and industrial by-products, the solid boron-containing Wastes (ΒW), which are created at various stages during the production process of concentrated and refined boron products, were investigated. The by-products under consideration, existing in five types SBW, DBW, SSBW, TBW and MBW, were used as additives and raw materials in clay mixes for the production of heavy clay ceramics (bricks, tiles and artificial lightweight aggregates). The BW were characterised in means of chemical and mineralogical composition and studied for their thermal behaviour. In addition, clay mixes with different amounts of BW (SBW and SSBW) were studied. A first approach was made on the formation of ceramic samples by dry pressing in order to minimize the borate migration towards the surface. The physical and mechanical properties, as well as the microstructure of the final products were studied. For 5 wt% SSBW addition and firing at 900-950°C, the sintered bodies presented comparable or improved physical and mechanical properties with respect to the reference formulation. Moreover, SBW or a commercial available borate solution (Evansite®) were introduced in a clay-based mix aiming to investigate their behaviour during a processing cycle comparable with that followed in the heavy clay industry. For firing at 1000°C, water absorption was reduced and bending strength increased for the samples with borates, compared to the reference samples. The addition of borates resulted in the formation of new crystalline phases during firing at high temperatures. BW consist of important fluxing oxides as well as of gas producing minerals during firing. Tests were performed on the bloating behavior of BW and mixes of them with other materials, by means of heating microscopy, aiming at the laboratorial production of lightweight aggregates (LWA). A new mix was proposed, according to the obtained results, consisting of 70wt% BW, 20wt% clay mixture and 10wt% quartz sand, for the formation of pellets. Abrupt heating of the dry pellets at 760°C, for 5min, resulted in porous LWA with bulk density <1g/cm3. The process was less energy demanding as the temperature was reduced compared to the one of the synthetic aggregates produced, where the firing temperature is 1100°C. During drying a white layer of boron salts was formed on the surface of the green pellets, which result on firing of a glassy layer causing alterations of their surface and experimental difficulties. In order to address this problem, fructose was added in the raw mix as a migration inhibitor. Addition of even 0.5wt% fructose inhibited the salt formation and a glassy impervious layer was formed after firing. The physical properties of the final samples meet the requirements of LWA according to the standard ASTM 330-97. The results of this thesis, led to the development of an optimised process for the production of artificial LWA with BW, as an alternative raw material.