V.A. Kreisberg, V.P. Rakcheev, Yu.G. Kolmogorov, A.F. Papeta, R.I. Zalomova
Gas evolution from Brazilian rock crystal and quartz as raw materials for producing silica glass
Procedings of Annual Meeting of the International Commission on Glass, Campos do Jordao, Brazil, 21-25 September 2003, The Brazilian Glass Industry Technical Association, p. 58-71 (2003)

ABSTRACT
Gas content, along with the content of element impurities and the content of mineral inclusions, is one of the most important characteristics of quality for quartz raw materials in producing high-quality silica glass. Gas evolution from rock crystal and vein quartz from Brazil as quartz raw materials for producing high-quality silica glass has been studied. Gas content and gas evolution of quartz raw materials have been investigated by kinetic thermodesorption mass spectrometry. Gas release on thermal treatment has been made without the gas accumulation in an enclosed volume. The quantity of evolved gases has been calculated by integration of releasing flows with respect to the time with due regard for sensitivity coefficients of mass spectrometer to different gases. It has been detected that water and carbon-containing impurities (CO₂, CO, hydrocarbons) are predominant in gas-liquid impurities of quartz raw materials from the deposits of Brazil. H₂, HCl, H₂S, SO₂, NO, N₂, O₂ et al. have been also registered in content of evolving gases. Gas evolution can be varied on thermal and chemical treatment of quartz raw materials. The evolution of water, carbon dioxide and hydrocarbons decreases after heat treatment and leaching. Rock crystals from Brazil are characterized by very low content of water. Water is basically released at low temperatures (up to 600 C) through dehydration and decrepitation of quartz material. The most important characteristic for the melting silica glass technology is the content of high temperature form of water (as well as other impurities) what is evolved at temperatures higher than 600 C by diffusion mechanism. Coefficients of water diffusion out of Brazilian quartz are close to values for nondefective quartz. Increasing amount of structural defects in quartz by either high temperature treatment or contribution of intergranular diffusion leads to rising diffusion coefficient. Quality of silica glass melted from quartz raw materials gets worse with increasing content of "high temperature" water. The content of high temperature form of water in rock crystals from Brazil amounts only several ppmm (up to 10 ppmm) (parts per million by mass) what is lower than in standard raw materials of "Unimin" and is an order lower than in transparent vein quartz commonly used. Enhanced content of carbon-containing impurities (graphite, coke-like substances, hydrocarbons) in quartz raw materials causes CO evolution at high temperatures, formation of bubbles and arising dark regions in silica glass.