Impact of furnace atmosphere and organic contamination of recycled cullet on redox State and fining of glass melts

Abstract

The onset temperature of fming and the quantity of fming gases is not only determined by the amount of fming agents in sodalime- silica batches, but also by the level of organic contaminants in the cullet or normal batch and the water vapor pressure in the furnace atmosphere. These conditions will also determine the redox State of the glass and residual sulfate or sulfide concentrations in the glass. Organic contaminants will form char during headng of the batch. This char partly reacts with CO₂ Coming from the decomposition of the soda, limestone or dolomite forming carbon monoxide. Stable types of char or cokes or cuUet-rich batches with only small amounts of carbonates will result in some carbon residues after the CO₂ evolution. This carbon partly reduces Sulfates and ferric iron in the fresh melts. This results in sulfide and ferrous iron formation in these glass melts. At increasing temperatures in the melt, the sulfides and Sulfates react together forming sulfur-containing gases between 1000 to 1250°C. The Sulfate retention decreases, finally the glass even may contain sulfur only in the sulfide form under very reduced conditions. In batches without reducing agents, Sulfates in the melt Start to decompose at temperatures exceeding 1400 °C. Small amounts of carbon and water vapor reduce the fining onset temperature. Water vapor from the furnace atmosphere predominantly Infiltrates the batch blanket during melting and foaming. The water will enhance the bubble and seed growth during fining. Water in the melt will influence the redox State of the final glass. Only in batches containing coarse raw materials or cullet, reducing or oxidizing gases from the furnace atmosphere Infiltrate the batch blanket and these gases will respectively reduce and oxidize components like iron oxides, sulfate/sulfide or chromium oxides in the batch blanket interior.