In industrial glass melting processes, the conversation of raw material mixtures into a homogenous molten glass without inclusions, involves several process steps, preferably taking place after each other. The performance of each step depends on the heating rate or heat transfer, achieved temperature level, the local chemistry in the batch or obtained melt, the flow or convection conditions and residence times. Different process steps require different circumstances. Heat transfer rates to and into the raw material batch determine the initial stages of batch melting-in and the contact between different raw material grains strongly influence the rate of reactive melting. The next stage, in soda-lime-silica glasses mainly starting at about 950-1000 deg C, is the dissolution of sand grains in the alkali alkali-earth-silicate melt. Essentially, the sand grains should be completely dissolved before the onset of fining, starting with the steady release of fining gases from the melt. After primary fining, usually taking place in the hot spot area of the melting tank, the molten glass should be cooled down, with rather slow temperature decrease in the secondary fining range. After chemical homogenisation of the melt and removal of inclusions, the glass melt has to be conditioned to a uniform viscosity (temperature) to control the forming processes. In this presentation, the different process steps and their optimum conditions will be described and some experimental methods will be demonstrated to study these process steps on a laboratory scale. Understanding of the mechanisms of each individual process step and conditions for optimum performance will support the development of new glass melting technologies with lower average residence times and improved energy efficiency.
