Studies on Lead Silicate Melts

Studies on the constitution of silicate melts are very important not only in the fields of metallurgical processing or the glass industry, but also in dealing with the properties or constitution of liquids. For this reason a PbO-SiO2 system was chosen as a model of silicate or slag because of its low melting point and glass forming properties.As a result electrical conductivity measurements, it was found that the lead silicate melt had a positive temperature coefficient and ionic conductance like molten NaCl, which was verified by fused electrolysis. It should be noted that the curves showing the relation between electrical conductivity and composition have breaks at the compounds corresponding to 2PbO·SiO2 and PbO·SiO2. This irregularity was more clearly shown on the curves of activation energy of electrical conductance versus composition. This tendency was also observed in density and viscosity measurements.The decomposition voltage of the melt was measured by using a Pt anode and molten Pb as a cathode. The electro-motive force of the cell −Pb/PbO(in silicate)/Pt:O2+ was also studied. There was a general decrease of e.m.f. with increasing temperature and concentration of PbO. The activity of PbO in the melts obtained from e.m.f. was compared with values from Richardson’s experiments. On the decomposition voltage versus composition curves there are also two breaks at 2PbO·SiO2 and PbO·SiO2.The differential thermal analysis with the aid of X-ray analysis was conducted in order to find out the mechanism of crystalization of lead silicate glass. The results indicate that even the stoichimetric compounds have different kinds of silicate anion, for example, 2PbO·SiO2 has Si2O76− and others as well as SiO44−. A new crystal or compound was observed at the composition point corresponding to 3PbO·2SiO2, but it was also found that this compound was decomposed to a mixture of 2PbO·SiO2 and PbO·SiO2 above 650°C. The results from the X-ray analysis of lead silicate glass, show that there are three similiar groups on the patterns, which coincide with the above mentioned discontinuity on the properties versus composition curves.