Optimization of Cerium Oxide and Potassium Nitrate for Lead Crystal Glass Fining Process

1. IS CeO2 FORLEAD CRYSTAL AN „ECOLOGICAL“ ALTERNATIVE TO FINING WITH As2O3 OR Sb2O3 ? Miroslav RADA, Petr FABIÁN Institute of Chemical Technology Department of Glass and Ceramics Technická 1905/5, 166 28 Prague 6, Czech Republic Tel: 00420224311139, Fax: 00420224313200 e-mail: miroslav.rada@vscht.cz

2. GOALS Investigate the influence of CeO2 only or in a combination with potassium nitrate on thefinal degree of fining, the viscosity and the surface tension for lead crystal glass containing 24 % by weight of PbO or more. Compare the results with the corresponding data obtained for lead crystal fined with the combinations of As2O3 + KNO3 and Sb2O3 + KNO3 at their optimum compositions.

3. Testing conditions The investigation was performed for lead crystal containing more than 24% by weight of PbO and fined by CeO2 only or in a combination with KNO3.The resultswere always compared with two optimum mixtures - 0.25 g As2O3 + 1.0 g KNO3 and 0.52 g Sb2O3 + 3.64 g KNO3 in a batch, corresponding to 100 g of glass. The theoretical composition of the glass and the total content of K2O (from K2CO3 and KNO3) were kept constant during all tests.Cerium oxide added to the glasses was introduced to the detriment of SiO2. The batches with the increasing content of CeO2, respectively KNO3, and yielding 100 g of melt were melted in PtRh crucibles at the temperature of 1420 degC for 112 minutes.

4. Parameters determined : • degree of fining • viscosity • surface tension

5. FIG. 1: Dependence of the degree of fining of the 24-percent lead crystalon the increasing content of CeO2 As2O3+KNO3: 202 seeds/100 g of glass Sb2O3+KNO3: 550 seeds/100 g of glass

6. FIG. 2:Dependence of the glass melting temperaturecorrespondingto log  = 2 (in oC)of the 24-percent lead crystalon the increasing content of CeO2 As2O3+KNO3: 1524°C Sb2O3+KNO3: 1511°C

7. FIG. 3:Dependence of the surface tension of the 24-percent lead crystalat a temperature of 1100 degC on the increasing content of CeO2 As2O3+KNO3: 290,7 mN.m-1 Sb2O3+KNO3: 301,6 mN.m-1

8. FIG. 4: Dependence of the degree of fining of the 24-percent lead crystalon the KNO3 content in the batch As2O3+KNO3: 202 seeds/100 g of glass Sb2O3+KNO3: 550 seeds/100 g of glass As2O3+KNO3: 202 seeds/100 g of glass Sb2O3+KNO3: 550 seeds/100 g of glass

9. FIG. 5:Dependence of the glass melting temperaturecorrespondingto log  = 2 (in oC) of the 24-percent lead crystal on the KNO3 content in the batch As2O3+KNO3: 1524°C Sb2O3+KNO3: 1511°C

10. FIG. 6:Dependence of the surface tension of the 24-percent lead crystalat a temperature of 1100 degC on the KNO3 content in the batch As2O3+KNO3: 290,7 mN.m-1 Sb2O3+KNO3: 301,6 mN.m-1 As2O3+KNO3: 290,7 mN.m-1 Sb2O3+KNO3: 301,6 mN.m-1

11. Table I: Final assessment of the effect of fining mixes on investigated parameters

12. CONCLUSIONS The influence of the content of cerium oxide only and the effect of potassium nitrate in the batch at a constant content of 0.5 percent by weight of CeO2 on various glass parameters were investigated for a 24-percent lead crystal. Following results could be obtained: • The optimum fining was obtained at a content of 1.5 percent by weight of CeO2 in the glass when the lead glass was fined with the aid of cerium oxide only. In this case, the degree of fining approached the values characterizing the glass fined with a combination of Sb2O3+KNO3. However, the values of the degree of fining are worse if they are compared to the glass fined with the As2O3+KNO3 mix. As regards the glass melting temperature, the glass fined with cerium oxide only is comparable to the glass fined with the optimum Sb2O3+KNO3 mix, respectively better than the optimum As2O3+KNO3 mix. A poorer fining

13. ability of cerium oxide is obviously due to the surface tension that increases with the increasing CeO2 content and the value of which is higher than that of glasses fined with As2O3+KNO3 and Sb2O3+KNO3. The fining with cerium oxide only would approach that of existing fining mixes only if higher CeO2 contents in the glass were used. Nevertheless, the introduction of such a large quantity of this raw material into the batch is hardly economically viable at present because of the price of the raw material itself and the additional costs associated with the glass decolorizing. 2. By taking into consideration the above reasons we decided to focus on the investigation into the effect of the KNO3 content on glass parameters while the CeO2 content was maintained constant at 0.5 percent by weight. At this CeO2 content, the decrease in the seed count and in the value of the glass melting temperature was the most effective. All the investigated parameters exhibited certain minimum values at varying KNO3 contents in the glass batch when a pronounced decrease in the number of seeds, the glass melting temperature and the surface tension could be recorded. Hence, despite of the fact that KNO3 does not affect directly the fining process, the optimum

14. adjustment of the redox state of glass gives the possibility to optimize both the degree of fining and the closely related viscosity and surface tension. 3. The investigations demonstrated that the optimum KNO3 content in the batch exerts a pronounced influence on the final glass properties. The resulting redox state should also be taken into consideration. 4. The efficiency of fining mixes under comparable conditions and at optimum compositions decreases in the following sequence: As2O3+KNO3, Sb2O3+KNO3, CeO2, respectively CeO2+KNO3. As regards the fining degree, the combination of Sb2O3+KNO3 and CeO2 are characterized by about the same results which are, however, about 2.5 times worse than those of As2O3+KNO3. The combination of CeO2+KNO3 with the optimum composition of the mixes (i.e. 0.5 percent by weight of CeO2 + 1 g KNO3 and 0.5 percent by weight of CeO2 + 3.5 g KNO3) used for fining the lead crystal exhibits the values of fining degree that are many times worse (the seed count per 100 g of glass was about 11 times, respectively 9 times larger than that of the fining combination of As2O3+KNO3).

15. 5. The complex assessment of the variations in the glass parameters corresponding to the optimum composition of the fining mix indicates that the viscosity plays an important role in the fining mechanism of cerium oxide only. The decrease in the glass viscosity with the growing CeO2 concentration makes the fining easier even if the surface tension increases. 6. However, the predominant effect of the surface tension on the fining process is evident from the comparison of the investigated parameters characterizing the fining mixes of As2O3+KNO3, Sb2O3+KNO3, CeO2 and CeO2+KNO3. This finding is demonstrated by the fact that the resulting degree of fining obtained with the As2O3+KNO3 mix is much better that that characterizing all other combination even if the value of the melting temperature is higher; however, the value of the surface tension of the lead glass fined with As2O3+KNO3 is lower. 7. The results that would be comparable to those characterizing the fining with the combinations of As2O3+KNO3 and Sb2O3+KNO3 used at present could be achieved only if the CeO2 contents in the batch were very high. The glass parameters can be improved

16. significantly if optimum amounts of nitrate are added to the batch. • There are no doubts about the ecological significance of cerium oxide even if its use as a fining agent might result in the increase in the production costs at present. The application of the above finding in the near or more distant future would contribute significantly to the reduction of the negative impact of the glass industry on the environment. • Acknowledgement: • This work was part of the research project„Preparation and Properties of AdvancedMaterials - Modelling, Characterization,Technology“ supported by Czech Ministry of Education, Youth and Sports under Contract No. MSM223100002