REDOX STATE AND ITS EFFECT ON SOME SELECTED PARAMETERS OF LEAD CRYSTAL

1. REDOX STATE AND ITS EFFECT ON SOME SELECTED PARAMETERS OF LEAD CRYSTAL Miroslav RADA, Michal NOVÁČEK, Jana STAŇKOVÁ 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. TOPIC Investigate the influence of the increasing nitrate content in the batch on the final redox state of 24-percent lead crystal by using a combination of 0.25 percent by weight of As2O3 + KNO3 and determine the changes in the values of selected parameters

3. Conditions of the tests The investigation was performed for lead crystal containing more than 24% by weight of PbO and fined by using a combination of 0.25 % by weight of As2O3 + potassium nitrate. The theoretical composition of the glass, the amount of As2O3 and the total content of K2O (from K2CO3 and KNO3) were kept constant during all tests. The batches with the increasing KNO3 content and yielding 100 g of melt were melted in PtRh crucibles at the temperature of 1420 degC for 112 minutes. During the melting process, the melt in the crucibles was always stirred by hand in the same manner.

4. Parameters determined : • degree of fining • surface tension • viscosity • content of bivalent iron and trivalent arsenic • content of trivalent and total arsenic • mass loss of the sample after the melting as compared to the theoretical quantity of 100 grams • partial pressure of oxygen • crystallization properties • mean coefficient of linear thermal expansion • chemical durability of cruhed glass in water at 98 degC

5. FIG. : 1Dependence of the number of seeds in 100 grams of glass on the increasing content of KNO3

6. FIG. 2:Dependence of the surface tension at 1100 degC on the increasing content of KNO3

7. FIG. 3:Dependence of the melting temperaturecorrespondingto log  = 2 (in oC)on the increasing content of KNO3

8. FIG. 4:Dependence of thegob temperaturecorrespondingto log  = 3 (in oC)on the increasing content of KNO3

9. FIG. 5:Dependence of the content of trivalent arsenic and bivalent iron on the increasing content of KNO3

10. FIG. 6:Dependence of the total content of As2O3on the increasing amount of KNO3

11. FIG. 7:Dependence of the mass losses of the glass sample after the melting on the increasing content of KNO3

12. FIG. 8:Temperaturedependence of the partial pressure of oxygen for the glass with optimum content and for glasses with a larger and smaller content of KNO3 than corresponds to the optimum

13. FIG. 9:Dependence of the liquidus temperature on the increasing content of KNO3

14. FIG. 10:Dependence of the temperature corresponding to the maximum crystallization rate on the increasing content of KNO3

15. FIG. 11:Dependence of the maximum crystallization rate on the increasing content of KNO3

16. FIG. 12:Dependence of the coefficient of thermal expansion on the increasing content of KNO3

17. FIG. 13:Dependence of theconsumption of 0.01-M hydrochloric acid (in ml) on the increasing content of KNO3

18. CONCLUSIONS • It was demonstrated that the content of KNO3 introducedinto the batch of lead crystal containing more than 24 percent by weight of PbO exerts an influence on the redox state during the glass melting process. The redox state is characterized by the respective concentration of the individual forms of polyvalent oxides as well as by the value of the partial pressure of oxygen. These two factors affect all the parameters investigated within the present paper. • At the certain same contents of KNO3 added to the batch all the investigated parameters are characterized by maxima and minima that repeat themselves periodically. Let’s call them optima from the viewpoint of the degree of fining.

19. Trivalent arsenic reduces the surface tension of the glass melt. • In the fining optima, the presence of polyvalent oxides of arsenic and iron occurring in their mostly oxidized forms together with the maximum partial pressure of oxygen bring about the maximum loosening and “fluffing-up” of the glass structure. • The first fining optimum characterized by the lowest content of KNO3 in the fining mix is the most favorable from the ecological point of view. • As compared with the worst or best value of the respective parameters in the whole investigated range, the following values characterize this optimum achieved under laboratory conditions:

20. The improvement in the degree of fining by 30 %. • The reduction in the surface tension by 25 mN/m. • The decrease in the glass melting temperature by 25 degC and in the gob temperature by 15 degC. • The increase in the ratio of As3+-to-Astotal contents from 0.15 to 0.20. • The decrease in the ratio of Fe2+- to-Fetotal theoretical contents from 0.32 to 0.12. • The increase in the volatilized amount of As2O3 by 0.03 percent by weight. • The increase in the losses due to the volatilization from 0.4 to 0.5 percent by weight. • The increase in the liquidus temperature by 45 degC, in the temperature of the maximum crystallization rate by 30 degC and the maximum crystallization rate by 0.03 μm/minute. • The increase in the coefficient of thermal expansion by 1.10-7 K-1. • The increase in the consumption of 0.01-M HCl by 10 %.

21. If the decision is taken to apply the above results in the commercial production, we should also take into consideration the fact that the resulting redox state is influenced by technological factors too. • The adoption of the optimum composition of the fining mix improves the quality of glass articles, lowers the consumption of energy, extends the furnace campaign and reduces NOx emissions. 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