Title of Invention

"A PROCESS FOR THE MANUFACTURE OF BARIUM CROWN GLASS"

Abstract

The present invention provides a process for the manufacture of barium crown glass.Large volumes of gases need to be expelled from the system during melting of most glasses. If some residual gas remains then it gives rise to seeds and bubbles. Therefore, it is necessary to either redissolve this or eliminate it otherwise. Barium containing glasses present special problems and it is generally very difficult to produce such glass without seeds and bubbles within acceptable limits. This write up describes a novel approach of using Barium silicate in the glass batch composition, which minimizes the presence of entrapped gases. This approach has been used in producing good quality LBC (Light Barium Crown) optical glass.

Full Text

The present invention relates to a process for the manufacture of barium crown glass. The main uses of barium crown glass is in making scientific and professional instruments such as analytical instruments, photographic cameras, projection instruments, office equipments, astronomical instruments, survey instruments, magnifiers, testing and measuring instruments, eye-precision instruments, medical and diagnostics instruments, night vision equipment, laser range finders, observation devices, protective glasses, night goggles, articulated sights, optical sights. Glass batch, originally a mixture of granular materials, responds to heating by producing new solid and liquid phases and by releasing gases. The reaction paths involve intermediate products and complex reaction mechanisms, in which gas evolution, liquid formation, wetting, surface phenomena and diffusion play a role. When the molten salt disappears, the glass forming melt, initially distributed in the form of drops or thin layers of coating solid particles, connects into islands that join into an interconnected body with open pores (vent holes), through which gases escape, or closed pores (bubbles) that either blow the mixture into a foam or, if viscosity is low move upwards due to buoyancy. Eventually the mixture becomes a liquid with suspended refractory grains and bubbles. In glass melting furnaces, the material is usually subjected to a complex temperature history, the temperature field is spatially nonuniform, and batch melting is generally nonlocal: percolation of hot gases, drainage of liquids, runoff of the molten materials from the top surface, and plunging of heavy drained melt into molten glass, make batch melting sensitive to geometrical configuration. Batch melting processes can be classified into three groups: particle melting, blanket melting, and pile melting. In the particle melting, although each batch particle undergoes the same temperature history; interaction with ambient gases may be a problem. In the blanket (cold cap) melting, segregation and percolation do not necessarily effect homogeneity and interaction between batch and atmosphere is minimized. Pile melting is nonuniform process in which heat transfer, flow, melting reactions, and bubble removal are coupled, and steep temperature gradients and cellular convection are typical. BaO containing glasses such as barium crown glass has several important critical properties such as constancy of refractive indices and Abbe vaules as referred by USP 5300467, USP 5316717, USP 5392816, USP 5390312. But BaO glasses generally show excessive foaming during melting. This foam causes several problems. A layer of foam on top adversely affects heat transfer from the flames and, thus, lower layers of glass may stay at lower temperatures, which cause refining problems. Moreover, stirring of glass melts with foam on top also leads to trapping of gas from the foam phase which again result in problems of seeds and bubbles. It is thus imperative that foam formation is reduced as far as possible if not eliminated altogether. In direct-fired glass melting furnaces normally a foam layer or blanket covers about one third of the bath surface reffered in J. Kappel and H. Roggendorf; Origin, Stability and Decay of foam on Glass Melts, pp.-351-359.Because of its low heat transfer coefficient the foam can reduce the heat flow from the overhead combustion to the underlying glass melt by as much as 60 %. It is said that in contrast to polyhedral soap foams, glass foams contain spherical bubbles, which are independent of each other with regard to size (1 to 10 mm) and to certain extent with regard to their relative motion. Two mechanisms contribute to the foam formation near the charging area. 1. The large amount is produced by the release of batch gases like CO2 and SO2 2. The smaller amount originates from gases like N2, O2 or H2O being entrapped in the batch. It has been reported that oxidized glass melts with a high sulfate content tends to foaming than reduced glass melts, Probably this is due to the lowering of the surface tension of the melt by SO42" ions. During melting of glass several transportation processes occur, such as the drainage of the liquids melt out of the foam lamellae, gas diffusion and material exchange between melt and the glass and thus foam stability. A measure for foam stability is the mean lifetime of the bubble. Lifetime ends by the tear of the bubble walls prior to thinning by the liquid drainage effect. Some gases have strong effect on shortening of life. The oxidation state of the melt has a significant influence on the lifetime of the bubbles. Drainage is not the rate determining step of foam decay. It is now established that oxidising atmospheres prevent foaming and reducing atmospheres destabilize existing foams. Thus the practice of "smoky fire" i.e. reducing flame is a good means to destabilize foams. Another method to decrease the oxygen content of the atmosphere near the foam surface is to spread rapidly combustible materials such as oil or carbonaceous powder on the foam layer. Oxygen depletion helps tearing of bubble films. An increase in partial pressure of water also helps in foam decay as it helps evaporation of liquid from foam films. Spurting of water can also destroy foam (J. Kappel and H. Roggendorf; 2nd International Conference on Advances in Fusion and Proceedings of Glass, Drusheldorf,1996 Origin Stability and Decay of foam on Glass Melts, pp.- 351-359). Earlier practice was to melt high barium containing batch or glasses in reducing flame and furnace atmosphere. It was felt that oxidising conditions would cause formation of barium peroxide (BaO2), which at a certain lower temperature may release oxygen resulting in formation of persistent seeds. Literature survey showed that barium peroxide is formed at 500°C and dissociates at 800°C(J.D.Lee, A New Concise Inorganic Chemistry, ELBS, (1975), pp. 144-145). The literature also indicates that barium containing glasses not only retains CO2 in the melt but also reacts with CO2 from the furnace atmosphere between 1100 to 1350°C. Barium glasses pose some more difficulties during melting because there may be an unfavourable reaction path which causes melt segregation. Although the composition may be in a homogeneous region of the phase diagram of the ternary SiO2-BaO-B2O3 system, the composition may be very close to zones of liquid - liquid immiscibility. An unfavourable chemical reaction path may lead to localized variation in meltcomposition, part of which could be within liquid-liquid immesibility zone of the phase diagram. This, in turn will be an incipient source of straie in glass. In barium borosilicates, melting reaction proceeds at very low temperature by the interaction between boric acid and barium carbonate. The reaction is exothermic and with moist batch it will occur at roomtemperatures with heat generation. Barium borate formed melts above 900°C to form a dense liquid, which tends to separate away from silica making subsequent formation of homogeneous glass difficult. The drawbacks of the hitherto known processes for making barium crown glass may thus be summerised as follows. 1. The glass shows excessive foaming during melting. 2. Heat transfer from the surface of the melt to the bulk is adversely affected. 3. BaO containing glasses retains excessive amount of CO2 4. BaO containing glasses causes melt segregation making the product in homogeneous. 5. BaO containing glasses contain more seeds and bubbles. The main object of the present invention is to provide a process for the manufacture of barium crown glass which obviate the drawbacks as mentioned above. Another object of the present invention is to produce homogeneous barium crown glass. Yet another object of the present invention is to produce glass with lesser number of seeds and bubbles. BaO containing glasses are often seen to contain many seed and bubbles on cooling. One reason for this is that barium glass can hold onto CO2, which is released only at around a particular temperature. One way to reduce the CO2 content would be to eliminate the gas at lower temperatures, through solid state reaction. BaCO3 (m.p. 1740°C) reacts with silica to form several silicates, which are normally as follows : BaSiO3 (BaO/SiO2molar ratio 1:1) melting point 1604°C BaSi2O5 i.e. BaO.2SiO2 (BaO/Si02 molar ratio0.5), melting point 1420°C Ba5Si8O21 i.e. 5BaO.8SiO2 (BaO/SiO2 molar ratio0.625), melting point 1446°C Ba2Si3O8 i.e. 2BaO.3SiO2 (BaO/SiO2 molar ratioO.666), melting point 1447°C It is also observed SiO2-Ba2Si2O5 eutectic at 1378°C (at 20 percent Ba2Si3O8 mole present in a SiO2- Ba2Si3O8 system) and another in the BaSi2O5-Ba2Si3O8 system that melts at 1410°C (~ 34.5 mole percent Ba2Si3O8). This implies that solid-state reactions must be done well below 1378°C. If a mixture of BaCO3 and SiO2 is heated then there can be no liquid phase upto 1378°C. BaC03 liberates CO2 at temperature much higher than CaC03 where pCO2 reaches 1 atm. at about 900 C.For BaC03 pco2 is 1 atm only at 1400°C or so9. BaC03, however, reacts with silica at 700-750°C If there is Na2CO3 in the batch then the reaction is initiated at as low as 400°C and the first melt may appear at 600°C. Ba0.2Si02 forms a eutectic with sodium di-silicate at 32 percent BaO.SiO2 and the eutectic melts at about 797°C. BaO retains CO2 in the melt in optical glasses. This CO2 is given off by decomposition of BaC03 and also by the furnace atmosphere. The dissolution reaction is said to specially occur in the range 1100-1350°C. BaO glasses is difficult to refine specialty when BaO >7pct. It may be possible to eliminate a good part of the gases through solid state decomposition and thus eliminate the possibility of those gases being entrapped by liquid glass. It appears that the major gas is CO2 from carbonates. Therefore, in order to reduce the foaming action during glass melting operation and to reduce the residual entrapped gas bubbles in the solid glass formed, BaC03 may be converted to barium silicates by solid-state sintering of a mixture of BaC03 + silica. This will reduced the amount of BaO in the glass melt which in effect will cause trapping of lesser number of gas bubble in the solidified glass body. In addition to this more homogeneity of the glass in term of optical properties will also be achieved. Thus the novelty of the process is production of seed and bubble free and optically homogeneous barium crown glass by hitherto unknown novel inventive step of using barium silicate instead of barium carbonte and silica in the glass batch composition. Accordingly, the present invention provides a process for the manufacture of barium crown glass which comprises mixing purified granulated silica sand 41.3-36%, Barium Nitrate (Ba (NO3)2) 4-6%, Borax (Na2B4O7) 7-8%, soda ash (Na2CO3) 5-6.5%, K2CO3 10-13.5%, ZnO 3-4%,As2O3 0.2-0.4%, Barium silicate 26-29%, charging these raw materials in the glass melting pot kept, at a temperature in the range of 400 to 500°C, characterized in that the use of barium silicate reduces seed and bubbles in the end product ,melting the raw materials at a temperature in the range of 1250 to 1450°C, stirring the melts at a temperature in the range of 1200 to 1400 C for a period in the range of 10 to 12 hours, followed by cooling to obtain the desired product. In an embodiment of the present invention 15 to 20 % glass cullet of the total batch, may be charged in the glass melting pot kept at a temperature in the range of 1000 to 1100°C before charging the sintered raw materials for melting. The followings examples are given by the way of illustration and therefore should not be construed to limit the scope of the present invention. Example -1 62.10 kg BaCO3 and 37.87kg Silica is mixed thoroughly in a ball mixer .The mixed material is sintered in a furnace at 1250°C for 8 hours. The sintered materials obtained was ground and identified as barium silicate. Silica sand 38.97%, Barium Nitrate (Ba (NO3)2) 4.34%, Borax (Na2B4O7) 7.21%, soda ash (Na2CO3) 5.26%, K2CO3 11.82%, ZnO- 3.65%,As2O3 0.25%, Barium silicate 28.5%.is mixed thoroughly and then charging the materials in a pot kept at 400°C, melting the raw materials at a temperature 1420°C,stirring the glass melts at a temperature 1220°C for a period of 10 to 12 hours, then the glass melting pot is taken out from the furnace and put into a cooling can to obtain the glass. Example - 2 61 kg BaCO3 and 38.97 kg Silica is mixed thoroughly in a ball mixer .The mixed material is sintered in a furnace at 1280°C for 8 hours. The sintered materials obtained was ground and identified as barium silicate. Silica sand 39.97%, Barium Nitrate (Ba (NO3)2) 5.34%, Borax (Na2B4O7) 7.25%, soda ash (Na2CO3) 5.22%, K2CO3 10.82%, ZnO- 3.65%,As2O3 0.25%, Barium silicate 27.5%.is mixed thoroughly and then charging the materials in a pot kept at 430°C, melting the raw materials at a temperature 1450°C,stirring the glass melts at a temperature 1250 °C for a period of 10 to 12 hours, then the glass melting pot is taken out from the furnace and put into a cooling can to obtain the glass. Example - 2 61 kg BaC03 and 38.97 kg Silica is mixed thoroughly in a ball mixer .The mixed material is sintered in a furnace at 1280°C for 8 hours. The sintered materials obtained was ground and identified as barium silicate. Silica sand 39.97%, Barium Nitrate (Ba (NO3)2) 5.34%, Borax (Na2B4O7) 7.25%, soda ash (Na2CO3) 5.22%, K2C03 10.82%, ZnO- 3.65%,As2O3 0.25%, Barium silicate 27.5%.is mixed thoroughly and then charging the materials in a pot kept at 430°C, melting the raw materials at a temperature 1450°C,stirring the glass melts at a temperature 1250 °C for a period of 10 to 12 hours, then the glass melting pot is taken out from the furnace and put into a cooling can to obtain the glass. Example -3 61.5 kg BaCO3 and 38.5 kg Silica is mixed thoroughly in a ball mixer .The mixed material is sintered in a furnace at 1250°C for 8 hours. The sintered materials obtained was ground and identified as barium silicate. Silica sand 38.97%, Barium Nitrate (Ba (NO3)2) 5.34%, Borax (Na2B4O7) 7.35%, soda ash (Na2CO3) 6.22%, K2CO3 12.82%, ZnO- 3.6%,As2O3 0.3%, Barium silicate 25.5%.is mixed thoroughly and then charging the materials in a pot kept at 430°C, melting the raw materials at a temperature 1425°C,stirring the glass melts at a temperature 1230 °C for a period of 10 to 12 hours, then the glass melting pot is taken out from the furnace and put into a cooling can to obtain the glass. Example - 4 61 kg BaCO3 and 40 kg Silica is mixed thoroughly in a ball mixer .The mixed material is sintered in a furnace at 1250°C for 8 hours. The sintered materials obtained was ground and identified as barium silicate. Silica sand 37.67%, Barium Nitrate (Ba (NO3)2) 5.84%, Borax (Na2B4O7) 7.55%, soda ash (Na2CO3) 5.22%, K2CO3 12.57%, ZnO- 3.50%,As2O3 0.4%, Barium silicate 27.25%.is mixed thoroughly and then charging the materials in a pot kept at 470°C, melting the raw materials at a temperature 1445°C,stirring the glass melts at a temperature 1235 °C for a period of 10 to 12 hours, then the glass melting pot is taken out from the furnace and put into a cooling can to obtain the glass. Example -5 62.5 kg BaCO3 and 37.48 kg Silica is mixed thoroughly in a ball mixer .The mixed material is sintered in a furnace at 1250°C for 8 hours. The sintered materials obtained was ground and identified as barium silicate. Silica sand 37.5%, Barium Nitrate (Ba (NO3)2) 5.54%, Borax (Na2B4O7) 7.6%, soda ash (Na2CO3) 5.22%, K2CO3 13.20%, ZnO- 3.85%,As2O3 0.38%, Barium silicate 26.71%.is mixed thoroughly and then charging the materials in a pot kept at 500°C, melting the raw materials at a temperature 1445°C,stirring the glass melts at a temperature 1235 °C for a period of 10 to 12 hours, then the glass melting pot is taken out from the furnace and put into a cooling can to obtain the glass. Advantages: The present method of making barium crown glass has following advantages: 1. There is less number of seeds and bubble in end product. 2. The intermediate compounds have better reactivity. 3. Most of the gases are expelled during sintering not during melting. 4. Foaming of glass becomes less. 5. Quality of the glass is improved. We claim: 1. A process for the manufacture of barium crown glass which comprises mixing purified granulated silica sand 41.3-36%, Barium Nitrate (Ba (NO3)2) 4-6%, Borax (Na2B4O7) 7-8%, soda ash (Na2CO3) 5-6.5%, K2CO3 10-13.5%, ZnO 3-4%,As2O3 0.2-0.4%, Barium silicate 26-29%, charging these raw materials in the glass melting pot kept, at a temperature in the range of 400 to 500°C, characterized in that the use of barium silicate reduces seed and bubbles in the end product ,melting the raw materials at a temperature in the range of 1250 to 1450°C, stirring the melts at a temperature in the range of 1200 to 1400°C for a period in the range of 10 to 12 hours, followed by cooling to obtain the desired product. 2. A process as claimed in claim 1 wherein 15 to 20 % glass cullet (recycled glass) of the total starting material, is charged in the glass melting pot kept at a temperature in the range of 1000 to 1100°C before charging the sintered raw materials for melting. 3. A process for the manufacture of barium crown glass substantially as herein described with reference to the examples.

Documents:

333-DEL-2002-Abstract-(17-09-2008).pdf

333-del-2002-abstract.pdf

333-DEL-2002-Claims-(17-09-2008).pdf

333-del-2002-claims.pdf

333-DEL-2002-Correspondence-Others-(17-09-2008).pdf

333-del-2002-correspondence-others.pdf

333-del-2002-correspondence-po.pdf

333-DEL-2002-Description (Complete)-(17-09-2008).pdf

333-del-2002-description (complete).pdf

333-DEL-2002-Form-1-(17-09-2008).pdf

333-del-2002-form-1.pdf

333-del-2002-form-18.pdf

333-DEL-2002-Form-2-(17-09-2008).pdf

333-del-2002-form-2.pdf

333-del-2002-form-3.pdf