Unit 14 Manufacture of Glass: Present Trend in Industrial Glass Melting
Introduction
The objective of industrial glass making is to produce the required quality at the lowest possible global cost. This has been achieved in the past by continuous evolution ever since the invention of the Siemens tank furnace. Technical innovations such as the fusion cast refractory and the substitution of oil or natural gas for producer gas simply accelerated progress and the trends were always in the same direction. What is new is that the cost of glass now includes the price of preserving the environment and, because of the uncertainty as to the most economical solutions to the environmental problems, there is also uncertainty about future trends.
Although these problems are the center of much attention the industry will continue to evolve by integration of technical developments and in reply to commercial requirements. This short review will begin with products and quality. A chapter on environmental considerations will then be proceeded by remarks on refractories and followed by a paragraph on modeling.
New Products
New products may require new compositions and in this case new or modified methods of melting may also be needed. The three following examples illustrate the range of the problems involved.
1. Anti Solar Glass
New colours for automobile or architectural glazing have in general been produced with little difficulty. However, when the product is required to be as opaque as possible to near infrared radiation it follows that melting and tank behavior will be distributed by the reduced radiant heat transfer through the molten glass.
Another difficulty is that to reach the required ferrous iron level, a reducer must be used and the usual sulfate additions limited. This entails the risk of inadequate silica digestion. In addition, a high ferrous iron content may be accompanied by the formation of traces of sulfide and unwanted amber coloration. The fact that it is not easy to obtain the desired characteristics in a normal furnace inspired the conception of an entirely new glass melting process: refining is carried out under reduced pressure which also removes the traces of sulfide. This technology remains, for now, the exception that proves the rule that progress in glass making has usually been achieved by evolution rather than revolution.
2. Zero Expansion Transparent Glass Ceramics
Glass ceramics designed to resist thermal shock are based on the lithia alumina silica system whose thermal expansivity, after ceramisation, is almost zero. The basic problems in the elaboration of the parent glass are its high viscosity, which is close to 400 poises at 1600C, and the digestion of the nucleant. Techniques for melting such glasses have however been developed over the last 40 years. Four products have been successively commercialized: opaque cooking ware, opaque cooker tops, transparent cooking ware and, more recently, the transparent cooker tops. Each of these new products has given rise to significant progress in glass quality. The transparent cooker top in particular implies not only absence of visible macroscopic defects but also the absence of even smaller inclusions that might prevent the mechanical shock resistance from reaching the particularly high level required.
3. Glass for Active Matrix Liquid Crystal Display Panels
For this application, thin sheets of glass are required containing no seed bigger than a few hundreds of a millimeter and with perfectly flat surfaces obtained with as little polishing as possible after forming. These two basic requirements imply unusually high degrees of refining and of homogeneity. Other properties are also essential: a high strain point (to avoid "compaction" during the heat treatments associated with the production of the grid of transistors) absence of alkali (which might diffuse into the liquid crystal and transistor layers) low expansivity (to match the amorphous silicon used) and resistance to acids (used during the process). The absence of alkali and relatively low expansion leads to the use of B2O3 which, being somewhat volatile, may eventually complicate the attainment of the required homogeneity.
In spite of these difficulties, such substrates are being produced in quantities sufficient for the expanding market for portable computers. Even more rapid expansion is expected in the future as the cathode ray tube gives way to the flat screen in new generation of television receivers.
Electric Melting
The cold top electric furnace is essentially non polluting and is chosen for melting glasses such as fluoride opal or lead crystal. Its greatest handicap is the cost of electric power which in Europe is more expansive than gas by a factor that varies from 3 in Switzland to 10 in Spain. Electric melting is not likely to be adopted simply to avoid dust, NOx and SOx emission. On the other hand, because the thermal efficiency of electric melting is less sensitive to furnace size, small electric furnaces can be used economically in countries where electricity is not too expensive.
In spite of the growing attention to emissions, global electricity consumption by the glass industry in Europe is probably decreasing. As the gap widens between electricity costs and fuel costs it has become more profitable to increase furnace size than to use electric boosting. This is particularly true for furnaces making containers, which represent 65% of all glass produced.
Selected from "Proceedings of XVII International Congress on Glass Vol. 1 Invited Lecture", Gong Fangtian, International Academic Publishers, 1995
Words and Expressions
global 全球的，全面的
anti solar glass 防晒玻璃
glazing 釉化
reducer 还原剂
entail 承担
digestion 消化
amber 琥珀
lithia alumina silica system 锂铝硅系统
ceramisation 陶瓷化
elaboration 精制
poise 泊
nucleant 核化剂
active matrix 活化阵列
seed 种子，小气泡
refine 细化
homogeneity 均化
grid 栅极
volatile 挥发的
attainment 获得
cold top electric furnace 冷顶电炉
handicap 缺陷
producer gas 发生炉煤气
