Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 141

Commonly used raw materials for E‐glass fiber include sand (SiO₂) as the primary silicon source, kaolin or china clay consisting of kaolinite (Al₂Si₂O₅(OH)₄) as the primary source of aluminum, limestone (CaCO₃) or quicklime (burnt lime, CaO) as that calcium, and boric acid (H₃BO₃) as that of boron. More complex minerals are often used in place of or in combination with these major components to manage cost, improve melting performance, or utilize locally sourced ingredients. Some examples of commonly used complex minerals include pyrophyllite (Al₂Si₄O₁₀(OH)₂), colemanite (Ca₂B₆O₈(OH)₆∙2H₂O), ulexite (NaCaB₅O₆(OH)₆∙5H₂O), borax (Na₂B₄O₅(OH)₄∙8H₂O), fluorspar (CaF₂), dolomite [(Mg, Ca)(CO₃)₂], spodumene (LiAl(SiO₃)₂), and soda ash (Na₂CO₃). For C‐glass fiber, albite (NaAlSi₃O₈) is commonly used to reduce the need for soda ash. For simplicity, primary phases of the above natural raw materials are used; all of them can contain other mineral phases and/or impurities, for example, iron from ore and grinding process.

Batch reaction chemistry and kinetics in glass melting have been widely studied with various experimental techniques, including differential thermal analysis (DTA), differential scanning calorimetry (DSC), X‐ray diffraction, electrical resistance and temperature distributions from molten glass to the batch pile, for laboratory batch sizes from as little as 0.1 g up to 10 kg in mass [12–14]. Models have also been developed from a thermodynamic approach [15].

As illustrated in Figure 3, the batch‐to‐melt conversion process of representative E‐glass and ECR‐glass batches can be characterized in the following steps:

1 De‐hydroxylation of batch ingredients, i.e. removal of both moisture water (120–150 oC) and crystalline water in the minerals, (colemanite, 350–450 oC; kaolinite or pyrophyllite, 500–600 oC).Figure 3 Differential scanning calorimetry output of E‐glass batches with and without boron from room temperature to 1400 oC (10 K/min heating rate), illustrating both endothermic and exothermic reaction events as batches are converted into glass through initial steps of dehydration and decarbonation. The intermediate silicates formed are anorthite (CaAl2Si2O8) and/or wollastonite (CaSiO3).Source: PPG, 2014.

2 Decarbonization, i.e. removal of carbonates (limestone, 700–800 oC).

3 Formation of intermediate crystalline phases (~1000 oC).

4 Liquid phase formation (1000–1400 oC).

5 Final dissolution of residual phases (sand, 1250 oC) [12].

The majority of the batch reactions are endothermic, the energy consumption of these steps constituting a significant portion of the total energy spent in the glass melting process. Finally, the glass fining step, which varies as a function of furnace throughput, consumes even more energy. The fiberglass industry is continuously searching for new raw materials to lower the energy consumption in batch melting [12] and improved technology to increase the furnace throughput without sacrificing glass quality. This in turn reduces the unit energy consumption of glass melted and glass fiber produced. In practical terms, successful implementation of these approaches can provide savings of up to 20% compared to typical E‐glass melting energy demands of 7000–8000 kJ/kg of glass [12].

One generally carries out glass melting and fining by controlling the melt temperature of the furnace hot spot near or higher than T_M at which the melt viscosity is 10 Pa∙s (Figure 4), which will ensure sufficient fluidity of the molten glass to promote both mixing of the batch components and removal of seeds (or bubbles). The latter is also aided by the use of fining agents such as sodium sulfate (Na₂SO₄), which is commonly used in making E‐glass fibers. However, because much higher melting temperatures are often required for specialty glass fibers, other fining agents such as manganese dioxide (MnO₂) or cerium dioxide (CeO₂) are also used with or without added sodium sulfate. Fining agents may also serve as oxidizing agents to control the iron oxidation states for both glass melting and fiber drawing process as will be discussed in more detail in a later section.

Figure 4 Viscosity curves of E‐glass, E‐CR‐glass, AR‐glass, D‐glass (derivate), S‐glass, and C‐glass, illustrating composition effects on reference temperatures of glass melting/fining and fiber drawing.

Source: PPG, 2014.

Fining action from the decomposition of Na₂SO₄ dissolved in the melt is created as the molten glass stream moves toward the higher temperature fining zone, or hot spot, of the furnace. The melt with dissolved sulfate becomes oversaturated at higher temperatures (>1350 ^oC) and sulfate decomposition and bubble formation begins to occur. The onset temperature of sulfate decomposition depends on many factors – water vapor pressure in the furnace combustion space, water dissolved in the glass, organic species in the batch materials, any addition of carbon to the batch, partial pressure of oxygen above the melt, and others [16]. Driven by buoyancy action, bubbles containing SO₂ and O₂ travel to the melt surface; along the way the sulfate bubbles expand, stripping smaller bubbles of CO₂ (primarily from decomposition of carbonates) and air (primarily N₂) trapped in the melt. The sulfate fining process can be described by the reaction

(1)

where O²⁻ is the free oxygen, which depends on glass composition. With other fining agents, for example CeO₂, oxygen fining gas is generated according to the reaction

(2)

Adequate use of fining agents is critical. The quantity needs to be optimized based on the furnace firing setup, types of fuel and air or oxygen used, glass composition, and types of fiber products to be made. Excessive use of fining agents can result in uncontrolled melt foaming. The foam layer reduces energy transfer from the combustion space to the glass melt beneath the foam blanket, resulting in energy waste because of excessive firing required to maintain the underglass temperature and in poor glass quality caused by inhomogeneous melting. Excessive foam can also lead to potential overheating of the furnace crown, shortening the furnace service life.