Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 82

As indicated by old glass specimens, bubbles cannot be completely eliminated with only physical fining. In a somewhat paradoxical way, better results are achieved if additional bubbles are produced within the melt at a sufficiently high, yet not too high, volume fraction to coalesce with the bubbles formed or entrapped during melting. The process is known as chemical fining as it involves reactions with gas‐releasing substances.

For reasons of cost, chemical compatibility, and effectiveness, the most widely used agent is sodium sulphate (Na₂SO₄). By experience, 4 kg of Na₂SO₄ are added per ton of produced glass. During the early stages of batch melting, the sulfate dissolves in the melt. Under oxidizing conditions, it decomposes at 1400–1450 °C according to the reaction

(10)

 where the braces {−} denote the state “dissolved in the melt.” Under reducing conditions, sodium sulphate reacts with the Na2S formed during primary batch melting as follows:(11)

The latter reaction already occurs at temperatures slightly below 1400 °C.

Oxygen fining is an alternative option. The agent typically used is Sb₂O₃; it is added to the batch in amounts of 3–5 kg per 1000 kg of sand, in combination with a four‐ to eightfold amount of NaNO₃ [5]. At the moderately low temperatures of primary batch melting, Sb₂O₃ converts to {Sb₂O₅} provided that a sufficiently high oxygen partial pressure in the batch is established (Figure 4, stage b). This is achieved by the action of NaNO₃, which decomposes at batch melting temperatures to release oxygen:

(12)

At increasing temperatures, the higher valences of polyvalent ions become increasingly unstable (see Chapter 5.6) so that the fining reaction actually reads

(13)

The release of oxygen bubbles reaches its maximum at about 1300 °C and extends beyond 1400 °C. The negative side effect of this procedure is the formation of the NO_x pollutant.

A simple calculation will finally explain why experience and empirical knowledge still play the predominant role in the allotment of fining agents. As used in the batch in Table 4, a mass of 4 kg of Na₂SO₄ represents 56.3 mol of SO₂, which, at 1400 °C, 1 bar, would fill a volume of 7.6 m³. Now, 1 ton of melt, by contrast, fills 0.4 m³ only. Obviously, only a very minor part of the nominal SO₂ ends up in gas bubbles otherwise a foam instead of a clear melt would be obtained. The major part of SO₂ is in fact lost during batch melting, by evaporation from the melt surface, or is retained in the glass. Thus, the proper allotment of fining rests on the small difference between sulfate input and the above losses. One of the rare attempts to perform a detailed sulfur balance of a glass furnace revealed that approximatively 0.25–0.3 kg of the sulfate added per t of glass are released in the form of fining bubbles [6].