07 - Use of Sulfur Dioxide in Chinese Float-Glass Production Lines - Am C Society Bulletin [PDF]

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07 Use of Sulfur Dioxide in Chinese FloatGlass Production Lines Am C Society Bulletin Dataset · October 2012

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Fig. 1 Sketch of a transitional roll desk used in float-glass production.

Use of Sulfur Dioxide in Chinese Float-Glass Production Lines Sulfur dioxide injected near the transitional roll desk in a float-glass production line can improve the physical, chemical and mechanical properties of the glass surface, but can cause pollution to the tin bath, production line and workplace. Liu Shimin, Qin Guoqiang and Li Dongchun

China has a great capacity for the use of float-glass and to supply the world market demand. About 145 float-glass production lines are currently in operation.1 Chinese float-glass technology (Luoyang float technology) is used in 85% of the lines. Other float techniques originated by Pilkington, PPG, Saint-Gobain and other companies also are used. However, many problems exist in the Chinese glass industry. Some of the problems are low product quality, high energy and resource consumption, serious environment pollution and lack of development of new techniques. These problems conflict with Chinese development policy and the world economic trend. Some modifications recently have been applied to many float lines that use the Luoyang float-glass technique to improve the glass quality. In the Luoyang technique, SO2 gas is injected near the transitional roll desk to eliminate tin pickup, bloom phenomenon and alkali oxides on the float-glass surface.2,3 The effect of SO2 gas on the tin permeation mechanism into the float-glass has been studied.4–6 The floatglass process is composed of many complex chemical reactions that relate to each other. Therefore, all the effects caused by a modification need to be investigated systematically before the modification is applied. The effects of the SO2 gas used in the float line are considered in this study. SO2 gas is injected at the transitional roll desk directly toward the under surface of the float-glass (Fig. 1) at temperatures of 550–600°C, which is near the float-glass transition temperature. The transitional roll desk is ~1.5 m long, and the velocity of float-glass here is ~250–390 m/h. Therefore, the duration of float-glass in the SO2 atmosphere is ~10 s. Because this is an open facility, SO2 gas inevitably enters the tin bath through

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Chinese Float-Glass Production diffusion, reacts with the hot roll and diffuses into the workplace. All these factors need to be investigated carefully.

Alkali Effects Alkali cations (K+, Na+ and other species) have high mobility and loose structure in the float-glass melt. Therefore, in the float-glass process, they concentrate at the surfaces (especially the under surface) of the floatglass at the annealing temperature. This forms an alkali-enriched layer, which is harmful to the float-glass.2,4 In a tin bath, the liquid tin is easily oxidized by oxygen at concentrations as low as 10 ppm. The reactions are Sn + 0.5O2 = SnO SnO + 0.5O2 = SnO2 Sn2+ and Sn4+ permeate into the interior of the float-glass in response to the electric field of the negatively charged O2–. At the same time, other small cations, i.e., Na+, K+, Ca2+ and Mg2+, diffuse to the surfaces at various migration rates because of the electric field. These cations form oxides and adhere to the surface of the float-glass because of van der Waals forces. This ion-exchange process occurs between the under surface of the float-glass and the liquid tin as well as between the top surface of the float-glass and the air. However, because of the strong charge compensation effect and the higher tin concentration in the liquid tin, much more alkali oxide is produced at the liquid-tin side than at the air side on the glass surfaces. The alkali oxide layer tends to react with water and CO2, which results in a sharp decrease in the chemical stability of the float-glass. After they are treated with SO2, the alkali oxides change to a loose sulfate layer that can be brushed off easily using water or cloth. Research has shown that removal of alkali oxide using the SO2 treatment leaves a surface that is enriched with SO2. This surface layer is chemically more stable and is harder and stronger compared with the original float-glass surface.7 Therefore, the SO2 treatment improves the physical, chemical and mechanical properties of the surface of float-glass by removal of alkali oxide.

Tin Doping The influence of SO2 on doping of tin into float-glass also has been studied using computer-modeling methods.5 A one-dimensional finite-difference computer model of the penetration of tin into float-glass has been built to investigate the tin concentration profile in the glass. The tin profile with a satellite peak is formed only when an adequate amount of Sn2+ cations are oxidized to Sn4+. The Sn4+ cations then diffuse into the glass while the glass ribbon passes through the SO2 atmosphere with a certain temperature distribution. The tin profile is little changed when the glass ribbon passes through a continuous lehr under air. However, it is changed when the glass ribbon passes through a lehr under an SO2 atmosphere. Therefore, it is concluded that SO2 treatment can eliminate the bloom phenomenon, which is caused by the oxidation of tin ions in the near surface of float-glass. Other researchers6 also have investigated the effect of SO2 on float-glass experimentally. Raw float-glass samples have been placed under a transitional roll desk for various times under SO2 atmosphere in a production line. No obvious variety in the tin depth profile or the existence of satellite peak at any depth from the glass surface has been found using X-ray fluorescence (XRF) and energy-dispersive spectroscopy (EDS).

Differences in Simulation and Experiment The different results between computer simulation and experimental method can be attributed to the differences in original conditions adopted in the two studies. The experiment was adopted in the actual production condition to seek the real consequences, whereas a simplified approximation was used in the computer model to describe the change trend. However, both studies affirm that the relative content and the distribution of tin ions, mainly existing in the form of Sn2+ in the fresh float-glass just from tin bath, change because of the presence of the active strong oxidant SO2. The effect of SO2 on transitional roll is significant.8 X-ray diffractometry (XRD) analysis on the sediment attached to the transitional roll indicates that sulfur content in the surface of the transitional roll increases to a saturation value under the treatment of SO2 gas. The formed loose-layered sulfide decreases dramatically the microhardness and the frictional coefficient of the transitional roll surface. This layered sulfide

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Chinese Float-Glass Production acts as a lubricant, which prevents the fresh float-glass from being damaged by the hard transitional roll. Moreover, the sulfate formed by the reaction between SO2 gas and the Na2O-enriched surface of the floatglass protects the glass during storage and portage.

SO2 Disadvantages The temperature of the glass at the transitional roll desk is in the range of 550–600°C, in which SO2 diffuses into the tin bath, even though the atmospheric pressure in the tin bath is positive (~20–30 Pa) compared with that outside the bath. SO2 is a typical active strong oxidizing agent. It intervenes with the reactions that occur in the tin bath and changes the chemical environment there. Therefore, excess SO2 causes pollution to tin bath. XRF analysis shows that the sulfur content in the tin bath increases near the exit of the tin bath. The sulfide is confirmed to be mainly SnS2 using XRD analysis.6 In addition to the sulfur that is introduced in the raw material, it has been determined that SO2 diffuses into the tin bath and reacts with the tin. This results in a larger tin consumption and a change in the chemical environment in the tin bath. The tin bath is the most important region for the float process, because the glass melt is formed as a result of the physical and chemical reactions there. If the tin bath operates in an atmosphere with a high sulfur concentration, the product quality deteriorates and the product cost increases. Moreover, hidden trouble may occur in the long term. Some measures, such as increasing the atmosphere pressure in tin bath and adjusting the position and direction of SO2 injector, should be taken to decrease the threat of SO2 on the tin bath. The transitional roll desk is open, which is different from other thermal facilities in the float line. The flux of SO2 injected here is ~0.05–0.1 L/min, with a maximum value of 0.5 L/min. Therefore, the SO2 gas diffuses into the surrounding workplace, which causes a waste of the SO2 gas, increase in product cost, pollution of the production line and workplace, and threat to the health of the workers. The effects of SO2 gas on the float-glass and the facilities have not been totally clarified. The inadequate knowledge obtained is mainly presumed from the observed results and lack of direct proof to support a strong conclusion. Therefore, it is difficult to understand the effects and mechanisms in a comprehensive manner. The disagreement between experiments and computer simulations on the effect of SO2 on penetration of tin clearly suggests the current status. A little is known about what needs to done, but exact reasons are unknown. This makes the application of SO2 gas in the float line tentative, and we cannot control its effects and consequences. Therefore, the removal of alkali oxide and the elimination of bloom on float-glass always must be emphasized. Large amounts of SO2 are used with total ignorance of the pollution problems, which are vital to the actual application. For example, heavy SO2 dosage aggravates the erosion on the surface of the rolls in the annealing furnace.9 Consider a float line that has a capacity of 500 ton/d. After 550,000 tons of float-glass have been produced, the surfaces of rolls from No. 1 to No. 7 become scraggy and coarse. This causes roller scratches on the under surface of the float-glass. However, unfortunately, the line usually has to run in this abnormal condition for 3–5 years. Moreover, a great deal of SnS is formed because of the SO2 pollution in the tin bath. Therefore, more SO2 is used to eliminate the tin pickup, which results in a dead loop. Generally, any modification on a complicated system, such as the float process, should be based on the deep understanding of its effects and consequences. Therefore, the proper use of a new technique can be achieved to exert its advantage and avoid its disadvantage. As far as SO2 is concerned, it should be used only with caution in the float line. A comprehensive investigation to clarify the effects of SO2 in the float line should be performed before massive applications of this technique are initiated. ■

About the Authors Liu Shimin, Qin Guoqiang and Li Dongchun are research staff members with the Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, Hebei, People’s Republic of China. Liu Shimin and Li Dongchun are Professors in the Dept. of Materials Science and Technology, Yanshan University. Qin Guoqiang is a Master at Yanshan University. Correspondence regarding this article should be addressed to Liu Shimin via e-mail at [email protected].

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