The content of SiO 2 in glass is very high, generally 50-80%. In ordinary bottles, containers and flat glass, the content is 70-75%. In quartz glass, the content is over 98%. SiO 2 forms a framework in glass, which gives glass good chemical stability, thermal stability, transparency, higher softening temperature, hardness and mechanical strength. However, with the increase of glass content, the melting temperature and the viscosity of glass liquid increase.
Na2O is a good flux. It can react with SiO 2 to form silicate at lower temperature. It can reduce the viscosity of glass liquid and accelerate the melting speed of glass. However, Na2O will weaken the structural strength of glass, increase the thermal expansion coefficient of glass, and reduce the thermal stability, chemical stability and mechanical strength of glass. Therefore, the total amount of Na2O and K2O in glass composition can not be higher than 16%. Soda soda and mirabilite Na2SO4 are the raw materials for introducing Na2O.
Sodium carbonate (Na2CO3) is a kind of fine white powder, easy to melt in water. The impurities include sodium chloride, sodium sulfate, iron oxide and so on. Soda ash is easy to deliquescence and agglomerate, which is not conducive to the mixing of batches. Therefore, it must be stored in a ventilated and dry warehouse. It is possible to form foam called "scum" on the surface of glass when melting. The quality requirements for soda ash are (%); Na2CO3 > 98, NaCl < 1, Na2SO4 < 0. 1, Fe2O3 < 0. 1.
Soda soda can be divided into light alkali and heavy alkali. Light alkali, light alkali, low bulk density (0. 61), fine particles. Mixed batch is prone to delamination during transportation. After entering the kiln, it is easy to be carried into the regenerator by air flow in the kiln, resulting in blockage and melting of lattice bricks. The heavy alkali content is significant (0. 94), coarse particles. Therefore, the use of heavy alkali is one of the measures to improve the quality of batch and reduce alkali dust. Mirabilite (Na2SO4) has a specific gravity of 2. No white powder of 7. It can be divided into water-free mirabilite and water-containing mirabilite (Na2SO4). 10H_2O) two kinds. Water-bearing mirabilite begins to precipitate crystalline water and become paste at above 35 C, which is inconvenient to use. Therefore, it is necessary to boil or bake beforehand. The melting point and boiling point of mirabilite are 884 ~C and 1430 ~C. The decomposition temperature of mirabilite is higher. If reducing agent exists in the melting process, the decomposition temperature of mirabilite can be greatly reduced. Therefore, coal powder must be added when mirabilite is used. The theoretical dosage of coal is 4% of mirabilite.
Mirabilite is not only a substitute for alkali, but also a common clarifier. The use of mirabilite also has the following disadvantages: compared with soda ash, it consumes more heat, because quartz sand and mirabilite react at a higher temperature and at a slower speed; the melted but unreacted mirabilite floats on the surface of glass liquid, which is prone to produce mirabilite bubbles; the erosion of refractories is also great, especially when mirabilite water exists; the melting of mirabilite mixture must be in a reducing atmosphere. However, when the amount of pulverized coal is too much, it will reduce Fe2O3 to Fe S and become brown. Therefore, the amount of mirabilite is limited. The quality requirements for mirabilite were (%); Na2SO4 > 85, NaCl < 2, CaSO4 < 4, Fe2O3 < 0. 3, H2O<5.