The chemical interaction between glass liquid and refractory in tin bath
In the float glass forming process, the main contact parts of the glass liquid and the refractory material are the flow channel and the flow channel. The runner and the runner are the entrance ends of the tin bath, which is closely related to the molding, so it is included in the tin bath for discussion.
The refractory materials in these parts are in contact with the glass liquid and are eroded. The refractory material on the contact surface is dissolved into the glass liquid, so that the concentration of silica or alumina in the glass liquid is locally increased. They are like unstable acids at high temperature, so they can replace less stable components and emit gas, causing bubbles in the glass. Such as:
Na2SO4+Si02=Na2SiO3+S03
2S03=2S02+O2
The ferrous oxide impurities formed in the refractories during the manufacturing process can reduce the residual sulfate in the glass and release sulfur dioxide:
Na2SO4+FeO=Na20+Fe203+SO2
If the refractory is fired in a reducing atmosphere, there may be fine carbon deposited in the pores, which can also separate the residual sulfate in the glass liquid to liberate the gas:
Na2SO4+C=Na20+CO+SO2
Therefore, the refractory material of the flow channel and the flow channel must have a high degree of refractoriness, excellent resistance to glass erosion, and the impurity content must be strictly controlled.
In addition, the expansion or nucleation of the gas in the pores of the refractory material may form large bubbles, and the capillary channels formed by the pores of the refractory material may generate bubbles for a long time, and air or other gases may also be along the cracks or bricks The gap rises to produce bubbles. In order to prevent such bubbles, the pores of refractory materials should be strictly required, and there should be no continuous pore channels; the brick surface must be processed to make the contact surface between the bricks smooth and tight.
Redox reactions
In order to meet the needs of glass molding, the tin liquid surface must be kept clean, so the tin bath space should be introduced with a reduced protective gas to prevent tin oxidation. The shielding gas is composed of most nitrogen and a small amount of reducing components such as hydrogen or carbon monoxide. In addition, there are trace oxygen, water vapor and carbon dioxide and other oxygen-containing components. Under such atmospheric conditions, oxidation and reduction reactions proceed at the interface of each phase.
There are trivalent and divalent iron ions in the glass liquid. In the reducing atmosphere, the trivalent iron ions are reduced to divalent iron ions, which makes the glass green darker.
Fe+3+e=Fe+2
Sulfur and oxygen (including oxygen in the atmosphere) in glass react with tin to form stannous sulfide, stannous oxide and tin oxide.
S02+Sn+2H2=SnS+2H20
Sn+1/2 02 =SnO
Sn+O2=SnO2
SnO+1/2 O2=SnO2
In addition, the oxidizing and reducing components in the atmosphere undergo redox reactions at a certain temperature:
H2+1/2 02=H20
CO+1/2 02=C02
Sn02+2H2=Sn+2H20
SnO+H2=Sn+H20
Sn02+2CO=Sn+2C02
SnO+CO=Sn+CO2
These reactions maintain a dynamic balance in different temperature regions of the tin bath. The state at equilibrium is closely related to the composition of the protective gas. The higher the content of the oxidizing component and the lower the content of the reducing component, the more intense the oxidation; on the contrary, the lower the content of the oxidizing component and the higher the content of the reducing component, the more beneficial it is to avoid the oxidation of the tin solution. In addition to oxygen, water vapor and carbon dioxide are also oxidizing components, and their content in the protective gas cannot be ignored either.
