Several chemicals that can erode aluminum silicate fiber include fluorine, fluoride, vanadium and other heavy metals, phosphoric acid, strong base, etc.
(1) Fluorine and fluoride. There is no doubt that fluoride is the most corrosive to aluminum silicate fiber. Under 100 ℃, the reaction of fluoride and water with aluminum silicate fiber will cause great damage to the fiber structure. Even at a low concentration, hydrofluoric acid will react with aluminum oxide to form AIF, AIF3 and H20, causing serious damage to the fiber structure. Hydrofluoric acid reacts most easily with SiO2. At a lower temperature (below 980 ℃), the fluorine rich environment may promote the low temperature recrystallization, which may lead to slight damage to the fiber structure. At the temperature where a large amount of recrystallization occurs, fluorine may form reactants on the fiber surface, thus causing changes in the fiber structure and obviously forming a thin hard shell layer on the fiber surface. Fluorine reacts with all aluminum silicate materials, including mullite. After reacting with fluorine, the reactants often vaporize, making it difficult to determine the cause of aluminum silicate fiber damage.
(2) Vanadium and other heavy metals. Vanadium and other heavy metals in inferior fuel will corrode aluminum silicate fiber when burning. This corrosive material comes from vanadium pentoxide (V203), which is a solid at room temperature but melts at about 690 ℃. The liquid slag is adsorbed in the pore structure of the fiber and reacts with aluminum silicate in the fiber. After reaction, a hardened shell will be formed on the hot side of the fiber lining. After a period of time, the hardened shell will fall off from the unreacted fiber layer of the fiber blanket, and the chemical reaction will continue on the exposed fresh fiber surface. The reaction rate depends on the following factors, such as the concentration of heavy metals, time, porosity of materials and temperature. There is no clear concentration boundary to define whether the reaction occurs or not. In general, it is high heavy metal concentration for a long time that shortens the life of refractories. When the temperature is higher than 690 ℃, vanadium pentoxide will react with the fiber, but the reaction speed is affected by the above factors. With the continuous chemical reaction, it is necessary to replace the hot surface materials regularly. In particular, vanadium pentoxide corrodes the fiber, but vanadium in the presence of alkaline compounds will form more corrosive slag at very low temperatures, resulting in faster degradation of aluminum silicate fibers. Generally, fuels with high heavy metal content also contain alkali.
(3) Sulphur and sulphuric acid. Aluminum silicate fiber has good resistance to sulfuric acid attack, but in rare cases, it will also produce some minor chemical reactions, and the typical reaction products are aluminum sulfate (Al2 (SiO4) 3) or aluminum sulfate hydrate. Generally speaking, this reaction will not cause fiber damage. In fact, the corrosion of metal riveting parts needs special attention. Asphalt coating or stainless steel film shall be used to protect metal riveting parts, or ensure that the temperature of metal riveting parts is higher than the dew point temperature of sulfuric acid, generally 121~177 ℃. When iron (Fe) and sulfuric acid (H2S04) coexist, aluminum silicate fiber will produce iron oxide aluminum oxide silicon oxide composite sulfate. This compound will “dissolve” the riveting parts and furnace shell, making the riveting parts become similar to gray rock. Generally, the fiber will turn yellow after contacting with sulfur. It is generally believed that the deposition of sulfur on the fiber surface will affect the fiber crystallization process at about 980 ℃.
The reaction of sulfur and aluminum silicate fiber is generally distributed on the fiber lining surface in a discontinuous manner. This reaction is considered to make the fiber surface form a uniform powdery layer, which will lead to the fall off of the lining material of the fiber lining hot surface under the condition of mechanical force (such as vibration) or airflow scouring. These materials gradually lost from the hot surface will expose the new surface, which is easy to be further eroded, so that the above reaction process can be self repeated. Only after a long time of use can these reactions cause substantial damage to the whole furnace lining. For the fiber lining of the layered structure, it may be necessary to regularly replace the fiber blanket on the hot side. If fiber components are used, a layer of ceramic coating material can be sprayed on the fiber surface.
(4) Alkali erosion. Alkali attack on aluminum silicate fiber is considered to mainly depend on time and temperature. The alkali metal reacts with the fiber to form a low melting compound, which makes the fiber shrink or sinter, and finally leads to the damage of the fiber lining. The presence of chemical substances such as V203, SO3, etc. will accelerate this damage behavior and cause the furnace lining damage earlier. Alkali erosion generally occurs on the hot surface of the fiber lining and forms a hard shell or “slag” layer after reaction. With the deepening of the reaction, the fiber surface is destroyed and loses its function, and the new surface is exposed to continuous chemical attack.
(5) Other acid attack. Aluminum silicate fiber is generally considered to have good resistance to hydrochloric acid (HCl), acetic acid (CH3COOH) and nitric acid, and can resist the attack of phosphoric acid at low temperature. However, when aluminum phosphate is formed at high temperature, the fiber will shrink significantly. When the temperature is higher than 540 ℃, try to avoid the contact between aluminum silicate fiber and phosphoric acid.