Common applications of fluorine-lined valves and ceramic valves in alumina manufacturing systems

Introduction to the Acid Production of Alumina from Fly Ash

In recent years, conducting research on the comprehensive utilization of high-value-added materials such as extracting alumina from high-alumina fly ash (with an alumina content greater than 30%) has become a hot topic in the coal power and alumina industries. The acid process for producing alumina from fly ash is a new technology for extracting alumina from high-alumina fly ash. The process consists of batching, dissolution, separation, washing and purification, evaporation and crystallization, calcination and acid recovery. The solid waste and wastewater generated during the production of alumina can be recycled and utilized, and various associated elements such as gallium, iron and silicon can also be used to produce a variety of products. The efficient and comprehensive utilization of fly ash has been achieved.

Dissolution is an important process in the acid production of alumina from fly ash. The fly ash slurry is mixed and heated with steam at a certain pressure and temperature in the dissolution tank, and the dissolution reaction can be carried out under medium temperature and low pressure conditions. In systems such as reaction vessels, flash tanks, and acidic fly ash slurry transportation, a large number of valves are needed for system switching, parameter adjustment, and other operations. Therefore, valves with high-temperature resistance, resistance to hydrochloric acid corrosion, and anti-erosion capabilities must be selected to meet the production requirements of the dissolution process. At present, the valves mainly used in the system are fluorine-lined valves and ceramic valves. Now, a brief analysis of the performance and usage of the valves is conducted.

Fluorine-lined valves provided by Viti Industry

Fluorine-lined valves adopt lining processes for all areas within the valve body that the medium can reach. The lining materials are generally fluoroplastics such as PTFE(F4), FEP(F46), and PFA, ensuring that the steel parts of the valve do not come into direct contact with corrosive media. This way, the problem that steel materials are not corrosion-resistant has been solved. At the same time, by taking advantage of the super corrosion resistance of fluoroplastics, fluorine-lined valves are widely used in industries such as petroleum and chemical engineering. The performance of fluorine-lined valves also varies to some extent due to the different fluoroplastics used.

PTFE (F4) is polytetrafluoroethylene. Polytetrafluoroethylene has a very strong resistance to chemical corrosion. It can resist the corrosion of almost any concentration of strong acids, strong alkalis, strong oxidants and solvents, and does not function even at high temperatures. Only molten alkali metals or their ammonia solutions, chlorine fluoride and elemental fluorine will have an effect on it. The advantages of polytetrafluoroethylene (PTFE) are its excellent anti-stain and anti-sticking properties, extremely low dynamic and static friction coefficients, and good anti-friction and lubricating performance. However, its disadvantages include the difficulty in molding and processing. It has no fluidity in the molten state and is hard to manufacture complex components using common processing methods such as molding and injection. Only the PTFE powder can be pre-pressed into the required shape and then sintered into shape.

F46 is FEP, and its Chinese name is perfluoroethylene-propylene copolymer. Among fluorine-lined valves, F46 is the most widely used fluoroplastic. It is a copolymer of tetrafluoroethylene and hexafluoropropylene and is a modified new type of fluoroplastic. Except for the operating temperature being approximately 50℃ lower than that of PTFE, all other excellent properties of PTFE/F4 are maintained. Due to the relatively low viscosity of the melt, it is easier to form compared with PTFE and can be processed by molding, injection molding and other methods. Its heat resistance is second only to polytetrafluoroethylene and it can be used continuously within the temperature range of -85 to +200℃. Even under extreme conditions of -200℃ and +260℃, its performance does not deteriorate and it can be used for a short period of time.

PFA, or polyperfluoroalkoxy, is a modification of polytetrafluoroethylene. It combines the advantages of F4 and F46, featuring excellent chemical resistance and high-temperature resistance, and can also be processed using the common methods for thermoplastics. The service temperature of PFA is the same as that of F4 (250℃), but its high-temperature strength is twice that of F4. The operating temperature of PFA-lined valves is slightly higher than that of F46-lined valves.

The fluorine-lined valves used in the dissolution system mainly include five types: stop valves, gate valves, check valves, safety valves and ball valves. The inner lining material is mainly F46. The medium temperature used should not exceed 150℃. They are mainly installed in the non-condensing gas discharge pipeline of the dissolution tank, steam pipeline, high-pressure water pipeline and acid gas absorption system. It performs well in low-temperature (below 130℃) systems such as high-pressure water pipelines and acid gas absorption. The failure rate of the safety door, steam door, exhaust door and check valve on the dissolution tank body is relatively high. The main phenomena are corrosion and perforation of the valve body, packing and material leakage, and internal leakage due to poor closure. Besides product quality issues, the main damage is caused by high-temperature (150-160℃) steam, acidic slurry and gas erosion. After selecting the PFA-lined valve with a higher working temperature, the failure rate was significantly reduced, basically meeting the production requirements.

The usage requirements of fluorine-lined valves

Check whether the parameters marked on the nameplate meet the working condition requirements of the equipment or system. The flow direction on the shell should be consistent with the flow direction of the medium in the pipeline. Try to install it horizontally in its natural state with the valve stem facing upwards to prevent excessive installation stress caused by pipe tension, which may affect the safe use of the valve. It is prohibited to forcibly open and close the fluorine-lined valve with an extension rod to avoid premature damage to the fluoroplastic sealing surface. When replacing the packing of fluorine-lined valves installed on pipelines carrying corrosive media such as acids and alkalis, isolation measures should be taken. Maintenance work can only be carried out after pressure relief and material discharge. It is strictly prohibited to operate under pressure at risk. When fluorine-lined valves are used to regulate flow, control valves should be selected. Using other valves will cause premature damage to the sealing surface. When fluorine-lined valves are connected to fluorine-lined pipes, gaskets are generally not added at the flange. If there is leakage at the middle flange of the valve body, the middle flange nut can be evenly tightened to eliminate the leakage.

Ceramic valve

Ceramic valves are made of high-performance structural ceramics, which feature excellent self-cleaning properties, super wear resistance, corrosion resistance, strong self-lubrication, and a long service life. The structural ceramic material has a small amount of deformation, and its tensile strength, compressive strength, and hardness are all very high. This can significantly enhance the safety and stability of the equipment operation system and is widely used in industries such as petroleum and chemical engineering. The valve body is designed with a three-section structure and an internal buffer compensation elastic gasket, which ensures stable and excellent sealing performance. It is less likely to get stuck and cause the ball to break in a medium containing impurities. Valve stems are made of various materials such as ceramic stems, metal-coated fluorine-coated stems, titanium alloy stems, and tantalum alloy stems, which can meet the requirements of mechanical properties and corrosion resistance under different medium conditions. The valve seat and inner lining materials are high-performance structural ceramics. The entire valve has light torque, low resistance and zero leakage. High-performance structural ceramic balls are adopted, which can achieve opening, closing and regulating functions according to usage requirements. The structural ceramic materials used in ceramic ball valves have excellent high-temperature resistance, corrosion resistance and wear resistance.

The usage requirements for ceramic valves: During the initial operation of the dissolution system, there may sometimes be some fiberglass reinforced plastic fragments, welding slag, rust and other debris in the slurry conveying pipeline, which can easily cause pipeline blockage or valve jamming, leading to the breakage of the valve core. Before use, the ceramic valve should be fully opened to flush the pipeline clean before it is put into normal operation. When using a ceramic control valve, the door should first be opened to the maximum extent as much as possible. After the impurities in the pipeline are completely removed, the control valve should be restored to a position with a slightly larger normal opening. When using ceramic valves, it is necessary to avoid thermal shock and keep the temperature difference between the valve and the medium within the specified range to prevent damage to the ceramic components. When a ceramic valve gets stuck when being switched on or off, do not force the switch. Instead, take measures such as reducing the pressure difference on both sides of the valve and performing backwashing before proceeding with the switch operation to prevent the valve stem from breaking or the valve core from cracking.

The ceramic valves used in the dissolution system are mainly domestic ceramic ball valves, and there are also a small number of imported ceramic valves. They are installed on the inlet and outlet pipes of the dissolution tank. The acidity and temperature are relatively high, and the working conditions are rather harsh. The main damage phenomena of ceramic valves are the fragmentation of the valve core, the breakage of the ceramic valve stem, the leakage of the packing material, the corrosion of the valve body caused by the cracking of the ceramic inner lining sleeve, and the leakage of the joint surface, etc. In comparison, the performance and service life of imported ceramic valves are significantly better than those of domestic valves. However, through the continuous improvement of domestic ceramic valves by manufacturers, the gap between the two has gradually narrowed, and domestic ceramic valves offer a higher cost performance.

Summary: Both fluorine-lined valves and ceramic valves have their own advantages and can meet the working conditions of the corresponding systems in the dissolution process. However, further improvements and upgrades are needed in terms of product materials, processes, and quality to better adapt to the process requirements of producing alumina from fly ash by acid method and ensure the safe and stable operation of the dissolution system.