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(1) Temperature and temperature difference
(3) The relationship between saturation concentration and temperature
(4) Material properties and surface conditions
(7) Interfacial stress and contact angle of nucleation
(8) Structure of the equipment
The Hazard and Control of Scaling in Desilication Process
Qin Guohong Ma Weimin
Abstract The processing of low-grade bauxite by Zhongzhou Branch Company adopts the sintering process to produce alumina. In order to reduce the hazard of impurities in raw materials and obtain qualified quality products, the desiliconization of materials is a unique process for the production of alumina by sintering. It is one of the main links in the energy consumption of the entire production process. At the same time, the performance of this process has a huge impact on the overall capacity of the entire sintering process. The desilication process is always accompanied by the generation of a large amount of fouling, which has a great adverse effect on the stable operation of the desiliconization unit and the control of production indicators. How to reduce and avoid the hazard of fouling in the production operation and process condition regulation has positive significance for improving the operation rate of the desiliconization unit and optimizing the process index.
Keywords: fouling desiliconization process water-containing sodium aluminosilicate water-containing calcium aluminosilicate
1 Causes, Mechanisms and Characteristics of Scale Formation
The sintering process for the production of alumina is a low-grade bauxite process with 5-6 g/l of SiO2 in the crude liquor. The impurities must be removed through a separate desiliconization process in order to obtain an economic decomposition rate. Alumina products, and reduce the scaling of the post-evaporation process, make evaporation work smoothly.
The desiliconization unit is refined by heating the material to 150° C. under medium pressure conditions and holding it for 1 hour to precipitate SiO 2 in the solution. The main reactions in this process are: 2Na2SiO3+2NaAl(OH)4+aq===Na2O•Al2O3•2SiO2•nH2O+4NaOH+aq
In order to further increase the purity of materials, Zhongzhou Branch Company has obtained higher decomposition rate and product grade. On this basis, it has developed a three-stage desiliconization process of pre-siliconization, depressurization and desilication at one time, and deep desilication by adding lime milk. The reaction of desiliconization with lime milk is:
3Ca(OH)2+2NaAl(OH)4+aq===3CaO•Al2O3•6H2O+2NaOH+aq
3CaO•Al2O3•6H2O+xNa2SiO3+aq===3CaO•AL2O3•xSiO2(6-2x)H2O+2xNaOH+aq
The major desilicated products are aqueous sodium aluminosilicates and water soluble calcium aluminosilicates with less solubility. The refined silicon content of the current material index is stable within the range of 600-750 g/l. From the above process conditions, the entire desiliconization process is a process in which the solid phase precipitates from the liquid phase. There are many factors that affect scaling, but there are mainly the following aspects:
The fouling process is affected by temperature on the one hand, and more critically on the other hand by the temperature difference between the wall temperature and the solution temperature. Usually the heat flux is proportional to the temperature difference. In industrial equipment, a large temperature difference is required to achieve a large heat flux density, and accordingly there is also a large concentration difference. Local supersaturation is conducive to the formation and growth of nuclei.
(2) Concentration of dissolved substances in the fluid
In the case of fouling by a chemical reaction, such as polymerization of the reaction wall surface, the concentration of the reaction material in the liquid plays a decisive role. For the crystallization process, the concentration difference (oversaturation) is critical as previously described, as the rate of mass transfer and scale generation increases with increasing concentration.
The change of saturation concentration with temperature directly affects the choice of operating conditions and the behavior of fouling. Various conditions may occur here, such as saturation concentrations rising, falling, or remaining constant with temperature.
The fouling conditions are generally related to the type of material and the surface condition. In general, smooth, non-corrosive material surfaces are less prone to fouling than rough, corroded surfaces.
(5) Flow rate of fluid
The flow rate of the fluid can affect scaling through the effects of heat transfer and mechanical forces. The flow rate can hinder the deposition of solid particles on the wall surface and make the crystals less likely to adhere, thereby reducing the scaling; however, if the wall surface is already fouled, the increase of the flow rate will promote the mass transfer and thus the crystallization process in the diffusion determination. When it is, it is conducive to the growth of scale.
(6) Scale thickness and shear strength
As the thickness of the scale increases, the flow rate increases, and accordingly the shear force increases, which helps to exfoliate the scale layer. If the scale strength of the scale layer is not so great, the two are in equilibrium.
As the interfacial stress decreases, the probability of nucleation increases. Due to the presence of impurity components on the interface, the interface stress can also be reduced, which is beneficial to the fouling process. Mersmann [4] studied the correlation between the success of crystal nuclei and the contact angle, and believed that the success of crystal nucleation increases with the increase of contact angle (the increase of contact angle indicates poor wetting), thus hindering the crystallization of generate.
The structure of the equipment has a certain influence on the fouling behavior, for example, the dead zone and the crevice part are most susceptible to fouling. For the desiliconization unit, there are a lot of solid phase output in the feeding system, inside the unit, and behind the unit in the liquid-solid separation system. It will inevitably cause scaling in the relevant process piping, equipment unit walls, control valves, etc., to a certain extent. The operation and production must be affected. In the event of serious damage, the Dused Pipeline, the reduction of equipment production capacity, and the abrasion of pipelines caused leakage, equipment accidents and safety accidents. How to reduce and avoid the impact of scaling on production has a positive significance.
2 Sites and hazards that fouling easily affects the process
Since fouling occurs throughout the desiliconization process, the accumulation of excessive fouling at the critical sites affecting the capacity of the desiliconization unit will directly affect the stability of the process. For the desiliconization unit, the characteristics of its process conditions determine that its capacity, heating efficiency, and effective volume are the main indicators affecting production capacity. At present, a single unit is required to have a feed of 400m3/h, a heating temperature of 150-160°C, and a single unit of φ2.8*12m and 73.85m3.
Influencing the capacity of the desiliconization unit: The first is the influence of the feeding system. The areas where the fouling easily accumulates at this location include coarse liquid tanks, crude liquid tank outlets, slag remover, and feed pump control valves. If the scaling is excessive, the feed pump will be insufficient for feeding, the feed pressure will be reduced, and feeding to the desiliconizer will not be normally performed, and serious accidents such as pressure drop will occur. In the desiliconization unit, the easy-to-aggregate areas include control valves, brush pot tubes, feed tubes, steam-water separators, and flash tank cone bottoms. There are discharge elbows, discharge tubes, and exhaust gas systems at the easily-abraded parts. In each control valve, fouling will cause the flow to fail, the unit pressure will be difficult to control, the heating conditions will be degraded, the internal blockage of the unit will be blocked, the material will not be discharged, the vapor and liquid will not be separated, and the normal operation of the steam heating and condensate system will be affected. . In the later part of the liquid-solid separation system, there are sedimentation tanks which are likely to cause significant influence. The fouling of this part mainly concentrates on the rakes, tank walls and rakes. Excessive fouling or abnormal detachment can cause the bottom of the tank to become blocked and the mechanical components are damaged. Interfere with liquid-solid sedimentation and separation, which is not conducive to the refining of sodium aluminate by leaf filtration. Influencing the heating capacity of the desiliconization unit, fouling is likely to affect production: coarse liquid heaters, casing heaters, fouling in the unit tanks, condensate systems, fouling in flash tanks, and separators for steam separators. Scale, due to fouling of the heat transfer surface, large thermal resistance to reduce the amount of heat transfer per unit area, time, so that the heater heat transfer efficiency is reduced or blocked, the pre-desilication temperature is not up to the requirements, increase the burden of desiliconization , The lack of steam can not be fully utilized, indirect heating pipeline scaling to reduce service life. The scaling of the inner wall of the desiliconization unit will directly occupy the effective volume of the unit. If too much, it will affect the desiliconization efficiency and safe operation of the unit.
3 How to reduce and avoid the impact of scaling on production in operations
The generation of fouling is unavoidable in the desiliconization process. To stabilize the operation of the desiliconization unit and achieve high production and operation rates, it is necessary to find the mechanism and regularity of scaling generation in daily operations as much as possible. During the operation, the influence of fouling on the process conditions is reduced as much as possible, and appropriate process conditions and operation methods are found for different operation steps.
3.1 Pre-Desiliconization System Operation Precautions
The pre-desilication system fouling mainly occurs in the crude liquid tank and adheres to the wall of the tank. The major impact on the production is that the scale is detached due to accumulation, and accumulation at the outlet of the tank or the deslagger affects the flow of the process. The key to the pre-siliconization process control is the temperature. To prevent the ambassador from scaling off due to temperature changes. At present, the pre-desilication temperature is generally 90-95°C. If the temperature is too low, the new steam should be turned on in a timely manner to ensure that the operating temperature of the pre-desilication system is within the proper range. In addition, we must also maintain thermal insulation, control leakage, reduce sewage, avoid the impact of lower temperature sewage and seed mother liquor on the system temperature; stabilize the amount of silicon residue in the control of 60-80g/l; continue to implement equipment technological transformation and process optimization, SiO2 is agglomerated on the seeds as much as possible to reduce the adhesion of the scale on the walls of the tank.
3.2 Precautions for operation of a desiliconization system (1) During the operation of desiliconization machine, the valve shall be inspected and controlled frequently, and each class shall be fully opened and closed at least twice, and it shall be flexible and easy to use, and the knot stuck in the valve shall be timely passed. Scale, to prevent fouling stuck in the valve and the valve control failure.
(2) Maintain a high flow rate, the flow rate should be greater than 350m3/h, the flow rate can hinder the deposition of solid particles on the wall surface, and make the crystal grains difficult to adhere, thus reducing the scaling.
(3) Reasonably adjust the heating range of each section of the heating sleeve, one section of the outlet is controlled at 115-120°C, the second section of the outlet is controlled at 130-135°C, and the third section of the outlet is controlled at 155-160°C to prevent heat transfer due to individual casing. Too fast and too much fouling of the heated surface affects the life cycle of the entire casing.
(4) Stable desilication temperature control, so that the steam system operating conditions remain stable in order to fully play the role of steam heating and energy saving, so that all levels of casing can be balanced heating.
(5) Maintain a good balance between steam and condensate, 1# condensate from evaporator pressure 0.35±0.03mpa, 2#, 3# condensate from evaporator pressure 0.20±0.03mpa, to prevent spent strips in the casing The formation of fouling to reduce the heat transfer efficiency affects the capacity of the equipment.
(6) Fouling in desiliconizer tanks should be carried out in the initial stage of operation with a greater strength into the steam brush pot, flushing away loosening scale adhered to the wall as much as possible; use in the middle of the operation The medium-intensity steam brush pot maintains sufficient agitation strength in the unit to prevent sedimentation and scaling of the materials. After a long period of operation, the unit must use a low-strength steam brush to scour the active scaling in the unit. , Stabilize the fouling on the wall, prevent the thick fouling that is adsorbed on the wall due to long-term operation and wash away and cause the plugging of the feed pipe, the discharge pipe and the bottom of the flash tank.
(7) For each level of flash tank, the opening of the spent steam should be adjusted according to the flow rate, pressure and temperature of the material in time during operation. The pressure of the first flash tank is controlled at 0.35±0.05 mPa, and the pressure of the secondary flash tank is controlled. 0.20±0.05mpa; By balancing the position of the liquid level in the stable flash drum from the discharge of the desiliconizer unit, the flash tank fall-off and the lack of vapors due to the discharge shock are reduced.
3.3 Deep Desilication System Operation Precautions
To stabilize the operation of the settler, the discharge of the desiliconizer should be kept stable and the impact on the settler should be reduced. In the operation of lifting the material to be slow, you can adjust more and less movement, brush pot block movements to a small extent, to prevent the sedimentation tank feeding the dumplings on the scale block, to create a good external conditions for the operation of the sedimentation tank.
3 Conclusion
Sintering has unique advantages in the treatment of low-grade bauxite and the overall recovery of resources. With the continuous development of the market, the demand for multi-aluminum oxide market is increasing, and further optimization of the sintering process to promote energy-saving enterprises. Market share is of great significance. Deepening tapping potentials and continuously improving the competitiveness of enterprises require that higher equipment operation rate and index control level be sought in all aspects of production process and process control. Through the exploration of the scaling mechanism and countermeasures in the desiliconization process, the desiliconization unit is effectively improved. The capacity and operating rate, indirect heating desiliconization unit from the date of just the average annual operating rate of about 73.6%, to the current average annual operating rate of 93.6%, a single monthly operating rate of up to 99.71%; single casing preheater cycle It has also increased from the beginning of less than two months to nearly four months now. As a result, the entire desiliconization process system reduces maintenance costs and reduces production costs. There are other factors in actual production that need further exploration to promote the sintering process alumina production process is more reasonable and mature.