The continuous electrothermal crystallizer is an advanced refining equipment. Since its successful development in 1975, it has not only been rapidly applied in various domestic tin-making plants, but also quickly exported to the world's major tin-producing countries. [1, the effectiveness and reliability of its refined metals have been highly recognized by international counterparts. Evaluation w. However, the understanding of this advanced equipment has yet to be deepened in many aspects. For example, although the application of continuous electrothermal crystallizer has been more than 20 years old, the operation and control of the electrothermal crystallizer in production practice still lacks theoretical understanding and guidance based on experience [3-5]. Therefore, it is clear that its operational control theory basis, to optimize the production process, maximize the role of equipment, improve production efficiency, especially for the development of continuous electrothermal crystallizer in the field of low melting point non-ferrous metal refining other than tin, with Important guiding significance. The following focuses on the basic principles that should be followed in actual operation control.
1 Continuous electrothermal crystallization machine working conditions In addition to good performance, an advanced equipment must meet certain external conditions in order to receive good results in the application.
For continuous electrothermal crystallization machines to refine metals, this external condition is the correct operation and control. However, proper operation and control must meet certain theoretical requirements.
After extensive research on various equipments for metal step-by-step crystal refining, it was found that China's continuous electrothermal crystallizer was so successful because it satisfies the following two conditions: (1) It can be well in the crystallizer tank. The temperature gradient and concentration gradient corresponding to the phase diagram are established in the melt; (2) The hydrodynamic difficulty of the solid-liquid two-phase material can be successfully overcome, and the two-phase flow and good separation are realized.
Both conditions are indispensable and are controlled in practice by the temperature of the melt in the tank, the speed of the agitator and the angle at which the crystallizer is tilted.
2 Temperature control of continuous electrothermal crystallization machine The process of stepwise crystallization of refined metal by continuous electrothermal crystallization machine is based on phase transformation: solid phase = liquid phase refining process mainly involves two solid and liquid aggregation states of metal.
According to the phase law, the number of degrees of freedom in the thermodynamic equilibrium state system, that is, the total number of variables required to describe the state of the system is: (1) It can be seen that under normal pressure, for a single-component single-phase system, F=1 That is, the system has an independently variable factor. For a single-component two-phase system, F =0, that is, the system is a non-variable system. For a two-component single-phase equilibrium system, F = 2 means that the system has two independently variable factors. For a two-component two-phase equilibrium system, F =1, that is, the system has only one independent variable, that is, only one between temperature and composition variables can be changed. If the temperature is specified, the composition cannot be specified, or the composition is determined by the temperature and cannot be arbitrarily specified. Conversely, if a composition is specified, the temperature cannot be arbitrarily specified and can only be specified by the balance. Metal step-by-step crystal refining is in this two-component two-phase system. Therefore, the temperature of the melt in the continuous electrothermal crystallizer tank must be controlled strictly in accordance with the corresponding phase diagram. Because the temperature is determined, the composition will depend on it. As shown, from the trough to the trough of the crystallizer, the melt temperature gradient must be greater than zero, and the melt temperature at the trough should be controlled to Te, and the temperature at which the trough temperature should be controlled should be in accordance with the Sn-Pb phase diagram () From the tail of the groove 186 * C gradually increased along the direction of the groove head to the 231 * C. When the lead is removed from silver, the temperature of the tail melt should be controlled to 304C according to the Pb-Ag phase diagram () and then along The direction of the trough head gradually rises to 327C. Since the temperature has a strict correspondence with the composition, and the temperature control is easier to realize than the control of the composition, in practice, the control of the composition is always achieved by adjusting the temperature.
Table i gives two sets of experimental data for the continuous electrothermal crystallization machine to remove silver from lead. When the other conditions are the same, when the tank head temperature Ta is equal to or slightly higher than 327C, the quality of the refined lead is obtained. When the tank head temperature Ta is lower than 327C, the quality of the refined lead is remarkably deteriorated. Experimental Table 1 The effect of the head temperature on the removal of lead by other materials is the same. Experimental material trough temperature / Silver-free lead de-silver lead desilver efficiency (%) Silver-containing lead de-silver lead desilver efficiency (%) continuous One of the important reasons for the failure of the electrothermal crystallizer refining product is that the heater cannot meet the melt temperature gradient in the continuous electrothermal crystallizer tank. When refining the metal with an electrothermal continuous crystallization machine, the selection of the feeding position should also follow the principle of the above temperature control, that is, the melt composition at the feeding position should be consistent or close to the composition of the material to be treated, which is the same as the chemical rectification process.
3 The solid-liquid separation in the continuous electrothermal crystallization machine creates the conditions for the movement and separation of the two solid and liquid relative flows, and the separation effect can be changed by adjusting the rotation speed of the spiral agitator and the inclined installation angle of the continuous electrothermal crystallizer, which is an electrothermal continuous crystallizer. Success. The success of continuous electrothermal crystallization machines has eclipsed many of the continuous electrothermal crystallization refining equipment that have appeared in the world.
The data in Table 2 shows that when the temperature of the crystallizer head is the same, the inclined installation angle and the screw stirrer speed have a great influence on the effect of refined lead. Within a certain range, when the rotation speed of the spiral agitator is constant, the angle of the tilting installation of the crystallizer tank is large to facilitate refining. Similarly, when the inclined installation angle of the tank is constant, the rotation speed of the screw can be increased, and the refining efficiency can also be improved.
Table 2: When the temperature of the trough head is the same, the influence of the installation angle of the crystallizer and the rotation speed of the screw stirrer on the silver removal of the lead material. The tilting speed of the screw, the rotation speed of the silver, the lead, the silver, the lead, the silver removal efficiency (%), the desilvering efficiency of the silver, lead, silver and lead (%) Silver-lead-free silver-lead desilvering efficiency (%) In the process of refining metal by continuous electrothermal crystallization machine, the two phases do not reach equilibrium completely, just as the principle of metal step-by-step crystallization refining [7] replaces the equilibrium partition coefficient To evaluate. The effective partition coefficient is: solidification speed, cm°s—the diffusion coefficient of impurities in the liquid phase, cm2.s*1; the thickness of the interfacial layer, cm. The equilibrium partition coefficient is a function of temperature and does not change with other experimental conditions. Three factors affect the effective partition coefficient. The solidification velocity V diffusion coefficient D and the concentration interface layer thickness 5 give the relationship between the effective partition coefficient ke and k()<1.
=1. That is, the effective partition coefficient ke bounds 1, which indicates that the actual effect of the continuous electrothermal crystallizer refining metal is lower than that under equilibrium conditions. The law of quantitative change between ke and ke has important guiding significance for the control of the metallizing process of the crystallizer. When ke=, the refining effect is the best. For a certain impurity, conditions should be created to promote the effective distribution coefficient as close as possible to the equilibrium. The partition coefficient can be achieved by strengthening natural convection or even forced convection. According to the mass transfer process principle, the thickness of the concentration interface layer becomes thinner with agitation and fluid flow rate. Convection also inhibits the formation of supercooling and dendritic formation, and promotes the heat transfer process between solid and liquid. The continuous electrothermal crystallizer uses a spiral agitator and a tilting installation of the tank to facilitate forced convection, which can promote mass transfer and separation between solid and liquid, and improve the refining effect. Adding the inclination angle of the crystallizer tank in a certain range can not only prolong the time of solid-liquid two-phase action, slow down the speed of crystallization process, but also increase the effect of reflux, enhanced washing and strong refining. The installation of the angle of inclination will result in a decrease in the production capacity of the continuous electrothermal crystallizer, and various factors must be considered in order to achieve a good level of quantity and quality of the product.
In the process of refining metal by continuous electrothermal crystallization machine, the installation angle of the machine body and the rotation speed of the spiral agitator play an important role in the refining effect, which is an important control factor in the production process. The theory of effective partition coefficients provides a theoretical basis for the control of these factors.
4 Conclusions The continuous electrothermal crystallization machine is macroscopically the machine of the famous Pattinson process, which is microscopically a continuous multi-stage regional refining device [1(). Its production efficiency and refining effect far exceeds that of old metal refining equipment. The control of the refining metal process of the continuous electrothermal crystallizer must follow certain physical and chemical principles. The temperature control must meet the requirements of the phase diagram. The adjustment and control of the inclination of the continuous electrothermal crystallizer installation and the speed of the screw agitator depends on the effective distribution. coefficient.
