As mentioned above, the gas content and slag content in the electrolytic aluminum body melt are far greater than those in the remelted aluminum liquid. The solubility of the gas dissolved in the aluminum liquid decreases as the temperature of the melt decreases. During the casting or rolling process, the melt cools rapidly and solidifies into a solid, and the gas dissolved in the aluminum may be seriously supersaturated. It is impossible for the gas precipitated during the crystallization process to gather into nuclei, diffuse, grow bubbles, and then float up and escape without external cores. The reasons are as follows: (1) During the casting and rolling process, the melt crystallizes very quickly, and the dissolved gas does not have time to nucleate and grow before the crystallization is completed; (2) Even if a core is formed, it is very unstable when the core size is in a critical state and may dissolve again in the melt; (3) Even if the core grows into a bubble, whether it can float up and escape depends on whether the buoyancy separating the bubble from the melt surface is greater than the surface tension of the bubble adsorbing it. That is:
(1)
(2)
Where P1 is the buoyancy of the bubble;
P2 is the surface tension of the interface between the bubble and the melt.
When p1 >p2, we get:
(3)
Where P1 is the melt density;
P2 is the gas density;
σ — is the surface tension of the melt;
d — is the bubble diameter;
B1 B2, A is a constant.
The minimum diameter d0 that a bubble should have when it leaves the melt surface can be determined by the following formula:
(4)
Where Q is the wetting angle, rad.
When the melt is at high temperature, the gas is always in a non-supersaturated state. The process of spontaneously forming a bubble core in a non-supersaturated state is a process of increasing free energy. However, the spontaneous process always reduces the free energy, so it is necessary to introduce an external bubble core to make the core diameter d≥0.002Q[σ/(p1-p2)] so that the bubble core can absorb hydrogen in the electrolytic aluminum body and grow and escape.
1 Basic principles of electrolytic aluminum body refining
The hydrogen dissolved in the electrolytic aluminum body, part of it is free in the aluminum liquid in the form of atoms or ions, and the other part diffuses into the oxide inclusions in the aluminum liquid and exists in the molecular state. Most of these inclusion particles are not dense, with holes in the middle. Their pseudo-density is similar to that of liquid aluminum, and they are suspended in the electrolytic aluminum body. The hydrogen partial pressure in the hole is equal to zero, and the hydrogen in the aluminum liquid diffuses into the hole until its partial pressure is the same as that in the aluminum liquid, thus establishing a dynamic balance.
Using the principle that hydrogen diffuses in phases with different partial pressures in order to establish a dynamic balance, some refining agents are added to the aluminum liquid, and some gases are input or generated. These gases are dispersed into the electrolytic aluminum body in the form of bubbles, and the hydrogen partial pressure in these bubbles is equal to 0. The hydrogen dissolved in the aluminum liquid diffuses into the bubbles, and the bubbles gradually grow and rise. As the bubbles slowly rise, more and more hydrogen diffuses into the bubbles, and finally the hydrogen is brought out of the liquid surface to achieve the purpose of dehydrogenation (see Figure 1).
Some molten salts are added to the refining agent, such as KCl, Na3AlF6, etc. These molten salts have two characteristics: they are easy to melt into liquids, and their density is lower than that of aluminum liquid in the molten state; the molten salt solution has a wetting and dissolving effect on non-metallic inclusions, which can improve the effect of degassing and slag removal.
Figure 1 Schematic diagram of bubble degassing
2 Selection and use of refining agents
In order to achieve good refining effects, remove most of the gas and slag in the electrolytic aluminum body, it is extremely important to select a suitable proportion of refining agents and adopt a suitable refining method.
There are many types of refining agents, which are divided into three categories: solid, liquid and gas. Solid refining agents include hexachloroethane, a mixture of sodium chloride, potassium chloride and cryolite, etc.; liquid refining agents mainly include carbon tetrachloride; gas refining agents include nitrogen, argon and a mixture of nitrogen and chlorine.
The solid refining agent used in the pure aluminum series is generally composed of 47% KCl + 30% NaCl + 23% Na3 AIF6, which can be made into powder and block. In order to improve the degassing effect, some powdered refining agents also add a certain proportion of hexachloroethane. Block refining agent is refined with a refining frame. Powdered refining agent uses inert gas as a carrier and blows it into the bottom of the melt. The amount used is 0.2%~0.3% of the aluminum liquid.
When these salt refining agents are added to the bottom of the electrolytic aluminum body, they will produce the following chemical reactions with aluminum:
AlCl3 and AIF3 are both gaseous substances, forming foreign bubble cores in the aluminum melt. Hydrogen dissolved in the aluminum melt diffuses into the bubbles, and the bubbles grow accordingly. When the bubbles grow to a certain extent, their rising buoyancy is greater than the surface tension formed between them and the melt, and they rise immediately. During the rising process of the bubbles, hydrogen continues to diffuse into the bubbles, and the slag particles in the melt are adsorbed on the surface when they come into contact with the bubbles, and finally the hydrogen and slag are taken out of the melt. The smaller the interfacial tension between the surface of the slag particles and the melt, the easier it is to be adsorbed and taken out by the bubbles.
Solid refining agent has a good slag removal effect while removing gas. This is because, as mentioned above, the characteristics of the molten salt in the refining agent are: easy to melt into liquid, low density, forming liquid droplets that float up; after Na3 AlF6 melts, it can dissolve the aluminum oxide in the aluminum melt and take out the oxide inclusions; the surface tension on non-metallic inclusions is small, The wetting angle of the liquid solvent to the solid is θ < 90°, which will wet the inclusions. When the molten salt droplets and bubbles float up and encounter inclusion particles, they can adsorb the inclusions and take them out of the liquid surface to achieve the purpose of slag removal (see Figure 2 and Figure 3).
Figure 2 Schematic diagram of the surface tension between oxide inclusions, aluminum liquid, and furnace gas
According to the laws of thermodynamics, the inclusions automatically adsorbed on the bubbles should meet the following conditions:
(5)
Where σ1 is the interfacial tension between the metal and the medium (gas);
σ2 is the interfacial tension between the metal and the impurity;
σ3 is the interfacial tension between the impurity and the medium.
Because the aluminum liquid and the oxide inclusions do not wet each other, that is, the contact angle θ Gold>90° (see Figure 2), the following relationship exists:
(6)
Because σ2 is always positive, so:
(7)
Figure 3 Schematic diagram of the flotation method for removing oxide inclusions
In the refining treatment of electrolytic aluminum, this condition can be well met by adding a certain proportion of KCI, NaCl and Na3 AlF6 to the refining agent, thereby achieving a better slag removal effect.
