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Flux refining of aluminum alloy melt
Source: | Author:aidaalloys | Published time: 2019-06-05 | 1536 Views | Share:
This paper introduces the role of flux refining in the purification process of aluminum alloy melt, the classification and requirements of flux, the composition of common flux, the scope of application and the method of use.
In the melting process of aluminum and aluminum alloys, hydrogen and oxidized inclusions are the main substances that contaminate the aluminum melt. Aluminum is very easy to form A1202 or secondary alumina with oxygen (Al2O and A10). At the same time, it is also very easy to absorb gas (H), which accounts for 70-90% of the total amount of gas in the aluminum melt, and the main defects in the cast aluminum alloy - pores and slag inclusions - are due to gas and oxidation remaining in the alloy. Caused by solid particles such as matter. Therefore, in order to obtain a high-quality melt, it is not only necessary to select a correct and reasonable smelting process, but also the refining and purifying treatment of the melt is important.
There are many refining and purifying methods for aluminum and aluminum alloy melts, mainly floating method, flux refining method, melt filtration method, vacuum method and combined method. This paper introduces the application of flux refining method in aluminum alloy smelting.
1 role of flux
Salt fluxes are widely used in the production of primary aluminum and recycled aluminum to improve melt quality and metal aluminum recovery [1. 2]. There are four functions of the flux: one, changing the wettability of the aluminum melt to the oxide (alumina), so that the aluminum melt is easily separated from the oxide (alumina), so that the oxide (alumina) is mostly Entry into the flux reduces the amount of oxide in the melt. Second, the flux can change the state of the oxide film on the surface of the melt. This is because it can break up the solid and dense oxide film on the surface of the melt into fine particles, thereby facilitating the hydrogen in the melt to escape from the particle gap of the oxide film layer and enter the atmosphere. Third, the presence of the flux layer can isolate the contact between the water vapor and the aluminum melt in the atmosphere, making it difficult for hydrogen to enter the aluminum melt, and at the same time preventing the melt oxidation loss. Fourth, the flux adsorbs oxides in the aluminum melt to purify the melt. In summary, the effect of flux refining to remove inclusions is mainly achieved by adsorption, dissolution and chemistry with oxide films and non-metallic inclusions in the melt.
2 Classification and selection of flux
2.1 Classification and requirements of flux
There are many kinds of fluxes used in aluminum alloy smelting, which can be divided into two types: covering agent (flux for preventing melt oxidation and inhalation) and refining agent (gas for removing gas and inclusions for inclusions). The covering agent is different from the refining agent. However, any flux used in the aluminum alloy smelting process must meet the following conditions [3. 8].
1 The melting point should be lower than the melting temperature of the aluminum alloy.
2 The specific gravity should be less than the specific gravity of the aluminum alloy.
8 can adsorb and dissolve inclusions in the melt and remove gases from the melt.
4 should not be chemically affected with metal and furnace lining. If it acts with metal, it should only produce inert gas that is insoluble in metal, and the flux should be insoluble in the molten metal.
5 The hygroscopicity is small and the evaporation pressure is low.
6 should not contain or produce harmful impurities and gases.
7 should have appropriate viscosity and fluidity.
8 easy to manufacture: the price is cheap.
2.2 The composition of the flux and the role of molten salt
The flux for aluminum alloy is generally composed of chlorides and fluorides of alkali metals and alkaline earth metals, and its main components are KCl, NaCl, NaF. CaF,. , Na3A1F6, Na2SiF6, etc. The physical and chemical properties of the flux (melting point, density, viscosity, volatility, hygroscopicity, and interfacial interaction with oxides) play a decisive role in the refining effect.
2.2.1. Chloride salt: Chloride salt is the most common basic component in aluminum alloy flux, while mixed salt of 45% NaCl + 55% KCl is the most widely used. Because of their strong wetting ability to solid Al2O3, inclusions and oxide film (the wetting angle with Al2O3 is more than 20 degrees) and the specific gravity of NaCl and KCl at the melting temperature is only 1.55g/cm3 and l. 50g/cm3, which is significantly smaller than the specific gravity of the aluminum melt, can be well spread on the surface of the aluminum melt to break and adsorb the oxide film on the surface of the melt. However, the flux containing only the chloride salt, the crushing and adsorption processes proceed slowly, and manual agitation must be performed to accelerate the above process. Chloride has a small surface tension and good wettability, and is suitable as a covering agent, wherein a chloride salt having a molecular crystal form such as CCl4
, SiCl4, A1C13, etc. can be used as a purifying agent alone, and a chlorine salt having an ionic crystal form such as LiCl, NaCl hair KCl, MgC12: or the like is suitable as a mixed salt flux.
2.2.2. Fluorine salt: Add NaF to the chloride salt mixture. Na3A1F6, CaF2. A small amount of fluoride salt, mainly for refining, such as adsorption, dissolution of Al2O3. The fluoride salt can also effectively remove the oxide film on the surface of the melt and improve the degassing effect. This is because: a) the fluorine salt can chemically react with the aluminum melt to form gaseous A1F, SiF4, BF3, etc., which mechanically promote the separation of the oxide film from the aluminum melt, and squeeze the oxide film and push Into the flux;
b) The current generated at the interface where the above reaction occurs also causes the oxide film to be "scoured" and broken. Therefore, the presence of the fluorine salt accelerates the destruction process of the oxide film on the surface of the aluminum melt, and the hydrogen in the melt can be easily escaped; c) the fluorine salt (especially CaF2:) can increase the mixed molten salt. The surface tension spheroidizes the molten salt of the adsorbed oxide, facilitates separation from the melt, reduces the loss caused by the slag-carrying aluminum, and accelerates the adsorption of the flux due to the increase of the flux-melt surface tension. process.
3 commonly used flux in aluminum alloy smelting
The flux refining method has a good effect on discharging non-metallic inclusions, but the degree of purification of non-metallic inclusions in the melt is not only related to the physical and chemical properties of the flux, but also depends to a large extent on the refining process conditions. For example, the amount of flux, the contact time of the flux with the melt, the contact area, the stirring condition, the temperature, and the like.
3.1 common flux
In order to refine aluminum alloy melts, hundreds of fluxes have been developed, and sodium and potassium-based chloride fluxes are the most widely used. For sodium alloys with low magnesium content, sodium chloride-based chloride refining agents are widely used, and aluminum alloys with high magnesium content are used to avoid sodium brittleness, and sodium-free carnallite-based refining fluxes are used.
The composition and function of the commonly used flux in the aluminum alloy smelting process are shown in Table 1 (4-7).
Table 1 Composition and application of common fluxes
Solvent type Component content, %
NaCl KCl MgCl2 Na3AlF6 Other ingredients Applicable alloy
Covering agent 39 50 6. 6 CaF2 4. 4 Al-Cu system, Al-Cu-Mg
Department, Al-Cu-Si system Al-Cu-Mg-Zn system
Na2CO385. CaF15 general aluminum alloy
50 50 general aluminum alloy
KCl, MgCl280 CaF220 Al-Mg based Al-Mg-Si alloy
31 14 CaF210 CaCL244 Al-Mg alloy
8 67 CaF210, MgF215 Al-Mg alloy
Refining agent 25-35 40-50 18-26 In addition to Al-Mg, other alloys other than Al-Mg-Si
8 67 MgF215, CaF210 Al-Mg alloy
KCl, MgCl260, CaF240 Al-Mg-based Al-Mg--Si alloy
42 46 Bacl26 (No. 2 flux) Al-Mg alloy
22 56 22 general aluminum alloy
50 35 15 general aluminum alloy
40 50 NaF10 general aluminum alloy
50 35 5 CaF210 general aluminum alloy
60 CaF220, NaF20 general aluminum alloy
36-45 50-55 3-7 CaF 21. 5-4 General aluminum alloy
Na2SiF630-50, C2Cl650-70 general aluminum alloy
40. 5 49. 5 KF10 can alloy
As can be seen from the above table, the content of some flux components varies widely, and can be determined according to actual conditions. First, it should be determined according to the content of alloying elements [8], because the content of main elements in most aluminum alloys can be varied within a certain range, and secondly, it is determined according to the impurity components and content. Therefore, in addition to using the flux produced by the flux factory, it is preferable to adjust the proportion of the flux component according to the composition of the smelted aluminum alloy to find the optimum flux composition.
It is not difficult to see the above various fluxes. When the composition of the aluminum alloy to be melted is determined, the design of the flux component is firstly the selection of the main component (such as chloride), followed by the addition of components (such as fluoride). select. After the flux is prepared, it is preferably smelted, condensed into pieces, and then pulverized, because the mechanical mixing state is not effective.
3. 2 flux dosage.
When smelting aluminum alloy scrap, the quality of the scrap is different, and the amount of the covering agent and the refining agent are also different.
3. 2. 1. Main covering agent dosage
a) Smelting good quality waste materials, such as block materials, tubes, and sheet covering agents (see Table 2). Table 2 Types and dosages of covering agents and materials Covering agent dosage (% of feed) Covering agent type Electric furnace melting: general products special products 0. 4-0. 5% 0. 5-0. 6% ordinary powder solvent Ordinary powder solvent gas furnace melting: raw aluminum ingot scrap 1-2% 2-4% KC1: NaC1 mixed 1:1 KC1: NaC1 mixed 1:1
Note: For high-magnesium aluminum alloys, it should be covered with a flux containing no sodium salt to avoid contact with sodium-containing flux.
b) Smelting of poor quality scraps, such as scraps from sawing, turning, milling, etc., and slag slag, etc., the amount of covering agent (see Table 3).
Table 3: Covering agent dosage
Category Usage (% of feed)
Small debris, debris, external slag 6-810-1515-20
3.2.2 Refining agent dosage
Different aluminum alloys, different products, and the amount of refining agent are also different (see Table 4).
Table 4 Refining agent dosage
Alloys and products, melting furnace, stationary furnace
High-magnesium alloy No. 2 flux 5-6kg/t No. 2 flux 5-6kg/t
Special products except high magnesium alloys Common flux 5-6kg/t ordinary flux 6-7kg/t
LT66, LT62, LG1, LG2, LG3, LG4 use ordinary flux, laminated flux dam
Other alloys Common flux 5-6kg/t
Note: 1 In wet areas and wet seasons, the amount of flux should be increased.
2 For large-size round ingots, the amount of flux should also be increased appropriately.
3. 3 flux usage
The following methods are commonly used in the production of smelting aluminum alloy by flux refining method.
1 The melt is refined in a ladle. First, a pack of flux is placed in the ladle, then the melt is injected and stirred thoroughly to increase the contact area between the two.
2 The melt is refined in an induction furnace. The flux is charged into the induction furnace, and the flux and the melt are thoroughly mixed by the stirring action of the induced magnetic field to achieve the purpose of refining.
3 Refining in a ladle or in a furnace with a mixer to mechanically disperse the flux in the melt.
4 The melt is refined in a magnetic field stirring device. The method relies on the action of electromagnetic force to continuously deliver the melt to the flux-metal interface to achieve active contact between the aluminum melt and the flux. The higher the melt rotation speed, the better the refining effect. 5 electric flux refining. This method is a continuous refining of a melt through a flux layer to which an electric field (at the metal-flux interface) is applied.
Among these five methods, the electric flux refining effect is the best.