Secondary aluminum production process (3)
4.3 secondary aluminum smelting
4.3.1 The purpose of smelting
The basic task of the metal alloy smelting is to charge a certain proportion of metal into the furnace, heated and melted to obtain a melt, then melt were melted component adjustment, an alloy liquid desirable. And take appropriate measures to control the content of the gas and the oxide inclusions in the smelting process, so that compliance component (including a main component or impurity elements), to ensure that the casting are properly organized (grain refinement) High quality alloy liquid.
Due to the nature of aluminum elements, aluminum has a strong tendency to porosity, but also easy to produce oxide inclusions. Therefore, to prevent oxidation and removal of gas and aluminum smelting process of inclusion has become the most prominent problem. For high-quality aluminum alloy liquid, its smelting process must be strictly and take measures to control all its aspects.
4.3.2 Smelting Process
Aluminum smelting process is as follows:
→ melting furnace installed (plus copper, zinc, silicon, etc.) → Cinder → plus magnesium, beryllium, etc. → stirring → sample → Adjust component → stirring → refining → → Converter → refining slag modification and stand → casting.
Installed furnace installed furnace proper method for reducing the burning metal and shorten the melting time is very important. For reverberatory furnace, bottom layer of aluminum ingots, into easy burning material, and then the pressure on the aluminum ingots. Back to charge a lower melting point means the top, making it the first melt, shed easy burning material will cover the following, in order to reduce the burning. Various charge a flat distribution should be uniform.
Melting the melting process and melting rate has an important influence on the quality of aluminum ingots. When the charge is heated to soften adequate coverage should stay flux, melting process should be taken to prevent overheating, after the charge was the level of the molten liquid surface, the temperature of the melt should be properly consistent agitation, but also helps to speed up the melting. Smelting furnace for too long not only reduces productivity, and increase the gas content of the melt, and hence, the melt when the melting time long secondary refining.
Cinder Cinder when you can charge all melting to the melting temperature. Preceded thrown into flux slag powder (high magnesium alloy should not be thrown into the sodium flux). Slag should be thorough, easy to pollution because of the presence of scum and increase the melt when the metal-containing gas.
Plus magnesium slag after adding beryllium, magnesium ingots can be added to the melt, and should add flux coverage. For high-magnesium alloy, to prevent burning magnesium, it should be added 0.002% to 0.02% beryllium. Beryllium metal reduction may be obtained from the use of fluorine sodium beryllium, beryllium, sodium fluorine is mixed with flux to join.
After stirring for sampling before and adjust the composition should be sufficient time for mixing. Stirring to a smooth, melt without damaging the surface oxide film.
After sampling the melt stir immediately sampled, and subjected ladle analysis.
Adjust the composition when ingredients do not meet the standard requirements, should be fed or watered down.
After the converter melt composition adjustment, when the melt temperature to meet the requirements, Bachu surface scum can converter.
Refining deterioration melt composition different purification methods are also different metamorphism.
4.3.3 component adjustment
In the smelting process, the metal elements are due to the oxidation and reduction of their own, they are much the degree of oxidation, not only with their affinity for oxygen related to the size, but also with the element concentration in the liquid alloy (live degrees), the nature of the oxides and in which the temperature and other factors. In general, the larger affinity for oxygen loss more elements, aluminum, magnesium, boron, titanium and zirconium strong affinity for oxygen; carbon, silicon, manganese followed; iron, cobalt, nickel, copper and lead relatively weak. Therefore, in the melting alloys strong affinity for oxygen elements to be "preferred oxidation" caused by excessive wear and tear; on the contrary, those of oxygen affinity weaker elements, can be relatively limited by "protection" and less wear and tear .
Through the melting, the alloy chemical composition of certain elements of loss due to oxidation and that the content is increased or decreased, depending on the relative loss should the elements and base metal elements may be. The relative loss of many elements of its content will decrease, known as "burning"; a relatively small loss element content increases, can be called "burn by"; to be able to properly control the chemical composition of the melt, when the metal charge in the Chooser It should take into account changes in the melting after the corresponding compensation amount on each element.
In the actual melting, the degree of burning of the alloy elements, but also by the quality of raw materials, flux and slag, operating techniques, particularly affect the nature of the oxide generated.
4.3.4 smelting process and oxide gas to prevent
Already mentioned, the main source of gas and liquid aluminum oxide inclusions is H2O, and H2O is stirred into the molten aluminum from the surface of the oxide film, the surface charge (particularly damp air corrosion of charge), melting and refining casting tools agent, deterioration agent into the molten aluminum. And stirred into molten aluminum oxide film and the inclusion of more low-grade charge (such as slag splashing, scraps remelted ingot) to form an oxide inclusions in liquid aluminum. To this end, the process should be melting and pouring attention to the following points:
① crucible and melt casting Tools
Should be carefully removed before using the dirt adhering to the surface rust, oxidation residue, the old paint layer and the like, and then coated with new paint, preheating and drying before use. Melting casting tools and transport of liquid aluminum crucible should be fully preheated before use.
② charge
Charge before use should be stored in a dry place, such as a charge already wet etching gas is carried out prior to dosing blow to remove surface corrosion layer. To charge the surface adhesion often sand (SiO2), part of the SiO2 and aluminum liquid will occur the following reaction:
4 Al + 3 SiO2 → 2 Al2O3 + 3 Si
The surplus generated Al2O3 and SiO2 are forming an oxide inclusions in molten aluminum, so before adding such material should also be used after the blowing sand. By the chip, splashing slag remelting ingots cast three back charge often contain more oxide inclusions and gas, so its usage should be strictly limited, usually no more than 15% of the total charge on important casting should not at all. Nor shall charge the surface of the oil, cutting coolants and other things, because all kinds of oil are having a hydrocarbon, oil heat into the complex structure and hydrogen.
When added to molten aluminum charge must be preheated to 150 ~ 180 ℃, preheat the one hand, when the purpose is to secure, to prevent aluminum liquid and condenses on the cold surface of the water charge meet and explosion; the other is for the to prevent gas and inclusion into the molten aluminum.
③ refining agent, deterioration agent
Some component because it is easy to absorb atmospheric moisture and deliquescence, and some crystal water itself. Therefore, it should be sufficiently dried before use, require the approval of certain substances such as ZnCl2 remelting to the water before using.
④ melting, casting process operation
Stirring the molten aluminum liquid should be smooth, try not to make the surface of the aluminum oxide film and the air stirred into the mixture. Explanatory should minimize the number of molten aluminum, the transfer note should reduce the flow of drop height and reduce splashing. When the ladle pouring nozzle as close as possible to reduce the flow of pouring cup drop height, and shall uniform casting, aluminum liquid splashing and vortex minimized. After casting After casting, aluminum spoon remaining solution should not be back into the crucible and poured into an ingot mold, or molten aluminum oxide inclusions will continue to increase. Deposited on the bottom of the crucible of about 50 ~ 100mm deep liquid aluminum has a relatively large amount of Al2O3 inclusions, etc., and therefore can not be used to pouring castings.
⑤ Duration melting temperature, melting and casting process
Increasing the temperature will accelerate the reaction between the aluminum solution and H2O, O2, the solubility of hydrogen in liquid aluminum melting temperature also rises with a sharp increase, when the temperature is higher than 900 ℃, aluminum surface oxide film becomes not dense, More significantly exacerbate the aforementioned reaction, so the melting temperature of not more than most aluminum alloys 760 ℃. As for the aluminum surface oxide protective film loose aluminum - magnesium alloy, aluminum fluid and H2O, the reaction between O2 more sensitive to temperature increases, thus limiting the melting temperature of the aluminum-magnesium alloy is more stringent (usually no more than 700 ℃) .
The duration of the melting and casting processes (especially to cast away the time after completion of refining) longer, the gas and oxide inclusions in the molten aluminum content is also higher. Therefore, we should try to shorten the duration of the melting and casting, in particular, to minimize further refinements to the completion of the pouring time, the general requirements set out in the factory within two hours after refining finished pouring, pouring endless as it should be re-refining, in wet weather Area and casting pinhole higher level, or easy to produce porosity, inclusions of alloy, the cast should be limited to shorter time.
4.4 Recycled aluminum refining
When the metal is molten composition adjustment is completed, the next step is refining of molten aluminum. Aluminum refining is intended to take to get through after degassing, cleaning measures high cleanliness, low alloy liquid gas content. Refining are the following methods:
Join chloride (ZnCl2, MnCl2, AlCl3, C2Cl6, TiCl4, etc.);
Blaine's method (through the N2,, Cl2 or N2 and Cl2 mixture);
Vacuum processing method;
Adding non-toxic method of refining agent;
Ultrasonic treatment
Its principle, the refining process has two functions: to dissolve hydrogen states, mainly rely on diffusion of hydrogen from liquid aluminum; oxide inclusions, mainly by the addition of flux medium surface adsorption or air bubbles are removed.
4.4.1 In addition to gas
Float method are generally used in addition to gas, the principle is in some kind of liquid aluminum through the bubbles of hydrogen gas, the use of these bubbles float process will be dissolved hydrogen with liquid aluminum, escaping into the atmosphere. In order to get a better refining result, it should make possible to import gas iron pipe pushed deep bath, iron pipe from the bottom of the lower crucible 100 to 150 mm, in order to make the trip longer bubbles rise to the surface at the same time not to sink inclusions in molten aluminum stir up the bottom. Through the iron pipe into the gas should be slowly moved laterally within the aluminum liquid to make bubbles around the pool were adopted. As far as possible to lower the pressure and aeration rate, such as the formation of small bubbles, the surface area of the bubbles to expand, and because small bubbles, floating speed is slow, which can remove more and gas inclusions. Meanwhile, to ensure good refining result, refining temperature should be selected appropriate temperature is too high the resulting larger bubbles and quickly go up, so that the refining effect of variation. The viscosity of liquid aluminum temperature is too low when large, is not conducive to liquid aluminum gas fully discharged, it will also reduce the refining effect.
Ultrasonic treatment with molten aluminum can be effectively degassed. Its principle is to pass into the liquid aluminum elastic wave, causing a "hole" phenomenon in the aluminum liquid, thus undermining the continuity of the liquid aluminum structure, resulting in numerous microscopic vacuum cave, dissolved in liquid aluminum Hydrogen can quickly escape into these cavities into bubbles core, continue to grow after the escape of liquid aluminum was bubbled to achieve the refining effect.
4.4.2 Impurity
For non-metallic inclusions, using a gas refining method can effectively remove, the higher the filter material may also be employed a method in the casting process or the melt through a mechanical filtering molten flux layer removed.
For metal impurities, the general approach is detrimental factors of favorable factors. That is, by alloying methods to turn it into a useful second phase, in order to facilitate the material performance of the play. If it must be removed, in most cases the boiling point of difference is the use of different elements of a high-temperature low-pressure selective distillation to achieve the purpose of removing metal impurities.
Containing aluminum smelting aluminum waste often contains excessive metal elements, it should be removed. Selective oxidation can be used, may be removed from the melt with the oxygen affinity of aluminum to oxygen ratio greater affinity for various metal impurities. For example, magnesium, zinc, calcium, zirconium and other elements, by stirring the melt is accelerated oxidation of these impurity elements, these metal oxides do not dissolve in liquid aluminum into the slag, so that you can and will by skimming it from the aluminum melt body removed.
The method can also be used to remove the differences in solubility of metal impurities in the alloy. E.g., an aluminum alloy to be contaminated with metal impurities can be well dissolved aluminum does not dissolve eutectic impurities, and then separated by filtration of liquid aluminum, and then removed by vacuum distillation to join metals. Usually by adding magnesium, zinc, mercury to remove aluminum iron, silicon and other impurities, then remove these added metal by vacuum distillation. After the contamination by impurities such as aluminum and magnesium eutectic 30%, in the past at the eutectic temperature alloy will stand for some time, filter out iron and silicon crystalline precipitate early phase, and then at 850 ℃ vacuum degassing magnesium, at this time a high vapor pressure impurities such as zinc, lead and magnesium are removed together, after the addition of magnesium to pure aluminum ingot.
In order to further improve the quality of molten aluminum, or some grade aluminum requires strict control of hydrogen content and inclusions, it can adopt a joint refining method, ie using both refining method. Such as chloride - filtration combined with refined argon - flux combined with refining methods can get better results than a single refinery.Prev News：>
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How to configure the aluminum melting furnace| Updated:2015.05.29 Source: Clicks:
