Treatment of molten aluminum
This article concludes our review of the remarkable British firm leadership Ashford Engineering Services «D.C. Casting Remelt Shop Handbook». It is a practical guide to the foundry independent plant for the production of extruded aluminum profiles. This time we will focus on the treatment of molten aluminum - furnace and ladle - before moving into the casting machine.
Shop for casting of aluminum ingots-pillars
Furnace for melting aluminum
Vertical casting machines for the casting of aluminum ingots
Systems casting aluminum ingots-pillars
Monitoring the furnace temperature
melt temperature control should be the determining factor. Excessive melt temperature leads to loss of the alloying elements and their requires additional administration, which increases the cost of the charge.
To be effective melting of aluminum, the kiln temperature must be at least 1100 oC. Temperature control is generally carried out at a temperature of the furnace roof or furnace temperature of flue gases. However, the dominant factor is the temperature of the melt in the bath furnace.
A thermocouple in the furnace bath
thermocouple, which is immersed in the furnace melt, the main issue is the service life of its refractory casing. Major problems - is "vmorazhivanie" casing thermocouple in the furnace bath and its mechanical damage when loaded in the furnace charge. Therefore, many founders thermocouple is removed from the bath furnace when loading it charge, cleaning furnace and the slag removed. At the beginning of the next cycle melting thermocouple was placed back to its place in the vessel of the furnace. Hole, into which is inserted a thermocouple should be sufficiently large, to allow a slight sticking of the metal and slag at the thermocouple housing.
Simple tools, which are required for furnace operation, shown in Figure 1. These tools need to work with aluminium scrap in the furnace, removing slag, cleaning ovens and sampling for the spectral analysis. All these instruments have a limited life and, therefore, are considered consumables. They are usually made at the factory.
Figure 1 - Simple tools and accessories for work
with a small melting furnace
Alloy preparation and sampling
When all melted scrap, and the oven is already loaded at its full capacity, melt sample taken and checked for its contents of the various alloying elements and impurities. The most effective method for this is the spectral analysis. Spectral analysis is performed on the light spectrometer. Light spectrometer - a device, wherein the arc vaporizes the metal sample, which is made from the sample. The light from the arc passes through a prism and a lens system. Photosensitive diodes measure the intensity of any given line light spectrum. Each element has its own wavelength and in the light spectrum. The intensity of each wavelength is proportional to the content of a given chemical element in the test sample. Calibrated photodiodes result in a percentage of the set of chemical elements in a sample.
According to the results of spectral analysis produce podshihtovku oven - add its various components - to achieve the specified chemical composition of molten aluminum in the furnace.
Removal of the slag from the melt in the furnace
After making sure the melt flux slag is removed from the melt surface. Slag - a mixture of various aluminum oxides, preferably with other impurities, which are collected on the surface of the aluminum melt. Before casting the melt transfer or for subsequent secondary treatment is necessary to remove slag.
In small furnaces, slag is removed from the metal surface with a long hand tool with a scraper at the end, rotated 90º (cm. Figure 1). Work-foundry slag collects on the surface of the melt and through the furnace window directs it into the vessel slag, which stands directly beneath the loading window. The figure below 2 shows the mechanical device for removing slag for large ovens.
Figure 2 - Examples of devices for mechanized removal of slag
in large smelters
Degassing and fluxing
Fluxing helps make metal cleaner. Fluxing produced using respective "powders" or "pills", who throw in the oven and carefully stirred melt. Another method is to purge the melt in the furnace with argon, nitrogen or a mixture of gases, which is fed through the silicon tube, immersed in the melt.
Metal requires some excerpts, to "degas" and allow hydrogen to stand out from the melt. Moreover, during fluxing pop-up on the surface of the melt gas bubbles assist in the removal of nonmetallic inclusions.
Even in the 1980-1990-s was used for degassing a 100% chlorine gas. However, this gas is very harmful to human health and, Moreover, causing corrosion of metal structures and equipment. Therefore were later used fluxing mixture of two gases with low content of chlorine. Examples of such gas mixtures are:
- 0,5-2,0 % chlorine and more 90 % nitrogen;
- 5-10 % chlorine and more 90 % argon;
- 5-10 % chlorine and more 90 % freon.
At large industries are often used in the process ladle degassing molten aluminum by melting the transmission line or the holding furnace to a casting machine. These systems are more efficient degassing, than degassing furnace. for instance, they are able to reduce the hydrogen content in the melt more, than 60 %. The figure 3 shows two types of such degassing systems - with rotary and blowing through a porous plug.
Figure 3 - Systems ladle degassing molten aluminum
Filtration of molten aluminum
Aluminum melt, which arrives from a melting furnace, contains various minor and major oxides, as well as other pollution, for example, fragments furnace lining. To eliminate problems with subsequent surface quality of extruded aluminum profiles prior to feeding the melt onto casting ingot-pillars must be a greater or lesser degree of filter.
Many plants cost at the melt filtration only by passing it through a folded in several layers of special glass fiber. In this case, the small non-metallic inclusions and other impurities inevitably fall first in bars, poles, and then extruded profiles.
At higher requirements for the surface of the melt extrusions apply filtering by filtration installation, main element of which is a ceramic filter. EXAMPLE simple filtration unit shown in Figure 4.
Figure 4 - Installation for filtering of molten aluminum
Source: D.C. Casting Remelt Shop Handbook, Ashford Engineering Services, 1997.