During smelting, aluminum melts are purified from dissolved metal impurities (sodium, magnesium, iron and zinc), suspended oxide (non-metallic) inclusions and dissolved hydrogen. In order to purify from dissolved metal impurities, degassing methods in casting, treatment with fluxes, and filtering is important. For these purposes, various methods of purging the melts with inert and active gases, sedimentation, treatment with chloride salts and fluxes, evacuation and filtering through mesh, granular and foam ceramic filters are used.
Degassing Methods In Casting
In Casting from aluminum and aluminum alloys AMg2 and AMgb can be carried out by blowing the melt with chlorine, a mixture of inert gas with chlorine (per 100 volume parts of inert gas, 1-10 volume parts of chlorine), chloride vapors (CrC1b. CC14, TiCl4), freon (CCI2F2 ) and filtration through granular filters from AlFj or ABO3 activated with chlorine or fluorine.
The consumption of refining gas is 0.2 … 0.5 m ‘per 1 ton of melt. Duration of blowing through ceramic nozzles or porous ceramic inserts 10 … 15 min at melt temperature.
Degassing Methods In Casting, impurities of lithium, potassium and calcium are also removed from the melt and up to 0.2% Mg is lost.
Melts are filtered through filters with a thickness of 150 … 200 mm from grains with a diameter of 4 … 6 mm.
The harmful effect of sodium on the technological properties of the AMgb alloy can be suppressed by introducing bismuth or antimony additives (0.2 … 0.3% of the mass of the melt) into the melt, forming intermegallides with sodium, melting at 775 and 856 ° С, respectively.
Gas purging is widely used for degassing melts and cleaning them from non-metallic inclusions.
Refining is carried out the more successfully, the smaller the size of the bubbles of the purged gas and the more uniform their distribution over the volume of the melt. Purge, as a rule, is carried out through porous inserts from sintered alumina, which ensure the production of gas bubbles with a diameter of 1.5 … 2.0 mm.
This refining method is widely used in foundries for the production of ingots.
The purge is carried out in special lined containers installed on the nougat of overflow of metal from the mixer into the mold or in the buckets.
A porous ceramic insert is installed in the bottom of the bucket or container.
For refining aluminum melts, nitrogen, argon, helium, chlorine and a mixture of nitrogen (90%) with chlorine are used, purified from moisture and oxygen.