When the novice hobby metal caster first thinks about melting metal, the immediate thought is to collect a big box of Al bottle tops and Al drink cans to melt. The most likely reason for this is that the material is relatively easy to collect and handle, and the thinking behind the idea is that because of its lightweight it should melt quite readily. Then why is it so difficult to melt & reclaim aluminium drink cans, bottle tops & swarf in a hobby gas fired crucible furnace?
To melt & reclaim light weight scrap aluminium requires the use of some specialised equipment. Most commercial scrap metal recovery foundries use what is known as a rotary type melting furnace. This type of furnace is designed in such a way that the flame actually strikes the rotating furnace lining, and the heat spreads quickly around the furnace walls, which absorb the heat. As the furnace rotates, the heat is also taken up or absorbed by the scrap metal.
It is essential to melt the metal under a cover of MOLTEN FLUX, otherwise very heavy metal oxidation results and subsequently very little metal is actually recovered. As each piece of small swarf or chip melts, it forms a globule of liquid metal surrounded by a shell of oxide. The skin tension of this oxidant around the globule prevents coalescence, i.e. (to grow together) and because of the large surface area presented by the mass of globules, with the increase of oxide formation loss of yield is bound to take place. The skin of the oxide on the molten globule has to be "ruptured" in order to allow coalescence, i.e. to allow the clusters of globules to actually join together in their molten state.
A molten flux encourages coalescence by chemical action.
While the rotation of the furnace provides a mechanical action. The special fluxes; coveral 48 & 57 provide the chemical action, they are the best type of fluxes to use when melting Al metal in the region of 590 to 600 C. (Coveral 48 & 57 products are copyright Foseco Pty Ltd) This flux may or may not provide similar results when used in a normal crucible gas fired furnace, but the yield loss will still be considerable. The hobby foundry worker would be better off directing his energy towards collecting easier to source, better quality scrap to melt, such as discarded cylinder heads, inlet manifold castings, auto pistons, etc. Or if the budget stretches that far, purchase commercially produced ingots, which are of a known quality. Commercial ingots will provide top quality metal right from the word go. Trying to melt lightweight Al drink cans and bottle tops is generally a waste of time for the hobby worker, unless you use the fluxes mentioned above, and use the correct furnace. Otherwise a lot of gas will be wasted heating the hobby crucible furnace for very little gain in metal yield. Col Coucher |
