2005-07-09
Reverberatory Furnace
A trip to the supermarket provided the required form work for this furnace. A common brand of plastic tub available from Woolworths comes in many useful sizes, but most importantly each width and length choice is available in two depths. The basic geometry seemed ideal for a reverb' furnace so about $15 was invested and construction begun.
The same refractory mix was used as in previous furnaces.
The lid was constructed first, half a bucket of mix went into the plastic tub and a smaller tub was pushed into it about two thirds of its total depth, then the edges smoothed by hand. The base construction was essentially identical, just deeper. As usual the excess refractory mix was made into bricks, plinths, etc.
After curing for 3 days the molds were stripped (which took some effort for the base because I used no release agent but the slope of the sides made it fairly easy for the lid). The still damp castings were then dried in the oven, following the usual scheme of soaking at about 120 C for several hours, then full-blast for six or more hours. Some scraping and filing made the seating between the lid and base near perfect.
Once dry I puzzled over how to turn these lumps of refractory into a usable furnace... Two weeks later and a browse of Bunnings gave some inspiration, a coil of hoop iron and some large nail plates were purchased, along with a box of 100 steel rivets. An hour or two of relatively enjoyable metal bashing yielding a somewhat ugly but quite usable little reverb furnace. A hole-saw and some filing was sufficient to make the hole for the burner/pouring spout.
I've never worked with hoop iron and rivets before, I love it already, so versatile and easy. Aviation tin snips and a drill make easy work of it. In many cases the holes in the nail plates and the hoop iron lined up. Murphy must have been on holiday or something? I only had to drill about 10 holes. I probably could have made a neater job by drill them all rather than being opportunistic, but the end result has a certain elegance for such a hacked together mess.
Burning it out was easy, the hoop iron folded over the underside of the lid made a perfect vent space, the lid was simply put flat on top of the base and the burner set a little higher than idle for about 10 minutes.
No structural failures, so the burner was cranked to 50 kPa and Aluminium was melting in another 8 minutes or so.
Some broken pieces of firebrick were used to prop-up the lid on the far side of the furnace so lengths of scrap Aluminium molding could be fed in. This worked quite a treat, easier with the very long lengths I have. In the past I've had to fold/break them up so they could be feed vertically into the crucible furnace. Once the furnace was near full the lid was removed briefly and the dross skimmed. The lid replaced (sans the firebrick wedges), burner removed and the entire furnace lifted over to the ingot mold and the pour made. After the pour the burner and wedges are replaced and more metal fed in.
This furnace totally rocks for breaking scrap down. I even melted down all the slag I've been collecting and recovered quite a bit of metal from it. The crucible-less construction is a great thing, I love it.
The coating on the Al I've got leaves lots of dross, slightly greenish (perhaps chromium oxides?), and burns off at first with a big smoky and smelly flame. The smoke has blackened the furnace a bit, but it is otherwise unharmed.
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