Backyard Foundry 

would you recommend going with refractory and perlite or kaowool with a refractory coating?

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Simpler: After pulling the valve, drop a few good-sized (or as good sized as you can through the valve hole) hunks of it into the tank, then toss in a cup or so of water. The "looks-like-smoke"/fog rolling out shortly afterwards will tell you when it's full of CO2 gas, which will make it about as safe as it's possible to be when cutting on a tank that used to hold flammables. (I have yet to meet a fuel-gas that can burn without oxygen, and since CO2 is heavier than air, letting a few chunks of dry ice melt (Actually, sublimate, but we won't nitpick, will we?) inside the tank will fill it with CO2, driving out both any final traces of fuel gas, and any oxygen from the air - the water just makes it act like a "fog machine" to give you a visual indicator of when it ought to be safe to start doing sparky/flamey things to the tank without worrying about causing a KABOOMy thing to happen - trust me... KABOOMy things are BAD when they aren't meant to happen!)

Yea that is pretty bad ass

Lid is on but the gas pressure is as low as you can set it without the flame blowing out. Allow an entire day and be very slow and deliberate.

Coat it with mixture of clay, sand and Portland cement. Cement 30% and sand 20% rest, all clay

Use a propane grill lighter which is about 10" long light it from the intake past the orifice. Takes a couple of tries adjusting the gas pressure and all but once you get it right you have it from then on.

All you have to do is fill it with water (or dry ice if you're fancy), wait a bit for the displacement. And that's about it.

I already have a video of how to do this safely. Too complex to cover all points Just watch the first part of the video starting at 2:28 Backyard foundry tutorial part1 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-gFOq88iQg60.html Pay special attention to how the valves have change over the years.

KS4 castable refractory. Check my foundry tutorial videos for more info.

thank you so muck I am trying to make a similar design

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My furnace looks very similar, with melting times around the 30 minute mark. I've done around 20 melts, and still had plenty left over in a typical 30lb propane tank. Orifice for my burner is around 1mm.

Kyoda Labs Thank you for the info...

I am not sure of the profitability since it uses propane. No problem melting gold as far as the temp. The limit is the liner. KS4 is rated to 2,550F after that it turns to glass and starts running down the sides in brilliant orange streaks. Looks the color of brown beer bottle when cooled. Mizzou can run up to 3000F and might be a better option. Both sold by Harbinson Walker now called ANH Refractories.. Propane burner can ramp past both if you are not careful.

Necessary to remove friability of material and add durability. not sure of safety of ceramic particles floating around. respiratory hazard. Asbestos I understand seeing it under microscope. Barbed fiber. Floating glass fibers suspicious. Raw wool eventually wears away. Ceramic coating little to none and safer.