Non-Chemical Gas Melting Oven

[markusm]

Dear Forum Members,

my name is Markus, I am 45 years old, and I do silver refining in my second year.
I love this Forum alot.
I thank all the Forum members because I am always learning here, so I would like to put a question.
I got myself a gas melting oven, because I got tired of the problems with my electric one. I have ordered the A10 crucibel (holding 10 kg of Silver) and 2 pairs of tongs - one for taking out and one for pouring. This oven has been used in the past for silver because I have found silver spatters inside the lid!
I know that with all the wealth of information, there is probably more than one Forum member who has worked with this equipment before. I am doing my initial tryouts until I have received the tongs. I am learning sofar everything by myself of the Internet. But I have an urgent question ´cause I want to get going as soon as possible with this oven. I have attached a propane cylinder to the tab closest to the oven and pressured air from a compressor to the tab on the far side. It is firing up and it is heating nicely. Will this setup with propane and pressured air be enough in this kind of oven, to get me to about 1100 °C. This is roughly what I need to get my material melted. Does the flame have to hit the crucibel directly? I think so, apparently otherwise the heat will not be enough. I made a tryout with a smaler crucibel, but all three flames did not hit it because the crucibel was too small. I tried to put the crucibel close to one of the three gas inlets and it began glowing dark red, but I suspect all three flames have to hit the crucibel in order to get to that yellow glow which I need. Also I suspect that more air would not harm, I would have to connect a second compressor for that. Or is it necessary to have a different kind of gas and maybe oxygene?
I have attached pictures of the oven, and I hope it can be seen what kind of oven I have here. He surely is almost 100 years old.
What do you think?

Looking forward to hear from you all,

Markus

 

[butcher]

Markus ,

Thanks for posting the picture of your new furnace.
In a burner you have to get the air to fuel ratio correct to burn efficiently.
Too little air or too much air in relation to the fuel can have a big effect on burner temperature, your crucible should also be sized for the furnace, as well as the size of your melt, a furnace will take a lot of heat to get it refractory up to temperature, also the type of refractory play a big role in a furnace, some types of refractory will absorb heat while other types do not absorb much heat and will actually reflect heat, it can also take time for a furnace even with a properly sized burner and fuel/air ratio come up to temperature.

Too little air is a reducing flame produces toxic carbon monoxide fumes, but can also be used to reduce metals in a melt, base metals will reduce to metal in a reducing flame, with gold this is unwanted, but with silver you will want a slightly reducing flame as silver will suck up excess air on melting, and bubble and spit it out on cooling, but then again we do not want to starve our burner for air to the point we loose efficiency or heat.

Too much air will give an oxidizing flame, this will cool the flame also, but the combustion gases of the fuel and air are a much safer CO2 gas produced if a slight excess air is used. We get a more complete combustion of fuel, but by no means are the gases from a furnace melting metals safe, and with an oxidizing flame we actually end up with more oxidized base metals in the flue gases. With gold we may want a slight oxidizing flame to help oxidize base metals in the melt.

Markus that looks like a nice furnace, I have not seen one that used compressed air, I really like it, but as stated you will need to regulate the air/fuel ratio, and also your furnace should be sized to the size of the metal you are melting, and the size of crucible (too small of a crucible gives a much larger inside area of the furnace to heat, and the burner may not of been designed to heat this much area. It would take an awful lot of fuel to melt a few grams in a furnace designed to melt several pounds of metals, and using a very large furnace getting it up to temperature, and if the crucible was way too small most of your heat would just be wasted passing right on through the furnace, fuel costing so much money nowadays that would be like watching dollar signs $$$ coming out of the flue of your furnace, while trying melt a few dollars of silver.

Some burners use fuel orifices and need a certain amount of air (within a range) to burn, changing fuel on this type of burner, this type of burner would need a different office size, and the fuel/air ratio adjusted for that type of fuel, I cannot tell from your picture if your burner uses orifices but I suspect it does not.

 

[markusm]

Hallo Butcher,
thank you for your reply.
Much technical terms, which I sound interseting to me, but I will have to look that up first.
I am planning to cast bars with 5 kg´s.
The material I am working with is cemented Silver.
I am glad the larg crucibel can hold more of the voluminous powder, because I think reloading during a melt would be difficult.
The thing you said about that the crucibel size must fit the oven was clear to me. How much space should I roughly have betwenn crucibel and refractory cement? My oven has an inner diameter of 24 cm. The crucibel which I have ordered now is 16 with at the Top and 12 at the bottom. I guess now that it will be too small for the oven, but I am still gald because I could fit in more powder.
You mentioned that you never saw an oven like this that worked with compressed air. I suppose oxygene should be used then instead?
Any more comments are very welcomend.

Kind Regards

Markus

 

[butcher]

Markus,
I have not seen that type of air setup on a burner, there are many ways to design burners.

But I do not now if your furnace was designed for air or oxygen, oxygen is so expensive, I suspect your furnace was designed to run on air.

Which you used air or oxygen would not matter in a burner as long as you used them in the correct proportion O2 to fuel ratio,or air to fuel ratio.
But oxygen will run a heck of a lot hotter than air.

Air is about 78% nitrogen and 20% oxygen, so you will need more air to supply the fuel with required oxygen to get the air to fuel ratio correct, the excess nitrogen that does not burn going through the furnace will cool the furnace (compared to the oxygen which burns) using pure oxygen you would need less volume, it could also be very costly, also if the furnace was designed to run on air, and you tried to run it on oxygen you could possibly over heat the furnace, as air with the nitrogen will run cooler than a fuel burnt with pure oxygen.

 

[markusm]

Hallo again,
thank you Butcher for pointing me the right direction.
I have removed the stop cock visible in the picture and attached a ventilator which I usually use on my fume hood.
It makes a huge wind and makes a much hotter flame. The crucible takes 10 minutes to get red hot now.
I am looking forward to my other equipment. I got the wrong idea with the compressor, and when you said you never saw an oven like that running with a compressor, my mistake became obvious to me. Thank you again for all the other chemistry bits and pieces on melting, I will take all of what you said and put it to work for me! Since this project seems well on its way now, I am starting now with my first electrolytic cell. What I want to get at is to cut out these redicolous high Refinery charges. It costs me an arm and a leg every time. How I would love to make my very own 999 Bars.
I will keep posting as I progress.


Regards
Markus