I'm having trouble getting the temperatures I need to pour iron. I'm using waste oil in a forced air burner with a preheater. In the last several attempts, the iron charge would melt, but only into a sort of slush. When pouring a small amount of liquid would come out, leaving a semi-solid mess in the crucible. Leaving it in the furnace longer doesn't help. It seems I'm getting close to temperature but just not hot enough for a clean melt. The weird thing is, I've used this furnace to melt iron consistently in the past. Some details: Furnace: 20 cm id, 23 cm inside height Crucible 1: 13 cm od, 16 cm height Crucible 2: 11 cm od, 14 cm height Lid is flat with 8 cm vent hole The tuyere enters at about the furnace floor. Plinth is 25mm. Things I've tried so far: - There seemed to be a cold spot on the furnace wall and one side of the crucible. I suspected it was taking the oil too long to vaporize, so I cut it with about 50% kerosene. The last burn was much cleaner, no evidence of a cold spot, and I could see flame starting right where the burner enters the tuyere. - I've tried two different size crucibles as listed above. - Increased the plinth from 25 mm to 50 mm - I'm mostly running a neutral to slightly rich flame. - To create the preheater, I split the burner tube into an array of 1/2" tubes. Flame exits the furnace impinging on these tubes, then they rejoin before the oil enters at the burner exit. I've tried supplementing this with a propane torch to heat longer and better sections of the tubes. Without airflow, the tubes get red hot, so there's considerable heat going into the air before it hits the burner. - Added a spacer to increase the furnace height by 25mm. - Leave the crucible in the furnace longer or reheat to solid mass after pouring of any liquid. Even 1/2 hour extra made no difference. So any ideas on why this is happening, and what to do about it?
I had similar issues with my oil burner furnace and had trouble getting the iron much above softening temperature. I did have one super fast dream run with an A25 crucible in a 28cm/11" bore and it turned out what I thought was kerosene/Jet A1 had some mystery additive that made the 20L plastic bucket go soft and sag a bit. I'm assuming some sort of solvent was tipped in and this burnt much faster. By monitoring the rate of temperature rise as the fuel air mix was varied I could get the furnace to at least 1350 deg C as that's the point that K type thermocouples melt. The A25 crucible was a snug fit to the furnace bore so I switched to a much smaller crucible like an A6 or similar to increase the volume available for combustion and this worked well. I'm now in the process of building a larger furnace that will have faster rate of burn due to the increased volume. So the upside down crucible on the right of the image is an A25 and iron only gets soft in it whereas the A6 gets plenty hot enough as there's more volume to support a higher rate.
Thank you for the reply. I did try another run using diesel instead of oil to get a more volatile fuel, and with a smaller crucible as you suggest. I ended up with a similar problem with a liquid/slush mixture, however letting it soak longer seemed to help. I did 3 batches and by the time I got to the third, it poured ok. Alas, it didn't quite hold enough iron to fill the mould, so I'll need to figure out something else. I did notice quite a bit of ash, which surprised me. In addition, there was a buildup of carbon on the side of the crucible opposite the tuyere, so possibly a cold spot in the furnace? Anyway, I seem to be getting closer; I just need some compromise of space in the furnace vs volume of the crucible, and stick with a lighter fuel.
The carbon deposits indicate a fuel/air mixture imbalance. After fitting a K type thermocouple I was able to get real time feedback on the best fuel/air mixture that gave the fastest rising temperature. I noticed that the ratio had to be adjusted several times during the run as temperatures increased and the furnace got hotter. Iron runs are typically longer than brass or aluminium by close to a factor of two. A thermocouple has to be removed at much above 1300 degrees C to prevent it melting but by that point the tuning is right and the furnace needs more run time to melt the iron. Fitted a K type thermocouple to my furnace to help fuel/air mix
