Recently we use a 1-atm gas mixture furnace to homogenize melt inclusions hosted in olivines. Firstly, we did the air tight test and found that there was slightly gas leak in this furnace at the joint between the glass cup and the furnace tube. We have rechecked the connections between different tubes and ensured that there was no gas leak. Then we did many experiments in different conditions but all of the heating olivine became black and opaque after heating 10 minutes at 1266oC. So that we can not find and polish melt inclusions under a binocular microscope. We use different thermocouple to measure inside of the furnace tube, no problem was found. The used gases are highly pure for Ar, H2, CO2.
The table below shows the detailed experiment conditions. Such experiments have done many times in different days. Experiments 1, 2 and 8 were repeated 3 times and experiments 3-7 were conducted for one time. I can not find the reason why the olivines are black after heating in all of the experiments, so I hope someone here can provide me some useful advice. Thank you very much.
Le Zhang:
My experience tells me that you are probably burning too rich and generating C and CO products which are being added to your final slurry. Either add more O2 or reduce the fuel flow rate. Another issue may be how well your furnace is mixing the O2 with the fuel. You may have a poor local mixing region in your furnace that burns too rich and causes unburned carbon particulates to settle out into the slurry. Poor mixing regions burn richer and colder and if you had several TCs located within the furnace volume, it will show you the “cold” spots. Fixing these spots requires that you clean up your fuel/O2 supply nozzles, making sure good mixing occurs throughout the entire furnace volume. Furnaces are fluidically designed to generate a strong flame recirculation zone near their center. Somehow your spray/mixing zone may have been disturbed and is now off center.
Le Zhang:
Argon is an inert gas, it can't burn in O2, it does nothing to the chemistry and can't cause you a problem. It is only used to displace and avoid other, active gases (H2, CO2, CO) from reacting with O2 and depositing excessive oxidations into your slurry to cause you problems. In fact using less Ar is like adding more CO2 fuel to create richer fuel zones of unwanted pure Carbon. Carbon is your only material that is black in color of all your gas constituants mentioned, and is your problem.
Hydrogen plus heat plus O2 creates water (H2O) and/or hydroxyl radicals (OH) which are all non black in color. Your gas feeds of Hydrogen and CO2 plus heat creates hydrocarbon radicals and other fuel formations that can further react and burn with O2 to create CO and pure C which are dark in color and will cause your problem.
Hi Vogel, if Ar is at 1L/min, the furnace tube (made of Al2O3, diameter is about 4 cm)should be full of Ar and without air. The problem may be that the air was not pushed out the tube completely. In our previous funace, everything is ok. The only difference of the new funace is that its tune has a larger diameter.
Sorry, but without actually being there to check out the hardware and make observations as the furnace is running, I can only give you general advice as above. Get an experienced operator and have him check furnace while running.
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