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Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
Some time ago I was producing calcium sulfide from scrap drywall (CaSO4 hydrates) and charcoal (for a process I was going to do but no longer have a need for). The process involved baking the drywall scrap to plaster of paris, powdering and mixing the materials with excess carbon, and forming it into a solid pellet. The pellet was then fully dehydrated and fired in my propane fired casting furnace for a couple hours at 1000-1200°C. I determined that after the process I had a CaS yield of something around 40-50% (measured by weight lost of H2S upon acidification of tiny samples), with the rest being primarily CaO. When the pellet was firing, there was quite a bit of SO2 release, and I attributed this to the reaction 3CaSO4 + CaS --> 4CaO + 4SO2, due to both carbon burnoff and the ability of SO2 to leave the porous pellet. Now however I am thinking that CO2 and H2O from the furnace flame may have simply reacted with some of the CaS, forming H2S that immediately burned to SO2.
Since the reaction mixture is porous and a poor conductor of heat anyway, a better way to carry out this process may be with a spherical pellet covered in insulation, and heated in a microwave oven. I have heard of people reaching very high temperatures in a microwave oven, even melting iron, by using a graphite absorber/crucible with a ceramic insert. I know that graphite is a good absorber of microwave radiation, and the pellet would contain about 35% of it by weight for calcium sulfide production. What I am now more interested in is lithium sulfide production as an improved precursor to lithium metal production in my calcothermic reduction cell. In this case the carbon would be mixed into the pellet at 44%. I would not use excess carbon in these, but rely on the reaction's ability to proceed either as CaSO4 + 4C --> CaS + 4CO or CaSO4 + 2C --> CaS + 2CO2 to ensure that there is enough carbon for reduction, but no solid carbon left after the reaction. Activated carbon would probably be the most suitable form since it is most reactive.
I suspect that the microwave heating will allow deeper penetration much faster than external heating would. I am not sure however how well the microwaves would permeate into the carbon bearing mix. Plates may be more efficient in this regard.
By the way for anyone interested, this is not a good reaction to carry out in your kitchen microwave unless you are ok with your house being filled with carbon monoxide and possibly SO2. Also the soluble metal sulfides are potent poisons, as well as H2S itself which is rapidly released when metal sulfides contact acids.
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Microwave Heating (for the production of metal sulfides and other coke reductions)
