Sciencemadness Discussion Board

Microwave Heating (for the production of metal sulfides and other coke reductions)

kilowatt - 8-3-2009 at 23:46

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.

chief - 9-3-2009 at 03:20

I'm working on similar things, but in the moment have no free-time to try this:
==> As vessel, use just simply and plainly a gypsum-crucible; if it takes part in the reaction, then that's what it should anyhow ...
==> in the center of the several cm thick gypsum-containment place the carbon-absorber with the gypsum-powder and some carbon-powder as well
==> then see what temp. will be reached, should be as high as the insulation permits at the given wattage ...

I read somewhere, that activated charcoal will work very well ; my own experiment with some raw pieces of charcoal showed heating, but far below the expectations, only to maybe 200 or 300 Celsius within 90 seconds ... ; and since I didn't want to blow the microwave then ....

[Edited on 9-3-2009 by chief]

not_important - 9-3-2009 at 06:28

You'll need excess carbon to work as an absorber as the reaction progresses, using a stoichiometric ratio will likely result in the temperature dropping as more of the carbon is consumed. If conditions are hot enough that the sulfide is liquid, or nearly so, it may be conductive enough to sustain heating, but getting the balance right could be difficult.

Chief - unfortunately calcium sulfate is not a good enough thermal insulator to serve for this. All of the successful microwave metal melting applications use an outer jacket of a low density highly insulating ceramic, and often ceramic fiber around that, at least if done with a home microwave. If you've a high power oven you can overcome the thermal losses, but this generally results in excessive heating of the microwave's internal walls.

Graphite can work better than charcoal as an absorber, although charcoal will be more reactive. You may find that using sulfate with some excess of activated carbon as the core of the pellets, with a outer shell high in graphite, gives the best results.

There's some modifications to the microwave that need to be done. Metal casting is usually for rather small amounts, so any rotating tray is removed and the optimal location in the oven determined. For heating of larger amounts of material, leaving the rotating plate in might be better as it will result in more uniform heating. The cooling air from the magnetron is redirected from venting into the oven to exiting directly out of the electronics enclosure, reducing cooling from air flow over the sample.

If you haven't read you might want to.

BTW, the old production of sulfide or carbonate from sulfate and carbon depended on the correct ration of sulfate and carbon for the desired product; this fr both calcium and sodium sulfates.

CaSO4 + C => CaCO3 + SO2

CaSO4 + 4C => CaS + 4CO (excess carbon needed)

[Edited on 9-3-2009 by not_important]

chief - 9-3-2009 at 07:55

Wouldn't the cooling air better be directed outwards ? After all the thing needs cooling, and how much is the highest possible temp the MW can stand in its cooking chamber ? At most somewhat more than 100 [Cels] ; at the desired end-temp of maybe 1300 [Cels] that "heat saving" just translates into a minor contribution ...
Also:Any fumes one would want to direct outwards, or the MV would be on its way to final corrosion soon ...

chief - 9-3-2009 at 09:16

Besides another question, at the occasion: When drying salts: Would the unfreezing-setting (1/3 of the full power) be the safe one, which doesn't burn the MW (when approaching dryness), or could one set it higher ?

[Edited on 9-3-2009 by chief]