Recycling Calcium Sulfate, is it Possible?

[megalomania]

A big concern in any lab is economical use of chemicals. Perhaps more so with myself I try to conserve as much chemical waste as possible. While review a number of chemical reactions that I perform, and hope to perform in greater quantities in the future, I observed that I will produce a rather large quantity of calcium sulfate waste. This is primarily due to the usefulness of sulfuric acid in many chemical reactions providing abundant sulfates.
Upon reviewing methods of converting calcium sulfate back into either sulfuric acid or obtaining the sulfur in some way I was distressed to note that it is largely insoluble and inert. Unless I want to add some to my garden, or plaster my walls there isn’t much I know to do with it.
Perhaps electrolysis to lime water and sulfuric acid could be possible, but would this be economical in any way? The solubility of calcium sulfate is, according to my calculations, about 2.5 g per gallon of water. This assumes I even have the specific hydrate that can achieve even this high solubility (compared to the other calcium sulfate solubilities). I wonder if a 55 gallon drum filled with 50 gallons of water and excess calcium sulfate could maintain a saturated solution of a whopping 125 g of the stuff. Would this be far to wasteful of electricity and time?
I wonder if molten electrolysis is out of the question. I found a melting point of 1450 degrees C and a boiling point of 1193 degrees C. Yes, I know the boiling point is lower than the melting point, strange. I did find that it could be decomposed at elevated temperatures, but not what that temperature may be. The solubility can be increased in nitric or hydrochloric acid which is a plus I suppose. On the other hand it is practically insoluble in sulfuric acid, which would be bad in aqueous electrolysis.
I have heard it can be a problem at landfills when calcium sulfate is subjected to a moist, anaerobic, low pH environment producing hydrogen sulfide gas. A biological approach maybe? I gather it can be burned to sulfur dioxide gas, so perhaps it can be directly reconverted into sulfuric acid, or at least an easier to deal with sulfate.
If anyone has any suggestions or novel ideas to help me as I begin this research project it will be appreciated.

[nbk2000]

Could it be used as a neutralizer for acid spills? The stuff is pretty useless except on an industrial scale, so why bother with trying to recylce it?

Wait, isn't Calcium Sulphate also known as "Plaster of Paris"? If so, then that'd be useful as molding material for casting things that go boom.

[rikkitikkitavi]

CaSO4 is a LARGE byproduct of the fertilizer industry , where phosphate fertilizers are produced- sofar no one have found a economically way of turning into more useful chemicals. Ergo it is used as plaster. In sweden a large fertilizer factory build a
island in the ocean with it and on that island now 20 windmills stand.
Probably as usefull as it gets.

The high melting point points out that is is very stable. Electrolysis of molten CaSO4 with graphite would probably lead to
formation of CaS and CO(CO2) since CaSO4 can be reduced to the sulfide with carbone at 1200+ K .
At even more extreme temperatures it decomposes into CaO and SO2 like all sulfates.

This could be the reason why the boiling point was stated to be lower than the melting point, since it is physically impossible for a substance to boil before it melts at the same pressure.It simply starts to decompose at the lower temperature and the decomposition of course accelerates at higher temperatures.

CaS can be used to genereate H2S of course .
Sulfides are strong bases (S-- + H2O = OH- + HS-)

Electrolysis in a aqeous solution would only yield
H2 and O2, and due to the low solubility conductivity is very low, hence demanding high voltage to get the current flowing.

I would just dump it. It is fortunately not harmful. It is a powerful drying agent however and can be used as one. In the CRC handbook there is a table of drying agents affinity for water, and I think that only P2O5 and H2SO4 had higher absorbtivity.

/rickard

<small>[ August 11, 2002, 12:43 PM: Message edited by: rikkitikkitavi ]</small>

[vulture]

Well, perchloric acid is a stronger acid than H2SO4 and approx. has the same volatility so conversion to Ca(ClO4)2 could be possible. I have however no idea how soluble that would be.

But I think using perchloric acid for this would be even more a waste of precious chemicals.

[Pu239 Stuchtiger]

It could be used as an exceptionally shitty dessicant.

It could be used to build a giant new lab separate from your house.

CaSO4 will dissolve in HCl solution for this reason:

CaSO4 + HCl ----&gt; CaClHSO4

CaSO4 dissolves in many acids due to very similar reactions.

[A-BOMB]

Or you could make some of that thermite that NBK was talking about a while ago. Or you could do some powdered fuel FAE studies, or you could just drive down rt 65 spraying it out to make a quasi-smoke screen causeing 15 car pile-up up and down rt 65.

[vulture]

CaSO4 won't dissolve in many acids simply because H2SO4 is a strong acid with a high boiling point.
Only acids which are stronger and have the same volatility or acids which have the same strenght but a much higher boiling point can do that. HCl is stronger but very volatile and is not easily obtainable in full strenght which would be required.

[megalomania]

I have made a modicum of progress on this front recently. Apparently you can react ammonium carbonate with calcium sulfate to swap the ol ions and get ammonium sulfate and calcium carbonate. A quick check of the chemistry leaves something to be desired though, I would suspect a bisulfate in there somewhere. Now I just have to figure out how to make and utilize ammonium carbonate. Since the Merck was kind enough to inform me ammonium carbonate doesn’t really exist, its actually a mix of ammonium bicarbonate and ammonium carbamate.
I have reason to believe from what I read that you can mix gypsum and ammonium (bi)carb together, dry, and heat strongly (red heat) to get this conversion. Otherwise the two are mixed in solution, a not altogether desirable proposition considering the low solubility of gypsum.
If successful the ammonium sulfate could be readily decomposed by heating or electrolytic methods. The ammonia could be recovered and processed back into a bicarbonate with CO2 (I still have to work on this part). That just leaves a bunch of calcium carbonate, which I could certainly use (I would probably turn it back into lime to neutralize the sulfuric acid that makes all the gypsum in the first place). This seems like a neat and tidy recycling project.

Of course my references are dated, I may be totally off base, and who knows if I have misinterpreted an ancient chemical name… I present this idea for peer review now while I figure out how to make ammonium carbonate for experiments in the future.

[rikkitikkitavi]

I believe I have read somewhere that it was a method in industrial scale where CaSO4 , NH3 and CO2 was reacted into CaO, [or Ca(OH)2],(NH4)2SO4 and CO2.

First
NH3 + CO2 + H2O =&gt; (NH4)2CO3
like megalomania wrote, it is not really NH4CO3 but it acts like it in solution:)

(NH4)2CO3 + CaSO4 =&gt; CaCO3(s) + (NH4)2SO4
the much lower solubility of CaCO3 than CaSO4 leads to the reaction progressing.

They simply had a slurry of CaSO4 in a pressurized wessel and lead CO2+NH3 into it (pressure increased the rate of gases reacting)
and letting it stand for a while.

Afterwards the CaCO3 was filterd off, the soltion evaporated and (NH4)2SO4 recovered. CaCO3 was converted into CaO by heating.

The energy for this came from coal , thus producing CO2 ,or recovering the CO2 from the CaCO3 decompostion.

/rickard

[Marvin]

Ive been thinking about this for some time. I think unless you plan to build a small chemical plant possibly needing high pressure ammonia and carbon dioxide it wont be much use. You could try it in small amounts using a sodastream type carbonator and aq ammonia. Solvay (Which produces Sodium carbonate and Calcium chloride from calcium carbonate and NaCl) only needs 880 amonia and CO2 bubbled in at neer atmospheric pressure to make the reaction work (These are intermediates). I suspect a sodastream, or similar home carbonator could be modified to produce quite high pressures of carbon dioxide if required.

Ultimatly recycling requires wasting one resource or other but if we can reduce it to a cycle requiring only electricity I think we can say we have succeeded. CaSO4 is reduced by carbon at high temp to give CaS, which unfortunatly has an ungodly melting point. If you had HCl you could produce H2S and CaCl2 both of which are useful. Turning CaCl2 into HCl and oxide or bleaching powerder, or using the chlorine in another way is promising, but H2S is, lets face it, rather poisonous, and liberated from HCl and CaS might be difficult to put to constuctive use.

CaCl2 cant be electrolysed directly, the sparingly soluable hydroxide clogs up the electrodes forcing the voltage up until its impractical. You can mix with sodium carbonate, and electrolyse the salt, and roast the calcium carbonate to CaO. If you can get mercury, which has long term toxicity problems to worry about, you can make something like a whiting cell, or a castner chlorine cell to decompose NaCl completely into sodium hydroxide and chlorine. The cell can be aranged similar to the castner chlorine cell such that the mercury is always covered by liquid and a rocking motion distributes the amalgam between the compartments, though I would advise against an exact copy of this cell which might alow mercury ions into the hydroxide product. With a purely passive reaction between the amalgam and water the product should be virtually free of mercury and assuming this cell is outside and doesnt have to be dissasembled on a regular basis mercury toxicity should be not a problem. Chlorine, sodium hydroxide and hydrogen have vast numbers of uses. You can easily make concentrated bleach, HCl directly and from those 5 chemicals the possabilities are vast. Salt is very very cheap. I'm ignoring how unpleasent/potentially fatal chlorine gas is but if you cant build equipment to use this safely, even if its only making bleach and venting the remainder, then you shouldnt be doing chemistry at all. Work outside and dont botch the job, build it to last or dont built it at all. If you need to dissasemble the cell, drain the water without losing any mercury, get as much of the mercury as possible into an air tight bottle (much easier said than done), and douse everywhere the mercury has touched with sulphur dust. Sweep up and discard apropriatly. This paragraph is much less a how to, and much more an idea in motion, needs to be expanded. Suggestions needed.

How about this idea for CaSO4 though. Reduce to CaS, add FeCl3, which is available everywhere for etching PCBs, obtain CaCl2 and iron sulphide Fe2S3 as a ppt. This turns into Ferrous sulphide and Iron disulphide (Iron pyrites) on heating at 60C, it can be dried and burned to produce the oxides and SO2 which can be used to make sulphuric acid. Iron oxides can be reduced by C in a very high temp furnace to make iron, this is certainly possible in a garden furnace specifically designed for iron and can be made out of clay. Recycling the CaSO4 requires vast amounts of power and carbon, and a lot of semi industrial process, but it can be done.