[chase]

Hi,
 
I am trying to clean up recycled gypsum (from plasterboard/drywall) and agricultural/farmer's gypsum. The recycled gypsum contains lots of fibres and other small contaminates. This gives it a greyish colour and I need it to be clean and white. As for the the farmer's gypsum, it contains silt/sand and seems to be coated in a muddy colour. I have tried sieving and filtration but the contaminates seem to be present at all levels even down to 2 microns. The colour does get better as you go down in size but its still not anywhere near white.
 
I've looked at solid-solid separation techniques but not sure what would be applicable here. Would a classier help? Should I look into micro/ultra filtration? Filter press? Any suggestions?
 
Thank you for your time,
 
Chase.

[curiouscat]

Could you burn the crap out? Is it organics? Would that dehydrate & destroy the Gypsum?

[curiouscat]

Could you try bleaching the organic matter white?

[Mitch]

Can you take some pictures? It might help.

[Arkcon]

It might help if we knew a bit more about your intended application.  Gypsum is the mined mineral form of calcium sulfate dihydate -- http://en.wikipedia.org/wiki/Gypsum  If its intended as a fertilizer, then roasting it to a dry powder, like curiouscat: said and then selling it as calcium sulfate anhydrous should work fine.  But that might ruin it as a building material.  And I don't know how to cheaply clean the  muddy samples.

[curiouscat]

Another option is flotation. Such impurities often have different densities.

Some gypsum too will be lost by solubility.

Is Gypsum stable to any of the strong acids? To oxidising agents? If so that sort of treatment might also work.

[chase]

Firstly, thank you all for your replies. I really appreciate it.

I have not tried burning it out but I will give it a go. It might work for recycled gypsum as it contains fibres but then it would leave ash. Don't think it will do anything to the farmer's gypsum. Also I'm trying to keep it energy efficient - might not be suitable for larger scale.

I have tried acid/base/detergent/bleach washing - no effect.

You can view images here:
https://drive.google.com/file/d/0B1FkNHc417hjSENwMWtrVW5IdjQ/edit?usp=sharing
https://drive.google.com/file/d/0B1FkNHc417hjUlJqSUJTLUY3R0U/edit?usp=sharing
Sorry about the quality but should give you a rough idea. The colour might look white but it isn't. The smallest grade you see is less than 63 microns.

The plan is to convert the Gypsum into Calcium Carbonate so I need it pretty white and clean.

Flotation? I looked into flocculation but that would not help as it won't distinguish between the gypsum and other contamination.

[Borek]

The plan is to convert the Gypsum into Calcium Carbonate so I need it pretty white and clean.

That's a plan that doesn't make sense to be honest. You can buy CaCO₃ much, much cheaper, plus I can't think of a method of conversion that doesn't require lots of energy.

[chase]

"I am known to be occasionally wrong."

I believe it's one of those occasions . I happen to be working on a process to produce PCC that is energy efficient.

Anyway, I converted some of the <63 microns gypsum samples to PCC and then added to it HCl, leaving behind the contaminants. The farmers gypsum contains about 11% of dirt and the recycled is around 1%. I burnt some of the recycled gypsum contamination - it burnt the fibres but there was still sand/silt/other crap left.

[Borek]

I happen to be working on a process to produce PCC that is energy efficient.

From CaSO₄? Can you explain the chemistry behind?

[curiouscat]

I happen to be working on a process to produce PCC that is energy efficient.

From CaSO₄? Can you explain the chemistry behind?

Hmm...I'm only guessing but once you get CaSO4 dissolved in H2O could you then add something like Na2CO3 or HCO3 or maybe bubble CO2 through the water?

Not sure.

[Borek]

I'm only guessing but once you get CaSO4 dissolved in H2O

Good luck dissolving CaSO₄.

[chase]

Reacting it with ammonia and carbon dioxide.

I was looking at ferrofluid and they use it in mining industry for separation. I was thinking maybe I could use that for flotation?

[curiouscat]

I'm only guessing but once you get CaSO4 dissolved in H2O

Good luck dissolving CaSO₄.

Yeah I'm  stupid. 

I misread a textbook solubility of 0.21g/100ml as 21 g/ 100 ml.

You are right.

[Borek]

I misread a textbook solubility of 0.21g/100ml as 21 g/ 100 ml.

CaSO₄ is a textbook example of a weakly soluble salt. It is also a textbok example of a weakly soluble salt that is not that weakly soluble

Reacting it with ammonia and carbon dioxide.

I guess it requires using powdered gypsum (to speed up the process), and it means converting solid to solid directly so there is no stage where the solution can be filtered (to get rid of insoluble impurities).

Interesting, especially taking into account fact how common limestone is. How is the PCC prepared using this way better than mined calcite?