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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: rwooduk on November 25, 2020, 06:27:09 AM

Title: Chemical to remove gypsum (calcium sulphate dihydrate)
Post by: rwooduk on November 25, 2020, 06:27:09 AM

I recently made a post about gypsum. I was able to make a nice solid gypsum layer, but now I am having some trouble removing it (it may have turned to y-anhydrite). So far I've tried, concentrated hydrochloric acid - which just fizzes away for hours but has little removal effect, high pressure water hose - again little removal, and manually with hammer and chisel (which works but takes a long time). Is there a chemical that could remove the gypsum more effectively i.e. dissolve it?

Thanks for any advice / help here.

Title: Re: Chemical to remove gypsum (calcium sulphate dihydrate)
Post by: AWK on November 25, 2020, 07:05:24 AM

How much gypsum would you like to dissolve?
In 1 liter of a solution containing 400 g of ammonium sulfate at 50°C (optimal temperature), you can dissolve 5 grams of gypsum (the solution must be mixed intensively to obtain a solution in a reasonable time). This is 3 times more than in cold water.

Title: Re: Chemical to remove gypsum (calcium sulphate dihydrate)
Post by: rwooduk on November 25, 2020, 08:01:59 AM

Quote from: AWK on November 25, 2020, 07:05:24 AM

How much gypsum would you like to dissolve?
In 1 liter of a solution containing 400 g of ammonium sulfate at 50°C (optimal temperature), you can dissolve 5 grams of gypsum (the solution must be mixed intensively to obtain a solution in a reasonable time). This is 3 times more than in cold water.

Thank you AWK. If there is more than 5g of gypsum (I have around ~50-100g remaining on my surface) could I leave it to soak overnight, or would 5g complete the reaction? Really looking for something that I can leave overnight which will steadily dissolve the layer.

Title: Re: Chemical to remove gypsum (calcium sulphate dihydrate)
Post by: rwooduk on November 25, 2020, 08:46:17 AM

Ok, I'll update this. I decided to put the metal block (with the gypsum layer on) into the oven at 250 °C, to see if I could convert it to β-anhydrite, then see if it could be more easily removed. I then carried it over to, and placed it in the sink, sounded like it was going to fall through the sink  :o, and cooled with water. The gypsum was then much easier to remove by hand. I'm thinking it lost its bond with the metal surface as it heated to β-anhydrite. But not sure.

If there are any ideas for other chemicals to remove please let me know because it was a bit dangerous to do what I did with the heating (and transfer) of the block to sink.