My source for calcium sulfate was DAP brand plaster of paris, which is the hemihydrate (CaSO4 * 1/2H2O). The ingredients listed on the back are calcium sulfate, calcium carbonate, and silicon dioxide. Assuming the latter two were only there in small amounts, I set off using this plaster straight out of the box. After heating some of it in my oven for 2 hours at 425 F to dehydrate it, I mixed up a batch of thermite according to blogfast’s specs:
TiO2 – 30g
CaSO4 – 25.5g
Al – 27g
CaF2 – 17.5g
Here’s how it went: http://www.youtube.com/watch?v=KfqKOaZyllQ
That video is actually my second try, which used the extra booster formula (see the video’s description for specifics). As you can see, the burn was very “sputtery.” While it did produce titanium metal, the pieces were small spheres that didn’t coalesce together like they should if the whole thing were molten. The slag was also very sparkly throughout, likely from tiny pieces of Ti that were trapped before they could flow together. You can see in the picture at the end spheres of white titanium embedded throughout the slag. Apparently the impurities, likely added as fire retardants or to improve its setting properties, were more detrimental than I thought. Here’s the Ti I recovered from this batch:
I looked up the MSDS for this particular brand of plaster: http://www.dap.com/docs/msds/00071008_english.pdf
Turns out, the impurities are pretty significant – CaCO3 at 15% – 25%, and SiO2 at 0% - 1%.
So, to remedy this I tried removing some of the additives. The silica isn’t worth the effort needed to remove it, but the CaCO3 is easy enough to destroy with a treatment in acid. As a bonus, by using sulfuric acid I make more of my desired reagent!
CaCO3 + H2SO4 == CaSO4 + H2O + CO2
I took some of the raw plaster, submerged it in a good amount of water (so it wouldn’t harden up on me), and added roughly 9M sulfuric acid slowly until the bubbling stopped. I filtered off the CaSO4, let it air dry, and then dehydrated the powder in a fused silica crucible over my small butane burner for about 45 minutes. Using this in the same thermite composition listed above, the reaction ran much smoother: http://www.youtube.com/watch?v=DSY4biEB88Y
When recovering the metal afterward, the pieces were much larger and I found 4 or 5 individual pieces about ½” in diameter. This is indicative of a much better reaction temperature, but there were still some problems with this batch. There were a number of large voids in the slag, the Ti metal was incredibly hard to separate from the adhering slag, and the metal pieces were much more tarnished.
My hypothesis is that all of these are due to water of crystallization still in the CaSO4, i.e. I didn’t heat it up enough. The water boiling off would cause the voids, and I suspect it would react with the hot Ti metal to tarnish its surface. The tarnish can be easily polished off, of course, but I feel that I can do better.
My next attempt was to make my own CaSO4, so I know it’s pure (or at least, I know for certain what’s in it!) I tried two methods to do this.
Method 1: CaCl2 + H2SO4 == CaSO4 + 2HCl
This uses calcium chloride and sulfuric acid, both of which are fairly OTC for me – CaCl2 is used in Damp Rid brand moisture absorbers, and my sulfuric acid is from Liquid Fire brand drain opener. It also produces hydrochloric acid as a byproduct, so this might be a handy way to get much higher purity acid than hardware store brand muriatic acid (which is highly contaminated with, mostly, iron).
Method 2: MgSO4 + CaCl2 == CaSO4 + MgCl2
I like this method a lot because it uses nothing hazardous at all, and both reactants are very easy to source: magnesium sulfate is Epsom salt and calcium chloride is used in Damp Rid.
I tried both methods, and they both work exceedingly well. As long as you use dilute solutions of everything (mine were < 5M), the precipitation doesn’t happen immediately. That means it is much less likely to form occlusions of impurities and you end up with a beautifully snow white product. I then heated both samples to 480F for two hours to dehydrate – here they are fresh out of the oven:
The Method 2 sulfate is on top, and Method 1 on bottom. The latter was quite a bit more powdery and a little purer white, which is interesting considering it was made from acid that has a dark red color to it.
I was able to use all the sulfate from Method 2 for a single 100g Ti thermite charge. The result can be seen here: http://www.youtube.com/watch?v=6TFty76NyHw
This one was a little slow to start, but burned fairly well once it got going. The slag was especially nice, conforming to the bottom of the flower pot very nicely with only one small void – meaning it was completely molten and (mostly) free of water. The pieces I recovered were about halfway between the first two reactions – larger spheres than the first run with their somewhat shiny cast, but smaller than the lumps of the second.
So that’s where I’m at now. Ideally, I want to produce large, shiny lumps of Ti for my element collection, but I think these will do nicely for now. If anyone else is trying this reaction, I hope my posting my efforts here was helpful!