I have seen the processes of welding rails together. My dad worked for the rail roads and did this allot. (He was a Welder and Black Smith by Trade.) Never any accidents from what I remember. My dad showed me how to make it if the the rail roads company didn't buy it. Iron oxide and aluminum. Basically rust and a 350 mesh aluminum powder. This was ignited with magnesium and I never seen it ignited any other way. I then joined the ARMY and went with 13M M.L.R.S. then went in to 21B Combat Engineer I was trained in 89D E.O.D but never could get the physic evaluation completed for 89D so stayed 21B while I was O.J.T. with 89D. We made thermite but for cutting cables, flange beams, rebar, metal plat, inch or larger bolts, and any thing metal. My understanding about thermite is that it is a pyrotechnic composition of an aluminum powder and an Iron oxide, which produces an aluminothermic reaction known as a thermite reaction. Stuff that can fuel thermite are magnesium, calcium, titanium, zinc, silicon, boron and water is nasty too. Most varieties are not explosive, but can create short bursts of extremely high temperatures focused on a very small area for a short period of time. Now add the particulates of flour, wood dust, house hold dust, fibers of cloth, resin, or any float-able burnable material, you know have an explosion.
In the shops while I was in the military for the belt and wheeled grinder stations we had PVC pips grounded and T spots with magnets installed in caps to remove Iron fillings before it got to the bath we had. The bath was some kind of nonflammable liquid that would cool and put out hot amber's of steel. The Vac system I could not explain since it hooked up to about 12 grinders that led to a cage and set about 50 ft away and we where never to mess with it. ??????
We did grind metals on those grinders of all sorts from mild steel, aluminum, brass, high carbon steel, copper and pot metals but never anything other than metal. Four years worth of that and never any accidents?
I not saying that this can not happen but best to be careful and stupid. I do like the fact that someone brings things up like this it is good to know too and good to keep safe.
Thanks
C.E.Sampel