Experiment 50: Silicon Thermite

A thermite is always a special reaction to perform for a 50th experiment!  I enjoy isolating elements at home for my periodic table collection, so I used silicon dioxide (common name: sand) and homemade ball-milled aluminum powder to make small beads of pure silicon.

Iron thermite uses aluminum powder and iron oxide, but silicon thermite needs an additional ingredient besides silicon dioxide to be successful.  Adding sulfur creates more heat in a side reaction with aluminum, thus helping the reaction keep going.  I used a 12:10:9 mass ratio of S:SiO2:Al.  All the materials were finely powdered, and the silicon dioxide came as 400 mesh chromatography media from some company online.  They had a free samples program, so I readily agreed to get free chemicals!

After mixing the ingredients, I placed them in a flowerpot and lit them with a magnesium ribbon fuse.  The reaction was extremely bright and wonderful, but I'll let the video speak for itself:
As seen in the video, the reaction also makes aluminum sulfide, which hydrolyzes in water to make toxic, smelly hydrogen sulfide gas.  This is the active ingredient in many stink bombs, so it really stinks!  Plan on taking a shower and washing your clothes before social interactions if you repeat this experiment.

When the reaction had cooled, I put the slag pieces in water and hydrolyzed off all the aluminum sulfide.  I then sifted the silicon beads out from the resulting aluminum oxide powder.  I may have lost some, but I still got a decent number of small beads of silicon.  They weren't very shiny, so I soaked them in dilute hydrochloric acid for an hour or so until I could see the pretty crystals inside.  They turned out quite nicely, and I was happy to isolate another element in my backyard!