This experiment performed 8/21/94. Danger Rating: 5 High temperatures, conc. HCl, and a pyrophoric gas warrant this.Background on silicon.
- Silicon
- Silicon is a metalloid element. It is usually associated with PC chips. In industrial processes relatively pure silicon is produced by electric furnace graphite reduction of sand. On smaller scales it is also produced by alumino/magnesothermic reduction.
Procedure
Preliminary experiments to test the feasibility of silicon production by a small scale magnesothermic reduction of silica were succesful. The form of magnesium available to me is Mg turnings. Magnesium is used as the limiting reagent to lessen formation of magnesium silicide. Any silicon converted to magnesium silicide is lost because the acid digestion used to remove the magnesium oxide and magnesium also converts the silicon in magnesium silicide to silane (silicon tetrahydride). Silane is a gas; moreso, it is pyrophoric. A gas that is pyrophoric ignites on contact with air. This rather complicates the digestion procedure, since digestion must be performed in a large pot as opposed to a small flask to prevent damage from the detonations. Silane itself doesn't detonate, rather burning; produced along with silane is hydrogen, though, which detonates violently when the silane ignites it. Thusly, some caution must be excersized in this experiment.Procedure
Ensure that both the magnesium turnings used in the reaction and the silica used are free of water. Burning magnesium reacts explosively with water. Place 3.0g Mg turnings and 3.8g silica powder in a test tube. Be careful to ensure the spread of reactants is even. Initiate the reaction by strongly heating the outer surface of the test tube with a propane torch. Commencement of reaction is quite apparent; the mixture will glow a bright cherry red, cooling to a black. The reaction occurs at about the melting point of glass, so the test tube will be completely deformed and quite useless. Internal stresses will have been set up in the glass and will cause the tube to shatter at the slightest provocation, so handle carefully. NOTE The contents of the test tube now consists of unreacted magnesium and silica, magnesium dioxide, silicon, and magnesium silicide. Magnesium, magnesium dioxide and magnesium silicide are all digestable with HCl. Silica can be removed with KOH solution. Silicon is insoluble in neither solvent. Because silane is pyrophoric, the acid digestion must be performed in a large open vessel. A one liter flask is sufficient, but must be filled to at least the 600cc mark with liquid to prevent too large of a closed space from being exposed to the shock of the detonations. The decomposition of silane produces extremely pure silicon and is used as the first purification step of industrially available silicon for use in integrated ciruits. Ultrapure silicon is produced by zone purification of a silicon melt. As the silane decomposes very pure silicon will deposit on the walls of the beaker as a brown powder. Separate the product mixture from the reaction tube in a mortar by breaking the test tube up and discarding the glass shards. Break up the larger black lumps to a fine powder. In a one liter flask place 500cc water and 100cc 31.45% acid. VERY SLOWLY add the powder to the acid solution. Let react outside overnight. This removes the magnesium, magnesium dioxide and magnesium silicide, leaving silicon and silica. Collect the fine black precipitate on a filter, washing with water. Let dry in the air. Charge a 40cc test tube with 10.0g KOH and 20cc water. Add the dry precipitate to this. After permitting overnight reaction, decant most of the KOH solution away from the black powder. Collect the powder via filtration and wash with water. Yield silicon: 0.5g Percent theoretical yield: 29.4% I would guess the low yield 1/3 of theoretical is due to the heterogenous reaction system (turnings/powder; as opposed to powder/powder).