[mchannon]

1 pound of metallurgical grade silicon takes about 1 pound of coal and 1 pound of wood chips.

1 pound of polysilicon takes about 2 pounds of metallurgical grade and even more energy.

1 pound of wafer takes about 2 pounds of polysilicon and even more energy.

Aluminum probably doesn't take as much energy but the difference is Silicon's energy is cheap (coal and wood chips for the first step).

[Joakal]

Isn't the reason that silicon is cheap to process is because there are high-silicon sands at Silicon Valley and other places?

By the way, would a solar furnace be feasible to replace your first step, eliminating CO2 result?

[mchannon]

I'm sure you didn't mean it as such, but that's actually kinda funny; no, Silicon Valley did not get its name from the manufacture of Silicon from sand. Most of the sand that is sourced to make your microchips and solar cells comes from quartz mines and sand pits in Appalachia; Alabama, SE Ohio, and West Virginia are probably the top producers. Silicon Valley imports wafers and has historically had very little to do with how those wafers got made.

In fact, according to the foreman at one of said plants I talked to, they use pieces of quartzite that are more like pebbles than sand; no point in crushing it further I guess.

The function of the carbon is not just to create the heat but also to give the oxygen some way to remove itself. In fact, I believe the oxygen would still rather bond to silicon than carbon but the carbon is able to pull enough physically away as gas to make the reaction work. The resultant gas is mostly CO, carbon monoxide, which somehow becomes CO2 after the plant's done with it.