So my kid is taking a Chemical Engineering course called Unit Operations and the Professor asked...

[NNadir]

6. Yes, it is. The initial reduction of SiO2 is carbothermic, usually with anthracite coal or coke.

Since it's coal or coke are usually contaminated with other elements, and because under these conditions, in a furnace in excess of 2000C, a certain amount of the wonderful refractory SiC forms, it is necessary to purify the crude silicon.

Silicon is generally resistant to attack by mineral acids with the exception of HF, but at elevated temperatures, it is attacked by gaseous HCl to form trichlorosilane, a liquid which is then distilled to purify it.

Trichlorosilane is explosive, and an explosion in Japan instanteously killed more people that were instantly killed by Fukushima's radiation releases, albeit not nearly as died in the earthquake from drowning, not that anyone is concerned with the safety of living on shorelines and thus proposes to ban coastal cities. Explosion at Mitsubishi polysilicon plant in Japan causes deaths

Generally risk is qualified in terms of DALY's, disability adjusted lost years, which accounts for how fast something kills victims of exposure.

Mid-career, I was involved heavily in the manufacturing side of the pharmaceutical industry, and as such toured chemical plants all around the world and became familiar with the general approach of the chemical industry.

It would seem, as you correctly note, that the output of the decomposition of trichlorosilane, pure silicon - but generally with some remaining impurities which often require zone refining via melting for further purification for use in chips - that HCl (along with chlorine gas) is produced in this reaction, and thus that the HCl could be recycled. However - and I'm speaking here not from direct knowledge, but by assumption - several factors may impact this. As noted, even distilled, the trichlorosilane might still contain minor impurities, hydrogen sulfide for example, other halogen acids such as HF etc, and the recycling might impact the purity, over time of the trichlorosilane. Trapping the gas to drive equilibrium may be into water, in which case it becomes problematic to reconvert the HCl into gas. The chlorine in the waste gas stream may be difficult to separate. Most importantly, the equipment may be fairly expensive - a common steel reactor is not going to be able to handle gaseous HCl at high temperature.

It's worth noting that the recent recalls of the "sartan" drugs apparently took place because Huahai decided to recycle solvents, leading to the accumulation of carcinogenic nitrosoamines in the product.

In the chemical industry, people speak of being "basic" in a commodity. A company can be "basic" in phosgene if they make their own phosgene on site from carbon monoxide and chlorine gas, and use the phosgene to make, say, isocyanates. Another option however is to buy phosgene in cylinders.

I suspect that the Huahai plant included specialized equipment utilized to make tetrazoles, which is not necessarily trivial chemistry, and probably they were "basic" in azides.

Whether or not to be basic in a commodity is described as a "make or buy" decision. My guess is that for the silicon commodity, basic plants for the manufacture of crude silicon, and then trichlorosilane, and then purified silicon plants using zone refining and finally chips or solar cells themselves are often far apart.

I've worked on projects in which oncology drugs were manufactured using over 25 steps. As I was familiar with the process, I had a pretty good sense of where the stuff was made. Intermediates from France, Germany and Japan were all shipped to the United States to make major intermediates which were then all shipped to Ireland to a high potency isolation plant - an expensive plant - to make about 10 kg of the drug, which, owing to its potency, was the world supply.

All this manufacturing was performed because the main other source of the drug was the bark of a rare tree in China, giving China control over the world supply and putting pressure on the tree population.

The reason for doing this was not to save energy and be energy efficiency. It was because it involve multiple specialized plants with specialized capabilities and experience in certain kinds of reactors.

I suspect that this situation is very similar in the semiconductor/solar cell industry.

Chemical plants, and the materials that go into them are often very expensive, and in order to maximize their economic viability, they need to run at high capacity, meaning pretty much that they have to handle all kinds of projects, exhibiting flexibility.

HCl is a cheap commodity, and often, frankly not worth recovering. Often it's cheaper to dump it, or otherwise get rid of it, than to build a plant to recycle it.

While looking into this a little bit, after writing this post, by the way, I came across a 1970's description and listing of deep welling chemical disposal sites in Alabama. If you're interested, drop me an email, and I'll send it to you.