I worked for a couple of years on the CUPROSUL process. This process used copper sulfate to scrub H2S out of various gas streams (the principal application was scrubbing geothermal steam). In the original process concept, the sulfur contain in the H2S ended up as ammonium sulfate. It was originally hoped that the ammonium sulfate could be sold as fertilizer. Unfortunately, ammonium sulfate is not widely used as fertilizer in the developed world…

And even worse, the ammonium sulfate that resulted from using the process to scrub geothermal steam contained contaminants that were unacceptable in a fertilizer. This meant that there was no market for the ammonium sulfate and that one would probably have to pay for its disposal.

So, what to do? I had done a literature survey on the reactions used to regenerate the copper sulfate from the copper sulfide produced in the scrubber. From reading the articles it was evident that scrubbed sulfur was briefly present as elemental sulfur in the stirred tank regen reactors but that it was quickly oxidized to the sulfate given the rather severe temperature and oxygen levels.

It occurred to me that, if we could somehow protect the elemental sulfur from further oxidation, we might be able keep it in its elemental form rather than end up with the sulfate form. So I went looking for good sulfur solvents that were immiscible in water and were poor solvents for oxygen. It turns out there are quite a number (including olive oil) but I decided to run an experiment with a chlorinated hydrocarbon which we had in the lab and that seemed to have the desired solvent characteristics.

I took a quantity of the aqueous copper sulfide slurry produced by the scrubber and I oxidized it in an agitated beaker in the presence of the chlorinated solvent. The slurry eventually disappeared and I stopped the agitation; allowing the two solvents to separate. I then decanted the chlorinated solvent into a pan and left it to evaporate in a fume hood. The next morning the pan was dry and covered in sulfur crystals.

That was very satisfying but, of course, we still had to look at the economics of an elemental sulfur by-product. A first pass analysis showed that elemental sulfur by-product did look more promising than sulfate but that the impact would vary by region. The cost of sulfur varies quite widely around the world. Some areas have vast amounts of mineral sulfur that is cheap to mine. Other areas have widely used processes that produce elemental sulfur as a by-product.

So the process modification was not a complete homerun but it did offer the prospect of changing the by-product produced to suit the local market.