Energy efficient electrolytic recovery of chlorine from waste HCl.

[NNadir]

Here's a nice paper I came across this afternoon on addressing a very serious industrial problem, what to do with waste HCl, hydrochloric acid: Sustainable Non-Noble Metal Bifunctional Catalyst for Oxygen-Depolarized Cathode and Cl2 Evolution in HCl Electrolysis (Tharamani C. Nagaiah et al, Chem. Mater., 2017, 29 (10), pp 4253–4264).

The authors state the problem very well in the introduction:

"Chlorine is a key building element for manufacturing important industrial chemicals and engineering materials,(1) such as polymers, resins, and elastomers. The requirement of chlorine has risen appreciably in the last few decades, because of its increased demand for the preparation of chlorine-free end materials, such as polyurethanes (PU) and polycarbonates (PC), which are produced using chlorine chemistry as well as chlorinated polymers (e.g., PVC). Generally, ca. 50%(1) of the Cl2 employed commercially ends up forming HCl and chloride salts as a byproduct during the course of their manufacturing, particularly those using phosgene and isocyanates as a carbonylating agent, which are key precursors for the production of PU and PC.

The increased production of excess HCl as a byproduct cannot be utilized effectively only by employing it in the production of PVC or for other small manufacturing industries. Moreover, many small-scale industries in India and other developing countries simply quench the produced HCl with lime. The option of neutralizing the excess HCl is inadmissible for obvious reasons. Therefore, an intelligent way of valorizing the HCl produced as a byproduct is the recycling strategy for its conversion into high-purity Cl2 in order to make the associated processes sustainable. Chlorine production at present is primarily based on HCl electrolysis, which is now a substantially imperative methodology that involves the generation of hydrogen at the cathode (E0 = 0.00 V) and chlorine at anode (E0 = 1.36 V) with an overall reaction potential of ?1.36 V.(2, 3)"

The authors don't note this in their paper, but a very common approach to chlorine production over the years has been to utilize mercury electrodes, which has lead to a very serious mercury contamination problem - commercial laundry bleach can often contain mercury - which along with medical waste and the worst environmental mercury release scheme of all, the use of dangerous fossil fuels as a source of energy, has contributed to serious mercury contamination of human beings.

(I sometimes speculate whether "mad hatter disease" is partially responsible for the willingness of a major North American country allowing itself to be "lead" by an inane and possibly insane corrupt orange monster of extremely low intelligence.)

In any case, they point out that one approach to regenerating chlorine is to oxidize it with oxygen with what is known as "an oxygen-consuming cathode known as oxygen depolarized cathode (ODC)" Here the oxygen gas is reduced to water with the addition of 4 protons and the release of 4 electrons, and at the anode, 4 chloride ions are oxidized to chlorine gas.

Unfortunately most of the ODC's contain expensive noble metals like platinum or rhodium. The authors exploit some very modern materials science to construct a new kind of ODC based on oxidized carbon nanotubes, zinc and tungsten.

Here's a picture of the structure of the catalyst:



The structure is a layered arrangement of polyvinyl imidazolium cationic polymers, oxidized carbon nanotubes and a zinc polyoxo tungstenate.

They claim that besides avoiding expensive metal catalysts (often not available in the third world as they describe) the approach saves about 30% of the energy required to regenerate chlorine from HCl using this system.

The disposal of HCl is a big problem in the first world as well. In many places HCl has been "deep welled" - that is dumped into deep bore holes.

This is hardly acceptable and if scalable, this is a very cool solution to the problem.

This is an obscure, but nonetheless important issue.

Have a nice Sunday evening.

[The Wielding Truth]

1. Is this approach in use now or just on the drawing board?

[NNadir]

2. I doubt it's been commercialized, maybe not even piloted. The commercial manufacture...

...of carbon nanotubes has only recently become mainstream, and specialized single layer coatings with the polyimidolium cation resin is probably not straight forward, although I'm hardly up to date with this sort of processing.

Any commercial activity based on a scientific discovery will originate with a lab scale analysis, but then face a more difficult challenge in scale up and piloting, a process which may not, in fact, succeed. Most projects that die, die here, assuming they get funded. Commercial launch may or may not succeed.

One hopes that all these hurdles will be overcome eventually in this case. This problem is very real and addressing it would go a long way. The authors point out that they have structured their research to find solutions for small operators of small plants who cannot afford anything but questionable solutions, in this case neutralization with lime. The elimination of expensive noble metals is an excellent step in this direction.

I could easily imagine this process being scalable to a very large plant or plants though. It's a non-trivial problem for large plants as well as small plants.

Thanks for your question.