Searching for Acidity or the Case of the Missing Chlorine: An Option for a Global Closed Loop Alkalinity–Acidity Cycle for Bauxite Residue Neutralization Based on HCl from PVC Recycling

The Bayer process depends on the large-scale use of caustic soda (NaOH)—produced at > 60 million tons/year from NaCl by the chlor-alkali process—and is thus one of the major consumers of the alkalinity generated by the latter. A part of this alkalinity then ultimately ends up in the bauxite residue from the Bayer process, arguably constituting one of the main chemical, technical, and environmental challenges for a valorization or long-term safe disposal and remediation of this material. By stoichiometric and chemical necessity, the complementary acidity resides in the Cl2 gas is also produced in the chlor-alkali process and is thus latent in the chlorinated compounds—notably polyvinyl chloride (PVC) and chlorinated solvents, such as dichloromethane or 1,1,1,-trichloroethane. Recapturing and recycling Cl2 from these uses in the form of hydrochloric acid (HCl) by means of a controlled thermal decomposition of the chlorinated hydrocarbons could—in principle—serve as a source of Brønsted acidity for the neutralization of bauxite residue (Red Mud) thereby transforming it into a nonhazardous material of much lower environmental concern limited to its NaCl content. This could establish a closed alkalinity–acidity cycle on a global scale while simultaneously addressing the end-of-life fate of environmentally persistent PVC that otherwise either is deposited in landfills or can end up in the oceans in form of dispersed microplastics.