There are 3 ways to make lye, NaOH, with a Chloralkali process.
Coca-Cola owns 9 currently operating plants in the USA that use the Mercury Cell Chloralkali method.
Here is an excerpt from a Mother Jones article dating back to 2004.
Here is a more scientific post in regards to the missing mercury.
Here is what the FDA had to say when fronted with the loss of mercury into the food supply and the runoff into local water supplies.
The EPA declared that "the fate of all the mercury consumed at mercury-cell chlor-alkali plants remains somewhat of an enigma."