Q1: What was mercury used for in the Minamata factory of Chisso Corp?

A1: It was used as a catalyst for the acetaldehyde production.

Commentary:
Mercury was used as a catalyst in the production of acetaldehyde.
The presence of mercury allowed effective generation of acetaldehyde in the manufacturing process.
Although a catalyst itself generally does not change during the chemical reaction, a reaction other than the targeted one (i.e., a side reaction) is assumed to have occurred in the Minamata factory, and produced methylmercury that flowed out into the sea mixed with the wastewater of the factory.