What does the mill do with the HOKX Acid or Caustic Soda car?That would be caustic soda (sodium hydroxide). It's the main ingredient in "white liquor", which is used to dissolve the lignin (essentially, the glue that holds the wood fibers together) in the pulping process.
PS Useless trivia: The HOKX mark denotes Oxy Chem, or Occidental Chemical, which seems pretty random. But "HOK" was Hooker Chemical's reporting mark; Oxy was formed ages ago when Occidental Petroleum purchased Hooker.