Mülheim an der Ruhr (mülˈhĪm än dĕr rōr) [key] or Mülheim, city (1994 pop. 177,175), North Rhine–Westphalia, W Germany, on the Ruhr River. It is an industrial center of the Ruhr district and a road and rail traffic hub. The city formerly produced mainly coal and steel, but in the mid-20th cent. its products were diversified to include machinery, electrical goods, and chemicals. At the city's noted institute for coal research, the Fischer-Tropsch process for coal liquification and the Ziegler process for the production of polyethylene plastics were discovered. Mülheim was chartered in 1808. There is a 12th-century castle in the city.