text: The New Jersey Zinc process is no longer used to produce primary zinc in the U.S., in Europe and Japan, but it still is used to treat secondary operations. This process peaked in 1960, when it accounted for 5% of world zinc production. A modified version of this process is still used at Huludao, China, which produced 65,000 metric tons per year.
  This process begins by roasting concentrates that are mixed with coal and briquetted in two stages. The briquettes are then heated in an autogenous coker at  and then charged into the retort. There are three reasons to briquette the calcine: to ensure free downward movement of the charge; to permit heat transfer across a practical size cross-section; to allow adequate porosity for the passage of reduced zinc vapour to the top of the retort. The reduced zinc vapour that is collected at the top of the retort is then condensed to a liquid.
  Overpelt improved upon this design by using only one large condensation chamber, instead of many small ones, as it was originally designed. This allowed for the carbon monoxide to be recirculated into the furnaces for heating the retorts.
  This process was licensed to the Imperial Smelting Corporation (ISC), based in Avonmouth, England, which had a large vertical retort (VR) plant in production for many years. It was used until the mid-1970s when it was superseded by the company's Imperial Smelting Furnace (ISF) plant. The VR plant was demolished in 1975.