Outotec open cycle (OOC) is a new low-energy process linking together production of hydrogen and sulfuric acid. While sulfuric acid is the world's most widely produced chemical by mass at approximately 200 Mt/a, the OOC gives the potential for making 4 Mt/a of hydrogen gas as a by-product. H 2 SO 4 manufacture requires a source of sulfur dioxide. 30% of world production of H 2 SO 4 is from the SO 2 by-product of pyrometallurgical processing of sulfur containing concentrates of metals such as copper, nickel and zinc. SO 2 can also be made by direct combustion of sulfur. In OOC, a divided electrochemical cell is used for SO 2 -depolarized electrolysis of water. SO 2 is fed to the anolyte and converted to H 2 SO 4 , while hydrogen gas is produced at the cathode. On the industrial scale, the equipment will be in the form of a membrane electrolyzer assembly or stack. A case is described where the OOC would be connected to a pyrometallurgical plant smelting 1 Mt/a of nickel and copper concentrate, producing 1 Mt/a of H 2 SO 4 and 20 kt/a of hydrogen.

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An empirical method of determination of the activity coefficients and activities was applied to the binary H2O–H2SO4 system. This component-based treatment was shown to give thermodynamically consistent definitions and satisfy the Gibbs–Duhem equation under the entire concentration range, unlike most molality-based and ionic interaction models. The selection of the reference state is important, and any derivations from the experimental data must follow thermodynamic rules. The empirical functions are extendable to the whole concentration range, including areas where water is not a solvent.