Abstract: Disclosed is a solid/liquid reaction process comprising the steps of: a) forming a slurry of a solid and a solution containing a species (SA) reactive with the solid and convertible to a species in the gas phase (SG); b) conducting a reaction between the solid and the species SA in a slurry in a vessel; and c) controlling a process condition of the slurry to minimise reactive conversion of species SA to species SG. Reactive conversion of species SA to species SG in the vessel is minimised by the placement of laminar slurry cover buoyant with respect to the slurry in the vessel. A process of particular importance is the process of cyanidation of gold ores in which reactive conversion of aqueous cyanide (CNaq?) to HCN (g) is a particular problem. A vessel (1, 2, 3) and leach circuit for conducting the solid/liquid process is also disclosed.