Traditionally, the design and evaluation of flotation equipment is substantially based on the mineral recovery reported to occur within the pulp and froth zones. This methodology fails to evaluate the flotation that occurs within the high intensity zone of a flotation machine. This thesis addresses the issue of where flotation occurs. By investigation of recovery within the high intensity zone of a Jameson Cell it provides additional knowledge to further the design and optimisation of flotation machines. The investigation is focussed upon the recovery within the Jameson Cell, specifically its high intensity zone - the downcomer. Within the Jameson Cell, the downcomer is largely separated from the main tank. This allows ready access to conduct measurements and calculated recovery within the high intensity zone, independent of the pulp zone. The primary aim of this thesis is to identify where flotation occurs within a Jameson Cell and identify what factors affect this flotation. This knowledge allows evaluation of the existing Jameson Cell design, to optimise throughput and recovery. It also allows discussion on the implications for the design of flotation machines in general. The operation of the Jameson Cell is evaluated from both hydrodynamic and flotation recovery perspectives, in five different experimental arrangements. The hydrodynamic evaluation of the downcomer in an air-water system was conducted in two test units, specifically built for the purpose. Existing methods used to measure air void fraction and bubble size in pulp zones were modified so that they were suitable for downcomer measurements. Evaluation of the effect of operating parameters and solids on downcomer hydrodynamics was conducted in production Jameson Cells, within the Mount Isa Mines Copper Concentrator. This required the development of procedures to measure the contact volume of the downcomer in an industrial environment. Lastly, flotation studies were conducted on two different coal types. Experimentation was undertaken with a standard Jameson Cell laboratory unit, as well as a purpose built, production scale test unit. Extensive modification was required to the laboratory Jameson Cell to fully evaluate the effect of parameters on flotation. Two methods of separating the recovery within the high intensity zone of the downcomer from overall recovery were developed. In addition, a methodology to equate a standard Jameson Cell laboratory test to the mode of operation in industrial Jameson Cells was generated. The thesis was developed along two lines of investigation - fundamental hydrodynamic studies within an air-water system, and evaluation of downcomer flotation in an air-water-solids system. The study of downcomer fundamentals was based upon the work of earlier researchers such as Jameson et al (1988), Evans (1990) and Atkinson (1994). The models produced by these researchers were tested and either validated, or extended, where experimental results indicated that this was necessary. It was noted that prior research largely centred on average downcomer performance and an extensive study was made on the measurement and mapping of bubble size, air void fraction and fluid velocity, within the downcomer. This allowed calculation of the corresponding bubble surface area flux. To further the knowledge of fundamental hydrodynamics within the downcomer, models generated within an air-water operating system were tested in the presence of solids. A number of authors have proposed that the high intensity zone of a flotation machine was the principal region of recovery (Davis, 1964a; Davis, 1964b; Cusack, 1968; Schubert and Heinrich,1998) and Schubert,1999). In addition a number of authors have discussed the possible collection mechanisms with the Jameson Cell downcomer (Jameson, 1993; Andreatidis, 1988; Atkinson 1993; Clayton, 1998; Mozaffari, 1998; Power, 2002). By undertaking flotation tests it was determined that recovery primarily occurred within the Jameson Cell downcomer. By linking the flotation study with the hydrodynamic study it is possible to determine what factors affect flotation performance. This thesis emphasises the central role that the high intensity zone of a flotation machine has in terms of flotation recovery. Emphasis is placed upon the small bubble size and high air void fractions that exist within the downcomer and lend themselves to rapid flotation. A geometric model was derived which supports the assertion that the high intensity zone of a flotation machine is a key to flotation recovery. This is further supported by calculation of recoveries within the downcomer.