For the improvement of safety and efficiency, fault diagnosis becomes increasingly important in mining industry. The expansion of flotation processes with high-tonnage cooper concentrators demands the use of large flotation circuits in which the large amount of instrumentation and interconnected subsystems (with coupled measured and non-measured variables) makes this process complex. Moreover, in a flotation process, any equipment failure can lead to a fault condition, which will affect the operation of this process. This paper proposes an approach for on-line fault diagnosis useful for a large flotation circuit based on a distributed architecture. In this approach, structural analysis is used for the design of the distributed fault diagnosis system. Finally, a procedure for the implementation of local diagnosers for on-line operation is presented and illustrated with an application to a flotation process.