PI controller design using a sampled-data fixed structure LQ optimization technique has been presented. The technique has been evaluated on a simulation model of the flotation series at Boliden's concentrator at Aitik in Sweden. It has been shown that when the existing controller architecture prevents utilization of LQ controllers it is still possible to optimize the parameters in the PI controllers using an LQ criterion. Several simulations results are included. An extension to the presented technique where online tuning of the PI controllers is considered in the design, is also discussed.