The design of bioreactors

A survey of the literature on biohydrometallurgical topics revealed that the papers devoted to bioreactors amount to less than 5% of the total and refer mainly to the analysis of the performance and to design guidelines of stirred tank and of bubble column machines, the so-called Pachuca tanks. These machines can be defined conventional in the sense that they have been borrowed from chemical engineering and hydrometallurgy and adapted to the requirements of biohydrometallurgical processes. However, past experience has shown that these types of reactors do not fully match the very particular conditions that exist in biohydrometallurgical systems that, quite correctly, have been qualified as “hybrid” owing to their specificity since they are characterized by many of the features of hydrometallurgical operations and of biological conversions. Literature data and the author's personal experience demonstrate that one of the present major drawbacks of these reactors is the power requirement that seriously affects the competitiveness of biohydrometallurgy with pyrometallurgy. The factors affecting the performance of biohydrometallurgical reactors are discussed with special reference to the process parameters and an analysis of the conditions to be satisfied by an ideal bioreactor is carried out. In the light of these considerations, the reactors currently operating in commercial plants are examined. The new prospects opened up by recent developments are finally discussed and, also on the grounds of experience recently gained on a laboratory scale, the potentials of machines tailor-designed for the conditions reigning in biohydrometallurgical systems are outlined.