Biolixiviacion

Hydrometallurgy 59 Ž2001. 217–231 www.elsevier.nlrlocaterhydromet

The design of bioreactors
G. Rossi )
Dipartimento Geoingegneria e Tecnologie ambientali, UniÕersita degli Studi, Piazza d’Armi, 19, 09123 Cagliari, Italy ` Received 19 October 1999; accepted 24 March 2000

Abstract A survey of the literature on biohydrometallurgical topics revealed that the papers devoted to bioreactorsamount 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, pastexperience 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 AhybridB 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 experiencedemonstrate 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. q 2001 Elsevier ScienceB.V. All rights reserved.
Keywords: Bioreactor; Biorotor; Stirred tank reactor; Pachuca; Solubilization rate; Design; Mass transfer coefficient

1. Introduction Compared to the great effort devoted worldwide to the biology and physiology of microorganisms and to biosolubilization kinetics in the light of microberminerals interactions—where the influence of reactors has unfortunately beenoverlooked—the

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Tel.: q 39-070-675-5528; fax: q 39-070-675-5523. E-mail address: grossi@vaxcal.unica.it ŽG. Rossi..

published results of investigations on reactors suitable to biohydrometallurgical processes only represent a small minority. In fact, out of the total number of papers published in the volumes of Symposia Proceedings and in the journals over the years, not even 5% have been devotedto bioreactor technology. Most of these papers provide very good design guidelines, and an indication of the excellent cultural and practical background of the authors in chemical engineering. However, they are based on the implicit

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G. Rossi rHydrometallurgy 59 (2001) 217–231

assumption that the microflora is kind of a biological catalyzer, whose overall performance is only moderately dependent on the operating characteristics of the machines where the process is carried out. In my mind this is probably the Achille’s heel of this approach. Hence, the reason to review and develop the subject of biohydrometallurgical reactor design was thedesire to discuss the state of the art and to point to the need for further research aimed at providing our technology with suitable machines where the potential of biohydrometallurgy can be fully exploited.

3. The factors affecting biohydrometallurgical reactor performance Biohydromellurgical processes take place in three-phase systems, consisting of Ži. an aqueous phase, that is a solution of...