Simultaneous Sulfate Reduction and Metal Precipitation in an Inverse Fluidized Bed Reactor

Abstract

The exploitation of minerals has contributed to the economic growth of many regions all over the world. In 1999, for example, Mexico produced 182202 tons of lead (Pb), 1485 tons of cadmium (Cd), 447948 tons of Zn and 402430 of copper (Cu) (www.inegi.gob.mx). Mining activities result in pollution problems due to the emissions of acid mine drainage (AMD) into the environment. AMD is one of the most widespread causes of pollution in the world (Gazea et al. 1996;. The principles governing the generation of AMD are relatively well understood.Upon exposure to oxygen and water and in the presence of oxidizing bacteria, pyrite and other sulfide minerals are oxidized to produce a leachate containing dissolved metals, sulfate and acidity . Agreements and legislationIn developed countries, like, Europe, USA and Australia, the source and amount of AMD generated as well as the damage caused to the environment are well described (GAO 1996; Harries 1997). In most developing countries these issues have not been well documented. Information on mining impact in these regions is limited and scattered widely in private and official files. The basic sources are reports produced by large mining firms, environmental organizations and government institutions which deal with environmental protection. As a consequence of this lack of information, the responsibility for the treatment of the wastes generated by the mining activities is also not clear. Small-scale and even some large-scale mining industries in developing countries seldom have their effluents properly treated before discharge into water bodies.More stringent legislation promoted by international agreements resulted in an increased interest in cost-effective technologies for the treatment of AMD. One example of these international agreements is the partnership for acid drainage remediation in Europe (PADRE), which promotes international best practice in the stewardship of waters and soils on European sites subject to the generation and migration of acidic drainage. Another example of these international agreements is the Commission for Environmental Cooperation (CEC) which is an international organization created by Canada, Mexico and the United States under the North American Agreement on Environmental Cooperation (NAAEC). This commission leads SRB can, in general, be divided into two major groups : those that oxidize the carbon source completely to CO 2 (i.e. Desulfobacter, Desulfococcus, Desulfosarcina, Desulfonema and Desulfobacterium) and those that oxidize the carbon source incompletely to acetate, CO 2 and H 2 (i.e. Desulfotomaculum, Desulfovibrio, Desulfomonas, Desulfobulbus and Termodesulfobacterium)Although some SRB are capable of fermentative and sulfidogenic growth on sugars and amino-acids, in anaerobic reactors and in the presence of sulfate, SRB are more likely to be involved in the last stages of mineralization rather than in the initial fermentative stage (Figure 1.1).