(TSE03-O) Bauxite Residue (Red Mud) Neutralization/Treatment Using Fly Ash and Direct Carbonization

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Safe treatment and storage of high volume industrial waste streams pose unique waste management challenges. Land disposal of these materials often has negative environmental impacts such as contamination of soil and groundwater, and consumes vast areas of land. Bauxite residue (Red Mud) from aluminum production, fly ash from coal combustion, and waste water treatment plant (WWTP) biosolids are three examples of high volume waste streams. Approximately 3 million tons of Red Mud are produced in the U.S. each year (30 M/yr globally), and are disposed in land-based impoundment reservoirs. Approximately 63 million and 7 million tons of fly ash and biosolids are generated annually in the U.S., respectively, much of which is disposed of in landfills. Another high volume (Gigatons/yr) waste stream with serious environmental impacts is CO2. Safe disposal of these wastes are a requisite. New methods for sustainable management and ways to find beneficial use for these wastes are highly desirable. Red Mud is usually managed by discharge into engineered or natural impoundment reservoirs, with subsequent dewatering by gravity consolidation and sometimes with capping for closure. Revegetation of dewatered Red Mud is not possible without addition of amendments because of the high pH, high salinity, and absence of nutrients and organic constituents. This proposal investigates the potential of using high volume waste materials, specifically CO2 and acidic fly ash, to neutralize Red Mud for the purpose of soil building and revegetation. The potential of Red Mud to safely sequester CO2 will also be determined. Bench scale batch experiments will be used to evaluate the rate and extent of Red Mud neutralization by contacting with dilute carbon dioxide waste streams, and by addition of acidic fly ash. The properties of the resulting mixture will be evaluated including pH, heavy metal leaching potential, carbonate mineral content, texture, bulk density, and water holding capacity. The additional neutralization capacity and reduction in heavy metal leaching afforded by WWTP biosolids, a common organic soil amendment needed to aid revegetation of surficial neutralized Red Mud, will also be determined. Various Red Mud to ash ratios, CO2 concentrations, and organic amendment concentrations will be used to determine which ratio provides optimal soil properties for revegetation. Lastly, engineering schemes to implement this technology will be evaluated for a possible Phase II field demonstration. Expected benefits and broader impacts. This project will provide new methods for neutralizing Red Mud, a high volume, highly alkaline residue from bauxite mining, with other high volume waste materials (CO2, acidic fly ash, biosolids). Treatment of Red Mud will make possible revegetation and environmental restoration of the large Red Mud disposal areas in the U.S. and elsewhere. The beneficial use of CO2, fly ash, and biosolids for this purpose will lower the environment impact of several industries. The potential exists to use millions of tons of CO2 or fly ash in Red Mud neutralization rather than emission to the atmosphere or disposal in a landfill, respectively. As much as 785% of the annual CO2 emissions from aluminum smelting can potentially be sequestered in the process of neutralizing Red Mud currently in storage. This project will also provide unique training opportunities for graduate and undergraduate students in the area of sustainable environmental management.