Aluminum hydroxide coatings in limestone drains
This paper describes a mixed flow reactor experiment and associated data analysis scheme that are well suited for studying the chemical and physical processes that occur in limestone drains used to treat acid mine drainage (AMD). The experiment simulates the slowly evolving, near steady state, reactions that form coatings on limestone. The resulting coatings can be recovered for analysis of their structure and composition. Analysis of the time evolution of the composition of the effluent solutions is used to isolate and understand key factors that affect limestone drain performance. The experiment investigated reactions between acidic aluminum sulfate solutions and calcite. The aluminum sulfate feed solutions contained 0.002-0.01 molal (32-329 mg/kg) Al and had pH values ranging from 3.7 to 4.2. At the beginning each experiment, the rate of H+ consumption by reaction with the calcite was fast causing a distinct increase of the effluent pH. The pH increase caused some of the dissolved Al to precipitate as a coating on the calcite surfaces. The coating blocked the transfer of ions to and from the calcite causing the reaction rates to be limited by ion diffusion through the coating. The continued growth of the coating caused it to become an increasingly effective barrier to ion transport, which caused the neutralization rate to slow and the effluent solution pH to decline toward that of the feed solution. Powder X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) suggested that the coatings were mostly poorly crystalline gibbsite. Effluent solutions were analyzed to determine pH along with Al, Ca and S concentrations. The coating thickness at each sample time was estimated from the amount of Al lost from the solution since the beginning of the experiment. This thickness and the Ca and H+ fluxes were used to find the apparent H+ diffusion coefficient in the coatings.