Publication:

Performance of a fixed bed reactor for mercury removal by biosortion using agricultural residue

Resumen en español

This research was aimed to evaluate the performance of a technology for mercury removal. This study was performed in a mining area located in Cauca, Colombia. The purpose of this project was to assess a low cost alternative for removal of mercury in the wastewater of a mining production unit (MPU) which produces gold. Activated carbons generated with coconut shells and coffee husks were evaluated as a treatment alternative for the removal of mercury released to the water after the amalgamation process. Thus, an evaluation of the MPU in terms of the wastewater characterization and the performance and conditions of the main processes used for gold production was carried out. Mercury losses were estimated throughout the units prior to the discharge point as an important factor contributing to the contamination pathways in the MPU. Coconut shells and coffee husk were ground and activated with a physical mechanism using a muffle furnace, nitrogen gas as the reducing atmosphere and steam as the activating agent. The activated biosorbents were characterized and compared based on their surface area, mesopore and micropore distribution, density and adsorption capacity by performing batch tests. Desorption test was applied for the biosorbent that showed the highest performance in the adsorption batch tests. A laboratory scale fixed bed reactor was evaluated for 10 days, in a semi continuous regimen, with the same conditions as that of the real MPU. Mercury, suspended solids and COD concentrations were found to exceed the Colombian national permissible limits of wastewater discharge with a maximum mercury, suspended solids and COD concentrations of 7.71, 18496, and 303 mg 1-1 O2, respectively. A primary sedimentation tank was designed as a starting point of the treatment train. The sedimentation process was used to remove the majority of solids before the use of the biosorbents for the mercury removal. Coconut shell and coffee husk were activated with maximum yields of 30.0 % and 23.7 %, respectively, obtained from the amount of precursor material introduced to the muffle furnace and the amount of activated carbon generated. Coconut shells activated at a carbonization temperature of 300 °C and an activation temperature of 600 °C showed the highest adsorption capacity and surface area with results of 0.0036 mg g-1 and 539.59 m² g-1, respectively. The fixed bed reactor did not achieve its breakthrough point or capacity after 10 days of operation and showed good mercury removal. The effluent quality complies with the national permissible limit of 0.002 mg 1-1 of mercury. Coconut shell activated carbon proved to offer a better alternative for the removal of mercury in effluents from the gold mining industry.