Development Of A Process For Treatment Of A Mine Water Containing Arsenic And Thiocyanate From Lab Scale To Pilot Plant Scale.
Vaxelaire, Stéphane; Battaglia-Brunet, Fabienne; Jacob, Jérôme; Jally, Bastien
A 8 million metric tons storage of cyanidation residue from a former French gold mine produces a 10 m3/h mine drainage that contains approx. 8 mg/L of As and 1 g/L of SCN- and is currently treated within a lime process. The treatment plant produces large amounts of sludge reflecting the large amounts of sulfate and calcium present in water.
The aim of the present study is to develop a semi-passive mine water treatment process that would produces a lower amount of sludge and improve treatment efficiency with target discharge level under 100 µg/L of Arsenic. After a review of processes used for arsenic and thiocyanate removal, the retained process is the adsorption of arsenic on ferrihydrite precipitate combined with the biodegradation of SCN-.
Laboratory experiments were conducted to complement and improve global knowledge:
- First, batch experiments were carried out to assess whether ferrous or ferric iron was more efficient in the process and to determine the optimal iron concentration.
- Then, continuous lab-scale experiments have been conducted. They consisted of mixing ferrous solution with contaminated water at the bottom inlet of a bio-filter column filled with pozzolana. The impact of hydraulic retention time has been evaluated and reduced from 30 h to 14 h, resulting in higher arsenic overall removal rate.
- Finally, a new and simpler process was tested. It consists on a mixing tube followed by a settling tank of rectangular design, fed with the contaminated drainage water. After 14 days of continuous operation, significant amount of arsenic was trapped into the ferrihydrite sludge.
Design of the pilot apparatus for on-site experiments
Lab-scale experimental results lead to modify the process design, using a settling step before the pozzolana filter. An on-site pilot experiment has been designed according to lab-scale results. This pilot plant will treat a 0.1 m3/h flow of contaminated drainage water. The pilot plant will consist of three steps: first an aerated pozzolana filter for bio-degradation of thiocyanate, followed by a settling tank fed with ferrous iron and last, a second pozzolana filter for treatment finishing. Experiments will be carried out in order to complete key data before final upscaling. Key-data to monitor are clogging of biofilters, impact of temperature over performances and rainfall dilution. Monitoring of arsenic speciation through the pilot operation (As III or V) will also be essential in the understanding of the process performance.