Treatment of Acid Mine Drainage with Coal Fly Ash: Exploring the Solution Chemistry and Product Water Quality

Abstract

A treatment process for Acid mine drainage (AMD) using coal fly ash (CFA) was developed. AMD was treated with CFA as the alkaline agent at different CFA: AMD ratios and pH, electrical conductivity (EC) evolution monitored over time. In a separate experiment two AMD sources with differing chemistry were treated with the same CFA to evaluate the impact of AMD chemistry on the treatment process and product water quality. Various CFA: AMD ratios were stirred in a beaker for a pre-set time and the process water chemistry determined. pH was observed to increase in a stepwise manner with buffer zones observed at 4-4.5, 4.5-7 and 6-8. AMD with low concentration of Al 3+ , Fe 2+ , Fe 3+ and Mn 2+ didn't exhibit these buffer zones. Two competing processes were observed to control the evolving pH of process water: dissolution of basic oxides (CaO, MgO) from CFA led to pH increase and hydrolysis of AMD species such as Al 3+ , Fe 2+ , Fe 3+ and Mn 2+ led to pH decrease. These processes initiated mechanisms such as precipitation, adsorption and ion exchange that led to decrease in inorganic contaminants as the treatment progressed. Inorganic contaminants removal was directly related to amount of CFA in reaction media. Precipitation of insoluble hydroxides and Al, Fe-oxyhydroxysulphates contributed to removal of major and minor chemical species. Precipitation of gypsum contributed to removal of sulphate. Na, K and Mg remained largely in solution after initial decrease. Significant leaching of B, Sr, Ba, and Mo from CFA was observed and was directly linked to amount of CFA in the reaction media. This will be a shortcoming of the treatment process since other processes may be required to polish up the product water. The treatment of AMD with CFA was observed to depend on CFA, AMD chemistry, treatment time and might therefore be site specific.

Synthesis of Poly-Alumino-Ferric Sulphate Coagulant from Acid Mine Drainage by Precipitation

“…The general characteristic of the raw AMD is presented in Table 2. The characteristics of AMD are typical of the South African coal AMD solutions [41,42]. As can be seen from the table, this included high concentration of Fe, Al, Ca, Mg and Mn with minor concentrations of Ni, Zn, Cu and Co.…”

Biomass Ashes for Acid Mine Drainage Remediation

“…Mn expects to precipitate mainly as oxide and hydroxide during AMD treatment by biomass ashes. However, the pH value required to remove Mn should exceed 8.4 [39,42,50].…”

“…8). According to Gitari et al [39], AMDs are highly reactive solutions that can dissolve most primary minerals from alkaline solid wastes (e.g., coal fly ash) and secondary phases can be formed subsequently. Dissolution of primary phases (e.g., portlandite) of the S-APCr biomass ash lead to pH increase and triggers hydrolysis, adsorption, precipitation, co-precipitation and ion exchange processes [39,51].…”

“…According to Gitari et al [39], AMDs are highly reactive solutions that can dissolve most primary minerals from alkaline solid wastes (e.g., coal fly ash) and secondary phases can be formed subsequently. Dissolution of primary phases (e.g., portlandite) of the S-APCr biomass ash lead to pH increase and triggers hydrolysis, adsorption, precipitation, co-precipitation and ion exchange processes [39,51]. The removal of Fe, Al and Mn could be mainly attributed to their hydroxide/oxyhydroxide precipitation/co-precipitation that depends on pH (the pH of minimum solubility of the hydroxides/oxyhydroxides of Fe 3+ , Fe 2+ , Al 3+ , Mn 2+ are 3.0, 6.0-8.0, 4.0-4.5, 8.4-9.0, respectively; [39]).…”

“…Also, coal fly ashes quite often contain high concentrations of potential pollutants such as heavy metals [32,33], comparing to biomass ashes, and may also have additional negative environmental impacts when using as neutralizing material for AMD treatment. The use of coal fly ashes from coal power plants for AMD treatment has been widely investigated [34][35][36][37][38][39][40][41]. However, there are few studies about AMD remediation by biomass ashes, particularly, from wooden chip and rice husk combustion [42][43][44].…”