Treatment of acid mine drainage through a ferrite formation process in central Hokkaido, Japan: Evaluation of dissolved silica and aluminium interference in ferrite formation

A two-step neutralization process, leading to the formation of ferrites has been developed for the treatment of acid mine drainage (AMD). In the first neutralization step magnesium oxide was used to raise the pH to around 4.5 to eliminate aluminium (Al) and to reduce the silica (Si) concentration. In the second neutralization step sodium hydroxide was used to complete the neutralization and to precipitate ferrous and ferric hydroxides, from which ferrite could be formed. The AMD of a closed gold mine in central Hokkaido, Japan was treated in a continuous-flow bench-scale plant of the process. During the first neutralization step the removal of Si was more difficult than the removal of Al; the presence of both Al and Si interfered in the ferritization of the sludge. When the Al removal decreased from 94.5% to 64.3% the magnetism per unit mass of the sludge was reduced by 36.7% and when the Al removal decreased to 21.8% the magnetism of the sludge was reduced by 51.5%. The interference of coexisting Si was even stronger: the sludge showed a 26% and 36.7% decrease in magnetism when the removal of Si was reduced by 15% and 18%, respectively.