Overburden is material that must be removed in order to mine the ore beneath it for extraction and processing to produce metals for commercial purposes. PTFI manages overburden according to a comprehensive Overburden Management Plan that has been approved by the Government of Indonesia.
Many metals occur in nature in the form of sulfide minerals. When the ore is mined and overburden containing sulfides is exposed to natural elements, sulfuric acid is potentially generated as a result of the natural actions of water, oxygen, and bacteria. Acid drainage may dissolve metals contained in overburden and carry them into the water drainage system, which if improperly managed could negatively impact the environment. This process is called acid mine drainage formation.
PTFI manages and monitors acid mine drainage resulting from its operation. Independent audits performed on PTFI’s environmental management system have concluded that PTFI’s overburden management programs are “well integrated” and “conform to international practices”. In accordance with the overburden management plan approved by the government, PTFI places overburden in managed areas near the Grasberg open pit. The PTFI plan to mitigate acid mine drainage encompasses the containment and treatment of existing acid mine drainage through limestone blending and limestone capping of overburden placement areas as part of future acid mine drainage formation management.