|An open pit mine.|
Monday, January 2, 2012
Closing a gold mine
The formal name for closing a gold mine is mine closure that occurs when the ore extracting activities at the gold mine have ceased, and ushers in the final decommissioning of the mine and its reclamation. This usually entails laying off most of the workers that can have a very significant impact on the community around the mine and the local economy. This is the time that the end of the life of a mine when the majority of reclamation work is completed so as to make the land safe and useful for other purposes again.
In many of the countries where mining activity is prevalent much of the closure planning has not yet been formalized leading to a floor of unintended consequences that have the potential of becoming a sovereign risk if not well managed.
Closure planning is a development in mine closure that continues throughout the life of the mine that usually starts before the mine is ever in production and is an integral part of the mines business plan. The closure plan undergoes periodic updates throughout the time of active mining, and incorporates a final decommissioning plan for the mine. The Australian model for my enclosure is also followed in many other countries with this model being the standard.
Most miners follow a progressive reclamation plants throughout the life of the mine that reduces the reclamation burden on the mine when it is finally closed.
There are two types of mines that require closure one of them is the open pit mine it is usually closed by a special type of explosive round is called a quarry closure shot. This is done by drilling a regular shot pattern only the shot holes are progressively shorter to further from the quarry wall that you get. What you are left with is a graded pattern of shot rock that leaves a slope down from the former quarry walls. This slope is often covered with topsoil and planted.
An underground mine is treated differently where in many cases a large concrete plug about 3 feet thick is placed over the shaft effectively limiting access to the mine. Some underground mines are capped using large metal grates that are usually used on deep level shafts that allows airflow to occur through the deep tunnels that helps prevent subsidence.
Older mines typically used wood beams, a foot or more thick, under loose rock or dirt as caps. Eventually the wood rots and sinkholes might develop. These mines are typically recapped with concrete, steel plates, expanding foam, or a combination.
Steel plates are used primarily on horizontal shafts, but are used on vertical shafts occasionally. Doors are added if access is needed. Holes are cut into the plate to allow for drainage as well as for a bat entrance.
Expanding foam is used mainly with vertical shafts where the use of the shaft is no longer needed. It forms a plug 6 feet or more thick. A 4-to-6-inch (100 to 150 mm) pipe through the plug allows for pressure equalization as well as drainage, or even as a bat entrance.
Closure planning has two different components. On-site planning is mostly about environmental rehabilitation and returning the landform to a reasonable condition. Off-mine closure is about loss of livelihood, which is a more complex issue. An element of this is generating an offset against the high cost of environmental remediation issues, which in certain cases can exceed the value of the mineral that was mined originally.