Open Pit / Cast Mining

Open Pit/Cast or surface mining is a method of extracting ore or mineralization that is found very close to the surface, with a sufficient quantity of ore within a close proximity to make it economically viable to extract, the extraction process is basically done by digging a large hole in the ground and thus removing the ore.  This sort of process even in its simplest form demands a high level of planning in maximising the extraction without removing to much waste rock or overburden that has no economic value to the operation, and as not to remove to much of the high grade ore in the early stages causing the operation to be left with a low grade of ore towards the end of the mine life cycle, the optimum extraction grade is usually planned to remain “at best” relatively constant throughout the extraction process.

 

One of the considerations that engineers face is not just the hardness of the rock being excavated but also the slope or angle of the walls of the pit usually around forty five degrees, in order to avoid any land slides which could be a danger to the mine workers as well as a hazard to heavy earth-moving machinery.  The walls of the mine are dug out in steps called benches, that provides a safe stable structure to the pit walls, and also allows earth moving machinery to have access when expanding the size of the pit.  In order to expand the size of the pit or to excavate in any certain direction, the rock is removed nearest the top of the pit, working sideways before moving down to the next bench thus obtaining a safe angle of the pit walls at all times.
 

Before the rock can be removed, it needs to be broken up into manageable pieces, this is done by drilling strategic holes into the rock, which are then filled with explosives.

The location and depth of the holes are very important as to fracture the rock in a way that allows the shape of the pit to remain constant, prior to any detonation of explosives (pictured right), a heavy steel woven mesh is placed over the detonation zone in order to minimise the danger of flying debris.



 

Around the pit is a declining road or haul road cut into the walls of the pit that allows large earth moving machinery and vehicles access to the bottom of the mine called the pit floor, the benches are arranged as steps, where the horizontal section of the step is called the berm, and the sloping section or wall of the pit is called the batter.




 

Exploration work is usually ongoing during the extraction process, in order to identify the direction, grade and depth of the ore, in order to expand on the data of the 3D image of the ore identified in the early exploration stages, samples of the rock are also analysed by the mine engineers that would determine the design and angles of the pits geometry.

The most common machinery used for the excavation process is the, Face Shovel (pictured right) costing $8 million, Haul Trucks costing $3 million as well as hole boring drilling rigs, where an open pit operation is located near a community, a water tanker with a water cannon would spray water around the machinery to keep the dust from blowing around

3D image showing open pit
& underground workings

Depending on the geometry and depth of the ore body, where possible, open pit mining is by far the most economical method of extraction for the recovery of low grade finely disseminated ore. The advantage of open-pit, as opposed to underground, mining is that it is usually easier, cheaper and quicker to bring into production. Open-pit mines in Nevada and in Western Australia have often been brought on-stream within a year at a cost far below the development of underground mines and in both regions account for virtually all gold production.
Open-pit deposits often have a relatively short life-span (on average, four to five years), after which it may become necessary to move to underground mining techniques to access deeper ores (if they are available and economic).  However, the continued identification of new surface deposits, such as those along the Carlin Trend in Nevada, USA, often provides replacement ore reserves.

Heap Leaching and carbon-in-pulp recovery have also resulted in the mining of large tonnage, low grade, near surface gold deposits.  Almost eighty per cent of all gold production in the United States comes from open-pit mining.
 

picture left:  an aerial photograph of an open pit operation called the 'Super Pit', located in Kalgoorlie, Western Australia which is owned and operated by KCGM (Kalgoorlie Consolidated Gold Mines).  It has now become the largest open pit operation in the southern hemisphere, measuring 3.2Km long, 1.4Km wide and 330m deep.  Approximately 85 million tonnes of ore is mined each year for a total yield of 800,000ozs of gold.

Gold was first discovered here in 1893 by Paddy Hannan and within 10 years, was known as the 'The Golden Mile' and was described as the richest square mile of earth in the world.  In 1989, all of the original mineral leases were amalgamated, and is now managed by KCGM.  They own 4 Komatsu PC800 685 tonne Face Shovels that can scoop 65 tonnes per load costing $10 million each, two 270 tonne Liebherr Haul Trucks, and 30 Caterpillar 225 tonne Haul Trucks costing $4 million per vehicle.

The wealth generated during the Kalgoorlie gold rush prompted the construction of a water pipeline from Perth, covering an uphill distance of 560Km into the desert,.  It was constructed by using 60,000 pipe sections using eight steam-powered pumping stations located along the pipeline and was officially opened on 22 January 1903 at a cost of £2.5 million and five years construction under the supervision of C.Y. O’Connor, the chief engineer.  Sadly, O’Connor committed suicide before completion of the project called the Goldfields Water Supply Scheme, or the 'Golden Pipeline'.