Print4.3.1: Surface Mining Methods
Surface mining methods are traditionally divided into two classes: mechanical and aqueous. Mechanical methods rely on breaking the ore by mechanical means, and aqueous methods rely on the use of water or another solvent, e.g. an acid, to break down the ore and facilitate its removal.
Mechanical Methods
Open pit mining
This type of mining is used for near-surface deposits, primarily metal and nonmetal. The overburden is hauled away to a waste area and a large pit is excavated into the orebody. The depth of the pit is increased by removing material in successive benches. A few examples of commodities mined by this method would include iron and diamonds.
Open cast mining
Open cast mining is also known as strip mining and is used for bedded deposits, and most commonly for coal. Although it is similar to open pit mining, the distinguishing characteristic is that the overburden is not hauled away to waste dumps; but rather, it is immediately cast directly into the adjacent mined-out cut. There are two important sub methods for open cast mining. One is known as area mining, and is applicable when the terrain is relatively flat; and the other is contour mining, better suited for mountains regions. A few examples of commodities mined by this method include coal and phosphate.
Quarrying
Quarrying is a method of extracting dimension stone. The term dimension stone encompasses certain stone products used for architectural purposes such as granite countertops, marble flooring, and monuments, among a few others. The goal in the mining of these products is to remove large slabs that can be cut and machined to exacting architectural applications. Unlike open pit mining in which benching is required to prevent failure of the sides or pit slopes, the high strength and competency of the rock mass in quarries is such that vertical walls of 1000’ or more can be excavated. Now that I’ve given you the classical mining engineering definition of quarrying, you should be aware that just about everybody uses this word, "quarry" to describe any open pit operation in stone! Oh well… A few examples of commodities mined by this method include Georgia marble and Vermont granite.
Auger mining
This is a method to recover additional coal from under the highwall of a contour mine, when the ultimate stripping ratio has been achieved in open cast operations. It is sometimes referred to as secondary mining because it is done after the open cast mine has reached an economic limit.
Aqueous Methods
Hydraulic mining
Hydraulic mining is used for a limited class of deposits that are characterized as loosely consolidated, such as placer-type deposits. A high-pressure water canon is used to dislodge the deposit, and the resulting solution is either pumped to a processing plant or a gravity separation is performed at the mine site using something like a sluice. A few examples of commodities mined by this method include gold and kaolin.
Dredging
This method is used for underwater recovery of loosely consolidated materials using a floating mining machine known as a dredge. In some cases, the deposits are naturally underwater, while in others the area is flooded, creating an artificial lake on which the dredge operates. A few examples of commodities mined by this method include sand and gravel.
Solution mining
Solution mining is used to recover deep deposits that would be uneconomical using underground methods, but only if the ore can be easily dissolved by a solvent. In this method, holes are drilled from the surface into the deposit. A solvent is pumped down one hole, and the resulting solution with the dissolved mineral is pumped out another hole. This solute or pregnant liquor, as it is often known, is processed to extract the mineral of interest. In some cases only one hole is used, but the hole has an inner and outer section to separate the in-going solvent from the out-coming solute. Water, acid, and steam are common solvents. A few examples of commodities mined by this method include uranium and sulfur.
Heap leaching
Heap leaching was used many years ago as a method to recover very low percentages of metal remaining in the tailings from mineral processing plants. Large piles, i.e., heaps, of the tailings of low-grade ore were created, a solvent was allowed to drip and percolate down through the heap, and then the pregnant liquor was recovered and processed. In this fashion, it is a secondary method. In recent years it has been used with increasing frequency to recover high-value metals such as gold from very low-grade ores. A few examples of commodities mined by this method include copper and gold.