For an introduction to metals and mining, please read the following sections for an overview to the industry:
Selecting a Mining Method
Selecting which mining method to use is one of the important decisions a mining company has to make. The mining method selected should maximize cost recovery and profitability of operations, minimize ore dilution, and permit the most efficient removal of ore.
Once an ore has been probed to be economically viable, and a pre-feasibility study has been conducted to classify the economically viable mineral volume as measured, indicated, and inferred resources, the most appropriate mining method needs to be selected based on the characteristics of the ore, such as dip size, shape of ore body, and strength of ore body.
At this stage, the selection is only serving as the basis for a project layout and feasibility study. While the basic principles of ore extraction should remain a part of the final project layout, it may be necessary to revise some details based on the characteristics of the ore being mined.
With respect to basic principles involved, and in the light of technological advancements, relatively few mining methods are used today. However, due to the uniqueness of each ore body, several variations of these methods are used. Before going into the details of the different mining methods, it is important to understand the main factors that determine the use of a particular method. The main factors to consider are the following:
Dip Size: It is a measure of the vertical inclination of the deposit, which identifies whether the broken ore will, without difficulty, fall to lower mining levels to be taken for processing
Shape Ore Body: There are three main types of ore bodies: vein-type, massive, or tubular
Vein-type: Vein type deposits are those that almost always have a dip at an angle greater than 50 degrees.
Massive: Massive ore bodies are those that spread over hundreds of meters.
Tubular: Tubular deposits are those that are relatively flat.
Strength of Ore Body: Is dependent on natural conditions such as jointing and faulting of the deposit. However, artificially created mine support also needs to be considered.
Self-supporting deposits: External support is not required as walls and pillars within the mine are sufficiently strong.
Supported deposits: Mines that require artificial support to ensure that working conditions are relatively safe.
Cut-and-fill stoping is a preferred method for high grade ore bodies with a steep dip size or irregular shape or vein structure. It is a versatile method that provides high ore recovery and is suitable for deposits that require mining ore out of selected ore pockets. Cut-and-fill allows selective mining and separate recovery of high grade sections and is therefore useful for ore bodies with irregular shapes and scattered minerals. It is also useful for deposits that have weak walls as the fill supports the slope walls and provides a platform for when the next slice is cut.
The development process for cut-and-fill mining begins with a haulage drive along the footwall of the ore body at the main level, after which the stope is undercut with drains for water. Once this is completed, a spiral ramp is built to provide access to the undercut. Also, for ventilation and filling material, a raise is made to connect to the above levels.
Starting from the bottom undercut and moving upwards, cut-and-fill stoping removes ore in horizontal slices. Ore is drilled and blasted, and the muck is removed from the stope. Once, the stope has been mined out, voids are back filled with waste rock and sand. This process is repeated continuously until the deposit is depleted. Each production level is accomplished by drifting until the entire slice has been mined. The slice is then back-filled and the fill becomes the working platform from which the next level is mined. Back slashing is done for providing access to the upper slices within the stope. When a stope is completed, a new access drift from the ramp is created to continue production within the upper stope. Operators continue to mine upwards until only a small pillar is left above. At this point, the roof becomes overly stressed and may burst and collapse on the workers below.
One of the advantages with cut-and-fill mining is the possibility to re-use waste for back-fill material such as tailing sand from the processing plant or waste rock from development. Also, since the mining can be tailored to suit the shape of the ore body, it is possible to minimize dilution of waste rock.
From a cost perspective, cut-and-fill mining is a relatively expensive process with a relatively low productivity. Typically, the direct mine operating costs for cut-and-fill range from $75-$250/tonne. Costs generally increase with increasing irregularity of the ore body and decreasing ground quality. An example of a mine which uses cut-and-fill mining is Goldcorp’s Red Lake mine.
Long-Hole Sublevel Stoping
Long-hole sublevel stoping, often referred to as sublevel open stoping and blast hole stoping is a commonly used method in large scale mining. It is a versatile and productive method that is primarily used for large ore bodies with a steep dip, regular shape, and defined ore bodies. It is used when the ore body is narrow in width (6-30 metres).
Development of sublevel stoping includes a haulage drift, raises to access sublevels, undercuts below the stope, draw points, and a slot raise at the end of the stope. The ore body is divided into parallel sublevels about 20 metres away from one another. Once this is completed, a slot raise is developed and subsequently increased by blasting and drilling accesses that are developed into the ore, filled with explosives and blasted sequentially.
There are different variations of the method depending on the shape of the ore body. However, the method is based on the principle of blasting out large stopes, which are back-filled to maximize recovery of the ore body. Drifts are created through the ore body to enable mining of stopes between sublevels. The ore body can be divided into primary and secondary stopes, where the primary stopes are mined first. The secondary stopes are left as pillars until the adjacent primary stopes are mined and back-filled. Stabilized fill in the primary stopes prevents side walls to cave in when secondary stopes are mined. Opening raises are required to accommodate the initial blast and start up of the stoping process.
Once ore from the first section has been blasted and mucked out, enough room is made to blast larger sections of the stope. Just like in cut-and-fill stoping, one of the advantages with back-filling is the possibility to reuse waste for back-fill material. Before excavating the secondary stope, cable bolts are often installed above the stope to provide additional support to the rock mass, preventing larger blocks from falling into the excavated stope. Since it is possible to mine the ore on different levels and stopes at the same time, productivity is maintained at a high rate. Ore recovery and dilution control tend to be excellent in long-hole sublevel stoping.
Compared to cut-and-fill stoping, direct mining costs are much lower and fall in the range of $20-$60/tonne. Costs tend to vary depending on drilling and blasting designs with more costly methods leading to higher ore recovery. Sublevel stoping (and its variations) is one of the most popular mining method and accounts for approximately 75% of the methods used in mines across Canada.
There are several variations of long-hole sublevel stoping. Some common ones include longitudinal stoping, transverse longitudinal stoping, big-hole stoping, and vertical crater retreat. Longitudinal stoping, also referred to as bench stoping or sublevel benching is a preferred method when the orebody is thin and narrow and steeply dipping and sheet-like. Transverse longitudinal stoping, also commonly referred to as primary- secondary sequence mining is useful in cases where the rock quality of the hanging wall limits the length of the open mining span. Big-hole stoping is a variation of sublevel stoping in which longer blast-holes with larger diameters are used. Vertical crater retreat is another variation of long-hole stoping and is used for ore bodies that are steeply dipping. Depending on the type of ore body, the appropriate variation is selected.
Shrinkage stoping is an overhand mining method that is used for self-supporting vertical ore bodies. In this method, ore is mined in horizontal slices, starting from the bottom to the top of the stope. The broken ore is left in the mined-out stope and serves as a support to the stope walls and a working platform for mining the ore above. Shrinkage stoping has a high recovery but requires steep and narrow ore bodies. Smaller ore bodies can be mined using a single stope, however, larger ore bodies need to be divided into separate stopes with intermediate pillars to stabilize the hanging wall; the pillars can generally be recovered upon completion of the mining. Shrinkage stoping can be used for ore bodies with steep dips, regular shapes, firm ore, and bodies that have a stable hanging wall and footwall.
The development for shrinkage stoping includes a haulage drift along the bottom of the stope with crosscuts into the ore underneath the stope, an undercut of the stope 5 to 10 m above the haulage drift, and a raise from the haulage level, passing through the undercut to the main level above to provide access and ventilation to the stope. Then, drilling and blasting is carried out. The main idea behind this method is that the muck takes up a greater volume than in-situ rock. Through blasting, the rock increases its volume by nearly 50%. Therefore, once a horizontal slice of stope is blasted, it drops on the mine floor as muck, approximately 35% of which is removed for processing, with the remaining left to serve as a working platform for the next cut. Once the stope is exhausted, all the blasted rock is cleaned out.
Shrinkage stoping was popular and important in the days when few machines were used in underground mining. It is of little importance today due to its drawbacks – that is, it is a labor-intensive method with dangerous working conditions and limited productivity. Due to increased mechanization, sublevel stoping, vertical crater retreat, sublevel caving, and cut-and-fill can be applied instead of shrinkage stoping at a relatively lower cost and with better safety in place.
Room-and-pillar mining is a method that is usually used for lower grade ore bodies that are relatively horizontal, with dip size of less than 30 degrees and thickness of no more than 30 metres. While this method is highly productive, it is feasible only when the ore, footwall, and hanging wall are of good quality.
Room-and-pillar mining is used to recover ore in open stopes. It leaves pillars to support the hanging wall; miners aim to leave the smallest possible pillars to recover the maximum amount of ore. Minerals contained in the pillars are non-recoverable and are therefore not included in the ore reserves of the mine. However, if the grade is sufficiently high, additional expense can be incurred to recover minerals contained in the pillars. While this would increase the mining cost on a per tonne basis, it may not impact mining cost on a per ounce basis if the ore grade is sufficiently high. Pillars are generally 11 to 15 meters in diameter and on average are spaced 11 metres apart with a pillar to ore ratio of approximately 1:5.
Depending on the thickness of the ore body, multiple cuts are carried out, with horizontal slices starting at the top and moving down in steps. The developmental sequence consists of an initial cutting, followed by overhand stoping of the ore in the back and horizontal benching of ore in the floor. The initial pass is initiated towards the top of the ore body below the hanging wall. Then, rock bolts are often installed to reinforce the rock strata. Further mining is carried out by horizontal drilling and blasting. As long as the stopes are sufficiently large, big drill jumbos can be used for drilling. The overhead stoping passes will continue upward until the top of the ore body is reached.
Compared to other mining methods, room-and-pillar mining is a low-cost method with a low recovery rate (since some ore is left behind in pillars). Yet, the method causes very little dilution as the contact between ore and waste is limited due to the waste being on top and ore being at the bottom. Direct operating costs for room-and-pillar method range from $5-$20/tonne.
Some variations of the room-and-pillar mining method include post room-and-pillar mining and step room-and-pillar mining. Post room-and-pillar mining is a combination of room-and-pillar mining and cut-and-fill stoping mining, whereas step room-and-pillar mining is a variation of room-and-pillar mining in which the footwall of an inclined ore body is adapted for use of trackless equipment.
Sublevel caving is a large-scale mining method, suitable for large ore bodies with a steep dip. The setup is similar to long-hole stoping except loft raises are not required. Also, mining starts at the top of the ore body and progresses downwards in a safe sequence. Sublevels are developed in the orebody at regular intervals, with each sublevel featuring a systematic layout with parallel drifts along or across the ore body. Sublevel caving is used in large and steep ore bodies as the rock mass must be stable enough to allow the sublevel drifts to remain open with just occasional rock bolting. The hanging wall must fracture and collapse and the ground on top of the ore body must be permitted to subside. The rock needs to be such that both the ore body and the host rock fracture under controlled conditions.
Since the mined-out area is not back-filled, the hanging wall keeps caving into the voids. The amount of developmental work involved with sublevel caving is significantly more as compared to other mining methods. However, development primarily involves drifting to prepare sublevels. Once the production drifts have been excavated and reinforced, the opening raise and long-hole drilling patterns are completed. Minimizing hole deviation when drilling is crucial as it will affect the fragmentation of the blasted ore and therefore also affect the flow of the caving rock mass. Ore handling involves mucking out the blasted material at the front, transporting it on the sublevels, and dumping the ore into process.
As compared to other mining methods, sublevel caving allows for a quicker and less expensive mining process. However, sublevel caving leads to greater dilution and is therefore used for lower ore bodies. The costs for sublevel caving range from $5-8$/tonne.
Block caving is the logical step-up of sublevel caving. As with sublevel caving, the ore is mined as one large stope, but instead of mining from the top, ore is mined using draw points from the bottom of the ore body. Due to this, upfront capital costs tend to be large. Ore is blasted in its lower portion to begin with, but after that, the rock is mostly broken under the stresses of its own mass. Since minimal blasting is required once production begins, operating costs for block caving are generally lower than that of other mining methods, ranging from $2.50-$5/tonne. Some block caving operations have mining costs that are less than those of open pit mining operations, which is considered to be the cheapest mining method.
Due to high dilution and low recovery, block caving should be used for large, steep, and low-grade ore bodies. Due to the low selectivity of the block caving method, it is suitable for ores that have uniform grade distribution. Also, high grade that does not cave well requires fracturing with explosives, which can increase developmental costs. The danger with poor caving is that a large cavity can form, which can result in sudden collapse of the rock mass, creating a potentially destructive blast.
Panel caving is very similar to block caving. The only difference is that a portion of the ore body is undercut and blasted at start up, such that caving propagates both horizontally and vertically. The advantage of panel caving is that it requires less capital prior to commencement of production.
The only operational block caving mine in Canada is New Gold’s New Afton mine located in British Columbia.
Open Pit Mining
Open pit mining method works best when the deposit is close to the surface as it is way more economical to use this method instead of underground mining. Open pit mining operations are the largest operations world-wide. Since this method allows for the use of large equipment, it results in huge economies of scale, enabling immense cost savings. It is interesting to note that the largest such operations could cost as low as $1/tonne and the smaller ones as much as $10/tonne. Cost variability in an open pit mine is measured by ‘strip ratio’, which is the ratio of mass of waste required to be moved to the mass of the ore moved. There is a direct correlation between the strip ratio and the cost of operation.
In order to ensure workplace safety, it is important to secure strong walls to ensure continuous production. Pit wall stability comes from the quality of rock and the slope of the pit wall. While a lower pit wall slope is more stable, it requires more waste rock to be removed, resulting in a higher strip ratio which translates into high operation costs. Hence, the design of such mines require delicate balance between stability, safety and costs of operations.
Having said that, open pit mining operations are by far the largest mining operations globally. Well known examples include Goldcorp’s Penasquito mine in Mexico and Freeport’s Grasberg mine in Indonesia.
A highly productive underground mining method, longwall mining is used in a large horizontal spread of thin-bedded deposits of uniform thickness usually ranging from 0.6m to 1m. Used a lot by South African gold mining companies, this method can be used in both soft and hard rocks as the working area along the mining face can be supported artificially in places where the hanging wall shows a tendency to collapse. Longwall mining helps to extract ore along a straight front with a large longitudinal extension. A network of haulage drifts is used to reach the production area and transport ore to shaft stations. In order to ventilate mine operations, haulage drifts are accompanied with parallel excavations. Haulage drifts are arranged in patterns and excavated in the deposit itself. The length of the longwall face is determined by the distance between two adjacent haulage drifts.
Longwall mining method is highly mechanized and can be used with utmost precision. The soft material can be cut loose mechanically as it does not require drilling or blasting. Special cutting machines or rotating drums with cutters move back and forth along the faces cutting off a slice of the seam. The chain conveyor carries the mineral to the haulage drift, from where it is transported out of the mine. Since the roof along the longwall is supported, as mining advances forward, the supports move forward and the roof behind is allowed to collapse.
This method is popular for mining thin reef deposits such as gold reef conglomerates that are hard to mine. Longwall mining accounts for over 90% of Australia’s underground coal production. While many mining companies have retrofitted technology, the challenge is to balance productivity with safety.
|Mining Method||Orebody Characteristics||Dilution||Recovery||Cost|
|Ore Strength||Waste Strength||Ore Dip||%, factor||%, factor||$/tonne|
|Open Pit||Moderate||Moderate||Flat||5||90||$1 – $10|
|Block Caving||Weak – Moderate||Weak – Moderate||Steep||15||95||$2.5 – $5|
|Cut-and-fill||Moderate – Strong||Weak – Moderate||Moderate – Steep||5||85||$75 – $200|
|Room-and-pillar||Moderate – Strong||Moderate – Strong||Flat – Moderate||5||90||$5 – $20|
|Skrinkage Stoping||Moderate – Strong||Moderate – Strong||Moderate – Steep||10||90||$125 – $200|
|Sublevel Longhole Stoping||Moderate – Strong||Strong||Steep||15||85||$20 – $60|
|Longwall||Weak – Moderate||Weak||Flat – Moderate||15||85||$100-$150|