Vertical crater retreat

From Queen's University Mine Design Wiki

This article is about the orebody requirements and developmental steps involved in Vertical Crater Retreat planning and mining.

Vertical crater retreat (VCR), also known as Vertical retreat mining, is an open stoping, bottom-up mining method that involves vertically drilling large-diameter holes into the orebody from the top, and then blasting horizontal slices of the orebody into an undercut.  Excavated stopes are most commonly backfilled to provide wall support for the blasting of successive stopes, however it is not always necessary. 

Similar to Sublevel open stoping and Blasthole stoping methods, VCR mining is used for steeply-dipping (>45º), or both vertically and horizontally large orebodies with competent ore and waste rock strength.  It differs from other open stoping methods in that it is a bottom-up method, as opposed to a left-to-right method, and it does not require the excavation of sublevel drifts before blasting and mucking can take place. The thickness of one horizontal slice varies between 2 and 5 meters in height.

Orebody Characteristics

VCR is a suitable mining method for orebodies that exhibit the following characteristics:

Characteristics	Requirements
'    'Orebody Dimensions	Steeply-dipping orebodies (>45º), with widths of no less than 12-15m, OR Very large both vertically and horizontally
Ore and Rock Strength	The strength of the waste rock must be competent in order to blast against it without having excessive amounts of dilution The ore deposit is to be of medium to competent strength
Grade	VCR is not a selective mining method, therefore the ore grade should be low to medium, and it should be relatively uniform throughout the entire orebody
Depth	VCR mining can occur at any depth.  Work is carried out in reinforced, small drifts; and given the nature of the mining method, no personnel has to work directly within the drift.  Therefore, safe execution of VCR mining can be carried out in deep mines
Oxidizing Ores	Given the small, localized stope size in VCR mining, ore can be recovered very soon after blasting occurs.  Therefore, oxidizing and self-cementing ores such as pyrrhotite can be mined using this method

Advantages

• Safety: Miners are working in drifts that are adequately ventilated and have fully supported roofs.  Furthermore, no workers are required to work inside the stope, minimizing the risk of unexpected injuries.  VCR mining also allows for the use of automated machinery, in which case workers are not at risk of equipment-related injuries
• Good Recoveries: Continuous mucking from the drawpoints can take place after blasting.  Furthermore, VCR mining can be used with a high degree of mechanization, generating a high level of productivity
• Cost: Once the pre-mining development is in place, mining has a low operating cost, as it is a bulk mining technique and employees are not required to manually operate the mucking machinery
• Wall Support: VCR stoping shares some great features with sublevel open and shrinkage stoping.  Good wall support is offered during the VCR stoping phase, using shrinkage  

Disadvantages

• Dilution of the ore can result if waste rock is less than competent in strength, or improper blasting techniques are carried out.  In addition, sorting is not possible
• Risk of drawpoint blockage: If improper blasting techniques are carried out, large rocks can get lodged in the drawpoint and arrest the movement of material through them.  This results in lost production
• A Large capital investment is required to establish the essential drift infrastructure required for proper VCR mining to take place.  Furthermore, the acquisition of equipment is necessary before mining can occur.
• Subsidence of overlying stope zones can be a problem as open stope expanses are left after mucking.  This can be avoided by using a backfill, which comes with additional costs

Overall Mining Process

The first step in designing a VCR mining operation is establishing that the orebody meets the required characteristics.  Not only must the dimensions, ore/rock strength, grade and depth be suitable for the bulk gravity teachnique, but the orientation of the orebody must be such that pre-mining infrastructure can be developed. A capital investment is required in order to establish the necessary infrastructure, such as shafts and drifts, as well as to acquire the necessary equipment. 

Planning Steps

The first characteristics to evaluate are the size, dip and plunge of the orebody, which is important because the installations of draw points are essential to the gravity flow of the blasted ore for collection.  The second element to assess is the shape and consistency of the orebody.  Two horizontal drifts are required before mining can take place, which are to have a very large vertical separation.  The distance between the two drifts depends on the consistency of the ore, the drilling accuracy, accessibility, and competency of the hanging wall.  These drifts are cut inside the orebdoy in order to minimize developmental costs.  The next step is assessing the blasting characteristics of the rock, which will help to determine the drilling pattern and stope sizing of the mine.  These tests can be done on similar ore blocks, or simply theoretically.  Early consideration of equipment selection can be done at this point, as they will be based on stope and block size, as well as production requirements, and most importantly availability. 

Once the essential planning is complete, the top and bottom drifts are drilled and blasted, and any necessary ground support is installed.

Mining

Holes are drilled vertically from the top drift through to the bottom drift.  Holes are charged such that blasting of horizontal slices of the orebody occurs, progressing from the bottom drift to the top drift.  In any new region of the mine, the ore zone is assessed as soon as possible, so that ore data can be collected and compared with the original estimates calculated by mine engineers.  This offers the engineers a chance to analyze the data and make any required modifications before the following stope layout is planned.  Extraction of the blasted material can now occur as fast as the system is designed, however just enough broken ore is mucked from the stope to create the required volume of space for successive blasts.  Blasted ore is collected at the drawpoints using LHD vehicles, and then transported to orepasses, sometimes to be crushed, before it is transported to surface for processing.  Upon completion of the ore extraction, the stope is often backfilled from the top drift, providing rock stability for upcoming blasts.  This process is repeated until the orebody is mined.   

Mining

Drilling

Holes are typically 165mm in diameter, allowing for a blast pattern spacing of 4.0 by 4.0m, and the thickness of one slice of ore varies between 2 and 5m .