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Pits & Quarries 

 

Summary

This review will first define the difference between open pit mines and quarries. It will then cover the details on open pit mining methods, design aspects, and gives an example of an open pit mine. In the later half, quarrying is covered with more focus on the sequence of mining in a quarry. Its characteristics, mining machineries involved, and publications are also mentioned. Finally, a few more topics are covered which include computer codes, laws, and codes of practice. Publications, organizations, and a list of companies supplying to the quarrying industry are also given.

INTRODUCTION

Open pit mining is the process of mining a near surface deposit by means of a surface pit excavated using one or more horizontal benches. The term open pit mining is usually used for metallic or non-metallic deposits and sparingly used for bedded deposits like coal. A quarry is a type of open pit mine used to mine building materials (construction aggregate, riprap, sand and gravel) and dimension stones usually at shallower depths. The term quarry has traditionally been used to mine stones.

OPEN PIT MINING

The ores in an open pit mine are covered by overburden. Both the ores and overburden are removed in benches ranging from height 9 m to 30 m. A thin deposit may require one or few benches but a thick deposit needs more number of benches and the pit in its production stage resembles like an inverted cone.

Design Aspects of Open Pit Mining:

1. Ultimate pit depth:
This is the maximum depth of the mine that it will reach at the end of its life. This is decided by economic reasons and a breakeven point is decided beyond which it is not economic to continue production. As this depends on economic conditions, a mine closed in 1980s can become economic now, as happened in case of Copper Mountain Project.

2. Bench Height:
The bench height is usually decided by the economic reach of the mining equipment used in the mine. As a thumb rule, a bench height is equal to the economic bucket height of an excavator (like shovel or loader) plus 3 m. The thickness of the orebody and its dip and the thickness overburden also play a deciding role in the bench height. As mentioned in Coal Trading, the bench height depends up on:
a) Deposit character and geology: selectivity
b) Production strategy: ore/waste ratios, blending requirements, no. of working faces, operating/capital costs, etc.
c) Slope stability considerations
d) Equipment set / equipment specific optimum geometry

3. Bench Slope:
Bench Slope is decided by the geo-technical conditions of the overburden and the orebody.

4. Overall pit slope:
This is also decided by the geo-technical stability of the rocks in the mine, bench height and bench slope. Overall pit slope is always less than the bench slope.

5. Bench Width:
This is usually decided based on the space required for the operation of the equipment on it.

6. Haul Road Width & Slope:
This depends on the operating parameters of the trucks and other hauling equipment used in the mine. It is always wider than the bench width.

7. Bench Length:
The length of the bench depends on the production rate. A bench will be longer for higher production rate. Usually, blasting at one bench is done not more than once a week. The extent of the orebody also has a deciding role here.

A simple diagram showing different design parameters is shown below:

A typical set of bench height, bench with and bench slope are given below:

Bingham Canyon Mine (1990), (source - Introductory Mining Engineering, Hartman)

  • Total production of waste and ore = 290,000 tonnes per day
  • Longest horizontal dimension of pit = 4,000 m
  • Pit Depth = 800 m
  • Drills - Roller bits rotary drills, 0.3 m dia
  • Shovel - 26 m3
  • Truck - 172 tonnes
  • Metals recovered: Cu, Mo, Au, Ag
  • Strip ratio - 0.83 / 1.0
  • Average grade - 0.6% Cu
  • Cut-off grade - 0.23% Cu

Wikipedia gives a very wide list of major open pit mines currently operating in the world.

Surface Mining by B. A. Kennedy provides an excellent source for mining engineers and students to know about open pit mining.


QUARRIES

Most quarries have smaller and vertical benches. Some of the quarries do not have any bench while some even have a highwall up to 300m.

The kinds of rocks extracted from quarries comprise:

  • Cinder
  • Chalk
  • China Clay
  • Clay
  • Coal
  • Coquina
  • Construction aggregate (sand and gravel)
  • Globigerina Limestone (Malta)
  • Granite
  • Gritstone
  • Gypsum
  • Limestone
  • Ores
  • Phosphate rock
  • Sandstone
  • Slate
  • Marble

SEQUENCE OF DEVELOPMENT

A quarry usually goes through the following stages before reaching the production stage:

1. Prospecting and Exploration:
The following properties of the rock are studied in the prospecting programs: physical & mineralogical properties, color and appearance, uniformity, strength, flaws, cracks, etc.

2. Development:
Along with the removal of overburden, development stage also involves developing the processing and other construction facility near the quarry. The main role of the processing facility is to cut and polish the rock. Large amount of wastes are generated in processing, so it is required to plan for some storage/dump space for the cuttings.

Limestone Quarrying (Photo source: Dexpan)

3. Production: The stones/materials in a quarry are excavated with any of these methods:

a) Drill and Blast:
Depending upon the quality and fineness of the material desired, the drilling and blasting requirements are set. The more uniformity is required in the rock excavated, the more drilling and controlled blasting will be required. Light gunpowder is used as explosive to avoid any shattering effect which can bring a significant destruction of the stone shape and quality. Two types of blasting methods are used for quarrying:
(i) Wedges method
(ii)Plug-and-feather method

b) Compressed Air/Water and Explosives:
A small charge of dynamite is used to create cracks in the stone, which is then cut with compressed air or water to widen the cracks and break the stone.

c) Channelling and Use of Saw:
Channelling involves cutting long and narrow channels into the rock to free up a slice from the large rocks. Three techniques are used:
(i) Large amount of single-line drilling with a small shattering
(ii) Single line drilling with use of expansion mortars (non-explosives) like Dexpan and Rockfrac
(iii) Single-line drilling with use of non-explosives like NonexTM and Magnum BusterTM, which uses non-detonating chemicals to break rocks.
(iv) Use of a saw to produce vertical or horizontal cuts in the rock itself. Three types of saw are mainly used: Disc/crosscut Saw, Chain Saw and Diamond Wire Saw

Disc/Crosscut Saw, Tanamerah Quarry

Disc/Crosscut Saw, Tanamerah Quarry


Chain Saw to produce vertical cuts (Albion Quarry, Isle of Portland)

Chain Saw to produce vertical cuts (Albion Quarry, Isle of Portland)


Diamond wire saw (Photo: Stonebtb.com)

Diamond wire saw (Photo: Stonebtb.com



4. Reclamation of quarries:

The main tasks involved in the reclamation of quarries are:

(i) Stability Monitoring - to analyse the long-term stability of areas or a particular landform in and around the quarry
(ii) Grassland Establishment - to restore on grasslands waste pile
(iii) Tree Establishment - to restore the quarry sites through trees plantation. Trees restoration in a quarry site is a long term process and usually does not provide any significant visual impact in 10 years. Selection of tree species is also important
(iv) Landscape Assessment for future application of the landforms for future application and habitat creation.

Reclamation of Limestone Quarries by Landform Simulation gives a very descriptive detail on the reclamation of a quarry.
An executive summary for the same can be found at the above link.


EQUIPMENT USED IN QUARRIES

Here are some of the equipment used in quarries:

CHARACTERISTICS

Here are the advatages and disadvantages of quarry mining that I took from Introductory Mining Engineering book, by Howard L. Hartman and Jan M. Mutmansky:

Advantages:

  • Low capital cost and low mechanization
  • Suited to some small deposits
  • Easily accessible; hoisting may complicate moving stones, supplies and workers
  • Stable wall and benches; generally no bank support required
  • High selectivity; can discard low quality stones
  • Good safety; little chance of slope failures

Disadvantages:

  • Somewhat limited by depth; usually less than 90m, can be up to 300m
  • Low productivity; high labor costs
  • Highest mining cost because of low productivity
  • Low production rate
  • Relatively skilled labor is required
  • Inflexible; cannot easily change the mining plan at depth
  • Mechanisation is limited by the nature of the method
  • Complicated and costly rock breakage method because of inability to use the maximum power of explosives
  • Waste can be 60% to 90%

PUBLICATIONS

Here are a few more valuable publications on quarries that I found on web:

Development & Reclamation Standards for Yukon Pits & Quarries

Reclamation of Limestone Quarries by Landform Simulation

The Arizona Marble Company Quarry in South-eastern Arizona

Aggregates Manager

Introductory Mining Engineering, 2nd Edition


VIDEOS



COMPUTER CODES

A computer code that appears to facilitate all aspects of pit and quarry investigation, design, and operation is named unsurprisingly Quarry. Here is the supplier's description:

Quarry has everything you need for quarry design and planning. To lay out quarry infrastructure, there are a multitude of 3D CAD functions plus a set of semi-automated pit designtools to build a surface model of the Quarry 'as-mined' face at any point in time. You will find that you can modify the bench widths and heights and automatically include ramps in your design. For those in hilly terrains, you can easily design to an existing seam or topographic surface and produce bench by bench volumes and grades from the proposed quarry.

Earthworks Corporation has the RM Manager code. It is described thus:

Earthworks RM Manager is an easy-to-use, menu driven system for monitoring and controlling the day-to-day operation of limestone quarries supplying raw material for cement manufacture. It has been designed for use by production staff with a good basic knowledge of quarry operations but does not assume skills in surveying, geology, mine planning or computing. The user is guided through a straightforward set of menu choices with online help at every stage. Menu choices are filled in automatically as processing proceeds so that the system can be operated with minimal training. Alternatively, full access to data files is provided by means of browse buttons so that more experienced staff can use the advanced options. Results may be output automatically to spreadsheets or customized graphs and tables.

SUPPLIERS

If you look in the Infomine.com databases you will find many suppliers of mining equipment. Most of these suppliers would be as happy to supply your pit or quarry as any other mining operation. Here are some of them:

Crushed limestone quarry near Bellefonte, Pennsylvania

I am told that Breaker Technology Inc. is a premium supplier of equipment to the pit & quarry market. Decide for yourself by examining their website. Be aware they are part of a larger group, the Astec International Aggregate and Mining Group whose purpose is "to grow and prosper by designing, manufacturing and selling the most innovative, productive and reliable equipment for building and restoring the world's infrastructure, coupled with unparalleled customer service." No demure claim this!


REFERENCE GUIDES

The only guide I will refer to here is the one that I found most interesting both because of its style, format, and approach as because of the location for which it is intended: the Yukon. I refer to the excellent volume, Development & Reclamation Standards for Yukon Pits & Quarries. Many others guides specific to particular states are available through a web search using the keywords "your-state-name pit or quarry regulations".


LAWS

In Ontario, Canada there is the Aggregate Resources Act. The purposes of the act include: manage aggregate resources; require rehabilitation of land from which aggregates are mined; and minimize adverse impact on the environment in respect of aggregate operations. And, as you would expect, the act requires you to obtain a licence before you develop a pit or quarry and mine for aggregate.


CODES OF PRACTICE

Alberta, Canada has a comprehensive 2004 Code of Practice for Pits. It basically sets out how you will operate, report, and reclaim.


ORGANIZATIONS

The Ontario Aggregate Resource Corporation site provides information for the holders of the 2,700 licenses and 2,900 permits issued in Ontario to the aggregate industry. But the information is of interest and possible use to all in the aggregate, pit, and quarry industry. For example, I very much enjoyed and liked their publications, including well-produced volumes on pit and quarry rehabilitation.

At this site is a long list of aggregate and construction resources associations, not all having to do with pits or quarries.

Construction aggregate quarry near Adelaide, South Australia

Here are a few links specific to the Canadian aggregates industry:


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