Wjec Mining Hazards

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School of somethingFACULTY OF OTHER

School of Process, Environmental and

Materials Engineering

Mining, Quarry & Mineral Engineering

Hazards Associated with Mining

Covering material required for WJEC AS-Level Geology:

GL3: Key Idea 3(c) Mining.

Prepared by Toby White, University of Leeds, Leeds, LS2 9JT


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SubsidenceRoom & Pillar (Pillar& Stall) workings in coal


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SubsidenceRoom & Pillar (Pillar& Stall) workings in coal


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SubsidenceUnderground Longwall Mining - Shearing

Coal carried away

on conveyor

Roof collapses

(goaf) behind

supports.

Roof supports

Coal

Coal shearer


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SubsidenceCoal Mining

Further examples of subsidence.


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Acid Mine Drainage


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Acid Mine DrainageIntroduction

Acid mine drainage (AMD) is polluted water that normally containshigh levels of iron, aluminum and sulphuric acid.

The contaminated water is often an orange or yellow-orange colour,indicating high levels of iron and has the smell of rotten eggs

(sulphate smell). AMD comes from pyrite (iron sulfide, fools gold, FeS2), a mineral

associated with coal and metal mining. When pyrite is disturbed, asit is during mining or highway construction, it weathers and reactswith oxygen and water to cause high levels of iron, aluminum, and

sulphate in run-off water.


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Acid Mine DrainageWheal Jane

AMD disaster (1991/92)

Wheal Jane mine (Cornwall)principally produced cassiterite, themain ore mineral for tin, but older

interconnecting workings alsoproduced pyrite and arsenopyrite.

Similar problems occur with coalwhere pyrite content is high.


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Methane

Methane in mines:

is the naturally occurring gas that most commonly appears in mines,

has the formula CH4and is associated with carbonaceous material,so is a particular problem in coal mines,

has been the cause of more explosions and related loss of life thanany other cause throughout the recorded history of mining,

forms a highly explosive mixture with air (fire-damp),

0.55 times the weight of air and rises,

can provide a fuel source in its own right through drainagetechnology.


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MethaneHow is methane released?

When the coal is disturbed by mining, a pressure gradient is createdwhich will result in flow through natural or stress-induced fractures.

The reduced gas pressure will promote desorption, which willrelease more methane into the system.

As coal is removed, methane will be released quickly at the face, aswell as the roof and floor.

This means there will be most methane where there is greatestrisk of sparks.

But methane will continue to escape; into collapsed (gob) areas behind longwall face,

into coal mass being transported.

Sudden gas outbursts can also occur and can be very destructive.


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MethaneControl of methane

The best way of controlling methane is by extracting it from thecarbonaceous strata in an organised way.

There is no single preferred method, but the choice depends on:

The permeability of the source seams,

The reason for drainage (as a fuel, or simply to protect mine)

The method of mining.

One example is given here.

Longwall mining

Two-entry retreat face

Horizontal boreholes

Some methane drained from seam

Will not remove all.

Ventilation

circuit

Clean air

Methane

carried away


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Instability


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InstabilityIntroduction

Failures of the rock mass can take place in a number ofdifferent ways, all of which are potentially dangerous.

Rockbursts in an underground mine

Collapse of a roof in an underground mine Collapse of a face on a quarry

Rockfall and toppling

Collapse of a slope

Original ground, overburden dump, spoil heaps, mine tailings(waste) dams.


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InstabilityCollapse of a face on a quarry

Slope & Face Failure

Translational Rotational/

circular

Toppling Rockfall


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Rockfall

There is a very real danger ofrockfall from the material at thetop of this face.

Faces are now generally kept toless than 15m high.

The face could be cleaned byan excavator

A buffer zone could be built asafe distance around the baseof the face.


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Toppling Failure

This differs to rockfall, in that a part of the face collapsesrather than a few stones or boulders. Note the cleanness of thenew face in the lower right, suggesting a weak plane (joint or small

fault).


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InstabilityCollapse of a slope

Planar/Slab failure

Planar instability along beddingwith loss of quarry crest andaccess to lower benches.


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Wedge failure

Wedge failure in a magnesianlimestone quarry along two welldefined joint planes.


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Rotational failure

This rotational failure took place in the tailings dam of the Crixasgold mine in South Africa.


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Bi/Multi-planar failure

This failure took place at St Aidans OCCS on at least two planes;the back plane (fault zone) and at least one basal plane (weakseatearths below the coal seams).


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Tailings Dam Failures


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Tailings Dam FailuresDams

Tailings material isusually highly toxic.

Dam types:

1. Centreline

2. Upstream

3. Downstream


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T ili D F il


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Tailings Dam FailuresTypes of Failures - Piping failure

Omai gold mine, Guyana.

No-one was killed, but 18 peopleneeded treatment for the effects ofcyanide poisoning.

T ili D F il


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Tailings Dam FailuresTypes of Failures - Piping failure

Omai gold mine, Guyana.

On August 19th 1995, the tailings dam at Guyana's Omai GoldMine cracked allowing 3.2 billion litres of cyanide-laced (28ppm)sludge to flow into the heart of the rainforest.

The current judgement is that the failure of the dam wascaused by massive loss of core integrity resulting from internalerosion of the dam fill (piping).

T ili D F il


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Tailings Dam FailuresTypes of Failures - Decant failure

The decant pipework collapsed and tailings escapedthrough the outflow.


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T ili D F il


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Tailings Dam FailuresTypes of Failures - Erosion

The photo shows repairs made at the Crown Mine tailingsdam by buttressing the toe with concrete.


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Tip Failure


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Mining at Leeds

The Mining and Mineral Engineering Programme is taught withinthe School of Process , Environmental and Materials Engineering

(otherwise known as SPEME), as part of the Process Engineeringgroup of subjects.

Most UK students are eligible for a bursary of 1,000 intheir 1styear.

For further information visit: www.canyoudigthis.com

Or go direct to: www.engineering.leeds.ac.uk

Contact: Toby White 0113 343 2784 T J White@leeds ac uk
http://www.canyoudigthis.com/http://www.engineering.leeds.ac.uk/mailto:[email protected]:[email protected]://www.engineering.leeds.ac.uk/http://www.canyoudigthis.com/

Wjec Mining Hazards

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