Mining

Mining

Mining involves extraction of metals or other resources from rocks Ores are mineral deposits that can be economically mined Typically, the extraction the metals involves grinding to sand-sized particles, and

separation using water and various chemicals

Tailings are what's left over, after extraction of resources The leftovers from one mine is called a tailing; from several mines, tailings. Most of the ore, sometimes as high as 97%, from a mining operation, ends up as tailings. It usually amounts to hundreds of thousands of tonnes a year at a mine site.

What to do with the tailings? Some mines do not use water extraction, (examples are coal and petroleum). The tailings

are simply piled up, and used to reclaim the site In mines where there is water extraction, have tailing dams which are used to store the

tailings Or they just let them they let them go into the sea. Both methods of disposal have advantages and disadvantages.

Types of extraction (concentration of the ore) Cyanide extraction for Gold Flotation with foaming agents – most metals Just plain digging and separation – coal, diamonds, gravel

METHODS OF MINING STRIP and OPEN PIT MINING

When ore bodies are close to surface, it is sometimes economical to remove the overlying rock to expose the ore body.

The surface soil is stripped off and stockpiled for later land reclamation.

ALLUVIAL MINING Often ore deposits, especially gold, are located in river

sediments These are recovered by dredges, or by hand panning or

sluice boxes HARD ROCK MINING

These are mined by underground tunnels Ore or coal brought to the surface by lifts Dangerous!

METHODS OF MININGOPEN PIT MINING and STRIP MINING

Open pit mining digs a large hole in the earth The mine goes down in layers, removing the soil

(overburden) until the ore layer in exposed The overburden is stockpiled for later mine reclamation The ore is processed , usually on site, producing tailings

and ore concentrate

Strip mining just goes down the face of a hill. Strip mining, - destroys whatever land it is on.

Both open pit mining and strip mining are can be very destructive. In the mountains strip mining will destroy vegetation and

lead to mud slides and severe soil erosion.

Typical strip mine operationsblastingblasting

overburdenoverburden Coal Coal removedremoved

removalremoval

Loose rockLoose rock

Open Pit, Strip Mine, Dredge

Methods of MiningDREDGING

Dredging is another mining process that affects our environment. The dredging machine goes along the bottom of a lake or

pond and scoops material onto a conveyer belt. Once the material is on the conveyer belt the dredger

sorts it and dumps the waste material behind it.

When the dredger dumps the waste product behind it, the sediments and large rock types pollute the water and kill more wild life and vegetation.

The only reason why the dredger does not receive as much criticism as the open pit and strip mines, is because you can't see it.

Cyanide extraction for Gold PROCESS: Preparation

ground into fine powder – mixed in solution of NaCN (sodium cyanide) & water referred as SLURRY

PROCESS: extraction Zinc powder is added to this gold-cyanide solution – which precipitates out the

gold - The mixture then goes thru a filter where the precipitate sticks to a heavy

canvas filter which is later cleaned to remove the gold. Extreme heat is applied which burns off the Zn

ENVIRONMENT: Gold mines make sure that cyanide doesn't escape, by using containment

systems and recycling the water. Entire operation must be kept on the alkaline side – HCN is volatile and

poisonous

ALTERNATIVE PROCESSES Gold extraction by Mercury. Elemental Hg forms an amalgam with many metals,

such silver and gold. Mercury boiled off, precious metals remain. Practised in Central America 1570-1900, and in Brazil and Africa until now.

Extraction via Flotation: Froth flotation - a process where valuable minerals are separated from

tailing by inducing them to gather on the surface of a froth layer. This process is based on the ability of certain chemicals to modify the surface

properties of the mineral(s). Other chemicals are used to generate the froth and still others are used to

adjust the pH. The process of froth flotation involves crushing and grinding the ore to a

fine size This separates the individual mineral particles from the waste rock and other

mineral particles. The grinding is normally done in water with the resultant slurry called the pulp.

The pulp is processed in the flotation cells. Here the pulp is agitated with a flotation mixture Fine bubbles are introduced

The ability of a mineral to float depends upon its surface properties. Chemical modification of these properties enables the mineral particles to

attach to an air bubble. The air bubble and mineral particles rise through the pulp to the surface of the

froth. Even though the air bubbles often break at this point, the mineral remains

on the surface of the froth. The mineral is physically separated from the remaining pulp material and

is removed for further processing.

Crushing

Toxicity of mine tailings

Tailings from copper mines generally have high levels of copper and other heavy metals.

Tailings derived from gold mining operations often contain high levels of many types of heavy metal such as arsenic, cadmium, mercury and lead

Toxicity in the marine environment can also result from the chemicals that are added in the ore refining process It is often the case that these chemicals are the most toxic

components of the tailings streams. Gold mines typically use cyanide - one of the most toxic

chemicals to marine life Nickel mines often use ammonia in the extraction process All refining operations use a variety of detergents, frothing

agents and diesel fuel in their operations. Some are inevitably lost to the environmnet

Submarine Tailings Discharge (STD)

Aim of STDs - is to deposit mine wastes in deep stratified waters It is likely the tailings will be trapped below the mixed surface

layer They and flow as a dense slurry to a deposition site on the

deep ocean floor. Recently new mines in the Asia-Pacific region have tended to

discharge tailings at a depth of around 80-100m although some mines in the Indonesia/Papua New Guinea region discharge below this level.

It has been often stated by the mining industry that in much of the Asia-Pacific Region, the land is unsuited to the construction of tailings dams due to rugged mountains, regular earthquakes and high rainfall. These factors increase the likelihood of a disastrous dam

failure. But as tailings dams require continuous monitoring and post-

mine remediation, STD is also cheaper in the long term.

Typical Submarine Tailings Discharge

STD operation

When tailings leave the STD pipe, the action of fast moving tailings entering stationary seawater generates turbulence and mixing. As a result a 'plume' of turbid, dirty water is formed. The dirty water from the plume can be expected to

spread for kilometers from the STD outfall. It may not be possible to see this from the surface, but

100-200m underneath, the water is dirty. For a certain distance from the outfall, the tailings

flow as a mass, without much mixing (only slightly) with the surrounding water. This is because the properties of the tailings are

different from the sea water, both in temperature and density.

Generally, water bodies with different properties do not mix well.

STD discharge – ideal case

Dangers in STD operation

In STD operations - tailings will enter the sea at different depths and will spread out for many kilometers. It is also possible that these tailings will not mix rapidly with the surrounding water, so will maintain their high concentrations of tailings for many kilometers from their source.

A concern - tailings can be carried into the surface waters through upwelling - a phenomena common in our region. Upwelling - the movement of water from the deep sea to the surface layers can be generated by tides, but is more commonly induced by winds and currents.

In regions prone to upwelling there is the potential for catastrophic quantities of tailings to be carried into the coastal near-shore environment.

Dangers in STD operation

In different regions there is often the potential for storms and other gradual processes of to bring tailings back to the surface. Recall, that stratification suppresses mixing Weak pycnocline or storms encourage mixing

It is important to carry out monitoring in waters receiving mine tailings

The main things to monitor include water quality toxicity benthos and plankton fish abundance

CASE STUDY – OK TEDI MINE IN PNG

Fly River – Ok Tedi Mine Site

Fly River

Fly River Estuary

Before

after

Ok Tedi River - After

Mammoth Mammoth trucks dump trucks dump 80,00080,000 tons of tons of waste every waste every day at the day at the edge of the Ok edge of the Ok Tedi mine in Tedi mine in the Star the Star Mountains, Mountains, forming a forming a huge heap, huge heap, then...then...

The heap of waste is designed to wash into the river as it rains. This is called an "erodible waste dump". A slurry of tailings, another kind of mine

waste left after the ore is processed, is also piped directly into the Ok Tedi catchment.

The murky, waste laden Ok Tedi runs down the mountain, then joins the larger Fly River. The slower moving lowland river allows the waste to settle, diverting the original course of the river and forming bars. As this photo shows, the river is choked and boat travel on the river becomes difficult or impossible. The waste has killed 70 to 90% of the fish.

Environmental effects on People who live here rely on fish they

catch in the river mouth and ocean. There is strong evidence that fish are

made sick by the mine waste, probably by heavy metals such as copper.

The number of fish caught has dropped severely.

Much of the waste washes down to the mouth of the Fly river.

• Sedimentation from increased waste rock/tailing

disposal BEFORE: Fly river carried about 100ppm natural

sediment NOW: Fly river carries about 450 –500ppm Aggradation – build up of sediment in river bed. This raises the level of river bed. Increased frequency in flooding of banks, forest floor buried, floods have deposited sediments up to 1m.

Flooding causes loss of vegetation Water logging of sediments causes reduced oxygen levels

in soil Starving the roots of oxygen, stressing vegetation and

eventually killing them.

Other Environmental effects

During times of high river water levels, the mine waste is transported up onto and over the river banks. At times like this when the river backs up into smaller creeks and swamps, the waste goes with it. The result is thick layers of sludge from the mine left in places which are normally high and dry

Effects of increased sedimentation

The mine waste smothers people's riverbank food gardens, limiting their capacity to grow food for their families.

It also kills trees and other vegetation wherever it forms a thick layer over their roots.

This is called "dieback" and affects a large area along the river system below the mine.

The floatation process extracts 80% copper and 70% gold – rest is discharged in tailings and waste rock.

Flotation chemicals are also discharged About 80,000 tonnes of tailing and 120,000 tonnes of

waste rock are discharged DAILY Acid drainage can be a problem

The ore body is a sulphide – creating potential for acid generation in the waste dumps.

This acid can accelerate the release of other metals (for example lead, zinc, arsenic mercury) from the waste dump and into ground and surface waters.

The Ok Tedi and Fly rivers are naturally alkaline from alkaline minerals draining from a predominantly limestone catchment. Acidification changes the ricer chemistry and biology.

Toxicity and water quality