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Metals, minerals, mining and (some of) its problems
A short series of lectures prepared for the London Mining Network
24 April 2009
by
Mark Muller
Acknowledgments:
I acknowledge gratefully the extent to which I have leant on the work contained in several good text books:
Mine Wastes: Characterization, Treatment and Environmental Impacts, 2nd Edition, by Bernd Lottermoser, 2007. Springer, Berlin Heidelberg.
Mining and the Environment: From Ore to Metal, by Karlheinz Spitz and John Trudinger, 2009. CRC Press, Leiden.
Introductory Mining Engineering, 2nd Edition, Howard Hartman and Jan Mutmansky, 2002. Wiley, New Jersey.
Thank you also to CAFOD, London, for suggesting and organising the workshop, and for covering my travelling expenses to London for the event.
Outline of lectures:
Topic 1: Ore mineralogy and orebodies
Topic 2: Mining
Topic 3: Ore processing and metal recovery
Topic 4: Mine wastes
Topic 5: Environmental and social concerns
Specific mining problems examined in some detail:
• Surface subsidence above underground longwall-mining panels
• Rockbursts in deep underground mines
• Tailings dam failures
• Riverine and submarine tailings disposal
• Cyanidation wastes and their attenuation (destruction)
• Radioactive uranium wastes and water contamination
• Sulphide wastes and acidification of waters
Topic 1: Ore mineralogy and orebodies
From a series of 5 lectures onMetals, minerals, mining and (some of) its problems
prepared for London Mining Networkby
Mark Muller [email protected]
24 April 2009
Outline of Topic 1:
• Elements and metals
• Types of minerals
• Radioactive elements, minerals and radioactive decay
• The process of oxidation
• Acids and alkalis
• Types of rocks and orebodies
• Examples of typical orebodies
e.g., iron, zinc, sulphur and oxygen are elements.
e.g., silicon-oxide (silica), iron-sulphide (pyrite) and tin-oxide (cassiterite) are minerals.
Metal-bearing minerals are the target of mining.Non metal-bearing minerals are referred to as gangue minerals.
e.g., granite, limestone, sandstone and gneiss are rocks.
“Orebodies” are rocks containing an enhanced percentage of metal-bearing minerals, high enough to be economic (i.e., mined at a profit), and a lower percentage of gangue minerals.
Elements are the building blocks of minerals
Minerals are the building blocks of rocks
Rocks are aggregates of minerals
Elements – minerals – rocks (orebodies)
MINING
LIBERATIONPROCESSING
METALLURGICALEXTRACTION
Recover target metal from mineral concentrate
Recover orebody from host rock
Liberate target minerals from rock and concentrate them
Metals enrichment factors
Metals require significant enrichment above their normal background levels in the Earth’s crust to form a mineable orebody.
Minerals are enriched to form orebodies through a wide range of different geological processes.
The enrichment factor required to make a mine viable (i.e., profitable – within today’s economic framework for minerals exploitation) will vary from time to time, depending on commodity prices, and the ease of extraction of both the orebody from the ground and the target metal from the orebody.
Figure from Spitz and Trudinger, 2009.
World production of non-fuel mineral commodities in 1999.
Table from Lottermoser, 2007.
Elements:
Elements are made up of atoms which consist of protons, neutrons and electrons. The number of protons (the “atomic number”) defines the “element”.
For example oxygen (O) has 8 protons, Uranium (U) has 92 protons.
In a well ordered fashion through the periodic table, the number of protons, neutrons and electrons increases, and the atoms (elements) become heavier and larger in diameter.
+
-
ElectronNegative electrical chargeNo mass
NeutronNo charge
ProtonPositive electrical charge
Hydrogen (H) atom
-
Lithium (Li) atom
+
+
-
-+
3 Electrons3 Protons4 Neutrons
1 “valence” electron in the outer electron “shell”
Periodic Table of Elements
“Metalloids”
“Rare Earth Elements”
Some definitions regarding metals:
• Metals are elements that are malleable, ductile, and conduct heat and electricity well – gold (Au), silver (Ag), copper (Cu), platinum (Pt) etc.
• Metalloids (or “semi-metals”) are elements with both metallic and non-metallic properties, and have a lower ability to conduct heat and electricity – boron (B), arsenic (As), antimony (Sb), bismuth (Bi), selenium (Se) and tellurium (Te).
• Heavy metals are those metals with a density greater than 6 g/cm3: Fe, Cu, Pb, Zn, Sn, Ni, Co, Mo, W, Hg, Cd, In, Tl. (Gold ~18 g/cm3)
• Base metals are those metals that tend to be used in industry by themselves, rather than alloyed with other metals – Cu, Pb, Zn, Sn.
Making minerals from elements:
The sharing of electrons by different elements forms the basis of the creation of compounds.
Minerals are compounds – combinations of elements held together by the forces established through the sharing of electrons.
Gold (Au) is stable and unreactive, and forms no compounds in nature.
An ion is an atom or molecule (compound) which has lost or gained one or more electrons, giving it a positive or negative electrical charge.
Anions a negative charge (e.g., CN-).
Cations have a positive charge (e.g., H+)
H+ cation lies at the root of acid mine drainage
CN- (cyanide) anion is the basis of cyanidation waste problems.
Minerals:
A mineral is often crystalline in form.
The crystal lattices of minerals hold metal elements very tightly.
Aggressive chemical means, or large amounts of thermal or electrical energy, are therefore required to liberate the metals from their host minerals
http://en.wikipedia.org/wiki/File:Pyrite_foolsgold.jpg
Model of the crystal form of the titanium oxide mineral rutile (TiO2)
http://www.hgs-model.com/gallery/index.html
Ti atoms
O atoms
The mineral pyrite (FeS2) in its characteristic cubic crystal form
“Classes” or groups of minerals:
Significant metal-hosting minerals
- Native metals: pure metals or metal alloys- Oxides: compounds with oxygen (O)- Sulphides: compounds with sulphur (S)
Minerals primarily of “industrial” interest
- Silicates: Si-O- Carbonates: CO3
- Halides (salts): Cl
Minerals hosting interesting metals (and with some industrial interest)
- Sulphates: SO4
- Phosphates: PO4
- Borates: B-O
Metal-bearing minerals:
Native metalsA native metal is a metal found in its metallic form in nature. Only gold, silver, copper and platinum metals occur in nature in
exploitable amounts.
All mined gold is native metal, alloyed with up to 15% silver. There are no common naturally occurring gold oxides, sulphides or other
minerals.
Prospector B. O. Holtermann with 286 kg solid gold nugget found in 1872 at Hill End, NSW, Australia. From Spitz and Trudinger, 2009.
http://en.wikipedia.org/wiki/File:SilverUSGOV.jpg
Native silver (Ag). Source: US Government.
Native copper (Cu) about 4 cm in size. Credit: Jonathan Zander.
http://en.wikipedia.org/wiki/File:Native_Copper_Macro_Digon3.jpg
Metal-bearing minerals:
Metal oxides Are simple compounds with the element oxygen (O). Metals are relatively easily extracted from oxide minerals. Examples include:
Hematite: Fe2O3 Ilmenite: FeTiO3 Rutile: TiO2 Cassiterite: SnO2
Coltan (Columbite-Tantalite): (Fe,Mn)(Nb,Ta)2O6
Cassiterite (SnO2). Source: US Government.
http://en.wikipedia.org/wiki/File:CassiteriteUSGOV.jpg
Hematite (FeO2) “kidney ore” from Michigan. The yellow is the reflection of a lamp used for lighting.
http://en.wikipedia.org/wiki/File:Hematite.jpg
Metal-bearing minerals:
Metal sulphidesAre simple compounds with the element sulphur (S). Metals are less easily extracted from sulphide minerals, and are often oxidised first, as the initial stage in metal recovery. Examples include:
Chalcocite: Cu2S Sphalerite: ZnS Galena: PbS
Pyrite: FeS2
http://en.wikipedia.org/wiki/File:Sphalerite4.jpg
Galena (PbS) crystal.
http://en.wikipedia.org/wiki/File:GalenaKansas.jpg
Aggregate of Sphalerite (ZnS) crystals. Credit: Andreas Früh
http://en.wikipedia.org/wiki/File:Cinnabar09.jpg
Cinnabar (HgS), Buckskin Mnts., Nevada. Credit: Chris Ralph.
“Rock-forming” minerals:
Silicates Are compounds with silicon-oxygen (Si-O) and occur in many different crystal forms.
Silicates all contain metallic elements, but it is currently not possible to extract the metals from them, so interest in silicate minerals lies in their industrial uses. Examples include:
Quartz (silica): SiO2 Beryl (emerald): Be3Al2(SiO3)6
Muscovite (mica): KAl2(AlSi3O10)(F,OH)2
http://en.wikipedia.org/wiki/File:Emerald_rough_300x422.jpg
Beryl Be3Al2(SiO3)6 variety emerald
Crocidolite (blue) asbestos Na2Fe2+
3Fe3+2Si8O22(OH)2
from the now closed mine at Wittenoom, Western Australia. Credit: John Hayman.
http://en.wikipedia.org/wiki/File:Blue_asbestos.jpg
Other minerals of interest:
SulphatesFormed with sulphur-oxygen (SO4).
Gypsum: CaSO4∙2H2O - used in cement
BoratesFormed with boron-oxygen (B-O) and are exploited for the metalloid element boron (B).
Borax: Na2B4O7∙10H2O Ulexite: NaCaB5O9∙8H2O
CarbonatesFormed with carbon-oxygen (CO3). They are easily dissolved in acids, and are able to neutralise acids.
Calcite, limestone: CaCO3 - limestone is the main component of cement
Other minerals of interest:
Phosphates Formed with phosphorous-oxygen (PO4).
Phosphates exploited for Rare Earth Element (REE) metals and thorium (Th):
Monazite: (Ce,La,Pr,Nd,Th,Y)PO4 - radioactive, due to thorium, and the most common ore of
thoriumApatite: (Ca,Sr,Ce,La)5(PO4)3(F,Cl,OH) - a non-radioactive source of
REEs
Apatite, variety fluorapatite (Ca5(PO4)3F from Mexico. Credit: Chris Ralph
http://en.wikipedia.org/wiki/File:Apatite09.jpg Phosphates are also mined to obtain phosphorus for use in agriculture and industry:
Phosphate: H3PO4
Other minerals of interest:
Halides (salts)Salts are “evaporite” minerals formed with chlorine (Cl).
They are easily dissolved in water and are often mined in-situ using solution-mining methods.
http://en.wikipedia.org/wiki/File:Mineral_Silvina_GDFL105.jpg
Sylvite (KCl). Credit: Luis Miguel Bugallo Sánchez.
Halite (rock salt): NaClSylvite: KCl - fertiliser industry
Oxidation:
Oxidation is a reaction with oxygen that results in the breakdown of minerals.
Metallic sulphide minerals (e.g., pyrite) oxidise in the presence of water and oxygen to:
• produce acids and • release dissolved metals into water.
Note:“Oxidised” sulphide minerals are not the same as “primary” oxide
minerals.
A primary oxide of iron is hematite: Fe2O3 Oxidation of pyrite (FeS2) produces iron-hydroxide: Fe(OH)3
Acids and alkalis and pH:
Anything that reacts with an “acid” is called an “alkali”.
pH is a measure of the acidity or alkalinity of a solution.
Acidic pH less than 7 (lemon juice = 2, battery acid = 0)Neutral pH equal to 7 (distilled water)Alkaline pH greater than 7 (household ammonia = 11)
They neutralise each other through the following reaction:
H+ + OH- H2O
Figure from: http://en.wikipedia.org/wiki/File:PH_scale.png. Credit Stephen Lower
ACID
ALKALIAcid Alkali Water
Rocks and orebodies:
Rocks and orebodies are aggregates of different minerals.
Orebodies have high concentrations of metal bearing minerals and are hosted in barren “country” rock.
Mined country rock is referred to as gangue or waste.
Volcanic, sedimentary and metamorphic processes form rocks and minerals.
Hydrothermal fluids associated with volcanic and metamorphic processes contain high concentrations of dissolved metals and also form ores
Igneous rocks
Igneous rocks are formed when molten magma cools and crystallises either on the surface or at depth in the crust.
Examples: granite, basalt, kimberlite.
An outcrop of orbicular granite. Locality: Orbicular Granite Nature Sanctuary, near Caldera, Chile.Photo credit: Herman Luyken
http://en.wikipedia.org/wiki/File:2005.11.08_005_Granito_Orbicular_Caldera_Chile.jpg
Sedimentary rocks
Sedimentary rocks are formed by deposition of
• clastic sediments derived from the erosion of other rocks (mud, gravel, sands)• organic matter• chemical precipitates (evaporites)
followed by burial and compaction of the material.
Examples: Sandstone, conglomerate, limestone, coal, potash.
An outcrop of conglomerate overlying sandstone. Locality: Point Reyes, Marin County, California.
http://en.wikipedia.org/wiki/File:Conglomeratereyes.jpg
Metamorphic rocks
Metamorphic rocks are formed when any rock type is subjected to high temperature and pressure.
Examples: marble (from limestone precursor), quartzite (from sandstone precursor), gneiss (from granite precursor).
Banded gneiss, formed by high pressure compression that aligned minerals, forming a layered fabric. Locality: Skagit Gneiss Complex, North Cascades Range, Washington, USA.Credit: US Geological Survey
http://en.wikipedia.org/wiki/File:Conglomeratereyes.jpg
Ore genesis:
Enrichment of metal-bearing minerals occurs in specific geo-tectonic settings in response to specific geological processes.
These geological settings and processes produce different types of orebodies, with “classic” mineral assemblages/combinations, e.g.:
Massive iron-orePlacer (alluvial) goldMassive copper sulphide + goldMassive lead-zinc sulphideLayered igneous intrusions: platinum, palladium, chromiumNickel laterite and bauxiteDiamondiferous kimberliteAlluvial diamond Mineral sandsCoal
Massive sulphide lead-zinc deposit, Black Angel Mine, Greenland (1973 – 1991)
http://www.angusandross.com/AR-NEW/pages/proj-black-angel.htm
http://www.angusandross.com/AR-NEW/pages/proj-black-angel-phase1.htm
Black Angel Mine exploited a massive sulphide lead-zinc deposit (sphalerite, galena and pyrite) hosted in marble and metasediments. Ore-grades of 12.5% Zn, 4.1% Pb, 30 ppm (g/ton) Ag were reported (Asmund et al., 1994). The massive sulphide orebodies are developed sub-parallel to metamorphic banding in the country rock, and were mined using a room-and-pillar method.
Massive sulphide ore (dark band) showing in a support pillar left remnant after cessation of mining in 1990. (From: Black Angel News, 2005).
SIMPLIFIED CROSS-SECTION THROUGH BLACK ANGEL MINE
Massive sulphide
orebodies
Approx. 9 km
600 m
3 m
Cable car access point into mine
Kimberlite diamond deposits
Figure from McCarthy and Rubidge (2005)
Diagram showing the structure of a kimberlite volcanic pipe.
Kimberlite volcanic pipes are the hosts of “primary” diamond deposits.
Both the volcanic magmas and the contained diamonds originate at depths of about 170 to 200 km below the Earth’s surface, and are brought to surface during a very rapid and explosive eruption events.
Kimberlite pipes are subsequently eroded through geologic time, exposing deeper parts of the pipe, and developing “secondary” deposits of alluvial diamonds that are found in river beds, flood plains, and offshore as marine deposits.
Diamond grades in kimberlite pipes are highly variable, and some pipes are completely barren (for good geological reasons). Some reported grades lie in the range 0.28 – 7.5 carats per ton (Roberts, 2007, pg 68).
Secondary alluvial diamond deposits may be significantly enriched in diamonds as the process of erosion “concentrates” heavy, resistant minerals.
http://en.wikipedia.org/wiki/File:Udachnaya_pipe.JPG
Udachnaya Pipe, Sakha Republic, Russia, in the summer of 2004. Credit: Alexander Stepanov.
Palaeo-placer gold deposit - Witwatersrand Basin, South Africa
Gold and carbon nodules with “buckshot” pyrite in conglomerate reef from the Witwatersrand Basin, South Africa. Figure from McCarthy and Rubidge, 2005. Photo credit: Goldfields.
1 cm
The Witwatersrand Basin in an ancient (2.8 billion years old) palaeo-placer deposit, consisting of multiple stacked and alternating shale, sandstone and thin conglomerate sedimentary bands.
The gold mineralisation is found in the conglomerate bands (called “reefs”), typically between 5 to 100 cm thick. The gold was either introduced at the time the sediments were deposited, or was introduced later by gold-bearing hydrothermal fluids (or both).
The sedimentary basin subsequently suffered extensive deformation, producing folds and faults that disrupt the deposit. Faults impact significantly on safe (and efficient) mining.
Underground mines operate up to a maximum depth of about 4,000 meters. Mineable grades in a deep goldmine operations are of the order of 10 – 20 g/ton.
Many of the reefs contain accessory uranium, which is processed as by-product on several mines.
Geological cross-section through the Welkom Goldfield. Figure from McCarthy, 2006
Gold
Pyrite
Carbon
Quartz
Nickel laterite ore deposits are the surficial, deeply weathered residues formed on top of ultramafic rocks that are exposed at surface in tropical climates. They are found widely in New Caledonia, Cuba, Australia, Papua New Guinea, the Philippines, and Indonesia, and are estimated to comprise about 73% of the world continental nickel resource.
Two kinds of lateritic nickel ore can be distinguished: limonite (oxide) types and saprolite (silicate) types.
Nickel laterite deposits
http://en.wikipedia.org/wiki/File:River_South_New_Caledonia.JPG.JPG
A Creek in southern New-Caledonia. Red colours reveal the richness of the ground in iron oxides, and nickel.
Mg RICH “ULTRAMAFIC” ROCK0.3% Ni
Olivine and pyroxene
(silicate minerals)
SAPROLITEZONE1.5 - 2.5% Ni
Serpentine(hydrated silicate)
Goethite(hydrated oxide)
LIMONITE ZONE1- 2% Ni
Limonite zone
Deep downward penetration of water producing weathering
The process of oxidation and weathering depletes the original mafic rock of Mg and Si, and concentrates Fe and Ni in the weathered zone.
Near surface upward evaporation of water precipitates Fe, Ni oxide
OREBODY
Radioactive elements:
In radioactive elements, the configuration of the nucleus is unstable, and breaks down, emitting radioactive “decay” products:
alpha, beta and gamma radiation.
Isotopes of an element have nuclei with the same number of protons but different numbers of neutrons.
Some isotopes are stable, and others subject to radioactive decay.
Modified from http://en.wikipedia.org/wiki/Alpha_particle
Helium nucleus
Electron
Energy(electromagneticradiation)
Alpha radiation is readily stopped by a sheet of paper.
Beta radiation is halted by an aluminium plate.
Gamma radiation is eventually absorbed as it penetrates a dense material. Lead, being dense, is good at absorbing gamma radiation – several centimeters of thickness is needed.
Radioactive elements:
A parent nuclide is an element that undergoes radioactive decay, producing a daughter nuclide, that may be a different element.
Parent DaughterU-238 decays to form Th-234 by releasing an alpha particle.92 protons 90 protons146 neutrons 144 neutrons
The daughter nuclide may itself be stable or unstable (i.e., radioactive).
The half-life is the time taken for half the radionuclide's atoms to decay. Half-lives vary between more than 1019 years, for very nearly stable nuclides, to 10−23 seconds for highly unstable ones.
Uranium radioactive decay series – and half-lives
Table from Lottermoser, 2007, and references therein.
Series ends with stable lead isotope
Series starts with radioactive isotope
Uranium-238 (92 protons, 146 neutrons)
The SI unit of radioactive decay is the Becquerel (Bq).
One Bq is defined as one decay per second.
Radioactive uranium minerals:
The main “primary” ore in uranium deposits is
Uraninite: UO2
Other important “primary” uranium ore minerals are:
Brannerite: (U,Ca,Y,Ce)(Ti,Fe)2O6 – a mixed uranium, iron, titanium oxide mineral.
Coffinite: USiO4∙nH2O – a hydrated uranium silicate
Pitchblende – an amorphous, poorly crystalline mix of uranium oxides often including triuranium octoxide: U3O8.
“Daughter” nuclides are trapped in uranium minerals or escape
Uraninite: UO2
100% uranium
Uraninite: UO2
75% uranium has decayedto daughter radionuclides.
Some daughters will remain trapped in the mineral, or they migrate elsewhere in the orebody to form other minerals
At the time the mineral isformed in orebody 1 Billion years later
Radioactive minerals:
The “primary” uranium minerals weather and break down very easily when exposed to water and oxygen, to produce numerous “secondary” (oxidised) minerals, for example carnotite and autunite, which are often mined, but in significantly lower quantities that uraninite.
Uranium is also found in small amounts in other minerals: allanite, xenotime, monazite, zircon, apatite and sphene.
http://en.wikipedia.org/wiki/File:Pichblende.jpg http://en.wikipedia.org/wiki/File:Carnotite-BYU.jpg
Carnotite K2(UO2)2(VO4)2·3H2O, An important “secondary” uranium-vanadium bearing mineral, from Happy Jack Mine, White Canyon District, Utah, USA. Credit: Andrew Silver.Uraninite (pitchblende) UO2
