basics of mining and mineral processing

www.pwc.com

20 12 Am er i ca s Sch ool of Mines Basics of Mining and Mineral Processing

W Scott Dunbar University of British Columbia

Agenda Geological Concepts Mining Methods Mineral Processing Methods Mine Waste Management Mining and Money A Future of Mining

The m ain topic s Crushing and

Flotation of

Smelting and

grinding

sulfides

refining

Pressure oxidation of concentrate Solution extraction Electro‐winning

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Gold ore processing

Mineral Processing Methods

3

O th er topi cs



Coal

Diamonds

Oil sands

Bioleaching

Uranium

Physical separation

Industrial minerals PwC


4

All th e che m is tr y y ou n eed t o k n ow

M eet atom A electron

+ + + + + + ++ + + + +

nucleus with protons

In this case 12 electrons 12 protons

Nucleus contains positive charges Each electron has a negative charge Number of positive charges = number of negative charges PwC


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Th e pos i t iv e ion A

+

Take away one electron

+ + + + + + ++ + + + +


Atom A becomes a positive ion A+ A  A+ + e PwC


7

Th e n ega t i ve i on A

‒

Add one electron

+ + + + + + + ++ + + +


Atom A becomes a negative ion A‒ A + e  A‒ PwC


8

Similarly Take away two electrons ++

2+

A  A + 2e (or A + 2e) Add two electrons A + 2e  A2‒ Can be generalized to n electrons if atoms will allow it

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I ons exis t i n solu ti on (y ou c an ’t t ouc h t h em ) Salt or sodium chloride NaCl (s)  Na+(aq) + Cl‒(aq) s – solid aq – in aqueous solution

Na+ Cl

‒

Na  Na+ + e ‒

Cl + e  Cl

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The goal of pr

oc essin g a n d r efin in g m eta ls

is to get the metals into solution as positive ions Some examples:

These are easier to ionize

Copper

Cu +2

Gold

Au +

Lead

Pb +2

Zinc

Zn +2

This is hard to do

Some metals ionize more easily than others

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An d on ce th ey ar e in solu ti on … electricity can be used to add electrons to the metal ions and “plate” them as solids onto a solid surface

www.csiro.au/helix/sciencemail/activities/CopperCoat.html PwC


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K itc he n che m is tr y (y ou c an do thi s)

9V battery snap with alligator clips

Copper sulphate from garden stores

Glass container

http://www.csiro.au/helix/sciencemail/activities/CopperCoat.html/ PwC


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Cr us hin g and G r indin g

G y r ator y cr u she r – fir st the blas t, th en thi s The spindle of the crusher moves eccentrically about the vertical axis

Hydraulic hammer Top of spindle

The rock is crushed between the spindle and the inner shell

Result: 10‐50 mm size particles

www.sandvik.com PwC


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Note s: G y r ator y Cr us he r

Crushing is the second stage of rock breakage or comminution, the first stage being blasting. Primary crushing is often done in the pit or underground. For hard rock a gyratory crusher is often used. The goal is to reduce rock particles to 10 ‐50 mm size. The rotation speed of a gyratory crusher is 85 ‐100 rpm. The picture on the right shows the top of the spindle of a gyratory crusher. A pneumatic rock breaker is also shown. This is operated by a human whose job is to use the breaker to break up the large fragments. Blasting should have broken all the rock into a smaller size. Secondary or even tertiary crushing might be necessary in the mill to ensure that rock breakage occurs to the required size. Secondary and tertiary crushing would be done by a cone crusher (see picture at right) the operation of which is similar to a gyratory crusher except that the conical crushing head is supported from below rather than by an overhead spider. The feed to the crushing head is from a large bowl. Cone crushers operate at higher rotation speeds than gyratory crushers. www.metsominerals.com

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Bagd ad: I n -pi t c r u sh er , con ve y or , an d stockpile

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Tw in in -pit c r us he r s an d c onve y or s at H VC

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AG an d S AG M ills – th e coar se gr in d Autogenous (AG): ore tumbled in water to “self‐grind” the ore particles ‐autogenous (SAG): Semi ore particles and steel balls tumbled with water

Result: <10 mm size particles

SAG mill Huckleberry Mine

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Not es: AG an d S AG m ill s – th e coar se gr in d Autogenous (AG) mills use large particles of ore as grinding media. For an ore to successfully grind autogenously, the ore must be hard and it must break along boundaries between mineral grains to produce particles large enough to grind the remaining particles to sufficiently fine size. If an ore cannot be ground autogenously to sufficiently fine sizes, semi ‐autogenous grinding is used in which steel balls and the ore itself are tumbled to break the ore. Autogenous grinding has two advantages, (1) it reduces metal wear and (2) the use of large ore particles as grinding media means that the need for secondary and tertiary crushing stages is reduced or eliminated. AG and SAG mills are available for both wet and dry grinding. The diameter of AG and SAG mills is normally two to three times the length. Larger diameter mills are common in North America while longer mills are more common in Europe. A large diameter mill relies on the rocks and balls falling through a large distance to break up the ore while a long mill relies on longer residence time. The size of the feed to a AG/SAG mill can be large and is limited to that which can be fed to the mill by conveying systems. Because of this the need for secondary and tertiary crushing is often eliminated. AG/SAG mills can also grind ore with high moisture and clay content, which is otherwise difficult to do.

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I n sid e a la r ge SAG m ill

Liner replacement in Highland Valley SAG mill

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Ball M ill – the f in e gr in d

www.porcupinegoldmines.ca

Result: partícles of size ~0.075 mm PwC


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Note s: Ball m

ill – th e fine gr in d

Grinding mills break up the ore particles into finer particles with a range of sizes. A ball mill grinds material by rotating a cylinder with steel grinding balls, causing the balls to fall back into the cylinder and onto the material to be ground. Grinding action is by impact. Ball mills are used to grind material 0.25 inch and finer down to a particle size between 20 to 75 microns (0.0008 to 0.003 in). The rotation is usually between 4 to 20 revolutions per minute, depending on the diameter of the mill; the larger the diameter, the slower the rotation. If the peripheral speed of the mill is too great, the mill begins to act like a centrifuge and the balls do not fall back into the center of the mill, but stay on the perimeter.. The point where the mill becomes a centrifuge is called the “critical speed", and ball mills usually operate at 65% to 75% of the critical speed. The power requirements of ball mills depend on the energy required to grind the feed particles to a particular size and on the dimensions and operating conditions of the mill.

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Cy clon e – sepa r at e coar se fr om fin es Fines Inlet

Separates coarse‐grained particles from particles in a slurry. Also called classification.

fine ‐grained

Slurry pumped in at high pressure. Creates low pressure in center of the cyclone (as in a tornado) Fine‐grained particles to the top ‐ overflow Coarse‐grained particles to the bottom ‐ underflow

Coarse

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G r in din g C ir cuit at Bagdad Concentrator capacity 75,000 tpd

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Note s: G r in din g C ir cui t at B agdad

Crushers, AG mills and SAG mills, ball mills, and cyclone separators are configured into grinding circuits depending on the way the ore breaks up into finer sizes which depends mostly on the hardness of the ore. The distribution of the size of particles resulting from one component of a grinding operation governs configuration of thesecondary grinding crushing circuit and the equipment used in the grinding circuit. Grinding the circuits typically involve or regrinding, cycling particles from the output of one unit back to the input of the unit. At Bagdad five grinding circuits in the mill process about 3000 tons of ore per hour. The output of an AG mill is fed into a screen. The coarse material from the screen is passed to a cone crusher and fed back into the AG mill. The cone crusher is used to break up larger particles which would otherwise simply cycle through the AG mill. The fine material from the screen is fed into a closed ball mill circuit. The output of the ball mill is separated into coarse and fine fractions in a cyclone, the coarse fraction (underflow) is recycled and the fine fraction (overflow) is pumped to the flotation tanks. In the absence of AG or SAG mills, there would be a rod mill followed by a closed circuit ball mill. However, a rod mill is less efficient at grinding rock than an AG or SAG mill.

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Bagda d G r in ding Cir cuit – the pic tur e

Cyclones

Autogenous mill

Ball mill

Screen

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G r in din g C ir cuit at H ighland Valle

y

Concentrator capacity 135,000 tpd

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Note s: G r in din g C ir cui t at H ighla n d Valle y

At Highland Valley there are five parallel grinding lines which process a total of 5400 tonnes of crushed ore per hour. Two of the grinding lines employ autogenous mills (AG) and three employ semi ‐ autogenous (SAG) mills. Each mill feeds two closed ‐circuit ball mills which reduce the ore to sand‐sized particles which feed the flotation circuits. Each grinding circuit grinds and re ‐grinds to ensure that the entire feed is reduced to sand size. The ore exiting the AG or SAG mill is fed into vibratory grizzly feeders which separate the ore into undersize and oversize. The undersize goes to the ball mill circuit while the oversize returns to the AG or SAG mill. The ball mill circuits employ cyclones to separate sand from coarser particles. Coarse particles are returned to the ball mill while finer sand particles (the overflow) go to the flotation cells. It is usually not possible to distinguish an AG from a SAG mill based on its appearance. Why are there two ball mills at HVC and one at Bagdad. Partly this is related to the larger tonnage throughput at HVC, approx 1150 tons per hour versus 600 tons per hour at Bagdad. However, it is also related to the power required to grind the rock into particles fine enough for flotation, Since there is a limit to the size of a ball mill, the harder the rock, the more mills that are needed to deliver the power. This does not necessarily mean that the rock at HVC is harder than that at Bagdad. The mill at HVC is a combination of machinery from other mills and it may be that it was “good enough at the time”.

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A gr in din g c ir cuit at H ighland Valle

y

cyclones

ball mill

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SAG mill


30

E n er gy cons um ption of cr us hin g an d grinding Largest consumer of energy at a mine site is crushing and grinding ‐50 mm Crushing: Grinding: from from >50mm <10mm to to 10 0.075mm 30 25 n o t/ 20 s r u o 15 h tt a 10 w o il k 5

0 PwC

s v e i s lo p x E

)t i p n (i g n i h s ru c ry a irm P

g n i h s ru c y r a d n o c e S

g n i d n ri G

particularly grinding because of the larger size change

www.elorantaassoc.com


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Flota t i on of Su lfi de s

Flotat ion – th e bas ic id ea

Frother makes froth stiff and stable Frothers are alcohols Add collector

Add frother

Air Concentrate

Slurry from grinding

To next flotation cell

Collector makes sulfide particles hydrophobic

bubble

Collectors are like soaps sulfide particle PwC


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N ote s: Flotat

ion – th e bas ic ide a

Froth flotation is the most common method for separating sulfide minerals from each other and from waste minerals or gangue. The particles from the grinders are mixed with water to form a

pulp in a flotation cell. An organic

chemical a collector is added. It selectively theYou surface of the mineral of interest and renders itcalled , meaning literally “afraid of coats water”. all have used a collector called soap; hydrophobic soap coats dirt particles rendering them hydrophobic. A stream of air bubbles is passed through the pulp. Being hydrophobic, the particles attach to the bubbles which, of course, are filled with air. The bubbles float to the surface and collect in a froth layer that either flows over the top of the cell into a channel at the base of the cell. (Some froths are thick and may have to be skimmed.) A frother, such as a long chain alkyl alcohol, is added to stabilize the froth layer. The froth on a beer will float things (yuk!), but the froth is not stable so beer cannot be used in sulfide flotation. The first use of flotation to separate sulfides was at the Broken Hill mine in Australia where they used eucalyptus oil as a collector. Collector chemistry has advanced considerably since then so that different metal sulfides in an ore can be sequentially floated by the use of different types of collectors and adjustment of the chemistry (typically the acidity) of the cell.

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Soa ps an d c oll ect or s Soap and collector molecules have a similar structure – one end is hydrophilic, the other hydrophobic

Hydrophobic ends of soap molecule attach to dirt or grease

hydrophilic end hydrophobic end

Water flow

Hydrophilic ends of collector molecule attach to sulfide particle bubbles PwC


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The fr oth – coppe r conc en tr at e

Wet concentrate ~27% copper

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Not es: The fr oth – coppe r conce n tr at e A simple materials balancing can be used to determine the amount of ore, K, needed to produce one ton of concentrate. This is known as the concentration factor. At Highland Valley the ore grade is 0.43% Cu and the recovery of copper in the concentrator is 85%. The concentrate is 28% copper. Thus K(tons) × 0.0043 × 0.85 = 1(ton) × 0.28 From which K ~ 77 tons. This ignores ore dilution, d%, which adds a factor 1 ‐d to the left hand side of the above equation. If drilling and blasting are properly controlled, dilution at an open pit mine is small. There is an upper limit to the concentration of a metal in a concentrate depending on the mineral in the ore. This is the direct proportion by atomic weight of the metal to the molecular weight of the mineral. Some approximate atomic weights are given in the table below: Copper 64

I ron 56

Lead 207

Zinc 65

Sulfur 32

For a copper concentrate made from chalcopyrite (CuFeS 2), the copper concentration limit is 34.8%, i.e., 64/(64+56+2×32) = 0.348. Similarly the concentration limit of lead in a lead concentrate made from galena (PbS) is about 87% and for a zinc concentrate made from sphalerite (ZnS), the concentration limit is about 67%. A mine that has bornite (Cu 5FeS4) in its ore can achieve quite high copper concentrations; unfortunately bornite is relatively rare.

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F lotati on c ir cu it s Reagents (Collector)

Conditioner tank

Pulp slurry

Concentrate

1

3 Rougher cells 2

Scavenger cells

Tailings

Flotation cell banks at The rougher tailings are floated by the scavenger cell and re‐floated by the rougher cells

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Neves Corvo Copper/Zinc Mine Portugal


38

Not es: F lota ti on cir cu it s

On the left is a simple flotation circuit for mineral concentration. The numbered triangles show the direction of flow. In a conditioning tank the collector is added to the slurry (often called pulp) from the grinding circuit. The conditioned pulp [1] is fed to a bank of rougher cells which remove most of the desired minerals to produce a concentrate froth. The tails from the rougher flow [2] to a bank of scavenger cells where the pulp is re ‐floated and the froth is returned [3] to the rougher cells for additional treatment. The scavenger tailings is usually barren enough to be discarded as tails but in some cases may be sent to cleaner cells to be re‐floated. More complex flotation circuits have several sets of rougher, scavenger, cleaner and re‐cleaner cells, as well as intermediate re ‐grinding of pulp or concentrate. On the right is a picture of the bank of flotation cells (blue motor housings) at the Neves Corvo copper/zinc mine in Portugal. Recovery of metals by flotation varies depending on the complexity of the ore. For a simple ore containing only copper with some gold by ‐product recovery can be 90 ‐95%. Recovery is lower for polymetallic ores which may contain roughly equal proportions of desirable metals.

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Separ at ion of Cu an d M o c onc en tr at es In column vats at Bagdad (2005 quantities)

In column vats molybdenite concentrate 58% Mo Cu/Mo concentrate

Sodium Sodium hydrosulfide hydrosulfide

Pressure leach copper concentrate 27% Cu

Strips collector off chalcopyrite particles

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Smelter


40

Not es: S epar at ion of C u an d M o c on cen tr at es

Both copper and molybdenum minerals are floated in the first stage, leaving iron sulfides and other waste minerals behind as tailings. The concentrate is then sent to a column flotation vat and sodium hydrosulfide added to remove the collector from the surfaces of the chalcopyrite so that it sinks to the bottom of the vat. The molybdenite floats to the surface since it is naturally hydrophobic. The molybdenite (MoS 2) in the concentrate may be purified for use in lubricants. Almost all molybdenum ore is converted by roasting to molybdic oxide (MoO 3). The oxide may be added directly to steel to form a hard alloy that can withstand high temperatures; such alloys are used in making high ‐ speed cutting tools, aircraft and missile parts, and forged automobile parts. Other useful compounds of molybdenum include ammonium molybdat e, used in chemical analysis for phosphates; and lead molybdate, used as a pigment in ceramic glazes.

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Con cen t r at e logi st i cs i n BC New loader for copper concentrates

Bagged moly concentrate at HVC shipped east by rail PwC


Vancouver Wharves lead‐zinc concentrates in copper concentrates out www.pnwship.com/canada/concentrates 42

The gr ad e-r ecove r y bat tl e

Chalcopyrite particle

Chalcopyrite particle with non‐sulfide inclusion

Chalcopyrite particle with attached non‐sulfide crystal

Allow collector more time to adhere to chalcopyrite particles Result: increased recovery of all particles with chalcopyrite, but concentrate grade decreases PwC


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N ote s: Th e gr ad e-r ecove r y bat tl e

This is a common problem in all sulfide concentration processes. The flow rate and tank size are designed to give the minerals enough time to be coated with collector (commonly called activation). Recovery depends on the flow rate. As the input flow rate decreases, the sulfide particles have more chance to be exposed to the collector and adhere to the bubbles so that recovery increases. However, the grade of the concentrate decreases because more silicates are recovered along with the target sulfide. One solution is to use finer grinding. However, this can be costly and would only be done if there was the possibility of recovering valuable metals.

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Bagd ad coppe r conc en tr at e

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45

Sm eltin g an d Re fin in g

The com pe ti ti on for ele ctr on s Copper the nucleus

Solid copper has free electrons

(positive charge)

available. That’s why it’s a good conductor.

Sulphur wants the electrons

‒

Oxygen wants them more

an electron (negative charge)

Sulphur

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Oxygen


47

Sm elt in g of copp er con cen tr at e of copper concentrate Undo what Nature did when forming the sulfide sulfur dioxide

Add electrons to copper

oxygen Oxygen takes electrons off sulphur because oxygen wants them more

copper anode (95‐98% pure)

copper concentrate CuFeS 2

Add electrons to iron

Iron oxides (slag) PwC


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N ote s: S m elt in g of c opp er con cen tr at e Three chemical reactions involving copper sulfides occur in a smelter (1,200C). 1

chalcopyrite + oxygen 2CuFeS2

+

3O2

2

covellite + oxygen CuS + O 2

3

chalcocite + oxygen Cu2S + O2

 



iron oxide + covellite + sulphur dioxide



2FeO

+

CuS

+

SO

2

chalcocite + sulphur dioxide Cu2S + SO 2

 

copper + sulphur dioxide 2Cu + SO 2

The copper and iron oxide collect at the bottom of the furnace to form matte copper which is tapped off and burned in a converter furnace to remove iron oxides and sulphur resulting in blister copper. Oxygen in the blister is then burned off using natural gas to form anode copper which is 95 to 98% pure and must be refined to produce cathode copper which is 99.99% pure. Limestone (CaCO3) is added to the furnace. When heated it decomposes to calcium oxide (CaO) and carbon dioxide (CO2). Calcium oxide reacts with silica (SIO2) and iron oxide (FeO) which remain solid at 1,100 C to form calcium and iron silicates which melt to form a slag. The slag is lighter than matte so it floats on top of it from where it is removed and taken to a disposal site. PwC


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D ouble en tr y che m is tr y (in a s m elte r ) chalcopyrite + oxygen 2CuFeS2

+

5O2

 

copper + iron oxide + 2Cu

+

2FeO

sulfur +

dioxide 4SO2

Electrons Account  4S +4

Reaction

D e b it

(in 4SO2)

24

Credit

Sulfur

4S‐2

Copper

2Cu+1  2Cu (what is wanted)

2

Iron

2Fe+3  2Fe+2 (in 2FeO)

2

Oxygen 5O2  10O‐2 (in 2FeO and 4SO2)

20

Balance 24

24

Remember: You saw double entry chemistry here first!

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50

The Sm elte r at M iam i Ar iz ona Copper sulfides in concentrate Copper anode

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51

An d w hat about th

e sulph u r diox ide ?

That’s the SO2 that results from smelting a sulphide It’s a poisonous gas but can be converted to sulphuric acid

Sulphuric acid is used in car batteries, the paper and fertilizer industries. It can also be used to leach copper sulphides (see later) Vitriol – the historic name of sulphuric acid PwC


52

H o w to m ake sulphur

ic ac id f r o m sulphur

dio x ide

The diagram on the previous slide shows the contact process which starts with the following reaction: 2SO2(g) + O2(g)  2SO3(g) in the presence of vanadium oxide catalyst at 400‐450C The sulphur trioxide gas could be bubbled through water but that results in an uncontrollable reaction. Instead the gas is absorbed into a highly concentrated solution of sulphuric acid to form a liquid called oleum (or fuming sulphuric acid) and then the oleum is mixed with water to produce sulphuric acid H2SO4(l) + SO3(g)  H2S2O7(l) H2S2O7(l) + H2O(l)  2H2SO4(l) Note that twice as much sulphuric acid is made as was srcinally used to make the oleum. PwC


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Ele ctr o- Re fin in g of C opp er An ode Use electrical energy to force copper ions off anode

Power supply

electron flow

Copper ion Cu+2

++ Anode from smelter 95‐98% copper

++ ++

++

Cathode 99.99% copper

++

Insoluble impurities form slimes on anode (could include gold, silver, platinum, palladium) PwC

Mineral Processing and Refining

54

N ote s: Ele ctr o- Re fin in g of Copp er An ode The anode copper plates from the smelter are placed on one side of a tank filled with sulphuric acid and cooper sulphate as an electrolyte. The power supply forces the copper +2 ions. The atoms in the anode to give up two electrons each (to oxidize) forming Cu electrons flow through the circuit and end up at the negatively charged cathode while the copper ions flow through the electrolyte toward the cathode. The electrons and ions combine at the cathode to produce 99.99% pure copper, hence the name cathode copper. After about two weeks in the cells the cathodes are harvested. At the anode

At the cathode

Cu  Cu+2 + 2e oxidation of copper

Cu+2 + 2e  Cu reduction of copper

Impurities, which may include gold, silver, platinum and palladium depending on the srcin of the concentrate, form slimes on the decomposed anode. They are extracted later by a variety of processes.

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Co ppe r r ef in er y at H ar javalta Finland

sm elte r ,

www.boliden.com PwC


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L eac h in g Re ac t ion s & H eap L eac h in g

L eac h in g of c opp er oxid es an d su lfi de s With dilute acid Each reaction produces copper sulfate. Recovery may be poor.

Azurite Tenorite Chalcopyrite

+

lixiviant

Copper Sulfate

Dilute acid

Water

Sulfuric Acid

Carbon Dioxide

Chalcocite

Sulfur Dioxide

Low grade oxides and sulfides PwC

Sulfur


58

L eachi n g of c opp er oxid es an d s u lfid es If you really must know the chemistry ... azurite

+

sulfuric acid



copper sulfate

2CuCO3Cu(OH)2

+

6H2SO4



3CuSO4 +

tenorite + CuO chalcopyrite + CuFeS2

+



copper + sulfate

H2SO4



CuSO4



copper sulfate

H2SO4



CuSO4

Cu2S

2CO

sulfuric acid

sulfuric acid

chalcocrite

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+

+

2

+ water +

4H2O

water

+

H2O

iron sulfur + + sulfur + water sulfate dioxide

+ FeSO

4

+

sulfuric + acid



copper sulfate

+



2CuSO4 +

H2SO4

carbon dioxide

+


SO2

+

2S

+ 4H

2O

+ sulfur + water S

+ 2H

2O

59

D u m p le ac h pa ds at M or en ci, Ar iz ona

Low grade ore ~0.2%

Pregnant leach solution (PLS) with copper sulfate CuSO4 www.geomineinfo.com/mining_photos.htm PwC


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H eap le ac hi n g

Leach pads can be divided into four categories: conventional or “flat” pads, dump leach pads, valley fills and on/off pads. Conventional leach pads are relatively flat, either graded smooth or terrain contouring on alluvial fans such as in the Chilean Atacama desert, Nevada and Arizona, and the ore is stacked in relatively thin lifts (5 to 15 m typically). The lifts in dump leach pads are much thicker (up to 50m). Valley fill systems are leach “pads” designed in natural valleys using either a buttress dam at the bottom of the valley, or a leveling fill within the valley. On/off pads (also known as dynamic heaps) are hybrid systems. A flat pad is built with a robust liner system. Then a single lift of ore, from 4 to 10 meters thick, is loaded and leached. At the end of the leach cycle the spent ore is removed for disposal and the pad recharged with fresh ore. Usually loading is automated, using conveyors and stackers.

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Le ac hin g of go ld or e w ith cy an ide The Elsener reaction gold +

sodium + water + oxygen cyanide

4Au + 8NaCN + 2H 2O +

O2



sodium sodium + aurocyanide hydroxide



4NaAu(CN)2 +

4NaOH

Lixiviant

Cyanide + water

Leaching done in heap leach pads or tanks This is the basis of two processes for extracting gold: Merrill‐Crowe: uses zinc to precipitate gold Carbon adsorption: adsorb aurocyanide onto activated carbon PwC


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G old h eap le ac h pa d

Drip trickle irrigation system on top of pad

Ruby Hill Gold Mine, Nevada, USA www.mining‐technology.com/projects/rubyhill/rubyhill6.html

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Seep a ge i n a le a ch p a d Note difference in color at top of pad

continuous irrigation

less consolidated more flow mineral particle

more consolidated and more fines less flow

Leach pad, Anchor Hill pit, South Dakota Photo courtesy Robertson Geoconsultants

Recovery is uncertain and varies over the life of the pad Typical gold recoveries: 40‐70% PwC


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Seep ag e in a l each p ad Suppose a gold mine piled some ore into a 200m  200m pile and suppose the pile was 10 m high. Then there are 400,000 m 3 of ore in the pile. The density of the ore might be 2.0 tonnes/m3. That means there are 800,000 tonnes of ore in the pile. If the ore grade is 2 g/t there are 1.6 million grams of gold in the pile. That’s about 51,450 oz. However, recovery of gold in a leach pad is typically 40 ‐70%. Thus for the hypothetical leach pad above, the expected amount of recovered gold would be between 640,000 and 1,120,000 grams . The reason for the low recoveries is that not all of the leaching solution (acid in the case of copper, cyanide in the case of gold) can flow past the mineral particles. Flow paths to the particles may be blocked. In addition, as more ore is placed on top of the pad, the particles in the underlying ore become consolidated (closer together) and can block the flow of the leaching solution. For this reason, a layer of ore is placed on top of a pad only after the recovery from the lower layers begins to decrease. Improved recovery can be obtained by blasting techniques which break the ore to smaller particles, by crushing ore to smaller sizes, and by agglomeration of fine particles which inhibit the flow of lixiviant. However, the costs of these methods can be considerable. PwC


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Agglo m er ation of

go ld or e ore + cement + lixiviant

Rotating agglomeration drum

Fines plug voids between particles and cause a loss of permeability which prevents the flow of lixiviant. Agglomeration of the fines into larger particles creates larger voids through which the lixiviant can flow. PwC


66

Solut ion E xtr ac tion Ele ctr o- w in n in g


67

Solu ti on extr ac ti on (S X ) PLS from leach pad with low Cu concentration

Loaded organic with high Cu concentration

Organic solvent

Mixer

Sulfuric acid (from EW)

Settler Copper sulfate to electro‐winning (EW)

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68

N ote s: Solu ti on extr action

(S X )

The water and copper sulfate form a solution known as a pregnant leach solution or PLS. The PLS is pumped into a solvent extraction plant (the SX or extraction stage) where it is mixed with an organic solvent, an acid which we will label HR, to denote a hydrogen atom and a long chain hydrocarbon molecule R. (This is the oily stuff seen in the tanks.) The copper sulfate and HR react in the mixer as follows: Copper sulphate

+

CuSO4 +

Organic acid 2HR



Loaded organic

+

Regenerated Sulphuric acid



CuR2

+

H2SO4

The sulfuric acid goes back to the heap leach pad and the copper organic phase CuR 2 goes to the stripping stage where it is mixed with a stronger acid solution to strip the copper from the CuR 2 Loaded organic

+

Sulphuric acid



CuR2

+

H2SO4



Copper sulphate CuSO4 +

+

Regenerated organic acid 2HR

Now the copper sulfate solution is much richer in copper. The organic acid is recovered and reused.

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69

E le ctr o- W in n in g (E W ) Win the copper from the solution Power supply

Copper sulfate CuSO4 solution from SX plant

electron flow

Copper ion Cu+2

++ Anode (lead‐tin alloy)

++ ++

++

Cathode (starter plate)

++

Sulphuric acid H2SO4 to SX plant PwC


70

Note s: E le ctr o- W in n in g (E W ) The copper sulfate solution is called an electrolyte. At the anode, electrical energy splits water into hydrogen and oxygen to give a hydrogen ion, two electrons and oxygen. The power supply causes the electrons to flow through the circuit to the cathode. Being positively charged, the copper ions are attracted to the negatively charged cathode where they combine with the electrons to form copper metal. At the anode: H2O → 2H+1 + 0.5O2 + 2e (oxidation of hydrogen) At the cathode: Cu+2 + 2e → Cu (reduction of copper) Copper that is 99.999% pure (five nines) has been produced using the SX/EW process. In electro ‐winning the copper is in a solution (the electrolyte) whereas in electro ‐refining the copper from the smelter forms the anode of the cell. Electro ‐winning requires much more energy than electro‐refining because more energ y is required to break down water to provide electrons than to oxidize copper to the Cu+2 state and provide two electrons. +1, Note: Oxygen is formed at the anode and produces bubbles. In addition, the hydrogen ions, H ‐2 combine with the sulfate ion, (SO 4) , produce sulfuric acid in the tank, H 2SO4. When the bubbles reach the surface they burst, liberating an aerosol of sulfuric acid called acid mist. This is not good for the health of operators in the tank house. Chemical additives are used to reduce the size of the bubbles and to put a thin layer of foam over the electrolyte to keep the bubbles from reaching the surface.

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Bagdad:

SX / E W fac ility

Several solvent stages extraction and solvent stripping in parallel Electro‐winning plant

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E le ctr o- W in n in g P lan ts

Harvesting and washing cathodes at Bagdad Quebrada Blanca PwC


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An ode s an d Cat h ode s

Starters

Anode (lead‐tin alloy)

Cathode Copper

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P r essu r e le ac h in g of c on cen t r at e another way to oxidize sulfides

Experimental facility at Bagdad, Arizona

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Th e p r essu r e le a ch p r oc ess

Copper sulfate to electro‐winning Molybdenum oxide to steel companies

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N ot es: Th e p r essu r e le ach p r oce ss

In a stainless steel reactor vessel the concentrate slurry is agitated or stirred for about 30 minutes. The temperatures used range between 212‐450 F (100 ‐232 C) and the pressures used range between 200 ‐ 600 psi (1379‐4137 kPa) For chalcopyrite concentrate there are actually two chemical reactions: chalcopyrite + oxygen  copper sulfate + ferrous sulfate CuFeS2 + 4O2  CuSO4 + FeSO4 ferrous sulfate + oxygen + water  ferric oxide (rust) + sulfuric acid 4FeSO4 + O2 + 4H2O  2Fe2O3 + 4H2SO4 Iron: Fe+2 in sulfate oxidized to Fe+3 in iron oxide. Some copper concentrates are “dirty” and contain impurities such as antimony, bismuth, arsenic and mercury. These are found within the iron oxide (rust) that precipitates during the leach. Any precious metals in the concentrate would also be found in the iron oxide. These can be extracted using cyanide leach processes (see later). ˚

˚

For molybdenite concentrate the chemical reaction is Molybdenite + oxygen + water  Molybdenum oxide + sulfuric acid MoS2 + 4.5O2 + 2H2O  MoO 3 + 2H2SO4 Bagdad is currently using their autoclave to oxidize their molybdenite concentrate.

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P r oc essi n g of G old Or e


78

Bas ic ally w e w ill see h ow this is transformed to this

0.116 oz per ton ~3.97 gm per tonne PwC


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M er r ill- Cr ow e pr oc ess The Elsener reaction gold + sodium + water + oxygen cyanide



sodium + sodium aurocyanide hydroxide

4Au + 8NaCN + 2H 2O +



4NaAu(CN)2 +

zinc sodium + dust aurocyanide Zn

+ 2NaAu(CN) 2

O2

 

gold + 2Au +

4NaOH

sodium zinc cyanide complex Na

2Zn

(CN)4

Gold precipitate is filtered and then smelted to produce gold bar

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W hy zinc ? Because zinc gives up electrons (oxidizes) more readily than gold A gold ion will pick up any electrons zinc provides and precipitate Zinc solid Zn(s)

Zinc in solution Zn+2(aq)



Gold in solution + 2Au+(aq)

+

Two electrons

+

Two electrons

2e

2e



Gold solid



Au(s)

Zinc is used to precipitate metals from solution in the following order

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Iron

Cadmium

Cobalt

Nickel

Fe+2

Cd+2

Co+2

Ni+2

Tin Sn+2

Lead

Antimony

Pb+2


Sb+3

Copper Cu+2

Silver Ag+2

Gold Au+1

81

M er r ill- Cr ow e as a s y ste m zinc dust

sodium cyanide water and oxygen

Mine

De‐aeration

Gold ore

Leach pad

tailings dore ~90% gold

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Furnace 1200C

Drying oven

Filter press


82

N ote s: M er r il l- Cr ow e as a s y ste m Ore is first crushed and ground, then placed in leach pads. (It may also be crushed and ground and placed in stirred tanks for leaching.) A sodium cyanide solution is added to the ore which produces a solution of sodium aurocyanide and sodium hydroxide. gold + sodium cyanide + oxygen + water → sodium aurocyanide + sodium hydroxide 4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 + 4NaOH The aurocyanide complex involves Au+, gold with one electron missing.. When zinc dust is added to the solution, the gold is reduced and precipitated as a solid. This is known as zinc cementation and actually consists of two reactions: zinc + sodium cyanide + oxygen + water → sodium zinc cyanide + sodium hydroxide Zn + 4NaCN + ½O2 + H2O Na2Zn(CN)4 + 2NaOH zinc + sodium aurocyanide → gold + sodium zinc cyanide Zn + 2NaAu(CN)2 → 2Au + Na2Zn(CN)4 The aurocyanide is de‐aerated (oxygen removed) to stop the first reaction from producing sodium zinc cyanide which would force the second reaction to the left and re ‐dissolve the gold. The resulting solids are filtered producing a barren solution and then smelted to produce a gold bar. The Merrill ‐Crowe process is used when the ore has a high silver to gold ratio as silver cannot be recovered using activated carbon methods (see next slides). However, if the ore contains a large amount of clay, the filtering process in Merrill‐Crowe can become difficult.

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Ads or ption o f aur o cy anide

onto activated carbon

Produced by burning of carbon rich materials such as coal, wood or coconut shell. Steam or chemicals are used to develop microporosity. Enormous internal surface areas where adsorption can occur. (1 gm of activated carbon has 500 m2 of surface area.)

Activated carbon particle Result is loaded carbon 300‐20,000 g/t PwC


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Thr ee w ay s ads or b go ld on to c ar bon Carbon in Pulp (CIP)

Leach and adsorb in separate sets of tanks Carbon in Leach (CIL)

Leach and adsorb in the same tanks Carbon in Column (CIC)

Leach in heap and adsorb in tanks

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Car bon i n L eac h (C I L ) Leach and adsorb in the same tanks cyanide

Ore

Crush, grind, thicken

slurry

Barren leachate

Tailings



Loaded carbon

carbon

Regenerated carbon

Strip carbon and electro‐win

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N ote s: C ar bon in L each

Leaching and adsorbing in the same tanks has the advantage of lower capital costs. It is also used when the ore is naturally carbonaceous (“preg ‐robbing”) to force adsorption onto the activated carbon. However, leaching and adsorption in the same tank leads to concentration gradients which must be broken down. This is done using greater agitation than that required in CIP tanks. The result is loss of precious metals from the carbon and lower recovery than in CIP.

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Car bon in P u lp (C I P ) Leach and adsorb in separate sets of tanks Cyanide leach Ore

Crush, grind, thicken

Carbon adsorption

Barren leachate



Tailings 

Loaded carbon

Regenerated carbon

Strip carbon and electro‐win

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Not es: C ar bon in P u lp In CIP the gold ore is ground into fine particles and passed as a slurry into leaching tanks. The pregnant solution from the leach tanks is then pumped into tanks containing activated carbon particles. The activated carbon flows in the opposite direction to the leachate. The number of tanks may vary between 4 and 8 depending on the rate of production. The leaching and adsorption are done in separate sets of tanks. The advantage of this is simplicity and the recovery can be over 95%. However, naturally occurring carbon in the ore will compete with the activated carbon (“preg ‐robbing”) and any silver or copper present will compete with the gold during adsorption.

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Car bon i n Colum n (C I C)

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H eap L eac h P ads an d P r eg P on ds

Leach Pad – Initial stage Pierina Mine, Peru (Merrill‐Crowe Process) www.cosapi.com.pe

Heap leach pad and (empty) preg pond Cortez Mine, Nevada PwC


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Not es: H eap L eac h Pads an d Pr eg P ond s

Left: The initial stage of one of several leach pads at the Pierina mine in Peru. The pad is underlain by a polyethylene liner (HDPE). The pregnant solution collects in a sump and is piped to a pregnant solution pond, also underlain by a liner. The leach pad and the preg pond at Cortez are shown on the right. There were several preg ponds, each lined with HDPE. The pond shown was empty at the time. (Beautiful scenery, but it was very cold that day)

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CI C Ad sor pt ion Tan k s at C or te z Hard to get a picture of the whole adsorption tank facility at Cortez. You have to go there to really appreciate it.

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Not es: C ar bon in Colu m n (C I C) The carbon‐in‐column (CIC) process is often used in conjunction with heap leach of gold ores. The “pregnant” solution of sodium auric cyanide from the leach pad is collected in a pond and passed through tanks where the gold is adsorbed onto activated carbon particles. Activated carbon acts like a sponge to gold cyanide complexes in solution such as sodium auric cyanide. The leachate flows in the opposite direction to the carbon particles so that the gold concentration of leachate decreases downstream and the amount of gold on the carbon increases upstream. Gold is stripped (eluted) from the loaded carbon by a solution of cyanide and caustic soda. The stripped carbon particles are recycled.

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Str ip car bon an d e le ctr o- w in Sodium hydroxide Loaded carbon

Acid wash

90C Regenerated carbon

Stripping



Aurocyanide Au CN 2

Electro‐winning Doré ~40‐90% gold

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Au CN 

Furnace 1200C

Clean cathode dry slimes




e  Au

2CN



2

95

Not es: S tr ip car bon an d e le ctr o- w in The carbon is first washed with acid to remove calcium that has precipitated on the carbon, as well as to clean fines out of the carbon pores. Aurocyanide is then stripped (eluted) from the loaded carbon by a hot solution of caustic soda (NaOH) and sodium cyanide. This essentially reverses the Elsener equation to break up the sodium aurocyanide NaAu(CN)2



 Na Au



CN 2

The stripped carbon particles are recycled. The solution is pumped into electro ‐winning tanks where the gold is plated onto a cathode. The electro ‐winning chemical reaction is ‐

Au CN 2 e

 Au

2CN



where e is an electron. The reaction could go either direction, but the application of electric current forces it to the right causing a reduction of the gold ion in the aurocyanide complex. Other metal cyanide complexes may be present resulting in impurities on the cathode. After electro ‐winning the cathodes are cleaned and the resulting slurry is dried and then refined to produce a doré bar containing mostly gold. The electrolyte may contain other metal ions (e.g., copper) as well as the cyanide ion CN The electrolyte can be treated to recover the cyanide for reuse. Recovery of the other metals is also possible. PwC




.

96

E le ctr o- w in n in g c ells

stainless steel, rubber lined

Mt Rawdon gold mine, Queensland Source: Mintrex Pty Ltd http://mintrex.com.au PwC


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W as h off c at h ode s

Hemlo/David Bell mine (Barrick Gold) Photo courtesy Bern Klein, Dept of Mining Engineering, UBC

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An d fi n al ly – t h e dor épou r

Mt Rawdon gold mine, Queensland Source: Mintrex Pty Ltd http://mintrex.com.au PwC


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W h en t o u se t h ese m et h ods Merrill‐Crowe

Usedif silver dominant in ore Filtering difficult if clays present Heap leach

Carbon‐in‐Leach (CIL)

Low capital costs – one set of tanks Carbonaceous ores Lower recovery than CIP

Carbon‐in‐Pulp(CIP)

High capital costs – two sets of tanks Non‐carbonaceous ore High recovery (~95%)

Carbon‐in‐Column (CIC)

Lower operating costs Used for lower grade ores Heap leach

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Ne w m o n t M inin g: Ro as te r at C ar lin Nevada Some gold ores contain natural carbon Gold is adsorbed onto the carbon as in CIL process This reduces recovery

Roaster used to burn carbon and release gold

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Re fr ac tor y gold Gold mixed in with a sulfide, typically pyrite or arsenopyrite Cannot be leached ~450 microns free gold

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gold in arsenopyrite


102

An au toc lav e … … in which sulfides are broken down resulting in oxides and sulfuric acid

Used to release gold from refractory gold ore (It will not fly) Sulfides are first separated by flotation Source: www.metsoc.org PwC


103

N ote s: An au toclave … Autoclaving is used to process a variety of ores or metal products and is done in one of two ways: Pressure oxidation of minerals – high pressure and temperature (e.g., at Bagdad) Pressure leach – high pressure in acid or alkaline conditions For refractory gold ores where precious metals are locked within sulfide minerals such as pyrite, the sulfur in these minerals has to be oxidized so that the sulfide minerals are broken down and the gold can be released. Following oxidization Base metals are released into solution to be processed by electro ‐winning Precious metals are leached using cyanide In a pressure leach of sulfide minerals an autoclave operates at temperatures >175 C and pH < 2, the following chemical reactions oxidize the iron and sulfur in pyrite. First the sulfur is oxidized: 2FeS2 + 7O2 + 2H2O  2FeSO4 + 2H2SO4 (oxidize sulfur from S‐1 to S +6) Next, the iron loses an electron and forms an iron oxide which precipitates (down Sulfuric acid is also formed.

‐pointing

arrow).

2FeSO4 + O2 + H2O  Fe2O3 + 2H2SO4 (oxidize iron from Fe+2 to Fe+3) 

Electrons reactions.are taken from the sulfur and iron atoms. The oxygen atoms get all the electrons in these

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O th er M eth ods

G r avit y conc en tr ation – water

shak in g table

ore

heavier particles Reciprocating motor “middlings”

tailings

slimes

www.odm.ca/pages/heavy.html

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G r avity co n cen tr ation concentrator

cent r ifugal

Used to separate free gold particles Water cavity

Concentrating cone

www.knelson.com

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G r avity conc ent r ation

Shaking table

A shaking table consists of a sloping deck with a riffled surface. A motor drives a small arm that shakes the table along its length, parallel to the riffle and rifle pattern. The shaking motion consists of a slow forward stroke followed by rapid return stroke. Water is added to the top of the table perpendicular to the table motion. The heaviest and coarsest particles move to one end of the table while the lightest and finest particles tend to wash over the riffles and to the bottom edge. Intermediate points between these extremes provides recovery of the middling (intermediate size and density) particles. Centrifugal concentrator

A centrifugal concentrator consists of a riffled cone or bowl that spins at high speed to create forces in excess of 60 times that of gravity. Slurry is introduced into the cone; the centrifugal force produced by rotation drives the solids toward the walls of the cone. The slurry migrates up along the wall where heavier particles are captured within the riffles. Injecting water through the holes located in the back of the riffles fluidizes the riffled area. The fluidization process prevents compaction of the concentrated bed and allows for efficient separation of heavy minerals.

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Sl u i ce bo x water flow gravel & sand here

riffles catch heavier particles http://nevada‐outback‐gems.com/design_plans/DIY_equipment.htm PwC


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T r o m m el scr een Smaller size screen

Larger size screen

www.metso.com

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M agn eti c s epar ati on

Feed leveler Non‐magnetic shell Stationary permanent magnet

Non‐magnetic material

Magnetic material falls away at underside of drum

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M agne tic separ ation in ir on or e plan t

www.metso.com PwC


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P r oc essin g w it h bac te r ia Specific types of bacteria derive energy by oxidizing sulfide minerals

Break down sulfides in refractory gold ore Can also be applied to the extraction of base metals from sulfides Thiobacillus ferrooxidans

13,466 times www.personal.psu.edu/mah37/pictures/outreach04/thio.bmp

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N ote s: P r oc essin g w it h ba cte r ia Bio‐oxidation of sulfides in refractory gold ore Gold is often embedded in the crystal structures of pyrite and arsenopyrite. In the presence of bacteria, the following reactions oxidize the sulfur in these minerals and break them up to release the gold.

Pyrite:

FeS 2 + 14Fe+3 + 8H 2O  15Fe+2 + 2(SO4)‐2 + 16H + +3

Arsenopyrite: Fe+2 to Fe+3:



+2

‐3

(1) ‐2

2 2H 2O FeAsS Fe + 4H + 3O + (AsO 4) + 2(SO 4) + 4H +2 ++O + +4Fe +3 + 2H2Fe 4Fe 2 20

+

(3)

(2)

The Fe+3 generated in Reaction 3 is consumed in Reaction 1. Bio‐leaching for copper The speed of the oxidation of copper/iron sulfides (and other metal sulfides) is vastly increased by the introduction of Thiobacillus ferrooxidans bacteria to the system. In the presence of Thiobacillus ferrooxidans the chemical reaction is: 4CuFeS2 + 11O2 + 6H2O  4CuSO4 + 4Fe(OH)3 + 4S (oxidize iron from Fe+2 to Fe+3) Bio‐leaching vs Bio‐oxidation? Bioleaching refers to the use of bacteria, principally Thiobacillus ferrooxidans , Leptospirillum ferrooxidans and thermophilic species of Sulfobacillus, Acidianus and Sulfolobus, to leach metal such as

copper, zinc, uranium, nickel and cobalt from a sulfide mineral into solution (water). Metal is recovered from these solutions and the solid residue is discarded. Bio‐oxidation refers to a pretreatment process that uses the same bacteria as bioleaching to catalyze the degradation of mineral sulfides, usually pyrite or arsenopyrite, which host or occlude gold, silver or both. Biooxidation leaves the metal values in the solid phase and the solution is discarded. http://technology.infomine.com/biometmine/biopapers/biomet_bioleaching.asp PwC


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Biole ac h i n H eap or Tank s

Tank leach Ashanti Gold, Ghana Bio‐leaching of nickel/copper sulfides Titan Resources, Australia

of Lawrence Consulting Ltd 960Courtesy tpd pyrite/arsenopyrite

www1.titanresources.com.au

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Coal

F or m at ion of Coal – Ste p 1 Deposition of organic debris in a swamp  peat bog

Burn’s bog, Fraser Delta http://gsc.nrcan.gc.ca/urbgeo/vanland/delta_e.php PwC


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Not es: F or m at ion of C oal – Ste p 1 Step 1: The first step in coal formation is accumulation of organic debris in a peat swamp. In

most environments, such as the forest floor, plant material decays as fast as it is produced, so it does not accumulate. However, in a peat swamp, stagnant water that does not contain oxygen inhibits the decay of organic material allowing it to accumulate and form peat. Burying the peat with sediment further inhibits the decay of peat.

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Fo r m at ion of coal – Ste ps 2+ Successive sedimentary deposits cover peat and form coal

20

Peat 1

20:1 volume reduction  loss of water and gases  coal

Coal PwC


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N ote s: F or m at ion of C oal – Ste ps 2+ Steps 2+ : Over time (millions of years) the sea level may rise and fall allowing organics to

accumulate as peat A transgression is where the shoreline moves landward, often due to a relative rise in sea level, resulting in the land surface being covered by the sea. Plant life on land began to evolve about 450 million years ago and so there are no coal deposits older than that. Most coal deposits were formed during the warm Carboniferous period 360 to 290 million years ago. Burial of peat by overlying sediments results in an increase in the temperature and pressure. One change that happens is compaction; it is estimated that coal results from a 20 to 1 compaction of peat, i.e., the coal is 1/20 the thickness of the srcinal peat layer. In addition to compaction there is a loss of moisture and volatiles. Much of the water that is lost was trapped in pore spaces and is expelled during compaction. Some of the water, plus the volatiles (gases) are released due to chemical changes in the peat.

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Coal r an k u ses an d gr ade Increasing rank (carbon content) Increasing pressure of compaction

Peat

For the garden

Lignite

Sub‐bituminous

Thermal coal if sulfur content low

Bituminous

Metallurgical (coking)

Anthracite

Hand‐fired or automatic stoves

Coal grade refers to the amount of ash and sulfur content. Low grade coal has high ash and/or high sulfur content. Ash is non‐combustible and sulfur is just not good. Anthracite delivers high energy per unit weight and burns cleanly with little soot, making it ideal for heating. However, its high value makes it prohibitively expensive for power plant use. Other uses include the fine particles used as filter media. PwC


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Not es: I s coal a m in er al?

This question can lead to some heated debates. We could start with the idea (Skinner, 2005) that all solids are potential minerals and then see if coal fits the expanded definition of a mineral: An element or compound, amorphous or crystalline, formed through biogeochemical processes There are biogeochemical processes involved in the formation of coal. However, they lead to a solid which includes carbonized plant remains. There is a wide variety of compounds in these plant remains and for this reason it is difficult to define a characteristic chemical composition or set of compounds that make up coal. For this reason coal is usually referred to as a rock – a combination of minerals. “Coal is the official state mineral of Kentucky (even though coal is not a mineral) and the official state rock of Utah.” (Source: wikipedia) References: http://en.wikipedia.org/wiki/Coal Skinner, HCW, 2005. Biominerals,Mineralogical Magazine 69 (5): 621–641

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Can ad i an Coal Re sou r ces

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U S Coa l R esou r ces lignite s u o n i m u ti b b u s

bituminous

‐

http://en.wikipedia.org/wiki/Coal

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N ot es: C oal Re sou r ces an d Pr od u ct io n

Proven reserves of coal worldwide are about 845 billion tonnes. This is enough coal to last almost 120 years at current rates of consumption. The US has the largest reserves of coal in the world, about 237 billion tonnes, and produces about 1 billion tonnes of coal per year. (China produces 3.2 billion tonnes per year.) Canada has about 7 billion tonnes of reserves and produces about 75 million tonnes of coal per year. Canada is the second largest metallurgical coal exporter, Australia being the first largest. Current (2011) coal prices are about $200/tonne. References: http://en.wikipedia.org/wiki/Coal http://www.nrcan.gc.ca/eneene/sources/coacha‐eng.php

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Coa l Pr oc essi n g Purpose Remove incombustible material such as dirt and rock to increase the heating value or carbon content of the coal Incombustible mineral material referred to as “ash” Sometimes known as “coal washing” Methods used Screens Dense media separation Flotation Drying PwC


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W h at doe s coal look

li k e?

Spores from vegetation

Yellow and orange dots are spores or algae

Well‐preserved wood Black material is charcoal or minerals (e.g., silicates) 0

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2 mm


127

Coa l p r oc essi n g

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N ot es: C oa l p r oce ssi n g Coal processing is sometimes referred to as coal cleaning because it removes silicate minerals such as sands, silts, clays and ash from the coal. There are several types of breakers. A rotary breaker consists of an outer fixed shell and an inner rotating drum with perforations. Typical rotational speed of the drum is 12 ‐18 rpm. Lifter plates pick up the run ‐of‐mine coal which then falls onto the drum. The softer coal breaks and passes through the perforations while the harder rock is transported to the waste stream. In addition to the cleaning (removal of rock), a size reduction is also achieved. The total surface area of a volume of fine particles is larger than the surface area of a coarse particle of the same volume. Since heat release from a coal particle is proportional to surface area, fine particles are desired for both thermal and metallurgical applications. However, during processing and transport, only the surface of the coarse particles oxidizes whereas an entire fine particle may oxidize lowering its thermal value. Thus, both thermal and metallurgical coal are ground to fine sizes at the location where it is used. Usually the fine particles of thermal coal are so dirty that they cannot be cleaned. Often they are discarded but it might be possible to blend the fines with coarse coal to achieve an overall acceptable ash content. The fines of metallurgical coal (also known as coking coal) can usually be floated to obtain clean coal. The flotation is an added expense, but the value of the metallurgical fines is high. Sometimes the clean fines are agglomerated to form coarse particles.

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D en se m edi a s epa r at i on – t h e ba si c id ea Feed

Fluid medium SG = w Material with SG < w (coal or floats)

typically magnetite in water

Material with SG > w (sinks)

Used in drum for coarse particles Used in hydrocyclone for finer particles which may not separate easily PwC


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Coal pr oc essin g p la n t

Cyclones Dense media drums

Source: www.flsmidthminerals.com/Company/Press+Room/Product+Brochures/HMS+Drum+Plant.htm PwC


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E lk vie w M in e, Br itis h C olum bia Capacity: 5.6 mtpa Reserves: 376.1 mt

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Coal tr an spor tati on s y ste m in BC

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Tr ain loads an d boatloads

Coal loaded at Westshore

Train near Elkview loadout

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Roc k slid e on r ai l r out e in BC ear ly 20 11

Source: Teck 1st quarter 2011 presentation report This rock slide took 7‐10 days to clear up

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Cer r ejon coal m in e in C olo m bia

Resources: 2,193 mt Bituminous coal

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Coal tr an spor tat ion s y ste m in Colo m bia

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The str ip r ati o f or coal m in es The strip ratio of a coal mine may be very high (11

‐12

at Elkview)

and it can vary considerably during the mine life. The compensating factor is that the yield of one tonne of coal ore is much larger (~ 60%) than the yield of one tonne of a metal ore. Also processing coal ore costs much less than processing metal ores. Cross‐section of geology at Eagle Mountain, BC

www.mining‐technology.com/projects/fording/fording7.html PwC


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Diamonds

W he r e diam onds ar e fo un d Mostly in very old rocks in the center of continents

> 2.5 by 1.6‐2.5 by < 1.6 by

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Note s: W he r e diam ond s ar e foun d Diamond deposits are found in the oldest parts of continents called cratons, where the basement rocks are older than 1,500 million years. The most productive cratons are older than 2,500 million years located in the central parts of continents such as North America, Asia, India, and Australia. Less productive deposits are found in rocks 1,600–2,500 million years old. Other than that described above, the location of diamond deposits cannot be related to any plate tectonic activity within the last 100 ‐200 million years. This is because the formation of diamonds and diamond deposits more related to processes deep in the earth rather than the shallow crustal processes that lead to base and precious metal deposits. http://www.amnh.org/exhibitions/diamonds/

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H o w do di am o nds f o r m ?

Continental plate

Non‐diamond bearing

Diamond bearing

Upper mantle

150 ‐ 200 km

Kimberlite pipes

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Diamond formation

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Note s: H ow do diam

onds fo r m ?

Diamonds are formed by re‐crystallization of graphite (carbon) at high pressure and temperature (900 ‐ 1200C) at depths greater than 150 km in a region below the earth’s crust known as the mantle. They are transported to the surface by magma under considerable pressure. Dissolved gases in the magma expand and the magma combines with boiling groundwater to result in an explosive supersonic eruption at the surface. The high speed prevents the diamonds in the magma from re ‐crystalizing as graphite. The result is a carrot ‐shaped pipe or vent at the surface and a small volcanic cone. The pipes contain minerals such as garnets and pyroxenes which are formed in the mantle. Fragments of crustal rock are also present. The rock in the pipes is called kimberlite, after the city of Kimberley, South Africa, where pipes were first discovered in the 1870s. Pipes occur in clusters and the pipes in a cluster are typically at most tens of kilometres apart. http://www.amnh.org/exhibitions/diamonds/ Diamonds from kimberlite pipes have been age ‐dated and found to be between 3,300 million to 990 million years old. However, the kimberlite rock was intruded only about 100 million years ago. Given the age of the diamonds, the carbon source is most likely carbon trapped in Earth's interior at the time Earth formed 4,600 million years ago. (Kirkley, MB et al, 1991, Gems and Gemology, 27:2‐25) Two things which make diamonds rare: Only about 1 in 50 kimberlite pipes contain diamonds. Secondly explosive eruptions that produce kimberlite pipes seem to have stopped occurring. The youngest kimberlite pipe in the world is in the Lac de Gras area of Canada and is about 50 million years old. (Davis WJ and Kjarsgaard BA, 1997, Journal of Geology, 105:503‐510) PwC


143

H o w to f ind a kim

be r lite pipe in the Ar ctic

Count indicator minerals in the glacial till

Kimberlite pipe

Ice flow Pyrope Mg3Al2(SiO4)3 A type of garnet

# pyrope per 20 kg sample 0 1‐10 11‐50

Take samples of till Count # of indicator mineral grains in samples PwC


51‐150 >150 144

Note s: H ow to find a k

im be r lite pipe in the Ar ctic

Indicator minerals such as pyrope (garnet), chromite and ilmenite, are present in the mantle where diamonds are formed and are transported to the Earth’s surface in the kimberlite. They are easily recognized, mobile, and resistant to weathering. Thus the trail of indicator minerals in glacial till left by glacial erosion of a kimberlite pipe can lead to the location of the pipe beneath the till. Tracking this trail of indicator minerals in the glacial deposits was the key to the first diamond mine discovery, the Ekati mine, in the Northwest Territories. A good read about this diamond find is: Fire Into Ice: Charles Fipke and the Great Diamond Hunt

by Vernon Frolick. Raincoast Books,

Vancouver, 1999 (ISBN 1‐55192‐232‐0) A more recent (and colorful) story about Mr Fipke can be found at http://www.wired.com/print/science/planetearth/magazine/16‐12/ff_diamonds Currently the Ekati and Diavik mines are in operation. There are many other diamond pipes under investigation in northern Canada. Canada is the third largest producer, by value, of diamonds in the world, after Botswana and Russia. Till photo: http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=670

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The y r eally ar e in th er e (s om ew he r e)

Glacial till in Lac de Gras area, NWT Kimberlite boulder in till deposit

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http://gsc.nrcan.gc.ca/mindep/method/kimberlite/index_e.php#ind Mineral Processing Methods

146

Dia vi k Dia m o nd M ine On a sunny summer’s day

Seasonal ice road Open February to April June 9, 2011 In winter (‐35 C) ˚

The “Anti‐Bling” PwC


147

Note s: Diavik Diam

ond M ine

Photos courtesy of Diavik Diamond Mines Inc. Left: Pit formed inside a dam constructed in a lake called Lac de Gras. Construction during 2000 ‐2003

shown. Mining occurs year round. Top right: The ice road extends 600 km from Tibbitt Lake (outside Yellowknife) to the Jericho Diamond

Mine. Seventy ‐five per cent of the road is ice, built over frozen lakes. Diavik is about 370 km from Tibbitt Lake. Travel time to Diavik is 15‐19 hours depending on load weight. Bottom right: Rough diamonds. The larger diamond on the lower left of the picture weighs about 8 carats and is worth C$30,000. The manner in which these diamond s are separated from the waste is interesting. See part C. Diavik Mines data: 27.2 Mtonne reserves at 3.9 carats/tonne, four orebodies (pipes) Annual ore production: 1.5 to 2 million tonnes Annual diamond production: maximum 8 million carats Mine life: 16 to 22 years. Production began January 2003, capital cost: C$1.3 billion Underground operation under development in 2007, expected to begin in 2009. Capital cost of underground development as of November 2007 is US$787 million. Open pit operation will cease in 2012.

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H o w do y o u c r us h diam o nd or e? Very carefully – with a High Pressure Grinding Roll (HPGR)

Adjust gap between rollers to maximum expected diamond size

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Note s: H igh pr essur e gr in din g r olls

A High Pressure Grinding Roll (HPGR) machine consists of a pair of counter ‐rotating rolls, one fixed and the other floating. Ore feed is introduced into the gap between the rolls. The position of the floating roll can be adjusted. A hydraulic spring system maintains grinding pressure on the floating roll. The pressure and roll speed can be adjusted during the grinding to adapt to changing feed properties. Comminution in a HPGR is done virtually completely by compression. This results in a product that has a higher percentage of fines than can be achieved with a SAG or AG mill where comminution is done by a combination of compression and shear. Coarse particles in the HPGR product exhibit extensive cracking which reduces the amount of grinding work to be performed in a downstream ball mill. HPGR technology was srcinally developed for the cement industry. Diamond mines adopted the technology in the early 1980s for crushing kimberl ite ore. HPGRs are now being used or considered for use in crushing gold and base metal ores where they would replace SAG and AG mills in a grinding circuit. Base and gold metal ores are typically harder than kimberlite. HPGR units have a 6 ‐10% higher capital cost than SAG mills and an issue is wear of the roll surface (which is typically studded), particularly in gold and base metal ore processing. However, this is offset by the low cost replacing wearsignificantly surfaces, short equipment times, a high throughput Energy costs of of a HPGR are also lower – most ofdelivery the energy in and a SAG or AG mill circuitrate. is consumed moving the mill cylinder itself.

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H P G R te st fac ilit y at N BK (UB C)

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D en se m edia hy dr oc y clone plan t To separate kimberlite (light) from diamonds (heavy)

www.stornowaydiamonds.com

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D iam ond or e pr oc essin g – X -r ay separ ati on Dense media separation in cyclones results in diamond concentrate (diamonds heavy, kimberlite light)

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Note s: Dia m ond or e pr oc essin g – X r ay separ ati on The crushed ore is mixed with finely ground ferrosilicon (dense media) slurry at a density of approximately 2.65g/cm3, near the density of diamond. The resulting slurry is spun at high speeds in a cyclone, creating a density gradient in which lighter materials (kimberlite) rise to the top of the cyclone (the overflow) and are discarded as waste (or used as backfill in an underground mine). The higher density minerals, including diamonds, concentrate at the lower levels (the underflow) of the cyclone and are sent to the X ‐ray separator as a diamond concentrate. Using a magnet the ferrosilicon is recovered for re‐use. The X‐ray separator system acts on a thin stream of particles from the concentrate accelerated off a moving belt into the air, where they encounter an intense beam of X ‐rays. Any diamond fluoresces in the X ‐rays, activating a photomultiplier that triggers a jet of air, deflecting the diamonds (red) into a collector bin. Sources: Crusher and cyclone pctures: www.diavik.ca X‐ray separator: www.amnh.org/exhibitions/diamonds/process.html See video at http://www.debeerscanada.com/files_2/snap_lake/snap_animations/mining_method/mining.swf

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O il San ds

O r igin of oil s an ds Resources ~1.7 trillion barrels

Conventional marine organic srcin in the southwest of Alberta Oil flows to the northeast The lowering of the temperature to less 

‐

than 80oils C allowed bio degradation of the lighter The result: thick bitumen with sand Map by Norman Einstein, May 10, 2006 PwC


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Not es: O r igi n of oil s an ds

For the geologically inclined: The following author favors a coalification srcin for oil sands: http://www.searchanddiscovery.net/documents/2004/stanton/index.htm But this author (and others) favor a marine source similar to conventional oil: Hein, F. J., 2006. Heavy Oil and Oil (Tar) Sands in North America: An Overview & Summary of Contributions. Natural Resources Research, 15(2): 67‐84 It is clear that the oil sands could not flow to where they are in their current condition. The fluid oil, whatever its srcin, has been degraded by bacteria, a process which removes the lighter hydrocarbon molecules, leaving behind the bitumen in between grains in these oil sands. Some current research is directed toward enhancing this biodegradation process in deep oil sand deposits and collect the resulting gases as an energy source – ie to exploit the oil sands in situ.

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Separ ate bitu m en f r om san d an d w ate r

Add hot Transport slurry to water extraction plant

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F lotation

of bitu m en Air bubbles attach to bitumen Floats to surface as froth Bitumen Water Sand/Clay

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Su bs equ en t p r oc essi n g st a ge s

Add naptha to bitumen froth and centrifuge to further separate bitumen from solids Two tonnes oil sand

yield one barrel of oil

Remove naptha UPGRADING Remove sulfur and nitrogen compounds to produce synthetic crude PwC


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A P r oble m This pit is about 90 m deep

www.guardian.co.uk/environment/2011/oct/05/1

80% of the bitumen resource lies below 100 m But oil sands are unstable PwC


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One alt er n at iv e: S t eam As sis te d G r av it y Drainage BUT Considerable energy is required to form steam

Source: Energy Resources Conservation Board

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Steam can escape into the lower pressure gas pool making it unavailable for heating bitumen


162

SAG D in stallat ion in Albe r ta

Operating at 250C

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Uranium

Ur an iu m in Can ada – the Ath abas ca Bas in

~300 m depth

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Note s; Ur an iu m in Can ada – th e Ath abas ca Bas in

The Athabasca Basin is composed of a sedimentary deposit of sandstones overlying deformed metamorphic basement rocks. In geological terms the combination of these two rock types is an unconformity, a buried erosion surface separating rock units of different ages. It results when there is a hiatus between the deposition of older underlying rocks and younger overlying rocks, allowing erosion to occur. Uranium is a large atom and does not fit into the crystal structure of typical rock types. One theory is that magmatic activity deformed the underlying metamorphic rocks and hydrothermal fluids from the magma transported uranium and deposited it in large quantities at the base of the sandstones. (See next slide) Unconformity‐type uranium deposits host high grades relative to other uranium deposits and include some of the largest and richest deposits known. Other significant deposits occur in the MacArthur Basin in the Northern Territory, Australia.

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Cr os s-section at C igar L ak e u r an iu m m in e

(Sandstone) (Quartz cap) (Weathered sandstone) (Clay cover) (Uranium ore) (Metamorphic bedrock) http://commons.wikimedia.org/wiki/File:Uranium_deposit%28Cigar_Lake%29.png PwC


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Ur anium

M ining in

Sas katc he w an Remote mining methods to avoid exposure to radiation Ground freezing to control groundwater Jet boring ‐ essentially wash ore out of ground

Excellent animation at: http://www.cameco.com/mining/cigar_lake/jet_boring_animation/

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U r an iu m or e pr oc essin g Uranium 19‐24% uranium oxide ore Precipitation Solvent extraction Ion exchange

Crushing Grinding Acid or alkali leach

~85% uranium oxide For details See www.chemcases.com/nuclear/nc‐06.html

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I n situ ex tr ac tion of ur

aniu m

Oxygenated water with peroxide or carbonate

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Note s: I n -situ extr ac tion of ur an iu m Applicable to deposits of uranium in sandstone and confined between impermeable layers. Deposits in Australia, western US and Kazakhstan. Injection wells pump a chemical solution — typically sodium bicarbonate and oxygen — into the sandstone layer containing uranium ore. The solution dissolves the uranium from the deposit and is then pumped to the surface through recovery wells and passed through ion exchange columns to be converted into yellowcake.

www.powertechuranium.com/s/AboutISR.asp

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I ndu str ial M ine r al s

I ndu str ial M in er als and t he ir Applic ations

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Industrial Minerals

Applications

Limestone

Construction

Clays

Ceramics

Sand

Paints

Gravel

Electronics

Kaolin

Fertilizers

Bentonite

Filtration

Silica

Plastics

Barite

Glass

Gypsum Talc

Detergents Papercoatings

Potash

Lubricants


173

G la cie r s an d gla cia l ti ll

Source of a number of gravel deposits

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Canadian

M in er al P r o duc tion 28 26

Source: Natural Resources Canada

24 22

Metallic Non-metallic (excluding coal)

) 20 n o i 18 lil b $ 16 C ( 14 e u l 12 a V

10 8 6 4 5 9 9 1

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6 9 9 1

7 9 9 1

8 9 9 1

9 9 9 1

0 0 0 2

1 0 0 2

2 0 0 2

3 0 0 2

4 0 0 2


5 0 0 2

6 0 0 2

7 0 0 2

8 0 0 2

9 0 0 2

0 1 0 2

175

IM M ine s in BC

Source: http://geoscape.nrcan.gc.ca/nanaimo/sandgravel_e.php

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I n du str ial m in er als pr oc essin g Spiral separator test rig Almost all physical crushing, grinding, separation by screening or gravity

Screen particle separator

www.bateman.co.za PwC


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As pe cts of I n du str ial M in er als There are no industrial mineral markets • market for mineral must be developed, price negotiated

But production may be turned on and off depending on market demand Mineral usually of low unit value • transportation costs must be factored into price

Relatively low capital and operation costs Almost all material mined is used • overburden stripping may be necessary

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END OF PART 3

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basics of mining and mineral processing

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