Mine waste introduction

Question 1

Name some common ore types and metals they contain

Answer 1

o Sulphide ores -Base metals Cu, Pb, Zn
o Iron oxide ores -Fe
o Skarn - Wolfram
o Graphite - C
o Alum shales - V, Mo, can contain REE and uranium

Question 2

What is the main difference between open pit mining and underground mining?

Answer 2

Open pit mining is cheaper and for lower grade ore

Question 3

Name some processes used in enrichment plants

Answer 3

Crushing, milling, separation, flotation

Question 4

Describe waste rock

Answer 4

The excavated material needed to reach ore, therefore consisting of surrounding bedrock. Can be crushed and blasted. Has mixed grain sizes. Amount depends on ore, depth and type of mining. Can be used in contruction if inert. Difficult to determine average mineralogy.

Question 5

Describe co-disposal and blending

Answer 5

Tailings disposed together with waste rock to fill void space. Finer tailings with high moisture content limits transport of water and oxygen compared to the waste rock. Alternatively, mixing of waste with different geochemical properties to form secondary minerals on reactive mineral surfaces (Acid producing vs acid consuming (carbonate-bearing rock, alkaline industrial residuals etc) materials for example)
Amendments/reagents: Carbonate bearing waste rock or other alkaline amendments such as fly ash, cement, kiln dust, green liquid dregs, lime, limestone, mesa lime, phosphate minerals, red mud bauxite, red gypsum, slag, sugar foam

Question 6

What are the pros and cons with co-disposal and blending?

Answer 6

o Pros
Can control acidity and heavy metal concentrations of AMD
o Cons
-Failing if inadequate mixing, or if grain size is too coarse in the neutralent
-Requires homogeneous mixing
-AMD neutralization products can inhibit the dissolution of carbonate minerals through armoring and a substantial excess of neutralizing capacity is therefore required for the method to work in the long term
-Requires sufficient tonnage of carbonate-bearing waste rock material
-Acidification and neutralization potential has to be balanced in both directions

Question 7

Describe in-pit filling and backfilling

Answer 7

Into mining voids, declines, shafts, surface boreholes
Hydraulic sand filling, dry rock filling, paste tailings
Combined with other methods such as underwater disposal, alkaline addition, cover technologies, sulfate reduction
Amendments/reagents: Cementing binders

Question 8

What are the pros and cons with in-pit filling and backfilling?

Answer 8

o Pros
-Reduces surface environmental impact
-Brings underground support, reduces risk for rock bursting
-Binders helps to minimize groundwater contamination
-Can reduce oxidation rates (ARD formation)
o Cons
-High cost, especially with binders
-Hold ups in extraction and mine development strategies
-High need for dewatering of tailings, risk of liquefaction otherwise
-Risk for tailings effluent into groundwater
-Only possible far into mining operation phase

Question 9

Describe how tailings behave as a slurry

Answer 9

Rather homogenous mixed. Segregation during deposition possible due to different particle sizes and density - coarser sediments closer to discharge point.
Dewatering or separation of finer material might occur before deposition. Sometimes deposited on low permeability ground or on liners - risk for slippage.
Residual process chemicals might resist in the tailings. Lime is sometimes added.
Can be a security risk - for example seismic activity in Chile.
Consists of 35-40% solids.
Discharge point of slurry is moved periodically.

Question 10

Describe paste tailings

Answer 10

Water removed from the tailings, until coarse fraction of the slurry are filled with the fines resulting in a homogeneous mix. Surface disposal, underground cemented or uncemented or in-pit disposal.
Is a non-segregated slurry.
Amendments/reagents: A variety of additives depending on end use.

Question 11

What are the pros and cons of paste tailings?

Answer 11

o Pros
- Can remove risk of dam failure
- Reduce amount of space needed to store tailings
- Can save water
- Can reduce risk for AMD, it remains saturated
- Can be used in cement
o Cons
- Thickening technology evaluated case-to-case, base on geochemical and physical characteristics and varies in cost
- Surface paste disposal requires more long-term studies on chemical stability and environmental impact

Question 12

Describe dry-stacking

Answer 12

Tailings filtered and de-watered, to higher degree than paste. Water can be re-used in the process. Moisture content of less than 20% achieved by combination of belt, drum, horizontal and vertical stacked pressure plates and vacuum filtration systems

Question 13

What are the pros and cons of dry-stacking?

Answer 13

o Pros
 No need for tailings dam – eliminates risk of dam failure
 Can save money if cost of water is high
 Allows for partial filtering, a certain % of the mine tailings are filtered to lower the risk of the investment
 Reduces risk of groundwater contamination through seepage
 Better recovery of metals and process chemicals
 Generates a new type of material on site (Can be used for reducing sulphide oxidation in the waste itselt, as a cover, layer etc)

o Cons
 High capital and operating costs
 Few large-scale projects as filters couldn’t handle large quantities.
 Affects water balance since the stack cannot store water (Not suitable in climates with a lot of snow or rain)
 Oxidation of sulphides can create AMD, generally low amount
 Suitability depends on climate

Question 14

What does a geochemical characterization program consist of?

Answer 14

o Mineralogy studies, whole rock chemistry (elemental composition), ore and tailings investigated, leachability, mechanical properties, site specific parameters

Question 15

Mention some sources, pathways and recipients

Answer 15

o Sources:
 Tailings, waste rock piles, pit walls, heap leach piles etc
o Pathways:
 Runoff, infiltration, groundwater, surface water, biota, mine waters, air
o Receiving environment:
 Groundwater, surface water, air, soil, sediment

Question 16

Name some sulphides

Answer 16

o Pyrite (FeS2)
o Galena (PbS)
o Arsenopyrite (FeAsS)
o Pyrrhotite (FeS)
o Sphalerite (ZnS)
o Chalcopyrite (CuFeS2)

Question 17

Which are three typical mineral groups found in mine waste?

Answer 17

o Sulphides:
 Ex Pyrite (FeS2), sphalerite (ZnS)
o Silicates:
 Ex kalifältspat (KAlSi3O8), quartz (SiO2)
 70-99% of the waste
o Carbonates:
 Ex calcite (CaCO3)

Question 18

What is the chemical reaction of pyrite oxidation by oxygen?

Answer 18

4FeS2 + 14H2O + 15O2 -> 4Fe(OH)3 + 8SO42- + 16H+

Question 19

Describe pyrite oxidation

Answer 19

Oxidation of sulfur -> oxidation of ferrous (Fe(II)) iron to ferric (Fe(III) iron -> hydrolysis and precipitation of ferric complexes and minerals.
The oxidant Fe(III) dominates at low pH, 2-3 orders of magnitude faster than with oxygen.

Question 20

Which secondary elements can be formed after pyrite oxidation?

Answer 20

Precipitation to ferrihydrite, schwertmannite, goethite or jarosite depending on pH and key elements such as K and S.
Forms coatings and cemented layers -> decreases oxidation.

Question 21

What is the impact of microbial activity on pyrite oxidation?

Answer 21

They increase the oxidation rate. They obtain energy from oxidizing ferrous iron to ferric iron.
They can form microenvironments with low pH, even though the bulk water is neutral.

Question 22

Describe the iron speciation depending on pH

Answer 22

o pH > 3.5 Fe(II) dominates. Fe(III) relatively insoluble.
o pH 5-6 Fe-bearing solutions normally relatively reduced, acidic solutions more oxidizing
o pH < 3 high amounts of Fe(III) present

Question 23

Describe the oxidation of Sphalerite

Answer 23

ZnS (s) + 2O2 => Zn2+ + SO42-
No acid with oxygen as oxidant
Can contain Cd and thallium (Tl) which can be toxic
Zn can be substituted by Fe, and then it is acid generating similarly to pyrrhotite

ZnS (s) + 8Fe3+ + 4H2O => Zn2+ + 8Fe2+ +H+ + SO42-
Acid producing

Question 24

Describe the oxidation of Galena

Answer 24

o PbS (s) + 2O2 => Pb2+ + SO42-
Main source of Pb contamination in mining

o PbS(S) + 8Fe3+ + 4H2O => 8Fe2+ + SO42- + 8H+ + Pb2+

Question 25

Describe the oxidation of Pyrrhotite

Answer 25

o Faster than pyrite
o Less acidity forms
o Important in early stages of sulphide oxidation
o Dependant on Fe(III)

Question 26

Describe the oxidation of chalcopyrite

Answer 26

o CuFeS2(S) + 16Fe3+ + 8H2O => 17Fe2+ + 2SO42- + 16H+ +Cu2+
o Generates Fe(II) which can oxidize to Fe(III) and cause acidification
o One of the most resistant sulphides to oxidation

Question 27

Which sulphides can lead to acidic environments?

Answer 27

• Pyrite, marcasite, arsenopyrite, chalcopyrite, pyrrhotite, enargite can all produce acid

Question 28

Describe sulphide oxidation in tailings

Answer 28

o Minerals dissolve and metal concentrations increase - Until equilibrium is reached. Reaction speed decreases with depth
Affecting factors:
- Surface area (Larger surface areas with fine grained material)
- Porosity (oxygen and water infiltration)
- Hardpan formation
- Water transport
- Oxygen replenishment

Question 29

What effect does particle size have?

Answer 29

Smaller particles – increased weathering, water holding capacity, cementation. Decreased porosity, oxygen diffusion and infiltration of water.

Question 30

Which different oxygen transport mechanisms are there?

Answer 30

o Diffusion in pore spaces (Due to concentration differences)
o Advection in pore spaces (due to wind)
o Convection in pore spaces (due to temperature)
o Barometric pumping (air pressure)
o Infiltration of water into tailings

Question 31

Describe differences in solubility and diffusion of oxygen

Answer 31

o Lower in water than air
o Affected by temperature and salinity

Question 32

Describe how oxygen flux is determined by Fick’s law

Answer 32

It is the effective diffusion coefficient time the concentration difference divided by the oxidation front depth.
The implication is that the oxygen flux decreases with depth

Question 33

How is water content measured?

Answer 33

o Volumetric (Vw / Vtot)
o Gravimetric (mw/mtot)

Question 34

Which zones are found in a tailings profile?

Answer 34

Oxidized zone
Accumulation zone
Unoxidized zone

Question 35

What happens in an oxidized zone in a tailings profile?

Answer 35

 Water infiltration
 Oxygen diffusion
 Sulfide oxidation
 Carbonate dissolution
 Silicate weathering
 Coatings on minerals

Question 36

What happens in an accumulation zone in a tailings profile?

Answer 36

 Acid neutralization
 Secondary mineral formation (Deadpan formation of oxyhydroxides, gypsum etc) which adsorbes dissolved metals from the water migrating within the tailings pore space
 Metal accumulation (adsorption, co-recipitation)

Question 37

What happens in an unoxidized zone in a talings profile?

Answer 37

 Limited water movement
 Low oxygen diffusion
 Low weathering rate

Question 38

Describe buffering by silicates

Answer 38

 Slower process than carbonates and metal hydroxides.
 Congruent weathering (Complete dissolution)
 Incongruent weathering (Alteration of mineralogy)
 Major reservoir of buffering capacity
 Increased weathering in acidic conditions, results in release of elements associated with silicates and consumption of acid
 Al is generated from silicate weathering, could act as a secondary buffering mineral as Al(OH)3 in future evolution of sulphide oxidation
 Major elements: Mg, Fe, Ca, Na, K

Question 39

Describe buffering by carbonates

Answer 39

 Releases Ca, Mg, Fe, Mn
 The most important minerals for acid buffering reactions
 Calcite, dolomite, ankerite, magnesite, siderite, rhodochrosite
 Calcite most reactive (Buffers the pH to neutral values and at pH 7 HCO3 - is the dominant specie. For the neutralization of 1 mol H+ 1 mol calcite is necessary. At pH below pH 6.3 carbonate can neutralize two moles of protons, as H2CO3 is the dominant species.)
 Neutralizer at pH 6.5-7.5 mainly
 Iron containing carbonates (Siderite):
- In reducing conditions – dissolution, Fe(II) forms
- In oxidized conditions – Fe(II) oxidizes to Fe(III) which precipitates to Fe-hydroxides and generates acid

Question 40

Describe buffering by lime

Answer 40

 Lime formula Ca(OH)3
 Increases pH to 10.5
 Highly soluble in water, fast neutralization reactions

Question 41

Describe buffering by metal hydroxides

Answer 41

 Can dissolve and buffer pH at different levels. Takes up 3 H+ for every dissolution.
 Gibbsite at pH 4 – 4.3
 Ex. Goethite at pH below 3.5

Question 42

How is the calcite buffering in closed vs open systems?

Answer 42

o Closed system (Water saturated wastes)
CaCO3(s) + 2H+ -> Ca2+ +H2CO3
Or
CaCO3(s) + H+ -> Ca2+ + HCO3-

o Open system (Unsaturated waste)
CaCO3(s) + CO2(g)+ H2O -> Ca2+ + 2HCO3-

Question 43

Name some buffer reactions at different pH

Answer 43

 pH 7: CaCO 3 + H + <–> Ca 2+ + HCO3 –
 pH 5.5: FeCO 3 + H + <–> Fe2+ + HCO3-
 pH 4.5: Al(OH) 3 + 3H + <–> Al 3+ + 3H2O
 pH 4: Fe(OH) 3 + 3H + <–> Fe3+ + 3H2O

Question 44

What is the weathering rate affected by?

Answer 44

o Crystal size, shape, surface area
o pH
o Dissolved CO2
o Temperature
o Redox conditions
o Exposure of the mineral to oxygen
o Removal of products (For example if water at equilibrium is replaced by new water, then more dissolvement possible)
o Weathering resistance to acid and oxygen
o Total amount of accumulated material

Question 45

Describe gypsum formation

Answer 45

o One of the most common secondary minerals
o From neutralization of acid
- In tailings (presence of carbonates)
- Neutralization station (quick lime)
- Treatment in water systems
o CaCO3 + H2O + H2SO4 => CaSO4*2H2O + H2CO3