Mine waste introduction Flashcards
Name some common ore types and metals they contain
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
What is the main difference between open pit mining and underground mining?
Open pit mining is cheaper and for lower grade ore
Name some processes used in enrichment plants
Crushing, milling, separation, flotation
Describe waste rock
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.
Describe co-disposal and blending
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
What are the pros and cons with co-disposal and blending?
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
Describe in-pit filling and backfilling
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
What are the pros and cons with in-pit filling and backfilling?
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
Describe how tailings behave as a slurry
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.
Describe paste tailings
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.
What are the pros and cons of paste tailings?
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
Describe dry-stacking
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
What are the pros and cons of dry-stacking?
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
What does a geochemical characterization program consist of?
o Mineralogy studies, whole rock chemistry (elemental composition), ore and tailings investigated, leachability, mechanical properties, site specific parameters
Mention some sources, pathways and recipients
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
Name some sulphides
o Pyrite (FeS2)
o Galena (PbS)
o Arsenopyrite (FeAsS)
o Pyrrhotite (FeS)
o Sphalerite (ZnS)
o Chalcopyrite (CuFeS2)
Which are three typical mineral groups found in mine waste?
o Sulphides:
Ex Pyrite (FeS2), sphalerite (ZnS)
o Silicates:
Ex kalifältspat (KAlSi3O8), quartz (SiO2)
70-99% of the waste
o Carbonates:
Ex calcite (CaCO3)
What is the chemical reaction of pyrite oxidation by oxygen?
4FeS2 + 14H2O + 15O2 -> 4Fe(OH)3 + 8SO42- + 16H+
Describe pyrite oxidation
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.
Which secondary elements can be formed after pyrite oxidation?
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.
What is the impact of microbial activity on pyrite oxidation?
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.
Describe the iron speciation depending on pH
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
Describe the oxidation of Sphalerite
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
Describe the oxidation of Galena
o PbS (s) + 2O2 => Pb2+ + SO42-
Main source of Pb contamination in mining
o PbS(S) + 8Fe3+ + 4H2O => 8Fe2+ + SO42- + 8H+ + Pb2+
Describe the oxidation of Pyrrhotite
o Faster than pyrite
o Less acidity forms
o Important in early stages of sulphide oxidation
o Dependant on Fe(III)
Describe the oxidation of chalcopyrite
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
Which sulphides can lead to acidic environments?
- Pyrite, marcasite, arsenopyrite, chalcopyrite, pyrrhotite, enargite can all produce acid
Describe sulphide oxidation in tailings
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
What effect does particle size have?
Smaller particles – increased weathering, water holding capacity, cementation. Decreased porosity, oxygen diffusion and infiltration of water.
Which different oxygen transport mechanisms are there?
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
Describe differences in solubility and diffusion of oxygen
o Lower in water than air
o Affected by temperature and salinity
Describe how oxygen flux is determined by Fick’s law
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
How is water content measured?
o Volumetric (Vw / Vtot)
o Gravimetric (mw/mtot)
Which zones are found in a tailings profile?
Oxidized zone
Accumulation zone
Unoxidized zone
What happens in an oxidized zone in a tailings profile?
Water infiltration
Oxygen diffusion
Sulfide oxidation
Carbonate dissolution
Silicate weathering
Coatings on minerals
What happens in an accumulation zone in a tailings profile?
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)
What happens in an unoxidized zone in a talings profile?
Limited water movement
Low oxygen diffusion
Low weathering rate
Describe buffering by silicates
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
Describe buffering by carbonates
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
Describe buffering by lime
Lime formula Ca(OH)3
Increases pH to 10.5
Highly soluble in water, fast neutralization reactions
Describe buffering by metal hydroxides
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
How is the calcite buffering in closed vs open systems?
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-
Name some buffer reactions at different pH
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
What is the weathering rate affected by?
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
Describe gypsum formation
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
