Prevention

Question 1

In what stage of mining should prevention be the focus?

Answer 1

In the assessment and design phases
o Decreasing amount of options as time goes
o Increasing cost of prevention as time goes

Question 2

What is the primary approach of prevention?

Answer 2

Apply methods that minimize the supply of the primary reactants for sulfide oxidation, and/or maximize the amount and availability of acid neutralizing reactants
May include:
 Minimizing oxygen supply because of diffusion or advection
 Minimizing water infiltration and leaching (water acts as both a reactant and a transport mechanism)
 Minimizing, removing, or isolating sulfide minerals
 Maximizing availability of acid neutralizing minerals and pore water alkalinity
 Controlling bacteria and biogeochemical processes

Question 3

Which prevention methods are used pre-mining?

Answer 3

o Characterisation
 Both ore and waste rock
 Identifying and quantifying minerals
 Optical microscopy, sequential extraction etc

o Prediction
 Rough estimation
 Categories PAG, NAG, UC
 Static testing – no consideration of field measurements
 Kinetic testing – data available later during mining
 Rarely considers other factors such as microbiology, texture, hardness

o Modelling
 Combines investigation data, characterization and prediction to increase understanding of processes and reactions
 Only as good as the information it is based on

o Planning for avoidance
 The decision of not to extract a particular rock type which could be difficult to manage in the future e.g. being highly reactive or containing elements of potential concern
 could also include when/how to transcend from open pit mining to underground mining
 Can decrease the long-term risk for negative environmental impact
 Can decrease the cost for treatment and remediation
 Can impact short term profit

Question 4

Which menthods can be used for prevention during operation?

Answer 4

Segregation, layering, co-disposal and blending, desulfurization, inhibition of sulphide oxidation, paste tailings, dry stacking, integrated waste storage facility

Question 5

What is meant by segregation?

Answer 5

 Separation of NAG and PAG particles
 Decreases the measures and costs of preventing oxidation
 Allows for strategic deposition of PAG materials
 Could also include sorting of different size fractions to be used in cover systems or similar.
 Can largely limit the amount of waste needed to treat or placed in purpose-built repositories.
 Require very detailed characterization and prediction of the material with a mine plan based on it.

Question 6

What is meant by layering?

Answer 6

 Placement of acid generating and acid consuming materials in geometries designed to control and limit ARD formation. Layering can also include the introduction of low permeability layers.
 Relies on the segregation of waste materials.
 Low permeability and/or acid consuming materials (possibly alkaline industrial residue or similar) can be used
 Utilization of waste generated on site
 Effectiveness and cost depends on the availability of materials and the balance between acid producing and acid neutralizing materials
 Preferential flow and poor mixing of pore water can lead to acid generation although there theoretically is enough neutralization potential available

Question 7

What is meant by desulfurization?

Answer 7

 Flotation used to separate non-economic sulfide minerals into a low volume stream.
 Decreases amount which needs treatment
 If the sulfur content can be decreased down to 0.1% or 1% having an NPR >3, the material is considered inert and could be used for other applications and hypothetically be sold as a separate product
 Xanthates are associated with a number of toxicological, ecological, and health and safety issues.
 The geotechnical behavior of desulfurized tailings is different compared to “regular” tailings. Thus, new regimes for its usage may need to be developed.

Question 8

What is meant by inhibition of sulphide oxidation?

Answer 8

 Promotion of coatings on acid producing sulphides in an early stage
 Coatings (microencapsulation) could consist of Fe-phosphate, Fe-(oxy) hydroxides, carbonates
* Utilizes the mine wastes’ own ability to oxidize and a coating agent/buffer to raise and maintain the pH at near-neutral level to precipitate secondary minerals on reactive surfaces.
* Buffer/Coating agent: cement kiln dust, fly ashes (biomass, coal etc.), green liquor dregs, lime, limestone, lime kiln dust, mesa lime, phosphate minerals, red mud bauxite, red gypsum, slag, sugar foam.
* Natural rocks and industrial wastes with high neutralization potential can be used to control the acidity and concentrations of metals in the leachate.
* Potentially a lower amount of neutralizing material can be used compared to the net neutralization potential of the mine waste.
* Can be used for both tailings and waste rock
* Can be cost-effective depending on what amendment is used
* Takes time (years) to be considered a “permanent solution”
* Long-term performance is not well known

Question 9

What is meant by paste tailings?

Answer 9

 Can reduce the potential for acid mine drainage as it remains saturated
 Can be used in cement
 The thickening technology needs to be evaluated on a case to case basis based on geochemical and physical characteristics and varies in cost
 Surface paste disposal requires more long-term studies on chemical stability and environmental impact

Question 10

What is meant by dry stacking?

Answer 10

 Generate a new type of material on site which is possible to compact which has the potential to be used for reducing sulfide oxidation in both the waste itself but also to be used as a cover, layer or similar to prevent oxidation in other waste streams.
 Oxidation of sulfides can create ARD but the volumes are generally low.
 Associated with high capital and operating costs i.e. less suitable for development projects
 Historically few large-scale projects as the filters were not able to process large quantities. Consultant companies predict that this will change in the near future as better filters are now available.
 Upstream diversion systems and surface water is required to prevent inundation, ponding and erosion of the dry stack

Question 11

What is meant by integrated waste storage facility?

Answer 11

 The creation of an integrated waste storage facility where several preventive methods are utilized. Usually, a combination of selective handling methods, chemical barriers, physical encapsulation and oxygen consuming materials are used. Has the potential to use materials generated on site such as desulfurized tailings, dry stacked tailings, sludge, etc
 Improved control of air movement within the pile (advection, convection and diffusion).
 Encapsulate materials with high potential to form acid through low permeability layers.
 Coat sulfide grains (chemical barrier)
 Combine construction methods that perform different depending on internal and external factors (geochemistry, topography, climate, material properties etc.)
 The construction is highly site-specific depending on what material types are generated within the mine workings.
 Usually associated with high cost. Since basically all parameters are individual, rigorous testing need to be performed.
 Due to the integrated manner of the facility, not all features are optimized. E.g. chemical barriers are likely more prominent when not combined with low permeability layers

Question 12

Which intermediate prevention methods are there?

Answer 12

Bactericides, organic coatings, water control, in-pit disposal and filling, re-mining

Question 13

What is meant by bactericides?

Answer 13

 Sulfide oxidation is accelerated by the presence of acidophilic iron- and sulfuroxidizing bacteria. By removing or decreasing the amount of bacteria, the oxidation is substantially decreased
 Reduces the activity or exterminates bacteria present in the mine waste most commonly by disrupting the bacteria’s’ contact with the reactive mineral surface or by harming their protective cell membrane.
 Bactericides can effectively inhibit the growth of microorganisms, thereby suppressing ARD formation.
 Cannot permanently inhibit microbial activity and repetitive additions are required
 Should be considered as a last minute resort or “in waiting for”- method.
 Since the reagents are toxic to microorganisms, they could also be potentially toxic or harmful to other type of biota.
 Needs to be continually added

Question 14

What is meant by organic coatings?

Answer 14

 The organic substances used create a hydrophobic chemical barrier like microencapsulation on the mineral surface, but this coating consist of organic matter which makes its properties and behavior differ from the inorganic microencapsulation
 Coatings are capable of inhibiting/limit sulfide oxidation with or without the presence of bacteria.
 The long-term stability is questioned since microorganisms could potentially degrade the coatings.
 Some of the amendments are toxic to the environment (e.g. 8 hydroxyquinoline, polyethylene polyamine, and oxalic acid)
 Many of the amendments tested are associated with high costs (e.g. alkoxysilanes and 8-hydroxyquinoline)
 Some of the reagents are pH sensitive with decreased solubility at low pH which limits their use when ARD is forming (e.g. humic acid).
 Most suitable for non-oxidized mine waste.

Question 15

What is meant by water control?

Answer 15

 Water acts as a reactant and transport mechanism for ARD. Although the amount of water needed for sulfide oxidation is basically always present (except in extremely arid environments). Water control mainly aim to reduce infiltration and thereby reduce the ARD volume since water treatment costs are more related to volume of, rather than concentrations in, the ARD
 Diversion of site surface drainage ditching and groundwater (barriers and higher permeability features that help control the flow of groundwater) as well as dewatering.
 Reduce the water supply to/from potentially acid-generating materials
 This is a cheap technique, easy to apply and that works well in conjunction with other techniques.
 Depend on site conditions, including climate, topography, geology, hydrology, and hydrogeology.
 Drainage must be based on catchment hydrology and usually require ongoing maintenance (debris accumulation, sloughing, and animal activity etc.)

Question 16

What is meant by in-pit disposal and backfilling?

Answer 16

 May be combined with other strategies such as subaqueous or underwater disposal, alkaline addition, cover technologies, and sulfate reduction.
 Reduce surface environmental impact by storing the material underground.
 Can reduce oxidation rates (ARD formation)
 High cost (especially when binders are used)
 Hold ups in extraction and mine development strategies
 Tailings need to be highly dewatered. Risk of liquefaction of the tailings if saturation levels are high.
 Usually only possible after of far into the mine operation phase

Question 17

What is meant by re-mining?

Answer 17

 Increasing metal prices and new emerging technology for extraction allow for re-mining.
 Could include processing waste rock that now is considered ore.
 Good opportunity to improve waste disposal systems and reduce or remove sulfide minerals, thus giving value at a site that needs to be cleaned up.
 Secondary minerals formation and mineral weathering may require other type of extraction methods making few sites suitable for re-mining.
 Can lead to degraded water quality
 Resource efficiency is driven by“social license”and need of critical elements (e.g.EU,Northvolt)

Question 18

Which methods for prevention are done during closure?

Answer 18

Oxygen barrier, seals, flooding

Question 19

What is meant by oxygen barrier?

Answer 19

 Engineered cover consisting of fine grained materials, alkaline materials, organic substrates over reactive mine wastes. Dry and water covers limits water and/or supply of O2 to the mine waste
 Could utilize materials with high saturation/ good water holding capacity or submerging mine tailings under water.
 Could use natural till/soil, non-reactive mine residues, low sulfide/desulfurized tailings, ARD treatment sludge, ash, clay, industrial alkaline wastes, organic materials
 Organic material can help create highly reduced conditions
 Alkaline covers may enhance the solubility of some elements of potential concern
 Organic covers can cause reductive dissolution of iron precipitates
 Organic amendments degrade over time endangering the geotechnical structure and stability of the barrier
 Should be accompanied by other preventive methods during operation to minimize the accumulation of secondary minerals

Question 20

What is meant by seals?

Answer 20

 Hydraulic seals that limit the movement of water and air through mine workings.
 Seals can be used for reinforcement and to keep water out during operation but also opposite, to promote flooded conditions after decommissioning.
 Alter or restrict the movement of water which helps keep all or the most reactive minerals water covered.
 Rely on the knowledge of water ingress and discharge locations.
 Require rigorous engineering design

Question 21

What is meant by flooding?

Answer 21

 Stop pumping groundwater from underground or open workings, allowing the voids to fill with water can substantially inhibit the oxygen supply, preventing (further) ARD formation.
 The construction and raise of tailings dams to develop a water cover above.
 Flooding underground workings or open workings with water can substantially inhibit the oxygen supply, preventing (further) ARD formation.
 Flooding of pre-oxidized mine waste (backfilled), pit walls or other open workings may result in secondary mineral dissolution and deterioration of the water quality.
 Should be accompanied by other preventive method during operation to minimize the accumulation of secondary minerals.
 Adding water cover above a tailings pond is associated with risk if the dam construction fails
 First flush impact possible

Question 22

What is meant by monitoring and maintenance?

Answer 22

 The process of routinely, systematically, and purposefully gathering information.
 Monitoring includes both observations and investigations.
 Monitoring is performed throughout the mining life cycle, all from base line studies to post-closure. However, it is during the decommissioning that current environmental conditions are compared to the predictive models to assess the need for long-term monitoring.
 Delay times between implementation and monitoring or measurement of effects often result in the need for long periods of monitoring or testing to determine the outcomes of implemented methods

Question 23

How is geochemical stabilization done?

Answer 23

Addition of alkaline industrial remnants
* (bio) fly ash
* Blast furnace slag
* Granulated blast furnace slag
* Cement kiln dust
* Lime kiln dust
* Green liquor dregs