Prediction

Question 1

What is the definition of inert waste?

Answer 1

o No significant decay or dissolution with negative environmental impact
o < 0.1% sulphide-sulfur or max 1% sulphide
o No spontaneous ignition or burn
o No harmful process chemicals
o Low content of As, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, V och Z

Question 2

What is done during early-phase prediction?

Answer 2

Drill core evaluation:
 Characterization of acid producing and neutralizing minerals
 Visible signs of weathering
 Trace element concentrations
 3D model
 Portable field XRF
Geochemical tests:
 Basic test for leachability
 Paste pH
 ABA and NAG test
 Leaching test of selected minerals or whole rock

Question 3

What are the different types of prediction tests?

Answer 3

Static tests:
 Hours or days
 To predict production of acid
 ABA, NAG
Short-term leaching test:
 Of sulphidic waste to give indication of constituents of interest
Kinetic tests:
 Simulate weathering and oxidation of sulphidic waste samples over time (months – years)
 Usually done after static tests to measure the lag time and longevity of acidity

Question 4

Describe the paste pH method

Answer 4

o Solid waste mixed with a certain amount of water -> measuring of pH
o In field -> first indication of acid potential (first flush characteristics)
o pH < 4 indicates potential to produce acid
o Electrical conductivity > 20 μS/cm indicates high levels of dissolved species
o No standard

Question 5

Describe the Net Acid Generation (NAG) test

Answer 5

Quantifies acid potential by adding hydrogen peroxide to oxidize sample

Single:
 Hydrogen peroxide -> boiling to get rid of hydrogen peroxide -> pH -> possibly titration to pH 4.5 and 7
 NAG pH < 4.5 higher risk for acid generation
 NAG pH > 4.5 non-acid producing
 Limitation - Incomplete oxidation of sulphides

Sequential:
 Hydrogen peroxide added in sequences to minimize uncertainties from its dissociation.
 Titration similar to the single NAG
 Repeated until pH > 4.5

Kinetic:
 Differences in pH and temp are controlled and measured during a single NAG test
 Estimation of reaction time and acid generation

Question 6

What are the pros and cons with NAG test?

Answer 6

Pros:
 Industry-wide testing
 NAGpH versus results from ABA (net acid producing potential NAPP) commonly plotted for geochemical classification

Cons:
 If carbonate minerals are present, unrealistic alkaline NAG pH are produced
 Quality of hydrogen peroxide might be bad
 Organic matter interferes with measurements

Question 7

What is the Acid Base Accounting (ABA) test?

Answer 7

o Static test to predict the potential of a waste sample to produce acid
o Implemented in Swedish legislation
o Based on Net Neutralization Potential (NNP) = Neutralization Potential (NP) – Acid Potential (AP)
o Alternatively based on Neutralization Potential Ratio (NPR) = NP/AP

Question 8

What are the category limits for NNP?

Answer 8

 Unit kg CaCO3/ton
 NNP > 20 no acidification potential
 NNP < - 20 potentially acid generating
 NNP between -20 and 20 – uncertaing acid generating potential

Question 9

How is neutralization potential (NP) decided?

Answer 9

 Acid consumption by neutralizing minerals
 Acid added to pH of 2 -> titrating with NaOH to determine amount of acid consumed by the sample
 Problems with strength of acid, varying variables (particle size, back-titration end-point, temperature, digestion duration)

Question 10

How is the acid potential (AP) determined?

Answer 10

 Maximum amount of sulphuric acid produced per tonne of sulphidic waste
 Wt% S * 31.25kg CaCO3/ton

Question 11

What are the limits for the NPR?

Answer 11

 NPR = NP/AP > 3 no acidification potential (NAG)
 NPR = NP/AP < 3 potentially acid generating (PAG)

Question 12

Which assumptions are with the ABA test?

Answer 12

 Sulfur only as S2-
 Pyrite the only sulfide mineral (If pyrrhotite is present, the assumption of only pyrite could lead to an overestimation of AP up to 1.5 times the actual value)
 Oxygen the only oxidant
 All Fe oxidizes to Fe(III)
 All Fe precipitates

Question 13

What are the pros and cons with the ABA test?

Answer 13

Pros
 Quickest method
 Most cost-effective to quantify ARD
 Industry-wide practise, labs doing the testing -> easily interpretable results
Cons
 AP overestimations if based on total S values
 AP and NP are not performed on exactly the same sample
 Site-by-site comparisons not possible

Question 14

What are the principles of kinetic tests?

Answer 14

o Mimics cycles of wetting and drying
o Simulate weathering and oxidation
o Validate and interpret static methods
o Predict long-term weathering rates and the potential for mine wastes and geologic materials to release elements
o Could evaluate acid generation and metal leaching
o Minimum 20 weeks

Question 15

What are some different kinetic tests?

Answer 15

Humidity cell test (HCT)
Column leach test
Field pilot-scale tests

Question 16

Describe the humidity cell test (HCT)

Answer 16

 Three days of moist air and three days of dry air
 Known amount of water added
 Metals, sulfate, pH, EC analyzed
 Elemental concentrations vs time
 Loads can be estimated

Question 17

What are the pros and cons of HCT?

Answer 17

Pros
* More realistic indication of leaching than short-term tests
* Tries to mimic climate cycle
Cons
* Mineral precipitation not reflecting actual site
* Rinse at the end may remove all reaction products
* Equilibrium not reached due to short contact times
* No mineralogical and microtextural analysis

Question 18

Decribe the column leach test and its pros and cons

Answer 18

 Simulate site-specific climate conditions
 Leaching quality over time
 Simulate mitigation measures such as covers
 Could allow precipitation of secondary minerals

Pros
* More realistic indication on leaching than short-term tests
* Procedural flexibility
* Low maintenance
Cons
* Unrealistic replication of climate
* No mineralogical and microtextural analysis

Question 19

Describe field pilot-scale tests and its pros and cons

Answer 19

 Important to compare with lab results
 Hard to conduct at an early stage of mine development

 Pros
* Exposure to the sites climate and microbiological conditions
* Examination of sulphide and carbonate textures
 Cons
* Not standardized
* Expensive construction and maintenance

Question 20

How is mine waste classified and what is a limitation with the classification?

Answer 20

o Does not consider metals or salinity
o Potentially acid forming (PAF)
- Kinetic tests must be done
- ARD management implemented
o Non-acid forming (NAF)
- If high sulphur content: Check for soluble secondary sulphate minerals (gypsum). Neutral to alkaline drainage but possibly high salinity levels (Exceeding sulphur guidelines). Possibly high metal content (Zinc, molybdenum, cadmium, arsenic, antimony, selenium)
o Uncertain (UC)

Question 21

What are some overall shortcomings with prediction methods?

Answer 21

o Over-/underestimations of acid forming potential
o Lack of consideration to other ARD forming minerals than pyrite
o Silicates are not considered for long term neutralization
o Controls on AMD formation such as mineralogy, texture and microbiological activity not considered