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Static Geochemical Tests For Mine Drainage Prediction Acid Base Accounting Net Acid Generating Test Mineralogy- Optical, X-ray Diffraction Elemental – X-ray Florescence

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Static Geochemical Tests

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Page 1: Static Geochemical Tests

Static Geochemical Tests For Mine Drainage Prediction

• Acid Base Accounting• Net Acid Generating Test• Mineralogy- Optical, X-ray Diffraction• Elemental – X-ray Florescence

Page 2: Static Geochemical Tests

Sampling

Page 3: Static Geochemical Tests

Acid Base Accounting• Maximum Potential Acidity (MPA), also called Acid Production

Potential (APP)

• Neutralization Potential (NP), also called Acid Neutralizing Capacity (ANC)

• Net Neutralization Potential (NNP), also called Net Acid Production Potential (NAPP)

• NNP = NP - MPA

• Paste pH, Fizz

• Does not predict pH or concentrations of metals and sulfate

Page 4: Static Geochemical Tests

Acid Base Accounting Stoichiometry

FeS2 + 2 CaCO3 + 3.75 O2 + 1.5 H2O →

2 SO42- + Fe(OH)3 + 2 Ca2+ + 2 CO2 ↑

One mole of pyrite oxidizes to produce 4 moles of acidity, sulfate and Iron Hydroxide.

Two moles of calcium carbonate (calcite) are required to neutralize the acidity.

On a mass basis, 200 grams of calcium carbonate are required for 64 grams of sulfur from pyrite, or ratio of 3.125. When Acid Base accounting is expressed in parts per thousand, the mass ratio is 31.25.

Page 5: Static Geochemical Tests

++ == NeutralNeutralWaterWater

Acid Base Accounting Stoichiometry

One Mole Pyrite Two Moles Calcite

Page 6: Static Geochemical Tests

Maximum Potential Acidity• Calculated from total sulfur measurement. ABA assumes all

sulfur present as pyrite. For many rocks this is a valid assumption.

• Ore bodies and waste rock at metal mines usually contain different sulfide minerals such as sphalerite (ZnS), galena (PbS), and others, in addition to pyrite.

• Not all sulfide produce acidity when oxidized, so total sulfur will probably over estimate potential acidity. For these mines,identification of specific sulfide minerals is helpful, using X-ray diffraction (XRD) and x-ray florescence (XRF) or optical techniques. The samples may also be tested using kinetic methods

• If sulfate minerals or organic sulfur are present, fractionate into sulfide, sulfate and organic. Organic S considered non-acid forming

Page 7: Static Geochemical Tests

Maximum Potential Acidity• Sulfate minerals like gypsum CaSO4* 2 H2O do not form

acid drainage.

• Metal sulfate salts such as copiapite FeIIFeIII4(SO4)6(OH)2* 20

H2O, represent “stored acidity”. They generate acidity by dissolving and metal hydrolysis.

FeIIFeIII4(SO4)6(OH)2* 20 H2O → Copiapite dissolution

Fe2+ + 4 Fe3+ + 6 SO42- + 2 OH- + 20 H2O

Fe3+ + 3 H2O → Fe(OH)3 + 3 H+ Oxidation and Hydrolysis of 5 moles of Fe yields 14 moles H+ , minus 2 moles OH- .

• Sulfate fractionation does not identify the minerals.

Page 8: Static Geochemical Tests

Sulfate Salts on Coal

Page 9: Static Geochemical Tests

Neutralization Potential

• A measure of acid neutralizing capacity of a rock.

• NP represent mostly carbonates, and small amounts of exchangeable bases and soluble silicate minerals.

• Modification of a test method designed to measure the calcium carbonate content of agricultural lime

Page 10: Static Geochemical Tests

Neutralization Potential –Siderite interference

• The iron carbonate, siderite can interfere with the determination of neutralization potential. Siderite will produce a net neutralization of zero.

FeCO3 + 2 H+ → Fe2+ + CO2↑ + H2O (Neutralization)

Fe2+ + 0.25 O2 + H2 O + H+ → Fe3+ + 1.5 H2O (Oxidation)

Fe3+ + 3H2 O → Fe(OH)3 + 3H+ (Hydrolysis)

FeCO3 + 1/4 O2 + 3/2 H2O → Fe(OH)3 + CO2 ↑ Summary reaction

A modified test using H2O2 is used in some laboratories to correct for siderite

Page 11: Static Geochemical Tests

Effects of Siderite and Test Method on Neutralization Potential

92692690%Calcite

56648% Siderite55% Clays

167049% Siderite

NP (ppt)H2O2

Method

NP (ppt)Standard Method

Sample Composition

Data from Skousen et al, 1997

Page 12: Static Geochemical Tests

Example Acid Base Accounting Data

1.37Coal

-37.6513.651.251.640.36Sandstone, gray

-14.1615.2229.380.940.3Shale,black2.1618.2116.250.520.3Shale,black3.368.0517.50.560.8Shale,black5.8810.5715.940.510.9Shale,gray4.339.024.690.150.9Shale,gray696.37014.690.150.58Limestone760.37654.690.150.3Limestone816.38214.690.150.3Limestone

NNP(ppt)

NP(ppt)

MPA(ppt)

% SThickness (meter)

Rock Type

Page 13: Static Geochemical Tests

Summary Interpretation Acid Base Accounting

• Ratio of NP:MPA. – Ratio <1, likely acid producer– Ratio 1<Ratio<2, Variable, some acid, most alkaline – Ratio> 2, acid neutralizer, source of alkalinity

• Neutralization Potential– NP>20ppt, acid neutralizer, source of alkalinity– 10<NP<20, Variable, some acid, most alkaline– NP<10, likely acid producer

• Net Neutralization Potential– NNP>12, acid neutralizer, source of alkalinity– 0<NNP<12, Variable, some acid, most alkaline– NNP<0, likely acid producer

Page 14: Static Geochemical Tests

Net Acid Generating Test (NAG), Australia

• React sample with H2O2 overnight to oxidize pyrite. Acid formed should react with neutralizers.

• Measure pH, Acidity, sulfate, specific conductance and others. Titrate solution to pH 7.Calculate H2SO4 equivalent

• Repeat sequence for samples with more than 1.5% S

Suggested Interpretation of NAG TestIf pH=4.5 , NAG = 0, does not form acid

If pH<4.5 , NAG < 5, low acid formerIf pH<4.5 , NAG > 5, likely to form acid

Interpretation may vary by site conditions

Page 15: Static Geochemical Tests

Example NAG Data, Australia

0.44.01311240.8C

6.3781242026.6B

7.324320545014.7A

NAGNAG pH

NAPPANCMPA% SSample

From “ARD Test Handbook”, 2002, AMIRA International, Melbourne, Australia

Page 16: Static Geochemical Tests

NAG Sample Interpretations• Sample A. Mineralogy shows all S in pyrite, but oxidizing slowly.

Reactive neutralizers present. May generate acidity long term after carbonates are reacted.

• Sample B. Much of S present in Galena (PbS) and sphalerite (ZnS), which do not form acid. Sample has enough neutralizers present to produce non acid water.

• Sample C. Low acid forming potential, but also low acid neutralizing capability.

Page 17: Static Geochemical Tests

Mineralogy

• Identify specific minerals present using optical methods, X-ray diffraction, scanning electron microscope

Calcite and dolomite Illite