static geochemical tests
DESCRIPTION
Static Geochemical TestsTRANSCRIPT
Static Geochemical Tests For Mine Drainage Prediction
• Acid Base Accounting• Net Acid Generating Test• Mineralogy- Optical, X-ray Diffraction• Elemental – X-ray Florescence
Sampling
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
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.
++ == NeutralNeutralWaterWater
Acid Base Accounting Stoichiometry
One Mole Pyrite Two Moles Calcite
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
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.
Sulfate Salts on Coal
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
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
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
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
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
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
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
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.
Mineralogy
• Identify specific minerals present using optical methods, X-ray diffraction, scanning electron microscope
Calcite and dolomite Illite