Acid Base Accounting

Acid Base Accounting) Determining Organic, Pyritic and Sulfate sulfurWritten by Ankan Basu on 19 October 2010

October 19, 2010 (Coal Geology): Acid Base Accounting (ABA) tests are a widely used procedure to interpret acid producing potential or acid neutralization potential of the overburden rock materials. ABA method is very popular in Northern America. In Australia, we have the same method with little difference in nomenclature. However, another method named Net Acid Generation (NAG) tests are also widely used in Australia that is not so popular in the US coal industry.

This article aims to clarify the nomenclature issue between the Northern America and Australia as well as provide little insight about the organic sulfur determination for ABA tests. Thousands of pages worth of articles available online from various sources on ABA and NAG tests apart from numerous other journal articles. This article should not be used as a reference guide for ABA tests.Table showing the equivalent terms in USA and Australia:

North America Australia

Net Neutralization Potential (NNP)

Net Acid Production Potential (NAPP)Net Acid Generation Potential (NAGP)

Acid Potential (AP)=%S*31.25

Maximum Potential Acidity (MPA)=%S*30.59

Neutralizing Potential (NP)Acid Neutralizing Capacity (ANC)Neutralizing Potential (NP)

NPP=NP-AP NAPP=MPA-ANC

Unit: Kg CaCO3/tonne Unit: Kg H2SO4/tonne

Notes: The lab result that we FIRST look at is the NNP or NAPP value (NET

Value). That tells you if the rock has any acid producing capacity. Now, the value is CALCULATED off two other numbers. So, to rely on this number, the AP (or MPA) and NP (or ANC) must be accurate.

Often, total sulfur is analyzed for the sample and used for the AP/MPA calculations (ANC=%S*30.59). However, the total sulfur includes sulfate

http://coalgeology.com/aba-acid-base-accounting-determining-organic-pyritic-and-sulfate-sulfur/7582/

http://coalgeology.com/aba-acid-base-accounting-determining-organic-pyritic-and-sulfate-sulfur/7582/

sulfur and the organic sulfur in the sample. So, we are over estimating the MAXIMUM POTENTIAL ACIDITY (MPA) ignoring sulfur that will never lead to any acidity generation. In most of the sedimentary rocks pyrite is the dominant contributor of sulfide sulfur while organic and sulfate sulfur contributes very little to the total sulfur content. But, it may not be correct for all rock types. Especially in the coal seams and for many dark shale and carbonaceous shale, organic sulfur can contribute significant amount. So, we need to determine the organic sulfur, sulfate sulfur and pyritic sulfur separately to properly evaluate acid producing potential of the overburden strata.

While MPA is calculated using the amount of sulfur that could produce acidity, Acid Neutralizing Capacity (ANC) is derived from acid digestion. If siderite is present in the sample, special test is required for proper determination of ANC.

First Let us discuss how we normally determine the Organic Sulfur Content for the ABA samples. Three step method is required for proper determination of pyritic (sulfide) sulfur that should be used for MPA calculations.

The sample is usually crushed and split into three identical parts. LECO Induction furnace is used to burn the sample and calculate SO2 for the sample.

Treat With

What is removed in this step?

What do we Get from LECO Furnace result?

Split 1

not treated

S (Py+S04+Organic)

Split 2

Treated with HCL

Removes Sulfate Sulfur, does not react with Pyrite

S (Py+Organic)

Split 3

Treated with HNO3

Removes Pyritic and Sulfate Sulfur

S (Organic)

Note: Remember we are not analyzing the decant, we are analyzing the sample (residue) after the acid treatments.

If you have data for all three tests from the lab, you can now easily calculate you sulfur content in various forms as:

Total Organic Sulfur= S (Organic) Total Sulfate Sulfur = S (Py+S04+Organic) – S (Py+Organic) Total Pyritic Sulfur = S (Py+Organic) – S (Organic)

Typical ABA Suite + NAG in Australia and various reporting units

Method Name Analyte Name Units

Total Carbon/Sulphur in soil by LECO

Sulphur %

HCl Extractable S, Ca and Mg in Soil ICP OES

Acid Soluble Sulphur (SHCl)

%W/W


Acid Soluble Calcium (CaHCl)

%W/W


Acid Soluble Magnesium (MgHCl)

%W/W

Acid Neutralising Capacity or Neutralisation Potential(ANC/NP)

ANC as % CaCO? %CACO3


ANC as % CaMg(CO?)2

%W/W


Acid Neutralisation Capacity/Neutralisation Potential

KGCA/T


Acid Neutralisation Capacity/Neutralisation Potential kg H?SO?/t

KGH2SO4/T


Acid Neutralisation Capacity/Neutralisation Potential Siderite Corrected

KGCA/T


Acid Neutralisation Capacity/Neutralisation Potential kg H?SO?/t

KGH2SO4/T

Siderite Corrected

Net Acid Generation Potential (NAGP)

Total Oxidisable Sulphur

KGH2SO4/T

Net Acid Generation Potential (NAGP)

Net Acid Production Potential

KGH2SO4/T

Single Addition Net Acid Generation (NAG)

pHox (NAG pH) NOUNIT


ECox (NAG Conductivity)

US/CM


NAG as kg H?SO?/tonne to pH 4.5

KGH2SO4/T


NAG as kg H?SO?/tonne to pH 7

KGH2SO4/T


NAG as kg CaCO3/tonne to pH 4.5

KGCA/T


NAG as kg CaCO3/tonne to pH 7

KGCA/T

Useful unit conversions between North America and Australia standards:

kg H2SO4 = 0.98 x kg CaCO3 pyrite% = sulfur% x 120/64 sulfur% = pyrite% x 64/120 carbon% x 81.66 = kg H2SO4/t neutralizing capacity (assuming that all the

carbon is calciumcarbonate) MPA (kg H2SO4/t) = 30.59 x sulfur% (assuming the sulfide is pyrite) ANC (kg H2SO4/t) = 0.98 x kg CaCO3/t 1 ounce = 28.35 gram

1 kilogram = 2.2 pound 1 tonne = 1.1 ton 1 metre = 3.28 feet 1 kilometre = 0.62 mile 1 hectare = 2.47 acres 1 litre = 0.264 gallon 1 cubic metre = 35.3 cubic feet

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Acid Base Accounting