Proposed Limpopo West Mine Concept Study ... - SRK Consulting

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SRK Consulting: 447493: Limpopo West Geochem Page 235.3 Data analysis and interpretation5.3.1 Acid Base AccountingAcid Base Accounting (ABA) testing was undertaken using the Modified Sobek method. The testmethod identifies the potential for the tested material to generate net acidity by determining the acidgenerating and neutralizing potentials and then calculating parameters from these two values whichindicate net acid generating potential.The analysis included the determination of initial pH, final NAG pH and determination of reactivesulfate (kg/t) using hydrogen peroxide oxidation. This approach used by IGS assumes that thesulfate produced during complete oxidation is representative of the sulfuric acid. From the value ofthe sulfate using hydrogen peroxide it was then possible to determine the acid generating potential(AP) of each sample as follows:( ) ⁄The Neutralization Potential (NP) of each sample was determined by reaction of the sample withexcess sulfuric acid over 24 hours and subsequent back titration to pH 8.3 using sodium hydroxide.The Net Neutralizing Potential (NNP) is calculated as the difference between Neutralizing Potential(NP) and Acid Potential (AP) and the Net Potential Ratio (NPR) are calculated for each sample asfollows:( )( )The NNP provide a balance between the acid generating mineral phases and acid neutralisingminerals, and allows the classification of the samples either as potentially acid consuming or acidproducing.The MEND Prediction Manual for Drainage chemistry (W.A Price, 2009) and the Global Acid RockDrainage Guideline (INAP, 2009) have been used as criteria to assess the acid generating potentialof the geological materials. The methodology that has been used in these guidelines advocatebasing AP on:The oxidation of sulfide minerals,The dissolution of acidic sulfate minerals, such as jarosite, alunite and melanterite,The hydrolysis of metals from sulfide and sulfate minerals.These guidelines emphasise that there is no minimum concentration of sulfide responsible for acidgeneration, and base the acid drainage assessment on NPR as presented in Table 5-3.Table 5-3: ABA Criteria based on NPR (W.A Price, 2009 and INAP, 2009)Criteria NNP (kg CaCO 3 eq/t) NPRPotentially Acid Forming (PAF) NNP< -20 NPR < 1Non Acid Forming (NAF) NNP> +20 NPR > 3Area of Uncertainty (UNC) -20 < NNP < +20 1 < NPR < 3The Acid Potential of the samples have been calculated for both Open and Closed systems. Theoxidation of pyrite in the open system results in carbon dioxide being exsolve into the atmosphere. Inthe open system, the acidity produced from 1 mole of pyrite (FeS2) is neutralised by 2 moles ofCaCO 3 resulting in a mass ratio of 1.0 g pyrite: 3.125 g calcite. The following reaction illustrates theopen system:( )GOGP/LAKJ Report_April2013_19April2013 (1)_Final_Ludi_22052013 April 2013
Acid Generating Potential kg/t CaCO3SRK Consulting: 447493: Limpopo West Geochem Page 24The Closed Systems depicts acid generating potential of a certain material whereby carbon dioxidedissolves in the water resulting in the formation of carbonic acid (H2CO3) which in turns reacts withCaCO 3 to form HCO3. In this reaction, acidity produced by 1 mole of pyrite is neutralised by 4 molesof CaCO 3 resulting in a mass ratio of 1.0 g pyrite: 6.325 g calcite. The following reaction illustratesthe closed system:( )As the samples tested represent composites of a variety of lithologies, it is assumed that thematerials being characterised represent a mixture of ore (coal seams) and waste (overburden andinterburden). The summary of the ABA results received from IGS for the waste rock and coalsamples is included in Appendix B.Neutralising potential (NP) in the samples varies widely, ranging from 0.06 kg CaCO 3 eq/t to 132 kgCaCO 3 eq/t. The variation might be attributed to compositing approach taken by IGS which resultedin a wide variety of matrix being mixed. The ratio between NP and AP represented by the NPRvalues is presented in Figure 5-2. It is evident that some of the samples (34%) plot above the 1:1 line(red) indicating that these samples have the potential to generate acidity. A considerable portion ofthe samples plot below the 4:1 line (green), indicating that these samples have some neutralisationpotential available.Previous studies on the mineralogy of the samples in the vicinity the LPWM project area (SRK, 2011and Bester et. al, 2009), indicated that minerals such as calcite (CaCO 3 ) and ankerite (Ca (Fe, Mg,Mn) (CO 3 ) 2 ) are present in most of the lithologies and might therefore contribute to the NP.400.00350.00300.00OPEN SYSTEM250.00CLOSEDSYSTEM200.00150.001:1 Above thisacidification islikely2:1100.004:1 Below thisacidificationunlikely50.000.000.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 400.00Neutralising Potential kg/t CaCO3Figure 5-2: AP vs. NP (NPR)GOGP/LAKJ Report_April2013_19April2013 (1)_Final_Ludi_22052013 April 2013
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