Quantitative Tool for Managing Acid Mine Drainage and Salinity

The work described in this report was carried out by CSIRO Exploration & Mining, QCAT Laboratory in Brisbane. The objectives of the project were:

• To determine the accuracy of the Prompt Gamma Neutron Activation Analysis (PGNAA) logging technique for the measurement of sulphur in overburden rock.
• To develop the SIROLOG PGNAA tool for the measurement of chloride content for in-situ salinity measurements from boreholes as a practical logging tool for salinity investigations.
• To develop a practice for incorporating PGNAA logging in the environmental management of acid mine drainage and salinity.
• To commercialise the tool for routine usage.

The technique employed for this work was Prompt Gamma Neutron Activation Analysis (PGNAA). This technique was developed by CSIRO in the past for the delineation of the coal seams and in situ determination of ash, iron, silica, alumina, sulphur, calcium and density of coal seams (Borsaru et al. , 2001; Borsaru et al. , 2004). A 70 mm diameter SIROLOG PGNAA logging tool was manufactured for this project. The PGNAA logging tool is one of the logging probes developed by CSIRO under the name SIROLOG and is available commercially. The tool uses a 252Cf neutron source as the primary source of radiation and a 200 mm x 50 mm diameter BGO ?-ray detector to record the ?-rays produced from the interaction of the neutrons and the rock (or coal) surrounding the borehole. The strength of the neutron source used in this work was ~ 2 µg (40 MBq). This is not considered a high activity radioactive source and does not cause more problems than routine nuclear logging carried out by commercial logging companies.

The present work showed that the PGNAA technique has potential for environmental applications, in particular, acid mine drainage and salinity. This work established that:

• PGNAA can measure total sulphur in overburden rock. Total sulphur is an important parameter in the estimation of the net acid producing potential (NAPP) of the rock and its estimation has a substantial contribution for developing an acid rock drainage and acid spoil control management plan.
• A correlation between NAG (net acid generation) and the PGNAA log was established at CAPCOAL area Pit D. This is important especially when NAG and total sulphur in the rock are not correlated.
• Correlation between Electric Conductivity and pH measured in the laboratory and the PGNAA logs were established from chip samples collected at different intervals from holes at CAPCOAL area Pit D.
• A correlation between the conductivity and PGNAA logs was established in a hole logged with both techniques. The hole did not intersect coal seams.

It is recommended that CSIRO develops the next generation SIROLOG PGNAA logging tool and marketing of the SIROLOG logging system will have high priority in CSIRO Exploration and Mining.