Table 1 Results for Testing using QMR Reactive Ground Test
Table 2 Results for Testing using AEISG Code of Practice
Figure 1 – HSE-C1 under microscope show layer of pyrite
Figure 2 – Sample HSE A and HSE B under microscope showing areas of fine pyrite
Figure 3 - Log or Temperature for AEISG Screening Test
Figure 4:
The fine pyrite in the HSE A and B samples gave a small initial rise in temperature due to fine particles and acidic weathering agent used in the AEISG test. Note no reaction for larger pyrite grains in C sample
Prior to testing as per the AEISG Code of Practice each sample was assessed for reactivity risk based on pH, carbonate and the amount of reactive sulphide and coal using the QMR Reactive Ground Test (refer to Table 1 and Table 2 for results of both tests). All sample where alkaline and all samples contained carbonates which reduces the risk of reactivity. Four samples contained trace pyrite and one sample, HSE-C1, contained 1.76% pyrite (refer to Table 1, Figure 1, Figure 2 and Figure 3). Sample HSE-C1 also contained >8% coal which inhibited any reaction between sulphide and nitrate (refer to Appendix for case study). There was a small temperature isotherm for the samples with the fine pyrite due to the acidic nature of the weathering agent using in the AESIG,
Conclusion:
All samples were found not to be reactive when tested as per the AEISG Code of Practice and the QMR Reactive Ground Test. .
Assessed By:
Dr G. Cavanough
Date:
2nd September 2022
Data Files:
AEISG Temperature Logs:
Data24.xlsx
QMR Reactive Ground Test results tables and audited temperature logs:
Rga27.xlxs
templog24.xlxs
Appendix
Case Study: Coal as an Inhibiter to the Reaction of Sulphide
Purpose of Study
The purpose of this case study is to present an investigation of coal as an inhibiter to reactions with sulphide and nitrate
BAckground
Figure 5 shows a plot of the temperature during the AEISG Code of Practice Screening Test for a sample that has been inhibited by coal.
Figure 5:
Typical temperature graph for a sample that contained >10% reactive sulphide and 16.8% coal
Sample Containing Sulphide and Coal Tested as Per the AEISG Code of Practice
Samples containing mainly coal and some reactive sulphide when tested using the AEISG code of practice are classified as non-reactive as the coal inhibits the reaction of sulphide with nitrate. This is not the case for samples with some visible coal, with the result when tested as per the AEISG code of Practice being Marginal. The reaction is a rise in Temperature and then decrease in temperature over the time as the carbon neutralizes the reaction. (Refer Figure 8 and Figure 9).
Sample 3A5 was tested as per the QMR reactive ground test and found to contain 7 % pyrite.
Figure 6 – Sample 3A5 QMR Reactive Ground Test Temperature Log showing a 50°C rise in temperature.
Figure 7 – Sample 3A5 showing layers of Coal containing Pyrite
Figure 8 – Temperature of Sample 3A5 tested as per the AEISG Code of Practice at 55°C.
Figure 9 – Temperature of Sample 3A5 tested as per the AEISG Code of Practice at 90°C.
Figure 10 – Sample 3A5 with Coal replaced with Silica sand tested as highly reactive (AEISG test at 55°C)
Sample 3A5 was tested at per the AEISG Code of Practice at 55°C (refer to Figure 8) and 90°C (refer to Figure 9) , additionally the coal fraction was removed and replaced with silica sand and tested at 55°C (refer to Figure 10).
When the sample is tested in the “as received condition” it is Marginal at 55°C and generates a 5.3°C isotherm at 90°C. The sample is highly reactive when the coal fraction removed. Hence it is concluded that the the coal inhibits a reaction that will cause significant heating. The implication of this observation is that samples containing mostly coal may be classified as non-reactive with no testing. When tested for the coal content Sample 3A5 contained 8.7% coal. Therefore, for samples with visible coal, reactivity can be assessed by the coal and sulphide content of the samples (if the sample does not contain inhibiting carbonates). If the coal and sulphide are of similar concentrations the sample can be classed as not reactive.
adding coal to inhibit a reaction with TESTING AS per the AEISG Code of Practice
Samples of highly reactive black shale and a 5.2% pyrite sample were tested with added coal. It was found that the amount of coal must be approximately the same as the pyrite for the sample to be tested as not reactive. Silica sand was substituted for coal on a sample to confirm the reaction is due to coal and not dilution (Refer to Figure 11 and Table 1).
Figure 11 – Examples of temperature Isotherms
Table 1 – Results for Testing as per the AEISG Code of Practice
Mixtures of coal/acid and coal/weathering agent
The following experimental procedure was performed:
A sample of coal was crushed, and 9 grams was placed in 2 beakers labelled A and B.
The beakers were placed in an oven at 55°C.
Weather agent was prepared as per the AEISG Code of Practices.
The weathering agent solution was place in the oven at 55°C.
A solution of HCl was prepared placed in the oven at 55°C.
When both solutions were at 55°C water was added to the HCl so as both solutions had approximately the same pH.
2 grams of weathering agent was added to coal sample in beaker A, mixed thoroughly and the pH taken of a small subsample
2 grams of acid solution was added to coal sample in beaker B, mixed thoroughly and the pH taken of a small subsample
Both beaker A and B were placed back in the oven.
At 30-minute intervals subsamples were taken from each sample and the pH measure
After 90 minutes, as there was no change in the pH, 6 grams of water were added to each sample and the samples were thoroughly mixed and tested after 30 minutes.
Results
Table 2 – pH of Coal + Weathering Agent and Coal + acid.
Figure 12 – Samples in the oven
Figure 13 – At time 0 with pH indicator mixed with samples, weathering agent and acid solution.
Figure 14 – Comparison a pH test for sample at the start and end of the test. The pH of both samples increased from 4 to 5.
Comparison of coal and urea as inhibiters
A reactive sample was prepared by milling a section along a band of pyrite in a piece of black shale (refer to Figure 15).
Figure 15 – Black shale used to prepare reactive sample
The following experimental procedure was performed:
An 18 gram sample was prepared using 30% coal and 70% of the reactive sample.
An 18 gram sample was prepared using 1% Urea and 99% of the reactive sample.
Both samples were then tested as per the AEISG Code of Practice i.e., 4 grams of weathering agent and 18 grams of ammonium nitrate added, sample mixed and placed in reaction vessels and held at 55°C.
The pH of the samples was measured at the beginning the test and then after 24 hours.
Results
Figure 16:
Temperature of samples prepared as per AEISG Code of Practice over 24 hours showing temperature isotherm for the sample containing coal
Figure 17: Coal + Reactive sample pH at start and end of test showing pH decrease from 4.5 to 2.5.
Table 3 – pH of samples at the start of the test and after 24 hours.
Discussion
Results for testing of samples as per the AEISG Code of Practice indicates that coal:
Inhibits the reaction of sulphide and nitrates
The concentration of both coal and sulphide must be approximately equal for the reaction to be completed inhibited.
Increasing coal content increases the time for the onsite of a reaction and decreases the magnitude of the temperature isotherm decreases.
Results of testing a mixture of coal/acid and coal/weathering agent indicates that coal:
Increases the pH on initial mixing with both acid and weathering agent
The increase in pH is greater for the weathering agent.
If both heat and water are present, there is further increasing of the pH.
Results of comparison of coal and urea as inhibiters indicates that:
Urea is a more effective inhibiter than coal.
Reactive ground samples become more acidic when a reaction occurs.
Conclusion
These initial results indicate that the mechanism by which coal inhibits the reaction of sulphide and nitrates is similar to that of urea in that it inhibits acid production which is required for sulphide to react with nitrate.