Description : Acid producing potential static tests have been undertaken on sample lithologies from New Zealand coal mines, to rank the lithologies into acid producing waste rock and stable non-acid producing waste rock. Results indicated that the main acid producing waste rocks in the Waikato Coal Region are the marine Whaingaroa siltstone and the Glen Massey formations and . the main alkaline producing lithologies were the Tauranga group volcanic-derived sediments A&D and the Mesozoic basement. Coal ash analysis of 28 samples from 28 mines has also been analysed for reactive sulphur percentage and their . base potential has been quantified by the chemical index of alteration CIA . The results provide mine management in the Waikato Coal Region with a classification of problematic and stable waste rock types.
New Zealand coal ash has the potential to be a problematic acid generator if the CIA value is above 20, CNK proportion in an A–CNK–FM ternary plot is between 30% and 56%, reactive sulphur per tonne of coal is >0.5 kg and dry ash weight percent in the coal is >5%. XRF and CIA values determined from wash drill cuttings would provide a method of determining waste rock volumes and magnitude of palaeoweathering underground.