Acid generating potential of waste rock and coal ash in New Zealand coal mines

Abstract 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.