Environmental evaluation of Cypress mine proposed overburden, West Coast, New Zealand

Hughes, Joseph Benjamin.

The geochemical properties of Kaiata Formation and Brunner Coal Measures are evaluated to determine their suitability for placement at the proposed Cypress and existing Stockton mines. Correct scheduling of this waste material is needed to prevent sulfide oxidation and the generation of acid rock drainage. The Kaiata Formation has both acid-generating and acidneutralising properties. Results from this study and previous research undertaken by Kingett Mitchell Ltd (2003) indicate that within the Kaiata Formation the sulfur content decreases with distance away from the Brunner Coal Measures/Kaiata Formation contact, while the calcium carbonate content increases. Sulfur values range from 0.33 to 4.23 wt % and the acid neutralising capacity is equivalent to a calcite content ranging from 0 to 9.8 wt %. The Brunner Coal Measures have a low capacity to neutralise acid and their sulfur content is variable ranging from 0.25 to 3.5 wt %. Appropriate control measures are therefore needed for the Brunner Coal Measures and any Kaiata Formation within close proximity to its contact. Tests have been undertaken to develop a rapid field and a rapid laboratory procedure to define the potentially acid-forming from the non acid-forming Kaiata Formation material. Results indicate that Paste pH, Net Acid Generation (NAG) and Carbonate Bomb tests are suitable. Results from the Carbonate Bomb test show a strong correlation with the Acid Neutralising Capacity (ANC) and the Net Acid Production Potential (NAPP), indicating that the Carbonate Bomb test may efficiently determine non acid-forming material in the field within fifteen minutes. The alkaline products Karamea Limestone, Omya Aglime, Holcim Cement Kiln Dust (CKD) and Bauxsol have been evaluated for possible use in controlling the effects of acid rock drainage. The effectiveness of these materials in neutralising acid was tested by analysing leachate compositions after alkaline addition using shake extraction and column leaching procedures with potentially acid-forming waste rock. Results indicated that Karamea Limestone, OMY A Aglime and CKD can neutralise the acidity that can be potentially released from the Brunner Coal Measures, resulting in a reduction of metals being mobilised from this material. The acid neutralising capacity of Bauxsol was found to decrease over the duration of the leaching process, and is not recommended for use in mitigating acid rock drainage.

xiv, 150 leaves : ill. (some col.), col. maps, tables ; 30 cm.

2004Hughes