The Importance of Evaluating Grinding Performance

Previous papers at Metplant conferences have covered the options for crushing and grinding circuit selection. High Pressure Grinding Rolls (HPGR) may have considerable advantages over autogenous milling circuits but test work and evaluating different grinding circuits must be done to establish which circuit provides the best option.

The Hawsons low grade magnetite project provides a specific example in using thorough test work, careful simulations and detail in design to allow the evaluation of grinding circuit performance. Fortunately tools are available for this evaluation and the test work for the Hawsons iron ore is detailed in this paper. The results of the test work and simulation indicates that although the Hawsons iron ore is competent in terms of autogenous milling given the medium hardness of the ore in the Drop Tests, the HPGR testing indicates that the use of HPGR is likely to use half the energy requirement. This is a characteristic of the Hawsons iron ore and is likely to be valid for other iron ores in the Braemer Iron Formation which stretches from the Hawsons lease in New South Wales to Razor Back and South Dam in South Australia.The Hawsons magnetite iron ore resource has been known from the early 1960s when CRA explored the area. However, the Pilbara region was discovered in this era and this led to development of high grade direct shipping grade hematite ores. Carpentaria Exploration (Capex) rediscovered the magnetite whilst undertaking multi-commodity exploration in the Broken Hill district during 2009.The magnetite grade was found to be low at around 16% Davis Tube mass recovery (DTR) but despite this, the project provides a positive business case as a result of the following key factors:

  • The mineralisation is particularly soft for crushing and grinding with a Bond Work Index of 6.3.
  • The mineralised rock fractures easily preferentially along grain boundaries.
  • The location is close to mining infrastructure and a skilled workforce in Broken Hill, 60km to the North East.
  • It is close to road and rail infrastructure for access to the resource.
  • The only contaminant in concentrate is silica.
  • The resource contains many billions of tonnes of magnetite mineralisation.
  • The low Bond Work index is a key characteristic of the Hawsons iron ore and the use of autogenous milling and HPGR circuits is compared to other iron ores indicating why the plant energy requirement may be just as important as iron ore grades.