Underground processing plants have advantages related to a low environmental footprint, reduced mining costs, improved security and increased value of the product delivered to surface. This paper presents a case study of the world‟s first operational underground gravity gold plant at the former Gwynfynydd mine and innovation leading to a new „Python‟ modular underground system. The Gwynfynydd mine is located in north Wales, United Kingdom. In response to the environmental constraints of being located within a National Park, the operator designed and built a mill underground based on physical separation.
Many gold ores liberate favourably using fine-crushing techniques such as High Pressure Grinding Rolls or Vertical Shaft Impactor. Using comminution circuits for mineral liberation, rather than for final recovery process, and by utilising a relatively high-mass pull high-recovery process route, the Python pre-concentration plant was developed for underground operation with a capacity of 20 tonnes per hour. The Gekko Python is currently operating at the Central Rand gold project in South Africa.
Keywords: Gold ores; Crushing; Gravity concentration; Ore handling; Mining
The concept of processing gold ores on the surface is a traditional paradigm. A process of pre-concentration or full concentration of ore underground, if achievable, would result in a major shift in operating costs. In particular, run of mine ore or a large proportion of it will not need to be transported to the surface. As a result, considerable environmental and financial benefits are likely to be the outcome. Probably the first reported underground gravity gold processing plant was operated between 1991 and 1999 by Welsh Gold PLC at their Gwynfynydd gold mine in north Wales, United Kingdom.
The plant was permanently installed in old workings and was a traditional gold concentration process. The ore was found to be amenable to a simple gravity circuit due to the generally large (>100 μm) gold particle size in the ore. In 1996, Sedimentary Holdings Pty Limited applied to patent the concept of continuous mining by road header, crushing and sizing, conversion to a slurry and then concentration using InLine Pressure Jigs (Devereux & Gray, 1996). At that time the road header technology limited the invention”s applications.
In a recent review Lane, Fountain & La Brooy (2009) note that underground pre-concentration is likely to result in significant savings in ore hoisting and trucking costs if 40% of the ore (or concentrate thereof) could be taken to surface. Bamber et al. (2005) suggest a number of advantages for underground processing, including; reduced haulage and transport costs, lower cut-off grades due to reduced costs, increased reserve base due to lower cut-off, reduced mining selectivity needs, bulk mining approaches and improved ground conditions due to availability of coarse backfill from the plant. Bamber et al. (2006) describe previous work undertaken to evaluate the potential of underground pre-concentration, predominantly in South African gold mines.
They concluded that “the implementation of pre-concentration underground will result in substantial operating cost savings, thus lowering the cut-off grade and increasing the potential reserve, as well as increasing the value of ore delivered to the surface”.
The programme yielded the Python system, which consists of five key connecting components: 1) coarse and fine crushing; 2) screening; 3) gravity and/or flotation separation; 4) concentrate handling; and 5) tailings disposal systems. Traditional surface processing for gold recovery is normally designed to achieve recovery through whole ore leaching (e.g. CIL/CIP).
This involves grinding 100% of the ore into sub-50 μm to 75 μm particles. The Python relies on coarser grinding (i.e. fine crushing) and pre-concentration of the gold-bearing minerals rather than liberation of the gold itself. This potentially provides a substantially reduced feed to the final gold production process. At present, only pre-concentration is being proposed for underground use. Classical gravity concentration processes features high-grade, low-mass pull units. The new system will provide a mass pull of 10% to 35% with recoveries potentially greater than 90%, providing concentrate grades of three to ten times the mined grade.