Slime Dump Pre-concentration - "Build a smaller plant for the same gold recovery"

Falcon approached Golden Valley Gold Mine in Zimbabwe in May 1999 to introduce to them the idea of using a continuous concentrator to pre-concentrate old tailings material. Golden Valley has an existing mill that operates on a 24-hour basis and has done so for many years. In fact, Golden Valley is the oldest continuously operating gold mine in Zimbabwe, having been in operation since the early 1900s.

The use of Falcon's continuous concentrator for dump pre-concentration was a new topic and technology for Rob and Bill Teasdale (Mine Owners). The ability of the continuous concentrator to recover very fine particles at G-forces of up to 300Gs into a reduced fraction of the feed mass made the process attractive to Golden Valley. A test sample was sent to Canada and tested at Process Research Associates (P.R.A.) in Vancouver to determine its amenability to gravity concentration.

The results achieved were as follows:

· Metallurgical recovery: 80%
· Mass recovery: 30%
· Feed grade: 1.1g/t Au.
· Concentration ratio: 3:1
· Concentrate grade: 3.3g/t Au.

Additional work was carried out at P.R.A. in November of 1999 confirming the above results. Laboratory bottle roll leach tests were also conducted at P.R.A. on the Falcon concentrate indicating that 80% recovery could be achieved by simply allowing the material to leach in a standard leach circuit at normal cyanide concentrations. This gave an overall recovery of 64% with no regrinding. Additional material was reground and subjected to standard bottle roll tests achieving +90% recovery. This prompted Golden Valley to look at installing a small regrind mill, yielding a quicker payback and an overall recovery of close to 75%. A basic flowsheet below describes the front end of their circuit

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The following process advantages made the Falcon continuous circuit the chosen route:

• No process water is added to the continuous machine. This eliminates problems with the water balance.
• Concentrate is produced at greater than 70% solids by mass, making it simple to regrind without having a dewatering or thickening step in-between. The entire down stream circuit is 70% smaller - massive savings in capital expenditure and operating costs.
• Eliminating 70% of the mass prior to leach makes operation of the leach circuit far more cost effective. In fact, the entire installation will be paid back in 20 months on cyanide savings alone.
• Treating 3.3g/t through the leach is more financially viable than the 1.1g/t in the dump.