Crushed ore is reclaimed in a 3.7 m (12′) diameter tunnel with, two variable speed slot feeders. Mill feed is conveyed to a 3.7 m x 1.2 m (12′ x 4′) 250 hp semi-autogenous (SAG) mill. Lime is screw fed onto the conveyor inside the reclaim tunnel. The SAG mill discharge enters a common sump shared with a 2.1 m x 3.6 m (7′ x 12′) 200 hp ball mill. Discharge from the common sump is cycloned in closed circuit with the ball mill.
Cyclone overflow, at 75% minus 75 micron (200 mesh) crossing a trash screen before entering the first of seven 4.9 m x 5.5 m (16′ x 18′) agitated carbon-in-leach tanks. Cyanide is added in tank 1. The slurry, at 35% solids advances from tank 1 to tank 7 by gravity, first passing through an air agitated “V” carbon screen “in each tank. A 28 hour retention time is achieved resulting in a 92% plus recovery of gold values and approximately 60% recovery of silver values.
Activated carbon is added in tank 7 and transferred by air-lifts countercurrent to the slurry flow at a rate of approximately 0.25 tonnes/day ( 0.28 tons/day). Carbon loadings averaging 150 oz/tonne (165 oz/ton) are realized. Loaded carbon from tank 1 is screened from the slurry, washed, dewatered and stored in 0.5 tonne (0.55 ton) canisters. The canisters with loaded carbon are then trucked to Nevada Goldfields Kingston Mill for stripping of precious metals. Kingston regenerates the carbon if necessary and the truck returns with unloaded carbon for re-use. A sufficient inventory of carbon is maintained to allow immediate turn around of the transfer truck. The canisters are designed for easy handling and educting of the loaded carbon into Kingstons stripping vessel. Table I compares predicted mill performance obtained from metallurgical work verses actual results with the Aurora mill.
As Table I depicts the milling facility achieved relatively agreeable results compared to metallurgical predictions. The major difference was the actual abrasiveness of the ore and the energy necessary for comminution. This was offset by a higher realized grade and recovery.
Tailings from CIL tank $7 are discharged over a safety screen, to recover any carbon overflow, into a surge tank. Originally the tailings were then pumped to a 1.5 meter belt filter press tor dewatering, producing a cake at 65%-85% solids. A belt filter press consists of two tensioned filter belts which trap the flocculated and partially dewatered tails between them in a wedge zone, then expel the remaining solution as the belts move over a series of rollers.
Tailings Handling Selection
Nevada Goldfields initial contacts with belt filter press (BFP) manufactures suggested a significant capital saving couid be realized utilizing a BFF verses a tailings impoundment if the press was applicable to Auroras ore. To explore the potential application further, a bulk composite sample of Aurora ore was stage crushed and ground to 80% minus 75 micron (200 mesh), slurried to 40% solids and brought up to a pH of 11, typical of Auroras’ tailings. Four manufactures contacted showed an interest in evaluating the capability of their BFPs’ in handling Aurora tailings. Performance parameters were provided to each manufacturer along with 5 gallons of Aurora slurry. Performance parameters were: 11.5 metric tonnes per hour (12.5 tph) continuous throughput, production of a cake at 75-80% solids, a maximum allowable of 3% solids in underflow effluent. Manufacturers were asked to submit results along with flocculant recommendations and consumption requirements, belt requirements, belt life predictions, model recommendations and cost estimates, providing design parameters could be met.
- Preconditioning (Flocculation) – Most applications require addition of Polymers to the slurry prior to dewatering. The addition of these flocculants results in the agglomeration of solids, releasing free moisture.
- Feed Distribution – The slurry is fed to a specially designed distributor, where the material is equally distributed for more efficient use of the filtering area.
- Gravity Drainage – Free moisture is separated from the solids by drainage through the moving filter screen.
- Wedge Pressure – Material advancing from the drainage zone is deposited in the wedge zone. Pressure is gradually applied as the upper and lower filter screens converge to form a “cake sandwich.”
- Compression/Shear – The screens then follow an “S” path through a series of compression rollers. The surface pressure created by screen tension squeezes additional moisture from the solids. Simultaneous shearing of the material opens additional voids for moisture release. The screens part, leaving a dry cake, which is removed from the screens by plastic scraper blades.
All four manufactures were successful in meeting the required parameters, in fact results were extremely encouraging. The major variance between the results was flocculant consumption, which varied from .12 kg/tonne (.3 lbs/ton) to .2 kg/tonne (.5 lbs/ton).
Summary and Conclusions
Initial selection of a tails filtration system for Aurora met two original objectives, a significant reduction in start-up capital costs and facilitated a streamline permitting process. Operational costs of the belt filter press alone compared favorably with predicted costs, however excessive costs incurred in handling the dewatered tailings eventually resulted in making alternative disposal methods economically attractive. Adverse trafficability characteristics of the dewatered tailings were not effectively anticipated resulting in the implementation of more costly handling methods.
Due to it’s relatively new application in hard rock mining, technical .and operational experience employing a belt filter press and handling dewatered tailings is fairly limited. Operational experience gained at Aurora suggests some considerations future operators would want to incorporate into their evaluation, if considering a belt filter press as an alternate method for tailings disposal.
- Mechanically a belt filter press is an efficient and effective machine and could be made to work -on-most hard rock tails. However, ore characteristics can significantly effect the filter’s operational costs. Due to a high number of wear items on the BFP, the abrasiveness of the tailings product is a major consideration, as are the flocculant requirements.
- The combination of factors affecting the continuous performance and maintaining the water balance on the BFP requires a more skilled operator than with a conventional system.
- To maintain, plant availability when the BFP is down for services or repairs a reclaim pond, surge tank or additional BFP capacity is required.
- The dewatered tailing’s physical characteristics can significantly affect the economics and operating efficiencies of handling the cake. Some methods available for transporting the cake from the BFP to the final deposition site include conveyors,, trucking, scrapers and pumping, or a combination thereof.
- In most cases the final deposition site will need to be lined with either a natural or artificial liner.
Although at Aurora this method of tailings disposal eventually was replaced when the handling costs became too onerous, it may be an economic alternative for operators at new or existing cyanide facilities. The method should be considered an alternative if one or more of the following conditions are present:
- Topography limitations- require an expensive dike construction.
- Environmental constraints or restrictions limit available tailings disposal alternatives.
- Reagent consumptions are high and tails solution reclaim efficiencies are low.
- A small project where dike construction costs would constitute a major portion of capital costs.
Prior to selecting a tails filtration method of disposal as a viable alternative, a pilot scale test should be performed, giving special attention to the physical-characteristics of the slurry and the de-watered tailings product.