Table of Contents
The American Girl Mining Joint Venture is an equal partnership between Eastmaque Gold Mines, Ltd. of Vancouver, BC Canada and MK Gold Company, a wholly owned subsidiary of Morrison Knudsen Corporation of Boise, Idaho.
Underground mining and open pit methods are used to supply ore to the processing areas. Heap leaching of ore grades in the range of 0.5 to 3.5 g/mt Au (0.015 to 0.100 oz Au/ton) is the method of choice for ores from the surface mine and for some ores from underground workings. For the ore mined containing gold grades above 3.5 g/mt Au (0.100 oz Au/ton) is processed in the mill to provide optimum overall recovery and economic return.
In order to provide crushed materials for the surface heap leach operation, mill feed material, and crushed aggregate for the underground backfilling needs, several product sizes must be crushed. Several different crusher throughput rates are necessary. The primary stage crusher is a Hazemag horizontal shaft impact crusher, delivering a nominal 15 cm (minus 6 in) product to the Tyroc 2.4 m x 7.3 m (8 ft x 24 ft) double deck screen.
Mill ore is reclaimed from a kidney-shaped stockpile with the use of three gravity-fed reclaim points, serviced by air blasters for stubborn damp ores. A variable frequency drive on the mill feed conveyor is linked to the Ramsey Series 200 electronic weigh scale to provide regulated feed to the ball mill [Allis Chalmers 447 kW, 600 hp, 2.9 m x 4.6 m (9.5 ft by 15 ft) rubber lined]. A grind size of approximately 80% passing 75 microns (200 mesh) is sufficient to provide suitable cyanide recovery for the gold ore. .
Some variability in cyanide levels is seen according to mill feed grades, but cyanide levels are typically held high to achieve optimum recovery for high gold grades that frequently spike through the leach circuit as a result of variable underground ore grades. Leached slurry is pumped following approximately 48 hours of leaching to a 25-m (82 ft) diameter Eimco thickener for solid/liquid separation. Pregnant solution is recovered as thickener overflow and is slightly turbid.
Carbon Adsorption Electrowin Refining
Two independent carbon adsorption column circuits handle the pregnant solution from the heap and mill processes. Carbon is stripped in a standard pressure strip using only caustic soda, temperature, and pressure to elute the gold. A single- pass electrowin circuit removes gold from the strip pregnant solution before it returns to a strip barren tank for reheating and reuse.
Carbon Adsorption Details
The details of the carbon adsorption circuit are shown in the detailed block flow for the area, Figure 2,
The heap leach carbon adsorption circuit consists of 5 tanks 2.28 m (7.5 ft) diameter by 2.44 m (8 ft) tall containing 1.81 tonnes (2 tons) of activated carbon each and receiving a flow of 79 to 88 lps (1250 to 1400 gallons per minute) of pregnant solution.
Prior to hot pressure stripping, each tonne of mill or heap carbon (from the No. 1 or head tank) is subjected to an intensive ambient temperature cyanide strip, a so-called cold strip. The cold strip cycle lasts for approximately 1-½ hours with cyanide solution at 30-38 kg/mt (60-75 lb/ton) passing up-flow through the carbon bed, at a rate of 0.5 bed volumes/minute with recycle of the solution for a total of two to three bed volumes.
Hot Strip Acid Washing Regeneration
The carbon is then transferred to the hot strip pressure vessel. Hot caustic solution at 135-150°C is then passed through the carbon bed at 0.34-0.41 Pa (50-60 psi) at a rate of 2.75 bed volumes per hour. The total strip time is currently 6 hours, and stripped carbon tails assays average less than 17 g Au/mt carbon (0.5 oz/ton).
Electrowin sludge is pressure rinsed from the cathodes and filtered prior to retorting overnight. The 0.71 cu. meter (2.5 cubic foot) retort raises the temperature of the cake to approximately 425°C and holds temperature for 2-3 hours before cool down.
Heap leaching as a concept, however, may have initiated a portion of the problems associated with copper at the site. As heap leach stacks increase in height as a result of efforts to maximize capital utilization of the lined area, the oldest ore on the stack receives continual cyanide solution for several years. This ore was considered leached out at some point after fresh ore was placed on top of it.
Copper Recovery Methods
The Scada system was implemented on a test basis to treat high copper content sodium cyanide solutions in an electrolytic copper reduction cell. Insufficient copper removal and very significant HCN gas evolution precluded ongoing use of the system. The other available systems were not tested. High flow rates and handling of HCN gas generation can limit the applicability of efficient copper recovery systems, especially at the high pH required for safe cyanide use.
Carbon adsorption efficiency was unsatisfactory during the initial months of processing (see Figure 4.). Both mill and heap leach pregnant solutions were combined during initial start-up and pumped through a single bank of carbon adsorption columns.
Thermal regeneration of the carbon was taking place in the early stages of site operation using the regeneration kiln with a capacity of 0.907 mt/day (1 ton/day capacity, vertical kiln). Activities were nominally monitored. Any carbon in excess of the capacity of the regeneration feed hopper bypassed the regeneration step.
At this point copper became the primary focus, and all problem-solving efforts focused on copper. Copper-to-gold ratios were checked and demonstrated to be at a critical level (in excess of 600 Cu: 1 Au) in the head column and increasing to very high levels (several thousand to one) in the tail column.
A complicated arrangement was implemented, in which each tonne of carbon (from either mill or heap circuit) was cold stripped 3 times for 5 to 5½ hours per cycle. This was an excessive cyanide consumer, and was eventually optimized to a single batch cold strip for 1½ to 2 hours per mill or heap tonne.
Carbon Advance Rate
The cost of processing the carbon frequently in both strip circuits (cold and hot) at low gold loadings was determined to be less than the high solution gold losses when carbon advances occurred only every 24 hours, as practiced before this change. This bumping effect is similar to a common occurrence with silver in heaps approaching closure during attempts to remove the last gold from the heap pregnant solution.
Stringent acid washing procedures were implemented to provide the best carbon adsorption media for both mill and heap circuits. Removal of the copper in this manner again vacates a significant portion of the active sites on carbon surfaces. Although American Girl has not yet documented the copper removed in its acid wash circuit, there are strict guidelines for acid washing of carbon.
Electrowin voltage is strictly controlled to limit electro-deposition of copper onto the cathodes, even for low concentrations allowed to pass through the cold strip cycle. Instead of allowing the voltage or amperage to drift throughout the strip and electrowin cycle, the operator constantly adjusts the voltage to maintain 2.3 volts.
High Cyanide Addition
Prior testing had added sodium cyanide solution to the carbon adsorption circuit for short periods of time or in very limited doses. This test attempted addition equal to changing cyanide titration from 0.25 kg NaCN/mt (0.5 lb NaCN per ton) of solution to 0.625 kg NaCN/mt (1.25 lb NaCN per ton) of solution, an increase of 150%. The economics of the high cyanide addition were confirmed by comparing the decrease in barren solution grade to the cost of an additional 0.19 lps (3 gpm) of sodium cyanide solution required at the columns.