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New agglomeration aids help heap leaching.

Most heap-leach pads require agglomeration. Fine clays, if not agglomerated, will cause size segregation within the pad and channeling of solutions and isolation of pockets of ore from proper leach-solution contact. The most common agglomerating agent is cement which also provides protective alkalinity for the cyanide solutions. However, the amount of cement needed for pH control is usually more than that needed for agglomeration. Therefore, excess cement is often added which can lead to reduced gold recovery.

Physically, fine unagglomerated smectite-type clays are composed of silica tetrahedra and alumina octahedra sheets. These sheets attract cations such as sodium and potassium, and retain large amounts of water within the mineral structure. Unless broken up, these saturated layers effectively resist leach solution penetration.

Lime or cement creates a new environment around the clay particles, raising the pH and causing the single valence sodium and potassium ions in the clay layers to be replaced by divalent calcium ions. The clay particles thereby change their orientation from a layered to a more multi-directional structure and release the held water. The alkalinity also marginally solubilizes the surfaces of ore particles releasing silica and alumina ions which interact to form cement-like minerals which stabilize the leach pad.

There is an ongoing search for improved agglomerating agents. Two that have recently come to E & MJ's attention are Leach-It and polymers.

LEACH-IT

The Chemstar Lime Co. of Phoenix, Arizona, recently introduced Leach-It, which according to independent tests by McClelland Laboratories, offers the following advantages compared with cement.

* Increased gold recovery of up to 20% compared with that achieved by the use of cement.

* The area under leach could be reduced, possibly by half.

* Pregnant solution volume would be less, and at a higher grade. Potentially half the volume at twice the grade. This in turn would reduce the size of the solution recovery circuit and pumping and storage requirements.

* Cyanide consumption and water evaporation should decrease due to less solution exposure to the environment.

Leach-It (U.S. Patent No. 5,116,417) is a lime-based combination of minerals which provides alkalinity for the necessary ion-exchange reactions, and for the development of cementitious materials such as tobermorite and ettringite. These minerals have a dendritic structure which provides innumerable passageways for leach solutions to penetrate and allows oxygen to circulate freely.

The dendritic structure can be clearly seen in electron-microscope pictures. Similar pictures of ores treated with cement show no dendritic minerals. Instead there are a lot of round balls which appear to be either clay particles coated with cement, or just cement. In some ores, gold could be bound in these balls making penetration and extraction by leach solutions difficult.

Initial tests were run at Chemstar's laboratories on a Nevada ore assaying 0.04 oz/st gold. The 50-lb samples agglomerated with cement, lime, Leach-It, and a blank, were leached with 0.05% sodium cyanide at a flowrate 200 ml/hr in 1,830 x 133-mm-dia columns. The untreated column dropped 190 mm as the ore became saturated but volume changes in the other columns were insignificant.

Times for first liquid to discharge from the columns were respectively: cement 1/2hr, lime 4 hr, untreated 4 1/2hr, Leach-It 5 hr. The fast time for the cement column indicated channeling and slow dispersion of the solution through the ore. The Leach-It treated column had the slowest time indicating complete saturation.

Maximum gold recovery was reached after 30 hr in the Leach-It column but 50 hr in the cement column. Similar results were obtained with eight different gold ores tested by McClelland Laboratories. Chemstar claim that as little as 0.5% increase in overall recovery will typically pay for the increased cost of Leach-It.

POLYMERS

Betz Industrial has been pursuing polymers as substitutes for cement, an approach based on pioneering work in agriculture on their use to maintain the permeability of high-clay soils. Betz's tests show that polymer-agglomerated gold ores have leach rates and ultimate recoveries equal to those of cement.

Drain-rate tests showed that acrylamide acrylic acid (AM/AA) copolymers were the most effective. Efficiency depends on the AM/AA ratio, molecular weight, and dosage. For a given ratio, effectiveness increases with molecular weight, the best agents being those with 70/30 to 90/10 ratios and 1 to 10 MM molecular weights. The most effective combination is also ore dependent.

Polymer produced agglomerates are open high-surface-area structures compared with those produced by cement at 3 kg/mt. However, the difference virtually disappears at polymer dosages below 40 g/mt. At these dosages percolation rates approach those of cement-agglomerated ores. Low initial percolation rates indicate that polymer treatment produces better agglomerates which present a more tortuous path for the circulating solutions.

Comparative leach tests were performed on an Idaho gold ore by an independent laboratory in two 6,100 x 380-mm-dia columns. One sample was agglomerated with 8.75 kg/mt cement and the other with 50 g/mt 70/30 AM/AA medium-molecular-weight polymer and 2.5 kg/mt lime to provide protective alkalinity. Rate of recovery and ultimate recovery for the two systems were comparable.

The results of field trials for both Leach-It and polymers are eagerly awaited but in the end all will depend on relative costs and recoveries.

References

1.) Leach-It, A New Agglomeration Agent for Increasing Leach Rate Efficiencies, Dan Walker, VP Technical Services, Chemstar Lime Co.

2.) Polymeric Agglomerating Agents for the Gold Mining Industry, D.M. Polizzotti & J.J. Robertson, Betz Industrial.

For further information contact:

Chemstar Lime Co., 2800 North 44th Street, Suite 400, Phoenix, AZ 85008-1557. Tel (602) 955-5711; Fax (602) 468-0488.

Betz Industrial, 1 Quality Way, Trevose, PA 19053.

Gene Gorsky, Al Paul Lefton Co. Inc., Rohm & Haas Building, Independence Mall West, Philadelphia, PA 19106. Tel (215) 923-9600.
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Publication:E&MJ - Engineering & Mining Journal
Date:Nov 1, 1992
Words:959
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