USGoldAndSilverLeaching.pdf

b Bureau of Mines Ination Circular/l984 Gold and Silver Leaching Practices in the United States By Peter G. Chamberlain and Michael G. Pojar UNITED STATES DEPARTMENT OF THE INTERIOR . Ination Circular 8969 Gold and Silver Leaching Practices in the United States By Peter G. Chamberlain and Michael G. Pojar UNITED STATES DEPARTMENT OF THE INTERIOR William P. Clark, Secretary BUREAU OF MINES Robert C. Horton, Director As the Nations principal conservation ogency, the Department of the Interior has responsibility fa most of our nationally owned public lands and natural resources. This includes fostering the wisest use of our land and water re- sources, protecting our fish and wildlife, preserving the environmental and cultural values o f our national parks and historical places, and providing for the enjoyment of life through outdoa recreation. The Department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also has a major re- sponsibility for American Indian reservation communities and fa people who Live in Island Territories under US. administration. Library of Congress Cataloging in Publication Data Chamberlain, Peter G., 1942- Gold and silver leaching in rhe United Scares. Bureau of Mines ination circular ; 8969 Bibliography p. 36-38. Supr. pf Docs. y. 1 28.278969. 1. Gold mines and mining-United Stares. 2. Silver mines and mining-United Scares. 3. Solution mining-United Stares. I. Pojar. Michael G. 11. Title. 1 1 1 . Series Lnfomation circular United States. Bureau of Mines ; 8969. For sale by the Superintendent ot Documents, US. Government Printing Office Washington, D.C. 20402 CONTENTS Abstract ....................................................................... Introduction ................................................................... Acknowledgments ................................................................. ..................................................................... Mineralogy ............................................................ Leaching technology ................................................ Heap leaching ore preparation Dump leaching ore preparation ................................................ In situ leaching ore preparation ............................................. Leaching process ............................................................. -each solutions ............................................................ Solution distribution ...................................................... Recovery ..................................................................... ................................................... Comparison of techniques Zinc precipitation ......................................................... Charcoal adsorption ........................................................ ............................................................ Leaching operations ...................................................................... Arizona California ................................................................... ..................................................................... Colorado ........................................................................ Idaho ...................................................................... Montana ....................................................................... Nevada New Mexico ................................................................... South Dakota ................................................................. Permitting regulations ......................................................... Federal regulations .......................................................... State regulations ............................................................ Leaching problems and research ................................................. Percolation .................................................................. Temperature .................................................................. .......... Solution loss ................................................. Calcium salt scale ...................................................... Research on novel solution mining s ............................... In situ leaching ...................................................... ......................................................... Leach farming ................................................... Thin-layer leaching Summary ................................................................... References ..................................................................... Appendix.--Gold and silver leaching bibiliography .............................. ILLUSTRATIONS 1 . Heap leaching system ....................................................... 2 . Typical pilot heap leaching operation ...................................... .............. 3 . “vat“ leaching accompanying conventional mill- ing operations is not considered in this publication. If ore is mined or if it is gathered from old nine waste-rock piles and hauled to specially prepared pads lined with clay, tar, or a l o n for leaching, the is “heap“ leach- ing figs. 1-2. The rock is frequently crushed before being placed on the pad. If mine waste-rock piles or dumps are judged to contain sufficient mtneral value to justify leaching and the solutions can be controlled dthout 3ef erence to specific products does not imply endorsement by the Bureau of Mines. Barren s o l u t i o n from processing p l a n t Influent l e a c h i n g s o l u t i o n S o l u t i o n makeup t a n k S o l u t i o n Pregnant e f f l u e n t s o l u t i o n - sprays - - FIGURE 1. - Heap leaching system. Sotutcon makeup tank appreciable losses, the pile is “dump“ leached without any preparation fig. 3. Although this technique is rarely used for leaching gold and silver, it is com- mon in the copper industry. Finally if the ore is broken and left in place or i f it conducts fluids flow without blasting, it can be leached “in situ“ or in-place fig. 4. An exposed ore body can be leached in situ by spray- EXPOSED ORE ing solution on the surface and collect- BODY ing it in recovery wells after it has percolated down through the ore. For buried ore bodies, the solution must be injected into the ation through in- jection wells and recovered from adjacent recovery wells. Although copper and ura- n i m have been leached in situ, there have been only sporadic attempts to de- velop such operations for leaching gold and silver. One attempt to in situleach gold a t the Ajax Mine near Victor, CO, i q described later. Since many operators are considering leaching gold and silver for the first time or are experiencing problems in es- tablishing leaching operations, this re- BURIED ORE port summarizes leaching principles and practices, discusses problem that may be encountered, and lists sources of addi- tional ination in a bibliographic appendix. FIGURE 4. - In situ leaching systems. A C K N O W L E D G M E N T S The authors wish to express apprecia- that provided engineering data on their tion to the following mining companies leaching experiments or operations Company American Selco, Inc.................. Can-American Mining Co............... Carlin Gold Mining Co................ Congress Consolidated Coal Mining Co. Cyprus Exploration Co................ D Z Exploration Co................... Gold Creek Corp...........,.......... Gold Resources Joint Venture......... Golden Arrow, Inc.................... Hildebrand Drilling.................. Landusky Mining Co................... Location kno, NV Tombstone, AZ Carlin. NV Phoenix, AZ Carson City, NV Lovelock, NV Eureka and Ely, NV Cripple Creek, CO Las Vegas, NV Phoenix. AZ Landusky, MT Occidental Minerals C o r p . . . . . . . . . . . . . Placer Amex I n c . . . . . . . . . . . . . . . . . . . . . . Scholz Minerals Engineering. Z n c . . . . . Silver Ridge Mining C o . . . . . . . . . . . . . . . Smoky Valley Mining C o . . . . . . . . . . . . . . . State of Maine Mining C o . . . . . . . . . . . . . Tombstone Exploration. I n c . . . . . . . . . . . Vekol Mlne Development C o . . . . . . . . . . . . Windfall V e n t u r e . . . . . . . . . . . . . . . . . . . . . Zortman Mlning C o . . . . . . . . . . . . . . . . . . . . MINEUALOGY Since many good references are avail- able on the geology of gold deposits, such ination is not provided herein. Of particular importance however, in uating the leachability of gold- silver deposits is their mineralogy. Gold is usually deposited as native or free gold associated with pyrite 5 . 4 - Occasionally, as at Cripple Creek, CO, the gold is deposited as a telluride. Various heavy metal compounds are fre- quently associated with the gold. Silver is usually deposited in its compound . Besides native silver Ag, those minerals containing leach- able silver are argentite silver sul- fide, Ag2S, cerargyrite silver chlo- ride, AgCl, embolite AgC1, AgBr, and bromyrite silver bromide, A g B r . Other silver minerals are not readily leachable 12, 24. Ore can be economically leached at grades about an order of magnitude lower than they are commonly milled. Current leaching operations are producing gold from ores containing as little as 0.03 ozlton with cutoff grades down to 0.01 oz/ton. Most silver leaching operations produce from ores grading 1 to 4 ozlton. The easiest ores to leach are those that have been weathered or oxidized, liberating the gold or silver from pyrite or other encapsulating minerals. dunderlined numbers in parentheses re- fer to items in the list of references preceding the appendix. Location--Con. liawthorne, NV San Francisco. CA Helena, MT Tombstone, AZ Round Mountain, NV Tombstone, AZ Tombstone, AZ Chandler, AZ Eureka, NV Mica, WA A variety of mineralogical conditions can hamper or prevent leaching of an ore. For example, deposits that contain organic carbon are not suitable because the carbon prevents much of the gold from dissolving and adsorbs any dissolved met- al before the leach solutions are recov- ered. Other refractory ores are those in which the gold or silver is totally en- cased by an impervious matrix material such as quartz so that the leaching solu- tions cannot contact the metal. Copper, cobalt, and zinc in the ore may preferen- tially take the place of gold and silver in the leaching reaction and greatly reduce the reaction with the desired metal. Some ores, such as tellurides or those containing arsenopyrite or antimony, must be roasted before they can be leached with cyanide and so are not amenable to heap leaching 1 2 . Although other leaching solutions hGe been investigated for use with telluride deposits, no com- mercial heap leaching operations have resulted. Pyrrhotite is another mineral that com- plicates leaching. Decomposition of pyr- rhotite in cyanide produces ferrocyanide, which removes free cyanide from solution and prevents its reaction with the gold or silver. This decomposition also re- moves oxygen from the solution, which further decreases the reaction with gold and silver. If manganese occurs with silver ores, its higher order oxidation products can refractory compounds of silver and Clay minerals i n the ore pose a major manganese 8, 38. Many silver deposits problem i n leaching operations. The clay were l e f t unmined throughout the Western particles block the leaching solutions United States because of this particu- flow through the ore and isolaee large l a r problem. Some of these deposits may portions of ore from the solution. Some be amenable to dual leaching, f i r s t heaps contain so much clay that the solu- with aqueous SO2 to recover manganese, tion perches on top with negligible down- followed by neutralization and leaching ward percolation. with cyanide to recover silver. LEACHING TECHNOLOGY HEAP LEACHING O R E PREPARATION Ore preparation for heap leaching con- sists of 1 preparing an impervious pad, 2 mining or gathering ore from dumps, 3 crushing the ore optional, and 4 placing the ore on the pads. Pads are usually situated i n f l a t ter- rain that is graded to provide gently sloping surface for drainage 2 t o 5 pct. Next the pad s i t e must be lined to prevent solution seepage losses. If plastic liners are used, the ground is covered with a layer of sand or tailing that is rolled t o provide a cushion. The l i n e r sections are laid out and cemented together, then covered with sand t o pro- vide additional cushioning. Heaps that are subjected to much vehicular traffic a r e often lined with asphalt. Some pads are merely lined with a thick clay or tailing layer that becomes essentially impervious when wet. A dam or berm with a drain is constructed on the downstream end of the pad to direct the solution into a holding pond. Size distribution of ore placed on heaps ranges from run-of-mine to minus 114 i n 6 mm. Many miners crush the ore t o significantly improve total recovery and recovery rates. Operating costs are, of course, higher when ore is crushed. Before an operator selects a particular particle size, the ore should be tested t o determine the trade-off between miner- a l recovery and crushing costs a t several sizes. O r e is spread on the prepared pads by scrapers, front-end loaders, trucks, and bulldozers; conveyors and stackers can also be used. A unique gantry system has been employed a t one operation i n New Mexico 22. Since solution percolation i n t o a h Z p is severely affected by any packing from driving on the surface of the heap, the specific technique by which the ore is placed i n heaps profoundly in- f luences precious metal recovery. Tech- niques that eliminate t r a f f i c on the im- placed ore are strongly urged. Each layer, called a l i f t , is usually placed 5 to 10 f t 2 to 3 m deep; the range encountered a t current operations was 2 f t 1 m t o 20 f t 7 m. Although it has long been f e l t that after solu- tions percolate through a heap greater than about 10 f t 3 m deep, they may be- come oxygen deficient, the rate of oxygen depletion has not been accurately mea- sured. Recent experiments with hydrogen peroxide additives have shown no in- creased metal recovery 40, which would indicate that the c r i t i c a oxygen content is not as high as originally envisioned. After the f i r s t l i f t has been leached, either it is removed from the pad, or the next l i f t is placed on top of it so that subsequent applications of leaching solu- tions w i l l percolate through both. A third and fourth l i f t can be added later. Operations using fine ore size generally leach with only one l i f t because a t the end of the leaching cycle the ore is virtually depleted and m