O f «:l°<*. ^v %.^^^'\-^^' %.^^^'\^' %<^fr^\r %^^-^/ A. ■^0' V. c'?^*^' .^ O < '-^^o^ °.^»'". ^otf :^&'- "^^^0^" r^^tt '-h^^ :^^-^ ^^^S '^^^^ ^ov o « o " ^*' *\^° .. v^^^\^^ "-^*^-^/ %'^^^\r \'^''\^^ %<^^\r Vv * ' -^^ ro*,/^ \.^^'^^^o'> -^^^^^^-^^ ^o^*^^*^/ ^^ * ..■^' %_^^t*' o'J '^^ ""'' a"^ > < .» - « • /> "^v fV> « • • » "^^ ^ > « « ,« *>^ 4.0 "7*. >> c • • • •<^^ i > %. r^'^ . 4 • • , ^: ^0^^ .40^ ^°'%.. -4 CK " V y^-^iX ^°^^^%°- ./ .•i°<. « 4 Pa 5 ► -^. \>1> O H O ^^ / .^9^ ^^^ '^0^ A^^ ^"•^^^ '^d' .^^ ^ ^ h ^ aV rK . «\ "ovjf^** A:^ "^ ""SW^*" '^'''\, v^f^*' A:^^'%. "-SiB^*" ^^'''V. "W^^^ a.^^'^. '"•«li»*'' .^^^ .^r."% ^^, ,^\^^'Ai^.%. J-.^yi'^^. .To-- .0 .-^^^ IC 8969 Bureau of Mines Information Circular/1984 Gold and Silver Leaching Practices in the United States By Peter G. Chamberlain and Michael G. Pojar UNITED STATES DEPARTMENT OF THE INTERIOR Information 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 Nation's principal conservation agency, the Department of the Interior has responsibility for 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 of our national parks and historical places, and providing for the enjoyment of life through outdoor 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 for people who live in Island Territories under U.S. administration. Library of Congress Cataloging in Publication Data; Chamberlain, Peter G., 1942- Gold and silver leaching practices in the United States. (Bureau of Mines information circular ; 8969) Bibliography: p. 36-38. Supt. of Docs, no.: I 28.27:8969. I. Gold mines and mining— United States, 2. Silver mines and mining— United States. 3. Solution mining— United States. I. Pojar, Michael G. II. Title. III. Series: Information circular (United States. Bureau of Mines) ; 8969. ;E^J-2«57CJ?' [TN423.A51 622s [622'.3422l 83-600316 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Abstract 1 Introduction 2 Acknowledgments 4 Mineralogy 5 Leaching technology 6 Heap leaching ore preparation 6 Dump leaching ore preparation 7 In situ leaching ore preparation 7 Leaching process 8 i^each solutions 8 Solution distribution 8 Recovery 12 Comparison of techniques 12 Zinc precipitation 12 Charcoal adsorption 14 Leaching operations. 15 Arizona 22 California 23 Colorado 23 Idaho 25 Montana 25 Nevada 26 New Mexico 28 South Dakota 29 Permitting regulations 29 Federal regulations 29 State regulations 30 Leaching problems and research 33 Percolation 33 Temperature 33 Solution loss 33 Calcium salt scale 34 Research on novel solution mining methods 34 In situ leaching 34 Leach farming 35 Thin-layer leaching 35 Summary 36 References 36 Appendix. — Gold and silver leaching blblllography 39 ILLUSTRATIONS 1 . Heap leaching system 2 2 . Typical pilot heap leaching operation 3 3. Dump leaching system 3 4. In situ leaching systems 4 5 . Fixed-spray solution distribution. 9 6 . Ralnblrd sprinkler 9 7. Bagdad wlggler 10 8. Solution distribution by ponding 11 9. Pregnant effluent solution collecting pond 11 li ILLUSTRATIONS—Continued Page 10. 11. 12. 13. 1. 2. 3. 4. 5. 6. Zinc precipitation recovery system 13 Charcoal adsorption recovery system 14 Leaching operation locations in the United States 22 In situ leaching test at Ajax Mine 24 TABLES Gold and silver heap and dump leaching operations in the Western United States 16 Location data for key operations 19 Ore mineralization and process characteristics 20 Leach solution treatment 21 Environmental Protection Agency regional offices 30 State permitting agencies 31 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT A ampere L/s liter per second A/ft2 ampere per square foot L/8*cm"2 liter per second per square centimeter Btu/h British thermal unit per hour m meter "C degree Celsius mi mile cm centimeter mln minute ft foot m2/kg square meter per kilogram ft2/lb square foot per pound mL/s*cm"2 milliliter per second per square centimeter g gram TiiTn millimeter gal/min gallon per minute mol/mol mole per mole gal/min'ft"2 gallon per minute per square foot oz ounce g/kg gram per kilogram oz/ton ounce per ton h hour Pa pascal in inch pet percent kg kilogram ton/d ton per day kg/d kilogram per day ton/yr ton per year kg/m^ kilogram per cubic meter V volt km kilometer W watt lb pound wk week lb/ft' pound per cubic foot wt pet weight percent lb/in2 pound per square inch yr year lb/ton pound per ton 1 GOLD AND SILVER LEACHING PRACTICES IN THE UNITED STATES By Peter G, Chamberlain^ and Michael G» Pojar ABSTRACT The surge in gold and silver prices during the 1970' s attracted many new mining operators and has rekindled Interest among experienced ones. With Its low capital Investment requirements and fast payout, leaching has attracted many operators — particularly those with small or low- grade deposits. Although In certain situations leaching offers many advantages over conventional mining methods , many operators are uncer- tain how these relatively new techniques should be Implemented. Conse- quently the Bureau of Mines has prepared this circular to disseminate information on gold and silver leaching practices, techniques, and problems. Engineering data gathered from 26 operations indicate that most ores are leached in heaps following crushing and distribution on pads. Metal values are recovered from cyanide leach solutions using either zinc precipitation or charcoal adsorption. Potential problems that may hamper on block development of a leaching operation are poor percolation characteristics of the ore, calcium salt buildup, low tem- peratures, and solution losses. An extensive bibliography on gold and silver leaching is appended. ^Supervisory mining engineer. ^Mining engineer. Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. INTRODUCTION Climbing gold and silver prices during the 1970' s awakened dormant interest in mining these metals in many districts. Since gold and silver deposits frequently can be profitably mined by small opera- tors, a mix of mining activity has been created, comprised of large and small companies, both experienced and unexperi- enced. A key factor in the profitability of small mining operations is the advent of an alternative to conventional mining and milling operations — solution mining (leaching). Basically gold and silver leaching involves spraying a cyanide so- lution on the ore to dissolve the metal values, collecting the solution contain- ing the dissolved metals , and recovering the metal from the solution. By elimi- nating milling, leaching reduces capital cost and startup time for new operations. Operating costs are likewise signif- icantly lower. The disadvantages of leaching are lower recovery and greater difficulty in controlling the cyanidation process. There are three types of leaching systems — heap, dump, and in situ; "vat" leaching accompanying conventional mill- ing operations is not considered in this publication. If ore is mined or if it is gathered from old mine waste-rock piles and hauled to specially prepared pads lined with clay, tar, or Hypalon^ for leaching, the method 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 mineral value to justify leaching and the solutions can be controlled without ^Reference to specific products does not imply endorsement by the Bureau of Mines. \ \ Barren solution from processing plant Solution sprays i^ To processing plont Pump FIGURE 1. - Heap leaching system. FIGURE 2. - Typical pilot heap leaching operation. J2T \ Waste rock dump Pregnant solution drainage Barren solution from processing j 1 plant ^ A ^ Solution makeup tank To processing i ^ plant ) Pump FIGURE 3. - Dump leaching system. 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 if it conducts fluids flow without blasting, it can be leached "in situ" or in-place (fig. A). An exposed ore body can be leached in situ by spray- ing solution on the surface and collect- ing it in recovery wells after it has percolated down through the ore. For buried ore bodies, the solution must be injected into the formation through in- jection wells and recovered from adjacent recovery wells. Although copper and ura- nium 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 situ "leach gold at the Ajax Mine near Victor, CO, is 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- port summarizes leaching principles and practices, discusses problems that may be encountered, and lists sources of addi- tional information in a bibliographic appendix. Solution sprays 1 7i-rote55ing . Darren plant \ makeup Barren solution makeup tank / / Solution -T^ flow Perforated casing Recovery well o EXPOSED ORE BODY Pump BURIED ORE BODY Pump FIGURE 4. - In situ leaching systems. ACKNOWLEDGMENTS The authors wish to express apprecia- tion to the following mining companies that provided engineering data on their leaching experiments or operations: Company Location American Selco, Inc Reno, NV Can-American Mining Co Tombstone, AZ Carlin Gold Mining Co Carlin, NV Congress Consolidated Coal Mining Co. Phoenix, AZ Cyprus Exploration Co Carson City, NV D Z Exploration Co Lovelock , NV Gold Creek Corp Eureka and Ely , NV Gold Resources Joint Venture Cripple Creek, CO Golden Arrow, Inc Las Vegas, NV Hildebrand Drilling Phoenix, AZ Landusky Mining Co Landusky , MT Company — Con , Loca t ion — Con . Occidental Minerals Corp Hawthorne , NV Placer Amex Inc San Francisco , CA Scholz Minerals Engineering, Inc Helena, MT Silver Ridge Mining Co Tombstone , AZ Smoky Valley Mining Co Round Mountain, NV State of Maine Mining Co Tombstone , AZ Tombstone Exploration, Inc Tombstone, AZ Vekol Mine Development Co Chandler , AZ Windfall Venture Eureka, NV Zor tman Mining Co Mica, WA MINERALOGY Since many good references are avail- able on the geology of gold deposits, such information is not provided herein. Of particular importance however, in evaluating the leachability of gold- silver deposits is their mineralogy. Gold is usually deposited as native or free gold associated with pyrite (_5).'* 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 form. Besides native silver (Ag), those minerals containing leach- able silver are argentite (silver sul- fide, Ag2S), cerargyrite (silver chlo- ride, AgCl), embolite (AgCl, AgBr), and bromyrite (silver bromide, AgBr). Other silver minerals are not readily leachable (JL2, 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 oz/ton with cutoff grades down to 0.01 oz/ton. Most silver leaching operations produce from ores grading 1 to 4 oz/ton. The easiest ores to leach are those that have been weathered or oxidized, liberating the gold or silver from pyrite or other encapsulating minerals. ^Underlined numbers in parentheses re- fer to items in the list of references preceding the appendix. 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 (12) . Although other leaching solutions have 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 form refractory compounds of silver and manganese (8^, 38) . Many silver deposits were left unmined throughout the Western United States because of this particu- lar problem. Some of these deposits may be amenable to dual leaching, first with aqueous SO2 to recover manganese, followed by neutralization and leaching with cyanide to recover silver. Clay minerals in the ore pose a major problem in leaching operations. The clay particles block the leaching solution's flow through the ore and isolate large portions of ore from the solution. Some heaps contain so much clay that the solu- tion perches on top with negligible down- ward percolation. LEACHING TECHNOLOGY HEAP LEACHING ORE 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. Fads are usually situated in flat ter- rain that is graded to provide gently sloping surface for drainage (2 to 5 pet). Next the pad site 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 to provide a cushion. The liner sections are laid out and cemented Together, then covered with sand to pro- vide additional cushioning. Heaps that are subjected to much vehicular traffic are 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 1/4 in (6 mm). Many miners crush the ore to 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 to determine the trade-off between miner- al recovery and crushing costs at several sizes. Ore 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 at one operation in New Mexico ( 22 ) , Since solution percolation into a heap is severely affected by any packing from driving on the surface of the heap, the specific technique by which the ore is placed in heaps profoundly in- fluences precious metal recovery. Tech- niques that eliminate traffic on the im- placed ore are strongly urged. Each layer, called a lift, is usually placed 5 to 10 ft (2 to 3 m) deep; the range encountered at current operations was 2 ft (1 m) to 20 ft (7 m) . Although it has long been felt that after solu- tions percolate through a heap greater than about 10 ft (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 critical oxygen content is not as high as originally envisioned. After the first lift has been leached, either it is removed from the pad, or the next lift is placed on top of it so that subsequent applications of leaching solu- tions will percolate through both, A third and fourth lift can be added later. Operations using fine ore size generally leach with only one lift because at the end of the leaching cycle the ore is virtually depleted and may as well be removed. Operations with larger sized ore particles that take longer to leach generally use multiple lifts. The solu- tion can then pick up extra mineral val- ues from the lower lifts until they are depleted without wasting valuable pad space. Placing ore on heaps so as to prevent surface packing is a problem. If vehicu- lar traffic packs the surface into a hard pan, the leaching solution will not per- colate uniformly downward; in fact, stag- nant puddles or ponding on the surface may occur. Although packed material can be loosened with a ripper, several compa- nies are considering conveyor systems to eliminate vehicular traffic on heaps. Pushing the material into place with a front end loader after it is dumped on the pad also minimizes surface packing. Where clay in the ore causes percola- tion problems, a relatively new technique of agglomerating the fines can be used to prepare the ore. Devised by the Bu- reau of Mines' Reno Research Center, the technique dramatically improved percola- tion rates, prevented channeling, and im- proved leaching rates in pilot tests (26- 28 ) . Several companies have used the technique in full-scale operations since 1980. At one of these sites, recovery improved from 37 to 90 pet as a result of agglomeration. The technique involves mixing the dry crushed ore with 5 to 15 lb/ ton (2.5 to 7.5 g/kg) port land cement, wetting with 8 to 16 wt pet water, me- chanically tumbling the wetted feed to effect agglomeration, and curing the ag- glomerated material for 72 h prior to leaching. The lime in the port land ce- ment reduces the amount of lime that must be added to the leaching solution to maintain the proper pH; see the following section on leaching solutions. Even greater benefits accrue if, instead of using water, the cement-ore mixture is wetted with relatively strong cyanide so- lution during agglomeration. This cya- nide solution can then begin precious metal dissolution while the pellets are curing so that the heap can be leached with water that can be recirculated throughout the leaching cycle. Prelimi- nary tests resulted in reduced cyanide consumptions and reduced leaching time. DUMP LEACHING ORE PREPARATION Although waste rock from either under- ground or open pit mines is of too low a grade to warrant conventional mill- ing, some gold or silver may be recovered by leaching it. If dumps are leached without additional ore preparation or transporting the rock to prepared pads, the operation is termed a dump leach. Caution must be exercised in selecting dumps to insure that no leach solutions can escape into ground water or surface water drainage. A heavy clay soil under- lying the dump is necessary to prevent solutions from percolating to the ground water. Dams are generally constructed to trap and hold the leach solutions. IN SITU LEACHING ORE PREPARATION There are no commercial-scale in situ gold or silver leaching operations in the United States. If such leaching is un- dertaken, ore will probably be prepared by blasting the formation to rubblize it. For shallow deposits [less than 300 ft (100 m)] the deposits would be rubblized with explosives placed in vertical holes patterned after techniques used for in- place copper leaching (9-10) . Deep de- posits would require blasting methods similar to those employed at conventional underground mines. For instance, if a deposit would be developed by a certain stoping configuration for conventional mining, the same configuration would likely be the most efficient for prepar- ing the ore for in situ leaching. The only difference would be that in conven- tional operations all of the rock from development openings and ore from the stopes is hauled to the surface, whereas in leaching operations only the 20 to 25 pet of this material would be hauled to the surface to provide "swell" space for the rubblization blast. Certain shallow underground mines in well-fractured formations may be amenable to in situ leaching without any ore prep- aration. Access to the deposit would be gained via the mine development openings; solution injection and recovery wells could be drilled from any of these open- ings at possible cost savings (by eli- minating drilling through barren over- burden) . Placer gold deposits would probably be permeable enough to permit leaching from vertical wells without blasting (33). LEACHING PROCESS Leach Solutions Although it is possible to leach gold and silver with several types of solu- tions, all current operations use sodi- um cyanide (NaCN) , mixed in water at strengths of about I lb/ton (0.5 g/kg) of solution, or 0.05 pet. Strengths encoun- tered in current operations range from 0.3 to 5.0 lb/ton (0.15 to 2.5 g/kg). The higher strengths are generally used on ores with high silver content. When the cyanide solution contacts free gold or silver, leaching occurs according to the following reactions (2^, 12) ; 2Au + ANaCN + O2 + 2H2O ->■ 2NaAu(CN)2 + H2O2 + 2NaOH and 4Au + 8NaCN + O2 + 2H2O -»■ 4NaAu(CN)2 + 4NaOH. The reaction depends strongly on oxygen, which is added by bubbling air through the solution and/or by spraying the solu- tion onto the heaps. When other silver minerals are leached, the silver similarly combines with the cyanide ion except that silver chloride apparently will combine without oxygen (24) ; for instance, AgCl + 2NaCN ->■ NaAg(CN)2 + NaCl. Leach solutions are effective at pH 9.5 to 11, although both lower and higher al- kalinities have been successfully used. Lower pH may result in decomposition of the cyanide by hydrolysis or by reaction with carbon dioxide in the air. Low pH can also permit gasification (and hence loss) of the cyanide if the solution con- tacts natural ground acids (19) . Con- versely, excessively high alkalinity seems to retard the reaction. The de- sired pH is maintained by adding lime (CaO) or caustic soda [sodium hydroxide (NaOH)] at about 0.5 to 1.0 lb/ton (0.25 to 0.5 g/kg) tic soda is maintenance scale. Some the cyanide be higher for of solution. Although caus- more expensive, it reduces problems caused by lime operators claim that both concentration and pH should silver ores than for gold ores , but no trend could be discerned from the engineering data available. Solution Distribution Leach solutions are pumped from a mix- ing pond to the distribution site after the cyanide and lime (or caustic soda) have been added. Upon reaching the site, the solution travels through a grid of distribution lines (usually 3/4- to 2-in- diam plastic tubing) deployed across the top of the heap or dump. Spray nozzles, sprinkler heads, or wriggler tubing con- nected at various intervals into the main distribution line apply the solution. Fixed-spray systems are the cheapest and easiest to install; some operators merely punch holes in the distribution lines at fixed intervals to create a spray (fig. 5). Although these require little maintenance, channeling is a com- mon problem. Other operators may attach short feeder lines connected to fixed lawn-type sprinklers, but these do not distribute the solution as uniformly as do other systems and frequently result in channeling. The most common solution distribution system is the oscillating lawn sprinkler, usually referred to as the rainbird sprinkler (fig. 6). This sprinkler gives a more uniform coverage than fixed sprays. These sprinklers are, however, susceptible to calcium salt scale, which restricts the flow. When the scale severely restricts solution flow, the sprinklers must be removed and soaked in hydrochloric acid to dissolve the calcium salts. Some operators have experimented with modifying the sprinkler orifice in an attempt to reduce this problem and to distribute the solution without the ring of more concentrated sprinkling that seems common from off-the-shelf sprin- klers. Sprinklers are generally operated FIGURE 5. = Fixed=spray solution distribution. FIGURE 6. = Rainbird sprinkler. 10 with 20 to 40 lb/in2 (0.14 to 0.28 x 10^ Pa) line pressure, which yields a radius of coverage averaging 35 to 50 ft (11 to 15 m). The so-called Bagdad wiggler, named for its development at Cyprus' Bagdad Mine in Arizona, has recently become popular. A wiggler is constructed by cutting a 9-in (23-cm) long segment of 1/4-in (6-mm) thick-walled gum rubber tubing and forc- ing one end over a hose connection at- tached to the feeder lines (fig. 7). As the solution passes through the tubing, the free end wiggles and sprays the solu- tion around in a circle. Wigglers are generally placed on 10-ft (3-m) centers. If the wiggler flops in a figure 8 in- stead of a circular path, the ends are sliced with three short spiral cuts. Proponents argue that wigglers have bet- ter distribution patterns than sprin- klers. Maintenance is relatively easy; when calcium salt scale appears in the lines, the wiggler can be stretched and shaped to dislodge the buildup. Wigglers are operated at around 20- to 40-lb/in2 (0.14 to 0.28 X 10^ Pa) line pressure. An even newer but similar solution dis- tribution technique employs the use of a wobbler. Wobblers are made by attaching lengths of thin plastic (Tygon) tubing to the solution feeder lines. At normal operating pressures the wobblers will produce a 4-ft (1.2-m) high spray with a radius of 9 ft (3 m). At least two operators are applying solutions by ponding (fig. 8). Leach so- lutions are directed over the top of the heaps, where berms and dams hold the so- lution in a pond. The pond is kept sev- eral inches deep as the solution perco- lates downward. A major disadvantage is that appreciable clay-size particles in the heap or dump material will thwart percolation and will promote channeling drastically. Another problem with pond- ing is maintaining sufficient oxygen in the solution for efficient dissolution of gold and/or silver. Regardless of the distribution method, after percolating through the ore the so- lution drains off the pad into a holding pond (fig. 9). The pond is lined with FIGURE 7. - Bagdad wiggler. u FIGURE 8. - Solution distribution by ponding. -^~ /%i, >fit^' 3"" "i, r™" ?^ ■ ■»gM^iii»iiii i )t'ii'wyW i i i > > /'/*^i ^/"^^^ Ore heap C regeneration Au- bearing solution Stripped C Au-loaded C L Recycled stripping solution Electrolytic cell Au to refinery FIGURE 11.- Charcoal adsorption recovery system (after Potter (36)). fluidized condition range from 15 to 25 gal/min*ft~2 (1 to 1.7 L/s*cm~2), depend- ing on the size of activated carbon used. The columns of charcoal are arranged in countercurrent series so that the fresh solution first enters the column that contains the charcoal with the most ad- sorbed precious metals. As the solution flows through the charcoal, gold and sil- ver are adsorbed onto its surface. The solution passes through columns contain- ing charcoal with successively less ad- sorbed metals until it emerges as a bar- ren solution from the last one. After emerging from the last column, the barren solution is pumped to the makeup pond. When the front column of carbon (or a portion of it) reaches its desired load- ing capacity, the carbon is removed for stripping. An identical amount of carbon is then removed forward in each column. 15 and a fresh charge is added to the last. Charcoal loading formerly ranged from 400 to 800 oz of metal per ton of charcoal (14 to 27 g/kg) ; a recent trend towards lower loading and more frequent stripping commonly results in loading levels of 150 to 250 oz/ton carbon (5 to 9 g/kg). Fac- tors that affect charcoal loading are so- lution grade, flow rate, gold-to-silver ratio, solution pH, charcoal type, and impurity concentrations. The loaded charcoal removed from the columns must be stripped of the precious metals. Stripping is accomplished with a hot caustic soda solution. The tradi- tional Zadra process involves soaking the loaded charcoal at 93° C in a 1.0 pet NaOH-0.1 pet NaCN solution for 24 to 48 h (42-43). By adding 20 pet ethanol or methanol to the solution, the time can be reduced to 5 or 6 h (20) . The stripping time and chemical consumption can be fur- ther reduced by pressure stripping (37) with a 0.4-pct NaOH solution (without NaCN) at 150° to 200° C and 50 to 90 lb/ in (0.3 to 0.6 10 Pa). Gold or silver are next removed from the stripping solution by electrowinning. Typical electrowinning cells contain a stainless steel anode plus a stainless steel wool cathode for plating the metal. The steel wool cathode is usually packed to a density of 1 lb/ft (16 kg/m ) and provides a cathode surface area of 10 ft /lb (2 m /kg). Operating voltages run from 2.5 to 3.5 V. Significantly higher voltages can break down the solution and generate hydrogen or ammonia gas, which blocks the plating action. Current ranges from 20 to 30 A, which provides a current density of 3 to 3.5 A/ft at the cathode. The solution should be retained in the cell at least 15 min, preferably 30, to win the gold and silver. The stripped carbon is regenerated by heating in a kiln or chamber at approxi- mately 700° C (1,300° F) in a reduc- ing atmosphere such as steam. Although carbon can be used two or three times without regeneration, it is simpler to regenerate it each time rather than try- ing to keep track of which batch needs regenerating. The life expectancy of carbon has not been well documented. Some operators have experienced a 25-pct reduction in adsorptive capacity after eight or nine cycles, while others have found only a 33-pct reduction after 8 or 9 yr of continuous use. A few operators simply sell the gold- and/or silver-laden charcoal for smelting rather than strip- ping it. That charcoal is, of course, destroyed during the process. LEACHING OPERATIONS Over 80 operations have been identified that have experimented with leaching, were actively leaching, or were seriously planning on leaching (fig. 12). Although many of these reported activities could not be verified, table 1 presents avail- able information on their status along with a general location, mine name, and operating company. The many new permits being granted each year indicate the great interest in leaching but make it impossible to generate a totally up-to- date list of operations. The 26 operations for which the most data were available were selected for presentation of geologic and operational parameters. Table 2 provides the loca- tion information for each, table 3 the ore characteristics and leaching data, and table 4 the extraction data. While the data contained in these tables will change, it is felt that operators or po- tential operators can use them to develop a feel for the range of conditions that can be expected at a particular site. The tables can also indicate possible solutions to site-specific problems. A State-by-State summary of leaching opera- tions follows. 16 CO 0) 4-1 CO 4-1 W 9) U •H c U (U 4J 03 CO 5 CO < • rH CO d CO 1 1 • o 3 a » a • • • 0) • • iH > • 4J • > • t: 1 4 t; • > X) « •« 1 1 • > X rj k 4-1 C •o CO 0) •r- a a a a f cu a 1 0) a 1 a> •H a 7 a 1 C : ^ «) 4-) > < 4- c > e S> > 4- c > P • d t- o • > k 4J c o > • a> o • • d Crt •H C t ) c •T- c 1 c •r- t » c C » 1- d -p- c d o •pH c » y c d f I o > d o o d 4J C CC 1 cc 4- c 1 cc t i. cc 1 CO c 4. 1 CO 4. c 1 ^ O 4J C 1 <0 cc ^£ u o d M Q a CO O c r— C r— * c ) c iH o iH y d u d r- d y o c ! pH • u •> ' • X '■' • • •C3 ' •o • a • a > a • . n) r- • • r-t < k rH H • u « > > 4. 0) • 4J C ) • • O ' k 0) O CO • "H ► *^ 4J • -H bO • • bO 4J bO M • f. > f »- •H > u • • T ( u bo C Cf *- bO >% ^ :^. ^ • • • • #k 4 u m" s a 1- a »^ a f- e a « c o c ' c ) u a o o o c (U • o > 0) • o > CI t> t; > cc 1 o ti • t; •c •o T •O CO > X -O t3 X> X > X I -ci t3 CO > X) tJ iH «- 1— r~ «- 1 M t— » l-l .- t • • ^ r- • ^■ ) rH ,-t rH • •H a •p- c •p- a ) ^ •r- i§ • OJ T- "c » « • •H C CO c < Q ) CO < •H 8 : w t; cr e cr CJ > V . u V o • • > Z: O CJ) Z CO 1 1 1 1 r-i CU • • c 1 tH M > cc 1 y . CO • bO •a ►-s • e > 1 M » T' •H d 1 ^ 1 PQ CO > 4- o • 4- > » ^—\ •O iJ ' PC c > U < •p^ < s « c CO ' CO (U IK » > k CO ' . CO c • C » U ' l-l rH s • e ^ 1 o < O 4J » "r- t O ' X < ' Cf cc 1 >^^ « • ot) y a ' CC 1— PC 0! ^ 1 d ! cu • 4J ^> c > € • > (I • •p- k 0) o < CO C T- ' cc rH pt ► •!- • cc 1 » • Cf • W OD < » 4- ) d >» o cc I ^ (1) c L, > cr • • ot » • "H d - » CO O rH k 0) (U 1- > V l-l c; » Ot ) • C > CO j= ' k r- pt. 1 O r-t k CO 4-) ► *J Cf 1— > a CO c > TJ U O ' k r- S O • 3 CO U- • V W < c Ct h- ) 4-) c c. > 3 C k 1 bO CO ot >-) ' • -p- c O >. o 4J C > cc 1 ^B ot CO c ) i- 1 O c ' d bO = > tt o «K s-g c M T- < u 1 a a a '1 . .H 1- X k tJ, O "r- ts • k 4- 4-1 • (U a • "c o ff a ) i- 1— JS c » u a t ' J= 4-> 1- o « > a » k 4- o ce a X < > X 1 X : CC bO M XJ 4J C cc ' t 4J > •>^ ( c 4- p X > c T3 r- t I • CO u c Li > • > c cc 1 I- T3 -H rH 4J 8^. < 2 Crt ^ a > 1- !(5 s c p: IS? •P" a i^ CO E CO (i: 8^ > -^ ir S" 5*^ • b } «f • • • • 4 > X- C •r- E k • • 1 »« • 4 > cr •p- k • • c <4- • ^ • * » 1— c 4- a k • • c k > 4- k Q, U < > cc c • "Z C, I • • >. k (4^ • 1- k (U c > ^ 4- CC c > eu • > • a • c Eo • > u M C a 2 \l t- > M • a a Cf 4- bO • CJ> C c ! 1 4- 14- « o :€ ' > > 3 CC e d •I 1- c • C 4- C c ' o k ^ a >t •r- •r^ k q) C b !I CO c e ' x c • 1 k ct • S2 a c d k rH O C 1 (U a • « a c; > ' X > s • CO • b 3 bO "H -o a 1 E- t— g c < • a •H 1- cc bO O b ) 1 O • 4- b i) • 4. 1- k rH o e d X 4J l-l • 1 1- a >% 4-1 C i- c ^ C LO > cc • ^ X 1 c ► H^ ci . CO d T- •p^ -H iJ o c > V c •H c CO T- t- T- 3 •H c • x . o c f- •^ • u M 1- M n o c Ok Cf a» c l-l CO a c o c r- bO • "T" > -H cc <- bO M X • 0) a 0) X •> 1- t- a t^ . O i* O f- •n CO X a c c > "CS d QC f . d « a 0) a CO CO C c 3 1 ^ bO r-i a « •H c "c V. l-l b 0^ •p^ a ct :^ 00 c • d CB CO CO oc • CJ X 4J d MH o a c Nw / Pi 0. c 1- ce a 3 e t- d -p- X cu -H O •pH o 0) OJ y T3 4J d ^■ •H O u X •.- a c c s Cs e 1— PQ •p- c gt bO d b )0 bO » u s «- d 4J a •H (V a d W cc X 1— bO-cJ cc c 4- cr 5 • d TS ^^ d oo o 1^ M 1-3 ^" c( > c S3 t«« t: c c a > >.c CC CO O cc 3 CO ^ DC Pd U o e ct 3 c CO a i- a •H JQ h- ) >- a W c a O b E )•« PQ c 1 "c C b 3 4. T" X < rH f-t CO 0. CO 9) 3 T3 X d T3 X U CJ d •r- xj rH y a ct c 1— •H e M 3 3 C • b 0- •o c; a o <« O »-i c (U d CO O rH rH •rl ft r- d u CO ;i '<5 i S 1 S SI ^ o U ^ c;§2 PO ss CO 5 ^ 5:2^ c 3 O f « (U •C3 CO r- CO . d TJ 0) . •p- •H cc • » p-( • • f- CO c .. (0 . cc d •H o rH d • CO M M • CO CO M-l • r^ c > < u 1^ T3 •H -H • u O • c c c c o J <4-l u 1.1 CO r-K o d o o rH Q o p Q o O O 4J N O •H cc 3 ■ ►" z f-l M fH O O S Pi H 4-1 b CO O CO • • • • TS C • TJ • p • ? •H T3 • •o • TJ • 0) 0) ? 01 C O > • c > • • • O !> O • O C !> • • c > • c > • • • • O -H C C T-l o c •H O o o C 1-1 c o rt C tH O o C -H O c: th o o o o O 4J rt a! •tJ o « W O o o ^ *J M o c n) 4J o o «) W Q . 1 • » • » • • • • • , , 1 > • » • • • • • • c • • » • • • • >. > • » • t • • • o > 73 t- M • • • m > 1- • >-l • t-i • I-I • • e > ,-4 Q) . . . (U » 2 • a> . O > > . . . > > c 1 • > • > < > • • u . bOr- > rH > t » r-i > u « ^ • r-{ > ,-{ • • c » •!- > -H > • • -H • a 1 > -H > T-t ' •H • . « . .. cr > CO > • . CO ! > CO > CO • CO • • > i-i . >- • o •> c • 0) pt c » ff^ • O O •> C c > c »» > c > « - . O 6 - o •O "^ t: t: ' -c 1 > -c x "C •o t: 1 TS TS "O X X t: •C 1 X X X X I X) P' T3 XJ XJ t3 X) X> • iH 1— rH r- r- rH r- 1 • • rH » r-* < » ^ ' rH r- rH rH • f-t • :5. 8 :g6 •H C c5 . 88 , : 18 : .5 Z S ) :c5 > 8 ) I O cr o o • 8 : > • B • Q) ft . Q) ^ » • M C cr • B • o b ) > a » "r- > O • w c > u • C . (U •H ' «- ' CO • c T . a . CJ • ^J c > a • 03 '^ > c • d) . o « »- > p. « to • u • T3 u • [a • C 5 \8 • 4- • Pd > o 1 9* "C > ) « CO > 4-1 c > c • a > a. « 0) » m «t > C3 fe ) > u • cc • cr ) • M • (U 4- > V • c ) > y ; 3 OJ • 0) ^ (£ • > c • 4- • 1- 1 • 0) < e :^:: 5 - > r - X 1 • cr > P- > rH rH • 1 «K s T > •r" > 4- ^ ' ^ • Xi T3 i-H • • M > 4J > ' C >> ' a • Q> rH ss • c • 0) "e > X .5 «0 ,•< T" : CJ * "^ c 1 »- »- J « u > Pm CJ) ^ M > Ph •c « -H W » r- cn r- . c < > ^ • V. 1 CJ > cc ) B CJ > 0) CO CO ' > •H > rH > e |e 4J ' b 3 w £ :^ : « c > »■ < > t > CJ "« c n a & o c « B N CO & • o > ^ < cc P M ' « ' c aM- 1- •r- J3 1- • O (U V (U • rH c « u -c < ' > ■^ ^ •1- 0) t: 1 4-t C < CO ? • > c 1 c ) X ) r- bOX 1 ^ ^ 1 • CO 4-) Q> X3 • iH c ' 3 r- ' ' X 5 ) Ji CO r- > OE > e c u bOr- 1 o w ' B M u 3 -H ^^ 3 W3 < 2 ^,5 2 P£ < T- > ^^ iSS^^ 1 < < 2 1- 2 ^ !S cS 11 » 3 n ) PQ T 1 < -H Q ' 2 Pu O •r-l ;gcS » • 1 » • 1 • • • 1 • 4- > • « • • N • • » I > • • • a • c > Q) X » 4 » 4 ► . • o ^ > c • c • 4J • cr 2 1 • • t > ce < JC 1 2 1 • o B • 1 T3 OJ • c < cr . . . c • «- < cr • > CJ o bO C T3 > o < . V «t c . . . o < > c ■ c • 2 r-\ • C <0 C > -H < • C ct b c < > . O -H . > 4- C • 1 « rH Ui y • •H : > 4J • O h 5 (U T- < . . C 4-1 . > c u cJ? » > r-{ 1 B • C C J3 to . •H OJ P- 4J < > . M CO • ct 1 ' c . o ' :a • t-H • g^ EH • 1^ • 4J a 1 c . > T3 »J V >- « bO • CJ • c y • 1 CJ o < CO 1 • • ^ . •4-1 •> O 4J C . M B ^ • 4J c rH ' u < •o c T3 V- bo . h3 fSrH iJ > ^4 Pu Ct c c •H c bo » « > u M CO • >-i n) 5 M a « o c 4J u 4-1 4J c < c •H e c . c . ► c. . • a; 0) u O M (U X • rH CJ hJ iJ CC T-t . • •> 0) X -H CJ > T- •H ' S C I C3 ji •^ y B •H O rH •^ cr u • a b e c < • bO B U C ^ 4. >- c . CO H- XI O B CO M M M ^ T) I-H X b ► e •^ <5 2 • • C "H C b Oi It ^ > o 4J 3 0) 3 > iH MH B cr e v. u u u • iH ► hJ bOPO y O •> ' a X 4J u 8 t o < •r e c c p C 0) •<-* V H Pi CO < S 4-t •H c c c > M cr B 1 M • CO bO • 3 W c 01 01 O X » c o s o 1 u c c OJ a; r- X > V r- -iH X 3 X3 (U B > w 5! Pli c r-i OJ O -r C^^ tH tH cc CO cr: s c c C . > a • o X CO B 0) O 4J Pi ^ > CO a CO 2 51 4J (0 > 4J ^ CO c CO 6 « 4-1 4J 3 *- s H Pt: rH i • <1> H d B S < CO 0) ►J B CO vH to S > tC 0) o 1- cfl 1 c M J»i U PC c B .H g c > ■\ c • r-( bO ^ B M • pc: »« > -H JS .-) •H u X «0 »- 0) CC rH E O (K O 4J C( S^ g 15 rJ CO C Ct c B > u CO CO > T3 O (1) rH C « C OJ f-i 3 rH cr U • r-( u 0) c l«H •H •H c c C J- O 0) XJ X 0) P^ B ' CO C > rH 3 iJ (0 C M *^ aT) a E 0) J.i CO 0) c a; y .fl 1 B C X r^ B X X 15 B CO y rH 0) B o > C (0 0) •H O •H y <5 O >^ X C X ct OJ cr 0) bO 4J cc z B c« T! •H r- C |J iH O B CO O B •H & u ^3* o £ a :§ »2 h! iH Pm ^9 w cfl w 3 at o ■a to <^ O "4H U 0) PO >-) CO a. O rH o O -H Q Ph o o u to rH CJ o o M to •O* ... ...4J tO» .(0» ...rH«»U |-l« .Jii. ...0»»(U OJO 00)0 000^30073 6Q QMQ QQQSQQB CO 3 3 CO o o 18 73 3 C C o o I I CO U (U a o bO c •H o ca 9) c o. CO 0) 0) > B CO s O O 0) > o o o o o o o o O P P O O Q O o o p p o c c «3 • c o CO P 0- M 0) c • c o CO P • • » o • • • » • • • • • • » • • 1 1 • • u •o . u > > • M u • (U iH . O • > > • > > . 1—1 . bO > • 1— 1 • .H < .H . •H > • •r^ " > • -H < •H • CO •K • CO . CO < CO • kJ k • M * • < > • i- • ' O CD O ►OOOIUOOOO . O « • a « • •O tJ t) T3 > t; T3T)T3T3'C3 >TJ'n'0'0'0'0T3'C • > •o t: iH ' I-^ iH .- • tH » f— • iH 1- • r- CO rH c5«»c5cno 'O •H CO :5 O O Z CO ^5 • iH 0) • O > • bOiH • -H • "CO • 1-1 O 0) « "O > t3 73 73 • rH iH i-l • • -H • CO O ■O "O "O So- s s o o CO CJ - o O Ou C CO CO tH O M CO «-i iH aT3 o. c O CO CO ^ s e rH o c (U 4-1 (U CO iH bOrH C CO S 3 H J= -a CO CO o a CO Ui c CO r-t u cu i 4J )-l o z CO o T3 73 c CO a> c 73 73 o o bO bO O 5 O C 73 U CO CJ CD 73 (0 % CO c o 4-1 >% CO P o o I c CO >> c M O CO CJ F>4 Fi^ O 73 >> O •H iH >i •> a) 73 iH > CO CO i-l »J 1-1 -H M CO .> % 0.73 M S C (U M O » I a O C CO rH O tH Fti O P bO -H 73 0) O U 4J 3 CO CO bC CO -H « .H U r-l <: <5 Z Z a o y 3 C M O CJ 4-l c o CJ < < z z o u (U r-4 N I-l -H 0) 4J ^ M < W O Z 0) 0) bO 0) 73 M Ed CJ C5 >^ C CO o. a o CJ 4J O CO o c u CO (U rH 4-1 a CO c: >-l 4J M O 0) :j S 0) iH bO c u o. 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No further information is available at this time. IDAHO Several operations in Idaho have con- ducted tests on the heap leaching poten- tial of lean ore in and around previous mining districts. Information on these tests and any full-scale activities re- sulting from them has been very scant; only two are discussed, Canadian Superior Mining, Ltd, , has been conducting experimental heap leach- ing of ores from the Stibnite area since 1974. Although the Yellow Pine Mine was originally an open pit operation for an- timony and tungsten, gold was encountered at one end of the pit. Recent drilling has delineated two nearby ore bodies — the West End and the Garnet Creek — that ap- pear favorable for leaching. In the fall of 1978, Canadian Superior conducted a 500-ton (0,45 x 10^ kg) heap test on crushed ore; two other tests were con- ducted during the summer of 1979, These tests led to a leaching plan whereby the ore would be mined from two open pits , placed into 45,000-ton (41 x 10^ kg) heaps, and leached. Production began late in 1982 after completion of an Envi- ronmental Impact Statement by the U,S. Forest Service, Canadian Superior had also obtained the rights to explore and develop a gold min- ing property owned by Thunder Mountain Gold, Inc, , in the Thunder Mountain dis- trict near McCall (31), After Canadian Superior terminated its agreement with Thunder Mountain, a new agreement was reached with Phillips Petroleum for de- veloping the property with Coeur d'Alene Mines Corp. as the operator. The gold apparently occurs at a shallow depth that would permit open pit mining. The opera- tion would probably be patterned after plans developed for the proposed opera- tion in nearby Stibnite, where ore would be heap-leached and recovered by charcoal adsorption, MONTANA Heap leaching for gold and silver is rapidly emerging in Montana, Of the sev- eral operations that have expressed in- terest, 2 have reached full commercial production, and 1 is conducting large- scale field tests. The two operations are on opposite sides of a peak in the Little Rocky Moun- tains ( 15 , 6). Landusky Mining, Inc, and Zortman Mining, Inc, , are both owned by Pegasus Gold, Ltd, One mine is a few miles north of Landusky near the old Au- gust and Gold Bug Mines, An open pit mine produces 24,000 ton/d (22 x 10^ kg/d) to supply the heaps, Blastholes are drilled on a 10- by 8-ft (3- by 2-m) pattern to depths of approximately 40 ft (12 m) , Tlie other mine is north of Zort- man near the old Ruby Gulch Mine, Here two open pits produce 17,000 ton/d (15 X 10^ kg/d) of ore, Blastholes are on an 8- by 8-ft (2- by 2-m) pattern. Together the two operations were estimated to con- tain 50 million tons of reserves at ore grades favorable for heap leaching and are annually producing 40,000 oz (1.2 X 10^ g) of gold and 90,000 oz (2,8 x 10^ g) of silver. The Golden Sunlight Property 6 miles east of Whitehall hosted extensive un- derground mining for gold through the early 1950' s. As part of its Heavy Met- als Program, the Bureau of Mines and oth- er agencies conducted detailed economic analysis of conventional mining on the property (_1^), The present operator, Placer-Amex, first experimented with leaching on a 26,000-ton (24 x 10^ kg) test heap. The heap was segregated into five segments (each containing a differ- ent size distribution of ore) that pro- vided solution for five separate charcoal adsorption units. The heap was then ex- panded to 40,000 tons (35 x 10^ kg) for a full production test. Feed for this effort was run-of-mine rock. High clay 26 content in the ore caused percolation troubles; the heaps had to be periodical- ly ripped. These troubles led to a re- cent decision to construct a conventional mill complex for processing the ore. Heap leaching has been dropped from imme- diate consideration. In 1978 a heap leaching test was car- ried out on ore from the Tourmaline Queen Mine east of Boulder, Approximately 25,000 tons (23 x 10^ kg) were satisfac- torily leached, A 30,000-ton (27 x 10^ kg) heap was constructed, and caustic was added for a new test to be conducted when the snow melted in the spring of 1979, When this test also proved successful, ore was obtained from an open pit mine for a commercial-scale heap. Since re- covery from this heap was lower than an- ticipated, the ore was crushed and re- placed on the heaps for leaching in 1981, Approximately 3 million tons (2,7 X 10^ kg) of ore graded at 0.07 oz/ton (0,024 g/kg) gold were delineated from open pit mining, NEVADA Nevada is currently the center of heap leaching activity. Numerous operations have been or are actively considering leaching. The Smoky Valley, the Cortez, and the several Carlin operations have all been well documented in mining liter- ature. Enough data are available on 11 of these operations to warrant their in- clusion in tables 2 to 4, The Carlin Gold Mining Co, operation pioneered solution mining systems tar- geted for low-grade, disseminated, oxi- dized gold ore bodies. Commercial heap leaching operations began in 1971, fol- lowing laboratory and pilot scale tests. About 10,000 tons (9 x 10^ kg) of low- grade oxidized ore were leached each month between April and October, Four leaching pads were eventually placed in operation. Although the pads were gen- erally leached about 7 days before re- placing the ore, solutions were applied until their gold grade dropped below 0,015 oz/ton (0,005 g/kg). Although the ore from this mine is now processed in a conventional mill, portions of the ore body are amenable to future heap leaching (35) , Exploration activities have uncov- ered two other ore deposits near the mine — the Maggie Creek and Gold Quarry Prospects — portions of which will proba- bly be heap-leached. Contracts have been awarded to construct mining and heap leaching facilities at the Maggie Creek Deposit, and approximately 2,5 million tons (2.3 X 10^ kg) of ore had been leached at the mine by the end of 1980. Carlin also has been dump-leaching at the Bootstrap Mine north of its Carlin pit. The Bootstrap Mine was a surface operation from the late 1960's until 1978. A sizable waste rock dump on the property is being leached. The dump was originally placed on a compacted ancient lakebed, which makes an impervious pad suitable for leach solution containment. The dumps are merely leveled to facili- tate solution distribution. Numerous small, underground mines around Round Mountain produced gold from the turn of the century until the late 1930' s. Now Smoky Valley has established an open pit mining and heap leaching operation near Round Mountain ( 25 , 41 ) . By 1977, the operation was producing 85,000 oz (2.6 X 10^ g) of dore bullion per year consisting of two-thirds of gold and one-third of silver. To achieve this rate, about 8,000 tons (7 x 10^ kg) of ore and 7,000 tons (6 x 10^ kg) of waste rock are mined per day. The ore is crushed, placed on a pad, and leached in one lift. The spent ore is then rinsed and moved off the pad to make room for the next batch of ore. Five pads are in continuous operation; four are being leached and one is being reconstituted at any given time. Three new pads were con- structed during 1980. Another leaching operation that is well documented is Placer-Amex's Cortez Mine (13-14) . The Cortez Mine opened as a surface operation for a conventional mill in 1969. Although the ore body was ex- hausted by spring of 1973, the mill con- tinued to process ore trucked from the nearby Gold Acres Mine. During mining 27 operations, marginal ore (below mill grade of 0.08 oz/ton [0.03 g/kg]) was stockpiled. When tests run on this mate- rial indicated that it might be suitable, a heap leaching operation began in 1971. In 1976 the conventional zinc precipita- tion mill circuit was converted to an activated carbon adsorption system. Placer-Amex also operated the nearby Gold Acres leaching operation. Although currently idle, the operation has been well publicized (13) . The site of an early mine, Gold Acres was open-pit-mined beginning in 1973 to feed the Cortez mill as a replacement for the depleted Cortez Mine ore. When this operation proved successful, a new leaching plant was con- structed at the Gold Acres site. It was one of the first, if not the first, com- mercial applications of the charcoal ad- sorption system in the country. The high clay content of Gold Acres ore created lower percolation rates and recoveries than were obtained from the nearby Cortez Mine. The clayey Gold Acres ore was not suitable for leaching from lifts placed on top of each other, and periodic rip- ping was needed to prevent ponding. Fur- thermore the ore was highly carbonaceous and thus very refractive below the oxi- dized zone. Although approximately 5 million tons (4,5 x 10^ kg) of ore had been heap-leached at Gold Acres and Cor- tez by the summer of 1977, the only cur- rent plans center around leaching waste dump rock from the idle Gold Acres prop- erty, Cortez has also announced plans to develop the nearby Horse Canyon Deposit, but it is anticipated that conventional milling rather than heap leaching will be used. South of Eureka, a Vieap leaching oper- ation has been in production several years. Near the original Windfall Mine underground workings, Idaho Mining Co, has produced ore from an open pit mine to feed its heap leaching operation. The operation is one of two that uses a pond- ing technique for solution distribution on the pads (fig. 8). Ore is hauled from the mine to the pad area and stacked. Berms are laid out on the surface of the pad to control the solution flow. Since the ore is a sandy dolomite, essentially clay free, percolation rates remain ac- ceptable. Idaho Mining also opened a new pit on the property in a zone that con- tains some clay. It will be interesting to see if adequate percolation can be maintained on ore from the new pit. In January 1980, Windfall Venture purchased the operation from Idaho Mining. Occidental Minerals Corp. dedicated its Candelaria operation in November 1980. At one time many underground silver mines were operating in the Candelaria dis- trict. Two of these mines — Lucky Hill and Mt. Diablo — are being stripped and open-pit mined to provide feed for a large heap leaching operation. Although the overall ore grade is good, the manga- nese oxide content makes it somewhat re- fractory, A number of experiments were carried out at the site, ranging from 500-lb (200-kg) "barrel" tests up to a pilot heap leach on 11,000 tons (10 x 10^ kg) of ore. Based on the successful pi- lot test, plans were made to mine the two deposits at a rate of 8,000 tons (7 x 10^ kg) per day along with 16,000 tons (14 X 10^ kg) per day waste rock. Stripping began in the spring of 1980, and the first silver was poured in the fall of 1980. In 1981, its first year of produc- tion, the operation produced 1.7 million oz (53 X 10^ g) of silver and 9,200 oz (0.28 X 10^ g) of gold. Tlie company pre- ferred not to disclose engineering data from this stage of development. In June 1982, the operation closed because of the decline in silver prices. In 1983 Nerco Metals bought Occidental Minerals and re- opened the Candelaria operation, American-Selco and Occidental Minerals combined on a successful heap leaching experiment on Alligator Ridge about 60 mi (96 km) northwest of Ely. First a 5,000- ton (4.5 X 10^ kg) heap containing 0,18 oz/ton (0,06 g/kg) gold was leached for 2 months, with an 80-pct recovery. Next the same amount of ore was leached after the fines were agglomerated, follow- ing the Bureau's technique (see section on ore preparation) , and the same re- covery was obtained in only half a month. Following these successful tests. 28 construction began in the spring of 1980, and the operation is now producing about 60,000 oz (180,000 g) of gold per year. D Z Exploration has been conducting ex- periments at the Packard Mine in an old silver mining district northeast of Love- lock for two seasons. The first experi- ments were on run-of -mine-sized material from the waste dumps. Since this materi- al had weathered, enough fines were cre- ated to restrict percolation through the reconstituted heaps. Attention focused in 1979 on the Bureau's agglomeration technique as a means to improve percola- tion. To test the technique on this rock, ore was crushed to three sizes — 2 in (5 cm), 7/8 in (2 cm), and 9/16 in (1.4 cm) — and agglomerated using 10 lb (4.5 kg) of cement per ton (90 kg) of ore. Cement was added to the crushed ore by a unique auger arrangement. Water spray was directed at the mixture, which then tumbled down an incline to complete the agglomeration. An 8-ft (2-m) high heap containing several hundred tons of each size material was then leached. Based on the results and additional drilling, full-scale production was ex- pected to begin in 1981, but no updated information is available, Pinson Mining Co. opened a major open pit mining and milling complex in Hum- boldt County during 1981 (39-40). Ap- proximately 500,000 tons (0.45 x 10^ kg) of material below the 0.05-oz/ton milling cutoff grade were stockpiled for future leaching. Pads were built in 1982 for several heap leaching tests involving ag- glomerated and unagglomerated ore, scale inhibitors, oxidizers, solution applica- tion techniques, etc. These pilot tests then formed the basis for designing a full-scale heap leaching facility that began operation in December 1982. Tests are now being conducted on material from the nearby Preble ore body. Other operations that have been dis- cussed by the mining press include the United Hearne Resources, Ltd. , operation near Hamilton, the Getchell property recently purchased by First Missis- sippi Corp. , the Bald Mountain property northeast of Ely, the Relief Canyon prop- erty being tested by Lacana Mining north- east of Reno, Dee Gold Mining Co.'s Boul- der Creek property near Carlin, Gila Mines Corp.'s mine near Reveille, the Borealis Mine managed by Houston Interna- tional Minerals Division of Tenneco, Inc. , the Tuscarora operation that be- longs to Tuscarora Associates, and the Gold Crown Mine in the Current Creek dis- trict operated by Great American Gold Co. The large number of leaching operations emerging in Nevada each year makes it im- possible to accurately track all of them. Since this is not the purpose of the report, other Nevada operations are not discussed. NEW MEXICO Solution mining operations have been relatively rare in New Mexico; only three have been identified. One of these is now inactive, while two are currently producing gold. All three are listed in the engineering data tables. Gold Fields Mining Corp. has brought the Ortiz Mine, 35 miles (56 km) south- east of Albuquerque, back into production with a heap leaching operation (22) . The activity stems from a mining and leaching plan submitted in 1978 to the New Mexico Health and Environment Department. Ore is being produced from a 3,000-ton/d (2.7 X 10^ kg/d) open pit mine, which is be- ing dewatered by a series of peripheral wells. After the ore is crushed to less than 3/8 in (9.5 mm), it is placed on a pad with a traveling gantry system. Peb- ble lime is added to the ore to control pH yet minimize scale problems that are normally encountered when "milk-of-lime" is added to the makeup solution. Gold recovery has averaged 78 pet with a high of 87 pet since the operation opened. During the mid-1970' s interest revived in the old Cooney mining district around Mogollon. Mine dumps from the Confidence Mine and surface vein ore from the Eberle Mine were tested by Challenge Mining Co. for leachability with favorable results. The company has developed a 2-yr plan for heap leaching at a rate of 34,000 ton/yr 29 (31 X lOf' kg/yr) ( LS ) . After 2 yr this material will be depleted, which will re- quire underground mining in the old Eber- le workings. An estimated 300,000 tons (0.27 X 109 kg) grading 4 oz/ton (1.4 g/kg) silver and 0.08 oz/ton (0.027 g/kg) gold remains in the mine. In 1975, Canorex opened a 500-ton/d (450 X 10^ kg/d) open pit gold mine to provide feed for a heap leaching opera- tion (_7 ) . Two adits in the ore body had proven ore reserves of 1 million tons (0.9 X 109 kg), grading 0.23 oz/ton (0.08 g/kg) gold and 0.63 oz/ton (0.21 g/kg) silver. A plant was constructed to han- dle solution from the heaps. The opera- tion apparently went well for a while but has not produced recently. SOUTH DAKOTA Two heap leaching operations have been identified in South Dakota. One functioned at the pilot-scale level in 1980; the other has applied for the per- mits necessary to construct a pilot facility. Cyprus Exploration is operating the pilot-scale leaching test at the Gilt Edge property 4 miles (6 km) southeast of Lead in the Black Hills. At the site of early mining activity, a pad and solution ponds were constructed for a 1,900-ton (1.7 X 10^ kg) heap. Further information is unavailable at this time. Tiaga Gold Corp. -Wharf Resources, Inc., has delineated a 5-million-ton (4.5 x 109 kg) ore body at 0.04 oz/ton (0.014 g/kg) gold on their Anne Creek property 4 miles (6 km) southwest of Lead. Plans are be- ing made to heap-leach the ore with a system patterned after the Zortman- Landusky operations in Montana. PERMITTING REGULATIONS FEDERAL REGULATIONS The Resource Conservation and Recovery Act of 1976 (RCRA) has been the Federal regulatory action recently attracting the most attention from miners (34) . Aimed at improving resource conservation and recovery through land use control, the act establishes a Federal-State permit system for hazardous waste management. Originally the mining operators were con- sidered to be producers of hazardous waste and operators of hazardous waste treatment, storage, and disposal facili- ties. So many problems arose in attempt- ing to apply the regulations to mining operations, however, that an eimendment was passed in October 1980 to exempt min- ing from the law while studies were con- ducted. It is safe to assume, however, that this act will eventually impose some control. Since the material on a leach- ing pad and any leachate emanating from it could be considered hazardous wastes, leaching operations will fall under the purview of the act. Ground water monitoring programs will be required to assure that the uppermost aquifer will not be harmed. Although the U.S. Envi- ronmental Protection Agency (EPA) is charged with administering the permit and enforcement provisions of the act, EPA intends to pass on the permit system to the States. EPA is also charged with implementing the Safe Drinking Water Act and, partic- ularly interesting to in situ leaching operators, its Underground Injection Con- trol Program. This program establishes a permitting system for five classes of wells by which fluids are injected or disposed into the ground. EPA also regu- lates surface discharge permits under the National Pollutant Discharge Elimination System (NPDES) and the Prevention of Sig- nificant Deterioration Program, which may affect a potential leaching system. Operators should contact the closest re- gional EPA office for clarification on permits required (table 5) . 30 TABLE 5. - Environmental Protection Agency regional offices issuing hazardous waste and surface discharge permits Office Coverage Office Coverage Region IV Region IX Enforcement Division New Mexico Enforcement Division Arizona 1st International Bldg. Texas 215 Fremont St. California 1201 Elm St. San Francisco, CA 94105 Nevada Dallas, TX 75270 (415) 556-2320 (214) 749-1983 Region X Region VIII Colorado Enforcement Division Idaho Enforcement Division Montana 1200 6th Ave. Oregon Suite 900 North Dakota Seattle, WA 98101 Washington 1860 Lincoln St, South Dakota (206) 442-1220 Denver, Co 80203 Utah (303) 837-3868 Wyoming Leaching operations targeted for U.S. Forest Service, Bureau of Land Manage- ment, or other Federal lands will proba- bly require an environmental assessment before permission to proceed is granted. A prospective operator should clear this through the district office of the Fed- eral agency controlling the land. A helpful discussion of regulations and procedures is contained in "Forest Ser- vice Current Information Report No. 14," available at Forest Service offices. In addition, recent Bureau of Land Man- agement (BLM) regulations (43 CFR 3800) on Surface Management of Public Lands Un- der U.S. Mining Laws went into effect January 1, 1981, for the purpose of pre- venting undue degradation and requiring reasonable reclamation of BLM lands dis- turbed by any mining. Depending on the level of activity and size of the dis- turbance, a plan must be filed and some- times approved by BLM before mining can proceed. STATE REGULATIONS Permits required for leaching vary con- siderably from State to State. Table 6 lists a few key permits and gives the as- sociated State agencies to be used as initial contacts. These contacts can provide details on which permits are re- quired and the regulations and require- ments pertaining to each. It is interesting to note that Califor- nia and Colorado have moved toward con- solidating permit requirements into one office. In California, the central con- tact is the Department of Economic and Business Development, which provides any new business venture with a complete list of necessary permits and directions for obtaining them. The county commissions assume leadership in California during the permitting process. Colorado has developed a voluntary pro- cess, tailored primarily for large opera- tions, whereby a company can request a Joint Review of a proposed mining proj- ect. If a company requests this Joint Review, the State coordinates interests of all Involved Federal, State, and local agencies and develops a statement of re- sponsibilities and problems that must be resolved and a schedule of activities during the regulatory time period. Com- panies that do not wish to use the Joint Review process may proceed as before, by interacting with the permitting agencies, a list of which can be obtained from the Colorado Department of Natural Resources (table 6). For the other States where leaching is practiced, table 6 lists the prime agency contacts. 31 TABLE 6. - State permitting agencies Agency Action Remarks ARIZONA Department of Natural Resources Booklet on laws and Mineral Bldg. , Fairgrounds regulations. Phoenix, AZ 85007 (602) 255-3791 State Office Bldg. 415 West Congress, Room 190 Tucson, AZ 85701 (602) 882-5399 State Mine Inspector Mining code and Must be notified of commencement or 705 West Wing regulations. suspension of operation. Regulates Capitol Bldg. health and safety. Phoenix, AZ 85007 (602) 255-5971 Bureau of Water Quality Control Water discharge Arizona Department of Health permit. 1740 West Adams Phoenix, AZ 85007 (602) 255-1252 Department of Water Resources New water supply 99 East Virginia permit. Phoenix, AZ 85004 (602) 255-1550 CALIFORNIA Department of Economic and All necessary Provides all permitting information Business Development permits. for any business. 1120 N St. Box 1499 Sacramento, CA 95805 (916) 322-1394 COLORADO Department of Natural Resources Joint review The joint review process is an op- Executive Director's Office process. tional consolidated review proce- 1313 Sherman St., Room 723 dure for major energy and mineral Denver, CO 80203 resource development projects. (303) 839-3337 Colorado permit Operator may elect to obtain permit directory. directory and deal with permitting agencies. IDAHO Department of Lands Mining permit Bureau of Minerals State House Boise, ID 83720 (608) 334-3569 Manager Source Control Section Waste water treat- Review, no permit. Water Quality Bureau ment review. Division of Environment Deapartment of Health & Welfare State House Boise, ID 83720 (208) 334-4059 32 TABLE 6. - State pennltting agencies — Continued Agency Actiou Remarks MONTANA Department of State Lands Operating permit... If over 36,000 ton/yr and/or 5 acres Reclamation Division disturbance; otherwise small mines' Hard Rock Bureau exclusion. Capitol Station Helena, MT 59601 (406) 449-2074 Department of Health and Water discharge Environmental Sciences permit. Water Quality Bureau Room 206 Cogswell Bldg. Helena, MT 59601 (406) 449-2406 NEVADA Administrator Mining permit Also will provide a list of State Division of Mineral Resources and Federal permits required before Department of Conservation and a mining permit can be granted. Natural Resources Nye Bldg. 201 South Fall St. Carson City, NV 89710 (702) 885-4368 NEW MEXICO Bureau Chief Ground water qual- Administer the Water Quality, Con- Health and Environment ity permit. trol Commissions regulations. Department Environmental Improvement Division Water Pollution Bureau Box 968 Santa Fe, NM 87503 (505) 827-5271 SOUTH DAKOTA Land Management Specialist Reclamation permit. Lead agency; coordinates input from Department of Agriculture other agencies. Conservation Division Surface Mining Program Anderson Bldg. Pierre, SD 57514 (605) 773-4201 Department of Health Solid waste dispo- Division of Environment and sal permit. Health Joe Foss Bldg. Pierre, SD 57514 (605) 773-3361 Department of Water and Water quality Natural Resources permit. Division of Water Quality Joe Foss Bldg. Pierre, SD 57514 (605) 773-3351 33 LEACHING PROBLEMS AND RESEARCH PERCOLATION Several problems hamper broader appli- cation of leaching methods for recovering gold and silver. A prime problem is the predominance of silt- and clay-size par- ticles in some ores that prevent uniform leaching solution percolation through the ore heaps. A high clay content will cause ponding and/or channeling, which blocks much of the ore from contact with the solution. Since small particles leach faster and more thoroughly than large ones, operators frequently crush the ore before leaching. Unfortunately crushing also can produce the extremely fine sizes that promote blockage and channeling. Even without crushing, some ores have a strong tendency to disinte- grate into clay through natural weather- ing agents and/or action of leaching solutions. Tests should be run on the ore to determine the percolation rate and metal recovery for various size distributions. Bureau of Mines research on agglomera- tion techniques provides the most promis- ing solution to clay problems; this was described in the section on Leaching Operations and in references 27 and 28. The several operators who have tried this technique have observed rather spectacu- lar inceases in percolation and recovery rates. Tombstone Exploration's Conten- tion Mine agglomerates its ore, as do the Alligator Ridge, Packard, and Candelaria operations. An extremely low permeability (tight) matrix presents a problem to leaching operators, again because the solution cannot reach the minerals. Weathered and oxidized ores generally leach better than unoxidized ones because oxidation breaks down an impeirmeable matrix. Heavier pow- der factors during mining may fracture the ore better, increasing the solution's access. A fresh, tight ore may, however, yield its values only if it is crushed and ground to a small enough size to ex- pose the metallic mineral grain. Crush- ing and in particular grinding, however, greatly boost the costs of preparing the ore for leaching. As previously men- tioned, it is imperative that laboratory tests be conducted to determine the most economical crushing size range. TEMPERATURE Solution temperature is a significant factor in leaching reactions. The chemi- cal reaction between gold and silver and cyanide proceeds much faster in warm so- lutions. Below 10° C (50° F) the reac- tion is appreciably slower than it is at normal summer operating temperatures. In practice, however, most operators work in cold weather until the solutions begin to freeze. Depending on its location, a mine can lose several months per year of potential leaching time in cold weather. A successful research program on meth- ods to heat solutions for leaching would provide a valuable improvement in the technology. A solar heating system would seem to be an ideal candidate since most leaching operations are located in arid regions with little cloud cover. Saline evaporation ponds may also be a possible source of heat for leaching solutions. The only reported research on solution heating is by Smoky Valley at its Round Mountain operation, where a 25-million- Btu/h (7.3-million-W) submersible kero- sine heater installed in the leach solu- tion pond has proven successful (26). SOLUTION LOSS At most leaching sites, 10 to 25 pet of the leaching solution is lost by evapora- tion. Since the leaching solution must contain high oxygen levels and since this is most easily achieved through spraying, evaporative losses seem inevitable. Im- portant research topics would be to de- termine this loss and to measure dis- solved oxygen in solutions as a function of the application method. Besides the evaporative losses, gangue minerals in the ore consume both the 34 cyanide and the lime or caustic soda. Consumption may run as high as 25 to 50 pet. Since the Interactions between cya- nide solutions and most gangue minerals are well established, no new research is recommended for cyanide leaching. For those deposits that exhibit high cyanide consumption, research on alternative lix- iviants may be warranted. Principal efforts have been directed to- ward the subeconomic, lower grade ores that are unminable and/or untreatable using traditional methods. Although ma- jor gold and silver projects have been investigated by various Bureau Research Centers, most current activity is con- centrated at the Reno and Twin Cities Research Centers. CALCIUM SALT SCALE Calcium salts in distribution lines pose a major problem with most systems, particularly those that use lime for al- kalinity control. At least one operation also experienced problems with barium salts clogging spray nozzles. The miner- al scale prevents operators from using some small-orifice components in their distribution systems, such as the drip irrigation tubes used in copper leaching operations, and is particularly trouble- some in sprinklers; frequent soaking in hydrochloric acid is about the only cure. Several operators mix a scale inhibitor, such as the Surfo-H35 organic phosphate marketed by Baroid, with the leaching so- lution to keep the calcium minerals dis- solved. Some additives, however, occa- sionally form colloidal suspensions that plug filtration systems. Bagdad wigglers, first used to distri- bute solutions at the Bagdad copper mine, offer a possible means to combat calcium salt scale. Simple to construct (fig. 7), the large-diameter tubing will remain open much longer than the small orifices of oscillating sprinklers and is easy to free from lime scale. Many operators have experimented with NaOH instead of lime to control pH with considerable reduction in the rate of lime scale. The relative costs must be carefully weighed, however, since NaOH is more expensive. RESEARCH ON NOVEL SOLUTION MINING METHODS The Bureau of Mines has actively inves- tigated new techniques in recovering and processing of gold and silver ores. In Situ Leaching In situ (in-place) leaching offers at- tractive benefits for producing metals at a minimum of capital investment and oper- ating cost, with low environmental degra- dation. The technique has not been ap- plied to gold and silver ores, primarily because of fear over public reaction to using cyanide solutions where ground wa- ter contamination is possible. Only one operation is known to have experimented with in-place gold and/or silver leach- ing, and that was with a noncyanide leach in an old underground mine, (See section on Colorado operations.) In this mine it was theorized that solutions could be in- jected into the fracture zone comprising the "vein" and that these solutions would migrate down the zone to drifts where they could be collected. To test the theory, water was injected into the vein and collected in a drift in a poured concrete trough (fig. 13). The experi- menters concluded that the trough (and drift) were not wide enough to encompass the vein-fracture system; excessive solu- tion losses occurred when the trough was bypassed. The experiment was never ex- panded to verify this conclusion because other metallurgical laboratory tests failed to resolve the chemistry of leach- ing that particular ore. Several companies have expressed a willingness to try leaching gold and silver in-place. Successful commercial leaching operations for uranium and cop- per have demonstrated that in-place leaching is economically feasible and environmentally safe. If a system can be developed for in-place-leaching gold and silver that will be publicly acceptable, small and/or low-grade deposits can be mined more cheaply and easily than 35 previously possible. The Bureau of Mines Twin Cities Research Center recently funded a contract study to evaluate the feasibility of such a system and to de- velop a conceptual design for It. The most likely system Involved blasting via vertical holes drilled from the surface to fragment shallow ore bodies , followed by solution application from the surface (36) . After the solution percolated down through the fragmented ore. It could be pumped to the surface from vertical re- covery wells or from underground solution collection drifts. Such a system would have a very good dls count ed-cash-f low re- turn on Investment with payout of less than 1 yr. The leached ore, which still contains the pregnant leaching solution. Is pushed by bulldozer or front-end loader to a sluice box where It Is slurried and pumped to a settling pond. After the spent ore has settled In the pond, the pregnant solution Is pumped from the pond to a recovery plant for removing the met- als. Meanwhile, the next layer of ore Is being cultivated as the cycle begins again. Although the field experiments to veri- fy this method were not conducted on gold and silver ores, laboratory experiments on them and field experiments on other ores worked well. Underground mines offer potential for In situ leaching. Ore would be blasted and left In stopes for leaching. After percolating through the rubbllzed ore, pregnant solution would be pumped to the surface for extraction of the metals. Placer deposits offer Intriguing possi- bilities for In-place leaching, and sev- eral years ago the Bureau conducted labo- ratory tests on cyanide placer deposits (33) . At some future date It Is reason- able to assume that placers could be leached by vertical Injection and recov- ery wells like those used for uranium In roll-front deposits. Leach Farming A patent granted In 1973 ( 23 ) describes a technique for leaching friable exposed ore bodies In place or conventionally mined ore that Is placed In extremely shallow heaps. With this technique the ore Is cultivated with standard farm equipment to loosen It to a depth of 6 to 12 In (15 to 30 cm). Leaching solution Is added to the loosened ore, which Is periodically cultivated and kept moist for the next several days. This cultiva- tion assures uniform contact between leaching solution and the ore and pre- vents downward escape of the solution. Thin-Layer Leaching Thin-layer leaching, somewhat a variant of the leach farming method, was de- veloped for the copper mines of South America (^-A^) . It offers potential for leaching gold and silver ores , particu- larly If combined with the Bureau's ag- glomeration process. The first step In the method involves crushing the ore; minus 1/2 in (1.2 cm) was sufficient for the copper ores tested. Next, a concentrated leaching solution is mixed with the ore in a ro- tating drum until the ore contains 10 pet of solution. The ore is then placed in piles while the solution reacts with the ore. After 24 h the ore is transported to an Impervious pad, where it is spread out in layers approximately 3 ft (1 m) deep. The ore is then sprayed with di- lute leaching solution and/or water un- til the dissolved metals are rinsed out. This rinse solution is collected in ponds in the same manner as with any other heap leach and then pumped to a recovery plant for metals extraction. The damp tailing is then removed from the pad and piled in the disposal area. 36 SUMMARY Gold and silver leaching operations have blossomed in many mining districts of the Western United States, Most are associated with old "vein" mines where the elements were deposited in fractures near volcanic or other igneous activity. A few large operations have also been es- tablished on oxidized sedimentary depos- its containing very fine, disseminated gold. In surveying current operations, over 80 were found that had conducted tests or were actively leaching on a commercial scale. Sufficient engineering data were obtained for 26 of these operations to justify their inclusion in tables that cover location, geology, heap configura- tion, influent solutions, effluent solu- tions, and extraction. One of the first problems encountered by a hopeful leaching operator is obtain- ing the necessary permits from Federal, State, and frequently local agencies. To provide a start, key Federal and State agencies are identified in tables 5 and 6. The main problems hampering leaching operations are unfavorable mineralogy and poor solution percolation owing to high clay content. Cold temperature effects on gold and silver solubility and rapid lime buildup in the distribution system also severely hamper the operations. Re- cent and ongoing Bureau research and con- tinued work at various mining operations are helping improve the techniques. REFERENCES 1. Ageton, R. W. , G. T. Krempasky, and W. L. Rice. A Systems Approach to Recov- ering Gold Resources in Jefferson County, Mont. 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