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IC s9^^ 
 
 Bureau of Mines Information Circular/1983 
 
 Agglomeration-Heap Leaching 
 Operations in the Precious 
 Metals industry 
 
 By G. E. McClelland, D. L. Pool, and J. A. Eisele 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 

Information Circular 8945 
 
 Agglomeration-Heap Leaching 
 Operations in the Precious 
 Metals Industry 
 
 By G. E. McClelland, D. L. Pool, and J. A. Eisele 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 James G. Watt, Secretary 
 
 BUREAU OF MINES 
 Robert C. Norton, Director 
 
Library of Congress Cataloging in Publication Data: 
 
 Xt^ 
 
 
 
 McClelland, G. E 
 
 
 
 
 Agglomeration-heap 
 
 eaching operations in the 
 
 precious metals 
 
 industry. 
 
 » 
 
 
 
 (Information circular 
 
 / United States 
 
 Department of the Interior, 
 
 Bureau of Mines ; 8945) 
 
 
 
 
 Bibliography: p. l6. 
 
 
 
 
 Supt. of Docs, no.: 1 
 
 28.27:8945. 
 
 
 
 1. Precious metals 
 
 —Metallurgy. 2. 
 
 Agglomeration. 3. Spoil 
 
 banks— Leaching. I. Poc 
 
 1, D. L. (Danny 1 
 
 ..). II. Eisele, J. A. (Judith 
 
 A.). III. Title. IV. Scries: Information 
 
 circular (U 
 
 nited States. Bu- 
 
 reau of Mines) ; 8945. 
 
 
 
 
 -T-?f30^vU4- [TN759] 
 
 622s [669'. 
 
 22'028] 
 
 83-600179 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Agglomeration concept 2 
 
 Commercial agglomeration-heap leaching operations 3 
 
 Arizona gold heap leaching , 20 tpd 3 
 
 Colorado gold vat leaching , 1 , 500 tpd 4 
 
 Arizona silver heap leaching, 2,000 tpd 6 
 
 Northern Nevada gold heap leaching , 2 , 500 tpd 7 
 
 Eastern Nevada gold heap leaching, 3,000 tpd 10 
 
 Summary and conclus ions 14 
 
 References 16 
 
 ILLUSTRATIONS 
 
 1 . Small Arizona gold agglomeration-heap leaching operation 3 
 
 2. Discharge end of the drum agglomerator used at the Colorado gold 
 
 agglomeration-vat leaching operation 5 
 
 3. Leaching vat at the Colorado gold agglomeration-vat leaching operation.... 6 
 
 4. Agglomeration at the Arizona silver heap leaching operation showing the 
 
 reverse belt agglomerator 8 
 
 5. Agglomerated ore stockpile at the Arizona silver heap leaching operation.. 8 
 
 6. Agglomerated heaps during leaching at the Arizona silver operation 9 
 
 7. Gold agglomeration-heap leaching operation in northern Nevada 10 
 
 8. Heap leaching gold ore in northern Nevada before agglomeration pretreat- 
 
 ment was adopted 11 
 
 9. Agglomerated gold ore being heap leached in northern Nevada 11 
 
 10. Overall view of the eastern Nevada agglomeration-heap leaching operation. . 12 
 
 11. Drum agglomerator used in the eastern Nevada operation 12 
 
 12. Agglomerated heaps being leached in the eastern Nevada gold heap leaching 
 
 operation 13 
 
 TABLE 
 
 1. Summary of agglomerating conditions of the five commercial operations 
 
 described 15 
 

 UNIT OF MEASURE ABBREVIATIONS USED 
 
 IN THIS REPORT 
 
 °c 
 
 degree Celsius 
 
 pet 
 
 percent 
 
 ft 
 
 foot 
 
 psig 
 
 pound per square inch, 
 gauge pressure 
 
 gal 
 
 gallon 
 
 
 
 
 
 rpm 
 
 revolution per minute 
 
 gpm 
 
 gallon per minute 
 
 
 
 
 
 sec 
 
 second 
 
 gpm/ft^ 
 
 gallon per minute 
 
 
 
 
 per square foot 
 
 tpd 
 
 ton avoirdupois per day 
 
 hr 
 
 hour 
 
 tph 
 
 ton avoirdupois per hour 
 
 in 
 
 inch 
 
 tr oz 
 
 troy ounce 
 
 lb 
 
 pound 
 
 
 
 lb/ton 
 
 pound per ton 
 avoirdupois 
 
 
 
AGGLOMERATION^HEAP LEACHING OPERATIONS 
 IN THE PRECIOUS METALS INDUSTRY 
 
 By G. E. McClelland/ D. L. Pool,^ and J. A. Eiseie^ 
 
 ABSTRACT 
 
 During the 1970' s, the Bureau of Mines investigated a particle ag- 
 glomeration technique for improving the flow of leaching solution 
 through heaps of clayey or crushed, low-grade gold-silver ores, wastes, 
 and tailings. This technology has been adopted on a broad scale by the 
 precious-metals-processing industry. This report presents information 
 on five commercial operations that have benefited from agglomeration 
 technology and that represent a cross section of current heap leaching 
 practice. The technology is cost effective because of decreased leach 
 times and improved precious metal recoveries. 
 
 ^Metallurgist. 
 ^Research chemist. 
 -'Supervisory chemical engineer. 
 All authors are with the Reno Research Center, Bureau of Mines, Reno, Nev. 
 
INTRODUCTION 
 
 Exploration during the mid to late 
 1970's identified numerous low-grade pre- 
 cious metal deposits, mine waste mate- 
 rials, and tailings piles throughout the 
 Western United States. The increase in 
 precious metal prices during this period 
 generated interest in processing these 
 low-grade feed materials by low-cost heap 
 leaching technology. Heap leaching with 
 cyanide was applied to many of the mate- 
 rials; however, many conventional oper- 
 ations were unsuccessful because exces- 
 sive amounts of clay in the feed or fines 
 generated during crushing prevented a 
 uniform flow of cyanide solution through 
 the heaps. 
 
 As part of its research program to im- 
 prove the recovery of gold and silver 
 from low-grade domestic resources, the 
 Bureau of Mines investigated agglomera- 
 tion pretreatment to overcome the perco- 
 lation problems associated with heap 
 leaching. Agglomeration of the clays and 
 fines before heap building permitted 
 a uniform and rapid flow of cyanide 
 
 leaching solution through the heaps. 
 Previous Bureau publications (l.~2^)^ de- 
 scribe bench- and pilot-scale experiments 
 showing the advantages of particle ag- 
 glomeration as a pretreatment for mate- 
 rials that are difficult to treat by 
 standard heap leaching techniques. 
 
 This report shows how the Bureau's re- 
 search on agglomeration-heap leaching has 
 been adopted and applied by the precious- 
 metals-processing industry. It describes 
 five commercial operations, ranging in 
 size from 20 to 3,000 tpd and represent- 
 ing a cross section of current heap 
 leaching practice, that have benefited 
 from agglomeration technology. The prob- 
 lems they experienced in trying to apply 
 conventional heap leaching to ores con- 
 taining clay ranged from complete plug- 
 ging of the heap, with no solution flow, 
 to slow solution flow, with long leach- 
 ing times for precious metal recovery. 
 Agglomeration pretreatment effectively 
 resolved the heap permeability problems 
 encountered by these operators. 
 
 AGGLOMERATION CONCEPT 
 
 Most precious metal ores require crush- 
 ing to minus 1 in or finer before agglom- 
 eration. Crushing to these sizes liber- 
 ates precious metal values and improves 
 overall recovery. Crushed ores can be 
 agglomerated by mixing 5 to 10 lb port- 
 land cement per ton of dry feed, wetting 
 with 8 to 16 pet moisture as either water 
 or strong cyanide solution, mechanically 
 tumbling the wetted mixture, and curing 
 the agglomerated feed for a minimum of 8 
 hr before heap leaching. The quantity of 
 cement added during agglomeration usually 
 provides the protective alkalinity re- 
 quired for cyanide leaching. After ag- 
 glomeration and heap building, leaching 
 is conducted with conventional heap 
 leaching techniques. 
 
 During agglomeration, the clay and fine 
 particles contained in the ore adhere 
 to the coarser particles and create a 
 
 coating of fines around the coarse parti- 
 cles. The agglomerates produced are of 
 sufficient green strength after curing to 
 withstand wetting with minimal degrada- 
 tion. Agglomeration overcomes the major 
 problems associated with particle segre- 
 gation during heap building O) , fines 
 migration, and solution channeling during 
 leaching by producing a porous, permeable 
 feed (_4). 
 
 A permeable feed material stacked in a 
 heap permits the uniform flow of leaching 
 solution and contact of the cyanide 
 leaching solution with the exposed pre- 
 cious metal particles and decreases the 
 leaching time required to obtain targeted 
 precious metal recovery, 
 
 '^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this report. 
 
COMMERCIAL AGGLOMERATION-HEAP LEACHING OPERATIONS 
 
 ARIZONA GOLD HEAP LEACHING, 20 TPD 
 
 A small Arizona operation (fig. 1) pro- 
 cessing 20 tpd of gold ore changed to ag- 
 glomeration technology after conventional 
 heap leaching was unsuccessful because of 
 inadequate leaching solution percolation. 
 
 The oxidized disseminated deposit con- 
 tains an average 0.2 tr oz of gold per 
 ton. The host rock is porous and the 
 gold particles are exposed. The ore is 
 mined from an open pit, transported to 
 the crushing plant by a front-end loader, 
 crushed to a nominal 3/8-in feed size, 
 and conveyed to a drum agglomerator. 
 
 The drum agglomerator is 3 ft in diame- 
 ter and 7 ft in length. A static scrap- 
 er, located lengthwise in the top portion 
 of the drum, prevents excessive buildup 
 
 of clayey material. A spray bar adjacent 
 to the scraper emits a spray that covers 
 a 5-ft length of the drum. The drum 
 slopes 5° from feed to discharge end and 
 rotates at 17 rpm. Feed retention time 
 is 20 sec. The drum can agglomerate 10 
 tph of dry feed. 
 
 The minus 3/8-in feed material is mixed 
 with 10 lb Portland cement (type II) per 
 ton of dry feed before discharge from the 
 feed conveyor, and the mixture is blended 
 in the first foot of the drum. The ore- 
 binder mixture is wetted by the spray 
 bar with a solution containing 2 lb NaCN 
 per ton. Total moisture required for ag- 
 glomeration is 10 pet. On discharge from 
 the drum, the agglomerated ore is con- 
 veyed to an impervious leaching pad and 
 stacked into a 7-ft-high heap that con- 
 tains 100 tons. The agglomerated ore is 
 
 FIGURE 1. - Small Arizona gold agglomeration-heap leaching operation. Shown are the open pit 
 mine, the crushing circuit, the agglomeration equipment, and a heap being leached. 
 
cured in the heap for 48 hr before 
 leaching. 
 
 The heap is leached in 5 days by spray- 
 ing solution containing 1 lb NaCN per ton 
 over the heap at a rate of 0.033 gpm/ft^. 
 The cement added during agglomeration is 
 sufficient for protective alkalinity. 
 The dissolved gold is recovered from the 
 pregnant solution by passing the solution 
 upward through four 14-in by 5-ft carbon 
 columns at a rate of 20 gpm/ft^. The 
 barren solution is recycled to the heap. 
 
 The loaded carbon is stripped with an 
 alkaline-alcohol solution. The precious 
 metal values are electrowon from solution 
 on steel wool cathodes in a cylindrical 
 electrowinning cell. The gold-laden 
 cathodes are treated with nitric acid to 
 dissolve excess iron, and the resultant 
 sludge is fire refined to produce a dore 
 bullion. 
 
 Gold recoveries obtained by agglomera- 
 tion and heap leaching average 90 pet. 
 Agglomeration pretreatment improved heap 
 permeability, percolation rate, gold re- 
 covery, and decreased the leaching period 
 required to obtain the targeted recovery. 
 
 COLORADO GOLD VAT LEACHING, 1,500 TPD 
 
 In this operation, the agglomerated 
 feed is leached in large vats rather than 
 in heaps. The operation is located in 
 the Colorado Rocky Mountains southwest of 
 Denver at an elevation of 10,000 ft (_5 ) . 
 
 The gold ore occurs in an oxidized dis- 
 seminated deposit and averages 0.06 tr oz 
 of gold per ton with trace amounts of 
 silver. The host rock is porous and the 
 gold is liberated by crushing. The ore 
 is mined from an open pit and is friable. 
 Seventy-five percent of the deposit can 
 be mined with dozers and front-end load- 
 ers. The remaining 25 pet must be 
 drilled and blasted. The operation mines 
 and processes 1,500 tons of ore per day. 
 All the material mined is ore; there is 
 no waste rock. 
 
 The mined ore is transported by front- 
 end loader to the in-pit crushing plant 
 where it is crushed to a 1/2-in size by 
 two-stage crushing. The minus 1/2-in 
 feed is conveyed uphill one-fourth mile 
 to the processing plant. The ore trans- 
 port conveyor system is cheaper to oper- 
 ate than truck transport, which keeps the 
 mining costs extremely low. 
 
 The transport conveyor discharges the 
 crushed ore onto a surge pile outside the 
 agglomeration-vat leaching building. The 
 enclosed building permits year-round 
 operation. From the surge pile the ore 
 is fed to an 8-1/2- by 32-ft rotating 
 drum (fig. 2). The drum slopes 8°, with 
 the feed end elevated, and rotates at 10 
 rpm. The walls of the drum are not 
 scraped because the agglomerated feed 
 does not build up excessively. 
 
 The ore is mixed with 10 lb of binder 
 (3 lb Portland cement (type II), 7 lb 
 fly ash) per ton of feed in the first 
 few feet of the drum. The material is 
 sprayed with a pH 12 solution containing 
 10 lb NaCN per ton. A 20-ft spray bar 
 inside the driom is used to increase the 
 moisture content of the feed to 13 pet. 
 Mechanical tumbling occurs along the 
 length of the drum. The agglomerated 
 feed is conveyed to a stockpile from 
 which the leaching vats are loaded by a 
 front-end loader. Curing occurs while 
 the vats are being loaded. 
 
 The feed is placed into one of four 
 inclined vats measuring 80 by 50 ft. The 
 deepest portion of the vat is 9 ft. Each 
 vat contains 1,000 tons of agglomerated 
 feed and requires 4 hr to load. 
 
 The ore is leached by spraying solution 
 at the rate of 0.05 gpm/ft^ until the vat 
 is full. The solution in the vat is cir- 
 culated and sprayed by pumps throughout 
 the leaching cycle to insure an adequate 
 dissolved oxygen content (fig. 3). 
 
 Gold recovery is 90 pet in a 3-day 
 leaching and washing cycle. The vats 
 
drain in approximately 8 hr. The mois- 
 ture retained by the leached residue is 
 13 pet, the same amount of moisture re- 
 quired for agglomeration, so there is no 
 entrained gold-bearing solution in the 
 residue. The plant capacity was designed 
 for 1,000 tpd, but changing the leaching- 
 washing cycle from 4 to 3 days allows 
 
 1,500 tons of agglomerated ore a day to 
 be processed. 
 
 The pregnant solution from the vats is 
 pumped to a carbon adsorption system for 
 gold recovery. The carbon system con- 
 sists of four adsorption tanks, each con- 
 taining 1.5 tons of 6- by 12-mesh coconut 
 
 FIGURE 2. - Discharge end of the drum agglomerator used at the Colorado 
 gold agglomeration-vat leaching operation. Drum diameter is 8=1/2 ft. 
 
FIGURE 3. = Leaching vat at the Colorado gold agglomeration=vat leaching operation. After flood- 
 ing the vat, the solution is circulated through the agglomerated ore by pumping and spraying to main- 
 tain an adequate dissolved oxygen content throughout the leaching period. 
 
 shell activated carbon. When the lead 
 carbon is loaded to 200 tr oz of gold per 
 ton, it is educted to the desorption cir- 
 cuit. Fresh carbon is kept in the last 
 tank in the solution flow sequence to in- 
 sure a good barren solution. The barren 
 solution is pumped to a holding tank for 
 recycle to the leaching vats. All rea- 
 gent makeup is done during agglomeration. 
 
 The loaded carbon is desorbed in 8 hr 
 by caustic cyanide solution at 116° C and 
 25 psig pressure. The pregnant solution 
 from desorption is passed through one of 
 four cylindrical electrowinning cells 
 where the gold is deposited on steel wool 
 cathodes. The cathodes are directly fire 
 
 refined to produce dore bullion contain- 
 ing 80 pet gold and 17 pet silver. 
 
 Preliminary metallurgical testing had 
 shown conventional heap leaching to be 
 unsatisfactory. Agglomeration increased 
 permeability, percolation rate, and gold 
 recovery. Agglomeration permits a very 
 short leaching cycle, which allows for 
 increased gold production. 
 
 ARIZONA SILVER HEAP LEACHING, 2,000 TPD 
 
 A silver agglomeration-heap leaching 
 operation is located near Tombstone, 
 Ariz. (6). Two types of materials are 
 mined: old waste material that was used 
 
for mine backfill and virgin ore adjacent 
 to the waste material. The waste mate- 
 rial is mined with front-end loaders to 
 expose the virgin ore. The virgin ore is 
 drilled and blasted and moved by front- 
 end loader. The silver content of the 
 two feed materials varies. The cutoff 
 grade of feed to the heaps is 1.0 tr oz 
 of silver per ton. Approximately 2,000 
 tpd of ore is mined and agglomerated. 
 
 The mined ore and waste are moved from 
 a stockpile to the crushing plant where 
 they are crushed to a nominal 1/2 in. 
 Lime (7 lb/ton of ore) is used as the 
 binder for agglomeration and is mixed 
 with the ore during secondary crushing. 
 The crushed ore-lime mixture is conveyed 
 to an underground ore stockpile. Some 
 moisture is sprayed onto the ore on the 
 crusher discharge conveyor to decrease 
 dusting. 
 
 The ore from the underground stockpile 
 is agglomerated on a reverse belt con- 
 veyor designed by the operators (fig. 4), 
 The 4- by 25-ft belt agglomerates ore at 
 a rate of 200 tph. The agglomerating 
 conveyor can be set at an angle between 
 35° and 45° , and the belt travels upward 
 while the ore moves down the belt. The 
 angle and speed of the belt can be varied 
 to provide the desired retention time of 
 ore on the belt. Water is sprayed at 
 several locations along the length of the 
 belt and gives the agglomerated feed a 
 moisture content of between 10 and 12 
 pet. A small amount of moistened fines 
 adheres to the belt and rides up the con- 
 veyor. A scraper was placed on the bot- 
 tom side of the drive roller to eliminate 
 excessive fines buildup on the belt. 
 
 The agglomerated ore is transported to 
 a stockpile by a radial arm stacker (fig. 
 5), The material from the stockpile is 
 trucked 500 ft to a 3/4-acre impervious 
 leaching pad and is allowed to cure dur- 
 ing heap building. Five heaps, each con- 
 taining 6,000 tons of agglomerated ore 
 stacked 10 to 11 ft high, are leached on 
 the pad (fig, 6). Three heaps are at 
 different stages of leaching, while the 
 other two are either being prepared for 
 leaching or being removed from the pad. 
 
 The heaps are sprayed with pH 10,5 so- 
 lution containing 2 lb NaCN per ton at a 
 rate of 0,0075 gpm/ft^. The leaching so- 
 lution percolates through the heap, is 
 collected on the impervious leaching pad, 
 and drains into plastic-lined solution 
 trenches , The leaching and washing cycle 
 is 7 days. The leached residue is trans- 
 ferred to an auxiliary leaching pad 
 and sprayed with cyanide solution 1 day 
 per month for additional precious metal 
 extraction. 
 
 Precious metal values in the pregnant 
 solution are recovered by Merrill-Crowe 
 zinc precipitation technology. The 
 precious-metal-bearing zinc precipitates 
 are refined on site and yield dore bul- 
 lion. The dore'' is shipped to another 
 facility for refining. The barren solu- 
 tions are recycled to the heaps. 
 
 Heap leaching was unsuccessful before 
 agglomeration pretreatment was applied to 
 the ore. Conventional heap leaching re- 
 covered only 37 pet of the leachable sil- 
 ver from 3-in feed material treated in 
 90-day leaching cycles. Severe percola- 
 tion problems were encountered. Agglom- 
 eration permitted finer crushing, which 
 liberated additional silver values for 
 dissolution by cyanide. Agglomeration 
 heap leaching increased silver recovery 
 to 90 pet of the leachable silver and de- 
 creased the leaching time to 7 days, 
 
 NORTHERN NEVADA GOLD HEAP 
 LEACHING, 2,500 TPD 
 
 An operation in northern Nevada (fig, 
 7) , which has produced gold by agitation 
 cyanidation and countercurrent decanta- 
 tion for several years , discovered a new 
 ore deposit several miles from the work- 
 ing mine. Higher grade ore from the new 
 deposit is transported to the original 
 mill for gold recovery. Ore containing 
 less than 0.07 tr oz gold per ton is heap 
 leached at the site. Average ore grade 
 for the heap material is 0.034 tr oz gold 
 per ton. The ore is mined by open pit 
 methods and is trucked to the heap leach- 
 ing site approximately one-half mile from 
 the new pit. About 2,500 tpd of ore is 
 mined and heap leached. 
 
FIGURE 4. - Agglomeration at the Arizona silver heap leaching operation showing the reverse belt 
 agglomerator. 
 
 FIGURE 5. - Agglomerated ore stockpile at the Arizona silver heap leaching operation. Regions of 
 fine material in the right portion of the photograph indicate insufficient moisture was added during 
 agglomeration. 
 
FIGURE 6. - Agglomerated heaps during leaching at the Arizona silver operation. In the foreground 
 is agglomerated ore after leaching. 
 
 The ore is crushed to minus 5/8 in by a 
 primary jaw crusher and a secondary cone 
 crusher. Portland cement (type II), at 
 the rate of 7 to 10 lb/ton of ore, is 
 added to the ore at the jaw crusher dis- 
 charge conveyor and is mixed with the 
 ore during secondary crushing. The ore- 
 binder mixture from secondary crushing is 
 conveyed to a radial arm stacker. Water 
 is sprayed onto the mixture at the dis- 
 charge end of the stacker and gives a 
 final moisture content of 9 to 13 pet. 
 The ore is agglomerated by cascading down 
 the sides of the conical-shaped agglom- 
 erated ore stockpile. Additional tum- 
 bling, sufficient to effect agglomera- 
 tion, occurs when the front-end loader 
 loads the rear dump truck with agglom- 
 erates from the stockpile and when the 
 
 truck dumps the agglomerated feed onto 
 the impervious asphalt leaching pad. The 
 agglomerated ore is cured for 2 to 3 days 
 while the heap is being built. 
 
 Five 17,000-ton heaps, approximately 12 
 ft high, are built on the leaching pad. 
 Three heaps are in different stages of 
 the leaching cycle while the remaining 
 two are either being prepared for leach- 
 ing or being removed from the pad. 
 
 The agglomerated heaps are leached by 
 spraying 0.004 to 0.005 gpm/ft^ of pH 
 10-11 solution containing 1.0 lb NaCN per 
 ton of solution. The portland cement 
 added during agglomeration provides most 
 of the alkalinity during leaching, but 
 small quantities of NaOH are added to the 
 
10 
 
 FIGURE 7. = Gold agglomeration-heap leaching operation in northern Nevada. Shown are agglomeration 
 equipment at center, agglomerated ore stockpile at right, and heaps being leached at left. 
 
 barren solution to maintain the desired 
 value. The leaching and washing cycle is 
 20 days. Leached residues are trans- 
 ported to a tailings disposal area. 
 
 The pregnant solution draining from the 
 leaching pad flows by gravity to a reser- 
 voir and is pumped through a series of 
 five carbon adsorption tanks for recov- 
 ery of dissolved gold values. The gold- 
 loaded, 12- by 30-mesh, coconut shell ac- 
 tivated carbon is transported to the 
 millsite for desorption by an alkaline 
 alcohol solution. The values desorbed 
 from the carbon are electrowon on steel 
 wool cathodes, which are refined on site 
 to produce dore'' bullion. 
 
 This operation initially tried conven- 
 tional heap leaching to process the low- 
 grade ore. Conventional heap leaching 
 was unsuccessful because of the high clay 
 
 content of the ore, particle segregation, 
 and fines migration, which resulted in 
 leaching solution channeling (fig. 8). 
 Agglomeration pretreatment increased gold 
 recovery by 60 pet while decreasing the 
 leaching cycle from 50 days to 20 days 
 (fig. 9). Even with agglomeration, some 
 ore retains approximately 30 pet mois- 
 ture, and long washing periods are re- 
 quired to recover the dissolved values 
 from the agglomerates. The longer wash- 
 ing cycle extends the total leaching cy- 
 cle to 30 days for some heaps. 
 
 EASTERN NEVADA GOLD HEAP 
 LEACHING, 3,000 TPD 
 
 The largest of the operations surveyed 
 is located in eastern Nevada and pro- 
 cesses approximately 3,000 tpd of ore {1^ 
 8^) (fig. 10). The ore is a silicified 
 siltstone containing finely disseminated 
 
11 
 
 FIGURE 8. - Heap leaching gold ore in northern Nevada before agglomeration pretreatment was 
 adopted. Notice the ponding on the heaps and the low rate of solution application. 
 
 FIGURE 9. - Agglomerated gold ore being heap leached in northern Nevada. The solution applica- 
 tion rate is greatly increased and no ponding is observed. 
 
12 
 
 FIGURE 10. = Overall view of the eastern Nevada agglomeration-heap leaching operation showing 
 agglomeration equipment building, crushed ore stockpile in front center, and heaps in different stages 
 of construction in the background. 
 
 FIGURE 11- Drum agglomerator (9 by 21 ft) used in the eastern Nevada operation. 
 
13 
 
 I 
 
 free gold. Gold-bearing carbonaceous ore 
 occurs at depth in the deposit. The 
 average grade of the deposit is 0.10 tr 
 oz gold per ton. The ore is mined by 
 open pit methods and is trucked a short 
 distance to the crushing circuit. 
 
 The run-of-mine ore is crushed to a 
 nominal 3/4 in by a primary jaw crusher, 
 a standard cone crusher, and a short-head 
 cone crusher. The crushed ore is con- 
 veyed into a building where it is 
 agglomerated. 
 
 Two types of ore are processed. Some 
 of the oxidized ore contains no clayey 
 material, and crushing it generates very 
 few fines. Only 6 pet of the material is 
 minus 200 mesh ( 4_) . Consequently, it 
 does not require a cement binder for ag- 
 glomeration. This portion of the ore is 
 agglomerated with cyanide solution to 
 start the leaching process during heap 
 building. 
 
 The second type of crushed ore is mixed 
 with 4 to 10 lb of Portland cement (type 
 II) per ton of feed in the first portion 
 of a 9- by 21-ft rotating drum agglom- 
 erator (fig. 11). Cyanide solution is 
 
 sprayed onto the mixture in the middle 
 portion of the drum and gives a final 
 moisture content of approximately 8 pet. 
 The mixture agglomerates in the last por- 
 tion of the drum. The design capacity of 
 the drum agglomerator is 215 tons of dry 
 ore per hour, but it has been operated at 
 450 tph. 
 
 The agglomerated ore is conveyed to a 
 stockpile where it is loaded by front-end 
 loader into trucks for haulage to the im- 
 pervious leaching pad. The agglomerates 
 cure during the heap building process. 
 
 The permanent heaps are built by driv- 
 ing the truck onto the heap and dumping 
 the feed over the edge. A tracked dozer 
 levels the heap , and the surface is 
 ripped before leaching. This heap build- 
 ing procedure is not recommended for 
 agglomeration-heap leaching of ores that 
 are susceptible to compaction (4^). Each 
 heap contains approximately 120,000 tons 
 of agglomerated ore stacked 15 ft high 
 (fig. 12). 
 
 The heaps are leached by sprinkling a 
 pH 11.5 solution containing 0.5 lb NaCN 
 per ton of solution at a rate of 0.003 to 
 
 FIGURE 12. - Agglomerated heaps being leached in the eastern Nevada gold heap leaching operation. 
 
14 
 
 0.005 gpm/ft^. The total leaching and 
 washing cycle is from 20 to 80 days and 
 depends on the ore being processed. The 
 pregnant solution collects on the imper- 
 vious pads and drains to plastic-lined 
 ditches. The solution flows by gravity 
 to a 7-million-gal pregnant solution 
 pond. 
 
 Pregnant solution from the pond is 
 pumped through a series of five activated 
 carbon tanks at a rate of 1,000 gpm. The 
 barren solution exiting the fifth carbon 
 tank, is pumped to the barren solution 
 pond for reagent makeup and recycle to 
 the heap. The loaded carbon from the 
 lead tank is stripped with a caustic cya- 
 nide solution at 120° C and 35 psig pres- 
 sure. The gold-bearing solution is 
 cooled to 85° C before electrowinning. 
 The solution is pumped through two 
 
 rectangular electrowinning cells where 
 the gold is deposited on steel wool cath- 
 odes. The cathodes are refined on-site 
 to produce dore bullion containing 92 pet 
 gold and 6 pet silver. The dore bars 
 are shipped to a custom refinery for 
 processing. 
 
 Recoveries by agglomeration-heap leach- 
 ing range from 70 to 90 pet, and depend 
 on the ore being processed. Conventional 
 heap leaching was used before the agglom- 
 erating equipment was on-line. Gold re- 
 coveries were less than 50 pet because of 
 fines migration and the high clay con- 
 tent , which caused blinding and channel- 
 ing of leaching solution. Agglomeration 
 technology has helped maintain targeted 
 recoveries in shorter leaching periods 
 for heaps with high clay content. 
 
 SUMMARY AND CONCLUSIONS 
 
 The precious metals industry has rapid- 
 ly adopted agglomeration-heap leaching 
 technology. The five operations de- 
 scribed in this report demonstrate the 
 versatility and applicability of the 
 technology to heap leachable ore types 
 for different size operations. A summary 
 of agglomerating conditions for the oper- 
 ations is shown in table 1. In the West- 
 ern United States, there are 36 commer- 
 cial operations using agglomeration-heap 
 leaching technology. 
 
 For the operations described, agglomer- 
 ation pretreatment technology effectively 
 improved heap permeability and permitted 
 better contact of the cyanide leaching 
 solution with the precious metal. As a 
 result, percolation rates increased, 
 leaching periods decreased, and precious 
 metal recoveries were improved over those 
 of conventional heap leaching technology. 
 Agglomeration pretreatment provides cer- 
 tain other advantages. Since the heaps 
 
 are more permeable and drain rapidly, 
 soluble losses are minimized and inter- 
 mittent sprinkling is permitted. This is 
 important in arid climates because solu- 
 tion loss by evaporation is minimized by 
 spraying the heaps during night hours 
 when winds and ambient temperature are 
 low. During the period that the heap is 
 not being sprayed, the residual cyanide 
 in the heap continues to dissolve pre- 
 cious metal and enriches the pregnant 
 solution when spraying is resumed. The 
 greater permeability of the heaps permits 
 better washing of dissolved values from 
 the heap and decreases the residual cya- 
 nide content of the washed, spent heap. 
 
 Even though more reagents and equipment 
 are required for agglomeration-heap 
 leaching than for conventional heap 
 leaching, it is cost effective because of 
 the decrease in leaching time and im- 
 proved precious metal recoveries. 
 
15 
 
 TABLE 1. - Summary of agglomerating conditions of the five commercial 
 operations described 
 
 
 Process- 
 ing rate, 
 tpd 
 
 Feed 
 
 size, 
 
 in 
 
 Binder 
 
 added 
 
 Agglomeration 
 
 Curing 
 time, 
 hr 
 
 Leach- 
 ing 
 
 cycle, 
 days 
 
 Recov- 
 
 Operation 
 
 to ore 
 
 Equip- 
 ment 
 used 
 
 Mois- 
 ture, 
 pet 
 
 ery, 
 
 
 Type 
 
 lb/ton 
 
 pet 
 
 Arizona 
 
 20 
 
 3/8 
 
 Cement . 
 
 10 
 
 Drum. . . 
 
 10 
 
 48 
 
 5 
 
 90 
 
 gold heap 
 leaching. 
 
 
 
 
 
 
 
 
 
 
 Colorado 
 
 1,500 
 
 1/2 
 
 . .do ... 
 
 3 
 
 . .do. . . 
 
 13 
 
 8 
 
 3 
 
 90 
 
 gold vat 
 
 
 
 
 
 
 
 
 
 
 leaching. 
 
 
 
 Fly ash 
 
 7 
 
 
 
 
 
 
 Arizona 
 
 2,000 
 
 1/2 
 
 Lime ... 
 
 7 
 
 Reverse 
 
 10-12 
 
 72 
 
 7 
 
 I90 
 
 silver heap 
 leaching. 
 
 
 
 
 
 belt. 
 
 
 
 
 
 Northern 
 
 2,500 
 
 5/8 
 
 Cement . 
 
 7-10 
 
 None^ . . 
 
 9-13 
 
 48-72 
 
 20 
 
 (^) 
 
 Nevada 
 
 
 
 
 
 
 
 
 
 
 gold heap 
 leaching. 
 
 
 
 
 
 
 
 
 
 
 Eastern 
 
 3,000 
 
 3/4 
 
 . .do. . . 
 
 4-10 
 
 Drum . . . 
 
 8 
 
 (M 
 
 20-80 
 
 70-90 
 
 Nevada 
 
 
 
 
 
 
 
 
 
 
 gold heap 
 leaching. 
 
 
 
 
 
 
 
 
 
 
 ^90 pet of the leachable silver is recovered. 
 
 ^Agglomeration occurs as the wetted feed cascades dovm the conical stockpile. A 
 drum agglomerator is planned for this operation. 
 
 ^Percent recovery is proprietary company information. Agglomeration increased the 
 recovery by 60 pet. 
 
 ^While heap is built. 
 
16 
 
 REFERENCES 
 
 1. Heinen, H. J., G. E. McClelland, 
 and R. E. Lindstrom. Enhancing Percola- 
 tion Rates in Heap Leaching of Gold- 
 Silver Ores. BuMines RI 8388, 1979, 20 
 pp. 
 
 2. McClelland, G. E., and J. A. 
 Eisele. Improvements in Heap Leaching To 
 Recover Silver and Gold From Low-Grade 
 Resources. BuMines RI 8612, 1982, 26 
 pp. 
 
 3. Johanson, J. R. Particle Segrega- 
 tion ... and What To Do About It. Chem. 
 Eng., (N.Y.), V. 85, No. 11, May 1978, 
 pp. 183-189. 
 
 4. Chamber lin, P. D. Heap Leaching 
 and Pilot Testing of Gold and Silver 
 Ores. Min. Cong. J., v. 67, No. 4, Apr. 
 1981, pp. 47-52. 
 
 5. Lewis, A. Producing Gold for $160/ 
 Tr Oz in Victor, Colorado. Eng. and Min. 
 
 J., v. 183, No. 10, Oct. 1982, pp. 102- 
 105. 
 
 6. Engineering and Mining Journal. 
 Pelletizing Aids Tombstone Leaching Oper- 
 ation. V. 182, No. 1, Jan. 1981, pp. 94- 
 95. 
 
 7. . This Month in Mining. Al- 
 ligator Ridge Uses Heap Leaching To Pro- 
 duce Gold Bullion Bars. V. 182, No. 8, 
 Aug. 1981, pp. 35-37. 
 
 8. McQuiston, F. W. , Jr., and R. S. 
 Shoemaker. Heap Leaching Gold and Silver 
 Ores. Ch. in Gold and Silver Cyanidation 
 Plant Practice. American Institute of 
 Mining, Metallurgical, and Petroleum En- 
 gineers, Inc., New York, v. 2, 1981, pp. 
 13-18. 
 
 *U.S. GOVERNMENT PRINTING OFFICE: 1983-605-015/61 
 
 INT.-BU.OF MINE5,PGH.,P A. 27064 
 
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