Agglomeration-Heap Leaching Operations in the Precious Metals Industry

Home , Heap leaching

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. Horton, Director Library of Congress Cataloging in Publication Data:

Agglomeratim-heap leaching operarions in the precious merals indusrry.

(Informution circular / United Stares Deparrrnenr of the Interior, Burcau of Mines ; 8945) Bibliography: p. 16. Supr. of Docs. no.: 1 28.2753945. 1. Precious metals-Metallurgy. 2. Agglomeration. 3. Spoil hanks-Leaching. I. Pool, D. L. (Danny I..). 11. Eisele, J. A. (Judirh A. 111. Tirle. IV. Series: Information circular (United Scares. Bu- reau of Mines) ; 8945. CONTENTS .Page Abstract ...... Introduction ...... Agglomeration concept ...... Commercial agglomeration-heap leaching operations ...... Arizona gold heap leaching. 20 tpd ...... Colorado gold vat leaching. 1. 500 tpd ...... Arizona silver heap leaching. 2. 000 tpd ...... Northern Nevada gold heap leaching. 2. 500 tpd ...... Eastern Nevada gold heap leaching. 3. 000 tpd ...... Summary and conclusions...... References ...... ILLUSTRATIONS Small Arizona gold agglomeration-heap leaching operation ...... Discharge end of the drum agglomerator used at the Colorado gold agglomeration-vat leaching operation ...... Leaching vat at the Colorado gold agglomeration-vat leaching operation.... Agglomeration at the Arizona silver heap leaching operation showing the reverse belt agglomerator...... Agglomerated ore stockpile at the Arizona silver heap leaching operation .. Agglomerated heaps during leaching at the Arizona silver operation...... Gold agglomeration-heap leaching operation in northern Nevada ...... Heap leaching gold ore in northern Nevada before agglomeration pretreatment was adopted ...... Agglomerated gold ore being heap leached in northern Nevada ...... Overall view of the eastern Nevada agglomeration-heap leaching operation .. Drum agglomerator used in the eastern Nevada operation ...... Agglomerated heaps being leached in the eastern Nevada gold heap leaching operation ......

TABLE

1. Summary of agglomerating conditions of the five commercial operations described ...... UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT degree Celsius Pet percent foot Pa pound per square inch, gauge pressure gallon rPm revolution per minute gallon per minute sec second gallon per minute per square foot tpd ton avoirdupois per day hour t ph ton avoirdupois per hour inch tr oz troy ounce pound pound per ton avoirdupois AGGLOMERATION-HEAP LEACHING OPERATIONS IN THE PRECIOUS METALS INDUSTRY

By G. E. McClelland,' D. L. Pool,' and J. A. Eisele3

ABSTRACT

During the 19701s, the Bureau of Mines investigated a particle agglomeration 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.

'~etallur~ist. '~esearch chemist. 3~upervisorychemical engineer. All authors are with the Reno Research Center, Bureau of Mines, Reno, Nev. 2

INTRODUCTION

Exploration during the mid to late leaching solution through the heaps. 1970's identified numerous low-grade pre- Previous Bureau publications (1-214 de- cious metal deposits, mine waste mate- scribe bench- and pilot-scale experiments rials, and tailings piles throughout the showing the advantages of particle ag- Western United States. The increase in glomeration as a pretreatment for mate- precious metal prices during this period rials that are difficult to treat by generated interest in processing these standard heap leaching techniques. low-grade feed materials by low-cost heap leaching technology. Heap leaching with This report shows how the Bureau's re- cyanide was applied to many of the mate- search on agglomeration-heap leaching has rials; however, many conventional oper- been adopted and applied by the precious- ations were unsuccessful because exces- metals-processing industry. It describes sive amounts of clay in the feed or fines five commercial operations, ranging in generated during crushing prevented a size from 20 to 3,000 tpd and represent- uniform flow of cyanide solution through ing a cross section of current heap the heaps. leaching practice, that have benefited from agglomeration technology. The prob- As part of its research program to im- lems they experienced in trying to apply prove the recovery of gold and silver conventional heap leaching to ores con- from low-grade domestic resources, the taining clay ranged from complete plug- Bureau of Mines investigated agglomera- ging of the heap, with no solution flow, tion pretreatment to overcome the perco- to slow solution flow, with long leach- lation problems associated with heap ing times for precious metal recovery. leaching. Agglomeration of the clays and Agglomeration pretreatment effectively fines before heap building permitted resolved the heap permeability problems a uniform and rapid flow of cyanide encountered by these operators.

AGGLOMERATION CONCEPT

Most precious metal ores require crush- coating of fines around the coarse parti- ing to minus 1 in or finer before agglom- cles. The agglomerates produced are of eration. Crushing to these sizes liber- sufficient green strength after curing to ates precious metal values and improves withstand wetting with minimal degrada- overall recovery. Crushed ores can be tion. Agglomeration overcomes the major agglomerated by mixing 5 to 10 lb port- problems associated with particle segre- land cement per ton of dry feed, wetting gation during heap building (3), fines with 8 to 16 pct moisture as either water migration, and solution channeling during or strong cyanide solution, mechanically leaching by producing a porous, permeable tumbling the wetted mixture, and curing feed (4).- the agglomerated feed for a minimum of 8 hr before heap leaching. The quantity of A permeable feed material stacked in a cement added during agglomeration usually heap permits the uniform flow of leaching provides the protective alkalinity re- solution and contact of the cyanide quired for cyanide leaching. After ag- leaching solution with the exposed pre- glomeration and heap building, leaching cious metal particles and decreases the is conducted with conventional heap leaching time required to obtain targeted leaching techniques. precious metal recovery.

During agglomeration, the clay and fine 4~nderlinednumbers in parentheses re- particles contained in the ore adhere fer to items in the list of references at to the coarser particles and create a the end of this report. ARIZONA GOLD HEAP LEACHING, 20 TPD

The nxidLzed The ainus 3/8-in fee material 9% mixed tains an average 0,2 tr a2 land cement (~ype12) per tofl, The h$>%t rock kg P6rQ befo~edischarge from the rtides are expose The om is feed eanveyar, and the mixture is bkn he first foot of the r The ore- er aixture 9s wetted by the spray to a nsainal 3/8-irx feed size, bar with a sela%ion containing 2 1 and conveyed to a drum agglsacrador, pcr ton, Tatal maisture required for age- glomeratlsn is I0 pet, Ch discharge frsa The drum aggloaeratot is 3 ft in diaae- the agglomerated e is con- ter and 7 ft in length, A sgatic scrap- an impervious leaching pa lengthwise in the top into a 7-ft-high heap that con- of the dram, prc5vex4it~ exees%he tains 100 tons, T~Eagglomera~ed ore Es cured in the heap for 48 hr before The mined ore is transported by front- leaching. end loader to the in-pit crushing plant where it is crushed to a 112-in size by The heap is leached in 5 days by spray- two-stage crushing. The minus 112-in ing solution containing 1 lb NaCN per ton feed is conveyed uphill one-fourth mile over the heap at a rate of 0.033 gpm/ft2. to the processing plant. The ore trans- The cement added during agglomeration is port conveyor system is cheaper to oper- sufficient for protective alkalinity. ate than truck transport, which keeps the The dissolved gold is recovered from the mining costs extremely low. pregnant solution by passing the solution upward through four 14-in by 5-ft carbon The transport conveyor discharges the columns at a rate of 20 gpm/ftZ. The crushed ore onto a surge pile outside the barren solution is recycled to the heap. agglomeration-vat leaching building. The enclosed building permits year-round The loaded carbon is stripped with an operation. From the surge pile the ore alkaline-alcohol solution. The precious is fed to an 8-112- by 32-ft rotating metal values are electrowon from solution drum (fig. 2). The drum slopes 8", with on steel wool cathodes in a cylindrical the feed end elevated, and rotates at 10 electrowinning cell. The gold-laden rpm. The walls of the drum are not cathodes are treated with nitric acid to scraped because the agglomerated feed dissolve excess iron, and the resultant does not build up excessively. sludge is fire refined to produce a dore' bullion. The ore is mixed with 10 lb of binder (3 lb portland cement (type 11), 7 lb Gold recoveries obtained by agglomera- fly ash) per ton of feed in the first tion and heap leaching average 90 pct. few feet of the drum. The material is Agglomeration pretreatment improved heap sprayed with a pH 12 solution containing permeability, percolation rate, gold re- 10 lb NaCN per ton. A 20-ft spray bar covery, and decreased the leaching period inside the drum is used to increase the required to obtain the targeted recovery. moisture content of the feed to 13 pct. Mechanical tumbling occurs along the COLORADO GOLD VAT LEACHING, 1,500 TPD length of the drum. The agglomerated feed is conveyed to a stockpile from In this operation, the agglomerated which the leaching vats are loaded by a feed is leached in large vats rather than front-end loader. Curing occurs while in heaps. The operation is located in the vats are being loaded. the Colorado Rocky Mountains southwest of Denver at an elevation of 10,000 ft (5).- The feed is placed into one of four inclined vats measuring 80 by 50 ft. The The gold ore occurs in an oxidized dis- deepest portion of the vat is 9 ft. Each seminated deposit and averages 0.06 tr oz vat contains 1,000 tons of agglomerated of gold per ton with trace amounts of feed and requires 4 hr to load. silver. The host rock is porous and the gold is liberated by crushing. The ore The ore is leached by spraying solution is mined from an open pit and is friable. at the rate of 0.05 gpm/ft2 until the vat Seventy-five percent of the deposit can is full. The solution in the vat is cir- be mined with dozers and front-end load- culated and sprayed by pumps throughout ers. The remaining 25 pct must be the leaching cycle to insure an adequate drilled and blasted. The operation mines dissolved oxygen content (fig. 3). and processes 1,500 tons of ore per day. All the material mined is ore; there is Gold recovery is 90 pct in a 3day no waste rock. leaching and washing cycle. The vats washing c Ba" en the lead r~ff ned 8a product dore bullion cant afn- ing 80 pet gold and 17 pet silver, cuit, Presh carbon 9s kept in the Last PreL9~4nary metallurgical test f rig had lutisn flow seque n conventPonaL heap leaching to arren solatian, eisfactory, Agglomeratfon Snerens percolation rate, and gold glamera$ion permits a very e during aggP ng cycle, wl2ich al$cms for

The pregnant solution passed through one of for mine backfill and virgin ore adjacent The heaps are sprayed with pH 10.5 so- to the waste material. The waste mate- lution containing 2 lb NaCN per ton at a rial is mined with front-end loaders to rate of 0.0075 gpm/ft2. The leaching so- expose the virgin ore. The virgin ore is lution percolates through the heap, is drilled and blasted and moved by front- collected on the impervious leaching pad, end loader. The silver content of the and drains into plastic-lined solution two feed materials varies. The cutoff trenches. The leaching and washing cycle grade of feed to the heaps is 1.0 tr oz is 7 days. The leached residue is trans- of silver per ton. Approximately 2,000 ferred to an auxiliary leaching pad tpd of ore is mined and agglomerated. and sprayed with cyanide solution 1 day per month for additional precious metal The mined ore and waste are moved from extraction. a stockpile to the crushing plant where they are crushed to a nominal 112 in. Precious metal values in the pregnant Lime (7 lblton of ore) is used as the solution are recovered by Merrill-Crowe binder for agglomeration and is mixed zinc precipitation technology. The with the ore during secondary crushing. preciousmetal-bearing zinc precipiJates The crushed ore-lime mixture is conveyed are refined on site and yield dore bul- to an underground ore stockpile. Some lion. The dorg is shipped to another moisture is sprayed onto the ore on the facility for refining. The barren solu- crusher discharge conveyor to decrease tions are recycled to the heaps. dusting. Heap leaching was unsuccessful before The ore from the underground stockpile agglomeration pretreatment was applied to is agglomerated on a reverse belt con- the ore. Conventional heap leaching re- veyor designed by the operators (fig. 4). covered only 37 pct of the leachable sil- The 4- by 25-ft belt agglomerates ore at ver from 3-in feed material treated in a rate of 200 tph. The agglomerating 90-day leaching cycles. Severe percola- conveyor can be set at an angle between tion problems were encountered. Agglom- 35" and 45", and the belt travels upward eration permitted finer crushing, which while the ore moves down the belt. The liberated additional silver values for angle and speed of the belt can be varied dissolution by cyanide. Agglomeration to provide the desired retention time of heap leaching increased silver recovery ore on the belt. Water is sprayed at to 90 pct of the leachable silver and de- several locations along the length of the creased the leaching time to 7 days. belt and gives the agglomerated feed a moisture content of between 10 and 12 NORTHERN NEVADA GOLD HEAP pct. A small amount of moistened fines LEACHING, 2,500 TPD adheres to the belt and rides up the conveyor. A scraper was placed on the bot- An operation in northern Nevada (fig. tom side of the drive roller to eliminate 7), which has produced gold by agitation excessive fines buildup on the belt. cyanidation and countercurrent decanta- tion for several years, discovered a new The agglomerated ore is transported to ore deposit several miles from the work- a stockpile by a radial arm stacker (fig. ing mine. Higher grade ore from the new 5). The material from the stockpile is deposit is transported to the original trucked 500 ft to a 314-acre impervious mill for gold recovery. Ore containing leaching pad and is allowed to cure dur- less than 0.07 tr oz gold per ton is heap ing heap building. Five heaps, each con- leached at the site. Average ore grade taining 6,000 tons of agglomerated ore for the heap material is 0.034 tr oz gold stacked 10 to 11 ft high, are leached on per ton. The ore is mined by open pit the pad (fig. 6). Three heaps are at methods and is trucked to the heap leach- different stages of leaching, while the ing site approximately one-half mile from other two are either being prepared for the new pit. About 2,500 tpd of ore is leaching or being removed from the pad. mined and heap leached.

The ore is crushed to minus 5$8 In by a truck dmps eh~agglomerated feed onto primary jaw crush crusher, Portlan the rate of 7 t lb/ton of ore, is nhile the heap i~$&zing built, ta the or@ at the jaw e sanveyar and is m9xe urlng secondary crushin binder aixgure Eesm secoa ary crushing Ls csnveyed to a radial am%tacker, Wafxr is %prayed onto the mixttlre at the dis- ~llki~ge elld the 8t8~kE~an ti~+Lm-$$$Estar~ cax~texa of 9 The are is aggloaerated by casca The agglam~ratedItcap% are leastxed by es af the cmxiea spraying U.1104 ta 0,005 gpm/fr2 of pii r s tg~ckpile, 10-1 1 sobutf+oncantainf-ng I,U fb NaCN per $Ling, sllfficfent ta effect a tan of solugion, The portlan t Eon, occur% wh during agglame laads the rear he alkalinity &?rates f rorn the stc)skj~PLe and when the smlL gunnt f tie% af NaOII are add barren solution ta maintain the ~$1w The Leaching washing cycle is 20 days, I,c;~ched r dues are trans- parted to a tailings

The prcgnane goiution draining from thc s by i;ravS&y to a msar- voir and is pumped throut;h a series of n tanks fur recov- valucs, The gold- h, cclconut shell ae- orted ta the y an alkaline alcoktal sokutfon. The vaftxes desorbed from the carbsti-i,are elee:trowon ox% steel wsoi cathsdcs, ~IxEchare refined an sitc ta produce dore& The Largest sf the operations surveyed Ss laoa~~din e;asturn Nevada snd pro- cegses approxi arely 3,000 tpd of ore (7- 8 fig 0,The are i a siLicif4 gi itstone containing finely disseminnted

th in %he deposit, The

e ore is mine

stance to the crushing cireait,

The run-of-aine ore 99 crush cone crusher

he per~anentheaps are ing the truck onto rhe feed over the e levels th~heap, leaching, Th e i~nat re %%fan-heap leaching action (41, Each heap contains of agglomera

The heaps are leached 11,5 solution contain er ton of ~olutionat a r 0.005 gpm/ft2. The total leaching and rectangular electrowinning cells where washing cycle is from 20 to 80 days and the gold is deposited on steel wool cath- depends on the ore being processed. The odes. The cathodes are refined on-site pregnant solution collects on the imper- to produce dore'bullion containing 92 pct vious pads and drains to plastic-lined gold and 6 pct silver. The dore' bars ditches. The solution flows by gravity are shipped to a custom refinery for to a 7-million-gal pregnant solution processing. pond. Recoveries by agglomeration-heap leach- Pregnant solution from the pond is ing range from 70 to 90 pct, and depend pumped through a series of five activated on the ore being processed. Conventional carbon tanks at a rate of 1,000 gpm. The heap leaching was used before the agglom- barren solution exiting the fifth carbon erating equipment was on-line. Gold re- tank is pumped to the barren solution coveries were less than 50 pct because of pond for reagent makeup and recycle to fines migration and the high clay con- the heap. The loaded carbon from the tent, which caused blinding and channel- lead tank is stripped with a caustic cya- ing of leaching solution. Agglomeration nide solution at 120° C and 35 psig pres- technology has helped maintain targeted sure. The gold-bearing solution is recoveries in shorter leaching periods cooled to 85' C before electrowinning. for heaps with high clay content. The solution is pumped through two

SUMMARY AND CONCLUSIONS

The precious metals industry has rapid- are more permeable and drain rapidly, ly adopted agglomeration-heap leaching soluble losses are minimized and inter- technology. The five operations de- mittent sprinkling is permitted. This is scribed in this report demonstrate the important in arid climates because solu- versatility and applicability of the tion loss by evaporation is minimized by technology to heap leachable ore types spraying the heaps during night hours for different size operations. A summary when winds and ambient temperature are of agglomerating conditions for the oper- low. During the period that the heap is ations is shown in table 1. In the West- not being sprayed, the residual cyanide ern United States, there are 36 conner- in the heap continues to dissolve pre- cia1 operations using agglomeration-heap cious metal and enriches the pregnant leaching technology. solution when spraying is resumed. The greater permeability of the heaps permits For the operations described, agglomer- better washing of dissolved values from ation pretreatment technology effectively the heap and decreases the residual cya- improved heap permeability and permitted nide content of the washed, spent heap. better contact of the cyanide leaching solution with the precious metal. As a Even though more reagents and equipment result, percolation rates increased, are required for agglomeration-heap leaching periods decreased, and precious leaching than for conventional heap metal recoveries were improved over those leaching, it is cost effective because of of conventional heap leaching technology. the decrease in leaching time and im- Agglomeration pretreatment provides cer- proved precious metal recoveries. tain other advantages. Since the heaps TABLE 1. - Summary of agglomerating conditions of the five commercial operations described - - - Process- Binder added Agglome -ltion Leach- Recov- Operation ing rate, t 0 Equip- lois- ing ery, tpd Type ment ure, cycle, Pet used pct -days - Arizona 20 Cement. Drum.. . 10 5 90 gold heap leaching.

Colorado 1,500 ..do.. . ..do.. . 13 3 . 90 gold vat leaching. Fly ash

Arizona 2,000 Lime. .. Reverse 0-12 7 '90 silver heap belt. leaching.

. Northern 2,500 Cement. one^. . 9-13 20 C3) Nevada gold heap leaching. Eastern 3,000 ..do.. . Drum.. . 8 20-80 70-90 Nevada gold heap leaching. I '90 ~ctof the leach;tbl 2~ggiomeration occurs as the wetted feed cascades down the conical stockpile. A drum agglomerator is planned for this operation. 3~ercentrecovery is proprietary company information. Agglomeration increased the recovery by 60 pct. 4While heap is built. REFERENCES

1. Heinen, H. J., G. E. McClelland, J., v. 183, No. 10, Oct. 1982, pp. 102- and R. E. Lindstrom. Enhancing Percola- 105. tion Rates in Heap Leaching of Gold- Silver Ores. BuMines RI 8388, 1979, 20 6. Engineering and Mining Journal. PP - Pelletizing Aids Tombstone Leaching Oper- ation. V. 182, No. 1, Jan. 1981, pp. 94- 2. McClelland, G. E., and J. A. 95. Eisele. Improvements in Heap Leaching To Recover Silver and Gold From Low-Grade 7. This Month in Mining. Al- p. Resources. BuMines RI 8612, 1982, 26 ligator Ridge Uses Heap Leaching To Pro- PP duce Gold Bullion Bars. V. 182, No. 8, Aug. 1981, pp. 35-37. 3. Johanson, J. R. Particle Segrega- tion ... and What To Do About It. Chem. 8. McCpiston, F. W., Jr., and R. S. Eng., (N.Y.), v. 85, No. 11, May 1978, Shoemaker. Heap Leaching Gold and Silver pp. 183-189. Ores. Ch. in Gold and Silver Cyanidation Plant Practice. American Institute of 4. Chamberlin, P. D. Heap Leaching Mining, Metallurgical, and Petroleum En- and Pilot Testing of Gold and Silver gineers, Inc., New York, v. 2, 1981, pp. Ores. Min. Cong. J., v. 67, No. 4, Apr. 13-18. 1981, pp. 47-52.

5. Lewis, A. Producing Gold for $1601 Tr Oz in Victor, Colorado. Eng. and Min.

*US. GOVERNMENT PRINTING OFFICE: 1983-605015l61