:iC 8945

Bureau of Mines Inrormat;on 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 INTERIORJames G. Watt. Secretary

BUREAU OF MINESRobert C. Horton. Director

Library of Congress Cataloging in Publication Data:

McClelland, G. E

Aggiomcratioii~heap leaching operarions in the precious metalsindustry.

(Information circular I United States Department of the Interior)Bureau of Mines; 8945)

Bibliography: p. 16.Supt. of Docs. no.: r 28.27:8945.

1. Precious metals-Mctal1iigy. 2. Agglomeration. 3. Spoilbanks-Leaching. I. Pool, D. L. (Danny L.). n. Eisele, J. A. (JudithA.). III. Title. iv. Series: Information circular (United States. fiu-

reau of Mines) ; 8945.

TN295.U4 (TN759) 6228 (669' .22'0281 83-600179

CONTENTSPage

Aba tract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Introduction. . . . . . . .. . .. . . . . . . . . .. . . . .. . . .. . .. . . . . .. . . . . . . . . . . . .. . . . .. . .. . .. . .. 2Agglomeration concept.......................................................... 2Commercial agglomeration-heap leaching operations.............................. 3

Arizona gold heap leaching, 20 tpd........................................ 3Colorado gold vat leaching, i, 500 tpd..................................... 4Arizona silver heap leaching, 2,000 tpd................................... 6Northern Nevada gold heap leaching, 2,500 tpd............................. 7Eastern Nevada gold heap leaching, 3,000 tpd.............................. 10

Sumry and conclusions........................................................ 14References..................................................................... 16

ILLUSTRATIONS

1.2.

Small Arizona gold agglomeration-heap leaching operation..................Discharge end of the drum agglomerator used at the Colorado goldagglomeration-vat leaching operation....................................

Leaching vat at the Colorado gold agglomeration-vat leaching operation....

Agglomeration at the Arizona silver heap leaching operation showing thereverse belt agglom.rator...............................................

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 pretreat-

3.4.

5.6.7.8.

ment was adopted........................................................9.

10.11.12.

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 leachingoperation. . . . . . . . . . . . . . . . . . .. . . .. . . . .. . . . . . . . . . . . . . . .. . . .. . . . . . .. . . .. . . . 13

TABLE

1. Summry of agglomerating conditions of the five commrcial operationsdescribed............................................................... 15

3

56

889

10

11111212

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

°c degree Celsius pct percent

ft foot psig pound per square inch,gauge pressure

gal gallonrpm revolution per minute

gpm gallon per minutesec second

gpm/ft2 gallon per minuteper 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 tonavoirdupois

AGGLOMERATION.HEAP LEACHING OPERATIONSIN THE PRECIOUS METALS INDUSTRY

By G. E. McClelland, 1 D. L. Paal,2 and J. A. Eisele3

ABSTRACT

During the 1970' s, the Bureau of Mines investigated a particle ag-glomeration technique for improving the flow of leaching solutionthrough heaps of clayey or crushed, low-grade gold-silver ores, wastes,and tailings. This technology has been adopted on a broad scale by theprecious-metals-processing industry. This report presents informationon five commrcial operations that have benefited from agglomerationtechnology and that represent a cross section of current heap leachingpractice. The technology is cost effective because of decreased leachtimes and improved precious metal recoveries.

'Metallurgist.2Research chemist.3supervisory chemical engineer.All authors are with the Reno Research Center, Bureau of Mines, Reno, Nev.

2

INTRODUCTION

Exploration during the mid to late1970' s identified numerous low-grade pre-cious metal deposits, mine waste mate-rials, and tailings piles throughout theWestern United States. The increase inprecious metal prices during this periodgenerated interest in processing theselow-grade feed materials by low-cost heapleaching technology. Heap leaching withcyanide 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 finesgenerated during crushing prevented auniform flow of cyanide solution throughthe heaps.

As part of its research program to im-prove the recovery of gold and silverfrom low-grade domestic resources, theBureau of Mines investigated agglomera-tion pretreatment to overcome the perco-lation problems associated with heapleaching. Agglomeration of the clays andfines before heap building permtteda uniform and rapid flow of cyanide

leaching solution through the heaps.Previous Bureau publications (!-1)4 de-scribe bench- and pilot-scale experimentsshowing the advantages of particle ag-glomeration as a pretreatment for mate-rials that are difficult to treat bystandard heap leaching techniques.

This report shows how the Bureau's re-search on agglomeration-heap leaching hasbeen adopted and applied by the precious-metals-processing industry. It describesfive commercial operations) ranging insize from 20 to 3,000 tpd and represent-ing a cross section of current heapleaching practice, that have benefitedfrom agglomeration technology. The prob-lems they experienced in trying to applyconventional 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 effectivelyresolved the heap permeability problemsencountered 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 improvesoverall recovery. Crushed ores can beagglomerated by mixing 5 to 10 lb port-land cement per ton of dry feed, wettingwith 8 to 16 pct moisture as either wateror strong cyanide solution, mechanicallytumbling the wetted mixture, and curingthe agglomerated feed for a minimum of 8hr before heap leaching. The quantity ofcement added during agglomeration usuallyprovides the protective alkalinity re-quired for cyanide leaching. After ag-glomeration and heap building, leachingis conducted with conventional heapleaching techniques.

During agglomeration, the clay and fineparticles contained in the ore adhereto the coarser particles and create a

coating of fines around the coarse parti-cles. The agglomerates produced are ofsufficient green strength after curing towithstand wetting with minimal degrada-tion. Agglomeration overcomes the majorproblems associated with particle segre-gation during heap building (l), finesmigration, and solution channeling duringleaching by producing a porous, permeablefeed (i).

A permeable feed material stacked in aheap permts the uniform flow of leachingsolution and contact of the cyanideleaching solution with the exposed pre-cious metal particles and decreases theleaching time required to obtain targetedprecious metal recovery.

4Underlined numers infer to items in the listthe end of this report.

parentheses re-of references at

4

cured in the heap for 48 hr beforeleaching.

The heap is leached in 5 days by spray-ing solution containing 1 lb NaCN per tonover the heap at a rate of 0.033 gpm/ft2.The cement added during agglomeration issufficient for protective alkalinity.The dissolved gold is recovered from thepregnant solution by passing the solutionupward through four 14-in by 5-ft carboncolumns at a rate of 20 gpm/ft2. Thebarren solution is recycled to the heap.

The loaded carbon is stripped with analkaline-alcohol solution. The preciousmetal values are electrowon from solutionon steel wool cathodes in a cylindricalelectrowinning cell. The gold-ladencathodes are treated with nitric acid todissolve excess iron, and the resultantsludge is fire refined to produce a dortbullion.

Gold recoveries obtained by agglomera-tion and heap leaching average 90 pct.Agglomeration pretreatment improved heappermeability, percolation rate, gold re-covery, and decreased the leaching periodrequired to obtain the targeted recovery.

COLORAO GOLD VAT LEACHING, 1,500 TPD

In this operation, the agglomeratedfeed is leached in large vats rather thanin heaps. The operation is located inthe Colorado Rocky Mountains southwest ofDenver at an elevation of 10,000 ft (~).

The gold ore occurs in an oxidized dis-seminated deposit and averages 0.06 tr ozof gold per ton with trace amounts ofsilver. The host rock is porous and thegold is liberated by crushing. The oreis mined from an open pit and is friable.Seventy-five percent of the deposit canbe mined with dozers and front-end load-ers. The remaining 25 pct must bedrilled and blasted. The operation minesand processes 1,500 tons of ore per day.All the material mined is ore; there isno was te rock.

The mined ore is transported by front-end loader to the in-pit crushing plantwhere it is crushed to a 1/2-in size bytwo-stage crushing. The minus 1/2-infeed is conveyed uphill one-fourth mileto the processing plant. The ore trans-port conveyor system is cheaper to oper-ate than truck transport, which keeps themining costs extremely low.

The transport conveyor discharges thecrushed ore onto a surge pile outside theagglomeration-vat leaching building. Theenclosed building permits year-roundoperation. From the surge pile the oreis fed to an 8-1/2- by 32-ft rotatingdrum (fig. 2). The drum slopes 8°, withthe feed end elevated, and rotates at 10rpm. The walls of the drum are notscraped because the agglomerated feeddoes not build up excess i vely.

The ore is mixed with 10 lb of binder(3 lb portland cement (type II), 7 lbfly ash) per ton of feed in the firstfew feet of the drum. The material issprayed with a pH 12 solution containing10 lb NaCN per ton. A 20-ft spray barinside the drum is used to increase themoisture content of the feed to 13 pct.Mechanical tumbling occurs along thelength of the drum. The agglomeratedfeed is conveyed to a stockpile fromwhich the leaching vats are loaded by afront-end loader. Curing occurs whilethe vats are being loaded.

The feed is placed into one of fourinclined vats measuring 80 by 50 ft. Thedeepest portion of the vat is 9 ft. Eachvat contains 1,000 tons of agglomeratedfeed and requires 4 hr to load.

The ore is leached by spraying solutionat the rate of 0.05 gpm/ft2 until the vatis full. The solution in the vat is cir-culated and sprayed by pumps throughoutthe leaching cycle to insure an adequatedissolved oxygen content (fig. 3).

Goldleaching

recovery is 90 pct inand washing cycle.

aThe

3-dayvats

;ii j due'

iTI II

I,"

'"

ff

,000

for mine backfill and virgin ore adjacentto the waste material. The waste mate-rial is mined with front-end loaders toexpose the virgin ore. The virgin ore isdrilled and blasted and moved by front-end loader. The silver content of thetwo feed materials varies. The cutoffgrade of feed to the heaps is 1.0 tr ozof silver per ton. Approximately 2,000tpd of ore is mined and agglomerated.

The mined ore and waste are moved froma stockpile to the crushing plant wherethey are crushed to a nominal 1/2 in.Lime (7 lblton of ore) is used as thebinder for agglomeration and is mixedwith the ore during secondary crushing.The crushed ore-lime mixture is conveyedto an underground ore stockpile. Somemoisture is sprayed onto the ore on thecrusher discharge conveyor to decreasedusting.

The ore f rom the underground stockpileis agglomerated on a reverse belt con-veyor designed by the operators (fig. 4).The 4- by 25-ft belt agglomerates ore ata rate of 200 tph. The agglomeratingconveyor can be set at an angle between35° and 45°, and the belt travels upwardwhile the ore moves down the belt. Theangle and speed of the belt can be variedto provide the desired retention time ofore on the belt. Water is sprayed atseveral locations along the length of thebelt and gives the agglomerated feed amoisture content of between 10 and 12pet. A small amount of moistened finesadheres to the belt and rides up the con-veyor. A scraper was placed on the bot-tom side of the drive roller to eliminateexcessive fines buildup on the belt.

The agglomerated ore is transported toa stockpile by a radial arm stacker (fig.5). The material from the stockpile istrucked 500 f t to a 3/4-acre imperviousleaching pad and is allowed to cure dur-ing heap building. Five heaps, each con-taining 6,000 tons of agglomerated orestacked 10 to 11 ft high, are leached onthe pad (fig. 6). Three heaps are atdifferent stages of leaching, while theother two are either being prepared forleaching or being removed from the pad.

7

The heaps are sprayed with pH 10.5 so-lution containing 2 lb NaCN per ton at arate of 0.0075 gpm/ft2. The leaching so-lution percolates through the heap, iscollected on the impervious leaching pad,and drains into plastic-lined solutiontrenches. The leaching and washing cycleis 7 days. The leached residue is trans-ferred to an auxiliary leaching padand sprayed with cyanide solution 1 dayper month for additional precious metalextraction.

Precious metal values in the pregnantsolution are recovered by Merrill-Crowezinc precipitation technology. Theprecious-metal-bearing zinc precipitatesare refined on site and yield dorE! bul-lion. The doré is shipped to anotherfacility for refining. The barren solu-tions are recycled to the heaps.

Heap leaching was unsuccessful beforeagglomeration pretreatment was applied tothe ore. Conventional heap leaching re-covered only 37 pct of the leachable sil-ver from 3-in feed material treated in90-day leaching cycles. Severe percola-tion problems were encountered. Agglom-eration permitted finer crushing, whichliberated additional silver values fordissolution by cyanide. Agglomerationheap leaching increased silver recoveryto 90 pct of the leachable silver and de-creased the leaching time to 7 days.

NORTHERN NEVADA GOLD HEAPLEACHING, 2,500 TPD

An operation in northern Nevada (fig.7), which has produced gold by agitationcyanidation and countercurrent decanta-tion for several years, discovered a newore deposit several miles from the work-ing mine. Higher grade ore from the newdeposit is transported to the originalmill for gold recovery. Ore containingless than 0.07 tr oz gold per ton is heapleached at the site. Average ore gradefor the heap material is 0.034 tr oz goldper ton. The ore is mined by open pitmethods and is trucked to the heap leach-ing site approximately one-half mile fromthe new pit. About 2,500 tpd of ore ismined and heap leached.

i ii iI; !\

'"',

) ~

, i

"

10) ~

9,

r'

Jlii dr IF

'Piit,+ f'I. 'ii

.1 Ii

\')

II,

\:C'l'ini l'

hi.iJ ,j i rid

1,..)',,:1:

1 ,. in' ,'1

¡,Il 'h'\

oj)

i111.i

'. (

i (!

ul L ¡ \Ii'

'1

:,1.

'ri

i I () ~

N

14

0.005 gpm/ft2. The total leaching andwashing cycle is from 20 to 80 days anddepends on the ore being processed. Thepregnant solution collects on the imper-vious pads and drains to plastic-linedditches. The solution flows by gravityto a 7-mllion-gal pregnant solutionpond.

Pregnant solution from the pond ispumped through a series of five activatedcarbon tanks at a rate of 1,000 gpm. Thebarren solution exiting the fifth carbontank is pumped to the barren solutionpond for reagent makeup and recycle tothe heap. The loaded carbon from thelead tank is stripped with a caustic cya-nide solution at 1200 C and 35 psig pres-sure. The gold-bearing solution iscooled to 850 C before electrowinning.The solution is pumped through two

rectangular electrowinning cells wherethe gold is deposited on steel wool cath-odes. The cathodes are refined on-siteto produce dore/ bullion containing 92 pct/gold and 6 pct silver. The dore barsare shipped to a custom refinery forprocessing.

Recoveries by agglomeration-heap leach-ing range from 70 to 90 pct, and dependon the ore being processed. Conventionalheap leaching was used before the agglom-erating equipment was on-line. Gold re-coveries were less than 50 pct because offines migration and the high clay con-tent, which caused blinding and channel-ing of leaching solution. Agglomerationtechnology has helped maintain targetedrecoveries in shorter leaching periodsfor heaps with high clay content.

SUMY AND CONCLUSIONS

The precious metals industry has rapid-ly adopted agglomeration-heap leachingtechnology. The five operations de-scribed in this report demonstrate theversatility and applicability of thetechnology to heap leachable ore typesfor different size operations. A summryof agglomerating conditions for the oper-ations is shown in table 1. In the West-ern United States, there are 36 commr-cial operations using agglomeration-heapleaching technology.

For the operations described, agglomer-ation pretreatment technology effectivelyimproved heap permeability and permittedbetter contact of the cyanide leachingsolution with the precious metal. As aresult, percolation rates increased,leaching periods decreased, and preciousmetal recoveries were improved over thoseof 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 permtted. This isimportant in arid climates because solu-tion loss by evaporation is minimized byspraying the heaps during night hourswhen winds and ambient temperature arelow. During the period that the heap isnot being sprayed, the residual cyanidein the heap continues to dissolve pre-cious metal and enriches the pregnantsolution when spraying is resumed. Thegreater permeability of the heaps permitsbetter washing of dissolved values fromthe heap and decreases the residual cya-nide content of the washed, spent heap.

Even though more reagents and equipmentare required for agglomeration-heapleaching than for conventional heapleaching, it is cost effective because ofthe decrease in leaching time and im-proved precious metal recoveries.

15

TABLE 1. - Summry of agglomerating conditions of the five commrcialoperations described

Process- Feed Binder added Agglomeration Curing Leach- RecovOperation ing rate, size, to ore Equip- Mois- time, ing ery,

tpd in Type lb/ton ment ture, hr cycle, pctused pct days

Arizona 20 3/8 Cement. 10 Drum... 10 48 5 90gold heapleaching.

Colorado 1,500 1/2 ..do.. . 3 . .do... 13 8 3 90gold vatleaching. Fly ash 7

Arizona 2,000 1/2 Lime... 7 Reverse 10-12 72 7 '90silver heap belt.leaching.

Northern 2,500 5/8 Cement. 7-10 None2 . . 9-13 48-72 20 (3 )Nevadagold heapleaching.

Eastern 3,000 3/4 ..do.. . 4-10 Drum.. . 8 (4 ) 20-80 70-90Nevadagold heapleachinl! .190 pct of the leachable silver is recovered.2Agglomeration occurs as the wetted feed cascades down the conical stockpile. A

drum agglomerator is planned for this operation.3percent recovery is proprietary company information. Agglomeration increased the

recovery by 60 pct.4While 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. BuMnes RI 8388, 1979, 20pp.

2. McClelland, G. E., andJ. A.Eisele. Improvements in Heap Leaching ToRecover Silver and Gold From Low-GradeResources. BuMines RI 8612, 1982, 26pp.

3. Johanson, J. R. Particle Segrega-t ion ... and What To Do About It. Chem.Eng., (N.Y.), v.8S, No. 11, May 1978,pp. 183-189.

4.andOres.1981,

Chamberlin, P.Pilot Tes ting ofMin. Congo J.,

pp. 47-52.

D. Heap LeachingGold and Silverv. 67, No.4, Apr.

5. Lewis, A. Producing Gold for $160/Tr Oz in Victor, Colorado. Eng. and Min.

"u.s. GOVERNMENT PRINTING OFFICE: 19836015/61

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 SilverOres. Ch. in Gold and Silver CyanidationPlant Practice. American Institute ofMining, Metallurgical, and Petroleum En-gineers, Inc., New York, v. 2, 1981, pp.13-18.

INT.-8U.OF MINES,PGH.,PA. 27064