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. 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 pretreat- ment 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 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. '~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