SPECIAL PUBLICATION 87 By Michael Silva 1986 CALIFORNIA DEPARTMENT OF CONSERVATION DIVISION OF MINES AND GEOLOGY GORDON K. VAN VLECK, Secretary THE RESOURCES AGENCY GEORGE DEUKMEJIAN, Governor STATE OF CALIFORNIA DON L. BLUBAUGH, Director DEPARTMENT OF CONSERVATION PLACER GOLD RECOVERY METHODS PLACER GOLD RECOVERY METHODS DIVISION OF MINES AND GEOLOGY JAMES F. DAVIS STATE GEOLOGIST SPECIAL PUBLICATION 87 PLACER GOLD RECOVERY METHODS By Michael Silva 1986 CALIFORNIA DEPARTMENT OF CONSERVATION DIVISION OF MINES AND GEOLOGY 801 K Street Sacramento, California 95814 CONTENTS INTRODUCTION 1 CONCENTRATION OF PLACER GOLD ORE 2 SMALL SCALE RECOVERY EQUIPMENT 2 Gold Pan 2 Rocker 3 Construction 3 Assembly 5 Operation 5 Sluices 6 Long Tom 7 Dip-Box 8 Shaking Tables 8 Portable Processing Equipment 10 Amalgamation 10 DRY PLACERS 10 Dry Washers 10 Air Tables (Oliver Gravity Separator) 11 MODERN RECOVERY EQUIPMENT 12 Pinched Sluice Systems 12 Spiral Concentrators 13 Rotating Spirals 15 Helixes 15 Jigs 16 Fine Material Separators 20 Bartles-Mozely Separator 20 Bartles CrossBelt Separator 21 Centrifugal Concentrators 21 Bowls 21 Knelson Concentrator 22 SUMMARY 22 OPERATING MINES 23 Hammonton Dredge 23 Hansen Brothers - Hugh Fisher 25 Bear River 26 Greenhorn Creek 26 TRI-R Engineering - Stinson Mine 27 SELECTED ANNOTATED REFERENCES 29 APPENDIX: LIST OF EQUIPMENT MANUFACTURERS AND SUPPLIERS 31 TABLE Table 1. Range of particle sizes effectively treated by various types of separation equipment 23 DISCLAIMER COMPANY NAMES AND PRODUCTS DESCRIBED IN THIS PUBLICATION ARE FOR DESCRIPTIVE PURPOSES ONLY AND DO NOT IMPLY ENDORSEMENT BY THE STATE OF CALIFORNIA, DE- PARTMENT OF CONSERVATION, DIVISION OF MINES AND GEOLOGY. CONVERSELY, THE OMIS- SION OF A COMPANY OR PRODUCT DOES NOT IMPLY REJECTION BY THE DEPARTMENT OF CONSERVATION, DIVISION OF MINES AND GEOLOGY. Figure 1. Rocker washer 3 Figure 2. Rocker parts and construction 4 Figure 3. Classifying action of sluice riffles 6 Figure 4. Hungarian riffle arrangement 6 Figure 5. Detail of Hungarian riffles 7 Figure 6. Side and plan views of a long tom 7 Figure 7. Shaking table concentrator 8 Figure 8. Mineral separation on a shaking table 9 Figure 9. Stratification of minerals along shaking table riffles 9 Figure 10. Denver Gold Saver 9 Figure 11. Dry washer 11 Figure 12. Separation on an air table or pneumatic shaking table 11 Figure 13. Oliver gravity separator 12 Figure 14. Mineral separation on an Oliver gravity separator 12 Figure 15. Cross section and plan view of pinched sluice 12 Figure 16. Schematic diagram of a single Reichert cone 13 Figure 17. Humphries spiral concentrator 14 Figure 18. Cross section of spiral stream flow 14 Figure 19. Components of a conventional jig 16 Figure 20. Overhead view of conventional 2 x 4 cell rectangular jig 16 Figure 21. Physical processes involved in jigging 17 Figure 22. Water flow velocities through the jig bed of conventional jigs and an IHC sawtooth drive jig. 18 Figure 24. Modular jig and circular jig composed of 12 modular jigs 18 Figure 23. Comparison of jigging process for conventional and IHC sawtooth drive jigs 19 Figure 25. Operation of the Bartles-Mosley concentrator 20 Figure 26. Mineral separation on a Bartles crossbelt concentrator 21 Photos Photo 1. Gold pans. 3 Photo 2. Rockers in operation 5 Photo 3. Sluice box in operation 6 Photo 4. Modern sluice lined with screening and rubber matting 6 Photo 5. Long tom in operation 7 Photo 6. Concentrate splitters in Reichert spiral 14 Photo 7. PMX rotary concentration table 15 Photo 8. PMX test plant with helix and rotary tables 15 Photo 9. TRI-R Engineering helix concentrator 16 Photo 10. Bartles crossbelt separator 21 Photo 11. Knelson concentrator 22 Photo 12. Yuba-Placer Gold Company’s Hammonton Dredge 24 Photo 13. Amalgam weighing on the dredge. 24 Photo 14. Retort used to by Yuba-Placer 25 Photo 15. Gold recovery system at the Hansen Brothers Bear River plant Photo 16. Deister shaking table in operation 26 Photo 17. Gold recovery system at Hansen Brothers Greenhorn Creek plant 26 Photo 18. Pump and concentrate barrels located inside shed beneath spiral assembly. 27 Photo 19. Gold recovery system at the Stinson Mine 27 Photo 20. Primary concentrators in recovery system at Stinson Mine 28 Photo 21. Helix separator in recovery system at Stinson Mine 28 ILLUSTRATIONS Figures PLACER GOLD RECOVERY METHODS By Michael Silva INTRODUCTION This report provides practical, timely information on meth- ods and equipment used in placer gold recovery. Included is detailed information on equipment, practices, recovery fac- tors, efficiency, design, and, where available, costs. Selected gold recovery operations are described in detail. In addition, the reported efficiency and reliability of various types of equip- ment used today is presented. One notable method not described is the cyanide process, the recovery of gold through leaching with cyanide, a hazardous substance that must be handled with great care. The information presented herein applies to small as well as large placer mining operations. Recreational and indepen- dent miners will find information on available equipment and designs with some suggestions for improving recovery. Those intending to mine small to medium-sized placer deposits will find detailed descriptions of suitable equipment and recovery methods. Finally, those interested in byproduct gold recovery from sand and gravel operations and other large placer depos- its will find descriptions of appropriate equipment and byproduct recovery installations. There is also a list of manu- facturers and suppliers for much of the described equipment. Production Gold has been mined from placer gold deposits up and down the state and in different types of environment. Initially, rich, easily discovered, surface and river placers were mined until about 1864. Hydraulic mines, using powerful water cannons to wash whole hillsides, were the chief sources of gold for the next 20 years. In 1884, Judge Lorenzo Sawyer issued a decree prohibiting the dumping of hydraulic mining debris into the Sacramento River, effectively eliminating large-scale hydrau- lic operations. For the next 14 years, drift mining placer gold deposits in buried Tertiary channels partially made up for the loss of placer gold production, but overall production declined. Production rose again with the advent of large-scale dredging. The first successful gold dredge was introduced on the lower Feather River near Oroville in 1898. Since then, dredging has contributed a significant part of California’s total gold pro- duction. The last dredge to shut down was the Yuba 21 dredge at Hammonton in 1968 (Clark, 1973). It is fitting that the 1981 revival of major placer gold production in California started with the reopening of this same dredge. Over 64% of the gold produced in California has come from placer deposits. The reason so much of it has been mined from placers is that placer deposits are usually easier to locate than lode deposits. A lone prospector with a gold pan can verify the existence of a placer gold deposit in a short period of time. Small placers are also relatively easy to mine, and the ore usually requires less processing than ore from lode mines. The same holds true for large placers other than drift mines. To- day, placer gold production comes from the dredge operating at Hammonton, from large placer mines employing the cya- nide process, from byproduct recovery in sand and gravel plants, from small placer mines, and from small dredging op- erations in rivers and streams. With placer mining, recovery of the gold from the ore is usually the most expensive phase of the mining operation and can be the most difficult to implement properly. The value of gold deposits is based on the amount of gold that can be re- covered by existing technology. Failure to recover a high per- centage of the gold contained in the deposit can affect the value of the deposit. Gravity separation remains the most widely used recovery method. Gravity recovery equipment, including gold pans, sluice boxes, long toms, jigs, and amalgamation devices, has been used since the time of the California gold rush, and many present day operations still employ the same equipment. The major flaw of the gravity separation method is that very fine gold, referred to as flour, flood, or colloidal gold, is lost in processing. Early miners recovered no more than 60% of as- sayed gold values, and as late as 1945 recovery of free gold averaged only 70-75% (Spiller, 1983). Moreover, it is likely that most remaining placer deposits have a higher percentage of fine gold than placers worked during the gold rush. It is understandable, then, that today more care is given to the re- covery of fine gold. In recent times a number of changes and new designs in gravity separation equipment have been developed. Most of these were developed outside the United States for the recov- ery of materials other than gold. Some of the new equipment has been successfully used to recover gold and some older designs have been modified and improved. Today, many types of equipment exist for the efficient recovery of placer gold. It is important to note that recovery techniques are often very site specific. A recovery system that collects a high per- centage of fine gold from one deposit may not perform effec- tively with ore from a different deposit. Many factors, such as particle size, clay content, gold size distribution, mining meth- ods, and character of wash water, affect the amount of gold recovered. Extensive experimentation and testing is usually required to design an optimum gold recovery system. 1 2 DIVISION OF MINES AND GEOLOGY SP 87 CONCENTRATION OF PLACER GOLD ORE The recovery of placer gold involves processing similar to the processing of most ores. First, the valuable material is sepa- rated from the valueless waste through concentration. The fi- nal concentrate, usually obtained by repeated processing, is smelted or otherwise refined into the final product. This report focuses on the equipment and methods used for initial pro- cessing, or concentration. As in other processing applications, many specialized terms are used to describe the phases of min- eral concentration. Although these terms are described herein as they relate to the processing of placer gold ores, most of the terms identified apply to mineral processing in general. The concentration of placer gold ore consists of a combina- tion of the following three stages: roughing, cleaning, and scav- enging. The object of concentration is to separate the raw ore into two products. Ideally, in placer gold recovery, all the gold will be in the concentrate, while all other material will be in the tailings. Unfortunately, such separations are never perfect, and in practice some waste material is included in the concen- trates and some gold remains in the tailings. Middlings, par- ticles that belong in either the concentrate or the tailings, are also produced, further complicating the situation. Roughing is the upgrading of the ore (referred to as feed in the concentration process) to produce either a low-grade, pre- liminary concentrate, or to reject tailings that contain no valu- able material at an early stage. The equipment used in this ap- plication are referred to as roughers. Roughers may produce a large amount of concentrate, permit the recovery of a very high percentage of feed gold, produce clean tailings, or produce a combination of the above. Roughers include jigs, Reichert cones, sluices, and dry washers. The next stage of mineral processing is referred to as clean- ing. Cleaning is the re-treatment of the rough concentrate to remove impurities. This process may be as simple as washing black sands in a gold pan. Mineral concentrates may go through several stages of cleaning before a final concentrate is pro- duced. Equipment used for cleaning is often the same as that used for roughing. A sluice used for cleaning black sand con- centrates is one example of a rougher used as a cleaner. Other devices, such as shaking tables are unsuitable for use as roughers and are used specifically for cleaning. Concentrates are cleaned until the desired grade (ore concentration) is ob- tained. The final stage is known as scavenging. Scavenging is the processing of tailings material from the roughing and cleaning steps before discarding. This waste material is run through equipment that removes any remaining valuable product. Scav- enging is usually performed only in large operations. Where amalgamation is practiced, scavenging also aids in the removal of mercury and prevents its escape into the environment. Equip- ment used in both roughing and cleaning may be used for scav- enging, depending on the amount of tailings to be processed. Any piece of equipment used in this latter capacity is termed a scavenger. Specific terms are also used to describe the efficiency of the concentration process. Recovery refers to the percentage of gold in the ore that was collected in the concentrate. A recov- ery of 90% means that 90% of the gold originally in the ore is in the concentrate and the remaining 10% is in the tailings and/ or middlings. The concentrate grade is the percentage of gold in the concentrate. A concentrate grade of 10% indicates the concentrate contains 10% gold by weight. The ratio of con- centration (or concentration ratio) is the ratio of the weight of the feed to the weight of the concentrates. For example, if 1,000 pounds of feed are processed and 1 pound of concentrate is recovered, the ration of concentration would be 1,000. The value of the ratio of concentration will generally increase with the concentrate grade. There is a general inverse relationship between recovery and concentrate grade in mineral concentration. Usually, the higher the concentrate grade, the lower the total recovery. Some valuable material is lost in producing a high grade concen- trate. In such cases, the higher grade concentrate is easier to refine than a lower grade concentrate, reducing refinery costs. The savings in refining costs is usually greater than the cost of recovering the small amount of remaining gold from the tail- ings. For each mining operation, a carefully determined com- bination of grade and recovery must be achieved to yield maxi- mum profitability. The best recovery systems will collect a maximum amount of placer gold in a minimum amount of con- centrate. SMALL SCALE RECOVERY EQUIPMENT Much of the equipment described in this section has been used for centuries. Many variations of the basic designs have been used throughout the years. Some are more efficient than others. Most have low capacity and do not efficiently recover fine gold. Only the most useful, simple, inexpensive, or easily constructed of these old but practical devices are described. Gold Pan Perhaps the oldest and most widely used gold concentrator is the gold pan. Although available in various shapes and sizes, the standard American gold pan is 15 to 18 inches in diameter at the top and 2 to 2 1/2, inches in depth, with the sides sloping 30-45 degrees. Gold pans are constructed of metal or plastic (Photo 1) and are used in prospecting for gold, for cleaning gold-bearing concentrates, and rarely, for hand working of rich, isolated deposits. A gold pan concentrates heavy minerals at the bottom while lighter materials are removed at the top. The basic operation of a pan is simple, but experience and skill are needed to process large amounts of material and achieve maximum recovery. Pan- ning is best learned from an experienced panner, but the gen- eral principles and steps are outlined below. 3 PLACER GOLD RECOVERY METHODS 1986 For maximum recovery, the material to be panned should be as uniform in size as possible. Panning is best done in a tub or pool of still, clear water. First, fill the pan one-half to three- fourths full of ore or concentrate. Add water to the pan or care- fully hold the pan under water and mix and knead the material by hand, carefully breaking up lumps of clay and washing any rocks present. Fill the pan with water (if not held underwater) and carefully remove rocks and pebbles, checking them before discarding. Tilt the pan slightly away and shake vigorously from side to side with a circular motion while holding it just below the surface of the water. Removal of lighter material is facili- tated by gently raising and lowering the lip of the pan in and out of the water. The pan may be periodically lifted from the water and shaken vigorously with the same circular motion to help concentrate materials. Large pebbles should be periodi- cally removed by hand. Panning continues until only the heavi- est material remains. Gold may be observed by gently swirling the concentrate into a crescent in the bottom of the pan. Coarse nuggets are removed by hand, while finer grained gold may be recovered by amalgamation. An experienced panner can pro- cess one-half to three-quarters of a cubic yard in 10 hours. Panning was widely used as a primary recovery method in the early days of mining. However, the process is extremely limited, as only coarse gold is recovered, while very fine par- ticles are usually washed away with the gravel. Only small amounts of gravel can be processed, even by the most experi- enced panners. Today the gold pan is used mostly for pros- pecting or for cleaning concentrate. Its low price, immediate availability, and portability make it an essential tool for the prospector or miner. Photo 1. Metal and plastic gold pans. Note 18-inch ruler for scale. Rocker One of the first devices used after the gold pan was the rocker. The rocker allowed small operators to increase the amount of gravel handled in a shift, with a minimum investment in equip- ment. Rockers vary in size, shape, and general construction, depending upon available construction materials, size of gold recovered, and the builder’s mining experience. Rockers gen- erally ranged in length from 24 to 60 inches, in width from 12 to 25 inches, and in height from 6 to 24 inches. Resembling a box on skids or a poorly designed sled, a rocker sorts materials through screens. (Figure 1). Figure 1. A simple rocker washer. From Sweet, 1980. Construction. Rockers are built in three distinct parts, a body or sluice box, a screen, and an apron. The floor of the body holds the riffles in which the gold is caught. The screen catches the coarser materials and is a place where clay can be broken up to remove all small particles of gold. Screens are typically 16 to 20 inches on each side with one-half inch openings. Fine material is washed through the openings by water onto an in- clined apron. The apron is used to carry all material to the head of the rocker, and is made of canvas stretched loosely over a frame. It has a pocket, or low place, in which coarse gold and black sands can be collected. The apron can be made of a variety of materials: blanket, carpet, canvas, rubber mat, burlap or amalgamated copper plate. Riffles below the apron help to collect gold before discharge. 4 DIVISION OF MINES AND GEOLOGY SP 87 Figure 2. Diagram of rocker and rocker parts. Reprinted from California Division of Mines and Geology Special Publication 41, “Basic Placer Mining.” [...]... The efficiency of recovery circuits at these plants is difficult to evaluate since the gold content of the ore is not recorded or calculated All recovery figures are estimates by Hugh Fisher based on the performance of the equipment and speculation as to original gold content of the feed Gold recovery in sand and gravel plants presents problems not associated with placer gold mines Recovery systems... source of information on the evaluation and examination of placer gold deposits Wenqian, W., and Poling, G.W., 1983, Methods for recovering fine placer gold: CIM Bulletin, v 76, no 860, December 1983, p 4756 Excellent overview of the difficulties involved in recovering very fine placer gold West, J.M., 1971, How to mine and prospect for placer gold: U.S Bureau of Mines Information Circular 8517, 43 p... environment Experience and concern are necessary for the safe and efficient use of mercury in placer gold recovery DRY PLACERS Placer deposits have been mined in the desert regions of southeastern California where very little water is available Since conventional wet methods cannot be used to recover gold in these areas, dry methods using air have been devised Dry concentration is much slower and less efficient... to facilitate fine gold recovery, but its escape into the environment must be prevented In addition to riffles, other materials are used to line sluices for enhanced recovery In the past, carpet, courdoroy, burlap, and denim were all used to line sluices to aid in the recovery of Figure 4 Usual arrangement of Hungarian riffles in a sluice From Cope, 1978 1986 PLACER GOLD RECOVERY METHODS 7 Disadvantages... consequently, reduces recovery In addition, extreme variations in feed rate occur because sand and gravel plants operate in response to demands for sand and gravel, not gold Variable feed rates may reduce gold recovery by causing recovery equipment to function erratically Finally, in most sand and gravel operations, the material mined has not been evaluated for gold content In these cases, gold recovery cannot... Small-scale placer mining methods: U.S Bureau of Mines Information Circular 6611, 18 p Dated publication outlining placer gold mining areas for the western states and basic mining methods Macdonald, E.H., 1983, Alluvial mining: Chapman and Hall, New York, 662 p Review of current practices of exploration, mining, and recovery of placer minerals McClure, S., 1982, The amazing Knelson concentrator: Gold Prospector,... and good gold recovery Disadvantages include a fairly high initial cost ($2,000 to $8,000 depending on manufacturer) and low processing rates Overall, these machines are simple, workable gold recovery units Amalgamation Although amalgamation is not strictly a recovery technique, it is used in many operations to increase gold recovery Basically, amalgamation is the practice of bringing free gold into... of flour gold is desired, pockets to hold mercury are constructed in front of the riffles A power washer of this type can 1986 PLACER GOLD RECOVERY METHODS process up to 21 cubic feet (approximately 0.8 cubic yards) of screened material an hour Hand-powered washers operated by two men can process 1 or more cubic yards per 8 hours, depending on the size of the material handled For recovery of gold, the... basic placer mining equipment and amalgamation Cassell, R., 1981, Unpublished laboratory test results: Golden State Minerals, Inc., Auburn, California, 2 p Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, 186 p Excellent report on location and history of lode and placer gold mines and mining districts in California Cope, L.W., 1978, Gold recovery. .. total gold recovered is less than -400 mesh (38 microns) According to the company, approximately 94% of the gold entering as feed is recovered Photo 12 A view of the dredge operated by Yuba -Placer Gold Company The hull is 223 feet long and the total length is 453 feet Recovery system Most literature states that jigs are effective at recovering gold down to a minimum of 200 mesh (75 microns) The recovery . CONSERVATION PLACER GOLD RECOVERY METHODS PLACER GOLD RECOVERY METHODS DIVISION OF MINES AND GEOLOGY JAMES F. DAVIS STATE GEOLOGIST SPECIAL PUBLICATION 87 PLACER GOLD. Yuba -Placer Gold Company’s Hammonton Dredge 24 Photo 13. Amalgam weighing on the dredge. 24 Photo 14. Retort used to by Yuba -Placer 25 Photo 15. Gold recovery