This report presents the processing and recovery methods proposed for the Lincoln Mine Project, including metallurgical testing summaries and a conceptual mill flowsheet.
Report context
This updated technical report for the Lincoln Mine Project, Amador County, California, was prepared with an effective date of July 2, 2015. The report describes a processing facility designed for gravity concentration and flotation of gold-bearing ore, with supporting metallurgical testwork conducted at multiple laboratories since 1989. The report also documents construction of a mill on the project property and identifies necessary modifications following initial operations.
Processing route
Crushing and screening
The proposed processing route begins with mine trucks delivering run-of-mine material to drive-over truck dump bins of approximately 400 tons live capacity. From these bins, feed and transfer belts deliver material at 100% minus 6 inches to a single-deck vibrating screen, which removes minus 0.5-inch material ahead of crushing. Oversize material reports to a 12-inch by 24 to 36-inch jaw crusher set at a 1-inch discharge gap.
The screening and crushing section will be completely enclosed and fitted with a water mist dust suppression system to eliminate airborne particulates. Discharge from the jaw crusher is transported to a 400 to 500-ton live-capacity crushed ore bin by covered conveyor belts. The first conveyor run is 80 feet long at no greater than a 17-degree angle, transferring to a 90-foot belt, also at no greater than a 17-degree angle.
The report notes that TPH, as part of a mill audit conducted for SGM, has suggested adding a secondary crusher to the plant prior to screening the product. Construction of a new crushing system is to be inside a new building on the north side of the mill, and the existing jaw crusher would be relocated to that new building. Final screen product is designed to be 100% passing 0.5 inches.
Grinding circuit
A variable-speed belt feeder supplies crushed material from the bottom of the crushed ore bin to a 5-foot-diameter by 10-foot-long rod mill operating in closed circuit. The rod mill discharges to an 8-mesh trash screen. Ground material flows by gravity to a centrifugal concentrator fitted with a 10-mesh feed screen, which produces a rougher gold concentrate and rougher gravity tailing. The rougher gravity tailing discharges to a sump and is pumped to a hydrocyclone. Cyclone oversize, coarser than 100 mesh, returns to the rod mill, closing the grinding circuit.
Cyclone overflow at 80% passing 100 mesh reports to the rougher flotation cells. The mill is designed to operate at a rate of 210 tons per day on a five-day week, or 150 tons per day equivalent operating seven days per week.
The report notes that during 2013 to 2014 operations, approximately 1,000 tons of low-grade material was processed, but the rod mill produced excessive fines and the processing rate through the grinding circuit was much lower than expected. A revised grinding circuit was identified as a required modification.
Gravity concentration
The gravity circuit uses a centrifugal concentrator (Knelson type) followed by a finishing table. Rougher concentrates from the Knelson concentrator discharge to a dewatering screw feeder and feed a small regrind mill, 24 inches in diameter by 36 inches long. Discharge from the regrind mill feeds a finishing table for production of a high-grade final gold concentrate.
The mill is expected to produce approximately 10 pounds of gravity final product per day. This high-grade concentrate is amenable to direct smelting and will be processed onsite into doré bars for sale to a refinery. Tailings from the finishing table return to the main grinding circuit sump for recycling, or to the flotation concentrate if grade is sufficient.
Planned modifications to the gravity circuit include changes to the coarse gold jig, relocation of the Knelson concentrator, and changes to the Wilfley table. A gold room expansion is needed to accommodate installation of an existing 6 by 16 Wilfley table, automate processing, and provide increased security.
Flotation circuit
The flotation circuit consists of rougher and one-stage cleaner flotation cells, specifically Denver #21 Sub-A or equivalent cells purchased on the used market. The circuit is designed to recover fine gold that the gravity circuit misses and to eliminate arsenic in final tailings to a level below limits consistent with California Class B solid waste requirements. Cleaner tailings may be returned to the grinding circuit for improved separation of middlings particles, though this is not shown on the flowsheet.
The Lincoln-Comet ores contain arsenic as arsenopyrite with minor pyrite, approximately 2.1 percent or less of total material. Pyrite is floated with amyl-xanthate after activation with copper sulfate at pH 7 to 8. A collector, specifically Aerofloat 208 or 3477 or equivalent, is used to optimize free gold recovery. Pine oil serves as the frother. Copper sulfate is added directly to the rod mill along with pH modification as necessary. Xanthate is also added to the rod mill, eliminating the need for a conditioner tank. Frother and Aero promoter are added directly to the feed to the rougher flotation circuit.
Flotation concentrate is transferred directly to a 4-foot, two-disk rotary disk filter, which dries the concentrate to approximately 12% moisture on a water-weight basis. Thickening of the product prior to drying is considered unnecessary at this plant size. The product discharges from the disk filter into sack-type containers and is stored on-site until at least one truckload accumulates. The approximately 22-ton shipment is transported for further processing by a second party.
Planned flotation modifications include float flow reorganization and addition of a conditioning tank. Process control and instrumentation additions are described as essential to maintain process control and determine metallurgical balances.
Tailings handling
Final flotation tailings are pumped to a 50 to 60-foot-diameter thickener for reclamation of process water and production of mine hydraulic sand backfill. The thickener provides approximately one day of storage of thickened tailings for direct placement as fill underground. Alternatively, thickened tailings are filtered into cake for storage under cover for later use as fill, or permanently impounded in the dry tailings surface fill unit.
Tailings from flotation assayed 377 ppm arsenic in testing at Kappes, Cassiday and Associates in 1997, below the 500 ppm limit for Class B solid waste imposed by the State of California. The report notes this must be monitored closely with alternative plans developed should arsenic levels exceed the limit.
Required tailings modifications include rebuilding tails handling using a new slimes thickener and filter press at the sand building, with the old thickener decommissioned and used as a process water storage tank.
Products
Two products will be created by the proposed mine. The greater proportion of gold will be recovered in the gravity circuit, estimated at 84%. This product is of extremely high gold grade and will be smelted onsite to produce doré. The remaining recoverable gold will be recovered in the flotation circuit and report to the bulk flotation concentrate.
Doré is generated at less than one pound per day and can be shipped via U.S. Postal Service or secure private courier. Johnson-Matthey was contacted regarding potential sale of this product; quoted terms are a treatment charge of $1.00 per ounce with a $1,000.00 minimum charge. Payment of gold content is 99.75% of spot price on the day of settlement.
Flotation concentrate will be dried, bagged, and transported via truck to either a pressure-oxidation treatment facility in Nevada or a pyrometallurgical smelter elsewhere. Newmont Mining Corp. indicated interest in purchasing flotation concentrate at approximately 85% of contained gold at the 30-day LME PM spot price, and payment of 85% of contained gold is used for economic assessment purposes.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Design mill throughput | 210 tpd (5-day week); 150 tpd (7-day week) | Proposed design |
| Gravity concentrate production | Approximately 10 lb/day | Expected |
| Flotation concentrate shipment size | Approximately 22 tons | Design |
| Grind size to flotation | 80% passing 100 mesh | Design |
| Overall gold recovery, gravity plus flotation (testwork) | 97.9% | Testwork, Lincoln-Comet material |
| Gravity recovery range (testwork) | 72.4% to 82.1% of gold in whole ore | Testwork |
| Assumed gravity recovery for economic analysis | 70% of mill head gold | Assumption |
| Assumed flotation recovery for economic analysis | 26% of mill head gold | Assumption |
| Total combined recovery used in PEA | 96% | Assumption |
| Rougher gravity concentration ratio | 370:1 | Testwork/assumption |
| Flotation concentration ratio | 54:1 | Testwork/assumption |
| Gravity concentrate grade range (testwork) | 49.738 to 72.874 oz Au/ton | Testwork, bulk ore grade sample |
| Gravity cleaner concentrate grade (confirmatory test, drill cores) | 863.351 to 1195.858 oz Au/ton | Testwork |
| Coarse gold particle size | Up to approximately 10M | Microscopic examination |
| Flotation tailings arsenic level | 377 ppm | Testwork, 1997 |
| Estimated mill repair cost | $1,853.0 thousand | Cost estimate |
Project website: https://seduli.com.au/lincoln-mine/
Technical qualifications
A number of technical limitations and qualifications apply to the information in this report.
Allihies Engineering Incorporated does not confirm or take responsibility for any past, current or future operations or detailed designs. The proposed mill of the 2011 technical report was constructed, but after processing about 1,000 tons of low-grade material it was determined that modifications to the existing mill would be required to achieve production goals.
Gravity recoveries obtained during laboratory testing using centrifugal concentrators are normally significantly higher than would be expected by gravity concentration in a commercial processing plant, and reported recoveries do not include discounts for possible gold losses during subsequent concentrate processing.
The grade of concentrates produced in the proposed mill will vary from testwork values depending on the head grade mined.
Arsenic levels in flotation tailings must be monitored closely, and alternative plans developed should levels exceed the 500 ppm limit for Class B solid waste in California.
Gold recovery during the 2013-2014 processing of approximately 1,000 tons of low-grade material was very low due to excessive fines produced by the rod mill. Mill throughput was much lower than expected, and various process upsets were experienced. Modifications are required including new tailings handling, secondary crushing, grinding circuit revisions, gravity and flotation circuit changes, gold room expansion, and process control instrumentation.
Source: Updated Technical Report on the Lincoln Mine Project, Amador Co., CA, prepared by Mine Development Associates, July 2, 2015, Section 17.0 Recovery Methods and associated subsections.
