This report describes the processing of South Vein mineralized material at the New Jersey Mill in 2012 and early 2013, as part of the Preliminary Economic Assessment for the Crescent Silver Project.
Report context
This section of the NI 43-101 Technical Report, dated September 24, 2013, documents recovery methods, processing plant design, and metallurgical results from the Crescent Silver Project near Kellogg, Idaho. Mineralized material from the South Vein was mined from three sublevels at approximately 3,940, 3,890, and 3,840 ft amsl, stockpiled separately by sublevel, and hauled to the New Jersey Mill. A total of 12,607 dry tons of South Vein material was processed from June 2012 through April 2013 under a joint venture between New Jersey Mining Company (NJMC) and USC.
Processing route
Plant Design and Expansion
In early 2011, NJMC and USC formed a joint venture to expand and operate the New Jersey Mill to a capacity of 440 t/d (18.4 t/h). Construction started in spring 2011 and was completed in June 2012. The new mill was designed to produce a single flotation concentrate and included a new crushing plant, larger mill building, new fine mill feed bin, new ball mill, refurbished large rougher flotation cell, new cleaner flotation cells, new paste thickener, concentrate filter, new pumps, and associated conveyors and piping. The expanded mill was commissioned in late June 2012 with South Vein mill feed.
Crushing Circuit
Run-of-mine material is fed by front-end loader into a bin with a belt feeder that feeds into a 15 inch by 24 inch jaw crusher. Feed is crushed to minus 3 inches and conveyed to a vibratory screen with a 0.5 inch by 2 inch slotted screen cloth. Screen undersize drops into a 385 t fine mill feed bin while oversize is conveyed to a Metso HP100 cone crusher with a closed-side setting of about 0.5 inch. The cone crusher product is conveyed back to the screen in closed loop for rescreening. Crushing plant throughput averaged about 66 t/h.
Grinding Circuit
The fine mill feed bin discharges onto a bin discharge belt conveyor, which discharges onto the ball mill feed belt conveyor. A weightometer on the ball mill feed belt displays real-time feed rate in tons per hour. A variable-frequency drive (VFD) on the under-bin conveyor allows operators to manually adjust feed rate. A screw-type lime feeder, also VFD-controlled, adds lime at about 1.12 lb/t to the ball mill feed to depress pyrite in flotation.
The ball mill is a 250 kW Marcy-type mill, 8 ft in diameter and 13 ft long. Mill feed enters via a spout feeder. A trommel screen on the discharge removes pebbles, wood, and debris. Ball mill discharge is pumped to a 10 inch Warman hydrocyclone; underflow returns to the mill for additional grinding while overflow flows by gravity in a 3 inch diameter pipeline to rougher flotation cells. A mill circuit survey determined the Operating Work Index to be 9.65 kWh/t. At the time of survey, the ball mill produced cyclone overflow with 80% passing 72 µm from feed that was 74% passing 0.25 inch. The grind achieved during the South Vein campaign was significantly finer than previous testing and bulk sampling (80% passing 72 µm versus 80% passing 86 to 100 µm).
Flotation Circuit
Cyclone overflow slurry is sampled every two minutes by an automatic sampler (3 ml sample size). Slurry is fed to a vibratory screen for trash removal before entering the rougher flotation circuit. Flotation reagents are fed via metering pumps: the collector Z-55 and a 10% NaCN solution to depress pyrite. The rougher circuit consists of one Wemco 144 and five Wemco 66D flotation cells. Rougher concentrate feeds two banks of three Wemco 36 cleaner flotation cells operated in series for two stages of cleaning. No reagents are added at the cleaner cells.
Two collectors, Z-55 and Aerofloat 242, were tested during the campaign with no significant difference in silver recovery or concentrate grade. Z-55 was selected as primary collector because it does not require MIBC for frothing, resulting in cost savings. Lime was the most important reagent for optimizing concentrate grade. When circuit pH fell below 8.0, more pyrite reported to final cleaner concentrate, reducing silver grade. When pH exceeded about 8.5, froth had darker color and silver grade increased by about 63%.
Both series and parallel modes of cleaner cell operation were tested. Series operation produced higher silver concentrate grade (414 oz/t) compared to parallel operation (358 oz/t) at approximately the same head grade, so series operation was adopted.
Concentrate Handling and Tailings Disposal
Final concentrate from the second stage of cleaner cells is piped to a 5 ft diameter thickener, then transferred to agitated storage tanks. Concentrate is either pumped into a concrete mixing truck for delivery to a local refinery or filtered with a plate and frame pressure filter and placed in 1 t supersacks for delivery to a foreign smelter.
Tailings from the rougher flotation circuit are pumped into two 12 ft diameter Deep Cone Thickeners (DCT) operated in parallel to produce paste tailings. Diluted flocculant (Z-Floc 565, selected from bench testing) is added at 0.102 lb of dry flocculant per ton of mill feed. Overflow is recycled to the mill circuit with no adverse metallurgical effects. Underflow density ranged from 60% to 70% solids, classified as paste at the high end. Underflow is pumped via peristaltic hose pumps to a paste tailings stack about 500 ft east of the mill building, forming a slope of about 6% with very little bleed water. This is the first application of paste tailings disposal in the Coeur d’Alene mining district.
Operating Results and Reconciliation
The overall silver recovery to final concentrate was 89.5% with an average concentrate grade of 340.67 oz/t. Recovery varied significantly by mining sublevel and increased with decreased oxidation at depth. The 3940 sublevel had the highest visible oxidation and 85.3% recovery; the 3840 level had the least oxidation and 95.1% recovery. Head grades were similar between these levels. Other process variables, such as grind and flotation reagents, remained relatively constant during processing of the sublevels.
A total of 250.6 t of concentrate were produced from 12,607 t of mill feed for a ratio of concentration of 50.3. Concentrate was sold to Essential Metals of Kellogg, Idaho and Ocean Partners of Wilton, Connecticut. Copper content was not included as a saleable product; copper assays varied from 13% to 15%.
Mine to mill reconciliation showed the mill reported significantly lower grade and contained ounces than calculated from chip samples, especially for the 3940 I-drift. After improving the materials handling process, the discrepancy reduced to 13% of contained ounces for the 3890 and 3840 I-drifts. The remaining discrepancy is believed due to sampling bias, loss of fines in handling, incorrect tonnage factor, or a combination.
In September 2012, SRK Consulting reviewed USC’s sampling procedures and recommended individual chip samples be limited to no more than 2.0 ft in width and weigh at least 5 lb, and that test drifts be mapped in greater detail each day.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Total South Vein mill feed processed | 12,607 dry tons | Historical operating data, 2012-2013 |
| Overall silver recovery to concentrate | 89.5% | Historical operating data |
| Average concentrate silver grade | 340.67 oz/t | Historical operating data |
| Ratio of concentration | 50.3 | Historical operating data |
| Total concentrate produced | 250.6 dry tons | Historical operating data |
| Mill design capacity | 440 t/d (18.4 t/h) | Proposed design |
| Crushing plant throughput | ~66 t/h | Historical operating data |
| Ball mill Operating Work Index | 9.65 kWh/t | Mill circuit survey |
| Cyclone overflow grind (80% passing) | 72 µm | Historical operating data |
| Lime addition rate | 1.12 lb/t | Historical operating data |
| Z-55 collector addition rate | 0.040 lb/t | Historical operating data |
| Sodium cyanide addition rate | 0.015 lb/t | Historical operating data |
| Steel (grinding balls) consumption | 1.50 lb/t | Historical operating data |
| Makeup water consumption | 102 gallons/t | Historical operating data |
| Flocculant dosage (Z-Floc 565) | 0.102 lb/t mill feed | Testwork and historical operation |
| Paste tailings underflow density | 60% to 70% solids | Historical operating data |
| Tailings stack repose angle | ~6% | Historical operating data |
| Silver recovery by sublevel: 3940 | 85.3% | Historical operating data |
| Silver recovery by sublevel: 3890 | 89.6% | Historical operating data |
| Silver recovery by sublevel: 3840 | 95.1% | Historical operating data |
Project website: https://www.mining.com/united-silver-reorganizes-says-expansion-on-target-for-q4/
Technical qualifications
This technical report section is based on a Preliminary Economic Assessment (PEA) and includes proposed design parameters for the expanded mill, historical operating data from 2012-2013, and testwork results from bench testing and mill surveys. The report acknowledges several limitations: copper assays were not obtained for the final 30% of concentrate tonnage, so copper head grade was not calculated; copper content was not a saleable product; electron microprobe analysis of tailings was recommended to confirm if oxidation was the primary factor in variable recovery; and mine-to-mill reconciliation showed discrepancies attributed to sampling bias, fines loss, or tonnage factor issues.
Source: Crescent Silver Project , NI 43-101 Technical Report, Preliminary Economic Assessment, September 24, 2013. Relevant sections: 17 Recovery Methods, 17.1 Operation Results, 17.2 Processing Methods, 17.3 Plant Design and Equipment Characteristics, 17.4 Consumable Requirements.
