AMC's September 2015 technical report for Hecla's San Sebastian silver-gold property in Durango, Mexico reviews a toll treatment plan for oxide ore, with eight leach tanks, four CCD thickeners and a new Caro's acid cyanide destruction step.
The San Sebastian Ag-Au Property lies in Durango, Mexico. AMC Mining Consultants (Canada) Ltd. prepared the Preliminary Economic Assessment technical report for Hecla Canada Ltd. on behalf of Hecla Mining Company, with an effective date of 8 September 2015. San Sebastian mineralized material will be processed under a toll treatment arrangement in the existing Velardeña Mill. The mill equipment consists of primary and secondary crushing, grinding, agitated cyanide leach, counter-current decantation (CCD), cyanide destruction, Merrill-Crowe, and smelting. The design basis is 400 dry tonnes per day of oxide material, or 146,000 dry tonnes a year, hauled by highway from the San Sebastian mine. The oxide material comes from the Francine Extension, North Vein and Middle Vein, and metallurgical testing found it responsive to cyanide leaching for gold and silver. The word "ore" is used in the section for metallurgical convenience, and the report says no Mineral Reserves have been estimated.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Design throughput | 400 | dry t/d | 146,000 dry t/y |
| Crushing availability | 75 | % | 66.7 t/h average |
| Grinding availability | 92 | % | 18.1 t/h |
| Grind size | 74 | µm P80 | Single-stage ball mill |
| Ball mill circulating load | 300 | % | Closed circuit |
| Bond ball mill work index | 17.8 | kWh/t | 16.1 kWh/short ton |
| Leach tanks | 8 | tanks | 8 m x 8.5 m, 2,893.8 m3 |
| Leach time, required / actual | 96 / 113.9 | h | Previous operation 131 h |
| Cyanide strength | 1.5 | g/L NaCN | Previously 3.5 g/L |
| CCD thickeners | 4 | units | 18 m diameter |
| Gold recovery | 90.8 | % | Deposit average, 5.0 g/t |
| Silver recovery | 85.6 | % | Deposit average, 819 g/t |
| Cyanide destruction feed / discharge | < 500 / < 50 | ppm CNwad | Caro's acid |
| Power / fresh water | 2.3 / 1.9 | MW / L/s | Not applicable |
Overview
The Velardeña Mill already has crushers, a grinding circuit, leach tanks, CCD thickeners, a Merrill-Crowe circuit and a smelting furnace. AMC's equipment review concludes that it appears capable of processing 400 t/d of San Sebastian oxide ore, though M3 Engineering made no site visit to confirm installed equipment or hydraulics. Capital is limited to a cyanide destruction circuit, missing equipment and a lift to the tailings storage facility.
Key Process Stages
Crushing and grinding
A front-end loader reclaims oxide material from a run-of-mine stockpile. A jaw crusher and a cone crusher, each with a screen at the feed end to scalp material, make the product for a single-stage ball mill in closed circuit with a hydrocyclone, grinding to 80% passing 74 µm. The installed equipment is a 24 in by 36 in Allis Chalmers jaw crusher of 100 hp, a 4 ft Symons standard cone crusher of 150 hp, a 6 ft by 10 ft sizing screen, a 1,000 t fine ore bin and a 10.5 ft by 13 ft, 800 hp Allis Chalmers grinding mill with a Krebs cyclone cluster. The report puts required grinding power at 530 hp against 800 hp installed.
Leaching
Cyclone overflow goes to a pre-leach thickener. Underflow is pumped to the first of eight agitated leach tanks in series, where sodium cyanide and lime are added, and thickener overflow goes straight to the Merrill-Crowe circuit. The tanks operate at 50% solids and pH 10.5 to 11.5. The proposed cyanide strength is 1.5 g/L against 3.5 g/L in Hecla's earlier operation, and the actual leach time of 113.9 hours exceeds the 96 hours required. Gravity concentration tests did not improve 96-hour recovery for either the Middle Vein or the East Francine Vein.
CCD and Merrill-Crowe
Leached slurry feeds four 18 m thickeners in counter-current decantation, with reclaim solution as wash in the last one. The wash ratio rises to 4 from 2 in the earlier operation. Pregnant solution is clarified and de-aerated, and zinc dust added to it produces a precious metal precipitate. The precipitate is filtered on two Perrin precipitate filters and batch smelted to make silver and gold doré. The Merrill-Crowe flow rises from 64 to 103 m3/h, and the unit flow rate on clarification filters goes from 0.60 to 0.97 m3/h per m2.
Cyanide destruction and tailings
Final CCD underflow feeds a cyanide destruction tank. Testwork showed both SO2-air and Caro's acid to be viable, and the report selects Caro's acid with lime for reagent consumption purposes, with a design residence time of 0.5 hours at pH 9. Water from the tailings pond is reclaimed to the mill. The capital estimate covers an SO2-air circuit using existing tanks, agitators and compressor, plus an electromagnet, samplers, a ball mill gear lubrication system and new pumps, with a new lift for tailings storage facility No. 3 designed from Stage 1 and Stage 2 models by Golder Associates.
Additional Interesting Data and Summary
Minera Hecla's recovery projections for the three veins are 90.3% gold and 85.6% silver for the Middle Vein, 90.8% and 85.2% for the North Vein, and 91.3% and 85.6% for the East Francine Vein. The tonnages behind them are 125 kt, 62 kt and 61 kt, or 248 kt in total, at average grades of 5.0 g/t gold and 819 g/t silver. The QP judged the projections reasonable. Cyanide demand matters: silver recovery from the North Vein depended heavily on cyanide concentration, and finer grinding improved recovery for the North and East Francine Veins.
Estimated consumables per tonne of feed are 5.79 kg of hydrogen peroxide, 5.79 kg of sulphuric acid, 3.38 kg of lime, 1.50 kg of sodium cyanide, 0.48 kg of zinc dust, 0.25 kg of flocculant and 2.05 kg of grinding balls. In 2003, the previous Hecla operation at Velardeña treated an average of 374.5 t/d, or 136,693 t/y.
Key Processes
- Jaw and cone crushing with scalping screens
- Single-stage ball milling in closed circuit with cyclones
- Eight-tank agitated cyanide leaching
- Four-stage counter-current decantation
- Zinc dust precipitation in a Merrill-Crowe circuit
- Cyanide destruction with Caro's acid and lime
Source: Technical Report for the San Sebastian Ag-Au Property, Durango, Mexico for Hecla Mining Company, September 8, 2015.
Project website: San Sebastian Ag-Au Property
