A 2012 Preliminary Economic Assessment (PEA) for remediation of a gold residue stockpile using bacterial oxidation (BACOX) technology, producing a bio-oxidized precious-metal concentrate for off-site extraction and a neutralized ferric arsenate residue for on-site impoundment.
Report context
This profile is drawn from the NI 43-101 PEA for the Snow Lake Reclamation Project, effective 27 August 2012, prepared by BacTech Environmental Corporation. The study addresses processing of the Gold Residue Stockpile (GRS), a highly refractory sulphide material with high arsenic content located at Snow Lake, Manitoba. Metallurgical testwork was conducted in two stages: Stage I (April–September 2010) and Stage II (June 2011–March 2012). The processing route employs BACOX bacterial oxidation to liberate gold from refractory sulphides, followed by off-site shipment of the oxidized residue for precious metals extraction and on-site neutralization of bioleach liquors.
Processing route
Material handling and preparation
The GRS material, containing approximately 16% moisture, is excavated at 135–150 dry t/d over eight months to build a stockpile sufficient for 12 months of continuous plant operation. The material is transported 600 m by end-dump truck to a weather-proof storage structure. A front-end loader feeds the process at 100–125 dry t/d. Feed passes a grizzly screen for debris removal before entering a repulp tank where wash water is added to achieve 40% wt pulp. The pulped material reports to a polishing regrind mill in closed circuit with two cyclones, targeting a particle size P₈₀ of 75 µm. Cyclone overflow feeds a high-rate thickener; thickener overflow recycles to the mill process water pond. Thickened underflow feeds a 30‑hour buffer storage tank ahead of bioleaching. The wash-regrind step freshens particle surfaces, reduces coarse fraction, and removes residual cyanide species that could interfere with bacterial oxidation.
Bacterial oxidation circuit
The BACOX circuit comprises six identical reactors. Dilution water is added automatically to achieve 10% solids feed density. Pulp is divided equally via a splitter header box among three primary reactors operating in parallel, whose combined product feeds a train of three reactors in series. Pulp flows between stages by gravity using open launders. Total residence time is 5–6 days, targeting minimum 95% pyrite oxidation and complete arsenopyrite oxidation. Acid is added to primary reactors for acid balance; a nutrient mix of milligram-level nitrogen, phosphorus, and potassium is also fed.
Reactors are agitated with fixed-speed drives and hydrofoil impellers for three-phase mixing. Air is sparged through a ring main from two low-pressure blowers. The exothermic process is maintained at 40°C by regulating cooling water through tube bundles that also serve as baffles. Each reactor has a working volume of 979 m³ (diameter 10.8 m, height 11.8 m). Total reactor working volume is 5,877 m³. Two blowers deliver 25,156 Nm³/hr each at 629 kW, with total agitator power of 1,293 kW across six units.
Oxidized pulp is pumped from the final reactor to a high-rate thickener. Thickener overflow reports to neutralization; underflow solids are washed and filtered on a belt filter (3:1 wash water ratio to solids, 95% wash efficiency). The filter cake,oxidized residue containing gold and silver,is shipped off-site for precious metals extraction.
Neutralization and tailings
Bioleach liquor from the thickener overflow and filtrate from residue filtration,containing soluble iron, arsenic, and acid,are neutralized in four agitated tanks (75 m³ each) operating in series with six hours design residence time. Limestone slurry is added to raise pH to 6.5, forming ferric arsenate in a gypsum and ferric hydroxide matrix. Neutralized pulp feeds a precipitate thickener; thickened underflow is pumped 1.4 km via HDPE pipelines to a clay- and HDPE-lined impoundment adjacent to the existing QMX tailings pond. Return water from the impoundment and clean thickener overflow recycle to the process water pond for reuse as bioleach dilution water.
Metallurgical testwork findings
Diagnostic leaching confirmed refractory gold: direct CIL cyanidation yielded only 9.4% gold extraction versus 96.5% after hot nitric acid digestion followed by cyanidation. In BACOX testwork at 10% pulp solids, 95% sulphide oxidation rendered 88.6% of gold cyanide-leachable, with BacTech indicating that further work could approach 96.5%. A test at 5% solids achieved 74% sulphide oxidation with 66.8% gold recovery by cyanidation. Batch bio-oxidation at 10% pulp density reached 94.9% in 50 days, equating to approximately 6 days continuous residence time.
Key issues identified: the GRS contains significant thiocyanate (mostly water-extractable), which exacerbates negative effects of As(III) on bacterial activity. The low Fe/As ratio (1.0:1 in head) increased to 3.5:1 during bioleaching via arsenic re‑precipitation. A hydrochloric acid wash suggested the Fe(III)/As(V) precipitate would be very stable. However, TCLP leachates exceeded US EPA arsenic limits even at Fe/As molar ratio 5.5. Solution changes, oxidation prior to neutralization, or increasing Fe/As ratio to ≥3 in bioleaching were recommended.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Feed gold grade | 9.68 | g/t | Design (from testwork) |
| Feed silver grade | 2.17 | g/t | Design |
| Feed iron grade | 25.18 | % | Design |
| Feed arsenic grade | 21.86 | % | Design |
| Annual feed rate (dry) | 39,186 | tpa | Design |
| Reclamation rate (dry) | 107 | t/d | Annual average design |
| Regrind mill feed (dry) | 4.72 | t/h | Design |
| Bioleach feed solids | 10 | % | Design |
| Total pyrite oxidation | 95 | % | Design target |
| Total arsenopyrite oxidation | 100 | % | Design target |
| Total residence time | 5–6 | days | Design |
| Reactors | 6 | , | Design (3 parallel primary, 3 series) |
| Individual reactor working volume | 979 | m³ | Design |
| Reactor temperature | 40 | °C | Design |
| Blower air delivered (total) | 25,156 | Nm³/hr | Design |
| Total agitator power | 1,293 | kW | Design |
| Neutralization residence time | 6 | hours | Design |
| Neutralization pH | 6.5 | , | Design |
| Neutralization reactors | 4 | , | Design (75 m³ each) |
| Filtered washed solids density | 90 | % | Design |
| Wash efficiency | 95 | % | Design |
| Oxidized residue cake (dry) | 56.61 | t/d | Design |
| Ferric arsenate product (wet) | 360.22 | t/d | Design |
| Gold extraction (diagnostic leach) | 96.5 | % | Testwork (hot nitric + cyanidation) |
| Gold extraction (direct CIL) | 9.4 | % | Testwork |
| Gold extraction (BACOX 10% solids) | 88.6 | % | Testwork (95% sulphide oxidation) |
| Gold extraction (BACOX 5% solids) | 66.8 | % | Testwork (74% sulphide oxidation) |
| Fe/As ratio increase during bioleach | 1.0:1 → 3.5:1 | , | Testwork (10% pulp solids) |
Technical qualifications
This profile is based solely on the 2012 PEA Mineral Processing and Metallurgical Testing (Section 13) and Recovery Methods (Section 17). The study is a preliminary economic assessment; it is not a feasibility study. All processing parameters are proposed design criteria developed from batch testwork and are subject to confirmation through pilot-scale testing. TCLP results on neutralized residues exceeded regulatory limits, indicating that further flowsheet development is required for arsenic stabilization. The bio-oxidized gold-bearing residue is shipped off-site; no on-site cyanidation or gold refining is described. The report date is 2012; this is a historical snapshot and does not show current operational status, ownership, or subsequent testwork.
Source: BacTech Environmental Corporation, “NI 43-101 Preliminary Economic Assessment Study for the BacTech Snow Lake Reclamation Project, Snow Lake, Manitoba, Canada,” effective date 27 August 2012, report date 11 October 2012, Project Report #1204. Sections 13 and 17. of the original technical report.
