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.
Consolidated report summaries
The following former standalone pages about this operation were consolidated here in October 2026. They are retained for source traceability; dates and figures reflect each report’s own effective date and may be superseded by the current information above.
BacTech Planned to Bioleach Snow Lake’s Arsenical Gold Residue and Lock Up the Arsenic
The 2012 PEA for the Snow Lake reclamation project in Manitoba set out a small bacterial oxidation plant that would turn a 9.7 g/t gold residue stockpile into a gold-bearing filter cake and stable ferric arsenate.
The BacTech Snow Lake reclamation project sits within the town of Snow Lake in west-central Manitoba, about 685 km north of Winnipeg, on the former New Britannia and Nor-Acme mine site owned by QMX Gold Corporation. Its target is a gold residue stockpile (GRS) of refractory, arsenic-rich mill tailings that was kept apart from other tailings in the hope that new technology would one day recover the gold. The Province of Manitoba took ownership of the residue in 2004. On April 25, 2011, the province gave BacTech Environmental Corporation the right to remediate it, and BacTech would keep the recovered gold as payment. The preliminary economic assessment, with an effective date of 27 August 2012, describes a development project using BacTech's licensed bioleaching technology. At that date, none of the required permits had been obtained, and the report gives no construction date.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Measured resource | 265,000 | tonnes | 9.7 g/t gold and 2.17 g/t silver |
| Feed gold grade | 9.68 | g/t | Process design criteria |
| Feed arsenic content | 21.86 | % | Process design criteria |
| Feed iron content | 25.18 | % | Process design criteria |
| Stockpile moisture | 16 | % | Measured in 2010 drilling |
| Annual feed rate | 39,186 | dry tpa | 46,650 wet tpa |
| Plant feed rate | 100 to 125 | t/d | Front-end loader to feed hopper |
| Regrind target | 75 | µm P80 | Polishing mill and cyclones |
| Bioleach feed density | 10 | % solids | After in-line dilution |
| Bioleach reactors | 6 | Not applicable | 979 m3 working volume each |
| Bioleach residence time | 5 to 6 | days | Total |
| Pyrite oxidation | 95 | % | Minimum design level |
| Arsenopyrite oxidation | 100 | % | Design criteria |
| Reactor temperature | 40 | °C | Controlled by cooling water |
| Neutralization endpoint | 6.5 | pH | Limestone slurry |
| Process building footprint | 600 | m2 | 30 m by 20 m |
Overview
The project has three parts: the stockpile itself, a process facility and a lined storage impoundment for the neutralized arsenic precipitate. The stockpile covers about 180 m by 105 m. In May 2011 BacTech drilled 33 holes into it on a 20 m grid and sent core to Inspectorate in Vancouver for bioleach test work.
The process does not recover gold on site. Bacteria oxidize the sulphides that lock the gold, the oxidized solids are washed and filtered, and the filter cake is shipped elsewhere for precious metals extraction. The arsenic released into solution is precipitated with limestone as ferric arsenate in a matrix of gypsum and ferric hydroxide, and that precipitate goes to the impoundment.
Key Process Stages
Reclaiming the stockpile
A cap of coarse, angular mine waste covers the stockpile. It would be removed in sections so that only part of the residue is exposed to the weather at any time. The 16% moisture in the residue should limit dust, with dust control added if needed. Excavation runs at roughly 135 to 150 t/d for eight months a year to build enough stock for twelve months of plant operation. An end-dump truck carries the material about 600 m to the plant, where it is held in a weatherproof building. Both sites have water management plans to collect snow melt and rain.
Material preparation
The process building holds two grinding mills plus tanks, agitators, pumps and piping in five circuits. A front-end loader feeds residue into a hopper at 100 to 125 t/d. A grizzly removes debris before a repulp tank brings the pulp to 40% solids with wash water. A polishing regrind mill runs in closed circuit with two cyclones. Regrinding freshens the particle surfaces, brings the coarse fraction down to a P80 of 75 µm and removes residual cyanide from the earlier operation, which could otherwise interfere with the bacteria. Cyclone overflow goes to a high rate thickener, and its underflow fills an agitated feed tank with 30 hours of buffer capacity.
Bacterial oxidation
The thickened pulp is diluted in line to 10% solids and split equally between three primary reactors running in parallel. Their combined product then passes through three more reactors in series, with gravity flow in open launders between stages. All six reactors are the same size, about 10.8 m in diameter and 11.8 m high. Acid goes to the primary reactors to balance arsenopyrite oxidation, along with milligram levels of nitrogen, phosphorus and potassium as nutrients.
Fixed speed agitators with hydrofoil impellers mix the three phases, and two low pressure blowers sparge air through a ring main. The reaction gives off heat, so cooling tube bundles in each reactor hold the temperature at 40 °C and also act as baffles. Residence time totals five to six days. The design achieves at least 95% pyrite oxidation and complete oxidation of arsenopyrite and pyrrhotite.
Residue separation
Oxidized pulp goes to a residue thickener. The underflow solids are washed on a belt filter, at a wash water ratio of 3 to 1 and 95% wash efficiency. The cake, about 57 t/d at 90% solids, carries the gold and silver and is shipped off site.
Neutralization and impoundment
Thickener overflow and filtrate carry soluble iron, arsenic and acid. Four 75 m3 agitated tanks in series neutralize this liquor with limestone slurry over six hours, raising it to pH 6.5. The ferric arsenate precipitate is thickened. Clear overflow returns to the process water pond, and the underflow is pumped through one of two HDPE pipelines to a clay and HDPE lined pit next to the existing QMX tailings pond, about 1.4 km northeast. A third line returns reclaimed water to the plant for reuse as bioleach dilution water.
Additional Interesting Data and Summary
The design bioleach liquor reaches about 21.22 g/L arsenic and 23.03 g/L iron, with only 0.04 g/L copper. Two blowers rated at 629 kW each supply about 25,156 Nm3/h of air, assuming 35% oxygen use. Agitators need about 1,293 kW in total.
The precipitate is bulky. The design pumps about 515 t/d of slurry to the impoundment and generates about 3.19 t of precipitate for every tonne of feed. Limestone arrives at 12.5 mm lump size, with four days of storage.
The regrind and bioleach circuits run about 49 weeks a year, while reclamation from the stockpile runs 35 weeks. A winter stockpile of about 11,600 dry tonnes keeps the plant fed when reclamation stops.
Besides the measured resource, the 2011 estimate includes an indicated resource of 9,300 tonnes at 9.2 g/t gold and an inferred resource of 28,000 tonnes at 7.0 g/t gold. The Town of Snow Lake receives a CAD$5/oz gold royalty, and the province takes a 2% NSR once initial capital is repaid. All environmental liability stays with the Province of Manitoba.
Key Processes
- Selective excavation and covered storage of arsenical tailings
- Repulping and polishing regrind to 75 µm P80
- Thickening ahead of a 30 hour feed tank
- Six-reactor bacterial oxidation at 40 °C
- Thickening, washing and belt filtration of oxidized residue
- Off-site extraction of gold and silver from filter cake
- Limestone neutralization to ferric arsenate and gypsum
- Disposal in a lined impoundment with water reclaim
Source: NI43-101 Preliminary Economic Assessment Study for the BacTech Snow Lake Reclamation Project, Snow Lake, Manitoba, Canada, 27 August, 2012.
Project website: BacTech Snow Lake Reclamation Project
