This NI 43-101 feasibility study update describes the proposed gold processing plant design for the Hardrock Project, based on extensive testwork and industry-standard criteria.
Report context
The Hardrock Project, located in Ontario, Canada, is the subject of a NI 43-101 Technical Report Feasibility Study Update dated January 2021. This report presents the recovery methods section (Section 17) outlining the proposed process plant design criteria, flowsheet, equipment selection, and supporting testwork results. The design is based on testwork outcomes, trade-off studies, client and vendor recommendations, and industry practices.
Processing route
Proposed Process Plant Design Criteria
The plant is designed to ramp up to a nameplate capacity of 27,000 t/d in approximately one year at a grind size of 80% passing (P80) 90 µm. The mill operation schedule is 24 h/d, 365 d/y with an overall availability of 92%. Crushing plant and processing plant equipment design factors allow for a margin of error in equipment sizing, used in calculations of equipment feed rates and residence times. Key general process design criteria are presented in Table 17.1 of the report.
The comminution testwork program determined grinding characteristics for various lithologies. Based on the run-of-mine expected composition, weighted averages were calculated to establish plant feed grindability parameters. The 75th percentiles of hardness are used for design purposes to ensure sufficient equipment capacity to handle process variations.
The ore hardness data available when the ball mill design was selected was measured on composite samples made up of a blend of 53 different core intervals from different lithologies representing the entire deposit. The weighted average of the composite samples Bond ball mill work index (BWI) obtained was 15.5 kWh/t, which was used for the design of the ball mills. Modified BWIs, considered more accurate for prediction of grinding circuit behavior, were also measured on samples from various lithologies. The overall 75th percentile result obtained was 15.5 kWh/t, confirming the design value as satisfactory and conservative.
The HPGR also generates microcracks on ore particles which typically reduces the power required at the ball mill. The microcrack effect was not considered when designing the ball mills, providing additional contingency.
Grind Size Determination
Cyanidation testwork established a correlation between grind size and gold recovery whereby a finer grind results in higher recovery. The Global Composite, considered the most representative of run-of-mine over the life-of-mine, was used to determine the optimal grind size. Analyses were also conducted on results from leach tests on Variability Composites A to I, the Master Composite (representing the feasibility study mine design for the first three years of operation), and Low Grade Composites to evaluate the impact of ore variability.
Economic evaluations completed in 2014 showed that a grind of P80 = 72 µm corresponds to the highest net revenue of additional recovery over incremental costs but is constrained at 24,000 t/d with the ball mills selected. At 27,000 t/d, a compromise is made between grind fineness and throughput and a 90 µm grind is considered optimal. The lower recovery between 72 µm and 90 µm is offset by the higher revenue with increased throughput. Lower throughputs during production ramp-up will allow for finer grinds and higher recovery in this period.
Impact of Mineralogical Composition on Leach Performance
Leach testing indicated that refractory arsenopyrite content in the deposit may correlate to recovery. A multivariate linear regression analysis was used to determine the correlation between leach residue grade and mineralogical composition. The results of leach tests conducted during the feasibility study stage and the basic engineering stage were used as the basis for the analysis.
The residual gold grade from the leach testwork was found to be highly correlated to the gold, arsenic, and sulfur head sample grades. The strong correlation between residual gold grade and arsenic and sulfur head grades suggests that arsenopyrite (FeAsS) contains refractory gold which is not recovered via leaching.
A weaker correlation between leach residue grade and grind size was also established. The weak correlation was from the lack of variability in the tested grind sizes (most tests were conducted around the optimal grind size).
The impact of grind size along with gold, arsenic, and sulfur head grades on the residual gold grade was modeled via a multivariate linear regression equation. This formula has been used in the block model and open pit optimization process to calculate the gold recovery of each individual block based on chemical composition and grind size. The overall gold recovery can then be computed for a determined time period.
Proposed Crushing Circuit
The crushing plant is a two-stage circuit consisting of a primary gyratory crusher and a secondary cone crusher, with a design availability of 67%. A 20% design factor has been selected such that crushing circuit equipment is sized to handle up to 2,025 t/h.
Run-of-mine ore is delivered by mine haulage trucks to a steel dump pocket feeding the primary crusher. The 1,300 mm x 1,800 mm 450 kW gyratory crusher crushes the ore from a 1,000 mm top size (275 mm P80) to a P80 of 120-160 mm product. The primary crusher operates with an open side setting of 160 to 200 mm.
The secondary crusher is a 950 kW standard cone crusher with a 45-60 mm closed side setting, installed in closed-circuit with a double deck screen to control the top size feeding the HPGR. The crushing circuit produces a final crushed product with a 50 mm top size and a 35 mm P80.
Proposed HPGR/Grinding and Gravity Recovery Circuit
The HPGR/grinding circuit crushes and grinds ore to the optimal grind size to maximize gold recovery in the leach and carbon-in-pulp circuit. The grinding and gravity circuit consists of two parallel operating lines, each consisting of a wet screen, ball mill, and gravity concentrator in closed circuit with cyclones.
A detailed comminution trade-off study recommended a two-stage crushing circuit followed by HPGR and ball milling circuit over other typical comminution flowsheets such as crushing followed by SAG milling and ball milling, to reduce throughput risk and increase energy efficiency from the high hardness levels measured in testing.
The HPGR is equipped with two 2,650 kW motors for a total of 5,300 kW. The HPGR roll dimensions are 2.2 m in diameter by 2.0 m in length with a rotating speed of 22 rpm. The HPGR circulating load is expected to be nominally 85%, with a design value of 110%.
The grinding mills are twin pinion ball mills equipped with motors totaling 10,500 kW per mill. Both mills are 6.7 m in diameter (inside liners) by 12.3 m in length (EGL). The ball milling circuit recirculating load is estimated at 300%, with a design value of 350% for pump selection.
Two gravity screens and two gravity concentrators are installed to process material for recovery of gravity recoverable gold. The gravity concentrate from both concentrators is transferred to a single gravity concentrate intensive leaching circuit, where 98% dissolution efficiency is expected.
Proposed Pre-Leach, Leach and Carbon-In-Pulp Circuit
The circuit consists of a pre-leach thickener, a series of leach tanks (one pre-leach and seven leach tanks) followed by seven CIP tanks. The 50 m diameter thickener increases slurry density from 35% to 55% solids and recycles water to the grinding circuit.
The leach circuit has a total residence time of 28 hours with an additional four hours available in the pre-aeration tank. Pre-aeration provides passivation of reactive sulphide minerals, minimizing their impact on cyanide consumption. Oxygen from the on-site oxygen plant is injected to reach a targeted 15 mg/L concentration of dissolved oxygen.
The CIP circuit is composed of seven CIP tanks (six operational) with a total retention time of 1.5 hours. The circuit is designed and operated in carrousel mode. Carbon concentration is maintained at 50 g/L in each operational tank.
Proposed Cyanide Destruction
The cyanide destruction circuit consists of two agitated reactors operating in parallel using the SO₂/air process to reduce weak acid dissociable cyanide concentration in the tailings slurry to less than 10 mg/L. Total retention time of 138 minutes is provided.
Proposed Elution and Carbon Regeneration
Loaded carbon is stripped using the pressure Zadra process with a heated diluted caustic (1.0% NaOH) and cyanide (0.1% NaCN) solution circulated through the column at a temperature of 140°C. A carbon elution cycle is completed within eight to twelve hours.
Proposed Power and Water Systems
The annual power consumption for the mill and ancillaries is estimated to be 299.1 GWh at 27,000 t/d. Power is to be supplied from an on-site power plant at 13.8 kV, consisting of seven gas-fired engine generating units in N+2 configuration.
Most of the water required for process plant operations is recycled from the Tailings Management Facility. No water is planned to be withdrawn from Kenogamisis Lake. Fresh water requirements come from underground workings collected in a water equalization pond.
Key reported parameters
| Parameter | Units | Value | Basis |
|---|---|---|---|
| Throughput – Design | t/y | 9,855,000 | Design |
| Throughput – Design | t/d | 27,000 | Design |
| Throughput – Design | t/h | 1,223 | Design |
| Design Grind Size (P80) | µm | 90 | Optimal based on testwork |
| Crusher Utilization | % | 67 | Design |
| Process Plant Availability | % | 92 | Design |
| Au Feed Grade – Average | g/t | 1.34 | Design |
| Au Feed Grade – Design | g/t | 2.10 | Design |
| Gold Recovery | % | 91.0 | Design |
| Bond Ball Mill Work Index (BWI) 75th percentile | kWh/t | 15.5 | Testwork weighted average |
| Bond Rod Work Index (RWI) 80th percentile | kWh/t | 16.8 | Testwork |
| HPGR Specific Grinding Force | N/mm² | 3.2 | Testwork |
| HPGR Circulating Load – Nominal | % | 85 | Design |
| HPGR Circulating Load – Design | % | 110 | Design |
| Ball Mill Recirculating Load – Estimated | % | 300 | Design |
| Ball Mill Recirculating Load – Design | % | 350 | Design for pump selection |
| Leach Circuit Residence Time | hours | 28 | Design |
| Pre-Aeration Tank Residence Time | hours | 4 | Design |
| CIP Circuit Retention Time | hours | 1.5 | Design |
| Annual Power Consumption | GWh | 299.1 | Estimated at 27,000 t/d |
| Cyanide Destruction Retention Time | minutes | 138 | Design |
| CN WAD Target | mg/L | <10 | Design |
| Gravity Concentrate Leach Dissolution Efficiency | % | 98 | Expected |
Project website: http://www.ontario.ca/page/hardrock-gold-mine
Technical qualifications
The report notes that the limits of application for the multivariate linear regression model used to calculate gold recovery are set based on the range of arsenic and sulfur grades of the samples tested. When the Au(g/t)/As(%) ratio is less than 8 or the Au(g/t)/S(%) ratio is less than 0.4, corrected values must be calculated using the minimum ratio and the head gold grade, as anything below these limits is considered outside the model's range.
The design assumes that run-of-mine ore follows the global composite composition. Ore variability is evaluated using variability composites, but the relationship between mineralogical composition and leach performance was established from specific testwork composites that may not capture all deposit variability.
The HPGR microcrack effect on ball mill power reduction was not considered in ball mill design calculations, providing additional contingency but meaning actual power requirements may be lower than design values.
Source: Hardrock Project NI 43-101 Technical Report Feasibility Study Update, January 2021, Section 17: Recovery Methods.
