Sugar Zone Mine — 2021 Feasibility Study for Expansion to 1,200 t/d

Technical report examines expansion of the Harte Gold Sugar Zone process plant from 800 t/d to 1,200 t/d throughput, addressing comminution bottlenecks and product options.

Report context

This article is based on the March 2021 National Instrument (NI) 43-101 Technical Report , Feasibility Study for Expansion to 1,200 t/d for the Sugar Zone Mine, located in the Sault Ste. Marie Mining Division, Ontario, Canada. The study reviews the impact of expanding throughput to 1,200 t/d while modelling current performance using operating data from August 2020. The report evaluates options for continuing production of gold doré bar combined with a minimum 100 g/t Au-grade flotation concentrate, or producing only gold doré bar through on-site leaching.

Processing route

Existing process overview

The existing Harte Gold Sugar Zone process plant currently produces gold doré bar and a bagged gold concentrate through gravity concentration and flotation circuits, respectively. Process stages include primary jaw crushing, secondary cone crushing with intermediate screening, fine ore feed storage, grinding circuit with classification tower, gravity concentration circuit, gold room with furnace, flotation circuit, concentrate thickening and filtration with transportation loadout, tailings thickening circuit, reagents area, and associated services.

Crushing circuit upgrades for 1,200 t/d

The crushing area consists of three modules: a jaw/primary crusher unit, a sizing screener unit, and a cone/secondary crusher unit. ROM material from underground feeds directly to the Metso C80 primary jaw crusher through a grizzly and ROM bin. Material passes through a Metso ES202 double-deck vibrating screen with 38 mm top deck and 15 to 16 mm bottom deck apertures; undersize reports to the crushed fine ore feed stockpile, and oversize reports to the secondary crusher.

The study identified that bottlenecks for the 1,200 t/d throughput expansion will be the HP100 secondary crusher and the existing 800 kW ball mill. To achieve 1,200 t/d, an HP200 is required to replace the existing HP100 secondary crusher. A fourth module will be added for the new HP200, with modification at the oversize screen chute capable of feeding the HP200 module or diverting to the existing HP100 module only when the HP200 is down for maintenance.

A Bruno simulation was performed at a nominal capacity of 76.9 t/h, predicting secondary crusher load of 48%. A higher loading of 85% is recommended to choke-feed the cone crusher, predicted at 120 t/h fresh feed. The C80 jaw crusher closed side setting of 55 mm will serve the nominal feed rate; it is recommended to open it to 75 to 85 mm to choke feed the secondary cone crusher. The ES202 screen at 120 t/h shows low loading and will not become a constraint, so a screen upgrade is not required. Minor motor upgrades will be required to materials-handling conveyors. The two mill feed conveyor feeders within the tunnel under the fine ore feed stockpile will also need upgrading to slider-bed types.

Grinding circuit design

The existing grinding area comprises a single 3.05 × 5.79 m (10 ft diameter × 19 ft long), 800 kW ball mill in closed circuit with a cyclone pack designed to produce a P80 75 µm grind size.

A simulation revealed that the existing 800 kW ball mill could match the predicted crushing circuit capacity of 918 t/d only with a BWi of less than 13.5 kWh/t, and would need significantly softer ore to operate at 1,200 t/d. A second ball mill (BM2) operating with the existing ball mill (BM1) will provide expansion capacity.

Three grinding circuit configurations were studied: a new ball mill in parallel, a new ball mill in series, and a combination of parallel and series. Based on metallurgical performance, construction complexity and capital cost considerations, the new ball mill in series was selected as the preferred option for the 1,200 t/d expansion.

The series configuration will have ore feed delivered to the BM1 feed chute with process water. BM1 will work in open circuit, discharging into the existing gravity-feed pump box together with BM2 discharge slurry. The gravity circuit and classification circuit routing will be the same as the present circuit. Cyclone underflow will be directed to BM2's feed chute, with BM2 operating in closed circuit with the cyclones. Cyclone overflow will report to flotation at the desired grind target.

A 3.09 m diameter × 4.60 m EGL ball mill with a 550-kW variable-speed drive (VSD) was chosen as the second ball mill, operating in series downstream of the existing 3.09 m diameter × 5.6 m EGL ball mill with 800 kW VSD. The selection of a VSD for BM2 will help optimize power draw in case of feed-blend hardness changes. It is recommended that the mill be operated at a higher-than-26% ball charge, with the mill slowed to cater to required power. In this new configuration, milling efficiency improvement can be expected when selecting a 75 mm ball for BM1 and a 50 mm ball for BM2.

A simulation done for current operation data of the existing mill showed that the existing 800 kW drive on BM1 is oversized, as a 46% ball charge would be required to draw its maximum power at 80% of critical speed. This is explained in part due to the lower 9 to 10 kWh/t BWi ore currently being treated compared to the 16.1 kWh/t used for initial design. The geometry of BM1 may require a ball retainer if its ball charge exceeds 34%.

The option to replace the secondary crusher with a SAG mill was considered but ruled out due to previous testwork and high required capital costs. Using a rod mill was also eliminated due to operational complexities and more favourable alternative options.

The expansion requires an additional cyclone, for a total of three cyclones, with two online and one on standby. A new cyclone pack consisting of a 4-cyclone carousel will replace the existing double configuration.

The existing pair of Falcon SB 1350 gravity concentrators and existing guard screen have capacity to accept the expanded throughput with a drop in the recovery percentage expected accordingly.

Flotation circuit modifications

Currently, the overflow from cyclones is directed to a flotation area conditioning tank, then to six 10 m³ Outotec TC10 flotation cells, providing roughly 40 minutes of retention time at 800 t/d. The primary flotation circuit comprises two rougher cells only, with rougher concentrate reporting directly to the existing 4 m concentrate thickener. Primary rougher tailings report to four rougher scavenger cells, with scavenger concentrate sent back to the conditioning tank for reprocessing.

To determine the best product combination for the 1,200 t/d expansion, three options were studied: increasing capacity to the existing gravity/flotation concentrate circuit, constructing a continuous carbon-in-leach (CIL) circuit, and batch leaching followed by a Merrill-Crowe (MC) circuit. After evaluation, it was decided to increase capacity to the existing gravity/flotation concentrate circuit with the addition of a new cleaner flotation stage, producing a combination of gold doré bar and a 100 to 150 g/t Au flotation concentrate.

For the expanded circuit, it is recommended to keep the flotation circuit together in one area, use the existing conditioning tank providing 4 minutes retention time, and add two new 20 m³ rougher flotation cells. The original six 10 m³ cells will all act as scavenger flotation cells. In addition, a single cleaner column cell is recommended, expected to increase final concentrate grade from 100 g/t Au to 130 g/t Au while maintaining similar overall flotation mass pull between 1.5% and 3%.

From a process design flow, the first rougher cell concentrate will combine with cleaner concentrate and be sent to the concentrate thickener. The second rougher concentrate will feed the cleaner-column cell. Rougher tailings will feed scavenger flotation cells, with scavenger concentrate and cleaner tailings returned to the conditioning tank. Scavenger tailings will be sent to the tailings-thickener feed box.

Space for new flotation equipment will be created by removing the existing control room. An additional blower will be required for the two new 20 m³ rougher flotation cells.

Concentrate dewatering

Final flotation concentrate is pumped to the 4 m diameter high-rate concentrate thickener along with filtrate return. Flocculant is added, and thickener overflow gravitates to the adjacent water tank. The concentrate is thickened to produce a 60–65% w/w slurry underflow, then pumped to agitated filter-feed storage tank prior to concentrate filtration through a single Andritz pressure filter.

No changes are required to the concentrate thickener and load-out facility. Existing concentrate thickener underflow pumps, filter feed tank, and concentrate filter press will be sufficient for the 1,200 t/d expansion. This is possible due to a reduction in flotation mass pull design criteria from 4.5% for 800 t/d to 2.7%, based on operational data. A cleaner design mass pull of 50% was considered for design. Halyard recommends adding two extra plates to the existing filter press if the required 10 to 12 cycles per shift cannot be achieved.

Tailings handling

At 800 t/d, tailings from flotation report to a 12 m high-compression thickener producing 65–70% w/w slurry before being pumped to the paste plant or tailings storage facility. For the 1,200 t/d expansion, additional flotation tailings thickening capacity will be required. A new 9 m diameter high-compression thickener will be added adjacent to the existing thickener. Tailings from flotation will be fed into a new tailings thickener feed box distributing flow to both thickeners by gravity.

The existing thickener will be fed at 34.2 t/h, with underflow sent to the paste plant's feed tank. When the paste plant is not operating, overflow will be sent to the North TMF. The new thickener will be fed at 18.4 t/h, with new pumps and an insulated HDPE pipeline conveying tailings to the North TMF.

Reagents and services

Reagent types include MIBC (frother), KAX51-33% strength (collector), Solvay A407 (promoter), Solenis Zetag 4105 (flocculant), and Solenis Amerfloc 485 (coagulant). Reagent manufacturers and consumption rates were updated based on operational data. Recent testwork confirmed tailings can achieve 72.6% solids w/w at the thickener underflow using 37 g/t coagulant and 10 g/t flocculant at 0.45 t/(m²·h) solids loading. Rougher concentrate testwork showed 80.1% solids w/w using 20 g/t flocculant at 0.08 t/(m²·h) loading.

Existing xanthate preparation plant will not be used as the reagent is now bought in liquid form. New storage tanks were being sized by the client's operations team. The existing flocculant preparation plant will be used for the expansion. One additional dosing pump for xanthate, frother, promoter, and flocculant will be required.

A second process-water tank with 100 m³ working volume will be required, along with a new process-water pump and a new gland water pump. One new air compressor, dryer, and receiver will be required for the new cleaner flotation column cell and plant instrumentation air.

Key reported parameters

Technical qualifications

Comminution testwork limitations: Although recent testwork samples of ball mill feed showed significantly lower BWi of 9.9 kWh/t (106 µm screen) to 11.6 kWh/t (150 µm screen) for the Sugar Zone, future ore zones including the proximal Middle Zone and deeper reaches of the North and South Zones have yet to undergo robust comminution testwork. Hence a higher design BWi of 16.1 kWh/t provides a conservative basis for the design.

Mill geometry constraints: The geometry of BM1, according to modelling, may require a ball retainer if its ball charge exceeds 34%. Care should be taken if a higher ball charge is attempted; the structural ball charge maximum stated by the mill manufacturer should be reviewed.

Concentrate filter press contingency: The concentrate system is expected to allow operation at 10 to 12 cycles per shift required for expansion. In case this is not operationally feasible, Halyard recommends adding two extra plates to the existing filter press.

Power distribution limitations: The existing 4.16 kV overhead distribution line conductors supplying the process plant building are undersized to power the proposed mill expansion. The line will be modified to a double-circuit line, or a new overhead line will have to be constructed if modification is not possible.

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*Source: National Instrument (NI) 43-101 Technical Report , Feasibility Study for Expansion to 1,200 t/d for the Sugar Zone Mine, Sault Ste. Marie Mining Division, Ontario, Canada, March 2021, Sections 17.0–17.3*