The Holt Mill, constructed in 1988 and expanded in 1988 and 2001, employs a grinding circuit with three mills in series followed by carbon-in-leach recovery and Zadra elution.
Report context
This section summarizes the recovery methods described in the NI 43-101 technical report. The SAS Metallurgist confirmed that no significant changes have been made to the Mill or Mine site assay laboratory since the previous technical report.
Processing route
Surface ore storage
Surface ore storage totals 4,900 t in three silos: the Holt headframe bin (900 t) and two other separate storage bins (1,000 t and 3,000 t). Ore can be delivered to the mill from the Holt Mine by conveyor or from a separate surface dump that enters a 100 tonne hopper, and then can be fed to either of the two storage bins.
Grinding circuit
The grinding circuit consists of a 5 m diameter by 6.1 m long Allis Chalmers ball mill, converted to a SAG mill, a 4 m diameter by 5.5 m long Allis Chalmers ball mill (a converted rod mill) and a 3.6 m diameter by 4.9 m long tertiary ball mill, all operating in series and in closed circuit. The grinding circuit is controlled by a Wonderware system and Modicom programmable logic.
Classification and thickening
The primary cyclone cluster consists of six 381 mm (15") Krebs D15B cyclones. A secondary cyclone cluster consists of twelve 254 mm (10") Krebs gMAX cyclones with an Outokumpu PSI-200 online analyzer. The secondary cyclone cluster feeds a 27 m (90 ft) Eimco thickener.
Carbon-in-leach and stripping
The thickener underflow feeds a pre-aeration tank, which gravity feeds five carbon-in-leach (CIL) tanks in series. The tank system is conventional gravity flow for slurry with counter-current carbon advancement.
Precious metal stripping is performed in batch operations, advancing 2.7 t of loaded carbon through a 1.2 m by 2.4 m (4 ft x 8 ft) Simplicity screen. Carbon is transferred to an adsorption column where a Zadra process is utilized as the gold elution method. Barren solution is circulated through two shell and tube heat exchangers and a 360 kW electric inline heater.
Refining and carbon regeneration
The resulting pregnant solution is pumped from the solution tank to an electro-winning cell. The gold precipitate is further refined using a 125 kW Inductotherm furnace and the doré bars are poured in a seven mould cascade arrangement. After stripping, the carbon is regenerated in a rotary kiln, quenched, screened and returned to the process. Carbon fines are collected in a tank, filtered in a Perrin press, and packaged for sale.
Reagent storage
Reagents and operating supplies for the mill, such as process chemicals and grinding steel, are stored in the reagent storage building attached to the concentrator at the south end of the building.
Tailings disposal
The tailings storage facility is located 2.5 km south of the process facilities. Tailings are pumped (at about 50% solids) via primary and booster pumps in a 254 mm HDPE pipe. The tailings pipe is equipped with two magnetic flow meters , one at the mill and the other at the tailings pond to monitor pressure differential. Excessive differences in flow will trigger an audible alarm. The tailings pipe is laid in a secondary containment capable of handling rock slurry in the event of a line burst.
The tailings storage facility is subdivided into four separate areas to receive the mill tailings and treat the solution that results from the milling of up to 3,000 tpd.
The quality of water discharge relies on natural degradation of cyanide and precipitants of other metals through the use of ferric sulphate. The annual discharge to the environment is closely monitored. The annual allowable discharge is a function of the water quality in the polishing pond and the flow rate in the receiving waters.
All of the tailings dams were designed by Golder Associates, and Golder has carried out the dam safety inspections. The tailings management facility (TMF) is divided into a series of four basins: the North basin, the Southwest Basin, the Southeast Basin and the Polishing Pond. The North Basin was originally formed in a valley by the construction of perimeter dams. The capacity of the North Basin was increased in 1995 by raising the dams and constructing additional dams. The Southwest Basin, Southeast Basin and the Polishing Pond are all contained by dams.
To date all of the tailings deposition has been into the North and Southwest basins. The current practice is to transfer tailings contact water from the North and Southwest basins into the Southeast Basin by pumping. Water is then transferred to the Polishing Pond from which it is released to the Magusi River. The effluent discharge is not permitted to exceed one-tenth of the flow in the Magusi River in the summer months and one-fifth during spring and in the fall.
There are five stages of TMF development, with associated permitted water levels and tailings levels. Up to 3.6 Mt of the tailings will be deposited in the Southwest and Southeast basins. Tailings deposition design is based upon an in situ void ratio of 1.0, a specific gravity of 2.65 and an inferred dry density of 1.33 t/m³. Slopes of 0.6% were assumed for sub-aerial deposition and slopes underwater were assumed to be 2.5% slopes.
The ultimate storage capacity of the Southwest Basin is calculated to be 1.43 Mt of tailings. There is space for the deposition of 0.7 Mt before the dam heights must be increased. The Southeast Basin has the capacity for 2.17 Mt of tailings. Golder has also developed the water management plan to be used with the deposition plan.
Laboratory
The assay laboratory is located at the Holt site in an area near but separate from the mill and previously used as an assay lab. The building was renovated and a sample preparation area, fire assay facilities and an AA facility were established to provide analytical services for the site.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Original design throughput | 1,360 | tpd | Design |
| Throughput after 1988 expansion | 2,500 | tpd | Historical operating data |
| Throughput after 2001 expansion | 3,000 | tpd | Historical operating data |
| SAG mill dimensions | 5 m diameter x 6.1 m long | , | Design |
| Ball mill (converted rod mill) dimensions | 4 m diameter x 5.5 m long | , | Design |
| Tertiary ball mill dimensions | 3.6 m diameter x 4.9 m long | , | Design |
| Primary cyclones | Six 381 mm Krebs D15B | , | Design |
| Secondary cyclones | Twelve 254 mm Krebs gMAX | , | Design |
| Thickener diameter | 27 (90) | m (ft) | Design |
| CIL tanks | Five in series | , | Design |
| Loaded carbon per batch | 2.7 | t | Historical operating data |
| Carbon screen dimensions | 1.2 m by 2.4 m (4 ft x 8 ft) | , | Design |
| Elution heater power | 360 | kW | Design |
| Furnace power | 125 | kW | Design |
| Tailings solids content | 50 | % | Historical operating data |
| Tailings pipe diameter | 254 | mm | Design |
| Southwest Basin capacity | 1.43 | Mt | Design |
| Southeast Basin capacity | 2.17 | Mt | Design |
| Tailings deposition void ratio | 1.0 | , | Design |
| Tailings specific gravity | 2.65 | , | Design |
| Tailings inferred dry density | 1.33 | t/m³ | Design |
| Sub-aerial deposition slope | 0.6 | % | Design |
| Underwater deposition slope | 2.5 | % | Design |
Project website: https://mininglifeonline.net/company/holloway-holt-mine/449
Technical qualifications
The Qualified Person stated that in their opinion, there are no processing factors or deleterious elements that could have a significant effect on potential economic extraction at the Holt-Holloway mines.
The description of the current milling process is summarized from the previous NI 43-101 technical report, and the SAS Metallurgist confirmed that no significant changes have been made to either the Mill or Mine site assay laboratory since the last technical report.
Source: Holt Mine , NI 43-101 Technical Report, sections 16 Recovery Methods and 16.1 Process Plant Flow Sheet, and Tailings Disposal.
