Otjikoto Gold Project — NI 43-101 Technical Report

This report details the proposed gravity/whole ore leach processing design for the Otjikoto Gold Project, including process design criteria, unit operations, and reagent consumption estimates.

Report context

The Otjikoto Gold Project NI 43-101 Technical Report describes the processing facility design developed for the Otjikoto deposit. The design basis is a nominal treatment rate of 2.5 Mtpa with a plant availability of 94%. The process design criteria were finalized using metallurgical testing data and the Definitive Feasibility Study (DFS). The mill is designed to process three major ore types: XR1 (oxide), XR2 (pyrite dominant), and XR3 (pyrrhotite dominant), with the XR3 ore type used as the primary basis for most mill design calculations due to its physical, chemical, and mineralogical characteristics and its predominance in the deposit.

Processing route

Crushing

The crushing circuit comprises a 42 inch by 65 inch (1,067 mm by 1,651 mm) gyratory crusher with a nominal throughput rate of 440 tph. Run-of-mine ore will be delivered from the open pit operations by 100 tonne trucks. The crushed product at a P80 of 150 mm will be conveyed to a 7,000 tonne live capacity mill feed stockpile.

A 25% design factor was included for sizing the primary crusher, conveyors, ball mill, thickeners, cyanide destruction circuit, reagent systems, and mainstream pumps to facilitate a future expansion.

Grinding circuit

The grinding circuit consists of a 24 ft diameter by 14.5 ft EGL (7.11 m diameter by 4.42 m EGL) SAG mill operated in closed circuit with a trommel for pebble screening on the mill discharge and a vibrating screen to produce ball mill circuit feed. The SAG mill product is combined with tailings from the gravity concentration circuit before being pumped to a classification cyclone cluster. Cyclone underflow feeds a 16.5 ft diameter by 28 ft EGL (5.03 m diameter by 8.53 m EGL) ball mill. Both grinding mills are powered with 4,000 kW (5,364 HP) synchronous motors. The circuit is designed to produce an 80% passing product size (P80) of 75 µm. The SAG mill circuit design includes a future allowance for installation of a pebble crusher.

Gravity concentration and intensive leaching

The gravity concentration circuit comprises four Knelson 48 inch (1,219 mm) concentrator units operated in parallel. Gravity concentrate collected from the Knelson units will be stored and processed by intensive cyanide leaching in a ConSep CS8000 Acacia Reactor on a batch basis. The design allows for processing of batches on either a 24 hour or 48 hour basis.

Cyanide leach circuit

Cyclone overflow from the grinding circuit is thickened in a pre-leach thickener to achieve a solids content of 45% in the underflow product prior to cyanidation. The leach circuit comprises one pre-aeration tank with a residence time of 6.9 hours and seven leach tanks with a total residence time of 48 hours.

Carbon-in-Pulp circuit

The CIP circuit consists of six tanks in series with a design residence time of one hour per tank. The circuit is designed for carbon concentrations in the range of 15–25 g/L with loaded carbon batch transfers of 5 tonnes every 24 hour cycle.

Cyanide destruction

The cyanide destruction circuit is based on the SO₂-Air process and comprises a single tank with a design residence time of 90 minutes. The circuit is designed to reduce weak acid dissociable cyanide (CNwad) concentrations to less than 10 ppm.

Tailings thickening and disposal

Product slurry from the cyanide destruction circuit is thickened in a tailings thickener to achieve an underflow density of 55%–65% solids. Thickened tailings are pumped to a tailings impoundment and water reclamation facility situated approximately 3 km from the mill.

Acid wash, elution, and carbon regeneration

Loaded carbon is processed in 5 tonne batches on a 24 hour cycle for gold recovery. The loaded carbon is treated in an acid wash followed by elution and regeneration of barren carbon in a rotary kiln. The elution circuit design is based on the pressure split Anglo-American Research Laboratories process (Pressure split “AARL”). Gold is recovered in the eluate stream during the elution process.

Electrowinning and gold room

The electrowinning circuit treats eluate from the carbon elution circuit and pregnant solution from the Acacia Reactor on a batch basis for the recovery of gold from solution, using atmospheric sludging electrowinning cells. Electrowinning sludge will be fluxed and smelted in an induction furnace to produce gold bars for shipment to a refinery.

Reagent make-up and distribution

The reagent make-up and distribution facilities cater for the make-up, storage, and distribution of sodium cyanide, caustic soda, hydrated lime, lead nitrate, hydrochloric acid, sodium metabisulphite, copper sulphate, flocculant, elution de-scalant, and process water anti-scalant.

Plant services

The design includes facilities for the generation, storage, and distribution of plant and instrument air at a design pressure of 7.5 bar, and leach air and cyanide destruction air supply at a design pressure of 3.0 bar. Plant water services include process water, reclaim water, fresh water, fire water, potable water, and gland service water. Mill fresh water consumption is projected at 1 million m³/a at the mill design throughput of 2.5 Mtpa. Fresh water will be supplied from wells on the Otjikoto property for both potable and process needs. Average overall plant power consumption during steady state mill operation will be approximately 32 kWh/t of ore processed. Electrical power will be generated on site using heavy fuel oil generators.

Laboratory design

A laboratory was designed to support the exploration, mining, and milling functions of the Otjikoto Project operation. The scope of work included laboratory layout, staffing requirements, equipment selection and costing, and design of the ventilation system. The lab design provides for analysis of up to 250 samples per day, with gold as the primary element and iron, sulfur, and silver as secondary elements. Analysis consists of preparation equipment and methods that take into consideration the occurrence of free gold, gravimetric methods, wet chemistry procedures, and instrument methods using Atomic Absorption, MP-AES, and LECO. Major components of the laboratory are the sample preparation lab, fire assay lab, wet chemistry lab, metallurgical lab, low level lab (environmental), and the instrument room. Support areas include rooms for mechanical, electrical, gas bottle storage, washrooms, and offices.

Key reported parameters

Reported reagent consumptions by ore type: sodium cyanide (XR1: 0.55 kg/t, XR2: 0.69 kg/t, XR3: 0.59 kg/t), caustic soda (30 kg/day for all ore types), hydrated lime (XR1: 1.29 kg/t, XR2: 1.18 kg/t, XR3: 0.82 kg/t), lead nitrate (100 g/t for all ore types), hydrochloric acid (0.83 t/d for all ore types), sodium metabisulphite (XR1: 0.70 kg/t, XR2: 0.83 kg/t, XR3: 0.75 kg/t), copper sulphate (XR1: 50 g/t, XR2: 126 g/t, XR3: 155 g/t), flocculant (XR1: 40 g/t, XR2: 20 g/t, XR3: 20 g/t), elution de-scalant (5 g/t for all ore types), and process water anti-scalant (14.8 g/t for all ore types).

Technical qualifications

The process design criteria presented in this report are based on metallurgical testing and the DFS. The XR3 pyrrhotite dominant ore type is listed for the crushing, grinding, gravity recovery, leach, and cyanide consumption criteria as this ore type was used as the basis for most of the mill design because of its physical, chemical, and mineralogical characteristics and its predominance in the Otjikoto deposit. The detailed process design criteria are presented in the Process Design Criteria in the DFS. The detailed laboratory design report can be found in the Laboratory Design Report of the DFS.

Source: Otjikoto Gold Project NI 43-101 Technical Report, Section 17 “Recovery Methods,” pages 251–258.