The Terakimti oxide deposit is planned to be processed using heap leaching with Merrill Crowe recovery, based on metallurgical test work by BCR in 2013 and SGS in 2016.
The proposed recovery method for the Terakimti oxide deposit involves heap leaching technology followed by Merrill Crowe processing for gold and silver recovery. The design follows metallurgical test work conducted by BCR in 2013 and SGS in 2016 which indicated the oxide zone is amenable to conventional cyanide leaching. The processing facilities are designed to treat 260 kilotonnes per annum of oxide material, equivalent to 715 tonnes per day. The recovery process is designed to operate year-round with defined availabilities for the crushing and agglomeration circuits and the leaching and Merrill Crowe treatment circuits.
Run-of-mine material will be delivered from the open pit to the crushing plant by haul trucks, with a front-end loader used to reclaim stocked material into the dump pocket according to the mine plan. The material will be reduced in a two-stage crushing circuit to a particle size of 100% passing 20 mm. Screen undersize and cone crusher discharge will be conveyed to an agglomerator where cement will be added to agglomerate fine particles with coarser particles. Lime will also be added to the crushed material prior to placement to control the alkalinity of the heap leach. The agglomerated material will discharge onto a crushed material stockpile adjacent to the crushing plant for curing.
The crushed and agglomerated material will be reclaimed from the stockpile with a front-end loader and loaded into a haul truck for transport to the heap leach pad. Material will be stacked in 5 m lifts and spread evenly by bulldozer. The final heap is expected to cover approximately 5.0 ha with an average vertical height of 20 m. Solution will be distributed over the leach pad at an overall solution feeding rate of approximately 8 to 10 L/h/m². Barren leach solution will be pumped to the top of the heap and distributed through pipe headers, with gold and silver bearing pregnant leach solution collected in a dedicated pond.
The pregnant leach solution will be processed in a Merrill Crowe facility where dissolved gold and silver will be recovered. The barren leach solution discharged from the Merrill Crowe facility will be pumped back to the barren leach solution pond for reuse in the heap leaching. The final product from the mine site will be doré bars produced via on-site smelting of the zinc precipitate; these bars will be sent to an off-site refinery to produce separate high purity gold and silver products.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Operating Year | 365 | d | Design criteria |
| Crushing/Agglomeration Circuit Availability | 70 | % | Design criteria |
| Overall Plant Availability | 90 | % | Design criteria |
| Annual Processing Rate | 260 | kt | Design criteria |
| Daily Processing Rate | 715 | t/d | Design criteria |
| Agglomerated Bulk Density – Initial | 1.40 | t/m³ | Design criteria |
| Crushed Particle Size, 100% Passing | 20 | mm | Design criteria |
| Moisture Content of Agglomerated Feed | 8 | % | Design criteria |
| Total Loading/Curing/Leaching/Rinsing Cycle | 100 | d | Design criteria |
| Cyanide Solution Strength | 0.5-1.0 | g/L | Sodium cyanide, pH 10.5 or higher |
| Heap Area | 5.0 | ha | Approximate, equivalent to 50,000 m² |
| Heap Height | 20 | m | Average vertical height, proposed |
| Stacking Lift Height | 5 | m | Not stated |
Overview
The Terakimti oxide deposit is planned to be developed as an open pit operation with a heap leach processing facility. The project is located in the Tigray National Regional State, Ethiopia. The processing route selected is heap leaching over agitated tank leaching, with the key factors being satisfactory gold extraction and rapid leach kinetics, lower capital and operating costs, and reduced project complexity. The recovery process is designed for gold and silver, with the final product being doré bars produced on site and then sent off-site for refining.
Key Process Stages
The process comprises several key stages. First, run-of-mine material is crushed in a two-stage crushing circuit, then agglomerated with cement and lime. The agglomerated material is cured on a stockpile before being transported to the heap leach pad. The heap is stacked in lifts and irrigated with barren leach solution. The gold and silver bearing pregnant leach solution is collected in a pond, then processed through a Merrill Crowe facility. The resulting doré bars are the final product from the mine site.
Additional Interesting Data and Summary
The heap leach pad will be an engineered structure with a gravel or sand base covered by a clay liner and an impermeable synthetic geomembrane. The pad is designed to withstand the loading of crushed leach material and the movement of heavy equipment. Four lined solution ponds are planned: one pregnant leach solution pond, one barren leach solution pond, one event or overflow solution pond, and one polishing pond. The event pond will temporarily store excess process solution from upset conditions, freshet, and excess precipitation events, with the solution recycled back into the heap leach circuit when normal operation resumes. The overflow solution pond will also supply makeup water to the process by pumping water back to the barren leach solution pond.
Process water will be required for agglomeration and heap leach pad irrigation. A fresh water supply system will be provided for fire water, gland service for pumps, and reagent makeup. The fresh/fire water tank will hold a sufficient supply equivalent to a 2-hour fire water supply. Potable water will be supplied from a nearby river, treated, and stored separately. The proposed heap height of 20 m would require geotechnical verification and further metallurgical test work confirmation.
Key Processes
- Two-stage crushing and screening
- Agglomeration with cement and lime addition
- Heap stacking and leaching
- Merrill Crowe recovery of gold and silver
- On-site smelting to produce doré bars
Source: NI 43-101 Technical Report and Preliminary Economic Assessment for the Terakimti Oxide Deposit, Harvest Project, Tigray National Regional State, Ethiopia, 2023.
Editorial Note: The source document refers to processing "oxide tailings" in one instance while the overall context describes processing oxide material; the article shows the consistent description of oxide material feed. The proposed heap height of 20 m is stated to require geotechnical verification and further metallurgical test work confirmation; this qualification is noted in the design criteria.
Project website: NI 43-101 Technical Report, Terakimti Oxide Deposit, Harvest Project
