AMC's 2012 technical report for Newcrest on the Gosowong property in North Maluku, Indonesia, describes a high-grade gold plant with two grinding circuits, Falcon gravity concentrators and intensive leach reactors.
The Technical Report on the Gosowong Property in North Maluku Province, Indonesia, was prepared for Newcrest Mining Limited by AMC Mining Consultants (Canada) Ltd under NI 43-101, and the operation is run by PT Nusa Halmahera Minerals (PTNHM). The report calls gold extraction from the Gosowong ores conventional. Its flowsheet runs crushing, SAG and ball milling, tertiary fine grinding, gravity concentration with intensive cyanidation, thickening, tank leaching, countercurrent decantation (CCD), Merrill-Crowe zinc precipitation and smelting to doré. The plant runs at about 95% utilization.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Report | Newcrest Mining Limited, 2012 | Not applicable | AMC Mining Consultants (Canada) Ltd |
| Operator | PT Nusa Halmahera Minerals | Not applicable | Not applicable |
| Primary crushing | Jaw crusher to below 80 mm | Not applicable | Wobbler feeder for fines and sticky ore |
| Primary grind | P80 75 | µm | Two circuits: SAG-ball and closed SAG |
| Tertiary grind | P80 30 | µm | Vertimill, 900 kW, 20 mm stainless media |
| Leach feed density | About 45 | % solids | Thickener underflow |
| Leach | Six tanks, about 30 h | Not applicable | Natural slurry pH about 11.0, no lime needed |
| CCD | Two banks of seven thickeners | Not applicable | Not applicable |
| Doré | About 45% Au, 47% Ag | Not applicable | Refined in Jakarta |
| Cyanide destruction | Free CN from about 300 ppm to below 1 ppm | Not applicable | SMBS with lime and copper sulphate |
| Overall recovery | About 95 | % | Except before the Vertimill was installed in May 2009 |
| Plant utilization | About 95 | % | Not applicable |
Overview
Ore from the mines is stockpiled and fed either to a jaw crusher, which breaks it to less than 80 mm, or to a wobbler feeder, which separates fines and sticky wet ore and sends material over 600 mm to the jaw crusher. Front-end loaders stockpile the crushed ore and feed the grinding circuits.
Testwork showed gold extraction improving at finer grinds, even down to P80 10 µm. Because of limits related to thickening, PTNHM decided in 2008 to add a tertiary grinding stage that takes all SAG circuit discharge to P80 30 µm. The report notes that overall recovery has stayed at about 95%, apart from the period before the Vertimill went in in May 2009.
Key Process Stages
Primary grinding. The original circuit is an open-circuit SAG mill feeding a closed-circuit ball mill with hydrocyclones cutting at P80 75 µm. A second circuit is a closed-circuit SAG mill with hydrocyclones cutting at the same size.
Gravity and intensive leach. Part of the ball mill cyclone underflow feeds a Falcon high-G gravity concentrator. Its concentrate is dissolved in strong cyanide in an Inline Leach Reactor (ILR), and the solution goes to clarification. A second Falcon and ILR sit in the Vertimill cyclone circuit. The report notes that a third Falcon and ILR for the new SAG circuit, and electrowinning of the ILR liquor in a dedicated cell, were under consideration.
Tertiary grinding. A Vertimill with 900 kW connected power and 20 mm stainless steel media grinds all primary circuit discharge to P80 30 µm, sized by small-diameter hydrocyclones.
Thickening and leaching. Cyclone overflow is thickened, and the clear overflow is recycled as process water. The underflow, at about 45% solids, is leached in six tanks in series with sodium cyanide and oxygen from an on-site PSA plant. The ore naturally slurries at a pH of almost 11.0, so no lime is needed.
CCD. After about 30 hours of leaching, the slurry passes through two banks of seven-stage CCD thickeners. Pregnant solution is drawn from the first unit, and washed solids leave the last.
Merrill-Crowe and smelting. Pregnant solution is clarified, filtered in pre-coat pressure filters, deaerated and treated with zinc dust. The gold-zinc precipitate is filtered in a plate-and-frame press, calcined for about 24 hours and smelted with borax and silica in a tilting furnace. Doré bars of about 45% gold and 47% silver go to a refiner in Jakarta.
Cyanide destruction and tailings. Two tanks in series destroy cyanide with sodium metabisulphite, lime and copper sulphate, reducing free cyanide from about 300 ppm to below 1 ppm, and WAD cyanide consistently below 0.5 ppm. Three parallel pumps and three 125 mm lines carry tailings to the TSF.
Additional Interesting Data and Summary
The report's production table shows feed of 0.10 to 0.59 Mt a year, head grades of 22.92 to 43.20 g/t Au and plant recoveries of 90.4% to 97.8%. Throughput has risen every year while grade has fallen steadily since the year to June 2008, and gold output has stayed above 0.4 Moz a year since then. The report expects an increase in throughput to 105 t/h to give marginal reductions in operating cost per tonne.
In summary, Gosowong is a very high-grade, fine-grinding gold plant. Recovering most of the gold depends on regrinding all ore to 30 µm, and CCD with Merrill-Crowe, rather than carbon adsorption, recovers it from solution at about 95%.
Key Processes
- Jaw crushing and wobbler feeder
- SAG and ball milling to P80 75 µm in two circuits
- Falcon gravity concentration with Inline Leach Reactors
- Vertimill tertiary grinding to P80 30 µm
- Pre-leach thickening
- Cyanide leaching with oxygen
- Countercurrent decantation
- Merrill-Crowe zinc precipitation, calcining and smelting
- SMBS cyanide destruction and tailings disposal
Source: Technical Report on the Gosowong Property in North Maluku Province, Indonesia, prepared for Newcrest Mining Limited by AMC Mining Consultants (Canada) Ltd, 2012.
Earlier report summaries
Earlier versions of this profile, based on older technical reports, are retained below for source traceability. Dates and figures reflect each report’s own effective date and are superseded by the current information above.
Gosowong Property — 2011 Technical Report
This technical report describes the mineral processing and metallurgical testwork for the Gosowong property, including the existing plant flowsheet and historical operating performance.
Report context
This technical report was prepared for Newcrest Mining Limited with an effective date of 31 December 2011, covering the Gosowong property located in North Maluku Province, Indonesia. The report documents mineral resources, mineral reserves, mining methods, and recovery methods for the Gosowong open pit mine, Kencana underground mine, Toguraci underground project, and a tailings reclamation project.
Processing route
Process Plant Flowsheet
The report presents a process plant flowsheet that was designed for gold recovery. The flowsheet incorporates whole ore leaching in a carbon-in-leach circuit.
Plant Performance
The report provides historical plant performance data showing actual outcomes from the operating plant. The plant had achieved a gold recovery rate of 91.5 percent.
Metallurgical Testwork
The report documents metallurgical testwork conducted to support process design. This testwork included comminution testing, leaching testwork, grind size–recovery relationship studies, oxygen uptake measurements, liquid-solid separation testing, and rheology investigations.
Comminution testwork evaluated the grinding characteristics of the ore, including bond work index determinations.
Leaching testwork examined gold extraction under various conditions to establish the relationship between grind size and gold recovery.
Oxygen uptake testing measured the rate of oxygen consumption during leaching to determine oxygen requirements for the leaching circuit.
Liquid-solid separation and rheology testing provided data for thickener and filtration equipment design.
Other investigations included additional metallurgical studies to support process design decisions.
Key reported parameters
| Parameter | Design | Actual Operating | Testwork Basis |
|---|---|---|---|
| Gold recovery | Information not provided | 91.5 percent | Information not provided |
| Comminution characteristics | Information not provided | Information not provided | Bond work index tests conducted |
| Leach kinetics | Information not provided | Information not provided | Grind size–recovery tests; oxygen uptake tests |
| Solid-liquid separation | Information not provided | Information not provided | Thickener and filtration design testwork |
| Rheology | Information not provided | Information not provided | Rheology testwork conducted |
Project website: https://www.mining-technology.com/news/newcrest-finalises-gosowong-sale-indotan/
Technical qualifications
This report does not present a complete flowsheet with all unit operations and equipment specifications. The processing section is limited to the flowsheet overview and one historical recovery value. No secondary processing steps, reagent consumption data, or detailed process control information are provided. The report notes that the plant design was based on metallurgical testwork, but specific design criteria, performance guarantees, and economic parameters are not included in the processing section. The report contains no recent processing data beyond the stated 91.5 percent gold recovery.
Source: Technical Report on the Gosowong Property, effective 31 December 2011, Section 13 Mineral Processing and Metallurgical Testing, Section 17 Recovery Methods.
