Overview
The mine started production in August 2010 and Vale holds a 62% percent interest in the Goro project.
Located on the South Pacific island of New Caledonia, the Goro Nickel-Cobalt Mine has one of the best undeveloped laterite orebodies in the world having excellent average grades, 55 million tonnes of estimated measured and indicated mineral reserves, and a large resource base.
Planned annual output capacity is 60,000 metric tonnes of nickel and 4,300 to 5,000 metric tonnes of cobalt.
The mine is expected to generate approximately 800 jobs once operational. It will have a positive impact on the economy of the island whose population is 250,000. The Mine is the largest development in New Caledonia's history.
Location
Located approximately 1,500km east of Australia and approximately 2,300km northwest of New Zealand, The Goro mine is located on the southeast end of Grande Terre in Province Sud, about 60km east of Noumea, the capital of New Calidonia.
The island's ecosystem is distinctive: it includes the world's largest lagoon, the second largest barrier reef, and one of the richest known tropical forests. The vegetation surrounding the mine is diverse in terms of endemic plant species. Both geological isolation and nutritional deficiencies have contributed to the development of a specific flora tolerant to the nickel-cobalt-iron rich soil.
Geology and Mineralization
The Goro mine comprises 6,042 hectares in 7 mining concessions. Four of the Goro mine mining concessions are perpetual and three others may be renewed for an additional 25 years. Goro Nickel also holds an additional 62 nickel, cobalt and chromium mining concessions and approximately 13,450 ha of surface rights in Province.
The Goro Deposits occur in nickel laterite soils that form over ultramafic rocks in a tropical to semitropical climate in areas of heavy rainfall. The deposits are located within the Kwe basin, an area of approximately 30 square kilometres. Lateralization is a chemical weathering process whereby atmospheric, groundwater and biological processes interact on exposed rock surfaces and fractures in the bedrock to decompose the primary rock minerals and form chemically stable mineral phases in the weathering zone.
The silicate nickel content in the original ultrafamic rock is about 0.3% or less, contained within the mineral olivine. The nickel is liberated during the leaching process and migrates downward, carried by the groundwater, in the laterite profile. The laterite profile thickness is approximately 40 metres but is extremely variable, ranging from 20 to 90 metres or more. The laterite profile is well developed on plateaus and is protected from erosion by an iron cap. However, in some places the iron cap has been breached by active drainage exposing the underlying laterite.
Mining and operation
The Goro mine is a conventional open pit mine using hydraulic excavators and rigid frame trucks. The mining rate will be approximately 4.8 million dry tonnes of ROM with an average waste to ore stripping ratio of 1:1. The ROM will be delivered to the Feed Preparation Plant (FPP) located at the north-western edge of the mining area where the oversize material is wet screened. The limonite and saprolite ROM will be either directly dumped into the FPP or placed on separate stockpiles. It is anticipated that a certain amount of short term and long term blending could be required as the mined out material available at the face may not meet the process plant requirement.
During stripping, topsoil removal will be followed by recovery of iron cap and iron shot. Generally, bulldozer ripping of the iron cap layer will be sufficient to loosen the material ahead of loading by a backhoe excavator into haul trucks, although a limited small diameter drilling and blasting may be necessary to break the hard Iron Cap material into a manageable size.
All ore zones will be mined by using hydraulic excavators. While the limonite ore has a homogenous and generally fine-grained texture, the saprolite ore is characterized by the boulder content. Some drilling and blasting of oversize boulders and bedrock pinnacles could be required to facilitate mining and provide excavator access to saprolite that would otherwise be inaccessible.
Mine layout and the direction and sequence of mining are largely dictated by the requirement to focus initial mining on Zone 1, to maximize in-pit waste disposal and to minimize resource sterilization. The top of ore will be defined by in fill core drilling. From this horizon, the limonite ore will be mined through the transition zone and into the saprolite until bedrock is reached. Within the limits of the bulk mining method and equipment, the limonite and saprolite will be mined and handled separately. Overburden removal and limonite and saprolite mining will occur simultaneously at various parts of the pit to maintain an approximately constant limonite to saprolite ratio to the autoclave feed.
Planned operations will use hydraulic excavators in the 13.6 cubic meter range for loading 100 tonne haul trucks. A fleet of two 4.4 cubic meters backhoes loading 50 tonne haul trucks will excavate some of the pit material as well as excavate and maintain pit drainage structures. Both overhand and underhand mining methods will be planned as necessary for the local conditions prevailing in the pit. Overhand mining by hydraulic excavators will face up to benches not exceeding 10 metres in height, while underhand mining is not planned to exceed 5 metres in depth.
Processing
The ore preparation plant is located very close to the mine. The laterite ores (limonite) and saprolite ores (garnierite) are mixed with water, sifted and ground to form a sludge known as slurry. This slurry is piped to the treatment plant. Next the slurried ore is preheated by steam and injected continuously into an autoclave with sulphuric acid. Leaching once again washes the ore with sulphuric acid. The role of the acid is to dissolve certain metals which are extracted in this way from the solid ore and transferred into the liquid solution. The high temperature in the autoclave (270°C) permits the acceleration of this extraction and therefore allows a greater quantity of ore to be processed in a small autoclave. However, this high temperature requires operation under high pressure so as to prevent the liquid from boiling.
The leached slurry thus obtained contains solids (mainly iron oxide) and a liquid solution containing the dissolved metals including nickel and cobalt but also metals not recoverable for exploitation (magnesium, aluminium, chromium, zinc, copper, etc.). This slurry is then cooled again and depressurised. This operation generates steam which is recycled upstream in the slurry heating circuit before its injection into the autoclave.
The leached and cooled slurry passes through a decantation circuit designed to separate and wash the solid residues from the liquid solution called mother liquor. In other words, the solids settle at the base of the decanter from which they are pumped (the underflow), while the excess liquid is collected (overflow). To wash the solids well, the operation is repeated six times in six successive decanters. By the end of the operation, the mother liquor has recovered 98% of the nickel and cobalt contained in the leached slurry. The solids are sent in the form of a thick paste to the unit for treating solid residues, where they are neutralised.
The mother liquor contains not only cobalt and nickel, but also other metals, present in the original ore (aluminium, iron, chromium, zinc, silica, copper and manganese), considered impurities (since not destined to be recovered and processed), as well as sulphuric acid not used during the leaching process. The acid and some of the metal impurities are eliminated through the addition of limestone and lime to form solid gypsum (plaster), separated from the solution by decantation and filtration operations. The gypsum and metal hydroxides form a thick paste which is sent to the waste processing plant. Copper is removed by precipitation then by absorption in an ion exchange circuit to remove the last traces. It is, in its turn, sent to the waste processing plant.
The mother liquid is injected into a first extraction circuit in which an organic solvent captures the nickel, cobalt and zinc, leaving manganese, magnesium and calcium in the liquor. This solution is sent to the liquid residue processing unit. A second extraction, on contact with a small quantity of hydrochloric acid, releases the nickel, cobalt and zinc. This solution, whose volume is between 20 and 30 times less than that of the mother liquor, gives a concentrate of nickel, cobalt and zinc chlorides. Passage through a selective resin enables the zinc to be retained. Finally, a concentrated hydrochloric solution is obtained, cleansed of nickel and cobalt.
The nickel chloride solution is processed in a fluidized bed oven, heated to a high temperature (800°C) through the combustion of a mixture of air and natural gas. The nickel chloride is then broken down into nickel oxide and hydrochloric acid which is reconstituted and recycled for the extraction process. The particles of nickel oxide thus formed are in the form of spherical granules, comprising successive layers, something like pearls, producing small grey balls, round and solid. The hydrochloric acid is recycled to the first solvent extraction stage. The cobalt chloride is neutralised by adding sodium carbonate (soda) to form a pulp of cobalt carbonate crystals, recovered, after decantation and filtration, as a purple powder.
