Hydrometallurgy, or “hydromet” for short, is a metal processing technology that uses a chemical process combining water, oxygen or other substances in a pressurized or other vessel to dissolve a metal from its ore, concentrate or an intermediate product (such as matte). Further processing is required to produce high purity metal.
The nickel industry worldwide has traditionally smelted concentrates produced from nickel, copper and cobalt sulphide ores to make an intermediate sulphide product called matte. Hydrometallurgy has been used for refining the matte to produce high purity nickel, copper and cobalt for the market. Thus, traditionally production of these metals has occurred in two steps: smelting and refining.
The new hydrometallurgical process that Vale developed will be able to process the nickel concentrate directly to metal products without first having to smelt the concentrate. It will be more economical and environmentally friendly since the sulphur dioxide and dust emissions associated with a smelter are eliminated. The process will also yield more of the valuable cobalt which is lost to a great extent in the smelting process.
This type of hydrometallurgical process is not entirely new. Many operations, some of them in Canada, have used hydrometallurgical processes for years for extraction of zinc and copper from sulphide concentrates. However, it was not until 1990’s that an acid-oxidative hydromet technology was developed to process nickel sulphide concentrates.
In the hydromet process for sulphide concentrate, a finely ground nickel-cobalt-copper concentrate will be processed in a pressurized vessel where it will react with oxygen and sulphuric acid to produce an impure solution of nickel, cobalt and copper. This solution will pass through a number of chemical purification steps ending with removal of impurities and separation of nickel, copper and cobalt. The copper and cobalt will be recovered as by-products. The nickel will be recovered by electrolysis as high quality electronickel product suitable for market. The waste solids from the process will be neutralized with lime and will be deposited in a specially designed disposal facility. All water leaving the plant, including precipitation run-off water will be processed to remove contaminants.
Developing a Hydrometallurgical Facility
In developing the hydromet process for Voisey’s Bay concentrate, Vale is using a step-by-step research and development approach.
- The first step was to prove that each separate chemical process works individually at a laboratory scale. This has been successfully completed.
- The second step was to operate a mini-pilot plant at our laboratory facilities to ensure that the steps can be interconnected into a continuous process. The mini-pilot plant began operating in 2003 and the program was completed in May 2005.
- The next step was to build a demonstration plant at Argentia to allow for fine-tuning and optimization of the process, using actual Voisey’s Bay concentrate. Construction of the demonstration plant began in May 2004. Operations began in October 2005 and were completed in June 2008.
The key mission of the demonstration plant was to prove out and fine-tune all processing steps to confirm technical and economic viability and assist designers in the selection of the most appropriate materials of construction and specification of major pieces of equipment. In November 2008, Vale made the decision to use hydromet technology in the processing plant.
Vale has selected a site near Long Harbour, Newfoundland, as the location for the processing plant. Engineering, procurement and environmental planning for the processing plant in Long Harbour began in 2006. Initial construction began in April 2009 and is expected to be completed by February 2013.
Hydrometallurgy has a major environmental advantage over conventional smelting since most of the waste products from the process are not produced as air emissions, but rather as solids, which can be easily contained.
In conventional pyrometallurgical smelting, sulphides are burnt off, creating SO2 gas, which is released as air emissions. With hydrometallurgy, there are no air emissions. The sulphides are transformed into elemental sulphur, which together with iron oxide form the solid tailings. The tailings will also contain gypsum and unleached rock minerals. Tailings will be neutralized to form a stable residue.