Cobalt in Idaho

Materials World magazine
,
1 Jun 2010

Michael Forrest reports on a new venture to source cobalt for the USA's green economy.

Cobalt is an essential metal in a number of high-tech applications where it imparts
hardness and temperature resistance to the alloyed metal. It is also used in catalysts, batteries, pigments and a variety of specialised chemicals. The global mine production in 2009 was estimated by the United States Geological Survey as 62,000t, of which ~6,700t was used in the country. None of this was mined or refined in the USA, although some negligible amounts were obtained as a by-product from a few mining operations.

This lack of domestic production and reliance on imports does not sit well with the USA, as it is a major producer of advanced superalloys used in propulsion systems, turbine blades and jet engines. In addition, rechargeable batteries, memory chips, satellites, hard disk drives, permanent magnets and hybrid/electric vehicles all use high purity cobalt. Less pure versions can be used in hard metals, solar and fuel cells, and in catalytic converters for the oil and gas industries.

Straight to market

Recognising both a strategic and commercial demand for the element Formation Metals Inc, Vancouver, Canada, is developing in Idaho, USA, a geological resource and a processing plant capable of meeting some of the demand. According to its CEO, Mari-Ann Green, the state offers a unique opportunity, not only to begin mine production, but also to offer a refined product direct to market.

The geological resource is hosted in the Idaho cobalt belt, a 30-35-mile-long metallogenic district characterised by stratiform copper-cobalt deposits.

The host geology is a thick, dominantly clastic, sequence of Middle Proterozoic age, sandwiched between late Proterozoic quartz monzonite intrusions. The clastic sediments were deposited in a large fault-bounded basin, probably as large submarine fan complexes and/or deltaic aprons that were frequently drowned by continuing subsidence within the basin. All significant copper-cobalt deposits and occurrences are found in the Apple Creek Formation (ACF), which constitutes the base of this sequence. The formation age is interpreted from dates of 1.37Ga for adamellites intruding the sequence, and 1.7Ga from mafic dykes and sills emplaced along the basin margin faults. The ACF structure is dominated by a regional rift. Cobalt-copper-gold mineralisation lies along a northwest-southeast trending structure, parallel to and west of the central axis of the rift.

On a local scale the ACF is over 16,000m thick with a 1,200m middle unit of argillite, siltite and quartzite containing the mineralisation. The mineralised district has been known since 1892 and initially mined for cobalt in the First World War. Since that date, a number of companies have explored and mined in the belt but none survived more than few years during times of strategic demand or high commodity prices. In 1994, Formation Metals Inc staked ground that had formerly been subject to a feasibility study.

Work by the company to determine the resources on its licence areas included the Ram and Sunshine deposits. However, the Ram deposit has sufficient reserves that the company decided to base its business model on this, keeping the adjacent Sunshine deposits in reserve. Although the analysis of trench samples has helped to delineate the mineralisation, core drilling has been the principal resource tool. Using 47-64mm diameter core for resource estimation and 85mm holes for metallurgical work, over 16,700m have been recovered on the Ram property. Aggregating the mineralised horizons within the Ram deposit, the total measured and indicated resource is 2.39Mt with an average cobalt grade of 0.63%, copper 0.65% and gold 0.45g/t.

Mining methods

The Ram deposit will be mined by underground methods, using mechanised cut-and-fill for narrower stopes, and longhole stoping for the thicker portions of the deposit. The southern one-third of the 3023 horizon can be mined by this method if the portions of the footwall that dip at angles less than 55 degrees are steepened.

Once the working level of the stope has been mined, it will be filled by paste backfill material that is fortified with two to four per cent cement. The fill will be in the form of a paste made from dewatered mill tailings. The paste will be piped from the paste plant located near the process plant and distributed down mine vent raises or the portal into the mine paste distribution system. The Ram deposit is accessed by a decline driven from the 7,060 elevation near the top of the measured mineralisation. The decline serves as the main access and haulage way for the deposit, with ore transported up the main decline to the portal pad. Here it is loaded into an aerial tramway for transport to the plant site.

In 2002, Formation had the opportunity to purchase the Sunshine precious metals refinery
located near Kellogg, Idaho. ‘We are currently building a hydro-metallurgical plant adjacent to the refinery to produce high purity cobalt to supply the US market that consumes 60% of the world’s supply,’ states Green.

Test work on the low-sulphide ore from the Ram deposit has shown that grinding the ore followed by flotation would result in a concentrate of 14.4% cobalt, 7.41% copper and 11g/t gold, with recoveries for the base metals of 93% and 73% for gold. The flotation circuit comprises two banks, each with four rougher and three cleaner cells. The volume of each rougher cell is 100ft3 which provides a total residence time of approximately 11 minutes. The cleaner cells have a volume of 24ft3, with a total residence time of 21 minutes. The final dewatered concentrate is sent to the hydro-metallurgical plant. Further testwork on the large-scale samples from the large diameter cores indicates that gravity separation on the flotation tailings would recover 94%, 93% and 78% of the cobalt, copper and gold, respectively.

Testwork by the Center for Advanced Mineral and Metallurgical Processing, Montanna, USA, and by SGS Lakefield, Ontario, Canada, in support of the feasibility study has demonstrated that a nitrogen-species catalysed leach is effective in recovering 99% of the cobalt and 93.4% of the copper from the flotation concentrate. The metallurgical test was based on the parameters shown in the table above and optimised for acid use.

Further tests by SGS, based on flotation, lowering the leach temperature to 50°C, and increasing oxygen overpressure to 8.5bars, is a route to continuous operation when combined with flash cooling and heat recovery. Continuous autoclave operation, incorporating nitrogen species as a catalyst, together with flash cooling, has significant advantages. The leach retention time is extended while throughput is increased. Operations are simplified and a continuous source of steam is recovered for future use. Full details of the process are proprietary and currently in the patent process.

‘We are on track to become the first high purity cobalt producer in the USA. The Ram deposit mine will produce around 800t/day of ore, that in turn will feed 58t/day of copper/cobalt concentrate to the Big Creek Hydro-metallurical plant. This will result in annual production in 2011 of 1,650 high purity tonnes of cobalt and 1,500t of copper,’ claims Green. ‘The estimated cash costs are under US$10/lb with copper credits against a recent LME price of over US$19/lb.’

 


Metallurgical estimated parameters

Parameter Value
Pre-leach temperature 90°C
Pre-leach retention time 60 minutes
NSC leach retention time 120 minutes
Acid addition (sulphuric) 50g/l
Nitrogen species addition 4g Na NO2/l
Sulphur dispersant addition 5kg/t
Slurry density 7.4% solids
Oxygen overpressure 15lbs/inch2 at 160°C

Further information: Formation Metals Inc