The mining area includes three major resource deposits to date. The size and position of the deposits are well-suited to conventional open pit drill and blast operations. The ore geometry results in a particularly low stripping ratio of 0.72 (waste:ore), with 12 metre benches and a final pit wall slope of approximately 48-50 degrees depending on deposit area selected for the proposed production rates. It is currently anticipated that ore production will commence simultaneously at Rönnbäcksnäset and Vinberget, with full production expected to be achieved in Year 2. Mining at Sundsberget is then forecasted to commence in Year 5 and reach full production by the time the Vinberget deposit is depleted.
Mining transport options to be considered during project development include:
- bulk movement options such as trucking, in-pit crushing with conveying or trolley assisted haulage
- utilisation of waste in several alternative uses.
Historical testwork was initially carried out by Boliden during the 1970s where a lab and large scale 4000 tonne pilot program, facilitated by test pit mining, achieved 26 to 34% nickel grade and 67 to 73% sulphide nickel recovery.
Metallurgical testwork has been undertaken on samples, extracted from drill cores, of the nickel sulphide ores from Rönnbäcken to determine the mineralogical, comminution and metallurgical properties of the various mineralized zones within the deposits. The purpose of the test work program was to develop a process flow sheet that maximises recovery of nickel and cobalt whilst minimizing the incorporation of penalty elements.
Through minipilot operation it was demonstrated that a nickel feed grade of 0.17% total nickel produced a concentrate with a grade of 22% nickel at 80% recovery. Follow-up laboratory-scale testwork indicated that concentrates with an increased grade of 28% nickel at 80% sulphide nickel recovery could be produced from the same composite samples used in the minipilot operation.
Proven and conventional technology
Based on the testwork performed, the flowsheet developed for the Rönnbäcken concentrator consists of conventional crushing, grinding, flotation, and dewatering steps, typical of many concentrator operations elsewhere in Sweden and Finland, as shown in the figure below. Two processing lines will be used in grinding and flotation to facilitate high reliability in operations. The conceptual concentrator design has been provided by Outotec (Sweden) AB.
Opportunities for value-added products
Metallurgical testwork has been performed to evaluate the potential for the recovery of a saleable magnetite concentrate from the nickel flotation tailings stream. Open circuit batch tests performed on the tailings from the mini-pilot plant work achieved a magnetite concentrate grading 66.2% iron with a magnetite recovery of 90.3%. Such a concentrate can be produced using flowsheets comprising desliming, multiple stages of low intensity magnetic separation, concentrate regrinding, flotation and product classification. While a 66% iron containing concentrate could be produced, the occurrence of higher levels of nickel, chrome and zinc and the fine particle size of the concentrate indicated that the product would not be attractive for carbon steel applications. Nickel Mountain is considering conversion of the magnetite concentrate to direct reduced iron/hot briquetted iron (DRI/HBI) for use as a scrap substitute or supplement in low alloy and stainless steel manufacture. A potential target market for sales of a high chrome/nickel DRI/HBI product is in relative proximity with a number of low alloy and stainless steel producers located in Sweden or within the European region.It has also been identified that the magnetite concentrate could potentially be suitable as dense medium (DMS) in coal processing applications. Initial testwork has indicated that the magnetic properties were promising and that the size distribution of the concentrate was acceptable. Further dedicated testwork and market research are required to assess the potential of a DMS application.
Similarly, Nickel Mountain is assessing the production of ferronickel, a value-added product, from further processing of the Ronnbacken nickel concentrate. An initial desktop study has indicated the possibility of producing either an intermediate calcine product for sale to ferronickel smelters, or a final ferronickel product for sale to stainless steel mills.
While all of the above-mentioned product options are encouraging, further technical-economic studies will need to be conducted to generate more detailed operating costs and capital expenditure estimates and to confirm the viability of such process routes.