Novel approach for processing complex carbonate-rich copper-cobalt mixed ores via reverse flotation

Abstract

The vast majority of current global production of cobalt from primary sources originates from extraction of Cu-Co sediment-hosed deposits in the Democratic Republic of Congo. With the progressive depletion of oxidic supergene mineralisation, many operations are progressively starting to extract sulphide ore at depth. Ore in the transition zone between oxides and sulphides usually hosts both oxide and sulphide mineralisation which is extremely difficult to process by conventional flotation. These mixed ores are complex to process by flotation alone because carbonate minerals, such as dolomite and magnesite, may represent a significant proportion of the gangue. Hence, mixed ores are usually processed through flotation of sulphides followed by oxide flotation using controlled potential sulphidisation (CPS) and further processing via acid leaching. The main challenges associated with carbonate-rich mixed ores are the low recovery of cobalt, mostly due to cobalt present in oxide minerals, the floatability of the carbonate gangue minerals, which can make up to 50% of the concentrate, and technical difficulties with regards to industrial application of CPS. This study investigates alternative options to the classic two-step CPS flotation route, including direct reverse flotation of the carbonate gangue at neutral or acid pH, as well as a two-step combined sulphide-reverse flotation route. The latter, including a sulphide flotation stage at neutral pH, followed by an acidic reverse flotation stage at pH 4.5–5, increased the overall copper and cobalt recoveries. The best performance is achieved with reverse flotation, using Na-Oleate and a mixture of phosphoric and sulphuric acid (ratio of 4:1). Overall Cu-Co recoveries of 93.5% and 85.1% respectively were achieved. It is postulated that phosphoric acid acts as a depressant of the Cu-Co oxide minerals through surface passivation while enabling selective flotation of magnesite and dolomite. Further investigation into the effect of phosphoric acid on flotation of these minerals is required. Overall, the results suggest that, following flotation of sulphides, an acidic reverse flotation stage increases the global Cu-Co recovery while producing a concentrate which is suitable for further processing by flotation or acid leaching. Hence, the suggested combined approach could be a viable alternative to aforementioned CPS stage in Cu-Co oxide-sulphide mixed ores flotation plants, especially in operations where the mixed ore contains significant proportions of carbonate gangue minerals.