Earlier articles have illustrated how automated scanning electron microscopes can be used to help with the choice of reagents to solve metallurgical problems in tailings. Here, we will discuss how the process can be used to reduce gangue minerals in concentrate streams. A-SEM data is now regularly used to identify the gangue minerals in flotation concentrates and can be used to focus efforts on their removal.
In the Betachem/Nasaco process mineralogy laboratory in Johannesburg, the team develops chemical solutions to the standard problems of poor concentrate grades. Client samples are scanned using one of the lab’s three A-SEM units (MLA from FEI). Gangue minerals are identified and particle size measured. Treatment methods are then suggested with reagent testing as appropriate.
The following text explores the five basic reasons why gangue minerals report to concentrate.
Naturally floating gangue mineral
In the platinum flotation concentrate shown in Fig.1. the PGMs are present as discrete PGMs and also locked with the three sulphide minerals. Talc and pyroxene are present in the final concentrate as naturally floating gangue. 69% of the talc and pyroxene are present as fully liberated 10-212µm particles. The talc and pyroxene were eliminated with the use of a specific talc depressant.
Gangue locked in valuable mineral
In Fig 2. this polymetallic flotation plant was producing poor zinc concentrate grades. 57% of the dolomite gangue is locked in the sphalerite grains. The only way to improve this situation is to adopt a regrind step to liberate the spaherite prior to recleaning.
Recovered as entrained fines
It is important to minimise Cr content in flotation concentrate in South African UG2 Platinum flotation operations. In this concentrate the Cr is present as chromite and 92% of the chromite is fully liberated. 73% is smaller than 10 micron and 20% is in the 10 to 212 µm fraction. The problem was improved with the choice of a different collector and specific depressant.
Recovered as larger particles in a non-draining froth
A gold flotation plant floats pyrite to recover the gold locked in the pyrite. The concentrate is leached to recover gold. The volume of concentrate needs to be reduced to increase leach residence time. Quartz was found to be the main gangue mineral, was well sized and liberated. Changing the frother gave a more mobile drainable froth and reduced the gangue content in the concentrate.
Other reasons, which may include process efficiency
A South African UG2 reef PGM flotation plant experienced poor concentrate grades. MLA analysis showed that 83% of chromite was present as well sized liberated particles. This was a recent phenomenon as earlier analysis had shown 33% being present as such. It was concluded that the process was out of balance and steps were taken to control process flows which resulted in a return to normal levels of Cr in the flotation concentrate.