Gravity-based procedures have been used in the processing of REE minerals for many years, especially when the REE minerals are found in alluvial or sand deposits. These procedures are applicable for REE ores that show a difference in specific gravities of the contained minerals, i.e. bastnäsite, monazite, and xenotime all have specific gravities of approximately 4.5-5.0, while common gangue minerals such as quartz, calcite, and dolomite have specific gravities less than 2.8. This difference allows for an efficient separation to be possible. Other important factors are the size and size distribution of the REE minerals, with the coarser grain size being more efficiently recovered. However, there are enhanced gravity machines that can recover high specific gravity minerals down to 10um.
The most common use of gravity processing is for pre-concentrating ores in the initial stages of mineral processing by removing as much gangue mass as possible. Also note that this process has the lowest operating cost, requires no reagents, and can lower the capital and operating cost of all downstream processing.
There are a few process requirements, outlined here, which are necessary to ensure high process efficiency with hard rock ores:
- The amount of grinding or size reduction is to provide the appropriate amount of REE mineral liberation that shows the particles have the necessary specific gravity difference. This implies that size reduction at this stage is to provide just enough so that the process is efficient. After the gravity stage, additional grinding can occur to suit the subsequent process step.
- The ground material needs to be size-separated, with each size fraction being processed with dedicated size appropriate equipment. Processing of un-sized material leads to low process recovery, and is due to the processing equipment efficiencies having high dependence upon grain size.
- The viscosity of the mineral slurry needs to be minimized to allow appropriate mineral movement, which involves de-sliming or adequate amounts of water use.
The most common types of standard gravity equipment are shaking tables, spirals, and Reichert cones. Enhanced gravity procedures utilize Kelsey jigs, Knelson concentrators, Falcon concentrators and Mozley Separators.
Spirals at work!