Simplistically, the yield stress provides a measurement for how thick a material is, or, practically, its resistance to flow. It measures internal pressure and is quantified by pascal units. For slurries of the same particle size distribution (PSD) and mineralogy, the yield stress is proportional to the solids content.
However, if you’re comparing two materials with different PSD, the yield stress curve will retain its shape but move along the x-axis. For example, mineral sands slimes with a large portion of less than 2um particles are seen as a paste at 20% weight per weight (w/w) solids, but coarse copper tailings with a large portion of particles at 250um will not exhibit paste-like behavior until approximately 70% w/w solids.
Yield stress and Thickener types
Underflow from a High-Rate Thickener will have a yield stress of 10-30 Pa and look like tomato ketchup, while underflow from a High-Density Thickener will have a yield stress of 30-100 Pa and resemble molasses. The Paste Thickener underflow will have a yield stress of 200-250 Pa and look like toothpaste.
How to select a Thickener
What you want to do with the thickener underflow will influence your selection. If you want to pump a reasonable distance with a centrifugal pump, then a High-Rate Thickener is the obvious choice since the underflow will be less viscous and easier to pump.
If you want to minimize the Tailings Storage Facility (fancy words for a pond) with a non-segregating thick deposit, you may opt for a Paste Thickener. However, to get the paste output, you need a tall tank for lots of compression and a really robust rake drive to move the material. You’ll also likely need specialized pumping and piping, and you guessed it, this all this comes at a cost.
The High-Density Thickener is for an in-between case. With the High-Density Thickener, you can:
- improve water recovery when it is important but not critcal
- keep capital costs reasonable
- produce the thickest mud that can be managed with centrifugal pumps
- when using for filter feed, reduce capacity and costs by making the most of gravity and creating a material that will still flow well enough for even distribution inside the chambers
With these benefits in mind, a High-Density Thickener is the technology of choice for:
- surface disposal to confined space, such as a worked-out area of surface mining
- filter feed scenarios where filtered tailings may be desirable and overall dewatering plant costs should be considered
- leach feed applications where it's important (or at least cost-effective) not to dilute leach reagents
High-Density Thickener design
What components make up a High-Density thickener? It’s important to include features of a Paste Thickener, such as dewatering pickets and a steeper floor slope, along with a higher torque mechanism at the drive to cater for the increased yield stress in the underflow mud.
The top part of the High-Density Thickener, however, is going to look much the same as its cousins the High-Rate Thickener and the Paste Thickener. Here, creating the optimal feed solids to promote the maximum free-settling flux and mixing the flocculant and/or coagulant at the right place and in the right shear environment remain critical to the High-Density Thickener’s effective operation.