The last thing that I would like to mention about thickeners is to do with their safety. In some plants fire a worker who wears his hard hat or takes foreign objects onto the catwalks above the thickeners.
The reason is that if, by accident, his hard hat ended up in the thickener that thickener may be down for days because of it. The only way any object can be removed from the cone is by shutting down the thickener and emptying it.
With a hard hat they tend to enter the cone upside down. This means that they will seal the only place that the thickener can be emptied from. The alternative is to empty the thickener with a pump which is a long slow job.
As you can imagine, a hard-hat or tool or large rock would easily plug the thickener underflow and piping shown below.
