Applying the force

Remember how in the last article I belittled the earth's G? Well, I am sorry and I take it back. It turns out, you see, that the natural product has a very special quality - it acts at once uniformly and on all particles. Like the ethereal force of a magnet, small, large, hidden or obstructed particles all are acted upon, whether they are in the depths of a viscous slurry or in thin air. The problem with induced centripetal force is that we have to impart G to the particles. Suddenly a whole family of forces and their effect comes back into the equation - drag, impact, friction, etc. In the course of imparting the forces, the whole system is altered and this can have a profound effect on the dynamics at play.

Let's illustrate this with our very own Knelson concentrator - the homogeneous slurry entering the machine vertically down the feed tube encounters the rotating deflector pad at the base and through friction with the spinning surface the slurry is given a radial acceleration. Not all gets the same crack, though, the material closest to the pad will receive the greatest boost and the momentum will be transferred to the rest by frictional drag. Then the whole mess encounters the cunningly angled cone wall and the slurry begins to segregate, becoming denser and denser the closer it gets to the wall as the water squeezes out. Imagine the task of a fine gold particle encountering this thick, viscous, coarse goo and having to muscle its way in to the cone wall to find refuge! So at least initially it looks like the Knelson we have come to love and trust hasn't a hope. All is not lost, however, the beast is even cleverer than we thought. Enter Byron's famous fluidization water and things loosen up a little, just enough to allow the small, light gold particles to burrow in. Consolidation trickling the boffins call it, and bless them for that.

Once our gold particle halts at the wall, well relative to the cone that is, the business of concentration is pretty much over and we are into the realm of capture. In fact the batch machine is not a concentrator but a capturer. Knelson Capturers Inc, sounds like a bunch of security hoods. A subtle difference, but a very important one, as it happens, because the emphasis changes from getting our particle to stay, to that of getting everything else to go! Now drag on the surface of all those big bad gangue particles comes into action and sweeps them to oblivion. Collisions, jostling and rollover take their toll too, and as the slurry is diluted towards the top of the cone by that same fluidization water, the whole effect becomes frantically successful. And all the while the little gold particle is firmly and safely tucked away in its bed watching the world rush by and looking forward to a flush.

Bear with me on one more grueling paragraph while we take a closer look at this rushing slurry business. The particles stuck to the wall are moving radially with the same velocity of the cone and have no vertical component. The further out we move into the mainstream, the greater the vertical velocity component and there is a progression between the two extremes. So the particles at the cone surface have a much longer residence time than those flowing further out, and lo and behold, this too assists in our wonderful recovery as each ring progressively has a chance to sedately offer itself up.

So, an artificial "G force" that looked none too promising ends up making amends in a machine that is too smart by far, nice colour too. Byron of course knew all of this before the paint on old Number One was even dry. As with all good inventions, the PhD's come later.