The Knudsen bowl
The Knudsen bowl was invented by George Knudsen of California and patented in 1942 (US #2,272,675). It eclipsed the first rubber-riffled centrifuge – the Ainlay bowl invented by Thomas Ainlay of Nebraska and patented in1928 and 1932 (US #1,658,874 and #1,853,249). The Knudsen bowl has particular advantages:
- Bowl of non-magnetizable material e.g. aluminium:
- Resistant to wear, and easily, completely and quickly cleaned;
- Tough rubber riffles as liner, easily and quickly removed; and
- Separation helped by “…agitation resulting from the wobbling action of the bowl and by the kneading action of the soft flexible rubber ribs of the riffle member”
The Knudsen bowl is typically 12 to 36-inch indiameter and mounted on a vertical drive shaft. The wall slopes outward to create a gradient of g forces increasing upwards, inducing the slurry to climb the wall. The need for fine pre-screening makes the Knudsenbowl unsuitable for wash-plants, but it has merit for upgrading if two or even three bowls are put in series.
Pre-screened feed enters from above via a slurry pipethat discharges in the centre of the spinning bowl about 1-inch from the base. The slurry is flung sideward to slam against the spinning wall. The climbing film of slurry is impeded by concentric ribs of tough rubber riffles. Dense particles slam in the grooves between the riffles; Light particles climb the riffles to escape as tailings. Lab tests by Mark Anthony [57,58] show the inventor’sinstructions to be in error. With <3/8 inch gravel “the rocks could not be washed out of the riffles with a high-pressure hose and had to be removed by pulling them out of the rubber riffles with a pair of pliers.” This was solved bylimiting the feed to <1/8
th inch. The rotation speed has tobe at least 80 rpm or “…the bowl started dumping large quantities of slurry directly onto the ground and drive mechanism.”
Optimum recovery of gold >65 Tyler mesh(about >0.2mm) is at 105 rpm with 68 gallons/minute of water and a pulp density of 5-20% solids. Tests by ‘Dredger’ of Alaska Gold Forum in NewSouth Wales, Victoria, Tasmania and New Zealand suggest Knudsen bowls lose about 20% gold, mainly invisible fine gold whose recovery can be much improved by:
- using a binocular microscope to detect and monitor fine gold;
- screening the feed at minus 250 mesh (e.g. a second pass);
- adding 2 variable speed v-belt pulleys, with lockable leve rspeed control, enabling precise adjustment of the bowl’s rpm;
- ensuring a person watches the feed rate;
- attaching a good small test sluice to the outlet of the bowl;
- checking tailings with a x40 to x80 binocular microscope; and
- cleaning the bowl regularly.
The Kelsey centrifugal jig
The Kelsey centrifugal jib was invented by Christopher G. Kelsey of Australia and patented in Australia in 1985 (PH9037, PG0122) and 1990 in the USA (US #4,898,666). The first commercial sale was of a lab-sized machine in 1989. Geo Logics Inc developed the device, and automatic screen cleaning was introduced in1999. In 2001 Geo Logics was bought by Roche Mining who continues to make the jig (www.rochemt.com.au). The Kelsey centrifugal jig maximises its effectiveness and efficiency by combining the pulsation principle of a jig with the high apparent gravitational field of a centrifuge. The main models and operating variables are:
- J200 KCJ – lab test unit, 15-100 kg/hour of solids;
- J1300 MkII KCJ – smallest commercial unit, 2-30 tons/hour;
- J1800 KCJ – largest commercial unit, 5-60 tons/hour.
- feed-related variables – feed type; feed density; feed rate;feed size; and density difference between minerals.
- jig set-up variables – screen aperture size; ragging type,ragging density, ragging size distribution and depth.
- jig control variables – rotational speed (induced gravity field);pulsing frequency; pulse amplitude (distance travelled duringeach pulse stroke); and rate of adding hutch water.
The slurry feed is 25-40% solids and enters a chamber consisting of a circular screen mounted vertically, plus a particle bed. The chamber rotates at 30-45 rpm to create a force of up to 50g. Water injected into the inner chamber subjects the slurry to a jig-like pulsation force to alternately expand and compress the particle bed. This stimulates denser minerals to move towards the bed and so be collected after passing through the screen into an inner chamber and discharged via small holes. Lighter particles over flow the chamber as tailings. According to the maker,“the ability to change the apparent gravitational field, up to 50 times gravity, results in a major improvement in separation efficiency, particularly of very fine minerals, by significantly reducing the effect of forces that hinder fine particle separation.”
Hindered settling is accentuated by the centrifugal force together with the pulsing of the ragging bed. The pulsing is via pulse arms connected to pads to work against the jig’s flexible diaphragm.
“Water contained within the concentrate hutches presses against the diaphragm, at a frequency and amplitude set by the operator, thus dilating the ragging bed. The level of dilation impacts on the amount of material able to pass to concentrate.”
The pulsed shockwaves have two effects:
- dilating the ragging bed to allow minerals to enter it; and
- accentuating differences in acceleration between particles of different density. Separation of particles of similar size andshape but different density slows at their terminal velocity. The shockwaves repeatedly stop the particles, limiting their time at terminal velocity to maintain a high rate of separation