Knelson CVD - Analysis of Operating Variables
M. McLeavy, B.A.Sc. University of British Columbia
B.Klein, University of British Columbia
I. Grewal, M.A.Sc. P.Eng; Knelson
July 25th, 2001
The Knelson Continuous Variable Discharge (CVD) is a continuous centrifugal gravity concentrator for high-mass yield recovery applications. It has four operating variables that enable control of mass yield, product grade and recovery namely: fluidisation water, bowl speed, pinch valve open time and pinch valve closed time. Plant testing of a pilot scale Knelson CVD6 was conducted to evaluate the effects of operating variables on mineral separation. The analysis of operating variable effects has provided a framework for the development of operating guidelines for the CVD.
The evolution of gravity concentration from batch to continuous centrifugal machines has extended performance to higher recoveries and higher upgrade ratios at finer particle sizes. Semi-continuous units, such as the Batch Knelson and Falcon SB, have been widely accepted for recovery of free gold within grinding circuits. Fully continuous machines (Kelsey Jig, Falcon C, Knelson CVD) that can recover mass yields as high as 50 per cent have yet to find their niche. Extending the particle size limits of spirals, improved metallurgical performance over non- sulphide mineral flotation and pre-concentration are incentives to develop these devices further. Issues that need to be addressed are mechanical reliability, water use, and separation performance (Burt, 1999; Brewis, 1995; Clifford, 1999; Holland-Blatt, 1998).
The selection of appropriate operating variable levels for ores with varying particle sizes, mineral compositions and pulp densities represents a significant challenge. In the Falcon C, feed enters through the top and travels down to the bottom of a rotating bowl where centrifugal acceleration forces particles to the wall. The particles travel up the bowl section where heavy particles displace light particles along the bowl wall. At the top of the bowl is a concentrate collection ring with valves positioned radially at the back of the ring. The valve aperture size is controllable and remains open. The tailing material forms the innermost layer on the bowl wall and overflows the bowl to a tailings launder (Falcon, 1999; Silva, Santos and Torres, 1998). The Falcon Model C is the simplest machine as it has only two operating variables; bowl speed and valve aperture size. No additional water is required to operate the machine.
In the Kelsey Jig, particles are fed into the top and enter the vertical jig bed. The water pulsation cycles in the hutch, behind the bed, facilitates particle separation. The centrifugal acceleration imparted on the heavy particles causes them to travel radially outward through ragging and into a hutch. Low density particles travel upwards across the bed and overflow to a launder. The three controllable variables in the Kelsey Jig are bowl speed, ragging size/density and pulsation stroke length (Geologics, 1999; Silva, Santos, Torres, 1998; Wyslouzil, 1990).
In the Knelson CVD, feed is introduced to the top of the machine through a feed tube into the centre of the bowl section. The feed hits a plate at the bottom of the bowl section and is dispersed radially to the bowl wall. The particles are accelerated to a g-force defined by the bowl speed and travel up the wall towards the ring. The partially upgraded slurry enters the separation ring where fluidisation water, supplied through holes in the ring wall, is added to fluidise the bed of packed particles. Concentrate is extracted through pinch valves at the back of the ring. The pinch valve timing (open/closed) can be adjusted. Light particles overflow the bowl into a tailings launder (Knelson, 2001).
Learn more on the CVD experimental program, results and discussions, operating variable performance ratio and conclusions by downloading the full technical paper.
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