There are two centrifugal concentrators employed in metallurgical tests and industrial plants, the Knelson and Falcon. The first one is a centrifugal concentrator can recover fine particles of free gold. The pulp is screened in order to avoid coarse particles such as 4 mm or more. The material in slurry form is fed centrally to the bottom of a ribbed inner cone rotating at a high speed. Centrifugal force causes the first feed to fill the bottom rib with solids and the next higher slightly larger diameter rib with solids and the next higher slightly larger diameter rib and subsequent ones are sequentially filled in few seconds. After that all the feed moves upward as thin film over the conical surface of the sand filled ribs and out. Compaction of solids trapped in the ribs of the cone is prevented by adding water through a series of holes in the cone wall. In this way, the trapped solids are fluidized.
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| Laboratory Knelson Concentrator | Laboratory Falcon Concentrator |
The Knelson concentrator employs the principles of hindered settling and centrifugal force. A central perforated cone containing horizontal ribs is rotated in order to get a centrifugal force of 60g (g is gravity acceleration). The cone is surrounded by a pressurized water jacket that forces water through holes to keep the bed of heavy particles fluidized. In this way, the force of the water acts against the centrifugal force of the rotating cone.
If there are free gold particles or particles transporting gold in the film of slurry that is passing over the fluidized solids trapped in the ribs of the cone, they will penetrate the fluidized bed under enhanced gravity situation and displace lighter particles of similar volume trapped in the rib. This process is repeated several times until saturate the ribs with heavy particles. In other words, this is a trading process that exchange light particles for heavy particles.
The Falcon concentrator consists of a fluidized bed spinning bowl gravity concentrator. It is has been designed primarily for gold recovery from grinding circuit hydrocyclone underflows or alluvial operations. The concentrator is capable of fine and coarser feed applications. Enhanced gravity concentrators accelerate feed slurries in a rotating cone shaped bowl to take advantage of the difference in density of valuable minerals and gangue in order to perform a separation.
The material is stratified according to specific gravity and then passed over a concentrate bed fluidized from behind by pressure water. The fluidized bed is required to retain coarse particles. Free gold or gold bearing minerals are retaining in the concentrate bed in preference to light minerals. Initially the design considered non-continuous operation, but with the years the design was improved and this centrifugal concentrator can work in continuous or not. There are several types of equipments to each need. The equipment functions and rising cycle are automated with variable frequency drive device.
Generally, the higher the field or the stronger separation gravity forces between different particles of different density, the more fast and efficient is the separation. Within limits, an enhanced gravity concentrator can treat more material and recover finer particles if it is spun faster. Falcon concentrators are able of centrifugal fields of 300g. The equipment can change this value due to the unit is equipped with a variable speed motor. Concentrating surface area is the single most important determinant for enhanced gravity concentrator. Similarly to jigs or shaking tables, the larger the surface area, the higher the overall capacity since performance is a function of residence time.
Many metallurgical laboratories have both concentrators and ideally the testing program can evaluate the performance obtained by using a Knelson or Falcon Concentrator according to the ore to be tested. It is important to mention that minerals present in different samples modify the results to be obtained. Results give a preliminary idea and the final test must be carried out with 20 – 30 kilos whose feed was done at steady state. Laboratory tests are done with 3-5 kilos and the reproducibility of the results is sometimes difficult. For this reason the final test is done with more material.
Centrifugal Concentrate cleaned in vanning dish