In recent years, the Knelson Concentrator has become the predominant unit used for primary gold recovery by gravity. However, its application potential as a cleaner in the final gravity concentration stage and its separation mechanisms have not been well studied. In most gold mills, rougher gravity concentrates produced by the Knelson (operating at 60 gravity acceleration or ‘g’) are further upgraded with shaking tables. Non-centrifuge units operating at one ‘g’. As a result, tabling results in significant gold losses (mainly fines). It is also labour intensive and represents a security risk because of the high gold content of the material being processed. The main objectives of this research, therefore, are to study the separation mechanisms of the Knelson Concentrator and to explore its potential application as a cleaner to supplement, or even to replace tabling.
A methodology based on the use of synthetic feeds (tungsten. silica and magnetite) was designed to study the performance and mechanisms of a 7.5 cm Knelson. Four types of gangue were used, both fine and coarse silica and magnetite. The fluidization water flowrate was optimized for each gangue type, and the Knelson was then overloaded by feeding a high-grade feed. The extent of overload was found to be highly dependent on the size distribution and density of the gangue processed, from severe and immediate with coarse magnetite to non-existent with fine silica. A two-parameter model to describe the overload was derived. which showed a good fit with the experimental data.
Two separable bowls were designed to analyze the nature of the material recovered in the five grooves of the Knelson bowl. It was found that the densest minerals, such as tungsten and gold, were mainly recovered in the inner groove sections. closest to the rotating axis. The outer sections packed at the beginning of the recovery cycle and served mostly to distribute fluidization water evenly.
Three modified Knelson bowls. a cleaner bowl equipped with a filter at the bottom of each groove, the other two with different groove depth and fluidization water flowrate distribution, were tested using the synthetic feed methodology. Results generally confirmed the soundness of the basic designs of the conventional and cleaning bowls. The importance of superficial fluidization water velocity was also demonstrated.
Recovery of gold with the 7.5 cm Knelson from seven samples of gold table rails from different gold mills was tested. Recovery significantly increased when the coarser, largely barren fraction was removed prior to processing. Concentrate grade significantly increased, up to 60% Au, by feeding a magnetite “filler” prior to the recovery cycle, and removing it magnetically at the end of the test, to mimic the effect of the cleaner bowl.