Dry separation equipment first appeared over a century ago when Edwin Steele and Henry Sutton, principals at Sutton, Steele & Steele, Dallas, Texas, invented the fluidized-bed separator, then called the specific-gravity separator. The firm held dozens of patents on dry concentrating methods that used static electricity, air, vibration and combinations of the three to accomplish their purpose.
Originally developed to concentrate gold and other metallic ores without using water, by 1919, this new separator had found its way into many other markets, including field seeds, peanuts, peas, beans and corn, beach sands, coal, cork, chemicals, and many other industries processing dry bulk materials.
The above drawing is a hand-drawn patent application from the specific-gravity separator from 1919. In 1931, Sutton, Steele & Steele was recognized by the Franklin Institute of Philadelphia with the John Price Wetherill medal for Discovery, Invention or Development in the Physical Sciences.
The gravity separator, also known as fluidized-bed separator, air table, or density separator, makes a highly sensitive dry separation on the basis of one of three particle characteristics – density, size or shape. When two of these characteristics are controlled within certain limits, the gravity separator is unmatched in its ability to separate a complex mixture into a continuous gradation across the range of differentiating characteristics (light to heavy; fine to coarse, or platy to (granula), while permitting the isolation of many intermediate fractions between the two extremes.
The ability to produce intermediate or "middling" fractions distinguishes it from other kinds of dry separation equipment. For example, when processing copper wire, a gravity separator will divide into copper, insulation and copper in insulation so the latter can be reduced further, then brought back to the gravity separator. This property and this property alone, permit the development of high-purity concentrations without loss of efficiency in recovery.
In addition to material density, the relative size and shape of each component of the mixture also bear on the efficiency of the separation. Wide variations in these material characteristics can dramatically affect the separation results. Where a wide range of particle sizes is present, screening may be required to segregate materials into manageable size ranges prior to separation. Where significant variations in shape are found to be detrimental to separation efficiency, size reduction may be added to the process to reduce the range of variation. These factors become more important as the densities of the materials to be separated become closer.
In operation, the material is fed onto the narrow side of a flat porous deck, sloped in two directions and vibrated with a straight-line reciprocating motion.
Low-pressure air, blown upward through the deck, fluidizes and stratifies the material according to differences in the terminal velocity of the particles. Heavy particles sink to the bottom of the stratified bed and are conveying upward toward the high or "heavy" side by the deck’s vibration. Light particles, lifted by the fluidizing air, flow down slope toward the light-end discharge. Particles with intermediate characteristics form a mixture between the light and heavy fractions and may be drawn off for retreatment. Affected by both the vibration and air flow, the material bed thins as the deck broadens toward the discharge face. Here the material is arrayed from heaviest to lightest in a thin layer, which can be precisely and easily divided into multiple fractions. Adjustable cutting fingers, positioned to make the final selection between separated fractions, direct each fraction to a separate discharge spout.
Gravity separators are generally available in two basic designs—rectangular-deck models and the more common trapezoidal-deck models. Rectangular-deck separators are recommended strictly for light-end separations where the objective is to separate a clean, light tailing from a larger amount of heavy material. Conversely, trapezoidal-deck separators are recommended for heavy-end separations, requiring the removal of a relatively small amount of heavy material.
Fundamentally, the operation is the same as it was at the turn of the century.
Part I of this series provides a high-level overview of how gravity separators work.
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