Chapter 15. Gravity SeparationIS. INTRODUCTION Separation by density difference is a process that is as old as recorded history. Separation of gold by density difference dates back to at least 3,000 BC as depicted in writings from ancient Egypt. The principle employed in gravity separation goes back further in time to the formation and weathering of the rocks and the releasing of the minerals they contain and the transport of the mineral grains by heavy rains. It is the driving force for the formation of the alluvial deposits of precious metals and gemstones that have been worked since beyond recorded history as they still are today. Archaeological excavations have discovered mineral concentration activities such as the lead-silver concentrating plant in Attica, Greece, dating from 300-400 BC. So gravity separation has a long history as a mineral concentration process. Not all mineral combinations are amenable to this type of concentration technique. To determine the suitability of gravity separation processes to a particular ore type, a concentration criterion is commonly used. A concentration criterion (CC) can be defined as : . . . SG of heavy mineral-SG of fluid Concentration Criterion = SG of light mineral - SG of fluid where SG ,
C oncentration Criterion
O 3.0 3.0
c 2.0 .2 2.01.5 1.5-
- - -
> 2.5 1.75 3.0
FeSi 212 m
Media viscosity, Pa.s
0.04 0.03 0.02 0.01 0 1 2 3
FeSi - atomised
FeSi 150 m
l e ad
Fig. 15.16. Viscosity of the heavy media produced from selected solids .
should be low since the viscosity of a suspension of solids is dependant on the solids concentration and the particle size. For efficient separation of a feed containing a wide size range, the media volume concentration should be kept to 35-38%. For a narrow size feed, the media can be up to 45-48% solids by volume. Fig. 15.16 shows the viscosity versus medium density for a number of common media solids. The dotted line in Fig. 15.16 represents an approximate critical concentration of solids in water above which the media viscosity is too high for efficient separation.
Example 15.4 How much 10% FeSi needs to be added to 1L of water to make a heavy liquid of S.G. 2.8? What is the maximum media S.G. that can be used with this solid? Density of FeSi = 7000 kg/m3 Solution Step 1: Calculate the mass of solid Density of water = 1000 kg/m3
Volume of water = 1L = 0.001m3 Mass of water = 0.001 x 1000 = 1.0 kg _. . ,,, ,. mass of medium Density of heavy medium = volume of medium Let the Mass of solid = X kg then volume of solid = X/7000m3 Therefore, density of medium = 2800 =X
mass of solid + mass of water volume of solid + volume of water
X + 1.0 7000 + 0.001
and solving for X:x X
(2800x0.001)-! _ ^
17000JStep 2: Calculate the maximum medium density. Assume that the viscosity limit of the medium occurs at a volume % solids of 35%. Let the volume of the liquid medium = 100 then the volume of solid = 35 and the volume of water = 100-35 = 65 The density of the medium then is given by : media density =massofmedium =
volume of medium
_ (35x7000) + (65x1000) _ 245000 + 65000 _
This calculation assumes that the solid is immiscible in the liquid and hence no volume change will occur. In the case of a soluble solid or two soluble liquids, the volumes will not be additive.
75.6.3. Types of Dense Medium Separators The aim of the dense medium separation is to produce a float product of lower density and a sink product of higher density than the medium. In some instances a third, middlings product is also produced. A variety of equipment is used to bring about this separation and they are
usually classified Into bath or trough separators and centrifugal separators, depending on the separation forces employed. Gravity Dense Medium Separators In gravity dense medium separators the minerals and dense medium are fed into a large quiescent pool or tank or trough of the medium. Particles denser the medium will sink and the low specific gravity particles float. The floating material overflows or is removed from the bath by scrapers while the sink material falls to the bottom of the tank and is removed by some means. The many types of static bath separators include those used for coal separation and mineral separation. Since coal separation has a very high floats content in the feed (up to 95%), the separator will need a high floats capacity whereas separators for the mineral industry may require a high sinks capacity depending on the ores being treated (up to 80% for iron ore). Operating requirements therefore differ depending on the type of ore being treated. The types of separators include: 1. drum separator consisting of a cylindrical rotating drum, used for mineral and coal separation. The floating product overflows from a weir at the opposite end to the drum feed. The size of the drums range up to about 4.6 m diameter by 7.0 m long, with capacities up to 800 t/h. The drums may consist of a single compartment, producing two products from a single dense medium suspension or consist of two compartments with two baths of different density to produce three or four products. Drum feed size ranges from 6 mm to 30 cm. Drewboy bath separator is used widely in the cleaning of coal. The coal is fed into the bath at one end and the floats scraped from the opposite end while the sinks are lifted out from the bottom of the bath by the vanes of a slowly revolving inclined wheel. The Drewboy bath has a high floats capacity and handles a feed from 12.7-600 mm at up to 820 t/h for a 4 m diameter bath. Cone Separator is used for ore treatment since it has a relatively high sinks capacity. The feed is dropped into a gently agitated media bath. The floats overflow a weir while the sinks product is removed directly from the bottom of the cone shaped vessel by pump or by an air lift. Cone separators are available in diameters from 0.9 to 6.1 m, with capacities up to 450 t/h. The method of sink discharge limits the maximum feed particle size to about 10 cm. The Chance cone is a similarly shaped vessel to the cone separator but differs from the normal dense medium methods in that it uses a rising flow of water to fluidise a bed of sand to simulate a dense fluid. The sand used is sized between 150-600 \an and when fluidised by water rising at 6-12 mm/s the density is in the range 1.4-1.7 and hence is suitable only for coal/shale separation. Gravity control is achieved by varying the water flowrate. Shale or refuse is discharged periodically through a double gate arrangement at the bottom of the cone.
Centrifugal Dense Medium Separators The buoyant forces acting on the light particles in a dense medium cause them to rise to the surface but the dense particles, being heavier than the liquid they displace, sink to the bottom. The magnitude of the gravitational and buoyant forces that separate the particles is a primary consideration because it governs the velocity with which the particles separate, which it turn determines the capacity of the separating vessel, hi a static bath the net gravitational force minus buoyant force may be written as follows;
Fg = ( M s - M F ) g where Fg Ms MF = gravitational force, = mass of solid and = mass of fluid displaced by the particle.
For particles which float, Fg will have a negative value and for sink particles, it will be positive. In a centrifugal separator, specific gravity separations result from application and utilisation of similar forces except that the acceleration of gravity is substituted by a centrifugal acceleration. The equation then becomes: .v 2 Fc = (M S -M F )IV
Fc v R
= centrifugal force, = tangential velocity and = radius of the centrifugal separator.
The centrifugal force will be balanced by the resistance of the liquid when the terminal velocity is reached. For small forces, as experienced by particles with a specific gravity near that of the medium, the particles fall in the Stokes range where the fluid resistance is essentially due to viscosity. However, for large forces, the particles will fall in Newton's range where the fluid resistance is primarily inertial and substantially independent of viscosity. It is, therefore, not possible to write an exact equation for the terminal velocity that would be applicable for all particles. Nevertheless, it is apparent that the forces causing the particles to separate in a static bath are proportional to g, whereas in a centrifugal separator they are proportional to v2/R which is much larger. Cyclones can be used to develop this centrifugal force. In a typical cyclone the centrifugal force acting on a particle in the inlet region is 20 times greater than the gravitational force in a static bath. In the conical section of the cyclone, v is further increased according to the relationship: vVR = constant (15.27)
At the apex of the cyclone where R decreases, the acceleration increases to over 200 times greater than gravity. Thus, the forces tending to separate the light and heavy particles are much greater in a cyclone than in a static bath. This offers two advantages: 1. 2. a relatively high capacity and Because the forces acting on the small particles are also much larger than static separations, the cyclone is much more applicable to the separation