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An Introduction to Flowing Film Concentration

Launderers, spirals and tables fall into the general class of flowing film concentrators were the primary means of separation is a flowing film combined with stratification. They utilize the principal of flowing film separation, and combine this with bed stratification to enhance recovery and increase capacity.  The actual separation mechanism varies by device, but the general principal is having a feed stream a rate where the coarse heavy particles settle to form a bed which then aids the concentration of finer heavy particles.  The lighter particles are then carried away by the flow of water.  In the case of spirals, centripetal forces aid in the separation.

 FLOWING FILM SEPARATIONS

A flowing film in a gravity separation device introduces additional forces, besides what is discussed in An Introduction to Gravity Concentration. The principal effect encountered comes from the flow usually is not uniform from one place to another. As a result, the shearing forces between adjacent layers of fluid produce rotation and other effects on various particles.

 To make this clear, consider a fluid, such as water, flowing over a surface. If the velocities at different levels above the surface are indicated by arrows of proportionate length, the picture will be similar to that shown in Figure 1.

Figure 1 – Velocity Distribution in Flowing Current

 Taking the stream from Figure 1, and introducing particles into it , such as in Figure 2.  One of the effects of the flowing stream will be to induce rotation in the body as shown, since the top portion is acted on by currents of greater velocity than the lower. If the particle rests on the bottom, it will progress by rolling along the surface, and if it is higher in the stream it will combine a rotary motion with its forward one.

 The force encountered by a particle as a result of its contact with the underlying surface or bottom of a stream, or with other particles resting on the bottom, plays an important part in flowing stream separations.  If you consider a coefficient of friction existing between such a particle and the surface,  the force on the rolling particle would be the coefficient times the component of the vertical force between the surface and the particle. This would in turn depend on the force of gravity, the buoyancy force and the force due to vertical currents that may be present. These are the forces considered in the discussion of classification and it is evident that if these are the factors to be considered light material will be carried faster by a flowing stream than heavy.

 Figure 2 – Rotational Impact on Particles from Velocity Distribution

 This is a very oversimplified analysis, however, and it is easier to accept the findings of experiment, which show that:

1.    Material of low density is carried faster than material of high density.

2.    In mixtures of particles of a range of sizes coarse particles in general are carried along the bottom of a stream faster than fine ones.

 These generalizations are based on observation of material carried by a flowing stream. They appear to depend not only on the factors previously considered but also on an additional effect; namely, the ability of some particles to hide behind others and thus avoid the effect of the flowing current. This is primarily the reason that coarse particles move faster than small ones.  This generalization is subject to the reservation that extremely large particles-out of the range of sizes otherwise present-may not move at all.

 LAUNDERS

Launders, in principle at least, represent an ideal method of gravity concentration. In theory the bed density in a launder stream increases from the top to the bottom, a desirable condition from the standpoint of selectivity in the separation. The mobility decreases with depth. If heavy material is withdrawn from the bottom of a flowing stream, it is evident that it will at some time have passed through a zone where the bed density is as high and the mobility as low as practicable - condition already recognized as most desirable in gravity concentration processes. Launders have the additional advantage of requiring a minimum number of moving parts in their construction.

 In the launder, a stream of fluid carries the material to be separated down a channel provided with draws for separating a heavy-gravity product and means for overflowing a lighter one. If properly constructed and operated, a comparatively solid bed of material will form on the bottom of the launder. Above this bed there will be found a layer of particles moved along by the stream at a comparatively slow speed. Above this successive layers will move with greater and greater velocity.

 It is evident that in the flowing mass of material hindered-settling conditions will prevail and will govern the fall of particles to the bottom, where the selective effect of the flowing stream will be effective in transporting lighter material faster and consequently further down the launder.

 This will be made clear by noting in sequence the effects of hindered settling and of stream selection.

 In hindered settling, particles settle in the following order:

1.    Large heavy particles.

2.    Large light particles and small heavy ones.

3.    Small light particles.

 If the launder is operated so that only the first two reach the bottom where stream selection is apparent, these will be subjected to removal in the following order:

1.    Large light particles.

2.    Large heavy particles.

3.    Small heavy particles.

 If this process can be stopped with the removal of large light particles, the separation will be a perfect one.

 Practically it has not been possible to accomplish this completely. Turbulence in the flowing stream, the lessening of the density of the bed by the act of withdrawing products and entrapment of light particles by the bed arise to defeat the perfect operation of the process. It would seem that research could do much to overcome these difficulties and a closer realization of the theoretical advantages of launder processes might be achieved.

 SPIRALS

Spirals behave in general like a launderer, but with the addition of centrifugal force (actually centrifugal force is an apparent force, and the actual action is a "fictious force" caused by the inertia of the particles and is a reaction to being confined by the outer wall).  This causes the water to pile up on the outer edge with the lighter and finer particles and the larger heavier particles to report to the inside. A cross section of this action is shown in Figure 3. 

 Figure 3 – Spiral Flow

Beyond this the actions are very similar to a launder.

 TABLES

Concentrating tables (or shaking tables) introduce no principles of separation that have not been considered. The sequence of hindered settling followed by flowing stream selection is frequently utilized by submitting material to be concentrated to a preliminary classification. This is so common a practice that classification and tabling has almost come to be recognized as a unitary process.

 Particles in a bed of material on a shaking table are subjected to the action of hindered  settling in the way already discussed. A differential motion of the table suffices to cause heavy-gravity material to move along the table between riffles, if these are present, while a cross stream carries low-gravity material transversely.

Figure 4 – Table

A theoretical objection to shaking tables is found in the fact that mobility is imparted to the bed of material from beneath, with the result that a zone of low bed density will be found immediately adjacent to the surface. The separation is thus not as sharp as could be desired at any one place. In effect, however, the separation occurring between any two riffles is supplemented by the re-treatment between the adjacent pair and the large number of successive separations produces an acceptable end result.

 AIR TABLES

In discussing the mechanics of hindered settling (in An Introduction to Gravity Concentration), nothing was said about the nature of the fluid beyond the fact that it possessed viscosity and was capable of exerting a force against particles moving relative to it. In air tables the fluid in question has the property of being compressible. This property in itself introduces no new forces into the behavior of a bed of particles being separated. The principles are the same. Hindered settling occurs in the same way as though the fluid were incompressible. When operating with deep beds of material, however, the air in the lower portion is under the pressure of the bed and undergoes considerable change in volume in rising to the top. Frequently this causes geysering in an improperly operating device and is overcome by maintaining the bed in a state of partial mobility through shaking, flowing, or other movements.

 Another feature that differentiates air tables and air classification from processes using water is the density of the fluid. Air being substantially weightless compared with most solids, the density of the bed must be built up from the material itself. This apparent disadvantage of air processes is outweighed by the avoidance of the necessity for drying the products. Thickening difficulties are also avoided, though dust may become a problem.