6.3 Separation of Solids from other Solids

Many times, we are faced with a need to separate particles from a mixture of particles. The separation criteria include differences in size, or differences in physico-chemical characteristics. We now want to learn the solid-solid separation processes that are employed in industry.

6.3.1. Screening

This is the separation of particles according to size. Screening objective is to separate a feed which contains a mixture of particles into the underflow and overflow fractions A and B respectively when the feed dropped is dropped on to the screen by gravity.

Below is a schematic illustration of a screen showing how a feed is separated into two products, the oversize (overflow) and the undersize (underflow or fines).

The two products are unsized fraction.  By this, we mean that for the oversize product, only  the minimum size is known. This is the size that is just slightly bigger than the size of the screen holes. All other sizes are larger than this minimum. The maximum size or the size of other particles in that product are unknown. Similarly for the undersize product, only the maximum size is known but not the minimum size or intermediate sizes.  To get the maximum, minimum and other particle sizes, you would need to pass the material through a series of screens as shown in Fig 6.2. The amount retained in each screen, that is size fraction, is weighed and its percentage calculated from the total mass of sample. This operation is called screen analysis.

Industrial screening can be done using structures made up of any of the following:

  • spaced metal bars
  • perforated or slotted plates
  • woven wire or fabric screens

When metal screening material is to be used, selection must be based on how compatible that metal is with the material being screened and also on the strength  of screen required For example, if you are screening very heavy feed, you  will need a strong screen. Commonly used metals include steel, stainless steel, bronze, copper and nickel.

Screening can be done wet or dry.  Screen structures include:  metal bars, perforated or slotted plates, woven wire or fabric.  Metals used include steel, stainless steel, bronze, copper, nickel.  Selection of material is based on compatibility with materials being screen and strength required.

6.3.1.1. Woven screen sizes

Woven screens are commonly used in industry. To describe the size of woven screen material, two terminologies used:

Aperture:  This is the minimum clear space in mm or mm between the edges of the opening. This is shown in Fig 6.3.

Mesh: This is the number of apertures per linear inch, i.e. number of apertures in 25.4mm along the wire. If you count the number of openings from one wire along the inch perpendicular to the wire, the number you get is the mesh of that screen. Screen analysis data is given in either mesh or aperture sizes.

Fig. 6.3 Screen aperture and mesh

In Table 6.1, we illustrate what we have learnt on screen analysis and woven screen sizes using data for a sample of Magadi soda ash.

Table 6.1 Screen analysis for Magadi soda ash

Mesh

Aperture (mm)

Fraction size %w/w

Cumulative size %w/w

+12

1.4

1.93

1.93

+16

1.0

15.41

17.34

+72

0.210

65.65

82.99

+120

0.125

9.49

92.48

+240

0.063

6.0

98.48

-240

Under 0.063 or -0.063

1.52

1.52

 

Practical activity: Draw a 3x3 inch screen whose size is 10 mesh

6.3.1.2. Screening effectiveness

Screen effectiveness is the measure of success in closely separating overflow A from underflow B.

Screen capacity is the mass of feed per unit time per unit surface area. e.g. tons hr-1.m-2?

Capacity and effectiveness are opposing factors which need reasonable balance.  When capacity is increased, screen effectiveness drops. Particles which can pass through the screen are hindered from doing so as a result of high capacity.

The overall chance for particle to pass through screen is a function of the number of times particle hits the screen and the probability of passage during a single hit.

A particle has the greatest chance of passing through the screen if:

1. The number of times the particle hits the screen is increased

2. The particles minimum dimension approaches parallel to the screen

3. The particle approaches perpendicularly

4. The particle is not impended by others.

5. The particle does not wedge into screen

6. The screen is not overloaded because this reduces contact of particle with screen

7. The particles are not likely to cause blinding (plugging or blockage) of the screen.

Blinding: Sticky, flaky and soft particles cause blinding as opposed to hard and rounded particles which. The latter are able to drop quickly and easily through the screen. Blinding occurs as a result of cohesion of particles with each other, adhesion of particles to screen surface or the segregation of particles with the large particles on the screen surface. Blinding reduces both screen capacity and effectiveness. There is least blinding if particles are carried in a stream of water. This is referred to as wet screening 

In most screens, particles drop through the screen opening by gravity. Coarse (large) particles unlike fine particles, drop quickly through large openings in a stationary surface. So, particles which are too large to pass through the screen form a layer on the screen surface. They therefore block the fine particles from passing through. This is the segregation we just mentioned about. To solve this problem and therefore increase screening effectiveness, industrial screen are operated in either of the following modes:

Shaking: This a vertical up-down motion of the screen

Vibration: This is a sideways motion on a horizontal plane

Gyration: This is combined horizontal and vertical motion around an axis

Brushing: A brush is used to sweep through the screen surface remove blocking particles from screen surface.

6.3.2. Grizzly screen

A grizzly screen consists of equally spaced bars set in an inclined stationary frame.  The bars are wide at the top and narrow at the bottom. This tapering prevents blocking and provides enough depth for mechanical strength of the bars. Small particles pass between the bars while large particles roll or slide off the screen. Fig. 6.4 is a schematic diagram of a grizzly screen. 

Fig 6.4. Schematic diagram of a grizzly screen

Grizzly screens are used for coarse feeds for example from a jaw crusher. The spacing may range from 50 to 200mm.

6.3.3. Jigging

A jig is a mechanical device used for the separation of materials of different specific gravities by pulsation of a stream of liquid flowing through the bed of materials. Liquid pulsates or "jigs" up and down causing heavy materials to move down to bottom while light particles rise to the top. Pulsation is caused by alternately applying and exhausting air pressure. Each product is then drawn separately. Jigging is illustrated in Fig 6.5.

Fig 6.5. Jigging operation

6.3.4. Magnetic Separation

If a mixture containing magnetic materials and non-magnetic materials is subjected to a magnetic field, there is competition for the particles between several forces.

These forces are:

1. Magnetic forces

2. Inertia

3. Gravity

4. Interparticle forces.

The feed will be separated as shown in Fig 6.6.

Fig 6.6. How a mixed feed of particles separates out in the presence of a magnetic field.

Three products can be obtained during magnetic separation. These are:

1. strongly magnetic product

2. weakly magnetic (middlings) product

3. non-magnetic (tailings) product

6.3.4.1. Belt Lifting magnets

This method which is used for dry particles is illustrated schematically in Fig 6.7.

Fig 6.7. Dry magnetic separation

Material to be separated is fed into the first conveyor. Above this conveyor is another conveyor with an electromagnet inside. The electromagnetic field decreases towards the right. Strongly and weakly magnetic materials are attracted and picked by the magnet. The non-magnetic materials continue to be conveyed by the bottom conveyor and drop in the first bin. As the strength of the electromagnet weakens towards the right, the middlings i.e. the weakly magnetic materials lose attachment and drop in the middle bin. The strongly magnetic materials drop off at the end of the electromagnet into the third bin.

The applications of this technology include the following:

  • Recovery of metal objects from refuse for recycling
  • Removal of metal objects from food products
  • Protection of machinery from metal objects
  • Mineral ore processing

6.3.4.2. Wet magnetic separation

Magnetic separation can also be carried out in wet state.  See Fig 6.8. The feed in slurry form is fed into compartment A which overflows into compartment B.  In compartment B, there is rotating drum DM, with a strong magnet.  Weakly and strongly magnetic materials are attracted to the surface of the drum while non-magnetic materials drop to the bottom and are discharged.  Compartment C has another drum DM2 with a weak magnet which is unable to attract weakly magnetic materials.  These materials are discharged at the bottom of C.  The strongly magnetic materials are scrapped off the drum and discharged at D.

Fig: 6.8. Wet magnetic drum separation

6.3.4.3. Biomagnetic Separation of Heavy Metal Ions from Solution

Research has shown that magnetic separation can be used with microbiology in the recovery of metal ions from solution.  The metal is taken up by a microorganism either intracellularly or  it is deposited onto the surface of the microorganism.

In one method, glycerol-3-phosphate is used as culture media.  This substrate does not enter the cell. It is cleaved by an extracellular enzyme phosphatase thereby giving a high concentration of phosphate ions at the cell surface.  The phosphate ions react with metal ions present in solution to form a precipitate on the cell surface of the microorganism. The microorganism then behaves like a paramagnet. The cells are concentrated by high gradient magnetic separation. Let us look at two examples that illustrate the use of this technology:

Example 1:     Candida utilis and Bacillus subtilis have been studied as Scavengers of uranyl ions.

Uranyl ions are strongly paramagnetic.  They get attached to the cells as a result of the phosphatase

activity thus coating the organism are separated manetically with the following yields:

Wt of UO2 / weight of cell

C. utilis                                                            0.44

B. subtilis                                                         0.31

Example 2:     A number of micro-organisms such as desulfonibrio strains produce sulphide ions.  If the organisms are grown in a media containing ferrous and sulphate ions.  Iron sulphide is ppted on the cell surface.  As a result heavy metal ions are scavenged to very low residual levels in the solution.  The separation is better then through the phosphate route as shown by the following results:

Residual levels

CO2+                                                    1.77 x 10-16ppm

Hg2+                                                    8.02 x 10-43ppm

Biomagnetic technology can therefore be  applied in the removal of toxic metal ions from industrial effluents including effluents from nuclear plants.  The ions are collected and concentrated to high levels.  Another application is in the mineral processing industry.

6.3.5. Electrostatic Separation

Minerals have a wide range of electrical conductivity.  Therefore use of high tension voltage to impart charge on a feed can effect separation of particles of varying electrical conductivity. The process stages are shown in Fig. 6.9.

Charging:  Ion bombardment is one of the methods used to charge particles.  In this method, particles are bombarded with ions of atmospheric gases generated from electric discharge of high voltage electrode placed some distance from the particles.

It is important to ensure that the electrostatic force imparted is strong enough to overcome other forces such as gravity, and inertia of conducting particles.  Charge increases with surface area.  We know that the smaller the particles, the larger the surface area.  Therefore in electrostatic separation, there is an upper limit of particle size.  Particles should be less than 1.5mm.  The feed is spread thinly in order to get maximum charge.

Discharging:

Particles are pinned to a conducting surface that drains the charge.  Once discharged, contact is broken.  Conducting particles lose charge fastest, hence they are the first to be separated.

Non-conductors are left clinging on conductor longest, hence last to be discharged.  The discharge for weak conductors is in-between.  This principle is illustrated further in Fig. 6.10. By  referring to this diagram, the following is the step by step process:

1. Mixture of particles of varying electrical conductivity is given electrostatic charge.

2. They are brought into contact with a rotating electrical conductor at earth potential

3. particles cling (or pinned) to electrical conductor by electrostatic attraction as long as charge remaining

4. Charge leaks away from good conductors much more rapidly than from poor conductors.

5. Weakly conducting materials remain attached to conductor longer than the good conductors.

6. Separation of good conductors from poor conductors is effected.

Fig 6.10. Electrostatic separation

Application of Electrostatic Separation

1. Mineral Beneficiation: separation of metals from non-metals

2. Metal powder processing: removal of non-metallic impurities to obtain high purity metal or alloy powders.

3. Food processing: Most food products are moist hence behave as conductors thus enabling  the removal of stones which are non-conductors, from the food product.  In the same way, dry leafy material (non-conducting) is removed from moist leafy one (conducting).

4. Waste processing: e.g in the separation of scrap wire from plastics; separation of glass cullet (broken glass) from metallic impurities and stones.

6.3.6. Flotation

This is a process in solids-liquids separation technology that uses differences in wettability of various materials. All naturally occurring solid particles and most inorganic chemical substances such as mineral ores have surfaces that have a strong affinity for water, i.e. they are hydrophilic. The surface properties of components in a mineral ore may vary within a very narrow range.  These small differences can be amplified by selective adsorption that makes some of the particles hydrophobic. The size of particles to be floated ranges between10 and 250mm. Such particles in a water suspension are recovered by means of their attachment to air bubbles.  Aggregates of particles and bubbles with densities less than the suspension itself rise to the surface and are removed. This flotation operation is the opposite of solid-liquid separation by means of sedimentation as we shall see in the next topic.

But how do we make particles hydrophobic (water repellent)  and flotable?. This is how we do that.

6.3.6.1 Collector attachment

A special surface active agent (surfactant) called collector or promoter is added to the suspension. Collectors are usually C2 to C6 compounds containing polar groups and include fatty acids, fatty acid amines and sulphonates among others. Selection of collector depends on the material being separated.  The collector molecule adsorbs on to the solid surface via the polar (charged) group.  This reaction is known as chemisorption. The hydrocarbon chain is facing the aqueous phase. This is shown in Fig.6.11. A layer probably, a monolayer of the collector molecules become attached to the surface of the particle.   Because the hydrocarbon chain and the water do not mix, the coated particle surface becomes hydrophobic. By being hydrophobic, a particle repels water. This results in the weakening of the forces acting between the particle surface and water and hence the diminishing of surface-water interactions at solid-surface interface. This causes the displacement of water film from the wetted solid surface by air.

6.3.6.2. Modifiers

In addition to the use of collectors to change the surface property of the particles, other chemicals

may be added to further modify either the particles to be floated, or the particles that are to remain

in the suspension or the aqueous phase itself. The chemical substances are called modifiers and are

of the following types:

1.  Activators modify surface of mineral to enhance collector attachment.

2.  Depressants inhibit collector attachment on materials which are not to be floated.

3.  pH regulators control pH to help in achieving selectivity.  The modify surface and also control ionization e.g. of collectors.

4.  Flocculating agents flocculate fine particles that are to be floated.

5.  Dispersing agents prevent flocculation in the flotation cell.

6.3.6.3. Bubble attachment.

We have so far carried out the surface preparation in readiness for bubble attachment. We shall now look at how a bubble is attached to the hydrophobic particle. We have mentioned that repulsion of water makes room for air to replace water at the particle surface.

Thermodynamically, the lower the free energy change ∆G  of a process, the easier it is for that process to take place. For the bubble-particle attachment:

∆G  =  gGL (cos q -1) (for a flat surface º ideal max).

where ∆G  = free energy per unit area of surface

gGL = gas-liquid interfacial tension

q        = gas bubble-solid surface contact angle measured through the liquid

Fig.___

Fig. How contact angle influences bubble attachment: (a) large contact φ, (b), medium  φ, (c) small φ

The steps involved in bubble attachment are the following:

Step 1:             collision between bubble and particle

Step 2:             thinning of the liquid film between bubble and particles.

Step 3:             rupture of the liquid film

Step 4:             Rapid expansion of the air meniscus over the particle so that a stable attachment achieved.

6.3.6.4. Bubble Generation

You may be wondering where the swarm of bubbles needed for flotation come from. They are produced by any of the following methods:

1. Mechanical agitation of the suspension of particles in water: This produces bubbles of about 1mm in diameter.

2. Nucleation of a gas from a solution: This is done by either applying a vacuum to the suspension (vacuum flotation) or by injecting air under pressure into the suspension

3. By electrolysis of the aqueous phase.

6.3.6.5. Attachment by electrostatic attraction

This is an alternative method to collector attachment by chemisorption. It is used for flotation of any charged surface. The particle surface is charged in a similar way as during electrostatic separation (section 6.5). The charge imparted may be either positive or negative. If the particle is positively charged, it attracts an anionic collector. If it is negatively charged, it attracts a cationic collector. The collectors used are colloidal electrolytes with a fairly large hydrocarbon chain ranging from C12 to C18. Consumption is about 10 times more than that of chemisorption collectors. Industrial application is therefore small.

6.3.6.6. Flotation cell

The flotation cell is shown in Fig. 6.12.  The material is ground in water to a maximum 250mm.  It is introduced into the flotation cell. A frothing agent is added to create a generous supply of fine bubbles when air is sparged.  Examples of frothers include pine oil, cresylic acid, polypropylene glycol ethers and methyl amyl alcohol. They need to be inexpensive and are consumed at the rate of (0.05 to1.0) x 10-4kg/kg of solids. The collector is also added at a level of 0.01-0.05kg collector/ton solids. Modifiers (see section 6.6.6) may also be added.Hydrophobic particles are collected at the air-bubble interface.  The bubbles with attached mineral particles rise to the surface where the material is removed. Particles that are readily wetted by water (hydrophilic) tend to remain in the water suspension. 

Fig 6.12. A flotation cell