Chem-Engine : Unit operations of particulate solids

12.1 Storage of solids

The frequently faced problems associated with the storage of bulk solids in bins and silos can be avoided if they are designed with respect to the flow properties of the bulk solid, which has to be stored.

12.1.1 Bins, silos, hoppers

Bins and silos provide economical storage of a large volume of material with minimum floor space. Mass flow requires that the hopper walls be sufficiently steep and smooth such that the stored material slides down the sloping walls instead of funneling (“rat-holing”) through the center core of the bin.

12.1.2 Long bins

Reclaiming via Long Feeders with the “Moving Hole” feeder system, the hopper and feeder can be made 30 m+ long and still discharge material effectively and uniformly along the full length.

This feature makes our system well suited to reclaiming from under large storage piles, domes and bulk cargo ships and barges.

For domes and ships, “funnel flow” hopper design is used to maximize storage capacity and at the same time, have a self-emptying hopper without manual intervention long feeder under an open storage pile can provide significant amount of storage. By exceeding the “piping” dimension for the stored material, a shallow “draw-down” angle of material in the pile is obtained.

Feeder length of over 30 m (100’) can be used for significant amount of “live” storage.

In addition, several feeders can be installed end-to-end, as is done on a ship, to cover a long length of pile.

12.1.3 Bulk storage domes

“Effective” discharge from a long opening makes the “Moving Hole” feeder well suited to reclaiming from under a storage dome.

The dome can have a flat floor or sloping walls as illustrated in the figure 12.4, depending on the “live” storage desired, and material flow characteristics.

12.1.4 Hoppers

Bulk storage hoppers

Vibratory bulk storage hoppers are used to load parts into a bowl feeder, giving large storage capacities and providing several hours running time. The hoppers can also be used to trickle-feed awkward components (i.e. components that easily tangle) into bowl feeders, therefore increasing feed rates and efficiency from the feeder. These units are best applied when the bowl feeder is situated at comparatively low level.

Elevating hopper loaders

Elevating hopper loaders are used to load heavy (or where appropriate light) parts to bowl feeders that because of design layout parameters is situated at a high level. The storage bin is floor mounted and therefore at a low convenient level for refilling. The storage bin can be either static or driven by vibration or a conveyor belt and these options are selected according to the components handling characteristics.

12.2 Feeders

Feeder selection is important for consistent material flow. “The Fix” usually entails wither retrofitting an existing funnel-flow bin or designing a new bin to ensure a mass-flow pattern. This fix can be an expensive liner or steeper hopper angles and as such, you can destroy this effort simply by selecting an improperly designed feeder.

Bin and feeder design go hand-in-hand. The feeder must work in unison with the bin and:

Suit the materials flow properties
Work with the bin outlet shape
Withdraw material uniformly across the outlet’s entire cross-sectional area
Minimize the vertical loads applied to the feeder
Accurately control the discharge rate

There are many types of feeders available to handle bulk solids and they can be divided into two categories:

Volumetric feeder discharges a volume of material as a function of time
Gravimetric feeder weighs material

12.2.1 Volumetric feeders

Volumetric feeding is adequate for many solids feeding applications. Feed accuracy in the range of 2-5% can be achieved with most volumetric designs.

Volumetric feeding becomes inaccurate if the bulk density of the solid that is being handled varies. The feeder cannot recognize a density change because it simply discharges a certain volume per unit time. Examples of volumetric type feeders are: screws, belts, rotary valves, louvered type, and vibratory.

12.2.2 Gravimetric feeders

A gravimetric feeder relies on weighing the material to achieve a required discharge rate or batch weight.

This approach should be used when:

Accuracy of less than 5% is required
The material’s bulk density varies
A record the weight of material used for a particular process
Feed accuracy of 0.25% is sometimes obtainable with a properly designed gravimetric feed system

A disadvantage of a feeder that weighs material is that it is usually more expensive than a volumetric device

There are two ways to feed gravimetrically:

Continuous system controls the weight/unit time such as lbs./hr or kg/hr
Batch system controls simply the weight of material such as 50 lbs. of material to a mixer

Examples of gravimetric feeders are:

Weigh-belts
Loss-in-weight systems
Gain-in-weight systems

12.3 Crushers and mills

Crushers and mills are typical process equipment for reducing solid chemicals, materials and other solid products to a desired particle or aggregate size range in dry or wet (slurry) forms. Mills are also utilized for mixing or dispersing solids in liquids. Feed size, material and hardness are some of the factors utilized in selecting the proper crusher or mill.

12.3.1 Jaw crushers

They pulverize feed materials between fixed and reciprocating plates, producing coarse granules. Blake, swing, overhead eccentric and Dodge jaw crushers are common variations.

A Jaw Crusher is one of the main types of primary crushers in a mine or ore processing plant. The size of a jaw crusher is designated by the rectangular or square opening at the top of the jaws (feed opening). For instance, a 24 × 36 jaw crusher has an opening of 24” by 36”; a 56 × 56 jaw crusher has an opening of 56” square. Primary jaw crushers are typically of the square opening design, and secondary jaw crushers are of the rectangular opening design. However, there are many exceptions to this general rule.

A Jaw Crusher reduces large size rocks or ore by placing the rock into compression. A fixed jaw, mounted in a “V” alignment is the stationary breaking surface, while the movable jaw exerts force on the rock by forcing it against the stationary plate. The space at the bottom of the “V” aligned jaw plates is the crusher product size gap, or the size of the crushed product from the jaw crusher. The rock remains in the jaws until it is small enough to pass through the gap at the bottom of the jaws.

12.3.2 Roll crushers

Roll crushers crush feed between the nip of two rolls or between a single roll and a fixed surface. They are used for intermediate grinding. Rolls crushers tend to produce weaker shaped product particles then impact mills.

12.3.3 Cone and gyratory crushers

They comprise of a cone shape bowl with a gyrating central head. Feed is crushed between the cone and head.

12.3.4 Ball and media mills

They reduce material to particle size by tumbling the feed with grinding media such as balls, rods or other shapes. Ball mills are typically wet, batch units. Water or another liquids and additives aid the grinding process by reducing friction, deflocculating or cooling. Media mills are also employed to disperse a powder into a liquid product such as pigment in a paint base. Motion is imparted to the media through tumbling or rotating the vessel, stirring rods or vibration. They are also known as pebble, rod, tube, compartment, tumbling, vibratory, stirred, dispersion, conical or tri-cone mills.

12.3.5 Disk attrition mills

They (including double disk mills) are modern versions of the ancient buhrstone mill where the stones are replaced with opposing disks or plates. The disks may be grooved, serrated or spiked.

12.3.6 Colloid and roll mills

They emulsify and disperse media by using high-speed rotors within a liquid media. The rotors often have a serrated outer surface. Some dispersion mills with larger gaps also use fine beads within the liquid to enhance dispersion. Roller mills or 3-roll mills disperse and refine a fine powder or pigment into a liquid by passing the paste between a series of rolls rotating at different speeds. Three-roll, colloid or other dispersion mills are commonly applied in paint, resin and adhesive applications.

12.3.7 Screen mill

They produce a uniformly sized product or granules. Some granulators use a rotating knifes while other types employ a crushing or shearing action against an integrated screen or grate to control product granule size.

12.3.8 Pin and mills

Use a rotor with one or more rows of rods that impact and/or propel particles into stationary pins or surfaces. Pin mills (including cross beaters and universal mills) fall into the category of high-speed rotor pulverizers or disintegrators. They tend to produce a finer product then coarse crushers or impactors.

12.3.9 Impact mills

They crush feed material by forcing it against a breaking surface. The feed material is propelled by gravity or by a rotating impeller or rotor. The impellers or rotors may be vertically or horizontally orientated. Vertical impact mills, cage mills, Bradford breakers, hammer mills, granulators are types of impact mills

High-speed impact mills

These include hammer mills, pin mills, counter-rotating pin mills, cage mills, turbo mills, and universal mills. A high-speed impact mill (figure 12.17) reduces non-friable and friable materials such as wood waste, sheet pulp, plastics, coal, chemicals, limestone, and fertilizer to medium-fine and fine (10- to 200-mesh) pieces. The material, which is to be reduced, enters the mill’s housing, which is impacted by a rotating assembly of hammers, pins, or cages. As it rotates, the assembly throws the material centrifugally outward where the hammers, pins, or cages grind it against a perforated screen for further size reduction. The final product’s size is controlled by the assembly’s rotating speed and the perforated screen at the discharge port. A high-speed impact mill is available in several sizes ranging from small laboratory equipment up to large production machines.

Hammer and cage impact mills

Use fixed or swinging hardened steel hammers, chain or a cage for coarse crushing to fine milling. Hammer crushers and cage mills are available in vertical and horizontal rotor configurations with one or many rows of hammers.

12.3.10 Jet and fluid energy mills

They function by impacting a stream or jet of feed particles against a wall or an opposing jet of particles.

12.3.11 Disc mills

Disc mills are used for shredding fibrous or tough materials such as wood products, cellulose, rubber or polymers.

12.3.12 Vertical roller

Vertical roller and dry pan crushers and mills use a vertically orientated crushing wheel or muller that revolves around a solid or perforated pan, or screen. Alternately, the pan can rotate or both the rollers and pan or grinding table can rotate. These mills are often used in foundries, and mineral and ore processing applications. They can reduce relatively coarse feed to a coarse powder in one step (e.g., minus 2 inch feed to -20 mesh product).

**Figure 12.22**
*Vertical roller mills*

12.4 Cutting machines

12.4.1 Rotary knife cutting

Rotary knife cutters include precision cutters, granulators, blow-through cutters, pelletizers, and guillotine cutters. A rotary knife cutter reduces large thin pieces or small thick pieces of non-friable materials such as paper, plastics, and rubbers to medium-coarse (1/8- to 1- inch) pieces.

The rotary knife cutter typically employs a shaft with a mounted knife (or knives) that rotates toward a stationary bed knife (or knives) to cut and shear materials between the blades. A perforated metal screen, located below the knives, retains oversized material until it’s processed to the proper size. Various screen mesh sizes allow particles to be reduced to multiple size ranges. The number of rotating knives and fixed knives depends on the machine’s size and function. The rotary knife cutter is available in several sizes (listed as knife tip-to-tip length by shaft length) ranging from small laboratory equipment up to large production machines and can be powered by a motor ranging from 2 horsepower up to hundreds of horsepower. It can be used in applications as varied as recycling thin plastic film and reducing full bales of rubber.

12.5 Crystallization

12.5.1 Tank crystallizers

This is probably the age old and most basic method of crystallization. In fact, the “pot of salt water” is a good example of tank crystallization. Hot, saturated solutions are allowed to cool in open tanks. After crystallization, the mother liquor is drained and the crystals are collected. Controlling nucleation and the size of the crystals is difficult. The crystallization is essentially just “allowed to happen”. Heat transfer coils and agitation can be used. Labor costs are high, thus this type of crystallization is typically used only in the fine chemical or pharmaceutical industries where the product value and preservation can justify the high operating costs.

12.5.2 Scraped surface crystallizers

A classic example may be the Swenson-Walker crystallizer consisting of a trough about two feet wide with a semi-circular bottom. The outside is jacketed with cooling coils and an agitator blade gently passes close to the trough wall removing crystals that grow on the vessel wall.

Advantages of Scraped Surface Continuous Crystallizers over other methods of Crystallization:

Smaller equipment, which generally means less expensive installations, less floor space needed, less operator labor, and no duplication of instrumentation, pipe, etc
Better process control, less upsets of hazardous or expensive materials and less peak utility demand
Modular design allows for easy expansion with growth in demand
Simple, self contained construction with minimum instrumentation and auxiliaries such as condensers, vacuum systems, etc
May be run for extended periods between hot washings whereas many shell and tube exchangers would plug up in minutes
May be run at much higher process fluid-coolant temperature differences than could shell and tube equipment without serious fouling or plugging
May be used over an extremely wide temperature range (-75 C to +100 C).
May be used with high percentages of solids (as high as 65% solids as slurry)
High viscosities are not a problem (has been used with mother liquor viscosities of 10,000 cp or higher)
Flow pattern in once-through operation closely approaches plug flow so conversion from batch operation is easy and virtually any desired time/temperature pattern is possible
In small capacity cases, a scraped surface crystallizer will be very inexpensive. This is also true in cases where, for much larger installations, vacuum crystallization may seem most attractive

12.5.3 Forced circulating liquid evaporator-crystallizer

These crystallizers combine crystallization and evaporation, thus the driving forces toward super saturation. The circulating liquid is forced through the tube side of a steam heater. The heated liquid flows into the vapor space of the crystallization vessel. Here, flash evaporation occurs, reducing the amount of solvent in the solution (increasing solute concentration), thus driving the mother liquor towards super saturation. The supersaturated liquor flows down through a tube, then up through a fluidized area of crystals and liquor where crystallization takes place via secondary nucleation. Larger product crystals are withdrawn while the liquor is recycled, mixed with the feed, and reheated.

**Figure 12.26**
*Continuous crystallizer*

12.6 Mixers

Mixers serve to put liquid in motion in order to achieve homogeneity of composition and eliminate the sedimentation process. They are driven by auxiliary equipment, such as a shaft, speed reducer or electric motor, to provide mixing action. They function by forcing sediment to flow in one direction and overcome the resistance during a liquid circulation flow in open reservoirs, ditches and canals. Mixers are also used to intensify physical and chemical processes in liquids, particularly the processes of gas and solid dissolution. Gas dissolution is usually used in sediment / waste water / anaerobic process. The intensified mixing operation is applied in order to lengthen the distance covered by gas bubbles and to prevent smaller bubbles from joining into bigger ones.

Direct drive, fast rotating mixers may also be employed to prevent surface scum from coming into existence and to destroy any surface scum that has already appeared.

Mixers are commonly categorized by:

The flow pattern they produce relative to the shaft centerline or the impeller axis in a fully baffled tank
Relative shear produced, the resulting divisions are: axial flow, radial flow, hi-shear, low-shear or high flow and specialized impellers
Specific action, or the action the cause within the liquid media

12.6.1 Types of mixers

Agitators

Agitators are used for mixing a product inside a vessel.

Static or motionless mixers

They are fins, obstructions, or channels mounted in pipes, designed to promote mixing as fluid flows through the mixer. Most static mixers first divide the flow, then rotate, channel or divert it, before recombining the flow. Some static mixers create additional turbulence to enhance mixing

Extruder feed mixers

They have an integral extruder screw to mix and then extrude its contents.

Turbine mixers

They include a wide range of general purpose mixing equipment, operating at reduced speeds provided by an enclosed gear drive with one or more multi-bladed impellers mounted on an overhung shaft. These mixers may be used on open tanks, when supported by a beam structure, or in closed tanks with a variety of seal and support arrangements. Because of the general-purpose capabilities of turbine mixers they may be used on almost any shape tank, of any size, with other drives or impellers

Magnetic drive mixers

They are ideally suited for Continuous Stirred Reactors (CSTR) and batch reactors where mixing and agitation must be contamination-free and leakage cannot be tolerated. The magnetic drive eliminates seals, and the problems associated with rotating seals, such as leakage, contamination, and constant maintenance.

Impeller/propeller mixers

They pump out in a radial direction generating a re-circulating mixing pattern above and below the disc. This high shear, high power design is stable under varying liquid depths and is an excellent choice as a rapid mixer in shallow basins, solids suspensions in shallow or varying water depths, and is often used as the lower impeller in a multiple impeller design.

Kneaders

They provide a kneading motion to mix the contents of the mixer.

**Figure 12.28**
*Kneader & disperser blades*

Screw mixers

They use a rotating screw that progresses around the periphery of a conical hopper. The screw lifts solids from the bottom of the hopper to the top, where the mixture flows by gravity back into the screw. Mixing occurs around the open screw, where the solids are transported by the screw exit at various levels and are replaced by other solids at that level. The screw’s shearing action also intimately mixes the various components. Gross mixing action also occurs within the mixture by the velocity profile created in the conical hopper as it feeds the screw. This gross mixing action is most effective when the solids move along the conical hopper walls.

Planetary mixers

They have two mixing blades that rotate around individual shafts and the two blades further rotate around a center axis. The net effect is intermixing, stirring, and shear.

Turbo mixers

They have a circular trough with a housing in the center around which revolves a spider or a series of legs with plow shares or mold boards on each leg. This type of mixer is also known as a plow mixer.

12.7 Mechanical separation

Screeners, classifiers, shakers and separators are all used for classification of powders or other bulk materials by particle size as well as separation of particles by density, magnetic properties or electrical characteristics. Round and rectangular screeners, magnetic separators, electrostatic separators, rotary sifters, wet or concentrating tables, rake classifiers, classifying hydro cyclones, floatation systems and trammels are included in the category.

12.7.1 Screeners

They are sifting units that are rotated as powder is fed into their interior. The finer particles fall through the sieve opening and oversized particles are ejected off the end. Rotary sifters or drum screeners are often used for de-agglomerating or de-lumping type operations.

Screeners are available in three main types:

Drum sifter
Rectangular deck
Round deck

12.7.2 Classifiers

Depending on your application’s requirements, a processor can achieve particle classification by:

Sieves

Sieves, primarily for coarse through fine grades of material use screens with a specified mesh size to separate the particles, and vibration or air fluidization is applied to the sieves to maintain particle flow through the screens. The toll processor can stack the sieves to classify a range of particle sizes greater than 100 microns.

Air classifier

An air classification machine uses air velocity to separate materials based on particle weight and size. It classifies particles ranging from 1 to 100 microns. Many air classifiers use a vane wheel to control the particle size distribution.

Air classifiers, cones or cyclones employ the spiral airflow action or acceleration within a chamber to separate or classify solid particles. Powders suspended in air or gas enters the cyclone and the heavier particles spiral out and down where they are collected. The air and finer particles flow up to the top where they may be passed to another cyclone with finer classification capability. A cyclone is essentially a settling chamber where the effects of gravity (acceleration) have been replaced with centrifugal acceleration. An air-classifying mill reduces friable materials such as polyesters, epoxies, acrylics, and sugar to fine and superfine (1 50- to 400-mesh) pieces. The material to be reduced first enters the mill’s high-speed impact grinding chamber where a fixed-speed, rotating grinding plate with fixed hammers reduces it.

Air moving through the mill then carries the particles to the classifying chamber where the classifier wheel rejects oversized particles and directs them back to the grinding chamber for further size reduction. The material circulates through this closed-loop environment until it’s been reduced to the appropriate particle size. The classifier wheel’s speed and the mill’s airflow rate are adjustable to allow for a wide range of particle sizes. Heated or chilled air can enhance an air-classifying mill’s performance.

Water classifiers

Water classifiers such as elutriators and classifying hydro cyclones use settling or flow in water or a liquid to separate or classify powdered materials based on particle size or shape.

Rake, spiral and bowl classifiers use mechanical action to dewater, de-slime or separate coarse bulk materials from finer materials or liquids. Rake classifiers lift solid-liquid mixtures up onto a plate with a screen or rake.

Spiral classifiers use an Archimedes pump screw to lift solid-liquid mixtures up onto a screen for dewatering. Drag classifiers consist of a chain-link conveyor or endless belt that is dragged through a solid-liquid mixture.

12.7.3 Separators

Electrostatic separators

They employ preferential ionization or charging of particles to separate conductors from dielectrics (nonconductors). The charged dielectric particles are attracted to an oppositely charged electrode and collected. The particles may be charged through contact electrification, conductive induction or high tension (ion bombardment).

**Figure 12.37**
*Collecting electrode in the ESP*

**Figure 12.38**
*Electrostatic separators*

Concentrating tables or density separators

They screen bulk materials or minerals based on the density (specific gravity), size and shape of the particles. This group includes jigging equipment, hindered-bed settling devices, shaking table, spiral concentrators, concentrating or wet tables, hydraulic concentrating tables, constriction plate separators or specialized settling vessels. Most concentrating or density separation equipment are hydraulic or water-based, although pneumatic or air-based systems are also available.

Magnetic separators

They use powerful magnetic fields to separate iron, steel, ferrosilicon or other ferromagnetic materials from non-magnetic bulk materials. The magnetic field may be generated by permanent magnets or electromagnets.

Trommel

Trommels are large rotary drum shaped with a grate-like surface with large openings. Trommels are used to separate very coarse materials from bulk materials such as coarse plastics from finer aluminum recycled material, coarse inorganic materials from organic wastes or large ore chunks from finer minerals.

12.8 Powder compacting equipments

These are used to shape powders as part of a forming process as well as to compress a wide range of materials into compact shapes for ease of transportation and ease of handling. Materials compressed by powder compacting equipment include powdered metals, ceramics, carbides, composites, pharmaceuticals, carbon/graphite, ferrites, explosives, chemicals, foods, nuclear fuel or other materials into compact shapes. Metal or ceramic powder compacts require additional processing such as sintering or forging to provide a finished part.

There are six main configurations of powder compacting equipment. These types are defined either by the shape of the product they produce or the technology used to process materials.

12.8.1 Briquetters

Briquetters and roll compactors turn fine, powdered materials into a briquettes, chunks, or sheets to improve handling, transportation, scrap disposal, storage or secondary processing. Briquetters often consist of a roll compactor with a serrated roll or a smooth roll combined with a granulator / chopper. Briquetters that form discrete cylindrical compacts also exist. Roll compactors with smooth rolls compact a powdered material into a sheet for the continuous production of ceramic or metal powder sheet or strip for filter applications or for clad / bimetal production. Some briquetters are used for fluid extraction and recovery.

12.8.2 Cold Isostatic Presses (CIP)

Cold Isostatic Presses (CIP) use a chamber to compact the powder or material placed in a sealed tool, bag or other flexible tooling. CIP use an oil-water mixture pressurized up to 100,000 psi. Flexible rubber or plastic tooling and steel mandrels are used in CIPing to produce perform with shapes that are more complex. CIP applications include refractory nozzles, blocks, and crucibles; cemented carbides, isotropic graphite, ceramic insulators, tubes for special chemical applications, ferrites, metal filters, pre-forms, and plastic tubes and rods.

12.8.3 Hot Isostatic Presses (HIP)

Hot Isostatic Presses (HIP) use an argon atmosphere or other gas mixtures heated up to 3000° F and pressurized up to 100,000 psi. Evacuated steel or metal cans or a sintered surface is used to contain and maintain a seal during HIPing. HIPs are used for densifying high performance ceramics, ferrites and cemented carbides, net-shape forming of nickel-base super alloy and titanium powders, compacting of high-speed tool steel, diffusion bonding of similar and dissimilar materials, and eliminating voids in aerospace castings or creep damaged blades.

12.8.4 Pellet mills

Pellet mills compress or extrude particles or fibrous materials into a cavity or die to form uniform cylindrical pellets. Compacted pellets are also formed using briquetters or tableting presses. Extruding pelletizers generate discrete and uniformly sized particles from a melt or a polymer (reclaimed scrap, post consumer or virgin plastic), liquid-solid pastes with a binder or other melting materials. The melt or paste is extruded through a die with multiple orifices. The pellet is sheared off or chopped after cooling / drying. Several types of pelletizers are available such as hot face, air, and cold cutting and underwater.

12.8.5 Rotary and multi-station tableting presses

Rotary and multi-station tableting presses have multiple stations or punches for compacting pharmaceuticals into tablets or metal powders into simple flat or multilevel shaped parts like gears, cams, or fittings. Rotary types have a series of stations or tool sets (dies and punches) arranged in a ring in a rotary turret. As the turret rotates, a series of cams and press rolls control filling, pressing and ejection. Pharmaceutical tablet and high volume metal part production facilities often use high-speed automatic rotary presses.

**Figure 12.45**
*Rotary and multi-station tableting press*

12.8.6 Single station presses

Single station presses are a type of powder compacting equipment that use a single action ram press with a die on both upper and lower punches. Single station powder compacting presses are available in several types basic types such as cam, toggle / knuckle and eccentric / rank presses with varying capabilities such as single action, double action, floating die, movable platen, opposed ram, screw, impact, hot pressing, coining or sizing.

12.9 Filtration

12.9.1 Liquid-solid filtration

Liquid-solid filtration equipment is normally used to filter, thicken or clarify a mixture of different elements. Examples of liquid-solid filtration and separation equipment types include sedimentation equipment, gravity filtration equipment, vacuum filtration equipment, pressure filtration equipment, thickeners, clarifiers, and centrifugal separators. Sedimentation is a gravitational or chemical process that causes particles to settle to the bottom. Sedimentation equipment includes gravity sedimentation filters and flocculation systems.

12.9.2 Gravity filtration

Gravity filtration uses the hydrostatic pressure of the pre-filter column above the filter surface to generate the flow of the filtrate. Gravity filtration equipment includes bag filters, gravity nutshces and sand filters.

Thickeners

Thickeners are used to separate solids from liquids by means of gravity sedimentation. Most thickeners are larger, continuous operation pieces of equipment. They are used for heavy-duty applications such as coal, iron ore taconites, copper pyrite, phosphates and other beneficiation processes. Common thickener types include conventional thickeners, high rate thickeners, lamella thickeners and tray thickeners.

12.9.3 Vacuum filters

Vacuum filters are available in batch (vacuum notches and vacuum leaf filters) and continuous (drum filters, disk filters and horizontal filters) operating cycles. Continuous vacuum filters are widely used in the process industry. The three main classes of continuous vacuum filters are drum, disk, and horizontal filters.

All of these vacuum filters have the following common features:

A filtering surface that moves from a point where a cake is deposited under a vacuum to a point of solids removal, where the cake is discharged through mechanical or pneumatic means, and then back to the point of slurry application
A valve to regulate pressure below the surface
An apparently continuous operating cycle that is actually a series of closely spaced batch cycles

Vacuum filtration equipment includes disc filters, horizontal belt filters, rotary drum filters (including pre-coat varieties), table filters, tilting pan filters, tray filters, and vacuum nutsche filters. A typical rotary vacuum filter is given in figure 12.49.

12.9.4 Pressure filters

Pressure filters operate at super atmospheric pressures at the filtering surface. The media is fed to the machine by diaphragm, plunger, screw and centrifugal pumps, blow cases and streams from pressure reactors. Most pressure filters are batch, or semi-continuous, machines. Rotary drum pressure filters and some others have continuous operating cycles. Continuous machines are more expensive and less flexible than batch machines. Pressure filtration equipment includes automatic pressure filters, candle filters, filter presses, horizontal plate pressure filters, nuts he pressure filters and vertical pressure leaf filters.

**Figure 12.52**
*Typical installation layout*

The candle filter

The Candle Filters are, as all pressure filters, operating on a batch cycle and may be seen in process lines handling titanium dioxide, flue gas, brine clarification, red mud, china clay, fine chemicals and many other applications that require efficient low moisture cake filtration or high degree of polishing.

Candle Filters are also used for thickening to produce a concentrated flowable slurry by partial removal of the liquid phase as filtrate. This mode of operation is possible since Candle Filters may operate on very short cycle times taking advantage of the high filtration rates whilst the cakes are still thin.

12.10 Cryogenic grinding

This technology can be used to most of the size reduction equipment. It’s most commonly used with high-speed impact mills and attrition mills. Cryogenic grinding reduces heat-sensitive and non-friable materials such as spices, plastics, organic dyes, and rubbers to medium-fine and fine (20- to 200-mesh) pieces. When reducing a heat-sensitive, non-friable material, a toll processor either mixes cryogenic fluid directly with the material in the grinding chamber during grinding or embrittles the material by exposing it to a cryogenic fluid prior to grinding. The most frequently used cryogenic fluids (called cryogens) are liquid nitrogen and liquid carbon dioxide. A cryogen can lower material and grinding temperatures to -300 F, which increases the machine’s particle size reduction capabilities by making a non-friable material friable and minimizing the heat generated during grinding. Cryogenic grinding technology is also used in size reduction operations requiring inert atmospheres, such as those handling explosive or flammable materials.

Raw material passing along a conveyor is cooled using controlled amounts of liquid nitrogen, which allows for finer grinding and increased throughputs

12.11 Blending

The type of equipment used in mixing and blending operations depends on the materials to be combined. Certain blenders cause more degradation or generate more fines than is acceptable in a particular application, while others generate friction that can be detrimental to heat-sensitive materials. That’s why a toll processor generally has several types of mixing and blending equipment available. For most powder blending applications, a toll processor uses either mechanical agitation blenders or rotating vessel blenders. A mechanical agitation blender uses motor-driven agitators to agitate the materials in its stationary vessel until they are mixed together.

Examples are ribbon blenders and conical-screw blenders, which both can die cohesive materials, such as plastics, pharmaceuticals, and spices. A rotating vessel blender has a rotating vessel that spins until the materials are mixed together. Examples are double-cone mixers (also called V-mixers) and drum tumblers, which both typically operate in batch mode and handle materials such as chemical powder blends, fertilizers, and plastic, compound pre-blends.

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Sunday 5 July 2020

Unit operations of particulate solids