11 Waste Processing – Size Reduction

Dr. J. Rajesh Banu

Objectives:

• The learning objectives of this module are
• To dispose the solid waste safely, without causing any environmental problems to the community, other living beings and the surroundings.
• To employ 3R principle / concept, i.e., ‘Reduce, Reuse and Recover’ strategy to solid waste.
• To reduce the particle size of solid waste for disposal.
• To recover reusable substances from wastes and to employ them as feedstocks for generating new useful products.
• To generate energy from wastes and utilize the energy for other beneficial processes. Ultimately, to make ease the waste disposal process and
• To employ it as an environment friendly and cost-effective process

1.      Size reduction- Definition

Size reduction is the process in which the particle size of a substance is reduced to a finer state (smaller particles to crude particles or to fine particles) of desired shape and size by using external forces. Size reduction process is also known as comminution and grinding.

2.   Classification of size reduction

On the basis of feed and size of the particles, size reduction is classified into 3 types namely, coarse size, intermediate size and fine size reduction. They are as follows:

2.1 Coarse Size Reduction

Coarse size reduction is mainly employed for reducing the size of rigid and unevenly formed solid substances to about 2 – 96 inches or higher. The equipments used for coarse size reduction are jaw crusher, gyratory crusher, toothed roll crusher, hammer mill.

2.2 Intermediate Size Reduction

Intermediate size reduction is mainly employed for reducing the size of rigid and unevenly formed solid substances to about 1 – 3 inches. The equipments used for intermediate size reduction are cone crusher, crushing rolls, stamp hill, disintegrator.

2.3 Fine Size Reduction

Fine size reduction is mainly employed for reducing the size of rigid and unevenly formed solid substances to about 0.25 – 0.5 inches. The equipments used for fine size reduction are ball mill, tube mill and rollers mill

3.   Factors affecting size reduction

The various factors that affect size reduction are hardness, robustness, abrasiveness, glueyness, softening temperature, particle nature, moisture level, purity requirement, ratio of feed size to product ratio, bulk density and physical effect

3.1 Hardness

The surface property of a material is hardness. Moh’s scale is considered as a random scale of hardness. The hardness number ranges between 1 and 10 for a number of mineral substances (for instance from graphite to diamond). Materials with hardness number ranging between 1-3 are called as soft and could be spotted with a fingernail. Materials with hardness number beyond 7 are hard and could not be spotted with a blade. The materials having hardness number ranging between 4-6 are designated as intermediate. Normally, when the material is harder, then it is complicated to reduce the size of the material.

3.2 Robustness

Robustness of a material is also an important factor that affects size reduction. In some instance, it is considered as more important than hardness of a material. A soft and tough material may lead to many problems in size reduction when compared to hard and delicate material. For instance, it is harder to fracture rubber than a blackboard chalk. Toughness creates difficulty in many materials, particularly in fibrous materials and is often connected with moisture content of a material.

3.3 Abrasiveness

Abrasiveness is an important property of hard materials (principally those of mineral origin) and could limit the efficiency of equipment that could be employed. During the milling of some very rough materials, the final fine particles may be polluted with in excess of 0.1 % of metal tattered from the pulverizing grinder.

3.4 Glueyness

Stickiness of a material is another property that leads to substantial complexity in size reduction of a material. The sticky substance may stick to the milling plane, or on the lattice of the screen could turn to be choked. If heat is produced during any size reduction process, then the sticky and resin like materials may cause problems to the entire process. By adding inert substances it can provide assistance for such process (For example, kaolin to sulphur).

3.5 Softening temperature

In size reduction process, heat is produced occasionally. The produced heat may lead to softening of some substances. The temperature at which the heat generating process happens is very essential. Waxy substances, such as stearic acid, and oils or fats containing substances are some of the substances that may be influenced by this temperature. To overcome these drawbacks, some process can be used to surmount this effect such as cooling of milling equipment either by using water jacket or allowing a stream of air to pass through the equipment. Another option is to employ liquid nitrogen.

3.6 Particles nature

Some of the materials are homogenous in nature. However, many materials possess various unique structure, for instance, mineral substances found to be weak in nature and these substances crack to produce flake-like elements, while other substances such as vegetable wastes being cellular nature have fibrous like structure.

3.7 Moisture level

Moisture level is considered as an essential parameter that posses some major characteristics which can affect size reduction, for instance, hard nature, robustness or glueyness. Normally substances must be dried out or damp. Substances that possess less than 5 percent of moisture content are considered to be appropriate for dry grinding process. Greater than 5 percent of moisture content leads to agglomeration or pasting. In the same way, substances that have more than 50 percent of moisture content are considered to be appropriate for wet grinding process. In case of low moisture content materials, coarse and intermediate size reduction process is suitable. In case of high moisture content particle, greater than 50 percent, fine size reduction process is applicable.

3.8 Purity requirement

Some of the size reduction equipments make the crushing or milling surfaces to worn and those processes must be avoided in case of high purified product requirement. Likewise, some equipment is considered to be inappropriate if the cleaning of various materials is complicated.

3.9 Ratio of feed size to product ratio

It is the ratio of feed to particle size (diameter) of product. Equipments that are used for producing very small sized particles could be essential to perform the size reduction process in various phases with the help of diverse machines, for instance, primary process such as crushing. Subsequently the crushed materials are subjected to coarse grinding process and finally subjected to fine grinding. For coarse size reduction process, the reduction ratio should be between 3 to 7. For grinding mill, the reduction ratio should be greater than 100.

3.10 Bulk density

The potency of many batch mill processes relies on volume. These processes normally require solid substances on the basis of weight. Therefore, all the parameters are kept equal and the substances that are produced from the equipments after the completion of process is connected with the bulk density of the materials.

3.11 Physical effect

Some of the materials are very effective and powerful. Minute quantities of dust particles can influence the operators. To eliminate these drawbacks, milling equipments must be covered. Systems that can extort air are attractive and beneficial. Wet grinding is appropriate as it eliminates this issue completely.

4. Energy and force applied for size reduction process

In size reduction process, the increment in surface area is enhanced by the applied energy. The input energy is utilized to cause the fracture and defects in the materials. The remaining energy is utilized for elastic twisting, transportation of materials for generating friction forces between the grind mill and material and fractioning forces and noise. About 0.1-2% of the energy supplied to the equipments is arose as the increased energy of the subjected material. Surface energy estimates the extent of intermolecular bonding destruction of a material during the formation of new surface. This can be estimated through a number of methods by contact angle goniometer. Effectiveness of size reduction process depends on the load application step and its extent. Extent and nature of force applied is also an important factor to be taken into account during size reduction process. For example, when the force input is inadequate for the elastic limit to be prevailed and the particles are compacted, then the energy will be piled up in the particle. Once the applied load is removed, then the particle subjected to size reduction process come to its original condition. In that case, the energy is generated as heat. Application of greater force will cause the particles to get crack. To utilize the energy adequately, the force applied should be slightly greater than the grinding potency of the substances. During size reduction process, the surface of the particles is generally uneven in nature and therefore the force is mainly focused on the elevated spots. As a result, greater pressure and elevated temperature is experienced by the material closely. When the particle is cracked slightly then the spot at which the force applied is get altered. Additionally, the kinetic energy is related with the rapid acceleration of material experiences a crack. The degree of surface fracture energy extent per unit volume of various materials are glass 1–10 J/m2, plastics 10–103 J/m2 and metals 103–105 J/m2. All these surface fracture energy values are found to be greater than the degree of thermodynamic surface energy extent which is approximately 10-1 J/m2.

5. Mechanism of size reduction

During size reduction process, smaller particles are produced from larger particles of the same material. Size reduction leads to the formation of crack in the larger particles. The basic three mechanisms involved in size reduction are cracking, crushing and grinding of solid particles (Figure 1). The particles are fractured through external shear forces (mechanical energy). Then the particles are crushed into smaller coarse particles. Finally, the crushed particles are grinded through grinders. During this process, by the application of mechanical energy the particles are distorted, loosened and cracked.

There are various mechanisms involved in size reduction process (Figure 2). They are impact, compression, shear and attrition.

Figure 2 Different mechanism of size reduction

(i) Impact

Impact is a mechanism which occurs when a solid particle (almost immobile) is beaten by a highly accelerated moving substance or when a moving substance hits an immobile surface.

(ii) Compression

Compression is a mechanism which occurs when a solid particle is disrupted by two rigid surfaces.

(iii) Shear

Shear is a mechanism which occurs when a solid particle is compressed between rollers through mechanical pressure.

(iv) Attrition

Attrition is a mechanism which occurs when the materials are subjected to force similar to shear but the surfaces scrapped with one another. As a result, shear forces are generated which shatter the materials.

6. Size reduction laws

The energy requisite of size reduction process mainly depends upon the parameters such as input and output of particle size, rigidity, force and other properties of solids. The different laws that describes energy need of particle size reduction process are:

• Kicks law
• Rittinger law
• Bond’s law

Kicks Law

Energy needed to crush a particular material is invariable for the similar reduction ratio irrespective of its original size (Energy is directly proportional to ratio of size change).

Hp = Energy, K= constant, D= initial diameter, d = final diameter

Energy required to crush material of 1 inch into 0.5 inch = Energy required to crush material of 3 inch into 1.5 inch.

Application: Kick’s theory is suitable for compression of larger materials

Rittinger Law

Energy needed to crush a particular material is proportional to the produced new surface area. (Energy is directly proportional to the new surface area produced).

Application: Rittinger’s theory is suitable for delicate materials which undergoes fine grinding

Bond law

Energy needed to crush a particular material into a desired particle size is proportional to the square root of product’s surface area to volume ratio. (Energy utilized in fracture propagation is directly proportional to produced fracture extent)

Ei = Bond’s working index

Application: Bond theory is applicable to particles which undergoes rough grind sizing. In bond theory, the working index is used for evaluating the effect of grinding functions.

6 Size reduction equipments

The machines used for reducing the particle size of materials are called as size reduction equipments. They are of various types. The most common types of size reduction equipments are shredders, glass crushers and wood grinders (Figure 3). Table 1 summarizes the different equipment used for size reduction.

6.1 Shredders

Shredders are the equipments used to shred the feed materials with the help of rotating shaft or sharp knives. These equipments are mainly used to reduce the size of municipal solid wastes. Examples of shredding equipments include: hammer mill, flail mill, shear shredder, cutting mills, cage disintegrators, roller mills and hydro pulpers. The hammer mill is the most commonly used shredding device.

6.1.1 Hammer mill

Hammer mill is a type of shredding device that works under the principle of collision between the fastly moving hammer located on a rotor and the solid material. During operation, the hammers of hammer mill which is fitted to a rotary component, hit the solid materials that is introduced into the device and shred it. In hammer mills, the rotor rotates at a speed of 8000 -15000 rpm. Once the waste material is shredded, they are pulled via the exit of the device. The exit section (discharge unit) may or may not consist of bottom grates. The hammer mills are of two types namely, One-way type (Figure 4) and reversible type. In one-way type hammer mills, the breaker plates are replaceable. In reversible type, the breaker plates are reversible (adjustable)

Figure 4. Hammer Mill

6.1.2 Flail mill

The flail mills are solitary pass shredding equipment works similar to the hammer mill, however afford only coarse tearing of materials, since the hammers are spaced further apart. During operation, the waste materials stay in the hammer till it moves via the hole in the bottom grate. They are often utilized as bag breakers.

6.1.3 Shear shredders

Shear shredders are the equipments in which the shredding of waste materials is take place by the application of shear forces. The shear shredder consists of two analogous counter rotary shafts with a sequence of discs located vertically which serve as shears. The solid particle to be shattered is moved to the middle of the counter rotary shafts. The magnitude of the solid particle is reduced by the shear action of the shattering discs. The shredded particles is forced via the outer exit section. Shear shredders are also used as bag breakers.

6.1.4 Hydropulpers

Hydropulper is also a type of shredding device mainly used for reducing the size of municipal solid wastes (Figure 5). During operation, the municipal solid waste material and water to be shredded are moved to the device. The fastly revolving shaft part slices and transfers the solid content into pulpable finish products. The resulting pulpable end products consist of 2.5 to 3.5 percent of solid particles. The non-pulpable substances are collected from the side tank of hydropulper unit. The remaining slurry is discharged through the base of the unit with the help of a pump and subjected to further processes. The waste effluent substances are settled down in the base of unit by gravity with bucket elevator.

6.2 Glass crushers

Glass crushers are the devices employed to mash glass vessels and other glass materials present in municipal solid wastes (Figure 6). The glass crushers are works under the principle of compression by the application of stress forces. The stress forces are applied by rotator wheels or rollers. The glass crushers consist of two rollers, front and back roller. A compression spring in the unit provides stress forces.

Figure 5. Hydropulper

During operation, the rollers are allowed to rotate. The materials to be compressed are introduced through the gap between the rollers. Through the application of high pressure, the materials are trampled. Glass is always compressed once it is segregated to minimize the cost incurred towards storage and shipping. In some mechanical segregation process, glass is trampled, subsequent to several segregation phases, to influence its elimination by screening. Crushed glass can be removed optically based on colour.

6.3 Wood grinders

In general, most of the wood grinders are wood chippers and are used to grate large wooden logs into chips (Figure 7). This could be utilized as a fuel and fine substances. The finer material in turn can be composted easily.

Figure 6. Glass breaker

Wood grinders consist of a rotating apex part (rotating arm), and a immobile base part holding a hammer mill. The wooden pieces to be grinded are fed through a hopper and the rotating force of the rotating arm makes certain that the substances run incessantly to the hammer mill. The incessant flow of grinded substances is pulled speedily from the grinder through a conveyor at the bottom.

7. Selection of size reduction equipment

The following aspects should be taken into account while selecting size reduction equipment:

The nature and properties of materials should be considered. Size requirements for tattered particles should be considered. The process involved in feeding section, condition of suitable milling ability should be considered. The operation condition (whether it is continuous or intermittent) should be taken into account. Operational characteristics such as energy requirement, permanent finely equipped maintenance constraints, ease mode of action, consistency, noise creation, air and water decontamination should be properly maintained. Land accessibility, height, noise and environmental restrictions should be considered. Storage of ferrous materials for additional processes should be considered.

8. Safety issues

The equipments employed for tear up action contain volatile substances like solvents and gasoline. The volatile substances will generate highly dangerous exhaust. The hammer mill shredder could produce sparks, when operating with high revolution on metals. It leads to a serious explosive issue. These are the major issues related with size reduction process.

9. Safety systems

Size reduction equipment (Shredder) should be placed in a segregated way when compared to other operating sections. Volatile proof substances must be used for electrical switches, mains, wiring and lightings to circumvent short circuiting. The room that contains shredder device must have an explosion ventilator to protect the building. An automatic fire controlling device should be installed to minimize the explosion level at initial stage of operating the devices. During explosion, the inert gases such as nitrogen or carbondioxide must be sprayed for a fraction of seconds to avoid fire problems. Halogens can also be used as the fire extinguishing agent.

10.  Summary

In this lecture we learnt about:

• Various Size reduction techniques in waste processing Various shredder unit functions
• Size reduction laws
• Size reduction equipment operation and usage Selection criteria for size reduction unit
• Safety measurements in shredder units.

References:

• George Tchobanoglous, Hilary Theisen, Samuel Vigil, “Integrated Solid Waste Management Engineering Principles and Management Issues”, (1993), referred page 544 – 551.