October 5, 2017 | Author: Jayant Swami | Category: Mill (Grinding), Industries, Chemistry, Materials, Nature
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Solid Solid-Fluid Operations...


Solid and Fluid – Solid Operations Size reduction

Size reduction Examples:

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Crude ore crushed to small & workable size Synthetic chemicals are grounded into powder Plastic sheets are cut into tiny cubes

Commercial requirements to meet specific size and shape Reduced particle size increases the reactivity of solids Reduction can enhance the separation of unwanted ingredients by mechanical means

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Methods of size reduction Compression (nutcrackers)


For coarse reduction of hard solids to give fines

Impact (hammer)


Gives fine, medium or coarse products

Attrition or rubbing (file)


Yields very fine particles from soft, nonabrasive materials Size reduction can occur from attrition of one particle by one or more other particles

 

Cutting (pair of shears)

4. 

Definite size and shape of particles with very few fines






Crushing rolls



Hammer mill



Disc attrition mill



Rotary knife cutter


Characteristics of comminuted products 

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Crushing or grinding is to produce small particles because of their large surface or their shape, size and number In mechanical separations, the energy required to create new surface is a measure of efficiency Irrespective of uniformity of feed, most actual crushers or grinders does not yield uniform product  the product particle size distribution is very wide Some grinders can control the magnitude of largest particles in their product but not the fines Some grinders may minimize fines, but can not eliminate them

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If the feed is uniform (both physical and chemical structure), then shapes of individual units in the product may be quite uniform Ratio between diameters of largest and smallest particles in a comminuted product is of the order of 104 relationships adequate for uniform sizes must be modified when applied to such mixtures 

Because of extreme variation in sizes of the individual particles,

After crushing, unless the particles are smoothed by abrasion, comminuted particles resemble to polyhedrons with nearly plane faces and sharp edges and corners These particles may be compact, with length, breadth and thickness nearly equal; or they may be plate-like or needle-like


Energy and power requirements in comminution Major expense in crushing and grinding is the power cost

Factors that control power cost are very important

During size reduction, the particles of feed material are first distorted and strained

Work necessary to strain the particles is stored temporarily in the solids as mechanical energy of stress (as in coiled spring)

Then additional force is applied to the stressed particles so that they distorted beyond their ultimate strength and suddenly ruptured into fragments and new surfaces are generated Since a unit area of solid has a definite amount of surface energy, the creation of new surfaces requires work This work is supplied by release of energy of stress when the particles break By conservation of energy, all energy of stress in excess of the new surface energy created must appear as heat

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Efficiency Size reduction is one of the least energy-efficient of all the unit operations Studies reveal that < 1% of energy applied to the solids is used to create new surface

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Rest is dissipated as heat

In operating size reduction machines, energy must also be supplied to overcome friction in the bearings and moving parts Mechanical efficiency is the ratio of the energy delivered to the solids to the total energy input to the machine This efficiency ranges from 25-60%

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Crushing laws and work index 

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Various laws and theories are proposed for predicting power requirements for size reduction of solids do not apply well in practice Approximate calculations give actual efficiencies of about 0.1-2% Assumption: the energy required to produce a change dDp in a particle of size Dp is a power function of Dp:

dP 1  n dDp D p

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Dp: particle size, mm P: Power required, kW m: mass flow rate, tons per hour

 1 p 1  Rittinger’s law: n = 2   K R     m  Dpb Dpa 

This law implies that the same energy required to produce a material from 100 mm to 50mm as is needed to reduce the same material from 50 mm to 33.3 mm

Kick’s law: n = 1


 Dpa p   K K ln   Dpb m 

  

This law implies that the same energy required to produce a material from 100 mm to 50 mm as is required to reduce the same material from 50 mm to 25mm

Bond’s law 

More realistic way of estimating power required for crushing and grinding of material This law postulates that the work required to form particles of size Dp from a very large feed  the square root of the surface-to-volume ratio of product (Sp/Vp) Sp p p 6     m Vp m  s Dp  


K p  B m Dp

KB is a constant depends on the type of the machine and on the material crushed

Work index is defined as the gross energy requirement in kilowatthour per ton of feed needed to reduce a very large feed to such a size that 80% of the product passes a 100µm screens

 p  K B  100 10    0.3162Wi m 3

  


Dp in mm P in kW m in tons per hour

If 80% of feed passes a mesh size of Dpa (mm) and 80% of product passes a mesh of Dpb (mm)  1 p 1   0.3162Wi   m Dpa  Dpb

 

   

Wi includes the friction in the crusher and the power achieved by above equation is the gross power Wi is available for many standard solid materials (both wet grinding and dry crushing) such as bauxite, coal, coke, cement clinker, clay, granite, limestone, etc.


Work indices for dry crushing and grinding S. No


Specific gravity

Work index, Wi






Cement clinker




Cement raw material




















Gypsum rock




Hematite (iron ore)








Phosphate rock







For dry grinding, multiply by 4/3. 13

Ex. Calculate power required to crush 100 ton/h of limestone if 80% of feed passes a 2” screen and 80% of the product passes a (1/8)” screen m  100 ton/h D pa  2"  2  25.4  50.8mm

 8  "  3.175mm

D pb  1

 1 1 p  0.3162mWi    D pb D pa  p  169.6kW


  ; Wi for limestone is 12.74  

Equipment for size reduction 

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Divided into four types: crushers, grinders, ultrafine grinders, cutting machines Crushers employ compression; grinders employ impact and attrition (sometimes combined with compression); ultrafine grinders operate by attrition Feed size

Product size

Coarse crushers



Intermediate crushers



Fine crushers



Colloid crushers


Down to 0.01µm


Crushers  Breaking large pieces of solid material into small lumps  Primary crusher operates on run-of-mine material, accepting anything that comes from the mine face and breaking it into 150250mm lumps  Secondary crusher reduces these lumps to particles ≈ 6 mm in size Grinders  Reduce crushed feed to powder  Product from an intermediate grinder might pass a 40-mesh screen  Product from a fine grinder may pass a 200-mesh with a 74µm opening Ultrafine grinder  Accepts feed no larger than 6mm  Product size is approximately 1-50µm Cutters  Give particles of definite size and shape, 2-10mm in length

Size reduction machines used in food processing engineering Range of reduction

Generic equipment name

Type of equipment



Jaw crushers Gyratory crushers Crushing rolls



Roller mills Hammer mills Tumbling mills Disc attrition mills


Ultrafine grinders

Hammer mills with internal classification Fluid-energy mills

Agitation mills 17

Methods of operating crushers 

Two methods of feeding material to a crusher 

Free crushing: feeding the material at a comparatively low rate so that product can readily escapes 

Residence time in the machine is short and production of appreciable quantities of undersize material is avoided

Choke feeding: machine is kept full of material and discharge of product is impeded so that the material remains in the crusher for a longer period  

Higher degree of crushing but capacity is reduced Energy consumption is high because of accumulated product inside machine 


 used only for small amounts of materials and when it is desired to complete the whole of size reduction in one operation

Crushers  

Slow-speed machines for coarse reduction of large quantities of solids Types of crushers 

  


Jaw crushers Gyratory crushers Smooth-roll crushers Toothed-roll crushers

Jaw, gyratory and smooth-roll crushers operate by compression, for instance, primary and secondary reduction of rocks and ores Such primary crushers are often used in mining, cement manufacture industries, etc.

Jaw crushers 

Feed is admitted between two jaws, set to form a V open at top One jaw is stationary; the other driven by eccentric, reciprocates in a horizontal plane and crushes lumps caught between jaws Advantages:      20

high and constant capacity, high operational reliability, long lifetime, easy replacement of wear and spare parts, low maintenance requirements

Gyratory grinders 

Conical crushing head gyrates inside a funnelshaped casing, open at the top Eccentric drives the shaft carrying the crushing head Solids caught between the head and the casing are broken and rebroken until they pass out the bottom 21

Smooth-roll crushers 

These are secondary crushers, producing a product 1-12mm in size Limited by the size of particle that can be nipped by the rolls to feed that range in size from 12-75mm


Toothed-roll crushers 

Roll faces carry corrugations, breaker bars or teeth May contain two rolls, or only one roll working against a stationary curved breaker plate Not limited by the problem of nip inherent with smooth rolls Operate by compression, impact and shear, not by compression alone Handle softer materials such as coal, bone and soft shale 23

Grinders   

Size reduction machines for intermediate duty Crusher products are often fed to grinder for further reduction Commercial grinders  

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Haller mills and impactors Rolling-compression machines Attrition mills Tumbling mills

Hammer mills  

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Contain high-speed rotor turning inside a cylindrical casing Feed dropped into the top of the casing is broken and falls out through a bottom opening Particles are broken by sets of swing hammers pinned to a rotor disk Particle entering the grinding zone cannot escape being struck by the hammers Particle shatters into pieces, which fly against a stationary plate inside the casing and break into still smaller fragments These in turn are rubbed into powder by hammers and pushed through a grate or screen that covers the discharge opening


 


Several rotor disks, 150-450 mm in diameter and each carrying four to eight swing hammers, are often mounted on the same shaft Hammers may be straight bars of metal with plain or enlarged ends or with ends sharpened to a cutting edge Intermediate hammer mills yield a product 25mm to 20-mesh in particle size Hammer mills for fine production, the peripheral speed of the hammer tips may reach 110m/s and reduce 0.1-15 tons/h to sizes finer than 200mesh (74µm)

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Hammer mills can grind anything – tough fibrous solids like bark or leather, steel turnings, soft wet pastes, sticky clays, hard rock For fine production, they are limited to softer materials Capacity and power requirements of a hammer mill vary greatly with the nature of the feed and cannot be estimated with confidence from theoretical considerations They may be found from small-scale or full-scale tests of the mill with a sample of the actual material to be ground Commercial mills typically reduce 60 – 240 kg of solid per kilowatthour of energy consumed

Impactors 

Impactors resembles s heavy-duty hammer mill except that contains no grate or screen Particles are broken by impact alone, without the rubbing action characteristics of hammer mill These are often primary reduction machines for rock and ore, processing up to 600 tons/h Rotor in an impactor may be run in either direction to prolong the life of hammers 28

Roller mills 

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Solids are caught and crushed between vertical cylindrical rollers and a stationary anvil ring or bull ring Rollers are driven at moderate speeds in a circular path Plows lift the solid lumps from the floor of the mill and direct them between the ring and the rolls where the reduction takes place Product is swept out of the mill by a stream of air to a classifier separator, from which oversize particles are returned to the mill for further reduction Application: in reduction of limestone, cement clinkers and coal They pulverize up to 50tons/h If the classifier is used, the product may be as fine as 99% through a 200-mesh


Attrition mills 

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Particles of soft solids are rubbed between the grooved flat faces of rotating circular disks In a single runner mill one disk is stationary and one rotates In double runner machine both disks are driven at high speed in opposite directions Feed enters through an opening in the hub of one of the disks Then feed passes outward though narrow gap between the disks and discharges from the periphery into a stationary casing The width of the gap, within limits, is adjustable 30

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At least one grinding plate is spring-mounted so that the disks can separate if unbreakable material gets into the mill Mills with different patterns of grooves, corrugations, or teeth on the disks perform a variety of operations, including grinding, cracking, granulating, shredding and sometimes blending Attrition mills grind from 0.5 to 8 tons/h to products that will pass a 200-mesh screen Energy required depends strongly on the nature of the feed and the degree of reduction accomplished and is much higher than in any other crushers and grinders considered so far Energy requirement is typically between 8 – 80 kWh per ton of product


Single runner attrition mill

Double runner attrition mill

Contain disks of buhrstone or rock emery for reducing solids such as clay and talc, or metal disks for solids such as wood, starch, insecticide powders, and carnauba wax Metal disks are usually of white iron, although for corrosive materials disks of stainless steel are necessary Disks are 250-1400mm in diameter turning at 350700 rpm

In general, grind to finer products than single runner mills but process softer feeds Air is often drawn through the mill to remove the product and prevent choking Disks may be cooled with water or refrigerated brine Turn faster at 12007000 rpm

Tumbling mills 

A cylindrical shell slowly turning about a horizontal axis and filled to about one-half its volume with a solid grinding medium forms a tumbling mill Shell is usually steel, lined with high carbon steel plate, porcelain, silica rock or rubber Grinding medium is metal rods in a rod mill, lengths of chain or balls of metal, rubber, or wood in a ball mill, flint pebbles or porcelain or zirconia spheres in a pebble mill 33

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For intermediate and fine reduction of abrasive materials, tumbling mills are unequalled Unlike other mills seen so far (require continuous feed), tumbling mills can be continuous or batch In a batch machine, a measured quantity of solid to be ground is loaded into the mill through an opening in the shell The opening is then closed and the mill turned on for several hours; it is then stopped, and the product is discharged In a continuous mill, the solid flows steadily through the revolving shell In a tumbling mill, the grinding elements are carried up the side of the shell nearly to the top, from whence they fall on the particles underneath Energy expended in lifting the grinding units is utilized in reducing the size of the particles

Rod mill 

Much of the reduction is done by rolling compression and by attrition as the rods slide downward and roll over one another Grinding rods are usually steel, 25125mm in diameter with several sizes present at all times in any given mill Rod mills are intermediate grinders, reducing a 20mm feed to perhaps 10-mesh Often preparing product from a crusher for final reduction in a ball mill They yield a product with little oversize and a minimum of fines 35

Ball mill or pebble mill 


Most of the reduction done by impact as the balls or pebbles drop from the top of the shell In a large ball mill the shell might be 3m in diameter and 4.25m long Balls are 25-125mm in diameter; the pebbles in pebble mill are 50175mm

Tube mills and compartment mills 

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Tube mill is a continuous mill with a cylindrical shell, in which material is ground for 2-5 times as long as in the shorter ball mill Tube mills are excellent for grinding to very fine powders in a single pass where the amount of energy consumed is not of primary importance Putting slotted transverse partitions in a tube mill converts it into a compartment mill One compartment mill may contain large balls, another small balls, and a third pebbles This segregation of the grinding media into elements of different size and weight aids considerably in avoiding wasted work, for the large, heavy balls break only the large particles, without interference by the fines

Critical speed of rotating mills 

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Faster the mill is rotated, the farther the balls are carried up inside the mill and greater the power consumption and the capacity of the mill If the speed is too high, the balls are carried over and the mill is said to be centrifuging The speed at which centrifuging occurs is called the critical speed From a balance between the gravitational and centrifugal forces, the critical speed nc may be founds as below nc    

  38

1 2

g Rr

g is the acceleration of gravity R is the radius of the mill r is the radius of the grinding elements

Operating speed must be less than nc Tumbling mills run at 65-80% of the critical speed, with lower values for wet grinding in viscous suspensions

Ultrafine grinders 

 

Many commercial powders must contain particles averaging 1-20µm with substantially all particles passing 325-mesh screen that has opening 44µm Ultrafine grinders can reduce particles to such fine size Ultrafine grinding of dry powder is done by high speed hammer mills, provided with internal or external classification, and by fluid-energy or jet mills Ultrafine wet grinding is done in agitated mills


Classifying hammer mills 

In a hammer mill with internal classification a set of swing hammers is held between two rotor disks as in a conventional machine But in addition to the hammers the rotor shaft carries two fans, which draw air through the mill inward toward the drive shaft and then discharge into ducts leading to collectors for the product On the rotor disks, there are short radial vanes for separating oversize particles from the required product size


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 

Fine particles are carried past the radial vanes as product Particles which are too large are thrown back for further reduction in the grinding chamber Maximum particle size of the product is varied by changing the rotor speed or by the size and no. of separator vanes Capacity: 1 – 2 tons/h to an average size of 1 – 20 µm Energy requirement: 40 kWh/ton


Fluid energy mills    

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Particles are suspended in a high velocity gas stream Gas may flow in a circular or elliptical path Gas flow may act as jets which rigorously agitate a fluidized bed Some reduction may occur when particles strike or rub against the walls of the confining chamber But most of reduction is caused by interparticle attrition Internal classification keeps the larger particles in the mill until they are reduced to desired size Suspending gas is usually compressed air or super heated steam admitted at 7atm through energizing nozzles 42

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Grinding chamber is an oval loop of pipe 25-200mm Feed enters near the bottom of the loop through a venturi injector Classification of ground particles takes place at the upper bend of the loop As gas stream flows around this bend at high speed, coarser particles are thrown outward against the outer wall while fines congregate at the inner wall 43

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Discharge opening in the inner wall at this point leads to a cyclone separator and a bag collector for product Classification is aided by the complex pattern of swirl generated in the gas stream at the bend in the loop of pipe Fluid energy mills can accept feed particles as large as 12mm but more effective when the feed particles are no larger than 100-mesh screen They reduce up to 1 ton/h of nonsticky solid to particles of average size 0.5-10µm in diameter using 1 – 4 kg of steam or 6 – 9 kg of air per kg of product Loop mills can process up to 6000 kg/h

Agitated mills    

 

Small batch non-rotating mills containing solid grinding medium are available Grinding medium consists of hard solid elements such as balls, pellets, or sand grains These mills are vertical vessel 4 – 1200l in capacity, filled with liquid in which the grinding medium is suspended The charge is agitated with multiarmed impellers Also reciprocating central column vibrates the vessel contents at about 20 Hz Concentrated feed slurry is admitted at the top and product (with some liquid) is withdrawn through a screen at the bottom Useful in producing particles < 1µm 45

Colloid mills 

    

Intense fluid shear in a high velocity stream is used to disperse particles or liquid droplets to form a stable suspension or emulsion Final size of particles or droplets is usually < 5µm Often there is a little actual size reduction in the mills Principal action is the disruption of lightly bonded clusters or agglomerates Syrups, milk, purees, ointments, paints, and greases are typical products using colloid mills Chemical additives are often useful for stabilizing suspensions 46

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The feed liquid is pumped between closely spaced surfaces, one of which is moving relative to the other at speeds of 50m/s or more In some design, liquid passes through the narrow spaces between a diskshaped rotor and its casing This clearance are adjustable down to 25µm Often cooling is required to remove the heat generated Capacity is relatively low up to 2-3 l/min for small mills and up to 440 l/m for largest mill

Cutting machines     

In some size reduction problems, the feed stocks are too resilient to be broken by compression, impact or attrition In other problems the feed must be reduced to particles of fixed dimensions These requirements can be met by machines known as granulators which yield more or less irregular pieces Other machines can meet these requirements are cutter which produces cubes, thin squares or diamonds These devices find application in many manufacturing processes but are especially well adapted to size reduction problems in making rubber and plastics They find important applications in recycling paper and plastic materials 48

Rotary knife cutters 


Contain a horizontal rotor turning at 200 – 900 rpm in a cylindrical chamber On the rotor 2 – 12 flying knives with edges of tempered steel , passing with close clearance over 1 to 7 stationary bed knives Feed particles entering from above may be cut several times before they are small enough to pass through a bottom screen with 5 – 8 mm openings

Criteria for size reduction process 

A crusher, grinder or cutter cannot be expected to perform satisfactorily unless   

The feed is of suitable size and enters at a uniform rate The product is removed asap after the particles are of desired size achieved Unbreakable material is kept out of machine In the reduction of low melting or heat sensitive products, the heat generated in the mill is removed

Therefore heaters and coolers, metal separators, pumps and blowers and constant-rate feeders are important adjuncts to the size reduction unit 50

Open- and close-circuit operations  


In many mills, the feed is broken into particles of satisfactory size by passing it once through the mill When no attempt is made to return oversize particles to the machines for further reduction, the mill is said to be operating in open-circuit This may require excessive amounts of power, for much of the energy is wasted in regrinding particles that are already fine enough Thus it is often economical to remove partially ground materials from the mill and pass it through a size separation device The undersize becomes the product and the oversize is returned to be grounded The separation device is sometimes inside the mill, as in ultrafine grinders; more commonly it is outside the mill

Close-circuit operation is applied to the action of a mill and separator connected so that the oversize particles are returned to the mill Energy must be supplied to drive the conveyors and separators in a closedcircuit system But despite this, the reduction in total energy requirement over opencircuit grinding often reaches 25%


Preliminary guide for selecting size reduction equipment Equipment

Max. Feed size (mm)

Min. Prod. Capacity Applications examples size (mm) (ton/day)

Jaw crushers



103 Metallic and nonmetallic minerals

Gyratory crushers




Metallic and nonmetallic minerals

Roller mill



1 - >103

Cereals, vegetables, calcite, kaolin

Hammer mill



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