mill performance data for grinding specific products 1. 3. · gypsum rock iron oxide limestone...

12.5. PARTICLE SIZE ENLARGEMENT 351

agglutination:

1. Superphosphate, sulfates, and NPK fertilizers. 2. Carbamide and diammophosphate. 3. Ammonium phosphate, potassium chloride, potassium bicarbon-

ate, and salt.

TABLE 12.10. Mill Performance Data for Grinding of Specific Products

Material Equipment Handbook Table No.

Anthracite Barite Cement clinker

Fertilizers Fuller‘s earth Grain Gypsum rock Iron oxide Limestone Limestone Metal stearates Oyster shells Phosphates Quicklime Rubber Seed cake Siliceous

Slate

Sodium carbonate Sulfur

refractories

ball mill CC wet Hardinge ball mill three-compartment

hammer mill roller attrition ring-roller ring-roller ring-roller wet Hardinge ball mill hammer mill hammer mill ball mill ball mill CC roller mill hammer mill pebble mill

three-compartment

roller ring-roller

wet tube mill

wet tube mill

46 35 42

41 48 32 45 47 34 35 50 38 39-40 44 51 33 36

43

48 49

Note: CC is closed circuit grinding; the ring-roller mill has built-in

(From Chemical Engineers‘ Handbook, McGraw-Hill, New York, air classification.

1984, pp. 8.48-8.60).

flatter surface. When the rotating surface is cylindrical and the flat surface is horizontal, the equipment is called a dry pan mill. The equipment shown throws the crushed material outwards where it is picked up and removed with an air stream. Table 12.5(c) is a list of materials that are being ground in roller mills. The ring-roller mill of Figure 12.4(a) is in this class.

g. Fluid jet pulverizers have opposed high speed gas jets that cause collision and disintegration of the particles. A size classifier and fan return larger sizes to the jet stream. The “Majac” jet mill of Figure 12.6(g) is a related kind of device; it has a horizontal section in which high speed gas jets act on the particles. These mills are used primarily for specialty fine grinding of high-value materials. Performance data of Micronizers are in Table 12.9(c); those of the Majac pulverizer are expected to be similar.

12.5. PARTICLE SIZE ENLARGEMENT

For many purposes, lumps of materials of intermediate sizes are the most desirable forms, neither too small nor too large. For instance, beds of overly small granules of catalysts exhibit too great resistance to flow of reacting fluids, and too small particles in suspensions settle out or filter too slowly. Other situations that benefit from size enlargement of particles are listed in Table 12.11.

Because of adhesive forces, particles tend to stick together, particularly small particles that have a large ratio of surface to mass. If a mass is vibrated or shaken lightly, for instance, smaller particles penetrate the interstices between larger ones with increase of contact area and adhesion of the mass. Substances differ naturally in their tendency to agglutinate; as examples, the following groups of materials are listed in the order of increasing tendency to

Adhesion of any mass of particles can be developed by sufficiently high pressure, but lower pressures suffice upon addition of liquid or syrupy binders. Table 12.12 is a list of some commercial agglomerations and the binders that they employ, and Table 12.13 shows how much moisture is needed.

The main types of processes used industrially for particle size enlargement are five in number, defined as follows:

1. Compaction is achieved either by compression or extrusion. Compression is done either into a mold to give a final shape or into a sheet or block that is later broken up to proper sizes. Extrudates are formed under pressure in dies of a variety of cross sections; as they leave the die they are broken up or cut to size.

2. Agglomeration is accomplished under tumbling or otherwise agitated conditions, with or without binding agents. Size is controlled by adjusting the residence time and by gradual addition of feed and binder, slurry or solution.

3. Globulation is the formation of droplets of solution, slurry, or melt followed by solidification by prilling, spray drying, or fluidized bed operation. Control of particle size is best achieved in fluidized beds.

4. Heat bonding is of two types: nodulization in which material is tumbled while heated to give hard rounded granules and sintering in which the product is an integrated mass that is subsequently broken to size.

5 . Flocculation, coagulation and growth of particles in dilute slurries, to assist in subsequent sedimentation and filtration.

A particular industry may employ more than one of these techniques, for instance the manufacture of solid catalysts. Spherical catalysts are made in rotating pan granulators (Fig. 12.7). If the rheological properties are suitable, the material can be extruded (Fig. 12.8), then cut into short cylinders, and subsequently tumbled (Fig. 12.9) into rounded shapes. Smaller spherical beads, for instance, of catalysts for moving bed processes, are made by precipitation or coagulation in an immiscible fluid. Pellets or rings are made on tabletting machines (Fig. 12.10). Although the process is more expensive than extrusion, the product is more nearly uniform. Both extrusion and tabletting result in diffusion resistant skins that, however, usually are eliminated on drying or calcination of the catalyst. Ammonia synthesis catalyst is made by sintering (Fig. 12.11) or fusion of the several ingredients, then crushed and used as irregular lumps of size ranges 1.5-3, 6-10, and 12-21 mm.

In the following, the main equipment for particle size enlargement will be illustrated and discussed.

TUMBLERS

The particles of a granular mass will cohere when they are tumbled and sprayed lightly with a liquid binder which often is water or a concentrated solution of the material being agglomerated. The growth may be due to agglomeration of small particles or to layering of material evaporated from the sprayed solution. Rotary kilns of the kind used for drying or chemical reaction (cement or lime burning, for instance) are adapted to size enlarging service. Usually the tumbling action is less intense, only enough to expose the material to sprays. The sprays are fine and are applied to the

352 DISINTEGRATION, AGGLOMERATION, AND SIZE SEPARATION OF PARTICULATE SOLIDS

surface of the bed of particles. The tumbling action then distributes the liquid uniformly through the mass.

The dkk granulator of Figure 12.7 is a shallow pan, inclined 45-70" to the horizontal and rotating at speeds of 10-30rpm. The ratio of pan diameter to collar height is 3-5. The variety of materials to which this equipment is applied is indicated by the listing of part (e) with this figure and in Table 12.14. As the rotation proceeds, fresh solids and spray are charged continually. The finer particles settle to the bottom, the largest remain at the top and then overflow the collar and constitute the product. Because of the size stratification, the product of disk granulation is more uniform in size than of drum granulators which

Feed 7 1

discharge a mixed product. Some

Product

(a)

performance data in addition to those shown with Figure 12.7 are:

Material Diameter (mm) kg/(min)(m2)

Iron ore 10-25 Cement flour Fertilizer 1.6-3.3

The data of Figure 12.7(c) for cement 44 kg/( min)(m*) .

Pans also are made with height more

11.4 18 14.3

kiln feed are 42-

nearly equal to the diameter. In one such device the material is continually-lifted onto

m

Feed inlet 1-AGagebar

I -

f I \ rc Product outlet

1

Grinding disks "1 Product outlet

(C)

Feed inlet -

Figure 12.6. Examples of mostly less common devices for size reduction. (a) Schematic of a gyratory crusher for very large lumps. (b) Squirrel-cage disintegrator with four cages. (c) Disc-type attrition mill, rotating at 1200-7000 rpm, clearances adjustable by increments of 0.001 in. (d) Schematic of colloid mill, clearance adjustable between 0.001 and 0.050 in., peripheral speeds to 10,000 ft/min. (e) Buhrstone attrition mill, used for making flour and grinding paints, printers inks and pharmaceuticals. (f) Roller or spindle mill; the crushed material is thrown outwards and removed with an air stream. (9) Majac fluid energy mill making a -200 mesh product; opposed air jets cause high speed collisions and disintegration of the material.


Finished A Feed to classifier coal feed Coarse particles

(f)

Figure l2.&(continued)

TABLE 12.11. Benefits of Size Enlargement and Examples of Such Applications

Benefit Examples of Application

1. Production of useful structu- ral forms and shapes

2. Preparation of definite quan- tity units

3. Reduced dusting losses 4. Creation of uniform, non-

segregating blends of fine materials

5. Better product appearance 6. Prevention of caking and

7. Improvement of flow

8. Greater bulk density to im-

lump formation

properties

prove storage and shipping of particulates

zards with irritating and ob- noxious materials

10. Control of solubility 11. Control of porosity and

surface-to-volume ratio 12. Increased heat transfer rates

13. Removal of particles from

9. Reduction of handling ha-

liquids

14. Fractionation of particle mix-

15. Lower pressure drop in tures in liquids

packed beds

pressing of intricate shapes in powder metallurgy; manufacture of spheres by planetary rolling

metering, dispensing, and ad- ministering of drugs in pharmaceutical tablets

briquetting of waste fines sintering of fines in the steel

industry

manufacture of fuel briquets granulation of fertilizers

granulation of ceramic clay for pressing operations

pelleting of carbon black

flaking of caustic

production of instant food products pelleting of catalyst supports

agglomeration of ores and glass

pellet flocculation of clays in batch for furnace feed

water using polymeric bridging agents

selective oil agglomeration of coal particles from dirt in water

reactors with granular catalysts

Fan

Compressed \

air, steam, Or gas Opposed jets

(9)

Feed

an internal sizing screen from which the oversize is taken off as product (Sherrington and Oliver, 1981, p. 69).

Rotating Drum Granulators. The equipment of Figure 12.9 is largely free of internals to promote mixing, but provides just sufficient turnover to effect redistribution of the spray throughout the mass. With heavy sprays and little tumbling action, excess and nonuniform agglomeration will occur. Granules 4-6 mm dia commonly are made by layering from the sprayed solutions. Fertilizer granules made this way are larger, more dense, and harder than those made by prilling. The trend in the industry has been for prilling towers to be replaced by drum granulators and for those in turn to be replaced by fluidized bed granulators in which dusting problems are most controllable.

A pitch of as much as lo" is used to assist material transport over the length of the drum. Length to diameter ratios of 2-3 are used and speeds of 10-20rpm. Recommended speeds are about 50% of the critical speed for the dry material; then adequate cascading occurs and the range of particle size distribution is narrowed. Figure 12.9(b) shows the results of such tests on small scale granulation of a fertilizer.

Another application of tumblers is to the manufacture of mixed fertilizers, in which solid ammonium nitrate, liquid ammonia, liquid phosphoric acid, and liquid sulfuric acid are charged separately and reacted. The incidental agglomeration is excessive, however, and the process must be followed by appropriate crushing and size classification.

Various designs of powder blenders can be equipped with sprays and used as granulators. Figure 12.12 is of a trough equipped with two sets of paddles that rotate in opposite directions and throw the particles to the center where they are wetted. Since the mixing is predominantly lateral rather than axial, a measure of plug flow exists and results in a narrower distribution of sizes than from an empty drum, and may approach that from inclined disk granulators. The shape is not quite as rounded, but can be improved if desired by tumbling subsequently in a dry drum. Dimensions and performance of commercial units are shown with Figure 12.12.


TABLE 12.12. List of Agglomerated Products and Their Binders

Material Binder Agglomeration Equipment Activated Charcoal Alumina Animal Feed Boric Acid

Carbon Black & Iron Powder Carbon, Synthetic Graphite Cement, Raw Mix Cement Kiln Dust

Charcoal Chrome Carbide Clay, Attapulgite Clay, Bentonite

Coal, Anthracite Coal, Bituminous Coal Dust Coke. Petroleum

Continuous Casting Flux Copper Smelter Dust Copper Sulrhite Concentrare Detergent Dust

Dolomite Kiln Dust Dye Pigment Electric Furnace Dust Fertilizer

Flourspar Flourspar Flyash (boiler) Flyash (high carbon)

Glass Batch Glass Batch Herbicide Herbicide

Iron Ore Lignite Limestone Manganese Ore

Manganese Oxide Phosphate Rock Plastic Powder Potash Finn

Sodium Borate Sulfur Powder Tungsten Carbide Zeolite

Lignosulfonate Water Molasses Water

Alcohol-Carbowax Sodium Silicatr Water Water

Starch Gel Alcohol Water Water

Pitch Lignosulfonate Water Pitch

Water Sodium Silicate Sodium Silicate Water

Water Lignosulfonate Water Ammonia

Sodium Silicate Lime-Molasses Water Lignosulfonate

Caustic Soda Water Lignosulfonate- Water Clay-Carbowax

Bentonite-water Gilsonite-Water Clay- Water Lime-Molasses

Sulfuric Acid Phosphoric Acid Alcohol Water

Sulfuric Acid Clay Alcohol Clay-Water

Turbulator* Turbulator*/Disc Ring Extruder Disc Pelletizer

Turbulator* Turbulator*/Disc Disc Pelletizer lbrbulator* / Disc

Briquetter Disc Pelletizer Tbrbulator*/Disc Turbulator*

Briquetter Disc Pelletizer Turbulator. Briquetter

Turbulator*/Disc Turbulator. Disc Pelletizer Disc Pelletizer

'I\trbulator*/Disc Turbulator*/Disc Turbulator*/Disc Drum

Disc Pelletizer Briquetter lhrbulator*/Disc Briquetter

Disc Pelletizer Briquetter Turbulator*/Disc Briquetter

Drum Turbulator*/Disc Tbrbulator* Briquet let

Turbulator*/Disc Tbrbulator'/Disc Disc Peiletizer Disc Pelletizer

Tbrbulator*/Disc Compactor Disc Pelletizer lbrbulator*/Disc

(Koerner and MacDougal, 1983).

Sticky, very fine, and highly aerated materials can be granulated in drums with high speed impellers with pegs or pins instead of paddles. In Figure 12.13, the material enters at one end, is immediately wetted, and emerges as pellets at the other end. Residence times are under a minute. The data with this figure show that the bulk density of carbon black is increased by a factor of 11, although with about 50% binder in the product.

As mentioned, other powder blending devices can be adapted to granulation, but unless most of the equipment is on hand, it is best to adopt a proven design with which some manufacturer has experience. If the stakes are high enough, the cost of a development program with other equipment may be justifiable.

ROLL COMPACTING AND BRIQUETING

Agglomeration of finely divided materials is accomplished at high rates and low costs by compression between rolls. The form of product may be continuous sheets that subsequently are broken up to desired size or it may be lumps or briquets of finished form and size. A few shapes are shown in Figure 12.14, but a great variety of simple geometrical shapes can be made with readily available rolls. Briquets are a low cost product, rough in shape, and not of highly uniform weight. When smooth appearance and weight uniformity are demanded, tabletting is the process to be used. The great variety of materials that have been compacted with rolls is indicated


TABLE 12.13. Moisture Requirements for Successful Granulation in Tumbling Machines

Approximate size

material, less than analyses of raw Moisture content

of ballad product Raw material indicated mesh (% HZO)

Precipitated calcium carbonate 200 29.5-32.1 Hydrated lime 325 25.7-26.6 Pulverized coal 48 20.8-22.1 Calcined ammonium metavananiate 200 20.9-21.8 Lead-zinc concentrate 20 6.9-7.2 Iron pyrite calcine 100 12.2-1 2.8

Taconite concentrate 150 9.2-10.1 Magnetic concentrate 325 9.8-10.2 Direct shipping open pit ores 10 10.3-10.9

Basic oxygen converter fume 1 Pm 9.2-9.6 Row cement meal 150 13.0-13.9 Utilities-fly ash 150 24.9-25.8 Fly ash-sewage sludge composite 150 25.7-27.1

Specular hematite concentrate 150 9.4-9.9

Underground iron ore 0.25 in. 10.4-10.7

Fly ash-clay slurry composite 150 22.4-24.9 Coal-limestone composite 100 21.3-22.8 Coal-iron ore composite 48 12.8-13.9 Iron ore-limestone composite 100 9.7-10.9 Coal-iron ore-limestone comDosite 14 13.3-14.8

Courtesy McDowell Wellman Company.

Concentrated ,Recycle fines

(a)

Dish size Motor Capacity '(4 lkW) (kg s-') Material Remarks

0.36 0.36 0.99 0.99 0.99 0.99 1.37 2.59

2.59 2.59 3.05 3.05 3.66

4.27 5.49

0.18 0.18 0.55 0.55 0.55 0.55 2.2

11

1 1

15 18 22

37 44

7.3

0.0 13 0.0044 0.13 0.076 0.076 0.15 0.28 8.5

0.85 0.93

1-7-2.3 2.8 3.4

11 11

Tungsten carbide Alumina Phosphate rock Bituminous coal filter cake Beryl ore mix Copper precipitate Frit enamel mix Zinc concentrate sinter mix

Chromate Bituminous coal fines Raw shale fines Bituminous coal filter cake Zinc sulphide ore

Nitrogen fertilizer material Magnetite ore

16 x 60 mesh micropellets

85% 4 x 30 mesh product Feed to pan dryer Feed to sinter belt

Feed to furnace Micropelletized sinter

machine feed For electric ore furnace For coking furnace For expanding in rotary kiln

For fluid bed roasting of

Feed: hot melt and recycle Feed to travelling grate -

4 x 30 mesh pellets

indurating section

(C)

Figure 12.7. Rotating dish granulator applications and performance (Sherrington and Oliver, 1981). (a) Edge and face view of a dish granulator, diameters to 25 ft, Froude no. n2D/g, = 0.5-0.8. (b) Stratification of particle sizes during rotation. (c) Typical applications of dish granulation (Drauo Corp.). (d) Capacity and power ( D r a m Corp.). (e) Performance on cement kiln feed.


~~ I 170 Ib./cu. ft material/ 125 lb /cu. ft. I Pelletizing

Diameter (m) 3.6 4 Depth (cm) 91 91 Speed (rpm) 17.5 14.0 Drive (kW)

Installed 30 37 Used 26 25-30

Feed rate (kg 9') 7.1 8.5-10.1 Moisture (%) 12.5-1 3.5 12.5-13.5

Granule compressive strength (kg) 2.7-6.7 2.7-6.7 Granule porosity (%) 26 26

Powder feed position Bottom centre Water feed positions

Main Jets above powder feed Secondary Fine sprays in top section of pan

Number of tooling stations 27 33 45

i9600-4*500 1,200-3,300 :z0-2y700 1fa Output (tablets/min) Max. tablet diameter (in.) Fill depth (in.):

Standard 0 4 0-H 0-ii ta-10 ii-13 Optional 44-10

&-I%

10 10 Max. operating pressure (tons) 10 Pressure release adjustment (tons) Upper punch entrance (in.)

0-10

(b)

Series 37 45 55 61 Number of stations 37 45 55 61 Max. operating pressure (tons) Max. depth of f i i (in.) r

23 k

2,775-11,100 25

r 2,500-10,000

1 i

Max. tablet diameter (in.) 1 Output (tablets/min) 888-3,552 2.050-8,200

(C)

Figure U.8. Operation and specifications of rotary tabletting machines (Curstensen, 1984). (a) Action of the punches of a rotary tabletting machine. (b) Specifications of a Sharples Model 328 (Stokes-Pennwult Co.). (c) Specifications of a Manesty Rotapress Mk 11 (Manesty Machines Ltd. and Thomas Engineering Znc.).


(a) Granule diameter (mm)

(b)

Installed Approximatea Diameter Length Power Capacity

Application (ft) (ft) (HP) rpm (tondhr)

Fertilizer granulation 5 10 15 10-17 7.5

balling 9 31 60 12-14 54

8' 16 75 8-14 40 Iron ore

12 33 75 10 98

*Capacity excludes recycle; actual drum throughput may be much highEr.

Inclination 2".

Figure 12.9. Rolling drum granulator sketch and performance. (a) Sketch of a rolling drum granulator (Sherringfon and Oliver, 1981). (b) Effect of rotational speed on size distribution: (1) at 20% of critical speed; (2) at 50%. (c) Performance data on commercial units (Capes and Fouda, 1984).

by the listing of Tables 12.15 and 12.16. A survey of equipment currently marketed worldwide is made by Pietsch (1976); an excerpt is in Table 12.16.

Compacting of specific materials can be facilitated with certain kinds of additives. Binders are additives that confer strength to the agglomerates, and lubricants reduce friction during the operation. Some additives may function both ways. A few of the hundreds of binders that have been tried or proposed are listed in Table 12.12. Lubricants include the liquids water, glycerine, and lubricating oils; and typical solids are waxes, stearic acid, metallic stearates, starch, and talc.

Successful compacting has been accomplished at temperatures as high as 1OOO"C. Extrudates are 1-10 mm thick. The information of commercially available equipment of Table 12.17 is repre- sentative. Rolls range in size from 130mmdia by 50mm wide to 910mmdia by 550mm wide. Capacities are 10-6000kg/h, and energy requirements, 2-16 kWh/ton. Compacting of mixed fertilizers and similar materials is accomplished by pressures of 30-1200 atm, of plastics and resins by 1200-2500 atm, and of metal powders above 5000atm. Feed supply may be in the vertical or horizontal direction, by gravity or forced feed. Horizontal feeding is less bothered by entrapped air.

Figures 12.10(a) and (b) show product in sheet form which is subsequently broken down to size. Pellets of large size also may be made for subsequent crushing. For instance, pellets 5cmdia by 1 cm thick are made for the pharmaceutical trade for breaking up to serve as coarse granular feed to tabletting.

TABLElTlNG

Rotary compression machines convert powders and granules into hard tablets of quite uniform weight, notably of pharmaceuticals, but also of some solid catalyst formulations. The process is illustrated in Figure 12.8(a). A powder is loaded into a die where it is retained by a lower punch; then it is compressed with an upper punch, and the tablet is ejected by raising both punches.

Most tablets are small, the largest shown in Figure 12.8 is 1-3/16in. dia and the greatest depth of fill is 1-3/8in., but other machines make tablets 4 in. dia and exert forces of 100 tons. The degree of weight uniformity normally aimed at is indicated by the specifications of the U.S. Pharmacopeia. This states that, of a sample of 20 tablets, only two may differ from the mean by the percentage stated following and only one may deviate by twice the percentage stated:

Weight of Tablet (mg)

Equal to or less than 13

% Deviation

15 13-130 10

130-324 7.5 More than 324 5

Greater weight uniformity is achieved with coarse powders or granules as feed. Too large a proportion of fines may cause the tablets to come apart upon ejection. Satisfactory feed can be prepared by first making large tablets in another machine and then

Fine mater ia l O A

,---+-I t

I I Compac ing

T r e t u r n e d

1' Flake breaker

T I

Vertical Screw M I Pre-compresses and deaerates/producl

Pressure Applied 4 Air to hydraulic actuator reaulates pressure eterted on rolls Prebreak Breaks sheets into chips and flakes

Sizes compacted material to desired particle size

Granulator

Screener F+ Overs Oversized granules recycled

Recycle Finished

controlled partlcle size

- Upper Feed Hopper Collects virgin and recycle product

Horizontal Feed Screw Controlled feed

-Feed and Recycle

Oversize and fines I for reprocessing

c- Recycle System

Original Powder Feed

+Lower Hopper

(C) (d) (e)

Figure 12.10. Equipment for compacting, briquetting, and pelleting. (a) Flowsketch of a process for compacting fine powders, then granulating the mass (Allis-Chalmers Co.). (b) Integrated equipment for roll compacting and granulating (Fitzpatrick Co.). (c) A type of briquetting rolls. (d) A gear pelleter. (e) A double roll extruder.

breaking them into coarse granules, or by batch-fluidized bed granulation. For pharmaceuticals the range of allowable additives to facilitate tabletting is limited. Magnesium stearate is a common lubricant to the extent of 0-2%, and corn starch is a common binder in the range of 0-5%. Disintegrants and fillers also are used. Preparation of such mixes is accomplished in powder blenders and fixed by granulation.

the large Manesty at 45-180 rpm. Maximum forces for these sizes of

tablets are 10 tons, but up to 100 tons may be needed for tablets 2.5-4 in. dia. The largest machines shown can be driven with about 50 HP.

PROCESSES

Powders, pastes and melts are pelleted by extrusion through a die The Stokes machines of Figure 12.8 operate at 35-100 rpm and followed by cutting. Binders and lubricants may need to be

incorporated in the feed, but the process usually is not feasible for

Ignition ~ Grnace /@ Grate

travel

Figure 12.11. Flowsketch and operation of a sintering process.

screens

m Cooler

TABLE 12.14. Industries that Employ Disk Granulators and Some of the Products They Process

Industry Typical Application

Steel

Foundry Ferroalloy

Copper LeadIZinc Other Metals

Glass Ceramics

Refractories Cement/Lime Chemicals

Ag-Chemicals

Foods

coal Power

Nonmetallic Minerals

Pulp, Paper, Wood Solid Waste

Electric Furnace Baghouse Dust, BOF Dust, OH Dust, Coke Fines. Raw Materials, Iron Ore Pellet- izing Baghouse Dust, Mold Sand Fines Silicon, Ferrosilicon, Ferromanganese. Ferro- chrome Concentrates, Smelter Dust, Precipitates Concentrates, Sinter Mix. Flue Dust, Drosses nngsten, Molybdenum, Antimony, Brass, Tin. Berrylium. Precious Metals, Aluminum, Silicon. Nickel Glass Rawmix, Furnace Dust, Glass Powder Alumina, Catalyst, Molecular Sieves, Substrates, Insulator Body, Tilemix, Press Feed, Proppants, Frits, Colors Bauxite, Alumina, Kiln Dust, Blends Raw Meal, Kiln Dust Soda Ash, Sodium Sulfate, Detergents, Cleaners, Zinc Oxide, Pigments, Dyes, Pharmaceutical Compounds, Industrial Carbons, Carbon Black Fertilizers, Pesticides, Herbicides, Insecticides. Soil Conditioners, Aglime, Dolomite, Trace Min- erals. N-P-K raw Materials Instant Drink Mix, Powdered Process Foods, Sugar, Sweetners, Confectionary Mix Coal Fines Coal Fines, Fly Ash, FGD Sludge, Boiler Ash, Wood Ash Clay, Talc, Kaolin, Fluorspar, Feldspar, Diatoma- ceous Earth, Fullers Earth, Perlite Paper Dust, Wood Fines, Sander Dust, Boiler Ash Incinerator Ash. Refuse Fines, Mixed Refuse,

Hopper Feeder

Dried Sludge

(Koerner and MacDougal, 1983).


Blunger dimensions

Length (m) Width (m) Height (m) Screw diameter (m) Pitch (m) Shaft speed (rpm) Capacity (kg s-') Installed power (kW)

4.5 1.4 1.07 0.8 1.7

5 5 23-25

104

APPROXIMATE SIZE MATERIAL BULK CAPACITY (WIDTH X LENGTH) SPEED DRIVE

MODEL DENSITY LB/FT3 (TONS/HR) (FT) (RPM) (H P)

A 25 8 2 X 8 56 15 5 0 15 2 x 8 56 20 75 22 2 x 8 56 25

100 30 2 x 8 56 30 25 30 4 x 8 56 30 50 60 4 x 8 5 6 50 75 90 4 x 8 56 75

100 120 4 x 8 56 100 25 30 4 x 12 56 50 50 6 0 4 x 12 56 100 75 90 4 x 12 5 6 150

100 120 4 x 12 56 200 125 180 4 x 12 56 300

(b)

Figure 12.12. Paddle blending granulator and typical performance. (a) Sketch of a double paddle trough granulator (Sherrington and Oliver, 1984). (b) Performance in granulation of fertilizers (Feeco International).

abrasive materials. Economically feasible power requirements correspond to the range of 100-200 lb/HP hr. The main types of machines are illustrated in Figures 12.10(e) and 12.15.

Screw extruders usually are built with a single screw as shown, but may have as many as four screws and the die may have multiple holes of various cross sections. An 8 in. dia screw can have a capacity of 2000 lb/hr of molten plastics. Tubing can be extruded at 150-300 ft/min. To make pellets, the extrudate goes to cutting machines in which parts as small as washers can be made at rates as high as 8000/min. The extrusion of plastics is described at length by Schwartz and Goodman (Plastics MateriaLr and Processes, Van

Nostrand Reinhold, New York, 1982); such equipment is applicable to other situations.

Ring pellet mills consist of a power driven rotating ring with radial holes, friction driven rolls to force the material through the holes and knives to cut the extrudate to desired lengths. The feed is charged with screw feeders into the spaces between the rolls and the feed distributor flights. Hole diameters range from 1.6 to 32mm. The force of compaction is due to flow friction through the die. Differing flow and compression characteristics are accommodated by varying the thickness of the ring. A production rate of 200 lb/HP hr has been quoted for a normal material through 0.25 in.


Cbrbon black feed Rate, Mg/day 26.Y Bulk density, kg/m3 5 1.3

Wet basis Production rate, kg h 2108.3

562.3 11.0

Buik density, kg/m Densification ratio Dry basis Production rate, kg h 1096.3 Bulk density, kg/m l 394.1

Pellets produced

4

Binder Specific gravity 1.05 Injection rate, kg/h 101 1.5 Use ratio, weight of binder to

weight of wet pellets 0.92 Power Consumptionb

Rate, kW 18.5 Per Mg of wet pellets, kWh

Rotap test (5 min), % 1.4 (avg. of 45 samples) Crushing strength, g 25 (avg. of 73 samples)

15.0' hduction quality

:Average from S d a y test, plus subsequent production.

'Cold shell. Ammeter readings.

Figure 12.13. Pin mixers which operate at high speed for granulation of fine and aerated powders. (a) Pinmixer for the granulation of wetted fine powders. (b) Performance of a pinmixer, dimensions 0.67 m dia by 2.54 m, for pelleting a furnace oil carbon black (Capes, 1980).

holes. The life of a die is measured in hours. Units of 300HP are made. Some applications are cited in Table 12.17.

The large tonnage application is to the preparation of animal feeds but many smaller scale applications also are being made. A survey of this literature is made by Sherrington and Oliver (1981).

PRILLING

In the process a molten material is disintegrated into droplets which are allowed to fall and to solidify in contact with an air stream. The

equipment is similar to that for spray drying, but the mechanism is simpler in that no evaporation occurs and as one consequence the product is less porous and stronger. A sketch of a prilling process is in Figure 12.16. At one time the chief application was to fertilizers, but a list of many other prillable materials is given in Table 12.18.

Dimensional and some operating data for prilling of urea and ammonium nitrate also are in Table 12.18. Towers as high as 60 m have been installed. Because of the expense of towers, prilling is not competitive with other granulation processes until capacities of 200-400 tons/day are reached.

( c)

& b e SIDE VIEW SIDE VIEW SIDE VIEW

CIRCULAR CROSS- SECTIONS - flat face bevel standard convex deep convex

(d) ( e )

362 DISINTEGRATION. AGGLOMERATION, AND SIZE SEPARATION OF PARTICULATE SOLIDS

Materials suitable for prilling are those that melt without decomposition, preferably have a low heat of solidification and a high enough melting point to permit the use of ambient air for cooling. Because of high viscosities, spray wheels are preferred to spray nozzles. The wheels often are equipped with scrapers to prevent clogging. Since several wheels are needed for capacity, they are often arranged in line and the cross section of the tower is made rectangular.

The density of the prills is reduced substantially when much evaporation occurs: with 0.2-0.5% water in the feed, ammonium nitrate prills have a specific gravity of 0.95, but with 3-5% water it falls to 0.75. Prilled granules usually are less dense than those made by layering growth in drum or fluidized bed granulators. The latter processes also can make larger prills economically. To make large prills, a tall tower is needed to ensure solidification before the bottom is reached. The size distribution depends very much on the character of the atomization but can be made moderately uniform. Some commercial data of cumulative % less than size are:

% Lessthansize 0 5 50 95 100 1.2 1.6 2.4 3.5 4.0 Dia (mm)

Cooling of the prills can be accomplished more economically in either rotary drums of fluidized beds than in additional tower height. Fluidized bed coolers arexheaper and better because more easily dust controllable, and also because they can be incorporated in the lower section of the tower. After cooling, the product is screened, and the fines are returned to the melter and recycled.

FLUIDIZED AND SPOUTED BEDS

In fluidized bed granulation, liquid or solution is sprayed onto or into the bed, and growth occurs by agglomeration as a result of binding of small particles by the liquid or by layering as a result of evaporation of solution on the surfaces of the particles. The granules grown by layering are smoother and harder. Some attrition also occurs and tends to widen the size distribution range of the product. Larger agglomerates are obtained when the ratio of droplet/granule diameters decreases. Increase in the rate of the fluidizing gas and in the temperature of the bed decreases penetration and wetting of the bed and hence leads to smaller granule sizes. A narrower and more concentrated spray wets a smaller proportion of the particles and thus leads to larger size product. The bed is often made conical so that the larger particles are lifted off the bottom and recirculated more thoroughly.

Initial particle size distribution often is in the range of 50-250 pm. The product of Table 12.19(a) is 0.7-2.4 mm dia.

A wide range of operating conditions is used commercially. Performance data are in Table 12.19. Gas velocities cover a range of 3-20 times the minimum fluidizing velocity or 0.1-2.5m/sec. Bed expansion ratios are up to 3 or so. As in fluidized bed drying, bed depths are low, usually between 12 and 24 in. Evaporation rates are in the range 0.005-1.0 kg/(sec)(mz).

Batch fluidized bed granulation is practiced for small production rates or when the residence time must be long. Figure 12.17(a) and Table 12.19(a) are of an arrangement to make granules as feed to pharmaceutical tabletting. A feature of this equipment is the elaborate filter for preventing escape of fine particles and assuring their eventual growth. A continuous process for recovery of pellets of sodium sulfate from incineration of paper mill wastes is the subject of Figure 12.17(b) and Table 12.19(b).

0 TOP VIEW TOP VIEW

SIDE VIEW S!,DE VIEW oblong foot ball

Figure 12.14. Common shapes and sizes of pellets made by some agglomeration techniques. (a) Sizes and shapes of briquets made on roll-type machines. (b) Catalyst pellets made primarily by extrusion and cutting (Imperial Chemical Industries). (c) Some of the shapes made with tabletting machines.


TABLE 12.15. Alphabetical List of S o m e of t h e Materials that Have Been Successfully Compacted by Roll Presses

Acrylic ra ins , activated carbon. adipic acid. alfalfa. alga powder. alumina, aluminium. ammonium chloride, animal feed. anthracite. asbestos

Barium chloride. barium sulfate, battery masses, bauxite. bentonite, bitumen, bone meal, borax, brass turnings

Cadmium oxide, calcined dolomite. calcium chloride. calcium oxide, carbomethylcellulose (CMC). carbonates. catalysts, cellulose acetate, ceramics, charcoal. clay, coal. cocoa powder. coffee powder, coke, copper. corn starch

Detergents. dextrine, dimethylterephthalate (DMT). dolomite, ductile metals, dusts, dyes Earthy ores, eggshells. elastomers, emulsifiers, epoxy resins Feldspar, ferroalloys. ferrosilicium, fertilizers, flue dusts, fluorspar, fly ash, foodstuffs, fruit powden,

Gipsum, glass making mixtures, glass powder, grain waste, graphite, gray iron chips and turnings Herb teas, herbicides, hops, hydrated lime Ice. inorganic salts, iron oxide, iron powder, insecticides Kaolin, kieselgur, kieserite h a d , lead oxide, leather wastes, LD-dust. lignite, lime, limestone, lithium carbonate, lithium fluoride,

Magnesia, magnesium carbonates, magnetite, maleic anhydrate. manganese dioxide. metal powden,

Naphthalene. nickel pwder s , nickel ores, niobium oxide Ores, organic chlorides, organic silicates, oil shale, oyster shells Pancreas powder, penicillin. pharmaceuticals, phosphate ores, plastics, polyvinylchloride (PVC),

Raisin seeds, reduced ores, refractory materials. rice starch, rock salt Salts, sawdust, scrap metals, shales, silicatcs. soda ash, sodium chloride. sodium compounds, sodium

Teas, tin, titanium sponge, turnings Urea, urea formaldehyde Vanadium, vermiculite, vitamins Waxes, welding powder, wood dust, wood shavings Yeast (dry) Zinc oxide, zirconium sand

fruit wastes. fungicides

lithium hydroxide

molding compounds, molybdcnum. monocalciumphosphate (MCP)

potash, potassium compounds, protein pigments, pyrites, pyrocatechol

cyanide, sponge iron, steel turnings, stone wool, sugar, sulfur

(Pietsch. 1976).

The multicompartment equipment of Figure 12.17(c) permits improved control of process conditions and may assure a narrower size distribution because of the approach to plug flow.

Some of the fluidized bed dryers of Figure 9.13 could be equipped with sprays and adapted to granulation. The dryer performance data of Tables 9.14 and 9.15 may afford some concept of the sizes and capacities of suitable granulators, particularly when the sprays are somewhat dilute and evaporation is a substantial aspect of the process.

Spouted beds are applicable when granule sizes larger than those that can be fluidized smoothly. Above l m m or so, large bubbles begin to form in the bed and contacting deteriorates. Two arrangements of spraying into a spouted bed are shown in Figure 12.17(d). Particles grow primarily by deposition from the evaporated liquid that wets them as they flow up the spout and down the annulus. As the performance data of Table 12.19(c) indicate, particles up to 5 mm dia can be made and even quite dilute solutions can be processed. The diameter of the spout can be deduced from the given gas rates and the entraining velocities of the particles being made. Figure 9.13(f) is a more complete sketch of a spouted bed arrangement. Example 9.9 is devoted to sizing a fluidized bed dryer, but many aspects of that design are applicable to a granulation process.

SINTERING AND CRUSHING

This process originally was developed to salvage iron ore fines that could not be charged directly to the blast furnace. Although other applications appear to be feasible, the original application to iron ore fines seems to be still the only one, possibly because the scale of the operation is so great. Figure 12.11 is a sketch of the process. A mixture of ore fines, some recycle, 14-25% of calcite or dolomite as fluxes, and 2.5-5% solid fuel is placed on a conveyor to a depth of 12in. or so. It is conveyed into an ignition furnace, burned and fused together, and then cooled and crushed to size. Fines under 6 mm are recycled. Sinter feed to blast furnaces is about 40-50 mm in the major dimension. The equipment is very large. One with a conveyor 5 m wide and 120m long has a capacity of 27,000 tons/day .

Nodulizing is another process of size enlargement by fusion. This employs a rotary kiln like those used for cement manufacture. The product is uniform, about 0.5in. dia, and more dense than sinter.

Sintering of powdered metals such as aluminum, beryllium, tungsten, and zinc as well as ceramics under pressure is widely practiced as a shaping process, but that is different from the sintering process described here.

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TABLE 12.17. Some Applications of Rotating Ring Pelletizers (see Figure 12.15(b))

Mrterlrl Rearon To

Pellet

LBIHPMR Pellet Sire (KQIKW/HR) flnChe8 Diameter)

(Miillmeter Dis.)

Asbestos Shorta

Clay Beso Material

Cryolite Filter clk4 Domollte

Herbicide

Inaecticide

Iron Oxide

Lignite

Nylon Film Scrap

Paper Scrap

Phenolic Molding Compound

Polyethelyme Film

POly8tyf8M Foam

Polypropylene Film

Rubber Accelerator

Starch

sawdust

Salt

Densify, Reduce Dust In at 20ib1ft3 (320kg1m3) Out at 651blft3 (1041kglm3)

Handling

Densify, Reduce Dust In at 5ibfft3 (80kglm3) Out at 30ib1ft3 (480kglm3)

Handilng

Densify, Handllng In at 13iWft3 (208kg1m3) Out et 36ibfft3 (577kgIm3)

Handllng, Densify, Calcine

Handilng

Handling

Handling, Control, Solubiilty

Defined Form, Reduce Dust

Calcining, Reduce Dust

Eliminate Fines

Densify

Densify

Reduce Dust, Handling

Densify from 51bltt3 (80kglm3)

Denslfy from 41Wft3 (64kglm3)

Densify

Reduce Dust, Handling

Handling

Bum

Hmdllng, Reduce Dust

to 20 lbfft3 (320kglm3)

to 241bn13 (384kglm3)

45 (27)

80 (49) 170 (103)

80 (4s)

300 (1W

150 (91)

100-300 61 -182)

100 (61)

200 (122)

1% (el)

120 (73)

50-1 00 (30- 61)

100 (61)

60 (36)

83 (50)

60 (36)

30 (18)

164 (100)

40 (24)

192 (117)

75 (46)

60 (36)

70 (43)

3/8 a

(9.5mm)

114" (6.4mm)

318' (9.5mm)

112' (12.8mm)

114' (6.4rnrn)

16" to 314" (3.2mm to 19mm)

318" (9.5mm)

114" to 3110" 6Amm to 48mm)

12/64" (4.8mm)

118" (3.2mm)

118" to 114" (32mm to 6.4mm)

118" to 114' (32mm to 6.4mrn)

118" (3.2mm)

1/2' (12.7mm)

118" (3.2mm)

118" to 3116" (3.2mm to 4.8mm)

118" (3.2mm)

118" (3.2mm)

12164" (4.8rnm)

12/04" (4.8mm)

114" (6.4mm)

118" (3.2mm)

(Sprout Waldron Co.).


ADAPTER T PI E RM O COUPLES

, BREAKER MELT

'\/------ EACKHEAT ZONE FRONTHEAT ZONE

(a)

Figure 12.15. Two types of extrusion pelleting equipment. (a) Screw-type extruder for molten plastics: The die is turned 90" in the illustration from its normal position for viewing purposes. The extruded material is cooled and chopped subsequently as needed (U.S. Industrial Chemical Co.). (b) Ring extruders: material is charged with screw conveyors to the spaces between the inner rolls and the outer perforated ring, the ring rotates, material is forced through the dies and cut off with knives.

PRILLING TOWER

-! t- COOLING AIR 1; Figure 12.16. A prilling tower for ammonium nitrate, product size range 0.4-2.0 mm. The dryer is not needed if the moisture content of the melt is less than about 0.5%.

SCREENS

AMMONIUM NITRATE PRILLS TD COATING AH) BAGGING

N I TRATE MELT FINES


TABLE 12.18. List of Typical Prillable Materials and Performances of Some Prilling Operations

(a) List of Typical Prillable Materials

Adherlm Adipic Acid Alpha Nphthol Ammonium nitrate md additives Asphalt Birphenol-A Bitumen Carbon pitch Caustic soda Cetyl alcohol Cod-derived waxes Cod tar pitch Dichlorebenzidine Fatty acids Fatty alcohols Epoxy resins Hydrocarbon resins Highmelting inorganic salts Ink formulations Lauric acid Myristic acid Myristyl alcohol Paraffms

Pentrhlomphenol Petroleum wax Phen~lic &-NOW& resin Fine rosin Polyethylene rains Polyrtynm resins Polypropylene-mrleic anhydride Poturlum nitrate Rethu Sodium Jycob Sodium nitrate Sodium nitxite Sodium sulphate

S t a y 1 d c o h d Substituted diphatics Substituted 8mida Sulphur Urea and additives Urea-sulphur mix Wax-rain blends

S t d C acid

(b) Data for the Prilling of Urea and Ammonium Nitrate

Tower size Prill tube height, ft 130 Rectangular cross section. ft

rate, Ib/h inlet temperature tempcrature rise, "I; 15

1 1 by 21.4

360.000 ambient

Cooling air

Melt TY PC Urea Ammonium Nitrate rate, Ib/h 35.200 (190 Ib HzO) 43,720 (90 Ib Hz0) inlet temperature, O I; 2 75 365

Prills outlet temperature, "F 120 225 size. mm approximately 1 t o 3

(HPD Inc.).


TABLE 12.19. Performance of Fluidized Bed and Spouted Bed Granulators

(a) Batch Fluidized Bed Granulator to Make Feed to Pharmaceutical Tablets; the Sketch Is in Figure 12.17(a)

APPROXIMATE RAXGE

Batch load, dry basis, Ib 20 to 4004 Volume of container for static bed, ft3 2 to 15 Fluidizing air fan, hp 5 to 25 Air (Steam) heating capacity, Btu/h 70,000 to 600,000 Drying air temperature, OC 40 to 80 Granulating liquid sprayb Two fluid nozzle

Air volume f to 2 SCFM Liquid volume 500 to 1500cm3/min

Batch processing time, min 30 to 50 Average granule size 24 to 8 mesh

'Batch capaaty exceeds 1500 Ib in the lugest modem units. bTypical granulating liquids are gelatin M sodium carboxymethyl cellulose solutions.

(Capes and Fouda, 1984).

(b) Performance of Fluidized Bed Granulation of Two Waste Products; Sketch IS in Figure 12.17(b) for Paper Mill Waste

BED GRANULAR PRODUCT TYPE OFSLUDGE INCINERATOR SIZE TEMPERATURE CAPACITY COMPOSITION

~~~

Oil refinery waste sludge 40 ft high; 1330'F 31 X lo3 lb/hr (85-95% water) 20 ft ID at of sludge

base increasing to 28 ft at top

Paper mill waste liquor' 20 ft ID 135@F 31 X 1031b/k (4056 solids) at top

(Capes and Fouda, 1984).

(c) Applications of Spouted Bed Granulations

- -

Statt-up matuial was silica sand; replaced by nodules of various uh components

silicates, A1103 after operation d incinerator.

Sulfur added to produce

Such I S CrsO4. N4 Ck Mg

90-9596 N8aS04 m d some NaaC03

Feed solution Product Cas temperature

Moisture Temperature Size Moisture Inlet Outlet Gaa flow rote Cawcity

9500 IWO1

Material content (%) ( "C) (mm) (I) /"C) ("C) (m s-') (k8 h-')

Complex fertilizer 27 15 3-3.5 2.4 170 70 13-9 4000

Sulphur - I 3 5 2-5 - IS - 1.1 x lo-' 4 0 Inorganic pigments,

Organic dyes, e.g. acid

Potassium ch I oride 68 15 4-5 - 200 60 13-9 Ammonium nitrate 4 175 2 . 5 4 0- 2 15 55 13.9

1 e.8. natural sienna 45 - 3 -5 - 280 100

blue black 63 - 1-3 6-5 226 154 Ammonium sulphate 60 70 - 2 - 190 83 -1.3 X lo-' -2-7 Sodium chloride 77 - -4.5 - 120 70 -1-8 X io-' -1.2

(Sherrington and Oliver, 1981 1.


Fan Exha

Sol ids

'i? ::3 F l u i d i z i n g a i r

FluoSolids reactor

pellet product air blower

Feed granulating liquid

,Atomizing nozzle

-Spout

-Annulus

'Spouting gos 'I'

Feed granulating liquid

,Atomizing nozzle

-Spout

-Annulus

spouting gos

.Particle fountain

.spout

.Annulus

Figure 12.17. Fluidized bed and spouted bed granulators. (a) A batch fluidized bed granulator used in the pharmaceutical industry; performance data in Table 12.19(a). (b) Part of a fluidized bed incineration process for paper mill waste recovering sodium sulfate pellets; performance data in Table 12.19(b). (c) A three-stage fluidized bed granulator for more complete control of process conditions and more nearly uniform size distribution. (d) Two modes of injection of spray to spouted beds, into the body on the left and at the top on the right; performance data in Table 12.19(c).

370 DISINTEGRATION. AGGLOMERATION, AND SIZE SEPARATION OF PARTICULATE SOLIDS

REFERENCES

Size Reduction and Separation

1. W.L. Badger and J.T. Banchero, Innoduction to Chemical Engineering, McGraw-Hill, New York, 1955.

2. W.M. Goldberger, Solid-solid systems, in Chemical Engineers’ Handbook, McGraw-Hill, New York, 1984.

3. E.G. Kelly and D.J. Spottiswood, Introduction to Mineral Processing, Wiley, New York, 1982.

4. W.J. Mead (Ed.), Balling devices, Briquet machines, Grinding, Mills colloid, Mills roller, Screening, in Encyclopedia of Chemical Process Equipment, Reinhold, New York, 1964.

5. A.L. Mular and R.B. Bhappu (Eds.), Mineral Processing Plant Design, AIMME, New York, 1980.

6. E.J. Pryor, Mineral Processing, Elsevier, New York, 1965. 7. E.R. Riegel, Chemical Process Machinery, Reinhold, New York, 1953. 8. G.C. Sresty, Crushing and grinding equipment, in Chemical Engineers’

9. A.F. Taggart (Ed.), Handbook of Mineral Dressing, Wiley, New York, Handbook, McGraw-Hill, New York, 1984, pp. 8.9-8.59.

1945. 10. A.F. Taggart, Elements of Ore Dressing, Wiley, New York, 1951. 11. B.A. Wills, Mineral Processing Technology, Pergamon, New York, 1985.

Size Enlargement

U. C.E. Capes, Particle Size Enlargement, Elsevier, New York, 1980. l3. C.E. Capes, Size englargement, in Chemical Engineers Handbook,

McGraw-Hill, New York, 1984, pp. 8.60-8.72. 14. C.E. Capes and A.E. Fouda, Agitation methods, in Ref. 17, pp.

286-294. 15. A.E. Capes and A.E. Fouda, Prilling and other spray methods, in Ref.

17, pp. 294-307. 16. J.T. Carstensen, Tabletting and pelletization in the pharmaceutical

industry, in Ref. 17, pp. 252-268. 17. M.E. Fayed and L. Otten, (Eds.), Handbook of Powder Science and

Technology, Van Nostrand Reinhold, New York, 1984. 18. R.M. Koerner and J.A. MacDougal (Eds.), Briquetting and

Agglomeration, Institute for Briquetting and Agglomeration, Erie, PA, 1983.

19. R.A. Limons, Sintering iron ore, in Ref. 17, pp. 307-331. 20. W. Pietsch, Roll Pressing, Heyden, London, 1976. 21. P.J. Sherrington and R. Oliver, Granulation, Heyden, London, 1981. 22. N.E. Stanley-Wood (Ed.), Enlargement and Compaction of Particulate

Solids, Butterworths, London, 1983.

mill performance data for grinding specific products 1. 3. · gypsum rock iron oxide limestone...

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