lime manual.pdf

7/29/2019 Lime Manual.pdf

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Lime Handling

Systems


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Executive Summary

Lime Handling Systems is a general article dealing with the storage and feeding of lime slurries.Design considerations involved in selection of equipment are discussed in addition to generalinformation on lime.

Recommendations for system designs are discussed with information presented on equipmentoptions.

The article is designed to acquaint the reader with the considerations involved in achieving anefficient and practical lime system.


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CHAPTER 1: GENERAL INFORMATION ON LIME 4Lime 5Lime Uses 5

Types of Lime 6Quicklime 7Hydrated Lime 8Selecting what type of Lime to use 10Lime Deliver 11

CHAPTER 2: FACTORS AFFECTING LIME SLAKING 13Factors Affecting Lime Slaking 14Lime Slaking 14Quicklime Properties 14Water Properties 15Water to Lime Ratio 16

CHAPTER 3: LIME SYSTEM COMPONENTS 18

Bulk Storage 19Typical Silo Configurations 21Dust Control 22Silo Flow Promotion 23Flooding Control 25Chemical Feeders 26

CHAPTER 4: LIME SLAKERS 30Lime Slakers 31Paste Type Slakers 31Detention Slakers Conventional 32Detention Slakers Grinding Mill Type 33Factors Affecting Slaker Selection 34

CHAPTER 5: SYSTEM ACCESSORIES 37Slaker Accessories 38Grit Removal 38Slurry Tanks 38Slurry Pumps and Piping 39

CHAPTER 6: CONTROL PANELS & INSTRUMENTATION 42Control Panels 43Instrumentation 43Silo Level and Inventory Controls 43Slurry Measurement 44

CHAPTER 7: LIME ADDITION CONTROL 45Lime Addition Control 46

Good Design Practices 49Other Considerations 49

CHAPTER 8: SMALL FEED SYSTEMS 50Small Feed Systems 51Pre-Made Lime Slurries 53

CHAPTER 9: OTHER CHEMICALS 54Other Chemicals 55Soda Ash 55Powdered Activated Carbon (PAC) 57


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Chapter 1: General Information on Lime

CHAPTER 1: GENERAL INFORMATION ON LIME

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Lime

The term lime is used to refer to a variety of calcium based substances, which include or may be

derived from limestone. Except for sulfuric acid, limestone and its derivatives are the most widelyused chemicals in the United States. Approximately 25 million tons of lime is used each year for avariety of different applications such as steel making, acid neutralization, water treatment,wastewater treatment, and air pollution control.

Lime Uses

FIGURE 1

Currently lime usage is growing rapidly with the major investments being made upon acid raincontrol and environmental protection, both of which use lime in large quantities. In addition, newleaching processes for certain types of mineral ores and new sludge composting requirements willrequire large new quantities of lime.

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This introduction to lime handling equipment is intended to acquaint users with the designconsiderations involved in setting up a lime handling facility for maximum efficiency and minimummaintenance considerations.

Types of Lime

The two most common derivatives of limestone are quicklime and hydrated lime. For substances,properties such as particle size, purity, and chemical reactivity vary considerably between grades.Following is a more complete definition of each lime type:

Limestone: Limestone is a naturally occurring substance, which consists primarily of calciumcarbonate (CaCO3). Limestone is typically quarried for use in its native form or for furtherprocessing to produce derivative substances such as quicklime. One form of limestone is marble.Figure 2 shows the types and properties of limestones.

FIGURE 2. Limestone Names, Grades, and Properties

Common Name andFormula

Grades or FormsAvailable 1

Appearance andProperties

Bulk Density

High CalciumLimestone CaCO3

Large LumpPebble or Crushed

Ground, Screened orGranular

Pulverized

White rock with intermediatehardness [Mohs hardness of3-5], product is quarried,typical purity is 95% CaCO3

90-115 lbs/cf

Dolomitic LimestoneCaCO3 & MgCO3 Large LumpPebble or CrushedGround, Screened or

GranularPulverized

White rock with intermediatehardness [Mohs hardness of3-5], product is quarried.

90-115 lbs/cf

1 Limestone is available in a large range of sizes often specified as percentages passing standard sieve sizes. Sizes listed inthe table are defined by ASTM C 51 as Large Lump [8 and smaller], Pebble or Crushed [2.5 and smaller], Ground, Screenedor Granular [1/4 and smaller], and Pulverized [100 % Passing a No 20 sieve].

The purity of limestone varies widely. The purity is measured by the percentage of available CaCO3and MgCO3. Limestone is primarily used for flue gas desulfurization, acid neutralization, and toproduce quicklime. Limestone delivery is available in barges, dump trucks, rail car and bulk trucks.

Once mined and processed for size, purity, and chemical constituents, the raw material is subjectedto a variety of mechanical processes to produce various grades of lime.

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Aerial photo compliments of Graymont Lime

ExshawExshaw

Lime Kiln

FIGURE 3: TYPICAL PRODUCTION PLANTQuicklime:

Quicklime consists primarily of calcium oxide (CaO) and is produced from limestone in aprocess known as calciningas defined below:

High Calcium Quicklime

CaCO3 (Calcium Carbonate) + Heat CaO (Calcium Oxide) + CO2 (gas)

Dolomitic Quicklime

CaCO3 MgCO3 (Calcium Carbonate and Magnesium Carbonate) + Heat

CaO MgO (Calcium Oxide and Magnesium Oxide) + CO2 (gas)

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Figure 4 below defines the different types of quicklimes and the basic properties of each type:

FIGURE 4. Quicklime Types, Grades, and Properties

Common Nameand Formula

Grades or FormsAvailable

Appearance andProperties

Bulk Density

High CalciumQuicklime CaO

Pebbled [3/4 1/4 ]Granular [1/4 minus]

Pulverized [100 %Passing # 20 sieve]

Soft [Moh hardness lessthan 3], white pebbles topowder, typical purity isabove 90% available CaO

48-70 lbs/cf

Dolomitic QuicklimeCaO & MgO

Pebbled [3/4 1/4 ]Granular [1/4 minus]

Pulverized [100 %Passing # 20 sieve]

Soft [Moh hardness lessthan 3], yellowish whitepebbles to powder, contains5 to 40% MgO

49-72 lbs/cf

The quality of quicklimes varies greatly. The quality of the lime is determined by the purity andreactivity of the quicklime. Purity is a measurement of the quantity of calcium oxide (CaO) in agiven sample that is chemically available. The purity of commercially available quicklime in theUnited States typically ranges from 85% to 95% available CaO.

Reactivity is a measurement of the heat released in the reaction between quicklime and water. Thisreaction is commonly known as lime slakingorhydration, the product of which is hydrated lime. Asnoted in Figure 5 below, reactivity is determined by slaking quicklime (at specific ratios of water toquicklime) and measuring the rate at which the product temperature rises.

It should be noted that while purity and reactivity are related, they are not the same; that is, a highpurity does not always indicate a highly reactive quicklime.

Quicklime is shipped via in barges, railcar, bulk truck, sea containers, bulk bags and paper bags.The most common delivery is in 25 Te bulk trucks.

Quick lime is either sold as quick lime containing approx 85-95% CaO or is further treated byhydration to produce hydrated lime Ca(OH)2 which contains approx. 72-74% CaO

Hydrated Lime:

Hydrated lime consists primarily of calcium hydroxide, Ca(OH2) and is the product of thereaction between quicklime and water as defined below:

High Calcium Hydrated Lime

CaO (Calcium Oxide) + H2O Ca(OH)2 (Calcium Hydroxide) + Heat

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Dolomitic Hydrate (type N)

CaO MgO (Calcium Oxide, Magnesium Oxide) + H2O Ca(OH)2 MgO (type N hydrate) +

Heat

Dolomitic Hydrate (type S)

CaOMgO (Calcium Oxide, Magnesium Oxide) + 2H2O Ca(OH)2 Mg(OH)2 (type S hydrate)+Heat

Each pound of calcium oxide will react with 0.32 pounds of water to form 1.32 pounds of calciumhydroxide. Commercial hydration of quicklime occurs in hydrators, which control the water tolime ratio precisely to this ideal ratio to produce a dry powdered product. Where quicklime isreacted with water in excess of the ideal ratio, the process is known as lime slakingwhich

produces a slurry of water and solid hydrated lime particles.

Figure 5 below defines the different types of hydrated limes and the basic properties of eachtype:

FIGURE 5. Hydrated Lime Names, Grades, and Properties

Common Name andFormula

Grades orForms Available

Appearance and Properties Bulk Density

High Calcium HydratedLime Ca(OH)2

Powder [80%passing 200 mesh

size}

Soft [Moh hardness less than 3]white powder, dusty, typicalpurity is 97% Ca(OH)2

25-40 lbs/cf

Normal Dolomitic

Hydrated Lime (TypeN) Ca(OH)2 & MgO

Powder [80%

passing 200 meshsize}

Soft [Moh hardness less than 3]

white powder, comes fromdolomitic quicklime, typicalcomposition is 46 to 48% CaO,33 to 34% MgO and 15 to 17%Ca(OH)2

30-40 lbs/cf

Pressure Dolomitic(Type S) Hydrated LimeCa(OH)2 Mg(OH)2

Powder [80%passing 200 mesh

size}

Soft [Moh hardness less than 3]white powder, comes fromdolomitic quicklime, typicalcomposition is 7% CaO andMgO and 90% Ca(OH)2 andMg(OH)2

30-40 lbs/cf

The properties of hydrated lime from different sources in the United States are fairly consistent with

those properties described previously. The purity of the hydrated lime is expressed as a percentageof the material that is calcium hydroxide (Ca(OH)2). The typical range of purity in the United Statesis above 95% calcium hydroxide (Ca(OH)2).

Hydrated lime is available in bulk trucks, bulk bags, and paper sacks.

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Selecting what type of Lime to use

The type of lime is to be utilized in the process is dependent on a variety of factors, chiefly theanticipated consumption of lime (as CaO) to be used. As consumption increases the overall savingsin chemical costs will justify cost increases in capital equipment. Keep in mind that if quick lime isused it has to be converted to hydrated lime prior to its use in the process in almost all cases.

Pebbled Crushed Pulverized

HYDRATED LIMEHYDRATED LIME

QUICKLIMEQUICKLIME

Choice of Lime to Use Lime Particle Size

Quicklime available in variety of particleQuicklime available in variety of particle

sizes from pulverized to pebble limesizes from pulverized to pebble lime

(usually 3/4minus)(usually 3/4minus)

Hydrated Lime - usually pulverizedHydrated Lime - usually pulverized

2 x 3/4 3/4 x 3/8 3/8 minus

FIGURE 6 FIGURE 7

Lime in either form is available in either bags or in bulk. As a rule of thumb the following differentialsapply to the various types of lime.

Cost difference between bagged hydrated lime and bulk hydrated lime Approx. $ 90.00 per ton(Based on unit costs, trucking, unloading, housekeeping, space requirements and labour costs for batch makeup)

Cost difference between bulk hydrated and bulk quicklime Approx. $ 40.00 per ton(Based on unit costs and the differential between CaO content in hydrated and quick lime plus trucking and storagecosts).

In general the use rate will dictate the form of lime used as illustrated in the following (Figure 8)

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Recommended Selections

0

1

4

ApproximateSavings

TonsperDay

> $90/Ton

> $40/Ton

Lime Types

FIGURE 8

Lime Deliveries

Lime is shipped in a variety of containers as illustrated below. Bulk deliveries are either inrailcars (100 tons) or in self-unloading bulk trailers (20-25 tons). Rail cars are either the hoppertype, which unload by dumping by gravity to a storage pit (See Fig.10) or unloading device or a

PD car, which is unloaded by pneumatic transfer. Truck trailers are usually equipped with apneumatic unload system which will unload 15 - 20 TPH through a 4" line using up to 600-1100cfm of convey air. (See Fig.11)

Sh ipm en t Ch o ice s

B u l k T r u c k

B a g

R a i l C a r

T o t e

FIGURE 9

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Rail Car Unload

FIGURE 10

Truck Delivery

FIGURE 11


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Chapter 2: Factors Affecting Lime Slaking

CHAPTER 2: FACTORS AFFECTING LIME SLAKING

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Factors Affecting Lime Slaking

The lime slaking process and product quality depend on three primary factors: quicklime properties,

water properties, and water to lime ratio. In most applications, water to lime ratio is the single mostimportant factor in lime slaking; as this ratio is decreased, product quality increases.

Lime Slaking

The term lime slaking refers to the reaction between calcium oxide (as a component of quicklime)and water to form calcium hydroxide. This process is represented by the following equation:

CaO + H2O Ca(OH)2 + Heat(Calcium Oxide) (Water) (Calcium Hydroxide)

In this reaction, each 1.0 lbs of calcium oxide combines with 0.32 lbs of water to yield 1.32 lbs ofcalcium hydroxide. The reaction releases heat (490 btu/lb of CaO) and is the same as that which

takes place in a lime hydrator to produce commercial hydrated lime. Commercial hydrationprocesses typically yield a dry product, whereas lime slaking occurs with an excess of water toproduce a slurry of hydrated lime solids in water.

Quicklime Properties

Three properties of quicklime: type, purity, reactivity, and particle size should be considered in limeslaking.

Type:

The grade of quicklime, high calcium or dolomitic effects the slaking rate. A dolomitic lime will slakeat a slower rate than high calcium quicklime. In the United States, the most common form ofquicklime is high calcium quicklime.

Purity:

Purity describes of the amount of quicklime that is chemically available as Calcium Oxide (CaO).The purity of quicklime affects the reactivity and determines the purity of the hydrated lime product.The purity of commercially available high calcium quicklime in the United States typically rangesfrom 85% to 95% available CaO.

Reactivity:

Reactivity describes the relative capacity of quicklime to reciprocate chemical change with water.Reactivity is a function of purity, particle size and other factors such as particle porosity. The slakingrate is a measurement of the time for the slaking process to reach completion. Thereaction is considered complete when the temperature of a given sample reaches a maximum. Asreactivity increases, the slaking rate, ultimate temperature rise, and surface area of hydrated limeincrease also.

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Particle size:

Quicklime particle size and mixing methods have a significant affect on slaking rate. Commercialgrade quicklime is available in a variety of sizes including pulverized (powder), granular (1/4minus), pebble (1/4-3/4 minus) and lump (2 minus). When adequately mixed with water,quicklime reactivity increases as particle size decreases. Ultimate temperature rise and productquality are not significantly affected by particle size with pebble or finer grades. The smaller thesize of quicklime the more susceptible the lime is to air slaking.

Water Properties

Two properties of water: dissolved solids and temperature should be considered in lime slaking.

Dissolved Solids: Water containing sulfites or sulfates in excess of 400 ppm should not be used forlime slaking as they inhibit the slaking process. Water containing bicarbonate hardness will notaffect the slaking process, however it should be noted that bicarbonate ions will precipitate (scale)as calcium carbonate on contact with slaked lime. Softened process water or potable water isrecommended for lime slaking.

FIGURE 12 FIGURE 13Temperature:


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In general, higher incoming water produces better product quality. Water that is too cold maydrown the quicklime and slow the slaking reaction down. Water that is too hot may boil during theslaking reaction. For reliable slaking with most lime types and slakers, water temperature should bebetween 55 deg F and 70 deg F.

Water to Lime Ratio

In most applications for slaked lime, surface area of the hydrated lime particles is of paramountimportance. Water to lime ratio is the principal determinant of product surface area; as this ratio isdecreased, surface area increases.

The water to lime ratio is defined as the ratio of water mass to quicklime mass. This ratio does notinclude dilution or process water, which may be added to slaked lime downstream of the slakingcompartment. In conventional lime slakers, water to lime ratio varies from 2.5:1 (for paste typeslakers) to 6.0:1 (for detention type slakers). The water should be evenly distributed into the slaker.Surges or pulsating of the water flow may cause improper slaking.

As shown in Figure 14, specific surface area of slaked lime particles is highly dependent on thewater to lime ratio, with surface area increasing as water to lime ratio decreases. As the graphshows, this increase is greatest when water to lime ratio varies in the region between 2.5:1 and6.0:1.

Figure 8. - Specific Surface of Calcium Hydroxide with Different Water

to Lime Ratios [68 Deg. F Water]

35000

40000

45000

50000

55000

2 4 6 8 10 12 14

Water to Quicklime Ratio

Blaine

SpecificSurface(sqcm/

g)

Paste

Detention

Source: A Study of the Reaction Between Calcium Oxide and Water, T.C. Miller, Published by the National Lime

FIGURE 14

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FIGURE 15 illustrates the temperature rise of varying Water to Lime Ratios

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Chapter 3: Lime System Components

CHAPTER 3: LIME SYSTEM DESIGN

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Bulk Storage

Storage silos are generally vertical cylindrical units with a straight side to diameter ratio of between

3:1 and 4:1 for best flow characteristics. A conical bottom is generally supplied with a minimumcone angle of 60 degrees.

Sizing the silo is a function of the usage rate. A 7-10 day minimum storage is typically used up to amaximum of 6 months. The minimum storage time may be altered depending on lime availability,transportation constraints or special problems. Minimum recommended silo size is 1-1/2 truckloadsto allow delivery of a full truckload while still maintaining function of the lime system during the re-order process.

Silo design should account for local seismic, wind and snow loadings as well as potential mass flowof the product. For quicklime 65 pcf is commonly used for structural calculations and 55 pcf for

volumetric calculations. For hydrated lime use 35 pcf structurally; 25 pcf for volumetric purposes. Anallowance should be made for some freeboard in the silo; typically 3 to account for the angle ofrepose of the product. This freeboard also increases the efficiency of the bin vent filter operation.

Material of construction for the silos include welded or bolted steel, concrete, or stainless steel.In all cases the material of construction for the silo must be compatible with the material stored.Steel construction is the most popular for these systems. Shop fabricated welded steel silos areeconomic up to 14 diameter. Beyond this size shipping constraints preclude their use and fieldfabricated silos are used. Bolted steel factory coated steel silos offer an attractive option atdiameters >12. They are shipped knocked down and can be quickly erected on site with locallabour.

Steel silos are generally supplied with epoxy or inorganic zinc coated exteriors. Interior productareas can be left uncoated but are more commonly coated with an epoxy primer. For difficultproducts the cone area can be coated with special superslick Teflon based coatings, whichgreatly improve flow in this area.

The silos can be supported in a variety of methods. Typical supports include skirt supported, legsupported or using structural steel. Some of these are illustrated below.

Si lo D esignSilo support

Skirt , s t ructure or leg

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A variety of discharge arrangements can be used depending on the application. For systemsrequiring duplicate trains for the lime make down equipment, silos can be equipped with abifurcated cone outlet which provides dual outlets or a splitter valve can be installed on the conedischarge to direct product to two or more trains. Some examples are shown below.

Silo Cone Design

In the case of a duplicate train system, if it is anticipated that both sides will be in use most ofthe time, then either a splitter valve arrangement or a bifurcated cone arrangement can be used.

If only one side is in use, as is the case of a backup system, then it is recommended to use thesplitter arrangement. In this case the idle side will tend to compact in the silo discharge andcreate flow discharge problems.

Silo accessories should include the following:

Access ladder (caged) with rest platforms (if required) and perimeter guard rail Vacuum pressure relief hatch 4" fill line with long radius elbows typically of Sch. 40 or 80 carbon steel c/w compression

couplings and grounding strips. Pipe restraints are recommended for operator safety.

High and low level probes (capacitance type) Knife gate shut off valve for maintenance Other accessories as desired for inventory control. With regard to silo coatings we recommend that the silo interior be epoxy primed after

surface preparation to SSPC-SP6 with special anti friction coatings applied to assist inflow promotion. The exterior and skirt interior area are usually epoxy primed with anacrylic finish coat.

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Typical Silo Configurations

PARTIALLY INSULATED DRIVE THROUGH INTERMEDIATE WALKWAY

MULTIPLE SYSTEMSSTRUCTURE SUPPORTED STAND ALONE

INSIDE BUILDING

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Dust Control

It is necessary to provide dust control on the silos to enable the conveying air to exit the siloduring offloading. Bin vent filters of the reverse air purge type are recommended with 150-250sq. ft of filter area, which will handle the average truck delivery system. For quick lime withlarger particle sizes a filter of this type with 150 sq. ft. is satisfactory. For hydrated lime orpulverized quick lime the filter area should be approx. 250 sq. ft. which results in an acceptablesurface to air ratio of < 4:1.Shaker type filters can also be utilized if compressed air is not available. With this type of filteradditional filter area is required as the dust builds up on the filter bags during the off-load cyclethereby increasing the pressure drop across the bags. This type of filter is generally mechanicallycleaned at the end of the of unload cycle. A minimum of 400 sq. ft. of filter area is recommended inaddition to an exhaust fan to assist air flow.In rail car unload systems additional filter capacity will be required sized to match the conveying airflow.

FIGURE 16Shown: WAM Reverse Pulse Bin Vent Filter

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FIGURE 16A

Dust Filters

eed to vent and clean theeed to vent and clean thepneumatic convey airpneumatic convey air

Usually located on roof of siloUsually located on roof of silo

Design for 3-4Design for 3-4 cfmcfm//sqsq. ft. filter area. ft. filter area

i.e. Truck 200 - 250i.e. Truck 200 - 250 sqsq.ft. filter area.ft. filter area

Rail 300 - 500Rail 300 - 500 sqsq.ft. filter area.ft. filter area

Mechanical shaker or reverse airMechanical shaker or reverse air

pulse typepulse type

Silo Flow Promotion

Once the lime is in the silo it is necessary to ensure a continuous smooth flow from the silo to thedownstream equipment. Bulk products stored in silos can be subject to bridging, rat-holing andflooding as illustrated below. If not addressed, these problems can cause catastrophic failure of thesilo structure.

Every bulk product stored in cylindrical silos will have its own unique flow promotion characteristic.

N ecessary to avoid brid gin g or rat-holingN ecessary to avoid brid gin g or rat-holing

Efficient use of silo volum eEfficient use of silo volume

Silo Flow Prom otion

FIGURE 17

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Bin activatorBin activator Imparts vibration energyImparts vibration energy

to productto product

Excellent for coarseExcellent for coarseproductsproducts

Interlock with feederInterlock with feeder

Do not use for continuousDo not use for continuousfeed of small amountsfeed of small amounts

Silo Flow Promotion

FluidizationFluidization Fluidizes product with airFluidizes product with air

pads, air cone, impactors topads, air cone, impactors topromote flowpromote flow

Excellent for powderedExcellent for powderedproductsproducts

Used only on refilling feederUsed only on refilling feederhopperhopper

Necessary to control floodingNecessary to control flooding

Silo Flow Promotion

FIGURE 19FIGURE 18

For pebble lime and other products shipped as a larger particle size, a vibrating bin activator sizedat a minimum of 1/3 the diameter of the silo is recommended.(See Figure 19) These devices impartvibration energy to the product and promote smooth flow. Caution is required when using thesedevices in situations where a small quantity of product is removed continuously as in a direct feedapplication. In these cases it is recommended to use a transition hopper above the feeder and refill

this hopper periodically.

For hydrated lime, pulverized quick lime and other powdered products, a fluidization system worksvery well. This system consists of an aeration cone on the hopper outlet plus air pads assisted bylow frequency, air operated impactors on the cone. (See Figure 18)

Both of the above flow promoters can be used on either type of lime but some restrictions applydepending on particle size, air dryness, etc.

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These devices should be interlocked with the feeding equipment to allow operation only whenfeeding.

Flooding Control

Flooding of the material can be a problem with hydrated lime due to material size or excessaeration. An anti-flood system should be used which generally consists of a rotary valve above atransition feed hopper. The rotary valve is used to maintain the level in the feed hopper betweentwo set points. This arrangement prevents flooding of the product through the feeder andprovides a constant head of product on the feeder for repeatable flow characteristics.(Figure.20)

An electromagnetic vibrator is generally installed on this hopper to provide flow promotion to thefeeder. (See Figure 21)

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Typical Feed Hopper Setup

FIGURE 20

Feed Hopper Flow Problems

Used on Hoppers

Electric Vibrator

FIGURE 22FIGURE 21

Chemical Feeders

A variety of feeding devices is available for controlled metering of lime to the slurry tank. For limeservice a volumetric screw feeder provides an economical, accurate device with minimummaintenance requirements. The accuracy of this device is approx. +/- 2%, which is normallysatisfactory on lime service.

Alternately a loss-in-weight feeder can be used which will give accuracy to +/- 0.2- 0.5% and also

provide instantaneous and totalized product usage for inventory control. Other types of feedersinclude gravimetric belt feeders and rotary airlock feeders.

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Feeders with accuracy to +/- 0.2% (FIGURE 23)

B E L TS P E E DP I C K U P

I N F E E D

S H E A R G A T E

M O T O R

M O T O RS P E E D

C O N T R O L L E R

D I S C H A R G E

B E L T

S I N G L E L O A D C E L L

M I C R O - C O M P U T E RC O N T R O L L E R

C O U N T E R B A L A N C E DW E I G H P L A T F O R M

G ra v im e tr ic F e e d e r

FIGURE 26 Belt FeederShown: Merrick Belt Feeder

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Volumetric Screw Feeder

FIGURE 25 Volumetric Screw Feeder

Shown: Metalfab DB-1 Screw Feeder

L o s s -In -

W e i g h tF e e d e r

Feeders with accuracy of +/- 2% (FIGURE 24)

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LossLoss--InIn--WeightWeight

FeederFeeder

FIGURE 27

Rotary Valve

FIGURE

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Chapter 4: Lime Slakers

CHAPTER 4: LIME SLAKERS

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Lime Slakers

The objective in slaking quick lime is to thoroughly hydrate the CaO to form Ca(OH)2 and produce avery small particle size with a large surface area. The slaking reaction produces a great deal ofheat, which is useful in sustaining and initiating further reactions.There are two types of lime slaker commonly used today: paste type slakers and detention typeslakers. The principal difference between paste slakers and detention slakers is the ratio at whichwater and lime are mixed during the reaction phase of the slaking process; paste type slakers reactquicklime with water at a ratio of approximately 2.5:1 whereas detention slakers react quicklimewith water at ratios of 3.5:1 or greater. Paste and detention type slakers come in a variety ofdifferent configurations and sizes, each suited to specific applications.

Paste Type Slakers

Paste slakers automatically control the water to quicklime ratio and rely on the heat released fromthe reaction as energy to complete the slaking. By controlling this ratio the slakers are able toeffectively handle frequent changes in the quicklime feed rate for both batch and continuousprocess applications. Optimum paste slaker performance is designed for short reaction times andmaximum utilization of the lime. For these reasons, the paste slaker is best suited for applicationsusing high quality quicklime. Paste slakers also consume substantially less power than comparablyrated detention type slakers.

Paste Slaker(.5, 1, 2, 4 TPH)

1000, 2000, 4000, 8000, Lbs./Hr.1000, 2000, 4000, 8000, Lbs./Hr.

Operate by mixing lime & water inOperate by mixing lime & water in1:2 ratio1:2 ratio

Heat of reaction breaks particlesHeat of reaction breaks particlesapartapart

Very efficient slakingVery efficient slaking

Low hp, low speed, minimalLow hp, low speed, minimalmaintenancemaintenance

Able to operate without additionalAble to operate without additionalwater heatingwater heating

FIGURE 28Shown: Merrick Paste Slake

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The paste slaker consists of two horizontal paddle shafts, which counter-rotate to mix water withquicklime. The mixture is driven toward the discharge (weir) end by means of pitched paddlesattached to the shafts. Water to lime ratio in the slaking compartment is automatically maintained atan user adjustable ratio between 2.5:1 and 2.8:1. Beyond the weir end, slaked lime is furtherdiluted by means of manually controlled spray jets integral to the slaker assembly. Grit (inertmaterial in quicklime) is removed either by screening or inclined classifier.

Paste type slakers are available in quicklime slaking capacities of 500, 1000, 2000, 4000 and 8000lbs/hr. They operate at water to lime ratios (in the slaking compartment) between 2.5:1 and 2.8:1which produces specific surface area (hydrated lime) of approximately 53,000 cm2 per gram.Typical operating temperature rise of a paste type slaker is approximately 140 deg F above thetemperature of the incoming water and steady state is usually reached within 5 minutes of startup.Standard slurry discharge is 18-20% solids at the slakers rated maximum capacity.

Detention Slakers - Conventional

Conventional detention type slakers are best suited to applications in which the lime quality ismedium to high and the lime feed rate changes neither frequently nor automatically. They are theleast expensive of the slaker types and are well suited for batch type operations where lime qualityis reliable.

As shown in Figure 29, a typical detention slaker consists of two slaking compartments, each

agitated by a conventional slow speed mixing impeller. Water and lime are fed into the firstcompartment and then overflow a weir into the second compartment. Additional agitation takesplace in the second compartment after which the product discharges into the grit removal system.The water to lime ratio in the slaking compartment is adjusted and monitored manually with thehand valves and flowmeters integral to the slaker assembly. Grit is removed either by screening orinclined classifier.

Conventional detention type slakers are available in a variety of quicklime slaking capacities up to16,000 lbs/hr. They operate at water to lime ratios (in the slaking compartment) between 3.5:1 to6.0:1which produces specific surface area (hydrated lime) of approximately 47,000 cm2 per gram.Typical operating temperature rise of a detention type slaker is approximately 100 deg F above the

temperature of the incoming water and steady state operation is usually reached within 15 minutesof startup. Standard slurry discharge is 18-25% solids at the slakers rated maximum capacity.With slakers of this type it is necessary to incorporate heating of the slaking water for efficientoperation in cold weather locations.

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Detention S laker 500, 1250, 2500, 5000,500, 1250, 2500, 5000,

10,00010,000 LbsLbs .. // H r.H r.

Op erated by mixing ratioOp erated by mixing ratio

of 1:3.5 - 4.0of 1:3.5 - 4.0

M echanical agitation atM echanical agitation at

high speed drives Slakinghigh speed d rives Slaking

reaction.reaction.

Higher H P, higher speedHigher HP , higher speed Re quires additional heating of makeu p wa ter inReq uires additional heating of makeu p water in

w inter conditionswinter conditions

FIGURE 29Shown: BIF Slurry Slaker

Detention Slakers - Grinding Mill Type

Grinding mill type detention slakers are best suited to applications where the lime quality is poor, grithandling is costly or when the processing rate exceeds that achievable in a conventional slaker.Grinding mill slakers operate on the same principle as conventional detention slakers, however theyhave the ability to grind inert materials (grit) contained in the quicklime and generally require little orno grit removal or disposal facilities. Grinding mill slakers are horizontal or vertical ball mills (ascommonly used in the mineral processing industry) modified to withstand the temperature of limeslaking. They have the advantage of being relatively simple and rugged. However, grinding millslakers are the most expensive of the slaker types, have limited capacity for turndown, andconsume the most energy.

As shown in Figure 30, a typical grinding mill slaker (horizontal type) consists of a rotating drumsupported by external rollers. The mill contains a grinding media (typically steel balls) agitated bymeans of horizontal lifting bars attached to the interior of the drum assembly. Slaked lime productdischarges from the drum and is pumped to a hydrocyclone, which classifies the slurry to recirculateoversize particles to the mill inlet. As with a conventional detention slaker, water to lime ratio in theslaking compartment is adjusted and monitored manually with hand valves and flowmeters.

Grinding mill type detention type slakers are made to order in quicklime slaking capacities up to60,000 lbs/hr. Operating parameters and characteristics are similar to conventional detentionslakers.

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An alternate version of the ball mill is a Vertimill which is basically a ball mill operated in a verticalmanner. This type is useful where very fine grinding is desired. (Figure 31)

Ball Mill 1 TPH and up1 TPH and up

Operates as aOperates as a

slurryslurryslakerslaker

Ball impacts andBall impacts and

rotation drivesrotation drives

slaking reactionslaking reaction

Very High HPVery High HP Requires water heating for winter operationRequires water heating for winter operation Especially suitable for poor quality limes &Especially suitable for poor quality limes &

High use ratio > 2- 4 TPHHigh use ratio > 2- 4 TPH

FIGURE 30

Shown: Svedala SRR Ball Mill SlakerFIGURE 31

Factors Affecting Slaker Selection

Many factors other than price and operating costs will influence the selection of a slaker. Someof the factors are outlined in the following:

Product Quality

Surface area of the hydrated lime product is of paramount importance in many applications. Testsshow a correlation between the diameter of slaked lime particles and the water to lime ratio usedduring slaking. As the water to lime ratio is reduced, the diameter is reduced also, resulting in a

substantial increase in surface area of the slaked lime product. Due to the difference in water tolime ratios, paste type slakers deliver more neutralizing capacity (as measured by free basetitration) than comparable detention slakers. For certain applications, this difference can result insubstantial improvements to the process as well as chemical cost savings.

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Grit Disposal Costs

The cost of grit disposal can be a significant factor in the selection of lime slakers. The inert portionof quicklime which is captured by a screen or inclined classifier is relative to the purity and particlesize quicklime. This grit must be transferred from the grit removal device to a bin, which is emptiedperiodically. While the grit itself is not a hazardous material (as defined by EPA), however, the pHexceeds 12.0 and caution must be exercised in handling and disposal. The grinding mill slakereliminates the grit disposal problem by grinding the grit to very fine particles, which are carried alongwith the lime slurry. This type of slaker should be considered where grit disposal costs are high.

Slaking Water Quality

For reliable operation of any lime slaker type, water used for lime slaking should be at atemperature not less than 55 F / 12 C. For slakers, which are expected to start and stopfrequently, the time to reach operating temperature (with associated hydrated lime quality) may be afactor. Paste type slakers reach their steady state operating temperature in approximately 1/3rd thetime required by comparable detention type slakers. In general, paste slaker performance is lesssusceptible to cold slaking water (less than 55 F / 12 C) than detention and grinding mill slakers.Water heaters may be added to raise the temperature of the slaking water for all types of slakers.

Space Constraints

Like all process equipment, lime slakers require regular cleaning and maintenance. They are oftenplaced within the skirt support structure of the quicklime storage vessel where space is at apremium. The size and shape of the slaker may be a determining factor in slaker selection; ingeneral, paste slakers are the most compact (in plan view). Some conventional slakers areconfigured with vertical mixing chambers and require less horizontal space, however headroomrequirements are greater. For grinding mill slakers, the vertical type require the least space in planview, however they can be up to 30 ft in height depending on the processing capacity.

Capital Cost

Budget constraints frequently determine slaker selection. Conventional detention type slakers arethe least expensive of the three type considered, with paste slakers priced approximately 15%higher for comparable capacities. The higher price of the paste slakers can often be recoveredquickly by the more efficient slaking and lower chemical usage. Grinding mill slakers are typically 3-5 times the price of comparable conventional slakers, however this premium can also be offset bythe reduction in grit disposal cost.

Operator Experience

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The users level of experience with lime slaking and prior experience with specific types of slakersshould be a factor in slaker selection. For safe and effective lime slaking, operators should haveknowledge of the theory and practice of lime slaking. In general, detention type slakers are moreforgiving of inaccuracies in lime and water feedrate; however manual slaking water control requirescalculation of slaking/dilution water flowrates each time the lime feed rate is changed. In caseswhere quicklime feed rate is changing frequently, the automatic water proportioning system in pasteslakers should be considered. In addition to knowledge of lime slaking, grinding mill slakers requirea knowledge of the grinding process and related equipment. For this reason; grinding mill systemsrequire more operator knowledge than conventional slakers and appropriate consideration shouldbe made in the selection process.

A summary of selection criteria and comparative features is presented in Figure 32 below.

FIGURE 32 Lime Slaker Selection Criteria and ComparisonCondition Paste Type Detention Type

ConventionalDetention TypeGrinding Mill

Maximum QuicklimeSlaking Rate (lb/hr)

8000 16,000 60,000

Minimum QuicklimeQuality

85% CaO with highreactivity OR 80% CaOwith medium reactivity

85% CaO withmedium reactivity

75% CaO with lowreactivity

Process Applications Batch or Continuous Batch Only Batch or ContinuousWater to Lime Ratio(Typical)

2.5:1 4.5:1 4.5:1

Slaking WaterControl Method Automatic Manual Manual

Turndown Ratio(Typical)

20:1 5:1 2:1

Horsepower Required0.5 Hp per Ton/hr of

Quicklime1.7 Hp per Ton/hr

of Quicklime10 Hp per Ton/hr of

QuicklimePercent Free Base(Typical)

81% 75% 75%

Ca(OH2) ProductSurface Area(Typical)

53,000 Cm2/Gram 47,000 Cm2/Gram 47,000 Cm2/Gram

Capital Expense

Typically 15% higher

than conventionaldetention type slakers Least

2-5 times higher than

conventionaldetention type slakers

Grit DisposalScreen or Inclined

ScrewScreen or Inclined

ScrewNot normally required

Operator Experience Important Important Critical

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Chapter 5: System Accessories

CHAPTER 5: SYSTEM ACCESSORIES

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Slaker Accessories

For efficient operation of the slaker additional equipment is required to monitor and control thesystem. These include the following:

Grit removal devices

Vent exhaust fansDust control devicesTemperature controls and alarmsWarning devicesIn line water heaters

Grit Removal

In order to extend the life of slurry piping, valves and pumps it is recommended that grit be removedfrom the lime slurry. This is accomplished in either a screen type, screw type of conveyor type gritremover.

Slurry Tanks

Sizing:

In batch makeup systems, slurry tanks should be sized for a minimum of 15 minutes retention timeof the lime slurry to allow the slurry to stabilize and complete the softening precipitation with thedilution water. This will minimize scaling on slurry pipelines. The slurry tanks should be circular withintegral baffles and an adequately sized low speed agitator. For optimum mixing the tank diameterto height ratio should be kept close to 1:1.For continuous feed applications, smaller tanks are used to minimize the lag time when lime feedsettings are changed. These tanks are generally 50-100 usg capacity and are quipped with highspeed agitators. The water level in the tanks is maintained by either float valves or with a controlvalve reacting to tank level indication.Tanks can be fabricated of Carbon steel, FRP or polyethylene construction.

Agitation:

Agitators for lime service can be either belt or gear drive units designed for low speed operation withradial flow impellers. For hydrated lime service the agitators need to be designed for wettingservice and generally are of the two blade design. Suspension applications common in quick limeslaking service are usually of the single prop design.

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Agitation

Low speed agitator ( 350 rpm)Low speed agitator ( 350 rpm)

Gear or Belt DriveGear or Belt Drive

Dual prop for WettingDual prop for Wetting

Single prop for SuspensionSingle prop for Suspension

CS or SS materialCS or SS material

FIGURE 32

Slurry Tank Level Controls:

Non-contacting devices are strongly recommended for lime and/ or slurry service as they avoid thebuild up problem common with any item immersed in the slurry. Ultrasonic level controls, airbubblers or load cells have all given excellent service.

Slurry Pumps and Piping

Pump selection is critical in lime service. A properly designed lime slurry addition system willoperate virtually maintenance free whereas poorly designed systems will create extensive

maintenance and control problems.Pumps should be designed for slurry service. Gland seals should be avoided as the seal watercreates scaling problems in the packing. Recessed impeller pumps with expeller options havegiven excellent service. Other choices include peristaltic hose pumps, progressive cavity designsand air operated diaphragm pumps.

Wilfley Pump

Centrifugal slurry pump

FIGURE 33


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Lime slurry piping should be designed to ensure a minimum flow velocity of 4/sec. with a minimumline size of 1-1/2. Care should be taken to eliminate short radius elbows. Avoid in line dilution of thelime slurry. Pump suction lines should be as short as possible with no bends. Piping materials canbe PVC, ABS or carbon steel.

Valves should be chosen with care to avoid problems. For pump isolation valving straight throughdiaphragm valves are recommended. Lime slurry take off valves should be of the pinch valve type,(Fig 33) which are self-cleaning with a 1 minimum diameter. Pinch valves can be of the straightthrough, reduced port or tapered style operating in a timer controlled on-off manner. Alternately theycan be operated in a modulating manner provided an intermittent fully open pulse is provided by thecontrol scheme.

FIGURE 34

TYPICAL BATCH SYSTEM SHOWING FEEDER SURGE HOPPER, SLURRY TANK, RECYCLE PUMP

FIGURE 35

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Shown: Red Valve Pneumatic Pinch Valve


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Pressure gauges should be of the isolation sleeve type, as shown below, to avoid pluggingproblems.

PressureGauge

Isolation Sleeve TypeIsolation Sleeve Type

Housing

SensingFluid

IntegrallyMoldedFlangeGasket

FlexibleElastomerElement

Where possible lime slurry should be added to the process with an air gap to avoid scalingproblems at the injection point. If this is not possible (i.e. Injection into a pipeline) then a retractableinjection lance assembly should be used to allow periodic cleaning.

AddingLi

metoProc

ess

FIGURE 35A

Note: Addition of lime to process using an air gap to minimize scaling.


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Chapter 6: Control Panels & Instrumentation

CHAPTER 6: CONTROL PANELS & INSTRUMENTATION

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Control Panels

Generally the lime system supplier will supply a complete control panel to control and monitor the

lime slurry preparation process.

These panels can be supplied utilizing either relay logic or programmable controllers.

Panels should be designed to NEMA 4 or 4X standards and a separate truck fill panel should beincorporated for the silo filling process to avoid having the truck driver access the main controlpanel. In cases where the silo bin vent filter is of the mechanical shaker type, the motor starters forthis device should be located in the main panel or in an MCC section for overall economics.

Control Panels

NEMA 4 or 4XNEMA 4 or 4X

designdesign

Usually withUsually with

integral PLCintegral PLC

Can haveCan have

integral MCCintegral MCC

sectionsection

Control Panels

RecommendRecommend

separate truckseparate truck

fill panelfill panel

FIGURE 36 FIGURE 37

Instrumentation

A variety of instrumentation is available to suit individual plant requirements.

Silo Level and Inventory Controls

Digital point sensors

Capacitance, radio frequency and vibrating tuning fork probes have all been usedsuccessfully. Rotating paddle type sensors, although the most economical devices,have resulted in higher maintenance costs than the other types.

Analog level sensorsRadar reflex units, capacitance cable probes and retractable Yo-Yo probes havegiven good service. These devices all react to the shape of the product stored in thesilo and hence are subject to some inaccuracies relating to the varying profile of thechemical as it is withdrawn from the silo.

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Load cells or strain gauges installed on the silo supports avoid this problem and willgive accurate total weights.

Another method of accomplishing inventory control is by means of loss-in-weight feeders, whichwill give instantaneous as well as totalized usage readouts, which can be used to calculateinventory levels.

Slurry Measurement

In general, lime slurry concentrations and flows are not measured. Instrumentation for this purposethat works effectively is limited and is quite expensive. It is usually preferable and easier to measurethe dry product feed rather than the liquid feed. If absolutely necessary for process controlmeasurement of slurry density can be accomplished with nuclear density meters and flow withcoriolis meters.

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Chapter 7: Lime Addition Control

CHAPTER 7: LIME ADDITION CONTROL

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Lime Addition Control

Lime can be added to the process in a dry form direct from the silo or as a slurry. For mostapplications a slurry feed is preferred as it ensures higher reactivity, better dispersion and the mostefficient use of chemical.

Forslaking systems it is advisable to control the lime slaking process as follows:

Set slaking rate at approx. 120% of maximum process requirements Operate the slaker as required to maintain the level of slurry in the slurry tank between the

high and low set points Re-circulate the lime slurry in a recycle loop with the lime being added to the process

through a pinch valve takeoff Adjust lime slurry feed by varying the timer cycle of the feed line pinch valve ON time

Note: It is not advisable to vary the feed rate of quick lime to the slaker as a means of controlling thelime addition rate to the process as the lag time through the slaking process is too long for effectiveresponse time.

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For hydrated lime systems either of the following methods can be used:

A) Variable Volume, Constant Strength (Batch Method)

Make up batches of hydrated lime slurry to pre -set concentrations asrequired and adjust the slurry feed rate as above for the slaking systems.

B) Constant Volume, Variable Strength (Direct Feed)

Feed a constant volume of dilute slurry to the process through a gravity,pump or eductor system.

Maintain a constant level of slurry in the slurry tank by introducing waterthrough a modulating control valve or float valve.

Vary the feed of dry lime to the slurry tank in response to the processdemands through a variable speed feeder.

Note:1. Method A is useful when using an eductor feeding mechanism.2. Method B should only be used when makeup water hardness and alkalinity are low.

FIGURE 38

METHOD A

Batch Makeup Process

Constant solution strengthConstant solution strength

Variable volumeVariable volume

Usually 5Usually 5 -- 20% slurry20% slurry

Feed to Process throughFeed to Process through

recirculating loop system.recirculating loop system.

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FIGURE 39

METHOD B

Continuous Makeup Process

Variable lime feedVariable lime feed

Constant lime slurry feedConstant lime slurry feed

Use only with softened makeupUse only with softened makeup

High water requirements ~ min. 30High water requirements ~ min. 30--

35 usgpm35 usgpm

Very low slurry strength 0.1Very low slurry strength 0.1 -- 1%1%


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Good Design Practices

Good design practices dictate that lime slurry piping and pump facilities be duplicated to ensurecontinuous operation. The level of this redundancy is predicated on the actual plant processrequirements.

Other Considerations

A large variety of accessories can be incorporated into the lime system package to suit customersindividual requirements and other process needs such as remote instrumentation and alarms,inventory control, heating and insulation of the silo skirt area, etc.

As a minimum a safety shower and eyewash should be incorporated at the working level for

operator safety when using quick lime in addition to safety glasses and protective clothing asrecommended when using any chemical. Safety showers should be equipped with a flow switch toalarm in the event of use to protect workers working alone.Lime preparation areas should be equipped with wash down provisions and a sump area withpump.

In summary, if careful attention is given at the design stage to the lime handling and feedingsystem, then an efficient, automated, low maintenance facility can be provided for the end userat an economical cost.

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Chapter 8: Small Feed Systems

CHAPTER 8: SMALL FEED SYSTEMS

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Small Feed Systems

Bulk Bag Systems

For intermediate use applications lime may be purchased in 1 ton bulk bags. We havedeveloped a system to handle these effectively as shown below to avoid the common flowproblems associated with using these bags as storage bags for a feeder. The bulk bags aredumped into the hopper and chemical is withdrawn as required to make up batches. Flowpromotion equipment is incorporated into the design. These skid mounted systems are shippedcompletely assembled and are custom designed for each application.

FIGURE 40Shown: Stanco Prepackaged Skid System

Bulk Bag Systems

For small to medium use applications


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Bag Systems

For low use applications utilizing 50# bags of chemical, a system illustrated below can besupplied to handle these effectively. The chemical bags are dumped into the sack dump systemand then transferred to a batching system. The resulting slurries/solutions can be added to theprocess through a variety of pumping methods. Shown in the slide are metering pumps typicallyused for adding solutions.

Bag Systems

For small applications

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Pre-Made Lime Slurries

For smaller use applications several lime manufacturers have developed delivery systems to handlehigh strength lime slurries approaching 35% solids. These slurries are shipped in bulk from themanufacturer to liquid tanks at the use point.This method of delivery saves on the expense of a dry lime makedown system at the cost of thehigher freight costs involved in shipping water.An illustration of a typical system from Chemical Lime is outlined below.

FIGURE 41

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Chapter 9: Other Chemicals

CHAPTER 9: Other Chemicals

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Other Chemicals

All chemicals have their own particular design considerations that should be addressed in orderto provide trouble free feeding systems. Some design aspects are addressed for common watertreatment plant chemicals in the following sections.

Soda Ash

Special considerations for soda ash include its tendency to form hard lumps if exposed tomoisture and its complex phase diagram in water.When designing soda ash systems special care should be taken to avoid long, complexpneumatic fill lines. Soda ash is vulnerable to product size degradation if handled extensively oroff loaded at too high a pressure. The resultant fine powder is especially sensitive to moistureand will cause flow problems in the silo.Designers should note that there are two common forms of soda ash in industrial use. Lightsoda ash is generally used as a filler in compounds and will have a product weight of 25-35 pcf.Dense soda ash is generally used in water treatment and has a bulk density of 65 pcf. Thestorage silo size and design should take the form of soda ash into account.Consideration should also be given to the use of a lump breaker at the discharge of the silocone. This can be used in conjunction with or in place of bin activators and will reduce anylumps that form in storage.

Soda ash dissolves in water to form a variety of compounds depending on the strength of thesolution and the temperature. The phase diagram is quite complex. Special care should betaken to ensure that the decahydrate form of soda ash cannot form as it forms a hard glassy

solid that is difficult and dangerous to remove from pipelines and pumps. By maintaining thetemperature and solution concentration within defined limits to ensure an unsaturated solution,this situation can be avoided. A phase diagram is shown below which illustrates the variousforms that can be produced.

Feeding soda ash solutions to the process can be accomplished using a batch tank makeupsystem with the solution fed to the process with metering pumps. Alternatively soda ash can bemetered continuously in dry form using a volumetric feeder. This feeder adds the soda ash to awetting cone/eductor system for direct feed to the process using the transport water as a carrier.Control over the addition is maintained by controlling the feeder speed in response to plantflow/ph signals. A typical flow diagram is shown below to illustrate the direct feed system

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E------------EA---------------------A

F---------------------------------------------F

G----------------------------------------------G

D------------------------------------D

H-------------------H

B--------------------

C-----------------------C

A-------

B-------

C-------

D-------

--------E

--------F

--------G

--------H


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Powdered Activated Carbon (PAC)

Special considerations for PAC include the type of carbon used and the flow promotion in thesilo.Designers should note that there are two common forms of PAC in water treatment plant use.PAC used in water treatment is produced from coal or wood each with its own characteristics.Coal based PAC is generally heavier with a bulk density ranging from 25-35 pcf. Wood basedPAC is lighter with a bulk density ranging from 15-25 pcf. The storage silo size and designshould take these ranges into account.

Flow promotion devices are required to ensure smooth even flow form the silo. Fluidizationsystems are very effective when using intermediate feed hoppers above the feeding device. Theuse of bin activators is not recommended as these devices tend to compact the PAC and createadditional flow problems.

Many jurisdictions also require that any electrical devices used in feeding or storing PAC berated for explosion potential. Local regulations should be checked to determine theirapplicability. The cost impact of this requirement can be minimized by locating many of thedevices such as control panels in an adjacent building as shown below.

Feeding PAC to the process can be accomplished using a batch tank or continuous makeupsystem with the slurry fed to the process with slurry pumps. Alternatively PAC can be meteredcontinuously in dry form using a volumetric feeder to a wetting cone/eductor system for directfeed to the process using the transport water as a carrier. Control over the addition ismaintained by controlling the feeder speed in response to plant flow signals.

Note that the volumetric feeder design should incorporate a conditioning screw around the feedscrew to avoid bridging and arching flow problems. Care should be taken to ensure that floodingof the PAC through the feeder is effectively prevented.A typical direct feed system utilizing a loss-in-weight feeder is shown in the following pictures.This system also has a separate building to house some of its electrical equipment.

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Summary

In conclusion, if the potential hazards and problems of feeding lime and other chemicals arerecognized and dealt with at the design stage, then effective, efficient and minimal maintenancesystems can be provided to the end users.

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