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Transcript of Lime Manual

Lime Handling

Systems

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 general information on lime. Recommendations for system designs are discussed with information presented on equipment options. The article is designed to acquaint the reader with the considerations involved in achieving an efficient and practical lime system.

CHAPTER 1: GENERAL INFORMATION ON LIME Lime Lime Uses Types of Lime Quicklime Hydrated Lime Selecting what type of Lime to use Lime Deliver CHAPTER 2: FACTORS AFFECTING LIME SLAKING Factors Affecting Lime Slaking Lime Slaking Quicklime Properties Water Properties Water to Lime Ratio CHAPTER 3: LIME SYSTEM COMPONENTS Bulk Storage Typical Silo Configurations Dust Control Silo Flow Promotion Flooding Control Chemical Feeders CHAPTER 4: LIME SLAKERS Lime Slakers Paste Type Slakers Detention Slakers Conventional Detention Slakers Grinding Mill Type Factors Affecting Slaker Selection CHAPTER 5: SYSTEM ACCESSORIES Slaker Accessories Grit Removal Slurry Tanks Slurry Pumps and Piping CHAPTER 6: CONTROL PANELS & INSTRUMENTATION Control Panels Instrumentation Silo Level and Inventory Controls Slurry Measurement CHAPTER 7: LIME ADDITION CONTROL Lime Addition Control Good Design Practices Other Considerations CHAPTER 8: SMALL FEED SYSTEMS Small Feed Systems Pre-Made Lime Slurries CHAPTER 9: OTHER CHEMICALS Other Chemicals Soda Ash Powdered Activated Carbon (PAC)

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Chapter 1: General Information on Lime

CHAPTER 1: GENERAL INFORMATION ON LIME

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Chapter 1: General Information on Lime

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 widely used chemicals in the United States. Approximately 25 million tons of lime is used each year for a variety 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 rain control and environmental protection, both of which use lime in large quantities. In addition, new leaching processes for certain types of mineral ores and new sludge composting requirements will require large new quantities of lime.

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Chapter 1: General Information on Lime

This introduction to lime handling equipment is intended to acquaint users with the design considerations involved in setting up a lime handling facility for maximum efficiency and minimum maintenance 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 calcium carbonate (CaCO3). Limestone is typically quarried for use in its native form or for further processing to produce derivative substances such as quicklime. One form of limestone is marble. Figure 2 shows the types and properties of limestones.FIGURE 2. Common Name and Formula High Calcium Limestone CaCO3 Limestone Names, Grades, and Properties Appearance and Properties White rock with intermediate hardness [Mohs hardness of 3-5], product is quarried, typical purity is 95% CaCO3 White rock with intermediate hardness [Mohs hardness of 3-5], product is quarried. Bulk Density 90-115 lbs/cf

Dolomitic Limestone CaCO3 & MgCO3

Grades or Forms Available 1 Large Lump Pebble or Crushed Ground, Screened or Granular Pulverized Large Lump Pebble or Crushed Ground, Screened or Granular Pulverized

90-115 lbs/cf

1

Limestone is available in a large range of sizes often specified as percentages passing standard sieve sizes. Sizes listed in the table are defined by ASTM C 51 as Large Lump [8 and smaller], Pebble or Crushed [2.5 and smaller], Ground, Screened or 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 CaCO3 and MgCO3. Limestone is primarily used for flue gas desulfurization, acid neutralization, and to produce 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 subjected to a variety of mechanical processes to produce various grades of lime.

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Chapter 1: General Information on Lime

ExshawLime Kiln

Aerial photo compliments of Graymont LimeFIGURE 3: TYPICAL PRODUCTION PLANT

Quicklime: Quicklime consists primarily of calcium oxide (CaO) and is produced from limestone in a process known as calcining as defined below: High Calcium Quicklime CaCO3 (Calcium Carbonate) + Heat Dolomitic Quicklime CaCO3 MgCO3 (Calcium Carbonate and Magnesium Carbonate) + Heat CaO MgO (Calcium Oxide and Magnesium Oxide) + CO2 (gas) CaO (Calcium Oxide) + CO2 (gas)

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Chapter 1: General Information on Lime

Figure 4 below defines the different types of quicklimes and the basic properties of each type:FIGURE 4. Common Name and Formula High Calcium Quicklime CaO Dolomitic Quicklime CaO & MgO Quicklime Types, Grades, and Properties Appearance and Properties Soft [Moh hardness less than 3], white pebbles to powder, typical purity is above 90% available CaO Soft [Moh hardness less than 3], yellowish white pebbles to powder, contains 5 to 40% MgO Bulk Density 48-70 lbs/cf

Grades or Forms Available Pebbled [3/4 1/4 ] Granular [1/4 minus] Pulverized [100 % Passing # 20 sieve] Pebbled [3/4 1/4 ] Granular [1/4 minus] Pulverized [100 % Passing # 20 sieve]

49-72 lbs/cf

The quality of quicklimes varies greatly. The quality of the lime is determined by the purity and reactivity of the quicklime. Purity is a measurement of the quantity of calcium oxide (CaO) in a given sample that is chemically available. The purity of commercially available quicklime in the United States typically ranges from 85% to 95% available CaO. Reactivity is a measurement of the heat released in the reaction between quicklime and water. This reaction is commonly known as lime slaking or hydration, the product of which is hydrated lime. As noted in Figure 5 below, reactivity is determined by slaking quicklime (at specific ratios of water to quicklime) 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 high purity 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 by hydration 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 the reaction 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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Chapter 1: General Information on Lime

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 calcium hydroxide. Commercial hydration of quicklime occurs in hydrators, which control the water to lime ratio precisely to this ideal ratio to produce a dry powdered product. Where quicklime is reacted with water in excess of the ideal ratio, the process is known as lime slaking which 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 each type:FIGURE 5. Common Name and FormulaHigh Calcium Hydrated Lime Ca(OH)2 Normal Dolomitic Hydrated Lime (Type N) Ca(OH)2 & MgO

Hydrated Lime Names, Grades, and Properties Appearance and PropertiesSoft [Moh hardness less than 3] white powder, dusty, typical purity is 97% Ca(OH)2 Soft [Moh hardness less than 3] white powder, comes from dolomitic quicklime, typical composition is 46 to 48% CaO, 33 to 34% MgO and 15 to 17% Ca(OH)2 Soft [Moh hardness less than 3] white powder, comes from dolomitic quicklime, typical composition is 7% CaO and MgO and 90% Ca(OH)2 and Mg(OH)2

Grades or Forms AvailablePowder [80% passing 200 mesh size} Powder [80% passing 200 mesh size}

Bulk Density25-40 lbs/cf 30-40 lbs/cf

Pressure Dolomitic (Type S) Hydrated Lime Ca(OH)2 Mg(OH)2

Powder [80% passing 200 mesh size}

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 percentage of the material that is calcium hydroxide (Ca(OH)2). The typical range of purity in the United States is above 95% calcium hydroxide (Ca(OH)2). Hydrated lime is available in bulk trucks, bulk bags, and paper sacks.

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Chapter 1: General Information on Lime