Lime Types Uses Composition And Treatment

Acidic industrial wastewater can be neutralized with slaked lime [Ca(OH)2], caustic soda (NaOH), or soda ash (Na2C03). Since slaked lime is less expensive than other bases, it is the most commonly used chemical for acidic neutralization [5].

Two types of commercial lime with their associated hydrates are high calcium quicklime, otherwise known as calcium oxide (CaO), and dolomitic quicklime (CaO and MgO). The composition of these will depend on the source of the parent limestone and the method of lime production, particularly the type of fuel used to convert the limestone to quicklime [6].

A high calcium quicklime will produce a high calcium hydrated lime containing 72-74% calcium oxide and 23-24% water in chemical combination with calcium oxide. A dolomitic quicklime will produce a dolomitic hydrate. Under normal hydrating conditions the calcium oxide fraction of the dolomitic quicklime completely hydrates, but generally only a small portion of the magnesium oxide hydrates. The composition of a normal dolomitic hydrate will be 46-48% calcium oxide, 33-34% magnesium oxide, and 15-17% water in chemical combination with the calcium oxide.

Typical compositions of commercially available lime are given in Tables 1 and 2. Note the differences between the compositions of high calcium hydrated lime and pebble quicklime.

Slaked lime, commonly known as hydrated lime, is obtained by adding enough water to quicklime to satisfy its affinity for water. Wetting or dissolving tanks are usually designed for 5 min of detention time with 0.5 lb/gal (0.06 kg/L) of water or 6% slurry at the highest feed rate. Hydrated lime is often used where the maximum feed rates are less than 250 lb/hr (113.3 kg/hr) [7],

A general reaction of hydrated lime with acidic wastewater is

A common system for the storage, transfer, and reaction of hydrated lime is diagramed in Figure 2. Hydrated lime is loaded into a storage silo and metered out using a lime screw/auger, rotary valve, or other metering device. The lime is then transferred by either a lime auger or pneumatic line to the mix tank. The mix tank or other device should provide the retention time necessary for complete reaction of the lime. Neutralization is not instantaneous, and contact time must be provided, usually not to exceed 5 min, depending on the wastewater composition.

It should also be noted that it is very difficult to hold pH values between 6 and 8, since slight changes in lime feed causes wide swings in pH as neutrality is approached [8].

In one industrial wastewater treatment center the lime storage silo was used for limestone, and a pneumatic transfer system was used to transfer the lime from the silo to a mix tank. When

Table 1 High-Calcium Hydrated Lime Analysis

Chemical analysis test results

Table 1 High-Calcium Hydrated Lime Analysis

Chemical analysis test results

Total calcium oxide, CaO

72.5

Available calcium oxide, CaO

70.6

Calcium hydroxide, Ca(OH)2

95.8

Magnesium oxide, MgO

1.02

Silicon dioxide, Si02

1.5

Total sulfur, S

0.01

Iron oxide, Fe203

0.33

Aluminum oxide, Al203

0.37

Free moisture

0.97

Physical analysis test results

% Passing 100 mesh screen

99.5

% Passing 200 mesh screen

98.8

% Passing 325 mesh screen

98.7

Bulk density

Loose

27 PFC

Settled

40 PFC

Table 2 Pebble Quicklime Chemical Analysis—

Test Results (%)

Total, CaO

98.8

Available calcium oxide, CaO

93.7

Magnesium oxide, MgO

1.9

Silicon, Si

1.36

Aluminum oxide, A1203

0.64

Iron oxide, Fe203

0.34

Total sulfur, S

0.035

Zinc, Zn

0.013

Lime Schematic Diagram
Figure 2 Schematic diagram of lime transfer and use.

the change was made to hydrated lime it quickly became apparent that the "dusty" nature of the hydrated lime made it very difficult to handle with the off-gas collection system. At the exit from mix tank was a wet scrubber, meant to scrub out any chlorine that may have been released during the neutralization process and to knock out any dust particles entrained in the air. This scrubber was overwhelmed by the magnitude of dust particles and could not effectively remove them from the air. The change was then made to a lime auger, which eliminated the majority of the "dusting" problems while still providing effective transfer.

Also particular to this system is the mix tank. This mix tank is operated as a slurry tank for the hydrated lime. Lime introduced to this tank is combined with water exiting the scrubber, and the mixture is gravity fed to a process trench. The process trench and subsequent basins provide adequate retention time for complete reaction of the lime. It should be noted that at this time the neutralization requirements are not completely satisfied by hydrated lime. A two-stage neutralization system is used. This type of system uses lime to raise the pH initially and finish the neutralization process using caustic soda (NaOH). This process is detailed in Section V.

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Responses

  • robert
    What is mean by lime polution?
    3 years ago
  • Negisti
    Does lime make pollution?
    2 years ago
  • karri sj
    What is pH of soda ash, hydrated lime and quick lime?
    11 months ago
  • Senja
    What does screen passing for pebble lime mean?
    11 months ago
  • Nick
    Does hydraded lime produce ammonia?
    10 months ago
  • topias ruotsalainen
    What kind of lime is used for water treatment?
    9 months ago
  • Sabrina Bader
    Is quick lime a pollutant?
    3 months ago

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