Quicklime systems

Aeration of ferric sulfate should be held to a minimum because of the hygroscopic nature of the material, particularly in damp atmospheres. Mixing of ferric sulfate and quicklime in conveying and dust vent systems should be avoided as caking and excessive heating can result. The presence of ferric sulfate and lime in combination has been known to destroy cloth bags in pneumatic unloading devices. Because ferric sulfate in the presence of moisture will stain, precautions similar to those discussed for ferric chloride should be observed. 

Hard and soft acid and base (HSAB) principle, 16 780 Hard blacks, 21 775 Hard-burned quicklime, 15 28 Hard coals, 6 703 classification, 6 712 Hard copper alloys, 7 723t relief annealed, 7 723t Hard copy systems, 9 513-514 Hard core repulsion, 23 93 Hard-elastic olefin fibers, 11 242 Hardenability, of steel, 23 283—284 Hardened MF resins, analysis of,... 

Method I.—The apparatus described in Preparation 1 is fitted up, a 500-c.c. flask being used, and 30 gms. (1 mol.) of finely powdered tartaric acid, 150 gms. (excess) of absolute alcohol, and 50 gms. of crystallised benzene placed in the flask. The object of the benzene is to help to volatilise the water produced by forming with it and the alcohol the low boiling ternary system—alcohol-benzene-water. The iron tube is packed with small lumps of good quicklime, and is heated to a temperature of 90°. The mixture in the flask is boiled, a few pieces of porous porcelain being added to promote steady ebullition. Esterification proceeds almost to completion, owing to the removal by the quicklime of the water formed. After 6 hours, the liquid in the flask, which will have become quite viscid owing to the formation of the ester, is distilled on a water bath until all the benzene and excess of alcohol have been removed the residue is fractionated from a metal bath under reduced pressure. 

As an example of this, we may first consider the system formed by the two components CaO and CO2, which can combine to form the compound CaCO. This substance, on being heated, dissociates into calcium oxide and carbon dioxide, and gives rise to the equilibrium CaCOa CaO + COg, In accordance with the definition given on p. 6, there are present here two solid phases, the carbonate and the quicklime, and one vapour phase the system is therefore univariant. To each temperature, therefore, there will correspond a certain definite maximum pressure of carbon dioxide (dissociation pressure) and this... 

The reaction is reversible, and CaO readily combines with CO2 to form CaC03. The pressure of CO2 in equilibrium with CaC03 and CaO increases with temperature. In the industrial preparation of quicklime, the system is never maintained at equilibrium rather, CO2 is constantly removed from the kiln to shift the equihbrium from left to right, promoting the formation of calcium oxide. 

Slaked lime is produced by reacting, or slaking quicklime with water, and consists mainly of calcium hydroxide. The term includes hydrated lime (dry calcium hydroxide powder), milk of lime and lime putty (dispersions of calcium hydroxide particles in water). Slaked lime is widely used in aqueous systems as a low-cost alkali. 

The quicklime needs to have an adequate resistance to crushing, impact and abrasion to avoid producing excessive fines within the kiln and in the lime handling system. As mentioned above, fines can reduce the porosity of the bed, increase pressure drop and reduce heat transfer. 

All grinding systems use air to convey the ground quicklime and, although part of the air is recycled, the remainder is purged via a bag filter to remove fines and heat. This inevitably leads to a limited amount of air slaking in addition to that which already occurs in the cooling zone of the lime kiln. Under typical conditions, the measured combined water may rise from 0.5 % in the feed to 0.7 % in the ground quicklime. 

Skip hoists can be used for all granular and lump grades but are more suitable for particles greater than 100 mm. Elevators — both belt-and-bucket and chain-and-bucket types — have been used for all grades of quicklime. The system should minimise spillage within the equipment and ensure that wear on linkages caused by the quicklime is minimised. 

A modem design of batch slaker is described in [22.2], It consists of a horizontal cylinder agitated along its length with paddles. Quicklime (up to 20 mm in size) is blown at a controlled rate into the cylinder and water is added to maintain the temperature at 85 to 95 °C. The unit is able to slake a truckload of 201 in 1 to 1 /2 hours. It can be fitted with a wet scrubber for dust control and with a de-grit-ting system. 

Table 27.1. Ground quicklime used for injection systems...

The chemical requirements for the injection grade quicklime used in steelmaking processes such as the LD-AC and the Q-BOP, are similar to those in Table 27.3. The lime is groimd to suit the requirements of the injection system (e.g., Table 27.1). 

In this technique, hydrated lime is fluidised in air and injected into the boiler or kiln at temperatures in excess of 850 °C. A system designed for injecting hydrated lime is described in [29.1]. The hydrated lime decomposes within 30milliseconds [29.5] to a porous and extremely reactive form of quicklime. 

Slaking quicklime is not tm inherently dusty operation and the volume of gaseous effluent is small. It is good practice to keep slakers under slight suction to ensure that dust and steam do not escape into the plant, or into the lime feed system. One technique employs a water spray in a pipe, which acts as an ejector and keeps the hydrator under slight suction. The spray removes the dust, and the collected dilute milk of lime is used for slaking. 

Finely divided products, such as ground quicklime and hydrated lime, are loaded into air pressure discharge tankers for transport. A flexible chute is used, which fits the loading hatch of the vehicle, and incorporates an extraction hood connected to a vacuum filter system. The hood removes both displaced and conveying air emerging from the hatch. 

Portland cement, invented in 1892 by Joseph Aspdin of Leeds, was so called because it resembled expensive Portland stone (at least to the eye of the inventor). It is made from about 80 % limestone and about 20 % clay. It was widely adopted because it possessed superior qualities to the older quicklime-based material, including the especially important property of being able to harden in damp conditions. This latter property was especially valuable at a time when tunnel constmction was widespread, including amongst other projects, the London underground system. 

The addition of either lime, Ca(OH)2, or quicklime, CaO, to water is used to raise the pH to about 11.5 and cause the precipitation of CaCOj, along with metal hydroxides and phosphates. Calcium carbonate is readily recovered from lime sludges and can be recalcined to produce CaO, which can be recycled through the system. 

Lime for pH adjustment and softening is a major dry chemical used in water treatment. The two forms commonly used are hydrated lime (Ca[OH]2) and quicklime (CaO). Quicklime is slaked with a small volume of water (converted into hydrated lime) to become a slurry that is delivered to the water. Quicklime reacting with water produces much heat. Operators must take precautions to avoid burns when working with quicklime. Figure 5-2 shows a typical quicklime feed system. 

Quicklime slaker S3rstems (Figure 5-2) require special handling. Hydrated lime is first formed by slaking quicklime with a small amount of water. When water is added to quicklime, it produces heat, which helps dissolve (convert) the unreacted quicklime more efficiently to hydrated lime. The hydrated lime formed is mixed with more water to form a slurry that is delivered to the point of application. Huge lime softening plants in the midwestern United States use quicklime and this particular chemical feed system. 

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