Hard-burned quicklime

Caustic soda is removed from the carbonate—bicarbonate solution by treating with a slight excess of hard-burned quicklime (or slaked lime) at 85—90°C in a stirred reactor. The regenerated caustic soda is separated from the calcium carbonate precipitate (lime mud) by centrifuging or rotary vacuum filtration. The lime mud retains 30—35% Hquid and, to avoid loss of caustic soda, must be weU-washed on the filter or centrifuge. Finally, the recovered caustic solution is adjusted to the 10% level for recycle by the addition of 40% makeup caustic soda. 

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,... 

Table 15.4 Typical properties of soft medium and hard-burned quicklimes.

Hard-burned quicklime with a preferred particle size of 2 mm, is used in the United States to control infestations of starfish on oyster beds. When a particle contacts a starfish, it causes a lesion which kills it. The quicklime is applied at approximately 150 kg per hectare [30.1]. 

The product was developed in Japan in the 1970s [32.37]. Initially, mixes containing hard-burned quicklime were used. However, the expansive stress, which can exceed 5000 t/m after 48 hours was sometimes relieved by blow-outs (i.e. ejection of the lime from the hole), rather than causing splitting. 

Hard-burned quicklime has been sintered as a result of over-burning at high temperatures. It slakes very slowly. 

Solid-burned quicklime see hard-burned quicklime. 

Hard-burned lime is a quicklime that is calcined at high temperature and is generally characterized by relatively high density and moderate-to-low chemical reactivity. 

Hardness. Most limestone is soft enough to be readily scratched with a knife. Pure calcite is standardized on Mohs scale at 3 aragonite is harder, 3.5—4. Dolomitic limestone is generally harder than high calcium. Dead-burned or sintered limes are 3—4 on this scale, whereas most commercial soft-burned quicklimes are 2—3 (see Hardness). 

Many aircrete producers specify low reactivity quicklime produced in mixed-feed kilns, fired with coke. The additional costs per tonne of lime, arising from the use of coke, and from operating at higher temperatures to produce hard-burned lime, are substantial, but are accepted as being necessary. 

Another aspect of reactivity and mean apparent density is that, as reported in [27.17], at the final temperature in the BOF of 1,600 °C, lightly burned quicklime sinters to medium burned within 5 min. and to hard burned within 10 min. (Table 27.5). 

There are at least two other critical variables that should be considered when purchasing lime for a chemical plant. Quicklime is usually classed as soft burned, medium burned, and dead or hard burned. The higher the temperature limestone is converted to quicklime, the less reactive it becomes. It can become almost impossible to slake (hydrate) quicklime to calcium hydroxide, if it is cooked at too high temperatures. Cooking limestone is an art and operators become very skill at their trade. 

The rotary kiln has become extremely important in lime processing, and is used to burn a large portion of dolomitic quicklime production. More than half of all industrial quicklime is made in rotary kilns. The rotary is particularly useful for making the hard burned grade of quicklime that must contain a minimum amount of unburned limestone. 

Calcium Oxide. Also called lime or quicklime (4,5), calcium oxide [1305-78-8] (Class 1, nonregenerative), is relatively iaexpensive. It is prepared by roasting calcium carbonate (limestone) and is available ia a soft and a hard form according to the way ia which it was burned. For desiccant service, soft-burned lime should always be used. Calcium oxide is most commonly used to dehydrate Hquids and is most efficient when it can be heated to speed the reaction rate. The reaction product is calcium hydroxide, which cmmbles as it picks up moisture. 

Lime (CaCOj is a common material used for making a hard and water-resistant cement. It is found in many parts of the earth as a component of limestone. It can be chemically altered by heating to produce stronger binding agents and it can be rather easily ground to a fine powder. Lime burned at 900°C turns into quicklime (CaCOj), which can be combined with water in the absence of carbon dioxide to produce slaked lime (CaCOH) ). This wet mass sets slowly with the loss of water to produce interwoven crystals of CaCOH) and then combines with carbon dioxide in the air to produce lime (Fig. 3.12). 

Hardness. Most commercial quicklime products have a hardness of 2 to 3 Mohs. The value for dead-burned dolomite is in the range 3 to 5 Mohs [13.1]. 

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