Hard-burning

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. 

Hard-burned magnesia is characterized by moderate crystaUite size and moderately low chemical reactivity. Hard-burned magnesia is readily soluble only in concentrated acids. 

Hard-burned magnesias may be used in a variety of appHcations such as ceramics (qv), animal feed supplements, acid neutralization, wastewater treatment, leather (qv) tanning, magnesium phosphate cements, magnesium compound manufacturing, fertilizer, or as a raw material for fused magnesia. A patented process has introduced this material as a cation adsorbent for metals removal in wastewater treatment (132). 

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,... 

Clean the entire exterior of the damaged section around the full circumference and for 4 in. each side of the fracture, and then sand it carefully to remove all the hard burned surface of the pipe. 

The reaction between APP and hard-burned MgO is strongly exothermic, with the cement setting within 3 min. The early strength of the cement was 13.8 MPa (2000 psi) after 1 h and over 20 MPa (2900 psi) after 5 h (Sugama and Kukacka, 1983b). Struvite was identified as the major cementitious phase by X-ray diffraction. 

The reaction between hard-burned MgO, water, and potassium dihydrogen phosphate produces a quick setting cementitious mass, KMgP04-6H20. This cement system was originally developed at Argonne National Laboratory for the stabilization and encapsulation of hazardous and radioactive wastes (Wagh et al., 1998). 

To date, there have been several unsuccessful attempts to fit these results to a simple model—for example, one based on a shrinking unreacted core or on reaction of a porous solid. The apparent role of water in the mechanism suggests that sulfur dioxide may be oxidized to sulfur trioxide on the surface and that sulfur trioxide diffuses through a product layer to react with calcium carbonate. This concept would be consistent with the similar kinetics observed for half- and fully calcined stone since the rate-determining step would presumably be the same in either case. This view is supported by the observation that reactivity in a fluidized bed decreases somewhat above about 850 °C because the thermodynamics of sulfur dioxide oxidation become less favorable. On the other hand, Borgwardt s observations with fully calcined stone (1) suggest that the decreased reactivity is caused by hard-burning of the stone. 

Hard-burning (also known as caustic-burned), in which the hydroxide or carbonate is heated to somewhat lower temperatures, between i,ooo°C and i,5oo°C (2,ooo°F and 3,ooo°F), allowing the compound to retain some of its chemical reactivity ... 

Hard-burned In the production of fertilizers and animal feed, in the extraction of uranium oxide from uranium... 

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

Hard-burned dolomite is generally produced in mixed-feed shaft kilns, operating under reducing conditions. Hard-burned dolomite is generally produced in mixed-feed shaft kilns, operating under reducing conditions. 

As described in section 20.3.1, other causes of grit are calcium carbonate and hard-burned lime. The production of grit is minimised by pulverising the lime and many processes reduce the particle size to below 10 mm and, in some cases, to below 5 mm. Dolomitic lime is generally ground more finely (see section 20.7). 

Expansive components. Expansion in the autoclave is generally caused by hard-burned calcium oxide and magnesium oxide. As described in section 15.4, during the calcination of limestone, the magnesium oxide component of lime becomes over-burned. Practical experience has shown that MgO levels up to 2 % can be tolerated. The amount of hard-burned calcium oxide generally increases as the average reactivity of the lime decreases. It also depends on the type of kiln and fuel used. In addition, malfunctions of the kiln can increase the proportion of the hard-burned fraction. 

The relative amount of hard-burned lime from a particular kiln can be monitored by study of the time-temperature curve produced by the reactivity test (Fig. 26.7). Any increase in the time at which the maximum temperature begins to fall (by 0.2°C) indicates that the amount of slow-slaking calcium oxide has increased. 

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). 

---