LIME-BURNING. 101 



the stone nearest the dome arch in the kiln was liable to become in- 

 jured by overburning before the top portions of the charge where tor- 

 oughly calcined." 



(2) Vertical kilns with mixed feed. In kilns of this type the lime- 

 stone and fuel are charged into the kiln in alternate layers. As the 

 burning progresses burned lime is drawn from the bottom of the kiln, 

 while fresh layers of limestone and fuel are added at the top. 



The advantages of mixed-feed kilns, as compared with the separate 

 feed-kilns described below, are (a) that they are cheaper to construct, 

 (b) that they are somewhat more economical of fuel, and (c) that they 

 give for the same size of kiln a larger output in the same time. These 

 advantages are partly counterbalanced by the disadvantages to which 

 they are subject, these being (a) that the burned lime is discolored 

 to some extent by its contact with the fuel, (6) that the ashes of the 

 fuel cannot readily be separated from the burned lime, thereby lower- 

 ing the quality of the product, and (c) that a part of the fuel ashes 

 may clinker on the outside of the lumps of lime, preventing even and 

 satisfactory burning. 



To sum up the advantages and disadvantages: the "mixed-feed" 

 kiln is cheaper both in first cost and in operating expanses; its, product 

 is good enough for most ordinary purposes, but is not as evenly burned 

 or as white as is the product of a "separate-feed" kiln. 



At a small Pennsylvania lime-plant three vertical mixed-feed kilns 

 are in use. Each of these kilns takes about 24 tons of stone per day, 

 requiring the services of six quarrymen to keep the three kilns supplied. 

 Bituminous slack is used for fuel, the consumption being 26 Ibs. slack 

 per bushel (75 Ibs.) of lime, equivalent to a fuel consumption of 34.7 per 

 cent on the weight of lime produced. This ratio if correct is enormous 

 compared to natural- or even Portland-cement plants, and points to 

 unusually inefficient management. As a general rule, a vertical mixed- 

 feed kiln may be expected to produce lime with a fuel consumption of 

 from 15 to 25 per cent of the weight of clean product. The cause of 

 this apparently high consumption is that so much of the product is 

 usually unfit for use. 



The Aalborg or Schofer kiln, one of the best types of stationary 

 kilns for cement practice, has been employed in a somewhat modified 

 form for burning lime and hydraulic lime. The lime-kiln of this type 

 shown in Fig. 21. The limestone is fed in at the charging door B, 

 rhile the fuel is charged through the chutes //. The mass of limestone 



the preheating chamber D is dried, heated, and partly decarbonated 



fore it enters the burning-zone, when the decarbonation is complete. 



