Shaft kiln utilized for lime production

Apparatus for calcining materials comprising:

• (a) an upstanding hollow shaft kiln, through which the material to be calcined passes downwardly by the force of gravity, defining a plurality of zones including an upper preheating zone, a first calcining zone having a defined length, a second calcining zone having a defined length relative to the length of said first calcining zone with said defined length being approximately twice the length of said first calcining zone, and a material cooling zone;
• (b) air tight charging means at the top of said kiln communicating with supply means for charging said kiln with said material to be calcined;
• (c) means for selectively introducing combustion air into a lower portion of said cooling zone from outside said kiln comprising a plurality of adjustable ducts located at the bottom of said material cooling zone;
• (d) a collector channel adjacent the upper portion of said cooling zone and the lower portion of said second calcining zone;
• (e) a plurality of burners located intermediate said first calcining zone and said second calcining zone in position to inject burnt gases therefrom into said kiln;
• (f) first flow means communicating said collector channel with said burner and including gas conveying means for conveying gases from said collector channel to said burners and creating a negative pressure within said collector channel to convey a minor portion of said burnt gases injected into said kiln downwardly through said second calcining zone in co-current flow with said material to said collector channel for intermingling with said combustion air and recycling to said burners, said minor portion comprising about 20 to 25% of the burnt gases injected to the kiln from said burner;
• and
• (g) second flow means communicating with an upper portion of said preheating zone for exhausting burnt gases therefrom and creating a negative pressure within said upper portion of said preheating zone to convey a major portion of said burnt gases injected into said kiln upwardly through said first calcining zone and said preheating zone in counter current flow to said material.