The lime recovery kiln is a long, rotating cylinder set on a slight incline in which lime mud is fed to the uphill feed end. The rotation of the kiln, along with gravity, works the lime mud down toward the burner at the far downhill end of the kiln. The final product exiting the lime recovery kiln is calcium oxide, which undergoes a number of other chemical treatments before being used to regenerate white pulping liquor.
Lime recovery kilns can be as long as 380 feet and as wide as 15 feet in diameter. These vessels are refractory lined and typically zoned into five major sections including the chain section, tumbler section, intermediate zone, burning zone, and refractory dam. The operating environment in each section varies and dictates refractory selection.
The chain section is designed to act as a heat exchange device by increasing the surface area of lime mud exposed to hot gases. The operating temperature in the chain section is relatively low. Monolithic refractories are typically used to line chain sections of lime recovery kilns and must be able to withstand significant impact and abrasion from the chains and incoming lime mud. High-cement, coarse-aggregate castables such as HarbisonWalker International’s GREENCAST®-12 GR PLUS, VERSAFLOW® 45 C PLUS, and LO-ABRADE® GR PLUS have shown continued success in the chain section.
Tumblers, also referred to as lifters, are also designed to act as heat exchange devices. Tumblers are raised sections of refractory that lift and mix the lime feed as it works its way down the kiln. This mixing process helps to ensure that the lime feed sees a more even distribution of heat during the calcination process. Refractory tumblers are often precast monolithic shapes. HWI’s VERSAFLOW® 45 C PLUS has a proven record of success in precast refractory tumblers and utilizes HWI’s proprietary anchoring system, providing a secure fixture to the kiln shell.
Operating temperatures become more moderate as the lime feed enters the intermediate zone. Historically, fireclay, 50 percent alumina, and 60 percent alumina refractory bricks have all been used in the intermediate zone. HarbisonWalker International’s CLIPPER® DP, KALA®, and UFALA® bricks have proven histories of success in intermediate zone service. In addition, intermediate zones are often insulated in order to reduce heat losses. HWI’s GREENLITE®-HS is a high-strength insulating refractory brick that can provide reliable service life as a backup insulator while reducing heat losses. GREENLITE®-HS is roughly half the density of historical intermediate-zone brick but still strong enough to tolerate mechanical stresses introduced by the rotary movement of the kiln.
Refractories in the burning zone of a lime recovery kiln are subject to the highest temperatures and are most susceptible to chemical attack. Temperatures in the burning zone can reach as high as 2500°F. Alkali attack occurs when alkalis, typically in the form of soda and potash entrained in the feed material, react with the lime mud feed and free alumina present in an alumina-silica-based refractory. For many years the product of choice in this application has been HWI’s UFALA®. UFALA® is a high-fired, 60 percent alumina refractory brick based on high-purity bauxitic kaolins. UFALA® is mullite bonded and contains no free alumina, which gives UFALA® superior alkali resistance.
HWI also provides KRUZITE®-70, a bauxite-based, 70 percent alumina brick for mills that still prefer to use a 70 percent alumina refractory in the burning zone. Another option for those mills that have experienced aggressive wear of the refractory lining due to alkali attack at elevated temperatures is MAGNEL® RS. MAGNEL® RS is a magnesia-spinel refractory brick that forms no eutectic with lime and does not react with alkalis.
Refractory retention dams have become much more common in modern lime recovery kilns. Dams increase the retention time of the feed material and make the calcination process more efficient. Both UFALA® and KRUZITE®-70 have proven service records in lime recovery kiln dams. Some dams have been formed, and many repaired, using monolithic refractories. Materials such as HWI’s VERSAFLOW® 60 PLUS, ULTRA-GREEN® 60 PLUS, EXCELERATE® ABR PLUS, and ON-LINE® 60G AR have been used with success.