Rotary kiln with a hollow brick insulating lining

Details Rotary kiln with a hollow brick insulating lining Rotary kiln with a hollow brick insulating lining

2003-08-25

2004-10-12

Wilson, Gregory A. (Department: 3749)

Heating

Tumbler-type rotary - drum furnace

Having brick-type lining

Reexamination Certificate

active

06802709

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of rotary kilns and means of thermally insulating it. More particularly, this invention relates to a rotary kiln having a hollow-brick insulating lining between the kiln shell and the work lining, used for pyro-processing materials such as cement, lime, lime sludge, clay, shale, refractory, bauxite, garbage, ores, fuels and minerals. Such kilns normally vary in diameter between 6 and 25 ft. and in length between 200 and 600 ft., and are supported along their length by pairs of support rollers and riding rings secured to the outer surface of the shell. In U.S. Pat. No. 4,344,596, Hjaeresen provides a detailed description of the kiln mechanics. In U.S. Pat. No. 4,200,469, Touborg provides a description of how rotary kilns are fired.
Conventional insulated rotary kilns used for pyro-processing a wide variety of materials, are commonly lined with two concentric layers of refractory brick or castable. The insulating layer is installed directly on the inner surface of the shell, and the dense or work layer is installed on top and concentric with the insulating layer. The bricks in the work layer are dense and of a heat resistant refractory composition such as magnesia, dolomite, alumina, or clay, which sufficiently resist the intense heat produced in the kiln. Heat resistant refractory materials are usually good heat conductors and relatively poor heat insulators. In the rotary kiln field, one of the main problems is to contain heat within the processing zone to prevent the fast overheating and destruction of the kiln shell. Another common problem is to protect the kiln shell against permanent damage when the work lining collapses. Due to the very high temperatures maintained inside the kiln and the high thermal conductivity of the work lining, part of the thermal energy inside the kiln is lost through the work lining and the kiln shell, thus requiring that the shell be thermally insulated. Besides the heat loss, which translates into additional fuel expenditure and additional combustion gas emissions, the temperature gradient between the inside hot face and the cold face of the refractory lining develops thermo-mechanical stress within the work lining. When said stress exceeds the ultimate strength of the brick, the lining fails. Therefore, it is important to use an insulating lining between the kiln shell and the work lining.
Rotary kilns are sometimes exposed to strong winds, snowstorms and rainstorms. When the hot kiln shell is suddenly quenched, it shrinks and the refractory lining is exposed to hoop stress that can cause its rupture. For such reason it is important to insulate the inside of the kiln shell in order to reduce its outside temperature.
Several patents propose ways to insulate the inside of the rotary kiln shell, behind the work lining. The prior art normally employs two concentric layers of bricks, or two concentric layers of castable or a brick layer concentric with an insulating refractory layer. Insulating materials are normally lightweight fiber, brick or castable with thermal conductivity below 10 B.t.u./hr./sq.ft./deg. F./in. thickness. In insulated rotary kilns the typical work lining thickness varies between 6 in. and 9 in., and the typical insulating lining thickness varies between 0.25 in. and 3 in. In order to accommodate the inner kiln shell curvature the brick shapes have different tapers that, when combined in the proper ratio, can line any kiln radius. Sometimes the back up insulating brick can be either tapered or straight since it is held in place by the work lining.
2. Description of the Prior Art
Many prior art patents cover the field of rotary kiln shell insulation between the work lining and the kiln shell. Prior art for rotary kiln shell insulation comprises a wide variety of dense or work linings installed on top of fiber refractory, steel cladded fiber refractory, solid insulating brick, solid insulating castable, solid fireclay brick, solid ceramic tiles, grooved brick and tiles, brick with recesses or pockets on the outside face, and bricks with pockets on the outside face filled with fiber. For example, in U.S. Pat. No. 1,410,729 Balz invented a dense brick attached to an insulating refractory by a tongue and groove system. In U.S. Pat. No. 1,701,287 Waite invented a static furnace wall construction containing air channels behind the work lining to insulate the furnace shell. In U.S. Pat. No. 1,622,431 Feigenbaum invented an air cooled static vertical incinerator. In U.S. Pat. No. 1,674,422 Allen invented an air cooled wall. In U.S. Pat. No. 1,688,321 Ser. No. 10/23/1928 Abott invented an air cooled furnace wall. In U.S. Pat. No. 2,641,205 Dolezal invented a cooling wall for heated chambers. In U.S. Pat. No. 5,695,329 Orcutt invented a way to insulate the kiln shell with fiber material inserted between the work lining and the kiln shell. In U.S. Pat. No. 1,936,635 Lee proposed a special-shape solid brick that, when put together, creates an air gap between parts of the brick and the kiln shell. In U.S. Pat. No. 2,230,141 Heuer proposed a dense brick cemented to a solid block of insulating brick or asbestos. In U.S. Pat. No. 2,635,865 Brumbaugh invented a layer of insulating concrete anchored to the kiln shell between the work lining and the shell. In U.S. Pat. No. 3,343,824 Schneider used multiple layers of asbestos sheets as an insulator between the work lining and the kiln shell. In U.S. Pat. No. 3,528,647 Hyde employed diatomaceous earth as an insulator between the shell and the work lining of a blast furnace, a vertical, non-rotary cylindrical furnace. In U.S. Pat. No. 4,289,479 Johnson utilized solid blocks of lightweight insulating material attached to the kiln shell as insulator. In U.S. Pat. No. 4,499,134 Whitely used steel cladded fiber as an insulator behind the work lining. In U.S. Pat. No. 4,582,742 Gilhart used blocks of high-temperature fiber insulation as a back up lining in a furnace. In U.S. Pat. No. 4,803,933 Carey invented a rotary kiln brick with recessed chambers filled with insulating pads on the outside face. In U.S. Pat. No. 5,033,959 Bernt proposed cladded insulating fiber behind the work lining. In U.S. Pat. No. 5,695,329 Orcutt invented a way to insulate the rotary kiln shell with layers of reinforced insulating fiber installed behind the work lining.
The prior art of insulating rotary kilns presents some specific problems such as: asbestos is practically banned as an industrial thermal insulator because of its well-proven carcinogenic properties; ceramic fibers shrink when heated above 2000° F., causing the service lining to collapse; solid insulating and semi-insulating bricks have relatively low compressive strength, usually lower than 3,000 p.s.i.; solid insulating and semi-insulating bricks shrink when overheated, which could cause the work lining to collapse; solid fireclay or alumina brick and tile have good compressive strength and good thermal stability but their thermal conductivity is higher than insulating fiber's and insulating brick's; bricks with air gaps or pockets on the outside face help reduce heat transfer from the kiln processing zone to the shell, but they have reduced contact with the shell, a mechanical disadvantage over solid brick alternatives; bricks with air gaps or pockets on the outside face, filled with insulating materials, help reduce heat transfer from the kiln processing zone to the shell, but these bricks have reduced contact with the shell, a mechanical disadvantage over solid brick alternatives; dual bricks, comprised of a dense brick cemented or sintered to a lightweight brick, have the disadvantage of the differential thermal expansion coefficient between the two materials. Such difference creates a shear plane within the brick that causes it to crack; forced air or convective air channels between the work lining and the shell are not applicable to rotary kilns due to their constantly turning motion and their length.
One of the main disadvantages of most prior art pa

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