Clamps consist merely of heaps composed of alternate layers of limestone and coal, having a fire-hole below, and covered with clay or sods to prevent the escape of heat.
This is a very wasteful method of burning, and should only be used where limestone and fuel are abundant.
Very similar arrangements for burning lime are in some parts of the country called Sow Kilns.
Lime Kilns are divided into two classes, Tunnel Kilns and Flare Kilns.
Tunnel Kilns are those in which the fuel and stone are placed in alternate layers.
Flare Kilns have the fuel below, so that the flame only reaches the stone in the kiln above.
Either form of kiln may be worked on the continuous or on the intermittent system.
The Continuous system is that in which the lime is gradually removed from the bottom of the kiln in small portions, fresh limestone being added at the top to make up for the burnt lime removed at the bottom.
The Intermittent system consists in burning and discharging a whole kiln-ful at a time. After the stone is well burnt through, the kiln is allowed tc cool down, and the burnt lime is removed. The empty kiln is then recharged, and the operation repeated.
The continuous system is most generally applied to tunnel kilns.
The lime so produced is likely to be unequally burnt, but the process is a cheaper one.
By the intermittent system, in which the whole kilnful is burnt at once, the lime is more uniformly calcined throughout.
Tunnel Kilns, called also Continuous, "Running," "Perpetual," or "Draw-Kilns."
A kiln of this class is shaped internally either like a cylinder, an inverted cone, or a pair of vertical cones base to base. It is lined with firebrick, and has an opening below, generally protected from the weather by a shed.
At the lower extremity of the cone is a grating, upon which is placed a layer of brushwood, and then alternate layers of coal and moistened stone, reaching to the top, the largest pieces being in the middle, where they will get most heat.
As the lime becomes burnt it is withdrawn through the grating, and fresh stone and fuel are added at the top.
This kiln is economical in fuel, requiring only about 1/5 the weight of the lime produced, but the lime is not equally well burnt throughout, and it requires great experience to manage the kiln properly.
Fig. 94 is a section of the form of kiln frequently erected as a temporary arrangement to burn lime during the progress of large works.
The kiln may be built of either bricks, stone, or concrete, or sunk into the ground.
The interior is lined with firebricks, a hollow space being left behind the lining.
The fuel and broken stone are thrown in at the top of the kiln, and lie in alternate layers., the thickness of each layer of stone being from six to eight times that of the fuel.
At the lower end of the kiln lies the wood for kindling the fire, resting upon a grate of loose bars, which can be drawn out one at a time.
The fire having been lighted at the bottom below the grating, the heat passes through the layers; those nearest the bottom are burnt first, and are withdrawn through the grating by removing one or more of its bars.
As the burnt lime is taken out at the bottom, the bulk of the contents of the kiln slide down, and the space thus left at the top is filled with fresh layers of fuel and stone.
It is convenient to have a shed in front of the drawhole, to secure the freshly burnt lime from the weather.
The size of the kiln varies according to the supply required. A kiln of the form shown in Fig. 94, 16 feet high, 4 feet wide at the bottom, and 9 feet at the top, will hold "about 25 tons of limestone, and will burn sufficient lime to keep twenty bricklayers constantly supplied with mortar."1
Fig. 95 shows in section a form of kiln largely used in the Midland counties for burning Lias lime.
The conical mound on the top is composed of layers of fuel and stone, plastered over with clay.
"Care is taken that the clay plastering covering the conical mound does not give too much vent in any one part to the products of combustion, lest too strong a draught should be set up toward such orifice, and cause overburning of the lime in its course.
"The fuel is made to burn in a smouldering fashion throughout its operation.
"At the opening of the drawhole, in order to ignite the contents of the kiln, a few large pieces of coal are built up.
"The fuel layers vary from 6 to 3 inches in thickness, those at the bottom being the thickest. The layers of mineral vary from 10 inches at the bottom to 18 inches at the top."2
Flare Kilns, called also Intermittent Kilns, are generally in the form of a cylinder, surmounted in most cases by a conical vault. The broken limestone rests upon arches, roughly formed from large pieces of the same material.
1 Hill's Lectures on Machinery.
2 Cooke's Aide Memoire.
Fig. 96. Plan.
Fig. 97. Section.
These rough arches must be carefully built, and the heat applied gradually so as not to split the stones.
The fire is lighted below, only the flame being in contact with the stone, thus producing much cleaner lime than that obtained by the methods in which they are mixed together.
Such a kiln is more easily managed than the kinds which are worked continuously, and the lime produced is more uniform in quality. The necessity of letting the fire out after each charge is burnt is a great inconvenience, and also causes waste of fuel.
For the same kind of lime this kiln requires about 5/3 (i.e. nearly double) as much coal as does the tunnel kiln. Moreover, the intermittent kiln requires relining every twelve months, which is a source of great expense.
Fig. 96 is the plan, and Fig. 97 a section of a pair of flare kilns, such as are used for burning grey chalk into lime. L is the hole through which the lower part of the kiln is loaded, I that for the higher levels. The draw-holes D D open into a central passage P.
In the section the kiln to the right is shown as loaded, the other as empty. The rough arches of limestone are shown in the former. The fire bars for the fuel are shown in plan and section, the spaces C C are packed with broken chalk, c c with chalk dust. The firebrick lining is hatched with broken lines, the ordinary brickwork with continued and broken lines alternately.
Fig. 98 is the section of a simpler flare kiln in common use.