FUEL OIL IN CERAMIC INDUSTRIES In the manufacture of clay products any fuel which causes discoloration by uneven heating, soot or smoke, is undesirable and unprofitable. Coal is out of the running in the manufacture of ceramic products, such as vases and dishes, and oil is the preferable fuel even in manufacturing enameled, vitrified, fire and common brick: Many enamel ware manufacturers use the muffle kiln. When ever it is necessary to treat the ware with two or more coats of enamel, it is necessary to apply all but the first coat at a higher temperature. In burning common brick about five days are re quired to water smoke and burn and 35 to 50 gallons of crude oil per thousand bricks are required. A longer time, higher tempera ture and greater consumption of oil per thousand bricks are neces sary in the burning of fire bricks, but the process is similar to that used for common brick.
In burning brick with oil the amount of fuel required varies with the quality of clay or shale used. One large plant in Kansas is burning brick using 100 gallons of oil per 1,000 brick. This includes fuel for running their boilers to operate tTie plant. The presence of carbon in clay is always a serious problem where coal is the fuel, because in case the carbon is ignited and burns freely, the fires in the furnace have to be drawn, all air supply shut off and the carbon allowed to smolder until completely burned out. In pulling a coal fire, the doors must be open and an excess of air rushes into the kiln before it can be daubed, not only checking the ware but supplying large quantities of oxygen for combustion of the carbon in the clay which might overburn the entire kiln. An oil fire does away with these dangers. It can be instantly turned off or turned down and the air inlets closed with out loss of fuel or danger to kilns. Fig. 94 shows an oil-burning brick kiln of a capacity of 500,000 brick.
Limestone as quarried is calcium carbonate, and its com position expressed chemically is To make quicklime, which is CaO, it is necessary that the carbon dioxide, be driven off by heat. Carbon dioxide begins to come off at a tem perature of about 750 degrees F., but a temperature of over 1,300 degrees is required to completely reduce the stone to calcium oxide. There are always some impurities present in the original limestone and the actual yield of quicklime varies from 30 to 55 percent of the limestone. The different quarries produce lime
stone of different densities and consequently the difficulty of re ducing the stone to quicklime varies. The dense and compact stones yield the best quality of lime.
The old method of burning lime in the "periodic" kilns is wasteful of fuel and time. This type of kiln is shown in fig. 95.
The kiln is made of large blocks of limestone or of brick. Two or three feet from the ground an arch (A) of large blocks of limestone is turned. The fire is built under the arch and the lime stone is piled on top of the arch, the lumps varying in size from that of a cocoanut just above the arch to that of a goose egg at the top of the kiln. After the fire is started, the temperature is raised very slowly for six or eight hours to prevent the limestone arch from crumbling. After this interval the temperature is kept at a full red heat for two days or more when the fire is allowed to burn out and the kiln cools. During the time of cooling, dis charging and recharging, the kiln is idle and much time is lost. Moreover a large amount of fuel is wasted in heating the walls of the kiln after each recharging.
When fuel oil is used in burning lime all the disadvantages of the periodic kiln are eliminated because the production is con tinuous. Furthermore, oil burned lime commands a ready market because of its greater cleanliness. There are two types of kiln which have proven remarkably successful with fuel oil. One, the "continuous" kiln, is vertical and this kiln should be charged with lumps of stone about the size of a man's head. If the temperature is too low or the lumps too large, the stone will not be calcined to the center and the lumps,will not slake. At the burning zone the width of the kiln should not exceed eight feet, because with a greater width the heat may not penetrate to the center of the charge. The combustion chamber should be large enough to allow combustion to take place before the oil enters the kiln, thus insuring a soft, long flame and permitting the gases to pass readily to the center of the kik. A low pressure air burner is preferable for this purpose and it should be so constructed as to thoroughly atomize the oil. Air should be admitted around the burner, giving complete combustion by passing through the flame.