Dense, compact stone is reduced with great difficulty, but the best quality of lime is produced from such stone. The lime-kiln should be charged with lumps of stone uniform in size, about as large as a man's head. If the pieces are too large, or the temperature of the kiln too low, the lumps will not be calcined to the center, and the lime will not slake.
The diameter or width of the kiln when oil fuel is used, should not exceed eight feet at the burning zone; if it is greater than this the heat of the flame may not penetrate to the center of the rock. Care must be taken in arranging the burners and fire box. The chamber should be large enough to allow combustion to take place before the oil enters the kiln. This in sures a soft, long, flame, permitting the gases to pass readily to the center of the kiln. If the burner is placed too far in, carbon is liable to form as a result of the flame impinging on the rocks. Furthermore, the lime near the flame is overburnt, while the lime in the center of the kiln is not burnt enough. The burner should be so constructed as to thoroughly atomize the oil, and where possible a low pressure air burner should be used.
Air for combustion should be admitted around the burner, so that it will pass through the flame and thus insure complete combustion. Some lime kilns have been arranged to take the air for combustion from the stack by means of a steam siphon. Good results have been obtained by this method, as the flame is much softer and longer, and the volume of gases increased. These gases pass through to the center of the kiln, and when they carry a sufficient amount of carbon monoxide, secondary combustion takes place there.
Lime burning has been carried on in many types of kilns, most of which can be readily converted to the use of oil fuel. Oil burnt lime finds a ready mar -het because of its cleanliness. The continuous type of lime kiln has proven to be the most successful with oil fuel ; the. rotary kiln has also been used with good results; but in this kiln the stone is broken into small pieces with the result that the lime produced is not adaptable for building purposes. On the other hand, when lime has to be ground and hydrated, this type of kiln saves considerable power that would be con sumed in grinding.
The following reports illustrate clearly the differ ence in the nature of the rock obtained from different localities.
The term "cement," as used in building operations, signifies a compound of lime and other substances that hardens under water or in contact with water. It is a mixture of lime, or lime and magnesia, with clay, or silicia, or both, It differs from common quicklime, in that it does not slake, expand or crumble, nor give off heat when wet, but chemically combines with part of the water to form a firm, solid rock.
Portland -cement may be defined as a compound consisting chiefly of silicates and aluminates of lime, produced by the calcination to incipient vitrification of a mechanical mixture of calcareous and argillaceous materials. The clinker thus produced is subsequently ground to a powder. The exact chemical composition of Portland cement varies considerably, its prin cipal constituents being lime, silicia, aluminum, and oxide of iron, mixed roughly in the following pro portions : lime, 60 to 67 per cent ; silicia, 20 to 27 per cent ; alumina 6 to 10 per cent ; iron oxide 3 to 5 per cent. These four constituents, as a rule amount to about 96 per cent; the remainder con sists of small quantities of sulphuric anhydride, mag nesia, and alkalies.
There are two general methods of preparing and mixing the raw materials. One is known as the wet process, and the other as the dry or semi-wet process. The dry process is generally used because it is more economical. To illustrate this economy, the records of two factories, located within two miles of each other, are given in the following report : Dry Process. Wet Process.
Duration of test 24 hours 24 hours Type of burner used Inside mixer Inside mixer Atomizing agent Steam High pressure air Pressure of agent at burner 90 pounds 80 pounds Pressure of oil at burner 40 pounds 60 pounds Temp. of oil at burner 110° F. 100° F.
Cement in metric tons 43 tons 31 tons Equiv. in bbl. of 380 240 bbl. 180 bbl.
Oil used per ton 58 gals. 70 gals.