The use of a mixture to bind materials—called “mortar”—to hold rocks together in a building or wall was developed in the Stone Age, and, by 1811, burnt lime had been used as a cement for more than 2,000 years. In that year, Joseph Aspdin, an English mason, burned a mixture of lime and clay and obtained a better cement, which he called portland cement, because it resembled rocks found at Portland, England. It proved to be an excellent material for making concrete for building pathways, streets, and floors in domestic and commercial buildings. Aspdin obtained a patent in 1824. He used a bottle kiln with a capacity of 90 barrels to produce the cement, but the burn took several days to complete so the production rate of finished cement was very low. In 1848, his son William Aspdin built a kiln in which sintering occurred, and this was the start of the production of portland cement as it is known today.
The demand for portland cement grew rapidly, requiring better kilns to increase production. In 1864, the continuous-operation annular kiln, based on kilns used for making bricks, was introduced. Annular kilns were horizontal cylinders into which the material to be treated was fed continuously to the kiln. As the material flowed through the kiln, it was burned and then discharged. To improve the burning of the material in the kiln and improve the flow rate, the annular kiln was made to rotate at a few revolutions per minute. Lifting the material at a very slow speed dropped it through the flame from the burner. Rotating the kiln gave it the name rotary kiln, which was patented in 1877 and is still the major tool for producing portland cement today. The bottle kiln was adapted in the 1880s to continuous operation as a shaft kiln.
In 1898, F. H. Lewis wrote that the invention of the rotary cement kiln could be compared with the invention of the Bessemer process for steel production in terms of higher output and decreasing cost (Lewis 1898). He compared the production rates of the kilns in barrels per day:
■ Intermittent bottle kiln (Aspdin’s type): 15-30
■ Continuous shaft kiln: 40-80
■ Early rotary kiln: 120-180
But better kilns were not enough. The clinker still had to be ground to -100 pm in efficient fine-grinding mills. Making portland cement involved two stages of fine grinding. In the first stage, cement raw material containing 85%-90% limestone and 10%- 15% clay and minor minerals was ground either wet or dry to a fineness of about 80% passing 150-200 mesh to make kiln feed. In the second stage, the kiln product—cement clinker—was ground dry to the fineness that, when mixed with water, aggregates, and gypsum and allowed to set, formed a hard cement.
Portland cement is ground to a fineness necessary to obtain its specified strength in a specific length of time. The fineness is defined by the Blaine surface area. For type 1 cement, a Blaine surface area of 3,060 cm2/g is required to obtain full strength in 28 days, and for type 3 high-early-strength cement, a Blaine surface area of 4,950 cm2/g is required to obtain full strength in 7 days.
In some parts of the world cements of various colors are made. The coloring agent can either be added to the finishing mill feed to obtain accurate blending in the grinding mill or can be added to the concrete mixer with the sand and aggregates. To prevent discoloration from the fine iron from abraded grinding balls and mill liners, either natural or manufactured pebbles are used as grinding media instead of iron or steel balls.
Cement clinker is always ground dry and must be kept dry until mixed with water to form concrete. The grindability (work index) of cement clinker is relatively constant even from different plants. Cement clinker stored over the winter in cold climates requires less energy for grinding than fresh clinker, particularly when the sides of the storage area are open. The change in power required is probably due to the clinker cracking during the cold weather with continued thawing and freezing. Stored clinker when ground, however, makes poorer quality cement. In some cement plants located in cold climates, clinker is made and stored at the start of winter when there is a much slower demand for cement, and the kilns are then shut down for the winter. In the spring clinker is available for grinding until the kilns are operating after maintenance and start-up time.
Cement kiln feed can be ground wet or dry. With the increase in demand for portland cement, the emphasis has been to reduce operating costs and the trend has been to dry grinding. Wet kiln feed contains 20%-30% by weight free water that has to be driven off in the kiln, which increases the fuel consumption in the kiln. Wet grinding is usually done in two-compartment, open-circuit mills to eliminate the water that is added to the classifiers if closed circuits are used. The grinding media in the first compartment can either be large balls or grinding rods. The use of rods in the first compartment was developed during the 1960s to give a more suitable feed for the open-circuit, fine-grinding, ball-milling compartment. It spread to other applications of multiple stages of wet grinding, such as grinding bauxite in a caustic solution to be fed to an alumina process plant.
Cement kiln feed is much less abrasive than cement clinker, which is the kiln product. With the use of wear-resistant materials in vertical-roller-mill grinding chambers and integral air classifiers from which the oversize is returned to the mill, vertical roller mills are becoming popular machines for dry-grinding cement kiln feed.
In the late 1950s and 1960s, with the introduction of single-stage ball mills for clinker grinding and the change to dry-grinding ball mills for cement kiln feed, the use of the same size ball mills for both purposes entered the picture. Often two ball mills of the same size as three clinker-grinding mills would be installed. The air separators would be the same size but the dry mills grinding cement raw material would be modified so that hot air would dry the ball mill feed before it entered the ball mill. Limestone is considerably less abrasive than cement clinker so ball life and liner wear in raw material grinding mills was very low. The use of ball mills for both clinker grinding and raw grinding decreased as the diameter of larger ball mills became too large for shipping to cement plants. Multiple — compartment mills capable of drawing more power than the same diameter, shorter length, single-compartment mills returned.
The need for more energy for grinding clinker and the limits on the size of mills that cement plants wanted to install led the way to the installation of high-pressure, doubleroll crushers to crush cement clinker to ball-mill feed size. With better wearing parts and the internal classifiers, the same opportunity may be available to vertical roller mills.
The dry-grinding closed circuit used for grinding cement raw material caught the attention of the steel industry. A few steel mills started buying on the open market iron ore concentrates and high-grade iron ores that were too fine for direct feed to blast furnaces. They built pelletizing plants to make pellets from mixtures of concentrates and high-grade fines to feed to their blast furnaces. The experience of the cement industry in using dry-grinding closed circuits for making cement kiln feed was applied to dry-grinding iron ore concentrates and fine ore to pelletizing feed, which was about 80% passing 325 mesh.
The factors for converting wet-grinding energy-calculated from grindability tests to dry grinding, from closed-circuit to open-circuit grinding, and comparing grinding media and liner wear between wet and dry grinding-were developed from the grinding of cement raw material. Limestone, the main ingredient in portland cement, does not vary widely between plants and became the base for making these comparisons.
The cement industry’s important contributions to fine-grinding technology were
■ Establishing the tumbling mill as the most efficient fine-grinding machine available, an industry position that continued for more than 100 years
■ Introducing the concept of closed grinding circuits in which dry mills were used in combination with air classifiers
The fineness of grind for the feed to cement kilns is about the same for all cement plants, as is the chemistry of the feed, with the base ingredient being limestone. The fineness of the grind for cement clinker is the surface area that meets the specifications for strength and days to set. Cement clinker is the product from the cement kiln and is, within narrow limits, uniform in grindability.