The Cement Manufacturing Process

 

Cement is a finely ground powder which, when mixed with water, forms a hardening paste of calcium silicate hydrates and calcium aluminate hydrates. Cement is used in mortar (to bind together bricks or stones) and concrete (bulk rock-like building material made from cement, aggregate, sand, and water). By modifying the raw material mix and the temperatures utilized in manufacturing, compositional variations can be achieved to produce cements with different properties. In the U.S., the different varieties of cement are denoted per the American Society for Testing and Materials (ASTM) Specification C-150.

Cement is produced from raw materials such as limestone, chalk, shale, clay, and sand. These raw materials are quarried, crushed, finely ground, and blended to the correct chemical composition. Small quantities of iron ore, alumina, and other minerals may be added to adjust the raw material composition. The fine raw material is fed into a large rotary kiln (cylindrical furnace) which rotates while the contents are heated to extremely high temperatures. The high temperature causes the raw material to react and form a hard nodular material called “clinker”. Clinker is cooled and ground with approximately 5 percent gypsum and other minor additives to produce Portland cement.

The heart of clinker production is the rotary kiln where the pyroprocessing stage occurs. The rotary kiln is approximately 20 to 25 feet (ft) in diameter and from 150 ft to well over 300 ft long; the kiln is set at a slight incline and rotates one to three times per minute. The kiln is most often fired at the lower end (sometimes, mid-kiln firing is used and new units incorporate preheating as well as precalcining), and the raw materials are loaded at the upper end and move toward the flame as the kiln rotates. The materials reach temperatures of 2500°F to well above 3000°F in the kiln. Rotary kilns are divided into two groups, dry-process and wet-process, depending on how the raw materials are prepared.

In wet-process kilns, raw materials are fed into the kiln as a slurry with a moisture content of 30 to 40 percent. To evaporate the water contained in the feedstock, a wet-process kiln requires additional length (in comparison to a dry kiln). Additionally, to evaporate the water contained in the slurry, a wet kiln consumes nearly 33 percent more kiln energy when compared to a dry kiln. Wet-process kilns tend to be older operations as compared to dry-processes where raw materials are fed into the process as a dry powder. There are three major variations of dry- process kilns in operation in the U.S.: long dry (LD) kilns, preheater (PH) kilns, and preheater/precalciner (PH/PC) kilns. In PH kilns and PH/PC kilns, the early stages of pyroprocessing occur before the materials enter the rotary kiln. PH and PH/PC kilns tend to have higher production capacities and greater fuel efficiency compared to other types of cement kilns.

Three important processes occur with the raw material mixture during pyroprocessing. First, all moisture is driven from the materials. Second, the calcium carbonate in limestone dissociates into CO2 and calcium oxide (free lime); this process is called calcination. Third, the lime and other minerals in the raw materials react to form calcium silicates and calcium aluminates, which are the main components of clinker. This third step is known as clinkering or sintering. The formation of clinker concludes the pyroprocessing stage.

Once the clinker is formed in the rotary kiln, it is cooled rapidly to minimize the formation of a glass phase and ensure the maximum yield of alite (tricalcium silicate) formation, an important component for the hardening properties of cement. The main cooling technologies are either the grate cooler or the tube or planetary cooler. In the grate cooler, the clinker is transported over a reciprocating grate through which air flows perpendicular to the flow of clinker. In the planetary cooler (a series of tubes surrounding the discharge end of the rotary kiln), the clinker is cooled in a counter-current air stream. Reciprocating type grate coolers can also be used to cool the clinker. The cooling air is used as secondary combustion air for the kiln to improve efficiency since the cooling air has been preheated during the process of cooling the clinker.

After cooling, the clinker can be stored in the clinker dome, silos, bins, or outside in storage piles. The material handling equipment used to transport clinker from the clinker coolers to storage and then to the finish mill is similar to that used to transport raw materials (e.g. belt conveyors, deep bucket conveyors, and bucket elevators). To produce powdered cement, the nodules of clinker are ground to the consistency of powder. Grinding of clinker, together with additions of approximately 5 percent gypsum to control the setting properties of the cement can be done in ball mills, ball mills in combination with roller presses, roller mills, or roller presses. While vertical roller mills are feasible, they have not found wide acceptance in the U.S. Coarse material is separated in a classifier that is re-circulated and returned to the mill for additional grinding to ensure a uniform surface area of the final product.

Equipment Used in Crushed Stone Processing

Major rock types processed by the crushed stone industry include limestone, granite, dolomite, traprock, sandstone, quartz, and quartzite. Minor types include calcareous marl, marble, shell, and slate. Major mineral types processed by the pulverized minerals industry, a subset of the crushed stone processing industry, include calcium carbonate, talc, and barite. Industry classifications vary considerably and, in many cases, do not reflect actual geological definitions.

Rock and crushed stone products generally are loosened by drilling and blasting and then are loaded by power shovel or front-end loader into large haul trucks that transport the material to the processing operations. Techniques used for extraction vary with the nature and location of the deposit. Processing operations may include crushing, screening, size classification, material handling and storage operations. All of these processes can be significant sources of PM and PM-10 emissions if uncontrolled.

Quarried stone normally is delivered to the processing plant by truck and is dumped into a bin. A feeder or screens separate large boulders from finer rocks that do not require primary crushing, thus reducing the load to the primary crusher. Jaw, impactor, or gyratory crushers are usually used for initial reduction. The crusher product, normally 7.5 to 30 centimeters (3 to 12 inches) in diameter, and the grizzly throughs (undersize material) are discharged onto a belt conveyor and usually are conveyed to a surge pile for temporary storage or are sold as coarse aggregates.

The stone from the surge pile is conveyed to a vibrating inclined screen called the scalping screen. This unit separates oversized rock from the smaller stone. The undersized material from the scalping screen is considered to be a product stream and is transported to a storage pile and sold as base material. The stone that is too large to pass through the top deck of the scalping screen is processed in the secondary crusher. Cone crushers are commonly used for secondary crushing (although impact crushers are sometimes used), which typically reduces material to about 2.5 to 10 centimeters (1 to 4 inches). The material (throughs) from the second level of the screen bypasses the secondary crusher because it is sufficiently small for the last crushing step. The output from the secondary crusher and the throughs from the secondary screen are transported by conveyor to the tertiary circuit, which includes a sizing screen and a tertiary crusher.

Tertiary crushing is usually performed using cone crushers or other types of impactor crushers. Oversize material from the top deck of the sizing screen is fed to the tertiary crusher. The tertiary crusher output, which is typically about 0.50 to 2.5 centimeters (3/16th to 1 inch), is returned to the sizing screen. Various product streams with different size gradations are separated in the screening operation. The products are conveyed or trucked directly to finished product bins, to open area stock piles, or to other processing systems such as washing, air separators, and screens and classifiers (for the production of manufactured sand).

Some stone crushing plants produce manufactured sand. This is a small-sized rock product with a maximum size of 0.50 centimeters (3/16th inch). Crushed stone from the tertiary sizing screen is sized in a vibrating inclined screen (fines screen) with relatively small mesh sizes.

Oversized material is processed in a cone crusher or a hammermill (fines crusher) adjusted to produce small diameter material. The output is returned to the fines screen for resizing.

In certain cases, stone washing is required to meet particulate end product specifications or demands.

For more information on equipment designed for processing crushed stone, visit Process Systems Design at http://www.processsystemsdesign.com or call (410) 861-6437.