Manufacturing Processes - An Overview

Portland cements are hydraulic cements composed primarily of hydraulic calcium silicates. Hydraulic cements set and harden by reacting chemically with water. During this reaction, called hydration, cement combines with water to form a stonelike mass. When the paste (cement and water) is added to aggregates (sand and gravel, crushed stone, or other granular material) it acts as an adhesive and binds the aggregates together to form concrete, the world's most versatile and most used construction material.

Although there are several variations of commercially manufactured portland cement, they each share many of the same basic raw materials and chemical components. Limestone, clay and sand are the most common raw materials, representing calcium, silica, alumina and iron components. Calcium is derived from limestone, marl or chalk, while silica, alumina and iron come from the sands, clays and iron ore sources. Other raw materials may include shale, shells and industrial byproducts such as mill scale.

The modern manufacturing processes required to produce portland cement are numerous and complex, requiring enormous equipment and machinery, tremendous heat and energy consumption and sophisticated computer and instrumentation technology. Throughout the process, each step is carefully monitored and tested to ensure that the finished product meets our quality specifications and that the environmental impact of our operation complies with governmental regulations.

Throughout North America and in most industrialized countries, two different manufacturing processes have become prevalent in the cement industry - "Wet Process" and "Dry Process".

In most respects, these processes are the same, or very similar. However, "Wet Process" technology is older and begins with the raw materials being ground and mixed with water, in a thick, liquid "slurry". In "Dry Process" technology, the crushed limestone and raw materials are ground and mixed together without the addition of water.

Regardless of whether a cement plant is engineered for wet or dry processing, the raw material mix is pumped or conveyed to a huge rotating furnace, known as the kiln. Essentially the heart and soul of the cement manufacturing process, the kiln is a horizontal steel cylinder, lined with firebrick and sloped slightly downward, from the raw material feed to the fuel feed supply and burner pipe. Typically, kilns will rotate at one to three revolutions per minute, heating the raw material mix to a red-hot, 2,500-plus degree molten form that emerges from the lower end of the kiln in the form of very hot, marble-sized chunks known as "clinker". Once out of the kiln, the clinker goes through a cooling process. When cool, the clinker is ready to pass through a series of grinding and milling processes that result in the gray powder we know as cement.

Modern cement plants featuring dry process technology are usually easy to recognize, for their shorter kilns and vertical pre-heating towers that serve to efficiently heat the raw materials in cyclone chambers before entering the kiln. Wet process plants, by contrast, feature longer kilns and large slurry tanks near the upper end of the kiln.

The Kiln

A cement kiln is a big rotating oven, typically 12 feet in diameter and 160 to 500 feet long depending upon the process used. It can process up to 150 tons of raw material an hour while burning 12 tons of fuel an hour.

Cement is the key ingredient of concrete, which in turn is a key ingredient of the world's infrastructure - as well as your home, your office and your kids' playgrounds.

Raw materials - limestone, clay and sand - are fed to the kiln here.

Fossil fuels - coal, oil or natural gas - are fed to the kiln here. Liquid waste-derived fuel is also fed to the kiln at this point.

As the raw materials move through the inclined, rotating kiln, they're heated to extremely high temperatures - 2,700 degrees Fahrenheit in this zone - and chemical reactions occur. The molecules become unglued and recombine to form new compounds called "clinker."

When solid rather than liquid waste-derived fuel is used, it's dropped into the kiln here without being removed from the containers that are used to transport it.*

These containers disintegrate and combine with the raw materials to become part of the clinker.

The waste-derived fuel breaks apart chemically, too. Organic compounds are destroyed and inorganic components bond with the raw materials.

The clinker leaves the kiln here to be cooled, mixed with gypsum, ground into a fine powder (cement), and then trucked to the customer.
Gases produced by the process move in this direction, where they're used to preheat the raw materials.

Powdered limestone (90% of the raw material) scrubs the hot gases as they pass through the preheating section, helping neutralize acid gases.

Kiln dust (partially processed raw material) is collected here and either fed back into the kiln with the raw materials, recycled in other processes or disposed of.

Common gases which come mainly from the raw materials used in this process include carbon dioxide, nitrogen and water vapor. Pollution control equipment is used to monitor and minimize the amount of emissions that result from the burning process.

Emissions of pollutants (such as sulfur dioxide and nitrogen oxides) can actually be lower when waste-derived fuels are used.

*U.S. Patent 4850290. Patent held jointly by Ash Grove Cement Company and Cadence Chemical Resources, Inc.