Portland cement production begins with the manufacture of clinker followed by its fine grinding with gypsum and other additives to make the finished cement product.

Grinding can occur on site or at offsite grinding plants known as cement mills.

Clinker manufacture involves the quarrying, crushing, and proportioning of raw materials to produce either a raw meal for the dry and semi-dry processes or a slurry for the wet and semi-wet processes.

Once the material is prepared, the raw mix is fed into a kiln where it is heated as it moves through a number of chemical and physical processes necessary for forming the clinker.

In the kiln, the raw meal, or slurry in the wet process, is subjected to a thermal treatment process consisting of the consecutive steps of “drying/preheating”, “calcining”, and “sintering” (also known as “burning” or “clinkering”); the various reactions zones are depicted in Figure 1A.

The first drying and preheating zone, occurs in a temperature range of from <100 to 750°C. Here, residual or free water is evaporated from the raw meal feed, the clay materials begin to decompose and the bound water is removed so that they become dehydrated.

Next is the calcining zone, where materials temperatures range from 750 to 1000°C. The material is “calcined”, where the calcium carbonate (CaCOa) in the limestone is dissociated producing calcium oxide (CaO, lime) and liberating carbon dioxide (CO,) gas.

Finally, in the burning zone, calcium oxide reacts with silicates, iron, and aluminium to form dicalcium silicate, tricalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite, denoted in shorthand as CS, CaS, Ca, and C4AF respectively.

In addition, clinker nodules, typically 3 to 20 mm in diameter, are formed in a semi-solid state in the burning zone, and solidify completely on cooling, which begins in a short cooling zone within the kiln, and continues in a cooler, outside of the cement kiln.

Figure 1A – Diagram of “reaction” zones for different kiln technologies

In the clinker burning process, it is essential to maintain kiln charge temperatures in the sintering zone between 1400 and 1500°C to convert the raw meal to clinker. To reach these temperatures, flame temperatures of about 2000C are necessary.

Also, for reasons of clinker quality, excess air is required in the sintering zone to maintain oxidizing conditions. Otherwise, if insufficient oxygen is present, tetracalcium alumino ferrite does not form instead Fe,O3 is reduced to Fe0. This leads to a clinker product that produces a quick setting cement with decreased final strength.

Additionally, the presence of unburned carbon in the burning region produces a clinker with an undesirable brown colour.

To complete the production of Portland cement, the cooled clinker is ground with a small amount of gypsum or anhydrite.

Clinker can be made either in energy-intensive and small-scale vertical kilns or in more efficient, larger scale rotary kilns.

With the exception of vertical shaft kilns (VSK) still used in certain geographical areas (mainly China and India), cement clinker is predominantly burnt in rotary kilns.

For the manufacture of cement using rotary kilns, heating of the raw meal to produce cement clinker can take place in one of four different types of arrangements: the “dry”, “semi-dry”, “semi-wet”, or “wet” processes:

(a) Dry process: Dry raw meal is fed to a cyclone preheater or precalciner kiln or, in some cases, to a long dry kiln with internal chain preheater.

(b) Semi-dry process: Dry raw meal is pelletised with water and fed to a travelling grate preheater prior to the rotary kiln or in some cases, to a long kiln equipped with internal cross preheaters.

(c) Semi-wet process: Raw slurry is first dewatered in filter presses. The resulting filter cake is either extruded into pellets and fed to a travelling grate preheater or fed directly to a filter cake drier for (dry) raw meal production prior to a preheater/precalciner kiln.

(d) Wet process: The raw slurry is fed either directly to a long rotary kiln equipped with an internal drying/preheating system (conventional wet process) or to slurry drier prior to a preheater/precalciner kiln (modem wet process).