Page 45 - CAPE Green Engineering Syllabus 2016.docx_Neat

2.0 Manufacture of Portland cement

The manufacturing is simple, although high temperatures are involved. Initially chalk and clay are
reduced to particle sizes of 75micron or less. The three main processes for manufacturing cement are:

1. wet process;

2. dry process; and,

3. semi-dry process.

2.1 Wet Process

If limestone is used, it is quarried and then crushed in a number of stages. In the final grinding stage
the limestone which has been ground to a fine ‘flour’ consistency in a ball mill is mixed with clay and
water. The resultant slurry is then pumped into storage tanks to await further processing. If chalk is
used, the chalk and clay are initially broken down separately in wash mills before combining together
in specific proportions to be stored as slurry prior to transfer to the kiln.

2.2 Dry Process

In this process, the raw materials are mixed together, dried and reduced in size via grinding to a fine
powder, called ‘raw meal’. Further blending, mixing and adjustment of the proportions occur in the
blending silo using of compressed air. After which it is moved to a storage silo prior to transfer to the
kiln.

2.3 Semi-Dry Process

This process is somewhat similar to the dry process. Instead of the blended raw material being
transferred to the kiln it is transferred into a ‘granulator’ (a rotating dish) where water, approximately
12 per cent of meal weight, is added. This leads to the formation of pellets approximately 15mm in
diameter. These pellets are then baked hard using pre-heated gases from the kiln prior to transfer into
the kiln.

At this point the remaining process is the same for all three types.

The material (slurry or meal) is preheated before entering the kiln. The kiln is an inclined a rotary kiln.
These kilns can be up to 7m in diameter and as long as 230m (dependent upon the process), kilns for
the dry and semi dry processes are shorter due the smaller amount of water needed to be driven off.
The meal or slurry is fed into the upper end of the kiln where, as it moves down the kiln it is heated to
progressively higher temperatures, culminating at approximately 1450oC – 1500oC at the lower end.

3.0 Sustainability of the production of OPC

Sustainability of the cement manufacturing process is important to the well-being of our environment,
continued growth, and human development. The most energy intensive stage of the Portland cement
production is the clinker production stage. It accounts for most of the energy used and nearly all of
the greenhouse gases (GHGs) released during cement production. Kiln systems evaporate inherent
water from the raw meal and calcine the carbonate constituents during clinker pre-processing stage.

The production of one tonne of Portland cement produces about one tonne of CO2 and other GHGs.

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