The beating heart of the cement manufacturing process – the kiln

There are around 13 million tonnes of cement produced per year in South Africa and behind each and every bag or tonnes of cement supplied, is a meticulous cement manufacturing process that would not be complete if we did not consider the work done by the kiln.

But what is a kiln in a cement plant and why is it important?

The cement rotary kiln, often known as the beating heart of the cement manufacturing process, was first developed in 1890, in Pennsylvania. This thermally insulated chamber is responsible for the pyroprocessing of what is known as hydraulic cements, which when mixed with water creates the binding material used on construction projects. 

By delving into how the cement kiln works, not only will you have a greater understanding of how the bags or tonnes of cement you are using on your project were produced, but why the cement kiln plays such a critical role in the production of that cement.

You’ll also gain a healthy dose of respect for this massive piece of equipment too, if you ever get to stand next to it.

In the 1950s and 60s, a typical cement plant would use a grate preheater with a system typically having a grate of 4m wide by 28m long and a rotary kiln of diameter 3.9 by 60m long. These systems would churn out 1 050 tons daily which was nearly tripled by new systems.

At Sephaku Cement, our 5-stage preheater kiln has a diameter of 5.4 m, and length of 74 m. This kiln has a design output of 6000 tons of clinker per day.

Located between Lichtenburg and Mahikeng in the North West Province in South Africa, our Aganang plant is capable of producing 1.9 million tons of clinker and our Delmas plant produces approximately 1.4 million tons of cement every year.

Aganang consists of a limestone open-cast quarry and an integrated cement production plant that mines the limestone with other ingredients and processes it to clinker. Approximately 45% of the clinker is blended with other raw materials to produce bagged and bulk cement. The remaining 55% is transferred to the Delmas plant for further processing into the different strength classes of cement.

Where is the kiln in the process?

Looking broadly, there is generally a 6-step process that unfolds in the production of cement. These are: 1) raw material extraction 2) grinding, proportioning and blending of raw mix 3) pre-heat phase 4) clinkering phase 5) cooling and grinding, and blending 6) packing and shipping.

While this process varies with every cement plant, there will almost always be a phase that revolves around the kiln. In the case of Portland cement the most common hydraulic cement, a rotatory kiln is used.

The cement rotatory kiln is a large rotating tube made from steel. The kiln tube slopes at angle of 1 to 4 degrees and rotates on an axis between 30 and 250 revolutions per hour allowing the raw mix to feed into it and progress slowly down towards the burn pipe.

What happens with our materials once in the kiln?

The clinkering phase of the cement manufacturing process begins by feeding the raw mix which typically comprised a ground mixture of limestone, clay, iron and alumina into the kiln. 

The raw mix, now called kiln feed, cascades through the preheater system including a pre-calciner and enters the kiln. In the kiln, the bed of material moves toward the burning zone where a reaction takes place and clinker is formed. The hot clinker then passes through a grate cooler where it is cooled and transferred to a storage facility.

The clinker which is a granular substance containing spherical particles of diameter 3-25 mm is the key agent used to bind building materials. Without this, your construction project would not be possible.

Owing to the fact that the kiln plays a critical role in converting raw mix to clinker, it becomes evident why this phase in the production process is known as the beating heart of the cement manufacturing process and why it is so important for the production of hydraulic cements.

To find out more about the different Portland cement options available at Sephaku Cement.

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