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We were discussing “Cements and its types” and “Safety requirement in cement industries” in our previous posts. We have also seen the concept of “Cement kiln firing system” industry in our recent post. 

Today we are going to start here one very important topic i.e. clinker cooling technology. We will understand the various terminologies and engineering concepts used in cement technology with the help of this category.

Further we will go for discussion of clinker storage and grinding, cement packing and dispatch, pollution control and much more facts about cement technology in our next post in this category of cement technology.

Clinker Cooling

The hot clinker discharged from the kiln at a temperature of about 1300 C is further treated in clinker cooler. The cooling of clinker influences its structure, main composition, grindability and consequently, the quality of the resulting cement.

Clinker cooling is necessary because

1.      Hot clinker is difficult to convey.
2.      Hot clinker has a negative effect on the grinding process.
3.      Reclaimed heat content of the hot clinker is an important factor in lowering the production cost
4.      Proper cooling improves the quality of cement.

Clinker Quality

The soundness, chemical resistance and strength of cement, as well as grindability of clinker are affected by the rate of cooling applied to the clinker. Rapid cooling of clinker is done because of the following reasons.

1.      In slow cooling, MgO crystals are formed which, during hydration of cement, form Mg (OH) 2. The hydroxide thus formed occupies 39 times the volume of MgO and hence causing expansion cracks. Rapid cooling will maintain MgO in a glassy state that does not form the hydroxide.
2.      On slow cooling, conversion of C2S from beta- to the hydraulically inactive gamma form takes place with an increase in volume by 10 %. This causes the nodules to burst destroying the clinker.
3.      Slow cooling will impair the soundness of cement as large crystals of Periclase (~60u) are formed Periclase of size ~ 5 - 8  does not impair soundness.
4.      Rapid cooling increases the sulphate resistance of cement. In rapid cooling, C3A is present in glassy state and is then much less susceptible to attack by Na, Mg sulphates.
5.      Slowly cooled clinker requires higher specific power for grinding.

The main types of clinker coolers are

Direct Coolers
Grate cooler
Planetary cooler
Rotary cooler
Shaft cooler

Indirect Coolers
Gravity cooler

In all coolers, the air flows directly - in counter or cross current through the clinker and some or all of the heated air is fed as combustion air to the kiln.

Grate cooler is the most widely used cooler because of the following reasons:

1.      Highest recuperation of heat from the clinker resulting in high secondary air temperature and lowest clinker temperature
2.      Enables efficient and steady kiln plant operation
3.      Low operating cost considering the total energy consumption ( thermal and electrical)
4.      Better grindability of clinker reducing the power requirement for cement grinding.
5.      Improved cement quality.

In recent years, considerable improvements in design of grate coolers have been made to improve heat recuperation and equipment reliability. These are as follows:

1.      Thicker beds of clinker to get better heat recuperation and to protect the grates from mechanical and thermal damages
2.      Cooling air fans with higher pressure to penetrate the thicker beds.
3.      Improved grate design to distribute clinker more uniformly across the cooler - grate area.
4.      Improved mechanical components, particularly grate plates, to give less wear and tear resulting in better service.
5.      Better inter - compartments seals to keep air of higher pressure in each compartment without any unwanted leakage and achieve a better cooling and thermal energy recovery effect.

Final Cooling

Cooling with air as the sole cooling medium cannot achieve so low a temperature in the Planetary, Rotary or Shaft coolers in which all the cooling air has to be used as combustion air supplied to the kiln. With grate cooler, temperature as low as 80 C can be easily attained, whereas the corresponding temperature for planetary and rotary coolers is 150 C and for shaft cooler, it is above 300 C. Hence, in these coolers, after - cooling is employed or alternatively, supplementary cooling with water is done. 

Do you have any suggestions? Please write in comment box.

We will see other topic i.e. “Clinker storage in cement industries” in our next post in the category of Steel and cement technology. 

I am very thankful to Mr. Subrata Bhaumik, Independent cement consultant, for providing such beautiful information and contents about cement technology. 

Mr. Subrata Bhaumik has more than 50 (Fifty) Years (1965 - 2016) of Experience in Cement and other related Industry covering more than 100 assignments in cement plants with capacities ranging from 100 tpd to 10,000 tpd in India and abroad involving visit to 25 countries overseas in connection with work.

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