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Friday, 16 December 2016

CEMENT MANUFACTURING PROCESS: RAW GRINDING PLANT

We were discussing “Cements and its types” and " Cement manufacturing technology: storage and pre-blending” and “Material preparation technology in cement manufacturing” in our previous posts. We have also see the concept of cement manufacturing process and its selection in our recent post.

Today we are going to start here one very important topic i.e. Process technology of cement manufacturing: Raw grinding plant. 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 properties of burning technology, firing technology, clinker cooling 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.

Raw Grinding Plant

The raw materials in predetermined quantities and proportions are inter - ground to a desired fineness either in
1.      Wet grinding mills such as ball mills operating either in close or in open circuit. These are also called slurry mills which are used in both wet process as well as semi - wet process plants.
2.      Dry grinding mills, operating only in closed circuit, are used in dry and semi dry process cement plants. The hot kiln exit gas is used for drying the raw materials.

About 85% of the total energy expended in cement production is used for size – reduction; 75 % in grinding operation alone and 2 - 20 % of the energy supplied to the mill is used for size reduction, the remaining is lost in various activities such as friction between particles, friction between particles and mill elements, generation of heat and sound, vibration and material turbulence etc.

The efficiency of the mill depends on

1.      Optimum revolutions
2.      Amount, type and size of grinding media
3.      Correct size of grinding compartments
4.      Grindability of mill feed
5.      L / D ratio of the mill
6.      Mill system
7.      Kind of mill lining

Material Drying and Grinding

The combined drying and grinding of the raw materials is done closed circuit only and it may be in either of the following:
1.      Horizontal ball mill
2.      Vertical roller mill
3.      Roller press with or without ball mill
  
With ball mill and roller press, a High efficiency separator (HES) is used to;

 -          improve the granulometric composition through narrower particle size distribution resulting in better raw mix control and kiln feed burnability.

 -          have lesser rejects resulting in increased mill feed rate and hence considerable reduction in specific power consumption.

Ball Mill Systems

Some of the drying systems of the ball mill are as follows
1.      Air swept mill
2.      Center discharge mill with bucket elevator and separator
3.      End discharge mill with bucket elevator and separator
4.      Pre - drying in impact crusher followed by conventional ball mill
5.      Drying in air separator and ball mill simultaneously

During normal plant operations, hot exit gas as available from the kiln plant is used for raw material drying. However, the entire quantity of the gas cannot be taken into the mill itself due to the limited cross - sectional area of mill bearings.

Vertical Roller Mill Systems

VRM offers simplicity by combining the functions of secondary / tertiary crushing, drying, grinding and classifying in one compact machine. In VRM, the entire amount of hot kiln gas can be taken in. Broadly, there are two systems. 

Three fan systems where the raw meal is mainly collected in an external cyclone separator and the mill fan is placed between the cyclone separator and the main kiln - raw mill ESP.
 Two fan systems where raw meal is collected in the kiln - raw mill ESP and separate mill fan is not used. The ESP fan is used as the mill fan.

Roller Press Systems

For raw mill drying and grinding, the following two systems are in operation in the cement plants
1.      Grinding plant with roller press, flush drier and ball mill
2.      Grinding plant with roller press and flush drier

High efficiency separators are universally used in the new installations with the roller press. For increased grinding capacity, roller press with HES of higher capacity are also used in the existing raw material grinding plants and the roller press is functioning as the pre - grinding unit.

Out of the three systems, the Vertical roller mill is usually used because:
  1. It has the highest grinding capacity (> 500 t / hr.)
  2. It has the highest drying capacity (~ 12 - 15 %) and the maximum acceptable moisture content is independent of the mill size.
  3. It can take mill feed of size as large as 100 - 150 mm which increases with increase in the mill size.
  4. It has the lowest specific power consumption (14 - 16 kwh / t) as compared to the ball mill which requires 17 - 20 kwh / t.
  5. The installation cost is lower than the BM system and other alternative processes.
  6. It has high mechanical reliability
However, it has a high maintenance cost and only a moderate tolerance for sticky and abrasive materials.

Metal wear and grinding

Dry grinding requires approximately 30 % more energy / wt. of ground material. Dry mills need more mill volume than comparable wet mills. However, the increased metal wear of the grinding media and the mill liners for wet grinding, equalizes the additional energy costs of dry grinding procedures ~ 30 - 40 % of wear costs in wet grinding.

Mechano - chemical reactions

Principal metal wear in the wet grinding procedure results from dissolving of the metal in water. Water acts on freshly exposed metal surfaces of grinding balls and liners. The dissolved iron forms iron hydroxides. Also, during wet grinding, hydrogen is generated by decomposition of water. At pH < 5, the dissolution increases rapidly as the acidity if the medium increases.

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We will see other topic i.e. “Process technology of cement manufacturing: Raw Meal Homogenizing and Storage” 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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