We were discussing “Cements and
its types” and “Material
preparation technology in cement manufacturing” in our previous
posts. We have also seen 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. clinker grinding process in cement manufacturing unit. We will
understand the various terminologies and engineering concepts used in cement
technology with the help of this category, but first read this article “Coal grinding and drying in cement industries”.
Further we will go for discussion of clinker 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 Grinding
Clinker and gypsum, being mixed together in
predetermined proportions, are fed to the grinding mills. Chemical gypsum, if
used instead of natural gypsum, needs to be dried either by natural sun drying
or by mechanical drying, before it is used with cooled and dried clinker for
inter - grinding in mills.
In clinker grinding, gypsum being the more readily
grindable, tends to be concentrated in the finer particle size fraction of the
product. Strength development improves if cement is more closely graded,
provided the specific surface of cement is not reduced. Improvement in strength
is due to faster rate of hydration.
Grindability Index of Clinker
This varies with its mineralogical composition and
depends, above all, upon its di - calcium silicate content. Higher the content
of C2S, the more difficult it is to grind the clinker.
Fresh clinker is harder to grind than clinker which
has been stored for 2 - 3 weeks
Methods of Grinding
There are two types of grinding systems
- Open circuit
- Closed circuit
Open Circuit Mills
The cement grinding units, operating in open
circuit, are used in some plants because of the following advantages
1. Low capital investment requirement
2. Simple in operation
3. Ease of maintenance
4. Better capability to use dusty clinker.
Close Circuit Mills
The cement grinding units operating in closed
circuit with air - separators are widely used in cement industries because of
the following reasons
1. Lower specific power consumption in kwh / t of
cement ground.
2. Lower consumption of mill internals
3. Superior grinding of materials having different
grindability characteristics
4. Higher compressive strength for same fineness of
cement
There are six grinding systems available for the grinding of clinker
- Conventional closed circuit system consisting of ball mill, bucket elevator and high efficiency separator.
- Pre - grinding with a roller press and subsequent finish grinding in a ball mill operating in either open or close circuit.
- Hybrid grinding in a roller press operating in closed circuit with separator and subsequent finish in a conventional closed circuit ball mill, BE and separator.
- Semi - finish grinding with a roller press operating in closed circuit with separator and subsequent finish grinding in a conventional closed circuit ball mill with BE and separator.
- Finish grinding with a roller press operating in closed circuit with disagglomerator and separator.
- Finish grinding with vertical roller mill.
Selection Criteria for the Cement Grinding System
- Availability of grinding capacity to be required
- Physical characteristics of the material to be ground
- Quality of cement including fineness to be required
- Cost of capital investment
- Operating costs including cost of electrical energy and plant maintenance
Finish Grinding
This is one of the basic technical operations of
cement production. Technology of cement grinding is based on the following
observations
The particle size fraction between 3 - 30 u is
conducive to strength development.
The particle size fraction < 3 u contributes to
the initial strength only. It hydrates fast and after one day, it results in
highest compressive and flexural strength.
The fraction above 60 u contributes little to the
strength.
Higher fineness (> 5000 cm2 / g) has
no influence on strength, in fact, lowers it.
Grinding Aids
These are materials which facilitate grinding in
ball and tube mills, by eliminating ball coating or by dispersing the ground
material. These may be added in the form of solution or as solids to the feed
or directly to the mill.
The addition of grinding aids improves mill
efficiency and hence, reduces power costs. They do not have a major effect on
the cement strength. They may reduce the early strength, but 28 days strengths
are about normal.
Grinding Aids used in cement industry are - Amine
acetate, Ethylene glycol, Propylene glycol.
Mill Atmosphere (Generation of Heat)
Heat is generated in the grinding of clinker,
resulting in a rise of temperature which in some instances exceeds 120 c. The
water content in gypsum is driven out, slowly at first, but above 80 C, at a
rapid rate, as a result of which, gypsum is dehydrated and transformed into its
hemi - hydrate or anhydrite form.
Thus, gypsum loses its property as a retarder and on
mixing with water, quickly forms a super - saturated solution of CaSO4,
from which gypsum is precipitated, causing a stiffening of the mass, called ‘
False set’. This is, however, a temporary phenomenon and can be reversed by
further mixing.
Cooling Procedures
For best results, cooling must be done in the
grinding mill circuit, often in the air separator. Closed circuit grinding
provides better temperature control over the complete grinding circuit. Several
cooling concepts have been introduced, like
- Mill ventilation
- Water cooling of mill - shell by spraying water over the shell using a sprinkler which lowers the temperature by 50 - 70 F
- Water injection into the mill. Water is injected into the hottest part of the mill. It evaporates instantaneously, and is carried into the mill by compressed air
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We will see other topic i.e. “
Cement storage, packing and dispatch” 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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Author of this content, please click the Author profile link below.
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