Recent Updates

Tuesday, 20 December 2016

CLINKER GRINDING UNIT PROCESS


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
  1. Open circuit
  2. 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

  1. Conventional closed circuit system consisting of ball mill, bucket elevator and high efficiency separator.
  2. Pre - grinding with a roller press and subsequent finish grinding in a ball mill operating in either open or close circuit.
  3. 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.
  4. 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.
  5. Finish grinding with a roller press operating in closed circuit with disagglomerator and separator.
  6. Finish grinding with vertical roller mill.

Selection Criteria for the Cement Grinding System  

  1. Availability of grinding capacity to be required
  2. Physical characteristics of the material to be ground
  3. Quality of cement including fineness to be required
  4. Cost of capital investment
  5. 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 

  1. Mill ventilation
  2. Water cooling of mill - shell by spraying water over the shell using a sprinkler which lowers the temperature by 50 - 70 F
  3. 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
Do you have any suggestions? Please write in comment box.

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.

For more detailed information about the original Author of this content, please click the Author profile link below.

Author Profile

Image courtesy: Google

Also read

No comments:

Post a Comment