CEMENT GRINDING SYSTEMS

This article is a part of ASEC ACADEMY training course

INTRODUCTION

 

The final manufacturing stage at a cement plant is the grinding of cement clinker from the kiln, mixed with 4-5% gypsum and possible additives, into the final product, cement .

 

In a modern cement plant the total consumption of electrical energy is about 100 kWh/t . The cement grinding process accounts for approx. 40% if this energy consumption. Further the quality of the final cement is very dependent on the operation mode and product quality as well as consumption of electrical energy it is important that the cement grinding plant is adequately designed , and is operated properly .

 

FINENESS

The cement has to be ground fine enough to meet the requirements for strength properties specified in current standards or, more often, requirements from the customers .

 

As it takes quite a long time to determine especially the late strength, the-hour –to hour and day-to-day control of cement grinding has to be based on the cement fineness . Over longer periods, however, it is the strength figures that determines to which fineness the cement is to be ground

 

The cement fineness can be determined by various methods. So far, determination of the specific surface, for instance, according to the Blaine permeability method, has been commonly used. However it is getting more common to determine the finesse by sieve,analysis on one or more sieves or by a complete particle size, since fineness figures based on these methods usually have a better correlation to the cement strength properties

GRINDABILITY

 

Experience from laboratory grinding tests, as well as from a great number of cement mills, show that the specific power consumption required to obtain a certain fineness, for instance 3000 Blaine , varies from 25% below 50% above the average .

 

Cement clinker with a low lime saturation factor has a high content of C₂ S and generally difficult to grind. There are, however, many other factors that seem to influence the grindablity. For instance, clinker with a high content of very small pores will be easy to grind, Other factors, as crystal size, cooling velocity after burning, and age of clinker after burning, may exert some influence on the grindability.

 

So far, it is not possible to calculate clinker grindability from the chemical and mineralogical composition. The proper dimensioning of cement mill must, therefore be based on laboratory grinding test, or in the case of a new cement plant where the clinker is not available, on certain assumptions as far as grindability is concerned.

 

When grinding to OPC fineness, the specific power consumption in closed circuit grinding is about 10% lower than in open circuit grinding This applies for grinding. This applies for grinding to Blaine surface. However, when grinding to equal residues or to equal strength the savings in closed – circuit grinding are higher, as will be described later.

1.       OVERVIEW GRINDING SYSTEMS

The common cement grinding systems are summarized in Figure 1 and can be divided into three main groups:

 

Tube Mill Only

 

All grinding work is done in the tube mill and the set-up is:

 

• ¨ Tube mill in open circuit
• ¨ Tube mill in closed circuit with
• Mechanical air separator
• Rotor type separator

 

Tube Mill with Pregrinding Unit

 

Part of the grinding work is done in a pregrinding unit, which can be:

• ¨ Roller press
• ¨ Vertical shaft impact crusher
• ¨ Vertical roller mill (without separator, only grinding tool is used)

Depending on the selected system, the tube mill is in an open or closed circuit.

 

Finish Grinding System

 

The tube mill is omitted and all grinding work is done in a more efficient comminution machine. The possible systems are:

 

• ¨ Roller press with desagglomerator and rotor type separator
• ¨ Vertical roller mill with integrated rotor type separator

2.       REQUIREMENTS

The requirements of a cement grinding system are manifold and a profound investigation and evaluation of all criteria is also necessary in order to find the appropriate solution (technically and economically) for:

 

• ¨ Procuring a new grinding system
• ¨ Expanding and optimizing an existing system

 

The common criteria are summarized in Figure 2 and the most important will be further discussed.

PRODUCTION – CEMENT TYPES

 

Elements essential for the process are:

 

• ¨ Portland cement                                                                2’500 – 6’000 [cm2/g1
• ¨ Portland cement with minor additive £ 5 [%] 2’500 – 6’000 [cm2/g
• ¨ Composite cement, additive > 5 [%]   2’500 – 9’000 [cm2/g]

 

Possible additives for composite cements are: Fly ash, pozzolana, blast furnace slag, limestone, silica fume, calcined clay etc.

 

A suitable system is required to grind one or several of the above cement types to obtain the required fineness at the lowest possible energy consumption.

 

In case of Portland cement, cooling must be performed within the system.

In case of composite cements drying within the system is often needed and components with different grindabilities must be interground.

 

PRODUCTION – CEMENT PROPERTIES

 

The cement properties have an influence on:

 

• ¨ Workability of concrete
• ¨ Early and late strength

 

The achievable values are given by the market requirements and the standards.

Cement properties are mainly influenced by:

 

• ¨ Composition of cement
• ¨ Chemical and mineralogical composition of components
• ¨ Reactivity of clinker
• ¨ Particle size distribution, particle shape
• ¨ Sulphate agent:
• Distribution and fineness
• Modification and its solubility

 

The grinding system has influence on:

 

Particle Size Distribution and Shape

 

The particle size distribution is presented in the RRSB-diagram as shown in Figure 3 and characterized by:

 

• ¨ Steepness of curve n (tand)
• ¨ Limiting particle d’ (36,8 [%] R)

 

The steepness depends on the type of grinding system, as shown in Fig. 3, and can be adjusted within a certain range.

 

Without considering the chemical-mineralogical influences, the tendency often goes from wide to narrow distribution:

 

• ¨ Early strength ® decrease
• ¨ Late strength ® increase
• ¨ Workability ® worsening

The particles shape can be characterized (simplified):

 

• ¨ Round shape ®        Tube mill system
• ¨ Angular shape ®        Finish grinding system

 

Angular shape could result in a higher water requirement.

 

Sulphate Agent

 

In order to achieve the optimal workability of concrete an adaptation of clinker reactivity, particle size distribution and soluble calcium sulphate is a need. The relation can be simplified as follows:

 

Clinker reactivity	Particle size distribution	Easily soluble CaSO4
Low	Wide	Low
Low	Narrow	Medium
High	Wide	High
High	Narrow	Very high

 

Clinker reactivity low                     C3A < 8 [%], Na2Oeqv. < 0,6, SR > 2,7

Clinker reactivity high                    C3A < 11 [%], Na2Oeqv < 0,9, SR < 2,3

Particle size distribution                 wide n < 0,9

Particle size distribution                 narrow n > 1,0

Solubility of sulphate agent:

 

Modified types of sulphate agent is :

• ¨ Dehydrate fed to the mill as “gypsum”

 

• ¨ Hemihydrate Produced in the mill under high

temperature from dehydrate

• ¨ Soluble anhydrite }

 

• ¨ Natural anhydrite fed to the mill as “gypsum”

 

Developmental stage of solubility:

 

1) Natural anhydrite

2) Dehydrate

3) Soluble anhydrite / hemihydrate

 

If supply of easily soluble sulphate is short ® flash set can occur

 

If oversupply exists ® false set can occur

 

If high amounts of easily soluble sulphate is required, cement must be ground at high temperature.

The following grinding systems produce a narrow particle size distribution at a low grinding temperature:

 

• ¨ Tube mill in closed circuit with a rotor type and a bag filter for product collection.
• ¨ Finish grinding system with a roller press.
• ¨ Finish grinding system with a vertical mill

 

If clinker has a high reactivity, the system must be laid out in such a way that:

 

• ¨ Steepness of particle size distribution can be adjusted
• ¨ Cement can be ground at a high temperature

 

If clinker is very reactive, finish grinding systems should not be selected.

 

Operation – Specific Energy Consumption

 

The total specific energy consumption of a grinding system depends on a multitude of factors:

 

Type of grinding system

To get the highest influence on energy consumption, a special  type of system is used. Figure 4 gives a comparison between tube mill open circuit, closed circuit, tube mill with pregrinding unit and finish grinding system. The given [kWh/t] in relation to fineness [cm2/g] should only the level of individual systems. But values are not absolute.

Further factors are:

 

• ¨ Mill feed
• ¨ Composition
• ¨ Grindability of components
• ¨ Grain size distribution
• ¨ Moisture content of additive

 

• ¨ Technical condition

 

• ¨ Main machines
• ¨ Wear parts in general
• ¨ Mil1 internals, ball charge etc.

 

• ¨ Mill control
• ¨ Manual
• ¨ Automatic (High Level Control

 

• ¨ Use of grinding aid

3.       GRINDING SYSTEMS

 

3.1     Tube Mill Systems

 

3.1.1   Tube Mill in Open Circuit

(Fig. 5)

 

Main Elements of System

1. Feed bins with weigh feeders
2. Tube mill with two or three compartments
3. Mill filter: Electrostatic precipitator or bag filter
4. Metal trap : Vibratory screen
5. Cement cooler : Not Standard outfit

 

JUDGEMENT

 

• ¨ Production
• Suited for Portland cement with low fineness.
• Not recommended if several types must be ground.
• Product characterized by wide particle size distribution.

 

• ¨ Operation
• Specific energy consumption high for fineness 3 3’000 [cm2/g].
• Mill output up to 200 [t/h].
• Product temperature high. Cooling by mill ventilation and water injection:
• Due to limited ventilation, cement coolers are often required.
• Sensitive to variation in mill feed grindability and granulometry.

Maintenance

• A system should be available.
• Simple to maintain and proven wear parts.

 

COMMENT

The tube mill in open circuit is not recommended for new installations.

Conversion to closed circuit is often recommended for its flexibility and reduced energy consumption.

 

Advantages

• Low investment cost and low space requirement .

Uncomplicated system for grinding of mass cements with a fineness of 2800 – 3500 Cm² / g

Disadvantages

• Tendency of formation of coating and agglomerations .
• Oversize particles in the final product .
• Very high power consumption when grinding to a fineness higher than 3500 Cm²/g .
• High cement temperature due to limited air flow through the mill.
• More sensitive against variations in grindability of the material .
• Grinding of only product fineness possible.

 

 

3.1.2   Tube Mill in Closed Circuit

System with  mechanical air separator (Fig. 6 )

Main Elements

1. Feed bins with weigh feeders .
2. Tube mills with two compartments
3. Mechanical air separator .

Configurations : 1 or 2 mechanical air separators

Separator with / without dedusting .

4. Mill filter : Electrostatic precipitator or bag filter with / without

prededusting in static separator or cyclone 4a

5. Cement coolers : Not standards outfit .

 

JUDGEMENT

• Production

Suited for Portland cement, Portland cement with minor additive and composite cements ( if equipped accordingly e.g. hot gas generator ) .

Product characterized by wide to medium particles size distribution .

• Operation

Specific energy consumption medium to high depending on product fineness .

Mill output up to 200 [t/h] .

Product temperature medium. Cooling by mill ventilation and water injection : Optionally by fresh air in separator and / or cement cooler

Variations in mill feed grindability can be equalized .

• Maintenance

High availability of system

Simple the maintain and proven wear parts .

 

COMMENT

Most commonly used system for cement grinding .

Not recommended for new installations due to separator design

Main Elements

1. Feed bins with weigh feeders
2. Tube mill with two compartments .
3. Rotor type separator

Configurations : 3 rotor type separator with bag filter

3a rotor type separator  with cyclones and small dedusting bag filter

 

4. Mill filter :  Bag filter

Cement cooler:    Not standard outfit and only combination with rotor type separator with cyclones .

 

 

JUDGEMENT

 

• Production

Suited for Portland cement, Portland cement with minor additives and composite cements ( if equipped accordingly ) .

Product characterized by narrow particle size distribution .

 

• Operation

Specific energy consumption medium to low.

Mill output  > 200 [t/h] .

Product temperature medium :

• à Configuration 3 – product respectively grinding temperature low . Cooling by mill ventilation and fresh air supply to separator .
• à Configuration 3a – product temperature medium . Cooling by mill ventilation and water injection and optionally be cement cooler .

• Maintenance

High availability of system.

Simple to maintain and proven wear parts.

 

 

COMMENT

Suitable for new installations.

Possibility for conversion of old systems from open to closed or for replacement of mechanical air separators .

 

Advantages

• * Less coating because of coarser grinding and lower retention time .
• * Less oversize in final product .
• * Reasonable power consumption also for high specific surfaces .
• * Cooler grinding , thus avoiding risk of false set .
• * Quick adaptability to changed operation conditions
• * Better control facilities.
• * Possibility to produce several cement qualities with different fineness up to 6000 Cm²/g

 

Disadvantages

• High investment cost and high space requirement
• Complex system

3.2     Tube Mill Systems with Pregrinding Unit

 

3.2.1   Roller Press

The roller press in front of the tube mill takes over a part of the mill’s grinding work. Size reduction in a press is more efficient than in a tube mill.

 

Basic configurations are (Fig. 8):

 

• Roller press in pregrinding mode

 

Press independent of tube mill, can serve several mills.

Type of tube mill: Closed circuit with mechanical air separator or rotor type separator.

Power relation: Press : mill ~ 20 : 80 [%]

• Hybrid system

 

Press linked together with tube mill, can serve only one mill.

Type of tube mill: Closed circuit with mechanical air separator or rotor type separator.

Power relation: Press : mill ~ 30 : 70 [%].

 

• Two stage grinding system

 

Press equipped with separator and desagglomerator, with/without intermediate storage, press could serve several mills.

 

Type of tube: Closed or open circuit.

Power relation: Press : tube mill ~50 : 50 [%]·

 

Finish grinding system

Press equipped with separator and desagglomerator.

Cement is ground in roller press only.

 

Rule of Thumb

1 [kW] absorbed in the press replaces 2 – 2,5 [kW] in the tube mill.

If roller press is used as a retrofit for existing systems, capacity increase of

25 – 50 [%] can be achieved.

 

3.2.1.1    Hybrid / Pregrinding System

Main Elements (Figure 9)

1. Press feed bins with weigh feeders (e.g. clinker, gypsum)
2. Roller press
3. Overflow bin or dividing gate for slabs
4. Additional mill feed bin (e.g., for wet additive)
5. Tube mill with two compartments
6. Rotor type separator: With bag filter or with cyclones
7. Mill filter: Bag type
8. Overflow bin or dividing gate for material separator coarse (not required in pregrinding mode)

Operation of Roller Press

The press is fed by fresh feed, slabs and coarses from the separator. If the latter is set zero the press works as a pregrinding system with slab recirculation only (future installations will be with no or low coarse recirculation).

 

JUDGEMENT

Production

Suited for all cement types if equipped accordingly.

Product characterized by narrow particle size distribution.

• Operation
• Specific energy consumption low.
• Mill output > 200 [t/h].
• Product temperature depends on selected separator (See chapter 3.1.2: System with rotor type separator).
• System can be operated also without roller press.

 

Maintenance

• Roller press : Low wear but very sensitive to foreign matters (metal etc.)

Wear protection reconditioning rollers still in a

development phase.

• System: Availability depends on roller press.

COMMENT

Suitable for new installations.
Possibility of capacity increase in existing systems

3.2.1.2    Two Stage Grinding System

Main Elements (Figure 10)

1. Press feed bins with weigh feeders
2. Roller press
3. Rotor type separator with desagglomerator Separator, bag filter or cyclones
4. Intermediate bin with weigh feeder
5. Tube mill with one compartment
6. Mill filter: Bag type
7. Metal trap
8. Bin for ground additive with weigh feeder

JUDGEMENT

 

• Production
• Suited mainly for Portland cement and with limitations for composite cement; the ground and dried additive is fed to the tube mill or mill outlet.
• In the first stage (press with separator) the cement is ground 1’000 – 1’500 [cm2/g] below desired product fineness and in the tube mill to the end fineness.
• Product characterized by medium to narrow steep grain size distribution.

 

• Operation
• System can not be operated without roller press.
• Specific low energy consumption.
• mill output given by fineness and available press size ~150 [t/h].
• Product temperature medium. Cooling by mill ventilation and water injection. Optional with cement cooler.

 

• Maintenance
• Roller press: Low wear but very sensitive to foreign matters.
  • Wear protection reconditioning rollers still in a
  • development phase.
• System:              Availability depends on roller press.

 

COMMENT

Tube mill can also be equipped with separator.

Suitable for new installations.

Possibility of capacity increase in existing systems.

3.2.2   Vertical Shaft Impact Crusher

The vertical shaft impact crusher in front of the tube mill takes over a part of the grinding work from the first compartment of a tube mill. The material is crusher to £ 3[mm] .

 

Basic configurations are ( Figure 11)

• Vertical shaft impact crusher in open circuit.

Material passes only once through the crusher.

Crusher independent of mill, could serve several mills.

Type of tube mills  : Any type of mill

 

• Vertical shaft impact crusher in closed circuit

System works in closed circuit with vibratory screen. Due to screen sensitive against moisture in feed .

Crusher independent of mill, could serve several mills .

Type of tube mill: Any type of mill.

COMMENT

The utilization of the crusher is mainly seen as a retrofit for existing systems in order to increase grinding capacity and not for new installations .

Achievable increase :: 15 – 20 [%] .

Reduction specific energy consumption  :; 2-3 [ kWh/t] .

The application of the crusher is only sporadically .

3.2.3   Vertical Roller Mill (without Separator)

 

The vertical roller mill, as known from the raw grinding, can be used as a pregrinding machine. The basic design of the mill is as for raw grinding, however, the mill does not have the built in separator and the grinding tools are adjusted for clinker grinding. The material fed to the tube mill is £ 3 [mm].

Possible configurations are (Figure 12):

 

• Closed circuit with recirculation

Comparable with roller press and slab recirculation.

 

• Closed circuit with screen

In both cases, the vertical mill can serve one or several tube mills.

Type of tube mill: Any type of mill.

 

COMMENT

The achievable results are comparable with the roller press.

The utilization of the vertical mill is mainly seen as a retrofit for existing systems and not for new installations.

Achievable capacity increases: 25 – 50 [%].

Reduction of specific energy consumption: 4 – 5 [kWh/t].

The application of the vertical roller mill can mainly be found in the Far East (Japanese suppliers).

3.3     Finish Grinding Systems

 

3.3.1   Roller Press

(Fig. 13)

 

Main Elements

1. Feed bins with weigh feeders
2. Roller press
3. Desagglomerator
4. Options: Desagglomerator combined with separator or separate desagglomerator
5. Rotor type separator

Configurations:        Separator with bag filter

• Separator with cyclones and dedusting bag filter

6. Slab recalculation (if required by process)
7. Cement cooler: Not standard outfit

 

JUDGEMENT

• Production
• Suited for Portland cement with or without minor additives.
• Product characterized by narrow particle grain size distribution (particles angular shape instead of round shape as from tube mill).

 

• Operation
• Very low Specific energy consumption.
• Mill output limited by max. available press size and fineness 100 [t/h]
• Product temperature medium to low depending on separator setup. Cooling only by fresh air supply to separator. Optionally cement cooler for product.

 

• Maintenance

Since new developed system, long term availability figures are not assured. Reconditioning wear protection rollers are items which should be further developed.

COMMENT

• Future cement grinding systems, due to simplicity and low energy
• Not suited for all clinker types due to possible quality problems.
• Critical clinker with high C₃

Each new installation needs detailed investigations regarding product quality.

3.3.2   Vertical Roller Mill

Main Elements (Fig. 14)

1. Feed bins with weigh feeders
2. Vertical roller mill with integrated rotor type separator
3. Mill filter: Bag type
4. Auxiliary furnace: No standard outfit, only used for composite cement

 

JUDGEMENT

• Production
• Suited for Portland cement with or without minor additives and composite cements (if equipped accordingly).
• Product characterized by narrow particle size distribution (particles angular shape).

• Operation
• Very low specific energy consumption.
• Medium to low product. Cooling is by fresh air supply to mill.
• Mill output is strongly dependent on wear rate of liners table and rollers (output decrease with increasing wear rate).

 

• Maintenance
• Expensive wear parts table and liners.
• Long term availability figures are not assured.

 

COMMENT

Alternatives to roller press system in finish grinding mode.

Not suited for all clinker types due to possible quality problems.

COMPARISON

 

For selecting a new system basically the following possibilities are given:

 

1. Tube mill closed circuit with rotor type separator
2. Tube mill closed circuit with roller press
3. Hybrid pregrinding system
4. Two stage system
5. Finish grinding system
6. Roller press
7. Vertical roller mill

 

The following table summarizes important criteria in order to facilitate the selection:

4.       GRINDING AIDS

 

Grinding aids are materials which facilitate grinding in ball or tube mills, by eliminating ball coating or by dispersing the ground material. When grinding cement, the additive must also have been shown not to be harmful to the finished cement

 

Grinding aids may be added in solution, as solids to the mill feed or directly to the mill itself .

 

The addition of a fluid may be more readily controlled than the addition of a small amount of granular material . Grinding aids are metered in quantities from 0.006 to 0.08 % of the clinker weight .

 

The majority of grinding aids are substances which become strongly absorbed by the ground particles, so the surface energy requirements are satisfied and no bonds remain to attract other particles and cause agglomeration.

 

Grinding aids prevent ball coating consequently mill efficient is increased. Grinding aids reduce power costs thus paying their way. Saving of – 2.5 US cents per tone of cement, depending upon the specific surface, are reported .

 

Grinding aids also increase the efficiency of air separators by dispersing the particles so that the smaller ones are not carried along by the circulating load as a result of more fines being released as finished product .

 

Grinding aids in themselves do not have a major effect strength .

 

Although they may reduce early strengths 28 days strength are about normal

By elimination of the surface energy forces which normally cause interparticle attraction, grinding aids improve cement flowability after grinding .

ASTM standard Specification for Portland Cement permit the use of two commercial aids “TDA” and 109B”

TDA               = TRIETHANOLAMINE

109 – B            = 2 – METHYL, 2-4 PENTANE DIOL

 

In Germany, the following grinding aids are employed in the grinding of finished cements:

Amine acetate

Ethylene glycol .

Propylene Glycol

 

Grinding with propylene glycol generates approximately 800 Cm²/gram cement more surface than grinding without an aid, given the same energy consumption.

the most popular grinding aids manufacturers are Sika Grinding aids 

more information about Cedex Cement additives

 

The cost of these grinding aids in about 0.25 – 0.40 U.S. Dollars / kg. The grinding aid is added to the cement in an amount of approximately 1 kg/t

Check list “ abnormal operation”

 

INDICATION	POSSIBLE REASON	ACTION
Product:
Mill:
– sound dull	– Mill overfilled	– Lower feed rate
– sound loud	– Mill underloaded	– Increase mill feed rate
Cement temperature:
– too high	–         Clinker temp. too high –         Low mill/separator venting –         Lack of water injection –         Cement cooler failure	–         Check clinker cooling –         Adjust venting rate   –         Adjust flow rate –         Mechanical replacement –         Repair

INDICATION	POSSIBLE REASON	ACTION
Product:
Mill Output:
– too low	–         Unsuitable ball charge composition or badly worn balls –         Diaphragm unsuitable or clogged –         Unsuitable or worn liners –         Change of feed grindability –         Coating of balls   –         Adjustment of separator –         Operators error	–         Change ball charge     –         Replace or clean diaphragm –         Replace liners   –         Further investigations required –         Improve cooling/use grinding aids –         Adjustment –         Instruction/training
Mill Exhaust Pressure:
– too high	–         Inlet/outlet clogged –         Diaphragm clogged	–         Clean –         Clean diaphragm
– too low	–         Diaphragm broken –         Major false air penetration	–         Replace broken plate –         Seal sources

 

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