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Roller mills can typically handle raw materials with an aggregate moisture of up to 15%. Ballmills, providing they are equipped with a drying compartment and are adequately air sweptwith hot gas (2.5-3.5M/sec above the ball charge), can handle 8%. Centre discharge mills(Double Rotator) and fully air-swept mills (5-6M/sec) can dry to 12-14% moisture. Normallydrying is effected by ducting part of the kiln exhaust gas through the mill with inlettemperatures of up to 300°C. Obviously a high drying requirement may be inconsistent withmaximizing the thermal efficiency of the kiln; generally five and six stage preheaters are onlyemployed where subsequent drying by the exhaust gas is minimal. Alternatively, but moreexpensively, dedicated hot gas generators can be used for drying in the raw mill. Drying isalso aided by heat dissipation from mill drive power which equates to approximately 1tmoisture per 1000kWh.

In the absence of pre-blend/storage of limestone and shale, or if the materials as fed to theraw mill are effectively dry, silos are appropriate for the feed materials. Where wet materialsare fed for drying in the raw mill, silos are unnecessary.

Ball mill operation is described in more detail under finish milling (Section 5.2).

Roller mills have a lower specific power consumption than ball mills. Loesche mills (Figure 3.1)comprise 2-4 conical rollers which are hydraulically pressed onto a horizontal rotating grindingtable. The roller axis is inclined at 15° to the table and, as axes of rollers and table do notintersect in the plane of the table, the relative motion involves both rolling and sliding whichenhances comminution. Feed material is directed onto the centre of the table and is thrownoutward by rotation under the rollers and into a rising air current at the periphery which isdirected by means of a louvre ring. The air sweep passes through an integral rotary classifier;fines pass out with the air current while coarse material falls back onto the feed table.

Material drying occurs in air suspension between table and classifier. Circulating load istypically 800%. Roller mills are prone to vibration due to an unstable grinding bed. A majorcause of material instability is fine, dry mill feed which can usually, be mitigated by sprayingwater directly onto the bed.

A recent innovation, the LV high efficiency classifier (ICR; 7/2003, pg 49), gives a highervelocity profile above the grinding table which effectively reduces the concentration ofsuspended material and the pressure drop across the mill. Significantly coarser classifier rejectsalso result in a more stable grinding bed. Mill throughput increases of 12-30% and systempower savings of 1.5-5kWh/t are claimed. The technology has now been successfully appliedto vertical coal mills (Nielsen; WC; 1/2002, pg 75) and to ball mill high-efficiency separators(ICR; 7/2003, pg 49). Other means of increasing roller mill capacity are described by Jung(CI; 2/2004, pg 52).

3.1 Raw MillingMore than 80% of new raw mills are vertical roller mills, though many ball millsare still in use. Roll presses are also used, particularly in upgrading existing ballmill circuits either to increase production or to reduce specific power consumption.The presses are employed as pre-grinders with or without a disagglomerator tostrip fines from the pressed cake before feeding to a ball mill.

3 Raw Milling and Blending

3.RawMilling&Blending 2/2/05 2:24 pm Page 2

The mill is started either with the rollers lifted away from the table, or with the hydro-pneumatic system at low pressure. In grinding mode, actual metal to metal contact should beprevented by limit switches or a mechanical stop and by consistent feed. Material which is notcarried upwards by the air stream falls from the table to a rejects trap, but every effort shouldbe made to exclude tramp metal which can damage the grinding surfaces. Some designsincorporate an external circulating system to elevate and return rejects to mill feed and thesedeliberately reduce the gas velocity through the peripheral inlet around the mill table from80-85M/s to 45-60M/s. The benefit of this system is a reduced mill pressure drop.

It is important that a roller mill be capable of drawing its designed power and this iscontrolled by adjustment of the roll pressure and of the height of the dam ring holdingmaterial on the table. Excessive rejects may be the result of too low a dam ring. Theinherently high static pressure in the roller mill system requires tight control of air in-leakage;no more than 10-15% in-leakage between mill inlet and dust collector should be permitted.


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Figure 3.1: Roller Mill


Loesche mills are defined by grinding table diameter (dM) and number of grinding rollers; egLM46.4 is 4.6M in diameter with 4 roller modules.

Primary roller mill controls are:� Mill drive power or mill differential pressure which control mill feed rate� inlet gas temperature (up to 600°C)� outlet gas temperature� outlet gas flow

It should be noted that the material cycle time of a roller mill is usually less than a minuteagainst several minutes for a ball mill. Thus, control response time should be appropriatelyfaster. In addition, the roller mill feed system must be capable of responding to low materialflow within no more than 45 seconds or excessive vibration will cause mill shut down.

Optimum fineness of kiln feed must be determined empirically and should be as coarse as thekiln will tolerate. Typically, raw meal should be ground to 15% +170# (88µ) and 1.5 – 2.5%+50# (300µ). A narrow particle size distribution is optimal because fines tend to increase dustloss by entrainment in exhaust gas, while coarse particles are harder to react in the kiln andresult in high free-lime, and/or excessive fuel consumption.

Specific power consumption depends upon material hardness and mill efficiency. For ball mills,the range is approximately 10kWh/t (mill drive only) for soft, chalky limestone to 25kWh/tfor hard materials. For roller mills the range may be 4.5 – 8.5kWh/T and, although ID fanpower is increased, system power is generally some 30% lower than for ball mills.

Mill product is monitored either by continuous, on-line analysis or by laboratory analysis ofhourly grab or composite samples. Computer control is employed to effect feed corrections inorder to maintain the desired optimum average composition.

Raw mills are monitored by:Production rate, tonnes/hourOperating hoursInvoluntary downtime hourskWh/tonne (mill motor)% of Connected Power (relative to mill motor rating)Product fineness, -170# & -50#Feed moisture, %Product moisture, %Limestone, % Clay/shale, % Additives, %

Additional operating parameters required periodically include:Circulating load, %Ball usage, g/t (for ball mills)Chemical analysis of +170# fraction


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The coarse (+170#) fraction may be lime-rich or, more likely, silica-rich relative to the totalsample but should show constant bias. Coarse particles should be limited to avoid burningproblems:

Separators are covered in more detail under finish milling (Section 5.3).

3.2 BlendingThere are various blending silo designs. The two major types involve turbulence (in which thematerial is tumbled about by the injection of high volume air through air-pads on the silofloor) and controlled flow (where sequenced light aeration of segments of air-pads causeslayers of material in the silo to blend by differential rates of descent within the silo).Controlled flow silos may have multiple discharge chutes, or an inverted cone over a centredischarge within which the meal is fluidised. Compressor power consumption isapproximately:

Turbulent mixing can be operated batch-wise or continuously. The former involves either afilling cycle corrected progressively to average the target mix, or a sequence of filling, mixing,sampling and analysing, correcting, remixing, and then feeding to kiln. Continuous blendinginvolves simultaneous feeding of the silo, overflow to a second silo and discharge to kiln feed.

Modern blending silos are generally of continuous, controlled flow type with each silo havingcapacity of more than 24 hours kiln feed and yielding a blending ratio of 3-5; older silos aremore like 2-3. Note that a given silo will show a lower blending efficiency if the feed is itselfconsistent. Apart from power savings, the effective capacity of a CF silo is some 20% greaterdue to the higher bulk density of meal which is not heavily aerated. The design of modernblending silos is described by Halbleib (ZKG; 10/2003, pg 44).

Blending silos should be monitored by:Blending ratio (sfeed / sproduct)Compressor kWh/tonne throughput

Blending silos are prone to internal build-up of dead material, particularly if raw meal is wetor if aeration is defective, and periodic (1-2 years) internal inspections and maintenance arenecessary. As raw meal is liable to solidify if left inactive (during a kiln shut-down forexample), blending silos may require emptying or re-circulating when not in use.

With the availability of real-time, on-line analysis of mill feed or product, it is possible tomaintain chemistry within narrow limits and modern plant designs frequently dispense withkiln feed blending.


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Silica >200µ Not more than 0.5% of KF90-200µ Not more than 1.0%> 45µ Not more than 2.0%

Calcite >125µ Not more than 5.0%

Turbulent mixing (airmerge) 1.5 - 2.5 kWh/tControlled flow, inverted cone 0.25 - 0.50

multi outlet 0.10 - 0.13(Bartholomew; ICR; 9/1995, pg 66)


3.3 Kiln FeedBoth the chemical composition and the rate of feed of raw meal to the kiln must beconsistent to avoid kiln instability and to minimize fuel consumption. Short term feedfluctuation (eg hunting of feeder control) as well as average feed rate should be monitored.

Air-suspension preheater kilns lose a fraction of kiln feed by entrainment in exhaust gas. Asthis fine fraction is usually of atypical composition, kiln feed analysis must be biased to yieldthe desired clinker composition. The dust loss, some 5-12% of kiln feed, is not usuallycollected until after the exhaust gas passes through a raw mill or dryer, so that dust catch isnot the same quantity or composition as preheater dust loss. Thus, even if the dust collectorcatch is returned directly to the kiln, it must still be compensated. Likewise, care must beexercised to minimize the chemical disturbance due to dust return.

If the kiln exhaust passes directly and continuously to dust collection, then the dust may bereturned directly to the kiln with kiln feed or, sometimes, by insufflation at the hood or at thefeed-end of the kiln which minimizes re-entrainment of the fines.

Kiln feed is monitored by:Chemical analysis on 4- or 8-hourly grab samples to determine statistical variation(see Section 6.5). Analysis is conventionally for major oxides with variation monitoredstatistically in terms of C3S or LSF.

Kiln feed should typically have an estimated standard deviation for grab samples of less than3% C3S or 1.2% LSF (Halbleib; ZKG; 10/2003, pg 44). It should be born in mind thatstandard deviation is not a perfect measure of variation as, simply applied, it does notdistinguish between a steady trend and constant fluctuation.

Kiln feed is normally conveyed by bucket elevator to the top of the preheater to minimizepower consumption. If this conveying is effected pneumatically, de-aeration is desirablebefore injection as the entraining air otherwise adds to the kiln ID fan load and may reducekiln capacity.

Although about 1.55t raw materials are required to produce 1t clinker, kiln feed to clinkerratio is typically 1.65 - 1.75 as weighed due to the loss of dust entrained with exhaust gas.This dust, however, is collected and returned. The ratio should be periodically reconciled withclinker and cement inventories and with measured dust loss in the preheater exhaust.

Kiln feed = Clinker + LoI + Bypass dust + Downcomer dust - Coal ash

where both bypass dust and downcomer dust are converted to ignited basis.

� � � � �


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The concise guide to cement manufacture

Fourth Edition January 2005

THECEMENT PLANT

OPERATIONS HANDBOOK

This important 300 page handbook

is available toall current

ICR subscribers free of charge

Philip A Alsop, PHD

CPOH Advert 2005 31/1/05 9:46 am Page 1

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