Grinding.docx

GRINDING Mill Speed Critical Speed = 42.3/sqrt(diameter) % Critical speed < 70 – “slow” >75% – fast 70-75% – “normal

Mill L/D and Chamber Lengths L/D Kurt Breitschmid from ??? report: The mill with a L/D ratio of 2,9 [-] is rather short for grinding cement with a fineness >3’500 [cm2/g] according to Blaine.

Lafarge: Raw Mills:

L/D = 1.5 – 3.2

Cement Mills L/D = 2.8 – 3.2 HGRS: Typical L/D is 3.2.

Chamber Lengths Lafarge: For blaine 3000, chamber 1

: 32-34%

For blaine 4000, chamber 1

: 26-28%.

For Raw Mill, chamber 1

: 35-45%

FLS Combidan Design Philosophy

FLS philosophy is of long 1st chamber and short 2nd chamber. In the second chamber there is a narrow range of ball charge and so there is no possiblity for classifying the charge. Therefore non-classifying (dragpeb) style liners are used. These liners should NOT be installed in a long second chamber or where there is a wide range of ball sizes. The maximum ball size that can be used with these liners is 50mm. FLS also supply traditional classifying liners on request. According to FLS, the corrugated 2nd chamber liner provides the necessary lifting and also prevents slippage between liners and balls. Also maximises power uptake due to the thin profile (+2.7% kW compared to traditional classifying liner for a 4.6m diameter mill) FLS mills have large centre openings. They allow the ball charge to be higher than the centre screen level when the mill is at stop because when the mill is running, the ball make a banana shape around the centre screen, therefore the screen is not damaged. Dear Mr Ortega,

For closed circuit mills grinding OPC as well as CEM II type products, the FLSmidth default CII media charge is 40% 25 mm, 40% 20 mm and 20% 15mm (~38 m^2/t). This charge is combined with a relatively low design circulation factor, reflecting that the rather fine media charge works with good grinding efficiency up to ~2000-2200 Blaine at the mill exit. This also means that we would use the same design charge for say 12% R45 µm and 5% R45 µm, as the design circulation factor would be adjusted for maintainng an approximately unchanged fineness at the mill outlet. A coarser CII charge wil have lower grinding efficiency and require a higher design circulation factor. This will give a coarser product at the mill outlet and thus reduce dry coating. The balance between lower grinding efficiency and reduced coating will depend both on the material and the Length/Diameter ratio of the mill.

Hoping that you will find my remarks useful, Ole S. Rasmussen FLSmidth A/S Process Design Vigerslev alle 77 DK-2500 Valby Phone +45 3618 2356 Fax +45 3617 1091

Quality criteria in mill and at mill exit CEMENT Ball Mill At Intermediate Diaphragm

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0% water at the intermediate diaphragm. 15 – 25% R0.5 mm (Old Holderbank Manual) HGRS / LAFARGE <5% R2mm at the end of chamber 1 is the general fineness target. 

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OR (LAFARGE) 86-92% passing 1 mm, 80-90% passing 0.6 mm, 75-85% passing 0.5mm Another is <50% R90mm sieve (this is also what we use for the grindability test).

Inlet 2nd Chamber 

<5% R1.2mm at inlet to the second chamber is necessary for a ball charge of 30mm.

At Discharge Diaphragm 

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<0.35% water in the mill exit product since values greater than this may lead to hydration à reduction in strength. Keep dew point under 60°C and there will be no hydration ?? Holderbank: 15-25% R90, max 5% R200 Lafarge: <5% R0.5mm, <30% R0.2mm before the discharge diaphragm. Blaine at the end of C2 is usually half that of the final blaine, or 40-60% for typical circulating loads.

RAW ball mill   

<5% R 4mm at intermediate diaphragm (Lafarge), <5% R 2mm (Hanspeter). 50-60% R90 at the mill discharge (Hanspeter), 30% (Karl-Heinz). Typical raw meal blaine fineness 4200 cm²/g (Karl-Heinz). Isn’t it much lower??

Ball charge filling level LAFARGE Recommended Ball Charge Filling Levels

Relationship Mill kWh/t and Ball Charge filling level 1. Austin, Klimpel and Luckie – „Process Engineering of Size Reduction: Ball Milling“. A. U = volume of material / empty volume between balls B. X-axis Ballcharge filling level C. Y-axis specific electrical energy consumption of mill kWh/t mill.

1. Energy Input for Cement Grinding, von H.-G. Ellerbrock and B. Schiller, Dusseldorf, ZKG – Nr. 2/1988)

2. Review of 11 mills showed the following results. They used kWh/t of mill throughput as a bench mark given that they were grinding to a mill exit of 1800 cm2/g. All mills were running at 55% to 75% of critical speed.

Practical results: Azergues. 100% > 72% of the charge (= 30% filling à 21.6%). Throughput 100% >80%. KWh/t mill 100% > 91%.

Relationship Throughput and ball charge filling Level 1. Austin, Klimpel and Luckie – „Process Engineering of Size Reduction: Ball Milling“.

Conclusions:  

40% filling gives maximum production. 40% gives 3.8% more production than 30% if the material level is optimised.

Lafarge best practice: in a non-sold out market, it is typical to maintain the 1st level charge at 26-28% filling in order to optimise kWh/t without penalising production too heavily. The second chamber is usually 2% higher than the 1st chamber level i.e. 28-30%. Loading can be reduced to 22% filling for OPTIMUM kWh/t if production really isn’t needed. This is a STRICT MINIMUM due to increased likelihood of damage on shell liners in C2. For high market demand, increase the filling level in the C1 to 32-34%, C2 34-26% according to:    

Mill installed power Stress on gearing Stress on mill shell Drying capacity

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Separator efficiency Other constraints

Ball level in the inlet trunnion max 50-75 mm above the level. When filling levels in C2 are >34%, the effect of classifying liners in often lost.

Material filling level Relationship Crushing Efficiency and Material Filling Level Optimimum filling level occurs between 0.6 and 1.1. This is independent of the amount of balls used. BUT: SEE Austin, Klimpel and Luckie which suggest the opposite.

Ball Charge Classification Ball charge classification works best in small mills. (stéphane)

Swelling of charge / Reverse Classification Robert’s Experiences:

1. Joliette: 2 different liners used in a monochamber mill. At this point, the ball charge size distribution is mixed 2. La Réunion. The plant used 2 very similar classifying liners of the same dimensions and wear profile. Nonetheless, where the 2 liner types met, there was a strict line defining the loss of the classifying action 3. ?? Can’t remember plant: The mounting of the classifying liners was slightly wrong – there was a 2 cm gap between liners in the middle of the chamber, which was filled with a ring of 20 mm balls. At this point, as strict disruption of the classifying effect was observed 4. Many mills: where unground clinker particles get through to the 2nd chamber, they cause localised swelling of the charge. Often, the smaller balls then migrate to the middle of the chamber leading to swollen mounds in the middle of the chamber.

Others’ Experiences Hanspeter example: Germany? During performance test material filling level was high and ball charge reverse classified. Reduced feed and filling level and after 1 day the ball charge was reclassified.

LAFARGE: Reverse classification occurs when particles greater than 1/3 the diameter of the void space between balls are NOT crushed immediately. The reasons are:  

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Feed granulometry or hardness too much for ball charge In the first chamber, inefficient crushing due to ball charge too small or liners too worn Ball charge too large in the 1st chamber combined with diaphragm slots too large à large particles get into the 2nd

This problem can be detected by:

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An overall filling level more than 3% greater than the calculated filling level  In C1 often the balls are visible but the overall level is more than 3% above the calculated value. Alternatively the first part of the C1 might be swollen (especially with high returns load) and the zones behind are empty because the retention is greater in the swollen area. In such cases there may be an inverse classification of the charge (see next point) Reverse classification in 1st or 2nd In this case the material is carried by gravity down the “slope” along the length of the compartment. In addition it is noticed that the balls in this area migrate down the slope too and they may stop in the middle or the end of the compartment. Often it is the BIGGER BALLS THAT MIGRATE à reverse classification in the C1 and C2. High variations in power draw at the mill motor

Residence Time Open Circuit:

>=12 minutes

Closed Circuit:

>=5 minutes

Test using fluorosceine, mixed in a bag with some clinker, then added to the feed belt. Sample at mill exit every 30 seconds for a period of ~25 minutes after adding the clinke+fluorosceine. Need specialised measuring equipment in lab.

Ball Coating Ball and liner coating can occur in raw as well as finish grinding. Ball coating can be a result of the following conditions: 1. 2. 3. 4. 5.

Inadequate Grinding Aid Poor Mill Ventilation Mill Overheating Too Much Moisture Input Mill Overloading

Adequate ventilation will help alleviate persistent problems. Mill ventilation can serve two purposes to reduce ball coating in mill temperature problem situations. It can take heat away from the mill as it is created by the grinding, and it can take water vapor away from introduction with the material or from water sprays which provide additional cooling.

The present theory concerning ball coating is that as the particles of feed shatter under the impact of the ball charge, the surface equilibrium becomes unbalanced. This unbalanced condition causes an attraction between the individual particles of feed and between the feed particles and balls resulting in ball and liner coating, material agglomeration in the mill and “pack set”. Ball and liner coating cushions the impact of balls, and in severe cases can affect lift. Material agglomeration affects grinding as well as separation.

Pack set is that condition where bulk cement, after being compacted by vibration, requires considerable mechanical effort to start initial flow. Actually, pack set bears much the same relationship with cement flowability, as starting friction bears to moving friction. Pack set, like starting friction, requires more force to start material flow than that required to keep the material flowing. Pack set has become a serious problem in recent years because of the cement industry’s use of large diameter mills which causes a higher material surface unbalance and the increased transportation of cement in bulk quantities.

Grinding Aids The addition of a grinding aid spreads the feed particles farther apart, thus exposing more surface area to the grinding media, resulting in increased production. The grinding aid adheres to the individual particles, thereby restoring the particles to a balanced state, thus reducing agglomeration, ball coating and pack set.

The quantity of grinding aid used in a particular mill is dependent on the size of the mill, feed size, product size, type of clinker being ground, etc. For instance, in large diameter ball mills, the impact force of the grinding media is so great, that a high material surface unbalance prevails in the mill when grinding all types of clinker, thus requiring a grinding aid at all times. The higher the fineness, the more grinding aid that must be used to restore the feed particles to a balanced state. Normally, grinding aids are added in a 15% concentrated solution (1:7 grinding aid to water – helps the dispersal of grinding aid in mill), at approximately 2-2.5 Lb./Ton for medium fineness and 3-4 Lb./Ton for high fineness cement. (Check with your supplier).

Willi Suter Presentation Price 1300 – 2000 $/t Dosage rate 200-400 g/t 0.26 – 0.8 $/t clinker.

Effect of grinding aid on kWh/t 

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Reduces coating onliners Reduces blaine for the same residue 45mm Enhances flowability, reduces agglomeration > better separation, less bypass

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Gives quality impact on aggregates in concrete as well as cement.

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Rochefort €150,000/ year on grinding aid. Also useful for slag. Also work in Vertical Roller Mills. Good ventilation helps dispersion. Typical dosing range 100-300 g/t (pure), optimal 180-260 g/t. Threshold value 150-200 g/t

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Typically 300-400 for a cement 3000 to 4000 blaine, 350-400 for higher blaines.

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Effect of ground limestone important?? XEU242P Never go below 350 g/t.

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Effect of grinding aid on product quality   

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Depends according to the grinding aid and cements concerned Flowability is improved à improvements or problems for transport and packing May help reduce the water requirement by acting as plasticisers. The reduction in water demand consequently leads to a higher compressive strength. Mortar and concrete may have an increased tendency to bleeding.

Spitzers (Unground Clinker) Spitzers can cause the production of superfines which in turn affects blaine. See Lafarge high blaine, high rejects problem. Note that hydrated clinker e.g. after long period of storage outside, can cause a large number of hydrated superfines which add nothing to strength development.

GRINDING TOOLS Ball Charge The reasons for the optimum are as follows. If the balls are too small then they don’t have the inertia to properly nip the material and break it. For example; grits (or spitzers) in the second compartment, the cascading small balls don’t hit with enough force to fracture the grains so they are worn smooth at a slow rate and retained in the mill, causing material transport problems. If the balls are bigger than the optimum for the feed size then the lower number of contact (nip) points reduces the specific breakage rate. Note that if a particle size is smaller than either the optimum for both the smaller or larger ball the specific breakage

rate of the smaller ball is superior, this is why finer balls in the second compartment work well. Relation ball size to particle size. For clinker:

Ball weights sizes and densities Ball weight = [diameter (mm)]3 / 250

LAFARGE Ball Size

Density

3.5 – 2.5 inch

4550 kg/m3

2.5 – 3/4 inch

4650 kg/m3

HOLCIM Ball Size

Density

90-60 mm

4.4 t/m³

50-30 mm

4.6 t/m³

30-20mm

4.7

1st chamber: 4.4, 2nd chamber 4.65 used by Holcim

WARNING: Depending on the source foundry bulk density values have been known to vary considerably.

Ball Charge Specific Surface = 785 / d (mm) m2/t

Ball Charge Condition Stefan Evers and Stephane: There is no contractual specification for the condition of ball charge There is a general contractual specification that <3% of charge breaks after being in the mill.

Example of delivered ball charge in La Réunion from a South African foundry (license(?) Maggotteaux.):    

Mixed 17/20mm balls Recycled drums 60mm with lumps and seam Small balls still with mould defects

Monochamber and Raw Mill Ball Charge Design

LAFARGE: Polysius model suits these mills well: D = 9.6e^(-0.13x) where x is the effective mill length. BALL WEIGHTS AND SIZES (New Grinding Media) High Chrome Cast Iron Grinding Balls

Data supplied by ME International (1990) Data based on steel density of 7559 kg/m3 or 471 lbs/ft3

WARNING: values may vary from supplier to supplier. The above table should be considered as approximate values. Sampling is recommended for bulk density and porosity index calculations.

1st Chamber Ball Charge Design Biggest ball size – bond formula 100mm balls 

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The use of 100mm balls is possible, but should be avoided as wear and liner damages will increase. In this case, the usage of 60mm balls should be reduced to a minimum. Big balls climb to the highest position à increased wear and breakage of liners and other balls. Bigger balls means less balls which means less contacts which means less crushing

90 mm

3 kg/ball

100mm

4 kg/ball

110mm

5.5 kg/ball

Raw mill

For the RM it is considered to have 40% – 45% of the total mill power consumption in the first compartment. For raw mills it is most common to use a ball charge with the coarse grading (up to 50% of 90 mm balls) – see table below for “coarse” grading.

Cement Mill The ball charge tendency in the 1st compartment is to use the coarser of the gradings available. When producing high Blaine Cement it is the objective to use less tonnage. Achieve nearer 8-9 kWh/t at the target mill output instead to go for a more fine ball charge (??)

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Standard design is adapted for 3200 cm²/g. Slegten Ball charge is same as Polysius for the 1st chamber, and finer for the second chamber. Smallest size = 2x diaphragm slot width Standard optimisation is to reduce the power drawn in the first chamber by liner and ball selection (coarser / finer)

Lafarge Design vs. Holcim

Using a pre grinding circuit the maximum ball size could be as low as 60 mm. These cases are very specific and can not be published as a common solution. Some plants use on 90, 80, 70 mm balls in 1st chamber. E.e. San Sebastien.

2nd Chamber Ball Charge Design Discussion IF THE SEPARATOR IS OVERLOADED, USE A FINER BALL CHARGE SO THAT THERE WILL BE LESS CIRCULATING LOAD AND THEREFORE LESS SEPARATOR BYPASS.

Transition zone charge ONLY FUNCTIONS WITH A CLASSIFIED CHARGE

The purpose of the transition zone is to grind the oversize that manages to get by the partition which is too large for the main second compartment charge to grind. Usually it is composed of 2, (sometimes 3 sizes). In metric, we usually pick 50 and 40 mm Ø’s. In U.S. units we pick 2″ and 1.5″ Ø’s. The transition zone constitutes the first part of the second compartment. In any case, for each size in the transition zone we extend the value (N) and the use the exact same number of balls for each size. Back-calculating we can then determine the weight and the percentage that represents. Note that transition zone design can only be used effectively if there is a good classifying action from the liners. Most people also get a little nervous with such a small tonnage in the transition. Thus in practice they tend to be larger than what is recommended by the Slegten model.

2nd Chamber Ball Charge – Bombled Versus Polysius and Slegten Comparing the two curves we can observe that the 1989 design has a slightly coarser ball charge in the first compartment than what the Polysius model would suggest using, (average ball weights were calculated to be 1.65 kg/ball vs. 1.5 kg/ball respectively). However it’s the reverse in the second. In fact Polysius model does not recommend any 0.75″Ø balls at all.

Comments: In general the Polysius model is simplistic – one curve fits all mills. As a rule of thumb, it suits raw mills and especially monochambers very well. Solutions will tend to have coarse second compartment ball charges. The Slegten model divides the mill ball charge into 3 parts: first compartment, transition zone and the second compartment. The approach in designing the ball charge is different in each. However, a recommendation from Slegten will vary from their own model

since they take into consideration field inspections, their own experience, and their own rules of thumb for mill idiosyncrasies. (They are not all that different from the rules discussed in “Practical Fundamentals …. Volume 1”.) This is after all how they make money on consulting. In general the Slegten model is compatible only with mills with classifying linings in the second compartment. Notes   

non-classifying liners only permit 3 ball sizes in C2. Lafarge recommended top-size 30-40 mm (2002). Standard optimisation is to maximise fine grinding using balls 25mm – 15mm.

Bombled distribution is standard for closed circuit coarse (OPC) cement: Is based on assumptions: 1) Optimum ball sizes are related to particle sizes according to the following equation: d1/d2 = (D1/D2)1.7 d

= particle diameter

D

= ball diameter 

Particle size distribution along the mill axis can be expressed as a function of % residue at particle size di:

Raw Mill It will be beneficial using a finer ball charge. The limit is given by the slot size of the partition wall.

Cement Mill With Classifying Liners SLEGTEN

The second chamber Slegten standard ball charge composition for an OPC cement varies with the cement fineness as indicated in the table: The repositions to keep the ball charge parameters constant are calculated according to Slegten criteria. As an indication the HMC standard ball charge for 3’000 [cm2/g] has 62 [g/u] and 29.4 [m2/t] as average ball weight and charge specific surface respectively. HOLCIM Closed circuit for clinker cements ( > 90 % clinker):

Cement Mill With Non-Classifying Liners Here, the grinding efficiency and the manufacturing technology for the balls do not allow one to use a large variety of ball sizes. Usually, the range in Ø, (from largest to smallest) should not exceed 0.75 inches (~20 mm). One must make a compromise then, between using very large Ø’s or very small Ø’s in the second compartment. Your choice must take into account the effect of porosity will have on throughput and the separator. If you work with too many sizes in such a compartment, then you’ll experience reverse or double reverse classification and a drop in production. It is better to work with a fairly flat gradation of ball sizes. Ball charge design for non-classifying liners

Open circuit cement mill SLEGTEN The charge composition is also adapted to the cement fineness:

HOLCIM

Calculation Procedure For A Composite Cement Composite cement equivalent OPC fineness calculation: When composite cements are ground the fineness according to Blaine is not anymore as reliable as with OPC. A fineness correction must be introduced. According to Slegten laboratory to calculate the equivalent OPC fineness of composite cement the following values: Ø

Gypsum: 125 * (actual gypsum content [%] – 5 ),

Ø

Limestone: 50 * actual limestone content in [%],

Ø

Puzolanna: 40 * actual Puzolana content in [%],

Ø

Fly ash: Fly ash fineness [cm2/g] * actual fly ahs content in [%].

must be deducted from the composite cement actual Blaine value.

Ball charge calculation:

Ø

The OPC equivalent fineness must be calculated for every cement type ground in

the mill being studied.

Ø

The ball charge will be chosen from table 1 or 2 (closed or open circuit mill)

according to the smallest OPC equivalent fineness calculated.

Ø

If the OPC equivalent fineness is, for example, 3’700 [cm2/g] interpolation of the

ball composition given on the tables must be done.

Ø

The [%] of balls ranging from 60 [mm] to 40 [mm] has to be adapted to the

operation condition of the first chamber (specific energy given to the material, state of the liner, of the ball charge and of the diaphragm): ©

If the first chamber supplies no coarse material to the second chamber, a

smaller amount of these balls than reported on the tables will be required, even the suppression of the biggest diameters. ©

On the contrary, when coarse material can be supplied to the second

chamber the amount of these balls reported on the tables might be

increased. Ø

The calculated ball charge must always be compared to the actual ball charge

because: ©

a coarser ball charge might led to a loss in grinding efficiency,

©

if the actual ball charge is finer than the calculated one the interpretation of

the process parameters of the grinding circuit will help to decide if a finer ball charge composition will allow an even better mill operation.

Ball Charge Wear Rates Net wear rate = process wear rate (loss of tonnage inside the mill during grinding) Gross wear rate = purchasing wear rate (takes into account the balls thrown away during sorting, either too small for compartment or scrap), is therefore more than the net wear.

A rough estimation can be calculated considering a global net wear rate of 1.1 [g/kWh] (Stephane). Gross wear rate 1.5 g/kWh (my estimate). Wear rate is the same for raw material and cement. Different for slurry grinding (6 times more). This value can change a lot depending on the cement composition and the abrasivity of these components. For example, for pure slag grinding the value can be doubled.

Specific ball wear costs Ball charge metallurgy

Compartment 1 Low Abrasive Material:

2.5% Carbon,17.5% Chromium

Medium Abrasive Material:

3.0%, 20%

High Abrasive Material:

3.0+%, 20%+

Compartment 2 Low / Medium Abrasive Material:

3,0% Carbon,12% Chromium

High Abrasive Material:

3.0%, 20+%

Application: The working conditions of a liner are “soft” when:

– the filling degree of balls in the chamber is above 28 %, – the level of material in the chamber is good, – the mill critical speed is below 76 %.

In any other case the working conditions are “hard”. If more than one of the abovementioned points are given during a mill operation the working conditions are very hard.

If the operating conditions are much more favourable than the one above (speed far below 76 % and/or filling degree >>28, the material level in the chamber is good) the working conditions are very soft.

Restrictions and comments:

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To avoid changes in the steel structure and therefore damage of the liner, the indicated operating temperatures (T operation) must not be exceded. Also heating up or cooling must be done according to the indicated gradient temperature. For high alloyed steels a perfect support of the plates must be granted to avoid plates breakage. For medium and high alloyed steels the hardness can be modified through specific heat treatment to reach a better compromise between wear and impact resistance.

Note: for identical liner design, alloy composition and heat treatment the lifetime reached will be the same and independent from the liner manufacturer.

Estanda: In first chamber design for maximum 3% expansion of the ball charge. (Also Lafarge) Estanda: In second chamber design for a maximum of 2% expansion in ball charge.

Liners Impact on Process LAFARGE: 8 – 10% production loss when lifting liners worn

Process Design Correct design is a function of the expansion of the charge which allows material to penetrate the charge, and the lifting angle defined as the angle between the vertical and the line between the top ball and the centre of the mill.

1st compartment – low hardness, high impact strength. Opposite for 2nd compartment (abrasion resistant).

Manganese liners – small mills, more deformation Low Chromium – big and small liners, less deformation High Chromium alloyed cast steel – Most Used, impact strength 4-10 J/cm² Classifying: every row for small mills, every two rows for medium mills, every 3 for large mills. Classifying angle increasing with mill size.

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Classifiying liners only work if ball filling level is <35% and chamber L/D is >1.5. The flat parts are corrugated, and the sloping parts are not. Therefore……… Above volume load of 34-36%, the traditional classifying liner breaks down. NOTE: The X-Class classifying effect breaks down at critical speeds >80%. Typical production rate increase from non-classified to classified à 6%. This is because the classifiying liner allows the efficient use of the existing fine ball charge, and further, allows a further increase in the amount of fine ball charge.

Grooving of liners is due to bolting (?? Info Magotteaux).

Lifetimes:

OPC 25000+ hours 1st chamber, 50000+ 2nd chamber. Raw Meal: 35000 hours 1st chamber, 70000 hours 2nd chamber. Rubber Liners – Wet grinding (steel corrosion, good abrasion resistance for balls<65mm) Conversion from non-classifying to classifying liners in short mills can reduce filling capacity by up to 5% (check!)

Liner Bolting

Diaphragms Diaphragm Design 10-20 cm2/tph

Selecting Diaphragms The type of diaphragm that suits a defined application can be found based on the following criteria: Mill type: cement mill, dry process raw mill, wet process raw mill.

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Diaphragm location:  Raw mill: transfer diaphragm at the drying chamber outlet, intermediate, peripheral discharge for a double rotator mill and outlet diaphragm.  For a cement mill: intermediate or outlet diaphragm. For the case of

cement mills with a drying chamber see raw mills.

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Mill working conditions:  ventilation air speed through the mill. Bigger or smaller than 1.2 [m/s].

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Material abrasivity. The [%] of slag or pouzzolana content on the fresh mill feed for cement grinding. The [%] of free silica at mill feed for raw material grinding.

Moisture content of the mill fresh feed. Above or below 2 [%H2O] content. The diaphragm type: the name indicated refer to Magotteaux – Slegten denomination. For details about each type of diaphragm, please, see B14.2 reg. 8 and B14.1 reg. 2. Diaphragm characteristics  The diaphragm frame can be manufactured or cast. The second one is only use in special applications where wear is very high.

Drying Compartment and 1st Chamber Needs to be shock resistant up to 400°C Needs to resist forces of ball charge in C1 Tend to be very open to allow passage of the gas. Machined plates have a greater lifetime than cast plates of the same slot width.

Intermediate: Chekka > trunpet at the exit of diaphragm lets material flow out of diaphragm outside the air stream BUT also accelerates the material in the gas stream > uses less of C2 charge. Most important design criteria are the slot width and total slot area. From Lafarge best practice papre, experience shows a 5% increase in mill throughput when installing a flow control diaphragm after a non-flow controlling diaphragm. Tangential slots – better material flow Radial slots- reduce probability of clogging

Slot plates made of up to 27% Chromium Pfeiffer design makes the material flow out of 3 and 9 O’clock positions thereby not entraining the material in the air-flow > increase in effective grinding length + reduced wear of balls near diaphragm. Weight of airfeel diaphragm in mill 5.2 metres diameter > 30 tonnes. Front plates 

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Cement slot width: Slot width normally 6mm, but may be 8mm in the case of moisture content > 2% in order to avoid clogging. Raw Meal slot width: 8mm intermediate Concentric slots lead to less blocking Thickness is 40-60mm when new, 15-30mm when worn out, Pfeiffer 8-10mm. Inside rings wear faster than outer rings.

Back Plates  

Thickness is 40-80mm when new, 15-30mm when worn out, Pfeiffer 8-10mm. Plates with slots provide less resistance to air flow, but impact negatively the material flow control.

Central Opening 

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Should be as large as possible for minimum air resistance, may be adaptable to the filling level. Grate is only a security measure against balls passing between chambers in the event of a blockage.

End diaphragm Centre discharge mill – single diaphragms, slots 2mm larger on fine compartment side.

Low wear rate. Slot width: OPC 7-10 mm Raw Meal 12-18 mm

Mill Head Tendency to use slide-shoe bearings on large mills. There are too many stresses on large mills which have trunnions. For trunnion mills, the end plate is either welded directly onto the shell or is bolted. The end plate is usually bolted to the trunnion unless it is cast as one piece if small. For proper direct welding a steel with low sulphur content is required. Conical with trunnions. Flat with shoe bearings. Mill head plates which get damaged > plates circulate in the mill and damage the liners.

Mill Shell  

Mill Shell – thicker in the middle. Trunnions afford greater bending stress (12 – 18 N/mm²) compared to 9-10 for shoe bearings. 6-8 for centre discharge mills.

Mill Ball Charge and Internals Lifetimes Ball Charge Magotteaux Rules:

OPC 2800-3200 cm2/g

lifetime 100%

OPC 4000-5000 cm2/g

-15%

Slag Cement 2800-3200 cm2/g

-20%

Slag Cement 4000-5000 cm2/g

-30%

FLS wear rates for ball charge:

Liners

Hanspeter Library Card:

The graphic below gives the distribution of the gross and net wear rate in the Group for cement grinding based on ATR 1’998. The average net wear is 14.9 [g/t] while 28.7 [g/t] for the gross wear rate. This means that about 50 [%] of the liner weight is not being used. Known cases show that within 15 – 20 [%] of the liner initial weight can be worn before the liner looses its function.

MILL VENTILATION, DEDUSTING and Filters

Pressure Drops Turbulent Flow: P2 = P1.(V2/V1)^2

Laminar Flow: P2 = P1.(V2/V1)^1

In a vertical mill: P2 = P1.(V2/V1)^1.5 (hence some laminar flow through nozzle ring)

In a bag filter: P2 = P1.(V2(v1)^1.4 (due to laminar flow through bags. Only turbulent at inlet and outlet)

Mill ventilation Objectives: 1. Primary objective is the removal of fines 2. Secondary is the removal of heat (a heat carrier) 3. Third is fluidisation or movement of material through the mill

Need >30°C above dew point in mill filter. Target 100°C at the intermediate diaphragm, and less than 90 at the mill outlet? Drying must be completed by the intermediate diaphragm.

Air temperature is generally 5°C lower than clinker temperature. If even lower à false air.

Notes from Alesd:       

FALSE AIR RAW MILL Best practice Jerez: 35% Addition of flaps at raw mill exit reduces false air 5-10% (Walter) Most important is reduction of underpressure at the mill entrance / exit (Alesd) Cost of rotary valve feeder: 52000 euros ex works for 200 t/h (Polysius) Estimated 70000 euros for 300 t/h

Mill ventilation velocities To fine dust at the mill filter can lead to to quick bag penetration with fine dust resulting in shortened bag lifetime and increased pressure drop Bag wear is not only a function of dust load but also depending on ducting and filter inlet design as well as air speed at the ilnlet of the filter. Lafarge allow up to 3 m/s above the ball charge in raw mills.

Above the Ball Charge Typically we target 1 m/s above the ball charge or greater. However this is very difficult to achieve. When determining this value watch out for inleakage around the hood and back drafting through the discharge airslide. They tend to inflate mill sweep velocity estimates. As a general rule old mills (pre 1980) tend to have small trunnions and are impossible to vent

above 1 m/s. Figure 1: Typical Mill Vent Velocities

Discharge Trunnion As a rule of thumb, the maximum vent velocity through the discharge trunnion is 23 to 25 m/s. To achieve higher usually requires such a large pressure drop that most dust collectors and fans are not designed to make this practical. To estimate the maximum venting volume flow, take the smallest available cross sectional area; deduct 40% to allow for material in the trunnion; multiply by 25 m/s.

Discharge Hood Normal maximum dust density to a filter is 200-300 g/m³. This equates to a gas speed at the exit of the drop-out box of 4-5 m/s. Older mill systems of course may not respect this, however this is the design value for new mills or for modifications. For higher velocities there is a tendency to entrain too much dust. This usually leads to a variety of operational and maintenance problems in the ducting and the dust collector. A gas speed of 4-5 m/s exit drop-out box and a mill speed of 1.5 m/s generally gives a blaine of the aspirated dust of ~3000 cm²/g (Beat Stocker). A gas speed of 2-2.5 m/s exit drop-out box (mill speed 0.5 m/s) gives a blaine of ~4000 (Beat).

Discharge Duct

Typically we find that the discharge vent duct is sized for about 10 to 15 m/s. We surmise that this is only to prevent settling. If the plant wishes to install a drop out chamber, to create a poor man’s static separator then design for 1.5 to 2 m/s for normal cement.

Static Separators on the Mill Ventilation These units work just like the two separators in series in one respect and like two separators in parallel in another. All of the feed that goes to the static separator (generated by mill sweep) is essentially double-classified. First by the static and then again in the dynamic separator (static rejects go to the elevator). The second nice thing is that the feed to the dynamic is reduced by the amount of product pulled off by the static. Thirdly, the fineness of the feed to the dynamic is reduced, this appears to improve its performance too. Fourthly, if your dynamic is a Sturtevant you will likely find the quality of the product from the grit is better than your separator fines, RosinRammler speaking.

The trick here is to get as much material as possible to the static. This means maximum mill sweep. Demopolis gets half of their mill exit material to the grit first, but it takes 1.6 m/s above the balls ( and a 400 HP fan) to do it.

If you have an H.E.S., the value of adding a static is questionable since the separator is so efficient already – it won’t know the difference. Quality may suffer too. The static’s generally produce cement superior to a Sturtevant, similar to a Raymond, and worse than an H.E.S. based on Rosin-Rammler. In short follows these rules of thumb: Pursue a static if you have the following combination; 

Sturtevant or Raymond

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potentially good mill sweep

Don’t bother if you have;   

an H.E.S. an F.L.S. mill with little trunnions other mill sweep limits (D.C.?)

As an alternative consider also a large dropout box (poor man’s static) if you have the room, but not the money. (Exshaw FM2)

Ventilation Limits  

Abrasion of mill internals High drag force leads to empty areas at entry of C1 and C2.

Mill Fan (Ball Mill) 

(BCI) Benchmark fan kW is 15-25% mill kW for sufficient RAW MILL drying. WHAT!! Alesd = 320/5200 = 6%.

Auxilliary Dedusting

Ducting Bhatpara, ducting at the exit of the 51 Dorol is about 15mm thick. Delivered with spiders inside in orderto maintain the shape of the parts. Dedusting ductwork design. 16 m/s for slag and clinker, 18 m/s for the rest.

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2000 m3/h for average point, no more than 2750 m3/h 18 m/s for ductwork airspeed – air quantity comes from air slide injection quantity etc. 1.5 m/s for hood airspeed Dedust blaine 5000 for cement transfer point Dedust blaine 3000 for clinker transfer point Flowrates of different points need to be balanced, or at least a system to balance.

Rochefort example (Beat Stocker): For 5 dedusting points

= 5 * 2000 m3/h

New filter and dedusting

= €80,000

Redo ducting and hoods

= €3000 per point

New hoods only, around

= €1000 per point

Example Stéphane cement course. Elevator dedusting located in wrong position + incorrect hooding à extraction of large quantity of fines from elevator. Then discharged from auxillary filter onto VRM feed belt à mill vibrations and mill stop.