grinding technology of limestone with ball mills

7/30/2019 Grinding Technology of Limestone With Ball Mills

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The Cement Grinding Office

www.thecementgrindingoffice.com Page 1

Grinding technology of limestone with ball mills

(Marc Piccinin)

Table of contents

1. The limestone2. Different applications of the limestone powder3. Different types of grinding4. Different types of mills and circuits5. Mill internals6. Ball charge7. Ventilation8. Heat balance

9. Drying


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1. The limestone:

Calcium carbonate - CaCO3

Limestone can be:

* rich: 90-100 of CaCO3* poor

Other elements: SiO2 (quartz), Al2O3, Fe2O3

Importance of SiO2 content (free silica) for:

* wear rate - choice of alloy (balls and linings)* hardness - ball charge gradation

2. Different applications of the limestone powder:

The main applications to grind limestone are:

- For chemical industry, cosmetics, pharmaceutical industry, agriculture

=> fine grinding (less than 1% residue on 90m)

- For cement industry (to produce clinker)

=> coarse grinding (10-20% residue on 90m)

3. Different types of grinding:

The 2 types of grinding are: wet and dry process.

Wet process:

- The advantages are:

* lower specific energy* simplicity of installation

- In Open circuit:

* the finished product is the mill discharge product* to adopt mill L/D = 3.5-4 in order to have a longer retention time of the material insidethe mill

- In closed circuit:* with hydrocyclones

Dry process:

Generally adopted all over the world.At this point, we only will develop dry process circuits.


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4. Different types of mills and circcuits:

4.1. Classical ball mill circuit:

See typical flow-sheet herebelow.

Main characteristics:

- Ball mill with 2 compartments- L/D = 3- Closed circuit with bucket elevator- Dynamic separator of 1st, 2nd or 3rd generation- Filter (Bag or EP)- Granulometry of fresh feed: normally 35-40mm- Moisture of fresh feed: maximum 1%H2O

This is the circuit to grind fine product.


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4.2. Air-swept mill circuit:

See the flow-sheet herebelow.


- Ball mill with 1 or 2 compartments* 1 compartment = 1 grinding chamber* 2 compartments = 1 drying and 1 grinding chamber- L/D = 2-2.5 (short mill)- Closed circuit with pneumatic system- Static separator and cyclones for cleaning- Filter (mainly the kiln filter which is an EP filter)- Granulometry of fresh feed: 5%R30mm (residue between 30 and 40mm)- Moisture of fresh feed: maximum 8%H2O with gas kiln at 350C (15% if additionalheater-temperature 600C)Without drying chamber: 4-6%H2O- Power requirements for pneumatic conveying: 4-6 kWh/t

This is one of the circuits used in the cement plants to grind the raw material .


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This circuit is recommended in case of big moisture content and for a fineness of theproduct around 20%R90m.

4.3. Birotator circuit:



- Ball mill with 3 compartments: 1 drying and 2 grinding- L/D = more than 3- Peripheral discharge (2 feeds)- Closed circuit with static separator for gas and dynamic separator for material- With bucket elevator- Filter (mainly the kiln filter which is an EP filter)- Granulometry of fresh feed: maximum 50 to 80mm- Moisture of fresh feed: maximum 7%H2O with gas kiln at 350C (15% if additionalheater)


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4.4. Tandem circuit:



- Ball mill with 1 grinding compartment- L/D = 2-2.5

- Drying in the crusher and in the mill- Static separator for mill and crusher- Pneumatic conveying system- Filter (mainly the kiln filter which is an EP filter)- Granulometry of fresh feed: up to 80mm at the crusher- Granulometry of the mill feed: 10-12mm is an optimum- Moisture of fresh feed: maximum 5%H2O with gas kiln at 350C (10% if additionalheater)


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4.5. Compound mill circuit:



- Ball mill with 2 grinding compartments or 1 drying and 1 grinding- L/D = 3- Closed circuit with bucket elevator- Dynamic separator of 1st, 2nd or 3rd generation- Filter (mainly the EP kiln filter)- Granulometry of fresh feed: normally 30mm if drying chamber and 50mm if there are2 grinding compartments- Moisture of fresh feed: maximum 2%H2O with 350C without drying compartmentWith drying compartment, 3-4%H2O with 350C and 6-8%H2O with 600C- It is difficult to have 3 compartments (passage of gas - high pressure drop)


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5. Mill internals:

5.1. Compound mill:

This kind of mill has normally 2 compartments.

The coumpound mill is generally used to grind cement and pure limestone in somecement plants.

5.1.1. First compartment:

The first compartment is to prepare the material before to go in the second chamber.Generally, the lenght is the third of the total usefull lenght. But it can be different due tothe granulometry of the fresh feed.The target is to have at the end of the chamber: maximum 5% residue on 2.5mm.

Lining:

The lining is a lifting effect lining.The shape can be different according:* the granulometry of the limestone* the hardness of the limestone* the ball charge* the length* the speed of rotation

Ball charge:

The ball charge is depending of:* the granulometry of the limestone* the hardness of the limestone* the liningGenerally, the ball charge is a gradation of balls diameter: 90mm, 80mm, 70mm,60mm.

Intermediate diaphragm:

Very often, the first chamber is empty of material, especially near the diaphragm.By laboratory trials and industrial tests, we got the following diagram:


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This diagram shows that the optimum of efficiency is reached when the ratio weight ofmaterial/weight of balls is around 18%.Practically, it means that the material must be at the level of the balls (to fill the voidswhich represent 42%).

The diaphragm must also be designed in order to allow the ventilation of the mill (bigcenter).The slots of the grate plates must have 6mm width if the moisture content of thelimestone is lower than 1%, if this content is higher, slots than 8mm must be forecast.

5.1.2. Second compartment:

The second chamber is the finishing chamber. It means an attrition work in order toproduce fineness.

Lining and ball charge:

For that reason, the lining is a classifying lining. The biggest balls are at the beginningof the compartment where the material coming from the first chamber is coarser, andthe finest balls at the end of the chamber where the material is the finest.

Laboratory trials were realized in the past and the conclusion is that it is better toclassify the balls by dimension.

Also, finer is the ball, better is the efficiency. But a fine ball charge can only be used ifthe lining is classifying!

According to these trials, balls of 10mm diameter are the best. But it is inacceptable toput these balls in an industrial ball mill because the retention time of the material wouldbe excessive.Then balls of 15mm minimum are used in the cement mills in open circuit, balls of17mm or 20mm in closed circuit.Concerning the limestone, it is better to limit the smallest dimension at 20mm becauseproblems of coating (see hereafter).

The ball charge will consist in a gradation of balls: 60mm, 50mm, 40mm, 30mm, 25mm

and 20mm diameter.


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15% of balls 60, 50 and 40mm are used to break the last big particles coming from the1st chamber.The rest of the ball charge depends of:* the fineness of the finished product* the kind of separator

* other factors like: ventilation, temperature, compartment length

Outlet diaphragm:

The slots much be bigger than the ones of the intermediate diaphragm.

5.2. Double rotator mill:

This kind of mill is used in the raw mill section of a cement plant.Limestone is prepared in order to produce the clinker in the kiln.The fineness requested is generally between 10 and 20%R90m and 1-2%R210m.

The double rotator mill is composed of:

- one drying chamber with lifters- one transfer diaphragm which allows important ventilation- one coarse grinding chamber with a lifting lining and a ball charge with 90-80-70-60-50mm.- one peripheral discharge diaphragm.- one fine grinding chamber on the other side with a classifying lining and a ball chargeof 50-40-30-25mm.

The proportion of the lengths are generally 40% for the coarse grinding chamber and

60% for the fine grinding chamber. The volume of the drying chamber depends of theoutput of the mill and the moisture of the limestone.

5.3. Air-swept mill:

This mill has only one grinding chamber with or without drying chamber.

The configuration is the following:

- 40% of the length with a lifting lining- 60% of the rest with a classifying lining- From time to time, it is interesting to add a retaining ring between both linings.

- The ball charge is composed of: 90-80-70-60-50-40-30mm diameter balls.- The filling degree is often low in order to insure a large passage for the hot gas.

6. Ball charge of the raw mills:

For the producer, the most important is to have a certain residue on 1 or 2 sieves.For example, 15%R90m.It means that we don't need fineness (surface - Blaine) => no need to producesuperfine particles.For that, the ball charge are relatively coarse, the retention time of the material insidethe mill is very short and the circulating load of the separator is high.

There is not method to calculate the ball charge of a raw mill.All is based on the experience.


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The total ball charge is normally between 300 gr/ball and 400gr/ball average.

A typical airswept mill coarse ball charge is the following:

90mm 16% 80mm 17% 70mm 11% 60mm 13% 50mm 14% 40mm 13% 30mm 16%

A typical double rotator mill fine ball charge will be the following:

1st chamber: 80mm 20%

70mm 13% 60mm 8% 50mm 8%

2nd chamber: 50mm 11% 40mm 19% 30mm 21%

Naturally, the ball charge optimisation depends directly of a mill survey.

7. Ventilation:

7.1. Introduction:

The ventilation has 3 objectives:

- to insure the cooling of the mill and the material- to dedust the mill- to remove the fine particles from the mill

7.2. Cooling of the mill:

A good cooling of the mill and the material inside the mill is necessary to maintain aproper working temperature.The temperature of reference is the one of the material at the mill outlet.For the cement mills, one can not exceed a temperature of 105-110C according thetype of circuit.For the limestone, a temperature lower of 100C should be better.If one goes higher, we will probably observe a coating on the lining and the balls.

7.3. Problem of coating:

The coating is an agglomeration of material on the lining and/or grinding media. It isdue to the superficial cohesion forces and the forces generated by the static electricity.


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ventilation air speed in m/sec

air speed = 0.8 to 1.2 m/sec

air speed = 1.2 to 1.5 m/sec

As limestone has the particular propriety to produce a big amount of very fine particles(less than 2 microns) when grounded, the problem of coating can become crucial.

Coating is an agent of efficiency loss which can be very important (up to 30%)because:

- coating is disturbing the segregation of the ball charge in case of classifying lining.- coating is also reducing the attrition's efficiency of the small balls which are not morein contact with the material.

7.4. Dedusting of the mill:

This function seems to be evident if we want to avoid the accumulation of dust in thewhole system.

7.6. Ventilation usual values:

When we want to define the necessary ventilation in a grinding installation, it iscalculated in terms ofair velocity in the free section of the mill, i.e:

The reference values which are generally admitted are divided in two cases and arethe following:

- Open circuit:

- Closed circuit:

Remark: we can also define the ventilation in term of Nm3/kg of finished product.Normally: 0.3 to 0.45 Nm3/kg.But this method is less usual.

7.7. Air speeds into the mill:

Besides the basis figures given hereabove, it is very important to respect somevelocities in other parts of the mill in order to:

- allow the passage of the required air- decrease as far as possible the pressure drop

These velocities are:

- mill inlet: 20-25 m/sec- intermediate and outlet diaphragm: 15-20 m/sec- mill outlet: 20-25 m/sec


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small diameters (< 2.8m): 120-160mm WG

medium diameters (3 to 3.8m): 150-200mmWG

large diameters (> 4m): 180-250mm WG

compound mills with lateral drive(s): 7.5-10%

compound mills with central drive: 15-20%

birotator mills: 30-40%

7.8. Mill's pressure drops:

The less pressure drops are important and the more:

- it will be easy to let the required air through the mill

- the energetic consumption will decrease (kW of the fan)

The standard pressure drops if the mill is well equiped are:

7.9. False air:

The false air is an air entry at mill outlet (at the outlet hood or at the peripheraldischarge for the birotator mills) which is added to the air really passing through themill.

This supplementary air does not bring anything, overloads the fan or reduces theventilation of the mill if the existing fan is too short. This false air is coming from somebadly adjusted airtightness or from deficient discharge devices.

We evaluate the false air percentage in function of the type of mill and its transmission.These values are given in percentage of the mill's required air:

In bad condition, a birotator mill can have up to 60% false air!

7.10. Calculation of the necessary ventilation:

If: diameter = D in mvolume load = J in %lining thickness = e in mmair speed = v in m/secfalse air = f in %

Q = quantity of air in m3/h


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we have:

Example:

D = 4.2me = 80mmj = 33%f = 10%

we want v = 1.5 m/sec

=> Q = 51 107 m3/h (we will choose a fan of 55 000 m3/h)

7.11. Calculation of the fan:

In the example hereabove, we need a fan of 55 000 m3/h.But the supplier needs also to know the pressure drops in the circuit to size the fan.The following figures must be taken into consideration:

- For the mill, to see before- static separator: 100 mmWG- cyclone: 80 mmWG- bag filter: 150 mmWG

- electroprecipitator: 40 mmWG- dynamic pressure: 20 mmWG- ducts: 80 mmWG

Example:

For the mill here above, equiped with a static separator, a cyclone and a bag filter, weforesee:

200mm pressure drop for the mill,

Total pressure drop = (mill + static + cyclone + dynamic + ducts + bag filter) pressure

drops

Then, 200 + 100 + 80 + 20 + 80 + 150 = 630 mmWG available to the fan

8. Heat balance:

The biggest part of the energy introduced in a ball mill is converted into heat.As a consequence, this heat can induce very high temperatures.These temperatures will affect the grinding process.Then, it is important to make the heat balance of the installation.

For all heat balances, there must be an equilibrium between what goes in and what

goes out of the system.


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Regardless to the characteristics and production data of the installation that we have tointroduce, the heat balance turns on 3 values:

- temperature of the product at mill outlet- amount of air- water to inject

9. Problems of drying:

In all the cement plants, the most important problem of the raw mill section is the dryingof the limestone and the other components.In dry process, when the material has more than 1% moisture, the efficiency of the milldrops drastically.Some theories say that the lost of efficiency is 10% every 1% moisture higher than 1%.In consequence, it is very important to get the drying capacities to avoid that.

In the cement plants, it is always possible to recuperate the heat of the kiln and all theball mills receive hot gas at 350C.

An additional burner can be indispensable in some cases.It is also possible to dry in a separate dryer but the cost is higher.Very often, the drying is realized in the ball mill where there is or not a drying chamber.

9.1. The drying chamber:

This chamber helps the drying of the material with the help of lifters. The material islifted and after falls in the hot gas stream.

Which length must have a drying chamber?

According to industrial experiences, it is possible to dry a maximum of220-230 l/m3.hof water to be efficient. This figure is generally accepted to size the drying chamber.

Example:

- Mill diameter: 4.6 m- Mill output: 230 t/h- Limestone moisture: 4%

=> quantity of water: 230 * 1000 * 4% = 9200 l/h

=> volume requested: 9200 / 225 = 40.89 m3


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=> length of the drying chamber: 40.35 / (p * 4.62 / 4) = 2.46 m (with the transferdiaphragm)

We will take a length of 2.50 m as it is DIN.

9.2. Temperature and quantity of gas:

The drying effect is more efficient with a higher temperature than with a higher quantityof gas.But due to mechanical problems (bearings), we can be limited in temperature.On the other side, the quantity of gas must respect some limitations in term of gasvelocity inside the mill.

These limitations are:

- Compound mill: 1.5-2 m/sec- Birotator mill: 3-4 m/sec

- Airswept mill: 6-9 m/sec

Then, the drying capacities must be a compromise between temperature and quantityof gas in order to run in good conditions of efficiency.

grinding technology of limestone with ball mills

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