Ball charge optimization

Annex to Best Practice:

DESCRIPTION

Ball charge optimization TYTP Power Best Practice

1             How to use the BP

The optimization of the Ball charge is divided in three grinding area’s according to the three zones rather similar to Slegten.

• 1st compartment:

Sufficient crushing behavior indicated by < 5% residue on the 2mm/4mm sieve before partition wall.

• Transition zone (beginning of 2nd comp.)

Depending mainly on the oversized material passing the partition wall.

• 2nd compartment (3rd comp.)

Specific surface as a function of the mill product fineness expected on the mill end.

2         Description of the main indicators used in this BP

Material load of separator

The optimum separator performance is achieved by maintaining a certain specific load.

Most common is to use the circulating load (CL). But the optimum CL (from 100% up to 400%) is different for each separator size and therefore grinding circuit. Thus this BP is focused on the separator loading (Qf/Qa) in kg/m³ (kg feeding material / gas flow separator). The optimum for the separator loading is depending on the separator generation:

• 1st generation: 1,0 kg/m³ (difficult to measure the airflow) or

6 t/m²   [depending on separator-surface (separator diameter); see also particular advise in the manual, as specific value depends also from product fineness]

• 2nd generation: 1,6 kg/m³ ; +/- 0,2 kg/m³

• 3rd generation: 1,8 kg/m³ ; +/- 0,2 kg/m³, higher if the fine material load remains< 1,0 kg/m³

A higher dust load will lead to a higher separator bypass and therefore over grinding in the ball mill. The opposite is a low circulating load with a high retention time in the ball mill which also leads to over grinding. Both cases have to be avoided.

It is assumed that all the transport components are able to handle the circulating load and therefore are not considered as a bottle neck. The raw mill is not limited by drying capacity.

The residue on 2mm/4mm before partition wall

Maintaining a proper preparation of the material before passing to the next compartment is the highest objective.

• Cement mill: target is to reach < 5% residue on the 2mm sieve.
• Raw mill: target is to reach < 5% residue on the 4mm sieve.

Signs to correct the 1^st compartment ball charge are dams, accumulated oversized material of material > 6mm, as well as reverse classification in the 2^nd compartment. The shape of the liners should also be considered.

Material filling level and ball charge expansion

Determine the permeability of the ball charge in the 1st and 2nd compartment:

An empty compartment will be assumed when no material can be seen on the surface. An overfilled compartment is considered if the material level is as high or higher than the ball level and the ball charge is expanded by more then 3% absolute points (free height level measurement with and without material).

In order to increase the material level in an empty mill, one will adjust the permeability of the charge by increasing the proportion of smallest ball size.

1st compartment: It is recommended to increase the amount of the smallest balls ( eg. 50 – 60mm) by up to max. 5% of the total ball charge each step, as this action can very easily overload the mill (back spilling)

2nd compartment: In 2^nd compartments often up to 100mm material layers are found, but without major consequences to the ball charge expansion. An expanded ball charge can result in a high circulating load.

Cement mill specific power 1st compartment

The specific power in the first compartment should be depending on cement composition within 8 – 12 kWh/t to provide sufficient crushing. If the drawn power is below 8 kWh/t the 1st compartment could be a bottle neck for the grinding system. In this case, if no compartment enlargement is executed, it is recommended to reduce the volume loading in the 2nd compartment to reduce power consumption. Make sure you do not reduce production.

The opposite case is given when the drawn power is higher than necessary (> 12 kW/t) or the mill shop afterwards equipped by an pre-grinding system. In this case the 2^nd compartment will be the bottle neck and the 1st compartment volume loading should be reduced.

The position of the partition wall is mainly depending on the precondition of the fresh material and the product fineness. By changing one of these items, using a pre grinding system, the partition wall may have to be removed to adapt the mill to the new conditions.

Raw mill specific power 1st compartment

For the RM it is considered to have 40% – 45% of the total mill power consumption in the first compartment.

• Most common ball charges within the group

The following ball charges are the most common in our group and can be used as starting point to design a new ball charge. This approach is recommended  if the existing ball charge failed completely.

There are several tools to calculate a ballcharge mainly based under the name “Slegten approach”. These tools are published in the “VADE-MECUM, CTS 2000” and the training documentation “Lafarge Mill grinding reference, 2nd edition, Volume 3: Process Methods and Theories, Section C: Ball Charge Design.”

Always start with 80 % as reference (see BP ball charge management).

There is a different approach for grinding circuits with bigger sized separators (mainly in the US). This grinding circuits are operating with a higher circulating load and need a higher permeability of the ball charge. The approach of this BP will be to use the more coarse ball charge.

The smallest ball size could be limited by the slot size of the discharge wall. In this case target the “average ball weight”  or “specific surface” indicators.

3.1      1^st  compartment CM and RM

The biggest ball size is calculated by the Bond formula and depends mainly from feed size and mill dynamics. (See also “BP Selecting Shell Liners in ball mills” to avoid 100mm balls). Most common is the 90 mm ball as biggest size but even 80 mm is possible depending on the clinker size.

Given parameters are for two different ball charges depending on the crushability. It is considered, that the material feed size is in between the recommended range .

Clinker               < 5%  > 25mm

Raw material     < 5%  > 30 mm; 100 % < 50 mm

• For raw mills it is most common to use a ball charge with the coarse grading (up to 50% of 90 mm balls)

Table 3.1; Example 1^st Compartment ball charge for different grindabilities

The use of 100mm balls is possible, but should be avoided as wear and liner damages will increase. The usage of 60mm balls should be reduced to a minimum. 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.

• The ball charge tendency in the 1^st 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.

3.2       Transition zone (2^nd compartment) CM and RM

The transition zone depends on the preparation in the first compartment as well the conditions of the partition wall. It is the zone after the partition wall in the beginning of the 2^nd compartment (in a 3 compartment mill it is the 2^nd compartment). The purpose is to grind oversized material passing the partition wall.

Cement mill:

The approach is to avoid the transition zone by better maintaining the partition wall and providing sufficient crushing in the 1^st compartment.

• The transition zone consists of 40mm and 50 mm balls. The basis is a 40 mm ball size for 5% residue on the 2 mm sieve. In case bigger grains are by-passing the partition through the center grate, even 60mm balls can be considered.

Raw mill:

The raw mill will be operate with a much coarser ball charge than the cement mill mainly because of the bigger slot sizes of the partition wall.

• The transition zone consists of 50 mm and 60 mm balls. The basis is a 50 mm ball size for 5% residue on the 4 mm sieve. In case bigger grains are by-passing the partition through the center grate even 70mm balls can be considered.

On either cement or raw milling it is preferable to fix the partition or 1^st compartment performance before adding bigger balls.

The ball charges given in chapter 3.3 include the transition zone.

3.3      2^nd compartment (3^rd compartment)

A common way to design the ball charge of the 2nd compartment is an exponential method (Polysius, Slegten, Gaitsch) over the mill length, for mills with classifying liners.

The following examples are given as a start point when no experience is available.

3.3.1    Cement Mill

The approach is to use a ball charge as fine as possible in the 2^nd compartment for all cement types as long as the criteria for size reduction is met in chamber 1. For mills producing several types of cement, we optimize for the main type.

It is recommended to start first with the coarse grading.

Mills having non classifying liners should be limited by three different ball sizes to avoid reverse classification.

Closed circuit for clinker cements ( > 90 % clinker):

Table 3.2; Example 2^nd Compartment ballcharge

Open circuit

Table 3.3; Example 2^nd Compartment ballcharge for open circuits

• Raw mill

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

Table 3.4; Example 2^nd Compartment ballcharge for raw mill