Design and Motor Rating Calculations for Bauxite Grinding Mills

Hi Friends,

In continuation to our earlier post on Efficient grinding system for Bauxite in Alumina refinery, we will discuss the methodology along with design calculations for sizing of Ball mill including the estimated motor rating with reference to a known mill operative in identical process conditions.The cross section of a typical Ball mill has been shown below just to have the physical feel of the main components of Ball mill and movement of grinding media inside in running condition. I am thankful to my friend who has developed this animation for schematic cross section of the ball mill in operation-

Schematic Cross section of Ball mill in Operation

It is evident from the above schematic diagram that size reduction of bauxite takes place due to impact action of balls and rubbing action of particles as well. The actual operating rotational speed of ball mill is controlled at around 60 to 80% of the critical speed of the mill derived on the basis of effective diameter of the mill as elaborated below. We have already discussed about the material of construction for ball mill liner and balls used as grinding media. We are presenting here the systematic approach to work out the dimensions (Diameter and Length) and motor rating of ball mill for successful operation for a typical capacity Alumina refinery as given below-

We have put all out efforts to present the calculations in simplified manner for easy understanding by one and all related to the subject. Hope, you will appreciate our attempts in preparing the concise and systematic derivations for such a complicated topic. Please put your views / remarks / suggestions / comments, if any, for further improvement.
Regards.

Rajendra Kunwar
rajendra@ceti.co.in
www.ceti.co.in

Guidelines for Ball Charge to Ball Mills for Wet Grinding of Bauxite

Hi Friends,

In earlier posts, we have discussed many design calculations for various stationary and rotating equipment used in Alumina refinery. We have also covered in Detail design calculations for sizing and motor rating of Ball mill for required grinding output.

The product fineness in the Ball mill is dependent on the size of balls used as grinding media. Here, we will present the basis for working out the estimated quantity of total ball charge to the mill required for wet grinding of Bauxite in Bayer process for alumina production. The figure may vary marginally depending on the hardness or work index of Bauxite to be processed.

Generally, following norms are followed as guidelines-

• Total ball charge is taken as 20 to 25% of total effective mill volume,
• Packing density of balls inside the mill is taken as 55 t0 60%,
• Preferred material of balls is forged Manganese steel,
• Sizes of balls are 100 mm, 80 mm and 50 mm in diameter,
• Mass ratio of ball charge for larger to smaller diameter balls is 54% : 36% : 10%,

Hope, the above guidelines give the clear understanding to arrive at ball charge as grinding media to Ball mill for Bauxite grinding.  To compensate for the ball consumption during grinding, only 100 mm diameter balls are charged to Ball mill periodically, generally half yearly basis or as and when felt necessary, @ about 15 to 20 kg balls per thousand tonnes of dry bauxite processed. However, ball shorting is essentially done at an interval of  three years duration and new balls are added depending on total requirement of grinding media for desired throughput rate and product fineness.

Have you ever seen such unique information in any published document so far? I know, your answer will be - 'No'. Please put your views / suggestions / remarks / comments, if any.

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Thanks and regards.

Kunwar Rajendra

Procedures for Systematic Start up and Stoppage of Ball Mill

Hi Friends,

In earlier post, we have developed the Design and motor rating calculations for Ball mill. In present post, we will discuss the systematic procedure for starting a Ball mill for taking into operation as well as stopping the Ball mill for taking out of circuit in Alumina refinery. Ball mill is one of the very critical equipment installed in Alumina refinery. Since the mass of the rotating cylinder with liners and balls is very high, thus the starting torque for the mill is enormously high. The drive motor for the Ball mill ranges fro 0.5 MW to 2.5 MW depending on the hardness of Bauxite, feed size and operating capacity of the mill for meeting the hourly alumina production requirement as described in earlier post. Generally squirrel cage induction motors are installed as drive motor for the Ball mill.  The mill is also provided with hydraulic pumps for lifting the Ball mill to provide a jerk to swing so that inertia of rest of the mill is broken for its rotation. The hydraulic pumps of Ball mills in Alumina refinery are popularly known as inching device for the mill. Since the drive motor rating of the mill is the highest among all the high tension (HT) motors of Alumina refinery, so it is connected with high voltage electrical supply line at about 6.60 kV. Hence all precautionary measures are taken to start the Ball mill for normal operation. Sudden start of the mill by direct power supply to its drive motor may cause serious damage to the equipment and power supply system as well.

Ball mill start up procedure: The systematic start up procedures for Ball mill are briefly outlined here under-

1. Check the readiness of upstream and down stream equipment in the process circuit,
2. Inform the turbine house of Co-generation plant to ensure the stable operation of Turbo-Generators (TGs) and connected boilers. During steam pressure fluctuations in boilers, the ball mill should not be started.
3. Check hydraulic oil level in oil tank,
4. Start hydraulic pumps for lifting the mill to swing,
5. When hydraulic oil pressure reached to around 2 kg/cm2, start the drive motor of the mill. The temperature of hydraulic oil should remain less than 40 degree centigrade,
6. Keep close watch on voltage, amperage, temperature and sound of drive motor,
7. Open aluminate liquor to the mill at controlled rate,
8. Start the ground slurry pump,
9. Start bauxite weigh feeder with low feeding rate to Ball mill,
10. If everything is normal, put the operation of grinding circuit in auto mode with normal grinding rate.

Ball mill stoppage procedure: The sequence of operation for stopping the Ball mill from normal running condition are given below-

1. Stop the bauxite feeder for stopping the feed to Ball mill first,
2. Continue flow of digestion liquor to mill for a few minutes to ensure flushing of slurry from the Ball mill,
3. Stop the Ball mill,
4. Stop the slurry pump,
5. Drain the slurry from pump including its suction and delivery lines.

Broadly, the above sequence of operations are followed for starting and stoppage of ball mills. Grinding section operators must be provided with written down procedures in order to avoid any confusion and thereby mishaps. Also, while starting the mill, presence of Area supervisor is considered essential as any small lapse during start up may cause serious damage to equipment as well as unsafe to the working personnel. Sudden start up may also cause damage to mill foundation and its alignment creating acute crisis for Alumina refinery. 

Hope, all major points have been covered under the topic. Please put your views / suggestions / remarks / comments, if any. We will always welcome your suggestions and recommendations.

If you like this article, then please press your rating as  +1  .
Thanks and regards.

Kunwar Rajendra