Notes on Tube mills
Tube mills are introduced in the last quarter of 18th century. The low speed mills are known as Tumbling or gravity mills.
The earlier version of a tube mill is of a close circuit with a bucket elevator. Now Air swept mills have come up with combined drying & grinding in addition to the elevator which suits best for raw grinding.
• High Utilisation factor.
• Low susceptibility to wear.
• Low cost for ‘core parts as well as replacement of worn out parts’ compared to other grinding units.
• Long residence time to ensure thorough grinding even for harder particles.
• Unaffected by any kind of foreign bodies that may enter.
• Minimum downtime for grinding media renewal.
Design of Tube mill:
To determine the exact power requirement, certain mathematical empirical correlations are to be obtained based on the following:
> Diameter , length , arrangement of compartments , type of lining
plates , internal fittings , size and distribution of grinding media , operating speed …etc.,and
> Also based on the properties of feed material, desired characteristics of the finished product and throughput.
An optimum performance of a grinding mill means minimum power consumption with maximum process engineering efficiency at the lowest operating cost.
Therefore getting ‘optimum design & operating conditions’ is a laborious and time consuming process. Always it has been necessary to proceed experimentally since mathematical models have not yet gained a footing in actual practice so far due to the non-availability of the accurate numerical values. Hence a detailed study of any mill with respect to the specific application is a must before proceeding for a major renovation. The following are the basic TEN FACTORS which are having a direct influence on the mill performance. Therefore knowing them is of the primary duty for the person assigned for “Mill Optimization”.
Grindability – normally varies in the range of +/- 4 %. Clinker and cement raw materials are heterogeneous in structure, their work index values are always higher, but below their natural grain size than above it.
The Grindability index is based on the following;
Moisture content – high means low performance
Hardness & Brittleness – relates to fineness
Porosity – more means high performance
Physical & Chemical properties
- Free Mgo is found to have negative effect whereas high free lime indicates positive influence.
- Increase in SR (Silica ratio) decreases the grindability & vice versa in the case of specific gravity.
- – Increase in C3S & Decrease in C2S increase the overall grindability.
- Effect may arise, not only on the raw materials but also due to the burning & cooling methods.
Mill L / D ratio:
The ratio, however varies with the circuit used, material type, feed Size and grinding requirements.
Generally there is no fixed rule; however tube mills have the ratio of 3 to 6: 1 and ball mills are of 2: 1.
Residence time depends on mill size, grindability of feed and conditions of grinding.
Power input >>> Fraction of mill volume occupied by the grinding charge.
>>> 1/ ¨ critical speed
>>> Interior diameter … etc
I = k D2.5 L
Where k is a constant
The above relation indicates that increasing ‘dia’ gives higher power than increasing ‘length’
If the mill is allowed to run at low speed it results into ‘rolls back down’ of grinding media, known as ‘cascading’.
In contrast if it runs at high speed, the grinding media will be thrown and results in to an effect known as ‘cataracting’.
Hence the optimum speed must be less than the critical speed. Critical speed occurs when the centrifugal force is equal to the weight of the grinding body at the highest point on the lining.
Ie ..By formula, ηC = 42.3 / √D
Increased speed increases the mill capacity & power consumption. It also increases the metal wear as well as the maintenance cost per ton of ground.
Mill Media size:
In general optimum ball diameter is proportional to the square root of the particle size.
The ball size is directly proportional to the function of the properties of the materials…ie. mainly the initial and final size of the material to be ground.
If ball is too large, the number of breaking contacts reduced or in other words the extreme fines made by each contact increased.
If ball is too small, the wasted contacts increased instead of breaking the particles.
Media size distribution:
Eventhough a lot of methods developed, the optimum media size distribution still remains a domain of long practical experience.
A good grinding media distribution increases the mill throughput at lesser power consumption.
> Finer grading results into finer residues and higher throughput.
> Coarser grading results into coarser product at reduced throughput.
> For mill first compartment, the result of ‘chamber sampling’
will be of a straight line whereas for second compartment the same will be of a straight line tends to show as a curve.
Mill Loading :
In any Tube mill, the grinding action is determined by the charge volume. The power input for moving the mill charge at a given speed is directly proportional to the charge weight (Grinding media charge plus weight of material) and the distance between the centres of gravity to the charge.
If the quantity and / or grading of the media are changed and the power consumption does not change proportionately, it indicates that the centre of gravity has shifted.
In general, the lower and upper values of ‘Loading’ are as
Charge volume < 25 % Sliding of the media results and Charge volume > 45 % Indicates disturbances along the Trajectories of the grinding Media. Therefore, for balls the charge volume will be in the range of 26 to 45 % and for cylpebs it is of the order of 25 to 33 %.
Mill Compartment and Diaphragm :
The sole purpose of Mill compartment is to segregate the grinding media according to size for efficient performance.
The length of the compartment is estimated based on
a) The power assigned to each compartment.
b) The degree of grinding to be achieved.
The material loading factor and energy utilisation are favourably influenced by the Mill diaphragm. In general it is of single, double wall design with lifters to regulate the flow of material through the mill. To increase the grinding efficiency, the fine grinding compartment is equipped with the dam-ring ribs. Further with the technology advancement, Controlled flow diaphragms find an important role in Tube mills design and optimization techniques. The optimum performance is directly proportional to the level of material in each compartment. This is controlled by the size, the number of slots and the ratio of plain area to allotted area in the diaphragm.
Also in a mill diaphragm,
Width of slot
∝ Length of the compartment
∝ Quality of feed
∝ Size of grinding media
Normal opening of slots or the active surface of partition varies between 6 to 20%
Mill liners have the direct roll on the power efficiency and increased reliability.
It has two direct functions, viz.
a) To increase the strength of mill shell
b) To lower the maintenance cost
Mill linings are subjected to severe abrasive action by the feed material and the grinding media due to continuous operation under excessive heat.
Therefore the following important parameters are to be considered before selection:
Size of Feed & Product:
If the feed size is too large it results into more loss in grinding efficiency than desired. Even 2 to 3 % of the large size that matters than 90% passing size. Since if this 2 to 3% of the large size feed cannot be crushed within the time it remains in the zone of large balls, it can rapidly build up in the mill necessitating a drastic setback.
Also the decrease in mill capacity caused by oversize grinding media is lower than that caused by oversize feed. Similarly the size of the product has probably the greatest effect than any single variable on the capacity of the mill.
…Ie., to say that even small changes in limiting the size of product can cause considerable influence on the mill capacity. Mill capacity decreases as the demand in fineness increases.
Mill Air flow :
Normally Flow across Mill is kept as
2 to 3.0 m/sec for raw materials grinding
1 to 1.5 m/sec for close circuit of cement grinding
1 to 1.2 m/sec for open circuit of cement grinding
Gas or air that flows through the mill performs a number of functions and the very important ones are:
First being to dry out the raw materials
Secondly , It influences the size reduction
Thirdly it is for the throughput improvement
Quantity of gas / air to be calculated is on the basis of material and heat balances.
The three parameters such as Feed rate , Mill differential pressure & Gas flow rate are inter linked with each other.
The maximum moisture that can be handled in an air-swept / semi air-swept mill is restricted to perhaps a maximum of 8% for better performance.
In closed circuit grinding, the circulating load ratio or the bucket elevator factor is another important parameter for achieving the desired capacity. The most attractive range of circulating load in raw material and in clinker grinding even with maximum sharpness of size separation appears to be in the range of 100 to 300 %.
In the operation of closed circuit grinding, it is important to keep the circulating load as constant. It is done by adjusting the feed rate but the variations in grindability as well as the counter active behaviour of the mill & the air separator give rise to oscillations. To overcome these phenomena, a very recent practice introduced is to alter the size output of the separator while keeping the fresh feed material constant.
The above points give us a clear picture about the basics of a tube mill in respect to the grinding operation of a cement plant. These tools will help in identifying the nature of a problem if a non-performance of a mill is reported. However solving the real problem is the expertise of the individual in consultation with the designers.