The ball mill’s basic concept dates back to prehistoric times when it was used to grind flint for pottery. A pharmaceutical ball mill is a type of grinder that is used to grind and combine materials in the production of different dosage forms. Impact reduces the size of the balls as they fall from near the top of the shell. Ball mills are commonly used in single stage fine grinding, regrinding, and as the second stage in two-stage grinding circuits. Ball mills are available in both wet and dry designs, depending on the application. Ball mills have been constructed with standard final product diameters ranging from 0.074 mm to 0.4 mm in diameter.
Principle of Ball Mill
The size reduction in the ball mill is a result of fragmentation mechanisms (impact and attrition) as the balls drop from near the top of the shell. Mixing of feed is achieved by the high energy impact of balls. The energy levels of balls are as high as 12 times the gravitational acceleration. The rotation of the base plate provides the centrifugal force to the grinding balls and the independent rotation of the shell to make the balls hit the inner wall of the shell. Since the shell is rotating in alternate (one forward cycle and one reverse cycle) directions, a considerable amount of grinding takes place in addition to homogenous mixing. The operating principle of the ball mill consists of the following steps: In a continuously operating ball mill, the feed material is fed through the central hole into the drum (shell) and moves there along with the grinding media (balls).
The material to be ground is fed at a 60° angle from the hopper, and the finished product is ejected at a 30° angle. The balls are raised up on the rising side of the shell and cascade down (or drop down on to the feed) from near the top of the shell as the shell rotates. Material grinding takes place due to the impact of falling grinding balls and the abrasion of particles between the balls. The ground material is discharged either through the grid or through the central hole in the discharge cap (mills with center emptying the milled product) (mills with unloading the milled product through the grid).In the ball mill depending on the rotational speed following possible modes of the grinding media motion could be achieved.
(a) Low speed: Speed mode with a rolling of grinding balls without flight.
(b) Mixed mode (Cascade mode motion): Speed mode with a partial rolling and a partial flight of grinding balls.
(c) High speed: Speed mode with the circular motion of balls with no fall.
In ball milling, the rotational speed is very significant. The mass of the ball slides or rolls over each other with inefficient output at low speeds, as shown in Fig. 2.3(a). The balls are thrown out to the walls by centrifugal force at a high speed, as shown in Fig. 2.3(b). There is no grinding at this speed since there is no impact or attrition. The ball’s compression on the shell wall is insufficient for comminution. However, at 2/3rds of the critical speed (50 to 80% of the critical speed), Fig. 2.3(c), the centrifugal speed force only occurs, causing the balls to be transported almost to the top of the mill before falling to the bottom. The impact of particles between the balls, as well as attrition between the balls, affect the maximum size decrease in this way. After the appropriate amount of time has passed, the material is removed and sieved to obtain a powder of the desired size. Ball mills are particularly useful for wet grinding smooth, aqueous, or oily dispersions since they produce particles of 10 microns or less.
Construction of Ball Mill
A shell, balls, and motor are the essential components of a ball mill. Figure 2.4 A pebble mill or tumbling mill is another name for a ball mill. It is made out of a hollow cylindrical shell (drum) with balls set on a metallic frame that allows it to revolve around its longitudinal axis. The shell’s axis can be horizontal or at a little inclination to the horizontal. It has a partial filling of balls. The balls, which can be made of chrome steel, stainless steel, or ceramic, are the grinding media. The balls, which can be of various diameters, take up 30 to 50% of the mill capacity, and their size is determined by the feed and mill size. The larger balls break down the coarse feed materials, while the smaller balls aid in the formation of the fine product by eliminating void areas between the balls. The weight of the grinding media balls is usually kept constant. The size of the balls is determined by the feed and mill diameter. An abrasion-resistant substance, such as manganese steel or rubber, is commonly used to line the inside surface of the cylindrical shell. Rubber-lined mills experience less wear. The metallic cylinder, which is covered with various materials, aids in the attrition mechanism. The mill’s length is roughly equal to, or slightly bigger than, its diameter.
An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously to fed at one end and discharge at the other. Large to medium ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts. High-quality ball mills are potentially expensive and can grind mixture particles to as small as 0.0001 mm, enormously increasing surface area and reaction rates.
Factors determining efficiency of ball mill
The degree of milling in a ball mill is influenced by;
(i) Residence time of the material in the mill chamber.
(ii) The size, density, and the number of the balls.
(iii) The nature (hardness) of the balls and material to be ground.
(iv) Feed rate and feed level in the vessel.
(v) Rotation speed of the cylinder.
Working of Ball Mill
Several types of ball mills exist. They differ to an extent in their operating principle. They also differ in their maximum capacity of the milling shell, ranging from 0.010 liters for planetary ball mill, mixer mill, or vibration ball mill to several 100 liters for horizontal rolling ball mills. The steps involved in the working process of ball mill are as follows:
(i) Initial stage: The powder particles are get flattened by the collision of the balls. It leads it changes in the shapes of individual particles or clusters of particles being impacted repeatedly by the milling balls with high kinetic energy.
(ii) Intermediate stage: Significant changes occur in comparison with those in the initial stage.
(iii) Final stage: Reduction in particle size takes place. The microstructure of the particle also appears to be more homogenous on a microscopic scale than those at the initial and intermediate stages.
(iv) Completion stage: The powder particles possess an extremely deformed metastable structure.
There are various types of ball mills used for different applications amongst which the first two are commonly used in pharmaceutical practice. These include Pebble ball mill, Vibrating ball mill, Drum ball mills, Jet-mills, Bead-mills, Horizontal rotary ball mills,
(i) Pebble ball mill
Pebble mills are sometimes called jar mill or pot mill which works on the principle of attrition and impact. The grinding is effected by placing the substance in the cylindrical vessel or jar vessels that are lined by porcelain or other hard substance containing pebbles or balls. The cylindrical vessel revolves horizontally on their long axis and the tumbling of the pebbles over one another and against the sides of the cylinder produce pulverization with a minimum loss of material.
(ii) Vibrating ball mill
The vibrating ball mill also works on the principle of attrition and impact. It consists of a mill shell containing a charge of balls similar to that of ball mills. In this case, the shell vibrates due to some frequency rather than rotated.
(i) Small capacity ball mills are used for the final grinding of drugs or for grinding suspensions.
(ii) The high-capacity ball mills are used for milling ores prior to the manufacture of pharmaceutical chemicals.
(iii) Ball mills are an efficient tool for grinding many brittle and sticky materials into fine powder.
(iv) The hard and abrasive as well as wet and dry materials can be ground in the ball mills for pharmaceutical purposes.
(v) Powders for ophthalmic and parenteral products can be reduced in size.
(vi) Ball mill is used for the milling of pigments and insecticides for industrial purposes.
(vii) Ball mills are also used in the manufacture of black powder.
(viii) Blending of explosives is an example of an application for rubber balls.
(ix) For systems with multiple components, ball milling has been shown to be effective in increasing solid-state chemical reactivity.
(x) Ball milling has been shown effective for the production of amorphous materials.
Merits of Ball Mill
(i) It produces very fine powder (particle size less than or equal to 10 microns).
(ii) It is suitable for milling toxic materials because of its design as a completely enclosed form.
(iii) It is used in milling highly abrasive materials.
(iv) Strong adaptability to the fluctuation of the physical property of the materials such as granularity, water content, and hardness.
(v) Ball mill has a big crushing ratio and high production capacity.
(vi) It has a simple design, ease of examination, and change of abraded spare parts.
(vii) Reliable operation, simple maintenance, and management.
(viii) It can be used for continuous operation if a sieve or classifier is attached to the mill.
(ix) It is capable of grinding a large variety of materials of different characters and different degrees of hardness.
(x) It is suitable for wet as well as dry grinding processes.
(xi) The cost of installation, power, and grinding medium is low.
(xii) It is suitable for both batch and continuous operation.
(xiii) Suitable for grinding material with high hardness.
(xiv) The shape of the final products is circular.
(xv) No contamination in the powder with ceramic ball.
(xvi) The capacity and fineness can be adjusted by adjusting the diameter of the ball.
Demerits of Ball Mill
(i) Contamination of product may occur as a result of wear and tear of the balls and partially from the casing.
(ii) High machine noise level especially if the hollow cylinder is made of metal, but much less if the rubber is used.
(iii) It has a relatively long milling time due to low rotary speed and thus has low working efficiency.
(iv) It is difficult to clean the machine after use.
(v) High production cost and high unit electricity consumption.
(vi) Heavy equipment so very high one-time capital investment.
(vii) Some raw materials may become damaged by steel balls.
(vii) Not suitable for sensitive and flammable substances.
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