Characteristics of Milling Process

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Characteristics of Milling Process

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Characteristics of Milling Process

Types of mills Characteristics

Characteristics of Milling Process

In this brief we categorise mills in 3 groups:
1. Low-speed tumbling mills
2. Roller mills
3. Very fine grinding mills, which include the following types of mill:

• High speed pulveriser or hammer mill
• The vibrating mill
• Pin mill
• Turbo mill
• Fluid energy mill
• Stirred media mill

There is also a section which looks at traditional mills used in developing countries and other forms of size reduction other than milling:

• Attrition mills e.g. stone milling
• Cutting machines
• Cryogenic comminution

Glossary for the milling process

Milling circuit – open and closed. The milling circuit is the complete mill system from beginning to end, including feed mechanism, mill, classifier, separator, product collector, etc. In a closed mill circuit the oversize particles are returned from the post milling processes to be remilled (see figure below) whereas with an open circuit the process has no feedback loop.

Air classification. Classification or sizing of particles using a mechanical air separator.
Batch mills. Mills which receive a discrete quantity of charge which is milled and then discharged. The process is then repeated.

Continuous mills. A mill which can accept a continuous flow of feedstock and hence can operate on a continuous basis. Both batch and continuous mills have their relative merits.
Peripheral and trunnion discharge. For cylindrical mills which are continuously fed, the discharge of the final product can be either through the periphery of the mill (peripheral discharge) or through the far end of the mill (trunnion discharge).
In this section we will now look in more detail at the mill types mentioned above.

Tumbling Mills

Autogenous mills
This type of mill consists of a large diameter, short length cylinder fitted with lifting bars. The cylinder is fed with a coarse feedstock of up to 250mm in size and in rotating the feedstock is lifted and then allowed to drop through a significant height. Three significant mechanisms cause the breakdown of the mineral; impact due to the fall of the mineral onto the charge below causes a reduction in the size of the feedstock; attrition of smaller particles between larger grinding bodies; abrasion or rubbing off of particles from the larger bodies. Steel or ceramic balls are often added to aid with the reduction process (the mill is then referred to as a semi-autogenous mill). The process can be carried out wet or dry. Removal of the final product can be carried out using air (where the process is dry) removing only the fines. Rotational speed is usually fairly low, about 80% of critical speed (critical speed is the speed at which the charge will be pinned to the rotating drum and does not drop) and typical drum diameter ranges from 2 to 10 metres. This type of mill is often used as a single stage process, providing sufficient size reduction in a single process. Alternatively, it can be part of a two stage process where further size reduction is required.

Characteristics
This type of mill is only suited to certain kinds of mineral – one which has a fairly coarse nature but once it is broken will disintegrate readily into a small size. In certain circumstances this type of mill can deliver a product with a fineness of less than 0.1mm. Testing is required beforehand to determine the suitability of a mineral for processing in an autogenous mill.

Suitable minerals such as copper or iron ore are listed in table 4. This type of mill has the distinct advantage of accepting coarse feedstock and supplying a relatively fine finished product, often sufficient as an end product. This can provide a reduction in plant costs if a single mill is used as a substitute for two or more stages. There is little wear as the grinding is often carried out by the mineral itself. Autogenous mills are most suited to large installations i.e. more than 50 tonnes per hour and have a power requirement ranging from 40 kW up to hundreds of kW.

Rod Mills
The rod mill is another tumbling mill but having a large percentage of its volume (30 – 40%) loaded with steel rods. The rods are placed axially in the mill and are loose and free to move within the mill. The internal lining of the drum has a series of lifters which raise the rods and drop them at a predetermined point. The mineral is fed in at one end with a maximum size of about 25mm. The rods crush the rock and as the charge passes through the mill it is reduced in size to approximately 2mm to 0.1mm.. The mill can be fed from one end with the product removed from the other end or, alternatively, the mill can be fed from both ends with the discharge at the centre. The process can be wet or dry but is more commonly carried out wet. Maximum rod length is about 6 to 7 metres, otherwise there is a risk of the rods bowing. The drum diameter is limited to 0.6 or 0.7 times the length of the mill.

Characteristics
Rod mills are used for grinding hard minerals. This type of mill is usually used as the first stage of a milling process to provide a reduced size feedstock for a further milling process.

Ball Mills
Ball mills are similar in concept to the rod mill but are charged with steel balls in place of the rods. The mill consists of a cylindrical drum, sometimes tapered at one end, and usually has a charge of steel balls (up to 40% by volume) ranging in size up to 125mm for larger mills. Product size can be as small as 0.005mm, but product size is dependant upon the time the charge spends in the grinding zone and therefore the reduction rate is a function of the throughput. The lining material is of great importance as there is a significant amount of wear taking place due to the action of the steel balls. The speed of rotation is optimum at about 75% of critical speed. Some mills are compartmentalised with each subsequent section having a smaller ball size.

The mineral can pass through to the proceeding section, but the balls cannot. This ensures that the smaller particles are attacked by the smaller grinding media.

Characteristics
It is a versatile grinding mill and has a wide range of applications. The mill can vary in size from small batch mills up to mills with outputs of hundreds of tonnes per hour. They are the most widely used of all mills. Small hand operated ball mills are used in Bolivia for preparation of ore, sand and gravel.

Roller Mills
There are two distinct type of roller mill. The first is a series of rollers which rotate around a central axis within a drum. The reduction takes place between the rollers and the drum. The second is where there are a series of fixed rollers and a rotating table. The milling takes place between the rollers and the table. This type of mill is used for dry grinding only and accepts only relatively soft minerals. Small machines can have a throughput of only a few tens of kg per hour whereas larger machines are capable of handling up to 40 or 50 tons per hour and occasionally more. Feed size varies according to the machine. The machines are often fitted with screens for closed-circuit grinding. Product size can be controlled by changing screens.

Very fine grinding mills

Hammer mills
These are high-speed mills operating at speeds of between 2000 and 6000 rpm A set of ‘hammers’ rotate about a central axis in a vertical or horizontal plane. The hammers can either be fixed or can swing freely, in which case the mill is termed a swing-hammer mill (see figure below). The whole system is enclosed in a housing and the outlet for the product is usually via a screen which sieves the product and allows only the required size of particle to pass.

Characteristics
The product size can be extremely fine – talc can be reduced to a size of 0.0025mm (40%), although an air classifier is required when such product size is required. Maximum capacity is in the order of 10 tons per hour and power consumption is relatively high. Rotating hammer mills are suited to the milling of sifter materials and this type of mill is often used to mill grain and other food stuffs.
Pin, air classifying and turbo mills

Description
A pin mill comprises two discs, one rotating and one stationary which are fitted with intermeshing pins set in a concentric pattern. The charge is fed into the centre of the discs and is broken down as it moves outwards through the pins which are moving at very high speed – up to 20,000 rpm The air classifying mill is similar in construction to the pin mill but incorporates a built-in classifier. This type of mill produces a significant airflow through the machine to aid with keeping temperature as low as possible. Oversize grains which pass through the mill have to be recycled. Turbo mills use a similar concept but the rotating disc is fitted with paddles or bars rather than pins. This rotating disc sits within a cage which is fitted with grids, screens or breaker plates. The mill is configured in such a way as to produce the desired particle size.

Characteristics
Pin mills are capable of very fine grinding without the need for screens and provide a uniform product size. Air classifying mills are used where the product is temperature sensitive. They are widely used in the pharmaceutical and fine chemical industries. They are suitable for relatively soft materials (below Moh 3) and for small quantities of material. Wear on the pins is significant if used continuously.

Vibratory mills
Description
This is the first mill we will consider which does not rely on rotation for the main grinding action. The vibrating mill is a grinding chamber which is filled to about 65 – 80% of its capacity with grinding media such as balls or rods. The chamber is vibrated at a frequency of between 1000 and 1500 times per minute (can be variable speed) by cams or imbalanced weights.

The grinding action is efficient and thorough. Grinding media material and chamber lining can vary depending on application.

Characteristics
Vibrating mills can grind hard or soft materials. Maximum throughput is in the order of 20 t.p.h. but feed size should be kept fairly small. Although final product size can be as low as 0.005mm this type of mill is often used for less fine applications. Product size and shape is a function of the time spent in the mill, media type and size, and frequency of vibration. Commonly functions as a batch mill.

Stirred media mills

Description
Stirred media mills are usually constructed in the form of a cylindrical drum inside which there are a series of rods, arms or perforated discs which are rotated on a central shaft. The drum is loaded with grinding media, such as metal balls or glass sand. The media and the charge is ‘stirred’ together and thus the grinding takes place.

Characteristics
Suited primarily to the very fine grinding of soft materials. Usually used with wet grinding but can be used for dry grinding also. Product size is as small as 0.005mm.

Fluid energy mills

Description
The general principle of operation in a fluid energy mill is that the mineral to be ground is fed into a grinding chamber in a high speed, high pressure and, often, high temperature jet of air (or other gas). The particles collide violently and this causes comminution to take place. Various designs of fluid energy mill exist, the most common being the microniser. This mill has a shallow circular grinding chamber and a series of peripheral jets set tangentially to a common circle. The turbulence causes bombardment which effects a rapid reduction in particle size. A centrifugal classification system keeps larger particles within the chamber while allowing fine particles to leave. In a well designed fluid energy mill there will be almost no contact between the charge and the mill lining.

Characteristics
Suitable for hard or soft materials to be reduces to 0.02mm or less. This method of milling tends to be energy intensive and slow but is suitable where the product is highly sensitive to heat or contamination from grinding media.

Other mills and reduction processes

Attrition mills
Attrition mills are the most common type of mill found in developing countries. The traditional grain mills of many regions of the world are based on attrition grinding between two circular stones, one rotating and the other stationary. Plate mills use a similar principle but are constructed of steel or ceramic plates and used more commonly in the vertical plane. Output from such a mill is low and only for small scale milling is such a mill of practical use.

Cutting machines
Many ductile or fibrous materials such as plastics, rubbers and miscellaneous chemicals cannot be milled using conventional milling equipment. Instead they are often cut or shredded. High speed rotating knife blades set in cutting mills will reduce such materials to a usable size. In certain cases reduction can be down to 0.25mm.

Cryogenic comminution
Ductile materials such as steel, plastics and rubber which cannot be milled easily, can be embrittled by lowering the temperature of the material. Once embrittled the material will lend itself more easily to comminution by conventional methods, usually with the use of a hammer mill. Liquid nitrogen is a gas used for this purpose. The process is expensive due to the cost of the gas but is used for some specialised applications.

Traditional mills in developing countries
As mentioned in the previous section there are a number of traditional mills in use throughout the world. Some of these mills date back thousands of years and have changed little in design. Many are precursors to modern mills. They are usually constructed from materials found locally by indigenous craftsmen. Often the quality of the product varies considerably and the throughput for such a mill is low, but in many circumstances, where the fineness is not critical and the quantity to be milled is low, choosing a traditional mill can be the best option. They are often simple and cheap to construct and can be powered by one of a wide variety of power sources. Some examples are given below.

• The Chilean Edge Mill.
Used commonly in Chilean gold ore processing, the edge mill has two large steel rimmed concrete wheels (these would have been stone in previous centuries) which roll around a circular concrete track and grind the gold ore beneath them.
Final product size can be very small and the final size is a function of the time in the crusher. Grinding is usually carried out wet, the ore being washed in and out of the circular track by the water.

• The See-Saw Crusher.
Very heavy stone or steel ‘see-saw’ crushers are rolled over the material which is to be crushed with the aid of a lever arm. The material is placed on a flat stone or steel surface and the product is similar to that from a roller crusher

• Other traditional mills include pestle and mortar type mills and stamp mills, amongst many others.

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