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 Description
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.
Autogenous mills 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 Description
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.
Rod Mills 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.
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Glossary for the milling process,Tumbling Mills,Autogenous mills,rod mills
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 Description
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.
Autogenous mills 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 Description
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.
Rod Mills 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.