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.
Suitability of different mills to different operations
Some of the characteristics and requirements to be considered when selecting a mill are given below. The mill manufacturer can usually be consulted concerning the application of a particular mill or for sourcing a mill which is suitable for a particular application.
• Mineral properties. The choice of mill type is primarily dependent upon the properties of the material it will be used process. It is vitally important to match the mill and material characteristics properly.
• Capacity. The scale of the operation will determine the size of the mill which is required. Throughput or capacity is often given in tonnes per hour (or kg per hour for small mills). Always check capacities with as many sources as possible as sales information can often be biased to encourage sales.
• Reduction ratio and final size requirement. This parameter will dictate whether a single mill will be sufficient for final product requirements or if a multi-stage plant will be needed. Generally speaking, the greater the reduction ratio, the larger the likelihood of a multi-stage process being required.
• Power requirements and type of power supply. Access to a power supply of suitable capacity is essential. Types of power supply for remote applications is discussed in a later chapter in this section. The power requirement for a given mill will be given in the mill specification document provided by the manufacturer. Specific power consumption (eg kilowatt hours per tonne) is often quoted and is a good comparative guide.
• Wet or dry product. Products which can be accepted in a wet state, such as slurries, can be milled wet which will often save power and reduce dust related problems. As a general rule, only tumbling mills are used for wet grinding, although other mills can be used for wet grinding in certain circumstances.
• Continuous or batch operation. Some mills can be designed in such a way as to enable continuous milling. This is important where the throughput is high, as well as making loading and emptying easier within the process. Some mills will only accept batch loads.
• Portable or stationary equipment required. Depending on the nature of the operation, the equipment can be sited permanently or can be portable. Portable equipment is useful for operations which move frequently due to the dispersed siting of the raw material or where a mobile milling service is offered.
• Classification. When considering a mill for a particular application, one needs to consider the classification mechanism that will be required for the process and whether this will have to be purchased separately or if it will be an integral part of the mill.
• Cost. Obviously cost is an important factor. It is important to consider all the costs beforehand. For an accurate analysis of the economic viability of a mill to be carried out the following costs need to be considered:
1. Capital costs of mill (and capital depreciation against the useful life of the mill)
2. Capital costs of peripherals, such as feeding and classification equipment, power supply, etc.
3. Transport costs
4. Running costs for fuel or electricity, labour, etc.
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Grinding mills and reduction processes,Suitability of different mills to different operations
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.
Suitability of different mills to different operations
Some of the characteristics and requirements to be considered when selecting a mill are given below. The mill manufacturer can usually be consulted concerning the application of a particular mill or for sourcing a mill which is suitable for a particular application.
• Mineral properties. The choice of mill type is primarily dependent upon the properties of the material it will be used process. It is vitally important to match the mill and material characteristics properly.
• Capacity. The scale of the operation will determine the size of the mill which is required. Throughput or capacity is often given in tonnes per hour (or kg per hour for small mills). Always check capacities with as many sources as possible as sales information can often be biased to encourage sales.
• Reduction ratio and final size requirement. This parameter will dictate whether a single mill will be sufficient for final product requirements or if a multi-stage plant will be needed. Generally speaking, the greater the reduction ratio, the larger the likelihood of a multi-stage process being required.
• Power requirements and type of power supply. Access to a power supply of suitable capacity is essential. Types of power supply for remote applications is discussed in a later chapter in this section. The power requirement for a given mill will be given in the mill specification document provided by the manufacturer. Specific power consumption (eg kilowatt hours per tonne) is often quoted and is a good comparative guide.
• Wet or dry product. Products which can be accepted in a wet state, such as slurries, can be milled wet which will often save power and reduce dust related problems. As a general rule, only tumbling mills are used for wet grinding, although other mills can be used for wet grinding in certain circumstances.
• Continuous or batch operation. Some mills can be designed in such a way as to enable continuous milling. This is important where the throughput is high, as well as making loading and emptying easier within the process. Some mills will only accept batch loads.
• Portable or stationary equipment required. Depending on the nature of the operation, the equipment can be sited permanently or can be portable. Portable equipment is useful for operations which move frequently due to the dispersed siting of the raw material or where a mobile milling service is offered.
• Classification. When considering a mill for a particular application, one needs to consider the classification mechanism that will be required for the process and whether this will have to be purchased separately or if it will be an integral part of the mill.
• Cost. Obviously cost is an important factor. It is important to consider all the costs beforehand. For an accurate analysis of the economic viability of a mill to be carried out the following costs need to be considered:
1. Capital costs of mill (and capital depreciation against the useful life of the mill)
2. Capital costs of peripherals, such as feeding and classification equipment, power supply, etc.
3. Transport costs
4. Running costs for fuel or electricity, labour, etc.
5. Maintenance costs