MINERAL MINERAL PROCESSING MILLING
Milling, sometimes also known as fine grinding, pulverising or comminution, is the process of reducing materials to a powder of fine or very fine size. It is distinct from crushing or granulation, which involves size reduction to a rock, pebble or grain size. Milling is used to produce a variety of materials which either have end uses themselves or are raw materials or additives used in the manufacture of other products. A wide range of mills has been developed for particular applications. Some types of mills can be used to grind a large variety of materials whereas others are used for dykrt6rdeyk 1 certain specific grinding requirements. This brief aims to present the factors to consider when choosing particular grinding applications and to give an overview of the equipment which is available. Material grinding is quite often an integral part of an industrial process, whether carried out on a large or small scale and in some cases the grinding mill may be the single most costly item for the production operation. Installing a grinding mill which is suitable for the purpose would be one of the main requirements for cost-effective and trouble-free material processing if a grinding stage is involved. Abrasives Animal products Brewing industry Chemical Confectionery Food processing Fuel preparation Metal power Mineral preparation Paint preparation Paper Pigments for colour industry Abrasives for grinding Cement and Limestone Grain milling Laboratory milling
Figure 1: Swing hammer mill being used for lime milling in Malawi. ©Practical Action.
Petroleum products Pharmaceuticals Plastics Printing ink Rubber Textiles Sintering Refractory materials for investment casting Tungsten power and dry lubricants Dry powder opacifiers for ceramics industry Carbon black for rubber Powders for the detergent industry Colour coating of polymers for the plastics industry Aggregates for the construction industry Fertilisers
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Mineral processing – Milling
Pulverised coal for power generation Glass, sand, lead oxide, potash and arsenic for glass making Table 1: Some applications of the milling process
Salt Charcoal for briquetting
In this brief we will concentrate on mineral grinding, rather than the grinding of grain and other foodstuffs, because a comprehensive publication already exists in the area of grain milling (see final section on resources).
Material Characteristics Material
When a material is to be milled there are certain characteristics which have to be taken into account. These include the following • Hardness • Brittleness • Toughness • Abrasiveness • Stickiness • Softening and melting temperature • Structure (e.g. close grained or cellular) • Specific gravity • Free moisture content • Chemical stability • Homogeneity • Purity The hardness of a material is probably the most important characteristic to consider when deciding on what type of mill to choose. Trying to grind a material which is too hard, such as sand in most types of beater mill, will result, either in costly damage to the mill or an expensive maintenance requirement. Most types of readily available hammer mills for agricultural grinding are not suitable for grinding most types of minerals. Hardness of minerals is expressed on Mohs scale - a numerical index ranging from 1 for talc (the softest mineral) to 10 for diamond (the hardest known material). Table 2 below shows Mohs’ scale of hardness.
Hardness No. 1 2 3 4 5 6 7 8 9 10 Mineral (example) Talc or graphite Rock salt or gypsum Calcite Fluorspar Apatite Felspar Quartz Topaz Sapphire Diamond Common practical test Marks paper - like a pencil Can be marked with fingernail Can mark a copper coin Can mark window glass Can mark a knife blade -
Table 2: Mohs’ scale of hardness
Mineral processing – Milling
In general, the harder the material, the more specialised and expensive the type of mill used has to be. In addition if a particular mill can be used over a range of hardness scales, the harder the material the lower the throughput for a given size requirement. Another characteristic of a material to be aware of is brittleness, which is the degree to which a material will easily break. Most minerals are brittle, as opposed to metals which are ductile, although some to a greater degree than others. Brittleness does not equate with hardness as brittle materials can be hard or not particularly hard. Materials which are not brittle to some degree, metals or soft plastics for example, cannot easily be milled.
Free moisture content of a material should be as low as possible for dry milling. In practice this can be a problem, especially in humid regions where the moisture can cause the material to stick to the grinding media. Different mills behave in different ways with moist materials and in some cases drying of the raw materials is required.
Also important is the final size of the material in question. Table 3 below gives details of some materials which are milled and the degree of fineness required. Specifiers may stipulate that a proportion of the material is finer than a particular size. Usually this proportion is 90% or 95% but may be 99% for particularly demanding applications. In certain applications a particular range of particle sizes may be required.
Material and Application Feldspar - (flux in ceramics) Talc - (paper making and cosmetics) Limestone - (agricultural lime) Ordinary Portland Cement Chalk Powdered charcoal or coal for fuel briquettes Pigments for Paints (various materials) Silica quartz (glass making) Phosphate (fertiliser) Iron Ore Lime (industrial applications such as detergents) China clay Alumina Particle Size in mm 0.075 0.01 1.2 < 0.10 0.05 <0.10 ~ 0.005 0.01 0.075 0.20 0.10 0.002 0.005
Table 3: Material Particle Size
Characteristics Characteristics of Mills
Types Types of mills 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
Mineral processing – Milling
• 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 for milling again (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 Tumbling Mills
Autogenous 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 the 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
Mineral processing – Milling
i.e. more than 50 tonnes per hour and have a power requirement ranging from 40 kW up to hundreds of kW.
Iron ore Limestone Copper ore Uranium
Phosphate Bauxite Slags Niobium ores
Table 4: Material Suitable for Autogenous Mills 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 between 2mm and 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.
Coke Cement clinker (needs to be dry)
Products for the glass industry
Table 5: Material Suitability for Rod Mills
Figure 2: Rod Mill
By Otto Ruskulis, Published by Practical Action on 08/01/10
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