grinding - mineral processing
The final fineness of the product mainly depends on the number of times the ore particles pass through the grinder. The longer the grinding, the smaller the particle size. Separate crushing and grinding steps are necessary, the ball mill can only receive the broken ore particle, and then grind to the grinding fineness required for flotation.
In order to separate the concentrate from the ore, the ore should be ground fine enough to release the target mineral from the non-mineral grains. The degree of grinding required for this depends on the size of the mineral particles in the ore. A laboratory-scale flotation test is usually required on materials of different particle sizes to determine the grinding particle size required to release the target minerals.The fineness of the ore particles produced by grinding is crucial to recover the minerals by flotation. The most common grinding machines are semi-automatic (SAG) and automatic (AG) mills and ball mills.
Determining an optimal grinding size can maximize the recovery of target minerals in the subsequent flotation process.The grinding size is too large, and some ore particles and non-ore particles cannot be separated, thus preventing their flotation. If the particle size is too fine, the bubbles that rise during the flotation will push the very fine ore-containing particles away, preventing them from contacting the bubbles, thereby reducing their ability to be recovered into the concentrate.In addition, extremely fine rock and iron sulfide particles may agglomerate with extremely fine sulfide ore particles, preventing the ore particles from floating.
According to the test, the particles usually need to be ground to a diameter of about 100 mm to release minerals from each other. When the particles are less than about 10 mm, this is not conducive to the flotation effect.Grinding operations are very power-hungry, which is another reason to avoid excessive grinding.
The crushed products are ground in SAG or AG mills. The self-grinding machine can grind ore without grinding media such as iron ball, or steel rod, as long as the hardness of the ore is sufficient for the rolling ore to grind by itself.A large vibrating screen is used to sieve the ground products to separate the oversized particles. A small cone crusher to recover the oversized material, and then sent them return to the SAG or AG mill for re-grinding. The correct size material is sent to the ball mill for final grinding.
The ball mill is the fine grinding machine connect the SAG or AG mill and flotation machine. Ball mills produce fine particles with a uniform size for flotation, its grinding medias commonly are steel ball. The ball mill rolls grinding media together with the ore, as the ore grinds, these balls initially 5-10 cm in diameter but gradually wear out.Grinding is always carried out under wet conditions, with about 70% solid mixture in water.This procedure maximizes ore production and minimizes power consumption.
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closed circuit grinding vsopen circuit grinding
The simplest grinding circuit consists of a ball or rod mill in closed circuit with a classifier; the flow sheet is shown in Fig. 25 and the actual layout in Fig. 9. This single-stage circuit is chiefly employed for coarse grinding when a product finer than 65 mesh is not required, but it can be adapted for fine grinding by substituting a bowl classifier for one of the straight type so as to enable the W/S ratio of the overflow to be kept below the 4/1 limit usually necessary for flotation. On account of the greater efficiency of the bowl classifier the trend of practice is towards its installation in plants grinding as coarse as 65 mesh.
Single-stage grinding is generally to be recommended for small plants on account of its simplicity. Variations in the size and character of the ore are unavoidable in most plants, but they are, as a rule, very much more noticeable when operations are on a small than when they are on a large scale. Multi-stage grinding as practised in large installations may, therefore, prove impossible to control on a very small scale, and for this reason the simplicity of single-stage grinding is likely to result in a greater overall efficiency than would be obtained with a multi-stage arrangement. When, however, the capacity of a plant approaches or exceeds 1,000 tons per day, two-stage grinding becomes preferable because the effect of normal variations of the ore is less marked and control becomes correspondingly easier.
The usual type of two-stage circuit is shown in Fig. 26, and is one that can be employed for any degree of grinding, although a straight must be substituted for a bowl classifier in the second stage when a 48-mesh product is required. It used to be the practice at one time to omit the first classifier and to pass the feed straight through the primary mill to the secondary circuit, but it was not a good method because either the secondary mill received pieces of ore that were too big or else the primary mill overground a large proportion of the feed. Much better results are obtainable by keeping the coarse ore circulating round the primary circuit, which is set for the efficient grinding of such material, until it is fine enough to be sent to the secondary circuit where the machines are set to grind fine ore more efficiently than coarse. It should be noted that the overflow of the primary classifier is sent to the secondary classifier, not direct to the mill, in order that all material which has been ground fine enough in the primary circuit can be discharged immediately without any chance of its entering and being overground in the secondary ball mill.
So important is it from the point of view of efficiency to get the undersize out of the circuit at the earliest possible moment, whether it is produced in the primary or in the secondary mill, that a special intermediate bowl classifier is often installed between the two stages. Such an arrangement has been found very useful in plants in which improvements in dry crushing practice have resulted in a reduction in the size of the feed to the grinding mills with the result that they have been able to take larger tonnages; the classifiers have then becomeoverloaded, especially in the case of older installations in which both stages were equipped with straight classifiers.
The method of installing a bowl classifier to overcome the difficulty is shown in Fig. 27. This circuit is usually adopted in modern practice, but with a bowl instead of a straight classifier, if necessary, in the closed circuit of the secondary ball mill.
Any increase in the efficiency of classification gives greater economy of power by reducing the amount of ore that is overground, so making a larger proportion of the power required to turn the mill available forgrinding the particles that are still too large. From a theoretical point of view, the ideal method of grinding would consist of a series of ball mills, each in closed circuit with a classifier and each so short that the ore in its passage through the mill would be struck only two or three times by the balls, any undersize produced being removed at once by the classifier ; in this way the chance of a particle being struck again after it had reached the required size would be reduced to a minimum. Practice circuit design approaches the ideal:
Open circuit grinding consists of one or more grinding mills, either parallel or in series, that discharges a finalground product without classification equipment and no return of coarse discharge back to the mill. Some very simplistic examples of open circuit grinding are see below and are made of aRod mill,Ball Mill or aRod mill, ball mill combination.
Not all ores can be ground in an open circuit type ofarrangement. Some conditions which do favor open circuit grinding such assmall reduction ratios,reduction of particles to a coarse, natural grainsize,recirculation of cleaner flotation middlings forregrinding anda non-critical size distribution of the final groundproduct.
Closed circuit grinding consists of one or more mills discharging ground product to classifiers which in turn return the coarse product from the size separation back to the mill for further grinding. In this circuit, grinding efficiency is very dependent upon the size separation effected so care should be exercised in selecting the type and size of classifier used to close the system.
This type of grinding is the most common circuit found in mineral processing facilities, mainly because a lot of ores and product requirements are not suitable for open circuit grinding. Some advantages presented by grinding in closed circuit are that this arrangement usually results in higher mill capacity and lower power consumption per ton of product, it eliminates overgrinding by removing fines early and it avoids coarse material in the final ground product by returning this material to the mill.
Although closed circuit grinding offers many choices for arrangement of the equipment as well as combinations of equipment, some of the more common circuits arerod mill/classifier,Ball mill/Classifier,Rod mill/Ball mill/Classifier and Rod mill/Classifier/Ball mill/Classifier.
The importance of the grinding circuit to overall production in any facility should be obvious by now. Because of the responsibilities assigned to grinding it becomes essential that a grinding mill accepts a certain required tonnage of ore per day while yielding a product that is of a known and controllable particle size. This leads to the conclusion that close control over the grinding circuit is extremely important.
There are many factors which can contribute to fluctuationsin performance of a mill, but some of the most common found inindustrial practice are thechanges in ore taken from different parts of the mine,changes in crusher settings,wear in the crushers,screen damage in the crusher circuit.
These are a few things that operators should look for when changes in mill performance are noticed. Stockpiling of ore ahead of the mill can aid in smoothing out some of the fluctuations although it must be stored in such a manner that no segregation occurs.
The reduction ratio in the grinding section is so much greater than in the crushing plant that labour becomes a relatively small item and the power and steel consumption the largest items of cost. Table 18 gives the average total consumption of power that may be expected in modern ball mill installations of various capacities up to 4,000 tons per day, the figures being based on an average medium-hard ore.
The cost of grinding is more difficult to predict than that of crushing because variations in the hardness and toughness of the ore produce proportionately wider variations in the consumption of power and steel. An approximate guide to grinding costs; they are direct costs and include no overhead charges. Power is assumed to cost 0.075 per kilowatt-hour in the case of the smallest plant and to decrease to a minimum per kilowatt-hour for the largest.
As the tonnage rises up to 1,000 tons per day the costs fall rapidly. In plants of greater capacity, however, they do not decrease in the same proportion with increase of tonnage, because the extra capacity is not obtained by increasing the size of the individual machines but by installing two or more similar units side by side, each of equal efficiency.Reduction of costs then becomes more a matter of organization than of plant design.
As already stated, the power and steel costs are the two largest items, those of labour and supplies being small by comparison ; it is on this account that recent progress has been mainly directed towards reducing the consumption of power and steel by means of greater efficiency in classification and by the use of mills of larger diameter.
The way in which the efficiency of classification has been increased has been described in the paragraph headed Grinding Circuits. An increase in the diameter of a mill gives greater economy in two ways : In the first place, the balls do more effective work in a large than in asmall mill, because, falling from a greater height, they shatter the pieces of ore with greater force ; in the second place, the ratio of the deadweight of the mill to the weight of the ball charge decreases as the diameter increases and thus in a large mill the useless weight to be moved is distributed over a greater weight of useful ball load than in a small mill, with the result that a larger proportion of the total power consumption is available to give the balls the cascading and rolling action necessary to break up the ore.
It is essential for the grinding and flotation sections of a plant to be run continuously. It takes nearly half an hour to clear the circuit of even a small grinding unit preparatory to stopping it, and often an hour is necessary to get the circuit fully loaded after restarting ; most of the power required to keep the machines running during the stopping and starting periods is wasted. Moreover, the operation of the flotation machines is poor during these periods so that much of the power required to keep them running is also wasted. In addition, modern practice aims at the elimination of everything likely to cause fluctuating conditions. For these reasons it is the universal custom to run the grinding and flotation plants for 24 hours per day.
fine grinding: how mill type affects particle shape characteristics and mineral liberation - sciencedirect
Particle shape characteristics compared after grinding in ball mill or stirred mill.Differences observed at laboratory scale but not at plant scale.Particle shape and liberation determined by mineralogy rather than breakage mechanisms.
In minerals beneficiation applications, the main function of comminution circuits is to liberate valuable minerals to facilitate downstream separation processes such as flotation. Traditionally, design and optimisation of comminution circuits was based on the production of a target particle size distribution at an optimised throughput with consideration of energy efficiency and equipment wear rates. However, research in flotation and process mineralogy is leading to queries as to whether more should be demanded from the comminution circuit in terms of particle preparation. For example if particle shape affects hydrophobicity, can mill operating conditions be adjusted to produce particles with shape characteristics which are more amenable to flotation? For this work UG2 ore (a South African platinum group mineral ore) was milled in a laboratory ball mill and stirred mill. To supplement the laboratory study, samples were taken from the fine grinding circuit of an operational UG2 concentrator. Automated Scanning Electron Microscopy with Energy Dispersive X-ray Spectrometry (Auto-SEM-EDS) was used for mineralogical and particle shape analysis of feed and product samples. Although the laboratory test work indicated a significant difference in product particle shape characteristics for the two mill types, the difference was not evident in the plant data.
ball mill for sale | grinding machine - jxsc mining
Ball mill is the key equipment for grinding materials. those grinding mills are widely used in the mining process, and it has a wide range of usage in grinding mineral or material into fine powder, such as gold, ironzinc ore, copper, etc.
JXSC Mining produce reliable effective ball mill for long life and minimum maintenance, incorporate many of the qualities which have made us being professional in the mineral processing industry since 1985. Various types of ball mill designs are available to suit different applications. These could include but not be restricted to coal mining grate discharge, dry type grinding, wet mineral grinding, high-temperature milling operations, stone & pebble milling.
A ball mill grinds ores to an end product size of thirty-five mesh or finer.
The feeding material to a ball mill is treated by:
Single or multistage crushing and screening
Crushing, screening, and/or rod milling
Primary crushing and autogenous/semi-autogenous grinding.
Normal feed sizes:
eighty percent of six millimeters or finer for hard rocker
eighty percent of twenty-five millimeters or finer for fragile rocks (Larger feed sizes can be tolerated depending on the requirements).
The ratio of machine length to the cylinder diameter of cylindrical type ball mills range from one to three through three to one. When the length to diameter ratio is two to one or even bigger, we should better choose the mill of a Tube Mill.
Grinding circuit design
Grinding circuit design is available, we experienced engineers expect the chance to help you with ore material grinding mill plant of grinding circuit design, installation, operation, and optimization.
The automatic operation has the advantage of saving energy consumption, grinding media, and reducing body liner wear while increasing grinding capacity. In addition, by using a software system to control the ore grinding process meet the requirements of different ore milling task.
The ball mill is a typical material grinder machine which widely used in the mineral processing plant, ball mill performs well in different material conditions either wet type grinding or dry type, and to grind the ores to a fine size.
Main ball mill components: cylinder, motor drive, grinding medium, shaft. The cylinder cavity is partial filling with the material to be ground and the metal grinding balls. When the large cylinder rotating and creating centrifugal force, the inner metal grinding mediums will be lifted to the predetermined height and then fall, the rock material will be ground under the gravity force and squeeze force of moving mediums. Feed material to be ground enters the cylinder through a hopper feeder on one end and after being crushed by the grinding medium is discharged at the other end.
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