Ball mills are employed in the comminution stage as grinding machines (size reduction). The purpose of grinders in the mining industry is to reduce the feed material size in order to liberate the minerals from the barren rock. Ball mills are the most common grinding machine employed in the mining industry.
Grinding occurs in a single stage, or multiple stages. Multiple stages may include a rod mill followed by a ball mill (two stage circuit), or a semi-autogenous grinding (SAG) mill followed by a ball mill (two stage circuit). Smaller plants tend to add extra crushing stages in order to operate a single grinding stage only.
The following process description is based upon a ball mill used in the hard rock mining industry for liberating minerals from ore, but the operating principle for ball mills used in other industries is the same.
For both wet and dry ball mills, the ball mill is charged to approximately 33% with balls (range 30-45%). Pulp (crushed ore and water) fills another 15% of the drums volume so that the total volume of the drum is 50% charged. Pulp is usually 75% solid (crushed ore) and 25% water; pulp is also known as slurry.
An electric motor is used to rotate the ball mill. As the ball mill rotates, the balls stick to the inner surface of the drum due to the centrifugal force created within the drum. At a certain angle, the weight of the balls overcomes the centrifugal force holding them against the drum and they begin to tumble back to the centre line of the ball mill (this area is known as the toe). In this manner, the ore is reduced in size by both attrition (ore rubbing against other bits of ore) and impact (balls impacting with the ore).
The ore moves gradually through the mill then exits through the discharge port. The discharge port may be covered by a grate to prevent oversized ore exiting the mill, or it may have no grate (overflow type ball mill).
Ball mills may operate in a closed-circuit, or open-circuit. Closed circuits return a certain amount of the ball mills output back to the ball mill for further size reduction. A typical closed system grinds the ore between two to three times.
Hydro-cyclones installed directly after the ball mill ensure only over-sized material is returned to the ball mill. Other types of classifiers can be used (rake and spiral classifiers), but the hydro-cyclone is now one of the most common.
Critical speed is defined as the point at which the centrifugal force applied to the grinding mill charge is equal to the force of gravity. At critical speed, the grinding mill charge clings to the mill inner surface and does not tumble.
Most ball mills operate at approximately 75% critical speed, as this is determined to be the optimum speed. The true optimum speed depends upon the drum diameter. Larger drum diameters operate at lower than 75% critical speed whilst smaller drum diameters operate at higher than 75% critical speed.
Irrespective of the type of grinding machine employed, grinding is a low efficiency and power intensive process. For this reason, the grinding stage of a mineral processing plant may account for up to 40% of total operating costs.
As a general rule of thumb, the larger the diameter of the ball mill drum, the more efficient the grinding process will be. This rule of thumb stops though once the diameter of the drum reaches approximately 4m (13.1 feet).
Dr Steve Morrell is a minerals processing engineer with over 40 years of specialist experience in the resources industry. Steve has been involved in the circuit design of the majority of major comminution circuits internationally, through which he has established himself as a world leading expert in comminution design and optimisation.Steve graduated with a Bachelor's Degree (Honours) in Engineering Science majoring in Metallurgy from Imperial College in London and spent the next 7 years gaining practical, real-world experience working on mines throughout Africa. In the late 1980's, Steve moved to Australia and completed Masters and Doctorate theses in grinding mill simulation and power draw modelling at the Julius Kruttschnitt Mineral Research Centre (JKMRC). Up untill 2000, when he left the JKMRC to start his own consulatnacy (SMCC Pty Ltd), Steve also led world-leading research projects such as the AMIRA P9, High Pressure Grinding Rolls, Fine Grinding and Mine-to-Mill programs and contributed significantly to many industry breakthroughs that helped improve our understanding of modern comminution processes and technologies.
In 2003, based on his extensive practical and research-based experience, Steve founded SMC Testing Pty Ltd to license the innovative SMC Test a laboratory comminution test which provides a range of information on the breakage characteristics of rock samples for use in the mining/minerals processing industry. To date, over 50,000 tests have been conducted globally, supporting that the SMC Test is one of the most useful, versatile and cost-effective rock breakage test procedures available.
In 2016, the Global Mining Standards and Guidelines Group (GMSG) recognised the SMC Test through the adoption of its guideline Morrell method for determining comminution circuit specic energy and assessing energy utilization eciency of existing circuits
In 2011 Steve was recognised for his significant contribution to the field of comminution when he was awarded the prestigious Art MacPherson Award by the Canadian Mineral Processors Society of CIM. In 2018 he was inducted into International Mining Technologys hall of Fame and in 2020 it was announced by the AusIMM that he had been given its Professional Excellence award.
3. MORRELL, S., 2010, Predicting the specific energy required for size reduction of relatively coarse feeds in conventional crushers and high pressure grinding rolls, minerals engineering Volume 23, Issue 2, January, Pages 151-153
5. MORRELL, S., 2008, Predicting the Overall Specific Energy Requirements of AG/SAG, Ball Mill and HPGR Circuits on the Basis of Small-Scale Laboratory Ore Characterisation Tests, Proceedings Procemin Conference, Santiago, Chile
8. MORRELL, S., 2006., AG/SAG Mill Circuit Grinding Energy Requirement - How to Predict it from Small Diameter Drill Core Samples Using the SMC Test, Advances in Comminution (Ed. Komar Kawatra), Society for Mining, Metallurgy and Exploration, pp 115-130
13. MORRELL, S. (2003). The influence of feed size on autogenous and semi- autogenous grinding and the role of blasting in its manipulation. Proceedings of the XXII International Mineral Processing Congress, Cape Town, South Africa, pp 526-533, SAIMM.
14. MORRELL, S. (2003). Grinding mills: how to accurately predict their power draw. Proceedings XXII International Mineral Processing Congress, Cape Town, South Africa, 29 September - 3 October 2003, pp 50-59, SAIMM.
16. MORRELL, S. (2000). Examining recent developments in the design and optimisation of crushing and grinding circuits to reduce costs and improve throughput and quality. Crushing and Grinding Technologies for Mineral Extraction, Perth, pp IIR Pty Ltd.
20. MORRELL, S. (1996). Power draw of wet tumbling mills and its relationship to charge dynamics. Part 1 a continuum approach to mathematical modelling of mill power draw. Tran Inst. Min. Met Vol 105: C43-53.
24. MORRELL, S., FINCH, W. M., KOJOVIC, T. and DELBONI, H. (1996). Modelling and simulation of large diameter autogenous and semi autogenous mills. Inernational Journal of Mineral Procesing Vol 44-45: 289-300.
27. MORRELL, S., JOHNSON, G. and REVY, T. (1991). A comparison through observation and simulation of the power utilisation and performance of two dissimilar comminution plants. Fourth Mill Operators Conf, Burnie, Tasmania, pp 157-160,
37. MORRELL, S. and LATCHIREDDI, S. (2000). The operation and interaction of grates and pulp lifters in autogenous and semi-autogenous mills. Seventh Mill Operator's Conference, Kalgoorlie, WA., pp 13-19, AusIMM.
41. MORRELL, S., VALERY, W., BANINI, G. and LATCHIREDDI, S. (2001). Developments in AG/SAG mill modelling. SAG 2001, Vancouver, pp 71-84, Dept of Mining and Mineral Process Engineering, Univ of British Columbia.
43. CLEARY, P., MORRISON, R. and MORRELL, S. (2001). DEM validation for a scale model SAG mill. SAG 2001, Vancouver, B.C., pp 191-206, Dept of Mining and Mineral Process Engineering, Univ of British Columbia.
51. DUNNE, R., MORRELL, S., LANE, G., VALERY, W. and HART, S. (2001). Design of the 40 foot diameter SAG mill installed at the Cadia gold copper mine. SAG 2001, Vancouver, pp 43-58, Dept of Mining and Mineral Process Engineering, Univ of British Columbia.
52. GOTTLEIB, P., MORRELL, S. and WELLER, K. R. (1994). Modelling grinding and liberation in tower mill regrind circuits used in lead/zinc concentrators. 8th European Symposium on Comminution, Stockholm, pp 761-772,
54. JANKOVIC, A. and MORRELL, S. (2000). Scale-up of tower mill performance using modeling and simulation. Proceedings of the XXI International Processing Congress, Rome,Italy, pp 1-7, Elsevier Science B. V.
57. KARAGEORGOS, J., BURFORD, B., VALERY, W., ROHNER, P., JOHNSON, G. and MORRELL, S. (1996). Copper concentrator autogenous grinding practices at Mount Isa Mines Ltd. Proc SAG '96, Vancouver, pp 145-163,
67. LYNCH, A. J. and MORRELL, S. (1992). The understanding of comminution and classification and its application in plant design and operation. Comminution - Theory and Practice. S. K. Kawatra pp 405-426.
73. NICOLI, D., MORRELL, S., CHAPMAN, B. and LATCHIREDDI, S. (2001). The development and installation of the twin chamber pulp lifters at Alcoa. SAG 2001, Vancouver, pp 240-255, Dept of Mining and Mineral Process Engineering Univ of British Columbia`.
80. TONDO, L. A., MORRELL, S. and JOHNSON, G. (1995). A study of the energy consumption for grinding an Australian gold ore in a high pressure roll machine. Proc XVI Encontro Nacional de Tratmento De MInerios E Hidrometalurgia, Rio, Brazil, pp 65-79,
82. VALERY, W., KOJOVIC, T., TAPIA-VERGARA, F. and MORRELL, S. (1999). Optimisation of blasting and sag mill feed size by application of on line size analysis. IIR Crushing and Grinding Conference, Perth (29-31 March), pp
83. VALERY, W., MORRELL, S., KOJOVIC, T., KANCHIBOTLA, S. S. and THORNTON, D. M. (2001). Modelling and simulation techniques applied for optimisation of mine to mill operations and case studies. VI Southern Hemisphere Meeting on Mineral Technology, Rio de Janeiro, Brazil, pp 107-116, CETEM/MCT.
84. WELLER, K. R., MORRELL, S. and GOTTLEIB, P. (1996). Use of grinding and liberation models to simulate tower mill circuit performance in lead/zinc concentrator to increase flotation recovery. Inernational Journal of Mineral Procesing Vol 44-45: 683-702.
Ball mill is a type of grinder machine which uses steel ball as grinding medium, can crush and grind the materials to 35 mesh or finer, adopted in open or close circuit. The feed materials can be dry or wet, they are broken by the force of impact and attrition that created by the different sized balls.Types of ball milldry grinding ball mill and wet grinding ball mill; grate discharge ball mill and overflow ball mill.Applicationsmining, chemical, glass, ceramics, etc.Suitable MaterialsCement, silicate products, new building materials, refractory materials, fertilizers, mineral processing and glass ceramics.
Ball mill is a horizontal machine, contains a hollow cylindrical shell that rotates around its axis, Inside the cylinder, there are many different sized stainless steel balls. As the the cylinder rotates, the mill balls lifts and then drops, strikes the materials, that is the impact and attrition take place.The cylinder chamber which turning around the horizontal axis is partially filled with grinding mediums: mostly are steel balls, cast iron or porcelain balls. Filling rate best at 40%, steel balls diameter with 30 to 80mm.These grinding balls are initially 3-10 cm in diameter, but gradually became smaller as grinding progressed. So we usually just refill the big balls.The chamber is lined with a wear resistant material, such as manganese-steel or high quality rubber, to extend the service time.Thanks to the closed grinding chamber, the dust and pollution generated in the grinding process are avoided to emit to air.
Eastman provides you with complete rock crushers and full list of replacement parts, original ball mill parts, form and function are a perfect fit.If your equipment breaks down, the productivity of the whole factory will be threatened. Critical wear parts are shipped with the goods to ensure they are available when you need them and to reduce maintenance time.
Eastman is a crushing manufacturer with more than 30 years of experience, produces hammer crusher used for a variety of applications.We not only can provide you with various types of rock crusher, but also can design reasonable crushing process for you free.
Factors of ball mill product sizeWithin the rotating chamber the grinding balls rub and strike against each other.The final discharge size can be changed by changing the number and size of the steel balls, the material of the ball, rotate speed, and the what material to be ground. Besides, the ball mill production rate is directly proportional to the drum rotation speed. Check the ball mill critical rotation speed which indicated in the manufacturers technical specifications.
Since the early days, there has been a general consensus within the industry and amongst grinding professionals that classification efficiency and circulating load both have a major effect on the efficiency of closed circuit ball mills. However, the effect of each is difficult to quantify in practice as these two parameters are usually interrelated. Based on experience acquired over the years and the investigative work conducted by F.C. Bond, it was established that the optimum circulating load for a closed ball mill cyclone circuit is around 250%. This value is used as guideline for the design of new circuits as well as to assess the performance of existing circuits.
The role of classification in milling appears to have been neglected in the current efforts to reduce the energy consumption of grinding. Two past approaches, experimental and modelling, for quantifying the effects of classification efficiency and circulating load on the capacity of closed ball mill circuits, are revisited and discussed in this paper. Application to the optimisation of existing circuits and design of new circuits is also discussed, with special attention to the development of more energy efficient circuits.
Circulating load and classification efficiency effect on ball mill capacity revisited. Relative capacity model introduced and validated. Relationship between circulating load and classification efficiency verified by industrial data. Existing fine screening technology could increase ball mill circuit capacity 1525%.
There are many factors that may affect the ball mills working efficiency and product quality during the operation. In this article, we will discuss the measures that can improve the ball mills performance.
The particle size of the feed material is an important process parameter that restricts the grinding efficiency of the ball mill. Due to the different physical and chemical properties and microhardness of the materials (the grindability of materials in raw meals decreases in clinkers), the clinker discharged from the cement kiln must be pretreated to reduce its particle size so as to increase the output and reduce the power consumption of the ball mill.
From table 1 we can learn that if the particle size of the feed material is reduced from 25 mm to less than 2 mm, the mill output can be increased by at least 60%, which is relatively consistent with the actual production.
There are two methods for clinker pretreatment: pre-crushing and pre-grinding. 1) The pre-crushing uses a crusher to crush the clinker before grinding, which can reduce the diameter of clinker particle to 5 ~ 8mm. 2) The pre-grinding adds a roller press to the cement grinding system. In this system, the clinker is extruded circularly, dispersed and separated, and becomes powder with diameter less than 2 mm;
The gradation of grinding media is also an important factor in improving the efficiency of ball mills. A reasonable gradation can only be calculated after analyzing the performance of the mill, the property of the feed material, and equipment layout in the closed-circuit grinding system.
The size of the grinding media is calculated based on the grinding capacity of the mill and the size of the feed material. Because of the complex movement of the grinding media and the material in the mill, and because the actual production situation of each cement plant is different, it is difficult to determine a universally applicable grading rule. Only through long-term production practice can we get the appropriate gradation scheme.
The gradation of grinding media is constantly changing in the process of mill operation, and the wear law of different size of grinding media is also different. Therefore, the supplementary of grinding media can only keep the loading capacity relatively balanced, but can not keep the gradation consistent.
The stable grinding process largely depends on the material of grinding medium. Different materials of grinding media lead to different wear consumption. If the hardness and wear resistance of the grinding media are poor, it is easy to deform and crack during the operation, which not only affects the grinding efficiency and blocks the grate gap, but also makes the partition device difficult to discharge material, and finally leads to the deterioration of the mill operation. Therefore, improving the quality of the grinding media is an effective way to ensure the long-term stable operation of the mill, otherwise, no matter how reasonable the grading scheme is, it is difficult to ensure that the expected grinding effect can always be achieved.
Once the grinding media and other equipment is properly selected for the grinding system, then the gradation can be determined according to the particle size of the feed material. However, no matter how reasonable the grading scheme is, it is always relative.
The ball bearing height of ball mills can be different due to different specifications, diameters, rotating speeds and liner forms of the ball mills. And the potential energy produced by different height of the ball is completely different. Therefore, the reasonable grinding ball diameter should not only match with the mill specifications, but also adapt to the liner form of the mill.
Large size mills with lifting liner bring grinding balls to higher heights and generate stronger impact force, so the diameter of grinding balls can be smaller. The ball diameter should be different according to the aging degree of the inner liner: new liners bring grinding balls to higher height, so the ball size can be smaller.
It can be seen from the experiment that when a grinding ball with a diameter of 70 mm falls freely from the height of 40 cm, its potential energy can completely crush a clinker particle with a diameter of 25 mm. Therefore, the minimum ball diameter should be selected on the premise of sufficient impact energy to increase the number of grinding balls, increase the impact times of balls on materials, and improve the grinding efficiency.
Table 2 and table 3 show the relationship between the material particle size and the grinding ball diameter for reference only. When determining the ball diameter, it is necessary to adjust it according to the cement plants own situation.Get in Touch with Mechanic