ball mill design parameters

how to size a ball mill -design calculator & formula

A) Total Apparent Volumetric Charge Filling including balls and excess slurry on top of the ball charge, plus the interstitial voids in between the balls expressed as a percentage of the net internal mill volume (inside liners).

B) Overflow Discharge Mills operating at low ball fillings slurry may accumulate on top of the ball charge; causing, the Total Charge Filling Level to be higher than the Ball Filling Level. Grate Discharge mills will not face this issue.

C) This value represents the Volumetric Fractional Filling of the Voids in between the balls by the retained slurry in the mill charge. As defined, this value should never exceed 100%, but in some cases particularly in Grate Discharge Mills it could be lower than 100%. Note that this interstitial slurry does not include the overfilling slurry derived from the difference between the Charge and Ball Filling.

D) Represents the so-called Dynamic Angle of Repose (or Lift Angle) adopted during steady operation by the top surface of the mill charge (the kidney) with respect to the horizontal. A reasonable default value for this angle is 32, but may be easily tuned to specific applications against any available actual power data.

The first step in mill design is to determine the power needed to produce the desired grind in the chosen ore. The most used equation, for this purpose, is the empirical Bond equation (Bond, 1960, 1961; Rowland and Kjos, 1978).

In this equation, E is the specific energy required for the grind, and F80 and P80 are the sizes in micrometers that 80% of the weight passes of the mill feed and product respectively. The parameter Wi, known as the work index of the ore, is obtained from batch bench tests first devised by Bond (1961). The power calculated on using equation 1, (Bond, 1961; Rowland and Kjos, 1978), relates to:

1) Rod milling a rod mill with a diameter of 2.44 meters, inside new liners, grinding wet in open circuit. 2) Ball milling a ball mill with a diameter of 2.44 meters, inside new liners, grinding wet in open circuit.

When the grinding conditions differ from these specified conditions, efficiency factors (Rowland and Kjos, 1978) have to be used in conjunction with equation 1. In general, therefore, the required mill power is calculated using the following equation

where n is the number of efficiency factors, EFi, used and fo is the feed rate of new ore to the mill. The power calculated from equation 2 can be looked up in published tables (Rowland and Kjos, 1978) and the correct mill size and type can be selected.

The philosophy in the development of the MRRC grinding simulation package was to build interactive software that could be used as an inexpensive means of providing a semi-quantitative check on a grinding mill design. In addition the software is designed to slot in to a general mineral processing package now undergoing development at the MRRC.

ball mill parameter selection & calculation - power, critical speed | jxsc

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The production capacity of the ball mill is determined by the amount of material required to be ground, and it must have a certain margin when designing and selecting. There are many factors affecting the production capacity of the ball mill, in addition to the nature of the material (grain size, hardness, density, temperature and humidity), the degree of grinding (product size), the uniformity of the feeding material, and the portion of loaded, , and the mill structure (the mill barrel length, diameter ratio, the number of bins, the shape of the partition plate and the lining plate). It is difficult to theoretically determine the productivity of the mill. The grinding mills production capacity is generally calculated based on the newly generated powder ore of less than 0.074 mm (-200 mesh). V Effective volume of ball mill, m3; G2 Material less than 0.074mm in product accounts for the percentage of total material, %; G1 Material less than 0.074mm in ore feeding accounts for 0.074mm in the percentage of the total material, %; qm Unit productivity calculated according to the new generation grade (0.074mm), t/(m3.h). The values of qm are determined by experiments or are calibrated in production with similar ore physical properties and the same equipment and working conditions. When there is no test data and production calibration value, it can be calculated by formula (1-3). Di1- Standard mill diameter, m; K4 feed size and product size coefficient of mill. G3 G4 The production capacity of existing or experimental mills with newly designed and parameters (feed size or product size calculated according to the new generation 0.074mm level) is shown in Table 1-6. The values of G1 and G2 above should be calculated according to actual data. If there is no actual data, they can be selected according to tables 1-7 and 1-8.

When the filling rate of grinding medium is less than 35% in dry grinding operation, the power can be calculated by formula (1-7). n - mill speed, r/min; G - Total grinding medium, T; - Mechanical efficiency, when the center drive, = 0.92-0.94; when the edge drive, = 0.86-0.90.

\ Critical Speed_ When the ball mill cylinder is rotated, there is no relative slip between the grinding medium and the cylinder wall, and it just starts to run in a state of rotation with the cylinder of the mill. This instantaneous speed of the mill is as follows: N0 - mill working speed, r/min; Kb speed ratio, %. There are many layers of grinding media in the mill barrel. It is assumed that the media will be concentrated in one layer, called the polycondensation layer, so that the grinding media of this layer will be in the maximum drop, i.e. the calculating speed of the mill when the total impact energy is the largest nj. Therefore, it is theoretically deduced that the reasonable working speed is The working speeds of various mills are shown in Table 1-10. Table 1-10 Working speeds of various mills

In production practice, there are many factors affecting the motion state of grinding media. Therefore, the appropriate working speed should be selected according to the actual situation. In determining the actual working speed of the mill, the influences of the mill specifications, production methods, liner forms, grinding media types, filling rate, physical and chemical properties of the ground materials, particle size of the grinding materials and grinding fineness of the products should be taken into account. The actual working speed of the mill should be determined by scientific experiments, which can reflect the influence of these factors more comprehensively.

Ball loading capacity The volume of the grinding medium is the percentage of the effective volume of the mill, which is called the filling rate of the grinding medium. The size of filling directly affects the number of shocks, the area of grinding and the load of grinding medium in the grinding process. At the same time, it also affects the height of the grinding medium itself, the impact on the material and the power consumption. A kind of The ball loading capacity of the mill can be calculated according to the formula (1-14). Gra Quantity of Grinding Medium, T. Rho s loose density of grinding medium, t/m3. Forged steel balls; P=s=4.5-4.8t/m3 cast steel balls P=4.3-4.6t/m3; rolling steel balls P=6.0-6.8t/m3; steel segments P=4.3-4.6t/m3_-filling ratio of grinding medium, When wet grinding: lattice ball mill pi = 40% 45%; overflow ball mill phi = 40%; rod mill phi = 35%. Dry grinding: When material is mixed between grinding media, the grinding medium expands, and when dry grinding is adopted, the material fluidity is relatively poor, material flow is hindered by abrasive medium, so filling rate is low, and the filling rate is between 28% and 35%. The pipe mill is 25%-35%. The void fraction of grinding medium_k=0.38-0.42 and the quality of crushed material accounts for about 14% of the quality of grinding medium.

Size and Proportion of Grinding Medium In the ball mill, the size and proportion of steel balls have a great influence on the productivity and working efficiency of the mill. For coarse and hard materials, larger steel balls should be selected, for fine and brittle materials, with smaller diameter steel balls, the impact times of steel balls in the mill increase with the decrease of ball diameter, and the grinding between balls increases. The clearance is dense with a decrease of spherical diameter. Therefore, it is better to choose the ball with a larger mass and smaller diameter (loose density) as the grinding medium. The size of the ball mainly depends on the particle size of the material to be ground, and the diameter and speed of the mill can be considered appropriately. Formula (1-15) is an empirical formula for spherical diameter and feed size. dmax The maximum diameter of steel ball, mm; amax the maximum size of feeding granularity, mm. After calculating the maximum steel ball diameter, the steel ball ratio in the mill can be calculated with reference to Fig. 2-1 (suitable for cement mill, other mills can refer to). After choosing the maximum diameter and minimum diameter of steel balls according to technological requirements, material properties, mill specifications and various parameters, and then matching grade, using curves, the accumulative percentage of the mass of each corresponding steel balls loaded into the mill can be found, the actual percentage of the mass can be calculated, and the loading quality of steel balls at all levels can be obtained. According to the production practice of production enterprises, the relationship between ball diameter and material size is shown in Table 1-11. A kind of Steel balls are gradually worn out in the process of grinding materials. The wear of drop steel ball is related to its impact force. The wear of grinding steel balls is related to the surface area of steel balls. In general, the steel ball in the grinder has both impact and abrasion effects, so the wear is proportional to the n power of the diameter of the steel ball, and the value of n is between 2 and 3. Table 1-11 The Relation between Steel Ball Diameter and Material Size

The quality and surface area of forged steel balls of various sizes are shown in Table 1-12. A kind of Because of the wear of steel balls in the mill production process, in order to keep the mill stable. Steel balls need to be added regularly. The maximum diameter of additional steel balls is still determined by the method mentioned above. In addition to the addition of additional steel balls, several smaller diameter steel balls should be added according to production experience.

ball mills: design and operating principle - strommashina

The Samara Strommashina plant has been manufacturing ball mills since the middle of the twentieth century. Hundreds of mills of most diverse modifications have been manufactured over 70 years of the plant operation. Currently the Strommashina manufactures the main types industrial ball mills designed for long trouble-free operation.

Ball mill design is rather simple. This grinding equipment is a drum filled with metal or cast iron balls (grinding media). It is used mostly for production of bulk construction materials, paints, pyrotechnical devices, ceramics and in other industries.

The ball mill patent was sealed more than 180 years ago. It is mentioned in "Industry and engineering" encyclopaedia dated 1896. This mechanism was described as an efficient device for grinding sand in glass production.

As a result of rotation, balls on the internal drum surface lift and then fall down under the gravity action. The source material is permanently fed through one of spigots. The raw material particles are ground by means of smashing, abrasion and collision. The drum is discharged through other spigot.

If the mill is designed for dry grinding, then the processed raw material is discharged using air flow. Air is delivered by drawing it off from the drum. If wet grinding is carried out, then material is carried by a water flow.

As rotation frequency grows, the drum mill capacity grows first. It is because the balls lift to significant height first. However, soon, if the speed continues to grow, the balls begin to "stick" to the internal drum surface. In this case the crushing plant capacity s drops down abruptly.

Upon expiry of some mill operation time the mill operating elements (grinding balls) get worn. In this case they are just added into the drum together with source material. The maximum filling level of impact elements is 50 mm below the inlet nozzle circumference.

Ball diameter is 30 to 60 mm. The maximum size of source material particles is 10 to 50 mm. The drum length is 1500 to 10000 mm The drum diameter is 900 to 4000 mm Rotation frequency is 10 to 40 rpm. Motor power is at least 22 kW. Capacity is at least 2 t per hour.

Such crushing equipment has its drawbacks which are usually specified as high price, large dimensions and high power consumption. However, the Strommashina has improved the ball mill design over the past decades. It eventually enabled optimising power consumption and reducing the mill manufacture cost. In addition, the plant engineering department is available for calculating the mill dimensions for customised production needs, and for performing all necessary equipment installation and start-up works.

ball mills

In all ore dressing and milling Operations, including flotation, cyanidation, gravity concentration, and amalgamation, the Working Principle is to crush and grind, often with rob mill & ball mills, the ore in order to liberate the minerals. In the chemical and process industries, grinding is an important step in preparing raw materials for subsequent treatment.In present day practice, ore is reduced to a size many times finer than can be obtained with crushers. Over a period of many years various fine grinding machines have been developed and used, but the ball mill has become standard due to its simplicity and low operating cost.

A ball millefficiently operated performs a wide variety of services. In small milling plants, where simplicity is most essential, it is not economical to use more than single stage crushing, because the Steel-Head Ball or Rod Mill will take up to 2 feed and grind it to the desired fineness. In larger plants where several stages of coarse and fine crushing are used, it is customary to crush from 1/2 to as fine as 8 mesh.

Many grinding circuits necessitate regrinding of concentrates or middling products to extremely fine sizes to liberate the closely associated minerals from each other. In these cases, the feed to the ball mill may be from 10 to 100 mesh or even finer.

Where the finished product does not have to be uniform, a ball mill may be operated in open circuit, but where the finished product must be uniform it is essential that the grinding mill be used in closed circuit with a screen, if a coarse product is desired, and with a classifier if a fine product is required. In most cases it is desirable to operate the grinding mill in closed circuit with a screen or classifier as higher efficiency and capacity are obtained. Often a mill using steel rods as the grinding medium is recommended, where the product must have the minimum amount of fines (rods give a more nearly uniform product).

Often a problem requires some study to determine the economic fineness to which a product can or should be ground. In this case the 911Equipment Company offers its complete testing service so that accurate grinding mill size may be determined.

Until recently many operators have believed that one particular type of grinding mill had greater efficiency and resulting capacity than some other type. However, it is now commonly agreed and accepted that the work done by any ballmill depends directly upon the power input; the maximum power input into any ball or rod mill depends upon weight of grinding charge, mill speed, and liner design.

The apparent difference in capacities between grinding mills (listed as being the same size) is due to the fact that there is no uniform method of designating the size of a mill, for example: a 5 x 5 Ball Mill has a working diameter of 5 inside the liners and has 20 per cent more capacity than all other ball mills designated as 5 x 5 where the shell is 5 inside diameter and the working diameter is only 48 with the liners in place.

Ball-Rod Mills, based on 4 liners and capacity varying as 2.6 power of mill diameter, on the 5 size give 20 per cent increased capacity; on the 4 size, 25 per cent; and on the 3 size, 28 per cent. This fact should be carefully kept in mind when determining the capacity of a Steel- Head Ball-Rod Mill, as this unit can carry a greater ball or rod charge and has potentially higher capacity in a given size when the full ball or rod charge is carried.

A mill shorter in length may be used if the grinding problem indicates a definite power input. This allows the alternative of greater capacity at a later date or a considerable saving in first cost with a shorter mill, if reserve capacity is not desired. The capacities of Ball-Rod Mills are considerably higher than many other types because the diameters are measured inside the liners.

The correct grinding mill depends so much upon the particular ore being treated and the product desired, that a mill must have maximum flexibility in length, type of grinding medium, type of discharge, and speed.With the Ball-Rod Mill it is possible to build this unit in exact accordance with your requirements, as illustrated.

To best serve your needs, the Trunnion can be furnished with small (standard), medium, or large diameter opening for each type of discharge. The sketch shows diagrammatic arrangements of the four different types of discharge for each size of trunnion opening, and peripheral discharge is described later.

Ball-Rod Mills of the grate discharge type are made by adding the improved type of grates to a standard Ball-Rod Mill. These grates are bolted to the discharge head in much the same manner as the standard headliners.

The grates are of alloy steel and are cast integral with the lifter bars which are essential to the efficient operation of this type of ball or rod mill. These lifter bars have a similar action to a pump:i. e., in lifting the product so as to discharge quickly through the mill trunnion.

These Discharge Grates also incorporate as an integral part, a liner between the lifters and steel head of the ball mill to prevent wear of the mill head. By combining these parts into a single casting, repairs and maintenance are greatly simplified. The center of the grate discharge end of this mill is open to permit adding of balls or for adding water to the mill through the discharge end.

Instead of being constructed of bars cast into a frame, Grates are cast entire and have cored holes which widen toward the outside of the mill similar to the taper in grizzly bars. The grate type discharge is illustrated.

The peripheral discharge type of Ball-Rod Mill is a modification of the grate type, and is recommended where a free gravity discharge is desired. It is particularly applicable when production of too many fine particles is detrimental and a quick pass through the mill is desired, and for dry grinding.

The drawings show the arrangement of the peripheral discharge. The discharge consists of openings in the shell into which bushings with holes of the desired size are inserted. On the outside of the mill, flanges are used to attach a stationary discharge hopper to prevent pulp splash or too much dust.

The mill may be operated either as a peripheral discharge or a combination or peripheral and trunnion discharge unit, depending on the desired operating conditions. If at any time the peripheral discharge is undesirable, plugs inserted into the bushings will convert the mill to a trunnion discharge type mill.

Unless otherwise specified, a hard iron liner is furnished. This liner is made of the best grade white iron and is most serviceable for the smaller size mills where large balls are not used. Hard iron liners have a much lower first cost.

Electric steel, although more expensive than hard iron, has advantage of minimum breakage and allows final wear to thinner section. Steel liners are recommended when the mills are for export or where the source of liner replacement is at a considerable distance.

Molychrome steel has longer wearing qualities and greater strength than hard iron. Breakage is not so apt to occur during shipment, and any size ball can be charged into a mill equipped with molychrome liners.

Manganese liners for Ball-Rod Mills are the world famous AMSCO Brand, and are the best obtainable. The first cost is the highest, but in most cases the cost per ton of ore ground is the lowest. These liners contain 12 to 14% manganese.

The feed and discharge trunnions are provided with cast iron or white iron throat liners. As these parts are not subjected to impact and must only withstand abrasion, alloys are not commonly used but can be supplied.

Gears for Ball-Rod Mills drives are furnished as standard on the discharge end of the mill where they are out of the way of the classifier return, scoop feeder, or original feed. Due to convertible type construction the mills can be furnished with gears on the feed end. Gear drives are available in two alternative combinations, which are:

All pinions are properly bored, key-seated, and pressed onto the steel countershaft, which is oversize and properly keyseated for the pinion and drive pulleys or sheaves. The countershaft operates on high grade, heavy duty, nickel babbitt bearings.

Any type of drive can be furnished for Ball-Rod Mills in accordance with your requirements. Belt drives are available with pulleys either plain or equipped with friction clutch. Various V- Rope combinations can also be supplied.

The most economical drive to use up to 50 H. P., is a high starting torque motor connected to the pinion shaft by means of a flat or V-Rope drive. For larger size motors the wound rotor (slip ring) is recommended due to its low current requirement in starting up the ball mill.

Should you be operating your own power plant or have D. C. current, please specify so that there will be no confusion as to motor characteristics. If switches are to be supplied, exact voltage to be used should be given.

Even though many ores require fine grinding for maximum recovery, most ores liberate a large percentage of the minerals during the first pass through the grinding unit. Thus, if the free minerals can be immediately removed from the ball mill classifier circuit, there is little chance for overgrinding.

This is actually what has happened wherever Mineral Jigs or Unit Flotation Cells have been installed in the ball mill classifier circuit. With the installation of one or both of these machines between the ball mill and classifier, as high as 70 per cent of the free gold and sulphide minerals can be immediately removed, thus reducing grinding costs and improving over-all recovery. The advantage of this method lies in the fact that heavy and usually valuable minerals, which otherwise would be ground finer because of their faster settling in the classifier and consequent return to the grinding mill, are removed from the circuit as soon as freed. This applies particularly to gold and lead ores.

Ball-Rod Mills have heavy rolled steel plate shells which are arc welded inside and outside to the steel heads or to rolled steel flanges, depending upon the type of mill. The double welding not only gives increased structural strength, but eliminates any possibility of leakage.

Where a single or double flanged shell is used, the faces are accurately machined and drilled to template to insure perfect fit and alignment with the holes in the head. These flanges are machined with male and female joints which take the shearing stresses off the bolts.

The Ball-Rod Mill Heads are oversize in section, heavily ribbed and are cast from electric furnace steel which has a strength of approximately four times that of cast iron. The head and trunnion bearings are designed to support a mill with length double its diameter. This extra strength, besides eliminating the possibility of head breakage or other structural failure (either while in transit or while in service), imparts to Ball-Rod Mills a flexibility heretofore lacking in grinding mills. Also, for instance, if you have a 5 x 5 mill, you can add another 5 shell length and thus get double the original capacity; or any length required up to a maximum of 12 total length.

On Type A mills the steel heads are double welded to the rolled steel shell. On type B and other flanged type mills the heads are machined with male and female joints to match the shell flanges, thus taking the shearing stresses from the heavy machine bolts which connect the shell flanges to the heads.

The manhole cover is protected from wear by heavy liners. An extended lip is provided for loosening the door with a crow-bar, and lifting handles are also provided. The manhole door is furnished with suitable gaskets to prevent leakage.

The mill trunnions are carried on heavy babbitt bearings which provide ample surface to insure low bearing pressure. If at any time the normal length is doubled to obtain increased capacity, these large trunnion bearings will easily support the additional load. Trunnion bearings are of the rigid type, as the perfect alignment of the trunnion surface on Ball-Rod Mills eliminates any need for the more expensive self-aligning type of bearing.

The cap on the upper half of the trunnion bearing is provided with a shroud which extends over the drip flange of the trunnion and effectively prevents the entrance of dirt or grit. The bearing has a large space for wool waste and lubricant and this is easily accessible through a large opening which is covered to prevent dirt from getting into the bearing.Ball and socket bearings can be furnished.

Scoop Feeders for Ball-Rod Mills are made in various radius sizes. Standard scoops are made of cast iron and for the 3 size a 13 or 19 feeder is supplied, for the 4 size a 30 or 36, for the 5 a 36 or 42, and for the 6 a 42 or 48 feeder. Welded steel scoop feeders can, however, be supplied in any radius.

The correct size of feeder depends upon the size of the classifier, and the smallest feeder should be used which will permit gravity flow for closed circuit grinding between classifier and the ball or rod mill. All feeders are built with a removable wearing lip which can be easily replaced and are designed to give minimum scoop wear.

A combination drum and scoop feeder can be supplied if necessary. This feeder is made of heavy steel plate and strongly welded. These drum-scoop feeders are available in the same sizes as the cast iron feeders but can be built in any radius. Scoop liners can be furnished.

The trunnions on Ball-Rod Mills are flanged and carefully machined so that scoops are held in place by large machine bolts and not cap screws or stud bolts. The feed trunnion flange is machined with a shoulder for insuring a proper fit for the feed scoop, and the weight of the scoop is carried on this shoulder so that all strain is removed from the bolts which hold the scoop.

High carbon steel rods are recommended, hot rolled, hot sawed or sheared, to a length of 2 less than actual length of mill taken inside the liners. The initial rod charge is generally a mixture ranging from 1.5 to 3 in diameter. During operation, rod make-up is generally the maximum size. The weights per lineal foot of rods of various diameters are approximately: 1.5 to 6 lbs.; 2-10.7 lbs.; 2.5-16.7 lbs.; and 3-24 lbs.

Forged from the best high carbon manganese steel, they are of the finest quality which can be produced and give long, satisfactory service. Data on ball charges for Ball-Rod Mills are listed in Table 5. Further information regarding grinding balls is included in Table 6.

Rod Mills has a very define and narrow discharge product size range. Feeding a Rod Mill finer rocks will greatly impact its tonnage while not significantly affect its discharge product sizes. The 3.5 diameter rod of a mill, can only grind so fine.

Crushers are well understood by most. Rod and Ball Mills not so much however as their size reduction actions are hidden in the tube (mill). As for Rod Mills, the image above best expresses what is going on inside. As rocks is feed into the mill, they are crushed (pinched) by the weight of its 3.5 x 16 rods at one end while the smaller particles migrate towards the discharge end and get slightly abraded (as in a Ball Mill) on the way there.

We haveSmall Ball Mills for sale coming in at very good prices. These ball mills are relatively small, bearing mounted on a steel frame. All ball mills are sold with motor, gears, steel liners and optional grinding media charge/load.

Ball Mills or Rod Mills in a complete range of sizes up to 10 diameter x20 long, offer features of operation and convertibility to meet your exactneeds. They may be used for pulverizing and either wet or dry grindingsystems. Mills are available in both light-duty and heavy-duty constructionto meet your specific requirements.

All Mills feature electric cast steel heads and heavy rolled steelplate shells. Self-aligning main trunnion bearings on large mills are sealedand internally flood-lubricated. Replaceable mill trunnions. Pinion shaftbearings are self-aligning, roller bearing type, enclosed in dust-tightcarrier. Adjustable, single-unit soleplate under trunnion and drive pinionsfor perfect, permanent gear alignment.

Ball Mills can be supplied with either ceramic or rubber linings for wet or dry grinding, for continuous or batch type operation, in sizes from 15 x 21 to 8 x 12. High density ceramic linings of uniform hardness male possible thinner linings and greater and more effective grinding volume. Mills are shipped with liners installed.

Complete laboratory testing service, mill and air classifier engineering and proven equipment make possible a single source for your complete dry-grinding mill installation. Units available with air swept design and centrifugal classifiers or with elevators and mechanical type air classifiers. All sizes and capacities of units. Laboratory-size air classifier also available.

A special purpose batch mill designed especially for grinding and mixing involving acids and corrosive materials. No corners mean easy cleaning and choice of rubber or ceramic linings make it corrosion resistant. Shape of mill and ball segregation gives preferential grinding action for grinding and mixing of pigments and catalysts. Made in 2, 3 and 4 diameter grinding drums.

Nowadays grinding mills are almost extensively used for comminution of materials ranging from 5 mm to 40 mm (3/161 5/8) down to varying product sizes. They have vast applications within different branches of industry such as for example the ore dressing, cement, lime, porcelain and chemical industries and can be designed for continuous as well as batch grinding.

Ball mills can be used for coarse grinding as described for the rod mill. They will, however, in that application produce more fines and tramp oversize and will in any case necessitate installation of effective classification.If finer grinding is wanted two or three stage grinding is advisable as for instant primary rod mill with 75100 mm (34) rods, secondary ball mill with 2540 mm(11) balls and possibly tertiary ball mill with 20 mm () balls or cylpebs.To obtain a close size distribution in the fine range the specific surface of the grinding media should be as high as possible. Thus as small balls as possible should be used in each stage.

The principal field of rod mill usage is the preparation of products in the 5 mm0.4 mm (4 mesh to 35 mesh) range. It may sometimes be recommended also for finer grinding. Within these limits a rod mill is usually superior to and more efficient than a ball mill. The basic principle for rod grinding is reduction by line contact between rods extending the full length of the mill, resulting in selective grinding carried out on the largest particle sizes. This results in a minimum production of extreme fines or slimes and more effective grinding work as compared with a ball mill. One stage rod mill grinding is therefore suitable for preparation of feed to gravimetric ore dressing methods, certain flotation processes with slime problems and magnetic cobbing. Rod mills are frequently used as primary mills to produce suitable feed to the second grinding stage. Rod mills have usually a length/diameter ratio of at least 1.4.

Tube mills are in principle to be considered as ball mills, the basic difference being that the length/diameter ratio is greater (35). They are commonly used for surface cleaning or scrubbing action and fine grinding in open circuit.

In some cases it is suitable to use screened fractions of the material as grinding media. Such mills are usually called pebble mills, but the working principle is the same as for ball mills. As the power input is approximately directly proportional to the volume weight of the grinding media, the power input for pebble mills is correspondingly smaller than for a ball mill.

A dry process requires usually dry grinding. If the feed is wet and sticky, it is often necessary to lower the moisture content below 1 %. Grinding in front of wet processes can be done wet or dry. In dry grinding the energy consumption is higher, but the wear of linings and charge is less than for wet grinding, especially when treating highly abrasive and corrosive material. When comparing the economy of wet and dry grinding, the different costs for the entire process must be considered.

An increase in the mill speed will give a directly proportional increase in mill power but there seems to be a square proportional increase in the wear. Rod mills generally operate within the range of 6075 % of critical speed in order to avoid excessive wear and tangled rods. Ball and pebble mills are usually operated at 7085 % of critical speed. For dry grinding the speed is usually somewhat lower.

The mill lining can be made of rubber or different types of steel (manganese or Ni-hard) with liner types according to the customers requirements. For special applications we can also supply porcelain, basalt and other linings.

The mill power is approximately directly proportional to the charge volume within the normal range. When calculating a mill 40 % charge volume is generally used. In pebble and ball mills quite often charge volumes close to 50 % are used. In a pebble mill the pebble consumption ranges from 315 % and the charge has to be controlled automatically to maintain uniform power consumption.

In all cases the net energy consumption per ton (kWh/ton) must be known either from previous experience or laboratory tests before mill size can be determined. The required mill net power P kW ( = ton/hX kWh/ton) is obtained from

Trunnions of S.G. iron or steel castings with machined flange and bearing seat incl. device for dismantling the bearings. For smaller mills the heads and trunnions are sometimes made in grey cast iron.

The mills can be used either for dry or wet, rod or ball grinding. By using a separate attachment the discharge end can be changed so that the mills can be used for peripheral instead of overflow discharge.

analysis of ball mill grinding operation using mill power specific kinetic parameters - sciencedirect

Effect of operating variables on the energy efficiency of ball mill analyzed.Rates of particle breakage and production of fines per unit power input considered.Both the parameters exhibit significant variation with operating conditions.Effect of variables found to be different under different operating conditions.Rate of production of fines parameter better suited for mill design and scale-up.

With a view to developing a sound basis for the design and scale-up of ball mills, a large amount of data available in the literature were analyzed for variation of the two key mill performance parameters: power specific values of the absolute breakage rate of the coarsest size fraction, S*, and absolute rate of production of fines, F*, with some of the important operating and design variables such as the mill speed, ball load, particle load, ball diameter and mill diameter. In general, values of both the mill performance parameters were found to vary significantly with the mill operating conditions. The nature and relative magnitude of variation for the two parameters also differed significantly. Moreover, the effect of any particular variable on the S* and F* values was found to be significantly different for different sets of operating conditions. It has been emphasized that, as the purpose of grinding is to produce fine particles, the mill design and scale-up work should be based mainly on the F* parameters. Moreover, it is not correct to regard the S* values to be independent of the mill design and operating variables as a general rule, especially for a fine analysis of the performance of the grinding systems.

optimization of ball milling parameters to produce centella asiatica herbal nanopowders | springerlink

Nanopowders of Centella asiatica was produced using planetary ball mill by varying milling parameters such as milling time, mass concentration, and bead amount. Particle size analysis employing photon correlation spectroscopy was carried out to record the effect of milling parameters on the particle size produced. The morphology of milled powders was also analyzed using a field emission scanning electron microscope. The bioactive component, asiatic acid, was extracted from various sizes of C. asiatica powders, and its extraction yield at different powder size was calculated by high-performance liquid chromatography. Optimization of milling parameters was found to be a crucial step in determining the content of asiatic acid extracted. In this study, the highest amount of asiatic acid extracted was 25.4 mg/g, obtained at moderate conditions of the following milling parameters: 4 h of milling time, 1.2% (w/v) concentration of powder, and 25 g of bead load.

Centella asiatica is a perennial plant that mostly grows in the tropical and subtropical climate. It is known as mandukarpani (Indian), gotu kala (Chinese), or pennywort (English). It is commonly used as a wound-healing agent and for treatment of mental disorders and skin cancer. Studies also show that the herb extract could increase collagen production and strengthen tensile strength [1]. Thus, this herb is widely used in formulations of cosmetic and pharmaceutical products and possesses high commercial value. All these therapeutic effects are mainly contributed by the triterpene glycosides, mainly asiatic acid and asiaticoside (Figure1). Asiatic acid, a pentacyclic triterpene, is the aglycone form of asiaticoside which consists of a sugar moiety: a trisaccharide containing -L-rhamnopyranoside linked to the disaccharide gentiobiose [2]. Although the pharmacological effect of asiaticoside has been well known, some data suggest that asiatic acid is the actual component responsible for the plant's therapeutic effects since asiaticoside is being hydrolyzed to asiatic acid in vivo[35]. Thus, asiatic acid can be considered as the bioactive compound of C. asiatica. [4]. Recent studies showed that asiatic acid could treat liver fibrosis [6], protect primary neurons against C2-ceramide-induced apoptosis [7], inhibit acetylcholinesterase [8], and induce neurotoxicity. Furthermore, modification of asiatic acid functional groups have been carried out with the aim of improving the compound's potential as a hepatoprotective agent [9, 10].

Natural products have been important sources of drugs and will continue to play an important role as a major source of new drugs in the years to come [1113]. Normally, bioactive components exist in a low concentration; thus, an effective method is urgently needed to retrieve the analyte from their cell matrix. Many methods have been introduced such as ultrasonic extraction (UE), microwave-assisted extraction, accelerated solvent extraction, pressurized hot water extraction [1416], and supercritical/subcritical extraction [17, 18]. However, the extraction efficiency of each method is greatly dependent on the particle size of plant material itself [19, 20]. Among the methods, UE has been known for their extraction efficiency with reduced processing time and solvent consumption [14].

Ball milling is a common method in the size reduction of material. The milling process produces nanopowders through the impact forces generated by action of centrifugal forces. Milling parameters such as milling time, mass concentration, and bead amount are important parameters that need to be considered in producing superfine powders. In this research, our aim is to investigate the effect of particle size on the amount of asiatic acid extracted employing high-performance liquid chromatography (HPLC).

To obtain a uniform distribution of particle size in the nanometer range, milling parameters such as milling time, mass concentration, and bead amount were varied. The results as shown in Figure2 indicate that each set of milling condition produced different particle sizes of powder. Varying milling time, mass concentration, and bead amount from 2 to 8 h, 0.4% to 2.8% (w/v), and 5 to 100 g varied the particle size from 408.8 to 191.1, 237.1 to 581.1, and 442.6 to 212.8 nm, respectively. Statistically, increasing milling time from 2 to 4 h significantly reduced the particle size since sufficient energy was provided for size reduction. However, no significant reduction in particle size was observed after an extended milling time of 4 to 8 h. This is due to the fibrous structure of plant material itself which limited further size reduction [21]. An increase of particle size from 237.1 to 581.13 nm was observed as mass concentration is increased from 0.4% to 2.8% (w/v). Increase of mass concentration led to inefficient energy distribution in the milling process caused by the weak interaction between bead media and particles [22]. At a higher mass concentration, the viscosities of slurries increase and the distance between individual particles is reduced leading to ineffective particle capture by the grinding beads. The higher viscosities will then weaken the motion of grinding media; thus, velocity of grinding media and kinetic energy of bead decreased tremendously leading to ineffective milling operation [23]. The study on the effect of bead amount on particle size showed an inverse correlation. The smallest particle size (212.8 nm) was achieved with the highest amount (100 g) of bead load, and the largest particles size (442.6 nm) was found with the lowest bead amount (5 g). This can be understood by the limited amount of beads present in the reaction. With a low amount of bead load, the limiting reactant is the bead itself since the low numbers of bead present in the physical reaction will cause a reduction in the collision frequency due to limited amounts of bead per mass powder and reduced energy efficiency. Similarly, with higher bead amount, the energy efficiency would be higher, as reported by He et al. [22].

As shown in Figure3, the microstructures of coarse and milled powders showed a huge reduction of the size of nanopowders from the micropowder. It proves that the impact of collision in the milling process has tremendously broken down the micropowder to nanosize powder. The nanopowders exist in random structure in an agglomerated form. This is common since a higher surface energy possessed by the nanoparticles caused them to aggregate once it goes into a solid-state condition [24].

To evaluate the amount of asiatic acid extracted, each powder from each set of milling condition was extracted and analyzed using HPLC. From the HPLC chromatogram, asiaticoside was eluted for 8 min (tR = 8 min), while asiatic acid had a tR of 23 min. This method shows a good separation of the two compounds for both micro- and nanosamples (Figure4). As shown in Figure4, all milled samples showed no presence of asiaticoside, and we believe that asiaticoside have been hydrolyzed to asiatic acid during the milling process. As shown in Figure5, increasing the milling time, mass concentration, and bead amount from 2 to 8 h, 0.4% to 2.8% (w/v), and 5 to 100 g during grinding significantly varied the amount of asiatic acid extracted from 4.5 to 25.4 mg/g, from 2.3 to 25.4 mg/g, and from 4.1 to 25.4 mg/g, respectively. The difference in the amount of asiatic acid extracted could be caused by the degree of efficiency of extraction due to the difference in particle size of the powder and the ability of asiatic acid to retain their structure during the milling process. Each set of milling time (2, 4, 6, and 8 h) showed a reduction of particles to nanosize range. We further observed that a shorter milling time of 2 h produced a lower yield of asiatic acid content compared to 4 h of milling which could be explained by the difference in particle size of the powder as shown in Figure2. Interestingly, after 6 h of milling, the content of asiatic acid reduced tremendously as compared to after 4 h of milling, and the amount is further reduced after 8 h of milling. At this time, further nanosize reduction has occurred, and the energy the milling process may have degraded the asiatic acid and consequently reduce the amount of asiatic acid extracted. Kormin also reported that the asiatic acid content in C. asiatica juice extract reduced significantly at higher temperatures [25]. Other findings have also showed that heat is an important factor that should be considered in extracting bioactive compound from plants. Effect of heat in the extraction of saponins from ginseng was clearly significant as the level of ginsenosides Rg1, Re, Rb1, Rc, R2, Rb3, and Rd decreased after the steaming process [26]. The same trend was also observed in experiments that investigate the effect of bead amounts on asiatic acid content. Effect of the nanopowders in improving extraction of asiatic acid can be clearly observed by varying mass concentrations. The parameter values range from 0.4% to 2.8% (w/v) because our preliminary study showed that at this range, nanopowders range from 100 to 1,000 nm could be produced. Above these values, the milled product produced a greater size of particle (microrange). In this experiment, samples that were ground at 0.4% and 1.2% (w/v) showed a high amount of asiatic acid (24.5 and 25.4 mg/g) extracted due to increased extraction efficiency caused by smaller particle size that resulted from efficient energy distribution in the milling process. When milled at a higher concentration (2.0% (w/v)), the asiatic acid content was reduced to 13 mg/g, and further milling at 2.8% (w/v) caused a further drop in the extracted asiatic acid content (2.3 mg/g). The sudden drop in the amount of extracted asiatic acid amount was probably due a lower extraction efficiency caused by a larger particle size (500.7 and 581.7 nm). In this study, optimized milling parameters for the extraction of asiatic acid from C. asiatica is 4 h of milling time, 1.2% (w/v) mass concentration, and 25 g of bead load. To our knowledge, this method yields the highest amount of extracted asiatic acid from the plant compared to other methods previously reported which ranges from 7 to 14 mg/g [15, 17].

Generally, extraction of plant material involved two physical phenomena: diffusion through the cell wall and washing out the analyte into extraction solvent. Using nanopowders in the extraction of bioactive compound uses the same principles as using micron powders, but the only difference lies in the particle size used in the extraction. In this section, we discuss how the unique properties of nanopowders results in higher extraction yields.

Size reduction prior to extraction has been reported to be a crucial step to obtain from the plant matrix [15, 19, 27, 28]. The reduction in size is reported to enhance the contact ratio between plant cell and the surrounding solvent [28]. In our study, the higher amount of asiatic acid extracted from milled samples can be explained by the amazing properties of nanoparticles contributed by an increase in surface area and the reduction of diffusional distance of the particles (Figure6) [29]. Decreasing the particle size to the nanometer range caused an increase of surface area, leading to a higher number of cells directly exposed to the extraction by the solvent [28]. Based on Noyes-Whitney equation [30], the dissolution velocity of particles increases due to surface area enlargement as described in Equation1:

where dc/dt is the rate of dissolution velocity, D is the diffusion coefficient, A is the surface area of the solid, C s is the concentration of the solid in the diffusion layer surrounding the solid, and C x is the concentration of the solid in the bulk dissolution medium. The dissolution velocity dc/dt described in the above equation also depends on the concentration gradient (C s - C x )/h where h, the diffusional distance is reduced for smaller particles [31]. Due to the reduced h, the concentration gradient of nanoparticles is large (Figure5) [29, 32] which causes the dissolution velocity to increase. This will enhance the diffusion of solvent into the cell wall, which in turn will allow the rapid release of active ingredients from the cell matrix of the plant. This is also supported by the phenomena where the disrupted cell wall structure of the plant material itself cause the active compound in the inner cytoplasm to be exposed directly while not being released from cell walls making effective ingredients dissolve out rapidly [33]. Past studies have shown that nanonization will allow the rapid release of active component from their cytoplasm matrix [3436] and help improve the extraction of bioactive components of plants.

In general, we have provided further proof that nanonization of plant material played an important role in contributing to the extraction yield of bioactive compounds from plant materials. In this research, we have successfully produced nanopowders of C. asiatica by planetary ball milling. Our findings showed that a smaller particle size gave a higher extraction yield of asiatic acid compared to a bigger particle size. However, if the particle size is too small, the extraction yield of the compound will be reduced due to the effect of heat and pressure in the milling jar. As a conclusion, optimized milling parameters for high extraction yield of asiatic acid from C. asiatica was found to be 4 h of milling time, 1.2% (w/v) concentration of powder, and 25 g of bead load.

Asiaticoside and asiatic acid standards (Biopurify Chegdu, Sichuan, China) were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA), acetonitrile (LC grade) was purchased from Fisher Chemicals, and methanol (HPLC grade) was purchased from Friedemann Schmidt (Berlin, Germany). Distilled water was purified using a Milli-Q water purification system (Millipore, Bedford, MA, USA).

The plant material was washed with running tap water and rinsed with distilled water to remove dirt and contaminants. The clean plant material was stored in an oven at 50C and was left to dry for 3 days. The dried plant material was ground using a conventional rotor mixer. The ground powder was then sieved through a 250-m sieve and stored at 4C prior to the ball milling process.

Ball milling is a common mechanical process to produce superfine powders. In this research, a planetary ball mill was chosen as a grinding tool due to its simplicity. The sieved powder was first mixed with Pluronic F127 and milled at different mass concentrations (0.4%, 1.2%, 2.0%, and 2.8% (w/v)) at a constant milling time (4 h) and fixed bead amount (25 g). The resulting milling product was freeze-dried and analyzed for their physical characteristics. The best condition of milling (% (w/v)) was determined by the size of powder yielding the highest amount of asiatic acid extracted. This condition was then applied at different milling times (2, 4, 6, and 8 h) and different bead amounts (5, 25, 50, and 100 g), and the sample was analyzed for their physical characteristic and amount of asiatic acid extracted.

Prior to the measurement, nanosuspensions of each sample were taken and adjusted to 0.01% (w/v) and introduced into a disposable cuvette to be measured in triplicates. z-Average and polydispersity were recorded. Field emission scanning electron microscopy (FESEM; SMT SUPRA 40VP, Carl Zeiss AG, Oberkochen, Germany) imaging was carried out by placing 10 L of the nanosuspension onto a glass slide and stored in an electronic desiccator (temperature 20C, humidity 18% RH) for drying purposes. The dried samples were then coated with gold (approximately 10-nm thick) and were placed onto an adhesive tape on the FESEM stub.

Coarse powder (0.5 g) was weighed and sonicated for 1 h using an ultrasonic processor (UP400S Ultrasonic Processor, Hielscher Ultrasonics GmbH, Teltow, Germany) at 60% amplitude and 1 cycle. The solids were removed by gravitational filtration, and the filtrate was collected. The nanopowders were also extracted in the same manner as above. The extract was dissolved in methanol (1 mg/mL) before being subjected to HPLC analysis.

The chromatographic separation was based on a modified method of Inamdar et al. [37] using a Zorbax Eclipse XDB-C18 4.6 150 mm (Agilent Technologies, Inc., Santa Clara, CA, USA), 5 m at 25C. The mobile phase, solvent A (ultrapure water) and solvent B (acetonitrile), was delivered at a flow rate of 1.0 mL min-1. Gradient elution was employed with the ratio of A/B and varied as follows: 0 min, 80:20 and 35 min, 45:55. Standards or samples were introduced into the HPLC using an Agilent 1200 G1367B auto sampler, injection volume was 40 L, and the detection wavelength was 206 nm. The amount of asiatic acid was determined using a standard calibration curves.

Hereby I, Muhammad Zuhairi Bin Borhan declare that as the author of the manuscript, have or do not have a direct financial relation with the commercial identities mentioned in my paper that might lead to a conflict of interest for any of the authors.

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MZBB: Mainly involve in all laboratory works; Experimental Design, Sample preparation, Analysis sample; Mainly involve in preparation of manuscript. RA: Assisting in all laboratory works; Extraction of compound, HPLC Analysis; Assisting in preparation of manuscript. MR: Assisting in all laboratory works; FESEM Analysis, Particle Size Analysis; Assisting in preparation of manuscript. SA: Assisting in all laboratory works; FESEM Analysis, Particle Size Analysis, HPLC Analysis; Assisting in preparation of manuscript.

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Borhan, M.Z., Ahmad, R., Rusop, M. et al. Optimization of ball milling parameters to produce Centella asiatica herbal nanopowders. J Nanostruct Chem 3, 79 (2013).