ball mill heat

ball mills - an overview | sciencedirect topics

A ball mill is a type of grinder used to grind and blend bulk material into QDs/nanosize using different sized balls. The working principle is simple; impact and attrition size reduction take place as the ball drops from near the top of a rotating hollow cylindrical shell. The nanostructure size can be varied by varying the number and size of balls, the material used for the balls, the material used for the surface of the cylinder, the rotation speed, and the choice of material to be milled. Ball mills are commonly used for crushing and grinding the materials into an extremely fine form. The ball mill contains a hollow cylindrical shell that rotates about its axis. This cylinder is filled with balls that are made of stainless steel or rubber to the material contained in it. Ball mills are classified as attritor, horizontal, planetary, high energy, or shaker.

Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.

By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:

where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.

The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.

The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.

where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.

A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.

Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction, and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles as well as collision energy. These forces are derived from the rotational motion of the balls and the movement of particles within the mill and contact zones of colliding balls.

By the rotation of the mill body, due to friction between the mill wall and balls, the latter rise in the direction of rotation until a helix angle does not exceed the angle of repose, whereupon the balls roll down. Increasing the rotation rate leads to the growth of the centrifugal force and the helix angle increases, correspondingly, until the component of the weight strength of balls becomes larger than the centrifugal force. From this moment, the balls are beginning to fall down, describing certain parabolic curves during the fall (Fig. 2.10).

With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls remain attached to the wall with the aid of centrifugal force is:

where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 65%80% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.

where db.max is the maximum size of the feed (mm), is the compression strength (MPa), E is the modulus of elasticity (MPa), b is the density of material of balls (kg/m3), and D is the inner diameter of the mill body (m).

The degree of filling the mill with balls also influences the productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 30%35% of its volume.

The productivity of ball mills depends on the drum diameter and the relation of drum diameter and length. The optimum ratio between length L and diameter D, L:D, is usually accepted in the range 1.561.64. The mill productivity also depends on many other factors, including the physical-chemical properties of the feed material, the filling of the mill by balls and their sizes, the armor surface shape, the speed of rotation, the milling fineness, and the timely moving off of the ground product.

where D is the drum diameter, L is the drum length, b.ap is the apparent density of the balls, is the degree of filling of the mill by balls, n is the revolutions per minute, and 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.

A feature of ball mills is their high specific energy consumption. A mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, that is, during the grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.

Milling time in tumbler mills is longer to accomplish the same level of blending achieved in the attrition or vibratory mill, but the overall productivity is substantially greater. Tumbler mills usually are used to pulverize or flake metals, using a grinding aid or lubricant to prevent cold welding agglomeration and to minimize oxidation [23].

Cylindrical Ball Mills differ usually in steel drum design (Fig. 2.11), which is lined inside by armor slabs that have dissimilar sizes and form a rough inside surface. Due to such juts, the impact force of falling balls is strengthened. The initial material is fed into the mill by a screw feeder located in a hollow trunnion; the ground product is discharged through the opposite hollow trunnion.

Cylindrical screen ball mills have a drum with spiral curved plates with longitudinal slits between them. The ground product passes into these slits and then through a cylindrical sieve and is discharged via the unloading funnel of the mill body.

Conical Ball Mills differ in mill body construction, which is composed of two cones and a short cylindrical part located between them (Fig. 2.12). Such a ball mill body is expedient because efficiency is appreciably increased. Peripheral velocity along the conical drum scales down in the direction from the cylindrical part to the discharge outlet; the helix angle of balls is decreased and, consequently, so is their kinetic energy. The size of the disintegrated particles also decreases as the discharge outlet is approached and the energy used decreases. In a conical mill, most big balls take up a position in the deeper, cylindrical part of the body; thus, the size of the balls scales down in the direction of the discharge outlet.

For emptying, the conical mill is installed with a slope from bearing to one. In wet grinding, emptying is realized by the decantation principle, that is, by means of unloading through one of two trunnions.

With dry grinding, these mills often work in a closed cycle. A scheme of the conical ball mill supplied with an air separator is shown in Fig. 2.13. Air is fed to the mill by means of a fan. Carried off by air currents, the product arrives at the air separator, from which the coarse particles are returned by gravity via a tube into the mill. The finished product is trapped in a cyclone while the air is returned in the fan.

The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.

Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.

When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.

In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).

Modern ball mills consist of two chambers separated by a diaphragm. In the first chamber the steel-alloy balls (also described as charge balls or media) are about 90mm diameter. The mill liners are designed to lift the media as the mill rotates, so the comminution process in the first chamber is dominated by crushing. In the second chamber the ball diameters are of smaller diameter, between 60 and 15mm. In this chamber the lining is typically a classifying lining which sorts the media so that ball size reduces towards the discharge end of the mill. Here, comminution takes place in the rolling point-contact zone between each charge ball. An example of a two chamber ball mill is illustrated in Fig. 2.22.15

Much of the energy consumed by a ball mill generates heat. Water is injected into the second chamber of the mill to provide evaporative cooling. Air flow through the mill is one medium for cement transport but also removes water vapour and makes some contribution to cooling.

Grinding is an energy intensive process and grinding more finely than necessary wastes energy. Cement consists of clinker, gypsum and other components mostly more easily ground than clinker. To minimise over-grinding modern ball mills are fitted with dynamic separators (otherwise described as classifiers or more simply as separators). The working principle is that cement is removed from the mill before over-grinding has taken place. The cement is then separated into a fine fraction, which meets finished product requirements, and a coarse fraction which is returned to mill inlet. Recirculation factor, that is, the ratio of mill throughput to fresh feed is up to three. Beyond this, efficiency gains are minimal.

For more than 50years vertical mills have been the mill of choice for grinding raw materials into raw meal. More recently they have become widely used for cement production. They have lower specific energy consumption than ball mills and the separator, as in raw mills, is integral with the mill body.

In the Loesche mill, Fig. 2.23,16 two pairs of rollers are used. In each pair the first, smaller diameter, roller stabilises the bed prior to grinding which takes place under the larger roller. Manufacturers use different technologies for bed stabilisation.

Comminution in ball mills and vertical mills differs fundamentally. In a ball mill, size reduction takes place by impact and attrition. In a vertical mill the bed of material is subject to such a high pressure that individual particles within the bed are fractured, even though the particles are very much smaller than the bed thickness.

Early issues with vertical mills, such as narrower PSD and modified cement hydration characteristics compared with ball mills, have been resolved. One modification has been to install a hot gas generator so the gas temperature is high enough to partially dehydrate the gypsum.

For many decades the two-compartment ball mill in closed circuit with a high-efficiency separator has been the mill of choice. In the last decade vertical mills have taken an increasing share of the cement milling market, not least because the specific power consumption of vertical mills is about 30% less than that of ball mills and for finely ground cement less still. The vertical mill has a proven track record in grinding blastfurnace slag, where it has the additional advantage of being a much more effective drier of wet feedstock than a ball mill.

The vertical mill is more complex but its installation is more compact. The relative installed capital costs tend to be site specific. Historically the installed cost has tended to be slightly higher for the vertical mill.

Special graph paper is used with lglg(1/R(x)) on the abscissa and lg(x) on the ordinate axes. The higher the value of n, the narrower the particle size distribution. The position parameter is the particle size with the highest mass density distribution, the peak of the mass density distribution curve.

Vertical mills tend to produce cement with a higher value of n. Values of n normally lie between 0.8 and 1.2, dependent particularly on cement fineness. The position parameter is, of course, lower for more finely ground cements.

Separator efficiency is defined as specific power consumption reduction of the mill open-to-closed-circuit with the actual separator, compared with specific power consumption reduction of the mill open-to-closed-circuit with an ideal separator.

As shown in Fig. 2.24, circulating factor is defined as mill mass flow, that is, fresh feed plus separator returns. The maximum power reduction arising from use of an ideal separator increases non-linearly with circulation factor and is dependent on Rf, normally based on residues in the interval 3245m. The value of the comminution index, W, is also a function of Rf. The finer the cement, the lower Rf and the greater the maximum power reduction. At C = 2 most of maximum power reduction is achieved, but beyond C = 3 there is very little further reduction.

Separator particle separation performance is assessed using the Tromp curve, a graph of percentage separator feed to rejects against particle size range. An example is shown in Fig. 2.25. Data required is the PSD of separator feed material and of rejects and finished product streams. The bypass and slope provide a measure of separator performance.

The particle size is plotted on a logarithmic scale on the ordinate axis. The percentage is plotted on the abscissa either on a linear (as shown here) or on a Gaussian scale. The advantage of using the Gaussian scale is that the two parts of the graph can be approximated by two straight lines.

The measurement of PSD of a sample of cement is carried out using laser-based methodologies. It requires a skilled operator to achieve consistent results. Agglomeration will vary dependent on whether grinding aid is used. Different laser analysis methods may not give the same results, so for comparative purposes the same method must be used.

The ball mill is a cylindrical drum (or cylindrical conical) turning around its horizontal axis. It is partially filled with grinding bodies: cast iron or steel balls, or even flint (silica) or porcelain bearings. Spaces between balls or bearings are occupied by the load to be milled.

Following drum rotation, balls or bearings rise by rolling along the cylindrical wall and descending again in a cascade or cataract from a certain height. The output is then milled between two grinding bodies.

Ball mills could operate dry or even process a water suspension (almost always for ores). Dry, it is fed through a chute or a screw through the units opening. In a wet path, a system of scoops that turn with the mill is used and it plunges into a stationary tank.

Mechanochemical synthesis involves high-energy milling techniques and is generally carried out under controlled atmospheres. Nanocomposite powders of oxide, nonoxide, and mixed oxide/nonoxide materials can be prepared using this method. The major drawbacks of this synthesis method are: (1) discrete nanoparticles in the finest size range cannot be prepared; and (2) contamination of the product by the milling media.

More or less any ceramic composite powder can be synthesized by mechanical mixing of the constituent phases. The main factors that determine the properties of the resultant nanocomposite products are the type of raw materials, purity, the particle size, size distribution, and degree of agglomeration. Maintaining purity of the powders is essential for avoiding the formation of a secondary phase during sintering. Wet ball or attrition milling techniques can be used for the synthesis of homogeneous powder mixture. Al2O3/SiC composites are widely prepared by this conventional powder mixing route by using ball milling [70]. However, the disadvantage in the milling step is that it may induce certain pollution derived from the milling media.

In this mechanical method of production of nanomaterials, which works on the principle of impact, the size reduction is achieved through the impact caused when the balls drop from the top of the chamber containing the source material.

A ball mill consists of a hollow cylindrical chamber (Fig. 6.2) which rotates about a horizontal axis, and the chamber is partially filled with small balls made of steel, tungsten carbide, zirconia, agate, alumina, or silicon nitride having diameter generally 10mm. The inner surface area of the chamber is lined with an abrasion-resistant material like manganese, steel, or rubber. The magnet, placed outside the chamber, provides the pulling force to the grinding material, and by changing the magnetic force, the milling energy can be varied as desired. The ball milling process is carried out for approximately 100150h to obtain uniform-sized fine powder. In high-energy ball milling, vacuum or a specific gaseous atmosphere is maintained inside the chamber. High-energy mills are classified into attrition ball mills, planetary ball mills, vibrating ball mills, and low-energy tumbling mills. In high-energy ball milling, formation of ceramic nano-reinforcement by in situ reaction is possible.

It is an inexpensive and easy process which enables industrial scale productivity. As grinding is done in a closed chamber, dust, or contamination from the surroundings is avoided. This technique can be used to prepare dry as well as wet nanopowders. Composition of the grinding material can be varied as desired. Even though this method has several advantages, there are some disadvantages. The major disadvantage is that the shape of the produced nanoparticles is not regular. Moreover, energy consumption is relatively high, which reduces the production efficiency. This technique is suitable for the fabrication of several nanocomposites, which include Co- and Cu-based nanomaterials, Ni-NiO nanocomposites, and nanocomposites of Ti,C [71].

Planetary ball mill was used to synthesize iron nanoparticles. The synthesized nanoparticles were subjected to the characterization studies by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques using a SIEMENS-D5000 diffractometer and Hitachi S-4800. For the synthesis of iron nanoparticles, commercial iron powder having particles size of 10m was used. The iron powder was subjected to planetary ball milling for various period of time. The optimum time period for the synthesis of nanoparticles was observed to be 10h because after that time period, chances of contamination inclined and the particles size became almost constant so the powder was ball milled for 10h to synthesize nanoparticles [11]. Fig. 12 shows the SEM image of the iron nanoparticles.

The vibratory ball mill is another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vials capacities in the vibratory mills are smaller (about 10 ml in volume) compared to the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6) at very high speed, as high as 1200 rpm.

Another type of the vibratory ball mill, which is used at the van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig. 1.7).

The mill is evacuated during milling to a pressure of 106 Torr, in order to avoid reactions with a gas atmosphere.[44] Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.

In spite of the traditional approaches used for gas-solid reaction at relatively high temperature, Calka etal.[58] and El-Eskandarany etal.[59] proposed a solid-state approach, the so-called reactive ball milling (RBM), used for preparations different families of meal nitrides and hydrides at ambient temperature. This mechanically induced gas-solid reaction can be successfully achieved, using either high- or low-energy ball-milling methods, as shown in Fig.9.5. However, high-energy ball mill is an efficient process for synthesizing nanocrystalline MgH2 powders using RBM technique, it may be difficult to scale up for matching the mass production required by industrial sector. Therefore, from a practical point of view, high-capacity low-energy milling, which can be easily scaled-up to produce large amount of MgH2 fine powders, may be more suitable for industrial mass production.

In both approaches but with different scale of time and milling efficiency, the starting Mg metal powders milled under hydrogen gas atmosphere are practicing to dramatic lattice imperfections such as twinning and dislocations. These defects are caused by plastics deformation coupled with shear and impact forces generated by the ball-milling media.[60] The powders are, therefore, disintegrated into smaller particles with large surface area, where very clean or fresh oxygen-free active surfaces of the powders are created. Moreover, these defects, which are intensively located at the grain boundaries, lead to separate micro-scaled Mg grains into finer grains capable to getter hydrogen by the first atomically clean surfaces to form MgH2 nanopowders.

Fig.9.5 illustrates common lab scale procedure for preparing MgH2 powders, starting from pure Mg powders, using RBM via (1) high-energy and (2) low-energy ball milling. The starting material can be Mg-rods, in which they are processed via sever plastic deformation,[61] using for example cold-rolling approach,[62] as illustrated in Fig.9.5. The heavily deformed Mg-rods obtained after certain cold rolling passes can be snipped into small chips and then ball-milled under hydrogen gas to produce MgH2 powders.[8]

Planetary ball mills are the most popular mills used in scientific research for synthesizing MgH2 nanopowders. In this type of mill, the ball-milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial and the effective centrifugal force reaches up to 20 times gravitational acceleration. The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed.

In the typical experimental procedure, a certain amount of the Mg (usually in the range between 3 and 10g based on the vials volume) is balanced inside an inert gas atmosphere (argon or helium) in a glove box and sealed together with certain number of balls (e.g., 2050 hardened steel balls) into a hardened steel vial (Fig.9.5A and B), using, for example, a gas-temperature-monitoring system (GST). With the GST system, it becomes possible to monitor the progress of the gas-solid reaction taking place during the RBM process, as shown in Fig.9.5C and D. The temperature and pressure changes in the system during milling can be also used to realize the completion of the reaction and the expected end product during the different stages of milling (Fig.9.5D). The ball-to-powder weight ratio is usually selected to be in the range between 10:1 and 50:1. The vial is then evacuated to the level of 103bar before introducing H2 gas to fill the vial with a pressure of 550bar (Fig.9.5B). The milling process is started by mounting the vial on a high-energy ball mill operated at ambient temperature (Fig.9.5C).

Tumbling mill is cylindrical shell (Fig.9.6AC) that rotates about a horizontal axis (Fig.9.6D). Hydrogen gas is pressurized into the vial (Fig.9.6C) together with Mg powders and ball-milling media, using ball-to-powder weight ratio in the range between 30:1 and 100:1. Mg powder particles meet the abrasive and impacting force (Fig.9.6E), which reduce the particle size and create fresh-powder surfaces (Fig.9.6F) ready to react with hydrogen milling atmosphere.

Figure 9.6. Photographs taken from KISR-EBRC/NAM Lab, Kuwait, show (A) the vial and milling media (balls) and (B) the setup performed to charge the vial with 50bar of hydrogen gas. The photograph in (C) presents the complete setup of GST (supplied by Evico-magnetic, Germany) system prior to start the RBM experiment for preparing of MgH2 powders, using Planetary Ball Mill P400 (provided by Retsch, Germany). GST system allows us to monitor the progress of RBM process, as indexed by temperature and pressure versus milling time (D).

The useful kinetic energy in tumbling mill can be applied to the Mg powder particles (Fig.9.7E) by the following means: (1) collision between the balls and the powders; (2) pressure loading of powders pinned between milling media or between the milling media and the liner; (3) impact of the falling milling media; (4) shear and abrasion caused by dragging of particles between moving milling media; and (5) shock-wave transmitted through crop load by falling milling media. One advantage of this type of mill is that large amount of the powders (100500g or more based on the mill capacity) can be fabricated for each milling run. Thus, it is suitable for pilot and/or industrial scale of MgH2 production. In addition, low-energy ball mill produces homogeneous and uniform powders when compared with the high-energy ball mill. Furthermore, such tumbling mills are cheaper than high-energy mills and operated simply with low-maintenance requirements. However, this kind of low-energy mill requires long-term milling time (more than 300h) to complete the gas-solid reaction and to obtain nanocrystalline MgH2 powders.

Figure 9.7. Photos taken from KISR-EBRC/NAM Lab, Kuwait, display setup of a lab-scale roller mill (1000m in volume) showing (A) the milling tools including the balls (milling media and vial), (B) charging Mg powders in the vial inside inert gas atmosphere glove box, (C) evacuation setup and pressurizing hydrogen gas in the vial, and (D) ball milling processed, using a roller mill. Schematic presentations show the ball positions and movement inside the vial of a tumbler mall mill at a dynamic mode is shown in (E), where a typical ball-powder-ball collusion for a low energy tumbling ball mill is presented in (F).

heat treatment process for high chromium grinding ball

Ball mill is widely used in building materials, mineral processing, cement, thermal power generation and fertilizer production of the main grinding equipment, grinding ball is the largest consumption of wear-resistant parts. Therefore, with the continuous development of key industries such as building materials, the demand for grinding balls will increase greatly. At present, ordinary forged steel balls are mainly used, with an average consumption of one kilogram of steel balls per ton of cement, and about two kilograms of steel balls per ton of metal ore ground This was nearly ten times more relevant than at the time. Especially as the third generation of anti-wear Alloy High Chromium White cast iron developed successfully, so that more than 4.5 m of large-scale cement mill ball consumption to 30 ~ 50 G / T, medium and small-scale cement mill ball consumption also reduced to only 100 G / T or so The new material revolution has greatly improved productivity and economic returns. It was only in the mid-1970s that I began my research on the development of cr-based white cast iron. The Xian Jiaotong University of Mn-Cr-Cu-Mo series and high-cr-based wear-resistant white cast iron were selected and developed. Since the 1980s, a variety of new wear-resistant materials developed in China have been gradually applied to the actual abrasive wear operations, some of which have reached the level of similar products abroad.

The good wear resistance of high chromium cast iron mainly depends on its matrix structure and the type and distribution of carbides. High Chromium cast iron is a kind of chromium white cast iron with chromium content between 12% -28%. This alloy of carbon and hard compounds gives high chromium cast iron good wear resistance. On the other hand, during solidification, M7C3 type carbides distribute in a rod-like shape, which can improve the toughness of high chromium cast iron to some extent. The production of high chromium cast ball is mainly composed of casting and heat treatment. Casting is mainly composition design, the casting method, carbide, and matrix structure directly, affect the wear resistance and toughness. Carbides and Metal Matrix Organization is first, by its chemical composition, so must be the reasonable design of its chemical element, content, followed by the heat treatment process

As mentioned before, high chromium grinding ball has excellent wear resistance in abrasive wear and has been widely used in many industries of national economy This is mainly because the high-chromium cast ball can obtain strong and tough matrix structure and high hardness M7C3 carbide by proper heat treatment and alloy modification. Many units at home and abroad began to choose high-chromium cast ball as a cement, mining, electricity and other industries as Abrasives, so high-chromium cast ball production technology has been the main topic of study at home and abroad.

Heat treatment of high chromium grinding ball is the main method to obtain good wear resistance. The research on high chromium cast iron ball mainly focuses on chemical composition selection, heat treatment process determination, modifier selection, good carbide type and wear mechanism. Foreign foundry researchers are more committed to the high-chromium cast iron microstructure, wear-resistant mechanism, and new preparation process development. Here are the specifications for the High Chromium Grinding Ball:

From the above indexes, it can be seen that the hardness of the grinding ball before and after quenching is quite different. The heat treatment process of high chromium cast iron grinding ball includes wind quenching, oil quenching, and special quenching solution treatment. After repeated test and mining, the oil quenching can fully meet all its technical specifications The consumption of iron ore is twice that of iron ore. The heat treatment process is the use of two-stage heat quenching and low-temperature tempering. At the rate of less than 70 C / h, the temperature is increased to 650-680 C for 2 hours, at the temperature increasing to 950 C for 4 hours, forced air-cooling, oil-cooling, and special quenching liquid cooling respectively, and tempering is carried out at 200 C or 420 C According to the ball diameter was 4 hours, 6 hours, 7 hours, out of slow cooling.

From the above indexes, it can be seen that the hardness of the grinding ball before and after quenching is quite different. The heat treatment process of high chromium cast iron grinding ball includes wind quenching, oil quenching, and special quenching solution treatment. After repeated test and mining, the oil quenching can fully meet all its technical specifications The consumption of iron ore is twice that of iron ore. The heat treatment process is the use of two-stage heat quenching and low-temperature tempering. At the rate of less than 70 C / h, the temperature is increased to 650-680 C for 2 hours, at the temperature increasing to 950 C for 4 hours, forced air-cooling, oil-cooling, and special quenching liquid cooling respectively, and tempering is carried out at 200 C or 420 C According to the ball diameter was 4 hours, 6 hours, 7 hours, out of slow cooling.

After reasonable matching of all elements, ratio casting qualified grinding ball blank, adopt continuous heat treatment furnace to ensure the uniformity of furnace temperature, select the appropriate quenching medium and tooling, the equipment to ensure the consistency of the process, to ensure the stability of product quality. Problems such as low wear resistance of high chromium grinding balls, high rate of breakage and large roundness loss were solved.

ball mill - retsch - powerful grinding and homogenization

Ball mills are among the most variable and effective tools when it comes to size reduction of hard, brittle or fibrous materials. The variety of grinding modes, usable volumes and available grinding tool materials make ball mills the perfect match for a vast range of applications.

RETSCH is the world leading manufacturer of laboratory ball mills and offers the perfect product for each application. The High Energy Ball Mill Emax and MM 500 were developed for grinding with the highest energy input. The innovative design of both, the mills and the grinding jars, allows for continuous grinding down to the nano range in the shortest amount of time - with only minor warming effects. These ball mills are also suitable for mechano chemistry. Mixer Mills grind and homogenize small sample volumes quickly and efficiently by impact and friction. These ball mills are suitable for dry, wet and cryogenic grinding as well as for cell disruption for DNA/RNA recovery. Planetary Ball Mills meet and exceed all requirements for fast and reproducible grinding to analytical fineness. They are used for the most demanding tasks in the laboratory, from routine sample processing to colloidal grinding and advanced materials development. The drum mill is a type of ball mill suitable for the fine grinding of large feed sizes and large sample volumes.

ball milling | material milling, jet milling | aveka

Ball milling is a size reduction technique that uses media in a rotating cylindrical chamber to mill materials to a fine powder. As the chamber rotates, the media is lifted up on the rising side and then cascades down from near the top of the chamber. With this motion, the particles in between the media and chamber walls are reduced in size by both impact and abrasion. In ball milling, the desired particle size is achieved by controlling the time, applied energy, and the size and density of the grinding media. The optimal milling occurs at a critical speed. Ball mills can operate in either a wet or dry state. While milling without any added liquid is commonplace, adding water or other liquids can produce the finest particles and provide a ready-to-use dispersion at the same time.

Grinding media comes in many shapes and types with each having its own specific properties and advantages. Key properties of grinding media include composition, hardness, size and density. Some common types include alumina, stainless steel, yttria stabilized zirconia and sand. Ball milling will result in a ball curve particle size distribution with one or more peaks. Screening may be required to remove over or undersized materials.

temperature progression in a mixer ball mill | springerlink

The influence of the operating frequency, the milling ball and grinding stock filling degree, the material of the milling balls and beakers, the milling ball diameter and the size of the milling beakers on the temperature increase inside the milling beakers in a mixer ball mill was investigated. These parameters influence the temperature progression and the equilibrium temperature of the system. The grinding stock filling degree with regard to the void volume in the milling ball package showed huge influence on the heating rate and the equilibrium temperature. In this context, the behavior of the temperature progression changes if the complete void volume is filled with the grinding stock.

Mechanochemistry using ball mills is a promising technique with applications in organic and inorganic chemistry as well as material sciences [14]. During ball milling, up to 80% of the energy that is generated in the mill is dissipated as heat [5]. This is why measurement and control of the temperature in the ball mills is important, for example, if heat-sensitive products are formed that would be degraded, or side reactions that would be favored at high temperatures [6, 7]. Furthermore, liquefaction of organic substrates because of a temperature increase can diminish the energy transfer and may disturb the reaction [8].

Ball milling procedures are often referred to as milling at room temperature [9, 10]. This term should be used carefully, as even within short milling times a temperature increase can be observed, if there are no precautions for temperature control. For example, it was shown that milling in a mixer ball mill (MBM) for 10min with two milling balls in a 10ml beaker raised the temperature from 25 to 30C, and Colacino and co-workers measured a temperature increase of approximately 14K after 30min milling in an MM200 mixer mill [6, 11]. McKissic et al. observed a temperature of 50C after 1h milling in a Spex mixer mill and Takacs and McHenry reported milling ball temperatures of 66C [12, 13]. Comparably higher temperatures can be reached in planetary ball mills (PBMs), where temperatures from 60 to 600C can be measured, depending on the type of PBM, the grinding stock, the grinding material and the filling degree [5, 13, 14].

The temperature of the milling beaker is often determined on the surface of the milling beaker, which can be done for example with temperature data loggers [12, 15] or thermocouples [13, 16]. However, the temperature on the surface is not necessarily the temperature inside the milling beaker, and a temperature difference between milling beaker and balls of 25K was reported [13]. For determination of the milling balls temperature, calorimetric measurements can be performed [5, 13]. The Lamaty group developed a mathematical model for the prediction of the milling walls temperature. Experimental and simulated results (MBM MM200, operated at 30s1 with 10ml steel vessels) were in good accordance. The calculated temperature difference of the inner and outer side of the vessel was negligible [11].

Temperature control and more importantly temperature regulation in MBMs and PBMs can be challenging. One option is the integration of milling pauses to the milling cycle that allow a cooling down of the milling beakers, but also increase the total reaction time [10, 17, 18]. Technical solutions for a temperature-controlled milling are: cryogenic milling, where the beakers are cooled with liquid nitrogen [19]; water cooling of the vessels [5] and of the milling beaker holder [20]; (forced) air cooling [21]; use of heating tapes [22]; the application of double-walled milling beakers, which are equipped with an inlet and outlet for a circulating liquid that can be tempered by a thermostat [2325].

All chemicals were purchased from Sigma Aldrich or Alfa Aesar and used as received. The reactions were accomplished in a Retsch MM400 Mixer Mill. If not stated otherwise, milling beakers made of stainless or tempered steel with a volume of 35mL and steel milling balls with a diameter of 5mm were used. The temperature was measured with a K-type thermocouple.

The temperature was determined inside of the milling beaker (T milling bed) in the milling ball/grinding stock mixture with a thermocouple. The milling beakers were equipped with the respective number of milling balls; the grinding stock was added and milling was accomplished at the respective frequency, osc, and milling time t. Afterward, the beaker was opened and the thermocouple placed in the middle of the beaker. The time for measurement was<1min. If not stated otherwise, quartz sand was used as grinding stock in the basic experiments to avoid interference with mechanochemical reactions.

The following formulas were used for the calculation of the milling ball filling degree MB and the grinding stock filling degrees GS and GS,rel; with the milling ball volume V MB, the grinding stock bulk volume V GS, the vessel volume V MV, the diameter of the milling balls d MB, the number of milling balls n MB and the porosity of the milling ball packing .

The amount of heat that is dissipated is strongly dependent on the milling parameters that influence the energy input in the milling beaker. These parameters are the frequency osc, the milling ball diameter d MB, the milling ball filling degree MB (Eq.1), the milling beaker size, the grinding stock filling degree GS (Eq.2) and material properties of the grinding stock as well as of the grinding tools, such as Youngs modulus, density, hardness and heat capacity [12, 26].

Temperature measurements at the surface and inside of the milling beaker showed that the temperature difference is negligible at low osc, but becomes larger for higher osc (T up to 15K, Online resource 1). Temperatures recorded with surface measurements for example with temperature data loggers are therefore only an imprecise indicator for the internal temperature conditions [12]. A more precise, alternative method would be the temperature measurement inside the wall with embedded thermocouples [27], but even in this case a temperature gradient form the middle of the milling bed to the wall should occur.

For measurements on the surface, the thermocouple position was reported to be of slight influence [16]. We measured the temperature difference between the surface of the wall and the cap and found differences between 0.2 and 1.4K (Online resources 1, 2). Thus, for further investigations, we determined the temperature inside the milling beaker (T milling bed) in the milling ball/grinding stock mixture with a thermocouple.

The first investigated parameter was the operating frequency. As shown in Fig.1, the temperature increased at osc=15s1 within 90min from 22 to 30C. With higher osc, the heating rate as well as the final temperature was higher. Thus, at osc=30s1, a temperature of 87C was measured. The energy that is dissipated at higher osc is raised because of the higher kinetic energy of the milling balls. This led to an enhanced energy input.

Influence of the operating frequency on the temperature measured in the milling bed. Conditions: MBM MM400, 35ml steel beaker, steel balls, d MB=5mm, MB=0.24, m quartz sand=15.36g ( GS=0.31)

Figure2 illustrates the change in the temperature progression, if the milling ball filling degree MB is varied. An increase of MB from 0.06 to 0.36 resulted in a higher end temperature and heating rate, whereas the differences in T milling bed for 0.06 MB0.18 are small compared to experiments at higher filling degrees (for discussion, see next chapter). At MB=0.45, the temperature reached after 90 min was considerably reduced. For values of MB<0.36, the increase of milling balls number resulted in more ballball and ballwall collisions and therefore led to a higher energy input and heat dissipation. The ball movement for MB>0.36 is hindered because of the reduced space for acceleration and less energy is dissipated, which justifies the lower final temperature [28, 29]. A similar result was reported by Fang et al. for the temperature that was generated in a lysis mill [30]. They observed the highest temperature if 60% of the beaker was filled with milling balls, which approximately corresponds to MB of 0.4. A higher heating rate was also found with an increased number of milling balls in a Spex mixer mill [13]. The observed dependency of the temperature from MB is also in good agreement with experimental results for the reaction of vanillin and barbituric acid in an MBM, in which the yield increased for 0.06< MB<0.30 and strongly decreased at MB=0.45 [24]. This indicates the influence of the temperature on an organic reaction in a ball mill and the demand for a temperature control for organic syntheses in ball mills.

Influence of the milling ball filling degree on the temperature measured in the milling bed. Conditions: MBM MM400, osc=20s1, 35ml steel beaker, steel balls, d MB=5mm, m quartz sand=7.48g ( GS=0.15)

Interestingly, if MB was changed from 0.21 to 0.3, a huge difference in the measured heating curve was observed. As the amount of grinding stock was kept constant at GS=0.15, we calculated GS,rel (Eq.3) defined as the grinding stock filling degree with regard to the void volume in the milling ball packing. The complete void volume is filled with the grinding stock if GS,rel=1. Results show that the change in the heat up curve occurs if GS, rel is approximately at this value. The milling beakers warm up significantly slower for GS,rel>1 (see Online resource 1). The grinding stock overfills the void volume, the velocity of the milling balls is reduced and thus less heat is generated.

Next, we varied GS at a constant MB (Fig.3). The highest end temperature of 77C was measured without any grinding stock. By adding even small amounts of quartz sand, the temperature decreased considerably [13]. The grinding stock influences the elasticity of the collisions; the velocity and motion of the milling balls are reduced and as a consequence less energy is dissipated as heat [29]. Thus, with grinding stock a damping effect can be observed. The grinding stock acts as a heat sink as the energy is dissipated in more material and the average temperature of the milling bed is lower. Furthermore, the convection of heat from the milling balls to the milling beaker wall is improved if a grinding stock is loaded, as the main mechanism for heat transfer between a moving ball and the wall is forced convection [5]. As observed for the variation of MB (see Fig.2), a significant change in the heating curve was observed if GS was varied. Temperatures obtained for GS0.20 are nearly in the same range, independent of GS, but considerably higher temperatures were measured if GS is equal or smaller than 0.15. The point of change is, as observed in the investigation of MB (Fig.2), if GS,rel reaches a value of 1.0 (Online resource 1). Thus, the effect of the amount of grinding stock on the temperature is little for values of GS,rel>1 ( GS0.20). Obviously, a further addition of grinding stock to the milling balls has a negligible effect on the temperature progression in the milling bed. A similar effect was found for changing MB0.20 as shown in Fig.2, which indicates a less pronounced temperature increase for 1.3 GS,rel3.87 (0.18 MB0.06), similar to the results summarized in Fig.3.

Influence of the grinding stock filling degree on the temperature measured in the milling bed. Conditions: MBM MM400, osc=20s1, 35ml steel beaker, steel balls, d MB=5mm, MB=0.24, quartz sand

The material of the milling balls is an important factor for the energy input in the milling beaker. We performed reactions with milling balls made of steel (=7.8gcm1), zirconium oxide (=5.9gcm1), sintered corundum (=3.8gcm1), silicon nitride (=3.25gcm1) and agate (=2.65 gcm1). From Fig.4, it becomes clear that the higher the density of the milling balls, the higher is the temperature. A linear increase with the density of the milling balls was found. Milling balls made of a high density material are heavier. Thus, the kinetic energy of the milling balls is elevated. This led to a higher energy that is provided in the collision.

Influence of the milling ball density on the temperature measured in the milling bed. Conditions: MBM MM400, osc=20s1, 35ml steel beaker, d MB=10mm, MB=0.24, m quartz sand=7.48g ( GS=0.15)

The milling beaker material affects the temperature of the milling beakers as well. The temperature in zirconium oxide milling beakers was approximately 6K higher as in steel beakers (with zirconium oxide balls in both types of milling beakers) because of the lower thermal conductivity of zirconium oxide.

As shown in Fig.5, the final temperature depends on d MB: T milling bed passes through a maximum for 7mm balls. Milling balls with lower or larger diameter led to a reduced dissipation of heat. Aside from density and number, the diameter of the milling balls can affect the energy dissipation, because larger milling balls correspond to a higher kinetic energy of the single balls. However, the number of milling balls is affected if MB is kept constant, which influences the number of collisions [24]. In addition, d MB affects the friction coefficient and the frictional energy [31]. The results are in consensus with data published by Kwon et al. for mechanical alloying, who reported elevated temperatures in planetary ball mills if d MB was increased from 3 to 9mm, but a decreased temperature for larger milling balls [5]. In contrast, Takacs reported that the final temperature was relatively independent of the number and size of the milling balls [13]. For milling with a constant number of milling balls, a higher temperature can be assumed for larger milling balls (as long as MB is not too high) because of the higher kinetic energy with higher d MB. For instance, the temperature increase for milling with three balls with a diameter of 10mm resulted in a temperature increase of 3K after 30min, whereas with 15mm balls T was 20K.

Influence of the milling ball diameter on the temperature measured in the milling bed. Conditions: MBM MM400, osc=20s1, 35ml steel beaker, steel balls, MB=0.24, m quartz sand=7.48g ( GS=0.15)

Experiments in milling beakers with varied volume revealed that the size of the milling beaker has a strong influence on the temperature (Online resource 1). While in 10ml beakers after 90min a temperature of 35C was observed, T milling bed was 52 and 58C in 35 and 50ml beakers, respectively. On the one hand, the number of milling balls is higher in larger milling beakers, which results in an increased number of collisions and therefore a higher energy input. On the other hand, the larger milling beakers have a lower volume to surface ratio. The ratio is 1.45 times lower for 50ml beakers as for 10ml beakers. Thus, the energy dissipation from the beaker to the environment is slower, resulting in a higher T milling bed.

The temperature that was measured after 90min milling strongly depends on the loaded material. The highest temperature was measured without any grinding stock (compare section grinding stock filling degree, Fig.3). The addition of powder led to lower temperatures in every case. The temperature ranges between 40 and 63C for quartz sand and vanillin, respectively (Online resource 1). The material influence on the temperature seems to be a complicated interaction of material properties like Youngs modulus and hardness [13]. The kind of material influences the elasticity of the collision and the motion of the balls, as shown for planetary ball mills [32]. Thus, it acts on the temperature progression and on the heat transfer from the balls to the wall of the beaker [13]. Youngs modulus for example is insufficient to describe the increase of temperature. For example, with MgF2 and CaF2 as grinding stocks, final temperatures of 51C were measured, although Youngs modulus is considerably different with 139 and 76GPa, respectively [33]. In addition to the material properties of the grinding stock, physical phenomena like the compaction of the material on the milling wall influence T milling bed. If the grinding stock material adheres to the wall, less material is trapped when the balls collide. Furthermore, the compact layers reduce the free space for acceleration of the balls.

Several grinding stock materials were examined at constant GS or constant mass. The trend of the end temperatures was similar for both conditions (Online resource 1). Material properties seem to have strong influence, balancing the changes in the grinding stock filling degree.

In Table1 shows the maximal observed temperature difference that was obtained by variation of one parameter, while other parameters were kept constant. In the investigated range, the effect of the operating frequency was highest. A temperature difference of roughly 55K was detected between milling at the lowest and highest operating frequency. By variation of the filling degree of the grinding stock as well as of the milling balls, huge values for T maxmin could be found. The impacts of the beaker size, grinding stock material and grinding tool material were at the same level with approximately 23K. The lowest effect was observed for changes in the grinding stock material (at constant m GS) and for the milling ball diameter. Thus, the highest effects were induced by osc and the filling degree, which is important with regard to the design of the experiments.

The measurement of the temperature that was generated in a mixer ball mill indicates a strong dependence of T milling bed on several milling parameters. Higher temperatures were measured with increased operating frequency and in milling beakers with larger volume. The heat dissipation passes through a maximum for the milling ball filling degree and the milling ball diameter. Regarding the milling ball material, a linear correlation to the density of the milling balls was found. The results indicate that GS,rel is of great influence on the temperature progression. A changed behavior was observed if GS,rel was increased over GS,rel=1. These results can be helpful for the experimental design and for performing reactions in ball mills successfully. Furthermore, the results seem to be transferrable to other types of ball mills (for example, planetary ball mills) and are not restricted to the investigated type of mixer ball mills.

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how to make a ball mill: 12 steps (with pictures) - wikihow

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Ball mills are a special instrument used to break up hard solids into a fine powder. They are similar to rock tumblers in that the instrument is a rotating container filled with heavy balls to grind the substance into powder. Ceramic material, crystalline compounds, and even some metals can be ground up using a ball mill. Using a motor, container, belt, caster wheels, and some basic building supplies, you can make your own ball mill.[1] X Research source

To make a ball mill, start by building a wooden platform and attaching a motor underneath it. Then, cut a slit into the wooden platform for the belt to pass through and attach casters to the platform for the container to sit on. Next, thread the belt through the slit and position the container so the belt is pulled tight. Finish by connecting the motor to the power supply, and filling the cylinder with metal balls and the substance you want to grind. For tips on how to operate your ball mill, read on! Did this summary help you?YesNo

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Grinding balls making machine is widely used for grinding balls producing in foundries, including balls making machine: automatic casting grinding media molding line and manual metal mold; heat treatment furnace for grinding balls, contents quenching plant and tempering machine; the balls separator for separating the grinding balls with risers, runners and pouring gates; and then the lab testers, like spectrometer, Rockwell hardness testing machine, impact value.