CdS/TiO2 composite photocatalysts were made by the method of secondary ball milling at different ball milling speeds, milling time, and material ratios. After the secondary ball milling process, parts of the samples were calcined at high temperatures. X-ray diffraction (XRD) and UV-Vis diffuse reflectance spectroscopy (DRS) were used to observe the powder particle size, structural defect, bandgap, and absorption spectrum of the samples. Combined with the observation results, the effects of ball milling speed, time, material ratio, and high-temperature calcination on the photocatalytic performance of CdS/TiO2 composite samples were analyzed. Furthermore, the methyl orange (MO) was chosen to simulate pollutants, and the photocatalytic degradation rate of CdS/TiO2 composite photocatalysts for MO was evaluated under sunlight and UV irradiation conditions. The photocatalytic degradation efficiency of CdS/TiO2 photocatalyst under UV irradiation is much higher than that under sunlight irradiation. The experimental results reveal that secondary ball milling can effectively promote the formation of CdS/TiO2 composite nanostructure and the high-temperature calcination can reduce the bandgap width, which makes the samples easier to be excited. When the ball milling speed, time, and material ratio were respectively 400rpm, 10h, 25:75, and then calcined at high temperature, after 2h of irradiation under UV light, CdS/TiO2 composite photocatalysts exhibited maximum photocatalytic degradation efficiency of 57.84%.
Nowadays, with the speedy development of the economy and increasing awareness of environmental protection, people pay more and more attention to the utilization of solar energy and photocatalytic treatment of organic pollutants by semiconductors . Photocatalysis technology has been considered as one of the most efficient solutions to address air pollutants due to its preferable properties and complete degradation [2,3,4]. Among ZnO , CeO2, TiO2, etc, TiO2 has been the preferred photocatalyst on account of its excellent photocatalytic performance, environmental protection, stable chemical properties [6,7], inexpensiveness, nontoxicity, and long service life [8,9]. In particular, when compared with the traditional methods, it is of great significance to the degradation of pollutants in wastes and can even completely degrade pollutants in sewage [10,11]. Therefore, TiO2 has been used in extensive variety of applications, such as optics, self-cleaning, water splitting, catalysts, electrical porcelain [12,13], reducing environmental pollution , and other aspects [15,16,17], which has prompted many people to use various methods, such as microwave-assisted solvent heat method and ion exchange method to prepare TiO2 [18,19]. For example, Maruthapandi et al.  presented polyaniline composite formed with TiO2; SiO2 is an effective adsorbent for the degradation of the organic contaminant in water, and the adsorbed contaminant can be desorbed to achieve reuse. Zangeneh et al.  used l-histidine with TiO2/CdS photocatalytic nanocomposite materials to prepare a polyethersulfone membrane. The results showed that due to the addition of them, the surface roughness of the membrane had a good change and its porosity and hydrophilic energy were also improved, thus improving the antifouling performance, which is of great significance to the protection of water resources. In addition, activated carbon nanocomposites synthesized with TiO2 and other substances have been used to remove airborne pollutants such as xylene [22,23]. In 2020, Rambabu et al.  proposed a tricomponent photocatalyst consisting of TiO2 multileg nanotubes, CdS nanoparticles, and reduced graphene oxide, which has good light absorption performance and can efficiently generate easily separated carriers under light irradiation. Furthermore, TiO2 was modified with Cu(OH)(2) as a catalyst to make composite catalyst and used it for photocatalytic hydrogen production, which shows a great hydrogen yield , and Qin et al.  reached a similar conclusion through experiments. However, since TiO2 with a wide bandgap width of 3.2eV, especially the crystalline phase of anatase can merely be excited under UV light, resulting in about 50% visible light cannot be utilized. At the same time, some of the electron-hole pairs induced by UV are easy to recombine, which greatly limits its photocatalytic performance [27,28].
In recent years, more in-depth studies have been conducted at home and abroad, to conquer these defects of TiO2 mentioned earlier and improve its photocatalytic activity. A large number of studies have shown that modification methods such as ion doping, semiconductor compounding, noble metal deposition, and photosensitization can extend the optical photoabsorption wavelength of pure titanium dioxide to the visible light range and enhanced its catalytic activity to different degrees [29,30]. Trejo-Tzab et al.  proposed that the photocatalytic activity of TiO2 can get improvement by nitrogen (N2) doping and deposition of Cu nanoparticles, the degradation rate of MB solution was used to evaluate the photocatalytic activity of this improved TiO2. The experimental phenomenon indicated that under the condition of low-intensity gas plasma, incorporating Cu into TiO2 P25 to obtain nitrogen-doped TiO2/Cu had higher photocatalytic activity than pure anatase TiO2. Also, Tokmakci et al.  proved that the photocatalytic performance of titanium dioxide photocatalysts mixed with both boron (B) and zirconium (Zr) is higher than that of single element mixed TiO2 and pure anatase TiO2. This kind of phenomenon can attribute to the reduction in the size of the photocatalyst and the successful weaving of B and Zr into the crystal structure by the mechanical ball milling method. Similarly, the sulfur (S)-doped TiO2 photocatalysts newly designed by Jalalah et al.  extended their absorption edge to the visible light range via incorporating sulfur into the lattice structure of TiO2, revealing an excellent photocatalytic activity of the new photocatalysts to MB in the visible region. Petrovic et al.  synthesized TiO2/CeO2 photocatalyst by high energy ball milling method. According to the degradation efficiency for MO solution, one could see that TiO2/CeO2 composite material had higher photocatalytic activity than anatase TiO2, resulting from the effective separation of electron/hole pairs on TiO2 because of the addition of CeO2. Aysin et al.  prepared silver-loaded TiO2 photocatalyst, the number of photogenerated charge carriers involved in the degradation process greatly increased because of the introduction of Ag and gave rise to the improvement of photocatalytic activity. Even with very little Ag loading, the photocatalytic degradation efficiency for MO of the silver-loaded samples was 50% higher than that of the unloaded TiO2 after 1h of irradiation under UV light. Chen et al.  obtained TiN/TiO2 composite nanoscale photocatalyst by ball milling of TiO2 in TiN-doped aqueous solution, and when compared with pure anatase TiO2, TiN/TiO2 composite nanoscale photocatalyst has better photocatalytic performance under both sunlight and ultraviolet light irradiation. The enhancement in the photocatalytic performance of the TiN/TiO2 may well owe to the extension of photoabsorption wavelength of the photocatalyst and the raise of the Ti3+ reaction center on the surface. Moreover, Balakrishnan et al.  used methylene blue to simulate pollutants to investigate the photocatalytic performance of TiO2ZnO nanostructures. The result proves that TiO2ZnO nanostructures had good photocatalytic performance in both visible and ultraviolet light. Habib et al.  also conducted an in-depth study on the photocatalysis of TiO2ZnO nanocomposites.
It can be seen from the studies listed above that in recent years, although some effects have been achieved through the modification of TiO2, most modification methods are difficult to be applied and industrialized due to high preparation cost, complicated process, and limited performance improvement. In 2015, Zhou et al.  prepared CdS/TiO2 composite photocatalytic material by a simple mechanistic method to boost the photocatalytic activity of TiO2 by sensitization and surface hybridization of CdS. In the paper, CdS/TiO2 samples were made by similar mechanochemical secondary ball milling process (dry ball milling and wet ball milling), to explore the optimal ball milling process and the effect of high-temperature calcination on the photocatalytic performance of CdS/TiO2 composite photocatalyst produced by secondary ball milling process; methyl orange (MO) was used to simulate pollutants, and the photocatalytic activity of CdS/TiO2 composite photocatalyst was analyzed by UV-Vis diffuse reflectance spectroscopy (DRS) and X-ray diffraction (XRD). CdS is a kind of commonly used photosensitive resistor, which has a strong photoelectric effect on visible light. The bandgap width of it is relatively narrow, and the spectral response range is close to that of visible light. However, electron holes have strong redox ability and can oxidize S2 on the surface under light irradiation, resulting in severe photocorrosion in the use of CdS alone. After the combination of TiO2 and CdS with a large difference in bandgap width, electrons can transition within the visible region, leading to the high-efficiency separation of electron holes. Therefore, TiO2 can be excited in the visible range, and the photogenic holes of CdS can be neutralized, which can inhibit the photocorrosion phenomenon and enhance the photocatalytic performance of CdS/TiO2, thus achieving more efficient degradation of pollutants in water. Although there have been many solutions for organic pollutants in water, for example, Yi et al.  studied the adsorption performance of silica gel to organic pollutants in water but only at low concentrations of pollutants. The production method is complex and has strict requirements on precision and time, which is not suitable for large-scale production or industrial utilization.
The materials used in this experiment are nanoscale pure anatase titanium dioxide (TiO2, if used as an industrial application, Wang et al.  proposed large-scale synthesis of high purity TiO2 by ion-exchange method, cadmium sulfide (CdS) and MO, all of which are analytical pure drugs produced by China National Pharmaceutical Corporation.
CdS/TiO2 composite photocatalysts were made by the secondary balling milling method, which was carried out in two steps. The first step was to fully compound the raw materials evenly mixed by dry ball milling, and the second step was to refine the particle size of powders by wet ball milling. The detailed preparation processes of CdS/TiO2 composite photocatalysts are as follows: (1) according to the designed material ratios (i.e., the mass ratio of CdS to TiO2), the TiO2 and CdS powders were accurately weighed and added into the corundum ball mill tanks in turn. Corundum balls with diameters of 10, 5, and 2.5mm and a mass ratio of 1:3:6 with a total mass of 200g were selected for grinding ball beads. The mass ratio of corundum balls and materials was 20:1. Place the ball mill tanks in the ball mill after the ingredients are finished. (2) On the basis of ball milling speed and ball milling time designed in the experiment, the TiO2 and CdS powders were ground by dry ball milling. (3) After the completion of dry ball milling, anhydrous ethanol was added to make the mixed materials show sticky shape and then put it into the ball mill again with the same milling parameters for wet ball milling for 2h. (4) After the ball milling was finished, the products were collected immediately, then dried and put into the resistance furnace to calcine at 400C for 2h. (5) The materials prepared by the above processes were fully ground for 30min to make the samples.
The properties and structures of the samples were characterized by UV-Vis DRS and XRD (Cu K, scanning rate was 4/min, and the transport current and acceleration voltage. Respectively. were 40mA and 45kV). Based on the above analyses, the XRD patterns of the CdS/TiO2 composite photocatalysts and the average particle sizes were obtained. Moreover, the absorbance spectrum of the photocatalysts was acquired by utilizing the UV-Vis spectrophotometer (type LAMBDA 650), and the band gaps as well as the light absorption capacity were further analyzed.
MO is a common organic dye, which has no volatilization and is difficult to decompose and oxidize under light and can be used to simulate experimental contaminants. In this experiment, MO simulating pollutants were utilized to study the photocatalytic properties of CdS/TiO2 composite photocatalysts, and the degradation rate was calculated by UV-2600 UV-Vis spectrophotometer. MO is a state of sodium sulfonate dyes, with the extension of photocatalysis time; MO was adsorbed on the surface of CdS/TiO2 composite photocatalysts and eventually degraded to H2O CO2 and other inorganic substances. During the degradation, new substances appeared and the concentration of new substances was constantly increasing. However, due to the complexity of the products, limited test time, and instrument conditions, the intermediate products could not be completely separated.
UV-high-pressure mercury lamp and long-arc xenon lamp were used to simulate the environment of UV light and sunlight irradiation conditions. The experimental procedures are as follows: (1) 10mg/L mixture solution of MO was made by weighing a certain amount of MO and deionized water with a balance and a measuring tube, respectively. (2) Weighing 25mg of photocatalyst with a balance and weighing 250mL of MO solution with a measuring tube, both were placed in a container together, and 4mL solution was removed by a pipette and put into the centrifuge tube in the opaque collection box. (3) The photocatalyst and MO mixed solution was put into the dark box and stirred with a magnetic stirrer for 30min, then 4mL solution was removed with a pipette and placed in the centrifuge tube in the opaque collection box. (4) Turn on the lamp needed for the experiment to preheat for 10min. (5) The photocatalyst and MO mixture was irradiated with simulated UV light or sunlight for 2h, while the cooling cycle was turned on. Place the solution directly under a light source so that it is fully illuminated and then stirred with a magnetic stirrer. Every 30min, 4mL of the solution was removed by a pipette and placed it in a centrifuge tube in the opaque collection box. (6) The samples of the solution collected in steps (3) and (5) were centrifuged in a centrifuge, and the supernatant liquor was removed with a pipette. The supernatant liquor was then centrifuged until the centrifuged solution did not precipitate. (7) Turn on the UV-2600 spectrophotometer and set its parameters. The centrifugal fluid collected from step (6) was poured into the colorimetric dishes, respectively, and the absorbance was measured (the absorbance of the centrifugal fluid was recorded as A t according to the illumination time). After each measurement, the colorimetric dish was cleaned with deionized water. (8) In this experiment, the photocatalytic degradation efficiency (D) can be obtained by the following formula:
The CdS/TiO2 composite photocatalyst prepared by the ball milling method has an effective composite structure. During the formation of the composite structure, CdS has a larger negative conductive potential and a smaller bandgap width than TiO2, which results to the migration of the electrons (e) of CdS to the conductive band of TiO2, achieving the purpose of e and holes (h+) separation resulting in the enhancement of the photocatalytic capacity of CdS/TiO2 samples.
When the composite photocatalyst is irradiated with light with energy greater than the bandgap energy of it, electrons are excited and enter the conduction band across the forbidden band, producing negatively charged highly active electrons (e) in the conduction band, leaving positively charged holes (h+) in the valence band, thus producing highly active electronhole pairs (h+e) on the surface of the photocatalyst. Under the action of an electric field, electrons and holes separate and migrate to the particles surface. Holes on the surface of the composite photocatalyst can oxidize hydroxyl (OH) and water (H2O) adsorbed on its surface to hydroxyl radicals (HO), which is a kind of nonselective oxidant with strong oxidation capacity and can completely oxidize MO to CO2, H2O, and other inorganic substances.
The material ratio is the ratio of raw materials needed to prepare unit products. This experiment prepared seven groups of CdS/TiO2 samples with a material ratio of 10:90, 15:85, 20:80, 25:75, 30:70, 35:65, and 40:60, respectively, to explore the influence of different material ratios on the photocatalytic performance of CdS/TiO2 composite photocatalysts. Under the conditions of ball milling speed of 400rpm and ball milling time of 10h, the photocatalytic performance of samples prepared with different material ratios was evaluated.
Figure 1 shows the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts that of different material ratios after 2h of irradiation under UV light or sunlight. As can be seen from Figure 1 whose horizontal axis and the vertical axis, respectively, represent the material ratio and photocatalytic degradation efficiency of the samples, when the material ratio of CdS/TiO2 composite photocatalysts is 25:75, the photocatalytic degradation efficiency is the highest, which is 8.04% and 54.32%, respectively, under sunlight and UV light. It follows that the photocatalytic performance of CdS/TiO2 composite photocatalysts synthesized at a material ratio of 25:75 is higher than that of pure TiO2, with photocatalytic degradation efficiency of 3.75% and 49.04%, respectively, under the irradiation of sunlight and UV light. Figure 2 exhibits the UV-Vis diffuse reflectance spectra of the TiO2 and CdS/TiO2 composite photocatalysts. One can see that from the figure, when compared with pure TiO2 (whose light absorption edge is at 390nm in the ultraviolet light range), the absorption edge of CdS/TiO2 composite photocatalyst after ball milling extends to the vicinity of 525nm in the visible region, resulting in greatly enhancement of photocatalytic activity. Also, Figure 2 shows that the CdS/TiO2 composite photocatalyst with a material ratio of 25:75 has the largest absorption spectral area, that is, the best photocatalytic performance, which is consistent with the data measured in Figure 1. The improvement of photocatalytic degradation efficiency can be put down to the effective composite structure of CdS/TiO2 composite photocatalyst formed by ball milling under this material ratio. In addition, under the same conditions, the photocatalytic degradation efficiency of UV irradiation is much higher than that of sunlight irradiation, since the bandgap width reduces the utilization rate of sunlight. According to the XRD pattern shown in Figure 3, the characteristic diffraction peak value of TiO2 and CdS still exist in the crystal phase of the CdS/TiO2 composite photocatalysts after ball milling, and the decrease in crystallinity of the samples made by ball milling processes may be caused by the strain effect of lattice and the defects of lattice structure due to the action of mechanical force.
The ball milling speed is the speed of the stirring shaft of the ball mill. Under the condition of low speed, medium speed, and high speed, the ball milling process can mix, compound, and crush the sample powders, respectively. As the speed of ball milling increases gradually, the ball milling energy will gradually increase and the composite will be more sufficient, but the damage to the material structure and the polymerization of the powders will also be more violent. To avoid the effect of the powder polymerization on the photocatalytic performance, dry ball milling was first used in this experiment, and then wet ball milling was carried out to refine the powder particle size. The photocatalytic properties of CdS/TiO2 composite photocatalysts were evaluated at different ball milling speeds under the conditions of the preset material ratio of 25:75 and ball milling time of 10h.
Figure 4 shows the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts exposed to UV light and sunlight for 2h at different ball milling speeds. It is evident from Figure 4 that the speed of ball milling has an obvious effect on the photocatalytic performance of the samples, especially under UV irradiation. When the ball milling speed is no more than 400rpm, the photocatalytic degradation efficiency of the CdS/TiO2 composite photocatalysts exhibits a positive linear relationship with the ball milling speed, which may be due to the fact that the powders are not fully compounded, resulting in the low separation efficiency between holes and electrons; hence, the photocatalytic activity of the composite does not reach the maximum value. When the ball milling speed is 400rpm, the CdS/TiO2 composite photocatalyst has the highest photocatalytic degradation efficiency of 55.39%, indicating that the powders are most fully compounded and the composite structure has fewer defects under this ball milling speed. When the ball milling speed is higher than 400rpm, a lot of adverse defects occur in the composite structure, which is not conducive to carrier separation and transfer resulting in the decrease of photocatalytic activity of the samples.
Moreover, DebyeScherrer formula was employed to calculate the average particle sizes of CdS/TiO2 composite photocatalysts prepared at different ball milling speeds, to explore the relationship between powder particle size and photocatalytic performance. Table 1 shows the detailed particle size of CdS/TiO2 composite photocatalysts at different ball milling speeds. As the table indicates, with the gradual increasing ball milling speed, the average particle size of CdS/TiO2 composite photocatalyst gradually decreases from 42.3nm and finally stabilizes at about 7.4nm. When the ball milling speed increases from 200 to 400rpm, the average particle size of the sample decreases significantly by 32.1nm, 75.9% compared with the average particle size at 200rpm. However, when the ball milling speed increases from 400r/min to 600r/min, the particle size is reduced by only 2.8 mm. The photocatalytic degradation efficiency of CdS/TiO2 composite photocatalyst is the highest at the ball milling speed of 400rpm, corresponding to the nano-effect of nanoparticles and the trend of particle size reduction as shown in Figure 5.
The time of ball milling directly affects the particle size and purity of the product. During ball milling, due to the severe collision and friction of ball milling beads, part of the product will fall off. The longer of ball milling time, the more serious the pollution of the product will be. On the other hand, as mentioned above, with an increase in the ball milling time, the particle size of CdS/TiO2 composite photocatalysts will gradually decrease to a certain value and tend to be stable, even if the ball milling time still increases. Under such a condition, the increase in the ball milling time will only lead to the contamination of the CdS/TiO2 composite photocatalysts, but no longer improve the photocatalytic performance. Hence, one can see that finding the optimal ball milling time is quite crucial.
Under the conditions of the preset material ratio of 25:75 and ball milling speed of 400rpm, Figure 6 shows the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts prepared at different milling time after 2h of UV or sunlight irradiation. As is shown in the diagram, the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalyst with a ball milling time of 10h is better than that of other ball milling time under both UV light and sunlight irradiation, so it can be considered that 10h is the best ball milling time for preparing the CdS/TiO2 composite photocatalysts. Similarly, DebyeScherrer formula was also made use of calculating the average particle size of CdS/TiO2 composite photocatalysts prepared under certain ball milling time, which is revealed in Table 2. The particle size of the sample significantly decreases when the ball milling time is no more than 10h and gradually becomes stable after more than 10h, which is consistent with the changing trend of the photocatalytic performance of the composite photocatalysts, which one can see from the comparison between Figures 6 and 7.
To explore the influence of high-temperature calcination on the photocatalytic activity of CdS/TiO2 photocatalysts (ball milling time of 10h, i.e., optimal ball milling time) with different material ratios, the samples were put into a resistance furnace and calcined at 400C for 2h. Subsequently, the photocatalytic test was carried out under UV or sunlight irradiation for 2h, and the calculated photocatalytic degradation efficiency was compared with that of the uncalcined samples, as shown in Figure 8. On the whole, the variation trend of photocatalytic degradation efficiency of CdS/TiO2 photocatalysts with material ratio after calcination is approximately the same as that of uncalcined samples, but the photocatalytic degradation efficiency of calcined samples increases slightly, which may be owing to the reduction of adverse structural defects of CdS/TiO2 composite photocatalysts. In addition, the bandgap width of the samples prepared and calcined under the optimal ball milling parameters and that of TiO2 was calculated. As shown in Figure 9(a) and (b), the bandgap width of pure anatase TiO2 is 3.05eV, which is consistent with the research showing that the bandgap width of TiO2 is 3.2eV, while the bandgap width of CdS/TiO2 composite photocatalyst is 1.88eV. The wider the bandgap, the greater the energy required to excite the materials. In this experiment, the CdS/TiO2 composite photocatalysts obtained by the secondary ball milling and calcining method have smaller bandgap width, easier excitation, and stronger photocatalytic performance.
In this paper, CdS/TiO2 composite photocatalysts are made by the method of secondary ball milling, and the effects of material ratio, ball milling time, ball milling speed, and high-temperature calcination on the photocatalytic performance are investigated. The following conclusions can be drawn from the experimental results:
The mechanochemical action of secondary ball milling can promote the dispersion of CdS on the surface of TiO2 and the interaction between these two, forming an effective composite nanostructure with extended light absorption edge and small bandgap width, resulting in a significant improvement on the photocatalytic degradation rate of CdS/TiO2 composite photocatalysts.
High-temperature calcination of CdS/TiO2 composite photocatalysts with different material ratios did not change the variation trend of their photocatalytic degradation efficiency, but the photocatalytic degradation efficiency increased slightly after calcination in general, which may be because of the reduction in adverse structural faults of CdS/TiO2 composite photocatalyst after calcination.
The material ratio and ball milling process had an obvious influence on the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts when compared with high-temperature calcination. When the ball milling speed, time, and material ratio were 400rpm, 10h, and 25:75, respectively, the CdS/TiO2 composite photocatalysts obtained by calcining after ball milling had the strongest photocatalytic performance on MO.
The irradiation conditions also had a significant influence on the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts. The photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts under UV irradiation was much higher than that under sunlight irradiation.
As for the application, it is suggested that the CdS/TiO2 composite photocatalysts can be prepared under the optimum conditions of the ball milling process if it is used in the industry. Since high-temperature calcination has a relatively small effect on its photocatalytic performance, for the sake of economy and operability, this step is not performed unless it is particularly necessary.
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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 .
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 . 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 .
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 . 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. 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. and El-Eskandarany etal. 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. 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, using for example cold-rolling approach, 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.
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).
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.
Machining of carbon/carbon (C/C) composite materials is difficult to carry out due to its high specific stiffness, brittleness, anisotropic, non-homogeneous and low thermal conductivity, which can result in tear, burr, poor surface quality and rapid wear of cutters. Accurate and fast prediction of cutting forces is important for milling C/C composite materials with high quality. This paper presents an alternative cutting force model involving the influences of the directions of fiber. Based on the calculated and experimental results, the cutting forces coefficients of 2.5D C/C composites are evaluated using multiple linear regression method. Verification experiment has been carried out through a group of orthogonal tests. Results indicate that the proposed model is reliable and can be used to predict the cutting forces in ball-end milling of 2.5D C/C composites.
Shan Chenwei is an associate professor at School of Mechanical Engineering, Northwestern Polytechnical University, Xian, China. He received the Ph.D. degree from the same university in 2009. His main research interests are deformation prediction and control of thin-walled sculptured surface, CAD/CAM and C/C Composite structures CNC machining.
The structure and properties of composite materials based on copper, strengthened with diamond nanoparticles in an amount of 10 35 vol.%, are studied. the starting matrix raw material is coarse copper particles with a size of about 1000 m. Materials in the form of granules are prepared by mechanical alloying after treatment in a planetary ball mill for 1 10 h. Granules are hot pressed at 500C. The effect of prolonged mechanical alloying on copper diamond composite material microstructure is studied. The effect of diamond content on composite material microhardness and thermal expansion coefficient is determined.
Th. Schubert, B. Trindade, T.Weigrber, and B. Kieback, Interfacial design of Cu-based composites prepared by powder metallurgy for heat sink applications, Mat. Sci. Eng. A, No. 475 39 44 (2008).
V. A. Popov, E. A. Skryleva, A. Chuvilin, et al., in: Proc. 3rd Intern. Symp. Detonation Nanodiamonds: Technology, Properties and Application, Russia, St. Petersburg, 1 4 July, 2008 [in Russian], Joffe Physico-Technical Institute, St. Petersburg (2008).
P. Y. Detkov, V. A. Popov, V. G. Kulichikhin, and S. I. Chukhaeva, Molecular building blocks for nanotechnology: from diamondoids to nanoscale materials and applications, Top. Appl. Phys., 109, 29 43 (2007).
A. A. Aksenov, A. S. Prosviryakov, D. V. Kudashov, and I. S. Gershman, Structure and properties of composite materials based on the Cu Cr system prepared by mechanical alloying, Izv. Vyssh. Uchebn. Zaved., Tsvetn. Met., No. 6, 39 46 (2004).
V. A. Popov, V. A. Zaitsev, A. S. Prosviryakov, et al., Study of mechanical alloying in preparing composite materials with nanosize strengthening particles, Izv. Vyssh. Uchebn. Zaved., Poroshk. Met. Funkts. Pokr., No. 2, 48 52 (2010).
D. V. Kudaschov, A. A. Aksenov, V. Klemm, et al., Microstructure formations in copper silicon carbide composites during mechanical alloying in a planetary mill, Werkstoffwisenschaft und Werkstofftechnogie, 31, 1048 1055 (2000).
Prosviryakov, A.S., Samoshina, M.E. & Popov, V.A. Structure and properties of composite materials based on copper strengthened with diamond nanoparticles by mechanical alloying. Met Sci Heat Treat 54, 298302 (2012). https://doi.org/10.1007/s11041-012-9501-8
Carbon nanotubes reinforced pure Al (CNT/Al) composites were produced by ball-milling and powder metallurgy. Microstructure and its evolution of the mixture powders and the fabricated composites were examined and the mechanical properties of the composites were tested. It was indicated that the CNTs were gradually dispersed into the Al matrix as ball-milling time increased and achieved a uniform dispersion after 6h ball-milling. Further increasing the ball-milling time to 812h resulted in serious damage to the CNTs. The tensile tests showed that as the ball-milling time increased, the tensile and yield strengths of the composites increased, while the elongation increased first and then decreased. The strengthening of CNTs increased significantly as the ball-milling time increased to 6h, and then decreased when further increasing the ball-milling time. The yield strength of the composite with 6h ball-milling increased by 42.3% compared with the matrix.
The ball mill liners and grinding media are the largest consumption of wear-resistant iron and steel parts with an annual consumption of 2 million tons in China. With the development of Chinas economic construction, the demand for cement is increasing year by year, and the consumption of wear-resistant materials is also increasing correspondingly, which will consume more metals and increase the production cost of cement. For the cement with special requirements (such as white cement), the quality of cement will be reduced, and the production can not be carried out smoothly. According to the cement output in 2003, more than 60000 tons of high-quality wear-resistant steel are needed for mill liners only. Moreover, the materials used for lining board production in China are uneven, and the actual consumption can be nearly 100000 tons. According to the characteristics of the cement industry, this paper carefully analyzes the working environment and wear failure reasons of cylinder liner, studies and selects ball mill liners material, and carries out production application.
The main function of the ball mill liner is to protect the mill and use the convex peak of the liner to play the ball to grind and crush the material. Therefore, the main failure mode of the liner is abrasive wear under the repeated impact of small energy. Fig. 1 shows the motion diagram of grinding ball and material. In the movement of grinding bodies and materials, the grinding balls with large diameters are mainly distributed in the outer ring. Most of the grinding balls fall on the bottom of the material bed and only a small part on the liner plate. Due to the buffering effect of materials and the mutual impact between the materials and the grinding body, the running track of the grinding body is disturbed, and the falling point deviates, and the falling height is reduced. As a result, the impact of the abrasive body on the liner is greatly reduced, the impact times and the impact frequency are increased.
The impact of the grinding ball on the ball mill liner is shown in Fig. 2. The impact point diameter D0 of the ball mill is smaller than the effective diameter D of the mill. Through analyses, it is determined that most of the grinding bodies in the bin only hit the lining plate after several times of impact and folding. Therefore, the impact on the lining plate is far less than the impact energy produced by the vertical falling object of 0.95 D (usually the thickness of the lining plate accounts for 0.5 % of the effective diameter).
Figure 3 shows the relative position of grinding balls. When the ball is brought down to a certain height by the rotating ball mill liner, only when x > R1 + R2, the movement direction V0 of Q1 will not change and form a direct impact on the liner; when x = R1 + R2, Q1 and Q2 pass each other, and the movement direction V0 of Q1 will not change greatly, However, a small amount of friction will reduce the impact on the liner, and most of the cases of x < R1 + R2 will not form a direct impact on the liner.
The main force of liner fracture caused by the impact of the abrasive body on the liner is the vertical component of tangential force between the surface of the liner and the contact point of the abrasive body. The size of this force is affected by the shape of liner; the movement state, speed, and direction of the abrasive body, which greatly weakens the strength of direct connection and improves the impact degree of the liner.
In order to improve the service life of the liner, reduce the material consumption and production cost, it is very important to carry out reasonable structural design under the condition of meeting the requirements of the cement grinding process. The large mill imported by Jidong Cement Company, Hebei Taihang Cement Company, and other units has small lining plate size, large plate thickness and bolt free installation, which lays a good design foundation for the application of high hardness materials to lining plate, and makes it possible to greatly increase the service life of lining plate. At present, the design basis of high manganese steel is still used in the lining plate of ball mill in China. From the geometric structure, it is usually thin and large, and there is obvious stress concentration at the bolt installation hole. In order to ensure the stable application of the new type of wear-resistant casting material and give full play to its unique advantages, this point must be taken into account when adopting new high wear-resistant casting materials.
In addition, the installation quality also plays an extremely important role in the reliable service of high hardness materials. After a long time of experimental exploration, it is found that for the application of casting materials with high hardness and high wear resistance, the supplier and application unit of lining plate should cooperate well, so that the user can better understand the characteristics of the new material, and the lining plate can not be suspended or loosened during installation And can get a certain cushion effect. This is easy to do in the installation process of cement mill, which can effectively ensure the good wear resistance of the lining plate.
Up to now, nearly half of the lining materials of ball mill in the domestic cement industry are still made of ordinary high manganese steel. The main failure modes of high manganese steel ball mill liners are as follows:
Fracture failure: the ball mill liner is greatly impacted by the grinding body and materials, especially in the situation of large-scale development of high-efficiency and energy-saving cement mill, the impact energy of lining plate increases. Although the toughness of high manganese steel is very good, the current supply quality can not be guaranteed very well, for example, the carbon content is too high, the manganese carbon ratio is improper, and the water toughening treatment has problems, the fracture failure will occur.
Protrusion deformation: the volume of high manganese steel liner is increased due to the continuous impact of abrasives and materials. At the same time, due to the extension of plastic materials caused by impact, the thickness of mill liner decreases, and the circumferential dimension increases. However, the circumferential dimension of the liner is limited by the overall dimension of the ball mill, and there is not much expansion space to cause the liner to protrude, As a result, the fastening bolts are broken and some lining plates fall off. This phenomenon often occurs in larger mills and in smaller mills.
Wear failure: abrasive wear is one of the main forms of ball mill failure. Even if the lining plate working in the first chamber of large grinding mill bears large stress and the surface is easy to produce work hardening, the hardening layer is very thin. Under the repeated action of abrasive, the metal which is extruded and bulged and the hardened layer impacted by large abrasive edges and corners is easy to crack and peel off. It is found that there is more friction between the plow board and the surface of the plow board under the condition of grinding and hardening.
Through the analysis of the working condition of the liner and the failure analysis of the high manganese steel liner, we realize that it is safe and effective to select the material with high hardness and high wear resistance with good comprehensive mechanical properties according to the working conditions of the cement mill.
Most of the ball mills used for cement production are tube mills with length diameter ratio L / d > 2.5. The mill has a coarse grinding bin and a fine grinding bin (some of which are three bins). The coarse grinding bin is mainly used for crushing and the fine grinding bin is mainly for grinding. The liner plates of the primary mill are mainly stepped liner and wavy liner, and the liner plates of some new high efficiency and energy-saving mills are not separated from their basic forms. The main forms of the fine grinding bin are pattern lining plate, small corrugated lining plate, and flat-lining plate. The effect of the coarse grinding chamber and fine grinding chamber is different, and the ball diameter of the grinding body loaded is different, and the impact on the lining plate is also very different. With the same mill diameter, the diameter of the grinding body in the coarse grinding chamber is large, the material fragmentation is also large, the impact on the lining plate is large, and the wear speed is high; the situation of the fine grinding bin is much better. When choosing lining materials, different materials must be determined according to different conditions.
Industrial growth increases the consumption of wear-resistant parts and then stimulates and drives the development of the wear-resistant material industry. The production process, mechanical properties, and industrial application effect of 40-50 typical brands of wear-resistant steel materials have been industrialized, and the process characteristics of various varieties and small batch have been formed. After many years of production and research of wear-resistant materials, it is considered that the application of multi Alloying High Chromium Cast Iron in the coarse grinding bin of mill with size less than or equal to 3.0 m can be used safely and stably with excellent wear resistance effect and comprehensive economic effect; The application of medium carbon low alloy chromium manganese steel in the roughing bin of the mill with a diameter of fewer than 3 m has good wear resistance and appropriate comprehensive mechanical properties. In general, many kinds of wear-resistant steel materials can be selected for the fine grinding bin, and the medium carbon low-alloy chromium manganese steel developed and produced by us has achieved good results. If there is a strong investment ability, the application of high chromium cast iron can achieve the effect of no replacement for many years, and the comprehensive economic and social benefits are more prominent.
All brand names, model names or marks are owned by their respective manufacturers. MGS Casting has no affiliation with the OEM. These terms are used for identification purposes only and are not intended to indicate affiliation with or approval by the OEM. All parts are manufactured by, for and warranted by MGS Casting and are not manufactured by, purchased from or warranted by the OEM.
Electrodes are the key material in the electrolysis of non-ferrous metals, and their selection and preparation can be a difficult problem in the hydrometallurgical industry. In this paper, starting from the selection of electrode materials and structural design, an Al/TiB2+10%Ti4O7-coating composite material was prepared by plasma spraying technology, and a -PbO2 coating was prepared by electrochemical deposition. The phase composition of the coating was analyzed by X-ray diffractometer, and the structure of the coating was observed by scanning electron microscopy. Results show that the electrode prepared by electrodeposition at a current density of 0.03Acm2 has a more compact structure and more uniform grain size. Through steady-state polarization curve, cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy, the electrochemical performance of the electrode was studied. Through porosity measurements, it was found that the composite electrode material prepared by the plasma spraying method under the parameters of a spraying power of 36kW, powder feeding rate of 30g/min, spraying distance of 105mm, and argon gas flow rate of 2.6 m3/h greatly reduces the charge resistance in the double-layer structure on the electrode surface, thereby accelerating the charge transfer rate. Plasma spraying and electrochemical deposition have been used to successfully prepare Al/TiB2+10%Ti4O7/-PbO2 composite electrode materials with good corrosion resistance and electrochemical catalytic activity. Compared to Ti/-PbO2 and Pb-(0.5wt%)Ag/-PbO2, the corrosion resistance and polarization potential increased by 83.6% and 93.0% and negatively shifted by 517.37mV and 587.12mV, respectively, and the catalytic activity was also significantly improved.
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S. Chen, B. Chen, S. Wang, W. Yan, Y. He, Z. Guo, R. Xu, Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SnSb-RuOx/-PbO2/-PbO2 electrode in zinc electrowinning. J. Alloy. Compd. (2019). https://doi.org/10.1016/j.jallcom.2019.152551
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R. Cano-Crespo, B.M. Moshtaghioun, D. Gmez-Garca et al., High temperature creep of carbon nanofiber-reinforced and graphene oxide-reinforced alumina composites sintered by spark plasma sintering. Ceram. Int. 43, 71367141 (2017)
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The authors are grateful for the financial support from the National Natural Science Foundation of People's Republic of China (NSFC51761020 and NSFC51761021), Yunnan Ten Thousand Talents Plan Young & Elite talents Project (YNWR-QNJ-2018-044).
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