Influence of steel balls on grinding process

(1) Influence of steel ball filling rate
The grinding effect of the ball mill is done by the steel ball, and the filling of the steel ball in the mill naturally determines the strength of the grinding action. From the analysis of physical phenomena, there are many balls, many hits, large grinding area and strong grinding effect. Conversely, when the ball is loaded less, the number of hits is small, the grinding area is small, and the grinding effect is weak. On the analysis of mechanical phenomena, it takes a lot of work to load the ball, the productivity of the mill is also large, the work is small when the ball is loaded, and the productivity of the mill is low.
Since the ball filling rate and the mill rotation rate work together, the transfer rate cannot be discarded when analyzing the influence of the filling rate, and only the two can be combined. When the mill is at a lower speed, the ball load forms an inclined surface in the mill, and when the ball rises to a high position, it rolls down along the ball ramp to form a diarrhea movement state. As the filling rate of the ball load increases, the angle of the inclined surface of the ball increases, the sliding moment of the ball increases, and the power required by the mill increases. According to the theory of Davis and Levinson, the ball filling rate is up. At 50%, the mill power reaches a maximum, as shown in Figure 1. When the filling rate exceeds 50%, the mill power begins to decrease. This is because the ball load rises too high, the ball returns to a high point when it rolls down, and the energy is transmitted back to the full barrel, so the actual power required by the mill drops. . When the ball filling rate is 100%, the speed mill actually becomes a roller, and the mill only needs to maintain the energy required for the rotation of the drum, so the power of the mill is very low. If the mill contains ore and water, the actual curve of the mill power (solid line in Figure 1) is different from the theoretical curve (dashed line), and the fill rate of the maximum power is shifted down. This is due to the addition of ore and water. The actual filling rate of the Netherlands has risen.

Figure 1 Relationship between power consumption and ball loading rate

The ball load will be in a throw-down state at a higher rate of rotation. The situation is complicated under the falling state, as shown in Figure 3-5-6. With a certain filling rate, the ball load will change from the sloping state to the falling state with the increase of the turning speed, but the ball turning load will be different when the ball is turned from plunging to throwing at different filling rates. The higher the rate, the higher the speed required to switch to the throwing state.
The above analysis shows that whether the ball load is in a sloping state or a throwing state, there is a corresponding suitable filling rate at a certain transfer rate, and the higher the filling rate, the better. The best criterion for verifying the filling rate is the size of the mill's productivity. The filling rate corresponding to the maximum productivity through testing is the optimum filling rate. Of course, when the mill size is not too large and the ball load is in the throwing motion, the filling rate can be calculated using the Contonovic formula. However, the generally applicable method is still experimentally determined. In addition, from the principle that the grinding process is a process of functional transformation, it can be considered that the maximum productivity of the mill necessarily corresponds to the maximum grinding work. Therefore, the maximum grinding work can also be used as the criterion for the optimal filling rate.

Figure 2 Relationship between power consumption and cylinder speed

The ball filling rate of the large ball mill should be reduced. The larger the diameter of the mill, the lower the ball filling rate. Table 1 lists the relationship between the diameter and filling rate of the large foreign mills. Therefore, the large ball mill can simplify the production series, save capital investment and operation and maintenance costs, so it was widely used in the 1970s. However, with the production application, it is found that its grinding efficiency is low and the productivity per unit volume is low. This shortcoming is due to the fact that the ball filling rate of the large ball mill is too low. The filling rate of the ball is too low, resulting in a significant reduction in the number of hits per unit time and a significant reduction in the grinding area, so that the grinding effect is weakened and the mill productivity is lowered.

(II) Influence of steel ball size When the grinding machine speed and filling rate are certain, that is, when the ball load motion state is certain, the size of the steel ball seriously affects the grain size characteristics, dissociation degree and consumption index of the grinding product. There are mainly 6 points as follows:
(1) Affecting the productivity of the mill. When the ball diameter is too large, the number of strikes is small and the polishing area is small, so that the productivity is lowered. When the ball diameter is too small, the productivity is lowered due to insufficient striking force. When the ball diameter is refined, the mill productivity can be greatly improved. In the industrial test of several selected factories, the author proves that when the ball diameter is changed from too large to accurate, the utilization coefficient of the mill according to -0.074mm can be increased by 15%~40%.
(2) Affecting the uniformity of the particle size distribution of the grinding products. If the ball diameter is too large, the number of hits will be small, resulting in a coarser level of grinding. The excessive impact force will increase the crushing time. Therefore, the product size is not uniform under the large ball diameter, too thick and over-grinding, which is not good for sorting. Some industrial experiments by the author have proved that when the ball diameter is over-adjusted to be precise, the maximum particle size and average particle size of the grinding products are reduced, and the over-grinding is also reduced by 3% to 4%, and the product particle size is more uniform. Yi is selected from intermediate grain size increased, more favorable to beneficiation.
(3) Affect the degree of dissociation of mineral monomers in grinding products. The excessively large ball diameter causes the ore to break through due to the excessive impact force, but the particle size is mechanically thinned, and the mineral monomer dissociation degree is not high. After the ball diameter is refined, the probability of dissociation of the mineral along the bonding surface increases, and the dissociation degree of the mineral monomer in the product increases. The author's industrial test proves that the ball diameter can be refined to increase the monomer dissociation degree of useful minerals by 4% to 6%, thereby improving the concentrate grade and recovery rate.
(4) Affect the level of ball consumption. According to Davis's theory of steel ball wear, the wear rate of the ball is proportional to its weight, the large ball wears high speed and consumes a large amount; the small ball wears low speed and consumes low. This has already been collected for the af Taggart's Mineral Processing Handbook. The author's industrial test also proves this. Several industrial tests have shown that the ball can be reduced by 10% to 20% after the ball diameter is adjusted to be too large.
(5) Affect the power consumption. When the loading of the ball is constant, the power consumption of the ball is also lower than that of the big ball. This has been studied at home and abroad, and some monographs list the power required to input KW _b per ton of steel balls:

Where D is the effective diameter of the mill, m;
V _P — ball filling rate, %;
C _S - mill transfer rate, %;
S _S — the diameter coefficient of the steel ball, the value of which is:

B is the maximum ball diameter, mm.
From the formula (1) and S _S values, the input power per ton of large balls is larger than that of the small balls. The author's industrial test confirmed that the mill power can be reduced by 2% to 3% after the ball diameter is reduced.
(6) Affect the size of the working noise of the mill. Because of the large energy, the big ball has a large noise loss when it hits or strikes the lining, so the noise is large. After the ball diameter is refined, the noise can be reduced. According to the author's many industrial tests, the working noise of the mill can be reduced by 3~4dB after the ball diameter is adjusted to be accurate. It can be seen from the above that the size of the steel ball has a great influence on the various indexes of the grinding, and it is of great significance to accurately select the size of the steel ball.
(III) Influence of the quality of the steel ball The quality of the steel ball affects both the productivity and the ball consumption, which in turn affects the cost of the grinding media. Simply pursuing high hardness and low unit consumption is not right. High hardness and low unit consumption are not equal to low cost. Balls with high hardness and low unit consumption are often very expensive. High hardness does not necessarily increase productivity, or even decline. Only high productivity can reduce the unit consumption indicators. Therefore, the primary criterion for selecting steel balls should be the high productivity of the mill and the low cost of the grinding media. Only high productivity and low grinding media costs can have good economic benefits. Economic benefits are a necessary condition for the survival and development of enterprises.
When choosing a steel ball, two problems are often overlooked: 1 the steel ball is not as hard as possible, but has its proper hardness value; 2 steel ball density is also a problem that cannot be ignored. Regarding the influence of hardness, generally speaking, as the hardness increases, as long as no crushing occurs, the unit cost of the steel ball decreases; and the deformation of the sphere is small, the absorption energy of the sphere is small during the crushing, and the energy can be more used for the crushing ore. Granules can increase the productivity of the mill. However, the increase in the hardness of the steel ball can only be moderate, and there is an appropriate range, not the harder the better. If only the ball consumption is considered, the higher the hardness, the lower the consumption. However, for the mill productivity, the productivity increases with the increase of the hardness of the steel ball within a certain range, but when the hardness exceeds a certain range, the mill productivity is adversely affected, and the mill productivity is lowered. When the hardness of the steel ball is too high, there are two reasons for the disadvantage of grinding: 1 The rebound of the steel ball is serious, and some energy loss is caused in the rebound. Therefore, the energy of the steel ball is not used for crushing, so it affects the crushing; When the hardness of the steel ball is too high, the ball and the ball slide in contact with each other, and the spherical particles between the balls cannot be effectively caught, so that the grinding effect of the ore particles is weakened. When studying the influence of steel ball hardness on grinding index, AB Kyle Poschen (KирпоциН) pointed out that laboratory tests have shown that steel balls have a problem of optimum hardness for various types of ore. According to this statement, the optimum values ​​of hardness of various ores are different. This argument is justified and deserves further study. Our Shougang iron ore production Dashi powerful application illustrates this problem. The Dashihe Iron Mine used four kinds of steel balls of different hardness from 1981 to 1983. The utilization coefficient of the mills during various ball milling is shown in Table 2. [next]

Table 2 shows: 1 steel balls with different hardness have different grinding effects, but the effect is not the highest hardness, but the effect is best when the hardness is right. The 20MnV forged steel ball has the highest productivity, but the HRC is only 30~40, and the hardness is proper. 2 The mill utilization coefficient (ie, volumetric productivity) between different hardness steel balls can differ by 20%~40%, which indicates that the influence of hardness on the mill productivity is quite remarkable. Simply pursuing high hardness and low ball consumption and neglecting productivity reduction is not possible. Take it.
Regarding the influence of the density of the steel ball on the grinding, generally, the productivity of the ball having the same size is large, and the productivity with a small density is small. The density of steel balls is affected by three factors: 1 material influence, steel, cast iron, alloy steel, etc. The density of different materials is different, the density of steel is larger than that of cast iron, and the alloy steel varies according to the density and content of main alloying elements. 2 The influence of the steel ball manufacturing method, the rolling and forging steel balls are dense in structure, so the density is large, and the microstructure of the cast cast steel balls, cast iron balls or cast alloy balls is not dense, and even there are pores, so the density Smaller. The density of rolled steel balls and forged steel balls can reach 7.8g/cm ³ , the cast steel balls can only reach 7.5g/cm ³ , and the cast iron balls are lower, only 7~7.1g/cm ³ . 3 The influence of the metallographic structure of the steel ball, the density of different crystal structures such as martensite, austenite, bainite and ferrite are also different, which also affects the crystal fineness.
The effect of density on productivity is also not negligible. It is also a Φ100mm steel ball. The quality of different ball types will vary by 200~400g. In some gold mines in Yunnan, although the consumption of forged steel balls is higher, the productivity is also high. After changing to a certain wear-resistant ball, the ball consumption is reduced, but the productivity is also reduced by 10% to 15%. For the use of grinding balls, still forged steel balls are used.
(IV) Influence of the material composition of the steel ball on the beneficiation process The steel ball itself is also worn by the ore during the grinding process, and is ground into iron powder or iron pieces and left in the pulp. Although this amount is not large, according to the current level of China, grinding a ton of mined ball about 1.5kg, but it has an impact on some subsequent operations. If the next step of the grinding product is a chemical process treated with acid, the iron powder in the ground product will first consume sulfuric acid, which will increase the acid consumption. For this reason, some uranium or gold mines in South Africa and North America often use gravel mills for grinding purposes in order to reduce the impact of iron on the subsequent wet chemical process. Magnetic ball milling is used in the laboratory to reduce the effects of iron on the product. These are well known to the ore dressing engineering technicians.
The effect of iron on ore dressing in grinding is often overlooked. Many studies in recent years have shown that the iron powder that is worn out in the grinding will soon oxidize to consume oxygen in the slurry, and at the same time lead to changes in the surface potential of the slurry and minerals, which in turn affects the flotation behavior. Some studies have pointed out that in wet grinding, the electrochemical interaction between minerals and steel balls, the wear of iron inhibits the natural floatability of minerals, and consumes more collectors during flotation. RL Pozzo's research indicates that the current generated by the combination of two or three electrodes between mineral and grinding media is closely related to the floatability of the mineral under both ground and unground conditions. When not ground, electrochemical action to produce an iron hydroxide coating reduces mineral floatability. Under grinding conditions, the interaction between metal fragments and minerals produced by the grinding media plays a major role in the inhibition of minerals, especially for pyrrhotite. HWXiang's research indicates that when the grinding medium is in contact with the sulfide ore, a galvanic current is formed, and the redox reaction occurs due to the difference between the grinding medium and the open potential of the sulfide ore. This galvanic reaction can be controlled by the mixed potential principle, and the material having a lower open circuit potential acts as an anode and is subjected to surface oxidation, so the selectivity of sulfide ore sorting may be increased or decreased. The mechanical-chemical reaction of sulfides reduces the selectivity of sorting. Finally, it is pointed out that the sorting selectivity of sulfides can be achieved by selecting suitable grinding media and conditions. In order to reduce the influence of the iron content of the steel ball on the beneficiation process, most researchers use the anti-corrosion material to make the grinding ball. According to research by RH Saiors, cast high-chromium steel balls (including C2%~3%, Cr12%~25%) are widely used in South America. The wear rate of such steel balls is 75% lower than that of ordinary steel balls. ~80%. In the article, the corrosion current was measured by grinding test, scanning electron microscope structural analysis and electrochemical measurement. The corrosion resistance mechanism of the steel ball was studied. The results show that the steel ball has a carbonized structure and a martensitic structure, has high hardness and wear resistance, and contains high chromium and is not easily corroded. This feature makes it highly resistant to wear. JW Jane's research indicates that the wear behavior of grinding media is related to the chemical composition, hardness, phase structure and corrosion of the slurry. Previous studies have shown that the steel balls of the martensite structure are hard, and the high carbon steel balls of this structure are less worn. In a high-chromium steel ball, the wear of a single martensite structure is greater than the structure in which both martensite and ferrite coexist. The article reports the use of heat treatment processes to manufacture steel balls of three structural types: martensitic spheres, ferrite spheres, martensite + ferrite spheres. Through experiments, electrochemical measurements, and flotation, it is found that the high chrome balls with martensite and ferrite structure are less worn due to: martensitic structure hardness, abrasion resistance and impact resistance, ferrite and high chromium. The content is easy to form a passivation layer, which reduces abrasion wear. High chrome balls have the characteristics of corrosion resistance and wear resistance, but the price of chromium is not low, and China is a chromium-deficient country, which is not the direction of China's steel ball development. As also pointed out earlier, the influence of steel ball density cannot be ignored. The density of chrome metal is 7.4g/cm ³ and the density of nickel metal is 8.9g/cm ³ , so the density of high chrome balls is significantly lower than that of forged steel balls, while the density of hard nickel alloy steel balls is equivalent to that of forged steel balls, so the use is high. Chrome balls can lead to reduced productivity, while hard nickel alloy steel balls do not reduce productivity. This problem deserves to be observed and confirmed in production. We can also find wear-resistant and corrosion-resistant materials from other directions to make steel balls. V. Rajagopal's article reports that the addition of copper helps to reduce the rate of abrasion in wet milling. The hard nickel alloy steel ball mentioned above also has high hardness, high temperature resistance and corrosion resistance, and is also an ideal wear ball. At present, there is no production of hard nickel alloy steel balls and linings in China. The author is doing research and development to end the situation of China's non-hard nickel alloy steel balls and linings. In the past, nickel production was low and expensive. Nickel metal was used as a strategic material control and did not have the conditions for developing hard nickel alloy steel balls and linings. However, the current situation has changed greatly, nickel production has increased greatly, and the world has formed a supply exceeding demand. Nickel metal prices have fallen sharply, requiring the search for new nickel metal consumption channels. Moreover, China's second largest nickel mine, Yuanjiang Nickel Mine, is a nickel silicate mine. It is not feasible to produce nickel or nickel oxide. The production of ferronickel is feasible, and the produced ferronickel is looking for sales. The production of hard nickel alloy steel balls and linings with ferronickel should be an important direction for the development of wear-resistant and corrosion-resistant steel balls in China.

Oxygen Generator

Oxygen Generator,Oxygen Plant For Hospital,Oxygen Plant Compressor,Oxygen Plant Equipment

Jiangsu Minnuo Group Co.,Ltd , https://www.mingngroupcn.com