Development Of The Method Of Accurate Ball-load-addition For Single Stage Grinding And Analysis Of Its Crushing Mechanism And Study On Its Application Effects

The purpose of metalore grinding is to liberate minerals. In the ore grinding operation, the diameters of loaded media balls and their proportions in ball mill and the methods of adding balls not only determine the quality of the product, but also have a great deal to do with the grinding efficiency and productivity and, separation indices, the electricity and steel consumption. Thus studies on ball-load-addition methods in ore grinding are of key importance.The dissertation summarizes the developments and trends of ore grinding, the factors of affecting ore grinding effects in ball mill, the ways of improving ore grinding effects, status of ball-load-addition studies at home and abroad, the reason for oversize balls used in the ore dressing-works in China and its research background and significance. It expounds the influence of the mechanics properties of the ore on crushing effects from the macromechanics properties of the ore, including the hardness, roughness, cleavage and structure faults to relationship between the macromechanics properties of the ore and its micro crystallization. It analyses the difference of ground performance of the minerals with different mechanics properties. The differences of the destruction-resistant mechanics properties between standard and natural samples of ore blocks studied in the dissertation are tested. The test result shows that average compressive strength of the irregular block samples is merely that of 42.88 percent of the standard ones. It follows that it is reliable that the diameters of media balls for ore grinding are determined according to the average compressive strength of the regular standard ore block samples.The deformation and the characteristics of the destruction patterns of the ore, the movement states of balls in the ball mill and the relationship between the sizes of balls and the ground action are explicated, which illustrates that adopting fitted sizes of balls in ball mill can only make the destructed minerals present the destruction form of consuming least amount of energy, called brittle destruction. In turn, good ground effects can be achieved.The factors of affecting the determination of the diameters of the balls are analyzed and the deductive process of the formula of accurately calculating the diameters of the balls, Duan's semi-theory formula, is presented in detail to show that the formula is deduced in systematic and unassailable thought based on the principle of crushing mechanics and it basically contains all-round factors affecting the determination of the diameters of the balls. Consequently, the formula can be used to theoretically instruct the calculation of the fitted ball diameters needed in practice. In the meantime, the conclusions are drawn as follows:1) the fitted or accurate ball sizes can make the crushing energy, the normal kinetic energy falling to the linings in the ball mill fitted or accurate, that is, the crushing forces fitted or accurate.2) too big crushing force will make the energy wasted, as the same in the case of too small force because the force is too small to break the particles in one impact action and make them fatigue and broken by repeated impacts. These two cases should be avoided.3) accurate ball diameters ought to be used to make ore grinding process ideal, which will make the crushing energy or crushing forces accurate. Based on the theories and analyses mentioned above, aiming to the problems of oversize balls used in the coarse ore grinding operation over a long period of time in China and of having no practical and effective way of solving it and combining with the worse results of ore grinding in 3.6m diameter by 4.5m long ball mill by using original ball-load scheme on the spot in Dahongshan in Yunnan province, the dissertation studies on a new accurate ball-load method systematically: taking the mechanics properties of the ore and operation circumstances of the mill into consideration and calculating accurate ball diameters by adopting Duan's semi-theory formula, the ball-load scheme, which is rationed in accordance with the grain composition characteristics of the fed ore and in the guidance of the principle of crushing statistical mechanics, is used as accurate recommended initial ball-load scheme.Under the same ore grinding conditions as the accurate ball-load scheme, expanded experiment studies are conducted in the laboratory. The ball-load scheme of bigger balls prevailed, and of smaller balls prevailed, respectively, are designed, plus the scheme on the spot, their ground effects are contrasted. The ground results of four schemes show that the effect of the accurate ball-load scheme is the best.By using crushing statistical principle, the breakage probabilities of four ball-load schemes under the impact action in one crushing cycle of the balls are calculated. The contrast of the calculated results shows that the total breakage probability in the accurate scheme is the highest, that is, its total ground effect is the best, although among which, the numerical values of the breakage probabilities of the coarse and fine grains are between that of both the ball-load schemes of bigger balls prevailed and smaller balls prevailed. The total ground effect of the scheme on the spot is the worst. The reason why the ground effect of the accurate ball-load scheme is better than that of both bigger balls prevailed scheme and smaller balls prevailed scheme is that each kind of ball accounts for the proper percentage in the accurate ball-load scheme, all of the coarse, medium and fine grains can be ground effectively, both over-coarse and over-fine grains are not too much, the production rate of regular grains - remaining grains by deducting those unable to be ground fine and those over crushed - is the highest, that is, the total ground effect of this scheme is the best. Furthermore, in the accurate scheme, the case of the spaces between bigger and smaller balls is similar to the most compact pileup, so the actual contact areas between balls are increased, which in turn enhances the ground effect. As to the scheme on the spot, the oversize balls used cause that the breakage probabilities of the coarse grains are the highest. On the Other hand, since the total weight of the balls loaded in the ball mill is definite, the number of the balls of the scheme on the spot is the least, the total breakage probability of it is the least, and especially, the number of the balls fitted to crush the fine grains is the least, thus the crushed effect of the fine grains is the worst, which leads to the totally worst ground effects.In addition to the above mentioned reasons that the ground effect of the accurate ball-load scheme is the best compared to the other three schemes, the ground mechanism and laws of the accurate ball-load scheme are concluded as follows:1) Aiming at the mechanics properties of the specific mineral needed to be crushed, the accurate crushing force exerted by the ball can be controlled through determining the accurate ball diameter. Under the fitted crushing force, the ore may be split along the combining interface of different minerals and form ideal monomer minerals. Therefore, accurately determining the crushing force according to the need of the crushed ore, in other words, according to the crushability-resistance represented by the specific mineral can improve the liberation degrees and the productivity during ore grinding process.2)The factors of affecting the breakage probability are: the percentage of the solid volume of each grain size contained in the slurry in the mill(γ_solid,%), the number of the balls fitted to crush certain grain size(m_b), the selective crushing function (S) of each grain size and its breakage function (B). The law of their influences is that the breakages cannot occur if the actual ball diameters are less than the value of accurate ball diameter calculated in terms of Duan's semi-theory, and they can if the actual ball diameters are greater than the latter; the value of B corresponding to the coarse grain is greater, it is relative smaller corresponding to the fine grain; under the condition of considering the comprehensive actual influence of both S and B in practice, the ground effect is the best if m_b is suitable toγsolid.3) The law of the relationship between the mineral destruction state and the crushing force, the choice of the crushing force and the ground process followed is that under the premises of accurately determining the ball diameter needed by each grain size, matching the percentage of the coarse and fine particles respectively with the suitable number of the balls can make the breakage occur along the combining interface of different minerals with the accurate crushing force, which make the breakage well selective and the ground probability of each grain size high.A new simple, convenient and practical ball addition method - diagraphy is put forward in the dissertation: based on the initial ball load positive accumulation characteristics curve, to which parallel line is drawn, the ball addition accumulation characteristics curve can obtained by making the ball charge characteristics after the balls are worn and added keep that of the initial ball load. From the ball addition accumulative characteristics curve, the percentage of each kind of balls can be consulted very conveniently. The industrial practice shows that the new method is practical and can reflect the ball wear action.By applying the new ball-load-addition method of the dissertation to the 3.6m diameter by 4.5m long mill, the ore grinding effects are entirely improved: the processing capacity of the grinding mill per hour has been improved by 18.50 percent on the basis of 6.02 percent improvement of the fineness ( - 200 mesh percent) of the grinding product; and the mineral liberation degree of copper has been improved notably by 9.76 percent, and of iron by 10.83 percent; over crushing of the mineral is somewhat mitigated; the grades of copper concentrate and iron concentrate have 0.55 percent and 1.35 percent improvement respectively, and their recovery 1.41 percent and 7.09 percent increase respectively; the ball wear and the energy consumption of handling unit amount of mine are decreased markedly by 16.67 percent and 18.44 percent respectively.