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I. Overview
The ore grindability is the benchmark data necessary for calculating the industrial mill according to the principle of proportional enlargement. It is the ore characteristic constant that can be determined experimentally. The value of the ore is constant or varies only according to a certain proportion under different grinding conditions. With the different calculation methods of industrial mills, the methods of measuring the degree of grindability are also different. Common active index method, energy efficiency method, new-born surface method and unit volume production method.
The most widely used method in the world is the work index method proposed by FC Bond. Bond believes that “the total energy effectively consumed during grinding is inversely proportional to the square root of the grain size of the grinding product.†If the particle size is measured by d 80 (80% less than d), the ore is made of F (80% less than F micron) The work required to grind P (80% less than P microns) is (kWh/ton):
(1)
In the formula, W 1 is the Bond work index, which represents the work (kilowatt hours) required to grind each ton of ore from infinity to P = 100 microns. The value of W 1 can be determined experimentally by different methods.
Grinding energy efficiency e is the amount of mill production per kilowatt-hour of electrical energy consumed during the grinding process (based on the amount of ore or new raw material less than 75 microns).
Based on the theory that “the energy consumed by grinding depends on the amount of new surfaceâ€, Thompson has developed a specific surface grindability measurement program. The corresponding grinder work index calculation formula is:
WT=WT 0 .K GS .K F
(2)
Where WT and WT 0 — Thompson work index kW·h/t for designing ore and reference ore
K Gs - relative surface grindability coefficient;
K F - the grain size factor of the ore.
The task of the test is simply to determine the relative surface grindability factor. The benchmark Thompson work index and feedstock particle size coefficients of different mills at different feeds and product sizes have been measured by Thompson and no additional testing is required. The ore work index to be measured can be calculated from the above formula.
Long-term use in the country is the unit volume production method, which is usually measured by the amount of new-75 micron material produced by a unit volume mill. In the measurement, the reference ore is generally tested at the same time as the ore to be tested, and the relative grindability coefficient is obtained by the comparison method, and then the grindability of the ore to be tested is calculated according to the known reference ore grindability data.
Second, use the drop weight test to determine the work index
The drop weight test, also known as the impact crush test, is one of the early methods for determining the friability and grindability of ore. It has not been completely eliminated since it has been continuously improved. The basic components of the early use of the falling weight test device are an anvil with a concave sample chamber and a cylindrical die. The free-falling weight (ball) impacts the die to break the ore, and the wearability is measured per joule. A new surface metric produced by work. Bond has designed a double pendulum ball and double pendulum impact crushing test device, replacing the falling ball or the falling weight and the steel anvil with two oppositely oscillating balls or hammers, and calculating the work index according to the impact breaking strength of the sample. Recently, the improved ball drop test device of the British ME Asim (Asim, 1984) placed four square-arranged balls in the sample chamber, surrounded the sample layer at the center, and then used the fifth free fall along the guide tube. The ball directly impacts the sample layer to simulate the impact crushing process in the ball mill . Although this method is not as close to the actual production of laboratory grinding machine, it can study the change of ore particle size distribution (measured by selection function and breaking function) and energy consumption (measured by Bond Function Index) at each step of the ore gradual crushing process. ), and all samples can be taken out for particle size analysis without sampling.
Third, use a conventional grinding machine to determine the work index
The following is a description of Bond's standard rod mill and ball mill test procedures developed at Bond's Allis-Chalmers laboratory.
1. Standard rod mill test
The ore was crushed to less than 12.7 mm (0.5 inch) and sampled and sieved. Take 1250 cm 3 sample and put it into the measuring cylinder and weigh it, and put it into the grinding machine for closed-circuit dry grinding. The rod mill has a diameter of 305 mm (12 inches) and a length of 605 mm (24 inches) with a corrugated liner and adjustable tilt angle. It has 2 rods with a diameter of 31.75 mm (1.25 inches) and 6 rods with a diameter of 44.45 mm (1.75 inches). The rod has a length of 533.4 mm (21 inches) and a total weight of 33.38 kg.
In order to make the material distribution at both ends of the grinding machine uniform, the mill rotates 8 turns first in the horizontal state, then tilts up 5°, rotates one turn, then leans down 5°, turns one turn, and finally adjusts to the horizontal state and then turns 8 turns. Dump 45° to 30 rpm when discharging. The obtained product was sieved by a test sieve with a mesh size of 4 to 65 mesh (4,750 to 212 μm), the product under the sieve was weighed, and the amount of the product under the sieve which was ground per revolution was calculated. The on-screen product is returned as a cyclic load, and the new feed is equal to the amount of sieved, and is sent to the rod mill for the second cycle of grinding. The cyclic load is controlled at 100%, that is, the amount of product under the sieve per grinding is approximately equal to 50% of the ore. For this reason, the number of grinding revolutions for each subsequent period must be calculated based on the output per sieve of the previous cycle, and so on until the amount of the sieved product produced per revolution reaches a constant value. Take the average of the output of each sieve under the last three revolutions to measure the grinding efficiency, and calculate the Bond work index W i as follows: [next]
(3)
Wherein P 1 - closed sieve screening test sieve mesh size μm;
G—grinding efficiency calculated according to the amount of sieved product in the grinding product, g/r.
P and F have the same meaning as before. Since the Bond Achievement Index was originally calculated in kilowatt-hours per short ton of ore, the conversion factor of 1.1 was introduced in the formula in order to use the International System of Units in a unified manner and replace the (metric) tons with short tons. The calculated work index value is suitable for calculating the input power of the open-flow wet mill and the overflow rod mill with a diameter of 2 meters (in the liner). If the inner diameter is D, it should be multiplied by (2/D) 0.2 ; For dry grinding, the factor of 1.3 should be multiplied.
2. Standard ball mill test
Minerals crushed to less than 3,350 microns are used as standard feeds, and finer feeds may be used if necessary. The sample was first sampled, and then a 700 cm 3 ore sample was weighed into a ball mill for dry grinding. The first cycle of grinding continued for 100 revolutions. The sieve is sieved by the test sieve, and the sieved material is weighed and taken out, and the new ore is added to the ball mill for the second cycle of grinding, and the cyclic load is controlled to be 250% of the new feed. The rest of the operation is the same as the rod mill test.
The ball mill is 305 x 305 mm and is finished with a smooth lining. There are 43 36.6 mm balls, 67 30.2 mm balls, 10 25.4 mm balls, 71 19.1 mm balls, and 94 15.9 mm balls, totaling 285 and weighing 20.125 kg. The mill speed was 70 rpm.
The work index is calculated as follows:
(4)
This model is suitable for calculating the input power of a 2 meter overflow ball mill with closed-circuit wet grinding. When dry grinding, multiply by 1.3, the diameter conversion factor is the same as the standard rod mill test method.
Many improved or simplified test procedures have been proposed for decades and can only be considered as complementary to the Bond test procedures. For example, DG Armstrong believes that open-cell batch rod mill products have similar particle size distributions to industrial closed-circuit ball mill products. Therefore, it is recommended to use laboratory rod mill open-circuit wet grinding instead of dry ball mill closed-circuit grinding method to determine ball milling work index. . According to Niiti, the unit energy consumption is not fixed throughout the grinding process, so it is recommended to continuously measure and record changes in energy consumption during the test. TF Berry proposed a test procedure for determining the work index using the comparative method.
3. Comparison method to determine the Bond Achievement Index
This method is based on the following assumptions: If two equal weight samples are ground in the same mill, the required input is required when the ore size is approximately equal and the grinding time, slurry concentration and mill speed are the same. Also equal. At this time, according to formula (1):
(5)
In the formula, the meaning of the symbol is the same as before. The unmarked ore indicates the ore to be tested, and the foot with the "0" indicates the reference ore. If the work index of the reference ore is known, the task of the test is only to find the P and F of the two ore samples milled under the same conditions. Such tests can thus be carried out using conventional laboratory mills. The grinding time is determined by the ore test to be tested according to the required grinding fineness.
Fourth, the unit volume production method
The test is generally carried out under open circuit conditions. Only in special cases, such as the difference in the grindability of different sizes of ore particles and the large cyclic load, the laboratory closed-circuit grinding experiment is arranged. The test task is usually Find the relative grindability coefficient K.
q t 0
K=----=----
q 0 t
(6)[next]
Where q and q 0 — the unit volume production of the to-be-tested and reference ore, based on the amount of new-75 μm material at the time of grinding, t/m 3 ·h;
t and t 0 - the time required to test and ground the ore to the specified particle size, S.
The test procedure is as follows: crush the ore to be tested and the reference ore to -3 mm, and sieve the fine particles of -150 μm according to the specifications of the ball mill, and weigh several equal parts (500 to 1000 g) of the sample under the same conditions. Grinding at different times, grinding the product separately, and then plotting the relationship between the grinding time and the cumulative yield of the undersize in the product, and finding out from the above to grind the sample to the specified fineness (by -75 μm Level content) required inter-grinding t and t 0 (below).
The use of -75 micron (200 mesh) material to represent fineness is a method that has been used in China's mineral processing industry for many years. This representation error will be large when the grinding size is finer than 90% - 75 microns. At this time, other methods must be used to indicate fineness, such as -45 micron material percentage or d80 (80% material is less than d micron). The specific surface area can sometimes be used to measure product particle size.
Anyone who uses the comparative method to determine the grindability should always stock enough reference ore samples in the laboratory. The benchmark ore sample is generally taken from a large, stable production mine.
Figure  Relative grindability curve
1. The reference ore grindability curve; 2. The ore grindability curve to be measured