How does the size of the grinding back sand ratio affect the grinding process?

In closed-circuit grinding, the size of the sand-return ratio has a great influence on the efficiency of grinding technology and the grinding productivity.

According to the principle of grinding dynamics, the productivity calculated by the grinding machine according to the qualified product is proportional to the content of the coarse grade in the ore. Expressed in mathematical terms as:

Q-KY group (2-8)

Where Q is the mill productivity calculated from qualifying products;

K——proportion factor, which is the maximum productivity that the mill can achieve after it has formed an acceptable product under ideal conditions;

Group Y - the content of the coarse fraction (greater than the qualified particle size) in the mill feed.

As the ratio of sand return increases, the coarse grade content of the mill also increases, which also increases the amount of ore per unit time passing through the mill, which speeds up the discharge rate and shortens the ore grinding time, thus improving the qualified Granular productivity. At the same time, since the reclaimed sand relatively reduces the fine-grained content in the ore, the over-grinding phenomenon is also reduced, which is advantageous for improving the efficiency of the grinding technology. It can be seen that closed-circuit grinding is beneficial to increase the return-to-sand ratio. Practice has shown that increasing the sand return ratio within a certain range does not increase the power consumption, and even lowers in some cases.

According to the results of the experimental research, the mathematical formula for the relationship between the productivity of the mill and the sand return ratio is:

In the formula, Q1, Q2 - the amount of processing of the mill when the return ratio is c1 and c2.

If c2=100%, the productivity of the mill Q2 is 1.0. According to the above formula, the relative productivity k of the mill is calculated as follows:

The curve in Figure 2-14 is drawn according to the formula (2-6). The curve shows the relationship between the relative productivity of the mill and the sand return ratio.

When the sand return ratio varies from 100% to 500%, the relative productivity of the mill increases by about 10% to 30%. However, when the sand return ratio increases again, the productivity increase is very slow, and there is no significant increase in the end. Production practice proves that the larger the ratio of returning to mine, the better. If it is too large, it will cause the overload of the grinding machine and the classifier, so that the grinding process can not be carried out normally, such as the phenomenon of “swollen belly” of the grinding machine, lowering of classification efficiency, etc. . It should be noted that there are different ranges of suitable sanding ratios for different grinding conditions. Generally speaking, the coarse grinding back sand ratio is smaller, and the fine grinding back sand ratio is larger. Some production mine experience shows that the ball mill has a sand return ratio of about 250%, and the rod mill is lower, about 150%.

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