Circuit Milling

Closed-Circuit Milling In closed-circuit miUing the taiUngs from a classifier are mixed with fresh feed and recycled to the miU. Calculations can be based on a material balance and an expUcit sohi-... 

Typical separation efficiency curves of an air classifier versus particle size are given in Fig. 20-14. The amount of top size in the fines may be very low, but there is typically 10 to 30 percent fines in the coarse product that is, the low end of the cui ve tends to flatten out at 10 to 30 percent. In addition, the separation at the cut size is typically a gradu cui ve. Data of this sort, which are needed to evaluate closed-circuit mill performance, are seldom available. See subsection on characteristics of size classifiers for a testing method. 

The simplest technological systems with recycling flows are shown schematically in Fig. 1. In this, the scheme (a) corresponds to a closed milling circuit, and the scheme (b) to a network of classifiers, in which, the fine product of the first classifier has the repeated classification in the second apparatus. These basic elements can be a part of more complex circuits. First let us examine the closed circuit milling system. 

The chain model corresponding to the closed circuit milling system with localised models of all its elements is presented in Fig. 2. It can be constructed in different ways, but first let us examine the model shown in Fig. 2a. Every column of the set of cells corresponds to an element of the circuit a mill a classifier or an absorber. The cells within columns correspond to fraction numbers with the total number of fractions equal to r. The fraction size decreases with increasing fraction number. The state of the system is characterised by the set of probabilities f, to occupy the cells, every of which can be interpreted as the relative mass content of particles in the cell ij. In particular, the set fj, I = 1,2,.. .r, corresponds to the particle size distribution in the hold-up of the /111 element of the circuit. 

Fig. 2. Two variants of constructing the cell model of the closed circuit milling system.

WATER REQUIREMENTS FOR CLOSED-CIRCUIT MILL SYSTEM 13.10... 

Milling circuits are either open circuit or closed circuit. In open circuit milling (Figure 12.16) the material passes only once through the mill, and so the only controllable variable is the residence time of the material in the mill. Thus the... 

In closed circuit milling (Figure 12.17) the material leaving the mill is subjected to some form of classification (separation according to particle size) with the oversize being returned to the mill with the feed material. Such a system is far more flexible since both product mean size and size distribution may be controlled. 

They were originally installed in 1961 as slurry mills. No. 9 was converted to a cement mill in 1984 and No. 10 in 1987, both as open circuit mills (see Figure 1). 

Before conversion to closed circuit Mill 9 had a 1st Chamber length of 3.7 m, with 27 torme of grinding media 0 60 to 0 90. Chamber 11 was 7.6 m long with 57 torme of media 0 15 to 0 20. 

A. L. Hinde and B. J. D. Lloyd, Realtime Particle Size Analysis in Wet Closed Circuit Milling, Powder Technol. 12 (1975), 37—50. 

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