Ore Grinding cycle

1. Classifier in grinding cycle

In the grinding cycle, a classifier, a hydrocyclone or a screen are needed to connect with a mill to form a unit. The function of the grading equipment is similar to the effect of the sieves in the crushing process, such as pre grading and grading, and overflow control classification.

1.1. Spiral classifier

The most commonly used classifier is a spiral classifier, which is divided into three kinds: high weir, low weir and sunk type. According to the number of spiral, it can be divided into single spiral and double spiral classifier. Spiral classifier with a sloping semi cylindrical trough 2, tank equipped with one or two spiral 3, it is the role of the sand pulp stirring and transported to the upper end of the chute. The spiral blade is connected with the hollow shaft, and the hollow shaft is supported in the bearings at both ends of the upper and lower sides. The transmission device is installed in the upper trough, the spiral bevel gear to drive motor. The lower end bearing is installed at the bottom of the lifting mechanism 6, which can be rotated to make it rise or drop. The lifting mechanism is driven by a motor through a decelerator and a pair of bevel gears to lift the lower end of the spiral. When parking, the spiral can be extracted so as not to sink the sand to hold the spiral, so that the driving can not be overloaded.

Fig1. High weir spiral classifier

1. transmission; 2. tank; about 3. spiral shaft; 4. inlet; 5. release valve; 6. lifting mechanism

The overflow weir of the high type weir spiral classifier is higher than the lower end bearing, but it is lower than the upper edge of the lower end spiral. It is suitable for separating the particle size of 0.15-0.20 mm, usually used in the first stage grinding and cooperating with the grinding machine. The bottom spiral of the sunk type classifier has four to five circles, which is all immersed in the pulp. The classification area is large, and it is good for separating the particle size from 0.15 mm. It is commonly used in the second stage grinding and grinding machine. The low weir type overflow is lower than the center of the lower end bearing, and the classification area is small. It can only be used for ore washing or dehydration. It is not used now. The spiral classifier is superior to other classifiers, because it is simple in structure, stable and reliable in operation, easy to operate, low in water content and easy to connect with mill, so it is often used. Its disadvantage is that the lower end bearing is easy to wear and the area is large, so it has a tendency to be replaced by hydrocyclone.

Fig2. sunk type spiral classifier

1. transmission; 2. tank; 3.  spiral shaft; 4. inlet; 5. lower support; 6. lifting mechanism

1.2. Hydrocyclone

The upper part of the hydrocyclone is cylindrical, and the lower part is conical. The pulp is sent in the tangent direction from the ore pipe at the 0.4-3.5 atmospheric pressure and revolving at the high speed inside, resulting in a large centrifugal force. Under the action of centrifugal force and gravity, the coarser particles are thrown to the wall of the apparatus, which are spirally downward moving, and finally discharged by the sand draining nozzle. The finer particles and most of the water form a swirling flow that rises up the center and flows out of the overflow tube. The swirler is divided into two kinds, namely, grading and desliming. The former is used to separate 800 to 74 (or 43) micron particle size, and the latter is used to remove 74 (or 43) to 5 micron fine mud. The concentration of the hydrocyclone for classification is higher, the pressure of the ore feeding is larger, the diameter of the cylinder is coarser, and the situation of the cyclone used for deliming is the opposite. The advantages of a hydrocyclone are simple structure, small area and high productivity. The disadvantage is that it is easy to wear, especially when the sand drain mouth wears fast, and the work is not stable enough, which makes production indicators fluctuate. But these shortcomings can be overcome gradually by using wearable materials.

Fig3. Structure of Hydrocyclone

Fig4. Working principle of Hydrocyclone

2. Open circuit and closed circuit grinding

The work of grinding machine is divided into two kinds: open and closed. In the closed circuit, it is divided into closed and semi closed (or partial closed). As in Figure 5 (a), it is called open circuit grinding, when the material is grinded only once by the grinding machine. (b) to (e) in Figure 5, called complete closed grinding. At this time the grinding machine is divided into fine overflow and coarse sands by the classifier, and all the back sand is sent to the original mill and then grinded. In Figure 5, (b), only part of the return sand of the classifier 1 returns to the original mill regrinding, the other is sent to the next mill, so it is a partial closed circuit. There are three kinds of functions of the in the grinding cycle. The classifier 1 in Figure 5 plays the role of pre classification. In the graph 5 (b), the classifier in the (E), the classifier 1 and 2 in (1) and (f) are all checked and graded, and the classifier in the (c) is not only pre grading, but also checking and grading. The classifier in e (2) has the function of overflow control and classification. It classifying the overflow of classifier 1, and puts forward the coarse-grained particles to get finer and more uniform products.

Fig5.Flowsheet of open circuit and closed circuit grinding

Because during closed circuit grinding, most of the materials returned to the mill are close to the fineness of the qualified particles. After a little grinding, they can become qualified products, and the material in the mill increases, so the mill can be faster and shorter. Therefore, the productivity of the closed circuit grinding is larger, the over comminution is lighter, the product is fine, and the grain size characteristics are even more uniform. Open circuit grinding is mostly seen in the first section of the two stage grinding mill, grinding from 20 to 25 mm to about 3 millimeters, and then grinded with a ball mill. In this way, the breaking ratio of the first rod mill is small, the production capacity is high, and the process is simple.

3. Grinding cycle load

In closed circuit grinding, the amount of return sand from the grader to the mill begins to increase gradually. After a period of time, it tends to be stable and constant. The stable weight of return sand is called the cycle load. It can be expressed with the absolute value (t / h), and it can also be expressed by the ratio of it to the new amount of ore supply. The cycle load represented by the absolute value is called the amount of return sand, and the cycle load represented by the relative value is called the ratio of back to sand. The number of cycle loads may be several times larger than that of the new ore supply. It is usually not less than 200%, sometimes more than 1000%, but it should not be larger than the passing capacity of the new feed and more than the mill, otherwise the mill will be blocked.

Sinonine has more than 10 years experiences of ore processing and beneficiation plant engineering, such as lead-zinc mining and iron ore processing, such as lead-zinc mining and iron ore processing, sinonine provides customers with high efficiency, low cost of ores mine solutions.

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