# Grinding related basics

Keywords：

jinhao,According

Classification：

Beneficiation Process

Case Details

1、What are the types and significant features of mills?

A: According to the different mill media are divided into: media for steel balls, called ball mills; media for steel rods called rod mills; media for gravel called gravel mills; their own ore for media called self-mills, from the mill and then add the right amount of steel balls, constitute the so-called semi-self-mills.

According to the shell shape and ore discharge method, ball mill is divided into short cylindrical, tubular, conical overflow discharge and lattice discharge, rod mill is also divided into cylindrical overflow discharge and open low level discharge, self-mill or semi-self-mill is mainly short cylindrical.

2、What is the method of mill specification?

A: The expression method is: diameter × length, expressed by the symbol D × L, such as 2700 × 2100mm ball mill, then its diameter is 2700mm and length is 2100mm.

3、Briefly describe what is the crushing process of the mill to the ore?

A: When the mill rotates at a certain speed, centrifugal force is generated, which causes friction between the media and the cylinder, and the cylinder rotates together, and when it reaches a certain height, its own gravity is strongly centrifugal force, it will be thrown down from the cylinder, thus crushing the ore. At the same time, due to the sliding phenomenon inside the medium, the ore also has a grinding effect. Under the action of the impact and grinding force of the medium, the ore is also crushed.

4、How many kinds of trajectories are there for the media in the mill?

A: There are three types of trajectories: ①, lagging (at low speed); ②, throwing (at normal speed); ③, centrifugal operation (the media and cylinder trajectory is the same).

5、What is the law of motion of the medium in the mill?

A: There are four main points as follows: ①, when the mill is running at a certain speed, the media in the centrifugal force, gravity under the action of regular circular motion; ②, grinding media in the barrel of the trajectory, by the upward movement of the circular trajectory and parabolic trajectory of the downward movement composed of; ③, each layer of media rise to a different height. The outermost layer to the innermost layer of the media height in descending order; ④, each layer of the media rotation cycle is different, the closer to the inner layer, the shorter the rotation cycle.

6, what is the critical speed, its calculation method?

A: In production, the speed at which the outermost grinding media starts to "centrifugal operation" is usually called the "critical speed" of the mill. The calculation method is as follows

Picture 1 (rpm)

Where: D - the effective diameter of the mill cylinder, i.e. the cylinder specification diameter minus twice the average thickness of the liner (m).

7、What are the types and characteristics of ball mills?

A: There are two types of ball mills commonly used in processing plants: lattice type and overflow type, which differ only in that the overflow type has no fan-shaped lattice plate device in the ore discharge part, and the ore slurry of the overflow type ball mill is automatically discharged through the ore discharge port, while the lattice type has forced ore discharge because of the lattice plate, thus increasing the processing capacity and reducing ore over-crushing.

8、What are the components of ball mill?

A: It mainly consists of six parts, such as cylinder, ore feeding part, ore discharging part, transmission part, bearing part and lubrication system.

9、What is the composition of the ore feeding part?

A: The ore feeding part is composed of the end cover of the hollow journal and the ore feeder and the inner sleeve of the journal.

10、What are the types of ore feeder?

A: There are three types of feeders used in ball mill of mineral processing plant: drum, worm and joint feeder.

11、What are the components of the ore discharge part? (lattice type)

A: The ore discharge part consists of the end cover of the hollow journal, fan-shaped lattice plate, central liner and inner sleeve of the journal and other parts.

12、What are the working characteristics of rod mill?

A: Its working characteristics are: it crushes the ore by the crushing and grinding effect produced by the "line contact" of the steel rod. Therefore, it has a selective crushing effect, reducing the ore over-crushing, and the product size is uniform.

13、What are the process indexes of grinding and classifying unit?

A: The process indexes are: grinding fineness, mill capacity and operating rate, classification efficiency and grinding efficiency. The former is the quality indicator, the latter is the quantity indicator.

14、What is the method of indicating the fineness of grinding?

A: The fineness of the grinding product is usually sieved by a "standard sieve" with a 200 mesh sieve, and expressed as a percentage of the total product under the sieve ("mesh" is the number of holes per inch of screen mesh, and is used to name the sieve).

15、What is the definition of mill utilization factor and how is it calculated?

A: Mill utilization factor is the average amount of raw ore per cubic meter of effective mill volume per unit of time, expressed in tons/m3 - hour. That is, the mill utilization factor q=Q/V(t/m3-h).

16、Definition and expression of the mill's hourly production capacity?

A: Mill table capacity, i.e. the amount of raw ore that can be processed by the mill per unit time (hour) under certain feed and product size conditions, expressed in tons per table hour.

17、What are the operational factors affecting the work of the grinding-classification unit?

A: They can be broadly summarized into three categories as follows:

①, the nature of the ore. Including hardness, mud content, feed size and the required fineness of the grinding product.

②, mill structure. Including mill type, specifications, speed, etc.

③, operating conditions. Mainly grinding media, slurry, concentration, sand return ratio and classification efficiency.

18、How to choose the shape of grinding media?

A: According to the grinding purpose and test, spherical and P2 rod-shaped media grinding effect is the best; spherical media in the grinding collision for point contact, so high grinding efficiency, suitable for fine grinding, but compared with the rod mill, over-crushing phenomenon is serious, generally when the feed size of 30mm or less, the product size is not less than 1mm, the use of rod mill is more efficient.

19、What is the principle of choosing the material of grinding media?

A: When choosing the media material, the specific gravity of the material, hardness, wear resistance, price, processing and manufacturing conditions should be fully considered. At present, the general use of steel or iron materials. In production, the large ball commonly used steel, small ball commonly used iron, coarse grinding should be used steel, fine quality available iron. In recent years has been successful in making rare earth manganese ductile iron balls instead of steel balls.

20、What is the size system of grinding media loading?

A: The size of the ore is determined according to the nature of the ore, the harder the ore, the coarser the feed size, the larger the size of the media should be added to produce a larger impact grinding effect, and vice versa, the smaller size of the media available to enhance the grinding effect.

21、What is the definition of media filling factor and the relationship with mill rotation rate? (One of the principles of media loading).

A: The filling rate, which is the percentage of mill volume occupied by the grinding media, is related to the mill rotation rate as follows:

Media filling rate % Suitable mill rotation rate %

32～35 76～80

38～40 78～82

42～45 80～84

22、What is grinding efficiency? Which methods does it have?

A: It refers to the amount of ore that can be processed for every 1 kW-h (one degree) of electrical energy consumed, and there are several ways to express it as follows:

①, the number of tons of raw ore processed by each consumption of one degree of electricity, that is, tons / degree.

②, each consumption of one degree of electricity obtained by the specified level (often -200 mesh) to calculate the number of tons of grinding products.

③, calculated by surface area tons/kWh.

23、What is the technical efficiency of the mill?

A: It refers to the ratio between the percentage content of qualified particles in the product obtained after grinding and the percentage content of greater than qualified particles in the original feed ore.

24、What do you mean by qualified grade?

A: The so-called qualified particle size is the upper limit of particle size should be less than the maximum size, and the lower limit of particle size should be subtracted from the over-crushed part.

25、How to improve the technical efficiency of the mill? What should be noted in the mill operation?

A: To improve the technical efficiency of the mill, the following aspects can be considered:

①, the use of closed circuit grinding process.

②, in the choice of equipment, coarse grinding should use the rod mill with peripheral discharge, fine grinding should use the lattice ball mill.

The following aspects should be noted in the mill operation.

①, closed circuit grinding should be appropriate cycle load .

②, the mill speed should be controlled appropriately.

③, the ball loading system should be reasonable.

④, the grinding concentration should be appropriate.

26、What is the mill throughput capacity? How to calculate?

A: Mill throughput capacity refers to the total amount of ore that can be passed per unit time, which is calculated as follows

Picture 2

Where: q - mill throughput capacity, t/m3-h.

Q - the productivity of the mill according to the original ore, tons/hour.

C - sand return ratio, in multiples.

V - the effective volume of the mill, m3.

27、What are the phenomena when the mill is "bloated"?

A: Generally, the following phenomena will occur:

①, the main motor ammeter indicates that the current is falling.

②, the mill discharge spit large pieces of ore, pulp gushing out.

③, classifier overflow run coarse phenomenon is serious, sand return significantly increased.

④, grinding machine operation sound dull, call inaudible steel ball impact sound.

28、What are the methods to load the proportion of ball mill grinding media size?

A: There are two main methods:

①, according to the total feed size of the mill for ball loading, the principle is: the ball diameter and the feed size to adapt, the ball volume and the content of the grain level to adapt.

②, simple method: the ratio of various balls is proportional to the ball mill diameter.

29, the mill "bloating", how should be handled in operation?

A: First of all, we should analyze the changes in the nature of the ore, the amount of ore feed, water feed, sand return, overflow particle size and other normal, in general, the following measures can be taken in operation:

①, reduce the amount of ore feed to the mill or stop ore feed for a short period of time.

②, adjust the amount of water, that is, strict control of grinding concentration.

(③) Reasonable addition of grinding media.

30、Why does the current of the main motor drop when the mill is "bloated"?

A: The operation of the mill and the grinding of ore require energy consumption, and most of the energy is consumed in the grinding process of ore, due to "bloating", the grinding effect is greatly reduced. Therefore, the mechanical energy converted into grinding energy is greatly reduced, so the main motor current decreases; from the grinding media movement, due to "bloating", the material increases, the concentration increases, the copper ball is lifted to a lower height, the grinding force is small, so the kinetic energy is reduced and the current decreases.

31、What is the influence of the amount of balls loaded in the ball mill on the grinding process? How much filling rate is qualified?

A: The amount of loaded balls has a certain influence on the grinding efficiency, with less balls loaded, the mill efficiency is low, and too many balls loaded, the inner layer of balls will easily interfere during operation, destroying the normal cycle of balls. Grinding (crushing) efficiency is also reduced.

Practice has proved that: the amount of ball loading is related to the rotation rate of the mill. When the running rate of the mill is low, more balls can be installed, when the rate is high, less can be installed.

For lattice ball mill: filling rate φ=40-45%

For overflow ball mill: filling rate φ=35-40%

32、Why must the steel balls be loaded into the mill in a certain ratio according to the size?

A: The size and dimensions of steel balls mainly depend on the physical and mechanical properties of the ore and the particle size composition of the ore.

If the ore is hard and coarse in size, it needs to have larger impact, so it is necessary to add steel balls with larger size; if the ore is soft and the size of the ore is small, and the product size is fine, it should be mainly ground, so it can be loaded with smaller size steel balls.

The production site ball mill is loaded with a variety of ball size balls. According to a certain ratio, to deal with materials composed of different sizes of ore particles. Theoretically, only to ensure a certain proportion of various balls can be adapted to the particle size composition of the materials to be ground, in order to achieve good grinding effect.

33、What will happen if there are too few large balls or small balls in the mill?

A: When the material size or hardness is large, a small proportion of large balls will cause the grinding product to run coarse and fail to meet the fineness requirements. And the processing capacity is also limited, affecting the grinding efficiency.

When processing materials with smaller ore size and lower hardness, if the ratio of small balls is small, the product size will also appear coarse phenomenon. The small balls should be added appropriately, and the proportion of small balls should be increased to adapt to the nature of the ore to be processed, so as to obtain a better product fineness. Therefore, the proportion of balls must be strictly in accordance with the proportion, to prevent imbalance.

34、Mill lubrication parts and the type of lubricant? What is the cycle time?

A: ①, large and small gears: 40-50# mechanical oil, twice per shift, once every six months.

②, hollow bearing: one-half of 50# mechanical oil mixed with one-half of 24# or 28# mechanical oil, added once per shift, oil change in 3 months.

③, pinion rolling bearing: calcium-based grease drip T=90℃, added every shift, oil change in half a year.

④, reducer gear and bearing: 40-50# mechanical oil, added every shift, 3 months oil change.

⑤、Motor: calcium-based grease drip, T=120℃, half year oil change.

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