Crushing and Grinding

Published time:08 October 2019

The crushing and grinding operations are an important part of the processing of mineral resources, and it is also operation with high investment and high energy consumption. In the case of metal mines, equipment investment in crushing operations accounts for 65% to 70% of the total plant value, power consumption is about 50% to 65%, and steel consumption is as high as 50%.

Therefore, how to improve the performance of grinding equipment, research and development of high-efficiency energy-saving equipment, obtain a larger crushing ratio, achieve a finer crushed product granularity, reduce steel consumption, has become a common goal pursued by workers in various fields.

This paper analyzes the improvement of the grinding process and the development of the grinding equipment.

1. Crushing and grinding improvement

The ore size reduction process involves two steps: crushing and grinding. The grinding process is the final operation of making the mineral to dissociate from the monomer and making the particle size meet the selection requirements.
Grinding is a high-efficiency and low-efficiency operation. The power consumption of crushing operations only accounts for 8% to 12% of grinding operations. Improving the grinding process is an effective way to achieve high efficiency, low consumption and increase economic benefits.

1.1 More crushing and less grinding

The crushing of materials is mainly achieved by the extrusion and impact of the equipment on the minerals, and the grinding is mainly achieved by the impact, grinding and grinding of the equipment.
The energy utilization efficiency of the crushing operation is much higher than that of the grinding operation. More crushing and less grinding to achieve the best economic benefits.

—-Adopt high-efficiency fine crusher. Such as the single-cylinder hydraulic cone crusher, the HP series cone crusher ( Nordberg ), and the domestic JC56, JC4060 jaw crusher, SX series double-roller jaw crusher, etc.

—-Improve the crushing process. According to the plant scale, the ore nature, the ore size, the product size, etc., choose the appropriate crushing process.

1.2 Grinding – classification stag

The principle of mineral grinding-classification processing is to combine grinding and classification operation, cleans out the gangue minerals in time, reduces the grinding volume and improves the beneficiation efficiency.

1.3 Fine crusher replaces the conventional grinding machine

The crushing operation of the ore dressing plant is very inefficient, and the fine crusher replaces the conventional mill to produce fine products. For the hard rock crushing, the water punching cone crusher can gradually replace the conventional drum mill.

1.4 Transform the old crushing process

Some of the original mining plants have a large design scale, but for a variety of reasons, the production scale is only about half of its original design.
With the gradual reduction of mineral resources, these old factories can be improved in energy conservation and efficiency, perfecting their crushing process, ensuring their crushing granularity while achieving energy saving and efficiency.

1.4.1 Microwave-assisted grinding

In the current mining production, the crushing method of mineral materials is mainly mechanical crushing. In order to reduce the steel consumption of crushing operations and improve the efficiency of energy utilization, mining workers have developed a new method of crushing, in which microwave pretreatment is a promising method of crushing.

The microwave is an electromagnetic wave having a frequency of approximately 300 MHz to 300 GHz and a wavelength of 2500 px to 1 mm.
Microwave is a high-frequency electromagnetic wave that penetrates into the interior of a mineral to cause orientation polarization and deformation polarization of matter molecules. As the electrode changes, the direction of polarization also changes constantly, resulting in the self-heating effect of the mineral body, and the temperature rises. However, due to the different mineral properties of the ore, the absorbing properties are also different, resulting in ore. The temperature difference between each mineral in the mineral is different, and the thermal expansion coefficient of each mineral is also different. As a result, thermal cracking and the like occur, which causes microcracks in the mineral system and expands the original microcracks, thereby facilitating subsequent pulverization operations.
Although microwave heating treatment has the incomparable advantages of traditional heating methods, the current theoretical research on microwave grinding is not deep enough. In the near future, microwave will play a huge role in reducing the energy consumption and steel consumption of grinding operations.

1.4.2 Selective grinding

The so-called selective grinding is the use of selective dissociation of minerals and selective grinding of the grinding, the purpose is to cause some selectivity in grinding operations.
The main purpose of the grinding operation is not to reduce the ore particle size, but to dissociate the useful minerals from the gangue minerals. The ultimate goal of grinding is to obtain the highest monomer dissociation with minimal energy input.
Selective grinding is widely used in mining production such as metal ore, non-metallic minerals and coal mines, especially in the production practice of bauxite.

1.4.3 Micro-stage grinding

Because the ore materials of different particle sizes have different requirements on the grinding form, the coarse-grained materials are suitable for the grinding method based on impact crushing, and the fine materials should be ground by grinding.
Micro-stage grinding is to install barrel linings with different surface shapes along the axial direction of the barrel of the ball mill. The surface of the ball mill is installed with a non-smooth lining to form a high steel ball drop height, resulting in impact pulverization. .
A smoother cylinder liner is installed at the discharge end of the ball mill to form a lower steel ball drop height, resulting in grinding and pulverizing.

From the feeding end to the discharge end, the pulverized form gradually changes from impact pulverization to grinding pulverization and pulverization, so that the grinding form changes along the axis direction of the ball mill, and stage grinding is realized in a billiard mill.
This can better meet the different needs of the ore material in different stages of the grinding process, different particle size composition, and meet the ore crushing law, thus improving the grinding efficiency.
The implementation of the micro-stage grinding technology only requires the modification of the surface of the partial cylinder liner, which is simple and easy.

1.4.4 Ultrafine grinding

The cost of conventional grinding technology is quite high. Even if the grinding particle size can make useful minerals dissociate, the over-grinding phenomenon will occur, and many useful minerals will be lost in the slime.
After several years of continuous development, the ultra-fine grinding technology of the mill has become one of the important deep processing technologies of industrial minerals and raw materials, which is of great significance to the development of modern high-tech industries.

Ultra-fine grinding technology is applied in the pretreatment of refractory gold ore. The gold is coated with pyrite. The gold ore contained in microscopic gold, sub-microscopic gold or solid solution is a kind of gold that is extremely difficult to dissolve and extract gold. ore.
The key to gold extraction is to destroy the pyrite package and expose the gold to dissociation. The pyrite is very stable and difficult to decompose. With the development of ultra-fine grinding technology, it is possible to use an ultra-fine grinding to open the package of sulfide to dissociate the gold.

1.4.5 Pressure grinding

There are many types of internal stress of the ball mill, such as impact, extrusion, shearing, grinding, etc. Some of the stress electric energy consumption is large and the pulverizing efficiency is not high.
Studies have shown that when the compressive stress with high pulverization efficiency is selected as the main stress, since the pressure pulverization process conforms to the smashing law of the material layer, when the pressure is small, the free loose material is first sufficiently dense, and when the pressure is increased, the squeezing particles are mutually The stress is transmitted, and when the strength value of the particles is exceeded, the mineral particles are broken and a large number of microcracks are generated.
Adjusting and strengthening the energy input for different material characteristics, and also restraining the action area of ​​the stress, so that the material passes through the stress zone regularly, and the mechanical energy is effectively converted into the pulverization energy, so that the particle becomes a blank product with low porosity. Through the subsequent process, a product with qualified particle size is obtained, thereby achieving the goal of high production and energy saving.

The high-pressure roller mill developed based on this theory has been applied to large-scale industrial production and has achieved good economic benefits. It can significantly improve the processing capacity of equipment systems, reduce the power consumption per unit, and save infrastructure investment and simplify the process. , reduce the number of broken sections, general ore materials can be used, the feed water content can reach 15%.

2 Application and development of rock crusher

In the past 10 years, the new grinding equipment has been continuously introduced, with the aim of obtaining a larger crushing ratio and obtaining more fine-grained crushing products to reduce the particle size of the grinding material, save energy and reduce consumption, and at the same time carry out structural innovation, adopting new technologies, new materials improve traditional equipment to improve reliability, durability, performance and efficiency.

2.1 Rotary crusher

The development of the gyratory crusher has a history of 100 years. Due to its large processing capacity, large ore size, and ability to handle hard ore, it is still important equipment for crushing various hard materials in large mines and other industrial sectors.
The rotary crusher has large production capacity, low unit power consumption and stable operation. It is suitable for processing sheet materials. The crushed product has a relatively uniform particle size and can be widely used for coarse crushing and medium crushing of various hardness ores. However, compared with the jaw crusher, the structure is complicated, the price is high, the maintenance is difficult, the repair cost is high, and the capital construction cost is high.

2.2 Jaw Crusher

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With the adoption of large-scale carrying equipment, the crushing mill’s ore feeding size has reached 1.2 to 2 m, which has promoted the development of the jaw crusher to large-scale.
The compound pendulum crusher has the advantages of high efficiency and low price, occupies a large market share of the jaw crusher. With the promotion of conservation, energy-saving and efficient production methods, several new jaw crushers have also been successfully developed.

2.3 Cone crusher

The spring cone crusher has been in existence for a hundred years. It was invented by the American Symons brothers using the principle of a gyratory crusher. So far, its structure has not changed much, its performance is stable, and it has a certain market share.
In order to meet today’s high throughput production, achieve high energy, achieve higher crushing ratio and finer product granularity, the new cone crusher has also been continuously developed and applied to production practice. For example, a hydraulic cone crusher that uses hydraulic instead of a spring and an inertia cone crusher that can replace the rough grinding operation have achieved good economic benefits in production and operation.

2.4 Roller crusher

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The high-pressure roller mill, also known as the roller crusher, works on the principle of material layer pulverization. It is a new type of high-efficiency energy-saving grinding equipment, which is gradually being applied and popularized at home and abroad.
When the high-pressure roller mill was originally designed and applied, it was mainly used for the crushing of limestone and brittle metal ore with less hardness, and it was used for the middle and fine crushing of crushing operations. After years of promotion and development, it has been used in fine crushing of medium hardness and above, especially in the case of iron ore crushing, its technology has become increasingly mature, it has a large crushing ratio, fine product size, high efficiency, low energy consumption, etc. The utility model can also be applied to replace a rough grinding operation, and the ore can be crushed by a high-pressure roller mill to obtain a product of 3 to 10 mm in size, and the magnetite can be greatly improved after pre-magnetic separation. The grade of the mine has the characteristics of water saving, electricity saving and production increase.
At present, the high-pressure roller mill is developing in the direction of large-scale, the diameter of the roller and the roll surface are further increased, the grain size range of the grinding is larger, and the throughput is also increased. The production practice shows that the single-machine production capacity of the high-pressure roller mill can reach 1,500 to 2 000 t / h, and the energy consumption of the crushed metal ore is 1.2 to 2. 8 kWh / t. Under the same conditions, the unit energy consumption is broken than the conventional one. The machine is 20% to 50% lower, the roll surface wear resistance is good, the service life of the inlaid hard alloy grain nail roll surface can reach 8 500 h, and the automation level is high. With the improvement of the performance of the high pressure roll mill, it is necessary for the metal mine. There will be broad application prospects.

3. Grinding mill application and development

The mill is further developed to a large scale. The change of the diameter of the mill has obvious changes for the grinding process. The large mill usually has a high specific crushing rate and can handle coarser grade materials.
However, if the diameter of the mill is too large, the dead zone of the ball will increase. When the mill increases the processing capacity, it will also reduce the residence time of the mineral material, hindering the transfer of energy from the ball medium to the ore particles, resulting in a unit volume yield. The decline, the unit energy consumption of grinding products increased, so the development direction of the mill has been developed from large-scale to high-efficiency energy-saving.

3.1 Self-grinding, semi-autogenous grinding machine

Since the use of self-grinding and semi-self-grinding technology in the 1950s, it has grown into a mature, reliable and continuously applied technology.
In the self-grinding process, the ore larger than 100 mm in the mill acts as a grinding medium. The ore material with less than 80 mm and more than 20 mm has poor grinding ability, and it is not easily broken by large ore materials, sometimes When the material is crushed, a steel ball of about 4% to 8% of the volume of the mill is often added to the mill, which improves the grinding efficiency of the mill, and thus semi-self-grinding occurs.
The semi-autogenous mill belongs to a cylindrical mill with heavy load, low speed and large starting torque. Nowadays, both the new expansion and the renovation of the old factory, almost all use self-grinding, semi-self-grinding technology, self-grinding,
The semi-self-grinding technology eliminates the two-stage crusher and the screening equipment, simplifies the process and improves the operating conditions, which not only reduces the capital cost of the construction, but also reduces the production and operation costs, and also facilitates automation.

3.2 Ball mill

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The ball mill is a traditional material crushing device. It has a history of more than 100 years. It is still important equipment for fine powdering of solid materials. It is widely used in metallurgy, chemical industry, cement, ceramics, construction, electricity and In the industrial sectors such as national defense, dry and wet grinding of various ores and materials is possible.
In recent years, the development of ball mills has focused on energy saving and consumption reduction, continuously improving and perfecting the grinding machine transmission mode, researching and developing new lining plates and grinding media, striving to achieve automatic control of the grinding process, and improving on the premise of ensuring grinding grain size. The processing capacity and grinding efficiency of the mill.

As we all know, the lining of the ball mill is a key part of the mill that can achieve high efficiency, energy saving and consumption reduction. After research and development, it has made good progress.

—-The angle spiral lining is also called energy-saving lining. After using this lining, the unit output power consumption is reduced by 10% to 25%, the mill output is increased by 15% to 20%, and the unit output ball consumption is reduced by 10% to 20%. It has the advantages of stable operation, less product pulverization, less noise, etc., and is especially suitable for crushing operations in cement production;

—-The rubber lining is a corrosion-resistant and wear-resistant non-metallic material lining. Compared with the manganese steel lining, it has the advantages of a lightweight, low energy consumption, high output and low noise.

—-On the basis of the rubber lining, a composite magnetic lining has been developed. This lining is magnetically adsorbed on the surface of the lining to adsorb a layer of magnetic particles and dielectric fragments to form a protective layer to extend the service life of the lining. It is almost half lighter than ordinary manganese steel lining, and can be directly adsorbed on the inner surface of the mill barrel without bolt fixing, which greatly reduces the workload of installation and maintenance, not only reduces energy consumption, but also increases the processing capacity of the mill.

3.3 Rod mill

The rod mill is developed on the basis of the ball mill. It has the advantages of reliable processing technology, low investment, less auxiliary equipment and simple process flow. It can be combined with the ball mill to form a different grinding process.
The rod mill mainly grinds the ore by the pressure and the grinding force of the grinding rod. When the rod hits the ore, it first hits the coarser grade ore, and then pulverizes the smaller-sized material, between the rod and the rod. When the rod is in contact with the wall, the coarser-grained ore particles are mixed with it, which acts as a bar sieve. The finer-grained material can pass through the gap between the rod and the rod, which is beneficial to the clamp. The coarser-grained material also allows the coarser-grained ore particles to be concentrated in the place where the grinding media strikes.
Therefore, the rod mill has the function of selective grinding, and the product has a uniform particle size and less pulverization.

3.4 Vertical spiral mixing mill

The vertical spiral mixing crusher is a new type of high-efficiency energy-saving grinding equipment successfully developed by Changsha Research Institute of Mining and Metallurgy. Its grinding effect is mainly grinding and stripping, as well as a small amount of impact and shearing, so that the original material can be kept. Lattice shape, make full use of energy to effectively grind the material, because in the fine grinding and ultra-fine grinding, friction grinding is the most effective grinding method, and has been used in the regrind or fine grinding operation of metal mines.

3.5 High-speed impact pulverizer

High-speed impact pulverizer refers to an impact pulverizing device that strongly impacts the material around a horizontal or vertical high-speed rotating body (rotor, hammer, blade), which can crush materials below 8 mm to – 10 μm at 70%. the above.
The device can be applied to ultra-fine pulverization of non-metal such as talc, clay, barite, calcium carbonate, mica, and graphite.

4. Ways to improve grinding efficiency

There are many factors affecting the grinding efficiency, including the nature of grinding feed, the size of the ore, the filling rate of the steel ball, the size of the steel ball and the ratio, the ball filling system, the grinding system, the grinding process, the mill operation, and the classification. Factors such as efficiency and amount of sand return, but these factors are not independent of each other and have a certain impact on each other.

4.1 Grinding feed properties

The mechanical properties of the ore, such as hardness, toughness, dissociation and structural defects, determine the grindability of the ore, which determines the difficulty of grinding.
The small grinding degree indicates that the ore is easy to grind, the smaller the wear of the ore on the mill, the lining and the grinding medium, the smaller the power consumption is consumed; on the contrary, if the grinding degree is large, the wear of the mill And power consumption is big.
The nature of the ore will directly affect productivity and the impact on grinding operations is of paramount importance. In the modern grinding operation, a grinding aid process has been added to add some specific chemicals to the grinding process to reduce the grindability of the ore and increase the productivity of the mill.

4.2 Feed size

The grain size of the mill has a great influence on the grinding efficiency of the mill. Generally speaking, the smaller the grain size, the smaller the work done by the mill on the ore; on the contrary, the larger the grain size, the mill The work done on the ore is greater.
The crushing of ore by steel ball is a random crushing, and the crushing efficiency is very low. Some researches have pointed out that the crushing efficiency of the ball mill is only 6% to 9%. It can be seen that the grinding grain size has a great influence on the mill. In order to achieve the final grinding fineness, it will inevitably increase the workload of the ball mill, and the energy consumption and power consumption of the ball mill will also increase.

4.3 Mill rotation rate and filling rate

There is a close relationship between the rotation rate and the filling rate of the mill. The two are related to each other. Generally speaking, once the mill is installed, its rotation rate is fixed, it will not change easily, and the operation of changing the rotation rate is compared. It is cumbersome, so in actual production, the transfer rate is generally not analyzed as a factor affecting the grinding efficiency. It is only necessary to analyze the suitable ball filling rate at a certain speed.
When the transfer rate is constant, the filling rate is large, the steel ball hits the material more frequently, the grinding area is large, the grinding effect is strong, but the power consumption also increases, and the filling rate is too high, which also affects the steel. The movement state of the ball reduces the impact on the large material; on the contrary, if the filling rate is small, the grinding area is small, the grinding effect is relatively weak, but the power consumption and energy consumption are also small. Therefore, at the production site, whether the filling rate is appropriate has a great influence on the grinding efficiency of the plant.

4.4 Steel ball size and ball ratio

In the mill, the steel ball and the mill are in point contact. When the ball diameter is too large, the crushing force is also large, so that the ore is broken along the direction of the penetrating force, instead of being broken along the crystal plane of different minerals with weak bonding force, resulting in no fracture. Selective.
In the case of the same filling rate of the steel ball, the ball diameter is too large, resulting in too few steel balls, low breaking probability, severe crushing, and uneven product size; on the contrary, if the steel ball is too small, its crushing effect on ore is small. The grinding efficiency is low, so the precise steel ball size and ball ratio have a great influence on the grinding efficiency.

4.5 Liner

The main function of the ball mill liner is to protect the mill. When the mill is running, the steel balls and materials inside the mill are thrown or slid by the liner to a certain height, and the material is ground and pulverized. It will also be affected by the impact, sliding and rolling of steel balls and materials, and will also be affected by temperature. Therefore, the main form of wear of the lining plate is abrasive wear under a small number of times of energy, so which material lining is selected, Reducing its wear and tear is always an important issue for ball mills.
At present, there are three main types of lining materials widely used: high manganese steel; alloy wear-resistant steel; high chromium cast iron.

—–High manganese steel has good wear resistance and good economic applicability, but low yield strength, suitable for medium and Use under high impact load wear conditions.
—-Alloy wear-resistant steel has a higher comprehensive performance than high-manganese steel and is suitable for medium impact wear conditions.
—-High-chromium cast iron has a higher wear resistance than the former two and is more widely used.

When selecting the lining material, the use of the ball mill and the economical factors should be considered in order to prolong the service life of the mill lining and achieve the most ideal effect.

4.6 Grinding concentration

Grinding concentration is also an important factor affecting the grinding efficiency. Its size will affect the specific gravity of the slurry, the adhesion of the ore around the steel ball and the fluidity of the slurry.
When the grinding concentration is low, the fluidity of the slurry is fast, and the adhesion of the material around the steel ball is low, so that the impact and grinding effect of the steel ball on the material are weakened, and the grinding efficiency is low.
When the grinding concentration is high, the adhesion of the material around the steel ball is good, the impact and grinding effect of the steel ball on the material is better, but the slurry fluidity is poor, the over-grinding is more serious, and it is not conducive to improving the processing capacity of the mill. Therefore, determining the optimum grinding concentration will have an important impact on the grinding efficiency.

4.7 Grinding classification process

For a long time, people tend to pay attention to the realization of grinding purposes, while ignoring the means and methods of grinding, patronizing the grinding grain size of the pursuit of requirements, and neglecting the monomer solution of various useful materials of ore containing various metals. The difference in the degree of separation will cause some minerals to be pulverized and some minerals to be insufficiently pulverized. In this case, if the conventional rough grinding process is still used, the grinding and sorting effects will not be good.

The classifier and the grinding machine work in a closed circuit, which can control the grain size of the grinding product and increase the productivity of the mill. Therefore, the classification efficiency has a certain influence on the grinding efficiency. When the classification efficiency is high, the qualified grain grade products can be eliminated in time. Avoid over-pulverization and reduce energy consumption; when the classification efficiency is low, the products that reach the qualified size can not be discharged in time and returned to the mill for re-grinding, which can easily cause over-grinding and affect the later selection effect.

4.8 Return sand ratio

The return-sand ratio is the ratio of the amount of sand returned by the ball mill to the ore and ore. The effect of graded anti-sand is not only to return the unqualified coarse particles, but also another important role to make the ball mill’s ore thickening and let the steel ball High efficiency crushing over the entire axial length of the mill increases the productivity of the mill.
Under normal circumstances, the amount of sand return should not exceed 500%, and the second section should not exceed 690%.

4.9 Grinding circuit automatic control

There are many variables in the operation of the grinding classifier, and the change of one factor can cause successive changes of many factors. The manual operation can not keep up with this change, can not meet the requirements of the production process, and adopt automatic control to make The grinding grade is maintained in a stable and suitable state, thereby increasing productivity and reducing energy consumption.

The automatic control of the grinding and grading circuit can increase the production capacity by 2. 5% to 10%, the treatment of 1 t ore can save electricity consumption of 0.4 kWh.

Chinese crushing and grinding equipment

China’s grinding equipment has undergone considerable development through technology introduction, technical cooperation, digestion and absorption, and self-development. At present, China has a wide variety of grinding equipment, complete varieties, continuous improvement in manufacturing quality, and increasing production year by year. It has become one of the countries with the most productive grinding equipment in the world.

JXSC has been supplying the rock crushing and grinding equipment since 1985, contact us for model selection, installation guidance, crushing plant optimization, layout design and so on service.

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