The crushing and screening operation is the preparatory operation before the ore is grinding in beneficiation plant. In order to reduce the energy consumption, we should strive for "more crushing and less grinding" to minimize the final product size of crushing.
Crushing operation and screening work together to form a crushing section, the total of all crushed stages (sometimes including washing and other operations) constitute a crushing & screening process, as shown in Figure 1.
Figure1. Crushing & screening process
1.Crushing stage number
The number of crushing stages depends on the maximum grain size and the final product size of the raw ore in plant. The maximum grain size of beneficiation plant is related to the deposit condition, the mine scale, the mining method and the equipment for the shovel transportation, which can be referred to table 1.
Table1.Relationship between run-of-mine max.size and mining method
The final product granularity depends on the process requirements of grinding and beneficiation. Because the power consumption of grinding operation is large, the final product granularity should be reduced as much as possible in the design. At present, the most suitable ore granularity and ball mill is 10-15mm, and some mills have reduced the granularity of the ball mill to about 10mm. The rod mill is 15-20mm when it is open operation, and it can increase to 20-25mm when the ore contains much mud and water content.
The size range of the original mine is 200-1400mm, and the granularity range of the ball mill is 10-20mm, so the total crushing ratio is 10-140. The total crushing ratio is equal to the product of the fragmentation ratio of each stage. The fragmentation ratio of each section is related to the type of crusher, working conditions and ore properties. The crushing ratio range of various crusher under different working conditions is shown in table 2.
Table 2. crushing Ratio range of various crushers
From the above description, it is known that the minimum total crushing ratio is 10, and it is impossible to achieve by one crushing stage in general. The maximum total crushing ratio is 140, which can be completed with three stages. So the common crushing process can be met with two or three stages to meet the above requirements. If the ore is very hard and the ore is too large, in the design of a large beneficiation plant, four pieces of crushing process can also be considered. If the number of large blocks is not much, it is appropriate to set up the pneumatic or hydraulic crushing hammer for two breakages before the primary crushing, and still can be used in three pieces of crushing process.
2.Pre screening
The pre screening is the screening operation of the ore before entering the crusher. Pre screening the fine material in the ore can prevent the ore from smashing, reduce the amount of ore entering the crusher, increase the mobility of the ore in the crushing chamber, and improve the production capacity of the crusher. In particular, pre screening is often used to prevent fine particles from clogging the crusher before mid crushing and fine crushing. The first stage crusher has larger discharge port, and the fine material is easy to pass through the crusher. If the fine particle content of the raw ore is less, or the crusher is selected according to the granularity of the ore, and the production capacity is enough, it can not be pre screened.
3.Inspection screening
The purpose of inspection screening is to sift out the crushing products larger than a specific particle size and then return to the crusher for further crushing, so as to control the final particle size of the crushing products and give full play to the production capacity of the crusher.
The closed crushing process with inspection crushing is much more complex than the open crushing with only pre screening, it need to install more screens to the mine equipment and transportation equipment, and even to set the individual workshop, but in order to make the mill work effectively, in which the final crushing section shall be provided to check the screening operation.
4.Raw material and crushing discharge typical grain size characteristic curve
When selecting and calculating the crushing and screening process, it is necessary to use the grain size characteristic curve of the ore and the grain size characteristic curve of the broken products. These curves can be directly measured and drawn through industrial tests, and their workload is large and can also be borrowed from the actual data of similar production plants. When calculating the crushing process, we usually use the typical grain size characteristic curve of the raw ore and all kinds of crusher under different working conditions, or the actual production data of the same type of ore. The grain size characteristics of various standard ore and crusher products are shown in Figure 2 to figure 8.
Figure2. Raw material size characteristic curve
Ⅰ.hard crushing ores;Ⅱ.mid hard crushing ores;Ⅲ.easy crushing ores
Figure3. Gyratory crusher size characteristic curve
Ⅰ.hard crushing ores;Ⅱ.mid hard crushing ores;Ⅲ.easy crushing ores
Figure4. Jaw crusher size characteristic curve
Ⅰ.hard crushing ores;Ⅱ.mid hard crushing ores;Ⅲ.easy crushing ores
Figure5. Standard cone crusher size characteristic curve
Ⅰ.hard crushing ores;Ⅱ.mid hard crushing ores;Ⅲ.easy crushing ores
Figure6. Mid type cone crusher size characteristic curve
Ⅰ.hard crushing ores;Ⅱ.mid hard crushing ores;Ⅲ.easy crushing ores
Figure7. Short head cone crusher size characteristic curve
Ⅰ.hard crushing ores;Ⅱ.mid hard crushing ores;Ⅲ.easy crushing ores
Figure8. Short head cone crusher closed crushing size characteristic curve
Ⅰ.hard crushing ores;Ⅱ.mid hard crushing ores;Ⅲ.easy crushing ores
5. Common crushing and screening process
There are four commonly used crushing and screening processes in design and production, such as Figure 9. The pre screening diagram of stage 1 before crushing is expressed as a dotted line, and it is required to determine whether or not it is used in accordance with the specific circumstances. The process a and c are open circuit processes. The final products are coarser, but they can simplify the equipment configuration of the crushing plant, reduce the number of fines crushera and vibrating screens, and lower investment. Therefore, the opening process can be used when the granularity of the mill can be coarser and the ore with large mud content is treated.
The process b and d in the figure are closed circuit. Compared with open circuit flow ratio, closed circuit process requires more equipments, complex configuration and higher investment, but its crushing products are small, which is conducive to improving the production capacity of grinding mill and reducing total energy consumption and production cost of grinding process. Therefore, the closed circuit process should be used as far as possible. The two or three segment crushing process should be determined according to the ore hardness, the maximum size of the raw ore, the granularity of the grinding machine and the size of the beneficiation plant. In general, when the ore is hard and requires more crushing, three pieces of crushing process should be adopted.
Figure 9. Common crushing and screening process
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