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The Advantages of Roller Mill Combined Grinding
2011-06-02
The Advantages of Roller Mill Combined Grinding
----implements high quality, high output and low consumption of cement production
 
roller press/roller mill/grinding equipment/CKP pre-grinding/cement vertical mill/slag vertical mill/coal vertical mill/Jiangsu Pengfei Group Co., Ltd
 
Abstract: In recent years, complying with the shoreline hardening process and production process and gradual updating on technology of process control, cement grinding process and equipments is mainly based on ball mill, combined with high efficient vertical mill, roller mill, etc. multi-types new grinding equipment and this is the process combination of these equipments, meanwhile it is facing to upsizing of grinding equipment and the technology of process control of lifter is facing to intelligentization so that it could meet the requirements on upsizing on cement production. Roller mill grinding technology is a advanced and mature grinding technology, combined grinding system made up of V type static separator and roller mill has integrative advantages in high quality, high production and low consumption in various roller mill grinding process.
  
Key words: roller mill  new type grinding    combined grinding   grinding system    cement quality   grinding efficiency    integrative advantages
  
A. Advantages of combined grinding
  
a. energy saving, environmental protection to ensure cement quality
  
Grinding is very important in the cement manufacturing process, no mater it is raw meal (half-finished) or cement (finished product) need to be gained via grinding. One ton cement manufacturing will need to grinding about 3 or 5 ton various material, power consumption will be 100-110kW.h, among this 60-70% of the power consumption is used for grinding. Considering from hydration, hardening and effective utilization of cementitious character, strength especially strength in early time, more fine the cement is grinded, the better it is and this could improve its bleeding, meanwhile cement also need to consider the distribution on grain size of product which ensures cement quality via energy-saving and environmental protection.
  
b. implement grand objective
  
Energy-saving is the key point for promoting continuous development of the whole society and implementing the grand objective of moderately prosperous society. Industry is the main consumer of energy and raw material, while cement industry is a large energy consumer, thus energy saving & consumption reduction becomes the long-term and important tasks of our cement industry and the key point of the implement of this objective is based on increasing of grinding efficiency and decreasing of power consumption on grinding. In actual production, pre-grinding system representative of roller press is the main leading of grinding. Pre-grinding could be divided into circular pre-grinding, mixer grinding, combined grinding and half-grinding. Compared to the 1st class closed grinding process of ball mill, combined grinding and half-grinding process has obvious advantages. Though half-final grinding system has better effect in the production updating, but its effect on energy saving is a little lower than that of combined grinding and the choice on equipment has relevant limitation, thus during actual engineering design, combined grinding process has been widely used. Especially in the updating project on current production, due to limitation of original equipment capacity(especially capacity of original separator is not enough), thus if choose to utilize half-final grinding process has larger limitation, however circular pre-grinding and combined grinding process has larger feasibility, while combined grinding process could gain better energy-saving and production updating effect.
  
B. Main measures for production improvement and power consumption reduction of combined grinding
  
a. average cement grain size
  
In the cement grinding, it is not even single grain size but a particle group with different grain size, thus when describing the fineness of cement, if just utilize residue to simply indicate: it’s almost 90% cement particle could pass the screen, but the grain size of these material under screen is not clear, thus same residue, the Blaine could appear the disparity. The average grain size of cement particle is
    
b. cement Blaine
  
Foreign cement standard indicates the index of specific surface, normally they utilizes Blaine method to test specific surface of cement, our national standard of Portland cement and clinker has been the same as that of foreign countries. Cement Blaine has better relationship with cement performances. Specific surface of finished cement and the strength of its physical mechanics has existed better relationship, while the specific surface of finished product normally is not very high which limited the exertion of hydrated activation. In actual production process, utilizes following technical measures to update the Blaine of cement to over 350m2/kg.
 
c. Cement grain size graduation
  
Proved by long-term experiment in foreign & local market, the graduation of cement particle is the key factor of cement performance, at present the best graduation of worldwide cement particle could be 3-32μm, while 3-32μm particle has played a important role on the updating of strength, its distribution on grain size is continuous, and the total amount should not lower than 65%; 16-24μm particle has great effect on cement performance, the more contents, the better it is; fine particle less than 3μm which is easy to form caking, thus it is better not to exceed 10%; more than 64μm particle has less activity. If the distribution of cement particle (graduation of particle) is not good, this will affect the water demand (workability) on hydration of cement, if add the water usage in order to reach the standard consistency of cement mortar, finally it will reduce the strength of cement or surface concrete after hardening. Thus getting hold of the index of graduation of cement particle is very important. The roundness factor of cement in foreign and local market, most is about 0.67. The average value of roundness factory of large & medium cement measured by building material developing & research institute is 0.63, the fluctuation is in 0.51-0.73. Proved by experiment, the roundness factory of cement particle is improved from 0.67 to 0.85, the compressive strength of cement mortar 28d could improve 20-30%.
 
Reasonable particle composition of cement means this composition could exert the gelation capacity of cement clinker maximum and implement the most close bulk density of volume. The clinker gelation capacity relates to the hydrated speed of particle and the extent of hydration, while the bulk density will be decided by content of particle in different sizes. Utilizing 45μm residue could make the enterprise understand the effective particle content of cement, while utilizing specific surface could grasp the fine particle content related to cement water demand. The combination of these two could control the parameters of grinding process to optimize the cement performances best.  
  
The time of full hydration of clinker particle larger than 45μm is very long, the contribution of cement strength is very small; the hydration production of clinker and gleization is the key reason of the gelation capacity of cement. The extent of hydration of cement particle could decide the exertion of cement gelation capacity. The extent of hydration of clinker relates to the kinds of mineral and the dimension of particle.
  
At present, particle graduation of well-known with best performance is: total amount of 3-32μm particle can’t be lower than 65%, the fine particle less than 3μm couldn’t exceed 10%, particle more than 65μm had better be 0 and there had better be no particle less than 1μm. As the 3.32μm plays great role on the updating of strength, especially the 16-24μm particle is very important for cement performances, the more the content is, the better it is; fine particle less than 3μm is easy to be caking, small particle less than 1μm could be hydrated during the mixing with water added, the function on strength of concrete is very small, and it could affect on the suitability of cement and additive and it could effect the cement performance and lead to the cracking of concrete which seriously effect the durability of concrete; the hydration of particle larger than 65μm is very slow, the contribution on 28d strength is very small.

In the fixed process, the 45μm residue of cement and the Blaine should be controlled inside a reasonable range which could prevent the particle less than 3μm and over 45μm, thus this could gain good cement performance and lower production cost. This fineness control method has the advantages of easy operation and effective control compared with other methods. Just need to sample, then screen and experiment and measure the Blaine which could become the evidence of mill operation.
  
Improving the output of cement grinding system and reducing power consumption is the key point people concerns, especially after the implementation of ISO standard, for most cement enterprises feel that they not only need to make the product meet the quality requirements on new standard, but also they don’t want to affect the mill quality and add the production cost, thus optimization on cement grinding system is obvious measures.
    
d. grinding method
  
Pre-grinding is the main measures for greatly improving the output of grinding system, pre-grinding normally means to set a fine crusher before ball mill which reduce inlet grain size, part of grinding task of original rough grinding chamber of ball mill should be transferred to be finished by higher fine crusher, after setting a pre-crushing, inner structure of ball mill should also implement relevant adjustment, especially first chamber should set up the target of updating the grinding capacity. Analyzing in theory, the grain size of inlet material after pre-crushed could decrease, the crushing and grinding function of first chamber will retreat the backseat, utilizing pre-crushing system improves the updating of mill output, low investment has max. advantage, it is mainly suitable for auxiliary equipment and the conveying equipment with limited extra capacity meanwhile it could largely update the enterprise with unreasonable production cost and benefit.
  
 (1)utilizes roller mill for pre-grinding before mill
  
Utilizes roller press to be pre-grinding equipment. Suggest to utilize the half-final grinding process means pre-grinding ball mill and separator makes up of closed system which gains more even material particle of mill, normally the particle less than 2mm could occupies about 90%, the max. particle should be controlled less than 5mm, shorten the stop time of material inside of mill, avoid the full grinding phenomenon. The output of ball mill pre-grinding process could exceed over 50%.
  
(2) utilizes high efficient separator

Necessary equipment of closed grinding is separator. Function of separator is to separate the particle with specific grain size from outlet material in time, reduce the over grinding amount inside of mill so that the grinding efficiency could be improved. But the separator can’t produce fine power itself, the choice and improvement of separator should be implemented together with the combination of mill. Of course, the efficiency of separator is high; output of system is high, too. The key technology of separator is scattering, classifying and collecting. Scattering should throw the material of inlet of separator as enough as possible, material particle should form specific space between them. Five-stage separating high separator has the advantages of Japanese O-Sepa separator cage type separates, the cyclone separating of rotor separator, separating of subway dust-collecting and separating of auxiliary air inlet; the scattering, classifying and dust-collecting mechanism is very definite, especially its classifying mechanism and centrifugal, cyclone type has obvious changes compared with rotor separator meanwhile each section of separator has reached a very high level, thus classifying efficiency is as high as 85%.
 
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