Modify cement grinding process and enhance product quality
After cement new standard carried out officially, it is found that to adapt ISO test method and requirements of new standard, the cement fineness must be enhanced. The specific surface area of cement should not be less than 350㎡/kg. Numerous tests and researches confirm that add superfine mixture into cement will not reduce strength but enhance breaking and compression strength notably.
At present, cement companies in China general use tube (ball) mill with small length-diameter (L/D＜4.0). Because of using same grinding process, lager mill feed material size, pursuing high output, adding mixture excessively to reduce cost and short retention period of grinded material in the mill, the finished products is with coarse fineness, small specific surface area, unreasonable grain grading, slow hydration and hardening velocity of cement and low early strength. Scientific research personnel from South China University of Technology once used clinker for same kiln and mills with different diameter & length to make cement. After comparison test, it is found that the specific surface area of cement from 3.2*13m long mill is large (＞480㎡/kg) and content of grain ＜30 micron is high in finished products. The cement is with reasonable grain grading and fast cement setting. The early and later strength are enhanced notably. The physical mechanical property of cement form 3*9m short mill is obvious lower than products form long mill.
At present, in order to reduce the fineness of the cement, preserve output of the grinding system, to make physical performance of finished products achieve new standard, some enterprises added high efficiency separator for the open-flow process, such as NHX high efficiency rotor separator. But because some relevant technical measure haven’t completed perfectly, grinding condition inside the mill is bad, it can’t create conditions for the separator outside the mill, and there’s so much cement coarse powder discharging from the mill, it get half the result with twice the effort.
一、Technical route changing for cement grinding process
The large scale production practice showed that large sized particles directly going inside the mill without pre-treatment will lead to reduction in grinding capacity of the mill. Irregular particles discharging from the mill tail and most of them are clinker with high strength and high quality. Lack of good grinding of clinker will not only cause strength exertion abnormal, but also cause material losses. Due to financial reasons, some enterprises which can’t change for large sized vertical mill just can only make technical transformation of existing grinding process.
Set material pre-treatment process of the mill, and replace the function of some part or all part of coarse grinding chamber, to compensate for the deficiency of the length of the cement mill. It can shorten the coarse grinding chamber correspondingly, extend the length of finely grinding chamber, increase the time for ground material inside the mill, and increase the contact time with grinding body, so as to get ground level and higher hydration activity products. Shandong Building Material Institute Researchers have once made transformation for 3×9m circle flow cement grinding system. Firstly, set pre-treatment process before into the mill, and reduce average particle size into the mill from 9.7mm to 5.3mm, then design gradation of grinding body in the mill. Secondly, transform the internal structure of the origin separator, improve separator’s classification efficiency, and properly reduce the cyclic loading rate. As a result, besides improving output and saving energy at the same time, specific surface area of finished products increased 70%, compressive resistance increased 65% in 3 days, which achieves obvious result.
二、Pre-treating process & equipment of material before mill
Utilizes pre-crushing process, the initial investment outlay is small and the flow is simple. Fine crusher produced loacally at present is based on the crusher produced in 70s of 20th century, in order for getting small particle, the time for limestone crushing is longer meanwhile due to hammber, material quality of protection plate, clinker temperature, the effect for getting material less than 5mm in short period is good, however it is bad for long period. If it is expected to gain long-term reliable effect, crusher with larger capacity should be utilized meanwhile closed screening & separating system should be configured. Power consumption of curhser per unit material for pre-treating is 4-5kwh/t, the operating ratio is low, thus iron separator should be set before crusher.
Roller press that utilizes crushing principle of material bed to extrude material for pre-crushing on middle of 80s of 20th century, at present many technical details has been completed, but the disadvantages of existing edge effect and the fast abrasiveness on roller surface hasn’t been solved out, thus maintenance cost is high, operating rate is low while process flow is complicated and power consumption of crusher for pre-treating on each unit is 3.5-5.0kwh/t. Utilizes roller press to pre-treat material whose initial investment is too much, normal enterprise is hard to accept.
Pre-treating material before cement mill is suitable to utilizes short & rough rod mill pre-grinding process. The obvious characters of this process is that no steel ball is utilizing as grinder inside of mill, but utilizing abrsive steel rod with high quality, meanwhile the rod has special function on optional crushing for material. The rod group shows the linear contact status, incoming material will be laminated by steel rod, the max. outlet grain size will be less than 2mm meanwhile over 30% of them are finished product, power consumption for pre-grinding is less than 3kwh/t. Compared with crusher and roller press, treating efficiency of rod mill is high, investment is medium, long-term operation is reliable, maintenance is convenient and cost is low. Material is small and even after treated by rod mill, grindability has been largely improved, high production and high fineness has been implemented and power consumption of grinding system has benn reduced. After setting rod mill, it could not only pre-grind material individually, but also centralized process grinded material.
三、Improvement on cement grinding process
Cement grinding is the reactiviaction process that utilizes mechanical force to fine crush and fine grind grinded material, tube mill could tranmit power to grinder via liner to implement effectively grind on cement particle during the rotation, obviously updated the total surface area of action which make them become the micro-size powder with higher hydration of cementitious activity.
Must pay special attention on improvement of internal of mill when utilizes pre-treating process before mill, no matter it is open-circuit or closed circuit. According to comparision, under same grinding condition, cement with different Blaine has different strength of cement mortar, the more finely material is grinder, the more total surface area of reaction is, the faster hydration and hardner rate is and the higher strength of cement mortar is.
Configuring pre-grinding process before mill obviously shorten the inlet grian size, the function of rough grinding chamber of mill has been partly or wholly replaced, it could shorten length of rough grinding chamber and extend length of fine grinding chamber accordingly meanwhile optimal the design on grinder graduation and load, the filling rate of first chamber should be lower than that of second chamber. Reduce the average size of grinder to enlarge the contact between total surface area of grinder and material and grinding proability, improve grinding efficiency to make the material be adequately grinded and ensure that finished cement has higher Blaine(＞35m2/kg).In the open-circuit process, it may replace normal diaphragm with separating type screen diaphragm with rough/fine separating (filter) function which adequately develop special grinding function of grinder inside fine grinding chamber. If utilizes screening diaphragm without conditions, distance between diaphragm ( ≤6mm) should be furtherly shortened, or the casted blind plate and grate plate as a combination of diaphragm to seriously prevent rough particle from entering into fine grinding chamber, so that the mill will keep highly fine and efficient for long time. Utilizing normal diaphragm, focus on grinding process, its grate plat difference shall no more than 8mm to control flowing speed of material and improve the grade of fine grinding. Meanwhile properly reduce circular load (≤100%) and outlet fineness (residuum 80um) to create conditions for high efficient separator and promote good virtuous circle of grinding system.
According to calculation, the surface area of one ton 10x10mm micro-forging is 265 times than that of one ton 25x35mm forging. The more grinders of unit weightis, the more total surface area is, the higher the contact and grinding efficiency between grinder and material is. Utilizes small ball type or segement type grinder in fine grinding chamber, must pay specialy attention to that the surface shape of liner effect on capacity updating of grinder. Proved by production, when liner is abrasived, the surface will be too smooth, though inner diameter of mill has been enlarged, but grinding efficiency is obviously reduced, fineness of outlet material is rougher. Concerning this appearance, the writer developed”activation technology of liner for fine grinding chamber”. Utilizing this technology could efficiently enlarge the friction factor of liner and drivision capacity of grinder, overcome the tangential slide in the external layer of grinder, eliminate the skidding due to traditional arraying of liner, adequately activate and strengthen grinding capacity of micro-grinder inside of fine grinding chamber, so the production efficiency will be reliable and product quality will be furtherly improved. Writer used to utilizes this technology in one Dia. 204x8m open-circuit cement mill of plant A, control hydraution fineness ＜80% without pre-grinding process before mill, the output of mill has been improved from 19tph to 22.5tph; utilizes this technology in 2.2x6.5m closed-circuit cement mill of plant B, seriously control fineness of finished product ＜2.50%, the output of mill has been improved from 9tph to 11.5tph, the ISO strength of cement reachs the standard and technical & ecnomoic effect are good.
When the surface of grinder inside of mill produce static electricity absoption and reduce the grinding efficiency, high efficient grinding aid without damage on cement performances should be imported to eliminated the damage for staic electricity and improve dispersity and fineness of material.
Others, according to records, oval grinder could obviously improve grinding efficiency and fineness of material, enlarge Blaine of product, improve the hydraution activiation and add the particle content （less than 30um）which is good for updating strength of ISO cement, thus enterprise under conditions should actively utilize.
For the small rate between length and diameter of cement mill for vertical kiln enterprise, if they want to improve Blaine of grinded product (≥350m2/kg), they should actively utilizes pre-grinding process before mill which is suitable for rod mill pre-grinding process as it not only could treat clinker individually, but alos can centralized –grinding all material to ensure the outlet grian size of material is ＜2mm and design grinder granduation based on this optimal design.
During the improvement inside of mill, utilizes “screening diaphragm, micro-grinder(≤15mm), meanwhile utilizing “activation technology of liner for fine grinding chamber”, eliminate tangential slide of grinder, improve grinding efficiency of mill and the fineness due to grinder.
The enterprise under conditions could utilizes oval ball or grinding aid to improve the dispersity, Blaine and grinding fficienty of material.
The above mentioned solution is effiective not only for open-circuit, but also for closed circuit or other grinding process, and they could improve the partical content (less than 30um) in finished product and hydraution gelation activiation to meet the new requirements of cement industry.