Technical analysis of high production, energy-saving and lower consumption for cement grinding system
Key words: mill –energy-saving
Abstract
Cement particles is an artificial particle, the cement particles have two features, high specific surface area and polydispersity.
The general expression of the cement powder: Ground Level (fineness and specific surface area), particle distribution and particle shape.
1. cement fineness
Cement granularity is the cement fineness. Cement fineness directly affects the concrete condensation, hydration, hardening, strength and other series of physical properties.
Our national cement standard prescribes fineness of cement products 80μm square hole sieve shall not exceed 10%. Controlling method is simple, so on the certain condition of grinding condition, the concrete strength has a certain relationship with its fineness. The smaller the residue on sieve is, the finer the cement is, and the higher the strength is. However, controlling the quality of the cement by using this method may cause many problems as followings:
⑴ When the cement is ground finely, for example, if 80μm square hole screen residue is less than 1%, control is not much significance. Cement abroad is commonly ground finely, so this target is almost cancelled from foreign cement standards.
⑵ When grinding process changes, the fineness value also changes. If the screen residual of open circle mill is too large, and mill screen residual of close circle mill, sometimes it is difficult to control fineness of cement according to its fineness.
⑶ Fineness value is screen residual of 0.08mm sieve, that is, particles content of cement≥ 80μm (%). As is known, Hydration activity of cement particles ≥ 64μm is very low. So the number of particle concentration by ≥ 80μm quality control of cement can not fully reflect the real activity of the cement.
2、The average particle size of cement
During the cement grinding process, it is not an even single particle, but the granule containing different size of particles, so when commenting fineness of cement by using this simple way of screen residual, almost 90% more cement particles through the sieve into the next screen material, but these particle size of objects under screen is not clear, so when the screen residual is same, specific surface area is different. There are several representation of average size, such as, arithmetic average diameter, geometric average diameter and harmonic average diameter.
The average size of cement particles is an important characterization of cement particle geometric parameters, but it can provide very limited information on particle size characteristics because two particles with same average particle size, may be combined by two different granularity.(particle size distribution)
3、Specific surface area of cement
Most of the abroad cement standards stipulate the specific surface area, and generally Blaine specific surface area is used to measure specific surface. Standard of Portland cement and clinker in China is consistent with the foreign standards. There’s good relationship between specific surface area of cement and cement performance. However, the specific surface area controlled by the quality of cement, there are two aspects of deficiency:
⑴ Specific surface area is sensitive to the amount of fine particles of cement, and sometimes specific surface area is not very high, but due to the reasonable distribution for the cement particle size, strength of cement is very high.
⑵ Cement specific surface area containing with mixed materials can not reflect the total appearance area, if it is mixed with pozzolanic admixture, cement specific surface area tends to be high.
4、Cement particle size distribution (particle size)
As we all know, even if the same screening fineness or similar specific surface area, the performance of cement sometimes will show a great difference, which reason is different distribution of size (particle shape factor is also very important). As a result, it is significant to study the cement particle characteristics, exploration and more precise quantitative strength of cement relations.
Long-term experimental studies from home and abroad have shown that, the cement particle size distribution is a determinant of cement, and currently the best article size distribution of cement is recognized as:3-32μm particles plays a major role on the strength, and its size distribution is continuous, its total amount should be no less than 65%; cement performance of 16-24μm particles is particularly important, the more the better; fine particles of less than 3μm, easy to knot, not to exceed 10%; particle of more than64μm has small activity, it is better not.
In addition, the cement particle size distribution (particle size distribution) will also affect the improper water demand of cement hydration (workability), if the standard cement mortar in order to achieve consistency and improve water consumption, the end will reduce the hardened cement or concrete strength. Therefore master index of cement particle size distribution is very important. That the size distribution of cement particle size distribution methods are a list of method, mapping method, matrix method and function.
In the 1990s, people began to study the performance of cement appearance to cement particle performance. If the cement particles are put under the electron microscope, we can find it is not a round shape, like a heap of crushed limestone, small angular, large angular, flake-like, and acicular. Cement particles appearance is related with the grinding process. Cement particle appearance is usually expressed by roundness coefficient (f). Roundness coefficient of round particle is 1, while roundness coefficient of other shapes is less than 1. Roundness coefficient abroad is mostly about 0.67. China Building Materials Science Research Institute have determined cement roundness coefficient from some large and medium-sized cement companies, and the average coefficient is 0.63, fluctuation between 0.51-0.73. At the same time from the study of cement particle appearance, it is found that the stronger the grinding ability is, the larger the f value has, for high-fine grinding cement, value f is the largest. F value for cement ground by mill with a roller press pre-crushing is large too.
Experimental studies have shown that when the roundness coefficient of cement particle is increased from 0.67 to 0.85, the cement mortar 28d compressive strength can increase 20-30%. After implementation of the ISO strength method, the improvement of the fineness is by cement grinding process in most backward companies and the using 80μm square hole sieve fineness to control the fineness, and the majority of particles is in a state of irrational.
Combination of reasonable cement particle is to maximize the gelation capacity of cement clinker and the tightest bulk density.Clinker pelling property is relevant to hydration speed and hydration degree of granules, and packing density depends on granule size content ratio. Using 45μm screen rejects it can make enterprise get idea of content of efficient granule in the cement, while using specific surface area it can get hold of the content of micro-sized granule in time which is closely related to water requirement of cement. On their combining grinding process parameters are controlled, it will make cement performance reach optimization.
1. Clinker granule with fineness＞45μm which entire hydraulic time is very long, and has less contribution to the cement strength. Hydration products which produced by clinker with water reaction is the root cause of gelling property generated by cement. Cement gelling property capacity depends on hydration degree of cement granule. Hydration degree of clinker is relevant to mineral species and granule size. According to the research, the relation between hydration depth and time of silicate cement can be showed by the following formula:
X=2t0.25
in the formula: X- hydration depth, μm；
t- hydration time, d
Time of complete hydration for the particle which size is 20μm needs more than one year but only 1.5h for the particle which size is 2μm, and the quantity of particle which size is 45μm which is hydrated in 28d is percent 50 approximately. The particle which size is above 45μm has little contribution to the cement performance. At present more accepted cement optimum performance for particle size distribution is that: the particle which fineness is 3 to 32um should not be below 65 percent, the particle which fineness is less than 3um does not exceed 10 percent. The quantity of particle which size is above 65um is 0 the best, and there is not the particle which size is bellow 1um the best. The particle which size is 3-32μm pays a leading role to the strength increase. Especially the particle which size is 16-24μm especial is important for the cement performance, whose content is the more, the better; the fine particle which size is below 3μm is knotted regiment easily. The fine particle which size is below 1μm hydrated quickly with stirring in water, which has the little effect to the concrete strength and influences the adaptability of the cement and the additive. It can also influence cement performance that can result in crazing of concrete and durability of concrete. The particle which size is above 65μm has a slow hydration speed and little contribution for the 28d strength.
2、Specific surface area value reflects mainly content of particle which size is bellow 5μm
The operation is easy and simple, control is effective and a large number of trail investments are not need in fineness controlled by 45μm material retained and special surface area
Output increased and power consumption of cement grinding system are the focus which are paid attention to by people, after ISO standard being put into practice especially. For the most of cement enterprise, they feel that optimization improvement for cement mill system is the first measure.
1、The principle of improvement in grinding process
The research of grinding process originally mainly paid attention to mill output increased and grinding power consumption reduced. In fact, the grinding process has the great influence to the product quality. So the relation of output、quality and energy consumption shall be considered totally when the researching and grinding process improvement being proceeded in the future.
⑴ principle of energy conservation
For the low energy utilization rate of traditional ball mill grinding process and the percent 70 power consumption being used for the grinding of raw material and cement. For this reason the basis task of grinding process improved is energy conservation.
⑵ principle of good output
The general objectives of improvement and perfection for grinding process are output of grinding equipment enhanced.
⑶principle of superior quality
Certain fineness and specific surface area of product are achieved and the reasonable gradation and spherical particle with high proportion, which is the important task with improvement and perfection for grinding process.
2、 Adopting precrushing technology
Precrushing is the major measure for increasing outputs of ball mill grinding system by a large margin, and which can be divided into precrushing and pregrinding by crushing theory.
2.1 Precrushing
Precrushing in generally means setting a fine crusher in front of the ball mill, which makes feed size reduced and delivers part coarse crushing task by the original ball mill coarse grinding chamber to the efficient fine crusher finished, that it is so-called “much crushing less grinding”. There has been decades of history of equipping fine crusher in front of cement mill interiorly, but because of the limitation of equipment material, this technology has not been used widely until now. At present some machinery plant launched a new generation of fine crusher which service life improved at a certain degree, but the problem of wearing in key parts has not been fundamental improved.
Outbound material for removing iron must be attached attention, sometimes the briquette or the other metal impurities can cause fatal damage to the fine crusher. Ball mill inside construct should be adjusted correspondingly after adding the precrushing especially in first chamber which use improving grinding ability for target. Some factory once try to increase output by increasing rotating speed of mill but the effect was not good. The feed size reduced after adding precrushing by analyzing in theories as well as the crushing effect and the grinding effect played second fiddle of the first chamber. This apparently does not conform to the requirement of Grinding speed enhanced, hoisting height of grinding body increased, crushing capacity enhanced while grinding capacity reduced.
The improvement of mill output increased by the precrushing system which has the max superiority is low investment. The system is major suitable for the vender with limited ability but enough mill auxiliary equipment and conveying equipment and the unreasonable cost efficiency resulted in promotion by a wide margin.
2.2 Pregrinding
Pregrinding in generally means adding a unit grinding device, and it can make the output of original grinding system increased grandly.
The equipment which is used in dry pregrinding has short ball mill、rolling mill、press roller、drum mill etc. The energy availability factor of the four pregrinding equipment above is short ball mill, rolling mill, drum mill, press roller from low to high in sequence.
If the pregrinding equipment is used for ball mill, the semi-finished grinding system is advised to use, that is closed loop system which consists of pregrinding ball mill and powder concentrator and it make the material in the follow-up mill still more even. Generally the material which the size <2mm covers around 90 percent, and the max size is controlled under 5mm, which can shorten the retention time of material in the mill and avoid the phenomenon of saturation of ball mill. The rang of output increased in ball mill pregrinding process can reach percent 50 above, but it has the lower energy conservation effect. The process is suitable for the vender which has idle equipment.
For large investment and relative complex process, cement enterprises with vertical kiln rarely use roller mill, roller press and tube roller mill as pregrinding device.
3、Technical transformation of open-circuit mill
Open-circuit, high fineness and high output mill is mainly used in cement grinding. The transformation of the original mill should have following working conditions:
⑴ Range of the mill diameter is Φ1.5-3.8m. The mill length-diameter ratio should be ＞2.5 at least.
⑵ Comprehensive moisture of material into the mill is <2%.
⑶ Size of material into the mill, grinding medium loadage and mill operation are normal and stable.
⑷ Good mill ventilation, dust collection and metering device.
3.1 Main content of open-circuit mill technical transformation
⑴ lining plate
After long time production practice, 11 kinds of ball mill shell lining plates are still used at present. The lining plates made by foreign companies are unified gradually. The first chamber usually uses lifting lining plate that is stair lining plate. The second chamber uses grading lining plate. This grading lining plate is not the domestic common cone type or flat lining plate adding cone type lining plate. It is the combination or compound of two even three kinds lining plates. After optimal combination or compound, this lining plate can express advantages of different lining plates thus insure energy entering into ball area furthest and eliminate dead zone in the mill as much as possible. We suggest units concerned intensify study to provide more preferable lining plate for cement plants. When transform open-circuit mill technical at present, the section chamber is usually equipped with activate lining plate to eliminate detention zone effectively as well as arouse and strength the movement of the grinding medium.
⑵ diaphragm
For the transformation of diaphragm device, the domestic enterprises still pay attention to wear resistant, impact resistance and anti-plug of grate plate but not enough attention to material inside the mill, airflow influence and control function. Take Φ2.2m ball mill for example, the efficient ventilation area of diaphragm is 0.38m2, centre piece area is 0.33m3 and the efficient ventilation area of centre piece is 0.03m2. The area of centre piece is 87% of the efficient ventilation area of diaphragm device. It also shows that the efficient ventilation area of centre piece is quite small under this form. After analysis and comparison, the most feasible scheme is to enlarge the ventilation area of centre piece. Enlarge grate plate hole size or increase hole quantity will impact the strength of grate plate and its control function on pellet. Furthermore, another purpose of transforming outdated centre piece is to control material velocity, thus to adjust pellet ratio of each chamber in the mill expediently and control material retention time in the mill.
When transform the technical of open-circuit mill, change the diaphragm with inner screening device in tail chamber and control granule in to tail chamber strictly to make steel balls in front chamber and sections in tail chamber crush and grind furthest.
⑶ grinding medium
The size of grinding medium is basis on grinding capacity and feeding size. The common grinding medium is small ends and large medium. For different condition in every plant, complex movement of material & grinding medium inside the mill and performance difference of the material, it is hard to find applicative regularity of common uneven. Try to find suitable grading in long time practice is the efficient way. Stable grinding process condition to a large extend depends on the material of grinding medium. The grading of grinding medium constantly changes during mill operation due to wear consumption. Wear discipline is different for different size of grinding medium. Supplementary ball (section) can only keep relative balance of loading capacity not be consistent for grading. If grinding medium is with bad strength and wear resisting property, it will transform and break during the operation. It will affect grinding efficiency. The fragments will block grate plate hole, make the discharging of diaphragm device difficult and worsen operation in the mill. Therefore, enhancing the quality of grinding medium is the powerful guarantee for long-time stable operation of the mill. Otherwise, it is hard to reach expected effect by more reasonable grading scheme. From economy aspect, big consumption of grinding medium will affect grinding capacity. Frequent stop and supplementary ball will result in low operation ratio, unstable working condition and higher grinding cost. The largest consumption of common steel ball for grinding 1t cement is 1000g/t. The period of supplementing ball is half a month. Wear resistant ball such as bearing steel ball, high chromium ball, low content alloy ball etc will reduce the consumption to 30-40g/t only for 1/25-1/30 of common steel ball. The period of supplementing ball is more than half a year. The common steel ball is 4000yuan/t. The wear resistant ball is 7000-8000yuan/t. Though the investment is higher for using wear resistant ball, its excellent property can lighten working for cleaning chamber and supplanting ball, enhance grinding capacity, reduce grinding cost and bring considerable economic benefit.
During technical transformation of open-circuit mill, we adopt miniature grinding medium to strengthen grinding capacity of tail chamber. The diameter of the litter section is 8-12mm. Its quantity of unit mass is 20 times than common steel section. Its total specific surface area is 2.5 times than common steel section. The grinding efficiency is in proportion of 0.5-0.7 of grinding medium specific surface area. Adopting litter section can enhance output, increase specific surface area of the products and improve micro powder grain composition.
⑷ Material separating device
For miniature grinding medium, it is necessary to design the discharging grate plate device to discharge fineness powder but not miniature grinding medium out of mill.
⑸ Reasonable technological parameter set
After transformation, chamber position, grinding medium grading and fineness control of high fineness and output mill should be designed according to cement variety, clinker grindability, mixture variety, mixing ratio and mill specification etc.
3.2 Technical index after open-circuit mill transformation
⑴Output increases 20-35%, power saves 17-25%
⑵Cement specific surface area is 300-350m2/kg.
⑶Grinding medium consumption can be reduced more than 25%.
3.3 Miniature grinding medium eliminates malignance grinding appearance during high fineness grinding.
⑴Formation of malignance grinding appearance
During grinding of open-circuit cement mill, pellet (section) rate (material percentage of grinding medium) in the chamber reduces along with mill output reducing per hour. Specific surface area of grinding mill is higher, the mill output per out is lower. The pellet (section) rate is lower, in other words material in chamber is less. And the capacity of grinding medium is larger. As a result, the crash between ball and ball, section and section, ball and lining plate will be fiercer during the operation.
If the specific surface area of portland cement mill is more than 320m2/kg, there will be fish scale plates in cement powder. If specific surface area reaches to 350-400m2/kg, mill output per hour will reduce dramatically and the fish scale plates in cement will increase. The cement fineness and the increasing of specific surface area will be blocked. The temperature in the mill will increase rapidly. When it reaches to 400-500m2/kg, even spray the mill, the cement temperature out of mill is more than200℃ and gypsum dehydration is 30-50%. Cement fluidity and grain size grading property becomes weak observably. As the result, remain time of material in the mill is too long and grinding impact times increases manifold. Therefore, cement micro grain is grinded and compressed in overlong time and strong external force from grinding medium to cause malignance grinding appearance such as cement clot, concentration and rapid hardening as well as cement wrapping balls, section and clinging lining plate and grate plate.
（2）Miniature grinding medium can eliminate “Malignant grinding phenomenon” effectively.
Mr. Jiang Yongcai, deceased cement grinding expert, professor of Hefei Cement Research & Design Institute, inventor of high-fineness and high- capacity mill, during researching open-circuit high-fineness mill, when grinding the Portland cement with the specific surface area 400-500m2/kg, there is no “Malignant grinding phenomenon” in the mill. The best parameters of the chamber are: material section rate is 12%, fill rate is 23%, average dimension of mini-section is 12mm, grinding condition is normal and good.
The differences from the common open-circuit mill are: The common open-circuit mill only uses the large size grinding medium, mostly are Φ18X22mm, Φ20X25mm, Φ25X30mm, their single weight is 10-20 times heavier than the single weight of mini-section, even 30 times.
3.4 Technological transformation of close-circuit mill
Because the mill specification increases and the existing mill has the needs of energy-saving, high capacity & high quality, close-circuit is an inevitable trend in cement grinding process.
4.1 Separator
The necessary equipment in the close-circuit grinding is separator. The function of separator is: timely choose the particles with the certain size from the discharging materials of mill in order to reducer the over-grinding quantity in the mill, thus the efficiency of mill grinding system can be improved. But the separator cannot produce fine powders itself, so the separator shall be chosen and transformed combined with the mill transformation. Of course, in general, if the separator efficiency is high, the system output is high.
The key technologies of separator are: separation, classification and collection. Separation means that the materials entered into the separator must be splashed out as far as possible, and there must be the certain spatial distance between material particles. Thus, the structure of splashing plate, rotary speed, splashing space, material moisture and material flow will affect the scatter coefficient directly; Classification means that after the materials have been splashed, within the limit period staying in the separation chamber, make full use of separation function of air flow to separate the coarse particles and fine particles as far as possible and sent them to their respective outlet. Thus, the air flow, air velocity, airflow way, gas-solid intersection, flow field distribution, quantity and structure of separation room will affect the classifying efficiency seriously; Collection means the capacity of catching coarse powers and fine powders, and it is concerned with the collection way and the structure of collecting parts.
Japan Onoda Company developed O-Sepa separator in 1979, it not only keeps the advantage of outer circulation of cyclone separator, but also adopts the separating principle of cage rotor plane spiral airflow, so that the separating efficiency can be improved greatly. The cage separator taking it as a representative is called the high efficiency swirl separator, also named as the third generation of separator after centrifugal separator and cyclone separator. Normally, its efficiency is above 80%, compared with the centrifugal separator and cyclone separator, the high efficiency swirl separator can improve the mill output 15-40%, save energy 10-20%, it has the advantages of small volume, light weight, arranged flexibly, the specific surface area of products can be adjusted within 300-600m2/kg, the system is negative pressure operation and no dust pollution.
Because the O-Sepa separator has no fine powders collection device, it shall be equipped the large bag filter or electrical precipitator matching with its treatment air flow for collecting the finished products, thus the system investment must be increased greatly, and makes the process layout complicated, the operation control also becomes difficult, so its popularization and application will be restricted to some extent. Mr. Zhang Shaoming, the professor of Nanjing Institute of Chemical researched and developed the rotor cyclone separator in the 1990s, and it was called the rotor separator for short. It is the small & medium-sized high efficiency separator used in the vertical kiln cement plant, and it is formed by combining the principle of cage rotor separator with the cyclone separator. For three key technology “separation”, “classification” and “collection”, compared with the cyclone separator, its structure has a breakthrough improvement. In the same output, its efficiency is quite compared with the high efficiency swirl separator, but the system investment can be reduced 20-30%; compared with the cyclone separator and the high efficiency centrifugal separator, it can be not only reduce the equipment specification, but also improve the efficiency 20-40%.
Along with the implementation of cement new standards, each cement company improves the product fineness and specific surface area currently, put forward the higher requirements for the rotor separator used for cement mill, so all manufacturers also improve constantly. For example, the inner circulation separator developed and manufactured by Jiangsu Kehang Company, it has been through Jiangsu provincial technology and product identification in march this year, it is the five-stage high efficiency separator integrating the suspension separation of German dynamic separator, cage separation of Japan O-Sepa separator, cyclone separation of rotor separator, bypass dust-collecting separation and auxiliary air inlet separation; its mechanism of separation, classification and collection is very clear, especially its classification mechanism has a breakthrough change compared with the centrifugal separator, cyclone separator and rotor separator. Each link of the separator has reached a high level, so the classification efficiency can reach 85%; it can thoroughly overcome the traditional separator’s disadvantages of wear, vibration, wind unevenness, low cement strength in the early stage from the structure; it adapted well to high capacity mill, high fineness mill, short mill, long mill and high moisture material, thus it is the classification equipment having obvious advantages.
4.2 The process adjustment of changing the open circuit grinding to close circuit grinding: The necessary process adjustment after changing the open circuit grinding to close circuit grinding mainly includes:
(1) Grading of steel ball. The average ball diameter in No. 1 chamber will be increased properly.
(2) The grate hole clearance of chamber diaphragm can be enlarged properly to increase the flow velocity of the material in the mill.
(3) Enlarge the feeding reamer of hollow shaft in the mill head.
⑷ Fineness control, relax the raw mill properly, the screen residue of 80μｍ hole can be controlled below 10%. Improve the fineness of cement mill, and it will be about thinner than original open circuit grinding to ensure the cement strength.
4.3 Improve the specific surface area of close circuit mill
The relativity of the specific surface area and physical mechanical strength is quite good. In a certain sense, improving the specific surface area of cement and increasing the grinding fine degree is one of the effective ways to improve the cement strengthen. Because the particularity of close circuit grinding process, the classifying precision of separator, the grading of grinding medium, etc, the specific surface area of its finished products is not high, so that restrict the playing of hydration activity. During the actual production, the following technical measures can be adopted, improves the specific surface area of cement above 350m2/k.
⑴ Actively adopt the material preprocessing technology before the mill, strictly control the max. feeding size less than 5mm to reduce the load of No. 1 chamber, properly shorten the length of No. 1 chamber, and prolong the No. 2 chamber.
⑵ Optimize the grading of grinding medium according to the feed size, reduce the average size of grinding medium, and increase the contact surface between grinding medium and material to create more micro powders.
⑶ The fill ratio of No. 1 chamber shall be 2～3% lower than No. 2 chamber, and the lining boards in No. 2 chamber shall be activate arranged, such as using classifying lining boards, etc. Activate the grinding medium and make full use of the fineness function of grinding medium.
⑷ Properly reduce the circulating load of grinding system, and it is best to be controlled ≤150%. Meanwhile, the circulating air flow of separator can be reduced properly to make it separate the thinner products.
⑸Adopts powerful ventilating and dust removal measures, in the mill air speed should be controlled at 1.0～1.5m/s。Draws on the features of closed circuit grinding process，in recent years begin to research the new type closed circuit grinding system that is composed of open circuit high fine and high production mill and high efficient separator, after production practice the effects are very distinct, compared with open circuit grinding system and ordinary closed circuit grinding system its increasing production and energy saving can improve from 30% to 80%, and it supplies the new technological method for increasing production and energy saving in the grinding of cement plant. There are so many closed circuit cement mills that use old-fashioned separating system; the specific surface area of cement produced from this system is on the low side, micro powder of cement is small, and intensity in early stage is not enough. In addition length ratio of mill is not reasonable; the gap between diaphragm and discharge grate is large; materials that can not be super crushed in the crushing silo enter into grinding silo so that grinding capacity of the grinding silo is not enough and spitting slag at the mill rear is very serious. It pollutes the environment, increases labor intensity of the workers and cement output is very low. Therefore Hefei Cement Research and Design Institute have carried out the technological transformation for the current closed circuit cement mill by using the technology of high production and high super mill. Material flow in the closed circuit mill is large; meanwhile it is decided by the material undulation, therefore the sieving device between crushing chamber and grinding chamber should be suitable for this working condition. Not only control the size, but also guarantee the flow. When confirming sieving proposal, should suitably adjust the chamber place, optimize grade matching and fill rate under the condition of improving specific surface area and output of cement, meanwhile adopts special discharge device. According to efficient control condition for the material size passing the sieving device, in the grinding chamber mainly use mini type grinding body and intensify the grinding capacity to increase the micro powder in the cement and improve the acceptability of discharge fineness. After transformation it can make 5-25μm micro powder increase 10-15%，in three days compressive strength of cement can improve 3.9MPa，specific surface area of cement can increase 20m2/kg，and mill output can improve 10-15%。
1、Existing defects in old-fashioned mill
1. Adopts sliding bearing: friction factor is large; start-up is difficult; operating resistance is large; during the operating process main bearing produces many friction heat and oil and water is not enough so easily happen accidents, and these things lead operating rate of mill is low, maintenance is large and operating personnel increase.
2.High power consumption: friction factor of sliding bearing of old-fashioned mill is from 0.04 to 0.08, energy consumption of main bearing occupies 11-15% of installed capacity, because of grinding structure in the mill the grinding efficiency is low, power consumption is from 25 to 30KWh per ton cement, and meal materials is from 14 to 18Kwh per ton meal material.
3. High oil consumption: Under the effect of positive pressure between bearing bush and hollow shaft and high temperature lubricating oil of sliding bearing easily produces deterioration, lubricating oil consumption is high, some mill seal is not good and the oil consumption will further increase, about old-fashioned 2.2m mill annual oil consumption is from 1 ton to 2 tons. Large-size mill needs the matched lubricating station, the investment will increase, in the operation it will have the cost, it has the large effects for the safe operation of the equipments, and the equipments usually need maintenance and repair. Oil stains produced by thin oil also influence the civilized production.
4. Low output: Friction factor of sliding bearing is large, stating the mill is difficult, current is high when start it, result in much more mills can not reach rated loading capacity; influenced by the backward grinding process, technology of the grinding structure in the mill is out of date, grinding efficiency is low, gradation of grinding body is not reasonable, all of these can lead low output of the mill. In the cement plant as usual outputs for closed circuit cement mill at Ф2.2x7m and Ф2.4x13m are 10--13t/h and 28—30t/h，modern mill can reach 18—22t/h and 32—36t/h。
5. Installation and maintenance is difficult: when install Babbitt metal bush need long time for scraping and grinding-in, meanwhile the maintenance and repair is large.
6. Grinding structure in the mill is out of date: many old-fashioned mills adopt ladder liner; some mills adopt annular groove liners and hyperbolical surface; diaphragm also adopts single, second chamber is not equipped with activating device, ad the chamber rear there is no discharging control device, so all these things have influenced the grinding efficiency of the mill.
2. Reformation for old-fashioned ball mill
1. Reformation for main bearing
2. Reformation in the mill: Liner, diaphragm, pellet balance device
3. Reformation for ball mill process: According to features of materials newly determines length ratio of chamber, adjusts
4. Transformation for diameter expansion: for the shells of Φ1.83m, Φ2.2m and Φ2.4m short mills should be over striking. The premise is that transform the main bearing, meanwhile original motor, reducer, bull gear and pinion, and foundation is not changed, the output can improve above 25%.
5.Transformation for lengthening: make the short shell of cement mill lengthen so that it is suitable for new standards of cement and improve the output.
Transformation for improving speed: rotary speed of old-fashioned mill is low, majority grinding body is not in the perfect working condition, after improving the speed grinding times are increased and energy of motion is increased. Transforming method: main bearing changes into rolling bearing, and after satisfying the power change the speed ratio of the reducer.
1、Differences of grinding functions between ball and bar
All the chambers of mill is provided with the crushing and grinding function,just the primary and secondary and degrees are different. The main function of fine grinding chamber is grinding, but grinding capacity of small ball and small steel bar is different. Materials are filled among grinding medias, the grinding efficiency is mainly decided by the connecting surface area between grinding media and materials. If the connecting surface area is large, the chances for grinding is much, production rate of finished products in unit time is high. Same quality ball compared with bar, because of line connecting method of the bar, it has much more connecting surface area than the ball. For single chamber under the same grinding loading capacity and feeding fine materials production rate of steel bar chamber is higher than ball chamber in the unit time, this is proved by the grinding theory and practices. It is pointed that at present compared with the materials the size of grinding media in the fine grinding chamber is too large, there are some reasons such as width of grate gap limit. High fine mill developed by Demark and our country is solved the problems better, fine grinding chamber successfully apply micro fine steel bar and obviously improve the grinding efficiency. Of course it is the key that adopts high efficient sieving diaphragm and rollback device at the mill rear. Therefore should be defined that for fine mill and super-fine mill the grinding efficiency of bar is higher than the ball’s.
2、Reasons for changing the grinding body in the cement fine grinding chamber into small ball
At present cement mill at home and abroad in the fine grinding chamber the small steel ball will be instead of steel bar, its reasons are:
⑴Energy consumption that use the small steel ball is lower than using small steel bar
⑵Grinding consumption of high quality small is smaller than steel bar
⑶Cement granule as the sphere ground by the small steel ball is much more than ground by the steel bar, but using the steel bar can make the material speed in the mill step up, it can prevent cement clot in the mill.
Generally speaking grinding function of mill includes crushing and grinding, optimization of operating condition of mill is that make crushing capacity and grinding capacity reach balance, and then grinding efficiency is improved, meanwhile output and fineness of finished products is even on the perfect level, this is the most essential principle for solving the grinding problem. Exactly analyze the matching conditions between crushing and grinding capacity under the different operation conditions, it is the judge to decide grinding body of fine grinding chamber to use steel bar or the steel ball.
3、Necessary and full conditions for fine grinding chamber selecting the small steel ball
⑴ Closed circuit grinding
For the open circuit grinding materials in the mill is passed one time, and out from the mill materials is finished products, so required grinding capacity is higher. In the closed circuit grinding certain material circulating load should be ensured, no matter adopt centrifugal separator or high efficient separator, generally speaking residue on sieve of discharge fineness at the mill rear is controlled in 30-40%，so for the required grinding capacity is relatively lower than open circuit mill. In order to guarantee the fineness of finished products, generally speaking fine grinding chamber of open circuit mill should adopt steel bar. Fine grinding chamber of closed circuit mill can adopt small steel ball, on one hand it can accelerate flow rate of materials and increase throughput; on the other hand residue on sieve of materials fed into fine grinding chamber should higher than open circuit open, there are some advantages to guarantee the impact from certain steel ball.
⑵Pre-crushing
The Pre-crushing has a class or multi-stage and streams or circle; it decides the feeding materials size. Present efficient silvery machine、roller presser and so on can obviously reduce the feeding size, even about 80% materials’ size under 2mm, actually this have completed large part of the function of mill Ⅰ position, relieving the burden of the mill. If the effect of pre-crushing is good, then the length of the Ⅰ position will cut down, and the steel balls’ general sphere diameter will descend. And that the descend of the steel balls’ general sphere diameter can enhance the grinding function of Ⅰ position, the materials down into the fine grinding storehouse relatively reduced, thus relieve the grinding burden. If the feeding size stability in the fine level, open-circuit mill’s fine grinding storehouse can use small steel ball. Both assure the fineness and increase the output. Conversely, if the pre-crushing is bad, the mill’s Ⅰ position completely be changed into crusher storehouse. Then enhance the fine grinding storehouse’s burden. Even if the closed-circuit mill can not use the small steel ball. Although adjust the air separate can control the fineness, but for the shortage of the grinding ability ,the output will reduce.
⑶length of mill
This is main point at the open-circuit mill. Presently, quite a few cement plant use open-circuit mil, and the open-circuit mil general be divided into three to four store house. The length of the mill decide the grinding time. The long mill’s grinding time is longer. Besides compare with the two-bin short mill, long mill’s reasonable long position makes the grinding function more clear. The grinding efficiency great improved, then use small steel ball is on premises. If use forging steel right now, both reduce the materials speed and cut down the output, and it will cause the more grinding event easily. To produce a paste and contrary to some effect, and reduced the grinding efficiency.
⑷scale of chamber length
This is main point at the closed-circuit mill. Presently, two-bin closed-circuit mill’s Ⅰ、Ⅱ storehouse’s length are not completely the same. There is the scale of 1:2, and near 1:1. The scale of 1:2 is normal range, now the Ⅱ storehouse use small steel balls. If the length of the two storehouses is close, then it will cause Ⅰ storehouse’s grinding ability excess and that Ⅱstorehouse’s not enough. If still use small steel balls, then Ⅱ storehouse can not finish the grinding task in the relative decrease grinding volume, at last result in a diminished output.
⑸variety of ground cement
This is main point at the cement. Deferent variety of the cement, the fineness of the grinding is also different. For example:
a. superhard （superfine）cement
It demands cement quick hydrate、high early strength. Besides the demand of the mineral composition, it’s rigid of the cement fineness. It set high demands to the mill’s grinding. Now whatever open-circuit long mill or closed-circuit mill should consider using the small forging steel in the fine grinding storehouse. And it should be careful of the use of the steel ball. From the present application practice, the effect of the superfine cement is good when use the forging steel grinding.
b. multiple composite materials cement
In order to reduce the production costs, the plant add more composite materials as far as possible. AS some plant add slag reach the ratio of 40-50%. Slag is hard to grind, for grinding in common, the grinding function of the mill should be powerful. When high-adding, the ratio of the epoxide of the slag and the fine materials is high, however the grinding function of these material is limited. Fine grinding storehouse should take priority use of small forging steel, or even if use the efficient separator is hard to improve the output. Because of the grinding ability is n, ot enough, the output in the mill tail unloading is limited, if improve the mill’s cycle load, thus the mill is not adapt for it.
⑹ the spherify of the cement grain
As mentioned, the higher degree of the ball shaped of the cement grain, the higher strength of the cement. In order to improve the cement strength and give full play to the clinker’s effect. It should create condition if the demand of the ball shaped of the cement grain is high, and use small steel in the fine grinding storehouse of the cement mill.
4、elliptical ball’s application in the cement grinding
We get enlightenment from the baseball mill can reduce the grinding’s power consumption, and then the elliptical ball appeared. It has the advantages that the round ball is not have, and the elliptical ball has used in the cement grinding.
⑴The round ball compared with the same diameter ball, weight increase, wallop enhance; compared with the same weight, low gravity of the elliptical ball ,cant stability is good. The hoisting height is higher than the round ball, and the crushing capacity is large. And it’s better stability make the grinding medium fall motion of diarrhea shearing action greatly enhanced.
⑵The different curvature radius of the round ball’s surface can form the different grain size’s contact angle. The chance of contracting with different grain size is greater than the sphere. The computer simulation results show that the efficiency contact area between oval ball and material is 30% larger than round ball，and makes arc-shaped clamp to different size material in the contact angle，thus it has selective grinding and good sieving function for material，makes grain composition and grain feature get effective improvement.
⑶Arc-shaped clamp makes the point contact of oval ball is no longer the point contact in geometrical significance when grinding material. And the function of contact is extended to curve line and curved surface. The special geometric shaped oval ball and its design parameter increases the grind capacity, enlarges the finished product’s specific surface area, and improves the cement’s quality. Practice has shown, when oval ball as grinding media, compare to round ball it enhances the cement specific surface area more than 20m2/kg.
⑷The electric charge has a character of marginal discharge，two ends of oval ball have charge，can reduces micro powder adsorption of big grind area of oval ball，thereby improve the grinding efficiency.
⑸Due to the area of contact material is increasing between oval balls and between oval ball and liner， makes less wear and tear and reduces noise.
5、Ball & forging mix load
It has been reported, ball and steel bars are mixed stowage and are adopted in the end chamber of circle flow cement mill which has better effect than simple using little steel ball or steel bar, not only ensure the reasonable cement specific surface area but also enhance mill’s output. However use little steel ball grind alone, cement specific surface area and compression strength are decreased, use steel bar alone decrease the output. In addition, it is observed and discovered from particle image ray observatory that round coefficient of cement particle also gets improved greatly. |
