Calcium carbide slag is a by-product of calcium carbide hydrolysis reaction in the production process of calcium chloride (PVC). With the development of the economy, China's PVC industry maintains a rapid growth momentum, the capacity of production equipment continues to expand, the scale of equipment is expanding, the structure is more reasonable, and the level of process technology is continuously improved. At present, more than 70% of the raw materials for producing PVC resin in China come from calcium carbide. Due to the development of PVC production by calcium carbide method, the discharge of calcium carbide slag is gradually increasing, and the pressure on the environment is increasing. Using the new dry process cement production technology, the use of calcium carbide slag for the production of cement clinker is an effective way to deal with calcium carbide slag in large quantities, but the characteristics of calcium carbide slag have a great impact on the cement production process, especially for the preparation and ripening of cement raw materials. The effect of material firing is very large. How to properly and effectively configure the cement production system for these characteristics is the key to this technology.
The basic characteristics of calcium carbide slag mainly include the important physical and chemical properties of calcium carbide slag dewatering performance, carbide slag particle size, bulk density, thermal properties, sintering properties, etc. The specific characteristics of the slag have been described in detail in the literature  . These characteristics have a great impact on the transportation, dehydration, drying, storage, measurement and other aspects, each process link will have a greater impact on the quality of raw materials. This paper will focus on the preparation of raw materials for the production of cement clinker using calcium carbide slag.
1 calcium carbide slag pretreatment process
The pretreatment system of calcium carbide slag generally includes different stages such as primary dewatering system, calcium carbide slag mechanical dewatering system, calcium carbide slag filter cake conveying system, calcium carbide slag drying and dust collecting system, and calcium carbide slag dry powder storage. The characteristics of calcium carbide slag have different effects on various stages of the pretreatment process.
1.1 Primary dewatering system
The large amount of calcium carbide slurry produced by the calcium carbide wet acetylene process has a low solid content and a moisture content of about 85% to 95% . In the preliminary dehydration process, it is generally treated by natural sedimentation and then enters. Mechanical dewatering system. After natural sedimentation, the solid content of the calcium carbide slurry will increase, and the moisture content is generally about 70% to 80%. The water retention of the calcium carbide slag in different factories is different, and the water content of the calcium carbide slurry after concentration is different. .
1.2 Mechanical dewatering system
The calcium carbide slag has many mechanical dewatering methods. Because of its good water retention, the plate and frame filter press is generally used to dehydrate the calcium carbide slag. When the pressure filtration system is well controlled, the calcium carbide slag filter cake can obtain lower moisture, and The moisture is relatively stable, which is beneficial to the subsequent conveying and drying processes; when the pressure filtration system is poorly controlled, the moisture fluctuation of the calcium carbide slag is large, and the moisture is high, which is unfavorable for the subsequent conveying and drying processes.
In the drying process of calcium carbide slag, the calcium carbide slag filter cake needs to be uniformly fed. Therefore, the discharge of the calcium carbide slag filter cake is required to be stable and the water is stable, and the working process of the plate and frame filter press is intermittent operation, so multiple filter press combinations are required. A working unit is formed for circulation to satisfy the continuous stable cutting of the calcium carbide slag cake. At present, the number of filter presses generally used in a filter press operation unit is 5 or 6 units, and a very small number of manufacturers choose 4 or 7 units. The difference in various options is due to the high failure rate of the plate and frame filter press, which requires frequent maintenance and the number of spare units. In general, five filter presses are selected as one operating unit, which requires a higher level of operation to meet production requirements. At present, there are also many chemical production enterprises increasing to 6 or even 7 units, increasing the number of spare filter presses, increasing the filter press time, and obtaining a filter cake with lower moisture, which is conducive to the subsequent conveying and drying process. There are also a few manufacturers who choose 4 filter presses as an operating unit, but the actual results are not ideal.
The water content of the calcium carbide slag filter cake after pressure filtration is greatly reduced. Due to the difference in the characteristics of calcium carbide slag in each factory, the water content of the final calcium carbide filter cake also has a certain difference. Under normal circumstances, the moisture content of the filter cake is reduced to about 35%. ~38%, there are also very few companies that calcium carbide filter cake moisture reached 25%.
1.3 calcium carbide slag filter cake conveying system
The calcium carbide slag after pressure filtration needs to be transported into the calcium carbide slag drying system, and can be transported by a chain plate conveyor for a short distance. At present, there is also the use of pipeline pumping calcium carbide slag, which has obvious advantages, but its failure rate is high, and the use effect is not ideal. Therefore, the transportation of calcium carbide slag filter cake is generally still carried by belt conveyor. During the transportation process, the selection of the belt and the angle of conveyance need to be carefully selected. The choice of the belt conveyor is different from that of the ordinary belt conveyor. In the process of selection, the conveyor capacity and material spillage of the belt conveyor should be considered.
1.4 calcium carbide slag drying system
The calcium carbide slag drying system has different choices depending on the production process. When the amount of calcium carbide slag is small, and the replacement rate of limestone is less than 15%, it can be directly fed into the raw mill to meet the drying requirements. When the scale of the use of carbide slag is expanded, it needs to be pre-baked. There are two main types of drying devices currently used: rotary dryers and hammer-type drying crushers .
The rotary dryer is a traditional drying method, and its advantages and disadvantages are obvious.
Its advantages are mainly manifested in:
(1) The calcium carbide slag drying system and the cement clinker firing system are independent of each other, and are not significantly interfered with each other, and are more suitable for the fluctuation of the feed amount of calcium carbide slag and the fluctuation of moisture;
(2) The rotary dryer is easy to operate and the operation mode is very mature.
Its shortcomings are mainly manifested in:
(1) The thermal efficiency is low and the overall heat consumption is increased;
(2) It is necessary to establish a separate exhaust gas treatment system, which increases equipment investment and corresponding civil engineering investment;
(3) The final moisture of drying can not meet the requirements of raw materials. Generally, the moisture of calcium carbide slag can only be dried to 10% to 15%. It is also necessary to carry out the second drying, increasing the number of times the calcium carbide slag dry powder is transferred. Not good for environmental protection;
(4) Large area;
(5) Bonding occurs during the drying process, and grinding is required in the raw material preparation.
The hammer-type drying crusher is specially developed for soft and abrasive materials such as high-moisture wet materials and filter cakes. When using a hammer-type drying crusher, it is generally combined with the kiln tail pre-decomposition system and uses the kiln tail. The discharged high-temperature flue gas is used as the heat source for drying. The number of preheater stages is adjusted according to the requirements of the flue gas temperature for drying. Generally, the design is two-stage, and the pre-decomposition system and the drying system use the same exhaust gas treatment system. In China, the first application in the wet grinding dry burning production line, the current technology has matured, but its advantages and disadvantages are also obvious.
Its main advantages are:
(1) The system has high heat utilization efficiency, and the probability of crust blockage in the pre-decomposition system is greatly reduced;
(2) Reduced a set of exhaust gas treatment devices, saving fixed investment;
(3) The drying effect is good, and the moisture content of the finished product is very low, generally about 1%, which can meet the requirements of raw materials;
(4) The system has a small footprint.
Its shortcomings are mainly manifested in:
(1) The operation requirements of the drying and crushing machine are relatively high, the feeding is relatively uniform, the water is relatively stable, and the upstream filter press workshop is required to be relatively stable;
(2) The system is associated with the kiln system. When the drying system fails, it will affect the kiln system at the same time. However, from the current wet grinding dry burning operation system, the problem has been solved.
1.5 calcium carbide slag dust collection system
The carbide slag particles are small, the specific gravity is light, and it is easy to fly in the air, which brings difficulties to dust collection. The dust collection system after the calcium carbide slag is generally dried adopts the two-stage dust collection method: firstly, the cyclone dust collector is used for preliminary dust collection, and then the bag dust collector or the electric dust collector is used for dust collection.
In the dust collection system, the primary cyclone dust collection system changes little, but the second stage dust collection system is related to the drying method of the calcium carbide slag. When the hammer type drying crusher is used for drying, the water vapor content in the smoke gas is used. Very high (up to 38%), the ordinary bag filter is difficult to meet the operational requirements, generally using a 5 electric field electric dust collector to collect dust.
2 raw material batching process
2.1 Influence of particle size change of calcium carbide slag on the process of raw meal preparation system
The original particle size of calcium carbide slag is fine, which meets the requirements of fineness of raw materials. However, during the drying process, coalescence occurs between the particles, resulting in coarser grain size of the calcium carbide slag after drying, which no longer satisfies the raw material. The particle size is required, so it is necessary to pulverize the carbide slag again.
When using the drying crusher to dry the calcium carbide slag, the problem of solving the coarser particle size of the calcium carbide slag after drying can be carried out from two aspects: First, the calcium carbide slag can be re-introduced into the raw material grinding to complete the grinding, and at the same time Preliminary mixing with other material components; second, sorting during the drying process, the coarse powder is returned to the drying crusher cycle, fine powder is collected as a product, and other materials are mixed with other mixing methods.
From the test of the influence of the fineness of calcium carbide slag on the burnability of raw materials, it can be known that when the fineness satisfies the raw material requirements, no grinding is required, and the raw material ingredients can be directly involved.
2.2 Influence of the fluidity of calcium carbide slag on storage and metering systems
The calcium carbide slag has good fluidity after drying, and the adhesion between the particles is strong, and it is easy to compact in the reservoir, which brings difficulties to stable discharge. When the level in the reservoir is too high, the lower layer of carbide slag is compacted, and it is difficult to discharge. If compressed air is used, it will rush out and cannot be discharged stably; when the level in the reservoir is too low The calcium carbide slag has not been compacted, its fluidity is not controlled, and it is unstoppable when unloading. Therefore, the storage of calcium carbide slag requires the selection of a suitable reservoir to ensure the stability of its discharge.
The proportion of calcium carbide slag in the raw material is very large, and its measurement has a great influence on the quality stability of the raw material. In the actual engineering design, it is necessary to select a measuring device with large measuring capacity, high precision and small dust, and the traditional belt scale measuring The method is not suitable in actual production. In addition, the instability of the material discharge causes great difficulty in the accurate measurement of the ingredients, so that the raw material quality fluctuates too much, the working condition is difficult to stabilize, and the quality of the clinker is not guaranteed.
2.3 Influence of carbide slag characteristics on conveying system
The calcium carbide slag dry powder particles are small and the bulk density is low (about 0.6 t/m3). The basic characteristics of the powder are also different from the limestone raw materials. The conveying method and conveying device need to be designed according to the characteristics of calcium carbide slag, and the traditional raw material conveying method cannot be applied. The calcium carbide slag powder has good fluidity, and the conventional belt conveying device cannot meet the requirements of the conveying amount, and the dust is too large, and the lower bulk density of the carbide slag requires a larger specification of the conveying device.
2.4 Selection of the raw material rate value of calcium carbide slag
The ratio of the batching rate of calcined cement clinker using calcium carbide slag as calcium raw material is basically the same as the conventional batching ratio value, generally: KH=0.9Â±
0.02, SM = 2.6 Â± 0.1, IM = 1.6 Â± 0.1.
2.5 Raw material homogenization method
The new dry process requires good homogeneity of the raw meal, and the uniformity of the raw meal is directly related to the stability of the system and the quality of the clinker. Raw material homogenization is generally carried out in two steps. In the current conventional production line, the first step of the homogenization process takes place in the raw material grinding process, and the second step is carried out in the raw material homogenization process.
In the calcium carbide slag cement production line, the raw material homogenization method is related to the raw material configuration process. When the fineness of the dried calcium carbide slag powder does not meet the fineness requirement of the raw material, grinding is required, and the calcium carbide slag can be combined with other components in the grinding system to complete the preliminary homogenization process, which is The raw meal preparation process is the same when using calcium raw materials.
When the fineness of the calcium carbide slag meets the requirements of the raw material, it is no longer necessary to enter the raw material grinding system. In the raw material grinding, only the other component materials after the compounding need to be ground, and then the ballast carbide slag after the compounding Mixing in the mixer, completing the first step of homogenization, and then completing the second step of homogenization in the raw material homogenization library to obtain the finished product.
The use of calcium carbide slag instead of limestone to prepare qualified cement raw materials must use scientific pretreatment process and batching process. These two points are inseparable from the in-depth study of the characteristics of calcium carbide slag, and also determine the replacement of limestone for the production of cement clinker by calcium carbide slag. Success or failure.
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