1、 外文资料翻译 Portland Cement Preparation of Raw Materials 1. Crushing The raw materials as received from the mines are in lump form and have to be crushed before fine grinding. The size of the lumps would depend upon theraw material and the method of mining, so the size of the crushing equipmentused woul
2、d depend primarily upon the size of the lumps received. In a moderncement factory, manufacturing 1000 to 2000 tonnes of cement per day, the limestone is mined by mechanical means and hence the size of individual pieces maybe 1 meter or so. In smaller factories, the mining of limestone may be manuala
3、nd the size of individual pieces may be 200 mm or so. Similarly, clay, shale,bauxite, laterite, sandstone, coal, gypsum, etc. may be received in sizes notlarger than 300 mm. (a) Limestone Crushers: The requirement of limestone per day for a 2000TPD factory may be about 2600 to 3000 TPD, for a 1000 T
4、PD factory 1300 to1500 TPD and for a 300 TPD factory 400 to 450 TPD. As the mining operationhas to be carried out in daylight hours only, the crushing operation should alsobe carried out in the 10 or 12 hours of light per day. The capacityof a limestone crusher for a 2000 TPD factory may be 300 TPH,
5、 for a 1000 TPD factory 150 TPH and for a 300 TPD factory 50 TPH. The decision on the type of limestonecrusher would depend upon the physical properties of the limestone； however,for smaller factories using manually mined limestone, single stage HammerCrushers are most suitable； but for larger facto
6、ries two stage crushing is alwaysmore economical and more efficient. The first stage may be a Jaw Crusher, Gyratory Crusher or Impact Crusher and the second stage a Hammer Crusher orCone Crusher. If possible, a screen of suitable size may be introduced after thesecondary crusher to allow only unders
7、ize to go for further processing whileoversize is returned to the crusher. A typical flowsheet for a limestone crushingplant in a 2000 TPD factory may be. Dumpers-Feed Hopper with Laminated conveyor at the bottom 300 TPHJaw Crusher (feed size 1000 mm- product size 200 mm ) 300 TPH HammerCrusher or I
8、mpact Crusher or Cone Crusher (feed size 200 mm - product size 20mm 300 THP Vibrating screen accepts minus 20 mm - oversize back to crusher. Proper dedusting arrangements must be made at the crushers, the vibratingscreen and the transfer points and a Dust Collector with fan installed. Powerconsumed
9、for the crushing operation may vary from 3 to 6 Kwh per tonne oflimestone. (b) Other Crushers. Crushers required for crushing shale, gypsum,coal, etc. may be of suitable design, preferably Hammer Mills, and of suitablesize. For the 2000 TPD factory requiring 10 % addition of shale to the raw mix,usi
10、ng the dry process and using 6 % gypsum for cement grinding, the shale required would be about 3000 TPD, coal 4000 TPD and gypsum 100 TPD. Thepreferable size of the crushers would be 30 TPH, 50 TPH and 20 TPH respectively. The feed size may be specified minus 300 mm, and product size 20 or 25mm. 2.S
11、torage of Raw Materials Limestone in lump form is generally not stored at the factory, it is crushedas soon as it is received at the factory or even crushed at the mine itself and onlycrushed limestone is store at the factory. Shale or other corrective materials,coal and gypsum are generally stored
12、in lump form as well as in crushed form. Itwould, therefore be advisable to have a common covered storage space for alltypes of crushed materials with proper partitions and proper reclaiming arrangements, Shale, gypsum, coal, etc., received in lump form, should be stored suitably and arrangements ma
13、de to feed the same to the respective crushers mechanically. Two types of storage for crushed material are common: (a) crane gantry with Electric Overhead Travelling (EOT) cranes and (b) Storage bins with overhead feed belt conveyors and underground reclaiming belt conveyors, the second alternative
14、may be more economical both in first costs and operating costs. Thelatest development in the storage of crushed raw materials, especially for factories with annual capacities of 0.5 million tonnes or more, is the Bed BlendingSystem in which crushed limestone from different faces of the mines and cor
15、rectives are spread in predetermined layers at one end and the material reclaimedwith the help of cutter blades at the other end, so that a preblended raw mix ofapproximately the desired chemical composition is fed to the raw mill hoppers.the system can be housed either in the open or under cover, d
16、epending upon theclimatic conditions of the location. Lump materials are generally stored in theopen and reclaimed with the help of Pay Loaders or Scraper Haulers or Bull Dozers. 3.Fine Grinding In the manufacture of Portland Cement types, the calcareous constituenthave to combine with the argillace
17、ous constituents are ground and the more intimately they are mixed, the more efficient would be combination. If the limestone is of cement grade composition, the argillaceous constituents are intimatelydis-seminated in the calcareous matrix, such a raw material can be groundcoarse； but if the calcar
18、eous constituent is very pure and considerable amount ofclay or shale has to be added, the raw mix must be ground much finer. A fewtests carried out in a pilot plant can indicate the optimum degree of grinding required for a particular raw mix. It must be remembered that the raw materials fed to the
19、 grinding equipment are generally minus 20 mm and the product sizemay be 13 7% on 90 micron sieve. Intimate mixing is another important factor； the raw mix has to be brought to a very precise oxide composition, which inthe manufacture of cement is generally gauged determining the total carbonates (T
20、C) by simple acid/alkali titration. For example, if at a factory the holdingpoint of TC is 78%, the TC from batch to batch should not vary more than 78 0.1%, and this can be achieved only by thorough mixing. 4.storage of Ground Material In the wet process the ground slurry is stored either in silos
21、or in basinswith continuous mixing. The storage capacity should be 2 to 3 days consumption for the factory. In the dry process, tall silos are used for storage of the dry raw meal. It is now the general practice that mixing silos or homogenising silos are placed above the storage silos, so that the
22、ready mixed and correctedraw mix is fed to the storage silos by gravity, result in a saving in power. The various methods of feeding the slurry and the dry raw meal to the burning equipment will be discussed in the next section, dealing with burning operation. 5.Coal Grinding As coal has been consid
23、ered a raw material, grinding of coal should alsoconsidered in this section. Coal has to be ground dry. Raw coal always contains2% to 10% moisture； during the rainy season the moisture content always goesup, sometimes even up to 15% to 20%. Therefore,coal has to be dried suitablybefore grinding. In
24、early days rotary driers were used, but some forty years ago,mills were designed in which the coal could be dried and ground simultaneously with the help of hot air, in air swept ball mills. In separate drying and grindingsystems the raw is dried in suitable driers with the help of hot air generated
25、 in anoil fired or coal fired or coal fired furnace； the spent air is vented to the atmosphere through suitable dust collectors. The dry coal is then ground, either in anopen circuit coal mill in which the ground coal is transported to the coal firinghopper with the help of a bucket elevator, other
26、dry coal may be ground in an airswept close circuit mill, and air used to transport the ground coal to the cycloneover the coal firing hopper； the same air is used as primary air in the coal firingpipe. In a simultaneous drying and grinding system, the front portion of themill； fitted with lifters a
27、nd chains, acts as the rotary drier and the back portion,filled with steel balls, acts as the closed circuit mill； hot air, either from the rotary kiln or from an auxiliary furnace, is used as air for drying and transportingthe coal. The spent air, in this case also, is used as primary air in the co
28、al firingpipe. The power consumption in separate drying and grinding units may workout to 25 to 30 kW h per tonne fine coal and in the simultaneous drying and grinding units to about 20 to 25 kW h per tonne fine coal. 6.Burning A mixture of pure lime and silica in the proportion of 2. 8:1 should giv
29、e100% tricalcium silicate on burning, but it will require a very prolonged heatingat 1600 or more, to obtain a perfect combination. It is not possible to obtainsuch prolonged heating conditions in actual plant practice； it is, therefore, necessary to add some fluxing materials to bring about the combination at a lowertemperature and in a shorter time. Alumina, iron oxide and, to some extent,magnesia and alkalies present in the raw mix act as the fluxing agents. By judicious inclusion of these oxides in the
