Limestone Crushing Made Easy
Present day mining equipment comfortably handles huge chunks of up to 1,800 mm size ROM stones to be fed as input to crushing system, says LN Mitra, while discussing size reduction equipment.
Sizing of limestone from Run of the Mine (ROM) to milling/process requirements, varies from industry to industry. Here we are more concerned about the limestone sizing requirements of the cement industry, although it must be recognised that there are vastly varied requirements where crushing and grinding of limestone to different sizes is required for different processes.
Present day mining equipment comfortably handles huge chunks of up to 1,800 mm size ROM stones to be fed as input to crushing system. The required feed size (product output size of crusher) to the final stage of sizing, viz., grinding mill, is either 95 per cent below 25 mm for ball mills and 95 per cent below 75 mm for the vertical mills. Development of mills in the last few decades and nature of limestone available in India, have brought in several types of crushers that are cost effective for such applications.
Conventionally, there are up to three stages of crushing employed for sizing limestone - primary crushing (with gyratory/double toggle jaw crushers), secondary crushing (with reversible hammer mills/impact hammer mills/cone crusher), tertiary crushing (with high speed hammer mills/cone crushers/impactors). In the cement industry, conventionally only up to two stages of crushing used to be required. However, the entry of large impactors (compound/single rotor) changed all that, quite drastically.
The primary gyratory/DTJ crushers:
These crushers reduce ROM of size 1,200 mm to an output of 300 mm. These are but of very robust designs with very low specific power consumption/low wear rate of crushing elements and called for least maintenance. Both prices and installation costs are very high for these crushers. In the event of very hard limestone in the deposit, often with high silica content, one has no choice but to go for these crushers as is very often the case in Japan. In India gyratory crushers are never employed due to prohibitive capital cost as mentioned earlier.
Some old plants with small capacity have DTJ crushers as primary crushing equipment. DTJ crushers with limited capacity is ruled out for modern day high capacity plants. Specific power consumption of DTJ crushers is very low but they offer poor top size control in the product.
The secondary crushers - hammer mills/cone crushers:
Crushed stone from primary crusher is fed to secondary crusher, either after prescreening or directly. Prescreening is cost effective if the primary crusher product contains 40 per cent or more undersize.
Hammer Mills were widely used for secondary crushing till end seventies of the last century. These were reasonably priced, but power consumption/wear rate of crushing element were high. Therefore, total cost of ownership was formidable. They were also vulnerable to ingress of uncrushables, damaging the internals.
Use of cone crushers in cement industry was limited again because of price and non-availability of manufacturers for large capacity machines in the country. Cone Crushers, inherently due to their crushing principle, consume very low power and have hydraulic system to protect itself form damage due to tramp uncrushable ingress. Performance of Cone crushers is unquestionably superlative, if the user can afford it's initial cost.
The million ton plant, impactor and vertical mills:
In India, in the early eighties of the last century, all the above conventional methods of preparing the raw meal (ground limestone) experienced revolutionary developments. The cement industry started building million tonne per annum capacity cement plants (which may however, not sound so revolutionary in today's context of 5 /7/10 MT plants). This called for high capacity crushers to feed the hungry mills or combination of mills. For Indian mining practices, crusher capacity was pegged at 1,800 TPH. On the other hand, ball mills on the milling side were slowly being replaced by newly developed Vertical Roller Mills for their superior all-around performance.
As we said earlier, the required product size from these crushers were 95 per cent below 25 mm for ball mill and 95 per cent below 75 mm for vertical mill. However very quickly the industry switched over to vertical mills only, as fast and effective developments of this genre of equipment gradually made ball mills redundant.
Single rotor and double rotor (compound) Impactors virtually replaced all other crushers employed in large capacity plants. Application of compound rotor eliminated complicated two stage crushing when Ball Mills were employed for grinding.
These Impactors came with heavy duty fabricated rotor and slid- in type blow bars as crushing elements. Product size could easily be adjusted with hydraulically operated impact arms while hydraulically actuated Grinding Path assembly fine tuned the top size of the product. Grinding path also protects the machine from damage due to uncrushable ingress.
Thus compound Impactors made two stages crushing irrelevant. These Impactors could produce minus 25 mm product from a ROM top size of 1,800 mm for the large capacity range of 1,600 to 2,000 TPH. In these machines, two rotors are mounted in such a manner that material flows from the primary rotor post crushing to the secondary rotor guided by the impact arms and gravity. The final product is obtained after secondary crushing and travel of the product over the grinding path.
By mid-eighties, Vertical Roller Mills virtually eliminated the Ball Mills for its multiple superior features and larger capacity obtainable. Single Rotor Impactors which can produce 95 per cent minus 75 mm output from ROM size of 0 to 1,800 mm, is presently the crusher choice for the fashionable VRMs which are considered state of the art technology. Thus, the theory and practice of sizing limestone for the cement kilns, underwent revolutionary changes in the last 30 years, even as the plant capacities went through the roof.
ABOUT THE AUTHOR:
LN Mitra is a mechanical engineer, and a retired technocrat, currently settled in Kolkata. He has worked for Larsen and Toubro Limited throughout his distinguished career, in various capacities, last of which was a very senior administrative assignment in South India. Mitra can be credited with introduction of Impactors in the Indian Cement Industry, and that of Secondary Impactors in particular. He has been considered as an accomplished specialist in the application of Impactors in not just cement industry, but all other core sector sizing applications, in the Indian capital goods market.