Adding a pre-grinder to a ball mill has become a well-accepted design for improving plant performance.
Due to two equally important reasons - reducing production costs and protecting the environment from generation of carbon dioxide, continuous effort is going on throughout the globe to reduce energy consumption in all sectors. The cement industry is not an exception.
The main consumer of electrical energy in the cement manufacturing process is grinding. As a result, improvement of energy efficiency in the grinding circuit of a cement plant is of high priority.
Though efficiency in grinding in ball mills is falling much behind other technologies, from operational ease, reliability, easy maintenance and investment point of view, the ball mill is the most preferred equipment, while being used either independently or integrated with other systems, especially in the cement grinding area.
Close Circuit Cement Grinding
In a conventional close circuit cement grinding mill, size reduction takes place in a two-compartment tube mill.
In the first compartment:
Size reduction is targeted from 25-30 mm particle size (max) to 2.0-2.5 mm (max)
Grinding takes place mainly by impact force
The section L/D is ~ 1.05
About 9-10 kWh/T energy is consumed
Larger balls (90, 80, 70, 60, 50 mm) and step type lining are used.
After the first compartment, the material passes through a diaphragm which separates the two compartments and allows only particles below a certain size (2.0-2.5 mm) to pass to the second compartment.
In the second compartment:
Size reduction is achieved from 2-2.5 mm (max) to a fineness from which a desired product fineness (for OPC normally, 3,000-3,200 Blaine) can be obtained from classifier
Grinding takes place mainly by attrition force and also a little by impact
The compartment L/D is ~ 2.2
The balance energy 20-23 kWh/T (OPC for product fineness of 3,000-3,200 Blaine) is used
Smaller balls (40, 30, 20 or 10 mm) or cylpebs (25x25, 22x22, 19x19 or even smaller) and wave type linings are used.
Ball mill efficiency is established to be very poor for particle size above ~ 2.5 mm. To make grinding more efficient, only the second compartment portion of ball mill is used, where particle size below 2.5 mm is handled and the first compartment is replaced with more efficient (for the size range) pre-grinder.
Vertical Roller Pre-grinding Mill
There is no doubt that presently the most preferred pre-grinder is the roller press. However, Vertical Roller Pre-grinder Mill (VRPM) is gradually gaining popularity, day by day. Clinker is caught between rotating table and roller and effectively ground by a combination of compressive force from roller and shear force generated by difference in circumferential speed of table and roller. Finally, ground material is extracted by gravity with the assistance of scrapers. The key difference in magnitude of applied pressure on rollers. In VRPM, the pressure is on the higher side.
Like roller press, VRPM can be used in various modes:
Pre-grinding mode where the feed material passes through VRPM only once and the product is fed to a ball mill. This option is not commonly used.
òModified pre-grinding mode where a static separator is introduced, coarse material is recycled back to the VRPM and the fines go to the ball mill as feed. I A typical particle size distribution of the feed to ball mill (from one installation) is given as under.
In front of, and inside the first compartment partition in a close circuit ball mill, the following particle size distribution is expected.
In VRPM, while the specific grinding energy of around 5.5 kWh/T is consumed, a much finer product than the first compartment of ball mill is achieved:
Semi-finish grinding mode in which a combination of static and dynamic separators are used in the VRPM circuit. In this case, part of the finished product is obtained from the dynamic separator of the VRPM circuit. The coarse material from static separator goes back to VRPM and the coarse material from dynamic separator goes to the ball mill circuit as feed.
Addition of fly ash may be made either at VRPM discharge or ball mill discharge depending on its fineness.
In an upgradation project, the whole ball mill is used as second compartment and existing full mill power is used only for fine grinding. Accordingly, the ball mill and hence the system capacity goes up. The grinding media pattern and liner plate type in the first compartment are changed to match fine grinding requirements. Over and above, as the feed size to ball mill is much finer than that of earlier second compartment feed, the grinding media pattern for the whole mill is designed carefully. In the upgradation project, sacrifice is made to an extent, for the efficiency of ball mill by using a higher L/D ratio (~ 3.2 in place of ~ 2.2) for fine grinding. For upgradation projects ~ 100 per cent increase in capacity (OPC) and a saving of 5-6 kWh/T are reported by cement plants.
During the initial days, after VRPM was introduced in the Indian market, the results were very discouraging. Improper system design, use of vibrating screen or Morgansen Sizer for separating fines from coarse in the VRPM product, which required heavy maintenance, even caused many projects to fail. However, in the recent past, due to the introduction of static separator in the VRPM circuit, intelligent system design and a few other modifications like providing speed regulation for the rotating table, VRPM has become a good success.
Unfortunately, no bold decision has yet been taken by cement plants - since they may be prejudiced by their initial experience - to exploit the full benefits of using VRPM in new cement grinding installations. It is expected that in case of new installations, a better performance can be achieved by optimum selection of L/D ratio for the ball mill.
Author: Jayanta Saha is a Cement Process Consultant based in Navi Mumbai.