Ball mills are here to stay
Tell us about the most efficient methods of cement grinding in today's context. At present, technologies, which are available in cement grinding, are from ball mill to vertical mill. That apart, we have the semi finished grinding mode, which is a high pressure grinding roll, with ball mill or roll pressed in finished grinding. This is somewhere comparable to vertical mill.
But these days, the cement manufacturers are opting for energy-efficient grinding mills as we have passed the stage of using ball mills. If you do a comparative analysis with vertical mills, semi finished grinding process has an intermediate fineness. Due to this, the industrial and cement users feel that the quality of cement-produced in a vertical mill-and the quality of cement-produced in a semi finished grinding system-is different. Let me put this straight. In semi-finished grinding system, the quality is better because you can control the percentage residues on 45 microns more effectively than in case of a vertical mill.
However, in case of energy efficiency, vertical mill is by far the best and most efficient method than the semi finished or the combicircuit. But if you take the finished grinding in roll press, then it will be more efficient as far as vertical mill is concerned. But the difficulties, now these are the perspective as far as the grinding is concerned in terms of power consumption etc. But if you look at fly ash addition, puzzolana addition, there are limitations to roll press mills where you cannot have wet materials added into roll press. That is where again vertical mill scores. I think vertical mill and semi finished grinding mode are the two best options available in terms of power consumption.
Any new technologies emerging or whether there is any research that is likely to materialise?
As far as I know, there have been attempts to use cement grinding with Horomill. However, all these variations have been tried and tested and the industry now has settled for high pressure grinding roll. This is simply because, if you see the process in a ball mill, it is by sheer and it is by impact. If you go to vertical mill, it is by friction and by compression. If you go to roll press, it almost becomes pure compression.
Pure compression is something that is probably the most efficient way of communicating with the given material.
I do not think (as far as I see) high pressure grinding roll or roll press are the most efficient. I am not really sure whether we are going beyond that at the moment. Present one, which is sustainable, has withstood the test of time is the high pressure grinding roll.
How does the quality of cement get affected by method of grinding?
Today, the quality of cement is spoken in terms of residuals in terms of 45 microns, 90 microns; because finally the good manufacturer, good quality cement will try to ensure that your cement is between a certain micron or fraction of 32 microns to 60 microns. Consumers are asking for guarantee for this types of particles band. This is not possible to be achieved in a vertical mill because by the very nature of grinding process the retention time in a vertical mill you are not in a position to grind this. But it is possible in a semi finished mode. In ball mill, if you finish, you have a very flat particle distribution. The moment you have flat particle distribution you can control it. If you ask me in terms of quality, I would say semi finished mode is the best.
How far India has progressed in domestic design and manufacturer of grinding units?
I presume you are referring to the fact that we have the technology and question is of manufacturability. Now in terms of all the three options that I mentioned, it is entirely indigenously produced, except for the gear boxes, because that is something which is not available in India and has to be imported. We call these as noble key components, which have to come from abroad. There are other options like hydraulics, which still comes from case of roll press, the roll body comes from abroad. But that is something I do not think will last long because that day is not very far off when we will be able to do it in India or source it within India. But with gear box, I am not too sure because these are the special gear boxes and especially when you go with high power ratings it needs to be still imported.
How does grinding of slag differ from grinding of fly ash in terms of energy consumption?
It is like this. Fly ash and slag as a material alone is a lot softer as compared to slag. But when you add it in conjunction with clinker, fly ash has a very peculiar behaviour. Fly ash is very porous material; it has a very hollow body and has fly ash particles that are very light and porous. Hence you do not get the right kind of separation and you end up having more fly ash in the product.
To overcome this situation, it is best advised to grind it separately. As far as I know, fly ash separately has not been tried so far in India. This is a very tricky material. So manufacturers apply the process of semi finished circuit where one can add fly ash in ball mill so it has one pass and then it goes to roll press. It is already grounded to a certain extent and then it becomes easier to grind. That process has to be adopted sooner or later.
What is the future of ball mill?
Ball mill will stay because it is the oldest and safest process; it is time tested and proven. Regions like Northeast or remote areas, where the right kind of skilled manpower is hard to find, ball mills can be a good success as it will produce cement of a very good quality. Large players like Lafarge still prefers a ball mill for grinding pet coke. Ball mill can stay for one more reason - they can go to small sized capacities also, which is not true for other technologies. Ball mills can be small but those model equipment cannot be small, they are of a minimum size.
- BS SRINIVASALU REDDY