Grinding operation still draws highest attention of the top management of plant in the entire plant operation cycle. No wonder when 35 per cent of energy is consumed by the cement grinding process alone. ICR had time to talk with Biswajit Dhar, who spoke on the present scenario of capex in the cement industry with respect to capacity enhancement in existing plants.
Please comment on the present scenario of CAPEX in the cement industry with respect to capacity enhancement in existing plants.
Currently, the cement industry is faced with a dilemma - cement industry in India is faced with a low demand scenario. On one hand, the capacity utilisation of the existing cement plants are between 65-70 per cent and the rate of growth of cement demand on year-to-year basis is not very encouraging. On the other hand, India-one of the fastest growing economies in the world-has relatively a very low per capita cement consumption - 200 kg, against the world average of 520 kg plus and China around 1,750 kg. Cost of capital, reduction of energy footprint and hence our commitment to CO2 abatement, long delivery time, are other factors which govern and guide towards capex, and this probably facilitates low cost capacity enhancement, like ball mills, with a better RoI than greenfield or even brownfield options.
Give an over view of various options available in the grinding operations. Please support your viewpoint with focus on improvement in energy efficiency and better product quality. (Comment on various combinations like mill with roll press or VRM with ball mill etc.)
There are various options available for cement grinding - ball mill, vertical roller mills (VRM), roller press (RP), Horo mill, etc. Choice in selection is based on capacity required and the choice of initial cost v/s energy consumption, etc. Whereas for capacity less than 100 tonnes per hour, ball mills open or closed circuit, latter being preferable, is the more favoured option in terms of low cost, lower time required from concept to commissioning, ease of operation, easy maintenance, lower operational skill requirement, VRM, RP find favour, in spite of the high capex requirement, particularly for higher capacities as the specific power consumption is lower less by around 40 per cent and that give a better payback, particularly in areas of high electricity cost.
Of course in terms of time required to install, ease of operation, maintenance friendliness, operational skills, ball mill is always an easier option. With the advent of latest generation separators, product quality, water demand, etc. doesn't concern RP/VRMs. For an incremental increase in capacity particularly with conventional ball mill systems, RP is widely added as a pre-grinder or hybrid or combi-systems resulting in 60 to 100 per cent capacity increase. This is a widely used model with low downtime and relatively reduced cost.
Do you think that our precious resources are diverted to produce more and more of 53 grade cement when majority of our jobs can be easily completed by 43 grade cement? It's influence on grinding operation. Kindly elaborate.
Grade 53 cement is normally preferred for application where high compressive strength is required for specialised applications, etc. in view of its higher late strength. However it is generally preferred, particularly in urban areas particularly to reduce construction time. Usage of Grade 53 for general construction activities should not be patronised, as they need increased lime content and is more energy intensive, thereby putting stress on the scarce quality limestone resource and depleting energy resources. Grade 53 generally calls for finer grinding which in turn ends up with increased wear and hence higher maintenance. Grade 43 is generally acceptable and sufficient for over all construction activities, except for cases as cited above.
While making blended cements we have option of co-grinding which is very common in fly ash based cement whereas for slag cement a route of separate grinding is taken. Why and what are advantages?
Co- grinding is generally widespread in Portland Pozzolana Cement manufacture, because it's a cheaper option and does not need separate infrastructure for separate grinding and mixing of fly ash with cement. Fly ash comes with various Blaine's and grindabilty and in cases where fly ash is distinctly coarse, separate grinding technically is a better solution as constituents in product will be of similar sizes rather than distinctly different sizes as seen in co- grinding. Separate infrastructure for fly ash grinding, is a costly option and moreover separate fly ash grinding has not been a real success in India or for that matter anywhere worldwide.
What is the role played by grinding aids? How does the cost economics work?
As we go for finer grinding, the phenomenon of agglomeration increases, resulting in decreased grinding efficiency. This is where grinding aids play a decisive role in enhancing mill performance. However not all grinding aids are compatible for all clinker and it must be carefully selected based on input and desired performance. Grinding aids have immense impact on quality, power consumption and handling, both in cement plant and at point of use. Grinding aids have been found to improve early/late strength of cement. Typically, grinding aids are costly and cost effectiveness should be carefully evaluated, in terms of performance improvements and cost.
There is a new thought on using ceramic grinding media for grey cement grinding. What about the cost?
Ceramic grinding media is a new entrant in the cement (ball mill) grinding circle. Although it is in use for some non cement applications, it is yet to be proven in cement. Suppliers claim low energy consumption with hardly any adverse effect on the productivity. This is yet to be established.
What is the role played by separators in the grinding operations? Are we able to really make full use to get optimum size particle size distribution (PSD)?
Separators have a very important role in the efficiency of any grinding systems. Design, selection and sizing of a separator in terms of its air to material ratio, circumferential velocity, rotor loading are some of the very important parameters which go in a long way for an efficient grinding system. A steep PSD improves cementitious properties and helps in marginally reducing Blaine's value.
Please advice on PSD with respect to strength development in cement.
Normally PSD gives a true reflection on the size distribution of particles. A steep PSD will definitely improve the strength developments and that's where the state-of-the-art separators are so important.
Various equipment manufacturers are promoting use of "Expert" system to run the mills on auto mode. How best these systems can be deployed?
Expert system bridges the difference between the lazy, unskilled operator and an agile, intelligent operator. Expert systems could be very useful if is well programmed and utilised with full conviction. It may require some tuning from time to time to adjust to varied conditions.
The conventional method of monitoring the grinding efficiency and product quality is checking the residue on sieve or mesh; do you think this is still relevant? How? Neither Blaine nor residue gives a complete picture of the particle size distribution. However Blaine and residue analysis are resorted to, as it can give feedback in a short span of time to take corrective action.
Biswajit Dhar, an M.Tech in Chemical Engg from I.I.T Kharagpur, has over 35 years of experience in plant operations, plant commissioning & energy optimisation at companies like ACC, ThyssenKrupp & UltraTech. Post retirement in 2017, he worked as a process consultant with various cement companies in India, Nepal and China, particularly in energy optimisation, capacity enhancement, de-bottlenecking, etc. He can be contacted on: Mob: 9702014464, Email: firstname.lastname@example.org | email@example.com