Clinker grinding technology is the most energy-intensive process in cement manufacturing. Traditionally, it was treated as "low on technology" and "high on energy" as grinding circuits use more than 60 per cent of total energy consumed and account for most of the manufacturing cost.

Since the increasing energy cost started burning the benefits significantly, the grinding technology came under radar of innovation and technology transformation. Focus shifted to being energy efficient and cost reduction. Population of traditional stand-alone two chamber and three chamber ball mills started slowing down getting replaced by vertical roller mills, high pressure grinding rolls (HPGR), pre-grinders, HoroMills along with high-efficiency dynamic separators, static separator (V-sep) with various innovative process combinations. Technology providers used this opportunity to create a business edge. As a result, substantial reduction in energy consumption levels was achieved successful.

Transformation in product portfolio emerged as blessing in disguise to the energy scenario. Change of traditional OPC into PPC, PSC and composite cement was a game changer. PPC captured widespread market giving breather to grinding energy situation as composition bears 35 per cent of fly ash whose grindability is much lower than clinker. This became profitable proposition not only for volume increase but also for reduced energy. Similarly, PSC proved profitable for the plants having resource logistic advantage. PSC manufacturing technology, varied from inter grinding to separate grinding, gave cost advantage due to clinker factor utilisation. The production of composite cements has been increasing for reasons concerned with process economics, energy reduction, ecology (mostly reduction of CO2 emission), conservation of resources and product quality/diversity. The most important properties of cement, such as strength and workability, are affected by its specific surface and by the fineness and width of the particle-size distribution. These can be modified to some extent by the equipment used in the grinding circuit, including its configuration and control.

Ball mill grinding
Evolution of ball mills was the starting point in communition theory adopted widely in cement manufacturing. Size reduction process of bulk solids acquires major part of cement process. This started from wet grinding in three chambers mills to latest mono chamber semi finished ball mills.

Most used systems are two chamber ball mills in finished mode. Grinding in these mills occur due to the effect of cataracting and cascading motion of grinding balls. Crucial factors that influence the grinding efficiency are:

• Liner design
• Circumferential velocity of the mill
• Shape, size and weight of the grinding media
• Friction between the lining and the grinding media
• Friction within the mill charge itself

Technology innovation made use of above factors; however results obtained were successful to the limited extent. Liners design advanced from Lorian liners to step liners to double wave to supplier customised patterns having less weight. Similarly classifiers changed from traditional deep wave, conveying pattern to thin design and bolt less, low weight liners. Wear rate of liners and grinding media is the most cost affecting factor. Improvements in wear rates and cost savings seen from Mn-steel to Hichrome to controlled metallurgy.

Higher the percentage chrome, better the wear resistance, however lowers the hardness normally. Hence, in order to make this system cost competitive and efficient, above factors must be used and leveraged for system selection and optimisation.

Vertical roller mill technology
Vertical Roller Mill (VRM) has been the most preferred technology over ball mill grinding in terms of various efficiency factors. VRM functions four main processes of grinding technology which are: drinding, drying, separation and transportation. Grinding efficiency is the energy utilised to create specific surface of material having same chemical/mineralogical composition. This can be expressed as: Eu = (Specific surface created/specific energy used). Energy utilisation in VRM grinding is better than ball mill technology. Theo power cons of VRM = (Sp. grinding press x roller area x No. of rollers x grinding track speed x friction factor). Friction factor differs from mill to mill and material to material.

Advancements in mill capacities and technology changing fast making VRM is most versatile and efficient. For the volume sensitive market, VRM started offering higher capacities, meeting expanding market requirements with single mill. At the same time, maintenance flexibilities proved this technology the most preferred choice. Various mill OEMs have their unique design features, offering advancements in technical features.

HPGR technology
High-pressure grinding rolls (HPGRs), with key process equipment as roller press in cement industry, have struggled and conquered for acceptance as finished mode operation in raw grinding and even as pregrinder in clinker grinding technology. Many of the issues that didn’t favour their widespread use have now been gradually gaining grounds. But still, it will remain subdued in selection arguments with VRM technology. Answer for this is again follows the similar requirement of customers, high capacity systems, low cost, high reliability, ease of operation and better wear resistance factors.

Most prevailing issue in the HPGR system is "high pressure" as the name suggests. High pressure has direct benefit on grinding efficiency however, it also has got immense impact on with standablity of grinding components, metallurgy of grinding profiles, mechanical stresses on drive components, etc.

In contemplating an answer to issue of the HPGR, the status of other accepted technologies like VRM must be examined. As an example, the latest and advanced VRM technology can be considered. When a plant operation / design is being considered, every well-equipped engineer will be able to turn to numerous rules of thumb associated with these factors:

Particle size distribution will be consistent in feed
Centralised and circumferentially distributed feed is required to extract the best performance.
Profile and condition of the HPGR is critical to deliver the best performance These issues prevail in VRM also. However VRM technology is well established to handle the variety of feed PSDs. A comparison between RP and VRM on some of the merits and demerits are as given:

Roller Press Parameters
Roller Press roller diameters typically vary from 0.5 m to 2.8 m, depending on the supplies, and roll widths vary from 0.2 m to 1.8 m. The aspect ratio of the rolls also varies as a function of manufacturer. Typical HPGR throughput rates range from 20 to 3,000 tph at different applications, with installed motor power as high as 3,000 kW per roll. The roller profile is the key deciding parameter on withstanding the high pressure and giving much needed life. This has been one of the significant characteristics of rollers / Roller Presses. but solutions are now in place for adequate life of the profiles. When operating an HPGR, the two most important operating parameters are:

Operating pressure
Roll speed

The two key operating parameters are inherently linked to the following:
RP throughput
Specific pressing force
Maximum pressure between the rolls
Specific energy input
RP Throughput: The throughput can also be calculated from the continuity equation as follows:
M = L x s x u x ?c x 3.6 (EQ 2) where
s = operating gap (mm)
?c = density of the product cake (t/m3)

Specific Pressing Force
The specific pressing force is defined as the grinding force applied to the rolls (kN), divided by the diameter (m) and width (m) of the rolls. The specific pressing force has the unit of N/mm2.Fsp = F/(1,000 x D x L) where,
Fsp = specific pressing force (N/mm2)
F = applied grinding force (kN)
D = roll diameter (m)
L = roll width (m)
Maximum achievable roller force must be at least
5500 kN/m2 for raw material
6000 kN/m2 for clinker and slag

Roller press design & operation
Similar to the VRM, there are few Roller Press Technology providers with different designs and principles. Mainly, they are: KHD, Koppern, Polysius. KHD has considerable roller press population for various applications. Their standard configurations are as below:

Stud lining roller is typical and popular solution for high life usage that KHD offers for. There are many advanced versions above this.

Similarly, Koppern roller press has got latest design features catering to the clinker grinding application. The roller profile is different than the other technology and offers high life hours of operation. Standard design feature and parameters that Koppern offers is as below:

POLYCOM is the established technology in roller press supplied by Thyssenkrupp Industries. Population spread across various application for clinker grinding, raw grinding and also in combi mode and finish mode.

Conclusion
Technology evolution has given user various options and alternatives. These options help user to focus on priorities. Volume is the main criteria. Mill capacities, especially in cement grinding are changing the scenario. There were no high capacity mills in the past for clinker grinding. Advanced VRM technology started offering higher capacity levels so that capex decisions became easy for the user.

At the same time, business expansion became visible in terms of stand-alone grinding station with high capacity mills. This is playing a major role in capturing various market demands and hence cement industry growth.

Design features of Gebr Pfeiffer
Design

• Roller and grinding bed inclined, concave grinding path
• Rollers suspended at a pressure frame
• Hydraulic actuated at a pressure frame
• Pull rods inclined to absorb torque, mill housing not affected with horizontal forces
• Swing lift device for retraction & servicing of roller
• High-efficiency separator type SLS

Operation

• Start-up with auxiliary drive, rollers statically on grinding table
• Normally large size mill casing and low pressure losses of gas flow

Design features of Loesche
Design

• Airlock in general for feeding
• High-efficiency separator, LDC
• Conical rollers 2, 3 according to mill capacity with S roller concept
• Rocker arm with hydraulic cylinders to exert the roller pressure
• Hydraulic cylinders with nitrogen accumulators
• Modular design; hydro pneumatic units can be combined with various mill sizes according to throughput or drying needs

Operation

• Rollers can be lifted from grinding track for low torque start-up of the mill
• Grinding pressure and counter pressures can be adjusted to maintain smooth operation

Design features of OK
Design

• Hydro pneumatic system for pressure exertion
• Multiple rollers, based on capacity
• Unique grooved roller profile
• Curved table profile
• Roller repositioning after wear

Operation

• Rollers will be lifter position during start up
• 2,4 rollers can be unloaded for partial capacity operation of special operating conditions
• Recirculation of hot exit gas to the Mill inlet

Design features of Ploysius Quadropol
Design

• 4 individual grinding rollers
• Cambered geometry of roller with better wear resistance
• Rollers can be swung out for maintenance
• Bearing assembly located outside the grinding chamber
• Ring craneway for maintenance works

Operation
Rollers can be unloaded for partial capacity operation of special operating conditions
Nozzle velocities can be adjusted during partial operations
High efficiency SEPOL separator
RP
Few applications in cement
grinding as semi finished mode
– Limited size?not beyond ~120 bar
+ Superior wear solutions
+ lower SEEC for dry and hard
material (Lower than VRM if in finished mode)
+ Market acceptance & early strength of cement
-Sensitive to feed variations
-More equipments & lay out space

VRM:
+ Proven solution, many applications
+ Large single units available
-Reliability of planetary reducer
+ Lower SEEC in compare with ball mill
+ Stable operation easy to achieve…
-… but often requiring water injection
-Support roller technology not successful

Feed chute placed above fixed roller, shut off gate and vertical flow control gate
Feature:

• Easy operation Only few moving parts
• Ensures stable operation even for high product fineness
• Easy start up procedure in
• combination with ROLCOX
• Roller profile is called Hexadur. It offers increased life of profile in compare with solid weld rollers.

About the authors:
Shreekant Hulgi Managing Partner, SPV Engineers
Vijaykumar Vemuri Managing Partner, SPV Engineers
Founded in October 2018, SPV Engineers is a partnership firm specialising in process safety (risk) assessments and solutions, plant performance improvement for productivity through "Mill & Kiln Modules", energy cost reduction through electrical and thermal energy optimisation, refractory design, selection and installation services, capex projects through process engineering, plant capacity debottlenecking and feasibility studies, fuel cost reduction through AFR projects and optimisation. SPV Engineers is a sales partner for latest and advanced SpectraFlow online analyser for minute-by-minute analysis and control. SpectraFlow is an analyser with no radiation risk and no statutory safety requirements. SpectraFlow works on unique principle of NIR (Near Infra Red) technology, which is first in the industry with minimum OPEX and cost of ownership.