Energy-Efficient Technologies in Cement Grinding

In this chapter an introduction of widely applied energy-efficient grinding technologies in cement grinding and description of the operating principles of the related equip‐ ments and comparisons over each other in terms of grinding efficiency, specific energy consumption, production capacity and cement quality are given. A case study per‐ formed on a typical energy-efficient Horomill® grinding technology, is explained. In this context, grinding circuit is introduced and explanations related to grinding and classification performance evaluation methodology are given. Finally, performance data related to Horomill® and high-efficiency TSV™ air classifier are presented.


Introduction
Cement is an energy-intensive industry in which the grinding circuits use more than 60 % of the total electrical energy consumed and account for most of the manufacturing cost [1]. The requirements for the cement industry in the future are to reduce the use of energy in grinding and the emission of CO 2 from the kilns. In recent years, the production of composite cements has been increasing for reasons concerned with process economics, energy reduction, ecology (mostly reduction of CO 2 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.
Performance of grinding circuits has been improved in recent years by the development of machinery such as high-pressure grinding rolls (HPGR) (roller presses), Horomills, highefficiency classifiers and vertical roller mills (VRM) for clinker grinding which are more energy efficient than machinery which has been in common use for many years such as tube mills. Energy-efficient equipments such as high-pressure grinding rolls, vertical roller mills, CKP pre-grinders, Cemex ® mills and Horomills ® are used at both finish grinding of cement and raw material-grinding stages due to higher energy consumption of conventional multi-compartment ball milling circuits. Multi-compartment ball mills can be classified as: • Single-compartment ball mills • Two-or three-compartment ball mills Multi-compartment ball mills and air separators have been the main process equipments in clinker grinding circuits in the last 100 years. They are used in grinding of cement raw materials (raw meal) (i.e. limestone, clay, iron ore), cement clinker and cement additive materials (i.e. limestone, slag, pozzolan) and coal. Multi-compartment ball mills are relatively inefficient at size reduction and have high specific energy consumption (kWh/t). Typical specific energy consumption is 30 kWh/t in grinding of cement. Barmac-type crushers found application as a pre-grinder in cement grinding circuits operating with ball mills to reduce the specific energy consumption of ball mill-grinding stage [2]. An overview of technical innovations to reduce the power consumption in cement plants was given by Fujimoto [1].
In this chapter, operating principles of high-pressure grinding rolls, Horomill ® , vertical roller mills, CKP pre-grinders and Cemex ® mills which are widely applied in finish grinding of cement are briefly explained in addition to the advantages and disadvantages over each other.

Barmac VSI crusher
The Barmac rock-on-rock crusher has a rotor that acts as a high-velocity, dry stone pump, hurling a continuous rock stream into a stone-lined crushing chamber. Broken rock about 30-50 mm in diameter enters the top of the machine from a feeder set and is accelerated in the rotor to be discharged into the crushing chamber at velocities of up to 85 m/s. Collision of highspeed rocks, with rocks falling in a separate stream or with a rock-lined wall, causes shattering. The product is typically gravel and sand-sized particles. Barmac crushers are available from 75 to 600 kW. The product-size distribution can be controlled by the rotor speed [3]. A schematic of a Barmac-type VSI crusher is given in Figure 1 [4].

High-pressure grinding rolls (HPGR)
High-pressure grinding rolls (roller presses) are used in both raw material and cement grinding. The principle of the HPGR is shown in Figure 2.  The material between the rolls is submitted to a very high pressure ranging from 100 to 200 MPa. Special hard materials are used as protection against wear, for example, Ni-hard linings to protect the rollers. During the process, cracks are formed in the particle, and fine particles are generated. Material is fed into the gap between the rolls, and the crushed material leaves as a compacted cake. The energy consumption is 2.5-3.5 kWh/t and about 10 kWh/t when recycling of the material is used. The comminution efficiency of a HPGR is better than ball mills such that it consumes 30-50 % of the specific energy as compared to a ball mill. Four circuit configurations of HPGR can be used in grinding of raw materials, clinker and slag such as [5]:   Application of HPGR in cement grinding circuits and the effects of operational and design characteristics of HPGR on grinding performance were discussed by Aydoğan [6]. HPGR arrangements and semifinish-grinding options are given in Figures 3 and 4.

Vertical roller mills (VRM)
Vertical roller mills have a lower specific energy consumption than tumbling mills and require less space per unit and capacity at lower investment costs. Vertical roller mills are developed to work as air-swept grinding mills. Roller mills are operated with throughput capacities of more than 300 t/h of cement raw mix (Loesche mill, Polysius® double roller mill, Pfeiffer® MPS mill). Loesche roller mill and Polysius® roller mills are widely applied in cement raw material grinding. Schematical view of a Pfeiffer MPS mill is given in Figure 5 [7], and a view from inside of a vertical roller mill is given in Figure 6.

Loesche vertical roller mill
A cross section of a Loesche mill with a conical rotor-type classifier is shown in Figure 7. The pressure arrangement of the grinding rolls is hydraulic. The mill feed is introduced into the mill from above, falling centrally upon the grinding plate; then it is thrown by centrifugal force underneath the grinding rollers. A retention ring on the periphery of the grinding table forms the mill feed into a layer called the grinding bed. The ground material spills over the rim of the retention ring. Here an uprising airstream lifts the material to the rotor-type classifier located at the top of the mill casing where the coarse particles are separated from the fines. The coarse particles drop back into the centre of the grinding compartment for further size reduction, whereas the fines together with the mill air leave the mill and the separator. The separator controls the product sizes from 400 to 40 μm. The moisture of the mill feed (cement raw material) can amount to 15-18 %. The fineness of the mill product can be adjusted in the range between 94 and 70 % passing 170 mesh. Capacities up to 400 t/h of cement raw mix are recorded [8].

Cement quality
Better product quality can be achieved as compared to the ball mill product due to the better options for separate grinding. For example, in additive cement production, the blast furnace slag has to be ground to Blaine values of 5,000 cm 2 /g. Water demand and setting times are similar to that of a ball mill cement under comparable conditions [9].

Polysius® vertical roller mill (drying grinding roller mill)
A mill feed arrangement conveys the raw material to the grinding bowl. Two double rollers (representing four grinding rollers) are put in motion by the revolving grinding bowl. The double rollers are independently mounted on a common shaft; they move and adjust them-selves to the velocity of the grinding bowl as well as to the thickness of the grinding bed. Thus, rollers are in permanent contact with the grinding bed. A hydropneumatic arrangement transfers the grinding pressure to the rollers. The disintegrated mill feed is shifted to the grinding bowl rim from where a gas stream emerging from the nozzle ring surrounding the grinding bowl carries the material upwards to the separator. The coarses precipitated in the separator gravitate centrally back to the grinding bowl, whereas the fines are collected in the electric precipitator. A raw material moisture of up to 8 % can be dried when utilizing the preheater exit gases only. If hot air from an air heater is also supplied, then a raw material moisture of up to 18 % can be handled [8]. The power requirement is 10-20 % lower than a ball mill, depending upon the grindability and moisture content of the raw material [10]. Other types of roller mills such as ball race mill (Fuller-Peters mill) and Raymond bowl-type ring mill are used in coal grinding.

CKP vertical pre-grinder
The CKP pre-grinder has been under development by Chichibu Cement and Kawasaki Heavy Industries since 1987. It has been commissioned by Technip under licence since 1993. The system is applied widely for clinker grinding and has also been used on raw material grinding.
In operation, material is fed through the inlet chute onto the grinding table centre, spread out to the grinding path by the centrifugal force arising from the table rotation, before being compressed and ground by the rollers. The preground material drops down out of the periphery of the table to the bottom of the casing and is discharged by the scrapers through the discharge chute. Grinding principle of the CKP system is shown in Figure 8. Typical CKP application is given in Figure 9 [11].  Main advantages of the CKP pre-grinders are stated by Dupuis and Rhin [11] as follows: • The grinding capacity can be increased up to 120 % for some raw materials.
• Installation is very easy due to the compact design as well as the possibility of installing the CKP outdoors.
• The energy consumption of the total grinding plant can be reduced by 20-30 % for cement clinker and 30-40 % for other raw materials.
• The overall grinding circuit efficiency and stability are improved.
• The maintenance cost of the ball mill is reduced as the lifetime of grinding media and partition grates is extended.

Cemex ® ring roller mill
F.L.Smidth has developed this cement grinding system which is a fully air-swept ring roller mill with internal conveying and grit separation. This mill is a major improvement of the cement grinding systems known today which are ball mill, roller press (HPGR)/ball mill, vertical roller mill and closed-circuit roller press for finish grinding. Views of mill interior are given in Figures 10 and 11. Cemex ® grinds the material by compressing it between a ring and a roller. The roller rotates between dam rings fitted on the sides of the grinding ring, ensuring uniform compaction and grinding. The mill rotates at a subcritical speed, and scooping devices at both ends of the ring ensure effective internal conveying of the material being ground. The material leaves the scooping devices at various points, which ensures good distribution of the material in the airstream between the air inlets and outlets. The process air enters through two inlets at either end of the mill and leaves through an outlet at either end of the mill. The air passes the falling material and carries the finer particles to Sepax ® separator, in which the final classification of the product takes place. The oversize particles are returned from Sepax ® to Cemex ® for further grinding. Due to this unique combination of internal grit separation and air-swept material conveying to Sepax ® , no external mechanical conveyor is needed, which makes the installation very compact and simple. The airflow rate through the mill is relatively low, the only lower limitation being the need for sufficient internal grit separation and conveying of the preseparated material to the final classification in Sepax ® separator [12].  Main purposes in designing of the ring roller mill (Cemex ® ) can be summarized as follows: • To reduce the specific energy consumption of grinding • To reduce the wear on the mill elements by applying pressures on the grinding bed • To reduce the energy consumption of the mill fan by reducing the air consumption in the grinding process • Simple mechanical design • Simple and compact design to reduce the external mill load recirculation • Simple and easy control of product quality and mill operation • Simple and easy change of product type Grinding tests by the F.L.Smidth company have shown that Cemex ® produces cement which meets the requirements of the standard specifications while enabling substantial savings in grinding energy consumption compared to the traditional ball mill systems. Due to the more energy-efficient grinding process, Cemex ® ground cement will usually have a steeper particlesize distribution curve than corresponding ball mill cements. Consequently, when ground to the same specific surface (Blaine), Cemex ® cement will have lower residues on a 32 or 45 μm sieve and tend to have a faster strength development. Grinding of cement to a lower Blaine value will reduce the specific power consumption [12]. A comparison of typical specific energy consumption of Cemex ® mill with conventional multi-compartment ball mill grinding and HPGR pre-grinding closed-circuit operations is given in Table 1 Some of the advantages of Cemex ® mill can be summarized as follows: • Up to 40 % lower energy costs compared with conventional grinding installations.
• Internal conveying and grit separation.
• No external mechanical conveyor.
• Low noise level.
• A third of the grinding pressure of the roller press and moderate grinding pressures.
• Long life of wear segments.
• Drying and cooling ability.
• Compact and simple design. • High grinding capacity.
• Cement quality meets prevailing standards.
• Same or better strengths than cement from ball mill.

Cement quality
As it was stated in the literature, grinding tests have shown that Cemex® produces cement which meets the requirements of standard specifications while enabling substantial savings in grinding energy consumption compared to the traditional ball mill systems. Due to the more energy-efficient grinding process, Cemex® ground cement will usually have a steeper particlesize distribution curve than corresponding ball mill cements. Consequently, when ground to the same specific surface (Blaine), Cemex® cement will have lower residues on a 32 or 45 μm sieve and tend to have a faster strength development. When grinding to a 28-day-strength target, Cemex® cement can be ground to a lower Blaine value, which further reduces specific power consumption [12].

Horomill®
Horomill® is a ring roller mill which is a joint development by the French plant manufacturer FCB Ciment and the Italian cement producer Buzzi Unicem Group [13]. Horomill® can be used in grinding of: • Cement raw materials (i.e. limestone, clay, iron ore, etc.) • Cement clinker and cement additive materials (i.e. limestone, slag, pozzolan, etc.) • Minerals and coal

Horomill® design and operational principle
The Horomill® (horizontal roller mill) consists of a horizontal shell equipped with a grinding track in which a roller exerts grinding force. The shell rotates faster than the critical speed which leads to centrifuging of the material. The main feature is the roller inside the shell which is rotated by the material freely on its shaft without a drive. Operating principle is schematically shown in Figure 12. Material is fed to the mill by gravity. There are scrapers located in the upper part of the shell. Scrapers cover the entire length of the mill and scrape off the material which falls onto the adjustable panel of the material advance system. Position of the material advance system which is sloping towards the discharge end could be changed in such a way that material could advance slower or faster, and thus it determines the number of passage of material under the roller which means the adjustment of circulating load. Grinding pressures change within a range of 500-800 bars. Concave and convex geometries of the grinding surfaces lead to angles of nip two or three times higher than in roller presses resulted in a thicker layer of ground material [14]. Horomill® mainly consists of three zones: • Feeding • Grinding

• Discharging
In the grinding zone, the cylindrical roller transfers the grinding power onto the material. Material bed in the mill is generated by the centrifugal effect.
As compared to hybrid systems, Horomilling resulted in lower energy consumptions with energy savings of 30-50 % for the same product quality. Noise generated is lower than conventional ball mill. They are smaller and compact units. Frictional forces in the Horomill grinding are kept at its minimum, and hence wear is due to the lack of differential speed between the material and the grinding ring. Horomill® is designed for closed-circuit finish grinding when compared with an HPGR. Bed thickness is two or three times the roll press (HPGR) [15].
It also has the flexibility of a vertical roller mill in grinding of different materials. A larger angle of nip draws the material bed into the grinding gap and reduces wear as compared to vertical roller mills. The recirculation of material within a vertical roller mill is very high. The recycle ratios are 15 or more, but it is practically impossible to measure the recycle ratios in a mill operating on the airflow principle. Material bed passes many times through the stressing gap, and it is possible to adjust the number of stressing during operation in a Horomill®. Also an internal bypass can be implemented if some of the ground material is returned from the mill outlet to the inlet. The external recycle ratio of a Horomill® connected in a closed circuit lies between four and eight and is therefore lower than with a roller press (HPGR) and vertical roller mill [14]. A comparison of the angles of nip of material is given in Figure 13 [15]. A photograph of an industrial scale Horomill® [13] is shown in Figure 14.

Typical Horomill® grinding application
Typical industrial scale Horomill® grinding and classification closed circuit are given in Figure 15. The circuit includes an elevator, a conveyor to the TSV™ classifier, a finishedproduct recovery filter at the TSV™ outlet and an exhauster. The rejects from the TSV™ classifier are returned by gravity to the mill inlet. The main features of the plant are as follows [15]: • Horomill®-installed power: 600 kW at variable speed

Typical Horomill® grinding and classification circuit (case study)
An industrial sampling survey was carried out during CPP-30R (pozzolanic portland cement) production around the Horomill® grinding and classification circuit given in Figure 16.
Sampling points of the circuit are shown in a simplified flowsheet (Figure 16). Horomill® was closed circuited with a TSV™-type dynamic separator in the circuit.

Performance evaluation methodology
Prior to sampling surveys, steady-state conditions were verified by examining the variations in the values of variables in the control room. When steady-state condition was achieved in the circuit, sampling was started, and sufficient amount of samples were collected from each point as shown in Figure 16. Due to the physical limitations, dried pozzolan stream was not sampled. Samples collected after stopping the belt conveyors by stripping the material from a length between 3 and 5 m is shown in Table 2. The operation during sampling was closed to steady-state conditions. Important variables of the operation were recorded in every 5 min in the control room. Average values of the control room data were used in the mass balance calculations. Mass balance calculations were carried out using JKSimMet computer program. Design parameters of the Horomill are presented in Table 3.

Laboratory studies
A combination of sieving and laser-sizing techniques was used for the determination of the whole particle-size distributions for each sample. SYMPATEC® dry laser sizer was used to determine the particle-size distribution of subsieve sample of 149 μm for each sample. Size distribution of +149 μm material was determined by dry sieving using a Ro-Tap. The entire size distribution for each sample was calculated using the sieving results obtained from the top size (50.8 mm) down to 149 μm and laser results obtained for the subsieve sample of −149 μm.

Mass balance calculations
Some errors are inevitable in any sampling operation. These errors result from dynamic nature of the system, physical conditions at particular point, random errors, measurement errors and human errors. Mass balancing involves statistical adjustment of the raw data to obtain the best fit estimates of flow rates. In this context, by using the particle-size distributions and the control room data, an extensive mass-balancing study was performed around Horomill®#3 circuit. Tonnage flow rates (t/h) and particle sizes of the streams are calculated by JKSimMet mass balance software. The success of the mass balance was checked by plotting the experimental and calculated (mass-balanced) particle-size distributions as shown in Figure 17. These results plotted in a 45° line indicate the quality of both sampling operation and laboratory studies. According to the result of mass balance calculations, if there had been a statistically significant difference between experimental and calculated values (scattering data), the data would have been rejected and not be used for performance evaluation studies. In this research, data obtained as a result of sampling and experimental studies were found to be in a satisfactorily good fit. Mass balance model of the circuit with the calculated tonnage flow rates (t/h) in every stream and fineness as 45 μm% residue is shown in Figure 18.  The performance of any classifier, in terms of size separation, is represented by an efficiency (TROMP) curve. An example for a classifier is shown in Figure 19. It describes the proportion of a given size of solids which reports to the coarse product. Mass-balanced particle-size distributions and tonnage flow rates around the separator were used to evaluate the performance of the separator. Percentage of any fraction in the feed pass to the coarse product (%) is defined as partition coefficient and expressed by Eq. (6): where U is the separator coarse tonnage (t/h), F the separator feed tonnage (t/h), u i is the % of size fraction (i) in separator coarse and f i is the % of size fraction (i) in separator feed.
Actual TROMP curve established for TSV® is presented in Figure 19. The d 50 size corresponds to 50 % of the feed passing to the coarse stream. It is therefore the size which has equal probability of passing to either coarse or fine streams. When this size is decreased, the fineness of the product increases. The operational parameters that affect the cut size are rotor speed and separator air velocity. Cut size for the TSV® was determined as 23.33 μm. The percentage of the lowest point on the tromp curve is referred as the bypass. It is the part of the feed which directly passes to the coarse stream (separator reject) without being classified. Bypass value is a function of the separator ventilation and separator feed tonnage. The bypass value of TSV® was 23.29 % which indicated a consistent performance for this separator. Fish-hook effect (β) is the portion of fines returning back into separator reject stream. When there is incomplete feed dispersion at the separator entry, or even within the classification zone, aggregates of fine particles may be classified as coarse particles and thus report to the coarse stream. Fish-hook amount of TSV® was 1.58 % which also indicated how effectively it is operating: Fish hook 24.87 23.29 1.58% -= -= The sharpness of separation was defined as d 25 /d 75 where d 75 is the particle size whose 75 % is reported to the separator reject and d 25 is the particle size whose 25 % is reported to the separator reject. 0.32 k = Usually, for TSV®-type separator, it is between 0.55 and 0.7. When the normal range for sharpness (k) parameter is considered, it is found to be not in the normal range [16]. When the normal range for sharpness (k) parameter is considered, it was found to be not in the normal range. The imperfection of separation is defined by Equation 8, and I was calculated as 0.47: The value of I indicated that separation performance is sufficiently good.

Operational results from an industrial scale Horomill® grinding and HPGR/two-compartment ball mill and classification closed circuit
Typical operating conditions for the Horomill® and two-compartment ball mill grinding with HPGR pre-crushing and classification circuits are compared in Table 4 for the same production type. As can be seen from Table 4, Horomill® production configuration has resulted in energy savings of 50 % as compared to HPGR/two-compartment ball milling configuration [16].

Comparison of different grinding technologies
Typical specific energy consumption comparison between Horomill® product and HPGR hybrid system for pozzolanic cement with a 4,200 Blaine is as follows [13]: It was also reported that concrete workability from a portland cement with a 3,200 Blaine which is a Horomill® product is equal or better than an equivalent ball mill product. Mortar and concrete strengths are always higher as shown in Figures 20 and 21. The closed-circuit recirculation factor is noted as about six in Horomill® grinding [17]. A comparison between the grinding systems and conventional ball mills applied in cement grinding circuits is given in Table 5. Grinding efficiencies of different systems in grinding of cement to a fineness according to a Blaine of 3,000 cm 2 /g were compared in Table 6. Figure 21. Compressive strength on concrete [15].  Table 5. Comparison between the grinding systems and conventional ball mills applied in cement grinding circuits.
Energy-Efficient Technologies in Cement Grinding http://dx.doi.org/10.5772/64427 The efficiency of a two-compartment ball mill is defined to be 1.0. This efficiency reflects the power consumption of the mill only and does not include any auxiliary equipment like conveyors and dust collectors nor the separator.

Conclusions
Comparisons between different energy-efficient grinding technologies and applications were presented for production of cement with energy savings. Industrial-scale data related to Horomill® and Polysius® HPGR/two-compartment ball mill circuit provided insights into the operational and size-reduction characteristics of Horomill® and HPGR/two-compartment ball mill-grinding process with indications that Horomill® application could produce the same type of pozzolanic portland cement at lower grinding energy requirement. The specific energy consumption figures indicated approximately 50 % grinding energy savings in Horomill® process.