Hakan Benzer,* Alex Jankovic,† and Levent Ergun*
A B S T R A C T
The current world consumption of cement is close to 2 billion tpa and is increasing by about 1% per annum. Conventional cement grinding circuits consist of two-compartment tube mills and air separators. Alternative mills such as high-pressure grinding rolls (HPRGs), vertical roller mills, and Horomills have been applied in recent times to improve grinding efficiency.
Air separators play a crucial role in improving the overall energy efficiency of a cement grinding circuit, and their design has been improving continuously over the decades.
The introduction of a clinker precrushing stage can significantly improve cement grind-ing energy efficiency. Due to the relatively low capital cost associated with installation of a Barmac crusher, it is proving an attractive upgrade option. Hybrid grinding circuits with HPGRs are being widely used, primarily to increase energy efficiency, with specific energy consumption reduced to almost 50% compared to some conventional circuits.
I N T R O D U C T I O N
The current world consumption of cement is close to 2 billion tpa. During the last 10 years, cement production has increased by 38%. Different types of portland cement are manu-factured to meet different physical and chemical specifications. The American Society for Testing and Materials (ASTM) has designated five types of portland cement, the charac-teristics of which are outlined in Table 1.
Portland cement is made from exact proportions of materials containing calcium, silica, alumina, and iron. Approximately 1.5 t of raw materials are required to produce 1 t of finished cement. Grinding is an important operation in the cement making process, occurring at the beginning and end of the production cycle. The last stage in the process of manufacturing portland cement is the finish grinding of clinker together with small amounts of gypsum and some admixtures. The principal objectives of clinker grinding are to promote the hydration of cement and to ensure complete coating of inert aggre-gates. The fineness of the cement affects the placeability, strength, and permeability of the concrete properties. The finer the grind, the more reactive the finished cement.
Therefore, every type of cement must exhibit a particular degree of fineness to meet its quality specification. In Figure 1, the particle-size distribution of the different cement types are presented.
* Hacettepe University, Ankara, Turkey
† Metso Minerals Process Technology (Asia Pacific), Brisbane, Australia
170 ADVANCES IN COMMINUTION COMMINUTION PRACTICES
The electrical energy consumed in the conventional cement making process is in the order of 110 kWh/t, about 30% of which is used for raw materials preparation and 40%
of which is used during final cement production by cement clinker grinding. Figure 2 shows the consumption of electrical energy by the different processes in a typical cement production plant (Fujimoto 1993). Minimizing production costs and increasing environ-mental concerns have emphasized the need to use less energy and therefore promoted the development of more-energy-efficient machines for grinding and classification.
E Q U I P M E N T U S E D F O R C L I N K E R G R I N D I N G
Tube Ball Mill
The continuous ball mill has been used for more than 100 years and is still the most widely installed grinding equipment for this application. Cement is ground in tube ball mills operating either in open or closed circuit. The tube mills are characterized by their length/diameter (L/D) ratio with a ratio of 3 found to be best to minimize energy expen-diture (Schnatz and Knobloch 2000). The tube ball mills can be operated with one, two, or three compartments, and the length of each compartment should be designed to achieve optimum size distribution variation from feed to the discharge end.
Special diaphragms divide the cylinders of multicompartment mills. The dia-phragms are primarily designed to prevent loss of the balls to the next compartment
TABLE 1 Portland cement classification with its constituents and fineness
Types Clinker, % Admixture, % Minor Component, % Fineness +45 μm, %
CEM I 95–100 — 0–5 11.4
CEM II 80–94 6–20 0–5 14.2
CEM III 35–64 36–65 0–5 5.9
CEM IV 65–89 11–35 0–5 11.6
CEM V 40–64 18–30 0–5 17.0
CEM I CEM II CEM III CEM IV CEM V
0.001 0.01 0.1 1
Particle Size, mm 100
90 80 70 60 50 40 30 20 10 0
Cumulative % Passing
FIGURE 1 Size distribution of different cement types
CEMENT CLINKER GRINDING PRACTICE AND TECHNOLOGY 171
while allowing the flow of ground material through the mill. The design of the dia-phragm influences the fineness of the ground material (Duda 1985).
Various shapes of mill liners have been developed for cement mills (see Figure 3).
The classifying liners for clinker grinding have a specific design. This lining causes a clas-sification of the grinding ball sizes down the length of the mill. The grooved liner is usually used in the second or third compartment of the cement mill to produce a cascading motion which promotes abrasion breakage.
Operation of the tube ball mills is relatively well understood with several design and operating parameters of the ball milling operation affecting the mill efficiency and the quality of the cement produced (Gouda 1981).
Vertical Roller Mill
Vertical roller mills (VRMs) have been used for limestone and coal grinding in the cement industry for many years due to their high drying capacity, low energy consump-tion, compactness, and reliability in operation. The largest mill in operation has an installed power of 6 MW and grinds 840 t/h of lump feed down to 85% passing 90 Pm.
Cement grinding by a VRM has found applications in pregrinding systems, advanced pre-grinding systems, and finish pre-grinding systems (Shimoide 1996).
Quarry Crushing and Pre-Homogenization,
5%
Raw Material Grinding, 24%
Feed Homogenization, 6%
Burning and Cooling, 22%
Conveying, Packing, and Loading, 5%
Finish Cement Grinding, 38%
FIGURE 2 Energy consumption for different stages of cement production
FIGURE 3 Example of mill liners in the first and second compartment of a cement ball mill
172 ADVANCES IN COMMINUTION COMMINUTION PRACTICES
In a VRM, the interparticle comminution takes place in a material filled gap between the rotating table and the grinding rollers. The mill feed is charged to the center of the table and moves, affected by centrifugal forces and friction, toward the table’s edge. On its way, it is nipped by two, three, four, or six conical rollers installed at the outside rim of the table. The rollers are attached to hydraulic cylinders that provide the grinding force for comminution of the material. The ground particles leave with the airstream and are taken up by the separator incorporated into the casing of the mill. The fine product reports to the mill discharge, and the coarse reject of the separator falls back onto the table as a recirculating load.
The VRM was first used in a commercial operation to finish-grind cement in 1984 (Shimoide 1996). Since then, however, further applications of this technology in the industry have been relatively limited. One reason is that a portion of the power savings achieved in the VRM (because of higher grinding efficiency) is lost due to additional power consumption by the fan. In addition, the VRM suffers from roller wear problems.
Recent plant trials, however, have indicated that the problem can be reduced with new roller designs. The wear rate and the throughput of the system depends very heavily on the consistency of the materials being ground (Nobis 2001). Effective comminution largely depends upon the formation of a stable grinding bed between the rollers and the grinding table.
The main operational bottleneck of the VRM is its high circulating load from the sepa-rator back to the table. This causes inefficient grinding operation because of the high load accumulation inside the mill. To overcome this problem, the roller mills can be operated with external material circulation. It has been reported that the specific power consump-tion involved in producing portland cement with external material circulaconsump-tion was 30%
less than for producing these cements in tube mills (Feige 1981). The Kawasaki Inc. CKP mill is an example of this type of machine and was developed based on the proven tech-nology of VRMs (Sutoh et al. 1992). In CKP systems, material is fed through a central chute. A centrifugal force, produced by rotation of the table, distributes the product over the table surface. After grinding, which is carried out between the table and rollers, the material is extracted from the CKP by gravity with the assistance of scrapers (Miranda et al. 1998). CKP mills are generally used as pregrinders, and the grinding energy effi-ciency of these mills as a pregrinder has resulted in grinding energy savings of 17%
(Dupuis and Rhin 2003).
Horizontal Roller Mill
The horizontal roller mill (Horomill) consists of a horizontal cylinder supported on slide-shoe bearings and driven through an open gear train. Terms used to describe the principles of operation of a Horomill include a bed material compression mill, a multi-compression mill, and a high-capacity mill (Cornille 1999). A simplified diagram outlining the principles of operation is shown in Figure 4.
The material passes into the mill at one end of the cylinder and, because of the cen-trifugal effect caused by operating the cylinder above the critical speed, is carried as a uniformly distributed layer of material on its inner surface. The finished product is col-lected in a dust filter, while the coarse particles are recycled to the mill. The grinding force is transmitted to the roller by hydraulic cylinders. Internal fittings are provided to control the material recirculation. It’s been reported that the grinding process based on multiple compressions gives the machine a high stability, and also the recirculating load can be adjusted to suit the quality target (Cordonnier 1994).
Compared to a ball mill, the Horomill operates with a larger grinding bed thickness and moderate pressures that lead to energy savings of 35% to 40% when used for cement grinding. In operation, the specific costs related to the liner and wear parts are
CEMENT CLINKER GRINDING PRACTICE AND TECHNOLOGY 173
higher than in an equivalent ball mill (Brunelli 2001). Mechanical problems with a Horomill have been reported in a Konya cement plant in Turkey (Fochardiere 1999).
High-Pressure Grinding Roll
The high-pressure grinding rolls (HPGRs) developed by Professor Schoenert have been offered as a comminution technology with claims of improved performance when com-pared to conventional grinding technology. In particular, it has been claimed that the HPRG has a lower specific energy consumption (Schoenert 1979).
The material to be ground in an HPGR is compressed in a gap between two counter-rotating grinding rolls (see Figure 5) with circumferential speed of 1 to 1.8 m/sec. The product from the HPGR is a compacted cake that contains fine particles and coarser par-ticles with a large number of incipient cracks and weak points that greatly reduce the energy expenditure during further comminution (Ellerbrock 1994).
An HPGR can be used at different stages in the cement grinding process: pre-crushed, finish grinding, hybrid grinding, and semifinish grinding. When an HPGR has been used in the precrushed stage, 20% reductions in overall energy consumption have been achieved (Kellerwessel 1996). Hybrid grinding involves splitting the coarse fraction from the air classifier to the HPRGs and ball mill, respectively. In the semifinish grinding application, the HPRG is operated in closed circuit with the air classifier, and the fines from the separator are finally ground in a tube mill circuit. In the finish-grinding applica-tion, the HPRGs operate with an air classifier in closed circuit. Using this finish-grinding configuration, the potential energy savings can be as high as 50% (Kellerwessel 1996), but the water requirements in the subsequent mortar production process are significantly higher due to the narrow size distribution produced (Roseman 1989; Odler and Chen 1995).
Air Classifier
Classification in the clinker grinding circuits is achieved using the air classifiers. Devel-opment of the air classifier was based on the operating principles of two devices, the sim-ple expansion chamber and the Mumford and Mood separator, patented in 1885 (Klumpar, Currier, and Ring 1986).
There are two types of air classifiers, dynamic and static. Static air classifiers are an old technology without moving parts. Classification is achieved by changes in air velocity and direction. The principle of operation is shown in Figure 6a. The airstream carrying the particles is converted from a directional flow through the outer cone into a rotating flow by guide vanes. The particles are subject to a centrifugal force—the coarse particles moving to the outer wall of the inner cone and collected in a bin, while the fine particles leave with the air and are sent to a dust collector. The product size can be altered to some
F
FIGURE 4 Comminution principle in a Horomill
174 ADVANCES IN COMMINUTION COMMINUTION PRACTICES
extent by changing the angle of the vanes, but the efficiency is low and static classifiers can be regarded more as grit separators than efficient classifiers.
Dynamic classifiers have both moving and fixed internal parts. The dynamic air clas-sifiers utilize a distribution plate to disperse the feed material into the separation zone.
Thus a particle of material is subjected to three forces: centrifugal force from the distri-bution plate, uplift from the air current, and gravity. Figure 6b indicates the forces acting on a particle in a dynamic air classifier.
Dynamic classifiers have evolved through three generations, each being significantly better than its predecessor. The first-generation classifier had a distributor plate, and the air circulation in the classifier was provided by a vertically supported rotor. The main problems with the first-generation classifiers were that the circulating air became very hot, fine particles were not removed from the recycling air, and the control of the product was very difficult. Figure 7a shows a simplified sketch of a first-generation air separator.
The second-generation classifier (see Figure 7b) is based on the same operating principles as the first but an external fan is used to circulate the air and a cyclone is used
Fixed Roll Feed
Moveable Roll Oil Cylinders
Product Nitrogen cylinder
FIGURE 5 The principle of operation of HPGRs
Adjustable Blades
Immersion Tube Fines
Tailings
(a) (b)
Particle Feed Direction of Rotation
To Coarse Particle Cone
To Fine-Particle Chamber fC
fD fG
R T
FIGURE 6 (a) Schematic of the static air classifier; and (b) separation mechanism in a dynamic air classifier
CEMENT CLINKER GRINDING PRACTICE AND TECHNOLOGY 175
to remove fine particles. Greater product control is possible due to the ability to adjust the rotor speed and air velocity separately.
The third-generation separators are highly efficient separator devices (see Figure 8).
The feed material to the separator is delivered as a dispersed curtain of particles, and the horizontal air flow to the separator results in uniform separation performance across the unit. The fine particles pass through a rotating cage before going to the fine product.
The bars of the cage assist in the performance of the separator.
C I R C U I T C O N F I G U R A T I O N F O R I M P R O V E D E N E R G Y E F F I C I E N C Y
For most of the twentieth century, the common dry-grinding circuits for the production of finished cement from cement clinker consisted of two-compartment tube mills with or without the air separators. The advantage of this circuit is its simplicity and ease of oper-ation; however, the energy consumption is high, especially for open-circuit operation.
One of the reasons that the two-compartment tube mill circuit has limited energy effi-ciency is due to the high reduction ratio that must be achieved in the single comminution/
classification step. Clinker feed size can vary from F80 = 10–40 mm and the final product size from P80 = 35–40 ȝm with the size reduction ratio being in the order of 250–1,000.
Large balls (up to 100 mm) are required in the first compartment of the tube mill to crush the coarse clinker. Ball mill grinding efficiency for feed sizes larger than F80 = 2–3 mm is particularly poor, and it should therefore be more energy efficient to precrush the clinker.
Recent work indicates that introduction of the Barmac crusher for clinker precrushing can increase the cement circuit throughput on the order of 10%–20%. Alternatively, the total energy consumption of the circuit can be reduced on the order of 5%–10% (Jankovic, Valery, and Davis 2004). This is an attractive upgrade option due to the relatively low capital investment involved in the installation of a Barmac crusher.
Clinker precrushing can be carried out with a variety of different crushers. Figure 9 shows the product size distributions from a Barmac and, alternatively, a high-performance (HP) cone crusher in closed circuit with a 4.75-mm screen at 2.3 kWh/t specific energy input. Although the 80% passing size for the HP cone crusher is finer, the Barmac product is potentially more favorable due to its higher content of fines. This advantage, however, is
Fresh Air
FIGURE 7 (a) First-generation dynamic air separator; and (b) second-generation dynamic air separator
176 ADVANCES IN COMMINUTION COMMINUTION PRACTICES
not crucial for the selection, as the clinker feed size, hardness, and abrasivity, as well as the required capacity, will have an effect on the particular crusher best suited to the par-ticular application.
In order to obtain the most efficient breakage in the first compartment of the ball mill after introduction of the precrushing stage, the ball size distribution should be changed to suit the new particle-size distribution of the material fed to the mill. An example of the measured particle-size distribution of the combined ball mill feed (new feed + 150%
recycle) with raw and precrushed clinker is shown in Figure 10. A significant fraction of the material in the feed containing the raw clinker is coarser than 5 mm. To effectively
Shaft
Fines Plus Air
Blade Vane
Feed Plus Air Inlet Spoke
Coarse Tertiary Air
Volute Annular Space
Plate Section
FIGURE 8 Schematic of a third-generation dynamic air separator
0.1
0.01 1 10 100
0 10 20 30 40 50 60 70 80 90 100
HP Cone, 2.3 kWh/t, Product Barmac, 2.3 kWh/t, Product HP Cone Feed
Barmac Feed
Size,mm
Cumulative % Passing
FIGURE 9 Product size distribution from the closed Barmac and HP cone crusher circuit
CEMENT CLINKER GRINDING PRACTICE AND TECHNOLOGY 177
grind this sized feed, the calculated top ball size required (using the Bond formula) would be 90–100 mm. For precrushed feed, the top ball size would be 35–40 mm due to the absence of coarse particles.
With precrushed feed, the optimum ratio in length between the first and second compartment would also be affected. Design of the transfer grate, mill liners, and sweep air velocity should also be reviewed to suit the new reduced ball size and provide effi-cient removal of fine particles.
In the last 20 years, HPGRs have been used extensively in cement grinding circuits due, primarily, to their higher grinding efficiency compared to the conventional two-compartment tube mills. HPGRs can be used for precrushing, finish grinding, hybrid grinding, and semifinish grinding. Table 2 shows the energy consumption of five cement-grinding circuits employing HPGR units in different applications (Aydo÷an, Ergün, and Benzer 2004). It can be observed that the overall circuit specific energy consumption decreases when a large portion of the size reduction (higher HPGR kWh/t) is performed by the HPGR. Circuits that employ HPGR mills can achieve in excess of 40% improvements in grinding energy efficiency, providing that the circuit is optimized and automated process control is employed.
In order to assess the performance of a particular cement-grinding circuit and to compare efficiency of different circuit configurations, complete audits are required. The audit includes monitoring and sampling of different circuit streams during steady-state operation, as well as mill inspection and sampling after a crash-stop. Based on informa-tion obtained from the audit, a mass balance can be carried out to determine material
Particle Size,
FIGURE 10 Combined ball mill feed-size distribution when processing raw and precrushed clinker
TABLE 2 Specific energy consumption in different cement grinding circuits utilizing HPGRs
Cement Grinding Circuit Description
HPGR Specific Energy Consumption, kWh/t
Circuit Overall Specific Energy Consumption, kWh/t
Open-circuit HPGR, closed-circuit ball mill 4.05 34.2
Open-circuit HPGR, closed-circuit ball mill 4.05 34.2