main trends concerning grinding processes in the cement industry

Summary: The main trends concerning grinding processes in the cement industry are still higher efficiency, reduction of the power consumption and system simplicity. In the case of new orders, vertical mills have increased their share to over 60  % and ball mills have fallen to less than 30  %. It is somewhat surprising that the number of different grinding processes and mill types used by the industry have increased rather than decreased. The throughput capacities of the employed mills have risen, but in some cases this has brought machines, e.g. gear units, to the limit of their capabilities. In the cement industry, the use of separate grinding plants is currently showing a two-digit annual growth rate. The number of mill vendors has also increased and the competitive pressure has intensified.

Introduction：

Worldwide, there are around 600 separate grinding plants in the cement industry. These receive their feed materials, such as clinker, slag, fly ash and other pozzolanas and intergrinding materials from various sources and are not preceded by an integrated clinker production line. This principle, known as the “split grinding” or “hub and spoke process”, is a practical method in cases where the raw materials for clinker manufacturing are only available locally, and particularly not at the locations where the cement is consumed. For this reason, the market for split plants has grown especially strongly in China, India, Vietnam, Brazil and Turkey. However, numerous split plants also exist in countries like Italy, Germany, France, Spain, South Korea, South Africa and Australia. One reason for this is that existing integrated plants often remain in operation only as grinding plants after clinker production has been discontinued, and the reason is that grinding plants enable newcomers to quickly establish themselves on a market.

The two foremost cement manufacturers, Lafarge and Holcim, increased the number of their grinding plants by 11 and 16 respectively in the period 2005 to 2008. Taken together, this makes up 16  % of all new grinding plants installed during this period. The variety of employed grinding processes already mentioned can already be seen in the case of the separate grinding plants (Fig. 1). Today, approx. 70  % of the installed mills are ball mills. This statistic includes the figures for China. In the case of new plants, the number of ball mills is even lower, at less than 50  %, and energy-efficient grinding processes are preferred. This trend is even clearer in integrated cement plants, where the worldwide percentage of new ball mills for raw material and coal grinding is now only approx. 10  % (excluding China) or 15  % (including China). The figures for clinker grinding are again comparable with those for separate grinding plants.

2Overview of grinding processes and applications

The cement industry makes use of four mill types: the ball mill, the vertical mill, the roller press (also known as high-pressure grinding roll) and the horizontal mill. A report on the development over time towards those mill types, which was largely determined by efforts to achieve higher energy efficiency, was already presented in [1]. However, it also has to be taken into consideration that the different mill feed raw material, coal, clinker and slag have different grindabilities, feed particle size ranges and moisture contents and also demand different throughput rates, fineness data and other quality parameters. Each of the mill types and the associated grinding process are therefore more suitable for some applications and requirements than for others, and there is no universal mill [2]. Instead, combination processes have been devised, or else certain mill types dominate individual applications.

Figure 2 presents an overview of new mill orders in the cement industry worldwide from 2007 to 2008. These figures do not include local supplies by Chinese vendors, as no precise data material is yet available for such cases. In the above two years, which will go down in history as boom years for mill suppliers, more than 660 mills were sold, 373 of them in 2007 and 289 in 2008 [3]. The above statistics take account of cancellations due to the impending world economic crisis. Of the more than 660 mills, 61 % (410 machines) were vertical mills. In the case of western vendors, 63 vertical mills were delivered to China (82  % for raw material grinding), while Chinese vendors delivered 5 units to clients outside China. During the stated period, an estimated 300 vertical mills were supplied by Chinese vendors to clients within China.

Aside from the 410 vertical mills in the above statistic, 189 ball mills (29  %) and 63 (10  %) other types of mill were sold. Of these 63 other mill types, 59 were roller presses and only 4 were horizontal mills, so that the last type of mill only had a total market share of less than 1 % in the period of time under consideration. Figure 3 depicts the percentages of the different applications. Clinker/cement grinding is the biggest market segment with 42  %, followed by raw material grinding with 34  %. Taken together, coal and slag grinding make up less than a quarter of all applications. All these figures have to be seen against the background of the order boom of the two years and particular account must be taken of the fact that vertical mills have the longest delivery times, other mills provide price advantages or have their own particular advantages and that therefore a decision in favour of vertical mills was made more difficult in specific cases [4].

2.1 Raw material grinding

Raw material grinding has the objective of producing a homogenous raw meal from a number of components that are sometimes variable in themselves. The feed moisture contents are generally between 3 and 8  %, but sometimes also over 20  % by weight. Fineness requirements are usually <  10  % to 15  % residue on the 90  µm screen (<  1– 2  % residue on the 200 µm screen) with feed particle sizes of 100-200 mm. Figure 4 shows that on this sector vertical mills were installed in 80  % of all new grinding plants, after 84  % in the preceding period of time. The advantage of vertical mills is their relatively low power consumption and the simultaneous grinding, drying and separation in the mill itself, together with a wide mass flow control range of 30 –100  %. This mill type achieves a specific power requirement of below 10  kWh/t at a medium raw material hardness and a medium product fineness of 12  % R 0.09 mm.

The increasing throughput rates of vertical mills kept pace with the rise in kiln line capacities. For instance, a Loesche LM 69.6 with its grinding track diameter of almost 7 m, six grinding rollers (Fig. 5) and a drive power of 6600 kW can achieve throughput rates of up to 1200 tph for medium-hard cement raw material. The example of a grinding plant at &shy;Grasim’s Tadripatri works shows that the mill capacity decreases significantly if the cement raw material is hard. At this plant, 16 000 tpd of raw material is ground by two LM 69.6 mills for a clinker output of 10 000 tpd [5]. Each mill runs at its warranted capacity of 420 tph. However, at a different plant in India the same size of mill achieves a grinding capacity of 745 tph. All the leading manufacturers of vertical mills now build machines with capacities exceeding 600 tph. Large vertical mills with several independent grinding rollers allow grinding operation to be maintained even if one roller or one pair of opposite rollers fails [5–6], so that at least partial-load operation of the kiln line is possible.

Ball mills are still used in 12–13  % of all Raw material grinding applications, such as the grinding of dry or abrasive raw materials.

However, the development potential of ball mills seems to be exhausted, even though some new concepts are under investigation, such as ball mills with a conical housing and mills with variable L/D ratios [7]. Although roller presses have so far generally been used in combination processes with ball mills, they have gained significance for the finish grinding process in recent years, partly due to new developments. Horizontal mills have so far little been able establish themselves in raw ma&shy;terial grinding. The growing success of roller presses is primarily due to improved separator concepts. KHD Humboldt Wedag brought the V-Separator onto the market in 1994. This separator has meanwhile become an important module in grinding systems and more than 135 units have been sold.

Such a system for raw material grinding (Fig. 6) consists of one or two parallel roller presses, a downstream static V-Separator and a preceding high-efficiency separator. The two separators are connected by a pneumatic conveyor. In the V-Separator, the material is dried and the coarse fraction is removed. The coarse material is then returned via a bucket elevator to the mill feed system for regrinding together with the fresh feed material. The fine material passes to the separator for finished product &shy;collection. Oversize material is also returned from there to the roller press. Throughput rates of up to 1000 tph can be achieved with this type of system. Particularly COMFLEX® compact systems exceed expectations with regard to system simplicity [8]. &shy;Polysius has implemented a similar concept, e.g. at TXI Oro Grande, with the difference that both separators precede the roller press [9].

2.2 Clinker/cement grinding

For cement grinding, normal mill capacities are between 100 and 200 tph. This means that even for medium-sized kiln lines of e. g. 5000 tpd, several mills usually have to be used. Cement grinding plants with capacities exceeding 300 tph have up to now only been installed at a handful of factories. Mill capacities depend greatly on the grindability of the clinker and of the intergrinding material, such as slag or limestone, as well as on the required cement fineness. The mill output decreases in line with increasing product fineness. The achieved mill capacities for different vertical mill sizes and cements are stated, for example, in [10 –11]. Developments during recent years have shown that there are now only small differences in the cement quality produced by different cement mills [12 –13].

Figure 7 shows that ball mills are still the type usually ordered for cement grinding applications, even though their market share has decreased from 59  % to 49  %. In contrast, vertical mills have significantly increased their share from 17  % to 39  %. The market shares of roller presses and horizontal mills have declined slightly. In view of the poor energy utilization of ball mills, one may wonder why they still account for almost half of all ordered cement mills. The reasons for this are complex. Firstly, ball mills are used in combination grinding systems and finish grinding systems because they are held in high regard due to easy operation and high availability (Fig. 8). Secondly, conversions to high-efficiency separators in closed circuits are often undertaken, often involving the purchasing of replacement ball mills or the modernization of existing ball mills [14]. Sometimes, ball mills are purchased as stand-by mills for vertical mills [15]. Furthermore, ball mills offer price advantages in comparison to other mill types, particularly if supplies from China are considered (Fig. 9).

In recent years, a number of developments have dealt with the improvement and optimization of ball mills. Such efforts are prompted by the fact that ball mills are still widely used and by the increasing specific grinding costs. Studies by the VDZ [16] or, for example, by CRH [17], show that existing cement mills possess a high optimization potential. One significant field is improvement of the grinding ball grading, which can reduce the energy requirement by around 10  %. Specific ma&shy;terial transfer diaphragms and discharge diaphragms with ma&shy;terial flow regulation bring a significant additional improvement. Modern noise sensors on the rotating mill shell can accurately measure the filling levels of the first and second grinding compartments independently of each other [18].

The flexibility of the HPGR/roller press allows it to be used in various system configurations as a “combi-mill” (Fig. 10)together with ball mills for primary grinding and semi-finish grinding, or as a single-stage grinding process for finish grinding. Particularly in the last few years, a trend towards finish grinding has become discernible. Three points are viewed as the main reasons for this [19]:

1. significantly improved grinding element service lives [20],

2. new separator technology with static-dynamic separators (Fig. 11),

3. the quality characteristics of cements from grinding machines other than the ball mill are no longer regarded as negative.

Finish grinding plants equipped with roller presses achieve a specific energy requirement of 21 kWh/t, the roller press being responsible for 15.5 kWh/t or approx. 79  % and the auxiliary units, separator, separator fan and bucket elevator accounting for 21  % [19].

The application of vertical mills for cement grinding is currently regarded as a real technological breakthrough [21]. However, this does not refer to so-called pregrinders [1], but to vertical mills with integral separator (Fig. 12) that are used for &shy;finish grinding. For the grinding of Portland Cement with 95  % &shy;clinker, mill capacities of more than 350 tph can be achieved [22], although the usual mill sizes offered by most vendors only permit outputs of around 200 tph (Fig. 13). With the aim of achieving higher capacities, a trend towards four or more grinding rollers is discernable, as this keeps roller dimensions small and thus minimizes the dynamic loadings and vibrations in the mill. The energy savings achieved with vertical mills in comparison to ball mills are primarily a function of the required fineness of grinding (Fig. 14).

All vertical mills provide the advantage of quick product type changeover because the short material residence times in the mill enable the attainment of quality and capacity parameters just a short time after a changeover [12, 22], without having to stop the grinding operation. On the other hand, it should not be forgotten that the grinding conditions (grinding speed, grinding pressure, water injection, grinding aids etc.) have to be optimized for each different mill feed material. Also, the height of the dam ring on the grinding table can only be optimally adjusted for one single product, or else compromises have to be made. During operation, the grinding bed depth can practically only be influenced by the grinding speed. This indicates the need for increased use of variable-speed drive units or the intro&shy;duction of completely new drive solutions for vertical mills in the near future.

Horizontal mills are suitable for use as a single-stage grinding process in cement finish grinding systems. After some teething problems, the scaled-up Horomill process is now regarded as technically mature. More than 15 years after its introduction onto the market, more than 40 Horomills are now in operation (Fig. 15), most of them for cement grinding. Compared to ball mills, an energy saving of up to 50  % can be achieved, and compared to vertical mills the saving is up to 20  % [23]. This mill type’s limitation to relatively low throughput rates is regarded as the biggest hurdle for its further marketing. The biggest mill available at present allows a throughput rate of 120 tph for normal Portland Cement. A report concerning a modified horizontal mill, called the BETA mill (Fig. 16), was presented in [24–25]. This grinding principle permits individual setting of grinding bed depth and grinding pressure, increasing the possibility that equally high or higher efficiencies could be achieved than with roller presses.

2.3 Slag grinding

The grindability of granulated blast furnace slag (GBFS) is up to 30  % worse than that of cement clinker, while the fineness requirements of 4200 cm2/g to 5600 cm2/g are comparatively high and the moisture content can reach 20  %. However, with 6  % of all applications, slag grinding only accounts for a minor proportion of grinding systems in the cement industry. For this reason, just a few orders suffice to alter the weighting of individual mill types. After a period of high growth rates, the market share of vertical mills recently declined from 59  % to the present 45  % (Fig. 17). While the share of ball mills has remained almost constant, the other grinding processes have nearly doubled their market share from 19  % to 34  %. However, this is practically restricted to roller presses, because horizontal mills have so far hardly become established for slag grinding.

The main advantage of vertical mills (Fig. 18) is their process simplicity with drying, grinding, separation and ma&shy;terial &shy;conveyance all taking place in one unit, combined with their very good energy utilization and low wear rates. In the meantime they achieve an equally high fineness of grinding >  6000  g/cm2 (Blaine) as ball mills. In slag-grinding systems, the external circulating material quantities are higher than those of raw material and clinker grinding systems because of the lower gas velocities in the mill [26]. Tramp metal is removed from the circulating material by magnetic drum separators and other separating devices. Vertical mills can handle a maximum moisture content of 20  %. Moist material has the effect of a grinding aid and actually assists in the grinding bed stabilization. Throughput rates depend greatly on the mill feed material and on the required product finenesses. Up to now, vertical mills for slag grinding have been designed for capacities of up to 300 tph [22].

Roller presses are employed for finish grinding and in combination grinding processes together with ball mills. Due to their high grinding pressures and short material compression time, the greatest energy savings are achieved in finish grinding. Finish grinding plants of the latest generation (Fig. 19) are very compact. Due to improved separator technology and longer grinding element service lives, roller press finish grinding plants have become an alternative to vertical mills. The separator technology and drying in an air stream allow the mill to be fed with GGBFS with moisture contents of up to 15%. Depending on the system configuration, the dried material from the external material circuit and the coarse material from the finished material separator can be mixed into the fresh feed material for the roller press. It is sensible to have a minimum feed material moisture content >  2  % in order to improve the material pull-in conditions for the roller press.

2.4 Coal grinding

Today, the product fineness demanded when grinding solid fuels is approx. <  0.5 – 5  % residue on the 90 µm screen for petroleum coke and anthracite and <  10 –15  % residue on the 90 µm screen for hard coal. In each case, the required residue on the 200 µm screen is 0  %. Finer grinding of the coal reduces the NOx emissions of a kiln line, but simultaneously decreases the throughput of the coal mill and raises the specific power consumption of the grinding process. Reduction of the residue on the 90 µm screen by two percentage points raises the electrical power intake by around 1 kWh/t. While the specific power consumption for grinding depends on the required product fineness and the grindability, the amount of wear in the mill is mainly determined by the quartz and pyrites content of the coals [1].

Fig. 20 shows that in recent years only two types of mill accounted for most coal grinding applications in the cement industry. Vertical mills have meanwhile achieved a share of almost 90  %, while ball mills are just over 10  %. Ball mills are almost exclusively employed for fuels with poor grindability. Up to now, the capacities of vertical mills for coal grinding in cement works have ranged up to 50 tph. Keeping pace with the trend towards higher kiln throughput rates has presented no problems. Vertical mills can cope quite flexibly with changes in the type of fuel, different grindabilities and varying fineness requirements, because the grinding pressures and, in some cases, the grinding table speeds can be adjusted fully automatically.

3Important mill developments

3.1 Vertical mills

In the raw material and coal grinding sectors, vertical mills have maintained their high market shares of 89 –90  % and have improved to almost 40  % in clinker grinding applications. With 410 vertical mills ordered, including slag grinding systems (Fig. 21), the number of different design versions has increased. Loesche successfully transferred the 2 +2 principle to larger mills and has applied the 3 + 3 principle, with a noticeable trend towards smaller roller modules with more grinding rollers. Thanks to the Quadropol from Polysius and the OK mill from FLSmidth, 4-roller mills have also gained ground. On the other hand, there is still a trend towards specialization, as illustrated by the Atox mill for raw material grinding and by the OK mill for clinker and slag grinding. In contrast, orders for the FRM mill from FLSmidth are declining. Polysius could experience a similar situation with the Dorol if the Quadropol finds more use in the clinker and slag grinding fields.

One fundamental development aim for vertical mills is to mini&shy;mize their maintenance requirements and make them easier to service [10]. Other developments concern optimization of the drive units, as in the case of the Quadropol (Fig. 22) from &shy;Polysius [27], the ATOX mill or the implementation of multidrive solutions as in the case of the MPS from Gebr. Pfeiffer [28]. A general aim for the near future is to improve the reliability and availability of the bevel planetary gear units used for vertical mills, with an additional planetary gear stage or helical gear stage for drive powers above 3600 kW. In addition to the already established manufacturers Loesche, Gebr. Pfeiffer, FLSmidth and Polysius, other vendors – particularly companies from China – are attempting to gain market shares. These companies sometimes copy mill designs of the established vendors or even use their proprietary names. Among the leading Chinese suppliers are the Hefei Cement Research & Design Institute (HCRDI) and the CNBM Group and their production firm Hefei Zhong Ya. With the HRM4800 (Fig. 23), this company has one 450 tph mill in operation for raw material grinding and states that a mill size for 750 tph is to follow shortly.

The list of other Chinese companies supplying vertical mills to the cement industry includes the CNBM Group, the design institutes TCDRI and CDI (both belonging to the Sinoma Group and working in conjunction with the manufacturing company CemTeck), CITIC Heavy Industries, Jiangsu &shy;Pengfei, SHMG/NHI, Zhengzhou Global HMG, CKM (Conch Kawasaki Joint Venture) etc.. While Chinese technology for the raw material and coal grinding sectors is already quite advanced, there has been little development for clinker and slag grinding, where only a few references are available. The joint venture of &shy;Kawasaki Plant Systems with the cement producer Anhui Conch shows that the Japanese VRM technology is now being increasingly marketed via joint venture partners or licensees. Some time earlier, Kawasaki had already granted Technip CLE a license for pregrinders. Onoda/Kobelco, Taiheiyo Engineering and UBE Machinery had also already followed this path with licenses granted, for example, to FLSmidth, Promac and WalchandNagar.

3.2 Roller presses

In the case of roller presses, two worldwide trends are discernible.Whereas in China the lack of alternatives and the widespread use of ball mills for cement grinding has limited roller presses almost exclusively to combination grinding processes [29], the trend in the rest of the world is towards single-stage grinding processes and the employment of roller presses for finish grinding. The increasing success in finish grinding applications is primarily due to improved plant concepts with static and dynamic separators and to longer grinding element lifetimes. This applies to roller presses in raw material, clinker and slag grinding applications. Thanks to cast iron &shy;materials with bainite, new compounds and Ni-Hard segments, the service lives of grinding elements has been increased to above 25 000 hours, even for relatively high grinding pressures in excess of 250 MPa.

To date, there are only three &shy;

established vendors of roller presses: KHD Humboldt Wedag, Polysius and &shy;Koeppern. Although FLSmidth has not developed this market segment in recent years, they now intend to resume roller press engineering. Their main reasons for doing so are the facts that roller presses are gaining in significance not only for the cement industry but also for the minerals sector, and that competition is becoming more intense on the mill sales market. The Chinese companies CNBM, Sinoma (CemTeck), CITIC and particularly Chengdu Leejun have meanwhile entered the roller press market [30]. The last-mentioned company has already supplied more than 300 units in China alone. They have received about 20 orders from customers outside China. While these Chinese vendors are said to still have their deficits, particularly in respect of documentation and service, the established vendors all provide a good level of service and thus assure the reliability and availability of their products.

4Prospects

Vendors of mills are now facing increasingly stiff competition in the cement industry. This is due firstly to the large number of technically mature plant solutions and secondly to the new competitors entering the market, especially from China. The established mill manufacturers are trying to maintain their technological competitive advantage by introducing various innovations. These include improved drive concepts, modular configuration of the grinding units, better wear protection, simpler maintenance and high-quality service. Chinese vendors are keen to obtain further reference installations outside China. Manufacturers find it difficult to establish totally new mill concepts on the market, even if they offer significant benefits. This was demonstrated by the introduction of the Horomill, and the BETA mill could suffer the same fate. All in all, it may be presumed that single-stage grinding processes with vertical mills or roller presses will continue to expand their market shares at the cost of ball mills.