Grinding Clinker Replacement Materials

Grinding of Clinker Replacement Materials Soeren Worre Joergensen, General Manager, Grinding Technology, F.L.SMIDTH

2

Grinding of Clinker Replacement Materials Introduction Blended cements have been produced for decades in some countries, and in recent years, a renewed interest in the production of such cements can be found all over the world. There are many reasons for this, with one of the main reasons being that the production of blended cements results in a lower overall production cost, a fact that has been strongly accentuated with the increase in fuel and power prices. The ability to have a partial substitution of clinker with replacement materials also results in an overall increased plant capacity without a large capital investment in terms of additional clinker production. Blended cements can have some advantages over pure Portland cement in end use properties such as lower heat development during hardening and an improved durability of the final concrete structures. Finally, saving of natural resources, utilization of industrial by-products and reduction of CO2 emissions are important contemporary issues favouring production of blended cements. There are a variety of solutions for the grinding of clinker replacement materials, whether by separate grinding or by intergrinding into blended cement. Selecting the correct solution for a specific application, whether it is a new grinding installation or an upgrade to an existing installation, can be a difficult task. In making the selection, one must consider not only the initial investment cost and the production cost, but also the simplicity of the operation, maintenance issues, drying issues, etc. This paper will present the experience of various grinding circuits for grinding clinker replacement materials, including ball mills, roller presses and vertical roller mills, and describe the benefits and comparisons of each system. Clinker Replacement Materials Blended cement is a mixture of separately ground or interground cement clinker, gypsum and mineral admixture(s). The mineral admixture(s), clinker replacement materials, will typically be one or more of the following materials: • • • •

Granulated Blastfurnace Slag (GBS) Natural Pozzolans Fly Ash (PFA) Limestone

The properties of those materials vary a lot as far as reactivity (i.e. clinker replacement potential), grindability, granulometri, humidity and abrasiveness is concerned. Granulated Blastfurnace Slag (GBS) This material is able to react in an alkaline or sulphatic medium to form hydrates of the same kind as formed by Portland cement hydration. This so-called latent hydraulic property is the basis for the use of GBS in blended cements and slag cements. The water quenching process applied to form GBS generates a product with a physical appearance like coarse sand. Hence the word “granulated” blast furnace slag or the German name “Hüttensand”. Just after water quenching the material can contain up to

3

30% of water. This may be reduced during transportation and storage to around 15 % of water. This is still a rather high moisture content that must be taken into consideration when grinding slag and slag cement. As GBS is dominated by a dense glassy structure with few large pores it is difficult to grind. Hence, on average it requires about 30-50% more grinding energy than clinker to reach the typical cement fineness of 300-500 m2/kg (Blaine). Further, the glassy structure of GBS makes it a very abrasive material to handle in the grinding circuit. The fairly high reactivity of ground GBS makes it possible to make blended cements with high clinker replacement level. Products with up to 80% slag are made. The standard specifications in many slag-cement producing countries make a distinction between slag and clinker dominated products. Products based on 80-100% slag are also produced. They are used for special purposes or – as for pure slag – for combinations with OPC during mixing of concrete. Slag cement can be ground in combined grinding or by separate grinding of slag and cement clinker with subsequent mixing. In combined grinding the slag component will end up in the coarser fraction of the product as slag usually is harder to grind than the cement clinker. This will in particular be the case in open circuit grinding. Although this phenomenon is somewhat reduced in closed circuit grinding systems, the hard slag component will still be concentrated in the circulation load, and hence have a steeper particle size distribution than the clinker component. The distribution of grinding energy to the slag component and to the clinker component respectively, which can not be controlled in combined grinding, may not be the optimum with respect to energy expenditure and product quality. More energy spent on the slag and less on the clinker would be more beneficial with respect to late strength and/or the grinding energy consumed. The benefits of combined grinding are good homogenization of the product, a reduced tendency of agglomeration and coating in the ball mill (grinding aid effect), and ability to use the heat from the clinker for drying the slag. Separate grinding of the slag and cement clinker is more economical in terms of energy consumption. The fineness of the separate components can be optimized to achieve the required product quality at minimized energy consumption. Further, with adequate storage and mixing facilities available a wide range of products can be offered to meet the end-users’ specific demands. In some countries separately ground slag is produced and sold directly to the concrete producers, who then use it together with Portland cement in their concrete mixes. Pozzolan A Pozzolan can be defined as a siliceous material that is non-hydraulic, but which is able to combine with lime and water at normal (ambient) temperatures to form hydrates with cementing properties Due to the microporous structure of many pozzolans they are generally easy to grind to products of high specific surface. Although easy to grind

4

pozzolans are often quite abrasive and they may often have a moisture content that requires provisions for drying in the grinding circuit. Pozzolans generally contribute more slowly to strength than GBS. For this reason, and sometimes also due to problems with water demand, the fraction of natural pozzolans in pozzolan cements is usually kept below 35%. In the product from combined grinding of pozzolan and cement clinker the pozzolan will be expected to be very fine and the clinker part to be very coarse due to the large difference in grindability of the two materials. This will in general terms result in a strength development with low early strengths and high late strengths. Further, PortlandPuzzolan cement from combined grinding will tend to have a higher water demand – due to the very fine pozzolan – than a product mixed of separately ground components with individually optimized particle size distributions. Fly Ash (PFA) Fly ash (or Pulverized Fuel Ash = PFA) is an artificial pozzolanic material with a reactivity similar to the reactivity of natural pozzolans. Hence, practical clinker replacements are normally below 40% with 20-30% as a typical range. The particles of PFA are mostly spherical, with a small fraction being hollow. The fineness of PFA usually varies between 200 and 500 m2/kg (Blaine), (or 45 µm residues of 10-40%), which means that it has a fineness similar to that of Portland cement. Therefore PFA is often introduced into blended cement with little or no grinding at all. If grinding is performed it will first result in crushing of the hollow and porous particles. A subsequent further grinding of the dense glassy material will tend to be rather energy consuming. PFA is usually available as a dry powder suitable for pneumatic transportation. A comparison of the three methods of making fly ash cement – combined grinding, separate grinding and direct intermixing of the PFA as it is received – indicated that the most favourable ratio between cement strength achieved and the corresponding grinding energy is obtained with combined grinding. The direct intermixing of the PFA offers the poorest solution of the three. In many plants partial combined grinding is implemented in the way that the PFA is introduced to the separator – irrespective of the type of grinding circuit – and only the coarse particles are returned to the mill for further grinding. Those particles and agglomerates are then reduced in size and the reactivity is improved. Mill systems for grinding of blended cements and clinker replacement materials As grinding of blended cement and slag has become more and more common a large number of existing ball mills have been brought into operation for these applications. Such mills, however, offer limited options for a plant that wishes to grind a blended cement or slag without investment in additional equipment. A main issue that then needs to be addressed is

5

how to dry the wet mill feed materials before grinding. Other mill types like roller presses and especially the vertical roller mill are much more suitable for grinding and drying of materials with considerable moisture content. Mill circuits with these types of mills are today often designed and installed specifically for grinding such materials. Further, the roller mills in comparison to a ball mill system offer much lower specific energy consumption. Ball mills As stated previously most existing ball mills designed for grinding Portland cement are not particularly suited for drying wet materials like, say many of the clinker replacement materials. However, the ability to dry the feed materials can be increased significantly if a hot gas generator is included in the mill circuit and the mill is provided with a drying compartment. An example of such mill is shown in the below FIGURE 1. FIGURE 1: Ball Mill with Drying Compartment

The mill shown has only one grinding compartment, which is typical for slag grinding applications, as the raw slag is fine enough to be ground with relatively small grinding balls, say less than 50 mm. Further, the flow of hot drying gas through the mill is facilitated without an intermediate diaphragm in the mill. However, for blended cement with clinker and lower moisture content in the mill feed material a two-compartment mill will usually be more suitable. A rather large number of those mills are in operation grinding various types of blended cement and slag. A mill of this type in closed circuit operation was commissioned recently in Poland and main data for the installed machinery, design conditions, and typically obtained performance figures appear from the below TABLE 1.

6

TABLE 1: Ball mill in closed circuit operation for slag grinding. Mill, type and size Mill Gear Power Separator Production, t/h Feed moisture, % Blaine, cm2/g Spec. energy consumption Mill, kWh/t Fan, kWh/t Separator, kWh/t Total, kWh/t

FLS UMS 5.0 x 15.0 + DC 5750 kW SEPAX 355 Achieved Design conditions performance 101 88 7.9 10 3794 3500 49.8 2.9 0.3 53.0

-

As seen from the table this mill provided with drying compartment and hot gas generator is able to grind and dry slag with up to 10% moisture. A ball mill offers a very well proven and sturdy technology for grinding of blended cements and slag. However, the ball mill has its limitations as regards drying of the wet clinker replacement materials, and further it has fairly high energy consumption. Consequently, other types of mills are becoming increasingly more common for grinding of these types of materials. Hydraulic Roller Press (HRP) It is a well-known fact that high-pressure roller presses are much more efficient in the comminution process than ball mills. For every 1 kWh/t that the HRP contributes to the grinding circuit, the ball mill power is reduced by approximately 2 kWh/t or even more, dependent on the type of material to be ground. Maximizing the work done by the HRP results in a more efficient overall grinding process. In order to increase the amount of work done by a HRP, it is necessary to recycle a portion of the pressed material back to the press for further grinding. From the mid eighties to the mid nineties a large number of roller presses were introduced for various applications, including grinding of blended cement. However, slag in particular is an ideal material for grinding in a roller press. The glassy and thus brittle structure of the slag makes grinding under high pressure very efficient. Further, with a suitable amount of moisture present slag forms a very stable grinding bed that allows a higher circulation of material back to the press without the associated operational instability as compared to clinker grinding. In many applications of a roller press for slag grinding at least a partial drying of the raw slag is required to attain a moisture content in the feed material that will provide adequate operating conditions for the press. Obviously the facilities for drying of the slag will usually add to the complexity of the installation.

7

In many cases the roll press is used as a pregrinder, with only a mechanical splitter to divide the crushed slag exiting the roller press and recirculate it back to the press for further processing. FIGURE 2 shows a roller press in a typical pregrinding arrangement. FIGURE 2:

The various suppliers of grinding equipment offer a variety of grinding systems designed for a larger utilization of the roller press than can be achieved in a pregrinding arrangement. Such systems, often named semi-finish systems, hybrid systems or the like, comprise a roller press, a ball mill, drying facilities and a separator arrangement that can be and have been combined in a multitude of arrangements. In particular the design of the separator arrangement is specific for the individual suppliers. Anyhow, whichever separator arrangement is applied they all have the ability to disagglomerate the product from the press and separate it in a course fraction recirculated to the press and a finer fraction for the ball mill and/or the fines separator. With the fines removed from the coarse material recirculated back to the roller press the operation is stabilized. Circulation factors around the press of over five can be achieved, resulting in a press energy input of >10 kWh/t, and significant reduction in the overall circuit power. In the extreme the ball mill can actually be eliminated – this is described as finish grinding in the roller press. An example of such arrangement is shown in FIGURE 3. Although not shown in the figure slag drying facilities are required in this arrangement as good control of the roller press feed moisture (1-2 %) is of significant importance to the operation of the roller press. Finish grinding operation results in very large savings in the specific energy consumption of the grinding system. One of the drawbacks to this type of system is the fact that the slag produced will have a very steep particle size distribution, which may not be optimum in terms of the effects of the setting time and the water requirements. However, it appears in some cases to have been possible to remedy this problem somewhat by recirculation of a portion of the pressed material back to the press for further grinding.

8

FIGURE 3:

TABLE 2 illustrates operating data from selected plants for both a pregrinding arrangement, and a finish grinding arrangement with a roller press. TABLE 2: Roll Press Systems for Slag Grinding

Grinding Arrangement Ball Mill Size Installed Power Roller Press Size Installed Power Capacity, t/h Blaine, cm2/g Mill kWh/t Press kWh/t Total Grinding kWh/t

Plant A Pregrinding 4.0M x 12.5M 2700 kW HRP 1.25 2 x 450 kW 55 4850 48.5 9.2 57.7

Plant B Finish N.A. HRC 60.38 2 x 900 kW 61 3100 18.5 18.5

The cost of installation of a HRP, along with the associated transport systems, can be quite high. However, the significant savings in power consumption as well as the increase in the output of the system can offer an attractive return. The maintenance of a roller press is much more critical than that required by a ball mill. Wear of the grinding rollers must be repaired. This maintenance is more expensive and requires a higher degree of sophistication and labour than a ball mill, if only for the reason that a roller press system comprises more equipment and equipment of a higher complexity than a ball mill circuit.

9

Vertical Roller Mills (VRM) Vertical roller mills have been employed for slag grinding for many years. Starting in the mid 1980’s, the Japanese machinery manufacturers, most notably Onoda-Kobe with the OK mill, began to market the vertical roller mill for cement and slag grinding. Today, VRM’s for cement and slag grinding are accepted by the industry as both proven machinery and process technology. The vertical roller mill is an ideal machine as it addresses all of the issues related to grinding and drying of blended cement and slag in a single integrated unit. The grinding economy of the vertical roller mill is far better than a ball mill. Typically, in slag grinding the grinding energy is 40-50% less for a vertical roller mill than the ball mill, depending on the required Blaine for the slag. Although the associated fan power for a vertical mill is higher than the ball mill, the overall system specific energy consumption is far lower. Not only are VRM’s very energy efficient, but also they are very versatile in terms of being able to handle wet raw materials as slag, and additives such as PFA, limestone, pozzolan, etc. The VRM can utilize much higher quantities of waste gases for drying than a ball mill; therefore the percent substitution of additives is not limited by the system drying capacity. FIGURE 4 illustrates the versatility of a vertical roller mill. The graphic represents a survey of the products ground in the first 10 OK mills that F.L.Smidth has supplied. (F.L.Smidth market and sell the OK mill under a license from Onoda Engineering and Kobe Steel). The product range comprises ordinary Portland cement, various types of blended cement and pure slag. FIGURE 4: OK mills - Composition of mill feed 100 90 80 70 60 50 40 30 20 10 0 A+B

C

C

D

D

D

E

F+G+ H

I+J

I+J

Plant Clinker, %

Gypsum, %

Limestone, %

Pozzolana, %

Slag, %

Fly ash, %

10

Many mill systems of this type are now operating worldwide. These mills are producing many different types of cements, with Blaine from 2800 to over 4400 cm2/g. Slag and slag cement are also produced with Blaine up to 6000 cm2/g. Two examples of experience from operation of OK mills grinding blended cement will be presented in the following. Cementos Progreso, Guatemala: (Ref. no. 1). In 1996-97 F.L.Smidth supplied an OK 33-4 mill to Cementos Progreso, Guatemala. This vertical mill for finish grinding was the first such installation in the Americas and one of the first in the world for grinding blended cement. The decision to select a vertical roller mill for grinding pozzolanic cement with a clinker factor of 75% and under variable conditions of feed moisture and temperature of clinker was taken after thorough evaluation of available technology for clinker grinding and considering the high cost of energy in Guatemala. A key criterion for selecting the new grinding system was among others the capability to achieve similar cement quality as achieved by the existing mill system comprising a roller press (pregrinder) and a ball mill. An example of product quality figures achieved appears from TABLE 3. The quantity of water required for the concrete was a concern during the study phase; however, the cement produced in the vertical mill showed no difference in this regard compared to the cement produced in the ball mills. This vertical mill option involving an OK mill had the advantage of accomplishing the drying, grinding and classifying in a single unit, providing operating cost savings of about 8 kWh/t and resulting in an excellent product. TABLE 3: Product quality: Vertical mill versus ball mill system Roller press + Ball Parameters OK Mill mill Fineness 93.2 92.3 % passing 45 µm Blaine, cm2/g 3601 4064 Concrete strength 3 day, psi 2774 7 day, psi 3734 3645 28 day, psi 5112 4982 56 day, psi 5867 5598 Cementos Minetti, Campana, Argentina: (Ref. no. 2) In 1998 Cementos Minetti, Argentina, decided to set up a new grinding station in Campana for grinding of mainly Portland cement and slag cement.

11

During evaluation of technologies prior to selection of process it soon became apparent that a vertical roller mill would offer a number of advantages. Producing and end product of similar quality compared to a ball mill system the vertical roller mill offered several benefits, such as lower energy consumption, lower maintenance costs and ability to dry the wet feed materials. Further, the OK mill was evaluated to offer a high flexibility in operation – for instance separate grinding practically without periods with transition products. The plant layout provides the options to co-grind the various feed components, i.e. clinker, gypsum, limestone and slag, or to grind Portland cement and slag separately involving facilities for mixing of the separate products. A comparison of the quality of slag cement produced in co-grinding in the OK mill and of a similar product ground in the company’s ball mill installation appears from TABLE 4. From the table the following can be stated regarding the quality of the product from the OK mill compared to the ball mill cement: • • • •

The OK roller mill allows a 4% higher clinker substitution Similar water demand Comparable setting time Almost identical compressive strength at similar sieve residue

Further and above all, for concrete a better workability is observed when using cement from the roller mill. TABLE 4: System Clinker Gypsum Limestone Slag Blaine Sieve residue Content of SO3 Water demand Setting time Comp. strength

Initial Final 2d 7d 28 d

% % % % cm2/g % R 32 µm % % min min MPa MPa MPa

Ball mill 74 5 21 3200 20.5 2.44 24.8 146 240 15.0 32.8 46.4

OK roller mill 70 5 5 20 3750 21.3 2.48 25.2 167 250 15.4 32.8 45.4

As mentioned previously, besides the new OK mill installation in Campana, Cementos Minetti also operates a ball mill installation grinding slag cement. A comparison of the specific energy consumption of the two grinding systems appears from TABLE 5.

12

TABLE 5: System Clinker % Gypsum % Slag % Blaine cm2/g Spec. energy consumption Mill kWh/t Separator kWh/t Fans kWh/t System kWh/t

Ball mill 78 4 18 3640 *

Roller mill OK 36-4 75 5 20 3640

34.4 0.4 5.2 40.0

17.2 0.1 8.4 25.7

*Adjusted to same fineness In this comparison the figures on the OK mill represents operation without limestone, which otherwise would result in a high Blaine surface compared to the sieve residue. The feed composition for the two mills may thus be considered similar and the energy figures may therefore reasonably be adjusted to represent operation with the same fineness of the end product. It appears that the specific energy consumption of the OK mill system is around 35% lower than that of the ball mill system. Similar energy savings are often observed in comparisons of OK mills and ball mills, in particular when grinding to high fineness or with a considerable fraction of slag in the feed material. A typical flow-sheet for an OK mill installation for grinding of slag and slag cement is shown in FIGURE 5. As can be seen a vertical roller mill offers a very simple arrangement with few machines in the grinding circuit, in particular compared to the roller press systems. FIGURE 5:

13

As the grinding, transport and separation processes in the vertical roller mill are all closely coupled, the internal circulation of material from the grinding bed to the separator is quite high. This can lead to a rather steep product particle size distribution, which can result in a high water demand. For this reason, it is imperative that there is control of the grinding process so that final product cement has the correct quality to satisfy the market demands. This actually is easily addressed in the operation of the OK mill. Adjustments in grinding pressure and airflow will influence the product particle size distribution, as will making physical changes to the dam ring. However, the correction of the particle size distribution comes with some price – the specific power consumption will be increased as the PSD is made wider. FIGURE 6 shows the relationship of the mill specific grinding power vs. adjusting the product size distribution by changes in the grinding process.

30,0

30,00

28,0

29,00

26,0

28,00

24,0

27,00

22,0

26,00

20,0

25,00

18,0 0,92

24,00 0,97

1,02

1,07

RRSB slope (3 - 30 µm) Specific energy

Water demand

Some of the clinker replacement materials, in particular slag, can be quite abrasive and cause a heavy wear of the grinding parts in a vertical roller mill. However, various technologies are available to facilitate the maintenance works on these parts and lower the corresponding costs. Some mills have rollers with segmented wear parts that fairly easily can be reversed for better

Water demand, %

Mill specific energy, kWh/t

FIGURE 6:

14

utilization of the wear parts before replacement with new ones. Special wear resistant materials have been developed for grinding parts. Further, it has become common practice, in particular for slag mills, to repair worn grinding parts by hardfacing. By regular hardfacing it is possible to minimize the deterioration of the mill performance usually associated with a progressing wear. Conclusion: Today there is a fairly wide range of mill arrangements available for grinding the various types of clinker replacement materials. All the systems described have been in the market for several years and with proper adjustments to the operational parameters or adjustments to the arrangement, each is capable of producing a product that satisfies the market’s requirements to end product quality. A ball mill and a vertical roller mill offer a simple plant arrangement, while arrangements with a roller press generally are somewhat more complicated with more equipment, which in many cases comprises a separate drying installation. A ball mill represents sturdy and well proven technology and it is easy to maintain. The roller press and the vertical roller mill are more complicated machines and maintenance work requires more skill. The maintenance costs may also be higher for the more complicated machines, to some extent depending on the materials ground. The specific energy consumption for a ball mill system is significantly higher than for systems with roller presses or vertical roller mills. The high energy consumption of the ball mill is actually the most decisive disadvantage of this technology. The vertical roller mill is the most versatile of the three grinding machines. Separate drying facilities are not required and it is easy to change from one product type to another product with practically no transition periods between products. Generally the ball mill system offers the lowest plant installation cost, including the costs for civil works. The total cost for a vertical roller mill installation is usually somewhat higher, and the cost for an installation with roller press will in most cases be the highest. However, the relative costs of the systems will obviously depend on specific plant arrangements and in particular on regional variations of costs for civil works. Today the vertical roller mill appears to become the most common mill for grinding of clinker replacement materials, or at least for slag grinding. Within the last two years the vertical roller mill has, based upon information available, acquired a market share of around 70% of new mills sold for slag grinding. The corresponding figures for roller presses and ball mills appear to be around 20% and 10% respectively.

15

References: 1. ZEPEDA, Erick, and IRELAND, Tomás, “A CENTURY OF CEMENT”, World Cement, March 1999. 2. FISCH, Hanspeter, and JOERGENSEN, Soeren Worre, “Erfahrungen mit einer FLS Wälzmühle (OK Mühle) für die Mahlung von Portlandzement, Kompositzement und Hüttensand”, VDZ Verfahrenstag 2001.