Cement Clinker Grinding

The main objective for cement clinker grinding is to increase the specific surface area of clinker to ensure fast hydration of cement when mixed with water.

Regarding this, the cement clinker is ground from 80 % passing between 10-20 mm to 100 % passing 90 µm (Jankovic et al., 2004). Normal conventional grinding equipment is a two-compartment ball mill. The compartments in the ball mill are separated by a diaphragm that allows only particles finer than a certain size to pass through to the second compartment. Then, coarse clinker is ground in the first compartment while the fine grinding is done in second compartment. However, in some plants, these circuits have been replaced with various HPGR-integrated ball

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mill arrangements as these circuits are found to be more energy-efficient than the conventional cement grinding circuits. Patzelt (1992) described four different HPGR-ball mill arrangements in cement grinding:

-Pre-grinding: HPGR and ball mill is operated in open circuit.

-Hybrid-Grinding: Raw feed of clinker, a split of the HPGR product and the oversize of the ball mill product is fed into HPGR. The ball mill is in closed circuit.

-Combi-Grinding: The oversize of HPGR product and raw feed of clinker is fed into HPGR. The undersize of HPGR product is sent to ball mill which might be either in open or closed circuit.

-Finish-Grinding: The HPGR product undersize is the final product. Yet, it is not commonly used for cement grinding. Instead, it is adapted to grinding of raw feed, and blast furnace slag.

The mode of operation of each HPGR-ball mill circuit is illustrated in Figure 2.7 below:

Figure 2.7. The modes of operation for cement grinding circuits (Patzelt, 1992)

21 2.4 Utilization of HPGR Prior To Ball Mill

In the literature, utilizing HPGR prior to ball mill was generally found to consume less energy than conventional ball mill grinding for the same degree of product fineness. Patzelt (1992) stated that various HPGR-ball mill arrangements use up to 30 % less energy than ball mills. Also, case studies obtained from cement grinding plants showed that use of HPGR with ball mill where both were in closed circuit use 55-70 % less energy than a conventional ball mill circuit (Von Seebach et al., 1996). The energy-efficiency of HPGR-ball mill arrangements were linked to the efficient breakage of particles in HPGR due to high interparticle stresses acted, and weakening of particles due to cracks imparted by HPGR which resulted less energy usage at downstream ball mills.

Fuerstenau et al. (1990) compared the reduction ratios and product size distributions of quartz, dolomite, limestone and hematite in ball milling and single particle high pressure roll mill comminution on an equal energy consumption basis. The results showed that high pressure roll mill is more energy-efficient than ball milling at low reduction ratios for the selected minerals and the range of energy levels tested.

These results supported the use of HPGR prior to ball mill for an initial low degree of size reduction.

Fuerstenau and Vazquez-Favela (1997) compared the relative energy efficiency for grinding narrowly-sized dolomite in hybrid grinding with respect to ball mill grinding for the same reduction ratio basis. The hybrid grinding experiments involved grinding material in laboratory-scale HPGR and then in ball mill where both HPGR and ball mill were in open circuit. It was found that hybrid grinding became more energy-efficient than ball mill at the same reduction ratio. This result was linked to weakening of particles in HPGR such that less energy was used in subsequent ball milling step.

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A detailed investigation for optimal use of various HPGR-ball mill arrangements on coal grinding was investigated by De (1995). In this research, it was shown that the utilization of HPGR prior to ball mill was more energy-efficient than ball mill for a given percentage of fines where HPGR and ball mill were in open circuit. However, any energy savings could reverse upon a threshold energy value in HPGR due to compaction and briquetting of coal bed which caused significant energy losses in terms of size reduction. Moreover, it was shown that high-pressure roll mill product exhibited faster rates of disappearance of a given size than fresh feed of coal when both are ground in a ball mill. This was linked to fracturing of coal by HPGR which in turn yielded faster grinding kinetics in subsequent ball milling.

Tavares (2005) studied particle weakening of copper and gold ores in high pressure grinding rolls. This study consisted of comparing primary fracture energy distribution and mean mass-specific fracture energies of narrowly-sized HPGR, hammer mill, and roll crusher products estimated by means of single particle breakage tests. Results show that there is a statistically significant weakening of HPGR product at coarse particles with respect to hammer mill and roll crusher products.

Fuerstenau et al. (1999) investigated the effect of ball size on the energy efficiency of pre-grinding of -3.4+2.4 mm dolomite. The pre-grinding experiments involved grinding material firstly in laboratory-scale HPGR, then in ball mill where both HPGR and ball mill are in open circuit. It was found that the reduction ratio of HPGR product in ball mill is higher at smaller ball sizes while the reduction ratio of fresh feed of dolomite in ball mill is higher at larger ball sizes. This result indicated that HPGR product contains internal damage to allow for the use of smaller balls.

Moreover, HPGR product was claimed to have higher breakage rates than fresh feed in ball milling due to weakening imparted to particles by HPGR.

23 CHAPTER 3

EXPERIMENTAL MATERIAL AND METHODS

3.1 Material

Portland cement clinker produced by Baştaş Cement Plant was used in this study.

Samples of clinker were taken from the feed and product ends of an open-circuit industrial-scale HPGR operated at a pressure of 100 bar. Samples of HPGR product and HPGR feed weighed approximately 90 kg. Narrow size fractions following nearly √2-order were obtained by screening the whole samples down to 106 µm.

The size distributions of HPGR feed and HPGR product are shown in Figure 3.1.

True density of samples was measured with a helium pycnometer in the Central Laboratory of METU and was found to be 3.19 g/cm³.

Figure 3.1. Particle size distributions of HPGR product and HPGR feed (Raw data at Table A.1 and Table A.2 in Appendix A)

24 3.2 Methods

The experimental methods involved single particle breakage tests and batch ball mill grinding tests with narrowly-sized samples of HPGR product and HPGR feed.

Single particle breakage tests were performed with size fractions above 3.35 mm while batch ball mill grinding tests were conducted with size fractions below 3.35 mm. Screen analyses of products after single particle breakage tests and batch grinding tests were carried out using dry sieving with a set of sieves progressing in

√2-order.

Drop weight test was utilized for single particle breakage tests. For each test, particles were stressed one by one with the drop head. Samples of HPGR product and HPGR feed taken from each size fraction were tested at the same specific impact energy. For each size fraction, 4-6 specific impact energy values were used.

Two laboratory drop weight testers were used, a larger one having a drop head of 20 kg, and the smaller one having a drop weight of 2 kg. Moreover, drop heads of 1.24 kg and 0.40 kg were used with small drop weight tester to achieve low specific energy levels where necessary. The specific energy levels and experimental conditions for each size fraction of HPGR product and HPGR feed tested are given in Table 3.1 through Table 3.12. Size fractions of HPGR product and HPGR feed coarser than 25.4 mm were not compared since the number of such particles was insufficient in HPGR product to perform drop weight tests at various energy levels.

Also, it should be noted that dust or material losses could occur during drop weight test and the subsequent screen analysis. In this case, care was taken during the experiment for minimizing the mass losses, in order to obtain correct test results.

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Table 3.1. Experimental conditions for drop weight testing of -4.7+3.35 mm of HPGR product

Table 3.2. Experimental conditions for drop weight testing of -4.7+3.35 mm of HPGR feed

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Table 3.3. Experimental conditions for drop weight testing of -6.35+4.7 mm of HPGR product

Table 3.4. Experimental conditions for drop weight testing of -6.35+4.7 mm of HPGR feed

Weight of the Drop Head

(kg) 0.40 1.24 1.24 2.00

Drop Height (cm) 5.1 3.2 6.3 7.5

Drop Energy (J) 0.20 0.39 0.77 1.47

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Table 3.5. Experimental conditions for drop weight testing of -9.53+6.35 mm of HPGR product Table 3.6. Experimental conditions for drop weight testing of -9.53+6.35 mm of

HPGR feed

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Table 3.7. Experimental conditions for drop weight testing of -12.7+9.53 mm of HPGR product

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Table 3.8. Experimental conditions for drop weight testing of -12.7+9.53 mm of HPGR feed

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Table 3.9. Experimental conditions for drop weight testing of -19.0+12.7 mm of HPGR product

Table 3.10. Experimental conditions for drop weight testing of -19.0+12.7 mm of HPGR feed

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Table 3.11. Experimental conditions for drop weight testing of -25.4+19.0 mm of HPGR product Table 3.12. Experimental conditions for drop weight testing of -25.4+19.0 mm of

HPGR feed

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different monosize balls (19.05 mm, 25.4 mm and 31.75 mm) were used. Sizes below 1.18 mm were not used in this study since there was insufficient amount below 1.18 mm in HPGR feed to perform batch grinding at the defined ball load and powder loading conditions.

For comparison purposes, the mass of balls and the mass of material were kept constant in grinding of each size fraction so that the power draw of the mill and the specific grinding energy would not differ significantly for any ball size and material size combination. Experimental grinding conditions for the batch ball mill tests are given in Table 3.13 through Table 3.15.

Samples were ground for cumulative times of 0.5, 1, 2, 4 and 8 minutes for -3.35+2.36 mm while cumulative grinding times of 0.25, 0.5, 1, 2, 4 and 8 min.

were chosen for -2.36+1.7 and -1.7+1.18 mm. Breakage distribution functions were estimated by the BII method using product size distributions at 0.5 min. grind time for -3.35+2.36 mm and 0.25 min for -2.36+1.7 mm and -1.7+1.18 mm size fractions. Breakage rates were estimated using the top size fraction remaining at all cumulative times.

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Table 3.13. Experimental conditions for batch ball mill grinding of HPGR product and HPGR feed (d_B = 19.05 mm)

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Table 3.14. Experimental conditions for batch ball mill grinding of HPGR product and HPGR feed (d_B = 25.4 mm)

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Table 3.15. Experimental conditions for batch ball mill grinding of HPGR product and HPGR feed (d_B = 31.75 mm)

36 CHAPTER 4