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The effect of anionic dispersants on grindability of different rank coals
Int. J. Miner. Process. 77 (2005) 199 – 207 www.elsevier.com/locate/ijminpro
The effect of anionic dispersants on grindability of different rank coals G. Atesok *, M. Ozer, F. Boylu, H. DVncer Istanbul Technical University, Mining Faculty, Mining Engineering Department, Mineral Processing Division, 34469, Maslak, Istanbul Received 5 October 2004; received in revised form 1 May 2005; accepted 9 May 2005 Available online 8 August 2005
Abstract The effect of PSS (Sodium Polystyrene Sulphonate) and NSF (Naphthalene Sulphonate Formaldehyde Condensate) chemicals used to control the pulp rheology on the grindability of coals was studied. Zonguldak region bituminous coal and Istanbul region brown coal samples that high and low rank were used. Wet grinding tests with two coal samples were performed with or without PSS and NSF under constant grinding conditions: solid ratios ranged from 50% to 60% with 0–60 min grinding periods. In the present study, grinding conditions of coal with high pulp solid ratio were improved through the lowering of the viscosity of the coal by using the dispersing agents, PSS and NSF, during the grinding stage and evaluation were made by using the befficiency factorQ. It is possible to increase the amount of finely ground bituminous coal from Zonguldak by 20% and 16%, if optimum concentrations of PSS (0.3%) and NSF (0.7%) are used, respectively. Whereas, the increase in the case of finely ground lignite from Istanbul was 32% and 20%, respectively. D 2005 Published by Elsevier B.V. Keywords: grinding; dispersant; viscosity; energy; coal
1. Introduction Crushing and grinding are size reduction process in minerals processing circuits. Comminution processes consume approximately 50% of energy used in mineral processing (Austin et al., 1984). Energy consumption is about 5–20 kW h/ton of ore in fine grinding, however, this figure increases 20–100 * Corresponding author. Tel.: +90 0212 285 61 78; fax: +90 0212 285 61 28. E-mail address: [email protected] (G. Atesok). 0301-7516/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.minpro.2005.06.004
kWh/ton of ore in ultra fine grinding. Therefore, it presents utmost importance to design size reduction especially grinding circuits (Austin et al., 1984). Pulp rheology plays a significant role in wet fine grinding processes and it affects the efficiency of grinding. Pulps show differences as pulp rheology depending on the different solids ratios (Ryncarz and Laskowski, 1977). Pulp viscosity, is especially, an important parameter in grinding. A number of investigators have studied the effect of pulp rheology in ball mill grinding for many years. Those investigations especially focused on the grinding of ceramic
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raw materials and clinker grinding. They reported up to 100% increase in grinding efficiency by the use of chemical additives (Somasundaran, 1978). The pulp rheology effects on wet grinding have been researching for several years. Previous studies (El-Shall and Somasundaran, 1984; Klimpel, 1982a,b, 1983, 1997a,b; Klimpel and Hansen, 1989) have shown that solids ratios in the pulp could be raised with no changes on yield stress value by using dispersing agents in grinding processes. Thus the grinding capacities could be increased. Some researchers (Wang and Forssberg, 1997) found that the polymeric type dispersing agents mixed with inorganic chemicals are more effective for fine grinding of bituminous coals. Investigations related to pulp rheology or the effect of additives on the grindability of coal, however, have been very limited. Those investigations have generally been conducted with bituminous coal after coal–water slurry became an important issue. In this study, the effects of anionic dispersants PSS (Sodium Polystyrene Sulphonate) and NSF (Naphthalene Sulphonate Formaldehyde Condensate) on pulp rheology and grindability when added to the medium during the wet grinding process to reduce the viscosity of coal–water slurries (grindability and the energy consumed) have been examined. Used in the experiments were bituminous coal from Zonguldak region in Turkey, which has a high degree of Table 1 Proximate and ultimate analyses of coal samples on dry bases Component
Proximate analyses
Ultimate analyses
O/C
Coal type
ash (%) total sulfur (%) volatile matter (%) fixed carbon (%) inherent moisture (%) upper calorific value (kcal/kg) C (%) H (%) O (%) N (%) S (%) Pyritic sulfur (%)
Istanbul (low rank coal)
Zonguldak (high rank coal)
40.36 7.19 41.58 18.06 32.00 3616
24.40 0.40 24.49 51.11 0.50 6218
Cummulative under size, %
200
100
10
1 0,1
1 Istanbul Brown Coal
56.45 4.73 10.50 1.30 0.42 0.20 0.186
Zonguldak Bituminous Coal
Fig. 1. Total sieve curves of coals when roller type grinders are used.
coalification, and lignite from Istanbul area, which has low coalification.
2. Experimental 2.1. Material The grinding tests were carried out with two different kinds of Turkish coals representing bituminous (Zonguldak) and brown coal (Istanbul). Table 1 shows the results of both proximate and ultimate analyses for these two coals. In crushing the coal samples; after jaw and conical crushers, roller type crusher was used and size of particles less than 3 mm was obtained. The fine sieve analyses of materials obtained with the roller type grinder are given in Fig. 1. Table 2 Physical properties and chemical structure of PSS and NSF
Items
Value
Molecular weight
14000
Sulphonation degree, % Specific gravity, g/cm3 Viscosity at 30o C, m. Pa. sec
26.70 5.1 15.53 1.40 7.20 4.01 0.580
10
Particle size, mm
Molecular weight
Chemical structure CH2
PSS CH
95 1.22 150 14000
SO3 Na n
NSF CH2
SO3 M
n
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Two different chemicals were used as dispersants: Sodium Polystyrene Sulphonate (PSS, a product of Japanese COM and Lion Corp.) and Naphthalene Sulphonate Formaldehyde Condensate (NSF, a product of Japanese COM and Lion Corp.). The physical and the chemical structures of both dispersants used in coal–water slurry technology are given in Table 2 (Ates¸ok et al., 2002a,b; Dinc¸er et al., 2003). The wet grinding tests were performed under the constant grinding conditions by employing the ball mill. Table 3 shows the ball mill characteristics used in this study. 2.2. Methods
201
The grinding experiments were carried out at intervals of 15 min. The mill was stopped after every 15 min and sufficient amount of the samples for determination of particle size and viscosity were removed without disturbing the pulp solid ratio and the grinding process was resumed. The surface area and viscosity measurements were carried out on samples obtained after grinding. Both measurements were repeated 3–4 times in parallel on each sample, and the average values were determined. The results are analysed according to both the viscosity and efficiency factors explained by the formula given below; Ef ¼ A1 =A2
In this study, the effects of solids ratio, the addition and concentration of PSS-NSF and grinding times on wet grinding were investigated. The wet grinding tests were performed with or without PSS and NSF in effective concentrations. For each coal, different solids ratios ranging from 55% to 60% were applied according to the slurry viscosities allowing easy rolling of the grinding media and the material in mill. In each experiment, 600 g coal samples completely ground to less than 3 mm were used in the grinding process. In experiments without dispersants, only sufficient water required to calculate the pulp solid ratio (PSR) was used. Whereas, in experiments including a dispersant, the dispersant was stirred for 5 min with the amount of pulp water required for solid ratio calculation. At the end, water or the mixture containing the dispersant was added to the mill.
where E f is the efficiency factor, A 1, is the surface area of the ground particles in a certain grinding time without dispersant, A 2 is the surface area of the ground particles in a certain grinding time with dispersant. The specific surface area of coal samples was measured by the BET method (with nitrogen). The zeta potential measurements were performed by Zeta Meter 3.0 type instrument; 0.1 g of the 38 Am coal sample was conditioned in 10 ml of solution for 10 min during which the pH was maintained at the desired value by adding either acid (HCl) or base (NaOH). Each data point is the mean of at least 10 measurements. The viscosity measurements were performed employing an RVD2-Brookfield rotating type viscometer.
Table 3 Ball mill characteristics
3. Results and discussion
Mill
Speed Lifters Media Charge
Material Charge
Internal diameter D, mm Length L, mm Volume V, cm3 Critical speed N c, rpm Operational speed, rpm Number Cross-section Material Diameter 1. d, mm Diameter 2. d, mm Loading for diameter 1, % Loading for diameter 2, % Specific gravity, g/cm3 Fractional ball loading, % Coal, g
225 235 8940 108 90 6 Semi-Circular Alloy Steel 30 40 46.5 53.5 7.387 33.9 600
3.1. Zeta potential measurements The zeta potential measurements taken at neutral pH on samples containing different concentration of dispersing agents are reported in Fig. 2. While the zeta potential of the both coals used in the experiments was around 20 mV at neutral pH, it shifted towards more negative values with the increasing of reagent amount. It is seen that PSS produces the highest negative potential at the lowest dispersant addition. While zeta potential of Zonguldak bituminous coal was 59 mV in 0.3% PSS concentration, zeta potential of Istanbul brown coal has been determined as 50
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0 PSS NSF PSS NSF
Zeta Potantial , mV
-10
ZONGULDAK ZONGULDAK ISTANBUL ISTANBUL
-20 -30 -40 -50 -60 -70 0
0,2
0,4
0,6
0,8
1
1,2
Dispersant Concentration, % Fig. 2. The zeta potentials of coals from Zonguldak and Istanbul containing different concentrations of dispersant.
mV. The highest absolute zeta potential values obtained with NSF were 54 mV for Zonguldak bituminous coal and 46 mV Istanbul brown coal at a concentration of 0.7%. Electrostatic bonding occurs between the positive sites of coal and the anionic head groups of dispersants. Hydrophobic bonding on the other hand takes places between the hydrophobic groups of coal and the hydrophobic chain of the polymer unit. The former mechanism is expected to result in higher negative zeta potential. As a result, coal particles hence negatively charged would repel with each other thus flocculation is prevented and dispersion is achieved.
3.2. Grinding experiments The results of grinding experiments carried out in the presence or absence of dispersants using coal samples of varying pulp solid ratios, both Zonguldak and Istanbul coals, are given together in Tables 4–7. In experiments where dispersants were used, the dispersion concentrations were determined in relation to adsorption and zeta potential characteristics of the coal samples. The grinding experiments were performed at optimum concentrations of 0.3% PSS and 0.7% NSF or above. As indicated in Tables 4–7 by increasing the solid ratio of the pulp from 50% to 55%, either in the
Table 4 The results of specific surface areas and efficiency factor measurements (Zonguldak bituminous coal and PSS) Dispersant and coal types
PSR* (%)
PSS Zonguldak
50
55
60
PSR: pulp solid ratio.
Specific Surface Area (m2/cm3)
Grinding time (min)
%0.0 Disp.
%0.3 Disp.
% 0.6 Disp.
% 0.3 Disp.
% 0.6 Disp.
15 30 45 60 15 30 45 60 15 30 45 60
0.6889 0.7875 0.8038 0.8641 0.7253 0.8669 0.9829 1.1163 0.6832 0.7364 0.7214 0.7058
0.6119 0.6951 0.6917 0.7303 0.6209 0.7279 0.8260 0.9443 0.5465 0.5883 0.5963 0.5872
0.6343 0.7436 0.7440 0.7642 0.6574 0.8091 0.9059 1.0797 0.5987 0.6664 0.6531 0.6513
1.1258 1.1330 1.1620 1.1832 1.1682 1.1910 1.1899 1.1821 1.2501 1.2518 1.2097 1.2019
1.0860 1.0590 1.0804 1.1307 1.1033 1.0714 1.0850 1.0339 1.1411 1.1051 1.1045 1.0836
Efficiency factor
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Table 5 The results of specific surface areas and efficiency factor measurements (Zonguldak bituminous coal and NSF) Specific surface area (m2/cm3)
Dispersant and coal types
PSR (%)
Grinding time (min)
%0.0 Disp.
%0.7 Disp.
%1.4 Disp.
%0.7 Disp.
%0.4 Disp.
NSF Zonguldak
50
15 30 45 60 15 30 45 60 15 30 45 60
0.6889 0.7875 0.8038 0.8641 0.7253 0.8669 0.9829 1.1163 0.6832 0.7364 0.7214 0.7058
0.6522 0.7489 0.7449 0.7856 0.6795 0.8141 0.9038 1.0013 0.5969 0.6627 0.6433 0.6055
0.6608 0.7750 0.8170 0.8848 0.6891 0.8309 0.9828 1.1176 0.6414 0.7186 0.7340 0.7162
1.0563 1.0516 1.0790 1.0999 1.0674 1.0649 1.0875 1.1149 1.1446 1.1112 1.1214 1.1657
1.0425 1.0161 0.9839 0.9766 1.0525 1.0433 1.0001 0.9988 1.0651 1.0247 0.9828 0.9855
55
60
presence or absence of dispersants, the grinding process is improved for both types of coal. However, a pulp solid ratio (PSR), which is greater than 55%, is favourable for coal from Zonguldak, but unfavourable for that from Istanbul. In the grinding experiments carried out using coal from Istanbul, it was observed that the value of the efficiency factor is lowered when the pulp solid ratio is increased to 60%. This is an indication of poor grinding. The solid ratio with optimum pulp was determined to be 60% for bituminous coal from Zonguldak and 55% for lignite from Istanbul. As is observed in related tables, the evaluations made using the parameter, the bEfficiency FactorQ, indicate that the polymeric dispersant PSS is more effective in the grinding process than NSF. The use of PSS with
Efficiency factor
coal from Zonguldak having 60% pulp solid ratio produced an efficiency factor of 1.25 after a 30-min grinding period. Whereas, when coal from Istanbul, which had a pulp solid ratio of 55%, was subjected to 60 min of grinding, the efficiency factor obtained was 1.32. The values of efficiency factors produced after grinding for 60 min in the presence of NSF were 1.16 and 1.20 for coal samples from Zonguldak (60% PSR) and Istanbul (55% PSR), respectively. Coal samples from Zonguldak and Istanbul, which had similar particle size distributions prior to grinding, exhibited different behaviours during the same grinding period. Istanbul coal is more readily ground than that from Zonguldak. As can be seen from Tables 4–7, the addition of dispersant to the grinding medium improves the grinding conditions both for coals
Table 6 The results of specific surface areas and efficiency factor measurements (Istanbul lignite and PSS) Specific surface area (m2/cm3)
Dispersant and coal types
PSR (%)
Grinding time (min)
%0.0 Disp.
%0.3 Disp.
%0.6 Disp.
%0.3 Disp.
%0.6 Disp.
PSS Istanbul
50
15 30 45 60 15 30 45 60 15 30 45 60
1.2447 1.3381 1.4817 1.5322 1.1822 1.2425 1.3436 1.3875 1.1244 1.1753 1.2648 1.3755
1.0354 1.1366 1.3026 1.2454 1.0194 1.0265 1.0756 1.0501 0.9588 0.9925 1.1273 1.2735
1.1062 1.1943 1.3288 1.4811 1.1135 1.1414 1.2156 1.2257 0.9733 1.0707 1.1943 1.3271
1.2022 1.1773 1.1375 1.2303 1.1597 1.2104 1.2492 1.3213 1.1727 1.1842 1.1220 1.0801
1.1252 1.1204 1.1151 1.0345 1.0617 1.0886 1.1053 1.1320 1.1552 1.0977 1.0590 1.0365
55
60
Efficiency factor
204
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Table 7 The results of specific surface areas and efficiency factor measurements (Istanbul lignite and NSF) Specific surface area (m2/cm3)
Dispersant and coal types
PSR (%)
Grinding time (min)
%0.0 Disp.
%0.7 Disp.
%1.4 Disp.
%0.7 Disp.
%1.4 Disp.
NSF Istanbul
50
15 30 45 60 15 30 45 60 15 30 45 60
1.2447 1.3381 1.4817 1.5322 1.1822 1.2425 1.3436 1.3875 1.1244 1.1753 1.2648 1.1505
1.1076 1.2149 1.3714 1.3158 1.0626 1.1064 1.1755 1.1415 0.9886 1.0304 1.0902 1.1407
1.2246 1.3027 1.4279 1.4268 1.1372 1.1948 1.2795 1.2520 1.0850 1.1198 1.1855 1.2448
1.1238 1.1014 1.0804 1.1645 1.1126 1.1230 1.1430 1.2155 1.1374 1.1406 1.1602 1.1955
1.0164 1.0272 1.0377 1.0739 1.0396 1.0399 1.0501 1.1082 1.0363 1.0496 1.0669 1.1050
55
60
from Zonguldak and Istanbul, and distributions of finer particle size are obtained. The optimum concentrations of NSF and PSS added to the grinding medium 0.7% and 0.3%, respectively. The addition of dispersants to the medium at concentrations indicated improves grinding whereas, if these are exceeded and excessive amounts of dispersant are added, poorer results are obtained in the grinding process. 3.3. Viscosity experiments The variations in the viscosity of the pulp versus the duration of grinding in experiments carried out with coal samples from Zonguldak and Istanbul, both in the presence and in the absence of dispersants, are given in Figs. 3 and 4, respectively. As shown in Fig. 4, the viscosity of Zonguldak coal increases as the pulp solid ratio changes from 50% to 60%. In the case of Istanbul coal, as the pulp solid ratio increases from 50% towards 60%, the viscosity also increases and reaches a maximum value when the pulp solid ratio is 55% (Fig. 3). When the viscosity values of coal samples from Zonguldak and Istanbul are compared, it observed that Istanbul coal has higher viscosity values depending on the pulp solid ratio of the medium as compared to coal from Zonguldak. When the relation between size and viscosity is considered, it is observed that coal particle sizes are large at lower viscosities and smaller at higher viscosities. At all pulp solid ratios, depending on the increasing grinding times, the viscosity values rise with diminishing size.
Efficiency factor
As can be seen in Figs. 3 and 4, in grinding experiments carried out with PSS or NSF at optimum dispersant concentrations, the viscosity increases depending upon the duration of grinding and the pulp solid ratio. In grinding experiments where coal from Istanbul were used without a dispersant, high viscosity values were obtained; whereas, these values were lower when a dispersant was used. While the viscosity of the medium was 2900 mPa s after grinding without a dispersant at 55% pulp solid ratio for 30 min, it was reduced to 460 mPa s when 0.3% PSS was used. The viscosity of the medium was observed to be 490 mPa s when 0.7% NSF was utilised. While grinding coal with 55% pulp solid ratio in absence of a dispersant for 60 min gave a pulp viscosity of around 5500 mPa s, the viscosity values obtained at optimum concentrations of PSS and NSF were 630 and 680 mPa s, respectively. In grinding experiments carried out with coal from Zonguldak where optimum concentrations of dispersants PSS and NSF were used, it was observed that the viscosity increases depending on the duration of grinding and the pulp solid ratio. However, the increase is less for coal from Istanbul. While high viscosity values were observed in the case of coal from Zonguldak when ground in the absence of a dispersant, these values were low when ground in the presence of a dispersant. While the viscosity of the medium was 2500 mPa s following a 60-min grinding process without a dispersant a pulp solid ratio of 60.5%, it was lowered to 560 mPa s by the use of 0.3% PSS. The viscosity of the grinding med-
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205
5000 4750 50% Pulp Density without Dispersant 55% Pulp Density
4500
60% Pulp Density
4250
50% Pulp Density %0.3 PSS 55% Pulp Density
4000
60% Pulp Density
3750
50% Pulp Density %0.7 NSF 55% Pulp Density
3500
60% Pulp Density
Visible Viscosity, m.Pa.s (Stirring Velocity: 100 RPM)
3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0 0
15
30
45
60
Grinding Time, Minute Fig. 3. The viscosity values of Istanbul coal after grinding under different conditions versus grinding times.
ium was observed to be 600 mPa s when 0.7% NSF was used.
4. Conclusion The following conclusions have been reached in the present study, which covered an investigation of
the effects of viscosity lowering dispersants on grinding; ! The conditions for grinding coal to fine particles have been improved for bituminous coal from Zonguldak, which has a high degree of coalification, and the lignite from Istanbul area, which have a low degree coalification, by addition of disper-
206
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2750
50% Pulp Density without Dispersant 55% Pulp Density
2500
60% Pulp Density
2250
50% Pulp Density %0.3 PSS 55% Pulp Density 60% Pulp Density
Visible Viscosity, m.Pa.s (Stirring Velocity: 100 RPM)
2000
50% Pulp Density %0.7 NSF 55% Pulp Density 60% Pulp Density
1750
1500
1250
1000
750
500
250
0 0
15
30
45
60
Grinding Time, Minute Fig. 4. The viscosity values of Zonguldak coal after grinding under different conditions versus grinding times.
sants (PSS and NSF) to the grinding medium. In experiments on both types of coal, which were carried out in the absence of dispersants, high viscosity values were obtained. On the other hand, in grinding experiment done with both types of coal in the presence of a dispersant, viscosity values were much lower than 1000 mPa s and depended on the utility of the coal–water slurries. ! The optimum concentrations for PSS and NSF added to the grinding medium were determined
to be 0.3% and 0.7%, respectively, which represent the maximum dispersant adsorption values for the two types of coal. In the grinding process of coal samples with high and low tendencies of conversion into fine coal, the effect of polymeric PSS is greater than NSF. Previously, in studies carried out using coal–water slurries, it was also observed that the viscosity lowering and dispersing effects of polymeric dispersants are greater than the surface-active materials (Nedo, 1997;
G. Atesok et al. / Int. J. Miner. Process. 77 (2005) 199–207
!
!
!
!
!
!
!
Ates¸ok et al., 2002a,b; Boylu et al., 2004; Dinc¸er et al., 2003). Grinding is improved by the addition of optimum amounts of dispersant to the medium. On the other hand, excessive amounts of dispersant have an adverse effect. The anionic dispersant in the medium increases negative charges on the coal surface and provides for better grinding through dispersion. However, excessive amounts of the dispersant in the pulp adversely affect dispersion and result in flocculation. The flocculation of coal particles in the medium has a cushioning effect and interferes with the grinding process. The solid ratio applied on grinding, differs on the coal type and is one of the important factors. The higher solids ratio in higher viscosity and hindered grinding conditions. The higher efficiencies were obtained on grinding with higher solids ratios, which means that the addition of dispersant agents to the pulp has no effect on grinding when solids ratio is too low. The higher pulp viscosities results in worse grinding conditions and can be overcome by dispersant addition. Dispersant addition on grinding shows different effects. For Zonguldak coals it effects the grinding at longer grinding times or finer final size, although it plays an important role in the initial grinding stages with Istanbul coals. If optimum amounts of PSS and NSF are used in 60-min grinding of bituminous coal from Zonguldak, having a pulp solid ratio of 60%, the amount of fine coal obtained increases by 20% and 16%, respectively. If optimum amounts of PSS and NSF are used in 60-min grinding of lignite from Istanbul, having a pulp solid ratio of 55%, the amount of fine coal obtained increases by 32% and 20%, respectively. The grinding experiments carried out using coal samples from Zonguldak and Istanbul regions, in media containing a dispersant, have shown that the grinding capacity is increased by lowering the viscosity; while reduced grinding times conserve important amounts of energy. The costs for grinding would be greatly diminished by the use of dispersants.
207
References Ates¸ok, G., Boylu, F., Sirkeci, A.A., Dinc¸er, H., 2002a. The effect of coal properties on the viscosity of coal–water slurries. Fuel 81, 1855 – 1858. Ates¸ok, G., Boylu, F., Sirkeci, A.A., 2002b. Rheological behaviour of low rank Turkish coal–water slurries. IXth Int. Mineral Processing Sym., Turkey, pp. 208 – 210. Austin, L.G., Klimpel, R.R., Luckie, P., 1984. Process Engineering of Size Reduction: Ball Milling. SME, New York. Boylu, F., Dinc¸er, H., Ates¸ok, G., 2004. Effect of coal particle size distribution, volume fraction and rank on the rheology of CWS. Fuel Processing Technology 85, 241 – 250. Dinc¸er, H., Boylu, F., Sirkeci, A.A., Ates¸ok, G., 2003. The effect of chemicals on the viscosity and stability of CWS. International Journal of Mineral Processing 70, 41 – 51. El-Shall, H., Somasundaran, P., 1984. Mechanism of grinding modification by chemical additives: organic reagents. Powder Technology 38, 267 – 273. Klimpel, R.R., 1982a. Laboratory studies of the grinding and rheology of coal water slurries. Powder Technology 32, 267 – 277. Klimpel, R.R., 1982b. Slurry rheology influence on the performance of mineral/coal grinding circuits. Mining Engineering, 1665 – 1668. Klimpel, R.R., 1983. Slurry rheology influence on the performance of mineral/coal grinding circuits—part 2. Mining Engineering, 21 – 26. Klimpel, R.R., 1997a. bThe Impact on Industrial Grinding Circuits of changing and/or Controlling the Slurry RheologyQ, Comminution Practices, Ed. S. Komar Kawatra, SME, Published by Society for Mining, Metallurgy, and Exploration, Inc. Littleton, Colorado, USA. Klimpel, R.R., 1997b. bIntroduction to the Principles of Size Reduction of Particles by Mechanical MeansQ, Instructional Modu¨le Series, Series Ed. R. Rajagopalan, Particle Science Technology (41 pages), Florida, USA. Klimpel, Rl., Hansen, R.T., 1989. The chemistry of mineral slurry rheology control grinding aids. Minerals and Metallurgical Processing 6 (1), 35 – 43. NEDO (New Energy and Industrial Technology Development Organization), CWN in Japan, March 1997. Ryncarz, A., Laskowski, J., 1977. Influence of flotation reagents on the wet grinding of quartz. Powder Technology 18, 179 – 185. Somasundaran, P., 1978. In: Onoda Jr., George, Hench, Larry (Eds.), Theories of Grinding, Ceramic Processing Before Firing. John Wiley and Sons, Inc. Wang, Y., Forssberg, E., 1997. Ultra-fVne Grinding and Classification of Minerals, Comminution Practices, Ed. S. Komar Kawatra, SME. Published by Society for Mining, Metallurgy, and Exploration, Inc. Littleton, Colorado, USA.
The effect of anionic dispersants on grindability of different rank coals G. Atesok *, M. Ozer, F. Boylu, H. DVncer Istanbul Technical University, Mining Faculty, Mining Engineering Department, Mineral Processing Division, 34469, Maslak, Istanbul Received 5 October 2004; received in revised form 1 May 2005; accepted 9 May 2005 Available online 8 August 2005
Abstract The effect of PSS (Sodium Polystyrene Sulphonate) and NSF (Naphthalene Sulphonate Formaldehyde Condensate) chemicals used to control the pulp rheology on the grindability of coals was studied. Zonguldak region bituminous coal and Istanbul region brown coal samples that high and low rank were used. Wet grinding tests with two coal samples were performed with or without PSS and NSF under constant grinding conditions: solid ratios ranged from 50% to 60% with 0–60 min grinding periods. In the present study, grinding conditions of coal with high pulp solid ratio were improved through the lowering of the viscosity of the coal by using the dispersing agents, PSS and NSF, during the grinding stage and evaluation were made by using the befficiency factorQ. It is possible to increase the amount of finely ground bituminous coal from Zonguldak by 20% and 16%, if optimum concentrations of PSS (0.3%) and NSF (0.7%) are used, respectively. Whereas, the increase in the case of finely ground lignite from Istanbul was 32% and 20%, respectively. D 2005 Published by Elsevier B.V. Keywords: grinding; dispersant; viscosity; energy; coal
1. Introduction Crushing and grinding are size reduction process in minerals processing circuits. Comminution processes consume approximately 50% of energy used in mineral processing (Austin et al., 1984). Energy consumption is about 5–20 kW h/ton of ore in fine grinding, however, this figure increases 20–100 * Corresponding author. Tel.: +90 0212 285 61 78; fax: +90 0212 285 61 28. E-mail address: [email protected] (G. Atesok). 0301-7516/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.minpro.2005.06.004
kWh/ton of ore in ultra fine grinding. Therefore, it presents utmost importance to design size reduction especially grinding circuits (Austin et al., 1984). Pulp rheology plays a significant role in wet fine grinding processes and it affects the efficiency of grinding. Pulps show differences as pulp rheology depending on the different solids ratios (Ryncarz and Laskowski, 1977). Pulp viscosity, is especially, an important parameter in grinding. A number of investigators have studied the effect of pulp rheology in ball mill grinding for many years. Those investigations especially focused on the grinding of ceramic
G. Atesok et al. / Int. J. Miner. Process. 77 (2005) 199–207
raw materials and clinker grinding. They reported up to 100% increase in grinding efficiency by the use of chemical additives (Somasundaran, 1978). The pulp rheology effects on wet grinding have been researching for several years. Previous studies (El-Shall and Somasundaran, 1984; Klimpel, 1982a,b, 1983, 1997a,b; Klimpel and Hansen, 1989) have shown that solids ratios in the pulp could be raised with no changes on yield stress value by using dispersing agents in grinding processes. Thus the grinding capacities could be increased. Some researchers (Wang and Forssberg, 1997) found that the polymeric type dispersing agents mixed with inorganic chemicals are more effective for fine grinding of bituminous coals. Investigations related to pulp rheology or the effect of additives on the grindability of coal, however, have been very limited. Those investigations have generally been conducted with bituminous coal after coal–water slurry became an important issue. In this study, the effects of anionic dispersants PSS (Sodium Polystyrene Sulphonate) and NSF (Naphthalene Sulphonate Formaldehyde Condensate) on pulp rheology and grindability when added to the medium during the wet grinding process to reduce the viscosity of coal–water slurries (grindability and the energy consumed) have been examined. Used in the experiments were bituminous coal from Zonguldak region in Turkey, which has a high degree of Table 1 Proximate and ultimate analyses of coal samples on dry bases Component
Proximate analyses
Ultimate analyses
O/C
Coal type
ash (%) total sulfur (%) volatile matter (%) fixed carbon (%) inherent moisture (%) upper calorific value (kcal/kg) C (%) H (%) O (%) N (%) S (%) Pyritic sulfur (%)
Istanbul (low rank coal)
Zonguldak (high rank coal)
40.36 7.19 41.58 18.06 32.00 3616
24.40 0.40 24.49 51.11 0.50 6218
Cummulative under size, %
200
100
10
1 0,1
1 Istanbul Brown Coal
56.45 4.73 10.50 1.30 0.42 0.20 0.186
Zonguldak Bituminous Coal
Fig. 1. Total sieve curves of coals when roller type grinders are used.
coalification, and lignite from Istanbul area, which has low coalification.
2. Experimental 2.1. Material The grinding tests were carried out with two different kinds of Turkish coals representing bituminous (Zonguldak) and brown coal (Istanbul). Table 1 shows the results of both proximate and ultimate analyses for these two coals. In crushing the coal samples; after jaw and conical crushers, roller type crusher was used and size of particles less than 3 mm was obtained. The fine sieve analyses of materials obtained with the roller type grinder are given in Fig. 1. Table 2 Physical properties and chemical structure of PSS and NSF
Items
Value
Molecular weight
14000
Sulphonation degree, % Specific gravity, g/cm3 Viscosity at 30o C, m. Pa. sec
26.70 5.1 15.53 1.40 7.20 4.01 0.580
10
Particle size, mm
Molecular weight
Chemical structure CH2
PSS CH
95 1.22 150 14000
SO3 Na n
NSF CH2
SO3 M
n
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Two different chemicals were used as dispersants: Sodium Polystyrene Sulphonate (PSS, a product of Japanese COM and Lion Corp.) and Naphthalene Sulphonate Formaldehyde Condensate (NSF, a product of Japanese COM and Lion Corp.). The physical and the chemical structures of both dispersants used in coal–water slurry technology are given in Table 2 (Ates¸ok et al., 2002a,b; Dinc¸er et al., 2003). The wet grinding tests were performed under the constant grinding conditions by employing the ball mill. Table 3 shows the ball mill characteristics used in this study. 2.2. Methods
201
The grinding experiments were carried out at intervals of 15 min. The mill was stopped after every 15 min and sufficient amount of the samples for determination of particle size and viscosity were removed without disturbing the pulp solid ratio and the grinding process was resumed. The surface area and viscosity measurements were carried out on samples obtained after grinding. Both measurements were repeated 3–4 times in parallel on each sample, and the average values were determined. The results are analysed according to both the viscosity and efficiency factors explained by the formula given below; Ef ¼ A1 =A2
In this study, the effects of solids ratio, the addition and concentration of PSS-NSF and grinding times on wet grinding were investigated. The wet grinding tests were performed with or without PSS and NSF in effective concentrations. For each coal, different solids ratios ranging from 55% to 60% were applied according to the slurry viscosities allowing easy rolling of the grinding media and the material in mill. In each experiment, 600 g coal samples completely ground to less than 3 mm were used in the grinding process. In experiments without dispersants, only sufficient water required to calculate the pulp solid ratio (PSR) was used. Whereas, in experiments including a dispersant, the dispersant was stirred for 5 min with the amount of pulp water required for solid ratio calculation. At the end, water or the mixture containing the dispersant was added to the mill.
where E f is the efficiency factor, A 1, is the surface area of the ground particles in a certain grinding time without dispersant, A 2 is the surface area of the ground particles in a certain grinding time with dispersant. The specific surface area of coal samples was measured by the BET method (with nitrogen). The zeta potential measurements were performed by Zeta Meter 3.0 type instrument; 0.1 g of the 38 Am coal sample was conditioned in 10 ml of solution for 10 min during which the pH was maintained at the desired value by adding either acid (HCl) or base (NaOH). Each data point is the mean of at least 10 measurements. The viscosity measurements were performed employing an RVD2-Brookfield rotating type viscometer.
Table 3 Ball mill characteristics
3. Results and discussion
Mill
Speed Lifters Media Charge
Material Charge
Internal diameter D, mm Length L, mm Volume V, cm3 Critical speed N c, rpm Operational speed, rpm Number Cross-section Material Diameter 1. d, mm Diameter 2. d, mm Loading for diameter 1, % Loading for diameter 2, % Specific gravity, g/cm3 Fractional ball loading, % Coal, g
225 235 8940 108 90 6 Semi-Circular Alloy Steel 30 40 46.5 53.5 7.387 33.9 600
3.1. Zeta potential measurements The zeta potential measurements taken at neutral pH on samples containing different concentration of dispersing agents are reported in Fig. 2. While the zeta potential of the both coals used in the experiments was around 20 mV at neutral pH, it shifted towards more negative values with the increasing of reagent amount. It is seen that PSS produces the highest negative potential at the lowest dispersant addition. While zeta potential of Zonguldak bituminous coal was 59 mV in 0.3% PSS concentration, zeta potential of Istanbul brown coal has been determined as 50
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0 PSS NSF PSS NSF
Zeta Potantial , mV
-10
ZONGULDAK ZONGULDAK ISTANBUL ISTANBUL
-20 -30 -40 -50 -60 -70 0
0,2
0,4
0,6
0,8
1
1,2
Dispersant Concentration, % Fig. 2. The zeta potentials of coals from Zonguldak and Istanbul containing different concentrations of dispersant.
mV. The highest absolute zeta potential values obtained with NSF were 54 mV for Zonguldak bituminous coal and 46 mV Istanbul brown coal at a concentration of 0.7%. Electrostatic bonding occurs between the positive sites of coal and the anionic head groups of dispersants. Hydrophobic bonding on the other hand takes places between the hydrophobic groups of coal and the hydrophobic chain of the polymer unit. The former mechanism is expected to result in higher negative zeta potential. As a result, coal particles hence negatively charged would repel with each other thus flocculation is prevented and dispersion is achieved.
3.2. Grinding experiments The results of grinding experiments carried out in the presence or absence of dispersants using coal samples of varying pulp solid ratios, both Zonguldak and Istanbul coals, are given together in Tables 4–7. In experiments where dispersants were used, the dispersion concentrations were determined in relation to adsorption and zeta potential characteristics of the coal samples. The grinding experiments were performed at optimum concentrations of 0.3% PSS and 0.7% NSF or above. As indicated in Tables 4–7 by increasing the solid ratio of the pulp from 50% to 55%, either in the
Table 4 The results of specific surface areas and efficiency factor measurements (Zonguldak bituminous coal and PSS) Dispersant and coal types
PSR* (%)
PSS Zonguldak
50
55
60
PSR: pulp solid ratio.
Specific Surface Area (m2/cm3)
Grinding time (min)
%0.0 Disp.
%0.3 Disp.
% 0.6 Disp.
% 0.3 Disp.
% 0.6 Disp.
15 30 45 60 15 30 45 60 15 30 45 60
0.6889 0.7875 0.8038 0.8641 0.7253 0.8669 0.9829 1.1163 0.6832 0.7364 0.7214 0.7058
0.6119 0.6951 0.6917 0.7303 0.6209 0.7279 0.8260 0.9443 0.5465 0.5883 0.5963 0.5872
0.6343 0.7436 0.7440 0.7642 0.6574 0.8091 0.9059 1.0797 0.5987 0.6664 0.6531 0.6513
1.1258 1.1330 1.1620 1.1832 1.1682 1.1910 1.1899 1.1821 1.2501 1.2518 1.2097 1.2019
1.0860 1.0590 1.0804 1.1307 1.1033 1.0714 1.0850 1.0339 1.1411 1.1051 1.1045 1.0836
Efficiency factor
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203
Table 5 The results of specific surface areas and efficiency factor measurements (Zonguldak bituminous coal and NSF) Specific surface area (m2/cm3)
Dispersant and coal types
PSR (%)
Grinding time (min)
%0.0 Disp.
%0.7 Disp.
%1.4 Disp.
%0.7 Disp.
%0.4 Disp.
NSF Zonguldak
50
15 30 45 60 15 30 45 60 15 30 45 60
0.6889 0.7875 0.8038 0.8641 0.7253 0.8669 0.9829 1.1163 0.6832 0.7364 0.7214 0.7058
0.6522 0.7489 0.7449 0.7856 0.6795 0.8141 0.9038 1.0013 0.5969 0.6627 0.6433 0.6055
0.6608 0.7750 0.8170 0.8848 0.6891 0.8309 0.9828 1.1176 0.6414 0.7186 0.7340 0.7162
1.0563 1.0516 1.0790 1.0999 1.0674 1.0649 1.0875 1.1149 1.1446 1.1112 1.1214 1.1657
1.0425 1.0161 0.9839 0.9766 1.0525 1.0433 1.0001 0.9988 1.0651 1.0247 0.9828 0.9855
55
60
presence or absence of dispersants, the grinding process is improved for both types of coal. However, a pulp solid ratio (PSR), which is greater than 55%, is favourable for coal from Zonguldak, but unfavourable for that from Istanbul. In the grinding experiments carried out using coal from Istanbul, it was observed that the value of the efficiency factor is lowered when the pulp solid ratio is increased to 60%. This is an indication of poor grinding. The solid ratio with optimum pulp was determined to be 60% for bituminous coal from Zonguldak and 55% for lignite from Istanbul. As is observed in related tables, the evaluations made using the parameter, the bEfficiency FactorQ, indicate that the polymeric dispersant PSS is more effective in the grinding process than NSF. The use of PSS with
Efficiency factor
coal from Zonguldak having 60% pulp solid ratio produced an efficiency factor of 1.25 after a 30-min grinding period. Whereas, when coal from Istanbul, which had a pulp solid ratio of 55%, was subjected to 60 min of grinding, the efficiency factor obtained was 1.32. The values of efficiency factors produced after grinding for 60 min in the presence of NSF were 1.16 and 1.20 for coal samples from Zonguldak (60% PSR) and Istanbul (55% PSR), respectively. Coal samples from Zonguldak and Istanbul, which had similar particle size distributions prior to grinding, exhibited different behaviours during the same grinding period. Istanbul coal is more readily ground than that from Zonguldak. As can be seen from Tables 4–7, the addition of dispersant to the grinding medium improves the grinding conditions both for coals
Table 6 The results of specific surface areas and efficiency factor measurements (Istanbul lignite and PSS) Specific surface area (m2/cm3)
Dispersant and coal types
PSR (%)
Grinding time (min)
%0.0 Disp.
%0.3 Disp.
%0.6 Disp.
%0.3 Disp.
%0.6 Disp.
PSS Istanbul
50
15 30 45 60 15 30 45 60 15 30 45 60
1.2447 1.3381 1.4817 1.5322 1.1822 1.2425 1.3436 1.3875 1.1244 1.1753 1.2648 1.3755
1.0354 1.1366 1.3026 1.2454 1.0194 1.0265 1.0756 1.0501 0.9588 0.9925 1.1273 1.2735
1.1062 1.1943 1.3288 1.4811 1.1135 1.1414 1.2156 1.2257 0.9733 1.0707 1.1943 1.3271
1.2022 1.1773 1.1375 1.2303 1.1597 1.2104 1.2492 1.3213 1.1727 1.1842 1.1220 1.0801
1.1252 1.1204 1.1151 1.0345 1.0617 1.0886 1.1053 1.1320 1.1552 1.0977 1.0590 1.0365
55
60
Efficiency factor
204
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Table 7 The results of specific surface areas and efficiency factor measurements (Istanbul lignite and NSF) Specific surface area (m2/cm3)
Dispersant and coal types
PSR (%)
Grinding time (min)
%0.0 Disp.
%0.7 Disp.
%1.4 Disp.
%0.7 Disp.
%1.4 Disp.
NSF Istanbul
50
15 30 45 60 15 30 45 60 15 30 45 60
1.2447 1.3381 1.4817 1.5322 1.1822 1.2425 1.3436 1.3875 1.1244 1.1753 1.2648 1.1505
1.1076 1.2149 1.3714 1.3158 1.0626 1.1064 1.1755 1.1415 0.9886 1.0304 1.0902 1.1407
1.2246 1.3027 1.4279 1.4268 1.1372 1.1948 1.2795 1.2520 1.0850 1.1198 1.1855 1.2448
1.1238 1.1014 1.0804 1.1645 1.1126 1.1230 1.1430 1.2155 1.1374 1.1406 1.1602 1.1955
1.0164 1.0272 1.0377 1.0739 1.0396 1.0399 1.0501 1.1082 1.0363 1.0496 1.0669 1.1050
55
60
from Zonguldak and Istanbul, and distributions of finer particle size are obtained. The optimum concentrations of NSF and PSS added to the grinding medium 0.7% and 0.3%, respectively. The addition of dispersants to the medium at concentrations indicated improves grinding whereas, if these are exceeded and excessive amounts of dispersant are added, poorer results are obtained in the grinding process. 3.3. Viscosity experiments The variations in the viscosity of the pulp versus the duration of grinding in experiments carried out with coal samples from Zonguldak and Istanbul, both in the presence and in the absence of dispersants, are given in Figs. 3 and 4, respectively. As shown in Fig. 4, the viscosity of Zonguldak coal increases as the pulp solid ratio changes from 50% to 60%. In the case of Istanbul coal, as the pulp solid ratio increases from 50% towards 60%, the viscosity also increases and reaches a maximum value when the pulp solid ratio is 55% (Fig. 3). When the viscosity values of coal samples from Zonguldak and Istanbul are compared, it observed that Istanbul coal has higher viscosity values depending on the pulp solid ratio of the medium as compared to coal from Zonguldak. When the relation between size and viscosity is considered, it is observed that coal particle sizes are large at lower viscosities and smaller at higher viscosities. At all pulp solid ratios, depending on the increasing grinding times, the viscosity values rise with diminishing size.
Efficiency factor
As can be seen in Figs. 3 and 4, in grinding experiments carried out with PSS or NSF at optimum dispersant concentrations, the viscosity increases depending upon the duration of grinding and the pulp solid ratio. In grinding experiments where coal from Istanbul were used without a dispersant, high viscosity values were obtained; whereas, these values were lower when a dispersant was used. While the viscosity of the medium was 2900 mPa s after grinding without a dispersant at 55% pulp solid ratio for 30 min, it was reduced to 460 mPa s when 0.3% PSS was used. The viscosity of the medium was observed to be 490 mPa s when 0.7% NSF was utilised. While grinding coal with 55% pulp solid ratio in absence of a dispersant for 60 min gave a pulp viscosity of around 5500 mPa s, the viscosity values obtained at optimum concentrations of PSS and NSF were 630 and 680 mPa s, respectively. In grinding experiments carried out with coal from Zonguldak where optimum concentrations of dispersants PSS and NSF were used, it was observed that the viscosity increases depending on the duration of grinding and the pulp solid ratio. However, the increase is less for coal from Istanbul. While high viscosity values were observed in the case of coal from Zonguldak when ground in the absence of a dispersant, these values were low when ground in the presence of a dispersant. While the viscosity of the medium was 2500 mPa s following a 60-min grinding process without a dispersant a pulp solid ratio of 60.5%, it was lowered to 560 mPa s by the use of 0.3% PSS. The viscosity of the grinding med-
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205
5000 4750 50% Pulp Density without Dispersant 55% Pulp Density
4500
60% Pulp Density
4250
50% Pulp Density %0.3 PSS 55% Pulp Density
4000
60% Pulp Density
3750
50% Pulp Density %0.7 NSF 55% Pulp Density
3500
60% Pulp Density
Visible Viscosity, m.Pa.s (Stirring Velocity: 100 RPM)
3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0 0
15
30
45
60
Grinding Time, Minute Fig. 3. The viscosity values of Istanbul coal after grinding under different conditions versus grinding times.
ium was observed to be 600 mPa s when 0.7% NSF was used.
4. Conclusion The following conclusions have been reached in the present study, which covered an investigation of
the effects of viscosity lowering dispersants on grinding; ! The conditions for grinding coal to fine particles have been improved for bituminous coal from Zonguldak, which has a high degree of coalification, and the lignite from Istanbul area, which have a low degree coalification, by addition of disper-
206
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2750
50% Pulp Density without Dispersant 55% Pulp Density
2500
60% Pulp Density
2250
50% Pulp Density %0.3 PSS 55% Pulp Density 60% Pulp Density
Visible Viscosity, m.Pa.s (Stirring Velocity: 100 RPM)
2000
50% Pulp Density %0.7 NSF 55% Pulp Density 60% Pulp Density
1750
1500
1250
1000
750
500
250
0 0
15
30
45
60
Grinding Time, Minute Fig. 4. The viscosity values of Zonguldak coal after grinding under different conditions versus grinding times.
sants (PSS and NSF) to the grinding medium. In experiments on both types of coal, which were carried out in the absence of dispersants, high viscosity values were obtained. On the other hand, in grinding experiment done with both types of coal in the presence of a dispersant, viscosity values were much lower than 1000 mPa s and depended on the utility of the coal–water slurries. ! The optimum concentrations for PSS and NSF added to the grinding medium were determined
to be 0.3% and 0.7%, respectively, which represent the maximum dispersant adsorption values for the two types of coal. In the grinding process of coal samples with high and low tendencies of conversion into fine coal, the effect of polymeric PSS is greater than NSF. Previously, in studies carried out using coal–water slurries, it was also observed that the viscosity lowering and dispersing effects of polymeric dispersants are greater than the surface-active materials (Nedo, 1997;
G. Atesok et al. / Int. J. Miner. Process. 77 (2005) 199–207
!
!
!
!
!
!
!
Ates¸ok et al., 2002a,b; Boylu et al., 2004; Dinc¸er et al., 2003). Grinding is improved by the addition of optimum amounts of dispersant to the medium. On the other hand, excessive amounts of dispersant have an adverse effect. The anionic dispersant in the medium increases negative charges on the coal surface and provides for better grinding through dispersion. However, excessive amounts of the dispersant in the pulp adversely affect dispersion and result in flocculation. The flocculation of coal particles in the medium has a cushioning effect and interferes with the grinding process. The solid ratio applied on grinding, differs on the coal type and is one of the important factors. The higher solids ratio in higher viscosity and hindered grinding conditions. The higher efficiencies were obtained on grinding with higher solids ratios, which means that the addition of dispersant agents to the pulp has no effect on grinding when solids ratio is too low. The higher pulp viscosities results in worse grinding conditions and can be overcome by dispersant addition. Dispersant addition on grinding shows different effects. For Zonguldak coals it effects the grinding at longer grinding times or finer final size, although it plays an important role in the initial grinding stages with Istanbul coals. If optimum amounts of PSS and NSF are used in 60-min grinding of bituminous coal from Zonguldak, having a pulp solid ratio of 60%, the amount of fine coal obtained increases by 20% and 16%, respectively. If optimum amounts of PSS and NSF are used in 60-min grinding of lignite from Istanbul, having a pulp solid ratio of 55%, the amount of fine coal obtained increases by 32% and 20%, respectively. The grinding experiments carried out using coal samples from Zonguldak and Istanbul regions, in media containing a dispersant, have shown that the grinding capacity is increased by lowering the viscosity; while reduced grinding times conserve important amounts of energy. The costs for grinding would be greatly diminished by the use of dispersants.
207
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