- Email: [email protected]
The effect of fines recycling on industrial grinding performance
Minerals Engineering 18 (2005) 1110–1115 This article is also available online at: www.elsevier.com/locate/mineng
The effect of fines recycling on industrial grinding performance J. Yianatos a
a,*
, N. Bergh a, R. Bucarey a, J. Rodrı´guez b, F. Dı´az
c
Department of Chemical Engineering, Santa Marı´a University, P.O. Box 110-V, Valparaı´so, Chile b Department of Metallurgical Engineering, Salvador Division, Codelco-Chile, Chile c Department of Radioactive Tracer Applications, Chilean Commission of Nuclear Energy, Chile Received 11 September 2004; accepted 1 March 2005 Available online 14 April 2005
Abstract The effect of fines recycling back to the rod mill in a conventional full-scale grinding circuit was evaluated. For this purpose, a fraction of the hydrocyclone feed stream was diverted to the rod mill feed. It was observed that operating with fines recycling allowed a decrease of 2–3% in the grinding product size, % +212 lm, at similar solid feed capacity, tph, and similar circulating load in the ball mills. This represents a potential increase of about 1.0–1.5% in the overall rougher flotation recovery. The effective pulp residence time in the ball mills was evaluated and was found to be 1.6–1.8 min. Thus, the effective volume of slurry inside the ball mills was estimated and was equal to 32.7%. The product size, % +212 lm, was correlated with the main operating variables such as Bond index, feed tonnage, power consumption and circulating load. The model was adjusted to describe normal operation without fines recycling. Then, the comparison of experimental data with fines recycling and those without it, showed a decrease of 2.5% in % +212 lm, for the same feed capacity tph. 2005 Elsevier Ltd. All rights reserved. Keywords: Grinding; Fine particle processing; Recycling; Mass balancing; Process optimisation
1. Introduction Slurry rheology affects the particle breakage, the mass transport and energy efficiency inside a grinding mill. The rheological effects are often interactive with each other, particularly in closed industrial grinding circuits, and sometimes may be cancelled out by each other, thus creating conflicting conclusions (Shi and Napier-Munn, 2002). In the case of ball mills, the bypass of fine particles, which return to the mill with the circulating load, is an issue of controversy. A significant amount of work has been done in pilot plant studies and field tests to investigate the effect of returning fines back to the mill. The *
Corresponding author. Tel.: +56 32 654 235; fax: +56 32 654 478. E-mail address: [email protected] (J. Yianatos).
0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.03.001
common concern is that fines should not be returned to the mill since these fines will (1) be subject to over grinding and (2) consume space that could be normally occupied by new mill feed. On the other hand, it is well known that the addition of fines in the rod mill feed act as a viscosity modifier, which helps the grinding efficiency (Napier-Munn et al., 1999). The fine solid improves the rheological properties of the pulp, reduces the hydraulic classification, helps the pulp transport and avoids rod mill tangles. Experience from plant operation has shown that the addition of fines, i.e. a small proportion of the hydrocyclone feed, to the rod mill allows the treatment capacity at similar product size and power consumption to increase (Zickar et al., 1981). Also, a similar effect was observed in SAG mills at Chuquicamata A2 concentrator, where a 8–12% increase in fresh mineral treatment
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
capacity was achieved by adding fines from the crushing stage, while the product particle size to flotation was kept approximately constant around 26.5% +212 lm (Yianatos and Pizarro, 1997). In addition, Zickar et al. (1981) discussed the importance of using a proper control system, for the fine recycling to the rod mill, in order to optimize the treatment capacity of the grinding circuit and decrease the potential over-grinding. In the present study, the effect of fines recycling to the rod mill feed was evaluated at the El Salvador concentrator. For this purpose, a small fraction of the hydrocyclone feed stream was recycled to create a fine circulating load back to the rod mill, while the ball mill circuit was operated under normal conditions. The rod mill discharge showed a 54.7 ± 0.5% solids by volume with a particle size of less than 18% 0.0038 mm. Under this condition, the rheological type of the slurry should have a pseudoplastic behavior, and then it can be expected that increasing fines increases grinding (Napier-Munn et al., 1999). In this study, the radioactive tracer technique was used to measure the rod mill recycle, as well as the effective mean residence time in ball mills.
1111
grinding circuit, a 12.5 cm (5 in) pipe was fitted. This pipe returns part of the hydrocyclone feed stream back to the rod mill, stream 2 in Fig. 1. 2.2. Mass flowrate measurement in fines recycling In order to optimize the amount of fine recycling, a measurement of the mass flowrate is required. In a previous work, Zickar et al. (1981) estimated the mass flowrate from the difference between pulp densities measured before and after adding a known amount of water to the recycling stream. In this study, a direct measurement of the slurry flowrate using a radioactive tracer (Br-82) was used. This technique consists of measuring the mean transport time, between external sensors located at a known distance, after injecting a pulse signal of tracer. Fig. 2 shows a diagram of the installation of three radioactive tracer detectors separated at distances of L1 = 6.5 m and L2 = 6.1 m, and located far from the tracer injection point. Fig. 3 shows the tracer time response observed, from which the mean transport time was measured at three testing points in the fine recycling stream. Table 1 shows a summary of the mass flow of recycled fines particles in grinding sections 1 and 4. The
2. Experimental 2.1. Grinding circuit
Br-82
The grinding circuit of the Salvador Division, CodelcoChile, consists of five sections operating in parallel. Sections 1–4 are formed by 1 rod mill (3.05 · 4.27 m) followed by 2 ball mills (3.05 · 4.27 m) in parallel, as it is shown in Fig. 1. Section 5 consists of 1 rod mill (4.11 · 5.49 m) followed by 1 ball mill (5.03 · 5.79 m). All ball mill circuits (1–5) operate in closed circuit. In order to study the effect of fines recycling to the rod mill feed, and how this influences the rest of the
Pulp L1
L2
Sensors
Fig. 2. Diagram of tracer system to measure the slurry velocity.
11000
1
3
4
2 7
11
10
activity [cps]
12
9
5
10000
Detector 1
9000
Detector 2
8000
Detector 3
7000 6000 5000 4000
8 6
3000 2000 1000 0 30
35
40
45
50
55
60
time [s] Fig. 1. Grinding circuit of sections 1–4, Salvador Division, CodelcoChile.
Fig. 3. Tracer time response in fine recycling stream returning to rod mill.
1112
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
Table 1 Fines recycle to the rod mill Recycled fines
Flow speed, m/s
Volumetric flow, m3/h
Solids conc. %
Mass flow, ton/h
Recycle, %
Section 1 Section 4
1.8 ± 0.2 1.5 ± 0.3
81.7 69.2
70.9 71.6
105.5 91.0
39.8 34.1
recycle from the sump box to the rod mill feed, stream number 2 in Fig. 1, was 39.8% and 34.1%, respectively.
The average residence time of the slurry in the sump box was around 30 s considering:
2.3. Residence time distribution measurement in ball mills
(a) the total volume of the sump box was 20 m3 and it was typically filled up to 50%; (b) the volumetric flowrate was 1233 m3/h, with an average solid mass flowrate of 1295 tph and 63% solids.
In order to study the hydrodynamic behavior of the ball mills during fines recycling in sections 1 and 4, the radioactive tracer technique was used to measure the residence time distribution of the liquid inside the mills. Thus, a radioactive tracer Br-82 solution, in water, was injected at the inlet of the ball mills, operating in closed loop. Then, the response time of the radioactive tracer was measured on-line using a non-invasive sensor located in the discharge of the mills sump box after the pump. Fig. 4 shows the closed loop response data in ball mill 2 (section 1). Here, the experimental data does not converge to zero rapidly, because a fraction of the radioactive tracer was recycled to the mill through the water circulating load in the ball mill circuit. The open loop impulse response was then estimated by de-convolution in order to obtain the actual transfer function of the system. From a mass balance adjustment the water circulating load in the ball mill was 38%. Thus, the total tracer feed entering to the ball mill was the initial tracer impulse at time zero plus a fraction (38%) of the actual mill response with a delay of 20 s (0.33 min), which approximately corresponds to the residence time in the hydrocyclone rack and piping. The shape of the response curve obtained for each mill was similar, with a pure delay, of 20–30 s followed by a sharp ascent and an exponential decay.
Thus, an effective mean residence time of 1.8 min was observed in mill 2 (section 1). 2.4. Estimation of the effective volume of slurry inside the ball mills In order to evaluate the hydrodynamic conditions inside the ball mills, an estimate of the effective volume of slurry inside the ball mills was developed. The total mills volume was 31.2 m3. The apparent volume of balls charge in the mills was 40%, with a void fraction of 42%, thus the total volume occupied by balls was 23.2%. In addition, the solid mass flowrate was 462.3 ± 11.3 tph, and the volumetric slurry flowrate was 340 m3/h, with a 73% solids and a solid density of 2.74 ton/m3. Then, for the effective residence time of 1.8 min measured in the ball mill 2, of section 1, the effective volume occupied by slurry was 32.7%. This result gives a total volume of slurry and balls of 56%, which is in agreement with the expected total volume for this type of operation (around 50%) (Gutie´rrez and Sepu´lveda, 1985; Napier-Munn et al., 1999). 2.5. Material balance of grinding circuit
0.009 Open loop
0.008
Closed loop
0.007
Data
E(t)
0.006 0.005 0.004 0.003 0.002 0.001 0 0
100
200
300
400
500
600
700
800
900
Time, s Fig. 4. RTD measured with radioactive tracer in ball mill 2, section 1.
Mass balance adjustments were developed in order to compare the operation with and without fines circulating load to the rod mill. This was made by measuring the rod mill fresh feed (stream 1 in Fig. 1), the fine recycle (stream 2) and the particle size distributions around the hydrocyclone (streams 6, 7 and 8). Then, from a mass balance adjustment per size class, the split of the hydrocyclone feed into the overflow and underflow streams was estimated. It was found that around 34–40% of the hydrocyclone feed was recycled to the rod mill feed. Table 2 shows a summary of the results for the streams 1–10, described in Fig. 1. Streams 11 and 12 have the same values of streams 9 and 10, respectively.
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
1113
Table 2 Solid mass balances in grinding sections 1 and 4 Stream no.
Stream, tph
Grinding section Section 1
1 2 3 4 5 6 7 8 9 10
Rod mill fresh feed Rod mill recycle Rod mill feed Rod mill discharge Sump box discharge Hydrocyclone feed Hydrocyclone overflow Hydrocyclone underflow Ball mill 1 feed Ball mill 2 feed
Section 4
T1
T2
T3
T4
T5
T6
265.2 0 265.2 265.2 1183.4 1183.4 265.2 918.2 467.6 450.6
260.4 0 260.4 260.4 1181.5 1181.5 260.4 921.1 443.3 477.8
265.0 102.7 367.7 367.7 1301.2 1198.5 265.0 933.5 472.3 461.3
265.1 106.2 371.3 371.3 1288.9 1182.7 265.1 917.6 458.0 459.6
265.0 91.0 356.0 356.0 1255.8 1164.8 265.0 899.8 447.8 452.1
269.8 91.3 361.1 361.1 990.0 898.8 269.8 628.9 295.2 333.7
2.6. Size distribution in grinding circuit Fig. 5 shows the particle size distribution around the hydrocyclone (a) working without fine circulating load to the rod mill and (b) with fine circulating load. Here a decrease in the size distribution at the hydrocyclone feed can be seen, as well as a decrease from 261 to 234 lm in the d80 passing size of the hydrocyclone overflow, while changing the operation without fine recycle to the operation with recycle.
3. Effect of fine recycling on particle size The main operating characteristics during sampling can be seen in Table 3. Under the same feed rate for the rod mills in sections 1 and 4 (264.1 ± 2.1 tph) and similar circulating load to the ball mills (347.4 ± 5.3%),
observed in tests T1, T2, T3, T4 and T5, a statistical difference between the operation with fines recycling (tests T3, T4 and T5) and those without recycle (tests T1 and T2) was observed. Thus, the use of recycle to the rod mill feed enabled a finer product size to be obtained, 23.7 ± 0.8% of particles larger than 212 lm, in comparison with the operation without recycle, 28.9 ± 1.9% of particles larger than 212 lm. On the other hand, the comparison between tests T5 and T6, both operating with fines recycle to the rod mill, in section 4, showed an increase from 23.9% to 29.5% of particles larger than 212 lm. This result was mainly due to the decrease in the circulating load to the ball mills, 340% and 233%, respectively. The same effect was observed in a previous work where the decrease (step change) of the water addition to the sump box decreased the circulating load to the ball mills, while increasing the particle size at the hydrocyclone overflow (Yianatos et al., 2002).
100
100
90
90 80
70
Cumulative % passing
Cumulative % passing
80
60 50 40 30
60 50 40 30
overflow
20
overflow
20
feed
10
feed
10
discharge
discharge
0
0 10
(a)
70
100
1000
Passing size, microns
10000
10
(b)
100
1000
Passing size, microns
Fig. 5. Size distribution of the hydrocyclone: (a) without fines recycles and (b) with fines recycle.
10000
1114
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
Table 3 Main operating characteristics Section 1
Section 4
T1
T2
T3
T4
T5
T6
Rod mill feed, tph % +12.7 mm (+1/2 in) % 0.038 mm
265.2 3.27 7.6
260.4 4.38 7.3
265.0 4.40 9.7
265.1 3.99 8.2
265.0 2.92 9.0
269.8 5.38 6.4
Rod mill discharge, % 0.038 mm Fines recycling, tph Recycle in rod mills, % Circulating load in ball mills, % Hydrocyclone overflow properties, d80 lm % +212 lm (% +65 mesh) % 12 lm
18.1 0 0 346 272 27.0 17.0
17.3 0 0 353 304 30.7 17.5
17.6 105.5 39.8 352 237 22.9 18.5
14.3 105.5 39.8 346 250 24.5 17.9
14.4 91.0 34.3 340 245 23.9 19.4
14.7 91.0 33.7 233 294 29.5 17.5
3.1. Estimation of the grinding product size % +212 lm
where
In order to evaluate the impact that the fines recycle to the rod mill feed has on the grinding product, a correlation was developed to describe the circuit behavior in terms of the main operating variables. The correlation was fitted to predict the operation without fines recycle and then the model was used to observe the differences between predictions without fines recycle and the actual operation using fines recycle. According to BondÕs law (Napier-Munn et al., 1999), 1 1 W ¼ 10 W i pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ð1Þ P 80 F 80
S Scorr
and P ¼T W
ð2Þ
% +212 lm S, corrected by the ball mill circulating load effect, % +212 lm circulating load in ball mill, % average circulating load in ball mills, % parameter for model fitting
L Lave a
Table 4 shows a summary of the mill parameters, for grinding sections 1–4 and section 5 (larger size), operating under normal conditions without fines recycle. The ‘‘a’’ parameter was derived by minimizing the sum of the square differences, between experimental data and model predictions (Scorr). Table 4 Mill characteristics and parameters for Eqs. (1)–(4)
where Mill power P, kW Lave, % a
work input, kWh/ton Bond Work Index, kWh/ton product size, lm feed size, lm feed throughput, tph power draw, kW
In addition, the following correlation, between P80 and the product particle size class used for control purposes, S (% +212 lm), was previously developed for the hydrocyclone overflow stream (Yianatos et al., 2000), P 80 ¼ 8:64 S þ 38:92
ð3Þ
Eqs. (1)–(3) allow for the estimation of the particle size class S, % +212 lm, in terms of power consumption, feed tonnage and particle size. Moreover, considering that the performance of the ball mills is influenced by the quantity and quality of the circulating load being returned to the mill, an empirical correction factor was defined to account for the effect of circulating load changes on the normal operating conditions in ball mills. S corr ¼ S ðL=Lave Þ
a
Section 5
2218 344 0.129
3270 344 0.129
35
ð4Þ
Predictive model, % +212 micron
W Wi P80 F80 T P
Sections 1–4
30
25
section 1-4
20
section 5 fine recycling 15 15
20
25
30
Experimental data, %+212 microns Fig. 6. Effect of fines recycling on grinding, sections 1–5.
35
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
Fig. 6 shows a comparison of the experimental values of S, %+212 lm, and those estimated by Eqs. (1)–(4), for sections 1–4 and section 5 (larger size milling circuit). The average F80 was 11516 ± 1191 and the Work Index Wi varied from 12.3 to 14.1 kWh/ton for the different data. From Fig. 6 it can be observed that the experimental values using fines recycling in sections 1 and 4 (triangles) are below those predicted for the normal operation without fine recycling. The difference of 2–3% less in S, %+212 lm, represents a potential increase of about 1– 1.5% in the rougher flotation recovery (Yianatos et al., 2000). 4. Conclusions The effect of fines recycling back to the rod mill in a conventional full-scale grinding circuit was evaluated. For this purpose a fraction of 34–40% of the hydrocyclone feed stream was diverted to the rod mill feed. It was observed that operating with fines recycling allows the grinding product size (%+212 lm) to decrease by 2–3% at similar feed capacity (solid tph). This effect represents an increase of about 1.0–1.5% in the overall rougher flotation recovery. A correlation was derived to describe the grinding product size (% +212 lm) in terms of the main operating variables such as Bond Work Index, feed tonnage, power consumption and circulating load in the ball mills. The model was adjusted to describe the operation without fines recycling. Thus, comparison of experimental data from fines recycling and those without recycling show a decrease of 2.5% in % +212 lm, for the same feed capacity (tph). The effective pulp residence time in ball mills was evaluated as 1.8 min in section 1 and 1.6 min in section 4. Thus, the effective volume of slurry inside the ball mills was estimated and was equal to 32.7%.
1115
It is also important to note that using fine particle recycling to the rod mill will require a good control system as the potential for over-grinding exists with the 12 lm fraction increasing leading to decrease the overall recovery.
Acknowledgements The authors are grateful to the El Salvador Division of Codelco-Chile, for permission to present this work. Funding for process modeling and control research is provided by Conicyt, project Fondecyt 1020215, and Santa Maria University, project 270322.
References Gutie´rrez, L., Sepu´lveda, J., 1985. Dimensionamiento y optimizacio´n de plantas concentradoras mediante te´cnicas de modelacio´n matema´tica. CIMM, Santiago, Chile, p. 72. Napier-Munn, T.J., Morrel, S., Morrison, R.D., Kojovic, T., 1999. Mineral comminution circuits. Their operation and optimisation. In: Napier-Munn, T.J. (Ed.) JKMRC Monograph Series in Mining and Mineral Processing, vol. 2, Australia, p. 269. Shi, F.N., Napier-Munn, T.J., 2002. Effects of slurry rheology on industrial grinding performance. International Journal of Mineral Processing 65, 125–140. Yianatos, J.B., Pizarro, E.A., 1997. Molienda de mineral de rechazo en molinos SAG concentradora A2, Division Chuquicamata, Codelco-Chile. Internal Report, Project No 219306-016. Yianatos, J.B., Bergh, L.G., Aguilera, J., 2000. The effect of grinding on mill performance at Divisio´n Salvador, Codelco-Chile. Minerals Engineering 13 (5), 485–495. Yianatos, J.B., Lisboa, M.A., Baeza, D.R., 2002. Grinding capacity enhancement by solid concentration control of hydrocyclone underflow. Minerals Engineering 15 (5), 317–323. Zickar, E.J., Gillett, F.G.M., Shore, J.R., 1981. The addition of fines to improve rod mill performance at Frood-Stobie Mill. In: Proceedings of the Canadian Mineral Processors Conference, Paper No. 4, pp. 52–76.
The effect of fines recycling on industrial grinding performance J. Yianatos a
a,*
, N. Bergh a, R. Bucarey a, J. Rodrı´guez b, F. Dı´az
c
Department of Chemical Engineering, Santa Marı´a University, P.O. Box 110-V, Valparaı´so, Chile b Department of Metallurgical Engineering, Salvador Division, Codelco-Chile, Chile c Department of Radioactive Tracer Applications, Chilean Commission of Nuclear Energy, Chile Received 11 September 2004; accepted 1 March 2005 Available online 14 April 2005
Abstract The effect of fines recycling back to the rod mill in a conventional full-scale grinding circuit was evaluated. For this purpose, a fraction of the hydrocyclone feed stream was diverted to the rod mill feed. It was observed that operating with fines recycling allowed a decrease of 2–3% in the grinding product size, % +212 lm, at similar solid feed capacity, tph, and similar circulating load in the ball mills. This represents a potential increase of about 1.0–1.5% in the overall rougher flotation recovery. The effective pulp residence time in the ball mills was evaluated and was found to be 1.6–1.8 min. Thus, the effective volume of slurry inside the ball mills was estimated and was equal to 32.7%. The product size, % +212 lm, was correlated with the main operating variables such as Bond index, feed tonnage, power consumption and circulating load. The model was adjusted to describe normal operation without fines recycling. Then, the comparison of experimental data with fines recycling and those without it, showed a decrease of 2.5% in % +212 lm, for the same feed capacity tph. 2005 Elsevier Ltd. All rights reserved. Keywords: Grinding; Fine particle processing; Recycling; Mass balancing; Process optimisation
1. Introduction Slurry rheology affects the particle breakage, the mass transport and energy efficiency inside a grinding mill. The rheological effects are often interactive with each other, particularly in closed industrial grinding circuits, and sometimes may be cancelled out by each other, thus creating conflicting conclusions (Shi and Napier-Munn, 2002). In the case of ball mills, the bypass of fine particles, which return to the mill with the circulating load, is an issue of controversy. A significant amount of work has been done in pilot plant studies and field tests to investigate the effect of returning fines back to the mill. The *
Corresponding author. Tel.: +56 32 654 235; fax: +56 32 654 478. E-mail address: [email protected] (J. Yianatos).
0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.03.001
common concern is that fines should not be returned to the mill since these fines will (1) be subject to over grinding and (2) consume space that could be normally occupied by new mill feed. On the other hand, it is well known that the addition of fines in the rod mill feed act as a viscosity modifier, which helps the grinding efficiency (Napier-Munn et al., 1999). The fine solid improves the rheological properties of the pulp, reduces the hydraulic classification, helps the pulp transport and avoids rod mill tangles. Experience from plant operation has shown that the addition of fines, i.e. a small proportion of the hydrocyclone feed, to the rod mill allows the treatment capacity at similar product size and power consumption to increase (Zickar et al., 1981). Also, a similar effect was observed in SAG mills at Chuquicamata A2 concentrator, where a 8–12% increase in fresh mineral treatment
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
capacity was achieved by adding fines from the crushing stage, while the product particle size to flotation was kept approximately constant around 26.5% +212 lm (Yianatos and Pizarro, 1997). In addition, Zickar et al. (1981) discussed the importance of using a proper control system, for the fine recycling to the rod mill, in order to optimize the treatment capacity of the grinding circuit and decrease the potential over-grinding. In the present study, the effect of fines recycling to the rod mill feed was evaluated at the El Salvador concentrator. For this purpose, a small fraction of the hydrocyclone feed stream was recycled to create a fine circulating load back to the rod mill, while the ball mill circuit was operated under normal conditions. The rod mill discharge showed a 54.7 ± 0.5% solids by volume with a particle size of less than 18% 0.0038 mm. Under this condition, the rheological type of the slurry should have a pseudoplastic behavior, and then it can be expected that increasing fines increases grinding (Napier-Munn et al., 1999). In this study, the radioactive tracer technique was used to measure the rod mill recycle, as well as the effective mean residence time in ball mills.
1111
grinding circuit, a 12.5 cm (5 in) pipe was fitted. This pipe returns part of the hydrocyclone feed stream back to the rod mill, stream 2 in Fig. 1. 2.2. Mass flowrate measurement in fines recycling In order to optimize the amount of fine recycling, a measurement of the mass flowrate is required. In a previous work, Zickar et al. (1981) estimated the mass flowrate from the difference between pulp densities measured before and after adding a known amount of water to the recycling stream. In this study, a direct measurement of the slurry flowrate using a radioactive tracer (Br-82) was used. This technique consists of measuring the mean transport time, between external sensors located at a known distance, after injecting a pulse signal of tracer. Fig. 2 shows a diagram of the installation of three radioactive tracer detectors separated at distances of L1 = 6.5 m and L2 = 6.1 m, and located far from the tracer injection point. Fig. 3 shows the tracer time response observed, from which the mean transport time was measured at three testing points in the fine recycling stream. Table 1 shows a summary of the mass flow of recycled fines particles in grinding sections 1 and 4. The
2. Experimental 2.1. Grinding circuit
Br-82
The grinding circuit of the Salvador Division, CodelcoChile, consists of five sections operating in parallel. Sections 1–4 are formed by 1 rod mill (3.05 · 4.27 m) followed by 2 ball mills (3.05 · 4.27 m) in parallel, as it is shown in Fig. 1. Section 5 consists of 1 rod mill (4.11 · 5.49 m) followed by 1 ball mill (5.03 · 5.79 m). All ball mill circuits (1–5) operate in closed circuit. In order to study the effect of fines recycling to the rod mill feed, and how this influences the rest of the
Pulp L1
L2
Sensors
Fig. 2. Diagram of tracer system to measure the slurry velocity.
11000
1
3
4
2 7
11
10
activity [cps]
12
9
5
10000
Detector 1
9000
Detector 2
8000
Detector 3
7000 6000 5000 4000
8 6
3000 2000 1000 0 30
35
40
45
50
55
60
time [s] Fig. 1. Grinding circuit of sections 1–4, Salvador Division, CodelcoChile.
Fig. 3. Tracer time response in fine recycling stream returning to rod mill.
1112
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
Table 1 Fines recycle to the rod mill Recycled fines
Flow speed, m/s
Volumetric flow, m3/h
Solids conc. %
Mass flow, ton/h
Recycle, %
Section 1 Section 4
1.8 ± 0.2 1.5 ± 0.3
81.7 69.2
70.9 71.6
105.5 91.0
39.8 34.1
recycle from the sump box to the rod mill feed, stream number 2 in Fig. 1, was 39.8% and 34.1%, respectively.
The average residence time of the slurry in the sump box was around 30 s considering:
2.3. Residence time distribution measurement in ball mills
(a) the total volume of the sump box was 20 m3 and it was typically filled up to 50%; (b) the volumetric flowrate was 1233 m3/h, with an average solid mass flowrate of 1295 tph and 63% solids.
In order to study the hydrodynamic behavior of the ball mills during fines recycling in sections 1 and 4, the radioactive tracer technique was used to measure the residence time distribution of the liquid inside the mills. Thus, a radioactive tracer Br-82 solution, in water, was injected at the inlet of the ball mills, operating in closed loop. Then, the response time of the radioactive tracer was measured on-line using a non-invasive sensor located in the discharge of the mills sump box after the pump. Fig. 4 shows the closed loop response data in ball mill 2 (section 1). Here, the experimental data does not converge to zero rapidly, because a fraction of the radioactive tracer was recycled to the mill through the water circulating load in the ball mill circuit. The open loop impulse response was then estimated by de-convolution in order to obtain the actual transfer function of the system. From a mass balance adjustment the water circulating load in the ball mill was 38%. Thus, the total tracer feed entering to the ball mill was the initial tracer impulse at time zero plus a fraction (38%) of the actual mill response with a delay of 20 s (0.33 min), which approximately corresponds to the residence time in the hydrocyclone rack and piping. The shape of the response curve obtained for each mill was similar, with a pure delay, of 20–30 s followed by a sharp ascent and an exponential decay.
Thus, an effective mean residence time of 1.8 min was observed in mill 2 (section 1). 2.4. Estimation of the effective volume of slurry inside the ball mills In order to evaluate the hydrodynamic conditions inside the ball mills, an estimate of the effective volume of slurry inside the ball mills was developed. The total mills volume was 31.2 m3. The apparent volume of balls charge in the mills was 40%, with a void fraction of 42%, thus the total volume occupied by balls was 23.2%. In addition, the solid mass flowrate was 462.3 ± 11.3 tph, and the volumetric slurry flowrate was 340 m3/h, with a 73% solids and a solid density of 2.74 ton/m3. Then, for the effective residence time of 1.8 min measured in the ball mill 2, of section 1, the effective volume occupied by slurry was 32.7%. This result gives a total volume of slurry and balls of 56%, which is in agreement with the expected total volume for this type of operation (around 50%) (Gutie´rrez and Sepu´lveda, 1985; Napier-Munn et al., 1999). 2.5. Material balance of grinding circuit
0.009 Open loop
0.008
Closed loop
0.007
Data
E(t)
0.006 0.005 0.004 0.003 0.002 0.001 0 0
100
200
300
400
500
600
700
800
900
Time, s Fig. 4. RTD measured with radioactive tracer in ball mill 2, section 1.
Mass balance adjustments were developed in order to compare the operation with and without fines circulating load to the rod mill. This was made by measuring the rod mill fresh feed (stream 1 in Fig. 1), the fine recycle (stream 2) and the particle size distributions around the hydrocyclone (streams 6, 7 and 8). Then, from a mass balance adjustment per size class, the split of the hydrocyclone feed into the overflow and underflow streams was estimated. It was found that around 34–40% of the hydrocyclone feed was recycled to the rod mill feed. Table 2 shows a summary of the results for the streams 1–10, described in Fig. 1. Streams 11 and 12 have the same values of streams 9 and 10, respectively.
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
1113
Table 2 Solid mass balances in grinding sections 1 and 4 Stream no.
Stream, tph
Grinding section Section 1
1 2 3 4 5 6 7 8 9 10
Rod mill fresh feed Rod mill recycle Rod mill feed Rod mill discharge Sump box discharge Hydrocyclone feed Hydrocyclone overflow Hydrocyclone underflow Ball mill 1 feed Ball mill 2 feed
Section 4
T1
T2
T3
T4
T5
T6
265.2 0 265.2 265.2 1183.4 1183.4 265.2 918.2 467.6 450.6
260.4 0 260.4 260.4 1181.5 1181.5 260.4 921.1 443.3 477.8
265.0 102.7 367.7 367.7 1301.2 1198.5 265.0 933.5 472.3 461.3
265.1 106.2 371.3 371.3 1288.9 1182.7 265.1 917.6 458.0 459.6
265.0 91.0 356.0 356.0 1255.8 1164.8 265.0 899.8 447.8 452.1
269.8 91.3 361.1 361.1 990.0 898.8 269.8 628.9 295.2 333.7
2.6. Size distribution in grinding circuit Fig. 5 shows the particle size distribution around the hydrocyclone (a) working without fine circulating load to the rod mill and (b) with fine circulating load. Here a decrease in the size distribution at the hydrocyclone feed can be seen, as well as a decrease from 261 to 234 lm in the d80 passing size of the hydrocyclone overflow, while changing the operation without fine recycle to the operation with recycle.
3. Effect of fine recycling on particle size The main operating characteristics during sampling can be seen in Table 3. Under the same feed rate for the rod mills in sections 1 and 4 (264.1 ± 2.1 tph) and similar circulating load to the ball mills (347.4 ± 5.3%),
observed in tests T1, T2, T3, T4 and T5, a statistical difference between the operation with fines recycling (tests T3, T4 and T5) and those without recycle (tests T1 and T2) was observed. Thus, the use of recycle to the rod mill feed enabled a finer product size to be obtained, 23.7 ± 0.8% of particles larger than 212 lm, in comparison with the operation without recycle, 28.9 ± 1.9% of particles larger than 212 lm. On the other hand, the comparison between tests T5 and T6, both operating with fines recycle to the rod mill, in section 4, showed an increase from 23.9% to 29.5% of particles larger than 212 lm. This result was mainly due to the decrease in the circulating load to the ball mills, 340% and 233%, respectively. The same effect was observed in a previous work where the decrease (step change) of the water addition to the sump box decreased the circulating load to the ball mills, while increasing the particle size at the hydrocyclone overflow (Yianatos et al., 2002).
100
100
90
90 80
70
Cumulative % passing
Cumulative % passing
80
60 50 40 30
60 50 40 30
overflow
20
overflow
20
feed
10
feed
10
discharge
discharge
0
0 10
(a)
70
100
1000
Passing size, microns
10000
10
(b)
100
1000
Passing size, microns
Fig. 5. Size distribution of the hydrocyclone: (a) without fines recycles and (b) with fines recycle.
10000
1114
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
Table 3 Main operating characteristics Section 1
Section 4
T1
T2
T3
T4
T5
T6
Rod mill feed, tph % +12.7 mm (+1/2 in) % 0.038 mm
265.2 3.27 7.6
260.4 4.38 7.3
265.0 4.40 9.7
265.1 3.99 8.2
265.0 2.92 9.0
269.8 5.38 6.4
Rod mill discharge, % 0.038 mm Fines recycling, tph Recycle in rod mills, % Circulating load in ball mills, % Hydrocyclone overflow properties, d80 lm % +212 lm (% +65 mesh) % 12 lm
18.1 0 0 346 272 27.0 17.0
17.3 0 0 353 304 30.7 17.5
17.6 105.5 39.8 352 237 22.9 18.5
14.3 105.5 39.8 346 250 24.5 17.9
14.4 91.0 34.3 340 245 23.9 19.4
14.7 91.0 33.7 233 294 29.5 17.5
3.1. Estimation of the grinding product size % +212 lm
where
In order to evaluate the impact that the fines recycle to the rod mill feed has on the grinding product, a correlation was developed to describe the circuit behavior in terms of the main operating variables. The correlation was fitted to predict the operation without fines recycle and then the model was used to observe the differences between predictions without fines recycle and the actual operation using fines recycle. According to BondÕs law (Napier-Munn et al., 1999), 1 1 W ¼ 10 W i pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ð1Þ P 80 F 80
S Scorr
and P ¼T W
ð2Þ
% +212 lm S, corrected by the ball mill circulating load effect, % +212 lm circulating load in ball mill, % average circulating load in ball mills, % parameter for model fitting
L Lave a
Table 4 shows a summary of the mill parameters, for grinding sections 1–4 and section 5 (larger size), operating under normal conditions without fines recycle. The ‘‘a’’ parameter was derived by minimizing the sum of the square differences, between experimental data and model predictions (Scorr). Table 4 Mill characteristics and parameters for Eqs. (1)–(4)
where Mill power P, kW Lave, % a
work input, kWh/ton Bond Work Index, kWh/ton product size, lm feed size, lm feed throughput, tph power draw, kW
In addition, the following correlation, between P80 and the product particle size class used for control purposes, S (% +212 lm), was previously developed for the hydrocyclone overflow stream (Yianatos et al., 2000), P 80 ¼ 8:64 S þ 38:92
ð3Þ
Eqs. (1)–(3) allow for the estimation of the particle size class S, % +212 lm, in terms of power consumption, feed tonnage and particle size. Moreover, considering that the performance of the ball mills is influenced by the quantity and quality of the circulating load being returned to the mill, an empirical correction factor was defined to account for the effect of circulating load changes on the normal operating conditions in ball mills. S corr ¼ S ðL=Lave Þ
a
Section 5
2218 344 0.129
3270 344 0.129
35
ð4Þ
Predictive model, % +212 micron
W Wi P80 F80 T P
Sections 1–4
30
25
section 1-4
20
section 5 fine recycling 15 15
20
25
30
Experimental data, %+212 microns Fig. 6. Effect of fines recycling on grinding, sections 1–5.
35
J. Yianatos et al. / Minerals Engineering 18 (2005) 1110–1115
Fig. 6 shows a comparison of the experimental values of S, %+212 lm, and those estimated by Eqs. (1)–(4), for sections 1–4 and section 5 (larger size milling circuit). The average F80 was 11516 ± 1191 and the Work Index Wi varied from 12.3 to 14.1 kWh/ton for the different data. From Fig. 6 it can be observed that the experimental values using fines recycling in sections 1 and 4 (triangles) are below those predicted for the normal operation without fine recycling. The difference of 2–3% less in S, %+212 lm, represents a potential increase of about 1– 1.5% in the rougher flotation recovery (Yianatos et al., 2000). 4. Conclusions The effect of fines recycling back to the rod mill in a conventional full-scale grinding circuit was evaluated. For this purpose a fraction of 34–40% of the hydrocyclone feed stream was diverted to the rod mill feed. It was observed that operating with fines recycling allows the grinding product size (%+212 lm) to decrease by 2–3% at similar feed capacity (solid tph). This effect represents an increase of about 1.0–1.5% in the overall rougher flotation recovery. A correlation was derived to describe the grinding product size (% +212 lm) in terms of the main operating variables such as Bond Work Index, feed tonnage, power consumption and circulating load in the ball mills. The model was adjusted to describe the operation without fines recycling. Thus, comparison of experimental data from fines recycling and those without recycling show a decrease of 2.5% in % +212 lm, for the same feed capacity (tph). The effective pulp residence time in ball mills was evaluated as 1.8 min in section 1 and 1.6 min in section 4. Thus, the effective volume of slurry inside the ball mills was estimated and was equal to 32.7%.
1115
It is also important to note that using fine particle recycling to the rod mill will require a good control system as the potential for over-grinding exists with the 12 lm fraction increasing leading to decrease the overall recovery.
Acknowledgements The authors are grateful to the El Salvador Division of Codelco-Chile, for permission to present this work. Funding for process modeling and control research is provided by Conicyt, project Fondecyt 1020215, and Santa Maria University, project 270322.
References Gutie´rrez, L., Sepu´lveda, J., 1985. Dimensionamiento y optimizacio´n de plantas concentradoras mediante te´cnicas de modelacio´n matema´tica. CIMM, Santiago, Chile, p. 72. Napier-Munn, T.J., Morrel, S., Morrison, R.D., Kojovic, T., 1999. Mineral comminution circuits. Their operation and optimisation. In: Napier-Munn, T.J. (Ed.) JKMRC Monograph Series in Mining and Mineral Processing, vol. 2, Australia, p. 269. Shi, F.N., Napier-Munn, T.J., 2002. Effects of slurry rheology on industrial grinding performance. International Journal of Mineral Processing 65, 125–140. Yianatos, J.B., Pizarro, E.A., 1997. Molienda de mineral de rechazo en molinos SAG concentradora A2, Division Chuquicamata, Codelco-Chile. Internal Report, Project No 219306-016. Yianatos, J.B., Bergh, L.G., Aguilera, J., 2000. The effect of grinding on mill performance at Divisio´n Salvador, Codelco-Chile. Minerals Engineering 13 (5), 485–495. Yianatos, J.B., Lisboa, M.A., Baeza, D.R., 2002. Grinding capacity enhancement by solid concentration control of hydrocyclone underflow. Minerals Engineering 15 (5), 317–323. Zickar, E.J., Gillett, F.G.M., Shore, J.R., 1981. The addition of fines to improve rod mill performance at Frood-Stobie Mill. In: Proceedings of the Canadian Mineral Processors Conference, Paper No. 4, pp. 52–76.