IMPC 2016: XXVIII International Mineral Processing ...

IMPC 2016: XXVIII International Mineral Processing Congress Proceedings - ISBN: 978-1-926872-29-2

THE MODELLING OF SCRUBBERS AND AG MILLS, WHEN TO USE THEM M. J. Daniel Consultant: CITIC-SMCC PT ABSTRACT One of the workhorses of the minerals industry is the autogenous grinding (AG) mill and its derivative the semi-autogenous (SAG) mill. These are relatively well understood, but not so Scrubbers. Scrubbers are mechanically of the same appearance/function, so what is the difference between an AG mill and a scrubber? When, or under what conditions should a scrubber be used and when should an AG mill be used? Can the processes within be mathematically modelled? In the past decades, the minerals industry has focused most of its attention on the modelling of unit processes such as cone crushers, HPGR’s, AG/SAG mills and ball mills within conventional processing circuits. Less attention has been placed on the modelling of scrubbers. This paper reviews the difference between scrubber and AG mill modelling, highlighting some of the similarities, with a view to confident comminution circuit designs. Engineering practitioners should be comfortable with these circuit designs and not shy away from considering AG mills for both small and large scale plants. AG mills can treat both harder or softer ore and scrubbers are for only softer ores. This paper differentiates scrubber and AG mill modelling in a similar way that overflow ball mills and grate discharge ball mills are analysed. Smaller scrubbers (600 kW) are suited to small scale soft ore (100-300 t/h) operations. Larger scrubbers up to 2,500 kW have the capability of very high throughput, in excess of 1,750 t/h with soft fine feed ore. AG mills are scalable up to 28 MW and are capable of treating harder ores at rates exceeding 2,000 to 4,500 t/h. Observations are that the specific energy requirements for the mill grinding duty is wide and varied. This is to a large degree the crux of the matter in sizing, as well as, differentiating, scrubbers from AG mills.

KEYWORDS AG mill modelling; Scrubber modelling; throughput; specific energy

Page 1 of 14 Published by the Canadian Institute of Mining, Metallurgy and Petroleum


INTRODUCTION AG milling is a mature technology that has been used in minerals processing applications for more than 100 years. In the last 50 years, harder metalliferous ores have seen the introduction of steel balls to assist in the grinding/milling action. Tumbling mills used in such a configuration are called semiautogenous mills (SAG). The greater minerals industry is no newcomer to the use of fully autogenous (AG) mills. Perhaps scrubbers are. What is the difference? When should a scrubber be used versus an AG mill? How are they mathematically modelled? AG mills operate with a low water addition relative to scrubbers and often have grate discharge mechanisms. AG mills promote a grinding environment that effectively generates a product with a very high proportion of fine material that can be rejected as a slurry from the mills for further downstream processing. Scrubbers, on the other hand, do not have a grate discharge mechanism, and they rely on high water content (low % solids) for slurred feed to overflow from the mill. This paper differentiates scrubber and AG mill modelling in a similar way to which overflow ball mills, versus grate discharge ball mills are analysed. Smaller scrubbers (600 kW) are suited to small scale soft ore (100–300 t/h) operations. Larger scrubbers up to 2,500 kW have the capability of very high throughput in excess of 1,750 t/h. AG mills are scalable up to 28 MW and are capable of treating harder ore. MODELLING SCRUBBERS AND AG MILLS FOR MINERALS PROCESSING Predicting the throughput capacity of tumbling mills is of utmost importance to the engineer designing a plant and also to the operational personnel who need to maintain certain production targets. New mines are now faced with; higher operating costs, lower grade ore deposits, and lower commodity prices. The future demands on the industry are to make more use of scrubbers and/or AG mills, as economies of scale drive down the capital and operating cost requirements. Engineering practitioners should be comfortable with these circuit designs and should not shy away from considering AG mills for both small, and large, scale plants. AG mills treat both harder or softer ore, and scrubbers for softer ores only. Predicting Throughput This paper differentiates scrubber and AG mill throughput/performance modelling in a similar way to that which standard overflow ball mills and grate discharge AG/SAG/ball mills are analysed. Smaller scrubbers 13×33 ft. (500–700 kW) are suited to small scale soft ore (100-400 t/h) operations. Larger scrubbers up to 2,500 kW have the capability of very high throughput in excess of 1750 t/h. AG mills are scalable up to 28 MW and are capable of treating harder ores at rates exceeding 2,000 to 4,500 t/h. Observations are that the specific energy requirements for the mill grinding duty is wide and varied. This is to a large degree the crux of the matter in sizing and differentiating scrubbers from AG mills. Throughput predictions are determined according to the size of the mill (power draw) and the nature of the ore properties (Specific energy). For scrubbers, AG mills, SAG mills and ball mills; the same modelling concept applies and requires equation 1 to predict throughput. Specific Energy of AG/SAG mills or scrubbers are very much dependent on ore feed size, ore hardness/competency, grate configuration, and pulp lifter design. The size of the mill’s pinion power is determined according to the charge motion and mill operating conditions (discussed next). Pinion power(kW) t … … … .. (1) Throughput = kWh h ) AG/SAG specific energy ( t



MILL POWER DRAW Charge Motion Modelling of the charge motion is based on the approach adopted by Morrell (1996a and 1996b) for predicting the power draw of wet tumbling mills. The model assumes the mill charge to be comprised of concentric shells. The slip between the shells causes a shearing motion and creates the conditions necessary for attrition breakage where relatively small particles are nipped between grinding media as they slide against one another (Figure 1). Impact breakage occurs predominantly in the vicinity of the toe of the charge where the grinding media impact after falling from the shoulder of the charge. Conceptually, any AG mill consists of a rotary cylindrical metallic chamber; into which the feed is continuously introduced through the hollow feed trunnion, at one end, and exits from the discharge trunnion at the other end. The discharge mode design can substantially vary between hollow discharge trunnion and screen plates (grates). These can be placed either at the periphery of the cylinder or radially to the trunnion. The mill shell is internally protected by means of steel liners. The modelling objective is to describe the influence of charge composition and motion on breakage performance, such that mill power draw under normal operating conditions can be predicted. A JKSimMet model was configured to study the breakage mechanisms involved in AG grinding. This model can incorporate a number of operating conditions, since these will influence the power draw and charge dynamics. SAG/AG mills have three types of breakage that impart the available mill energy to all particles in the mill. These are shown schematically in Figure 1 and described as follows: • Impact is the action of a falling charge element, (including balls or rocks), onto the particles lying on the exposed part of the charge. • Abrasion, which includes the rubbing action of rock particles against each other, against balls or mill liners. • Attrition, including the mechanism of particles being nipped between steel balls and larger ore particles rolling/sliding against each other.

Figure 1 – Types of breakage processes between ball-rock and rock-rock in AG/SAG mills AG or Scrubber Grinding Model Data Inputs The terminology, “Autogenous (AG)” or “Scrubber”, denotes generically the action of rocks grinding upon themselves and the interaction with the steel/rubber liners that put the charge in motion within the rotating mill. This complex process can be represented and quantified in mathematical models. These models require the following: • ore breakage properties, • knowledge of the mill specifications, and • the motion of the charge (mill content, consisting of rocks/minerals and water). The input data required for AG/scrubber mill models are shown in Figure 2.



Figure 2 – SAG/AG/scrubber simulation data input requirement (Morrell, 1996) Mill Products To maintain consistent grind, the comminution circuit is required to have; stable power draw, steady grinding media and/or rock load, high unit availability, and throughput. Processing of a competent ore at a coarse target grind size may require the inclusion of a recycle crusher to a single stage AG or SAG circuit to prevent over grinding. This inclusion may not always be required if the target grind is fine. At times, prevention of over grinding of valuable minerals can be difficult to factor into a circuit design. This is often a consequence of the use of gravity classification devices (i.e. cyclones, screw classifiers and etc.), rather than a result of the comminution equipment selected. Heavier minerals are generally classified at a finer cut point than lighter varieties, which can result in over-grinding. Scrubbers on the other hand, are cylindrical “overflow” configured rotating devices that rely on high water flows with a fine soft ore. The resultant very low % solids transports the fine slurry out of the scrubber mills. The specific energy is low; hence scrubbers have high volumetric capacity relative to the grate discharge mills. Scrubbers, as a result, are often specified using ore residence time as a design feature. In this paper, scrubber performance is based on an equivalent “overflow AG mill”, similar to the comparison of the grate discharge AG/SAG mills with the overflow ball mills as depicted in Figure 3. The examples presented in Figure 3 rely on the application of the Morrell C-power model for the prediction of mill power. This in conjunction with the mill/ore specific energy is used to determined AG or scrubber mill throughput (Equation 1).



Evaluating AG Mill Performance in Respect to Power Draw The AG/SAG mill power model accounts for the power required to put the load/charge into motion. This in turn leads to particle/rock breakage which accumulates as slurry. The slurry is removed from the mill via the grates/pulp lifters in grate discharge SAG/AG mills, or, in the case of scrubbers/ball mills via an overflow discharge mechanism (Figures 5 and 6). The dominant model parameters, which influence mill power draw and hence throughput performance are: • Mill size, in particular mill diameter (Figures 7 and 8), • Feed size and ore hardness (Figure 7), • Ball filling - SAG mills (5–15%) and Ball mills (30–40%) (Figure 6), • Ball filling is zero for AG mills and Scrubbers (Figure 6), and • AG/SAG mill discharge arrangement (overflow or grate discharge) (Figure 3).

Figure 3 – Charge shape for grate mills and overflow mills (Morrell 1996a, 1996b) The secondary parameters that influence mill performance are: • AG/SAG mill discharge mechanism (pulp lifter, radial, spiral, twin chamber etc.) (Figure 4), • AG/SAG mill discharge screen aperture (Figure 11), • Total charge or mill filling (Figures 3 and 6), • Increase in discharge end grate open area (Figure 4), • Pebble port size, number and position (Figure 4), • Slurry density within the mill (Figure 5), • Effects of scats crushing, and • Effects of scats crusher closed side setting, return to mill.



Karowe discharge grates with ports at circumference to promote large particle discharge

SAG mill spiral discharge grates, fully ported to promote pebble discharge for dedicated crushing

Figure 4 – Karowe AG mill grates (LHS) and spiral SAG mill grates (RHS) scrubber http://www.aatmining2015.ausimm.com.au/Media/AATMining2015/presentations/D2%20S5-1010.pdf http://www.ausimm.com.au/content/docs/branch/2013/lachlan_2013_05_presentation.pdf

65% solids

55% solids

Figure 5 – % solids in AG milling reduces slimes generation (Danoczi & Herbst, 2011) Evaluating Key Differences Between AG mills and Scrubbers Power draw modelling for AG/SAG mills is well established and benchmarked with industrial units that are in operation. Scrubbers are modelled as; overflow AG mills treating a fine feed, having a low or “soft” ore competency, and a low percent solids. Material transport in the scrubber is via a high flowing slurry medium, whereas in AG mills, the load is coarse, feed coarse and energy intensity high. The grinding action in an AG mill (Figure 5) results in the formation of a slurry which is removed via the pebble ports and grates and pulp lifters (Table 1). Power draw modelling is only part of Equation 1. The denominator, the AG/SAG specific energy is determined separately and is dependent on a number of process and equipment variables and ore properties. Detailed modelling of the charge motion, grate design, and lifter design, can be achieved through advanced DEM modelling using Rocky 3 software. A few examples of this are shown in Figure 6.



Figure 6 – Examples of Rocky DEM modelling capability for grate and liner design. Table 1 – Comparative features between Scrubbers and AG mills. Scrubber AG Mills examples Example provided Outotec Figure 8 Operating conditions and ore properties Mechprotech Figure 7 Citic Hic Figure 14

Mill size

low aspect ratio, mill diam up to 20 ft

Mills up to 40×33 ft

Mill speed

55–65%

65–80%

Mill filling

20–30%

30–40%

Discharge trunnion to mill diameter

> 0.25 (Figure 7)

3 g/cm3. Figures 9 and 10 overlay the “zones” in which AG/Scrubbers should be employed in relation to ore competency. The higher the DWi value necessitated AG milling plus pebble crushing, whereas low



DWi indicate zones where scrubbers would be better used. Ore variability can result in the mill either over performing or underperforming relative to a static set target throughput. This may have an impact on; financial and economic models, and revenue forecasts. Careful forecast modelling enables a project in the design phase to defer some of the capital expenditure, by developing the flowsheet in a phased approach. A common theme in mining operations is; as the resource is exploited, the deeper primary ores are often more competent and difficult to treat than the surface oxide/weathered ores. This contributes to s mills apparent difficulty in treating ore at a budgeted specified throughput rate. Once developed, the “site/unit” models (Table 2) should be revised on an ongoing basis, especially if new processes such as pebble crushers are introduced to the circuit. Also, the effect of blasting and overall transfer size could change, resulting in altered plant performance. Table 2 – Comparative throughput capacity for various Scrubbers and AG mills w.r.t. ore properties.

Unit

Size (ft)

Installed power (kW)

Scrubber

13x33

600

Pinion power (kW)

Dwi (kWh/m3)

Ore density (g/cm3)

Axb

AG SE (kWh/t)

Estimated t/h

504

1.6

2.5

156

1.7

296

Scrubber

13x33

600

504

2.1

2.5

119

2.0

252

Scrubber

20x37

2,500

2,100

0.7

2.9

414

1.2

1,750

Scrubber

20x37

2,500

2,100

2.5

2.9

116

2.8

750

AG mill

28x13

4,000

3,360

3.3

2.7

82

4.9

686

AG mill

28x13

4,000

3,360

6.2

2.7

44

7.5

448

AG mill

28x13

4,000

3,360

9.7

2.7

28

11.2

300

AG mill

40x33

28,000

23,520

4.8

3.3

69

5.2

4,523

AG mill

40x33

28,000

23,520

9.0

3.3

37

11.5

2,045

The site specific models may be further developed on the basis of ore variability over time, which can be used to predict monthly, quarterly and annual throughput, on the basis of measured ore competency and grindability tests. Design case scenarios and associated operating plant databases, cannot always rely on data as presented in Figures 9 and 10. The site specific models are presented as a comparison to the Dance database where known variations and static operating conditions, result in varied AG/Scrubber specific energy (Figure 12). For example, the 28 MW Sino Iron AG mills are illustrated as open circuit AG mills, where in practice (Figure 13), the Sino AG mills are operated as SS AG mills with a ~100 um transfer size to the magnetic separation processes.



Figure 12 – AG/Scrubber specific energy versus DWi, data from Table 2 overlay Figure 13

http://kerman.com.au/wp-content/uploads/2013/08/Sino-Iron-Lines-3-6-SMPElectrical-Works1640x426.jpg Figure 13 – Six Sino Iron SS AG mills (~1200 t/h), Citic-HIC, 40x33 ft (EGL), 28 MW each CONCLUSIONS Scrubber and AG mills are modelled in a similar way as to how overflow ball mills and grate discharge ball mills are analysed. The “database” examples presented in previous publications of Bueno (2013) and Dance et al. (2011) as shown in Figures 9 and 10 are useful and should be used as a guide only.



Small scrubbers (600 kW) are suited to small scale soft ore operations with a relatively high throughput (100-300 t/h). Larger scrubbers up to 2,500 kW have the capability of very high throughput in excess of 1,750 t/h. Larger AG mills such as the case of the 4 MW Karowe mill, treat 300–450 t/h. Much larger AG mills, that are scalable and proven up to 28 MW in the iron and copper processing, are capable of treating harder ore at rates in a range of 2,000 to 4,500 t/h. AG/Scrubber circuits differ depending on the parameters incorporated into Equation 2. These include design variables such as; aspect ratio, trommel screen aperture, mill speed, ball charge etc. Other process variables such as; blasting, primary and secondary crushing, feed size, and ore competency will influence the modelled outcomes as calculated in Equation 3. Overall the AG mill works: it is the understanding of their performance and the expectations that perhaps is the biggest risk to a project. REFERENCES Bailey, C., Lane, G., Morrell, S., & Staples, P. (2009). What can go wrong is comminution circuit design? Proceedings of the 10th Mill Operators Conference, Adelaide, SA. 12–14 October 2009, 143– 149. Bueno, M., Lane, G., & Foggiatto, B. (2013). Power based comminution calculations using Ausgrind. Proceeding of Procemin Conference, Santiago, Chile, 17 October 2013. Dance, A., Bisiaux, B., & Amonoo, G. (2011). Improvements in SAG mill throughput from finer feed size at the Newmont Ahafo Operation. Presented at SAG 2011, Vancouver, BC. Morrell, S. (1996a). Power draw of wet tumbling mills and its relationship to charge dynamics - Part 1: A continuum approach to mathematical modelling of mill power draw. Trans Inst Min Metal, Section C, 105, C43–53. Morrell, S. (1996b). Power draw of wet tumbling mills and its relationship to charge dynamics - Part 2: An empirical approach to modelling of mill power draw. Trans Inst Min Metal, C, 105, C54–62. Morrell, S. (2004a). An alternative energy- size relationship to that proposed by Bond for the design and optimisation of grinding circuits. International Journal of Mineral Processing, 74, 133–141. Morrell, S. (2004b). Predicting the specific energy of autogenous and semi-autogenous mills from small diameter drill core samples. Minerals Engineering, 17(3), 447–451. Morrell, S. (2006). Design of AG/SAG mill circuits using the SMC test. SAG 2006, Vancouver, BC. Morrell S, (2011). Mapping orebody hardness variability for AG/SAG/crushing and HPGR circuit. International Autogenous Grinding, Semi autogenous Grinding and High Pressure Roll Technology 2011, Paper #154, (Editors Major, K., Flintoff B C, Klein B, McLeod K). Morrell, S., & Valery, W. (2001). Influence of feed size on AG/SAG mill performance. SAG 2001, Vancouver, BC. 203–214 Underwood, G., Coetzaa, V., & Oosthuysen, H. (2015). Karowe comminution circuit upgrade project. Proceedings of the 2015 Africa Australia Technical Conference (11–12 June 2015). http://www.aatmining2015.ausimm.com.au/Media/AATMining2015/presentations/D2S51010.pdf
