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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. Rodrıguez b, F. Dıaz

c

´ ´ Department of Chemical Engineering, Santa Marıa University, P.O. Box 110-V, Valparaıso, Chile b Department of MetallurgicalEngineering, 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 hydrocyclonefeed 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 wasevaluated 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 ofexperimental data with fines recycling and those without it, showed a decrease of 2.5% in % +212 lm, for the same feed capacity tph. Ó 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. Therheological 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 inpilot plant studies and field tests to investigate the effect of returning fines back to the mill. The
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Corresponding author. Tel.: +56 32 654 235; fax: +56 32 654 478. E-mail address: juan.yianatos@usm.cl (J. Yianatos).

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 bynew 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 theaddition 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

0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.mineng.2005.03.001

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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...