Planeacion Empresarial

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I-204

INTRODUCTION

INFLUENCE OF FEED SIZE ON AG/SAG MILL PERFORMANCE
S.Morrell1 & W.Valery 1 1.JKMRC, Isles Road, Indooroopilly, Qld 4068, Australia e-mail : s.morrell@mailbox.uq.edu.au ABSTRACT

The introduction and subsequent ascendancy of autogenous and semiautogenous milling for comminution circuits has undoubtedly lead to many economic advantages. From a process viewpoint,however, at least one downside has been the sensitivity of these machines to process input variations. Foremost among these is ore competence, though feed size comes a close second. This sensitivity is due to the reliance of ag/sag mills on the feed ore for their grinding media competence and size. Ag mills are the most sensitive in this respect with sag mills being increasingly less so as theball charge is increased. This contrasts with ball mills whose grinding media are generally constant with respect to competence and size distribution and whose size reduction performance is relatively independent of feed size distribution. Although often considered to be a curse by some, the performance dependence of ag/sag mills on feed size can be turned to advantage through its manipulation. To doso, however, it is important to understand the relationship between feed size and performance. This paper illustrates some of the observed relationships between feed size and ag/sag mill response and provides examples of cases where feed size has been successfully manipulated.

MEASUREMENT OF FEED SIZE Fluctuations in feed size distribution are second only to ore competence variation in theirinfluence on autogenous and semi-autogenous mill performance. For some operations where feed size is not well controlled this creates significant problems with mill stability and is understandably seen as a major problem area. Some operations, however, have recognized an opportunity in the relationship between feed size and mill performance and manipulate feed size to obtain efficiencyimprovements. Examples include changes to blasting practices, ROM stockpiling, partial or fully secondary crushing and selective prescreening of sag mill feed. This paper presents and analyses a variety of plant data which illustrates the influence of feed size distribution variation on mill performance and discusses the reasons why ag and sag mills respond the way they do. From this analysis recommendationsfor optimum feed size distributions are made.

Successful manipulation of feed size starts with successful measurement. Over a relatively short time period taking a belt cut and sieving the material is the most accurate method of determining the size distribution of the ag/sag mill feed. However, fluctuations in feed size make such sizings valuable only if they can be done very frequently, whichis clearly impracticable in a production environment. To illustrate how much fluctuation may occur Table 1 page I-205 shows belt-cut data from a number of sites where repeat sizing were carried out over a relatively short period of time, during which it was expected that feed ore characteristics should not change. It can be seen that the repeat surveys all return standard deviations of 15-17mm inthe P80. The 95% confidence interval is therefore of the order of +/- 30mm – a sizeable variation.

I-205 Table 1: Variation in Measured Feed size Plant 1 2 3 No.Surveys Mean P80 (mm) 9 113 5 92 12 57 S.D (mm) 15 17 17

I-206 performance are being looked for. Such an example is shown in Figure 2, which contains a large volume of on-line data and shows the correlation between the F80 (asestimated using image analysis) and mill throughput and specific power. There is clearly a great deal of scatter in the data which is caused by variations in operating conditions, variations in ore competence and the imprecision in representing an entire size distribution by a single (F80) parameter. This latter effect is illustrated in Figure 3 page I-207, which shows two distributions, which have...