T.J. Napier-Munn

Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell. Part 5: validation of k-Sb relationship and effect of froth depth

Abstract A previous investigation carried out by the authors at the Hellyer concentrator, using a 3 m3 cell fitted with four different impellers treating plant zinc cleaner feed ore, suggested a linear correlation between flotation rate constant k and bubble surface area flux Sb. The relationship between k and Sb was found to be independent of impeller type. This paper describes an investigation at the Scuddles concentrator in Western Australia to validate the findings at Hellyer for a different ore. Unlike the Hellyer work for which only one froth depth was used, the Scuddles work was carried out at different froth depths using the same 3 m3 cell fitted in turn with three different impellers viz. Batequip, Dorr-Oliver and Outokumpu. The results confirmed the strong correlation between k and Sb at three different froth depths used for the study. Moreover, this relationship was found to be practically independent of impeller type. However, at shallow froth depth the kSb relationship was found to be linear, whereas at intermediate and deep froth depths the relationship was found to be non-linear with the froth playing an important role in the overall kinetics.

Detecting performance improvements in trials with time-varying mineral processes — Three case studies

Abstract Mineral processing engineers often conduct trials to improve the performance of their plants. A common problem in such trials is detecting real but relatively small improvements or changes in process performance against a background of very noisy data. This large data variance is frequently caused by (among other things) long-term time trends in performance, which can be a result of systematic changes in feed conditions. This paper describes two well-known statistical procedures for dealing with such situations, the paired t-test and the randomised block experiment. These methods are illustrated through their application to three real case studies in base metal flotation plants, all involving “yes-no” decisions, and all using metal recovery as the main performance criterion: 1. 1. The evaluation of a new flotation collector in a production plant. 2. 2. The assessment of two alternative flotation circuit configurations in a pilot plant. 3. 3. Determination of the value of introducing a regrind stage ahead of a flotation circuit in a production plant. The paper considers the practical problems encountered in these experiments, discusses the compromises sometimes required in analysing imperfect experiments, and shows how the statistical procedures can be used to make good decisions in the face of uncertainty. The formulae and computational procedures are given in full in full to encourage their application to similar situations in the practice of mineral processing.

Optimal experiments for time dependent mineral processes

Time dependence is an important characteristic of mineral processing plant data. This paper finds that the time dependence in the recovery data for an experiment at Bougainville Copper Limited (BCL) (Napier-Munn, 1995) can be described by an autoregressive-one process. The paper investigates the optimum form of experimental design for such data. Two intuitive approaches for the design of experiments involving time-dependent data have been disproved recently. Cheng & Steinberg (1991) showed that in some circumstances systematic experiments are preferable to replicated randomized block designs, and Saunders & Eccleston (1992) showed that rather than sampling at a frequency which ensures independent data, in some circumstances sampling intervals should be as small as possible. A third issue, raised in this paper, concerns the use of standard statistical tests when the data are serially correlated. It is shown that the simple paired t-test, suitably modified for time dependence, is appropriate and easily adapted to allow for a covariate and a sequential analysis. The results are illustrated using the BCL data and are already being used to design major experiments involving another mineral process.

The Value of Incremental Performance Improvement in Concentrators—How to Secure and Quantify Small Gains

There are many scales of innovation in the pursuit of concentrator performance improvement, including paradigm change, inventive change and incremental change. All are important, but the last is low risk, low cost and has a relatively high probability of success, given the right metallurgical skills. Seeking continuous improvement is also essential to ensure that performance does not decline over time due to ore type changes, personnel changes, equipment wear and other factors. When properly implemented, these small gains offer a very high rate of return for the project cost because they are made from marginal, not total, operating costs of the plant. To be effective this approach requires plant trials that use designed structure and appropriate statistics to test the observed differences arising from the old and new process treatments, be they a change in flotation reagent, in grind, pulp density, etc. However the pursuit of small but financially significant performance gains is often regarded as fruitless because of the range of variance in plant data, and the consequent difficulty of ‘proving’ that a benefit has been achieved. Opportunity for improvement is thus sometimes relegated to the realm of the impossible, or at best, treated with skepticism. Operations managers prefer to look for the larger gains, which are easier to demonstrate and prove. Unfortunately, these are seldom to be found; rather, a large gain can be more easily made by executing a series of small recovery improvements that are together equivalent to the ephemeral single large gain. In this paper we advocate a policy of continuing improvement driven by rigorous performance testing protocols and a formal ‘risk analysis’ approach to judging the efficacy of changes implemented in the plant. Following these simple procedures allows rational decisions to be made on the basis of quantifying risk and reward. In particular we consider the choice of hurdle rate for decision-making which best balances risk and reward. It is shown through case studies from actual industrial practice that this approach is practical and delivers significant financial value to the operation.