The calculation of the liberation spectrum in the product from a continuously operating ball mill is both important and difficult The liberation spectrum depends on the mineralogical texture of the ore from which the mill feed derives and also on the processing steps through which portions of the feed have passed prior to milling. These steps typically include classification by hydrocyclone or separation operations, such as flotation and gravity if the mill is regrinding tails or concentrate. The calculation procedure that is based on the population balance procedure is understood in principle, but the indiscriminate use of this method requires hundreds of parameters that must be estimateda hopeless task under most conditions.

The initial experimental work was aimed at characterizing the flow pattern and mass transport characteristics of the experimental mill. The residence-time distribution was established by an impulse test using a liquid-phase tracer. The measured response of the tracer indicated that the mill behaved as three mixed regions in series. The particle size distributions in the mill holdup and the mill product at steady state allowed the estimation of the selection, breakage and overflow classification processes that describe the comminution process.

A Denver 16 x 16 overflow ball mill was used. The feed was fed dry through a vibrating feeder. Water was supplied to the mill by a peristaltic pump which maintained a constant flow. The mill was charged with a mix of steel grinding balls having a size distribution given in Table 1.

The measured concentration response was found to fit the response to three perfectly mixed regions in series with a slight time delay resulting from the distance-velocity lag in the feed pipe from the injection point to the mill entrance.

The size distribution of the product and the mill holdup were simultaneously fitted and the following parameters minimized the sum of squared deviations from both measured distributions simultaneously.

An overfilling indicator for wet overflow discharge ball mills.Mathematical descriptions of volume-based slurry residence time in ball mills.121 sets of plant survey data used to establish the slurry residence time patterns.The residence time thresholds as the overfilling indicator defined from the database.

The lack of constraints in ball mill capacity in the published ball mill models may result in unrealistic predictions of mill throughput. This paper presents an overfilling indicator for wet overflow discharge ball mills. The overfilling indicator is based on the slurry residence time in a given mill and given operational conditions. Mathematical descriptions of the method to estimate the volume-based residence time of slurry are presented. A database consisting of 121 sets of industrial overflow ball mill surveys worldwide was used to establish the pattern of the slurry residence time in the full scale operational overflow ball mills. According to the pattern, the residence time thresholds beyond which overfilling a ball mill is likely to occur were defined. For a ball mill with an internal diameter smaller than 5.9m, the volume-based residence time threshold is set at 2min; and for a ball mill larger than 5.9m in diameter, the threshold is set at 1min. In addition to being incorporated in ball mill models to warn of any unrealistic simulations, the overfilling indicator can also be utilised at ball mill operation sites to guide the mill throughput control and optimisation.

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