It is known that in a tumbling mill, a material can be ground faster in water than in air. The influence of liquid physical properties such as density, surface tension and viscosity on the rate of grinding has been the subject of numerous investigations. In another approach, some investigators chose to study the effect of bulk slurry rheology on the efficiency of grinding. In all cases, the purpose was to elucidate the role and effectiveness of liquid medium in wet grinding processes. Apart from the overall increase in grinding efficiency over the dry systems, another unique characteristic associated with wet grinding systems is the increase in the specific breakage rate of a mono-sized feed (S value of the well-known batch grinding kinetic equation) as fines accumulate during batch and continuous ball milling . This type of grinding behavior was also noticeable even in a continuous industrial- scale ball mill, where size reduction and material transport through the mill occurred concurrently. However, there has been no effort to study this inherent behavior of wet grinding in a more systematic manner. This paper presents further results on the quantitative analysis of acceleration of breakage rates observed in a batch ball milling system as well as the possible mechanisms involved.
Grinding results and acceleration factor
the disappearance kinetics plots for dry grinding single size fractions of quartz and copper ore. It is clear that breakage of these materials follows the first-order law, that is, the accumulation of fines in the charge has no effect on the specific rate of breakage of the top size material (constant S value).
the corresponding first-order plots for wet grinding 20*30 mesh quartz over a range of slurry concentrations. It is noted that breakage of this top size fraction deviates from the normal first-order hypothesis previously observed with dry grinding, with acceleration in the specific breakage rate as the size environment becomes finer. With progressively higher solid concentrations the degree of rate acceleration tends to diminish, leading eventually to deceleration in the specific breakage rate at a very high slurry concentration.
Laboratory batch ball milling of 20*30 mesh quartz feed in water for a slurry concentration range of 20% to 56% solid by volume exhibited an acceleration of specific breakage rate of this size as fines accumulated in the mill. A quantitative measure of this acceleration effect was expressed in terms of the acceleration factor, defined as the ratio of the instantaneous and initial specific breakage rates. For a given slurry concentration, the acceleration factor was found to increase with fineness of grinding but the degree of increase became less as the slurry concentration was progressively increased. In addition, the variation of j with slurry concentration and viscosity followed a pattern that matched the various slurry flow regimes visually observed from the movement of mill charge. A general correlation was developed to enable the estimation of the acceleration factor from the product size distributions and slurry concentration. A qualitative model was proposed and experimental evidence given to support that the mechanism of acceleration in breakage rates of the top size materials involved the ability of larger particles to cling to the ball surfaces by virtue of liquid surface tension. Effect of liquid medium viscosity on the acceleration factor (20*30 mesh quartz; J = 0.3; U= 1.0; /C = 0.70; 40 vol.% solid). 74 C. Tangsathitkulchai / Powder Technology 124 (2002) 67–75 and consistency of the slurry pulp. The adherence of these larger particles can act to shield the smaller particles from receiving impact by the colliding balls. This shielding effect was further promoted by the degree of flow turbulence in the charge that caused some smaller size particles to be removed from the grinding zone.
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