J. Franzidis

Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell. Part 4: Effect of bubble surface area flux on flotation performance

The metallurgical performance of a 2.8m 3 portable industrial scale flotation cell was measured when treating zinc cleaner feed at Hellyer concentrator in Tasmania, Australia. The cell was fitted in turn with four different impeller-stator systems and operated over a wide range of air flow rates and impeller speeds. Bubble size, gas holdup and superficial gas velocity were measured at each of 64 different operating conditions along with the metallurgical performance of the cell. When metallurgical performance was expressed in terms of a kinetic constant, it was found that neither bubble size nor gas holdup nor superficial gas velocity could be related to flotation rate individually; but when taken together, they determine the bubble surface area flux in the cell, which could be related to flotation rate extremely well. A linear relationship between flotation rate and bubble surface area flux was found for all four impellers investigated: the slope of the line was independent of the type of impeller used. The linear relationship was verified for different size fractions of the ore: the slope of the straight line was different for different size fractions, values being greater for the smaller size particles. The relationship was also independently confirmed at another zinc cleaner operation. This finding has potential practical application in flotation plant modelling, design and optimisation.

An empirical model for entrainment in industrial flotation plants

Abstract The entrainment and drainage of particles in a flotation froth have long been recognised as important factors affecting both concentrate grade and recovery. In this paper, an empirical partition curve is proposed that describes the degree of entrainment within a conventional flotation cell. Two empirical parameters are defined: • • entrainment parameter (ζ)—the particle size for which the degree of entrainment is 20%. • • drainage parameter (δ)—related to the preferential drainage of coarse particles. The model is flexible enough to fit a broad data set published in the flotation literature. Results from a case study indicate that the model is sufficiently detailed to describe the entrainment and drainage mechanisms. This being so, the effect of process variables which affect the degree of entrainment can be properly quantified and then included in a simulation package, assisting in the optimisation of existing flotation circuits and the design of future ones.

Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell — Part 1: Effect on bubble size distribution

Abstract Bubble size distributions were measured at different locations in a 2.8 m3 portable industrial scale sub-aeration flotation cell, treating zinc cleaner feed in the Hellyer Concentrator in Tasmania, Australia. The cell was fitted in turn with four different impeller-stator systems, and operated over a range of air flow rates and impeller speeds. The mean bubble size was found to increase with increase in air flow rate at different locations in the cell, for all four impellers, and to decrease with increase in impeller speed. The mean bubble size was largest close to the impeller shaft and smallest at the impeller discharge point, for all the impellers. The shape of the bubble size distribution also changed with location in the cell. The “global mean” bubble size calculated by simple arithmetic average of the values at six locations in the cell coincided remarkably well with the mean bubble size measured halfway between the impeller shaft and the side of the cell, at the top of the pulp. In general, the impellers produced “global mean” values of 1.0 mm or less at the manufacturers recommended impeller speed.

Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell. Part 3: Effect on superficial gas velocity

Abstract Superficial gas velocity was measured at different locations in a 2.8 m 3 portable industrial scale sub-aeration flotation cell, treating zinc cleaner feed at Hellyer Concentrator in Tasmania, Australia. The cell was fitted in turn with four different impeller-stator systems, and operated over a range of air flow rates and impeller speeds. Superficial gas velocity values ranged from 0.29 cm/sec to 6.4 cm/sec at the air flow rates employed. The distribution of air in the cell was very dependent on the impeller type, as well as on the operating conditions in the cell. For a well dispersed condition, the value of superficial gas velocity was uniform at different locations, whereas for flooding conditions the superficial gas velocity was very high near to the impeller and much lower away from the impeller.

Review of froth modelling in steady state flotation systems

Abstract Froth models proposed in the literature are reviewed with the aim of identifying their significance and usefulness in the modelling and scale-up of the froth phase in steady state flotation systems. Literature indicates that froth phase performance is better understood in terms of froth recovery, the fraction of material presented at the pulp-froth interface that reports to the concentrate. This review suggests that froth recovery is a strong function of drainage rate of particles from the froth phase to the slurry phase. Drainage rate, in turn, is determined by physical factors, such as froth removal technique, geometry of the flotation cell, flux and distribution of air at the pulp-froth interface, the water content, particle size and solids content, and chemical factors, such as froth stability and froth loading. These factors influence the froth residence time, which has been identified as a key froth parameter. Finally, it is proposed that future work should focus largely on the development of a methodology to investigate froth performance based on the froth recovery in different flotation systems. This will enable generic relationships between the froth recovery and froth sub-processes and key froth parameters to be established, and make it possible to relate froth performance in different flotation systems.

Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell part 2: Effect on gas holdup

Abstract Gas holdup was measured at different locations in a 2.8 m 3 portable industrial scale subaeration flotation cell, treating zinc cleaner feed at Hellyer Concentrator in Tasmania, Australia. The cell was fitted in turn with four different impeller-stator systems, and operated over a range of air flow rates and impeller speeds. The gas holdup was found to increase with increase in impeller speed as well as with increase in air flow rate, the manner in which it increased depended on the impeller type. Values ranged from 2% to 33%, with the greatest values produced by the Outokumpu impeller.

Reverse flotation of coal—A novel way for the beneficiation of coal fines

Abstract South African coals have a much higher ash content than European coals. Consequently, ash entrainment is often a problem in the flotation of South African coal, since it adversely affects the concentrate grade. It has thus been very difficult to produce a low ash metallurgical export coal by this process. This paper presents a novel method aimed at reducing entrainment, viz. reverse flotation. The method involves the simultaneous flotation of the ash and the depression of the coal using cationic amine surfactants. Initial flotation testwork on liberated coal/ash mixtures has shown the excellent ash collecting ability of quaternary amines. For example, a concentrate ash recovery of 92% was achieved from a feed “coal” with an ash content of 54%. The tailings (ie product) ash was 12%. Coal depression was less successful; in the same experiment coal recovery into the concentrates was 27%. Improving the coal depression by altering the pH is being investigated.

Development of the reverse coal flotation process : application to column cells

Abstract This paper describes the continuation of the development of the reverse coal flotation process. Reverse and forward flotation are compared in a laboratory column cell using synthetic feed mixtures consisting of washed coal and quartz. Forward flotation gave slightly better separation performance than reverse flotation; however, the process was limited by the throughput capacity, which is determined by the column carrying capacity in the froth zone. The throughput capacity of reverse flotation was found to be 3 times that of forward flotation, with only a small loss in separation efficiency.

Development of the reverse coal flotation process: depression of coal in the concentrates

Abstract Reverse flotation for the beneficiation of coal fines is being developed as an alternative to conventional coal flotation in an attempt to reduce ash entrainment. Preliminary work identified quaternary amines as good ash collectors. This paper details the experimental work performed to attempt to improve the depression of coal in the concentrates using the quaternary amines. Adsorption of the amines onto low-ash, washed coal was investigated. It was found that the amines adsorbed readily onto the coal surface and it was concluded from pH studies that the adsorption was partly a hydrophobic mechanism which indicated that the amines should be able to function as coal depressants in addition to their role as ash collectors. This was confirmed by separate flotation experiments on coal and quartz samples which indicated that entrainment, rather than poor depression was responsible for coal loss to the concentrates. In the light of this, a staged-addition process was employed and was found to significantly reduce entrainment. For example, a product of 86% coal recovery containing 12% ash was obtained using a 3-stage addition process compared to a product of 79% coal recovery at 14% ash using a normal one stage process. The feed for both these runs was a pure quartz/washed coal mixture containing 54% ash.

An on-site evaluation of different flotation technologies for fine coal beneficiation

Abstract This paper presents the results of an investigation of the performance of a number of different flotation cell technologies for the beneficiation of fine coal. The work was conducted on-site at the Grootegeluk Colliery in the northern Transvaal province of South Africa, using a pilot-scale conventional column cell, a pilot-scale Jameson-type cell, and an air-sparged hydrocyclone (ASH). In addition, characterisation and conventional batch flotation tests were conducted in the laboratory in the Department of Chemical Engineering at the University of Cape Town. All three units tested on-site demonstrated improved selectivity compared to conventional subaeration flotation. In the column cell, optimum performance could only be achieved at very low throughputs. Substantial losses of coal occured in the coarser size fractions. The Jameson-type cell was able to operate effectively at about double the throughput of the column cell at similar recoveries. Coal recovery in the coarser size fractions was still poor, but better than that of the column cell. The ASH was characterised by a very high throughput, more than 150 times that of the column cell on the basis of solids capacity per unit cross-sectional area. However, the ASH required more than three times the reagent dosage of the other two units to achieve this. The ASH performed particularly well in the recovery of the coarser size fractions, but was less effective than the other cells on the finer size fractions. Overall, the best performance for this application was that of the Jameson cell, owing to its higher capacity in comparison to the column cell. The high reagent requirement of the ASH makes this technology uneconomic in this application.