- Title
- Studies of froth recovery in flotation
- Creator
- Rahman, Reza Mainur
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2014
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Flotation is a widely used separation process in the mineral industry. The successful operation of a flotation plant depends on a number of factors such as the suspension of mineral particles in the pulp, collector efficiency, bubble particle attachment and froth stability. In addition, the role of the froth zone is crucial in the flotation process as it plays a significant role in controlling the final product grade and recovery. The importance of the froth zone in both laboratory and plant scale flotation cells has been recognised by researchers for many years (Cutting et al., 1986; Feteris et al., 1987; Gorain et al., 1998b; Lynch et al., 1981). Savassi et al. (1997), using an indirect mass balancing technique on a plant scale flotation cell, estimated that the recovery of particles in the froth zone of a rougher cell can vary from 70% to 10% depending on the position of the cell in a rougher bank. As a result 50% of overall plant flotation cell efficiency can be lost due to poor froth performance.The froth recovery, the mass flowrate of particles reporting to the flotation product as a fraction of the mass flowrate that enters the froth layer, is a single parameter that can globally express the overall froth performance. If known, it could be a quantity of great utility in flotation plant design and the simulation of flotation circuits. However, the complexity of the froth zone, and interactions in the flow of solids and liquid between the pulp and froth zones, make it difficult to perform independent straightforward measurements of the froth recovery. At present, a number of indirect methods are available in the minerals industry to measure froth recovery (Alexander et al., 2003; Savassi et al., 1997; Vera et al., 1999). These methods are based on certain assumptions, which are not completely justified. As a result, it is quite important to develop a direct method to perform independent froth recovery measurements and investigation of froth behaviour. The overall objective of this research work is to study the froth zone and measure the froth recovery of a flotation cell A froth dropback method is used to directly measure froth recovery and experiments are performed with laboratory, pilot-scale and plantscale flotation cells. The effects of both physical and operating conditions (e.g. particle size, fine to coarse particles ratio in feed, air flowrate, froth depth, collector concentration and frother concentration) on froth recovery are investigated. In order to carry out the aforementioned works, a unique device has been developed to measure the dropback of particles from the froth zone. The aim is to decouple the froth zone from the pulp zone, to allow independent measurement of the froth recovery. The device consists of two concentric cylindrical tubes. The inner tube is denoted the dropback collection chamber or catcher. During the experiments, particleladen bubbles are allowed to pass through the annular gap between the two concentric tubes and a stable froth is formed at the top of the froth dropback (FDB) device. The collection chamber is mounted directly beneath the froth layer. Particles that fall off the froth are collected in the dropback chamber, and concentrate passes over the lip of the cell and is collected from a product launder. The mass flowrate into the device is estimated as the sum of the concentrate and dropback flowrates, and the froth recovery can then be calculated. In the first part of this thesis, the FDB device was used in a controlled laboratory environment to investigate the froth behaviour. Flotation experiments were performed using pure silica mineral. A mixture of finer 60G (d₈₀ = 72 μm) and coarser 50N (d₈₀ = 299 μm) silica particles with a wide particle size distribution was used as feed. The effect of important flotation parameters such as the ratio of fine-to-coarse particles in the feed, the collector concentration, the air flowrate, and froth depth on froth recovery were investigated. It was found that froth recovery can be as low as 60% in the laboratory flotation environment. It was also found that the froth recovery is a strong function of particle size. Finer particles have high froth recoveries whereas the coarse particles have low froth recoveries, and are more prone to detach and report in the dropback stream. One of the important findings of the investigation is that the presence of fine particles can have a very significant effect on the collection of coarse particles in the flotation cell. The investigation also found that an increase in collector concentration significantly increases the collection of coarse particles in the collection (pulp) zone. However, higher collector concentrations can lead to destabilisation of the froth zone, resulting in a significant loss of the coarse particles from the froth zone. Air flowrate was found to have a positive impact on both collection and froth zone recovery. It was found that the froth zone works as a barrier to the collection of very coarse particles. However, at shallow froth depths, a significant collection of coarse particles in the product is found, but the froth recovery drops quickly as froth height is increased. In the second part of the thesis, an investigation into froth zone recovery in a controlled plant environment is presented. The laboratory apparatus was taken to the Northparkes copper mine, near Parkes, NSW, and a feed of copper-enriched minerals was taken from the head of the cleaner scavenger bank in an operating circuit. The feed slurry had a relatively higher copper grade varying from 5.2% to 6.8%. During this investigation, the size and grade of the particles in the flotation samples were analysed to acquire in-depth knowledge about the froth dropback mechanism. It was found that the froth recovery could be as low as 70% but it is relatively easy to achieve values in the range 75 to 85% by the correct choice of operating variables. The results show trends similar to those found in the laboratory with silica. Froth recovery was higher for finer particles whereas for coarse particles it decreases. It was found that the air flowrate has a positive impact on both collection (pulp) and froth zone recoveries. However, the effect was more prominent in the froth zone. Froth depth was found to have a significant effect on froth recovery. A larger number of valuable mineral particles detach and report to the dropback stream, with increasing froth height. Increasing the collector concentration gave expected results in the collection (pulp) zone, with a significant increase in coarse particle recovery. However, at high collector concentrations, it was observed that there was an increase in bubble coalescence and froth instability as well as a slight decrease in froth recovery. Increasing the frother concentration gave a significant improvement in froth recovery and a slight increase in collection zone recovery was also found. This may be attributed to the formation of finer, more stable, bubbles and a corresponding increase in froth stability. Size and grade analysis of the samples suggest that the dropback particles are mainly composite or middling fractions. It appears that particles whose grade is higher than the feed may be collected into the froth zone, but some particles with grades lower than the concentrate may detach from the froth and report to the dropback stream. In the third part of the thesis, a study of froth recovery and dropback in an operating plant rougher cell is presented. The dropback device was modified to make it sufficiently robust for in-plant testing. It was then inserted into the rougher bank of the Northparkes copper concentrator and the froth recovery was measured on two consecutive days. The feed grade to the rougher cell was 0.4% Cu. During the test work, the air flowrate into the rougher cell was varied. Each run was repeated for three times to check the reproducibility of the procedure. From the experimental results of Day 1 and Day 2, it was found that the froth recovery of the rougher cell was very high, around 90%, and was a strong function of particle size. Increasing the air rate slightly increases the froth recovery overall, and also enhances the recovery of coarse particles. The dropback particles have higher grade than rougher cell feed but lower than concentrate grade. From grade vs particle size analysis, it was also found that for relatively coarser particles the dropback grade is much higher than the grades of both feed and rougher cell tailings. However, for relatively finer particles the dropback grade is lower than the feed grade, possibly as a result of entrainment. It was also found that the copper grade of concentrate collected from the dropback device was lower than that of the rougher cell itself, for the same froth height. This result could have come about because of a disparity in the froth residence time, which was lower in the dropback device than in the froth layer in the rougher cell. Alternatively, the finest gangue particles may not have had sufficient time to drain out of the froth. This thesis represents the first systematic attempt to measure froth dropback using feed samples drawn from an operating mineral concentrator. The froth recovery results are higher than had been expected from the few reported accounts in the literature. While the results are promising, it is clear however that the froth dropback device is still at an early development stage. There are few limitations in the current froth dropback device. However, one limitation is that entrainment is not eliminated and further research work should be carried out to minimise entrainment. It is worth noting that, because it is possible to physically capture samples of the particles that have dropped out of a froth, closer examination of the surfaces of these particles, and their fractional liberation using Mineral Liberation Analysis or QemScan, will give valuable information about the measures that could be taken to increase their overall recovery, especially relating to the flotation of coarse particles. In summary, the experimental results from the three phases of this investigation suggest that the froth recovery can be as low as around 60%, but it is relatively simple to achieve froth recoveries in the range 75% – 85% using standard operating variables. Froth dropback is a strong function of particle size. Coarser particles are more prone to detach from the froth zone and report as dropback particles. Results from the high grade copper flotation work suggest that composite or middling particles, having lower grades than concentrate but higher grades than feed, report as dropback particles. These findings provide valuable information on the behaviour of flotation froths in the laboratory and in an operating concentrator. The value gained from the findings of this research work in industrial plant operations, together with further research into managing the froth zone effectively and improving coarse particle recovery, has the potential to reduce energy consumption and therefore further improve productivity in the mining industry.
- Subject
- flotation; froth; froth recovery; froth dropback
- Identifier
- http://hdl.handle.net/1959.13/1047609
- Identifier
- uon:14808
- Rights
- Copyright 2014 Reza Mainur Rahman
- Language
- eng
- Full Text
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