Dee Bradshaw

A review of induction and attachment times of wetting thin films between air bubbles and particles and its relevance in the separation of particles by flotation

Bubble-particle attachment in water is critical to the separation of particles by flotation which is widely used in the recovery of valuable minerals, the deinking of wastepaper, the water treatment and the oil recovery from tar sands. It involves the thinning and rupture of wetting thin films, and the expansion and relaxation of the gas-liquid-solid contact lines. The time scale of the first two processes is referred to as the induction time, whereas the time scale of the attachment involving all the processes is called the attachment time. This paper reviews the experimental studies into the induction and attachment times between minerals and air bubbles, and between oil droplets and air bubbles. It also focuses on the experimental investigations and mathematical modelling of elementary processes of the wetting film thinning and rupture, and the three-phase contact line expansion relevant to flotation. It was confirmed that the time parameters, obtained by various authors, are sensitive enough to show changes in both flotation surface chemistry and physical properties of solid surfaces of pure minerals. These findings should be extended to other systems. It is proposed that measurements of the bubble-particle attachment can be used to interpret changes in flotation behaviour or, in conjunction with other factors, such as particle size and gas dispersion, to predict flotation performance.

The effect of chemical composition and molecular weight of polysaccharide depressants on the flotation of talc

Abstract Polysaccharide reagents are frequently used in the mineral processing industry to improve the grade of the concentrate by depressing the naturally floatable gangue. A microflotation study showed that at an ionic strength of 10 −3 M KNO 3 , the dosage, molecular weight, and chemical composition of polysaccharide depressants affect their depressing action on talc. Viscosity readings for four carboxymethylcellulose (CMC) reagents and three modified guar gum reagents are reported, and estimates of the molecular weight of the CMCs are calculated. The natural floatability of talc was diminished when guar-based polymers were added to the system. Moreover, the depression of talc was increased as the guar molecular weight was increased. Strong depression of talc was achieved at a relatively low guar dosage; higher dosages did not further increase this depression. At 10 −3 M KNO 3 , the CMCs were ineffective depressants of talc. No systematic change in talc depression for differing molecular weight CMCs was observed. A small increase in depression was observed when the concentration of CMC was increased. A mechanism is proposed for the polymer–talc interactions based on these preliminary studies and information from similar studies in literature.

Frother characterisation using dynamic surface tension measurements

There is as yet no accepted method of predicting flotation frother performance in plant processes based on laboratory scale characterisation techniques. This paper investigates the use of dynamic surface tension, using the maximum bubble pressure method, as a means of evaluating frother performance. The method has been refined to facilitate measurement at high bubble rates, an important criterion with respect to the analysis of small, fast adsorbing molecules at low concentration such as flotation frothers. An adsorption model is presented, which has been used to elucidate the role of frothers with respect to bubble size in a flotation cell, based on the correlation of model parameters to physical frother characteristics. The potential for the application of dynamic surface tension to the performance of the froth phase in a flotation system is also discussed.

Measurement of the sub-process of bubble loading in flotation

A method has been developed to measure bubble loading in a microflotation cell. Excellent reproducibility has been achieved. The number of particles loaded per bubble decreased as particle size increased. A good correlation between bubble loading and batch flotation tests was observed.

The flotation of pyrite using mixtures of dithiocarbamates and other thiol collectors

Abstract This paper presents results which show that when mixtures of dithiocarbamates are used to float pyrite each component has a synergistic effect on the performance of the others. In particular cyclo-hexyl dithiocarbamate which produced the lowest recovery when used as a pure collector had a significantly advantageous effect on recovery and grade when mixed in small quantities with another dithiocarbamate. Different molar ratios of cyclo-hexyl (oC6) and di-n-propyl (di-n-C3) dithiocarbamates (DTC) in the mixtures were tested. In all cases 90:10 and 10:90 mixtures produced higher recoveries and grades than did 50:50 mixtures or the pure components. Recoveries increased from 74.0% for pure oC6 DTC and 83.5% for pure di-n-C3 DTC to 89.5% for the 90:10 mixture. Grades increased from 14.5% for pure oC6 DTC and 18.5% for pure di-n-C3 DTC to 20.5% for the 50:50 mixture. The effect of mixtures was less marked at higher dosages indicating that for the same performance lower dosages of mixtures were required compared to those required by the pure components. The mixtures resulted in faster rates of flotation as well as a greater recovery of coarse particles. Increased recoveries and grades were obtained in the first minute due to an enhancement in the rate of flotation. When mixtures were used the p 50 of the concentrate increased showing that coarser material was collected. This effect was greater in the first concentrate than the final concentrate. Mixtures of dithiocarbamates and other thiol collectors, viz. xanthate, sodium mercaptobenzothiazole and dithiophosphate further enhanced recoveries and grades to a maximum recovery of 96.0% and grade of 21.0% for a mixture of 90% potassium n -butyl xanthate (PNBX) and 10% oC6 DTC. The method (premixing or separate) and sequence of collector addition had little effect on recoveries or grades. The effect of copper sulphate addition varied. In the case of cyclo-hexyl dithiocarbamate, the addition of copper sulphate dramatically reduced recoveries and grades but when other collectors were present this did not occur. Generally the addition of copper sulphate slightly enhanced recoveries and grades and greatly enhanced water recoveries probably due to a froth stabilization effect.

The flotation behaviour of chromite with respect to the beneficiation of UG2 ore

Abstract The UG2 reef of the Bushveld Igneous Complex in South Africa is a valuable source of platinum group metals (PGMs) that is beneficiated by flotation. Chromite is one of the main gangue consituents of UG2 ore and it is essential that the amount of chromite reporting to the concentrate during flotation be minimised since it is detrimental to the subsequent smelting operations. It is generally considered that chromite is naturally hydrophilic and only reports to the concentrate by entrainment. However, it is possible that, under certain conditions, the chromite can be activated by the typical reagent suite used in the flotation of UG2 ore, rendering it hydrophobic and amenable to true flotation. The aim of this work was to investigate the effect of copper sulphate and the collectors, sodium isobutyl xanthate and dithiophosphate on the flotability of chromite in a microflotation cell. The effect of copper sulphate on the zeta potential of chromite was also determined. It was found that collectors alone have an insignificant effect on the flotation of chromite whereas, in the presence of copper sulphate recoveries of above 60% were observed in mildly acid to mildly alkaline solutions decreasing 20% at a pH value of 10. The extent of activation depended on the copper sulphate dosage. Zeta potential measurements showed that, above pH 4 copper species were adsorbed on the chromite and between pH 5 and 9 the adsorption lead to charge reversal of the chromite particles to positive values. The speciation programme, MINTEQA2, was used to predict the copper species present over the pH range used. It was postulated that the activation is due to the adsorption of copper hydroxide species at the chromite surface, which then act as sites for collector adsorption.

The use of electrochemical measurements in the flotation of a platinum group minerals (PGM) bearing ore

Abstract Electrochemical potential is considered an important parameter for controlling the recovery and selectivity of sulphide minerals during flotation. The aim of this work was to use mineral potential measurements to gain an understanding of respective mineral flotation performance in the processing of a platinum group mineral (PGM) bearing ore, whilst manipulating the chemical conditions in the pulp. Sulphide mineral potentials were logged during the conditioning stage of the flotation tests. Varying the milling environment and conditioning pH affected the chemical conditions in the pulp. At any condition the mixed potential of the mineral electrodes varied, spanning a range of approximately 100 mV, and increased as follows: pyrrhotite, pentlandite, chalcopyrite, pyrite and platinum. The addition of CuSO 4 caused the potential to increase, whilst the addition of xanthate caused to the potential to decrease. The magnitude of the changes in potential for the various mineral electrodes on reagent addition was not the same and followed the recovery trend of the minerals viz. chalcopyrite>pentlandite>pyrrhotite. Chalcopyrite recovery was unaffected by differences in milling media and conditioning. Pyrrhotite flotation was inhibited when the ore was milled in the plant circuit even though the mixed potentials were similar to those measured in laboratory milled pulps. This was presumably due to excessive oxidation in the grinding and classification circuits of the plant. Acid conditioning improved the recovery of pyrrhotite through cleaning the mineral surface from hydrophilic iron oxy/sulphoxy species and also showed an enhanced reaction with xanthate. Pyrrhotite flotation was only slightly inhibited when its mixed potential was below the equilibrium potential for dixanthogen formation.

Pentlandite–feldspar interaction and its effect on separation by flotation

The major loss of PGM and base metals in beneficiation of the Merensky Reef ore occurs during the separation of the siliceous gangue from the base metal sulphides and PGM minerals by selective flotation. The predominant gangue minerals in Merensky ores fed to the PGM concentrators are pyroxene and feldspar. Other important gangue minerals are talc and chlorite. A significant percentage of these minerals reports to the concentrate, thus diluting the concentrate grade and ultimately increasing the transport and smelting costs. Feldspar is naturally hydrophilic and requires activation for flotation. The present work investigates how collector adsorption (sodium isobutyl xanthate), copper sulphate activation, and the distribution of ions on mineral surfaces may influence selectivity in the flotation of these ores. The possible chemical reactions taking place on the surface of synthetic pentlandite, natural feldspar, and a 1:1 mixture were investigated. In the pH range studied, zeta potential and time of flight secondary ion mass spectrometry (ToF-SIMS) analyses indicated the presence of xanthate and copper ions on pentlandite surfaces. Xanthate ions did not adsorb onto feldspar surfaces. However, at pH 9, feldspar surfaces became coated with copper species and the subsequent adsorption of xanthate ions caused the feldspar to float. The results are explained in terms of the interaction between various copper and xanthate species present and the zeta potentials of the minerals at the pH values of interest.

The use of dithiophosphates and dithiocarbamates for the flotation of arsenopyrite

Abstract The recovery of arsenopyrite from an arsenopyrite/pyrite ore is desirable for a number of reasons. This can be optimized using a two stage flotation process in which a dithiophosphate is added at pH =11 in the first stage and copper sulphate and a dithiocarbamate in the second stage. It was found that better separations were obtained when aged ore was used. It was possible to simulate this ageing process by heating. Various alkyl functional groups of the collectors were tested and the best separations were obtained using sodium isobutyl dithiophosphate and a mixture of 10% : 90% sodium cyclohexyl : n - propyl dithiocarbamate. This suite of collectors led to a recovery of 74.8% arsenopyrite and 8.4% pyrite with grades of 37.9% arsenopyrite and 11.6% pyrite respectively. The grades of gold in the feed, concentrate and tailings for these conditions were 8.10 g/ton, 271 g/ton and 2.55 g/ton respectively. This represented a gold recovery of 83.6%.

The flotation of fine pyrite using colloidal gas aphrons

Abstract Fine mineral flotation is known to be problematic, however there is evidence that flotation performance is improved by the use of fine bubbles. In this work, a comparison is made between the flotation performance of two flotation systems; a conventional batch flotation cell and flotation using a spinning disc impeller as is used to generate very small bubble suspensions called colloidal gas aphrons (CGAs). The comparison of the CGA and the standard impeller was made using an artificial ore consisting of gravity concentrated pyrite from Durban Roodepoort Deep (South Africa) and quartz from Delmas (South Africa) finer than 38 microns. The collector was sodium n-propyl xanthate, the frother Dow 400. The comparison of the flotation performance was made at the following conditions; pH 4 and 8 and copper sulphate addition nil and 50 g/ton. The froths found in the two systems had significantly different structures. The CGA impeller yielded a froths surface with considerably larger bubbles and a wider distribution of bubble sizes than the conventional batch cell. The CGA float yielded higher grades (50% sulphur) than the conventional float (35%), although the flotation rate was somewhat lower. The results obtained indicate that this flotation system may have application for the selective recovery of finely ground minerals.