L.J. Warren

The flotability of very fine particles — A review

Abstract The review is in three main sections. First, the observation that overall flotation performance deteriorates for the finest particles is confirmed, but the precise effects of particle size on recovery, grade, and flotation rate are shown to be more complex. In the second section an assessment is made of theories which have been proposed to explain the slower flotation rate of ultrafines, the effects of slime coating, and the flotation of slimes with cationic collectors. The last section of the paper gives a critical evaluation of methods which are claimed to improve the flotation of ultrafine particles.

Determination of the contributions of true flotation and entrainment in batch flotation tests

Abstract Laboratory batch flotation tests were carried out on a deslimed (+6 μm) sulfiderich cassiterite ore, an ultrafine fraction (−6 μm) of a cassiterite ore and a bituminous coal. Chemical conditions were kept constant but the water recovery was varied by changing the height of the froth column and the rate and depth of froth removed. The recovery of the floatable mineral in each system was then found to be linearly related to the weight of water recovered. The intercept of the regression line on the mineral recovery axis, where the water recovery is zero, was interpreted as the recovery due to true flotation. The entrainment contribution was proportional to the slope of the line. In this way the contributions of entrainment and true flotation to overall recovery were separated.

Shear-flocculation of ultrafine scheelite in sodium oleate solutions

Studies have been made of the aggregation of ultrafine scheelite (CaWO4) suspensions in turbulent flow. The turbulence was generated by a single bladed paddle stirrer in a baffled beaker, a system chosen to simulate industrial practice. Coagulation by an indifferent electrolyte and flocculation by a polyelectrolyte produced aggregates which were broken up by intense agitation. Quite different behavior was observed for scheelite suspended in dilute sodium oleate solutions. Aggregation occurred only at high stirring speeds, the extent of aggregation and the size of aggregates increasing the faster the suspension was stirred. Classical theories of colloid stability could not explain this “shear-flocculation” and other effects such as the higher energy of turbulent collisions and the energy of “hydrophobic association” are considered.

Surface titrations and electrokinetic measurements on stannic oxide suspensions

The electrokinetic and surface charge behavior of stannic oxide (SnO2) in KNO3 electrolyte solutions is reported as a function of pH and ionic strength. Potentiometric titrations of the surface groups showed that the point of zero charge occurred at pH 4.3 ± 0.1. The isoelectric point obtained from microelectrophoresis was at pH 4.5 ± 0.15. Stannic oxide behaved similarly to other insoluble metal oxides with respect to the effect of pH on its surface charge and zeta potential. Intrinsic equilibrium constants, which describe the ionization of surface hydroxyl groups and the ion-binding of electrolyte, were calculated using various estimates of the total hydroxyl site density and three different methods of calculation. The preferred values are NS = 11.2 OH groups per nm2, pKa1int = 0.7, pKa2int = 7.9, and p*KK+int = 6.3.

Flocculation of stirred suspensions of cassiterite and tourmaline

Abstract Relatively pure samples of the minerals cassiterite and tourmaline were separated from a tin ore, and were used to prepare suspensions containing particles between 2 and s μm diameter. These suspensions were stirred in a baffled beaker under various coalitions and the changes in the state of aggregation measured with a photosedimentometer. Zeta-potentials of the particles were also measured. It was found that both minerals coagulated spontaneously in the pH range of interest (pH 4–6) but that the degree of aggregation could be increased by the shear-flocculation effect to produce larger, stronger flocs. The shear-flocculation step was made selective by using sodium fluorosilicate as a selective dispersant for tourmaline. The mechanism of the shear-flocculation effect is discussed and compared with that of coagulation under shear.

Electrokinetic properties of scheelite

The electrophoretic mobility of scheelite (CaWO4) from King Island, Australia, has been measured in a variety of inorganic electrolytes, mostly at constant ionic strength and controlled pH. Scheelite develops a negative surface charge in its saturated solution, but the zeta-potential may be reduced to zero by increasing the calcium concentration to between 10−2 and 10−1 M, the exact concentration depending on the pH. Difficulties in interpreting results at the high electrolyte concentration required to produce this isoelectric point are discussed. The mechanism by which calcium reduces this negative potential could not be established with certainty. Hydrolytic processes involving specific adsorption have only a secondary effect, except below pH 4 or when both pH and calcium concentrations are high. In solutions of MgCl2 the zeta-potential of scheelite could be changed from negative to positive by raising the pH.

Selective flotation of apatite from iron oxides

Abstract The effects of the key variables on the selective flotation of Mt. Weld phosphate ore were studied with the aim of optimizing the flotation process to yield a concentrate of the following specifications: >36% P2O5, P 2 O 5 ( Fe 2 O 3 + Al 2 O 3 ) >10 at a P2O5 recovery of >70%. The main difficulty was to reduce the iron content in the apatite concentrate to the level required. Analysis of the QEM∗SEM results for two batch flotation tests gave useful insights into the flotation behaviour of the various types of minerals, particles and composite particles present in the ore. It was found that iron in the concentrate occurred mainly as hematite and goethite, which were collected partly due to the incomplete depression of the liberated iron oxides and partly due to composite particles containing iron oxides. Systematic microflotation tests on purified samples of apatite and hematite provided the basis for understanding the mechanism of the flotation process. A key feature of the flotation was the use of sodium silicate as a selective depressant for iron oxides. The flotation pH, the modulus of sodium silicate, the pH and the time of standing of the stock solution of sodium silicate and the presence of polyvalent metal ions in the silicate solution all affected the selectivity with which the iron oxides were depressed. A final improved flotation flowsheet was proposed which achieved 81% P2O5 recovery at an iron contamination level of 2.7% Fe2O3 in the concentrate.

Relation between the relative density of composite coaly grains and their flotation recovery

Abstract Batch flotation tests on low- and high-rank Australian coals showed that it was difficult to reduce the original ash yield substantially whilst maintaining high recovery. The ash yield of the flotation concentrates was found to be mainly the result of the recovery of composite coaly grains containing both carbonaceous matter and mineral matter. Even 50 μm grains were composite in nature, the mineral matter being very finely disseminated. Separation of the flotation products using the heavy-liquid density-gradient technique showed that flotation recovery of coaly grains of a given size decreased with increasing relative density of the grains. The effect was much greater for coal of low rank. The fall in recovery correlated with an increase in the proportion of hydrophilic mineral matter in the composite grains, rather than with the changing proportions of the macerals vitrinite and inertinite.

Diffuse reflectance infrared Fourier transform spectroscopic study of the adsorption mechanism of oleate on haematite

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) was found to be effective for analysing adsorbed oleate species on the highly absorbing mineral Haematite. The adsorption of oleate on haematite was studied by DRIFT at pH values from 6.0 to 9.8 and oleate concentrations from 5·10−5 to 5·10−4 mol dm−3. These concentrations are as low as those normally used in the froth flotation of ores containing haematite. The spectra suggest that chemisorption of oleate on haematite took place under all conditions and that the chemisorbed species were either chelating bidentate or bridged bidentate, corresponding to two peaks at 1580 and 1518 cm−1, respectively. Dimers of oleic acid, corresponding to a peak at 1713 cm−1, coexisted with the chemisorbed oleate. Oleic acid monomer, corresponding to a shoulder at 1738 cm−1, appeared on the haematite surface at pH 8.0 and relatively high oleate concentrations. At flotation concentrations of oleate, maximum adsorption of all species occurred at pH 8.0. The oleic acid monomer probably adsorbed via hydrogen bonding as well as hydrocarbon chain association and the monomer adsorption was stronger than that of oleic acid dimer, which probably occurred via hydrocarbon chain association only. The differences in the mechanisms of adsorption of oleate on haematite and on apatite are discussed.

The joint effect of rank and grain size on the flotation of Australian bituminous coals

Abstract Laboratory batch flotation tests were carried out to determine the effect of grain size on the recovery of four Australian bituminous coals of high and low rank. The flotation concentrates, taken over successive time intervals, and the tailings, were sized and the recovery-size curves plotted. A method was developed for determining the proportions of liberated and composite grains in the various flotation products. Flotation tests were carried out in the presence of methylisobutylcarbinol (MIBC) and MIBC + kerosene. The shapes of the recovery-size curves depended on the rank of the coal and whether or not kerosene was present. The variation of the recovery with size was explained in terms of the joint effect of rank and size on the floatability of composite grains.