Microemulsified collector and depressants of sulphide minerals for coal cleaning with froth floatation

Zhao, Xuemin (2022). Microemulsified collector and depressants of sulphide minerals for coal cleaning with froth floatation. University of Birmingham. Ph.D.

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Abstract

Flotation is the primary method for fine coal preparation with Kerosene as a collector. But low
recovery of clean coal, high sulfur content, poor selectivity and high consumption are among the
main factors that seriously affect the flotation efficiency. This Ph.D work aims to address some of
these challenges. First, a new type of nonionic collector was therefore developed in this work.
Such a new collector costs approximately 50% of that of the traditional collector with the same
combustible recovery. In order to solve the problem of high sulfur content in coal, the effects of
PAC (Polymeric aluminum chloride) and cooked corn starch on depressing pyrite were then
studied, which have rarely been reported in the literature. More details of this work are summarized
in the following:
Firstly, a new collector called MEC (Microemulsion Collector) composed of blending surfactants
of Span 80 (Sorbitol fatty acid ester) and Tween 80 (Sorbitan esters and polysorbates), Kerosene,
MIBC (Methyl iso-butyl carbinol) and distilled water was developed. The collector was then
characterized by Dynamic Light Scattering (DLS) for size analysis, Cryo-Transmission Electron
Microscope (Cryo-TEM) for morphology, electrical conductivity for structure and rheological
measurement for viscosity. The small average size of the MEC indicates that MEC could disperse
the oily collector and form small droplets in water to improve the combustible recovery of cleaning
coal. Electrical conductivity is used to confirm the W/O structure of the new collector. The
viscosity of the collector is important for collector molecue spreading on the sample surface in the
flotation progress. Flotation performance studies confirmed that when the CR (combustible
recovery) was 60% and A (Ash) was 16%, the dosage of MEC was ~1000 g/t and the cost was
~7400 RMB/(103 t) coal slime compared to ~1000 g/t Kerosene with 14220 RMB/(103 t) coal slime,
due to the fact that polar oxygen functional groups on coal surface and the hydrophilic sites in
MEC formed hydrogen bonding. The electrical conductivity of MEC increased with increasing
concentration of mixed surfactants and water, but decreased with increasing concentration of
cosurfactant. The rheological measurements of MEC indicated its Newtonian behavior with
constant viscosity, which was slightly higher than that of water. The well-known EDLVO theory
was then adopted to investigate the interaction mechanisms of coal samples without and with
collectors. The addition of MEC and K (Kerosene) could both reduce the repulsive electrostatic
interaction energies (VE) between particles, and the reduction effect was larger for MEC than K.
The hydrophobic interaction energy (VH) and the total interaction energy (VT) were both much
more negative for MEC than for K and that of particles without reagent, signifying that the particles
modified by MEC were prone to combine into larger flocs and into foam area.
Secondly, the depressants corn starch or PAC combined with the new collector were studied for
depressing the pyrite. Hence the depressing effects of corn starch and PAC on pyrite were first
optimized using Box-Behnken design (BBD). The optimized conditions for minimum pyrite
recovery were found for corn starch as the dosage was 1490 g/t; modified pH for corn starch was
4; the heated temperature was 100℃. Under the optimized conditions, the predicted yield by BBD
method was 13.92%, compared well to the actual yield of 14.01%. The optimized conditions for
PAC were the dosage of PAC was 400 mg/L flotation slurry; pH of the flotation slurry was pH =
3; the concentration of PAC was 110 mL water per 1 g PAC. Under this set of optimized conditions,
the predicted yield was 18.3% by BBD method, also compared well to the actual experimental
yield of 18.64%.
Subsequently, combined approaches including Zeta potential, contact angle, FT-IR (Fourier
Transform Infrared Spectroscopy), and XPS (X-ray Photoelectron Spectroscopy) analyses were
adopted to study the interaction mechanisms between the depressants and pyrite samples. The Zeta
potential showed a steady increase of negative values with the corn starch concentration increasing,
whereas the Zeta potential increased steadily from negative values to ~20 mV with the PAC
concentration increasing due to the adsorption of Al(OH)2+, Al(OH)2+, Al3(OH)45+, and Al2(OH)24+
on the surface of pyrite. The depressant-free pyrite presented a higher value of contact angle. And
the contact angle of pyrite with PAC was significantly lower than that with corn starch, indicating
that a much more hydrophilic surface was generated with PAC.
The XPS analyses showed that the strength of both FeOOH 2p3/2 and Fe(II)-S 2p1/2 peaks of pyrite
decreased upon interaction with cooked starch, suggesting that pyrite chemically interacted with
corn starch. The FeOOH concentration increased significantly from 24.46% to 41.46% with 900
g/t starch due to the formation of FeOOH coating. And the sulfate (SO42-) concentration also
increased from 4.91% to 17.09%, improving the hydrophilicity of the pyrite. Compared with the
pyrite untreated with PAC, the concentration of FeOOH and sulfate (SO42-) with 400 mg/L PAC
both increased. And the O concentration increased from 46.09% to 58.11%, which would also
improve the hydrophilicity of the pyrite surface.
The FT-IR results of pyrite after interaction with corn starch demonstrated that the bands at 2984cm-1 and 2941 cm-1 due to C-H stretching of the -CH2 groups increased intensity for the samples treated with corn starch. No bands were observed at 1122 and 1032 cm-1 for pyrite after corn starch adsorption because of the chemical interaction of corn starch with pyrite. And the results of pyrite interacting with PAC illustrated the band at 2941 cm-1 due to the C-H stretching of the -CH2 group was very strong at the same position as that of the PAC-free. And the bands at 927 and 3362 cm-1 owing to hydroxyl with stretching vibration for PAC were missing after adsorbing with pyrite caused by the chemical interactions of PAC and pyrite.