J.G. Dunn

Selective flocculation of ultrafine iron ore. 1. Mechanism of adsorption of starch onto hematite

Abstract The mechanism of adsorption of wheat starch and its components (amylopectin and amylose) onto hematite has been investigated. The nature of the species adsorbed onto hematite and the extent of adsorption were determined by diffuse reflectance infrared Fourier transform spectroscopy. Wheat starch and amylopectin were adsorbed strongly onto hematite and the results were consistent with the formation of a surface complex, rather than adsorption by hydrogen bonding. Supporting evidence was obtained from reversibility tests and complexation of amylopectin with iron((III) in solution.

The oxidation of sulphide minerals

The literature associated with the thermal behaviour of mineral sulphides has been selectively and critically reviewed. Particular attention has been paid to: • • the importance of characterising the starting material, as well as intermediate products • • the effect of experimental variables on the thermal analysis results The various reactions that sulphides can undergo in inert and oxidising atmospheres are presented. Under mild oxidising conditions, such as an air atmosphere and heating rates of 10–20°C min−1, the oxidation occurs as a sequence of reactions usually controlled by oxygen diffusion, although in some situations decomposition of the sulphide with evolution of sulphur can occur. Besides the formation of oxides and sulphates, and the subsequent decomposition of the latter, solid-solid reactions can occur between sulphates and unreacted sulphides. In ternary systems, such as the iron-nickel sulphides, considerable ion diffusion can take place. Under more vigorous oxidising conditions, such as an oxygen atmosphere with a heating rate in excess of 40°C min−1, some sulphides can be ignited. Under these conditions the relative ignition temperatures of sulphides can be measured, and the effects of variables such as particle size and stoichiometry on the ignition temperature examined. The oxidation of pyrite is presented as a case study of the effects of experimental variables on the results of thermal analysis. The application of the results of studies to the industrial processing of sulphides of economic importance has been discussed.

Optimisation of selective flocculation of ultrafine iron ore

Abstract A laboratory scale selective flocculation process capable of recovering iron oxide from a sample of ultrafine (less than 10 μm) iron ore tailings containing kaolinite as the main gangue mineral has been developed. Experiments were carried out (i) to select the most appropriate flocculant from a large range of commercially available starches and polyacrylamides and, (ii) to optimise the many parameters that affect selectivity and recovery. Under optimum conditions, a feed containing 46.6% iron was upgraded to a concentrate with 57–58% iron at 65–75% recovery. Wheat starch was shown to be the most appropriate selective flocculant.

The effect of experimental variables on the mechanism of the oxidation of pyrite: Part 1. Oxidation of particles less than 45 μm in size

Abstract The oxidation of pyrite of particle size less than 45 μm has been studied by simultaneous thermogravimetry- differential thermal analysis (TG- DTA) at a heating rate of 2.5 ° C min−1 in an air atmosphere. Sample masses of approximately 1.8 mg were contained in platinum crucibles. Partially oxidised samples were isolated and the phase composition determined by quantitative X-ray diffraction (XRD). Micrographs of these samples were obtained by scanning electron microscopy (SEM). Four effects were observed in the DTA record. At temperatures of 395–420 ° C there were several sharp exotherms. XRD analysis showed the presence of 90% pyrite and 6% hematite in samples heated to 428° C. Beyond this there was a broad exotherm, with a shoulder on the high temperature side, in the temperature range 420–490 ° C. By 470 ° C, 47% hematite was detected with 36% pyrite remaining. By 505 ° C 65% hematite was present with 10% pyrite unreacted. Beyond this temperature there was a weak endotherm which was complete by 610 °C. Only hematite was detected in a sample heated to 696 ° C. The corresponding TG curves showed two major weight losses, the first in the temperature range 440–480 ° C and the second between 550–605 ° C. The SEM results indicated that reacted particles had little general porosity. Increasing the heating rate progressively from 2.5 to 40 °C min−1 caused a significant change in the TG-DTA curve, with a decrease in the intensity of the major exothermic peak, and the appearance of a new peak between 530–550 ° C. The second weight loss decreased relative to the first weight loss. Changing the atmosphere from air to oxygen and heating at 40 °C min−1 produced a further significant change in the TG-DTA record. Only one intense exotherm and one rapid weight loss were observed between 475–500 ° C. The particles, when examined by SEM, had a very high porosity. The differences in appearance of the TG-DTA records have been attributed to changes in the reaction mechanism which occur as a result of variation in the experimental conditions.

Selective flocculation of ultrafine iron ore 2. Mechanism of selective flocculation

The mechanisms involved in the selective flocculation of hematite from kaolinite, using starch as the flocculant, have been investigated. Adsorption isotherms of starch on hematite and kaolinite were measured by a thermogravimetric method, supported by infrared analysis of the adsorbed species and zeta-potential changes on adsorption of starch. The preferential adsorption of starch onto hematite was proposed to be due to the higher concentration of hydroxylated metal adsorption sites. By comparing selective flocculation performance with adsorption results, it was established that flocculation occurs by the classical bridging mechanism.

The Decrepitation of Dolomite and Limestone

Abstract The degrees of decrepitation of a limestone and of a dolomite were determined by an industrially accepted procedure called the Pilkington test, and by a TG method. A good correlation in decrepitation trend with change in particle size between the two methods was apparent for the dolomite, and the maximum degree of decrepitation was found to be in the 180–250 μm (60–80 mesh) fraction by both methods. The correlation was less evident for the limestone, and the change in degree of decrepitation with particle size was different for the two tests. The TG method gave much larger values for the degree of decrepitation relative to the Pilkington test, and so can be considered a more sensitive test.

A Fourier transform infrared study of the oxidation of pyrite

Abstract The oxidation of pyrite in an air atmosphere has been studied using Fourier transform infrared (FTIR) spectroscopy to identify phases present in the temperature range 430–680°C. Evidence of the formation of sulfate was obtained; the species was identified mainly as anhydrous iron(II) sulfate by comparison with the spectra of various iron sulfates and data from the literature. Iron(III) sulfate also formed as a minor phase. The FTIR analysis was supported by chemical analysis, both methods giving a maximum of iron(II) sulfate formed between 500 and 550°C. The FTIR method could be used to follow quantitatively the disappearance of pyrite as the oxidation progressed.

The effect of experimental variables on the mechanism of the oxidation of pyrite: Part 2. Oxidation of particles of size 90–125 μm

The oxidation of pyrite of particle size 90–125 μ m has been studied by simultaneous thermogravimetry-differential thermal analysis (TG-DTA). Partially oxidised samples were characterized by qualitative and quantitative X-ray diffraction (XRD) and scanning electron microscopy (SEM). The first weight loss, of 4%, occurred at 425–435 °C, and was associated with a weak exotherm. XRD analysis indicated 11% hematite present at 453 °C, with 86% pyrite unreacted. The SEM micrographs of samples heated to 453 °C were visually identical to those of unreacted pyrite, although analysis by energy dispersive X-ray spectrometry showed some rim oxidation. Between 450 and 515 ° C there was a gradual weight loss of 55%, with an associated exothermic drift. The SEM micrographs revealed a pyrite core surrounded by a thick product coating. By 520 °C, 75% hematite was present as determined by XRD, with 16% pyrite remaining. The next weight loss, of 30%, occurred between 515 and 535 °C, and was coincident with the major exothermic peak observed. The hematite content increased to 89% by 550 °C, and no unreacted pyrite was detected. There was significant porosity apparent in the core of the particles. The final weight loss of 11% occurred above 565 ° C. No DTA response was evident. Increasing the heating rate between 2.5 and 40 °C min− increased the third weight loss at the expense of the second. A similar effect was noted on changing the atmosphere from air to oxygen. A comparison of the oxidation of pyrite of particle size < 45 and 90–125 μm has been made. The effects of changing the particle size, heating rate and atmosphere are discussed. Depending on the conditions chosen, one of the three following mechanisms of reaction operates: (i) slow combustion, controlled by oxygen diffusion; (ii) pyrolytic decomposition of pyrite and oxidation of the resulting pyrrhotite; and (iii) ignition. The TG-DTA records show distinct differences according to the prevailing mechanism.

A Fourier transform infrared study of the oxidation of pyrite: The influences of experimental variables

Abstract The influence of variables such as moisture in the air atmosphere, heating rate and particle size on the oxidation of pyrite has been investigated. FTIR spectrometry has been used to characterize the intermediates. All three variables had an influence on the extent of oxidation and the relative abundance of the phases formed.

The oxidation of violarite

Abstract A sample of violarite was synthesized from pure components and characterized. Samples of particle size 45–75 μm were oxidized in a TG-DTA apparatus at a heating rate of 10°C min −1 and the products characterized at various temperatures by X-ray diffraction (XRD), backscattered electron (BSE) images obtained on a Scanning Electron Microscope (SEM), Fourier transform infrared (FT-IR) spectroscopy and electronprobe microanalysis (EPMA). Only minor sulfation reactions occurred up to 405°C, but above this temperature the violarite decomposed to form a monosulfide solid solution (mss, (Fe,Ni) 1 − x S) and sulfur. The first reaction produced a mass loss, and oxidation of the evolved sulfur produced an exotherm. From 470°C, there was a mass gain due to the continued formation of sulfate species and an exotherm caused by the conversion of iron sulfide to hematite. From 585°C, a mass loss occurred as the iron sulfates decomposed. At 670°C decomposition of the mss took place, with the formation of heazlewoodite, (Ni, Fe) 3 ± x S 2 , and sulfur. The first reaction produced a mass loss, and oxidation of the evolved sulfur gave an exotherm. Trevorite, (Fe, Ni) 3 O 4 , was also formed above this temperature, from the oxidation of either mss or heazlewoodite, and this reaction also contributed to the mass loss. Above 725°C a mass loss and sulfur dioxide evolution was associated with the decomposition of nickel sulfate. This was followed by an endotherm at 785°C caused by the melting of the heazlewoodite, which immediately oxidized to produce a mass loss and exotherm. The final product above 860°C contained trevorite, nickel oxide and hematite. Extensive migration of iron occurred during the oxidation process.