N.J. Welham

Mechanical enhancement of the dissolution of ilmenite

Abstract An ilmenite concentrate has been ball milled for up to 100 h in a laboratory mill without any evidence of a phase change. The particle size reached steady-state within 10 h of milling with a concomitant maximum in the BET surface area. There was considerable damage to the crystal lattice with a decrease in the crystallite size which was a function of log (milling time) and an increase in the lattice strain. The resultant powders were leached in sulphuric acid at up to 120°C for up to 8 h. It was found that there were two stages of dissolution, a rapid stage which was mass transport controlled (E a = ∼15 kJ mol −1 ) and a second, chemically controlled stage (E a = ∼70 kJ mol −1 ). The fraction of dissolution during the initial stage increased with milling time until over 80 % of the ilmenite was solubilised within 1 h at 120°C for 100 h milled powder. The enhanced dissolution was attributed to the exposure of planes along which preferential dissolution of ilmenite has been shown to occur.

Morphology: an ambiguous indicator of biogenicity.

This paper deals with the difficulty of decoding the origins of natural structures through the study of their morphological features. We focus on the case of primitive life detection, where it is clear that the principles of comparative anatomy cannot be applied. A range of inorganic processes are described that result in morphologies emulating biological shapes, with particular emphasis on geochemically plausible processes. In particular, the formation of inorganic biomorphs in alkaline silica-rich environments are described in detail.

Extended milling of graphite and activated carbon

Graphite and activated carbon have been ball milled under vacuum for up to 1000 hours. The graphite became amorphous, with no evidence of recrystallisation after heating to 1200°C. There was little change in the structure of the activated carbon. Heating the carbons under argon showed that the two milled samples absorbed substantial gas, over 30% of their mass in some cases. The reactivity of both carbons during combustion increased with milling time, the greatest increase was concomitant with a decrease in graphite crystallinity. After milling for 1000 hours, graphite showed similar combustion characteristics to activated carbon. Although iron was present as a contaminant at appreciable levels, there was no evidence that it had any significant effect on the oxidation of the carbon.

Mechanical activation of the solid-state reaction between Al and TiO2

Titanium dioxide and aluminium have been mechanically milled together for times up to 100 h. The onset temperature of the main reaction was observed to decrease from 1050 to 660°C after 5 h of milling, increasing the milling time to 100 h decreased the onset to 560°C. Elemental maps showed that the major decrease was due to increased mixing of the phases. Beyond 5 h, the temperature at which the reaction was half completed was found to be a function of log (milling time) and seemed to be correlated with the decrease in crystallite size. There seemed to be three different rate determining steps, the importance of each changing with milling time. Long range solid state diffusion was predominant for milling times <5 h, short range diffusion for 5–50 h and, possibly, diffusion along lattice defects after 100 h milling. An onset temperature of 560°C was observed for milling times of ≥10 h. This was thought to be due to a change in the rate determining step from mass transport to chemical reaction control.

Effect of extended ball milling on graphite

Graphite has been milled for up to 1000 h in a laboratory scale tumbling ball mill under vacuum. Raman spectroscopy of the powders indicated the increasing dominance of D-type graphitic sp2 bonding over G-type bonding with increasing milling time. Diamond-like sp3 bonding and possibly fullerene-like bonding also became evident after milling. TEM of the 100 h sample showed the presence of ribbons which were composed of sheets showing defects, delamination, translation, warping and curvature. Interplanar spacings of 0.40–0.50 nm were measured with the spacing increasing towards the edge of the ribbons where delamination was evident. Thermogravimetric analysis in argon of the powder after exposure to air showed an increasing mass loss with milling time indicating the presence of chemisorbed gas. Using TG–FTIR the gas was found to be a mixture of CO2 and an unidentified gas (probably oxygen). BET surface area measurements showed a maximum in the surface area; however, this was shown to be massively in error for the longer milling times due to the presence of the chemisorbed gas.

The stability of iron phases presently used for disposal from metallurgical systems—A review

Abstract The current phases used for the disposal of iron and arsenical wastes, jarosite and scorodite, are examined from a thermodynamic viewpoint using critically assessed data. For scorodite, it is demonstrated that the differences in the solubility reported in the literature are due to starting phases of differing crystallinity. Contour plots of solubility vs pH and activity are presented for both scorodite and jarosite and show that the minimum solubility is achieved for pH 4–7. However, neither phase is stable towards transformation to goethite with scorodite only stable below pH 4 and jarosite below pH 7. The practicalities and implications of the disposal of these phases is discussed and recommendations for future disposal options are made.

A parametric study of the mechanically activated carbothermic reduction of ilmenite

Abstract The Becher process has been the commercial way to upgrade ilmenite for over twenty years and the basic process has changed surprisingly little. It has recently been shown that ball milling ilmenite and coal together prior to thermal processing leads to a decrease in both temperature and time of reaction compared with powders milled separately then mixed. In this paper, the effect of milling time and annealing conditions on the extent of reduction and leachability of products are examined to give an overview of potential improvements in the process. Reduction of ilmenite to rutile can be attained within an hour of annealing at temperatures below that used industrially. Leaching indicates that it may be possible to produce a higher grade of TiO 2 than the current processing technique. However, over-reduction leads to the formation of phases which cause the fraction of titanium leached to increase and the iron solubility to decrease.

Enhanced dissolution of tantalite/columbite following milling

Abstract A tantalum/niobium concentrate has been ball-milled for up to 50 h in a laboratory mill without apparent phase change, other than a decrease in unit cell symmetry and some amorphisation. The extent of dissolution in a mixture of hydrochloric acid and sodium fluoride increased with milling time. For a powder milled for 50 h, over 95% of the concentrate were solubilised in 24 h compared with ∼50% in 48 h for a 2-h milled powder. The kinetics of dissolution changed with the milling time, indicating a change in the rate-determining step due to milling. These effects were attributed to the decrease in crystallinity of the powder, giving a substantial increase in the lattice strain and in unsatisfied surface bonds where dissolution was thought to occur.

Activation of the carbothermic reduction of manganese ore

The effect of extended milling on the carbothermic reduction of a manganese ore has been examined using a combination of thermal analysis and X-ray diffraction (XRD). Thermodynamic modelling indicated that reduction of MnO2 to MnO was possible at 25 °C, although no reaction was found to occur during milling of the ore with graphite for up to 10 h. For a physical mixture, cryptomelane, KMn8O16, reduced at 500 °C and braunite, Mn7SiO12, at 700 °C after 10 h milling these temperatures were reduced by 200 °C. The initial product was Mn3O4, although in the 10-h-milled powder, the reduction of braunite may have been directly to MnO. Reduction at 600 °C only formed Mn3O4 in the unmilled powder but the major product in the 10-h-milled powder was MnO. The increased extent of reaction after premilling may allow current processing plants to expand their throughput without increasing the size of reduction kiln.

Increased chemisorption onto activated carbon after ball-milling

Abstract Activated carbon has been milled for up to 1000 h in a laboratory-scale tumbling ball mill under a vacuum. Thermogravimetric (TG) analysis of the powder in argon showed an increasing mass loss with milling time indicating the presence of chemisorbed gas. TG Fourier transform infrared spectrometry showed the gas was a mixture of water, CO2 and an unidentified gas (probably oxygen). BET surface area measurements showed a decreasing surface area with milling time, however, this was shown to be massively in error for the longer milling times due to the presence of the chemisorbed gas. The area occupied by the chemisorbed gas increased from 40 to 80% of the true surface area which was almost constant at 1258±27 m2 g−1 for all three powders. These results show that extremely large errors may be made when using BET analysis to determine the surface area of powders, especially those where the surface activity is substantially increased during processing.