Earth and Environmental Sciences

It is clear that earth and environmental sciences are of crucial importance to Australia. Environmentally, sustainable ore extraction, mineral processing, coal combustion, and soil use are merely a few of the areas that will continue to require significant research support. Synchrotron techniques are already opening up new ways to address the complex problems arising from earth resource utilization, and this contribution is expected to increase.

Evaluation of an ore body

The economic viability of a potential new ore body depends on many factors. One of the most important factors is the ease of mineral processing. Mineral bodies contain a number of crystalline and amorphous pases that contain complex distributions of metal cations. The optimizing of mineral processing conditions is often dependent on the precise composition of the ore and this generally requires very high resolution data. Although the presence of major phases is often readily established using conventional X-ray methods the quantification of all the species present, including establishing the distribution of the metal cations requires extremely high resolution coupled with high signal-to-noise data. The use of lower grade ores with increasing complexity in mineralogy is accelerating this requirement.

Often, an understanding of mineralogy changes that occur during processing is derived from equlibrium studies that only provide information about the final product in an idealized "steady state" operation. However, few mineral processing operations operate in this mode. In order to understand chemical and physical properties of minerals, it is important to obtain information under conditions that emulate the 'real' processing conditions. This information can be derived from so-called in situ experiments, where the sample is subjected to elevated temperature, pressure (typically hydrothermal pressure), sample pH and so on during powder X-ray diffraction data collection.

Two recent Australian expamples of the use of in situ XRD (X-ray Diffraction) experimentation in the mineral processing area are:

• the pressure acid leaching of Ni-laterites at real processing conditions of 250 C and 600 psi in H₂SO₄. The experimenta showed the formation of an intermediate phae kieserite MgSO₄.H₂O that cannot be observed in ex situ experiments due to its negative temperature coefficient of solubility.
• the formation of silico-ferrite of calcium and aluminium (SFCA). SFCA is the major bonding phase for the iron-ore sinters used in the production of iron and steel. The experiments have allowed study of the mechanism of SFCA formation, observation of intermediate phases directly with respect to time and temperature, and derivation of the order and comparative rates of phase formation throughout the experiments.

Mineral benification-flotation

The surface chemistry of metal sulfides is of major importance in the separation of the valuable and unwanted components of base metal ores, in the hydrometallurgical processing of a concentrate to produce the corresponding metal from the sulfide, and in the leaching of rejected material in waste heaps. Over the past thirty years, conventional X-ray photoelectron spectroscopy (XPS) has provided a wealth of information in these areas, but establishing the chemical nature of the true surface layer has proven to be difficult because of the several nanometre analysis depth. Since the application of synchrotron XPS to mineral fracture surfaces, the importance of surface chemical states arising from relaxation of the outermost layer following fracture has become evident. Enhanced surface sensitivity is achieved by determining the sulpfur 2p spectrum from a sulfide mineral fracture surface with ~200 eV synchrotron X-rays, compared with 1487 eV X-rays in conventional XPS. For example, in the case of pyrite, the additional states present at the surfaces are believed to be S^2-, arising from broken S-S bonds, and S₂^2-. The relevance of surface states to industrial-scale processes lies in their influence on surface reactivity, and this reactivity can be monitored by synchrotron XPS when fresh mineral fracture surfaces are subsequently exposed to different environments.

The enhanced surface sensitivity provided by synchrotron XPS, as well as the ability of angle-dependent NEXAFS (near edge X-ray absorption fine structure) to reveal orientation, have also assisted elucidation of the mechanism by which flotation reagents interact with the surface of minerals. By contrast, investigation of passivation layers that slow the dissolution kinetics in the hydrometallurgical processing of mineral concentrates is usually hindered not by the lack of surface sensitivity but because a near surface, yet buried, interfacial layer must be characterized. In that situation it is the non-destructive chemical depth profiling ability of NEXAFS spectroscopy that is exploited, and attempts are currently being made to augment the XAS data with threshold XAES (X-ray Auger-electron spectroscopy) measurements. The principle underpinning the threshold XAES approach is that, by incrementing the photon energy through the absorption edges for the different species present, it should be possible to identify an interfacial species under resonance conditions, even if that species might be present in only a thin interfacial layer.

Environmental Issues

Mineral weathering resulting in the release of acid mine drainage (AMD) is now of considerable environmental concern. It is estimated that in Australia remediation costs will be in the order of $900 million over the next 15 years. The dual XRF-XRD (X-ray fluorescence-X-ray difraction) mapping facility of the Microdiffraction and Fluorescence Probe will enable the identification of reaction and reprecipitation layering on mineral surfaces as a function of weathering. Understanding the evolution of these layers in terms of both their elemental composition and crystalline phase is important to the prediction and control of AMD. This combination of analyses will enable significant contributions to be made to the understanding of the release of toxic elements, which often accompanies AMD, and their bioavailability.

Another example is mine tailings containing arsenic. Arsenic (As) can exist structurally bound in compounds such as FeAsO₄ or adsorbed onto the surface of minerals such as goethite (FeOOH). The manner in which the As is incorporated has a strong impact on its 'availability' to the environment and hence on the steps that must be taken in the remediation of contaminated sites. The synergistic use of the XRF-XRD mapping facility together with X-ray absorption spectroscopy (EXAFS) is important in understanding the nature of contaminant metals at the molecular level, so that appropropriate action can be taken at the macro level.