Peter May

At the present state-of-the-art, the modelling of equilibria occurring in concentrated reactive electrolyte mixtures remains problematic: the theoretical deficiencies related to calculating the activity of species in multicomponent solutions seem unlikely to be overcome soon. However, thermodynamic data for reactive systems, e.g. solubility values, are widespread in the chemical literature and need to be better utilised. A key challenge for storing this information is expressing the complete chemistry of a large number of potentially complicated solutions – such as phosphate and borate buffers – in a compact, and ultimately machine-processable, form. The approach taken to date (e.g. [1]) records only the analytical concentrations of well-defined components. We are seeking to build the data repository which will be required in future to parameterise theoretical solution chemistry frameworks estimated to involve at least five million experimental data values. A major issue is that existing databases do not store information relating to the chemical behaviour of the solution components that is needed for processing experimental data. We find that defining all solution species and the set of allowed reactions is necessary and sufficient for describing the chemistry of the solution. A database system implementing this design is in development building upon our existing chemical reaction [2] and physicochemical property [3] databases. Accordingly, the new database can store a vast range of properties including solubility, pH, density, mean activity coefficients and equilibrium constants. Methods to harmonise these data, which is a preliminary step prior to extracting reliable thermodynamic parameters for modelling, are described.

The IUPAC Subcommittee on Solubility and Equilibrium Data has been selecting, editing and reviewing projects and manuscripts for publication as part of the IUPAC-NIST Solubility Data Series (SDS). This series provides comprehensive reviews of data from the primary literature for solubilities of gases, liquids and solids in liquids or solids. Data are compiled in a uniform format, critically evaluated and, where high-quality data from independent sources agree sufficiently, recommended values are proposed [1]. SDS Volume 102 was published recently [2]. A current IUPAC project is dealing with solubilities of subs tances related to urolithiasis [3]. Kidney stones consist of a range of inorganic and organic substances that are sparingly soluble in urine, thereby producing ionic and uncharged aqueous species which can undergo a wide variety of protonation and complexation reactions. Using these data as an example, this work explores the features of the JESS solubility database [4] that assist with the presentation, manipulation and critical evaluation of solubility data. In particular, the unique combination of databases available in JESS for speciation equilibria [5], physicochemical properties of solutions [6] and solubilities provides improved evaluation facilities, including the assessment of thermodynamic consistency among solubility, calorimetric and speciation data as described in a recent SDS volume [7]. Systems of different chemical complexity will highlight the benefits of applying the JESS software package to the production of future SDS volumes.

Reliable solubility data for oxides and hydroxides of heavy metals in alkaline solutions are relevant to a number of industrial and geochemical processes, including the production of high-purity alumina in the Bayer process, the formation of ores from geothermal solutions at high pH, hydrometallurgical leaching processes for the extraction of metal values from ores and plant residues, and the storage and processing of certain types of radioactive waste. Recent results obtained for various systems will be reported, including experimental and modelling aspects. A new facility for the storage, handling and processing of solubility data will be outlined.

The deportment and speciation of trace elements during mining and minerals processing is becoming an area of increasing concern with regards to potential health and environment risks within the immediate vicinity and surrounding area of mining and minerals processing operations. This concern also extends to the transport of concentrates and other products from site to other facilities for further processing. In many cases the hazards associated with trace elements in the ore are exacerbated through their concentration in the processing plant. In Australia, the National Pollutant Inventory (NPI, 2010) lists antimony, arsenic, beryllium cadmium, chromium (III&VI), cobalt, copper, cyanide, lead, manganese, mercury, nickel, selenium and zinc as monitoring targets. From NPI data, the main trace metal emissions to the environment from metal processing operations are generally lead, arsenic, antimony and cadmium. Other trace elements of possible concern are mercury, bismuth, selenium and tellurium. However, little is known about the deportment and speciation of these trace elements in gold processing solutions, and how they vary in the leaching, carbon adsorption and tailings disposal facilities. The use of chemical equilibrium studies can enhance our basic understanding of the deportment and speciation of trace metals under conditions present in gold processing solutions, and assist in laboratory and field investigations. This paper presents some initial results from an initial two-year study into the deportment and speciation of trace metals in gold processing solutions. Although equilibrium models can be very useful in determining the deportment and fate of trace metals in process solutions, it must be remembered that these models have a number of limitations. The accuracy and precision of the modelling is very dependent on the thermodynamic database of from which the models are generated, and the range of conditions for which the data is available. In addition, models based on thermodynamic data may in some instances be unrealistic for processing solutions which often have a short residence time, and for which the speciation of metals may be governed more by kinetic than thermodynamic factors. For these reasons, care must be used in the interpretation of the information generated by the models and should, where possible, be confirmed by plant data or by laboratory tests.