To address this need, the following studies are proposed.
1) Comprehensive Literature Review
Note that translation of foreign language papers will likely be necessary, and that such translations are considered to be part of the contract scope.
a) Simple rare earth sulphate salts
The published solubilities of the simple rare earth sulphate salts in water and in sulphuric acid media will be reviewed via a detailed comprehensive search of the literature. The published data will be compared and reported, as a function of temperature, and where relevant, as a function of the acid concentration. Based on the reviewed information, a decision will be made to report the data either as g of REE sulphate per litre or as g REE sulphate/100 g of saturated solution. All of the published data will be compared on a consistent basis and reported in tabular and graphical forms. Notes concerning residence time, possible solution impurity phases and whether the data were obtained at "equilibrium" must be included. The literature review is a significant undertaking which will be separately reported and will be of relevance and use to the rare earth element industry. The report will contain a listing of all the reviewed documents.
b) Sodium, potassium and ammonium rare earth double salts
The published solubilities of the Na, K and NH4 rare earth double sulphate salts in water and in the presence of excess alkali sulphate will be reviewed via a detailed comprehensive search of the literature. Only the disulphate species (e.g., NaNd(SO4)2.H2O) need be considered, and for the purpose of this study, the "rare earths" are assumed to include lanthanum and yttrium, but not promethium. It will likely may be necessary to obtain translations of relevant publications. The published data will be compared and reported in tabular and graphical form, as a function of temperature and alkali sulphate concentration. Based on the reviewed information, a decision will be made to report the data either as g of REE sulphate per litre or as g REE sulphate/100 g of saturated solution.
2) Solubility Determinations of Trivalent Rare Earth Sulphate Salts
Note that suppliers should bear in mind the cost of REE chemicals at the necessary purity when designing and scaling their experimental protocols. The cost of the required chemicals is included in the contract value.
To complement the published data, the solubilities of all the trivalent rare earth sulphate salts (of at least 99.9% purity) in water will be measured over the temperature range from 20 to 95oC. Note that solubility measurements of the alkali-REE double sulphate salts are NOT part of this request for proposals. The densities of the associated saturated solutions at each sampling temperature will also be determined. For the purpose of this study, the "rare earths" include cerium to lutetium, lanthanum and yttrium, but not promethium. It is important that the solubilities of all the simple rare earth sulphate salts be measured, as different rare earths are prevalent in different ores and concentrates and researchers often focus on a variety of rare earth species. Initial tests will determine the necessary saturation times at low (25oC), medium (50oC) and high temperatures (85oC). Subsequent measurements will be carried out at 10oC intervals, on heating and cooling, and similar results must be attained upon heating and cooling to demonstrate that equilibrium is attained. In addition, the effect of five concentrations of H2SO4 (0.05, 0.1, 0.2, 0.3, 0.5 M) on the solubility of one light rare earth (neodymium) and one heavy rare earth (holmium) will be ascertained. X-ray diffraction analyses of the saturating solid phase at the various temperatures will be carried out to assess the degree of hydration of the various rare earth sulphate salts as the temperature is increased and decreased. Variations in the degree of hydration may explain possible "breaks" in the solubility curves during heating and cooling. It is appreciated that the oxidation of Ce3+ some of the heavy rare earth simple sulphates are relatively high and may require different solubility apparatus than that used for the light rare earths. Methods to address these problems are required. The measured solubilities will be compared in tabular or graphical form with the data generated in the literature review above, and discussion of possible differences between the current and previously published results must be provided.
