I. Nirdosh

Radium in uranium mill tailings ― some observations on retention and removal

Abstract The various mechanisms by which radium can be retained by uranium mill tailings are discussed. These include coprecipitation (solid solutions), adsorption on silica and hydroxides, ion exchange in clay minerals, and retention in pores. Previous work on radium removal from mill tailings and on radioactive disequilibrium in uranium deposits is critically reviewed in relation to these mechanisms. Fresh data are presented, indicating the roles of charge-depressant ions and of organic complexing agents in radium removal. The addition of reducing agent has been found effective in the presence of complexing agents because it can convert some low-solubility metal hydroxides to their more soluble, lower valency form, thereby eliminating radium adsorption sites. A radium level of 30 pCi/g in the tailings is concluded to be environmentally safe for surface disposal of tailings.

FORCED CONVECTION MASS TRANSFER INSIDE LARGE HEMISPHERICAL CAVITIES UNDER LAMINAR FLOW CONDITIONS

Rates of mass transfer at hemispherical cavities machined in the wall of a vertical rectangular duct were measured by an electrochemical technique which involved measuring the limiting current of the cathodic reduction of K3Fe(CN)6 Variables studied were solution flow rate, physical properties of the solution and cavity diameter. The mass transfer coefficient inside the cavity was found to be less than the flat surface value (the duct wall), and decreased with an increase in the cavity diameter. Mass transfer data inside the cavity were correlated by the equation: where Sh and Sc and Re are Sherwood, Schmidt and Reynolds numbers, respectively; and de, and d are the duct equivalent diameter and cavity diameter, respectively. Practical implications of the present results in mass transfer limited processes such as diffusion controlled corrosion, electrochemical machining, electroplating and etching have been highlighted

Adsorption-desorption studies on the 226Ra-hydrated metal oxide systems

Abstract Adsorption of 226 Ra on freshly precipitated hydrous oxides of Fe, Mn, Zr and Ti are investigated at pH 1 and 10. Radium removal is found to be highly sensitive to the solution pH and all oxides effectively adsorb Ra 2+ at higher pH. Radium adsorption is observed even at pH 1 on these oxides even though the zeta potentials at this pH are unfavourable. However, significant radium adsorption is observed only on manganese dioxide. Desorption of 226 Ra from the oxides is found to be difficult in alkaline solutions of EDTA.

Bench scale investigations on the electrolytic recovery of zinc powder from Galvanizer's ash

Abstract Bench scale studies on the alkaline leaching of zinc from the waste solids from galvanizing operations are reported. A process is tested for obtaining a purified caustic leach liquor from which zinc can be recovered as powder by electrolysis. Ninety percent zinc dissolution is obtained by leaching 6–8 mesh particles at a caustic concentration of 250 g/L and at a gage pressure of 345 kPa. Electrolysis is done on a stationary zinc-plated cathode. The effect of current density on the quality of the deposit is described. A process flowsheet is suggested.

Application of topochemical, topostructural, physicochemical and geometrical parameters to model the flotation efficiencies of N-arylhydroxamic acids

Abstract Structure–activity relationships (SAR) modeling of separation efficiencies of 17 arylhydroxamic acids tested as mineral collectors is attempted using topological indices, partition coefficients and Connolly molecular surface areas and solvent-excluded volume. Several of the computed parameters are found to fit in a second-order polynomial regression with the separation efficiencies ( E s ) of the collectors. Soil–water partition coefficient, log K oc , gives the best correlation with a standard error of estimate (SEE) of 4.95 and correlation coefficient, r =0.9635. The data set is split into N -aryl– C -alkyl and N -aryl– C -aryl types based on the structure of the hydroxamic acids, and the regression analyses were repeated. In the case of N -aryl– C -alkyl compounds, Connolly solvent-excluded volume gives the best fit with an SEE of 2.88 and correlation coefficient of 0.9936, and several other variables used in the study are found to have r >0.97. First-order valence connectivity index and log K oc give the best fit for N -aryl– C -aryl compounds, and the SEE in either case is 4.14 and r =0.9756. These results indicate that any one of the four classes of parameters considered in the study can be used for the interpretation of variation of separation efficiencies with the change in the hydrophobic part of a series of flotation collectors. It is inferred that lower-order valence connectivity indices are the good starting point to SAR modeling of flotation collectors.

QSAR Modeling of Flotation Collectors Using Principal Components Extracted from Topological Indices

Several topological indices were calculated for substituted-cupferrons that were tested as collectors for the froth flotation of uranium. The principal component analysis (PCA) was used for data reduction. Seven principal components (PC) were found to account for 98.6% of the variance among the computed indices. The principal components thus extracted were used in stepwise regression analyses to construct regression models for the prediction of separation efficiencies (Es) of the collectors. A two-parameter model with a correlation coefficient of 0.889 and a three-parameter model with a correlation coefficient of 0.913 were formed. PCs were found to be better than partition coefficient to form regression equations, and inclusion of an electronic parameter such as Hammett sigma or quantum mechanically derived electronic charges on the chelating atoms did not improve the correlation coefficient significantly. The method was extended to model the separation efficiencies of mercaptobenzothiazoles (MBT) and aminothiophenols (ATP) used in the flotation of lead and zinc ores, respectively. Five principal components were found to explain 99% of the data variability in each series. A three-parameter equation with correlation coefficient of 0.985 and a two-parameter equation with correlation coefficient of 0.926 were obtained for MBT and ATP, respectively. The amenability of separation efficiencies of chelating collectors to QSAR modeling using PCs based on topological indices might lead to the selection of collectors for synthesis and testing from a virtual database.

Distribution of 230Th and other radionuclides in Canadian uranium mill streams

Abstract 230Th distribution in the process streams of three sulphuric acid-leach Canadian uranium mills has been studied. It was found that 70 to 90% of 230Th in the ore is dissolved uring leaching and remains in solution as long as the solution pH is below 1.4, above which it starts to precipitate. Almost the entire 230Th initially present in the ore is found in the barren liquor mixed with the neutralized tailings. Percentage dissolutions of 232Th and 228Th are much less than that of 230Th when 232Th is in a refractory mineral and the milling conditions are mild.

LIQUID-SOLID MASS TRANSFER AT HORIZONTAL WOVEN SCREENS WITH UPWARD COCURRENT GAS-LIQUID FLOW

Rates of liquid-solid mass transfer at horizontal single screens and an array of horizontal parallel separated screens were studied under upward cocurrent gas (N2)-liquid bubbly flow using the electrochemical technique. Variables studied were gas and liquid flow rates, and screen characteristics (e.g., mesh number and wire diameter) Under the present conditions where relatively low solution flow rates were used the rate of mass transfer was found to be mainly determined by the gas flow rate. For a given gas flow rate, the mass transfer coefficient decreased with increasing solution flow rate. The data for single screen were correlated with a dimensionless equation. Rates of mass transfer at an array of separated horizontal screens were lower than those at the single screen by an amount ranging from 3 to 45% depending on screen mesh number and flow conditions. The importance of the present study for building continuous high space time yield catalytic, and electrochemical reactors suitable for electrochemical air pollution control is highlighted.

Ferric nitrate leaching of uranium and radium from uranium ores

Abstract A dilute aqueous acidic ferric nitrate solution of a relatively low concentration of 0.02 M is found to remove up to 97% of the uranium and 93% radium from ores occurring in the Elliot Lake area of Canada, after an initial flotation-separation of the sulfide minerals from the ore. A temperature of 75°C, a contact time of 24 h and a liquid/solid ratio of 1 ml/g are found optimum for leaching giving tailings which are effectively sulfide-free and with radium levels approaching a low value of 25 pCi/g. Radium may be removed from the leachate by adsorption and uranium by solvent extraction. An amount of 80–87% of the ferric nitrate may be recirculated for further leaching. Because of the low reagent concentrations and the recycle, it is possible to keep nitrate ion levels in the effluent below the prescribed level of 10 mg/l.

The reducing-complexing treatment for the leaching of radium from uranium mill tailings

Abstract The removal of radium from acid-leach uranium mill tailings has been difficult due to its strong retention as mixed sulfates and in adsorbed forms. In this work, the leaching action of a complexing agent (reducing the free radium cation concentration in solution) is combined with that of a reducing agent. Thus high valency metal hydroxides and basic salts (notably Fe 3+ ) which tend to retain radium are converted to the lower valency from which is much more soluble in the complexing agent. The reducing agent employed is sodium hydrosulfite, in combination with various organic complexing agents; the preferred complexing agent is EDTA. Over 90% of the radium in uranium leach tailings from Elliot Lake can be removed in 1 h contact at room temperature with a solution 0.04 M in hydrosulfite and EDTA, with 1.0 M potassium chloride added as a surface charge depressant. The gradual addition of solids to liquid permits the use of low liquid/solid ratios, and a treatment scheme for the process is proposed.