Processing, Removal, Recovery and Immobilization of Heavy Metals and Radionuclides Wastes
(a) Roasting Characteristics of Refractory Gold Ores

Gold bearing deposits are geochemically classified as sedimentary and hydrothermal ores. The sedimentary gold deposits are free milling and pose many problems for gold recovery. The hydrothermal gold deposits are rock minerals and in these, gold is associated with sulfide carbonaceous and siliceous compounds and also pose considerable difficulties for recovery. These deposits are commonly known as refractory gold ores and in these ores gold is generally locked in sulfides or sulfide carbonate crystals, making it difficult for the cyanide solution to penetrate for leaching. Mainly for this reason, a pretreatment oxidation step is required to liberate precious metal content and make them amenable to leaching. Roasting is one such oxidation pretreatment technique. In the present research, chemical and structural changes occurring during the roasting of refractory gold ores were investigated. Effect of temperature on oxidation behavior of ores was analyzed using TGA, DTA and XRD methods. The experimental results showed that the carbon in the ore has to be oxidized to a minimum value before any appreciable oxidation of sulfur occurs. Oxidation rates as a function of temperatures were calculated. Optimum conditions under which a successful roasting of the refractory gold ores can be obtained are evaluated.


(b) Recycling and Safe Disposal of Spent Potliners

Spent potliner is generated in the electrolytic process of reducing alumina to make aluminum metal using molten fluoride electrolytes. Spent potliner (SPL), obtained at the end of the pots life contains a small amount of cyanide and fluoride which present environmental problems. The SPL also contains significant quantities of usable C, Na and Al. SPL became a listed hazardous waste under RCRA (KO88) in July, 1990. Several studies were conducted to explore the possibilities of safe storage, disposal, and recovery of spent potliner. The purpose of the present study is to fix the hazardous SPL in a glassy matrix so as to make it safely disposable. A low viscosity and a low melting point are prime requirements for accomplishing the above. Viscosity measurements were performed, and effect of various additives, such as sand, limestone, dicalcium silicate, MgO baghouse dust, on the viscosity of SPL was studied. The results showed that with the addition of sand and limestone, very high fluidity of SPL melt can be obtained at significantly low temperatures. The glass formation characteristics and the fluoride leachability of these fluxes were also studied. Optimum amounts of additives required to obtain the best glass formation characteristics, the lowest fluoride leachability and the lowest viscosity of melt were determined.

Also, gas and particulate exhaust compositions during high temperature calcination of SPL were determined. The effect of time and SPL particle size on weight loss was determined. The experimentally determined gas and particulate compositions were compared with the calculated equilibrium compositions.


(c) Evaluation of Radionuclides Removal/ Immobilization using Zeolites

The principal objective of this project is to evaluate the environmental problems caused by radionuclides migration at the Nevada Test Sites (NTS) and weapons detonation ares. Adsorption behavior of radionuclides species is needed to judge removal/immobilization of cationic radionuclides contaminants. Experiments on the reactions of rocks, soils and zeolites with radionuclides wastes are conducted to outline kinetic steps and equilibria. These studies include determination of data on solution reactions over the temperatures and concentrations at waste sites, solid phase physical and thermodynamic data, adsorption isotherms, solubilities of radionuclides in silicate-water systems. A mathematical model for the exchange process will be developed. A comparison of estimated concentrations with the experimental measurements will be made. A final assessment will be made for the development of technology for the removal/immobilization of radionuclides and establish recommended procedures for a scaled up use in the field tests.