Patent Number: 056132441
Section: summary

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to vitrifying or melting liquid wastes for which additional materials are needed to form a desired glass or slag composition, and more particularly to a process for vitrifying low-level radioactive high-sodium liquid wastes. BACKGROUND OF THE INVENTION Vitrification or melting of liquid Bastes requires that other materials be added to the waste, so that upon melting, a glass or slag material is formed that is resistant to natural forces such as leaching, decrepitation, and abrasion. These additional materials constitute a significant proportion of the final form, usually in the range of 70 to 80 percent by weight. The appropriate glass or slag formers, which are well known to those experienced in the art consist of metal oxides, such as boric, calcia, alumina, silica, magnesia, and others, such as titania and zirconia, to achieve special properties. Previous to the present invention, waste processors would feed glass or slag forming minerals and low-level radioactive high-sodium liquid wastes directly into the melting furnace for vitrification. This seemingly simpler procedure results in the formation of large volumes of gases containing nitrogen oxides formed by thermal decomposition of nitrates and nitrites in the waste. Nitrogen oxides pose a significant health hazard, and the gas thus generated must be treated to remove them. The present invention provides an improved technology wherein nitrates and nitrites are decomposed into nitrogen gas in a separate operation, and dry feed materials are processed by the melting furnace. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a process for preparing radioactive and other hazardous liquid wastes for treatment by the method of vitrification or melting. DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of the present process involves the following steps: 1. Mixing of finely divided dry material including glass-forming minerals, binders to impart physical strength to intermediate product pellets, and reductants to decompose nitrogenous species in the liquid waste. 2. Pelletizing of mixed dry materials with water to form wet pellets, bricks, briquettes, plates, extrudates, or other shape by conventional methods including mixing, rolling, compacting, extruding (ring pelletizer), agglomerating (disc pelletizer), or other technique. 3. Heating to 50.degree. to 120.degree. C. to remove free moisture and to form dry physically strong intermediate product pellets with capacity to absorb liquid waste. 4. Exposing dry pellets to liquid waste by spraying, dipping, or other means to prepare loaded pellets. Proportions are determined by the extent of waste loading desired in the final waste form and the concentrations of components within the liquid waste. Proportions appropriate to treat the subject liquid waste are cited in examples 1, 2, and 3. 5. Heating loaded pellets to 50.degree. to 120.degree. C. to remove free moisture and further heating to 150.degree. to 450.degree. C. to induce reaction between reductants in the pellets and nitrogenous species in the liquid waste to prepare a dry homogeneous product suitable for melting. The invention specifically is applicable to the low-level radioactive high-sodium liquid wastes, such as those currently stored in underground tanks at the Hanford Nuclear Reservation in Washington State, but it also is applicable to the vitrification or melting of other liquid wastes requiring the addition of glass-forming materials such as Hanford site high-level liquid wastes and liquid wastes hazardous by virtue of contained heavy metals and other RCRA-listed materials. However, it is understood that the invention is broad in scope and is neither dependent upon addition of materials to react with the wastes nor chemical reaction of materials within the substrate. That is, the substrate may function only as a carrier for the appropriate hazardous component or components in the liquid waste.