Provided herein are methods for the destruction of cyanide in cyanide-containing waste. In particular, methods for the destruction of cyanide in Spent Potliner are provided.
The Hall-Heroult process for the production of metallic aluminum dates from the 19th century. Many refinements to the process have been made, but processes that use the basic Hall-Heroult process remain the most common processes for aluminum production throughout the world. In these processes, the bottom and internal walls of a cathode of an aluminum pot are formed with a liner of carbon blocks joined by conductive carbonaceous binder and wrapped with refractory firebricks and insulating bricks. The resulting combination is referred to as xe2x80x9cpotliner.xe2x80x9d The insulating bricks and firebricks are composed of material such as silica and alumina (aluminum oxide).
During the production of aluminum, the aluminum reduction pot is filled with a bath of alumina and molten salts. Over the three to seven year life span of an aluminum reduction pot, bath salts migrate into the potliner, thereby resulting in the deterioration and eventual failure of the aluminum cell as a cathode. During its lifespan, a cathodic potliner may absorb its own weight in bath salt materials. The failed potliner material is referred to herein as xe2x80x9cspent potliner.xe2x80x9d
When an aluminum reduction cell is taken out of service, the spent potliner is cooled and fractured to facilitate handling and disposal. The fractured spent potliner is a nonhomogeneous material that contains carbon, silica and/or alumina from the insulating brick and firebricks, aluminum, significant quantities of sodium salts, aluminum salts and oxides, fluoride salts and traces of cyanides. On the average, a large aluminum smelter with a production capacity of 175,000 tons of aluminum per year will produce about 6,000-12,000 tons of spent potliner per year. The quantity of spent potliner generated annually in the United States exceeds approximately 230,000 tons per year.
The first cut of the spent potliner contains the carbon portion of the materials contained inside the two-electrode reduction cell. The second cut, which the U.S. Environmental Protection Agency (EPA) does not list as a hazardous waste, includes the other materials contained in the potliner. The spent first cut of the potliner (hereinafter referred to as xe2x80x9cSpent Potlinerxe2x80x9d) has long been listed as a hazardous waste by the EPA and state environmental authorities based on toxicity and the presence of cyanide complexes. Regulations require treatment of listed Spent Potliner to reduce cyanide concentrations and other constituent compounds before it can be disposed in a landfill.
Because of its cyanide content, its high concentrations of other constituent compounds, and the high volumes of Spent Potliner produced, Spent Potliner presents a potential environmental hazard and a major burden for aluminum producers, who remain ultimately liable for the proper disposition of Spent Potliner.
There are a variety of approaches for reducing the potential toxicities of Spent Potliner. One technique includes combustion or incineration of the Spent Potliner (see, e.g., U.S. Pat. Nos. 4,735,784; 4,927,459; 5,024,822; 5,164,174; 5,222,448 and 5,286,274). Most of these processes result in a product in the form of glassy slag material that still contains some hazardous components.
Other processes include chemical treatment (see, e.g., U.S. Pat. No. 4,113,831). In these processes, the initial Spent Potliner constituents are replaced with compounds that are less toxic, but which still include hazardous components at levels above those established by various environmental authorities.
Another treatment involves pyrohydrolysis of the Spent Potliner in conjunction with the introduction of water to create an off-gas containing fluoride materials present in the Spent Potliner (see, e.g., U.S. Pat. No. 4,113,832). Such pyrohydrolysis techniques may also be used with fluidized bed reactors (see, e.g., U.S. Pat. Nos. 4,158,701 and 4,160,808). These processes tend to produce large volumes of waste material that must be disposed in landfills and that may contain non-leachable hazardous waste.
Other methods for treating cyanide-containing wastes, such as Spent Potliner, can be divided into two groups: methods that destroy cyanide by breaking the ion to form simpler, non-hazardous compounds (such as carbon dioxide and nitrogen gas) by an oxidation or electrolytic decomposition; and methods that reduce the volume of cyanide waste by evaporation, reverse osmosis or ion exchange.
Oxidation has been used in the destruction of cyanide to form carbon dioxide and nitrogen. The process generally is effective in destroying free cyanide and some cyanide complexes. Oxidation of cyanide wastes is achieved through the addition of chlorine under alkaline pH conditions (alkaline chlorination), such as through the addition of sodium hypochlorite or through the addition of ozone (ozonation) or hydrogen peroxide (peroxidation) at elevated temperature and pressure. These processes involve handling large volumes of toxic and/or corrosive chemicals and generally are not effective on wastes that contain complexed cyanides, such as the cyanides present in Spent Potliner generated by many aluminum companies.
Electrolytic decomposition involves passing an electric current through the cyanide solution to break the cyanide ion. The process is effective in the destruction of free and complexed forms of cyanide. Electrolytic decomposition, however, is a very expensive process because of the large amounts of electrical energy consumed and is applicable only where the cyanide concentrations are at a relatively high level. High capital equipment costs are also associated with the process.
Hence, there is still a need to develop methods for treatment of Spent Potliner to produce waste authorized for disposal in a landfill. Accordingly, it is an object herein to provide methods for treatment of cyanide-containing wastes, particularly, Spent Potliner material from aluminum reduction cells, for disposal in a landfill.
It is another object herein to provide methods for the chemical destruction of cyanide (free or as a metal complex), such as that present in Spent Potliner, for disposal in a landfill. It is another object herein to provide methods for the destruction of cyanide (free or as a metal complex) that are safe and cost effective.
Provided herein are methods for treatment of cyanide-containing hazardous waste, particularly metal-containing waste, such as that generated by the aluminum industry. In particular, methods for treatment of Spent Potliner for destruction of cyanide are provided. The methods provided herein involve treating the waste, such as Spent Potliner, with an aqueous oxidizing solution that contains a sufficient amount of oxidizing agent to reduce the cyanide concentrations. The cyanide concentrations can be lowered to levels required by the EPA for landfill disposal. Hence the methods provided herein lower levels to about 590 parts per million (ppm) total cyanide or less and 30 ppm amenable cyanide or less, and preferably to 500 ppm total cyanide or less. These methods advantageously can be performed at ambient temperature and pressure. Typically the reaction is complete within an hour.
The time for completion will vary as a function of cyanide concentration, forms of cyanide present and reagent concentrations. The precise conditions and concentration reagents for particular types of waste and sources thereof may be readily determined empirically based upon the instant disclosure.
The aqueous oxidizing solution preferably contains a hypochlorite, a peroxide or a permanganate, and more preferably the aqueous oxidizing solution contains calcium hypochlorite or sodium hypochlorite, most preferably sodium hypochlorite. The effectiveness of the oxidizing solution is enhanced by adding an agent that increases the oxidation/reduction potential (ORP) of the oxidizing solution to enhance cyanide destruction.
The concentration or amount of oxidizing agent used depends upon the initial concentration of cyanide in the waste, and can be determined empirically or adjusted during the reaction. For example, the progress of a reaction can be assessed by monitoring the oxidation potential of a mixture of the waste, such as Spent Potliner, and aqueous oxidizing solution. The desired oxidation potential of an aqueous oxidizing waste mixture following the treatment of, for example Spent Potliner, is preferably greater than about 400 millivolts (mv). If there is remaining cyanide above the desired level, additional aqueous oxidizing solution is added.
Agents that enhance the ORP of the waste for destruction of cyanide in the waste, such as crushed Spent Potliner, include, but are not limited to, chlorine gas that is bubbled into the solution, hydrogen peroxide, ozone in solution, magnesium chloride, potassium chloride and also potassium permanganate. Magnesium chloride and potassium chloride are preferred; magnesium chloride is most preferred. If permanganate is used, the pH of the solution must be monitored to ensure that it remains alkaline.
A carbonate or other buffering agent can be optionally added for treatment of certain cyanide-containing waste. It has been found that for treatment of Spent Potliner, addition of a carbonate is not necessary.
Of the agents noted above that enhance the ORP, those that enhance the ORP sufficiently to permit destruction of complexed cyanides in the absence of an agent such as a carbonate are preferred. These agents include, but are not limited to, chlorine gas that is bubbled into the solution, hydrogen peroxide, ozone in solution, magnesium chloride and also potassium permanganate.
As used herein, an agent that enhances the ORP of the oxidizing solution is an agent that increases the oxidization strength of the oxidizing solution. Any such agent that achieves this result is contemplated herein. As noted, such agents include, but are not limited to, bubbled chlorine gas in the solution, hydrogen peroxide, ozone in the solution, magnesium chloride and potassium permanganate. Preferred are agents that enhance the oxidation strength sufficiently to destroy complexed cyanides, particularly in the absence of additional agents, such as a carbonate. Such agents include, but are not limited to magnesium chloride.
In an embodiment of the methods provided herein for the treatment of the waste, such as Spent Potliner, a metal chloride is also included in the aqueous oxidizing solution. In particular, the waste, such as the Spent Potliner, is contacted with a mixture containing the aqueous oxidizing solution and the metal chloride or other agent that increases the ORP of the aqueous oxidizing solution in amounts and for a time sufficient to reduce the cyanide levels, preferably to EPA required levels, particularly lower than 590 ppm total cyanide. As noted, the reaction can be performed at ambient temperature and pressure. In addition, the waste, such as the Spent Potliner is preferably crushed, to preferably fine powder containing pieces of a size from about 0.25 inch minus (i.e., less than xc2xc inch) to about 1.25 inch minus, prior to treatment.
The metal chloride is selected from alkali metal chlorides and alkaline earth metal chlorides. The preferred metal chloride is magnesium chloride, and preferably the aqueous oxidizing solution contains magnesium or calcium ions.