The present invention relates to a process for treating complex cyanide, especially cyanide associated with spent aluminum potliners.
Aluminum potliners are created in the smelting of aluminum metal and its alloys. They are the liners of the pots into which the molten aluminum is poured. A typical liner lasts about five years. The Pacific Northwest aluminum producers currently annually produce about 50,000-75,000 tons of spent potliners, which are currently classified in the U.S. as hazardous waste. They contain significant concentrations of several soluble pollutants, namely, cyanides, fluorides, and heavy metals (such as lead, beryllium, and cadmium). They may also include polynuclear aromatic hydrocarbons (PNAs).
U.S. patent application Ser. No. 09/254,000 (which I incorporate by reference) describes a treatment process for spent potliners that is technically feasible at a reasonable cost (with reasonable capital cost). That process produces disposable waste products that comply with all environmental control regulations. It is robust and able to tolerate variations in the feed because spent potliners from different sources differ significantly in their makeup (that is, the process can accept widely differing feedstreams rather than requiring a particular feedstock). The process initially reacts crushed potliner with a calcium chloride/hydrochloric acid (CaCl2/HCl) leach mill solution at elevated temperatures and pressures. Then, it oxidizes the PNAs with a strongly acidic FeCl3 leach mill solution in an oxygen-containing environment at elevated temperature and pressure. Cyanides are destroyed and complex fluorides converted to insoluble florspar by reacting a slurry of finely crushed potliner (xe2x85x9 inch minus) in the calcium chloride/hydrochloric acid leach mill solution at a pulp density of about 30-40%.
Comalco treats potliners using a two-step process involving a calciner to complex the cyanides followed by a hydrometallurgical step to recover the fluorides. Pechiney""s SPLIT process introduces ground potliner to a hot airflow vortex rotating at supersonic speeds to destroy the cyanides followed by reacting the fluorides with an additive to produce a disposable solid waste.
The xe2x80x9cLCLL process, xe2x80x9d Alcan""s Low Caustic Leaching and Liming process, digests (leaches) finely ground spent potliner in a dilute caustic solution at around 85xc2x0 C. for about one hour. Then, Alcan adds NaOH in a plugflow reactor at about 180xc2x0 C. and 160 psig to destroy the cyanide in the leach solution in about one additional hour of processing while producing sodium fluoride. Finally, Alcan adds more caustic (generally, lime) to the remaining fluoride liquor for a third hour of treatment in equipment comparable to aluminum smelter wet scrubbers to produce calcium fluoride and a recyclable, caustic leach solution.
The Adrien process for treating spent potliner uses five stages to recover calcium fluoride, aluminum fluoride suitable for smelting, and to leave a carbonaceous, solid waste. Adrien leaches the crushed potliner, washes the solid residue with two acid washes using NMF or another fluoroacid, and, finally, uses a water wash.
The process of the present invention destroys complex cyanides effectively and efficiently. In air at atmospheric pressure an effective amount of chloride salt solution, especially calcium chloride, magnesium chloride, or a mixture of these salts, and sodium or calcium hypochlorite (i.e., household bleach) react within about one hour with complex cyanide, such as cyanide complexed with iron or copper as is common with aluminum spent potliner, to reduce the cyanide. Optionally (and typically), the process also uses an effective amount of calcium carbonate as a source of carbon dioxide to accelerate the reaction, as illustrated in the formulae below. I believe that the following reactions are involved with my preferred process (using calcium salts for purposes of illustration):
CaCl(OCl)+CO2+H2O=CaCl(HCO3)+HClOxe2x80x83xe2x80x83(1)
HClO=HCl+Oxe2x80x83xe2x80x83(2)
2HCl+CaCO3=CaCl2+CO2+H2Oxe2x80x83xe2x80x83(3)
The combination of ingredients and reactants causes the cyanide to oxidize. I have not been able to destroy the cyanides in potliner using chloride salt solution alone or the hypochlorite alone, at least when the cyanide is bound with iron or copper. The chloride salt and hypochlorite together, however, provides an effective environment for destruction of the cyanides. The calcium carbonate forms carbon dioxide that appears to accelerate the destruction of the cyanide.