Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Reexamination Certificate
1999-02-22
2001-02-20
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
C588S253000, C588S253000, C588S253000, C252S182320
Reexamination Certificate
active
06190626
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydrometallurgical process for detoxifying spent aluminum potliners. The process destroys hazardous cyanides and polynuclear aromatics and converts hazardous fluorides to insoluble florspar to produce a solid waste that can be disposed of in a landfill.
BACKGROUND ART
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. (The production of aluminum generates about 35 kg of spent potliner per ton of metal.) Spent potliners are currently classified in the U.S. as hazardous waste. They contain significant concentrations of four soluble pollutants, namely, cyanides, fluorides, polynuclear aromatic hydrocarbons (PNAs), and heavy metals (such as lead, beryllium, and cadmium).
A treatment process for spent potliners must be technically feasible at a reasonable cost (with reasonable capital cost). It must produce disposable waste products that comply with all environmental control regulations. It must be robust to tolerate variations in the feed because spent potliners from different sources differ significantly in their makeup (that is, the process should be able to accept widely differing feedstreams rather than be limited to a particular feedstock). All spent aluminum potliners require treatment prior to landfill disposal, so aluminum producers are struggling to find an acceptable treatment process.
Reynold's proposed using a thermal treatment process involving high temperatures in a rotary kiln with the addition of sand and limestone to complex with the fluorides. Comalco proposed 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 introduced ground potliner to a hot airflow vortex rotating at supersonic speeds to destroy the cyanides while reacting the fluorides with an additive to produce a disposable solid waste.
Details for two other spent potliner treatment processes are presented in:
(1) J. Bernier et al., “The LCLL Process - Spent Potlining Recycling Solution,” 52nd Annual Conf. of Metallurgists, Quebec, Canada, Aug. 28-Sep. 2, 1993; and
(2) R. Adrien et al., “A Process for Treatment and Recovery of Spent Potliners,” Light Metals 1996, The Minerals, Metals, & Materials Soc. (1996) 1261-1263.
I incorporate these articles by reference. The “LCLL process” is Alcan's Low Caustic Leaching and Liming process which involves three steps that require the use of complicated reactors and associated transfer equipment First, the LCLL process digests (leaches) finely ground spent potlining in a dilute caustic solution at around 85° C. for about one hour. Then, Alcan adds NaOH in a plugflow reactor at about 180° 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 LCLL process requires a significant capital expenditure for the processing equipment.
The Adrien process uses five stages to recover calcium fluoride, aluminum fluoride suitable for smelting, and a disposal, carbonaceous, solid waste from the spent potliner. Adrien leaches the crushed potliner, washes the solid residue with two acid washes using NH
4
F or another fluoroacid, and, finally, uses a water wash.
SUMMARY OF THE INVENTION
The method of the present invention is a hydrometallurgical process that detoxifies spent aluminum potliners to produce a solid waste suitable for landfill disposal. The process destroys hazardous cyanides and polynuclear aromatics and converts hazardous fluorides to insoluble florspars in a single charge to a reactor involving calcium and iron processing stages. The method initially reacts crushed potliners with a calcium chloride/hydrochloric acid (CaCl
2
/HCl) leach mill solution at elevated temperatures and pressures. Then, the method involves oxidation of the PNAs with a strongly acidic FeCl
3
leach mill solution in an oxygen-containing environment at elevated temperature and pressure. By “strongly acidic,” I mean at a pH of less than or about 2.0, and, generally, at a pH of about 0.2-0.5 or lower. The reactions preferably occur at 90-120° C. and 70-90 psi, as described in greater detail in the Detailed Description. Following completion of the leach, I adjust the pH of the reaction liquor (with calcium additions, generally) to precipitate the iron at about pH 3.2. These reactions produce a disposable solid waste and a recyclable reaction liquor (the leach mill solution).
Detoxifying spent potliners removes hazardous metals and polynuclear aromatics (PNAs) to allow the landfill disposal of a solid waste residue. I also believe that it destroys cyanides by converting then to carbon dioxide and nitrogen. The method, preferably, comprises (a) crushing the spent potliners; (b) hydrometallurgically processing (i.e., leaching) a slurry of the crushed potliners in a reactor to place the metals in solution, to complex the fluorides as florspar, and to decompose the polynuclear aromatics (PNAs) and cyanides; (c) precipitating the metals from the solution to form a solid waste in the reactor; (d) separating the precipitated solid waste from the solution; and (e) disposing of the solid waste in a landfill. I add an effective amount of iron to the crushed potliners, if they lack iron, having found that iron will destroy the PNAs in my acidic leach conditions. I carry out the process in successive stages in the same reaction vessel. I treat the reaction liquor further to remove heavy metals and recycle it as a leach mill solution for forming the reaction slurry with fresh potliner, thereby minimizing the waste products and making the process economically and environmentally attractive.
In some cases, I complete the process by washing the solids from the reactor with one or more applications of acetic acid to ensure the complete destruction of the PNAs.
At the same time I destroy the cyanides and convert the complex fluorides to insoluble florspar by reacting the slurry of the finely crushed potliner (minus ⅛ in) in a calcium chloride/hydrochloric acid leach mill solution at a pulp density of about 30-40%. I make the slurry as thick as possible while allowing it to be readily pumped and processed; 40% pulp density is the maximum density I normally use. I add iron powder to the slurry at a concentration of about 5-10% by weight based on the weight of the potliner. I initially oxidize the slurry in a titanium pressure vessel for about one hour at a pH of about 6.0 and at about 90° C. and 70 psig. At the conclusion of this initial oxidation, the cyanides are destroyed (they enter solution during the ball milling of the slurry so that the solid waste even at that stage is essentially free from cyanides), and the fluorides are fixed with calcium in an insoluble waste.
The cyanides enter solution, I believe, during ball milling of the potliner with a calcium chloride solution (pH 7-12) without giving off cyanide gas. With PNAs present in the potliner (which is common), I must lower the pH of the milled potliner with sulfuric acid to carry out a combination reaction at about 120° C. and 90 psi in the presence of iron to destroy the PNAs and cyanides.
Then, I lower the pH in the same reactor following the initial oxidation usually by adding H
2
SO
4
(or another suitable acid) to achieve a pH of at least about 0.2 or below. The H
2
SO
4
reacts with CaCl
2
to produce HCl and hydrated gypsum (CaSO
4
). The gypsum precipitates and maintains a negative water balance for the reaction (i.e., water is consumed in the gypsum hydrate so that liquid wastes are not created in
Bos Steven
Hammar John C.
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