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Ind. Eng. Chem. Res. 2004, 43, 5828-5837
Update on Spent Potliners Treatments: Kinetics of Cyanides Destruction at High Temperature Yann Courbariaux, Jamal Chaouki,* and Christophe Guy Gas Technology Research Group, Department of Chemical Engineering, Ecole Polytechnique, P.O. Box 607, Succursale Centre-Ville, Montreal (Quebec), H3C 3A7 Canada
A review of the technologies available for treating the cyanide content of spent potliners is presented. The various strategies of treatment are compared. Processes involving the combustion of spent potliners at temperature close to 800 °C were selected as the most cost-effective approach. It was found that designers of such processes are confronted with a lack of data concerning the kinetics of cyanide destruction of spent potliners and the occurrence of agglomeration. This paper addresses both issues by showing that small amounts of additives are necessary for the smooth operation of combustion processes and by presenting a simple model for predicting the amount of additive needed in well-mixed conditions. In addition, for the first time, the apparent kinetics of the destruction of cyanides in spent potliners at high temperature is presented. Introduction Aluminum is found in nature only in an oxidized form and must be reduced before being processed. The reduction is carried out in cells about 1-1.5 m in height, 5-15 m in length, and 3-5 m in width. The production of aluminum involves alumina as the main source of aluminum. Carbon is the species being oxidized, and carbon dioxide is the main byproduct.
2Al2O3 + 3C f 4Al + 3CO2
(I)
The reduced aluminum metal generated is on the bottom of the cell and is siphoned out on a regular basis.1 The cells are filled with a molten bath of cryolite (Na3AlF6), which can be generated in two different ways
3NaF + AlF3 f Na3AlF6
(IIa)
6NaOH + Al2O3 + 12HF f 2Na3AlF6 + 9H2O (IIb) Alumina is a liquid at temperatures close to 2050 °C. However, operating the process at these temperatures would be too costly. Consequently, the alumina fusion temperature is lowered by using a mixture of cryolite (80-85%) and alumina (2-8%) to form an eutectic melting at 950 °C.1,2 The liquid bath in the reduction cell is submitted to a current between two electrodes, forcing reaction I toward the formation of aluminum. The anode is composed of carbon and hangs above the molten bath. The lower tip of the anode is in contact with the molten bath. As the reaction proceeds, the carbon is consumed, and the anode is regularly lowered into the bath to provide some new carbon for the reaction. The cathode is composed of steel bars protected by a carbon layer. It is not consumed during the reaction; nevertheless, during its lifetime, it undergoes a slow degradation. The content of the bath slowly infiltrates the cathodes and can crystallize, leading to the formation of cracks. As well, secondary products can * To whom correspondence should be addressed. Tel.: (1)514-340-4711. Fax: (1)-514-340-4159. E-mail: Jamal.chaouki@ polymtl.ca.
be generated by reactions involving chemical species adsorbed onto the carbon layer. In addition, nitrogen from the air above the cell can react with the carbon of the electrode and the sodium content of the bath to form some cyanide salts such as NaCN.
2Na + 2C + N2 f 2NaCN
(III)
After 3-8 years, the cathode must be replaced. The discarded cathodes are then given the name of “spent potliners”. The content of spent potliners is highly variable but usually includes aluminum in the 5-20% range, as well as refractory bricks (20-50%), carbon (5%-50%), sodium (7-20%), and calcium (1-3%). This waste is considered hazardous because it contains some highly toxic species such as cyanides (0.1-0.7%), fluorides (20%), and polycyclic aromatic hydrocarbons (PAHs) in high concentration. Moreover, in contact with water, cyanides can react to generate vapors of highly toxic hydrogen cyanide and a caustic solution containing dissolved cyanides. In the meantime, reactions occurring when cyanides make contact with water inside the spent potliners result in emissions of hydrogen and methane, which, in an unventilated area, can lead to explosions. Moreover, on a long-term basis, leaching of fluoride can translate into the contamination of rainwater runoff if left uncontrolled. After numerous studies and contradictory decisions, on Sep 28, 1998, the U.S. Environmental Protection Agency (EPA) ruled to forbid the landfill of spent potliners without adequate treatment. As a result, aluminum producers are faced with the problem of the disposal of huge amounts of spent potliners.3 There are about a dozen aluminum producers in the U.S.; Canada counts 12 producers, 11 of which are in the province of Quebec. On an annual basis, it is estimated that about 800 000 to 1 million tons of spent potliners are produced in the world. Quebec, alone, produces more than 50 000 tons of spent potliners per year. Over the years, most of this waste has been piling up in dedicated warehouses, awaiting treatment. Treatment of spent potliners represents a huge financial burden for most aluminum producers. In 1993, Pawlek evaluated the average cost of treatment per ton of spent
10.1021/ie049775x CCC: $27.50 © 2004 American Chemical Society Published on Web 08/25/2004
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5829 Table 1. Low-Temperature Processes Based on the Leaching of Cyanides and Their Subsequent Treatment
process
particle size (mm)
spent potliner treatment by steps
residence time
conditions
initial composition of cyanides (mass/mass)
efficiency of cyanide removal by steps
Alcan5,7-9
