Reduction of chlorine emissions using stack monitoring

Reynolds Metals Company has been awarded a contract by the Department of Energy to investigate techniques to reduce chlorine emissions in the processing of aluminum. One of the unique features of this program is the utilization of an on-line emissions monitor to control the fluxing process. The basic premise is that any chloride reaching the stack indicates either that too much chlorine is being used, or that chlorine is not being injected in an efficient manner. Although researchers have proposed the use of an emissions monitor as a means of closed loop control, this is the first time the procedure has been reduced to practice in the cast shop. The emissions monitor used is a Fourier Transform infrared (FTIR) analyzer. A 25-ton melting furnace and casting complex at the Reynolds Metals Company corporate research center are used in these trials. The furnace has porous plugs in the bottom. Gas fluxing is also done outside the furnace with an efficient rotating spinning degasser. The FTIR analyzer measures emissions from the furnace and the in-line treatment box, and so the efficiency of both injection devices is established. The program consists of four experiments. In each the rate of reduction in sodium, lithium and calcium, the hydrogen content, and the metal cleanliness are measured. The overall goal is to reduce chlorine emission without sacrificing metal quality. In experiment 1 a baseline, is established by melting without the use of chlorine, both in the furnace and in the treatment box. In experiment 2 a two-hour-long flux is used in the furnace by injecting nitrogen-5%Cl-2 via the porous plugs. In experiment 3 the fluxing time in the furnace is reduced to one hour, and chlorine is also introduced into the spinning degasser. In the fourth, and final, experiment no furnace fluxing is used, and all alkali metal reduction is accomplished via the in-line spinning degasser.