While it is generally acknowledged that dross generation should be kept to a minimum, too often, the importance of maximising the aluminium content of the dross is overlooked. Some mistakenly believe that a low metal content is a good thing and that the aluminium is being kept in the furnace. In reality, this metal is most likely being lost due to insufficient cooling and thermiting. Much can be gleaned from looking at the dross that is generated in a casthouse; in fact, the quality of dross can provide a good indication of the overall efficiency of the operation. Even with the very low aluminium prices of today, of circa. US$1400 per tonne, a reduction in dross generation within the furnace can provide huge savings per year. Effective dross management also results in better metal quality, improved fuel efficiency, prolonged refractory life and improved yield in the entire facility. This paper will look at how dross is generated within the furnace in the first place, followed by ways to minimize the dross generation within the furnace using continuous and sub-surface circulation which can also provide significant energy and CO2 reductions. A separate paper will discuss dross processing options and possibilities. In summary, by careful attention to the equipment and process techniques around the furnace and the follow-on dross management significant cost savings and environmental benefits can be realized by cast house operations.
While it is generally acknowledged that dross generation should be kept to a minimum, too often the importance of maximizing the aluminium content of the dross is overlooked. Some mistakenly believe that a low metal content is a good thing and that the aluminium is being kept in the furnace. In reality, this metal is most likely being lost due to insufficient cooling and thermiting. Much can be gleaned from looking at the dross that is generated in a casthouse; in fact, the quality of dross can provide a good indication of the overall efficiency of the operation. Even with the very low aluminium prices of today of about US$1400 per tonne, a recovery improvement of just 3% for a facility producing 500t of dross per month can provide savings in excess of $250.000 per year. Effective dross management also results in better metal quality, improved fuel efficiency, prolonged refractory life and improved profitability in the entire facility. Over the years, as facilities have focused on better dross cooling and handling techniques, dross recoveries have improved. Today, dross recoveries should be in the range of 60 – 70%. These numbers will raise debate but 30 years of experience give us deep insight into these results. The paper looks at the different techniques of handling the dross that is produced within the melting/casting operation with the objective of maximizing aluminium recovery. This paper will consider both the initial dross handling within the cast house but then also how secondary processors should be evaluated to maximize the value of the dross being processed. A company can lose as much dross recovery opportunity here as in their own facility. . In summary, by careful attention to the equipment and process techniques around the furnace and the follow-on dross management, significant cost savings and environmental benefits can be realized by cast house operations.
The handling of dross is an unfortunate but necessary activity in any aluminum cast house. One technology designed to cool dross and maximise metal recoveries is the dross press. Since its introduction to the industry in the mid 1990’s this technology has evolved to meet the demands of the modern cast house and ever more stringent environmental legislation. This paper describes developments that address the weaknesses and limitations of early systems and enhancements that have enabled the cooling of a wide variety of dross types including salt slag from rotary furnaces. The paper will discuss data and results from the first fully commercialized installation of a specially designed press for cooling salt slag from Tilt Type Rotary Furnaces. Finally the paper will look at some of the economical benefits of using a press through various economic models showing the positive effect small improvements in dross recoveries can have on plant profitability.
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