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Metal Casting Technologies : December 2010
16 www.metals.rala.com.au GREEN TECHNICAL FEATURE For case 2 a range of wrought alloy products is produced by direct chill casting (Figure 2). Four DC casting units and 10 furnaces are assumed. All these products are degassed and filtered. Sodium removal is carried out using furnace fluxing with MgCl2. Typical alloy compositions were assumed for the 6xxx alloys i.e. a mix of soft and hard alloys. The generic cast house life cycle systems (shown in Fig. 1 and 2) are considered to be applicable for casting aluminium ingots and extrusion wrought aluminium alloy within cast house. The emissions from production primary aluminium are not counted, as the aluminium is the primary product of the cast house operations. In line with the goal of the study only emissions due to activities to produce aluminium products within the cast house are counted including emissions from consumption of other materials such as lost alloying additions and auxiliary materials like refractory (for making furnaces), steel and iron, etc. are also counted. All energy required for production these materials and energy consumed within cast house are counted, as well as, required water (so called "virtual" water). The only transportation emissions are included only for transportation of aluminium dross to the dross processing plant and transportation back recycled aluminium, as dross processing is included into the system boundary. Assumptions To conduct the study, some simplifications have been made due to availability of data and the complexity of processes itself. It was assumed that all electricity is coming from coal fired power stations. Energy consumption is the only input assumed for recycling the aluminium from dross. Only internal transportation within plant and transportation for dross recycling operations has been taken into account with assumption of transport distance to dross recycling facilities of 300km (two ways) made by road trucks. The following emissions within the study are considered to be negligible: a) emissions due to inputs of auxiliary materials (such as chemicals for water treatment, etc.); and b) emission created from the construction, maintenance and renewal of facilities and equipment, except for refractory for furnace maintenance and replacement of casting moulds. Other assumptions are; 1. Magnesium alloy recovery is 85% and Chinese Pidgeon process magnesium is used. Silicon recovery is assumed to be 95% and 98% recovery is assumed for Mn, Cu and Cr. 2. Cast iron mould life on the ingot conveyors is 18 months. The moulds weigh 50kg and there are 260 moulds per casting line. 3. Siphoning is used to transfer metal from reduction crucibles to the furnaces. 4. Dross generation is assumed to be 1.5% of production for case 2 and 1.0% for case 1. The higher dross amount for case 2 is due to the high magnesium content in the wrought alloys, the additional fluxing operations and the higher scrap remelting operations. See  for typical data. 5. Metal recovery from the dross is 50% of the dross weight i.e. a typical figure reported in the industry. 6. For Case 1 the furnace energy consumption is assumed to be 300MJ/tonne. This would be a reasonable typical figure for the industry  considering a 1% scrap return. The theoretical figure for melting Al is 1.1 GJ/tonne. For Case 2 a higher figure of 500MJ/tonne is assumed because of the higher 7% scrap return and the longer residence in the furnaces due to alloying and fluxing operations. 7. Steel strapping is assumed for the remelt ingot product. 8. In the degassing operations, a typical figure of one litre of argon per kg of melt treated is assumed. 9. Refractory consumption is assumed to be higher for case 2 because of the presence of magnesium in the alloys, more furnaces with more furnace operations and more refractories used in the casting process. Reference  is used as a guide. In computing of the emissions, each process and material involved in production is considered bounded (i.e. treated in Figure 1. Generic "gate-to-gate" operations and material flows for aluminium ingot casting cast house 1. Figure 2. Generic "gate-to-gate" operations and material flow for cast house 2 producing wrought alloy products.