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1 mole of aluminum oxide has a molecular weight of 102 grams (Al=27 grams and O=16 grams). It means that for 1000 kg of compound you can get 529 kilograms of aluminum (in pure quality). The ratio is $=54/102=0.529$

For 6.7 tons it means $0.529*6.7=3.544$ tons or $3544$ kg of aluminum you can get.