Why smelters are power hun­gry

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‘‘HE in­her­ent chem­i­cal at­tributes of alu­minium aren’t com­pletely to blame for the en­ergy in­ten­sity of alu­minium pro­duc­tion, ac­cord­ing to a group of re­searchers from Swin­burne Univer­sity of Tech­nol­ogy and CSIRO. Al­though there are sev­eral fac­tors that con­trib­ute to the rel­a­tively high en­ergy con­sump­tion and high cap­i­tal cost per tonne, most of them can be traced back to ma­te­ri­als,’’ they said in a re­cent re­view of chal­lenges in light met­als pro­duc­tion.

Alu­minium is one of the more abun­dant el­e­ments in the earth’s sur­face rocks be­cause alu­minium is stable when com­bined with oxy­gen. Alu­minium’s knack for stay­ing bound to oxy­gen means the Hall-Her­oult process uses about 6 kilo­watt hours of en­ergy to strip oxy­gen away to cre­ate 1kg of alu­minium.

But to cre­ate that kilo­gram, an­other six or seven kilo­watt hours have been used as well, they said, lost to the sur­round­ings as heat. The rea­son for the heat loss is that the molten ma­te­rial used to dis­solve and host the alu­minium ox­ide — called cry­o­lite — is cor­ro­sive at the nec­es­sary tem­per­a­ture of about 960C.

Cry­o­lite will at­tack the inside walls of the Hal­lHer­oult re­ac­tion cell un­less pre­vented. The only cur­rently fea­si­ble way to do this is to freeze the cry­o­lite sit­ting next to the walls. Keep­ing the pe­riph­eral cry­o­lite frozen while the cen­tral por­tion is molten means a lot of heat must be con­tin­u­ally drawn off. The re­sult is high en­ergy con­sump­tion.

The re­searchers said that among CSIRO’s Hal­lHer­oult projects was work on de­vel­op­ing new wall ma­te­ri­als to help ad­dress the prob­lem. Other project work in­cluded try­ing to find ma­te­ri­als that can dis­solve alu­minium ox­ide at lower tem­per­a­tures.

Sarah Belfield

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