Revision as of 20:03, 13 June 2012

Aluminum making

Aluminum making needs huge amount electricity (15kwh/kg), some carbon source to smelter electrodes, little fluoride to lower melting point and lot of pure alumina (Al2O3). It is known Hall process http://en.wikipedia.org/wiki/Hall_process. There are no other more economical process.

Key material to aluminum is ultra pure alumina and cheap energy. There are many process to make pure alumina, we should select some best fit to small scale production. Clays are posible start material or feldspar mineral (weather to clay). Bauxite are rare.

There are some Hall process modifications, like titaniumborate bed help reduce electrode distance (normal 4.5cm = ~4.5vol). Graphite electrodes can maybe changed to hematite electrodes to overcome graphite consumption. There are also method to electrolyse aluminum chloride (no CO2 exhaust but Cl, maybe recycleable to clay extraction)

Diffrent Alumina extraction processes

Acid process from clay

Alumina (alumium ore) extraction from clay. Bauxite are rare minera, clay are common so chemical process based on clay- not bauxite. Aluminumcloride are leached out with hydrochloric acid: http://www.freepatentsonline.com/4388280.html.

- sulphurous acid process
- nitric acid process

Alkali process from clay

- bayer process (very limited, high viscosity if SiO2 present)
- Soda sintering process
- Lime sintering process

Carbon dioxide process from feldspar

- Feldspar mineral (sand) and high pressure carbon dioxide can be used to produce alumina. CaAl_2Si_2O_8 + 2CO_2 +4H_2O -> CaCO_3 + 2SiO_2 + 2Al(OH)_3

High temperature proceses

Kaolin clay is composed of aluminum and silicon (Si2Al2O5(OH)4). Reduction of Kaolin would then result in an aluminum/silicon alloy. In practice, carbides (usually SiC) resist further reduction except at very high temperatures (2050°C). Thus, the smelted product is likely some aluminum-rich alloy with various highly refractory simple and complex carbides. (Halman tests suggest that product are aluminum and silicon carbides, not pure metal/alloy)

Parent material is reduced carbothermically, that is, it is reduced in the presence of carbon and the absence of oxygen at high temperatures. The carbon is oxidized to produce carbon mon- and di-oxide and the metal oxides are reduced (but converted to carbides).

The paper carbothermal reduction of alumina by m.halmann (at) weizmann.ac.il describes the carbothermic reduction of alumina rich material in an atmosphere of methane gas at high temperatures. Internal copy of this paper: File:Halman.pdf (laboratory test at 2200C produced AlC, no test with methane atmosphere calculation show aluminum at 2800K)

Such temperatures could be produced with a point focusing solar collector (see Metal Refining for more)

Page 66 in this online book talks about a plant in the Ukraine that smelts kaolin into an aluminum/silicon alloy: The Handbook of Aluminum by George E Totten, D Scott MacKenzie. (are you sure, it is well know that soviet used kaoline/clays + lime smelt to produce alumina, feeded to normal Hall cells. I don't have such book to check.)

At temperatures below 2000°C, I assume aluminum metal may be separated from silicon carbide (SiC).