How to obtain aluminum – main methods

History and methods of producing aluminum

[Deposit Photos]

Alu­minum is a met­al that is used in many spheres of hu­man ac­tiv­i­ty. This el­e­ment is wide­spread in na­ture, but can­not be found in pure form. This is be­cause alu­minum has a high chem­i­cal ac­tiv­i­ty and can en­ter into chem­i­cal re­ac­tions with var­i­ous el­e­ments. Ow­ing to this fea­ture, ob­tain­ing free alu­minum is a la­bo­ri­ous and com­plex process. Let’s ex­am­ine it in de­tail.

Raw ma­te­ri­al for pro­duc­ing the met­al

Alu­minum is pro­duced from baux­ites, which are found in sig­nif­i­cant lay­ers on the earth’s sur­face, and con­tain around 50% of alu­minum ox­ides and ox­ide hy­drates. The chem­i­cal com­po­si­tion of baux­ites is com­plex; this alu­minum ore con­tains around 20% sil­i­con earth, from 30% to 70% alum shale, and up to 10% ti­ta­ni­um ox­ide. Iron ox­ides in baux­ites may make up from 2% to 50%. Alum shales, which con­tain alu­minum ox­ide, con­sist of kaoli­n­ite, corun­dum and alu­minum hy­drox­ide.

[Deposit Photos]

Since re­cent times alu­minum has been pro­duced from nephelines and alu­nites, con­sist­ing of sil­i­con, potas­si­um ox­ide and sodi­um ox­ide. To pro­duce one ton of pure alu­minum, two tons of alum shale must be pro­cessed. To pro­duce two tons of alum shale, 4.5 tons of baux­ite must be pro­cessed.

Sup­plies of baux­ites on Earth are rel­a­tive­ly scarce. Coun­tries with large baux­ites fields can pro­duce alu­minum in in­dus­tri­al scale – Brazil, Aus­tralia, Guinea, Chi­na, In­dia, Tur­key, Greece, Suri­nam, Venezuela, Ja­maica and Rus­sia.

How the pro­duc­tion of alu­minum was born


The first sci­en­tist to iso­late alu­minum in free form was the Dan­ish physi­cist Ørst­ed – in 1825. Ørst­ed car­ried out a chem­i­cal re­ac­tion of an amal­gam of potas­si­um and alu­minum chlo­ride. Two years lat­er the Ger­man physi­cist Wöh­ler re­placed the potas­si­um amal­gam with metal­lic potas­si­um. As potas­si­um is an ex­pen­sive ma­te­ri­al, which it is not prof­itable to use in in­dus­tri­al pro­duc­tion, sci­en­tists looked for a re­place­ment for this el­e­ment. In 1854, the French sci­en­tist Dev­ille be­gan to use sodi­um, and also a durable dou­ble chlo­ride of sodi­um and alu­minum.

Lat­er, pro­duc­tion of alu­minum was car­ried out us­ing mag­ne­sium from melt­ed cry­o­lite – this tech­nol­o­gy was in­vent­ed by the Rus­sian sci­en­tist Beke­tov, but was first used in in­dus­tri­al pro­duc­tion by the Ger­mans in the 1880s. In Ger­many, the first fac­to­ry for the pro­duc­tion of alu­minum was built, but pro­duc­tion of the met­al re­mained very ex­pen­sive.

In 1886, alu­minum was first pro­duced us­ing the elec­trol­y­sis re­ac­tion. This method was patent­ed prac­ti­cal­ly si­mul­ta­ne­ous­ly by two sci­en­tists – Héroult from France and Hall from the USA.

Hall-Heroult process for producing aluminium [Wikimedia]

Both physi­cists pro­posed to dis­solve alu­minum ox­ide in melt­ed cry­o­lite, af­ter which the alu­minum was to be pro­duced by elec­trol­y­sis. Thanks to Héroult and Hall, in 1886 the era of the in­dus­tri­al pro­duc­tion of alu­minum be­gan. Click here to find out more in­ter­est­ing facts about alu­minum.

Tech­nol­o­gy of alu­minum pro­duc­tion - stages

In the 21st cen­tu­ry, the tech­nol­o­gy of alu­minum pro­duc­tion de­vel­oped in 1886 is still used. The process in­cludes three stages: first alu­minum ox­ide is ob­tained from nephelines or baux­ites, then in­dus­tri­al alu­minum is ex­tract­ed from it, with a lev­el of pu­ri­ty of 99.5%. This is in­suf­fi­cient for some in­dus­tri­al pur­pos­es, so in the last stage the met­al is pu­ri­fied to 99.99%.

Ob­tain­ing alu­minum ox­ide


Alu­minum ox­ide can be ob­tained by three meth­ods:

  • elec­trolyt­ic;
  • acidic;
  • al­ka­line.

The third method is the most wide­spread, which was im­proved and elab­o­rat­ed on from the mo­ment it was in­vent­ed in the 18th cen­tu­ry. Baux­ite is crushed with lime and caus­tic al­ka­li, then melt­ed in an au­to­clave at a tem­per­a­ture of 250 °С, as a re­sult of which the chem­i­cal break­down of baux­ite takes place, and sodi­um alu­mi­nate forms. The ob­tained sub­stance is di­lut­ed with an al­ka­line so­lu­tion and sep­a­rat­ed from sludge, and rinsed in drain­ers. Af­ter sodi­um alu­mi­nate breaks down, it is pumped through fil­ters and mixed, adding sol­id alu­minum hy­drox­ide as a seed into the re­main­ing alu­minum hy­drox­ide residue from the so­lu­tion. Then the alu­minum hy­drox­ide is heat­ed to ob­tain alu­minum ox­ide. This is the first stage for ob­tain­ing metal­lic alu­minum.

Elec­trol­y­sis of alu­minum ox­ide

The re­ac­tion of elec­trol­y­sis is car­ried out in a spe­cial bath with an elec­tric cur­rent, lined with hy­dro­car­bon blocks. Coal an­odes are im­mersed in the bath, which burn with the re­lease of pure oxy­gen from the alu­minum ox­ide, and form car­bon monox­ide and diox­ide.

Elec­trol­y­sers are grad­u­al­ly in­clud­ed in the elec­tri­cal chain, form­ing a se­ries. An­odes can be cal­ci­nat­ed from large coal block with a mass of over one ton, and self-cal­ci­nat­ing, con­sist­ing of coal bricks in an alu­minum shell, which are baked at high tem­per­a­tures in the elec­trol­y­sis process. From the alu­minum ox­ide dis­solved in an al­loy with a cry­o­lite base, the liq­uid met­al set­tles on the coal base of the bath. This is alu­minum in the form in which it is used in in­dus­try.

Ob­tain­ing alu­minum of high pu­ri­ty


In or­der to ob­tain pure alu­minum, in­dus­tri­al alu­minum must be ad­di­tion­al­ly pu­ri­fied – shaped cast­ings are mold­ed from the ob­tained prod­uct. First in­dus­tri­al alu­minum is sort­ed, then melt­ed down in a fur­nace. Met­als with a high­er melt­ing point than alu­minum re­main in the fur­nace – they may be nick­el or iron. Ni­tro­gen or chlo­rine is bub­bled through molten alu­minum, re­mov­ing non-metal­lic in­clu­sions from it. To re­move low-melt­ing metal­lic mix­tures, al­loys of mer­cury, zinc or mag­ne­sium are used. Mag­ne­sium can be re­moved from the al­loy by chlo­rine. To ob­tain a cast al­loy of alu­minum, ad­di­tives are in­tro­duced that de­ter­mine its com­po­si­tion.