Production of pure alumina from iron contaminated sulfate liquors

Alumina is recovered from aluminum sulfate leach liquors by treatment of the liquor with HCl gas to the point of incipient crystallization of AlCl.sub.3.6H.sub.2 O, extraction of the resulting solution with a tertiary amine to remove iron, further treatment with HHl gas to precipitate AlCl.sub.3.6H.sub.2 O and roasting of the AlCl.sub.3.6H.sub.2 O to Al.sub.2 O.sub.3.

The leaching of calcined clay with various mineral acids to dissolve the 
aluminum content is well known. It is also known in the prior art to leach 
uncalcined material with hot concentrated sulfuric acid. The aluminum may 
then be separated from the sulfuric acid leach liquor, which also contains 
iron and other impurities, by sparging with HCl gas, causing AlCl.sub.3 
.multidot.6H.sub.2 O to crystallize. Impurities present in the mother 
liquor may be entrained in the AlCl.sub.3 .multidot.6H.sub.2 O crystals, 
or they may be present as a surface film on the crystals, in which case 
they can be removed by washing. In order to obtain a high degree of 
purity, especially from iron, dissolution of AlCl.sub.3 .multidot.6H.sub.2 
O crystals and recrystallization is necessary. The purified AlCl.sub.3 
.multidot.6H.sub.2 O is roasted to Al.sub.2 O.sub.3 and HCl. 
It is also known in the prior art to purify acid aluminum leach liquors 
from their iron contamination by solvent extraction of the iron. This is 
generally done by using a tertiary amine to extract iron from chloride 
liquors or a primary amine to extract iron from sulfate liquors. Iron 
extraction using a tertiary amine has advantages over using a primary 
amine, e.g., the extractant can be loaded to a higher iron level and phase 
separation is better in a shorter time. However, a tertiary amine will not 
extract iron from a sulfate solution since the iron must be in the form of 
an anionic complex, such as FeCl.sub.4.sup.-, in order to be extracted by 
the tertiary amine. 
It has now been found, according to the invention, that alumina of a high 
degree of purity, particularly with respect to iron, may be obtained by 
means of a process comprising (1) treatment of an aluminum sulfate leach 
liquor with HCl gas to the point of incipient crystallization of aluminum 
chloride hexahydrate, i.e., to the point where further addition of the HCl 
gas would result in crystallization of AlCl.sub.3 .multidot.6H.sub.2 O, 
whereby iron in the solution is converted to an anionic chloride complex, 
(2) extraction of the resulting solution with a tertiary amine to remove 
the iron, (3) further addition of HCl gas to precipitate AlCl.sub.3 
.multidot.6H.sub.2 O, and (4) roasting of the AlCl.sub.3 
.multidot.6H.sub.2 O to Al.sub.2 O.sub.3. 
The feed solution in the process of the invention consists of an 
iron-contaminated aluminum sulfate solution obtained by leaching an 
aluminum-bearing material with sulfuric acid according to conventional 
procedures. Such procedures typically consist of leaching an 
aluminum-bearing material such as clay with hot concentrated sulfuric 
acid, or leaching of calcined clay with concentrated sulfuric acid. The 
resulting solutions typically contain aluminum in an amount of about 10 
g/l to 60 g/l and iron in an amount of about 0.5 g/l to 5 g/l. 
Preferably, the feed solution is initially treated with an oxidizing agent 
to ensure complete oxidation of the iron to the ferric state. The 
preferred oxidizing agent is hydrogen peroxide, but other oxidizing agents 
such as chlorine or oxygen may be employed. Suitable amounts of oxidizing 
agent will obviously depend on the specific oxidant employed, and the 
amount of iron in solution. Generally, about 0.001 to .05 moles of H.sub.2 
O.sub.2 per liter of feed solution is sufficient to completely oxidize the 
iron. 
Gaseous HCl is then added to the feed solution, at ambient conditions of 
temperature and pressure, by any conventional means such as sparging. 
Addition of the HCl is continued to the point of incipient crystallization 
of AlCl.sub.3 .multidot.6H.sub.2 O, where substantially all of the iron 
contaminant is converted to an anionic complex, predominantly 
FeCl.sub.4.sup.-. Optimum amounts of gaseous HCl will depend on the 
specific concentrations of aluminum and iron in the feed solution, as well 
as other specific conditions, such as temperature, pressure, and pH that 
may effect the solubility of the AlCl.sub.3 .multidot.6H.sub.2 O. 
Accordingly, the optimum amount is usually best determined empirically. 
However, a range of about 0.04 to 0.4 moles of gaseous HCl per liter of 
feed solution is generally suitable. 
The HCl-treated feed solution is then extracted with a tertiary amine to 
remove the iron from the aqueous phase. The preferred tertiary amines are 
aliphatic tertiary amines having chain lengths of about 12 to 18 carbons. 
These are generally employed in solution in an inert solvent such as 
kerosene, benzene, or toluene, the concentration of the amine being about 
5 to 20 percent by volume of the solution. A co-solvent consisting of an 
aliphatic alcohol having a chain length of about 8 to 10 carbons may also 
be included in the extractant solution in order to improve extraction 
efficiency and prevent formation of a third phase during the extraction 
procedure. Suitable concentrations of the co-solvent in the extractant 
solution will generally be about 5 to 15 percent by volume. 
The extraction procedure, i.e., contacting of the HCl-treated feed solution 
and the extractant solution, and subsequent separation of raffinate and 
iron-loaded extractant phases, may be by means of any conventional batch 
or continuous process. The optimum amount of extractant employed may vary 
considerably depending on the concentration of iron in the feed solution, 
type of extraction procedure, etc. However, volume ratios of extractant 
solution to feed solution will generally be in the range of about 1:1 to 
1:5. 
The raffinate is then treated, as by sparging, with gaseous HCl at ambient 
conditions of temperature and pressure in order to crystallize high purity 
AlCl.sub.3 .multidot.6H.sub.2 O from the solution. The gaseous HCl is 
added in an amount sufficient to effect substantially complete 
precipitation of aluminum as AlCl.sub.3 .multidot.6H.sub.2 O, which may 
then be roasted to Al.sub.2 O.sub.3 by conventional procedures at a 
temperature of about 1000.degree. C.

The process of the invention will be more specifically illustrated by the 
following example: 
EXAMPLE 
A sulfate leach liquor was produced by leaching calcined clay with 50 
vol-pct H.sub.2 SO.sub.4. The filtered liquor, which contained 48 g/l Al 
and 1.37 g/l Fe, was reacted with a small amount of H.sub.2 O.sub.2 to 
insure complete oxidation of the iron. The liquor was then sparged with 
HCl gas to the point of incipient crystallization. The resulting solution 
was contacted with one-third its volume of a mixture of 15 vol-pct Alamine 
336 (a commercially available long chain tertiary aliphatic amine), 5 
vol-pct decyl alcohol and 80 vol-pct kerosene. The iron content of the 
raffinate was 0.01 g/l. Sparging of the raffinate with HCl gas resulted in 
crystallization of high purity AlCl.sub.3 .multidot.6H.sub.2 O.