Note: Descriptions are shown in the official language in which they were submitted.
- 10650~8
The present invention relates to a method of precipitating
aluminium and iron from acid solutions.
Such solutions, for example, may comprise wash solutions ob-
tained from gas cleaning plants or metal recovery plants. When
working-up iron-oxides obtained by roasting pyrites, for example,
in the manner described in Swedish Patent Specification 319 785 or
the Canadian Patent Specification 865 647, by purification through
chlorinating volatilization of the iron-oxides, the outgoing
chlorine-containing gas is washed, for instance, in washing -towers
10 in which the metal chlorides are quantitatively absorbed and dis-
solved in the acid washing liquid.
Acid solutions obtained by treating certain minerals or metal- ;
lurgical products and/or intermediate products hydro-metallurgically
can also be treated according to the inventionO Such acid solutions
may be obtained when, for example, ores, minerals or roasted pro-
ducts are leached, electrolyzed or treated with acids in such
manner that iron and aluminium is dissolved to a greater or lesser
extent together with other valuable metals, or in certain cases ~ -
in which substantially only iron and aluminium are dissolved out.
20 An example of the first type of acid solutions mentioned comprises ~~ -
leaching solutions obtained when working-up metal-containing
laterite ores or shales and residual solutions subsequent to re-
covering metals electrolytically, for example, such as is described
in the Swedish l~atent Specification 7507507-7 (Patent Application
1975-07-01~. An example of the other type of acid solution mention-
ed comprises solutions obtained when digesting or dissolving so-
called red sludge from the Bayer-process for the manufacture of
aluminium and when leaching aluminium-iron-silicate, for example
andalusite and kaoliniteO
30Thus, in respect of solutions of the aforementioned type
containing salts of aluminium and iron there is a need either to ~
separate these elements, so that pure iron and/or aluminium can be ;
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produced as a produc-t, or to conver-t the elements to a form which
will enable -them to be safely deposited.
As will readily be understood, it is necessary to be able to
treat such acid solutions so that they have no harmful effect on
the environment. Normally, -the elements contained in such solu-tions
are precipitated out as a mixed hydroxide precipitate by neutralizing
the solution to a pH lying between 4 and 7, using, for example,
lime-stone, milk of lime (slaked lime), sodium hydroxide or carborate.
In order to ensure that all the iron is precipitated out, it is
necessary to oxidize the iron -to its trivalent state.
Mixed precipitates of aluminium hydroxide and iron hydroxide
are bulky and have a gelatinous and slimy consistency. Consequently
such precipi-tates are difficult to filter and to free from any
liquid enclosed therein or adhering thereto. Because of thisJ such
mixed precipitates are both difficult to work-up and difficult to
dispose of.
It is a well known fact that aluminium hydroxides begin to
precipitate out at a pH of 4~3, using lime sodium hydroxide, etc.,
and that they have precipitated out quantitatively at a pH of 4.8.
By quantitative precipitation is meant here that all aluminium is
precipitated out to the maximum extent permitted by the solubility
product. The corresponding pH-values for the hydroxide precipitation
of tri-valued iron are 2.8 and 3.5. On the other hand, iron (II)
hydroxide will not precipitate ou-t until a pH of approximately 9 is
reached. See, for example, in this respect MODERN KEMI (No. 4)(1973),
page 30.
In many cases it is desirable to use the solution, subsequent
to precipitating out Al and Fe, in a closed hydro-metallurgical
process, for example, by returning residual solution to a leaching
step arranged upstream of the precipitation step. In such instances
a high pH is an obvious disadvantage, since the amount of acid used
during the leaching process will then be unnecessarily high~
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106~68
It has now been surprisingly found that when aluminium and
iron are slectively precipitated out from solutions having a pH
of less than 4, there are obtained precipitates which can be readily
separated by,for example, filtration and worked-up or disposed of
without, unless desired, it being necessary to completely neutralize
the solution or to make the solution basic, i.e. raising -the pH
to abou-t 7 or thereabove.
In accordance with the invention, a reductant is admixed with
the solution in a quantity sufficient to convert any ingoing tri-
valent iron present in the solution to a divalent sta-te by reduction,
whereafter a neutralizing agent is added until a pH which at least -
exceeds 4 is reached so that aluminium hydroxide is precipitated out
quantitatively, whereafter an oxidizing agent is added to the solu
tion so that the ingoing iron is oxidized to a trivalent state and
... ~ . ... .
so that the iron content is caused to precipitate out in a compact,
readily filtered form.
Thus, in accordance with the invention the iron content of the
solution shall be in a divalent state when the aluminium is to be
precipitated out. When the original solution contains ions of tri-
~0 valent ~nn, these are therefore reduced by adding a reductant priorto the precipitation of the aluminiumO Sulphur dioxide or metallic
iron, for example in the form of scrap iron, can be used to advantage
as a reductant. In the case of a sulphate solution, it is also suit-
able to effect the aforementioned reduction by adding a sulphidic
material, such as iron~sulphide for example.
Subsequent to the quantitative precipitation of aluminium from
the solution, the solution is oxidized by adding thereto a gas con-
taining free oxygen, such as air or oxygen gas or some other suit-
able oxidizing agent such as chlorine gas, nitric acid or an agent
which gives off chlorine or oxygen. In certain cases oxidation can ;
be effected, to advantage, by ano~ic oxidation in an electrolysis -
cell. The oxidation is conveniently carried out at ~evated pressure,
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1065068
for example in an autoclave or similar apparatus. During the oxida-
tion process, the iron is converted from a divalent to a trivalent
state.
The iron is caused to precipitate out in a trivalent state in
a compact, readily filtered form in conjunction with the oxidizing
process or subsequent to the said processO The precipitate may have
the form of a complex basic iron sulphate, basic iron oxides or
hematite. The most important parameters for obtaining the desired
precipitate are the pH value of the solution, the amount of iron
contained therein, the precipitation temperature and time.
When alkaliions or ammonium ions are present in the leaching
solution or are added thereto, the iron content of the solution can
be caused to precipitate out at a pH which is higher than 1 and
which is preferably lower than 3, as basic iron sulphate also in-
cluding alkali ions and/or ammonium ions and having a structure
which is principally A-Fe3-(S04)2 (OH)6, where A can be one or more
of the ions Na j K or NH4 . Such a structure coincides with the
mineral jarosite. When precipitating out jarosite, the temperature
is suitably maintained at at least 90G. A lower ^temperature will
extend the precipitation time.
In the absence of alkali ions or ammonium ions, it is also pos-
sible within the same pH range to precipitate the iron out as a ~
precipitate called hydronium-jarosite (corresponding to CarphosidOEite) -
having the same principle structure as that shown above for jarosite
but where A is H30 -ionO
The iron can also be precipitated out in an autoclave at
elevated pressure in the form of hematite. It is disclosed in the
literature, TMS Paper A73-65 (1973), that when oxidizing iron (II)
solutions, hematite can be precipitated out at an oxygen pressure of
20 k,p/cm2 and at a temperature of 200 C in solutionsh~ing a ~nal-acid
content of 60 g/1 H2S04~ -
The iron can also be precipitated out in the form of basic i~n
.: . ,,. ~ ... . .. . .
11065068
oxide having a composition corresponding to goethi-te at a pH
below approximately 6, suitably within a pH range of 2-4. In this
case no extra ~ditons to the solution are required, with the excep-
tion of a neutralizing agent and an oxidizing agent. A suitable pre-
cipitation temperature in this case is 85Co
OH-ions from the solustion ore consumed during -the iron
precipitation process, and hence the pH falls during said process.
During the precipita-tion process, the pH of the solution can be
regulated by adding protolytes, such as a base. When the iron is to
be precipitated ou-t as jarosite or hematite, pH values of the desired
low magnitude can be obtained by adding acid to the solution prior
to causing the iron to precipitate. Thus, the iron can be caused to
precipitate out in the desired form during the entire precipition
process by continuously measuring the pH of the solu-tion during said
process and adjusting the system in accordance with the measurements ,
recorded. -
The aluminium and iron precipltates obtained in accordance
wltn the invention can be readily worked-up and can be safely
disposed of.
When the original solution contains soluble salts of, for
example, potassium, sodium and magnesium in addition to iron and
aluminium, said salts can be readilyrecovered, for example, by
evap~rating the residual solution subsequent to precipitating out
the iron and aluminium therefrom. A residual solution having a low
pH is favourable in respect to the resovery of said salts, it being
assumed that the acidity is derived from hydrochloric acid in
ch~oride solutions and sulphuric acid in sulphate solutions. When,
subsequent to recovering the metal content of the solution, -the re-
sidual solution comprises a sulphate solution which shall be destroyed
in a roasting or combustion furnace and the S02-gas thus formed shall
be utilized, it is advantageous for the solution to be as acid as -
possible(having a high sulphate content or containing free sulphuric
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~ 1065068
acid). In such cases, also jarosite precipitated out of the solution
and containing sulphate ions may be charged to the furnace at the
same time, thereby to split-off and utilize the sulphur content.
