Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Iron group metal
Reexamination Certificate
1999-10-20
2001-08-07
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Iron group metal
C423S140000, C423S142000, C423S050000
Reexamination Certificate
active
06270738
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for preparing usable products, in particular a water treatment solution which contains ferric iron, from an impure ferric sulfate solution, in particular a ferric sulfate solution formed in the process for preparing hydronium jarosite.
STATE OF THE ART
The applicant's earlier patent application WO 97/38944 discloses a process for preparing a pure product which contains ferric iron. In this process, the initial substance is hydrous ferrous sulfate produced as a by-products in the process for preparing titanium dioxide. The ferrous sulfate is oxidized with oxygen in a pressurized vessel at an elevated temperature. During the oxidation the ferrous salt dissolves in its own crystal water, and simultaneously hydronium jarosite begins to precipitate. The reaction equation is
5 FeSO
4
×H
2
O+11/4 O
2
→H
3
OFe
3
(OH)
6
(SO
4
)
2
(s)+Fe
2
(SO
4
)
3
(1)
During the oxidation step, an equilibrium forms between the ferric sulfate solution and the hydroniumjarosite. At this time approximately one-half of the iron is in the solution and the other half is in the form of precipitated hydroniumnjarosite. The solid jarosite is filtered out, and it is used for the preparation of pure water treatment chemicals. A solution containing ferric sulfate is left. The use of this solution has proved to be problematic, since it forms in a considerable amount in proportion to the amount of hydroniumjarosite. Its storage is cumbersome and expensive. There is the additional difficulty that the solution contains most of the impurities of the original ferrous sulfate, in particular manganese.
OBJECT OF THE INVENTION
The problem is thus the further treatment of the impure ferric sulfate solution and its potential exploitation. It is therefore an object of the invention to find a method for the exploitation of the impure ferric sulfate solution in such a manner that no materials to be stored and/or eliminated are left from the process for the preparation of hydroniumjarosite. It is thus an object to improve the profitability of the process for the preparation of hydronium jarosite.
INVENTION
According to the invention, there is provided a process for preparing a usable product, in particular a water treatment solution which contains ferric iron, from an impure ferric sulfate solution which contains at least one other metal as an impurity, the process being characterized in that it comprises a first precipitation step, in which a base is added to the said impure ferric sulfate solution in order to raise the pH to approx. 2-5, preferably approx. 3-4, whereupon ferric hydroxide precipitates; following the first precipitation step, a second precipitation step, in which there are added to the solution an oxidant and a base in order to raise the pH to approx. 6-10, preferably approx. 8-9, whereupon the said impurity metal precipitates; and one or more separation steps for separating from the sulfate solution the solids precipitated during the first and the second precipitation steps; and possibly an additional step, in which the said separated solids, or a portion thereof, are treated further in order to form a usable product.
In the process according to the invention there is preferably used as the initial substance an impure ferric sulfate solution which is formed as a by-products of the process for the preparation of hydroniumjarosite and which contains as an impurity manganese, which precipitates as manganese oxide in the second precipitation step.
Thus the process according to the invention comprises two separate precipitation steps, ferric hydroxide being precipitated in the first precipitation step and the impurity metal, such as manganese dioxide, being precipitated in the second precipitation step.
Alongside or in addition to manganese the impurity metal may be, for example, nickel and/or zinc. At the pH concerned, these impurity metals precipitate as hydroxide.
According to one embodiment of the invention, the base added to the first precipitation step is added in the form of a solution or an aqueous suspension, which contains in part or entirely the sulfate solution obtained from the separation step. In this manner the amount of water circulating in the process can be regulated and thereby the concentration of the sulfate solution can be affected.
The base may be MgO, Mg(OH)
2
, MgCO
3
, NH
3
, NaOH, KOH. In this case the said sulfate solution respectively contains soluble MgSO
4
, (NH
4
)
2
SO
4
, Na
2
SO
4
or K
2
SO
4
.
One usable magnesium-containing base is an aqueous suspension prepared from calcined and ground magnesite. Another option is a suspension which contains magnesium hydroxide. As was pointed out, the suspending can be carried out in water or in the sulfate solution obtained as the final product of the process, or in a suitable mixture of these. Thus there is obtained a basic suspension, which is added to the ferric sulfate solution in order to precipitate iron. The pH of the ferric sulfate solution is originally approx. 1, and in the first precipitation step it is raised to the range of approx. 2-5, preferably approx. 3-4, at which iron precipitates in the form of ferric hydroxide. When magnesium oxide is used as the base in the first precipitation step, there thus occurs the reaction
The precipitated ferric hydroxide is in an amorphous state, in which state its solubility in acids is at its best.
In the second precipitation step, the pH of the solution is raised to approx. 6-10, preferably approx. 8-9, by adding a base, such as NaOH, KOH, Na
2
CO
3
or NH
3
, to the solution. In addition, an oxidant, such as H
2
O
2
, is added to the solution, whereupon the Mn
2+
oxidizes and precipitates as manganese dioxide. Thus the reaction occurring in the second precipitation step is:
Mn
2+
+2 H
2
O
2
→MnO
2
+2 H
2
O
As the oxidant it is possible to use oxygen oxidants such as hydrogen peroxide, other peroxy compounds, or ozone. In addition it is possible to use chlorine, chlorine dioxide, chlorite, hypochlorite or chlorate.
According to one embodiment of the invention, the process comprises a joint separation step, in which the solids formed in both the first and the second precipitation steps are separated together from the sulfate solution.
According to another embodiment of the invention, the process comprises a first separation step, in which the solids precipitated in the first precipitation step are separated from the sulfate solution, and a second separation step, in which the solids precipitated in the second precipitation step are separated from the sulfate solution.
The solids precipitated in the first and second precipitation steps are preferably separated by filtration, but also other separation methods can be used.
The filtrate is a Mg sulfate solution the Mg content of which ranges from 0.5 to 5%, depending on the composition of the basic suspension of the first step. Such a solution is suitable for use, for example, for the bleaching of cellulose. If the Mg content of the solution is sufficiently high, the magnesium sulfate can be separated from the solution by crystallization. Crystalline magnesium sulfate can be used, for example, as a trace element in fertilizers.
The separated solids contain as the principal component a ferric hydroxide precipitate which can be used for preparing an iron chemical suitable for the treatment of waste water. Since ferric hydroxide is a solid product, it can easily be transported to distant places, where it is dissolved in an acid in order to prepare a solution chemical suitable for waste water applications. Nitric acid is especially suitable. Ferric nitrate solutions are suitable for uses in which the water chemical is required to have oxidizing properties, for example, in the deodorization of waste waters which contain hydrogen sulfide.
In that embodiment of the process according to the invention which contains a joint separation step, the use of nitric acid is advantageous also for the reason that manganese dioxide present in
Jafverstrom Stefan
Jokinen Simo
Kenakkala Timo
Bos Steven
Greenberg & Traurig, LLP
Kemira Chemicals Oy
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