DE102014018131A1 - Process for the further processing of iron sulphate heptahydrate - Google Patents

Abstract

The invention is directed to a process for further processing iron sulfate heptahydrate into iron sulfate monohydrate. The process comprises the steps of: a) forming an aqueous solution or suspension of iron sulfate heptahydrate in a container (mixture I), b) conveying mixture I into a pressure vessel and heating the mixture (I) to a temperature above that Boiling temperature of the mixture (I) is at atmospheric pressure, forming iron sulfate monohydrate as a solid and a solution (II), c) separating the iron sulfate monohydrate solid from solution (II). Optionally, the remaining iron content in solution II is further recovered by reacting solution II in an oxidation reactor with air or oxygen and optionally additives and forming iron oxide and iron hydroxide in solid form.

Description

Field of the invention

The invention is directed to a process for the further processing of iron sulphate heptahydrate into iron sulphate monohydrate and optionally to iron oxides or iron hydroxides.

Technological background of the invention

In the production of titanium dioxide by the so-called sulfate process as well as the pickling of ferrous metals, large quantities of iron sulfate are produced. Ferrous sulphate is used in various fields, such as water treatment in sewage treatment plants, as a reducing agent for chromate in cement or as a raw material for iron oxide production.

In principle, iron sulfate is available from technical processes in 3 hydrate stages, as hepta-, tetra- or monohydrate. In the mentioned titanium dioxide production by the sulphate process and in the pickling of ferrous metals, the iron sulphate accumulates mainly as heptahydrate. Ferrous sulfate heptahydrate has the highest water content of the three hydrate stages and can be stored poorly at temperatures above 25 ° C, because it melts in its own water of crystallization. In the chromate reduction in cement, it shows less effectiveness than, for example, tetrahydrate and monohydrate.

Therefore, various processes have been developed to further process the iron sulfate heptahydrate to lower hydrate levels.

The conversion of Heptahydrats to tetrahydrate or monohydrate is nowadays industrially predominantly by thermal drying. The disadvantage of this method, however, is a relatively high energy consumption, since due to the heat yield of about 50% at least twice the heat of vaporization of the water of crystallization to be removed.

In the likewise energy-intensive cooling crystallization remain after separation of the iron sulfate due to the solubility curve in the system iron sulfate water at least 13 wt .-% iron sulfate in the solution (mother liquor).

Alternatively, a melting process according to DE 1 230 409 be applied, in which the iron sulfate heptahydrate is heated up to 95 ° C, so that it melts in its water of crystallization and iron sulfate monohydrate precipitates and is separated. This process is more energy-efficient than the thermal drying, but remain in accordance with the solubility in the system iron sulfate-water about 25 wt .-% iron sulfate in the mother liquor.

There is thus a need for a process for the further processing of iron sulfate heptahydrate, which overcomes the disadvantages of the prior art.

Task and brief description of the invention

The invention has for its object to provide a method for further processing of iron sulfate heptahydrate to iron sulfate monohydrate and optionally other iron compounds, which is energy efficient and is characterized by an increased recovery of iron compounds.

• a) Bilden einer wässrigen Lösung bzw. Suspension von Eisensulfat-Heptahydrat in einem Behälter (Mischung I),
• b) Fördern von Mischung I in ein Druckgefäß und Aufheizen der Mischung (I) auf eine Temperatur, die über der Siedetemperatur der Mischung (I) bei Atmosphärendruck liegt, wobei sich Eisensulfat-Monohydrat als Feststoff und eine Lösung (II) bilden,
• c) Abtrennen des Eisensulfat-Monohydrat-Feststoffs aus Lösung (II).

The object is achieved by a process for the further processing of iron sulfate heptahydrate characterized by the following process steps:

• a) forming an aqueous solution or suspension of iron sulfate heptahydrate in a container (mixture I),
• b) conveying mixture I into a pressure vessel and heating the mixture (I) to a temperature which is above the boiling point of the mixture (I) at atmospheric pressure, forming iron sulfate monohydrate as a solid and a solution (II),
• c) separating the iron sulfate monohydrate solid from solution (II).

Further advantageous embodiments of the invention are specified in the subclaims.

characters

1 Solubility curve of iron sulphate in water (after: Gmelin's Handbook of Inorganic Chemistry, System-No. 59 "Iron", Part B-Delivery 2, 8th ed. 1930, p. 401 )

2 : Schematic flow diagram of the process according to the invention

Description of the invention

All the details disclosed below with regard to temperature, pressure, concentration in% by weight, etc. are to be understood to mean that all values which are within the range of the respective measuring accuracy known to the person skilled in the art are included.

The inventive method is based on the finding that the solubility of iron sulfate in water above a temperature of about 60 ° C decreases. Studies in the context of the invention have shown that at a temperature above about 135 ° C, the solubility of Iron sulfate is lower than the solubility at 5 ° C, which represents the lower limit of practical cooling temperature in technical systems. Usually, with the help of evaporative cooling systems with reasonable energetic effort, however, minimum temperatures of about 15 ° C are reached, in which the iron sulfate solubility is about 18 wt .-% and thus 18 wt .-% iron sulfate in the solution (mother liquor) remain (see 1 ).

In accordance with the present invention, aqueous solutions of iron sulfate and / or suspensions are heated above the boiling temperature which sets at atmospheric pressure. The solubility of iron sulfate decreases with increasing temperature and iron sulphate monohydrate precipitates. As is known, the rising temperature is accompanied by an increase in pressure.

In a particular embodiment of the invention, the solution is heated above 110 ° C, in particular at 135 ° C to 220 ° C and particularly preferably at 140 ° C to 160 ° C. The temperature of 160 ° C is achieved for example at a pressure of about 6 bar. The investigations in the context of the present invention have shown that the iron sulphate solubility is about 12% by weight at 140 ° C. and about 6% by weight at 160 ° C. and is thus significantly lower than in the known processes based on a melt process.

A schematic flow diagram of the method according to the invention shows 2 , In a first step, in a container ( 1 ) an aqueous solution and / or suspension of iron sulfate heptahydrate (mixture I). The iron sulfate solution / suspension is preferably saturated or supersaturated.

The mixture I is placed in a heatable pressure vessel (autoclave) ( 2 ). Here, the mixture I is heated to a temperature above the boiling point at atmospheric pressure, preferably to above 110 ° C, in particular to 135 ° C to 220 ° C and particularly preferably to 140 ° C to 160 ° C. Due to the pressure-resistant design, a corresponding to the respective temperature overpressure forms. In accordance with the decreasing solubility of iron sulfate in water at high temperatures, iron sulfate monohydrate precipitates.

Subsequently, the separation takes place ( 3 ) of ferrous sulfate monohydrate ( 4 ), and there remains a low-iron sulfate solution, so-called mother liquor (solution II) ( 5 ).

In a particular embodiment, the precipitated iron sulfate monohydrate is withdrawn and fed to a cyclone or preferably a decanter centrifuge. The latter is preferably heated and also pressure-resistant, to allow separation of the iron sulfate monohydrate without redissolving.

Solution II ( 5 ) can in a particular embodiment at least partially in the container ( 1 ) to be led back.

In an alternative embodiment of the invention, in the solution II ( 5 ) remaining iron content also recovered to a large extent. For this purpose, solution II ( 5 ), which are not in the container ( 1 ) was recycled to an oxidation reactor. The oxidation reactor is pressure-resistant and is operated with air or oxygen. In addition, additives such as salts (eg NaCl) or alkaline substances (eg NaOH) may be added. The temperature is preferably adjusted to above 25 ° C, in particular to 70 ° C to 220 ° C and particularly preferably to 140 ° C to 160 ° C. As a result, iron oxide and iron hydroxide form in solid form and a solution III, which is almost free of iron. "Almost iron-free" in the context of the invention means a total iron content of less than 0.5 wt .-%.

The iron sulfate monohydrate obtained according to the invention can be used in a known manner, for example in water treatment, as a reducing agent in general and in particular for chromate reduction in cement. Furthermore, there is the possibility for further processing into iron oxide pigments or for the production of sulfuric acid via a roasting process.

The iron oxide and iron hydroxide produced according to the invention can generally be used in desulfurization processes and can furthermore be used for the production of iron oxide pigments or pig iron.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 1230409 [0007]

Cited non-patent literature

• Gmelin's Handbook of Inorganic Chemistry, System-No. 59 "Iron", Part B-Delivery 2, 8th Ed. 1930, p. 401 [0013]

Claims (7)

Process for the further processing of iron sulfate heptahydrate characterized by the following process steps: a) forming an aqueous solution or suspension of iron sulfate heptahydrate in a container (mixture I), b) conveying mixture I into a pressure vessel and heating the mixture (I) to a temperature which is above the boiling point of mixture I at atmospheric pressure, forming iron sulfate monohydrate as a solid and a solution II, c) separating the iron sulfate monohydrate solid from solution II. A method according to claim 1, characterized in that in step b) is heated to above 110 ° C, in particular to 135 ° C to 220 ° C, particularly preferably to 140 ° C to 160 ° C at a corresponding overpressure. A method according to claim 1 or 2, characterized in that solution II is at least partially recycled to the container of step a). Method according to one or more of claims 1 to 3, characterized in that following process step c) the following process steps follow: d) conveying solution II into a pressure-resistant oxidation reactor and supplying air or oxygen and optionally of additives, e) adjusting a Temperature in the range of above 25 ° C, in particular at 70 ° C to 220 ° C, particularly preferably at 140 ° C to 160 ° C and precipitating iron oxide and / or iron hydroxide and forming a nearly iron-free solution III, f) separating the iron oxide and iron hydroxide. A method according to claim 4, characterized in that salts or alkaline substances are used as the additive. Method according to one or more of claims 1 to 5, characterized in that the iron sulfate monohydrate is used in the following areas: water treatment, reducing agents, for example for chromate reduction in cement, production of iron oxide pigment, production of sulfuric acid by roasting. A method according to claim 4 or 5, characterized in that the iron oxide and iron hydroxide are used in desulfurization or further processed to iron oxide pigment or pig iron.

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