GB2569767A

GB2569767A - Red mud treatment

Info

Publication number: GB2569767A
Application number: GB1906289.2A
Authority: GB
Inventors: Edward Durrant Richard
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-05-03
Filing date: 2019-05-03
Publication date: 2019-06-26
Anticipated expiration: 2039-05-03
Also published as: GB2569767B; GB201906289D0

Abstract

A method of treating red mud comprising adding ferrous iron to red mud to produce magnetite. The ferrous iron may be in the form of a ferrous compound produced as a by-product of another industrial process , for example ferrous sulphate or ferrous chloride. The reaction mixture may be heated. The magnetite may be magnetically separated from the reaction mixture. The formation of particles of ferric oxyhydroxide comprising a surface layer of magnetite is also disclosed.

Description

Red mud is a highly alkaline waste product of the Bayer process comprising mainly iron oxide along with alkali such as sodium hydroxide. The Bayer process is responsible for over 95% of all alumina produced globally, and for every tonne of alumina produced via the Bayer process, approximately 1 to 1.5 tonnes of red mud is also produced. Red mud is therefore produced in immense quantities, which poses serious waste disposal and storage issues. In particular, red mud is an environmentally hazardous substance due to its high alkalinity, which is usually in excess of pH 12.

It is therefore desirable to provide methods of processing and treating red mud to reduce its environmental burden. It is also desirable to extract useful materials from red mud that may be economically valuable.

Summary of the Invention

According to a first aspect of the invention, there is provided a method of treating red mud, the method comprising adding ferrous iron to the red mud to produce magnetite.

The ferrous iron may be in the form of a ferrous compound.

The ferrous compound may be ferrous sulphate.

The ferrous compound may be ferrous chloride.

The ferrous compound may be in the form of a solution, for example an aqueous solution.

The ferrous iron may be added to the red mud so that the ferrous iron reacts with alkali, for example sodium hydroxide in the red mud to produce ferrous hydroxide.

The ferrous hydroxide may react with ferric oxyhydroxide, such as goethite and/or limonite, in the red mud to produce magnetite.

The ferrous hydroxide may react with ferric oxyhydroxide in the red mud to produce particles of ferric oxyhydroxide comprising the magnetite. This may make the particles magnetic

The magnetite may be formed on the surface of the particles, thus making the particle susceptible to magnetic separation/extraction from the reaction mixture.

The method may further comprise extracting at least a portion of the magnetite from the red mud.

At least a portion of the magnetite may be extracted from the red mud using a magnetic field.

The method may further comprise heating the reaction mixture formed by addition of the ferrous iron to the red mud.

The method may comprise heating the red mud and/or the ferrous iron before the addition of the ferrous iron to the red mud.

The method may facilitate the removal of water from the reaction mixture and may further comprise removing water from the reaction mixture once the ferrous iron has reacted with the red mud.

According to a second aspect of the invention, there is provided magnetite produced by the method of the invention.

According to a third aspect of the invention, there is provided particles of ferric oxyhydroxide, such as goethite and/or limonite, comprising magnetite produced by the method of the invention. The magnetite may be formed on the surface of the particles.

According to a fourth aspect of the invention, there is provided processed red mud produced by the method of the invention. The processed red mud has a reduced alkalinity compared to pre-processed or raw red mud. The processed red mud is therefore less toxic compared to pre-processed or raw red mud.

According to a fifth aspect of the invention, there is provided use of a ferrous iron compound in the treatment of red mud.

The use of the ferrous iron compound may be to produce magnetite from the red mud.

Detailed Description

The method of the invention involves treating red mud with ferrous iron, in particular in the form of a ferrous compound. The method of the invention advantageously produces magnetite, which is an exceptionally useful substance due to its excellent magnetic properties. The method of the invention also reduces the alkalinity and environmental burden of the red mud, and also aids water removal, for example by settling or filtration.

Red mud, which may be raw or freshly produced or may have been previously processed to some degree, may be treated with a ferrous compound, for example ferrous sulphate or ferrous chloride. The ferrous compound may be added to the red mud in the form of a solution, for example an aqueous solution. Although the term “adding” is used here, this is intended to mean any combination or mixing together of the red mud and the ferrous compound. Ferrous sulphate (otherwise known as iron(II) sulphate, with the formula FeSO₄) is available as a by-product of titanium dioxide manufacture. It is also a by-product of the pickling (rust removal) of steel with sulphuric acid. Ferrous chloride (otherwise known as iron(II) chloride, with the formula FeCl₂) is produced in the treatment of wastes from steel production, namely the treatment of spent acid, and is also a by-product of titanium production. Thus, the method of the invention advantageously utilises the by-products of other industrial processes to treat industrial waste resulting from the production of bauxite.

The ferrous compound reacts with the alkali in red mud (for example sodium hydroxide) to produce ferrous hydroxide. Ferrous hydroxide then reacts with ferric oxyhydroxide (such as goethite and/or limonite) in the red mud, which comprise FeO(OH), to produce magnetite according to the following reaction:

Fe(OH)₂ + 2FeO(OH) -+ Fe₃O₄ + 2H₂O, where the Fe(OH)₂ is the ferrous hydroxide, the FeO(OH) is provided by the ferric oxyhydroxide (i.e. iron(III) oxide-hydroxide), and the Fe₃O₄ is the resulting magnetite. The ferric oxyhydroxide may or may not be hydrated. Further details of this part of the reaction are found in “Effect of ferrous/ferric ions molar ratio on reaction mechanism for hydrothermal synthesis of magnetite nanoparticles”, Bull. Mater. Sci., Vol. 31, No. 5, 2008, pp.713-717. This reaction occurs slowly at ambient temperatures, but can be accelerated by heating the reaction mixture. For example, the reaction mixture that results from combining the ferrous compound and the red mud may be heated to accelerate the reaction. The reaction mixture may be heated to an elevated temperature above ambient or room temperature (e.g. above 25°C), for example to a temperature of 100°C or more. The heating may be performed under pressure. The reaction is preferably performed excluding air, in particular oxygen, for example under an inert atmosphere such as argon or nitrogen, because the ferrous hydroxide is readily oxidised by oxygen in the air.

Ferric oxyhydroxides, such as goethite, derived from bauxite generally contain alumina in the crystal structure. Only part of this alumina is removed during processing by the Bayer process. The ferrous hydroxide therefore typically reacts with only the surface of the aluminous ferric oxyhydroxide particles to produce a layer of magnetite, but this is sufficient to make the particles magnetic. The magnetic ferric oxyhydroxide particles and/or other magnetite may then be extracted from the reaction mixture, for example by magnetic extraction using a magnetic field, thereby producing a useful and valuable material from the red mud. The magnetite is also denser than ferric oxyhydroxides and therefore more readily extracted. Magnetite also does not contain water, in contrast to hydrated ferric oxyhydroxides. The method of the invention therefore removes water from the particles in the red mud, thereby reducing its tendency to flow and leak from dams. In particular, magnetite is hydrophobic and is therefore less prone to flow as a slurry because water will not readily adhere to the surface of the particles. This also aids filtering and settling of the particles from the reaction mixture.

The product of reacting red mud with a ferrous compound in this way is less noxious and less alkaline than raw red mud. In fact, the processed red mud that results from the method is nearly neutral in pH. The method of the invention also advantageously produces magnetite from red mud using the by-products of other industrial processes.

Claims

1. A method of treating red mud, the method comprising adding ferrous iron to the red mud to produce magnetite.

2. The method of claim 1, wherein the ferrous iron is in the form of a ferrous compound.

3. The method of claim 2, wherein the ferrous compound is ferrous sulphate.

4. The method of claim 2, wherein the ferrous compound is ferrous chloride.

5. The method of any preceding claim, wherein the ferrous iron is added to the red mud so that the ferrous iron reacts with alkali in the red mud to produce ferrous hydroxide.

6. The method of claim 5, wherein the ferrous hydroxide reacts with ferric oxyhydroxide in the red mud to produce the magnetite.

7. The method of claim 6, wherein the ferrous hydroxide reacts with particles of ferric oxyhydroxide in the red mud to produce particles of ferric oxyhydroxide comprising the magnetite.

8. The method of claim 7, wherein the magnetite is formed on the surface of the particles.

9. The method of claim 8, wherein the magnetite forms a layer on the surface of the particles.

10. The method of any preceding claim, further comprising extracting at least a portion of the magnetite from the red mud.

11. The method of claim 10, wherein at least a portion of the magnetite is extracted from the red mud using a magnetic field.

12. The method of any preceding claim, further comprising heating the reaction mixture formed by addition of the ferrous iron to the red mud.

13. The method of any preceding claim, wherein the method comprises heating the

5 red mud and/or the ferrous iron before the addition of the ferrous iron to the red mud.

14. The method of any preceding claim, wherein the method further comprises removing water from the reaction mixture once the ferrous iron has reacted with the red mud.

15. Magnetite produced by the method of any preceding claim.

16. Particles of ferric oxyhydroxide comprising magnetite produced by the method of any one of claims 1 to 14.

17. Processed red mud produced by the method of any one of claims 1 to 14.