WO2016023812A1

WO2016023812A1 - Process, assembly and plant using hydrogen peroxide

Info

Publication number: WO2016023812A1
Application number: PCT/EP2015/068138
Authority: WO
Inventors: Anthony J. Nugent
Original assignee: Solvay Sa
Priority date: 2014-08-14
Filing date: 2015-08-06
Publication date: 2016-02-18
Also published as: AU2015303354A1; AU2015303354B2; AR101573A1

Abstract

Process for oxidising species in solution using hydrogen peroxide, said process comprising the steps of: - agitating a solution or suspension of particles in a tank with an agitating device; - adding hydrogen peroxide to said solution or suspension; wherein the hydrogen peroxide is added to the solution or suspension in the tank and wherein said hydrogen peroxide addition is assisted by a device directing the solution or suspension to be treated where the hydrogen peroxide is being added. Assembly and plant suitable for performing said process.

Description

Process, assembly and plant using hydrogen peroxide

This application claims priority to European application No. EP

14181063.0 filed on August 14, 2014, the whole content of this application being incorporated herein by reference for all purposes.

The present invention relates to a process, assembly and plant using hydrogen peroxide.

More particularly: it relates to a process for the extraction (leaching) of a metal from its ore using hydrogen peroxide, to an assembly for performing said process and to a plant incorporating such an assembly. It relates more specifically to a U leaching process, assembly and plant.

Generally speaking, in hydrometallurgy, leaching can happen either in acid or in alkaline medium. The chemical nature of the ore usually determines the type of leach circuit and, in turn, the extent of grinding required. If there is significant carbonate in the ore body (as limestone or calcite), alkaline

(carbonate) leaching is generally used. Otherwise, acid (sulphate) leaching is generally better.

In the case of U (uranium), alkaline leaching is usually not as effective as acid leaching and is not used except in cases of high carbonate ores which render acid consumption uneconomic.

In a uranium leach circuit, the peroxygen compounds (hydrogen peroxide or Caro's acid) can be used to continuously oxidise iron which has been dissolved from gangue minerals or has been introduced during grinding, to the ferric state according to equations (1) and (2).

2Fe²⁺ + H₂0₂ + 2H⁺ -> 2Fe ³⁺ + 2H₂0 (1)

2Fe²⁺ + 2H⁺ + HSOs^" -> 2Fe ³⁺ + HS0₄ ^~ + H₂0 (2)

Ferric ion subsequently oxidises uranium (IV) by a surface electrochemical reaction to the acid soluble (+6) oxidation state according to equation (3).

U02(s) + 2Fe U0₂ ²⁺ (aq) + 2Fe²⁺ (3)

Unfortunately, both hydrogen peroxide and Caro's acid are destroyed by decomposition reactions (4) (5) which are catalysed by a number of metal ions present in uranium leach liquors:

2H₂0₂ 2H₂0 + 0₂ (g) (4) 2HS0₅ ^" -> 2 HS0₄ ^" + 0₂ (g) ( 5)

These reactions are undesirable because they consume oxidant that otherwise would be used to oxidise Fe²⁺.

Hydrogen peroxide was used as oxidant in acid leaching in the nineteen eighties. Lucas et al. (LUCAS, G.C., FULTON, E.J., VAUTIER, F.E.,

WATERS, D.J., and RING, R.J. Queensland Mines Plant Trial with Caro's Acid, Proc. Australas. Inst. Min. Metall., 287, September 1983) describe plant trials performed in Queensland where pyrolusite (Mn02) was replaced by Caro's acid as oxidant. De Vries (Hydrogen Peroxide in Sulfuric Acid Extraction of Uranium Ores, United States Patent 4,425,307, January, 10, 1984) patented a process that utilizes hydrogen peroxide in an acid leach of uranium ore.

Hydrogen peroxide can namely be used as is or can be converted to Caro's acid or peroxymono sulphuric acid (H2S05) by mixing with concentrated sulphuric acid. It is reported that using Caro's acid will reduce the hydrogen peroxide consumption as a result of more efficient use of the hydrogen peroxide.

The main benefits of using hydrogen peroxide (or Caro's acid) as oxidant were claimed to be:

• Better process control

• Oxidant savings

· Sulphuric acid savings

• Lime savings

• Cleaner oxidant handling

• Simplified effluent treatment.

On the downside, hydrogen peroxide will dissociate if it does not react rapidly with the iron in solution. The method of application of the hydrogen peroxide to the slurry is therefore a very important. This parameter is much less critical with other types of oxidants which do not exhibit this characteristic of catalytic decomposition.

This is confirmed by Secomb et al (482 p; ISBN 0 85825 029 2; 1985; p. 139-144; Institution of Chemical Engineers; St. Leonards (Australia); 13.

Australian chemical engineering conference; Perth (Australia); 25-28 Aug 1985). They concluded from continuous flow experiments that Caro's acid utilisation was greater than 95 per cent while hydrogen peroxide utilisation varied between 78 and 93 per cent. They also concluded that in large-scale uranium leaching circuits: - factors which will lessen the efficient use of oxidant are high pH, redox potential and temperature

- a good method of application is essential to ensure rapid reaction between the oxidant and ferrous ion. A poor of application will lead to hydrogen peroxide or Caro's acid decomposition by iron (III), vanadium, manganese (II) and other catalytic metals in solution or solids present in a slurry.

The article entitled "Determination of Flow Structure in a Gold.

Leaching Tank by CFD Simulation " (J C. P. K. Dagadul, Z. Stegowski, L. Furman, E. H. K. Akahol, K. A. Danso, Journal of Applied Mathematics and Physics, 2014, 2, 510-519), deals with the problems of mixing in stirred tanks. It gives preference to the use of computational fluid dynamics (CFD) over experimental residence time distribution measurements for simulating the flow distribution inside leaching tanks. It also shows that at the top of the simulated leaching tank, equipped with a double impeller rotating shaft, the flow velocity is the lowest.

The article entitled "Continuous Precipitation of Uranium with Hydrogen Peroxide" (Wrence E. Burkhart, Metallurgical Transactions B. Process

Metallurgy, Metallurgical Society of Aime. New York, US, vol. 21B, N°5, 1 October 1990, pp 819-826) describes the use of a draft tube - impeller as agitation device inside a Uranium leaching vessel having feed inlet tubes located in the outer region of the vessel from a position about an inch above the fluid surface.

In fact, in most of the hydrogen peroxide using leaching processes, said hydrogen peroxide is introduced at the top for practical reasons.

The present invention is based on the idea of directing the slurry to be treated to where the peroxide is being added, and of doing so either by using a pump but preferably, a combination of a draft tube and an impeller (often called "draft tube mixer"), which is more efficient, and less energy consuming. In fact, the same concept can apply to any process where hydrogen peroxide is used to oxidise species in solution.

Therefore, the present invention relates to a process for oxidising species in solution using hydrogen peroxide, said process comprising the steps of:

- agitating a solution or suspension in a tank with an agitating device;

- adding hydrogen peroxide to said solution or suspension;

wherein the hydrogen peroxide is added to the solution or suspension in the tank and wherein said hydrogen peroxide addition is assisted by a draft tube - impeller which is a device separate from the agitating device and which is directing the solution or suspension to be treated where the hydrogen peroxide is being added.

In one embodiment, said process can be used to destroy cyanide in a slurry before the slurry is deposited in a waste storage dam or to oxidise cyanide in a waste effluent stream before discharge to the environment.

However, in a preferred embodiment, the species to be oxidized comprises a metal from an ore, said process being hence a process for leaching a metal from an ore. Hence, in that embodiment, the present invention relates to a process for leaching a metal from an ore using hydrogen peroxide, said process comprising the steps of:

- agitating a suspension of particles of said ore in a leaching tank with an agitating device in order to keep the particles in suspension;

- adding hydrogen peroxide to said suspension;

wherein the hydrogen peroxide is added to the suspension in the leaching tank and wherein said hydrogen peroxide addition is assisted by a draft tube - impeller which is a device separate from the agitating device and which is directing the slurry to be treated where the hydrogen peroxide is being added.

By "leaching" is meant an extractive metallurgy technique (also called hydrometallurgy) which converts metals contained in an ore into soluble salts thereof generally in an aqueous media comprising a lixiviant, which is responsible for said conversion. Leaching is usually done in tanks or vessels which are generally cylindrical (horizontal or vertical) or of horizontal tube form, and which may work under pressure in which case they are generally known as autoclaves.

Leaching generally involves the use of aqueous solutions containing the lixiviant which is brought into contact with a material containing a valuable metal ("ore"). The lixiviant in solution may be acidic or basic in nature. The type and concentration of the lixiviant is normally controlled to allow some degree of selectivity for the metal or metals that are to be recovered. In the leaching process, oxidation potential, temperature, and pH of the solution are important parameters, and are often manipulated to optimize dissolution of the desired metal component into the aqueous phase. As explained above, acid leaching is more common. As acids, sulfuric, hydrochloric and nitric acid are the most commonly used, sulfuric acid being the most commonly used. There is no limitation to the kind of metal that can be leached of

(extracted) by the process of the invention. For example, a major target of hydrometallurgy is copper. Other examples of suitable metals are Co, Au, Mb and Ni. For instance, according to the invention, hydrogen peroxide can be used as an oxidant in the cyanidation of precious metals like Au, Cu, Zn and Ag, preferably Au (like for instance in the peroxide assisted leach process

(PAL) developed by Evonik-Degussa). However, the present invention is particularly suitable for extracting (leaching) U from U containing ores.

Before the "leaching" according to the invention, the ore has generally undergone pretreatment steps like size reduction and classification in order to be in the shape of particles of appropriate size. And after the leaching, the leached solids and pregnant solution are generally separated prior to further processing.

According to the invention, the tank is equipped with an agitator (agitating device, which is a device separate from the draft tube - impeller) to keep the solids in suspension and improve the solid to liquid contact. Generally, this device comprises at least one impeller mounted on a shaft. It is common practice to use a so called twin impeller i.e. two impellers mounted on a same vertical shaft and spaced from each other. The geometry and space between the impellers is preferably optimized, for instance using the above CFD mentioned method, in order to optimize the flow pattern and hence, homogenization of the suspension. The shaft can be mounted from the top or from the bottom of the tank but generally, it is top mounted, for instance by being fixed on a rail mounted on the top of said tank. It is namely so that most of the tanks are "open" (have no cover) or vented (atmospheric) so that this mounting arrangement is the easiest in practice, also for maintenance reasons.

According to the invention, hydrogen peroxide is added to the suspension or solution and it can be used as such (in aqueous solution) or it can be converted to Caro's acid. At least when U leaching is concerned, preferred concentration ranges are:

- for Η₂0₂: in the range l-70%(w/w) , more likely 3-35% and preferably 5- 20%(w/w;

- for Caro's acid: in the range l-45%(w/w), more likely 2-20% and preferably 5-10% (w/w).

As explained above, Caro's acid is particularly suitable. In that case, said Caro's acid is preferably made on site (in the leaching plant) by mixing hydrogen peroxide and concentrated sulphuric acid in appropriate concentrations and conditions.

The hydrogen peroxide used in the present invention may also be present in an equilibrium solution involving a peracid or any other liquid peroxide derivative.

Hence, the present invention may also be interpreted as follows: a process for oxidising species in solution using a liquid peroxide composition, said process comprising the steps of:

- agitating a solution or suspension of particles in a tank with an agitating device;

- adding the liquid peroxide composition to said solution or suspension;

wherein the liquid peroxide composition is added to the solution or suspension in the tank and wherein said liquid peroxide composition addition is assisted by a draft tube - impeller which is a device separate from the agitating device and which is directing the solution or suspension to be treated where the liquid peroxide composition is being added.

By "liquid peroxide composition" is meant any liquid composition (generally an aqueous one) comprising hydrogen peroxide. As explained above, it may be hydrogen peroxide in aqueous solution or converted to Caro's acid. It may also be a liquid composition comprising a peracid or any other peroxide derivative and which comprises hydrogen peroxide as a result from the equilibrium reaction of formation/decomposition of said peracid or other peroxide derivative.

For sake of simplicity, the terms "hydrogen peroxide" will be used below to designate more generally a liquid peroxide composition comprising hydrogen peroxide as defined above.

According to the invention, the addition of hydrogen peroxide in the tank can be made using any suitable and compatible injection device. Usually, it uses a valve and pipework. The injection device is positioned to allow the oxidant flow to enter the draft tube assembly, the case being. It can be directed onto the surface (or subsurface) of the slurry or solution in the tank.

In a preferred embodiment of the invention, the hydrogen peroxide is added in a region of the upper part of the tank where the flow is substantially vertical and downwards. As can be seen from the above mentioned article on CFD simulations, this is (at least for cylindrical tanks) generally close to the main central axis where the agitating device is generally located. Hence, in a preferred embodiment of the invention, the tank is cylindrical, the agitating device is located on or close to the central axis of said tank and the hydrogen peroxide is added close to said agitating device.

According to the invention, the hydrogen peroxide addition is assisted by a device directing the suspension or solution to be treated where the peroxide is being added i.e. generally to the top of the tank, as close as is practical, subject to engineering constraints to the addition (injection) device.

Hence, preferably, the agitating device and the device assisting the hydrogen peroxide addition are mounted close to each other, for instance by being fixed on a common rail mounted on the top of said tank.

Preferably, the device assisting the hydrogen peroxide addition comprises at least one impeller inside a draft tube. Such a device is also called a "draft tube - impeller". Preferably, the impeller is a twin impeller comprising an upper impeller of a high shear/dispersion type and a lower impeller of an axial/ pump type. Thereby, the upper impeller is to disperse the peroxide over the draft tube cross section and the lower is to push through the required volume of slurry/leach solution containing the necessary excess of ferrous for the peroxide to oxidize. Its top to bottom circulation behaviour is of particular value in deep tanks for suspension of solids and for dispersion of gases.

In the above, by "draft tube" is meant a cylindrical housing around and slightly larger in diameter than the impeller (s) and whose height may either be a little more than the diameter of the impeller, or it may extend the full depth of the suspension or solution, depending on the flow pattern that is required.

According to the invention, the device/valve for adding the hydrogen peroxide and the impeller-draft tube system are close to each other so that the latter can actually assist the former in adding hydrogen peroxide to the tank. In a preferred embodiment of the invention, the device/valve for adding the hydrogen peroxide and the impeller-draft tube system are integrated, since the best performance is achieved when the peroxide is being injected into the draft tube when it is being operated by the electric motor which turns the impellor shaft.

The present invention also concerns an assembly for performing the above described process, said assembly comprising a tank equipped with an agitating device, a hydrogen peroxide addition device and a draft tube - impeller device separate from the agitating device designed to assist the hydrogen peroxide addition by directing the suspension or solution to be treated where the peroxide is being added. Preferably, the agitating device comprises a twin impeller. Preferably as well, the draft tube- impeller device comprises a twin impeller on a single shaft, said twin impeller comprising an upper impeller of a high shear /dispersion type and a lower impeller of an axial/pump type.

In a preferred embodiment, the agitating device and the draft tube - impeller device are mounted on a rail on top of the tank.

The present invention also concerns a plant for performing the process as described above, said plant comprising an assembly as set forth above.

Figures 1 to 6 are meant to show some preferred embodiments of the invention without any intention of limiting its scope thereto.

Figure 1 is a section view of a impeller-draft tube system according to the invention, said section being through a vertical plane comprising the axis of said system; Figure 2 is a section view of the same impeller-draft tube system but through a vertical plane perpendicular to the one of Figure 1 ; Figure 3 is a top view of the same impeller-draft tube system; Figures 4, 5 and 6 are respectively the equivalents of Figures 1, 2 and 3 but extended to the entire leaching tank equipped with said impeller-draft tube system and with a main agitating device.

These Figures show a draft tube (1) inside which a twin impeller (2) is located, which twin impeller (2) comprises an upper impeller (4) which is a high shear/dispersion type impeller and a lower impeller (3) which is an axial/ pump type impeller. This impeller is moved using an electrical motor (6) and it is fixed to the rail (8) of a leaching tank (9) using a fixation device (5). This leaching tank (9) is also equipped with a main agitating device (7) close to which the impeller (2) - draft tube (1) system is located.

The hydrogen peroxide is added in the upper part of the draft tube (1) by means of a pumping device (not shown).

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

Claims

C L A I M S

1. - Process for oxidising species in solution using hydrogen peroxide, said process comprising the steps of:

- adding hydrogen peroxide to said solution or suspension; wherein the hydrogen peroxide is added to the solution or suspension in the tank and wherein said hydrogen peroxide addition is assisted by a draft tube - impeller which is a device separate from the agitating device and which is directing the solution or suspension to be treated where the hydrogen peroxide is being added.

2. - Process according to claim 1, in which the species to be oxidized comprises a metal from an ore, said process being hence a process for leaching a metal from an ore.

3. - Process according to claim 2, which involves the use of an aqueous media containing an acid as lixiviant.

4. - Process according to claim 3, wherein the acid is sulfuric acid.

5. - Process according to any of claims 2 to 4, wherein the metal is uranium.

6. - Process according to any of the preceding claims, wherein the agitating device comprises a twin impeller.

7. - Process according to any of the preceding claims, wherein the hydrogen peroxide is added in aqueous solution or as Caro's acid.

8. - Process according to any of the preceding claims, wherein the hydrogen peroxide is added in a region of the tank where the flow is substantially vertical and downwards.

9. - Process according to claim 8, wherein the tank is cylindrical, wherein the agitating device is located on or close to the central axis of said tank and wherein the hydrogen peroxide is added close to said agitating device.

10. - Process according to any of the preceding claims, wherein the impeller of the draft tube- impeller is a twin impeller comprising an upper impeller of a high shear/dispersion type and a lower impeller of an axial/ pump type.

11. - Assembly for performing the process according to any of the preceding claims, said assembly comprising a tank equipped with an agitating device, a hydrogen peroxide addition device and a draft tube - impeller device which is separate from the agitating device and designed to assist the hydrogen peroxide addition by directing the suspension or solution to be treated where the peroxide is being added.

12. - Assembly according to claim 11, wherein the agitating device comprises a twin impeller on a single shaft.

13. - Assembly according to claim 11 or 12, wherein the draft tube - impeller device comprises a twin impeller, said twin impeller comprising an upper impeller of a high shear/dispersion type and a lower impeller of an axial/ pump type.

14. - Plant for performing the process according to any of claims 1 to 10, said plant comprising an assembly according to any of claims 11 to 13.