PROCESS FOR THE PREPARATION OF A CONCENTRATE OF METALS, RARE METALS AND RARE EARTH METALS FROM RESIDUES OF ALUMINA PRODUCTION BY BAYER PROCESS OR FROM MATERIALS WITH A CHEMICAL COMPOSITION SIMILAR TO SAID RESIDUES, AND REFINEMENT OF THE CONCENTRATE SO OBTAINED

Abstract

Multistage process for the preparation of a concentrate of metals, rare metals and rare earth metals from residues of alumina production by Bayer process (red mud), or from materials with a chemical composition similar to red mud, and multistage process for separating the elements of interest, transforming them into single products to be re-used in the Bayer process and/or sending them to the respective reference markets.

The sole FIGURE appended shows the simplified block diagram of the invention, in terms of its most extensive definition.

Claims

1. A process for the concentration of metals, rare metals and rare earth metals, occurring in the powdery by-products resulting from the treatment of bauxite (red mud) and transformation thereof into individual products to be used in the Bayer process and/or to be sent to respective reference markets thereof, comprising the following stages: First concentrating step, for the separation of occurring iron, which essentially involves the following operations: Optional grinding and drying of the material to be treated material at a temperature from 60 to 250 C.; Optional roasting in order to eliminate also the crystallization water; Optional grinding of the resulting product; Mixing with a component, capable of modifying the basicity index of the mixture, called Fly Ash, that is a waste of other processes, so as to result in a basicity index with values ranging from 0.1 to 2.0; Further mixing with carbon-containing reducing substances to supplement the reducing action of the fly ash; Melting of the material in a first reactor, with the operating temperature of melting bath1300 C.; Obtaining a product enriched in aluminium, titanium and other metals, rare metals and rare earth metals merging into the overlying slag from which the underlying molten metallic product is separated in the form of an iron alloy with minimum quality equal to pig iron; Second concentrating step, for separating the compounds containing aluminium and silicon, which may involve a basic (alkaline) leaching or an acid leaching and, in the case of alkaline leaching, essentially comprises the following operations: Sending the molten slag, exiting from the reactor of the first concentrating step, into a second reactor where at a temperature of about 1000 C. an alkali and/or alkaline earth metal carbonate is added to it and then it is cooled to temperature500 C.; Alkaline leaching of the resulting product in order for the aluminium to be dissolved, in the form of hydrated salt of alkali and/or alkaline earth metal, and Si in the form of hydrated silica; Treatment of the liquid obtained with milk of lime in order to separate the silica and then with CO2 to precipitate Al(OH)3 that is separated and calcined to give Al2O3; Sending the residual liquid into the head of the basic leaching section; Obtaining, at the end of the leaching operations, a solid having a higher concentration of the rare and rare earth metal content.

2. The process according to claim 1, wherein the second concentrating step, in order to separate the aluminium and silicon containing compounds, and involving an acid leaching, essentially comprises the following operations: Cooling of the slag exiting from the reactor in the first concentrating step to a temperature below 100 C.; Fine grinding of the resulting product to maximize the specific surface area (average particle size lower than 0.2 mm); Acid leaching with aqueous solution of nitric acid 0.3 N, to extract rare and rare earth metals along with the titanium and other possibly occurring trace elements, with separation of the unsolubilized residue from the acid leaching and treatment for the extraction of Al and Si compounds.

3. The process according to claim 2, where there is provided for a Third concentrating step, which essentially comprises the following operations: Filtration of the liquid resulting from the acid leaching; Passage of the filtered liquid, which contains in solution rare, rare earth and undesirable metals, through selective ion exchange resins of cationic type, on the active sites of which the cations of the above metals are blocked; Selective removal, from the resulting cation exchange resin, only of the undesired metals, such as iron, aluminium, calcium, titanium, sodium, with HNO3 aqueous solution of between 1.25 N and 1.75 N; Subsequent extraction of the rare and rare earth metals from the cation exchange resin, deprived of the undesired elements, with HNO3 aqueous solution of between 3 N to 10 N, resulting in the regeneration of the cation resin and use thereof in a subsequent concentrating cycle; Selective separation, by known techniques, from the resulting solution, of the individual extracted rare and rare earth metals.

4. The process according to claim 1, wherein the basicity index of the mixture is changed by Coal Fly Ash containing a significant carbon percentage and a resulting reducing activity.

5. The process according to claim 1, wherein the first reactor for melting the material to be treated is selected from the group comprising reactors of the type EAF (Electric Arc Furnace), of the type plasma transferred arc, plasma not transferred arc, microwave plasma, Brown's gas reactor and electrolysis catalysed gas reactor.

6. The process according to claim 2, wherein the acid leaching is performed with a 0.3 N, preferably 0.6 N, aqueous diluted nitric acid solution.

7. The process according to claim 2, wherein the ratio (weight/volume) between the material to be treated and the acid solution is between 1/2 and 1/50.

8. The process according to claim 2, wherein the temperature during the acid leaching is between 40 C. and 95 C. at atmospheric pressure.

9. The process according to claim 2, wherein the duration of the acid leaching is between 15 minutes and 120 minutes.

10. The process according to claim 2, wherein the acid leaching operation is repeated at least once.

11. The process according to claim 2, wherein the unsolubilized residue is separated from the acid leaching liquid by the decantation or filtration technique.

12. The process according to claim 2, wherein the selective removal, in the third concentrating step, only of the undesirable metals is obtained by passing through the cation exchange resin a nitric acid aqueous solution from 1.25 N to 1.75 N.

13. The process according to claim 2, wherein the rare and rare earth metals are extracted, using a nitric acid aqueous solution of from 3 N to 10 N, from the cation exchange resin, already deprived of undesirable metals, which, being thus regenerated, is available for a subsequent resin concentration cycle.

14. The process according to claim 2, wherein the solution enriched in rare and rare earth metals is subjected to known techniques for selective separation to obtain separately the individual components of the enriched solution.

15. The process according to claim 14, wherein the selective separation is performed with organic solvents.

16. The process according to claim 1, wherein the organic solvent is DEHPA, di-(2-ethylhexil) phosphoric acid.

17. The process according to claim 1, wherein the rare and rare earth metals to be concentrated are scandium, yttrium and lanthanum.

Description

[0054] The sole FIGURE appended shows the simplified block diagram of the invention, in terms of its most extensive definition.

[0055] The description of the invention given above is of a general nature. A more detailed description of a relative embodiment will now be given, with the help of the example, aimed at achieving a better understanding of the objects, features and advantages of the invention.

EXAMPLE

[0056] The example illustrate an application of the process according to the invention.

[0057] A sample of red mud is dried to 250 C. and then ground. The following table shows the elementary analysis of the main elements of interest present on the sample of pre-treated red mud thus obtained, used in this example.

TABLE-US-00004 TABLE 4 Initial elementary analysis of the red mud used Unit of Parameter measurement Values Aluminium mg/kg 94,982.0 Iron mg/kg 165,967.0 Yttrium mg/kg 52.0 Lanthanum mg/kg 83.0 Scandium mg/kg 40.0

First Concentrating Step:

[0058] during this step the aim is to drastically reduce, selectively, the iron content present in the matrix, both to obtain a concentration effect of the elements of interest, and because the iron is an important interfering element for the processes used in the subsequent concentration and separation steps.

[0059] A basicity index corrector, containing silica and calcium oxide, is added to the sample of red mud, suitably pre-treated. In this test, the basicity corrector is added in order to obtain a binary basicity index value IB.sub.2 of approximately 0.6.

[0060] Moreover, an appropriate quantity of carbon is added, to give a suitable reducing potential to the load. A quantity of carbon equal to 11% of the weight of the sample of red mud is added in this test.

[0061] It should be noted that the quantity of the reducing agent and the basicity index corrector added is determined, each time, on the basis of the red muds used.

[0062] The mixture formed by the pre-treated red mud, the basicity index corrector and the carbon is loaded in a plasma transferred arc reactor, in which the plasmogenic gas is nitrogen. The system must be maintained in the reaction conditions, that is, at a temperature greater than 1300 C., until completion of the reduction reactions. In the case of the reactor used, this phenomenon occurred in approximately 60 minutes, but this time may vary on the basis of the type of technology used to reach the reaction conditions, the type of load (depending, for example, on the content of iron oxides and interfering elements present), the geometry of the reactor etc.

[0063] The slag and the pig iron produced are collected separately at the end of the reaction time. The slag, compared with the calcined red mud, is 55% by weight; this means that the rare and the rare earth metals of interest, in the slag, should have a concentration of approximately double, with respect to the pre-treated red mud. The following table shows the elementary analysis of the main elements of interest present on the sample of slag produced:

TABLE-US-00005 TABLE 5 Elementary analysis of the slag obtained after the first concentrating step Unit of Parameter measurement Values Aluminium mg/kg 122,555.0 Iron mg/kg 24,877.0 Yttrium mg/kg 102.0 Lanthanum mg/kg 150.0 Scandium mg/kg 80.0

[0064] As may be seen from the data shown in Table 4 and Table 5, the concentration di Yttrium, Lanthanum and Scandium has almost doubled compared with the sample of pre-treated red mud. As expected, the content of iron in the slag has considerably reduced, compared with the pre-treated red mud, falling from approximately 16.5% to approximately 2.5%. The iron removed has resulted in a ferrous-based metallic phase (iron content>92%), which is similar to pig iron in terms of quality. The content of aluminium before and after the treatment has a less regular trend, because the compounds containing this element also undergo reactions with the development of volatile compounds (flow managed separately and not included in the invention), so the concentration factor of this element in the slag is approximately 30%.

Second Concentrating Step (Alternative B), by Separating Compounds Containing Aluminium and Silicon:

[0065] the slag exiting from the melting in the first concentrating step is cooled to a temperature below 100 C. Fine grinding of the material is then carried out to obtain an average particle size lower than 0.2 mm. In this way, the specific surface area is maximized and the leaching reaction is improved. The ground material passes to the subsequent leaching in acid; the acid used is a diluted nitric acid solution, with a concentration of 0.6 N.

[0066] The ratio (weight/volume) between the solid and the acid liquid, used in the test, was 1/50; the reaction system was also maintained at 90 C. and with atmospheric pressure.

[0067] At the end of the reaction time, the liquid was separated from the solid by mechanical decantation, by centrifuging; the clarified liquid was then filtered and analysed. The solid treated was recovered for any extraction of aluminium and silicon compounds, according to the methods described in the case of alternative A of the second concentrating step.

[0068] The following table shows the percentage of extraction of the rare and rare earth metals of interest, calculated from the elementary analysis of these elements in the 0.6 N nitric acid solution, at the end of the leaching reaction:

TABLE-US-00006 TABLE 6 Elementary analysis of the 0.6N nitric acid solution, at the end of the leaching reaction Unit of Extraction after single Parameter measurement passage (1 hour) Aluminium % 60.0 Iron % 22.5 Yttrium % 65.8 Lanthanum % 18.7 Scandium % 50.0

[0069] The liquid resulting from the acid leaching is sent, after filtration, to the next third concentrating step.

[0070] Third Concentrating Step, by Separating Compounds Containing Aluminium:

[0071] the liquid coming from the acid leaching, and which contains, in solution, rare and rare earth metals, as well as a not insignificant quantity of other elements, is treated on selective ion exchange resins, of cationic type.

[0072] The resins capture the rare and the rare earth metals present in the alkaline solution (such as, for example, scandium, yttrium and lanthanum), and block them on the active sites of the resin, thereby concentrating them. The concentration factor of the rare and rare earth metals on the resins depends on the ratio between the volume of leaching liquid treated and the volume of the bed of selective resins. A typical concentration ratio is approximately 100 times.

[0073] The concentration ratio on the resins in the test in question was 20 times. A volume of 2 litres of acid solution has been treated with a volume of 0.1 litres of selective resins.

[0074] Together with the cations of the metals and rare metals, other cations of any undesired elements present, such as aluminium and iron, are also captured, if present, in the test described.

[0075] The 0.6 N nitric acid solution, after the passage on resins, is sent to the second step, for re-use in the process, if necessary with the addition of new solution.

[0076] After having captured the rare and rare earth metals of interest, together with the undesired elements, the bed of selective resins, of the cationic type, is treated with a solution of nitric acid with a concentration of between 1.25 N and 1.75 N. A nitric acid solution with this concentration frees from the resin only the undesired elements, leaving the rare and rare earth metals on the bed.

[0077] The bed of selective resins is then washed with a nitric acid solution with a concentration of between 3 N and 10 N; in these conditions, the rare and rare earth metals which were linked to the resins are removed from the bed. In this way, an acid solution is generated containing a concentrate of a mixture of rare and rare earth metals. The concentration of the rare and rare earth metals in the resulting acid solution depends on the content (in grams) of each element in the filtering bed (degree of saturation of the filtering bed) and on the ratio between the volume of the bed of resins and the total volume of the 3 N to 6 N nitric acid solution used. In the example in question, for instance, the 0.1 litre filter bed, containing 1.8 mg of scandium, has been washed with a volume of 0.5 litres of 6 N nitric acid solution, obtaining a concentration of 3.6 mg/litre of scandium in solution.

[0078] The rare and rare earth metals present in the nitric acid solution obtained in this way can be separated selectively by means of one of the prior art techniques.