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

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

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 bath≥1300° 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 temperature≤500° 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.