RU2840984C1 - Method of processing red mud to produce scandium oxide - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to processing red mud in order to obtain scandium oxide. Method involves filtration of red sludge from the liquid phase, repulping the red sludge cake with a soda-bicarbonate solution, bubbling the sludge pulp with a gas-air mixture to activate scandium compounds in the sludge, carbonizing leaching of slime pulp, filtration to produce production scandium-bearing solution. Obtained solution is mixed with alkali-aluminate solution to produce precipitate, which is filtered and repeatedly leached to obtain scandium-containing alkali-aluminate solution and insoluble precipitate. Precipitate is repeatedly leached to obtain scandium-containing soda-bicarbonate solution. Scandium-containing alkali-aluminate solution and the scandium-containing soda-bicarbonate solution are mixed to obtain a concentrate, from which scandium is extracted into the sulphuric solution by two-stage sulphuric acid leaching. From the scandium-containing solutions filtered after 1st and 2nd stages, scandium sulphate is deposited, which is treated with sulphuric acid solution to obtain basic scandium sulphate and then treated with oxalic acid. Obtained scandium oxalate precipitate is filtered and calcined to obtain scandium oxide.

EFFECT: high extraction of scandium.

7 cl, 4 tbl, 1 dwg, 21 ex

Description

Field of technology to which the invention relates

The invention relates to the field of non-ferrous metallurgy, namely, to the technology of complex processing of bauxites, in particular, waste generated during the production of alumina from bauxites using the Bayer method, the so-called red mud.

State of the art

It is known that scandium is a classic dispersed element and is not found in nature in a free state, and practically does not form deposits of Sc-containing minerals on an industrial scale. Scandium is obtained only in industrial production, mainly in the form of Sc ₂ O ₃ oxide, which has the appearance of a white powder, or its salts. Scandium is one of the most expensive metals on Earth, which hinders its active use in innovative and high technologies, as well as as a component of light alloys. At the same time, the demand for scandium is growing. First of all, scandium is of interest for the production of alloys used as a structural material in aircraft construction, rail, road and water transport, since it gives aluminum alloys improved properties. Several hundredths of a percent of metallic scandium in an aluminum alloy increases its electrical conductivity, strength, etc. torsion, improves plastic properties, increases resistance to stress corrosion cracking, ensures weldability of these alloys in an air atmosphere, while the strength of the welded seams is greater than the material.

The world's major commercial reserves of scandium are concentrated in red mud, waste from the processing of bauxite into alumina. The disposal of this environmentally harmful waste is a major problem for aluminum production. The development of scandium extraction technology allows us to improve the properties of red mud and reduce its hazard class, including significantly reducing the content of active caustic alkali and phytotoxic aluminum, and reducing the humidity of red mud. This reduces the hazard class of the mud from 3 to 4 and eliminates the need for expensive burial of red mud, environmental payments for waste disposal, and also creates a source of additional profit when using activated mud in the construction industry or for cleaning industrial waste from heavy metals, in particular, molybdenum, copper, strontium.

A method for obtaining scandium oxide is known from patent RU 2247788 of 10.03.2005, which includes multiple successive leaching of red mud with a mixture of sodium carbonate and sodium bicarbonate solutions, washing and separating the precipitate, introducing zinc oxide dissolved in sodium hydroxide into the resulting solution, holding the solution at an elevated temperature and stirring, separating the zinc and scandium precipitate, treating it with a sodium hydroxide solution at boiling temperature, separating, washing and drying the resulting product with subsequent extraction of scandium oxide by known methods. This method allows obtaining up to 58 g of rich scandium concentrate from 1 t of red mud with an average mass fraction of Sc ₂ O ₃ of 30.0%, with scandium oxide extraction of 13.9%. To obtain such a content of Sc ₂ O ₃ in the concentrate (about 30.0%), it is necessary to repeatedly, at least 10 times, turn over the primary Sc-containing solution for a new leaching cycle of a fresh portion of red mud, which reduces the productivity of the technological process as a whole. The main disadvantage of this method is the low extraction of scandium oxide from red mud (13.9%), caused by high secondary losses of scandium during the repeated use of Sc-containing soda-bicarbonate solution due to the binding of scandium from the solution at the beginning of the leaching process into poorly soluble AlScO ₃ . Aluminum in the solution appears due to the decomposition of part of the calcium hydrogarnets and sodium hydroaluminosilicates contained in the red mud during interaction with sodium bicarbonate NaHCO ₃ .

From patent RU 2536714 of 27.12.2014, a method for obtaining scandium concentrate from red mud is known, which ensures an increase in the degree of scandium extraction from red mud into a primary Sc-containing solution during carbonization leaching and the production of a productive solution purified from accompanying impurities and more concentrated in scandium before the precipitation of scandium concentrate. This method for obtaining scandium concentrate consists of sequential carbonization leaching of red mud with a carbonate solution with simultaneous gasification of the slurry pulp with a gas-air mixture containing CO ₂ , filtration of the carbonized slurry pulp to obtain a scandium-containing solution, sequential separation of scandium from accompanying impurity components with appropriate concentration, precipitation of poorly soluble scandium compounds from the purified solution, filtration, washing and drying of the sediment to obtain a scandium concentrate. In this case, carbonization leaching of red mud is carried out with initial vibration-cavitation treatment of the slurry pulp, separation of scandium from impurity components with appropriate concentration from the obtained scandium-containing solution is carried out by sorption on phosphate ion exchangers, desorption of scandium from the organic phase of ion exchangers. In this case, desorption is carried out with mixed carbonate-chloride solutions in a pulsating mode with the production of a scandium-containing eluate, from which the stepwise precipitation of poorly soluble scandium compounds is carried out. First, the precipitation of poorly soluble compounds of impurity components is carried out with the separation of the sediment, which is a titanium-zirconium concentrate, and then the precipitation of the scandium concentrate is carried out. Due to the features of the method, an increase in the extraction of scandium oxide into the target product (concentrate) is ensured on average up to 23.5%. The main disadvantage of this method is also the low extraction of scandium from red mud into the Sc-containing production solution.

It is known that the extractable scandium in the red mud of Russian alumina plants (Ural Aluminum Plant (UAZ) and Bogoslovsky Aluminum Plant (BAZ)) is about 70%, so it is important to create conditions under which the extraction of scandium will be increased.

A method for obtaining scandium oxide is known from patent RU 2257348 dated 27.07.2005, according to which a scandium-containing concentrate is dissolved in a mineral acid (hydrochloric, sulfuric, nitric), the scandium solution is purified from impurities by treating the solutions with a sulfate-containing inorganic compound and then with barium chloride, the purified scandium solution is treated with alkaline reagents, in particular, NH ₄ OH, to obtain poorly soluble scandium compounds: oxyhydroxide ScO(OH) or scandium hydroxocarbonate. _This is followed by filtering the pulp to separate the scandium precipitate from the solution, treating the precipitate with formic acid _H2CO2 , separating the scandium formate precipitate from the mother liquor, washing the precipitate with formic acid, drying and calcining the precipitate to obtain commercial scandium oxide with a purity of ≥ 99%. The disadvantages of the known method include its multi-stage nature, which requires a large amount of process equipment, increasing OPEX (operating expenses) and a high specific consumption of materials, increasing CAPEX (capital expenditures).

A method for extracting scandium oxide from a scandium-containing concentrate is known from patent RU 2478725 dated 10.04.2013. The technology involves dissolving the Sc-containing concentrate in sulfuric acid, removing the acid-insoluble precipitate, and converting scandium into a precipitate in the presence of an ammonium compound. Then, the precipitate is filtered, washed, dried, and calcined to obtain a scandium oxide precipitate. After removing the acid-insoluble precipitate, the sulfuric acid concentration in the filtrate is brought to 540-600 g/ ^dm3 , and ammonium chloride _NH4Cl is used as the ammonium compound, introduced into the solution in an amount of 26.7-53.5 g/ ^dm3 at a temperature of 50-70 °C, followed by holding for 1-2 hours with stirring. The resulting precipitate is washed with ethyl alcohol. The technical result is a simplification of the technology for obtaining commercial scandium oxide of high purity with a yield of up to 97-98% from poor scandium concentrate, for example, from production waste generated during the processing of bauxite into alumina.

The invention under patent RU 2647398 dated 04.08.2016 of RUSAL for a method for obtaining a scandium-containing concentrate, including single-stage soda-bicarbonate leaching of red mud to obtain a production scandium-containing solution, sorption of scandium from the solution on phosphonic resins, desorption of scandium with a concentrated soda solution at an elevated temperature to obtain a rich scandium-containing eluate, two-stage hydrolysis of the scandium-containing eluate to obtain a Sc-containing concentrate, is accepted as a prototype. The main disadvantages of this method are that its practical implementation results in a low degree of scandium extraction from red mud, no more than 20-25%, high cost of scandium due to high specific consumption of phosphonic resins, due to their intensive destruction during desorption with hot concentrated soda solution, as well as due to the need for periodic regeneration of phosphonic resin from iron and titanium with oxalic acid, poor reproducibility of the process when conducting a 2-stage hydrolysis of scandium-containing soda-bicarbonate solution, which is a buffer solution due to which the pH value changes abruptly, which leads to the impossibility of controlling the 1st stage of hydrolysis, as well as difficulties with separating the scandium-containing sediment from the liquid phase in the 2nd stage of hydrolysis.

Disclosure of the essence of the invention

The objective of the proposed invention is to create a technology that provides for the extraction of scandium from multi-ton waste from alumina production - red mud, which, according to the accepted classification, belongs to hazard class 3. Due to the high alkalinity of this waste, the areas near its storage site (sludge storage facilities) are not suitable for either construction or agriculture. As already noted, the demand for scandium is growing, scandium is of interest for the production of alloys used as a structural material in aircraft construction, railway, automobile and water transport, since it imparts improved properties to aluminum alloys.

The technical result is the solution of the set task and the achievement of the highest possible scandium extraction during the processing of red mud under modern technological conditions.

In the proposed method, several main technological stages can be distinguished for obtaining primary scandium-containing concentrate during processing of red mud using soda-bicarbonate leaching technology, and then re-cleaning the primary scandium-containing concentrate to scandium oxide with a purity of 2N (99% pure metal), given as an example of implementation below and shown in the process flow chart in Fig. 1.

First of all, this is the stage of filtration of red mud received from the stage of thickening and washing of alumina production, on the filter press from the liquid phase. The liquid phase contains dissolved soda Na ₂ CO ₃ , caustic alkali NaOH, sodium aluminate NaAlO ₂ . After filtration, a cake with a moisture content of 28-35% is obtained, and the liquid phase is returned to the main alumina production.

Next, it is necessary to note the stage of repulping the red mud cake with a recycled soda-bicarbonate solution containing at least 20 g/ ^dm3 in terms of _{Na2O bkb} (i.e. the amount of _Na2O contained in the solution in the form of sodium bicarbonate _NaHCO3 ) and at least 100 g/ ^dm3 in terms of _{Na2O carbon} (i.e. the amount of _Na2O contained in the form of sodium _{carbonate Na2CO3} ).

An important stage is the activation of the slurry pulp at a temperature of ≤ 40 °C for ≥ 10 hours by bubbling with a gas-air mixture (e.g. exhaust gases from sintering and/or alumina calcination furnaces and/or a boiler or plant thermal power plant and/or limestone kilns) containing at least 3 vol.% carbon dioxide, to achieve a pH of ≤ 10.5 in the liquid phase due to the conversion of part of the sodium carbonate to sodium bicarbonate (Na ₂ CO ₃ + CO ₂ + H ₂ O = 2NaHCO ₃ ). The use of the slurry pulp activation stage makes it possible to increase the scandium extraction from red mud at the subsequent leaching stage from 20-25% to 40-45%, incl. by opening the surface of the sludge particles due to the destruction of the so-called "network structure" of the red mud, as well as reducing secondary scandium losses due to the formation of insoluble double oxide of scandium and aluminum AlScO ₃ and other phases. It should be explained that the pH - hydrogen potential of a caustic alkali solution is about 14, the pH of a soda solution is about 12, the pH of a sodium bicarbonate solution is about 8.4. Thus, achieving pH ≤ 10.5 shows that in the solution after bubbling with a gas-air mixture containing carbon dioxide, due to the sorption of CO ₂ , all the caustic alkali has turned into alkali carbonate, and some of the alkali carbonate has turned into sodium bicarbonate and its content is ≥ 20 g / dm ³ in terms of Na ₂ O _bkb . The initial pH of the sludge pulp is ≥ 12.5. The minimum pH value of the slurry during bubbling is about 8.5, but achieving this value requires very long bubbling.

Next, the activated sludge pulp, consisting of red mud and a soda-bicarbonate solution of a given composition, selected experimentally, is heated to a temperature of 85-95 °C and subjected to soda-bicarbonate leaching to obtain a scandium-containing solution with a content of at least 10 mg/dm ³ Sc ₂ O ₃ . The pulp is filtered after soda-bicarbonate leaching on a filter press to obtain a production scandium-containing solution and a cake of leached red mud with a pH close to neutral and a moisture content of 28-32%. After this, the scandium-containing production solution is mixed with the alkali-aluminate solution obtained at the stage of precipitation of the hydrolysis _concentrate and its filtration, with the composition (g/ ^dm3 ): _Na2Ototal = 90-320 ( _Na2Ototal is the _sum of _Na2O contained _in the solution in various forms, including caustic alkali Na(OH), designated as _Na2Ocu , _sodium carbonate _Na2CO3 , designated as _Na2Ocoal , sodium bicarbonate _NaHCO3 , designated as _Na2Obi , _sodium aluminate _{NaAlO2 )} . _Na2Ocu = _70-290 , _{Al2O3 =} 35-150 in a ratio of 10:1 to 20:1 (by volume). Maintain with stirring for at least 3 hours at a temperature of 85-95 °C to obtain a precipitate of primary scandium-containing concentrate.

Depending on the ratio of the production scandium-containing solution to _{the alkali-aluminate solution, two types of primary scandium-containing concentrate are obtained: 1) with a Sc2O3 content} from ≥ 0.1 to < 1.0 wt.% and a low content of yttrium, thorium, but a high content of zirconium; 2) with a scandium oxide content from ≥ 1.0 to 4.0 wt.% and a high content of yttrium, thorium, REE (rare earth metals).

Next comes the filtration of the obtained primary scandium-containing concentrate and washing on a cloth from the alkaline solution.

Primary scandium-containing concentrate _{with Sc2O3} content from 0.1 to < 1 wt.% is used to obtain scandium oxide with purity of 2N. Primary scandium-containing concentrate _{with Sc2O3} content from ≥ 1 to 4.0 wt.% is a commercial product sold on the market and can also be used to obtain scandium oxide with purity of 2N.

Repeated leaching of primary scandium-containing _concentrate with _{Sc2O3 content} from 0.1 to 4.0 wt.% is carried out with alkali-aluminate solution of the composition (g/ ^dm3 ): _Na2Otot = 290-380 g/ ^dm3 ; _Na2Ocu = 260-350 g/ ^dm3 ; _Al2O3 = 130-190 g/ _dm3 , caustic modulus of about 3.5, at temperature from 60-95 °C and ratio L (liquid ₎ : S (solid) ^of not less than 3 : 1 (by weight) for ≥ 1 hour. This operation allows for maximum removal _{of Al2O3 from} primary scandium-containing concentrate.

The leached pulp is filtered to obtain a scandium-containing alkali-aluminate solution with a scandium concentration of ≥ 0.05 g/ ^dm3 , sent to the precipitation of the hydrolysis concentrate to reduce the consumption of caustic alkali and insoluble precipitate-I (see the process flow chart in Fig. 1). Precipitate-I contains a residual amount of unleached scandium, therefore, to increase the through yield of scandium in the finished product, it is sent for repeated leaching with a soda-bicarbonate solution to further extract scandium into the solution. Therefore, repeated leaching of the insoluble precipitate-I is carried out with a soda-bicarbonate solution of the composition (g/ ^dm3 ): _{Na2O total} = 120-140, _{Na2O bkb} = 10-20, at a temperature of 60-95 °C and a ratio of L:S ≥ 3:1 (by weight) for ≥ 1 hour, to obtain a scandium-containing soda-bicarbonate solution with a _Sc2O3 content of ≥ 0.05 g/ _dm3 . Two stages of alkaline-aluminate and soda-bicarbonate leaching allow extracting ^up to 90-95% of scandium into the solution and leaving up to 95% of the main impurities in the insoluble precipitate.

The next stage is the filtration of the leached pulp to obtain a scandium-containing alkali-aluminate solution and insoluble precipitate-II (see the process flow chart in Fig. 1). After several leaching stages, precipitate-II contains virtually no scandium and precipitate-II consists of aluminum-, zirconium-, and REE-containing phases, so it can be returned to the main alumina production to reduce secondary losses of alumina. The resulting insoluble precipitate-II is removed to the main alumina production or can be used to obtain zirconium and REE concentrate.

A scandium-containing alkali-aluminate solution and a scandium-containing soda-bicarbonate solution are mixed at room temperature and the mixture is maintained with stirring for 1 hour to obtain a hydrolysis scandium-containing concentrate with _{a Sc2O3} content of 4 to 12 wt.%. The resulting scandium-containing hydrolysis concentrate is filtered to obtain an alkali-aluminate solution used to precipitate the primary scandium-containing concentrate.

This is followed by a two-stage sulfuric acid leaching of the filtered scandium-containing hydrolysis concentrate: in the 1st stage at pH = 3.7-4.3 for at least 2 hours and in the 2nd stage at a temperature of at least 80 °C at pH = 2.0-2.3 for at least 2 hours with the extraction of at least 90% of scandium into the sulfuric acid solution. It should be explained that the value of the hydrogen potential pH in the 1st and 2nd stages of sulfuric acid leaching is obtained by adding the calculated amount of 72% technical sulfuric acid, the pH value is monitored with an industrial pH meter. After mixing the Sc-containing hydrolysis concentrate and the sulfuric acid solution, turning on the stirring and heating, the pH is measured and adjusted with sulfuric acid. Next, the scandium-containing sulfate solutions filtered after the 1st and 2nd stages with a Sc ₂ O ₃ content of 0.8 to 3 g/dm ³ are mixed, and an alkali solution is added to the mixed scandium-containing solution to achieve pH = 4.5-5.5. Scandium sulfate Sc ₂ (SO ₄ ) ₃ is precipitated from the sulfate solution at pH = 4.5-5.5, not containing impurities, including thorium, REE, hafnium, zirconium, yttrium, titanium and other rare and trace elements.

This is followed by a stage of repeated treatment of the sulfuric acid solution with caustic alkali to a pH of no more than 5.6 for selective precipitation of scandium, and after filtration, a zirconium concentrate and a scandium-containing solution are obtained. The obtained basic scandium sulfate is treated with oxalic acid at a ratio of Sc(OH)SO ₄ : H ₂ C ₂ O ₄ ×2H ₂ O = 1 : 1.2 (by weight) for at least 1 hour at a temperature of at least 60 °C for conversion to scandium oxalate (Sc ₂ (C ₂ O ₄ ) ₃ ). The resulting scandium oxalate precipitate is filtered and the scandium oxalate is calcined at a temperature of at least 860 °C for at least 2 hours to remove all the carbon from the scandium oxalate Sc ₂ (C ₂ O ₄ ) ₃ and obtain pure scandium oxide.

The proposed technology allows obtaining scandium oxide with a purity of ≥ 2N, namely, Sc ₂ O ₃ ≥ 99 wt.%.

The important differences of the new method include the stage of red mud activation before soda-bicarbonate leaching at a temperature of ≤ 40 °C and an activation time of at least 8 hours, which allows to increase scandium extraction by approximately 2 times from 20-25% to 40-45%. The activation operation opens the surface of red mud particles by destroying the "mesh" structure. The so-called "mesh" structure is formed in red mud during the production of alumina from bauxite using the Bayer method. For fast and effective separation of aluminate solution and red mud, special synthetic flocculants based on organic compounds are used in industry. Flocculants are long carbon chains of several million carbon units with active radicals at the ends of the chain. These flocculants, due to the chain length and active radicals, connect the main phases of the red mud into larger, well-precipitated particles in the form of a three-dimensional network, in the nodes of which there are large particles of iron oxide (hematite) with a low surface charge (zeta potential) and finely dispersed sodium hydroaluminosilicates with a high zeta potential that “crosslink” the hematite. The extracted scandium is absorbed on the surface of the red mud particles, but due to the “mesh” structure, the soda-bicarbonate solution does not have access to the surface. During the activation operation, the mesh structure is destroyed and the surface of the mud is exposed. The activation stage is important for the destruction of the “mesh” structure formed under the influence of synthetic flocculants in alumina production, and also reduces secondary scandium losses due to its binding at the beginning of the soda-bicarbonate leaching process into poorly soluble AlScO ₃ . Aluminum in the solution appears as a result of the decomposition of a portion of calcium hydrogarnets and sodium hydroaluminosilicates contained in the red mud, when interacting with sodium bicarbonate NaHCO ₃ . Thus, the activation stage allows separating the processes of decomposition of sodium hydroaluminosilicates and hydrogarnets and leaching of scandium into the liquid phase.

Another important difference of the method is the use of freshly precipitated aluminum hydroxide as a scandium "collector", obtained by mixing the production Sc-containing soda-bicarbonate solution with a high-modulus alkali-aluminate solution from alumina production. This operation made it possible to replace scandium sorption and/or two-stage scandium hydrolysis from the production soda-bicarbonate solution. The process of concentrating scandium on aluminum hydroxide makes it possible to control the impurity composition in the primary scandium-containing concentrate, i.e. to obtain two types of primary scandium-containing concentrate with different impurity compositions.

Depending on the ratio of the scandium-containing production solution to the alkali-aluminate solution, the following is obtained:

- concentrate with _Sc2O3 content from _0.1 to <1.0 wt.%, with low yttrium, thorium, and high zirconium content; mixing is carried out based on the content of _Na2O caustic after mixing 1-5 g/ ^dm3 ; this concentrate is used to obtain scandium oxide with a purity of 2N;

- concentrate with scandium oxide content from 1 to 4 wt.% and high content of yttrium, thorium, REE; mixing is carried out based on the content of Na ₂ O caustic after mixing 5.1-20 g/dm ³ ; the concentrate is a commercial product sold on the market, or can be used to obtain scandium oxide with a purity of 2N.

Two-stage leaching of primary scandium-containing concentrate with alkali-aluminate solution and soda-bicarbonate solution allows to get rid of more than 90% of impurities, including titanium, zirconium, iron, aluminum, calcium and others. Sulfuric acid re-cleaning of _concentrate with _Sc2O3 content from 0.1 to 0.3 wt.% at pH from 4.5 to 5.5 allows to purify scandium from thorium and other impurities and create conditions for further obtaining of zirconium concentrate, which increases the depth of bauxite processing.

Fig. 1 shows a general block diagram including the technology for obtaining scandium-containing concentrate with _{a Sc2O3} content of 0.1 to 4.0 wt.%, its refining to scandium oxide with a purity of 2N, and zirconium concentrate.

As noted above, the main objective of the invention and the scandium oxide production technology as a whole is to achieve the highest possible scandium extraction during red mud processing. For example, increasing scandium extraction from red mud obtained during processing of a mixture of bauxites from the North Ural bauxite region and the Middle Timan bauxite region from 20-25% to 40-45% reduces the specific amount of processed mud from 28 to 16 tons per 1 kg of the obtained commercial scandium oxide with a purity of 2N. Extraction of zirconium concentrate from red mud in addition to scandium allows to reduce the cost of scandium oxide production by 20% or more.

Implementation of the invention

Table 1 shows a typical composition of red mud used for soda-bicarbonate leaching of scandium.

Table 1

Sample name Content, mass% _{Sc2O3 ​ ZrO2 TiO2 P2O5 ​} Y Th ΣРЗМ
(amount of REE) Original
red mud 0,017 0.096 4.37 1,121 0.044 0,008 0.2

Table 2 shows examples of the influence of red mud activation modes on scandium extraction during soda-bicarbonate leaching (temperature, time, solution concentration by _{Na2O bkb} ) on scandium extraction during soda-bicarbonate leaching.

Table 2

No.
p/p Time
activation, hour Activation temperature, °C Solution composition, g/ ^dm3 Scandium extraction, wt.% Na ₂ O _bkb Na ₂ O _carbon 1 0 0 20 100 23 2 10 40 17 100 40 3 5 35 20 100 30 4 10 55 20 100 18 5 10 35 10 100 20 6 10 35 20 100 40

Examples 2 and 6 in Table 2 are the most effective, as they allow the maximum transfer of scandium from the solid phase to solution.

The optimum temperature and time for the red mud activation stage were selected experimentally and depended on the following conditions:

– the need to separate two processes: 1) the process of decomposition of tricalcium hydroaluminate 3CaO*Al ₂ O ₃ *6H ₂ O to calcium carbonate CaCO ₃ and calcium hydrocarboaluminate and 2) the process of leaching scandium from red mud into a soda-bicarbonate solution. The task was to prevent the binding of scandium released during the first process by aluminum into an inert compound - double oxide AlScO ₃ . Therefore, the activation process was carried out at a temperature of no more than 40 °C, and leaching at a temperature above 80 °C, selected experimentally as optimal;

– the activation time was also determined by the need to destroy the “mesh” structure of the red mud, which was formed due to the use of synthetic flocculants in the process to accelerate the thickening of the washing.

If the activation stage is not used, the scandium extraction decreases, as shown in Table 2.

Table 3 shows the effect of the mixing ratio of the production scandium-containing solution and the alkali-aluminate solution of the following composition (g/ ^dm3 ): _{Na2O total} = 90-320; _{Na2O ku} = 70-290; _Al2O3 = 35-150 in a ratio of ₁₀ :1 to 20:1 (by volume) and a holding time with stirring for at least 3 hours at a temperature of 50-98 °C to obtain a precipitate of primary scandium-containing concentrate.

Table 3

No.
p/p Product ratio
Sc-content of solution and alkali-aluminate solution (by volume) Process temperature,
°C _{Sc2O3 content}
mass % Sum of impurities ZrO ₂ , TiO ₂ , Y ₂ O ₃ , Th, REE, wt. % 1 20:1 55 4.0 45 2 10:1 60 0,1 5.0 3 8:1 60 0.04 3.5 4 10:1 80 0,1 5.0 5 25:1 60 The process does not proceed, Sc-containing sediment has not formed 6 20:1 40 The process does not proceed, the resulting gel-like Sc-containing sediment is not filtered 7 20:1 100 The process does not proceed, the resulting gel-like Sc-containing sediment is not filtered

Examples 1-4 in Table 3 show that mixing the production scandium-containing solution and the alkali-aluminate solution in the protected range of temperatures and volume ratios allows obtaining a filterable precipitate suitable for further processing into scandium oxide.

Table 4 shows the influence of hydrogen potential (pH) and temperature during two-stage selective sulfuric acid leaching of scandium-containing hydrolysis concentrate on the extraction of scandium into solution.

Table 4

No.
p/p pH 1st stage
sulfuric acid leaching pH 2nd stage
sulfuric acid leaching Process temperature on
1st and 2nd stages, °C Extraction _{of Sc2O3}
in solution, mass% 1 4.0 2.1 85 90 2 3.7 2.0 75 85 3 3.7 2,3 80 83 4 4.3 2.0 80 83 5 4.0 2.5 85 70 6 4.0 1.8 85 The pulp is not filtered 7 4.0 2.1 60 50 8 5.0 2.1 85 30

In examples 1-4 of Table 4, the extraction of scandium from the solid phase into solution was more than 80 wt.%, which confirms the achievement of advantages by the proposed method in comparison with analogues and the prototype.

Taking into account the above description and examples, the scope of legal protection is claimed for a method for processing red mud to obtain scandium oxide, including filtering red mud from liquid phase, repulping red mud cake with a soda-bicarbonate solution, bubbling the sludge pulp with a gas-air mixture containing carbon dioxide, carbonation leaching of the sludge pulp with a soda-bicarbonate solution to obtain a scandium-containing solution, filtering it on a filter to obtain a production scandium-containing solution, characterized in that bubbling is carried out to activate scandium compounds in the sludge and achieve a pH of ≤ 10.5 in the liquid portion, after which carbonation leaching is carried out, after filtration the production scandium-containing solution is mixed with an alkali-aluminate solution to obtain a primary precipitate scandium-containing aluminum hydroxide with a scandium oxide content of 0.1 to 4 wt.%, filtering the primary Sc-containing aluminum hydroxide and re-leaching with an alkali-aluminate solution, then filtering to obtain a scandium-containing alkali-aluminate solution with a scandium concentration of at least 0.05 g/dm ³ and an insoluble precipitate, filtering the insoluble precipitate, re-leaching the insoluble precipitate with a soda-bicarbonate solution to obtain a scandium-containing soda-bicarbonate solution with a Sc ₂ O ₃ content of at least 0.05 g/dm ³ , filtering the resulting solution, mixing the scandium-containing alkali-aluminate solution and the scandium-containing soda-bicarbonate solution to obtain, upon mixing, a hydrolysis scandium-containing concentrate with _{a Sc2O3} content of 4 to 12 wt.%, the filtered scandium-containing hydrolysis concentrate is subjected to a two-stage sulfuric acid leaching, wherein at the 1st stage the pH is 3.7-4.3, at the 2nd stage the pH is 2.0-2.3 with the extraction of scandium into a sulfuric acid solution, the scandium-containing sulfuric acid solutions filtered after the 1st and 2nd stages are mixed and an alkali solution is added to achieve a pH of 4.5-5.5 with the precipitation of scandium sulfate that does not contain impurity elements, then re-treated with caustic alkali to a pH of no more than 5.6 for the selective precipitation of scandium, after which scandium sulfate is treated with a sulfuric acid solution to obtain basic scandium sulfate, the resulting basic scandium sulfate is treated with oxalic acid for conversion into scandium oxalate, filter the resulting scandium oxalate precipitate, calcine the scandium oxalate to remove carbon and obtain scandium oxide containing at least 99.0 wt.% Sc ₂ O ₃ in the calcined product.

Preferably, bubbling is carried out at a temperature of no more than 40 °C for ≥ 10 hours to activate scandium compounds in the sludge and achieve a pH of ≤ 10.5 in the liquid portion, after which carbonation leaching is carried out at a temperature of no lower than 80 °C, after filtration, the scandium-containing production solution is mixed with an alkali-aluminate solution in a ratio of 10:1 to 20:1 by volume at a temperature of 50-98 °C to obtain a precipitate of primary scandium-containing aluminum hydroxide with a scandium oxide content of 0.1 to 4 wt.%, the primary Sc-containing aluminum hydroxide is filtered and re-leached with an alkali-aluminate solution at a temperature of 60-95 °C and a Liquid:Solid ratio of at least 3:1 by weight for at least 1 hour, filtered to obtain a scandium-containing alkali-aluminate solution with a scandium concentration of at least 0.05 g/ ^dm3 and an insoluble precipitate, filtered off the insoluble precipitate, re-leached the insoluble precipitate with a soda-bicarbonate solution of the composition (g/ ^dm3 ): _{Na2O total} = 120-140, _{Na2O bkb} = 10-20 at a temperature of 60-95 °C and a Liquid: Solid ratio of at least 3: 1 by weight for at least 1 hour to obtain a scandium-containing soda- _bicarbonate solution with a _Sc2O3 content of at least 0.05 g/ ^dm3 , while the resulting basic scandium sulfate is then treated with oxalic acid at a ratio of Sc(OH) _SO4 : _{H2C 2} O ₄ ×2H ₂ O = 1 : 1.2 by weight for at least 1 hour at a temperature of at least 60 °C for conversion to scandium oxalate, filter the resulting scandium oxalate precipitate, calcine the scandium oxalate at a temperature of at least 860 °C for at least 2 hours to remove carbon and obtain scandium oxide.

It is advisable to carry out bubbling at the activation stage with a gas-air mixture containing at least 3 vol.% carbon dioxide. At the stage of mixing the production scandium-containing solution and the alkali-aluminate solution, it is preferable to use the composition of the alkali-aluminate solution (g/dm ³ ): Na ₂ O _total = 90-320; Na ₂ O _ku = 70-290; Al ₂ O ₃ = 35-150, while the mixture is stirred and held for at least 3 hours. Repeated leaching of scandium-containing aluminum hydroxide is carried out with an alkali-aluminate solution of the composition (g/dm ³ ): Na ₂ O _total = 290-380 g/dm ³ ; Na ₂ O _ku = 260-350 g/dm ³ ; Al ₂ O ₃ = 130-190 g/dm ³ . The scandium-containing sulfate solution filtered after two-stage sulfuric acid leaching contains Sc ₂ O ₃ from 0.8 to 3 g/dm ³ . The concentration of sulfuric acid H ₂ SO ₄ during the processing of scandium sulfate is 100-350 g/dm ³ .

Claims (7)

1. A method for processing red mud to obtain scandium oxide, comprising filtering red mud from the liquid phase, repulping the red mud cake with a soda-bicarbonate solution, bubbling the sludge pulp with a gas-air mixture containing carbon dioxide, carbonation leaching of the sludge pulp with a soda-bicarbonate solution to obtain a scandium-containing solution, filtering it on a filter to obtain a production scandium-containing solution, characterized in that bubbling is carried out to activate scandium compounds in the sludge and achieve a pH of ≤ 10.5 in the liquid portion, after which carbonation leaching is carried out, after filtration the production scandium-containing solution is mixed with an alkali-aluminate solution to obtain a precipitate of primary scandium-containing aluminum hydroxide containing oxide scandium from 0.1 to 4 wt.%, filter the primary Sc-containing aluminum hydroxide and re-leach it with an alkali-aluminate solution, then filter it to obtain a scandium-containing alkali-aluminate solution with a scandium concentration of at least 0.05 g/dm ³ and an insoluble precipitate, filter off the insoluble precipitate, re-leach the insoluble precipitate with a soda-bicarbonate solution to obtain a scandium-containing soda-bicarbonate solution with a Sc ₂ O ₃ content of at least 0.05 g/dm ³ , filter off the resulting solution, mix the scandium-containing alkali-aluminate solution and the scandium-containing soda-bicarbonate solution to obtain, upon mixing, a hydrolytic scandium-containing concentrate with a Sc ₂ O ₃ content of 4 up to 12 wt.%, the filtered scandium-containing hydrolysis concentrate is subjected to a two-stage sulfuric acid leaching, wherein at the 1st stage the pH is 3.7-4.3, at the 2nd stage the pH is 2.0-2.3 with the extraction of scandium into a sulfuric acid solution, the scandium-containing sulfuric acid solutions filtered after the 1st and 2nd stages are mixed and an alkali solution is added to achieve a pH of 4.5-5.5 with the precipitation of scandium sulfate that does not contain impurity elements, then treated with caustic alkali to a pH of no more than 5.6 for the selective precipitation of scandium, after which scandium sulfate is treated with a sulfuric acid solution to obtain basic scandium sulfate, the resulting basic scandium sulfate is treated with oxalic acid for conversion to scandium oxalate, the resulting oxalate precipitate is filtered scandium, calcining scandium oxalate to remove carbon and obtain scandium oxide containing at least 99.0 wt.% Sc ₂ O ₃ in the calcined product. 2. The method according to claim 1, characterized in that bubbling is carried out at a temperature of no higher than 40 °C for ≥ 10 hours to activate scandium compounds in the sludge and achieve a pH of ≤ 10.5 in the liquid portion, after which carbonation leaching is carried out at a temperature of no lower than 80 °C, after filtration, the scandium-containing production solution is mixed with an alkali-aluminate solution in a ratio of 10:1 to 20:1 by volume at a temperature of 50-98 °C to obtain a precipitate of primary scandium-containing aluminum hydroxide with a scandium oxide content of 0.1 to 4 wt.%, the primary Sc-containing aluminum hydroxide is filtered and re-leached with an alkali-aluminate solution at a temperature of 60-95 °C and a Liquid:Solid ratio of at least 3:1 by weight for at least 1 hour, filtered to obtain a scandium-containing alkali-aluminate solution with a scandium concentration of at least 0.05 g/ ^dm3 and an insoluble precipitate, filtered off the insoluble precipitate, re-leached the insoluble precipitate with a soda-bicarbonate solution of the composition (g/ ^dm3 ): _{Na2O total} = 120-140, _{Na2O bkb} = 10-20 at a temperature of 60-95°C and a Liquid:Solid ratio of at least 3:1 by weight for at least 1 hour to obtain a scandium-containing soda-bicarbonate solution with a _Sc2O3 content of at least 0.05 g/ _dm3 , and then the resulting basic scandium sulfate is treated with oxalic acid at ^a ratio of Sc(OH) _SO4 : _{H2C2O4 × 2H2O} = 1: _1.2 by weight for at least 1 hour at a temperature of at least 60°C for conversion to scandium oxalate, filter the resulting scandium oxalate precipitate, calcine the scandium oxalate at a temperature of at least 860°C for at least 2 hours to remove carbon and obtain scandium oxide. 3. The method according to paragraph 1 or 2, characterized in that bubbling at the activation stage is carried out with a gas-air mixture containing at least 3 vol.% carbon dioxide. 4. The method according to paragraph 1 or 2, characterized in that at the stage of mixing the production scandium-containing solution and the alkali-aluminate solution, the composition of the alkali-aluminate solution is used (g/ ^dm3 ): _{Na2O total} = 90-320, _{Na2O ku} = 70-290, _Al2O3 = _35-150 , wherein the mixture is stirred and held for at least 3 hours. 5. The method according to item 1 or 2, characterized in that the repeated leaching of scandium-containing aluminum hydroxide is carried out _with an alkali-aluminate solution of the composition (g/ ^dm3 ): _{Na2O total} = 290-380 g/ ^dm3 , _{Na2O cu} = 260-350 g/ ^dm3 , _Al2O3 = 130-190 g/ ^dm3 . 6. The method according to item 1 or 2, characterized in that the scandium-containing sulfate solution filtered after two-stage sulfuric acid leaching contains Sc ₂ O ₃ from 0.8 to 3 g/dm ³ . 7. The method according to item 1 or 2, characterized in that the concentration of _sulfuric acid _H2SO4 during the processing of scandium sulfate is 100-350 g/ ^dm3 .