RU2563015C2 - Method of extraction of cerium from nitrate solutions containing sum of rare-earth elements - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention can be used for processing of concentrates of rare-earth metals. For extraction of cerium from the nitrate solution containing the sum of rare-earth elements, cerium is oxidised to tetravalent state by hydrogen peroxide and is settled by ammonia by simultaneous their adding in separate portions at continuous stirring of pulp. The completeness of behaviour of reactions is monitored. The pulp is heated and filtered, deposit is washed out and the obtained cerium hydroxide is dried. Oxidation and sedimentation operations are conducted at the temperature of solution 40-50°C at pH 5.8-6.0. The pulp is heated up to the temperature 75-85°C and maintained at pH 5.8 within 30-60 minutes with the subsequent adding to the pulp of nitric acid up to pH value 4.5. Before filtration the pulp is conditioned for pH stabilisation. The obtained deposit of cerium hydroxide is washed out by distilled water using repulping-filtration method.

EFFECT: invention allows to the degree of extraction of cerium from solution and purity of cerium hydroxide at smaller number of operations.

4 cl, 3 tbl

Description

The invention relates to a technology for the processing of concentrates of rare earth metals isolated from any type of natural and man-made raw materials and containing cerium in significant quantities, and can be used in various industries.

In the modern world, in accordance with the requirements of scientific and technological progress, a steady increase in the consumption and production of rare earth metals (REM) is maintained. The levels of REM production and consumption in industrialized countries for several decades have been clear indicators of economic development and national security.

According to apt expression of academician A.E. Fersman, REMs are “industry vitamins” and its important strategic potential.

According to estimates by the Industrial Mineral Company of Australia, the demand for rare earth oxides (REO) in 2014 should reach 200 thousand tons per year, significantly exceeding the previous years.

Transition metals of the lanthanum group, as well as scandium and yttrium, are used in various fields of modern technology: in electronics, instrumentation, nuclear engineering, mechanical engineering, chemical industry, metallurgy, etc.

With the development and expansion of the applications of rare-earth metals, cerium, including in the form of compounds of a sufficiently high degree of purity, acquires a growing independent significance. In addition, the significant content of this element in the majority of concentrates processed determines the advantages of its separation from the group of other REEs at the early stages of the technological process. This approach significantly reduces the volume of the processed substance, which entails a decrease in flows, volumes of technological equipment used and increases the economic efficiency of production.

Usually, differences in the properties of its tri- and tetravalent compounds are used to isolate cerium. For this, cerium is oxidized by chemical or electrochemical methods, and then separated from the trivalent REE using the low solubility of Ce (IV) hydroxide in mineral acids (CK Gupta, N. Krishnamurthy Extractive Metallurgy of Rare Earths. L, NY, W .: CRC Press, 2005, 522 p.).

Most methods for producing pure cerium are based on its preliminary conversion to the tetravalent state, followed by separation from the remaining rare earth elements by precipitation, ion exchange, or extraction methods. However, the known methods for the separation of cerium do not allow satisfactorily solving the problem, because either they do not allow sufficiently to extract cerium, or do not provide its sufficient purity, or they require the organization of multi-stage, apparatusally complex and energy-consuming processes.

A known method for the separation of cerium (RF patent No. 2085493, publ. 07/27/1997), the essence of which is that in a solution of rare earth chlorides is added in portions or continuously a solution of sodium, potassium or calcium hypochlorite (20-115 g / l) and 40-115 g / l of sodium, potassium or calcium hydroxide, intensively mixing the reaction mass, the process is carried out at 20-100 ° C and a pH of 1.7-5.5. Then, cerium hydroxide precipitate is separated by filtration.

The disadvantages of the method are the contamination of the final product by concomitant rare earth elements (from two to several percent) and the low extraction of cerium (95%) in those cases when the content of impurities is in the lower of the given ranges of values. In this case, up to 5% of cerium remains in the filtrate and is either lost aimlessly with subsequently separated lanthanum, or requires an additional stage of extraction separation from the latter.

A known method for the separation of cerium (+4) from a nitric acid solution of rare-earth elements, including dilution of a rare-earth solution with water, hydrolytic precipitation of cerium-containing sediment at 70-100 ° C, separation and washing of the precipitate from the mother liquor, and water for dilution of REE is taken in an amount corresponding to the ratio : [H] _ref +3 [Ce (+4)] _ref <(V _add + V _ref ) / V _ref <10 [Ce (+4)] _ref where [H] _ref , [Ce (+4)] _Ref - numerical values of the concentrations of free acid and cerium (+4) in the initial solution, expressed in mol / l; V _ref , V _add - numerical values of the volumes of the initial solution and the introduced water, expressed in l, and the deposition is carried out from the resulting solution for at least 30 minutes (RF patent No. 2162821 publ. 02/10/2001).

At the end of the process, the precipitate of the basic cerium salt (+4) formed is separated from the solution of trivalent REE by filtration. The precipitate is washed with a 0.3 M solution of ammonium nitrate. The washed precipitate is dried and calcined at a temperature of 900 ° C.

The proposed method allows to obtain a high-purity (> 99.9%) cerium product by hydrolytic precipitation of cerium (+4) from a solution of the amount of REE within 30 minutes, while when using the known method, it takes 3 hours to complete the process.

The disadvantage of this process is that the extraction of cerium from the solution is quite small (94.5%).

The closest technical solution, selected as a prototype, is an industrial method of oxidation - deposition of cerium using hydrogen peroxide (A.I. Mikhaylichenko, E. B. Mikhlin, Yu. B. Patrikeev. Rare-earth metals. M: Metallurgy, 1987, p. 120-121). The process is carried out as follows: a 30% hydrogen peroxide solution and a 25% ammonia solution are added to a weakly acid solution of REM nitrates at room temperature. The oxidation of cerium occurs according to the following reactions:

2Се (NO ₃ ) ₃ + H ₂ O ₂ + 6NH ₄ OH = 2Се (OH) ₄ + 6NH ₄ NO ₃

2Ce (NO ₃ ) ₃ + 7H ₂ O ₂ + 6NH ₄ OH = 2CeO ₂ (OH) ₄ + 6NH ₄ NO ₃ + 8H ₂ O

Hydrogen peroxide is taken in a 50% excess, the process is carried out with constant stirring at pH 5-6. Then the pulp is heated to 90 ° C (until the complete transition of the red-brown precipitate of cerium peroxide into pale yellow tetravalent cerium hydroxide) and filtered. Further isolation of cerium is carried out by extraction with tributyl phosphate. For this, the precipitate of Ce (IV) hydroxide is treated with tributyl phosphate (TBP), in equilibrium with 12M HNO ₃ . In this case, tetravalent cerium is extracted, and trivalent rare-earth metals transfer to raffinate. The organic phase is washed four times with 2-4 M HNO ₃ to wash off trivalent rare-earth metals and re-extracted from it cerium with an acid solution with the addition of hydrogen peroxide. In this way, sufficiently pure cerium hydroxide is obtained.

The process also has disadvantages: the frequency and duration of the process, the high consumption of reagents, the complexity, low extraction of cerium in the finished product, the formation of cerium peroxide compounds, which complicates the further dissolution of cerium hydroxide. We add that the achievement of acceptable purity is not achieved at the stage of oxidation - deposition of cerium and requires subsequent extraction purification.

The technical problem to which the present invention is directed is to obtain cerium hydroxide with a relative REE content of at least 96% and recovery of at least 99.9% (residual cerium content in the filtrate, where the remaining rare earth elements are concentrated no more than 0.05 g / l). Moreover, the result should be achieved already at the stage of oxidation - deposition and separation of the precipitate and not require additional operations to clean the filtrate from residual cerium.

The technical result is achieved due to the fact that in the known process of separating cerium from a nitrate solution, including the oxidation of cerium to tetravalent by simultaneous introduction of separate portions of hydrogen peroxide and ammonia with constant stirring of the pulp, monitoring the completeness of the reaction, heating the pulp to a predetermined temperature, subsequent filtration, washing the precipitate and drying the obtained cerium hydroxide, changes were made, namely:

- the oxidation operation is carried out at a solution temperature of 40-50 ° C;

- the oxidation of cerium is carried out by introducing a stoichiometric amount of hydrogen peroxide while neutralizing the solution with gaseous ammonia to pH 5.8-6.0, the end of the process is controlled by the color change of the clarified part of the pulp with the addition of excess hydrogen peroxide;

- after the end of oxidation, the pulp is heated to 75-80 ° C, kept for 30-60 minutes at a pH of 5.8, if necessary, correcting it with nitric acid and gaseous ammonia;

- after the end of the exposure, nitric acid is introduced into the pulp to a pH of 4.5,

- before filtration, the pulp is maintained to stabilize the pH to a predetermined value of 4.5;

- the obtained precipitate of cerium hydroxide Ce (OH) _{4 is} washed with water by the method of repulpation-filtration at a ratio of T: W = 1: 3 in two stages

Carrying out the oxidation process at a temperature of 40-50 ° C optimizes the process by increasing the rate of the oxidation reaction, however, a further increase in temperature above 50 ° C leads to the loss of an oxidizing agent - hydrogen peroxide, since its decomposition begins.

Oxidation is carried out in the pH range 5.8–6.0, which prevents the coprecipitation of trivalent REE hydroxides, the deposition pH of which varies from 6.3 (for Lu, Yb) to 8.35 (for La) with tetravalent cerium hydroxide, and ends when pH 5.8, while the pH of the onset of precipitation of Ce (OH) ₄ is in the range of 0.8-1.0, which allows the most complete separation of cerium from other REEs.

Exposure of the pulp at a temperature of 75-80 ° C, pH 5.8 for 30-60 minutes, is necessary to completely remove the excess hydrogen peroxide from the pulp, while the specified pH value of 5.8 is supported by gaseous ammonia and nitric acid. This means that the process is monitored by the acidity of the medium, i.e. by pH value. With an increase in this value, nitric acid is supplied, and with a decrease in pH, gaseous ammonia.

The introduction of nitric acid into the pulp until a pH of 4.5 is established allows one to effectively free the solid phase (cerium hydroxide) from impurities of trivalent REEs that inevitably co-precipitate with it due to local fluctuations in the hydrogen index, since their dissolution begins at pH less than 5.0. At pH 4.5, a guaranteed result is achieved.

Filtration with separation of Ce (OH) _{4 is} carried out only after stabilization of the pH value, for which the pulp is kept for 1-1.5 hours. Water washing the filtered precipitate of cerium hydroxide by repulpation-filtration method ensures that the bulk of the mother liquor of the precipitation located in the pores of the particles of cerium hydroxide is removed and replaced by water.

The above particular features of the invention were identified on the basis of an analysis of numerous studies to establish optimal values for the parameters of individual process operations, which allow the method to be carried out in the optimal mode in terms of increasing the extraction and purity of the resulting cerium hydroxide and the absence of the need for additional purification of a solution containing the remaining rare earth elements.

The combination of the above features is necessary and sufficient to achieve the technical result of the invention, which consists in improving the separation of cerium and other rare earth metals, increasing its purity and extraction, as well as in the absence of additional technically complex operations to achieve the goals.

The following are specific examples of the implementation of the proposed technical solution.

Example 1. In the initial nitrate solution having the composition shown in table 1, at a temperature of 45 ° C was introduced hydrogen peroxide in separate portions and constant stirring of the pulp.

The process of cerium oxidation with hydrogen peroxide is accompanied by the formation of cerium peroxide compounds CeO ₄ · 2H ₂ O or Ce (OH) ₂ · (O · OH) ₂ along with cerium (IV) hydroxide. The latter decompose upon heating to form Ce (OH) ₄ , therefore, the oxidation process is carried out at a temperature different from room temperature, as in the prototype. The first stage of oxidation ends when hydrogen peroxide is introduced into the solution in an amount corresponding to the stoichiometry of the reaction.

At the same time, gaseous ammonia is introduced to neutralize the free acid released during the hydrolysis of the tetravalent cerium salt in order to raise the pH to 5.8-6.0 and prevent the precipitation of trivalent rare-earth metals. Further, to control the end of the oxidation process, an excess of hydrogen peroxide is introduced in portions until the color of the solution disappears. The oxidation process is complete.

The pulp is then heated to a temperature of 75 ° C and maintained at this temperature and pH 5.8 with ammonia and nitric acid for 40 minutes to remove the excess hydrogen peroxide.

At the end of exposure to deoxidize trivalent rare-earth metals precipitated together with tetravalent cerium, nitric acid is introduced until the pulp pH reaches pH 4.5.

To stabilize the pH value, the pulp is kept for an hour, and then the pulp is filtered to separate the precipitate Ce (OH) ₄ .

The resulting precipitate is subjected to washing with distilled water at a ratio of T: W = 1: 2 by repulpation-filtration (repeated washing). Sludge washing is performed to remove the stock solution. It is conducted under certain conditions, i.e. with the ratio of T: W. Monitoring of the end of the washing process is carried out according to the chemical analysis of the precipitate for the content of the components of the mother liquor, i.e. ammonium nitrate. To ensure that the bulk of the trivalent REM of the nitric acid solution in the pores of the cerium hydroxide particles is removed and replaced with water, the pH should be 5.6.

The composition of the filtrate and sediment after washing the cake by the method of repulpation-filtration (analysis of the sample) are shown in table 2.

The precipitate is dried and further processed depending on its specific application.

The degree of extraction of cerium from the solution into the precipitate was calculated from the concentrations of cerium oxide in the solution before and after precipitation. The degree of extraction of cerium oxide is equal to:

where 25 is the initial volume of the solution before the deposition of cerium, l;

106.7 - concentration of cerium oxide in solution before precipitation, g / l;

24.2 - the final volume of the solution after deposition, l;

0.03 - the concentration of cerium oxide in solution after deposition, g / L.

Thus, according to the results of the experiment (example 1), the degree of extraction of cerium oxide was 99.97% (more than 99.9%, as stated in the technical task).

The results of the study of the process of extraction of cerium from nitrate solutions containing REE are shown in table 3. The same table shows the data on the extraction of cerium, carried out analogously to example 1, but differing in the values of individual parameters: pH, temperature and exposure time. The parameters were varied only for the oxidation process, since subsequent operations are identical and do not affect the degree of cerium evolution, but only its purity.

Respectively:

in example 2, the experiment was carried out at a temperature of 25 ° C;

in example 3, the exposure time of the pulp after heating was 75 minutes;

in example 4, nitric acid was introduced to a pH of 5.2;

in example 5, the exposure of the pulp before filtration was 1.5 hours, pH 4.5;

in example 6, the exposure of the pulp before filtration was 45 minutes, pH 4.5;

in example 7, the oxidation was carried out in the range of pH 6.0-6.2.

From table 3 it can be seen that when the process is carried out at a low temperature (Example 2), a significant amount of cerium remains in the solution in the trivalent form (this can be seen by the amount of ΣРЗО in the precipitate), which is explained by the ineffective effect of the oxidizing agent.

When holding the pulp after heating for 75 minutes (example 3), the composition of the cerium precipitate practically did not differ from the experiment with the exposure of the pulp for 1 hour. This is due to the fact that with this method of oxidation in the solution there is practically no excess of hydrogen peroxide, which must be decomposed, therefore a pulp exposure of 1 hour is sufficient.

When nitric acid is introduced to dissolve trivalent REEs to pH 5.2 (Example 4), it can be seen from the composition of the precipitate that, along with cerium, trivalent REEs that do not dissolve at this pH are also present.

When the pulp was aged for 1.5 hours (Example 5), the composition of the precipitate did not differ from the experiment with an exposure before filtration of 1 hour, however, at an exposure time of 45 minutes (Example 6), samarium was present in the precipitate, which had the closest precipitation pH. During the experiment, the pH before filtration was not stable, slightly shifting to the acidic region.

When conducting oxidation in the pH range 6.0–6.2 (Example 7), despite the subsequent introduction of nitric acid to pH 4.5, the precipitate contained significant amounts of trivalent REEs, which indicates their significant coprecipitation with cerium in the oxidation process.

An analysis of the results shows that the optimal conditions are the process described in example 1.

The advantages of the proposed method for the separation of cerium from nitrate solutions are that an improvement is achieved in the separation of cerium and other rare earth metals, an increase in its purity and extraction, as well as in the absence of additional technically complex operations to achieve the desired technical result.

In addition, the achieved reduction in the consumption of hydrogen peroxide, the extraction of cerium oxide from the nitrate solution is not less than 99.9%, fewer operations compared to the prototype.

Currently, pilot tests of the proposed method are being completed, and its implementation in the industry will be carried out from 2014.

Claims (4)

1. The method of separation of cerium from a nitrate solution containing the sum of rare-earth elements, including the oxidation of cerium to a tetravalent state with hydrogen peroxide, precipitation of cerium with ammonia by simultaneous introduction of them in separate portions with constant stirring of the pulp, monitoring the completeness of the reactions, heating the pulp to a predetermined temperature, followed by filtration washing the precipitate and drying the obtained cerium hydroxide, characterized in that the oxidation and precipitation operations are carried out at a solution temperature of 40-50 ° C. pH 5.8-6.0, then the pulp is heated to a temperature of 75-85 ° C and maintained at pH 5.8 for 30-60 minutes, followed by the introduction of nitric acid into the pulp to a pH value of 4.5; pulps to stabilize the pH, and the resulting cerium hydroxide precipitate is washed with distilled water by the method of repulpation-filtration. 2. The method according to p. 1, characterized in that at the stage of deposition of cerium, gaseous ammonia is used as a precipitant. 3. The method according to p. 1, characterized in that when heating and holding the pulp to remove excess hydrogen peroxide, the optimal pH of 5.8 is supported by ammonia and nitric acid. 4. The method according to p. 1, characterized in that the pulp exposure before filtration is selected in the range of 1-1.5 hours, and the washing of the resulting precipitate is carried out with distilled water at a ratio of T: L = 1: 2 or T: L = 1: 3 to a pH of 5.6.