Field of the invention.

The present invention relates to technologies for complex processing of apatite, in particular, technologies for obtaining a concentrate of rare-earth metals (REM) and gypsum plaster from phosphogypsum, a waste of sulphuric acid technology for producing phosphoric acid from apatite.

Background of the invention

Tens of millions of tons of phosphorus-containing minerals, such as apatite, rock phosphate, etc., are processed for producing phosphorus-containing fertilizers. Typically, the processing is carried out by treating these natural materials with concentrated nitric or sulphuric acid. During the treatment with sulphuric acid, apatite is decomposed with precipitation of calcium sulphate and formation of phosphoric acid solution. In this case, the main waste product is phosphogypsum (calcium sulphate contaminated with impurities of P2O5, F, Fe, Al, Sr, REM) which comprises most of the rare-earth metals contained in apatite. For example, apatite of the Kola Peninsula contains up to 1% of rare-earth metals, the 70 to 100 % of which are precipitated with calcium sulphate at apatite processing with sulphuric acid. Phosphogypsum constitutes whole mountains around the plants for processing of apatite. Every year millions of tons of phosphogypsum containing about 0.5 % REM in terms of oxides, which currently can not be extracted from it, are sent to dumps. Furthermore, the presence of such dumps containing toxic compounds including fluorine is an environmental problem. In this regard, numerous research projects have been conducted to develop processing technology to extract REM and remove toxic components.

The method for extracting rare-earth elements from phosphogypsum by treatment with nitric acid and subsequent extraction of rare earth elements (REE) by phosphine oxide is known (Martynova I.N. et al. Research of distribution of REE in the course of extraction from acidic nitrate-phosphate solutions. Collected articles "Processing and physico-chemical properties of compounds of rare elements. Apatity, 1984, pp. 6-8 (Rus)). The disadvantage of this method is the need for expensive trialkyl phosphine oxide and the impossibility of complete liquid-phase removal of REE from the organic phase. Furthermore, because of the high loss of trialkyl phosphine oxide with the aqueous phase, this method is uneconomical and requires additional facilities for trialkyl phosphine oxide utilization.

Nitric acid extraction technology for isolation of rare earth elements from apatite, giving up to 85% release in a solution also containing phosphorus and fluorine is known (Kosynkin V.D. et al. "Condition and perspective of rare earth industry in Russia", "Metals" (rus), No. 1 , 2001). The disadvantage of this method is the impossibility of using process solutions in a closed loop and the subsequent low recovery of REM in the process in closed loop.

The method for extracting rare earth elements from phosphogypsum (PCT publication WO201 1008137) is known. The method involves the acid extraction of rare earth element compounds from phosphogypsum using a solution consisting of a mixture of sulphuric acid and nitric acid in a ratio of 3.2: 1.2 with a concentration of 1-3 wt.% and a liquid to solid ratio of 4:5 over a period of 8-12 minutes, while the extraction suspension is agitated and subjected to a hydroacoustic effect. The insoluble gypsum residue is then separated from the extraction suspension and the rare earth element compounds are recovered from the extraction solution by cation exchange sorption with the extraction solution being passed through a cation exchange filter. The main disadvantages of this method are not sufficiently high enough degree of extraction of rare earth metals (up to 85%) with the high cost ion exchanger, long duration of the process and large material flows.

The method of recovering rare-earth elements from phosphogypsum disclosed in RU patent No. 2293781 seems to be the closest analog of the present invention. The method comprises treatment of phosphogypsum with sulphuric acid solution to recover rare-earth elements into solution, separation of gypsum precipitate, increasing of oversaturation rate of the solution in terms of rare-earth elements to crystalize rare-earth metal concentrate, and separation of the concentrate from mother liquor followed by concentrate processing. Phosphogypsum is treated with 22-30% sulphuric acid solution at liquids-to-solids ratio 1.8-2.2 during 20-30 min to prevent spontaneous crystallization of rare-earth element concentrate in solution before insoluble precipitate is separated. An increase of the oversaturation rate of the solution is achieved by means of providing sodium concentration 0.4-1.2 g/L. The disadvantage of this method is the use of additional reagents, high acid concentrations and significant amounts thereof, a large number of basic technological operations with incomplete extraction of rare earth elements (up to 71.4%) and the overall complexity of the process.

The present invention addresses the problem of highly efficient extraction of REM from phosphogypsum followed by obtaining REM concentrate, and simultaneous purification of calcium sulphate from phosphorus and fluorine impurities. In the present invention, the term "REM" is used to indicate lanthanides and yttrium. Also, the symbol "Ln" is used for these elements.

The problem is solved by the method of the present invention. REM extraction into a solution (REM leaching) is carried out by recrystallization of phosphogypsum from hemihydrate CaS0 ₄ » 0,5 H ₂ 0 or anhydride CaS0 ₄ into dihydrate CaS0 _{4 *} 2 H ₂ 0 in an acidic solution of calcium salts, wherein REM, fluorine impurities, phosphorus and alkali metals are extracted into the aqueous solution. Yield of REM extraction is 90-98 %, the residual content of phosphorus, fluorine and alkali metal in calcium sulphate dihydrate is no more than 0.3 wt.%, 0.1 wt.%, 0.05 wt.%, respectively. Isolation of REM concentrate from the aqueous solution can be effected by any suitable method described in the literature.

The essence of the present invention is set forth in details below.

The authors of the present invention have found that a significant part of REM in the phosphogypsum hemihydrate is present in the form of compounds MLn(SC>4) ₂ (wherein M - alkali metal atom, Na or K), forming a solid solution with the main phase of calcium sulphate. Thus, treatment of calcium sulphate which is not accompanied by changes in the crystal structure thereof will not provide a high degree of extraction of rare earth metals in the solution. That is, the best extraction of rare-earth metals is achieved by recrystallization of calcium sulphate hemihydrate CaS0 ₄ * 0,5 H ₂ 0 or anhydride CaS0 ₄ into dihydrate CaS0 _{4 *} 2 H ₂ 0.

It is shown for dihydrate CaS0 ₄ » 2 H ₂ 0 that REM form separate phases in the form of sulphates, while REM are not present in detectable amounts in crystals of CaS0 ₄ « 2 H ₂ 0 available (Bushuev N.N., Nabiev A.G., Petropavlovskiy I.A., Smirnov I.C. "The nature of inclusion of REE cerium subgroup in the structure of calcium sulphate crystalline hydrates", Journal of Applied Chemistry (rus), 1988, No. 10, V. 61 , pp. 2153-2158; Bobik V.M. Coprecipitation of rare earth elements in a system of three heterovalent ions with sulphates of alkali and alkaline earth elements. Radiochemistry (rus), 1977, No. 5, pp. 606-610).

Thus, REM extraction in the course of recrystallization of phosphogypsum hemihydrate can be described by the following equation:

MLn(S0 ₄ ) ₂ . n(2 CaS0 ₄ . H ₂ O) _S0lid solution + 2 Ca ²⁺ _SO | _Ution + (3n+4) H ₂ 0

(2n+2) Ca(S0 ) » 2 H ₂ O _crys tai + M ⁺ _so | _u tion + Ln ³⁺ _S0 | _u ,j ₀ n. This equation shows that the presence of calcium salts in the solution increases the yield of rare-earth metals into solution. Therefore, recrystallization is preferably carried out in the presence of readily soluble calcium salts: Ca(N03) ₂ , CaX ₂ (where X is CI, Br, I), Ca(C10 ₄ ) ₂ , CaSiF6 etc. The concentration of calcium salt is selected to allow: a) proceeding recrystallization of phosphogypsum (increasing calcium concentration makes dihydrate formation thermodynamically less favorable); and b) full recovering of the rare earth metals (as it is shown in the Examples below, increasing calcium concentration increases recovery rate). Use of CaCl ₂ and Ca(N0 ₃ ) ₂ at concentration of 10-300 g/L and 10-500 g/L, respectively, is especially preferred (0.075 - 3.75 M in terms of Ca ²⁺ ). The upper limit is determined by item "a" (the possibility of occurrence of recrystallization of hemihydrate), the bottom limit is determined by item "b" (the desired degree of rare earth metals extraction).

Due to the presence of various impurities in phosphogypsum, precipitation of REM can occurs as separate phases, such as MLn(S0 ₄ ) ₂ , LnP0 ₄ *nH ₂ 0, LnF ₃ *nH ₂ 0, M _x Ln _y (P0 ₄ )*nH ₂ 0 etc. Thus, recrystallization is preferably effected in the presence of a strong acid (pKa <0) forming soluble calcium salts. HX (where X is CI, Br, I), H O ₃ , etc. can be used. The acid dissolves all of the above-mentioned compounds of REM, as well as calcium phosphates and fluorides.

Ca _x (P0 ₄ ) _y F _z + n → x Ca ²⁺ + y H3PO4 + z HF.

LnP0 ₄ + 3 FT→ La ³⁺ + H3PO4.

The concentration of acid is selected to allow: a) proceeding recrystallization of phosphogypsum (increasing acid concentration makes dihydrate formation thermodynamically less favorable); b) full recovering of the rare earth metals (as it is shown in the Examples below, increasing acid concentration increases recovery rate); c) most complete dissolution of phosphorus, fluorine, alkali metal impurities contained in the phosphogypsum. g) The concentration of acid in the solution should prevent REM precipitation in form of phosphates, fluorides, double sulphates. Use of HC1 and HNO3 at concentration of 5-250 g/L and 5-300 g/L, respectively, is especially preferred (0,2-8 M in terms of H ⁺ ). The upper limit is determined by item "a" (the possibility of occurrence of recrystallization of hemihydrate), the bottom limit is determined by items "b"-"g" (the desired degree of rare earth metals extraction, degree of calcium sulphate purity).

The duration and temperature of the process is determined by the same demands, - the possibility of occurrence of recrystallization of hemihydrate into dihydrate and the completeness of recrystallization. The process is preferably carried out at 10-45 °C for 0.5-4 hours, most preferably - at 20-30 °C for 1-2 hours.

Conversion of hemihydrate into dihydrate and anhydride into dihydrate is thermodynamically possible, but due to the high stability of the calcium sulphate anhydride, conversion of anhydride into dihydrate takes too much time (under normal conditions the detectable conversion occurs within a few weeks). Conversion of hemihydrate into dihydrate is more preferable, since it occurs quickly.

f all the above conditions are fulfilled, the degree of extraction of rare earth metals in the solution is up to 98%, a residual content of phosphorus, fluorine and alkali metal impurities in calcium sulphate dihydrate does not exceed 0.3 wt.% , 0.1 wt.%, and 0.05 wt.%, respectively.

Isolation of REM concentrate from an aqueous solution is described in the literature (e.g. Chemistry and technology of rare and trace elements. Part 2. Ed. Bolshakov .A., Moscow, High School, 1976, p. 360 (rus)) and is not the subject of the present invention. As an example, a process comprising liquid extraction of REM into an organic extractant, REM re- extraction from the organic phase with concentrated acid, neutralizing the acid by carbonates, alkali metals or ammonia, and precipitating REM in the form of carbonate, hydroxide, etc. is well-known. For the extraction from nitrate solutions, organic extractants based on neutral organophosphorous compounds (preferably tributylphospate - TBP) are typically used. For the extraction from chloride solutions, extractants based on organophosphorous acids (preferably di-(2-ethylhexyl)phosphoric acid - DEHPA) are typically used. By choosing suitable process conditions (nature of the extraction system, the ratio of the organic extractant/aqueous solution, amount of extraction and re-extraction stages, REM precipitation mode), up to 98% REM contained in the solution after recrystallization of phosphogypsum can be extracted into concentrate.

Thus, the overall recovery of REM from phosphogypsum into the concentrate according to the invention is up to 95%.

The present invention is explained in more detail below using exemplary embodiments, serving solely for illustrative purposes and not intended to limit the scope of the present invention defined by the appended claims. Examples 1 -7.

Recrystallization of CaS0 » 2 H ₂ 0 in solutions x% HNO3 + y% Ca(N03) ₂ was carried out under the following conditions: temperature - 20-45 °C, total time of crystallization - 3 hours, the weight ratio of the solution (L, liquid) and loaded hemihydrate (S, solid), L/S = 1/1 to 3/1. Recrystallization was carried out by continuously stirring the suspension.

At the end of the recrystallization process, precipitate of dihydrate of calcium sulphate was filtered at a press-filter. Resulted dihydrate was washed on the filter with water in a five- stage counter-current mode. The amount of wash water was 70% by weight of the loaded phosphogypsum.

Table 1 shows the composition of the initial hemihydrate and the resulting recrystallized dihydrate. Concentrations of impurities are shown relative to anhydrous CaS0 ₄ .

Example 8.

Recrystallization of CaS0 ₄ .2 H ₂ 0 was carried out in solution 7% HC1 + 17% CaCl ₂ under the following conditions: temperature - 22-25 °C, total time of crystallization - 3 hours, the weight ratio of the solution (L, liquid) and loaded hemihydrate (S, solid), L/S = 3/1. Recrystallization was carried out at continuously stirring the suspension.

At the end of the recrystallization process, precipitate of dihydrate of calcium sulphate was filtered using a vacuum filter. Resulted dihydrate was washed on the filter with acetone followed by drying at 50-70 °C. The amount of acetone was 200% by weight of the loaded phosphogypsum.

Table 2 shows the composition of the initial hemihydrate and the resulting recrystallized dihydrate. Concentrations of impurities are shown relative to anhydrous CaS0 ₄ .

Example 9 (control).

Recrystallization of CaS0 ₄ » 2 H ₂ 0 was carried out in solution 20% H O3 + 30% Ca(N(¾)2 under the following conditions: temperature - 20-25 °C, total time of crystallization - 3 hours, the weight ratio of the solution (L, liquid) and loaded hemihydrate (S, solid), L/S = 3/1. Recrystallization was carried out by continuously stirring the suspension. After the end of the recrystallization process, precipitate of dihydrate of calcium sulphate was filtered using a vacuum filter. Resulted dihydrate was washed on the filter with acetone followed by drying at 50-70 °C. The amount of acetone was 200% by weight of the loaded phosphogypsum.

Table 3 shows the composition of the initial hemihydrate and the resulting recrystallized precipitate. Concentrations of impurities are shown relative to anhydrous CaS0 ₄ .

As can be seen from the data in Tables 1 -3, if recrystallization of hemihydrate into dihydrate was not fully completed, extraction of impurities into the solution is greatly reduced and, accordingly, the level of impurities in the residual insoluble precipitate is increased.

While the present invention is described in detail above, one skilled in the art will recognize that modifications and equivalent substitutions can be made, and such modifications and substitutions are within the scope of the present invention defined by the appended claims.

Table 1.

Table 2.

Table 3.