CA2930349A1 - Method for recovering rare earth metals from waste sulphates - Google Patents

Abstract

The present invention relates to a method for recovering rare earth metals from waste sulphate materials, such as waste gypsum, which is a known secondary resource of rare earth metals and widely present e.g. in areas, where industrial phosphate production takes place. The present invention combines sulphate reduction treatment, such as bioreduction with sulphate reducing bacteria, and magnetic separation, which is based on an exceptionally high magnetic susceptibility of rare earth compounds compared to e.g. calcium compounds in such reductively pretreated gypsum precipitate.

Description

METHOD FOR RECOVERING RARE EARTH METALS FROM WASTE
SULPHATES
Field of the Invention The present invention relates to recovery of rare earth metals from waste sulphate materials. In particular, the invention relates to reductive fractionation of waste gypsum, comprising sulphate salts of calcium and other metals to their dispersed sulphides, in which form the metal components with high magnetic susceptibility can be recovered by using magnetic separation. Sulphate reducing bacteria are preferably used for the reductive fractionation.
Description of Related Art Igneous apatite minerals, industrially utilized for manufacturing phosphate fertilizers, are a known secondary source of rare earth metals. The rare earth (RE) content of apatites varies between 0,5 to 1 % as oxides. Several pilot processes to recover the valuable rare earth metals in connection with the adjacent fertiliser production have been developed, so far without economic success (Jorjani et al., 2011; Al-Shawi et al., 2002).
The for long time leading fertilizer manufacturing process includes the leaching of the ore with sulphuric acid, which includes formation of phosphogypsum as Ca504=2H20 (dihydrate) as the side product. This process, used for many decades for example in the Finnish Siilinjarvi fertiliser plant has already produced ca. 45 million tonnes of phosphopgypsum, concurrently gathered in the storage pile in the fertiliser plant area. In the former studies performed by Kemira Oy it was concluded that even 80 % of the rare earth content of the phosphate raw material will be carried into the waste gypsum (Lounamaa et al., 1980). A similar experience is reported from several other sites in the world.
The rare earth metals are present in the gypsum as their respective sulphates, even though particulates of monazite, non-dissolved in the sulphuric acid process may also appear.
Typical techniques for the chemical fractionation of rare earths from phosphogypsum usually include leaching with dilute sulphuric acid solution, separation of rare earth concentrates from leaching sulphuric acid by pre-concentration via evaporation, liquid-liquid extraction or precipitation method and anhydrite production from purified

2 phosphogypsum by recrystallization of concentrated sulphuric acid solution.
All such methods have so far rendered complex and uneconomical (Preston et al., 1996;
WO
2011/008137 A3), due to ineffectiveness of the multi-stage procedures and low initial concentration of rare earths in the phosphogypsum.
On the other hand, a combined mechanical-magnetic separation method (FI 101787 B) has been proposed to purify phosphogypsum waste from its heavy metal impurities.
WO
2009/125064 Al discloses a respective method for purification of flue gas desulfurization (FGD) gypsum. In such techniques gypsum is subjected to grinding in various degrees of fineness, then slurried by water addition and finally led to high gradient magnetic separation (HGMS) to collect the magnetized fraction. The main goal of the method has been to purify waste gypsum for its possible future use as contaminant free filler in various components such as boards in construction industry or as pigment in paper-making, but the method has shown potential in using the magnetic separation to recover metals from waste gypsum. Yet the recovery of such metals is highly dependent on the consistency of the slurry as well as of the fineness of the gypsum stock and in average but 35 %
of the rare earth metals such as La, Nd, Ce and Y could be recovered. It is obvious that despite the rare earth metal sulphates generally have high magnetic susceptibilities, the crystallisation of the gypsum encapsulates the REs to a large amount within the Ca-sulphate granules, which has a low magnetisation and thus leads to high losses in the separation stage.
The use of sulphate reducing bacteria (SRB) for removing contaminants such as heavy metals from aqueous solutions is disclosed by Kaksonen and Puhakka (2007). The SRB
can be used for treating ground and surface waters contaminated with acid mine drainage (AMD), and for recovering metals from wastewater and process streams. The biologically produced H2S precipitates metals as metal sulfides, while biogenic bicarbonate alkalinity neutralizes acidic waters. In such method, the aqueous sulphate solution, provided with appropriate electron donor is inoculated with micro-organisms, which promote the reduction of the sulphate ion to hydrogen sulphide:
8H2 + 2S042- ¨> 0 H2S + HS- + 5H20 + 30H
Instead of hydrogen, organic compounds descending from e.g. fermentation processes or waste streams with anaerobic degradation stages and including e.g. organic acids or

3 alcohols can be used as electron donors. Thus, for example waste water from dairy industries or from farming have been used (Kaksonen and Puhakka, 2007;
Rzeczycka et al., 2010) for the treatment of phosphogypsum by SRB.
Kaufman et al. (1997) present a combined chemical and biological process for the recycling of flue gas desulfurization (FGD) gypsum into calcium carbonate and elemental sulphur. In this process, a mixed culture of sulfate-reducing bacteria (SRB) utilizes inexpensive carbon sources, such as sewage digestor synthesis gas, to reduce FGD gypsum to hydrogen sulfide. In the process concept, the sulphide is further oxidized to elemental sulfur via reaction with ferric sulfate, and accumulating calcium ions are precipitated as calcium carbonate using carbon dioxide. Employing anaerobically digested municipal sewage sludge (AD-MSS) medium as a carbon source, SRBs in serum bottles demonstrated an FGD gypsum reduction rate of 8 mg/L/h (109 cells) -1. A
chemostat with continuous addition of both AD-MSS media and gypsum exhibited sulfate reduction rates as high as 1,3 kg FGD gypsum/m3=d, with 100 % conversion of sulphate.
The sulphide ions generated by the SRB however further react with the metal cations in the solution producing low solubility metal sulphides:
H25 + M2'¨>MS(s) + 2H' EP 0844981 B1 proposes a biomagnetic separation method for the recovery of metals from an influent liquid containing e.g. radioactive heavy metal contaminants from waste water of a nuclear plant. The technique involves adding specific adsorbent material to the contaminated solution to attach the contaminants by chemical or electrostatic adsorption.
As the ferromagnetic adsorbent, bacterially generated ferrous sulphide from the respective sulphate is preferably used (Watson et al., 1996). The method is targeted to remove toxic heavy metals from the influent solution, and proved successful in dropping the concentrations of e.g. mercury, cadmium, chromium and lead contents of the solution by several orders of magnitude.
WO 2013/044376 Al relates to magnetic separation of different rare earth compounds, wherein a quantitative fractionation of various rare earth metal compounds is described in terms of their magnetic susceptibilities by using a separation channel rigged with magnets

4 arranged progressively from weakest to strongest along the length axis and respective output channels to fractionate compounds with various susceptibilities and specific gravities. This publication shows the feasibility of separating and refining rare individual rare earth compounds by HGMS techniques, yet chemical formulation of the rare earth compounds as a necessary pre-treatment before magnetic fractionation is not disclosed.
Thus, it has been estimated that even 60 to 80 % of rare earth metals, which are used as ingredients in phosphate production industry, end up to waste gypsum. Another recent analysis of the Finnish phosphogypsum gives the contents of La, Ce and Y in the 390, 1100 and 23 ppm, respectively. It would therefore be beneficial to develop an economic process to recover the valuable metal contents from waste gypsum.
Summary of the Invention The present invention is based on reductive and enriching treatment of sulphate materials combined with magnetic separation to recover rare earth metals.
Particularly, the present invention relates to a method for recovering rare earth metal enrichment from waste sulphates by first reducing rare earth metal sulphates to a metal sulphide precipitate and then separating a highly magnetized fraction of the metal sulphide precipitate with a magnetic separator.
In the present method sulphate reduction may be carried out for example by utilizing sulphate reducing bacteria, by applying thermal treatment or by using hydrometallurgical reduction with H2S.
More precisely, the method according to the present invention is characterized by what is stated in the characterizing part of claim 1. In addition, the use of said method is characterized in claim 16.
Considerable advantages are obtained with the method of the invention, which provides a cost-effective and environmentally friendly technical solution for recovering valuable rare earth metals for example from the wastes of phosphate production industry. In addition the method can be used for recycling of waste gypsum back to calcium carbonate and sulphuric acid.

Next, the invention will be described more closely with references to the attached drawing and detailed description.
Brief Description of the Drawings

5 Figure 1 is a schematic description of a process according to the present invention.
Numbers 1-5 are process steps, which are explained in the detailed description below.
Detailed Description of Embodiments Herein below the following short terms are used:
"SRB" as in sulphate reducing bacteria "RE" as in rare earth or rare earth metal "HGMS" as in high grade/gradient magnetic separation/separator Characterizing to the method of the present invention is to combine a reductive treatment of a waste sulphate material and a following magnetic separation to recover valuable rare earth metals. In one embodiment the waste sulphate material is waste gypsum, for example waste phosphogypsum.
In the method waste sulphates containing rare earth metal compounds are reduced in a liquid phase e.g. by sulphate reducing bacteria (SRB) to form a finely divided rare earth metal precipitate, followed by separation of the magnetized fraction of the precipitate by a magnetic separator, such as high grade magnetic separator (HGMS). Thus, the present invention is based on an enrichment of the rare earth metal content of e.g.
waste gypsum into a metal sulphide precipitate, and to a higher magnetic susceptibility of the RE
compounds in the precipitate compared to other substances present in said precipitate (such as calcium sulphate/sulphide/phosphate). Preferably, the method relates to a recovery of a rare earth metal enrichment, which comprises rare earth metals as their corresponding sulphides, oxides or phosphates, or as a combination thereof Said enrichment may also comprise small amounts of other compounds than rear earth metals compounds, for example K, Fe, Ca, Mg and Al sulphides.
Thus, the magnetized fraction of the metal sulphide precipitate comprises rare earth metals and has a higher magnetic susceptibility compared to other substances, such as calcium compounds, present in said precipitate. It has been discovered that he magnetic

6 susceptibility of e.g. rare earth metal sulphides is often exceptionally high, whereas that of calcium sulphide is low. The same applies to corresponding oxides and sulphates.
In a preferred embodiment the process comprises the following steps (numbers 1 to 5 are also correspondingly marked into Figure 1):
1. Dissolving a waste gypsum to a dilute sulphuric acid or water, 2. Inoculating SRB with appropriate nutrient solution (and pH), 3. SRB reducing process (for example with rates exceeding 10 kg/m3 of gypsum), 4. Recovery of the finely dispersed sulphide slurry by precipitation or filtration, 5. Use of HGMS for the recovery of the highly magnetised fraction of the fine sulphides.
One suitable sulphate reducing bacteria for use in the present method originate from genus Desulfovibrio. As an example bacterium such as Desulfovibrio desulfuricans can be used.
In addition, SRB belonging to the genera Desulfobulbus and Desulfotomaculum have shown to be promising. In order to carry out the reduction mechanism, SRB need some organic nutrients for their metabolism. Thereby SRB may use carbon sources, such as sewage digests, alcohol or synthesis gas, as microbial nutrients, and, also as electron donors. This biological reduction, i.e. bioreduction is preferably carried out in anaerobic reaction conditions and at temperatures between 20 C and 50 C, more preferably between 30 and 40 C and particularly about 37 C.
However, it is also possible to replace the steps 1 to 3 in the above process example by a thermal treatment of the gypsum by using e.g. syngas produced by gasification of biomass or by using hydrometallurgical reduction with hydrogen sulphide H25. In one possible aspect the sulphate reduction is thus performed with calcium sulphide received from thermal roasting or sulphidisation of waste gypsum.
According to one embodiment, the waste sulphate material is reduced to a finely divided precipitate having a maximum particle size of below 0,50 gm, such as between 0,10 and 0,50 gm. The precipitate is typically formed as an ultimately fine sludge, with low or negligible degree of co-precipitated granules. In addition, the sulphides have higher magnetic susceptibility than the corresponding sulphates. Thus, the enriched sludge of such

7 PCT/F12014/050891 rare earth metal sulphides, which have potentially high magnetic susceptibility, can be subjected to an effective fractionation process by applying high magnetic fields.
According to another embodiment of the present invention, the metal precipitate obtained by the bioreduction or such reductive treatment of waste gypsum, consists of elements, which are selected from the group of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ga, Ge, Ho, Nb, Sc, Ta, Th, U, Y, In, Al, Ca, Fe, K, Mg, Mn, Na, P and S, as their corresponding sulphides, phosphates or oxides, or as any combination thereof.
In a further embodiment, the formed non-rear earth metal comprising substances (i.e. non-magnetic e.g. calcium sulphate fraction from step 5) can be used for the treatment of acid mine waters to precipitate heavy metal sulphates or it may be recycled in a thermal process to recycle sulphur as sulphuric acid and calcium as quicklime.
As mentioned earlier, the sulphate reduction may also be performed by a chemical reaction or reactions in aqueous sulphate slurry by using hydrogen sulphide. Sulphate waste material can also be treated thermally to produce metal precipitates. Thereby, according to one embodiment of the invention, it is possible to combine the chemical reaction and the thermal treatment or carry out each presented reduction scheme solely.
HGMS equipment is preferred for an efficient separation of the finely dispersed magnetic REs. The equipment itself is usually rather simple and provides an easy flushing of magnetics. In addition the maintenance cost is low as well as the power consumption.
Herein it is preferred to choose HGMS equipment capable of recovering rare earth metals having a magnetic susceptibility x of at least 1 000, more preferably at least 5 000.
However, also lower (below 1 000) and higher (up to even 150 000) susceptibilities may exist after sulphate reduction, so the separator should preferably be adjustable or able to perform within a wide magnetic susceptibility scale.
Thus, the present invention wherein sulphate reduction is combined with magnetic separation provides an environmentally friendly and an efficient method for recovering valuable rare earth metals from waste sulphate materials. The said method is targeted to metal companies and is usable around the world, especially in areas where industrial phosphate production takes place.

8 Herein below the present invention is illustrated by a non-limiting working example. It should be understood, however, that the embodiments given in the description above and in the example are for illustrative purposes only, and that various changes and modifications are possible within the scope of the claims.
Example 1 Phosphogypsum samples were first dried in oven (105 C, 20 h). Gypsum leachate was then prepared by adding dry phosphogypsum powder to water (50 g/L), followed by 24 h mixing in Erlenmeyer glass. Obtained solution was filtered (0,45 gm) to remove solid phosphogypsum particles. Clear solution was used for sulphate reducing bacteria (SRB) studies. The phosphogypsum filtrate was rendered anaerobic by flushing with N2 gas through a 0,22 gm pore size filter for 1 hour, after which the flask containing the gypsum leachate was sealed with a gas tight butyl rubber stopper and open top screw cap. The phosphogypsum leachate was amended with 0,2 g yeast extract and 3,75 ml lactate L-1.
Pre-grown Desulfovibrio desulfuricans bacteria were added to the 2,5 L volume of phosphogypsum leachate.
The culture developed a precipitation, which was collected on a 0,22 gm pore size filter funnel by vacuum suction. The precipitate was rinsed from the filter with sterile double distilled water, collected in 50 ml cone tubes and dried prior to analysis.
The formed precipitate was analysed by using standard ICM-MS and ICP-OES methods. The contents of La, Ce and Y in the SRB precipitate were observed as 30 400, 66 200 and 8 800 ppm (mg/kg), respectively. The Nd-content of the SRB precipitate was 45 000 ppm.
The result indicates substantial enrichment of the said metals and also of other rare earth metals in the formed SRB precipitate.
The highly magnetized fraction of the precipitate was then recovered with high grade magnetic separation (HGMS), providing an enrichment, wherein the content of above mentioned rare earth metals was high, as disclosed in Table 1:

9 Table 1. Rare earth metal content of the enrichment after recovery.
Component ppm w-%
La 137290 13,7 Ce 263940 26,4 Y 25600 2,6 Nd 396610 39,7(*) (*) from separate experiment (FI 101787 B) Example 2 The experiment as described in Example 1 was identically repeated to test the reproducibility of the procedure. The contents of La, Ce and Y in the test 2 SRB precipitate were observed as 33 900, 77 300 and 5 200 ppm (mg/kg), respectively. The Nd-content of the SRB precipitate was 38 900 ppm.
By using the similar magnetic separation as in test 1, the final enrichment is as disclosed in Table 2:
Table 2. Rare earth metal content of the enrichment after recovery Component ppm w-%
La 153097 15,3 Ce 308196 30,8 Y 15127 1,5 Nd 342848 34,3(*) (*) from separate experiment (FI 101787 B) Example 3 Phosphogypsum samples of the same origin as used in the aforementioned patent FI
101787 B were dried in oven (105 C, 20 h). Gypsum leachate was prepared by adding dry phosphogypsum powder to water (50 g/L), followed by 24 h mixing in Erlenmeyer glass.
Obtained solution was filtered (0,45 gm) to remove solid phosphogypsum particles. Clear solution was used for sulphate reducing bacteria (SRB) studies.

The continuously operated sulphate reduction and REE precipitation experiment was done in 0,7-liter UASB (upflow anaerobic sludge blanket) column, equipped also with solution recycling line with a powerful pump to adjust the sludge fluidization and, if needed, to mix and homogenize the sludge in column. The column was inoculated with 500 ml of 5 -- anaerobic granular sludge from an operating waste water treatment plant, and filled up to a total volume of 700 ml with sulphate rich water. Microbial activity was ensured by continuing the sulphate rich water, ethanol and substrates pumping. When sulphate reduction was performing reliably, sludge inside the column was agitated with recycle line pumping (300 ml/h for 1 minute) and homogenized sludge sample was taken from the

10 -- column for elemental analysis, corresponding an initial situation of the sludge. Then phosphogypsum filtrate was pumped to the column.
The phosphogypsum filtrate used in the experiment was rendered anaerobic by flushing with N2 gas for 1 hour and pumped then to 0,7-liter column with the speed of 27 ml/h for -- 20 days. Simultaneously, substrate-nutrition solution was pumped to the column with the speed of 1,75 ml/h for providing following concentrations to the total feed:
ethanol (0,16 v-%), KH2PO4 (13,8 mg/1), (NH4)2SO4 (33,7 mg/1), ascorbic acid (2,7 mg/1), thioglycolic acid (2,7 mg/1) and yeast extract (2,7 mg/1). With these parameters, the hydraulic retention time (HRT) was maintained at 24 hours. During 20 days of running, the pH, ORP
and -- sulphate reduction rates were observed. The pH remained in the area of 5,5 ¨ 5,8, while ORP remained in values less than -200 mV (Ag/AgC1/ 3M KC1 electrode). Sulphate reduction rates were fluctuating from 38 to 80 %. After the 20 day experiment, sludge in column was again agitated with recycle line pumping (300 ml/h for 1 minute) and homogenized sludge sample was taken from the column for elemental analysis.
While the inoculated waste suspension dilutes the observed REE contents, the experiment yet indicated significant enrichment of those into the sludge. The following enrichment factors were found for La, Ce, Y and Nd during the treatment:
La initial 7,3 mg/kg; final 202,0 mg/kg (enrichment ratio of 28);
-- Ce initial 13 mg/kg; final 477 mg/kg (enrichment ratio of 37);
Y initial 3,6 mg/kg, final 48,8 mg/kg (enrichment ratio of 14) and Nd initial 7,2 mg/kg, final 295 mg/kg (enrichment ratio of 41).
Elemental contribution studies of phosphogypsum filtrate, fed to the column, revealed that precipitation rate was 100 % for La, Ce, Y and Nd.

11 While the above description and example show and describe and point out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the details of the method may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same operations or give substantially the same results as those achieved above are within the scope of the invention.
Substitutions of the elements from one described embodiment to another are also fully intended and contemplated. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.
Citation list - patent literature 1. WO 2011/008137 A3 2. FI 101787 B
3. WO 2009/125064 Al 4. EP 0844981 B1 5. WO 2013/044376 Al Citation list - non-patent literature Al-Shawi, A. W., Engdal, S. E., Jenssen, O. B., Jorgenssen, T. R., Rosaeg, M., The integrated recovery of rare earths from apatite in the Odda process of fertilizer production by solvent extraction. A plant experience, Proc. Int. Solvent Extraction Conf., ISEC 2002, Johannesburg.
Jorjani, E., Bagherieh, A. H., Chelgani, S. C., Rare earth elements leaching from Chadormalu apatite concentrate: Laboratory studies and regression predictions, Korean Journal of Chemical Engineering, Vol. 28, pp. 557-562, 2011.
Kaksonen, A. H., Puhakka, J. A., Sulfate Reduction Based Bioprocesses for the Treatment of Acid Mine Drainage and the Recovery of Metals, Engineering in Life Sciences, Vol. 7, pp. 541-564, 2007.

12 Kaufman, E. N., Little, M. H., Selvaraj, P., A biological process for the reclamation of flue gas desulfurization gypsum using mixed sulfate-reducing bacteria with inexpensive carbon sources, Applied Biochemistry and Biotechnology, Vol. 63-65, pp. 677-693, 1997.
Lounamaa, N., Mattila, T., Judin, V. P., Sund, H. E., Recovery of rare earths from phosphorus rock by solvent extraction, Proc. 2nd. Int. Congr. Phosphorus Compounds, Institut Mondial du Phosphate, Paris, 1980, pp. 759-768.
Preston, J. S., Cole, P. M., Craig, W. M., Feather, A. M., The recovery of rare earth oxides from a phosphoric acid by-product. Part 1. Leaching of rare earth values and recovery of a mixed rare earth oxide by solvent extraction, Hydrometallurgy, Vol. 41, pp.
1-19, 1996.
Rzeczycka, M., Miernik, A., Markiewicz, Z., Simultaneous Degradation of Waste Phosphogypsum and Liquid Manure from Industrial Pig Farm by a Mixed Community of Sulfate-Reducing Bacteria, Polish Journal of Microbiology, Vol. 59, pp. 241-247, 2010.
Watson, J. H. P., Ellwood, D. C., Duggleby, C. J., A chemostat with magnetic feedback for the growth of sulphate reducing bacteria and its application to the removal and recovery of heavy metals from solution, Minerals Engineering, Vol. 9, pp. 973-983, 1996.

Claims (17)

Claims

1. A method for recovering an enrichment of rare earth metal compounds from a waste sulphate material, characterized in that the method includes - reducing the waste sulphate material to a metal sulphide precipitate, and - recovering a magnetized fraction of the metal sulphide precipitate with a magnetic separator.

2. The method according to claim 1, characterized in that the waste sulphate material is selected from waste gypsum, such as waste phosphogypsum.

3. The method according to claim 1 or 2, characterized in that the waste sulphate material is reduced to the metal sulphide precipitate via bioreduction by using sulphate reducing bacteria in an aqueous sulphate solution.

4. The method according to claim 3, characterized in selecting the sulphate reducing bacteria from those originating from genus Desulfovibrio, the bacteria being for example Desulfovibrio desulfuricans.

5. The method according to any of the preceding claims, characterized in that the reduction is carried out as a bioreduction, wherein sulphate reducing bacteria use carbon sources, such as sewage digests, alcohols or synthesis gas, as microbial nutrients and electron donors.

6. The method according to any of the preceding claims, characterized in that the reduction is carried out as a bioreduction in anaerobic reaction conditions and at temperatures between 20 °C and 50 °C, preferably between 30 and 40 °C.

7. The method according to any of the preceding claims, characterized in that the waste sulphate material is dissolved into dilute sulphuric acid or water before reducing it to the metal sulphide precipitate.

8. The method according to any of the preceding claims, characterized in that sulphate reduction is performed with calcium sulphide received from thermal roasting or sulphidisation of waste gypsum.

9. The method according to claims 1, 2 or 8 characterized in that sulphate reduction is performed in aqueous sulphate slurry by using hydrogen sulphide.

10. The method according to claims 1, 2 or 8 characterized in that sulphate reduction is solely performed by thermal reduction of the sulphate waste material.

11. The method according to any of the preceding claims, characterized in that the waste sulphate material is reduced to a finely divided metal sulphide precipitate having a maximum particle size of 0,50 µm.

12. The method according to any of the preceding claims, characterized in that the metal sulphide precipitate consists of elements, selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ga, Ge, Ho, Nb, Sc, Ta, Th, U, Y, In, Al, Ca, Fe, K, Mg, Mn, Na, P and S, as their corresponding sulphides, phosphates or oxides, or as a combination thereof.

13. The method according to any of the preceding claims, characterized in that the magnetized fraction of the metal precipitate comprises sulphides, phosphates or oxides, or a combination thereof, of rare earth metals and has a higher magnetic susceptibility compared to other non-rare earth metal substances, such as calcium compounds, present in said precipitate.

14. The method according to any of the preceding claims, characterized in that the magnetized fraction preferably has a magnetic susceptibility .CHI. of at least 1 000.

15. The method according to any of the preceding claims, characterized in that a high gradient magnetic separator is used for separating the magnetized fraction of the metal sulphide precipitate.

16. Use of the method according to claims 1 to 15 for recovering rare earth metals from waste phosphogypsum.

17. Use of the method according to claims 1 to 15 for precipitating heavy metal sulphates of mine waters or for recycling sulphur as sulphuric acid and calcium as quicklime with the non-rare earth metal substances, such as calcium compounds, present in the metal sulphide precipitate.