DE102012017418B4 - Method for obtaining the double salt NaNd (SO4) 2 * H20 from a starting mixture - Google Patents

Abstract

Process for recovering the double salt NaNd (SO4) 2 · H2O from a pre-comminuted starting mixture containing NdFeB, which undergoes magnetic separation and comminution followed by hydrometallurgical digestion with the addition of sulfuric acid, while determining the hydrometallurgical digestion released hydrogen volume flow or the total volume of hydrogen released, wherein the released hydrogen volume flow or the total volume of hydrogen is used as a control variable for the amount of sulfuric acid to be added and / or used to determine the completion of the hydrometallurgical digestion, and then by adding a sodium-containing saline solution, of a sodium-containing salt and / or NaOH, the sparingly soluble double salt NaNd (SO4) 2 · H2O is precipitated after hydrometallurgical digestion.

Description

The invention relates to a process for obtaining the double salt NaNd (SO ₄ ) ₂ .H ₂ O and optionally from Nd ₂ O ₃ from starting mixtures / scrap, in which NdFeB is contained.

Neodymium is currently the rare earth metal with the highest demand. Above all due to its diverse field of application in powerful NdFeB permanent magnets for hard disks, loudspeakers as well as for efficient motors and generators, there is a great need. Furthermore, neodymium oxide (Nd ₂ O ₃ ) is used as a dopant for glasses, laser crystals, capacitors and as a polymerization catalyst for Polybutadienkautschukherstellung. Due to the increasing importance of electric drives and generators (electromobility, wind power) and the simultaneous depletion of neodymium as a raw material on the world market, an efficient, environmentally friendly and cost-effective recycling process of neodymium from scrap becomes an urgent task in the field of recycling of rare earths.

For the recovery of neodymium from unmixed NdFeB material, mainly powder metallurgical processes such as hydrogen embrittlement (HD or HDDR processes) have been used in the manufacture of NdFeB magnets. However, these processes require very high initial purities of the starting materials to be recycled, since otherwise the yield is insufficient and thus produced magnets achieve only inferior properties. In particular, there is a problem with partially corroded magnetic material, which requires extensive prior removal of oxide and hydroxide layers for recycling by hydrogen embrittlement. On sintered NdFeB magnets, moreover, a protective layer of tin, nickel, aluminum or a spray or dip is usually applied. This can mean very high process engineering requirements for recycling by means of hydrogen embrittlement and, in most cases, prevents a recycling process for magnetic material of different origin and composition. Another problem is bound magnets in which the unsintered NdFeB material in a plastic, such. B. an epoxy resin is embedded, which also prevents an efficient recycling process due to the limited possibilities for separation by hydrogen embrittlement.

In addition to powder metallurgy, various hydrometallurgical recycling processes are also known, which by principle offer better possibilities of substance separation by wet-chemical procedures. According to the disclosure of WO 94/26665 A1 The decomposition of the NdFeB material to be recycled is effected, for example, by superficial oxidation of NdFeB by means of NaOH to Nd ₂ O ₃ , which is then dissolved in acetic acid, selectively crystallized by concentration as neodymium acetate, fluorinated and reduced in the last step to elementary neodymium. In this process, among other things, the only superficial oxidation of NdFeB to Nd ₂ O ₃ , the resulting very slow digestion rate and the need for continuous mechanical comminution during the digestion process with simultaneous magnetic separation of elemental iron, disadvantageous.

In US 5,129,945 B1 it is described that the digestion of process waste of the rare earth magnet production can be done with 2 MH ₂ SO ₄ . From the solution, the double salt NaNd (SO ₄ ) ₂ .H ₂ O or NH ₄ Nd (SO ₄ ) ₂ .H ₂ O is precipitated selectively by addition of NaOH or NH ₄ OH. For the separation of magnetic scrap of different composition, this method indicates the possibility that, due to the different solubility product of the double salts, cerium earths (La, Ce, Pr, Nd, Sm, Eu, Gd) and iron earths (Tb, Dy, Ho, Er, Tm, Yb, Lu) can be separated from each other without much effort.

In US 5,129,945 B1 the double salt is fluorinated by the addition of hydrofluoric acid to NdF ₃ which, after thorough drying, is then calciothermally reduced back to elemental Nd. However, this approach is problematic due to the use and release of hydrogen fluoride (HF). It is therefore desirable to develop a simpler and more environmentally friendly processing of the precipitation products to a required raw material. In US 5,129,945 B1 also the reaction of the double salt with an oxalic acid solution to neodymium oxalate is described, which can be decomposed by thermal treatment at 900 ° C to Nd ₂ O ₃ , CO ₂ and H ₂ O. The problem is another required salt metathesis step in conjunction with the loss of oxalic acid used.

One of the main obstacles to the effective recycling of neodymium from waste magnetic materials by hydrometallurgical processes, however, is a lack of or insufficient control of the digestion of magnetically rich materials of varying origin with varying texture and composition. Thus, the time required for digestion, depending on the composition and nature of the material to be recycled, is very long (eg, by passivation and agglomeration, but also not otherwise separable inert foreign materials) and both the digestion itself and the subsequent precipitation require unnecessary high use of chemicals.

Furthermore, it is problematic that NdFeB-containing scrap / waste from different sources can contain very different impurities that do not occur in unmixed process waste during magnet production - including, for example, B. other rare earth magnets such as SmCo. According to the prior art, in this case, for example, only the precipitation of a mixed double salt such as NaNd _1-x Sm (SO ₄ ) ₂ .H ₂ O is carried out with subsequent solvent extraction or ion exchange for the separation of the rare earths. However, this procedure is very complicated and prevents a simple recycling process. Other separation paths during digestion, such. B. by hydrogen flotation ( US 5,238,489 B1 ), are in real occurring old materials due to other impurities (eg by plastics) not or difficult to implement.

None of the known recycling processes can efficiently recycle mixed, non-grade magnetic materials that are subject to variations in composition and texture.

Another method of recovering rare earth-nickel alloy elements by dissolving elements of waste in nitric acid.

Out EP 0 790 322 A1 For example, a method of recovering rare earths from rare earth nickel alloy scrap is known.

It is therefore an object of the invention to provide opportunities for effective recovery of neodymium oxide in order to achieve a high proportion of unsorted waste / scrap with reduced effort.

According to the invention, this object is achieved by a method having the features of the claim. Advantageous embodiments and further developments of the invention can be achieved with features described in the subordinate claims.

The present invention solves the recycling problem of non-grade NdFeB magnetic materials of variable composition and nature by a combination of various process steps, which are described in detail below.

First of all, pre-comminution of the material mixture to be recycled, in which NdFeB is contained, can take place.

In the process according to the invention, a starting mixture in which NdFeB is present is subjected to hydrometallurgical digestion with the addition of sulfuric acid. At the same time, the determination of the hydrogen volume flow released during the hydrometallurgical digestion or of the total released hydrogen volume is carried out, the released hydrogen volume flow or the entire volume of hydrogen being used as a controlled variable for the amount of sulfuric acid to be added and / or used to determine the completion of the hydrometallurgical digestion.

Thus, the increase of the released hydrogen volume flow can be determined and, if this increase has dropped to a predefinable threshold, can be reacted accordingly.

If the hydrogen volume flow falls below a predeterminable threshold value, the hydrometallurgical digestion can be ended, since at least the major part of a chemical reaction in which Nd ^{3+ can be} converted into a chemical compound in which neodymium is present has been reacted.

It is advantageously possible to control the amount of acid to be added by means of the increase of the released hydrogen volume flow and / or its integral.

In the invention, at least one magnetic separation can be performed. In this case, demagnetized NdFeB can be separated from a starting mixture / scrap. For this purpose, only ferro- and ferrimagnetic materials are separated.

Alternatively, a separation of all magnetized materials in the scrap can take place. For separation, magnetic separation technique can be used, which is preferably not selective for all ferromagnetic materials, but only for magnetized (remanent) ferromagnetic and ferrimagnetic materials, in particular NdFeB or SmCo magnets. Subsequent to this magnetic separation or, if appropriate, subsequent to a further magnetic separation, fine comminution and homogenization of the enriched and demagnetized and NdFeB-containing part of the starting mixture / scrap can be carried out with the subsequent hydrometallurgical digestion.

The magnetic separation can be realized by a technically comparatively simple adaptation of conventional magnetic separators by substitution of the separator magnets or coils by soft magnetic ferromagnetic separators (eg of iron). This pre-sorting of the metal-containing scrap also allows the separation of all unbound super-para-, para- and diamagnetic foreign matter from the magnets, such as plastics. The advantage of this As a result, NdFeB accumulates heavily, which significantly reduces the amount of chemicals required for subsequent hydrometallurgical digestion and significantly reduces its susceptibility to potentially interfering foreign materials. In addition, the time required for digestion is greatly reduced because less foreign matter and foreign matter must be digested with.

After the separated magnetized material has been heated above the Curie temperature of NdFeB (583 K), another magnetic separation can optionally be carried out analogously to the first method step in order to separate materials with a higher Curie temperature (eg SmCo, AlNiCo) from the demagnetized NdFeB and thus to further enrich the NdFeB content in the starting material mixture (scrap). Any buildup of NdFeB on magnetized materials will not interfere significantly with this process, since such materials are usually minority constituents of the feedstock mix / scrap. This physical separation of, in particular, SmCo even before the hydrometallurgical digestion is technically much easier to implement than a complex chemical separation of the various rare earths by means of solvent extraction or ion exchange after the digestion process. If no other rare earth magnets are present in the starting mixture / scrap apart from NdFeB, the demagnetization can also take place by means of an alternating electromagnetic field instead of by heating above the Curie temperature. However, demagnetization should be performed for the subsequent comminution and hydrometallurgical digestion to prevent coalescence of the ground particles.

In the next step, the fine comminution and homogenization of the enriched and demagnetized scrap can take place. In many experiments, the particle size distribution of the milled scrap was investigated for optimized digestion. Average particle sizes in the range from 100 μm to 250 μm have proved favorable for the hydrometallurgical digestion with regard to a short grinding time, a high digestion rate and an optimum yield. However, since the brittleness of the NdFeB in the starting mixture / scrap is very different from the brittleness of foreign materials (eg Ni), the grinding parameters should be adapted individually to the composition of the starting mixture / scrap for optimized digestion. In the present invention, the grinding parameters can be optimized for the first time in the current process for each current scrap composition. In this case, the control unit used may be the released hydrogen volume flow or the hydrogen volume flow (V _H2 ) integrated over a certain period of time from the hydrometallurgical pulping process described below.

A possible real-time reaction control of the acid hydrometallurgical digestion is advantageous. Depending on the chemical composition of the starting mixture / scrap (different proportions of NdFeB, Fe, Ni, plastics, etc.) suitable amounts of a strong acid are required for optimized digestion, and depending on the starting mixture / scrap quality (particle sizes, agglomeration behavior, etc.). but also environmental conditions (eg acid temperature), the hydrometallurgical digestion requires different periods of time. Although both the starting mixture / scrap composition and nature can vary greatly, a measure has been found which surprisingly permits reliable and reproducible control of many important process parameters. The continuous measurement of the hydrogen volume flow (dV _H2 / dt), the hydrogen released during the hydrometallurgical digestion, and the comparison of this variable with its time course can precisely control the respective amount and concentration of the acid required for the digestion. In this way, not only the amount of acid used can be greatly reduced, but also the time required for the hydrometallurgical digestion can be significantly reduced, since the reaction end of the digestion reaction can be determined. The performance of this process step is shown, for example, in direct comparison with the digestion process, which in US 5,129,945 B1 is described. There, the complete digestion according to the embodiments requires 24 hours, whereas in the present invention the total duration of mechanical pretreatment and digestion takes only a few minutes. The chemical background for the reaction control is the different reaction kinetics for the oxidation of the various occurring in the starting mixture / scrap constituents by acid under hydrogen release, for example: Nd ₂ Fe ₁₄ B + 34H ⁺ → 2Nd ³⁺ + 14Fe ²⁺ + B + 17H ₂ (very fast) Ni + 2H ⁺ → Ni ²⁺ + H ₂ (very slow)

In the case of hydrometallurgical digestion, H ₂ SO ₄ is added as acid.

After the end of the reaction, in the next step analogous to the prior art, undissolved constituents are separated from the reaction mixture and a double salt, for example the Composition NaNd (SO ₄ ) ₂ · H ₂ O, by adding a neutral sodium-containing salt solution and / or a sodium-containing salt, in particular saturated Na ₂ SO ₄ solution, and / or by addition of NaOH, preferably in excess, can be precipitated. The thus-separated double salt may be annealed in a heat treatment until Nd ₂ O _{3 is} obtained.

This double salt can also be converted to elemental neodymium by halogenation with HF or HCl followed by metallothermal, electrolytic or otherwise reduction of the neodymium halide.

As has been shown in many experiments, in contrast to the prior art in the present invention, a problematic measurement and targeted adjustment of the pH for the precipitation reaction is no longer necessary because the acid concentration by the previous in situ optimization of hydrometallurgical digestion already in the optimum range in terms of product purity and yield. Furthermore, the required amount of precipitating reagents (and acid concentration) can be calculated from the total volume of hydrogen measured at digestion, resulting in a further reduction in the amount of chemicals needed for the process. The quantitative precipitation can also be achieved at very low pH values.

Following the precipitation of the double salt this is separated. It can be washed and recrystallized for further purification depending on the required product purity.

In the last step, the prior art double salt can be reduced to elemental neodymium either by the multi-step work-up described above, but this implies the problems already described.

On the other hand, the direct thermal decomposition of NaNd (SO ₄ ) ₂ · H ₂ O to Nd ₂ O ₃ as well as Na ₂ O, SO ₃ and H ₂ O is much easier to realize in terms of process technology US 5,129,945 B1 can dispense with the additional Salzmetatheseschritt with oxalic acid without loss of quality in the product. The strongly different melting / resp. Sublimation temperatures of Nd ₂ O ₃ (melting point 3043 K), Na ₂ O (sublimation temperature 1548 K) and SO ₃ (decomposition α-SO ₃ : 323 K) not only allow separation of the decomposition by-products of Nd ₂ O ₃ already during the thermal decomposition , but by the targeted resublimation of Na ₂ O and by introducing SO ₃ in water can also recover for the process required NaOH and H ₂ SO ₄ , which makes the present recycling process even more efficient and environmentally friendly.

By a combination of a magnetic separation and final thermal magnetization stages in conjunction with a real-time control of the mechanical pre-treatment (refining) of the enriched starting mixture / scrap and the hydrometallurgical digestion process and the precipitation process (reaction times, chemical quantities, concentrations), wherein an in situ measurement of the hydrogen volume flow, the hydrogen generated in the digestion can be used particularly advantageously, a significant improvement over the prior art can be achieved. The effort can be reduced and the yield and product purity can be significantly increased.

The invention will be explained in more detail by way of example in the following. Individual features can be combined independently of each other. Characteristics are not necessarily bound to the respective example.

Showing:

1 : An example of a lifting magnetic separation for separating SmCo magnets from a mixture of different magnetic starting materials;

2 : Left - measured integrated hydrogen volume flow for the hydrometallurgical digestion of the enriched and demagnetized starting mixture / scrap after different grinding times. Right - the measured integrated hydrogen volume flow taking into account the grinding time to optimize the overall process time and

3 : Left - the measured integrated hydrogen volume flow for the digestion of the enriched and demagnetized starting mixture / scrap for different acid amounts. Right - the time course of the hydrogen volume flow during the hydrometallurgical digestion with the marking of possible control-technical threshold values for a further acid addition and for the determination of the end of the reaction.

1st example (magnetic separation):

To separate Sm from a starting mixture of various rare earth magnetic materials, a pre-shredded mixture consisting of 14.8 g of uncoated sintered SmCo magnets and 144.8 g of nickel-plated, sintered NdFeB magnets was heated to 375 ° C. under a nitrogen atmosphere (heating rate 3 K / min, holding time 60 minutes). The magnetized SmCo magnetic fragments thereafter were separated by reverse magneto-magnetic separation by means of a 100 × 100 × 2.5 mm ³ iron sheet behind a lift-off belt. 86.6 g of unmixed NdFeB magnets and 73.0 g of SmCo magnets with NdFeB adhesions were obtained.

2nd example (recycling):

200 g of nickel-coated, sintered NdFeB magnets from disk servos were demagnetized by heating to 375 ° C. (heating rate 3 K / min, holding time 60 min) under a nitrogen atmosphere. Twenty-five grams of the demagnetized magnets were then ground in a disk vibratory mill under air for 60 seconds. 1000 mg of the milled magnets were then digested by adding 6 ml of 2 MH ₂ SO ₄ at 24 ° C. while measuring the volume of hydrogen released. In each case when flattening the reaction (reduction of the released hydrogen volume flow to less than 20 ml / min) as long 1 ml 2 MH ₂ SO _{4 was} added until less than 1 ml / minute of hydrogen was released even after renewed addition of acid (V _ges [H ₂ ] = 440 ml, V _ges [H ₂ SO ₄ ] = 8 mL). The total time for milling and digestion was 5 min. The reaction mixture was filtered, and the sparingly soluble double salt NaNd (SO ₄ ) ₂ .H ₂ O was quantitatively crystallized out within 20 min by addition of saturated Na ₂ SO ₄ solution in excess (10 mL) at pH = 0.5. The double salt was filtered off, washed with water, predried at 80 ° C and then calcined at 1350 ° C for 10 hours in flowing air. 293 mg X-ray homogeneous Nd ₂ O _{3 were} obtained (yield 99% including 5 mass% nickel in the milled starting mixture). Ferrous impurities were less than 350 ppm.

Claims (12)

Process for obtaining the double salt NaNd (SO ₄ ) ₂ .H ₂ O from a pre-comminuted starting mixture containing NdFeB, in which a magnetic separation and a fine comminution are carried out and then a hydrometallurgical digestion with addition of sulfuric acid, with simultaneous determination of during the volume of hydrogen released or the total volume of hydrogen released is used as a controlled variable for the amount of sulfuric acid to be added and / or used to determine the completion of the hydrometallurgical digestion, and thereafter by addition of a hydrotreating agent sodium salt solution, a sodium-containing salt and / or NaOH, the sparingly soluble double salt NaNd (SO ₄ ) ₂ .H ₂ O is precipitated after the hydrometallurgical digestion. A method according to claim 1, characterized in that the amount of sulfuric acid to be added is controlled by means of the increase of the released hydrogen volume flow and / or its integral. A method according to claim 1 or 2, characterized in that in a magnetic separation demagnetized NdFeB is separated from a starting mixture / scrap and / or a magnetic separation only for magnetized (remanent) ferromagnetic and ferrimagnetic materials, in particular NdFeB or SmCo magnets are separated. Method according to one of the preceding claims, characterized in that before a magnetic separation, a demagnetization of the NdFeB with a heating up to above its Curie temperature and / or the use of electromagnetic alternating fields is performed. Method according to one of the preceding claims, characterized in that in a magnetic separation all unbound super-para, para- and diamagnetic foreign substances are separated from the magnets. Method according to one of the preceding claims, characterized in that in a magnetic separation materials with higher Curie temperature of demagnetized NdFeB are separated. Method according to one of the preceding claims, characterized in that a fine comminution to an average particle size in the range of 100 microns to 250 microns is performed. Method according to one of the preceding claims, characterized in that after precipitation, the double salt is separated. Method according to one of the preceding claims, characterized in that the double salt is annealed in a heat treatment until Nd ₂ O ₃ has been obtained. Method according to one of the preceding claims, characterized in that by addition of a neutral sodium-containing salt solution and / or a sodium-containing salt, in particular saturated Na ₂ SO ₄ solution, in excess and / or by addition of NaOH, the formed sparingly soluble double salt NaNd (SO ₄ ) ₂ · H ₂ O precipitated after hydrometallurgical digestion, separated and this double salt by halogenation with HF or HCl and then metallothermal, electrolytic or otherwise reduction of neodymium halide to elemental neodymium is converted. Method according to one of the preceding claims, characterized in that by targeted resublimation of Na ₂ O and by introducing SO ₃ in water for the hydrometallurgical pulping NaOH and H ₂ SO _{4 are} recovered. Method according to one of the preceding claims, characterized in that still magnetized SmCo magnetic fragments are separated by a reverse Aushebemagnetscheidung.

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