© The Minerals, Metals & Materials Society 2018
Boyd R. Davis, Michael S. Moats, Shijie Wang, Dean Gregurek, Joël Kapusta, Thomas P. Battle, Mark E. Schlesinger, Gerardo Raul Alvear Flores, Evgueni Jak, Graeme Goodall, Michael L. Free, Edouard Asselin, Alexandre Chagnes, David Dreisinger, Matthew Jeffrey, Jaeheon Lee, Graeme Miller, Jochen Petersen, Virginia S. T. Ciminelli, Qian Xu, Ronald Molnar, Jeff Adams, Wenying Liu, Niels Verbaan, John Goode, Ian M. London, Gisele Azimi, Alex Forstner, Ronel Kappes and Tarun Bhambhani (eds.)Extraction 2018The Minerals, Metals & Materials Serieshttps://doi.org/10.1007/978-3-319-95022-8_196

Extraction and Purification of Rare Earth Elements and Cobalt from NdFeB Magnet Wastes

Hoda Emami1   and Pouya Hajiani1
(1)
INNORD/GéoMégA Resources, 75 Boulevard de Mortagne, Boucherville, QC, J4B 6Y4, Canada
 
 
Hoda Emami
Email: hemami@geomega.ca

Abstract

In this study, a combined pyrometallurgical and hydrometallurgical technique has been designed and used to recover rare earth mixture elements (REE ) and cobalt from a NdFeB magnetic waste. while addressing the cost of reagent and process facility. This process can be adjusted to different magnetic wastes regardless of their compositions. Prior to the leaching , the magnet scrap was pretreated by two rounds of roasting (750°C in air) and a mechanical grinding in between, to convert iron metal to ferric oxide. Subsequent leaching was performed using optimized conditions (pH, temperature and retention time) where the extraction ratio of REE to Fe III is the highest. After removing iron content from the leachate (pH ≈ 3), REE was precipitated selectively and purified further. The produced REE concentrate reaches 99% purity with 85% recovery in a single run. The cobalt oxide was isolated from the PLS as a by-product with 99.8% purity.

Keywords

HydrometallurgyRare earth extractionRecycling magnetic waste

Introduction

Currently, extracting REE from secondary sources is gaining more attention than extraction from primary sources [1]. Potential secondary resources for REEs include permanent magnetic waste scraps, used batteries , circuit board scraps, etc. [2]. These wastes are either generated as scraps from manufacturing units or as wastes from industrial processes. Among these wastes, magnetic scraps are significantly more attractive for REE recovery due to their high REE content, particularly in Nd, Dy and Pr [3–5].

Industrial magnet residues have been identified to consist of an average REE grade of above 40% TREO and between 1% and 4% cobalt . Elemental analysis of a magnetic waste scrap provided by LCM Corp. is shown in Table 1. The residues typically contain up to 4 different REEs, the main ones being Nd and Dy, critical in the production of permanent magnets . Without any identified chemical plant to treat and recycle such a waste in North America, tons of magnet residue are piled or shipped to overseas SX plants. Having a competitive process to treat the residue and isolate valuable elements is of interest.
Table 1

Elemental analysis of magnetic waste scrap

Elements and compounds

Content (wt%)

Al

0.2

B

0.7

Co

2.6

Cu

0.1

Dy2O3

16.3

Fe

47.3

Ga

0.1

Nd2O3

32.1

Pr6O11

0.06

In this study, the whole process (Fig. 1) was designed to fully recover the rare earth metals contained in magnetic waste scrap. The REE in magnetic waste material was digested selectively in acid during leaching , and the main impurities were removed in the following stages. Cobalt oxide is isolated and precipitated in the last step. The final concentrate was a solid rare earth carbonate with 99% purity and 85% recovery in a single run.
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Fig. 1

Simplified flow sheet for hydrometallurgical process of REE and Co extraction from magnetic waste

Process Description

Mechanical and Heat Treatment

To selectively and efficiently recover REE while reducing acid consumption during leaching , the magnetic waste was passed through a heat treatment process to oxidize metallic elements. Specifically, iron (the main impurity) was oxidized to ferric iron oxide , as ferric iron oxide in roasted scrap is much less reactive to acid during leaching compared to iron metal.

Reaction 1 occurs during heat treatment of magnetic waste:
$$ xM + \frac{y}{2}O_{2} \to M_{x} O_{y} \left( {M:Fe,Co,Al\,and\,Ga} \right) $$
(1)

The magnet scrap was pretreated by two rounds of roasting (750 °C in air) for a total of 6 h with mechanical grinding in-between. Since the oxidation state of iron depends on the heat treatment conditions, retention time and heating rate were optimized in order to fully transform iron to ferric oxide.

At the end of calcination , the weight gain was 23%, with a rare earth oxide content of 32% in the final roasted concentrate. This step is crucial before the rest of process, as performing a proper heat treatment will reduce acid consumption considerably in leaching .

Leaching

During the heat treatment and grinding, the powder particle size decreases through the formation of micro cracks (through oxygen absorption) and mechanical stress and strains (through grinding). The decreasing particle size increases the accessible powder surface area for reaction with acid during leaching . The calcined powder contains metal oxide and RE oxide with some impurities which are leached by HCl (reaction 2).
$$ 2yHCl + M_{x} O_{y} \to xMCl_{2y/x} + yH_{2} O \left( {M:RE,Fe,Co,Al\,and\,Ga} \right) $$
(2)

The stoichiometric amount of HCl for RE oxide leaching with 20% excess was used to selectively leach RE oxide against iron . The leaching conditions (pH, retention time and temperature ) were monitored and optimized to enhance mild acid leaching and selectivity for RE oxide. Temperature was elevated to 90 °C to improve diffusion limit kinetics.

After leaching , REE recovery was 85% without leaching a significant amount of ferric iron into the leachate (Table 2). After filtration , the solid cake was transferred to the magnetic separation unit. The magnetic part of the residue (containing remaining rare earth oxide) is recycled to the feed stream for the leaching unit.
Table 2

Acid leaching recovery

Element

Recovery (wt%)

Fe

6.8

Al

57.9

Cu

51.3

Dy

86.2

Nd

83.8

Pr

87.3

Co

35.8

Precipitation and Purification for Recovering REE and Co Raw Materials

One of the major concerns in recycling magnetic scraps is the impurities, which may affect the magnetic properties. Thus, the major emphasis on recovering the raw materials was made on reaching to the purities for raw recycled materials as required by magnetic producer. Following the leach , the liquor was purified in two steps by selective precipitation . Iron was first precipitated below pH 3 by using sodium hydroxide, then copper was removed as copper sulfide at controlled pH conditions (pH = 3 to 4).

REE were precipitated by using sodium hydroxide and purified further in the next step to increase the REE purity to 99% in the final mixed concentrate. After washing and drying, the rare earth precipitate can be converted to oxides through thermal decomposition. The remaining Co in the liquor after REE precipitation was precipitated in the final step as a cobalt hydroxide by-product. Trace copper impurity was removed by precipitating copper sulfide , which has a lower solubility compared to cobalt sulfide , allowing for removal of Cu as CuS from the Co concentrate. The final purity of Co by-product after the precipitation was 99.8%.

Discussion

In this process, acid consumption was reduced considerably by heat treatment prior to selective acid leaching . REE extraction from magnetic scraps seeking full digestion through direct leaching requires three times more HCl, of which almost 68% will be used for iron leaching .

By applying heat treatment, iron was controlled without using significant amounts of reagent or energy . In fact, the main energy consumer in this process is the heat treatment, which operates at elevated temperature . This step should be optimized and properly designed. Considering the elemental recovery after leaching , acid consumption for 95% REE recovery decreased by 64% compared to full acid digestion. Acid consumption per element in Table 3 shows that almost 19% of the acid is used for iron leaching and 72% is related to extraction of REE . Further optimization of the heat treatment process and leach may allow for better selectivity of REE against iron .
Table 3

Acid consumption per element

Element

Acid consumption (%)

Fe

19.2

Al

2.53

Cu

0.27

Dy

24.11

Nd

48.21

Pr

0.26

Co

5.21

As a further attempt to develop a more cost-effective and efficient REE extraction process, we have modified the process by changing to full acid digestion via direct leaching and eliminating the heat treatment and grinding unit. Even though that full digestion of the scrap is not preferred due to its high acid consumption , this process will regenerate and recover the acid efficiently. An innovative acid regeneration technique integrated in this process plays a major role in the feasibility and simplifies the operation. Iron removal is performed without using any reagent, while it can recover the associated acid simultaneously. High REE recovery (99%) was obtained from this method compared to the aforementioned process. Acid recovery of 50% was achieved in the acid regeneration unit. This unit is under investigation to improve the acid recovery up to 95%.

Conclusion

The advantage of a magnet residue as a secondary source of REE is that it already includes high REE grades compared to raw minerals , while containing less impurities. Like many other REE sources, iron is the most important impurity in these residues, which forms about 50% of metals in the feed.

The industrial magnet residue has been processed using selective acid leaching . The residue was first roasted twice at 750 °C for a total of 6 h with a grinding round in between. Air roasting converts all metals into oxides (especially metal iron to ferric oxide). Then, leaching with diluted HCl extracted more than 85% of REEs without significant ferric iron extraction . Following iron removal at pH less than 3 by sodium hydroxide solution, REE was precipitated by using the same reagent with Co removed from the barren liquor . The recoveries and purities of products and by-product (cobalt ) were assessed to be used in a feasibility study.