Recovery of Valuable Metals from Acoustic Magnet Swarf Slurry

Acoustic magnet swarf slurry (AMSS) has 1900 tons production worldwide. The composition of AMSS contained 12.45% neodymium, 34.35% iron, 0.48% boron and 52.5% of cooling reagent. Removing cooling reagent and decreasing iron dissolution percent were achieved by oxidation roasting and selective leaching. Selective leaching removed 89% of iron with parameter of 0.5M HCl, solid-liquid ratio equal to 1:100, 4 hours at 95°C. The rest of iron was separated by solvent extraction with Aliquat 336 as extractant. The optimal parameters of extracting iron were 0.1M A336, 3M chloride ion, aqueous-organic ratio equal to 1:3 and mixed for 1 minute. Oxalic acid added to solution after solvent extraction to precipitate neodymium. The final product was neodymium oxidize with 99% of purity by calcined neodymium oxalate at 900°C for 0.5 hour. The recovery percent of neodymium was 99%.


Introduction
The production of acoustic magnet swarf slurry (AMSS) depended on the production of acoustic magnet.There are 1900 tons of AMSS production per year worldwide [1][2][3].AMSS was produced in molding processing of acoustic magnet [4].The demand of acoustic magnet will increase at percent of 20% for past 10 years [5][6].Due to the high demand of acoustic magnet and in view of economy and limited material, the recovery of valuable metals in AMSS is necessary.There are few literatures studied on AMSS; however, many kinds of method studied on recovering metals from permanent magnet.Compared to many kinds of methods [7][8][9][10], hydrometallurgy was a better one from energy perspective.
According to different acid we had been studied [11][12][13], HCl has a great effect on leaching iron.During iron removing process, 20-30% of metals present in the leachate will lose due to co-precipitation [14].Oxidation roasting is carried out to decrease iron leaching efficiency.In separation, solvent extraction is chosen because of highly selective on different elements.In recent years, different extractants had been studied [15][16][17][18].Phosphorous-based extractants play a key role in rare earth [19].However, phosphorous-based extractants was not a great choice on AMSS due to the co-extraction effect on iron and rare earth elements.Aliquat 336 (A336) is chosen because of high loading capacity of iron (III) in chloride-contained solution [20].In purification process, oxalate acid has high selective precipitation on rare earth.Hence, we will build a recovery process with the reagent above to achieve effective result.In this study, we focused on finding optimal experimental parameters and built a recycle process for AMSS.

Raw material analysis and pretreatment
AMSS dried by furnace and obtained moisture content equal to 50%.Crystal phase and condition of AMSS were analysed by X-ray diffraction (XRD, Dandong, DX-2700) and Thermogravimetric Analysis (TGA, Perkin Elmer, Pyris Diamond TG/DTA).After the oxidation roasting process, all the AMSS after oxidation roasting was sieved through a 38 mm mesh (#400 mesh) to liberate elements cladded in oxidized iron.The composition of AMSS dissolved by aqua regia and the composition of solution in this experiment were determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Varian, Vista-MPX).

Selective leaching
Main purpose of selective leaching is to dissolve the valuable metals and precipitated most iron into residue.The equilibrium time and temperature were fixed to 4 hours and 368 K. Other parameters were experimented like acidity (0.2-0.8 N), solid-liquid ratio (20-200 ml/g) were set for finding the optimal condition to leach valuable metals.

Solvent extraction
Aliquat 336 (A336) as an extarctant is able to separate iron from solution.Extraction of Iron into A336 was an effective reaction within 30 seconds, so we fixed mixing time as 1 minute.Concentration of chloride ion (0.1-3.0 N) and extarctant (0.2-2.0 mol/L) were adjusted by deionized water and kerosene.Aqueous-organic ratio (1-10 ml/ml) was also experimented to find out the optimal parameters.

Chemical precipitation and calcination
Oxalic acid was first dissolved by deionized water and adjusted into 1 mol/L solution.Different volume (1-20 ml) of oxalic acid added to stripping solution.Precipitate would turn into oxidation by 1073K calcination.The purity of final product was determined by XRD.

Raw material analysis and pretreatment
The composition of AMSS were shown in Table 1.The main character of AMSS was the cooling reagent which accounts for 52.5%.It was important to remove cooling reagent before leaching to avoid separating AMSS from acid.Therefore, oxidation roasting not only decreased iron leaching percent but also removed cooling reagent.Diffraction patterns (Fig. 1) showes the specific alloy (Nd 2 Fe 14 B) peak.It also revealed the AMSS containing oxidation like Nd 2 O 3 and Fe 3 O 4 .It was estimated that the friction between machine and raw magnet casting caused oxidation of AMSS.The main crystal phase and their leaching percent with same condition which AMSS was roasted on different temperature is presented in Table 2. FD in Table 2 indicated that AMSS was fully dissolved by aqua regia.In the Table 2 the increase of roasting temperature caused the increase of oxidized percent and the decrease the weight percent on each metals.Moreover, since the Fe2O3 phase participated in the reaction, the leaching percent of neodymium and iron decreased.In this process, leaching of neodymium and reducing iron dissolved percent were the main objective.Hence, we chose 600℃ as the optimal roasting temperature.

Selective leaching
Fig. 3 illustrates the effect of acid concentration in selective leaching process by increasing the acidity from 0.2 N to 0.8N and obtained 99% of neodymium dissolved on 0.5 N HCl.Since neodymium achieved equilibrium, more H + ion reacted with iron caused leaching of iron increasing continually.

Fig. 3. Efficiency of acidity on leaching of AMSS
The efficiency of solid-liquid ratio is illustrated on Fig. 4. Neodymium dissolved from 50% to 99% with solidliquid ratio from 1:20 to 1:100.However, leaching efficiency of iron under this condition increased to 25% which also needed to concern.Based on the literature, VanderHoogerstraete mentioned that the key point of removing iron in leaching process was acidity and time [21].Hence, the lower acidity could precipitate iron and higher solid-liquid ratio could dissolve iron cladded on the surface.Therefore, we chose 0.4 N HCl and solid-liquid ratio of 0.01 as optimal parameter.(0.5M, 368K, 5 hours) Fig. 4. The effect of solid-liquid ratio on leaching process

Solvent extraction
The extraction reaction of A336 with metal is presented as Eq. 1 [23]: Fig. 5 illustrates that 2 N chloride ion could extract 99% of iron.However, when chloride concentration more than 3 N, the extraction reaction could be improved by increasing [FeCl � � ] to make whole reaction stabilized [16].Extraction percent of iron decreased from 99% to 68.8% with aqua-organic from 1:1 to 1:10.As neodymium purity concern, we chosen 1:3 as optimal parameter of aquaorganic ratio.At the extraction process, we chosen 1:3 of aqua-organic ratio, 3 N of chloride ion and 0.1 N of A336 with 1 minute mixing.With the condition above, we separated 99% of iron in this process.

Chemical precipitation and calcination
The result of precipitating neodymium by using oxalic acid is illustrated in Fig. 8. Co-precipitation effect occurred on boron, however, the content of boron in AMSS was less than 1%.The co-precipitation only had a few effect to the neodymium purity.The neodymium

Conclusion
The suggested recovery process of AMSS is presented in Fig. 8.After roasting at 600℃ with 6 hours, selective leaching would separate 89% of iron.The concentration of chloride ion would adjust by HCl to prepare the aqueous phase of solvent extraction.The rest of iron was separated by solvent extraction with Aliquat 336.The mass of oxalic acid requested added was calculated by the experiment above.Each litre of solution had to mix 18 grams of oxalic acid to precipitate 99% of neodymium.The neodymium oxalate was calcined at 900℃ in 30 minutes to obtain neodymium oxide.The purity of final product was over 99% and recovery percent of neodymium was 99%.

Fig. 1 .
Fig. 1.The XRD pattern of raw material.The result of TG-DTA diagram is presented in Fig. 2. Three reverse points at 550, 850, 1000 ℃ revealed the different crystal phase generation including Fe 2 O 3 , Fe 3 O 4 , Nd 2 O 3 and NdBO 3 .

(Fig. 5 .Fig. 6 .
Fig. 5.The efficiency of HCl concentration on extractionIn Fig.6, the result shows that extraction percent of neodymium increased from 1.0 to 1.5 N but achieve equilibrium on 2.0 N A336.Because of the low distribution ratio of neodymium in A336, the extraction percent was less than 10% even with higher concentration of A336.Higher concentration of A336 increased extraction percent of iron and achieved equilibrium in 1.0 N.

Table 1 .
Composition of AMSS

Table 2 .
Crystal phase and leaching efficiency of different roasting temperature with fixed condition E3S Web of Conferences 53, 04015 (2018) https://doi.org/10.1051/e3sconf/20185304015ICAEER 2018 oxalate was calcined by furnace at 1173 K with 30 minutes.The purity of neodymium oxide product was over 99%.