TECHN ICAL FIELD The present specification relates to the field of extractive metallurgy, in particular to the field of mineral or ore processing. More particularly, the present specification relates to the recovery of rare earth elements in ore processing.

BACKGROU ND Rare earth elements (REEs) are typically used in modern devices such as high- strength magnets, batteries, displays, lighting, and high performance metal alloys. However, these elements are usually highly dispersed in ore deposits, and therefore are not found as rare earth minerals for extraction. REEs are rather present as side products in the processing of ores. Various methods for ore processing are known and often involve a series of dissolution, leaching, extraction, separation steps and the like. It has proven beneficial to recover valuable REEs during ore processing, both for improving the efficiency of the ore processing itself, as well as for the recovery of REEs for further uses.

Based on the high selectivity of oxalate to REEs, a common method for the recovery of REEs is oxalic acid precipitation. The REEs are most likely found dissolved in solutions from ore refining processes. By the addition of oxalic acid in said solutions comprising dissolved REEs, REEs are precipitated as REE salts and can be recovered.

For example, a method for recovering rare earth elements is disclosed in

International patent application WO 2016/128621. This reference teaches a pregnant leach solution from the leaching of clay which is treated with oxalic acid for precipitation of REE salts.

Chinese patent applications 1043768 and 1044499 provide other examples of methods for the recovery of RREs comprising oxalic acid precipitation on acid leaching solution from the leaching of clay. Another example of oxalic acid precipitation for the recovery of REEs can be found in US patent publication 2015/0354026.

Methods known in the art usually require high amounts of oxalic acid and the obtained REEs are often of low purity. To overcome this, it is typically suggested to pre-purify the acid leaching solution to potentially remove impurities that would increase the need for oxalic acid and provide for lower purity of the recovered REEs. See for example Xia, C. (2013), A review on iron separation in rare earths hydrometallurgy using precipitation and solvent extractions methods, Proceedings of the MS&T'13 Rare Earth Elements, 255-275, in which a sulphuric acid pregnant solution is initially purified for removal of iron, aluminum, and the like, by precipitation through neutralization. Other known pre-purification methods involve double sulfate precipitation on pregnant leaching solution prior to recovery of REEs. (Li, L. (2011). Rare Earths Extraction and Separation, China: Inner Mongolia Science and Technology Press). A known attempt of a direct oxalate precipitation for REEs recovery is described in Rare Earth resources (Roche Engineering 2014, Technical Report on the Mineral Reserves and Development of the Bull Hill Mine, Wyoming (Pre-feasibility Study Report)). This reference teaches hydrochloric acid leach solution which were directly submitted to oxalate precipitation. However, the oxalic acid consumption was very high (35-76 g/L / 121-296 kg/t) mainly caused by the presence of iron and aluminum in the leach solution.

Another example is found in Anvia, M., Ho, E., and Soldenhoff, K. (2013), Alternative process for rare earths recovery from bastnasite containing ore, Proceedings of the MS&T13 Rare Earth Elements, 153-166, where the authors reported a direct oxalate precipitation on a pregnant leaching solution with an oxalic acid demand of 10 times the stochiometric amount.

It can thus be seen that impurities found in ore refining solutions are a problem when it comes to recovering REEs and that pre-purification adds on the complexity of the recovery and overall refining process. The drawbacks of adding steps are both economical and environmental. Therefore, there is a need to develop more efficient methods for recovering REEs , to reduce environmental impacts and global costs.

SUMMARY Certain exemplary embodiments provide A method for recovering at least one rare earth element from an aqueous acid solution, the method comprising the steps of: a) treating the aqueous acid solution with oxalic acid or at least one oxalate salt, in the presence of at least one additive salt to precipitate the at least one rare earth element as at least one rare earth salt; and b) recovering the at least one rare earth salt from the treated aqueous acid solution as a solid product.

The embodiments of the present specification will be better understood by referring to the following detailed description and the attached drawings, in which:

BRIEF DESCRIPTION OF TH E DRAWINGS

Figure 1 represents a conceptual flow chart of a direct oxalate precipitation

Figure 2 is a graph of REE recovery at various oxalic dosage conditions.

Figure 3 is a graph of REE recovery at various pH.

Figure 4 is a graph of REE recovery at various temperatures

Figure 5 is a graph of REE recovery for different alkalis for adjusting pH.

Figure 6 is a graph of REE recovery when NaCI is used as an additive salt.

Figure 7 is a graph of REE recovery for the study of the effect of NaCI.

Figure 8 is a graph of REE recovery with reduced oxalic addition.

Figure 9 is a graph of REE recovery for different additive salts.

DETAILED DESCRIPTION OF SELECTED EM BODIMENTS

In the following detailed description section, specific embodiments are described. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present specification, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the specification is not limited to the specific embodiments described below, but rather, includes all alternatives, modifications, and equivalents falling within the scope of the present specification.

At the outset, for ease of reference, certain terms used in the present specification and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present specification is not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present specification.

DEFIN ITIONS:

"Rare earth element" or "REE" as used herein defines one of a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides, and scandium and yttrium which are considered rare earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties. Therefore, rare earth elements are cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm),

samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb) and yttrium (Y).

"Oxalate" as used herein refers to the dianion derived from oxalic acid, having the formula C ₂ 0 ₄ ^2~ , also written as (COO) ₂ ^2~ .

"Ore processing" refers to a term used in the field of extractive metallurgy. Also known as mineral processing, ore dressing, or ore refining, it is the process of separating commercially valuable minerals from their ores. Those terms may be used interchangeably herein.

"Leaching" as used herein refers to an extractive metallurgy process where ore is soluble and impurities are insoluble, and therefore which converts metals into soluble salts in aqueous media. "Dissolution" is generally the process by which a solid, gas, or liquid is dispersed homogeneously in a gas, solid, or, especially, a liquid. In metallurgy, it most often refers to the dispersion of valuable metals found in ores into an aqueous solution and /or a suitable solvent. "Pre-purification" as used herein typically refers to a purification step to potentially remove some undesired impurities, carried out before undergoing a further treatment of an ore processing process.

"Hydrolysis" is usually the cleavage of chemical bonds by the addition of water. In many cases of metal ions such as iron and aluminum, hydrolysis tends to proceed as pH rises leading to the precipitation of a hydroxide.

As used herein, "low grade" refers to a materiel or solution of inferior quality, potentially containing lesser amounts of valuable materials and/or higher amounts of impurities.

"Pregnant solution" as used herein defines a metal-bearing aqueous solution obtained after treatment such as leaching in an ore refining process, and before concentration and recovery of the metal.

As used herein, the expression "stoichiometric amount" of a reagent of a reaction is the optimum amount or ratio where, assuming that the reaction proceeds to completion: all of the reagent is consumed; there is no deficiency of the reagent; there is no excess of the reagent. In other words, the stoichiometry is used to find the right amount of one reactant to "completely" react with the other reactant in a chemical reaction - that is, the stoichiometric amounts that would result in no leftover reactants when the reaction takes place.

In the recovery of rare earth elements, there is a need for more efficient and cost effective methods. In particular, there is a need to improve oxalic acid precipitation processes currently used in the art. The present specification provides a direct oxalate precipitation for REEs recovery from a low-grade REE pregnant solution, without the need for pre-purification. The REEs may typically be found in aqueous pregnant acid solutions derived from various steps of an ore refining process, such as dissolution, leaching, or the like. The pregnant acid solution therefore comprises at least one dissolved REE. In some embodiments, the solution comprises multiple dissolved REEs. The aqueous pregnant solution may be obtained, for example, from a dissolution or leaching process with an acid solution, such as sulfuric acid, hydrochloric acid, or the like, or mixtures thereof.

The direct oxalate precipitation of the present specification may be conducted directly on aqueous pregnant acid solution without pre-purification such as iron hydrolysis, iron precipitation, double sulfate precipitation, solvent extraction or the like, and combinations thereof. Proceeding with direct oxalate precipitation would generally increase the oxalic acid consumption and provide low yield and low purity REEs. However, the present specification provides for the direct oxalate

precipitation comprising treatment of the aqueous pregnant acid solution with oxalic acid or an oxalate salt, or mixtures thereof, in the presence of at least one additive salt, to cause precipitation of at least one REE as at least one REE salt. In one embodiment, the reaction conditions which will be described below may be adjusted to selectively precipitate only one specific REE. In other embodiments some selected RREs may precipitate and others may remain in the solution. In yet other embodiments, all of the REEs dissolved in the solution are reacted to precipitate.

The amount of oxalic acid added to the pregnant solution may be around 5 times the stoichiometric amount. Preferably, the amount of oxalic acid is around 4 times and more preferably the amount of oxalic acid is less then 2 times the stoichiometric amount. The additive salt may be a chloride, a sulfate or a combination thereof. Preferably, the additive salt is selected from NaCI, Na ₂ S0 ₄ , KCI, K ₂ S0 ₄ , NH ₄ CI, (N H ₄ ) ₂ S0 _4j and combination thereof. The additive salt may be added as a solid or as an aqueous solution in an amount of at least 0.5 mol/L of total volume of the aqueous acid solution, preferably at least 1 mol/L. Also preferably, the additive salt is a chloride salt.

The treatment of the aqueous pregnant acid solution with oxalic acid and the additive salt may be carried out around 15°C or ambient temperature (25°C), or the solution may be heated up to 100°C. Preferably, the solution is heated to a temperature from 50°C to 100°C, and more preferably between 75°C and 100°C

The treatment step may also comprise adjustment of the pH of the solution.

Typically, the pH may be adjusted with a base, preferably a hydroxide, an oxide or a carbonate, or any combination thereof of potassium, ammonium, magnesium, or any combination of these bases. The pH may be adjusted to a range between 0.5 and 3.5. Preferably, the pH may be adjusted between 1 and 3, and more preferably around 1.5. The pH may be adjusted at any time during the process when necessary, for example before the addition of oxalic acid, simultaneously with the addition of oxalic acid, after the addition of oxalic acid, or a combination thereof. Following the treatment step described above in the prescribed conditions, the solution may be left to react for a certain period of time to allow the REE salts to form. Preferably, the reaction temperature may be maintained during this aging time. The solution may then be left for the REE salts to settle for a period of time. The temperature may also be maintained during the settling time. The precipitated REE salts may then be recovered from the treated solution as a solid product. This step may be achieved by filtration or any suitable conventional methods.

The recovered REE salts may be submitted to additional treatments to further purify, separate, recover specific REEs, or the like. For example, the recovered precipitate may be calcinated, dissolved in an acid solution for further treatment, converted to hydroxides for acid dissolution, or any suitable recovery process commonly known in the art.

The solution obtained after the recovery of REEs precipitated salts may be further treated for recycling in any appropriate ore processing step. In one embodiment, the solution may be treated to raise the pH and therefore precipitate impu rities that may remain. For example, the pH can be raised from 1 to less than 3, or from 3 to 3.5. The precipitated impurities may then be recovered, for example by filtration to provide a cleaned or partially cleaned solution. This cleaned or partially cleaned solution may be subjected to further refining to recover possible remaining valuables metals or other valuable materials, for example, in a second leaching of dissolution step.

A complete exemplary flowsheet of a direct oxalate precipitation of the present specification is provided in Figure 1. It may be modified to comprise any suitable modifications according to the state of the art and provide different conceptual flowsheets within the scope of the present specification

While the present specification may be susceptible to various modifications and alternative forms, the embodiments will now be described by way of examples. However, it should again be understood that the specification is not intended to be limited to the particular examples disclosed herein. Indeed, the present

specification includes all alternatives, modifications, and equivalents falling within the scope of the present specification.

EXAMPLES

All experimental work used a pregnant leaching solution (PLS) collected from an acid baking water leach operation. The average pH of the PLS was 0.34. The REE contents of this PLS were weighted. The solution compositions are listed in Table 1.

Table 1 - Elemental composition of the PLS sample used as the feed of DOP

Element Concentration Element Concentration

(ppm) (ppm)

La 103.4 Nb 39.1

Ce 311.0 Zr 978.8

Pr 29.5 K 16.7

Nd 107.6 Na 118.7

Sm 32.8 S 26661.6

Eu 3.0 Si 179.7

Gd 35.9 Ti 155.8

Tb 8.8 Zn 96.0 Element Concentration Element Concentration

(ppm) (ppm)

Dy 65.4 Pb 11.5

Er 50.0 Al 263.3

Tm 9.4 Ca 1169.7

Yb 44.1 Fe 1558.6

Lu 5.7 Th 50.9

Y 425.5 U 3.4

All the chemicals in the present examples were of analytical grade. The oxalic acid was freshly prepared as 0.5 mol/L solution before each test. The solution pH was measured with Orion 8102 BNUWP Ross Ultra™ Combination pH probe. ATC (automatic temperature compensation) was used to properly read pH at elevated temperatures. The pH was controlled manually whenever required using NaOH (50% solution) and H ₂ S0 ₄ (10% solution). In one test, 14% ammonium hydroxide was used to adjust pH instead of NaOH. The reaction vessel was placed in a water bath sitting in a heating kettle (GLAS-COL), and the temperature was measured with a J-type thermal couple. Temperature control was realized through a GLAS-COL Digitrol™ II temperature controller.

Typically, the temperature could be controlled within a range of ±2°C. Ice pellets were used in the water bath to cool down the PLS temperature quickly, when required.

The reaction vessel containing 100 mL of PLS was first heated to the designated temperature and the pH was adjusted to the target level. Once the temperature of the solution was stabilized, the oxalic acid solution was slowly added through a micro pump at a constant rate, or through a glass pipette. The oxalic acid was delivered over a 30 minute period and agitation was provided by overhead stirrers. Unless otherwise stated, all experiments were executed in open vessels with a 30 minute reaction time (oxalic acid addition period). Following that, a 90 minute aging time was applied using the same agitation method and strength. During the aging period, the pH was monitored but not adjusted and the temperature was maintained. The solution was then allowed to settle for 20 hours. The settling temperature was maintained at the reaction temperature. To avoid excessive evaporation, the PLS vessel was capped loosely during the aging and settling stages. At the end of the settling stage, the product was filtered through a microfilter set using a Millipore™ filter paper. The solid was collected and oven-dried at 90°C to 100°C. The barren leaching solution (BLS) was measured for its volume and then samples were taken for chemical analysis by Can met MINING'S Analytical Service Group using both Inductively coupled plasma atomic emission spectroscopy (ICP AES) and Inductively coupled plasma mass spectrometry (ICP MS) methods.

REE recovery yield and oxalic stoichiometric consumption data were calculated from the analytical results. The recoveries of metallic elements were calculated using the metal concentration in the BLS and PLS. Metal balance calculations were practiced for REEs to locate significant errors in experimental or analytical procedures.

The oxalic acid consumption was defined as the addition amount of this reagent. The reagent regenerated from the barren solutions or oxalate products was not counted in the calculation of reagent consumptions. The oxalic acid consumption was counted as the times of stoichiometric amounts of oxalic acid required for all the REEs to precipitate as oxalate (times of stoichiometric demand - TSD).

RESULTS AN D DISCUSSION The majority of the direct oxalate precipitation (DOP) tests were conducted at 75°C to allow better crystallization and better rejection of impurity elements. As shown in Figure 2, the maximum TREE (total rare earth element) recovery (at 98.6%) was achieved when the oxalic acid addition was above 24 TSD. HREE (heavy rare earth element) recovery shows a slightly different trend. The maximum recovery of HREE (at 99.6%) was obtained at 18 TSD. The HREE recovery at 12 and 18 TSD was about 4-5% higher than LREE, indicating that HREE (yttrium included) was comparatively easier to precipitate than LREE (light rare earth elements) when oxalic acid was excessive. The solution pH is an important factor for oxalate precipitation to properly proceed, since the pH is directly influencing the activity of the dissociated oxalate anion which is the direct precipitation reactant. It is known in the art that between pH 0 and pH 7, the higher the pH the higher the concentration of ionized oxalate. As a result, the increase of pH from 1 to 4 could theoretically reduce the demand of total oxalic acid addition.

As shown in Table 2, the activity (a) of the oxalate anion increases from 0.77 to 177 with an increase in pH (between pH 0.8 and pH 2.5). Yttrium oxalate solubility (measured) increases slightly with the elevation of pH between 0.8 and 1.5. Above pH 1.5 however the solubility of yttrium oxalate increases significantly. Based on this consideration coupled with the concerns of impurity precipitation at higher pH, pH 1.5 was recommended for operating REE oxalate precipitation. The significant increase of yttrium solubility could possibly be a result of the formation of yttrium oxalate complex at higher oxalate activity at higher pH. Table 2 - Effect of pH on Y oxalate solubility (oxalate 0.1 mol/L)

PH (C204 ) Y oxalate

solubility (mg/L)

0.8 0.77 7.9

0.9 1.1 7.3

1.0 1.65 7.1

1.1 2.34 6.5

1.2 3.3 6.3

1.3 4.6 7.1

1.4 6.2 7.5

1.5 8.4 8.1

1.7 15 8.9

2.0 33 14

2.5 177 22

As shown in Figure 3, the REE recovery increases with increasing of the pH. Four precipitation tests were conducted at pH 0.5, 1.0, 1.5 and 2.1 respectively. For each tests, the pH was continuously and manually controlled throughout the reacting and aging stage (90 minutes) at a constant value. When the pH was above 1.5, the HREE recovery dropped slightly but the LREE recovery increased. A possible reason for this split trend is that with the addition of sodium hydroxide as the pH adjusting alkali, more sodium ion becomes available and this is in favor of precipitation of LREE. At the same oxalic acid addition conditions, HREE precipitation becomes less favorable.

Temperature has an impact on oxalate anion activity, oxalic acid solubility and oxalate metal complex stability. More importantly, an elevated temperature (e.g. 70°C-80°C) is recommended for producing strong and fully formed crystals that are easy to filter. Between 75 and 100°C, TREE recovery increased with increasing temperature, but all REE recoveries observed at high temperature were lower than that at 25°C (Figure 4). Such an observation indicates that the room temperature and the high temperature oxalate precipitates have different chemistries. The response of oxalate precipitation at elevated temperature (i.e. 70-100°C) appeared to fit into the trend observed in known double-sulphate salt precipitation, i.e., the solubility of double sulphate salts drops with the elevation of reacting temperature. In the present examples, the temperature conditions that favour the formation of double salt may also increase the REE recovery in oxalate precipitation process.

A comparison study was done with one test using sodium hydroxide to adjust pH, and another using ammonium hydroxide, which is less favorable in forming double sulphate salts with REEs when comparing to sodium ion. This is mostly because the solubility of REE double salts differs, i.e., xRE2(S04)3.y(N H4)2S04.zH20 > xlRE2(S04)3.ylNa2S04.zlH20 > x2RE2(S04)3.y2K2S04.z2H20 (Li, 2011). Li 2011 also reported that these differences were used in practice for the preliminary separation of LREE and HREE. As such, using ammonium hydroxide to adjust the pH is expected to produce a lower REE recovery. As shown in Figure 5, the REE recovery was apparently higher when NaOH was used to adjust the pHof the PLS. H REE recovery was 1% higher when NaOH was used. LREE recovery was 10% higher when NaOH was used compared to NH ₄ OH. Using NaOH resulted in an elevation of the sodium concentration in the PLS, and obviously, with significant presence of su lphate anion, such an increase of sodium concentration favours the formation of double salts and potentially enhanced the REE precipitation in DOP. This observation further supports the concept that double salt formation may play a positive role in oxalate precipitation.

If the positive effect of double salt formation is real, adding more of the additive salt should result in even higher REE recovery. A study on REE recovery at different NaCI addition conditions is shown in Figure 6. By adding 120 g/L NaCI, the REE recovery increased significantly. LREE recovery increased by 5.6%, while HREE recovery increased by 2.0%. Under these operating conditions, further increases of NaCI addition seems to have no further positive effects on the REE recovery.

At high oxalic acid addition conditions, the effect of NaCI might be masked. A set of tests was carried out at with 10 TSD oxalic acid added as reported in Table 3. Lower REE recoveries but more significant differences in REE recovery results were expected at this rate. The results are shown in Figure 7. By increasing the temperature from 25 to 75°C the TREE recovery is reduced. The significant drop of HREE recovery (by 17.9%) is notably responsible for the reduced TREE recovery. On the other hand, the LREE recovery increased indicating that the addition of NaCI at higher temperature plays a role in forming LREE double salts and enhanced the LREE precipitation in DOP. When reducing the addition of NaCI in Test 3 (NaCI 120 g/L rather than 240 g/L), the gap between HREE and LREE recovery was also largely reduced. HREE precipitation was more favourable at lower NaCI, indicating that with limited addition of oxalic acid, more oxalate was used to precipitate LREE at higher NaCI condition, leaving less oxalate to react with HREE.

Table 3 - Test conditions summary for Figure 7

Test Oxalic acid PH Additive Temperature

number addition (NaCI g/L) (°C)

1 10 TSD 1.5 240 25

2 10 TSD 1.5 240 75

3 10 TSD 1.5 120 75

A trial to further cut the oxalate addition to 4.7 TSD was conducted at 25°C. The NaCI addition amounts were 240 g/L and 300 g/L respectively. The purpose of this test at 25°C was to examine whether the additive salt could enhance the REE oxalate recovery at room temperature. The results (Figure 8) show that the TREE recovery reached 96.8% at this low oxalic acid addition condition when 300 g/L of NaCI was added. With 240 g/L of NaCI, the TREE recovery was 94.7%.

Other additive salts were tested for their effects on rare earth precipitation with oxalic acid. The results are demonstrated in Figure 9. With no additive salt, the precipitation of TREE was about 80%. The addition of NaCI, KCI and N H ₄ CI at 4.1 mol/L, all had a substantial positive effect on the recovery of TREE, H REE and LREE. KCI as the additive salt had greatest impact on REE recovery (>98%). NH ₄ CI was weaker compared to the positive effects of NaCI and KCI. The addition of a same amount of Na ₂ S0 ₄ however showed a very different trend than the other three additive salts. The addition of sodium sulphate largely inhibited the precipitation of HREE but increased the precipitation of LREE. Such a phenomenon indicates that the use of sodium, potassium and ammonium ions as additive salt could enhance the LREE precipitation. However, to obtain an increased recovery of HREE, it is preferred that the additive salt contains chloride ions. A hypothesis offering an over simplified explanation to this result may be that the presence of excessive chloride anion triggers a competition between oxalate and chloride anions in the complexing of iron, which is the main reason for the high oxalic acid consumption.

In this proposed DOP flowsheet (see Figure 1), the addition of large amount of additive salts is a challenge to the total operating cost unless a large portion of these reagents are recycled from the barren solution. Obviously, the majority of impurities in the barren solution must be removed before the barren solution can be reused and recycled in this flowsheet.

Direct oxalate precipitation of REEs from low grade PLS is challenged by high oxalic acid consumption. Preliminary research was conducted on dilute acid baking water leach PLS. Complete precipitation of REEs required 18 TSD of oxalic acid. Increasing the reaction pH from 0.5 to 2.1 increased the recovery of both LREE and H REE.

Within the range of 75 to 100°C, the elevation of temperature results in increased REE recovery. However, higher REE recovery was observed at 25°C, which indicates that the effect of temperature on the reaction chemistry has to be further investigated. Using NaOH to adjust pH appeared to have higher REE recovery than with the use of ammonia. The addition of NaCI, KG, and N H ₄ CI as additive salt all substantially enhanced the LREE and H REE recovery. When using NaCI as additive salt (240 g/L or 4.1 mol/L), the minimum oxalic demand for recovery of 95% REE was reduced substantially to 4.7 TSD. With an addition of 4.1 mol/L of KCI, the REE recovery could reach 98% with the same oxalic acid addition (4.7 TSD). Sodium sulphate salt was also found to enhance the LREE recovery but HREE recovery was depressed, indicating the important role of chloride anion in the additive salts. All results in the present examples indicated that the formation of double salt plays an important role in oxalic precipitation from sulphated PLS. Utilizing this side reaction chemistry provided by the use of an additive salt, good REE recovery by oxalic precipitation (>95%) could be realized at comparatively lower addition amounts of oxalic acid reagent.