Preparation for Samples Containing Rare Earth Metals

Issue

How to prepare for samples containing rare earth metals

Environment

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• iCAP TQ

Resolution

The following procedure has been used for the preparation of silicate mineral samples for ICP-OES and ICP-MS measurement: 

1. Weigh ~ 20 grams of sample into a 500 mL Erlenmeyer flask. 
2. Add 100 mL of 1:1 HNO₃. 
3. Place flask on a hot plate and bring to a gentle boil for 2 hours. 
4. Cool, filter and transfer to a 200 mL volumetric flask and dilute to volume with DI water. 

The accuracy of the extraction on specific minerals would need to be confirmed by analyzing the same sample (same aliquot that was extracted) using a complete digestion approach after extraction.
This approach is considered better than comparison of independent samples prepared by the two methods that suggest that sample homogeneity issues complicate extraction studies where recoveries between 60 and 100% have been found on samples thought to be the same.
The measurement technique suggested is ICP-MS. 

Catalysts 

Zeolites and, to a lesser extent, silica gel are common supports for the RE elements.

The following procedure may prove useful: 

1. Weigh 0.1 to 0.5 grams of sample into a Pt crucible or dish. 
2. Add 20 mL of HClO4 and 20 to 50 mL of HF depending upon sample size. 
3. Heat on a hot plate to the dense white fumes of HClO4. 
4. Cool and add 15 mL of a 4% H3BO3 solution. 
5. Quantitatively transfer to a quartz or Vycor beaker and heat to heavy dense fumes. 
6. Transfer to a volumetric flask (250 to 1000 depending), add dilute with DI water and HCl to 10 % v/v HCl. 

The above procedure is presented only as a guide into the basis theory behind the acid digestion of silica containing catalysts where HF is used to break apart and dissolve the entire zeolites, heating to fumes of HClO4 then rids the sample of excess HF, and very importantly the addition of boric acid and heating to fumes results in the dissolution of any insoluble RE fluorides.
Si is lost during the preparation. 

Organic Matrices 

• Samples can be digested with nitric/perchloric.  
• It is also very acceptable to dry ash organic samples for RE elemental analysis in a Pt crucible and then bring the resulting RE oxides into solution using a sodium carbonate fusion. Or if the RE element alone is sought, dissolution in dilute nitric or HCl.  

The following is a general guide for carbonate fusions: 

1. Make certain that the sample is well mixed with the sodium carbonate. 
2. A 5-9s pure sodium carbonate is recommended. 
3. Mix the sample with the flux at no more than a 1:20 ratio. Typical sample to flux ratios are in the 1:10 area. 
4. If organic matter is present, either the sample is mixed with the flux initially and heated slowly to 500 °C for ~ 2 hours before bring up to full temperature, or the sample can be pre-ashed at 500 °C and then the ash mixed with the flux. 
5. Use Pt as the crucible container material. 
6. Perform the fusion at 1000 °C in a muffle furnace. Avoid flames since this fusion is difficult to perform in a flame due to the high melting point of the sodium carbonate. 
   NOTE: Most fusions are complete in 15 minutes and some require up to 45 minutes. 
7. Dissolve the fuseate in dilute HCl (1:1). 

Hydrolytic Stability and Preferred Matrices 

• The hydrolytic instability of the RE elements increases with increasing atomic number.
• The hydrolytic stability is related to the ionic radius, which decreases with atomic number (the lanthanide contraction).
• None of the RE elements are as prone to hydrolysis as is Sc, which has a smaller ionic radius than any of the REEs.
• For comparison purposes, Sc begins to precipitate from solution at a pH of between 2 (high conc. Sc) to 4 (low conc. Sc), whereas Ce begins to precipitate from pH 6 (high conc Ce) to 8 (low conc Ce) and Yb begins to precipitate from pH 5 (high conc Yb) to pH 7 (low conc Yb).
• The RE element oxides, hydroxides, carbonates, fluorides, oxalates, sulfates, and phosphate are all insoluble in water and neutral to basic media. The fluorides and oxalates are insoluble in dilute nitric acid. The fluorides may be dissolved in 1:4 nitric acid: water that is saturated with boric acid. The chlorides are soluble in water. 
• The RE elements can be mixed with any of the elements at high concentrations (200 to 2000 µg/mL), with the exception of the fluoride containing elements (Ti, Zr, Hf, Nb, Ta, W, Si, Ge, Sn, Sb, Mo). Low levels (≤ 1 µg/mL) can be mixed with all of the elements, provided the nitric acid content is 5 to 10 % v/v. In general, the RE elements are more stable in relatively high (> 5% v/v) levels of acid. 

ICP Measurement 

• The RE elements have very complex emission spectra and are difficult to measure in the presence of one another using ICP-OES. For this reason, ICP-MS is the measurement technique of choice by many analysts. 
• Naturally, the RE elements occur in the presence of one another where as a general rule:
• La, Ce, Nd, Pr and, Sm are most abundant;
• Dy, Er, Eu, Gd, and Tb are moderately abundant, and;
• Ho, Lu, Yb, and Tm are least abundant. 
• The groups are separated in concentration by one or more orders in magnitude, i.e., if Ce is low (1-4) % level, then Dy is hundreds of ppm and Tm is sub ppm.

Analysis

• The Rare Earth (RE) elements include Y, La, and the Lanthanides (Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu).
• The chemistry of these elements are sufficiently similar that they can be treated as a group rather than individually.
• RE elements are technically the elements grouped as the 'Lanthanides', which are Ce thru Lu.
• However, chemists generally think of the Rare Earths as Sc, Y, La and the Lanthanides (Ce thru Lu) while others exclude Sc but include all of the others.

The following table shows the source of the names of the RE elements:

The Metals 

• The metals for these elements: are all attacked by moist air; are insoluble in NaOH or HN4OH; slowly react with H2O liberating H2; and are readily soluble in HNO3 and HCl. 

• The most common oxidation state formed upon acid dissolution is +3. Ce can have a valence of +4 and Eu, Yb, and Sm can have a valence of +2. 

Oxides, Hydroxides, Carbonates 

• The hydroxides are soluble in acids and the oxides are soluble provided they have not been ignited to high temperatures.
• Therefore, when ashing samples, attempt to keep the ashing temperature at 450 to 500 deg °C.
• Heating the oxides with 1:1 nitric acid is common with dissolution taking several minutes to hours depending upon sample size and oxide thermal history.
• The carbonates are insoluble in water and soluble in dilute nitric and HCl. 

Minerals 

• The most common minerals are silicates, phosphates, tantalates, and niobates that are summarized as follows: 
  • Silicates - Gadonite (contains Fe, Be, Y and Gd thru Lu plus small amounts of Ce); Cerite (silicates of La, Ce, Pr, Nd, Sm and Eu); Allanite (contains Ca, Fe, Al, La, Ce, Pr, Nd, Sm and Eu).
  • Phosphates - Monazite (contains mainly phosphates of Ce and La, Pr, Nd, Sm and Eu plus small amounts of the other RE as well as Th). 
  • Tantalates and Niobates - Fergusonite (contains Ta, Nb, Y, Gd thru Lu, Ce and Tb); Euxenite (contains Nb, Ti, Ce, U, Y, and Gd thru Lu); Samarskite (contains Nb, Ta, Y, Fe, Ca, U, and Tb). 

• Since the RE minerals contain such a wide assortment of other elements, a variety of sample preparation methods exist, including fusions with Na2O2, Na2CO3, Li2CO3 and Li2B4O7, as well as digestions with strong mineral acids such as HF/Perchloric /nitric/hydrochloric acid combinations.
• Of these fluxes, the use of lithium tetraborate (Li2B4O7) has proven to be a very useful way of opening out minerals associated with the REs. 
• A procedure reported by Knaack, C., Cornelius, S.B. and Hooper, P.R., GeoAnalytical Lab, Washington State University, December, 1994 involves fusion of the sample (2 grams) with an equal amount of the lithium tetraborate flux in a carbon crucible at 1000 °C in a muffle furnace for 30 minutes.
• The fuseate is then treated with HF, nitric, and then perchloric acids with a repetition of the perchloric acid treatment to dissolve fluorides. The final solution is made to 60 mL and contains nitric acid, hydrogen peroxide and HF, where In, Re, and Ru are added as internal standards for an ICP-MS measurement.
• This method is reported to be applicable for the lanthanides (La thru Lu) together with Ba, Rb, Y, Nb, Cs, Hf, Ta, Pb, Th, Sr, and Zr. 
• Many analysts prefer acid digestions using mineral acids singularly or in combinations depending upon the mineral involved.
• For example 1:1 HNO3 can be used to extract the RE from the sample leaving behind silica and other residues.
• The extraction is typically performed for 2-4 hours using heat.
• Both glass and Teflon vessels are commonly used. An acid digestion/extraction is particularly useful if silica is absent.