There are many kinds of minerals containing rare earth elements, and their compositions are also very complex. Rare earth analysis includes very rich contents, involving almost all fields of chemical analysis and instrumental analysis, and is a difficult point in analytical chemistry. The analysis of rare earth elements can be divided into two categories: one is the determination of the total amount of rare earth elements, including the determination of the group content of rare earth elements; The second is the determination of single rare earth element content. To master the analysis of rare earth elements, we must have a comprehensive understanding of the basic properties of rare earth elements, the characteristics of rare earth ores and the analysis methods of rare earth elements, so that after receiving rare earth samples, we can choose reasonable analysis methods according to the characteristics of the samples and their analysis tasks, and correctly issue analysis and inspection sheets.
task analysis
1. Distribution, occurrence and classification of rare earth minerals in the crust.
The total mass fraction of rare earth elements in the earth's crust is 0.0 153%, of which cerium is the largest (accounting for 0.0046%), followed by yttrium, neodymium and lanthanum. The lowest content is promethium, then thulium, lutetium, terbium, europium, holmium, erbium, ytterbium and so on. Rare earth elements mainly exist in three states in the earth's crust:
(1) exists in ores as a single rare earth mineral, such as monazite, bastnaesite and xenotime.
(2) Calcium, strontium, barium, manganese, zirconium and thorium in isomorphic replacement minerals exist in rock-forming minerals, other metallic minerals and nonmetallic minerals, such as fluorite, apatite and ilmenite.
(3) It is adsorbed on the particle surface or interlayer of some minerals in the form of ions, such as rare earth ions adsorbed on the particle surface or interlayer of clay minerals and mica minerals, forming ion-adsorbed rare earth deposits.
Ion adsorption ore is a rare earth ore with important industrial value unique to China. 75% ~ 95% of rare earth elements in ion-adsorbed rare earth minerals are adsorbed in kaolin and mica in ionic state, and the rest 10% of rare earth elements exist in mineral phase (bastnaesite, monazite, xenotime, etc.). ), isomorphism (mica, feldspar, fluorite, etc. ) and solid dispersed phase (timely, etc. ). The content of rare earth oxides in ion-adsorbed rare earth minerals is generally around 0.65438 0%, and some can be as high as 0.3%. According to the distribution value of rare earth elements in ionic rare earth minerals, they can be divided into the following types: yttrium-rich heavy rare earth minerals, europium-rich medium yttrium light rare earth minerals, medium yttrium heavy rare earth minerals, lanthanum-rich neodymium light rare earth minerals, medium yttrium light rare earth minerals and non-selective distribution rare earth minerals. Ionic rare earth ore can be leached with NaCl, (NH4)2SO4, NH4Cl and other solutions without beneficiation, and then the rare earth in the solution can be converted into oxalate or carbonate, and finally the rare earth oxide can be obtained by burning.
Second, the analytical chemical properties of rare earth elements
(a) A brief description of the chemical properties of rare earth elements
Rare earth elements are located in group Ⅲ B of the periodic table of elements, including scandium (Sc), yttrium (Y) and lanthanides lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (nd), promethium (Pm), samarium (Sm), europium (Eu) and gadolinium (Gd). Their atomic numbers are 2 1, 39 and 57 ~ 7 1 respectively. Lanthanum, cerium, praseodymium, neodymium, promethium, samarium and europium are light rare earths, while gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and yttrium are heavy rare earths. Rare earth elements are typical metal elements, and their metal activity is second only to alkali metals and alkaline earth metals, and similar to aluminum. Rare earth metals are unstable in the air, and will oxidize and change color when they contact with humid air, so they need to be preserved in kerosene. Rare earth metals can decompose water, acting slowly in cold water and rapidly in hot water, releasing hydrogen. Rare earth metals do not work with alkali.
(2) Properties of main compounds of rare earth elements
(1) rare earth oxide. Rare earth oxides are very important compounds in rare earth analytical chemistry. The standard solutions of various rare earth elements are basically prepared with high-purity rare earth oxides. Rare earth oxides can be obtained by burning rare earth hydroxides, oxalates, carbonates, nitrates and rare earth metals in air. Most rare earth elements generate trivalent oxides after combustion, cerium is tetravalent oxide CeO2, praseodymium is Pr6O 1 1, and terbium is Tb4O7. Rare earth oxides are insoluble in water and alkaline solutions, but soluble in inorganic acids (except hydrofluoric acid and phosphoric acid).
(2) rare earth oxalate. The solubility of rare earth oxalate is small, which is the basis for the determination of total rare earth by oxalate gravimetric method. With the increase of atomic number, the solubility of rare earth oxalate increases, so the error of gravimetric determination of heavy rare earth elements is greater than that of light rare earth elements. Rare earth oxalate can be completely converted into rare earth oxide when burned at 800 ~ 900℃.
(3) rare earth hydroxide. Generally speaking, rare earth hydroxides are colloidal precipitates. The pH value of different rare earth hydroxides at the beginning of precipitation is different, which decreases with the increase of atomic number and becomes weaker and weaker. Rare earth hydroxide is mainly used to separate rare earth elements from copper, zinc, nickel, calcium and magnesium.
(4) Rare earth halides. Fluoride is insoluble in rare earth halides and can be used for the separation and enrichment of rare earth elements. Other halides have great solubility in water and are easily deliquescent. Rare earth fluoride can be dissolved in H2SO4 or HNO3-HClO4.
Third, the decomposition method of rare earth ore
(1) acid decomposition method. Due to the diversity and complexity of rare earth minerals, their decomposition methods are different. Most rare earth minerals can be decomposed by sulfuric acid or acidic solvents. For example, beryl and cerium oxide can be decomposed by hydrochloric acid, while monazite and xenotime can't be decomposed completely by concentrated hydrochloric acid, so hot sulfuric acid must be used. Insoluble rare earth niobate minerals can be decomposed by hydrofluoric acid and acid sulfate.
Sealing or microwave digestion is a very effective method to decompose rare earth minerals, which has the advantages of fast speed, complete decomposition, low blank and low loss. Microwave digestion generally uses nitric acid+hydrofluoric acid.
(2) Alkali melting decomposition method. Alkaline melting decomposition method is suitable for almost all rare earth ores, and generally sodium peroxide or sodium hydroxide (or sodium hydroxide plus a small amount of sodium peroxide) is used. Its advantages are short melting time, and anions such as phosphate, silicate, aluminate and fluoride can be separated after water immersion, which simplifies the subsequent analysis process.
(3) Salt leaching of ionic rare earth minerals. In addition to mixed rare earth oxides extracted by chemical methods and mixed rare earth oxides obtained by other treatment processes, some samples of ionic rare earth ores are also raw rare earth ores. Ionic rare earth mines generally require the determination of the total amount of ionic rare earth and the total amount of all-phase rare earth (ionic phase and mineral are equal). The sample decomposition method is the same as other rare earth ores that determine the total amount of rare earth. However, the determination of the total amount of ionic rare earth has its unique sample treatment method-salt leaching method.
Leaching agents used in ionic rare earth ore leaching are various electrolyte solutions, and the leaching process is an ion exchange process, which follows the general law of ion exchange. The essence of salt leaching method is to use a certain concentration of salt solution as leaching agent (in fact, it is an analytical agent) to desorb rare earth cations adsorbed in mineral soil and then transfer them to leaching solution. Various electrolyte (acid, alkali, salt) solutions with appropriate concentration can be used as leaching agents for ionic rare earth ores. Commonly used leaching agents are: ammonium chloride, sodium chloride, ammonium sulfate, hydrochloric acid, sulfuric acid and so on.
The main factors affecting the leaching rate are the type, concentration and pH value of leaching agent. The leaching rate of rare earth increases with the increase of leaching agent concentration. However, at this time, the leaching rate of non-rare earth impurities also increases accordingly, and it is necessary to choose the appropriate leaching agent concentration through experiments.
The pH value of hydrolysis of rare earth ions in water is 6 ~ 7.5. Therefore, the pH value of rare earth leaching solution must be less than 6. If the pH value is too low, the acidity of the leaching agent is too high. At this time, although a high leaching rate of rare earth can be obtained, the leaching rate of non-rare earth impurities also increases accordingly, which may interfere with the subsequent determination. On the contrary, if the pH value of the leaching solution is too high, rare earth ions will hydrolyze and precipitate, reducing the leaching rate. Generally, the pH value of leaching solution is controlled in the range of 4.5 ~ 5.5, and ideal results can be obtained.
In rare earth analysis, ammonium sulfate (2%) is generally chosen as the leaching agent of ionic rare earth ore, considering the leaching rate of rare earth, the leaching rate of impurities and the difficulty of controlling the pH value of leaching solution.
Fourth, the separation and enrichment methods of rare earth elements
The main separation and enrichment methods of rare earth elements are shown in Table 6- 1.
Table 6- 1 Main separation and enrichment methods of rare earth elements
Five, the analysis method of rare earth elements
The main task of rare earth analysis is to determine the total amount of rare earth, the content of single rare earth element in mixed rare earth and the amount of cerium group rare earth or yttrium group rare earth. Because the chemical properties of rare earth elements are very similar, rare earth analysis is one of the most difficult and complicated topics in inorganic analysis. In order to determine the total amount of rare earth elements in various content ranges and different forms and various single rare earth elements, almost all analytical methods are used. The following introduces the most commonly used analysis methods in rare earth analysis.
(1) chemical analysis method
The chemical analysis methods of rare earth elements include gravimetric method and titration method, which are mainly used to determine the total amount of rare earth elements.
1. gravimetric method
Gravimetric method is used to analyze samples with rare earth content greater than 5%, which is an ancient and classic analysis method for determining the total amount of rare earth. Although this method has a long process and complicated operation, its accuracy and precision are superior to other methods, so the arbitration analysis or standard analysis methods of the total amount of constant rare earths at home and abroad are gravimetric methods.
Oxalic acid, diphenylglycolic acid, cinnamic acid and mandelic acid can be used as rare earth precipitants, among which oxalate gravimetric method has high accuracy, easy filtration and wide application. In this method, the precipitate obtained by oxalate precipitation separation is burned into oxide and weighed.
Step 2 Titration
Titration analysis is mainly based on redox reaction and coordination reaction. For the analysis of rare earth mineral raw materials, the process control of rare earth metallurgy and the analysis of some rare earth materials, coordination titration is often used to determine the total amount of rare earth. Redox titration is often used to determine cerium, europium and other valence-changing elements. The range and accuracy of single rare earth titration are equivalent to that of gravimetric method, but the operation steps are simpler than gravimetric method, which is often used to determine the total amount of rare earth in samples with simple components. For the determination of the total amount of mixed rare earth, it is difficult to calibrate the standard solution because the distribution of rare earth in the sample is unclear or changeable, which leads to errors. Therefore, the titration of total mixed rare earth is mainly used for the control and analysis of production process. Redox titration of rare earth elements is mainly used for the determination of Ce4 ++ and Eu2++. Because other rare earth elements and other constant elements do not interfere with the determination, this method has good selectivity.
The general procedure of redox titration of total cerium is to oxidize Ce3+ to Ce4+ first, and then titrate Ce4+ with standard reducing titrant. Commonly used oxidants for oxidizing Ce3+ are ammonium persulfate, perchloric acid and potassium permanganate. Fe2 ++ is a commonly used reducing agent for Ce4 ++ titration, and phenanthroline and benzo-aminobenzoic acid or their mixture are the most commonly used indicators. Nitro-phenanthroline and mixed indicators of phenanthroline and 2,2'-bipyridine are also useful. Due to the redox nature of the above indicator itself, attention should be paid to the blank value of the indicator. The redox titration of europium is generally to reduce Eu3+ to Eu2+ with zinc amalgam in hydrochloric acid medium, quantitatively oxidize Eu2+ to Eu3+ with Fe3+ in carbon dioxide or other inert atmosphere, and then titrate the generated Fe2+ with potassium dichromate. Or directly titrate eu2++ with FeCl3. Someone quantitatively oxidized Eu2 ++ to Eu3+ with potassium dichromate, and then titrated the remaining potassium dichromate with ferrous. Among these methods, the quantitative reduction of Eu3+ is the key to affect the results. In addition, only by controlling the size and purity of zinc particles and mastering the flow rate of solution through zinc column can ideal results be obtained.
Coordination titration of rare earth elements uses amino carboxyl complexing agent as titrant, which forms a stable complex with trivalent rare earth ions. The EDTA complex of rare earth elements is relatively stable, and its lgK value is between 15 ~ 19. The stability constants of rare earth complexes are not much different from each other, and generally only the total amount of rare earth can be titrated.
Xylenol orange, arsenazo ⅲ, arsenazo ⅰ, chrome black T, ammonium cyanurate, PAN, PAR, methylene blue, bromopyrogallol and some mixed indicators can be used as indicators for the determination of rare earth by coordination titration. Among them, xylenol orange is the most commonly used, and the suitable acidity for titration is pH 5 ~ 6.
(2) Instrumental analysis
The instrumental analysis methods of rare earth elements mainly include visible spectrophotometry, inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence spectrometry (XRF). Their respective applications are shown in Table 6-2.
Table 6-2 Application of Instrumental Analysis in Determination of Rare Earth Elements
The Analysis Task of Rare Earth Minerals and the Choice of Analysis Methods
There are two main tasks in the analysis of rare earth minerals: the determination of the total amount of rare earth and the determination of the content of each single rare earth. The samples mainly include the following categories: rare earth ore, rare earth concentrate, rare earth oxide, rare earth slag, rare earth oxalate, rare earth carbonate, rare earth chloride and rare earth fluoride.
For rare earth minerals, the sample treatment methods can be alkali dissolution, complex acid dissolution or microwave digestion, and the determination methods mainly include spectrophotometry, ICP -AES, ICP -MS, XRF, INAA and so on. General spectrophotometry can only determine the total rare earth, cerium group rare earth or yttrium group rare earth, but not single rare earth. Other methods can easily determine the content of each single rare earth, and the sum of each single rare earth content is the total amount of rare earth. Among them, ICP-MS and INAA have the highest sensitivity, ICP-AES is in the middle, followed by XRF. Although ICP-MS and INAA have good analytical performance, they are still difficult to popularize, especially in small and medium-sized enterprises, because of their expensive equipment and high operating costs. The disadvantage of XRF is poor sensitivity and it is difficult to determine trace rare earth elements. In contrast, ICP-AES has been widely used in the field of rare earth analysis, and it is becoming more and more popular in China. This method has the advantages of high sensitivity, easy establishment and fast analysis speed. However, the determination of trace rare earths must adopt certain enrichment methods. It is worth mentioning that, for the South ionic rare earth ore unique to China, the detection items also include the determination of the rare earth content in ionic phase and the determination of the rare earth content in each phase (ionic phase and mineral phase).
The total amount of rare earth in rare earth concentrate, rare earth oxide, rare earth oxalate, rare earth carbonate, rare earth chloride and rare earth fluoride is basically determined by oxalate gravimetric method. Titration is not commonly used in the determination of total mixed rare earth. According to the nature of the sample, rare earth concentrate can be decomposed by alkali dissolution or acid dissolution. Generally speaking, rare earth oxalate and rare earth carbonate should be burned in muffle furnace at 900℃ before analysis. Rare earth oxides can be completely decomposed by hydrochloric acid and nitric acid. Rare earth chloride can be decomposed directly with hydrochloric acid, while rare earth fluoride must be treated with perchloric acid to be completely decomposed by acid. The determination of rare earth content in high-content rare earth minerals is a very important work. At present, ICP-AES and XRF methods can be used to determine the content of rare earth. XRF is regarded as a standard analytical method and arbitration method because of its accuracy, rapidity and direct analysis. ICP-AES has the advantages of simple sample preparation, fast analysis speed and wide linear range, which has been widely used and become another important analysis technology comparable to XRF.
To sum up, for the determination of rare earth elements in rare earth minerals, due to the comprehensive consideration of sample properties, rare earth content range, analysis purpose, analysis cost and other factors, combined with the laboratory's own conditions, the appropriate analysis method should be selected.
Skills training
Exercises under combat conditions
1. During training, 5-8 people in each group are divided into several groups.
2. Each group plays a role, using the knowledge they have learned, inquiring about relevant information on the Internet, and completing the work of entrusted samples of rare earth mines from sample acceptance to sample inspection sheet issuance.
3. Fill in Quality Form 1 and Form 2 in Appendix 1.