Foreword:
In X-ray fluorescence spectrometry, borate melting sample preparation technology of oxides is considered to be the most accurate sample preparation method in X-ray fluorescence spectrometry because it completely eliminates the mineral effect and particle size effect of samples, and the matrix effect caused by * * elements can be reduced to some extent after samples are diluted with flux. Melting sample preparation method was first proposed by Claisse, and the first paper on melting sample preparation was published in 1956 (Claisse,? f？ Department? Yes? Mines? Quebec? Canada? p？ r？ 327,? 1956; ? That? Norelco？ Report? 4(3),? 95,? 1957)。 Since then, this technology has gradually matured and has been adopted by a large number of laboratories around the world, becoming an important part of industrial testing and scientific research. This topic takes this as the breakthrough point to talk about the application of melting sample preparation method in XRF analysis.
Directory:
I. Basic conditions for preparing glass frits
1. Flow
crucible
3. Release agent
4. Sample melting furnace
Secondly, the solubility characteristics of elements in glass plates.
3. Retention of sulfur and fluorine in glass frit
1. How to preserve sulfur in glass melt?
2. Behavior of fluorine in glass frit
4. Preparation of melting samples of reduced materials
I. Basic conditions for preparing glass frits
1. Flow
The flux used for melting sample preparation is mainly borate. It can be said that the history of melting sample preparation is accompanied by the use of borate. In the earliest glass bead experiment designed by Claisse, anhydrous borax (Na2B4O7) was used. At present, sodium tetraborate, as a flux, has withdrawn from the mainstream stage of melting sample preparation, and has been replaced by lithium borate flux with better performance. In addition to lithium borate, sodium metaphosphate flux is sometimes used for special samples.
1) lithium tetraborate flux (Li2B4O7)
Lite, usually abbreviated as LiT, is a weakly acidic flux with good compatibility with alkaline samples. The melting point of lithium tetraborate is 9 17 degrees, and its chemical composition is 17.7% Li2O and 82.3% B2O3. It hardly crystallizes after melting, and it is the most important and commonly used flux in melting sample preparation method, with excellent performance.
2) Lithium metaborate flux (LiBO2)
Generally abbreviated as LiM, it is an alkaline flux with good compatibility with acidic oxides. The melting point of lithium metaborate is 849 degrees. The chemical components are 30.0% Li2O and 70.0% B2O3, which have good fluidity after melting, but are easy to crystallize when cooling. It is generally not recommended to use lithium metaborate as a flux alone.
3) mixed solvent
The mixed solvent is a mixture of lithium tetraborate and lithium metaborate in different proportions, the chemical composition is between LiT and LiM, and the melting point is basically between them. In most cases, the use of mixed flux will achieve better melting effect, and the glass melt will not break or adhere to the crucible.
4) sodium tetraborate flux
Anhydrous borax is the earliest used flux. The melting point of Na2B4O7 is 74 1 degree, which is suitable for most oxides, and the glass sheet will not crystallize and crack. However, his shortcomings are also obvious, such as strong hygroscopicity and difficult long-term storage of glass melt, which is not conducive to calibrating samples.
5) sodium metaphosphate flux
Sodium metaphosphate began to melt at about 600 degrees. Because of its low melting temperature, it can be used as a good flux for high-temperature volatile substances, while sodium metaphosphate has good solubility for chromium oxide, which makes up for the poor solubility of chromium oxide in lithium borate flux to some extent. Sodium metaphosphate has good water solubility and can sometimes be used as a flux to prepare aqueous solution for ICP and AA analysis.
crucible
Platinum is the only practical metal found so far that can be used in smelting operations. On the one hand, the addition of gold increases the strength of platinum crucible and is not easy to deform; On the other hand, crucibles are more difficult to penetrate. At present, the commonly used crucible material is an alloy composed of 95%Pt-5%Au. The thicker the wall, the less likely it is to deform and the more durable it is, but the cost will be higher. Now it is generally made into a round bottom shape with a diameter of 32mm, which is also the most economical. According to the use method, it can be divided into direct molding crucible and casting crucible. For the casting crucible, there will be a special supporting mold.
Figure 1 shows the matching crucible of Claisse furnace, which is equipped with a special mold for casting. Fig. 2 is a crucible made by a domestic company for heating the sample melting furnace of silicon carbide rod, in which the glass plate is directly molded.
Figure 1? Crucible for melting furnace
Figure 2? Crucible for heating silicon carbon rod
Crucible maintenance should pay attention to three aspects.
1) to reduce corrosion.
This is also the most important. Any substance with incomplete oxidation may cause corrosion to the crucible, such as sulfide, ferroalloy and Cu2O. So don't take any chances, you need to have a full understanding of the sample before melting.
2) Cleaning
During the use of the crucible, some special samples may lead to poor demoulding effect and glass residue will stick to the crucible. The quick cleaning method of residue is generally boiling with dilute acid. It should be emphasized that single acid, nitric acid or hydrochloric acid must be used, and mixed acid of nitric acid and hydrochloric acid is not allowed. Another method is to add flux to melt again, and then pour out the glass plate after demoulding.
3) polishing
For some slight corrosion, such as when the crucible loses its luster, try to polish the crucible once. Generally, flannel or fine-grained sandpaper (such as 800 mesh) can be used to rub the crucible at high speed to make the crucible smooth again. Be careful in polishing operation, and never rub the crucible directly with coarse sandpaper. If the corrosion is too serious, it is best to recast the crucible once.
3. Release agent
The molten glass is easy to adhere to or soak the platinum crucible and mold, which makes the melt easy to adhere to the crucible or mold and difficult to pour. Therefore, the release agent must be used to help the melt demould from the crucible or mold smoothly. At present, it is found that only halides have such characteristics and can be used as release agents, such as bromide LiBr, NH4Br and iodide KI, NH4I, etc. As for the mechanism of release agent, it is generally believed that release agent will form a coating on the glass surface, so that the melt can be peeled off from the crucible or mold smoothly.
The dosage of release agent is small, generally 20~50mg, and it can be added in solid or solution form. It should be noted that halogen elements are volatile at high temperature, so melting temperature and melting time will also affect the dosage of release agent. When the release agent is added in the form of solution, it has the advantages of no need to weigh and good accuracy. The release agent can be added into the crucible before melting or injected into the crucible before casting. The latter can only add solids, but the amount of release agent is small. When using release agent, the interference of Br and I on analytical elements should be considered, and the interference between elements should be avoided as far as possible.
Figure 3? Relationship between iodine and bromine volatilization and time
4. Sample melting furnace
Generally speaking, the melting reaction needs a temperature above 65,438+0,000 degrees. In the early days, melting flakes were usually made with the help of gas lamps or muffle furnaces. Now there are a large number of melting sample preparation furnaces with strong professionalism and high degree of automation. According to the characteristics of use, it can be generally divided into the following four types.
1) muffle furnace. Completely manual smelting equipment can't be completely replaced now. It can be used in the case of few molten samples, but how to ensure the uniformity of glass frit is its biggest challenge. Generally speaking, the manufacture of repeatable glass frit requires certain operating skills and proficiency.
2) Silicon carbon rod heating furnace. At present, there are many companies producing silicon carbon rod heating melting furnaces in China, and the degree of specialization is also very high. Now the furnace type has basically realized automation, safe operation, good reproducibility of the produced samples and longer service life of the crucible. Generally, it can accommodate 4~6 samples at a time, and the work efficiency is relatively high.
3) gas stove. Using high calorific value gas to realize melting sample preparation, the temperature control accuracy may be slightly lower than that of silicon carbon rod heating sample melting furnace. When using, attention should be paid to avoid contact between crucible and reducing flame of gas, and pay attention to the safety of crucible use.
4) High frequency induction furnace. The crucible in the furnace is under the electromagnetic condition of high frequency oscillation, and melting is realized by self-heating. It has the characteristics of high speed, low energy consumption and safe operation. However, because the heating mode is realized by atomic vibration and collision of crucible, the loss of crucible may be large for a long time, and the temperature control is not as good as that of silicon-carbon rod melting furnace.
Secondly, the solubility characteristics of elements in glass plates.
In the periodic table of elements, the elements that can form glass are B, Si, P, As, Sb, O, S, Se and Te, which are located in main groups III to VI in the periodic table. Therefore, for the elements of main groups III to VI, the formation of glass is hardly a problem, so there is basically no refractory problem, and the only possibility is the dissolution and volatilization of sulfur. Similarly, fluorine has such a problem. Another problem is the melting of transition elements, especially chromium oxide, which is reflected in its low solubility. Copper-containing materials also have refractory problems, one is crucible corrosion, the other is glass plate adhesion.
In the periodic table below, we divide it into several parts. First, the commonly used elements in industry include those that are not commonly used; The second is rare earth elements; Thirdly, volatile elements include halogen elements and sulfur. For most commonly used industrial elements, except for the melting of chromium and copper, it is basically no problem for other elements to melt glass plates. Bromine and iodine are volatile elements and are often used as release agents, which has been discussed. For sulfur and fluorine, due to their industrial importance, the accuracy of measurement is inevitable, which will be discussed in the next chapter. Because the content of rare earth elements in the sample is not too high, it is not difficult to melt except the detection limit.
The solubility of oxides in borate flux is emphasized.
Although in the whole periodic table of elements, oxides of almost all elements can be dissolved in borate flux except those that produce gas oxides, the only difference is solubility. The following figure clearly shows this relationship. Generally speaking, lithium tetraborate has a better melting effect on basic oxides, while lithium metaborate has a greater solubility on acidic oxides. The following solubility diagram (drawn by Claisse) can be used as a guide for daily fusing work, so avoid detours. In terms of solubility, only chromium oxide is a special case at present, and its solubility is extremely small. 7g borate can only dissolve 0. 15g Cr2O3 at most. It is reported in the literature that the solubility of chromium oxide can be appropriately increased by adding appropriate sodium salt. According to this theory, is the solubility of sodium borate flux to chromium oxide better?
Another thing that needs to be explained separately is the melting of copper-containing materials. Copper oxide will slightly plate copper on the platinum crucible, which will lead to crucible corrosion, and at the same time, the copper-containing glass sheet will greatly adhere to the crucible, which may also be related to copper precipitation. But for copper sulfide ore, it seems to be easier to melt than pure copper oxide on the premise of full oxidation. The accurate determination of copper-bearing materials, especially copper ores, by XRF with melting sample preparation, and the determination of elements in copper concentrate by melting method have been reported a few times, which is a very wide application field.
Figure 4 Periodic Table of Elements
Figure 5? Oxide solubility table