Task description
ICP-AES, ICP-MS and visible spectrophotometry are the main methods to determine low-content tungsten in ores, but ICP-AES and ICP-MS are expensive and expensive to operate, so they have not been fully popularized at present. At present, thiocyanate colorimetry is still the most widely used method. This method has the advantages of rapidity, simplicity, stability and wide application range. The purpose of this topic is to learn how to determine tungsten in ore by thiocyanate colorimetry, learn the basic operation of melting samples and master the matters needing attention in photometric operation through practical training. The original records can be recorded truly and normally, and the results can be calculated according to the effective figures.
Task implementation
I. Preparation of Instruments and Reagents
(1) hydrogen peroxide.
(2) Potassium thiocyanate (50%), filtered and used.
(3) hydrochloric acid (2.2+3).
(4) Mixed solution of hydrochloric acid (2.2+3)- titanium trichloride (0.038%): Take 0.25mL of commercially available titanium trichloride (15%) and dilute it to 100mL with hydrochloric acid (2.2+3).
(5) Tungsten trioxide standard solution: ① Weigh 1.0000g of tungsten trioxide (high purity reagent) which was burned at 750℃ in advance, put it in a 200mL beaker, add 20 ml of 20% sodium hydroxide solution, heat and dissolve it, cool it to room temperature, transfer it to a 1000mL volumetric flask, and use 2% sodium hydroxide solution. 1 ml This solution contains 1.000 mg of tungsten trioxide. ② Take 100.0mL of the above solution ①, put it in a 1000mL volumetric flask, dilute it to scale with 2% sodium hydroxide solution, and shake well. 1 ml This solution contains 100.0 microgram of tungsten trioxide.
(6) Visible spectrophotometer, muffle furnace and iron crucible.
Second, the analysis steps
Weigh 0.5 ~ 1.0 g sample (accurate to 0.000 1 g) and put it in a 30mL iron crucible (with the sample as a reagent blank), add 5 g sodium peroxide, stir it evenly with a small wire hook, cover it with a thin layer of sodium peroxide, and put it in a muffle furnace at 750℃ to melt until it is red, transparent and uniform. Take it out, cool it slightly, put it in a 250 ml beaker with 50 ml of water in advance, and drain off the frit. Wash the crucible with water and transfer it to a 100mL volumetric flask. Cool tap water to room temperature, dilute with water to scale, shake well, dry and filter or let stand for clarification.
Transfer 2.00 ~ 10.00 ml of test solution into a 50mL colorimetric tube. When the test solution is insufficient 10mL, make it up to 10mL with water. Add 2.0 ml of 50% potassium thiocyanate solution, shake well, dilute to scale with mixed solution of hydrochloric acid and titanium trichloride, and shake well. After 20min minutes, use 1 cm cuvette, and measure its absorbance at 430 nm wavelength on spectrophotometer with reference to reagent blank. Find out the corresponding amount of tungsten trioxide from the working curve.
Working curve drawing:
Transfer 0.0, 1.00, 2.00, 4.00, 6.00, 8.00 and 10.00mL of tungsten trioxide standard solution ② into a group of 50mL volumetric flasks, and when the test solution is insufficient 10mL, make it up to 10mL with water.
Third, the calculation of analysis results
Calculate the mass fraction of WO3 according to the following formula:
Rock mineral analysis
Where: w(WO3) is the mass fraction of WO3,%; M 1 is the WO3 mass of the test solution in the working curve, μ g; V is the total volume of test solution, ml; V 1 is the volume of test solution after subpackaging, ml; M is the mass of the weighed sample, g.
Fourth, fill in the quality form
After the task is completed, fill in quality tables 3, 4 and 8 in appendix 1.
task analysis
First, the principle of determination of tungsten by potassium thiocyanate colorimetry
Potassium thiocyanate method is based on the formation of yellow-green complex between pentavalent tungsten and potassium thiocyanate. Generally speaking, tungsten in ores is usually hexavalent. In order to reduce tungsten to pentavalent, titanium trichloride or stannous chloride is usually used as reducing agent. The reduction of hexavalent tungsten by stannous chloride is slow and the color development process is long. The reduction reaction of titanium trichloride to hexavalent tungsten is very fast, generally it only takes 5 ~ 10 min, and the color can reach the maximum strength and be stable within a few hours. However, due to the purple color of trivalent titanium, the mixed reductant of stannous chloride and titanium trichloride is often used at present.
Second, the color conditions
(1) acidity: generally, hydrochloric acid is used as the chromogenic medium, and the acidity is 3 ~ 6 mol/L. The test solution should be consistent with the standard. In order to weaken the interference of molybdenum, the acidity should not be less than 3 mol/L.
(2) Concentration of potassium thiocyanate: Methods It is stipulated that 2ml of 50% potassium thiocyanate solution should be added into 50mL colorimetric tube. The test shows that the concentration is a little small, as long as the amount of standard substance and test solution is the same, it has no effect on the results.
(3) Relationship between color development and stabilization time and temperature: Temperature has little influence on color, so colors can be compared 20min minutes after color development, although room temperature changes greatly. The color can be stable for 3 h below 30℃.
(4) Reducing agent dosage: Trivalent titanium itself is purple. Too much titanium will weaken the color intensity of K [wo (SCN) 4] (when the amount of tungsten is low) or make the colorimetric solution black (when the amount of tungsten is high). However, when the amounts of interfering substances such as Fe (Ⅲ) and Mo (ⅵ) are different, the residual amount of Ti (Ⅲ) is also different, so titanium trichloride should be added as appropriate. If only titanium trichloride is used as reducing agent, in general, 0.035mL concentrated titanium trichloride solution can be added to 25mL colorimetric solution. At present, it is common to reduce Fe(ⅲ) with stannous chloride first, and then add a certain amount of titanium trichloride to make tungsten quickly reduce and develop color. Generally, the solution diluted to110 with concentrated titanium trichloride is 0. 15 ~ 0.2 ml (about 3 drops). The concentration of stannous chloride has no obvious effect.
Third, the basic knowledge of melting decomposition method
(A) the basic principle of melting
Melting in sample treatment refers to the process of mixing samples with some solid reagents and heating them to a temperature above the melting point of these reagents, and heterogeneous chemical reaction occurs between the solid samples and the flux, and the samples are decomposed into compounds soluble in water or acid, which is convenient for the next step of leaching components. This method is an efficient decomposition method, which is mainly used for samples that cannot be decomposed by acid or can not be completely decomposed by acid, such as complex ores and alloys.
(2) Common fluxes for smelting and matters needing attention
Melting decomposition requires the use of excessive flux (usually 4 ~ 8 times the sample amount), and the amount of flux added is very important. Impurities in the reagent may become potential pollution sources for trace element analysis, so the reagent must be of high purity. Considering that the addition of flux may introduce new matrix elements and thus become new interference sources, the initial matrix may change greatly. This requires more care in handling. In addition, the composition of the container material corroded by flux during melting may contaminate the analyte and interfere with the determination of the elements to be measured. Therefore, the application of this method in trace analysis is limited.
Although there is no essential difference in mechanism between melting and dissolution, its remarkable feature is that the heterogeneous reaction between sample and flux solves some problems that cannot be solved by dissolution at high temperature, which is an indispensable means of sample dissolution. The melting operation is more complicated than the general dissolving operation, so it is necessary to choose the appropriate flux and pay attention to the dosage of flux. Pay attention to the heating mode, temperature and time, and the reagents and methods used in leaching should be appropriate. One disadvantage of melting is that the flux used must be excessive enough, which is easy to cause pollution, and various salts will be produced during the reaction, which makes the subsequent treatment complicated.
Fluxes can be divided into acid flux, alkaline flux, coordination flux and reduction flux. Table 3-4 lists the commonly used fluxes.
Table 3-4 Common Fluxes for Melting Samples
sequential
(3) Selection of melting crucible and matters needing attention in its use
1. Selection of crucible
The selection of crucible is a crucial link in smelting. The selection of crucible needs to consider the properties of the sample, the items to be tested and the influence of crucible material on the determination. Table 3-5 lists the characteristics of crucibles commonly used to melt samples.
Table 3-5 Characteristics of Common Crucibles for Melting Samples
2. Usage and precautions of common crucibles
(1) iron crucible: it is mainly used for melting sodium peroxide, and can also be used for sintering silicate samples by Smith method to determine alkali metals. In addition to the serious pollution of iron, the pollution of other transition metals can not be ignored. Suitable for treating rare metal samples.
(2) Nickel crucible: commonly used for melting sodium peroxide and sodium hydroxide. Among them, the Ni-Cr alloy crucible is not easy to oxidize and has high temperature resistance. Transition metals, especially manganese, will enter the melt of the sample in large quantities when melting. Nickel becomes brittle when heated with sulfur or sulfide. Compounds of silver, mercury and vanadium should not be treated in crucibles. Borax is not suitable as flux, and nickel crucible is only suitable for treating rare elements (such as tantalum and niobium) besides analyzing main silicates.
(3) Zirconium crucible: It can be used for sodium peroxide melting, and the temperature is usually 550℃. At this time, about 2mg of zirconium enters the melt at a time. Its pollution is lower than that of iron, nickel and silver crucibles, and even lower than that of platinum crucibles. Generally, a crucible can melt 100 times. Zirconium crucible is a good sample processing crucible because it is rarely polluted by common impurities.
(4) Silver crucible: commonly used for melting sodium hydroxide, with low melting point. When 8.5g sodium hydroxide melts 10min, the loss of silver is10.8mg. The melting of silver crucible will lead to the pollution of precious metals and heavy metals, but it is resistant to alkali solution corrosion, which is one of its great advantages.
(5) Platinum crucible: Platinum crucible plays an important role in sample processing. Besides aqua regia and sodium hydroxide seriously corrode platinum when heated, it can also resist the corrosion of various reagents, especially molten sodium carbonate and hydrofluoric acid. Although platinum has the least pollution to samples in various metal products, it will still cause different degrees of harm in many cases. Platinum disk adopts platinum-iridium alloy (containing 0.3% ~ 1% iridium), which will cause pollution of such precious metals. Iron pollution is often found in the application of platinum, which may be introduced by iron crucible pliers. Too much iron will make the crucible black and brittle. The iron on the surface can be removed by hot concentrated hydrochloric acid or potassium pyrosulfate, but the iron inside will slowly disperse and continue to be polluted, which is difficult to eliminate. Therefore, it is not suitable to make platinum crucible contact with iron, and crucible pliers should be made of nickel-chromium alloy and wrapped with platinum foil. With the increase of iron content in the sample, the pollution of platinum on the sample is more serious. Therefore, platinum crucible is not suitable for treating samples with high iron content. The melting of potassium pyrosulfate and potassium hydroxide, especially sodium hydroxide, seriously corrodes the platinum crucible, and the leaching amount of platinum is 1 ~ 100 mg. Platinum crucibles are also forbidden to be heated with various chemically inert metals (such as aluminum and copper) and many oxidants (such as bromine and aqua regia).
Fourth, the application of visible spectrophotometry
(a) the application of visible spectrophotometry.
Spectrophotometry is to measure the absorbance of a series of standard solutions with the help of a spectrophotometer, draw a standard curve, and then calculate the concentration or content of the tested substance from the standard curve according to the absorbance of the tested solution. Spectrophotometry is widely used in metallurgy, medicine, chemical industry, materials, environment, electronics and other fields because of its simple operation and accurate results. Scientists in China have done a lot of work in the synthesis of inorganic metal ion chromogenic reagent, and achieved universally recognized results.
The core of spectrophotometry is color reaction, so the selection of color reagent is very important. Color developers can be divided into inorganic color developers and organic color developers. Inorganic chromogenic reagent is not widely used in photometric analysis, mainly because the generated complex is not stable enough, and its sensitivity and selectivity are not high. Organic chromogenic reagents are widely used in photometric analysis. Table 3-6 lists the commonly used organic color developers.
Table 3-6 Several Important Organic Chromogenic Agents
Table 3-7 lists the commonly used inorganic color developers.
Table 3-7 Several Important Inorganic Chromogenic Agents
(2) Selection of color developing conditions of visible spectrophotometry.
The color developing conditions mainly include the amount of color developing agent, acidity, color developing temperature, color developing time and so on. These conditions have a great influence on the analysis results, and they must be carefully selected through experiments.
1. amount of developer
The appropriate dosage of developer is usually determined by experiments. The method is as follows: different amounts of chromogenic agents are added to a series of solutions containing the same concentration of components to be detected, and then the absorbance is determined under the same conditions. In the actual analysis, the dosage of color developer in the stable absorbance region is selected as the dosage of color developer.
2. Acidity
The influence of acidity on the color development system is mainly manifested in the following three aspects:
The influence of (1) on color developer. Many color developers are organic acids (bases), and the change of medium acidity will directly affect the degree of dissociation of color developers and whether the color reaction can be completed.
(2) The influence on the measured metal ions. When the acidity of the medium decreases, many metal ions will hydrolyze to form various types of hydroxyl complexes, and even precipitate, which makes the color reaction impossible.
(3) Influence on colored complexes. For some color reactions that can form step-by-step complexes, the composition of the products will change with the acidity of the medium.
It can be seen that the acidity of the medium is an important factor affecting the color reaction. The optimum acidity of color reaction can be determined by experiments. The method is to fix the concentration of ions and chromogenic agent in the solution, change the acidity of the solution, measure the absorbance of each solution, draw an A-pH curve, and find out the best pH range.
3. Color temperature
Most color reactions can be carried out quickly at room temperature, but some reactions need to be heated properly. For example, when silicon is measured by silicon molybdenum blue method, the reaction of producing silicon molybdenum yellow takes tens of minutes at room temperature, but it can be completed in boiling water bath for 30 s. For some color reactions, the increase of temperature will reduce the stability of colored complexes. For example, the thiocyanate complex of molybdenum can be stable for 40 hours at 15 ~ 20℃, and completely discolor at 12h when it exceeds 40℃.
4. Color development time
Due to the different reaction speed, the time to complete the color reaction is also different. Some reactions are instantaneous, and the colored complexes can be stable for a long time after completion, such as the color reaction of arsenazo ⅲ with rare earth. Some reactions progress slowly, and once completed, the stability time is also long, such as the color reaction of tiron with titanium. Although some color reactions can be completed quickly, the products will decompose quickly, such as the color reaction of dimethylglyoxime with nickel. Therefore, the formation and stabilization time of colored complexes should be determined by experiments. The method is as follows: prepare the color-developing liquid, calculate the time from adding the color-developing agent, measure the absorbance every few minutes, and then draw an A-t curve to determine the color-developing time and the time for measuring the absorbance.
Experimental guide and safety tips
Samples with high arsenic content will precipitate free arsenic in the chromogenic solution, and the solution will be turbid, which will hinder the determination of tungsten trioxide. Before adding sodium peroxide to melt, add 0.5 g ammonium chloride to mix with the sample, put it in a muffle furnace at 300 ~ 400℃ for 20 minutes (until white smoke is discharged), and drive away arsenic in the form of arsenic chloride.
The addition of sodium peroxide is 6 ~ 8 times of the sample. When melting the sample, the crucible should be sent to a high temperature furnace for 3 ~ 5 minutes, otherwise it is difficult to clean the crucible. The melting temperature must be controlled at about 750℃. If the temperature is too low and the melting time is too long, it is difficult to completely leach the melt.
The leaching solution is permanganate purplish red or manganate green, and a small amount of diammonium sulfate is added, which greatly reduces the price and separates the precipitate from tungsten.
Generally, 10mL water can be used to replace the blank of iron crucible.
For the sample with high copper content, it is necessary to melt 1g anhydrous sodium carbonate and sodium peroxide, dry the test solution and filter it with medium-speed filter paper.
It must be leached while it is hot. Too cold leaching is not complete, and the hydrogen peroxide generated by the reaction is not easy to decompose. Because hydrogen peroxide can destroy tungsten thiocyanate complex, the results are often very low.
When the sample contains a small amount of fluoride, the colorimetric solution is blue-green, which often leads to high results. Aluminum (Ⅲ) can form a complex (AlF3) which is difficult to dissociate with fluoride ions. The experiment shows that the interference of fluorine can be eliminated by adding an appropriate amount of aluminum chloride to the alkaline test solution. However, it is not advisable to add too much aluminum chloride, otherwise it will consume potassium thiocyanate and weaken the color of tungsten complex. Generally, 50% aluminum chloride 1mL can be added.
If the colorimetric solution is amber, appropriate amount of titanium trichloride should be added until it disappears.
Trivalent iron can form red iron thiocyanate complex with potassium thiocyanate. Although it can be reduced to low-cost fading, when the amount is too large, the reduction of tungsten is incomplete due to the consumption of reducing agent, and the result is low. In this case, an appropriate amount of reducing agent can be added, or ferric iron can be reduced with stannous chloride first.
Calcium precipitates into calcium hydroxide in alkaline solution. When the amount is too large, the result is sometimes low, which may be caused by the adsorption of calcium hydroxide. In this case, EDTA can be added to form a complex to eliminate the interference.
Stannous chloride is toxic, irritating to respiratory tract, gastrointestinal tract, skin and eyes, and harmful to liver and kidney; Repeated contact with the skin may cause a rash; Inhaling a large amount of its dust will cause pneumoconiosis. When it is decomposed by heating, it will produce corrosive toxic smoke.
Titanium trichloride is corrosive and will cause harm if inhaled, ingested or absorbed through the skin. Strong irritation to mucosa, upper respiratory tract, eyes and skin. Titanium trioxide is easy to oxidize. After opening the bottle, add a small amount of zinc particles to reduce it.
Potassium thiocyanate is irritating to eyes and skin, and a large amount of inhalation and swallowing will cause certain harm to human body. This substance is harmful to the environment and will pollute the water body. Nonflammable, but it can release toxic cyanide and sulfide smoke when decomposed at high temperature.
case analysis
When an employee in the detection center of Ganzhou Huaxing Tungsten Products Co., Ltd. used thiocyanate colorimetry to detect tungsten in ore, it was found that the solution was green after color development. Please consult relevant information to help him analyze the possible causes and solutions.
Expansion and improvement
Thiocyanate differential spectrophotometry
Spectrophotometry can only determine the low content of tungsten in ores, while differential spectrophotometry can also determine the high content of tungsten. The specific method is as follows.
I. Reagents
(1) Sodium peroxide, hydrazine sulfate.
(2) hydrochloric acid (4+6)- stannic chloride (0.5%): 400 ml concentrated hydrochloric acid (ρ = 1. 19g/ml), add 50g stannic chloride (sncl2h2o), and after the reagent is dissolved, dilute it to10000 with water.
(3) Titanium trichloride (3%): Take 100mL of calibrated commercially available titanium trichloride (the content is not lower than 15%), dilute it to 500mL with hydrochloric acid (4+6), add some zinc amalgam and store it in a brown bottle.
(4) Oxalic acid (saturated solution), potassium thiocyanate (40%) and disodium EDTA (3%).
(5) Hydrochloric acid-tin dichloride-titanium trichloride mixed solution: mix 90mL of hydrochloric acid (4+6)- tin dichloride (0.5%) solution, 3.0 ml of 40% potassium thiocyanate solution and1.0 ml of 3% titanium trichloride solution evenly.
(6) Tungsten trioxide standard solution: Weigh 0.2000g, 0.4000g, 0.6000g and 0.8000g of tungsten trioxide (high purity reagent) burnt at 750℃ respectively, put them in a group of 250ml beakers, add 50ml of 20% sodium hydroxide solution, heat them until they are completely dissolved, take them out, cool them to room temperature, and move them into a group of 200000 respectively. These solutions 1 ml contain 0. 100 mg, 0.200 mg, 0.300 mg and 0.400 mg of tungsten trioxide, respectively.
Second, the analysis steps
Weigh 0.2500~0.5000 g sample, put it in 30mL iron crucible, add 4 g sodium peroxide, stir with small iron wire, and cover it with a thin layer of sodium peroxide. Put it into a muffle furnace at 750℃ and melt it until it is red, transparent and uniform. Take it out and cool it slightly. Leach the glass frit in a 250mL beaker filled with 100mL warm water in advance, rinse the crucible with water, and transfer to a 250mL volumetric flask. Cool the tap water to room temperature, dilute it with water to scale and shake well. Standing for clarification or drying and filtering.
Transfer 5.00 ~ 10.00 ml of test solution and put it in a 100mL volumetric flask filled with10 ml of oxalic acid saturated solution in advance (add 5.0mL of water when transferring 5.00mL of test solution). Dilute to scale with mixed solution of hydrochloric acid-tin dichloride-potassium thiocyanate-titanium trichloride, and shake well. After 20min minutes, the absorbance of the standard solution without tungsten or the standard solutions of 1.00mg, 2.00mg and 3.00mg of tungsten oxide was measured on a spectrophotometer at the wavelength of 430nm with an automatic liquid tank of 1cm.
Determination of absorbance of tungsten trioxide standard solution;
Transfer 10.00 ml standard solutions containing 0. 100 mg, 0.200 mg, 0.300 mg and 0.400 mg of tungsten trioxide and10.0 ml of water per ml respectively, and put them into a group of/kloc-0 containing10.0 ml of oxalic acid saturated solution. Follow the following analysis steps. At the same time as the test solution, the absorbance of the standard solution of 1.00mg, 2.00mg, 3.00mg of tungsten trioxide was measured with reference to the standard solution of 1.00mg, 2.00mg, 4.00mg of tungsten trioxide.
Third, the calculation of analysis results
Calculate the mass fraction of tungsten trioxide according to the following formula:
Rock mineral analysis
Where: w(WO3) is the mass fraction of tungsten trioxide,%; Ax is the measured absorbance of the test solution; A is the measured absorbance of 1mg tungsten trioxide standard solution; B is the content of tungsten trioxide in the used reference solution, mg; V is the total volume of test solution, ml; V 1 is the volume of test solution after subpackaging, ml; M is the mass of the weighed sample, g.
Fourth, matters needing attention
(1) The key problem of this method is that the whole operation process must be careful, because the absorbance of 0.00 1 is equivalent to about 0. 1% of tungsten trioxide in the sample, which is easy to cause errors.
(2) The volumetric flask and pipette used must be calibrated in advance before use.