Iron is one of the most widely distributed metal elements in the earth, with an average content of 5% in the earth's crust, ranking fourth after oxygen, silicon and aluminum in the element abundance table. There are more than 300 kinds of iron minerals known in nature, but under the current technical conditions, magnetite (Fe3O4 containing 72.4% iron), hematite (Fe2O3 containing 70.0% iron), siderite (FeCO3 containing 48.2% iron), limonite (Fe2O3 containing 48% ~ 62.9% iron) and so on.
Iron ore is the basic raw material of iron and steel industry, which can be smelted into pig iron, wrought iron, ferroalloy, carbon steel, alloy steel and special steel. Iron ore for blast furnace ironmaking requires TFe (total iron content) ≥ 50%, S ≤ 0.3%, P ≤ 0.25%, Cu ≤ 0.2%, Pb ≤ 0. 1%, Zn ≤ 0. 1% and Sn≤0.08%%. China imports a large amount of commercial iron ore from abroad every year.
Conventional analysis of iron ore is simple analysis, that is, determination of total iron (TFe), ferrous iron, soluble iron, silicon, sulfur and phosphorus. Currency analysis also needs to determine: alumina, calcium oxide, magnesium oxide, manganese oxide, arsenic, potassium, sodium, vanadium, iron, chromium, nickel, cobalt, bismuth, silver, barium, strontium, lithium and rare dispersed elements. Absorbed water, bound water, loss on ignition and carbon dioxide, etc. This section focuses on the determination of total iron.
I. Decomposition of iron ore samples
Iron ore is a kind of mineral which is difficult to decompose, and the decomposition speed is very slow. The analysis sample should pass through a 200-mesh sieve, or the particle size of the sample should not be greater than 0.074 mm.
Iron ore can generally be decomposed with hydrochloric acid in a low-temperature electric furnace. If the residue is white, the sample is completely decomposed. If the residue is black or other colors, it is because iron silicate is insoluble in hydrochloric acid. Hydrofluoric acid or ammonium fluoride can be added and heated to completely decompose the sample. Magnetite is decomposed very slowly, so it can be decomposed with mixed acid of sulfur and phosphorus (1+2) in high temperature electric furnace. However, care should be taken not to heat for too long to prevent the generation of pyrophosphate.
Some iron ore samples are difficult to decompose by acid, so it is advisable to decompose the samples by alkali melting. Commonly used fluxes are sodium carbonate, sodium peroxide, sodium hydroxide and mixed flux of sodium peroxide and sodium carbonate (1+2), which are carried out in silver crucible, nickel crucible, high aluminum crucible or graphite crucible. After alkali melting decomposition, it is leached with hydrochloric acid solution.
Two. Summary of analysis methods of iron in iron ore
Iron content in iron ore is relatively high, generally between 20% and 70%, and its analysis methods include stannous chloride-mercuric chloride-potassium dichromate volumetric method, titanium trichloride-potassium dichromate volumetric method and stannous chloride-mercuric chloride-cerium sulfate volumetric method.
The first method (also known as potassium mercuric dichromate method) is a classic method for the determination of iron in iron ore, which has the advantages of simplicity, rapidity, accuracy, stability and easy mastery. It has been widely used in practical work, and has become one of the national standard methods-stannous chloride-mercuric chloride-potassium dichromate volumetric method to determine the total iron content (GB/T 6730.4-65438). The basic principle is as follows: in hot concentrated hydrochloric acid medium, Fe (Ⅲ) in the test solution is reduced by stannous chloride, and the excess stannous chloride is removed by mercuric chloride oxidation. In the presence of mixed acid of sulfur and phosphorus, with sodium diphenylamine sulfonate as indicator, all Fe (Ⅱ) was titrated with potassium dichromate standard titration solution until the solution showed a stable purple color, and the iron content in the sample was calculated according to the consumption of potassium dichromate standard solution.
(1) In practical work, in order to reduce Fe (Ⅲ) quickly and completely, when the prepared solution is heated to a small volume, SnCl2 solution is added dropwise while it is hot until the yellow color fades. SnCl2 solution is added while it is hot, because the reduction of Fe (Ⅲ) by Sn (Ⅱ) is very slow at room temperature. Increasing the temperature to near boiling can accelerate the reaction. Concentrating to a small volume not only improves the acidity and prevents SnCl2 _ 2 from hydrolysis, but also increases the concentration of reactants, which is beneficial to the observation of the reduction of Fe (Ⅲ) and the color change when the reduction is complete.
(2) Adding mercuric chloride to remove excess stannous chloride must be carried out in cold solution, and its oxidation is slow. After adding mercuric chloride solution, it needs to stand for 2 ~ 3min before titration. Because HGCl _ 2 can oxidize Fe (Ⅱ) in hot solution, the determination result is low: after adding HGCl _ 2 solution, it is not placed, or the reaction is incomplete, and Sn (Ⅱ) is not completely removed, which makes the result high; If it is left for too long, the reduced Fe (Ⅱ) will be oxidized by oxygen in the air, which makes the result low.
(3) The function of adding mixed acid of sulfur and phosphorus before titration: it is the acidity required to ensure the oxidation capacity of K2Cr2O7; The other is that H3PO4 and Fe (Ⅲ) form a colorless complex ion [Fe(HpO4)2], which can not only eliminate the influence of FeCl3 yellow on the color change at the end point, but also reduce the potential of Fe (Ⅲ)/Fe (Ⅱ) pair and broaden the range of titration jump. However, it must be noted that the stability of Fe (Ⅱ) in H3PO4 medium is poor, and it should be titrated as soon as possible after adding mixed acid of sulfur and phosphorus.
(4) The reaction speed between sodium diphenylamine sulfonate and K2Cr2O7 was originally slow, but the reaction between sodium diphenylamine sulfonate and K2Cr2O7 was very fast due to the catalysis of trace Fe (Ⅱ), and the color changed dramatically. Therefore, a certain amount of ammonium ferrous sulfate solution should be added in the blank test. Since K2Cr2O7 will also be consumed when the indicator is oxidized, the dosage of indicator should be strictly controlled.
The second method (also called mercury-free potassium dichromate method) is because mercury salt is toxic, pollutes the environment and harms human health, so people put forward an improved method to avoid using mercury salt. This method is widely used, and it is also one of the national standard analysis methods for the determination of total iron content by titanium trichloride-potassium dichromate volumetric method (GB/T6730.5- 1986). The basic principle is that Fe (Ⅲ) in the test solution is reduced to Fe (Ⅱ) with titanium trichloride solution in hydrochloric acid medium. Using sodium tungstate solution (methyl orange, neutral red, methylene blue, etc.) to indicate the end point of complete reduction of Fe (Ⅲ). ). When the colorless sodium tungstate solution turns blue (tungsten blue), it means that Fe (Ⅲ) has been completely reduced. Oxidizing excess titanium trichloride with potassium dichromate solution until tungsten blue disappears, then adding mixed acid of sulfur and phosphorus, and titrating with potassium dichromate standard titration solution with sodium diphenylamine sulfonate as indicator to generate all Fe (Ⅱ) until the solution appears stable purple.
On the basis of the first method, the third method only uses cerium sulfate standard titration solution instead of potassium dichromate standard titration solution as oxidant to titrate Fe (Ⅱ). It is suitable for the determination of samples with high arsenic and antimony content.