GB/T 6730.73-2024Iron ores - Determination of total iron content - EDTA photometric titration method (English PDF)

1 Scope

Warning — persons using this document need practical experience of work in a proper laboratory. This document does not set out every possible safety problem. It is the responsibility of the user to take appropriate safety and health measures and to ensure compliance with the conditions laid down in the relevant national regulations.

This document describes a method for the determination of total iron content in iron ores by EDTA photometric titration.

This document applies to the determination of total iron content in natural iron ore, iron ore concentrate and man-made lump ore, including sinter and pellets. Determination range, as a mass fraction: 30.0 percent to 72.0 percent.

2 Normative references

The contents of the following documents constitute indispensable provisions of this document through normative reference in the text. For dated references, only the edition corresponding to that date applies to this document; for undated references, the latest edition, including all amendments, applies to this document.

GB/T 6730.1 Iron ores — Preparation of predried test samples for chemical analysis

GB/T 6730.3 Iron ores — Determination of hygroscopic moisture in analytical samples — Gravimetric, Karl Fischer and mass loss methods

GB/T 8170 Rules of rounding off for numerical values and expression and judgement of limiting values

GB/T 10322.1 Iron ores — Sampling and sample preparation procedures

GB/T 12805 Laboratory glassware — Burettes

GB/T 12806 Laboratory glassware — One-mark volumetric flasks

GB/T 12808 Laboratory glassware — One-mark pipettes

3 Terms and definitions

There are no terms and definitions that need to be defined in this document.

4 Principle

4.1 Method 1 — microwave digestion. The test portion is decomposed by microwave digestion in a mixed solution of hydrochloric acid, nitric acid and hydrofluoric acid. The solution is diluted with water and adjusted with sodium hydroxide to pH 1. Sulfuric acid is used to remove the interference of titanium and boric acid to reduce the effect of fluoride. The solution is titrated with standard disodium ethylenediaminetetraacetate solution and the total iron content is calculated.

4.2 Method 2 — alkali fusion. The test portion is fused with sodium peroxide and sodium hydroxide. The fused mass is dissolved in hydrochloric acid and nitric acid. Sulfuric acid is used to remove the interference of titanium, ammonium fluoride to mask aluminium, and boric acid to remove excess fluoride. The solution is diluted with water and adjusted with sodium hydroxide to pH 1, then titrated with standard disodium ethylenediaminetetraacetate solution and the total iron content is calculated.

5 Reagents

Unless otherwise stated, only reagents confirmed to be of analytical grade and distilled water, deionized water or water of equivalent purity are used in the analysis.

5.1 High purity iron powder, of purity (mass fraction) not less than 99.98 percent. 5.2 Sodium hydroxide. 5.3 Disodium ethylenediaminetetraacetate (C10H14N2Na2O8, EDTA). 5.4 Sodium peroxide. 5.5 Ammonium fluoride. 5.6 Boric acid. 5.7 Hydrochloric acid, of density 1.19 g/mL. 5.8 Nitric acid, of density 1.42 g/mL. 5.9 Hydrofluoric acid solution, made up 1 to 9 by volume. 5.10 Sulfuric acid solution, made up 1 to 1 by volume.

5.11 Boric acid solution: weigh 100 g of boric acid (5.6) into a 1 L beaker, add 25 g of sodium hydroxide (5.2), make up to 1 L with water and mix. Store in a plastic bottle. The mass concentration of this solution is 100 g/L.

5.12 Sodium hydroxide solution: weigh 100 g of sodium hydroxide (5.2) into a 1 L beaker, add water to 500 mL and mix. The mass concentration of this solution is 200 g/L. Store in a plastic bottle.

5.13 Iron standard solution: weigh 2.792 g of high purity iron powder (5.1) into a 250 mL beaker and add 5 mL of hydrochloric acid (5.7) and 5 mL of nitric acid (5.8). Cover with a watch glass and heat on a hot plate at 80 degrees Celsius until dissolution is complete. After cooling, transfer to a 500 mL volumetric flask and dilute to the mark with water. The concentration of this solution is 0.100 0 mol/L.

5.14.1 Preparation of the standard EDTA titration solution: weigh 40 g of disodium ethylenediaminetetraacetate (5.3), add 800 mL of water, heat to dissolve, cool, dilute to 1 000 mL and mix. Store in a plastic bottle. The concentration of this solution is 0.1 mol/L.

5.14.2.1 Dissolution of the high purity iron powder for standardisation: weigh 0.20 g of high purity iron powder (5.1), to the nearest 0.000 2 g, into a 250 mL beaker and add 5 mL of hydrochloric acid (5.7) and 5 mL of nitric acid (5.8). Cover with a watch glass and heat on a hot plate at 80 degrees Celsius until dissolution is complete. While stirring with a glass rod, add 2 mL of sulfuric acid solution (5.10), 15 mL of sodium hydroxide solution (5.12) and 5 mL of sulfosalicylic acid solution (5.15), rinse off the glass rod and dilute to 150 mL with water.

5.14.2.2 Titration: cover with a watch glass and heat to boiling on a hot plate at 210 degrees Celsius. Take the beaker off and rinse the watch glass and the inner wall of the beaker. While the solution is still hot, above 80 degrees Celsius, place the beaker on the titration stand of the automatic potentiometric titrator and insert the combination glass pH electrode (6.3). Stir with a magnetic stirrer or with the rod stirrer supplied with the instrument and, with continuous stirring, add sodium hydroxide solution (5.12) to adjust the acidity of the solution to pH 1 plus or minus 0.05. Insert the photometric electrode (6.4) and the titration tip, check that both are immersed in the solution, and make sure that there are no bubbles in the light path of the photometric electrode. Run the titration programme and titrate with the standard EDTA titration solution (5.14.1) under continuous stirring. Repeat the determination four times and record the four end points and the volumes of standard EDTA titration solution consumed. Examples of titration curves are given in Annex B. If bubbles appear during the titration, the titration may be paused and the photometric electrode withdrawn from the solution and reinserted to clear them. Note: temperature compensation is not used when adjusting the acidity; at the end of the titration the temperature of the solution shall be kept above 50 degrees Celsius.

5.14.2.3 The EDTA concentration without blank correction is calculated by formula (1) and expressed in mol/L, to four decimal places. In the formula the symbols have the following meanings: the number of the determination, one to four; the mass of high purity iron powder, in grams; the volume of standard EDTA titration solution consumed at the end point, in millilitres; and 0.055 847, the conversion factor for iron. If the range of the four results, maximum minus minimum, is equal to or less than 0.000 2 mol/L, the mean of the four results is taken as the final result; if the range is greater than 0.000 2 mol/L, the median is taken as the final result.

5.14.3 Reagent blank: pipette 1.00 mL of the iron standard solution (5.13) into a 250 mL beaker and add 5 mL of hydrochloric acid (5.7), 5 mL of nitric acid (5.8) and 2 mL of sulfuric acid solution (5.10). While stirring with a glass rod, add 15 mL of sodium hydroxide solution (5.12) and 5 mL of sulfosalicylic acid solution (5.15), rinse off the glass rod and dilute to 150 mL with water. Proceed as in 5.14.2.2, recording the four end points and the volumes of standard EDTA titration solution consumed. If the range of the four volumes is equal to or less than 0.02 mL, the mean is taken as the final result; if the range is greater than 0.02 mL, the median is taken. The reagent blank value is then calculated by formula (2) as the final volume result minus the volume of standard EDTA titration solution equivalent to 1.00 mL of the iron standard solution, the result being calculated to two decimal places; that equivalent volume is itself obtained by formula (3) from the volume and concentration of the iron standard solution and the uncorrected concentration of the EDTA solution.

If there is no iron in the solution, the iron sulfosalicylate complex cannot form and photometric titration is not possible. Iron is therefore added when the blank test is carried out, and the added iron is converted into the equivalent volume of standard EDTA titration solution. A 1 mL single-mark pipette shall first be calibrated by weighing the water it delivers and converting that mass to a volume.

5.14.4 Calculation of the concentration of the standard EDTA titration solution: the concentration is calculated by formula (4), expressed in mol/L to four decimal places, from the mass of pure iron powder, the volume of standard EDTA titration solution consumed at the end point, the reagent blank value obtained in 5.14.3, and the conversion factor for iron. If the range of the four results is equal to or less than 0.000 2 mol/L, the mean of the four is taken as the final result; if the range is greater than 0.000 2 mol/L, the median is taken.

5.15 Sulfosalicylic acid solution: weigh 10 g of sulfosalicylic acid, dissolve it in 100 mL of water and keep it in a 125 mL reagent bottle. The mass concentration of this solution is 100 g/L.

6 Apparatus

Unless otherwise specified, ordinary laboratory apparatus is used. Burettes, one-mark volumetric flasks and one-mark pipettes shall comply with GB/T 12805, GB/T 12806 and GB/T 12808 respectively.

6.1 Analytical balance, with a scale interval of 0.1 mg.

6.2 Automatic potentiometric titrator, able to deliver both at fixed intervals and dynamically.

6.3 Combination glass pH electrode, able to measure pH 0 to 14.

6.4 Photometric electrode, acid resistant, with a measuring wavelength of 520 nm, connected to the automatic titrator by a shielded cable.

6.5 Microwave digestion system, fitted with a temperature sensor and a temperature control system able to control the digestion parameters such as temperature and time accurately, controllable to 250 degrees Celsius with an accuracy of plus or minus 5 degrees Celsius. The temperature measuring system shall be calibrated regularly, following the calibration procedure given by the manufacturer of the microwave digestion system for the temperature sensor.

6.6 Modified polytetrafluoroethylene (PTFE-TFM) digestion vessel, of 100 mL capacity.

6.7 Corundum crucible, of 20 mL to 30 mL capacity.

6.8 High temperature furnace, with a working temperature of not less than 800 degrees Celsius. At 650 degrees Celsius the deviation in furnace temperature uniformity shall not exceed 10 degrees Celsius and the deviation in furnace temperature stability shall not exceed 10 degrees Celsius.

7 Sampling and sample preparation

7.1 Laboratory sample. Sampling and sample preparation are carried out in accordance with GB/T 10322.1. The particle size of the laboratory sample for analysis shall be less than 100 micrometres. Where the sample has a high content of combined water or of readily oxidisable matter, the particle size shall be less than 160 micrometres.

7.2.1 Ores with a higher content of combined water or readily oxidisable matter. For the following types of ore, an air-equilibrated sample is prepared in accordance with GB/T 6730.3: processed ore containing metallic iron; natural or processed ore with a sulfur content greater than 0.2 percent; and natural or processed ore with a combined water content greater than 2.5 percent.

7.2.2 Natural or processed ore outside the scope of 7.2.1. Mix the laboratory sample thoroughly and take the sample by increment division. In accordance with GB/T 6730.1, dry the test sample at a temperature of 105 degrees Celsius plus or minus 2 degrees Celsius.

8 Analytical procedure

8.1 Number of determinations. In accordance with Annex C, at least two independent determinations are made on the same predried test sample. Note: independent means that the result of any second or subsequent determination is not influenced by the results of the preceding determinations.

8.2 Test portion. For an air-equilibrated sample prepared according to 7.2.2, weigh 0.25 g of sample to the nearest 0.000 2 g. At the same time determine the hygroscopic moisture content in accordance with GB/T 6730.3 and calculate the mass of the dry test portion. For other samples, weigh 0.25 g of sample into a weighing bottle. Open the lid of the weighing bottle and dry the bottle and its lid for 2 h at 105 degrees Celsius plus or minus 2 degrees Celsius. Put the lid on the weighing bottle, transfer it to a desiccator and cool it to room temperature, about 20 min. Weigh the weighing bottle with its lid on, to the nearest 0.000 2 g, to obtain the first mass. Transfer the test portion into the modified polytetrafluoroethylene digestion vessel (6.6) and weigh the weighing bottle with its lid on again to obtain the second mass. The mass of the dry test portion is the difference between the two.

8.3 Blank test. 8.3.1 A blank test is carried out alongside the test portion. 8.3.2 The blank test value represents the reagent blank taken from the same operating procedure and the same bottle of reagent, and it may be used for calculating the results of several samples tested at the same time. At least two independent determinations are made.

8.4.1.1 Decomposition, method 1 — microwave digestion. Transfer the test portion (8.2) into a 100 mL PTFE-TFM digestion vessel (6.6) and add 5 mL of hydrochloric acid (5.7), 5 mL of nitric acid (5.8) and 0.5 mL of hydrofluoric acid solution (5.9). For samples with a total iron content of less than 40 percent, add 1 mL of hydrofluoric acid solution (5.9). If there is sample on the wall of the vessel, add the acid dropwise so as to wet the sample. Shake the PTFE-TFM digestion vessel (6.6) gently so that the sample and the acid are thoroughly mixed. Check whether a reaction occurs; if it does, wait until it is complete before closing the PTFE-TFM digestion vessel (6.6). Load the sample vessels into the microwave digestion system (6.5). Set the digestion temperature to 210 degrees Celsius, the ramp time to 20 min and the digestion hold time to 30 min. Press the start key to begin the digestion. When digestion is complete, take out the PTFE-TFM digestion vessel (6.6), cool it below 50 degrees Celsius and open it at the temperature specified by the manufacturer of the microwave digestion system. Open the PTFE-TFM digestion vessel (6.6) carefully and slowly so as to avoid loss of test solution. Rinse the digestion lid with water and collect the washings in a 250 mL beaker. Add 2 mL of sulfuric acid solution (5.10) to the PTFE-TFM digestion vessel (6.6), rinse out the vessel and collect the washings in the 250 mL beaker. While stirring with a glass rod, add 30 mL of boric acid solution (5.11), 5 mL of sulfosalicylic acid solution (5.15) and 15 mL of sodium hydroxide solution (5.12), rinse off the glass rod and dilute to 150 mL with water.

The microwave digestion oven shall be operated as required by the manufacturer. So that the course of the reaction can be monitored and the temperature controlled, the contents of the control vessel shall be similar in nature to the contents of the other sample vessels in the same run. For samples with a total iron content of less than 40 percent, if too much white precipitate (silica) is seen in the solution, the precipitate shall be filtered off on a fast filter paper before the 30 mL of boric acid solution (5.11) is added; the beaker and the precipitate are washed with warm 2 percent hydrochloric acid until the yellow colour of ferric chloride is no longer seen, then washed six to eight times with warm water, and the filtrate and washings are collected in a 250 mL beaker.

8.4.1.2 Decomposition, method 2 — alkali fusion. Place the test portion (8.2) in a corundum crucible (6.7) containing 2 g of sodium hydroxide (5.2), add 1.5 g of sodium peroxide (5.4) and mix. Hold the corundum crucible (6.7) at the mouth of the high temperature furnace (6.8) at 650 degrees Celsius for 1 min to 2 min and then place it inside the furnace for 30 min; alternatively place it in the furnace at room temperature, raise the temperature to 650 degrees Celsius and hold for 30 min. Take the corundum crucible (6.7) out of the high temperature furnace (6.8) and rotate it so that the melt is spread evenly over the wall, then let it cool. Place the corundum crucible (6.7) in a 250 mL beaker, add 100 mL of warm water and stir the crucible with a glass rod so that the fused mass dissolves. Add 2 mL of sulfuric acid solution (5.10), 5 mL of hydrochloric acid (5.7) and 5 mL of nitric acid (5.8), add one or two glass beads to prevent bumping, cover with a watch glass and heat slowly until the fused mass has dissolved. Rinse the corundum crucible (6.7) with water and remove it. Add 0.5 g of ammonium fluoride (5.5) and 3 g of boric acid (5.6). While stirring with a glass rod, add 5 mL of sulfosalicylic acid solution (5.15) and 5 mL of sodium hydroxide solution (5.12), rinse off the glass rod and dilute to 150 mL with water.

8.4.2 Titration. Proceed as in 5.14.2.2 and record the volume of EDTA consumed at the end point of the titration.

8.5 Determination of the blank test value. 8.5.1 The blank test is analysed at the same time as the test portion, using the same procedure and the same quantities of reagents. After heating to boiling on the hot plate, until no small bubbles remain, as described in 5.14.2.2, add 1.00 mL of the iron standard solution (5.13) and proceed as in 5.14.2.2. Carry out two parallel tests and record the end points and the volumes of standard EDTA titration solution consumed. 8.5.2 If the absolute difference between the two volumes is equal to or less than 0.02 mL, their mean is taken as the final result; otherwise the determination is repeated four times as in 8.5.1 and the median of the four results is taken as the final result. 8.5.3 The blank test value is calculated by formula (5), to two decimal places, from the volume obtained in 8.5.2 less the volume of EDTA equivalent to the 1.00 mL of iron standard solution added, that equivalent being obtained from the volume and concentration of the iron standard solution and the concentration of the standard EDTA titration solution.

9 Calculation and expression of analytical results

9.1 Calculation of the analytical result. The total iron content, expressed as a mass fraction, is calculated by formula (6); the numerical value is rounded in accordance with GB/T 8170 and the result is calculated to two decimal places. In the formula the symbols have the following meanings: the concentration of the EDTA solution, in mol/L; the volume of EDTA solution consumed by the sample, in millilitres; the blank test value obtained in 8.5.3, in millilitres; the mass of the test portion on a dry basis, in grams; and 0.055 847, the factor derived from the atomic weight of iron.

9.2 Determination and expression of the analytical result. If the absolute value of the difference between two independent analytical results on the same sample is not greater than the repeatability limit given in 9.3, their arithmetic mean is taken as the analytical result. If the absolute value of the difference is greater than the repeatability limit, further determinations are added and the analytical result is determined in accordance with Annex C.

9.3 Precision. The precision obtained by statistical analysis is given in Table 1. For method 1, microwave digestion, over a mass fraction range of 30.0 percent to 72.0 percent, the repeatability limit is 0.21 percent and the reproducibility limit is 0.28 percent. For method 2, alkali fusion, over the same range, the repeatability limit is 0.22 percent and the reproducibility limit is 0.36 percent.

10 Test report

The test report shall include the following information: the name and address of the laboratory; the number of the test report and its date of issue; the number of this document; the pre-treatment method used in the test; any necessary detail of the test sample itself; the analytical result; and any abnormal circumstances arising during the determination, together with any operation not specified in this document that may have affected the analytical result of the test sample.