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A rapid titrimetric determination of iron in copper concentrates and copper-bearing leach solutions
54
SHORT
Tofanta, Vol. 25, pp. 54-55.
Pergamon
Press, 1978. Printed
COMMUNICATIONS
in Great Britain
A RAPID TITRIMETRIC DETERMINATION OF IRON IN COPPER CONCENTRATES AND COPPER-BEARING LEACH SOLUTIONS
v. s. sASllU Metallurgical Chemistry Section, Physical Sciences Laboratory, Mineral Sciences Laboratories, CANMET, Ottawa, Canada
(Received 11 May 1977. Accepted 25 June 1977)
Summary-A This method mate. It has concentrates
titrimetric method for the determination of iron(II1) in the presence of copper is presented. involves the reduction of iron(II1) with titanous sulphate followed by titration with dichrobeen successfully used for the determination of iron in both leach solutions and copper with a relative error of 1%.
The standard analytical method’ for the determination of total iron in leach liquors involves the reduction of iron(II1) to iron(I1) with stannous chloride, the excess of which is oxidized by mercuric chloride, followed by titration of ferrous ion with standard dichromate solutions, with diphenylamine as indicator. This method, when applied to solutions containing copper, presents a problem in that copper (II) is reduced to copper (I) which is not oxidized by mercuric chloride, but is titrated with dichromate, so the total iron determined will be high. One method of elimination of the interference due to copper is to use either metallic copper or test lead as the reducing agent.’ Titanous sulphate completely reduces iron(II1) to iron(I1) and can be used for the determination of total iron, provided that all the iron is first oxidized to iron(II1). However, copper(I1) is reduced to copper metal along with the reduction of iron(II1) by titanous sulphate in sulphate medium and prevents visual determination of the end-. point when potassium thiocyanate is used as indicator. On the other hand, the reduction to metallic copper can be used as an indication that the ion(II1) has all been reduced. The metallic copper can then be dissolved by the addition of mercuric perchlorate solution, followed by the titration of the iron(I1) with dichromate solution. A detailed study was therefore undertaken for the determination of iron in the presence of copper, with titanous sulphate as the reducing agent. EXPERIMENTAL
Reagents Ferric sulphate solution. Dissolve 1.2500 g of high-purity iron in dilute sulphuric acid with hydrogen peroxide as the oxidizing agent and make up to 250 ml in a standard flask. Mercuric perchlorate solution, 8%. Treat 40 g of mercuric nitrate with 25 ml of cont. nitric acid and 10 ml of cont. perchloric acid and evaporate the solution on a hot-plate to perchloric acid fumes. Repeat with another 10 ml of nerchloric acid. Dissolve the resulting salt in water and -make up to 500 ml. Sulohuric-ohomhoric acid mixture. Mix 260 ml of cont. sulphuric acid and 740 ml of syrupy orthophosphoric acid. Titanous sulphate solution. Dissolve 5 g of titanium metal in 500 ml of dilute sulphuric acid (1 + 3). Separate the
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undissolved metal and dilute the solution with enough distilled water to dissolve all the titanous sulphate. Store the solution in a brown bottle in a dark place. Stannous chloride solution. Dissolve 15 g of SnCl,. 2H,O in 100 ml of cont. hydrochloric acid. Titration procedure
Pipette an aliquot of the sample solution (5540 ml, depending on the iron content of the sample, 330.25 g/l.) into a 5C0-ml conical flask and add 5 or 2 ml of 20% copper sulphate solution (depending upon the amount of copper present in the sample) followed by 5-10 drops of 10% sulphosalicylic acid solution. Then add titanous, sulphate solution dropwise from a burette, with shaking, until the disappearance of the violet colour. On appearance of the grey-blue colour, add titanous sulphate solution in 75100% excess (or until the appearance of a turbidity due to the precipitated copper). Next add 10 ml of mercuric perchlorate solution, with shaking, followed by 20 ml of sulphuric acid (1 + I), 5 ml of sulphuric-phosphoric acid mixture, 150 ml of distilled water and some oxidized sodium diphenylamine sulphonate indicator. Titrate with potassium dichromate solution to the appearance of a permanent blue-violet. Dissolution procedure for copper concentrate
Treat a l-g sample of the copper concentrate with 25 ml of cont. nitric acid and heat on a hot-plate. When all the sample has dissolved (about 1 hr) evaporate the solution nearly to dryness. Add 2 ml of sulphuric acid (1 + 1) and 40 ml of distilled water and heat for about 30 min. Filter the solution into a lOO-ml standard flask and wash the residue several times with hot water transferring the washings to the filter funnel. Make up the solution to 100 ml. RESULTS
AND
DISCUSSION
In the well-established method for the redox titration of iron with dichromate, stannous chloride is used for initial reduction of the iron(III), but when copper(I1) is present this method give results high by as much as 60% because of the copper(I) produced in the reduction process. This prompted the search for other reducing agents. Titanous sulphate was found to give satisfactory results, but a detailed study of the conditions was necessary. During the reduction of iron(II1) in the presence of copper, the excess of titanous sulphate reduces the cupric ion
55
SHORT COMMUNICATIONS
Table 2. Analysis of bacterial leach solutions
Table 1. Effect of HCI concentration on the determination of 26.98 mg of iron
CHCU M
Fe found, mg
Error, %
28.03 29.81 29.00 27.84 41.35 43.87
+3.8 + 10.5 +7.5 + 3.2 + 53.3 -l-62.6
Fe:Cu 0.05 0.50 1.00 2.00 3.00 4.50
to metallic copper, and this can be taken as an indication that the iron has been completely reduced It was found advantageous, however, to use sulphosalicylic acid as indicator for this purpose, the accuracy being better. Ferric iron is reduced with reagents such as amalgamated zinc, hydrogen sulphide or sulphurous acid in l-2N sulphuric acid, and the effect of the acid concentration on the reduction with titanium(II1) was therefore studied. At higher acidities the iron-sulphosalicylic acid complex is unstable and it is found difficult to decide when the reduction is complete. For this reason further work was done with 0.12N sulphuric acid medium. There seems to be no difference in the effect of excess of titanous sulphate in the range 20-100x and a 75% excess was chosen for further work. The concentration of cupric ion was varied to determine the effect of the copper. The results (see Table 4) show that a Cu:Fe ratio of at least 5 was optimal. Solutions containing low concentrations of copper should have more added copper. Nickel, copper and lead in 1: 1 w/w ratio to iron were found to have no effect on the results. The applicability of the method in chloride medium was also investigated. It was found that completion of the reduction step was difficult to judge visually and the results are seriously in error because of precipitation and subsequent titration of cuprous chloride (Table 1). PRACl’ICAL
APPLICATIONS
The method has been applied to the determination of iron in bacterial leach solutions containing copper and various amounts of iron (0.28-3.4 g/l.) in sulphate medium. The results presented in Table 2 show that the values of iron determined by the proposed method compare favourably with the values obtained by atomic-absorption spectrophotometry.
1:1.4
Analysis by atomic absorption, g/l. Fe cu 4.73
3.41
Proposed method, Fe, 811. 3.27 1 3.45 3.46 3.39 J
3.39
1.28 1.25
1.28
1:4
5.40
1.28
1.29 1.30 i
1:4
5.34
1.43
1.38 > 1.35 1.32 1.32 J
1.34
0.45 0.44 0.45
0.45
0.28 0.28 0.28 0.28
0.28
1:12
1:19
5.37
5.20
0.43
0.28
A copper concentrate (McIntyre Mines Ltd., Schumachar, Ontario) was also analysed by the proposed method (24.4, 23.0, 23.3, 23.6, 23.5%) and by dichromate titration after separation of iron from copper by double precipitation with ammonia (23.6%) The results are in good agreement, showing the method to be capable of general application in sulphate medium with a relative error of about 2%. Acknowledgement-Thanks are due to Mr. J. C. Ingles, former Head, Chemical Analysis Section, Extraction Metallury Division, for suggesting the problem. REFERENCES
1. A. I. Vogel, Textbook of Quantitative Inorganic Analysis, 3rd Ed., Wiley, New York, 1961. 2. R. J. C. MacDonald, Dept. Energy, Mines and Resources, Mines Branch, Ottawa, Canada, Internal Rept. EM1 73-12. October, 1973.
SHORT
Tofanta, Vol. 25, pp. 54-55.
Pergamon
Press, 1978. Printed
COMMUNICATIONS
in Great Britain
A RAPID TITRIMETRIC DETERMINATION OF IRON IN COPPER CONCENTRATES AND COPPER-BEARING LEACH SOLUTIONS
v. s. sASllU Metallurgical Chemistry Section, Physical Sciences Laboratory, Mineral Sciences Laboratories, CANMET, Ottawa, Canada
(Received 11 May 1977. Accepted 25 June 1977)
Summary-A This method mate. It has concentrates
titrimetric method for the determination of iron(II1) in the presence of copper is presented. involves the reduction of iron(II1) with titanous sulphate followed by titration with dichrobeen successfully used for the determination of iron in both leach solutions and copper with a relative error of 1%.
The standard analytical method’ for the determination of total iron in leach liquors involves the reduction of iron(II1) to iron(I1) with stannous chloride, the excess of which is oxidized by mercuric chloride, followed by titration of ferrous ion with standard dichromate solutions, with diphenylamine as indicator. This method, when applied to solutions containing copper, presents a problem in that copper (II) is reduced to copper (I) which is not oxidized by mercuric chloride, but is titrated with dichromate, so the total iron determined will be high. One method of elimination of the interference due to copper is to use either metallic copper or test lead as the reducing agent.’ Titanous sulphate completely reduces iron(II1) to iron(I1) and can be used for the determination of total iron, provided that all the iron is first oxidized to iron(II1). However, copper(I1) is reduced to copper metal along with the reduction of iron(II1) by titanous sulphate in sulphate medium and prevents visual determination of the end-. point when potassium thiocyanate is used as indicator. On the other hand, the reduction to metallic copper can be used as an indication that the ion(II1) has all been reduced. The metallic copper can then be dissolved by the addition of mercuric perchlorate solution, followed by the titration of the iron(I1) with dichromate solution. A detailed study was therefore undertaken for the determination of iron in the presence of copper, with titanous sulphate as the reducing agent. EXPERIMENTAL
Reagents Ferric sulphate solution. Dissolve 1.2500 g of high-purity iron in dilute sulphuric acid with hydrogen peroxide as the oxidizing agent and make up to 250 ml in a standard flask. Mercuric perchlorate solution, 8%. Treat 40 g of mercuric nitrate with 25 ml of cont. nitric acid and 10 ml of cont. perchloric acid and evaporate the solution on a hot-plate to perchloric acid fumes. Repeat with another 10 ml of nerchloric acid. Dissolve the resulting salt in water and -make up to 500 ml. Sulohuric-ohomhoric acid mixture. Mix 260 ml of cont. sulphuric acid and 740 ml of syrupy orthophosphoric acid. Titanous sulphate solution. Dissolve 5 g of titanium metal in 500 ml of dilute sulphuric acid (1 + 3). Separate the
Crown Copyrights reserved.
undissolved metal and dilute the solution with enough distilled water to dissolve all the titanous sulphate. Store the solution in a brown bottle in a dark place. Stannous chloride solution. Dissolve 15 g of SnCl,. 2H,O in 100 ml of cont. hydrochloric acid. Titration procedure
Pipette an aliquot of the sample solution (5540 ml, depending on the iron content of the sample, 330.25 g/l.) into a 5C0-ml conical flask and add 5 or 2 ml of 20% copper sulphate solution (depending upon the amount of copper present in the sample) followed by 5-10 drops of 10% sulphosalicylic acid solution. Then add titanous, sulphate solution dropwise from a burette, with shaking, until the disappearance of the violet colour. On appearance of the grey-blue colour, add titanous sulphate solution in 75100% excess (or until the appearance of a turbidity due to the precipitated copper). Next add 10 ml of mercuric perchlorate solution, with shaking, followed by 20 ml of sulphuric acid (1 + I), 5 ml of sulphuric-phosphoric acid mixture, 150 ml of distilled water and some oxidized sodium diphenylamine sulphonate indicator. Titrate with potassium dichromate solution to the appearance of a permanent blue-violet. Dissolution procedure for copper concentrate
Treat a l-g sample of the copper concentrate with 25 ml of cont. nitric acid and heat on a hot-plate. When all the sample has dissolved (about 1 hr) evaporate the solution nearly to dryness. Add 2 ml of sulphuric acid (1 + 1) and 40 ml of distilled water and heat for about 30 min. Filter the solution into a lOO-ml standard flask and wash the residue several times with hot water transferring the washings to the filter funnel. Make up the solution to 100 ml. RESULTS
AND
DISCUSSION
In the well-established method for the redox titration of iron with dichromate, stannous chloride is used for initial reduction of the iron(III), but when copper(I1) is present this method give results high by as much as 60% because of the copper(I) produced in the reduction process. This prompted the search for other reducing agents. Titanous sulphate was found to give satisfactory results, but a detailed study of the conditions was necessary. During the reduction of iron(II1) in the presence of copper, the excess of titanous sulphate reduces the cupric ion
55
SHORT COMMUNICATIONS
Table 2. Analysis of bacterial leach solutions
Table 1. Effect of HCI concentration on the determination of 26.98 mg of iron
CHCU M
Fe found, mg
Error, %
28.03 29.81 29.00 27.84 41.35 43.87
+3.8 + 10.5 +7.5 + 3.2 + 53.3 -l-62.6
Fe:Cu 0.05 0.50 1.00 2.00 3.00 4.50
to metallic copper, and this can be taken as an indication that the iron has been completely reduced It was found advantageous, however, to use sulphosalicylic acid as indicator for this purpose, the accuracy being better. Ferric iron is reduced with reagents such as amalgamated zinc, hydrogen sulphide or sulphurous acid in l-2N sulphuric acid, and the effect of the acid concentration on the reduction with titanium(II1) was therefore studied. At higher acidities the iron-sulphosalicylic acid complex is unstable and it is found difficult to decide when the reduction is complete. For this reason further work was done with 0.12N sulphuric acid medium. There seems to be no difference in the effect of excess of titanous sulphate in the range 20-100x and a 75% excess was chosen for further work. The concentration of cupric ion was varied to determine the effect of the copper. The results (see Table 4) show that a Cu:Fe ratio of at least 5 was optimal. Solutions containing low concentrations of copper should have more added copper. Nickel, copper and lead in 1: 1 w/w ratio to iron were found to have no effect on the results. The applicability of the method in chloride medium was also investigated. It was found that completion of the reduction step was difficult to judge visually and the results are seriously in error because of precipitation and subsequent titration of cuprous chloride (Table 1). PRACl’ICAL
APPLICATIONS
The method has been applied to the determination of iron in bacterial leach solutions containing copper and various amounts of iron (0.28-3.4 g/l.) in sulphate medium. The results presented in Table 2 show that the values of iron determined by the proposed method compare favourably with the values obtained by atomic-absorption spectrophotometry.
1:1.4
Analysis by atomic absorption, g/l. Fe cu 4.73
3.41
Proposed method, Fe, 811. 3.27 1 3.45 3.46 3.39 J
3.39
1.28 1.25
1.28
1:4
5.40
1.28
1.29 1.30 i
1:4
5.34
1.43
1.38 > 1.35 1.32 1.32 J
1.34
0.45 0.44 0.45
0.45
0.28 0.28 0.28 0.28
0.28
1:12
1:19
5.37
5.20
0.43
0.28
A copper concentrate (McIntyre Mines Ltd., Schumachar, Ontario) was also analysed by the proposed method (24.4, 23.0, 23.3, 23.6, 23.5%) and by dichromate titration after separation of iron from copper by double precipitation with ammonia (23.6%) The results are in good agreement, showing the method to be capable of general application in sulphate medium with a relative error of about 2%. Acknowledgement-Thanks are due to Mr. J. C. Ingles, former Head, Chemical Analysis Section, Extraction Metallury Division, for suggesting the problem. REFERENCES
1. A. I. Vogel, Textbook of Quantitative Inorganic Analysis, 3rd Ed., Wiley, New York, 1961. 2. R. J. C. MacDonald, Dept. Energy, Mines and Resources, Mines Branch, Ottawa, Canada, Internal Rept. EM1 73-12. October, 1973.