POLYFERRIC SULPHATE SOLUTION

Abstract

A process for production of polyferric sulphate (PFS) solution comprises: a) digesting iron oxide in aqueous sulphuric acid, to form an initial reaction solution including ferrous sulphate; b) subsequently oxidising obtained ferrous ions (Fe.sup.2+) with nitric acid to convert ferrous ions to ferric ions(Fe.sup.3+); and c) oxidising evolving nitrogen oxides to nitric acid to recover nitric acid; d) wherein the molar ratio of iron to sulphuric acid is such that the obtained PFS product will be sulphate deficient.

Claims

1. A process for production of polyferric sulphate (PFS) solution comprising: digesting iron oxide in aqueous sulphuric acid, to form an initial reaction solution including ferrous sulphate; subsequently oxidising obtained ferrous ions (Fe.sup.2+) with nitric acid to convert ferrous ions to ferric ions (Fe.sup.3+); and oxidising evolving nitrogen oxides to nitric acid to recover nitric acid; wherein the molar ratio of iron to sulphuric acid is such that the obtained PFS product will be sulphate deficient by 0.1% to 1%.

2. The process according to claim 1, wherein the iron oxide is selected from the group consisting of mill scale, magnetite, and mixtures thereof.

3. The process according to claim 1, wherein the aqueous sulphuric acid has a concentration in the range 45-60% mass.

4. (canceled)

5. (canceled)

6. The process according to claim 1, wherein the digestion is carried out at a temperature in the range 60 C.-115 C.

7. The process according to claim 6, wherein the digestion is carried out at a temperature in the range 80 C.-110 C.

8. The process according to claim 1, wherein the addition of nitric acid is started when the concentration of Fe.sup.2+ reaches 7-11 mass.

9. The process according to claim 8, wherein the addition of nitric acid is started when the concentration of Fe.sup.2+ reaches 9-10% mass.

10. The process according to claim 1, wherein the evolving NOx is oxidised by aerated water and/or aqueous hydrogen peroxide.

11. The process according to claim 1, wherein at least some of the nitric acid used for the oxidation of ferrous sulphate is recovered from the process of NOx oxidation.

12. The process according to claim 1, wherein the NOx oxidation is carried out at a temperature in the range 35-90 C.

13. The process according to claim 1, wherein the concentration of the added nitric acid is in the range 25-35% mass.

14. The process according to claim 1, wherein the ratio of Fe.sup.3+ to Fe.sup.2+ in the product is at least 50:1.

15. The process according to claim 1, wherein essentially all Fe.sup.2+ is oxidised to Fe.sup.3+.

16. The process according to claim 2, wherein the iron oxide is admixed with hematite.

17. The process according to claim 1, wherein the molar ratio of iron to sulphuric acid is such that the obtained product will be sulphate deficient by 0.1 to 3%.

18. The process according to claim 1, wherein the ratio of Fe.sup.3 to Fe.sup.2+ in the product is at least 60:1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The invention will now be further described, by way of example only, with reference to the following drawings in which:

[0055] FIG. 1 is a diagrammatic illustration of a process for the manufacture of poly ferric sulphate in accordance with an embodiment of the present invention;

[0056] FIGS. 2 and 3 are graphs of concentration against elapsed time for comparative experiments not in accordance with the present invention; and

[0057] FIGS. 4 to 7 are graphs of concentration against elapsed time for processes in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

[0058] Referring to FIG. 1 there is shown a process for the production of PFS solution in accordance with an embodiment of the present invention. Water is loaded into the reactor 1, then 98% sulphuric acid is added in such a ratio that the final concentration of diluted sulphuric acid is in the range of 45-60% mass. Then mill scale/magnetite is added in a near-stoichiometric amount over 4-5 hours.

[0059] After the concentration of ferrous iron reaches 8-10% a near-stoichiometric amount of nitric acid is added to the reactor. The reaction initially produces nitric oxide (NO):

3 Fe.sup.2++4H.sup.++NO.sub.3.sup..fwdarw.3 Fe.sup.3++2H.sub.2O+NO

[0060] The nitric oxide subsequently oxidises to nitrogen dioxide, and the evolving NOx is captured and oxidised to nitric acid in a three-stage scrubber, where a 35-50% aqueous solution of hydrogen peroxide is circulated.

[0061] The obtained finished PFS with Fe.sup.3+ concentration (8.5-12.8%), Fe.sup.2+ concentration (0.1-0.3%) and free acid (3.5 to 0.1%) may be stored in the reactor tank and supplied to customers.

[0062] Examples of the process in accordance with the invention which follow are for the purpose of demonstrating specific processes in accordance with the invention and are not intended to be limiting in scope on the invention or claims. The following examples are exemplary syntheses of numerous actual runs.

Comparative Example 1.

[0063] To a stirred 500 ml beaker are charged 132 ml water, 72 ml concentrated sulphuric acid (98%), and 69 g of magnetite. The reaction is carried out for 3 hours at 80 C. The results are shown in FIG. 2.

Comparative Example 2.

[0064] To a stirred 1000 ml beaker are charged 192 ml water, 83 ml of concentrated sulphuric acid (98%), 30 ml of 30% nitric acid and 96.5 g of mill scale, and the mixture is stirred at 100-110 C. The results are shown in a FIG. 3.

[0065] It is seen from the results above that in the absence of ferrous ion, resulting from oxidation by the nitric acid in Example 2, the digestion of iron oxides in sulphuric acid proceeds much slower even at temperatures that are significantly higher than in the Example 1.

[0066] The Examples 3 and 4 show the results of full scale plant trials in accordance with embodiments of the invention.

Examples 3 and 4.

[0067] A stirred tank 80 m.sup.3 volume was charged with 13,200 kg of 98% sulphuric acid, 19,600 kg of water and 6,900 kg of magnetite was added over 2-3 hours. The digestion continued for up to 5 hours at 85-110 C., and 1870 kg of 31.5% HNO.sub.3 was added. The reactor was constantly flashed with air at a rate of 1000-1200 m.sup.3/h and 675 kg of evolved NO.sub.x were oxidised to nitric acid. In order to prevent loss of heat, the reactor when necessary was fed up to 2300 kg/h of 7 barg steam. The reaction temperature for Example 3 was about 80 C., and for Example 4 was about 100 C.

[0068] Free sulphuric acid content of the final product was determined by titration based on EN 890: 2004. Details are summarised below.

[0069] 1.00 PURPOSE AND SCOPE

[0070] This procedure describes the method for determining the free acid content of all grades of Ferric Sulphate solution.

[0071] 2.00 DEFINITIONS

[0072] None.

[0073] 3.00 REFERENCES

TABLE-US-00001 EN 890: 2004 Chemical used for treatment of water intended for human consumption - Iron (III) sulphate liquid, Annex B.4 Determination of free acid

[0074] 3.01 Supporting Documents

[0075] None

[0076] 4.00 PROCEDURE

[0077] Principle

[0078] 4.01 An excess of Sulphuric Acid is accurately added, to the test solution. Fluoride ions are added in excess which form a strong complex with Fe.sup.3+ ions in the sample, thereby removing their acidic properties. The concentration of Hydrogen ions (from the added sulphuric acid plus that present in samples as free acid/less that reacted with samples that are sulphate/acid deficient) is determined by titration against Sodium Hydroxide, using Phenolphthalein as an indicator.

[0079] Reagents

[0080] 4.02

TABLE-US-00002 0.200M Sodium Hydroxide e.g. Fisher Catalogue No. J/7640 0.100M Sulphuric Acid e.g. Fisher Catalogue No. J/8450 (0.2N) Phenolphthalein indicator e.g. Fisher Catalogue No. P/2401L/08 solution, 1% Potassium Fluoride Dissolve 310 g of KF2H.sub.2O in 1000 ml of Solution, 200 g/l water. Adjust to pH 9.1 (the colour change of Phenolphthalein) by adding Sodium Hydroxide or Sulphuric Acid as appropriate (Add phenolphthalein indicator and titrate in just enough NaOH or H.sub.2SO.sub.4 as appropriate to cause the colour of the solution to change). Caution Potassium Fluoride is toxic. Wear rubber gloves

[0081] Method

[0082] 4.03 1. Tare a 250 ml volumetric flask, and weigh in approximately 20 g of sample. Record the sample mass taken (W). Dilute the sample to the mark with water and mix.*

[0083] 2. Pipette 10.0 ml of the dilute solution into a 500 ml Erlenmeyer flask (using a one mark pipette).* Add 25.0 ml of 0.100 M (0.2N) Sulphuric Acid (using a one mark pipette). Also add approximately 30 ml of water.

[0084] 3. In a separate Erlenmeyer flask, add 20 ml of 200g/1 KF solution, 50 ml of water, three drops of Phenolphthalein indicator, and 5 ml of 0.100M (0.2N) Sulphuric Acid. Titrate in 0.200M (0.2N) Sodium Hydroxide until the indicator just changes colour (to faint pink).

[0085] Note that this step is specified to ensure that the Potassium Fluoride used in each test is neutral to the perception of the person performing the test. Experience has shown that between operator variance is reduced significantly in this way.

[0086] 4. Pour the neutralised KF solution into the sample flask (from steps 2 and 3). Add three more drops of Phenolphthalein indicator, and titrate to the Phenolphthalein colour change. Record the titre, T.

[0087] Calculation 4.04

[00001]$\%.Math.H_2.Math.{\mathrm{SO}}_4=\frac{\left(T-25\right){24.5175}^{*}}{W}$

[0088] *Note that if a balance with sufficient precision (4 decimal place) is available the sample dilution stage of the test may be omitted by accurately weighing approximately 2.0 g of sample into the 500 ml Erlenmeyer the flask. The test will then continue from partway through step 2: i.e. Add 25.0 ml of 0.100M (0.2N) Sulphuric Acid (using a one mark pipette) . . . If using this method, 40 ml of KF solution should be used instead of 20 ml.

[0089] Without the dilution to 250 ml and pipetting 10 ml, the calculation becomes:

[00002]$\%.Math.H_2.Math.{\mathrm{SO}}_4=\frac{\left(T-25\right)0.9807}{W}$

[0090] The results are shown in FIGS. 4 and 5 respectively. The run in Example 3 produced about 40,000 kg of PFS solution with 12.8% concentration of Fe.sup.3+, 0.19% concentration of Fe.sup.2+ and 0.34% mass concentration of free acid.

[0091] The run in Example 4 produced about 40,000 kg of PFS solution with 12.36% concentration of Fe.sup.3+, 0.17% concentration of Fe.sup.2+ and 0.37% mass concentration of free acid.

Comparative Example 5

[0092] A stirred vessel was charged with 183 g of water and 133.9 g of 98% sulphuric acid. Magnetite (69 g) was added, and the reaction mixture was maintained at 80-90 C. After 2 hours 15 ml 30% nitric acid was added. The obtained product had 9.4% concentration of Fe.sup.3+, 0.11% concentration of Fe.sup.2+ and 0.02% concentration of free acid.

Comparative Example 6

[0093] A stirred vessel was charged with 183 g of water and, 127 g of 98% sulphuric acid. Magnetite (69 g) was added, and the reaction mixture was maintained heated at 80-90 C. After 4 hours 15 ml 30% nitric acid was added. The obtained product had 12.3% concentration of Fe.sup.3+, 0.21% concentration of Fe.sup.2+ and 4.99% concentration of free acid.

[0094] Results are shown in FIGS. 6 and 7 respectively. Later addition of nitric acid (Example 6) resulted in a higher concentration of ferric in the product than earlier addition (Example 5).

[0095] The foregoing describes exemplary embodiments of the present invention. It is understood that modifications to these embodiments can be made within the scope of the present invention. As such, the examples should not to be construed, in any way, as limiting of the scope of the present invention. The present invention should only be limited by the following claims.