GB2246561A

GB2246561A - Aqueous ferric sulphate solutions

Info

Publication number: GB2246561A
Application number: GB9110308A
Authority: GB
Inventors: William John Ferguson
Original assignee: Tioxide Group Services Ltd
Current assignee: Tioxide Group Services Ltd
Priority date: 1990-06-11
Filing date: 1991-05-13
Publication date: 1992-02-05
Anticipated expiration: 2011-05-13
Also published as: ES2040162B1; GB9012980D0; GB2246561B; GB9110308D0; ES2040162A1

Abstract

Aqueous solutions of ferric sulphate tend to be unstable when subjected to low temperatures; more stable solutions are obtained by oxidising ferrous sulphate with nitric acid and sulphuric acid followed by treatment with a peroxide eg H2O2. The products are free of ferrous ions and can have a range of free acid content while remaining stable. The products are flocculents used in the treatment of contaminated water.

Description

OXIDATION PROCESS This invention relates to an oxidation process and particularly to a process for the production of a solution of ferric sulphate.

Ferric sulphate has use in the treatment of contaminated water supplies as a flocculant but the concentration presently obtained by current production methods is limited by the tendency of the solutions to precipitate during storage at low temperature. It would also be advantageous if this tendency to freeze could be reduced and also if higher iron concentration could be obtained to improve coagulation performance in use and also to improve the economics of transportation to the customer.

According to the invention a process for the production of an aqueous solution of ferric sulphate comprises reacting in aqueous solution ferrous sulphate, nitric acid and sulphuric acid to oxidise ferrous sulphate to ferric sulphate and completing the oxidation by reacting residual ferrous sulphate with a peroxide oxidising agent so that the resultant product is substantially free of ferrous ions.

The process of the invention permits the total oxidation of ferrous ions to ferric ions thereby producing an aqueous solution substantially free of ferrous ions which allows the higher concentration solutions to be obtained and used particularly with a deficiency of 5042 ions with respect to Fe3+ ions. This also promotes an increase in the freeze stability of the solutions on storage outdoors in cold climates.

Generally speaking the process according to the invention involves two oxidation stages to produce the desired product. Firstly ferrous sulphate is oxidised with nitric acid in the presence of sulphuric acid. Secondly the aqueous solution is treated with a peroxide oxidising agent to complete the oxidation carried out in the first stage.

In the process of the invention any suitable form of ferrous sulphate can be used in the first stage such as that resulting from many industrial process e.g. the steel industry but preferably the ferrous sulphate produced as a by-product in the production of titanium dioxide is that chosen. This particular form of ferrous sulphate is that known as "copperas" which is FeSO4.7H2O and is obtained from the so-called "sulphate" process for the production of titanium dioxide pigment and usually it is contaminated with weak sulphuric acid. The presence of this sulphuric acid makes a contribution to the total sulphate ions in solution during operation of the process of the invention but further addition of concentrated sulphuric acid is usually necessary.

In the first stage the ferrous sulphate is usually added to nitric acid and sulphuric acid to form the aqueous solution and to commence oxidation of ferrous ions to ferric ions. Nitric acid of a suitable strength is employed and this may be obtained by using concentrated acid, suitably diluted or by using a mixture of concentrated nitric acid and a weaker acid perhaps recycled from another stage of an industrial process or obtained by regeneration of nitric acid from recovered nitric oxide gas by-products. For instance a mixture of weak nitric acid (28%) and concentrated nitric acid (60%) in volume ratio of about 1:6 has been found to be satisfactory as the oxidising agent of the first stage.

The sulphuric acid used in the first stage will usually be concentrated acid and in an amount so that the required total amount of sulphate ions will be present in the finally produced aqueous solution of ferric sulphate.

During oxidation in the first stage it is convenient to add the ferrous sulphate continuously to the mixed acids and a particular practical method of carrying this out is by using a screw conveyor.

The oxidation is carried out at an elevated temperature, usually above 500C and preferably from 700C to 800C during which nitric oxides substantially (NO) are evolved and removed by scrubbing the effluent gas stream to produce weak nitric acid for recycling.

When the ferrous sulphate has been initially reacted with the mixed acids, typically at least 40% of the Fe2+ will have been oxidised to Fe3+, then a peroxide is added to the solution to complete the oxidation of all the ferrous ions to ferric ions.

Any suitable peroxide oxidising agent can be used such as disodium peroxide or potassium persulphate but preferably hydrogen peroxide in aqueous solution is used. The amount of the oxidising agent, e.g. hydrogen peroxide, should be sufficient to complete the oxidation of all remaining ferrous ions in solution and preferably is added in excess.

For the production of the most preferred freeze -stable aqueous solutions i.e. those of an increased concentration compared to those currently produced by nitric acid oxidation, it has been observed that the amount of sulphuric acid present should be such as to produce a ratio of SO42-:Fe3+ in the final product of at least 2.7:2.

The S04:Fe3+ ratio can be up to 2.97:2 for a product containing 50% solids and up to 2.91:2 for a product containing 52.5% solids.

According to the invention also a composition suitable for use in the treatment of water containing contaminates comprises an aqueous solution of ferric sulphate substantially free of ferrous ions containing up to 53% by weight solids and having a ratio of SO4:Fe3+ of from 2.7:2 to 2.97:2.

Preferably the free acid content of the solution is from -5% to +3% H2SO4 and more preferably from -5% to -1%.

Compositions of the invention and those made by the process of the invention are useful in treating water in purification plants.

The most preferred compositions of the invention are those which contain no detectable ferrous ions using conventional analytical techniques.

The invention is illustrated in the following Examples.

Example 1 Recycled nitric acid (28% by weight) (221 litres) and nitric acid (60% by weight) (1300 litres) were added to sulphuric acid (98% by weight) (730 litres) in a lagged stainless steel tank of volume 21000 litres and the temperature raised to 75 C. Commercial copperas (FeSO4.7H2O) (12.45 tonnes) was added to the acid mixture using a screw conveyor at a rate of 5 tonnes per hour. The mixture was stirred and the nitric oxide evolved was passed to a scrubbing system to generate weak nitric acid.

After all the copperas had been added to the acid hydrogen peroxide (90 litres) was added to the steel tank.

Samples of the solution were analysed to determine the content of Fe2+ ions and Fe3+ ions by the following procedures: Ferrous Ion Pipette the original sample (5 cm3) into a conical flask (250 cm3) containing distilled water (ca 100 cm3) and Knopps Reagent (ca 50 cm3), add diphenylamine indicator (ca 1 cm3) and titrate to a permanent bluish end-point against potassium dichromate solution (0.01 N).

CALCULATION OF FERROUS IRON Ferrous iron = Titre x 0.00056 x 100 Analytical sg x 5 Ferric Ion Pipette the 'stock' solution (5 cm3) into a conical flask (250 cm3) containing distilled water (100 cm3), add pyridine 2-3 diol indicator solution (0.5%, ca 1 cm3) and titrate against EDTA solution (0.1M), to a yellow end-point.

The colour change is very slow and great care must be taken as the end-point is approached.

CALCULATION OF FERRIC IRON Ferric iron = Titre x 0.056 x 100 x 100 Analytical sg x 5 x 10 i.e. Titre x 1.12 Analytical sg The solution contained 13.96% by weight Fe3+ which is equivalent to 49.9% Fe2(SO4)3 and had an Fe3+:SO42- ratio of 2/2.904. The solution was free of ferrous Fe2+ ions.

The usefulness of the solution obtained was tested as described below A conventionally available solution of ferric sulphate (12.5% total Fe and 0.25% Fe2+) was used to treat impure water at a level of the equivalent of 20 mg/litre Fe2(SO#3. Treatment was adequate to coagulate impurities in the water. The residual iron content of the treated water was 0.15 mg/litre.

Using the product of this Example as described above the same coagulation of impurities was achieved when the amount used was equivalent to 12 mg/litre Fe2(SO4)3. The residual iron content of the water was 0.06 mg/litre. Thus the product of the invention is advantageous in enabling a reduced amount to be used as well as producing purified water with a lower impurity content.

EXAMPLE 2 Aqueous solutions of ferric sulphate with a range of free acid contents were prepared by the following general method.

Sulphuric acid (98% by weight) and 170 ml of nitric acid (60% by weight) were added to a 2000 ml glass beaker and heated to 75 C.

The beaker was equipped with a stirrer and the contents were stirred throughout the preparation.

Copperas (ferrous sulphate heptahydrate) was added to the acid mixture at a sufficiently slow rate to minimise the rate of evolution of nitric oxide. Water was added to maintain the viscosity of the mixture at a value sufficient to permit efficient stirring. After all the copperas had been added a sample of the solution was analysed and the amount of ferrous iron (Fe2+) calculated.

Hydrogen perioxide (600 ml) was added to the beaker to convert the remaining ferrous ion to ferric ion. A sample of the mixture was analysed again and if any ferrous ion was detected then a further quantity of hydrogen peroxide was added. A sample of the solution was analysed and the total ferric iron content calculated. The solution was diluted with a further quantity of water to the desired ferric ion content.

The solution was allowed to cool and samples stored in a freezer at a temperature of -5 C. Solutions in which precipitation occurred were deemed to be unstable. The condition of the solutions and the evaluation of their stability is shown in Table 2 following.

In Table 1 the actual amounts of ingredients used in the general method are set out to produce the desired solutions. The amount of water is the total of that added to maintain the viscosity of the mixture and that to dilute the final solution to the desired ferric ion content.

TABLE 1

Free Acid Content % %Fe2(SO4)3 10 -5 -3 -1 0 +1 +3 50 80 80 100 120 135 140 ml H2SO4 40 170 165-170 170-175 100-120 120 100 100 ml H20 2000 2000 1900 1850 1800 1600 1400 g FeS04.7H20 50 70 90 100 120 135 140 m1H2504 45 140-150 100-150 140-180 100-140 100-120 100 100 ml 1120 2100 2000 2000 1900 1850 1650 1600 gFeS04.7H2O 50 70 100 105 120 130 135-140 ml H2S 4 47.5 150 150 120-140 90-100 100 90-100 100 ml H2O 2000 2000 1850 1850 1800 1800 1800 g FeS04.7H20 70 80 100 100-120 120 130 140 ml mlH2SO4 50 50 100 100-120 80 70 30 30 ml H20 2000 1900 1850 1700 1700 1600 1400 g FeSO4.71120 60 90 100 120 120 130 140 mlH2SO4 52.5 50 50 20-50 20-40 30 30 20-30 ml H20 2000 1800 1700 1600 1500 1500 1200 g FeS04.7H2O 90 ml H2SO4 55 2080 ml H20 1650 g FeSO4.7H2O 575 20-60 ml H2O 1800 g FeSO4.7H2o 60 90 ml H2SO4 60 40 20 ml H2O 1800 1800 g FeSO4.7H2o TABLE 2 Free Acid Content % %Fe2(SO4)3 -10 -5 -3 -1 0 +1 +3 40 solid stable stable stable stable stable stable 45 solid stable stable stable stable stable stable 47.5 solid stable stable stable stable stable stable 50 solid stable stable stable solid solid solid 52.5 solid stable solid solid solid solid solid 55 solid - - - - - 57.5 solid - - - - - 60 solid solid - - - - - The results show the importance of free acid content and hence Fe/SO4 ratio in controlling stability. Those solutions classed as stable could be used as coagulants in water treatment.

EXAMPLE 3 The general method of Example 2 was repeated employing 100 ml of H2SO4, 100 to 120 ml of H2O and 1850g copperas. In this experiment 30g of disodium peroxide was used instead of hydrogen peroxide and a vigorous reaction took place and a gas was given off.

Complete conversion to ferric ion occurred.

EXAMPLE 4 Example 3 was repeated but using 30g of potassium persulphate instead of the disodium peroxide. The reaction was less vigorous than that in Example 3 and complete conversion to ferric ion took place. The product was a useful flocculant for water treatment.

EXAMPLE 5 A series of different solutions were made using the general method of Example 2. All the solutions had a final ferric sulphate content of 50%. Differing ferrous ion contents in the final solution were obtained by employing appropriate amounts of hydrogen peroxide and three basic free acid contents were chosen.

The constitution of each solution prepared is set out in Table 3 below.

SOLUTION FREE ACID % FERROUS ION % Al -5 0.68 A2 -5 0.34 A3 -5 0.20 A4 -5 0.12 As -5 0 B1 -3 0.78 B2 -3 0.48 B3 -3 0.20 B4 -3 0 C1 -1 0.58 C2 -1 0.35 C3 -1 0.21 C4 -1 0.10 C5 -1 0 The turbidity of each solution produced was measured using the following general procedure.

A sample of raw water was obtained with a turbidity of 10 (assessed using a HACH turbidity tester) and the pH adjusted to 7.0.

Known quantities of each solution were mixed with the raw water for one minute whilst stirred at 150 revolutions per minute and then for 20 minutes at 10 revolutions per minute. The supernatant liquid was filtered through filter paper (Whatman Grade 1) and its turbidity assessed using the turbidity tester.

Graphs were plotted of turbidity against the quantity of solution used (dose rate). These graphs are shown as Figures 1, 2 and 3 attached hereto: Figure 1 is the graph for soultions having a free acid content of -5% Figure 2 is the graph for solutions having a free acid content of -3% and Figure 3 is the graph for solutions having a free acid content of -1%.

These graphs show the importance in reducing turbidity of the amount of residual ferrous ions in solution.

Claims

1. A process for the production of an aqueous solution of ferric sulphate which comprises reacting in aqueous solution ferrous sulphate, nitric acid and sulphuric acid to oxidise ferrous sulphate to ferric sulphate and completing the oxidation by reacting residual ferrous sulphate with a peroxide oxidising agent so that the resultant product is substantially free of ferrous ions.

2. A process according to claim 1 in which the ferrous sulphate is FeSO47ll2O.

3. A process according to claim 1 or 2 in which prior to the addition of said peroxide oxidising agent at least 40% of the Fe2+ has been oxidised to Fe3+.

4. A process according to claim 1, 2 or 3 in which the reaction is conducted at a temperature of above 50 C.

5. A process according to claim 4 in which the temperature is from 700C to 80 C.

6. A process according to any one of the preceding claims in which the oxidising agent is disodium peroxide.

7. A process according to any ony of claims 1 to 5 in which the oxidising agent is potassium persulphate.

8. A process according to any one of claims 1 to 5 in which the oxidising agent is hydrogen peroxide.

9. A process according to any one of the preceding claims in which the amount of oxidising agent is in excess of that which is required to complete the oxidation of all remaining ferrous ions in solution.

10. A process according to any one of the preceding claims in which the amount of sulphuric acid present is such as produces a ratio of SO42-:Fe3+ in the resultant product of at least 2.7:2.

11. A process according to claim 10 in which the ratio is up to 2.97:2.

12. A process according to claim 10 in which the ratio is up to 2.91:2.

13. A composition suitable for use in the treatment of water containing contaminates which comprises an aqueous solution of ferric sulphate substantially free of ferrous ions containing up to 53% by weight solids and having a ratio of SO42-:Fe3+ of from 2.7:2 to 2.97:2.

14. A composition according to claim 13 in which the free acid content of the solution is from -5% to +3%.

15. A composition according to claim 14 in which the free acid content is from -5% to -1%.

16. A process according to claim 1 substantially as described in the foregoing Examples.

17. An aqueous solution of ferric sulphate when prepared by a process according to any one of claims 1 to 12 and 16.

18. A method of treating contaminated water which comprises adding an aqueous solution according to claim 17 to contaminated water to flocculate any solid particles therein.

AMENDMENTS TO THE CLA't;1S l-;A.VE CEEl1 r-IL=-~D AS FOLLGv Jv 9. A process according to any one of the preceding claims in which the amount of oxidising agent is in excess of that which is required to complete the oxidation of all remaining ferrous ions in solution.

11. A process according to claim 10 in which the ratio is up to 2.97:2.

12. A process according to claim 10 in which the ratio is up to 2.91:2.

13. A composition obtained by a process according to claim 1 suitable for use in the treatment of water containing contaminates which comprises an aqueous solution of ferric sulphate substantially free of ferrous ions containing up to 53% by weight solids and having a ratio of SO42-:Fe3+ of from 2.7:2 to 2.97:2.