GB1561024A

GB1561024A - Production of solid ferric sulphate

Info

Publication number: GB1561024A
Application number: GB1811277A
Authority: GB
Original assignee: Tioxide Group Ltd
Current assignee: Tioxide Group Ltd
Priority date: 1977-04-29
Filing date: 1977-04-29
Publication date: 1980-02-13
Also published as: DE2818098A1; NL7803927A; FR2388766A1

Description

(54) PRODUCTION OF SOLID FERRIC SULPHATE (71) We, TIOXIDE GROUP LIMITED, a British Company, of 10 Stratton Street, London W1A 4XP, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to an improved form of ferric sulphate which is suitable, for example, for use in the treatment of water containing suspended solids.

It is known to treat impure water with ferric sulphate in order to flocculate therefrom suspended solids. Hitherto, the ferric sulphate has normally been available in the form of an aqueous solution, for example one containing between 40% and 50% by weight of ferric sulphate. One reason for this is that ferric sulphate, when suspended as a dust in the atmosphere, attacks the mucous membranes, particularly those of the nose, causing bleeding and discomfort. It has, in the past, proved difficult to provide ferric sulphate in a solid form which is substantially dust-free.

It is an object of this invention to provide a process whereby such a product can be produced.

Accordingly, the present invention is a process for the production of solid ferric sulphate comprising forming a hot aqueous acidic solution of ferric sulphate containing a concentration of ferric sulphate in the range 50% to 70% by weight, seeding the solution with finely divided solid ferric sulphate, subdividing the product formed and thereafter drying the subdivided product.

The hot aqueous solution of ferric sulphate is conveniently prepared by the oxidation of an aqueous solution of ferrous sulphate.

This oxidation may conveniently be carried out by the addition to the ferrous sulphate solution of a mixture of nitric and sulphuric acids or by any other suitable method.

Where the ferrous sulphate solution is derived from the effluent of the so-called "sulphate" process for the production of pigmentary titanium dioxide, the ferric sulphate solution produced is normally in the range 40% to 50% and particularly in the range 40% to 45%, by weight.

The aqueous acidic ferrous sulphate solution is concentrated if necessary, or heated if concentration is not required, to give a hot solution containing a concentration of ferric sulphate in the range of 50% to 70% and preferably one in the range 55% to 65% by weight and at a temperature in excess of 70"C, preferably one in the range 100"C to 1200C. The temperature at the boiling point of the solution, after concentration when necessary, is very convenient.

It has been found advantageous to add the seed to the solution at such an elevated temperature since these temperatures, eg in excess of 70"C, aid the dispersion and solution of the seed.

The subdivision of the material may be accomplished by any suitable method.

For example the seeded mass, after stirring but while still fluid and preferably at a temperature above about 60"C, may be run out of the vessel into suitably shaped moulds wherein the contents solidify. The moulds may conveniently be heated, for example by means of a band drier, or they may be allowed to stand at room temperature or at an elevated temperature, to complete the solidification process.

The moulds are, of course, chosen to provide the desired shape for use or for future treatment of the material. For example, slabs may be formed having a weight in the range of about 2 to 25 kilograms and may be used in this form or, alternatively, the slabs may be broken up and sieved to give smaller pieces of relatively uniform size.

Where the material it cast into slabs it is desirable to reduce at least a proportion of the slabs to very finely-divided material, for example to particles in the size range 1/3O" to 4tB, and preferably in the range 1/1,, to i". Such material is also suitable as seed material for recycling to unseeded aqueous acidic ferric sulphate solutions to be used in the present invention.

An alternative method of subdivision is to cool the ferric sulphate solution after seeding for example to a temperature in the range 0 C to 40"C and preferably to one in the range 15"C to 25"C allow the liquid to form a deformable mass having a putty-like consistency. The time taken for this to occur may vary, for example from 10 minutes to 6 hours. When this stage is reached it is advisable to subdivide the material as quickly as possible, for example within 10 minutes. In a continuous process the size of the seeding and mixing vessel is chosen to allow the appropriate residence time during cooling so that the material is discharged as a deformable mass substantially as soon as the correct consistency is reached.

The preferred method of subdivision of the deformable material is by extrusion through appropriately sized holes to give elongated particles of high surface area to volume ratio. It has been found convenient to subject these particles, after drying, to light milling and sieving to ensure particles of suitably uniform size/shape characteristics. For example, extrusion through holes having a diameter in the range I 1/1G" to '-" and preferably through holes in the range l/,G" to 3/,", prior to drying, has been found convenient. The particles thus produced may then be subjected to light milling and sieving to provide a majority of pieces having a length between about l" and 1".

Alternatively, the deformable mass can be subjected to pelletisation, for example in a trundler, to produce spherical pellets, for example having a diameter in the range /1G to -2 As before, seed material for recycle is conveniently obtained by reducing the required amount of product to a very finelydivided material of the appropriate particle size.

After subdividing (by either method) the material may be dried, for example to a product having a solids content in the range 60% to 80%, and particularly one in the range 70% to 75%, by weight, of which at least 95% of the iron content is in the ferric state and, more usually, at least 99% is in this state.

It is believed that where the initial aqueous acidic ferric sulphate solution contains free sulphuric acid, the concentration of this should be controlled to obtain the best product, for example to a free sulphuric acid content in the range 20% more, to 10% less, than the molar equivalent. The molar equivalent is that wherein the amount of sulphate ions from free sulphuric acid is equal to that from the ferric sulphate. A preferred range appears to be from 5 % more than the molar equivalent to 5% less than the molar equivalent.

Although the seed for unseeded aqueous ferric sulphate solution is conveniently obtained as described previously, it may also be made by allowing an acidic ferric sulphate to stand for a prolonged period of time in air until solid ferric sulphate is precipitated from solution.

A proportion of seed in the range 1% to 50% by weight may be added (based on the ferric sulphate content of the unseeded solution) but it is preferred to add an amount in the range 10% to 25%.

Ferric sulphate produced by the present process has a high density, for example a specific gravity of about 1.9, and this forms a very conveniently-handlable material in the quantities likely to be required.

Examples of the process of the present invention are given below.

EXAMPLE 1 An aqueous acidic solution of ferric sulphate (obtained by the oxidation of an aqueous acidic ferrous sulphate solution with a mixture of nitric and sulphuric acids) and containing 42% by weight of ferric sulphate and 0.4% of free sulphuric acid was taken and concentrated by boiling until the ferric sulphate content reach 60% by weight and the free sulphuric acid content was 0.9%. The solution was seeded with ferric sulphate crystals obtained by allowing an aqueous ferric sulphate solution (containing 56% ferric sulphate and 2.5% sulphuric acid) to stand in an open vessel for 6 days to allow the ferric sulphate crystals to form and precipitate.

Twenty-five per cent by weight of ferric sulphate crystals were added as seed when the temperature of the aqueous acidic ferric sulphate solution was at 1100 C. The seed dispersed readily and the mixture was cooled to 21 0C and maintained at that temperature for 180 minutes when a plastic deformable mass had formed. This was extruded through a metal plate containing a number of holes of 1/11 inch diameter to produce 'worms' of solid ferric sulphate. This material was dried at 650C to 700C to produce a hardened material of similar physical shape containing 72% solids and which was dust-free and free-flowing. This was very suitable for adding to water containing suspended solids since it dissolved therein very quickly and presented no problem in environmental pollution during handling. If desired, the dried material could be subjected to light milling and screening to produce particles of more uniform size and shape.

EXAMPLE 2 The process described in Example 1 was repeated except that the hot aqueous acidic ferric sulphate solution was seeded with the plastic deformable product produced as described above prior to drying. Sufficient material was added as seed to provide an additional 25% by weight of ferric sulphate in the seeded solution.

Again, on cooling as described in Example 1, a plastic deformable mass formed which was treated as described in Example 1 to give an end product which was indistinguishable from the product of that Example.

EXAMPLE 3 Starting material similar to that of Example 1 was concentrated until the concentration of ferric sulphate in the liquor was 65% by weight.

The solution at 110 C was then seeded with sufficient finely-divided solid ferric sulphate to constitute 15% by weight (on the ferric sulphate in solution).

The mixture was stirred for 10 minutes and the resulting liquid poured into a series of moulds to give, on cooling and solidification, blocks weighing approximately 6 kilograms.

The greater proportion of the blocks were then passed through a drier at 1050C.

The final product had a solids content of 67%. Some blocks were subjected to milling to provide sufficient finely-divided material for recycle in the process as seed material.

The blocks were very suitable for addition to a potable water treatment plant in which they dissolved at a suitable rate to cause flocculation of suspended solids during the passage of water through the purification plant.

WHAT WE CLAIM IS:- 1. A process for the production of solid ferric sulphate comprising forming a hot aqueous acidic solution of ferric sulphate containing a concentration of ferric sulphate in the range 50% to 70% by weight, seeding the solution with finely-divided solid ferric sulphate, subdividing the product formed and thereafter drying the subdivided product.

2. A process as claimed in claim 1 wherein the aqueous solution of ferric sulphate is formed by oxidising an aqeous solution of ferrous sulphate.

3. A process as claimed in claim 2 wherein the oxidation is by means of a mixture of nitric and sulphuric acids.

4. A process as claimed in any one of the preceding claims wherein the aqueous solution of ferric sulphate, after concentration if necessary, contains an amount of ferric sulphate in the range 55% to 65%, by weight.

5. A process as claimed in any one of the preceding claims wherein the seed is added to the aqueous ferric sulphate solution when the latter is at a temperature in excess of 70"C.

6. A process as claimed in any one of the preceding claims wherein an amount of seed in the range 1 ' to 50% by weight (based on the ferric sulphate content of the unseeded solution) is added.

7. A process as claimed in claim 6 wherein the amount of seed added is in the range 10% by 25%, by weight.

8. A process as claimed in any of the preceding claims wherein the subdivided material is dried to a solids content in the range 60% to 80%, by weight.

9. A process as claimed in claim 8 wherein the subdivided material is dried to a solids content in the range 70% to 75%, by weight.

10. A process as claimed in any of the preceding claims wherein at least 99 % of the iron content of the material is in the ferric state.

11. A process as claimed in any of the preceding claims wherein the aqueous ferric sulphate solution contains free sulphuric acid in an amount in the range 20% more, to 10% less, than the molar equivalent (as herein before defined).

12. A processs as claimed in claim 11 wherein the amount of free sulphuric acid is in the range 5% more, to 5% less, than the molar equivalent.

13. A process as claimed in claim 1 and substantially as set out in Example 1, 2 or 3.

14. Solid ferric sulphate when prepared by a process as claimed in any of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. EXAMPLE 2 The process described in Example 1 was repeated except that the hot aqueous acidic ferric sulphate solution was seeded with the plastic deformable product produced as described above prior to drying. Sufficient material was added as seed to provide an additional 25% by weight of ferric sulphate in the seeded solution. Again, on cooling as described in Example 1, a plastic deformable mass formed which was treated as described in Example 1 to give an end product which was indistinguishable from the product of that Example. EXAMPLE 3 Starting material similar to that of Example 1 was concentrated until the concentration of ferric sulphate in the liquor was 65% by weight. The solution at 110 C was then seeded with sufficient finely-divided solid ferric sulphate to constitute 15% by weight (on the ferric sulphate in solution). The mixture was stirred for 10 minutes and the resulting liquid poured into a series of moulds to give, on cooling and solidification, blocks weighing approximately 6 kilograms. The greater proportion of the blocks were then passed through a drier at 1050C. The final product had a solids content of 67%. Some blocks were subjected to milling to provide sufficient finely-divided material for recycle in the process as seed material. The blocks were very suitable for addition to a potable water treatment plant in which they dissolved at a suitable rate to cause flocculation of suspended solids during the passage of water through the purification plant. WHAT WE CLAIM IS:-

1. A process for the production of solid ferric sulphate comprising forming a hot aqueous acidic solution of ferric sulphate containing a concentration of ferric sulphate in the range 50% to 70% by weight, seeding the solution with finely-divided solid ferric sulphate, subdividing the product formed and thereafter drying the subdivided product.