GB1599716A

GB1599716A - Process for producing a pigment

Info

Publication number: GB1599716A
Application number: GB2025878A
Authority: GB
Original assignee: Redland Roof Tiles Ltd
Current assignee: Redland Roof Tiles Ltd
Priority date: 1978-05-17
Filing date: 1978-05-17
Publication date: 1981-10-07
Also published as: DE2919854A1

Description

(54) A PROCESS FOR PRODUCING A PIGMENT (71) We, REDLAND ROOF TILES LIMITED, a British Company, of Redland House, Castle Gate, Reigate, Surrey, 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: - This invention relates to a process for producing a black iron oxide pigment.

In our Patent Specification No. 1534196 we described and claimed a method of manufacturing black iron oxide pigment comprising the steps of taking a first aqueous solution of ferrous sulphate at a starting temperature of 64"C or more, taking a second aqueous solution of an alkali metal hydroxide or ammonia, adding the second solution to the first solution, while main taining the temperature of the combined solutions above 64"C, until the pH of the combined solution is in the range from 8.5 to 10.0 and oxidizing the combined solutions to deposit black iron oxide.

The present specification is concerned with an invention which can be regarded as a modification or improvement of that inven tion.

The importance of preheating the ferrous sulphate solution is critical. Various com plex (and not fully understood) transitions occur in ferrous sulphate solutions between about 56"C and about 68"C. However, to obtain as near to reproducible results as possible, the present method must be oper ated with an initial ferrous sulphate solu tion temperature of at least 680C. The tem perature of the ferrous sulphate solution before and during the reaction is thought to be important because of the dependency upon temperature of the degree of solvation (hydration) of the iron ions.

Thus. the present invention provides a method of manufacturing black iron oxide pigment comprising the steps of takina a first aqueous solution of ferrous sulphate at a starting temperature above 68"C, taking a second aqueous solution of an alkali metal hydroxide or ammonia, adding the second solution to the first solution while maintaining the temperature of the combined solutions above 68"C, the addition being preferably, but not essentially, effected in bursts until the pH of the combined solutions is from 8.5 to 10, oxidizing the combined solutions so as to deposit black iron oxide from the resulting mixture, and during the oxidation and separation ensuring that insufficient cooling occurs to result in sulphate crystallisation.

Control of pH in the mixing stage of the present method is also important. Although a result can be attained outside the limits of from pH 8.5 to pH 12, it is highly desirable to employ a pH as the end figure achieved after mixing which is within the range about 8.5 to 10.0.

It is preferred that the pH of the combined solutions is in the range from 9.0 to 9.5 and that the temperature of the first solution is in the range from 68"C to 100"C.

It is further preferred that the temperature of the first solution is in the rané 70"C to 80"C and, in one particular example, the temperature is 75 C.

The first solution may be prepared by dissolving ferrous sulphate heptahydrate in water and this first solution preferably has a concentration of ferrous sulphate of at least 25% w/w, e.g. from 25% to 30% w/w.

The second aqueous solution is preferably sodium hydroxide, which may be commercially available caustic soda.

The second solution is preferably added to the first solution with continuous stirring to prevent substantial local pH variations and the rate of addition is preferably such that local variations in viscosity do not arise.

In general, the quantity of alkali and its rate of addition should be such that the overall viscosity should not exceed 2500, say 1500, preferably 1000 centipoises. Preferably this viscosity of the combined solutions is maintained at below 500 centipoises, and ideally below 250 centipoises. This may be achieved by stirring in regulated amounts of the second solution to form a homogeneous solution while breaking down and dissipating any gelatinous frameworks of ferrous hydroxide. During this process, the pH of the combined solutions may be continually monitored by a sensor in the tank in which the mixing takes place and the viscosity of the mixed solution may also be monitored.

The oxidation may preferably be achieved by bubbling air into oxygen through the combined solutions.

For details of the techniques employed in the mixing stage and/or the oxidation stage the reader is referred for example, to the information given m our Patent Specification No. 1534196. Indeed, with appropriate modification to take account of the additional information contained herein and the suggested modifications and improvements, the skilled man would have no difficulty in adapting the specific Example given in said prior specification to the present invention.

It should be noted that the tank employed for effecting the reaction in the present method is ideally a tank which is neither too squat nor too tall. A tank which is approximately as tall as it is wide is ideally suited to the present process since it allows the best possible compromise between aeration efficiency and the efficiency of mixing (bearing in mind viscosity considerations).

Furthermore, if an iron tank is employed a liner of some description should be used since the presence of metallic iron is highly undesirable in the present process. Suitable liners are butyl rubber or polyester liners.

In preparing the ferrous sulphate solution for use in the present method, it is convenient to load cold water into a reaction vessel or tank and to apply live steam thereto as the heating means to raise it to a temperature above about 68"C. Upon addition of ferrous sulphate the water is preferably agitated and extra heat applied to ensure that solution is achieved with relative ease (the solution of ferrous sulphate in water is an endothermic process). The ferrous sulphate applied is also preferably in powdered form and should ideally be light green in colour (i.e. a material which has not been subjected to appreciable oxidation-the lower the content of ferric ion in the material the better).

The aqueous solution of hydroxide which is added to the ferrous sulphate solution results in an exothermic reaction and the addition has to be very carefully controlled so as to avoid the production of so viscous a solution that adequate mixing is difficult.

Suitable relative quantities of the hydroxide solution and the ferrous sulphate are. for example. 150 Kg of caustic soda liquor 46.8% w/w) for 270 Kg of ferrous sulphate heptahydrate. With such amounts, the hydroxide solution is preferably added in 10 bursts of 15 Kg amounts, the bursts being separated by two minute intervals with continuous stirring. Continuous stirring and the use of discontinuous addition are, of course, techniques which can be applied at any level of process dimensions.

When the hydroxide solution is added in bursts its temperature is in a sense immaterial since the heat of reaction is such that a temperature above the required level of 68"C is maintained.

As indicated earlier, pH control in the present process is important, a preferred pH range for the desired end product after mixing is from 9 to 10. Below pH 9 the solution has a tendency to turn brown and is easily oxidised and above 10 the solution becomes too viscous.

One means of oxidizing the mixture produced by the first stages of the present method is to pass oxygen, air or an oxygencontaining gas through a ceramic aerator which is positioned within the reaction vessel or tank. To prevent the blocking of the pores in such an aerator during the period of addition of the hydroxide solution a slight water pressure may be applied thereto in order that the reactants cannot enter and block the pores of the aerator.

It is highly preferred that in the present process no excess of hydroxide solution should be employed. Naturally, using an excess of hydroxide will raise the pH to a value substantially above the upper value referred to above as preferred for the pH of the mixture produced from the hydroxide solution and the ferrous sulphate solution.

During the oxidation stage, it is convenient to pass, for example, air through the aeration equipment at a pressure of 10 p.s.i.g. or more. It has been discovered that the efficiency of absorption of the oxygen is remarkably high in the present method. Up to 60% of the oxygen passed into the mixture is absorbed.

Naturally, as with the process of our prior Specification No. 1534196, the aeration may ideally be continued until viscosity of the mixture falls to about 20 centipoise.

After oxidation, the resulting reaction mixture may be passed to a filter, e.g. a filter press. The resulting filter cake may be washed with water at, for example, a pressure of 20 to 30 p.s.i.g. to remove excess sulphate and other soluble impurities (the principal impurity being solium sulphate).

In carrying out the present method as previously indicated, it is essential not to allow too much cooling to occur in the latter stages or sodium sulphate will crystallize out in many cases.

If the pigment produced by the present process is desired in dry from a hydrocarbon oil may desirably be incorporated therewith to inhibit oxidation and to give some stability to the pigment. It is, however, preferred to employ the pigment in paste form since it has been found that the paste form (with water) disperses better in cementitious mixes than a dry powder and there are great difficulties in re-dispersing a dry powder in water to give an adequate uniform dispersion once the initial paste material has been dried.

As in our earlier invention, the ferrous sulphate heptahydrate utilised in the method is that commercially available as a by-product of titanium dioxide manufacture and such copperas will typically contain up to 6% by weight of water and 0.6% of weight in total of trace elements such as first transi- tion series metals and small quantities of heavy metals. Traces of these elements are coprecipitated with the pigment.

The pigment produced by the above process is an intense black but when dispersed as 1 %2% w/w in concrete has a blue-grey shade.

Thus, said pigment may be used in the production of concrete building products such as roof tiles and in this use the good staining power of the pigment is particularly important.

Naturally, the pigment may be used in any other applications where good staining power is desirable.

We make no claim herein to a method of manufacturing black iron oxide pigment as described in the specific Example of our prior Specification No. 1534196.

Subject to the foregoing disclaimer, WHAT WE CLAIM IS:- 1. A method of manufacturing black iron oxide pigment comprising the steps of taking a first aqueous solution of of ferrous sulphate at a starting temperature above 68"C, taking a second aqueous solution of an alkali metal hydroxide or ammonia, adding the second solution to the first solution while maintaining the temperature of the combined solutions above 68"C, the addition being preferably, but not essentially, effected in bursts until the pH of the combined solutions is from 8.5 to 10, oxidizing the combined solutions so as to deposit black iron oxide from the resulting mixture, and during the oxidation and separation ensuring that insufficient cooling occurs to result in sulphate crystallisation.

2. A method as claimed in claim 1, in which the pH of the combined solutions is from 9.0 to 9.5.

3. A method as claimed in claim 1 or claim 2, in which the ferrous sulphate solution is at least 25% w/w FeSO4.7H20/ water.

4. A method as claimed in any one of claims 1 to 3, in which the temperature of the first solution is from 68"C to 100"C.

5. A method as claimed in any one of claims 1 to 4, in which the temperature of the first solution is from 70"C to 80"C.

6. A method as claimed in any one of claims 1 to 5, in which the temperature of the first solution is 75"C.

7. A method as claimed in any one of claims 1 to 6, in which the first solution is prepared by dissolving ferrous sulphate heptahydrate in water.

8. A method as claimed in any one of claims 1 to 7, in which the second solution is added to the first with continuous stirring to prevent substantial local pH variations and the rate of addition is such that the viscosity of the combined solution does not rise above 1000 centipoises.

9. A method as claimed in claim 8, in which the viscosity of the combined solutions does not rise above 500 centipoises.

10. A method as claimed in claim 9, in which the viscosity of the combined solutions does not rise above 250 centipoises.

11. A method as claimed in any one of claims 1 to 10, in which the second aqueous solution is sodium hydroxide.

12. A method as claimed in any one of claims 1 to 11, in which the combined solutions are oxidized by bubbling air through the solutions.

13. A method of manufacturing black iron oxide pigment as claimed in claim 1 and substantially as hereinbefore described.

14. A black iron oxide pigment which has been produced by a method as claimed in any one of claims 1 to 13.

15. A concrete building product including a pigment as claimed in claim 14.

16. A method of producing a concrete product which comprises dispersing a pigment as claimed in claim 14 in a cementitious mix and setting the resulting mixture in a desired shape.

17. A method as claimed in claim 16, wherein the desired shape is a roof tile.

18. A method as claimed in claim 16 or claim 17, wherein the pigment is used at a concentration of from 1% to 2% w/w in the cementitious mix.

19. A concrete product which has been produced by a method as claimed in any one of claims 16 to 18.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. stability to the pigment. It is, however, preferred to employ the pigment in paste form since it has been found that the paste form (with water) disperses better in cementitious mixes than a dry powder and there are great difficulties in re-dispersing a dry powder in water to give an adequate uniform dispersion once the initial paste material has been dried. As in our earlier invention, the ferrous sulphate heptahydrate utilised in the method is that commercially available as a by-product of titanium dioxide manufacture and such copperas will typically contain up to 6% by weight of water and 0.6% of weight in total of trace elements such as first transi- tion series metals and small quantities of heavy metals. Traces of these elements are coprecipitated with the pigment. The pigment produced by the above process is an intense black but when dispersed as 1 %2% w/w in concrete has a blue-grey shade. Thus, said pigment may be used in the production of concrete building products such as roof tiles and in this use the good staining power of the pigment is particularly important. Naturally, the pigment may be used in any other applications where good staining power is desirable. We make no claim herein to a method of manufacturing black iron oxide pigment as described in the specific Example of our prior Specification No. 1534196. Subject to the foregoing disclaimer, WHAT WE CLAIM IS:-

1. A method of manufacturing black iron oxide pigment comprising the steps of taking a first aqueous solution of of ferrous sulphate at a starting temperature above 68"C, taking a second aqueous solution of an alkali metal hydroxide or ammonia, adding the second solution to the first solution while maintaining the temperature of the combined solutions above 68"C, the addition being preferably, but not essentially, effected in bursts until the pH of the combined solutions is from 8.5 to 10, oxidizing the combined solutions so as to deposit black iron oxide from the resulting mixture, and during the oxidation and separation ensuring that insufficient cooling occurs to result in sulphate crystallisation.

15. A concrete building product including a pigment as claimed in claim 14.

17. A method as claimed in claim 16, wherein the desired shape is a roof tile.