GB2161494A

GB2161494A - Precipitation process

Info

Publication number: GB2161494A
Application number: GB08516813A
Authority: GB
Inventors: Anthony Airey; Andrew Spiller
Original assignee: British Ceramic Research Association Ltd
Current assignee: British Ceramic Research Association Ltd
Priority date: 1984-07-13
Filing date: 1985-07-03
Publication date: 1986-01-15
Also published as: GB8417943D0; GB8516813D0

Abstract

A process, especially an industrial process for producing a homogeneous mixture of acidic and alkaline solutions for use, for example, in the manufacture of a protected pigment, comprises simultaneously introducing the solutions into a mixing vessel while continuously mixing them, a diluent, for example water, being added to the vessel simultaneously with or prior to the addition of the mixture. Preferably the pH of the mixture is controlled to a value of between 6 and 10. The process may produce a precipitate of a pigment e.g. cadmium sulphide by simultaneously mixing an acidic solution of the pigment or its precursor and a zirconium compound and an alkaline solution of the pigment or its precursor. The precipitated pigment may then be treated by precipitating a silicon compound from a solution of its salt, mixing the resulting precipitate with a fluoride, firing the mixture and cleaning the protected pigment so formed.

Description

SPECIFICATION Precipitation process This invention relates to precipitation processes and particularly to processes for obtaining a precipitate from a mixture of alkaline and acidic solutions. The invention is especially but not exclusively useful in relation to the commercial preparation of pigments of the kind disciosed in our co-pending Patent Application No. 8225482.

In the technique disclosed in Application 8225482 pigments are produced by a twostage precipitation process, the first stage of which involves the mixing of acidic and alkaline solutions to produce a precipitate which is then further processed to produce a pigment.

In the commercial application of this process problems can arise in effecting sufficiently rapid homogenious mixing of the relatively large volumes of solution employed. It has now been found that improved results can be attained by employing a modified mixing process particularly suited to larger volumes and to continuous production.

In its broader aspects the invention provides a process for producing a homogenous mixture of acidic and alkaline solutions comprising introducing the acidic solution into a mixing vessel while simultaneously introducing alkaline solution into the mixing vessel and continuously mixing the solutions in the vessel.

Preferably a diluent, for example water, is introduced into the mixing vessel along with or prior to the introduction of the solution.

The diluent is preferably agitated as by stirring during the introduction of the solutions.

Preferably the volume of diluent is such that efficient mixing can be achieved, for example, it may be at least equal to the volume of the combined solutions.

The invention also provides a method of producing a precipitate from a mixture of an acidic solution of a pigment or its precursor and a zirconium compound and an alkaline solution of the pigment or its precursor, the method comprising introducing the acidic solution into a mixing vessel while simultaneously introducing alkaline solution into the mixing vessel and continuously mixing the solutions in the vessel.

Preferably a diluent, for example water, is introduced into the mixing vessel with or prior to the introduction of the solutions.

Preferably the pH of the mixture is controlled during addition of the solutions to a value of between 4 and 12, preferably between 6 and 10.

The invention also provides a method of producing a pigment comprising introducing an acidic solution of cadmium salts and zirconium salts into a mixing vessel while simultaneously introducing an alkaline solution of sodium sulphide into the mixing vessel and continuously mixing the solutions in the mixing vessel to cause precipitation, in the presence of the precipitate so formed, precipitating a silicon compound from a solution of its salts, mixing the resulting precipitate with a fluoride, firing the mixture and cleaning the protected pigment so formed.

The invention also provides precipitates of pigment and zirconium compounds and protected pigments produced by the aforesaid method.

Examples of the present invention will now be described. The invention is not restricted to the method described in the examples which are given by way of illustration only.

In the production of a protected ceramic pigment described in our co-pending U.K.

Patent Application 8225482 a mixture of an aqueous solution of cadmium sulphate and a solution containing zirconium acetate was mixed with a solution comprising selenium powder dissolved in a sodium sulphide solution which had been made alkaline by the addition of ammonium hydroxide solution.

In view of the relatively small volumes of solution utilised in the laboratory a rapid and efficient mixing of the two components was achieved and the precipitate so formed proved to provide a protected ceramic pigment of an acceptably high quality after it had been further processed.

In commercial conditions it was found that equally acceptable results were not obtained when the laboratory process was scaled up and the reason for this was identified as being the inability of the commercial plant to obtain a rapid homogenous mix of the relatively large volumes of solutions being handled.

In the commercial method according to the present example both solutions are mixed together in water, the volume of water employed being sufficient to ensure efficient mixing and precipitation. This resulted in a relatively rapid homogenous mixing of the solutions and no loss in product quality or yield was detected.

By utilising this method it was possible to maintain certain pH levels, for example, by altering the addition rates of the two solutions. This control of pH level is relatively easy when relatively small amounts of solution are added to large volume of water diluent as each drop of solution does not change the pH of the total volume significantly before it reacts. It has been found that maintaining the pH level within the range 4 to 1 2 produces satisfactory products but a preferable range is 6 to 1 0. Typical examples of the process are now set out.

Example 1 160g of a solution of cadmium sulphate containing 1469/1Cd was mixed with 2059 of a solution of zirconyl chloride containing 29% by weight of ZrOC12 and made up to 500 ml with water.

5.lug of selenium powder was dissolved in 101g of a sodium sulphide solution containing 154gel Na2S and then made strongly alkaline by adding 26.59 of sodium hydroxide. This solution was then made up to 500 ml with water.

The two solutions were added simulataneously over a period of two minutes to 800 mls water with continuous stirring maintaining a pH of between 10 and 1 2 throughout the addition.

In accordance with further process steps described in our co-pending Application No.

8225482 a further precipitation step was carried out followed by the addition of a fluoride and firing to provide a protected ceramic pigment. Using the precipitate provided by the process described in Example 1 the following process was carried out to obtain the protected ceramic pigment.

The precipitate obtained was washed, filtered and dried and the dried powder was mixed with a solution containing 829 sodium silicate (S.G. 1.50) in 820 mls water. The slurry obtained was then acidified with 46 mls of 5M acetate to produce a gel which was washed, filtered and dried at 130"C to give a fine orange-brown powder. This powder was dry mixed with 3% by weight of lithium fluoride and fired at 1050"C for 20 minutes.

The resulting pigment composition ground, washed consecutively with nitric acid, water, 5M sodium hydroxide solution and water to give a bright red pigment powder.

Example 2 The method used was the same as described in Example 1 except that an addition time of 37 minutes and a pH of 8 was used.

Example 3 The method was the same as Example 1 except that the addition time and pH were 37 minutes and 10 respectively.

Example 4 The method used was the same as Example 1 except that the addition time was 37 minutes and the pH was 1 2.

Example 5 2689 of cadmium sulphate solution containing 146g/l Cd was mixed with 3429 of zirconyl chloride solution containing 29% by weight of ZrOC12 and made up to 500 ml with water.

8.59 of selenium powder were dissolved in 178.89 of sodium sulphide solution contain ing 154gel Na2S and then made strongly alkaline by adding 44.29 of sodium hydroxide. This solution was then made up to 500 mls with water.

These two solutions were then added simultaneously to 500 mls water with continuous stirring over a period of 37 minutes and maintaining a pH of 8 throughout the addition.

To obtain the protected ceramic pigment from this precipitate the following process was followed.

The precipitate obtained was washed, filtered and dried and the dried powder was added to a solution of 1359 sodium silicate in 1350 mls and gelled with 77 mls of 5M acetic acid.

The precipitate obtained was then treated in the manner described in Example 1.

A bright red pigment was obtained which was suitable for all ceramic applications.

Example 6 The method used was the same as Example 5 except that the addition was controlled at a pH of 10.

Example 7 The method used was the same as Example 5 except that the addition was controlled at a pH of 12.

Example 8 The method used was the same as Example 5 except that the addition time was 1 2 minutes and was controlled at a pH of 4.

Example 9 The method used was the same as Example 5 except that the addition time was 1 2 minutes and was controlled at a pH of 6.

Example 10.

The method used was the same as Example 5 except that the addition time was 1 2 minutes and was controlled at a pH of 8.

Example 11 The method used was the same as Example 5 except that the addition time was 1 2 min utes and was controlled at a pH of 10.

Various modifications can be made to the present invention, for example as opposed to a batch process the mixing process could be continuous. In such a process the solutions could be separately added at a predetermined rate to a predetermined flow of water. Ad ditionally, to obtain alternative precipitates dif ferent solutions could be employed and could be added to liquids other than water, depend ing upon the nature of the solutions.

The method of the invention thus provides a commercially viable process for mixing an acidic solution with an alkaline solution to provide a precipitate as the pH level during precipitation can be controlled closely at any desired level throughout the addition. In addi tion, mixing rates can be slowed to more practical rates without affecting product qual ity and hence more consistent homogenous mixing can be achieved. Anion interference is greatly reduced so that the concentrations of solutions can be increased without affecting product quality. This means a greater product yield for a given volume can be achieved.

Clearly, the method of the invention is more amenable to continuous production.

Claims

CLAIMS:

1. A process for producing a homogeneous mixture of acidic and alkaline solutions comprising introducing the acidic solution into a mixing vessel while simultaneously introducing alkaline solution into the mixing vessel and continuously mixing the solutions in the vessel.

2. A process as claimed in claim 1, in which a diluent, for example water, is introduced into the mixing vessel simultaneously with the introduction of the solutions.

3. A process as claimed in claim 1, in which a diluent, for example water, is introduced into the mixing vessel prior to the introduction of the solutions.

4. A process as claimed in claim 2 or claim 3, in which the diluent is agitated by stirring during the introduction of the solutions.

5. A process as claimed in any one of claims 2 to 4, in which the volume of diluent is such that efficient mixing can be achieved.

6. A process as claimed in claim 5, in which the volume of diluent is at least equal to the volume of the combined solutions.

7. A method of producing a precipitate from a mixture of an acidic solution of a pigment or its precursor and a zirconium compound and an alkaline solution of the pigment or its precursor, the method comprising introducing the acidic solution into a mixing vessel while simultaneously introducing alkaline solution into the mixing vessel and continuously mixing the solutions in the vessel.

8. A method as claimed in claim 7, in which a diluent, for example water, is introduced into the mixing vessel simultaneously with the introduction of the solutions.

9. Method as claimed in claim 7, in which a diluent, for example water, is introduced into the mixing vessel prior to the introduction of the solutions.

10. A method as claimed in any one of claims 7 to 9, in which the pH of the mixture is controlled during addition of the solutions to a value of between 4 and 1 2.

11. A method as claimed in claim 10, in which the pH of the mixture is controlled during addition of the solutions to a value of between 6 and 10.

1 2 A method of producing a pigment comprising introducing an acidic solution of cadmium salts and zirconium salts into a mixing vessel while simultaneously introducing an alkaline solution of sodium sulphide into the mixing vessel and continuously mixing the solutions in the mixing vessel to cause precipitation, in the presence of the precipitate so formed, precipitating a silicon compound from a solution of its salts, mixing the resulting precipitate with a fluoride, firing the mixture and cleaning the protected pigment so formed.

1 3. A precipitate of pigment and zirconium compound when produced by the method of claims 7 to 11.

14. A protected pigment when produced by the method of claim 1 2.

15. A process for producing a homogenous mixture of acidic and alkaline solutions substantially as hereinbefore described.

16. A method of producing a precipitate substantially as hereinbefore described.

1 7. A method of producing a protected pigment substantially as hereinbefore described.

1 8. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.