GB2168367A

GB2168367A - Cadmium pigments with reduced solubility of the cadmium in dilute hydrochloric acid

Info

Publication number: GB2168367A
Application number: GB08525318A
Authority: GB
Inventors: Dr Lothar Kaufmann; Klaus Dieter Hellwig; Herbert Thoma
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1984-12-14
Filing date: 1985-10-15
Publication date: 1986-06-18
Also published as: FR2574807A1; GB8525318D0; FR2574807B1; ES8605556A1; DE3445569C1; GB2168367B; ES547138A0

Abstract

This invention relates to cadmium pigments, which owing to a content of from 0.2 to 5% by weight of the zinc salt of 2,4,6-trismercapto-s-triazine, are protected against dissolution of the cadmium by dilute hydrochloric acid, as well as to a process for the production of such protected cadmium pigments.

Description

SPECIFICATION Cadmium pigments with reduced solubility of the cadimum in dilute hydrochloric acid This invention relates to cadmium pigments with reduced solubility of the cadmium in dilute hydrochloric acid as a result of containing a difficulty soluble sulphur-containing zinc compound.

The term cadmium pigments refers to cadmium sulphides or cadmium sulphoselenides, which have the crystal structure of a hexagonal wurzite lattice and have the particle size distribution which is characteristic of pigments. The cadmium sulphide is yellow; from a formal point of view, mixed crystals occur as a result of the incorporation of Se2- ions into the cadmium sulphide, with the colour changing from orange and red to magenta as the selenium content increases.

Such cadmium pigments are preferably used for colouring plastics materials. In view of the toxicity of cadmium, it is desirable to reduce substantially the acid-soluble proportion of these finely-divided pigments, especially in dilute hydrochloric acid and thus also in gastric acid, so that when pigment dust is taken orally during handling of the pigments, damage to health can be substantially ruled out.

In DE-PS 24 48 338, pigments with reduced cadmium solubility in dilute hydrochloric acid consisting of cadmium chalcogenides have already been described, in which the acid-solubility is reduced by precipitated difficulty soluble inorganic sulphides, including zinc sulphide. The precipitation takes place by introducing hydrogen sulphide gas or by addition of H2S-yielding organic compounds into an aqueous pigment suspension containing the zinc salt.

A disadvantage of this process is that either toxic hydrogen sulphide gas is directly used or it is necessary to work with substances which yield sulphide ions with the result that in the same way a certain exposure to hydrogen sulphide-containing gases cannot be avoided. Since hydrogen sulphide has to be used in excess, particular protective measures during operation are necessary.

For the same reason, costly measures for purifying the resulting outgoing air are indispensible and further environment-protective precautions are necessary since the H2S-saturated filtrate has to be detoxified before it can be released.

The technical cost and time requirement for treating the pigments according to this process in thus high.

These inadequacies can be overcome according to the present invention by cadmium pigments with a reduced solubility of the cadmium in dilute hydrochloric acid resulting from a content of a difficulty soluble sulphur-containing zinc compound, which are characterized in that that contain as the sulphur-containing zinc compound the zinc salt of the 2,4,6-trismercapto-s-triazine having the formula Zn3(C3N3S3)2 in an amount of from 0.2 to 5%, by weight. Preferred cadmium pigments according to the invention have a content of this zinc compound of from 0.4 to 0.8%, by weight.

The invention further provides a process for the production of cadmium pigments containing the sulphur-containing zinc compound. The process comprises thoroughly mixing a stirrable aqueous suspension of the cadmium pigment in each case with a corresponding suspension of the compound Zn3(C3N3S3)2, maintaining a weight ratio between the cadmium pigment and the zinc compound of 100:0.2 to 5, preferably 100:0.4 to 0.8, and then filtering off and drying the pigment. The aqueous suspensions of the pigment and the organic zinc compound are neutral and practically electrolytefree, so that the acid-stabilized pigment separated from the united and mixed suspensions maintains its original purity and thus a washing process is not necessary.

The process according to the invention has the following advantages: - The substances used do not release sulphide ions and are non-toxic; - no materials capable of endangering the environment escape from the process, so that corresponding protective measures are not necessary; - all commercial cadmium pigments can be positively influenced in relation to their acid-solubility by simple mixing with the neutral inhibitor suspension and without a special washing process, subsequent filtration and drying. (determination of acid solubility according to DIN 53 770); - it is not necessary to carry out a precipitation operation on the cadmium pigment; - the process requires only low handling and energy expenditure.

The zinc compound of trismercapto-s-triazine (TMT) used for the stabilization according to the invention can be produced in known manner by double decomposition of the TMT with a zinc salt, in which a solution of the sodium salt of TMT is reacted with a zinc sulphate solution and the precipitate is released from the main quantity of the resulting sodium sulphate in solution by washing twice in a decantation process.

The cadmium pigments to be treated according to the process of the invention may be produced in the usual manner: A cadmium yellow pigment may be produced by reacting cadmium sulphate with sodium sulphide in an aqueous solution, washing and drying the corresponding precipitate and subsequently annealing at 530 C. The cadmium red pigment may be produced by reacting cadmium carbonate with sodium sulphoselenide solution, washing and drying the resulting precipitate and subsequently annealing at 5800C. The yellow and red pigments produced in this manner are subsequently subjected to a treatment with dilute sulphuric acid.

Pigments produced in this manner contain from 0.6 to 0.1% by weight of soluble cadmium according to DIN 53 770. According to these DIN specifications 10 g of chalcogenide are shaken with 150 ml of 0.1 N HCI in an Erlenmeyer flask with a capacity of 250 ml for 15 min at 23"C by overhead rotation. The suspension is then allowed to stand for 10 min. The solution is then filtered by decanting over a membrane filter (0.15 lim pore size). After 5 min the filtration is ended. The Cd in the filtrate is determined with an atom absorption photometer.

Production of the suspension of the zinc compound of the TMT, the use of which in the process according to the invention is described in the following Examples, may be undertaken as follows: 100 í of cold, completely demineralised, water are placed in a 300 1 special steel container and 50 1 of zinc sulphate solution with a content of 100 g of zincil, and 74 1 of a commercial 15% solution of the trisodiu m salt of 2,4,6-trismercapto-s-triazine are introduced with intense stirring.

A difficulty-soluble precipitate, which within technically conventional tolerances corresponds to the composition Zn3(C3N3S3)2, and sodium sulphate in solution are formed as reaction products.

To remove the main quantity of sodium sulphate, the precipitate is washed by decanting twice with fully demineralised water and then filled up to a total volume of 280 1.

The suspension produced in this manner has a solids content of Zn3(C3N3S3)2 of 50 g/l. The suspension is homogenised by intensive stirring before removing the proportion of Zn3(C3N3S3)2 required for the treatment of the pigment in each case.

The invention is illustrated by the following Examples: Example 1 16 1 of a Zn3(C3N3S3)2 suspension with a solids content of 50 gil are added at a temperature of from 20 to 25"C to a neutral aqueous suspension which contains 400 kg of cadmium red pigment in a total volume of 1.6 m3.

Both suspensions are intensively stirred together for 30 min, the solids material is filtered off on a filter press and dried at 1100C.

The acid-soluble cadmium proportion according to DIN 53 770 is 0.007% by weight of the finished product.

Example 2 28 1 of an aqueous Zn3(C3N3S3)2 suspension with a solids content of 50 gll are added at a temperature of from 20 to 25"C to a neutral aqueous suspension which contains 350 kg of cadmium yellow pigment in a total volume of 1.6m3.

Both suspensions are intensively stirred together for 30 min and further processed as in Example 1.

The acid-soluble cadmium proportion according to DIN 53 770 is 0.006% by weight of the finished product.

Example 3 48 1 of an aqueous Zn3(C3N3S3)2 suspension with a solids content of 50 g/l are added at a temperature of from 20 to 25"C to a neutral aqueous suspension which contains 400 kg of cadmium red pigment in a total volume of 1.6 m3.

The acid-soluble cadmium proportion according to DIN 53 770 is 0.002% by weight of the finished product.

Example 4 42 1 of an aqueous Zn3(C3N3S3)2 suspension with a solids content of 50 g/l are added at a temperature of from 20 to 25"C to a neutral aqueous suspension which contains 350 kg of cadmium yellow pigment in a total volume of 1.6 m3.

The acid-soluble cadmium proportion according to DIN 53 770 is 0.003% by weight of the finished product.

Example 5 64 1 of an aqueous Zn2 (C2NaS2)2 suspension with a solids content of 50 g/l are added at a temperature of from 20 to 250C to a neutral aqueous suspension which contains 400 kg of cadmium red pigment in a total volume of 1.6 m3.

The acid-soluble cadmium proportion according to DIN 53 770 is 0.001% by weight of the finished product.

Example 6 56 1 of an aqueous Zn3(C3N3S3)2 suspension with a solids content of 50 g/l are added at a temperature of from 20 to 250C to a neutral aqueous suspension which contains 350 kg of cadmium yellow pigment in a total volume of 1.6m3.

Both suspensions are thoroughly stirred together for 30 min and further processed as in Example 1.

Example 7 350 1 of an aqueous Zn3(C3N3S3)2 suspension with a solids content of 50 g/l are added at a temperature of from 20 to 25"C to a neutral aqueous suspension which contains 350 kg of cadmium yellow pigment in a total volume of 1.6 m3.

Both suspensions are thoroughly stirred together and further processed as in Example 1.

Claims

1. A cadmium pigment having reduced solubility of the cadmium in dilute hydrochloric acid, the pigment containing from 0.2 to 5% by weight of the zinc salt of the 2,4,6-trismercapto-s-triazine of the formula ZN3(C3N3S3)2.

2. A cadmium pigment as claimed in claim 1, containing from 0.4 to 0.8% by weight of the compound Zn3(C3N3S3)2.

3. A cadmium pigment substantially as described with particular reference to any of the examples.

4. A process for the production of a cadmium pigment as claimed in any of claims 1 to 3 which comprises thoroughly mixing a stirrable aqueous suspension of the cadmium pigment with a corresponding suspension of the compound Zn3(C3N3S3)2, maintaining a weight ratio between the cadmium pigment and the zinc compound of 100:0.2 to 5, and then filtering off and drying the pigment.

5. A process as claimed in claim 4 wherein the weight ratio between the cadmium pigment and the zinc compound is maintained between 100:0.4 to 0.8.