UNITED STATES PATENT OFFICE 2,643,196 PROCESS for PREPARING PURE CADMUM RED PGMENT Benjamin W

Patented June 23, 1953 2,643,196 UNITED STATES PATENT OFFICE 2,643,196 PROCESS FOR PREPARING PURE CADMUM RED PGMENT Benjamin W. Allan and Frank O. Rummery, Baltimore, Md., assignors to The Glidden Com pany, Cleveland, Ohio, a corporation of Ohio No Drawing. Application March 11, 1949, Serial No. 81,023 5. Claims. (C. 106-301) 2 This invention relates to pure cadmium sul to coprecipitate an oxidic cadmium compound foSelenide red pigments, and to an improved and a crude cadmium sulfoSelenide pigment. process for preparing such red pigments. It is a further object, to provide an improved In preparing cadmium red pigments it is cus Cadmium red pigment by calcining a crude red tomary to dissolve selenium in an alkali metal pigment in the presence of an intimately dis or barium sulfide solution, and then to react persed OXidic cadmium compound, the intimate this selenium-containing solution with a soluble dispersion being achieved by coprecipitation of cadmium salt. When the cadmium Salt is Suit the respective components from aqueous solution. ably selected in view of the metal ions present It is yet another object to reduce or avoid in the sulfide Solution, an unextended (i.e., pure) Selenium losses during the calcination of cad crude pigment may be formed by the reaction. mium red pigments by having an oxidic cad The crude pigment is separated from the bal mium compound more intimately dispersed ance of the reaction mass, Washed and dried, if throughout the calcination mass than hereto desired, and then calcined under non-oxidizing fore. conditions, During the calcination the cad 5 These and other objects will be perceived from mium, sulfur and selenium are induced to unite the following description of the invention. into a pigmentary product which exhibits the We have found that numerous advantages ac desired shades of red. Crue from calcining a crude cadmium red pig During the calcination it is desirable to have ment maSS which has had an oxidic cadmium present an oxidic cadmium compound, such as 20 Compound coprecipitated with it and hence in cadmium oxide, hydroxide, carbonate, etc., timately dispersed through it. Selenium losses since the presence of such compound has been during calcination are reduced or substantially found to effect a reduction in the amount of eliminated, lower calcining temperatures are selenium which is otherwise copiously volatilized possible, and greater uniformity and control out of the calcination charge. In One proposal 25 of Shade from batch to batch are achieved. of the prior art the oxidic cadmium compound These advantages spring largely from the inti was prepared separately from the crude cad macy of the mixture and the carefully selected mium sulfoselenide pigment, and was then mixed proportions of oxidic cadmium compound to or blended into the crude pigment before the Selenium, and are in contrast with results se latter was calcined. The separate preparation cured When the oxidic compound is prepared of the oxidic cadmium compound in pigmentary 30 Separately and then mixed or blended with the grades and States, and the blending thereof crude pigment. We have found that coprecipi with the crude SulfoSelenide pigment involves tion of the oxidic cadmium compound and the considerable expense in time, labor and equip Cadmium Sulfide, preferably in the presence of ment, and in addition has not been found to Selenium, produces the necessary intimacy of produce as uniform a calcination mixture as is 35 mixture, and is furthermore a practical, com deemed desirable. This invention is directed to mercial method which may be adapted to the the avoidance of the separate preparation of the present commercial methods for producing crude oxidic cadmium compound and to the avoid cadmium pigments. Coprecipitation eliminates ance of a separate mixing or blending step. In the former need of separately preparing the accordance with the preferred embodiment of 40 Oxidic compound in a pigmentary quality and this invention, the oxidic cadmium compound State, and avoids the expenses incident to the is formed in the presence of, and simultaneously time, labor and equipment for so preparing and with, the crude cadmium sulfoselenide as a re mixing the oxidic cadmium compound with a Sult of its coprecipitation therewith from aque calcination maSS. In view of these numerous ous solutions. By So coprecipitating these de 45 advantages of a coprecipitation method, it is sired compounds to produce the desired calcina apparent that the latter method offers many tion mixture, intimate dispersion of the oxidic features and effects which are commercially compound throughout the crude cadmium pig desirable. ment is obtained, and the separate preparation The invention is applicable particularly to and subsequent intermixing steps of the desired 50 the unextended or pure cadmium red pigments. ingredients of the calcination mass are avoided. AS is well known, such pigments are produced By calcining Such intimate mixture, an improved by incorporating a proportion of selenium with cadmium pigment is obtained, and losses of a crude cadmium sulfide pigment mass. The selenium are minimized. Selenium may be incorporated in such mass by Accordingly, it is an object of this invention 55 two alternative methods, one being to dissolve 2,643,198 3 4. the selenium in an alkali-metal or barium Sul The portions enclosed in brackets represent a ?ide solution and then to strike this solution single strike solution, but in the interest of clar with an aqueous solution of a cadmium Salt to ity, the portions thereof which are involved in produce a pure cadmium sulfoSelenide precipi the formation of the oxidic cadmium compound tate. The other method involves first preparing are represented separately from the portions a pure cadmium sulfide precipitate and Subse which react with the bulk of the cadmium Salt quently, but prior to calcination, adding thereto to form cadmium sulfoSelenide. In all of the and mixing therein the desired quantity of above equations, the selenium may be present in Selenium. The present invention may be ap a variable amount; accordingly, the symbol Se plied to each of these methods, but for reasons O is used to signify that selenium is present but is of commercial expediency we particularly pre not necessarily present in atomic proportions. fer the first of said methods; namely, that in The above equations illustrate that the various which the selenium is dissolved in the aqueous reactants are selected so that cadium Sulfo sulfide strike liquor, so that the oxidic compound Selenide and the oxidic cadmium compounds are is coprecipitated with the cadmium sulfide in the 5 the only insoluble products which are formed. presence of Selenium. The other reaction products, being Soluble, can In accordance with our preferred method of be removed from the precipitates by washing. operation, we strike a soluble cadmium salt solu During the Subsequent calcination of the crude tion with a previously prepared aqueous alkaline pigment obtained from the strike, other reactions Sulfide Solution containing Seleniuin and an 20 occur between the oxidic cadmium compound, oxidic compound of an alkali or alkali-earth part to all of the selenium and part of the cad metal, the latter oxidic compound being chosen mium sulfide to produce the final pigment. in view of the cadmium Salt which is being used Without being restricted to the theory there indi So that cadmium SulfoSelenide and an oxidic cated, we believe that the following reactions are cadmium compound are the only in Soluble prod 25 typical: ucts of the subsequent strike reactions. For ex acCdS--2Se--CdCO3-> ample, if a cadmium nitrate solution is to be 2CdSe--SO2-CO-- (ac-1) CdS struck, the sulfide solution may be composed of acCdS-3Se--2CdCO3-> sodium, potassium or lithium sulfide containing . 3CaSe--SO2-4-2CO2-- (ac-1) CaS Selenium dissolved therein, and either or any of 30 sodium, potassium, lithium or barium hydroxide, acCdS--2Se--Cd(OH)2-> or sodium, potassium or lithium carbonate. 2CdSe--SO2--H2-- (ac-1) CdS Barium carbonate or mixtures of oxidic com acCdS-4-3Se--2Cd(OH)2 > pounds of alkali metal and barium which by 3CdSe--SO2-2H2O-- (ac-1) CdS metathesis yield barium carbonate cannot be It will be understood that exceSS Selenium Over used because barium carbonate is in Soluble and the molar proportions indicated in these re hence unable to effect the precipitation of the actions may be used. The reactions of other desired oxidic cadmium compound. When a Oxidic cadmium compounds, such as cadmium solution of cadmium sulfate is to be struck to oxalate, cadmium formate, cadmium tartrate, make a pure red pigment, alkali-earth metal ions etc., are comparable, and the fundamental con must be essentially absent in both the Sulfide Sideration common to all is the concept of co component and the oxidic component since even precipitating cadmium sulfide and an oxidic Small amounts of alkali-earth metal ions from cadmium compound in the presence of selenium, such sources or any other sources may produce and then calcining the coprecipitate to bring a precipitation of alkali-earth metal Sulfate about a reaction between the three components along with the desired crude cadmium pigment. (1) the oxidic cadmium compound, (2) part of The following tabulation shows the various oxidic the cadmium sulfide, and (3) part to all of the compounds which can be used in the indicated selenium. During the calcination a part of the strike systems for producing pure red pigments: cadmium sulfide may not enter into the afore mentioned reaction directly, as the equations Cd(NO3)2--alkali metal Sulfide Solution con 50 illustrate, but such portions may nevertheless be taining Se: influenced by the calcination and by the result (a) NaOH, Na2CO3, KOH, K2CO3, Li2CO3 or ing reaction products in Such manner that pig mixtures mentary and desired color qualities are imparted (b) NaOH, KOH, LiOH, Ba(OH)2 Ca(OH)2 to the entire calcination charge.

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