Patented June 1-1, 1946 2,402,103 r

l l UNITED STATE s PATENT OFF ICE 2,402,103 ' PRODUCTION OF CHROMIUM COMPOUNDS Marvin J. Udy, Niagara Falls, N. Y. No Drawing. Application March 19, 1942, Serial No. 435,438 ‘ 6 Claims. (CI. 23-56) 1 This invention relates to the production of hex 2 .. avalent chromium compounds and has for an ob calcium chromate, all of the sodium oxide or the ject the provision of improved methods of ex sodium chromate is wasted and additional base i“ the form of lime (CaO) in amount equivalent tracting hexavalent chromium from crude ma to the sodium oxide of the sodium chromate must terials containing chromium in the hexavalent be provided. ‘ state combined with oxygen and a base in the 1 Calcium chromate can be produced quite readi form of chromate. - More particularly, the inven y tion contemplates the provision of improved by roasting chromium-bearing materials such . as ferrochromium, oxidized ferrochromium and methods of extracting hexavalentchromium from chromite are in the presence of .lime or in the crude materials containing chromium in the form lo presence of lime and a. small amount of an oxida , of calcium chromate. The invention further con tion promoter such as soda ash. Its recovery templates the provision of improved methods of from products obtained in such roasting opera producing calcium chromate and sodium dichro tions is rather di?lcult and expensive because of mate. ' I its relatively low solubility in water. Therefore, Chromate for industrial uses is prepared and 15 calcium chromate is produced, according to some marketed largely as sodium dichromate. The present practices, by ?rst producing sodium chro chromate is produced initially as sodium chro mate and thereafter treating the sodium chro mate but conversion to the dichromate‘is desir mate in aqueous solution with a soluble calcium able because of di?iculties involved in obtaining compound to form the relatively insoluble cal a suitable solid sodium chromate product for 20 cium chromate which is :ecovered by evapora marketing. Chromate in the form of calcium tion and crystallization. , _ ‘chromate would be desirable if calcium chromate Procedures involving the production of calcium could be produced. and recovered directly from chromate by ?rst producing sodium chromate crude chromium-bearing materials at a recovery may be wasteful of reagents.‘ The production of cost of the order‘ of the cost of recovering sodium 25 the sodium chromate involves roasting of chro chromate, since calcium chromate contains rela mium-bearing' material with air in the presence tively cheap lime (CaO) as the basic constituent, of a su?lcient amount of a sodium compound such‘ whereas sodium chromate contains relatively ex as sodium carbonate to form sodium chromate pensive sodium oxide as the basic constituent, and with all of the chromium of the chromium-bear calcium chromate can be crystallized in the form 30 ing material. The conversion of all of the chro of desirable solid products. No commercially mium to sodium chromate [may require the use satisfactory procedure for producing and recov of a substantial excess of the sodium compound ering calcium chromate has been proposed by which excess may be wasted, or it may be recov others heretofore. Accepted commercial practices ered at considerable expense. Conversion of the for producing both calcium chromate and-sodium 35 sodium chromate to calcium chromate by treat dichromate involve the use-of sodium chromate ment of the sodium chromate in aqueous'solution as the source of chromate radical. ‘ with a soluble calcium compound results in the These procedures have been adopted because production of a sodium compound of the type of sodium chromate can be produced readily by di soluble calcium compound employed, and the so rect oxidation of chromium-bearing materials, 40 dium compound thus formed ‘is practically worth and, being very soluble in water, it can be sepa rated readily from solid impurities, whereas, so less commercially. Thus, for example. sodium dium dichromate can not be produced readily by carbonate may be employed in the roasting opera tion to produce chromate. Any excess sodium direct oxidation of chromium-bearing materials, carbonate employed at this'point is wasted ‘as a.“ and calcium chromate, being relatively slightly 45 soluble in water, is di?lcult to separate from solid , result of combining with‘ one or more components impurities. Both procedures result in the wasting of the chromium-bearing material to form in of sodium'oxidc oi’ the sodium chromate. The soluble compounds. The sodium chromate solu production and marketing of chromatelas so tion, after concentration. may be treated with dium dichromate rather than as calcium chro 50 calcium chloride to form calcium chromate in ac mateis explained by the fact that in the produc~ cordance with the following equation: tion of sodium dichromate one-half of the so dium oxide of the sodium chromate is utilized, The sodium chromate and calcium chloride react only'one-hali being wasted, and no additional to .form the relatively ‘slightly‘soluble calcium base is required, whereas, in the production of 55 chromate. The chromium of‘the sodium chro 2,402,108 ' 3 a compounds less soluble in water than calcium mate can be obtained largely as calcium chro _‘ chromate or the compounds may be free of con mate by repeated evaporation and crystallization stituents capable of combining with calcium to treatments. The sodium chloride produced is form compounds less soluble in water than cal wasted because the commercial value of sodium cium chromate. Whenthe compounds contain- . chloride is so low that its recovery in a suitably ing. the bases as constituents are free of constitu pure commercial formwould not be economical. _ ents capable of combining with calcium to form Thus, the process‘ results in a substantially com compounds less soluble in water than calcium _ plete loss of the‘sodium compound employed in chromate, they are employed in conjunction with the initial roasting treatment. 7 When sodium dichromate (NazCiuOr) is to be 10 acids having constituents which are capable of prepared in, accordance with some heretofore combining with calcium to form compounds less customary procedures, chromium-bearing mate soluble in water than calcium chromate. Thus, rial is roasted as described above to produce, for example, the base may be ‘provided in the sodium chromate (NazCrOO, a sodium chromate form of an acid carbonate or acid sulphate which solution ‘is formed by leaching the roasted mar 15 compounds are capable‘ of reacting with calcium , terialwith ‘water, and sodium dichromate "is chromate to form compounds (calcium carbonate formed by treating the‘ sodium chromate'solu- v and calcium sulphate) less soluble in water than tion with an acid such assulphuric acid. Sodium calcium chromate, or,'the base maybe provided chromate and sulphuric acid react to produce in the form '01 a chromate in which case the com- - sodium dichromate and sodium sulphate as indi pound should be employed in conjunction with 20 an acid, suchv as carbonic acid or sulphuric acid, cated by the following equation: capable of combining with calcium to form a ‘ compound less soluble in water than calcium The sodium sulphate and-the sodium dichromate ' chromate. , may be separated by'evaporation and crystalliza 25 Processes of the invention are based on reac tion, the sodium sulphate being wasted. tions of the following types: " The present invention provides methods of pro I ducing calcium chromate by means of which the loss of relatively expensive sodium compounds In processes employing the above reactions, cal cium chromate of the crude calcium chromate may be reduced or avoided. 80 In accordance with the invention, chromate bearing material provides the base for combining compounds are recovered in relatively pure forms with the chromate ionsto form the soluble di from crude chromate-bearing materials contain chromate compound, calcium dichromate, and carbonic acidformed by reaction of carbon -di-‘ ing calcium chromate and which may contain, oxide with water provides the acid constituent - also, an alkali metal clu' omate such as sodium 35 chromate. A complete process of'the invention capable of combining with calcium to form a may include oxidation of chromium-bearing ma gompound of calcium (CaCOs) less soluble in terial containing chromium in the elemental ater than calcium chromate. state or in the trivalent stateunder such condi tions as to produce the crude/chromate-bearing 40 material containing calcium chromate or con In processes employing the above reactions, taining calci'um chromate and alkali metal chro calcium chromate is decomposed by sodium acid mateinvarieus proportions. ' carbonate with the production of highly soluble ' In carrying out a process of the invention for‘ recovering relatively pure chromate compounds _45 sodium dichromate and slightly, soluble calcium from crude chromate-bearing material containing > carbonate. calcium chromate, the crude chromate-bearing ' material is digested in the form of a pulp with an aqueous liquid containing in solution an acid or In processes employing the above reactions, an acid salt capable of reacting with calcium sodium acid carbonate formed by reaction 0! . chromate to effect its decomposition and to form carbonic acid with sodium chromate a compound of calcium less soluble in water than calcium chromate. The composition 01’ the pulp preferably is so regulated as to effect conversion‘ probably forms-the active agent for decomposi to, dichromate of the chromium of the decomposed 55 tion of the-calcium chromate as in Equation 2 calcium chromate. Conversion of the chromium of the decomposed calcium chromate to dichro above.Crude calcium chromate-bearing. - _ material for mate is controlled by establishing in the pulp treatment in the process of the invention may be - a controlled amount of one or more bases capable obtained from any suitable source. Such ma o1 combining with chromate ions to produce one terial may be produced, for example, by roasting or more soluble dichromate compounds. ‘ in air ferrochromium or chromite ore in ?nely The bases employed may be metallic elements ' divided condition and in the presence of lime to or_ radicals which function as metallic elements oxidize elemental chromium ‘or trivalent chro in forming chemical ‘compounds. Thus, for ex mium contained therein to the hexavalent state ample, the bases which are suitable for use in (i5 with the production of calcium chromate. The practicing the invention include calcium (Ca), roasting charge may contain- an oxidation pro sodium (Na), potassium (K) and the. ammonium moter such as sodium carbonate or other sodium radical (NHO . ~ ‘ compound in which case sodium chromate may The bases may be employed in the form of also be formed, The amount of sodium chromate ._ any suitable saltsor other compoimds, that is, 70 produced in any roasting operation will depend in chemical combination with non-metallic ele upon the relative quantities oi! sodium and chro ments or with base or acid forming radicals. The compounds containing the bases as constitu-_ mium present in the roasting charge. When so ents_may be salts of acids whose acid radicals I dium dichromate or calcium chromate is the end ‘are capable of combining with calcium to form 75 product sought, roasting charges should be pre e _' ‘9,402,108 ' 5 6 . 4 . pared with ‘sodium present in amounts not ex-v treated with calcium hydroxide as described‘ ceeding that required to form'sodium chromate above to form calcium chromate and sodium with half of the chromium present and lime chromate. Sodium chromate thus formed may should be provided in amounts suiiicient to form ‘be separated from the calcium chromate. and ~ calcium chromate with the remainder of the chro recovered as such or converted to sodium di ' mium. chromate or it may be used in conjunction with when sodium dichromate is the end product carbon’ dioxide to treat additional calcium chro sought. roasting preferably is so conducted as to » mate-bearing material. When sodium chromate - form a product containing sodium chromate and solution is employed to treat calcium chromate calcium chromate in about equi-molecular pro bearing material containing sodium chromate, portions. The resulting. product may be digested .it is advisable to bleed from the leach solution in an aqueous pulp with carbon dioxide to convert > containing sodium dichromate 'an amount of so substantially all of the chromate to sodium di-v dium dichromate equivalent to the sodium chro chromate in accordance with the following equa- ' mate contained in the calcium chromate bear ing material to be treated before returning it to the process. ' When carbon dioxide and water are employed _ V _ HaaCnOr+CaCOa+HaO - to digest calcium chromate-bearing material in The liquid containing in solution the sodium di- ' order to produce calcium dichromate, digestion chromate thus formed may be separated from in~ 20 preferably is carried out under-pressure and‘at ' soluble material of the roasted charge and evap an elevated-temperature. Grinding of the cal orated to produce solid sodium dichromate. cium chromate-bearing material during digestion when ‘calcium chromate is an end product may be carried out advantageously. ~Grinding sought, roasting preferably is conducted in the prevents accumulation of precipitated calcium absence of any sodium compound or in the pres as carbonate on the surfaces of particles of calcium encelof the minimum amount of sodium com chromate with resulting reduction in the rate of pound which will promote e?ective oxidation of solution of the calcium chromate. Pressure; of the chromium. If calcium chromate is substan 40 pounds per square inch and higher may be tially the only chromate formed in roasting, it employed advantageously. An elevated tempera may be separated and recovered by digesting the 80 ture of about 70° C. or higher prevents the for roasting charge in an aqueous pulp with carbon mation and solution of ‘calcium bicarbonate ' dioxide to produce a solution of calcium dichro (camwonn, which ‘forms at lower tempera ‘mate and treating the solution of calcium di tures but which decomposes at temperatures chromate' with calcium hydroxideto form cal ‘ above about 70° C. Digestion may be carried out cium'chromate in accordance with the following at lower temperatures and the solution contain equations: ' _ I ‘ ; I ing calcium dichromate and calcium bicarbonate (5) 2CaCrOt+C0z+HzO-> . ' may be heated after separation from the solid CaCraO1+CaCO:+HaO material to decompose the calcium bicarbonate. Calcium carbonate‘ produced by decomposition of ’ (e) oacnoq+cuomr+zcscroi+rno ' 4n the calcium bicarbonate may be separated from The relatively insoluble calcium chromate thus the calcium dichromate solution by filtering. formed may be precipitated by boiling or evap-> when sodium chromate is employed in con crating the solution. Anhydrous‘ calcium chroq Junction with carbon dioxide and water for leach mate may be precipitated Jrom solution by heat ing calcium chromate-bearing material, lower ing the solution to a temperature of about 180° C. 45 pressures and temperatures may be employed or higher under pressure of about 150 pounds per satisfactorily. ' The reactions take place rapidly square inch or higher. . 4 at atmospheric pressure. p‘ . > when some'sodium chromate is formed inop I claim: . e ' ' erations designed for the production of‘ calcium . i. The method of. recovering chromium from chromate, the roasted charge may be digesteddi chromium-bearing material which comprises sub rectly with carbon dioxide and water with the jecting the’ material to an oxidizing treatment in ' production ‘of a mixture of sodium dichromate the presence of lime and soda ash to form an and calcium dichromate; it may be leached ?rst oxidized product containing calcium chromate with water to remove the soluble sodium chro and "sodium chromate, digesting the oxidized mate and then digested with water and carbon product in' the form of a pulp with an aqueous dioxide to produce calcium‘ dichromate; or it may liquid containing in solution sodium chromate . be digested with carbon dioxide and a solution. and carbon dioxide to decompose calcium chro of sodium chromate to miOduce a solution of so mate and form’ sodium dichromate and insolu dium dichromate. The solution of sodium di ble calcium carbonate, the composition of the chromate may he treated with calcium hydroxide pulp being so regulated as to etlect conversion to to form calcium chromate and sodium chromate sodium dichromate. of a substantial amount of .inaceordance with the following equation: . ' the chromium of the decomposed calcium chro mate, separating a sodium dichromate solution (7) NaaCnOr+Ca(OH)-.--> from insoluble material of the oxidized product, naicrol-i-cacroi-tmo treating the sodium dichromate solution with a‘ The calcium chromate thus formed may be pre calcium compound under pressure at a tempera cipitated from solution in the hydrated state or ture not. lower than aboutv 180° C. to form a solu the anhydrous state, and the solution of sodium tion of sodium chromate and precipitate anhy chromate may be separated from the precipitate drous calcium chromate, separating the solution and used for leaching additional calcium chro 70 of sodium chromate and the anhydrous calcium - mate-bearing material. A solution of calcium chromate precipitate, and utilizing the solution dichromate may be treated with calcium hy of sodium chromate in the digestion of additional - droxide as described above to produce calcium calcium chromate-bearing material. chromate. A solution containing calcium di- " -2. The method of recoveringchromium from chromate and sodium dichromate may also be w chromite ore which comprises subjecting the ore secures “g? . solution from insoluble materiel oi the oxi?sed to an oxidizing treatment in the presence of lime product, treating the sodium dichromnte solution and soda ash to form on oxidized product con with as calcium compound under pressure at as taming calcium chromate and sodium‘ chi-ornate, temperature not lower than about 189° C. to term digesting the oxidized product in the form of a. at solution oi’ sodium chromete end pre?zoitste pulp with an aqueous‘ liquid containing in solu anhydrous calcium chromete, separating the tionedsodium chromste and carbon dioxide to de compose culclum chromute and form sodium di solutioncalcium oi’chromate sodium precipitate,chromete end and the utilizing the chromete and insoluble calcium carbonate, the solution of sodium chromete in the digestion e53 composition of the pulp being so regulated us to additional calcium chromete-besring materiel. effect conversion to sodium dichrom to of 2. sub 6i. The method of producing anhydrous eel“ stsntial amount of the chromium of the decomc cluxn chromete which comprises trecting an posed calcium chromote, separating as sodium dis aqueous solution conteining sodium dlchro-y chromete solution from insoluble material of the with calcium hydroxide, heating the solution to e oxidized product, treating the sodium dichrornute temperature not lower then about 180° it, under ' solution with e, calcium compound under pressure pressure to form and precipitate slnhydrous cel= at e temperature not lower than about 180° C. cium chromete, and separating the anhydrous to form e. solution of sodium chi-ornate and pre calcium chromcte from the accompanying liquid. clpitate anhydrous calcium chromate, separating 5. The method of producing anhydrous cel= the solution of ‘sodium chromcte and the cnhy» cium chromste which comprises trectina an drous calcium chromete precipitate, and utilizing aqueous solution containing calcium dichromote the solution of sodium. chromste in the digestion v of additional calcium chromatic-bearing material. with calcium hydroxide, heating the solution to a temperature not/lower than about let° Q. under 3. The method of recovering chromium irom pressure to form and preclpitete anhydrous cal ferrochromium which comprises subjecting the cium chromate, and separating the anhydrous ' ferrochromium in ?nely divided form to an oxi 25 calcium chromate from the accompanying liquid. dizing treatment in the presence of lime and soda 6. The method of producing anhydrous cal ash to form an oxidized product containing cal cium chromate ‘which comprises treating an cium chromate and sodium chromate, digesting‘ aqueous solution containing a dichromate of the the oxidized product in the form of a pulp with group consisting of sodium dichromate and col an aqueous liquid containing in solution sodium clum dichromste with calcium hydroxide, hect chromete and carbon dioxide to decompose cal irig the solution to a. temperature not lower than - cium chromete and form sodium dichromate and insoluble calcium carbonate, the composition of shout 180° C. under pressure to form and precipi the pulp being so regulated as to e?ect conver- , tote anhydrous calcium chrome-to, end seperut $5 ing the anhydrous calcium chromete from the sion to sodium dichromcte of a. substantial accompanying liquid. amount of the chromium of the decomposed cal MARVE? J. UJDY. clum chromate, separating e sodium dichromate