US2275054A - Electrolytic production of lead sulpho-chromates - Google Patents

Description

March 3, 1942.

S. W..STOCKDALE ETAL ELECTROLYTIC PRODUCTION OF LEAD SULPH-CHROMATES Filed July 18, 1939 INVENToRs 67E/pfff muraria/uf 15557 E WfA VER ATTORNEYS Patented Mar. 3, .1942

ELECTROLYTIC PRODUCTION OF LEAD SULPHO-CHROMATES Stephen W. Stockdale, Hammond, Ind., and Elbert F. Weaver, Calumet City, Ill., assignors to International Smelting and Refining Company, a corporation of Montana Application July 18, 1939, Serial No. 285,104

-3 Claims.

This invention relates to the production of sulpho-chromates, and is concerned more particularly with the provision of a new and improved method for vproducing lead sulphochromate pigments electrolytically. The lead sulpho-chromate" products referred to herein are known as such in the color industry and comprise complex salts of lead sulphate and lead chromate.

The method of the invention comprises passing an electric current through a bifluid electrolytic cell comprising a lead anode, an anolyte containing a substance capable of serving during electrolysis as a solvent for lead, a diaphragm, a catholyte comprising an aqueous solution containing sulphate ions and chromate ions, and an insoluble cathode. Sulpho-chromates, having the y desired chemical composition and physical characteristics, are produced in accordance with this method by controlling the composition of the electrolyte as hereinafter more fully described.

In carrying out a process embodying the new method, a lead anode is immersed in an anolyte containing a substance capable of serving upon electrolysis as a, solvent for lead. An aqueous solution ora substance capable of yielding nitrate or acetate ions, such, for example, as sodium nin trate or sodium acetate generally is preferred for use as the anolyte. An insoluble cathode (of iron, for example) is capable of yielding sulphate and chromate ions such, for example, as sodium sulphate and sodium chromate. The anolyte and catholyte are physically separated by means of a permeable diaphragm which prevents the two solutions from intermingling, but enables an electric current to pass between anode and cathode and permits the migration of ions from one electrolyte to the other.

A biiiuiiA electrolytic cell of conventional design, equipped with one or more diaphragms of the type usually employed in producing lead salts electrolytically, may be employed in carrying out the process. The complete apparatus preferably includes means for continuously withdrawing and circulating both the anolyte and the catholyte through a circuit outside the cell during the course of the electrolytic operation. The lead sulpho-chromate product forms in the anolyte and may be separated therefrom in any suitable immersed in acatholyte comprising an aqueous solution of a substance' manner after withdrawal of the anolyte from the cell and before its return thereto. 'I'he sulphate and chromate concentrations in the catholyte are maintained at desired values by the addition of sulphuric acid and chromic acid to the catholyte at a point outside the cell.

In operating the cell to produce a lead sulphochrornate product, an electric current is passed from the lead anode through the anolyte, the

diaphragm, and the catholyte to the cathode. Satisfactory operation is secured with a potential dilerence of about 3 to 5 volts between the anode and th cathode, and with a current density of Sodium sulphate and sodium chromate diffuse through the diaphragm from the catholyte into the anolyte and there react with the lead nitrate to precipitate lead sulpho-chromate and to regenerate the sodium nitrate:

The tendency for the alkalinity of the catholyte to increase as a result of the migration of sodium ions thereto and the production of sodium hydroxideat the cathode is compensated for by the addition of sulphuric acid and chromic acid or their equivalents to the catholyte externally of this cell:

heretofore known processes for producing sulphochromate pigments.

In some cases it is advantageous to maintain the catholyte at a slightly greater hydrostatic head than the anolyte. In this manner any tendency of the sodium nitrate to diffuse through the diaphragm from the anolyte into the catholyte is diminished and at the same time diffusion of sodium sulphate and sodium from the catholyte into the anolyte is aided.

The chemical composition and physical properties of the product of the cell may be varied considerably by varying the conditions prevailing in the cell during electrolysis. 'I'he factors which exert particular influence in this direction and which should be properly controlled to secure a lead sulpho-chromate product having optimum physical properties for use as a pigment and to facilitate operation of the electrolytic cell are the following:

l. Concentration of salts in the electrolyte Concentration of sulphate in the anolyte should be maintained at about 0.075% to O.l% by Weight NazSO/i, and concentration oi chromate in the anolyte should be maintained at about 0.675% to .l5% by weight 'liiazCrOra Corrosion oi the anode is facilitated by maintaining these concentration values, and accumulation of soluble lead in the anolyte is thereby prevented. Soluble lead, if present in appreciable quantities in the anolyte, will contaminate the cell product and render it unsuitable for use as a pigment. The concentration of sulphate in the catholyte preferably is maintained at about 1% to 3% by weight Na2SO4, and the concentration of chromate in the catholyte preferably is maintained at about 2.5% to 3% by Weight NazCrUr. By maintaining these concentration values in the catholyte, the rate of transfer of chromate and sulphate ions to the anolyte is easily established at a value to maintain proper ion concentrations in the anolyte.

2. .alkalinity of the electrolyte .linity preferably is maintained by adding sulphurc acid and chromlc acid to the catholyte in suitable proportions and in suitable total amounts, as this procedure provides a very precise method of controlling the concentration of sodium chromate and sodium sulphate in the catholyte, either to maintain an established concentration of these salts or to eifectv any desired increase or reduction in the concentration of either constituent.

3. Temperature of the electrolyte The temperature of the electrolyte preferably is maintained at about C. to 25 C.` Within this temperature range transition of the generally preferred light yellow rhombiccrystals into the darker yellow monoclinic crystals is inhibited.

A preferred process in accordance with the invention is illustrated in the accompanying dlagrarnmatic flow sheet. An electrolytic cell tank I0 is substantially llled with an anolyte l'l comprising an aqueous solution of salt such as sodium nitrate or sodium acetate capable of serving during electrolysis as a solvent for lead. For example, an aqueous solution containing 2% to 8% by weight of sodium nitrate may be employed as the chromate anolyte. Immer-sed in the anolyte is a lead anode l2.

suitably spaced from the anode and also immersed in the anolyte is a diaphragm i3, which advantageously is in the form of a fabric bag. rli'he diaphragm bag i3 contains a catholyte lil comprising an aqueous solution containing sodium sulphate, preferably in a concentration between about 1% to 3% by Weight Na2SO4, and

sodium chromate, preferably in a concentration between about 2.5% and 3% by weight NazCrOr. As indicated above, the catholyte may be maintained under a slight hydrostatic head with respect to the anolyte. An insoluble cathode l5, for example a cathode of iron, is immersed in the catholyte.

lin order to provide for circulating of catholyte through the catholyte compartment within the diaphragm bag i3, catholyte is continuously introduced from a storage tank ld through a conduit l'l extending to a point adjacent the bottom of the catholyte compartment. Catholyte overflows continuously from the catholyte compartment through a spout (not shown) and passes through a conduit iii to a pump it, by means of which it is returned to the catholyte storage tanlr it.

Sulphuric acid and chromic acid are added to the catholyte outside the catholyte compartment (preferably to the catholyte in the storage tank ifi) in. such relative proportions and in such total amounts as are required to maintain the desired concentrations of chromate and sulphate in the catholtye and to maintain the pH thereof at a proper value..

The cathode iii and the anode i2 are connected Y to s. suitable source oi electric current, and upon closing the electrical circuit the reactions described above take place with the formation of lead. sulpho-chromate in the arrolyte. The anolyte with the insoluble lead sulpho-chromate suspended therein is continuously withdrawn from the bottom of the cell tank it through a conduit The solid sulpho-chromate productl is separated from the withdrawn anolyte in any suitable manner, ior example by a decantation operation or a filtration operation. The separated lead sulpho-chromate is dried in preparation for marketing, and the clarified anolyte is forced by a pump 2l to an anolyte storage tank 22. Anolyte from the storage tank is fed continuously through a conduit 23 into the anolyte compartment of the cell tank iii at a rate sumcient to maintain a constant level of anolyte therein;

is pointed out above, the anolyte necessarily contains a small percentage of dissolved chromate and dissolved sulphate which enters the anolyte from the catholyte chiefly by migration through the diaphragm i3 during electrolysis. lf desired, in order to insure maintenance of proper sulphateand chromate concentrations in the anolyte, small quantities of catholyte may be introduced into the anolyte outside the cell, as by passing regulated small quantities of catholyte through a conduit 2t from the catholyte storage tank l@ to the anolyte storage tank 22.

The sulpho-chromate product of the cell may be produced in desired tones of color from light yellow to lemon yellow, depending upon the relative proportions of sulphate to chromate established and maintained in the anolyte. A product of light shade is obtained when a relatively high proportion of sulphate is present in the anolyte, and a product of relatively dark" shade is produced when the chromate concentration in the anolyte is relatively high. Other factors, as indicated above, also influence the color and tone of the product.

We claim:

1. The method of producing a lead sulphochromate product electrolyticallyY which comprises passing an electric current through a biiluid electrolytic cell comprising a lead anode, an anolyte containing a substance capable of serving during electrolysis as a solvent for lead, a diaphragm, a catholyte comprising an aqueous `solution of sodium Vsulphate and sodium chromate, and an insoluble cathode, and controlling the chemical composition and physical characteristics of the sulpho-chromate product by maintaining in the catholyte sodium sulphate in an amount equal to about 1% to 3% by Weight Na2SO4 a-nd sodium chromate in an amount equal to about 2.5% to 3% by weight Na2CrO4.

2. The method of producing a lead sulphochromate product electrolytically which comprises passing an electric current through a biuid electrolytic cell comprising a lead anode, an anolyte containing a substance capable of serving during electrolysis as a solvent for lead, a diaphragm, a catholyte comprising an aqueous solution of sodium sulphate and sodium chromate, and an insoluble cathode, and controlling the chemical composition and physical characteristics of the.sulphochromate product by maintaining in the anolyte sodium sulphate in an amount equal to about 0.075% to 0.15% by weight NazSOi' and sodium chromate in an amount equal to about 0.075% to 0.15% by weight NazCrOi.

3. The method of producing a lead sulphochromate product electrolytically which comprises passing an electric current through a biuid electrolytic cell comprising a lead anode, an anolyte containing a substance capable of serving during electrolysis as a solvent for lead, a diaphragm, a catholyte comprising an aqueous solution of sodium sulphate and sodium chromate, and an insoluble cathode, and adding sulphuric acid and chromic acid to the catholyte in such relative proportions and in such total amount as to maintain in the catholyte a concentration of sodium sulphate of about 1% to 3% by weight Na2SO4, a concentration of sodium chromate of about 2.5% -to 3.0% by weight Na2CrO4, and a hydrogen ion concentration equivalent to a pH of about 7 to 8.

STEPHEN W. STOCKDALE. ELBERT F. WEAVER.

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