Document 2Jpvr14qg6ozvb62mY6zvZKg7

ANACONDA ELECTROLYTIC WHITE LEAD Anaconda Electrolytic White Lead Bv R. G. BOWMAN, Assistant General Superintendent W. J. KNOX, White Lead Plant Superintendent Discussions of processes tor the manufacture of white lead generally open with the statement that white lead is the oldest chemical pigment known to man. This fact is of more than historical interest; :n the light of the present extensive use of white lead, it indicates that the compound possesses characteristics that make it unique among white pigments, and superior to all in its particular field. A greater variety of processes have been proposed for manufacturing white lead than for any other one chem ical compound. Only a few of these processes have come into commercial use. however, and a large part of the present production here and abroad is made by the process used in ancient times. The outstanding disadvantages of the old process are that it is slow and laborious, affords no control of the product, and yields at best an impure material lacking in uniformity. Improved processes have had as their main object the shortening of the time required. Most of them accomplish this and some have yielded a superior product as well, but only two or three are now in use commercially. For centuries the manufacture and mixing of paints has been treated as an art, and the practice has been much influenced by tradition and prece dent. There is a reluctance to abandon old materials for this purpose; the quaint clumsiness of the ancient methods of manufacture has long been, credited with imparting to the product a homespun quality of punty and integrity, and the adoption of the modem processes has been slow. The electrolytic production of white lead is not new, a large number of pr>ce*se> and forms of apparatus have been proposed and patented within the Z* years, preceding the introduction of the Sperry process, all directed toward the pro duction of white lead, wholly or in part, by electrolysis. One or two vf these processes were tried commercially but proved unsuccessful. Process The Sperry process, used by the Anaconda Lead Products Co. is in%*ention of Elmer A. Sperry, and was worked out to a commercial ba*: the East Chicago plant. It resembles, in a general way, earlier meth but embodies the features that make it possible to operate continuous!-.to produce a uniform product, the character and composition of wh:cn under complete control. It has made possible the rapid production ;f v. lead of a degree of chemical purity and brilliant "whiteness'* surer any produced by other methods. These properties of the product arc . pendent of the punty of the metallic lead used. J PNYC00000032 ne<rr POSSIBLE COP# The Sperry process is not a process for the production of -..h:te lead alone, but a basic method cf producing in insoluble salt of any metal by a continuous electrolytic process. Application of the method to lead saits, and to white lead m particular, was selected on account of the staple character of the product, calling for production on a tonnage basis. The process is a combination of electrolysis and chemical precipitation, m which the precipi tation is a secondary reaction and the composition of the reagents and of the precipitate are controlled electreivtically. It is unique among elec:rcivt:ck processes, in that it yields a solid product that is neither an anoce nor a cathode deposit and is removed continuously. As applied to white lead, the process is earned on in a ceil having a lead anode and an iron cathode, separated by a porous diaphragm. The anode is surrounded by an electrolyte--the "anolyte"--containing sodium acetate and a very small amount of sodium carbonate. The cathode is sur rounded by a similar electrolyte--the "catholyte"--containing sodium ace tate and a relatively large amount of sodium carbonate. Each -electrolyte :s maintained in rapid circulation about its electrode. The circulation systems of the two electrolytes are entirely independent and no communication exists between catholyte and anolyte save through the diaphragm in the ceil. The anolyte in passing through the cell removes the product being precipitated. After passing through a settler to remove the solids it is re turned continuously to the cell. The catholyte in its circulation external to the cell is replenished with the ions of which it was depleted by migration through the cell diaphragm. In so far as the reactions are concerned the concentration of the precipitants in the catholyte and of the electrolyte in the anolyte- are unimportant. The concentrations are maintained as high as is practical to reduce the electrical resistance. White Lead--Electrolytic CeU Reactions In practice a small amount of Na2C03, not exceeding .O'fr, is added to the anolyte before placing the electrolytic cells in operation. When the current is turned on the lead dissolves from the surface of the anode and is immediately precipitaeed as White Lead at a slight distance from the anode surface. The precipitate is carried down through the cell by the descending current of anolyte in a thick cloud without depositing on the anode surface. Hydrogen and the hydroxide ions arc liberated at the cathode. The hydroxide and carbonate ions of the hydrolyzed NsaCOj of the catholyte migrate through the diaphragm towards the anode replenishing the anions removed from the anolyte by precipitation of the lead ions. By this electrolytic mi gration the electro-chemical equivalent amount of anions required to pre cipitate the lead cations as they are dissolved at the anode are transported to the anolyte. Slight traces of carbonate can be detected m the anolyte but it is entirely free of dissolved lead salts. However, there is no Na2CC>3 present in the solution film in contact with the anode. The foregoing outlines the major reaction. In practice, in order to replenish and conserve chemicals and to restore the loss in volume of eacholyte due to decomposition of water at the cathode, evaporation and mechan ical losses, anolyte is added to the catholyte in a small continuous stream. This introduces NaAc into the catholyte which is also acted upon by the current, the acetate ions replacing a part of the hydroxide and carbonate ions chat migrate to the anolyte, This is compensated :cr bv certr.itur.g a certain amount of seepage of cathoiyte into anolyte through :he diaphragm. The seepage is controlled by selecting a diaphragm fabric of the desired porosity for a given hydrostatic solution head of cathoiyte in the cathode compartment. The addition of relatively small amounts of ar.oivte to catho iyte and cathoiyte to anolyte permits adjusting the sodium concentration of the wo electrolytes and the conservation of chemicals, since ail make-up solution can then be added to the anolyte in the form of filtrate from the niters. All anolyte in the filter cakes ts replaced by water by a ccuncer current washing operation and returned to the anolyte circulation svstem. In addition wash water filtrate is also returned as required to replenish the losses enumerated above. As the volume of solutions transferred between the two electrolytes is very small the anolyte as previously stated is essen tially free of the reacting salts and they are only brought into the cone where the white lead is precipitated in electro-chemical equivalent amounts, there is never an excess present to alter the reaction or sr.fiuer.ee the sice of the particle. The White Lead which is removed continuously from the cell bv the circulation of the anolyte, is removed from the anolyte by settling and filter ing. The filter pulp is washed, dned, ground and air floated, and :s then barrelled in a dry pulverulent form. The clear anolyte is recirculated through the cell. The composition of the anolyte during its passage through the cell remains unchanged except for a smail increase in the acetate concentration. This is controlled by dilution with wash water from the filter. The cathoiyte is depleted in carbonate ions and a small percent of acetate ions, and en nched in hydroxide ions during its passage through the cathode compart ment of the cell. The excess of hydrate ions is neutralised and the loss in carbonate ions restored by passing the cathoiyte through carbonation towers, in which a stream of descending solution passes through a rising stream of CO, gas from a coke burning boiler. The acetate loss is replenished as noted by running a small stream of anolyte into the cathoiyte after it is discharged from the cells. White lead is a basic carbonate of lead. The actual composition of the compound may therefore be varied. When a solution of a carbonate salt is added to a solution containing a soluble lead salt and White Lead is pre cipitated the empirical equation for the reaction that takes place *.n the for mation of White Lead of theoretical composition is generally given as: 3 Pb(G.HsO)- 4- 2 tfa-CO, - 2 NaOH = 2 PbC03 - Pb(OH)2'-f6 NaC3H302 The composition of the electrolytic cell solution would indicate that the equation should be written: 3 Pb(C*H30)2 4- a Na>C03 4- 2 H0 = 2 PbCO-, Pb(OH)2 -f 2 NaHCOa -r <5 NaC2 H3CU Since analysis of the anolyte of the electro-chemical cell shows that NaHC03 is produced at the time that White Lead is precipitated, the re- action indicated, therefore, is that the >si2C03 u hydrclv:ed to fora NaHC03 and NaOH, and that the intermediate compounds Pb (C03H)2 and Pb(OH)a are first formed, which in turn reacc to produce the com" pound Pb(C03Pb0H)2. This formula is preferred as it indicates the for mation of a compound and not a mixture, as 2 PbC03 Pb(OH)2 would indicate. Characteristics of Product The outstanding characteristics of the White Lead produced by the electrolytic process are exceptional puncy, brilliant whiteness, and uni formity. The exceptional purity and freedom from contamination are the results of the initial exclusion from the white lead of all metallic impurities con tained in the original lead, exclusion of the impurities in the chemicals em ployed and the elimination of all external sources of contamination. The metallic lead undergoes a refining action in the electrolytic ceil, rr.iy leid being dissolved. The metallic impurities commonly found in lead, such u bismuth, antimony, and silver, together with small particles of ur.corrcced lead, remain as a firmly adhering slime on the surface of the ancde Other impurities that could be introduced with the ocher raw materials used ire excluded by adding these raw materials to the catholyte; the fabric dia phragm, which separates this solution from the solution-in which the white lead is formed, is an efficient filter. Water soluble salts are removed by washing the product with hot water. There is no soluble basic lead acetate present and the washed product is neutral or slightly alkaline. Proper selec tion of materials of construction and the exclusion of atmospheric dust have brought about the elimination of the third source of contamination. The brilliant whiteness is the result of the exceptional purity thus obtained. The characteristic uniformity of the physical and chemical properties of this white lead and the ease with which these properties can ve virted are the result of the positive control of the chemical reactions * * piace in the electrolytic cell. Automatic recording and regulating instruments are used throughout the plant to insure complete records, to perm.: ;f all operations, and to maintain uniform operating conditions. T prac- cically no variation in any of the operating factors, such as u j j m* position of the electrolytes, current density, temperature of r.s. ar race of flow of solutions. The product is the true basic cariv:: := usually given the formula 2 PbC03 Pb(OH)2 in text books. * nirly Pb (CO3 PbOH)2 since it is a single compound and not 1 t ; :hc older formula would suggest. The physical properties of electrolytic white lead ;in ~ rather a wide range to meet changing specifications. This .* by changing the operating constants of the cell. Following * showing the range over which the physical properties in ducing the various grades of electrolytic white lead now m cases these figures are not strictly comparable with ocher p .r . i ver -- r..?hed .-.i.ttion 1 pro- -'me PNYCOCG00035 BEST POSSiBLE COPY .is the method or making the determination may vary .;r i i:rr.rr. :r. ::ar.c*:i for comparison may be necessary. Relative Range in Physical Properties of Various Grades of Electrolytic White Lead Flow (Gardner Flowmeter)* 34 6<`e Oil--67.4r^ Pigment, Oil Absorption. (Gardner-Coleman). per * vO grr.s........... I: to :> 7 * cc Oil Absorption. (Standard Rufcouc Method), per TX* ^ms., ........................................................................................ " * 5s :: U --? Hiding Power. (Pfund Cryptometer), per pound......... '.3 50 to It 3 3 ?c Tinting Strength (Ultramarine), DuPont Numerical Strength Scale.........................................................................."'G :c 13 :G: Particle sice. Weighted average...................... .................. 3 95 to 1 i* m.:cr:?r.s Refined linseed oil, acid number 5, is used. Miscellaneous Data--Electrolytic White Lead Brightness Blue Green Red Pfund Colorimeter................................ (430 uu.) (4*0 uu.) (5)0 uu) Percent Diffused Reflection.................. 37.4 $9.2 59.3 Specific Gravity.......................................................................................... 5.SI Residue on 325 mesh................................................................................. 0.105r7 Insoluble.... ..................................................................................................j 01 *C Free Acetic Acid.... ......-........................................................................... None Moisture....................................................................................................... 9 27/c Wt. per Solid Gallon.... ......................................................................... f 6.T3 Lbs. One Pound Bulks--Gallons................ 0.0t"63 The development of the Sperry electrolytic white lead process has cre ated much interest m the paint industry and among eiectrochemucs Several descriptive articles have been published and are listed below The above description represents a consolidation of material from a number of these articles, with some additions to bring the whole up to dace. "Anaconda Electrolytic White Lead," by R. G. Bowman. Transactions A. I. M. E. Vol. 73, 1926. pp. 146-170. "Electrolytic White Lead," Paint, Oil August 25, 1927. Chemical Review, Chicago. "Pioneer Work in Development of White Lead Manufacture by Sperry Electrolytic Process," by R. G. Bowman. Engineering cr Mmmg Journal. Vol. 123. No. 8. page 318. "Electrolytic White Lead/* by W. J. Knox. Paint tP Varnish Produc tion Manager, Vol. 34, No. 5, November, 1929, pp. 22-24. "The Manufacture of Electrolytic White Lead," by 1. G. Kate. The Armour Engineer, Vol. 21, No. 3, March, 1930, pp. 88, 39, 116. BEST POSSIBLE COPT