US2383674A - Process for production of sodium bicarbonate - Google Patents

Description

Patented Aug. 28, 1945 UNITED STATES PATENT OFFICE PROCESS FOR PRODUCTION OF SODIUM BICARBONATE Sidney G. Osborne, Saint Davids, Ontario, Canada, assignor to Hooker Electrochemical Company, Niagara Falls, N. Y., a corporation of New York No Drawing. Application August 21, 1941, Serial No. 407,756

2 Claims.

upon brine that has been made up at ordinary temperatures and contains about 290 grams of NaCl per liter, and usually show a yield or current efliciency of about 90 per cent. In Patent No.

2,173,986, there is disclosed a method of heating brine to 60 to 90 C. and resaturating it at these temperatures. Brine that has been saturated in this manner contains 310 to 325 grams of sodium chloride per liter. In Patent No. 1,866,065, there is disclosed an electrolytic alkali-chlorine diaphragm cell, which, when supplied with brine that has been saturated while hot in accordance with patent No. 2,173,986, produces an efliuent containing 11 to 12 per cent NaOH. When producing ll per cent NaOH solution, which is preferred practice, the current efficiency of this cell is ordinarily 95 per cent. The relatively concentrated product and high efiiciency obtainable from cells constructed and operated in accordance with these patents will hereinafter be considered as typical of best modern practice and referred to accordingly, to distinguish these results from those characteristic of general practice. It is desirable, and one of the objects of myinvention, to be able to adapt the cells of best modern practice to the production of sodium carbonate and bicarbonate.

It is known that alkali metal carbonates can be produced from electrolytic cell eflluent by admitting carbon dioxide directly to the cathode compartment of the cell. In the case of'the electrolytic cell and process illustrated and described in Hargreaves-Bird British Patent No. 18,039 of 1892, this is accomplished by admitting steam and gases containing carbon dioxide directly to the cathode compartment of the cell. This compartment be- Hence the cell eflluent is quite unsaturated and as will be shown by example, the carbonation products are in solution. In the cell of the Patent No.

1,866,065, the cathode compartment is normally comes completely filled with an atmosphere of the gases, so that the sodium hydroxide produced in the cell comes into contact with the carbon dioxide as fast as formed. The efliciency of such a cell is typically 88 per cent and, as the contact of the hydroxide with the CO2 is imperfect, the product is mostly the carbonate. The concentration of the hydroxide must be presumed to be of the order of that mentioned above as characteristic of the early art or later general practice.

almost completely filled with eflluent and thisis one of the conditions which give this cell its high efliciency. Toreplace this liquid with an atmosphere of CO2 would impair the efliciency of the cell. It would of course be possible to bubble 002 through the cathode compartment, but since this compartment is not designed for this purpose the reaction would be incomplete. Moreover, the hydrogen evolved in the cell, which is a valuable by product, would be contaminated by C02.

It has also been proposed in Craney Patent No.

- 552,955, 1896, and'Suchy, Patent No. 1,477,086, 1923, to produce precipitated alkali metal bicarbonate by reacting carbonate, produced in the cathode compartment of an electrolytic cell, with C02. For this purpose, the filtrate remaining after the removal of the precipitated bicarbonate, containing chloride and residual carbonate and bicarbonate, is returned to the cathode compartment of the cell, where the bicarbonate reacts with fresh caustic alkali produced in the cell and is converted to carbonate. This is then reacted with CO2 as before. In the process of the Craney patent the solution of alkali produced in the cell must be presumed to be of the concentration characteristic of the art of that time. In 1896 electrolytic alkali chlorine cells were quite primitive, by modern standards, and the concentration of caustic alkali even lowe than the lowest modem practice. In fact, it was commonly of the order of 6 to 6 per cent. The carbonate and bicarbonate produced from such solution would be of a correspondingly low concentration. A considerable part of the bicarbonate would be in solution. Alkali metal carbonate and bicarbonate of bicarbonate in accordance with his process would be far greater than could be tolerated in the anode compartment of the cell. The Suchy patent has todo with potassium bicarbonate, which is considerably more soluble than sodium bicarbonate, so that even when precipitating potassium bicarbonate by carbonation theanode compartment of the ce 2 [Q gsaaon' oi'asawrated solutionothydroxidedmtasslumchlorideameportofthehicarithrinssinwaterwithit. Waterisoicourse or the water is used up in nevertheless the' quanflty brought ehlorideisiargreaterthanthatusedup. In

carriedbyitintheiormoi'alkalimetaichloride, 30

carbonate and bicarbonate are too great to be allowed to go to waste. An evaporation step not mentioned in these patents is thereiore an essen- 1 tialportoithese. Theprocessesof thesepatentscouldnotbeoarriedoutinthecell o1 Patent'No. 1,886,065 even if it were desirable emcleney.

"Ihave found thatit istodisturb 7 to do so-(which it is not), without impairing its 1 the operation and reduce the eiiicieney of the cell byreactingtbecausticalkaliwithcarbondioxide or the bicarbonate in the cathode compartment ottheeeliitselnoslntheprocessesoithethree prior art patents rei'erred to. On the contrary,

, it is far preferable to remove the eflluent and earhonate it outside the-cell. thus avoiding any in- 'terrerence withthenormal operationoithecell. WhenthisisdonethecaustioalkalicanJldesired.belorselytothebicarbonatein -a continuous operation. Ir-the canstic'alkali be thatoisodiumandittheiscarried out in an emcient absorption apparatus, very littleearbonateneedreniain. Iithecaustiesoda solution is brought to saturation by addition of chloride before or after carbonation- (preferably before), a very large part of the bi carbonate'will be inthe precipitate and only a negllslhlepartwillremaininthe solution. This isespeelallytrueirthesolutionwasorllinallyot olbest modem practice-Jo fact, I find that under these'conditions the carbonation products remaininginthe'lolutiohatterremovalorthe tolerated in the-onodecompartment oi the cell.

Afterremovaloitheflfipfiatedticarbonatel, thereiorereeyehtheiiltrate nottotheoathode inacoordanoewlththecompartment oi the cell practice orthepriorartbuttotbesnodecompariment. Binoethewaterthmrecyeledreplaces vmter that would otherwisebeintrodueed with.

thebrinethereisnolaiildingupoiwaterinthetorproductionoltheeausticalkaliandr atthesametime. Itisonlynecessarytoadd waterandsodiumchloride inquautitysuiiloient mpi bthewaterandchloridein thecell. Inthlswaylamabletoproduoesodimn I bicarhonateinsolidiomiromtheeluentoi electrolytic caustic soda-chlorine cellawithdut any expensive evaporation'step. and Iithvery highyieldespeciallyiitheeiluentheothish withbestmodefnp .lfsodimncarhonateistheproductmteithis -isproducedrromthebiearbonatchyroosting.-

.mthatcasethecozdrlvenoflisotcom'serecycled.

1s Thecompositionottheeiliuentoi bestmodern practice and that of general practice are as follows:

' mm W Bmmodc'n Omani 2o PM I m I practice, poetics,

percent poi-mt mob mob NaOH ii 2.0 20

NoOL. 15% 13% 1.02 1.9,

H30 73%} v 77 J0. 8.1!)

Itwlllbeseeni'romthesengm'esthattheefllu- .ent of best modern practice is considerably more nearlysaturatedthanthatoftheearlyart. Tim hearing of this upon my invention appear from theexamples tobegivenlater. .r

The reaction of CO: with NaOH is" as follows:

NBOH-I-COI-PNBICOS-I-Hb (1) micos+n=o+cor maaooi 2) Itwillbeseenfromthese reaotionsthatllaaCOa is an intermediate product in the carbonation o! NaOHand thatwaterisformedintheflmtpartotthereactionandtakenuoaaalndurlngthe 4 second-part. Thisisone oithereaiiomivivhythe proportion of the X8200: that is precipitated is so much less than the proportion oiNaHCO: that is precipitated. 'Ihecarbonationofweakcell eiiluentin accordancewiththeHarsreaves-Birdprocessmaybe as follows:

mliaicoaort'li'isreactionwuibemtireiyin solution. .1hiswillbeshownlaterhyanenmple.

'mecarbonationoi'theweakliasomsflutionf oi Equation s-in accordance with the Oraney potentmuhewriwenasfollows: I ss Na:CO:+1.94NaCi+3'iHaO+OOammcowmmaawmio to Averyccnsimucpartormemnoowrmiu' reaction'willbeinsolutiopmswillbeshownlsterthe relatively hlah concentration characteristic 'co 'lheearbonationofthetveokcellemuentoithe prior art, saturated by addition oi No.01 thereto,

withcaretoavoidgoinslieyondsotui'ationuwith Namrnaybewrittenasiollows:

theprecipltata This aolnwlllheillush-ated r m. or tumour ll M 0! recent practice-alter satnration'by'addiflon 0! mi, ms; be written so follows:

2la03+1938s01+83fi0+200a a The NaHCOa of this reaction will be almost entirely in the precipitate, only a negligible proportion remaining in the filtrate. This will like This result is typical of the Hargreaves-Bird process and shows why the product is a solution. Example II.The product of Example I was carbonated as far as possible to NaHCO in accordance with Reaction 4. The composition of the resulting product was as follows:

Filtrate Solids This result istypical of the Craney process. The analysis of the filtrate shows that it contains I a substantial part of the product and prove that the filtrate remaining after removal of the precipitated solids in this process is far too alkaline to be recycled to the anode compartment of the cell, and must therefore be recycled to the cathode compartment, necessitating an evaporation step to prevent accumulation of water and NaCl in the system.

' Example [IL-9V per cent NaOH solution was brought to saturation by addition of NaCl, care being exercised to avoid adding more NaCl than just suflicient for saturation. The solution was then carbonated as far as possible to NaHCOa, in accordance with Reaction 5. The composition of the product was as follows:

Solids Filtrate 20. 9 3. 08 208. 7. 50 None 301-. 90 None 1, 000. 00

This represents a yield of NazCOa and NaHCOs Solids Filtrate amounting to 93.0 per cent of the theoretical yield, based on the current supplied to the cell, assumf ing the eificiency of the cell at 95 per cent asbe-f fore, or a yield of 89 per cent in the precipitate. The filtrate is obviously fit to be recycled to the anod compartment of the cell without appreciably affecting its efficiency. v

A comparison'of this example with Example III shows that the increased concentration of amounting to 84.2 per cent of the theoretical yield,

based on the current supplied to the cell, assuming the efllciency of the latter at 90. per cent, or a yield of 80 per cent in the precipitate. The filtrate of this example is low enough in NazCOa and NaHCOz to be recycled to the anode compartment of the cell without seriously ailecting its efliciency,

Example IV. 1l per cent NaOH solution saturated by addition of NaCl, with care to avoid an excess of NaCl, was carbonated as far as possible to NaI-ICOa in accordance with Reaction 6. The resulting composition was as follows:

Filtrate This represents a yield of NaaCOa and NaHCOa NaOH obtainable by best modern practice results in an increase in yield of NazCOa and NaHCOa in the precipitate from per cent to 89 per cent. This example represents 'what I consider good practice of my invention.

Example V.-11 per cent NaOH solution, satu- ',rated by addition of NaCl with care to avoid an excess thereof, was carbonated as far as possible to NaHCOa in accordance with Reaction 6 as in the previous example, except that the reaction was carried on undera pressure of 2 mm. of mercury, in order to promote absorption of the C02. The analysis of the product was as follows:

Solids Filtrai-e This result represents a yield of NazCO: and NaHCOa amounting to 96 per cent of the theoretical yield, assuming the cell efilciency at per cent as before, or a yield of 92.7per cent in the precipitate.

A comparison of this result with that of the preceding example shows that a slight pressure assists the absorption of C02 sumciently to raise the yield of Na2COa and NaHCOa in the precipitate from 89 per cent to 92.7 per cent. This example represents what I at present consider best practice of my invention.

It should be noted that the yield of 92.7 per cent .of solid dry NAaCOs and NaHCOa obtained in Example V compares favorably with the yield of 88 per cent of NazCOa obtained by the Hargreaves- Bird process Moreover, in the Hargreaves-Bird process, as shown above, the product is a solution and the cost of its recovery is a serious item of expense, which it is one of the objects of my in-.- vention toavoid.

In carrying out the carbonation step of my process the liquid may be sprayed downward through anatmosphere of CO2; or the CO2 may be bubbled through successive batches of the liquid so that a very large excess of 002 will be contacted very vigorously with'each batch as it nears the finish of the carbonation, in order to carry the carbonation as nearly as possible completely to the bicarbonate. Pressure in excess of the 2 mm. of mercury of Example V may also be used.

In carrying out the saturation of the cell eiiluent with sodium chloride, the addition of excess chloride beyond that which will go into solution can be avoided by passing the liquid upward through a bed of finely divided salt at a velocity below that at which salt is carried upward,

By my process I am therefore able to produce dry sodium bicarbonate from the efliuent of caustic. soda-chlorine cells, without the expensive evaporation step heretofore believed to be unavoidq able, beyond the slight cost of drying; and, by tak-V ing advantage of the high concentration of NaOH but with an actual improvement in that respect.

separate steps for evaporation of water or flltration of caustic soda solution which comprises supplying a substantially saturated solution of sodium chloride to the anodic zone of an electrolytic diaphragm cell and therein elmolyaing it to decompose a part 01' the sodium chloride, thereby producing inthe cathode zone of the cell an unsaturated solution containing caustic soda and sodium chloride; removing the solution from the cell; passing the solution through a bed of sodium 20 chloride, thereby bringing :the solution to substantial saturation with respectto sodium chloride and caustic soda, while leaving behind any insoluble impurities carried by the solution or sodium chloride; passing carbon dioxide into the saturated solution until the caustic soda therein is carbonated as far as practicable to sodium bicarbonate; filtering out from the solution the resulting precipitated sodium bicarbonate; adding water to the filtrate to replace tlm water decomposed in the electrolysis step; resaturating the reaseaeie y suiting solution: and recycling the augmented soiutiontotheanodicaoneofthecell. Y

2. The cyclic process for production of precipitated sodium bicarbonate oi. high purity andwith a high overall yield'thereoi and avoidance 01 any separate steps ior evaporation oi water or filtration of caustlc soda solution which comprises supplying a substantially saturated solution of sodium chloride to the anodic none oi an electrolytic diaphragm cell and therein electrolyzing it to decompose a part of the sodium chloride, therebyproducinginthecathodezoneotthe cell an unsaturated solution containing causticsoda and sodium chloride; removing the solution from the cell; passing the solution upwards throu h a bed orsodium chloride at a velocity below the predetermined velocity at which particles oi salt are carried upward, thereby bringing the solution to substantial saturation; with respect to sodium chloride and caustic soda, while leaving behind the insoluble impurities carried by the solution bicarbonate; filtering out from the solution the' resulting precipitated sodium bicarbonate; adding water to the filtrate to replace the water decom posed in the electrolysis step; resaturatlng the resulting solution; and recycling the augmented so- 1 3 lution to the anodic zone of the cell. SIDNEY G. OSBORNE.