Dec. 11, 1956 v_ w. KREMER ETAI 2,773,752 cRYs'rALLIzATIoN oF somma CYANIDE: Filed July 20, 1953

INVENTORJ` Vince-,w+ W. Kœmer Charles H. Lemke.

ATToRp/:y fr* f 1C@ 2,773,752 United States Patent O Patented Dec. 1 1, 4 1956 1 2 solution to increase, resulting eventually in a product containing more hydroxide than any specifica tion for sodium cyanide could tolerate. An additional 2,173,752 object of the invention is development of a continuous CRYSTALLIZATION oF soDIUM CYANIDE process for recovering white sodium cyanide from aque ous solutions in which the concentration of sodium Vincent W. Kremer and Charles H. Lemke, Niagara Falls, N. Y., assignors to E. I. du Pont de Nemours & Com hydroxide in the solutions can be easily controlled. pany, Wilmington, Del., a corporation of Delaware The above-enumerated and other objects of thein vention may be accomplished by a process in which a Application July zo, 1953, serial No. 369,034 solution of sodium cyanide is continuously concentrated 4 Claims. (Cl. 2.3-302) in a vacuum evaporator within which the caustic con tent is maintained between critical limits and the re sultant crystals Withdrawn from the solution at a rela tively low temperature. Generally the solutions treated This invention relates to the crystallization of sodium in this manner will have been made by neutralizing cyanide and more particularly to the crystallization of with hydrocyanic acid. The invention sodium cyanide from solutions made up by neutralizing is, however, not restricted in applicability to solutions caustic with cyanide. prepared in this manner but may be extended to cover Wet methods for making sodium cyanide, that is meth solutions made in other Ways as well. The reaction of ods employing gaseous hydrogen cyanide to neutralize '_ cyanide with sodium carbonate in solution may aqueous caustic solutions, have been known for many for example also be used to prepare the sodium cyanide. years. Thus Roessler (U. S. P. 716,350) developed such Any carbonate or other precipitates formed should be a process las long ago as 1902. Although this patent removed as by filtration before the sodium cyanide is and others directed to the same type of process are quite recovered. old, the sodium cyanide of commerce has generally Whatever the source of the sodium cyanide solution, been prepared by the Castner process employing am it should be carefully adjusted to a sodium hydroxide monia introduced into a mixture of carbon and metallic content of at least (L2-1.0% by weight before it is passed sodium at an elevated temperature. The Castner process into the evaporator. The caustic content in the evapo has the advantage over the wet processes­ that it does rator may then be allowed to build up to about 2-3% not require separation of product sodium cyanide from 30 but this ñgure should not be exceeded or the final prod water. This separation is difficult because the cyanides uct will contain an undue percentage of diluent sodium tend to hydrolyze in aqueous solution yielding a variety hydroxide. Consequently, no large bath of original of decomposition products. A major object of this in mother liquor should be evaporated to complete dry vention is, therefore, development of a novel and useful ness. This diñìculty may be overcome by withdrawing , method for recovering sodium cyanide from aqueous some of the mother liquor and treating the withdrawn solution. portions with additional hydrogen cyanide. Preferably Among the hydrolysis products that may be formed a slurry containing some of the crystallized material in aqueous sodium cyanide solutions, especially when together with its mother liquor is removed from solu any attempt is made to crystallize the solid from the tion and filtered. At least part of the mother liquor solutions, is hydrogen cyanide. Formation of this com 40 comprising the filtrate may be then neutralized with pound obviously detracts from the over-all yield ob hydrogen cyanide and returned to the evaporator. tainable. Additionally, hydrogen cyanide has a very If the proper concentration of hydroxide is maintained marked tendency to polymerize into a series of com at the evaporator, hydrolysis of the sodium cyanide into pounds most of which are colored. These polymeriza hydrocyanic acid and polymerization of the latter into` tion products may appear as yellow or red solutes or discoloring impurities can be avoided. Temperature as heavy brown to black ñocculent precipitates. What and pressure conditions, however, still require careful ever the particular color formed in an evaporating solu control. Normal pressures require such a high boiling tion, it is generally carried over into thev crystalline temperature that noticeable decomposition and product product. Another object of this invention is consequently discoloration may occur even at high alkalinity. Excess development of a process for recovering white solid 50 alkalinity must at any rate be avoided during evaporation sodium cyanide from aqueous solution. Ito avoid serious impairment in lthe value of the sodium Despite the technological diiñculties inherent in the cyanide obtained. A causticr concentration of about wet processes, attention has recently been again drawn 2-3% can, as noted, insure high-test sodium cyanide in to them because of the increased demand for cyanides 55 good yield but only if the evaporation is carried out originating particularly in the artificial fibre industry. under about 40­l()0 mm. of mercury pressure and a These processes afford opportunity for the continuous temperature of 50-70" C. A preferred pressure range. reactions, as opposed to batch reactions, essential if the is between about 40 and 80 mm, of mercury. Lower increased demand is to be met. A further object of this pressures can, of course, be used but are not necessary. invention is development of a method for continuously 60 Details of the invention will be more readily under recovering white sodium cyanide from aqueous solution. stood from the following examples and from: lt has been found that decomposition of sodium The drawing, which shows schematically apparatus cyanide in aqueous solution can be substantially re for carrying out a preferred embodiment of the inven

pressed, even at _the relatively high temperatures and tion. - ' y ’ ~ » Í concentrations required for crystallizing the solid, by Example 1 maintaining a quantity of free sodium hydroxide in the 65 D evaporating solution. Free alkali has the further ad ' A 'control run was made to determine thev eiîects of vantage of preventing the polymerization of hydrogen evaporation on aqueous sodium cyanide. A solution of cyanide as long as the alkali is present in excess. Con 98.3% sodium cyanide in water with an initial concen trol of the amount of free sodium hydroxide in solution, tration of around 35% by weight was boiled to dryness however, presents a problem, particularly where a con 70 in 40 minutes at 40•70° C. and 28-50 mm. of mercury tinuous process is involved. In such a process there is pressure. The recovered product contained 93.4% a tendency for the caustic content of the evaporating sodium cyanide and 3.47% water. Removal of the 2,773,752 Y water gave sodium cyanide of only 96.5% purity. These yields an acceptable product with very slight decomposi ii'guresshow about 1.8% decomposition of the cyanide tion losses. Somel modifications in’theprocess are, m the relatively short period required for evaporation nevertheless, possible. Recîirculation of mother liquor and _at .the low vtemperatures Aand 4pressures employed., for neutralization may, for instance, be increased or de creased as desired. Decreasing the recirculation Will Example 2 ’ Y generally> increase the hydroxide content of the evapo A series' of runs was made to show the effect of rator while increasing the recirculation will reduce it. ^ sodium hydroxide additions on the decomposition rates In practice the hydroxide content ofthe dried cyanide. . ofnearly saturated (4U-42%) solutions of sodium cya is about one-third that of the mother liquor. Around nide.. Pressures utilized were atmospheric, A solution lO V1% is the maximum sodium hydroxide tolerable inthe f Ycontaining 0.5% sodium hydroxide in addition to the solid cyanide. The concentration of sodium hydroxide sodium cyanide showed, in one hour, 0.12% decomposi in the evaporator should not therefore be built up above tion at 60° C., 0.24% a't 70° C. and 0.85% at 80° C. about a,3% maximum with„2-2.5% Preferred.. A 'solution containing 1.0% hydroxide in addition to Having now described our invention, we claim: cyanide gave 0.1% decomposition in one hour­ at 60° C., 1. The method of producingwhite crystalline sodium 0.2%` in one hour at 70° C; and about 0.75% inone cyanide which comprises continuously feeding a con hour at 80°C; Use of,2.0% caustic in the saturated centrated solution of sodium cyanide into an evaporator V"sodiur'ri cyanide solutions reduced the decomposition in held at a temperature of 50_°-70° VC. and a pressure of one hour to about 0.05%v at 60°' C., 0.14% at 70° C., 40-80 mm. of mercury, forming a slurry in saidevapo and 0.58%"at 80° C. ' ' 20 rator comprising saturated mother liquor and crystals of sodium cyanide, maintaining a concentration of sodium . . Y Exàmpleâ v hydroxide of between >0.2 and 3% in said mother liquor A cyclic process was carried out in which a concen and continuously separating a portion of said crystals ‘ trated‘ solution of sodium hydroxide was continuously from said mother liquor and recycling ,a portion of said neutralized With'hydrogen cyanide to a content of 'about liquor. )38%V sodium cyanide and y0.2-l.0% sodium hydroxide. 2. The Jmethod of producing white crystalline sodium After neutralization, the alkaline cyanide solution was cyanide which comprises y continuously feeding a con continuously passed into an evaporator and concentrated centrated solution of sodium cyanide into an evaporator at a pressure of> about >60 mm. of mercury and a tem held `at a temperature of 50°-70° C. and a pressure of perature of 60-70° C. The slurry which formed in the 30 40-80 mm. of mercury, forming a slurry in said evapo evaporator was continuously Withdrawn and filtered. rator comprising saturated- mother liquor andy crystals The wet‘white product was removed anddried while a of sodium cyanide, maintaining a concentration of portion of the‘ñltrate was returned to the evaporator. sodium hydroxide of between 0.2 and¿3% in saidmother The remainder of the filtrate was recycledto the original liquor, continuously withdrawing part of the slurry from sodium hydroxide Asolution and again neutralized with the evaporator, ñltering >the crystals from the withdrawn the latter. By means of this recycling step the caustic portion of the slurry and recycling the ñltrateto the Í concentration in the evaporator was maintained at around evaporator. 2% andI prevented lfrom building up to higher levels. 3. The method of claim 2 in which the vconcentration . IV,The drawing’ shows schematically conventional ap of the sodium hydroxide is maintained at thedesired n paratus in which the process described'` here may be car 40 level. by reacting part of said ñltrate with hydrocyanic ried'out. Storage tank 10'is provided to retain and mix acid before returning the rsame tothe evaporator. the " required alkaline reagents. Line 11> introduces 4. The methodV of forming white crystalline sodium sodium hydroxide solution, partially neutralized yby hy cyanide which comprises continuously neutralizing a drogen' cyanide into tank 10. Through line 12 the concentrated solution of caustic soda with hydrocyanic liquid reactants are conducted ­to reactor 13 into which acid to a caustic:V content of 0.2-1.0%, continuouslyy Agaseous hydrogen cyanide is fed" through line 14. This feeding the resultant sodium cyanide solution into an :reactor 4may be of any convenient -form although a jet evaporator held at a temperature of 50-70° C. anda type *(not shown), is preferred. Sodium cyanide solution pressure of 40-80 mm. of mercury, maintaining a caustic containing (l2-‘1.0% sodium hydroxide is fed through concentration in the evaporator .of V0.2-3%, continuously line 15 into vevaporator 16 and thence through line 17 to 50 producing sodium» cyanide crystals in the evaporator, ñlter’lS. Solid is conveyed through line 19 to a dryer continuously removing .the crystals together with at and the ñltrate'through lines V20 and 21 to the evapo tendant mother liquor, iiltering the crystals from, ,the 'rator-16'and storage-tank 20 respectively. Line 21> per mother 'liquor and drying them with heat, recycling a mitsk the'r‘ecycling step necessaryl to prevent the caustic first portion of the mother liquor to the evaporator, content of the'evaporator from building' up to an yex Ul Ur neutralizing a second .portion _of said Ynriother liquor with ' treme level. Conventional pumps (not shown) are, of hydrocyanic acid to a caustic content of 0.2-1.0% and course, located to provide Vthe necessary liquid flow. returning-the secondìportion of.` neutralizedmother. liquor In one run of the process of this example, the feed'to to the evaporator. vthe'evaporator was at a rate of 4 parts per hour of whichabout 1.5 `parts consists of ñltrate recycled direct 60 RefereneesCìted inthe ñle of. this patent ' Y ly tothe evaporator, and the remainder of'fresh material. UNITED S'i‘ATE-SV PATENTS Slurry was ­continuously withdrawn ata rate of- about 3.4 parts _per hour and filtered to yield 0.5 part of 'white 716,350 Roessler '______- Dec. 16, 1902 crystals'carrying about one-third their Weight of mother 859,482 VBueb ______.. Iuly 9, 1907 liquor. " These crystals were then dried to givev a white ' ' 1,531,123 `Mittasch et al, ______Mar. 24, 1925 product analyzing about r96--98% sodium cyanide, less v2,365,417 Kusman _t ______Dec. 19, 1944 than 1% sodium hydroxide and less than 2% sodium 2,616,782' vvCain et> al. ______Nov. 4, ’1952 carbonate. '2,708,151 McMinn ___.. ______-_ May 10, ‘1955 It will be seen thatthe-cyclieprocess of-Exainple 3