Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
AN IMPROVED PROCESS FOR P~EPARING ANHYDROUS HCN
FIELD OF THE INVENTION
~ This invention relates to a process for producing anhydrous hydrogen cyanide and
r~ 5 more particularly to a batch or continuous process for producing anhydrous hydrogen
cyanide from sodium cyanide and sulfuric acid.
BACKGROUND OF THE INVENTION
A laboratory method for preparing HCN from NaCN and sulfuric acid followed by
batch distillation using a reactor/still vessel can be found in The Handbook of P~ .~d~i~e
Inorganic Chemistry 1965, p. 65g. In the above handbook disclosure, the HCN was made
anhydrous by removal of the water fraction in the vapor stream by absorption in conc~ d
sulfuric acid rather than by ~ till~tion.
There is a need for a convenient way to prepare anhydrous HCN economically in
quantities lower than could be Justified by large standard commercial methods such as the
Andrussow process which uses arnmonia, m~th~ne and catalyst at high temperature. Such a
process becomes increasingly attractive for small quantity HCN users as commercial
m~nnf~ctllrers of HCN become more reluctant to ship the m~t~.rj~l in ~ny kind of container.
The process invented here provides such needs and also permits production of the anhydrous
HCN as needed for direct use in commercial processes (eg. pesticides, ph~rm~ceuticals) and
subst~nti~lly lowers the arnount of on-site storage ofthe product.
SUMMARY O~ THE INVENTION
A batch or continuous process has been discovered for the preparation of anhydrous
HCN by reacting alkali metal or ~Ik~lin~-earth metai salts of cyanide with mineral acids in a
solvent at a temperature of from 10 to 60~C and a pH from 0 to 4 to form a reaction product
comprising HCN in solution, transferring the liquid reaction product to a fractional
tion column and recovering tlle anhydrous llydrogen cyanide from the fractionating
column. This process results in low polymer formation and high product yield.
DETAILS OF T~IE INVENTION
The batch and continuous processes of the invention have been developed to offer safe,
practical methods of producing anhydrous HCN from its sodium salt and results in low
polymer formation and high chemical yield on a scale of .05-5MM lb/yr. The low polymer
formation is key to being able to achieve practical, cornmercial scale fractional ~ till~ion of
the resulting aqueous solution in high yield. The following reaction conditions further defne
the invention.
The process of the invention can be carried out batchwise or c-mtin~lously. In the batch
mode, all the acid can be charged first~ or it can be co-fed with the cyanide salt solution. The
pH should be acidic throughout the reaction from pH ~ to 4 preferably below 3 to prevent
HCN polymer formation. In canying out the continuous process, the cyanide salt solution
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and acid are co-fed in the mole ratio described herein to a reactor with agitation. The
discharge from the reactor is controlled by its level. Since the reaction is immediate in both
the batch and continuous process, from a practical standpoint, there is no upper or lower
residence time on this reactor.
In the process of the invention, the only known suitable solvents are water or an
aqueous reaction product recycled from the fractio~iating column which is typically present
at between about 50-99% by weight water. The reaction temperature should be m~int~;ned
between 10 and 60~C, with the preferred temperature range between 20 and 40~C toI"inimi7f HCN hydrolysis, and to maintain solubility of the HCN and salt by-products in the
10 reaction mass.
The mineral acids of the invention are sulfuric and phosphoric. However, sulfuric acid
is preferred for economy and environmental reasons. Before feeding into the reactor, the
mineral acid is diluted below 90% with water, or the recycled reaction product from the
bottom of the distillation column. Preferably, the sulfuric or phosphoric acid concentration is
15 25-45% on a salt-free basis or 15-35% including the salts in the recycled reaction product
from the bottom of the fii~ tion column. A lower concentration can be used, but aqueous
waste increases and the batch reactor and the distillation column become larger as a result of
higher dilution. Somewhat higher concentrations can be tolerated, up to 70%, but increased
yield loss to formate results. It may be necessary to add NaOH to stabilize aqueous NaCN.
20 Anyone skilled in the art would know the amount of NaOH to add. For reasons of safety and
economy tlle dilution of the mineral acid is preferably below a concentration of 50% with
the water or the stripped salt solution recycled from the bottom of the fractionating column.
The major consideration here is the amount of water to be introduced to the reactor. There
must be sufficient water to prevent large amounts of the by-product salts from precipitating
25 out of solution during the reaction and prior to distillation.
Alkali metal and alkaline-earth metal salts of cyanide can be used, however, thepreferred salt is sodium cyanide for reasons of availability and economy. The cyanide salt
should be dissolved completely in water before introduction into the reaction process.
The mole ratio of cyanide salt to acid is related to the number of available acidic
30 protons. In theory there are two available protons for (H2SO4) and three available protons for
(H3PO4). For example, the mole ratio of salt (NaCN) to acid (H2SO4) is less than 2 and the
mole ratio of salt (NaCN) to (H3PO4) is less than 3. Using the preferred salt (NaCN) and
acid (H~SO4~, the preferred mole ralio should be between 1.5 and 1.85 for environmental
reasonst economy and safety. The preferred mole ratio for NaCN and H3PO4 should be
35 between 2.5 and 2.85.
The cyanide is added to the acidic reactor employillg typical techniques that ensure
good micro-mixing at the addition poinl. It is importallt lo minillli~c llle lillle tllat tllc
hydrogen cyanide might "see" a pH of abo~e 4 to minimi~ HCN polymer formation.
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Addition time is not important and is only limited by the capacity of tlle equipment to
remove tl1e heat generated by the reaction. The reaction occurs immediately at all
temperatures outlined above.
The water/HCN/salt reaction product feed to the- fractionating column must be low in
S HCN polymer for the purpose of reliability. A dark (high polymer) reaction product will
Iead to fouling and pluggage of the column that will require clean-out more often. Typically
the ~CN concentration in the reactor before transfer to the fractionating column is 1-10%
preferably 3-7%. Higher concentrations of HCN can lead to increased vaporization loss and
polymer formation at the operating te~ c;ldLures.
Preferred is the process wherein anhydrous HCN is prepared in a batch or continuous
process by reacting sodium cyanide with sulfuric acid in a reactor where the solvent is water,
the reaction temperature range is between 20 and 40.~C, the mole ratio of sodium cyanide to
sulfuric acid is between 1.5 and 1.85, the cyanide is added employing good micro-mixing
teclmiques followed by transferring the water/HCN/salt reaction product to a fractionating
column and separating the desired anhydrous HCN. What is meant by anhydrous HCN is at
least 98.5% HCN.
The process of the invention produces less HCN polymer and results in less hydrolysis
of the HCN and is therefore superior to processes known l3resently.
The process of this invention can be further illustrated by the following examples. In
~he examples the slightly yellow tinted solution indicates low HCN polymer content.
EXAMPLE I
Preparation of HCN - Batch Process
To a one liter resin kett~e reactor equipped with mechanical stirring were charged 153.5
g 98% H2SO4 and 382.0 g water. In a separate vessel, 126 g of 99% NaCN were totally
dissolved in 495.4 g water and 5.0 g 50% NaOH at room temperature. Using a 250 ml
reservoir and a low flow charging pump, the NaCN solution was charged to the reactor
subsurface over the period of one hour while m~int~ining the reactor temperature at 25 ~C.
The resulting water/salt/HCN slightly yellow tinted solution was fractionally distilled by
feeding to the middle of a 20 plate jacketed Oldershaw column equipped with an electrically
heated round bottom flask reboiler with discharge pump, refrigerated glycol cooled
condenser and timed (li~till:~te reflux controller and round bottom distillate receiver. HCN
distillate obtained: 57.3 g, 83.2% crude yield, water content 0.06%. HCN yield from
cyanide mass balance in liquid streams: 86.9%. Chemical yield loss to measurable by-
products in liquid streams (formate): <1.0%.
E7~AMPLE 2
Preparation of HCN - Batcil Process
A series of six batch reactions were carried out in aiacketed, mechanically agitated,
3000 gallon (11,350 liter) glass lined steei reactor. The reactor was equipped with an
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external circulating cooler loop and feed lines for aqueous sodium cyanide, sulfuric acid, and
water/recycle brine from the previous distillation batch. Provisions were included to allow
premixing and cooling in-line of the sulfuric acid and water/recycle brine streams while
charging the reactor. The reactor was inerted and vented to an aqueous scrubber.The 66 degree Baume (93.7%) sulfuric acid charge of 2970 lb (1348 Kg) was mixed
in-line with 9735 Ib (4420 Kg) of recycle brine (20-25 wt % salts) and cooled in line to 35
~C while feeding the reactor over 7~ to 80 minutes. With agitation, and cooling via the
reactor jacket, 15,397 Ib (6990 Kg) of prechilled (15 ~C) 14.3 wt % aqueous sodium cyanide
was metered in over 80 to 90 minutes while m~int~ning the reactor temperature at 35 C.
} 0 Tlle sodium cyanide feed was introduced subsurface near the agitator to ensure good mixing
with an endpoint pH of less than 3Ø
Tlle resulting 4.3wt% HCN slightly yellow aqueous brine mixture was then fed to the
middle of a packed glass lined fractional distillation column equipped with a stearn-heated
reboiler, refrigerated brine condenser, and a reflux accumulator with reflux feed controls.
Distillate hydrogen cyanide was fed directly to the user. Approximately 30 to 40% of the
stripped bottoms brine was accl-ml]1zlted and recycled for sulfuric acid dilution in the next
batch. HCN crude yield basis distillate (6 batch ave.): 91.9%, 1115 Ib, 507 kg, bp 26-27 ~C.
HCN yield basis complete cyanide balance on all liquid streams including liold-ups and
distillation bottoms, (6 batch ave.): 99.6% (1208 Ib, 549 kgj. Average HCN yield loss to by-
products ~formate) 0.4%. Yield loss to distillation in bottoms is 2.4 to 6.0%, 5.5% average
and varies with the particular experimental equipment used.
EXAMP'LE 3
Prel~aration of HCN - Continuous Process
An apparatus consisting of a reaction and distillation system in series was set up. The
reaction system was made up of a 1 liter resin llask, with cooling, mechanical stirring, two
charging reservoirs with their accompanying charging metering pumps feeding to the reactor
subsurface, dry ice condenser, and a reactor discharge metering pump for transfer to the
distillation system. The distillation system consisted of a 20 plate Oldershaw column, round
bottoln flask reboiler with discharge pump, timed distillate reflux control, refrigerated glycol
3G cooled condenser, and fli~til1~1e receiver. Both the reactor and (1i~t~ tion systems were
vented to a common aqueous scrubber.
The reactor and distillation column reboiler each were charged with 256.4 g of a 2.5%
H~SO4 solutio1l. In separate flasks, reactant solutions were made up: 1) NaCN solution
consisted of 281.3 g 99% NaCN, 1114.7 g water and 11.3 g 50% NaO~I, and 2) a 43%sulfuric acid solution consisted of 331.2 g 98% H2SO4 and 423.8 g water. The NaCN
solution and the 43% sulfuric acid solution were partially charged separately to the two
charging reservoirs which were then started simult.lllcously, mahltaining cvcn llows and a
reactant mole ratio of 1.7 moles cyanide per mole sulf~lric acid over a period o~about 4 hrs.
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Botll additiolls were made subsurface near the agitator for good mixing. Cooling was
applied to the reactor as needed to mailltaill the 50 ~C temperature. The reactor charging
reservo;rs were replenished with solutions as needed until they were consumed. A volume of
approximately 250 ml was maintained in the reactor and the slightly yellow tinted reactor
discharge was pumped to the midpoint of the distillation column and distilled in a continuous
manner. The distillate was collected in two fractions. The HCN ~ tiIl~te obtained: 1st cut
41.3 g HCN, .205% water, 2nd cut 69.2 g HCN, 1.33% water. Total: 110.5 g ~ii.ctill~t~.
Crude yield: 71.2% HCN. Yield calculated from cyanide balance of liquid streams: 74.6%
HCN. Chemical yield loss from measurable by-products in liquid streams (formate): < 1.0%
~XAMPLE 4
Preparation of HCN - Batch Process
A series of 20 batch reactions were carried out in the reactor and distillation equipment
as described in Example 2.
The reaction and distillation systems were water flushed after each batch, and this flush
15 (average 3259 lb; 1480 kg) was returned to the reactor. To the flush the following charges
were made; a) 3030 lb ~1375 kg) average of 93% sulfuric acid co-fed with 9465 Ib (4297 kg)
average recycle brine cooled in-line as previously described; b) 13144 Ib (5967 kg) average
of 17 wt% prechilled aqueous sodium cyanide metered as previously described while
m~int~inillg the 35 degree C reaction temperature, with an endpoint pH of less than 3Ø
The resulting 4.3 wt% ~ICN slightly yellow aqueous brine was then distilled as
previously described with 30 to 40% of the stripped bottoms brine recycled into the next
batch for sulfuric acid dilution.
