DECOMPOSITION DU THIOCYANATE D'AMMONIUM

DECOMPOSITION OF AMMONIUM THIOCYANATE

Abrégé anglais

7 TB2/88
ABSTRACT
A process for decomposing ammonium thiocyanate and
equivalent compounds comprising mixing the compounds with
molten ammonium sulfate at atmospheric pressure and about
310°C. The compounds decompose giving off carbon dioxide,
sulfur dioxide, ammonia and sulfur and leave a residue of
ammonia salts but no organic materials.

Revendications

TB2/88
CLAIMS:
1. A process for decomposing a compound selected
from the group consisting of ammonium thiocyanate, thiourea
or mixtures thereof comprising maintaining a quantity of
ammonium sulfamate in a molten state adding a less than molar
equivalent of the compound to said ammonium sulfamate and
maintaining the resulting mixture at atmospheric pressure and
a temperature greater than 250°C but less than 444.6°C until
completion of the reaction.
2. A process as claimed in Claim 1 wherein the mixture
is maintained at the specified temperature until no further
evolution of gases is evident.
3. The process as claimed in Claim 1 wherein the
compound is ammonium thiocyanate.
4. The process as claimed in Claim 2 wherein the
compound is ammonium thiocyanate.
5. The process as claimed in Claim 3 wherein the
temperature is maintained at about 310°C until no further
evolution of gas is evident.
6. The process as claimed in Claim 1 wherein the
compound is a mixture of ammonium thiocyanate and thiourea.
7. The process as claimed in Claim 6 wherein the
temperature is maintained at about 310°C until no further
evolution of gases is evident.

Description

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DECOMPOSITION OF AMMONIUM THIOCYANATE TB2/88
FIELD OF THE INVENTION:
This invention relates to the processing of coke
oven gases and, in particular, to the decomposition of ammonium
thiocyanate, thiourea, their equivalents and mixtures thereof
produced during the removal of hydrogen cyanide from coke
oven gas by aqueous ammonium polysulfide.
DESCRIPTION OF THE PRIOR ART:
It is clearly undesirable to release ammonium thio-
cyanate into the environment due to its toxic properties and it
has been proposed in the past to decompose the ammonium thio-
cyanate with various processes. These processes generally
were carried out at high pressure in an acidic aqueous medium.
An alternative process employed a reductive combustion process
which is difficult to control and expensive.
None of the preceding processes yielded any useful
products or, if so, only a limited amount of useful product.
The water processes required high pressure and expensive equip-
ment.
Typical of such prior processes is that of United
States Patent 1,984,757 issued December 18, 1934 which uses
sulfuric acid in an aqueous solution at temperatures of 150-
180C with the resultant pressure. The apparatus must be
constructed to withctand such pressure and also withstand the
corrosive action of the mixture.
An alternative process is disclosed in United States

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2 T~2/88
Patent 2,524,054 issued October 1950 which includes heating
the ammonium thiocyanate in the presence of alkali metal
sulfamates or ammonium sulfate to a temperature of 80 - 245C
to produce salts of guanidine. The reaction does decompose
the ammonium thiocyanate, but the remaining products are still
dangerous and undesirable.
SUMMARY OF THE INVENTION:
It is the purpose of this invention to provide a process
for decomposing ammonium thiocyanate or similar compounds at
atmospheric pressure with a reliable process that yields useful
output products. In the process, the ammonium thiocyanate
either as a solid or as a solution is heated with ammonium
sulfamate to a temperature at which the ammonium sulfamate
liquifies. The reaction proceeds to decompose the ammonium
thiocyanate producing sulfur, ammonia, carbon dioxide, sulfur
dioxide leaving a residue including ammonium salts.
BRIEF DESCRIPTION OF THE DRAWINGS:
The single figure of the drawings presents a schematic
block diagram of a process utilizing this reaction and producing
the desired output products.
DESCRIPTION OF THE PREFERRED PROCESS:
Considering the single figure, it is seen that the input
materials, ammonium thiocyanate and ammonium sulfamate, are
introduced into the reactor 1 where they are heated. The gases,
ammonia, sulfur dioxide and carbon dioxide bubble off freely.
With temperatures above the melting point of ammonium sulfamate,
the solid materials form a liquid as long as the temperature
does not exceed the boiling point of sulfur (444.6C). The

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3 TB2/88
gases evolved may be re-introduced by passing the gases
through bubbler 2 which contains sulfuric acid which reacts
with the ammonia to produce ammonium sulfate which can be
separated and re-introduced to reactor 1.
The remaining sulfur dioxide and carbon dioxide can
be bubbled through the incoming aqueous solution of ammonium
thiocyanate and polysulfide in bubbler 3. The remaining carbon
dioxide can be released to the atmosphere while the ammonium
thiocyanate is boiled in evaporator 4 to remove the water and
then introduced into reactorl as molten NH4SCN. The liquid
sulfur and ammonium salts may be tapped out of reactor 1 and
separated in separator 5. The reaction in the reactor will be
as shown in the following equation:
NH4SCN + (NH4) 2S04~~~S + 4NH3 2 2
The ammonium salt remaining in reactor 1 will be
primarily ammonium imido disulfate.
In the following examples the reaction was carried
out in a batch process in a reactor. Yields reported were
less than actual, because they were based on the weight change
of the reaction vessel plus contents, caused by the reaction.
The vessel retained sublimed material which reduced the change
in weight and thus the apparent yield.

4 Ts2/88
EXAMPLE #l
84 parts of ammonium sulfamate (NH4S03NH2) and 58
parts of ammonium thiocyanate by weight were heated to 370 C
over a 1.5 hr. period. The residue was free of thiocyanate
and organic matter and the apparent yield was 86%.
EXAMPLE #2
To amelt of 25 parts by weight of ammonium sulfamate
at 370C was gradually added, 6 parts of ammonium thiocyanate
(a molar ratio of 2 2/3 : 1). Additions were made at a rate
to maintain the melt above 280C. The reaction was complete in
less than lO minutes as indicated by the absence of further gas
production. The apparent yield was 70~.
In each of the examples the residue in the reaction
vessel was found to consist of ammonium sulfamate, ammonium
imido disulfate and sulfur. It will be noted that no hazardous
products remained, the organic material having been completely
dissociated.
While the examples given were batch processes, it
will be appreciated that in a commercial situation the continu-
ous process is to be preferred if possible. Example #2 isclosest to such a continuous process.
It will be understood that ammonium thiocyanate when
in liquid form will normally result in a equilibrium mixture
of ammonium thiocyanate and thiourealwhich may be termed an
equivalent of ammonium thiocyanate.
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