Tetrathiocarbonate batch process

Salts of tetrathiocarbonic acid are produced by a batch process in which a hydroxide, hydrogen sulfide, sulfur and carbon disulfide are reacted sequentially. The salts are produced as aqueous solutions having concentrations of upwards of 30 percent by weight.

FIELD OF THE INVENTION 
This invention relates to the manufacture of salts of tetrathiocarbonic 
acid. In one of its more particular aspects this invention relates to a 
batch process for manufacturing aqueous solutions of tetrathiocarbonates 
on a commercial scale. 
BACKGROUND OF THE INVENTION 
The chemistry of thiocarbonic acids and their salts has been studied in 
some detail, as indicated by O'Donoghue and Kahan, Journal of the Chemical 
Society, Vol. 89(II), pages 1812-1818 (1906); Yeoman, Journal of the 
Chemical Society, Vol. 119, pages 38-54 (1921); Mills and Robinson, 
Journal of the Chemical Society, Vol. 128(II), pages 2326-2332 (1928) and 
by Stone et al. in U.S. Pat. No. 2,893,835, dated Jul. 7, 1959. 
According to O'Donoghue and Kahan, as far back as 1826 derivatives of 
thiocarbonic acid were prepared by Berzelius, who reacted aqueous 
solutions of hydrosulfides with carbon disulfide to give unstable 
solutions which yielded unstable crystalline salts in accordance with the 
following reaction: 
EQU 2 KSH+CS.sub.2 =&gt;K.sub.2 CS.sub.3 +H.sub.2 S (1) 
Other thiocarbonates were prepared and further characterized by O'Donoghue 
and Kahan. Their paper, at page 1818, reports the formation of ammonium 
thiocarbonate by reacting liquid ammonia with cold alcoholic thiocarbonic 
acid prepared by dropping a solution of calcium thiocarbonate into 
concentrated hydrochloric acid to produce free thiocarbonic acid (H.sub.2 
CS.sub.3). The calcium thiocarbonate utilized by the authors is described 
as a double salt, including the calcium cation in combination with both 
the hydroxide and the trithiocarbonate anions. In addition to free 
thiocarbonic acid, other compounds prepared by O'Donoghue and Kahan 
included the sodium, potassium, zinc and lead salts. However, regardless 
of which of these salts were prepared, a common characteristic was their 
relative instability, with the prepared compounds breaking down and 
releasing carbon disulfide and hydrogen sulfide and/or a metal sulfide, 
often in a matter of minutes. 
The noted paper by Yeoman reports a further study of thiocarbonates (called 
trithiocarbonates therein) and also reports the preparation and properties 
of perthiocarbonates (or tetrathiocarbonates), derivatives of 
tetrathiocarbonic acid (H.sub.2 CS.sub.4). Yeoman reports on methods of 
preparing the ammonium, alkali metal and alkaline earth metal salts of 
these acid species. For example, Yeoman prepared ammonium trithiocarbonate 
by saturating an alcoholic ammonia solution with hydrogen sulfide and then 
adding carbon disulfide to precipitate the product salt. Ammonium 
perthiocarbonate was prepared in a similar manner, except that after 
reacting the ammonia and hydrogen sulfide, elemental sulfur was added to 
form the disulfide, (NH.sub.4).sub.2 S.sub.2 ; adding carbon disulfide 
immediately precipitated the product. 
Yeoman states that solutions of both ammonium trithiocarbonate and 
perthiocarbonate are very unstable due both to decomposition to form 
thiocyanate as a product, and to complete dissociation back into ammonia, 
hydrogen sulfide and carbon disulfide. 
Considerable explanation is provided concerning the stability of 
thiocarbonates, as exemplified by sodium trithiocarbonate and 
perthiocarbonate. Sodium trithiocarbonate solutions in water are said to 
remain stable only if oxygen and carbon dioxide are rigidly excluded; the 
presence of oxygen causes decomposition to form carbon disulfide and 
thiosulfates, while carbon dioxide decomposes the solution to form a 
carbonate, elemental sulfur, carbon disulfide and hydrogen sulfide. 
Potassium trithiocarbonate behaves similarly, according to Yeoman. 
Yeoman also attempted to prepare and characterize the stability of 
thiocarbonate salts of four of the alkaline earth metals. Yeoman was 
unable to prepare a pure calcium tri- or tetrathiocarbonate, but did 
observe that the double salt of calcium trithiocarbonate which he prepared 
was more stable (probably because it was less hygroscopic) than the sodium 
or potassium thiocarbonates. The barium salt of tetrathiocarbonic acid 
could not be isolated, although Yeoman believed it existed in solution. 
Solid barium trithiocarbonate could not be isolated, although it was 
alleged to behave like sodium trithiocarbonate when dissolved in water. 
The preparation of aqueous solutions of the tri- and tetrathiocarbonates 
of magnesium and strontium was alleged, but the magnesium thiocarbonates 
were not isolated. 
The previously noted paper by Mills and Robinson shows the preparation of 
ammonium thiocarbonate by digesting ammonium pentasulfide (obtained by 
suspending sulfur in aqueous ammonia, then saturating with hydrogen 
sulfide) with carbon disulfide. A crystalline residue from the reaction 
was found to be ammonium perthiocarbonate. The authors prepared a "better" 
ammonium perthiocarbonate product, however, by extracting the ammonium 
pentasulfide with carbon disulfide in a Soxhlet apparatus. 
Stone et al. disclose several methods for preparing solid ammonium, alkali 
and alkaline earth metal salts of tri- and tetraperoxythiocarbonates, 
hereinafter referred to simply as "tetrathiocarbonates." One such method 
involves the solution of an active metal such as sodium in anhydrous 
ethanol to form an ethoxide which, in turn, is reacted with hydrogen 
sulfide and carbon disulfide to form sodium trithiocarbonate. They report, 
however, that the trithiocarbonates tend to be quite soluble in ethanol, 
and if it is desired to recover the solid material from the solution, it 
is necessary to treat the reaction mixture with a "displacing agent" such 
as ether, in which case the thiocarbonates frequently separate, not as 
solids, but as difficultly crystallizable oils which appear to be 
saturated aqueous solutions of the trithiocarbonate salt. Consequently, 
such a procedure is not considered feasible for use on a commercial scale. 
Similar problems were reported with tetrathiocarbonate salts, which were 
prepared using procedures analogous to those for the trithiocarbonates. 
These problems were reportedly solved by carrying out the preparation 
reaction in a medium which is composed of a major part of a nonsolvent for 
the reaction components and a minor proportion of a liquid which is 
miscible with the nonsolvent and which is a solvent, to a measurable 
degree, for inorganic sulfides. The preferred nonsolvents used were 
relatively low boiling hydrocarbon materials such as hexane, cyclohexane 
and benzene. The second solvent was preferably ethanol, isopropanol or 
dioxane. 
Basic physical and chemical properties of these materials and a number of 
methods for making them are summarized in considerable detail, starting at 
page 154 in "Carbon Sulfides and their Inorganic and Complex Chemistry" by 
G. Gattow and W. Behrendt, Volume 2 of "Topics in Sulfur Chemistry", A. 
Senning, Editor, George Thieme Publishers, Stuttgart, 1977. 
What is needed is a process for the manufacture of salts of 
tetrathiocarbonic acid which is less cumbersome than the processes 
previously used. Such process should be capable of providing aqueous 
solutions of tetrathiocarbonates on a commercial scale. The present 
invention provides such a process. 
SUMMARY OF THE INVENTION 
The present invention provides a batch process for the production of salts 
of tetrathiocarbonic acid which is capable of providing aqueous solutions 
of tetrathiocarbonates in concentrations useful for various commercial 
applications, such as in the control of nematodes and other soil-borne and 
water-borne pathogens. 
Although it might be expected that hydrogen sulfide and carbon disulfide 
would react to form trithiocarbonic acid according to the reaction: 
EQU H.sub.2 S+CS.sub.2 =&gt;H.sub.2 CS.sub.3 ( 2) 
this does not occur. The present invention provides a process which is less 
cumbersome than prior processes and which can be readily practiced in a 
simple straightforward manner. According to the process of the pressent 
invention, it has been found that tetrathiocarbonates can be produced in 
concentrations of upwards of 30 percent in water by means of a batch 
process in which, for example, sodium hydroxide reacts with hydrogen 
sulfide to produce sodium sulfide in an exothermic reaction; the sodium 
sulfide thereby produced reacts with elemental sulfur in an endothermic 
reaction to produce sodium disulfide; and the sodium disulfide thereby 
produced reacts with carbon disulfide to produce sodium tetrathiocarbonate 
in an exothermic reaction. The reaction sequence is as follows: 
EQU 2 NaOH+H.sub.2 S=&gt;Na.sub.2 S+2 H.sub.2 O (3) 
EQU Na.sub.2 S+S=&gt;Na.sub.2 S.sub.2 ( 4) 
EQU Na.sub.2 S.sub.2 +CS.sub.2 =&gt;Na.sub.2 CS.sub.4 ( 5) 
Adding the reactants shown in Reactions (3), (4) and (5) above sequentially 
under controlled conditions results in a product which comprises aqueous 
solutions of tetrathiocarbonates in concentrations of upwards of 30 
percent by weight. These tetrathiocarbonate solutions are directly toxic 
to many plant pathogens, breaking down in soil to release carbon 
disulfide, which acts as a fumigant. Tetrathiocarbonates are 
biodegradable, producing sulfates and carbonates, and leave no residue in 
soil or plants treated with tetrathiocarbonates.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the process of the present invention, a hydroxide, hydrogen sulfide, 
sulfur and carbon disulfide are reacted in approximately stoichiometric 
quantities in a water medium to produce aqueous tetrathiocarbonate 
solutions having concentrations of 30 percent by weight or more, 
preferably concentrations of about 31 percent to about 35 percent by 
weight. 
The description of the invention will proceed using sodium 
tetrathiocarbonate as an example of the tetrathiocarbonates to which the 
present invention is directed. It should be understood, however, that 
other tetrathiocarbonates, such as potassium tetrathiocarbonate, ammonium 
tetrathiocarbonate, lithium tetrathiocarbonate, calcium tetrathiocarbonate 
and magnesium tetrathiocarbonate can be similarly prepared by using the 
corresponding hydroxide. 
The process can be conducted in any convenient reaction vessel in which the 
reactants can be thoroughly mixed and which can be heated or cooled to 
control the reaction temperature. Pressure is not a major consideration 
since pressures in the range of about 15-30 psig. are sufficient for the 
process. Heating and cooling can be provided by either external or 
internal heat exchangers. A stirred tank reactor, for example, is 
satisfactory for conducting the process of the present invention. 
In order to ensure that the reaction path followed in the batch process of 
the present invention is the desired path illustrated in Reactions (3), 
(4) and (5), it is essential that the reactants be introduced into the 
reactor in the proper order in the proper quantities and at the optimum 
temperatures for the reactions to proceed as desired. The following 
description of a typical run outlines the reaction conditions and other 
considerations which are important in achieving the results desired. 
A 6000 gallon stirred tank reactor is flushed with nitrogen to provide an 
inert atmosphere essentially free of oxygen. The oxygen level is usually 
less than about 1.0 percent by weight and preferably less than about 0.3 
percent by weight. Water is then added to the reactor at a rate of 30,000 
lbs./hr. for a period of 46 minutes. Sodium hydroxide is added to the 
reactor in about a 5 percent to about a 15 percent excess, preferably 
about a 10 percent excess. The sodium hydroxide is added as a 50 weight 
percent solution at a rate of 22,500 lbs./hr. for a period of 43 minutes 
in the first stage of the process. This results in a concentration of 
about 25 percent by weight. Concentrations of about 10 percent to about 50 
percent and preferably about 15 percent to about 35 percent by weight can 
be used. The sodium hydroxide solution is preferably introduced into the 
reactor above the liquid surface. During this time the temperature rises 
by about 40.degree. F. 
In the second stage of the process, which is exothermic, hydrogen sulfide 
is added to the sodium hydroxide solution at a rate of 1,600 lbs./hr. for 
about 2 hours to provide no more than about a 5 percent excess. The 
hydrogen sulfide, which is added as a gas, is preferably introduced as 
near the bottom of the reactor as possible to allow the hydrostatic head 
of the reactor contents and the agitation to be effective in reacting the 
hydrogen sulfide with the sodium hydroxide (Reaction 3). An excess of 
hydrogen sulfide over the 5 percent excess mentioned above should be 
avoided, since the excess hydrogen sulfide will eventually cause a 
pressure build-up in the reactor. Typically, hydrogen sulfide gas may have 
up to about 1-2 percent by weight of inerts, which will simply cause a 
pressure build-up in the reactor and can be removed by venting. A 
continuous flow of about 16-33 lbs./hr. to an external scrubber, for 
example, is sufficient to vent inerts and relieve pressure build-up. 
Alternatively, pressure build-up due to inerts can be relieved by venting 
at the end of the hydrogen sulfide addition. The reaction between sodium 
hydroxide and hydrogen sulfide, as pointed out above, is exothermic. A 
temperature rise of about 35.degree. F. results. 
Since sodium sulfide, which is formed upon the addition of hydrogen sulfide 
to the diluted sodium hydroxide solution in the second stage of the 
process, begins to precipitate at temperatures below about 90.degree. F., 
the reactor should be maintained at a temperature of at least about 
110.degree. F. Superatmosphere pressures of about 2.5 psig. to about 10 
psig are adequate. The heat produced upon mixing the sodium hydroxide 
solution with water and the exothermic reaction with hydrogen sulfide is 
usually sufficient to prevent precipitation of the sodium sulfide product. 
However, if the temperature of the reactor following the addition of 
hydrogen sulfide to the sodium hydroxide solution is insufficient to keep 
sodium sulfide in solution, heat may be added to the reactor by means of a 
heater or steam jacket to maintain a temperature of about 110.degree. F. 
Agitation of the reactants is essential during this and succeeding stages. 
For beginning the third stage of the process, the addition of sulfur, the 
temperature should be above about 140.degree. F. to assure reaction of the 
sulfur with the sodium sulfide. Temperatures of about 140.degree. to about 
170.degree. F. are desirable. Sulfur is added in the molten state at a 
temperature of about 280.degree. F., preferably by spraying into the vapor 
space above the liquid contents of the reactor. The particle size of the 
sprayed sulfur particles is preferably less than 1/8 inch in diameter. 
Contact between molten sulfur droplets and metal surfaces inside the 
reactor should be avoided. Sulfur is added at a rate of 1,500 lbs./hr. for 
about 2 hours. The reaction of sulfur with sodium sulfide (Reaction 4) is 
endothermic, resulting in about a 5.degree. F. temperature drop. It is 
essential that there be no unreacted sulfur present when carbon disulfide 
is added in the fourth stage of the process. Since sulfur is extremely 
soluble in carbon disulfide, any unreacted sulfur will preferentially be 
held in the carbon disulfide phase rather than being available for 
reaction with the sodium sulfide in accordance with Reaction 4, thereby 
reducing the yield of product. 
For the reaction between sodium disulfide and carbon disulfide (Reaction 5) 
to proceed at a reasonable rate, a temperature of about 
135.degree.-140.degree. F. has been found optimum. Temperatures of about 
120.degree. to about 160.degree. F. can be used. The reactor pressure is 
typically about 5 psig to about 20 psig, preferably about 10 psig to about 
15 psig. Carbon disulfide is added below the surface of the liquid reactor 
contents at a rate of 2,800 lbs./hr. for about 2.5 hours. The temperature 
can be maintained at about 135.degree. to about 140.degree. F., with 
cooling if necessary, since the reaction is exothermic. Venting is 
undesirable, since the carbon disulfide must be prevented from leaving the 
reactor in order to insure an optimum yield of sodium tetrathiocarbonate. 
Consequently, agitation and recirculation of the resulting solution should 
continue until all the carbon disulfide has reacted, which could take as 
much as several hours. During the reaction the pressure may rise by about 
10 psig to about 20 psig. 
The resulting product is an absolutely clear solution, containing neither 
unreacted sulfur, which would result in a cloudy product, nor unreacted 
carbon disulfide, which would appear either as a separate phase or as 
bubbles of cloudiness. The product is orange-red in color and has a slight 
sulfur odor. The specific formulation described above produces 5000 
gallons of 31.8 percent by weight of sodium tetrathiocarbonate in water 
and has a specific gravity of about 1.20 to about 1.30, typically 1.26 at 
70.degree. F. The slight excesses of sodium hydroxide and hydrogen sulfide 
utilized in the process of the present invention have been found to help 
hold the active carbon disulfide component more tightly in solution, 
thereby reducing odor and making the product more stable. 
Thus there has been provided a batch process for producing salts of 
tetrathiocarbonic acid as aqueous solutions having concentrations in the 
range of about 30 percent by weight to about 35 percent by weight which 
are relatively stable and yet capable of releasing carbon disulfide under 
conditions of use. 
The invention may be embodied in other forms without departing from the 
spirit or essential characteristics thereof. For example, as pointed out 
above, other salts of tetrathiocarbonic acid than sodium 
tetrathiocarbonate can be prepared using the process of the present 
invention. Consequently, the present embodiments are to be considered only 
as being illustrative and not restrictive, with the scope of the invention 
being indicated by the appended claims. All embodiments which come within 
the scope and equivalency of the claims are therefore intended to be 
embraced therein.