TETRATHIOCARBONATES PROCESS

TETRATHIOCARBONATE PROCESS 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 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 byO'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. Patent 2,893,835, dated July 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: 2 KSH + CS2 = > K2CS3 + H2S (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 (H2CS3). 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.STDC0405In 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 (H2CS4). 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 perthiocar bonate was prepared in a similar manner, except that after reacting the ammonia and hydrogen sulfide, elemental sulfur was added to form the disulfide, (NH4)2S2; 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 andRobinson 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.STDC0236Consequently, 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.STDC0358 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 ComplexChemistry" 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 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: H2S + CS2 = > H2CS3 (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, straight-forward manner.

According to one of the processes of the present 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.STDC0387The reaction sequence is as follows: 2 NaOH + H2S = > Na2S + 2 H20 (3) Na2S + S = > Na2S2 Na2S2 + CS2 = > Na2CS4 (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.

According to another of the processes of the present invention aqueous solutions of tetrathiocarbonates can be produced in a continuous process in relatively high concentrations, such as about 30 percent by weight or more by continuously feeding water, a hydroxide, sulfur, carbon disulfide and hydrogen sulfide; continuously reacting the hydroxide, sulfur, carbon disulfide and hydrogen sulfide; and continuously recovering an aqueous solution of a salt of tetrathiocarbonic acid.

The reactions occurring simultaneously in a continuous process for the production of sodium tetrathiocarbonate are the following: 2NaOH + H2S = > Na2S + 2 H2O (3) Na2S + S = > Na2S2 (4) Na2S2 + CS2 = > Na2CS4 (5) Na2S + CS2 = > Na2CS3 (6) Na2CS3 + S = > Na2CS4 (7) Adding water, sodium hydroxide, molten sulfur, carbon disulfide and hydrogen sulfide simultaneously but separately to a reactor and continuously reacting under controlled conditions results in a product which comprises an aqueous solution having a concentration of 30 percent by weight or more of sodium tetrathiocarbonate. These tetrathiocarbonate solutions are stable and directly toxic to many plant pathogens, breaking down in soil to release carbon disulfide, which acts as a fumigant.STDC0155Tetrathiocarbonates are biodegradable, producing sulfates and carbonates, and leave no residue in the soil or in plants treated with tetrathiocarbonates.

BRIEF DESCRIPTION OF THE DRAWING The sole figure of the drawing is a schematic representation of a flowsheet in partial elevation showing a system for conducting the continuous process of the present invention including a continuous stirred tank reactor and its auxiliary equipment.

BATCH PROCESS In the batch 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 tetrathio-carbonate, ammonium tetrathiocarb onate, 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 intro duced 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.STDC0284Concentrations 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 400 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.STDC0260A 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 350 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 900F., the reactor should be maintained at a temperature of at least about 1100 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 1100 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 1400 F. to assure reaction of the sulfur with the sodium sulfide. Temperatures of about 1400 to about 1700 F. are desirable. Sulfur is added in the molten state at a temperature of about 2800 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 50 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 135-1400 F. has been found optimum. Temperatures of about 1200 to about 1600 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 1350 to about 1400 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 700 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.

CONTINUOUS PROCESS In the continuous process of the present invention, a hydroxide, hydrogen sulfide, sulfur and carbon disulfide are reacted continuously in approximately stoichiometric quantities in a water medium to produce aqueous tetrathiocarbonate solutions preferably having concentrations of about 15 percent by weight or more, more preferably concentrations of about 30 to about 55 percent, and most preferably about 40 to about 50 percent by weight of the tetrathiocarbonate salt.

Higher concentrations are particularly desirable because increased throughputs are possible and shipping costs are lower. Although the reactants are typically present in stoichiometric quantities, an excess of about 5 percent to about 10 percent by weight of the hydroxide and hydrogen sulfide have been found desirable to stabilize the product tetrathiocarbonate solutions. Especially preferred are an excess of about 10 percent of the hydroxide and about 5 percent of the hydrogen sulfide.STDC0451 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 introduced simultaneously and continuously and thoroughly mixed, and in which the reaction temperature can be maintained as desired. Temperature control can be provided by either external or internal heat exchangers. Pressure is not a major consideration. Superatmospheric pressures, usually in the range of about 5 psig. to about 30 psig. are typically employed. A continuous stirred tank reactor (CSTR), for example, is ideally suited for conducting the process of the present invention.

A continuous process for the manufacture of sodium tetrathiocarbonate requires that all chemical reactants be essentially completely reacted in order to ensure that the reactions depicted in Reactions (3), (4), (5), (6) and (7) are effective to produce the desired product and to avoid severe operating problems.

For instance, if sulfur is not completely reacted, it will remain as a solid and plug product filters and piping. If CS2 is not completely reacted, it will vaporize and significantly increase the reactor pressure. Moreover, CS2 will also compete with Na2S and Na2 CS3 for sulfur, resulting in incomplete reaction and plugging of piping and equipment.

In a continuous process all possible reaction paths are competing. The dominant reaction path is the one having the fastest global kinetics. Which reaction path will predominate can, to some extent, be determined by choice of reaction conditions, such as temperature, concentration and residence time.

For example, at low temperatures and low concentrations the reaction path which predominates is the path defined by reactions (3), (6) and (7), whereas at high temperatures the predominant reaction path is the path defined by reactions (3), (4) -and (5).

Reaction (3), which is common to both paths, is an ionic reaction and is very fast. This reaction, which is the fastest reaction in the entire sequence of reactions, is instantaneous and complete.

If the temperature and the product concentration are both low, the reaction of CS2 with Na2S (Reaction 6) and the reaction of CS2 with Na2S2 (Reaction 5) have the next higher reaction rates. The reaction rate of Reaction 5 is mass transfer limited and depends upon the concentration of CS2 in the aqueous phase, in which the reaction occurs. This reaction is slower at higher temperatures because the solubility of CS2 in water decreases with increasing temperature.

The reaction of Na2S with CS2 (Reaction 6) is kinetically limited at low temperatures and increases with increasing temperature until the solubility becomes limiting.

The slowest reactions and those most sensitive to temperature are the addition of sulfur to Na2S (Reaction 4) and the addition of sulfur to Na2CS3 (Reaction 7). Normally, molten sulfur is present in the form of an eight-membered ring structure, S8, as monoclinic sulfur. The effect of heat and the presence of hydroxide ions is to open the sulfur ring, thereby transforming the ringed monoclinic sulfur to a linear form. The resulting linear form is either a linear organized structure, rhombic sulfur, or a linear unor ganized structure, amorphous sulfur. Either Na2S or Na2 CS3 will swallow the sulfur molecules at one or both ends of the sulfur chain to form, in the case of Na2S, a polysulfide, Na2Sx such as Na2S2 or, in the case ofNa2CS3, the corresponding tetrathiocarbonate, Na2CS4.

Increasing the temperature increases the rate of both sulfur reactions, but particularly the rate of the reaction of sulfur with Na2CS3, which is the slowest reaction and is rate limiting under most operating conditions. The reaction path which predominates at lower temperatures and lower concentrations is therefore the path defined by Reactions 3, 6 and 7.

At higher temperatures and concentrations the reactions of CS2 become rate limiting because the solubility of CS2 in the aqueous reaction phase is depressed by the high temperature and the high concentration of Na2CS4. As the temperature increases, the rate of Reaction 6 increases until it is limited by the solubility of CS2. At this time Reaction 6 proceeds at the same rate as Reaction 5. As a result, for higher concentrations and lower residence times, the maximum temperature is limited. At higher temperatures, the sulfur reactions proceed at a faster rate, so that the reaction path which predominates at higher temperatures is the path defined by Reactions 3, 4, and 5.

In general, a continuous process requires thatreactor volume, reactor temperature, reactant feed rates and product compositions be correlated so that sufficient residence time is provided for complete conversion to the desired product. Compared to a batch process, where the required reaction time can be readily provided, in a continuous process the residence time determines the reaction time.

A continuous process is decidedly advantageous, first and foremost because a continuous process is generally the most convenient and efficient way to conduct chemical reactions, assuming the intended chemical reaction or reactions can be run continuously.

Production rates of continuous processes can be several times those of batch processes. For example, whereas a batch process may be capable of producing about 4,000 pounds per day of an aqueous solution of sodium tetrathiocarbonate having a concentration of about 30 percent to about 35 percent by weight, the continuous process of the present invention can produce about 15,000-20,000 pounds of product solution per day using the same reactor size.STDC0258 Another advantage of the continuous process of the present invention is that it is capable of producing concentrations of sodium tetrathiocarbonate higher than those produced in a batch process, that is, concentrations of up to about 55 percent by weight.

Even higher concentrations can be produced using the process of the present invention. However, at concentrations over about 55 percent by weight the reaction rate falls off because the solubility of CS2 in the aqueous reaction phase is reduced and because carbonate contaminants introduced with the water begin to precipitate. The latter problem can be circumvented by using carbonate-free and bicarbonate-free water.

Referring now to the drawing a continuous stirred tank reactor 10 is equipped with a sulfur feed conduit 12, a water supply conduit 14, a caustic (NaOH) conduit 16, a carbon disulfide conduit 18, a nitrogen (N2) conduit 20 and a hydrogen sulfide conduit 22.

Conduit 18 extends well into reactor 10. Conduit 20 contains a valve 24 and conduit 22 contains a valve 26.

Reactor 10 is also equipped with a double stirrer 28 attached to a motor 30. Reactor 10 is shown containing a solution 32. Reactor 10 is fitted with a sight level gauge 34. A heat exchanger 36 is connected to reactor 10 via a 3-way temperature control valve 37, a conduit 38, filters 40, a conduit 42, a pump 44 and a conduit 46. A conduit 48, also connected to 3-way valve 37, serves as a bypass line for heat exchanger 36. A conduit 50 connects heat exchanger 36 to a conduit 52, which serves as a recycle line and a conduit 54, which is connected to a level control valve 55, and serves as a connection to storage ctanks, not shown. A scrubber 56 is equipped with a caustic tank 58 and a recycle line 60, which is provided with a pump 62. Scrubber 56 is connected to reactor 10 by means of a conduit 64.STDC0141An emergency scrubber 66 is connected to reactor 10 by means of a conduit 68 equipped with a rupture disk type of pressure relief valve 70.

In carrying out the process of the present invention, water is introduced into reactor 10 by means of water supply conduit 14. Although shown entering the top of the reactor, water can be introduced into the reactor at either the top or the bottom. Water is usually introduced at ambient conditions. However, under low temperatures it may be necessary to heat the water in order to prevent freezing.

The hydroxide is introduced into the reactor at the top or bottom as desired. In the drawing caustic is shown entering reactor 10 at the top via caustic conduit 16. Depending upon the particular tetrathiocarbonate being produced, the corresponding hydroxide such as sodium hydroxide, potassium hydroxide, ammonia, lithium hydroxide, calcium hydroxide or magnesium hydroxide, for example, is introduced as an aqueous solution. Concentrations of about 10 percent to about 70 percent by weight are typically used. Preferably concentrations of about 20 percent to about 60 percent and more preferably about 40 percent to about 50 percent are used. The hydroxide can be introduced under ambient conditions. However, it is important that the temperature of the hydroxide feed be kept above its freezing temperature.

Sulfur is fed into reactor 10 at the top via sulfur feed conduit 12. Sulfur in the molten state is sprayed into the reactor through the vapor space above the surface of the liquid. Preferably the molten sulfur is normally introduced through a heated nozzle to prevent sulfur from plugging the nozzle in the relatively colder operating temperature of the reactor.

The molten sulfur is normally introduced at a temperature of about 2500 F. to about 3000 F., preferably about 2750 F. to about 2850 F. Typically, particle sizes of the sprayed molten sulfur droplets of about 1/32 inch to about 3/8 inch in diameter are used.

Particle sizes of less than about 1/8 inch in diameter have been found to produce the best results. Care should be taken that the molten sulfur particles do not contact any metal surface within the reactor in order that the sulfur in the molten state is available for reaction instead of solidifying on metal surfaces within the reactor.

Carbon disulfide is introduced into reactor 10 via carbon disulfide conduit 18. It is introduced below the surface of the liquid in the reactor in order to make it available for reaction in solution 32 and to prevent its contacting sulfur particles in the vapor space above solution 32. Preferably the carbon disulfide is introduced close to stirrer 28 to ensure that the carbon disulfide is dispersed in the solution quickly and evenly. The carbon disulfide can be introduced at ambient conditions.

Hydrogen sulfide gas is introduced into reactor 10 via hydrogen sulfide conduit 22. The volume introduced is regulated by means of valve 26. The hydrogen sulfide is preferably sparged into solution 32 below stirrer 28 to ensure thorough mixing within the liquid phase and to take advantage of the hydrostatic head of the liquid within the reactor in effecting complete reaction of the hydrogen sulfide.

The process is commenced by filling the reactor with a heel of product and purging the vapor space above the solution with nitrogen gas introduced into reactor 10 via nitrogen conduit 20. The flow of nitrogen is controlled by means of valve 24. Nitrogen exits reactor 10 via conduit 64 which connects reactor 10 with scrubber 56. When the oxygen content in the reactor 10 is less than about 0.3 weight percent, valve 24 is closed to stop the flow of nitrogen. Simultaneously water, a hydroxide, hydrogen sulfide, molten sulfur and carbon disulfide are fed into the reactor with the result that the tetrathiocarbonate product is produced continuously as long as reactant feed rates are maintained.

Product is pumped out of reactor 10 by means of pump 44 through conduits 46 and 42 to filters 40 which function to remove any solids such as sulfur from the product stream. Filtered product is flowed to 3way temperature control valve 37 which-maintains the temperature of reactor 10 by directing the product stream either wholly or partly to heat exchanger 36 or bypass conduit 48. Most of the cooler product recycles back to reactor 10 via conduit 52. The rest of the product flows through conduit 54 to level control valve 55 to storage. Level control valve 55 maintains the liquid within reactor 10 at the desired operating level.STDC0358 Gases collected in the vapor space of the reactor above the liquid level are vented from the reactor via conduit 64 through caustic scrubber 56. An emergency scrubber is provided as shown in the drawing. A rupture disk type of pressure relief valve 70 activates emergency scrubber 66 when the pressure within reactor 10 builds up to an unacceptable level.

As pointed out above, in order for the process of the present invention to produce desired product, reactor volume, reactor temperature, reactant feed rates and product composition are correlated. It is usual to express feed rates in terms of residence times, since residence time is inversely proportional to feed rate. The correlation between residence time and temperature of reaction, measured as temperature of the liquid phase, for various product compositions is shown in Table 1.

Table 1 ProductConcentration Residence Time TemperatureWeight Percent Hours OF.

31.8 1.25 180 2.8 160 3.4 140 4.2 130 40 3.0 140 4.5 130 48 3.8 130 4.7 120 6.3 120 52 3.6 130 In general, temperatures of about 1100 F. to about 1800 F. at residence times of about 1 to about 7 hours are effective to produce concentrations of product tetrathiocarbonate in the range of about 30 percent by weight to about 55 percent by weight.

Especially preferred are temperatures of about 1300 F.

to about 1600 F. and residence times of about 2.5 to about 4.5 hours. Most preferred are temperatures of about 1400 F. to about 1500 F. and residence times of about 3 to about 4 hours.

The following example describes a process for manufacturing a 31.8 percent by weight aqueous solution of sodium tetrathiocarbonate. It is intended for purposes of illustration only and is not to be construed as implying any limitation on the scope of the present invention, which is defined in the appended claims.

EXAMPLE A quantity of 140 gallons of a 31.8 percent by weight aqueous solution of sodium tetrathiocarbonate was introduced as a heel into a 200 gallon continuous stirred tank reactor. The reactor was purged with nitrogen and heated to 1400 F. Water at a feed rate of 226 lbs./hr., a 50 percent by weight aqueous solution of sodium hydroxide at a feed rate of 151.1 lbs./hr., molten sulfur at a feed rate of 27.4 lbs./hr., carbon disulfide at a feed rate of 65 lbs./hr. and hydrogen sulfide gas at a feed rate of 30.5 lbs./hr. were continuously fed into the reactor. The molten sulfur was introduced at a temperature of 2850 F. The reactor was maintained at a temperature of 1450 F. and a pressure of 15 psig. A 31.8 percent by weight aqueous solution of sodium tetrathiocarbonate at a rate of 500 lbs./hr.STDC0028was continuously produced.

The present invention thus provides a continuous process for producing salts of tetrathiocarbonic acid conveniently as stable aqueous solutions of high concentrations, for example, in the range of about 30 to about 55 percent by weight.

The invention may be embodied in other forms without departing from the spirit or essential charac teristics 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.