WO1996011165A1

WO1996011165A1 - Process for synthesizing thiocarbonates

Info

Publication number: WO1996011165A1
Application number: PCT/US1995/010105
Authority: WO
Inventors: Donald C. Young
Original assignee: Entek Corporation
Priority date: 1994-10-11
Filing date: 1995-08-09
Publication date: 1996-04-18
Also published as: ZA957035B; AU3279295A; TW366326B; IL115009A0

Abstract

Aqueous thiocarbonate solutions are synthesized in a batch or continuous reactor at a pressure below the critical pressure of carbon disulfide and at a temperature (at the pressure of the reaction) above the boiling point of carbon disulfide. The reactor is fitted with a condenser for preventing the escape of carbon disulfide vapor from the reaction zone. The batch synthesis procedure yields a product solution having a carbon disulfide equivalent concentration of about 26 weight percent and takes about 62.5 minutes to complete (not including the time required to add the reactants to and remove the final product from the reactor).

Description

PROCESS FOR SYNTHESIZING THIOCARBONATES

BACKGROUND The present invention relates to the synthesis of thiocarbonates in an aqueous reaction medium.

The chemical reaction employed in a commercial batch process for producing an aqueous sodium

tetrathiocarbonate solution takes about 6 hours to complete (not including the amount of time required to add the reactants to and remove the final product from the reactor) and yields a product having a carbon

disulfide equivalent concentration of about 13 weight percent. (As used in the specification and claims, the "carbon disulfide equivalent concentration" of an aqueous solution is determined by the following equation

wherein [T₁] is the concentration of a first

thiocarbonate in the aqueous solution, mwT₁ is the molecular weight of the first thiocarbonate, [T_n] is the concentration of the n^tn thiocarbonate in the aqueous solution, mwT_n is the molecular weight of the n^th

thiocarbonate, and mwCS₂ is the molecular weight of carbon disulfide (namely, about 76.14). For example, in an aqueous solution containing about 31.8 weight percent sodium tetrathiocarbonate (Na₂CS₄) and having no other thiocarbonate present in the solution, n equals 1, mwT₁ equals about 186.25 (i.e., the molecular weight of sodium tetrathiocarbonate), and the carbon disulfide equivalent concentration in the solution is about 13 weight percent.)

SUMMARY OF THE INVENTION In order to increase agronomic profitability, there is a need to increase the carbon disulfide

equivalent concentration of aqueous thiocarbonate

solutions made by a batch process. In addition, there is a need to decrease the batch reaction time.

The present invention satisfies both needs by providing a method for synthesizing thiocarbonates where the chemical reaction of the batch process (not including the time required to add the reactants to and remove the final product from the reactor) is generally completed in less than about two hours, and the resulting aqueous thiocarbonate solution typically has a carbon disulfide equivalent concentration of at least about 15 percent weight percent. The method comprises reacting carbon disulfide with at least one additional reactant in an aqueous medium present in a reactor or reaction zone and is characterized in that the reaction is conducted at a pressure below the critical pressure of carbon disulfide and at a temperature (at the pressure of the reaction) above the boiling point of carbon disulfide.

DRAWINGS

The decreased reaction time of the batch thiocarbonate synthesis procedure of the present

invention and the increased carbon disulfide equivalent concentration of the resulting aqueous thiocarbonate solution, as well as other features, aspects, and

advantages of the invention will become better understood with reference to the following description, appended claims, and accompanying drawing where:

Figure 1 is a schematic representation of an apparatus employed in the current thiocarbonate synthesis process and embodying features of the present invention; and

Figure 2 is a schematic representation of a pilot plant apparatus employed in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

The thiocarbonates synthesized by the process of the present invention generally have the formula I

where M₁, M₂, and M_z are each independently selected from the group consisting of inorganic and organic cations; al is the valence of M^* a2 is the valence of M₂; az is the valence of M_z; each of b1, b2, and bz is 0 or a positive integer (i.e., 1 or a whole number greater than 1), provided that bl + b2 + ... + bz equals a positive integer; R^ and R₂ are each independently selected monovalent organic groups; each of cl and c2 is 0 or 1; d is at least 3; and e is a positive integer, provided that (al•b1 + a2•b2 + ... + az•bz) equals (2 - c1 - c2)•e.

(As used in the specification and claims, the term

'"inorganic cation" means a cation devoid of even one carbon atom; the term "organic cation" means a cation containing at least one carbon atom; and the term

"organic group" means a group containing at least one carbon atom.) More typically, the thiocarbonates

prepared by the process of the present invention have the formula II b )

where M₁, M₂, M_Z, a1, a2, az, b1, b2, bz, d, and e are as defined above. Most often, the process of the present invention is employed to synthesize thiocarbonates having the formula III

(M^+a)_b(CS_d)^-2 (III) where M is selected from the group consisting of

inorganic and organic cations; a is the valence of M; b is a positive integer; d is as defined above; and a_*b equals 2. (The discussion in remaining portion of the specification regarding M is equally applicable to M₁, M₂, and M_z.)

Preferred inorganic cations for M are ammonium ion, alkali metal cations (especially sodium and

potassium), alkaline earth metal cations (especially magnesium and calcium), and transition metal cations (especially the micronutrients zinc, iron, manganese, copper, and molybdenum).

Organic cations include, but are not limited to, compounds having the formula IV

where X₁, X₂, X₃, X₄, and X_n are independently selected from the group consisting of hydrogen and C₁-C₅₀ organic groups; f1 is the valence of X₁; f2 is the valence of X₂; f3 is the valence of X₃; f4 is the valence of X₄; fn is the valence of X_n; Q is selected from the group

consisting of nonmetals, semi-metals, and metalloids; each of g1, g2, g3, g4 , and gn is 0 or 1; and h is the valence of Q, provided that at least one of X₁, X₂, X₃, X₄, or X_n is an organic group, the total number of carbon atoms in the organic cation is l to 60, and

f1 + f2 + f3 + f4 + ... + fn is a positive integer less than h (generally equal to h - 1 or h - 2) .

The most suitable organic cations have the formula V

where X₁, X₂, X₃, X₄, Q, and h are as defined above; and each of f1, f2, f3, f4 , g1, g2, g3, and g4 is 1. The organic groups employed for R₁, R₂, X_χ, X₂,

X₃, X₄, and X_n can be derived from aliphatic, alicyclic, or aromatic compounds, and include straight chain, branched chain, and cyclic structures. The organic groups can be, for example, substituted or unsubstituted alkyl, alkenyl, alkynyl, aryl, arylalkyl, or alkylaryl groups, and can include heteroatoms such as oxygen, sulfur, nitrogen, and phosphorus. Generally, each R± , R₂, X₁, X₂, X₃, X₄, and X_n comprises up to about 20, preferably up to 12, more preferably up to 8, even more preferably up to 6, and most preferably up to 4, carbon atoms. Each of R₁, R₂, X₁, X₂, X₃, X₄, and X_n is also preferably a hydrocarbyl group (e.g., methyl, ethyl, n- propyl, isopropyl, n-butyl, sec-butyl, t-butyl, isobutyl, 'hexyl, octyl, phenyl, and benzyl), with the most

preferred hydrocarbyl groups being alkyl groups.

The total number of carbon atoms in the organic cation is usually 1 to 40, preferably 1 to 20, more preferably 1 to 10, and most preferably 1 to 8. Q is preferably nitrogen, phosphorus, arsenic, or antimony, with the corresponding organic cations being primary, secondary, tertiary, and quaternary ammonium, phosphonium, arsonium, and stibonium cations. More preferably, the organic cation is a quaternary ammonium cation.

While the thiocarbonate is preferably a tetrathiocarbonate (d = 4), the thiocarbonate can also be a pentathiocarbonate (d = 5), a hexathiocarbonate (d = 6), and a septathiocarbonate (d - 7). In fact, d can be 8 or more.

The thiocarbonate synthesis process of the present invention is carried out at a temperature above the boiling point of carbon disulfide. For example, the following Table I indicates several reaction pressures and their corresponding minimum reaction temperatures, i.e., their respective carbon disulfide boiling points.

Generally, the reaction is conducted at a temperature at least about 5°C (9°F), more often at least about 10°C (18°F), even more often at least about 20°C (36ºF), and most often at least about 30ºC (about 54ºF), above the boiling point of carbon disulfide. Preferably, the thiocarbonate synthesis is performed at a temperature at least about 35°C (63°F), more preferably at least about 40°C (72ºF), and even more preferably at least about 45°C (81°F), and most preferably at least about 50°C (about 90ºF), above the carbon disulfide boiling point. The maximum reaction temperature is the

decomposition temperature of the thiocarbonate being synthesized. To illustrate, for the preparation of sodium tetrathiocarbonate at atmospheric pressure, the maximum reaction temperature is about 150°C (302°F).

The thiocarbonate synthesis process of the present invention is conducted at a pressure equal to or greater than about atmospheric pressure, but below the critical pressure of carbon disulfide, i.e., below about 7386.6 kpascal (72.9 atm). Because of convenience, it is preferred to run the reaction at about atmospheric pressure.

With reference to the Figure 1, the system 10 is employed in one batch synthesis process of the present invention. More specifically, (a) water and (b) sulfur and/or one or more sulfur-containing compounds are initially charged to a reactor 12 fitted with a stirrer 14 or other mixing device. The sulfur-containing

compounds have a formula selected from the group

consisting of H₂S_i, HR₁S_i, R₁R₂S_i MHS_i , (M+^a)_bS_i, and 'MR₁S_i, where M, R₁ , R₂, a, and b are as defined above (provided that a_*b equals 2) and i is at least 1. When the sulfide-containing compound is a monosulfide, i is 1, and when the sulfide-containing compound is a polysulfide i is greater than 1. Generally, i is from about 1 to about 5, and preferably 2 to about 5. In order to further stabilize the resulting aqueous thiocarbonate solution, the total amount of sulfur and sulfur-containing compound charged to the reactor 12 is generally in excess of the stoichiometric amount required to react with the carbon disulfide to form the thiocarbonate. Preferably, the total amount of sulfur and sulfur-containing compound charged to the reactor 12 is at least about 102, more preferably at least about 104, even more preferably at least about 106, and most preferably at least about 108, percent of the amount required for a stoichiometric reaction.

One or more bases are also initially charged to the reactor 12 when at least one of the charged sulfur-containing compounds is selected from the group

consisting of H₂S_i, HR₁S_i, and MHS_i, where R₁, M, and i are as defined above. In addition, even when sulfur or one of the sulfur-containing compounds selected R₁R₂S_i, (M^+a)_bS_i, and MR₁S_i is employed in the reaction, it is desirable to initially charge a base to the reactor to stabilize the resulting thiocarbonate solution.

Preferably, the amount of base charged to the reactor 12 is at least about 102, more preferably at least about 104, even more preferably at least about 106, and most preferably at least about 108, percent of the amount required for a stoichiometric reaction.

Virtually any organic or inorganic base is 'used. Exemplary organic and inorganic bases are listed in the Handbook of Chemistry and Physics, 65th Edition, Weast el al. Editors, CRC Press, Inc., Boca Raton,

Florida (1984), pages D-163 to D-165 (hereinafter

referred to as the "Handbook"), which Handbook is

incorporated herein in its entirety by reference.

Preferably, the base employed in the present invention is selected from the group consisting of alkali metal

hydroxides (especially, sodium hydroxide and potassium hydroxide), alkaline earth metal hydroxides, and ammonium hydroxide.

After charging the water, sulfur and/or the sulfur-containing compound, and, if used, the base to the reactor 12, a heat exchanger 16 then adjusts contents of the reactor to a temperature, at the corresponding reaction pressure, above the boiling point of carbon disulfide. The heat exchanger 16, which can either heat or cool the contents of the reactor, is in thermal communication with the reactor 12. For example, as shown in Figure 1, a portion of the liquid phase 18 present in the reactor 12 is withdrawn through a conduit 20 and passes through the heat exchanger 16 where its

temperature is adjusted to the desired level. The temperature adjusted portion of the liquid phase 18 next passes through another conduit 22 to the intake side of a pump 24 and, after being discharged from the pump 24, enters the reactor 12 via a return conduit 26.

After the reactor 12 reaches the desired temperature, carbon disulfide is then fed into the reactor 12 through a conduit 28 that is in fluid

communication with the return conduit 26. The carbon disulfide is fed into the reactor 12 at a rate that does not exceed the capacity of a condenser 30 to condense substantially all the carbon disulfide vapor attempting to escape the reactor 12 through the condenser 30. The condenser 30 is maintained in fluid communication with the reactor 12 by a conduit 32. The condenser 30 is maintained at a temperature that condenses carbon

disulfide vapor to liquid at the operating temperature of the reactor 12. Gases (e.g., air, oxygen) and volatile materials having a boiling point less than carbon

disulfide exit the condenser 30 through a conduit 34 and enter a scrubber (not shown). The scrubber removes the volatile materials from the gaseous effluent, while allowing innocuous gases, such as air and oxygen, to vent to the atmosphere.

To ensure that the carbon disulfide feed rate remains within the condensation capacity of the condenser 30, a thermocouple (not shown) is inserted into the condenser 30 to measure the temperature within the condenser 30. The carbon disulfide feed rate is adjusted as needed to maintain the temperature measured by the thermocouple slightly below the boiling point of carbon disulfide so that carbon disulfide vapor entering the condenser 30 is condensed and returned to the reactor 12 (via the conduit 32) where it revaporizes. When the temperature measured by the thermocouple rises and before it reaches the carbon disulfide boiling point, the carbon disulfide feed rate is reduced so that the condensation capacity of the condenser 30 is not exceeded.

It is preferred that the thiocarbonate synthesis process of the present invention be conducted under a substantially oxygen free atmosphere. Generally, the oxygen level in the vapor phase or atmosphere 36 in the reactor 12 is less than about 1, preferably less than about 0.75, more preferably less about 0.5, and most preferably less than about 0.25, weight percent. The oxygen content in the vapor phase 36 is reduced as the carbon disulfide fed into the reactor 12 displaces the oxygen from the reactor 12. As noted above, oxygen leaving the reactor 12 through the conduit 34 vents to the outside atmosphere after passing through the

condenser 30 and the scrubber. Another batch synthesis process of the present invention differs from the preceding batch process in that sulfur is fed into the reactor 12 mixed with the carbon disulfide. Accordingly, in this batch version, water and one or more sulfur-containing compounds having a formula selected from the group consisting of H₂S_i, HR₁S_i, R₁R₂S_i, MHS_i, (M^+a)_bS_i, and MR₁s_i, wherein M, R₁, R₂, a, b, and i are as defined above, are initially charged to the reactor 12. At least one base is also initially charged to the reactor 12 when either one or more of the charged sulfur-containing compounds is selected from the group consisting of H₂S_i, HR₁S_i, and MHS₁, wherein M, R₁, and i are as defined above, and/or a base is employed to stabilize the final aqueous

thiocarbonate solution. Next, the reactor 12 is heated by means of the heat exchanger 16 to a temperature, at the corresponding reaction pressure, above the boiling point of carbon disulfide. After the contents of the reactor 12 reach the desired temperature, the sulfur- carbon disulfide mixture is fed into the reactor 12 via the conduit 28 at a rate not exceeding the condensation capacity of the condenser 30.

The reaction employed in the batch

thiocarbonate processes of the present invention is preferably completed (not including the time required to add the reactants to and remove the final product from the reactor 12) in less than about 1.75 hours. More preferably, the reaction is finished in less than about 1.5, even more preferably less than about 1.25, and most preferably less than about 1.1, hours.

The resulting aqueous thiocarbonate solutions prepared by the batch processes often have a carbon disulfide equivalent concentration of at least about 16, more often at least about 17, even more often at least about 18, and most often at least about 19, weight percent. Because agronomic profitability increases as the carbon disulfide equivalent concentration of the solution increases (all other factors being held

constant), the carbon disulfide equivalent concentration in the aqueous thiocarbonate solutions prepared by the batch processes of the present invention is preferably at least about 20, more preferably at least about 21, even more preferably at least about 22, and roost preferably at least about 23 weight percent. Typically, the aqueous thiocarbonate solutions prepared by the batch processes of the invention have a carbon disulfide equivalent concentration less than about 35, more typically less than about 33, and most typically less than about 30, weight percent.

The reduced reaction times and higher carbon disulfide equivalent concentrations, in comparison to the above-noted commercial batch process, achieved by the batch process of the present invention are attributed to the fact that the reaction medium employed in the present process consists of a substantially homogenous single aqueous phase. The presence of a substantially

homogeneous single aqueous phase is due to the reaction being run at a temperature above the boiling point of carbon disulfide (at the pressure of the reaction) .

Accordingly, the liquid phase 18 or aqueous reaction medium contains little, if any, carbon

disulfide. More specifically, the concentration of carbon disulfide in the aqueous reaction medium at any time during the synthesis procedure (i.e., when the reaction temperature (at the pressure of the reaction) is above the boiling point of carbon disulfide) is generally less than about 0.1, preferably less than about 0.05, more preferably less than about 0.01, even more

preferably less than about 0.005, and most preferably less than about 0.001, weight percent (based on all the ingredients present in the reaction zone at the time of measurement).

In contrast, since the commercial batch reaction is run at a temperature less than the boiling point of the carbon disulfide (at the pressure of the reaction), the reaction medium of the commercial batch process is a non-homogenous medium having, at a minimum, a carbon disulfide liquid phase and an aqueous phase.

Since sulfur that dissolves in the carbon disulfide phase is not readily available to react with at least one reagent (e.g., sulfides) present in the aqueous phase, the presence of a separate carbon disulfide phase

adversely affects the reaction rate. In addition, aqueous thiocarbonate solutions prepared using a non- homogenous reaction medium have less than 15 weight percent carbon disulfide equivalent concentrations.

The system 10 of Figure 1 can also be employed in a continuous thiocarbonate synthesis process. For example, in a continuous thiocarbonate synthesis process within the scope of the present invention, water and the various reactants are added to the reactor 12 at a predetermined stoichiometric rate and the resultant aqueous thiocarbonate solution is removed from the reactor 12 through a conduit (not shown) at an equivalent rate. Water and liquid reactants are typically

separately added to the reactor 12 above the liquid phase 18 through one or more conduits (such as conduits 38, 40, and 42). Any solid reactant is generally admixed with a liquid reagent, with the combination being introduced to the reactor 12. Gaseous reactants are preferably

introduced through one or more conduits (such as conduits 44, 46, and 48) which are in fluid communication with the conduit 26.

EXAMPLE

The following examples, which are intended to illustrate, and not limit, the invention, describe the preparation of concentrated sodium tetrathiocarbonate solutions in a laboratory (Example 1) and in a pilot plant (Example 2).

EXAMPLE 1

Laboratory Preparation Of A Concentrated Sodium Tetrathiocarbonate Solution

A three-neck, round-bottom flask was fitted with a heating mantle, a gas inlet tube, a liquid

addition funnel, a solids addition funnel, and a reflux condenser cooled with dry ice/isopropanol. The entire apparatus was maintained under a nitrogen pressure of about 5.08 cm (two inches) of water.

Water (about 335.1 g) and solid sodium hydroxide pellets (about 252.6 g) were added to flask. After the sodium hydroxide had dissolved (approximately 3 'minutes), hydrogen sulfide gas (about 102.1 g) was added to flask at about 1.5 g per minute. The temperature of the solution in the flask was maintained between about 65° to about 95°C. After completion of the hydrogen sulfide addition, crystalline sulfur (about 91.8 g) was added to the flask. The reaction of the sulfur was complete after about one minute to produce a dark red solution. Liquid carbon disulfide (about 210.1 g) was added to the flask at a rate of about 0.64 g per minute. The carbon disulfide was instantly vaporized on entering the flask. Any unreacted carbon disulfide was condensed in the reflux condenser and returned to the flask. After the reaction was complete, the product was a deep red liquid and an analysis of the product gave the results listed in the following Table A.

EXAMPLE 2

Pilot Plant Preparation Of A Concentrated

Sodium Tetrathiocarbonate Solution

The pilot plant consisted of a stainless steel reactor (about 1140 1 (300 gallon)) schematically shown 'in Figure 2 where the numerals refer to the parts listed below in Table B.

The reactor was charged with about 62.2 kg (137 pounds) of water, about 316 kg (697 pounds) of a 45 percent sodium hydrosulfide solution, about 193 kg (425 pounds) of a 50 percent sodium hydroxide solution, and about 76.3 kg (168 pounds) of crystalline sulfur. The mixture was reacted for about two hours at about 90°C. Then, about 183 kg (404 pounds) carbon disulfide was added to the reaction mixture over a period of about 90 minutes. The reaction temperature was maintained at about 80° to about 95°C during the carbon disulfide

'addition. The resulting product was a dark red solution and an analysis of the product gave the results shown in the following Table C.

Although the present invention has been

described in considerable detail with reference to some preferred versions, other versions are possible. For example, the heat exchanger 16 can be located on the discharge side of the pump 24 and/or the carbon disulfide feed conduit 28 can be located on the intake side of the pump 24. Alternatively, the carbon disulfide can be fed into the vapor phase 36 of the reactor 12 through one of the conduits 38, 40, or 42. In addition, instead of or in addition to the heat exchanger 16, the temperature in the reactor 12 can be adjusted by means of a coiled tubing heat exchanger (not shown) located inside the reactor 12. Therefore, the spirit and scope of the appended claims should not necessarily be limited to the preferred versions described herein.

Claims

1. A method for synthesizing a thiocarbonate comprising the step of reacting (I) carbon disulfide and (II) sulfur and/or at least one sulfur-containing

compound in an aqueous medium in a reaction zone,

where

the thiocarbonate has at least three sulfur atoms per thiocarbonate group;

the sulfur-containing compound has a formula

selected from the group consisting of H₂S_i, HR₁S_i, R₁R₂S_i, HMS_i, (M^+a)_bS_i' and MR₁S_i,

at least one base is present in the aqueous medium when at least one of the sulfur-containing compounds is selected from the group consisting of H₂S_i,

HR₁S_i, and MHS_i;

M is selected from the group consisting of inorganic and organic cations;

a is the valence of M;

b is a positive integer;

R₁ and R₂ are each independently selected from monovalent organic groups;

i is at least 1;

the reaction is conducted at a pressure below the critical pressure of carbon disulfide and at a temperature which, at the pressure of the reaction, is above the boiling point of carbon disulfide; and during at least a portion of the reaction at least a portion of any carbon disulfide vapor leaving the reaction zone is condensed and returned to the reaction zone.

2. The method of claim 1 where the thiocarbonate has of the formula ,

where

M_l, M₂, and M_z are each independently selected from the group consisting of inorganic and organic

cations;

a1 is the valence of M^

a2 is the valence of M₂;

az is the valence of M_z;

each of bl, b2, and bz is 0 or a positive integer, provided that bl + b2 + ... + bz equals a positive integer;

each of cl and c2 is 0 or 1;

d is at least 3; and

e is a positive integer, provided that (al«bl + a2•b2 + ... + az•bz) equals (2 - c1 - c2)•e.

3. The method of claim 2 where the

thiocarbonate has the formula

.

4. The method of claim 2 where the

thiocarboante has the formula (M₁ ^+a1)_b1(CS_d)^-2.

5. The method of any one of claims 2-4 where d is at least 4.

6. The method of any one of claims 1-5 where 'i is from 1 to about 5.

7. The method of any one of claims 2-6 where M₁, M₂, and M_z are each independently selected from the group consisting of alkali metals, alkaline earth metals, and ammonium.

8. The method of any one of claims 1-7 where R₁ and R₂ are independently selected from monovalent organic radicals containing up to about 12 carbon atoms per radical.

9. The method of any one of claims 1-7 where R₁ and R₂ are independently selected from monovalent organic radicals containing up to about 6 carbon atoms per radical.

10. The method of any one of claims 1-9 where the thiocarbonate comprises a tetrathiocarbonate.

11. The method of any one of claims 1-10 further comprising the step of condensing at least a portion of any carbon disulfide vapor leaving the

reaction zone and returning at least a portion of the condensed carbon disulfide to the reaction zone.

12. The method of any one of claims 1-11 where the reaction is conducted at a temperature that is at least about 5°C (9ºF) above the boiling point of carbon disulfide.

13. The method of any one of claims 1-12 where the reaction is conducted at about or above ambient pressure.

14. The method of any one of claims 1-13 where the synthesis is performed as a batch reaction.

15. The method of any one of claims 1-13 where the synthesis is performed as a continuous reaction.

16. The method of any one of claims 1-15 where the carbon disulfide equivalent concentration in the resulting product is at least about 15 weight percent.

17. The method of any one of claims 1-16 where the synthesis method comprises the step of reacting in the aqueous medium carbon disulfide, sulfur, H₂S, and a baβe.

18. The method of any one of claims 1-16 where the synthesis method comprises the step of reacting in the aqueous medium carbon disulfide, sulfur, MHS, and a base.

19. The method of any one of claims 1-18 further comprising the step of separately adding each reactant to the reaction zone.

20. The method of any one of claims 1-18 further comprising the step of adding a composition comprising sulfur and carbon disulfide to the reaction zone.

21. The method of any one of claims 1-20 where the concentration of carbon disulfide in the aqueous medium at any time during the synthesis procedure is less than about 0.1 weight percent based on all ingredients present in the reaction zone at that time.

22. The method of any one of claims 1-21 where the total amount of sulfur and sulfur-containing compound in the aqueous medium in the reaction zone is in excess of the stoichiometric amount required to react with the carbon disulfide present in the reaction zone to from the compound.

23. The method of any one of claims 1-22 where the amount of base present in the reaction zone is greater than the amount required for a stoichiometric reaction to form the compound.

24. The method of any one of claims 1-23 where the oxygen level in the vapor phase of the reaction zone is less than about 1 weight percent.