GB1584626A - Production of anhydrous sodium dithionite - Google Patents

Description

(54) PRODUCTION OF ANHYDROUS SODIUM DITHIONITE (71) We, VIRGINIA CHEMICALS INC., of 3340 West Norfolk Road, Portsmouth, Virginia 23703, United States of America, a corporation organized under the laws of the State of Maine, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for producing anhydrous sodium dithionite.

Hydrosulfites, also termed dithionites, are in demand as bleaching agents, such as for bleaching groundwood pulps. Zinc dithionite is being replaced by sodium dithionite because of the shortage and increasing cost of zine dust to produce zinc dithionite and because of ecological objections to disposal of zinc-containing wastes.

Sodium dithionite can be produced by electrolytic and borohydride procedures, but the most economical procedures for making a high-quality solid product increasingly use the formate radical as a means for reducing the valence of the sulfur atom.

This development began in 1933 with U.S. Patent No. 2,010,615 which discloses a method for producing anhydrous alkali metal dithionites by introducing gaseous sulfur dioxide into an aqueous methanol solution, which contains sodium formate and sodium carbonate and is held at a temperature below 30"C, and then bringing the SO2-methanol solution to the temperature at which sodium dithionite formation begins. In one Example, Na2CO3 is 19.0% of the sodium formate. This process requires a considerable excess of sodium formate to buffer the acidity of the solution and produces crystals of excessive fineness and low stability.

More than 30 years later, a succession of improvements, based on sodium hydroxide as the source of alkali, were disclosed, particularly including U.S. Patent Nos. 3,411.875, 3,576,598, 3,714, 340, 3,718, 732, 3,872, 221, 3,887,695, and 3,897,544; Japanese Patent Nos. 1003/68 and 2,405/71; and Belgium Patent No. 698,247.

These improvements comprise the addition of sulfur dioxide-containing methanol and an alkaline agent to an aqueous solution of an alkali metal formate, the resulting aqueous methanol solution being held at a reaction temperature above the dehydration point of a hydrated alkali metal dithionite in order to prevent the formation of crystals having water of crystallization occluded therewithin. The rate of addition must correspond to the rate of production of dithionite; if too rapid, the dithionite ion decomposes, thus reducing yield.

The improvements comprise absorbing sulfur dioxide in a water-miscible alcohol as a first feed solution, dissolving sodium hydroxide and sodium formate outside of the reactor in very hot water as a second feed solution. and feeding these two solutions into a reactor which is held at 60-90"C and contains a small amount of the alcohol under superatmospheric pressure.

High reactor concentrations are used to obtain high production per unit of reactor volume and per unit of alcohol volume, and methyl formate (which can be a by-product of a previous reaction) is dissolved in the methyl alcohol used as recipient in the reactor for the added solutions. particularly as taught in U.S. Patent No. 3.887,695.

Although each of these newer processes uses sodium hydroxide as the source of alkali and barely mentions sodium carbonate therefor, it is noteworthy that Japanese Patent Number 7.003/68 teaches absorption of sulfur dioxide in methanol to a suitable concentration and then gradually adding an alkaline aqueous solution including both sodium formate and sodium carbonate which is 33% by weight of the sodium formate. The yield is about 56% based on sulfur dioxide and 54% based on sodium.

Tables I and II give comparative experimental data and results for the 15 examples in the four most pertinent United States prior art patents. Data on dust characteristics of the products are not available.

From the examples showing experimental results in the prior art, it appears that: (a) sodium hydroxide has been routinely selected as the alkali source; (b) sodium carbonate has seldom been used but is often mentioned; and (c) in the few available examples that have employed sodium carbonate as the alkali source, the yield is decidedly inferior to sodium hydroxide in the production of sodium dithionite. Nevertheless, simply because sodium carbonate is considerably cheaper than sodium hydroxide, its use is highly desirable.

Moreover, production rate per unit of reactor volume is a critically important economic TABLE I U.S. 3,411,875 Ex. 1 Ex. 2 Ex. 3 Ex. 4 NaOH, parts 23 21 20 30 Na2CO3, parts -- -- -- - H2O, parts 200 240 136 100 HCOONa, parts 80 90 70 75 CH30H, parts 470 500 386 424 HCOOCH3, parts SO2, parts 80 87 72 100 Na2S204 (gross/100% basis) 4/76.0 90/78.3 72/64.1 104/94.2 Assay, % 90.5 91.2 89.0 90.6 Temp Range, "C. 60.70 70 70 70 Total Reaction Time, Hrs. 4.8 5.3 5.0 5.3 Productivity (lbs/hr/gal) * -- -- -- 0.191 Total Na equivalents 1.751 1.848 1.529 1.853 Total formate equivalents 1.176 1.323 1.029 1.103 Total SO2 equivalents 1.249 1.358 1.124 1.561 Total CH3OH equivalents 14.699 15.605 12.047 13.233 *The "gal" is U.s. gal.

TABLE II U.S.3,887,695 U.S.3,887,695 U.S.3,897,544 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 1 Ex. 2 Ex. 1 Ex. 2 Ex.3 Ex. 4 Ex. 5 NaOH, parts 23.2 30.0 23.2 23.2 550 650 28 32 - - 28 Na2CO3, parts - - - - - - - - 28 - H2O. parts 108.8 98 100 133.7 880 880 150 125 120 120 130 NCOONa, parts 108 77.6 108 90 1360 1190 100 117 78 131 100 CH3OH, parts 326.3 425 325 401.3 3800 3650 450 375 480 480 390 NCOOCH3, parts - - - - - 150 - - - - SO2, parts 103.4 100 103.4 98.2 1920 1920 110 128 85 85 110 Na2S2O4 (gros basis) 119.4 108.7 119.2 95.5 2269 2265 112 131 87 80 110 Na2S2O4 (100% basis) 110.4 100.0 110.0 86.9 2087.5 2083.8 101.9 117.9 74.8 72.8 97.9 Assay, % 92.5 92.0 92.3 91.0 92.0 92 91 90 86 91 89 Temp. Range. C 70-82 70-82 70-82 70 83 83 72 74 71 - Total Reaction Time, Hrs, 5.5 4.5 4.0 6.0 4.0 4.0 7.0 8.0 7.0 8.0 7.0 Productivty (Lbs/hr/gal)* 0.215 - 0.293 - - 0.65 - 0.158 - - Total Na equivlents 2.168 1.891 2.168 1.903 33.747 33.747 2.17 2.58 1.675 1.926 2.17 Total formate equivalents 1.588 1.141 1.588 1.323 19.997 19.995 1.470 1.720 1.147 1.926 1.470 Total SO2 equivalents 1.614 1.561 1.614 1.533 29.972 29.972 1.717 1.998 1.327 1.327 1.717 Total CH3OH equivalents 10.184 13.265 10.456 12.525 118.602 116.418 14.045 11.704 14.981 14.981 12.172 * The "gal" is U.S. gal.

consideration in view of the reaction being relatively slow and requiring repetitive batch pressure reactions. Also as commented upon in German Patent No. 2,000,877 and in U.S.

Patent No. 3,887,695, it is difficult to enhance the productivity of the reaction system, particularly by decreasing the volume of alcohol with which the sulfur dioxide is added to the system, partly because water both accompanies caustic soda (generally added as a concentrated solution) and is chemically bound therewith and because the ratio of alcohol to water cannot be lowered without promoting the solubility of dithionite (dissolved dithionite immediately decomposes at the reaction pH, thereby causing a reduction in both yield and efficiency of the process.

According to the present invention, there is provided a process for producing anhydrous sodium dithionite, comprising providing in a reactor, water, sodium formate, sulfur dioxide, methanol, and sodium carbonate, wherein in said reactor are methanol and methyl formate and said providing comprises adding thereto in any order: a a solution comprising water and sodium formate, b) a solution comprising sulfur dioxide and methanol, sodium carbonate; and wherein said providing gives a reaction mixture having any weight ratio of total-methanol to water in the range (4.2 to 5.2)/i.

For the purpose of calculating the weight ratio of total-methanol to water, regard is had to methanol added as such to the process, and to methanol corresponding to the resolution of methyl formate into components thereof.

Preferably, said process of the present invention satisfies any one or more of the following preferences: (i) said providing gives a reaction mixture having any sulfur dioxide-to-water weight ratio in the range (1.7 to 2.4)/1; (ii) said providing gives a reaction mixture having any sulfur dioxide-to-methanol equivalence ratio in the range (0.2 to 0.25)/ 1; (iii) said providing gives a reaction mixture having any sulfur dioxide-to-formate ion equivalence ratio that is substantially 1.4/1; (iv) said solution comprising sulfur dioxide and methanol contains methyl formate; (v) said addition of sodium carbonate is in an amount that is substantially 60% by weight of the weight of said sodium formate;; (vi) said addition of sodium carbonate is in any proportion that is at least 50% by weight of the weight of said sodium formate when said sodium carbonate is used as 100%by weight of the needed alkali for said process.

In carrying out the present invention, said sodium carbonate can comprise powder sodium carbonate, e.g. dry powder sodium carbonate, or it can be added as a slurry in methanol to said reactor. Preferably, sulfurdioxide is prereacted with sodium carbonate in such a slurry. Additional sulfur dioxide can be dissolved in the methanol. In utilizing said slurry obtained by treating sodium carbonate with a solution comprising sulfur dioxide and methanol, this solution optionally containing methyl formate, 80 to 85% by weight of this solution can be used for said treating of sodium carbonate; and the balance of that solution can be used for said purpose of being added to said reactor. Said balance can be introduced into said reactor within the second 1/3 of the total reaction time required by said process to produce said anhydrous sodium dithionite.Said addition of said slurry can take place during substantially the first 1/3 of the total reaction time required by said process to produce said anhydrous sodium dithionite. Said addition of said solution comprising water and sodium formate can take place substantially simultaneously with said addition of said slurry.

Said process of the present invention can also comprise recovering said anhydrous sodium dithionite produced by that process.

In carrying out said process of the present invention, the yield of the anhydrous sodium dithionite produced can be expressed in various ways and have various values. On a sulfur dioxide basis, the yield of anhydrous sodium dithionite can be in the range 74 to 82% to 82% by weight. On a formate basis, the yield of anhydrous sodium dithionite can be in the range 53 to 59% by weight. On a sodium basis, the yield of anhydrous sodium dithionite can be in the range 65 to 75% by weight. On a basis of reactor volume, the yield of anhydrous sodium dithionite can be substantially 2.3 pounds per US gallon, and substantially 0.6 pound per hour per US gallon.

It was found that soda ash (i.e. one example of sodium carbonate) can be at least 7% more efficient than caustic soda with respect to usage of Na2O, which is an improvement on U.S. Patent No. 3.887,695. Although the reasons are not entirely understood, it is thought that perhaps the very strong alkalinity of NaOH, which is present when an increment of NaOH solution contacts the reaction medium and dissolves, results in the formation of some by-product. such as Na2SO3, which is insoluble and largely unreactive. Soda ash, on the other hand. is not immediately dissolved when added and therefore tends to release its alkalinity at a slower rate.

However, a comparative calculation of prior ait results for Na2O, on the basis of alkaline Na versus total Na added, can be misleading for those examples (such as Example 4 in United States Patent 3,897,544) wherein no caustic soda is added, all sodium being supplied by sodium formate. The reason therefor is that the process cannot be continued with recycled alcohol.To explain, the desired reaction requires one mol of formic acid and two mols of bisulfite: HCOOH + 2NaHSO3e Na2S204 + CO2 + 2H2O when one adds SO2 to sodium formate as in Example 4 of United States Patent Number 3,897,544, one produces two mols of formic acid for two mols of NaHSO3: H2O +2SO2+ 2HCOONa-32HCOOH + 2NaHSO3 The extra formic acid must react with excess methanol to produce methyl formate.

Otherwise, the excess formic acid would cause the hydrosulfite to decompose. The problem occurs when the methanpl is recovered for the next batch. It will now contain much methyl formate, and could result in a run with severe decompositidn if used according to Example 4 of United States 3,897,544 because of an excess of formic acid produced, which cannot be converted to methyl formate due to the latter already being present.

The formulations in the examples given hereinafter, in which sodium formate is balanced with either NaOH or Na2CO3, are based on the assumption that the methanol, containing methyl formate, will be recovered from a previous run so that the methyl formate which is added is neither consumed nor produced. The examples thus simulate normal commercial production in which the methyl alcohol is recycled over and over.

In virgin batches such as the demonstrative examples of the prior art, in which caustic soda is used, methyl formate is not fed. Formate ion must consequently be supplied entirely from HCOONa, and NaOH must be correspondingly reduced.

As a result of this, the recycle formulations in the following examples cannot be literally compared with the virgin formulations of the prior art examples. For this reasonj comparisons are hereinafter made on a basis of total sodium use even though the consumption of less total Na is less apparent because the sodium formate tends to "dilute" the effect.

In carrying out the present invention, the following steps can be used: (A) -dissolving sodium formate in water at elevated temperatures and up to maximum concentration to form an aqueous formate. solution; (B) dissolving sulfur dioxide in methanol; (C) providing a recipient or puddle methanol solution within a fixed reactor and adding thereto:: (1) sodium carbonate, preferably as a dry powder, according to a first specific schedule to provide at least a substantial proportion of the alkali needed for the reaction, this sodium carbonate being at least 50% by weight of the sodium formate when used as 100% of the needed alkali; (2) the aqueous formate solution according to a second specific schedule, and (3) the SO2-methanol solution according to a third specific schedule in which about 80% by weight is added as a "fast" SO2 feed, the remainder as a "slow" SO2 feed'at progressively slower addition rates.

As an improved soda ash embodiment, even better results are obtained by pre-reacting the sulfur dioxide with the sodium carbonate suspended in the methanol and then adding the resultant slurry of sodium bisulfite in methanol along with the concentrated sodium formate solution to the main reactor at a controlled rate. This embodiment produces sodium dithionite having acceptable particle size and dust properties and an assay that is at least as good as the product of the standard caustic process as disclosed in U.S. Patent No.

3;887,695 at a yield which is 22-25 % greater than the yield of the standard caustic process while consuming about 3.0% by weight less sulfur dioxide and about 8.6% by weight less equivalent NaOH.

- An additional soda ash embodiment comprises slurrying sodium metabisulfite (representing the stoichiometric reaction product of all of the soda ash with about half of the sulfur dioxide) with most of the methanol, and dissolving 60% of the other half of the SO2 in the methanol. and then adding this metabisulfite slurry to the main reactor along with the concentrated aqueous solution of sodium formate. This second improvement produces sodium dithionite with excellent particle size and dust properties at about 20% improved yield as compared to the standard caustic process according to U.S. Patent No. 3,897,544 while consuming about 5.4% by weight less equivalent sodium hydroxide.

In contrast to prior art experimentation with Na2CO3, these results demonstrate that an optimum ratio of reactants and an optimum experimental procedure have been discovered for using sodium carbonate as the source of a substantial amount of added alkali in the incremental reduction of sulfur dioxide with formate ion in aqueous methanolic solution.

An advantage of the present invention is the relatively low cost of sodium carbonate.

Investigation of the feasibility of substituting sodium carbonate for sodium hydroxide in a standardized process as disclosed in U.S. Patent No. 3,887,695 proceeded by making a direct substitution of an anount of Na2CO3 equivalent to the NaOH while maintaining the same amount of water, including the chemically bound water in the NaOH. Undesirable amounts of dust were obtained and the product was of lower purity. Next, the amount of soda ash was reduced, all other variables being constant. This improved the purity of assay of the product and lowered the dust.

In one set of experiments, dust was reduced from 350 to 307 (as hereinafter defined) by adding 45 g less Na2CO3 and was further reduced to 215 by a five-minute delay in feeding the Na2CO3, but the most effective variable for decreasing dust was found to be the SO2 as shown by the following results for another set of experiments: SO2 Dust Number 81/19 260 82/18 196 83/17 147 84/16 97 The dustiness of production batches of sodium dithionite is checked with a colorimetric analytical method that uses a rubine solution to measure the sodium dithionite dust in a sample. In this method, 0.0256 grams Nazi204 react with and decolorize 25 milliliters of rubine solution for determining the dust rating of each production batch.

A dustometer apparatus is assembled for carrying out this dust rating procedure. A carefully cleaned and dried elutriator tube, one inch in internal diameter and 34 inches in length and having a ball joint at each end thereof, is mounted in upright position. A carefully cleaned and dried elutriator sample container tube, one inch in internal diameter and 8 inches in length, with a layer of glass wool about 1 inch thick packed into the bottom thereof, is connected in upright position to the bottom of the elutriator tube. A nitrogen source at 5-10 psig is connected through a needle control valve and a rotameter to the bottom of the sample container tube.A J-shaped lead-off tube of 4-5 mm internal diameter is connected in inverted position to the upper ball joint of the elutriator tube, the straight side of the "J" extending to within one inch of the bottom of a 500 ml graduated cylinder containing 450 ml of water. A door bell buzzer is attached to the curved portion of the "J" and connected to a suitable source of power. A 50 ml burette containing the rubine solution is mounted above the top of the graduated cylinder.

To begin the test, a bottle containing a portion of a production batch is thoroughly mixed by gently rolling and tumbling the bottle (when the contents of the bottle are shaken, the dust cloud that settles on the top interferes with reproducibility of the test). A 50 + 0.1 gram sample is removed from the bottle and carefully poured into the elutriator sample tube, care being exercised to prevent the loss of dust during the transfer and a camel's hair brush being used to transfer all particles into the sample container tube.

3.0 ml of rubine solution are added to the water in the graduated cylinder. The vibration is started and adjusted by having the vibrator bar striking sharply against the underside of the J-shaped lead-off tube in the proximate center of the arc. When the buzzer is operating properly and suitably controlled by the variac, the operator can feed the vibrations when he places a finger on the tube about 3 or 4 inches from the point where the vibrator bar strikes it. If the apparatus is clamped too firmly, the vibrations are dampened and ineffective for disloding the dust that settles out and within the lead-off tube.

The needle valve which controls the nitrogen flow through the elutriator column is opened carefully and rapidly and adjusted to a flow rate of 0.09425 cubic feet per minute.

This flow rate is held constant throughout the run. As soon as the nitrogen flow into the bottom of the elutriator column and through the sodium dithionite sample is high enough to form a dust cloud, a timer is started, At the instant that the rubine solution is decolorized, another few milliliters of rubine solution are added. The number of milliliters of rubine solution are noted at one-minute intervals. The elutriator running is continued for approximately 5 minutes (+ one-half minute). The volume of rubine solution is determined to the nearest 0.1 milliliter, and the time is determined to the nearest 0.1 minute.

The dust index or dust rating is calculated as follows: Milliliters of rubine solution x 30 = dust number Elapsed minutes In general, SO2 addition is divided into two portions. The "fast" SO2, comprising 80-85% of the total, is fed during the first 80 minutes; "slow" SO2, comprising the remainder, is fed during the second 80 minutes; and the reaction is allowed to "cook", with no additional feed being added other than the "scrub" alcohol to prevent loss of volatile reactants, during the third 80 minutes.

Next, the extra water and methanol associated with the use of NaOH are reduced, and, finally, the amounts of reactants are scaled up to utilize the full volume of the reactor and to obtain maximum productivity per unit of available volumetric capacity. The laboratory and pilot plant experimental data for Na2CO3 are presented in Examples 2, 3, 4, 6 and 7 and compared with Examples 1 and 5 for NaOH and with 15 examples for NaOH and Na2CO3 in the prior art.

Examples 1-4 (Example lisa comparative Example notfthe presentinventiòn) Data and results for Examples 1-4 are listed in Table all1. Example 1 presents the average for seven runs using caustic soda at 99% purity in a 73 % solution and no sodium carbonate, according to a standard laboratory procedure with laboratory scale equipment, using the process described in U.S. Patent No. 3,887,695. Example 2 presents the results for an equivalent amount of sodium carbonate and the same amount of water as in Example 1.

Example 3 gives results for a lesser quantity of sodium carbonate and the same amount of water as in Example 2. Example 4 presents results for a still lesser amount of sodium carbonate and decreased amounts of both water and methanol.

As a typical procedure, Example 2 was prepared according to the following detailed steps.

To a stirred reactor is added 851 g. of methanol plus 38 g. of methyl formate. This charge is heated to.70"C and maintained under a pressure of 35 PSIG with nitrogen.

A mixture of 828 g of 96% sodium formate is mixed with 727 g. water and heated to the boiling point. The mixture is transferred to a stainless steel cylinder, which is jacketed with 70 psig steam to maintain the solution at about 300"C. The level in the feeder is measured by a float to which is attached a metal rod which protrudes from the top of the cylinder into a sight glass. The sight glass is calibrated in millimeters, so that the volume in the feeder is known with a good degree of accuracy. The feed rate is controlled by reading the feeder level every millimeter and comparing this with the calculated value.

A mixture of 1702 g. methanol, 76 g. methyl formate and 1307 g. of sulfur dioxide is placed in another stainless steel cylinder equipped with a sight glass and meter stick, which also enables the volume to be known with good accuracy. The feed rate of the mixture is contrplled by a rotameter.

Five hundred and fifty-seven grams of sodium carbonate are weighed TABLE III Laboratory Scale Dithionite Preparations Example No. 1 2 3 4 NaOH, g. * 424 -- -- - H2O, g. 155 -- -- - Na2CO3, g. -- 557 517 501 HCOONa (96%), g. 787 828 828 819 H2O, g. 455 727 727 595 CH3OH. g. 1702 1702 1702 1206 HCOOCH3, g. 76 76 76 76 SO2, g. 1282 1307 1282 1263 CH3OH (puddle), g. 851 851 851 851 HCOOCH3 (puddle), g 38 38 38 38 HCOONa (puddle), g. 41 -- -- - H2O (puddle), g. 22 -- -- - CH3OH (scrub), g. 450 500 450 450 Na2S204- g. 1429 1462 1416 1432 Assay. % 90.9 89.8 91.6 90.9 Pure Nail204. g. 1299 1313 1297 1301 Dust No. 175 453 97 238 Total Na equivalents 22.775 22.341 21.931 21.496 Total formate equivalents 14.073 14.073 14.073 13.94 Total SO2 equivalents 20.012 20.401 20.012 19.716 Total CH3OH equivalents 95.625 97.210 95.625 80.144 * 99% purity 73% solution into five beakers of 100 g. each plus one beaker of 57 g. The feeding mechanism for soda ash consists of two valves and a small hopper. The hopper easily holds 100 g. The space between the valve holds 11.1 g. Since 557 g are to be fed over 79 minutes, 7.05 g. should be fed per minute. At 11.1 g. per dump, the valve system should be operated over every 1.57 minutes.A bleed of nitrogen is maintained at the soda ash inlet to prevent condensation from the reaction plugging the inlet (In later experiments, the nitrogen bleed was omitted, and the valve and fittings heated with electrical tape to prevent condensation).

With the reactor contents at 700C, the SO2-methanol feed is started. After the SO2 concentration in the pot has reached 1% (by calculation), the timer is started and the feeds of sodium formate solution and solid soda ash begun. Five percent of the sodium formate solution is fed in the first minute, and the remaining 95 % fed over 79 minutes. The soda ash is fed over 79 minutes as a "fast SO2" feed rate. Eighty-one percent of the SO2-methanol is fed over 80 minutes, and the remaining 19% fed over the following 80 minutes at progressively slower rates. At the end of 80 minutes, the SO2-methanol feed rate is reduced to about 1/3 that of the fast SO2 feed rate, and this flow rate is maintained for 20 minutes.The rate is then reduced to 26% of the fast SO2 feed rate for another 15 minutes and finally to 17% of the fast rate until the SO2-methanol solution runs out, which is generally at 160 minutes.

The reaction temperature, which is at 70% at the start, is allowed to reach 83"C after about 5 minutes. A temperature of 83"C is maintained thereafter until the end of the run.

After the slow SO2 feed is finished (at about 160 minutes), the run is allowed to "cook" for another 80 minutes, or until a total of 240 minutes of run time.

During the entire run, a feed (see Table III) of methanol is fed to the scrubber to lessen the loss of volatile reactants such as methvl formate and SO Samples are taken of the reaction filtrate after tha fast SO2 feed (80 minutes), after the slow SO2 feed (160 minutes), and at the end of the run (240 minutes). A ten ml sample is mixed with an alkaline formaldehyde solution (to tie up bisulfite) and is then titrated with 0.1 N standard iodide solution. This "titer" is a measure of the sodium thiosulfate content of the solution and is therefore an indication of the extent of decomposition of hydrosulfite.

The contents of the reactor are filtered through a glass-fritted Buchner funnel, maintaining an atmosphere of nitrogen above the product to prevent contact with air. The product is washed with methanol and dried in a vacuum flask, under vacuum while heated in a hot water bath. The dried product is weighed to determine the yield, assayed for hydrosulfite purity. and a dust number is run plus a screen analysis.

Examples 5-7 (Example 5 is a comparative Examples not of the present invention) Data and results for Examples 5-7 are listed in Table IV which shows pilot plant results for a standard run (Example 5) using sodium hydroxide and for two runs (Examples 6 and 7) using sodium carbonate. The reactor is equipped and operated as described in U.S.

Patent Number 3,887,695. The sodium carbonate Is 63% by weight of the sodium formate.

In Example 6, the amount of methanol is the same as in Example 5, but the amount of water that is present has been slightly increased. In Example 7, the amount of water has been reduced by 25% and the amount of methanol has been reduced by 20% with respect to Example 6.

Comparative Ratios and Productivity Calculation for Laboratory and Pilot Plant Examples and for Prior Art Examples In Tables V(a), V(b). calculated ratios, based on equivalence values in Tables I-IV, and calculated yields, based on SO2, formate ion, and sodium ion, are also given for both experimental runs and prior art examples. Finally, calculated productivity in terms of reactor volume as pounds per hour per gallon are listed for one example and for each of the four prior art patents. In general, productivity increased 18% by laboratory data and 20% by pilot plant data.

TABLE IV Pilot Plant Dithionite Preparations Example Number 5 6 7 NaOH, parts 71 -- - Na2CO3, parts -- 83 83 H2O, parts 98 116 87 HCOONa, parts 138 138 138 CH3OH, parts 501 501 400 HCOOCH3, parts 18 18 18 SO2, parts 211.2 205.2 205.2 Na2S204 (gross basis) 246 239 247.5 Na2S204 (100% basis) 226 222 228.7 Assay, % 92 92.7 92.4 Dust No. 100 206 102 Total Reaction Time, Hrs. 4.0 4.0 4.0 Productivity, pounds/gal. * 2.26 2.26 2.70 Total Na equivalents 3.704 3.514 3.514 Total formate equivalents 2.247 2.247 2.247 Total SO2 equivalents 3.30 3.206 3.206 Total CH3OH equivalents 15.937 15.937 12.784 * The "gal" is U.S. gal TABLE V(a) Experimental Results Example Number 1 2 3 4 5 6 7 Sodium Equiv./SO2Equiv. 1.138 1.133 1.096 1.090 1.119 1.093 1.093 Sodium Equiv./Formate Equiv.* 1.618 1.612 1.558 1.542 1.648 1.564 1.564 Wt.CH3OH/Wt.H2O*** 4.842 4.283 4.209 4.310 5.204 4.396 4.702 SO2Equiv./CH3OH Equiv.** 0.209 0.206 0.209 0.246 0.207 0.202 0.252 Wt. SO2/Wt. H2O 2.027 1.797 1.762 2.121 2.161 1.774 2.366 SO2 Equiv./Formate Equiv.* 1.422 1.422 1.422 1.414 1.473 1.431 1.431 Yield: SO2 Basis, % 74.6 74.0 74.4 75.8 78.7 79.5 81.9 Formate*basis, % 53.0 53.6 52.9 53.6 57.8 56.7 58.5 Sodium Basis, % 65.5 67.5 67.9 69.5 70.1 72.6 74.8 Productivity (lbs/hr/gal)**** 0.49 0.49 0.49 0.57 0.49 0.49 0.57 * Includes formate equivalents in sodium formate and methyl formate. The experimental results are some examples of the ratio substantially 1.4/1 for SO2 Equiv./Formate Equiv.

** Includes methanol equivalents in methyl formate.

*** The value "5.204" in Example 5 is in substance "5.2". Thus, as a practical matter, the "5.204" can be regarded as "5.2",i.e. "5.204" can be regarded "5.2" correct to the first significant decimal figure **** The "gal" is U.S. gal TABLE V(b) Prior Art Results U.S.3,887,695 U.S.3,411,875 U.S.3,714,340 U.S.3,879,544 Sodium Equiv./SO2Equiv. 233 1.126 1.187-1.402 1.211-1.343 1.261-1.451 Sodium Equiv./Formate Equiv.* 1.688 2.397-1.680 1.365-1.657 1.000-1.476 Wt.CH3OH/Wt.H2O 4.233-4.313 2.081-4.235 2.995-4.331 2.996-3.995 SO2Equiv./CH3OH Equiv.** 0.253-0.257 0.085-0.118 0.118-0.158 0.089-0.171 Wt. SO2/Wt. H2O 2.180 0.399-0.999 0.734-1.033 0.708-1.023 SO2Equiv./Formate Equiv.* 0.253-0.257 1.026-1.415 0.118-0.158 0.089-0.171 Yield: SO2Basis, % 80.0 65.5-70.0 65.1-78.6 63.0-68.3 Formate*Basis, % 60.0 34.0-49.0 37.7-50.4 21.7-39.9 Sodium Basis, % 70.9-71.1 48.1-58.4 52.4-60.8 43.4-54.0 Productivity (lbs/hr/gal)*** 0.475 0.191 0.215-0.293 0.158 * Includes formate equivalents in sodium formate and methyl formate.

** Includes methanol equivalents in methyl formate.

*** The "gal" is U.S. gal.

It is clear that, in terms of sodium equivalence per SO2 equivalence, there is a slight difference between the two sodium hydroxide examples (1 and 5), the three sodium carbonate examples (4, 6 and 7), and the four prior art patents. Employment of soda ash enables use of 3% less total sodium equivalent/ SO2 equivalents over the best results in the prior art.

Although the laboratory and pilot plant examples, using either sodium hydroxide or sodium carbonate, utilize a higher weight ratio of methanol to water than the four prior art examples, the five sodium carbonate ratios are somewhat lower than the two sodium hydroxide ratios among the laboratory and pilot plant examples. As to the weight ratio of sulfur dioxide to water, the laboratory and pilot plant examples possess higher values than three of the four prior art patents. As to the equivalence ratio of sulfur dioxide to formate, the laboratory and pilot plant examples have distinctly higher values relative to prior art, for both sodium hydroxide and sodium carbonate.

It is on a yield basis and on a process productivity basis that the invention is most striking.

In the laboratory series, substituting sodium carbonate for sodium hydroxide in Example 4 compared to Example 1 increased the efficiency of the process when based on all three raw materials. The productivity, as measured by the pounds of product prepared per hour per gallon of reaction solution, is also increased by the substitution. A similar increase in process efficiencies and productivity is observed in Example 7 vs. Example 5 in the pilot plant.

These results are believed to be noteworthy in view of the high assay and low dust numbers for the products of Examples 6 and 7, as given in Table IV, which are highly useful as industrial bleaching agents.

Additional pilot plant results are presented in Table VI for a standard run with caustic soda and for two runs with soda ash in which the Na2CO3 is 62.9% by weight of the HCCOONa. Significantly improved productivities, measured as pounds of pure Na2S204 per gallon of reactor volume, were obtained in Examples 9 and 10. Even though the amount of water which was used for dissolving the increased amount of sodium formate was slightly greater than the total amount of water used in Example 8 with the NaOH and with the HCOONa, the subsitution of Na2CO3 for NaOH enabled the methanol to be reduced even though additional sulfur dioxide was used. Because of the relatively low density of methanol, this reduction saves considerable valuable space in a reactor.

In general, use of sodium carbonate as the principle source of supply for alkali in the process of this invention, as demonstrated in Examples 9-10, (Example 8 is a comparative Example not of the present invention) enables less solvent and larger quantities of reactants to be used in a reactor so that productivity is significantly increased as compared with the worldwide standard productivities obtained with sodium hydroxide as the principle source of supply for alkali. Less total sodium is thus required when soda ash is employed.

TABLE VI Pilot Plant Dithionite Preparations Example Number 8 9 10 Na2CO3 lb fed - 100 100 NaOH2 lb fed* 70 - HCOONa, lb fed** 132.5 159 159 SO2,lb fed 211.2 243.2 243.2 CH3OH,lbfed 501 483 483 (Initial Puddle) (142) (164) (164) (With SO2) (284) (244) (244) (Scrub) ( 75) ( 75) ( 75) H2O,lb fed 100 104 104 (With NaOH) ( 26) - (With HCOONa) ( 74) (104) (104) HCOOCH3 18 16 16 Total Vol., gal*** 100 100 100 Filtrate Vol,, gal*** 87 84 84 Product, 1b 247 302 296 Product, %Na2S204 91.5 90.3 90.3 Pure Na2S204 226 273 267 Productivity, lb/gal*** 2.26 2.73 2.67 Yield:SO2 Basis ;78.7 82.5 80.8 Formate Basis -66.6 67.0 65.7 Sodium Basis 70.2 74.2 72.7 Productivity(lb/gal/hr)*** 0.57 0.68 0.68 * 100%pure ** 100%pure *** The "gal" are U.S. gal.

Although the Examples 11 and 12 which follow are directed towards the use of soda ash as a solid, and are both within the present invention it is to be understood that the present invention encompasses the addition of water to soda ash with a corresponding reduction of water in other feed streams. - Example 11 This example illustrates the pre-reaction of sodium carbonate and sulfur dioxide to form a feed material.

Three separate feeds were prepared. Feed "A" was made by suspending 83 parts by weight of sodium carbonate in 167 parts of methyl alcohol containing 9 parts methyl formate, and adding to the suspension 167 parts of sulfur dioxide. Feed "B" was made by dissolving 131 parts of sodium formate of approximately 96% purity in 93 parts water. Feed "C" was made by dissolving 35 parts sulfur dioxide in 35 parts methyl alcohol containing 2 parts methyl formate.

An initial charge consisting of 115 parts methyl alcohol containing 6 parts methyl formate was placed in the reactor. This charge was agitated and heated to a temperature of 65 C and at a pressure of 20 psig. Then Feed "A" and Feed "B" were started simultaneously and at such a rate that the specified quantity of each would be fed to the reactor in an 80-minute period. Heating of the reactor contents continued until a temperature of 83 C was reached, at which time the heat was reduced to maintain a controlled reaction temperature of 83 C.

The time period from 65 to 83 was approximately 10 minutes. Also after this same 10 minutes. the reactor pressure had reached 50 psig owing to the release of carbon dioxide gas from the reaction. The reaction pressure was thereafter maintained at 50 psig by controlled release of the carbon dioxide formed in the reaction. The released gas left the reactor through first a water-cooled condenser (35 C) followed by a chilled condenser (-10 C).

then a chilled scrubber fed with methyl alcohol at a rate of 0.26 parts per minute. The condensates from the two condensers plus the scrubber effluent containing methanol re entered the reactor.

At the end of the 80 minute period of feeding Feed "A'' and Feed "B". Feed "C" was started at a rate of 1.5 parts per minute. The rate was reduced to 1.0 parts per minute after 15 minutes, and further reduced to 0.7 parts per minute after another 15 minutes. The entire 72 parts of Feed "C" was consumed in 80 minutes. During this time, the temperature and pressure within the reactor were maintained by 83"C and 50 psig, by carrying out controlled release of gas as mentioned above.

These same conditions were maintained for an additional 70-minute period after the completion of Feed "C". At this time, 230 minutes from the beginning in all, the reactor contents were cooled to 60"C and filtered. Then the filter cake was washed with 240 parts methyl alcohol, dried under vacuum to yield a crystalline product of 240.5 parts by weight and 92.3 % assay as sodium hydrosulfite.

Example 12 This example illustrates the use of sodium metabisulfite as a feed material. Example 12 was carried out in a manner similar to that of Example 11.

Again, three separate feeds were prepared. Feed "A" was made by suspending 150 parts of sodium metabisulfite in 167 parts of methyl alcohol containing 9 parts of methyl formate, and adding to the suspension 67 parts of sulfur dioxide. Feed "B" and Feed "C" were identical to those described in Example 11.

As has hereinbefore been stated, the process of this invention results in increased productivity for a given reaction vessel per unit of time. This increased productivity results from the fact that it is possible using the teachings of this invention, to utilize more of the reactor volume for the production of sodium dithionite than has heretofore been possible.

The present invention can enable the use of less water which in turn permits one to use less methanol while still keeping appropriate ratios of methanol to water in the reactor. By the use of less methanol, there is more room in the reactor so that more product can be produced per batch.

The decimal figures given in the above Tables are rounded to the nearest thousandth for ratios and to the nearest tenth for yields and in assays. However, the decimal figures are generally rounded in the present description and appended claims to the nearest tenth of hundredth for ratios, and, in the present description, to the nearest whole number for yields.

WHAT WE CLAIM IS: 1. A process for producing anhydrous sodium dithionite, comprising providing a reactor, water. sodium formate, sulfur dioxide, methanol, and sodium carbonate, wherein in said reactor are methanol and methyl formate and said providing comprises adding thereto in any order: (a) a solution comprising water and sodium formate, (b) a solution comprising sulfur dioxide and methanol, (c) sodium carbonate; and wherein said providing gives a reaction mixture having any weight ratio of total-methanol to water in the range (4.2 to 5.2)/ 1.

2. A process as claimed in claim 1, wherein said providing gives a reaction mixture having any sulfur dioxide-to-water weight ratio in the range (1.7 to 2.4)/ 1.

3. A process as claimed in claim 1 or 2, wherein said providing gives a reaction mixture having any sulfur dioxide-to-methanol equivalence ratio in the range (0.2 to 0.25)/ 1.

4. A process as claimed in any one of claims 1 to 3. wherein said providing gives a reaction mixture having any sulfur dioxide-to-formate ion equivalence ratio that is substantially 1.4/1.

5. A process as claimed in any one of claims 1 to 4, wherein said solution comprising sulfur dioxide and methanol contains methyl formate.

6. A process as claimed in any one of claims 1 to 5. wherein said addition of sodium carbonate is in an amount that is substantially 60% by weight of the weight of said sodium formate.

7. A process as claimed in any one of claims 1 to 5, wherein said addition of sodium carbonate is in any proportion that is at least 50% by weight of the weight of said sodium formate when said sodium carbonate is used as 100% by weight of the needed alkali for said process.

8. A process as claimed in any one of claims 1 to 7, wherein said sodium carbonate comprises powder sodium carbonate.

9. A process as claimed in claim 8, wherein said powder sodium carbonate comprises dry powder sodium carbonate.

10. A process as claimed in any one of claims 1 to 8, wherein said sodium carbonate is added as a slurry in methanol to said reactor.

11. A process as claimed in claim 10. wherein said slurry contains a sodium bisulfite.

12. A process as claimed in claim 1 wherein said slurry contains sodium metabisulfite.

13. A process as claimed in claim any one of claims 10 to 12 wherein sulfur dioxide is prereacted with sodium carbonate in said slurry.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (29)

**WARNING** start of CLMS field may overlap end of DESC **. started at a rate of 1.5 parts per minute. The rate was reduced to 1.0 parts per minute after 15 minutes, and further reduced to 0.7 parts per minute after another 15 minutes. The entire 72 parts of Feed "C" was consumed in 80 minutes. During this time, the temperature and pressure within the reactor were maintained by 83"C and 50 psig, by carrying out controlled release of gas as mentioned above. These same conditions were maintained for an additional 70-minute period after the completion of Feed "C". At this time, 230 minutes from the beginning in all, the reactor contents were cooled to 60"C and filtered. Then the filter cake was washed with 240 parts methyl alcohol, dried under vacuum to yield a crystalline product of 240.5 parts by weight and 92.3 % assay as sodium hydrosulfite. Example 12 This example illustrates the use of sodium metabisulfite as a feed material. Example 12 was carried out in a manner similar to that of Example 11. Again, three separate feeds were prepared. Feed "A" was made by suspending 150 parts of sodium metabisulfite in 167 parts of methyl alcohol containing 9 parts of methyl formate, and adding to the suspension 67 parts of sulfur dioxide. Feed "B" and Feed "C" were identical to those described in Example 11. As has hereinbefore been stated, the process of this invention results in increased productivity for a given reaction vessel per unit of time. This increased productivity results from the fact that it is possible using the teachings of this invention, to utilize more of the reactor volume for the production of sodium dithionite than has heretofore been possible. The present invention can enable the use of less water which in turn permits one to use less methanol while still keeping appropriate ratios of methanol to water in the reactor. By the use of less methanol, there is more room in the reactor so that more product can be produced per batch. The decimal figures given in the above Tables are rounded to the nearest thousandth for ratios and to the nearest tenth for yields and in assays. However, the decimal figures are generally rounded in the present description and appended claims to the nearest tenth of hundredth for ratios, and, in the present description, to the nearest whole number for yields. WHAT WE CLAIM IS:

1. A process for producing anhydrous sodium dithionite, comprising providing a reactor, water. sodium formate, sulfur dioxide, methanol, and sodium carbonate, wherein in said reactor are methanol and methyl formate and said providing comprises adding thereto in any order: (a) a solution comprising water and sodium formate, (b) a solution comprising sulfur dioxide and methanol, (c) sodium carbonate; and wherein said providing gives a reaction mixture having any weight ratio of total-methanol to water in the range (4.2 to 5.2)/ 1.

2. A process as claimed in claim 1, wherein said providing gives a reaction mixture having any sulfur dioxide-to-water weight ratio in the range (1.7 to 2.4)/ 1.

3. A process as claimed in claim 1 or 2, wherein said providing gives a reaction mixture having any sulfur dioxide-to-methanol equivalence ratio in the range (0.2 to 0.25)/ 1.

4. A process as claimed in any one of claims 1 to 3. wherein said providing gives a reaction mixture having any sulfur dioxide-to-formate ion equivalence ratio that is substantially 1.4/1.

5. A process as claimed in any one of claims 1 to 4, wherein said solution comprising sulfur dioxide and methanol contains methyl formate.

6. A process as claimed in any one of claims 1 to 5. wherein said addition of sodium carbonate is in an amount that is substantially 60% by weight of the weight of said sodium formate.

7. A process as claimed in any one of claims 1 to 5, wherein said addition of sodium carbonate is in any proportion that is at least 50% by weight of the weight of said sodium formate when said sodium carbonate is used as 100% by weight of the needed alkali for said process.

8. A process as claimed in any one of claims 1 to 7, wherein said sodium carbonate comprises powder sodium carbonate.

9. A process as claimed in claim 8, wherein said powder sodium carbonate comprises dry powder sodium carbonate.

10. A process as claimed in any one of claims 1 to 8, wherein said sodium carbonate is added as a slurry in methanol to said reactor.

11. A process as claimed in claim 10. wherein said slurry contains a sodium bisulfite.

12. A process as claimed in claim 1 wherein said slurry contains sodium metabisulfite.

13. A process as claimed in claim any one of claims 10 to 12 wherein sulfur dioxide is prereacted with sodium carbonate in said slurry.

14. A process as claimed in claim 13, wherein 80 to 85 % by weight of said solution (b)

is prereacted with sodium carbonate; and the balance of that solution is used for said purpose of being added to said reactor.

15. A process as claimed in claim 14, wherein said balance is introduced into said reactor within the second 1/3 of the total reaction time required by said process to produce said anhydrous sodium dithionite.

16. A process as claimed in any one of claims 10 to 15, wherein said addition of said slurry takes place during substantially the first 1/3 of the total reaction time required by said process to produce said anhydrous sodium dithionite.

17. A process as claimed in any one of claims 10 to 16, wherein said addition of said solution comprising water and sodium formate takes place substantially simultaneously with said addition of said slurry.

18. A process as claimed in any one of claims 1 to 17 comprising recovering said anhydrous sodium dithionite.

19. A process as claimed in claim I, substantially as hereinbefore described.

20. A process as claimed in claim 1, substantially as described in Example 2.

21. A process as claimed in claim 1, substantially as described in Example 3.

22. A process as claimed in claim 1, substantially as described in Example 4.

23. A process as claimed in claim 1, substantially as described in Example 6.

24. A process as claimed in claim 1, substantially as described in Example 7.

25. A process as claimed in claim1, substantially as described in Example 9.

26. A process as claimed in claim 1, substantially as described in Example 10.

27. A process as claimed in claim 1, substantially as described in Example 11.

28. A process as claimed in claim 1, substantially as described in Example 12.

29. Anhydrous sodium dithionite, produced by a process according to any one of claims 1 to 28. - - - -