DE2728316B2 - Batch process for the production of anhydrous sodium dithionite (Na2 S2 O4) - Google Patents

Description

Alkali metal dithionites, especially sodium dithionite, are widely recognized as technical Bleaching agents used, for example to bleach ground wood pulps.

Sodium dithionite can be produced electrolytically as well as using borohydride. However, processes in which the formation is used to reduce the Reduce the value of the sulfur atom. In this way, one can enter in a particularly economical manner produce solid end product of high quality. ω

This process development began in 1933 with US Pat. No. 2010 615, in which a process for the production of anhydrous alkali metal dithionites is described, according to which gaseous sulfur dioxide is introduced into an aqueous methanol solution containing sodium formate and sodium carbonate and at a temperature below 3O ⁰ C. The temperature of the methanolic SO ₂ solution is then increased to such an extent that the formation of sodium dithionite begins. In one embodiment, the amount is «; only 19.0% of the sodium carbonate used, based on the sodium formate used show only poor stability.

About 30 years later a number of improvements have been described, the major ones being are based on the fact that sodium hydroxide is used as the alkali supplier References referenced: US-PS 34 11 875, 35 76 598, 37 14 340, 37 18 732, 38 72 221, 38 87 695 and 38 97 544; JA-PS 7003/68 and 2405/71 as well as BE-PS 6 98 247.

In these improved methods, the SOi-containing methanol solution and an alkali become one added aqueous solution of an alkali metal formate and the aqueous methanol solution obtained is at a reaction temperature above the dewatering point of hydrated alkali metal dithionite held so as to prevent the formation of crystals that contain water of crystallization. The rate at which the reactants are added must correspond to the rate at which the dithionite is produced correspond. If the additional speed is too high, the just formed decomposes Dithionition and the yield decreases accordingly.

Further contemplated improvements to the process are that sulfur dioxide in one water-miscible alcohol is absorbed as the first feed solution, that also sodium hydroxide and sodium formate dissolved in very hot water outside the reactor as a second feed solution and that both solutions are fed into the reactor, which is at a temperature im Range of 60 to 90 ° C is kept and a small Contains amount of alcohol under superatmospheric pressure

According to US Pat. No. 3,887,695, higher concentrations of the reactants are used in the reactor also a higher yield of the desired dithionite per unit volume of the reactor and per unit volume of the alcohol used. For this purpose, methyl formate is used as a by-product may arise in a previous reaction cycle in those! Part of the methyl alcohol dissolved, which is presented in the reactor and to which the feed solutions are then added will.

Although in the more recent improved processes, sodium hydroxide is essentially the sole source is used for the supply of the required alkali and hardly any mention of sodium carbonate finds, it should be pointed out that the JA-PS 7003/68 teaches, initially sulfur dioxide in appropriate concentration to dissolve in methanol and then gradually an aqueous alkaline solution add, which contains both sodium formate and sodium carbonate, namely the latter compound in an amount of 33 percent by weight, based on the sodium formate. The yield of sodium dithionite in this process is 56%, based on the sulfur dioxide used and 54%, based on the

total sodium used, d. H. both used Reaction substances are not used satisfactorily

Tables I and II below show the most important data and results for the 15th Embodiments compiled, which in the

Table I.

four major US patents are included as prior art. Data relating to the properties of the dust (Dust number) of these products are not accessible.

U.S. Patent 3,411,875 2 3 4th example 21 20th 30th 1 - - - NaOH, parts by weight 23 240 136 100 Na ₂ CO ₃ , parts by weight - 90 70 75 H ₂ O, weight parts 200 500 386 424 HCOONa, parts by weight 80 - - - CH ₃ OH, parts by weight 470 87 72 100 _{HCOOCH 3} , parts by weight - ■ 90 / 78.3 72 / 64.1 104 / 94.2 SO ₂ , parts by weight 80 91.2. 89.0 90.6 Na ₂ S ₂ O ₄ , gross / 100% basis 84 / 76.0 70 70 70 Purity,% 90.5 5.3 5.0 5.3 Temperature range, ⁰ C 60-70 - - 0.0228 Total reaction time, hours 4.8 1,848 1.529 1.853 Spreading rate, kg / h / l - 1.323 1.029 1.103 Total Na equivalents 1.751 1.358 1.124 1.561 Total formate equivalents 1.176 15.605 12,047 13.233 Total SO ₂ equivalents 1.249 Total CH ₃ OH equivalents 14.669

Table II

I. U.S. Patent 37 14 340 3 4th U.S. Patent 3,887,695 2 example 23.2 23.2 example 650 t 1 2 - - 1 - j NaOH, parts by weight
I Na ₂ CO ₃ , parts by weight 23.2 30.0 100 133.7 550 880 IH ₂ O, parts by weight - - 108 90 - 1190 I HCOONa, parts by weight 108.8 98 325 401.3 880 3650 I CH ₃ OH, parts by weight 108 77.6 - - 1360 150 I HCOOCH ₃ , parts by weight 326.3 425 103.4 98.2 3800 1920 I SO ₂ , parts by weight - - 119.2 95.5 - 2265 I Na ₂ S ₂ O ₄ , gross amount, 103.4 100 1920 I parts by weight 119.4 108.7 110.0 86.9 2269 2083.8 I Na ₂ S ₂ O ₄ , 100% basis 92.3 91.0 92 I purity,% 110 * 4 100.0 70-82 70 2087.5 83 \ Temperature range, "C 92.5 92.0 4.0 6.0 92.0 4.0 Total reaction time, hours 70-82 70-82 0.0351 - 83 0.0569 Spreading rate, kg / h / l 5.5 4.5 2.168 1.903 4.0 33.747 Total Na equivalents 0.0257 - 1,588 1.323 - 19,995 Total formate equivalents 2.168 1,891 1.614 1.533 33.747 29.972 : ■ total SO ₂ equivalents 1,588 1.141 10.456 12.525 19,997 116.418 \ Total CHjOH equivalents 1.614 1.561 29.972 10.184 13.265 118,602

Table II (continued) 27 28 316 3 6 - 5 5 - 28 28 - U.S. Patent 38,97 544 120 130 NaOH, parts by weight example 78 4th 100 Na ₂ CO ₃ , parts by weight 1 2 480 _ 390 H ₂ O, parts by weight 28 32 - - - HCOONa, parts by weight - - 85 120 110 CH ₃ OH, parts by weight 150 125 87 131 110 _{HCOOCH 3} , parts by weight 100 117 74.8 480 97.9 SO ₂ , parts by weight 450 375 86 - 89 Na ₂ S ₂ O ₄ , gross amount, parts by weight - - 71 85 - Na ₂ S ₂ O ₄ , 100% basis 110 128 7.0 80 7.0 Purity,% 112 131 - 72.8 - Temperature range, ⁰ C 101.9 117.9 1.675 91 2.17 Total reaction time, hours 91 90 1.147 - 1,470 Spreading rate, kg / h / l 72 74 1.327 8.0 1.717 Total Na equivalents 7.0 8.0 14.981 - 12,172 Total formate equivalents - 0.0189 1.926 Total SO ₂ equivalents 2.17 2.52 1.926 Total CH ₃ OH equivalents 1,470 1,720 1.327 1.717 1.998 14.981 14.045 11,704

The following conclusions can be drawn from the examples of the prior art:

(a) Sodium hydroxide is routinely used as a supplier for the required alkali has been used;

(b) Sodium carbonate is often used in the relevant Prior publications mentioned but has rarely been used in practice;

(c) in the few exemplary embodiments in which Sodium carbonate has been used as a supplier for the required alkali is the yield with regard to the production of sodium dithionite, significantly less than when using Sodium hydroxide.

Nevertheless, it would be extremely desirable to have sodium carbonate as the main supplier for such a process to be able to use for the required alkali, if only because this compound is much cheaper than sodium hydroxide. In addition, the rate of production of sodium dithionite is per unit of reactor volume an extraordinarily critical size and of particular economic importance because the The reaction in question proceeds relatively slowly and therefore several approaches under pressure one after the other in the intermittent processes must be brought to reaction. In DE-PS 2000877 and in US-PS 38 87 695 has also already been explained that it is very difficult to measure the performance of such a To improve the reaction system decisively, in particular by reducing the amount of alcohol, which is fed into the reactor along with the sulfur dioxide, in part because Water gets into the reaction mixture together with sodium hydroxide, partly as solution water and partly in chemically bound form, while, on the other hand, the weight ratio of alcohol to water is not significantly lowered can without significantly increasing the solubility of the dithionite. However, dissolved dithionite decomposes immediately at the pH values prevailing in the reaction, so that this results in a reduction in yield and a

Reduction in performance of the overall procedure follows

Accordingly, it is an object of the present invention to provide an improved process for the preparation of anhydrous sodium dithionite in which substantial amounts of the required sodium ion are provided by sodium carbonate. In addition, this improved process is intended to increase the production of dithionite per unit volume of the reactor.
The batchwise process according to the invention for the production of anhydrous sodium diihionite (Na ₂ S ₂ O ₄ ), in which a solution of SO ₂ in methanol is reacted with a solution of sodium formate in water and with sodium carbonate at given weight ratios of methanol to water, is characterized that dry powdered sodium carbonate in an amount of at least 50 percent by weight of the sodium formate used, based on ratios where the sodium formate would provide 100% of the required alkali, fed directly into the reactor or _{slurried in the SO 2} methanol solution, water only in the the amount required to dissolve the sodium formate is used and the weight ratio of methanol: water is _{adjusted to a value in the range from about 4.2 to 5.2 and the weight ratio of SO 2} : water is adjusted to a value in the range from 1.7 to 2.4

According to the method of the invention, therefore, sodium dithionite is made using sodium carbonate produced, which is fed into the reactor in the form of a dry powder or as a slurry will. Methanol and water are regulated to a predetermined weight ratio and that Water is only used in the minimum amount

det, which is necessary to the sodium formate in maximum concentration at elevated temperature dissolve. The amounts of sulfur dioxide, methanol, sodium formate, water and sodium carbonate will be preferably positioned one on top of the other in such a way that the specified reactor volume is fully utilized will. Each methanol solution contains approximately 4% methyl formate. An additional amount of methanol is fed in via the reflux condenser in order to prevent losses of methyl formate and to keep other by-products as low as possible. This additional amount of methanol is approximately 15% of the total methanol.

The equivalent ratio of sulfur dioxide to methanol is expediently in the range of about 0.20 up to 0.25. The equivalent ratio of sulfur dioxide to formiation is expediently about 1.4. the The amount of sodium carbonate used is suitably about 60 percent by weight of the amount of the sodium formate used. The equivalent ratio from sodium carbonate to sodium formate is therefore appropriately about 0.8.

The yields of anhydrous sodium dithionite that can be achieved according to the invention are relatively high and are _{about 74 to 82%, based on SO 2} , about 53 to 59% based on formate and about 65 to 75% based on sodium. Based on the reactor volume, the production of dithionite is about 0.2755 kg / l or about 0.0718 kg / h / l.

With regard to the utilization of the total Na ₂ O used, the process according to the invention is at least 7% more effective than the processes according to the prior art which work with sodium hydroxide, as described, for example, in US Pat. No. 3,887,695

A comparative calculation of the prior art results using Sodium hydroxide (Na2O) based on the alkaline However, sodium in relation to the total sodium added can lead to incorrect conclusions, for example when using example 4 of US Pat. No. 3,897,544, in which no sodium hydroxide is used is added, but all of the sodium is only supplied in the form of the sodium formate. The reason this consists in the fact that such a process can not be continued without alcohol in the Cycle is returned. To explain it, it should be noted that the desired implementation a total of 1 mole of formic acid and 2 moles of bisulfite consumed, according to the equation below:

HCOOK + 2NaHSO ₃ - Na ₂ S ₂ O ₄ + CO ₂ + 2H ₂ O

_{However, if SO 2 is added} to the sodium formate solution in accordance with Example 4 of US Pat.

H ₂ O + SO ₂ + 2HCOONa - »2HCOOH + 2NaHSO ₃

The formic acid formed in excess must therefore with excess methanol with the formation of Formic acid methyl ester react. If that reaction didn't take place, it would be the Excess formic acid decompose the freshly formed dithionite immediately. The real problem arises when the methanol is recovered for a next batch. It now contains the Formic acid methyl ester is formed and can lead to serious decomposition in a subsequent test run of the dithionite lead if you use such a methyl formate-containing methanol for the approach of Example 4 of US-PS 38 97 544 used when namely in such a further test run Excess formic acid is formed, it can no longer be converted to methyl formate because the corresponding methyl ester is already present in the methanol.

ίο The following in the exemplary embodiments reaction approaches used, in which sodium formate either with sodium hydroxide or with sodium carbonate is used together are based on the assumption that the methyl formate containing Methanol comes from a previous test run, so that the added methyl formate neither consumed is still generated again. The exemplary embodiments therefore simulate the conditions of a usual production on a commercial scale, in which the methyl alcohol is repeatedly recycled will.

In the original approaches, as they result from the exemplary embodiments of the prior art and in those cases where sodium hydroxide is used, no methyl formate is therefore fed either The formation must therefore be completely supplied by the sodium formate used and the The amount of sodium hydroxide must be decreased accordingly.

As a result, the approaches used in the following examples that involve a cycle or recovery of methanol correspond, not literally with the original approaches according to the embodiments of the prior art are compared. For this reason, below comparisons were also made on the basis of total sodium used, although the Consumption of less total sodium is less noticeable because of the presence of sodium formate this effect somewhat diluted

The method according to the invention is therefore carried out essentially in the following way:

a) Sodium formate is dissolved in water at an elevated temperature up to its maximum Concentration and thereby an aqueous formate solution obtained;

b) sulfur dioxide is dissolved in methanol to form a methanol solution;

c) a methanol solution is placed in a reactor and the methanol solution is added to the the following additives added:

1) Sodium carbonate in the form of a dry powder, according to a regulated program, thereby creating a substantial amount of the for the

Reaction required alkali available

is provided and the sodium carbonate is at least 50 percent by weight of the used Sodium formate makes up, based on proportions where the sodium formate is 100% of what is needed Alkali would provide;

2) the aqueous formate solution according to a second regulated program and

3) the SOj-methanol solution according to a third regulated program, with about 80% of the

Solution can be added as a quick feed

and the remaining amount of sulfur dioxide from a slow supply but increasingly less Additional speed is added.

According to a further embodiment of the method according to the invention, even more favorable Results obtained by mixing the sulfur dioxide with the sodium carbonate suspended in methanol allowed to pre-react and then the slurry formed together with the concentrated sodium formate solution fed into the main reactor at a controlled rate. In this embodiment becomes sodium dithionite with favorable particle size and favorable dust properties and degree of purity which is at least as good as that according to the standard method using sodium hydroxide obtained product as described in US Pat. No. 3,887,695. The yield in the However, the method according to the invention is about 22 to 25% greater than this standard method, while at the same time about 3.0% less sulfur dioxide and about 8.6% less equivalents of sodium hydroxide are consumed.

In contrast to the information in the prior art, when using solid Sodium carbonate achieve an optimal ratio of the individual reactants and an optimal working method, wherein the sodium carbonate provides a substantial amount of the alkali required for the reaction

The best results in terms of a reduction in the dust number of the sodium dithionite formed achieve by taking the total amount of sulfur dioxide in terms of the addition rate to the reaction batch, as can be seen from the result of the following comparative examples:

Distribution ratio of SO2 dust number

81/19
82/18
83/17
84/16

260

196

147

97

The dust properties of the final sodium dithionite products obtained in such a batch process is determined by means of a colorimetric analysis method in which a fuchsine solution the powdered sodium dithionite is used for this to be determined in a sample. With this analysis method, each Production sample 25 ml of a fuchsine solution reacted with 0.0256 g of sodium dithionite and decolorized

A special device is used to carry out this evaluation method. It consists of a carefully cleaned and dried mud pipe with an inner diameter of 2.4 cm and a length of 8636 cm, with a ball screw at each end, which is clamped in a vertical position. An equally carefully cleaned and dried Schlänunrohr for the sample to be examined with an inside diameter of 2 ^ 4 cm and a length of 2032 cm, which has a glass wool pack of about 24 cm in length at the lower end, is connected in an upright position to the bottom of the slurry tube The bottom of the tube containing the sample is connected to a nitrogen source, which is under an overpressure of 035 to 0.70 kg / cm ² , via a needle control valve and a rotameter.

In addition, a J-shaped drainage tube (4 to 5 mm internal diameter) is connected in an inverted configuration to the upper ball-lock end of the slurry tube, with the straight leg of the "J" extending into a graduated cylinder within one inch of the floor of 500 ml capacity containing 450 ml of water. A doorbell buzzer is connected to the bent end of the J-shaped discharge pipe and is connected to a power source via a control device (variac). A 50 ml burette containing the fuchsine solution is placed at the top of the graduated cylinder
At the beginning of the examination, a bottle

is which a sample of the product to be examined contains, gently rolled and twisted around that way to mix the contents carefully. If, on the other hand, the contents of the bottle are shaken too vigorously, they will sit down Dust clouds from the top of the bottle, which then produce reproducible results during the examination impede. A 50 g sample (error limit ± 0.1 g) is then taken from the bottle and carefully transferred into the receiving tube for the slurry experiment, being careful

that there are no dust particles during the transfer get lost, and a camel hair brush is used to remove any remaining particles into the sample tube convict.

Then 3.0 ml of the fuchsine solution are added

to the water in the graduated cylinder. Then you set the vibration device in motion and regulate it one that the vibrating plunger of the doorbell buzzer sharply against the underside of the J-shaped discharge pipe if the buzzer is set correctly and through the Is controlled by the control unit, the examining laboratory technician can feel the vibrations directly if he puts his finger on the pipe about three to four inches from where If the entire assembly is clamped too tightly, however, the Vibrations are dampened and are then no longer effective around the inside and outside of the discharge pipe to set the settling dust in motion again.

Then carefully open the needle valve which allows the nitrogen gas to flow through the slurry column regulates, setting a flow rate of 26683 cmVminute This flow rate is maintained during the entire test run. As soon as the nitrogen flow, the enters at the bottom of the slurry tube and passes through the sodium dithionite sample sufficiently strongly is to form a cloud of dust becomes one

Timing device set

By the time the fuchsine solution is decolorized, A few more milliliters of the fuchsine solution are added at one-minute intervals the number of milliliters of fuchsine solution recorded. The slurry tube is left for about 5 minutes running (deviation ± 0.5 minutes). The total volume of fuchsine solution is determined to be about 0.1 ml and the lapse of time is found to be 0.1 minute. The dust number is determined according to the following equation calculated

Milliliters of fuchsine solution, _ _. , ..

- - - - χ 30 = dust number.

Time in minutes

In general, the amount of methanolic SC> 2 solution added is separated into two partial amounts The amount of SO2 solution added quickly is about 80 to 85% of the total amount and is used in the Added over the course of the first 80 minutes. H. the rest, will be added within the next 80 minutes. The reaction mixture is then left without react further additive during the next 80 minutes, with only the rinse alcohol which is used to prevent loss of volatile reactants.

In Examples 2, 3, 4, 6 and 7 below are the results of experiments with sodium carbonate on a laboratory scale and in technical work explained and the results are compared with those according to Examples 1 and 5 under Use of sodium hydroxide can be achieved, as well as with the results of the prior art (15 Examples) where sodium hydroxide and sodium carbonate have been used together.

The individual conditions and results for Examples 1 to 4 are shown in Table III below in summary Example 1 corresponds to the average values of 7 runs using of sodium hydroxide with a degree of purity of 99% in the form of a 73% solution without use of sodium carbonate according to a method as described in US-PS 38 87 695, in which im Laboratory scale example 2 shows the results using equivalent Amounts of sodium carbonate and the same amounts of water as in Example 1. Example 3 shows the results when using a smaller amount of sodium carbonate and the same amount of water as in example 2. In example 4 there are still smaller amounts of sodium carbonate, but also smaller amounts of Water and methanol.

A typical procedure, as used inter alia for Example 2, is described below in individually explained:

A stirred reactor is charged with a mixture of 851 g of methanol and 38 g of methyl formate. This charge is heated to 70 ° C. and maintained under a nitrogen pressure of 2.45 kg / cm ² .

In addition, a mixture of 828 g of sodium formate (degree of purity 96%) and 727 g of water is heated practically to the boiling point. cm ² ) in order to keep the solution at a temperature above 148.9 ° C in this way. The liquid level in this storage vessel is determined by means of a float to which a metal rod is attached, which protrudes from the cylinder head into a sight glass.This sight glass is calibrated in millimeters so that the liquid volume in the storage vessel is known with a high degree of accuracy of the liquid level per millimeter and comparison with a calculated value

In addition, another cylinder equipped in the same way with a sight glass and indicator rod is switched off stainless steel with a mixture of 1702 g of methanol, 76 g of methyl formate and 1307 g Filled with sulfur dioxide The volume of liquid in this second storage container can also be changed determine good accuracy. The feed rate of the mixture is determined by means of a rotameter regulated.

100 g of sodium carbonate are placed in each of 5 beakers and 75 g of sodium carbonate are placed in a sixth beaker weighed in. The total amount of sodium carbonate is therefore 557 g. The feeding device for this Sodium carbonate consists of a filling funnel with two valves. This funnel holds a good 100 g Material. The space between the two valves contains about 11.1 g of material. Since a total of 557 g If sodium carbonate has to be fed in over 79 minutes, 7.05 g sodium carbonate should be fed in per minute are fed. Since 11.1g per batch Sodium carbonate can be discharged, the valve system should be started every 1.57 minutes. At the inlet opening for the sodium carbonate, a side stream of the nitrogen is fed in so as to prevent condensation products from clogging this inlet during the reaction. At later Attempts are made to omit this nitrogen sidestream and instead use the valve and connectors heated by means of an electrically heated belt to prevent condensation.

As soon as the reactor contents have reached a temperature of 70 ° C, the SCh methane solution is fed in.After the reaction mixture has reached a calculated SO2 concentration of 1%, a timer is started and the sodium formate solution and the solid sodium carbonate are then fed in . 5% of the sodium formate solution is fed in within the first minute and the remaining 95% is fed in during the remaining 79 minutes. The sodium carbonate is fed in over 79 minutes at a rate which _{corresponds to the rapid SO 2} feed. 81% of the SO ₂ methanol solution is fed within of the first 80 minutes and the remaining 19% are fed in at an increasingly slower rate in the subsequent 80 minutes

At the end of this second 80 minute period, the SCV methanol solution feed rate will increase about a third of that of the fast feed rate and this feed rate is maintained for the next 20 minutes. Then the feed speed is increased to 26% the fast feed rate and held there for about 15 minutes. In the end there is a reduction to 17% of the fast feed rate until all of the SC ^ -MethanoI solution is fed in. This generally takes about 160 minutes

The reaction temperature is at the beginning of Implementation 70 ° C and you leave them within about 5 Minutes to 83 ° C. This temperature value

of 83 ° C is then maintained until the end of the test run.

After finishing the period of the slow

With the addition of the SO ₂ methanol solution (ie about 160 minutes after the start of the reaction), the reaction mixture is left to stand for a further 80 minutes until a total of 240 minutes have passed since the start of the experiment.

During the entire test run, 500 g of methanol are fed to a washing device in order to reduce the loss of volatile reactants, e.g. B. of methyl formate and SO ₂ .

After the first 80 minutes portion, that is, after completion of the rapid supply of SO2, and after 160 minutes, ie after the end of the slow addition of SO _2, and after completion of the run, ie after 240 minutes, respectively samples tested of the filtrate of the reaction mixture . A 10 ml sample is mixed with an alkaline formaldehyde solution in order to bind the bisulfite present therein and then titrated with a 0.1 N standard iodine solution. The titer value determined is a measure of the sodium thiosulphate content of the solution and thus a measure of the degree of decomposition of the formed

Dithionite.

The contents of the reactor are filtered through a Buchner funnel fitted with a glass frit, a Protective atmosphere of nitrogen is maintained to avoid any Konf.kt of the reaction product with air to prevent. The reaction product is then washed with methanol and in a Vacuum bottle dried by heating in a hot water bath under vacuum. The dried product will weighed and the yield as well as the degree of purity and the number of dusts determined. You also run a Sieve analysis through.

Table III

Production of sodium dithionite on a laboratory scale

example 2 3 4th 1**) - _ - NaOH, g *) 424 - - - H ₂ O, g 155 557 517 501 Na ₂ CO ₃ , g - 828 828 819 HCOONa (96%), g 787 in 727 595 H ₂ O, g 455 1702 1702 1206 CH ₃ OH, g 1702 76 76 76 HCOOCH ₃ , g 76 1307 1282 1263 SO ₂ , g 1282 851 851 851 CH ₃ OH (submitted), g 851 38 38 38 HCOOCH ₃ (submitted), g 38 - - - HCOONa (submitted), g 41 - - - H ₂ O (submitted), g 22nd 500 450 450 CH ₃ OH (scrubber), g 450 1462 1416 1432 Na ₂ S ₂ O ₄ , g 1429 89.8 91.6 90.9 Degree of purity,% 90.9 1313 1297 1301 Pure Na ₂ S ₂ O ₄ , g 1299 453 97 238 Dust number 175 22,341 21.931 21,496 Total Na equivalents 22.775 14.073 14.073 13.94 Total formate equivalents 14.073 20.401 20.012 19.716 Total SO ₂ equivalents 20.012 97.210 95.625 80.144 Total CH ₃ OH equivalents 95.625

*) 99% purity,

73 percent solution.
**) According to US-PS 38 87 695.

Examples 5 to 7

These examples show the results of experiments which have been carried out on a semi-industrial scale, namely an experiment using Sodium hydroxide and two experiments using sodium carbonate The results obtained are summarized in Table IV. For carrying out the tests, the method described in US Pat. No. 3,887,695 reactor described used and the experiments are carried out in the manner described there The amount of sodium carbonate used is 63 percent by weight of the sodium formate used. In Example 6, the amount of methanol is the same as in Example 5, but the amount of water is opposite to Example 5 slightly increased In Example 7, the amount of water is reduced by 25% compared to Example 6 and the The amount of methanol has been reduced by 20%.

Table IV

Semi-technical try 7th example - 5 *) 6th 83 NaOH, parts by weight 71 _ 87 _{Nl 2} CO ₃ , parts by weight - 83 138 H ₂ O, parts by weight 98 116 400 HCOONa, parts by weight 138 138 18th CH ₃ OH, parts by weight 501 501 205.2 _{HCOOCH 3} , parts by weight 18th 18th 247.5 SO ₂ . Parts by weight 211.2 205.2 228.7 Na ₂ S ₂ O ₄ , gross amount, parts by weight 246 239 92.4 Na ₂ S ₂ O ₄ , 100% basis 226 222 102 Degree of purity,% 92 92.7 4.0 Dust number 100 206 0.3234 Total reaction time, hours 4.0 4.0 3.514 Yield, kg / 1 0.2707 0.2707 2.247 Total Na equivalents 3.704 3.514 3.206 Total formate equivalents 2.247 2.247 12.784 Total SOy equivalents 3,300 3.206 Total CH, OH equivalents 15.937 15.937

*) According to US-PS 38 87 695.

Tables Va and Vb show calculated ratios of the reactants used from the experiments which have been described in Tables I to IV. In addition, yield values are given based on SO ₂ or on formiation or on sodium. These calculations were carried out both for the exemplary embodiments and for the examples according to the prior art. Finally, there are also the calculated bets for the yield based on the

Unit volume of the reactor per hour, indicated. in the

in general it can be stated that with the irr

Tests carried out on a laboratory scale Productivity has been increased by 18% and the im

semi-industrial scale performed testers by 20%.

Table Va

Experimental results

example

1 ***) 2 3

Equivalent ratio Na: SO ₂
Equivalent ratio Na: formate *)
_{CH 3} OH: H ₂ O weight ratio
Equivalent ratio SO ₂ : CH ₃ OH **)
Weight ratio SO ₂ : H ₂ O
Equivalent ratio SO ₂ : formate *)

yield

SO ₂ base,%
Formate basis,%
Sodium base,%
Spreading rate, kg / h / l

*) Including the formate equivalents in Na formate and methyl anic acid. **) Including the methanol equivalents in the formic acid meter. ***) According to the state of the art.

1.138 1.133 1,096 1.090 1.119 1.093 1.093 1.618 1.612 1.558 1,542 1.648 1.564 1.564 4,842 4,283 4.209 4,310 5.204 4,396 4.702 0.209 0.206 0.209 0.246 0.207 0.202 0.252 2.027 1.797 1,762 2.121 2.161 1,774 2,366 1.422 1.422 1.422 1.414 1.473 1.431 1.431 74.6 74.0 74.4 75.8 78.7 79.5 81.9 53.0 53.6 52.9 53.6 57.8 56.7 58.5 65.5 67.5 67.9 69.5 70.1 72.6 74.8 0.0587 0.0587 0.0587 0.0682 0.0587 0.0587 0.0682

030 143/270

Table Vb

Prior Art US Patent Results

38 87 695 3411875 3714340 38 97 544 1.126 1.187-1.402 1,211-1,343 1,261-1,451 1.688 1.397-1.680 1.365-1.657 1,000-1,476 4,233-4,313 2.081-4.235 2.995-4.331 2.996-3.995 0.253-0.257 0.085-0.118 0.118-0.158 0.089-0.171 2.180 0.399-0.999 0.734-1.033 0.708-1.023 0.253-0.257 1,026-1,415 0.118-0.158 0.089-0.171 80.0 65.5-70.0 65.1-78.6 63.0-68.3 60.0 34.0-49.0 37.7-50.4 21.7-39.9 70.9-71.1 48.1-58.4 52.4-60.8 43.4-54.0 0.0569 0.0228 0.0257-0.0351 0.0189

Equivalent ratio Na: SO ₂
Equivalent ratio Na: Fonniat *)
_{CH 3} OH: H ₂ O weight ratio
Equivalent ratio SO ₂ : CH ₃ OH **)
Weight ratio SO ₂ : H ₂ O
Equivalent ratio SO ₂ : formate *)

yield

SO ₂ -BaSiS,%

Formate basis,%

Sodium base,%
Spreading rate, kg / h / l

*) Including the formate equivalents in Na formate and methyl formate. **) Including the methanol equivalents in the methyl formate.

From the table values it can be seen that with regard to the equivalent ratio of sodium to SO _{2 there is} a small difference between Examples 1 and 5, which work with sodium hydroxide, the three Examples 4, 6 and 7, which work with sodium carbonate, and the examples of mainly There are four US patents to be used for the prior art. The use of sodium carbonate enables a 3% lower equivalent ratio of total sodium to SO ₂ compared with the best results of the prior art. Although a higher weight ratio of methanol to water was used in the laboratory-scale or pilot-scale examples using either sodium hydroxide or sodium carbonate than in the fifteen prior art examples, the corresponding ratios for sodium carbonate are in the five concerned Examples slightly lower than the corresponding ratios for sodium hydroxide in the remaining two examples, both on a laboratory and pilot scale. With regard to the weight ratio of sulfur dioxide to water, it is found that the examples on the laboratory or semi-industrial scale have higher values than three of four publications according to the prior art. The equivalent ratios of sulfur dioxide to formate in the laboratory tests and the semi-technical tests are markedly higher than in the prior art, both when using sodium hydroxide and also

Table VI sodium

However, the particular advantages of the process according to the invention become clear when one considers the Take into account yield figures and the calculated product yield for those on a laboratory scale Examples carried out lead to the replacement of sodium hydroxide by sodium carbonate in Example 4 compared to Example 1 to an increased performance in terms of the formation of the desired product.

This increase in performance is measured as the amount of dithionite product produced per hour and per liter of reactor solution and this amount of product produced is due to the replacement of sodium hydroxide also improved by sodium carbonate Corresponding improvements in the effectiveness of the Procedure and yield are compared with the pilot run of Example 7 observed with Example 5

The results are particularly convincing with regard to the high and low purity values Dust numbers of the products of Examples 6 and 7, which result from Table IV, since such properties for industrial use as bleaching agents are highly desirable.

Additional pilot results are summarized in Table VI for one Trial run using sodium hydroxide and two trials using Sodium carbonate, the proportion of sodium carbonate being 62.9 percent by weight of the amount used Sodium formate

Trials on a semi-industrial scale
example

9

(Comparison) 10

Na ₂ CO ₁ , kg fed
NaOH, kg added *)

45.36

45.36

31.75

HCOONa, kg added **) SO ₂ kg fed in

CH ₃ OH, kg fed
(amount presented)
(together with SO ₂ )
(Washer)

H ₂ O, kg fed
(together with NaOH)
(together with HCOONa)

HCOOCH ₃ , kg

Total volume, 1 Filtrate volume, 1

Product, kg

Product,% Na ₂ S ₂ O ₄ Pure Na ₂ S ₂ O ₄ , kg Yield, kg / 1

yield

SO ₂ base,%
Formate basis,%
Sodium base,%
Spreading rate, kg / h / l

*) 100% pure.
**) 100% pure.

Try in semi-technical scale 10 example 8th 9 72.12 (Comparison) 110.22 60.10 72.12 219.09 95.71 110.22 (74.39) 227.25 219.09 (110.68) (64.41) (74.39) (34.02) (128.82) (110.68) 47.17 (34.02) (34.02) - 45.36 47.17 (47.17) (11.79) - 7.71 (33.57) (47.17) 378.53 8.16 7.71 317.97 378.53 378.53 134.26 329.32 317.97 90.3 112.04 136.98 121.11 91.5 90.3 0.3198 102.51 123.83 80.8 0.2707 0.327 65.7 78.7 82.5 72.7 66.6 67.0 0.0814 70.2 74.2 0.0682 0.0814

In Examples 9 and 10, significant improvements with regard to the increase in yield are achieved as can be seen from the values given for the production of pure sodium dithionite per liter Reactor volume result. Although the amount of water which is needed to dissolve the increased amount of Sodium formate was used, is only slightly higher than the total amount used in Example 8 Water, in which sodium hydroxide has been used in combination with sodium formate, enables the Replacing sodium hydroxide with sodium carbonate will decrease the amount of methanol, though additional sulfur dioxide was applied. Since methanol has a comparatively lower density than Having water, this reduction in the amount of methanol results in a significant amount of valuable reactor space is saved.

In general, it enables sodium carbonate to be used as the main source of the alkali in the Process according to the invention (cf. Examples 8 to 10), fewer solvents and larger amounts of Using reactants in a reactor of a given capacity will result in performance of a reactor compared to known standard processes that require sodium when one Sodium carbonate is used as a reaction component.

The following example also explains

in case the use of sodium carbonate in solid form.

In the process according to the invention, it is therefore essential that the reactor as a whole have as much as possible small amount of water is supplied, and that a predetermined methanol: water ratio is maintained is, but the amount of water can be unevenly distributed over the individual feed streams.

Example 11

This example illustrates an embodiment in which sodium carbonate is allowed to prereact with a portion of the sulfur dioxide, while the remaining methanolic SO ₂ solution is fed in as a separate feed.

Three different feeds are made Feed "A" is obtained by adding 83 Parts by weight of sodium carbonate suspended in 167 parts by weight of methyl alcohol, the 9 parts by weight of methyl formate and then adding 167 parts by weight of sulfur dioxide. Feed "B" is obtained by adding 131 parts by weight of sodium formate dissolves with a degree of purity of about 96% in 93 parts by weight of water. Feed-in "C" becomes obtained by dissolving 35 parts by weight of sulfur dioxide in 35 parts by weight of methyl alcohol, the still contains 2 parts by weight of methyl formate.

A charge of 115 parts by weight of methanol containing 6 parts by weight of methyl formate is placed in a reactor. This charge is stirred and heated to a temperature of 65 ° C. ^{under an overpressure of 1.4 kg / cm 2} "B" fed in at the same time, and at such a rate as each feed would be fed in individually within the first 80 minute period. The reactor contents are heated until a temperature of 83 ° C is reached, whereupon the heating is reduced and the temperature is kept at 83 ° C. Approx. 10 minutes are required to heat the reactor contents from 65 ° C to 83 ° C ^{Reactor pressure increased to an overpressure of 3.50 kg / cm 2} in the course of the reaction as a result of the formation of CO2. The overpressure in the reactor is then kept at this value by letting the carbon dioxide formed out of the reactor in a controlled manner refrigerated scrubber, which is charged with methanol at a rate of 0.26 parts by weight per minute. The condensates from the two condensers and the effluent from the methanol washer are returned to the reactor together

After the first 80 minute period, feed "C" is also fed at a rate of 1.5 parts by weight per minute. After 15 minutes, the feed rate is reduced to 1.0 parts by weight per minute and after a further 15 minutes the feed rate is 0.7 Parts by weight per minute reduced It therefore takes 80 minutes to feed the 72 parts by weight of feed "C". During the addition of this feed (C), too, the temperature in the reactor is kept at 83 ° C. and the overpressure is kept at 3.50 kg / cm ² .

The same conditions are maintained for an additional 70 minutes period after completing the feed of the Zufhr (C) After the lapse of 230 minutes reckoned from the start of the reaction, the reactor contents is cooled to 6O ⁰ C and filtered Subsequently, the Filter cake washed with 240 parts by weight of methanol and dried in vacuo. In this way, 240.5 parts by weight of a crystalline product are obtained which consists of the desired sodium dithionite with a degree of purity of 923%

As already mentioned above, the method according to the invention enables the performance the reaction per unit volume of a given reactor per hour to increase significantly. These Performance improvement is based on the fact that a larger proportion of the total reactor volume for production of sodium dithionite can be exploited than was previously possible.

According to the invention, less water is used and therefore less methanol can also be used while maintaining the required ratio of water to methanol in the reactor can be. As a result of the use of less methanol, there is more space in the reactor Available, so that a larger amount of end product can be generated per reaction batch.

Claims (7)

Patent address: 1. Stepwise process for the production of anhydrous sodium dithionite (Na ₂ S ₂ O ₄ ), in which a solution of SO ₂ in methanol is reacted with a solution of sodium formate in water and with sodium carbonate at given weight ratios of methanol to water, characterized in that, that dry powdered sodium carbonate in an amount of at least 50 percent by weight of the sodium formate used, based on ratios where the sodium formate would provide 100% of the required alkali, is fed directly into the reactor or slurried in the SOrMethanoUösung, water only in the one used to dissolve the sodium formate required amount is used and the weight ratio of methanol: water to a value in the range from about 4.2 to 5.2 and the weight ratio of SO2: water to a value in the range from 1.7 to 2.4 2. The method according to claim 1, characterized in that a solution of SO ₂ in methanol is used which contains methyl formate 3. The method according to claim 1 and 2, characterized in that methanol and methyl formate are fed into the reactor before introducing the methanolic SO ₂ solution, the aqueous sodium formate solution and the sodium carbonate 4. The method according to claim 1 to 3, characterized in that there is an equivalent ratio used from SO2 to formate of about 1.4 5. The method according to claim 1 to 4, characterized in that the sodium carbonate is slurried in only a portion of the methanolic SO ₂ solution 6. The method according to claim 5, characterized in that the subset is about 80 to 85 Makes up percent by weight of the total methanolic SO2 solution. 7. ■ The method according to claim 6 and 7, characterized in that the aqueous sodium formate solution feeds into the reactor at about the same time as the slurry. «