DD146817A1 - PROCESS FOR THE PRODUCTION OF SODIUM CYANIDE - Google Patents

Abstract

The invention relates to a process for the preparation of sodium cyanide with low sodium hydroxy content and good flowability while avoiding the Blausaeurepolymerisation and formate formation. The essence of the process according to the invention consists in the lowering of the sodium hydroxide content by an additional rinsing of the crystals with a 0.2 to 2% blue water solution having a temperature of 290 to 320 K and a heating of the dried NaCN at 470 to 540 K. The invention can be used in continuous and batch plants for the production of sodium cyanide.

Description

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Method r Herste l development of sodium cyanide

IPK C01C / 3-10 Anwendungsge Bidding of the invention :.

The invention relates to a process for the preparation of sodium cyanide with low Natriümhydroxidgehalt and better flowability while avoiding hydrogen cyanide and Pormiatbildung.

Characteristics of the known technical solutions.

Sodium cyanide is mainly produced by neutralization of hydrocyanic acid with caustic soda.

In high-percentage sodium hydroxide solution, hydrocyanic acid is introduced in liquid or gaseous form when the reaction vessels are cooled. To avoid the Blausäurepolymerisation, which is cause of darkening of the product, the introduction of a sodium hydroxide solution Blaus'äure in excess of 0.2 - 3 Io aborted (USP 2,993,754). Particularly continuous neutralization processes (German Pat. No. 1,103,905 and German Pat. No. 1,115,116) work with an excess of sodium hydroxide up to 3>. Precise adherence to a specific residual alkali content can only be achieved with a high technical outlay on measuring and control technology ,

However, the excess alkali content not only prevents the hydrocyanic acid, it also causes a suppression of the hydrolysis in the next stage of the process "evaporation", so that only a small amount of sodium cyanide is converted to Natriurnformiat.

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By evaporating in a vacuum crystallizer, a crystal slurry is produced, wherein there is still a concentration of the free sodium hydroxide solution.

The observance of certain evaporation temperatures and residence times as well as the residual alkali content are crucial in the process for the production of large crystals and the suppression of hydrolysis »(USP 4 083 935). In filter apparatus or centrifuges, the crystals are separated from the saturated sodium cyanide solution. The crystals with 4 - 12 ίο water must then be dried. The saturated sodium cyanide solution adhering to the crystals contains, among other impurities, free sodium hydroxide solution, which remains in the solution during incomplete neutralization. · The dried sodium cyanide is contaminated with 0.5-1.5% sodium hydroxide. This content depends on the neutralization process and the separation process to which the crystal slurry is subjected. If the neutralization is carried out with a slight excess of sodium hydroxide, the NaOH content in the NaCN drops; but the content of sodium formate increases, so far only compromise solutions were possible.

The sodium hydroxide content has, in addition to sodium formate, effects on the hygroscopic properties and the tendency to harden the technical sodium cyanide. Drying at 470-500 K is said to cause flowability (DE Pat. Nos. 1,150,367 and 1,089,738).

Zi el the invention :

According to the invention, a cost effective process for removing sodium hydroxide in sodium cyanide has been found to provide significant advantages in the known production process.

Explanation of the invention of the invention :

The inventive method is essentially characterized in that sodium cyanide, which is prepared by the known neutralization process, after crystallization and crystal separation by centrifuging or filtering in the centrifuge or filter rinsed with a dilute aqueous solution of hydrogen cyanide and thus the adhering sodium hydroxide is neutralized.

The washing process follows the pilation process after the liquid has been separated. The hydrocyanic acid solution is used at a concentration of 0.2-2 % .

Since only the adhering to the crystals sodium hydroxide solution must be neutralized little hydrogen cyanide is required. This is the decisive advantage of the process. Due to the low hydrocyanic acid concentration, a polymerization ,; even when using an excess of hydrocyanic acid prevented. In contrast, an overdose in the normal neutralization process certainly leads to discoloration of the sodium cyanide solution and of the salt. Experience has shown, however, that it is sufficient if the crystals are treated with less than the equivalent amount of hydrocyanic acid, since in addition to the neutralization a 'Vascheffekt occurs, which also leads to the slight lowering of other foreign salts.

The neutralization liquid used is the most suitable vapor condensate from the evaporator stage which has been treated with liquid hydrocyanic acid. The hydrocyanic acid solution should be used with about 290 K to 320 K. The neutralization solution is recycled after passing through the filter or the centrifuge together with the mother liquor,

so that no additional wastewater incurred or product losses occur.

If a sodium cyanide is to be generated which is free from sodium hydroxide and sodium formate, the hydroxide-free NaCN is treated at 470-540 K after the usual drying. It has proven expedient to carry out this high-temperature treatment with well pre-dried and post-neutralized NaCN (water content -0.3 $) at about 520 K.

The method according to the invention offers the following technological and qualitative advantages:

It is possible to take advantage of incomplete neutralization with a residual alkali content of 0.5-2% and at the same time produce a sodium cyanide with less than 0.1 % NaOH.

The risk of a Pehlcharge by exceeding the Feutralisationspunktes can be kept low, hydrolysis and Formiatbildung can be suppressed. Maintaining a constant residual alkalinity is difficult especially with continuous neutralization processes. The subsequent neutralization of the crystals allows these irregularities to compensate.

- The drying process is positively influenced because free sodium hydroxide is more hygroscopic than NaCN. Shortening the residence time or lowering the drying temperature are possible, or with the same drying parameters, the water content in the final product is lower than for comparable untreated samples.

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- The final product shows significant qualitative improvements.

The NaCN content is 0.5 to 1.5 # higher than sodium cyanide, which was prepared by the usual methods.

The product is less hygroscopic and the Riesel ability remains in the same storage conditions longer.

When treating sodium cyanide above 470K to remove sodium formate, it is also advantageous to use alkali-free product. At temperatures above 470 K, the following reaction possibilities are possible:

1) HCOONa + UaOH> Na ₂ CO ₃ + H ₂

2) 2HCOO x Na > Na ₂ CO ₃ + CO + Hg.

In case 1) - i. a temperature treatment without pretreatment-all the Pormiat and hydroxide, converted to carbonate. The NaCN content is practically not increased.

In case 2) - i.e. a temperature treatment of the hydroxide-free NaCN-, the NaCN content is increased by the neutralization with hydrogen cyanide and undergoes a further increase by the release of CO and the loss of mass is seen from equation 2.

Hrun exemplary U gs b ice jjj el e:

example 1

From a sodium cyanide solution of the following composition:

% λ- 2 41 , 4 NaCN 1 , 9 # NaOH 1 ,8th % Na CHOO • 0 % Na ₂ CO ₃

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was made by evaporation under vacuum, a crystal slurry.

In a centrifuge, the crystals were separated from the mother liquor. It had a residual water content of 5.2 fo.

From the same starting material, a part was placed untreated in the drying apparatus. The other part was treated with a 1% cyanide solution in the centrifuge. In this case, 10 1 of hydrogen cyanide solution per 100 kg of NaCN moisturizing salt were used.

The products dried at 400 K had the following composition:

not treated Prod, with 1 oiger product HCN neutralized (*) (*) 97.4 98.4 0.8 0.1 0.9 0.9 0.6 0.4 '0.2 0.1

NaCN NaOH NaCHOO Na ₂ CO ₃ H ₂ O

after a temperature treatment at 520 K, the following compositions of the products were determined:

untreated product neutralized with 1% HCN ( ίο )

Prod. (#)

NaCN 97.5 98.7

.1- 2ί

NaOH NaCHOO Na ^ CO H ₂ O

0.2 0.1 0.1 0.1 2.2 1.2 0.1 0.1

example 2

From a sodium cyanide solution of the following composition: 40.5 l NaCN

0.9 l NaOH

1, 8 Io NaCHOO

0.9 # Na ₂ CO ₃

was obtained as in Example 1, crystals, also a part untreated in the drying apparatus and the other part (100 kg) with 10 1 1iger liger solution of cyanide washed. The dried products had the following composition:

Prod, neutralized with 0.7% HCN

  98.3 0.1 0.8 0.6 0.1

after a temperature treatment at 520 IC, the following compositions of the products were determined:

untreated with 1 $ HCN new product (#) 'tralized

NaCN 97.6 98.6

NaOH 0.1 "0.1

not treated Product (#) NaCN 97.6 NaOH 0.7 NaCHOO 0.9 Na ₂ CO ₃ East ', 8 H ₂ O 0.1

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WaCHOO

H ₂ O

0.1 2.2 0.1 0.1 1.2 0.1

After 3 months of storage, the products showed the following phenomena:

1. untreated ETaCK dried at 400 K continuous solidification of powder-surface encrustation

2. NaNN neutralized with HCN dried at 400 K solidification significantly less than 1. no superficial crusts

3. untreated NaCN dried at 400 K at 520 K no solidification no crusts

4. HON neutralized NaCN no solidification at 400 K dried at 520 K post-treated no crusts

Claims (2)

"3- 2 1 He fi ndun ^ sans prüche 1. A process for the preparation of sodium cyanide by neutralization of hydrocyanic acid and sodium hydroxide solution and subsequent crystallization in the suppression of formate formation by a sodium hydroxide excess, characterized in that the sodium cyanide after crystallization following the separation of the mother liquor with a dilute : aqueous hydrogen cyanide solution with a concentration of 0.2 to 2 Io and a temperature of 290-320 K is rinsed. 2. A process for the preparation of sodium cyanide according to item 1, characterized in that the dried sodium cyanide is heated at a temperature range of 470 K to 540 K.