DE942506C - Process to prevent corrosion - Google Patents

Description

Stainless steels are corroded by chlorite or Chlordioxydlösungen and by moist Chlordioxyddämpfe, in particular in the acidic p _H range, strong. Since many apparatuses are made entirely or partially from this material, the use of these bleaching and oxidizing agents is confronted with considerable difficulties. It is already known to use solutions of chlorites or chlorine dioxide for the purpose of preventing corrosion ίο of. To add stainless steel acids of nitrogen oxides or their salts. However, up to now it has not been possible to protect the stainless steel against corrosion by chlorine dioxide in gaseous form, especially in the presence of acid vapors.

Chlorine dioxide in gas form enters treatment apparatus not only when working with this gas itself, but also arises when Chloritr-solutions values and high temperature are used at low p _H. Chlorine dioxide formation occurs to a greater extent when the item to be treated is soaked in a chlorite solution and this is then activated by the action of volatile acids and heat. Similar conditions exist when the item to be treated with

one. Solution of chlorine it. and impregnated with an acid-releasing product and then heated will.

It has now been found that the corrosion of stainless steel by chlorine dioxide in gaseous form in Can prevent the presence of acid vapors if one is made of stainless steel or such apparatus containing in addition to chlorodipoxide or chlorites a by evaporation of a ίο Mixture of nitric acid and with water vapor introduces vapor mixture produced by volatile acids.

Acids that are volatile with steam are, for example: acetic acid, formic acid, ' Lactic acid, propionic acid, ketobutyric acid, hydrochloric acid and others. Chlorites come in water soluble products such as alkali or alkaline earth chlorites for use. In the case of 'use of chlorites, for example with acidic steaming with neutral or alkaline Treatment material soaked in chlorite solution is to be used more on the volatile acid than 'for the implementation, the amount of chlorite present is required.

The nitric acid, from which the acid, which is volatile with water vapor, is evaporated, is effective _{in many cases even at a very low concentration, for example of ι g / l · HNO 3.} In the case of continuous evaporation of acids into the reaction space, 3 to 5% strength nitric acid solutions are expediently used. It is noted that the consumption of nitric acid is very low. However, higher concentrations can also be used without further ado, as long as no NO ₂ evolution occurs, since the latter gas has a reducing effect on chlorite or chlorine dioxide and thus oxidant losses can occur. " ¹ NO ₂ evolution can also occur if the volatile one used Acid has a reducing effect on nitric acid, e.g. formic acid. In such cases it is advisable to add urea or uric acid and / or their salts to the nitric acid solution. In general, g of these substances per liter is sufficient. Instead of starting from free nitric acid, you can also use mixtures of nitrates and sulfuric acid, for example.

In the case of continuous evaporation, it is also expedient to use steam to volatilize the evaporation Replace acid on an ongoing basis according to the withdrawal. As a rule, the addition takes place in dilution, namely - this depends on the amount of evaporated water. if you can also use the chlorite solution alone or together with a volatile acid in the diluted Run in nitric acid and develop chlorine dioxide during evaporation. That Evaporation of the mixture of acids and nitric acid, which are volatile with water vapor, for example by direct heating or by introducing steam, can be done in the reaction chamber itself will. Man. but can also carry out the evaporation in a special apparatus.

The process is particularly advantageous when bleaching fibrous materials such as cellulose, Textile fibers made from natural or regenerated cellulose or from synthetic material in loose form In the form or in the form of yarn or woven fabric. You can also use the method, for example also when bleaching other materials such as flour, oils, fats, etc., with disinfectants or Use oxidation processes.

example 1

Chlorine dioxide gas from a generator, diluted with an inert gas, is mixed in a pipeline with vaporous acetic acid, which had evaporated from a 0.2% boiling nitric acid solution with a content of 1 to 2% acetic acid, and the gas mixture is _t then passed into a reaction chamber made of stainless steel, in which dry, powdery organic bodies, for example flour, are treated. The stainless steel does not suffer any corrosion.

Example 2

In a 3% nitric acid one leaves at boiling temperature slowly a 10% solution of sodium chlorite and run in a 100% formic acid solution. The escaping gas mixture of chlorine dioxide and formic acid are fed into a mixing drum made of stainless steel, in which sulphite pulp (100%> Humidity). From there, the pulp arrives after being saturated with the gas mixture a heated bleaching tower made of ceramic material or stainless steel and remains in it about one Hour. The pulp is then removed from the bottom and washed. The white obtained is 88% without a noticeable drop in the degree of polymerization has occurred. The stainless steel parts show no corrosion.

Example 3

roo kg may contain cotton fabric is treated with a 2.5%> by weight solution of calcium chloride which still sodium nitrate, padded on a pad-mangle, ioo squeezed to ⁰ / "humidity and rolled up. The roll of fabric is then placed on a closed jigger, which is provided with overhead guide rollers. In the jig trough there are 100 liters of boiling 3% nitric acid, which still contains ι g / l urea. In the boiling solution is allowed to gradually 900 cm ³ of formic acid (90 ⁰ mg) were run, as the fabric in the vapor space (outside the liquor) runs 4 passages in the change-speed. A well-bleached fabric is obtained and no corrosion is observed.

Example 4

Viscose staple fiber is continuously on Edel- iao desulphurized steel mesh belts, washed and then bleached. On the running main web is the fiber fleece is soaked by sprinkling with a 0.25% sodium chlorite solution "and to 130% Moisture is squeezed off .. On the following sieve belt

steam that was developed in a special vessel from a 5% nitric acid, Steamed for 4 minutes. Washing and finishing is carried out on further sieve belts. You get one high degree of whiteness without a drop in the degree of polymerisation and without corrosion of the stainless steel parts.

Claims (1)

PATENT CLAIM: Method of preventing the corrosion of ίο Stainless steel through chlorine dioxide in gaseous form in Presence of acid vapors, thereby marked. draws that one in the existing or containing stainless steel apparatus in addition to chlorine dioxide or chlorites, one produced by evaporation of a mixture of nitric acid and introduces vapor mixture produced with water vapor volatile acids. Referred publications:
British Patent No. 696967;
Belgian patent specification No. 500 190. © 509 703 4.56