CS252620B1 - A method for removing chromium from an amalgam electrolysis system - Google Patents

Abstract

The method solves the problem of deep removal of chromium from the amalgam electrolysis system by introducing iron ions and hydrochloric acid into the brine stream before alkalization or dechlorination in an amount sufficient to ensure that the iron concentration in the brine after its addition reaches 100 to 1,000 times the chromium concentration and the pH is in the range < 0.5 to 1.5.

Description

(54) A method for removing chromium from an amalgam electrolysis system

The method solves the problem of deep chromium removal from the amalgam electrolysis system by introducing into the brine stream prior to alkalization or respiration. prior to dechlorination, it introduces iron ions and hydrochloric acid in an amount sufficient to cause the iron concentration in the brine after dosing to reach 100 to 1000 times the chromium concentration and a pH in the range <0.5 to 1.5.

The invention relates to a process for removing chromium from an amalgam electrolysis system.

One known process for the industrial production of alkali metal, chlorine and hydrogen hydroxides consists in the electrolysis of an aqueous solution of alkali metal chlorides, the so-called brine, in mercury cathode electrolysers, on which the alkali metal forming amalgam is precipitated, leaving chlorine on the anode. The resulting amalgam is decomposed with water into the corresponding hydroxide and hydrogen in a separate reactor. The alkali-depleted brine is continuously removed from the electrolysers, acidified to a pH of about 2.5, frees most of the dissolved chlorine by vacuum or purging (so-called dechlorination), drains with solid salt (NaCl or KCl) and frees Mg ²⁺ , Ca ²⁺ , Fe ³⁺ , SO ₄ ^2- etc.

These ions are removed by alkalinizing the brine with an alkali metal hydroxide and adding other chemicals such as sodium carbonate, resp. Potassium chloride, barium chloride or carbonate, calcium chloride, etc. Depending on the purification option selected. The resulting precipitates of insoluble hydroxides, carbonates and sulphates are removed in the next step by settling and filtration. The brine is then usually acidified to a pH of 4 to 6 and fed again to the electrolysers. The brine flow is continuous.

However, this commonly used procedure does not sufficiently remove the ions of tremor from the brine, which are reduced to a metal that does not dissolve in the mercury at the cathode. Undesirable hydrogen is deposited on the surface of the reduced chromium particles during electrolysis, since the hydrogen overvoltage on the chromium is considerably lower than on the alkali metal amalgam. Hydrogen excretion at the cathode has a number of known adverse consequences, the most important of which is the increased specific energy consumption (kwh / t NaOH and kwh / t KOH).

Some published procedures, such as US Patent No. 4,155,819 and USSR AO 643,430, suggest the removal of harmful metals by iron ions. According to the US patent, Fe ²⁺ is added to the brine in an amount of 1 to 10 times greater than the concentration of harmful metals, then the brine is acidified to a pH of 1.5 to 4.5 and then basified to a pH of 6 to 8. when alkalinized, it does not admit sufficiently high pH necessary to precipitate Mg ²⁺ (pH 10).

Indeed, at a higher pH, the heavy metal removal efficiency is lost in this process. Practical use of this method would require the introduction of an additional separation node. According to a second invention, the harmful metals are removed by Fe ³⁺ ions by precipitation at a pH of 3 to 6. The ferric hydroxide formed absorbs the harmful metals. By this procedure, a concentration of Cr 10 ppb is obtained, which is still too high for some electrolysis.

Another possible way to remove the tremorous ions from the brine is by the formation of poorly soluble chromium hydroxide CrlOHlj. However, due to the oxidative action of chlorine and chlorine compounds present in brine, most of the chromium ions are present in the form of CrO 4. Therefore, these ions must first be reduced to Cr ³ ions. The use of reducing agents, such as soluble SO ² , S ² O ² , S ² ions, is very costly due to the large volumes of brine to be treated and the considerable concentrations of chlorine and chlorine oxidants present in the brine.

The solution of the above mentioned problem appears to be a method of removing chromium from the amalgam electrolysis system, in which the circulating aqueous solution of alkali metal chloride, i.e. brine, is acidified, dechlorinated, saturated with alkali metal chloride, alkalized after leaving the electrolysers, separated and the brine returned back to the electrolysers of the present invention, which comprises introducing into the brine stream, prior to alkalization, iron ions and hydrochloric acid in an amount sufficient to bring the iron concentration in the brine to 100-1000 times the concentration of astonishment and pH was in the range of (0.5 to 15). The iron ions may be introduced into the brine stream in the form of chlorides or sulfate of Fe ²⁺ and / or Fe ³⁺ , either continuously or for a limited time depending on the nature of the chromium input into the system.

The iron ions can also be released into the brine stream from the steel bottom of the cells, which are for this purpose grouped out individually or individually from the direct current electrical circuit by external electrical connection of their cathodes to the anodes for a limited period of time. Iron ions and hydrochloric acid can be introduced into the brine stream before deohlorination.

The advantage of the invention is that without the inclusion of others, a sufficient reduction in the chromium content of the brine is achieved without compromising the subsequent purification processes.

investment-intensive operations with this technology were

The process according to our invention uses to reduce CrO ₄ chloride ions, which are always present in brine in high concentration. As can be seen from equation (1)

CrO 4 + 6 Cl + 16 H ₃ O ⁺ = 2 Cr ³⁺ + 3 Cl ₂ + 24 H ₂ O (1) must be acid, resp. a concentration of H ₃ O ⁺ ions sufficient to produce a concentration of Cr ³⁺ ions which, in the subsequent alkalization, exceeds the solubility product of chromium hydroxide. The reaction is also facilitated by reducing the chlorine concentration in the brine. Practical tests have shown that the required brine pH is in the range <0.5 to 1.5).

It is probable that both xFe ³⁺ + Cr ³⁺ + 3 (x + 1) OH = / Fe (OH) ₃ / _x plays a significant role in both acidification according to reaction (1) and subsequent alkalinization of reaction (2). Cr (OH) ₃ (2) role of reaction rate as well as increased acidity, respectively. the alkalinity of the brine at the dosing point of the concentrated acid or hydroxide.

In order to increase the efficiency of chromium hydroxide separation, the concentration of iron ions in the acidified brine is adjusted to 100 to 1000 times the chromium concentration according to our invention.

The alkalization then co-precipitates the iron and chromium hydroxides according to equation (2). The coprecipitate to some extent prevents the oxidation of chromium hydroxide back to CrO ₄ ions, which can occur according to equation (3)? / Fe (OH) ₃ / _x . Cr (OH) ₃ J + 3 CIO + 4 OH = 2 x Fe (OH) ₃ + 2 CrO + 3 Cl + 5 H ₂ O (3)

The necessary iron ions can be introduced into the brine in the form of chlorides or sulphates. Whether the salts of Fe ²⁺ or Fe ³⁺ ions are involved, since the oxidizing capacity of the brine is sufficient to always produce ferric hydroxide.

If the chromium removal is carried out only occasionally, it is advantageous to use as a source of Fe ions the steel bottom of the electrolysers, which are discarded by short-circuit couplers from the electric circuit while stopping the mercury circulation. This will expose the bottom of the cells and anodically dissolve the iron. The depletion of the bottom material is negligible due to its thickness.

The formed concentration of Fe ions in the brine is sufficient to co-precipitate the astonishment eventually released from the mercury after its circulation has been restored. It is advisable to carry out the shutdown of the electrolysers in succession in groups for a limited period of time and to restart the shutdown of the electrolysers in such a way that all electrolysers are subject to this procedure. In this case too, it is necessary to acidify the brine to a pH <0.5 to 15 before alkalizing the brine.

Examples

Example 1

NaCl electrolysis, current density 5 kA / m ² , Na concentration in Hg 0,5% w / w, Cl ₂ content in 0,02 g / l dechlorine brine, ClO-j "content in brine 1 to 2 g / l, brine flow 250 m ³ / h, anode partly Ti, partly graphite, Cr content in brine 10 ppb, H2 content in Cl2 2% vol. Cr input with NaCl salt. At a reduced brine flow rate of 120 m ³ / h to increase settling efficiency, 7 kg of FeCl ₃ .6H ₂ O was metered into the brine over 75 minutes, while maintaining the pH of the brine at pH 1 to 1.4 for 6 hours prior to dechlorination. After a previous increase in H ₂ content in Cl ₂ to 2.5% by volume, the H ₂ content decreased after 3 hours to 0.7% by volume.

The Cr content in the brine dropped from the original 10 ppb to 2 ppb in 24 hours. The voltage on the graphite anode electrolysers dropped by 0.1 V, on the Ti anode electrolysers by 0.03 V. The current yield increased by 3%. Specific electricity consumption decreased by approximately 5%.

Example 2

Electrolysis of KCl, current density 4 kA / m ² , K concentration in Hg 0.2% by weight, Cl2 content in brine after dechlorine 0.1 g / l, ClO3 content in brine 4 to 6 g / l, brine flow 70 m ³ / h, graphite anodes, Cr content in brine 20 ppb, H2 content in Cl ₂ 3% Cr by volume permanently present in KCl salt.

Dosing 1.5 kg of FeCl3 is continuously administered. 6 H ₂ O in 24 hours into the brine at the outlet of the electrolysers, the pH of the brine before dechloro is maintained at 1.3 at normal flow. The concentration of Fe in the brine at the outlet of the cells after two days was set at values of 0.5 to 5 mg Fe / l and the H ₂ content in Cl ₂ to 1-2% by volume. The Cr content in the brine decreased from the original 10 ppb to 5 ppb . The voltage on the electrolysers dropped by 0.1 and the cathodic current yield increased by 6%. Specific electricity consumption decreased by 7%. The rate of loss of the graphite anode thickness was about half. The consumption of hydrochloric acid increased from the usual 80 t / month to 120 t / month.

Claims (4)

OBJECT OF THE INVENTION A process for removing chromium from an amalgam electrolysis system, in which the circulating aqueous solution of an alkali metal chloride, i.e. brine, is acidified, dechlorinated, saturated with an alkali metal chloride, alkalized, the resulting entrapment is separated and the brine is returned to the electrolysers, by introducing into the brine stream, prior to alkalization, iron ions and hydrochloric acid in an amount sufficient to achieve an iron concentration in the brine of 100 to 1000 times the chromium concentration after its dosing and a pH in the range of ¢ 0.5 to 1, 5). 2. A process according to claim 1, characterized in that the iron ions are introduced into the brine stream in the form of Fe ²⁺ and / or Fe ³⁺ chlorides or sulfates continuously or for a limited time depending on the nature of the chromium input into the system. 3. A process according to claim 1, characterized in that the iron ions are released into the brine stream from the steel bottom of the electrolytic cells which are grouped or individually discarded from the direct current electrical circuit by external electrical connection of their cathodes to the anodes for a limited time. it also interrupts the mercury circulation. ⁴ 4. The process of claim 1 wherein the iron ions and hydrochloric acid are introduced into the brine stream prior to dechlorination.