CS258716B1 - Method of salt brine treatment for electrolysis - Google Patents

Abstract

The solution relates to the chemical treatment of brine, which results in the precipitation of undesirable ones ions which are waterborne substances cooking salt from which the brine is being prepared. Precipitated substances are subsequently removed along with mechanical impurities. On the precipitation is used with barium compounds, wherein the adjustment takes place in two stages, first in an acidic environment, and consequent in an alkaline environment. The solution according to the invention is suitable for brine treatment before electrolysis, in which it is made into lye sodium and chlorine.

Description

The invention relates to the treatment of an aqueous solution

NaCl - brine for electrolysis to produce chlorine and NaOH.

The production of chlorine and NaOH by electrolysis of brine is one of the oldest electrochemical products and is still one of the basic production. In the course of the technology machine, it has been found that the brine must be treated prior to the electrolysis to remove some metal cations such as Fe, Ca, Mg, Al and the like from the solution. and also to adjust the sulfate ion concentration to the desired level. This is usually done by deposition of reagents, in particular NaOH, Na ₂ CO ₃ , CaCl ₂ and BaCl ₂ . Alkaline reagent systems with sulphate ion precipitators, e.g. a combination of Na ₂ CO ₃ and BaCO ₃ , the resulting pH of the solution being in the alkaline range.

From the standpoint of separation of precipitated substances and impurities, it is important that the precipitated substances are removed as completely as possible from the brine and that the precipitated particles are well filtered. It is important that these requirements are met with a relatively low consumption of the required chemicals. In addition to the effect of temperature, which is generally in the range of 60 to 70 ° C, it has been shown that increasing the alkalinity has a positive effect on these requirements. These requirements are all the more difficult because modern electrolysis processes operate with brine recirculation, while only the necessary technological quantity is taken from the recirculated brine to prevent the concentration of undesirable substances from increasing to a technologically acceptable level. For better separation of the precipitates, adjuvants such as gelatin, agar-agar or flocculants such as polyacrylamide are added to the brine.

According to the present invention, the electrolysis brine is treated by barium salt precipitation agents and subsequent separation of the precipitated substances together with mechanical impurities such that the treated brine solution, preferably by saturation of the recirculated brine, is treated at pH below 6.5 with stirring at least once barium compound in an amount of 0.7 to 1.1 moles of barium compounds, calculated on the sulphate ions present at 40 to 85 ° C and after distribution, the system is made alkaline by treatment with NaOH and 0.5 to 4 hours after addition of the precipitating agent the solid phase it is usually separated from the system by filtration.

An advantage of the process according to the invention is that the cost of precipitating unwanted ions from the brine is reduced, while the high separation of precipitated particles is achieved in a relatively short time and the treated brine has very good properties even in the electrolysis process, especially in terms of current yields.

The term brine refers to an aqueous solution of sodium chloride, sodium salt, irrespective of its geological and / or geological properties. the geographical provenance as obtained by mining or, if necessary, after its further processing. The brine mentioned in the description is intended for electrolytic treatment into sodium hydroxide and chlorine. The concentration of sodium chloride in the brine to be prepared is usually in the range of 250 to 350 g / l, but these limitations are not limiting. For the preparation of brine, it is also advisable to use recycled brine electrolysis waste, which is saturated with common salt before being used for technological purposes, although a part of it is usually discharged before its saturation in order to avoid an undesirable increase of impurities.

The salt used for the preparation of brine is neither physically nor chemically homogeneous, but contains mechanical impurities but also chemically different substances, which need to be removed from the electrolysis, or their concentration to a technologically permissible concentration level. In the process according to the invention, this treatment is effected by precipitation and subsequent separation of the solid phase. It has been shown that by using barium compounds such as barium chloride, barium hydroxide, barium carbonate, a remarkable reduction in the consumption of these auxiliary chemicals can be achieved if precipitation takes place first in an acidic environment formed by hydrochloric acid and only after distribution. into the alkaline region. Either individual barium compounds may be used for this purpose or may be combined with each other. Their dosing is usually carried out in a slurry form, the liquid phase being brine. The mixing of the brine in the treatment with the added reagent is carried out by blowing with air or other inert gas, but it can also be solved using mechanical stirrers.

The amount added is calculated from the amount of dissolved sulfates in brine, taking into account their desired final concentration.

After stirring the precipitant in brine, the system is rendered alkaline by treatment with NaOH and / or sodium carbonate, preferably resulting in a pH of 10.

After the appropriate time, usually 0.5 to 4 hours, the solid phase is separated from the system, most often by filtration, using hydrocyclones, or by simple sedimentation in conical bottom tanks. The treated brine is either led directly to the electrolysis or stored in reservoirs.

The process of the invention may conveniently be combined with some other known measures, such as e.g. the use of flocculants, the use of agglomerating agents or combinations of alkaline agents.

Specific embodiments of the process according to the invention are illustrated by the following examples.

Example 1

For electrolysis, the recycled brine is saturated to a NaCl concentration of 300 g / l. Artemian salt from the USSR is used as feedstock for saturation. The analysis of the salt used is as follows:

insoluble residue 0.08 wt.

% R ₃ O ₃ 0.023 wt.

CaO 0.293 wt.

MgO 0.034 wt.

CaSO ₄ 0.588 wt.

humidity 0.030% wt.

NaCl 98.95 wt.

To the saturated solution after heating to 70 ° C, technical hydrochloric acid is added in an amount such that the resulting solution has a concentration of C - 0.07 moles of HCl / L solution.

After this treatment, the precipitating agent is added in the form of a 3: 7 suspension of BaCl ₂ and BaCO ₃ in a total amount of 25% by weight. in brine (NaCl). The reagent is added in a molar ratio so that, after precipitation, the concentration of sulfate ions remains in an amount corresponding to 3 g of Na ₂ SO ₄ / l. After homogenization, the resulting system is basified with NaOH and Na ₂ CO ₃ to pH 10 with stirring by air purging. The solid phase from this system was completely removed by filtration. The filtrate obtained contained 3 g of Na ₃ SO ₄ / l sulfate ions and the use in electrolysis reduces the formation of solid amalgam.

Example 2

To model the efficacy of the brine treatment, a solution containing 300 g / l NaCl pa and 0.01056 g SO ₄ ² / ml is prepared. Adjust the solution to pH 4.9 with HCl. After adjusting the pH, add an equivalent amount of Ba (OH) ₂ .8 H ₂ O to precipitate 1/3 of the sulphates present. Heat the solution under reflux at 70 ° C for half an hour. After heating, determine the non-precipitated sulphates, calculated with the amount of solution C = - 0.1 mol / l BaCl ₂ and determine its excess by comploxometry. The determined value of precipitated sulphates is 120% of the theoretical amount (formation of double salts is assumed). The experiment demonstrates a reduction in the precipitation agent consumption to the desired conc. SO ₄ - ^! 20 wt.

After adjusting the pH to the alkaline region and separating the solid phase, the brine can be used for electrolysis.

Claims (2)

Process for treating brine for electrolysis by barium salt precipitation agents and subsequent separation of precipitates together with mechanical impurities, characterized in that the treated brine solution, preferably by saturation of the recirculated brine, is sieved at pH below 6.5 with stirring at least once barium compound of the invention in an amount of 0.7 to 1.1 moles of barium compounds, calculated on the sulphate ions present, at a temperature of 40 to 85 ° C and after distribution, the system is made alkaline by treatment with NaOH and 0.5 to 4 hours after addition of the precipitating agent. the solid phase is usually separated from the system by filtration.