RU2100288C1 - Method of removing arsenic from waste waters and aqueous solutions of alkali metals - Google Patents

Abstract

FIELD: water treatment. SUBSTANCE: arsenic-containing solutions are treated in vortex electromagnetic field with iron salt in corresponding alkali solution and simultaneously introducing them into reaction chamber of velocity layer apparatus, which results in formation of difficultly soluble precipitate containing ferric arsenate and ferric hydroxide wherein content of arsenic ranges from 3.5 to 24.5 wt %. Dissolved arsenic compounds are isolated from water by means of adsorption of ferric oxohydrate formed in water being treated in velocity layer. Once settled down and coagulated, solid and liquid phases are separated. EFFECT: improved procedure. 1 dwg

Description

 The invention relates to chemical technology, in particular to the purification of solutions of salts of alkali metals and aqueous effluents from arsenic and can be used to obtain technically pure sodium chloride, suitable for electrolysis, as well as to return the effluents to the process.

In the chemical and other related industries, the formation of significant amounts of arsenic-contaminated liquid and solid wastes, such as NaCl, KCl, etc., which are included in the range of secondary material resources and require deep cleaning for further use, not only deep (at the level of 10 ^-6 wt.) the degree of purification from arsenic, but also the reliable fixation of arsenic-containing waste to prevent the emission of arsenic into the environment.

Known methods for the extraction of arsenic from solutions, such as electrolysis, flotation, sorption, extraction, reagent deposition, etc. The literature review [1] describes in sufficient detail methods that allow you to remove arsenic from aqueous solutions to a MPC of 0.05 mg / l (which corresponds to 10 ^-6 wt.) at certain values of the initial concentrations of arsenic in solution, pH and other parameters. For the purification of aqueous solutions containing a small amount of arsenic (<1 g / l), methods of reagent precipitation of it in the form of sparingly soluble compounds: arsenites and arsenates of polyvalent metals Fe, Ca, Ni, Al, Pb, Zn have gained practical importance. The neutralization process is carried out, as a rule, in two stages: first, the bulk of the arsenic is removed (most often by the calcareous method), and then the arsenic is precipitated to a MAC value in some other way. The method of reactive precipitation with iron salts is used, usually at the stage of post-treatment, when the effluent contains a small amount of arsenic. This is explained by poor filtration of arsenic-iron-containing sediments [5]
Currently known and widely used is the method of coprecipitation of impurities with organic and inorganic collectors, that is, substances, during the deposition of which simultaneously precipitates and removes impurities [2] This method provides a very high degree of purification, unattainable in conventional deposition. The most frequently used and one of the best is coprecipitation with iron hydroxide [4] It is obvious that the highest rate of a chemical process involving a solid phase corresponds to the case of the maximum contact area of the phases (ceteris paribus), that is, effective mixing is necessary.

 There is a method [3] for cleaning aqueous solutions of arsenic by treatment with chlorine (in the form of chlorine water) and a metal salt solution forming hydroxide flakes in a weakly alkaline medium, for example, with ferrous sulfate, followed by stirring and separating the precipitate formed by filtration.

 In [6], a method was described for isolating arsenic from acidified aqueous solutions with a solution of iron compounds containing ≅ 10 wt.h. Fe per 1 part by weight As, which in the process is oxidized with air or another oxidizing agent to Fe (III) at a pH of 1.5 to 3 (optimally 2 to 2.6). The process is conducted in the apparatus and with a mixer, where arsenic is precipitated in the form of high-quality iron arsenate.

The method [7] allows you to remove arsenic from the solution with a salt of iron (III) in an amount of 0.3 to 3 parts by weight with respect to As, followed by neutralization with alkali to pH 4 6 and further flotation separation of the precipitate, which consists of iron arsenate FeAsO ₄ and its hydroxide Fe (OH) ₃ .

A known method [8] for the removal of arsenic compounds from industrial wastewater with ferrous salt and an oxidizing agent (H ₂ O ₂ and others) with the aim of coprecipitation of impurities and oxidation of As (III) to pH ≅5. Arsenic compounds co-precipitate together with Fe (OH) ₂ and Fe (OH) ₃ . There is a high economic efficiency of the method due to the low consumption of chemicals and the small amount of precipitate formed.

 All the methods described above have disadvantages, the main of which are the complexity of the hardware design and their specificity with respect to wastes of a certain composition, an excess of the precipitating reagent with respect to stoichiometry, in addition, a constant cleaning efficiency: often a sufficient degree of purification is not provided and the solution requires additional deposition.

 Closest to the described method in its technical essence and the achieved effect is the method [9] for the treatment of wastewater containing arsenic and organic pollution. The method involves reactive precipitation of arsenic (V) with an iron (III) salt in the presence of alkali at pH 6 8. As a result, a precipitate is formed containing arsenic. The liquid phase does not contain arsenic. The order of operations is as follows: first, an iron (II) or (III) salt is added to the aqueous solution containing arsenic, then an oxidizing agent to convert As (III) to As (V) and oxidize Fe (II) to Fe (III), then after The oxidation process is added to the water with an alkali to pH 6 8. The neutralized water is fed into the process of separation of solid and liquid phases.

The described method is simple, but periodic and time-consuming (approximately 30-60 minutes). The salt content of the liquid phase is increased as a result of the formation of NaCl or Na ₂ SO ₄ (depending on which acid the pH was adjusted), an increased consumption of iron-containing reagent compared with stoichiometric.

 The objective of the invention is the development of an optimal economic process for the maximum removal of arsenic from aqueous solutions of any concentration, as a result of which the effluents discharged or returned to the technological process do not contain arsenic, reducing the duration of the process, the consumption of reagents, immobilizing the arsenic-containing precipitate formed.

The essence of the proposed method for the purification of aqueous solutions of arsenic is that for maximum extraction of arsenic from solutions with any initial concentration of As, they are added depending on the initial pH value:
1) for acidic solutions, an iron (II) salt and an oxidizing agent (for example, H ₂ O ₂ , etc.), the corresponding acid (for example, HCl, H ₂ SO ₄ ) to adjust the pH to 4 6. The resulting solution is fed into the reaction chamber of the apparatus vortex layer simultaneously with the stoichiometrically necessary amount of alkali (NaOH, KOH); moreover, As Fe 1 5-9;
2) for alkaline solutions, an oxidizing agent (for example, H ₂ O ₂ ), acid (for example, HCl, H ₂ SO ₄ ) to adjust the pH to 8 10, the stoichiometrically necessary amount of alkali (NaOH, KOH). The resulting alkaline solution simultaneously with the required amount of iron-containing reagent (solution of iron (III) salt (for example, FeCl ₃ • 6H ₂ O) is fed into the reaction chamber of the vortex layer apparatus. Moreover, the atomic ratio between arsenic and iron is selected As Fe 1 5-17.

As a result of intense mixing, induction heating to 70 - 90 ^o C and exposure to all positive factors of the vortex layer, all reactions: oxidation-reduction, substitution, neutralization, partial electrolysis of water, etc. go to the end. Directly in the system, iron (III) hydroxide is formed with a molecular degree of dispersion, and then a gel of a given structure with the necessary properties is obtained, which allows almost all arsenic to be deposited. Reagents can be used both in solid form and in the form of aqueous solutions. To separate the solid and liquid phases, the suspension enters the sump, then centrifugation. The precipitate contains FeAsO ₄ and Fe (OH) ₃ . The proposed method involves the preliminary oxidation of As (III) to As (V) s and sorption coprecipitation of already pentavalent arsenic together with iron hydroxide. To intensify the implementation of the method, a method for processing solutions in a vortex layer of ferromagnetic particles was used, which is created by exposing them to a rotating magnetic field. It is known to use vortex layer devices (ABC) as chemical reactors for the intensification of a number of technological processes [10] in particular, for wastewater treatment from Ni, Cr, Pb due to the complex effect of intensive mixing and dispersion on the processed substances, acoustic and electromagnetic processing friction, electrolysis. ABC is a steel flow apparatus with a capacity of 15–40 m ³ / h (depending on size). Under the action of the vortex layer, intensive mixing, heating and grinding of the reagents to a colloidal degree of dispersion occurs, which allows maximum purification in a short time and continuous processing. The resulting metal hydroxides are more dispersed than the hydroxides obtained in apparatuses with mechanical stirring.

An increase in the dispersion of sediments does not slow down their sedimentation. Conversely, the precipitation of solid particles after the reaction in ABC occurs at 1.5
2 times faster than after a reaction in an apparatus with a stirrer [10]
In the studies, real technological effluents and water-salt solutions of sodium chloride formed in the technology of decontamination and processing of lewisite were subjected to purification from arsenic by the proposed method and used ABC type B 150 K-07 with a productivity of 15 m ³ / h with a working area diameter for this purpose 126 mm manufactured by the Poltavkhimmash plant (GOST 15150-69). The table shows the results of purification of real technological solutions of an alkaline nature from arsenic using ABC.

 Tests of the cleaning method using ABC showed that at the sensitivity level of the analysis procedure (GOST 4152-81), residual arsenic is not detected in the liquid phase, and beyond the proposed parameters (processing time, reagent ratio), a complete positive effect is not achieved. Thus, the distinguishing features of the proposed method are: the order of operations, temperature, processing time, pH, the use of the vortex layer apparatus, the simultaneous supply of the initial solution, iron-containing reagent and alkali to the reactor.

 The quality of solution purification depends on the exact stoichiometric ratio of reagents relative to arsenic in the initial solution, as well as on the exposure time of the gel-like suspension after ABC in the sump for the formation of a precipitate structure (≈6 8 h) and, of course, on the sorption capacity of the resulting precipitate in arsenic (≈30 mg / g).

 It must be emphasized that the method of producing hydroxide, temperature, pH, and other factors are so important that changing one or more of them leads to the production of products that differ markedly in their physical and physicochemical properties, in particular, in sorption capacity.

The most effective is the use of the proposed purification method for the extraction of arsenic from solutions of alkali metal salts (NaCl, HCl, Na ₂ SO ₄ , K ₂ SO ₄ ), since a salt (for example, additionally formed as a result of the formation of Fe (OH) ₃ hydroxide) NaCl) does not interfere, but simply increases the concentration of the initial saline solution.

For the purification of alkali metal salts (in particular, NaCl), it was proposed to dissolve them in weakly alkaline aqueous solutions and to further process already aqueous solutions. The procedure is as follows: dissolve the initial NaCl salt in water or in 1-2% alkali to approximately 15% concentration, then oxidize the As (III) solution in solution to As (V) with 32% hydrogen peroxide H ₂ O ₂ , then adjust the pH of the solution to 8 with alkali NaOH or HCl, if necessary. In the next step, the necessary amount of alkali is added stoichiometrically, and then the prepared alkaline salt solution and the iron-containing reagent (FeCl ₃ • 6H ₂ O) are simultaneously fed into the working chamber of the vortex layer apparatus for processing at 70–90 ^° C (for ABC type B 150 K-07 processing time is 30 to 40 s).

After settling and centrifugation, the liquid phase (approximately 25 wt. NaCl brine) contains virtually no arsenic, iron and organic contaminants. According to its indicators, after partial evaporation to 30% concentration, it meets the requirements for NaCl brine for the diaphragm electrolysis method (STP 5-12-82) VPO "Caustic", Volgograd:
Mass concentration:
NaCl 310 315 g / dm ³
Ca ^{2+ ions} not more than 5 mg / dm ³
Mg ^{2+ ions} not more than 1 mg / dm ³
Sulfate ion SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ no more than 5 mg / dm ³
Fe ³⁺ ions not more than 0.1 mg / dm ³
Transparency not less than 96%
The optimal conditions for conducting the process in a vortex electromagnetic field were found.

The processing time is limited by the time during which the reaction mass is heated to 70 90 ^o C (optim. 80 ^o C) in the working chamber of the vortex layer apparatus. For ABC type B 150 K-07, this time is 30 40 s. For comparison: in devices with mixers, this time is 30-60 minutes or more. When conducting the deposition process in devices with mixers in most cases, the deposition of 1 wt. fractions of arsenic consumed from 100 to 250 wt. fractions of an iron-containing reagent [4] In our case, the reagents are consumed according to an exact stoichiometric calculation, with As Fe 1 5-17 depending on the initial concentration of arsenic in the solution.

The use of devices with mechanical mixing devices require large production areas and significant capital costs. In contrast, the complete set with ABC for wastewater and aqueous solutions by coprecipitation on metal hydroxides includes only tanks for reagents (NaOH, HCl, FeCl ₃ • 6H ₂ O), one ABC and a sludge collector. A special feature is the simultaneous supply of reagents to the ABC working area. The processing time can be from hundredths of a second to several tens of seconds.

 Schematic diagram of the installation using ABC for the treatment of wastewater and aqueous solutions by reagent deposition is shown in the drawing.

 The installation consists of a collection-averager 1, pump 2, metering tanks (3-5), a device with a vortex layer 6, and a sludge collector-settler 8.

 The application of the proposed method can significantly simplify the process, reducing the time by approximately 240 times, thereby allowing the process to be conducted continuously.

 The proposed cleaning method is the most rational of the known and can be effectively used in industry. Its undoubted advantage is the simplicity and speed of the required cleaning depth.

The operations used are easily realized in a continuous mode, and the method itself is favorably distinguished by the possibility of immobilization and further utilization of the formed iron arsenate, which can be easily converted to Na ₃ AsO ₄ by leaching with NaOH and then through an ion exchange column to technical arsenic acid H ₃ AsO ₄ [11] being a commercial product. In addition, the precipitate may be suitable for the preparation of zeolites [12] and ion exchangers [13]
The precipitate after ABC contains, by weight.

3,5≅As≅24,5
37≅Fe≅43
depending on the deposition conditions. The method is applicable equally effectively for both concentrated and weakly concentrated solutions with respect to arsenic.

 The deposition of arsenic from aqueous solutions is carried out by the adsorption mechanism, while the possibility of the reverse transition of arsenic to solution under favorable conditions is excluded. From this it is clear that complete immobilization of arsenic is achieved and the storage of such sediments is almost safe.

The data [14] confirm this: the solubility product for salt
FeAsO ₄ • 8Fe (OH) ₃ : PR [Fe ³⁺ ] · [AsO $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4}}$ ] = (5.7 ± 4.2) • 10 ^-21
This means that this sediment can be buried, and in this case, the pollution of natural waters with arsenic due to leaching it from the sediments is minimized.

 According to the “Interim Classifier of Toxic Industrial Wastes" [15] Similar precipitation can be attributed to hazard class 4.

Example 1. In the initial aqueous solution of NaCl (15 wt.) 0.5 l with an arsenic content of 2520 mg add H ₂ O ₂ peroxide (32%) in an amount of 2.7 ml, adjust the pH of the solution with alkali NaOH or hydrochloric acid to pH 8 NaOH is added, 20 g, then the solution is fed into the working chamber of the vortex layer apparatus simultaneously with the FeCl ₃ • 6H ₂ O iron salt (46 g). Processing in ABC type B 150 K-07 is carried out for 30-40 s, then the suspension is poured into a sludge collector, where coagulation of sediment occurs. The clarified solution (NaCl brine approximately 25%) contains 1 • 10 ^-6 % As, not more.

The precipitate contains
As≥10
Fe≥45
NaCl≈2
Example 2. To 270 g of an initial aqueous solution of NaCl with an arsenic content of 850 g, 4 g of NaOH (dry) and 1 ml of H ₂ O ₂ (32%) are added, the pH of the solution is adjusted to 8 with HCl-acid, and then fed to the working chamber ABC type B 150 K-07, simultaneously with the iron salt FeCl ₃ • 6H ₂ O (27.5 g) and alkali NaOH (12.2 g). After treatment for 30 s, the suspension is poured into a sludge collector for settling coagulation. The clarified solution (NaCl brine approximately 25%) contains 1 • 10 ^-6 % arsenic. Sludge dehydrated (sediment) contains,
As≥7
Fe≥48
NaCl≈2
Example 3. To 0.5 l of wastewater with an arsenic content of 700 mg / l add 0.6 g NaOH (dry) and 1 ml of H ₂ O ₂ (32%), adjust the pH of hydrochloric acid (33%) to 8, then served in a working chamber ABC type B 150 K-07 simultaneously with the iron salt FeCl ₃ • 6H ₂ O (11.5 g) and alkali (5 g). After treatment for 35 s, the suspension is poured into a sludge collector-settler for coagulation of sediment. The clarified layer contains 1 • 10 ^-6 % of arsenic, not more. The precipitate contains
As≥7
Fe≥48
Example 4. To 0.5 l of acidic wastewater (pH 4) with an arsenic content of 500 mg / l, 9.33 g of FeSO ₄ • 7H ₂ O and 0.6 ml of H ₂ O ₂ peroxide (32%) are added, then the resulting solution is fed into the working chamber of ABC type B 150 K-07 simultaneously with alkali NaOH (42%) 8 g. After processing for 35 s, the suspension is poured into a sludge collector for sediment maturation. The clarified layer contains 1 • 10 ^-6 wt. arsenic, no more.

The precipitate contains
As≥6
Fe≥48
Thus, the use of the proposed method provides stable extraction of arsenic from aqueous solutions from selected conditions and the overall intensification of the process, reduces the duration of the reagent deposition process from several hours to several tens of seconds and simplifies the design of the process by replacing the stirred reactor with a modern production vortex layer apparatus continuous action. Using this method together allows you to get the gels of iron hydroxide (III) with a given structure and a given sorption capacity depending on the pH and concentration of arsenic in the treated solution.

 Literature.

 1. Concern L. N. Gulkin L. N. Nuzhdin A.I. Chemical Industry, 1988, N 10, p. ten.

 2. Gorshtein G.I. Kumanev G.A. Kifarova G. / Transactions of IREA, 1963, v. 25, p. 104.

 3. Patent GDR N 69089, class. 85 B 2/01, 1969.

 4. Nemodruk A.A. Analytical chemistry of arsenic. M. Science, 1976.

 5. UK patent N 1 502 775.

 6. Application of Japan 59-164639, 1983; ISM N 4, 1988, RZH, 1985, 20L109P.

 7. Japanese application 55-41863, 1978; RZH 1981, 3I443P; ISR N 1, 1984.

 8. Japanese application 55-31437, 1978; RZH 1981, 10I438P.

 9. Application of Japan 59-189972, 1984, ISM N 4, 1990.

 10. Logvinenko D.D. Shelyakov O.P. Intensification of technological processes in devices with a vortex layer. Kiev: Engineering, 1976.

 11. US patent N 4244927, RJ, 1981, 18L105P.

 12. USA patent N 3981970, IPM, no. 23, No. 2, 1977.

 13. Pakholkov V.S. et al. // Journal of Applied Chemistry, 1980, vol. LIII, no. 2, p. 280-285.

 14. Chukhlantsev V.G. // Journal of analytical chemistry, 1956, v. 11, p. 529.

 15. Temporary classifier of toxic industrial waste and guidelines for determining the toxicity class of industrial waste. M. Publ. Standards, 1987.

Claims (1)

The method of purification of wastewater and aqueous solutions of alkali metal salts from arsenic, including treatment with an oxidizing agent and an iron-containing reagent-precipitant in the presence of alkali, followed by separation of the precipitate, characterized in that the precipitation of the iron salt is carried out in a vortex electromagnetic field in a solution of the corresponding alkali during induction heating to 70 - 90 ^o C and pH 4 10, with an atomic ratio of arsenic and iron As Fe 1 5 17 and with the simultaneous supply of an arsenic-containing solution and an iron-containing reagent in a vortex layer, where we co-precipitate shyak in the form of iron arsenate together with iron hydroxide containing arsenic in the sediment 3.5 ≅ As ≅ 24.5 wt.

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